1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2 @c 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
3 @c This is part of the GCC manual.
4 @c For copying conditions, see the file gcc.texi.
11 @c man begin COPYRIGHT
12 Copyright @copyright{} 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
13 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
15 Permission is granted to copy, distribute and/or modify this document
16 under the terms of the GNU Free Documentation License, Version 1.2 or
17 any later version published by the Free Software Foundation; with the
18 Invariant Sections being ``GNU General Public License'' and ``Funding
19 Free Software'', the Front-Cover texts being (a) (see below), and with
20 the Back-Cover Texts being (b) (see below). A copy of the license is
21 included in the gfdl(7) man page.
23 (a) The FSF's Front-Cover Text is:
27 (b) The FSF's Back-Cover Text is:
29 You have freedom to copy and modify this GNU Manual, like GNU
30 software. Copies published by the Free Software Foundation raise
31 funds for GNU development.
33 @c Set file name and title for the man page.
35 @settitle GNU project C and C++ compiler
37 gcc [@option{-c}|@option{-S}|@option{-E}] [@option{-std=}@var{standard}]
38 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
39 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
40 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
41 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
42 [@option{-f}@var{option}@dots{}] [@option{-m}@var{machine-option}@dots{}]
43 [@option{-o} @var{outfile}] [@@@var{file}] @var{infile}@dots{}
45 Only the most useful options are listed here; see below for the
46 remainder. @samp{g++} accepts mostly the same options as @samp{gcc}.
49 gpl(7), gfdl(7), fsf-funding(7),
50 cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
51 and the Info entries for @file{gcc}, @file{cpp}, @file{as},
52 @file{ld}, @file{binutils} and @file{gdb}.
55 For instructions on reporting bugs, see
56 @w{@uref{http://gcc.gnu.org/bugs.html}}.
59 See the Info entry for @command{gcc}, or
60 @w{@uref{http://gcc.gnu.org/onlinedocs/gcc/Contributors.html}},
61 for contributors to GCC@.
66 @chapter GCC Command Options
67 @cindex GCC command options
68 @cindex command options
69 @cindex options, GCC command
71 @c man begin DESCRIPTION
72 When you invoke GCC, it normally does preprocessing, compilation,
73 assembly and linking. The ``overall options'' allow you to stop this
74 process at an intermediate stage. For example, the @option{-c} option
75 says not to run the linker. Then the output consists of object files
76 output by the assembler.
78 Other options are passed on to one stage of processing. Some options
79 control the preprocessor and others the compiler itself. Yet other
80 options control the assembler and linker; most of these are not
81 documented here, since you rarely need to use any of them.
83 @cindex C compilation options
84 Most of the command line options that you can use with GCC are useful
85 for C programs; when an option is only useful with another language
86 (usually C++), the explanation says so explicitly. If the description
87 for a particular option does not mention a source language, you can use
88 that option with all supported languages.
90 @cindex C++ compilation options
91 @xref{Invoking G++,,Compiling C++ Programs}, for a summary of special
92 options for compiling C++ programs.
94 @cindex grouping options
95 @cindex options, grouping
96 The @command{gcc} program accepts options and file names as operands. Many
97 options have multi-letter names; therefore multiple single-letter options
98 may @emph{not} be grouped: @option{-dr} is very different from @w{@samp{-d
101 @cindex order of options
102 @cindex options, order
103 You can mix options and other arguments. For the most part, the order
104 you use doesn't matter. Order does matter when you use several options
105 of the same kind; for example, if you specify @option{-L} more than once,
106 the directories are searched in the order specified.
108 Many options have long names starting with @samp{-f} or with
109 @samp{-W}---for example,
110 @option{-fmove-loop-invariants}, @option{-Wformat} and so on. Most of
111 these have both positive and negative forms; the negative form of
112 @option{-ffoo} would be @option{-fno-foo}. This manual documents
113 only one of these two forms, whichever one is not the default.
117 @xref{Option Index}, for an index to GCC's options.
120 * Option Summary:: Brief list of all options, without explanations.
121 * Overall Options:: Controlling the kind of output:
122 an executable, object files, assembler files,
123 or preprocessed source.
124 * Invoking G++:: Compiling C++ programs.
125 * C Dialect Options:: Controlling the variant of C language compiled.
126 * C++ Dialect Options:: Variations on C++.
127 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
129 * Language Independent Options:: Controlling how diagnostics should be
131 * Warning Options:: How picky should the compiler be?
132 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
133 * Optimize Options:: How much optimization?
134 * Preprocessor Options:: Controlling header files and macro definitions.
135 Also, getting dependency information for Make.
136 * Assembler Options:: Passing options to the assembler.
137 * Link Options:: Specifying libraries and so on.
138 * Directory Options:: Where to find header files and libraries.
139 Where to find the compiler executable files.
140 * Spec Files:: How to pass switches to sub-processes.
141 * Target Options:: Running a cross-compiler, or an old version of GCC.
142 * Submodel Options:: Specifying minor hardware or convention variations,
143 such as 68010 vs 68020.
144 * Code Gen Options:: Specifying conventions for function calls, data layout
146 * Environment Variables:: Env vars that affect GCC.
147 * Precompiled Headers:: Compiling a header once, and using it many times.
148 * Running Protoize:: Automatically adding or removing function prototypes.
154 @section Option Summary
156 Here is a summary of all the options, grouped by type. Explanations are
157 in the following sections.
160 @item Overall Options
161 @xref{Overall Options,,Options Controlling the Kind of Output}.
162 @gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
163 -x @var{language} -v -### --help --target-help --version @@@var{file}}
165 @item C Language Options
166 @xref{C Dialect Options,,Options Controlling C Dialect}.
167 @gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
168 -fno-asm -fno-builtin -fno-builtin-@var{function} @gol
169 -fhosted -ffreestanding -fopenmp -fms-extensions @gol
170 -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
171 -fallow-single-precision -fcond-mismatch @gol
172 -fsigned-bitfields -fsigned-char @gol
173 -funsigned-bitfields -funsigned-char}
175 @item C++ Language Options
176 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
177 @gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
178 -fconserve-space -ffriend-injection @gol
179 -fno-elide-constructors @gol
180 -fno-enforce-eh-specs @gol
181 -ffor-scope -fno-for-scope -fno-gnu-keywords @gol
182 -fno-implicit-templates @gol
183 -fno-implicit-inline-templates @gol
184 -fno-implement-inlines -fms-extensions @gol
185 -fno-nonansi-builtins -fno-operator-names @gol
186 -fno-optional-diags -fpermissive @gol
187 -frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
188 -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ @gol
189 -fno-default-inline -fvisibility-inlines-hidden @gol
190 -Wabi -Wctor-dtor-privacy @gol
191 -Wnon-virtual-dtor -Wreorder @gol
192 -Weffc++ -Wno-deprecated -Wstrict-null-sentinel @gol
193 -Wno-non-template-friend -Wold-style-cast @gol
194 -Woverloaded-virtual -Wno-pmf-conversions @gol
197 @item Objective-C and Objective-C++ Language Options
198 @xref{Objective-C and Objective-C++ Dialect Options,,Options Controlling
199 Objective-C and Objective-C++ Dialects}.
201 -fconstant-string-class=@var{class-name} @gol
202 -fgnu-runtime -fnext-runtime @gol
203 -fno-nil-receivers @gol
204 -fobjc-call-cxx-cdtors @gol
205 -fobjc-direct-dispatch @gol
206 -fobjc-exceptions @gol
208 -freplace-objc-classes @gol
211 -Wassign-intercept @gol
212 -Wno-protocol -Wselector @gol
213 -Wstrict-selector-match @gol
214 -Wundeclared-selector}
216 @item Language Independent Options
217 @xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
218 @gccoptlist{-fmessage-length=@var{n} @gol
219 -fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]}} @gol
220 -fdiagnostics-show-option
222 @item Warning Options
223 @xref{Warning Options,,Options to Request or Suppress Warnings}.
224 @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
225 -w -Wextra -Wall -Waggregate-return -Walways-true -Wno-attributes @gol
226 -Wc++-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment @gol
227 -Wconversion -Wno-deprecated-declarations @gol
228 -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels @gol
229 -Werror -Werror-* -Werror-implicit-function-declaration @gol
230 -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol
231 -Wno-format-extra-args -Wformat-nonliteral @gol
232 -Wformat-security -Wformat-y2k @gol
233 -Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
234 -Wimport -Wno-import -Winit-self -Winline @gol
235 -Wno-int-to-pointer-cast @gol
236 -Wno-invalid-offsetof -Winvalid-pch @gol
237 -Wlarger-than-@var{len} -Wunsafe-loop-optimizations -Wlong-long @gol
238 -Wmain -Wmissing-braces -Wmissing-field-initializers @gol
239 -Wmissing-format-attribute -Wmissing-include-dirs @gol
240 -Wmissing-noreturn @gol
241 -Wno-multichar -Wnonnull -Wno-overflow @gol
242 -Woverlength-strings -Wpacked -Wpadded @gol
243 -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast @gol
244 -Wredundant-decls @gol
245 -Wreturn-type -Wsequence-point -Wshadow @gol
246 -Wsign-compare -Wstack-protector @gol
247 -Wstrict-aliasing -Wstrict-aliasing=2 @gol
248 -Wstring-literal-comparison @gol
249 -Wswitch -Wswitch-default -Wswitch-enum @gol
250 -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
251 -Wunknown-pragmas -Wno-pragmas -Wunreachable-code @gol
252 -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
253 -Wunused-value -Wunused-variable -Wvariadic-macros @gol
254 -Wvolatile-register-var -Wwrite-strings}
256 @item C-only Warning Options
257 @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
258 -Wmissing-prototypes -Wnested-externs -Wold-style-definition @gol
259 -Wstrict-prototypes -Wtraditional @gol
260 -Wdeclaration-after-statement -Wpointer-sign}
262 @item Debugging Options
263 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
264 @gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
265 -fdump-noaddr -fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
266 -fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
267 -fdump-ipa-all -fdump-ipa-cgraph @gol
269 -fdump-tree-original@r{[}-@var{n}@r{]} @gol
270 -fdump-tree-optimized@r{[}-@var{n}@r{]} @gol
271 -fdump-tree-inlined@r{[}-@var{n}@r{]} @gol
272 -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias @gol
274 -fdump-tree-ssa@r{[}-@var{n}@r{]} -fdump-tree-pre@r{[}-@var{n}@r{]} @gol
275 -fdump-tree-ccp@r{[}-@var{n}@r{]} -fdump-tree-dce@r{[}-@var{n}@r{]} @gol
276 -fdump-tree-gimple@r{[}-raw@r{]} -fdump-tree-mudflap@r{[}-@var{n}@r{]} @gol
277 -fdump-tree-dom@r{[}-@var{n}@r{]} @gol
278 -fdump-tree-dse@r{[}-@var{n}@r{]} @gol
279 -fdump-tree-phiopt@r{[}-@var{n}@r{]} @gol
280 -fdump-tree-forwprop@r{[}-@var{n}@r{]} @gol
281 -fdump-tree-copyrename@r{[}-@var{n}@r{]} @gol
282 -fdump-tree-nrv -fdump-tree-vect @gol
283 -fdump-tree-sink @gol
284 -fdump-tree-sra@r{[}-@var{n}@r{]} @gol
285 -fdump-tree-salias @gol
286 -fdump-tree-fre@r{[}-@var{n}@r{]} @gol
287 -fdump-tree-vrp@r{[}-@var{n}@r{]} @gol
288 -ftree-vectorizer-verbose=@var{n} @gol
289 -fdump-tree-storeccp@r{[}-@var{n}@r{]} @gol
290 -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
291 -feliminate-unused-debug-symbols -femit-class-debug-always @gol
292 -fmem-report -fprofile-arcs @gol
293 -frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
294 -ftest-coverage -ftime-report -fvar-tracking @gol
295 -g -g@var{level} -gcoff -gdwarf-2 @gol
296 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
297 -p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
298 -print-multi-directory -print-multi-lib @gol
299 -print-prog-name=@var{program} -print-search-dirs -Q @gol
302 @item Optimization Options
303 @xref{Optimize Options,,Options that Control Optimization}.
304 @gccoptlist{-falign-functions=@var{n} -falign-jumps=@var{n} @gol
305 -falign-labels=@var{n} -falign-loops=@var{n} @gol
306 -fbounds-check -fmudflap -fmudflapth -fmudflapir @gol
307 -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize @gol
308 -fbranch-target-load-optimize2 -fbtr-bb-exclusive @gol
309 -fcaller-saves -fcprop-registers -fcse-follow-jumps @gol
310 -fcse-skip-blocks -fcx-limited-range -fdata-sections @gol
311 -fdelayed-branch -fdelete-null-pointer-checks -fearly-inlining @gol
312 -fexpensive-optimizations -ffast-math -ffloat-store @gol
313 -fforce-addr -ffunction-sections @gol
314 -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload @gol
315 -fcrossjumping -fif-conversion -fif-conversion2 @gol
316 -finline-functions -finline-functions-called-once @gol
317 -finline-limit=@var{n} -fkeep-inline-functions @gol
318 -fkeep-static-consts -fmerge-constants -fmerge-all-constants @gol
319 -fmodulo-sched -fno-branch-count-reg @gol
320 -fno-default-inline -fno-defer-pop -fmove-loop-invariants @gol
321 -fno-function-cse -fno-guess-branch-probability @gol
322 -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
323 -funsafe-math-optimizations -funsafe-loop-optimizations -ffinite-math-only @gol
324 -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss @gol
325 -fomit-frame-pointer -foptimize-register-move @gol
326 -foptimize-sibling-calls -fprefetch-loop-arrays @gol
327 -fprofile-generate -fprofile-use @gol
328 -fregmove -frename-registers @gol
329 -freorder-blocks -freorder-blocks-and-partition -freorder-functions @gol
330 -frerun-cse-after-loop @gol
331 -frounding-math -frtl-abstract-sequences @gol
332 -fschedule-insns -fschedule-insns2 @gol
333 -fno-sched-interblock -fno-sched-spec -fsched-spec-load @gol
334 -fsched-spec-load-dangerous @gol
335 -fsched-stalled-insns=@var{n} -fsched-stalled-insns-dep=@var{n} @gol
336 -fsched2-use-superblocks @gol
337 -fsched2-use-traces -fsee -freschedule-modulo-scheduled-loops @gol
338 -fsection-anchors -fsignaling-nans -fsingle-precision-constant @gol
339 -fstack-protector -fstack-protector-all @gol
340 -fstrict-aliasing -ftracer -fthread-jumps @gol
341 -funroll-all-loops -funroll-loops -fpeel-loops @gol
342 -fsplit-ivs-in-unroller -funswitch-loops @gol
343 -fvariable-expansion-in-unroller @gol
344 -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize @gol
345 -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts @gol
346 -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-sink @gol
347 -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize @gol
348 -ftree-vect-loop-version -ftree-salias -fipa-pta -fweb @gol
349 -ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop -fwhole-program @gol
350 --param @var{name}=@var{value}
351 -O -O0 -O1 -O2 -O3 -Os}
353 @item Preprocessor Options
354 @xref{Preprocessor Options,,Options Controlling the Preprocessor}.
355 @gccoptlist{-A@var{question}=@var{answer} @gol
356 -A-@var{question}@r{[}=@var{answer}@r{]} @gol
357 -C -dD -dI -dM -dN @gol
358 -D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
359 -idirafter @var{dir} @gol
360 -include @var{file} -imacros @var{file} @gol
361 -iprefix @var{file} -iwithprefix @var{dir} @gol
362 -iwithprefixbefore @var{dir} -isystem @var{dir} @gol
363 -imultilib @var{dir} -isysroot @var{dir} @gol
364 -M -MM -MF -MG -MP -MQ -MT -nostdinc @gol
365 -P -fworking-directory -remap @gol
366 -trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
367 -Xpreprocessor @var{option}}
369 @item Assembler Option
370 @xref{Assembler Options,,Passing Options to the Assembler}.
371 @gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
374 @xref{Link Options,,Options for Linking}.
375 @gccoptlist{@var{object-file-name} -l@var{library} @gol
376 -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic @gol
377 -s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
378 -Wl,@var{option} -Xlinker @var{option} @gol
381 @item Directory Options
382 @xref{Directory Options,,Options for Directory Search}.
383 @gccoptlist{-B@var{prefix} -I@var{dir} -iquote@var{dir} -L@var{dir}
384 -specs=@var{file} -I- --sysroot=@var{dir}}
387 @c I wrote this xref this way to avoid overfull hbox. -- rms
388 @xref{Target Options}.
389 @gccoptlist{-V @var{version} -b @var{machine}}
391 @item Machine Dependent Options
392 @xref{Submodel Options,,Hardware Models and Configurations}.
393 @c This list is ordered alphanumerically by subsection name.
394 @c Try and put the significant identifier (CPU or system) first,
395 @c so users have a clue at guessing where the ones they want will be.
398 @gccoptlist{-EB -EL @gol
399 -mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
400 -mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
403 @gccoptlist{-mapcs-frame -mno-apcs-frame @gol
404 -mabi=@var{name} @gol
405 -mapcs-stack-check -mno-apcs-stack-check @gol
406 -mapcs-float -mno-apcs-float @gol
407 -mapcs-reentrant -mno-apcs-reentrant @gol
408 -msched-prolog -mno-sched-prolog @gol
409 -mlittle-endian -mbig-endian -mwords-little-endian @gol
410 -mfloat-abi=@var{name} -msoft-float -mhard-float -mfpe @gol
411 -mthumb-interwork -mno-thumb-interwork @gol
412 -mcpu=@var{name} -march=@var{name} -mfpu=@var{name} @gol
413 -mstructure-size-boundary=@var{n} @gol
414 -mabort-on-noreturn @gol
415 -mlong-calls -mno-long-calls @gol
416 -msingle-pic-base -mno-single-pic-base @gol
417 -mpic-register=@var{reg} @gol
418 -mnop-fun-dllimport @gol
419 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
420 -mpoke-function-name @gol
422 -mtpcs-frame -mtpcs-leaf-frame @gol
423 -mcaller-super-interworking -mcallee-super-interworking @gol
427 @gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
428 -mcall-prologues -mno-tablejump -mtiny-stack -mint8}
430 @emph{Blackfin Options}
431 @gccoptlist{-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer @gol
432 -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly @gol
433 -mlow-64k -mno-low64k -mid-shared-library @gol
434 -mno-id-shared-library -mshared-library-id=@var{n} @gol
435 -mlong-calls -mno-long-calls}
438 @gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
439 -mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
440 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
441 -mstack-align -mdata-align -mconst-align @gol
442 -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
443 -melf -maout -melinux -mlinux -sim -sim2 @gol
444 -mmul-bug-workaround -mno-mul-bug-workaround}
447 @gccoptlist{-mmac -mpush-args}
449 @emph{Darwin Options}
450 @gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
451 -arch_only -bind_at_load -bundle -bundle_loader @gol
452 -client_name -compatibility_version -current_version @gol
454 -dependency-file -dylib_file -dylinker_install_name @gol
455 -dynamic -dynamiclib -exported_symbols_list @gol
456 -filelist -flat_namespace -force_cpusubtype_ALL @gol
457 -force_flat_namespace -headerpad_max_install_names @gol
458 -image_base -init -install_name -keep_private_externs @gol
459 -multi_module -multiply_defined -multiply_defined_unused @gol
460 -noall_load -no_dead_strip_inits_and_terms @gol
461 -nofixprebinding -nomultidefs -noprebind -noseglinkedit @gol
462 -pagezero_size -prebind -prebind_all_twolevel_modules @gol
463 -private_bundle -read_only_relocs -sectalign @gol
464 -sectobjectsymbols -whyload -seg1addr @gol
465 -sectcreate -sectobjectsymbols -sectorder @gol
466 -segaddr -segs_read_only_addr -segs_read_write_addr @gol
467 -seg_addr_table -seg_addr_table_filename -seglinkedit @gol
468 -segprot -segs_read_only_addr -segs_read_write_addr @gol
469 -single_module -static -sub_library -sub_umbrella @gol
470 -twolevel_namespace -umbrella -undefined @gol
471 -unexported_symbols_list -weak_reference_mismatches @gol
472 -whatsloaded -F -gused -gfull -mmacosx-version-min=@var{version} @gol
473 -mkernel -mone-byte-bool}
475 @emph{DEC Alpha Options}
476 @gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
477 -mieee -mieee-with-inexact -mieee-conformant @gol
478 -mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
479 -mtrap-precision=@var{mode} -mbuild-constants @gol
480 -mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
481 -mbwx -mmax -mfix -mcix @gol
482 -mfloat-vax -mfloat-ieee @gol
483 -mexplicit-relocs -msmall-data -mlarge-data @gol
484 -msmall-text -mlarge-text @gol
485 -mmemory-latency=@var{time}}
487 @emph{DEC Alpha/VMS Options}
488 @gccoptlist{-mvms-return-codes}
491 @gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
492 -mhard-float -msoft-float @gol
493 -malloc-cc -mfixed-cc -mdword -mno-dword @gol
494 -mdouble -mno-double @gol
495 -mmedia -mno-media -mmuladd -mno-muladd @gol
496 -mfdpic -minline-plt -mgprel-ro -multilib-library-pic @gol
497 -mlinked-fp -mlong-calls -malign-labels @gol
498 -mlibrary-pic -macc-4 -macc-8 @gol
499 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
500 -moptimize-membar -mno-optimize-membar @gol
501 -mscc -mno-scc -mcond-exec -mno-cond-exec @gol
502 -mvliw-branch -mno-vliw-branch @gol
503 -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
504 -mno-nested-cond-exec -mtomcat-stats @gol
508 @emph{GNU/Linux Options}
509 @gccoptlist{-muclibc}
511 @emph{H8/300 Options}
512 @gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
515 @gccoptlist{-march=@var{architecture-type} @gol
516 -mbig-switch -mdisable-fpregs -mdisable-indexing @gol
517 -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
518 -mfixed-range=@var{register-range} @gol
519 -mjump-in-delay -mlinker-opt -mlong-calls @gol
520 -mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
521 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
522 -mno-jump-in-delay -mno-long-load-store @gol
523 -mno-portable-runtime -mno-soft-float @gol
524 -mno-space-regs -msoft-float -mpa-risc-1-0 @gol
525 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
526 -mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
527 -munix=@var{unix-std} -nolibdld -static -threads}
529 @emph{i386 and x86-64 Options}
530 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
531 -mfpmath=@var{unit} @gol
532 -masm=@var{dialect} -mno-fancy-math-387 @gol
533 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib @gol
534 -mno-wide-multiply -mrtd -malign-double @gol
535 -mpreferred-stack-boundary=@var{num} @gol
536 -mmmx -msse -msse2 -msse3 -m3dnow @gol
537 -mthreads -mno-align-stringops -minline-all-stringops @gol
538 -mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
539 -m96bit-long-double -mregparm=@var{num} -msseregparm @gol
541 -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs @gol
542 -mcmodel=@var{code-model} @gol
543 -m32 -m64 -mlarge-data-threshold=@var{num}}
546 @gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
547 -mvolatile-asm-stop -mregister-names -mno-sdata @gol
548 -mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
549 -minline-float-divide-max-throughput @gol
550 -minline-int-divide-min-latency @gol
551 -minline-int-divide-max-throughput @gol
552 -minline-sqrt-min-latency -minline-sqrt-max-throughput @gol
553 -mno-dwarf2-asm -mearly-stop-bits @gol
554 -mfixed-range=@var{register-range} -mtls-size=@var{tls-size} @gol
555 -mtune=@var{cpu-type} -mt -pthread -milp32 -mlp64 @gol
556 -mno-sched-br-data-spec -msched-ar-data-spec -mno-sched-control-spec @gol
557 -msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec @gol
558 -msched-ldc -mno-sched-control-ldc -mno-sched-spec-verbose @gol
559 -mno-sched-prefer-non-data-spec-insns @gol
560 -mno-sched-prefer-non-control-spec-insns @gol
561 -mno-sched-count-spec-in-critical-path}
563 @emph{M32R/D Options}
564 @gccoptlist{-m32r2 -m32rx -m32r @gol
566 -malign-loops -mno-align-loops @gol
567 -missue-rate=@var{number} @gol
568 -mbranch-cost=@var{number} @gol
569 -mmodel=@var{code-size-model-type} @gol
570 -msdata=@var{sdata-type} @gol
571 -mno-flush-func -mflush-func=@var{name} @gol
572 -mno-flush-trap -mflush-trap=@var{number} @gol
576 @gccoptlist{-mcpu=@var{cpu} -msim -memregs=@var{number}}
578 @emph{M680x0 Options}
579 @gccoptlist{-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
580 -m68060 -mcpu32 -m5200 -mcfv4e -m68881 -mbitfield @gol
581 -mc68000 -mc68020 @gol
582 -mnobitfield -mrtd -mshort -msoft-float -mpcrel @gol
583 -malign-int -mstrict-align -msep-data -mno-sep-data @gol
584 -mshared-library-id=n -mid-shared-library -mno-id-shared-library}
586 @emph{M68hc1x Options}
587 @gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
588 -mauto-incdec -minmax -mlong-calls -mshort @gol
589 -msoft-reg-count=@var{count}}
592 @gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
593 -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
594 -m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
595 -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
596 -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
599 @gccoptlist{-EL -EB -march=@var{arch} -mtune=@var{arch} @gol
600 -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
601 -mips16 -mno-mips16 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
602 -mshared -mno-shared -mxgot -mno-xgot -mgp32 -mgp64 @gol
603 -mfp32 -mfp64 -mhard-float -msoft-float @gol
604 -msingle-float -mdouble-float -mdsp -mpaired-single -mips3d @gol
605 -mlong64 -mlong32 -msym32 -mno-sym32 @gol
606 -G@var{num} -membedded-data -mno-embedded-data @gol
607 -muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
608 -msplit-addresses -mno-split-addresses @gol
609 -mexplicit-relocs -mno-explicit-relocs @gol
610 -mcheck-zero-division -mno-check-zero-division @gol
611 -mdivide-traps -mdivide-breaks @gol
612 -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls @gol
613 -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp @gol
614 -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 @gol
615 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 @gol
616 -mfix-sb1 -mno-fix-sb1 @gol
617 -mflush-func=@var{func} -mno-flush-func @gol
618 -mbranch-likely -mno-branch-likely @gol
619 -mfp-exceptions -mno-fp-exceptions @gol
620 -mvr4130-align -mno-vr4130-align}
623 @gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
624 -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
625 -melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
626 -mno-base-addresses -msingle-exit -mno-single-exit}
628 @emph{MN10300 Options}
629 @gccoptlist{-mmult-bug -mno-mult-bug @gol
630 -mam33 -mno-am33 @gol
631 -mam33-2 -mno-am33-2 @gol
632 -mreturn-pointer-on-d0 @gol
636 @gccoptlist{-mno-crt0 -mbacc -msim @gol
637 -march=@var{cpu-type} }
639 @emph{PDP-11 Options}
640 @gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
641 -mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
642 -mint16 -mno-int32 -mfloat32 -mno-float64 @gol
643 -mfloat64 -mno-float32 -mabshi -mno-abshi @gol
644 -mbranch-expensive -mbranch-cheap @gol
645 -msplit -mno-split -munix-asm -mdec-asm}
647 @emph{PowerPC Options}
648 See RS/6000 and PowerPC Options.
650 @emph{RS/6000 and PowerPC Options}
651 @gccoptlist{-mcpu=@var{cpu-type} @gol
652 -mtune=@var{cpu-type} @gol
653 -mpower -mno-power -mpower2 -mno-power2 @gol
654 -mpowerpc -mpowerpc64 -mno-powerpc @gol
655 -maltivec -mno-altivec @gol
656 -mpowerpc-gpopt -mno-powerpc-gpopt @gol
657 -mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
658 -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd @gol
659 -mnew-mnemonics -mold-mnemonics @gol
660 -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
661 -m64 -m32 -mxl-compat -mno-xl-compat -mpe @gol
662 -malign-power -malign-natural @gol
663 -msoft-float -mhard-float -mmultiple -mno-multiple @gol
664 -mstring -mno-string -mupdate -mno-update @gol
665 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
666 -mstrict-align -mno-strict-align -mrelocatable @gol
667 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
668 -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
669 -mdynamic-no-pic -maltivec -mswdiv @gol
670 -mprioritize-restricted-insns=@var{priority} @gol
671 -msched-costly-dep=@var{dependence_type} @gol
672 -minsert-sched-nops=@var{scheme} @gol
673 -mcall-sysv -mcall-netbsd @gol
674 -maix-struct-return -msvr4-struct-return @gol
675 -mabi=@var{abi-type} -msecure-plt -mbss-plt @gol
676 -misel -mno-isel @gol
677 -misel=yes -misel=no @gol
679 -mspe=yes -mspe=no @gol
680 -mvrsave -mno-vrsave @gol
681 -mmulhw -mno-mulhw @gol
682 -mdlmzb -mno-dlmzb @gol
683 -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double @gol
684 -mprototype -mno-prototype @gol
685 -msim -mmvme -mads -myellowknife -memb -msdata @gol
686 -msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
688 @emph{S/390 and zSeries Options}
689 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
690 -mhard-float -msoft-float -mlong-double-64 -mlong-double-128 @gol
691 -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack @gol
692 -msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
693 -m64 -m31 -mdebug -mno-debug -mesa -mzarch @gol
694 -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd @gol
695 -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard}
698 @gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
699 -m4-nofpu -m4-single-only -m4-single -m4 @gol
700 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al @gol
701 -m5-64media -m5-64media-nofpu @gol
702 -m5-32media -m5-32media-nofpu @gol
703 -m5-compact -m5-compact-nofpu @gol
704 -mb -ml -mdalign -mrelax @gol
705 -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave @gol
706 -mieee -misize -mpadstruct -mspace @gol
707 -mprefergot -musermode -multcost=@var{number} -mdiv=@var{strategy} @gol
708 -mdivsi3_libfunc=@var{name} @gol
709 -madjust-unroll -mindexed-addressing -mgettrcost=@var{number} -mpt-fixed @gol
713 @gccoptlist{-mcpu=@var{cpu-type} @gol
714 -mtune=@var{cpu-type} @gol
715 -mcmodel=@var{code-model} @gol
716 -m32 -m64 -mapp-regs -mno-app-regs @gol
717 -mfaster-structs -mno-faster-structs @gol
718 -mfpu -mno-fpu -mhard-float -msoft-float @gol
719 -mhard-quad-float -msoft-quad-float @gol
720 -mimpure-text -mno-impure-text -mlittle-endian @gol
721 -mstack-bias -mno-stack-bias @gol
722 -munaligned-doubles -mno-unaligned-doubles @gol
723 -mv8plus -mno-v8plus -mvis -mno-vis
724 -threads -pthreads -pthread}
726 @emph{System V Options}
727 @gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
729 @emph{TMS320C3x/C4x Options}
730 @gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
731 -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
732 -mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
733 -mparallel-insns -mparallel-mpy -mpreserve-float}
736 @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
737 -mprolog-function -mno-prolog-function -mspace @gol
738 -mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
739 -mapp-regs -mno-app-regs @gol
740 -mdisable-callt -mno-disable-callt @gol
746 @gccoptlist{-mg -mgnu -munix}
748 @emph{x86-64 Options}
749 See i386 and x86-64 Options.
751 @emph{Xstormy16 Options}
754 @emph{Xtensa Options}
755 @gccoptlist{-mconst16 -mno-const16 @gol
756 -mfused-madd -mno-fused-madd @gol
757 -mtext-section-literals -mno-text-section-literals @gol
758 -mtarget-align -mno-target-align @gol
759 -mlongcalls -mno-longcalls}
761 @emph{zSeries Options}
762 See S/390 and zSeries Options.
764 @item Code Generation Options
765 @xref{Code Gen Options,,Options for Code Generation Conventions}.
766 @gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
767 -ffixed-@var{reg} -fexceptions @gol
768 -fnon-call-exceptions -funwind-tables @gol
769 -fasynchronous-unwind-tables @gol
770 -finhibit-size-directive -finstrument-functions @gol
771 -fno-common -fno-ident @gol
772 -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
773 -fno-jump-tables @gol
774 -freg-struct-return -fshort-enums @gol
775 -fshort-double -fshort-wchar @gol
776 -fverbose-asm -fpack-struct[=@var{n}] -fstack-check @gol
777 -fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
778 -fargument-alias -fargument-noalias @gol
779 -fargument-noalias-global -fargument-noalias-anything
780 -fleading-underscore -ftls-model=@var{model} @gol
781 -ftrapv -fwrapv -fbounds-check @gol
786 * Overall Options:: Controlling the kind of output:
787 an executable, object files, assembler files,
788 or preprocessed source.
789 * C Dialect Options:: Controlling the variant of C language compiled.
790 * C++ Dialect Options:: Variations on C++.
791 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
793 * Language Independent Options:: Controlling how diagnostics should be
795 * Warning Options:: How picky should the compiler be?
796 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
797 * Optimize Options:: How much optimization?
798 * Preprocessor Options:: Controlling header files and macro definitions.
799 Also, getting dependency information for Make.
800 * Assembler Options:: Passing options to the assembler.
801 * Link Options:: Specifying libraries and so on.
802 * Directory Options:: Where to find header files and libraries.
803 Where to find the compiler executable files.
804 * Spec Files:: How to pass switches to sub-processes.
805 * Target Options:: Running a cross-compiler, or an old version of GCC.
808 @node Overall Options
809 @section Options Controlling the Kind of Output
811 Compilation can involve up to four stages: preprocessing, compilation
812 proper, assembly and linking, always in that order. GCC is capable of
813 preprocessing and compiling several files either into several
814 assembler input files, or into one assembler input file; then each
815 assembler input file produces an object file, and linking combines all
816 the object files (those newly compiled, and those specified as input)
817 into an executable file.
819 @cindex file name suffix
820 For any given input file, the file name suffix determines what kind of
825 C source code which must be preprocessed.
828 C source code which should not be preprocessed.
831 C++ source code which should not be preprocessed.
834 Objective-C source code. Note that you must link with the @file{libobjc}
835 library to make an Objective-C program work.
