1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2 @c 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007
3 @c Free Software Foundation, Inc.
4 @c This is part of the GCC manual.
5 @c For copying conditions, see the file gcc.texi.
12 @c man begin COPYRIGHT
13 Copyright @copyright{} 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
14 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006,
15 2007 Free Software Foundation, Inc.
17 Permission is granted to copy, distribute and/or modify this document
18 under the terms of the GNU Free Documentation License, Version 1.2 or
19 any later version published by the Free Software Foundation; with the
20 Invariant Sections being ``GNU General Public License'' and ``Funding
21 Free Software'', the Front-Cover texts being (a) (see below), and with
22 the Back-Cover Texts being (b) (see below). A copy of the license is
23 included in the gfdl(7) man page.
25 (a) The FSF's Front-Cover Text is:
29 (b) The FSF's Back-Cover Text is:
31 You have freedom to copy and modify this GNU Manual, like GNU
32 software. Copies published by the Free Software Foundation raise
33 funds for GNU development.
35 @c Set file name and title for the man page.
37 @settitle GNU project C and C++ compiler
39 gcc [@option{-c}|@option{-S}|@option{-E}] [@option{-std=}@var{standard}]
40 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
41 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
42 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
43 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
44 [@option{-f}@var{option}@dots{}] [@option{-m}@var{machine-option}@dots{}]
45 [@option{-o} @var{outfile}] [@@@var{file}] @var{infile}@dots{}
47 Only the most useful options are listed here; see below for the
48 remainder. @samp{g++} accepts mostly the same options as @samp{gcc}.
51 gpl(7), gfdl(7), fsf-funding(7),
52 cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
53 and the Info entries for @file{gcc}, @file{cpp}, @file{as},
54 @file{ld}, @file{binutils} and @file{gdb}.
57 For instructions on reporting bugs, see
61 See the Info entry for @command{gcc}, or
62 @w{@uref{http://gcc.gnu.org/onlinedocs/gcc/Contributors.html}},
63 for contributors to GCC@.
68 @chapter GCC Command Options
69 @cindex GCC command options
70 @cindex command options
71 @cindex options, GCC command
73 @c man begin DESCRIPTION
74 When you invoke GCC, it normally does preprocessing, compilation,
75 assembly and linking. The ``overall options'' allow you to stop this
76 process at an intermediate stage. For example, the @option{-c} option
77 says not to run the linker. Then the output consists of object files
78 output by the assembler.
80 Other options are passed on to one stage of processing. Some options
81 control the preprocessor and others the compiler itself. Yet other
82 options control the assembler and linker; most of these are not
83 documented here, since you rarely need to use any of them.
85 @cindex C compilation options
86 Most of the command line options that you can use with GCC are useful
87 for C programs; when an option is only useful with another language
88 (usually C++), the explanation says so explicitly. If the description
89 for a particular option does not mention a source language, you can use
90 that option with all supported languages.
92 @cindex C++ compilation options
93 @xref{Invoking G++,,Compiling C++ Programs}, for a summary of special
94 options for compiling C++ programs.
96 @cindex grouping options
97 @cindex options, grouping
98 The @command{gcc} program accepts options and file names as operands. Many
99 options have multi-letter names; therefore multiple single-letter options
100 may @emph{not} be grouped: @option{-dr} is very different from @w{@samp{-d
103 @cindex order of options
104 @cindex options, order
105 You can mix options and other arguments. For the most part, the order
106 you use doesn't matter. Order does matter when you use several
107 options of the same kind; for example, if you specify @option{-L} more
108 than once, the directories are searched in the order specified. Also,
109 the placement of the @option{-l} option is significant.
111 Many options have long names starting with @samp{-f} or with
112 @samp{-W}---for example,
113 @option{-fmove-loop-invariants}, @option{-Wformat} and so on. Most of
114 these have both positive and negative forms; the negative form of
115 @option{-ffoo} would be @option{-fno-foo}. This manual documents
116 only one of these two forms, whichever one is not the default.
120 @xref{Option Index}, for an index to GCC's options.
123 * Option Summary:: Brief list of all options, without explanations.
124 * Overall Options:: Controlling the kind of output:
125 an executable, object files, assembler files,
126 or preprocessed source.
127 * Invoking G++:: Compiling C++ programs.
128 * C Dialect Options:: Controlling the variant of C language compiled.
129 * C++ Dialect Options:: Variations on C++.
130 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
132 * Language Independent Options:: Controlling how diagnostics should be
134 * Warning Options:: How picky should the compiler be?
135 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
136 * Optimize Options:: How much optimization?
137 * Preprocessor Options:: Controlling header files and macro definitions.
138 Also, getting dependency information for Make.
139 * Assembler Options:: Passing options to the assembler.
140 * Link Options:: Specifying libraries and so on.
141 * Directory Options:: Where to find header files and libraries.
142 Where to find the compiler executable files.
143 * Spec Files:: How to pass switches to sub-processes.
144 * Target Options:: Running a cross-compiler, or an old version of GCC.
145 * Submodel Options:: Specifying minor hardware or convention variations,
146 such as 68010 vs 68020.
147 * Code Gen Options:: Specifying conventions for function calls, data layout
149 * Environment Variables:: Env vars that affect GCC.
150 * Precompiled Headers:: Compiling a header once, and using it many times.
151 * Running Protoize:: Automatically adding or removing function prototypes.
157 @section Option Summary
159 Here is a summary of all the options, grouped by type. Explanations are
160 in the following sections.
163 @item Overall Options
164 @xref{Overall Options,,Options Controlling the Kind of Output}.
165 @gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
166 -x @var{language} -v -### --help@r{[}=@var{class}@r{]} --target-help @gol
167 --version @@@var{file}}
169 @item C Language Options
170 @xref{C Dialect Options,,Options Controlling C Dialect}.
171 @gccoptlist{-ansi -std=@var{standard} -fgnu89-inline @gol
172 -aux-info @var{filename} @gol
173 -fno-asm -fno-builtin -fno-builtin-@var{function} @gol
174 -fhosted -ffreestanding -fopenmp -fms-extensions @gol
175 -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
176 -fallow-single-precision -fcond-mismatch -flax-vector-conversions @gol
177 -fsigned-bitfields -fsigned-char @gol
178 -funsigned-bitfields -funsigned-char}
180 @item C++ Language Options
181 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
182 @gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
183 -fconserve-space -ffriend-injection @gol
184 -fno-elide-constructors @gol
185 -fno-enforce-eh-specs @gol
186 -ffor-scope -fno-for-scope -fno-gnu-keywords @gol
187 -fno-implicit-templates @gol
188 -fno-implicit-inline-templates @gol
189 -fno-implement-inlines -fms-extensions @gol
190 -fno-nonansi-builtins -fno-operator-names @gol
191 -fno-optional-diags -fpermissive @gol
192 -frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
193 -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ @gol
194 -fno-default-inline -fvisibility-inlines-hidden @gol
195 -fvisibility-ms-compat @gol
196 -Wabi -Wctor-dtor-privacy @gol
197 -Wnon-virtual-dtor -Wreorder @gol
198 -Weffc++ -Wno-deprecated -Wstrict-null-sentinel @gol
199 -Wno-non-template-friend -Wold-style-cast @gol
200 -Woverloaded-virtual -Wno-pmf-conversions @gol
203 @item Objective-C and Objective-C++ Language Options
204 @xref{Objective-C and Objective-C++ Dialect Options,,Options Controlling
205 Objective-C and Objective-C++ Dialects}.
206 @gccoptlist{-fconstant-string-class=@var{class-name} @gol
207 -fgnu-runtime -fnext-runtime @gol
208 -fno-nil-receivers @gol
209 -fobjc-call-cxx-cdtors @gol
210 -fobjc-direct-dispatch @gol
211 -fobjc-exceptions @gol
213 -freplace-objc-classes @gol
216 -Wassign-intercept @gol
217 -Wno-protocol -Wselector @gol
218 -Wstrict-selector-match @gol
219 -Wundeclared-selector}
221 @item Language Independent Options
222 @xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
223 @gccoptlist{-fmessage-length=@var{n} @gol
224 -fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]} @gol
225 -fdiagnostics-show-option}
227 @item Warning Options
228 @xref{Warning Options,,Options to Request or Suppress Warnings}.
229 @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
230 -w -Wextra -Wall -Waddress -Waggregate-return -Warray-bounds @gol
231 -Wno-attributes -Wc++-compat -Wc++0x-compat -Wcast-align -Wcast-qual @gol
232 -Wchar-subscripts -Wclobbered -Wcomment @gol
233 -Wconversion -Wcoverage-mismatch -Wno-deprecated-declarations @gol
234 -Wdisabled-optimization -Wno-div-by-zero @gol
235 -Wempty-body -Wno-endif-labels @gol
236 -Werror -Werror=* @gol
237 -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol
238 -Wno-format-extra-args -Wformat-nonliteral @gol
239 -Wformat-security -Wformat-y2k -Wignored-qualifiers @gol
240 -Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
241 -Wimport -Wno-import -Winit-self -Winline @gol
242 -Wno-int-to-pointer-cast -Wno-invalid-offsetof @gol
243 -Winvalid-pch -Wlarger-than-@var{len} -Wunsafe-loop-optimizations @gol
244 -Wlogical-op -Wlong-long @gol
245 -Wmain -Wmissing-braces -Wmissing-field-initializers @gol
246 -Wmissing-format-attribute -Wmissing-include-dirs @gol
247 -Wmissing-noreturn @gol
248 -Wno-multichar -Wnonnull -Wno-overflow @gol
249 -Woverlength-strings -Wpacked -Wpadded @gol
250 -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast @gol
251 -Wredundant-decls @gol
252 -Wreturn-type -Wsequence-point -Wshadow @gol
253 -Wsign-compare -Wsign-conversion -Wstack-protector @gol
254 -Wstrict-aliasing -Wstrict-aliasing=n @gol
255 -Wstrict-overflow -Wstrict-overflow=@var{n} @gol
256 -Wswitch -Wswitch-default -Wswitch-enum @gol
257 -Wsystem-headers -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized @gol
258 -Wunknown-pragmas -Wno-pragmas -Wunreachable-code @gol
259 -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
260 -Wunused-value -Wunused-variable @gol
261 -Wvariadic-macros -Wvla @gol
262 -Wvolatile-register-var -Wwrite-strings}
264 @item C-only Warning Options
265 @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
266 -Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs @gol
267 -Wold-style-declaration -Wold-style-definition @gol
268 -Wstrict-prototypes -Wtraditional -Wtraditional-conversion @gol
269 -Wdeclaration-after-statement -Wpointer-sign}
271 @item Debugging Options
272 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
273 @gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
274 -fdbg-cnt-list -fdbg-cnt=@var{counter-value-list} @gol
275 -fdump-noaddr -fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
276 -fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
277 -fdump-ipa-all -fdump-ipa-cgraph @gol
279 -fdump-tree-original@r{[}-@var{n}@r{]} @gol
280 -fdump-tree-optimized@r{[}-@var{n}@r{]} @gol
281 -fdump-tree-inlined@r{[}-@var{n}@r{]} @gol
282 -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias @gol
284 -fdump-tree-ssa@r{[}-@var{n}@r{]} -fdump-tree-pre@r{[}-@var{n}@r{]} @gol
285 -fdump-tree-ccp@r{[}-@var{n}@r{]} -fdump-tree-dce@r{[}-@var{n}@r{]} @gol
286 -fdump-tree-gimple@r{[}-raw@r{]} -fdump-tree-mudflap@r{[}-@var{n}@r{]} @gol
287 -fdump-tree-dom@r{[}-@var{n}@r{]} @gol
288 -fdump-tree-dse@r{[}-@var{n}@r{]} @gol
289 -fdump-tree-phiopt@r{[}-@var{n}@r{]} @gol
290 -fdump-tree-forwprop@r{[}-@var{n}@r{]} @gol
291 -fdump-tree-copyrename@r{[}-@var{n}@r{]} @gol
292 -fdump-tree-nrv -fdump-tree-vect @gol
293 -fdump-tree-sink @gol
294 -fdump-tree-sra@r{[}-@var{n}@r{]} @gol
295 -fdump-tree-salias @gol
296 -fdump-tree-fre@r{[}-@var{n}@r{]} @gol
297 -fdump-tree-vrp@r{[}-@var{n}@r{]} @gol
298 -ftree-vectorizer-verbose=@var{n} @gol
299 -fdump-tree-storeccp@r{[}-@var{n}@r{]} @gol
300 -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
301 -feliminate-unused-debug-symbols -femit-class-debug-always @gol
302 -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs @gol
303 -frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
304 -ftest-coverage -ftime-report -fvar-tracking @gol
305 -g -g@var{level} -gcoff -gdwarf-2 @gol
306 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
307 -fdebug-prefix-map=@var{old}=@var{new} @gol
308 -femit-struct-debug-baseonly -femit-struct-debug-reduced @gol
309 -femit-struct-debug-detailed@r{[}=@var{spec-list}@r{]} @gol
310 -p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
311 -print-multi-directory -print-multi-lib @gol
312 -print-prog-name=@var{program} -print-search-dirs -Q @gol
313 -print-sysroot-headers-suffix @gol
316 @item Optimization Options
317 @xref{Optimize Options,,Options that Control Optimization}.
319 -falign-functions[=@var{n}] -falign-jumps[=@var{n}] @gol
320 -falign-labels[=@var{n}] -falign-loops[=@var{n}] -fassociative-math @gol
321 -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize @gol
322 -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves @gol
323 -fcheck-data-deps -fcprop-registers -fcrossjumping -fcse-follow-jumps @gol
324 -fcse-skip-blocks -fcx-limited-range -fdata-sections -fdce -fdce @gol
325 -fdelayed-branch -fdelete-null-pointer-checks -fdse -fdse @gol
326 -fearly-inlining -fexpensive-optimizations -ffast-math @gol
327 -ffinite-math-only -ffloat-store -fforward-propagate @gol
328 -ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm @gol
329 -fgcse-sm -fif-conversion -fif-conversion2 -finline-functions @gol
330 -finline-functions-called-once -finline-limit=@var{n} @gol
331 -finline-small-functions -fipa-cp -fipa-marix-reorg -fipa-pta @gol
332 -fipa-pure-const -fipa-reference -fipa-struct-reorg @gol
333 -fipa-type-escape -fivopts -fkeep-inline-functions -fkeep-static-consts @gol
334 -fmerge-all-constants -fmerge-constants -fmodulo-sched @gol
335 -fmodulo-sched-allow-regmoves -fmove-loop-invariants -fmudflap @gol
336 -fmudflapir -fmudflapth -fno-branch-count-reg -fno-default-inline @gol
337 -fno-defer-pop -fno-function-cse -fno-guess-branch-probability @gol
338 -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
339 -fno-sched-interblock -fno-sched-spec -fno-signed-zeros @gol
340 -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss @gol
341 -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls @gol
342 -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays @gol
343 -fprofile-generate -fprofile-use -fprofile-values -freciprocal-math @gol
344 -fregmove -frename-registers -freorder-blocks @gol
345 -freorder-blocks-and-partition -freorder-functions @gol
346 -frerun-cse-after-loop -freschedule-modulo-scheduled-loops @gol
347 -frounding-math -frtl-abstract-sequences -fsched2-use-superblocks @gol
348 -fsched2-use-traces -fsched-spec-load -fsched-spec-load-dangerous @gol
349 -fsched-stalled-insns-dep[=@var{n}] -fsched-stalled-insns[=@var{n}] @gol
350 -fschedule-insns -fschedule-insns2 -fsection-anchors -fsee @gol
351 -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller @gol
352 -fsplit-wide-types -fstack-protector -fstack-protector-all @gol
353 -fstrict-aliasing -fstrict-overflow -fthread-jumps -ftracer -ftree-ccp @gol
354 -ftree-ch -ftree-copy-prop -ftree-copyrename -ftree-dce @gol
355 -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-loop-im @gol
356 -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize @gol
357 -ftree-parallelize-loops=@var{n} -ftree-pre -ftree-reassoc -ftree-salias @gol
358 -ftree-sink -ftree-sra -ftree-store-ccp -ftree-ter @gol
359 -ftree-vect-loop-version -ftree-vectorize -ftree-vrp -funit-at-a-time @gol
360 -funroll-all-loops -funroll-loops -funsafe-loop-optimizations @gol
361 -funsafe-math-optimizations -funswitch-loops @gol
362 -fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb @gol
364 --param @var{name}=@var{value}
365 -O -O0 -O1 -O2 -O3 -Os}
367 @item Preprocessor Options
368 @xref{Preprocessor Options,,Options Controlling the Preprocessor}.
369 @gccoptlist{-A@var{question}=@var{answer} @gol
370 -A-@var{question}@r{[}=@var{answer}@r{]} @gol
371 -C -dD -dI -dM -dN @gol
372 -D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
373 -idirafter @var{dir} @gol
374 -include @var{file} -imacros @var{file} @gol
375 -iprefix @var{file} -iwithprefix @var{dir} @gol
376 -iwithprefixbefore @var{dir} -isystem @var{dir} @gol
377 -imultilib @var{dir} -isysroot @var{dir} @gol
378 -M -MM -MF -MG -MP -MQ -MT -nostdinc @gol
379 -P -fworking-directory -remap @gol
380 -trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
381 -Xpreprocessor @var{option}}
383 @item Assembler Option
384 @xref{Assembler Options,,Passing Options to the Assembler}.
385 @gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
388 @xref{Link Options,,Options for Linking}.
389 @gccoptlist{@var{object-file-name} -l@var{library} @gol
390 -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic @gol
391 -s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
392 -Wl,@var{option} -Xlinker @var{option} @gol
395 @item Directory Options
396 @xref{Directory Options,,Options for Directory Search}.
397 @gccoptlist{-B@var{prefix} -I@var{dir} -iquote@var{dir} -L@var{dir}
398 -specs=@var{file} -I- --sysroot=@var{dir}}
401 @c I wrote this xref this way to avoid overfull hbox. -- rms
402 @xref{Target Options}.
403 @gccoptlist{-V @var{version} -b @var{machine}}
405 @item Machine Dependent Options
406 @xref{Submodel Options,,Hardware Models and Configurations}.
407 @c This list is ordered alphanumerically by subsection name.
408 @c Try and put the significant identifier (CPU or system) first,
409 @c so users have a clue at guessing where the ones they want will be.
412 @gccoptlist{-EB -EL @gol
413 -mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
414 -mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
417 @gccoptlist{-mapcs-frame -mno-apcs-frame @gol
418 -mabi=@var{name} @gol
419 -mapcs-stack-check -mno-apcs-stack-check @gol
420 -mapcs-float -mno-apcs-float @gol
421 -mapcs-reentrant -mno-apcs-reentrant @gol
422 -msched-prolog -mno-sched-prolog @gol
423 -mlittle-endian -mbig-endian -mwords-little-endian @gol
424 -mfloat-abi=@var{name} -msoft-float -mhard-float -mfpe @gol
425 -mthumb-interwork -mno-thumb-interwork @gol
426 -mcpu=@var{name} -march=@var{name} -mfpu=@var{name} @gol
427 -mstructure-size-boundary=@var{n} @gol
428 -mabort-on-noreturn @gol
429 -mlong-calls -mno-long-calls @gol
430 -msingle-pic-base -mno-single-pic-base @gol
431 -mpic-register=@var{reg} @gol
432 -mnop-fun-dllimport @gol
433 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
434 -mpoke-function-name @gol
436 -mtpcs-frame -mtpcs-leaf-frame @gol
437 -mcaller-super-interworking -mcallee-super-interworking @gol
441 @gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
442 -mcall-prologues -mno-tablejump -mtiny-stack -mint8}
444 @emph{Blackfin Options}
445 @gccoptlist{-mcpu=@var{cpu}@r{[}-@var{sirevision}@r{]} @gol
446 -msim -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer @gol
447 -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly @gol
448 -mlow-64k -mno-low64k -mstack-check-l1 -mid-shared-library @gol
449 -mno-id-shared-library -mshared-library-id=@var{n} @gol
450 -mleaf-id-shared-library -mno-leaf-id-shared-library @gol
451 -msep-data -mno-sep-data -mlong-calls -mno-long-calls @gol
452 -mfast-fp -minline-plt}
455 @gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
456 -mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
457 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
458 -mstack-align -mdata-align -mconst-align @gol
459 -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
460 -melf -maout -melinux -mlinux -sim -sim2 @gol
461 -mmul-bug-workaround -mno-mul-bug-workaround}
464 @gccoptlist{-mmac -mpush-args}
466 @emph{Darwin Options}
467 @gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
468 -arch_only -bind_at_load -bundle -bundle_loader @gol
469 -client_name -compatibility_version -current_version @gol
471 -dependency-file -dylib_file -dylinker_install_name @gol
472 -dynamic -dynamiclib -exported_symbols_list @gol
473 -filelist -flat_namespace -force_cpusubtype_ALL @gol
474 -force_flat_namespace -headerpad_max_install_names @gol
476 -image_base -init -install_name -keep_private_externs @gol
477 -multi_module -multiply_defined -multiply_defined_unused @gol
478 -noall_load -no_dead_strip_inits_and_terms @gol
479 -nofixprebinding -nomultidefs -noprebind -noseglinkedit @gol
480 -pagezero_size -prebind -prebind_all_twolevel_modules @gol
481 -private_bundle -read_only_relocs -sectalign @gol
482 -sectobjectsymbols -whyload -seg1addr @gol
483 -sectcreate -sectobjectsymbols -sectorder @gol
484 -segaddr -segs_read_only_addr -segs_read_write_addr @gol
485 -seg_addr_table -seg_addr_table_filename -seglinkedit @gol
486 -segprot -segs_read_only_addr -segs_read_write_addr @gol
487 -single_module -static -sub_library -sub_umbrella @gol
488 -twolevel_namespace -umbrella -undefined @gol
489 -unexported_symbols_list -weak_reference_mismatches @gol
490 -whatsloaded -F -gused -gfull -mmacosx-version-min=@var{version} @gol
491 -mkernel -mone-byte-bool}
493 @emph{DEC Alpha Options}
494 @gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
495 -mieee -mieee-with-inexact -mieee-conformant @gol
496 -mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
497 -mtrap-precision=@var{mode} -mbuild-constants @gol
498 -mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
499 -mbwx -mmax -mfix -mcix @gol
500 -mfloat-vax -mfloat-ieee @gol
501 -mexplicit-relocs -msmall-data -mlarge-data @gol
502 -msmall-text -mlarge-text @gol
503 -mmemory-latency=@var{time}}
505 @emph{DEC Alpha/VMS Options}
506 @gccoptlist{-mvms-return-codes}
509 @gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
510 -mhard-float -msoft-float @gol
511 -malloc-cc -mfixed-cc -mdword -mno-dword @gol
512 -mdouble -mno-double @gol
513 -mmedia -mno-media -mmuladd -mno-muladd @gol
514 -mfdpic -minline-plt -mgprel-ro -multilib-library-pic @gol
515 -mlinked-fp -mlong-calls -malign-labels @gol
516 -mlibrary-pic -macc-4 -macc-8 @gol
517 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
518 -moptimize-membar -mno-optimize-membar @gol
519 -mscc -mno-scc -mcond-exec -mno-cond-exec @gol
520 -mvliw-branch -mno-vliw-branch @gol
521 -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
522 -mno-nested-cond-exec -mtomcat-stats @gol
526 @emph{GNU/Linux Options}
527 @gccoptlist{-muclibc}
529 @emph{H8/300 Options}
530 @gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
533 @gccoptlist{-march=@var{architecture-type} @gol
534 -mbig-switch -mdisable-fpregs -mdisable-indexing @gol
535 -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
536 -mfixed-range=@var{register-range} @gol
537 -mjump-in-delay -mlinker-opt -mlong-calls @gol
538 -mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
539 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
540 -mno-jump-in-delay -mno-long-load-store @gol
541 -mno-portable-runtime -mno-soft-float @gol
542 -mno-space-regs -msoft-float -mpa-risc-1-0 @gol
543 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
544 -mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
545 -munix=@var{unix-std} -nolibdld -static -threads}
547 @emph{i386 and x86-64 Options}
548 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
549 -mfpmath=@var{unit} @gol
550 -masm=@var{dialect} -mno-fancy-math-387 @gol
551 -mno-fp-ret-in-387 -msoft-float @gol
552 -mno-wide-multiply -mrtd -malign-double @gol
553 -mpreferred-stack-boundary=@var{num} -mcx16 -msahf -mrecip @gol
554 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 @gol
555 -msse4a -m3dnow -mpopcnt -mabm -msse5 @gol
556 -mthreads -mno-align-stringops -minline-all-stringops @gol
557 -mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
558 -m96bit-long-double -mregparm=@var{num} -msseregparm @gol
559 -mveclibabi=@var{type} -mpc32 -mpc64 -mpc80 -mstackrealign @gol
560 -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs @gol
561 -mcmodel=@var{code-model} @gol
562 -m32 -m64 -mlarge-data-threshold=@var{num} @gol
563 -mfused-madd -mno-fused-madd}
566 @gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
567 -mvolatile-asm-stop -mregister-names -mno-sdata @gol
568 -mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
569 -minline-float-divide-max-throughput @gol
570 -minline-int-divide-min-latency @gol
571 -minline-int-divide-max-throughput @gol
572 -minline-sqrt-min-latency -minline-sqrt-max-throughput @gol
573 -mno-dwarf2-asm -mearly-stop-bits @gol
574 -mfixed-range=@var{register-range} -mtls-size=@var{tls-size} @gol
575 -mtune=@var{cpu-type} -mt -pthread -milp32 -mlp64 @gol
576 -mno-sched-br-data-spec -msched-ar-data-spec -mno-sched-control-spec @gol
577 -msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec @gol
578 -msched-ldc -mno-sched-control-ldc -mno-sched-spec-verbose @gol
579 -mno-sched-prefer-non-data-spec-insns @gol
580 -mno-sched-prefer-non-control-spec-insns @gol
581 -mno-sched-count-spec-in-critical-path}
583 @emph{M32R/D Options}
584 @gccoptlist{-m32r2 -m32rx -m32r @gol
586 -malign-loops -mno-align-loops @gol
587 -missue-rate=@var{number} @gol
588 -mbranch-cost=@var{number} @gol
589 -mmodel=@var{code-size-model-type} @gol
590 -msdata=@var{sdata-type} @gol
591 -mno-flush-func -mflush-func=@var{name} @gol
592 -mno-flush-trap -mflush-trap=@var{number} @gol
596 @gccoptlist{-mcpu=@var{cpu} -msim -memregs=@var{number}}
598 @emph{M680x0 Options}
599 @gccoptlist{-march=@var{arch} -mcpu=@var{cpu} -mtune=@var{tune}
600 -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
601 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 @gol
602 -mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020 @gol
603 -mnobitfield -mrtd -mno-rtd -mdiv -mno-div -mshort @gol
604 -mno-short -mhard-float -m68881 -msoft-float -mpcrel @gol
605 -malign-int -mstrict-align -msep-data -mno-sep-data @gol
606 -mshared-library-id=n -mid-shared-library -mno-id-shared-library}
608 @emph{M68hc1x Options}
609 @gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
610 -mauto-incdec -minmax -mlong-calls -mshort @gol
611 -msoft-reg-count=@var{count}}
614 @gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
615 -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
616 -m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
617 -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
618 -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
621 @gccoptlist{-EL -EB -march=@var{arch} -mtune=@var{arch} @gol
622 -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
623 -mips16 -mno-mips16 -mflip-mips16 @gol
624 -minterlink-mips16 -mno-interlink-mips16 @gol
625 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
626 -mshared -mno-shared -mxgot -mno-xgot -mgp32 -mgp64 @gol
627 -mfp32 -mfp64 -mhard-float -msoft-float @gol
628 -msingle-float -mdouble-float -mdsp -mno-dsp -mdspr2 -mno-dspr2 @gol
629 -msmartmips -mno-smartmips @gol
630 -mpaired-single -mno-paired-single -mdmx -mno-mdmx @gol
631 -mips3d -mno-mips3d -mmt -mno-mt -mllsc -mno-llsc @gol
632 -mlong64 -mlong32 -msym32 -mno-sym32 @gol
633 -G@var{num} -mlocal-sdata -mno-local-sdata @gol
634 -mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt @gol
635 -membedded-data -mno-embedded-data @gol
636 -muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
637 -mcode-readable=@var{setting} @gol
638 -msplit-addresses -mno-split-addresses @gol
639 -mexplicit-relocs -mno-explicit-relocs @gol
640 -mcheck-zero-division -mno-check-zero-division @gol
641 -mdivide-traps -mdivide-breaks @gol
642 -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls @gol
643 -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp @gol
644 -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 @gol
645 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130 @gol
646 -mfix-sb1 -mno-fix-sb1 @gol
647 -mflush-func=@var{func} -mno-flush-func @gol
648 -mbranch-cost=@var{num} -mbranch-likely -mno-branch-likely @gol
649 -mfp-exceptions -mno-fp-exceptions @gol
650 -mvr4130-align -mno-vr4130-align}
653 @gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
654 -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
655 -melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
656 -mno-base-addresses -msingle-exit -mno-single-exit}
658 @emph{MN10300 Options}
659 @gccoptlist{-mmult-bug -mno-mult-bug @gol
660 -mam33 -mno-am33 @gol
661 -mam33-2 -mno-am33-2 @gol
662 -mreturn-pointer-on-d0 @gol
666 @gccoptlist{-mno-crt0 -mbacc -msim @gol
667 -march=@var{cpu-type} }
669 @emph{PDP-11 Options}
670 @gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
671 -mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
672 -mint16 -mno-int32 -mfloat32 -mno-float64 @gol
673 -mfloat64 -mno-float32 -mabshi -mno-abshi @gol
674 -mbranch-expensive -mbranch-cheap @gol
675 -msplit -mno-split -munix-asm -mdec-asm}
677 @emph{PowerPC Options}
678 See RS/6000 and PowerPC Options.
680 @emph{RS/6000 and PowerPC Options}
681 @gccoptlist{-mcpu=@var{cpu-type} @gol
682 -mtune=@var{cpu-type} @gol
683 -mpower -mno-power -mpower2 -mno-power2 @gol
684 -mpowerpc -mpowerpc64 -mno-powerpc @gol
685 -maltivec -mno-altivec @gol
686 -mpowerpc-gpopt -mno-powerpc-gpopt @gol
687 -mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
688 -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd @gol
689 -mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp @gol
690 -mnew-mnemonics -mold-mnemonics @gol
691 -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
692 -m64 -m32 -mxl-compat -mno-xl-compat -mpe @gol
693 -malign-power -malign-natural @gol
694 -msoft-float -mhard-float -mmultiple -mno-multiple @gol
695 -mstring -mno-string -mupdate -mno-update @gol
696 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
697 -mstrict-align -mno-strict-align -mrelocatable @gol
698 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
699 -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
700 -mdynamic-no-pic -maltivec -mswdiv @gol
701 -mprioritize-restricted-insns=@var{priority} @gol
702 -msched-costly-dep=@var{dependence_type} @gol
703 -minsert-sched-nops=@var{scheme} @gol
704 -mcall-sysv -mcall-netbsd @gol
705 -maix-struct-return -msvr4-struct-return @gol
706 -mabi=@var{abi-type} -msecure-plt -mbss-plt @gol
707 -misel -mno-isel @gol
708 -misel=yes -misel=no @gol
710 -mspe=yes -mspe=no @gol
712 -mvrsave -mno-vrsave @gol
713 -mmulhw -mno-mulhw @gol
714 -mdlmzb -mno-dlmzb @gol
715 -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double @gol
716 -mprototype -mno-prototype @gol
717 -msim -mmvme -mads -myellowknife -memb -msdata @gol
718 -msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
720 @emph{S/390 and zSeries Options}
721 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
722 -mhard-float -msoft-float -mlong-double-64 -mlong-double-128 @gol
723 -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack @gol
724 -msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
725 -m64 -m31 -mdebug -mno-debug -mesa -mzarch @gol
726 -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd @gol
727 -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard}
730 @gccoptlist{-meb -mel @gol
734 -mscore5 -mscore5u -mscore7 -mscore7d}
737 @gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
738 -m4-nofpu -m4-single-only -m4-single -m4 @gol
739 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al @gol
740 -m5-64media -m5-64media-nofpu @gol
741 -m5-32media -m5-32media-nofpu @gol
742 -m5-compact -m5-compact-nofpu @gol
743 -mb -ml -mdalign -mrelax @gol
744 -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave @gol
745 -mieee -misize -minline-ic_invalidate -mpadstruct -mspace @gol
746 -mprefergot -musermode -multcost=@var{number} -mdiv=@var{strategy} @gol
747 -mdivsi3_libfunc=@var{name} @gol
748 -madjust-unroll -mindexed-addressing -mgettrcost=@var{number} -mpt-fixed @gol
752 @gccoptlist{-mcpu=@var{cpu-type} @gol
753 -mtune=@var{cpu-type} @gol
754 -mcmodel=@var{code-model} @gol
755 -m32 -m64 -mapp-regs -mno-app-regs @gol
756 -mfaster-structs -mno-faster-structs @gol
757 -mfpu -mno-fpu -mhard-float -msoft-float @gol
758 -mhard-quad-float -msoft-quad-float @gol
759 -mimpure-text -mno-impure-text -mlittle-endian @gol
760 -mstack-bias -mno-stack-bias @gol
761 -munaligned-doubles -mno-unaligned-doubles @gol
762 -mv8plus -mno-v8plus -mvis -mno-vis
763 -threads -pthreads -pthread}
766 @gccoptlist{-mwarn-reloc -merror-reloc @gol
767 -msafe-dma -munsafe-dma @gol
769 -msmall-mem -mlarge-mem -mstdmain @gol
770 -mfixed-range=@var{register-range}}
772 @emph{System V Options}
773 @gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
775 @emph{TMS320C3x/C4x Options}
776 @gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
777 -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
778 -mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
779 -mparallel-insns -mparallel-mpy -mpreserve-float}
782 @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
783 -mprolog-function -mno-prolog-function -mspace @gol
784 -mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
785 -mapp-regs -mno-app-regs @gol
786 -mdisable-callt -mno-disable-callt @gol
792 @gccoptlist{-mg -mgnu -munix}
794 @emph{VxWorks Options}
795 @gccoptlist{-mrtp -non-static -Bstatic -Bdynamic @gol
796 -Xbind-lazy -Xbind-now}
798 @emph{x86-64 Options}
799 See i386 and x86-64 Options.
801 @emph{Xstormy16 Options}
804 @emph{Xtensa Options}
805 @gccoptlist{-mconst16 -mno-const16 @gol
806 -mfused-madd -mno-fused-madd @gol
807 -mtext-section-literals -mno-text-section-literals @gol
808 -mtarget-align -mno-target-align @gol
809 -mlongcalls -mno-longcalls}
811 @emph{zSeries Options}
812 See S/390 and zSeries Options.
814 @item Code Generation Options
815 @xref{Code Gen Options,,Options for Code Generation Conventions}.
816 @gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
817 -ffixed-@var{reg} -fexceptions @gol
818 -fnon-call-exceptions -funwind-tables @gol
819 -fasynchronous-unwind-tables @gol
820 -finhibit-size-directive -finstrument-functions @gol
821 -finstrument-functions-exclude-function-list=@var{sym},@var{sym},@dots{} @gol
822 -finstrument-functions-exclude-file-list=@var{file},@var{file},@dots{} @gol
823 -fno-common -fno-ident @gol
824 -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
825 -fno-jump-tables @gol
826 -frecord-gcc-switches @gol
827 -freg-struct-return -fshort-enums @gol
828 -fshort-double -fshort-wchar @gol
829 -fverbose-asm -fpack-struct[=@var{n}] -fstack-check @gol
830 -fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
831 -fno-stack-limit -fargument-alias -fargument-noalias @gol
832 -fargument-noalias-global -fargument-noalias-anything @gol
833 -fleading-underscore -ftls-model=@var{model} @gol
834 -ftrapv -fwrapv -fbounds-check @gol
839 * Overall Options:: Controlling the kind of output:
840 an executable, object files, assembler files,
841 or preprocessed source.
842 * C Dialect Options:: Controlling the variant of C language compiled.
843 * C++ Dialect Options:: Variations on C++.
844 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
846 * Language Independent Options:: Controlling how diagnostics should be
848 * Warning Options:: How picky should the compiler be?
849 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
850 * Optimize Options:: How much optimization?
851 * Preprocessor Options:: Controlling header files and macro definitions.
852 Also, getting dependency information for Make.
853 * Assembler Options:: Passing options to the assembler.
854 * Link Options:: Specifying libraries and so on.
855 * Directory Options:: Where to find header files and libraries.
856 Where to find the compiler executable files.
857 * Spec Files:: How to pass switches to sub-processes.
858 * Target Options:: Running a cross-compiler, or an old version of GCC.
861 @node Overall Options
862 @section Options Controlling the Kind of Output
864 Compilation can involve up to four stages: preprocessing, compilation
865 proper, assembly and linking, always in that order. GCC is capable of
866 preprocessing and compiling several files either into several
867 assembler input files, or into one assembler input file; then each
868 assembler input file produces an object file, and linking combines all
869 the object files (those newly compiled, and those specified as input)
870 into an executable file.
872 @cindex file name suffix
873 For any given input file, the file name suffix determines what kind of
878 C source code which must be preprocessed.
881 C source code which should not be preprocessed.
884 C++ source code which should not be preprocessed.
887 Objective-C source code. Note that you must link with the @file{libobjc}
888 library to make an Objective-C program work.
891 Objective-C source code which should not be preprocessed.
895 Objective-C++ source code. Note that you must link with the @file{libobjc}
896 library to make an Objective-C++ program work. Note that @samp{.M} refers
897 to a literal capital M@.
900 Objective-C++ source code which should not be preprocessed.
903 C, C++, Objective-C or Objective-C++ header file to be turned into a
908 @itemx @var{file}.cxx
909 @itemx @var{file}.cpp
910 @itemx @var{file}.CPP
911 @itemx @var{file}.c++
913 C++ source code which must be preprocessed. Note that in @samp{.cxx},
914 the last two letters must both be literally @samp{x}. Likewise,
915 @samp{.C} refers to a literal capital C@.
919 Objective-C++ source code which must be preprocessed.
922 Objective-C++ source code which should not be preprocessed.
927 @itemx @var{file}.hxx
928 @itemx @var{file}.hpp
929 @itemx @var{file}.HPP
930 @itemx @var{file}.h++
931 @itemx @var{file}.tcc
932 C++ header file to be turned into a precompiled header.
935 @itemx @var{file}.for
936 @itemx @var{file}.FOR
937 Fixed form Fortran source code which should not be preprocessed.
940 @itemx @var{file}.fpp
941 @itemx @var{file}.FPP
942 Fixed form Fortran source code which must be preprocessed (with the traditional
946 @itemx @var{file}.f95
947 Free form Fortran source code which should not be preprocessed.
950 @itemx @var{file}.F95
951 Free form Fortran source code which must be preprocessed (with the
952 traditional preprocessor).
954 @c FIXME: Descriptions of Java file types.
961 Ada source code file which contains a library unit declaration (a
962 declaration of a package, subprogram, or generic, or a generic
963 instantiation), or a library unit renaming declaration (a package,
964 generic, or subprogram renaming declaration). Such files are also
967 @itemx @var{file}.adb
968 Ada source code file containing a library unit body (a subprogram or
969 package body). Such files are also called @dfn{bodies}.
971 @c GCC also knows about some suffixes for languages not yet included:
983 Assembler code which must be preprocessed.
986 An object file to be fed straight into linking.
987 Any file name with no recognized suffix is treated this way.
991 You can specify the input language explicitly with the @option{-x} option:
994 @item -x @var{language}
995 Specify explicitly the @var{language} for the following input files
996 (rather than letting the compiler choose a default based on the file
997 name suffix). This option applies to all following input files until
998 the next @option{-x} option. Possible values for @var{language} are:
1000 c c-header c-cpp-output
1001 c++ c++-header c++-cpp-output
1002 objective-c objective-c-header objective-c-cpp-output
1003 objective-c++ objective-c++-header objective-c++-cpp-output
1004 assembler assembler-with-cpp
1012 Turn off any specification of a language, so that subsequent files are
1013 handled according to their file name suffixes (as they are if @option{-x}
1014 has not been used at all).
1016 @item -pass-exit-codes
1017 @opindex pass-exit-codes
1018 Normally the @command{gcc} program will exit with the code of 1 if any
1019 phase of the compiler returns a non-success return code. If you specify
1020 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
1021 numerically highest error produced by any phase that returned an error
1022 indication. The C, C++, and Fortran frontends return 4, if an internal
1023 compiler error is encountered.
1026 If you only want some of the stages of compilation, you can use
1027 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
1028 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
1029 @command{gcc} is to stop. Note that some combinations (for example,
1030 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
1035 Compile or assemble the source files, but do not link. The linking
1036 stage simply is not done. The ultimate output is in the form of an
1037 object file for each source file.
1039 By default, the object file name for a source file is made by replacing
1040 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
1042 Unrecognized input files, not requiring compilation or assembly, are
1047 Stop after the stage of compilation proper; do not assemble. The output
1048 is in the form of an assembler code file for each non-assembler input
1051 By default, the assembler file name for a source file is made by
1052 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
1054 Input files that don't require compilation are ignored.
1058 Stop after the preprocessing stage; do not run the compiler proper. The
1059 output is in the form of preprocessed source code, which is sent to the
1062 Input files which don't require preprocessing are ignored.
1064 @cindex output file option
1067 Place output in file @var{file}. This applies regardless to whatever
1068 sort of output is being produced, whether it be an executable file,
1069 an object file, an assembler file or preprocessed C code.
1071 If @option{-o} is not specified, the default is to put an executable
1072 file in @file{a.out}, the object file for
1073 @file{@var{source}.@var{suffix}} in @file{@var{source}.o}, its
1074 assembler file in @file{@var{source}.s}, a precompiled header file in
1075 @file{@var{source}.@var{suffix}.gch}, and all preprocessed C source on
1080 Print (on standard error output) the commands executed to run the stages
1081 of compilation. Also print the version number of the compiler driver
1082 program and of the preprocessor and the compiler proper.
1086 Like @option{-v} except the commands are not executed and all command
1087 arguments are quoted. This is useful for shell scripts to capture the
1088 driver-generated command lines.
1092 Use pipes rather than temporary files for communication between the
1093 various stages of compilation. This fails to work on some systems where
1094 the assembler is unable to read from a pipe; but the GNU assembler has
1099 If you are compiling multiple source files, this option tells the driver
1100 to pass all the source files to the compiler at once (for those
1101 languages for which the compiler can handle this). This will allow
1102 intermodule analysis (IMA) to be performed by the compiler. Currently the only
1103 language for which this is supported is C@. If you pass source files for
1104 multiple languages to the driver, using this option, the driver will invoke
1105 the compiler(s) that support IMA once each, passing each compiler all the
1106 source files appropriate for it. For those languages that do not support
1107 IMA this option will be ignored, and the compiler will be invoked once for
1108 each source file in that language. If you use this option in conjunction
1109 with @option{-save-temps}, the compiler will generate multiple
1111 (one for each source file), but only one (combined) @file{.o} or
1116 Print (on the standard output) a description of the command line options
1117 understood by @command{gcc}. If the @option{-v} option is also specified
1118 then @option{--help} will also be passed on to the various processes
1119 invoked by @command{gcc}, so that they can display the command line options
1120 they accept. If the @option{-Wextra} option has also been specified
1121 (prior to the @option{--help} option), then command line options which
1122 have no documentation associated with them will also be displayed.
1125 @opindex target-help
1126 Print (on the standard output) a description of target-specific command
1127 line options for each tool. For some targets extra target-specific
1128 information may also be printed.
1130 @item --help=@var{class}@r{[},@var{qualifier}@r{]}
1131 Print (on the standard output) a description of the command line
1132 options understood by the compiler that fit into a specific class.
1133 The class can be one of @samp{optimizers}, @samp{warnings}, @samp{target},
1134 @samp{params}, or @var{language}:
1137 @item @samp{optimizers}
1138 This will display all of the optimization options supported by the
1141 @item @samp{warnings}
1142 This will display all of the options controlling warning messages
1143 produced by the compiler.
1146 This will display target-specific options. Unlike the
1147 @option{--target-help} option however, target-specific options of the
1148 linker and assembler will not be displayed. This is because those
1149 tools do not currently support the extended @option{--help=} syntax.
1152 This will display the values recognized by the @option{--param}
1155 @item @var{language}
1156 This will display the options supported for @var{language}, where
1157 @var{language} is the name of one of the languages supported in this
1161 This will display the options that are common to all languages.
1164 It is possible to further refine the output of the @option{--help=}
1165 option by adding a comma separated list of qualifiers after the
1166 class. These can be any from the following list:
1169 @item @samp{undocumented}
1170 Display only those options which are undocumented.
1173 Display options which take an argument that appears after an equal
1174 sign in the same continuous piece of text, such as:
1175 @samp{--help=target}.
1177 @item @samp{separate}
1178 Display options which take an argument that appears as a separate word
1179 following the original option, such as: @samp{-o output-file}.
1182 Thus for example to display all the undocumented target-specific
1183 switches supported by the compiler the following can be used:
1186 --help=target,undocumented
1189 The sense of a qualifier can be inverted by prefixing it with the
1190 @var{^} character, so for example to display all binary warning
1191 options (i.e. ones that are either on or off and that do not take an
1192 argument), which have a description the following can be used:
1195 --help=warnings,^joined,^undocumented
1198 A class can also be used as a qualifier, although this usually
1199 restricts the output by so much that there is nothing to display. One
1200 case where it does work however is when one of the classes is
1201 @var{target}. So for example to display all the target-specific
1202 optimization options the following can be used:
1205 --help=target,optimizers
1208 The @option{--help=} option can be repeated on the command line. Each
1209 successive use will display its requested class of options, skipping
1210 those that have already been displayed.
1212 If the @option{-Q} option appears on the command line before the
1213 @option{--help=} option, then the descriptive text displayed by
1214 @option{--help=} is changed. Instead of describing the displayed
1215 options, an indication is given as to whether the option is enabled,
1216 disabled or set to a specific value (assuming that the compiler
1217 knows this at the point where the @option{--help=} option is used).
1219 Here is a truncated example from the ARM port of @command{gcc}:
1222 % gcc -Q -mabi=2 --help=target -c
1223 The following options are target specific:
1225 -mabort-on-noreturn [disabled]
1229 The output is sensitive to the effects of previous command line
1230 options, so for example it is possible to find out which optimizations
1231 are enabled at @option{-O2} by using:
1234 -O2 --help=optimizers
1237 Alternatively you can discover which binary optimizations are enabled
1238 by @option{-O3} by using:
1241 gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
1242 gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
1243 diff /tmp/O2-opts /tmp/O3-opts | grep enabled
1248 Display the version number and copyrights of the invoked GCC@.
1250 @include @value{srcdir}/../libiberty/at-file.texi
1254 @section Compiling C++ Programs
1256 @cindex suffixes for C++ source
1257 @cindex C++ source file suffixes
1258 C++ source files conventionally use one of the suffixes @samp{.C},
1259 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
1260 @samp{.cxx}; C++ header files often use @samp{.hh}, @samp{.hpp},
1261 @samp{.H}, or (for shared template code) @samp{.tcc}; and
1262 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
1263 files with these names and compiles them as C++ programs even if you
1264 call the compiler the same way as for compiling C programs (usually
1265 with the name @command{gcc}).
1269 However, the use of @command{gcc} does not add the C++ library.
1270 @command{g++} is a program that calls GCC and treats @samp{.c},
1271 @samp{.h} and @samp{.i} files as C++ source files instead of C source
1272 files unless @option{-x} is used, and automatically specifies linking
1273 against the C++ library. This program is also useful when
1274 precompiling a C header file with a @samp{.h} extension for use in C++
1275 compilations. On many systems, @command{g++} is also installed with
1276 the name @command{c++}.
1278 @cindex invoking @command{g++}
1279 When you compile C++ programs, you may specify many of the same
1280 command-line options that you use for compiling programs in any
1281 language; or command-line options meaningful for C and related
1282 languages; or options that are meaningful only for C++ programs.
1283 @xref{C Dialect Options,,Options Controlling C Dialect}, for
1284 explanations of options for languages related to C@.
1285 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
1286 explanations of options that are meaningful only for C++ programs.
1288 @node C Dialect Options
1289 @section Options Controlling C Dialect
1290 @cindex dialect options
1291 @cindex language dialect options
1292 @cindex options, dialect
1294 The following options control the dialect of C (or languages derived
1295 from C, such as C++, Objective-C and Objective-C++) that the compiler
1299 @cindex ANSI support
1303 In C mode, support all ISO C90 programs. In C++ mode,
1304 remove GNU extensions that conflict with ISO C++.
1306 This turns off certain features of GCC that are incompatible with ISO
1307 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
1308 such as the @code{asm} and @code{typeof} keywords, and
1309 predefined macros such as @code{unix} and @code{vax} that identify the
1310 type of system you are using. It also enables the undesirable and
1311 rarely used ISO trigraph feature. For the C compiler,
1312 it disables recognition of C++ style @samp{//} comments as well as
1313 the @code{inline} keyword.
1315 The alternate keywords @code{__asm__}, @code{__extension__},
1316 @code{__inline__} and @code{__typeof__} continue to work despite
1317 @option{-ansi}. You would not want to use them in an ISO C program, of
1318 course, but it is useful to put them in header files that might be included
1319 in compilations done with @option{-ansi}. Alternate predefined macros
1320 such as @code{__unix__} and @code{__vax__} are also available, with or
1321 without @option{-ansi}.
1323 The @option{-ansi} option does not cause non-ISO programs to be
1324 rejected gratuitously. For that, @option{-pedantic} is required in
1325 addition to @option{-ansi}. @xref{Warning Options}.
1327 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1328 option is used. Some header files may notice this macro and refrain
1329 from declaring certain functions or defining certain macros that the
1330 ISO standard doesn't call for; this is to avoid interfering with any
1331 programs that might use these names for other things.
1333 Functions which would normally be built in but do not have semantics
1334 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1335 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1336 built-in functions provided by GCC}, for details of the functions
1341 Determine the language standard. This option is currently only
1342 supported when compiling C or C++. A value for this option must be
1343 provided; possible values are
1348 ISO C90 (same as @option{-ansi}).
1350 @item iso9899:199409
1351 ISO C90 as modified in amendment 1.
1357 ISO C99. Note that this standard is not yet fully supported; see
1358 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1359 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1362 Default, ISO C90 plus GNU extensions (including some C99 features).
1366 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1367 this will become the default. The name @samp{gnu9x} is deprecated.
1370 The 1998 ISO C++ standard plus amendments.
1373 The same as @option{-std=c++98} plus GNU extensions. This is the
1374 default for C++ code.
1377 The working draft of the upcoming ISO C++0x standard. This option
1378 enables experimental features that are likely to be included in
1379 C++0x. The working draft is constantly changing, and any feature that is
1380 enabled by this flag may be removed from future versions of GCC if it is
1381 not part of the C++0x standard.
1384 The same as @option{-std=c++0x} plus GNU extensions. As with
1385 @option{-std=c++0x}, this option enables experimental features that may
1386 be removed in future versions of GCC.
1389 Even when this option is not specified, you can still use some of the
1390 features of newer standards in so far as they do not conflict with
1391 previous C standards. For example, you may use @code{__restrict__} even
1392 when @option{-std=c99} is not specified.
1394 The @option{-std} options specifying some version of ISO C have the same
1395 effects as @option{-ansi}, except that features that were not in ISO C90
1396 but are in the specified version (for example, @samp{//} comments and
1397 the @code{inline} keyword in ISO C99) are not disabled.
1399 @xref{Standards,,Language Standards Supported by GCC}, for details of
1400 these standard versions.
1402 @item -fgnu89-inline
1403 @opindex fgnu89-inline
1404 The option @option{-fgnu89-inline} tells GCC to use the traditional
1405 GNU semantics for @code{inline} functions when in C99 mode.
1406 @xref{Inline,,An Inline Function is As Fast As a Macro}. This option
1407 is accepted and ignored by GCC versions 4.1.3 up to but not including
1408 4.3. In GCC versions 4.3 and later it changes the behavior of GCC in
1409 C99 mode. Using this option is roughly equivalent to adding the
1410 @code{gnu_inline} function attribute to all inline functions
1411 (@pxref{Function Attributes}).
1413 The option @option{-fno-gnu89-inline} explicitly tells GCC to use the
1414 C99 semantics for @code{inline} when in C99 or gnu99 mode (i.e., it
1415 specifies the default behavior). This option was first supported in
1416 GCC 4.3. This option is not supported in C89 or gnu89 mode.
1418 The preprocessor macros @code{__GNUC_GNU_INLINE__} and
1419 @code{__GNUC_STDC_INLINE__} may be used to check which semantics are
1420 in effect for @code{inline} functions. @xref{Common Predefined
1421 Macros,,,cpp,The C Preprocessor}.
1423 @item -aux-info @var{filename}
1425 Output to the given filename prototyped declarations for all functions
1426 declared and/or defined in a translation unit, including those in header
1427 files. This option is silently ignored in any language other than C@.
1429 Besides declarations, the file indicates, in comments, the origin of
1430 each declaration (source file and line), whether the declaration was
1431 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1432 @samp{O} for old, respectively, in the first character after the line
1433 number and the colon), and whether it came from a declaration or a
1434 definition (@samp{C} or @samp{F}, respectively, in the following
1435 character). In the case of function definitions, a K&R-style list of
1436 arguments followed by their declarations is also provided, inside
1437 comments, after the declaration.
1441 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1442 keyword, so that code can use these words as identifiers. You can use
1443 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1444 instead. @option{-ansi} implies @option{-fno-asm}.
1446 In C++, this switch only affects the @code{typeof} keyword, since
1447 @code{asm} and @code{inline} are standard keywords. You may want to
1448 use the @option{-fno-gnu-keywords} flag instead, which has the same
1449 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1450 switch only affects the @code{asm} and @code{typeof} keywords, since
1451 @code{inline} is a standard keyword in ISO C99.
1454 @itemx -fno-builtin-@var{function}
1455 @opindex fno-builtin
1456 @cindex built-in functions
1457 Don't recognize built-in functions that do not begin with
1458 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1459 functions provided by GCC}, for details of the functions affected,
1460 including those which are not built-in functions when @option{-ansi} or
1461 @option{-std} options for strict ISO C conformance are used because they
1462 do not have an ISO standard meaning.
1464 GCC normally generates special code to handle certain built-in functions
1465 more efficiently; for instance, calls to @code{alloca} may become single
1466 instructions that adjust the stack directly, and calls to @code{memcpy}
1467 may become inline copy loops. The resulting code is often both smaller
1468 and faster, but since the function calls no longer appear as such, you
1469 cannot set a breakpoint on those calls, nor can you change the behavior
1470 of the functions by linking with a different library. In addition,
1471 when a function is recognized as a built-in function, GCC may use
1472 information about that function to warn about problems with calls to
1473 that function, or to generate more efficient code, even if the
1474 resulting code still contains calls to that function. For example,
1475 warnings are given with @option{-Wformat} for bad calls to
1476 @code{printf}, when @code{printf} is built in, and @code{strlen} is
1477 known not to modify global memory.
1479 With the @option{-fno-builtin-@var{function}} option
1480 only the built-in function @var{function} is
1481 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1482 function is named this is not built-in in this version of GCC, this
1483 option is ignored. There is no corresponding
1484 @option{-fbuiltin-@var{function}} option; if you wish to enable
1485 built-in functions selectively when using @option{-fno-builtin} or
1486 @option{-ffreestanding}, you may define macros such as:
1489 #define abs(n) __builtin_abs ((n))
1490 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1495 @cindex hosted environment
1497 Assert that compilation takes place in a hosted environment. This implies
1498 @option{-fbuiltin}. A hosted environment is one in which the
1499 entire standard library is available, and in which @code{main} has a return
1500 type of @code{int}. Examples are nearly everything except a kernel.
1501 This is equivalent to @option{-fno-freestanding}.
1503 @item -ffreestanding
1504 @opindex ffreestanding
1505 @cindex hosted environment
1507 Assert that compilation takes place in a freestanding environment. This
1508 implies @option{-fno-builtin}. A freestanding environment
1509 is one in which the standard library may not exist, and program startup may
1510 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1511 This is equivalent to @option{-fno-hosted}.
1513 @xref{Standards,,Language Standards Supported by GCC}, for details of
1514 freestanding and hosted environments.
1518 @cindex openmp parallel
1519 Enable handling of OpenMP directives @code{#pragma omp} in C/C++ and
1520 @code{!$omp} in Fortran. When @option{-fopenmp} is specified, the
1521 compiler generates parallel code according to the OpenMP Application
1522 Program Interface v2.5 @w{@uref{http://www.openmp.org/}}. This option
1523 implies @option{-pthread}, and thus is only supported on targets that
1524 have support for @option{-pthread}.
1526 @item -fms-extensions
1527 @opindex fms-extensions
1528 Accept some non-standard constructs used in Microsoft header files.
1530 Some cases of unnamed fields in structures and unions are only
1531 accepted with this option. @xref{Unnamed Fields,,Unnamed struct/union
1532 fields within structs/unions}, for details.
1536 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1537 options for strict ISO C conformance) implies @option{-trigraphs}.
1539 @item -no-integrated-cpp
1540 @opindex no-integrated-cpp
1541 Performs a compilation in two passes: preprocessing and compiling. This
1542 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1543 @option{-B} option. The user supplied compilation step can then add in
1544 an additional preprocessing step after normal preprocessing but before
1545 compiling. The default is to use the integrated cpp (internal cpp)
1547 The semantics of this option will change if "cc1", "cc1plus", and
1548 "cc1obj" are merged.
1550 @cindex traditional C language
1551 @cindex C language, traditional
1553 @itemx -traditional-cpp
1554 @opindex traditional-cpp
1555 @opindex traditional
1556 Formerly, these options caused GCC to attempt to emulate a pre-standard
1557 C compiler. They are now only supported with the @option{-E} switch.
1558 The preprocessor continues to support a pre-standard mode. See the GNU
1559 CPP manual for details.
1561 @item -fcond-mismatch
1562 @opindex fcond-mismatch
1563 Allow conditional expressions with mismatched types in the second and
1564 third arguments. The value of such an expression is void. This option
1565 is not supported for C++.
1567 @item -flax-vector-conversions
1568 @opindex flax-vector-conversions
1569 Allow implicit conversions between vectors with differing numbers of
1570 elements and/or incompatible element types. This option should not be
1573 @item -funsigned-char
1574 @opindex funsigned-char
1575 Let the type @code{char} be unsigned, like @code{unsigned char}.
1577 Each kind of machine has a default for what @code{char} should
1578 be. It is either like @code{unsigned char} by default or like
1579 @code{signed char} by default.
1581 Ideally, a portable program should always use @code{signed char} or
1582 @code{unsigned char} when it depends on the signedness of an object.
1583 But many programs have been written to use plain @code{char} and
1584 expect it to be signed, or expect it to be unsigned, depending on the
1585 machines they were written for. This option, and its inverse, let you
1586 make such a program work with the opposite default.
1588 The type @code{char} is always a distinct type from each of
1589 @code{signed char} or @code{unsigned char}, even though its behavior
1590 is always just like one of those two.
1593 @opindex fsigned-char
1594 Let the type @code{char} be signed, like @code{signed char}.
1596 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1597 the negative form of @option{-funsigned-char}. Likewise, the option
1598 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1600 @item -fsigned-bitfields
1601 @itemx -funsigned-bitfields
1602 @itemx -fno-signed-bitfields
1603 @itemx -fno-unsigned-bitfields
1604 @opindex fsigned-bitfields
1605 @opindex funsigned-bitfields
1606 @opindex fno-signed-bitfields
1607 @opindex fno-unsigned-bitfields
1608 These options control whether a bit-field is signed or unsigned, when the
1609 declaration does not use either @code{signed} or @code{unsigned}. By
1610 default, such a bit-field is signed, because this is consistent: the
1611 basic integer types such as @code{int} are signed types.
1614 @node C++ Dialect Options
1615 @section Options Controlling C++ Dialect
1617 @cindex compiler options, C++
1618 @cindex C++ options, command line
1619 @cindex options, C++
1620 This section describes the command-line options that are only meaningful
1621 for C++ programs; but you can also use most of the GNU compiler options
1622 regardless of what language your program is in. For example, you
1623 might compile a file @code{firstClass.C} like this:
1626 g++ -g -frepo -O -c firstClass.C
1630 In this example, only @option{-frepo} is an option meant
1631 only for C++ programs; you can use the other options with any
1632 language supported by GCC@.
1634 Here is a list of options that are @emph{only} for compiling C++ programs:
1638 @item -fabi-version=@var{n}
1639 @opindex fabi-version
1640 Use version @var{n} of the C++ ABI@. Version 2 is the version of the
1641 C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
1642 the C++ ABI that first appeared in G++ 3.2. Version 0 will always be
1643 the version that conforms most closely to the C++ ABI specification.
1644 Therefore, the ABI obtained using version 0 will change as ABI bugs
1647 The default is version 2.
1649 @item -fno-access-control
1650 @opindex fno-access-control
1651 Turn off all access checking. This switch is mainly useful for working
1652 around bugs in the access control code.
1656 Check that the pointer returned by @code{operator new} is non-null
1657 before attempting to modify the storage allocated. This check is
1658 normally unnecessary because the C++ standard specifies that
1659 @code{operator new} will only return @code{0} if it is declared
1660 @samp{throw()}, in which case the compiler will always check the
1661 return value even without this option. In all other cases, when
1662 @code{operator new} has a non-empty exception specification, memory
1663 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1664 @samp{new (nothrow)}.
1666 @item -fconserve-space
1667 @opindex fconserve-space
1668 Put uninitialized or runtime-initialized global variables into the
1669 common segment, as C does. This saves space in the executable at the
1670 cost of not diagnosing duplicate definitions. If you compile with this
1671 flag and your program mysteriously crashes after @code{main()} has
1672 completed, you may have an object that is being destroyed twice because
1673 two definitions were merged.
1675 This option is no longer useful on most targets, now that support has
1676 been added for putting variables into BSS without making them common.
1678 @item -ffriend-injection
1679 @opindex ffriend-injection
1680 Inject friend functions into the enclosing namespace, so that they are
1681 visible outside the scope of the class in which they are declared.
1682 Friend functions were documented to work this way in the old Annotated
1683 C++ Reference Manual, and versions of G++ before 4.1 always worked
1684 that way. However, in ISO C++ a friend function which is not declared
1685 in an enclosing scope can only be found using argument dependent
1686 lookup. This option causes friends to be injected as they were in
1689 This option is for compatibility, and may be removed in a future
1692 @item -fno-elide-constructors
1693 @opindex fno-elide-constructors
1694 The C++ standard allows an implementation to omit creating a temporary
1695 which is only used to initialize another object of the same type.
1696 Specifying this option disables that optimization, and forces G++ to
1697 call the copy constructor in all cases.
1699 @item -fno-enforce-eh-specs
1700 @opindex fno-enforce-eh-specs
1701 Don't generate code to check for violation of exception specifications
1702 at runtime. This option violates the C++ standard, but may be useful
1703 for reducing code size in production builds, much like defining
1704 @samp{NDEBUG}. This does not give user code permission to throw
1705 exceptions in violation of the exception specifications; the compiler
1706 will still optimize based on the specifications, so throwing an
1707 unexpected exception will result in undefined behavior.
1710 @itemx -fno-for-scope
1712 @opindex fno-for-scope
1713 If @option{-ffor-scope} is specified, the scope of variables declared in
1714 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1715 as specified by the C++ standard.
1716 If @option{-fno-for-scope} is specified, the scope of variables declared in
1717 a @i{for-init-statement} extends to the end of the enclosing scope,
1718 as was the case in old versions of G++, and other (traditional)
1719 implementations of C++.
1721 The default if neither flag is given to follow the standard,
1722 but to allow and give a warning for old-style code that would
1723 otherwise be invalid, or have different behavior.
1725 @item -fno-gnu-keywords
1726 @opindex fno-gnu-keywords
1727 Do not recognize @code{typeof} as a keyword, so that code can use this
1728 word as an identifier. You can use the keyword @code{__typeof__} instead.
1729 @option{-ansi} implies @option{-fno-gnu-keywords}.
1731 @item -fno-implicit-templates
1732 @opindex fno-implicit-templates
1733 Never emit code for non-inline templates which are instantiated
1734 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1735 @xref{Template Instantiation}, for more information.
1737 @item -fno-implicit-inline-templates
1738 @opindex fno-implicit-inline-templates
1739 Don't emit code for implicit instantiations of inline templates, either.
1740 The default is to handle inlines differently so that compiles with and
1741 without optimization will need the same set of explicit instantiations.
1743 @item -fno-implement-inlines
1744 @opindex fno-implement-inlines
1745 To save space, do not emit out-of-line copies of inline functions
1746 controlled by @samp{#pragma implementation}. This will cause linker
1747 errors if these functions are not inlined everywhere they are called.
1749 @item -fms-extensions
1750 @opindex fms-extensions
1751 Disable pedantic warnings about constructs used in MFC, such as implicit
1752 int and getting a pointer to member function via non-standard syntax.
1754 @item -fno-nonansi-builtins
1755 @opindex fno-nonansi-builtins
1756 Disable built-in declarations of functions that are not mandated by
1757 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1758 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1760 @item -fno-operator-names
1761 @opindex fno-operator-names
1762 Do not treat the operator name keywords @code{and}, @code{bitand},
1763 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1764 synonyms as keywords.
1766 @item -fno-optional-diags
1767 @opindex fno-optional-diags
1768 Disable diagnostics that the standard says a compiler does not need to
1769 issue. Currently, the only such diagnostic issued by G++ is the one for
1770 a name having multiple meanings within a class.
1773 @opindex fpermissive
1774 Downgrade some diagnostics about nonconformant code from errors to
1775 warnings. Thus, using @option{-fpermissive} will allow some
1776 nonconforming code to compile.
1780 Enable automatic template instantiation at link time. This option also
1781 implies @option{-fno-implicit-templates}. @xref{Template
1782 Instantiation}, for more information.
1786 Disable generation of information about every class with virtual
1787 functions for use by the C++ runtime type identification features
1788 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1789 of the language, you can save some space by using this flag. Note that
1790 exception handling uses the same information, but it will generate it as
1791 needed. The @samp{dynamic_cast} operator can still be used for casts that
1792 do not require runtime type information, i.e. casts to @code{void *} or to
1793 unambiguous base classes.
1797 Emit statistics about front-end processing at the end of the compilation.
1798 This information is generally only useful to the G++ development team.
1800 @item -ftemplate-depth-@var{n}
1801 @opindex ftemplate-depth
1802 Set the maximum instantiation depth for template classes to @var{n}.
1803 A limit on the template instantiation depth is needed to detect
1804 endless recursions during template class instantiation. ANSI/ISO C++
1805 conforming programs must not rely on a maximum depth greater than 17.
1807 @item -fno-threadsafe-statics
1808 @opindex fno-threadsafe-statics
1809 Do not emit the extra code to use the routines specified in the C++
1810 ABI for thread-safe initialization of local statics. You can use this
1811 option to reduce code size slightly in code that doesn't need to be
1814 @item -fuse-cxa-atexit
1815 @opindex fuse-cxa-atexit
1816 Register destructors for objects with static storage duration with the
1817 @code{__cxa_atexit} function rather than the @code{atexit} function.