838 Objective-C source code which should not be preprocessed.
842 Objective-C++ source code. Note that you must link with the @file{libobjc}
843 library to make an Objective-C++ program work. Note that @samp{.M} refers
844 to a literal capital M@.
847 Objective-C++ source code which should not be preprocessed.
850 C, C++, Objective-C or Objective-C++ header file to be turned into a
855 @itemx @var{file}.cxx
856 @itemx @var{file}.cpp
857 @itemx @var{file}.CPP
858 @itemx @var{file}.c++
860 C++ source code which must be preprocessed. Note that in @samp{.cxx},
861 the last two letters must both be literally @samp{x}. Likewise,
862 @samp{.C} refers to a literal capital C@.
866 Objective-C++ source code which must be preprocessed.
869 Objective-C++ source code which should not be preprocessed.
873 C++ header file to be turned into a precompiled header.
876 @itemx @var{file}.for
877 @itemx @var{file}.FOR
878 Fixed form Fortran source code which should not be preprocessed.
881 @itemx @var{file}.fpp
882 @itemx @var{file}.FPP
883 Fixed form Fortran source code which must be preprocessed (with the traditional
887 @itemx @var{file}.f95
888 Free form Fortran source code which should not be preprocessed.
891 @itemx @var{file}.F95
892 Free form Fortran source code which must be preprocessed (with the
893 traditional preprocessor).
895 @c FIXME: Descriptions of Java file types.
902 Ada source code file which contains a library unit declaration (a
903 declaration of a package, subprogram, or generic, or a generic
904 instantiation), or a library unit renaming declaration (a package,
905 generic, or subprogram renaming declaration). Such files are also
908 @itemx @var{file}.adb
909 Ada source code file containing a library unit body (a subprogram or
910 package body). Such files are also called @dfn{bodies}.
912 @c GCC also knows about some suffixes for languages not yet included:
923 Assembler code which must be preprocessed.
926 An object file to be fed straight into linking.
927 Any file name with no recognized suffix is treated this way.
931 You can specify the input language explicitly with the @option{-x} option:
934 @item -x @var{language}
935 Specify explicitly the @var{language} for the following input files
936 (rather than letting the compiler choose a default based on the file
937 name suffix). This option applies to all following input files until
938 the next @option{-x} option. Possible values for @var{language} are:
940 c c-header c-cpp-output
941 c++ c++-header c++-cpp-output
942 objective-c objective-c-header objective-c-cpp-output
943 objective-c++ objective-c++-header objective-c++-cpp-output
944 assembler assembler-with-cpp
952 Turn off any specification of a language, so that subsequent files are
953 handled according to their file name suffixes (as they are if @option{-x}
954 has not been used at all).
956 @item -pass-exit-codes
957 @opindex pass-exit-codes
958 Normally the @command{gcc} program will exit with the code of 1 if any
959 phase of the compiler returns a non-success return code. If you specify
960 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
961 numerically highest error produced by any phase that returned an error
962 indication. The C, C++, and Fortran frontends return 4, if an internal
963 compiler error is encountered.
966 If you only want some of the stages of compilation, you can use
967 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
968 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
969 @command{gcc} is to stop. Note that some combinations (for example,
970 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
975 Compile or assemble the source files, but do not link. The linking
976 stage simply is not done. The ultimate output is in the form of an
977 object file for each source file.
979 By default, the object file name for a source file is made by replacing
980 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
982 Unrecognized input files, not requiring compilation or assembly, are
987 Stop after the stage of compilation proper; do not assemble. The output
988 is in the form of an assembler code file for each non-assembler input
991 By default, the assembler file name for a source file is made by
992 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
994 Input files that don't require compilation are ignored.
998 Stop after the preprocessing stage; do not run the compiler proper. The
999 output is in the form of preprocessed source code, which is sent to the
1002 Input files which don't require preprocessing are ignored.
1004 @cindex output file option
1007 Place output in file @var{file}. This applies regardless to whatever
1008 sort of output is being produced, whether it be an executable file,
1009 an object file, an assembler file or preprocessed C code.
1011 If @option{-o} is not specified, the default is to put an executable
1012 file in @file{a.out}, the object file for
1013 @file{@var{source}.@var{suffix}} in @file{@var{source}.o}, its
1014 assembler file in @file{@var{source}.s}, a precompiled header file in
1015 @file{@var{source}.@var{suffix}.gch}, and all preprocessed C source on
1020 Print (on standard error output) the commands executed to run the stages
1021 of compilation. Also print the version number of the compiler driver
1022 program and of the preprocessor and the compiler proper.
1026 Like @option{-v} except the commands are not executed and all command
1027 arguments are quoted. This is useful for shell scripts to capture the
1028 driver-generated command lines.
1032 Use pipes rather than temporary files for communication between the
1033 various stages of compilation. This fails to work on some systems where
1034 the assembler is unable to read from a pipe; but the GNU assembler has
1039 If you are compiling multiple source files, this option tells the driver
1040 to pass all the source files to the compiler at once (for those
1041 languages for which the compiler can handle this). This will allow
1042 intermodule analysis (IMA) to be performed by the compiler. Currently the only
1043 language for which this is supported is C@. If you pass source files for
1044 multiple languages to the driver, using this option, the driver will invoke
1045 the compiler(s) that support IMA once each, passing each compiler all the
1046 source files appropriate for it. For those languages that do not support
1047 IMA this option will be ignored, and the compiler will be invoked once for
1048 each source file in that language. If you use this option in conjunction
1049 with @option{-save-temps}, the compiler will generate multiple
1051 (one for each source file), but only one (combined) @file{.o} or
1056 Print (on the standard output) a description of the command line options
1057 understood by @command{gcc}. If the @option{-v} option is also specified
1058 then @option{--help} will also be passed on to the various processes
1059 invoked by @command{gcc}, so that they can display the command line options
1060 they accept. If the @option{-Wextra} option is also specified then command
1061 line options which have no documentation associated with them will also
1065 @opindex target-help
1066 Print (on the standard output) a description of target specific command
1067 line options for each tool.
1071 Display the version number and copyrights of the invoked GCC@.
1073 @include @value{srcdir}/../libiberty/at-file.texi
1077 @section Compiling C++ Programs
1079 @cindex suffixes for C++ source
1080 @cindex C++ source file suffixes
1081 C++ source files conventionally use one of the suffixes @samp{.C},
1082 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
1083 @samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
1084 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
1085 files with these names and compiles them as C++ programs even if you
1086 call the compiler the same way as for compiling C programs (usually
1087 with the name @command{gcc}).
1091 However, the use of @command{gcc} does not add the C++ library.
1092 @command{g++} is a program that calls GCC and treats @samp{.c},
1093 @samp{.h} and @samp{.i} files as C++ source files instead of C source
1094 files unless @option{-x} is used, and automatically specifies linking
1095 against the C++ library. This program is also useful when
1096 precompiling a C header file with a @samp{.h} extension for use in C++
1097 compilations. On many systems, @command{g++} is also installed with
1098 the name @command{c++}.
1100 @cindex invoking @command{g++}
1101 When you compile C++ programs, you may specify many of the same
1102 command-line options that you use for compiling programs in any
1103 language; or command-line options meaningful for C and related
1104 languages; or options that are meaningful only for C++ programs.
1105 @xref{C Dialect Options,,Options Controlling C Dialect}, for
1106 explanations of options for languages related to C@.
1107 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
1108 explanations of options that are meaningful only for C++ programs.
1110 @node C Dialect Options
1111 @section Options Controlling C Dialect
1112 @cindex dialect options
1113 @cindex language dialect options
1114 @cindex options, dialect
1116 The following options control the dialect of C (or languages derived
1117 from C, such as C++, Objective-C and Objective-C++) that the compiler
1121 @cindex ANSI support
1125 In C mode, support all ISO C90 programs. In C++ mode,
1126 remove GNU extensions that conflict with ISO C++.
1128 This turns off certain features of GCC that are incompatible with ISO
1129 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
1130 such as the @code{asm} and @code{typeof} keywords, and
1131 predefined macros such as @code{unix} and @code{vax} that identify the
1132 type of system you are using. It also enables the undesirable and
1133 rarely used ISO trigraph feature. For the C compiler,
1134 it disables recognition of C++ style @samp{//} comments as well as
1135 the @code{inline} keyword.
1137 The alternate keywords @code{__asm__}, @code{__extension__},
1138 @code{__inline__} and @code{__typeof__} continue to work despite
1139 @option{-ansi}. You would not want to use them in an ISO C program, of
1140 course, but it is useful to put them in header files that might be included
1141 in compilations done with @option{-ansi}. Alternate predefined macros
1142 such as @code{__unix__} and @code{__vax__} are also available, with or
1143 without @option{-ansi}.
1145 The @option{-ansi} option does not cause non-ISO programs to be
1146 rejected gratuitously. For that, @option{-pedantic} is required in
1147 addition to @option{-ansi}. @xref{Warning Options}.
1149 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1150 option is used. Some header files may notice this macro and refrain
1151 from declaring certain functions or defining certain macros that the
1152 ISO standard doesn't call for; this is to avoid interfering with any
1153 programs that might use these names for other things.
1155 Functions which would normally be built in but do not have semantics
1156 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1157 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1158 built-in functions provided by GCC}, for details of the functions
1163 Determine the language standard. This option is currently only
1164 supported when compiling C or C++. A value for this option must be
1165 provided; possible values are
1170 ISO C90 (same as @option{-ansi}).
1172 @item iso9899:199409
1173 ISO C90 as modified in amendment 1.
1179 ISO C99. Note that this standard is not yet fully supported; see
1180 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1181 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1184 Default, ISO C90 plus GNU extensions (including some C99 features).
1188 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1189 this will become the default. The name @samp{gnu9x} is deprecated.
1192 The 1998 ISO C++ standard plus amendments.
1195 The same as @option{-std=c++98} plus GNU extensions. This is the
1196 default for C++ code.
1199 Even when this option is not specified, you can still use some of the
1200 features of newer standards in so far as they do not conflict with
1201 previous C standards. For example, you may use @code{__restrict__} even
1202 when @option{-std=c99} is not specified.
1204 The @option{-std} options specifying some version of ISO C have the same
1205 effects as @option{-ansi}, except that features that were not in ISO C90
1206 but are in the specified version (for example, @samp{//} comments and
1207 the @code{inline} keyword in ISO C99) are not disabled.
1209 @xref{Standards,,Language Standards Supported by GCC}, for details of
1210 these standard versions.
1212 @item -aux-info @var{filename}
1214 Output to the given filename prototyped declarations for all functions
1215 declared and/or defined in a translation unit, including those in header
1216 files. This option is silently ignored in any language other than C@.
1218 Besides declarations, the file indicates, in comments, the origin of
1219 each declaration (source file and line), whether the declaration was
1220 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1221 @samp{O} for old, respectively, in the first character after the line
1222 number and the colon), and whether it came from a declaration or a
1223 definition (@samp{C} or @samp{F}, respectively, in the following
1224 character). In the case of function definitions, a K&R-style list of
1225 arguments followed by their declarations is also provided, inside
1226 comments, after the declaration.
1230 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1231 keyword, so that code can use these words as identifiers. You can use
1232 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1233 instead. @option{-ansi} implies @option{-fno-asm}.
1235 In C++, this switch only affects the @code{typeof} keyword, since
1236 @code{asm} and @code{inline} are standard keywords. You may want to
1237 use the @option{-fno-gnu-keywords} flag instead, which has the same
1238 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1239 switch only affects the @code{asm} and @code{typeof} keywords, since
1240 @code{inline} is a standard keyword in ISO C99.
1243 @itemx -fno-builtin-@var{function}
1244 @opindex fno-builtin
1245 @cindex built-in functions
1246 Don't recognize built-in functions that do not begin with
1247 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1248 functions provided by GCC}, for details of the functions affected,
1249 including those which are not built-in functions when @option{-ansi} or
1250 @option{-std} options for strict ISO C conformance are used because they
1251 do not have an ISO standard meaning.
1253 GCC normally generates special code to handle certain built-in functions
1254 more efficiently; for instance, calls to @code{alloca} may become single
1255 instructions that adjust the stack directly, and calls to @code{memcpy}
1256 may become inline copy loops. The resulting code is often both smaller
1257 and faster, but since the function calls no longer appear as such, you
1258 cannot set a breakpoint on those calls, nor can you change the behavior
1259 of the functions by linking with a different library. In addition,
1260 when a function is recognized as a built-in function, GCC may use
1261 information about that function to warn about problems with calls to
1262 that function, or to generate more efficient code, even if the
1263 resulting code still contains calls to that function. For example,
1264 warnings are given with @option{-Wformat} for bad calls to
1265 @code{printf}, when @code{printf} is built in, and @code{strlen} is
1266 known not to modify global memory.
1268 With the @option{-fno-builtin-@var{function}} option
1269 only the built-in function @var{function} is
1270 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1271 function is named this is not built-in in this version of GCC, this
1272 option is ignored. There is no corresponding
1273 @option{-fbuiltin-@var{function}} option; if you wish to enable
1274 built-in functions selectively when using @option{-fno-builtin} or
1275 @option{-ffreestanding}, you may define macros such as:
1278 #define abs(n) __builtin_abs ((n))
1279 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1284 @cindex hosted environment
1286 Assert that compilation takes place in a hosted environment. This implies
1287 @option{-fbuiltin}. A hosted environment is one in which the
1288 entire standard library is available, and in which @code{main} has a return
1289 type of @code{int}. Examples are nearly everything except a kernel.
1290 This is equivalent to @option{-fno-freestanding}.
1292 @item -ffreestanding
1293 @opindex ffreestanding
1294 @cindex hosted environment
1296 Assert that compilation takes place in a freestanding environment. This
1297 implies @option{-fno-builtin}. A freestanding environment
1298 is one in which the standard library may not exist, and program startup may
1299 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1300 This is equivalent to @option{-fno-hosted}.
1302 @xref{Standards,,Language Standards Supported by GCC}, for details of
1303 freestanding and hosted environments.
1307 @cindex openmp parallel
1308 Enable handling of OpenMP directives @code{#pragma omp} in C/C++ and
1309 @code{!$omp} in Fortran. When @option{-fopenmp} is specified, the
1310 compiler generates parallel code according to the OpenMP Application
1311 Program Interface v2.5 @w{@uref{http://www.openmp.org/}}.
1313 @item -fms-extensions
1314 @opindex fms-extensions
1315 Accept some non-standard constructs used in Microsoft header files.
1317 Some cases of unnamed fields in structures and unions are only
1318 accepted with this option. @xref{Unnamed Fields,,Unnamed struct/union
1319 fields within structs/unions}, for details.
1323 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1324 options for strict ISO C conformance) implies @option{-trigraphs}.
1326 @item -no-integrated-cpp
1327 @opindex no-integrated-cpp
1328 Performs a compilation in two passes: preprocessing and compiling. This
1329 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1330 @option{-B} option. The user supplied compilation step can then add in
1331 an additional preprocessing step after normal preprocessing but before
1332 compiling. The default is to use the integrated cpp (internal cpp)
1334 The semantics of this option will change if "cc1", "cc1plus", and
1335 "cc1obj" are merged.
1337 @cindex traditional C language
1338 @cindex C language, traditional
1340 @itemx -traditional-cpp
1341 @opindex traditional-cpp
1342 @opindex traditional
1343 Formerly, these options caused GCC to attempt to emulate a pre-standard
1344 C compiler. They are now only supported with the @option{-E} switch.
1345 The preprocessor continues to support a pre-standard mode. See the GNU
1346 CPP manual for details.
1348 @item -fcond-mismatch
1349 @opindex fcond-mismatch
1350 Allow conditional expressions with mismatched types in the second and
1351 third arguments. The value of such an expression is void. This option
1352 is not supported for C++.
1354 @item -funsigned-char
1355 @opindex funsigned-char
1356 Let the type @code{char} be unsigned, like @code{unsigned char}.
1358 Each kind of machine has a default for what @code{char} should
1359 be. It is either like @code{unsigned char} by default or like
1360 @code{signed char} by default.
1362 Ideally, a portable program should always use @code{signed char} or
1363 @code{unsigned char} when it depends on the signedness of an object.
1364 But many programs have been written to use plain @code{char} and
1365 expect it to be signed, or expect it to be unsigned, depending on the
1366 machines they were written for. This option, and its inverse, let you
1367 make such a program work with the opposite default.
1369 The type @code{char} is always a distinct type from each of
1370 @code{signed char} or @code{unsigned char}, even though its behavior
1371 is always just like one of those two.
1374 @opindex fsigned-char
1375 Let the type @code{char} be signed, like @code{signed char}.
1377 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1378 the negative form of @option{-funsigned-char}. Likewise, the option
1379 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1381 @item -fsigned-bitfields
1382 @itemx -funsigned-bitfields
1383 @itemx -fno-signed-bitfields
1384 @itemx -fno-unsigned-bitfields
1385 @opindex fsigned-bitfields
1386 @opindex funsigned-bitfields
1387 @opindex fno-signed-bitfields
1388 @opindex fno-unsigned-bitfields
1389 These options control whether a bit-field is signed or unsigned, when the
1390 declaration does not use either @code{signed} or @code{unsigned}. By
1391 default, such a bit-field is signed, because this is consistent: the
1392 basic integer types such as @code{int} are signed types.
1395 @node C++ Dialect Options
1396 @section Options Controlling C++ Dialect
1398 @cindex compiler options, C++
1399 @cindex C++ options, command line
1400 @cindex options, C++
1401 This section describes the command-line options that are only meaningful
1402 for C++ programs; but you can also use most of the GNU compiler options
1403 regardless of what language your program is in. For example, you
1404 might compile a file @code{firstClass.C} like this:
1407 g++ -g -frepo -O -c firstClass.C
1411 In this example, only @option{-frepo} is an option meant
1412 only for C++ programs; you can use the other options with any
1413 language supported by GCC@.
1415 Here is a list of options that are @emph{only} for compiling C++ programs:
1419 @item -fabi-version=@var{n}
1420 @opindex fabi-version
1421 Use version @var{n} of the C++ ABI@. Version 2 is the version of the
1422 C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
1423 the C++ ABI that first appeared in G++ 3.2. Version 0 will always be
1424 the version that conforms most closely to the C++ ABI specification.
1425 Therefore, the ABI obtained using version 0 will change as ABI bugs
1428 The default is version 2.
1430 @item -fno-access-control
1431 @opindex fno-access-control
1432 Turn off all access checking. This switch is mainly useful for working
1433 around bugs in the access control code.
1437 Check that the pointer returned by @code{operator new} is non-null
1438 before attempting to modify the storage allocated. This check is
1439 normally unnecessary because the C++ standard specifies that
1440 @code{operator new} will only return @code{0} if it is declared
1441 @samp{throw()}, in which case the compiler will always check the
1442 return value even without this option. In all other cases, when
1443 @code{operator new} has a non-empty exception specification, memory
1444 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1445 @samp{new (nothrow)}.
1447 @item -fconserve-space
1448 @opindex fconserve-space
1449 Put uninitialized or runtime-initialized global variables into the
1450 common segment, as C does. This saves space in the executable at the
1451 cost of not diagnosing duplicate definitions. If you compile with this
1452 flag and your program mysteriously crashes after @code{main()} has
1453 completed, you may have an object that is being destroyed twice because
1454 two definitions were merged.
1456 This option is no longer useful on most targets, now that support has
1457 been added for putting variables into BSS without making them common.
1459 @item -ffriend-injection
1460 @opindex ffriend-injection
1461 Inject friend functions into the enclosing namespace, so that they are
1462 visible outside the scope of the class in which they are declared.
1463 Friend functions were documented to work this way in the old Annotated
1464 C++ Reference Manual, and versions of G++ before 4.1 always worked
1465 that way. However, in ISO C++ a friend function which is not declared
1466 in an enclosing scope can only be found using argument dependent
1467 lookup. This option causes friends to be injected as they were in
1470 This option is for compatibility, and may be removed in a future
1473 @item -fno-elide-constructors
1474 @opindex fno-elide-constructors
1475 The C++ standard allows an implementation to omit creating a temporary
1476 which is only used to initialize another object of the same type.
1477 Specifying this option disables that optimization, and forces G++ to
1478 call the copy constructor in all cases.
1480 @item -fno-enforce-eh-specs
1481 @opindex fno-enforce-eh-specs
1482 Don't generate code to check for violation of exception specifications
1483 at runtime. This option violates the C++ standard, but may be useful
1484 for reducing code size in production builds, much like defining
1485 @samp{NDEBUG}. This does not give user code permission to throw
1486 exceptions in violation of the exception specifications; the compiler
1487 will still optimize based on the specifications, so throwing an
1488 unexpected exception will result in undefined behavior.
1491 @itemx -fno-for-scope
1493 @opindex fno-for-scope
1494 If @option{-ffor-scope} is specified, the scope of variables declared in
1495 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1496 as specified by the C++ standard.
1497 If @option{-fno-for-scope} is specified, the scope of variables declared in
1498 a @i{for-init-statement} extends to the end of the enclosing scope,
1499 as was the case in old versions of G++, and other (traditional)
1500 implementations of C++.
1502 The default if neither flag is given to follow the standard,
1503 but to allow and give a warning for old-style code that would
1504 otherwise be invalid, or have different behavior.
1506 @item -fno-gnu-keywords
1507 @opindex fno-gnu-keywords
1508 Do not recognize @code{typeof} as a keyword, so that code can use this
1509 word as an identifier. You can use the keyword @code{__typeof__} instead.
1510 @option{-ansi} implies @option{-fno-gnu-keywords}.
1512 @item -fno-implicit-templates
1513 @opindex fno-implicit-templates
1514 Never emit code for non-inline templates which are instantiated
1515 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1516 @xref{Template Instantiation}, for more information.
1518 @item -fno-implicit-inline-templates
1519 @opindex fno-implicit-inline-templates
1520 Don't emit code for implicit instantiations of inline templates, either.
1521 The default is to handle inlines differently so that compiles with and
1522 without optimization will need the same set of explicit instantiations.
1524 @item -fno-implement-inlines
1525 @opindex fno-implement-inlines
1526 To save space, do not emit out-of-line copies of inline functions
1527 controlled by @samp{#pragma implementation}. This will cause linker
1528 errors if these functions are not inlined everywhere they are called.
1530 @item -fms-extensions
1531 @opindex fms-extensions
1532 Disable pedantic warnings about constructs used in MFC, such as implicit
1533 int and getting a pointer to member function via non-standard syntax.
1535 @item -fno-nonansi-builtins
1536 @opindex fno-nonansi-builtins
1537 Disable built-in declarations of functions that are not mandated by
1538 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1539 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1541 @item -fno-operator-names
1542 @opindex fno-operator-names
1543 Do not treat the operator name keywords @code{and}, @code{bitand},
1544 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1545 synonyms as keywords.
1547 @item -fno-optional-diags
1548 @opindex fno-optional-diags
1549 Disable diagnostics that the standard says a compiler does not need to
1550 issue. Currently, the only such diagnostic issued by G++ is the one for
1551 a name having multiple meanings within a class.
1554 @opindex fpermissive
1555 Downgrade some diagnostics about nonconformant code from errors to
1556 warnings. Thus, using @option{-fpermissive} will allow some
1557 nonconforming code to compile.
1561 Enable automatic template instantiation at link time. This option also
1562 implies @option{-fno-implicit-templates}. @xref{Template
1563 Instantiation}, for more information.
1567 Disable generation of information about every class with virtual
1568 functions for use by the C++ runtime type identification features
1569 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1570 of the language, you can save some space by using this flag. Note that
1571 exception handling uses the same information, but it will generate it as
1572 needed. The @samp{dynamic_cast} operator can still be used for casts that
1573 do not require runtime type information, i.e. casts to @code{void *} or to
1574 unambiguous base classes.
1578 Emit statistics about front-end processing at the end of the compilation.
1579 This information is generally only useful to the G++ development team.
1581 @item -ftemplate-depth-@var{n}
1582 @opindex ftemplate-depth
1583 Set the maximum instantiation depth for template classes to @var{n}.
1584 A limit on the template instantiation depth is needed to detect
1585 endless recursions during template class instantiation. ANSI/ISO C++
1586 conforming programs must not rely on a maximum depth greater than 17.
1588 @item -fno-threadsafe-statics
1589 @opindex fno-threadsafe-statics
1590 Do not emit the extra code to use the routines specified in the C++
1591 ABI for thread-safe initialization of local statics. You can use this
1592 option to reduce code size slightly in code that doesn't need to be
1595 @item -fuse-cxa-atexit
1596 @opindex fuse-cxa-atexit
1597 Register destructors for objects with static storage duration with the
1598 @code{__cxa_atexit} function rather than the @code{atexit} function.
1599 This option is required for fully standards-compliant handling of static
1600 destructors, but will only work if your C library supports
1601 @code{__cxa_atexit}.
1603 @item -fno-use-cxa-get-exception-ptr
1604 @opindex fno-use-cxa-get-exception-ptr
1605 Don't use the @code{__cxa_get_exception_ptr} runtime routine. This
1606 will cause @code{std::uncaught_exception} to be incorrect, but is necessary
1607 if the runtime routine is not available.
1609 @item -fvisibility-inlines-hidden
1610 @opindex fvisibility-inlines-hidden
1611 This switch declares that the user does not attempt to compare
1612 pointers to inline methods where the addresses of the two functions
1613 were taken in different shared objects.
1615 The effect of this is that GCC may, effectively, mark inline methods with
1616 @code{__attribute__ ((visibility ("hidden")))} so that they do not
1617 appear in the export table of a DSO and do not require a PLT indirection
1618 when used within the DSO@. Enabling this option can have a dramatic effect
1619 on load and link times of a DSO as it massively reduces the size of the
1620 dynamic export table when the library makes heavy use of templates.
1622 The behaviour of this switch is not quite the same as marking the
1623 methods as hidden directly, because it does not affect static variables
1624 local to the function or cause the compiler to deduce that
1625 the function is defined in only one shared object.
1627 You may mark a method as having a visibility explicitly to negate the
1628 effect of the switch for that method. For example, if you do want to
1629 compare pointers to a particular inline method, you might mark it as
1630 having default visibility. Marking the enclosing class with explicit
1631 visibility will have no effect.
1633 Explicitly instantiated inline methods are unaffected by this option
1634 as their linkage might otherwise cross a shared library boundary.
1635 @xref{Template Instantiation}.
1639 Do not use weak symbol support, even if it is provided by the linker.
1640 By default, G++ will use weak symbols if they are available. This
1641 option exists only for testing, and should not be used by end-users;
1642 it will result in inferior code and has no benefits. This option may
1643 be removed in a future release of G++.
1647 Do not search for header files in the standard directories specific to
1648 C++, but do still search the other standard directories. (This option
1649 is used when building the C++ library.)
1652 In addition, these optimization, warning, and code generation options
1653 have meanings only for C++ programs:
1656 @item -fno-default-inline
1657 @opindex fno-default-inline
1658 Do not assume @samp{inline} for functions defined inside a class scope.
1659 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1660 functions will have linkage like inline functions; they just won't be
1663 @item -Wabi @r{(C++ only)}
1665 Warn when G++ generates code that is probably not compatible with the
1666 vendor-neutral C++ ABI@. Although an effort has been made to warn about
1667 all such cases, there are probably some cases that are not warned about,
1668 even though G++ is generating incompatible code. There may also be
1669 cases where warnings are emitted even though the code that is generated
1672 You should rewrite your code to avoid these warnings if you are
1673 concerned about the fact that code generated by G++ may not be binary
1674 compatible with code generated by other compilers.
1676 The known incompatibilities at this point include:
1681 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1682 pack data into the same byte as a base class. For example:
1685 struct A @{ virtual void f(); int f1 : 1; @};
1686 struct B : public A @{ int f2 : 1; @};
1690 In this case, G++ will place @code{B::f2} into the same byte
1691 as@code{A::f1}; other compilers will not. You can avoid this problem
1692 by explicitly padding @code{A} so that its size is a multiple of the
1693 byte size on your platform; that will cause G++ and other compilers to
1694 layout @code{B} identically.
1697 Incorrect handling of tail-padding for virtual bases. G++ does not use
1698 tail padding when laying out virtual bases. For example:
1701 struct A @{ virtual void f(); char c1; @};
1702 struct B @{ B(); char c2; @};
1703 struct C : public A, public virtual B @{@};
1707 In this case, G++ will not place @code{B} into the tail-padding for
1708 @code{A}; other compilers will. You can avoid this problem by
1709 explicitly padding @code{A} so that its size is a multiple of its
1710 alignment (ignoring virtual base classes); that will cause G++ and other
1711 compilers to layout @code{C} identically.
1714 Incorrect handling of bit-fields with declared widths greater than that
1715 of their underlying types, when the bit-fields appear in a union. For
1719 union U @{ int i : 4096; @};
1723 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1724 union too small by the number of bits in an @code{int}.
1727 Empty classes can be placed at incorrect offsets. For example:
1737 struct C : public B, public A @{@};
1741 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1742 it should be placed at offset zero. G++ mistakenly believes that the
1743 @code{A} data member of @code{B} is already at offset zero.
1746 Names of template functions whose types involve @code{typename} or
1747 template template parameters can be mangled incorrectly.
1750 template <typename Q>
1751 void f(typename Q::X) @{@}
1753 template <template <typename> class Q>
1754 void f(typename Q<int>::X) @{@}
1758 Instantiations of these templates may be mangled incorrectly.
1762 @item -Wctor-dtor-privacy @r{(C++ only)}
1763 @opindex Wctor-dtor-privacy
1764 Warn when a class seems unusable because all the constructors or
1765 destructors in that class are private, and it has neither friends nor
1766 public static member functions.
1768 @item -Wnon-virtual-dtor @r{(C++ only)}
1769 @opindex Wnon-virtual-dtor
1770 Warn when a class appears to be polymorphic, thereby requiring a virtual
1771 destructor, yet it declares a non-virtual one. This warning is also
1772 enabled if -Weffc++ is specified.
1774 @item -Wreorder @r{(C++ only)}
1776 @cindex reordering, warning
1777 @cindex warning for reordering of member initializers
1778 Warn when the order of member initializers given in the code does not
1779 match the order in which they must be executed. For instance:
1785 A(): j (0), i (1) @{ @}
1789 The compiler will rearrange the member initializers for @samp{i}
1790 and @samp{j} to match the declaration order of the members, emitting
1791 a warning to that effect. This warning is enabled by @option{-Wall}.
1794 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
1797 @item -Weffc++ @r{(C++ only)}
1799 Warn about violations of the following style guidelines from Scott Meyers'
1800 @cite{Effective C++} book:
1804 Item 11: Define a copy constructor and an assignment operator for classes
1805 with dynamically allocated memory.
1808 Item 12: Prefer initialization to assignment in constructors.
1811 Item 14: Make destructors virtual in base classes.
1814 Item 15: Have @code{operator=} return a reference to @code{*this}.
1817 Item 23: Don't try to return a reference when you must return an object.
1821 Also warn about violations of the following style guidelines from
1822 Scott Meyers' @cite{More Effective C++} book:
1826 Item 6: Distinguish between prefix and postfix forms of increment and
1827 decrement operators.
1830 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
1834 When selecting this option, be aware that the standard library
1835 headers do not obey all of these guidelines; use @samp{grep -v}
1836 to filter out those warnings.
1838 @item -Wno-deprecated @r{(C++ only)}
1839 @opindex Wno-deprecated
1840 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
1842 @item -Wstrict-null-sentinel @r{(C++ only)}
1843 @opindex Wstrict-null-sentinel
1844 Warn also about the use of an uncasted @code{NULL} as sentinel. When
1845 compiling only with GCC this is a valid sentinel, as @code{NULL} is defined
1846 to @code{__null}. Although it is a null pointer constant not a null pointer,
1847 it is guaranteed to of the same size as a pointer. But this use is
1848 not portable across different compilers.
1850 @item -Wno-non-template-friend @r{(C++ only)}
1851 @opindex Wno-non-template-friend
1852 Disable warnings when non-templatized friend functions are declared
1853 within a template. Since the advent of explicit template specification
1854 support in G++, if the name of the friend is an unqualified-id (i.e.,
1855 @samp{friend foo(int)}), the C++ language specification demands that the
1856 friend declare or define an ordinary, nontemplate function. (Section
1857 14.5.3). Before G++ implemented explicit specification, unqualified-ids
1858 could be interpreted as a particular specialization of a templatized
1859 function. Because this non-conforming behavior is no longer the default
1860 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
1861 check existing code for potential trouble spots and is on by default.
1862 This new compiler behavior can be turned off with
1863 @option{-Wno-non-template-friend} which keeps the conformant compiler code
1864 but disables the helpful warning.
1866 @item -Wold-style-cast @r{(C++ only)}
1867 @opindex Wold-style-cast
1868 Warn if an old-style (C-style) cast to a non-void type is used within
1869 a C++ program. The new-style casts (@samp{dynamic_cast},
1870 @samp{static_cast}, @samp{reinterpret_cast}, and @samp{const_cast}) are
1871 less vulnerable to unintended effects and much easier to search for.
1873 @item -Woverloaded-virtual @r{(C++ only)}
1874 @opindex Woverloaded-virtual
1875 @cindex overloaded virtual fn, warning
1876 @cindex warning for overloaded virtual fn
1877 Warn when a function declaration hides virtual functions from a
1878 base class. For example, in:
1885 struct B: public A @{
1890 the @code{A} class version of @code{f} is hidden in @code{B}, and code
1898 will fail to compile.
1900 @item -Wno-pmf-conversions @r{(C++ only)}
1901 @opindex Wno-pmf-conversions
1902 Disable the diagnostic for converting a bound pointer to member function
1905 @item -Wsign-promo @r{(C++ only)}
1906 @opindex Wsign-promo
1907 Warn when overload resolution chooses a promotion from unsigned or
1908 enumerated type to a signed type, over a conversion to an unsigned type of
1909 the same size. Previous versions of G++ would try to preserve
1910 unsignedness, but the standard mandates the current behavior.