1818 This option is required for fully standards-compliant handling of static
1819 destructors, but will only work if your C library supports
1820 @code{__cxa_atexit}.
1822 @item -fno-use-cxa-get-exception-ptr
1823 @opindex fno-use-cxa-get-exception-ptr
1824 Don't use the @code{__cxa_get_exception_ptr} runtime routine. This
1825 will cause @code{std::uncaught_exception} to be incorrect, but is necessary
1826 if the runtime routine is not available.
1828 @item -fvisibility-inlines-hidden
1829 @opindex fvisibility-inlines-hidden
1830 This switch declares that the user does not attempt to compare
1831 pointers to inline methods where the addresses of the two functions
1832 were taken in different shared objects.
1834 The effect of this is that GCC may, effectively, mark inline methods with
1835 @code{__attribute__ ((visibility ("hidden")))} so that they do not
1836 appear in the export table of a DSO and do not require a PLT indirection
1837 when used within the DSO@. Enabling this option can have a dramatic effect
1838 on load and link times of a DSO as it massively reduces the size of the
1839 dynamic export table when the library makes heavy use of templates.
1841 The behavior of this switch is not quite the same as marking the
1842 methods as hidden directly, because it does not affect static variables
1843 local to the function or cause the compiler to deduce that
1844 the function is defined in only one shared object.
1846 You may mark a method as having a visibility explicitly to negate the
1847 effect of the switch for that method. For example, if you do want to
1848 compare pointers to a particular inline method, you might mark it as
1849 having default visibility. Marking the enclosing class with explicit
1850 visibility will have no effect.
1852 Explicitly instantiated inline methods are unaffected by this option
1853 as their linkage might otherwise cross a shared library boundary.
1854 @xref{Template Instantiation}.
1856 @item -fvisibility-ms-compat
1857 @opindex fvisibility-ms-compat
1858 This flag attempts to use visibility settings to make GCC's C++
1859 linkage model compatible with that of Microsoft Visual Studio.
1861 The flag makes these changes to GCC's linkage model:
1865 It sets the default visibility to @code{hidden}, like
1866 @option{-fvisibility=hidden}.
1869 Types, but not their members, are not hidden by default.
1872 The One Definition Rule is relaxed for types without explicit
1873 visibility specifications which are defined in more than one different
1874 shared object: those declarations are permitted if they would have
1875 been permitted when this option was not used.
1878 In new code it is better to use @option{-fvisibility=hidden} and
1879 export those classes which are intended to be externally visible.
1880 Unfortunately it is possible for code to rely, perhaps accidentally,
1881 on the Visual Studio behavior.
1883 Among the consequences of these changes are that static data members
1884 of the same type with the same name but defined in different shared
1885 objects will be different, so changing one will not change the other;
1886 and that pointers to function members defined in different shared
1887 objects may not compare equal. When this flag is given, it is a
1888 violation of the ODR to define types with the same name differently.
1892 Do not use weak symbol support, even if it is provided by the linker.
1893 By default, G++ will use weak symbols if they are available. This
1894 option exists only for testing, and should not be used by end-users;
1895 it will result in inferior code and has no benefits. This option may
1896 be removed in a future release of G++.
1900 Do not search for header files in the standard directories specific to
1901 C++, but do still search the other standard directories. (This option
1902 is used when building the C++ library.)
1905 In addition, these optimization, warning, and code generation options
1906 have meanings only for C++ programs:
1909 @item -fno-default-inline
1910 @opindex fno-default-inline
1911 Do not assume @samp{inline} for functions defined inside a class scope.
1912 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1913 functions will have linkage like inline functions; they just won't be
1916 @item -Wabi @r{(C++ only)}
1918 Warn when G++ generates code that is probably not compatible with the
1919 vendor-neutral C++ ABI@. Although an effort has been made to warn about
1920 all such cases, there are probably some cases that are not warned about,
1921 even though G++ is generating incompatible code. There may also be
1922 cases where warnings are emitted even though the code that is generated
1925 You should rewrite your code to avoid these warnings if you are
1926 concerned about the fact that code generated by G++ may not be binary
1927 compatible with code generated by other compilers.
1929 The known incompatibilities at this point include:
1934 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1935 pack data into the same byte as a base class. For example:
1938 struct A @{ virtual void f(); int f1 : 1; @};
1939 struct B : public A @{ int f2 : 1; @};
1943 In this case, G++ will place @code{B::f2} into the same byte
1944 as@code{A::f1}; other compilers will not. You can avoid this problem
1945 by explicitly padding @code{A} so that its size is a multiple of the
1946 byte size on your platform; that will cause G++ and other compilers to
1947 layout @code{B} identically.
1950 Incorrect handling of tail-padding for virtual bases. G++ does not use
1951 tail padding when laying out virtual bases. For example:
1954 struct A @{ virtual void f(); char c1; @};
1955 struct B @{ B(); char c2; @};
1956 struct C : public A, public virtual B @{@};
1960 In this case, G++ will not place @code{B} into the tail-padding for
1961 @code{A}; other compilers will. You can avoid this problem by
1962 explicitly padding @code{A} so that its size is a multiple of its
1963 alignment (ignoring virtual base classes); that will cause G++ and other
1964 compilers to layout @code{C} identically.
1967 Incorrect handling of bit-fields with declared widths greater than that
1968 of their underlying types, when the bit-fields appear in a union. For
1972 union U @{ int i : 4096; @};
1976 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1977 union too small by the number of bits in an @code{int}.
1980 Empty classes can be placed at incorrect offsets. For example:
1990 struct C : public B, public A @{@};
1994 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1995 it should be placed at offset zero. G++ mistakenly believes that the
1996 @code{A} data member of @code{B} is already at offset zero.
1999 Names of template functions whose types involve @code{typename} or
2000 template template parameters can be mangled incorrectly.
2003 template <typename Q>
2004 void f(typename Q::X) @{@}
2006 template <template <typename> class Q>
2007 void f(typename Q<int>::X) @{@}
2011 Instantiations of these templates may be mangled incorrectly.
2015 @item -Wctor-dtor-privacy @r{(C++ only)}
2016 @opindex Wctor-dtor-privacy
2017 Warn when a class seems unusable because all the constructors or
2018 destructors in that class are private, and it has neither friends nor
2019 public static member functions.
2021 @item -Wnon-virtual-dtor @r{(C++ only)}
2022 @opindex Wnon-virtual-dtor
2023 Warn when a class has virtual functions and accessible non-virtual
2024 destructor, in which case it would be possible but unsafe to delete
2025 an instance of a derived class through a pointer to the base class.
2026 This warning is also enabled if -Weffc++ is specified.
2028 @item -Wreorder @r{(C++ only)}
2030 @cindex reordering, warning
2031 @cindex warning for reordering of member initializers
2032 Warn when the order of member initializers given in the code does not
2033 match the order in which they must be executed. For instance:
2039 A(): j (0), i (1) @{ @}
2043 The compiler will rearrange the member initializers for @samp{i}
2044 and @samp{j} to match the declaration order of the members, emitting
2045 a warning to that effect. This warning is enabled by @option{-Wall}.
2048 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
2051 @item -Weffc++ @r{(C++ only)}
2053 Warn about violations of the following style guidelines from Scott Meyers'
2054 @cite{Effective C++} book:
2058 Item 11: Define a copy constructor and an assignment operator for classes
2059 with dynamically allocated memory.
2062 Item 12: Prefer initialization to assignment in constructors.
2065 Item 14: Make destructors virtual in base classes.
2068 Item 15: Have @code{operator=} return a reference to @code{*this}.
2071 Item 23: Don't try to return a reference when you must return an object.
2075 Also warn about violations of the following style guidelines from
2076 Scott Meyers' @cite{More Effective C++} book:
2080 Item 6: Distinguish between prefix and postfix forms of increment and
2081 decrement operators.
2084 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
2088 When selecting this option, be aware that the standard library
2089 headers do not obey all of these guidelines; use @samp{grep -v}
2090 to filter out those warnings.
2092 @item -Wno-deprecated @r{(C++ only)}
2093 @opindex Wno-deprecated
2094 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
2096 @item -Wstrict-null-sentinel @r{(C++ only)}
2097 @opindex Wstrict-null-sentinel
2098 Warn also about the use of an uncasted @code{NULL} as sentinel. When
2099 compiling only with GCC this is a valid sentinel, as @code{NULL} is defined
2100 to @code{__null}. Although it is a null pointer constant not a null pointer,
2101 it is guaranteed to of the same size as a pointer. But this use is
2102 not portable across different compilers.
2104 @item -Wno-non-template-friend @r{(C++ only)}
2105 @opindex Wno-non-template-friend
2106 Disable warnings when non-templatized friend functions are declared
2107 within a template. Since the advent of explicit template specification
2108 support in G++, if the name of the friend is an unqualified-id (i.e.,
2109 @samp{friend foo(int)}), the C++ language specification demands that the
2110 friend declare or define an ordinary, nontemplate function. (Section
2111 14.5.3). Before G++ implemented explicit specification, unqualified-ids
2112 could be interpreted as a particular specialization of a templatized
2113 function. Because this non-conforming behavior is no longer the default
2114 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
2115 check existing code for potential trouble spots and is on by default.
2116 This new compiler behavior can be turned off with
2117 @option{-Wno-non-template-friend} which keeps the conformant compiler code
2118 but disables the helpful warning.
2120 @item -Wold-style-cast @r{(C++ only)}
2121 @opindex Wold-style-cast
2122 Warn if an old-style (C-style) cast to a non-void type is used within
2123 a C++ program. The new-style casts (@samp{dynamic_cast},
2124 @samp{static_cast}, @samp{reinterpret_cast}, and @samp{const_cast}) are
2125 less vulnerable to unintended effects and much easier to search for.
2127 @item -Woverloaded-virtual @r{(C++ only)}
2128 @opindex Woverloaded-virtual
2129 @cindex overloaded virtual fn, warning
2130 @cindex warning for overloaded virtual fn
2131 Warn when a function declaration hides virtual functions from a
2132 base class. For example, in:
2139 struct B: public A @{
2144 the @code{A} class version of @code{f} is hidden in @code{B}, and code
2152 will fail to compile.
2154 @item -Wno-pmf-conversions @r{(C++ only)}
2155 @opindex Wno-pmf-conversions
2156 Disable the diagnostic for converting a bound pointer to member function
2159 @item -Wsign-promo @r{(C++ only)}
2160 @opindex Wsign-promo
2161 Warn when overload resolution chooses a promotion from unsigned or
2162 enumerated type to a signed type, over a conversion to an unsigned type of
2163 the same size. Previous versions of G++ would try to preserve
2164 unsignedness, but the standard mandates the current behavior.
2169 A& operator = (int);
2179 In this example, G++ will synthesize a default @samp{A& operator =
2180 (const A&);}, while cfront will use the user-defined @samp{operator =}.
2183 @node Objective-C and Objective-C++ Dialect Options
2184 @section Options Controlling Objective-C and Objective-C++ Dialects
2186 @cindex compiler options, Objective-C and Objective-C++
2187 @cindex Objective-C and Objective-C++ options, command line
2188 @cindex options, Objective-C and Objective-C++
2189 (NOTE: This manual does not describe the Objective-C and Objective-C++
2190 languages themselves. See @xref{Standards,,Language Standards
2191 Supported by GCC}, for references.)
2193 This section describes the command-line options that are only meaningful
2194 for Objective-C and Objective-C++ programs, but you can also use most of
2195 the language-independent GNU compiler options.
2196 For example, you might compile a file @code{some_class.m} like this:
2199 gcc -g -fgnu-runtime -O -c some_class.m
2203 In this example, @option{-fgnu-runtime} is an option meant only for
2204 Objective-C and Objective-C++ programs; you can use the other options with
2205 any language supported by GCC@.
2207 Note that since Objective-C is an extension of the C language, Objective-C
2208 compilations may also use options specific to the C front-end (e.g.,
2209 @option{-Wtraditional}). Similarly, Objective-C++ compilations may use
2210 C++-specific options (e.g., @option{-Wabi}).
2212 Here is a list of options that are @emph{only} for compiling Objective-C
2213 and Objective-C++ programs:
2216 @item -fconstant-string-class=@var{class-name}
2217 @opindex fconstant-string-class
2218 Use @var{class-name} as the name of the class to instantiate for each
2219 literal string specified with the syntax @code{@@"@dots{}"}. The default
2220 class name is @code{NXConstantString} if the GNU runtime is being used, and
2221 @code{NSConstantString} if the NeXT runtime is being used (see below). The
2222 @option{-fconstant-cfstrings} option, if also present, will override the
2223 @option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
2224 to be laid out as constant CoreFoundation strings.
2227 @opindex fgnu-runtime
2228 Generate object code compatible with the standard GNU Objective-C
2229 runtime. This is the default for most types of systems.
2231 @item -fnext-runtime
2232 @opindex fnext-runtime
2233 Generate output compatible with the NeXT runtime. This is the default
2234 for NeXT-based systems, including Darwin and Mac OS X@. The macro
2235 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
2238 @item -fno-nil-receivers
2239 @opindex fno-nil-receivers
2240 Assume that all Objective-C message dispatches (e.g.,
2241 @code{[receiver message:arg]}) in this translation unit ensure that the receiver
2242 is not @code{nil}. This allows for more efficient entry points in the runtime
2243 to be used. Currently, this option is only available in conjunction with
2244 the NeXT runtime on Mac OS X 10.3 and later.
2246 @item -fobjc-call-cxx-cdtors
2247 @opindex fobjc-call-cxx-cdtors
2248 For each Objective-C class, check if any of its instance variables is a
2249 C++ object with a non-trivial default constructor. If so, synthesize a
2250 special @code{- (id) .cxx_construct} instance method that will run
2251 non-trivial default constructors on any such instance variables, in order,
2252 and then return @code{self}. Similarly, check if any instance variable
2253 is a C++ object with a non-trivial destructor, and if so, synthesize a
2254 special @code{- (void) .cxx_destruct} method that will run
2255 all such default destructors, in reverse order.
2257 The @code{- (id) .cxx_construct} and/or @code{- (void) .cxx_destruct} methods
2258 thusly generated will only operate on instance variables declared in the
2259 current Objective-C class, and not those inherited from superclasses. It
2260 is the responsibility of the Objective-C runtime to invoke all such methods
2261 in an object's inheritance hierarchy. The @code{- (id) .cxx_construct} methods
2262 will be invoked by the runtime immediately after a new object
2263 instance is allocated; the @code{- (void) .cxx_destruct} methods will
2264 be invoked immediately before the runtime deallocates an object instance.
2266 As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
2267 support for invoking the @code{- (id) .cxx_construct} and
2268 @code{- (void) .cxx_destruct} methods.
2270 @item -fobjc-direct-dispatch
2271 @opindex fobjc-direct-dispatch
2272 Allow fast jumps to the message dispatcher. On Darwin this is
2273 accomplished via the comm page.
2275 @item -fobjc-exceptions
2276 @opindex fobjc-exceptions
2277 Enable syntactic support for structured exception handling in Objective-C,
2278 similar to what is offered by C++ and Java. This option is
2279 unavailable in conjunction with the NeXT runtime on Mac OS X 10.2 and
2288 @@catch (AnObjCClass *exc) @{
2295 @@catch (AnotherClass *exc) @{
2298 @@catch (id allOthers) @{
2308 The @code{@@throw} statement may appear anywhere in an Objective-C or
2309 Objective-C++ program; when used inside of a @code{@@catch} block, the
2310 @code{@@throw} may appear without an argument (as shown above), in which case
2311 the object caught by the @code{@@catch} will be rethrown.
2313 Note that only (pointers to) Objective-C objects may be thrown and
2314 caught using this scheme. When an object is thrown, it will be caught
2315 by the nearest @code{@@catch} clause capable of handling objects of that type,
2316 analogously to how @code{catch} blocks work in C++ and Java. A
2317 @code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
2318 any and all Objective-C exceptions not caught by previous @code{@@catch}
2321 The @code{@@finally} clause, if present, will be executed upon exit from the
2322 immediately preceding @code{@@try @dots{} @@catch} section. This will happen
2323 regardless of whether any exceptions are thrown, caught or rethrown
2324 inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
2325 of the @code{finally} clause in Java.
2327 There are several caveats to using the new exception mechanism:
2331 Although currently designed to be binary compatible with @code{NS_HANDLER}-style
2332 idioms provided by the @code{NSException} class, the new
2333 exceptions can only be used on Mac OS X 10.3 (Panther) and later
2334 systems, due to additional functionality needed in the (NeXT) Objective-C
2338 As mentioned above, the new exceptions do not support handling
2339 types other than Objective-C objects. Furthermore, when used from
2340 Objective-C++, the Objective-C exception model does not interoperate with C++
2341 exceptions at this time. This means you cannot @code{@@throw} an exception
2342 from Objective-C and @code{catch} it in C++, or vice versa
2343 (i.e., @code{throw @dots{} @@catch}).
2346 The @option{-fobjc-exceptions} switch also enables the use of synchronization
2347 blocks for thread-safe execution:
2350 @@synchronized (ObjCClass *guard) @{
2355 Upon entering the @code{@@synchronized} block, a thread of execution shall
2356 first check whether a lock has been placed on the corresponding @code{guard}
2357 object by another thread. If it has, the current thread shall wait until
2358 the other thread relinquishes its lock. Once @code{guard} becomes available,
2359 the current thread will place its own lock on it, execute the code contained in
2360 the @code{@@synchronized} block, and finally relinquish the lock (thereby
2361 making @code{guard} available to other threads).
2363 Unlike Java, Objective-C does not allow for entire methods to be marked
2364 @code{@@synchronized}. Note that throwing exceptions out of
2365 @code{@@synchronized} blocks is allowed, and will cause the guarding object
2366 to be unlocked properly.
2370 Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
2372 @item -freplace-objc-classes
2373 @opindex freplace-objc-classes
2374 Emit a special marker instructing @command{ld(1)} not to statically link in
2375 the resulting object file, and allow @command{dyld(1)} to load it in at
2376 run time instead. This is used in conjunction with the Fix-and-Continue
2377 debugging mode, where the object file in question may be recompiled and
2378 dynamically reloaded in the course of program execution, without the need
2379 to restart the program itself. Currently, Fix-and-Continue functionality
2380 is only available in conjunction with the NeXT runtime on Mac OS X 10.3
2385 When compiling for the NeXT runtime, the compiler ordinarily replaces calls
2386 to @code{objc_getClass("@dots{}")} (when the name of the class is known at
2387 compile time) with static class references that get initialized at load time,
2388 which improves run-time performance. Specifying the @option{-fzero-link} flag
2389 suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
2390 to be retained. This is useful in Zero-Link debugging mode, since it allows
2391 for individual class implementations to be modified during program execution.
2395 Dump interface declarations for all classes seen in the source file to a
2396 file named @file{@var{sourcename}.decl}.
2398 @item -Wassign-intercept
2399 @opindex Wassign-intercept
2400 Warn whenever an Objective-C assignment is being intercepted by the
2404 @opindex Wno-protocol
2405 If a class is declared to implement a protocol, a warning is issued for
2406 every method in the protocol that is not implemented by the class. The
2407 default behavior is to issue a warning for every method not explicitly
2408 implemented in the class, even if a method implementation is inherited
2409 from the superclass. If you use the @option{-Wno-protocol} option, then
2410 methods inherited from the superclass are considered to be implemented,
2411 and no warning is issued for them.
2415 Warn if multiple methods of different types for the same selector are
2416 found during compilation. The check is performed on the list of methods
2417 in the final stage of compilation. Additionally, a check is performed
2418 for each selector appearing in a @code{@@selector(@dots{})}
2419 expression, and a corresponding method for that selector has been found
2420 during compilation. Because these checks scan the method table only at
2421 the end of compilation, these warnings are not produced if the final
2422 stage of compilation is not reached, for example because an error is
2423 found during compilation, or because the @option{-fsyntax-only} option is
2426 @item -Wstrict-selector-match
2427 @opindex Wstrict-selector-match
2428 Warn if multiple methods with differing argument and/or return types are
2429 found for a given selector when attempting to send a message using this
2430 selector to a receiver of type @code{id} or @code{Class}. When this flag
2431 is off (which is the default behavior), the compiler will omit such warnings
2432 if any differences found are confined to types which share the same size
2435 @item -Wundeclared-selector
2436 @opindex Wundeclared-selector
2437 Warn if a @code{@@selector(@dots{})} expression referring to an
2438 undeclared selector is found. A selector is considered undeclared if no
2439 method with that name has been declared before the
2440 @code{@@selector(@dots{})} expression, either explicitly in an
2441 @code{@@interface} or @code{@@protocol} declaration, or implicitly in
2442 an @code{@@implementation} section. This option always performs its
2443 checks as soon as a @code{@@selector(@dots{})} expression is found,
2444 while @option{-Wselector} only performs its checks in the final stage of
2445 compilation. This also enforces the coding style convention
2446 that methods and selectors must be declared before being used.
2448 @item -print-objc-runtime-info
2449 @opindex print-objc-runtime-info
2450 Generate C header describing the largest structure that is passed by
2455 @node Language Independent Options
2456 @section Options to Control Diagnostic Messages Formatting
2457 @cindex options to control diagnostics formatting
2458 @cindex diagnostic messages
2459 @cindex message formatting
2461 Traditionally, diagnostic messages have been formatted irrespective of
2462 the output device's aspect (e.g.@: its width, @dots{}). The options described
2463 below can be used to control the diagnostic messages formatting
2464 algorithm, e.g.@: how many characters per line, how often source location
2465 information should be reported. Right now, only the C++ front end can
2466 honor these options. However it is expected, in the near future, that
2467 the remaining front ends would be able to digest them correctly.
2470 @item -fmessage-length=@var{n}
2471 @opindex fmessage-length
2472 Try to format error messages so that they fit on lines of about @var{n}
2473 characters. The default is 72 characters for @command{g++} and 0 for the rest of
2474 the front ends supported by GCC@. If @var{n} is zero, then no
2475 line-wrapping will be done; each error message will appear on a single
2478 @opindex fdiagnostics-show-location
2479 @item -fdiagnostics-show-location=once
2480 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
2481 reporter to emit @emph{once} source location information; that is, in
2482 case the message is too long to fit on a single physical line and has to
2483 be wrapped, the source location won't be emitted (as prefix) again,
2484 over and over, in subsequent continuation lines. This is the default
2487 @item -fdiagnostics-show-location=every-line
2488 Only meaningful in line-wrapping mode. Instructs the diagnostic
2489 messages reporter to emit the same source location information (as
2490 prefix) for physical lines that result from the process of breaking
2491 a message which is too long to fit on a single line.
2493 @item -fdiagnostics-show-option
2494 @opindex fdiagnostics-show-option
2495 This option instructs the diagnostic machinery to add text to each
2496 diagnostic emitted, which indicates which command line option directly
2497 controls that diagnostic, when such an option is known to the
2498 diagnostic machinery.
2500 @item -Wcoverage-mismatch
2501 @opindex Wcoverage-mismatch
2502 Warn if feedback profiles do not match when using the
2503 @option{-fprofile-use} option.
2504 If a source file was changed between @option{-fprofile-gen} and
2505 @option{-fprofile-use}, the files with the profile feedback can fail
2506 to match the source file and GCC can not use the profile feedback
2507 information. By default, GCC emits an error message in this case.
2508 The option @option{-Wcoverage-mismatch} emits a warning instead of an
2509 error. GCC does not use appropriate feedback profiles, so using this
2510 option can result in poorly optimized code. This option is useful
2511 only in the case of very minor changes such as bug fixes to an
2516 @node Warning Options
2517 @section Options to Request or Suppress Warnings
2518 @cindex options to control warnings
2519 @cindex warning messages
2520 @cindex messages, warning
2521 @cindex suppressing warnings
2523 Warnings are diagnostic messages that report constructions which
2524 are not inherently erroneous but which are risky or suggest there
2525 may have been an error.
2527 The following language-independent options do not enable specific
2528 warnings but control the kinds of diagnostics produced by GCC.
2531 @cindex syntax checking
2533 @opindex fsyntax-only
2534 Check the code for syntax errors, but don't do anything beyond that.
2538 Inhibit all warning messages.
2542 Make all warnings into errors.
2546 Make the specified warning into an error. The specifier for a warning
2547 is appended, for example @option{-Werror=switch} turns the warnings
2548 controlled by @option{-Wswitch} into errors. This switch takes a
2549 negative form, to be used to negate @option{-Werror} for specific
2550 warnings, for example @option{-Wno-error=switch} makes
2551 @option{-Wswitch} warnings not be errors, even when @option{-Werror}
2552 is in effect. You can use the @option{-fdiagnostics-show-option}
2553 option to have each controllable warning amended with the option which
2554 controls it, to determine what to use with this option.
2556 Note that specifying @option{-Werror=}@var{foo} automatically implies
2557 @option{-W}@var{foo}. However, @option{-Wno-error=}@var{foo} does not
2560 @item -Wfatal-errors
2561 @opindex Wfatal-errors
2562 This option causes the compiler to abort compilation on the first error
2563 occurred rather than trying to keep going and printing further error
2568 You can request many specific warnings with options beginning
2569 @samp{-W}, for example @option{-Wimplicit} to request warnings on
2570 implicit declarations. Each of these specific warning options also
2571 has a negative form beginning @samp{-Wno-} to turn off warnings; for
2572 example, @option{-Wno-implicit}. This manual lists only one of the
2573 two forms, whichever is not the default. For further,
2574 language-specific options also refer to @ref{C++ Dialect Options} and
2575 @ref{Objective-C and Objective-C++ Dialect Options}.
2580 Issue all the warnings demanded by strict ISO C and ISO C++;
2581 reject all programs that use forbidden extensions, and some other
2582 programs that do not follow ISO C and ISO C++. For ISO C, follows the
2583 version of the ISO C standard specified by any @option{-std} option used.
2585 Valid ISO C and ISO C++ programs should compile properly with or without
2586 this option (though a rare few will require @option{-ansi} or a
2587 @option{-std} option specifying the required version of ISO C)@. However,
2588 without this option, certain GNU extensions and traditional C and C++
2589 features are supported as well. With this option, they are rejected.
2591 @option{-pedantic} does not cause warning messages for use of the
2592 alternate keywords whose names begin and end with @samp{__}. Pedantic
2593 warnings are also disabled in the expression that follows
2594 @code{__extension__}. However, only system header files should use
2595 these escape routes; application programs should avoid them.
2596 @xref{Alternate Keywords}.
2598 Some users try to use @option{-pedantic} to check programs for strict ISO
2599 C conformance. They soon find that it does not do quite what they want:
2600 it finds some non-ISO practices, but not all---only those for which
2601 ISO C @emph{requires} a diagnostic, and some others for which
2602 diagnostics have been added.
2604 A feature to report any failure to conform to ISO C might be useful in
2605 some instances, but would require considerable additional work and would
2606 be quite different from @option{-pedantic}. We don't have plans to
2607 support such a feature in the near future.
2609 Where the standard specified with @option{-std} represents a GNU
2610 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
2611 corresponding @dfn{base standard}, the version of ISO C on which the GNU
2612 extended dialect is based. Warnings from @option{-pedantic} are given
2613 where they are required by the base standard. (It would not make sense
2614 for such warnings to be given only for features not in the specified GNU
2615 C dialect, since by definition the GNU dialects of C include all
2616 features the compiler supports with the given option, and there would be
2617 nothing to warn about.)
2619 @item -pedantic-errors
2620 @opindex pedantic-errors
2621 Like @option{-pedantic}, except that errors are produced rather than
2626 This enables all the warnings about constructions that some users
2627 consider questionable, and that are easy to avoid (or modify to
2628 prevent the warning), even in conjunction with macros. This also
2629 enables some language-specific warnings described in @ref{C++ Dialect
2630 Options} and @ref{Objective-C and Objective-C++ Dialect Options}.
2632 @option{-Wall} turns on the following warning flags:
2634 @gccoptlist{-Waddress @gol
2635 -Warray-bounds @r{(only with} @option{-O2}@r{)} @gol
2637 -Wchar-subscripts @gol
2639 -Wimplicit-function-declaration @gol
2642 -Wmain @r{(only for C/ObjC and unless} @option{-ffreestanding}@r{)} @gol
2643 -Wmissing-braces @gol
2649 -Wsequence-point @gol
2650 -Wsign-compare @r{(only in C++)} @gol
2651 -Wstrict-aliasing @gol
2652 -Wstrict-overflow @gol
2655 -Wuninitialized @r{(only with} @option{-O1} @r{and above)} @gol
2656 -Wunknown-pragmas @gol
2657 -Wunused-function @gol
2660 -Wunused-variable @gol
2663 Note that some warning flags are not implied by @option{-Wall}. Some of
2664 them warn about constructions that users generally do not consider
2665 questionable, but which occasionally you might wish to check for;
2666 others warn about constructions that are necessary or hard to avoid in
2667 some cases, and there is no simple way to modify the code to suppress
2668 the warning. Some of them are enabled by @option{-Wextra} but many of
2669 them must be enabled individually.
2674 This enables some extra warning flags that are not enabled by
2675 @option{-Wall}. (This option used to be called @option{-W}. The older
2676 name is still supported, but the newer name is more descriptive.)
2678 @gccoptlist{-Wclobbered @gol
2680 -Wignored-qualifiers @gol
2681 -Wmissing-field-initializers @gol
2682 -Wmissing-parameter-type @r{(C only)} @gol
2683 -Wold-style-declaration @r{(C only)} @gol
2684 -Woverride-init @gol
2687 -Wuninitialized @r{(only with} @option{-O1} @r{and above)} @gol
2688 -Wunused-parameter @r{(only with} @option{-Wunused} @r{or} @option{-Wall}@r{)} @gol
2691 The option @option{-Wextra} also prints warning messages for the
2697 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2698 @samp{>}, or @samp{>=}.
2701 (C++ only) An enumerator and a non-enumerator both appear in a
2702 conditional expression.
2705 (C++ only) A non-static reference or non-static @samp{const} member
2706 appears in a class without constructors.
2709 (C++ only) Ambiguous virtual bases.
2712 (C++ only) Subscripting an array which has been declared @samp{register}.
2715 (C++ only) Taking the address of a variable which has been declared
2719 (C++ only) A base class is not initialized in a derived class' copy
2726 Inhibit warning messages about the use of @samp{#import}.
2728 @item -Wchar-subscripts
2729 @opindex Wchar-subscripts
2730 Warn if an array subscript has type @code{char}. This is a common cause
2731 of error, as programmers often forget that this type is signed on some
2733 This warning is enabled by @option{-Wall}.
2737 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
2738 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
2739 This warning is enabled by @option{-Wall}.
2743 @opindex ffreestanding
2744 @opindex fno-builtin
2745 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
2746 the arguments supplied have types appropriate to the format string
2747 specified, and that the conversions specified in the format string make
2748 sense. This includes standard functions, and others specified by format
2749 attributes (@pxref{Function Attributes}), in the @code{printf},
2750 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
2751 not in the C standard) families (or other target-specific families).
2752 Which functions are checked without format attributes having been
2753 specified depends on the standard version selected, and such checks of
2754 functions without the attribute specified are disabled by
2755 @option{-ffreestanding} or @option{-fno-builtin}.
2757 The formats are checked against the format features supported by GNU
2758 libc version 2.2. These include all ISO C90 and C99 features, as well
2759 as features from the Single Unix Specification and some BSD and GNU
2760 extensions. Other library implementations may not support all these
2761 features; GCC does not support warning about features that go beyond a
2762 particular library's limitations. However, if @option{-pedantic} is used
2763 with @option{-Wformat}, warnings will be given about format features not
2764 in the selected standard version (but not for @code{strfmon} formats,
2765 since those are not in any version of the C standard). @xref{C Dialect
2766 Options,,Options Controlling C Dialect}.
2768 Since @option{-Wformat} also checks for null format arguments for
2769 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2771 @option{-Wformat} is included in @option{-Wall}. For more control over some
2772 aspects of format checking, the options @option{-Wformat-y2k},
2773 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2774 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2775 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2778 @opindex Wformat-y2k
2779 If @option{-Wformat} is specified, also warn about @code{strftime}
2780 formats which may yield only a two-digit year.
2782 @item -Wno-format-extra-args
2783 @opindex Wno-format-extra-args
2784 If @option{-Wformat} is specified, do not warn about excess arguments to a
2785 @code{printf} or @code{scanf} format function. The C standard specifies
2786 that such arguments are ignored.
2788 Where the unused arguments lie between used arguments that are
2789 specified with @samp{$} operand number specifications, normally
2790 warnings are still given, since the implementation could not know what
2791 type to pass to @code{va_arg} to skip the unused arguments. However,
2792 in the case of @code{scanf} formats, this option will suppress the
2793 warning if the unused arguments are all pointers, since the Single
2794 Unix Specification says that such unused arguments are allowed.
2796 @item -Wno-format-zero-length
2797 @opindex Wno-format-zero-length
2798 If @option{-Wformat} is specified, do not warn about zero-length formats.
2799 The C standard specifies that zero-length formats are allowed.
2801 @item -Wformat-nonliteral
2802 @opindex Wformat-nonliteral
2803 If @option{-Wformat} is specified, also warn if the format string is not a
2804 string literal and so cannot be checked, unless the format function
2805 takes its format arguments as a @code{va_list}.
2807 @item -Wformat-security
2808 @opindex Wformat-security
2809 If @option{-Wformat} is specified, also warn about uses of format
2810 functions that represent possible security problems. At present, this
2811 warns about calls to @code{printf} and @code{scanf} functions where the
2812 format string is not a string literal and there are no format arguments,
2813 as in @code{printf (foo);}. This may be a security hole if the format
2814 string came from untrusted input and contains @samp{%n}. (This is
2815 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2816 in future warnings may be added to @option{-Wformat-security} that are not
2817 included in @option{-Wformat-nonliteral}.)
2821 Enable @option{-Wformat} plus format checks not included in
2822 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2823 -Wformat-nonliteral -Wformat-security -Wformat-y2k}.
2827 Warn about passing a null pointer for arguments marked as
2828 requiring a non-null value by the @code{nonnull} function attribute.
2830 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2831 can be disabled with the @option{-Wno-nonnull} option.
2833 @item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
2835 Warn about uninitialized variables which are initialized with themselves.
2836 Note this option can only be used with the @option{-Wuninitialized} option,
2837 which in turn only works with @option{-O1} and above.
2839 For example, GCC will warn about @code{i} being uninitialized in the
2840 following snippet only when @option{-Winit-self} has been specified:
2851 @item -Wimplicit-int
2852 @opindex Wimplicit-int
2853 Warn when a declaration does not specify a type.
2854 This warning is enabled by @option{-Wall}.
2856 @item -Wimplicit-function-declaration
2857 @opindex Wimplicit-function-declaration
2858 @opindex Wno-implicit-function-declaration
2859 Give a warning whenever a function is used before being declared. In
2860 C99 mode (@option{-std=c99} or @option{-std=gnu99}), this warning is
2861 enabled by default and it is made into an error by
2862 @option{-pedantic-errors}. This warning is also enabled by
2867 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2868 This warning is enabled by @option{-Wall}.
2870 @item -Wignored-qualifiers
2871 @opindex Wignored-qualifiers
2872 @opindex Wno-ignored-qualifiers
2873 Warn if the return type of a function has a type qualifier
2874 such as @code{const}. For ISO C such a type qualifier has no effect,
2875 since the value returned by a function is not an lvalue.
2876 For C++, the warning is only emitted for scalar types or @code{void}.
2877 ISO C prohibits qualified @code{void} return types on function
2878 definitions, so such return types always receive a warning
2879 even without this option.
2881 This warning is also enabled by @option{-Wextra}.
2885 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2886 function with external linkage, returning int, taking either zero
2887 arguments, two, or three arguments of appropriate types.
2888 This warning is enabled by @option{-Wall}.
2890 @item -Wmissing-braces
2891 @opindex Wmissing-braces
2892 Warn if an aggregate or union initializer is not fully bracketed. In
2893 the following example, the initializer for @samp{a} is not fully
2894 bracketed, but that for @samp{b} is fully bracketed.
2897 int a[2][2] = @{ 0, 1, 2, 3 @};
2898 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2901 This warning is enabled by @option{-Wall}.
2903 @item -Wmissing-include-dirs @r{(C, C++, Objective-C and Objective-C++ only)}
2904 @opindex Wmissing-include-dirs
2905 Warn if a user-supplied include directory does not exist.
2908 @opindex Wparentheses
2909 Warn if parentheses are omitted in certain contexts, such
2910 as when there is an assignment in a context where a truth value
2911 is expected, or when operators are nested whose precedence people
2912 often get confused about.
2914 Also warn if a comparison like @samp{x<=y<=z} appears; this is
2915 equivalent to @samp{(x<=y ? 1 : 0) <= z}, which is a different
2916 interpretation from that of ordinary mathematical notation.
2918 Also warn about constructions where there may be confusion to which
2919 @code{if} statement an @code{else} branch belongs. Here is an example of
2934 In C/C++, every @code{else} branch belongs to the innermost possible
2935 @code{if} statement, which in this example is @code{if (b)}. This is
2936 often not what the programmer expected, as illustrated in the above
2937 example by indentation the programmer chose. When there is the
2938 potential for this confusion, GCC will issue a warning when this flag
2939 is specified. To eliminate the warning, add explicit braces around
2940 the innermost @code{if} statement so there is no way the @code{else}
2941 could belong to the enclosing @code{if}. The resulting code would
2958 This warning is enabled by @option{-Wall}.
2960 @item -Wsequence-point
2961 @opindex Wsequence-point
2962 Warn about code that may have undefined semantics because of violations
2963 of sequence point rules in the C and C++ standards.
2965 The C and C++ standards defines the order in which expressions in a C/C++
2966 program are evaluated in terms of @dfn{sequence points}, which represent
2967 a partial ordering between the execution of parts of the program: those
2968 executed before the sequence point, and those executed after it. These
2969 occur after the evaluation of a full expression (one which is not part
2970 of a larger expression), after the evaluation of the first operand of a
2971 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2972 function is called (but after the evaluation of its arguments and the
2973 expression denoting the called function), and in certain other places.
2974 Other than as expressed by the sequence point rules, the order of
2975 evaluation of subexpressions of an expression is not specified. All
2976 these rules describe only a partial order rather than a total order,
2977 since, for example, if two functions are called within one expression
2978 with no sequence point between them, the order in which the functions
2979 are called is not specified. However, the standards committee have
2980 ruled that function calls do not overlap.
2982 It is not specified when between sequence points modifications to the
2983 values of objects take effect. Programs whose behavior depends on this
2984 have undefined behavior; the C and C++ standards specify that ``Between
2985 the previous and next sequence point an object shall have its stored
2986 value modified at most once by the evaluation of an expression.
2987 Furthermore, the prior value shall be read only to determine the value
2988 to be stored.''. If a program breaks these rules, the results on any
2989 particular implementation are entirely unpredictable.
2991 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2992 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2993 diagnosed by this option, and it may give an occasional false positive
2994 result, but in general it has been found fairly effective at detecting
2995 this sort of problem in programs.
2997 The standard is worded confusingly, therefore there is some debate
2998 over the precise meaning of the sequence point rules in subtle cases.
2999 Links to discussions of the problem, including proposed formal
3000 definitions, may be found on the GCC readings page, at
3001 @w{@uref{http://gcc.gnu.org/readings.html}}.
3003 This warning is enabled by @option{-Wall} for C and C++.
3006 @opindex Wreturn-type
3007 @opindex Wno-return-type
3008 Warn whenever a function is defined with a return-type that defaults
3009 to @code{int}. Also warn about any @code{return} statement with no
3010 return-value in a function whose return-type is not @code{void}
3011 (falling off the end of the function body is considered returning
3012 without a value), and about a @code{return} statement with a
3013 expression in a function whose return-type is @code{void}.
3015 For C++, a function without return type always produces a diagnostic
3016 message, even when @option{-Wno-return-type} is specified. The only
3017 exceptions are @samp{main} and functions defined in system headers.
3019 This warning is enabled by @option{-Wall}.
3023 Warn whenever a @code{switch} statement has an index of enumerated type
3024 and lacks a @code{case} for one or more of the named codes of that
3025 enumeration. (The presence of a @code{default} label prevents this
3026 warning.) @code{case} labels outside the enumeration range also
3027 provoke warnings when this option is used.
3028 This warning is enabled by @option{-Wall}.
3030 @item -Wswitch-default
3031 @opindex Wswitch-switch
3032 Warn whenever a @code{switch} statement does not have a @code{default}
3036 @opindex Wswitch-enum
3037 Warn whenever a @code{switch} statement has an index of enumerated type
3038 and lacks a @code{case} for one or more of the named codes of that
3039 enumeration. @code{case} labels outside the enumeration range also
3040 provoke warnings when this option is used.
3044 Warn if any trigraphs are encountered that might change the meaning of
3045 the program (trigraphs within comments are not warned about).
3046 This warning is enabled by @option{-Wall}.
3048 @item -Wunused-function
3049 @opindex Wunused-function
3050 Warn whenever a static function is declared but not defined or a
3051 non-inline static function is unused.
3052 This warning is enabled by @option{-Wall}.
3054 @item -Wunused-label
3055 @opindex Wunused-label
3056 Warn whenever a label is declared but not used.
3057 This warning is enabled by @option{-Wall}.
3059 To suppress this warning use the @samp{unused} attribute
3060 (@pxref{Variable Attributes}).
3062 @item -Wunused-parameter
3063 @opindex Wunused-parameter
3064 Warn whenever a function parameter is unused aside from its declaration.
3066 To suppress this warning use the @samp{unused} attribute
3067 (@pxref{Variable Attributes}).
3069 @item -Wunused-variable
3070 @opindex Wunused-variable
3071 Warn whenever a local variable or non-constant static variable is unused
3072 aside from its declaration.
3073 This warning is enabled by @option{-Wall}.
3075 To suppress this warning use the @samp{unused} attribute
3076 (@pxref{Variable Attributes}).
3078 @item -Wunused-value
3079 @opindex Wunused-value
3080 Warn whenever a statement computes a result that is explicitly not
3081 used. To suppress this warning cast the unused expression to
3082 @samp{void}. This includes an expression-statement or the left-hand
3083 side of a comma expression that contains no side effects. For example,
3084 an expression such as @samp{x[i,j]} will cause a warning, while
3085 @samp{x[(void)i,j]} will not.
3087 This warning is enabled by @option{-Wall}.
3091 All the above @option{-Wunused} options combined.
3093 In order to get a warning about an unused function parameter, you must
3094 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
3095 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
3097 @item -Wuninitialized
3098 @opindex Wuninitialized
3099 Warn if an automatic variable is used without first being initialized or
3100 if a variable may be clobbered by a @code{setjmp} call.
3102 These warnings are possible only in optimizing compilation,
3103 because they require data flow information that is computed only
3104 when optimizing. If you do not specify @option{-O}, you will not get
3105 these warnings. Instead, GCC will issue a warning about @option{-Wuninitialized}
3106 requiring @option{-O}.
3108 If you want to warn about code which uses the uninitialized value of the
3109 variable in its own initializer, use the @option{-Winit-self} option.
3111 These warnings occur for individual uninitialized or clobbered
3112 elements of structure, union or array variables as well as for
3113 variables which are uninitialized or clobbered as a whole. They do
3114 not occur for variables or elements declared @code{volatile}. Because
3115 these warnings depend on optimization, the exact variables or elements
3116 for which there are warnings will depend on the precise optimization
3117 options and version of GCC used.
3119 Note that there may be no warning about a variable that is used only
3120 to compute a value that itself is never used, because such
3121 computations may be deleted by data flow analysis before the warnings
3124 These warnings are made optional because GCC is not smart
3125 enough to see all the reasons why the code might be correct
3126 despite appearing to have an error. Here is one example of how
3147 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
3148 always initialized, but GCC doesn't know this. Here is
3149 another common case:
3154 if (change_y) save_y = y, y = new_y;
3156 if (change_y) y = save_y;
3161 This has no bug because @code{save_y} is used only if it is set.
3163 @cindex @code{longjmp} warnings
3164 This option also warns when a non-volatile automatic variable might be
3165 changed by a call to @code{longjmp}. These warnings as well are possible
3166 only in optimizing compilation.
3168 The compiler sees only the calls to @code{setjmp}. It cannot know
3169 where @code{longjmp} will be called; in fact, a signal handler could
3170 call it at any point in the code. As a result, you may get a warning
3171 even when there is in fact no problem because @code{longjmp} cannot
3172 in fact be called at the place which would cause a problem.
3174 Some spurious warnings can be avoided if you declare all the functions
3175 you use that never return as @code{noreturn}. @xref{Function
3178 This warning is enabled by @option{-Wall} or @option{-Wextra} in
3179 optimizing compilations (@option{-O1} and above).
3181 @item -Wunknown-pragmas
3182 @opindex Wunknown-pragmas
3183 @cindex warning for unknown pragmas
3184 @cindex unknown pragmas, warning
3185 @cindex pragmas, warning of unknown
3186 Warn when a #pragma directive is encountered which is not understood by
3187 GCC@. If this command line option is used, warnings will even be issued
3188 for unknown pragmas in system header files. This is not the case if
3189 the warnings were only enabled by the @option{-Wall} command line option.
3192 @opindex Wno-pragmas
3194 Do not warn about misuses of pragmas, such as incorrect parameters,
3195 invalid syntax, or conflicts between pragmas. See also
3196 @samp{-Wunknown-pragmas}.
3198 @item -Wstrict-aliasing
3199 @opindex Wstrict-aliasing
3200 This option is only active when @option{-fstrict-aliasing} is active.
3201 It warns about code which might break the strict aliasing rules that the
3202 compiler is using for optimization. The warning does not catch all
3203 cases, but does attempt to catch the more common pitfalls. It is
3204 included in @option{-Wall}.
3205 It is equivalent to @option{-Wstrict-aliasing=3}
3207 @item -Wstrict-aliasing=n
3208 @opindex Wstrict-aliasing=n
3209 This option is only active when @option{-fstrict-aliasing} is active.
3210 It warns about code which might break the strict aliasing rules that the
3211 compiler is using for optimization.
3212 Higher levels correspond to higher accuracy (fewer false positives).
3213 Higher levels also correspond to more effort, similar to the way -O works.
3214 @option{-Wstrict-aliasing} is equivalent to @option{-Wstrict-aliasing=n},
3217 Level 1: Most aggressive, quick, least accurate.
3218 Possibly useful when higher levels
3219 do not warn but -fstrict-aliasing still breaks the code, as it has very few
3220 false negatives. However, it has many false positives.
3221 Warns for all pointer conversions between possibly incompatible types,
3222 even if never dereferenced. Runs in the frontend only.
3224 Level 2: Aggressive, quick, not too precise.
3225 May still have many false positives (not as many as level 1 though),
3226 and few false negatives (but possibly more than level 1).
3227 Unlike level 1, it only warns when an address is taken. Warns about
3228 incomplete types. Runs in the frontend only.
3230 Level 3 (default for @option{-Wstrict-aliasing}):
3231 Should have very few false positives and few false
3232 negatives. Slightly slower than levels 1 or 2 when optimization is enabled.
3233 Takes care of the common punn+dereference pattern in the frontend:
3234 @code{*(int*)&some_float}.
3235 If optimization is enabled, it also runs in the backend, where it deals
3236 with multiple statement cases using flow-sensitive points-to information.
3237 Only warns when the converted pointer is dereferenced.
3238 Does not warn about incomplete types.
3240 @item -Wstrict-overflow
3241 @itemx -Wstrict-overflow=@var{n}
3242 @opindex Wstrict-overflow
3243 This option is only active when @option{-fstrict-overflow} is active.
3244 It warns about cases where the compiler optimizes based on the
3245 assumption that signed overflow does not occur. Note that it does not
3246 warn about all cases where the code might overflow: it only warns
3247 about cases where the compiler implements some optimization. Thus
3248 this warning depends on the optimization level.
3250 An optimization which assumes that signed overflow does not occur is
3251 perfectly safe if the values of the variables involved are such that
3252 overflow never does, in fact, occur. Therefore this warning can
3253 easily give a false positive: a warning about code which is not
3254 actually a problem. To help focus on important issues, several
3255 warning levels are defined. No warnings are issued for the use of
3256 undefined signed overflow when estimating how many iterations a loop
3257 will require, in particular when determining whether a loop will be
3261 @item -Wstrict-overflow=1
3262 Warn about cases which are both questionable and easy to avoid. For
3263 example: @code{x + 1 > x}; with @option{-fstrict-overflow}, the
3264 compiler will simplify this to @code{1}. This level of
3265 @option{-Wstrict-overflow} is enabled by @option{-Wall}; higher levels
3266 are not, and must be explicitly requested.
3268 @item -Wstrict-overflow=2
3269 Also warn about other cases where a comparison is simplified to a
3270 constant. For example: @code{abs (x) >= 0}. This can only be
3271 simplified when @option{-fstrict-overflow} is in effect, because
3272 @code{abs (INT_MIN)} overflows to @code{INT_MIN}, which is less than
3273 zero. @option{-Wstrict-overflow} (with no level) is the same as
3274 @option{-Wstrict-overflow=2}.
3276 @item -Wstrict-overflow=3
3277 Also warn about other cases where a comparison is simplified. For
3278 example: @code{x + 1 > 1} will be simplified to @code{x > 0}.
3280 @item -Wstrict-overflow=4
3281 Also warn about other simplifications not covered by the above cases.
3282 For example: @code{(x * 10) / 5} will be simplified to @code{x * 2}.
3284 @item -Wstrict-overflow=5
3285 Also warn about cases where the compiler reduces the magnitude of a
3286 constant involved in a comparison. For example: @code{x + 2 > y} will
3287 be simplified to @code{x + 1 >= y}. This is reported only at the
3288 highest warning level because this simplification applies to many
3289 comparisons, so this warning level will give a very large number of
3293 @item -Warray-bounds
3294 @opindex Wno-array-bounds
3295 @opindex Warray-bounds
3296 This option is only active when @option{-ftree-vrp} is active
3297 (default for -O2 and above). It warns about subscripts to arrays
3298 that are always out of bounds. This warning is enabled by @option{-Wall}.
3300 @item -Wno-div-by-zero
3301 @opindex Wno-div-by-zero
3302 @opindex Wdiv-by-zero
3303 Do not warn about compile-time integer division by zero. Floating point
3304 division by zero is not warned about, as it can be a legitimate way of
3305 obtaining infinities and NaNs.
3307 @item -Wsystem-headers
3308 @opindex Wsystem-headers
3309 @cindex warnings from system headers
3310 @cindex system headers, warnings from
3311 Print warning messages for constructs found in system header files.
3312 Warnings from system headers are normally suppressed, on the assumption
3313 that they usually do not indicate real problems and would only make the
3314 compiler output harder to read. Using this command line option tells
3315 GCC to emit warnings from system headers as if they occurred in user
3316 code. However, note that using @option{-Wall} in conjunction with this
3317 option will @emph{not} warn about unknown pragmas in system
3318 headers---for that, @option{-Wunknown-pragmas} must also be used.
3321 @opindex Wfloat-equal
3322 Warn if floating point values are used in equality comparisons.
3324 The idea behind this is that sometimes it is convenient (for the
3325 programmer) to consider floating-point values as approximations to
3326 infinitely precise real numbers. If you are doing this, then you need
3327 to compute (by analyzing the code, or in some other way) the maximum or
3328 likely maximum error that the computation introduces, and allow for it
3329 when performing comparisons (and when producing output, but that's a
3330 different problem). In particular, instead of testing for equality, you
3331 would check to see whether the two values have ranges that overlap; and
3332 this is done with the relational operators, so equality comparisons are
3335 @item -Wtraditional @r{(C only)}
3336 @opindex Wtraditional
3337 Warn about certain constructs that behave differently in traditional and
3338 ISO C@. Also warn about ISO C constructs that have no traditional C
3339 equivalent, and/or problematic constructs which should be avoided.
3343 Macro parameters that appear within string literals in the macro body.
3344 In traditional C macro replacement takes place within string literals,
3345 but does not in ISO C@.
3348 In traditional C, some preprocessor directives did not exist.
3349 Traditional preprocessors would only consider a line to be a directive
3350 if the @samp{#} appeared in column 1 on the line. Therefore
3351 @option{-Wtraditional} warns about directives that traditional C
3352 understands but would ignore because the @samp{#} does not appear as the
3353 first character on the line. It also suggests you hide directives like
3354 @samp{#pragma} not understood by traditional C by indenting them. Some
3355 traditional implementations would not recognize @samp{#elif}, so it
3356 suggests avoiding it altogether.
3359 A function-like macro that appears without arguments.
3362 The unary plus operator.
3365 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
3366 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
3367 constants.) Note, these suffixes appear in macros defined in the system
3368 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
3369 Use of these macros in user code might normally lead to spurious
3370 warnings, however GCC's integrated preprocessor has enough context to
3371 avoid warning in these cases.
3374 A function declared external in one block and then used after the end of
3378 A @code{switch} statement has an operand of type @code{long}.
3381 A non-@code{static} function declaration follows a @code{static} one.
3382 This construct is not accepted by some traditional C compilers.
3385 The ISO type of an integer constant has a different width or
3386 signedness from its traditional type. This warning is only issued if
3387 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
3388 typically represent bit patterns, are not warned about.
3391 Usage of ISO string concatenation is detected.
3394 Initialization of automatic aggregates.
3397 Identifier conflicts with labels. Traditional C lacks a separate
3398 namespace for labels.
3401 Initialization of unions. If the initializer is zero, the warning is
3402 omitted. This is done under the assumption that the zero initializer in
3403 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
3404 initializer warnings and relies on default initialization to zero in the
3408 Conversions by prototypes between fixed/floating point values and vice
3409 versa. The absence of these prototypes when compiling with traditional
3410 C would cause serious problems. This is a subset of the possible
3411 conversion warnings, for the full set use @option{-Wtraditional-conversion}.
3414 Use of ISO C style function definitions. This warning intentionally is
3415 @emph{not} issued for prototype declarations or variadic functions
3416 because these ISO C features will appear in your code when using
3417 libiberty's traditional C compatibility macros, @code{PARAMS} and
3418 @code{VPARAMS}. This warning is also bypassed for nested functions
3419 because that feature is already a GCC extension and thus not relevant to
3420 traditional C compatibility.
3423 @item -Wtraditional-conversion @r{(C only)}
3424 @opindex Wtraditional-conversion
3425 Warn if a prototype causes a type conversion that is different from what
3426 would happen to the same argument in the absence of a prototype. This
3427 includes conversions of fixed point to floating and vice versa, and
3428 conversions changing the width or signedness of a fixed point argument
3429 except when the same as the default promotion.
3431 @item -Wdeclaration-after-statement @r{(C only)}
3432 @opindex Wdeclaration-after-statement
3433 Warn when a declaration is found after a statement in a block. This
3434 construct, known from C++, was introduced with ISO C99 and is by default
3435 allowed in GCC@. It is not supported by ISO C90 and was not supported by
3436 GCC versions before GCC 3.0. @xref{Mixed Declarations}.
3440 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
3442 @item -Wno-endif-labels
3443 @opindex Wno-endif-labels
3444 @opindex Wendif-labels
3445 Do not warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
3449 Warn whenever a local variable shadows another local variable, parameter or
3450 global variable or whenever a built-in function is shadowed.
3452 @item -Wlarger-than-@var{len}
3453 @opindex Wlarger-than
3454 Warn whenever an object of larger than @var{len} bytes is defined.
3456 @item -Wunsafe-loop-optimizations
3457 @opindex Wunsafe-loop-optimizations
3458 Warn if the loop cannot be optimized because the compiler could not
3459 assume anything on the bounds of the loop indices. With
3460 @option{-funsafe-loop-optimizations} warn if the compiler made
3463 @item -Wpointer-arith
3464 @opindex Wpointer-arith
3465 Warn about anything that depends on the ``size of'' a function type or
3466 of @code{void}. GNU C assigns these types a size of 1, for
3467 convenience in calculations with @code{void *} pointers and pointers
3468 to functions. In C++, warn also when an arithmetic operation involves
3469 @code{NULL}. This warning is also enabled by @option{-pedantic}.
3472 @opindex Wtype-limits
3473 @opindex Wno-type-limits
3474 Warn if a comparison is always true or always false due to the limited
3475 range of the data type, but do not warn for constant expressions. For
3476 example, warn if an unsigned variable is compared against zero with
3477 @samp{<} or @samp{>=}. This warning is also enabled by
3480 @item -Wbad-function-cast @r{(C only)}
3481 @opindex Wbad-function-cast
3482 Warn whenever a function call is cast to a non-matching type.
3483 For example, warn if @code{int malloc()} is cast to @code{anything *}.
3486 Warn about ISO C constructs that are outside of the common subset of
3487 ISO C and ISO C++, e.g.@: request for implicit conversion from
3488 @code{void *} to a pointer to non-@code{void} type.
3490 @item -Wc++0x-compat @r{(C++ and Objective-C++ only)}
3491 Warn about C++ constructs whose meaning differs between ISO C++ 1998 and
3492 ISO C++ 200x, e.g., identifiers in ISO C++ 1998 that will become keywords
3493 in ISO C++ 200x. This warning is enabled by @option{-Wall}.
3497 Warn whenever a pointer is cast so as to remove a type qualifier from
3498 the target type. For example, warn if a @code{const char *} is cast
3499 to an ordinary @code{char *}.
3502 @opindex Wcast-align
3503 Warn whenever a pointer is cast such that the required alignment of the
3504 target is increased. For example, warn if a @code{char *} is cast to
3505 an @code{int *} on machines where integers can only be accessed at
3506 two- or four-byte boundaries.
3508 @item -Wwrite-strings
3509 @opindex Wwrite-strings
3510 When compiling C, give string constants the type @code{const
3511 char[@var{length}]} so that
3512 copying the address of one into a non-@code{const} @code{char *}
3513 pointer will get a warning; when compiling C++, warn about the
3514 deprecated conversion from string literals to @code{char *}. This
3515 warning, by default, is enabled for C++ programs.
3516 These warnings will help you find at
3517 compile time code that can try to write into a string constant, but
3518 only if you have been very careful about using @code{const} in
3519 declarations and prototypes. Otherwise, it will just be a nuisance;
3520 this is why we did not make @option{-Wall} request these warnings.
3524 Warn for variables that might be changed by @samp{longjmp} or
3525 @samp{vfork}. This warning is also enabled by @option{-Wextra}.
3528 @opindex Wconversion
3529 @opindex Wno-conversion
3530 Warn for implicit conversions that may alter a value. This includes
3531 conversions between real and integer, like @code{abs (x)} when
3532 @code{x} is @code{double}; conversions between signed and unsigned,
3533 like @code{unsigned ui = -1}; and conversions to smaller types, like
3534 @code{sqrtf (M_PI)}. Do not warn for explicit casts like @code{abs
3535 ((int) x)} and @code{ui = (unsigned) -1}, or if the value is not
3536 changed by the conversion like in @code{abs (2.0)}. Warnings about
3537 conversions between signed and unsigned integers can be disabled by
3538 using @option{-Wno-sign-conversion}.
3540 For C++, also warn for conversions between @code{NULL} and non-pointer
3541 types; confusing overload resolution for user-defined conversions; and
3542 conversions that will never use a type conversion operator:
3543 conversions to @code{void}, the same type, a base class or a reference
3544 to them. Warnings about conversions between signed and unsigned
3545 integers are disabled by default in C++ unless
3546 @option{-Wsign-conversion} is explicitly enabled.
3549 @opindex Wempty-body
3550 Warn if an empty body occurs in an @samp{if}, @samp{else} or @samp{do
3551 while} statement. Additionally, in C++, warn when an empty body occurs
3552 in a @samp{while} or @samp{for} statement with no whitespacing before
3553 the semicolon. This warning is also enabled by @option{-Wextra}.
3555 @item -Wsign-compare
3556 @opindex Wsign-compare
3557 @cindex warning for comparison of signed and unsigned values
3558 @cindex comparison of signed and unsigned values, warning
3559 @cindex signed and unsigned values, comparison warning
3560 Warn when a comparison between signed and unsigned values could produce
3561 an incorrect result when the signed value is converted to unsigned.
3562 This warning is also enabled by @option{-Wextra}; to get the other warnings
3563 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
3565 @item -Wsign-conversion
3566 @opindex Wsign-conversion
3567 @opindex Wno-sign-conversion
3568 Warn for implicit conversions that may change the sign of an integer
3569 value, like assigning a signed integer expression to an unsigned
3570 integer variable. An explicit cast silences the warning. In C, this
3571 option is enabled also by @option{-Wconversion}.
3575 @opindex Wno-address
3576 Warn about suspicious uses of memory addresses. These include using
3577 the address of a function in a conditional expression, such as
3578 @code{void func(void); if (func)}, and comparisons against the memory
3579 address of a string literal, such as @code{if (x == "abc")}. Such
3580 uses typically indicate a programmer error: the address of a function
3581 always evaluates to true, so their use in a conditional usually
3582 indicate that the programmer forgot the parentheses in a function
3583 call; and comparisons against string literals result in unspecified
3584 behavior and are not portable in C, so they usually indicate that the
3585 programmer intended to use @code{strcmp}. This warning is enabled by
3589 @opindex Wlogical-op
3590 @opindex Wno-logical-op
3591 Warn about suspicious uses of logical operators in expressions.
3592 This includes using logical operators in contexts where a
3593 bit-wise operator is likely to be expected.
3595 @item -Waggregate-return
3596 @opindex Waggregate-return
3597 Warn if any functions that return structures or unions are defined or
3598 called. (In languages where you can return an array, this also elicits
3601 @item -Wno-attributes
3602 @opindex Wno-attributes
3603 @opindex Wattributes
3604 Do not warn if an unexpected @code{__attribute__} is used, such as
3605 unrecognized attributes, function attributes applied to variables,
3606 etc. This will not stop errors for incorrect use of supported
3609 @item -Wstrict-prototypes @r{(C only)}
3610 @opindex Wstrict-prototypes
3611 Warn if a function is declared or defined without specifying the
3612 argument types. (An old-style function definition is permitted without
3613 a warning if preceded by a declaration which specifies the argument
3616 @item -Wold-style-declaration @r{(C only)}
3617 @opindex Wold-style-declaration
3618 Warn for obsolescent usages, according to the C Standard, in a
3619 declaration. For example, warn if storage-class specifiers like
3620 @code{static} are not the first things in a declaration. This warning
3621 is also enabled by @option{-Wextra}.
3623 @item -Wold-style-definition @r{(C only)}
3624 @opindex Wold-style-definition
3625 Warn if an old-style function definition is used. A warning is given
3626 even if there is a previous prototype.
3628 @item -Wmissing-parameter-type @r{(C only)}
3629 @opindex Wmissing-parameter-type
3630 A function parameter is declared without a type specifier in K&R-style
3637 This warning is also enabled by @option{-Wextra}.
3639 @item -Wmissing-prototypes @r{(C only)}
3640 @opindex Wmissing-prototypes
3641 Warn if a global function is defined without a previous prototype
3642 declaration. This warning is issued even if the definition itself
3643 provides a prototype. The aim is to detect global functions that fail
3644 to be declared in header files.
3646 @item -Wmissing-declarations @r{(C and C++ only)}
3647 @opindex Wmissing-declarations
3648 Warn if a global function is defined without a previous declaration.
3649 Do so even if the definition itself provides a prototype.
3650 Use this option to detect global functions that are not declared in
3651 header files. In C++, no warnings are issued for function templates,
3652 or for inline functions, or for functions in anonymous namespaces.
3654 @item -Wmissing-field-initializers
3655 @opindex Wmissing-field-initializers
3658 Warn if a structure's initializer has some fields missing. For
3659 example, the following code would cause such a warning, because
3660 @code{x.h} is implicitly zero:
3663 struct s @{ int f, g, h; @};
3664 struct s x = @{ 3, 4 @};
3667 This option does not warn about designated initializers, so the following
3668 modification would not trigger a warning:
3671 struct s @{ int f, g, h; @};
3672 struct s x = @{ .f = 3, .g = 4 @};
3675 This warning is included in @option{-Wextra}. To get other @option{-Wextra}
3676 warnings without this one, use @samp{-Wextra -Wno-missing-field-initializers}.
3678 @item -Wmissing-noreturn
3679 @opindex Wmissing-noreturn
3680 Warn about functions which might be candidates for attribute @code{noreturn}.
3681 Note these are only possible candidates, not absolute ones. Care should
3682 be taken to manually verify functions actually do not ever return before
3683 adding the @code{noreturn} attribute, otherwise subtle code generation
3684 bugs could be introduced. You will not get a warning for @code{main} in
3685 hosted C environments.
3687 @item -Wmissing-format-attribute
3688 @opindex Wmissing-format-attribute
3690 Warn about function pointers which might be candidates for @code{format}
3691 attributes. Note these are only possible candidates, not absolute ones.
3692 GCC will guess that function pointers with @code{format} attributes that
3693 are used in assignment, initialization, parameter passing or return
3694 statements should have a corresponding @code{format} attribute in the
3695 resulting type. I.e.@: the left-hand side of the assignment or
3696 initialization, the type of the parameter variable, or the return type
3697 of the containing function respectively should also have a @code{format}
3698 attribute to avoid the warning.
3700 GCC will also warn about function definitions which might be
3701 candidates for @code{format} attributes. Again, these are only
3702 possible candidates. GCC will guess that @code{format} attributes
3703 might be appropriate for any function that calls a function like
3704 @code{vprintf} or @code{vscanf}, but this might not always be the
3705 case, and some functions for which @code{format} attributes are
3706 appropriate may not be detected.
3708 @item -Wno-multichar
3709 @opindex Wno-multichar
3711 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
3712 Usually they indicate a typo in the user's code, as they have
3713 implementation-defined values, and should not be used in portable code.
3715 @item -Wnormalized=<none|id|nfc|nfkc>
3716 @opindex Wnormalized
3719 @cindex character set, input normalization
3720 In ISO C and ISO C++, two identifiers are different if they are
3721 different sequences of characters. However, sometimes when characters
3722 outside the basic ASCII character set are used, you can have two
3723 different character sequences that look the same. To avoid confusion,
3724 the ISO 10646 standard sets out some @dfn{normalization rules} which
3725 when applied ensure that two sequences that look the same are turned into
3726 the same sequence. GCC can warn you if you are using identifiers which
3727 have not been normalized; this option controls that warning.
3729 There are four levels of warning that GCC supports. The default is
3730 @option{-Wnormalized=nfc}, which warns about any identifier which is
3731 not in the ISO 10646 ``C'' normalized form, @dfn{NFC}. NFC is the
3732 recommended form for most uses.
3734 Unfortunately, there are some characters which ISO C and ISO C++ allow
3735 in identifiers that when turned into NFC aren't allowable as
3736 identifiers. That is, there's no way to use these symbols in portable
3737 ISO C or C++ and have all your identifiers in NFC.
3738 @option{-Wnormalized=id} suppresses the warning for these characters.
3739 It is hoped that future versions of the standards involved will correct
3740 this, which is why this option is not the default.
3742 You can switch the warning off for all characters by writing
3743 @option{-Wnormalized=none}. You would only want to do this if you
3744 were using some other normalization scheme (like ``D''), because
3745 otherwise you can easily create bugs that are literally impossible to see.