1915 A& operator = (int);
1925 In this example, G++ will synthesize a default @samp{A& operator =
1926 (const A&);}, while cfront will use the user-defined @samp{operator =}.
1929 @node Objective-C and Objective-C++ Dialect Options
1930 @section Options Controlling Objective-C and Objective-C++ Dialects
1932 @cindex compiler options, Objective-C and Objective-C++
1933 @cindex Objective-C and Objective-C++ options, command line
1934 @cindex options, Objective-C and Objective-C++
1935 (NOTE: This manual does not describe the Objective-C and Objective-C++
1936 languages themselves. See @xref{Standards,,Language Standards
1937 Supported by GCC}, for references.)
1939 This section describes the command-line options that are only meaningful
1940 for Objective-C and Objective-C++ programs, but you can also use most of
1941 the language-independent GNU compiler options.
1942 For example, you might compile a file @code{some_class.m} like this:
1945 gcc -g -fgnu-runtime -O -c some_class.m
1949 In this example, @option{-fgnu-runtime} is an option meant only for
1950 Objective-C and Objective-C++ programs; you can use the other options with
1951 any language supported by GCC@.
1953 Note that since Objective-C is an extension of the C language, Objective-C
1954 compilations may also use options specific to the C front-end (e.g.,
1955 @option{-Wtraditional}). Similarly, Objective-C++ compilations may use
1956 C++-specific options (e.g., @option{-Wabi}).
1958 Here is a list of options that are @emph{only} for compiling Objective-C
1959 and Objective-C++ programs:
1962 @item -fconstant-string-class=@var{class-name}
1963 @opindex fconstant-string-class
1964 Use @var{class-name} as the name of the class to instantiate for each
1965 literal string specified with the syntax @code{@@"@dots{}"}. The default
1966 class name is @code{NXConstantString} if the GNU runtime is being used, and
1967 @code{NSConstantString} if the NeXT runtime is being used (see below). The
1968 @option{-fconstant-cfstrings} option, if also present, will override the
1969 @option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
1970 to be laid out as constant CoreFoundation strings.
1973 @opindex fgnu-runtime
1974 Generate object code compatible with the standard GNU Objective-C
1975 runtime. This is the default for most types of systems.
1977 @item -fnext-runtime
1978 @opindex fnext-runtime
1979 Generate output compatible with the NeXT runtime. This is the default
1980 for NeXT-based systems, including Darwin and Mac OS X@. The macro
1981 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
1984 @item -fno-nil-receivers
1985 @opindex fno-nil-receivers
1986 Assume that all Objective-C message dispatches (e.g.,
1987 @code{[receiver message:arg]}) in this translation unit ensure that the receiver
1988 is not @code{nil}. This allows for more efficient entry points in the runtime
1989 to be used. Currently, this option is only available in conjunction with
1990 the NeXT runtime on Mac OS X 10.3 and later.
1992 @item -fobjc-call-cxx-cdtors
1993 @opindex fobjc-call-cxx-cdtors
1994 For each Objective-C class, check if any of its instance variables is a
1995 C++ object with a non-trivial default constructor. If so, synthesize a
1996 special @code{- (id) .cxx_construct} instance method that will run
1997 non-trivial default constructors on any such instance variables, in order,
1998 and then return @code{self}. Similarly, check if any instance variable
1999 is a C++ object with a non-trivial destructor, and if so, synthesize a
2000 special @code{- (void) .cxx_destruct} method that will run
2001 all such default destructors, in reverse order.
2003 The @code{- (id) .cxx_construct} and/or @code{- (void) .cxx_destruct} methods
2004 thusly generated will only operate on instance variables declared in the
2005 current Objective-C class, and not those inherited from superclasses. It
2006 is the responsibility of the Objective-C runtime to invoke all such methods
2007 in an object's inheritance hierarchy. The @code{- (id) .cxx_construct} methods
2008 will be invoked by the runtime immediately after a new object
2009 instance is allocated; the @code{- (void) .cxx_destruct} methods will
2010 be invoked immediately before the runtime deallocates an object instance.
2012 As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
2013 support for invoking the @code{- (id) .cxx_construct} and
2014 @code{- (void) .cxx_destruct} methods.
2016 @item -fobjc-direct-dispatch
2017 @opindex fobjc-direct-dispatch
2018 Allow fast jumps to the message dispatcher. On Darwin this is
2019 accomplished via the comm page.
2021 @item -fobjc-exceptions
2022 @opindex fobjc-exceptions
2023 Enable syntactic support for structured exception handling in Objective-C,
2024 similar to what is offered by C++ and Java. This option is
2025 unavailable in conjunction with the NeXT runtime on Mac OS X 10.2 and
2034 @@catch (AnObjCClass *exc) @{
2041 @@catch (AnotherClass *exc) @{
2044 @@catch (id allOthers) @{
2054 The @code{@@throw} statement may appear anywhere in an Objective-C or
2055 Objective-C++ program; when used inside of a @code{@@catch} block, the
2056 @code{@@throw} may appear without an argument (as shown above), in which case
2057 the object caught by the @code{@@catch} will be rethrown.
2059 Note that only (pointers to) Objective-C objects may be thrown and
2060 caught using this scheme. When an object is thrown, it will be caught
2061 by the nearest @code{@@catch} clause capable of handling objects of that type,
2062 analogously to how @code{catch} blocks work in C++ and Java. A
2063 @code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
2064 any and all Objective-C exceptions not caught by previous @code{@@catch}
2067 The @code{@@finally} clause, if present, will be executed upon exit from the
2068 immediately preceding @code{@@try @dots{} @@catch} section. This will happen
2069 regardless of whether any exceptions are thrown, caught or rethrown
2070 inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
2071 of the @code{finally} clause in Java.
2073 There are several caveats to using the new exception mechanism:
2077 Although currently designed to be binary compatible with @code{NS_HANDLER}-style
2078 idioms provided by the @code{NSException} class, the new
2079 exceptions can only be used on Mac OS X 10.3 (Panther) and later
2080 systems, due to additional functionality needed in the (NeXT) Objective-C
2084 As mentioned above, the new exceptions do not support handling
2085 types other than Objective-C objects. Furthermore, when used from
2086 Objective-C++, the Objective-C exception model does not interoperate with C++
2087 exceptions at this time. This means you cannot @code{@@throw} an exception
2088 from Objective-C and @code{catch} it in C++, or vice versa
2089 (i.e., @code{throw @dots{} @@catch}).
2092 The @option{-fobjc-exceptions} switch also enables the use of synchronization
2093 blocks for thread-safe execution:
2096 @@synchronized (ObjCClass *guard) @{
2101 Upon entering the @code{@@synchronized} block, a thread of execution shall
2102 first check whether a lock has been placed on the corresponding @code{guard}
2103 object by another thread. If it has, the current thread shall wait until
2104 the other thread relinquishes its lock. Once @code{guard} becomes available,
2105 the current thread will place its own lock on it, execute the code contained in
2106 the @code{@@synchronized} block, and finally relinquish the lock (thereby
2107 making @code{guard} available to other threads).
2109 Unlike Java, Objective-C does not allow for entire methods to be marked
2110 @code{@@synchronized}. Note that throwing exceptions out of
2111 @code{@@synchronized} blocks is allowed, and will cause the guarding object
2112 to be unlocked properly.
2116 Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
2118 @item -freplace-objc-classes
2119 @opindex freplace-objc-classes
2120 Emit a special marker instructing @command{ld(1)} not to statically link in
2121 the resulting object file, and allow @command{dyld(1)} to load it in at
2122 run time instead. This is used in conjunction with the Fix-and-Continue
2123 debugging mode, where the object file in question may be recompiled and
2124 dynamically reloaded in the course of program execution, without the need
2125 to restart the program itself. Currently, Fix-and-Continue functionality
2126 is only available in conjunction with the NeXT runtime on Mac OS X 10.3
2131 When compiling for the NeXT runtime, the compiler ordinarily replaces calls
2132 to @code{objc_getClass("@dots{}")} (when the name of the class is known at
2133 compile time) with static class references that get initialized at load time,
2134 which improves run-time performance. Specifying the @option{-fzero-link} flag
2135 suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
2136 to be retained. This is useful in Zero-Link debugging mode, since it allows
2137 for individual class implementations to be modified during program execution.
2141 Dump interface declarations for all classes seen in the source file to a
2142 file named @file{@var{sourcename}.decl}.
2144 @item -Wassign-intercept
2145 @opindex Wassign-intercept
2146 Warn whenever an Objective-C assignment is being intercepted by the
2150 @opindex Wno-protocol
2151 If a class is declared to implement a protocol, a warning is issued for
2152 every method in the protocol that is not implemented by the class. The
2153 default behavior is to issue a warning for every method not explicitly
2154 implemented in the class, even if a method implementation is inherited
2155 from the superclass. If you use the @option{-Wno-protocol} option, then
2156 methods inherited from the superclass are considered to be implemented,
2157 and no warning is issued for them.
2161 Warn if multiple methods of different types for the same selector are
2162 found during compilation. The check is performed on the list of methods
2163 in the final stage of compilation. Additionally, a check is performed
2164 for each selector appearing in a @code{@@selector(@dots{})}
2165 expression, and a corresponding method for that selector has been found
2166 during compilation. Because these checks scan the method table only at
2167 the end of compilation, these warnings are not produced if the final
2168 stage of compilation is not reached, for example because an error is
2169 found during compilation, or because the @option{-fsyntax-only} option is
2172 @item -Wstrict-selector-match
2173 @opindex Wstrict-selector-match
2174 Warn if multiple methods with differing argument and/or return types are
2175 found for a given selector when attempting to send a message using this
2176 selector to a receiver of type @code{id} or @code{Class}. When this flag
2177 is off (which is the default behavior), the compiler will omit such warnings
2178 if any differences found are confined to types which share the same size
2181 @item -Wundeclared-selector
2182 @opindex Wundeclared-selector
2183 Warn if a @code{@@selector(@dots{})} expression referring to an
2184 undeclared selector is found. A selector is considered undeclared if no
2185 method with that name has been declared before the
2186 @code{@@selector(@dots{})} expression, either explicitly in an
2187 @code{@@interface} or @code{@@protocol} declaration, or implicitly in
2188 an @code{@@implementation} section. This option always performs its
2189 checks as soon as a @code{@@selector(@dots{})} expression is found,
2190 while @option{-Wselector} only performs its checks in the final stage of
2191 compilation. This also enforces the coding style convention
2192 that methods and selectors must be declared before being used.
2194 @item -print-objc-runtime-info
2195 @opindex print-objc-runtime-info
2196 Generate C header describing the largest structure that is passed by
2201 @node Language Independent Options
2202 @section Options to Control Diagnostic Messages Formatting
2203 @cindex options to control diagnostics formatting
2204 @cindex diagnostic messages
2205 @cindex message formatting
2207 Traditionally, diagnostic messages have been formatted irrespective of
2208 the output device's aspect (e.g.@: its width, @dots{}). The options described
2209 below can be used to control the diagnostic messages formatting
2210 algorithm, e.g.@: how many characters per line, how often source location
2211 information should be reported. Right now, only the C++ front end can
2212 honor these options. However it is expected, in the near future, that
2213 the remaining front ends would be able to digest them correctly.
2216 @item -fmessage-length=@var{n}
2217 @opindex fmessage-length
2218 Try to format error messages so that they fit on lines of about @var{n}
2219 characters. The default is 72 characters for @command{g++} and 0 for the rest of
2220 the front ends supported by GCC@. If @var{n} is zero, then no
2221 line-wrapping will be done; each error message will appear on a single
2224 @opindex fdiagnostics-show-location
2225 @item -fdiagnostics-show-location=once
2226 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
2227 reporter to emit @emph{once} source location information; that is, in
2228 case the message is too long to fit on a single physical line and has to
2229 be wrapped, the source location won't be emitted (as prefix) again,
2230 over and over, in subsequent continuation lines. This is the default
2233 @item -fdiagnostics-show-location=every-line
2234 Only meaningful in line-wrapping mode. Instructs the diagnostic
2235 messages reporter to emit the same source location information (as
2236 prefix) for physical lines that result from the process of breaking
2237 a message which is too long to fit on a single line.
2239 @item -fdiagnostics-show-option
2240 @opindex fdiagnostics-show-option
2241 This option instructs the diagnostic machinery to add text to each
2242 diagnostic emitted, which indicates which command line option directly
2243 controls that diagnostic, when such an option is known to the
2244 diagnostic machinery.
2248 @node Warning Options
2249 @section Options to Request or Suppress Warnings
2250 @cindex options to control warnings
2251 @cindex warning messages
2252 @cindex messages, warning
2253 @cindex suppressing warnings
2255 Warnings are diagnostic messages that report constructions which
2256 are not inherently erroneous but which are risky or suggest there
2257 may have been an error.
2259 You can request many specific warnings with options beginning @samp{-W},
2260 for example @option{-Wimplicit} to request warnings on implicit
2261 declarations. Each of these specific warning options also has a
2262 negative form beginning @samp{-Wno-} to turn off warnings;
2263 for example, @option{-Wno-implicit}. This manual lists only one of the
2264 two forms, whichever is not the default.
2266 The following options control the amount and kinds of warnings produced
2267 by GCC; for further, language-specific options also refer to
2268 @ref{C++ Dialect Options} and @ref{Objective-C and Objective-C++ Dialect
2272 @cindex syntax checking
2274 @opindex fsyntax-only
2275 Check the code for syntax errors, but don't do anything beyond that.
2279 Issue all the warnings demanded by strict ISO C and ISO C++;
2280 reject all programs that use forbidden extensions, and some other
2281 programs that do not follow ISO C and ISO C++. For ISO C, follows the
2282 version of the ISO C standard specified by any @option{-std} option used.
2284 Valid ISO C and ISO C++ programs should compile properly with or without
2285 this option (though a rare few will require @option{-ansi} or a
2286 @option{-std} option specifying the required version of ISO C)@. However,
2287 without this option, certain GNU extensions and traditional C and C++
2288 features are supported as well. With this option, they are rejected.
2290 @option{-pedantic} does not cause warning messages for use of the
2291 alternate keywords whose names begin and end with @samp{__}. Pedantic
2292 warnings are also disabled in the expression that follows
2293 @code{__extension__}. However, only system header files should use
2294 these escape routes; application programs should avoid them.
2295 @xref{Alternate Keywords}.
2297 Some users try to use @option{-pedantic} to check programs for strict ISO
2298 C conformance. They soon find that it does not do quite what they want:
2299 it finds some non-ISO practices, but not all---only those for which
2300 ISO C @emph{requires} a diagnostic, and some others for which
2301 diagnostics have been added.
2303 A feature to report any failure to conform to ISO C might be useful in
2304 some instances, but would require considerable additional work and would
2305 be quite different from @option{-pedantic}. We don't have plans to
2306 support such a feature in the near future.
2308 Where the standard specified with @option{-std} represents a GNU
2309 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
2310 corresponding @dfn{base standard}, the version of ISO C on which the GNU
2311 extended dialect is based. Warnings from @option{-pedantic} are given
2312 where they are required by the base standard. (It would not make sense
2313 for such warnings to be given only for features not in the specified GNU
2314 C dialect, since by definition the GNU dialects of C include all
2315 features the compiler supports with the given option, and there would be
2316 nothing to warn about.)
2318 @item -pedantic-errors
2319 @opindex pedantic-errors
2320 Like @option{-pedantic}, except that errors are produced rather than
2325 Inhibit all warning messages.
2329 Inhibit warning messages about the use of @samp{#import}.
2331 @item -Wchar-subscripts
2332 @opindex Wchar-subscripts
2333 Warn if an array subscript has type @code{char}. This is a common cause
2334 of error, as programmers often forget that this type is signed on some
2336 This warning is enabled by @option{-Wall}.
2340 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
2341 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
2342 This warning is enabled by @option{-Wall}.
2344 @item -Wfatal-errors
2345 @opindex Wfatal-errors
2346 This option causes the compiler to abort compilation on the first error
2347 occurred rather than trying to keep going and printing further error
2352 @opindex ffreestanding
2353 @opindex fno-builtin
2354 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
2355 the arguments supplied have types appropriate to the format string
2356 specified, and that the conversions specified in the format string make
2357 sense. This includes standard functions, and others specified by format
2358 attributes (@pxref{Function Attributes}), in the @code{printf},
2359 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
2360 not in the C standard) families (or other target-specific families).
2361 Which functions are checked without format attributes having been
2362 specified depends on the standard version selected, and such checks of
2363 functions without the attribute specified are disabled by
2364 @option{-ffreestanding} or @option{-fno-builtin}.
2366 The formats are checked against the format features supported by GNU
2367 libc version 2.2. These include all ISO C90 and C99 features, as well
2368 as features from the Single Unix Specification and some BSD and GNU
2369 extensions. Other library implementations may not support all these
2370 features; GCC does not support warning about features that go beyond a
2371 particular library's limitations. However, if @option{-pedantic} is used
2372 with @option{-Wformat}, warnings will be given about format features not
2373 in the selected standard version (but not for @code{strfmon} formats,
2374 since those are not in any version of the C standard). @xref{C Dialect
2375 Options,,Options Controlling C Dialect}.
2377 Since @option{-Wformat} also checks for null format arguments for
2378 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2380 @option{-Wformat} is included in @option{-Wall}. For more control over some
2381 aspects of format checking, the options @option{-Wformat-y2k},
2382 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2383 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2384 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2387 @opindex Wformat-y2k
2388 If @option{-Wformat} is specified, also warn about @code{strftime}
2389 formats which may yield only a two-digit year.
2391 @item -Wno-format-extra-args
2392 @opindex Wno-format-extra-args
2393 If @option{-Wformat} is specified, do not warn about excess arguments to a
2394 @code{printf} or @code{scanf} format function. The C standard specifies
2395 that such arguments are ignored.
2397 Where the unused arguments lie between used arguments that are
2398 specified with @samp{$} operand number specifications, normally
2399 warnings are still given, since the implementation could not know what
2400 type to pass to @code{va_arg} to skip the unused arguments. However,
2401 in the case of @code{scanf} formats, this option will suppress the
2402 warning if the unused arguments are all pointers, since the Single
2403 Unix Specification says that such unused arguments are allowed.
2405 @item -Wno-format-zero-length
2406 @opindex Wno-format-zero-length
2407 If @option{-Wformat} is specified, do not warn about zero-length formats.
2408 The C standard specifies that zero-length formats are allowed.
2410 @item -Wformat-nonliteral
2411 @opindex Wformat-nonliteral
2412 If @option{-Wformat} is specified, also warn if the format string is not a
2413 string literal and so cannot be checked, unless the format function
2414 takes its format arguments as a @code{va_list}.
2416 @item -Wformat-security
2417 @opindex Wformat-security
2418 If @option{-Wformat} is specified, also warn about uses of format
2419 functions that represent possible security problems. At present, this
2420 warns about calls to @code{printf} and @code{scanf} functions where the
2421 format string is not a string literal and there are no format arguments,
2422 as in @code{printf (foo);}. This may be a security hole if the format
2423 string came from untrusted input and contains @samp{%n}. (This is
2424 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2425 in future warnings may be added to @option{-Wformat-security} that are not
2426 included in @option{-Wformat-nonliteral}.)
2430 Enable @option{-Wformat} plus format checks not included in
2431 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2432 -Wformat-nonliteral -Wformat-security -Wformat-y2k}.
2436 Warn about passing a null pointer for arguments marked as
2437 requiring a non-null value by the @code{nonnull} function attribute.
2439 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2440 can be disabled with the @option{-Wno-nonnull} option.
2442 @item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
2444 Warn about uninitialized variables which are initialized with themselves.
2445 Note this option can only be used with the @option{-Wuninitialized} option,
2446 which in turn only works with @option{-O1} and above.
2448 For example, GCC will warn about @code{i} being uninitialized in the
2449 following snippet only when @option{-Winit-self} has been specified:
2460 @item -Wimplicit-int
2461 @opindex Wimplicit-int
2462 Warn when a declaration does not specify a type.
2463 This warning is enabled by @option{-Wall}.
2465 @item -Wimplicit-function-declaration
2466 @itemx -Werror-implicit-function-declaration
2467 @opindex Wimplicit-function-declaration
2468 @opindex Werror-implicit-function-declaration
2469 Give a warning (or error) whenever a function is used before being
2470 declared. The form @option{-Wno-error-implicit-function-declaration}
2472 This warning is enabled by @option{-Wall} (as a warning, not an error).
2476 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2477 This warning is enabled by @option{-Wall}.
2481 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2482 function with external linkage, returning int, taking either zero
2483 arguments, two, or three arguments of appropriate types.
2484 This warning is enabled by @option{-Wall}.
2486 @item -Wmissing-braces
2487 @opindex Wmissing-braces
2488 Warn if an aggregate or union initializer is not fully bracketed. In
2489 the following example, the initializer for @samp{a} is not fully
2490 bracketed, but that for @samp{b} is fully bracketed.
2493 int a[2][2] = @{ 0, 1, 2, 3 @};
2494 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2497 This warning is enabled by @option{-Wall}.
2499 @item -Wmissing-include-dirs @r{(C, C++, Objective-C and Objective-C++ only)}
2500 @opindex Wmissing-include-dirs
2501 Warn if a user-supplied include directory does not exist.
2504 @opindex Wparentheses
2505 Warn if parentheses are omitted in certain contexts, such
2506 as when there is an assignment in a context where a truth value
2507 is expected, or when operators are nested whose precedence people
2508 often get confused about. Only the warning for an assignment used as
2509 a truth value is supported when compiling C++; the other warnings are
2510 only supported when compiling C@.
2512 Also warn if a comparison like @samp{x<=y<=z} appears; this is
2513 equivalent to @samp{(x<=y ? 1 : 0) <= z}, which is a different
2514 interpretation from that of ordinary mathematical notation.
2516 Also warn about constructions where there may be confusion to which
2517 @code{if} statement an @code{else} branch belongs. Here is an example of
2532 In C, every @code{else} branch belongs to the innermost possible @code{if}
2533 statement, which in this example is @code{if (b)}. This is often not
2534 what the programmer expected, as illustrated in the above example by
2535 indentation the programmer chose. When there is the potential for this
2536 confusion, GCC will issue a warning when this flag is specified.
2537 To eliminate the warning, add explicit braces around the innermost
2538 @code{if} statement so there is no way the @code{else} could belong to
2539 the enclosing @code{if}. The resulting code would look like this:
2555 This warning is enabled by @option{-Wall}.
2557 @item -Wsequence-point
2558 @opindex Wsequence-point
2559 Warn about code that may have undefined semantics because of violations
2560 of sequence point rules in the C and C++ standards.
2562 The C and C++ standards defines the order in which expressions in a C/C++
2563 program are evaluated in terms of @dfn{sequence points}, which represent
2564 a partial ordering between the execution of parts of the program: those
2565 executed before the sequence point, and those executed after it. These
2566 occur after the evaluation of a full expression (one which is not part
2567 of a larger expression), after the evaluation of the first operand of a
2568 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2569 function is called (but after the evaluation of its arguments and the
2570 expression denoting the called function), and in certain other places.
2571 Other than as expressed by the sequence point rules, the order of
2572 evaluation of subexpressions of an expression is not specified. All
2573 these rules describe only a partial order rather than a total order,
2574 since, for example, if two functions are called within one expression
2575 with no sequence point between them, the order in which the functions
2576 are called is not specified. However, the standards committee have
2577 ruled that function calls do not overlap.
2579 It is not specified when between sequence points modifications to the
2580 values of objects take effect. Programs whose behavior depends on this
2581 have undefined behavior; the C and C++ standards specify that ``Between
2582 the previous and next sequence point an object shall have its stored
2583 value modified at most once by the evaluation of an expression.
2584 Furthermore, the prior value shall be read only to determine the value
2585 to be stored.''. If a program breaks these rules, the results on any
2586 particular implementation are entirely unpredictable.
2588 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2589 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2590 diagnosed by this option, and it may give an occasional false positive
2591 result, but in general it has been found fairly effective at detecting
2592 this sort of problem in programs.
2594 The standard is worded confusingly, therefore there is some debate
2595 over the precise meaning of the sequence point rules in subtle cases.
2596 Links to discussions of the problem, including proposed formal
2597 definitions, may be found on the GCC readings page, at
2598 @w{@uref{http://gcc.gnu.org/readings.html}}.
2600 This warning is enabled by @option{-Wall} for C and C++.
2603 @opindex Wreturn-type
2604 Warn whenever a function is defined with a return-type that defaults to
2605 @code{int}. Also warn about any @code{return} statement with no
2606 return-value in a function whose return-type is not @code{void}.
2608 For C, also warn if the return type of a function has a type qualifier
2609 such as @code{const}. Such a type qualifier has no effect, since the
2610 value returned by a function is not an lvalue. ISO C prohibits
2611 qualified @code{void} return types on function definitions, so such
2612 return types always receive a warning even without this option.
2614 For C++, a function without return type always produces a diagnostic
2615 message, even when @option{-Wno-return-type} is specified. The only
2616 exceptions are @samp{main} and functions defined in system headers.
2618 This warning is enabled by @option{-Wall}.
2622 Warn whenever a @code{switch} statement has an index of enumerated type
2623 and lacks a @code{case} for one or more of the named codes of that
2624 enumeration. (The presence of a @code{default} label prevents this
2625 warning.) @code{case} labels outside the enumeration range also
2626 provoke warnings when this option is used.
2627 This warning is enabled by @option{-Wall}.
2629 @item -Wswitch-default
2630 @opindex Wswitch-switch
2631 Warn whenever a @code{switch} statement does not have a @code{default}
2635 @opindex Wswitch-enum
2636 Warn whenever a @code{switch} statement has an index of enumerated type
2637 and lacks a @code{case} for one or more of the named codes of that
2638 enumeration. @code{case} labels outside the enumeration range also
2639 provoke warnings when this option is used.
2643 Warn if any trigraphs are encountered that might change the meaning of
2644 the program (trigraphs within comments are not warned about).
2645 This warning is enabled by @option{-Wall}.
2647 @item -Wunused-function
2648 @opindex Wunused-function
2649 Warn whenever a static function is declared but not defined or a
2650 non-inline static function is unused.
2651 This warning is enabled by @option{-Wall}.
2653 @item -Wunused-label
2654 @opindex Wunused-label
2655 Warn whenever a label is declared but not used.
2656 This warning is enabled by @option{-Wall}.
2658 To suppress this warning use the @samp{unused} attribute
2659 (@pxref{Variable Attributes}).
2661 @item -Wunused-parameter
2662 @opindex Wunused-parameter
2663 Warn whenever a function parameter is unused aside from its declaration.
2665 To suppress this warning use the @samp{unused} attribute
2666 (@pxref{Variable Attributes}).
2668 @item -Wunused-variable
2669 @opindex Wunused-variable
2670 Warn whenever a local variable or non-constant static variable is unused
2671 aside from its declaration.
2672 This warning is enabled by @option{-Wall}.
2674 To suppress this warning use the @samp{unused} attribute
2675 (@pxref{Variable Attributes}).
2677 @item -Wunused-value
2678 @opindex Wunused-value
2679 Warn whenever a statement computes a result that is explicitly not used.
2680 This warning is enabled by @option{-Wall}.
2682 To suppress this warning cast the expression to @samp{void}.
2686 All the above @option{-Wunused} options combined.
2688 In order to get a warning about an unused function parameter, you must
2689 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
2690 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
2692 @item -Wuninitialized
2693 @opindex Wuninitialized
2694 Warn if an automatic variable is used without first being initialized or
2695 if a variable may be clobbered by a @code{setjmp} call.
2697 These warnings are possible only in optimizing compilation,
2698 because they require data flow information that is computed only
2699 when optimizing. If you do not specify @option{-O}, you will not get
2700 these warnings. Instead, GCC will issue a warning about @option{-Wuninitialized}
2701 requiring @option{-O}.
2703 If you want to warn about code which uses the uninitialized value of the
2704 variable in its own initializer, use the @option{-Winit-self} option.
2706 These warnings occur for individual uninitialized or clobbered
2707 elements of structure, union or array variables as well as for
2708 variables which are uninitialized or clobbered as a whole. They do
2709 not occur for variables or elements declared @code{volatile}. Because
2710 these warnings depend on optimization, the exact variables or elements
2711 for which there are warnings will depend on the precise optimization
2712 options and version of GCC used.
2714 Note that there may be no warning about a variable that is used only
2715 to compute a value that itself is never used, because such
2716 computations may be deleted by data flow analysis before the warnings
2719 These warnings are made optional because GCC is not smart
2720 enough to see all the reasons why the code might be correct
2721 despite appearing to have an error. Here is one example of how
2742 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
2743 always initialized, but GCC doesn't know this. Here is
2744 another common case:
2749 if (change_y) save_y = y, y = new_y;
2751 if (change_y) y = save_y;
2756 This has no bug because @code{save_y} is used only if it is set.
2758 @cindex @code{longjmp} warnings
2759 This option also warns when a non-volatile automatic variable might be
2760 changed by a call to @code{longjmp}. These warnings as well are possible
2761 only in optimizing compilation.
2763 The compiler sees only the calls to @code{setjmp}. It cannot know
2764 where @code{longjmp} will be called; in fact, a signal handler could
2765 call it at any point in the code. As a result, you may get a warning
2766 even when there is in fact no problem because @code{longjmp} cannot
2767 in fact be called at the place which would cause a problem.
2769 Some spurious warnings can be avoided if you declare all the functions
2770 you use that never return as @code{noreturn}. @xref{Function
2773 This warning is enabled by @option{-Wall}.
2775 @item -Wunknown-pragmas
2776 @opindex Wunknown-pragmas
2777 @cindex warning for unknown pragmas
2778 @cindex unknown pragmas, warning
2779 @cindex pragmas, warning of unknown
2780 Warn when a #pragma directive is encountered which is not understood by
2781 GCC@. If this command line option is used, warnings will even be issued
2782 for unknown pragmas in system header files. This is not the case if
2783 the warnings were only enabled by the @option{-Wall} command line option.
2786 @opindex Wno-pragmas
2788 Do not warn about misuses of pragmas, such as incorrect parameters,
2789 invalid syntax, or conflicts between pragmas. See also
2790 @samp{-Wunknown-pragmas}.
2792 @item -Wstrict-aliasing
2793 @opindex Wstrict-aliasing
2794 This option is only active when @option{-fstrict-aliasing} is active.
2795 It warns about code which might break the strict aliasing rules that the
2796 compiler is using for optimization. The warning does not catch all
2797 cases, but does attempt to catch the more common pitfalls. It is
2798 included in @option{-Wall}.
2800 @item -Wstrict-aliasing=2
2801 @opindex Wstrict-aliasing=2
2802 This option is only active when @option{-fstrict-aliasing} is active.
2803 It warns about code which might break the strict aliasing rules that the
2804 compiler is using for optimization. This warning catches more cases than
2805 @option{-Wstrict-aliasing}, but it will also give a warning for some ambiguous
2806 cases that are safe.
2810 All of the above @samp{-W} options combined. This enables all the
2811 warnings about constructions that some users consider questionable, and
2812 that are easy to avoid (or modify to prevent the warning), even in
2813 conjunction with macros. This also enables some language-specific
2814 warnings described in @ref{C++ Dialect Options} and
2815 @ref{Objective-C and Objective-C++ Dialect Options}.
2818 The following @option{-W@dots{}} options are not implied by @option{-Wall}.
2819 Some of them warn about constructions that users generally do not
2820 consider questionable, but which occasionally you might wish to check
2821 for; others warn about constructions that are necessary or hard to avoid
2822 in some cases, and there is no simple way to modify the code to suppress
2829 (This option used to be called @option{-W}. The older name is still
2830 supported, but the newer name is more descriptive.) Print extra warning
2831 messages for these events:
2835 A function can return either with or without a value. (Falling
2836 off the end of the function body is considered returning without
2837 a value.) For example, this function would evoke such a
2851 An expression-statement or the left-hand side of a comma expression
2852 contains no side effects.
2853 To suppress the warning, cast the unused expression to void.
2854 For example, an expression such as @samp{x[i,j]} will cause a warning,
2855 but @samp{x[(void)i,j]} will not.
2858 An unsigned value is compared against zero with @samp{<} or @samp{>=}.
2861 Storage-class specifiers like @code{static} are not the first things in
2862 a declaration. According to the C Standard, this usage is obsolescent.
2865 If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
2869 A comparison between signed and unsigned values could produce an
2870 incorrect result when the signed value is converted to unsigned.
2871 (But don't warn if @option{-Wno-sign-compare} is also specified.)
2874 An aggregate has an initializer which does not initialize all members.
2875 This warning can be independently controlled by
2876 @option{-Wmissing-field-initializers}.
2879 An initialized field without side effects is overridden when using
2880 designated initializers (@pxref{Designated Inits, , Designated
2881 Initializers}). This warning can be independently controlled by
2882 @option{-Woverride-init}.
2885 A function parameter is declared without a type specifier in K&R-style
2893 An empty body occurs in an @samp{if} or @samp{else} statement.
2896 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2897 @samp{>}, or @samp{>=}.
2900 A variable might be changed by @samp{longjmp} or @samp{vfork}.
2903 Any of several floating-point events that often indicate errors, such as
2904 overflow, underflow, loss of precision, etc.
2906 @item @r{(C++ only)}
2907 An enumerator and a non-enumerator both appear in a conditional expression.
2909 @item @r{(C++ only)}
2910 A non-static reference or non-static @samp{const} member appears in a
2911 class without constructors.
2913 @item @r{(C++ only)}
2914 Ambiguous virtual bases.
2916 @item @r{(C++ only)}
2917 Subscripting an array which has been declared @samp{register}.
2919 @item @r{(C++ only)}
2920 Taking the address of a variable which has been declared @samp{register}.
2922 @item @r{(C++ only)}
2923 A base class is not initialized in a derived class' copy constructor.
2926 @item -Wno-div-by-zero
2927 @opindex Wno-div-by-zero
2928 @opindex Wdiv-by-zero
2929 Do not warn about compile-time integer division by zero. Floating point
2930 division by zero is not warned about, as it can be a legitimate way of
2931 obtaining infinities and NaNs.