3747 Some characters in ISO 10646 have distinct meanings but look identical
3748 in some fonts or display methodologies, especially once formatting has
3749 been applied. For instance @code{\u207F}, ``SUPERSCRIPT LATIN SMALL
3750 LETTER N'', will display just like a regular @code{n} which has been
3751 placed in a superscript. ISO 10646 defines the @dfn{NFKC}
3752 normalization scheme to convert all these into a standard form as
3753 well, and GCC will warn if your code is not in NFKC if you use
3754 @option{-Wnormalized=nfkc}. This warning is comparable to warning
3755 about every identifier that contains the letter O because it might be
3756 confused with the digit 0, and so is not the default, but may be
3757 useful as a local coding convention if the programming environment is
3758 unable to be fixed to display these characters distinctly.
3760 @item -Wno-deprecated-declarations
3761 @opindex Wno-deprecated-declarations
3762 Do not warn about uses of functions (@pxref{Function Attributes}),
3763 variables (@pxref{Variable Attributes}), and types (@pxref{Type
3764 Attributes}) marked as deprecated by using the @code{deprecated}
3768 @opindex Wno-overflow
3769 Do not warn about compile-time overflow in constant expressions.
3771 @item -Woverride-init
3772 @opindex Woverride-init
3775 Warn if an initialized field without side effects is overridden when
3776 using designated initializers (@pxref{Designated Inits, , Designated
3779 This warning is included in @option{-Wextra}. To get other
3780 @option{-Wextra} warnings without this one, use @samp{-Wextra
3781 -Wno-override-init}.
3785 Warn if a structure is given the packed attribute, but the packed
3786 attribute has no effect on the layout or size of the structure.
3787 Such structures may be mis-aligned for little benefit. For
3788 instance, in this code, the variable @code{f.x} in @code{struct bar}
3789 will be misaligned even though @code{struct bar} does not itself
3790 have the packed attribute:
3797 @} __attribute__((packed));
3807 Warn if padding is included in a structure, either to align an element
3808 of the structure or to align the whole structure. Sometimes when this
3809 happens it is possible to rearrange the fields of the structure to
3810 reduce the padding and so make the structure smaller.
3812 @item -Wredundant-decls
3813 @opindex Wredundant-decls
3814 Warn if anything is declared more than once in the same scope, even in
3815 cases where multiple declaration is valid and changes nothing.
3817 @item -Wnested-externs @r{(C only)}
3818 @opindex Wnested-externs
3819 Warn if an @code{extern} declaration is encountered within a function.
3821 @item -Wunreachable-code
3822 @opindex Wunreachable-code
3823 Warn if the compiler detects that code will never be executed.
3825 This option is intended to warn when the compiler detects that at
3826 least a whole line of source code will never be executed, because
3827 some condition is never satisfied or because it is after a
3828 procedure that never returns.
3830 It is possible for this option to produce a warning even though there
3831 are circumstances under which part of the affected line can be executed,
3832 so care should be taken when removing apparently-unreachable code.
3834 For instance, when a function is inlined, a warning may mean that the
3835 line is unreachable in only one inlined copy of the function.
3837 This option is not made part of @option{-Wall} because in a debugging
3838 version of a program there is often substantial code which checks
3839 correct functioning of the program and is, hopefully, unreachable
3840 because the program does work. Another common use of unreachable
3841 code is to provide behavior which is selectable at compile-time.
3845 Warn if a function can not be inlined and it was declared as inline.
3846 Even with this option, the compiler will not warn about failures to
3847 inline functions declared in system headers.
3849 The compiler uses a variety of heuristics to determine whether or not
3850 to inline a function. For example, the compiler takes into account
3851 the size of the function being inlined and the amount of inlining
3852 that has already been done in the current function. Therefore,
3853 seemingly insignificant changes in the source program can cause the
3854 warnings produced by @option{-Winline} to appear or disappear.
3856 @item -Wno-invalid-offsetof @r{(C++ only)}
3857 @opindex Wno-invalid-offsetof
3858 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
3859 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
3860 to a non-POD type is undefined. In existing C++ implementations,
3861 however, @samp{offsetof} typically gives meaningful results even when
3862 applied to certain kinds of non-POD types. (Such as a simple
3863 @samp{struct} that fails to be a POD type only by virtue of having a
3864 constructor.) This flag is for users who are aware that they are
3865 writing nonportable code and who have deliberately chosen to ignore the
3868 The restrictions on @samp{offsetof} may be relaxed in a future version
3869 of the C++ standard.
3871 @item -Wno-int-to-pointer-cast @r{(C only)}
3872 @opindex Wno-int-to-pointer-cast
3873 Suppress warnings from casts to pointer type of an integer of a
3876 @item -Wno-pointer-to-int-cast @r{(C only)}
3877 @opindex Wno-pointer-to-int-cast
3878 Suppress warnings from casts from a pointer to an integer type of a
3882 @opindex Winvalid-pch
3883 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
3884 the search path but can't be used.
3888 @opindex Wno-long-long
3889 Warn if @samp{long long} type is used. This is default. To inhibit
3890 the warning messages, use @option{-Wno-long-long}. Flags
3891 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
3892 only when @option{-pedantic} flag is used.
3894 @item -Wvariadic-macros
3895 @opindex Wvariadic-macros
3896 @opindex Wno-variadic-macros
3897 Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
3898 alternate syntax when in pedantic ISO C99 mode. This is default.
3899 To inhibit the warning messages, use @option{-Wno-variadic-macros}.
3904 Warn if variable length array is used in the code.
3905 @option{-Wno-vla} will prevent the @option{-pedantic} warning of
3906 the variable length array.
3908 @item -Wvolatile-register-var
3909 @opindex Wvolatile-register-var
3910 @opindex Wno-volatile-register-var
3911 Warn if a register variable is declared volatile. The volatile
3912 modifier does not inhibit all optimizations that may eliminate reads
3913 and/or writes to register variables.
3915 @item -Wdisabled-optimization
3916 @opindex Wdisabled-optimization
3917 Warn if a requested optimization pass is disabled. This warning does
3918 not generally indicate that there is anything wrong with your code; it
3919 merely indicates that GCC's optimizers were unable to handle the code
3920 effectively. Often, the problem is that your code is too big or too
3921 complex; GCC will refuse to optimize programs when the optimization
3922 itself is likely to take inordinate amounts of time.
3924 @item -Wpointer-sign
3925 @opindex Wpointer-sign
3926 @opindex Wno-pointer-sign
3927 Warn for pointer argument passing or assignment with different signedness.
3928 This option is only supported for C and Objective-C@. It is implied by
3929 @option{-Wall} and by @option{-pedantic}, which can be disabled with
3930 @option{-Wno-pointer-sign}.
3932 @item -Wstack-protector
3933 @opindex Wstack-protector
3934 This option is only active when @option{-fstack-protector} is active. It
3935 warns about functions that will not be protected against stack smashing.
3937 @item -Woverlength-strings
3938 @opindex Woverlength-strings
3939 Warn about string constants which are longer than the ``minimum
3940 maximum'' length specified in the C standard. Modern compilers
3941 generally allow string constants which are much longer than the
3942 standard's minimum limit, but very portable programs should avoid
3943 using longer strings.
3945 The limit applies @emph{after} string constant concatenation, and does
3946 not count the trailing NUL@. In C89, the limit was 509 characters; in
3947 C99, it was raised to 4095. C++98 does not specify a normative
3948 minimum maximum, so we do not diagnose overlength strings in C++@.
3950 This option is implied by @option{-pedantic}, and can be disabled with
3951 @option{-Wno-overlength-strings}.
3954 @node Debugging Options
3955 @section Options for Debugging Your Program or GCC
3956 @cindex options, debugging
3957 @cindex debugging information options
3959 GCC has various special options that are used for debugging
3960 either your program or GCC:
3965 Produce debugging information in the operating system's native format
3966 (stabs, COFF, XCOFF, or DWARF 2)@. GDB can work with this debugging
3969 On most systems that use stabs format, @option{-g} enables use of extra
3970 debugging information that only GDB can use; this extra information
3971 makes debugging work better in GDB but will probably make other debuggers
3973 refuse to read the program. If you want to control for certain whether
3974 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
3975 @option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
3977 GCC allows you to use @option{-g} with
3978 @option{-O}. The shortcuts taken by optimized code may occasionally
3979 produce surprising results: some variables you declared may not exist
3980 at all; flow of control may briefly move where you did not expect it;
3981 some statements may not be executed because they compute constant
3982 results or their values were already at hand; some statements may
3983 execute in different places because they were moved out of loops.
3985 Nevertheless it proves possible to debug optimized output. This makes
3986 it reasonable to use the optimizer for programs that might have bugs.
3988 The following options are useful when GCC is generated with the
3989 capability for more than one debugging format.
3993 Produce debugging information for use by GDB@. This means to use the
3994 most expressive format available (DWARF 2, stabs, or the native format
3995 if neither of those are supported), including GDB extensions if at all
4000 Produce debugging information in stabs format (if that is supported),
4001 without GDB extensions. This is the format used by DBX on most BSD
4002 systems. On MIPS, Alpha and System V Release 4 systems this option
4003 produces stabs debugging output which is not understood by DBX or SDB@.
4004 On System V Release 4 systems this option requires the GNU assembler.
4006 @item -feliminate-unused-debug-symbols
4007 @opindex feliminate-unused-debug-symbols
4008 Produce debugging information in stabs format (if that is supported),
4009 for only symbols that are actually used.
4011 @item -femit-class-debug-always
4012 Instead of emitting debugging information for a C++ class in only one
4013 object file, emit it in all object files using the class. This option
4014 should be used only with debuggers that are unable to handle the way GCC
4015 normally emits debugging information for classes because using this
4016 option will increase the size of debugging information by as much as a
4021 Produce debugging information in stabs format (if that is supported),
4022 using GNU extensions understood only by the GNU debugger (GDB)@. The
4023 use of these extensions is likely to make other debuggers crash or
4024 refuse to read the program.
4028 Produce debugging information in COFF format (if that is supported).
4029 This is the format used by SDB on most System V systems prior to
4034 Produce debugging information in XCOFF format (if that is supported).
4035 This is the format used by the DBX debugger on IBM RS/6000 systems.
4039 Produce debugging information in XCOFF format (if that is supported),
4040 using GNU extensions understood only by the GNU debugger (GDB)@. The
4041 use of these extensions is likely to make other debuggers crash or
4042 refuse to read the program, and may cause assemblers other than the GNU
4043 assembler (GAS) to fail with an error.
4047 Produce debugging information in DWARF version 2 format (if that is
4048 supported). This is the format used by DBX on IRIX 6. With this
4049 option, GCC uses features of DWARF version 3 when they are useful;
4050 version 3 is upward compatible with version 2, but may still cause
4051 problems for older debuggers.
4055 Produce debugging information in VMS debug format (if that is
4056 supported). This is the format used by DEBUG on VMS systems.
4059 @itemx -ggdb@var{level}
4060 @itemx -gstabs@var{level}
4061 @itemx -gcoff@var{level}
4062 @itemx -gxcoff@var{level}
4063 @itemx -gvms@var{level}
4064 Request debugging information and also use @var{level} to specify how
4065 much information. The default level is 2.
4067 Level 0 produces no debug information at all. Thus, @option{-g0} negates
4070 Level 1 produces minimal information, enough for making backtraces in
4071 parts of the program that you don't plan to debug. This includes
4072 descriptions of functions and external variables, but no information
4073 about local variables and no line numbers.
4075 Level 3 includes extra information, such as all the macro definitions
4076 present in the program. Some debuggers support macro expansion when
4077 you use @option{-g3}.
4079 @option{-gdwarf-2} does not accept a concatenated debug level, because
4080 GCC used to support an option @option{-gdwarf} that meant to generate
4081 debug information in version 1 of the DWARF format (which is very
4082 different from version 2), and it would have been too confusing. That
4083 debug format is long obsolete, but the option cannot be changed now.
4084 Instead use an additional @option{-g@var{level}} option to change the
4085 debug level for DWARF2.
4087 @item -feliminate-dwarf2-dups
4088 @opindex feliminate-dwarf2-dups
4089 Compress DWARF2 debugging information by eliminating duplicated
4090 information about each symbol. This option only makes sense when
4091 generating DWARF2 debugging information with @option{-gdwarf-2}.
4093 @item -femit-struct-debug-baseonly
4094 Emit debug information for struct-like types
4095 only when the base name of the compilation source file
4096 matches the base name of file in which the struct was defined.
4098 This option substantially reduces the size of debugging information,
4099 but at significant potential loss in type information to the debugger.
4100 See @option{-femit-struct-debug-reduced} for a less aggressive option.
4101 See @option{-femit-struct-debug-detailed} for more detailed control.
4103 This option works only with DWARF 2.
4105 @item -femit-struct-debug-reduced
4106 Emit debug information for struct-like types
4107 only when the base name of the compilation source file
4108 matches the base name of file in which the type was defined,
4109 unless the struct is a template or defined in a system header.
4111 This option significantly reduces the size of debugging information,
4112 with some potential loss in type information to the debugger.
4113 See @option{-femit-struct-debug-baseonly} for a more aggressive option.
4114 See @option{-femit-struct-debug-detailed} for more detailed control.
4116 This option works only with DWARF 2.
4118 @item -femit-struct-debug-detailed@r{[}=@var{spec-list}@r{]}
4119 Specify the struct-like types
4120 for which the compiler will generate debug information.
4121 The intent is to reduce duplicate struct debug information
4122 between different object files within the same program.
4124 This option is a detailed version of
4125 @option{-femit-struct-debug-reduced} and @option{-femit-struct-debug-baseonly},
4126 which will serve for most needs.
4128 A specification has the syntax
4129 [@samp{dir:}|@samp{ind:}][@samp{ord:}|@samp{gen:}](@samp{any}|@samp{sys}|@samp{base}|@samp{none})
4131 The optional first word limits the specification to
4132 structs that are used directly (@samp{dir:}) or used indirectly (@samp{ind:}).
4133 A struct type is used directly when it is the type of a variable, member.
4134 Indirect uses arise through pointers to structs.
4135 That is, when use of an incomplete struct would be legal, the use is indirect.
4137 @samp{struct one direct; struct two * indirect;}.
4139 The optional second word limits the specification to
4140 ordinary structs (@samp{ord:}) or generic structs (@samp{gen:}).
4141 Generic structs are a bit complicated to explain.
4142 For C++, these are non-explicit specializations of template classes,
4143 or non-template classes within the above.
4144 Other programming languages have generics,
4145 but @samp{-femit-struct-debug-detailed} does not yet implement them.
4147 The third word specifies the source files for those
4148 structs for which the compiler will emit debug information.
4149 The values @samp{none} and @samp{any} have the normal meaning.
4150 The value @samp{base} means that
4151 the base of name of the file in which the type declaration appears
4152 must match the base of the name of the main compilation file.
4153 In practice, this means that
4154 types declared in @file{foo.c} and @file{foo.h} will have debug information,
4155 but types declared in other header will not.
4156 The value @samp{sys} means those types satisfying @samp{base}
4157 or declared in system or compiler headers.
4159 You may need to experiment to determine the best settings for your application.
4161 The default is @samp{-femit-struct-debug-detailed=all}.
4163 This option works only with DWARF 2.
4165 @item -fdebug-prefix-map=@var{old}=@var{new}
4166 @opindex fdebug-prefix-map
4167 When compiling files in directory @file{@var{old}}, record debugging
4168 information describing them as in @file{@var{new}} instead.
4170 @cindex @command{prof}
4173 Generate extra code to write profile information suitable for the
4174 analysis program @command{prof}. You must use this option when compiling
4175 the source files you want data about, and you must also use it when
4178 @cindex @command{gprof}
4181 Generate extra code to write profile information suitable for the
4182 analysis program @command{gprof}. You must use this option when compiling
4183 the source files you want data about, and you must also use it when
4188 Makes the compiler print out each function name as it is compiled, and
4189 print some statistics about each pass when it finishes.
4192 @opindex ftime-report
4193 Makes the compiler print some statistics about the time consumed by each
4194 pass when it finishes.
4197 @opindex fmem-report
4198 Makes the compiler print some statistics about permanent memory
4199 allocation when it finishes.
4201 @item -fpre-ipa-mem-report
4202 @opindex fpre-ipa-mem-report
4203 @item -fpost-ipa-mem-report
4204 @opindex fpost-ipa-mem-report
4205 Makes the compiler print some statistics about permanent memory
4206 allocation before or after interprocedural optimization.
4208 @item -fprofile-arcs
4209 @opindex fprofile-arcs
4210 Add code so that program flow @dfn{arcs} are instrumented. During
4211 execution the program records how many times each branch and call is
4212 executed and how many times it is taken or returns. When the compiled
4213 program exits it saves this data to a file called
4214 @file{@var{auxname}.gcda} for each source file. The data may be used for
4215 profile-directed optimizations (@option{-fbranch-probabilities}), or for
4216 test coverage analysis (@option{-ftest-coverage}). Each object file's
4217 @var{auxname} is generated from the name of the output file, if
4218 explicitly specified and it is not the final executable, otherwise it is
4219 the basename of the source file. In both cases any suffix is removed
4220 (e.g.@: @file{foo.gcda} for input file @file{dir/foo.c}, or
4221 @file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
4222 @xref{Cross-profiling}.
4224 @cindex @command{gcov}
4228 This option is used to compile and link code instrumented for coverage
4229 analysis. The option is a synonym for @option{-fprofile-arcs}
4230 @option{-ftest-coverage} (when compiling) and @option{-lgcov} (when
4231 linking). See the documentation for those options for more details.
4236 Compile the source files with @option{-fprofile-arcs} plus optimization
4237 and code generation options. For test coverage analysis, use the
4238 additional @option{-ftest-coverage} option. You do not need to profile
4239 every source file in a program.
4242 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
4243 (the latter implies the former).
4246 Run the program on a representative workload to generate the arc profile
4247 information. This may be repeated any number of times. You can run
4248 concurrent instances of your program, and provided that the file system
4249 supports locking, the data files will be correctly updated. Also
4250 @code{fork} calls are detected and correctly handled (double counting
4254 For profile-directed optimizations, compile the source files again with
4255 the same optimization and code generation options plus
4256 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
4257 Control Optimization}).
4260 For test coverage analysis, use @command{gcov} to produce human readable
4261 information from the @file{.gcno} and @file{.gcda} files. Refer to the
4262 @command{gcov} documentation for further information.
4266 With @option{-fprofile-arcs}, for each function of your program GCC
4267 creates a program flow graph, then finds a spanning tree for the graph.
4268 Only arcs that are not on the spanning tree have to be instrumented: the
4269 compiler adds code to count the number of times that these arcs are
4270 executed. When an arc is the only exit or only entrance to a block, the
4271 instrumentation code can be added to the block; otherwise, a new basic
4272 block must be created to hold the instrumentation code.
4275 @item -ftest-coverage
4276 @opindex ftest-coverage
4277 Produce a notes file that the @command{gcov} code-coverage utility
4278 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
4279 show program coverage. Each source file's note file is called
4280 @file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
4281 above for a description of @var{auxname} and instructions on how to
4282 generate test coverage data. Coverage data will match the source files
4283 more closely, if you do not optimize.
4285 @item -fdbg-cnt-list
4286 @opindex fdbg-cnt-list
4287 Print the name and the counter upperbound for all debug counters.
4289 @item -fdbg-cnt=@var{counter-value-list}
4291 Set the internal debug counter upperbound. @var{counter-value-list}
4292 is a comma-separated list of @var{name}:@var{value} pairs
4293 which sets the upperbound of each debug counter @var{name} to @var{value}.
4294 All debug counters have the initial upperbound of @var{UINT_MAX},
4295 thus dbg_cnt() returns true always unless the upperbound is set by this option.
4296 e.g. With -fdbg-cnt=dce:10,tail_call:0
4297 dbg_cnt(dce) will return true only for first 10 invocations
4298 and dbg_cnt(tail_call) will return false always.
4300 @item -d@var{letters}
4301 @item -fdump-rtl-@var{pass}
4303 Says to make debugging dumps during compilation at times specified by
4304 @var{letters}. This is used for debugging the RTL-based passes of the
4305 compiler. The file names for most of the dumps are made by appending a
4306 pass number and a word to the @var{dumpname}. @var{dumpname} is generated
4307 from the name of the output file, if explicitly specified and it is not
4308 an executable, otherwise it is the basename of the source file. These
4309 switches may have different effects when @option{-E} is used for
4312 Most debug dumps can be enabled either passing a letter to the @option{-d}
4313 option, or with a long @option{-fdump-rtl} switch; here are the possible
4314 letters for use in @var{letters} and @var{pass}, and their meanings:
4319 Annotate the assembler output with miscellaneous debugging information.
4322 @itemx -fdump-rtl-bbro
4324 @opindex fdump-rtl-bbro
4325 Dump after block reordering, to @file{@var{file}.148r.bbro}.
4328 @itemx -fdump-rtl-combine
4330 @opindex fdump-rtl-combine
4331 Dump after the RTL instruction combination pass, to the file
4332 @file{@var{file}.129r.combine}.
4335 @itemx -fdump-rtl-ce1
4336 @itemx -fdump-rtl-ce2
4338 @opindex fdump-rtl-ce1
4339 @opindex fdump-rtl-ce2
4340 @option{-dC} and @option{-fdump-rtl-ce1} enable dumping after the
4341 first if conversion, to the file @file{@var{file}.117r.ce1}. @option{-dC}
4342 and @option{-fdump-rtl-ce2} enable dumping after the second if
4343 conversion, to the file @file{@var{file}.130r.ce2}.
4346 @itemx -fdump-rtl-btl
4347 @itemx -fdump-rtl-dbr
4349 @opindex fdump-rtl-btl
4350 @opindex fdump-rtl-dbr
4351 @option{-dd} and @option{-fdump-rtl-btl} enable dumping after branch
4352 target load optimization, to @file{@var{file}.31.btl}. @option{-dd}
4353 and @option{-fdump-rtl-dbr} enable dumping after delayed branch
4354 scheduling, to @file{@var{file}.36.dbr}.
4358 Dump all macro definitions, at the end of preprocessing, in addition to
4362 @itemx -fdump-rtl-ce3
4364 @opindex fdump-rtl-ce3
4365 Dump after the third if conversion, to @file{@var{file}.146r.ce3}.
4368 @itemx -fdump-rtl-cfg
4369 @itemx -fdump-rtl-life
4371 @opindex fdump-rtl-cfg
4372 @opindex fdump-rtl-life
4373 @option{-df} and @option{-fdump-rtl-cfg} enable dumping after control
4374 and data flow analysis, to @file{@var{file}.116r.cfg}. @option{-df}
4375 and @option{-fdump-rtl-cfg} enable dumping dump after life analysis,
4376 to @file{@var{file}.128r.life1} and @file{@var{file}.135r.life2}.
4379 @itemx -fdump-rtl-greg
4381 @opindex fdump-rtl-greg
4382 Dump after global register allocation, to @file{@var{file}.139r.greg}.
4385 @itemx -fdump-rtl-gcse
4386 @itemx -fdump-rtl-bypass
4388 @opindex fdump-rtl-gcse
4389 @opindex fdump-rtl-bypass
4390 @option{-dG} and @option{-fdump-rtl-gcse} enable dumping after GCSE, to
4391 @file{@var{file}.114r.gcse}. @option{-dG} and @option{-fdump-rtl-bypass}
4392 enable dumping after jump bypassing and control flow optimizations, to
4393 @file{@var{file}.115r.bypass}.
4396 @itemx -fdump-rtl-eh
4398 @opindex fdump-rtl-eh
4399 Dump after finalization of EH handling code, to @file{@var{file}.02.eh}.
4402 @itemx -fdump-rtl-sibling
4404 @opindex fdump-rtl-sibling
4405 Dump after sibling call optimizations, to @file{@var{file}.106r.sibling}.
4408 @itemx -fdump-rtl-jump
4410 @opindex fdump-rtl-jump
4411 Dump after the first jump optimization, to @file{@var{file}.112r.jump}.
4414 @itemx -fdump-rtl-stack
4416 @opindex fdump-rtl-stack
4417 Dump after conversion from GCC's "flat register file" registers to the
4418 x87's stack-like registers, to @file{@var{file}.152r.stack}.
4421 @itemx -fdump-rtl-lreg
4423 @opindex fdump-rtl-lreg
4424 Dump after local register allocation, to @file{@var{file}.138r.lreg}.
4427 @itemx -fdump-rtl-loop2
4429 @opindex fdump-rtl-loop2
4430 @option{-dL} and @option{-fdump-rtl-loop2} enable dumping after the
4431 loop optimization pass, to @file{@var{file}.119r.loop2},
4432 @file{@var{file}.120r.loop2_init},
4433 @file{@var{file}.121r.loop2_invariant}, and
4434 @file{@var{file}.125r.loop2_done}.
4437 @itemx -fdump-rtl-sms
4439 @opindex fdump-rtl-sms
4440 Dump after modulo scheduling, to @file{@var{file}.136r.sms}.
4443 @itemx -fdump-rtl-mach
4445 @opindex fdump-rtl-mach
4446 Dump after performing the machine dependent reorganization pass, to
4447 @file{@var{file}.155r.mach} if that pass exists.
4450 @itemx -fdump-rtl-rnreg
4452 @opindex fdump-rtl-rnreg
4453 Dump after register renumbering, to @file{@var{file}.147r.rnreg}.
4456 @itemx -fdump-rtl-regmove
4458 @opindex fdump-rtl-regmove
4459 Dump after the register move pass, to @file{@var{file}.132r.regmove}.
4462 @itemx -fdump-rtl-postreload
4464 @opindex fdump-rtl-postreload
4465 Dump after post-reload optimizations, to @file{@var{file}.24.postreload}.
4468 @itemx -fdump-rtl-expand
4470 @opindex fdump-rtl-expand
4471 Dump after RTL generation, to @file{@var{file}.104r.expand}.
4474 @itemx -fdump-rtl-sched2
4476 @opindex fdump-rtl-sched2
4477 Dump after the second scheduling pass, to @file{@var{file}.149r.sched2}.
4480 @itemx -fdump-rtl-cse
4482 @opindex fdump-rtl-cse
4483 Dump after CSE (including the jump optimization that sometimes follows
4484 CSE), to @file{@var{file}.113r.cse}.
4487 @itemx -fdump-rtl-sched1
4489 @opindex fdump-rtl-sched1
4490 Dump after the first scheduling pass, to @file{@var{file}.136r.sched1}.
4493 @itemx -fdump-rtl-cse2
4495 @opindex fdump-rtl-cse2
4496 Dump after the second CSE pass (including the jump optimization that
4497 sometimes follows CSE), to @file{@var{file}.127r.cse2}.
4500 @itemx -fdump-rtl-tracer
4502 @opindex fdump-rtl-tracer
4503 Dump after running tracer, to @file{@var{file}.118r.tracer}.
4506 @itemx -fdump-rtl-vpt
4507 @itemx -fdump-rtl-vartrack
4509 @opindex fdump-rtl-vpt
4510 @opindex fdump-rtl-vartrack
4511 @option{-dV} and @option{-fdump-rtl-vpt} enable dumping after the value
4512 profile transformations, to @file{@var{file}.10.vpt}. @option{-dV}
4513 and @option{-fdump-rtl-vartrack} enable dumping after variable tracking,
4514 to @file{@var{file}.154r.vartrack}.
4517 @itemx -fdump-rtl-flow2
4519 @opindex fdump-rtl-flow2
4520 Dump after the second flow pass, to @file{@var{file}.142r.flow2}.
4523 @itemx -fdump-rtl-peephole2
4525 @opindex fdump-rtl-peephole2
4526 Dump after the peephole pass, to @file{@var{file}.145r.peephole2}.
4529 @itemx -fdump-rtl-web
4531 @opindex fdump-rtl-web
4532 Dump after live range splitting, to @file{@var{file}.126r.web}.
4535 @itemx -fdump-rtl-all
4537 @opindex fdump-rtl-all
4538 Produce all the dumps listed above.
4542 Produce a core dump whenever an error occurs.
4546 Print statistics on memory usage, at the end of the run, to
4551 Annotate the assembler output with a comment indicating which
4552 pattern and alternative was used. The length of each instruction is
4557 Dump the RTL in the assembler output as a comment before each instruction.
4558 Also turns on @option{-dp} annotation.
4562 For each of the other indicated dump files (either with @option{-d} or
4563 @option{-fdump-rtl-@var{pass}}), dump a representation of the control flow
4564 graph suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
4568 Just generate RTL for a function instead of compiling it. Usually used
4569 with @samp{r} (@option{-fdump-rtl-expand}).
4573 Dump debugging information during parsing, to standard error.
4577 @opindex fdump-noaddr
4578 When doing debugging dumps (see @option{-d} option above), suppress
4579 address output. This makes it more feasible to use diff on debugging
4580 dumps for compiler invocations with different compiler binaries and/or
4581 different text / bss / data / heap / stack / dso start locations.
4583 @item -fdump-unnumbered
4584 @opindex fdump-unnumbered
4585 When doing debugging dumps (see @option{-d} option above), suppress instruction
4586 numbers and address output. This makes it more feasible to
4587 use diff on debugging dumps for compiler invocations with different
4588 options, in particular with and without @option{-g}.
4590 @item -fdump-translation-unit @r{(C++ only)}
4591 @itemx -fdump-translation-unit-@var{options} @r{(C++ only)}
4592 @opindex fdump-translation-unit
4593 Dump a representation of the tree structure for the entire translation
4594 unit to a file. The file name is made by appending @file{.tu} to the
4595 source file name. If the @samp{-@var{options}} form is used, @var{options}
4596 controls the details of the dump as described for the
4597 @option{-fdump-tree} options.
4599 @item -fdump-class-hierarchy @r{(C++ only)}
4600 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
4601 @opindex fdump-class-hierarchy
4602 Dump a representation of each class's hierarchy and virtual function
4603 table layout to a file. The file name is made by appending @file{.class}
4604 to the source file name. If the @samp{-@var{options}} form is used,
4605 @var{options} controls the details of the dump as described for the
4606 @option{-fdump-tree} options.
4608 @item -fdump-ipa-@var{switch}
4610 Control the dumping at various stages of inter-procedural analysis
4611 language tree to a file. The file name is generated by appending a switch
4612 specific suffix to the source file name. The following dumps are possible:
4616 Enables all inter-procedural analysis dumps; currently the only produced
4617 dump is the @samp{cgraph} dump.
4620 Dumps information about call-graph optimization, unused function removal,
4621 and inlining decisions.
4624 @item -fdump-tree-@var{switch}
4625 @itemx -fdump-tree-@var{switch}-@var{options}
4627 Control the dumping at various stages of processing the intermediate
4628 language tree to a file. The file name is generated by appending a switch
4629 specific suffix to the source file name. If the @samp{-@var{options}}
4630 form is used, @var{options} is a list of @samp{-} separated options that
4631 control the details of the dump. Not all options are applicable to all
4632 dumps, those which are not meaningful will be ignored. The following
4633 options are available
4637 Print the address of each node. Usually this is not meaningful as it
4638 changes according to the environment and source file. Its primary use
4639 is for tying up a dump file with a debug environment.
4641 Inhibit dumping of members of a scope or body of a function merely
4642 because that scope has been reached. Only dump such items when they
4643 are directly reachable by some other path. When dumping pretty-printed
4644 trees, this option inhibits dumping the bodies of control structures.
4646 Print a raw representation of the tree. By default, trees are
4647 pretty-printed into a C-like representation.
4649 Enable more detailed dumps (not honored by every dump option).
4651 Enable dumping various statistics about the pass (not honored by every dump
4654 Enable showing basic block boundaries (disabled in raw dumps).
4656 Enable showing virtual operands for every statement.
4658 Enable showing line numbers for statements.
4660 Enable showing the unique ID (@code{DECL_UID}) for each variable.
4662 Turn on all options, except @option{raw}, @option{slim} and @option{lineno}.
4665 The following tree dumps are possible:
4669 Dump before any tree based optimization, to @file{@var{file}.original}.
4672 Dump after all tree based optimization, to @file{@var{file}.optimized}.
4675 Dump after function inlining, to @file{@var{file}.inlined}.
4678 @opindex fdump-tree-gimple
4679 Dump each function before and after the gimplification pass to a file. The
4680 file name is made by appending @file{.gimple} to the source file name.
4683 @opindex fdump-tree-cfg
4684 Dump the control flow graph of each function to a file. The file name is
4685 made by appending @file{.cfg} to the source file name.
4688 @opindex fdump-tree-vcg
4689 Dump the control flow graph of each function to a file in VCG format. The
4690 file name is made by appending @file{.vcg} to the source file name. Note
4691 that if the file contains more than one function, the generated file cannot
4692 be used directly by VCG@. You will need to cut and paste each function's
4693 graph into its own separate file first.
4696 @opindex fdump-tree-ch
4697 Dump each function after copying loop headers. The file name is made by
4698 appending @file{.ch} to the source file name.
4701 @opindex fdump-tree-ssa
4702 Dump SSA related information to a file. The file name is made by appending
4703 @file{.ssa} to the source file name.
4706 @opindex fdump-tree-salias
4707 Dump structure aliasing variable information to a file. This file name
4708 is made by appending @file{.salias} to the source file name.
4711 @opindex fdump-tree-alias
4712 Dump aliasing information for each function. The file name is made by
4713 appending @file{.alias} to the source file name.
4716 @opindex fdump-tree-ccp
4717 Dump each function after CCP@. The file name is made by appending
4718 @file{.ccp} to the source file name.
4721 @opindex fdump-tree-storeccp
4722 Dump each function after STORE-CCP. The file name is made by appending
4723 @file{.storeccp} to the source file name.
4726 @opindex fdump-tree-pre
4727 Dump trees after partial redundancy elimination. The file name is made
4728 by appending @file{.pre} to the source file name.
4731 @opindex fdump-tree-fre
4732 Dump trees after full redundancy elimination. The file name is made
4733 by appending @file{.fre} to the source file name.
4736 @opindex fdump-tree-copyprop
4737 Dump trees after copy propagation. The file name is made
4738 by appending @file{.copyprop} to the source file name.
4740 @item store_copyprop
4741 @opindex fdump-tree-store_copyprop
4742 Dump trees after store copy-propagation. The file name is made
4743 by appending @file{.store_copyprop} to the source file name.
4746 @opindex fdump-tree-dce
4747 Dump each function after dead code elimination. The file name is made by
4748 appending @file{.dce} to the source file name.
4751 @opindex fdump-tree-mudflap
4752 Dump each function after adding mudflap instrumentation. The file name is
4753 made by appending @file{.mudflap} to the source file name.
4756 @opindex fdump-tree-sra
4757 Dump each function after performing scalar replacement of aggregates. The
4758 file name is made by appending @file{.sra} to the source file name.
4761 @opindex fdump-tree-sink
4762 Dump each function after performing code sinking. The file name is made
4763 by appending @file{.sink} to the source file name.
4766 @opindex fdump-tree-dom
4767 Dump each function after applying dominator tree optimizations. The file
4768 name is made by appending @file{.dom} to the source file name.
4771 @opindex fdump-tree-dse
4772 Dump each function after applying dead store elimination. The file
4773 name is made by appending @file{.dse} to the source file name.
4776 @opindex fdump-tree-phiopt
4777 Dump each function after optimizing PHI nodes into straightline code. The file
4778 name is made by appending @file{.phiopt} to the source file name.
4781 @opindex fdump-tree-forwprop
4782 Dump each function after forward propagating single use variables. The file
4783 name is made by appending @file{.forwprop} to the source file name.
4786 @opindex fdump-tree-copyrename
4787 Dump each function after applying the copy rename optimization. The file
4788 name is made by appending @file{.copyrename} to the source file name.
4791 @opindex fdump-tree-nrv
4792 Dump each function after applying the named return value optimization on
4793 generic trees. The file name is made by appending @file{.nrv} to the source
4797 @opindex fdump-tree-vect
4798 Dump each function after applying vectorization of loops. The file name is
4799 made by appending @file{.vect} to the source file name.
4802 @opindex fdump-tree-vrp
4803 Dump each function after Value Range Propagation (VRP). The file name
4804 is made by appending @file{.vrp} to the source file name.
4807 @opindex fdump-tree-all
4808 Enable all the available tree dumps with the flags provided in this option.
4811 @item -ftree-vectorizer-verbose=@var{n}
4812 @opindex ftree-vectorizer-verbose
4813 This option controls the amount of debugging output the vectorizer prints.
4814 This information is written to standard error, unless
4815 @option{-fdump-tree-all} or @option{-fdump-tree-vect} is specified,
4816 in which case it is output to the usual dump listing file, @file{.vect}.
4817 For @var{n}=0 no diagnostic information is reported.
4818 If @var{n}=1 the vectorizer reports each loop that got vectorized,
4819 and the total number of loops that got vectorized.
4820 If @var{n}=2 the vectorizer also reports non-vectorized loops that passed
4821 the first analysis phase (vect_analyze_loop_form) - i.e. countable,
4822 inner-most, single-bb, single-entry/exit loops. This is the same verbosity
4823 level that @option{-fdump-tree-vect-stats} uses.
4824 Higher verbosity levels mean either more information dumped for each
4825 reported loop, or same amount of information reported for more loops:
4826 If @var{n}=3, alignment related information is added to the reports.
4827 If @var{n}=4, data-references related information (e.g. memory dependences,
4828 memory access-patterns) is added to the reports.
4829 If @var{n}=5, the vectorizer reports also non-vectorized inner-most loops
4830 that did not pass the first analysis phase (i.e. may not be countable, or
4831 may have complicated control-flow).
4832 If @var{n}=6, the vectorizer reports also non-vectorized nested loops.
4833 For @var{n}=7, all the information the vectorizer generates during its
4834 analysis and transformation is reported. This is the same verbosity level
4835 that @option{-fdump-tree-vect-details} uses.
4837 @item -frandom-seed=@var{string}
4838 @opindex frandom-string
4839 This option provides a seed that GCC uses when it would otherwise use
4840 random numbers. It is used to generate certain symbol names
4841 that have to be different in every compiled file. It is also used to
4842 place unique stamps in coverage data files and the object files that
4843 produce them. You can use the @option{-frandom-seed} option to produce
4844 reproducibly identical object files.
4846 The @var{string} should be different for every file you compile.
4848 @item -fsched-verbose=@var{n}
4849 @opindex fsched-verbose
4850 On targets that use instruction scheduling, this option controls the
4851 amount of debugging output the scheduler prints. This information is
4852 written to standard error, unless @option{-dS} or @option{-dR} is
4853 specified, in which case it is output to the usual dump
4854 listing file, @file{.sched} or @file{.sched2} respectively. However
4855 for @var{n} greater than nine, the output is always printed to standard
4858 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
4859 same information as @option{-dRS}. For @var{n} greater than one, it
4860 also output basic block probabilities, detailed ready list information
4861 and unit/insn info. For @var{n} greater than two, it includes RTL
4862 at abort point, control-flow and regions info. And for @var{n} over
4863 four, @option{-fsched-verbose} also includes dependence info.
4867 Store the usual ``temporary'' intermediate files permanently; place them
4868 in the current directory and name them based on the source file. Thus,
4869 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
4870 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
4871 preprocessed @file{foo.i} output file even though the compiler now
4872 normally uses an integrated preprocessor.
4874 When used in combination with the @option{-x} command line option,
4875 @option{-save-temps} is sensible enough to avoid over writing an
4876 input source file with the same extension as an intermediate file.
4877 The corresponding intermediate file may be obtained by renaming the
4878 source file before using @option{-save-temps}.
4882 Report the CPU time taken by each subprocess in the compilation
4883 sequence. For C source files, this is the compiler proper and assembler
4884 (plus the linker if linking is done). The output looks like this:
4891 The first number on each line is the ``user time'', that is time spent
4892 executing the program itself. The second number is ``system time'',
4893 time spent executing operating system routines on behalf of the program.
4894 Both numbers are in seconds.
4896 @item -fvar-tracking
4897 @opindex fvar-tracking
4898 Run variable tracking pass. It computes where variables are stored at each
4899 position in code. Better debugging information is then generated
4900 (if the debugging information format supports this information).
4902 It is enabled by default when compiling with optimization (@option{-Os},
4903 @option{-O}, @option{-O2}, ...), debugging information (@option{-g}) and
4904 the debug info format supports it.
4906 @item -print-file-name=@var{library}
4907 @opindex print-file-name
4908 Print the full absolute name of the library file @var{library} that
4909 would be used when linking---and don't do anything else. With this
4910 option, GCC does not compile or link anything; it just prints the
4913 @item -print-multi-directory
4914 @opindex print-multi-directory
4915 Print the directory name corresponding to the multilib selected by any
4916 other switches present in the command line. This directory is supposed
4917 to exist in @env{GCC_EXEC_PREFIX}.
4919 @item -print-multi-lib
4920 @opindex print-multi-lib
4921 Print the mapping from multilib directory names to compiler switches
4922 that enable them. The directory name is separated from the switches by
4923 @samp{;}, and each switch starts with an @samp{@@} instead of the
4924 @samp{-}, without spaces between multiple switches. This is supposed to
4925 ease shell-processing.
4927 @item -print-prog-name=@var{program}
4928 @opindex print-prog-name
4929 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
4931 @item -print-libgcc-file-name
4932 @opindex print-libgcc-file-name
4933 Same as @option{-print-file-name=libgcc.a}.
4935 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
4936 but you do want to link with @file{libgcc.a}. You can do
4939 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
4942 @item -print-search-dirs
4943 @opindex print-search-dirs
4944 Print the name of the configured installation directory and a list of
4945 program and library directories @command{gcc} will search---and don't do anything else.
4947 This is useful when @command{gcc} prints the error message
4948 @samp{installation problem, cannot exec cpp0: No such file or directory}.
4949 To resolve this you either need to put @file{cpp0} and the other compiler
4950 components where @command{gcc} expects to find them, or you can set the environment
4951 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
4952 Don't forget the trailing @samp{/}.
4953 @xref{Environment Variables}.
4955 @item -print-sysroot-headers-suffix
4956 @opindex print-sysroot-headers-suffix
4957 Print the suffix added to the target sysroot when searching for
4958 headers, or give an error if the compiler is not configured with such
4959 a suffix---and don't do anything else.
4962 @opindex dumpmachine
4963 Print the compiler's target machine (for example,
4964 @samp{i686-pc-linux-gnu})---and don't do anything else.
4967 @opindex dumpversion
4968 Print the compiler version (for example, @samp{3.0})---and don't do
4973 Print the compiler's built-in specs---and don't do anything else. (This
4974 is used when GCC itself is being built.) @xref{Spec Files}.
4976 @item -feliminate-unused-debug-types
4977 @opindex feliminate-unused-debug-types
4978 Normally, when producing DWARF2 output, GCC will emit debugging
4979 information for all types declared in a compilation
4980 unit, regardless of whether or not they are actually used
4981 in that compilation unit. Sometimes this is useful, such as
4982 if, in the debugger, you want to cast a value to a type that is
4983 not actually used in your program (but is declared). More often,
4984 however, this results in a significant amount of wasted space.
4985 With this option, GCC will avoid producing debug symbol output
4986 for types that are nowhere used in the source file being compiled.
4989 @node Optimize Options
4990 @section Options That Control Optimization
4991 @cindex optimize options
4992 @cindex options, optimization
4994 These options control various sorts of optimizations.
4996 Without any optimization option, the compiler's goal is to reduce the
4997 cost of compilation and to make debugging produce the expected
4998 results. Statements are independent: if you stop the program with a
4999 breakpoint between statements, you can then assign a new value to any
5000 variable or change the program counter to any other statement in the
5001 function and get exactly the results you would expect from the source
5004 Turning on optimization flags makes the compiler attempt to improve
5005 the performance and/or code size at the expense of compilation time
5006 and possibly the ability to debug the program.
5008 The compiler performs optimization based on the knowledge it has of
5009 the program. Optimization levels @option{-O} and above, in
5010 particular, enable @emph{unit-at-a-time} mode, which allows the
5011 compiler to consider information gained from later functions in
5012 the file when compiling a function. Compiling multiple files at
5013 once to a single output file in @emph{unit-at-a-time} mode allows
5014 the compiler to use information gained from all of the files when
5015 compiling each of them.
5017 Not all optimizations are controlled directly by a flag. Only
5018 optimizations that have a flag are listed.
5025 Optimize. Optimizing compilation takes somewhat more time, and a lot
5026 more memory for a large function.
5028 With @option{-O}, the compiler tries to reduce code size and execution
5029 time, without performing any optimizations that take a great deal of
5032 @option{-O} turns on the following optimization flags:
5035 -fcprop-registers @gol
5038 -fdelayed-branch @gol
5040 -fguess-branch-probability @gol
5041 -fif-conversion2 @gol
5042 -fif-conversion @gol
5043 -finline-small-functions @gol
5044 -fipa-pure-const @gol
5045 -fipa-reference @gol
5047 -fsplit-wide-types @gol
5050 -ftree-copyrename @gol
5052 -ftree-dominator-opts @gol
5059 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
5060 where doing so does not interfere with debugging.
5064 Optimize even more. GCC performs nearly all supported optimizations
5065 that do not involve a space-speed tradeoff. The compiler does not
5066 perform loop unrolling or function inlining when you specify @option{-O2}.
5067 As compared to @option{-O}, this option increases both compilation time
5068 and the performance of the generated code.
5070 @option{-O2} turns on all optimization flags specified by @option{-O}. It
5071 also turns on the following optimization flags:
5072 @gccoptlist{-fthread-jumps @gol
5073 -falign-functions -falign-jumps @gol
5074 -falign-loops -falign-labels @gol
5077 -fcse-follow-jumps -fcse-skip-blocks @gol
5078 -fdelete-null-pointer-checks @gol
5079 -fexpensive-optimizations @gol
5080 -fgcse -fgcse-lm @gol
5081 -foptimize-sibling-calls @gol
5084 -freorder-blocks -freorder-functions @gol
5085 -frerun-cse-after-loop @gol
5086 -fsched-interblock -fsched-spec @gol
5087 -fschedule-insns -fschedule-insns2 @gol
5088 -fstrict-aliasing -fstrict-overflow @gol
5092 Please note the warning under @option{-fgcse} about
5093 invoking @option{-O2} on programs that use computed gotos.
5097 Optimize yet more. @option{-O3} turns on all optimizations specified by
5098 @option{-O2} and also turns on the @option{-finline-functions},
5099 @option{-funswitch-loops}, @option{-fpredictive-commoning} and
5100 @option{-fgcse-after-reload} options.
5104 Reduce compilation time and make debugging produce the expected
5105 results. This is the default.
5109 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
5110 do not typically increase code size. It also performs further
5111 optimizations designed to reduce code size.
5113 @option{-Os} disables the following optimization flags:
5114 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
5115 -falign-labels -freorder-blocks -freorder-blocks-and-partition @gol
5116 -fprefetch-loop-arrays -ftree-vect-loop-version}
5118 If you use multiple @option{-O} options, with or without level numbers,
5119 the last such option is the one that is effective.
5122 Options of the form @option{-f@var{flag}} specify machine-independent
5123 flags. Most flags have both positive and negative forms; the negative
5124 form of @option{-ffoo} would be @option{-fno-foo}. In the table
5125 below, only one of the forms is listed---the one you typically will
5126 use. You can figure out the other form by either removing @samp{no-}
5129 The following options control specific optimizations. They are either
5130 activated by @option{-O} options or are related to ones that are. You
5131 can use the following flags in the rare cases when ``fine-tuning'' of
5132 optimizations to be performed is desired.
5135 @item -fno-default-inline
5136 @opindex fno-default-inline
5137 Do not make member functions inline by default merely because they are
5138 defined inside the class scope (C++ only). Otherwise, when you specify
5139 @w{@option{-O}}, member functions defined inside class scope are compiled
5140 inline by default; i.e., you don't need to add @samp{inline} in front of
5141 the member function name.
5143 @item -fno-defer-pop
5144 @opindex fno-defer-pop
5145 Always pop the arguments to each function call as soon as that function
5146 returns. For machines which must pop arguments after a function call,
5147 the compiler normally lets arguments accumulate on the stack for several
5148 function calls and pops them all at once.
5150 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5152 @item -fforward-propagate
5153 @opindex fforward-propagate
5154 Perform a forward propagation pass on RTL. The pass tries to combine two
5155 instructions and checks if the result can be simplified. If loop unrolling
5156 is active, two passes are performed and the second is scheduled after
5159 This option is enabled by default at optimization levels @option{-O2},
5160 @option{-O3}, @option{-Os}.
5162 @item -fomit-frame-pointer
5163 @opindex fomit-frame-pointer
5164 Don't keep the frame pointer in a register for functions that
5165 don't need one. This avoids the instructions to save, set up and
5166 restore frame pointers; it also makes an extra register available
5167 in many functions. @strong{It also makes debugging impossible on
5170 On some machines, such as the VAX, this flag has no effect, because
5171 the standard calling sequence automatically handles the frame pointer
5172 and nothing is saved by pretending it doesn't exist. The
5173 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
5174 whether a target machine supports this flag. @xref{Registers,,Register
5175 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
5177 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5179 @item -foptimize-sibling-calls
5180 @opindex foptimize-sibling-calls
5181 Optimize sibling and tail recursive calls.
5183 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5187 Don't pay attention to the @code{inline} keyword. Normally this option
5188 is used to keep the compiler from expanding any functions inline.
5189 Note that if you are not optimizing, no functions can be expanded inline.
5191 @item -finline-small-functions
5192 @opindex finline-small-functions
5193 Integrate functions into their callers when their body is smaller than expected
5194 function call code (so overall size of program gets smaller). The compiler
5195 heuristically decides which functions are simple enough to be worth integrating
5198 Enabled at level @option{-O2}.
5200 @item -finline-functions
5201 @opindex finline-functions
5202 Integrate all simple functions into their callers. The compiler
5203 heuristically decides which functions are simple enough to be worth
5204 integrating in this way.
5206 If all calls to a given function are integrated, and the function is
5207 declared @code{static}, then the function is normally not output as
5208 assembler code in its own right.
5210 Enabled at level @option{-O3}.
5212 @item -finline-functions-called-once
5213 @opindex finline-functions-called-once
5214 Consider all @code{static} functions called once for inlining into their
5215 caller even if they are not marked @code{inline}. If a call to a given
5216 function is integrated, then the function is not output as assembler code
5219 Enabled if @option{-funit-at-a-time} is enabled.
5221 @item -fearly-inlining
5222 @opindex fearly-inlining
5223 Inline functions marked by @code{always_inline} and functions whose body seems
5224 smaller than the function call overhead early before doing
5225 @option{-fprofile-generate} instrumentation and real inlining pass. Doing so
5226 makes profiling significantly cheaper and usually inlining faster on programs
5227 having large chains of nested wrapper functions.
5231 @item -finline-limit=@var{n}
5232 @opindex finline-limit
5233 By default, GCC limits the size of functions that can be inlined. This flag
5234 allows the control of this limit for functions that are explicitly marked as
5235 inline (i.e., marked with the inline keyword or defined within the class
5236 definition in c++). @var{n} is the size of functions that can be inlined in
5237 number of pseudo instructions (not counting parameter handling). The default
5238 value of @var{n} is 600.
5239 Increasing this value can result in more inlined code at
5240 the cost of compilation time and memory consumption. Decreasing usually makes
5241 the compilation faster and less code will be inlined (which presumably
5242 means slower programs). This option is particularly useful for programs that
5243 use inlining heavily such as those based on recursive templates with C++.
5245 Inlining is actually controlled by a number of parameters, which may be
5246 specified individually by using @option{--param @var{name}=@var{value}}.
5247 The @option{-finline-limit=@var{n}} option sets some of these parameters
5251 @item max-inline-insns-single
5252 is set to @var{n}/2.
5253 @item max-inline-insns-auto
5254 is set to @var{n}/2.
5255 @item min-inline-insns
5256 is set to 130 or @var{n}/4, whichever is smaller.
5257 @item max-inline-insns-rtl
5261 See below for a documentation of the individual
5262 parameters controlling inlining.
5264 @emph{Note:} pseudo instruction represents, in this particular context, an
5265 abstract measurement of function's size. In no way does it represent a count
5266 of assembly instructions and as such its exact meaning might change from one
5267 release to an another.
5269 @item -fkeep-inline-functions
5270 @opindex fkeep-inline-functions
5271 In C, emit @code{static} functions that are declared @code{inline}
5272 into the object file, even if the function has been inlined into all
5273 of its callers. This switch does not affect functions using the
5274 @code{extern inline} extension in GNU C89@. In C++, emit any and all
5275 inline functions into the object file.
5277 @item -fkeep-static-consts
5278 @opindex fkeep-static-consts
5279 Emit variables declared @code{static const} when optimization isn't turned
5280 on, even if the variables aren't referenced.
5282 GCC enables this option by default. If you want to force the compiler to
5283 check if the variable was referenced, regardless of whether or not
5284 optimization is turned on, use the @option{-fno-keep-static-consts} option.
5286 @item -fmerge-constants
5287 @opindex fmerge-constants
5288 Attempt to merge identical constants (string constants and floating point
5289 constants) across compilation units.
5291 This option is the default for optimized compilation if the assembler and
5292 linker support it. Use @option{-fno-merge-constants} to inhibit this
5295 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5297 @item -fmerge-all-constants
5298 @opindex fmerge-all-constants
5299 Attempt to merge identical constants and identical variables.
5301 This option implies @option{-fmerge-constants}. In addition to
5302 @option{-fmerge-constants} this considers e.g.@: even constant initialized
5303 arrays or initialized constant variables with integral or floating point
5304 types. Languages like C or C++ require each non-automatic variable to
5305 have distinct location, so using this option will result in non-conforming
5308 @item -fmodulo-sched
5309 @opindex fmodulo-sched
5310 Perform swing modulo scheduling immediately before the first scheduling
5311 pass. This pass looks at innermost loops and reorders their
5312 instructions by overlapping different iterations.
5314 @item -fmodulo-sched-allow-regmoves
5315 @opindex fmodulo-sched-allow-regmoves
5316 Perform more aggressive SMS based modulo scheduling with register moves
5317 allowed. By setting this flag certain anti-dependences edges will be
5318 deleted which will trigger the generation of reg-moves based on the
5319 life-range analysis. This option is effective only with
5320 @option{-fmodulo-sched} enabled.
5322 @item -fno-branch-count-reg
5323 @opindex fno-branch-count-reg
5324 Do not use ``decrement and branch'' instructions on a count register,
5325 but instead generate a sequence of instructions that decrement a
5326 register, compare it against zero, then branch based upon the result.
5327 This option is only meaningful on architectures that support such
5328 instructions, which include x86, PowerPC, IA-64 and S/390.
5330 The default is @option{-fbranch-count-reg}.
5332 @item -fno-function-cse
5333 @opindex fno-function-cse
5334 Do not put function addresses in registers; make each instruction that
5335 calls a constant function contain the function's address explicitly.
5337 This option results in less efficient code, but some strange hacks
5338 that alter the assembler output may be confused by the optimizations
5339 performed when this option is not used.
5341 The default is @option{-ffunction-cse}
5343 @item -fno-zero-initialized-in-bss
5344 @opindex fno-zero-initialized-in-bss
5345 If the target supports a BSS section, GCC by default puts variables that
5346 are initialized to zero into BSS@. This can save space in the resulting
5349 This option turns off this behavior because some programs explicitly
5350 rely on variables going to the data section. E.g., so that the
5351 resulting executable can find the beginning of that section and/or make
5352 assumptions based on that.
5354 The default is @option{-fzero-initialized-in-bss}.
5356 @item -fmudflap -fmudflapth -fmudflapir
5360 @cindex bounds checking
5362 For front-ends that support it (C and C++), instrument all risky
5363 pointer/array dereferencing operations, some standard library
5364 string/heap functions, and some other associated constructs with
5365 range/validity tests. Modules so instrumented should be immune to
5366 buffer overflows, invalid heap use, and some other classes of C/C++
5367 programming errors. The instrumentation relies on a separate runtime
5368 library (@file{libmudflap}), which will be linked into a program if
5369 @option{-fmudflap} is given at link time. Run-time behavior of the
5370 instrumented program is controlled by the @env{MUDFLAP_OPTIONS}
5371 environment variable. See @code{env MUDFLAP_OPTIONS=-help a.out}
5374 Use @option{-fmudflapth} instead of @option{-fmudflap} to compile and to
5375 link if your program is multi-threaded. Use @option{-fmudflapir}, in
5376 addition to @option{-fmudflap} or @option{-fmudflapth}, if
5377 instrumentation should ignore pointer reads. This produces less
5378 instrumentation (and therefore faster execution) and still provides
5379 some protection against outright memory corrupting writes, but allows
5380 erroneously read data to propagate within a program.
5382 @item -fthread-jumps
5383 @opindex fthread-jumps
5384 Perform optimizations where we check to see if a jump branches to a
5385 location where another comparison subsumed by the first is found. If
5386 so, the first branch is redirected to either the destination of the
5387 second branch or a point immediately following it, depending on whether
5388 the condition is known to be true or false.
5390 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5392 @item -fsplit-wide-types
5393 @opindex fsplit-wide-types
5394 When using a type that occupies multiple registers, such as @code{long
5395 long} on a 32-bit system, split the registers apart and allocate them
5396 independently. This normally generates better code for those types,
5397 but may make debugging more difficult.
5399 Enabled at levels @option{-O}, @option{-O2}, @option{-O3},
5402 @item -fcse-follow-jumps
5403 @opindex fcse-follow-jumps
5404 In common subexpression elimination (CSE), scan through jump instructions
5405 when the target of the jump is not reached by any other path. For
5406 example, when CSE encounters an @code{if} statement with an
5407 @code{else} clause, CSE will follow the jump when the condition
5410 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5412 @item -fcse-skip-blocks
5413 @opindex fcse-skip-blocks
5414 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
5415 follow jumps which conditionally skip over blocks. When CSE
5416 encounters a simple @code{if} statement with no else clause,
5417 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
5418 body of the @code{if}.
5420 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5422 @item -frerun-cse-after-loop
5423 @opindex frerun-cse-after-loop
5424 Re-run common subexpression elimination after loop optimizations has been
5427 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5431 Perform a global common subexpression elimination pass.
5432 This pass also performs global constant and copy propagation.
5434 @emph{Note:} When compiling a program using computed gotos, a GCC
5435 extension, you may get better runtime performance if you disable
5436 the global common subexpression elimination pass by adding
5437 @option{-fno-gcse} to the command line.
5439 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5443 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
5444 attempt to move loads which are only killed by stores into themselves. This
5445 allows a loop containing a load/store sequence to be changed to a load outside
5446 the loop, and a copy/store within the loop.
5448 Enabled by default when gcse is enabled.
5452 When @option{-fgcse-sm} is enabled, a store motion pass is run after
5453 global common subexpression elimination. This pass will attempt to move
5454 stores out of loops. When used in conjunction with @option{-fgcse-lm},
5455 loops containing a load/store sequence can be changed to a load before
5456 the loop and a store after the loop.
5458 Not enabled at any optimization level.
5462 When @option{-fgcse-las} is enabled, the global common subexpression
5463 elimination pass eliminates redundant loads that come after stores to the
5464 same memory location (both partial and full redundancies).
5466 Not enabled at any optimization level.
5468 @item -fgcse-after-reload
5469 @opindex fgcse-after-reload
5470 When @option{-fgcse-after-reload} is enabled, a redundant load elimination
5471 pass is performed after reload. The purpose of this pass is to cleanup
5474 @item -funsafe-loop-optimizations
5475 @opindex funsafe-loop-optimizations
5476 If given, the loop optimizer will assume that loop indices do not
5477 overflow, and that the loops with nontrivial exit condition are not
5478 infinite. This enables a wider range of loop optimizations even if
5479 the loop optimizer itself cannot prove that these assumptions are valid.
5480 Using @option{-Wunsafe-loop-optimizations}, the compiler will warn you
5481 if it finds this kind of loop.
5483 @item -fcrossjumping
5484 @opindex fcrossjumping
5485 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
5486 resulting code may or may not perform better than without cross-jumping.
5488 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5490 @item -fauto-inc-dec
5491 @opindex fauto-inc-dec
5492 Combine increments or decrements of addresses with memory accesses.
5493 This pass is always skipped on architectures that do not have
5494 instructions to support this. Enabled by default at @option{-O} and
5495 higher on architectures that support this.
5499 Perform dead code elimination (DCE) on RTL.
5500 Enabled by default at @option{-O} and higher.
5504 Perform dead store elimination (DSE) on RTL.
5505 Enabled by default at @option{-O} and higher.
5507 @item -fif-conversion
5508 @opindex fif-conversion
5509 Attempt to transform conditional jumps into branch-less equivalents. This
5510 include use of conditional moves, min, max, set flags and abs instructions, and
5511 some tricks doable by standard arithmetics. The use of conditional execution
5512 on chips where it is available is controlled by @code{if-conversion2}.
5514 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5516 @item -fif-conversion2
5517 @opindex fif-conversion2
5518 Use conditional execution (where available) to transform conditional jumps into
5519 branch-less equivalents.
5521 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5523 @item -fdelete-null-pointer-checks
5524 @opindex fdelete-null-pointer-checks
5525 Use global dataflow analysis to identify and eliminate useless checks
5526 for null pointers. The compiler assumes that dereferencing a null
5527 pointer would have halted the program. If a pointer is checked after
5528 it has already been dereferenced, it cannot be null.
5530 In some environments, this assumption is not true, and programs can
5531 safely dereference null pointers. Use
5532 @option{-fno-delete-null-pointer-checks} to disable this optimization
5533 for programs which depend on that behavior.
5535 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5537 @item -fexpensive-optimizations
5538 @opindex fexpensive-optimizations
5539 Perform a number of minor optimizations that are relatively expensive.
5541 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5543 @item -foptimize-register-move
5545 @opindex foptimize-register-move
5547 Attempt to reassign register numbers in move instructions and as
5548 operands of other simple instructions in order to maximize the amount of
5549 register tying. This is especially helpful on machines with two-operand
5552 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
5555 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5557 @item -fdelayed-branch
5558 @opindex fdelayed-branch
5559 If supported for the target machine, attempt to reorder instructions
5560 to exploit instruction slots available after delayed branch
5563 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5565 @item -fschedule-insns
5566 @opindex fschedule-insns
5567 If supported for the target machine, attempt to reorder instructions to
5568 eliminate execution stalls due to required data being unavailable. This
5569 helps machines that have slow floating point or memory load instructions
5570 by allowing other instructions to be issued until the result of the load
5571 or floating point instruction is required.
5573 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5575 @item -fschedule-insns2
5576 @opindex fschedule-insns2
5577 Similar to @option{-fschedule-insns}, but requests an additional pass of
5578 instruction scheduling after register allocation has been done. This is
5579 especially useful on machines with a relatively small number of
5580 registers and where memory load instructions take more than one cycle.
5582 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5584 @item -fno-sched-interblock
5585 @opindex fno-sched-interblock
5586 Don't schedule instructions across basic blocks. This is normally
5587 enabled by default when scheduling before register allocation, i.e.@:
5588 with @option{-fschedule-insns} or at @option{-O2} or higher.
5590 @item -fno-sched-spec
5591 @opindex fno-sched-spec
5592 Don't allow speculative motion of non-load instructions. This is normally
5593 enabled by default when scheduling before register allocation, i.e.@:
5594 with @option{-fschedule-insns} or at @option{-O2} or higher.
5596 @item -fsched-spec-load
5597 @opindex fsched-spec-load
5598 Allow speculative motion of some load instructions. This only makes
5599 sense when scheduling before register allocation, i.e.@: with
5600 @option{-fschedule-insns} or at @option{-O2} or higher.
5602 @item -fsched-spec-load-dangerous
5603 @opindex fsched-spec-load-dangerous
5604 Allow speculative motion of more load instructions. This only makes
5605 sense when scheduling before register allocation, i.e.@: with
5606 @option{-fschedule-insns} or at @option{-O2} or higher.
5608 @item -fsched-stalled-insns
5609 @itemx -fsched-stalled-insns=@var{n}
5610 @opindex fsched-stalled-insns
5611 Define how many insns (if any) can be moved prematurely from the queue
5612 of stalled insns into the ready list, during the second scheduling pass.
5613 @option{-fno-sched-stalled-insns} means that no insns will be moved
5614 prematurely, @option{-fsched-stalled-insns=0} means there is no limit
5615 on how many queued insns can be moved prematurely.
5616 @option{-fsched-stalled-insns} without a value is equivalent to
5617 @option{-fsched-stalled-insns=1}.
5619 @item -fsched-stalled-insns-dep
5620 @itemx -fsched-stalled-insns-dep=@var{n}
5621 @opindex fsched-stalled-insns-dep
5622 Define how many insn groups (cycles) will be examined for a dependency
5623 on a stalled insn that is candidate for premature removal from the queue
5624 of stalled insns. This has an effect only during the second scheduling pass,
5625 and only if @option{-fsched-stalled-insns} is used.
5626 @option{-fno-sched-stalled-insns-dep} is equivalent to
5627 @option{-fsched-stalled-insns-dep=0}.
5628 @option{-fsched-stalled-insns-dep} without a value is equivalent to
5629 @option{-fsched-stalled-insns-dep=1}.
5631 @item -fsched2-use-superblocks
5632 @opindex fsched2-use-superblocks
5633 When scheduling after register allocation, do use superblock scheduling
5634 algorithm. Superblock scheduling allows motion across basic block boundaries
5635 resulting on faster schedules. This option is experimental, as not all machine
5636 descriptions used by GCC model the CPU closely enough to avoid unreliable
5637 results from the algorithm.
5639 This only makes sense when scheduling after register allocation, i.e.@: with
5640 @option{-fschedule-insns2} or at @option{-O2} or higher.
5642 @item -fsched2-use-traces
5643 @opindex fsched2-use-traces
5644 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
5645 allocation and additionally perform code duplication in order to increase the
5646 size of superblocks using tracer pass. See @option{-ftracer} for details on
5649 This mode should produce faster but significantly longer programs. Also
5650 without @option{-fbranch-probabilities} the traces constructed may not
5651 match the reality and hurt the performance. This only makes
5652 sense when scheduling after register allocation, i.e.@: with
5653 @option{-fschedule-insns2} or at @option{-O2} or higher.
5657 Eliminate redundant sign extension instructions and move the non-redundant
5658 ones to optimal placement using lazy code motion (LCM).
5660 @item -freschedule-modulo-scheduled-loops
5661 @opindex freschedule-modulo-scheduled-loops
5662 The modulo scheduling comes before the traditional scheduling, if a loop
5663 was modulo scheduled we may want to prevent the later scheduling passes
5664 from changing its schedule, we use this option to control that.
5666 @item -fcaller-saves
5667 @opindex fcaller-saves
5668 Enable values to be allocated in registers that will be clobbered by
5669 function calls, by emitting extra instructions to save and restore the
5670 registers around such calls. Such allocation is done only when it
5671 seems to result in better code than would otherwise be produced.
5673 This option is always enabled by default on certain machines, usually
5674 those which have no call-preserved registers to use instead.
5676 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5678 @item -ftree-reassoc
5679 @opindex ftree-reassoc
5680 Perform reassociation on trees. This flag is enabled by default
5681 at @option{-O} and higher.
5685 Perform partial redundancy elimination (PRE) on trees. This flag is
5686 enabled by default at @option{-O2} and @option{-O3}.
5690 Perform full redundancy elimination (FRE) on trees. The difference
5691 between FRE and PRE is that FRE only considers expressions
5692 that are computed on all paths leading to the redundant computation.
5693 This analysis is faster than PRE, though it exposes fewer redundancies.
5694 This flag is enabled by default at @option{-O} and higher.
5696 @item -ftree-copy-prop
5697 @opindex ftree-copy-prop
5698 Perform copy propagation on trees. This pass eliminates unnecessary
5699 copy operations. This flag is enabled by default at @option{-O} and
5703 @opindex ftree-salias
5704 Perform structural alias analysis on trees. This flag
5705 is enabled by default at @option{-O} and higher.
5707 @item -fipa-pure-const
5708 @opindex fipa-pure-const
5709 Discover which functions are pure or constant.
5710 Enabled by default at @option{-O} and higher.
5712 @item -fipa-reference
5713 @opindex fipa-reference
5714 Discover which static variables do not escape cannot escape the
5716 Enabled by default at @option{-O} and higher.
5718 @item -fipa-struct-reorg
5719 @opindex fipa-struct-reorg
5720 Perform structure reorganization optimization, that change C-like structures
5721 layout in order to better utilize spatial locality. This transformation is
5722 affective for programs containing arrays of structures. Available in two
5723 compilation modes: profile-based (enabled with @option{-fprofile-generate})
5724 or static (which uses built-in heuristics). Require @option{-fipa-type-escape}
5725 to provide the safety of this transformation. It works only in whole program
5726 mode, so it requires @option{-fwhole-program} and @option{-combine} to be
5727 enabled. Structures considered @samp{cold} by this transformation are not
5728 affected (see @option{--param struct-reorg-cold-struct-ratio=@var{value}}).
5730 With this flag, the program debug info reflects a new structure layout.
5734 Perform interprocedural pointer analysis.
5738 Perform interprocedural constant propagation.
5739 This optimization analyzes the program to determine when values passed
5740 to functions are constants and then optimizes accordingly.
5741 This optimization can substantially increase performance
5742 if the application has constants passed to functions, but
5743 because this optimization can create multiple copies of functions,
5744 it may significantly increase code size.
5746 @item -fipa-matrix-reorg
5747 @opindex fipa-matrix-reorg
5748 Perform matrix flattening and transposing.
5749 Matrix flattening tries to replace a m-dimensional matrix
5750 with its equivalent n-dimensional matrix, where n < m.
5751 This reduces the level of indirection needed for accessing the elements
5752 of the matrix. The second optimization is matrix transposing that
5753 attemps to change the order of the matrix's dimensions in order to
5754 improve cache locality.
5755 Both optimizations need fwhole-program flag.
5756 Transposing is enabled only if profiling information is avaliable.
5761 Perform forward store motion on trees. This flag is
5762 enabled by default at @option{-O} and higher.
5766 Perform sparse conditional constant propagation (CCP) on trees. This
5767 pass only operates on local scalar variables and is enabled by default
5768 at @option{-O} and higher.
5770 @item -ftree-store-ccp
5771 @opindex ftree-store-ccp
5772 Perform sparse conditional constant propagation (CCP) on trees. This
5773 pass operates on both local scalar variables and memory stores and
5774 loads (global variables, structures, arrays, etc). This flag is
5775 enabled by default at @option{-O2} and higher.
5779 Perform dead code elimination (DCE) on trees. This flag is enabled by
5780 default at @option{-O} and higher.
5782 @item -ftree-dominator-opts
5783 @opindex ftree-dominator-opts
5784 Perform a variety of simple scalar cleanups (constant/copy
5785 propagation, redundancy elimination, range propagation and expression
5786 simplification) based on a dominator tree traversal. This also
5787 performs jump threading (to reduce jumps to jumps). This flag is
5788 enabled by default at @option{-O} and higher.
5792 Perform dead store elimination (DSE) on trees. A dead store is a store into
5793 a memory location which will later be overwritten by another store without
5794 any intervening loads. In this case the earlier store can be deleted. This
5795 flag is enabled by default at @option{-O} and higher.
5799 Perform loop header copying on trees. This is beneficial since it increases
5800 effectiveness of code motion optimizations. It also saves one jump. This flag
5801 is enabled by default at @option{-O} and higher. It is not enabled
5802 for @option{-Os}, since it usually increases code size.
5804 @item -ftree-loop-optimize
5805 @opindex ftree-loop-optimize
5806 Perform loop optimizations on trees. This flag is enabled by default
5807 at @option{-O} and higher.
5809 @item -ftree-loop-linear
5810 @opindex ftree-loop-linear
5811 Perform linear loop transformations on tree. This flag can improve cache
5812 performance and allow further loop optimizations to take place.
5814 @item -fcheck-data-deps
5815 @opindex fcheck-data-deps
5816 Compare the results of several data dependence analyzers. This option
5817 is used for debugging the data dependence analyzers.
5819 @item -ftree-loop-im
5820 @opindex ftree-loop-im
5821 Perform loop invariant motion on trees. This pass moves only invariants that
5822 would be hard to handle at RTL level (function calls, operations that expand to
5823 nontrivial sequences of insns). With @option{-funswitch-loops} it also moves
5824 operands of conditions that are invariant out of the loop, so that we can use
5825 just trivial invariantness analysis in loop unswitching. The pass also includes
5828 @item -ftree-loop-ivcanon
5829 @opindex ftree-loop-ivcanon
5830 Create a canonical counter for number of iterations in the loop for that
5831 determining number of iterations requires complicated analysis. Later
5832 optimizations then may determine the number easily. Useful especially
5833 in connection with unrolling.
5837 Perform induction variable optimizations (strength reduction, induction
5838 variable merging and induction variable elimination) on trees.
5840 @item -ftree-parallelize-loops=n
5841 @opindex ftree-parallelize-loops
5842 Parallelize loops, i.e., split their iteration space to run in n threads.
5843 This is only possible for loops whose iterations are independent
5844 and can be arbitrarily reordered. The optimization is only
5845 profitable on multiprocessor machines, for loops that are CPU-intensive,
5846 rather than constrained e.g. by memory bandwidth. This option
5847 implies @option{-pthread}, and thus is only supported on targets
5848 that have support for @option{-pthread}.
5852 Perform scalar replacement of aggregates. This pass replaces structure
5853 references with scalars to prevent committing structures to memory too
5854 early. This flag is enabled by default at @option{-O} and higher.
5856 @item -ftree-copyrename
5857 @opindex ftree-copyrename
5858 Perform copy renaming on trees. This pass attempts to rename compiler
5859 temporaries to other variables at copy locations, usually resulting in
5860 variable names which more closely resemble the original variables. This flag
5861 is enabled by default at @option{-O} and higher.
5865 Perform temporary expression replacement during the SSA->normal phase. Single
5866 use/single def temporaries are replaced at their use location with their
5867 defining expression. This results in non-GIMPLE code, but gives the expanders
5868 much more complex trees to work on resulting in better RTL generation. This is
5869 enabled by default at @option{-O} and higher.
5871 @item -ftree-vectorize
5872 @opindex ftree-vectorize
5873 Perform loop vectorization on trees.
5875 @item -ftree-vect-loop-version
5876 @opindex ftree-vect-loop-version
5877 Perform loop versioning when doing loop vectorization on trees. When a loop
5878 appears to be vectorizable except that data alignment or data dependence cannot
5879 be determined at compile time then vectorized and non-vectorized versions of
5880 the loop are generated along with runtime checks for alignment or dependence
5881 to control which version is executed. This option is enabled by default
5882 except at level @option{-Os} where it is disabled.
5884 @item -fvect-cost-model
5885 @opindex fvect-cost-model
5886 Enable cost model for vectorization.
5890 Perform Value Range Propagation on trees. This is similar to the
5891 constant propagation pass, but instead of values, ranges of values are
5892 propagated. This allows the optimizers to remove unnecessary range
5893 checks like array bound checks and null pointer checks. This is
5894 enabled by default at @option{-O2} and higher. Null pointer check
5895 elimination is only done if @option{-fdelete-null-pointer-checks} is
5900 Perform tail duplication to enlarge superblock size. This transformation
5901 simplifies the control flow of the function allowing other optimizations to do
5904 @item -funroll-loops
5905 @opindex funroll-loops
5906 Unroll loops whose number of iterations can be determined at compile
5907 time or upon entry to the loop. @option{-funroll-loops} implies
5908 @option{-frerun-cse-after-loop}. This option makes code larger,
5909 and may or may not make it run faster.
5911 @item -funroll-all-loops
5912 @opindex funroll-all-loops
5913 Unroll all loops, even if their number of iterations is uncertain when
5914 the loop is entered. This usually makes programs run more slowly.
5915 @option{-funroll-all-loops} implies the same options as
5916 @option{-funroll-loops},
5918 @item -fsplit-ivs-in-unroller
5919 @opindex fsplit-ivs-in-unroller
5920 Enables expressing of values of induction variables in later iterations
5921 of the unrolled loop using the value in the first iteration. This breaks
5922 long dependency chains, thus improving efficiency of the scheduling passes.
5924 Combination of @option{-fweb} and CSE is often sufficient to obtain the
5925 same effect. However in cases the loop body is more complicated than
5926 a single basic block, this is not reliable. It also does not work at all
5927 on some of the architectures due to restrictions in the CSE pass.
5929 This optimization is enabled by default.
5931 @item -fvariable-expansion-in-unroller
5932 @opindex fvariable-expansion-in-unroller
5933 With this option, the compiler will create multiple copies of some
5934 local variables when unrolling a loop which can result in superior code.
5936 @item -fpredictive-commoning
5937 @opindex fpredictive-commoning
5938 Perform predictive commoning optimization, i.e., reusing computations
5939 (especially memory loads and stores) performed in previous
5940 iterations of loops.
5942 This option is enabled at level @option{-O3}.
5944 @item -fprefetch-loop-arrays
5945 @opindex fprefetch-loop-arrays
5946 If supported by the target machine, generate instructions to prefetch
5947 memory to improve the performance of loops that access large arrays.
5949 This option may generate better or worse code; results are highly
5950 dependent on the structure of loops within the source code.
5952 Disabled at level @option{-Os}.
5955 @itemx -fno-peephole2
5956 @opindex fno-peephole
5957 @opindex fno-peephole2
5958 Disable any machine-specific peephole optimizations. The difference
5959 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
5960 are implemented in the compiler; some targets use one, some use the
5961 other, a few use both.
5963 @option{-fpeephole} is enabled by default.
5964 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5966 @item -fno-guess-branch-probability
5967 @opindex fno-guess-branch-probability
5968 Do not guess branch probabilities using heuristics.
5970 GCC will use heuristics to guess branch probabilities if they are
5971 not provided by profiling feedback (@option{-fprofile-arcs}). These
5972 heuristics are based on the control flow graph. If some branch probabilities
5973 are specified by @samp{__builtin_expect}, then the heuristics will be
5974 used to guess branch probabilities for the rest of the control flow graph,
5975 taking the @samp{__builtin_expect} info into account. The interactions
5976 between the heuristics and @samp{__builtin_expect} can be complex, and in
5977 some cases, it may be useful to disable the heuristics so that the effects
5978 of @samp{__builtin_expect} are easier to understand.
5980 The default is @option{-fguess-branch-probability} at levels
5981 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5983 @item -freorder-blocks
5984 @opindex freorder-blocks
5985 Reorder basic blocks in the compiled function in order to reduce number of
5986 taken branches and improve code locality.
5988 Enabled at levels @option{-O2}, @option{-O3}.
5990 @item -freorder-blocks-and-partition
5991 @opindex freorder-blocks-and-partition
5992 In addition to reordering basic blocks in the compiled function, in order
5993 to reduce number of taken branches, partitions hot and cold basic blocks
5994 into separate sections of the assembly and .o files, to improve
5995 paging and cache locality performance.
5997 This optimization is automatically turned off in the presence of
5998 exception handling, for linkonce sections, for functions with a user-defined
5999 section attribute and on any architecture that does not support named
6002 @item -freorder-functions
6003 @opindex freorder-functions
6004 Reorder functions in the object file in order to
6005 improve code locality. This is implemented by using special
6006 subsections @code{.text.hot} for most frequently executed functions and
6007 @code{.text.unlikely} for unlikely executed functions. Reordering is done by
6008 the linker so object file format must support named sections and linker must
6009 place them in a reasonable way.
6011 Also profile feedback must be available in to make this option effective. See
6012 @option{-fprofile-arcs} for details.
6014 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
6016 @item -fstrict-aliasing
6017 @opindex fstrict-aliasing
6018 Allows the compiler to assume the strictest aliasing rules applicable to
6019 the language being compiled. For C (and C++), this activates
6020 optimizations based on the type of expressions. In particular, an
6021 object of one type is assumed never to reside at the same address as an
6022 object of a different type, unless the types are almost the same. For
6023 example, an @code{unsigned int} can alias an @code{int}, but not a
6024 @code{void*} or a @code{double}. A character type may alias any other
6027 Pay special attention to code like this:
6040 The practice of reading from a different union member than the one most
6041 recently written to (called ``type-punning'') is common. Even with
6042 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
6043 is accessed through the union type. So, the code above will work as
6044 expected. However, this code might not:
6055 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
6057 @item -fstrict-overflow
6058 @opindex fstrict-overflow
6059 Allow the compiler to assume strict signed overflow rules, depending
6060 on the language being compiled. For C (and C++) this means that
6061 overflow when doing arithmetic with signed numbers is undefined, which
6062 means that the compiler may assume that it will not happen. This
6063 permits various optimizations. For example, the compiler will assume
6064 that an expression like @code{i + 10 > i} will always be true for
6065 signed @code{i}. This assumption is only valid if signed overflow is
6066 undefined, as the expression is false if @code{i + 10} overflows when
6067 using twos complement arithmetic. When this option is in effect any
6068 attempt to determine whether an operation on signed numbers will
6069 overflow must be written carefully to not actually involve overflow.
6071 See also the @option{-fwrapv} option. Using @option{-fwrapv} means
6072 that signed overflow is fully defined: it wraps. When
6073 @option{-fwrapv} is used, there is no difference between
6074 @option{-fstrict-overflow} and @option{-fno-strict-overflow}. With
6075 @option{-fwrapv} certain types of overflow are permitted. For
6076 example, if the compiler gets an overflow when doing arithmetic on
6077 constants, the overflowed value can still be used with
6078 @option{-fwrapv}, but not otherwise.
6080 The @option{-fstrict-overflow} option is enabled at levels
6081 @option{-O2}, @option{-O3}, @option{-Os}.
6083 @item -falign-functions
6084 @itemx -falign-functions=@var{n}
6085 @opindex falign-functions
6086 Align the start of functions to the next power-of-two greater than
6087 @var{n}, skipping up to @var{n} bytes. For instance,
6088 @option{-falign-functions=32} aligns functions to the next 32-byte
6089 boundary, but @option{-falign-functions=24} would align to the next
6090 32-byte boundary only if this can be done by skipping 23 bytes or less.
6092 @option{-fno-align-functions} and @option{-falign-functions=1} are
6093 equivalent and mean that functions will not be aligned.
6095 Some assemblers only support this flag when @var{n} is a power of two;
6096 in that case, it is rounded up.
6098 If @var{n} is not specified or is zero, use a machine-dependent default.
6100 Enabled at levels @option{-O2}, @option{-O3}.
6102 @item -falign-labels
6103 @itemx -falign-labels=@var{n}
6104 @opindex falign-labels
6105 Align all branch targets to a power-of-two boundary, skipping up to
6106 @var{n} bytes like @option{-falign-functions}. This option can easily
6107 make code slower, because it must insert dummy operations for when the
6108 branch target is reached in the usual flow of the code.
6110 @option{-fno-align-labels} and @option{-falign-labels=1} are
6111 equivalent and mean that labels will not be aligned.
6113 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
6114 are greater than this value, then their values are used instead.
6116 If @var{n} is not specified or is zero, use a machine-dependent default
6117 which is very likely to be @samp{1}, meaning no alignment.
6119 Enabled at levels @option{-O2}, @option{-O3}.
6122 @itemx -falign-loops=@var{n}
6123 @opindex falign-loops
6124 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
6125 like @option{-falign-functions}. The hope is that the loop will be
6126 executed many times, which will make up for any execution of the dummy
6129 @option{-fno-align-loops} and @option{-falign-loops=1} are
6130 equivalent and mean that loops will not be aligned.
6132 If @var{n} is not specified or is zero, use a machine-dependent default.
6134 Enabled at levels @option{-O2}, @option{-O3}.
6137 @itemx -falign-jumps=@var{n}
6138 @opindex falign-jumps
6139 Align branch targets to a power-of-two boundary, for branch targets
6140 where the targets can only be reached by jumping, skipping up to @var{n}
6141 bytes like @option{-falign-functions}. In this case, no dummy operations
6144 @option{-fno-align-jumps} and @option{-falign-jumps=1} are
6145 equivalent and mean that loops will not be aligned.
6147 If @var{n} is not specified or is zero, use a machine-dependent default.
6149 Enabled at levels @option{-O2}, @option{-O3}.
6151 @item -funit-at-a-time
6152 @opindex funit-at-a-time
6153 Parse the whole compilation unit before starting to produce code.
6154 This allows some extra optimizations to take place but consumes
6155 more memory (in general). There are some compatibility issues
6156 with @emph{unit-at-a-time} mode:
6159 enabling @emph{unit-at-a-time} mode may change the order
6160 in which functions, variables, and top-level @code{asm} statements
6161 are emitted, and will likely break code relying on some particular
6162 ordering. The majority of such top-level @code{asm} statements,
6163 though, can be replaced by @code{section} attributes. The
6164 @option{fno-toplevel-reorder} option may be used to keep the ordering
6165 used in the input file, at the cost of some optimizations.
6168 @emph{unit-at-a-time} mode removes unreferenced static variables
6169 and functions. This may result in undefined references
6170 when an @code{asm} statement refers directly to variables or functions
6171 that are otherwise unused. In that case either the variable/function
6172 shall be listed as an operand of the @code{asm} statement operand or,
6173 in the case of top-level @code{asm} statements the attribute @code{used}
6174 shall be used on the declaration.
6177 Static functions now can use non-standard passing conventions that
6178 may break @code{asm} statements calling functions directly. Again,
6179 attribute @code{used} will prevent this behavior.
6182 As a temporary workaround, @option{-fno-unit-at-a-time} can be used,
6183 but this scheme may not be supported by future releases of GCC@.
6185 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
6187 @item -fno-toplevel-reorder
6188 @opindex fno-toplevel-reorder
6189 Do not reorder top-level functions, variables, and @code{asm}
6190 statements. Output them in the same order that they appear in the
6191 input file. When this option is used, unreferenced static variables
6192 will not be removed. This option is intended to support existing code
6193 which relies on a particular ordering. For new code, it is better to
6198 Constructs webs as commonly used for register allocation purposes and assign
6199 each web individual pseudo register. This allows the register allocation pass
6200 to operate on pseudos directly, but also strengthens several other optimization
6201 passes, such as CSE, loop optimizer and trivial dead code remover. It can,
6202 however, make debugging impossible, since variables will no longer stay in a
6205 Enabled by default with @option{-funroll-loops}.
6207 @item -fwhole-program
6208 @opindex fwhole-program
6209 Assume that the current compilation unit represents whole program being
6210 compiled. All public functions and variables with the exception of @code{main}
6211 and those merged by attribute @code{externally_visible} become static functions
6212 and in a affect gets more aggressively optimized by interprocedural optimizers.
6213 While this option is equivalent to proper use of @code{static} keyword for
6214 programs consisting of single file, in combination with option
6215 @option{--combine} this flag can be used to compile most of smaller scale C
6216 programs since the functions and variables become local for the whole combined
6217 compilation unit, not for the single source file itself.
6219 This option is not supported for Fortran programs.
6221 @item -fcprop-registers
6222 @opindex fcprop-registers
6223 After register allocation and post-register allocation instruction splitting,
6224 we perform a copy-propagation pass to try to reduce scheduling dependencies
6225 and occasionally eliminate the copy.
6227 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
6229 @item -fprofile-generate
6230 @opindex fprofile-generate
6232 Enable options usually used for instrumenting application to produce
6233 profile useful for later recompilation with profile feedback based
6234 optimization. You must use @option{-fprofile-generate} both when
6235 compiling and when linking your program.
6237 The following options are enabled: @code{-fprofile-arcs}, @code{-fprofile-values}, @code{-fvpt}.
6240 @opindex fprofile-use
6241 Enable profile feedback directed optimizations, and optimizations
6242 generally profitable only with profile feedback available.
6244 The following options are enabled: @code{-fbranch-probabilities}, @code{-fvpt},
6245 @code{-funroll-loops}, @code{-fpeel-loops}, @code{-ftracer}
6247 By default, GCC emits an error message if the feedback profiles do not
6248 match the source code. This error can be turned into a warning by using
6249 @option{-Wcoverage-mismatch}. Note this may result in poorly optimized
6253 The following options control compiler behavior regarding floating
6254 point arithmetic. These options trade off between speed and
6255 correctness. All must be specifically enabled.
6259 @opindex ffloat-store
6260 Do not store floating point variables in registers, and inhibit other
6261 options that might change whether a floating point value is taken from a
6264 @cindex floating point precision
6265 This option prevents undesirable excess precision on machines such as
6266 the 68000 where the floating registers (of the 68881) keep more
6267 precision than a @code{double} is supposed to have. Similarly for the
6268 x86 architecture. For most programs, the excess precision does only
6269 good, but a few programs rely on the precise definition of IEEE floating
6270 point. Use @option{-ffloat-store} for such programs, after modifying
6271 them to store all pertinent intermediate computations into variables.
6275 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations},
6276 @option{-ffinite-math-only}, @option{-fno-rounding-math},
6277 @option{-fno-signaling-nans} and @option{-fcx-limited-range}.
6279 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
6281 This option is not turned on by any @option{-O} option since
6282 it can result in incorrect output for programs which depend on
6283 an exact implementation of IEEE or ISO rules/specifications for
6284 math functions. It may, however, yield faster code for programs
6285 that do not require the guarantees of these specifications.
6287 @item -fno-math-errno
6288 @opindex fno-math-errno
6289 Do not set ERRNO after calling math functions that are executed
6290 with a single instruction, e.g., sqrt. A program that relies on
6291 IEEE exceptions for math error handling may want to use this flag
6292 for speed while maintaining IEEE arithmetic compatibility.
6294 This option is not turned on by any @option{-O} option since
6295 it can result in incorrect output for programs which depend on
6296 an exact implementation of IEEE or ISO rules/specifications for
6297 math functions. It may, however, yield faster code for programs
6298 that do not require the guarantees of these specifications.
6300 The default is @option{-fmath-errno}.
6302 On Darwin systems, the math library never sets @code{errno}. There is
6303 therefore no reason for the compiler to consider the possibility that
6304 it might, and @option{-fno-math-errno} is the default.
6306 @item -funsafe-math-optimizations
6307 @opindex funsafe-math-optimizations
6309 Allow optimizations for floating-point arithmetic that (a) assume
6310 that arguments and results are valid and (b) may violate IEEE or
6311 ANSI standards. When used at link-time, it may include libraries
6312 or startup files that change the default FPU control word or other
6313 similar optimizations.
6315 This option is not turned on by any @option{-O} option since
6316 it can result in incorrect output for programs which depend on
6317 an exact implementation of IEEE or ISO rules/specifications for
6318 math functions. It may, however, yield faster code for programs
6319 that do not require the guarantees of these specifications.
6320 Enables @option{-fno-signed-zeros}, @option{-fno-trapping-math},
6321 @option{-fassociative-math} and @option{-freciprocal-math}.
6323 The default is @option{-fno-unsafe-math-optimizations}.
6325 @item -fassociative-math
6326 @opindex fassociative-math
6328 Allow re-association of operands in series of floating-point operations.
6329 This violates the ISO C and C++ language standard by possibly changing
6330 computation result. NOTE: re-ordering may change the sign of zero as
6331 well as ignore NaNs and inhibit or create underflow or overflow (and
6332 thus cannot be used on a code which relies on rounding behavior like
6333 @code{(x + 2**52) - 2**52)}. May also reorder floating-point comparisons
6334 and thus may not be used when ordered comparisons are required.
6335 This option requires that both @option{-fno-signed-zeros} and
6336 @option{-fno-trapping-math} be in effect. Moreover, it doesn't make
6337 much sense with @option{-frounding-math}.
6339 The default is @option{-fno-associative-math}.
6341 @item -freciprocal-math
6342 @opindex freciprocal-math
6344 Allow the reciprocal of a value to be used instead of dividing by
6345 the value if this enables optimizations. For example @code{x / y}
6346 can be replaced with @code{x * (1/y)} which is useful if @code{(1/y)}
6347 is subject to common subexpression elimination. Note that this loses
6348 precision and increases the number of flops operating on the value.
6350 The default is @option{-fno-reciprocal-math}.
6352 @item -ffinite-math-only
6353 @opindex ffinite-math-only
6354 Allow optimizations for floating-point arithmetic that assume
6355 that arguments and results are not NaNs or +-Infs.
6357 This option is not turned on by any @option{-O} option since
6358 it can result in incorrect output for programs which depend on
6359 an exact implementation of IEEE or ISO rules/specifications for
6360 math functions. It may, however, yield faster code for programs
6361 that do not require the guarantees of these specifications.
6363 The default is @option{-fno-finite-math-only}.
6365 @item -fno-signed-zeros
6366 @opindex fno-signed-zeros
6367 Allow optimizations for floating point arithmetic that ignore the
6368 signedness of zero. IEEE arithmetic specifies the behavior of
6369 distinct +0.0 and @minus{}0.0 values, which then prohibits simplification
6370 of expressions such as x+0.0 or 0.0*x (even with @option{-ffinite-math-only}).
6371 This option implies that the sign of a zero result isn't significant.
6373 The default is @option{-fsigned-zeros}.
6375 @item -fno-trapping-math
6376 @opindex fno-trapping-math
6377 Compile code assuming that floating-point operations cannot generate
6378 user-visible traps. These traps include division by zero, overflow,
6379 underflow, inexact result and invalid operation. This option requires
6380 that @option{-fno-signaling-nans} be in effect. Setting this option may
6381 allow faster code if one relies on ``non-stop'' IEEE arithmetic, for example.
6383 This option should never be turned on by any @option{-O} option since
6384 it can result in incorrect output for programs which depend on
6385 an exact implementation of IEEE or ISO rules/specifications for
6388 The default is @option{-ftrapping-math}.
6390 @item -frounding-math
6391 @opindex frounding-math
6392 Disable transformations and optimizations that assume default floating
6393 point rounding behavior. This is round-to-zero for all floating point
6394 to integer conversions, and round-to-nearest for all other arithmetic
6395 truncations. This option should be specified for programs that change
6396 the FP rounding mode dynamically, or that may be executed with a
6397 non-default rounding mode. This option disables constant folding of
6398 floating point expressions at compile-time (which may be affected by
6399 rounding mode) and arithmetic transformations that are unsafe in the
6400 presence of sign-dependent rounding modes.
6402 The default is @option{-fno-rounding-math}.
6404 This option is experimental and does not currently guarantee to
6405 disable all GCC optimizations that are affected by rounding mode.
6406 Future versions of GCC may provide finer control of this setting
6407 using C99's @code{FENV_ACCESS} pragma. This command line option
6408 will be used to specify the default state for @code{FENV_ACCESS}.
6410 @item -frtl-abstract-sequences
6411 @opindex frtl-abstract-sequences
6412 It is a size optimization method. This option is to find identical
6413 sequences of code, which can be turned into pseudo-procedures and
6414 then replace all occurrences with calls to the newly created
6415 subroutine. It is kind of an opposite of @option{-finline-functions}.
6416 This optimization runs at RTL level.
6418 @item -fsignaling-nans
6419 @opindex fsignaling-nans
6420 Compile code assuming that IEEE signaling NaNs may generate user-visible
6421 traps during floating-point operations. Setting this option disables
6422 optimizations that may change the number of exceptions visible with
6423 signaling NaNs. This option implies @option{-ftrapping-math}.
6425 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
6428 The default is @option{-fno-signaling-nans}.
6430 This option is experimental and does not currently guarantee to
6431 disable all GCC optimizations that affect signaling NaN behavior.
6433 @item -fsingle-precision-constant
6434 @opindex fsingle-precision-constant
6435 Treat floating point constant as single precision constant instead of
6436 implicitly converting it to double precision constant.
6438 @item -fcx-limited-range
6439 @opindex fcx-limited-range
6440 When enabled, this option states that a range reduction step is not
6441 needed when performing complex division. The default is
6442 @option{-fno-cx-limited-range}, but is enabled by @option{-ffast-math}.
6444 This option controls the default setting of the ISO C99
6445 @code{CX_LIMITED_RANGE} pragma. Nevertheless, the option applies to
6450 The following options control optimizations that may improve
6451 performance, but are not enabled by any @option{-O} options. This
6452 section includes experimental options that may produce broken code.
6455 @item -fbranch-probabilities
6456 @opindex fbranch-probabilities
6457 After running a program compiled with @option{-fprofile-arcs}
6458 (@pxref{Debugging Options,, Options for Debugging Your Program or
6459 @command{gcc}}), you can compile it a second time using
6460 @option{-fbranch-probabilities}, to improve optimizations based on
6461 the number of times each branch was taken. When the program
6462 compiled with @option{-fprofile-arcs} exits it saves arc execution
6463 counts to a file called @file{@var{sourcename}.gcda} for each source
6464 file. The information in this data file is very dependent on the
6465 structure of the generated code, so you must use the same source code
6466 and the same optimization options for both compilations.
6468 With @option{-fbranch-probabilities}, GCC puts a
6469 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
6470 These can be used to improve optimization. Currently, they are only
6471 used in one place: in @file{reorg.c}, instead of guessing which path a
6472 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
6473 exactly determine which path is taken more often.
6475 @item -fprofile-values
6476 @opindex fprofile-values
6477 If combined with @option{-fprofile-arcs}, it adds code so that some
6478 data about values of expressions in the program is gathered.
6480 With @option{-fbranch-probabilities}, it reads back the data gathered
6481 from profiling values of expressions and adds @samp{REG_VALUE_PROFILE}
6482 notes to instructions for their later usage in optimizations.
6484 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
6488 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
6489 a code to gather information about values of expressions.
6491 With @option{-fbranch-probabilities}, it reads back the data gathered
6492 and actually performs the optimizations based on them.
6493 Currently the optimizations include specialization of division operation
6494 using the knowledge about the value of the denominator.
6496 @item -frename-registers
6497 @opindex frename-registers
6498 Attempt to avoid false dependencies in scheduled code by making use
6499 of registers left over after register allocation. This optimization
6500 will most benefit processors with lots of registers. Depending on the
6501 debug information format adopted by the target, however, it can
6502 make debugging impossible, since variables will no longer stay in
6503 a ``home register''.
6505 Enabled by default with @option{-funroll-loops}.
6509 Perform tail duplication to enlarge superblock size. This transformation
6510 simplifies the control flow of the function allowing other optimizations to do
6513 Enabled with @option{-fprofile-use}.
6515 @item -funroll-loops
6516 @opindex funroll-loops
6517 Unroll loops whose number of iterations can be determined at compile time or
6518 upon entry to the loop. @option{-funroll-loops} implies
6519 @option{-frerun-cse-after-loop}, @option{-fweb} and @option{-frename-registers}.
6520 It also turns on complete loop peeling (i.e.@: complete removal of loops with
6521 small constant number of iterations). This option makes code larger, and may
6522 or may not make it run faster.
6524 Enabled with @option{-fprofile-use}.
6526 @item -funroll-all-loops
6527 @opindex funroll-all-loops
6528 Unroll all loops, even if their number of iterations is uncertain when
6529 the loop is entered. This usually makes programs run more slowly.
6530 @option{-funroll-all-loops} implies the same options as
6531 @option{-funroll-loops}.
6534 @opindex fpeel-loops
6535 Peels the loops for that there is enough information that they do not
6536 roll much (from profile feedback). It also turns on complete loop peeling
6537 (i.e.@: complete removal of loops with small constant number of iterations).
6539 Enabled with @option{-fprofile-use}.
6541 @item -fmove-loop-invariants
6542 @opindex fmove-loop-invariants
6543 Enables the loop invariant motion pass in the RTL loop optimizer. Enabled
6544 at level @option{-O1}
6546 @item -funswitch-loops
6547 @opindex funswitch-loops
6548 Move branches with loop invariant conditions out of the loop, with duplicates
6549 of the loop on both branches (modified according to result of the condition).
6551 @item -ffunction-sections
6552 @itemx -fdata-sections
6553 @opindex ffunction-sections
6554 @opindex fdata-sections
6555 Place each function or data item into its own section in the output
6556 file if the target supports arbitrary sections. The name of the
6557 function or the name of the data item determines the section's name
6560 Use these options on systems where the linker can perform optimizations
6561 to improve locality of reference in the instruction space. Most systems
6562 using the ELF object format and SPARC processors running Solaris 2 have
6563 linkers with such optimizations. AIX may have these optimizations in
6566 Only use these options when there are significant benefits from doing
6567 so. When you specify these options, the assembler and linker will
6568 create larger object and executable files and will also be slower.
6569 You will not be able to use @code{gprof} on all systems if you
6570 specify this option and you may have problems with debugging if
6571 you specify both this option and @option{-g}.
6573 @item -fbranch-target-load-optimize
6574 @opindex fbranch-target-load-optimize
6575 Perform branch target register load optimization before prologue / epilogue
6577 The use of target registers can typically be exposed only during reload,
6578 thus hoisting loads out of loops and doing inter-block scheduling needs
6579 a separate optimization pass.
6581 @item -fbranch-target-load-optimize2
6582 @opindex fbranch-target-load-optimize2
6583 Perform branch target register load optimization after prologue / epilogue
6586 @item -fbtr-bb-exclusive
6587 @opindex fbtr-bb-exclusive
6588 When performing branch target register load optimization, don't reuse
6589 branch target registers in within any basic block.
6591 @item -fstack-protector
6592 @opindex fstack-protector
6593 Emit extra code to check for buffer overflows, such as stack smashing
6594 attacks. This is done by adding a guard variable to functions with
6595 vulnerable objects. This includes functions that call alloca, and
6596 functions with buffers larger than 8 bytes. The guards are initialized
6597 when a function is entered and then checked when the function exits.
6598 If a guard check fails, an error message is printed and the program exits.
6600 @item -fstack-protector-all
6601 @opindex fstack-protector-all
6602 Like @option{-fstack-protector} except that all functions are protected.
6604 @item -fsection-anchors
6605 @opindex fsection-anchors
6606 Try to reduce the number of symbolic address calculations by using
6607 shared ``anchor'' symbols to address nearby objects. This transformation
6608 can help to reduce the number of GOT entries and GOT accesses on some
6611 For example, the implementation of the following function @code{foo}:
6615 int foo (void) @{ return a + b + c; @}
6618 would usually calculate the addresses of all three variables, but if you
6619 compile it with @option{-fsection-anchors}, it will access the variables
6620 from a common anchor point instead. The effect is similar to the
6621 following pseudocode (which isn't valid C):
6626 register int *xr = &x;
6627 return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
6631 Not all targets support this option.
6633 @item --param @var{name}=@var{value}
6635 In some places, GCC uses various constants to control the amount of
6636 optimization that is done. For example, GCC will not inline functions
6637 that contain more that a certain number of instructions. You can
6638 control some of these constants on the command-line using the
6639 @option{--param} option.
6641 The names of specific parameters, and the meaning of the values, are
6642 tied to the internals of the compiler, and are subject to change
6643 without notice in future releases.
6645 In each case, the @var{value} is an integer. The allowable choices for
6646 @var{name} are given in the following table:
6649 @item salias-max-implicit-fields
6650 The maximum number of fields in a variable without direct
6651 structure accesses for which structure aliasing will consider trying
6652 to track each field. The default is 5
6654 @item salias-max-array-elements
6655 The maximum number of elements an array can have and its elements
6656 still be tracked individually by structure aliasing. The default is 4
6658 @item sra-max-structure-size
6659 The maximum structure size, in bytes, at which the scalar replacement
6660 of aggregates (SRA) optimization will perform block copies. The
6661 default value, 0, implies that GCC will select the most appropriate
6664 @item sra-field-structure-ratio
6665 The threshold ratio (as a percentage) between instantiated fields and
6666 the complete structure size. We say that if the ratio of the number
6667 of bytes in instantiated fields to the number of bytes in the complete
6668 structure exceeds this parameter, then block copies are not used. The
6671 @item struct-reorg-cold-struct-ratio
6672 The threshold ratio (as a percentage) between a structure frequency
6673 and the frequency of the hottest structure in the program. This parameter
6674 is used by struct-reorg optimization enabled by @option{-fipa-struct-reorg}.
6675 We say that if the ratio of a structure frequency, calculated by profiling,
6676 to the hottest structure frequency in the program is less than this
6677 parameter, then structure reorganization is not applied to this structure.
6680 @item max-crossjump-edges
6681 The maximum number of incoming edges to consider for crossjumping.
6682 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
6683 the number of edges incoming to each block. Increasing values mean
6684 more aggressive optimization, making the compile time increase with
6685 probably small improvement in executable size.
6687 @item min-crossjump-insns
6688 The minimum number of instructions which must be matched at the end
6689 of two blocks before crossjumping will be performed on them. This
6690 value is ignored in the case where all instructions in the block being
6691 crossjumped from are matched. The default value is 5.
6693 @item max-grow-copy-bb-insns
6694 The maximum code size expansion factor when copying basic blocks
6695 instead of jumping. The expansion is relative to a jump instruction.
6696 The default value is 8.
6698 @item max-goto-duplication-insns
6699 The maximum number of instructions to duplicate to a block that jumps
6700 to a computed goto. To avoid @math{O(N^2)} behavior in a number of
6701 passes, GCC factors computed gotos early in the compilation process,
6702 and unfactors them as late as possible. Only computed jumps at the
6703 end of a basic blocks with no more than max-goto-duplication-insns are
6704 unfactored. The default value is 8.
6706 @item max-delay-slot-insn-search
6707 The maximum number of instructions to consider when looking for an
6708 instruction to fill a delay slot. If more than this arbitrary number of
6709 instructions is searched, the time savings from filling the delay slot
6710 will be minimal so stop searching. Increasing values mean more
6711 aggressive optimization, making the compile time increase with probably
6712 small improvement in executable run time.
6714 @item max-delay-slot-live-search
6715 When trying to fill delay slots, the maximum number of instructions to
6716 consider when searching for a block with valid live register
6717 information. Increasing this arbitrarily chosen value means more
6718 aggressive optimization, increasing the compile time. This parameter
6719 should be removed when the delay slot code is rewritten to maintain the
6722 @item max-gcse-memory
6723 The approximate maximum amount of memory that will be allocated in
6724 order to perform the global common subexpression elimination
6725 optimization. If more memory than specified is required, the
6726 optimization will not be done.
6728 @item max-gcse-passes
6729 The maximum number of passes of GCSE to run. The default is 1.
6731 @item max-pending-list-length
6732 The maximum number of pending dependencies scheduling will allow
6733 before flushing the current state and starting over. Large functions
6734 with few branches or calls can create excessively large lists which
6735 needlessly consume memory and resources.
6737 @item max-inline-insns-single
6738 Several parameters control the tree inliner used in gcc.
6739 This number sets the maximum number of instructions (counted in GCC's
6740 internal representation) in a single function that the tree inliner
6741 will consider for inlining. This only affects functions declared
6742 inline and methods implemented in a class declaration (C++).
6743 The default value is 450.
6745 @item max-inline-insns-auto
6746 When you use @option{-finline-functions} (included in @option{-O3}),
6747 a lot of functions that would otherwise not be considered for inlining
6748 by the compiler will be investigated. To those functions, a different
6749 (more restrictive) limit compared to functions declared inline can
6751 The default value is 90.
6753 @item large-function-insns
6754 The limit specifying really large functions. For functions larger than this
6755 limit after inlining inlining is constrained by
6756 @option{--param large-function-growth}. This parameter is useful primarily
6757 to avoid extreme compilation time caused by non-linear algorithms used by the
6759 This parameter is ignored when @option{-funit-at-a-time} is not used.
6760 The default value is 2700.
6762 @item large-function-growth
6763 Specifies maximal growth of large function caused by inlining in percents.
6764 This parameter is ignored when @option{-funit-at-a-time} is not used.
6765 The default value is 100 which limits large function growth to 2.0 times
6768 @item large-unit-insns
6769 The limit specifying large translation unit. Growth caused by inlining of
6770 units larger than this limit is limited by @option{--param inline-unit-growth}.
6771 For small units this might be too tight (consider unit consisting of function A
6772 that is inline and B that just calls A three time. If B is small relative to
6773 A, the growth of unit is 300\% and yet such inlining is very sane. For very
6774 large units consisting of small inlininable functions however the overall unit
6775 growth limit is needed to avoid exponential explosion of code size. Thus for
6776 smaller units, the size is increased to @option{--param large-unit-insns}
6777 before applying @option{--param inline-unit-growth}. The default is 10000
6779 @item inline-unit-growth
6780 Specifies maximal overall growth of the compilation unit caused by inlining.
6781 This parameter is ignored when @option{-funit-at-a-time} is not used.
6782 The default value is 30 which limits unit growth to 1.3 times the original
6785 @item large-stack-frame
6786 The limit specifying large stack frames. While inlining the algorithm is trying
6787 to not grow past this limit too much. Default value is 256 bytes.
6789 @item large-stack-frame-growth
6790 Specifies maximal growth of large stack frames caused by inlining in percents.
6791 The default value is 1000 which limits large stack frame growth to 11 times
6794 @item max-inline-insns-recursive
6795 @itemx max-inline-insns-recursive-auto
6796 Specifies maximum number of instructions out-of-line copy of self recursive inline
6797 function can grow into by performing recursive inlining.
6799 For functions declared inline @option{--param max-inline-insns-recursive} is
6800 taken into account. For function not declared inline, recursive inlining
6801 happens only when @option{-finline-functions} (included in @option{-O3}) is
6802 enabled and @option{--param max-inline-insns-recursive-auto} is used. The
6803 default value is 450.
6805 @item max-inline-recursive-depth
6806 @itemx max-inline-recursive-depth-auto
6807 Specifies maximum recursion depth used by the recursive inlining.
6809 For functions declared inline @option{--param max-inline-recursive-depth} is
6810 taken into account. For function not declared inline, recursive inlining
6811 happens only when @option{-finline-functions} (included in @option{-O3}) is
6812 enabled and @option{--param max-inline-recursive-depth-auto} is used. The
6815 @item min-inline-recursive-probability
6816 Recursive inlining is profitable only for function having deep recursion
6817 in average and can hurt for function having little recursion depth by
6818 increasing the prologue size or complexity of function body to other
6821 When profile feedback is available (see @option{-fprofile-generate}) the actual
6822 recursion depth can be guessed from probability that function will recurse via
6823 given call expression. This parameter limits inlining only to call expression
6824 whose probability exceeds given threshold (in percents). The default value is
6827 @item inline-call-cost
6828 Specify cost of call instruction relative to simple arithmetics operations
6829 (having cost of 1). Increasing this cost disqualifies inlining of non-leaf
6830 functions and at the same time increases size of leaf function that is believed to
6831 reduce function size by being inlined. In effect it increases amount of
6832 inlining for code having large abstraction penalty (many functions that just
6833 pass the arguments to other functions) and decrease inlining for code with low
6834 abstraction penalty. The default value is 12.
6836 @item min-vect-loop-bound
6837 The minimum number of iterations under which a loop will not get vectorized
6838 when @option{-ftree-vectorize} is used. The number of iterations after
6839 vectorization needs to be greater than the value specified by this option
6840 to allow vectorization. The default value is 0.
6842 @item max-unrolled-insns
6843 The maximum number of instructions that a loop should have if that loop
6844 is unrolled, and if the loop is unrolled, it determines how many times
6845 the loop code is unrolled.
6847 @item max-average-unrolled-insns
6848 The maximum number of instructions biased by probabilities of their execution
6849 that a loop should have if that loop is unrolled, and if the loop is unrolled,
6850 it determines how many times the loop code is unrolled.
6852 @item max-unroll-times
6853 The maximum number of unrollings of a single loop.
6855 @item max-peeled-insns
6856 The maximum number of instructions that a loop should have if that loop
6857 is peeled, and if the loop is peeled, it determines how many times
6858 the loop code is peeled.
6860 @item max-peel-times
6861 The maximum number of peelings of a single loop.
6863 @item max-completely-peeled-insns
6864 The maximum number of insns of a completely peeled loop.
6866 @item max-completely-peel-times
6867 The maximum number of iterations of a loop to be suitable for complete peeling.
6869 @item max-unswitch-insns
6870 The maximum number of insns of an unswitched loop.
6872 @item max-unswitch-level
6873 The maximum number of branches unswitched in a single loop.
6876 The minimum cost of an expensive expression in the loop invariant motion.
6878 @item iv-consider-all-candidates-bound
6879 Bound on number of candidates for induction variables below that
6880 all candidates are considered for each use in induction variable
6881 optimizations. Only the most relevant candidates are considered
6882 if there are more candidates, to avoid quadratic time complexity.
6884 @item iv-max-considered-uses
6885 The induction variable optimizations give up on loops that contain more
6886 induction variable uses.
6888 @item iv-always-prune-cand-set-bound
6889 If number of candidates in the set is smaller than this value,
6890 we always try to remove unnecessary ivs from the set during its
6891 optimization when a new iv is added to the set.
6893 @item scev-max-expr-size
6894 Bound on size of expressions used in the scalar evolutions analyzer.
6895 Large expressions slow the analyzer.
6897 @item omega-max-vars
6898 The maximum number of variables in an Omega constraint system.
6899 The default value is 128.
6901 @item omega-max-geqs
6902 The maximum number of inequalities in an Omega constraint system.
6903 The default value is 256.
6906 The maximum number of equalities in an Omega constraint system.
6907 The default value is 128.
6909 @item omega-max-wild-cards
6910 The maximum number of wildcard variables that the Omega solver will
6911 be able to insert. The default value is 18.
6913 @item omega-hash-table-size
6914 The size of the hash table in the Omega solver. The default value is
6917 @item omega-max-keys
6918 The maximal number of keys used by the Omega solver. The default
6921 @item omega-eliminate-redundant-constraints
6922 When set to 1, use expensive methods to eliminate all redundant
6923 constraints. The default value is 0.
6925 @item vect-max-version-for-alignment-checks
6926 The maximum number of runtime checks that can be performed when
6927 doing loop versioning for alignment in the vectorizer. See option
6928 ftree-vect-loop-version for more information.
6930 @item vect-max-version-for-alias-checks
6931 The maximum number of runtime checks that can be performed when
6932 doing loop versioning for alias in the vectorizer. See option
6933 ftree-vect-loop-version for more information.
6935 @item max-iterations-to-track
6937 The maximum number of iterations of a loop the brute force algorithm
6938 for analysis of # of iterations of the loop tries to evaluate.
6940 @item hot-bb-count-fraction
6941 Select fraction of the maximal count of repetitions of basic block in program
6942 given basic block needs to have to be considered hot.
6944 @item hot-bb-frequency-fraction
6945 Select fraction of the maximal frequency of executions of basic block in
6946 function given basic block needs to have to be considered hot
6948 @item max-predicted-iterations
6949 The maximum number of loop iterations we predict statically. This is useful
6950 in cases where function contain single loop with known bound and other loop
6951 with unknown. We predict the known number of iterations correctly, while
6952 the unknown number of iterations average to roughly 10. This means that the
6953 loop without bounds would appear artificially cold relative to the other one.
6955 @item align-threshold
6957 Select fraction of the maximal frequency of executions of basic block in
6958 function given basic block will get aligned.
6960 @item align-loop-iterations
6962 A loop expected to iterate at lest the selected number of iterations will get
6965 @item tracer-dynamic-coverage
6966 @itemx tracer-dynamic-coverage-feedback
6968 This value is used to limit superblock formation once the given percentage of
6969 executed instructions is covered. This limits unnecessary code size
6972 The @option{tracer-dynamic-coverage-feedback} is used only when profile
6973 feedback is available. The real profiles (as opposed to statically estimated
6974 ones) are much less balanced allowing the threshold to be larger value.
6976 @item tracer-max-code-growth
6977 Stop tail duplication once code growth has reached given percentage. This is
6978 rather hokey argument, as most of the duplicates will be eliminated later in
6979 cross jumping, so it may be set to much higher values than is the desired code
6982 @item tracer-min-branch-ratio
6984 Stop reverse growth when the reverse probability of best edge is less than this
6985 threshold (in percent).
6987 @item tracer-min-branch-ratio
6988 @itemx tracer-min-branch-ratio-feedback
6990 Stop forward growth if the best edge do have probability lower than this
6993 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
6994 compilation for profile feedback and one for compilation without. The value
6995 for compilation with profile feedback needs to be more conservative (higher) in
6996 order to make tracer effective.
6998 @item max-cse-path-length
7000 Maximum number of basic blocks on path that cse considers. The default is 10.
7003 The maximum instructions CSE process before flushing. The default is 1000.
7005 @item max-aliased-vops
7007 Maximum number of virtual operands per function allowed to represent
7008 aliases before triggering the alias partitioning heuristic. Alias
7009 partitioning reduces compile times and memory consumption needed for
7010 aliasing at the expense of precision loss in alias information. The
7011 default value for this parameter is 100 for -O1, 500 for -O2 and 1000
7014 Notice that if a function contains more memory statements than the
7015 value of this parameter, it is not really possible to achieve this
7016 reduction. In this case, the compiler will use the number of memory
7017 statements as the value for @option{max-aliased-vops}.
7019 @item avg-aliased-vops
7021 Average number of virtual operands per statement allowed to represent
7022 aliases before triggering the alias partitioning heuristic. This
7023 works in conjunction with @option{max-aliased-vops}. If a function
7024 contains more than @option{max-aliased-vops} virtual operators, then
7025 memory symbols will be grouped into memory partitions until either the
7026 total number of virtual operators is below @option{max-aliased-vops}
7027 or the average number of virtual operators per memory statement is
7028 below @option{avg-aliased-vops}. The default value for this parameter
7029 is 1 for -O1 and -O2, and 3 for -O3.
7031 @item ggc-min-expand
7033 GCC uses a garbage collector to manage its own memory allocation. This
7034 parameter specifies the minimum percentage by which the garbage
7035 collector's heap should be allowed to expand between collections.
7036 Tuning this may improve compilation speed; it has no effect on code
7039 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
7040 RAM >= 1GB@. If @code{getrlimit} is available, the notion of "RAM" is
7041 the smallest of actual RAM and @code{RLIMIT_DATA} or @code{RLIMIT_AS}. If
7042 GCC is not able to calculate RAM on a particular platform, the lower
7043 bound of 30% is used. Setting this parameter and
7044 @option{ggc-min-heapsize} to zero causes a full collection to occur at
7045 every opportunity. This is extremely slow, but can be useful for
7048 @item ggc-min-heapsize
7050 Minimum size of the garbage collector's heap before it begins bothering
7051 to collect garbage. The first collection occurs after the heap expands
7052 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
7053 tuning this may improve compilation speed, and has no effect on code
7056 The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which
7057 tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
7058 with a lower bound of 4096 (four megabytes) and an upper bound of
7059 131072 (128 megabytes). If GCC is not able to calculate RAM on a
7060 particular platform, the lower bound is used. Setting this parameter
7061 very large effectively disables garbage collection. Setting this
7062 parameter and @option{ggc-min-expand} to zero causes a full collection
7063 to occur at every opportunity.
7065 @item max-reload-search-insns
7066 The maximum number of instruction reload should look backward for equivalent
7067 register. Increasing values mean more aggressive optimization, making the
7068 compile time increase with probably slightly better performance. The default
7071 @item max-cselib-memory-locations
7072 The maximum number of memory locations cselib should take into account.
7073 Increasing values mean more aggressive optimization, making the compile time
7074 increase with probably slightly better performance. The default value is 500.
7076 @item max-flow-memory-locations
7077 Similar as @option{max-cselib-memory-locations} but for dataflow liveness.
7078 The default value is 100.
7080 @item reorder-blocks-duplicate
7081 @itemx reorder-blocks-duplicate-feedback
7083 Used by basic block reordering pass to decide whether to use unconditional
7084 branch or duplicate the code on its destination. Code is duplicated when its
7085 estimated size is smaller than this value multiplied by the estimated size of
7086 unconditional jump in the hot spots of the program.
7088 The @option{reorder-block-duplicate-feedback} is used only when profile
7089 feedback is available and may be set to higher values than
7090 @option{reorder-block-duplicate} since information about the hot spots is more
7093 @item max-sched-ready-insns
7094 The maximum number of instructions ready to be issued the scheduler should
7095 consider at any given time during the first scheduling pass. Increasing
7096 values mean more thorough searches, making the compilation time increase
7097 with probably little benefit. The default value is 100.
7099 @item max-sched-region-blocks
7100 The maximum number of blocks in a region to be considered for
7101 interblock scheduling. The default value is 10.
7103 @item max-sched-region-insns
7104 The maximum number of insns in a region to be considered for
7105 interblock scheduling. The default value is 100.
7108 The minimum probability (in percents) of reaching a source block
7109 for interblock speculative scheduling. The default value is 40.
7111 @item max-sched-extend-regions-iters
7112 The maximum number of iterations through CFG to extend regions.
7113 0 - disable region extension,
7114 N - do at most N iterations.
7115 The default value is 0.
7117 @item max-sched-insn-conflict-delay
7118 The maximum conflict delay for an insn to be considered for speculative motion.
7119 The default value is 3.
7121 @item sched-spec-prob-cutoff
7122 The minimal probability of speculation success (in percents), so that
7123 speculative insn will be scheduled.
7124 The default value is 40.
7126 @item max-last-value-rtl
7128 The maximum size measured as number of RTLs that can be recorded in an expression
7129 in combiner for a pseudo register as last known value of that register. The default
7132 @item integer-share-limit
7133 Small integer constants can use a shared data structure, reducing the
7134 compiler's memory usage and increasing its speed. This sets the maximum
7135 value of a shared integer constant's. The default value is 256.
7137 @item min-virtual-mappings
7138 Specifies the minimum number of virtual mappings in the incremental
7139 SSA updater that should be registered to trigger the virtual mappings
7140 heuristic defined by virtual-mappings-ratio. The default value is
7143 @item virtual-mappings-ratio
7144 If the number of virtual mappings is virtual-mappings-ratio bigger
7145 than the number of virtual symbols to be updated, then the incremental
7146 SSA updater switches to a full update for those symbols. The default
7149 @item ssp-buffer-size
7150 The minimum size of buffers (i.e. arrays) that will receive stack smashing
7151 protection when @option{-fstack-protection} is used.
7153 @item max-jump-thread-duplication-stmts
7154 Maximum number of statements allowed in a block that needs to be
7155 duplicated when threading jumps.
7157 @item max-fields-for-field-sensitive
7158 Maximum number of fields in a structure we will treat in
7159 a field sensitive manner during pointer analysis.
7161 @item prefetch-latency
7162 Estimate on average number of instructions that are executed before
7163 prefetch finishes. The distance we prefetch ahead is proportional
7164 to this constant. Increasing this number may also lead to less
7165 streams being prefetched (see @option{simultaneous-prefetches}).
7167 @item simultaneous-prefetches
7168 Maximum number of prefetches that can run at the same time.
7170 @item l1-cache-line-size
7171 The size of cache line in L1 cache, in bytes.
7174 The size of L1 cache, in kilobytes.
7177 The size of L2 cache, in kilobytes.
7179 @item use-canonical-types
7180 Whether the compiler should use the ``canonical'' type system. By
7181 default, this should always be 1, which uses a more efficient internal
7182 mechanism for comparing types in C++ and Objective-C++. However, if
7183 bugs in the canonical type system are causing compilation failures,
7184 set this value to 0 to disable canonical types.
7186 @item max-partial-antic-length
7187 Maximum length of the partial antic set computed during the tree
7188 partial redundancy elimination optimization (@option{-ftree-pre}) when
7189 optimizing at @option{-O3} and above. For some sorts of source code
7190 the enhanced partial redundancy elimination optimization can run away,
7191 consuming all of the memory available on the host machine. This
7192 parameter sets a limit on the length of the sets that are computed,
7193 which prevents the runaway behaviour. Setting a value of 0 for
7194 this paramter will allow an unlimited set length.
7196 @item sccvn-max-scc-size
7197 Maximum size of a strongly connected component (SCC) during SCCVN
7198 processing. If this limit is hit, SCCVN processing for the whole
7199 function will not be done and optimizations depending on it will
7200 be disabled. The default maximum SCC size is 10000.
7205 @node Preprocessor Options
7206 @section Options Controlling the Preprocessor
7207 @cindex preprocessor options
7208 @cindex options, preprocessor
7210 These options control the C preprocessor, which is run on each C source
7211 file before actual compilation.
7213 If you use the @option{-E} option, nothing is done except preprocessing.
7214 Some of these options make sense only together with @option{-E} because
7215 they cause the preprocessor output to be unsuitable for actual
7220 You can use @option{-Wp,@var{option}} to bypass the compiler driver
7221 and pass @var{option} directly through to the preprocessor. If
7222 @var{option} contains commas, it is split into multiple options at the
7223 commas. However, many options are modified, translated or interpreted
7224 by the compiler driver before being passed to the preprocessor, and
7225 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
7226 interface is undocumented and subject to change, so whenever possible
7227 you should avoid using @option{-Wp} and let the driver handle the
7230 @item -Xpreprocessor @var{option}
7231 @opindex preprocessor
7232 Pass @var{option} as an option to the preprocessor. You can use this to
7233 supply system-specific preprocessor options which GCC does not know how to
7236 If you want to pass an option that takes an argument, you must use
7237 @option{-Xpreprocessor} twice, once for the option and once for the argument.
7240 @include cppopts.texi
7242 @node Assembler Options
7243 @section Passing Options to the Assembler
7245 @c prevent bad page break with this line
7246 You can pass options to the assembler.
7249 @item -Wa,@var{option}
7251 Pass @var{option} as an option to the assembler. If @var{option}
7252 contains commas, it is split into multiple options at the commas.
7254 @item -Xassembler @var{option}
7256 Pass @var{option} as an option to the assembler. You can use this to
7257 supply system-specific assembler options which GCC does not know how to
7260 If you want to pass an option that takes an argument, you must use
7261 @option{-Xassembler} twice, once for the option and once for the argument.
7266 @section Options for Linking
7267 @cindex link options
7268 @cindex options, linking
7270 These options come into play when the compiler links object files into
7271 an executable output file. They are meaningless if the compiler is
7272 not doing a link step.
7276 @item @var{object-file-name}
7277 A file name that does not end in a special recognized suffix is
7278 considered to name an object file or library. (Object files are
7279 distinguished from libraries by the linker according to the file
7280 contents.) If linking is done, these object files are used as input
7289 If any of these options is used, then the linker is not run, and
7290 object file names should not be used as arguments. @xref{Overall
7294 @item -l@var{library}
7295 @itemx -l @var{library}
7297 Search the library named @var{library} when linking. (The second
7298 alternative with the library as a separate argument is only for
7299 POSIX compliance and is not recommended.)
7301 It makes a difference where in the command you write this option; the
7302 linker searches and processes libraries and object files in the order they
7303 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
7304 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
7305 to functions in @samp{z}, those functions may not be loaded.
7307 The linker searches a standard list of directories for the library,
7308 which is actually a file named @file{lib@var{library}.a}. The linker
7309 then uses this file as if it had been specified precisely by name.
7311 The directories searched include several standard system directories
7312 plus any that you specify with @option{-L}.
7314 Normally the files found this way are library files---archive files
7315 whose members are object files. The linker handles an archive file by
7316 scanning through it for members which define symbols that have so far
7317 been referenced but not defined. But if the file that is found is an
7318 ordinary object file, it is linked in the usual fashion. The only
7319 difference between using an @option{-l} option and specifying a file name
7320 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
7321 and searches several directories.
7325 You need this special case of the @option{-l} option in order to
7326 link an Objective-C or Objective-C++ program.
7329 @opindex nostartfiles
7330 Do not use the standard system startup files when linking.
7331 The standard system libraries are used normally, unless @option{-nostdlib}
7332 or @option{-nodefaultlibs} is used.
7334 @item -nodefaultlibs
7335 @opindex nodefaultlibs
7336 Do not use the standard system libraries when linking.
7337 Only the libraries you specify will be passed to the linker.
7338 The standard startup files are used normally, unless @option{-nostartfiles}
7339 is used. The compiler may generate calls to @code{memcmp},
7340 @code{memset}, @code{memcpy} and @code{memmove}.
7341 These entries are usually resolved by entries in
7342 libc. These entry points should be supplied through some other
7343 mechanism when this option is specified.
7347 Do not use the standard system startup files or libraries when linking.
7348 No startup files and only the libraries you specify will be passed to
7349 the linker. The compiler may generate calls to @code{memcmp}, @code{memset},
7350 @code{memcpy} and @code{memmove}.
7351 These entries are usually resolved by entries in
7352 libc. These entry points should be supplied through some other
7353 mechanism when this option is specified.
7355 @cindex @option{-lgcc}, use with @option{-nostdlib}
7356 @cindex @option{-nostdlib} and unresolved references
7357 @cindex unresolved references and @option{-nostdlib}
7358 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
7359 @cindex @option{-nodefaultlibs} and unresolved references
7360 @cindex unresolved references and @option{-nodefaultlibs}
7361 One of the standard libraries bypassed by @option{-nostdlib} and
7362 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
7363 that GCC uses to overcome shortcomings of particular machines, or special
7364 needs for some languages.
7365 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
7366 Collection (GCC) Internals},
7367 for more discussion of @file{libgcc.a}.)
7368 In most cases, you need @file{libgcc.a} even when you want to avoid
7369 other standard libraries. In other words, when you specify @option{-nostdlib}
7370 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
7371 This ensures that you have no unresolved references to internal GCC
7372 library subroutines. (For example, @samp{__main}, used to ensure C++
7373 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
7374 GNU Compiler Collection (GCC) Internals}.)
7378 Produce a position independent executable on targets which support it.
7379 For predictable results, you must also specify the same set of options
7380 that were used to generate code (@option{-fpie}, @option{-fPIE},
7381 or model suboptions) when you specify this option.
7385 Pass the flag @option{-export-dynamic} to the ELF linker, on targets
7386 that support it. This instructs the linker to add all symbols, not
7387 only used ones, to the dynamic symbol table. This option is needed
7388 for some uses of @code{dlopen} or to allow obtaining backtraces
7389 from within a program.
7393 Remove all symbol table and relocation information from the executable.
7397 On systems that support dynamic linking, this prevents linking with the shared
7398 libraries. On other systems, this option has no effect.
7402 Produce a shared object which can then be linked with other objects to
7403 form an executable. Not all systems support this option. For predictable
7404 results, you must also specify the same set of options that were used to
7405 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
7406 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
7407 needs to build supplementary stub code for constructors to work. On
7408 multi-libbed systems, @samp{gcc -shared} must select the correct support
7409 libraries to link against. Failing to supply the correct flags may lead
7410 to subtle defects. Supplying them in cases where they are not necessary
7413 @item -shared-libgcc
7414 @itemx -static-libgcc
7415 @opindex shared-libgcc
7416 @opindex static-libgcc
7417 On systems that provide @file{libgcc} as a shared library, these options
7418 force the use of either the shared or static version respectively.
7419 If no shared version of @file{libgcc} was built when the compiler was
7420 configured, these options have no effect.
7422 There are several situations in which an application should use the
7423 shared @file{libgcc} instead of the static version. The most common
7424 of these is when the application wishes to throw and catch exceptions
7425 across different shared libraries. In that case, each of the libraries
7426 as well as the application itself should use the shared @file{libgcc}.
7428 Therefore, the G++ and GCJ drivers automatically add
7429 @option{-shared-libgcc} whenever you build a shared library or a main
7430 executable, because C++ and Java programs typically use exceptions, so
7431 this is the right thing to do.
7433 If, instead, you use the GCC driver to create shared libraries, you may
7434 find that they will not always be linked with the shared @file{libgcc}.
7435 If GCC finds, at its configuration time, that you have a non-GNU linker
7436 or a GNU linker that does not support option @option{--eh-frame-hdr},
7437 it will link the shared version of @file{libgcc} into shared libraries
7438 by default. Otherwise, it will take advantage of the linker and optimize
7439 away the linking with the shared version of @file{libgcc}, linking with
7440 the static version of libgcc by default. This allows exceptions to
7441 propagate through such shared libraries, without incurring relocation
7442 costs at library load time.
7444 However, if a library or main executable is supposed to throw or catch
7445 exceptions, you must link it using the G++ or GCJ driver, as appropriate
7446 for the languages used in the program, or using the option
7447 @option{-shared-libgcc}, such that it is linked with the shared
7452 Bind references to global symbols when building a shared object. Warn
7453 about any unresolved references (unless overridden by the link editor
7454 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
7457 @item -Xlinker @var{option}
7459 Pass @var{option} as an option to the linker. You can use this to
7460 supply system-specific linker options which GCC does not know how to
7463 If you want to pass an option that takes an argument, you must use
7464 @option{-Xlinker} twice, once for the option and once for the argument.
7465 For example, to pass @option{-assert definitions}, you must write
7466 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
7467 @option{-Xlinker "-assert definitions"}, because this passes the entire
7468 string as a single argument, which is not what the linker expects.
7470 @item -Wl,@var{option}
7472 Pass @var{option} as an option to the linker. If @var{option} contains
7473 commas, it is split into multiple options at the commas.
7475 @item -u @var{symbol}
7477 Pretend the symbol @var{symbol} is undefined, to force linking of
7478 library modules to define it. You can use @option{-u} multiple times with
7479 different symbols to force loading of additional library modules.
7482 @node Directory Options
7483 @section Options for Directory Search
7484 @cindex directory options
7485 @cindex options, directory search
7488 These options specify directories to search for header files, for
7489 libraries and for parts of the compiler:
7494 Add the directory @var{dir} to the head of the list of directories to be
7495 searched for header files. This can be used to override a system header
7496 file, substituting your own version, since these directories are
7497 searched before the system header file directories. However, you should
7498 not use this option to add directories that contain vendor-supplied
7499 system header files (use @option{-isystem} for that). If you use more than
7500 one @option{-I} option, the directories are scanned in left-to-right
7501 order; the standard system directories come after.
7503 If a standard system include directory, or a directory specified with
7504 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
7505 option will be ignored. The directory will still be searched but as a
7506 system directory at its normal position in the system include chain.
7507 This is to ensure that GCC's procedure to fix buggy system headers and
7508 the ordering for the include_next directive are not inadvertently changed.
7509 If you really need to change the search order for system directories,
7510 use the @option{-nostdinc} and/or @option{-isystem} options.
7512 @item -iquote@var{dir}
7514 Add the directory @var{dir} to the head of the list of directories to
7515 be searched for header files only for the case of @samp{#include
7516 "@var{file}"}; they are not searched for @samp{#include <@var{file}>},
7517 otherwise just like @option{-I}.
7521 Add directory @var{dir} to the list of directories to be searched
7524 @item -B@var{prefix}
7526 This option specifies where to find the executables, libraries,
7527 include files, and data files of the compiler itself.