2933 @item -Wsystem-headers
2934 @opindex Wsystem-headers
2935 @cindex warnings from system headers
2936 @cindex system headers, warnings from
2937 Print warning messages for constructs found in system header files.
2938 Warnings from system headers are normally suppressed, on the assumption
2939 that they usually do not indicate real problems and would only make the
2940 compiler output harder to read. Using this command line option tells
2941 GCC to emit warnings from system headers as if they occurred in user
2942 code. However, note that using @option{-Wall} in conjunction with this
2943 option will @emph{not} warn about unknown pragmas in system
2944 headers---for that, @option{-Wunknown-pragmas} must also be used.
2947 @opindex Wfloat-equal
2948 Warn if floating point values are used in equality comparisons.
2950 The idea behind this is that sometimes it is convenient (for the
2951 programmer) to consider floating-point values as approximations to
2952 infinitely precise real numbers. If you are doing this, then you need
2953 to compute (by analyzing the code, or in some other way) the maximum or
2954 likely maximum error that the computation introduces, and allow for it
2955 when performing comparisons (and when producing output, but that's a
2956 different problem). In particular, instead of testing for equality, you
2957 would check to see whether the two values have ranges that overlap; and
2958 this is done with the relational operators, so equality comparisons are
2961 @item -Wtraditional @r{(C only)}
2962 @opindex Wtraditional
2963 Warn about certain constructs that behave differently in traditional and
2964 ISO C@. Also warn about ISO C constructs that have no traditional C
2965 equivalent, and/or problematic constructs which should be avoided.
2969 Macro parameters that appear within string literals in the macro body.
2970 In traditional C macro replacement takes place within string literals,
2971 but does not in ISO C@.
2974 In traditional C, some preprocessor directives did not exist.
2975 Traditional preprocessors would only consider a line to be a directive
2976 if the @samp{#} appeared in column 1 on the line. Therefore
2977 @option{-Wtraditional} warns about directives that traditional C
2978 understands but would ignore because the @samp{#} does not appear as the
2979 first character on the line. It also suggests you hide directives like
2980 @samp{#pragma} not understood by traditional C by indenting them. Some
2981 traditional implementations would not recognize @samp{#elif}, so it
2982 suggests avoiding it altogether.
2985 A function-like macro that appears without arguments.
2988 The unary plus operator.
2991 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
2992 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
2993 constants.) Note, these suffixes appear in macros defined in the system
2994 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
2995 Use of these macros in user code might normally lead to spurious
2996 warnings, however GCC's integrated preprocessor has enough context to
2997 avoid warning in these cases.
3000 A function declared external in one block and then used after the end of
3004 A @code{switch} statement has an operand of type @code{long}.
3007 A non-@code{static} function declaration follows a @code{static} one.
3008 This construct is not accepted by some traditional C compilers.
3011 The ISO type of an integer constant has a different width or
3012 signedness from its traditional type. This warning is only issued if
3013 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
3014 typically represent bit patterns, are not warned about.
3017 Usage of ISO string concatenation is detected.
3020 Initialization of automatic aggregates.
3023 Identifier conflicts with labels. Traditional C lacks a separate
3024 namespace for labels.
3027 Initialization of unions. If the initializer is zero, the warning is
3028 omitted. This is done under the assumption that the zero initializer in
3029 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
3030 initializer warnings and relies on default initialization to zero in the
3034 Conversions by prototypes between fixed/floating point values and vice
3035 versa. The absence of these prototypes when compiling with traditional
3036 C would cause serious problems. This is a subset of the possible
3037 conversion warnings, for the full set use @option{-Wconversion}.
3040 Use of ISO C style function definitions. This warning intentionally is
3041 @emph{not} issued for prototype declarations or variadic functions
3042 because these ISO C features will appear in your code when using
3043 libiberty's traditional C compatibility macros, @code{PARAMS} and
3044 @code{VPARAMS}. This warning is also bypassed for nested functions
3045 because that feature is already a GCC extension and thus not relevant to
3046 traditional C compatibility.
3049 @item -Wdeclaration-after-statement @r{(C only)}
3050 @opindex Wdeclaration-after-statement
3051 Warn when a declaration is found after a statement in a block. This
3052 construct, known from C++, was introduced with ISO C99 and is by default
3053 allowed in GCC@. It is not supported by ISO C90 and was not supported by
3054 GCC versions before GCC 3.0. @xref{Mixed Declarations}.
3058 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
3060 @item -Wno-endif-labels
3061 @opindex Wno-endif-labels
3062 @opindex Wendif-labels
3063 Do not warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
3067 Warn whenever a local variable shadows another local variable, parameter or
3068 global variable or whenever a built-in function is shadowed.
3070 @item -Wlarger-than-@var{len}
3071 @opindex Wlarger-than
3072 Warn whenever an object of larger than @var{len} bytes is defined.
3074 @item -Wunsafe-loop-optimizations
3075 @opindex Wunsafe-loop-optimizations
3076 Warn if the loop cannot be optimized because the compiler could not
3077 assume anything on the bounds of the loop indices. With
3078 @option{-funsafe-loop-optimizations} warn if the compiler made
3081 @item -Wpointer-arith
3082 @opindex Wpointer-arith
3083 Warn about anything that depends on the ``size of'' a function type or
3084 of @code{void}. GNU C assigns these types a size of 1, for
3085 convenience in calculations with @code{void *} pointers and pointers
3088 @item -Wbad-function-cast @r{(C only)}
3089 @opindex Wbad-function-cast
3090 Warn whenever a function call is cast to a non-matching type.
3091 For example, warn if @code{int malloc()} is cast to @code{anything *}.
3094 Warn about ISO C constructs that are outside of the common subset of
3095 ISO C and ISO C++, e.g.@: request for implicit conversion from
3096 @code{void *} to a pointer to non-@code{void} type.
3100 Warn whenever a pointer is cast so as to remove a type qualifier from
3101 the target type. For example, warn if a @code{const char *} is cast
3102 to an ordinary @code{char *}.
3105 @opindex Wcast-align
3106 Warn whenever a pointer is cast such that the required alignment of the
3107 target is increased. For example, warn if a @code{char *} is cast to
3108 an @code{int *} on machines where integers can only be accessed at
3109 two- or four-byte boundaries.
3111 @item -Wwrite-strings
3112 @opindex Wwrite-strings
3113 When compiling C, give string constants the type @code{const
3114 char[@var{length}]} so that
3115 copying the address of one into a non-@code{const} @code{char *}
3116 pointer will get a warning; when compiling C++, warn about the
3117 deprecated conversion from string literals to @code{char *}. This
3118 warning, by default, is enabled for C++ programs.
3119 These warnings will help you find at
3120 compile time code that can try to write into a string constant, but
3121 only if you have been very careful about using @code{const} in
3122 declarations and prototypes. Otherwise, it will just be a nuisance;
3123 this is why we did not make @option{-Wall} request these warnings.
3126 @opindex Wconversion
3127 Warn if a prototype causes a type conversion that is different from what
3128 would happen to the same argument in the absence of a prototype. This
3129 includes conversions of fixed point to floating and vice versa, and
3130 conversions changing the width or signedness of a fixed point argument
3131 except when the same as the default promotion.
3133 Also, warn if a negative integer constant expression is implicitly
3134 converted to an unsigned type. For example, warn about the assignment
3135 @code{x = -1} if @code{x} is unsigned. But do not warn about explicit
3136 casts like @code{(unsigned) -1}.
3138 @item -Wsign-compare
3139 @opindex Wsign-compare
3140 @cindex warning for comparison of signed and unsigned values
3141 @cindex comparison of signed and unsigned values, warning
3142 @cindex signed and unsigned values, comparison warning
3143 Warn when a comparison between signed and unsigned values could produce
3144 an incorrect result when the signed value is converted to unsigned.
3145 This warning is also enabled by @option{-Wextra}; to get the other warnings
3146 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
3148 @item -Waggregate-return
3149 @opindex Waggregate-return
3150 Warn if any functions that return structures or unions are defined or
3151 called. (In languages where you can return an array, this also elicits
3155 @opindex Walways-true
3156 Warn about comparisons which are always true such as testing if
3157 unsigned values are greater than or equal to zero. This warning is
3158 enabled by @option{-Wall}.
3160 @item -Wno-attributes
3161 @opindex Wno-attributes
3162 @opindex Wattributes
3163 Do not warn if an unexpected @code{__attribute__} is used, such as
3164 unrecognized attributes, function attributes applied to variables,
3165 etc. This will not stop errors for incorrect use of supported
3168 @item -Wstrict-prototypes @r{(C only)}
3169 @opindex Wstrict-prototypes
3170 Warn if a function is declared or defined without specifying the
3171 argument types. (An old-style function definition is permitted without
3172 a warning if preceded by a declaration which specifies the argument
3175 @item -Wold-style-definition @r{(C only)}
3176 @opindex Wold-style-definition
3177 Warn if an old-style function definition is used. A warning is given
3178 even if there is a previous prototype.
3180 @item -Wmissing-prototypes @r{(C only)}
3181 @opindex Wmissing-prototypes
3182 Warn if a global function is defined without a previous prototype
3183 declaration. This warning is issued even if the definition itself
3184 provides a prototype. The aim is to detect global functions that fail
3185 to be declared in header files.
3187 @item -Wmissing-declarations @r{(C only)}
3188 @opindex Wmissing-declarations
3189 Warn if a global function is defined without a previous declaration.
3190 Do so even if the definition itself provides a prototype.
3191 Use this option to detect global functions that are not declared in
3194 @item -Wmissing-field-initializers
3195 @opindex Wmissing-field-initializers
3198 Warn if a structure's initializer has some fields missing. For
3199 example, the following code would cause such a warning, because
3200 @code{x.h} is implicitly zero:
3203 struct s @{ int f, g, h; @};
3204 struct s x = @{ 3, 4 @};
3207 This option does not warn about designated initializers, so the following
3208 modification would not trigger a warning:
3211 struct s @{ int f, g, h; @};
3212 struct s x = @{ .f = 3, .g = 4 @};
3215 This warning is included in @option{-Wextra}. To get other @option{-Wextra}
3216 warnings without this one, use @samp{-Wextra -Wno-missing-field-initializers}.
3218 @item -Wmissing-noreturn
3219 @opindex Wmissing-noreturn
3220 Warn about functions which might be candidates for attribute @code{noreturn}.
3221 Note these are only possible candidates, not absolute ones. Care should
3222 be taken to manually verify functions actually do not ever return before
3223 adding the @code{noreturn} attribute, otherwise subtle code generation
3224 bugs could be introduced. You will not get a warning for @code{main} in
3225 hosted C environments.
3227 @item -Wmissing-format-attribute
3228 @opindex Wmissing-format-attribute
3230 Warn about function pointers which might be candidates for @code{format}
3231 attributes. Note these are only possible candidates, not absolute ones.
3232 GCC will guess that function pointers with @code{format} attributes that
3233 are used in assignment, initialization, parameter passing or return
3234 statements should have a corresponding @code{format} attribute in the
3235 resulting type. I.e.@: the left-hand side of the assignment or
3236 initialization, the type of the parameter variable, or the return type
3237 of the containing function respectively should also have a @code{format}
3238 attribute to avoid the warning.
3240 GCC will also warn about function definitions which might be
3241 candidates for @code{format} attributes. Again, these are only
3242 possible candidates. GCC will guess that @code{format} attributes
3243 might be appropriate for any function that calls a function like
3244 @code{vprintf} or @code{vscanf}, but this might not always be the
3245 case, and some functions for which @code{format} attributes are
3246 appropriate may not be detected.
3248 @item -Wno-multichar
3249 @opindex Wno-multichar
3251 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
3252 Usually they indicate a typo in the user's code, as they have
3253 implementation-defined values, and should not be used in portable code.
3255 @item -Wnormalized=<none|id|nfc|nfkc>
3256 @opindex Wnormalized
3259 @cindex character set, input normalization
3260 In ISO C and ISO C++, two identifiers are different if they are
3261 different sequences of characters. However, sometimes when characters
3262 outside the basic ASCII character set are used, you can have two
3263 different character sequences that look the same. To avoid confusion,
3264 the ISO 10646 standard sets out some @dfn{normalization rules} which
3265 when applied ensure that two sequences that look the same are turned into
3266 the same sequence. GCC can warn you if you are using identifiers which
3267 have not been normalized; this option controls that warning.
3269 There are four levels of warning that GCC supports. The default is
3270 @option{-Wnormalized=nfc}, which warns about any identifier which is
3271 not in the ISO 10646 ``C'' normalized form, @dfn{NFC}. NFC is the
3272 recommended form for most uses.
3274 Unfortunately, there are some characters which ISO C and ISO C++ allow
3275 in identifiers that when turned into NFC aren't allowable as
3276 identifiers. That is, there's no way to use these symbols in portable
3277 ISO C or C++ and have all your identifiers in NFC.
3278 @option{-Wnormalized=id} suppresses the warning for these characters.
3279 It is hoped that future versions of the standards involved will correct
3280 this, which is why this option is not the default.
3282 You can switch the warning off for all characters by writing
3283 @option{-Wnormalized=none}. You would only want to do this if you
3284 were using some other normalization scheme (like ``D''), because
3285 otherwise you can easily create bugs that are literally impossible to see.
3287 Some characters in ISO 10646 have distinct meanings but look identical
3288 in some fonts or display methodologies, especially once formatting has
3289 been applied. For instance @code{\u207F}, ``SUPERSCRIPT LATIN SMALL
3290 LETTER N'', will display just like a regular @code{n} which has been
3291 placed in a superscript. ISO 10646 defines the @dfn{NFKC}
3292 normalization scheme to convert all these into a standard form as
3293 well, and GCC will warn if your code is not in NFKC if you use
3294 @option{-Wnormalized=nfkc}. This warning is comparable to warning
3295 about every identifier that contains the letter O because it might be
3296 confused with the digit 0, and so is not the default, but may be
3297 useful as a local coding convention if the programming environment is
3298 unable to be fixed to display these characters distinctly.
3300 @item -Wno-deprecated-declarations
3301 @opindex Wno-deprecated-declarations
3302 Do not warn about uses of functions (@pxref{Function Attributes}),
3303 variables (@pxref{Variable Attributes}), and types (@pxref{Type
3304 Attributes}) marked as deprecated by using the @code{deprecated}
3308 @opindex Wno-overflow
3309 Do not warn about compile-time overflow in constant expressions.
3311 @item -Woverride-init
3312 @opindex Woverride-init
3315 Warn if an initialized field without side effects is overridden when
3316 using designated initializers (@pxref{Designated Inits, , Designated
3319 This warning is included in @option{-Wextra}. To get other
3320 @option{-Wextra} warnings without this one, use @samp{-Wextra
3321 -Wno-override-init}.
3325 Warn if a structure is given the packed attribute, but the packed
3326 attribute has no effect on the layout or size of the structure.
3327 Such structures may be mis-aligned for little benefit. For
3328 instance, in this code, the variable @code{f.x} in @code{struct bar}
3329 will be misaligned even though @code{struct bar} does not itself
3330 have the packed attribute:
3337 @} __attribute__((packed));
3347 Warn if padding is included in a structure, either to align an element
3348 of the structure or to align the whole structure. Sometimes when this
3349 happens it is possible to rearrange the fields of the structure to
3350 reduce the padding and so make the structure smaller.
3352 @item -Wredundant-decls
3353 @opindex Wredundant-decls
3354 Warn if anything is declared more than once in the same scope, even in
3355 cases where multiple declaration is valid and changes nothing.
3357 @item -Wnested-externs @r{(C only)}
3358 @opindex Wnested-externs
3359 Warn if an @code{extern} declaration is encountered within a function.
3361 @item -Wunreachable-code
3362 @opindex Wunreachable-code
3363 Warn if the compiler detects that code will never be executed.
3365 This option is intended to warn when the compiler detects that at
3366 least a whole line of source code will never be executed, because
3367 some condition is never satisfied or because it is after a
3368 procedure that never returns.
3370 It is possible for this option to produce a warning even though there
3371 are circumstances under which part of the affected line can be executed,
3372 so care should be taken when removing apparently-unreachable code.
3374 For instance, when a function is inlined, a warning may mean that the
3375 line is unreachable in only one inlined copy of the function.
3377 This option is not made part of @option{-Wall} because in a debugging
3378 version of a program there is often substantial code which checks
3379 correct functioning of the program and is, hopefully, unreachable
3380 because the program does work. Another common use of unreachable
3381 code is to provide behavior which is selectable at compile-time.
3385 Warn if a function can not be inlined and it was declared as inline.
3386 Even with this option, the compiler will not warn about failures to
3387 inline functions declared in system headers.
3389 The compiler uses a variety of heuristics to determine whether or not
3390 to inline a function. For example, the compiler takes into account
3391 the size of the function being inlined and the amount of inlining
3392 that has already been done in the current function. Therefore,
3393 seemingly insignificant changes in the source program can cause the
3394 warnings produced by @option{-Winline} to appear or disappear.
3396 @item -Wno-invalid-offsetof @r{(C++ only)}
3397 @opindex Wno-invalid-offsetof
3398 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
3399 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
3400 to a non-POD type is undefined. In existing C++ implementations,
3401 however, @samp{offsetof} typically gives meaningful results even when
3402 applied to certain kinds of non-POD types. (Such as a simple
3403 @samp{struct} that fails to be a POD type only by virtue of having a
3404 constructor.) This flag is for users who are aware that they are
3405 writing nonportable code and who have deliberately chosen to ignore the
3408 The restrictions on @samp{offsetof} may be relaxed in a future version
3409 of the C++ standard.
3411 @item -Wno-int-to-pointer-cast @r{(C only)}
3412 @opindex Wno-int-to-pointer-cast
3413 Suppress warnings from casts to pointer type of an integer of a
3416 @item -Wno-pointer-to-int-cast @r{(C only)}
3417 @opindex Wno-pointer-to-int-cast
3418 Suppress warnings from casts from a pointer to an integer type of a
3422 @opindex Winvalid-pch
3423 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
3424 the search path but can't be used.
3428 @opindex Wno-long-long
3429 Warn if @samp{long long} type is used. This is default. To inhibit
3430 the warning messages, use @option{-Wno-long-long}. Flags
3431 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
3432 only when @option{-pedantic} flag is used.
3434 @item -Wvariadic-macros
3435 @opindex Wvariadic-macros
3436 @opindex Wno-variadic-macros
3437 Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
3438 alternate syntax when in pedantic ISO C99 mode. This is default.
3439 To inhibit the warning messages, use @option{-Wno-variadic-macros}.
3441 @item -Wvolatile-register-var
3442 @opindex Wvolatile-register-var
3443 @opindex Wno-volatile-register-var
3444 Warn if a register variable is declared volatile. The volatile
3445 modifier does not inhibit all optimizations that may eliminate reads
3446 and/or writes to register variables.
3448 @item -Wdisabled-optimization
3449 @opindex Wdisabled-optimization
3450 Warn if a requested optimization pass is disabled. This warning does
3451 not generally indicate that there is anything wrong with your code; it
3452 merely indicates that GCC's optimizers were unable to handle the code
3453 effectively. Often, the problem is that your code is too big or too
3454 complex; GCC will refuse to optimize programs when the optimization
3455 itself is likely to take inordinate amounts of time.
3457 @item -Wpointer-sign
3458 @opindex Wpointer-sign
3459 @opindex Wno-pointer-sign
3460 Warn for pointer argument passing or assignment with different signedness.
3461 This option is only supported for C and Objective-C@. It is implied by
3462 @option{-Wall} and by @option{-pedantic}, which can be disabled with
3463 @option{-Wno-pointer-sign}.
3467 Make all warnings into errors.
3471 Make the specified warning into an errors. The specifier for a
3472 warning is appended, for example @option{-Werror=switch} turns the
3473 warnings controlled by @option{-Wswitch} into errors. This switch
3474 takes a negative form, to be used to negate @option{-Werror} for
3475 specific warnings, for example @option{-Wno-error=switch} makes
3476 @option{-Wswitch} warnings not be errors, even when @option{-Werror}
3477 is in effect. You can use the @option{-fdiagnostics-show-option}
3478 option to have each controllable warning amended with the option which
3479 controls it, to determine what to use with this option.
3481 Note that specifying @option{-Werror=}@var{foo} automatically implies
3482 @option{-W}@var{foo}. However, @option{-Wno-error=}@var{foo} does not
3485 @item -Wstack-protector
3486 @opindex Wstack-protector
3487 This option is only active when @option{-fstack-protector} is active. It
3488 warns about functions that will not be protected against stack smashing.
3490 @item -Wstring-literal-comparison
3491 @opindex Wstring-literal-comparison
3492 Warn about suspicious comparisons to string literal constants. In C,
3493 direct comparisons against the memory address of a string literal, such
3494 as @code{if (x == "abc")}, typically indicate a programmer error, and
3495 even when intentional, result in unspecified behavior and are not portable.
3496 Usually these warnings alert that the programmer intended to use
3497 @code{strcmp}. This warning is enabled by @option{-Wall}.
3499 @item -Woverlength-strings
3500 @opindex Woverlength-strings
3501 Warn about string constants which are longer than the ``minimum
3502 maximum'' length specified in the C standard. Modern compilers
3503 generally allow string constants which are much longer than the
3504 standard's minimum limit, but very portable programs should avoid
3505 using longer strings.
3507 The limit applies @emph{after} string constant concatenation, and does
3508 not count the trailing NUL@. In C89, the limit was 509 characters; in
3509 C99, it was raised to 4095. C++98 does not specify a normative
3510 minimum maximum, so we do not diagnose overlength strings in C++@.
3512 This option is implied by @option{-pedantic}, and can be disabled with
3513 @option{-Wno-overlength-strings}.
3516 @node Debugging Options
3517 @section Options for Debugging Your Program or GCC
3518 @cindex options, debugging
3519 @cindex debugging information options
3521 GCC has various special options that are used for debugging
3522 either your program or GCC:
3527 Produce debugging information in the operating system's native format
3528 (stabs, COFF, XCOFF, or DWARF 2)@. GDB can work with this debugging
3531 On most systems that use stabs format, @option{-g} enables use of extra
3532 debugging information that only GDB can use; this extra information
3533 makes debugging work better in GDB but will probably make other debuggers
3535 refuse to read the program. If you want to control for certain whether
3536 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
3537 @option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
3539 GCC allows you to use @option{-g} with
3540 @option{-O}. The shortcuts taken by optimized code may occasionally
3541 produce surprising results: some variables you declared may not exist
3542 at all; flow of control may briefly move where you did not expect it;
3543 some statements may not be executed because they compute constant
3544 results or their values were already at hand; some statements may
3545 execute in different places because they were moved out of loops.
3547 Nevertheless it proves possible to debug optimized output. This makes
3548 it reasonable to use the optimizer for programs that might have bugs.
3550 The following options are useful when GCC is generated with the
3551 capability for more than one debugging format.
3555 Produce debugging information for use by GDB@. This means to use the
3556 most expressive format available (DWARF 2, stabs, or the native format
3557 if neither of those are supported), including GDB extensions if at all
3562 Produce debugging information in stabs format (if that is supported),
3563 without GDB extensions. This is the format used by DBX on most BSD
3564 systems. On MIPS, Alpha and System V Release 4 systems this option
3565 produces stabs debugging output which is not understood by DBX or SDB@.
3566 On System V Release 4 systems this option requires the GNU assembler.
3568 @item -feliminate-unused-debug-symbols
3569 @opindex feliminate-unused-debug-symbols
3570 Produce debugging information in stabs format (if that is supported),
3571 for only symbols that are actually used.
3573 @item -femit-class-debug-always
3574 Instead of emitting debugging information for a C++ class in only one
3575 object file, emit it in all object files using the class. This option
3576 should be used only with debuggers that are unable to handle the way GCC
3577 normally emits debugging information for classes because using this
3578 option will increase the size of debugging information by as much as a
3583 Produce debugging information in stabs format (if that is supported),
3584 using GNU extensions understood only by the GNU debugger (GDB)@. The
3585 use of these extensions is likely to make other debuggers crash or
3586 refuse to read the program.
3590 Produce debugging information in COFF format (if that is supported).
3591 This is the format used by SDB on most System V systems prior to
3596 Produce debugging information in XCOFF format (if that is supported).
3597 This is the format used by the DBX debugger on IBM RS/6000 systems.
3601 Produce debugging information in XCOFF format (if that is supported),
3602 using GNU extensions understood only by the GNU debugger (GDB)@. The
3603 use of these extensions is likely to make other debuggers crash or
3604 refuse to read the program, and may cause assemblers other than the GNU
3605 assembler (GAS) to fail with an error.
3609 Produce debugging information in DWARF version 2 format (if that is
3610 supported). This is the format used by DBX on IRIX 6. With this
3611 option, GCC uses features of DWARF version 3 when they are useful;
3612 version 3 is upward compatible with version 2, but may still cause
3613 problems for older debuggers.
3617 Produce debugging information in VMS debug format (if that is
3618 supported). This is the format used by DEBUG on VMS systems.
3621 @itemx -ggdb@var{level}
3622 @itemx -gstabs@var{level}
3623 @itemx -gcoff@var{level}
3624 @itemx -gxcoff@var{level}
3625 @itemx -gvms@var{level}
3626 Request debugging information and also use @var{level} to specify how
3627 much information. The default level is 2.
3629 Level 1 produces minimal information, enough for making backtraces in
3630 parts of the program that you don't plan to debug. This includes
3631 descriptions of functions and external variables, but no information
3632 about local variables and no line numbers.
3634 Level 3 includes extra information, such as all the macro definitions
3635 present in the program. Some debuggers support macro expansion when
3636 you use @option{-g3}.
3638 @option{-gdwarf-2} does not accept a concatenated debug level, because
3639 GCC used to support an option @option{-gdwarf} that meant to generate
3640 debug information in version 1 of the DWARF format (which is very
3641 different from version 2), and it would have been too confusing. That
3642 debug format is long obsolete, but the option cannot be changed now.
3643 Instead use an additional @option{-g@var{level}} option to change the
3644 debug level for DWARF2.
3646 @item -feliminate-dwarf2-dups
3647 @opindex feliminate-dwarf2-dups
3648 Compress DWARF2 debugging information by eliminating duplicated
3649 information about each symbol. This option only makes sense when
3650 generating DWARF2 debugging information with @option{-gdwarf-2}.
3652 @cindex @command{prof}
3655 Generate extra code to write profile information suitable for the
3656 analysis program @command{prof}. You must use this option when compiling
3657 the source files you want data about, and you must also use it when
3660 @cindex @command{gprof}
3663 Generate extra code to write profile information suitable for the
3664 analysis program @command{gprof}. You must use this option when compiling
3665 the source files you want data about, and you must also use it when
3670 Makes the compiler print out each function name as it is compiled, and
3671 print some statistics about each pass when it finishes.
3674 @opindex ftime-report
3675 Makes the compiler print some statistics about the time consumed by each
3676 pass when it finishes.
3679 @opindex fmem-report
3680 Makes the compiler print some statistics about permanent memory
3681 allocation when it finishes.
3683 @item -fprofile-arcs
3684 @opindex fprofile-arcs
3685 Add code so that program flow @dfn{arcs} are instrumented. During
3686 execution the program records how many times each branch and call is
3687 executed and how many times it is taken or returns. When the compiled
3688 program exits it saves this data to a file called
3689 @file{@var{auxname}.gcda} for each source file. The data may be used for
3690 profile-directed optimizations (@option{-fbranch-probabilities}), or for
3691 test coverage analysis (@option{-ftest-coverage}). Each object file's
3692 @var{auxname} is generated from the name of the output file, if
3693 explicitly specified and it is not the final executable, otherwise it is
3694 the basename of the source file. In both cases any suffix is removed
3695 (e.g.@: @file{foo.gcda} for input file @file{dir/foo.c}, or
3696 @file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
3697 @xref{Cross-profiling}.
3699 @cindex @command{gcov}
3703 This option is used to compile and link code instrumented for coverage
3704 analysis. The option is a synonym for @option{-fprofile-arcs}
3705 @option{-ftest-coverage} (when compiling) and @option{-lgcov} (when
3706 linking). See the documentation for those options for more details.
3711 Compile the source files with @option{-fprofile-arcs} plus optimization
3712 and code generation options. For test coverage analysis, use the
3713 additional @option{-ftest-coverage} option. You do not need to profile
3714 every source file in a program.
3717 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
3718 (the latter implies the former).
3721 Run the program on a representative workload to generate the arc profile
3722 information. This may be repeated any number of times. You can run
3723 concurrent instances of your program, and provided that the file system
3724 supports locking, the data files will be correctly updated. Also
3725 @code{fork} calls are detected and correctly handled (double counting
3729 For profile-directed optimizations, compile the source files again with
3730 the same optimization and code generation options plus
3731 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
3732 Control Optimization}).
3735 For test coverage analysis, use @command{gcov} to produce human readable
3736 information from the @file{.gcno} and @file{.gcda} files. Refer to the
3737 @command{gcov} documentation for further information.
3741 With @option{-fprofile-arcs}, for each function of your program GCC
3742 creates a program flow graph, then finds a spanning tree for the graph.
3743 Only arcs that are not on the spanning tree have to be instrumented: the
3744 compiler adds code to count the number of times that these arcs are
3745 executed. When an arc is the only exit or only entrance to a block, the
3746 instrumentation code can be added to the block; otherwise, a new basic
3747 block must be created to hold the instrumentation code.
3750 @item -ftest-coverage
3751 @opindex ftest-coverage
3752 Produce a notes file that the @command{gcov} code-coverage utility
3753 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
3754 show program coverage. Each source file's note file is called
3755 @file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
3756 above for a description of @var{auxname} and instructions on how to
3757 generate test coverage data. Coverage data will match the source files
3758 more closely, if you do not optimize.
3760 @item -d@var{letters}
3761 @item -fdump-rtl-@var{pass}
3763 Says to make debugging dumps during compilation at times specified by
3764 @var{letters}. This is used for debugging the RTL-based passes of the
3765 compiler. The file names for most of the dumps are made by appending a
3766 pass number and a word to the @var{dumpname}. @var{dumpname} is generated
3767 from the name of the output file, if explicitly specified and it is not
3768 an executable, otherwise it is the basename of the source file.
3770 Most debug dumps can be enabled either passing a letter to the @option{-d}
3771 option, or with a long @option{-fdump-rtl} switch; here are the possible
3772 letters for use in @var{letters} and @var{pass}, and their meanings:
3777 Annotate the assembler output with miscellaneous debugging information.
3780 @itemx -fdump-rtl-bbro
3782 @opindex fdump-rtl-bbro
3783 Dump after block reordering, to @file{@var{file}.148r.bbro}.
3786 @itemx -fdump-rtl-combine
3788 @opindex fdump-rtl-combine
3789 Dump after instruction combination, to the file @file{@var{file}.129r.combine}.
3792 @itemx -fdump-rtl-ce1
3793 @itemx -fdump-rtl-ce2
3795 @opindex fdump-rtl-ce1
3796 @opindex fdump-rtl-ce2
3797 @option{-dC} and @option{-fdump-rtl-ce1} enable dumping after the
3798 first if conversion, to the file @file{@var{file}.117r.ce1}. @option{-dC}
3799 and @option{-fdump-rtl-ce2} enable dumping after the second if
3800 conversion, to the file @file{@var{file}.130r.ce2}.
3803 @itemx -fdump-rtl-btl
3804 @itemx -fdump-rtl-dbr
3806 @opindex fdump-rtl-btl
3807 @opindex fdump-rtl-dbr
3808 @option{-dd} and @option{-fdump-rtl-btl} enable dumping after branch
3809 target load optimization, to @file{@var{file}.31.btl}. @option{-dd}
3810 and @option{-fdump-rtl-dbr} enable dumping after delayed branch
3811 scheduling, to @file{@var{file}.36.dbr}.
3815 Dump all macro definitions, at the end of preprocessing, in addition to
3819 @itemx -fdump-rtl-ce3
3821 @opindex fdump-rtl-ce3
3822 Dump after the third if conversion, to @file{@var{file}.146r.ce3}.
3825 @itemx -fdump-rtl-cfg
3826 @itemx -fdump-rtl-life
3828 @opindex fdump-rtl-cfg
3829 @opindex fdump-rtl-life
3830 @option{-df} and @option{-fdump-rtl-cfg} enable dumping after control
3831 and data flow analysis, to @file{@var{file}.116r.cfg}. @option{-df}
3832 and @option{-fdump-rtl-cfg} enable dumping dump after life analysis,
3833 to @file{@var{file}.128r.life1} and @file{@var{file}.135r.life2}.
3836 @itemx -fdump-rtl-greg
3838 @opindex fdump-rtl-greg
3839 Dump after global register allocation, to @file{@var{file}.139r.greg}.
3842 @itemx -fdump-rtl-gcse
3843 @itemx -fdump-rtl-bypass
3845 @opindex fdump-rtl-gcse
3846 @opindex fdump-rtl-bypass
3847 @option{-dG} and @option{-fdump-rtl-gcse} enable dumping after GCSE, to
3848 @file{@var{file}.114r.gcse}. @option{-dG} and @option{-fdump-rtl-bypass}
3849 enable dumping after jump bypassing and control flow optimizations, to
3850 @file{@var{file}.115r.bypass}.
3853 @itemx -fdump-rtl-eh
3855 @opindex fdump-rtl-eh
3856 Dump after finalization of EH handling code, to @file{@var{file}.02.eh}.
3859 @itemx -fdump-rtl-sibling
3861 @opindex fdump-rtl-sibling
3862 Dump after sibling call optimizations, to @file{@var{file}.106r.sibling}.
3865 @itemx -fdump-rtl-jump
3867 @opindex fdump-rtl-jump
3868 Dump after the first jump optimization, to @file{@var{file}.112r.jump}.
3871 @itemx -fdump-rtl-stack
3873 @opindex fdump-rtl-stack
3874 Dump after conversion from registers to stack, to @file{@var{file}.152r.stack}.
3877 @itemx -fdump-rtl-lreg
3879 @opindex fdump-rtl-lreg
3880 Dump after local register allocation, to @file{@var{file}.138r.lreg}.
3883 @itemx -fdump-rtl-loop2
3885 @opindex fdump-rtl-loop2
3886 @option{-dL} and @option{-fdump-rtl-loop2} enable dumping after the
3887 loop optimization pass, to @file{@var{file}.119r.loop2},
3888 @file{@var{file}.120r.loop2_init},
3889 @file{@var{file}.121r.loop2_invariant}, and
3890 @file{@var{file}.125r.loop2_done}.