7529 The compiler driver program runs one or more of the subprograms
7530 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
7531 @var{prefix} as a prefix for each program it tries to run, both with and
7532 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
7534 For each subprogram to be run, the compiler driver first tries the
7535 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
7536 was not specified, the driver tries two standard prefixes, which are
7537 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}. If neither of
7538 those results in a file name that is found, the unmodified program
7539 name is searched for using the directories specified in your
7540 @env{PATH} environment variable.
7542 The compiler will check to see if the path provided by the @option{-B}
7543 refers to a directory, and if necessary it will add a directory
7544 separator character at the end of the path.
7546 @option{-B} prefixes that effectively specify directory names also apply
7547 to libraries in the linker, because the compiler translates these
7548 options into @option{-L} options for the linker. They also apply to
7549 includes files in the preprocessor, because the compiler translates these
7550 options into @option{-isystem} options for the preprocessor. In this case,
7551 the compiler appends @samp{include} to the prefix.
7553 The run-time support file @file{libgcc.a} can also be searched for using
7554 the @option{-B} prefix, if needed. If it is not found there, the two
7555 standard prefixes above are tried, and that is all. The file is left
7556 out of the link if it is not found by those means.
7558 Another way to specify a prefix much like the @option{-B} prefix is to use
7559 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
7562 As a special kludge, if the path provided by @option{-B} is
7563 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
7564 9, then it will be replaced by @file{[dir/]include}. This is to help
7565 with boot-strapping the compiler.
7567 @item -specs=@var{file}
7569 Process @var{file} after the compiler reads in the standard @file{specs}
7570 file, in order to override the defaults that the @file{gcc} driver
7571 program uses when determining what switches to pass to @file{cc1},
7572 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
7573 @option{-specs=@var{file}} can be specified on the command line, and they
7574 are processed in order, from left to right.
7576 @item --sysroot=@var{dir}
7578 Use @var{dir} as the logical root directory for headers and libraries.
7579 For example, if the compiler would normally search for headers in
7580 @file{/usr/include} and libraries in @file{/usr/lib}, it will instead
7581 search @file{@var{dir}/usr/include} and @file{@var{dir}/usr/lib}.
7583 If you use both this option and the @option{-isysroot} option, then
7584 the @option{--sysroot} option will apply to libraries, but the
7585 @option{-isysroot} option will apply to header files.
7587 The GNU linker (beginning with version 2.16) has the necessary support
7588 for this option. If your linker does not support this option, the
7589 header file aspect of @option{--sysroot} will still work, but the
7590 library aspect will not.
7594 This option has been deprecated. Please use @option{-iquote} instead for
7595 @option{-I} directories before the @option{-I-} and remove the @option{-I-}.
7596 Any directories you specify with @option{-I} options before the @option{-I-}
7597 option are searched only for the case of @samp{#include "@var{file}"};
7598 they are not searched for @samp{#include <@var{file}>}.
7600 If additional directories are specified with @option{-I} options after
7601 the @option{-I-}, these directories are searched for all @samp{#include}
7602 directives. (Ordinarily @emph{all} @option{-I} directories are used
7605 In addition, the @option{-I-} option inhibits the use of the current
7606 directory (where the current input file came from) as the first search
7607 directory for @samp{#include "@var{file}"}. There is no way to
7608 override this effect of @option{-I-}. With @option{-I.} you can specify
7609 searching the directory which was current when the compiler was
7610 invoked. That is not exactly the same as what the preprocessor does
7611 by default, but it is often satisfactory.
7613 @option{-I-} does not inhibit the use of the standard system directories
7614 for header files. Thus, @option{-I-} and @option{-nostdinc} are
7621 @section Specifying subprocesses and the switches to pass to them
7624 @command{gcc} is a driver program. It performs its job by invoking a
7625 sequence of other programs to do the work of compiling, assembling and
7626 linking. GCC interprets its command-line parameters and uses these to
7627 deduce which programs it should invoke, and which command-line options
7628 it ought to place on their command lines. This behavior is controlled
7629 by @dfn{spec strings}. In most cases there is one spec string for each
7630 program that GCC can invoke, but a few programs have multiple spec
7631 strings to control their behavior. The spec strings built into GCC can
7632 be overridden by using the @option{-specs=} command-line switch to specify
7635 @dfn{Spec files} are plaintext files that are used to construct spec
7636 strings. They consist of a sequence of directives separated by blank
7637 lines. The type of directive is determined by the first non-whitespace
7638 character on the line and it can be one of the following:
7641 @item %@var{command}
7642 Issues a @var{command} to the spec file processor. The commands that can
7646 @item %include <@var{file}>
7648 Search for @var{file} and insert its text at the current point in the
7651 @item %include_noerr <@var{file}>
7652 @cindex %include_noerr
7653 Just like @samp{%include}, but do not generate an error message if the include
7654 file cannot be found.
7656 @item %rename @var{old_name} @var{new_name}
7658 Rename the spec string @var{old_name} to @var{new_name}.
7662 @item *[@var{spec_name}]:
7663 This tells the compiler to create, override or delete the named spec
7664 string. All lines after this directive up to the next directive or
7665 blank line are considered to be the text for the spec string. If this
7666 results in an empty string then the spec will be deleted. (Or, if the
7667 spec did not exist, then nothing will happened.) Otherwise, if the spec
7668 does not currently exist a new spec will be created. If the spec does
7669 exist then its contents will be overridden by the text of this
7670 directive, unless the first character of that text is the @samp{+}
7671 character, in which case the text will be appended to the spec.
7673 @item [@var{suffix}]:
7674 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
7675 and up to the next directive or blank line are considered to make up the
7676 spec string for the indicated suffix. When the compiler encounters an
7677 input file with the named suffix, it will processes the spec string in
7678 order to work out how to compile that file. For example:
7685 This says that any input file whose name ends in @samp{.ZZ} should be
7686 passed to the program @samp{z-compile}, which should be invoked with the
7687 command-line switch @option{-input} and with the result of performing the
7688 @samp{%i} substitution. (See below.)
7690 As an alternative to providing a spec string, the text that follows a
7691 suffix directive can be one of the following:
7694 @item @@@var{language}
7695 This says that the suffix is an alias for a known @var{language}. This is
7696 similar to using the @option{-x} command-line switch to GCC to specify a
7697 language explicitly. For example:
7704 Says that .ZZ files are, in fact, C++ source files.
7707 This causes an error messages saying:
7710 @var{name} compiler not installed on this system.
7714 GCC already has an extensive list of suffixes built into it.
7715 This directive will add an entry to the end of the list of suffixes, but
7716 since the list is searched from the end backwards, it is effectively
7717 possible to override earlier entries using this technique.
7721 GCC has the following spec strings built into it. Spec files can
7722 override these strings or create their own. Note that individual
7723 targets can also add their own spec strings to this list.
7726 asm Options to pass to the assembler
7727 asm_final Options to pass to the assembler post-processor
7728 cpp Options to pass to the C preprocessor
7729 cc1 Options to pass to the C compiler
7730 cc1plus Options to pass to the C++ compiler
7731 endfile Object files to include at the end of the link
7732 link Options to pass to the linker
7733 lib Libraries to include on the command line to the linker
7734 libgcc Decides which GCC support library to pass to the linker
7735 linker Sets the name of the linker
7736 predefines Defines to be passed to the C preprocessor
7737 signed_char Defines to pass to CPP to say whether @code{char} is signed
7739 startfile Object files to include at the start of the link
7742 Here is a small example of a spec file:
7748 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
7751 This example renames the spec called @samp{lib} to @samp{old_lib} and
7752 then overrides the previous definition of @samp{lib} with a new one.
7753 The new definition adds in some extra command-line options before
7754 including the text of the old definition.
7756 @dfn{Spec strings} are a list of command-line options to be passed to their
7757 corresponding program. In addition, the spec strings can contain
7758 @samp{%}-prefixed sequences to substitute variable text or to
7759 conditionally insert text into the command line. Using these constructs
7760 it is possible to generate quite complex command lines.
7762 Here is a table of all defined @samp{%}-sequences for spec
7763 strings. Note that spaces are not generated automatically around the
7764 results of expanding these sequences. Therefore you can concatenate them
7765 together or combine them with constant text in a single argument.
7769 Substitute one @samp{%} into the program name or argument.
7772 Substitute the name of the input file being processed.
7775 Substitute the basename of the input file being processed.
7776 This is the substring up to (and not including) the last period
7777 and not including the directory.
7780 This is the same as @samp{%b}, but include the file suffix (text after
7784 Marks the argument containing or following the @samp{%d} as a
7785 temporary file name, so that that file will be deleted if GCC exits
7786 successfully. Unlike @samp{%g}, this contributes no text to the
7789 @item %g@var{suffix}
7790 Substitute a file name that has suffix @var{suffix} and is chosen
7791 once per compilation, and mark the argument in the same way as
7792 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
7793 name is now chosen in a way that is hard to predict even when previously
7794 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
7795 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
7796 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
7797 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
7798 was simply substituted with a file name chosen once per compilation,
7799 without regard to any appended suffix (which was therefore treated
7800 just like ordinary text), making such attacks more likely to succeed.
7802 @item %u@var{suffix}
7803 Like @samp{%g}, but generates a new temporary file name even if
7804 @samp{%u@var{suffix}} was already seen.
7806 @item %U@var{suffix}
7807 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
7808 new one if there is no such last file name. In the absence of any
7809 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
7810 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
7811 would involve the generation of two distinct file names, one
7812 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
7813 simply substituted with a file name chosen for the previous @samp{%u},
7814 without regard to any appended suffix.
7816 @item %j@var{suffix}
7817 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
7818 writable, and if save-temps is off; otherwise, substitute the name
7819 of a temporary file, just like @samp{%u}. This temporary file is not
7820 meant for communication between processes, but rather as a junk
7823 @item %|@var{suffix}
7824 @itemx %m@var{suffix}
7825 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
7826 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
7827 all. These are the two most common ways to instruct a program that it
7828 should read from standard input or write to standard output. If you
7829 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
7830 construct: see for example @file{f/lang-specs.h}.
7832 @item %.@var{SUFFIX}
7833 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
7834 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
7835 terminated by the next space or %.
7838 Marks the argument containing or following the @samp{%w} as the
7839 designated output file of this compilation. This puts the argument
7840 into the sequence of arguments that @samp{%o} will substitute later.
7843 Substitutes the names of all the output files, with spaces
7844 automatically placed around them. You should write spaces
7845 around the @samp{%o} as well or the results are undefined.
7846 @samp{%o} is for use in the specs for running the linker.
7847 Input files whose names have no recognized suffix are not compiled
7848 at all, but they are included among the output files, so they will
7852 Substitutes the suffix for object files. Note that this is
7853 handled specially when it immediately follows @samp{%g, %u, or %U},
7854 because of the need for those to form complete file names. The
7855 handling is such that @samp{%O} is treated exactly as if it had already
7856 been substituted, except that @samp{%g, %u, and %U} do not currently
7857 support additional @var{suffix} characters following @samp{%O} as they would
7858 following, for example, @samp{.o}.
7861 Substitutes the standard macro predefinitions for the
7862 current target machine. Use this when running @code{cpp}.
7865 Like @samp{%p}, but puts @samp{__} before and after the name of each
7866 predefined macro, except for macros that start with @samp{__} or with
7867 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
7871 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
7872 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}),
7873 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
7874 and @option{-imultilib} as necessary.
7877 Current argument is the name of a library or startup file of some sort.
7878 Search for that file in a standard list of directories and substitute
7879 the full name found.
7882 Print @var{str} as an error message. @var{str} is terminated by a newline.
7883 Use this when inconsistent options are detected.
7886 Substitute the contents of spec string @var{name} at this point.
7889 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
7891 @item %x@{@var{option}@}
7892 Accumulate an option for @samp{%X}.
7895 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
7899 Output the accumulated assembler options specified by @option{-Wa}.
7902 Output the accumulated preprocessor options specified by @option{-Wp}.
7905 Process the @code{asm} spec. This is used to compute the
7906 switches to be passed to the assembler.
7909 Process the @code{asm_final} spec. This is a spec string for
7910 passing switches to an assembler post-processor, if such a program is
7914 Process the @code{link} spec. This is the spec for computing the
7915 command line passed to the linker. Typically it will make use of the
7916 @samp{%L %G %S %D and %E} sequences.
7919 Dump out a @option{-L} option for each directory that GCC believes might
7920 contain startup files. If the target supports multilibs then the
7921 current multilib directory will be prepended to each of these paths.
7924 Process the @code{lib} spec. This is a spec string for deciding which
7925 libraries should be included on the command line to the linker.
7928 Process the @code{libgcc} spec. This is a spec string for deciding
7929 which GCC support library should be included on the command line to the linker.
7932 Process the @code{startfile} spec. This is a spec for deciding which
7933 object files should be the first ones passed to the linker. Typically
7934 this might be a file named @file{crt0.o}.
7937 Process the @code{endfile} spec. This is a spec string that specifies
7938 the last object files that will be passed to the linker.
7941 Process the @code{cpp} spec. This is used to construct the arguments
7942 to be passed to the C preprocessor.
7945 Process the @code{cc1} spec. This is used to construct the options to be
7946 passed to the actual C compiler (@samp{cc1}).
7949 Process the @code{cc1plus} spec. This is used to construct the options to be
7950 passed to the actual C++ compiler (@samp{cc1plus}).
7953 Substitute the variable part of a matched option. See below.
7954 Note that each comma in the substituted string is replaced by
7958 Remove all occurrences of @code{-S} from the command line. Note---this
7959 command is position dependent. @samp{%} commands in the spec string
7960 before this one will see @code{-S}, @samp{%} commands in the spec string
7961 after this one will not.
7963 @item %:@var{function}(@var{args})
7964 Call the named function @var{function}, passing it @var{args}.
7965 @var{args} is first processed as a nested spec string, then split
7966 into an argument vector in the usual fashion. The function returns
7967 a string which is processed as if it had appeared literally as part
7968 of the current spec.
7970 The following built-in spec functions are provided:
7974 The @code{getenv} spec function takes two arguments: an environment
7975 variable name and a string. If the environment variable is not
7976 defined, a fatal error is issued. Otherwise, the return value is the
7977 value of the environment variable concatenated with the string. For
7978 example, if @env{TOPDIR} is defined as @file{/path/to/top}, then:
7981 %:getenv(TOPDIR /include)
7984 expands to @file{/path/to/top/include}.
7986 @item @code{if-exists}
7987 The @code{if-exists} spec function takes one argument, an absolute
7988 pathname to a file. If the file exists, @code{if-exists} returns the
7989 pathname. Here is a small example of its usage:
7993 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
7996 @item @code{if-exists-else}
7997 The @code{if-exists-else} spec function is similar to the @code{if-exists}
7998 spec function, except that it takes two arguments. The first argument is
7999 an absolute pathname to a file. If the file exists, @code{if-exists-else}
8000 returns the pathname. If it does not exist, it returns the second argument.
8001 This way, @code{if-exists-else} can be used to select one file or another,
8002 based on the existence of the first. Here is a small example of its usage:
8006 crt0%O%s %:if-exists(crti%O%s) \
8007 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
8010 @item @code{replace-outfile}
8011 The @code{replace-outfile} spec function takes two arguments. It looks for the
8012 first argument in the outfiles array and replaces it with the second argument. Here
8013 is a small example of its usage:
8016 %@{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)@}
8019 @item @code{print-asm-header}
8020 The @code{print-asm-header} function takes no arguments and simply
8021 prints a banner like:
8027 Use "-Wa,OPTION" to pass "OPTION" to the assembler.
8030 It is used to separate compiler options from assembler options
8031 in the @option{--target-help} output.
8035 Substitutes the @code{-S} switch, if that switch was given to GCC@.
8036 If that switch was not specified, this substitutes nothing. Note that
8037 the leading dash is omitted when specifying this option, and it is
8038 automatically inserted if the substitution is performed. Thus the spec
8039 string @samp{%@{foo@}} would match the command-line option @option{-foo}
8040 and would output the command line option @option{-foo}.
8042 @item %W@{@code{S}@}
8043 Like %@{@code{S}@} but mark last argument supplied within as a file to be
8046 @item %@{@code{S}*@}
8047 Substitutes all the switches specified to GCC whose names start
8048 with @code{-S}, but which also take an argument. This is used for
8049 switches like @option{-o}, @option{-D}, @option{-I}, etc.
8050 GCC considers @option{-o foo} as being
8051 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
8052 text, including the space. Thus two arguments would be generated.
8054 @item %@{@code{S}*&@code{T}*@}
8055 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
8056 (the order of @code{S} and @code{T} in the spec is not significant).
8057 There can be any number of ampersand-separated variables; for each the
8058 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
8060 @item %@{@code{S}:@code{X}@}
8061 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
8063 @item %@{!@code{S}:@code{X}@}
8064 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
8066 @item %@{@code{S}*:@code{X}@}
8067 Substitutes @code{X} if one or more switches whose names start with
8068 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
8069 once, no matter how many such switches appeared. However, if @code{%*}
8070 appears somewhere in @code{X}, then @code{X} will be substituted once
8071 for each matching switch, with the @code{%*} replaced by the part of
8072 that switch that matched the @code{*}.
8074 @item %@{.@code{S}:@code{X}@}
8075 Substitutes @code{X}, if processing a file with suffix @code{S}.
8077 @item %@{!.@code{S}:@code{X}@}
8078 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
8080 @item %@{,@code{S}:@code{X}@}
8081 Substitutes @code{X}, if processing a file for language @code{S}.
8083 @item %@{!,@code{S}:@code{X}@}
8084 Substitutes @code{X}, if not processing a file for language @code{S}.
8086 @item %@{@code{S}|@code{P}:@code{X}@}
8087 Substitutes @code{X} if either @code{-S} or @code{-P} was given to
8088 GCC@. This may be combined with @samp{!}, @samp{.}, @samp{,}, and
8089 @code{*} sequences as well, although they have a stronger binding than
8090 the @samp{|}. If @code{%*} appears in @code{X}, all of the
8091 alternatives must be starred, and only the first matching alternative
8094 For example, a spec string like this:
8097 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
8100 will output the following command-line options from the following input
8101 command-line options:
8106 -d fred.c -foo -baz -boggle
8107 -d jim.d -bar -baz -boggle
8110 @item %@{S:X; T:Y; :D@}
8112 If @code{S} was given to GCC, substitutes @code{X}; else if @code{T} was
8113 given to GCC, substitutes @code{Y}; else substitutes @code{D}. There can
8114 be as many clauses as you need. This may be combined with @code{.},
8115 @code{,}, @code{!}, @code{|}, and @code{*} as needed.
8120 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
8121 construct may contain other nested @samp{%} constructs or spaces, or
8122 even newlines. They are processed as usual, as described above.
8123 Trailing white space in @code{X} is ignored. White space may also
8124 appear anywhere on the left side of the colon in these constructs,
8125 except between @code{.} or @code{*} and the corresponding word.
8127 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
8128 handled specifically in these constructs. If another value of
8129 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
8130 @option{-W} switch is found later in the command line, the earlier
8131 switch value is ignored, except with @{@code{S}*@} where @code{S} is
8132 just one letter, which passes all matching options.
8134 The character @samp{|} at the beginning of the predicate text is used to
8135 indicate that a command should be piped to the following command, but
8136 only if @option{-pipe} is specified.
8138 It is built into GCC which switches take arguments and which do not.
8139 (You might think it would be useful to generalize this to allow each
8140 compiler's spec to say which switches take arguments. But this cannot
8141 be done in a consistent fashion. GCC cannot even decide which input
8142 files have been specified without knowing which switches take arguments,
8143 and it must know which input files to compile in order to tell which
8146 GCC also knows implicitly that arguments starting in @option{-l} are to be
8147 treated as compiler output files, and passed to the linker in their
8148 proper position among the other output files.
8150 @c man begin OPTIONS
8152 @node Target Options
8153 @section Specifying Target Machine and Compiler Version
8154 @cindex target options
8155 @cindex cross compiling
8156 @cindex specifying machine version
8157 @cindex specifying compiler version and target machine
8158 @cindex compiler version, specifying
8159 @cindex target machine, specifying
8161 The usual way to run GCC is to run the executable called @file{gcc}, or
8162 @file{<machine>-gcc} when cross-compiling, or
8163 @file{<machine>-gcc-<version>} to run a version other than the one that
8164 was installed last. Sometimes this is inconvenient, so GCC provides
8165 options that will switch to another cross-compiler or version.
8168 @item -b @var{machine}
8170 The argument @var{machine} specifies the target machine for compilation.
8172 The value to use for @var{machine} is the same as was specified as the
8173 machine type when configuring GCC as a cross-compiler. For
8174 example, if a cross-compiler was configured with @samp{configure
8175 arm-elf}, meaning to compile for an arm processor with elf binaries,
8176 then you would specify @option{-b arm-elf} to run that cross compiler.
8177 Because there are other options beginning with @option{-b}, the
8178 configuration must contain a hyphen.
8180 @item -V @var{version}
8182 The argument @var{version} specifies which version of GCC to run.
8183 This is useful when multiple versions are installed. For example,
8184 @var{version} might be @samp{4.0}, meaning to run GCC version 4.0.
8187 The @option{-V} and @option{-b} options work by running the
8188 @file{<machine>-gcc-<version>} executable, so there's no real reason to
8189 use them if you can just run that directly.
8191 @node Submodel Options
8192 @section Hardware Models and Configurations
8193 @cindex submodel options
8194 @cindex specifying hardware config
8195 @cindex hardware models and configurations, specifying
8196 @cindex machine dependent options
8198 Earlier we discussed the standard option @option{-b} which chooses among
8199 different installed compilers for completely different target
8200 machines, such as VAX vs.@: 68000 vs.@: 80386.
8202 In addition, each of these target machine types can have its own
8203 special options, starting with @samp{-m}, to choose among various
8204 hardware models or configurations---for example, 68010 vs 68020,
8205 floating coprocessor or none. A single installed version of the
8206 compiler can compile for any model or configuration, according to the
8209 Some configurations of the compiler also support additional special
8210 options, usually for compatibility with other compilers on the same
8213 @c This list is ordered alphanumerically by subsection name.
8214 @c It should be the same order and spelling as these options are listed
8215 @c in Machine Dependent Options
8221 * Blackfin Options::
8225 * DEC Alpha Options::
8226 * DEC Alpha/VMS Options::
8228 * GNU/Linux Options::
8231 * i386 and x86-64 Options::
8244 * RS/6000 and PowerPC Options::
8245 * S/390 and zSeries Options::
8250 * System V Options::
8251 * TMS320C3x/C4x Options::
8256 * Xstormy16 Options::
8262 @subsection ARC Options
8265 These options are defined for ARC implementations:
8270 Compile code for little endian mode. This is the default.
8274 Compile code for big endian mode.
8277 @opindex mmangle-cpu
8278 Prepend the name of the cpu to all public symbol names.
8279 In multiple-processor systems, there are many ARC variants with different
8280 instruction and register set characteristics. This flag prevents code
8281 compiled for one cpu to be linked with code compiled for another.
8282 No facility exists for handling variants that are ``almost identical''.
8283 This is an all or nothing option.
8285 @item -mcpu=@var{cpu}
8287 Compile code for ARC variant @var{cpu}.
8288 Which variants are supported depend on the configuration.
8289 All variants support @option{-mcpu=base}, this is the default.
8291 @item -mtext=@var{text-section}
8292 @itemx -mdata=@var{data-section}
8293 @itemx -mrodata=@var{readonly-data-section}
8297 Put functions, data, and readonly data in @var{text-section},
8298 @var{data-section}, and @var{readonly-data-section} respectively
8299 by default. This can be overridden with the @code{section} attribute.
8300 @xref{Variable Attributes}.
8305 @subsection ARM Options
8308 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
8312 @item -mabi=@var{name}
8314 Generate code for the specified ABI@. Permissible values are: @samp{apcs-gnu},
8315 @samp{atpcs}, @samp{aapcs}, @samp{aapcs-linux} and @samp{iwmmxt}.
8318 @opindex mapcs-frame
8319 Generate a stack frame that is compliant with the ARM Procedure Call
8320 Standard for all functions, even if this is not strictly necessary for
8321 correct execution of the code. Specifying @option{-fomit-frame-pointer}
8322 with this option will cause the stack frames not to be generated for
8323 leaf functions. The default is @option{-mno-apcs-frame}.
8327 This is a synonym for @option{-mapcs-frame}.
8330 @c not currently implemented
8331 @item -mapcs-stack-check
8332 @opindex mapcs-stack-check
8333 Generate code to check the amount of stack space available upon entry to
8334 every function (that actually uses some stack space). If there is
8335 insufficient space available then either the function
8336 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
8337 called, depending upon the amount of stack space required. The run time
8338 system is required to provide these functions. The default is
8339 @option{-mno-apcs-stack-check}, since this produces smaller code.
8341 @c not currently implemented
8343 @opindex mapcs-float
8344 Pass floating point arguments using the float point registers. This is
8345 one of the variants of the APCS@. This option is recommended if the
8346 target hardware has a floating point unit or if a lot of floating point
8347 arithmetic is going to be performed by the code. The default is
8348 @option{-mno-apcs-float}, since integer only code is slightly increased in
8349 size if @option{-mapcs-float} is used.
8351 @c not currently implemented
8352 @item -mapcs-reentrant
8353 @opindex mapcs-reentrant
8354 Generate reentrant, position independent code. The default is
8355 @option{-mno-apcs-reentrant}.
8358 @item -mthumb-interwork
8359 @opindex mthumb-interwork
8360 Generate code which supports calling between the ARM and Thumb
8361 instruction sets. Without this option the two instruction sets cannot
8362 be reliably used inside one program. The default is
8363 @option{-mno-thumb-interwork}, since slightly larger code is generated
8364 when @option{-mthumb-interwork} is specified.
8366 @item -mno-sched-prolog
8367 @opindex mno-sched-prolog
8368 Prevent the reordering of instructions in the function prolog, or the
8369 merging of those instruction with the instructions in the function's
8370 body. This means that all functions will start with a recognizable set
8371 of instructions (or in fact one of a choice from a small set of
8372 different function prologues), and this information can be used to
8373 locate the start if functions inside an executable piece of code. The
8374 default is @option{-msched-prolog}.
8377 @opindex mhard-float
8378 Generate output containing floating point instructions. This is the
8382 @opindex msoft-float
8383 Generate output containing library calls for floating point.
8384 @strong{Warning:} the requisite libraries are not available for all ARM
8385 targets. Normally the facilities of the machine's usual C compiler are
8386 used, but this cannot be done directly in cross-compilation. You must make
8387 your own arrangements to provide suitable library functions for
8390 @option{-msoft-float} changes the calling convention in the output file;
8391 therefore, it is only useful if you compile @emph{all} of a program with
8392 this option. In particular, you need to compile @file{libgcc.a}, the
8393 library that comes with GCC, with @option{-msoft-float} in order for
8396 @item -mfloat-abi=@var{name}
8398 Specifies which ABI to use for floating point values. Permissible values
8399 are: @samp{soft}, @samp{softfp} and @samp{hard}.
8401 @samp{soft} and @samp{hard} are equivalent to @option{-msoft-float}
8402 and @option{-mhard-float} respectively. @samp{softfp} allows the generation
8403 of floating point instructions, but still uses the soft-float calling
8406 @item -mlittle-endian
8407 @opindex mlittle-endian
8408 Generate code for a processor running in little-endian mode. This is
8409 the default for all standard configurations.
8412 @opindex mbig-endian
8413 Generate code for a processor running in big-endian mode; the default is
8414 to compile code for a little-endian processor.
8416 @item -mwords-little-endian
8417 @opindex mwords-little-endian
8418 This option only applies when generating code for big-endian processors.
8419 Generate code for a little-endian word order but a big-endian byte
8420 order. That is, a byte order of the form @samp{32107654}. Note: this
8421 option should only be used if you require compatibility with code for
8422 big-endian ARM processors generated by versions of the compiler prior to
8425 @item -mcpu=@var{name}
8427 This specifies the name of the target ARM processor. GCC uses this name
8428 to determine what kind of instructions it can emit when generating
8429 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
8430 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
8431 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
8432 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
8433 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
8434 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm7tdmi-s},
8435 @samp{arm8}, @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
8436 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
8437 @samp{arm920t}, @samp{arm922t}, @samp{arm946e-s}, @samp{arm966e-s},
8438 @samp{arm968e-s}, @samp{arm926ej-s}, @samp{arm940t}, @samp{arm9tdmi},
8439 @samp{arm10tdmi}, @samp{arm1020t}, @samp{arm1026ej-s},
8440 @samp{arm10e}, @samp{arm1020e}, @samp{arm1022e},
8441 @samp{arm1136j-s}, @samp{arm1136jf-s}, @samp{mpcore}, @samp{mpcorenovfp},
8442 @samp{arm1156t2-s}, @samp{arm1176jz-s}, @samp{arm1176jzf-s},
8443 @samp{cortex-a8}, @samp{cortex-r4}, @samp{cortex-m3},
8444 @samp{xscale}, @samp{iwmmxt}, @samp{ep9312}.
8446 @itemx -mtune=@var{name}
8448 This option is very similar to the @option{-mcpu=} option, except that
8449 instead of specifying the actual target processor type, and hence
8450 restricting which instructions can be used, it specifies that GCC should
8451 tune the performance of the code as if the target were of the type
8452 specified in this option, but still choosing the instructions that it
8453 will generate based on the cpu specified by a @option{-mcpu=} option.
8454 For some ARM implementations better performance can be obtained by using
8457 @item -march=@var{name}
8459 This specifies the name of the target ARM architecture. GCC uses this
8460 name to determine what kind of instructions it can emit when generating
8461 assembly code. This option can be used in conjunction with or instead
8462 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
8463 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
8464 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{armv6}, @samp{armv6j},
8465 @samp{armv6t2}, @samp{armv6z}, @samp{armv6zk}, @samp{armv7}, @samp{armv7-a},
8466 @samp{armv7-r}, @samp{armv7-m}, @samp{iwmmxt}, @samp{ep9312}.
8468 @item -mfpu=@var{name}
8469 @itemx -mfpe=@var{number}
8470 @itemx -mfp=@var{number}
8474 This specifies what floating point hardware (or hardware emulation) is
8475 available on the target. Permissible names are: @samp{fpa}, @samp{fpe2},
8476 @samp{fpe3}, @samp{maverick}, @samp{vfp}. @option{-mfp} and @option{-mfpe}
8477 are synonyms for @option{-mfpu}=@samp{fpe}@var{number}, for compatibility
8478 with older versions of GCC@.
8480 If @option{-msoft-float} is specified this specifies the format of
8481 floating point values.
8483 @item -mstructure-size-boundary=@var{n}
8484 @opindex mstructure-size-boundary
8485 The size of all structures and unions will be rounded up to a multiple
8486 of the number of bits set by this option. Permissible values are 8, 32
8487 and 64. The default value varies for different toolchains. For the COFF
8488 targeted toolchain the default value is 8. A value of 64 is only allowed
8489 if the underlying ABI supports it.
8491 Specifying the larger number can produce faster, more efficient code, but
8492 can also increase the size of the program. Different values are potentially
8493 incompatible. Code compiled with one value cannot necessarily expect to
8494 work with code or libraries compiled with another value, if they exchange
8495 information using structures or unions.
8497 @item -mabort-on-noreturn
8498 @opindex mabort-on-noreturn
8499 Generate a call to the function @code{abort} at the end of a
8500 @code{noreturn} function. It will be executed if the function tries to
8504 @itemx -mno-long-calls
8505 @opindex mlong-calls
8506 @opindex mno-long-calls
8507 Tells the compiler to perform function calls by first loading the
8508 address of the function into a register and then performing a subroutine
8509 call on this register. This switch is needed if the target function
8510 will lie outside of the 64 megabyte addressing range of the offset based
8511 version of subroutine call instruction.
8513 Even if this switch is enabled, not all function calls will be turned
8514 into long calls. The heuristic is that static functions, functions
8515 which have the @samp{short-call} attribute, functions that are inside
8516 the scope of a @samp{#pragma no_long_calls} directive and functions whose
8517 definitions have already been compiled within the current compilation
8518 unit, will not be turned into long calls. The exception to this rule is
8519 that weak function definitions, functions with the @samp{long-call}
8520 attribute or the @samp{section} attribute, and functions that are within
8521 the scope of a @samp{#pragma long_calls} directive, will always be
8522 turned into long calls.
8524 This feature is not enabled by default. Specifying
8525 @option{-mno-long-calls} will restore the default behavior, as will
8526 placing the function calls within the scope of a @samp{#pragma
8527 long_calls_off} directive. Note these switches have no effect on how
8528 the compiler generates code to handle function calls via function
8531 @item -mnop-fun-dllimport
8532 @opindex mnop-fun-dllimport
8533 Disable support for the @code{dllimport} attribute.
8535 @item -msingle-pic-base
8536 @opindex msingle-pic-base
8537 Treat the register used for PIC addressing as read-only, rather than
8538 loading it in the prologue for each function. The run-time system is
8539 responsible for initializing this register with an appropriate value
8540 before execution begins.
8542 @item -mpic-register=@var{reg}
8543 @opindex mpic-register
8544 Specify the register to be used for PIC addressing. The default is R10
8545 unless stack-checking is enabled, when R9 is used.
8547 @item -mcirrus-fix-invalid-insns
8548 @opindex mcirrus-fix-invalid-insns
8549 @opindex mno-cirrus-fix-invalid-insns
8550 Insert NOPs into the instruction stream to in order to work around
8551 problems with invalid Maverick instruction combinations. This option
8552 is only valid if the @option{-mcpu=ep9312} option has been used to
8553 enable generation of instructions for the Cirrus Maverick floating
8554 point co-processor. This option is not enabled by default, since the
8555 problem is only present in older Maverick implementations. The default
8556 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
8559 @item -mpoke-function-name
8560 @opindex mpoke-function-name
8561 Write the name of each function into the text section, directly
8562 preceding the function prologue. The generated code is similar to this:
8566 .ascii "arm_poke_function_name", 0
8569 .word 0xff000000 + (t1 - t0)
8570 arm_poke_function_name
8572 stmfd sp!, @{fp, ip, lr, pc@}
8576 When performing a stack backtrace, code can inspect the value of
8577 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
8578 location @code{pc - 12} and the top 8 bits are set, then we know that
8579 there is a function name embedded immediately preceding this location
8580 and has length @code{((pc[-3]) & 0xff000000)}.
8584 Generate code for the Thumb instruction set. The default is to
8585 use the 32-bit ARM instruction set.
8586 This option automatically enables either 16-bit Thumb-1 or
8587 mixed 16/32-bit Thumb-2 instructions based on the @option{-mcpu=@var{name}}
8588 and @option{-march=@var{name}} options.
8591 @opindex mtpcs-frame
8592 Generate a stack frame that is compliant with the Thumb Procedure Call
8593 Standard for all non-leaf functions. (A leaf function is one that does
8594 not call any other functions.) The default is @option{-mno-tpcs-frame}.
8596 @item -mtpcs-leaf-frame
8597 @opindex mtpcs-leaf-frame
8598 Generate a stack frame that is compliant with the Thumb Procedure Call
8599 Standard for all leaf functions. (A leaf function is one that does
8600 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
8602 @item -mcallee-super-interworking
8603 @opindex mcallee-super-interworking
8604 Gives all externally visible functions in the file being compiled an ARM
8605 instruction set header which switches to Thumb mode before executing the
8606 rest of the function. This allows these functions to be called from
8607 non-interworking code.
8609 @item -mcaller-super-interworking
8610 @opindex mcaller-super-interworking
8611 Allows calls via function pointers (including virtual functions) to
8612 execute correctly regardless of whether the target code has been
8613 compiled for interworking or not. There is a small overhead in the cost
8614 of executing a function pointer if this option is enabled.
8616 @item -mtp=@var{name}
8618 Specify the access model for the thread local storage pointer. The valid
8619 models are @option{soft}, which generates calls to @code{__aeabi_read_tp},
8620 @option{cp15}, which fetches the thread pointer from @code{cp15} directly
8621 (supported in the arm6k architecture), and @option{auto}, which uses the
8622 best available method for the selected processor. The default setting is
8628 @subsection AVR Options
8631 These options are defined for AVR implementations:
8634 @item -mmcu=@var{mcu}
8636 Specify ATMEL AVR instruction set or MCU type.
8638 Instruction set avr1 is for the minimal AVR core, not supported by the C
8639 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
8640 attiny11, attiny12, attiny15, attiny28).
8642 Instruction set avr2 (default) is for the classic AVR core with up to
8643 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
8644 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
8645 at90c8534, at90s8535).
8647 Instruction set avr3 is for the classic AVR core with up to 128K program
8648 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
8650 Instruction set avr4 is for the enhanced AVR core with up to 8K program
8651 memory space (MCU types: atmega8, atmega83, atmega85).
8653 Instruction set avr5 is for the enhanced AVR core with up to 128K program
8654 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
8655 atmega64, atmega128, at43usb355, at94k).
8659 Output instruction sizes to the asm file.
8661 @item -minit-stack=@var{N}
8662 @opindex minit-stack
8663 Specify the initial stack address, which may be a symbol or numeric value,
8664 @samp{__stack} is the default.
8666 @item -mno-interrupts
8667 @opindex mno-interrupts
8668 Generated code is not compatible with hardware interrupts.
8669 Code size will be smaller.
8671 @item -mcall-prologues
8672 @opindex mcall-prologues
8673 Functions prologues/epilogues expanded as call to appropriate
8674 subroutines. Code size will be smaller.
8676 @item -mno-tablejump
8677 @opindex mno-tablejump
8678 Do not generate tablejump insns which sometimes increase code size.
8681 @opindex mtiny-stack
8682 Change only the low 8 bits of the stack pointer.
8686 Assume int to be 8 bit integer. This affects the sizes of all types: A
8687 char will be 1 byte, an int will be 1 byte, an long will be 2 bytes
8688 and long long will be 4 bytes. Please note that this option does not
8689 comply to the C standards, but it will provide you with smaller code
8693 @node Blackfin Options
8694 @subsection Blackfin Options
8695 @cindex Blackfin Options
8698 @item -mcpu=@var{cpu}@r{[}-@var{sirevision}@r{]}
8700 Specifies the name of the target Blackfin processor. Currently, @var{cpu}
8701 can be one of @samp{bf522}, @samp{bf525}, @samp{bf527},
8702 @samp{bf531}, @samp{bf532}, @samp{bf533}, @samp{bf534},
8703 @samp{bf536}, @samp{bf537}, @samp{bf538}, @samp{bf539},
8704 @samp{bf542}, @samp{bf544}, @samp{bf548}, @samp{bf549},
8706 The optional @var{sirevision} specifies the silicon revision of the target
8707 Blackfin processor. Any workarounds available for the targeted silicon revision
8708 will be enabled. If @var{sirevision} is @samp{none}, no workarounds are enabled.
8709 If @var{sirevision} is @samp{any}, all workarounds for the targeted processor
8710 will be enabled. The @code{__SILICON_REVISION__} macro is defined to two
8711 hexadecimal digits representing the major and minor numbers in the silicon
8712 revision. If @var{sirevision} is @samp{none}, the @code{__SILICON_REVISION__}
8713 is not defined. If @var{sirevision} is @samp{any}, the
8714 @code{__SILICON_REVISION__} is defined to be @code{0xffff}.
8715 If this optional @var{sirevision} is not used, GCC assumes the latest known
8716 silicon revision of the targeted Blackfin processor.
8718 Support for @samp{bf561} is incomplete. For @samp{bf561},
8719 Only the processor macro is defined.
8720 Without this option, @samp{bf532} is used as the processor by default.
8721 The corresponding predefined processor macros for @var{cpu} is to
8722 be defined. And for @samp{bfin-elf} toolchain, this causes the hardware BSP
8723 provided by libgloss to be linked in if @option{-msim} is not given.
8727 Specifies that the program will be run on the simulator. This causes
8728 the simulator BSP provided by libgloss to be linked in. This option
8729 has effect only for @samp{bfin-elf} toolchain.
8730 Certain other options, such as @option{-mid-shared-library} and
8731 @option{-mfdpic}, imply @option{-msim}.
8733 @item -momit-leaf-frame-pointer
8734 @opindex momit-leaf-frame-pointer
8735 Don't keep the frame pointer in a register for leaf functions. This
8736 avoids the instructions to save, set up and restore frame pointers and
8737 makes an extra register available in leaf functions. The option
8738 @option{-fomit-frame-pointer} removes the frame pointer for all functions
8739 which might make debugging harder.
8741 @item -mspecld-anomaly
8742 @opindex mspecld-anomaly
8743 When enabled, the compiler will ensure that the generated code does not
8744 contain speculative loads after jump instructions. If this option is used,
8745 @code{__WORKAROUND_SPECULATIVE_LOADS} is defined.
8747 @item -mno-specld-anomaly
8748 @opindex mno-specld-anomaly
8749 Don't generate extra code to prevent speculative loads from occurring.
8751 @item -mcsync-anomaly
8752 @opindex mcsync-anomaly
8753 When enabled, the compiler will ensure that the generated code does not
8754 contain CSYNC or SSYNC instructions too soon after conditional branches.
8755 If this option is used, @code{__WORKAROUND_SPECULATIVE_SYNCS} is defined.
8757 @item -mno-csync-anomaly
8758 @opindex mno-csync-anomaly
8759 Don't generate extra code to prevent CSYNC or SSYNC instructions from
8760 occurring too soon after a conditional branch.
8764 When enabled, the compiler is free to take advantage of the knowledge that
8765 the entire program fits into the low 64k of memory.
8768 @opindex mno-low-64k
8769 Assume that the program is arbitrarily large. This is the default.
8771 @item -mstack-check-l1
8772 @opindex mstack-check-l1
8773 Do stack checking using information placed into L1 scratchpad memory by the
8776 @item -mid-shared-library
8777 @opindex mid-shared-library
8778 Generate code that supports shared libraries via the library ID method.
8779 This allows for execute in place and shared libraries in an environment
8780 without virtual memory management. This option implies @option{-fPIC}.
8781 With a @samp{bfin-elf} target, this option implies @option{-msim}.
8783 @item -mno-id-shared-library
8784 @opindex mno-id-shared-library
8785 Generate code that doesn't assume ID based shared libraries are being used.
8786 This is the default.
8788 @item -mleaf-id-shared-library
8789 @opindex mleaf-id-shared-library
8790 Generate code that supports shared libraries via the library ID method,
8791 but assumes that this library or executable won't link against any other
8792 ID shared libraries. That allows the compiler to use faster code for jumps
8795 @item -mno-leaf-id-shared-library
8796 @opindex mno-leaf-id-shared-library
8797 Do not assume that the code being compiled won't link against any ID shared
8798 libraries. Slower code will be generated for jump and call insns.
8800 @item -mshared-library-id=n
8801 @opindex mshared-library-id
8802 Specified the identification number of the ID based shared library being
8803 compiled. Specifying a value of 0 will generate more compact code, specifying
8804 other values will force the allocation of that number to the current
8805 library but is no more space or time efficient than omitting this option.
8809 Generate code that allows the data segment to be located in a different
8810 area of memory from the text segment. This allows for execute in place in
8811 an environment without virtual memory management by eliminating relocations
8812 against the text section.
8815 @opindex mno-sep-data
8816 Generate code that assumes that the data segment follows the text segment.
8817 This is the default.
8820 @itemx -mno-long-calls
8821 @opindex mlong-calls
8822 @opindex mno-long-calls
8823 Tells the compiler to perform function calls by first loading the
8824 address of the function into a register and then performing a subroutine
8825 call on this register. This switch is needed if the target function
8826 will lie outside of the 24 bit addressing range of the offset based
8827 version of subroutine call instruction.
8829 This feature is not enabled by default. Specifying
8830 @option{-mno-long-calls} will restore the default behavior. Note these
8831 switches have no effect on how the compiler generates code to handle
8832 function calls via function pointers.
8836 Link with the fast floating-point library. This library relaxes some of
8837 the IEEE floating-point standard's rules for checking inputs against
8838 Not-a-Number (NAN), in the interest of performance.
8841 @opindex minline-plt
8842 Enable inlining of PLT entries in function calls to functions that are
8843 not known to bind locally. It has no effect without @option{-mfdpic}.
8847 @subsection CRIS Options
8848 @cindex CRIS Options
8850 These options are defined specifically for the CRIS ports.
8853 @item -march=@var{architecture-type}
8854 @itemx -mcpu=@var{architecture-type}
8857 Generate code for the specified architecture. The choices for
8858 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
8859 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX@.
8860 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
8863 @item -mtune=@var{architecture-type}
8865 Tune to @var{architecture-type} everything applicable about the generated
8866 code, except for the ABI and the set of available instructions. The
8867 choices for @var{architecture-type} are the same as for
8868 @option{-march=@var{architecture-type}}.
8870 @item -mmax-stack-frame=@var{n}
8871 @opindex mmax-stack-frame
8872 Warn when the stack frame of a function exceeds @var{n} bytes.
8874 @item -melinux-stacksize=@var{n}
8875 @opindex melinux-stacksize
8876 Only available with the @samp{cris-axis-aout} target. Arranges for
8877 indications in the program to the kernel loader that the stack of the
8878 program should be set to @var{n} bytes.
8884 The options @option{-metrax4} and @option{-metrax100} are synonyms for
8885 @option{-march=v3} and @option{-march=v8} respectively.
8887 @item -mmul-bug-workaround
8888 @itemx -mno-mul-bug-workaround
8889 @opindex mmul-bug-workaround
8890 @opindex mno-mul-bug-workaround
8891 Work around a bug in the @code{muls} and @code{mulu} instructions for CPU
8892 models where it applies. This option is active by default.
8896 Enable CRIS-specific verbose debug-related information in the assembly
8897 code. This option also has the effect to turn off the @samp{#NO_APP}
8898 formatted-code indicator to the assembler at the beginning of the
8903 Do not use condition-code results from previous instruction; always emit
8904 compare and test instructions before use of condition codes.
8906 @item -mno-side-effects
8907 @opindex mno-side-effects
8908 Do not emit instructions with side-effects in addressing modes other than
8912 @itemx -mno-stack-align
8914 @itemx -mno-data-align
8915 @itemx -mconst-align
8916 @itemx -mno-const-align
8917 @opindex mstack-align
8918 @opindex mno-stack-align
8919 @opindex mdata-align
8920 @opindex mno-data-align
8921 @opindex mconst-align
8922 @opindex mno-const-align
8923 These options (no-options) arranges (eliminate arrangements) for the
8924 stack-frame, individual data and constants to be aligned for the maximum
8925 single data access size for the chosen CPU model. The default is to
8926 arrange for 32-bit alignment. ABI details such as structure layout are
8927 not affected by these options.
8935 Similar to the stack- data- and const-align options above, these options
8936 arrange for stack-frame, writable data and constants to all be 32-bit,
8937 16-bit or 8-bit aligned. The default is 32-bit alignment.
8939 @item -mno-prologue-epilogue
8940 @itemx -mprologue-epilogue
8941 @opindex mno-prologue-epilogue
8942 @opindex mprologue-epilogue
8943 With @option{-mno-prologue-epilogue}, the normal function prologue and
8944 epilogue that sets up the stack-frame are omitted and no return
8945 instructions or return sequences are generated in the code. Use this
8946 option only together with visual inspection of the compiled code: no
8947 warnings or errors are generated when call-saved registers must be saved,
8948 or storage for local variable needs to be allocated.
8954 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
8955 instruction sequences that load addresses for functions from the PLT part
8956 of the GOT rather than (traditional on other architectures) calls to the
8957 PLT@. The default is @option{-mgotplt}.
8961 Legacy no-op option only recognized with the cris-axis-aout target.
8965 Legacy no-op option only recognized with the cris-axis-elf and
8966 cris-axis-linux-gnu targets.
8970 Only recognized with the cris-axis-aout target, where it selects a
8971 GNU/linux-like multilib, include files and instruction set for
8976 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
8980 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
8981 to link with input-output functions from a simulator library. Code,
8982 initialized data and zero-initialized data are allocated consecutively.
8986 Like @option{-sim}, but pass linker options to locate initialized data at
8987 0x40000000 and zero-initialized data at 0x80000000.
8991 @subsection CRX Options
8994 These options are defined specifically for the CRX ports.
9000 Enable the use of multiply-accumulate instructions. Disabled by default.
9004 Push instructions will be used to pass outgoing arguments when functions
9005 are called. Enabled by default.
9008 @node Darwin Options
9009 @subsection Darwin Options
9010 @cindex Darwin options
9012 These options are defined for all architectures running the Darwin operating
9015 FSF GCC on Darwin does not create ``fat'' object files; it will create
9016 an object file for the single architecture that it was built to
9017 target. Apple's GCC on Darwin does create ``fat'' files if multiple
9018 @option{-arch} options are used; it does so by running the compiler or
9019 linker multiple times and joining the results together with
9022 The subtype of the file created (like @samp{ppc7400} or @samp{ppc970} or
9023 @samp{i686}) is determined by the flags that specify the ISA
9024 that GCC is targetting, like @option{-mcpu} or @option{-march}. The
9025 @option{-force_cpusubtype_ALL} option can be used to override this.
9027 The Darwin tools vary in their behavior when presented with an ISA
9028 mismatch. The assembler, @file{as}, will only permit instructions to
9029 be used that are valid for the subtype of the file it is generating,
9030 so you cannot put 64-bit instructions in an @samp{ppc750} object file.
9031 The linker for shared libraries, @file{/usr/bin/libtool}, will fail
9032 and print an error if asked to create a shared library with a less
9033 restrictive subtype than its input files (for instance, trying to put
9034 a @samp{ppc970} object file in a @samp{ppc7400} library). The linker
9035 for executables, @file{ld}, will quietly give the executable the most
9036 restrictive subtype of any of its input files.
9041 Add the framework directory @var{dir} to the head of the list of
9042 directories to be searched for header files. These directories are
9043 interleaved with those specified by @option{-I} options and are
9044 scanned in a left-to-right order.
9046 A framework directory is a directory with frameworks in it. A
9047 framework is a directory with a @samp{"Headers"} and/or
9048 @samp{"PrivateHeaders"} directory contained directly in it that ends
9049 in @samp{".framework"}. The name of a framework is the name of this
9050 directory excluding the @samp{".framework"}. Headers associated with
9051 the framework are found in one of those two directories, with
9052 @samp{"Headers"} being searched first. A subframework is a framework
9053 directory that is in a framework's @samp{"Frameworks"} directory.
9054 Includes of subframework headers can only appear in a header of a
9055 framework that contains the subframework, or in a sibling subframework
9056 header. Two subframeworks are siblings if they occur in the same
9057 framework. A subframework should not have the same name as a
9058 framework, a warning will be issued if this is violated. Currently a
9059 subframework cannot have subframeworks, in the future, the mechanism
9060 may be extended to support this. The standard frameworks can be found
9061 in @samp{"/System/Library/Frameworks"} and
9062 @samp{"/Library/Frameworks"}. An example include looks like
9063 @code{#include <Framework/header.h>}, where @samp{Framework} denotes
9064 the name of the framework and header.h is found in the
9065 @samp{"PrivateHeaders"} or @samp{"Headers"} directory.
9067 @item -iframework@var{dir}
9069 Like @option{-F} except the directory is a treated as a system
9070 directory. The main difference between this @option{-iframework} and
9071 @option{-F} is that with @option{-iframework} the compiler does not
9072 warn about constructs contained within header files found via
9073 @var{dir}. This option is valid only for the C family of languages.
9077 Emit debugging information for symbols that are used. For STABS
9078 debugging format, this enables @option{-feliminate-unused-debug-symbols}.
9079 This is by default ON@.
9083 Emit debugging information for all symbols and types.
9085 @item -mmacosx-version-min=@var{version}
9086 The earliest version of MacOS X that this executable will run on
9087 is @var{version}. Typical values of @var{version} include @code{10.1},
9088 @code{10.2}, and @code{10.3.9}.
9090 If the compiler was built to use the system's headers by default,
9091 then the default for this option is the system version on which the
9092 compiler is running, otherwise the default is to make choices which
9093 are compatible with as many systems and code bases as possible.
9097 Enable kernel development mode. The @option{-mkernel} option sets
9098 @option{-static}, @option{-fno-common}, @option{-fno-cxa-atexit},
9099 @option{-fno-exceptions}, @option{-fno-non-call-exceptions},
9100 @option{-fapple-kext}, @option{-fno-weak} and @option{-fno-rtti} where
9101 applicable. This mode also sets @option{-mno-altivec},
9102 @option{-msoft-float}, @option{-fno-builtin} and
9103 @option{-mlong-branch} for PowerPC targets.
9105 @item -mone-byte-bool
9106 @opindex mone-byte-bool
9107 Override the defaults for @samp{bool} so that @samp{sizeof(bool)==1}.
9108 By default @samp{sizeof(bool)} is @samp{4} when compiling for
9109 Darwin/PowerPC and @samp{1} when compiling for Darwin/x86, so this
9110 option has no effect on x86.
9112 @strong{Warning:} The @option{-mone-byte-bool} switch causes GCC
9113 to generate code that is not binary compatible with code generated
9114 without that switch. Using this switch may require recompiling all
9115 other modules in a program, including system libraries. Use this
9116 switch to conform to a non-default data model.
9118 @item -mfix-and-continue
9119 @itemx -ffix-and-continue
9120 @itemx -findirect-data
9121 @opindex mfix-and-continue
9122 @opindex ffix-and-continue
9123 @opindex findirect-data
9124 Generate code suitable for fast turn around development. Needed to
9125 enable gdb to dynamically load @code{.o} files into already running
9126 programs. @option{-findirect-data} and @option{-ffix-and-continue}
9127 are provided for backwards compatibility.
9131 Loads all members of static archive libraries.
9132 See man ld(1) for more information.
9134 @item -arch_errors_fatal
9135 @opindex arch_errors_fatal
9136 Cause the errors having to do with files that have the wrong architecture
9140 @opindex bind_at_load
9141 Causes the output file to be marked such that the dynamic linker will
9142 bind all undefined references when the file is loaded or launched.
9146 Produce a Mach-o bundle format file.
9147 See man ld(1) for more information.
9149 @item -bundle_loader @var{executable}
9150 @opindex bundle_loader
9151 This option specifies the @var{executable} that will be loading the build
9152 output file being linked. See man ld(1) for more information.
9156 When passed this option, GCC will produce a dynamic library instead of
9157 an executable when linking, using the Darwin @file{libtool} command.
9159 @item -force_cpusubtype_ALL
9160 @opindex force_cpusubtype_ALL
9161 This causes GCC's output file to have the @var{ALL} subtype, instead of
9162 one controlled by the @option{-mcpu} or @option{-march} option.
9164 @item -allowable_client @var{client_name}
9166 @itemx -compatibility_version
9167 @itemx -current_version
9169 @itemx -dependency-file
9171 @itemx -dylinker_install_name
9173 @itemx -exported_symbols_list
9175 @itemx -flat_namespace
9176 @itemx -force_flat_namespace
9177 @itemx -headerpad_max_install_names
9180 @itemx -install_name
9181 @itemx -keep_private_externs
9182 @itemx -multi_module
9183 @itemx -multiply_defined
9184 @itemx -multiply_defined_unused
9186 @itemx -no_dead_strip_inits_and_terms
9187 @itemx -nofixprebinding
9190 @itemx -noseglinkedit
9191 @itemx -pagezero_size
9193 @itemx -prebind_all_twolevel_modules
9194 @itemx -private_bundle
9195 @itemx -read_only_relocs
9197 @itemx -sectobjectsymbols
9201 @itemx -sectobjectsymbols
9204 @itemx -segs_read_only_addr
9205 @itemx -segs_read_write_addr
9206 @itemx -seg_addr_table
9207 @itemx -seg_addr_table_filename
9210 @itemx -segs_read_only_addr
9211 @itemx -segs_read_write_addr
9212 @itemx -single_module
9215 @itemx -sub_umbrella
9216 @itemx -twolevel_namespace
9219 @itemx -unexported_symbols_list
9220 @itemx -weak_reference_mismatches
9223 @opindex allowable_client
9224 @opindex client_name
9225 @opindex compatibility_version
9226 @opindex current_version
9228 @opindex dependency-file
9230 @opindex dylinker_install_name
9232 @opindex exported_symbols_list
9234 @opindex flat_namespace
9235 @opindex force_flat_namespace
9236 @opindex headerpad_max_install_names
9239 @opindex install_name
9240 @opindex keep_private_externs
9241 @opindex multi_module
9242 @opindex multiply_defined
9243 @opindex multiply_defined_unused
9245 @opindex no_dead_strip_inits_and_terms
9246 @opindex nofixprebinding
9247 @opindex nomultidefs
9249 @opindex noseglinkedit
9250 @opindex pagezero_size
9252 @opindex prebind_all_twolevel_modules
9253 @opindex private_bundle
9254 @opindex read_only_relocs
9256 @opindex sectobjectsymbols
9260 @opindex sectobjectsymbols
9263 @opindex segs_read_only_addr
9264 @opindex segs_read_write_addr
9265 @opindex seg_addr_table
9266 @opindex seg_addr_table_filename
9267 @opindex seglinkedit
9269 @opindex segs_read_only_addr
9270 @opindex segs_read_write_addr
9271 @opindex single_module
9273 @opindex sub_library
9274 @opindex sub_umbrella
9275 @opindex twolevel_namespace
9278 @opindex unexported_symbols_list
9279 @opindex weak_reference_mismatches
9280 @opindex whatsloaded
9282 These options are passed to the Darwin linker. The Darwin linker man page
9283 describes them in detail.
9286 @node DEC Alpha Options
9287 @subsection DEC Alpha Options
9289 These @samp{-m} options are defined for the DEC Alpha implementations:
9292 @item -mno-soft-float
9294 @opindex mno-soft-float
9295 @opindex msoft-float
9296 Use (do not use) the hardware floating-point instructions for
9297 floating-point operations. When @option{-msoft-float} is specified,
9298 functions in @file{libgcc.a} will be used to perform floating-point
9299 operations. Unless they are replaced by routines that emulate the
9300 floating-point operations, or compiled in such a way as to call such
9301 emulations routines, these routines will issue floating-point
9302 operations. If you are compiling for an Alpha without floating-point
9303 operations, you must ensure that the library is built so as not to call
9306 Note that Alpha implementations without floating-point operations are
9307 required to have floating-point registers.
9312 @opindex mno-fp-regs
9313 Generate code that uses (does not use) the floating-point register set.
9314 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
9315 register set is not used, floating point operands are passed in integer
9316 registers as if they were integers and floating-point results are passed
9317 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
9318 so any function with a floating-point argument or return value called by code
9319 compiled with @option{-mno-fp-regs} must also be compiled with that
9322 A typical use of this option is building a kernel that does not use,
9323 and hence need not save and restore, any floating-point registers.
9327 The Alpha architecture implements floating-point hardware optimized for
9328 maximum performance. It is mostly compliant with the IEEE floating
9329 point standard. However, for full compliance, software assistance is
9330 required. This option generates code fully IEEE compliant code
9331 @emph{except} that the @var{inexact-flag} is not maintained (see below).
9332 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
9333 defined during compilation. The resulting code is less efficient but is
9334 able to correctly support denormalized numbers and exceptional IEEE
9335 values such as not-a-number and plus/minus infinity. Other Alpha
9336 compilers call this option @option{-ieee_with_no_inexact}.
9338 @item -mieee-with-inexact
9339 @opindex mieee-with-inexact
9340 This is like @option{-mieee} except the generated code also maintains
9341 the IEEE @var{inexact-flag}. Turning on this option causes the
9342 generated code to implement fully-compliant IEEE math. In addition to
9343 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
9344 macro. On some Alpha implementations the resulting code may execute
9345 significantly slower than the code generated by default. Since there is
9346 very little code that depends on the @var{inexact-flag}, you should
9347 normally not specify this option. Other Alpha compilers call this
9348 option @option{-ieee_with_inexact}.
9350 @item -mfp-trap-mode=@var{trap-mode}
9351 @opindex mfp-trap-mode
9352 This option controls what floating-point related traps are enabled.
9353 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
9354 The trap mode can be set to one of four values:
9358 This is the default (normal) setting. The only traps that are enabled
9359 are the ones that cannot be disabled in software (e.g., division by zero
9363 In addition to the traps enabled by @samp{n}, underflow traps are enabled
9367 Like @samp{u}, but the instructions are marked to be safe for software
9368 completion (see Alpha architecture manual for details).
9371 Like @samp{su}, but inexact traps are enabled as well.
9374 @item -mfp-rounding-mode=@var{rounding-mode}
9375 @opindex mfp-rounding-mode
9376 Selects the IEEE rounding mode. Other Alpha compilers call this option
9377 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
9382 Normal IEEE rounding mode. Floating point numbers are rounded towards
9383 the nearest machine number or towards the even machine number in case
9387 Round towards minus infinity.
9390 Chopped rounding mode. Floating point numbers are rounded towards zero.
9393 Dynamic rounding mode. A field in the floating point control register
9394 (@var{fpcr}, see Alpha architecture reference manual) controls the
9395 rounding mode in effect. The C library initializes this register for
9396 rounding towards plus infinity. Thus, unless your program modifies the
9397 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
9400 @item -mtrap-precision=@var{trap-precision}
9401 @opindex mtrap-precision
9402 In the Alpha architecture, floating point traps are imprecise. This
9403 means without software assistance it is impossible to recover from a
9404 floating trap and program execution normally needs to be terminated.
9405 GCC can generate code that can assist operating system trap handlers
9406 in determining the exact location that caused a floating point trap.
9407 Depending on the requirements of an application, different levels of
9408 precisions can be selected:
9412 Program precision. This option is the default and means a trap handler
9413 can only identify which program caused a floating point exception.
9416 Function precision. The trap handler can determine the function that
9417 caused a floating point exception.
9420 Instruction precision. The trap handler can determine the exact
9421 instruction that caused a floating point exception.
9424 Other Alpha compilers provide the equivalent options called
9425 @option{-scope_safe} and @option{-resumption_safe}.
9427 @item -mieee-conformant
9428 @opindex mieee-conformant
9429 This option marks the generated code as IEEE conformant. You must not
9430 use this option unless you also specify @option{-mtrap-precision=i} and either
9431 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
9432 is to emit the line @samp{.eflag 48} in the function prologue of the
9433 generated assembly file. Under DEC Unix, this has the effect that
9434 IEEE-conformant math library routines will be linked in.
9436 @item -mbuild-constants
9437 @opindex mbuild-constants
9438 Normally GCC examines a 32- or 64-bit integer constant to
9439 see if it can construct it from smaller constants in two or three
9440 instructions. If it cannot, it will output the constant as a literal and
9441 generate code to load it from the data segment at runtime.
9443 Use this option to require GCC to construct @emph{all} integer constants
9444 using code, even if it takes more instructions (the maximum is six).
9446 You would typically use this option to build a shared library dynamic
9447 loader. Itself a shared library, it must relocate itself in memory
9448 before it can find the variables and constants in its own data segment.
9454 Select whether to generate code to be assembled by the vendor-supplied
9455 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
9473 Indicate whether GCC should generate code to use the optional BWX,
9474 CIX, FIX and MAX instruction sets. The default is to use the instruction
9475 sets supported by the CPU type specified via @option{-mcpu=} option or that
9476 of the CPU on which GCC was built if none was specified.
9481 @opindex mfloat-ieee
9482 Generate code that uses (does not use) VAX F and G floating point
9483 arithmetic instead of IEEE single and double precision.
9485 @item -mexplicit-relocs
9486 @itemx -mno-explicit-relocs
9487 @opindex mexplicit-relocs
9488 @opindex mno-explicit-relocs
9489 Older Alpha assemblers provided no way to generate symbol relocations
9490 except via assembler macros. Use of these macros does not allow
9491 optimal instruction scheduling. GNU binutils as of version 2.12
9492 supports a new syntax that allows the compiler to explicitly mark
9493 which relocations should apply to which instructions. This option
9494 is mostly useful for debugging, as GCC detects the capabilities of
9495 the assembler when it is built and sets the default accordingly.
9499 @opindex msmall-data
9500 @opindex mlarge-data
9501 When @option{-mexplicit-relocs} is in effect, static data is
9502 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
9503 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
9504 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
9505 16-bit relocations off of the @code{$gp} register. This limits the
9506 size of the small data area to 64KB, but allows the variables to be
9507 directly accessed via a single instruction.
9509 The default is @option{-mlarge-data}. With this option the data area
9510 is limited to just below 2GB@. Programs that require more than 2GB of
9511 data must use @code{malloc} or @code{mmap} to allocate the data in the
9512 heap instead of in the program's data segment.
9514 When generating code for shared libraries, @option{-fpic} implies
9515 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
9519 @opindex msmall-text
9520 @opindex mlarge-text
9521 When @option{-msmall-text} is used, the compiler assumes that the
9522 code of the entire program (or shared library) fits in 4MB, and is
9523 thus reachable with a branch instruction. When @option{-msmall-data}
9524 is used, the compiler can assume that all local symbols share the
9525 same @code{$gp} value, and thus reduce the number of instructions
9526 required for a function call from 4 to 1.
9528 The default is @option{-mlarge-text}.
9530 @item -mcpu=@var{cpu_type}
9532 Set the instruction set and instruction scheduling parameters for
9533 machine type @var{cpu_type}. You can specify either the @samp{EV}
9534 style name or the corresponding chip number. GCC supports scheduling
9535 parameters for the EV4, EV5 and EV6 family of processors and will
9536 choose the default values for the instruction set from the processor
9537 you specify. If you do not specify a processor type, GCC will default
9538 to the processor on which the compiler was built.
9540 Supported values for @var{cpu_type} are
9546 Schedules as an EV4 and has no instruction set extensions.
9550 Schedules as an EV5 and has no instruction set extensions.
9554 Schedules as an EV5 and supports the BWX extension.
9559 Schedules as an EV5 and supports the BWX and MAX extensions.
9563 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
9567 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
9570 @item -mtune=@var{cpu_type}
9572 Set only the instruction scheduling parameters for machine type
9573 @var{cpu_type}. The instruction set is not changed.
9575 @item -mmemory-latency=@var{time}
9576 @opindex mmemory-latency
9577 Sets the latency the scheduler should assume for typical memory
9578 references as seen by the application. This number is highly
9579 dependent on the memory access patterns used by the application
9580 and the size of the external cache on the machine.
9582 Valid options for @var{time} are
9586 A decimal number representing clock cycles.
9592 The compiler contains estimates of the number of clock cycles for
9593 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
9594 (also called Dcache, Scache, and Bcache), as well as to main memory.
9595 Note that L3 is only valid for EV5.
9600 @node DEC Alpha/VMS Options
9601 @subsection DEC Alpha/VMS Options
9603 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
9606 @item -mvms-return-codes
9607 @opindex mvms-return-codes
9608 Return VMS condition codes from main. The default is to return POSIX
9609 style condition (e.g.@: error) codes.
9613 @subsection FRV Options
9620 Only use the first 32 general purpose registers.
9625 Use all 64 general purpose registers.
9630 Use only the first 32 floating point registers.
9635 Use all 64 floating point registers
9638 @opindex mhard-float
9640 Use hardware instructions for floating point operations.
9643 @opindex msoft-float
9645 Use library routines for floating point operations.
9650 Dynamically allocate condition code registers.
9655 Do not try to dynamically allocate condition code registers, only
9656 use @code{icc0} and @code{fcc0}.
9661 Change ABI to use double word insns.
9666 Do not use double word instructions.
9671 Use floating point double instructions.
9676 Do not use floating point double instructions.
9681 Use media instructions.
9686 Do not use media instructions.
9691 Use multiply and add/subtract instructions.
9696 Do not use multiply and add/subtract instructions.
9701 Select the FDPIC ABI, that uses function descriptors to represent
9702 pointers to functions. Without any PIC/PIE-related options, it
9703 implies @option{-fPIE}. With @option{-fpic} or @option{-fpie}, it
9704 assumes GOT entries and small data are within a 12-bit range from the
9705 GOT base address; with @option{-fPIC} or @option{-fPIE}, GOT offsets
9706 are computed with 32 bits.
9707 With a @samp{bfin-elf} target, this option implies @option{-msim}.
9710 @opindex minline-plt
9712 Enable inlining of PLT entries in function calls to functions that are
9713 not known to bind locally. It has no effect without @option{-mfdpic}.
9714 It's enabled by default if optimizing for speed and compiling for
9715 shared libraries (i.e., @option{-fPIC} or @option{-fpic}), or when an
9716 optimization option such as @option{-O3} or above is present in the
9722 Assume a large TLS segment when generating thread-local code.
9727 Do not assume a large TLS segment when generating thread-local code.
9732 Enable the use of @code{GPREL} relocations in the FDPIC ABI for data
9733 that is known to be in read-only sections. It's enabled by default,
9734 except for @option{-fpic} or @option{-fpie}: even though it may help
9735 make the global offset table smaller, it trades 1 instruction for 4.
9736 With @option{-fPIC} or @option{-fPIE}, it trades 3 instructions for 4,
9737 one of which may be shared by multiple symbols, and it avoids the need
9738 for a GOT entry for the referenced symbol, so it's more likely to be a
9739 win. If it is not, @option{-mno-gprel-ro} can be used to disable it.
9741 @item -multilib-library-pic
9742 @opindex multilib-library-pic
9744 Link with the (library, not FD) pic libraries. It's implied by
9745 @option{-mlibrary-pic}, as well as by @option{-fPIC} and
9746 @option{-fpic} without @option{-mfdpic}. You should never have to use
9752 Follow the EABI requirement of always creating a frame pointer whenever
9753 a stack frame is allocated. This option is enabled by default and can
9754 be disabled with @option{-mno-linked-fp}.
9757 @opindex mlong-calls
9759 Use indirect addressing to call functions outside the current
9760 compilation unit. This allows the functions to be placed anywhere
9761 within the 32-bit address space.
9763 @item -malign-labels
9764 @opindex malign-labels
9766 Try to align labels to an 8-byte boundary by inserting nops into the
9767 previous packet. This option only has an effect when VLIW packing
9768 is enabled. It doesn't create new packets; it merely adds nops to
9772 @opindex mlibrary-pic
9774 Generate position-independent EABI code.
9779 Use only the first four media accumulator registers.
9784 Use all eight media accumulator registers.
9789 Pack VLIW instructions.
9794 Do not pack VLIW instructions.
9799 Do not mark ABI switches in e_flags.
9804 Enable the use of conditional-move instructions (default).
9806 This switch is mainly for debugging the compiler and will likely be removed
9807 in a future version.
9809 @item -mno-cond-move
9810 @opindex mno-cond-move
9812 Disable the use of conditional-move instructions.
9814 This switch is mainly for debugging the compiler and will likely be removed
9815 in a future version.
9820 Enable the use of conditional set instructions (default).
9822 This switch is mainly for debugging the compiler and will likely be removed
9823 in a future version.
9828 Disable the use of conditional set instructions.
9830 This switch is mainly for debugging the compiler and will likely be removed
9831 in a future version.
9836 Enable the use of conditional execution (default).
9838 This switch is mainly for debugging the compiler and will likely be removed
9839 in a future version.
9841 @item -mno-cond-exec
9842 @opindex mno-cond-exec
9844 Disable the use of conditional execution.
9846 This switch is mainly for debugging the compiler and will likely be removed
9847 in a future version.
9850 @opindex mvliw-branch
9852 Run a pass to pack branches into VLIW instructions (default).
9854 This switch is mainly for debugging the compiler and will likely be removed
9855 in a future version.
9857 @item -mno-vliw-branch
9858 @opindex mno-vliw-branch
9860 Do not run a pass to pack branches into VLIW instructions.
9862 This switch is mainly for debugging the compiler and will likely be removed
9863 in a future version.
9865 @item -mmulti-cond-exec
9866 @opindex mmulti-cond-exec
9868 Enable optimization of @code{&&} and @code{||} in conditional execution
9871 This switch is mainly for debugging the compiler and will likely be removed
9872 in a future version.
9874 @item -mno-multi-cond-exec
9875 @opindex mno-multi-cond-exec
9877 Disable optimization of @code{&&} and @code{||} in conditional execution.
9879 This switch is mainly for debugging the compiler and will likely be removed
9880 in a future version.
9882 @item -mnested-cond-exec
9883 @opindex mnested-cond-exec
9885 Enable nested conditional execution optimizations (default).
9887 This switch is mainly for debugging the compiler and will likely be removed
9888 in a future version.
9890 @item -mno-nested-cond-exec
9891 @opindex mno-nested-cond-exec
9893 Disable nested conditional execution optimizations.
9895 This switch is mainly for debugging the compiler and will likely be removed
9896 in a future version.
9898 @item -moptimize-membar
9899 @opindex moptimize-membar
9901 This switch removes redundant @code{membar} instructions from the
9902 compiler generated code. It is enabled by default.
9904 @item -mno-optimize-membar
9905 @opindex mno-optimize-membar
9907 This switch disables the automatic removal of redundant @code{membar}
9908 instructions from the generated code.
9910 @item -mtomcat-stats
9911 @opindex mtomcat-stats
9913 Cause gas to print out tomcat statistics.
9915 @item -mcpu=@var{cpu}
9918 Select the processor type for which to generate code. Possible values are
9919 @samp{frv}, @samp{fr550}, @samp{tomcat}, @samp{fr500}, @samp{fr450},
9920 @samp{fr405}, @samp{fr400}, @samp{fr300} and @samp{simple}.
9924 @node GNU/Linux Options
9925 @subsection GNU/Linux Options
9927 These @samp{-m} options are defined for GNU/Linux targets:
9932 Use the GNU C library instead of uClibc. This is the default except
9933 on @samp{*-*-linux-*uclibc*} targets.
9937 Use uClibc instead of the GNU C library. This is the default on
9938 @samp{*-*-linux-*uclibc*} targets.
9941 @node H8/300 Options
9942 @subsection H8/300 Options
9944 These @samp{-m} options are defined for the H8/300 implementations:
9949 Shorten some address references at link time, when possible; uses the
9950 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
9951 ld, Using ld}, for a fuller description.
9955 Generate code for the H8/300H@.
9959 Generate code for the H8S@.
9963 Generate code for the H8S and H8/300H in the normal mode. This switch
9964 must be used either with @option{-mh} or @option{-ms}.
9968 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
9972 Make @code{int} data 32 bits by default.
9976 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
9977 The default for the H8/300H and H8S is to align longs and floats on 4
9979 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
9980 This option has no effect on the H8/300.
9984 @subsection HPPA Options
9985 @cindex HPPA Options
9987 These @samp{-m} options are defined for the HPPA family of computers:
9990 @item -march=@var{architecture-type}
9992 Generate code for the specified architecture. The choices for
9993 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
9994 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
9995 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
9996 architecture option for your machine. Code compiled for lower numbered
9997 architectures will run on higher numbered architectures, but not the
10000 @item -mpa-risc-1-0
10001 @itemx -mpa-risc-1-1
10002 @itemx -mpa-risc-2-0
10003 @opindex mpa-risc-1-0
10004 @opindex mpa-risc-1-1
10005 @opindex mpa-risc-2-0
10006 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
10009 @opindex mbig-switch
10010 Generate code suitable for big switch tables. Use this option only if
10011 the assembler/linker complain about out of range branches within a switch
10014 @item -mjump-in-delay
10015 @opindex mjump-in-delay
10016 Fill delay slots of function calls with unconditional jump instructions
10017 by modifying the return pointer for the function call to be the target
10018 of the conditional jump.
10020 @item -mdisable-fpregs
10021 @opindex mdisable-fpregs
10022 Prevent floating point registers from being used in any manner. This is
10023 necessary for compiling kernels which perform lazy context switching of
10024 floating point registers. If you use this option and attempt to perform
10025 floating point operations, the compiler will abort.
10027 @item -mdisable-indexing
10028 @opindex mdisable-indexing
10029 Prevent the compiler from using indexing address modes. This avoids some
10030 rather obscure problems when compiling MIG generated code under MACH@.
10032 @item -mno-space-regs
10033 @opindex mno-space-regs
10034 Generate code that assumes the target has no space registers. This allows
10035 GCC to generate faster indirect calls and use unscaled index address modes.
10037 Such code is suitable for level 0 PA systems and kernels.
10039 @item -mfast-indirect-calls
10040 @opindex mfast-indirect-calls
10041 Generate code that assumes calls never cross space boundaries. This
10042 allows GCC to emit code which performs faster indirect calls.
10044 This option will not work in the presence of shared libraries or nested
10047 @item -mfixed-range=@var{register-range}
10048 @opindex mfixed-range
10049 Generate code treating the given register range as fixed registers.
10050 A fixed register is one that the register allocator can not use. This is
10051 useful when compiling kernel code. A register range is specified as
10052 two registers separated by a dash. Multiple register ranges can be
10053 specified separated by a comma.
10055 @item -mlong-load-store
10056 @opindex mlong-load-store
10057 Generate 3-instruction load and store sequences as sometimes required by
10058 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
10061 @item -mportable-runtime
10062 @opindex mportable-runtime
10063 Use the portable calling conventions proposed by HP for ELF systems.
10067 Enable the use of assembler directives only GAS understands.
10069 @item -mschedule=@var{cpu-type}
10071 Schedule code according to the constraints for the machine type
10072 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
10073 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
10074 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
10075 proper scheduling option for your machine. The default scheduling is
10079 @opindex mlinker-opt
10080 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
10081 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
10082 linkers in which they give bogus error messages when linking some programs.
10085 @opindex msoft-float
10086 Generate output containing library calls for floating point.
10087 @strong{Warning:} the requisite libraries are not available for all HPPA
10088 targets. Normally the facilities of the machine's usual C compiler are
10089 used, but this cannot be done directly in cross-compilation. You must make
10090 your own arrangements to provide suitable library functions for
10091 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
10092 does provide software floating point support.
10094 @option{-msoft-float} changes the calling convention in the output file;
10095 therefore, it is only useful if you compile @emph{all} of a program with
10096 this option. In particular, you need to compile @file{libgcc.a}, the
10097 library that comes with GCC, with @option{-msoft-float} in order for
10102 Generate the predefine, @code{_SIO}, for server IO@. The default is
10103 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
10104 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO@. These
10105 options are available under HP-UX and HI-UX@.
10109 Use GNU ld specific options. This passes @option{-shared} to ld when
10110 building a shared library. It is the default when GCC is configured,
10111 explicitly or implicitly, with the GNU linker. This option does not
10112 have any affect on which ld is called, it only changes what parameters
10113 are passed to that ld. The ld that is called is determined by the
10114 @option{--with-ld} configure option, GCC's program search path, and
10115 finally by the user's @env{PATH}. The linker used by GCC can be printed
10116 using @samp{which `gcc -print-prog-name=ld`}. This option is only available
10117 on the 64 bit HP-UX GCC, i.e. configured with @samp{hppa*64*-*-hpux*}.
10121 Use HP ld specific options. This passes @option{-b} to ld when building
10122 a shared library and passes @option{+Accept TypeMismatch} to ld on all
10123 links. It is the default when GCC is configured, explicitly or
10124 implicitly, with the HP linker. This option does not have any affect on
10125 which ld is called, it only changes what parameters are passed to that
10126 ld. The ld that is called is determined by the @option{--with-ld}
10127 configure option, GCC's program search path, and finally by the user's
10128 @env{PATH}. The linker used by GCC can be printed using @samp{which
10129 `gcc -print-prog-name=ld`}. This option is only available on the 64 bit
10130 HP-UX GCC, i.e. configured with @samp{hppa*64*-*-hpux*}.
10133 @opindex mno-long-calls
10134 Generate code that uses long call sequences. This ensures that a call
10135 is always able to reach linker generated stubs. The default is to generate
10136 long calls only when the distance from the call site to the beginning
10137 of the function or translation unit, as the case may be, exceeds a
10138 predefined limit set by the branch type being used. The limits for
10139 normal calls are 7,600,000 and 240,000 bytes, respectively for the
10140 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
10143 Distances are measured from the beginning of functions when using the
10144 @option{-ffunction-sections} option, or when using the @option{-mgas}
10145 and @option{-mno-portable-runtime} options together under HP-UX with
10148 It is normally not desirable to use this option as it will degrade
10149 performance. However, it may be useful in large applications,
10150 particularly when partial linking is used to build the application.
10152 The types of long calls used depends on the capabilities of the
10153 assembler and linker, and the type of code being generated. The
10154 impact on systems that support long absolute calls, and long pic
10155 symbol-difference or pc-relative calls should be relatively small.
10156 However, an indirect call is used on 32-bit ELF systems in pic code
10157 and it is quite long.
10159 @item -munix=@var{unix-std}
10161 Generate compiler predefines and select a startfile for the specified
10162 UNIX standard. The choices for @var{unix-std} are @samp{93}, @samp{95}
10163 and @samp{98}. @samp{93} is supported on all HP-UX versions. @samp{95}
10164 is available on HP-UX 10.10 and later. @samp{98} is available on HP-UX
10165 11.11 and later. The default values are @samp{93} for HP-UX 10.00,
10166 @samp{95} for HP-UX 10.10 though to 11.00, and @samp{98} for HP-UX 11.11
10169 @option{-munix=93} provides the same predefines as GCC 3.3 and 3.4.
10170 @option{-munix=95} provides additional predefines for @code{XOPEN_UNIX}
10171 and @code{_XOPEN_SOURCE_EXTENDED}, and the startfile @file{unix95.o}.
10172 @option{-munix=98} provides additional predefines for @code{_XOPEN_UNIX},
10173 @code{_XOPEN_SOURCE_EXTENDED}, @code{_INCLUDE__STDC_A1_SOURCE} and
10174 @code{_INCLUDE_XOPEN_SOURCE_500}, and the startfile @file{unix98.o}.
10176 It is @emph{important} to note that this option changes the interfaces
10177 for various library routines. It also affects the operational behavior
10178 of the C library. Thus, @emph{extreme} care is needed in using this
10181 Library code that is intended to operate with more than one UNIX
10182 standard must test, set and restore the variable @var{__xpg4_extended_mask}
10183 as appropriate. Most GNU software doesn't provide this capability.
10187 Suppress the generation of link options to search libdld.sl when the
10188 @option{-static} option is specified on HP-UX 10 and later.
10192 The HP-UX implementation of setlocale in libc has a dependency on
10193 libdld.sl. There isn't an archive version of libdld.sl. Thus,
10194 when the @option{-static} option is specified, special link options
10195 are needed to resolve this dependency.
10197 On HP-UX 10 and later, the GCC driver adds the necessary options to
10198 link with libdld.sl when the @option{-static} option is specified.
10199 This causes the resulting binary to be dynamic. On the 64-bit port,
10200 the linkers generate dynamic binaries by default in any case. The
10201 @option{-nolibdld} option can be used to prevent the GCC driver from
10202 adding these link options.
10206 Add support for multithreading with the @dfn{dce thread} library
10207 under HP-UX@. This option sets flags for both the preprocessor and
10211 @node i386 and x86-64 Options
10212 @subsection Intel 386 and AMD x86-64 Options
10213 @cindex i386 Options
10214 @cindex x86-64 Options
10215 @cindex Intel 386 Options
10216 @cindex AMD x86-64 Options
10218 These @samp{-m} options are defined for the i386 and x86-64 family of
10222 @item -mtune=@var{cpu-type}
10224 Tune to @var{cpu-type} everything applicable about the generated code, except
10225 for the ABI and the set of available instructions. The choices for
10226 @var{cpu-type} are:
10229 Produce code optimized for the most common IA32/AMD64/EM64T processors.
10230 If you know the CPU on which your code will run, then you should use
10231 the corresponding @option{-mtune} option instead of
10232 @option{-mtune=generic}. But, if you do not know exactly what CPU users
10233 of your application will have, then you should use this option.
10235 As new processors are deployed in the marketplace, the behavior of this
10236 option will change. Therefore, if you upgrade to a newer version of
10237 GCC, the code generated option will change to reflect the processors
10238 that were most common when that version of GCC was released.
10240 There is no @option{-march=generic} option because @option{-march}
10241 indicates the instruction set the compiler can use, and there is no
10242 generic instruction set applicable to all processors. In contrast,
10243 @option{-mtune} indicates the processor (or, in this case, collection of
10244 processors) for which the code is optimized.
10246 This selects the CPU to tune for at compilation time by determining
10247 the processor type of the compiling machine. Using @option{-mtune=native}
10248 will produce code optimized for the local machine under the constraints
10249 of the selected instruction set. Using @option{-march=native} will
10250 enable all instruction subsets supported by the local machine (hence
10251 the result might not run on different machines).
10253 Original Intel's i386 CPU@.
10255 Intel's i486 CPU@. (No scheduling is implemented for this chip.)
10256 @item i586, pentium
10257 Intel Pentium CPU with no MMX support.
10259 Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
10261 Intel PentiumPro CPU@.
10263 Same as @code{generic}, but when used as @code{march} option, PentiumPro
10264 instruction set will be used, so the code will run on all i686 family chips.
10266 Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
10267 @item pentium3, pentium3m
10268 Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set
10271 Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set
10272 support. Used by Centrino notebooks.
10273 @item pentium4, pentium4m
10274 Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
10276 Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction
10279 Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
10280 SSE2 and SSE3 instruction set support.
10282 Intel Core2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3
10283 instruction set support.
10285 AMD K6 CPU with MMX instruction set support.
10287 Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
10288 @item athlon, athlon-tbird
10289 AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions
10291 @item athlon-4, athlon-xp, athlon-mp
10292 Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE
10293 instruction set support.
10294 @item k8, opteron, athlon64, athlon-fx
10295 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets
10296 MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
10297 @item k8-sse3, opteron-sse3, athlon64-sse3
10298 Improved versions of k8, opteron and athlon64 with SSE3 instruction set support.
10299 @item amdfam10, barcelona
10300 AMD Family 10h core based CPUs with x86-64 instruction set support. (This
10301 supersets MMX, SSE, SSE2, SSE3, SSE4A, 3dNOW!, enhanced 3dNOW!, ABM and 64-bit
10302 instruction set extensions.)
10304 IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction
10307 IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW!
10308 instruction set support.
10310 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is
10311 implemented for this chip.)
10313 Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is
10314 implemented for this chip.)
10316 Embedded AMD CPU with MMX and 3dNOW! instruction set support.
10319 While picking a specific @var{cpu-type} will schedule things appropriately
10320 for that particular chip, the compiler will not generate any code that
10321 does not run on the i386 without the @option{-march=@var{cpu-type}} option
10324 @item -march=@var{cpu-type}
10326 Generate instructions for the machine type @var{cpu-type}. The choices
10327 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
10328 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
10330 @item -mcpu=@var{cpu-type}
10332 A deprecated synonym for @option{-mtune}.
10334 @item -mfpmath=@var{unit}
10336 Generate floating point arithmetics for selected unit @var{unit}. The choices
10337 for @var{unit} are:
10341 Use the standard 387 floating point coprocessor present majority of chips and
10342 emulated otherwise. Code compiled with this option will run almost everywhere.
10343 The temporary results are computed in 80bit precision instead of precision
10344 specified by the type resulting in slightly different results compared to most
10345 of other chips. See @option{-ffloat-store} for more detailed description.
10347 This is the default choice for i386 compiler.
10350 Use scalar floating point instructions present in the SSE instruction set.
10351 This instruction set is supported by Pentium3 and newer chips, in the AMD line
10352 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
10353 instruction set supports only single precision arithmetics, thus the double and
10354 extended precision arithmetics is still done using 387. Later version, present
10355 only in Pentium4 and the future AMD x86-64 chips supports double precision
10358 For the i386 compiler, you need to use @option{-march=@var{cpu-type}}, @option{-msse}
10359 or @option{-msse2} switches to enable SSE extensions and make this option
10360 effective. For the x86-64 compiler, these extensions are enabled by default.
10362 The resulting code should be considerably faster in the majority of cases and avoid
10363 the numerical instability problems of 387 code, but may break some existing
10364 code that expects temporaries to be 80bit.
10366 This is the default choice for the x86-64 compiler.
10369 Attempt to utilize both instruction sets at once. This effectively double the
10370 amount of available registers and on chips with separate execution units for
10371 387 and SSE the execution resources too. Use this option with care, as it is
10372 still experimental, because the GCC register allocator does not model separate
10373 functional units well resulting in instable performance.
10376 @item -masm=@var{dialect}
10377 @opindex masm=@var{dialect}
10378 Output asm instructions using selected @var{dialect}. Supported
10379 choices are @samp{intel} or @samp{att} (the default one). Darwin does
10380 not support @samp{intel}.
10383 @itemx -mno-ieee-fp
10385 @opindex mno-ieee-fp
10386 Control whether or not the compiler uses IEEE floating point
10387 comparisons. These handle correctly the case where the result of a
10388 comparison is unordered.
10391 @opindex msoft-float
10392 Generate output containing library calls for floating point.
10393 @strong{Warning:} the requisite libraries are not part of GCC@.
10394 Normally the facilities of the machine's usual C compiler are used, but
10395 this can't be done directly in cross-compilation. You must make your
10396 own arrangements to provide suitable library functions for
10399 On machines where a function returns floating point results in the 80387
10400 register stack, some floating point opcodes may be emitted even if
10401 @option{-msoft-float} is used.
10403 @item -mno-fp-ret-in-387
10404 @opindex mno-fp-ret-in-387
10405 Do not use the FPU registers for return values of functions.
10407 The usual calling convention has functions return values of types
10408 @code{float} and @code{double} in an FPU register, even if there
10409 is no FPU@. The idea is that the operating system should emulate
10412 The option @option{-mno-fp-ret-in-387} causes such values to be returned
10413 in ordinary CPU registers instead.
10415 @item -mno-fancy-math-387
10416 @opindex mno-fancy-math-387
10417 Some 387 emulators do not support the @code{sin}, @code{cos} and
10418 @code{sqrt} instructions for the 387. Specify this option to avoid
10419 generating those instructions. This option is the default on FreeBSD,
10420 OpenBSD and NetBSD@. This option is overridden when @option{-march}
10421 indicates that the target cpu will always have an FPU and so the
10422 instruction will not need emulation. As of revision 2.6.1, these
10423 instructions are not generated unless you also use the
10424 @option{-funsafe-math-optimizations} switch.
10426 @item -malign-double
10427 @itemx -mno-align-double
10428 @opindex malign-double
10429 @opindex mno-align-double
10430 Control whether GCC aligns @code{double}, @code{long double}, and
10431 @code{long long} variables on a two word boundary or a one word
10432 boundary. Aligning @code{double} variables on a two word boundary will
10433 produce code that runs somewhat faster on a @samp{Pentium} at the
10434 expense of more memory.
10436 On x86-64, @option{-malign-double} is enabled by default.
10438 @strong{Warning:} if you use the @option{-malign-double} switch,
10439 structures containing the above types will be aligned differently than
10440 the published application binary interface specifications for the 386
10441 and will not be binary compatible with structures in code compiled
10442 without that switch.
10444 @item -m96bit-long-double
10445 @itemx -m128bit-long-double
10446 @opindex m96bit-long-double
10447 @opindex m128bit-long-double
10448 These switches control the size of @code{long double} type. The i386
10449 application binary interface specifies the size to be 96 bits,
10450 so @option{-m96bit-long-double} is the default in 32 bit mode.
10452 Modern architectures (Pentium and newer) would prefer @code{long double}
10453 to be aligned to an 8 or 16 byte boundary. In arrays or structures
10454 conforming to the ABI, this would not be possible. So specifying a
10455 @option{-m128bit-long-double} will align @code{long double}
10456 to a 16 byte boundary by padding the @code{long double} with an additional
10459 In the x86-64 compiler, @option{-m128bit-long-double} is the default choice as
10460 its ABI specifies that @code{long double} is to be aligned on 16 byte boundary.
10462 Notice that neither of these options enable any extra precision over the x87
10463 standard of 80 bits for a @code{long double}.
10465 @strong{Warning:} if you override the default value for your target ABI, the
10466 structures and arrays containing @code{long double} variables will change
10467 their size as well as function calling convention for function taking
10468 @code{long double} will be modified. Hence they will not be binary
10469 compatible with arrays or structures in code compiled without that switch.
10471 @item -mmlarge-data-threshold=@var{number}
10472 @opindex mlarge-data-threshold=@var{number}
10473 When @option{-mcmodel=medium} is specified, the data greater than
10474 @var{threshold} are placed in large data section. This value must be the
10475 same across all object linked into the binary and defaults to 65535.
10479 Use a different function-calling convention, in which functions that
10480 take a fixed number of arguments return with the @code{ret} @var{num}
10481 instruction, which pops their arguments while returning. This saves one
10482 instruction in the caller since there is no need to pop the arguments
10485 You can specify that an individual function is called with this calling
10486 sequence with the function attribute @samp{stdcall}. You can also
10487 override the @option{-mrtd} option by using the function attribute
10488 @samp{cdecl}. @xref{Function Attributes}.
10490 @strong{Warning:} this calling convention is incompatible with the one
10491 normally used on Unix, so you cannot use it if you need to call
10492 libraries compiled with the Unix compiler.
10494 Also, you must provide function prototypes for all functions that
10495 take variable numbers of arguments (including @code{printf});
10496 otherwise incorrect code will be generated for calls to those
10499 In addition, seriously incorrect code will result if you call a
10500 function with too many arguments. (Normally, extra arguments are
10501 harmlessly ignored.)
10503 @item -mregparm=@var{num}
10505 Control how many registers are used to pass integer arguments. By
10506 default, no registers are used to pass arguments, and at most 3
10507 registers can be used. You can control this behavior for a specific
10508 function by using the function attribute @samp{regparm}.
10509 @xref{Function Attributes}.
10511 @strong{Warning:} if you use this switch, and
10512 @var{num} is nonzero, then you must build all modules with the same
10513 value, including any libraries. This includes the system libraries and
10517 @opindex msseregparm
10518 Use SSE register passing conventions for float and double arguments
10519 and return values. You can control this behavior for a specific
10520 function by using the function attribute @samp{sseregparm}.
10521 @xref{Function Attributes}.
10523 @strong{Warning:} if you use this switch then you must build all
10524 modules with the same value, including any libraries. This includes
10525 the system libraries and startup modules.
10534 Set 80387 floating-point precision to 32, 64 or 80 bits. When @option{-mpc32}
10535 is specified, the significands of results of floating-point operations are
10536 rounded to 24 bits (single precision); @option{-mpc64} rounds the the
10537 significands of results of floating-point operations to 53 bits (double
10538 precision) and @option{-mpc80} rounds the significands of results of
10539 floating-point operations to 64 bits (extended double precision), which is
10540 the default. When this option is used, floating-point operations in higher
10541 precisions are not available to the programmer without setting the FPU
10542 control word explicitly.
10544 Setting the rounding of floating-point operations to less than the default
10545 80 bits can speed some programs by 2% or more. Note that some mathematical
10546 libraries assume that extended precision (80 bit) floating-point operations
10547 are enabled by default; routines in such libraries could suffer significant
10548 loss of accuracy, typically through so-called "catastrophic cancellation",
10549 when this option is used to set the precision to less than extended precision.
10551 @item -mstackrealign
10552 @opindex mstackrealign
10553 Realign the stack at entry. On the Intel x86, the
10554 @option{-mstackrealign} option will generate an alternate prologue and
10555 epilogue that realigns the runtime stack. This supports mixing legacy
10556 codes that keep a 4-byte aligned stack with modern codes that keep a
10557 16-byte stack for SSE compatibility. The alternate prologue and
10558 epilogue are slower and bigger than the regular ones, and the
10559 alternate prologue requires an extra scratch register; this lowers the
10560 number of registers available if used in conjunction with the
10561 @code{regparm} attribute. The @option{-mstackrealign} option is
10562 incompatible with the nested function prologue; this is considered a
10563 hard error. See also the attribute @code{force_align_arg_pointer},
10564 applicable to individual functions.
10566 @item -mpreferred-stack-boundary=@var{num}
10567 @opindex mpreferred-stack-boundary
10568 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
10569 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
10570 the default is 4 (16 bytes or 128 bits).
10572 On Pentium and PentiumPro, @code{double} and @code{long double} values
10573 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
10574 suffer significant run time performance penalties. On Pentium III, the
10575 Streaming SIMD Extension (SSE) data type @code{__m128} may not work
10576 properly if it is not 16 byte aligned.
10578 To ensure proper alignment of this values on the stack, the stack boundary
10579 must be as aligned as that required by any value stored on the stack.
10580 Further, every function must be generated such that it keeps the stack
10581 aligned. Thus calling a function compiled with a higher preferred
10582 stack boundary from a function compiled with a lower preferred stack
10583 boundary will most likely misalign the stack. It is recommended that
10584 libraries that use callbacks always use the default setting.
10586 This extra alignment does consume extra stack space, and generally
10587 increases code size. Code that is sensitive to stack space usage, such
10588 as embedded systems and operating system kernels, may want to reduce the
10589 preferred alignment to @option{-mpreferred-stack-boundary=2}.
10623 These switches enable or disable the use of instructions in the MMX,
10624 SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4A, SSE5, ABM or 3DNow! extended
10626 These extensions are also available as built-in functions: see
10627 @ref{X86 Built-in Functions}, for details of the functions enabled and
10628 disabled by these switches.
10630 To have SSE/SSE2 instructions generated automatically from floating-point
10631 code (as opposed to 387 instructions), see @option{-mfpmath=sse}.
10633 These options will enable GCC to use these extended instructions in
10634 generated code, even without @option{-mfpmath=sse}. Applications which
10635 perform runtime CPU detection must compile separate files for each
10636 supported architecture, using the appropriate flags. In particular,
10637 the file containing the CPU detection code should be compiled without
10642 This option will enable GCC to use CMPXCHG16B instruction in generated code.
10643 CMPXCHG16B allows for atomic operations on 128-bit double quadword (or oword)
10644 data types. This is useful for high resolution counters that could be updated
10645 by multiple processors (or cores). This instruction is generated as part of
10646 atomic built-in functions: see @ref{Atomic Builtins} for details.
10650 This option will enable GCC to use SAHF instruction in generated 64-bit code.
10651 Early Intel CPUs with Intel 64 lacked LAHF and SAHF instructions supported
10652 by AMD64 until introduction of Pentium 4 G1 step in December 2005. LAHF and
10653 SAHF are load and store instructions, respectively, for certain status flags.
10654 In 64-bit mode, SAHF instruction is used to optimize @code{fmod}, @code{drem}
10655 or @code{remainder} built-in functions: see @ref{Other Builtins} for details.
10659 This option will enable GCC to use RCPSS and RSQRTSS instructions (and their
10660 vectorized variants RCPPS and RSQRTPS) with an additional Newton-Rhapson step
10661 to increase precision instead of DIVSS and SQRTSS (and their vectorized
10662 variants) for single precision floating point arguments. These instructions
10663 are generated only when @option{-funsafe-math-optimizations} is enabled
10664 together with @option{-finite-math-only} and @option{-fno-trapping-math}.
10665 Note that while the throughput of the sequence is higher than the throughput
10666 of the non-reciprocal instruction, the precision of the sequence can be
10667 decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
10669 @item -mveclibabi=@var{type}
10670 @opindex mveclibabi
10671 Specifies the ABI type to use for vectorizing intrinsics using an
10672 external library. Supported types are @code{acml} for the AMD
10673 math core library style of interfacing. GCC will currently emit
10674 calls to @code{__vrd2_sin}, @code{__vrd2_cos}, @code{__vrd2_exp},
10675 @code{__vrd2_log}, @code{__vrd2_log2}, @code{__vrd2_log10},
10676 @code{__vrs4_sinf}, @code{__vrs4_cosf}, @code{__vrs4_expf},
10677 @code{__vrs4_logf}, @code{__vrs4_log2f}, @code{__vrs4_log10f}
10678 and @code{__vrs4_powf} when using this type and @option{-ftree-vectorize}
10679 is enabled. A ACML ABI compatible library will have to be specified
10683 @itemx -mno-push-args
10684 @opindex mpush-args
10685 @opindex mno-push-args
10686 Use PUSH operations to store outgoing parameters. This method is shorter
10687 and usually equally fast as method using SUB/MOV operations and is enabled
10688 by default. In some cases disabling it may improve performance because of
10689 improved scheduling and reduced dependencies.
10691 @item -maccumulate-outgoing-args
10692 @opindex maccumulate-outgoing-args
10693 If enabled, the maximum amount of space required for outgoing arguments will be
10694 computed in the function prologue. This is faster on most modern CPUs
10695 because of reduced dependencies, improved scheduling and reduced stack usage
10696 when preferred stack boundary is not equal to 2. The drawback is a notable
10697 increase in code size. This switch implies @option{-mno-push-args}.
10701 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
10702 on thread-safe exception handling must compile and link all code with the
10703 @option{-mthreads} option. When compiling, @option{-mthreads} defines
10704 @option{-D_MT}; when linking, it links in a special thread helper library
10705 @option{-lmingwthrd} which cleans up per thread exception handling data.
10707 @item -mno-align-stringops
10708 @opindex mno-align-stringops
10709 Do not align destination of inlined string operations. This switch reduces
10710 code size and improves performance in case the destination is already aligned,
10711 but GCC doesn't know about it.
10713 @item -minline-all-stringops
10714 @opindex minline-all-stringops
10715 By default GCC inlines string operations only when destination is known to be
10716 aligned at least to 4 byte boundary. This enables more inlining, increase code
10717 size, but may improve performance of code that depends on fast memcpy, strlen
10718 and memset for short lengths.
10720 @item -minline-stringops-dynamically
10721 @opindex minline-stringops-dynamically
10722 For string operation of unknown size, inline runtime checks so for small
10723 blocks inline code is used, while for large blocks library call is used.
10725 @item -mstringop-strategy=@var{alg}
10726 @opindex mstringop-strategy=@var{alg}
10727 Overwrite internal decision heuristic about particular algorithm to inline
10728 string operation with. The allowed values are @code{rep_byte},
10729 @code{rep_4byte}, @code{rep_8byte} for expanding using i386 @code{rep} prefix
10730 of specified size, @code{byte_loop}, @code{loop}, @code{unrolled_loop} for
10731 expanding inline loop, @code{libcall} for always expanding library call.
10733 @item -momit-leaf-frame-pointer
10734 @opindex momit-leaf-frame-pointer
10735 Don't keep the frame pointer in a register for leaf functions. This
10736 avoids the instructions to save, set up and restore frame pointers and
10737 makes an extra register available in leaf functions. The option
10738 @option{-fomit-frame-pointer} removes the frame pointer for all functions
10739 which might make debugging harder.
10741 @item -mtls-direct-seg-refs
10742 @itemx -mno-tls-direct-seg-refs
10743 @opindex mtls-direct-seg-refs
10744 Controls whether TLS variables may be accessed with offsets from the
10745 TLS segment register (@code{%gs} for 32-bit, @code{%fs} for 64-bit),
10746 or whether the thread base pointer must be added. Whether or not this
10747 is legal depends on the operating system, and whether it maps the
10748 segment to cover the entire TLS area.
10750 For systems that use GNU libc, the default is on.
10753 @itemx -mno-fused-madd
10754 @opindex mfused-madd
10755 Enable automatic generation of fused floating point multiply-add instructions
10756 if the ISA supports such instructions. The -mfused-madd option is on by
10757 default. The fused multiply-add instructions have a different
10758 rounding behavior compared to executing a multiply followed by an add.
10761 These @samp{-m} switches are supported in addition to the above
10762 on AMD x86-64 processors in 64-bit environments.
10769 Generate code for a 32-bit or 64-bit environment.
10770 The 32-bit environment sets int, long and pointer to 32 bits and
10771 generates code that runs on any i386 system.
10772 The 64-bit environment sets int to 32 bits and long and pointer
10773 to 64 bits and generates code for AMD's x86-64 architecture. For
10774 darwin only the -m64 option turns off the @option{-fno-pic} and
10775 @option{-mdynamic-no-pic} options.
10777 @item -mno-red-zone
10778 @opindex no-red-zone
10779 Do not use a so called red zone for x86-64 code. The red zone is mandated
10780 by the x86-64 ABI, it is a 128-byte area beyond the location of the
10781 stack pointer that will not be modified by signal or interrupt handlers
10782 and therefore can be used for temporary data without adjusting the stack
10783 pointer. The flag @option{-mno-red-zone} disables this red zone.
10785 @item -mcmodel=small
10786 @opindex mcmodel=small
10787 Generate code for the small code model: the program and its symbols must
10788 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
10789 Programs can be statically or dynamically linked. This is the default
10792 @item -mcmodel=kernel
10793 @opindex mcmodel=kernel
10794 Generate code for the kernel code model. The kernel runs in the
10795 negative 2 GB of the address space.
10796 This model has to be used for Linux kernel code.
10798 @item -mcmodel=medium
10799 @opindex mcmodel=medium
10800 Generate code for the medium model: The program is linked in the lower 2
10801 GB of the address space but symbols can be located anywhere in the
10802 address space. Programs can be statically or dynamically linked, but
10803 building of shared libraries are not supported with the medium model.
10805 @item -mcmodel=large
10806 @opindex mcmodel=large
10807 Generate code for the large model: This model makes no assumptions
10808 about addresses and sizes of sections.
10811 @node IA-64 Options
10812 @subsection IA-64 Options
10813 @cindex IA-64 Options
10815 These are the @samp{-m} options defined for the Intel IA-64 architecture.
10819 @opindex mbig-endian
10820 Generate code for a big endian target. This is the default for HP-UX@.
10822 @item -mlittle-endian
10823 @opindex mlittle-endian
10824 Generate code for a little endian target. This is the default for AIX5
10830 @opindex mno-gnu-as
10831 Generate (or don't) code for the GNU assembler. This is the default.
10832 @c Also, this is the default if the configure option @option{--with-gnu-as}
10838 @opindex mno-gnu-ld
10839 Generate (or don't) code for the GNU linker. This is the default.
10840 @c Also, this is the default if the configure option @option{--with-gnu-ld}
10845 Generate code that does not use a global pointer register. The result
10846 is not position independent code, and violates the IA-64 ABI@.
10848 @item -mvolatile-asm-stop
10849 @itemx -mno-volatile-asm-stop
10850 @opindex mvolatile-asm-stop
10851 @opindex mno-volatile-asm-stop
10852 Generate (or don't) a stop bit immediately before and after volatile asm
10855 @item -mregister-names
10856 @itemx -mno-register-names
10857 @opindex mregister-names
10858 @opindex mno-register-names
10859 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
10860 the stacked registers. This may make assembler output more readable.
10866 Disable (or enable) optimizations that use the small data section. This may
10867 be useful for working around optimizer bugs.
10869 @item -mconstant-gp
10870 @opindex mconstant-gp
10871 Generate code that uses a single constant global pointer value. This is
10872 useful when compiling kernel code.
10876 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
10877 This is useful when compiling firmware code.
10879 @item -minline-float-divide-min-latency
10880 @opindex minline-float-divide-min-latency
10881 Generate code for inline divides of floating point values
10882 using the minimum latency algorithm.
10884 @item -minline-float-divide-max-throughput
10885 @opindex minline-float-divide-max-throughput
10886 Generate code for inline divides of floating point values
10887 using the maximum throughput algorithm.
10889 @item -minline-int-divide-min-latency
10890 @opindex minline-int-divide-min-latency
10891 Generate code for inline divides of integer values
10892 using the minimum latency algorithm.
10894 @item -minline-int-divide-max-throughput
10895 @opindex minline-int-divide-max-throughput
10896 Generate code for inline divides of integer values
10897 using the maximum throughput algorithm.
10899 @item -minline-sqrt-min-latency
10900 @opindex minline-sqrt-min-latency
10901 Generate code for inline square roots
10902 using the minimum latency algorithm.
10904 @item -minline-sqrt-max-throughput
10905 @opindex minline-sqrt-max-throughput
10906 Generate code for inline square roots
10907 using the maximum throughput algorithm.
10909 @item -mno-dwarf2-asm
10910 @itemx -mdwarf2-asm
10911 @opindex mno-dwarf2-asm
10912 @opindex mdwarf2-asm
10913 Don't (or do) generate assembler code for the DWARF2 line number debugging
10914 info. This may be useful when not using the GNU assembler.
10916 @item -mearly-stop-bits
10917 @itemx -mno-early-stop-bits
10918 @opindex mearly-stop-bits
10919 @opindex mno-early-stop-bits
10920 Allow stop bits to be placed earlier than immediately preceding the
10921 instruction that triggered the stop bit. This can improve instruction
10922 scheduling, but does not always do so.
10924 @item -mfixed-range=@var{register-range}
10925 @opindex mfixed-range
10926 Generate code treating the given register range as fixed registers.
10927 A fixed register is one that the register allocator can not use. This is
10928 useful when compiling kernel code. A register range is specified as
10929 two registers separated by a dash. Multiple register ranges can be
10930 specified separated by a comma.
10932 @item -mtls-size=@var{tls-size}
10934 Specify bit size of immediate TLS offsets. Valid values are 14, 22, and
10937 @item -mtune=@var{cpu-type}
10939 Tune the instruction scheduling for a particular CPU, Valid values are
10940 itanium, itanium1, merced, itanium2, and mckinley.
10946 Add support for multithreading using the POSIX threads library. This
10947 option sets flags for both the preprocessor and linker. It does
10948 not affect the thread safety of object code produced by the compiler or
10949 that of libraries supplied with it. These are HP-UX specific flags.
10955 Generate code for a 32-bit or 64-bit environment.
10956 The 32-bit environment sets int, long and pointer to 32 bits.
10957 The 64-bit environment sets int to 32 bits and long and pointer
10958 to 64 bits. These are HP-UX specific flags.
10960 @item -mno-sched-br-data-spec
10961 @itemx -msched-br-data-spec
10962 @opindex mno-sched-br-data-spec
10963 @opindex msched-br-data-spec
10964 (Dis/En)able data speculative scheduling before reload.
10965 This will result in generation of the ld.a instructions and
10966 the corresponding check instructions (ld.c / chk.a).
10967 The default is 'disable'.
10969 @item -msched-ar-data-spec
10970 @itemx -mno-sched-ar-data-spec
10971 @opindex msched-ar-data-spec
10972 @opindex mno-sched-ar-data-spec
10973 (En/Dis)able data speculative scheduling after reload.
10974 This will result in generation of the ld.a instructions and
10975 the corresponding check instructions (ld.c / chk.a).
10976 The default is 'enable'.
10978 @item -mno-sched-control-spec
10979 @itemx -msched-control-spec
10980 @opindex mno-sched-control-spec
10981 @opindex msched-control-spec
10982 (Dis/En)able control speculative scheduling. This feature is
10983 available only during region scheduling (i.e. before reload).
10984 This will result in generation of the ld.s instructions and
10985 the corresponding check instructions chk.s .
10986 The default is 'disable'.
10988 @item -msched-br-in-data-spec
10989 @itemx -mno-sched-br-in-data-spec
10990 @opindex msched-br-in-data-spec
10991 @opindex mno-sched-br-in-data-spec
10992 (En/Dis)able speculative scheduling of the instructions that
10993 are dependent on the data speculative loads before reload.
10994 This is effective only with @option{-msched-br-data-spec} enabled.
10995 The default is 'enable'.
10997 @item -msched-ar-in-data-spec
10998 @itemx -mno-sched-ar-in-data-spec
10999 @opindex msched-ar-in-data-spec
11000 @opindex mno-sched-ar-in-data-spec
11001 (En/Dis)able speculative scheduling of the instructions that
11002 are dependent on the data speculative loads after reload.
11003 This is effective only with @option{-msched-ar-data-spec} enabled.
11004 The default is 'enable'.
11006 @item -msched-in-control-spec
11007 @itemx -mno-sched-in-control-spec
11008 @opindex msched-in-control-spec
11009 @opindex mno-sched-in-control-spec
11010 (En/Dis)able speculative scheduling of the instructions that
11011 are dependent on the control speculative loads.
11012 This is effective only with @option{-msched-control-spec} enabled.
11013 The default is 'enable'.
11016 @itemx -mno-sched-ldc
11017 @opindex msched-ldc
11018 @opindex mno-sched-ldc
11019 (En/Dis)able use of simple data speculation checks ld.c .
11020 If disabled, only chk.a instructions will be emitted to check
11021 data speculative loads.
11022 The default is 'enable'.
11024 @item -mno-sched-control-ldc
11025 @itemx -msched-control-ldc
11026 @opindex mno-sched-control-ldc
11027 @opindex msched-control-ldc
11028 (Dis/En)able use of ld.c instructions to check control speculative loads.
11029 If enabled, in case of control speculative load with no speculatively
11030 scheduled dependent instructions this load will be emitted as ld.sa and
11031 ld.c will be used to check it.
11032 The default is 'disable'.
11034 @item -mno-sched-spec-verbose
11035 @itemx -msched-spec-verbose
11036 @opindex mno-sched-spec-verbose
11037 @opindex msched-spec-verbose
11038 (Dis/En)able printing of the information about speculative motions.
11040 @item -mno-sched-prefer-non-data-spec-insns
11041 @itemx -msched-prefer-non-data-spec-insns
11042 @opindex mno-sched-prefer-non-data-spec-insns
11043 @opindex msched-prefer-non-data-spec-insns
11044 If enabled, data speculative instructions will be chosen for schedule
11045 only if there are no other choices at the moment. This will make
11046 the use of the data speculation much more conservative.
11047 The default is 'disable'.
11049 @item -mno-sched-prefer-non-control-spec-insns
11050 @itemx -msched-prefer-non-control-spec-insns
11051 @opindex mno-sched-prefer-non-control-spec-insns
11052 @opindex msched-prefer-non-control-spec-insns
11053 If enabled, control speculative instructions will be chosen for schedule
11054 only if there are no other choices at the moment. This will make
11055 the use of the control speculation much more conservative.
11056 The default is 'disable'.
11058 @item -mno-sched-count-spec-in-critical-path
11059 @itemx -msched-count-spec-in-critical-path
11060 @opindex mno-sched-count-spec-in-critical-path
11061 @opindex msched-count-spec-in-critical-path
11062 If enabled, speculative dependencies will be considered during
11063 computation of the instructions priorities. This will make the use of the
11064 speculation a bit more conservative.
11065 The default is 'disable'.
11070 @subsection M32C Options
11071 @cindex M32C options
11074 @item -mcpu=@var{name}
11076 Select the CPU for which code is generated. @var{name} may be one of
11077 @samp{r8c} for the R8C/Tiny series, @samp{m16c} for the M16C (up to
11078 /60) series, @samp{m32cm} for the M16C/80 series, or @samp{m32c} for
11079 the M32C/80 series.
11083 Specifies that the program will be run on the simulator. This causes
11084 an alternate runtime library to be linked in which supports, for
11085 example, file I/O. You must not use this option when generating
11086 programs that will run on real hardware; you must provide your own
11087 runtime library for whatever I/O functions are needed.
11089 @item -memregs=@var{number}
11091 Specifies the number of memory-based pseudo-registers GCC will use
11092 during code generation. These pseudo-registers will be used like real
11093 registers, so there is a tradeoff between GCC's ability to fit the
11094 code into available registers, and the performance penalty of using
11095 memory instead of registers. Note that all modules in a program must
11096 be compiled with the same value for this option. Because of that, you
11097 must not use this option with the default runtime libraries gcc
11102 @node M32R/D Options
11103 @subsection M32R/D Options
11104 @cindex M32R/D options
11106 These @option{-m} options are defined for Renesas M32R/D architectures:
11111 Generate code for the M32R/2@.
11115 Generate code for the M32R/X@.
11119 Generate code for the M32R@. This is the default.
11121 @item -mmodel=small
11122 @opindex mmodel=small
11123 Assume all objects live in the lower 16MB of memory (so that their addresses
11124 can be loaded with the @code{ld24} instruction), and assume all subroutines
11125 are reachable with the @code{bl} instruction.
11126 This is the default.
11128 The addressability of a particular object can be set with the
11129 @code{model} attribute.
11131 @item -mmodel=medium
11132 @opindex mmodel=medium
11133 Assume objects may be anywhere in the 32-bit address space (the compiler
11134 will generate @code{seth/add3} instructions to load their addresses), and
11135 assume all subroutines are reachable with the @code{bl} instruction.
11137 @item -mmodel=large
11138 @opindex mmodel=large
11139 Assume objects may be anywhere in the 32-bit address space (the compiler
11140 will generate @code{seth/add3} instructions to load their addresses), and
11141 assume subroutines may not be reachable with the @code{bl} instruction
11142 (the compiler will generate the much slower @code{seth/add3/jl}
11143 instruction sequence).
11146 @opindex msdata=none
11147 Disable use of the small data area. Variables will be put into
11148 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
11149 @code{section} attribute has been specified).
11150 This is the default.
11152 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
11153 Objects may be explicitly put in the small data area with the
11154 @code{section} attribute using one of these sections.
11156 @item -msdata=sdata
11157 @opindex msdata=sdata
11158 Put small global and static data in the small data area, but do not
11159 generate special code to reference them.
11162 @opindex msdata=use
11163 Put small global and static data in the small data area, and generate
11164 special instructions to reference them.
11168 @cindex smaller data references
11169 Put global and static objects less than or equal to @var{num} bytes
11170 into the small data or bss sections instead of the normal data or bss
11171 sections. The default value of @var{num} is 8.
11172 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
11173 for this option to have any effect.
11175 All modules should be compiled with the same @option{-G @var{num}} value.
11176 Compiling with different values of @var{num} may or may not work; if it
11177 doesn't the linker will give an error message---incorrect code will not be
11182 Makes the M32R specific code in the compiler display some statistics
11183 that might help in debugging programs.
11185 @item -malign-loops
11186 @opindex malign-loops
11187 Align all loops to a 32-byte boundary.
11189 @item -mno-align-loops
11190 @opindex mno-align-loops
11191 Do not enforce a 32-byte alignment for loops. This is the default.
11193 @item -missue-rate=@var{number}
11194 @opindex missue-rate=@var{number}
11195 Issue @var{number} instructions per cycle. @var{number} can only be 1
11198 @item -mbranch-cost=@var{number}
11199 @opindex mbranch-cost=@var{number}
11200 @var{number} can only be 1 or 2. If it is 1 then branches will be
11201 preferred over conditional code, if it is 2, then the opposite will
11204 @item -mflush-trap=@var{number}
11205 @opindex mflush-trap=@var{number}
11206 Specifies the trap number to use to flush the cache. The default is
11207 12. Valid numbers are between 0 and 15 inclusive.
11209 @item -mno-flush-trap
11210 @opindex mno-flush-trap
11211 Specifies that the cache cannot be flushed by using a trap.
11213 @item -mflush-func=@var{name}
11214 @opindex mflush-func=@var{name}
11215 Specifies the name of the operating system function to call to flush
11216 the cache. The default is @emph{_flush_cache}, but a function call
11217 will only be used if a trap is not available.
11219 @item -mno-flush-func
11220 @opindex mno-flush-func
11221 Indicates that there is no OS function for flushing the cache.
11225 @node M680x0 Options
11226 @subsection M680x0 Options
11227 @cindex M680x0 options
11229 These are the @samp{-m} options defined for M680x0 and ColdFire processors.
11230 The default settings depend on which architecture was selected when
11231 the compiler was configured; the defaults for the most common choices
11235 @item -march=@var{arch}
11237 Generate code for a specific M680x0 or ColdFire instruction set
11238 architecture. Permissible values of @var{arch} for M680x0
11239 architectures are: @samp{68000}, @samp{68010}, @samp{68020},
11240 @samp{68030}, @samp{68040}, @samp{68060} and @samp{cpu32}. ColdFire
11241 architectures are selected according to Freescale's ISA classification
11242 and the permissible values are: @samp{isaa}, @samp{isaaplus},
11243 @samp{isab} and @samp{isac}.
11245 gcc defines a macro @samp{__mcf@var{arch}__} whenever it is generating
11246 code for a ColdFire target. The @var{arch} in this macro is one of the
11247 @option{-march} arguments given above.
11249 When used together, @option{-march} and @option{-mtune} select code
11250 that runs on a family of similar processors but that is optimized
11251 for a particular microarchitecture.
11253 @item -mcpu=@var{cpu}
11255 Generate code for a specific M680x0 or ColdFire processor.
11256 The M680x0 @var{cpu}s are: @samp{68000}, @samp{68010}, @samp{68020},
11257 @samp{68030}, @samp{68040}, @samp{68060}, @samp{68302}, @samp{68332}
11258 and @samp{cpu32}. The ColdFire @var{cpu}s are given by the table
11259 below, which also classifies the CPUs into families:
11261 @multitable @columnfractions 0.20 0.80
11262 @item @strong{Family} @tab @strong{@samp{-mcpu} arguments}
11263 @item @samp{51qe} @tab @samp{51qe}
11264 @item @samp{5206} @tab @samp{5202} @samp{5204} @samp{5206}
11265 @item @samp{5206e} @tab @samp{5206e}
11266 @item @samp{5208} @tab @samp{5207} @samp{5208}
11267 @item @samp{5211a} @tab @samp{5210a} @samp{5211a}
11268 @item @samp{5213} @tab @samp{5211} @samp{5212} @samp{5213}
11269 @item @samp{5216} @tab @samp{5214} @samp{5216}
11270 @item @samp{52235} @tab @samp{52230} @samp{52231} @samp{52232} @samp{52233} @samp{52234} @samp{52235}
11271 @item @samp{5225} @tab @samp{5224} @samp{5225}
11272 @item @samp{5235} @tab @samp{5232} @samp{5233} @samp{5234} @samp{5235} @samp{523x}
11273 @item @samp{5249} @tab @samp{5249}
11274 @item @samp{5250} @tab @samp{5250}
11275 @item @samp{5271} @tab @samp{5270} @samp{5271}
11276 @item @samp{5272} @tab @samp{5272}
11277 @item @samp{5275} @tab @samp{5274} @samp{5275}
11278 @item @samp{5282} @tab @samp{5280} @samp{5281} @samp{5282} @samp{528x}
11279 @item @samp{5307} @tab @samp{5307}
11280 @item @samp{5329} @tab @samp{5327} @samp{5328} @samp{5329} @samp{532x}
11281 @item @samp{5373} @tab @samp{5372} @samp{5373} @samp{537x}
11282 @item @samp{5407} @tab @samp{5407}
11283 @item @samp{5475} @tab @samp{5470} @samp{5471} @samp{5472} @samp{5473} @samp{5474} @samp{5475} @samp{547x} @samp{5480} @samp{5481} @samp{5482} @samp{5483} @samp{5484} @samp{5485}
11286 @option{-mcpu=@var{cpu}} overrides @option{-march=@var{arch}} if
11287 @var{arch} is compatible with @var{cpu}. Other combinations of
11288 @option{-mcpu} and @option{-march} are rejected.
11290 gcc defines the macro @samp{__mcf_cpu_@var{cpu}} when ColdFire target
11291 @var{cpu} is selected. It also defines @samp{__mcf_family_@var{family}},
11292 where the value of @var{family} is given by the table above.
11294 @item -mtune=@var{tune}
11296 Tune the code for a particular microarchitecture, within the
11297 constraints set by @option{-march} and @option{-mcpu}.
11298 The M680x0 microarchitectures are: @samp{68000}, @samp{68010},
11299 @samp{68020}, @samp{68030}, @samp{68040}, @samp{68060}
11300 and @samp{cpu32}. The ColdFire microarchitectures
11301 are: @samp{cfv1}, @samp{cfv2}, @samp{cfv3}, @samp{cfv4} and @samp{cfv4e}.
11303 You can also use @option{-mtune=68020-40} for code that needs
11304 to run relatively well on 68020, 68030 and 68040 targets.
11305 @option{-mtune=68020-60} is similar but includes 68060 targets
11306 as well. These two options select the same tuning decisions as
11307 @option{-m68020-40} and @option{-m68020-60} respectively.
11309 gcc defines the macros @samp{__mc@var{arch}} and @samp{__mc@var{arch}__}
11310 when tuning for 680x0 architecture @var{arch}. It also defines
11311 @samp{mc@var{arch}} unless either @option{-ansi} or a non-GNU @option{-std}
11312 option is used. If gcc is tuning for a range of architectures,
11313 as selected by @option{-mtune=68020-40} or @option{-mtune=68020-60},
11314 it defines the macros for every architecture in the range.
11316 gcc also defines the macro @samp{__m@var{uarch}__} when tuning for
11317 ColdFire microarchitecture @var{uarch}, where @var{uarch} is one
11318 of the arguments given above.
11324 Generate output for a 68000. This is the default
11325 when the compiler is configured for 68000-based systems.
11326 It is equivalent to @option{-march=68000}.
11328 Use this option for microcontrollers with a 68000 or EC000 core,
11329 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
11333 Generate output for a 68010. This is the default
11334 when the compiler is configured for 68010-based systems.
11335 It is equivalent to @option{-march=68010}.
11341 Generate output for a 68020. This is the default
11342 when the compiler is configured for 68020-based systems.
11343 It is equivalent to @option{-march=68020}.
11347 Generate output for a 68030. This is the default when the compiler is
11348 configured for 68030-based systems. It is equivalent to
11349 @option{-march=68030}.
11353 Generate output for a 68040. This is the default when the compiler is
11354 configured for 68040-based systems. It is equivalent to
11355 @option{-march=68040}.
11357 This option inhibits the use of 68881/68882 instructions that have to be
11358 emulated by software on the 68040. Use this option if your 68040 does not
11359 have code to emulate those instructions.
11363 Generate output for a 68060. This is the default when the compiler is
11364 configured for 68060-based systems. It is equivalent to
11365 @option{-march=68060}.
11367 This option inhibits the use of 68020 and 68881/68882 instructions that
11368 have to be emulated by software on the 68060. Use this option if your 68060
11369 does not have code to emulate those instructions.
11373 Generate output for a CPU32. This is the default
11374 when the compiler is configured for CPU32-based systems.
11375 It is equivalent to @option{-march=cpu32}.
11377 Use this option for microcontrollers with a
11378 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
11379 68336, 68340, 68341, 68349 and 68360.
11383 Generate output for a 520X ColdFire CPU. This is the default
11384 when the compiler is configured for 520X-based systems.
11385 It is equivalent to @option{-mcpu=5206}, and is now deprecated
11386 in favor of that option.
11388 Use this option for microcontroller with a 5200 core, including
11389 the MCF5202, MCF5203, MCF5204 and MCF5206.
11393 Generate output for a 5206e ColdFire CPU. The option is now
11394 deprecated in favor of the equivalent @option{-mcpu=5206e}.
11398 Generate output for a member of the ColdFire 528X family.
11399 The option is now deprecated in favor of the equivalent
11400 @option{-mcpu=528x}.
11404 Generate output for a ColdFire 5307 CPU. The option is now deprecated
11405 in favor of the equivalent @option{-mcpu=5307}.
11409 Generate output for a ColdFire 5407 CPU. The option is now deprecated
11410 in favor of the equivalent @option{-mcpu=5407}.
11414 Generate output for a ColdFire V4e family CPU (e.g.@: 547x/548x).
11415 This includes use of hardware floating point instructions.
11416 The option is equivalent to @option{-mcpu=547x}, and is now
11417 deprecated in favor of that option.
11421 Generate output for a 68040, without using any of the new instructions.
11422 This results in code which can run relatively efficiently on either a
11423 68020/68881 or a 68030 or a 68040. The generated code does use the
11424 68881 instructions that are emulated on the 68040.
11426 The option is equivalent to @option{-march=68020} @option{-mtune=68020-40}.
11430 Generate output for a 68060, without using any of the new instructions.
11431 This results in code which can run relatively efficiently on either a
11432 68020/68881 or a 68030 or a 68040. The generated code does use the
11433 68881 instructions that are emulated on the 68060.
11435 The option is equivalent to @option{-march=68020} @option{-mtune=68020-60}.
11439 @opindex mhard-float
11441 Generate floating-point instructions. This is the default for 68020
11442 and above, and for ColdFire devices that have an FPU. It defines the
11443 macro @samp{__HAVE_68881__} on M680x0 targets and @samp{__mcffpu__}
11444 on ColdFire targets.
11447 @opindex msoft-float
11448 Do not generate floating-point instructions; use library calls instead.
11449 This is the default for 68000, 68010, and 68832 targets. It is also
11450 the default for ColdFire devices that have no FPU.
11456 Generate (do not generate) ColdFire hardware divide and remainder
11457 instructions. If @option{-march} is used without @option{-mcpu},
11458 the default is ``on'' for ColdFire architectures and ``off'' for M680x0
11459 architectures. Otherwise, the default is taken from the target CPU
11460 (either the default CPU, or the one specified by @option{-mcpu}). For
11461 example, the default is ``off'' for @option{-mcpu=5206} and ``on'' for
11462 @option{-mcpu=5206e}.
11464 gcc defines the macro @samp{__mcfhwdiv__} when this option is enabled.
11468 Consider type @code{int} to be 16 bits wide, like @code{short int}.
11469 Additionally, parameters passed on the stack are also aligned to a
11470 16-bit boundary even on targets whose API mandates promotion to 32-bit.
11474 Do not consider type @code{int} to be 16 bits wide. This is the default.
11477 @itemx -mno-bitfield
11478 @opindex mnobitfield
11479 @opindex mno-bitfield
11480 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
11481 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
11485 Do use the bit-field instructions. The @option{-m68020} option implies
11486 @option{-mbitfield}. This is the default if you use a configuration
11487 designed for a 68020.
11491 Use a different function-calling convention, in which functions
11492 that take a fixed number of arguments return with the @code{rtd}
11493 instruction, which pops their arguments while returning. This
11494 saves one instruction in the caller since there is no need to pop
11495 the arguments there.
11497 This calling convention is incompatible with the one normally
11498 used on Unix, so you cannot use it if you need to call libraries
11499 compiled with the Unix compiler.
11501 Also, you must provide function prototypes for all functions that
11502 take variable numbers of arguments (including @code{printf});
11503 otherwise incorrect code will be generated for calls to those
11506 In addition, seriously incorrect code will result if you call a
11507 function with too many arguments. (Normally, extra arguments are
11508 harmlessly ignored.)
11510 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
11511 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
11515 Do not use the calling conventions selected by @option{-mrtd}.
11516 This is the default.
11519 @itemx -mno-align-int
11520 @opindex malign-int
11521 @opindex mno-align-int
11522 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
11523 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
11524 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
11525 Aligning variables on 32-bit boundaries produces code that runs somewhat
11526 faster on processors with 32-bit busses at the expense of more memory.
11528 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
11529 align structures containing the above types differently than
11530 most published application binary interface specifications for the m68k.
11534 Use the pc-relative addressing mode of the 68000 directly, instead of
11535 using a global offset table. At present, this option implies @option{-fpic},
11536 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
11537 not presently supported with @option{-mpcrel}, though this could be supported for
11538 68020 and higher processors.
11540 @item -mno-strict-align
11541 @itemx -mstrict-align
11542 @opindex mno-strict-align
11543 @opindex mstrict-align
11544 Do not (do) assume that unaligned memory references will be handled by
11548 Generate code that allows the data segment to be located in a different
11549 area of memory from the text segment. This allows for execute in place in
11550 an environment without virtual memory management. This option implies
11553 @item -mno-sep-data
11554 Generate code that assumes that the data segment follows the text segment.
11555 This is the default.
11557 @item -mid-shared-library
11558 Generate code that supports shared libraries via the library ID method.
11559 This allows for execute in place and shared libraries in an environment
11560 without virtual memory management. This option implies @option{-fPIC}.
11562 @item -mno-id-shared-library
11563 Generate code that doesn't assume ID based shared libraries are being used.
11564 This is the default.
11566 @item -mshared-library-id=n
11567 Specified the identification number of the ID based shared library being
11568 compiled. Specifying a value of 0 will generate more compact code, specifying
11569 other values will force the allocation of that number to the current
11570 library but is no more space or time efficient than omitting this option.
11574 @node M68hc1x Options
11575 @subsection M68hc1x Options
11576 @cindex M68hc1x options
11578 These are the @samp{-m} options defined for the 68hc11 and 68hc12
11579 microcontrollers. The default values for these options depends on
11580 which style of microcontroller was selected when the compiler was configured;
11581 the defaults for the most common choices are given below.
11588 Generate output for a 68HC11. This is the default
11589 when the compiler is configured for 68HC11-based systems.
11595 Generate output for a 68HC12. This is the default
11596 when the compiler is configured for 68HC12-based systems.
11602 Generate output for a 68HCS12.
11604 @item -mauto-incdec
11605 @opindex mauto-incdec
11606 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
11613 Enable the use of 68HC12 min and max instructions.
11616 @itemx -mno-long-calls
11617 @opindex mlong-calls
11618 @opindex mno-long-calls
11619 Treat all calls as being far away (near). If calls are assumed to be
11620 far away, the compiler will use the @code{call} instruction to
11621 call a function and the @code{rtc} instruction for returning.
11625 Consider type @code{int} to be 16 bits wide, like @code{short int}.
11627 @item -msoft-reg-count=@var{count}
11628 @opindex msoft-reg-count
11629 Specify the number of pseudo-soft registers which are used for the
11630 code generation. The maximum number is 32. Using more pseudo-soft
11631 register may or may not result in better code depending on the program.
11632 The default is 4 for 68HC11 and 2 for 68HC12.
11636 @node MCore Options
11637 @subsection MCore Options
11638 @cindex MCore options
11640 These are the @samp{-m} options defined for the Motorola M*Core
11646 @itemx -mno-hardlit
11648 @opindex mno-hardlit
11649 Inline constants into the code stream if it can be done in two
11650 instructions or less.
11656 Use the divide instruction. (Enabled by default).
11658 @item -mrelax-immediate
11659 @itemx -mno-relax-immediate
11660 @opindex mrelax-immediate
11661 @opindex mno-relax-immediate
11662 Allow arbitrary sized immediates in bit operations.
11664 @item -mwide-bitfields
11665 @itemx -mno-wide-bitfields
11666 @opindex mwide-bitfields
11667 @opindex mno-wide-bitfields
11668 Always treat bit-fields as int-sized.
11670 @item -m4byte-functions
11671 @itemx -mno-4byte-functions
11672 @opindex m4byte-functions
11673 @opindex mno-4byte-functions
11674 Force all functions to be aligned to a four byte boundary.
11676 @item -mcallgraph-data
11677 @itemx -mno-callgraph-data
11678 @opindex mcallgraph-data
11679 @opindex mno-callgraph-data
11680 Emit callgraph information.
11683 @itemx -mno-slow-bytes
11684 @opindex mslow-bytes
11685 @opindex mno-slow-bytes
11686 Prefer word access when reading byte quantities.
11688 @item -mlittle-endian
11689 @itemx -mbig-endian
11690 @opindex mlittle-endian
11691 @opindex mbig-endian
11692 Generate code for a little endian target.
11698 Generate code for the 210 processor.
11702 @subsection MIPS Options
11703 @cindex MIPS options
11709 Generate big-endian code.
11713 Generate little-endian code. This is the default for @samp{mips*el-*-*}
11716 @item -march=@var{arch}
11718 Generate code that will run on @var{arch}, which can be the name of a
11719 generic MIPS ISA, or the name of a particular processor.