3893 @itemx -fdump-rtl-sms
3895 @opindex fdump-rtl-sms
3896 Dump after modulo scheduling, to @file{@var{file}.136r.sms}.
3899 @itemx -fdump-rtl-mach
3901 @opindex fdump-rtl-mach
3902 Dump after performing the machine dependent reorganization pass, to
3903 @file{@var{file}.155r.mach}.
3906 @itemx -fdump-rtl-rnreg
3908 @opindex fdump-rtl-rnreg
3909 Dump after register renumbering, to @file{@var{file}.147r.rnreg}.
3912 @itemx -fdump-rtl-regmove
3914 @opindex fdump-rtl-regmove
3915 Dump after the register move pass, to @file{@var{file}.132r.regmove}.
3918 @itemx -fdump-rtl-postreload
3920 @opindex fdump-rtl-postreload
3921 Dump after post-reload optimizations, to @file{@var{file}.24.postreload}.
3924 @itemx -fdump-rtl-expand
3926 @opindex fdump-rtl-expand
3927 Dump after RTL generation, to @file{@var{file}.104r.expand}.
3930 @itemx -fdump-rtl-sched2
3932 @opindex fdump-rtl-sched2
3933 Dump after the second scheduling pass, to @file{@var{file}.150r.sched2}.
3936 @itemx -fdump-rtl-cse
3938 @opindex fdump-rtl-cse
3939 Dump after CSE (including the jump optimization that sometimes follows
3940 CSE), to @file{@var{file}.113r.cse}.
3943 @itemx -fdump-rtl-sched
3945 @opindex fdump-rtl-sched
3946 Dump after the first scheduling pass, to @file{@var{file}.21.sched}.
3949 @itemx -fdump-rtl-cse2
3951 @opindex fdump-rtl-cse2
3952 Dump after the second CSE pass (including the jump optimization that
3953 sometimes follows CSE), to @file{@var{file}.127r.cse2}.
3956 @itemx -fdump-rtl-tracer
3958 @opindex fdump-rtl-tracer
3959 Dump after running tracer, to @file{@var{file}.118r.tracer}.
3962 @itemx -fdump-rtl-vpt
3963 @itemx -fdump-rtl-vartrack
3965 @opindex fdump-rtl-vpt
3966 @opindex fdump-rtl-vartrack
3967 @option{-dV} and @option{-fdump-rtl-vpt} enable dumping after the value
3968 profile transformations, to @file{@var{file}.10.vpt}. @option{-dV}
3969 and @option{-fdump-rtl-vartrack} enable dumping after variable tracking,
3970 to @file{@var{file}.154r.vartrack}.
3973 @itemx -fdump-rtl-flow2
3975 @opindex fdump-rtl-flow2
3976 Dump after the second flow pass, to @file{@var{file}.142r.flow2}.
3979 @itemx -fdump-rtl-peephole2
3981 @opindex fdump-rtl-peephole2
3982 Dump after the peephole pass, to @file{@var{file}.145r.peephole2}.
3985 @itemx -fdump-rtl-web
3987 @opindex fdump-rtl-web
3988 Dump after live range splitting, to @file{@var{file}.126r.web}.
3991 @itemx -fdump-rtl-all
3993 @opindex fdump-rtl-all
3994 Produce all the dumps listed above.
3998 Produce a core dump whenever an error occurs.
4002 Print statistics on memory usage, at the end of the run, to
4007 Annotate the assembler output with a comment indicating which
4008 pattern and alternative was used. The length of each instruction is
4013 Dump the RTL in the assembler output as a comment before each instruction.
4014 Also turns on @option{-dp} annotation.
4018 For each of the other indicated dump files (either with @option{-d} or
4019 @option{-fdump-rtl-@var{pass}}), dump a representation of the control flow
4020 graph suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
4024 Just generate RTL for a function instead of compiling it. Usually used
4025 with @samp{r} (@option{-fdump-rtl-expand}).
4029 Dump debugging information during parsing, to standard error.
4033 @opindex fdump-noaddr
4034 When doing debugging dumps (see @option{-d} option above), suppress
4035 address output. This makes it more feasible to use diff on debugging
4036 dumps for compiler invocations with different compiler binaries and/or
4037 different text / bss / data / heap / stack / dso start locations.
4039 @item -fdump-unnumbered
4040 @opindex fdump-unnumbered
4041 When doing debugging dumps (see @option{-d} option above), suppress instruction
4042 numbers, line number note and address output. This makes it more feasible to
4043 use diff on debugging dumps for compiler invocations with different
4044 options, in particular with and without @option{-g}.
4046 @item -fdump-translation-unit @r{(C++ only)}
4047 @itemx -fdump-translation-unit-@var{options} @r{(C++ only)}
4048 @opindex fdump-translation-unit
4049 Dump a representation of the tree structure for the entire translation
4050 unit to a file. The file name is made by appending @file{.tu} to the
4051 source file name. If the @samp{-@var{options}} form is used, @var{options}
4052 controls the details of the dump as described for the
4053 @option{-fdump-tree} options.
4055 @item -fdump-class-hierarchy @r{(C++ only)}
4056 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
4057 @opindex fdump-class-hierarchy
4058 Dump a representation of each class's hierarchy and virtual function
4059 table layout to a file. The file name is made by appending @file{.class}
4060 to the source file name. If the @samp{-@var{options}} form is used,
4061 @var{options} controls the details of the dump as described for the
4062 @option{-fdump-tree} options.
4064 @item -fdump-ipa-@var{switch}
4066 Control the dumping at various stages of inter-procedural analysis
4067 language tree to a file. The file name is generated by appending a switch
4068 specific suffix to the source file name. The following dumps are possible:
4072 Enables all inter-procedural analysis dumps; currently the only produced
4073 dump is the @samp{cgraph} dump.
4076 Dumps information about call-graph optimization, unused function removal,
4077 and inlining decisions.
4080 @item -fdump-tree-@var{switch}
4081 @itemx -fdump-tree-@var{switch}-@var{options}
4083 Control the dumping at various stages of processing the intermediate
4084 language tree to a file. The file name is generated by appending a switch
4085 specific suffix to the source file name. If the @samp{-@var{options}}
4086 form is used, @var{options} is a list of @samp{-} separated options that
4087 control the details of the dump. Not all options are applicable to all
4088 dumps, those which are not meaningful will be ignored. The following
4089 options are available
4093 Print the address of each node. Usually this is not meaningful as it
4094 changes according to the environment and source file. Its primary use
4095 is for tying up a dump file with a debug environment.
4097 Inhibit dumping of members of a scope or body of a function merely
4098 because that scope has been reached. Only dump such items when they
4099 are directly reachable by some other path. When dumping pretty-printed
4100 trees, this option inhibits dumping the bodies of control structures.
4102 Print a raw representation of the tree. By default, trees are
4103 pretty-printed into a C-like representation.
4105 Enable more detailed dumps (not honored by every dump option).
4107 Enable dumping various statistics about the pass (not honored by every dump
4110 Enable showing basic block boundaries (disabled in raw dumps).
4112 Enable showing virtual operands for every statement.
4114 Enable showing line numbers for statements.
4116 Enable showing the unique ID (@code{DECL_UID}) for each variable.
4118 Turn on all options, except @option{raw}, @option{slim} and @option{lineno}.
4121 The following tree dumps are possible:
4125 Dump before any tree based optimization, to @file{@var{file}.original}.
4128 Dump after all tree based optimization, to @file{@var{file}.optimized}.
4131 Dump after function inlining, to @file{@var{file}.inlined}.
4134 @opindex fdump-tree-gimple
4135 Dump each function before and after the gimplification pass to a file. The
4136 file name is made by appending @file{.gimple} to the source file name.
4139 @opindex fdump-tree-cfg
4140 Dump the control flow graph of each function to a file. The file name is
4141 made by appending @file{.cfg} to the source file name.
4144 @opindex fdump-tree-vcg
4145 Dump the control flow graph of each function to a file in VCG format. The
4146 file name is made by appending @file{.vcg} to the source file name. Note
4147 that if the file contains more than one function, the generated file cannot
4148 be used directly by VCG@. You will need to cut and paste each function's
4149 graph into its own separate file first.
4152 @opindex fdump-tree-ch
4153 Dump each function after copying loop headers. The file name is made by
4154 appending @file{.ch} to the source file name.
4157 @opindex fdump-tree-ssa
4158 Dump SSA related information to a file. The file name is made by appending
4159 @file{.ssa} to the source file name.
4162 @opindex fdump-tree-salias
4163 Dump structure aliasing variable information to a file. This file name
4164 is made by appending @file{.salias} to the source file name.
4167 @opindex fdump-tree-alias
4168 Dump aliasing information for each function. The file name is made by
4169 appending @file{.alias} to the source file name.
4172 @opindex fdump-tree-ccp
4173 Dump each function after CCP@. The file name is made by appending
4174 @file{.ccp} to the source file name.
4177 @opindex fdump-tree-storeccp
4178 Dump each function after STORE-CCP. The file name is made by appending
4179 @file{.storeccp} to the source file name.
4182 @opindex fdump-tree-pre
4183 Dump trees after partial redundancy elimination. The file name is made
4184 by appending @file{.pre} to the source file name.
4187 @opindex fdump-tree-fre
4188 Dump trees after full redundancy elimination. The file name is made
4189 by appending @file{.fre} to the source file name.
4192 @opindex fdump-tree-copyprop
4193 Dump trees after copy propagation. The file name is made
4194 by appending @file{.copyprop} to the source file name.
4196 @item store_copyprop
4197 @opindex fdump-tree-store_copyprop
4198 Dump trees after store copy-propagation. The file name is made
4199 by appending @file{.store_copyprop} to the source file name.
4202 @opindex fdump-tree-dce
4203 Dump each function after dead code elimination. The file name is made by
4204 appending @file{.dce} to the source file name.
4207 @opindex fdump-tree-mudflap
4208 Dump each function after adding mudflap instrumentation. The file name is
4209 made by appending @file{.mudflap} to the source file name.
4212 @opindex fdump-tree-sra
4213 Dump each function after performing scalar replacement of aggregates. The
4214 file name is made by appending @file{.sra} to the source file name.
4217 @opindex fdump-tree-sink
4218 Dump each function after performing code sinking. The file name is made
4219 by appending @file{.sink} to the source file name.
4222 @opindex fdump-tree-dom
4223 Dump each function after applying dominator tree optimizations. The file
4224 name is made by appending @file{.dom} to the source file name.
4227 @opindex fdump-tree-dse
4228 Dump each function after applying dead store elimination. The file
4229 name is made by appending @file{.dse} to the source file name.
4232 @opindex fdump-tree-phiopt
4233 Dump each function after optimizing PHI nodes into straightline code. The file
4234 name is made by appending @file{.phiopt} to the source file name.
4237 @opindex fdump-tree-forwprop
4238 Dump each function after forward propagating single use variables. The file
4239 name is made by appending @file{.forwprop} to the source file name.
4242 @opindex fdump-tree-copyrename
4243 Dump each function after applying the copy rename optimization. The file
4244 name is made by appending @file{.copyrename} to the source file name.
4247 @opindex fdump-tree-nrv
4248 Dump each function after applying the named return value optimization on
4249 generic trees. The file name is made by appending @file{.nrv} to the source
4253 @opindex fdump-tree-vect
4254 Dump each function after applying vectorization of loops. The file name is
4255 made by appending @file{.vect} to the source file name.
4258 @opindex fdump-tree-vrp
4259 Dump each function after Value Range Propagation (VRP). The file name
4260 is made by appending @file{.vrp} to the source file name.
4263 @opindex fdump-tree-all
4264 Enable all the available tree dumps with the flags provided in this option.
4267 @item -ftree-vectorizer-verbose=@var{n}
4268 @opindex ftree-vectorizer-verbose
4269 This option controls the amount of debugging output the vectorizer prints.
4270 This information is written to standard error, unless
4271 @option{-fdump-tree-all} or @option{-fdump-tree-vect} is specified,
4272 in which case it is output to the usual dump listing file, @file{.vect}.
4273 For @var{n}=0 no diagnostic information is reported.
4274 If @var{n}=1 the vectorizer reports each loop that got vectorized,
4275 and the total number of loops that got vectorized.
4276 If @var{n}=2 the vectorizer also reports non-vectorized loops that passed
4277 the first analysis phase (vect_analyze_loop_form) - i.e. countable,
4278 inner-most, single-bb, single-entry/exit loops. This is the same verbosity
4279 level that @option{-fdump-tree-vect-stats} uses.
4280 Higher verbosity levels mean either more information dumped for each
4281 reported loop, or same amount of information reported for more loops:
4282 If @var{n}=3, alignment related information is added to the reports.
4283 If @var{n}=4, data-references related information (e.g. memory dependences,
4284 memory access-patterns) is added to the reports.
4285 If @var{n}=5, the vectorizer reports also non-vectorized inner-most loops
4286 that did not pass the first analysis phase (i.e. may not be countable, or
4287 may have complicated control-flow).
4288 If @var{n}=6, the vectorizer reports also non-vectorized nested loops.
4289 For @var{n}=7, all the information the vectorizer generates during its
4290 analysis and transformation is reported. This is the same verbosity level
4291 that @option{-fdump-tree-vect-details} uses.
4293 @item -frandom-seed=@var{string}
4294 @opindex frandom-string
4295 This option provides a seed that GCC uses when it would otherwise use
4296 random numbers. It is used to generate certain symbol names
4297 that have to be different in every compiled file. It is also used to
4298 place unique stamps in coverage data files and the object files that
4299 produce them. You can use the @option{-frandom-seed} option to produce
4300 reproducibly identical object files.
4302 The @var{string} should be different for every file you compile.
4304 @item -fsched-verbose=@var{n}
4305 @opindex fsched-verbose
4306 On targets that use instruction scheduling, this option controls the
4307 amount of debugging output the scheduler prints. This information is
4308 written to standard error, unless @option{-dS} or @option{-dR} is
4309 specified, in which case it is output to the usual dump
4310 listing file, @file{.sched} or @file{.sched2} respectively. However
4311 for @var{n} greater than nine, the output is always printed to standard
4314 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
4315 same information as @option{-dRS}. For @var{n} greater than one, it
4316 also output basic block probabilities, detailed ready list information
4317 and unit/insn info. For @var{n} greater than two, it includes RTL
4318 at abort point, control-flow and regions info. And for @var{n} over
4319 four, @option{-fsched-verbose} also includes dependence info.
4323 Store the usual ``temporary'' intermediate files permanently; place them
4324 in the current directory and name them based on the source file. Thus,
4325 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
4326 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
4327 preprocessed @file{foo.i} output file even though the compiler now
4328 normally uses an integrated preprocessor.
4330 When used in combination with the @option{-x} command line option,
4331 @option{-save-temps} is sensible enough to avoid over writing an
4332 input source file with the same extension as an intermediate file.
4333 The corresponding intermediate file may be obtained by renaming the
4334 source file before using @option{-save-temps}.
4338 Report the CPU time taken by each subprocess in the compilation
4339 sequence. For C source files, this is the compiler proper and assembler
4340 (plus the linker if linking is done). The output looks like this:
4347 The first number on each line is the ``user time'', that is time spent
4348 executing the program itself. The second number is ``system time'',
4349 time spent executing operating system routines on behalf of the program.
4350 Both numbers are in seconds.
4352 @item -fvar-tracking
4353 @opindex fvar-tracking
4354 Run variable tracking pass. It computes where variables are stored at each
4355 position in code. Better debugging information is then generated
4356 (if the debugging information format supports this information).
4358 It is enabled by default when compiling with optimization (@option{-Os},
4359 @option{-O}, @option{-O2}, ...), debugging information (@option{-g}) and
4360 the debug info format supports it.
4362 @item -print-file-name=@var{library}
4363 @opindex print-file-name
4364 Print the full absolute name of the library file @var{library} that
4365 would be used when linking---and don't do anything else. With this
4366 option, GCC does not compile or link anything; it just prints the
4369 @item -print-multi-directory
4370 @opindex print-multi-directory
4371 Print the directory name corresponding to the multilib selected by any
4372 other switches present in the command line. This directory is supposed
4373 to exist in @env{GCC_EXEC_PREFIX}.
4375 @item -print-multi-lib
4376 @opindex print-multi-lib
4377 Print the mapping from multilib directory names to compiler switches
4378 that enable them. The directory name is separated from the switches by
4379 @samp{;}, and each switch starts with an @samp{@@} instead of the
4380 @samp{-}, without spaces between multiple switches. This is supposed to
4381 ease shell-processing.
4383 @item -print-prog-name=@var{program}
4384 @opindex print-prog-name
4385 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
4387 @item -print-libgcc-file-name
4388 @opindex print-libgcc-file-name
4389 Same as @option{-print-file-name=libgcc.a}.
4391 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
4392 but you do want to link with @file{libgcc.a}. You can do
4395 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
4398 @item -print-search-dirs
4399 @opindex print-search-dirs
4400 Print the name of the configured installation directory and a list of
4401 program and library directories @command{gcc} will search---and don't do anything else.
4403 This is useful when @command{gcc} prints the error message
4404 @samp{installation problem, cannot exec cpp0: No such file or directory}.
4405 To resolve this you either need to put @file{cpp0} and the other compiler
4406 components where @command{gcc} expects to find them, or you can set the environment
4407 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
4408 Don't forget the trailing @samp{/}.
4409 @xref{Environment Variables}.
4412 @opindex dumpmachine
4413 Print the compiler's target machine (for example,
4414 @samp{i686-pc-linux-gnu})---and don't do anything else.
4417 @opindex dumpversion
4418 Print the compiler version (for example, @samp{3.0})---and don't do
4423 Print the compiler's built-in specs---and don't do anything else. (This
4424 is used when GCC itself is being built.) @xref{Spec Files}.
4426 @item -feliminate-unused-debug-types
4427 @opindex feliminate-unused-debug-types
4428 Normally, when producing DWARF2 output, GCC will emit debugging
4429 information for all types declared in a compilation
4430 unit, regardless of whether or not they are actually used
4431 in that compilation unit. Sometimes this is useful, such as
4432 if, in the debugger, you want to cast a value to a type that is
4433 not actually used in your program (but is declared). More often,
4434 however, this results in a significant amount of wasted space.
4435 With this option, GCC will avoid producing debug symbol output
4436 for types that are nowhere used in the source file being compiled.
4439 @node Optimize Options
4440 @section Options That Control Optimization
4441 @cindex optimize options
4442 @cindex options, optimization
4444 These options control various sorts of optimizations.
4446 Without any optimization option, the compiler's goal is to reduce the
4447 cost of compilation and to make debugging produce the expected
4448 results. Statements are independent: if you stop the program with a
4449 breakpoint between statements, you can then assign a new value to any
4450 variable or change the program counter to any other statement in the
4451 function and get exactly the results you would expect from the source
4454 Turning on optimization flags makes the compiler attempt to improve
4455 the performance and/or code size at the expense of compilation time
4456 and possibly the ability to debug the program.
4458 The compiler performs optimization based on the knowledge it has of
4459 the program. Optimization levels @option{-O} and above, in
4460 particular, enable @emph{unit-at-a-time} mode, which allows the
4461 compiler to consider information gained from later functions in
4462 the file when compiling a function. Compiling multiple files at
4463 once to a single output file in @emph{unit-at-a-time} mode allows
4464 the compiler to use information gained from all of the files when
4465 compiling each of them.
4467 Not all optimizations are controlled directly by a flag. Only
4468 optimizations that have a flag are listed.
4475 Optimize. Optimizing compilation takes somewhat more time, and a lot
4476 more memory for a large function.
4478 With @option{-O}, the compiler tries to reduce code size and execution
4479 time, without performing any optimizations that take a great deal of
4482 @option{-O} turns on the following optimization flags:
4483 @gccoptlist{-fdefer-pop @gol
4484 -fdelayed-branch @gol
4485 -fguess-branch-probability @gol
4486 -fcprop-registers @gol
4487 -fif-conversion @gol
4488 -fif-conversion2 @gol
4491 -ftree-dominator-opts @gol
4496 -ftree-copyrename @gol
4499 -funit-at-a-time @gol
4502 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
4503 where doing so does not interfere with debugging.
4507 Optimize even more. GCC performs nearly all supported optimizations
4508 that do not involve a space-speed tradeoff. The compiler does not
4509 perform loop unrolling or function inlining when you specify @option{-O2}.
4510 As compared to @option{-O}, this option increases both compilation time
4511 and the performance of the generated code.
4513 @option{-O2} turns on all optimization flags specified by @option{-O}. It
4514 also turns on the following optimization flags:
4515 @gccoptlist{-fthread-jumps @gol
4517 -foptimize-sibling-calls @gol
4518 -fcse-follow-jumps -fcse-skip-blocks @gol
4519 -fgcse -fgcse-lm @gol
4520 -fexpensive-optimizations @gol
4521 -frerun-cse-after-loop @gol
4524 -fschedule-insns -fschedule-insns2 @gol
4525 -fsched-interblock -fsched-spec @gol
4527 -fstrict-aliasing @gol
4528 -fdelete-null-pointer-checks @gol
4529 -freorder-blocks -freorder-functions @gol
4530 -falign-functions -falign-jumps @gol
4531 -falign-loops -falign-labels @gol
4535 Please note the warning under @option{-fgcse} about
4536 invoking @option{-O2} on programs that use computed gotos.
4540 Optimize yet more. @option{-O3} turns on all optimizations specified by
4541 @option{-O2} and also turns on the @option{-finline-functions},
4542 @option{-funswitch-loops} and @option{-fgcse-after-reload} options.
4546 Do not optimize. This is the default.
4550 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
4551 do not typically increase code size. It also performs further
4552 optimizations designed to reduce code size.
4554 @option{-Os} disables the following optimization flags:
4555 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
4556 -falign-labels -freorder-blocks -freorder-blocks-and-partition @gol
4557 -fprefetch-loop-arrays -ftree-vect-loop-version}
4559 If you use multiple @option{-O} options, with or without level numbers,
4560 the last such option is the one that is effective.
4563 Options of the form @option{-f@var{flag}} specify machine-independent
4564 flags. Most flags have both positive and negative forms; the negative
4565 form of @option{-ffoo} would be @option{-fno-foo}. In the table
4566 below, only one of the forms is listed---the one you typically will
4567 use. You can figure out the other form by either removing @samp{no-}
4570 The following options control specific optimizations. They are either
4571 activated by @option{-O} options or are related to ones that are. You
4572 can use the following flags in the rare cases when ``fine-tuning'' of
4573 optimizations to be performed is desired.
4576 @item -fno-default-inline
4577 @opindex fno-default-inline
4578 Do not make member functions inline by default merely because they are
4579 defined inside the class scope (C++ only). Otherwise, when you specify
4580 @w{@option{-O}}, member functions defined inside class scope are compiled
4581 inline by default; i.e., you don't need to add @samp{inline} in front of
4582 the member function name.
4584 @item -fno-defer-pop
4585 @opindex fno-defer-pop
4586 Always pop the arguments to each function call as soon as that function
4587 returns. For machines which must pop arguments after a function call,
4588 the compiler normally lets arguments accumulate on the stack for several
4589 function calls and pops them all at once.
4591 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4595 Force memory operands to be copied into registers before doing
4596 arithmetic on them. This produces better code by making all memory
4597 references potential common subexpressions. When they are not common
4598 subexpressions, instruction combination should eliminate the separate
4599 register-load. This option is now a nop and will be removed in 4.2.
4602 @opindex fforce-addr
4603 Force memory address constants to be copied into registers before
4604 doing arithmetic on them.
4606 @item -fomit-frame-pointer
4607 @opindex fomit-frame-pointer
4608 Don't keep the frame pointer in a register for functions that
4609 don't need one. This avoids the instructions to save, set up and
4610 restore frame pointers; it also makes an extra register available
4611 in many functions. @strong{It also makes debugging impossible on
4614 On some machines, such as the VAX, this flag has no effect, because
4615 the standard calling sequence automatically handles the frame pointer
4616 and nothing is saved by pretending it doesn't exist. The
4617 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
4618 whether a target machine supports this flag. @xref{Registers,,Register
4619 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
4621 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4623 @item -foptimize-sibling-calls
4624 @opindex foptimize-sibling-calls
4625 Optimize sibling and tail recursive calls.
4627 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4631 Don't pay attention to the @code{inline} keyword. Normally this option
4632 is used to keep the compiler from expanding any functions inline.
4633 Note that if you are not optimizing, no functions can be expanded inline.
4635 @item -finline-functions
4636 @opindex finline-functions
4637 Integrate all simple functions into their callers. The compiler
4638 heuristically decides which functions are simple enough to be worth
4639 integrating in this way.
4641 If all calls to a given function are integrated, and the function is
4642 declared @code{static}, then the function is normally not output as
4643 assembler code in its own right.
4645 Enabled at level @option{-O3}.
4647 @item -finline-functions-called-once
4648 @opindex finline-functions-called-once
4649 Consider all @code{static} functions called once for inlining into their
4650 caller even if they are not marked @code{inline}. If a call to a given
4651 function is integrated, then the function is not output as assembler code
4654 Enabled if @option{-funit-at-a-time} is enabled.
4656 @item -fearly-inlining
4657 @opindex fearly-inlining
4658 Inline functions marked by @code{always_inline} and functions whose body seems
4659 smaller than the function call overhead early before doing
4660 @option{-fprofile-generate} instrumentation and real inlining pass. Doing so
4661 makes profiling significantly cheaper and usually inlining faster on programs
4662 having large chains of nested wrapper functions.
4666 @item -finline-limit=@var{n}
4667 @opindex finline-limit
4668 By default, GCC limits the size of functions that can be inlined. This flag
4669 allows the control of this limit for functions that are explicitly marked as
4670 inline (i.e., marked with the inline keyword or defined within the class
4671 definition in c++). @var{n} is the size of functions that can be inlined in
4672 number of pseudo instructions (not counting parameter handling). The default
4673 value of @var{n} is 600.
4674 Increasing this value can result in more inlined code at
4675 the cost of compilation time and memory consumption. Decreasing usually makes
4676 the compilation faster and less code will be inlined (which presumably
4677 means slower programs). This option is particularly useful for programs that
4678 use inlining heavily such as those based on recursive templates with C++.
4680 Inlining is actually controlled by a number of parameters, which may be
4681 specified individually by using @option{--param @var{name}=@var{value}}.
4682 The @option{-finline-limit=@var{n}} option sets some of these parameters
4686 @item max-inline-insns-single
4687 is set to @var{n}/2.
4688 @item max-inline-insns-auto
4689 is set to @var{n}/2.
4690 @item min-inline-insns
4691 is set to 130 or @var{n}/4, whichever is smaller.
4692 @item max-inline-insns-rtl
4696 See below for a documentation of the individual
4697 parameters controlling inlining.
4699 @emph{Note:} pseudo instruction represents, in this particular context, an
4700 abstract measurement of function's size. In no way does it represent a count
4701 of assembly instructions and as such its exact meaning might change from one
4702 release to an another.
4704 @item -fkeep-inline-functions
4705 @opindex fkeep-inline-functions
4706 In C, emit @code{static} functions that are declared @code{inline}
4707 into the object file, even if the function has been inlined into all
4708 of its callers. This switch does not affect functions using the
4709 @code{extern inline} extension in GNU C@. In C++, emit any and all
4710 inline functions into the object file.
4712 @item -fkeep-static-consts
4713 @opindex fkeep-static-consts
4714 Emit variables declared @code{static const} when optimization isn't turned
4715 on, even if the variables aren't referenced.
4717 GCC enables this option by default. If you want to force the compiler to
4718 check if the variable was referenced, regardless of whether or not
4719 optimization is turned on, use the @option{-fno-keep-static-consts} option.
4721 @item -fmerge-constants
4722 Attempt to merge identical constants (string constants and floating point
4723 constants) across compilation units.
4725 This option is the default for optimized compilation if the assembler and
4726 linker support it. Use @option{-fno-merge-constants} to inhibit this
4729 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4731 @item -fmerge-all-constants
4732 Attempt to merge identical constants and identical variables.
4734 This option implies @option{-fmerge-constants}. In addition to
4735 @option{-fmerge-constants} this considers e.g.@: even constant initialized
4736 arrays or initialized constant variables with integral or floating point
4737 types. Languages like C or C++ require each non-automatic variable to
4738 have distinct location, so using this option will result in non-conforming
4741 @item -fmodulo-sched
4742 @opindex fmodulo-sched
4743 Perform swing modulo scheduling immediately before the first scheduling
4744 pass. This pass looks at innermost loops and reorders their
4745 instructions by overlapping different iterations.
4747 @item -fno-branch-count-reg
4748 @opindex fno-branch-count-reg
4749 Do not use ``decrement and branch'' instructions on a count register,
4750 but instead generate a sequence of instructions that decrement a
4751 register, compare it against zero, then branch based upon the result.
4752 This option is only meaningful on architectures that support such
4753 instructions, which include x86, PowerPC, IA-64 and S/390.
4755 The default is @option{-fbranch-count-reg}.
4757 @item -fno-function-cse
4758 @opindex fno-function-cse
4759 Do not put function addresses in registers; make each instruction that
4760 calls a constant function contain the function's address explicitly.
4762 This option results in less efficient code, but some strange hacks
4763 that alter the assembler output may be confused by the optimizations
4764 performed when this option is not used.
4766 The default is @option{-ffunction-cse}
4768 @item -fno-zero-initialized-in-bss
4769 @opindex fno-zero-initialized-in-bss
4770 If the target supports a BSS section, GCC by default puts variables that
4771 are initialized to zero into BSS@. This can save space in the resulting
4774 This option turns off this behavior because some programs explicitly
4775 rely on variables going to the data section. E.g., so that the
4776 resulting executable can find the beginning of that section and/or make
4777 assumptions based on that.
4779 The default is @option{-fzero-initialized-in-bss}.
4781 @item -fbounds-check
4782 @opindex fbounds-check
4783 For front-ends that support it, generate additional code to check that
4784 indices used to access arrays are within the declared range. This is
4785 currently only supported by the Java and Fortran front-ends, where
4786 this option defaults to true and false respectively.
4788 @item -fmudflap -fmudflapth -fmudflapir
4792 @cindex bounds checking
4794 For front-ends that support it (C and C++), instrument all risky
4795 pointer/array dereferencing operations, some standard library
4796 string/heap functions, and some other associated constructs with
4797 range/validity tests. Modules so instrumented should be immune to
4798 buffer overflows, invalid heap use, and some other classes of C/C++
4799 programming errors. The instrumentation relies on a separate runtime
4800 library (@file{libmudflap}), which will be linked into a program if
4801 @option{-fmudflap} is given at link time. Run-time behavior of the
4802 instrumented program is controlled by the @env{MUDFLAP_OPTIONS}
4803 environment variable. See @code{env MUDFLAP_OPTIONS=-help a.out}
4806 Use @option{-fmudflapth} instead of @option{-fmudflap} to compile and to
4807 link if your program is multi-threaded. Use @option{-fmudflapir}, in
4808 addition to @option{-fmudflap} or @option{-fmudflapth}, if
4809 instrumentation should ignore pointer reads. This produces less
4810 instrumentation (and therefore faster execution) and still provides
4811 some protection against outright memory corrupting writes, but allows
4812 erroneously read data to propagate within a program.
4814 @item -fthread-jumps
4815 @opindex fthread-jumps
4816 Perform optimizations where we check to see if a jump branches to a
4817 location where another comparison subsumed by the first is found. If
4818 so, the first branch is redirected to either the destination of the
4819 second branch or a point immediately following it, depending on whether
4820 the condition is known to be true or false.
4822 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4824 @item -fcse-follow-jumps
4825 @opindex fcse-follow-jumps
4826 In common subexpression elimination, scan through jump instructions
4827 when the target of the jump is not reached by any other path. For
4828 example, when CSE encounters an @code{if} statement with an
4829 @code{else} clause, CSE will follow the jump when the condition
4832 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4834 @item -fcse-skip-blocks
4835 @opindex fcse-skip-blocks
4836 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
4837 follow jumps which conditionally skip over blocks. When CSE
4838 encounters a simple @code{if} statement with no else clause,
4839 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
4840 body of the @code{if}.
4842 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4844 @item -frerun-cse-after-loop
4845 @opindex frerun-cse-after-loop
4846 Re-run common subexpression elimination after loop optimizations has been
4849 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4853 Perform a global common subexpression elimination pass.
4854 This pass also performs global constant and copy propagation.
4856 @emph{Note:} When compiling a program using computed gotos, a GCC
4857 extension, you may get better runtime performance if you disable
4858 the global common subexpression elimination pass by adding
4859 @option{-fno-gcse} to the command line.
4861 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4865 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
4866 attempt to move loads which are only killed by stores into themselves. This
4867 allows a loop containing a load/store sequence to be changed to a load outside
4868 the loop, and a copy/store within the loop.
4870 Enabled by default when gcse is enabled.
4874 When @option{-fgcse-sm} is enabled, a store motion pass is run after
4875 global common subexpression elimination. This pass will attempt to move
4876 stores out of loops. When used in conjunction with @option{-fgcse-lm},
4877 loops containing a load/store sequence can be changed to a load before
4878 the loop and a store after the loop.
4880 Not enabled at any optimization level.
4884 When @option{-fgcse-las} is enabled, the global common subexpression
4885 elimination pass eliminates redundant loads that come after stores to the
4886 same memory location (both partial and full redundancies).