11721 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
11722 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
11723 The processor names are:
11724 @samp{4kc}, @samp{4km}, @samp{4kp}, @samp{4ksc},
11725 @samp{4kec}, @samp{4kem}, @samp{4kep}, @samp{4ksd},
11726 @samp{5kc}, @samp{5kf},
11728 @samp{24kc}, @samp{24kf2_1}, @samp{24kf1_1},
11729 @samp{24kec}, @samp{24kef2_1}, @samp{24kef1_1},
11730 @samp{34kc}, @samp{34kf2_1}, @samp{34kf1_1},
11731 @samp{74kc}, @samp{74kf2_1}, @samp{74kf1_1}, @samp{74kf3_2},
11734 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
11735 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000},
11736 @samp{rm7000}, @samp{rm9000},
11739 @samp{vr4100}, @samp{vr4111}, @samp{vr4120}, @samp{vr4130}, @samp{vr4300},
11740 @samp{vr5000}, @samp{vr5400} and @samp{vr5500}.
11741 The special value @samp{from-abi} selects the
11742 most compatible architecture for the selected ABI (that is,
11743 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
11745 In processor names, a final @samp{000} can be abbreviated as @samp{k}
11746 (for example, @samp{-march=r2k}). Prefixes are optional, and
11747 @samp{vr} may be written @samp{r}.
11749 Names of the form @samp{@var{n}f2_1} refer to processors with
11750 FPUs clocked at half the rate of the core, names of the form
11751 @samp{@var{n}f1_1} refer to processors with FPUs clocked at the same
11752 rate as the core, and names of the form @samp{@var{n}f3_2} refer to
11753 processors with FPUs clocked a ratio of 3:2 with respect to the core.
11754 For compatibility reasons, @samp{@var{n}f} is accepted as a synonym
11755 for @samp{@var{n}f2_1} while @samp{@var{n}x} and @samp{@var{b}fx} are
11756 accepted as synonyms for @samp{@var{n}f1_1}.
11758 GCC defines two macros based on the value of this option. The first
11759 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
11760 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
11761 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
11762 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
11763 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
11765 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
11766 above. In other words, it will have the full prefix and will not
11767 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
11768 the macro names the resolved architecture (either @samp{"mips1"} or
11769 @samp{"mips3"}). It names the default architecture when no
11770 @option{-march} option is given.
11772 @item -mtune=@var{arch}
11774 Optimize for @var{arch}. Among other things, this option controls
11775 the way instructions are scheduled, and the perceived cost of arithmetic
11776 operations. The list of @var{arch} values is the same as for
11779 When this option is not used, GCC will optimize for the processor
11780 specified by @option{-march}. By using @option{-march} and
11781 @option{-mtune} together, it is possible to generate code that will
11782 run on a family of processors, but optimize the code for one
11783 particular member of that family.
11785 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
11786 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
11787 @samp{-march} ones described above.
11791 Equivalent to @samp{-march=mips1}.
11795 Equivalent to @samp{-march=mips2}.
11799 Equivalent to @samp{-march=mips3}.
11803 Equivalent to @samp{-march=mips4}.
11807 Equivalent to @samp{-march=mips32}.
11811 Equivalent to @samp{-march=mips32r2}.
11815 Equivalent to @samp{-march=mips64}.
11820 @opindex mno-mips16
11821 Generate (do not generate) MIPS16 code. If GCC is targetting a
11822 MIPS32 or MIPS64 architecture, it will make use of the MIPS16e ASE@.
11824 MIPS16 code generation can also be controlled on a per-function basis
11825 by means of @code{mips16} and @code{nomips16} attributes.
11826 @xref{Function Attributes}, for more information.
11828 @item -mflip-mips16
11829 @opindex mflip-mips16
11830 Generate MIPS16 code on alternating functions. This option is provided
11831 for regression testing of mixed MIPS16/non-MIPS16 code generation, and is
11832 not intended for ordinary use in compiling user code.
11834 @item -minterlink-mips16
11835 @itemx -mno-interlink-mips16
11836 @opindex minterlink-mips16
11837 @opindex mno-interlink-mips16
11838 Require (do not require) that non-MIPS16 code be link-compatible with
11841 For example, non-MIPS16 code cannot jump directly to MIPS16 code;
11842 it must either use a call or an indirect jump. @option{-minterlink-mips16}
11843 therefore disables direct jumps unless GCC knows that the target of the
11844 jump is not MIPS16.
11856 Generate code for the given ABI@.
11858 Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
11859 generates 64-bit code when you select a 64-bit architecture, but you
11860 can use @option{-mgp32} to get 32-bit code instead.
11862 For information about the O64 ABI, see
11863 @w{@uref{http://gcc.gnu.org/projects/mipso64-abi.html}}.
11865 GCC supports a variant of the o32 ABI in which floating-point registers
11866 are 64 rather than 32 bits wide. You can select this combination with
11867 @option{-mabi=32} @option{-mfp64}. This ABI relies on the @samp{mthc1}
11868 and @samp{mfhc1} instructions and is therefore only supported for
11869 MIPS32R2 processors.
11871 The register assignments for arguments and return values remain the
11872 same, but each scalar value is passed in a single 64-bit register
11873 rather than a pair of 32-bit registers. For example, scalar
11874 floating-point values are returned in @samp{$f0} only, not a
11875 @samp{$f0}/@samp{$f1} pair. The set of call-saved registers also
11876 remains the same, but all 64 bits are saved.
11879 @itemx -mno-abicalls
11881 @opindex mno-abicalls
11882 Generate (do not generate) code that is suitable for SVR4-style
11883 dynamic objects. @option{-mabicalls} is the default for SVR4-based
11888 Generate (do not generate) code that is fully position-independent,
11889 and that can therefore be linked into shared libraries. This option
11890 only affects @option{-mabicalls}.
11892 All @option{-mabicalls} code has traditionally been position-independent,
11893 regardless of options like @option{-fPIC} and @option{-fpic}. However,
11894 as an extension, the GNU toolchain allows executables to use absolute
11895 accesses for locally-binding symbols. It can also use shorter GP
11896 initialization sequences and generate direct calls to locally-defined
11897 functions. This mode is selected by @option{-mno-shared}.
11899 @option{-mno-shared} depends on binutils 2.16 or higher and generates
11900 objects that can only be linked by the GNU linker. However, the option
11901 does not affect the ABI of the final executable; it only affects the ABI
11902 of relocatable objects. Using @option{-mno-shared} will generally make
11903 executables both smaller and quicker.
11905 @option{-mshared} is the default.
11911 Lift (do not lift) the usual restrictions on the size of the global
11914 GCC normally uses a single instruction to load values from the GOT@.
11915 While this is relatively efficient, it will only work if the GOT
11916 is smaller than about 64k. Anything larger will cause the linker
11917 to report an error such as:
11919 @cindex relocation truncated to fit (MIPS)
11921 relocation truncated to fit: R_MIPS_GOT16 foobar
11924 If this happens, you should recompile your code with @option{-mxgot}.
11925 It should then work with very large GOTs, although it will also be
11926 less efficient, since it will take three instructions to fetch the
11927 value of a global symbol.
11929 Note that some linkers can create multiple GOTs. If you have such a
11930 linker, you should only need to use @option{-mxgot} when a single object
11931 file accesses more than 64k's worth of GOT entries. Very few do.
11933 These options have no effect unless GCC is generating position
11938 Assume that general-purpose registers are 32 bits wide.
11942 Assume that general-purpose registers are 64 bits wide.
11946 Assume that floating-point registers are 32 bits wide.
11950 Assume that floating-point registers are 64 bits wide.
11953 @opindex mhard-float
11954 Use floating-point coprocessor instructions.
11957 @opindex msoft-float
11958 Do not use floating-point coprocessor instructions. Implement
11959 floating-point calculations using library calls instead.
11961 @item -msingle-float
11962 @opindex msingle-float
11963 Assume that the floating-point coprocessor only supports single-precision
11966 @item -mdouble-float
11967 @opindex mdouble-float
11968 Assume that the floating-point coprocessor supports double-precision
11969 operations. This is the default.
11975 Use (do not use) @samp{ll}, @samp{sc}, and @samp{sync} instructions to
11976 implement atomic memory built-in functions. When neither option is
11977 specified, GCC will use the instructions if the target architecture
11980 @option{-mllsc} is useful if the runtime environment can emulate the
11981 instructions and @option{-mno-llsc} can be useful when compiling for
11982 nonstandard ISAs. You can make either option the default by
11983 configuring GCC with @option{--with-llsc} and @option{--without-llsc}
11984 respectively. @option{--with-llsc} is the default for some
11985 configurations; see the installation documentation for details.
11991 Use (do not use) revision 1 of the MIPS DSP ASE.
11992 @xref{MIPS DSP Built-in Functions}. This option defines the
11993 preprocessor macro @samp{__mips_dsp}. It also defines
11994 @samp{__mips_dsp_rev} to 1.
12000 Use (do not use) revision 2 of the MIPS DSP ASE.
12001 @xref{MIPS DSP Built-in Functions}. This option defines the
12002 preprocessor macros @samp{__mips_dsp} and @samp{__mips_dspr2}.
12003 It also defines @samp{__mips_dsp_rev} to 2.
12006 @itemx -mno-smartmips
12007 @opindex msmartmips
12008 @opindex mno-smartmips
12009 Use (do not use) the MIPS SmartMIPS ASE.
12011 @item -mpaired-single
12012 @itemx -mno-paired-single
12013 @opindex mpaired-single
12014 @opindex mno-paired-single
12015 Use (do not use) paired-single floating-point instructions.
12016 @xref{MIPS Paired-Single Support}. This option requires
12017 hardware floating-point support to be enabled.
12023 Use (do not use) MIPS Digital Media Extension instructions.
12024 This option can only be used when generating 64-bit code and requires
12025 hardware floating-point support to be enabled.
12030 @opindex mno-mips3d
12031 Use (do not use) the MIPS-3D ASE@. @xref{MIPS-3D Built-in Functions}.
12032 The option @option{-mips3d} implies @option{-mpaired-single}.
12038 Use (do not use) MT Multithreading instructions.
12042 Force @code{long} types to be 64 bits wide. See @option{-mlong32} for
12043 an explanation of the default and the way that the pointer size is
12048 Force @code{long}, @code{int}, and pointer types to be 32 bits wide.
12050 The default size of @code{int}s, @code{long}s and pointers depends on
12051 the ABI@. All the supported ABIs use 32-bit @code{int}s. The n64 ABI
12052 uses 64-bit @code{long}s, as does the 64-bit EABI; the others use
12053 32-bit @code{long}s. Pointers are the same size as @code{long}s,
12054 or the same size as integer registers, whichever is smaller.
12060 Assume (do not assume) that all symbols have 32-bit values, regardless
12061 of the selected ABI@. This option is useful in combination with
12062 @option{-mabi=64} and @option{-mno-abicalls} because it allows GCC
12063 to generate shorter and faster references to symbolic addresses.
12067 Put definitions of externally-visible data in a small data section
12068 if that data is no bigger than @var{num} bytes. GCC can then access
12069 the data more efficiently; see @option{-mgpopt} for details.
12071 The default @option{-G} option depends on the configuration.
12073 @item -mlocal-sdata
12074 @itemx -mno-local-sdata
12075 @opindex mlocal-sdata
12076 @opindex mno-local-sdata
12077 Extend (do not extend) the @option{-G} behavior to local data too,
12078 such as to static variables in C. @option{-mlocal-sdata} is the
12079 default for all configurations.
12081 If the linker complains that an application is using too much small data,
12082 you might want to try rebuilding the less performance-critical parts with
12083 @option{-mno-local-sdata}. You might also want to build large
12084 libraries with @option{-mno-local-sdata}, so that the libraries leave
12085 more room for the main program.
12087 @item -mextern-sdata
12088 @itemx -mno-extern-sdata
12089 @opindex mextern-sdata
12090 @opindex mno-extern-sdata
12091 Assume (do not assume) that externally-defined data will be in
12092 a small data section if that data is within the @option{-G} limit.
12093 @option{-mextern-sdata} is the default for all configurations.
12095 If you compile a module @var{Mod} with @option{-mextern-sdata} @option{-G
12096 @var{num}} @option{-mgpopt}, and @var{Mod} references a variable @var{Var}
12097 that is no bigger than @var{num} bytes, you must make sure that @var{Var}
12098 is placed in a small data section. If @var{Var} is defined by another
12099 module, you must either compile that module with a high-enough
12100 @option{-G} setting or attach a @code{section} attribute to @var{Var}'s
12101 definition. If @var{Var} is common, you must link the application
12102 with a high-enough @option{-G} setting.
12104 The easiest way of satisfying these restrictions is to compile
12105 and link every module with the same @option{-G} option. However,
12106 you may wish to build a library that supports several different
12107 small data limits. You can do this by compiling the library with
12108 the highest supported @option{-G} setting and additionally using
12109 @option{-mno-extern-sdata} to stop the library from making assumptions
12110 about externally-defined data.
12116 Use (do not use) GP-relative accesses for symbols that are known to be
12117 in a small data section; see @option{-G}, @option{-mlocal-sdata} and
12118 @option{-mextern-sdata}. @option{-mgpopt} is the default for all
12121 @option{-mno-gpopt} is useful for cases where the @code{$gp} register
12122 might not hold the value of @code{_gp}. For example, if the code is
12123 part of a library that might be used in a boot monitor, programs that
12124 call boot monitor routines will pass an unknown value in @code{$gp}.
12125 (In such situations, the boot monitor itself would usually be compiled
12126 with @option{-G0}.)
12128 @option{-mno-gpopt} implies @option{-mno-local-sdata} and
12129 @option{-mno-extern-sdata}.
12131 @item -membedded-data
12132 @itemx -mno-embedded-data
12133 @opindex membedded-data
12134 @opindex mno-embedded-data
12135 Allocate variables to the read-only data section first if possible, then
12136 next in the small data section if possible, otherwise in data. This gives
12137 slightly slower code than the default, but reduces the amount of RAM required
12138 when executing, and thus may be preferred for some embedded systems.
12140 @item -muninit-const-in-rodata
12141 @itemx -mno-uninit-const-in-rodata
12142 @opindex muninit-const-in-rodata
12143 @opindex mno-uninit-const-in-rodata
12144 Put uninitialized @code{const} variables in the read-only data section.
12145 This option is only meaningful in conjunction with @option{-membedded-data}.
12147 @item -mcode-readable=@var{setting}
12148 @opindex mcode-readable
12149 Specify whether GCC may generate code that reads from executable sections.
12150 There are three possible settings:
12153 @item -mcode-readable=yes
12154 Instructions may freely access executable sections. This is the
12157 @item -mcode-readable=pcrel
12158 MIPS16 PC-relative load instructions can access executable sections,
12159 but other instructions must not do so. This option is useful on 4KSc
12160 and 4KSd processors when the code TLBs have the Read Inhibit bit set.
12161 It is also useful on processors that can be configured to have a dual
12162 instruction/data SRAM interface and that, like the M4K, automatically
12163 redirect PC-relative loads to the instruction RAM.
12165 @item -mcode-readable=no
12166 Instructions must not access executable sections. This option can be
12167 useful on targets that are configured to have a dual instruction/data
12168 SRAM interface but that (unlike the M4K) do not automatically redirect
12169 PC-relative loads to the instruction RAM.
12172 @item -msplit-addresses
12173 @itemx -mno-split-addresses
12174 @opindex msplit-addresses
12175 @opindex mno-split-addresses
12176 Enable (disable) use of the @code{%hi()} and @code{%lo()} assembler
12177 relocation operators. This option has been superseded by
12178 @option{-mexplicit-relocs} but is retained for backwards compatibility.
12180 @item -mexplicit-relocs
12181 @itemx -mno-explicit-relocs
12182 @opindex mexplicit-relocs
12183 @opindex mno-explicit-relocs
12184 Use (do not use) assembler relocation operators when dealing with symbolic
12185 addresses. The alternative, selected by @option{-mno-explicit-relocs},
12186 is to use assembler macros instead.
12188 @option{-mexplicit-relocs} is the default if GCC was configured
12189 to use an assembler that supports relocation operators.
12191 @item -mcheck-zero-division
12192 @itemx -mno-check-zero-division
12193 @opindex mcheck-zero-division
12194 @opindex mno-check-zero-division
12195 Trap (do not trap) on integer division by zero.
12197 The default is @option{-mcheck-zero-division}.
12199 @item -mdivide-traps
12200 @itemx -mdivide-breaks
12201 @opindex mdivide-traps
12202 @opindex mdivide-breaks
12203 MIPS systems check for division by zero by generating either a
12204 conditional trap or a break instruction. Using traps results in
12205 smaller code, but is only supported on MIPS II and later. Also, some
12206 versions of the Linux kernel have a bug that prevents trap from
12207 generating the proper signal (@code{SIGFPE}). Use @option{-mdivide-traps} to
12208 allow conditional traps on architectures that support them and
12209 @option{-mdivide-breaks} to force the use of breaks.
12211 The default is usually @option{-mdivide-traps}, but this can be
12212 overridden at configure time using @option{--with-divide=breaks}.
12213 Divide-by-zero checks can be completely disabled using
12214 @option{-mno-check-zero-division}.
12219 @opindex mno-memcpy
12220 Force (do not force) the use of @code{memcpy()} for non-trivial block
12221 moves. The default is @option{-mno-memcpy}, which allows GCC to inline
12222 most constant-sized copies.
12225 @itemx -mno-long-calls
12226 @opindex mlong-calls
12227 @opindex mno-long-calls
12228 Disable (do not disable) use of the @code{jal} instruction. Calling
12229 functions using @code{jal} is more efficient but requires the caller
12230 and callee to be in the same 256 megabyte segment.
12232 This option has no effect on abicalls code. The default is
12233 @option{-mno-long-calls}.
12239 Enable (disable) use of the @code{mad}, @code{madu} and @code{mul}
12240 instructions, as provided by the R4650 ISA@.
12243 @itemx -mno-fused-madd
12244 @opindex mfused-madd
12245 @opindex mno-fused-madd
12246 Enable (disable) use of the floating point multiply-accumulate
12247 instructions, when they are available. The default is
12248 @option{-mfused-madd}.
12250 When multiply-accumulate instructions are used, the intermediate
12251 product is calculated to infinite precision and is not subject to
12252 the FCSR Flush to Zero bit. This may be undesirable in some
12257 Tell the MIPS assembler to not run its preprocessor over user
12258 assembler files (with a @samp{.s} suffix) when assembling them.
12261 @itemx -mno-fix-r4000
12262 @opindex mfix-r4000
12263 @opindex mno-fix-r4000
12264 Work around certain R4000 CPU errata:
12267 A double-word or a variable shift may give an incorrect result if executed
12268 immediately after starting an integer division.
12270 A double-word or a variable shift may give an incorrect result if executed
12271 while an integer multiplication is in progress.
12273 An integer division may give an incorrect result if started in a delay slot
12274 of a taken branch or a jump.
12278 @itemx -mno-fix-r4400
12279 @opindex mfix-r4400
12280 @opindex mno-fix-r4400
12281 Work around certain R4400 CPU errata:
12284 A double-word or a variable shift may give an incorrect result if executed
12285 immediately after starting an integer division.
12289 @itemx -mno-fix-vr4120
12290 @opindex mfix-vr4120
12291 Work around certain VR4120 errata:
12294 @code{dmultu} does not always produce the correct result.
12296 @code{div} and @code{ddiv} do not always produce the correct result if one
12297 of the operands is negative.
12299 The workarounds for the division errata rely on special functions in
12300 @file{libgcc.a}. At present, these functions are only provided by
12301 the @code{mips64vr*-elf} configurations.
12303 Other VR4120 errata require a nop to be inserted between certain pairs of
12304 instructions. These errata are handled by the assembler, not by GCC itself.
12307 @opindex mfix-vr4130
12308 Work around the VR4130 @code{mflo}/@code{mfhi} errata. The
12309 workarounds are implemented by the assembler rather than by GCC,
12310 although GCC will avoid using @code{mflo} and @code{mfhi} if the
12311 VR4130 @code{macc}, @code{macchi}, @code{dmacc} and @code{dmacchi}
12312 instructions are available instead.
12315 @itemx -mno-fix-sb1
12317 Work around certain SB-1 CPU core errata.
12318 (This flag currently works around the SB-1 revision 2
12319 ``F1'' and ``F2'' floating point errata.)
12321 @item -mflush-func=@var{func}
12322 @itemx -mno-flush-func
12323 @opindex mflush-func
12324 Specifies the function to call to flush the I and D caches, or to not
12325 call any such function. If called, the function must take the same
12326 arguments as the common @code{_flush_func()}, that is, the address of the
12327 memory range for which the cache is being flushed, the size of the
12328 memory range, and the number 3 (to flush both caches). The default
12329 depends on the target GCC was configured for, but commonly is either
12330 @samp{_flush_func} or @samp{__cpu_flush}.
12332 @item mbranch-cost=@var{num}
12333 @opindex mbranch-cost
12334 Set the cost of branches to roughly @var{num} ``simple'' instructions.
12335 This cost is only a heuristic and is not guaranteed to produce
12336 consistent results across releases. A zero cost redundantly selects
12337 the default, which is based on the @option{-mtune} setting.
12339 @item -mbranch-likely
12340 @itemx -mno-branch-likely
12341 @opindex mbranch-likely
12342 @opindex mno-branch-likely
12343 Enable or disable use of Branch Likely instructions, regardless of the
12344 default for the selected architecture. By default, Branch Likely
12345 instructions may be generated if they are supported by the selected
12346 architecture. An exception is for the MIPS32 and MIPS64 architectures
12347 and processors which implement those architectures; for those, Branch
12348 Likely instructions will not be generated by default because the MIPS32
12349 and MIPS64 architectures specifically deprecate their use.
12351 @item -mfp-exceptions
12352 @itemx -mno-fp-exceptions
12353 @opindex mfp-exceptions
12354 Specifies whether FP exceptions are enabled. This affects how we schedule
12355 FP instructions for some processors. The default is that FP exceptions are
12358 For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
12359 64-bit code, then we can use both FP pipes. Otherwise, we can only use one
12362 @item -mvr4130-align
12363 @itemx -mno-vr4130-align
12364 @opindex mvr4130-align
12365 The VR4130 pipeline is two-way superscalar, but can only issue two
12366 instructions together if the first one is 8-byte aligned. When this
12367 option is enabled, GCC will align pairs of instructions that it
12368 thinks should execute in parallel.
12370 This option only has an effect when optimizing for the VR4130.
12371 It normally makes code faster, but at the expense of making it bigger.
12372 It is enabled by default at optimization level @option{-O3}.
12376 @subsection MMIX Options
12377 @cindex MMIX Options
12379 These options are defined for the MMIX:
12383 @itemx -mno-libfuncs
12385 @opindex mno-libfuncs
12386 Specify that intrinsic library functions are being compiled, passing all
12387 values in registers, no matter the size.
12390 @itemx -mno-epsilon
12392 @opindex mno-epsilon
12393 Generate floating-point comparison instructions that compare with respect
12394 to the @code{rE} epsilon register.
12396 @item -mabi=mmixware
12398 @opindex mabi-mmixware
12400 Generate code that passes function parameters and return values that (in
12401 the called function) are seen as registers @code{$0} and up, as opposed to
12402 the GNU ABI which uses global registers @code{$231} and up.
12404 @item -mzero-extend
12405 @itemx -mno-zero-extend
12406 @opindex mzero-extend
12407 @opindex mno-zero-extend
12408 When reading data from memory in sizes shorter than 64 bits, use (do not
12409 use) zero-extending load instructions by default, rather than
12410 sign-extending ones.
12413 @itemx -mno-knuthdiv
12415 @opindex mno-knuthdiv
12416 Make the result of a division yielding a remainder have the same sign as
12417 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
12418 remainder follows the sign of the dividend. Both methods are
12419 arithmetically valid, the latter being almost exclusively used.
12421 @item -mtoplevel-symbols
12422 @itemx -mno-toplevel-symbols
12423 @opindex mtoplevel-symbols
12424 @opindex mno-toplevel-symbols
12425 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
12426 code can be used with the @code{PREFIX} assembly directive.
12430 Generate an executable in the ELF format, rather than the default
12431 @samp{mmo} format used by the @command{mmix} simulator.
12433 @item -mbranch-predict
12434 @itemx -mno-branch-predict
12435 @opindex mbranch-predict
12436 @opindex mno-branch-predict
12437 Use (do not use) the probable-branch instructions, when static branch
12438 prediction indicates a probable branch.
12440 @item -mbase-addresses
12441 @itemx -mno-base-addresses
12442 @opindex mbase-addresses
12443 @opindex mno-base-addresses
12444 Generate (do not generate) code that uses @emph{base addresses}. Using a
12445 base address automatically generates a request (handled by the assembler
12446 and the linker) for a constant to be set up in a global register. The
12447 register is used for one or more base address requests within the range 0
12448 to 255 from the value held in the register. The generally leads to short
12449 and fast code, but the number of different data items that can be
12450 addressed is limited. This means that a program that uses lots of static
12451 data may require @option{-mno-base-addresses}.
12453 @item -msingle-exit
12454 @itemx -mno-single-exit
12455 @opindex msingle-exit
12456 @opindex mno-single-exit
12457 Force (do not force) generated code to have a single exit point in each
12461 @node MN10300 Options
12462 @subsection MN10300 Options
12463 @cindex MN10300 options
12465 These @option{-m} options are defined for Matsushita MN10300 architectures:
12470 Generate code to avoid bugs in the multiply instructions for the MN10300
12471 processors. This is the default.
12473 @item -mno-mult-bug
12474 @opindex mno-mult-bug
12475 Do not generate code to avoid bugs in the multiply instructions for the
12476 MN10300 processors.
12480 Generate code which uses features specific to the AM33 processor.
12484 Do not generate code which uses features specific to the AM33 processor. This
12487 @item -mreturn-pointer-on-d0
12488 @opindex mreturn-pointer-on-d0
12489 When generating a function which returns a pointer, return the pointer
12490 in both @code{a0} and @code{d0}. Otherwise, the pointer is returned
12491 only in a0, and attempts to call such functions without a prototype
12492 would result in errors. Note that this option is on by default; use
12493 @option{-mno-return-pointer-on-d0} to disable it.
12497 Do not link in the C run-time initialization object file.
12501 Indicate to the linker that it should perform a relaxation optimization pass
12502 to shorten branches, calls and absolute memory addresses. This option only
12503 has an effect when used on the command line for the final link step.
12505 This option makes symbolic debugging impossible.
12509 @subsection MT Options
12512 These @option{-m} options are defined for Morpho MT architectures:
12516 @item -march=@var{cpu-type}
12518 Generate code that will run on @var{cpu-type}, which is the name of a system
12519 representing a certain processor type. Possible values for
12520 @var{cpu-type} are @samp{ms1-64-001}, @samp{ms1-16-002},
12521 @samp{ms1-16-003} and @samp{ms2}.
12523 When this option is not used, the default is @option{-march=ms1-16-002}.
12527 Use byte loads and stores when generating code.
12531 Do not use byte loads and stores when generating code.
12535 Use simulator runtime
12539 Do not link in the C run-time initialization object file
12540 @file{crti.o}. Other run-time initialization and termination files
12541 such as @file{startup.o} and @file{exit.o} are still included on the
12542 linker command line.
12546 @node PDP-11 Options
12547 @subsection PDP-11 Options
12548 @cindex PDP-11 Options
12550 These options are defined for the PDP-11:
12555 Use hardware FPP floating point. This is the default. (FIS floating
12556 point on the PDP-11/40 is not supported.)
12559 @opindex msoft-float
12560 Do not use hardware floating point.
12564 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
12568 Return floating-point results in memory. This is the default.
12572 Generate code for a PDP-11/40.
12576 Generate code for a PDP-11/45. This is the default.
12580 Generate code for a PDP-11/10.
12582 @item -mbcopy-builtin
12583 @opindex bcopy-builtin
12584 Use inline @code{movmemhi} patterns for copying memory. This is the
12589 Do not use inline @code{movmemhi} patterns for copying memory.
12595 Use 16-bit @code{int}. This is the default.
12601 Use 32-bit @code{int}.
12604 @itemx -mno-float32
12606 @opindex mno-float32
12607 Use 64-bit @code{float}. This is the default.
12610 @itemx -mno-float64
12612 @opindex mno-float64
12613 Use 32-bit @code{float}.
12617 Use @code{abshi2} pattern. This is the default.
12621 Do not use @code{abshi2} pattern.
12623 @item -mbranch-expensive
12624 @opindex mbranch-expensive
12625 Pretend that branches are expensive. This is for experimenting with
12626 code generation only.
12628 @item -mbranch-cheap
12629 @opindex mbranch-cheap
12630 Do not pretend that branches are expensive. This is the default.
12634 Generate code for a system with split I&D@.
12638 Generate code for a system without split I&D@. This is the default.
12642 Use Unix assembler syntax. This is the default when configured for
12643 @samp{pdp11-*-bsd}.
12647 Use DEC assembler syntax. This is the default when configured for any
12648 PDP-11 target other than @samp{pdp11-*-bsd}.
12651 @node PowerPC Options
12652 @subsection PowerPC Options
12653 @cindex PowerPC options
12655 These are listed under @xref{RS/6000 and PowerPC Options}.
12657 @node RS/6000 and PowerPC Options
12658 @subsection IBM RS/6000 and PowerPC Options
12659 @cindex RS/6000 and PowerPC Options
12660 @cindex IBM RS/6000 and PowerPC Options
12662 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
12669 @itemx -mno-powerpc
12670 @itemx -mpowerpc-gpopt
12671 @itemx -mno-powerpc-gpopt
12672 @itemx -mpowerpc-gfxopt
12673 @itemx -mno-powerpc-gfxopt
12675 @itemx -mno-powerpc64
12679 @itemx -mno-popcntb
12687 @itemx -mno-hard-dfp
12691 @opindex mno-power2
12693 @opindex mno-powerpc
12694 @opindex mpowerpc-gpopt
12695 @opindex mno-powerpc-gpopt
12696 @opindex mpowerpc-gfxopt
12697 @opindex mno-powerpc-gfxopt
12698 @opindex mpowerpc64
12699 @opindex mno-powerpc64
12703 @opindex mno-popcntb
12709 @opindex mno-mfpgpr
12711 @opindex mno-hard-dfp
12712 GCC supports two related instruction set architectures for the
12713 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
12714 instructions supported by the @samp{rios} chip set used in the original
12715 RS/6000 systems and the @dfn{PowerPC} instruction set is the
12716 architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and
12717 the IBM 4xx, 6xx, and follow-on microprocessors.
12719 Neither architecture is a subset of the other. However there is a
12720 large common subset of instructions supported by both. An MQ
12721 register is included in processors supporting the POWER architecture.
12723 You use these options to specify which instructions are available on the
12724 processor you are using. The default value of these options is
12725 determined when configuring GCC@. Specifying the
12726 @option{-mcpu=@var{cpu_type}} overrides the specification of these
12727 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
12728 rather than the options listed above.
12730 The @option{-mpower} option allows GCC to generate instructions that
12731 are found only in the POWER architecture and to use the MQ register.
12732 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
12733 to generate instructions that are present in the POWER2 architecture but
12734 not the original POWER architecture.
12736 The @option{-mpowerpc} option allows GCC to generate instructions that
12737 are found only in the 32-bit subset of the PowerPC architecture.
12738 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
12739 GCC to use the optional PowerPC architecture instructions in the
12740 General Purpose group, including floating-point square root. Specifying
12741 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
12742 use the optional PowerPC architecture instructions in the Graphics
12743 group, including floating-point select.
12745 The @option{-mmfcrf} option allows GCC to generate the move from
12746 condition register field instruction implemented on the POWER4
12747 processor and other processors that support the PowerPC V2.01
12749 The @option{-mpopcntb} option allows GCC to generate the popcount and
12750 double precision FP reciprocal estimate instruction implemented on the
12751 POWER5 processor and other processors that support the PowerPC V2.02
12753 The @option{-mfprnd} option allows GCC to generate the FP round to
12754 integer instructions implemented on the POWER5+ processor and other
12755 processors that support the PowerPC V2.03 architecture.
12756 The @option{-mcmpb} option allows GCC to generate the compare bytes
12757 instruction implemented on the POWER6 processor and other processors
12758 that support the PowerPC V2.05 architecture.
12759 The @option{-mmfpgpr} option allows GCC to generate the FP move to/from
12760 general purpose register instructions implemented on the POWER6X
12761 processor and other processors that support the extended PowerPC V2.05
12763 The @option{-mhard-dfp} option allows GCC to generate the decimal floating
12764 point instructions implemented on some POWER processors.
12766 The @option{-mpowerpc64} option allows GCC to generate the additional
12767 64-bit instructions that are found in the full PowerPC64 architecture
12768 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
12769 @option{-mno-powerpc64}.
12771 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
12772 will use only the instructions in the common subset of both
12773 architectures plus some special AIX common-mode calls, and will not use
12774 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
12775 permits GCC to use any instruction from either architecture and to
12776 allow use of the MQ register; specify this for the Motorola MPC601.
12778 @item -mnew-mnemonics
12779 @itemx -mold-mnemonics
12780 @opindex mnew-mnemonics
12781 @opindex mold-mnemonics
12782 Select which mnemonics to use in the generated assembler code. With
12783 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
12784 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
12785 assembler mnemonics defined for the POWER architecture. Instructions
12786 defined in only one architecture have only one mnemonic; GCC uses that
12787 mnemonic irrespective of which of these options is specified.
12789 GCC defaults to the mnemonics appropriate for the architecture in
12790 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
12791 value of these option. Unless you are building a cross-compiler, you
12792 should normally not specify either @option{-mnew-mnemonics} or
12793 @option{-mold-mnemonics}, but should instead accept the default.
12795 @item -mcpu=@var{cpu_type}
12797 Set architecture type, register usage, choice of mnemonics, and
12798 instruction scheduling parameters for machine type @var{cpu_type}.
12799 Supported values for @var{cpu_type} are @samp{401}, @samp{403},
12800 @samp{405}, @samp{405fp}, @samp{440}, @samp{440fp}, @samp{505},
12801 @samp{601}, @samp{602}, @samp{603}, @samp{603e}, @samp{604},
12802 @samp{604e}, @samp{620}, @samp{630}, @samp{740}, @samp{7400},
12803 @samp{7450}, @samp{750}, @samp{801}, @samp{821}, @samp{823},
12804 @samp{860}, @samp{970}, @samp{8540}, @samp{ec603e}, @samp{G3},
12805 @samp{G4}, @samp{G5}, @samp{power}, @samp{power2}, @samp{power3},
12806 @samp{power4}, @samp{power5}, @samp{power5+}, @samp{power6},
12807 @samp{power6x}, @samp{common}, @samp{powerpc}, @samp{powerpc64},
12808 @samp{rios}, @samp{rios1}, @samp{rios2}, @samp{rsc}, and @samp{rs64}.
12810 @option{-mcpu=common} selects a completely generic processor. Code
12811 generated under this option will run on any POWER or PowerPC processor.
12812 GCC will use only the instructions in the common subset of both
12813 architectures, and will not use the MQ register. GCC assumes a generic
12814 processor model for scheduling purposes.
12816 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
12817 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
12818 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
12819 types, with an appropriate, generic processor model assumed for
12820 scheduling purposes.
12822 The other options specify a specific processor. Code generated under
12823 those options will run best on that processor, and may not run at all on
12826 The @option{-mcpu} options automatically enable or disable the
12829 @gccoptlist{-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple @gol
12830 -mnew-mnemonics -mpopcntb -mpower -mpower2 -mpowerpc64 @gol
12831 -mpowerpc-gpopt -mpowerpc-gfxopt -mstring -mmulhw -mdlmzb -mmfpgpr}
12833 The particular options set for any particular CPU will vary between
12834 compiler versions, depending on what setting seems to produce optimal
12835 code for that CPU; it doesn't necessarily reflect the actual hardware's
12836 capabilities. If you wish to set an individual option to a particular
12837 value, you may specify it after the @option{-mcpu} option, like
12838 @samp{-mcpu=970 -mno-altivec}.
12840 On AIX, the @option{-maltivec} and @option{-mpowerpc64} options are
12841 not enabled or disabled by the @option{-mcpu} option at present because
12842 AIX does not have full support for these options. You may still
12843 enable or disable them individually if you're sure it'll work in your
12846 @item -mtune=@var{cpu_type}
12848 Set the instruction scheduling parameters for machine type
12849 @var{cpu_type}, but do not set the architecture type, register usage, or
12850 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
12851 values for @var{cpu_type} are used for @option{-mtune} as for
12852 @option{-mcpu}. If both are specified, the code generated will use the
12853 architecture, registers, and mnemonics set by @option{-mcpu}, but the
12854 scheduling parameters set by @option{-mtune}.
12860 Generate code to compute division as reciprocal estimate and iterative
12861 refinement, creating opportunities for increased throughput. This
12862 feature requires: optional PowerPC Graphics instruction set for single
12863 precision and FRE instruction for double precision, assuming divides
12864 cannot generate user-visible traps, and the domain values not include
12865 Infinities, denormals or zero denominator.
12868 @itemx -mno-altivec
12870 @opindex mno-altivec
12871 Generate code that uses (does not use) AltiVec instructions, and also
12872 enable the use of built-in functions that allow more direct access to
12873 the AltiVec instruction set. You may also need to set
12874 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
12880 @opindex mno-vrsave
12881 Generate VRSAVE instructions when generating AltiVec code.
12884 @opindex msecure-plt
12885 Generate code that allows ld and ld.so to build executables and shared
12886 libraries with non-exec .plt and .got sections. This is a PowerPC
12887 32-bit SYSV ABI option.
12891 Generate code that uses a BSS .plt section that ld.so fills in, and
12892 requires .plt and .got sections that are both writable and executable.
12893 This is a PowerPC 32-bit SYSV ABI option.
12899 This switch enables or disables the generation of ISEL instructions.
12901 @item -misel=@var{yes/no}
12902 This switch has been deprecated. Use @option{-misel} and
12903 @option{-mno-isel} instead.
12909 This switch enables or disables the generation of SPE simd
12915 @opindex mno-paired
12916 This switch enables or disables the generation of PAIRED simd
12919 @item -mspe=@var{yes/no}
12920 This option has been deprecated. Use @option{-mspe} and
12921 @option{-mno-spe} instead.
12923 @item -mfloat-gprs=@var{yes/single/double/no}
12924 @itemx -mfloat-gprs
12925 @opindex mfloat-gprs
12926 This switch enables or disables the generation of floating point
12927 operations on the general purpose registers for architectures that
12930 The argument @var{yes} or @var{single} enables the use of
12931 single-precision floating point operations.
12933 The argument @var{double} enables the use of single and
12934 double-precision floating point operations.
12936 The argument @var{no} disables floating point operations on the
12937 general purpose registers.
12939 This option is currently only available on the MPC854x.
12945 Generate code for 32-bit or 64-bit environments of Darwin and SVR4
12946 targets (including GNU/Linux). The 32-bit environment sets int, long
12947 and pointer to 32 bits and generates code that runs on any PowerPC
12948 variant. The 64-bit environment sets int to 32 bits and long and
12949 pointer to 64 bits, and generates code for PowerPC64, as for
12950 @option{-mpowerpc64}.
12953 @itemx -mno-fp-in-toc
12954 @itemx -mno-sum-in-toc
12955 @itemx -mminimal-toc
12957 @opindex mno-fp-in-toc
12958 @opindex mno-sum-in-toc
12959 @opindex mminimal-toc
12960 Modify generation of the TOC (Table Of Contents), which is created for
12961 every executable file. The @option{-mfull-toc} option is selected by
12962 default. In that case, GCC will allocate at least one TOC entry for
12963 each unique non-automatic variable reference in your program. GCC
12964 will also place floating-point constants in the TOC@. However, only
12965 16,384 entries are available in the TOC@.
12967 If you receive a linker error message that saying you have overflowed
12968 the available TOC space, you can reduce the amount of TOC space used
12969 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
12970 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
12971 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
12972 generate code to calculate the sum of an address and a constant at
12973 run-time instead of putting that sum into the TOC@. You may specify one
12974 or both of these options. Each causes GCC to produce very slightly
12975 slower and larger code at the expense of conserving TOC space.
12977 If you still run out of space in the TOC even when you specify both of
12978 these options, specify @option{-mminimal-toc} instead. This option causes
12979 GCC to make only one TOC entry for every file. When you specify this
12980 option, GCC will produce code that is slower and larger but which
12981 uses extremely little TOC space. You may wish to use this option
12982 only on files that contain less frequently executed code.
12988 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
12989 @code{long} type, and the infrastructure needed to support them.
12990 Specifying @option{-maix64} implies @option{-mpowerpc64} and
12991 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
12992 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
12995 @itemx -mno-xl-compat
12996 @opindex mxl-compat
12997 @opindex mno-xl-compat
12998 Produce code that conforms more closely to IBM XL compiler semantics
12999 when using AIX-compatible ABI. Pass floating-point arguments to
13000 prototyped functions beyond the register save area (RSA) on the stack
13001 in addition to argument FPRs. Do not assume that most significant
13002 double in 128-bit long double value is properly rounded when comparing
13003 values and converting to double. Use XL symbol names for long double
13006 The AIX calling convention was extended but not initially documented to
13007 handle an obscure K&R C case of calling a function that takes the
13008 address of its arguments with fewer arguments than declared. IBM XL
13009 compilers access floating point arguments which do not fit in the
13010 RSA from the stack when a subroutine is compiled without
13011 optimization. Because always storing floating-point arguments on the
13012 stack is inefficient and rarely needed, this option is not enabled by
13013 default and only is necessary when calling subroutines compiled by IBM
13014 XL compilers without optimization.
13018 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
13019 application written to use message passing with special startup code to
13020 enable the application to run. The system must have PE installed in the
13021 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
13022 must be overridden with the @option{-specs=} option to specify the
13023 appropriate directory location. The Parallel Environment does not
13024 support threads, so the @option{-mpe} option and the @option{-pthread}
13025 option are incompatible.
13027 @item -malign-natural
13028 @itemx -malign-power
13029 @opindex malign-natural
13030 @opindex malign-power
13031 On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
13032 @option{-malign-natural} overrides the ABI-defined alignment of larger
13033 types, such as floating-point doubles, on their natural size-based boundary.
13034 The option @option{-malign-power} instructs GCC to follow the ABI-specified
13035 alignment rules. GCC defaults to the standard alignment defined in the ABI@.
13037 On 64-bit Darwin, natural alignment is the default, and @option{-malign-power}
13041 @itemx -mhard-float
13042 @opindex msoft-float
13043 @opindex mhard-float
13044 Generate code that does not use (uses) the floating-point register set.
13045 Software floating point emulation is provided if you use the
13046 @option{-msoft-float} option, and pass the option to GCC when linking.
13049 @itemx -mno-multiple
13051 @opindex mno-multiple
13052 Generate code that uses (does not use) the load multiple word
13053 instructions and the store multiple word instructions. These
13054 instructions are generated by default on POWER systems, and not
13055 generated on PowerPC systems. Do not use @option{-mmultiple} on little
13056 endian PowerPC systems, since those instructions do not work when the
13057 processor is in little endian mode. The exceptions are PPC740 and
13058 PPC750 which permit the instructions usage in little endian mode.
13063 @opindex mno-string
13064 Generate code that uses (does not use) the load string instructions
13065 and the store string word instructions to save multiple registers and
13066 do small block moves. These instructions are generated by default on
13067 POWER systems, and not generated on PowerPC systems. Do not use
13068 @option{-mstring} on little endian PowerPC systems, since those
13069 instructions do not work when the processor is in little endian mode.
13070 The exceptions are PPC740 and PPC750 which permit the instructions
13071 usage in little endian mode.
13076 @opindex mno-update
13077 Generate code that uses (does not use) the load or store instructions
13078 that update the base register to the address of the calculated memory
13079 location. These instructions are generated by default. If you use
13080 @option{-mno-update}, there is a small window between the time that the
13081 stack pointer is updated and the address of the previous frame is
13082 stored, which means code that walks the stack frame across interrupts or
13083 signals may get corrupted data.
13086 @itemx -mno-fused-madd
13087 @opindex mfused-madd
13088 @opindex mno-fused-madd
13089 Generate code that uses (does not use) the floating point multiply and
13090 accumulate instructions. These instructions are generated by default if
13091 hardware floating is used.
13097 Generate code that uses (does not use) the half-word multiply and
13098 multiply-accumulate instructions on the IBM 405 and 440 processors.
13099 These instructions are generated by default when targetting those
13106 Generate code that uses (does not use) the string-search @samp{dlmzb}
13107 instruction on the IBM 405 and 440 processors. This instruction is
13108 generated by default when targetting those processors.
13110 @item -mno-bit-align
13112 @opindex mno-bit-align
13113 @opindex mbit-align
13114 On System V.4 and embedded PowerPC systems do not (do) force structures
13115 and unions that contain bit-fields to be aligned to the base type of the
13118 For example, by default a structure containing nothing but 8
13119 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
13120 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
13121 the structure would be aligned to a 1 byte boundary and be one byte in
13124 @item -mno-strict-align
13125 @itemx -mstrict-align
13126 @opindex mno-strict-align
13127 @opindex mstrict-align
13128 On System V.4 and embedded PowerPC systems do not (do) assume that
13129 unaligned memory references will be handled by the system.
13131 @item -mrelocatable
13132 @itemx -mno-relocatable
13133 @opindex mrelocatable
13134 @opindex mno-relocatable
13135 On embedded PowerPC systems generate code that allows (does not allow)
13136 the program to be relocated to a different address at runtime. If you
13137 use @option{-mrelocatable} on any module, all objects linked together must
13138 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
13140 @item -mrelocatable-lib
13141 @itemx -mno-relocatable-lib
13142 @opindex mrelocatable-lib
13143 @opindex mno-relocatable-lib
13144 On embedded PowerPC systems generate code that allows (does not allow)
13145 the program to be relocated to a different address at runtime. Modules
13146 compiled with @option{-mrelocatable-lib} can be linked with either modules
13147 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
13148 with modules compiled with the @option{-mrelocatable} options.
13154 On System V.4 and embedded PowerPC systems do not (do) assume that
13155 register 2 contains a pointer to a global area pointing to the addresses
13156 used in the program.
13159 @itemx -mlittle-endian
13161 @opindex mlittle-endian
13162 On System V.4 and embedded PowerPC systems compile code for the
13163 processor in little endian mode. The @option{-mlittle-endian} option is
13164 the same as @option{-mlittle}.
13167 @itemx -mbig-endian
13169 @opindex mbig-endian
13170 On System V.4 and embedded PowerPC systems compile code for the
13171 processor in big endian mode. The @option{-mbig-endian} option is
13172 the same as @option{-mbig}.
13174 @item -mdynamic-no-pic
13175 @opindex mdynamic-no-pic
13176 On Darwin and Mac OS X systems, compile code so that it is not
13177 relocatable, but that its external references are relocatable. The
13178 resulting code is suitable for applications, but not shared
13181 @item -mprioritize-restricted-insns=@var{priority}
13182 @opindex mprioritize-restricted-insns
13183 This option controls the priority that is assigned to
13184 dispatch-slot restricted instructions during the second scheduling
13185 pass. The argument @var{priority} takes the value @var{0/1/2} to assign
13186 @var{no/highest/second-highest} priority to dispatch slot restricted
13189 @item -msched-costly-dep=@var{dependence_type}
13190 @opindex msched-costly-dep
13191 This option controls which dependences are considered costly
13192 by the target during instruction scheduling. The argument
13193 @var{dependence_type} takes one of the following values:
13194 @var{no}: no dependence is costly,
13195 @var{all}: all dependences are costly,
13196 @var{true_store_to_load}: a true dependence from store to load is costly,
13197 @var{store_to_load}: any dependence from store to load is costly,
13198 @var{number}: any dependence which latency >= @var{number} is costly.
13200 @item -minsert-sched-nops=@var{scheme}
13201 @opindex minsert-sched-nops
13202 This option controls which nop insertion scheme will be used during
13203 the second scheduling pass. The argument @var{scheme} takes one of the
13205 @var{no}: Don't insert nops.
13206 @var{pad}: Pad with nops any dispatch group which has vacant issue slots,
13207 according to the scheduler's grouping.
13208 @var{regroup_exact}: Insert nops to force costly dependent insns into
13209 separate groups. Insert exactly as many nops as needed to force an insn
13210 to a new group, according to the estimated processor grouping.
13211 @var{number}: Insert nops to force costly dependent insns into
13212 separate groups. Insert @var{number} nops to force an insn to a new group.
13215 @opindex mcall-sysv
13216 On System V.4 and embedded PowerPC systems compile code using calling
13217 conventions that adheres to the March 1995 draft of the System V
13218 Application Binary Interface, PowerPC processor supplement. This is the
13219 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
13221 @item -mcall-sysv-eabi
13222 @opindex mcall-sysv-eabi
13223 Specify both @option{-mcall-sysv} and @option{-meabi} options.
13225 @item -mcall-sysv-noeabi
13226 @opindex mcall-sysv-noeabi
13227 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
13229 @item -mcall-solaris
13230 @opindex mcall-solaris
13231 On System V.4 and embedded PowerPC systems compile code for the Solaris
13235 @opindex mcall-linux
13236 On System V.4 and embedded PowerPC systems compile code for the
13237 Linux-based GNU system.
13241 On System V.4 and embedded PowerPC systems compile code for the
13242 Hurd-based GNU system.
13244 @item -mcall-netbsd
13245 @opindex mcall-netbsd
13246 On System V.4 and embedded PowerPC systems compile code for the
13247 NetBSD operating system.
13249 @item -maix-struct-return
13250 @opindex maix-struct-return
13251 Return all structures in memory (as specified by the AIX ABI)@.
13253 @item -msvr4-struct-return
13254 @opindex msvr4-struct-return
13255 Return structures smaller than 8 bytes in registers (as specified by the
13258 @item -mabi=@var{abi-type}
13260 Extend the current ABI with a particular extension, or remove such extension.
13261 Valid values are @var{altivec}, @var{no-altivec}, @var{spe},
13262 @var{no-spe}, @var{ibmlongdouble}, @var{ieeelongdouble}@.
13266 Extend the current ABI with SPE ABI extensions. This does not change
13267 the default ABI, instead it adds the SPE ABI extensions to the current
13271 @opindex mabi=no-spe
13272 Disable Booke SPE ABI extensions for the current ABI@.
13274 @item -mabi=ibmlongdouble
13275 @opindex mabi=ibmlongdouble
13276 Change the current ABI to use IBM extended precision long double.
13277 This is a PowerPC 32-bit SYSV ABI option.
13279 @item -mabi=ieeelongdouble
13280 @opindex mabi=ieeelongdouble
13281 Change the current ABI to use IEEE extended precision long double.
13282 This is a PowerPC 32-bit Linux ABI option.
13285 @itemx -mno-prototype
13286 @opindex mprototype
13287 @opindex mno-prototype
13288 On System V.4 and embedded PowerPC systems assume that all calls to
13289 variable argument functions are properly prototyped. Otherwise, the
13290 compiler must insert an instruction before every non prototyped call to
13291 set or clear bit 6 of the condition code register (@var{CR}) to
13292 indicate whether floating point values were passed in the floating point
13293 registers in case the function takes a variable arguments. With
13294 @option{-mprototype}, only calls to prototyped variable argument functions
13295 will set or clear the bit.
13299 On embedded PowerPC systems, assume that the startup module is called
13300 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
13301 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
13306 On embedded PowerPC systems, assume that the startup module is called
13307 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
13312 On embedded PowerPC systems, assume that the startup module is called
13313 @file{crt0.o} and the standard C libraries are @file{libads.a} and
13316 @item -myellowknife
13317 @opindex myellowknife
13318 On embedded PowerPC systems, assume that the startup module is called
13319 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
13324 On System V.4 and embedded PowerPC systems, specify that you are
13325 compiling for a VxWorks system.
13329 Specify that you are compiling for the WindISS simulation environment.
13333 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
13334 header to indicate that @samp{eabi} extended relocations are used.
13340 On System V.4 and embedded PowerPC systems do (do not) adhere to the
13341 Embedded Applications Binary Interface (eabi) which is a set of
13342 modifications to the System V.4 specifications. Selecting @option{-meabi}
13343 means that the stack is aligned to an 8 byte boundary, a function
13344 @code{__eabi} is called to from @code{main} to set up the eabi
13345 environment, and the @option{-msdata} option can use both @code{r2} and
13346 @code{r13} to point to two separate small data areas. Selecting
13347 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
13348 do not call an initialization function from @code{main}, and the
13349 @option{-msdata} option will only use @code{r13} to point to a single
13350 small data area. The @option{-meabi} option is on by default if you
13351 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
13354 @opindex msdata=eabi
13355 On System V.4 and embedded PowerPC systems, put small initialized
13356 @code{const} global and static data in the @samp{.sdata2} section, which
13357 is pointed to by register @code{r2}. Put small initialized
13358 non-@code{const} global and static data in the @samp{.sdata} section,
13359 which is pointed to by register @code{r13}. Put small uninitialized
13360 global and static data in the @samp{.sbss} section, which is adjacent to
13361 the @samp{.sdata} section. The @option{-msdata=eabi} option is
13362 incompatible with the @option{-mrelocatable} option. The
13363 @option{-msdata=eabi} option also sets the @option{-memb} option.
13366 @opindex msdata=sysv
13367 On System V.4 and embedded PowerPC systems, put small global and static
13368 data in the @samp{.sdata} section, which is pointed to by register
13369 @code{r13}. Put small uninitialized global and static data in the
13370 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
13371 The @option{-msdata=sysv} option is incompatible with the
13372 @option{-mrelocatable} option.
13374 @item -msdata=default
13376 @opindex msdata=default
13378 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
13379 compile code the same as @option{-msdata=eabi}, otherwise compile code the
13380 same as @option{-msdata=sysv}.
13383 @opindex msdata-data
13384 On System V.4 and embedded PowerPC systems, put small global
13385 data in the @samp{.sdata} section. Put small uninitialized global
13386 data in the @samp{.sbss} section. Do not use register @code{r13}
13387 to address small data however. This is the default behavior unless
13388 other @option{-msdata} options are used.
13392 @opindex msdata=none
13394 On embedded PowerPC systems, put all initialized global and static data
13395 in the @samp{.data} section, and all uninitialized data in the
13396 @samp{.bss} section.
13400 @cindex smaller data references (PowerPC)
13401 @cindex .sdata/.sdata2 references (PowerPC)
13402 On embedded PowerPC systems, put global and static items less than or
13403 equal to @var{num} bytes into the small data or bss sections instead of
13404 the normal data or bss section. By default, @var{num} is 8. The
13405 @option{-G @var{num}} switch is also passed to the linker.
13406 All modules should be compiled with the same @option{-G @var{num}} value.
13409 @itemx -mno-regnames
13411 @opindex mno-regnames
13412 On System V.4 and embedded PowerPC systems do (do not) emit register
13413 names in the assembly language output using symbolic forms.
13416 @itemx -mno-longcall
13418 @opindex mno-longcall
13419 By default assume that all calls are far away so that a longer more
13420 expensive calling sequence is required. This is required for calls
13421 further than 32 megabytes (33,554,432 bytes) from the current location.
13422 A short call will be generated if the compiler knows
13423 the call cannot be that far away. This setting can be overridden by
13424 the @code{shortcall} function attribute, or by @code{#pragma
13427 Some linkers are capable of detecting out-of-range calls and generating
13428 glue code on the fly. On these systems, long calls are unnecessary and
13429 generate slower code. As of this writing, the AIX linker can do this,
13430 as can the GNU linker for PowerPC/64. It is planned to add this feature
13431 to the GNU linker for 32-bit PowerPC systems as well.
13433 On Darwin/PPC systems, @code{#pragma longcall} will generate ``jbsr
13434 callee, L42'', plus a ``branch island'' (glue code). The two target
13435 addresses represent the callee and the ``branch island''. The
13436 Darwin/PPC linker will prefer the first address and generate a ``bl
13437 callee'' if the PPC ``bl'' instruction will reach the callee directly;
13438 otherwise, the linker will generate ``bl L42'' to call the ``branch
13439 island''. The ``branch island'' is appended to the body of the
13440 calling function; it computes the full 32-bit address of the callee
13443 On Mach-O (Darwin) systems, this option directs the compiler emit to
13444 the glue for every direct call, and the Darwin linker decides whether
13445 to use or discard it.
13447 In the future, we may cause GCC to ignore all longcall specifications
13448 when the linker is known to generate glue.
13452 Adds support for multithreading with the @dfn{pthreads} library.
13453 This option sets flags for both the preprocessor and linker.
13457 @node S/390 and zSeries Options
13458 @subsection S/390 and zSeries Options
13459 @cindex S/390 and zSeries Options
13461 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
13465 @itemx -msoft-float
13466 @opindex mhard-float
13467 @opindex msoft-float
13468 Use (do not use) the hardware floating-point instructions and registers
13469 for floating-point operations. When @option{-msoft-float} is specified,
13470 functions in @file{libgcc.a} will be used to perform floating-point
13471 operations. When @option{-mhard-float} is specified, the compiler
13472 generates IEEE floating-point instructions. This is the default.
13474 @item -mlong-double-64
13475 @itemx -mlong-double-128
13476 @opindex mlong-double-64
13477 @opindex mlong-double-128
13478 These switches control the size of @code{long double} type. A size
13479 of 64bit makes the @code{long double} type equivalent to the @code{double}
13480 type. This is the default.
13483 @itemx -mno-backchain
13484 @opindex mbackchain
13485 @opindex mno-backchain
13486 Store (do not store) the address of the caller's frame as backchain pointer
13487 into the callee's stack frame.
13488 A backchain may be needed to allow debugging using tools that do not understand
13489 DWARF-2 call frame information.
13490 When @option{-mno-packed-stack} is in effect, the backchain pointer is stored
13491 at the bottom of the stack frame; when @option{-mpacked-stack} is in effect,
13492 the backchain is placed into the topmost word of the 96/160 byte register
13495 In general, code compiled with @option{-mbackchain} is call-compatible with
13496 code compiled with @option{-mmo-backchain}; however, use of the backchain
13497 for debugging purposes usually requires that the whole binary is built with
13498 @option{-mbackchain}. Note that the combination of @option{-mbackchain},
13499 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
13500 to build a linux kernel use @option{-msoft-float}.
13502 The default is to not maintain the backchain.
13504 @item -mpacked-stack
13505 @item -mno-packed-stack
13506 @opindex mpacked-stack
13507 @opindex mno-packed-stack
13508 Use (do not use) the packed stack layout. When @option{-mno-packed-stack} is
13509 specified, the compiler uses the all fields of the 96/160 byte register save
13510 area only for their default purpose; unused fields still take up stack space.
13511 When @option{-mpacked-stack} is specified, register save slots are densely
13512 packed at the top of the register save area; unused space is reused for other
13513 purposes, allowing for more efficient use of the available stack space.
13514 However, when @option{-mbackchain} is also in effect, the topmost word of
13515 the save area is always used to store the backchain, and the return address
13516 register is always saved two words below the backchain.
13518 As long as the stack frame backchain is not used, code generated with
13519 @option{-mpacked-stack} is call-compatible with code generated with
13520 @option{-mno-packed-stack}. Note that some non-FSF releases of GCC 2.95 for
13521 S/390 or zSeries generated code that uses the stack frame backchain at run
13522 time, not just for debugging purposes. Such code is not call-compatible
13523 with code compiled with @option{-mpacked-stack}. Also, note that the
13524 combination of @option{-mbackchain},
13525 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
13526 to build a linux kernel use @option{-msoft-float}.
13528 The default is to not use the packed stack layout.
13531 @itemx -mno-small-exec
13532 @opindex msmall-exec
13533 @opindex mno-small-exec
13534 Generate (or do not generate) code using the @code{bras} instruction
13535 to do subroutine calls.
13536 This only works reliably if the total executable size does not
13537 exceed 64k. The default is to use the @code{basr} instruction instead,
13538 which does not have this limitation.
13544 When @option{-m31} is specified, generate code compliant to the
13545 GNU/Linux for S/390 ABI@. When @option{-m64} is specified, generate
13546 code compliant to the GNU/Linux for zSeries ABI@. This allows GCC in
13547 particular to generate 64-bit instructions. For the @samp{s390}
13548 targets, the default is @option{-m31}, while the @samp{s390x}
13549 targets default to @option{-m64}.
13555 When @option{-mzarch} is specified, generate code using the
13556 instructions available on z/Architecture.
13557 When @option{-mesa} is specified, generate code using the
13558 instructions available on ESA/390. Note that @option{-mesa} is
13559 not possible with @option{-m64}.
13560 When generating code compliant to the GNU/Linux for S/390 ABI,
13561 the default is @option{-mesa}. When generating code compliant
13562 to the GNU/Linux for zSeries ABI, the default is @option{-mzarch}.
13568 Generate (or do not generate) code using the @code{mvcle} instruction
13569 to perform block moves. When @option{-mno-mvcle} is specified,
13570 use a @code{mvc} loop instead. This is the default unless optimizing for
13577 Print (or do not print) additional debug information when compiling.
13578 The default is to not print debug information.
13580 @item -march=@var{cpu-type}
13582 Generate code that will run on @var{cpu-type}, which is the name of a system
13583 representing a certain processor type. Possible values for
13584 @var{cpu-type} are @samp{g5}, @samp{g6}, @samp{z900}, and @samp{z990}.
13585 When generating code using the instructions available on z/Architecture,
13586 the default is @option{-march=z900}. Otherwise, the default is
13587 @option{-march=g5}.
13589 @item -mtune=@var{cpu-type}
13591 Tune to @var{cpu-type} everything applicable about the generated code,
13592 except for the ABI and the set of available instructions.
13593 The list of @var{cpu-type} values is the same as for @option{-march}.
13594 The default is the value used for @option{-march}.
13597 @itemx -mno-tpf-trace
13598 @opindex mtpf-trace
13599 @opindex mno-tpf-trace
13600 Generate code that adds (does not add) in TPF OS specific branches to trace
13601 routines in the operating system. This option is off by default, even
13602 when compiling for the TPF OS@.
13605 @itemx -mno-fused-madd
13606 @opindex mfused-madd
13607 @opindex mno-fused-madd
13608 Generate code that uses (does not use) the floating point multiply and
13609 accumulate instructions. These instructions are generated by default if
13610 hardware floating point is used.
13612 @item -mwarn-framesize=@var{framesize}
13613 @opindex mwarn-framesize
13614 Emit a warning if the current function exceeds the given frame size. Because
13615 this is a compile time check it doesn't need to be a real problem when the program
13616 runs. It is intended to identify functions which most probably cause
13617 a stack overflow. It is useful to be used in an environment with limited stack
13618 size e.g.@: the linux kernel.
13620 @item -mwarn-dynamicstack
13621 @opindex mwarn-dynamicstack
13622 Emit a warning if the function calls alloca or uses dynamically
13623 sized arrays. This is generally a bad idea with a limited stack size.
13625 @item -mstack-guard=@var{stack-guard}
13626 @item -mstack-size=@var{stack-size}
13627 @opindex mstack-guard
13628 @opindex mstack-size
13629 If these options are provided the s390 back end emits additional instructions in
13630 the function prologue which trigger a trap if the stack size is @var{stack-guard}
13631 bytes above the @var{stack-size} (remember that the stack on s390 grows downward).
13632 If the @var{stack-guard} option is omitted the smallest power of 2 larger than
13633 the frame size of the compiled function is chosen.
13634 These options are intended to be used to help debugging stack overflow problems.
13635 The additionally emitted code causes only little overhead and hence can also be
13636 used in production like systems without greater performance degradation. The given
13637 values have to be exact powers of 2 and @var{stack-size} has to be greater than
13638 @var{stack-guard} without exceeding 64k.
13639 In order to be efficient the extra code makes the assumption that the stack starts
13640 at an address aligned to the value given by @var{stack-size}.
13641 The @var{stack-guard} option can only be used in conjunction with @var{stack-size}.
13644 @node Score Options
13645 @subsection Score Options
13646 @cindex Score Options
13648 These options are defined for Score implementations:
13653 Compile code for big endian mode. This is the default.
13657 Compile code for little endian mode.
13661 Disable generate bcnz instruction.
13665 Enable generate unaligned load and store instruction.
13669 Enable the use of multiply-accumulate instructions. Disabled by default.
13673 Specify the SCORE5 as the target architecture.
13677 Specify the SCORE5U of the target architecture.
13681 Specify the SCORE7 as the target architecture. This is the default.
13685 Specify the SCORE7D as the target architecture.
13689 @subsection SH Options
13691 These @samp{-m} options are defined for the SH implementations:
13696 Generate code for the SH1.
13700 Generate code for the SH2.
13703 Generate code for the SH2e.
13707 Generate code for the SH3.
13711 Generate code for the SH3e.
13715 Generate code for the SH4 without a floating-point unit.
13717 @item -m4-single-only
13718 @opindex m4-single-only
13719 Generate code for the SH4 with a floating-point unit that only
13720 supports single-precision arithmetic.
13724 Generate code for the SH4 assuming the floating-point unit is in
13725 single-precision mode by default.
13729 Generate code for the SH4.
13733 Generate code for the SH4al-dsp, or for a SH4a in such a way that the
13734 floating-point unit is not used.
13736 @item -m4a-single-only
13737 @opindex m4a-single-only
13738 Generate code for the SH4a, in such a way that no double-precision
13739 floating point operations are used.
13742 @opindex m4a-single
13743 Generate code for the SH4a assuming the floating-point unit is in
13744 single-precision mode by default.
13748 Generate code for the SH4a.
13752 Same as @option{-m4a-nofpu}, except that it implicitly passes
13753 @option{-dsp} to the assembler. GCC doesn't generate any DSP
13754 instructions at the moment.
13758 Compile code for the processor in big endian mode.
13762 Compile code for the processor in little endian mode.
13766 Align doubles at 64-bit boundaries. Note that this changes the calling
13767 conventions, and thus some functions from the standard C library will
13768 not work unless you recompile it first with @option{-mdalign}.
13772 Shorten some address references at link time, when possible; uses the
13773 linker option @option{-relax}.
13777 Use 32-bit offsets in @code{switch} tables. The default is to use
13782 Enable the use of the instruction @code{fmovd}.
13786 Comply with the calling conventions defined by Renesas.
13790 Comply with the calling conventions defined by Renesas.
13794 Comply with the calling conventions defined for GCC before the Renesas
13795 conventions were available. This option is the default for all
13796 targets of the SH toolchain except for @samp{sh-symbianelf}.
13799 @opindex mnomacsave
13800 Mark the @code{MAC} register as call-clobbered, even if
13801 @option{-mhitachi} is given.
13805 Increase IEEE-compliance of floating-point code.
13806 At the moment, this is equivalent to @option{-fno-finite-math-only}.
13807 When generating 16 bit SH opcodes, getting IEEE-conforming results for
13808 comparisons of NANs / infinities incurs extra overhead in every
13809 floating point comparison, therefore the default is set to
13810 @option{-ffinite-math-only}.
13812 @item -minline-ic_invalidate
13813 @opindex minline-ic_invalidate
13814 Inline code to invalidate instruction cache entries after setting up
13815 nested function trampolines.