4888 Not enabled at any optimization level.
4890 @item -fgcse-after-reload
4891 @opindex fgcse-after-reload
4892 When @option{-fgcse-after-reload} is enabled, a redundant load elimination
4893 pass is performed after reload. The purpose of this pass is to cleanup
4896 @item -funsafe-loop-optimizations
4897 @opindex funsafe-loop-optimizations
4898 If given, the loop optimizer will assume that loop indices do not
4899 overflow, and that the loops with nontrivial exit condition are not
4900 infinite. This enables a wider range of loop optimizations even if
4901 the loop optimizer itself cannot prove that these assumptions are valid.
4902 Using @option{-Wunsafe-loop-optimizations}, the compiler will warn you
4903 if it finds this kind of loop.
4905 @item -fcrossjumping
4906 @opindex crossjumping
4907 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
4908 resulting code may or may not perform better than without cross-jumping.
4910 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4912 @item -fif-conversion
4913 @opindex if-conversion
4914 Attempt to transform conditional jumps into branch-less equivalents. This
4915 include use of conditional moves, min, max, set flags and abs instructions, and
4916 some tricks doable by standard arithmetics. The use of conditional execution
4917 on chips where it is available is controlled by @code{if-conversion2}.
4919 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4921 @item -fif-conversion2
4922 @opindex if-conversion2
4923 Use conditional execution (where available) to transform conditional jumps into
4924 branch-less equivalents.
4926 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4928 @item -fdelete-null-pointer-checks
4929 @opindex fdelete-null-pointer-checks
4930 Use global dataflow analysis to identify and eliminate useless checks
4931 for null pointers. The compiler assumes that dereferencing a null
4932 pointer would have halted the program. If a pointer is checked after
4933 it has already been dereferenced, it cannot be null.
4935 In some environments, this assumption is not true, and programs can
4936 safely dereference null pointers. Use
4937 @option{-fno-delete-null-pointer-checks} to disable this optimization
4938 for programs which depend on that behavior.
4940 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4942 @item -fexpensive-optimizations
4943 @opindex fexpensive-optimizations
4944 Perform a number of minor optimizations that are relatively expensive.
4946 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4948 @item -foptimize-register-move
4950 @opindex foptimize-register-move
4952 Attempt to reassign register numbers in move instructions and as
4953 operands of other simple instructions in order to maximize the amount of
4954 register tying. This is especially helpful on machines with two-operand
4957 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
4960 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4962 @item -fdelayed-branch
4963 @opindex fdelayed-branch
4964 If supported for the target machine, attempt to reorder instructions
4965 to exploit instruction slots available after delayed branch
4968 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4970 @item -fschedule-insns
4971 @opindex fschedule-insns
4972 If supported for the target machine, attempt to reorder instructions to
4973 eliminate execution stalls due to required data being unavailable. This
4974 helps machines that have slow floating point or memory load instructions
4975 by allowing other instructions to be issued until the result of the load
4976 or floating point instruction is required.
4978 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4980 @item -fschedule-insns2
4981 @opindex fschedule-insns2
4982 Similar to @option{-fschedule-insns}, but requests an additional pass of
4983 instruction scheduling after register allocation has been done. This is
4984 especially useful on machines with a relatively small number of
4985 registers and where memory load instructions take more than one cycle.
4987 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4989 @item -fno-sched-interblock
4990 @opindex fno-sched-interblock
4991 Don't schedule instructions across basic blocks. This is normally
4992 enabled by default when scheduling before register allocation, i.e.@:
4993 with @option{-fschedule-insns} or at @option{-O2} or higher.
4995 @item -fno-sched-spec
4996 @opindex fno-sched-spec
4997 Don't allow speculative motion of non-load instructions. This is normally
4998 enabled by default when scheduling before register allocation, i.e.@:
4999 with @option{-fschedule-insns} or at @option{-O2} or higher.
5001 @item -fsched-spec-load
5002 @opindex fsched-spec-load
5003 Allow speculative motion of some load instructions. This only makes
5004 sense when scheduling before register allocation, i.e.@: with
5005 @option{-fschedule-insns} or at @option{-O2} or higher.
5007 @item -fsched-spec-load-dangerous
5008 @opindex fsched-spec-load-dangerous
5009 Allow speculative motion of more load instructions. This only makes
5010 sense when scheduling before register allocation, i.e.@: with
5011 @option{-fschedule-insns} or at @option{-O2} or higher.
5013 @item -fsched-stalled-insns=@var{n}
5014 @opindex fsched-stalled-insns
5015 Define how many insns (if any) can be moved prematurely from the queue
5016 of stalled insns into the ready list, during the second scheduling pass.
5018 @item -fsched-stalled-insns-dep=@var{n}
5019 @opindex fsched-stalled-insns-dep
5020 Define how many insn groups (cycles) will be examined for a dependency
5021 on a stalled insn that is candidate for premature removal from the queue
5022 of stalled insns. Has an effect only during the second scheduling pass,
5023 and only if @option{-fsched-stalled-insns} is used and its value is not zero.
5025 @item -fsched2-use-superblocks
5026 @opindex fsched2-use-superblocks
5027 When scheduling after register allocation, do use superblock scheduling
5028 algorithm. Superblock scheduling allows motion across basic block boundaries
5029 resulting on faster schedules. This option is experimental, as not all machine
5030 descriptions used by GCC model the CPU closely enough to avoid unreliable
5031 results from the algorithm.
5033 This only makes sense when scheduling after register allocation, i.e.@: with
5034 @option{-fschedule-insns2} or at @option{-O2} or higher.
5036 @item -fsched2-use-traces
5037 @opindex fsched2-use-traces
5038 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
5039 allocation and additionally perform code duplication in order to increase the
5040 size of superblocks using tracer pass. See @option{-ftracer} for details on
5043 This mode should produce faster but significantly longer programs. Also
5044 without @option{-fbranch-probabilities} the traces constructed may not
5045 match the reality and hurt the performance. This only makes
5046 sense when scheduling after register allocation, i.e.@: with
5047 @option{-fschedule-insns2} or at @option{-O2} or higher.
5051 Eliminates redundant extension instructions and move the non redundant
5052 ones to optimal placement using LCM.
5054 @item -freschedule-modulo-scheduled-loops
5055 @opindex fscheduling-in-modulo-scheduled-loops
5056 The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled
5057 we may want to prevent the later scheduling passes from changing its schedule, we use this
5058 option to control that.
5060 @item -fcaller-saves
5061 @opindex fcaller-saves
5062 Enable values to be allocated in registers that will be clobbered by
5063 function calls, by emitting extra instructions to save and restore the
5064 registers around such calls. Such allocation is done only when it
5065 seems to result in better code than would otherwise be produced.
5067 This option is always enabled by default on certain machines, usually
5068 those which have no call-preserved registers to use instead.
5070 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5073 Perform Partial Redundancy Elimination (PRE) on trees. This flag is
5074 enabled by default at @option{-O2} and @option{-O3}.
5077 Perform Full Redundancy Elimination (FRE) on trees. The difference
5078 between FRE and PRE is that FRE only considers expressions
5079 that are computed on all paths leading to the redundant computation.
5080 This analysis faster than PRE, though it exposes fewer redundancies.
5081 This flag is enabled by default at @option{-O} and higher.
5083 @item -ftree-copy-prop
5084 Perform copy propagation on trees. This pass eliminates unnecessary
5085 copy operations. This flag is enabled by default at @option{-O} and
5088 @item -ftree-store-copy-prop
5089 Perform copy propagation of memory loads and stores. This pass
5090 eliminates unnecessary copy operations in memory references
5091 (structures, global variables, arrays, etc). This flag is enabled by
5092 default at @option{-O2} and higher.
5095 Perform structural alias analysis on trees. This flag
5096 is enabled by default at @option{-O} and higher.
5099 Perform interprocedural pointer analysis.
5102 Perform forward store motion on trees. This flag is
5103 enabled by default at @option{-O} and higher.
5106 Perform sparse conditional constant propagation (CCP) on trees. This
5107 pass only operates on local scalar variables and is enabled by default
5108 at @option{-O} and higher.
5110 @item -ftree-store-ccp
5111 Perform sparse conditional constant propagation (CCP) on trees. This
5112 pass operates on both local scalar variables and memory stores and
5113 loads (global variables, structures, arrays, etc). This flag is
5114 enabled by default at @option{-O2} and higher.
5117 Perform dead code elimination (DCE) on trees. This flag is enabled by
5118 default at @option{-O} and higher.
5120 @item -ftree-dominator-opts
5121 Perform a variety of simple scalar cleanups (constant/copy
5122 propagation, redundancy elimination, range propagation and expression
5123 simplification) based on a dominator tree traversal. This also
5124 performs jump threading (to reduce jumps to jumps). This flag is
5125 enabled by default at @option{-O} and higher.
5128 Perform loop header copying on trees. This is beneficial since it increases
5129 effectiveness of code motion optimizations. It also saves one jump. This flag
5130 is enabled by default at @option{-O} and higher. It is not enabled
5131 for @option{-Os}, since it usually increases code size.
5133 @item -ftree-loop-optimize
5134 Perform loop optimizations on trees. This flag is enabled by default
5135 at @option{-O} and higher.
5137 @item -ftree-loop-linear
5138 Perform linear loop transformations on tree. This flag can improve cache
5139 performance and allow further loop optimizations to take place.
5141 @item -ftree-loop-im
5142 Perform loop invariant motion on trees. This pass moves only invariants that
5143 would be hard to handle at RTL level (function calls, operations that expand to
5144 nontrivial sequences of insns). With @option{-funswitch-loops} it also moves
5145 operands of conditions that are invariant out of the loop, so that we can use
5146 just trivial invariantness analysis in loop unswitching. The pass also includes
5149 @item -ftree-loop-ivcanon
5150 Create a canonical counter for number of iterations in the loop for that
5151 determining number of iterations requires complicated analysis. Later
5152 optimizations then may determine the number easily. Useful especially
5153 in connection with unrolling.
5156 Perform induction variable optimizations (strength reduction, induction
5157 variable merging and induction variable elimination) on trees.
5160 Perform scalar replacement of aggregates. This pass replaces structure
5161 references with scalars to prevent committing structures to memory too
5162 early. This flag is enabled by default at @option{-O} and higher.
5164 @item -ftree-copyrename
5165 Perform copy renaming on trees. This pass attempts to rename compiler
5166 temporaries to other variables at copy locations, usually resulting in
5167 variable names which more closely resemble the original variables. This flag
5168 is enabled by default at @option{-O} and higher.
5171 Perform temporary expression replacement during the SSA->normal phase. Single
5172 use/single def temporaries are replaced at their use location with their
5173 defining expression. This results in non-GIMPLE code, but gives the expanders
5174 much more complex trees to work on resulting in better RTL generation. This is
5175 enabled by default at @option{-O} and higher.
5178 Perform live range splitting during the SSA->normal phase. Distinct live
5179 ranges of a variable are split into unique variables, allowing for better
5180 optimization later. This is enabled by default at @option{-O} and higher.
5182 @item -ftree-vectorize
5183 Perform loop vectorization on trees.
5185 @item -ftree-vect-loop-version
5186 @opindex ftree-vect-loop-version
5187 Perform loop versioning when doing loop vectorization on trees. When a loop
5188 appears to be vectorizable except that data alignment or data dependence cannot
5189 be determined at compile time then vectorized and non-vectorized versions of
5190 the loop are generated along with runtime checks for alignment or dependence
5191 to control which version is executed. This option is enabled by default
5192 except at level @option{-Os} where it is disabled.
5195 Perform Value Range Propagation on trees. This is similar to the
5196 constant propagation pass, but instead of values, ranges of values are
5197 propagated. This allows the optimizers to remove unnecessary range
5198 checks like array bound checks and null pointer checks. This is
5199 enabled by default at @option{-O2} and higher. Null pointer check
5200 elimination is only done if @option{-fdelete-null-pointer-checks} is
5205 Perform tail duplication to enlarge superblock size. This transformation
5206 simplifies the control flow of the function allowing other optimizations to do
5209 @item -funroll-loops
5210 @opindex funroll-loops
5211 Unroll loops whose number of iterations can be determined at compile
5212 time or upon entry to the loop. @option{-funroll-loops} implies
5213 @option{-frerun-cse-after-loop}. This option makes code larger,
5214 and may or may not make it run faster.
5216 @item -funroll-all-loops
5217 @opindex funroll-all-loops
5218 Unroll all loops, even if their number of iterations is uncertain when
5219 the loop is entered. This usually makes programs run more slowly.
5220 @option{-funroll-all-loops} implies the same options as
5221 @option{-funroll-loops},
5223 @item -fsplit-ivs-in-unroller
5224 @opindex -fsplit-ivs-in-unroller
5225 Enables expressing of values of induction variables in later iterations
5226 of the unrolled loop using the value in the first iteration. This breaks
5227 long dependency chains, thus improving efficiency of the scheduling passes.
5229 Combination of @option{-fweb} and CSE is often sufficient to obtain the
5230 same effect. However in cases the loop body is more complicated than
5231 a single basic block, this is not reliable. It also does not work at all
5232 on some of the architectures due to restrictions in the CSE pass.
5234 This optimization is enabled by default.
5236 @item -fvariable-expansion-in-unroller
5237 @opindex -fvariable-expansion-in-unroller
5238 With this option, the compiler will create multiple copies of some
5239 local variables when unrolling a loop which can result in superior code.
5241 @item -fprefetch-loop-arrays
5242 @opindex fprefetch-loop-arrays
5243 If supported by the target machine, generate instructions to prefetch
5244 memory to improve the performance of loops that access large arrays.
5246 This option may generate better or worse code; results are highly
5247 dependent on the structure of loops within the source code.
5249 Disabled at level @option{-Os}.
5252 @itemx -fno-peephole2
5253 @opindex fno-peephole
5254 @opindex fno-peephole2
5255 Disable any machine-specific peephole optimizations. The difference
5256 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
5257 are implemented in the compiler; some targets use one, some use the
5258 other, a few use both.
5260 @option{-fpeephole} is enabled by default.
5261 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5263 @item -fno-guess-branch-probability
5264 @opindex fno-guess-branch-probability
5265 Do not guess branch probabilities using heuristics.
5267 GCC will use heuristics to guess branch probabilities if they are
5268 not provided by profiling feedback (@option{-fprofile-arcs}). These
5269 heuristics are based on the control flow graph. If some branch probabilities
5270 are specified by @samp{__builtin_expect}, then the heuristics will be
5271 used to guess branch probabilities for the rest of the control flow graph,
5272 taking the @samp{__builtin_expect} info into account. The interactions
5273 between the heuristics and @samp{__builtin_expect} can be complex, and in
5274 some cases, it may be useful to disable the heuristics so that the effects
5275 of @samp{__builtin_expect} are easier to understand.
5277 The default is @option{-fguess-branch-probability} at levels
5278 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5280 @item -freorder-blocks
5281 @opindex freorder-blocks
5282 Reorder basic blocks in the compiled function in order to reduce number of
5283 taken branches and improve code locality.
5285 Enabled at levels @option{-O2}, @option{-O3}.
5287 @item -freorder-blocks-and-partition
5288 @opindex freorder-blocks-and-partition
5289 In addition to reordering basic blocks in the compiled function, in order
5290 to reduce number of taken branches, partitions hot and cold basic blocks
5291 into separate sections of the assembly and .o files, to improve
5292 paging and cache locality performance.
5294 This optimization is automatically turned off in the presence of
5295 exception handling, for linkonce sections, for functions with a user-defined
5296 section attribute and on any architecture that does not support named
5299 @item -freorder-functions
5300 @opindex freorder-functions
5301 Reorder functions in the object file in order to
5302 improve code locality. This is implemented by using special
5303 subsections @code{.text.hot} for most frequently executed functions and
5304 @code{.text.unlikely} for unlikely executed functions. Reordering is done by
5305 the linker so object file format must support named sections and linker must
5306 place them in a reasonable way.
5308 Also profile feedback must be available in to make this option effective. See
5309 @option{-fprofile-arcs} for details.
5311 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5313 @item -fstrict-aliasing
5314 @opindex fstrict-aliasing
5315 Allows the compiler to assume the strictest aliasing rules applicable to
5316 the language being compiled. For C (and C++), this activates
5317 optimizations based on the type of expressions. In particular, an
5318 object of one type is assumed never to reside at the same address as an
5319 object of a different type, unless the types are almost the same. For
5320 example, an @code{unsigned int} can alias an @code{int}, but not a
5321 @code{void*} or a @code{double}. A character type may alias any other
5324 Pay special attention to code like this:
5337 The practice of reading from a different union member than the one most
5338 recently written to (called ``type-punning'') is common. Even with
5339 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
5340 is accessed through the union type. So, the code above will work as
5341 expected. However, this code might not:
5352 Every language that wishes to perform language-specific alias analysis
5353 should define a function that computes, given an @code{tree}
5354 node, an alias set for the node. Nodes in different alias sets are not
5355 allowed to alias. For an example, see the C front-end function
5356 @code{c_get_alias_set}.
5358 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5360 @item -falign-functions
5361 @itemx -falign-functions=@var{n}
5362 @opindex falign-functions
5363 Align the start of functions to the next power-of-two greater than
5364 @var{n}, skipping up to @var{n} bytes. For instance,
5365 @option{-falign-functions=32} aligns functions to the next 32-byte
5366 boundary, but @option{-falign-functions=24} would align to the next
5367 32-byte boundary only if this can be done by skipping 23 bytes or less.
5369 @option{-fno-align-functions} and @option{-falign-functions=1} are
5370 equivalent and mean that functions will not be aligned.
5372 Some assemblers only support this flag when @var{n} is a power of two;
5373 in that case, it is rounded up.
5375 If @var{n} is not specified or is zero, use a machine-dependent default.
5377 Enabled at levels @option{-O2}, @option{-O3}.
5379 @item -falign-labels
5380 @itemx -falign-labels=@var{n}
5381 @opindex falign-labels
5382 Align all branch targets to a power-of-two boundary, skipping up to
5383 @var{n} bytes like @option{-falign-functions}. This option can easily
5384 make code slower, because it must insert dummy operations for when the
5385 branch target is reached in the usual flow of the code.
5387 @option{-fno-align-labels} and @option{-falign-labels=1} are
5388 equivalent and mean that labels will not be aligned.
5390 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
5391 are greater than this value, then their values are used instead.
5393 If @var{n} is not specified or is zero, use a machine-dependent default
5394 which is very likely to be @samp{1}, meaning no alignment.
5396 Enabled at levels @option{-O2}, @option{-O3}.
5399 @itemx -falign-loops=@var{n}
5400 @opindex falign-loops
5401 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
5402 like @option{-falign-functions}. The hope is that the loop will be
5403 executed many times, which will make up for any execution of the dummy
5406 @option{-fno-align-loops} and @option{-falign-loops=1} are
5407 equivalent and mean that loops will not be aligned.
5409 If @var{n} is not specified or is zero, use a machine-dependent default.
5411 Enabled at levels @option{-O2}, @option{-O3}.
5414 @itemx -falign-jumps=@var{n}
5415 @opindex falign-jumps
5416 Align branch targets to a power-of-two boundary, for branch targets
5417 where the targets can only be reached by jumping, skipping up to @var{n}
5418 bytes like @option{-falign-functions}. In this case, no dummy operations
5421 @option{-fno-align-jumps} and @option{-falign-jumps=1} are
5422 equivalent and mean that loops will not be aligned.
5424 If @var{n} is not specified or is zero, use a machine-dependent default.
5426 Enabled at levels @option{-O2}, @option{-O3}.
5428 @item -funit-at-a-time
5429 @opindex funit-at-a-time
5430 Parse the whole compilation unit before starting to produce code.
5431 This allows some extra optimizations to take place but consumes
5432 more memory (in general). There are some compatibility issues
5433 with @emph{unit-at-a-time} mode:
5436 enabling @emph{unit-at-a-time} mode may change the order
5437 in which functions, variables, and top-level @code{asm} statements
5438 are emitted, and will likely break code relying on some particular
5439 ordering. The majority of such top-level @code{asm} statements,
5440 though, can be replaced by @code{section} attributes. The
5441 @option{fno-toplevel-reorder} option may be used to keep the ordering
5442 used in the input file, at the cost of some optimizations.
5445 @emph{unit-at-a-time} mode removes unreferenced static variables
5446 and functions. This may result in undefined references
5447 when an @code{asm} statement refers directly to variables or functions
5448 that are otherwise unused. In that case either the variable/function
5449 shall be listed as an operand of the @code{asm} statement operand or,
5450 in the case of top-level @code{asm} statements the attribute @code{used}
5451 shall be used on the declaration.
5454 Static functions now can use non-standard passing conventions that
5455 may break @code{asm} statements calling functions directly. Again,
5456 attribute @code{used} will prevent this behavior.
5459 As a temporary workaround, @option{-fno-unit-at-a-time} can be used,
5460 but this scheme may not be supported by future releases of GCC@.
5462 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5464 @item -fno-toplevel-reorder
5465 Do not reorder top-level functions, variables, and @code{asm}
5466 statements. Output them in the same order that they appear in the
5467 input file. When this option is used, unreferenced static variables
5468 will not be removed. This option is intended to support existing code
5469 which relies on a particular ordering. For new code, it is better to
5474 Constructs webs as commonly used for register allocation purposes and assign
5475 each web individual pseudo register. This allows the register allocation pass
5476 to operate on pseudos directly, but also strengthens several other optimization
5477 passes, such as CSE, loop optimizer and trivial dead code remover. It can,
5478 however, make debugging impossible, since variables will no longer stay in a
5481 Enabled by default with @option{-funroll-loops}.
5483 @item -fwhole-program
5484 @opindex fwhole-program
5485 Assume that the current compilation unit represents whole program being
5486 compiled. All public functions and variables with the exception of @code{main}
5487 and those merged by attribute @code{externally_visible} become static functions
5488 and in a affect gets more aggressively optimized by interprocedural optimizers.
5489 While this option is equivalent to proper use of @code{static} keyword for
5490 programs consisting of single file, in combination with option
5491 @option{--combine} this flag can be used to compile most of smaller scale C
5492 programs since the functions and variables become local for the whole combined
5493 compilation unit, not for the single source file itself.
5496 @item -fno-cprop-registers
5497 @opindex fno-cprop-registers
5498 After register allocation and post-register allocation instruction splitting,
5499 we perform a copy-propagation pass to try to reduce scheduling dependencies
5500 and occasionally eliminate the copy.
5502 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5504 @item -fprofile-generate
5505 @opindex fprofile-generate
5507 Enable options usually used for instrumenting application to produce
5508 profile useful for later recompilation with profile feedback based
5509 optimization. You must use @option{-fprofile-generate} both when
5510 compiling and when linking your program.
5512 The following options are enabled: @code{-fprofile-arcs}, @code{-fprofile-values}, @code{-fvpt}.
5515 @opindex fprofile-use
5516 Enable profile feedback directed optimizations, and optimizations
5517 generally profitable only with profile feedback available.
5519 The following options are enabled: @code{-fbranch-probabilities}, @code{-fvpt},
5520 @code{-funroll-loops}, @code{-fpeel-loops}, @code{-ftracer}
5524 The following options control compiler behavior regarding floating
5525 point arithmetic. These options trade off between speed and
5526 correctness. All must be specifically enabled.
5530 @opindex ffloat-store
5531 Do not store floating point variables in registers, and inhibit other
5532 options that might change whether a floating point value is taken from a
5535 @cindex floating point precision
5536 This option prevents undesirable excess precision on machines such as
5537 the 68000 where the floating registers (of the 68881) keep more
5538 precision than a @code{double} is supposed to have. Similarly for the
5539 x86 architecture. For most programs, the excess precision does only
5540 good, but a few programs rely on the precise definition of IEEE floating
5541 point. Use @option{-ffloat-store} for such programs, after modifying
5542 them to store all pertinent intermediate computations into variables.
5546 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations}, @*
5547 @option{-fno-trapping-math}, @option{-ffinite-math-only},
5548 @option{-fno-rounding-math}, @option{-fno-signaling-nans}
5549 and @option{fcx-limited-range}.
5551 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
5553 This option should never be turned on by any @option{-O} option since
5554 it can result in incorrect output for programs which depend on
5555 an exact implementation of IEEE or ISO rules/specifications for
5558 @item -fno-math-errno
5559 @opindex fno-math-errno
5560 Do not set ERRNO after calling math functions that are executed
5561 with a single instruction, e.g., sqrt. A program that relies on
5562 IEEE exceptions for math error handling may want to use this flag
5563 for speed while maintaining IEEE arithmetic compatibility.
5565 This option should never be turned on by any @option{-O} option since
5566 it can result in incorrect output for programs which depend on
5567 an exact implementation of IEEE or ISO rules/specifications for
5570 The default is @option{-fmath-errno}.
5572 On Darwin systems, the math library never sets @code{errno}. There is therefore
5573 no reason for the compiler to consider the possibility that it might,
5574 and @option{-fno-math-errno} is the default.
5576 @item -funsafe-math-optimizations
5577 @opindex funsafe-math-optimizations
5578 Allow optimizations for floating-point arithmetic that (a) assume
5579 that arguments and results are valid and (b) may violate IEEE or
5580 ANSI standards. When used at link-time, it may include libraries
5581 or startup files that change the default FPU control word or other
5582 similar optimizations.
5584 This option should never be turned on by any @option{-O} option since
5585 it can result in incorrect output for programs which depend on
5586 an exact implementation of IEEE or ISO rules/specifications for
5589 The default is @option{-fno-unsafe-math-optimizations}.
5591 @item -ffinite-math-only
5592 @opindex ffinite-math-only
5593 Allow optimizations for floating-point arithmetic that assume
5594 that arguments and results are not NaNs or +-Infs.
5596 This option should never be turned on by any @option{-O} option since
5597 it can result in incorrect output for programs which depend on
5598 an exact implementation of IEEE or ISO rules/specifications.
5600 The default is @option{-fno-finite-math-only}.
5602 @item -fno-trapping-math
5603 @opindex fno-trapping-math
5604 Compile code assuming that floating-point operations cannot generate
5605 user-visible traps. These traps include division by zero, overflow,
5606 underflow, inexact result and invalid operation. This option implies
5607 @option{-fno-signaling-nans}. Setting this option may allow faster
5608 code if one relies on ``non-stop'' IEEE arithmetic, for example.
5610 This option should never be turned on by any @option{-O} option since
5611 it can result in incorrect output for programs which depend on
5612 an exact implementation of IEEE or ISO rules/specifications for
5615 The default is @option{-ftrapping-math}.
5617 @item -frounding-math
5618 @opindex frounding-math
5619 Disable transformations and optimizations that assume default floating
5620 point rounding behavior. This is round-to-zero for all floating point
5621 to integer conversions, and round-to-nearest for all other arithmetic
5622 truncations. This option should be specified for programs that change
5623 the FP rounding mode dynamically, or that may be executed with a
5624 non-default rounding mode. This option disables constant folding of
5625 floating point expressions at compile-time (which may be affected by
5626 rounding mode) and arithmetic transformations that are unsafe in the
5627 presence of sign-dependent rounding modes.
5629 The default is @option{-fno-rounding-math}.
5631 This option is experimental and does not currently guarantee to
5632 disable all GCC optimizations that are affected by rounding mode.
5633 Future versions of GCC may provide finer control of this setting
5634 using C99's @code{FENV_ACCESS} pragma. This command line option
5635 will be used to specify the default state for @code{FENV_ACCESS}.
5637 @item -frtl-abstract-sequences
5638 @opindex frtl-abstract-sequences
5639 It is a size optimization method. This option is to find identical
5640 sequences of code, which can be turned into pseudo-procedures and
5641 then replace all occurrences with calls to the newly created
5642 subroutine. It is kind of an opposite of @option{-finline-functions}.
5643 This optimization runs at RTL level.
5645 @item -fsignaling-nans
5646 @opindex fsignaling-nans
5647 Compile code assuming that IEEE signaling NaNs may generate user-visible
5648 traps during floating-point operations. Setting this option disables
5649 optimizations that may change the number of exceptions visible with
5650 signaling NaNs. This option implies @option{-ftrapping-math}.
5652 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
5655 The default is @option{-fno-signaling-nans}.
5657 This option is experimental and does not currently guarantee to
5658 disable all GCC optimizations that affect signaling NaN behavior.
5660 @item -fsingle-precision-constant
5661 @opindex fsingle-precision-constant
5662 Treat floating point constant as single precision constant instead of
5663 implicitly converting it to double precision constant.
5665 @item -fcx-limited-range
5666 @itemx -fno-cx-limited-range
5667 @opindex fcx-limited-range
5668 @opindex fno-cx-limited-range
5669 When enabled, this option states that a range reduction step is not
5670 needed when performing complex division. The default is
5671 @option{-fno-cx-limited-range}, but is enabled by @option{-ffast-math}.
5673 This option controls the default setting of the ISO C99
5674 @code{CX_LIMITED_RANGE} pragma. Nevertheless, the option applies to
5679 The following options control optimizations that may improve
5680 performance, but are not enabled by any @option{-O} options. This
5681 section includes experimental options that may produce broken code.
5684 @item -fbranch-probabilities
5685 @opindex fbranch-probabilities
5686 After running a program compiled with @option{-fprofile-arcs}
5687 (@pxref{Debugging Options,, Options for Debugging Your Program or
5688 @command{gcc}}), you can compile it a second time using
5689 @option{-fbranch-probabilities}, to improve optimizations based on
5690 the number of times each branch was taken. When the program
5691 compiled with @option{-fprofile-arcs} exits it saves arc execution
5692 counts to a file called @file{@var{sourcename}.gcda} for each source
5693 file The information in this data file is very dependent on the
5694 structure of the generated code, so you must use the same source code
5695 and the same optimization options for both compilations.
5697 With @option{-fbranch-probabilities}, GCC puts a
5698 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
5699 These can be used to improve optimization. Currently, they are only
5700 used in one place: in @file{reorg.c}, instead of guessing which path a
5701 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
5702 exactly determine which path is taken more often.
5704 @item -fprofile-values
5705 @opindex fprofile-values
5706 If combined with @option{-fprofile-arcs}, it adds code so that some
5707 data about values of expressions in the program is gathered.
5709 With @option{-fbranch-probabilities}, it reads back the data gathered
5710 from profiling values of expressions and adds @samp{REG_VALUE_PROFILE}
5711 notes to instructions for their later usage in optimizations.
5713 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
5717 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
5718 a code to gather information about values of expressions.
5720 With @option{-fbranch-probabilities}, it reads back the data gathered
5721 and actually performs the optimizations based on them.
5722 Currently the optimizations include specialization of division operation
5723 using the knowledge about the value of the denominator.
5725 @item -frename-registers
5726 @opindex frename-registers
5727 Attempt to avoid false dependencies in scheduled code by making use
5728 of registers left over after register allocation. This optimization
5729 will most benefit processors with lots of registers. Depending on the
5730 debug information format adopted by the target, however, it can
5731 make debugging impossible, since variables will no longer stay in
5732 a ``home register''.
5734 Enabled by default with @option{-funroll-loops}.
5738 Perform tail duplication to enlarge superblock size. This transformation
5739 simplifies the control flow of the function allowing other optimizations to do
5742 Enabled with @option{-fprofile-use}.
5744 @item -funroll-loops
5745 @opindex funroll-loops
5746 Unroll loops whose number of iterations can be determined at compile time or
5747 upon entry to the loop. @option{-funroll-loops} implies
5748 @option{-frerun-cse-after-loop}, @option{-fweb} and @option{-frename-registers}.
5749 It also turns on complete loop peeling (i.e.@: complete removal of loops with
5750 small constant number of iterations). This option makes code larger, and may
5751 or may not make it run faster.
5753 Enabled with @option{-fprofile-use}.
5755 @item -funroll-all-loops
5756 @opindex funroll-all-loops
5757 Unroll all loops, even if their number of iterations is uncertain when
5758 the loop is entered. This usually makes programs run more slowly.
5759 @option{-funroll-all-loops} implies the same options as
5760 @option{-funroll-loops}.
5763 @opindex fpeel-loops
5764 Peels the loops for that there is enough information that they do not
5765 roll much (from profile feedback). It also turns on complete loop peeling
5766 (i.e.@: complete removal of loops with small constant number of iterations).
5768 Enabled with @option{-fprofile-use}.
5770 @item -fmove-loop-invariants
5771 @opindex fmove-loop-invariants
5772 Enables the loop invariant motion pass in the RTL loop optimizer. Enabled
5773 at level @option{-O1}
5775 @item -funswitch-loops
5776 @opindex funswitch-loops
5777 Move branches with loop invariant conditions out of the loop, with duplicates
5778 of the loop on both branches (modified according to result of the condition).
5780 @item -ffunction-sections
5781 @itemx -fdata-sections
5782 @opindex ffunction-sections
5783 @opindex fdata-sections
5784 Place each function or data item into its own section in the output
5785 file if the target supports arbitrary sections. The name of the
5786 function or the name of the data item determines the section's name
5789 Use these options on systems where the linker can perform optimizations
5790 to improve locality of reference in the instruction space. Most systems
5791 using the ELF object format and SPARC processors running Solaris 2 have
5792 linkers with such optimizations. AIX may have these optimizations in
5795 Only use these options when there are significant benefits from doing
5796 so. When you specify these options, the assembler and linker will
5797 create larger object and executable files and will also be slower.
5798 You will not be able to use @code{gprof} on all systems if you
5799 specify this option and you may have problems with debugging if
5800 you specify both this option and @option{-g}.
5802 @item -fbranch-target-load-optimize
5803 @opindex fbranch-target-load-optimize
5804 Perform branch target register load optimization before prologue / epilogue
5806 The use of target registers can typically be exposed only during reload,
5807 thus hoisting loads out of loops and doing inter-block scheduling needs
5808 a separate optimization pass.
5810 @item -fbranch-target-load-optimize2
5811 @opindex fbranch-target-load-optimize2
5812 Perform branch target register load optimization after prologue / epilogue
5815 @item -fbtr-bb-exclusive
5816 @opindex fbtr-bb-exclusive
5817 When performing branch target register load optimization, don't reuse
5818 branch target registers in within any basic block.
5820 @item -fstack-protector
5821 Emit extra code to check for buffer overflows, such as stack smashing
5822 attacks. This is done by adding a guard variable to functions with
5823 vulnerable objects. This includes functions that call alloca, and
5824 functions with buffers larger than 8 bytes. The guards are initialized
5825 when a function is entered and then checked when the function exits.
5826 If a guard check fails, an error message is printed and the program exits.
5828 @item -fstack-protector-all
5829 Like @option{-fstack-protector} except that all functions are protected.
5831 @item -fsection-anchors
5832 @opindex fsection-anchors
5833 Try to reduce the number of symbolic address calculations by using
5834 shared ``anchor'' symbols to address nearby objects. This transformation
5835 can help to reduce the number of GOT entries and GOT accesses on some
5838 For example, the implementation of the following function @code{foo}:
5842 int foo (void) @{ return a + b + c; @}
5845 would usually calculate the addresses of all three variables, but if you
5846 compile it with @option{-fsection-anchors}, it will access the variables
5847 from a common anchor point instead. The effect is similar to the
5848 following pseudocode (which isn't valid C):
5853 register int *xr = &x;
5854 return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
5858 Not all targets support this option.
5860 @item --param @var{name}=@var{value}
5862 In some places, GCC uses various constants to control the amount of
5863 optimization that is done. For example, GCC will not inline functions
5864 that contain more that a certain number of instructions. You can
5865 control some of these constants on the command-line using the
5866 @option{--param} option.
5868 The names of specific parameters, and the meaning of the values, are
5869 tied to the internals of the compiler, and are subject to change
5870 without notice in future releases.
5872 In each case, the @var{value} is an integer. The allowable choices for
5873 @var{name} are given in the following table:
5876 @item salias-max-implicit-fields
5877 The maximum number of fields in a variable without direct
5878 structure accesses for which structure aliasing will consider trying
5879 to track each field. The default is 5
5881 @item salias-max-array-elements
5882 The maximum number of elements an array can have and its elements
5883 still be tracked individually by structure aliasing. The default is 4
5885 @item sra-max-structure-size
5886 The maximum structure size, in bytes, at which the scalar replacement
5887 of aggregates (SRA) optimization will perform block copies. The
5888 default value, 0, implies that GCC will select the most appropriate
5891 @item sra-field-structure-ratio
5892 The threshold ratio (as a percentage) between instantiated fields and
5893 the complete structure size. We say that if the ratio of the number
5894 of bytes in instantiated fields to the number of bytes in the complete
5895 structure exceeds this parameter, then block copies are not used. The
5898 @item max-crossjump-edges
5899 The maximum number of incoming edges to consider for crossjumping.
5900 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
5901 the number of edges incoming to each block. Increasing values mean
5902 more aggressive optimization, making the compile time increase with
5903 probably small improvement in executable size.
5905 @item min-crossjump-insns
5906 The minimum number of instructions which must be matched at the end
5907 of two blocks before crossjumping will be performed on them. This
5908 value is ignored in the case where all instructions in the block being
5909 crossjumped from are matched. The default value is 5.
5911 @item max-grow-copy-bb-insns
5912 The maximum code size expansion factor when copying basic blocks
5913 instead of jumping. The expansion is relative to a jump instruction.
5914 The default value is 8.
5916 @item max-goto-duplication-insns
5917 The maximum number of instructions to duplicate to a block that jumps
5918 to a computed goto. To avoid @math{O(N^2)} behavior in a number of
5919 passes, GCC factors computed gotos early in the compilation process,
5920 and unfactors them as late as possible. Only computed jumps at the
5921 end of a basic blocks with no more than max-goto-duplication-insns are
5922 unfactored. The default value is 8.
5924 @item max-delay-slot-insn-search
5925 The maximum number of instructions to consider when looking for an
5926 instruction to fill a delay slot. If more than this arbitrary number of
5927 instructions is searched, the time savings from filling the delay slot
5928 will be minimal so stop searching. Increasing values mean more
5929 aggressive optimization, making the compile time increase with probably
5930 small improvement in executable run time.
5932 @item max-delay-slot-live-search
5933 When trying to fill delay slots, the maximum number of instructions to
5934 consider when searching for a block with valid live register
5935 information. Increasing this arbitrarily chosen value means more
5936 aggressive optimization, increasing the compile time. This parameter
5937 should be removed when the delay slot code is rewritten to maintain the
5940 @item max-gcse-memory
5941 The approximate maximum amount of memory that will be allocated in
5942 order to perform the global common subexpression elimination
5943 optimization. If more memory than specified is required, the
5944 optimization will not be done.
5946 @item max-gcse-passes
5947 The maximum number of passes of GCSE to run. The default is 1.
5949 @item max-pending-list-length
5950 The maximum number of pending dependencies scheduling will allow
5951 before flushing the current state and starting over. Large functions
5952 with few branches or calls can create excessively large lists which
5953 needlessly consume memory and resources.
5955 @item max-inline-insns-single
5956 Several parameters control the tree inliner used in gcc.
5957 This number sets the maximum number of instructions (counted in GCC's
5958 internal representation) in a single function that the tree inliner
5959 will consider for inlining. This only affects functions declared
5960 inline and methods implemented in a class declaration (C++).
5961 The default value is 450.
5963 @item max-inline-insns-auto
5964 When you use @option{-finline-functions} (included in @option{-O3}),
5965 a lot of functions that would otherwise not be considered for inlining
5966 by the compiler will be investigated. To those functions, a different
5967 (more restrictive) limit compared to functions declared inline can
5969 The default value is 90.
5971 @item large-function-insns
5972 The limit specifying really large functions. For functions larger than this
5973 limit after inlining inlining is constrained by
5974 @option{--param large-function-growth}. This parameter is useful primarily
5975 to avoid extreme compilation time caused by non-linear algorithms used by the
5977 This parameter is ignored when @option{-funit-at-a-time} is not used.
5978 The default value is 2700.
5980 @item large-function-growth
5981 Specifies maximal growth of large function caused by inlining in percents.
5982 This parameter is ignored when @option{-funit-at-a-time} is not used.
5983 The default value is 100 which limits large function growth to 2.0 times
5986 @item large-unit-insns
5987 The limit specifying large translation unit. Growth caused by inlining of
5988 units larger than this limit is limited by @option{--param inline-unit-growth}.
5989 For small units this might be too tight (consider unit consisting of function A
5990 that is inline and B that just calls A three time. If B is small relative to
5991 A, the growth of unit is 300\% and yet such inlining is very sane. For very
5992 large units consisting of small inlininable functions however the overall unit
5993 growth limit is needed to avoid exponential explosion of code size. Thus for
5994 smaller units, the size is increased to @option{--param large-unit-insns}
5995 before applying @option{--param inline-unit-growth}. The default is 10000
5997 @item inline-unit-growth
5998 Specifies maximal overall growth of the compilation unit caused by inlining.
5999 This parameter is ignored when @option{-funit-at-a-time} is not used.
6000 The default value is 50 which limits unit growth to 1.5 times the original
6003 @item max-inline-insns-recursive
6004 @itemx max-inline-insns-recursive-auto
6005 Specifies maximum number of instructions out-of-line copy of self recursive inline
6006 function can grow into by performing recursive inlining.
6008 For functions declared inline @option{--param max-inline-insns-recursive} is
6009 taken into account. For function not declared inline, recursive inlining
6010 happens only when @option{-finline-functions} (included in @option{-O3}) is
6011 enabled and @option{--param max-inline-insns-recursive-auto} is used. The
6012 default value is 450.
6014 @item max-inline-recursive-depth
6015 @itemx max-inline-recursive-depth-auto
6016 Specifies maximum recursion depth used by the recursive inlining.
6018 For functions declared inline @option{--param max-inline-recursive-depth} is
6019 taken into account. For function not declared inline, recursive inlining
6020 happens only when @option{-finline-functions} (included in @option{-O3}) is
6021 enabled and @option{--param max-inline-recursive-depth-auto} is used. The
6022 default value is 450.
6024 @item min-inline-recursive-probability
6025 Recursive inlining is profitable only for function having deep recursion
6026 in average and can hurt for function having little recursion depth by
6027 increasing the prologue size or complexity of function body to other
6030 When profile feedback is available (see @option{-fprofile-generate}) the actual
6031 recursion depth can be guessed from probability that function will recurse via
6032 given call expression. This parameter limits inlining only to call expression
6033 whose probability exceeds given threshold (in percents). The default value is
6036 @item inline-call-cost
6037 Specify cost of call instruction relative to simple arithmetics operations
6038 (having cost of 1). Increasing this cost disqualifies inlining of non-leaf
6039 functions and at the same time increases size of leaf function that is believed to
6040 reduce function size by being inlined. In effect it increases amount of
6041 inlining for code having large abstraction penalty (many functions that just
6042 pass the arguments to other functions) and decrease inlining for code with low
6043 abstraction penalty. The default value is 16.
6045 @item max-unrolled-insns
6046 The maximum number of instructions that a loop should have if that loop
6047 is unrolled, and if the loop is unrolled, it determines how many times
6048 the loop code is unrolled.
6050 @item max-average-unrolled-insns
6051 The maximum number of instructions biased by probabilities of their execution
6052 that a loop should have if that loop is unrolled, and if the loop is unrolled,
6053 it determines how many times the loop code is unrolled.
6055 @item max-unroll-times
6056 The maximum number of unrollings of a single loop.
6058 @item max-peeled-insns
6059 The maximum number of instructions that a loop should have if that loop
6060 is peeled, and if the loop is peeled, it determines how many times
6061 the loop code is peeled.
6063 @item max-peel-times
6064 The maximum number of peelings of a single loop.
6066 @item max-completely-peeled-insns
6067 The maximum number of insns of a completely peeled loop.
6069 @item max-completely-peel-times
6070 The maximum number of iterations of a loop to be suitable for complete peeling.
6072 @item max-unswitch-insns
6073 The maximum number of insns of an unswitched loop.
6075 @item max-unswitch-level
6076 The maximum number of branches unswitched in a single loop.
6079 The minimum cost of an expensive expression in the loop invariant motion.
6081 @item iv-consider-all-candidates-bound
6082 Bound on number of candidates for induction variables below that
6083 all candidates are considered for each use in induction variable
6084 optimizations. Only the most relevant candidates are considered
6085 if there are more candidates, to avoid quadratic time complexity.
6087 @item iv-max-considered-uses
6088 The induction variable optimizations give up on loops that contain more
6089 induction variable uses.
6091 @item iv-always-prune-cand-set-bound
6092 If number of candidates in the set is smaller than this value,
6093 we always try to remove unnecessary ivs from the set during its
6094 optimization when a new iv is added to the set.
6096 @item scev-max-expr-size
6097 Bound on size of expressions used in the scalar evolutions analyzer.
6098 Large expressions slow the analyzer.
6100 @item vect-max-version-checks
6101 The maximum number of runtime checks that can be performed when doing
6102 loop versioning in the vectorizer. See option ftree-vect-loop-version
6103 for more information.
6105 @item max-iterations-to-track
6107 The maximum number of iterations of a loop the brute force algorithm
6108 for analysis of # of iterations of the loop tries to evaluate.
6110 @item hot-bb-count-fraction
6111 Select fraction of the maximal count of repetitions of basic block in program
6112 given basic block needs to have to be considered hot.
6114 @item hot-bb-frequency-fraction
6115 Select fraction of the maximal frequency of executions of basic block in
6116 function given basic block needs to have to be considered hot
6118 @item max-predicted-iterations
6119 The maximum number of loop iterations we predict statically. This is useful
6120 in cases where function contain single loop with known bound and other loop
6121 with unknown. We predict the known number of iterations correctly, while
6122 the unknown number of iterations average to roughly 10. This means that the
6123 loop without bounds would appear artificially cold relative to the other one.
6125 @item tracer-dynamic-coverage
6126 @itemx tracer-dynamic-coverage-feedback
6128 This value is used to limit superblock formation once the given percentage of
6129 executed instructions is covered. This limits unnecessary code size
6132 The @option{tracer-dynamic-coverage-feedback} is used only when profile
6133 feedback is available. The real profiles (as opposed to statically estimated
6134 ones) are much less balanced allowing the threshold to be larger value.
6136 @item tracer-max-code-growth
6137 Stop tail duplication once code growth has reached given percentage. This is
6138 rather hokey argument, as most of the duplicates will be eliminated later in
6139 cross jumping, so it may be set to much higher values than is the desired code
6142 @item tracer-min-branch-ratio
6144 Stop reverse growth when the reverse probability of best edge is less than this
6145 threshold (in percent).
6147 @item tracer-min-branch-ratio
6148 @itemx tracer-min-branch-ratio-feedback
6150 Stop forward growth if the best edge do have probability lower than this
6153 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
6154 compilation for profile feedback and one for compilation without. The value
6155 for compilation with profile feedback needs to be more conservative (higher) in
6156 order to make tracer effective.
6158 @item max-cse-path-length
6160 Maximum number of basic blocks on path that cse considers. The default is 10.
6163 The maximum instructions CSE process before flushing. The default is 1000.
6165 @item global-var-threshold
6167 Counts the number of function calls (@var{n}) and the number of
6168 call-clobbered variables (@var{v}). If @var{n}x@var{v} is larger than this limit, a
6169 single artificial variable will be created to represent all the
6170 call-clobbered variables at function call sites. This artificial
6171 variable will then be made to alias every call-clobbered variable.
6172 (done as @code{int * size_t} on the host machine; beware overflow).
6174 @item max-aliased-vops
6176 Maximum number of virtual operands allowed to represent aliases
6177 before triggering the alias grouping heuristic. Alias grouping
6178 reduces compile times and memory consumption needed for aliasing at
6179 the expense of precision loss in alias information.
6181 @item ggc-min-expand
6183 GCC uses a garbage collector to manage its own memory allocation. This
6184 parameter specifies the minimum percentage by which the garbage
6185 collector's heap should be allowed to expand between collections.
6186 Tuning this may improve compilation speed; it has no effect on code
6189 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
6190 RAM >= 1GB@. If @code{getrlimit} is available, the notion of "RAM" is
6191 the smallest of actual RAM and @code{RLIMIT_DATA} or @code{RLIMIT_AS}. If
6192 GCC is not able to calculate RAM on a particular platform, the lower
6193 bound of 30% is used. Setting this parameter and
6194 @option{ggc-min-heapsize} to zero causes a full collection to occur at
6195 every opportunity. This is extremely slow, but can be useful for
6198 @item ggc-min-heapsize
6200 Minimum size of the garbage collector's heap before it begins bothering
6201 to collect garbage. The first collection occurs after the heap expands
6202 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
6203 tuning this may improve compilation speed, and has no effect on code
6206 The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which
6207 tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
6208 with a lower bound of 4096 (four megabytes) and an upper bound of
6209 131072 (128 megabytes). If GCC is not able to calculate RAM on a
6210 particular platform, the lower bound is used. Setting this parameter
6211 very large effectively disables garbage collection. Setting this
6212 parameter and @option{ggc-min-expand} to zero causes a full collection
6213 to occur at every opportunity.
6215 @item max-reload-search-insns
6216 The maximum number of instruction reload should look backward for equivalent
6217 register. Increasing values mean more aggressive optimization, making the
6218 compile time increase with probably slightly better performance. The default
6221 @item max-cselib-memory-locations
6222 The maximum number of memory locations cselib should take into account.
6223 Increasing values mean more aggressive optimization, making the compile time
6224 increase with probably slightly better performance. The default value is 500.
6226 @item max-flow-memory-locations
6227 Similar as @option{max-cselib-memory-locations} but for dataflow liveness.
6228 The default value is 100.
6230 @item reorder-blocks-duplicate
6231 @itemx reorder-blocks-duplicate-feedback
6233 Used by basic block reordering pass to decide whether to use unconditional
6234 branch or duplicate the code on its destination. Code is duplicated when its
6235 estimated size is smaller than this value multiplied by the estimated size of
6236 unconditional jump in the hot spots of the program.
6238 The @option{reorder-block-duplicate-feedback} is used only when profile
6239 feedback is available and may be set to higher values than
6240 @option{reorder-block-duplicate} since information about the hot spots is more
6243 @item max-sched-ready-insns
6244 The maximum number of instructions ready to be issued the scheduler should
6245 consider at any given time during the first scheduling pass. Increasing
6246 values mean more thorough searches, making the compilation time increase
6247 with probably little benefit. The default value is 100.
6249 @item max-sched-region-blocks
6250 The maximum number of blocks in a region to be considered for
6251 interblock scheduling. The default value is 10.
6253 @item max-sched-region-insns
6254 The maximum number of insns in a region to be considered for
6255 interblock scheduling. The default value is 100.
6258 The minimum probability (in percents) of reaching a source block
6259 for interblock speculative scheduling. The default value is 40.
6261 @item max-sched-extend-regions-iters
6262 The maximum number of iterations through CFG to extend regions.
6263 0 - disable region extension,
6264 N - do at most N iterations.
6265 The default value is 0.
6267 @item max-sched-insn-conflict-delay
6268 The maximum conflict delay for an insn to be considered for speculative motion.
6269 The default value is 3.
6271 @item sched-spec-prob-cutoff
6272 The minimal probability of speculation success (in percents), so that
6273 speculative insn will be scheduled.
6274 The default value is 40.
6276 @item max-last-value-rtl
6278 The maximum size measured as number of RTLs that can be recorded in an expression
6279 in combiner for a pseudo register as last known value of that register. The default
6282 @item integer-share-limit
6283 Small integer constants can use a shared data structure, reducing the
6284 compiler's memory usage and increasing its speed. This sets the maximum
6285 value of a shared integer constant's. The default value is 256.
6287 @item min-virtual-mappings
6288 Specifies the minimum number of virtual mappings in the incremental
6289 SSA updater that should be registered to trigger the virtual mappings
6290 heuristic defined by virtual-mappings-ratio. The default value is
6293 @item virtual-mappings-ratio
6294 If the number of virtual mappings is virtual-mappings-ratio bigger
6295 than the number of virtual symbols to be updated, then the incremental
6296 SSA updater switches to a full update for those symbols. The default
6299 @item ssp-buffer-size
6300 The minimum size of buffers (i.e. arrays) that will receive stack smashing
6301 protection when @option{-fstack-protection} is used.
6303 @item max-jump-thread-duplication-stmts
6304 Maximum number of statements allowed in a block that needs to be
6305 duplicated when threading jumps.
6307 @item max-fields-for-field-sensitive
6308 Maximum number of fields in a structure we will treat in
6309 a field sensitive manner during pointer analysis.
6314 @node Preprocessor Options
6315 @section Options Controlling the Preprocessor
6316 @cindex preprocessor options
6317 @cindex options, preprocessor
6319 These options control the C preprocessor, which is run on each C source
6320 file before actual compilation.
6322 If you use the @option{-E} option, nothing is done except preprocessing.
6323 Some of these options make sense only together with @option{-E} because
6324 they cause the preprocessor output to be unsuitable for actual
6329 You can use @option{-Wp,@var{option}} to bypass the compiler driver
6330 and pass @var{option} directly through to the preprocessor. If
6331 @var{option} contains commas, it is split into multiple options at the
6332 commas. However, many options are modified, translated or interpreted
6333 by the compiler driver before being passed to the preprocessor, and
6334 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
6335 interface is undocumented and subject to change, so whenever possible
6336 you should avoid using @option{-Wp} and let the driver handle the
6339 @item -Xpreprocessor @var{option}
6340 @opindex preprocessor
6341 Pass @var{option} as an option to the preprocessor. You can use this to
6342 supply system-specific preprocessor options which GCC does not know how to
6345 If you want to pass an option that takes an argument, you must use
6346 @option{-Xpreprocessor} twice, once for the option and once for the argument.
6349 @include cppopts.texi
6351 @node Assembler Options
6352 @section Passing Options to the Assembler
6354 @c prevent bad page break with this line
6355 You can pass options to the assembler.
6358 @item -Wa,@var{option}
6360 Pass @var{option} as an option to the assembler. If @var{option}
6361 contains commas, it is split into multiple options at the commas.
6363 @item -Xassembler @var{option}
6365 Pass @var{option} as an option to the assembler. You can use this to
6366 supply system-specific assembler options which GCC does not know how to
6369 If you want to pass an option that takes an argument, you must use
6370 @option{-Xassembler} twice, once for the option and once for the argument.
6375 @section Options for Linking
6376 @cindex link options
6377 @cindex options, linking
6379 These options come into play when the compiler links object files into
6380 an executable output file. They are meaningless if the compiler is
6381 not doing a link step.
6385 @item @var{object-file-name}
6386 A file name that does not end in a special recognized suffix is
6387 considered to name an object file or library. (Object files are
6388 distinguished from libraries by the linker according to the file
6389 contents.) If linking is done, these object files are used as input
6398 If any of these options is used, then the linker is not run, and
6399 object file names should not be used as arguments. @xref{Overall
6403 @item -l@var{library}
6404 @itemx -l @var{library}
6406 Search the library named @var{library} when linking. (The second
6407 alternative with the library as a separate argument is only for
6408 POSIX compliance and is not recommended.)
6410 It makes a difference where in the command you write this option; the
6411 linker searches and processes libraries and object files in the order they
6412 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
6413 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
6414 to functions in @samp{z}, those functions may not be loaded.
6416 The linker searches a standard list of directories for the library,
6417 which is actually a file named @file{lib@var{library}.a}. The linker
6418 then uses this file as if it had been specified precisely by name.
6420 The directories searched include several standard system directories
6421 plus any that you specify with @option{-L}.
6423 Normally the files found this way are library files---archive files
6424 whose members are object files. The linker handles an archive file by
6425 scanning through it for members which define symbols that have so far
6426 been referenced but not defined. But if the file that is found is an
6427 ordinary object file, it is linked in the usual fashion. The only
6428 difference between using an @option{-l} option and specifying a file name
6429 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
6430 and searches several directories.
6434 You need this special case of the @option{-l} option in order to
6435 link an Objective-C or Objective-C++ program.
6438 @opindex nostartfiles
6439 Do not use the standard system startup files when linking.
6440 The standard system libraries are used normally, unless @option{-nostdlib}
6441 or @option{-nodefaultlibs} is used.
6443 @item -nodefaultlibs
6444 @opindex nodefaultlibs
6445 Do not use the standard system libraries when linking.
6446 Only the libraries you specify will be passed to the linker.
6447 The standard startup files are used normally, unless @option{-nostartfiles}
6448 is used. The compiler may generate calls to @code{memcmp},
6449 @code{memset}, @code{memcpy} and @code{memmove}.
6450 These entries are usually resolved by entries in
6451 libc. These entry points should be supplied through some other
6452 mechanism when this option is specified.
6456 Do not use the standard system startup files or libraries when linking.
6457 No startup files and only the libraries you specify will be passed to
6458 the linker. The compiler may generate calls to @code{memcmp}, @code{memset},
6459 @code{memcpy} and @code{memmove}.
6460 These entries are usually resolved by entries in
6461 libc. These entry points should be supplied through some other
6462 mechanism when this option is specified.
6464 @cindex @option{-lgcc}, use with @option{-nostdlib}
6465 @cindex @option{-nostdlib} and unresolved references
6466 @cindex unresolved references and @option{-nostdlib}
6467 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
6468 @cindex @option{-nodefaultlibs} and unresolved references
6469 @cindex unresolved references and @option{-nodefaultlibs}
6470 One of the standard libraries bypassed by @option{-nostdlib} and
6471 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
6472 that GCC uses to overcome shortcomings of particular machines, or special
6473 needs for some languages.
6474 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
6475 Collection (GCC) Internals},
6476 for more discussion of @file{libgcc.a}.)
6477 In most cases, you need @file{libgcc.a} even when you want to avoid
6478 other standard libraries. In other words, when you specify @option{-nostdlib}
6479 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
6480 This ensures that you have no unresolved references to internal GCC
6481 library subroutines. (For example, @samp{__main}, used to ensure C++
6482 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
6483 GNU Compiler Collection (GCC) Internals}.)
6487 Produce a position independent executable on targets which support it.
6488 For predictable results, you must also specify the same set of options
6489 that were used to generate code (@option{-fpie}, @option{-fPIE},
6490 or model suboptions) when you specify this option.
6494 Pass the flag @option{-export-dynamic} to the ELF linker, on targets
6495 that support it. This instructs the linker to add all symbols, not
6496 only used ones, to the dynamic symbol table. This option is needed
6497 for some uses of @code{dlopen} or to allow obtaining backtraces
6498 from within a program.
6502 Remove all symbol table and relocation information from the executable.
6506 On systems that support dynamic linking, this prevents linking with the shared
6507 libraries. On other systems, this option has no effect.
6511 Produce a shared object which can then be linked with other objects to
6512 form an executable. Not all systems support this option. For predictable
6513 results, you must also specify the same set of options that were used to
6514 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
6515 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
6516 needs to build supplementary stub code for constructors to work. On
6517 multi-libbed systems, @samp{gcc -shared} must select the correct support
6518 libraries to link against. Failing to supply the correct flags may lead
6519 to subtle defects. Supplying them in cases where they are not necessary
6522 @item -shared-libgcc
6523 @itemx -static-libgcc
6524 @opindex shared-libgcc
6525 @opindex static-libgcc
6526 On systems that provide @file{libgcc} as a shared library, these options
6527 force the use of either the shared or static version respectively.
6528 If no shared version of @file{libgcc} was built when the compiler was
6529 configured, these options have no effect.
6531 There are several situations in which an application should use the
6532 shared @file{libgcc} instead of the static version. The most common
6533 of these is when the application wishes to throw and catch exceptions
6534 across different shared libraries. In that case, each of the libraries
6535 as well as the application itself should use the shared @file{libgcc}.
6537 Therefore, the G++ and GCJ drivers automatically add
6538 @option{-shared-libgcc} whenever you build a shared library or a main
6539 executable, because C++ and Java programs typically use exceptions, so
6540 this is the right thing to do.
6542 If, instead, you use the GCC driver to create shared libraries, you may
6543 find that they will not always be linked with the shared @file{libgcc}.
6544 If GCC finds, at its configuration time, that you have a non-GNU linker
6545 or a GNU linker that does not support option @option{--eh-frame-hdr},
6546 it will link the shared version of @file{libgcc} into shared libraries
6547 by default. Otherwise, it will take advantage of the linker and optimize
6548 away the linking with the shared version of @file{libgcc}, linking with
6549 the static version of libgcc by default. This allows exceptions to
6550 propagate through such shared libraries, without incurring relocation
6551 costs at library load time.
6553 However, if a library or main executable is supposed to throw or catch
6554 exceptions, you must link it using the G++ or GCJ driver, as appropriate
6555 for the languages used in the program, or using the option
6556 @option{-shared-libgcc}, such that it is linked with the shared
6561 Bind references to global symbols when building a shared object. Warn
6562 about any unresolved references (unless overridden by the link editor
6563 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
6566 @item -Xlinker @var{option}
6568 Pass @var{option} as an option to the linker. You can use this to
6569 supply system-specific linker options which GCC does not know how to
6572 If you want to pass an option that takes an argument, you must use
6573 @option{-Xlinker} twice, once for the option and once for the argument.
6574 For example, to pass @option{-assert definitions}, you must write
6575 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
6576 @option{-Xlinker "-assert definitions"}, because this passes the entire
6577 string as a single argument, which is not what the linker expects.
6579 @item -Wl,@var{option}
6581 Pass @var{option} as an option to the linker. If @var{option} contains
6582 commas, it is split into multiple options at the commas.
6584 @item -u @var{symbol}
6586 Pretend the symbol @var{symbol} is undefined, to force linking of
6587 library modules to define it. You can use @option{-u} multiple times with
6588 different symbols to force loading of additional library modules.
6591 @node Directory Options
6592 @section Options for Directory Search
6593 @cindex directory options
6594 @cindex options, directory search
6597 These options specify directories to search for header files, for
6598 libraries and for parts of the compiler:
6603 Add the directory @var{dir} to the head of the list of directories to be
6604 searched for header files. This can be used to override a system header
6605 file, substituting your own version, since these directories are
6606 searched before the system header file directories. However, you should
6607 not use this option to add directories that contain vendor-supplied
6608 system header files (use @option{-isystem} for that). If you use more than
6609 one @option{-I} option, the directories are scanned in left-to-right
6610 order; the standard system directories come after.
6612 If a standard system include directory, or a directory specified with
6613 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
6614 option will be ignored. The directory will still be searched but as a
6615 system directory at its normal position in the system include chain.
6616 This is to ensure that GCC's procedure to fix buggy system headers and
6617 the ordering for the include_next directive are not inadvertently changed.
6618 If you really need to change the search order for system directories,
6619 use the @option{-nostdinc} and/or @option{-isystem} options.
6621 @item -iquote@var{dir}
6623 Add the directory @var{dir} to the head of the list of directories to
6624 be searched for header files only for the case of @samp{#include
6625 "@var{file}"}; they are not searched for @samp{#include <@var{file}>},
6626 otherwise just like @option{-I}.
6630 Add directory @var{dir} to the list of directories to be searched
6633 @item -B@var{prefix}
6635 This option specifies where to find the executables, libraries,
6636 include files, and data files of the compiler itself.
6638 The compiler driver program runs one or more of the subprograms
6639 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
6640 @var{prefix} as a prefix for each program it tries to run, both with and
6641 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
6643 For each subprogram to be run, the compiler driver first tries the
6644 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
6645 was not specified, the driver tries two standard prefixes, which are
6646 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}. If neither of
6647 those results in a file name that is found, the unmodified program
6648 name is searched for using the directories specified in your
6649 @env{PATH} environment variable.
6651 The compiler will check to see if the path provided by the @option{-B}
6652 refers to a directory, and if necessary it will add a directory
6653 separator character at the end of the path.
6655 @option{-B} prefixes that effectively specify directory names also apply
6656 to libraries in the linker, because the compiler translates these
6657 options into @option{-L} options for the linker. They also apply to
6658 includes files in the preprocessor, because the compiler translates these
6659 options into @option{-isystem} options for the preprocessor. In this case,
6660 the compiler appends @samp{include} to the prefix.
6662 The run-time support file @file{libgcc.a} can also be searched for using
6663 the @option{-B} prefix, if needed. If it is not found there, the two
6664 standard prefixes above are tried, and that is all. The file is left
6665 out of the link if it is not found by those means.
6667 Another way to specify a prefix much like the @option{-B} prefix is to use
6668 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
6671 As a special kludge, if the path provided by @option{-B} is
6672 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
6673 9, then it will be replaced by @file{[dir/]include}. This is to help
6674 with boot-strapping the compiler.
6676 @item -specs=@var{file}
6678 Process @var{file} after the compiler reads in the standard @file{specs}
6679 file, in order to override the defaults that the @file{gcc} driver
6680 program uses when determining what switches to pass to @file{cc1},
6681 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
6682 @option{-specs=@var{file}} can be specified on the command line, and they
6683 are processed in order, from left to right.
6685 @item --sysroot=@var{dir}
6687 Use @var{dir} as the logical root directory for headers and libraries.
6688 For example, if the compiler would normally search for headers in
6689 @file{/usr/include} and libraries in @file{/usr/lib}, it will instead
6690 search @file{@var{dir}/usr/include} and @file{@var{dir}/usr/lib}.
6692 If you use both this option and the @option{-isysroot} option, then
6693 the @option{--sysroot} option will apply to libraries, but the
6694 @option{-isysroot} option will apply to header files.
6696 The GNU linker (beginning with version 2.16) has the necessary support
6697 for this option. If your linker does not support this option, the
6698 header file aspect of @option{--sysroot} will still work, but the
6699 library aspect will not.
6703 This option has been deprecated. Please use @option{-iquote} instead for
6704 @option{-I} directories before the @option{-I-} and remove the @option{-I-}.
6705 Any directories you specify with @option{-I} options before the @option{-I-}
6706 option are searched only for the case of @samp{#include "@var{file}"};
6707 they are not searched for @samp{#include <@var{file}>}.
6709 If additional directories are specified with @option{-I} options after
6710 the @option{-I-}, these directories are searched for all @samp{#include}
6711 directives. (Ordinarily @emph{all} @option{-I} directories are used
6714 In addition, the @option{-I-} option inhibits the use of the current
6715 directory (where the current input file came from) as the first search
6716 directory for @samp{#include "@var{file}"}. There is no way to
6717 override this effect of @option{-I-}. With @option{-I.} you can specify
6718 searching the directory which was current when the compiler was
6719 invoked. That is not exactly the same as what the preprocessor does
6720 by default, but it is often satisfactory.
6722 @option{-I-} does not inhibit the use of the standard system directories
6723 for header files. Thus, @option{-I-} and @option{-nostdinc} are
6730 @section Specifying subprocesses and the switches to pass to them
6733 @command{gcc} is a driver program. It performs its job by invoking a
6734 sequence of other programs to do the work of compiling, assembling and
6735 linking. GCC interprets its command-line parameters and uses these to
6736 deduce which programs it should invoke, and which command-line options
6737 it ought to place on their command lines. This behavior is controlled
6738 by @dfn{spec strings}. In most cases there is one spec string for each
6739 program that GCC can invoke, but a few programs have multiple spec
6740 strings to control their behavior. The spec strings built into GCC can
6741 be overridden by using the @option{-specs=} command-line switch to specify
6744 @dfn{Spec files} are plaintext files that are used to construct spec
6745 strings. They consist of a sequence of directives separated by blank
6746 lines. The type of directive is determined by the first non-whitespace
6747 character on the line and it can be one of the following:
6750 @item %@var{command}
6751 Issues a @var{command} to the spec file processor. The commands that can
6755 @item %include <@var{file}>
6757 Search for @var{file} and insert its text at the current point in the
6760 @item %include_noerr <@var{file}>
6761 @cindex %include_noerr
6762 Just like @samp{%include}, but do not generate an error message if the include
6763 file cannot be found.
6765 @item %rename @var{old_name} @var{new_name}
6767 Rename the spec string @var{old_name} to @var{new_name}.
6771 @item *[@var{spec_name}]:
6772 This tells the compiler to create, override or delete the named spec
6773 string. All lines after this directive up to the next directive or
6774 blank line are considered to be the text for the spec string. If this
6775 results in an empty string then the spec will be deleted. (Or, if the
6776 spec did not exist, then nothing will happened.) Otherwise, if the spec
6777 does not currently exist a new spec will be created. If the spec does
6778 exist then its contents will be overridden by the text of this
6779 directive, unless the first character of that text is the @samp{+}
6780 character, in which case the text will be appended to the spec.