13816 This option has no effect if -musermode is in effect and the selected
13817 code generation option (e.g. -m4) does not allow the use of the icbi
13819 If the selected code generation option does not allow the use of the icbi
13820 instruction, and -musermode is not in effect, the inlined code will
13821 manipulate the instruction cache address array directly with an associative
13822 write. This not only requires privileged mode, but it will also
13823 fail if the cache line had been mapped via the TLB and has become unmapped.
13827 Dump instruction size and location in the assembly code.
13830 @opindex mpadstruct
13831 This option is deprecated. It pads structures to multiple of 4 bytes,
13832 which is incompatible with the SH ABI@.
13836 Optimize for space instead of speed. Implied by @option{-Os}.
13839 @opindex mprefergot
13840 When generating position-independent code, emit function calls using
13841 the Global Offset Table instead of the Procedure Linkage Table.
13845 Don't generate privileged mode only code; implies -mno-inline-ic_invalidate
13846 if the inlined code would not work in user mode.
13847 This is the default when the target is @code{sh-*-linux*}.
13849 @item -multcost=@var{number}
13850 @opindex multcost=@var{number}
13851 Set the cost to assume for a multiply insn.
13853 @item -mdiv=@var{strategy}
13854 @opindex mdiv=@var{strategy}
13855 Set the division strategy to use for SHmedia code. @var{strategy} must be
13856 one of: call, call2, fp, inv, inv:minlat, inv20u, inv20l, inv:call,
13857 inv:call2, inv:fp .
13858 "fp" performs the operation in floating point. This has a very high latency,
13859 but needs only a few instructions, so it might be a good choice if
13860 your code has enough easily exploitable ILP to allow the compiler to
13861 schedule the floating point instructions together with other instructions.
13862 Division by zero causes a floating point exception.
13863 "inv" uses integer operations to calculate the inverse of the divisor,
13864 and then multiplies the dividend with the inverse. This strategy allows
13865 cse and hoisting of the inverse calculation. Division by zero calculates
13866 an unspecified result, but does not trap.
13867 "inv:minlat" is a variant of "inv" where if no cse / hoisting opportunities
13868 have been found, or if the entire operation has been hoisted to the same
13869 place, the last stages of the inverse calculation are intertwined with the
13870 final multiply to reduce the overall latency, at the expense of using a few
13871 more instructions, and thus offering fewer scheduling opportunities with
13873 "call" calls a library function that usually implements the inv:minlat
13875 This gives high code density for m5-*media-nofpu compilations.
13876 "call2" uses a different entry point of the same library function, where it
13877 assumes that a pointer to a lookup table has already been set up, which
13878 exposes the pointer load to cse / code hoisting optimizations.
13879 "inv:call", "inv:call2" and "inv:fp" all use the "inv" algorithm for initial
13880 code generation, but if the code stays unoptimized, revert to the "call",
13881 "call2", or "fp" strategies, respectively. Note that the
13882 potentially-trapping side effect of division by zero is carried by a
13883 separate instruction, so it is possible that all the integer instructions
13884 are hoisted out, but the marker for the side effect stays where it is.
13885 A recombination to fp operations or a call is not possible in that case.
13886 "inv20u" and "inv20l" are variants of the "inv:minlat" strategy. In the case
13887 that the inverse calculation was nor separated from the multiply, they speed
13888 up division where the dividend fits into 20 bits (plus sign where applicable),
13889 by inserting a test to skip a number of operations in this case; this test
13890 slows down the case of larger dividends. inv20u assumes the case of a such
13891 a small dividend to be unlikely, and inv20l assumes it to be likely.
13893 @item -mdivsi3_libfunc=@var{name}
13894 @opindex mdivsi3_libfunc=@var{name}
13895 Set the name of the library function used for 32 bit signed division to
13896 @var{name}. This only affect the name used in the call and inv:call
13897 division strategies, and the compiler will still expect the same
13898 sets of input/output/clobbered registers as if this option was not present.
13900 @item -madjust-unroll
13901 @opindex madjust-unroll
13902 Throttle unrolling to avoid thrashing target registers.
13903 This option only has an effect if the gcc code base supports the
13904 TARGET_ADJUST_UNROLL_MAX target hook.
13906 @item -mindexed-addressing
13907 @opindex mindexed-addressing
13908 Enable the use of the indexed addressing mode for SHmedia32/SHcompact.
13909 This is only safe if the hardware and/or OS implement 32 bit wrap-around
13910 semantics for the indexed addressing mode. The architecture allows the
13911 implementation of processors with 64 bit MMU, which the OS could use to
13912 get 32 bit addressing, but since no current hardware implementation supports
13913 this or any other way to make the indexed addressing mode safe to use in
13914 the 32 bit ABI, the default is -mno-indexed-addressing.
13916 @item -mgettrcost=@var{number}
13917 @opindex mgettrcost=@var{number}
13918 Set the cost assumed for the gettr instruction to @var{number}.
13919 The default is 2 if @option{-mpt-fixed} is in effect, 100 otherwise.
13923 Assume pt* instructions won't trap. This will generally generate better
13924 scheduled code, but is unsafe on current hardware. The current architecture
13925 definition says that ptabs and ptrel trap when the target anded with 3 is 3.
13926 This has the unintentional effect of making it unsafe to schedule ptabs /
13927 ptrel before a branch, or hoist it out of a loop. For example,
13928 __do_global_ctors, a part of libgcc that runs constructors at program
13929 startup, calls functions in a list which is delimited by @minus{}1. With the
13930 -mpt-fixed option, the ptabs will be done before testing against @minus{}1.
13931 That means that all the constructors will be run a bit quicker, but when
13932 the loop comes to the end of the list, the program crashes because ptabs
13933 loads @minus{}1 into a target register. Since this option is unsafe for any
13934 hardware implementing the current architecture specification, the default
13935 is -mno-pt-fixed. Unless the user specifies a specific cost with
13936 @option{-mgettrcost}, -mno-pt-fixed also implies @option{-mgettrcost=100};
13937 this deters register allocation using target registers for storing
13940 @item -minvalid-symbols
13941 @opindex minvalid-symbols
13942 Assume symbols might be invalid. Ordinary function symbols generated by
13943 the compiler will always be valid to load with movi/shori/ptabs or
13944 movi/shori/ptrel, but with assembler and/or linker tricks it is possible
13945 to generate symbols that will cause ptabs / ptrel to trap.
13946 This option is only meaningful when @option{-mno-pt-fixed} is in effect.
13947 It will then prevent cross-basic-block cse, hoisting and most scheduling
13948 of symbol loads. The default is @option{-mno-invalid-symbols}.
13951 @node SPARC Options
13952 @subsection SPARC Options
13953 @cindex SPARC options
13955 These @samp{-m} options are supported on the SPARC:
13958 @item -mno-app-regs
13960 @opindex mno-app-regs
13962 Specify @option{-mapp-regs} to generate output using the global registers
13963 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
13966 To be fully SVR4 ABI compliant at the cost of some performance loss,
13967 specify @option{-mno-app-regs}. You should compile libraries and system
13968 software with this option.
13971 @itemx -mhard-float
13973 @opindex mhard-float
13974 Generate output containing floating point instructions. This is the
13978 @itemx -msoft-float
13980 @opindex msoft-float
13981 Generate output containing library calls for floating point.
13982 @strong{Warning:} the requisite libraries are not available for all SPARC
13983 targets. Normally the facilities of the machine's usual C compiler are
13984 used, but this cannot be done directly in cross-compilation. You must make
13985 your own arrangements to provide suitable library functions for
13986 cross-compilation. The embedded targets @samp{sparc-*-aout} and
13987 @samp{sparclite-*-*} do provide software floating point support.
13989 @option{-msoft-float} changes the calling convention in the output file;
13990 therefore, it is only useful if you compile @emph{all} of a program with
13991 this option. In particular, you need to compile @file{libgcc.a}, the
13992 library that comes with GCC, with @option{-msoft-float} in order for
13995 @item -mhard-quad-float
13996 @opindex mhard-quad-float
13997 Generate output containing quad-word (long double) floating point
14000 @item -msoft-quad-float
14001 @opindex msoft-quad-float
14002 Generate output containing library calls for quad-word (long double)
14003 floating point instructions. The functions called are those specified
14004 in the SPARC ABI@. This is the default.
14006 As of this writing, there are no SPARC implementations that have hardware
14007 support for the quad-word floating point instructions. They all invoke
14008 a trap handler for one of these instructions, and then the trap handler
14009 emulates the effect of the instruction. Because of the trap handler overhead,
14010 this is much slower than calling the ABI library routines. Thus the
14011 @option{-msoft-quad-float} option is the default.
14013 @item -mno-unaligned-doubles
14014 @itemx -munaligned-doubles
14015 @opindex mno-unaligned-doubles
14016 @opindex munaligned-doubles
14017 Assume that doubles have 8 byte alignment. This is the default.
14019 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
14020 alignment only if they are contained in another type, or if they have an
14021 absolute address. Otherwise, it assumes they have 4 byte alignment.
14022 Specifying this option avoids some rare compatibility problems with code
14023 generated by other compilers. It is not the default because it results
14024 in a performance loss, especially for floating point code.
14026 @item -mno-faster-structs
14027 @itemx -mfaster-structs
14028 @opindex mno-faster-structs
14029 @opindex mfaster-structs
14030 With @option{-mfaster-structs}, the compiler assumes that structures
14031 should have 8 byte alignment. This enables the use of pairs of
14032 @code{ldd} and @code{std} instructions for copies in structure
14033 assignment, in place of twice as many @code{ld} and @code{st} pairs.
14034 However, the use of this changed alignment directly violates the SPARC
14035 ABI@. Thus, it's intended only for use on targets where the developer
14036 acknowledges that their resulting code will not be directly in line with
14037 the rules of the ABI@.
14039 @item -mimpure-text
14040 @opindex mimpure-text
14041 @option{-mimpure-text}, used in addition to @option{-shared}, tells
14042 the compiler to not pass @option{-z text} to the linker when linking a
14043 shared object. Using this option, you can link position-dependent
14044 code into a shared object.
14046 @option{-mimpure-text} suppresses the ``relocations remain against
14047 allocatable but non-writable sections'' linker error message.
14048 However, the necessary relocations will trigger copy-on-write, and the
14049 shared object is not actually shared across processes. Instead of
14050 using @option{-mimpure-text}, you should compile all source code with
14051 @option{-fpic} or @option{-fPIC}.
14053 This option is only available on SunOS and Solaris.
14055 @item -mcpu=@var{cpu_type}
14057 Set the instruction set, register set, and instruction scheduling parameters
14058 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
14059 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
14060 @samp{f930}, @samp{f934}, @samp{hypersparc}, @samp{sparclite86x},
14061 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc},
14062 @samp{ultrasparc3}, @samp{niagara} and @samp{niagara2}.
14064 Default instruction scheduling parameters are used for values that select
14065 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
14066 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
14068 Here is a list of each supported architecture and their supported
14073 v8: supersparc, hypersparc
14074 sparclite: f930, f934, sparclite86x
14076 v9: ultrasparc, ultrasparc3, niagara, niagara2
14079 By default (unless configured otherwise), GCC generates code for the V7
14080 variant of the SPARC architecture. With @option{-mcpu=cypress}, the compiler
14081 additionally optimizes it for the Cypress CY7C602 chip, as used in the
14082 SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
14083 SPARCStation 1, 2, IPX etc.
14085 With @option{-mcpu=v8}, GCC generates code for the V8 variant of the SPARC
14086 architecture. The only difference from V7 code is that the compiler emits
14087 the integer multiply and integer divide instructions which exist in SPARC-V8
14088 but not in SPARC-V7. With @option{-mcpu=supersparc}, the compiler additionally
14089 optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
14092 With @option{-mcpu=sparclite}, GCC generates code for the SPARClite variant of
14093 the SPARC architecture. This adds the integer multiply, integer divide step
14094 and scan (@code{ffs}) instructions which exist in SPARClite but not in SPARC-V7.
14095 With @option{-mcpu=f930}, the compiler additionally optimizes it for the
14096 Fujitsu MB86930 chip, which is the original SPARClite, with no FPU@. With
14097 @option{-mcpu=f934}, the compiler additionally optimizes it for the Fujitsu
14098 MB86934 chip, which is the more recent SPARClite with FPU@.
14100 With @option{-mcpu=sparclet}, GCC generates code for the SPARClet variant of
14101 the SPARC architecture. This adds the integer multiply, multiply/accumulate,
14102 integer divide step and scan (@code{ffs}) instructions which exist in SPARClet
14103 but not in SPARC-V7. With @option{-mcpu=tsc701}, the compiler additionally
14104 optimizes it for the TEMIC SPARClet chip.
14106 With @option{-mcpu=v9}, GCC generates code for the V9 variant of the SPARC
14107 architecture. This adds 64-bit integer and floating-point move instructions,
14108 3 additional floating-point condition code registers and conditional move
14109 instructions. With @option{-mcpu=ultrasparc}, the compiler additionally
14110 optimizes it for the Sun UltraSPARC I/II/IIi chips. With
14111 @option{-mcpu=ultrasparc3}, the compiler additionally optimizes it for the
14112 Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
14113 @option{-mcpu=niagara}, the compiler additionally optimizes it for
14114 Sun UltraSPARC T1 chips. With @option{-mcpu=niagara2}, the compiler
14115 additionally optimizes it for Sun UltraSPARC T2 chips.
14117 @item -mtune=@var{cpu_type}
14119 Set the instruction scheduling parameters for machine type
14120 @var{cpu_type}, but do not set the instruction set or register set that the
14121 option @option{-mcpu=@var{cpu_type}} would.
14123 The same values for @option{-mcpu=@var{cpu_type}} can be used for
14124 @option{-mtune=@var{cpu_type}}, but the only useful values are those
14125 that select a particular cpu implementation. Those are @samp{cypress},
14126 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
14127 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc},
14128 @samp{ultrasparc3}, @samp{niagara}, and @samp{niagara2}.
14133 @opindex mno-v8plus
14134 With @option{-mv8plus}, GCC generates code for the SPARC-V8+ ABI@. The
14135 difference from the V8 ABI is that the global and out registers are
14136 considered 64-bit wide. This is enabled by default on Solaris in 32-bit
14137 mode for all SPARC-V9 processors.
14143 With @option{-mvis}, GCC generates code that takes advantage of the UltraSPARC
14144 Visual Instruction Set extensions. The default is @option{-mno-vis}.
14147 These @samp{-m} options are supported in addition to the above
14148 on SPARC-V9 processors in 64-bit environments:
14151 @item -mlittle-endian
14152 @opindex mlittle-endian
14153 Generate code for a processor running in little-endian mode. It is only
14154 available for a few configurations and most notably not on Solaris and Linux.
14160 Generate code for a 32-bit or 64-bit environment.
14161 The 32-bit environment sets int, long and pointer to 32 bits.
14162 The 64-bit environment sets int to 32 bits and long and pointer
14165 @item -mcmodel=medlow
14166 @opindex mcmodel=medlow
14167 Generate code for the Medium/Low code model: 64-bit addresses, programs
14168 must be linked in the low 32 bits of memory. Programs can be statically
14169 or dynamically linked.
14171 @item -mcmodel=medmid
14172 @opindex mcmodel=medmid
14173 Generate code for the Medium/Middle code model: 64-bit addresses, programs
14174 must be linked in the low 44 bits of memory, the text and data segments must
14175 be less than 2GB in size and the data segment must be located within 2GB of
14178 @item -mcmodel=medany
14179 @opindex mcmodel=medany
14180 Generate code for the Medium/Anywhere code model: 64-bit addresses, programs
14181 may be linked anywhere in memory, the text and data segments must be less
14182 than 2GB in size and the data segment must be located within 2GB of the
14185 @item -mcmodel=embmedany
14186 @opindex mcmodel=embmedany
14187 Generate code for the Medium/Anywhere code model for embedded systems:
14188 64-bit addresses, the text and data segments must be less than 2GB in
14189 size, both starting anywhere in memory (determined at link time). The
14190 global register %g4 points to the base of the data segment. Programs
14191 are statically linked and PIC is not supported.
14194 @itemx -mno-stack-bias
14195 @opindex mstack-bias
14196 @opindex mno-stack-bias
14197 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
14198 frame pointer if present, are offset by @minus{}2047 which must be added back
14199 when making stack frame references. This is the default in 64-bit mode.
14200 Otherwise, assume no such offset is present.
14203 These switches are supported in addition to the above on Solaris:
14208 Add support for multithreading using the Solaris threads library. This
14209 option sets flags for both the preprocessor and linker. This option does
14210 not affect the thread safety of object code produced by the compiler or
14211 that of libraries supplied with it.
14215 Add support for multithreading using the POSIX threads library. This
14216 option sets flags for both the preprocessor and linker. This option does
14217 not affect the thread safety of object code produced by the compiler or
14218 that of libraries supplied with it.
14222 This is a synonym for @option{-pthreads}.
14226 @subsection SPU Options
14227 @cindex SPU options
14229 These @samp{-m} options are supported on the SPU:
14233 @itemx -merror-reloc
14234 @opindex mwarn-reloc
14235 @opindex merror-reloc
14237 The loader for SPU does not handle dynamic relocations. By default, GCC
14238 will give an error when it generates code that requires a dynamic
14239 relocation. @option{-mno-error-reloc} disables the error,
14240 @option{-mwarn-reloc} will generate a warning instead.
14243 @itemx -munsafe-dma
14245 @opindex munsafe-dma
14247 Instructions which initiate or test completion of DMA must not be
14248 reordered with respect to loads and stores of the memory which is being
14249 accessed. Users typically address this problem using the volatile
14250 keyword, but that can lead to inefficient code in places where the
14251 memory is known to not change. Rather than mark the memory as volatile
14252 we treat the DMA instructions as potentially effecting all memory. With
14253 @option{-munsafe-dma} users must use the volatile keyword to protect
14256 @item -mbranch-hints
14257 @opindex mbranch-hints
14259 By default, GCC will generate a branch hint instruction to avoid
14260 pipeline stalls for always taken or probably taken branches. A hint
14261 will not be generated closer than 8 instructions away from its branch.
14262 There is little reason to disable them, except for debugging purposes,
14263 or to make an object a little bit smaller.
14267 @opindex msmall-mem
14268 @opindex mlarge-mem
14270 By default, GCC generates code assuming that addresses are never larger
14271 than 18 bits. With @option{-mlarge-mem} code is generated that assumes
14272 a full 32 bit address.
14277 By default, GCC links against startup code that assumes the SPU-style
14278 main function interface (which has an unconventional parameter list).
14279 With @option{-mstdmain}, GCC will link your program against startup
14280 code that assumes a C99-style interface to @code{main}, including a
14281 local copy of @code{argv} strings.
14283 @item -mfixed-range=@var{register-range}
14284 @opindex mfixed-range
14285 Generate code treating the given register range as fixed registers.
14286 A fixed register is one that the register allocator can not use. This is
14287 useful when compiling kernel code. A register range is specified as
14288 two registers separated by a dash. Multiple register ranges can be
14289 specified separated by a comma.
14293 @node System V Options
14294 @subsection Options for System V
14296 These additional options are available on System V Release 4 for
14297 compatibility with other compilers on those systems:
14302 Create a shared object.
14303 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
14307 Identify the versions of each tool used by the compiler, in a
14308 @code{.ident} assembler directive in the output.
14312 Refrain from adding @code{.ident} directives to the output file (this is
14315 @item -YP,@var{dirs}
14317 Search the directories @var{dirs}, and no others, for libraries
14318 specified with @option{-l}.
14320 @item -Ym,@var{dir}
14322 Look in the directory @var{dir} to find the M4 preprocessor.
14323 The assembler uses this option.
14324 @c This is supposed to go with a -Yd for predefined M4 macro files, but
14325 @c the generic assembler that comes with Solaris takes just -Ym.
14328 @node TMS320C3x/C4x Options
14329 @subsection TMS320C3x/C4x Options
14330 @cindex TMS320C3x/C4x Options
14332 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
14336 @item -mcpu=@var{cpu_type}
14338 Set the instruction set, register set, and instruction scheduling
14339 parameters for machine type @var{cpu_type}. Supported values for
14340 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
14341 @samp{c44}. The default is @samp{c40} to generate code for the
14346 @itemx -msmall-memory
14348 @opindex mbig-memory
14350 @opindex msmall-memory
14352 Generates code for the big or small memory model. The small memory
14353 model assumed that all data fits into one 64K word page. At run-time
14354 the data page (DP) register must be set to point to the 64K page
14355 containing the .bss and .data program sections. The big memory model is
14356 the default and requires reloading of the DP register for every direct
14363 Allow (disallow) allocation of general integer operands into the block
14364 count register BK@.
14370 Enable (disable) generation of code using decrement and branch,
14371 DBcond(D), instructions. This is enabled by default for the C4x. To be
14372 on the safe side, this is disabled for the C3x, since the maximum
14373 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
14374 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
14375 that it can utilize the decrement and branch instruction, but will give
14376 up if there is more than one memory reference in the loop. Thus a loop
14377 where the loop counter is decremented can generate slightly more
14378 efficient code, in cases where the RPTB instruction cannot be utilized.
14380 @item -mdp-isr-reload
14382 @opindex mdp-isr-reload
14384 Force the DP register to be saved on entry to an interrupt service
14385 routine (ISR), reloaded to point to the data section, and restored on
14386 exit from the ISR@. This should not be required unless someone has
14387 violated the small memory model by modifying the DP register, say within
14394 For the C3x use the 24-bit MPYI instruction for integer multiplies
14395 instead of a library call to guarantee 32-bit results. Note that if one
14396 of the operands is a constant, then the multiplication will be performed
14397 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
14398 then squaring operations are performed inline instead of a library call.
14401 @itemx -mno-fast-fix
14403 @opindex mno-fast-fix
14404 The C3x/C4x FIX instruction to convert a floating point value to an
14405 integer value chooses the nearest integer less than or equal to the
14406 floating point value rather than to the nearest integer. Thus if the
14407 floating point number is negative, the result will be incorrectly
14408 truncated an additional code is necessary to detect and correct this
14409 case. This option can be used to disable generation of the additional
14410 code required to correct the result.
14416 Enable (disable) generation of repeat block sequences using the RPTB
14417 instruction for zero overhead looping. The RPTB construct is only used
14418 for innermost loops that do not call functions or jump across the loop
14419 boundaries. There is no advantage having nested RPTB loops due to the
14420 overhead required to save and restore the RC, RS, and RE registers.
14421 This is enabled by default with @option{-O2}.
14423 @item -mrpts=@var{count}
14427 Enable (disable) the use of the single instruction repeat instruction
14428 RPTS@. If a repeat block contains a single instruction, and the loop
14429 count can be guaranteed to be less than the value @var{count}, GCC will
14430 emit a RPTS instruction instead of a RPTB@. If no value is specified,
14431 then a RPTS will be emitted even if the loop count cannot be determined
14432 at compile time. Note that the repeated instruction following RPTS does
14433 not have to be reloaded from memory each iteration, thus freeing up the
14434 CPU buses for operands. However, since interrupts are blocked by this
14435 instruction, it is disabled by default.
14437 @item -mloop-unsigned
14438 @itemx -mno-loop-unsigned
14439 @opindex mloop-unsigned
14440 @opindex mno-loop-unsigned
14441 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
14442 is @math{2^{31} + 1} since these instructions test if the iteration count is
14443 negative to terminate the loop. If the iteration count is unsigned
14444 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
14445 exceeded. This switch allows an unsigned iteration count.
14449 Try to emit an assembler syntax that the TI assembler (asm30) is happy
14450 with. This also enforces compatibility with the API employed by the TI
14451 C3x C compiler. For example, long doubles are passed as structures
14452 rather than in floating point registers.
14458 Generate code that uses registers (stack) for passing arguments to functions.
14459 By default, arguments are passed in registers where possible rather
14460 than by pushing arguments on to the stack.
14462 @item -mparallel-insns
14463 @itemx -mno-parallel-insns
14464 @opindex mparallel-insns
14465 @opindex mno-parallel-insns
14466 Allow the generation of parallel instructions. This is enabled by
14467 default with @option{-O2}.
14469 @item -mparallel-mpy
14470 @itemx -mno-parallel-mpy
14471 @opindex mparallel-mpy
14472 @opindex mno-parallel-mpy
14473 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
14474 provided @option{-mparallel-insns} is also specified. These instructions have
14475 tight register constraints which can pessimize the code generation
14476 of large functions.
14481 @subsection V850 Options
14482 @cindex V850 Options
14484 These @samp{-m} options are defined for V850 implementations:
14488 @itemx -mno-long-calls
14489 @opindex mlong-calls
14490 @opindex mno-long-calls
14491 Treat all calls as being far away (near). If calls are assumed to be
14492 far away, the compiler will always load the functions address up into a
14493 register, and call indirect through the pointer.
14499 Do not optimize (do optimize) basic blocks that use the same index
14500 pointer 4 or more times to copy pointer into the @code{ep} register, and
14501 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
14502 option is on by default if you optimize.
14504 @item -mno-prolog-function
14505 @itemx -mprolog-function
14506 @opindex mno-prolog-function
14507 @opindex mprolog-function
14508 Do not use (do use) external functions to save and restore registers
14509 at the prologue and epilogue of a function. The external functions
14510 are slower, but use less code space if more than one function saves
14511 the same number of registers. The @option{-mprolog-function} option
14512 is on by default if you optimize.
14516 Try to make the code as small as possible. At present, this just turns
14517 on the @option{-mep} and @option{-mprolog-function} options.
14519 @item -mtda=@var{n}
14521 Put static or global variables whose size is @var{n} bytes or less into
14522 the tiny data area that register @code{ep} points to. The tiny data
14523 area can hold up to 256 bytes in total (128 bytes for byte references).
14525 @item -msda=@var{n}
14527 Put static or global variables whose size is @var{n} bytes or less into
14528 the small data area that register @code{gp} points to. The small data
14529 area can hold up to 64 kilobytes.
14531 @item -mzda=@var{n}
14533 Put static or global variables whose size is @var{n} bytes or less into
14534 the first 32 kilobytes of memory.
14538 Specify that the target processor is the V850.
14541 @opindex mbig-switch
14542 Generate code suitable for big switch tables. Use this option only if
14543 the assembler/linker complain about out of range branches within a switch
14548 This option will cause r2 and r5 to be used in the code generated by
14549 the compiler. This setting is the default.
14551 @item -mno-app-regs
14552 @opindex mno-app-regs
14553 This option will cause r2 and r5 to be treated as fixed registers.
14557 Specify that the target processor is the V850E1. The preprocessor
14558 constants @samp{__v850e1__} and @samp{__v850e__} will be defined if
14559 this option is used.
14563 Specify that the target processor is the V850E@. The preprocessor
14564 constant @samp{__v850e__} will be defined if this option is used.
14566 If neither @option{-mv850} nor @option{-mv850e} nor @option{-mv850e1}
14567 are defined then a default target processor will be chosen and the
14568 relevant @samp{__v850*__} preprocessor constant will be defined.
14570 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
14571 defined, regardless of which processor variant is the target.
14573 @item -mdisable-callt
14574 @opindex mdisable-callt
14575 This option will suppress generation of the CALLT instruction for the
14576 v850e and v850e1 flavors of the v850 architecture. The default is
14577 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
14582 @subsection VAX Options
14583 @cindex VAX options
14585 These @samp{-m} options are defined for the VAX:
14590 Do not output certain jump instructions (@code{aobleq} and so on)
14591 that the Unix assembler for the VAX cannot handle across long
14596 Do output those jump instructions, on the assumption that you
14597 will assemble with the GNU assembler.
14601 Output code for g-format floating point numbers instead of d-format.
14604 @node VxWorks Options
14605 @subsection VxWorks Options
14606 @cindex VxWorks Options
14608 The options in this section are defined for all VxWorks targets.
14609 Options specific to the target hardware are listed with the other
14610 options for that target.
14615 GCC can generate code for both VxWorks kernels and real time processes
14616 (RTPs). This option switches from the former to the latter. It also
14617 defines the preprocessor macro @code{__RTP__}.
14620 @opindex non-static
14621 Link an RTP executable against shared libraries rather than static
14622 libraries. The options @option{-static} and @option{-shared} can
14623 also be used for RTPs (@pxref{Link Options}); @option{-static}
14630 These options are passed down to the linker. They are defined for
14631 compatibility with Diab.
14634 @opindex Xbind-lazy
14635 Enable lazy binding of function calls. This option is equivalent to
14636 @option{-Wl,-z,now} and is defined for compatibility with Diab.
14640 Disable lazy binding of function calls. This option is the default and
14641 is defined for compatibility with Diab.
14644 @node x86-64 Options
14645 @subsection x86-64 Options
14646 @cindex x86-64 options
14648 These are listed under @xref{i386 and x86-64 Options}.
14650 @node Xstormy16 Options
14651 @subsection Xstormy16 Options
14652 @cindex Xstormy16 Options
14654 These options are defined for Xstormy16:
14659 Choose startup files and linker script suitable for the simulator.
14662 @node Xtensa Options
14663 @subsection Xtensa Options
14664 @cindex Xtensa Options
14666 These options are supported for Xtensa targets:
14670 @itemx -mno-const16
14672 @opindex mno-const16
14673 Enable or disable use of @code{CONST16} instructions for loading
14674 constant values. The @code{CONST16} instruction is currently not a
14675 standard option from Tensilica. When enabled, @code{CONST16}
14676 instructions are always used in place of the standard @code{L32R}
14677 instructions. The use of @code{CONST16} is enabled by default only if
14678 the @code{L32R} instruction is not available.
14681 @itemx -mno-fused-madd
14682 @opindex mfused-madd
14683 @opindex mno-fused-madd
14684 Enable or disable use of fused multiply/add and multiply/subtract
14685 instructions in the floating-point option. This has no effect if the
14686 floating-point option is not also enabled. Disabling fused multiply/add
14687 and multiply/subtract instructions forces the compiler to use separate
14688 instructions for the multiply and add/subtract operations. This may be
14689 desirable in some cases where strict IEEE 754-compliant results are
14690 required: the fused multiply add/subtract instructions do not round the
14691 intermediate result, thereby producing results with @emph{more} bits of
14692 precision than specified by the IEEE standard. Disabling fused multiply
14693 add/subtract instructions also ensures that the program output is not
14694 sensitive to the compiler's ability to combine multiply and add/subtract
14697 @item -mtext-section-literals
14698 @itemx -mno-text-section-literals
14699 @opindex mtext-section-literals
14700 @opindex mno-text-section-literals
14701 Control the treatment of literal pools. The default is
14702 @option{-mno-text-section-literals}, which places literals in a separate
14703 section in the output file. This allows the literal pool to be placed
14704 in a data RAM/ROM, and it also allows the linker to combine literal
14705 pools from separate object files to remove redundant literals and
14706 improve code size. With @option{-mtext-section-literals}, the literals
14707 are interspersed in the text section in order to keep them as close as
14708 possible to their references. This may be necessary for large assembly
14711 @item -mtarget-align
14712 @itemx -mno-target-align
14713 @opindex mtarget-align
14714 @opindex mno-target-align
14715 When this option is enabled, GCC instructs the assembler to
14716 automatically align instructions to reduce branch penalties at the
14717 expense of some code density. The assembler attempts to widen density
14718 instructions to align branch targets and the instructions following call
14719 instructions. If there are not enough preceding safe density
14720 instructions to align a target, no widening will be performed. The
14721 default is @option{-mtarget-align}. These options do not affect the
14722 treatment of auto-aligned instructions like @code{LOOP}, which the
14723 assembler will always align, either by widening density instructions or
14724 by inserting no-op instructions.
14727 @itemx -mno-longcalls
14728 @opindex mlongcalls
14729 @opindex mno-longcalls
14730 When this option is enabled, GCC instructs the assembler to translate
14731 direct calls to indirect calls unless it can determine that the target
14732 of a direct call is in the range allowed by the call instruction. This
14733 translation typically occurs for calls to functions in other source
14734 files. Specifically, the assembler translates a direct @code{CALL}
14735 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
14736 The default is @option{-mno-longcalls}. This option should be used in
14737 programs where the call target can potentially be out of range. This
14738 option is implemented in the assembler, not the compiler, so the
14739 assembly code generated by GCC will still show direct call
14740 instructions---look at the disassembled object code to see the actual
14741 instructions. Note that the assembler will use an indirect call for
14742 every cross-file call, not just those that really will be out of range.
14745 @node zSeries Options
14746 @subsection zSeries Options
14747 @cindex zSeries options
14749 These are listed under @xref{S/390 and zSeries Options}.
14751 @node Code Gen Options
14752 @section Options for Code Generation Conventions
14753 @cindex code generation conventions
14754 @cindex options, code generation
14755 @cindex run-time options
14757 These machine-independent options control the interface conventions
14758 used in code generation.
14760 Most of them have both positive and negative forms; the negative form
14761 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
14762 one of the forms is listed---the one which is not the default. You
14763 can figure out the other form by either removing @samp{no-} or adding
14767 @item -fbounds-check
14768 @opindex fbounds-check
14769 For front-ends that support it, generate additional code to check that
14770 indices used to access arrays are within the declared range. This is
14771 currently only supported by the Java and Fortran front-ends, where
14772 this option defaults to true and false respectively.
14776 This option generates traps for signed overflow on addition, subtraction,
14777 multiplication operations.
14781 This option instructs the compiler to assume that signed arithmetic
14782 overflow of addition, subtraction and multiplication wraps around
14783 using twos-complement representation. This flag enables some optimizations
14784 and disables others. This option is enabled by default for the Java
14785 front-end, as required by the Java language specification.
14788 @opindex fexceptions
14789 Enable exception handling. Generates extra code needed to propagate
14790 exceptions. For some targets, this implies GCC will generate frame
14791 unwind information for all functions, which can produce significant data
14792 size overhead, although it does not affect execution. If you do not
14793 specify this option, GCC will enable it by default for languages like
14794 C++ which normally require exception handling, and disable it for
14795 languages like C that do not normally require it. However, you may need
14796 to enable this option when compiling C code that needs to interoperate
14797 properly with exception handlers written in C++. You may also wish to
14798 disable this option if you are compiling older C++ programs that don't
14799 use exception handling.
14801 @item -fnon-call-exceptions
14802 @opindex fnon-call-exceptions
14803 Generate code that allows trapping instructions to throw exceptions.
14804 Note that this requires platform-specific runtime support that does
14805 not exist everywhere. Moreover, it only allows @emph{trapping}
14806 instructions to throw exceptions, i.e.@: memory references or floating
14807 point instructions. It does not allow exceptions to be thrown from
14808 arbitrary signal handlers such as @code{SIGALRM}.
14810 @item -funwind-tables
14811 @opindex funwind-tables
14812 Similar to @option{-fexceptions}, except that it will just generate any needed
14813 static data, but will not affect the generated code in any other way.
14814 You will normally not enable this option; instead, a language processor
14815 that needs this handling would enable it on your behalf.
14817 @item -fasynchronous-unwind-tables
14818 @opindex fasynchronous-unwind-tables
14819 Generate unwind table in dwarf2 format, if supported by target machine. The
14820 table is exact at each instruction boundary, so it can be used for stack
14821 unwinding from asynchronous events (such as debugger or garbage collector).
14823 @item -fpcc-struct-return
14824 @opindex fpcc-struct-return
14825 Return ``short'' @code{struct} and @code{union} values in memory like
14826 longer ones, rather than in registers. This convention is less
14827 efficient, but it has the advantage of allowing intercallability between
14828 GCC-compiled files and files compiled with other compilers, particularly
14829 the Portable C Compiler (pcc).
14831 The precise convention for returning structures in memory depends
14832 on the target configuration macros.
14834 Short structures and unions are those whose size and alignment match
14835 that of some integer type.
14837 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
14838 switch is not binary compatible with code compiled with the
14839 @option{-freg-struct-return} switch.
14840 Use it to conform to a non-default application binary interface.
14842 @item -freg-struct-return
14843 @opindex freg-struct-return
14844 Return @code{struct} and @code{union} values in registers when possible.
14845 This is more efficient for small structures than
14846 @option{-fpcc-struct-return}.
14848 If you specify neither @option{-fpcc-struct-return} nor
14849 @option{-freg-struct-return}, GCC defaults to whichever convention is
14850 standard for the target. If there is no standard convention, GCC
14851 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
14852 the principal compiler. In those cases, we can choose the standard, and
14853 we chose the more efficient register return alternative.
14855 @strong{Warning:} code compiled with the @option{-freg-struct-return}
14856 switch is not binary compatible with code compiled with the
14857 @option{-fpcc-struct-return} switch.
14858 Use it to conform to a non-default application binary interface.
14860 @item -fshort-enums
14861 @opindex fshort-enums
14862 Allocate to an @code{enum} type only as many bytes as it needs for the
14863 declared range of possible values. Specifically, the @code{enum} type
14864 will be equivalent to the smallest integer type which has enough room.
14866 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
14867 code that is not binary compatible with code generated without that switch.
14868 Use it to conform to a non-default application binary interface.
14870 @item -fshort-double
14871 @opindex fshort-double
14872 Use the same size for @code{double} as for @code{float}.
14874 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
14875 code that is not binary compatible with code generated without that switch.
14876 Use it to conform to a non-default application binary interface.
14878 @item -fshort-wchar
14879 @opindex fshort-wchar
14880 Override the underlying type for @samp{wchar_t} to be @samp{short
14881 unsigned int} instead of the default for the target. This option is
14882 useful for building programs to run under WINE@.
14884 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
14885 code that is not binary compatible with code generated without that switch.
14886 Use it to conform to a non-default application binary interface.
14889 @opindex fno-common
14890 In C, allocate even uninitialized global variables in the data section of the
14891 object file, rather than generating them as common blocks. This has the
14892 effect that if the same variable is declared (without @code{extern}) in
14893 two different compilations, you will get an error when you link them.
14894 The only reason this might be useful is if you wish to verify that the
14895 program will work on other systems which always work this way.
14899 Ignore the @samp{#ident} directive.
14901 @item -finhibit-size-directive
14902 @opindex finhibit-size-directive
14903 Don't output a @code{.size} assembler directive, or anything else that
14904 would cause trouble if the function is split in the middle, and the
14905 two halves are placed at locations far apart in memory. This option is
14906 used when compiling @file{crtstuff.c}; you should not need to use it
14909 @item -fverbose-asm
14910 @opindex fverbose-asm
14911 Put extra commentary information in the generated assembly code to
14912 make it more readable. This option is generally only of use to those
14913 who actually need to read the generated assembly code (perhaps while
14914 debugging the compiler itself).
14916 @option{-fno-verbose-asm}, the default, causes the
14917 extra information to be omitted and is useful when comparing two assembler
14920 @item -frecord-gcc-switches
14921 @opindex frecord-gcc-switches
14922 This switch causes the command line that was used to invoke the
14923 compiler to be recorded into the object file that is being created.
14924 This switch is only implemented on some targets and the exact format
14925 of the recording is target and binary file format dependent, but it
14926 usually takes the form of a section containing ASCII text. This
14927 switch is related to the @option{-fverbose-asm} switch, but that
14928 switch only records information in the assembler output file as
14929 comments, so it never reaches the object file.
14933 @cindex global offset table
14935 Generate position-independent code (PIC) suitable for use in a shared
14936 library, if supported for the target machine. Such code accesses all
14937 constant addresses through a global offset table (GOT)@. The dynamic
14938 loader resolves the GOT entries when the program starts (the dynamic
14939 loader is not part of GCC; it is part of the operating system). If
14940 the GOT size for the linked executable exceeds a machine-specific
14941 maximum size, you get an error message from the linker indicating that
14942 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
14943 instead. (These maximums are 8k on the SPARC and 32k
14944 on the m68k and RS/6000. The 386 has no such limit.)
14946 Position-independent code requires special support, and therefore works
14947 only on certain machines. For the 386, GCC supports PIC for System V
14948 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
14949 position-independent.
14951 When this flag is set, the macros @code{__pic__} and @code{__PIC__}
14956 If supported for the target machine, emit position-independent code,
14957 suitable for dynamic linking and avoiding any limit on the size of the
14958 global offset table. This option makes a difference on the m68k,
14959 PowerPC and SPARC@.
14961 Position-independent code requires special support, and therefore works
14962 only on certain machines.
14964 When this flag is set, the macros @code{__pic__} and @code{__PIC__}
14971 These options are similar to @option{-fpic} and @option{-fPIC}, but
14972 generated position independent code can be only linked into executables.
14973 Usually these options are used when @option{-pie} GCC option will be
14974 used during linking.
14976 @option{-fpie} and @option{-fPIE} both define the macros
14977 @code{__pie__} and @code{__PIE__}. The macros have the value 1
14978 for @option{-fpie} and 2 for @option{-fPIE}.
14980 @item -fno-jump-tables
14981 @opindex fno-jump-tables
14982 Do not use jump tables for switch statements even where it would be
14983 more efficient than other code generation strategies. This option is
14984 of use in conjunction with @option{-fpic} or @option{-fPIC} for
14985 building code which forms part of a dynamic linker and cannot
14986 reference the address of a jump table. On some targets, jump tables
14987 do not require a GOT and this option is not needed.
14989 @item -ffixed-@var{reg}
14991 Treat the register named @var{reg} as a fixed register; generated code
14992 should never refer to it (except perhaps as a stack pointer, frame
14993 pointer or in some other fixed role).
14995 @var{reg} must be the name of a register. The register names accepted
14996 are machine-specific and are defined in the @code{REGISTER_NAMES}
14997 macro in the machine description macro file.
14999 This flag does not have a negative form, because it specifies a
15002 @item -fcall-used-@var{reg}
15003 @opindex fcall-used
15004 Treat the register named @var{reg} as an allocable register that is
15005 clobbered by function calls. It may be allocated for temporaries or
15006 variables that do not live across a call. Functions compiled this way
15007 will not save and restore the register @var{reg}.
15009 It is an error to used this flag with the frame pointer or stack pointer.
15010 Use of this flag for other registers that have fixed pervasive roles in
15011 the machine's execution model will produce disastrous results.
15013 This flag does not have a negative form, because it specifies a
15016 @item -fcall-saved-@var{reg}
15017 @opindex fcall-saved
15018 Treat the register named @var{reg} as an allocable register saved by
15019 functions. It may be allocated even for temporaries or variables that
15020 live across a call. Functions compiled this way will save and restore
15021 the register @var{reg} if they use it.
15023 It is an error to used this flag with the frame pointer or stack pointer.
15024 Use of this flag for other registers that have fixed pervasive roles in
15025 the machine's execution model will produce disastrous results.
15027 A different sort of disaster will result from the use of this flag for
15028 a register in which function values may be returned.
15030 This flag does not have a negative form, because it specifies a
15033 @item -fpack-struct[=@var{n}]
15034 @opindex fpack-struct
15035 Without a value specified, pack all structure members together without
15036 holes. When a value is specified (which must be a small power of two), pack
15037 structure members according to this value, representing the maximum
15038 alignment (that is, objects with default alignment requirements larger than
15039 this will be output potentially unaligned at the next fitting location.
15041 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
15042 code that is not binary compatible with code generated without that switch.
15043 Additionally, it makes the code suboptimal.
15044 Use it to conform to a non-default application binary interface.
15046 @item -finstrument-functions
15047 @opindex finstrument-functions
15048 Generate instrumentation calls for entry and exit to functions. Just
15049 after function entry and just before function exit, the following
15050 profiling functions will be called with the address of the current
15051 function and its call site. (On some platforms,
15052 @code{__builtin_return_address} does not work beyond the current
15053 function, so the call site information may not be available to the
15054 profiling functions otherwise.)
15057 void __cyg_profile_func_enter (void *this_fn,
15059 void __cyg_profile_func_exit (void *this_fn,
15063 The first argument is the address of the start of the current function,
15064 which may be looked up exactly in the symbol table.
15066 This instrumentation is also done for functions expanded inline in other
15067 functions. The profiling calls will indicate where, conceptually, the
15068 inline function is entered and exited. This means that addressable
15069 versions of such functions must be available. If all your uses of a
15070 function are expanded inline, this may mean an additional expansion of
15071 code size. If you use @samp{extern inline} in your C code, an
15072 addressable version of such functions must be provided. (This is
15073 normally the case anyways, but if you get lucky and the optimizer always
15074 expands the functions inline, you might have gotten away without
15075 providing static copies.)
15077 A function may be given the attribute @code{no_instrument_function}, in
15078 which case this instrumentation will not be done. This can be used, for
15079 example, for the profiling functions listed above, high-priority
15080 interrupt routines, and any functions from which the profiling functions
15081 cannot safely be called (perhaps signal handlers, if the profiling
15082 routines generate output or allocate memory).
15084 @item -finstrument-functions-exclude-file-list=@var{file},@var{file},@dots{}
15085 @opindex finstrument-functions-exclude-file-list
15087 Set the list of functions that are excluded from instrumentation (see
15088 the description of @code{-finstrument-functions}). If the file that
15089 contains a function definition matches with one of @var{file}, then
15090 that function is not instrumented. The match is done on substrings:
15091 if the @var{file} parameter is a substring of the file name, it is
15092 considered to be a match.
15095 @code{-finstrument-functions-exclude-file-list=/bits/stl,include/sys}
15096 will exclude any inline function defined in files whose pathnames
15097 contain @code{/bits/stl} or @code{include/sys}.
15099 If, for some reason, you want to include letter @code{','} in one of
15100 @var{sym}, write @code{'\,'}. For example,
15101 @code{-finstrument-functions-exclude-file-list='\,\,tmp'}
15102 (note the single quote surrounding the option).
15104 @item -finstrument-functions-exclude-function-list=@var{sym},@var{sym},@dots{}
15105 @opindex finstrument-functions-exclude-function-list
15107 This is similar to @code{-finstrument-functions-exclude-file-list},
15108 but this option sets the list of function names to be excluded from
15109 instrumentation. The function name to be matched is its user-visible
15110 name, such as @code{vector<int> blah(const vector<int> &)}, not the
15111 internal mangled name (e.g., @code{_Z4blahRSt6vectorIiSaIiEE}). The
15112 match is done on substrings: if the @var{sym} parameter is a substring
15113 of the function name, it is considered to be a match.
15115 @item -fstack-check
15116 @opindex fstack-check
15117 Generate code to verify that you do not go beyond the boundary of the
15118 stack. You should specify this flag if you are running in an
15119 environment with multiple threads, but only rarely need to specify it in
15120 a single-threaded environment since stack overflow is automatically
15121 detected on nearly all systems if there is only one stack.
15123 Note that this switch does not actually cause checking to be done; the
15124 operating system must do that. The switch causes generation of code
15125 to ensure that the operating system sees the stack being extended.
15127 @item -fstack-limit-register=@var{reg}
15128 @itemx -fstack-limit-symbol=@var{sym}
15129 @itemx -fno-stack-limit
15130 @opindex fstack-limit-register
15131 @opindex fstack-limit-symbol
15132 @opindex fno-stack-limit
15133 Generate code to ensure that the stack does not grow beyond a certain value,
15134 either the value of a register or the address of a symbol. If the stack
15135 would grow beyond the value, a signal is raised. For most targets,
15136 the signal is raised before the stack overruns the boundary, so
15137 it is possible to catch the signal without taking special precautions.
15139 For instance, if the stack starts at absolute address @samp{0x80000000}
15140 and grows downwards, you can use the flags
15141 @option{-fstack-limit-symbol=__stack_limit} and
15142 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
15143 of 128KB@. Note that this may only work with the GNU linker.
15145 @cindex aliasing of parameters
15146 @cindex parameters, aliased
15147 @item -fargument-alias
15148 @itemx -fargument-noalias
15149 @itemx -fargument-noalias-global
15150 @itemx -fargument-noalias-anything
15151 @opindex fargument-alias
15152 @opindex fargument-noalias
15153 @opindex fargument-noalias-global
15154 @opindex fargument-noalias-anything
15155 Specify the possible relationships among parameters and between
15156 parameters and global data.
15158 @option{-fargument-alias} specifies that arguments (parameters) may
15159 alias each other and may alias global storage.@*
15160 @option{-fargument-noalias} specifies that arguments do not alias
15161 each other, but may alias global storage.@*
15162 @option{-fargument-noalias-global} specifies that arguments do not
15163 alias each other and do not alias global storage.
15164 @option{-fargument-noalias-anything} specifies that arguments do not
15165 alias any other storage.
15167 Each language will automatically use whatever option is required by
15168 the language standard. You should not need to use these options yourself.
15170 @item -fleading-underscore
15171 @opindex fleading-underscore
15172 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
15173 change the way C symbols are represented in the object file. One use
15174 is to help link with legacy assembly code.
15176 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
15177 generate code that is not binary compatible with code generated without that
15178 switch. Use it to conform to a non-default application binary interface.
15179 Not all targets provide complete support for this switch.
15181 @item -ftls-model=@var{model}
15182 @opindex ftls-model
15183 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
15184 The @var{model} argument should be one of @code{global-dynamic},
15185 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
15187 The default without @option{-fpic} is @code{initial-exec}; with
15188 @option{-fpic} the default is @code{global-dynamic}.
15190 @item -fvisibility=@var{default|internal|hidden|protected}
15191 @opindex fvisibility
15192 Set the default ELF image symbol visibility to the specified option---all
15193 symbols will be marked with this unless overridden within the code.
15194 Using this feature can very substantially improve linking and
15195 load times of shared object libraries, produce more optimized
15196 code, provide near-perfect API export and prevent symbol clashes.
15197 It is @strong{strongly} recommended that you use this in any shared objects
15200 Despite the nomenclature, @code{default} always means public ie;
15201 available to be linked against from outside the shared object.
15202 @code{protected} and @code{internal} are pretty useless in real-world
15203 usage so the only other commonly used option will be @code{hidden}.
15204 The default if @option{-fvisibility} isn't specified is
15205 @code{default}, i.e., make every
15206 symbol public---this causes the same behavior as previous versions of
15209 A good explanation of the benefits offered by ensuring ELF
15210 symbols have the correct visibility is given by ``How To Write
15211 Shared Libraries'' by Ulrich Drepper (which can be found at
15212 @w{@uref{http://people.redhat.com/~drepper/}})---however a superior
15213 solution made possible by this option to marking things hidden when
15214 the default is public is to make the default hidden and mark things
15215 public. This is the norm with DLL's on Windows and with @option{-fvisibility=hidden}
15216 and @code{__attribute__ ((visibility("default")))} instead of
15217 @code{__declspec(dllexport)} you get almost identical semantics with
15218 identical syntax. This is a great boon to those working with
15219 cross-platform projects.
15221 For those adding visibility support to existing code, you may find
15222 @samp{#pragma GCC visibility} of use. This works by you enclosing
15223 the declarations you wish to set visibility for with (for example)
15224 @samp{#pragma GCC visibility push(hidden)} and
15225 @samp{#pragma GCC visibility pop}.
15226 Bear in mind that symbol visibility should be viewed @strong{as
15227 part of the API interface contract} and thus all new code should
15228 always specify visibility when it is not the default ie; declarations
15229 only for use within the local DSO should @strong{always} be marked explicitly
15230 as hidden as so to avoid PLT indirection overheads---making this
15231 abundantly clear also aids readability and self-documentation of the code.
15232 Note that due to ISO C++ specification requirements, operator new and
15233 operator delete must always be of default visibility.
15235 Be aware that headers from outside your project, in particular system
15236 headers and headers from any other library you use, may not be
15237 expecting to be compiled with visibility other than the default. You
15238 may need to explicitly say @samp{#pragma GCC visibility push(default)}
15239 before including any such headers.
15241 @samp{extern} declarations are not affected by @samp{-fvisibility}, so
15242 a lot of code can be recompiled with @samp{-fvisibility=hidden} with
15243 no modifications. However, this means that calls to @samp{extern}
15244 functions with no explicit visibility will use the PLT, so it is more
15245 effective to use @samp{__attribute ((visibility))} and/or
15246 @samp{#pragma GCC visibility} to tell the compiler which @samp{extern}
15247 declarations should be treated as hidden.
15249 Note that @samp{-fvisibility} does affect C++ vague linkage
15250 entities. This means that, for instance, an exception class that will
15251 be thrown between DSOs must be explicitly marked with default
15252 visibility so that the @samp{type_info} nodes will be unified between
15255 An overview of these techniques, their benefits and how to use them
15256 is at @w{@uref{http://gcc.gnu.org/wiki/Visibility}}.
15262 @node Environment Variables
15263 @section Environment Variables Affecting GCC
15264 @cindex environment variables
15266 @c man begin ENVIRONMENT
15267 This section describes several environment variables that affect how GCC
15268 operates. Some of them work by specifying directories or prefixes to use
15269 when searching for various kinds of files. Some are used to specify other
15270 aspects of the compilation environment.
15272 Note that you can also specify places to search using options such as
15273 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
15274 take precedence over places specified using environment variables, which
15275 in turn take precedence over those specified by the configuration of GCC@.
15276 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
15277 GNU Compiler Collection (GCC) Internals}.
15282 @c @itemx LC_COLLATE
15284 @c @itemx LC_MONETARY
15285 @c @itemx LC_NUMERIC
15290 @c @findex LC_COLLATE
15291 @findex LC_MESSAGES
15292 @c @findex LC_MONETARY
15293 @c @findex LC_NUMERIC
15297 These environment variables control the way that GCC uses
15298 localization information that allow GCC to work with different
15299 national conventions. GCC inspects the locale categories
15300 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
15301 so. These locale categories can be set to any value supported by your
15302 installation. A typical value is @samp{en_GB.UTF-8} for English in the United
15303 Kingdom encoded in UTF-8.
15305 The @env{LC_CTYPE} environment variable specifies character
15306 classification. GCC uses it to determine the character boundaries in
15307 a string; this is needed for some multibyte encodings that contain quote
15308 and escape characters that would otherwise be interpreted as a string
15311 The @env{LC_MESSAGES} environment variable specifies the language to
15312 use in diagnostic messages.
15314 If the @env{LC_ALL} environment variable is set, it overrides the value
15315 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
15316 and @env{LC_MESSAGES} default to the value of the @env{LANG}
15317 environment variable. If none of these variables are set, GCC
15318 defaults to traditional C English behavior.
15322 If @env{TMPDIR} is set, it specifies the directory to use for temporary
15323 files. GCC uses temporary files to hold the output of one stage of
15324 compilation which is to be used as input to the next stage: for example,
15325 the output of the preprocessor, which is the input to the compiler
15328 @item GCC_EXEC_PREFIX
15329 @findex GCC_EXEC_PREFIX
15330 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
15331 names of the subprograms executed by the compiler. No slash is added
15332 when this prefix is combined with the name of a subprogram, but you can
15333 specify a prefix that ends with a slash if you wish.
15335 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
15336 an appropriate prefix to use based on the pathname it was invoked with.
15338 If GCC cannot find the subprogram using the specified prefix, it
15339 tries looking in the usual places for the subprogram.
15341 The default value of @env{GCC_EXEC_PREFIX} is
15342 @file{@var{prefix}/lib/gcc/} where @var{prefix} is the prefix to
15343 the installed compiler. In many cases @var{prefix} is the value
15344 of @code{prefix} when you ran the @file{configure} script.
15346 Other prefixes specified with @option{-B} take precedence over this prefix.
15348 This prefix is also used for finding files such as @file{crt0.o} that are
15351 In addition, the prefix is used in an unusual way in finding the
15352 directories to search for header files. For each of the standard
15353 directories whose name normally begins with @samp{/usr/local/lib/gcc}
15354 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
15355 replacing that beginning with the specified prefix to produce an
15356 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
15357 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
15358 These alternate directories are searched first; the standard directories
15359 come next. If a standard directory begins with the configured
15360 @var{prefix} then the value of @var{prefix} is replaced by
15361 @env{GCC_EXEC_PREFIX} when looking for header files.
15363 @item COMPILER_PATH
15364 @findex COMPILER_PATH
15365 The value of @env{COMPILER_PATH} is a colon-separated list of
15366 directories, much like @env{PATH}. GCC tries the directories thus
15367 specified when searching for subprograms, if it can't find the
15368 subprograms using @env{GCC_EXEC_PREFIX}.
15371 @findex LIBRARY_PATH
15372 The value of @env{LIBRARY_PATH} is a colon-separated list of
15373 directories, much like @env{PATH}. When configured as a native compiler,
15374 GCC tries the directories thus specified when searching for special
15375 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
15376 using GCC also uses these directories when searching for ordinary
15377 libraries for the @option{-l} option (but directories specified with
15378 @option{-L} come first).
15382 @cindex locale definition
15383 This variable is used to pass locale information to the compiler. One way in
15384 which this information is used is to determine the character set to be used
15385 when character literals, string literals and comments are parsed in C and C++.
15386 When the compiler is configured to allow multibyte characters,
15387 the following values for @env{LANG} are recognized:
15391 Recognize JIS characters.
15393 Recognize SJIS characters.
15395 Recognize EUCJP characters.
15398 If @env{LANG} is not defined, or if it has some other value, then the
15399 compiler will use mblen and mbtowc as defined by the default locale to
15400 recognize and translate multibyte characters.
15404 Some additional environments variables affect the behavior of the
15407 @include cppenv.texi
15411 @node Precompiled Headers
15412 @section Using Precompiled Headers
15413 @cindex precompiled headers
15414 @cindex speed of compilation
15416 Often large projects have many header files that are included in every
15417 source file. The time the compiler takes to process these header files
15418 over and over again can account for nearly all of the time required to
15419 build the project. To make builds faster, GCC allows users to
15420 `precompile' a header file; then, if builds can use the precompiled
15421 header file they will be much faster.
15423 To create a precompiled header file, simply compile it as you would any
15424 other file, if necessary using the @option{-x} option to make the driver
15425 treat it as a C or C++ header file. You will probably want to use a
15426 tool like @command{make} to keep the precompiled header up-to-date when
15427 the headers it contains change.
15429 A precompiled header file will be searched for when @code{#include} is
15430 seen in the compilation. As it searches for the included file
15431 (@pxref{Search Path,,Search Path,cpp,The C Preprocessor}) the
15432 compiler looks for a precompiled header in each directory just before it
15433 looks for the include file in that directory. The name searched for is
15434 the name specified in the @code{#include} with @samp{.gch} appended. If
15435 the precompiled header file can't be used, it is ignored.
15437 For instance, if you have @code{#include "all.h"}, and you have
15438 @file{all.h.gch} in the same directory as @file{all.h}, then the
15439 precompiled header file will be used if possible, and the original
15440 header will be used otherwise.
15442 Alternatively, you might decide to put the precompiled header file in a
15443 directory and use @option{-I} to ensure that directory is searched
15444 before (or instead of) the directory containing the original header.
15445 Then, if you want to check that the precompiled header file is always
15446 used, you can put a file of the same name as the original header in this
15447 directory containing an @code{#error} command.
15449 This also works with @option{-include}. So yet another way to use
15450 precompiled headers, good for projects not designed with precompiled
15451 header files in mind, is to simply take most of the header files used by
15452 a project, include them from another header file, precompile that header
15453 file, and @option{-include} the precompiled header. If the header files
15454 have guards against multiple inclusion, they will be skipped because
15455 they've already been included (in the precompiled header).
15457 If you need to precompile the same header file for different
15458 languages, targets, or compiler options, you can instead make a
15459 @emph{directory} named like @file{all.h.gch}, and put each precompiled
15460 header in the directory, perhaps using @option{-o}. It doesn't matter
15461 what you call the files in the directory, every precompiled header in
15462 the directory will be considered. The first precompiled header
15463 encountered in the directory that is valid for this compilation will
15464 be used; they're searched in no particular order.
15466 There are many other possibilities, limited only by your imagination,
15467 good sense, and the constraints of your build system.
15469 A precompiled header file can be used only when these conditions apply:
15473 Only one precompiled header can be used in a particular compilation.
15476 A precompiled header can't be used once the first C token is seen. You
15477 can have preprocessor directives before a precompiled header; you can
15478 even include a precompiled header from inside another header, so long as
15479 there are no C tokens before the @code{#include}.
15482 The precompiled header file must be produced for the same language as
15483 the current compilation. You can't use a C precompiled header for a C++
15487 The precompiled header file must have been produced by the same compiler
15488 binary as the current compilation is using.
15491 Any macros defined before the precompiled header is included must
15492 either be defined in the same way as when the precompiled header was
15493 generated, or must not affect the precompiled header, which usually
15494 means that they don't appear in the precompiled header at all.
15496 The @option{-D} option is one way to define a macro before a
15497 precompiled header is included; using a @code{#define} can also do it.
15498 There are also some options that define macros implicitly, like
15499 @option{-O} and @option{-Wdeprecated}; the same rule applies to macros
15502 @item If debugging information is output when using the precompiled
15503 header, using @option{-g} or similar, the same kind of debugging information
15504 must have been output when building the precompiled header. However,
15505 a precompiled header built using @option{-g} can be used in a compilation
15506 when no debugging information is being output.
15508 @item The same @option{-m} options must generally be used when building
15509 and using the precompiled header. @xref{Submodel Options},
15510 for any cases where this rule is relaxed.
15512 @item Each of the following options must be the same when building and using
15513 the precompiled header:
15515 @gccoptlist{-fexceptions -funit-at-a-time}
15518 Some other command-line options starting with @option{-f},
15519 @option{-p}, or @option{-O} must be defined in the same way as when
15520 the precompiled header was generated. At present, it's not clear
15521 which options are safe to change and which are not; the safest choice
15522 is to use exactly the same options when generating and using the
15523 precompiled header. The following are known to be safe:
15525 @gccoptlist{-fmessage-length= -fpreprocessed -fsched-interblock @gol
15526 -fsched-spec -fsched-spec-load -fsched-spec-load-dangerous @gol
15527 -fsched-verbose=<number> -fschedule-insns -fvisibility= @gol
15532 For all of these except the last, the compiler will automatically
15533 ignore the precompiled header if the conditions aren't met. If you
15534 find an option combination that doesn't work and doesn't cause the
15535 precompiled header to be ignored, please consider filing a bug report,
15538 If you do use differing options when generating and using the
15539 precompiled header, the actual behavior will be a mixture of the
15540 behavior for the options. For instance, if you use @option{-g} to
15541 generate the precompiled header but not when using it, you may or may
15542 not get debugging information for routines in the precompiled header.
15544 @node Running Protoize
15545 @section Running Protoize
15547 The program @code{protoize} is an optional part of GCC@. You can use
15548 it to add prototypes to a program, thus converting the program to ISO
15549 C in one respect. The companion program @code{unprotoize} does the
15550 reverse: it removes argument types from any prototypes that are found.
15552 When you run these programs, you must specify a set of source files as
15553 command line arguments. The conversion programs start out by compiling
15554 these files to see what functions they define. The information gathered
15555 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
15557 After scanning comes actual conversion. The specified files are all
15558 eligible to be converted; any files they include (whether sources or
15559 just headers) are eligible as well.
15561 But not all the eligible files are converted. By default,
15562 @code{protoize} and @code{unprotoize} convert only source and header
15563 files in the current directory. You can specify additional directories
15564 whose files should be converted with the @option{-d @var{directory}}
15565 option. You can also specify particular files to exclude with the
15566 @option{-x @var{file}} option. A file is converted if it is eligible, its
15567 directory name matches one of the specified directory names, and its
15568 name within the directory has not been excluded.
15570 Basic conversion with @code{protoize} consists of rewriting most
15571 function definitions and function declarations to specify the types of
15572 the arguments. The only ones not rewritten are those for varargs
15575 @code{protoize} optionally inserts prototype declarations at the
15576 beginning of the source file, to make them available for any calls that
15577 precede the function's definition. Or it can insert prototype
15578 declarations with block scope in the blocks where undeclared functions
15581 Basic conversion with @code{unprotoize} consists of rewriting most
15582 function declarations to remove any argument types, and rewriting
15583 function definitions to the old-style pre-ISO form.
15585 Both conversion programs print a warning for any function declaration or
15586 definition that they can't convert. You can suppress these warnings
15589 The output from @code{protoize} or @code{unprotoize} replaces the
15590 original source file. The original file is renamed to a name ending
15591 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
15592 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
15593 for DOS) file already exists, then the source file is simply discarded.
15595 @code{protoize} and @code{unprotoize} both depend on GCC itself to
15596 scan the program and collect information about the functions it uses.
15597 So neither of these programs will work until GCC is installed.
15599 Here is a table of the options you can use with @code{protoize} and
15600 @code{unprotoize}. Each option works with both programs unless
15604 @item -B @var{directory}
15605 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
15606 usual directory (normally @file{/usr/local/lib}). This file contains
15607 prototype information about standard system functions. This option
15608 applies only to @code{protoize}.
15610 @item -c @var{compilation-options}
15611 Use @var{compilation-options} as the options when running @command{gcc} to
15612 produce the @samp{.X} files. The special option @option{-aux-info} is
15613 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
15615 Note that the compilation options must be given as a single argument to
15616 @code{protoize} or @code{unprotoize}. If you want to specify several
15617 @command{gcc} options, you must quote the entire set of compilation options
15618 to make them a single word in the shell.
15620 There are certain @command{gcc} arguments that you cannot use, because they
15621 would produce the wrong kind of output. These include @option{-g},
15622 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
15623 the @var{compilation-options}, they are ignored.
15626 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
15627 systems) instead of @samp{.c}. This is convenient if you are converting
15628 a C program to C++. This option applies only to @code{protoize}.
15631 Add explicit global declarations. This means inserting explicit
15632 declarations at the beginning of each source file for each function
15633 that is called in the file and was not declared. These declarations
15634 precede the first function definition that contains a call to an
15635 undeclared function. This option applies only to @code{protoize}.
15637 @item -i @var{string}
15638 Indent old-style parameter declarations with the string @var{string}.
15639 This option applies only to @code{protoize}.
15641 @code{unprotoize} converts prototyped function definitions to old-style
15642 function definitions, where the arguments are declared between the
15643 argument list and the initial @samp{@{}. By default, @code{unprotoize}
15644 uses five spaces as the indentation. If you want to indent with just
15645 one space instead, use @option{-i " "}.
15648 Keep the @samp{.X} files. Normally, they are deleted after conversion
15652 Add explicit local declarations. @code{protoize} with @option{-l} inserts
15653 a prototype declaration for each function in each block which calls the
15654 function without any declaration. This option applies only to
15658 Make no real changes. This mode just prints information about the conversions
15659 that would have been done without @option{-n}.
15662 Make no @samp{.save} files. The original files are simply deleted.
15663 Use this option with caution.
15665 @item -p @var{program}
15666 Use the program @var{program} as the compiler. Normally, the name
15667 @file{gcc} is used.
15670 Work quietly. Most warnings are suppressed.
15673 Print the version number, just like @option{-v} for @command{gcc}.
15676 If you need special compiler options to compile one of your program's
15677 source files, then you should generate that file's @samp{.X} file
15678 specially, by running @command{gcc} on that source file with the
15679 appropriate options and the option @option{-aux-info}. Then run
15680 @code{protoize} on the entire set of files. @code{protoize} will use
15681 the existing @samp{.X} file because it is newer than the source file.
15685 gcc -Dfoo=bar file1.c -aux-info file1.X
15690 You need to include the special files along with the rest in the
15691 @code{protoize} command, even though their @samp{.X} files already
15692 exist, because otherwise they won't get converted.
15694 @xref{Protoize Caveats}, for more information on how to use
15695 @code{protoize} successfully.