6782 @item [@var{suffix}]:
6783 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
6784 and up to the next directive or blank line are considered to make up the
6785 spec string for the indicated suffix. When the compiler encounters an
6786 input file with the named suffix, it will processes the spec string in
6787 order to work out how to compile that file. For example:
6794 This says that any input file whose name ends in @samp{.ZZ} should be
6795 passed to the program @samp{z-compile}, which should be invoked with the
6796 command-line switch @option{-input} and with the result of performing the
6797 @samp{%i} substitution. (See below.)
6799 As an alternative to providing a spec string, the text that follows a
6800 suffix directive can be one of the following:
6803 @item @@@var{language}
6804 This says that the suffix is an alias for a known @var{language}. This is
6805 similar to using the @option{-x} command-line switch to GCC to specify a
6806 language explicitly. For example:
6813 Says that .ZZ files are, in fact, C++ source files.
6816 This causes an error messages saying:
6819 @var{name} compiler not installed on this system.
6823 GCC already has an extensive list of suffixes built into it.
6824 This directive will add an entry to the end of the list of suffixes, but
6825 since the list is searched from the end backwards, it is effectively
6826 possible to override earlier entries using this technique.
6830 GCC has the following spec strings built into it. Spec files can
6831 override these strings or create their own. Note that individual
6832 targets can also add their own spec strings to this list.
6835 asm Options to pass to the assembler
6836 asm_final Options to pass to the assembler post-processor
6837 cpp Options to pass to the C preprocessor
6838 cc1 Options to pass to the C compiler
6839 cc1plus Options to pass to the C++ compiler
6840 endfile Object files to include at the end of the link
6841 link Options to pass to the linker
6842 lib Libraries to include on the command line to the linker
6843 libgcc Decides which GCC support library to pass to the linker
6844 linker Sets the name of the linker
6845 predefines Defines to be passed to the C preprocessor
6846 signed_char Defines to pass to CPP to say whether @code{char} is signed
6848 startfile Object files to include at the start of the link
6851 Here is a small example of a spec file:
6857 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
6860 This example renames the spec called @samp{lib} to @samp{old_lib} and
6861 then overrides the previous definition of @samp{lib} with a new one.
6862 The new definition adds in some extra command-line options before
6863 including the text of the old definition.
6865 @dfn{Spec strings} are a list of command-line options to be passed to their
6866 corresponding program. In addition, the spec strings can contain
6867 @samp{%}-prefixed sequences to substitute variable text or to
6868 conditionally insert text into the command line. Using these constructs
6869 it is possible to generate quite complex command lines.
6871 Here is a table of all defined @samp{%}-sequences for spec
6872 strings. Note that spaces are not generated automatically around the
6873 results of expanding these sequences. Therefore you can concatenate them
6874 together or combine them with constant text in a single argument.
6878 Substitute one @samp{%} into the program name or argument.
6881 Substitute the name of the input file being processed.
6884 Substitute the basename of the input file being processed.
6885 This is the substring up to (and not including) the last period
6886 and not including the directory.
6889 This is the same as @samp{%b}, but include the file suffix (text after
6893 Marks the argument containing or following the @samp{%d} as a
6894 temporary file name, so that that file will be deleted if GCC exits
6895 successfully. Unlike @samp{%g}, this contributes no text to the
6898 @item %g@var{suffix}
6899 Substitute a file name that has suffix @var{suffix} and is chosen
6900 once per compilation, and mark the argument in the same way as
6901 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
6902 name is now chosen in a way that is hard to predict even when previously
6903 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
6904 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
6905 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
6906 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
6907 was simply substituted with a file name chosen once per compilation,
6908 without regard to any appended suffix (which was therefore treated
6909 just like ordinary text), making such attacks more likely to succeed.
6911 @item %u@var{suffix}
6912 Like @samp{%g}, but generates a new temporary file name even if
6913 @samp{%u@var{suffix}} was already seen.
6915 @item %U@var{suffix}
6916 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
6917 new one if there is no such last file name. In the absence of any
6918 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
6919 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
6920 would involve the generation of two distinct file names, one
6921 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
6922 simply substituted with a file name chosen for the previous @samp{%u},
6923 without regard to any appended suffix.
6925 @item %j@var{suffix}
6926 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
6927 writable, and if save-temps is off; otherwise, substitute the name
6928 of a temporary file, just like @samp{%u}. This temporary file is not
6929 meant for communication between processes, but rather as a junk
6932 @item %|@var{suffix}
6933 @itemx %m@var{suffix}
6934 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
6935 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
6936 all. These are the two most common ways to instruct a program that it
6937 should read from standard input or write to standard output. If you
6938 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
6939 construct: see for example @file{f/lang-specs.h}.
6941 @item %.@var{SUFFIX}
6942 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
6943 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
6944 terminated by the next space or %.
6947 Marks the argument containing or following the @samp{%w} as the
6948 designated output file of this compilation. This puts the argument
6949 into the sequence of arguments that @samp{%o} will substitute later.
6952 Substitutes the names of all the output files, with spaces
6953 automatically placed around them. You should write spaces
6954 around the @samp{%o} as well or the results are undefined.
6955 @samp{%o} is for use in the specs for running the linker.
6956 Input files whose names have no recognized suffix are not compiled
6957 at all, but they are included among the output files, so they will
6961 Substitutes the suffix for object files. Note that this is
6962 handled specially when it immediately follows @samp{%g, %u, or %U},
6963 because of the need for those to form complete file names. The
6964 handling is such that @samp{%O} is treated exactly as if it had already
6965 been substituted, except that @samp{%g, %u, and %U} do not currently
6966 support additional @var{suffix} characters following @samp{%O} as they would
6967 following, for example, @samp{.o}.
6970 Substitutes the standard macro predefinitions for the
6971 current target machine. Use this when running @code{cpp}.
6974 Like @samp{%p}, but puts @samp{__} before and after the name of each
6975 predefined macro, except for macros that start with @samp{__} or with
6976 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
6980 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
6981 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}),
6982 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
6983 and @option{-imultilib} as necessary.
6986 Current argument is the name of a library or startup file of some sort.
6987 Search for that file in a standard list of directories and substitute
6988 the full name found.
6991 Print @var{str} as an error message. @var{str} is terminated by a newline.
6992 Use this when inconsistent options are detected.
6995 Substitute the contents of spec string @var{name} at this point.
6998 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
7000 @item %x@{@var{option}@}
7001 Accumulate an option for @samp{%X}.
7004 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
7008 Output the accumulated assembler options specified by @option{-Wa}.
7011 Output the accumulated preprocessor options specified by @option{-Wp}.
7014 Process the @code{asm} spec. This is used to compute the
7015 switches to be passed to the assembler.
7018 Process the @code{asm_final} spec. This is a spec string for
7019 passing switches to an assembler post-processor, if such a program is
7023 Process the @code{link} spec. This is the spec for computing the
7024 command line passed to the linker. Typically it will make use of the
7025 @samp{%L %G %S %D and %E} sequences.
7028 Dump out a @option{-L} option for each directory that GCC believes might
7029 contain startup files. If the target supports multilibs then the
7030 current multilib directory will be prepended to each of these paths.
7033 Process the @code{lib} spec. This is a spec string for deciding which
7034 libraries should be included on the command line to the linker.
7037 Process the @code{libgcc} spec. This is a spec string for deciding
7038 which GCC support library should be included on the command line to the linker.
7041 Process the @code{startfile} spec. This is a spec for deciding which
7042 object files should be the first ones passed to the linker. Typically
7043 this might be a file named @file{crt0.o}.
7046 Process the @code{endfile} spec. This is a spec string that specifies
7047 the last object files that will be passed to the linker.
7050 Process the @code{cpp} spec. This is used to construct the arguments
7051 to be passed to the C preprocessor.
7054 Process the @code{cc1} spec. This is used to construct the options to be
7055 passed to the actual C compiler (@samp{cc1}).
7058 Process the @code{cc1plus} spec. This is used to construct the options to be
7059 passed to the actual C++ compiler (@samp{cc1plus}).
7062 Substitute the variable part of a matched option. See below.
7063 Note that each comma in the substituted string is replaced by
7067 Remove all occurrences of @code{-S} from the command line. Note---this
7068 command is position dependent. @samp{%} commands in the spec string
7069 before this one will see @code{-S}, @samp{%} commands in the spec string
7070 after this one will not.
7072 @item %:@var{function}(@var{args})
7073 Call the named function @var{function}, passing it @var{args}.
7074 @var{args} is first processed as a nested spec string, then split
7075 into an argument vector in the usual fashion. The function returns
7076 a string which is processed as if it had appeared literally as part
7077 of the current spec.
7079 The following built-in spec functions are provided:
7082 @item @code{if-exists}
7083 The @code{if-exists} spec function takes one argument, an absolute
7084 pathname to a file. If the file exists, @code{if-exists} returns the
7085 pathname. Here is a small example of its usage:
7089 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
7092 @item @code{if-exists-else}
7093 The @code{if-exists-else} spec function is similar to the @code{if-exists}
7094 spec function, except that it takes two arguments. The first argument is
7095 an absolute pathname to a file. If the file exists, @code{if-exists-else}
7096 returns the pathname. If it does not exist, it returns the second argument.
7097 This way, @code{if-exists-else} can be used to select one file or another,
7098 based on the existence of the first. Here is a small example of its usage:
7102 crt0%O%s %:if-exists(crti%O%s) \
7103 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
7106 @item @code{replace-outfile}
7107 The @code{replace-outfile} spec function takes two arguments. It looks for the
7108 first argument in the outfiles array and replaces it with the second argument. Here
7109 is a small example of its usage:
7112 %@{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)@}
7118 Substitutes the @code{-S} switch, if that switch was given to GCC@.
7119 If that switch was not specified, this substitutes nothing. Note that
7120 the leading dash is omitted when specifying this option, and it is
7121 automatically inserted if the substitution is performed. Thus the spec
7122 string @samp{%@{foo@}} would match the command-line option @option{-foo}
7123 and would output the command line option @option{-foo}.
7125 @item %W@{@code{S}@}
7126 Like %@{@code{S}@} but mark last argument supplied within as a file to be
7129 @item %@{@code{S}*@}
7130 Substitutes all the switches specified to GCC whose names start
7131 with @code{-S}, but which also take an argument. This is used for
7132 switches like @option{-o}, @option{-D}, @option{-I}, etc.
7133 GCC considers @option{-o foo} as being
7134 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
7135 text, including the space. Thus two arguments would be generated.
7137 @item %@{@code{S}*&@code{T}*@}
7138 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
7139 (the order of @code{S} and @code{T} in the spec is not significant).
7140 There can be any number of ampersand-separated variables; for each the
7141 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
7143 @item %@{@code{S}:@code{X}@}
7144 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
7146 @item %@{!@code{S}:@code{X}@}
7147 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
7149 @item %@{@code{S}*:@code{X}@}
7150 Substitutes @code{X} if one or more switches whose names start with
7151 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
7152 once, no matter how many such switches appeared. However, if @code{%*}
7153 appears somewhere in @code{X}, then @code{X} will be substituted once
7154 for each matching switch, with the @code{%*} replaced by the part of
7155 that switch that matched the @code{*}.
7157 @item %@{.@code{S}:@code{X}@}
7158 Substitutes @code{X}, if processing a file with suffix @code{S}.
7160 @item %@{!.@code{S}:@code{X}@}
7161 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
7163 @item %@{@code{S}|@code{P}:@code{X}@}
7164 Substitutes @code{X} if either @code{-S} or @code{-P} was given to GCC@.
7165 This may be combined with @samp{!}, @samp{.}, and @code{*} sequences as well,
7166 although they have a stronger binding than the @samp{|}. If @code{%*}
7167 appears in @code{X}, all of the alternatives must be starred, and only
7168 the first matching alternative is substituted.
7170 For example, a spec string like this:
7173 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
7176 will output the following command-line options from the following input
7177 command-line options:
7182 -d fred.c -foo -baz -boggle
7183 -d jim.d -bar -baz -boggle
7186 @item %@{S:X; T:Y; :D@}
7188 If @code{S} was given to GCC, substitutes @code{X}; else if @code{T} was
7189 given to GCC, substitutes @code{Y}; else substitutes @code{D}. There can
7190 be as many clauses as you need. This may be combined with @code{.},
7191 @code{!}, @code{|}, and @code{*} as needed.
7196 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
7197 construct may contain other nested @samp{%} constructs or spaces, or
7198 even newlines. They are processed as usual, as described above.
7199 Trailing white space in @code{X} is ignored. White space may also
7200 appear anywhere on the left side of the colon in these constructs,
7201 except between @code{.} or @code{*} and the corresponding word.
7203 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
7204 handled specifically in these constructs. If another value of
7205 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
7206 @option{-W} switch is found later in the command line, the earlier
7207 switch value is ignored, except with @{@code{S}*@} where @code{S} is
7208 just one letter, which passes all matching options.
7210 The character @samp{|} at the beginning of the predicate text is used to
7211 indicate that a command should be piped to the following command, but
7212 only if @option{-pipe} is specified.
7214 It is built into GCC which switches take arguments and which do not.
7215 (You might think it would be useful to generalize this to allow each
7216 compiler's spec to say which switches take arguments. But this cannot
7217 be done in a consistent fashion. GCC cannot even decide which input
7218 files have been specified without knowing which switches take arguments,
7219 and it must know which input files to compile in order to tell which
7222 GCC also knows implicitly that arguments starting in @option{-l} are to be
7223 treated as compiler output files, and passed to the linker in their
7224 proper position among the other output files.
7226 @c man begin OPTIONS
7228 @node Target Options
7229 @section Specifying Target Machine and Compiler Version
7230 @cindex target options
7231 @cindex cross compiling
7232 @cindex specifying machine version
7233 @cindex specifying compiler version and target machine
7234 @cindex compiler version, specifying
7235 @cindex target machine, specifying
7237 The usual way to run GCC is to run the executable called @file{gcc}, or
7238 @file{<machine>-gcc} when cross-compiling, or
7239 @file{<machine>-gcc-<version>} to run a version other than the one that
7240 was installed last. Sometimes this is inconvenient, so GCC provides
7241 options that will switch to another cross-compiler or version.
7244 @item -b @var{machine}
7246 The argument @var{machine} specifies the target machine for compilation.
7248 The value to use for @var{machine} is the same as was specified as the
7249 machine type when configuring GCC as a cross-compiler. For
7250 example, if a cross-compiler was configured with @samp{configure
7251 arm-elf}, meaning to compile for an arm processor with elf binaries,
7252 then you would specify @option{-b arm-elf} to run that cross compiler.
7253 Because there are other options beginning with @option{-b}, the
7254 configuration must contain a hyphen.
7256 @item -V @var{version}
7258 The argument @var{version} specifies which version of GCC to run.
7259 This is useful when multiple versions are installed. For example,
7260 @var{version} might be @samp{4.0}, meaning to run GCC version 4.0.
7263 The @option{-V} and @option{-b} options work by running the
7264 @file{<machine>-gcc-<version>} executable, so there's no real reason to
7265 use them if you can just run that directly.
7267 @node Submodel Options
7268 @section Hardware Models and Configurations
7269 @cindex submodel options
7270 @cindex specifying hardware config
7271 @cindex hardware models and configurations, specifying
7272 @cindex machine dependent options
7274 Earlier we discussed the standard option @option{-b} which chooses among
7275 different installed compilers for completely different target
7276 machines, such as VAX vs.@: 68000 vs.@: 80386.
7278 In addition, each of these target machine types can have its own
7279 special options, starting with @samp{-m}, to choose among various
7280 hardware models or configurations---for example, 68010 vs 68020,
7281 floating coprocessor or none. A single installed version of the
7282 compiler can compile for any model or configuration, according to the
7285 Some configurations of the compiler also support additional special
7286 options, usually for compatibility with other compilers on the same
7289 @c This list is ordered alphanumerically by subsection name.
7290 @c It should be the same order and spelling as these options are listed
7291 @c in Machine Dependent Options
7297 * Blackfin Options::
7301 * DEC Alpha Options::
7302 * DEC Alpha/VMS Options::
7304 * GNU/Linux Options::
7307 * i386 and x86-64 Options::
7320 * RS/6000 and PowerPC Options::
7321 * S/390 and zSeries Options::
7324 * System V Options::
7325 * TMS320C3x/C4x Options::
7329 * Xstormy16 Options::
7335 @subsection ARC Options
7338 These options are defined for ARC implementations:
7343 Compile code for little endian mode. This is the default.
7347 Compile code for big endian mode.
7350 @opindex mmangle-cpu
7351 Prepend the name of the cpu to all public symbol names.
7352 In multiple-processor systems, there are many ARC variants with different
7353 instruction and register set characteristics. This flag prevents code
7354 compiled for one cpu to be linked with code compiled for another.
7355 No facility exists for handling variants that are ``almost identical''.
7356 This is an all or nothing option.
7358 @item -mcpu=@var{cpu}
7360 Compile code for ARC variant @var{cpu}.
7361 Which variants are supported depend on the configuration.
7362 All variants support @option{-mcpu=base}, this is the default.
7364 @item -mtext=@var{text-section}
7365 @itemx -mdata=@var{data-section}
7366 @itemx -mrodata=@var{readonly-data-section}
7370 Put functions, data, and readonly data in @var{text-section},
7371 @var{data-section}, and @var{readonly-data-section} respectively
7372 by default. This can be overridden with the @code{section} attribute.
7373 @xref{Variable Attributes}.
7378 @subsection ARM Options
7381 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
7385 @item -mabi=@var{name}
7387 Generate code for the specified ABI@. Permissible values are: @samp{apcs-gnu},
7388 @samp{atpcs}, @samp{aapcs}, @samp{aapcs-linux} and @samp{iwmmxt}.
7391 @opindex mapcs-frame
7392 Generate a stack frame that is compliant with the ARM Procedure Call
7393 Standard for all functions, even if this is not strictly necessary for
7394 correct execution of the code. Specifying @option{-fomit-frame-pointer}
7395 with this option will cause the stack frames not to be generated for
7396 leaf functions. The default is @option{-mno-apcs-frame}.
7400 This is a synonym for @option{-mapcs-frame}.
7403 @c not currently implemented
7404 @item -mapcs-stack-check
7405 @opindex mapcs-stack-check
7406 Generate code to check the amount of stack space available upon entry to
7407 every function (that actually uses some stack space). If there is
7408 insufficient space available then either the function
7409 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
7410 called, depending upon the amount of stack space required. The run time
7411 system is required to provide these functions. The default is
7412 @option{-mno-apcs-stack-check}, since this produces smaller code.
7414 @c not currently implemented
7416 @opindex mapcs-float
7417 Pass floating point arguments using the float point registers. This is
7418 one of the variants of the APCS@. This option is recommended if the
7419 target hardware has a floating point unit or if a lot of floating point
7420 arithmetic is going to be performed by the code. The default is
7421 @option{-mno-apcs-float}, since integer only code is slightly increased in
7422 size if @option{-mapcs-float} is used.
7424 @c not currently implemented
7425 @item -mapcs-reentrant
7426 @opindex mapcs-reentrant
7427 Generate reentrant, position independent code. The default is
7428 @option{-mno-apcs-reentrant}.
7431 @item -mthumb-interwork
7432 @opindex mthumb-interwork
7433 Generate code which supports calling between the ARM and Thumb
7434 instruction sets. Without this option the two instruction sets cannot
7435 be reliably used inside one program. The default is
7436 @option{-mno-thumb-interwork}, since slightly larger code is generated
7437 when @option{-mthumb-interwork} is specified.
7439 @item -mno-sched-prolog
7440 @opindex mno-sched-prolog
7441 Prevent the reordering of instructions in the function prolog, or the
7442 merging of those instruction with the instructions in the function's
7443 body. This means that all functions will start with a recognizable set
7444 of instructions (or in fact one of a choice from a small set of
7445 different function prologues), and this information can be used to
7446 locate the start if functions inside an executable piece of code. The
7447 default is @option{-msched-prolog}.
7450 @opindex mhard-float
7451 Generate output containing floating point instructions. This is the
7455 @opindex msoft-float
7456 Generate output containing library calls for floating point.
7457 @strong{Warning:} the requisite libraries are not available for all ARM
7458 targets. Normally the facilities of the machine's usual C compiler are
7459 used, but this cannot be done directly in cross-compilation. You must make
7460 your own arrangements to provide suitable library functions for
7463 @option{-msoft-float} changes the calling convention in the output file;
7464 therefore, it is only useful if you compile @emph{all} of a program with
7465 this option. In particular, you need to compile @file{libgcc.a}, the
7466 library that comes with GCC, with @option{-msoft-float} in order for
7469 @item -mfloat-abi=@var{name}
7471 Specifies which ABI to use for floating point values. Permissible values
7472 are: @samp{soft}, @samp{softfp} and @samp{hard}.
7474 @samp{soft} and @samp{hard} are equivalent to @option{-msoft-float}
7475 and @option{-mhard-float} respectively. @samp{softfp} allows the generation
7476 of floating point instructions, but still uses the soft-float calling
7479 @item -mlittle-endian
7480 @opindex mlittle-endian
7481 Generate code for a processor running in little-endian mode. This is
7482 the default for all standard configurations.
7485 @opindex mbig-endian
7486 Generate code for a processor running in big-endian mode; the default is
7487 to compile code for a little-endian processor.
7489 @item -mwords-little-endian
7490 @opindex mwords-little-endian
7491 This option only applies when generating code for big-endian processors.
7492 Generate code for a little-endian word order but a big-endian byte
7493 order. That is, a byte order of the form @samp{32107654}. Note: this
7494 option should only be used if you require compatibility with code for
7495 big-endian ARM processors generated by versions of the compiler prior to
7498 @item -mcpu=@var{name}
7500 This specifies the name of the target ARM processor. GCC uses this name
7501 to determine what kind of instructions it can emit when generating
7502 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
7503 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
7504 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
7505 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
7506 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
7507 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm7tdmi-s},
7508 @samp{arm8}, @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
7509 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
7510 @samp{arm920t}, @samp{arm922t}, @samp{arm946e-s}, @samp{arm966e-s},
7511 @samp{arm968e-s}, @samp{arm926ej-s}, @samp{arm940t}, @samp{arm9tdmi},
7512 @samp{arm10tdmi}, @samp{arm1020t}, @samp{arm1026ej-s},
7513 @samp{arm10e}, @samp{arm1020e}, @samp{arm1022e},
7514 @samp{arm1136j-s}, @samp{arm1136jf-s}, @samp{mpcore}, @samp{mpcorenovfp},
7515 @samp{arm1176jz-s}, @samp{arm1176jzf-s}, @samp{xscale}, @samp{iwmmxt},
7518 @itemx -mtune=@var{name}
7520 This option is very similar to the @option{-mcpu=} option, except that
7521 instead of specifying the actual target processor type, and hence
7522 restricting which instructions can be used, it specifies that GCC should
7523 tune the performance of the code as if the target were of the type
7524 specified in this option, but still choosing the instructions that it
7525 will generate based on the cpu specified by a @option{-mcpu=} option.
7526 For some ARM implementations better performance can be obtained by using
7529 @item -march=@var{name}
7531 This specifies the name of the target ARM architecture. GCC uses this
7532 name to determine what kind of instructions it can emit when generating
7533 assembly code. This option can be used in conjunction with or instead
7534 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
7535 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
7536 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{armv6}, @samp{armv6j},
7537 @samp{iwmmxt}, @samp{ep9312}.
7539 @item -mfpu=@var{name}
7540 @itemx -mfpe=@var{number}
7541 @itemx -mfp=@var{number}
7545 This specifies what floating point hardware (or hardware emulation) is
7546 available on the target. Permissible names are: @samp{fpa}, @samp{fpe2},
7547 @samp{fpe3}, @samp{maverick}, @samp{vfp}. @option{-mfp} and @option{-mfpe}
7548 are synonyms for @option{-mfpu}=@samp{fpe}@var{number}, for compatibility
7549 with older versions of GCC@.
7551 If @option{-msoft-float} is specified this specifies the format of
7552 floating point values.
7554 @item -mstructure-size-boundary=@var{n}
7555 @opindex mstructure-size-boundary
7556 The size of all structures and unions will be rounded up to a multiple
7557 of the number of bits set by this option. Permissible values are 8, 32
7558 and 64. The default value varies for different toolchains. For the COFF
7559 targeted toolchain the default value is 8. A value of 64 is only allowed
7560 if the underlying ABI supports it.
7562 Specifying the larger number can produce faster, more efficient code, but
7563 can also increase the size of the program. Different values are potentially
7564 incompatible. Code compiled with one value cannot necessarily expect to
7565 work with code or libraries compiled with another value, if they exchange
7566 information using structures or unions.
7568 @item -mabort-on-noreturn
7569 @opindex mabort-on-noreturn
7570 Generate a call to the function @code{abort} at the end of a
7571 @code{noreturn} function. It will be executed if the function tries to
7575 @itemx -mno-long-calls
7576 @opindex mlong-calls
7577 @opindex mno-long-calls
7578 Tells the compiler to perform function calls by first loading the
7579 address of the function into a register and then performing a subroutine
7580 call on this register. This switch is needed if the target function
7581 will lie outside of the 64 megabyte addressing range of the offset based
7582 version of subroutine call instruction.
7584 Even if this switch is enabled, not all function calls will be turned
7585 into long calls. The heuristic is that static functions, functions
7586 which have the @samp{short-call} attribute, functions that are inside
7587 the scope of a @samp{#pragma no_long_calls} directive and functions whose
7588 definitions have already been compiled within the current compilation
7589 unit, will not be turned into long calls. The exception to this rule is
7590 that weak function definitions, functions with the @samp{long-call}
7591 attribute or the @samp{section} attribute, and functions that are within
7592 the scope of a @samp{#pragma long_calls} directive, will always be
7593 turned into long calls.
7595 This feature is not enabled by default. Specifying
7596 @option{-mno-long-calls} will restore the default behavior, as will
7597 placing the function calls within the scope of a @samp{#pragma
7598 long_calls_off} directive. Note these switches have no effect on how
7599 the compiler generates code to handle function calls via function
7602 @item -mnop-fun-dllimport
7603 @opindex mnop-fun-dllimport
7604 Disable support for the @code{dllimport} attribute.
7606 @item -msingle-pic-base
7607 @opindex msingle-pic-base
7608 Treat the register used for PIC addressing as read-only, rather than
7609 loading it in the prologue for each function. The run-time system is
7610 responsible for initializing this register with an appropriate value
7611 before execution begins.
7613 @item -mpic-register=@var{reg}
7614 @opindex mpic-register
7615 Specify the register to be used for PIC addressing. The default is R10
7616 unless stack-checking is enabled, when R9 is used.
7618 @item -mcirrus-fix-invalid-insns
7619 @opindex mcirrus-fix-invalid-insns
7620 @opindex mno-cirrus-fix-invalid-insns
7621 Insert NOPs into the instruction stream to in order to work around
7622 problems with invalid Maverick instruction combinations. This option
7623 is only valid if the @option{-mcpu=ep9312} option has been used to
7624 enable generation of instructions for the Cirrus Maverick floating
7625 point co-processor. This option is not enabled by default, since the
7626 problem is only present in older Maverick implementations. The default
7627 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
7630 @item -mpoke-function-name
7631 @opindex mpoke-function-name
7632 Write the name of each function into the text section, directly
7633 preceding the function prologue. The generated code is similar to this:
7637 .ascii "arm_poke_function_name", 0
7640 .word 0xff000000 + (t1 - t0)
7641 arm_poke_function_name
7643 stmfd sp!, @{fp, ip, lr, pc@}
7647 When performing a stack backtrace, code can inspect the value of
7648 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
7649 location @code{pc - 12} and the top 8 bits are set, then we know that
7650 there is a function name embedded immediately preceding this location
7651 and has length @code{((pc[-3]) & 0xff000000)}.
7655 Generate code for the 16-bit Thumb instruction set. The default is to
7656 use the 32-bit ARM instruction set.
7659 @opindex mtpcs-frame
7660 Generate a stack frame that is compliant with the Thumb Procedure Call
7661 Standard for all non-leaf functions. (A leaf function is one that does
7662 not call any other functions.) The default is @option{-mno-tpcs-frame}.
7664 @item -mtpcs-leaf-frame
7665 @opindex mtpcs-leaf-frame
7666 Generate a stack frame that is compliant with the Thumb Procedure Call
7667 Standard for all leaf functions. (A leaf function is one that does
7668 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
7670 @item -mcallee-super-interworking
7671 @opindex mcallee-super-interworking
7672 Gives all externally visible functions in the file being compiled an ARM
7673 instruction set header which switches to Thumb mode before executing the
7674 rest of the function. This allows these functions to be called from
7675 non-interworking code.
7677 @item -mcaller-super-interworking
7678 @opindex mcaller-super-interworking
7679 Allows calls via function pointers (including virtual functions) to
7680 execute correctly regardless of whether the target code has been
7681 compiled for interworking or not. There is a small overhead in the cost
7682 of executing a function pointer if this option is enabled.
7684 @item -mtp=@var{name}
7686 Specify the access model for the thread local storage pointer. The valid
7687 models are @option{soft}, which generates calls to @code{__aeabi_read_tp},
7688 @option{cp15}, which fetches the thread pointer from @code{cp15} directly
7689 (supported in the arm6k architecture), and @option{auto}, which uses the
7690 best available method for the selected processor. The default setting is
7696 @subsection AVR Options
7699 These options are defined for AVR implementations:
7702 @item -mmcu=@var{mcu}
7704 Specify ATMEL AVR instruction set or MCU type.
7706 Instruction set avr1 is for the minimal AVR core, not supported by the C
7707 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
7708 attiny11, attiny12, attiny15, attiny28).
7710 Instruction set avr2 (default) is for the classic AVR core with up to
7711 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
7712 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
7713 at90c8534, at90s8535).
7715 Instruction set avr3 is for the classic AVR core with up to 128K program
7716 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
7718 Instruction set avr4 is for the enhanced AVR core with up to 8K program
7719 memory space (MCU types: atmega8, atmega83, atmega85).
7721 Instruction set avr5 is for the enhanced AVR core with up to 128K program
7722 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
7723 atmega64, atmega128, at43usb355, at94k).
7727 Output instruction sizes to the asm file.
7729 @item -minit-stack=@var{N}
7730 @opindex minit-stack
7731 Specify the initial stack address, which may be a symbol or numeric value,
7732 @samp{__stack} is the default.
7734 @item -mno-interrupts
7735 @opindex mno-interrupts
7736 Generated code is not compatible with hardware interrupts.
7737 Code size will be smaller.
7739 @item -mcall-prologues
7740 @opindex mcall-prologues
7741 Functions prologues/epilogues expanded as call to appropriate
7742 subroutines. Code size will be smaller.
7744 @item -mno-tablejump
7745 @opindex mno-tablejump
7746 Do not generate tablejump insns which sometimes increase code size.
7749 @opindex mtiny-stack
7750 Change only the low 8 bits of the stack pointer.
7754 Assume int to be 8 bit integer. This affects the sizes of all types: A
7755 char will be 1 byte, an int will be 1 byte, an long will be 2 bytes
7756 and long long will be 4 bytes. Please note that this option does not
7757 comply to the C standards, but it will provide you with smaller code
7761 @node Blackfin Options
7762 @subsection Blackfin Options
7763 @cindex Blackfin Options
7766 @item -momit-leaf-frame-pointer
7767 @opindex momit-leaf-frame-pointer
7768 Don't keep the frame pointer in a register for leaf functions. This
7769 avoids the instructions to save, set up and restore frame pointers and
7770 makes an extra register available in leaf functions. The option
7771 @option{-fomit-frame-pointer} removes the frame pointer for all functions
7772 which might make debugging harder.
7774 @item -mspecld-anomaly
7775 @opindex mspecld-anomaly
7776 When enabled, the compiler will ensure that the generated code does not
7777 contain speculative loads after jump instructions. This option is enabled
7780 @item -mno-specld-anomaly
7781 @opindex mno-specld-anomaly
7782 Don't generate extra code to prevent speculative loads from occurring.
7784 @item -mcsync-anomaly
7785 @opindex mcsync-anomaly
7786 When enabled, the compiler will ensure that the generated code does not
7787 contain CSYNC or SSYNC instructions too soon after conditional branches.
7788 This option is enabled by default.
7790 @item -mno-csync-anomaly
7791 @opindex mno-csync-anomaly
7792 Don't generate extra code to prevent CSYNC or SSYNC instructions from
7793 occurring too soon after a conditional branch.
7797 When enabled, the compiler is free to take advantage of the knowledge that
7798 the entire program fits into the low 64k of memory.
7801 @opindex mno-low-64k
7802 Assume that the program is arbitrarily large. This is the default.
7804 @item -mid-shared-library
7805 @opindex mid-shared-library
7806 Generate code that supports shared libraries via the library ID method.
7807 This allows for execute in place and shared libraries in an environment
7808 without virtual memory management. This option implies @option{-fPIC}.
7810 @item -mno-id-shared-library
7811 @opindex mno-id-shared-library
7812 Generate code that doesn't assume ID based shared libraries are being used.
7813 This is the default.
7815 @item -mshared-library-id=n
7816 @opindex mshared-library-id
7817 Specified the identification number of the ID based shared library being
7818 compiled. Specifying a value of 0 will generate more compact code, specifying
7819 other values will force the allocation of that number to the current
7820 library but is no more space or time efficient than omitting this option.
7823 @itemx -mno-long-calls
7824 @opindex mlong-calls
7825 @opindex mno-long-calls
7826 Tells the compiler to perform function calls by first loading the
7827 address of the function into a register and then performing a subroutine
7828 call on this register. This switch is needed if the target function
7829 will lie outside of the 24 bit addressing range of the offset based
7830 version of subroutine call instruction.
7832 This feature is not enabled by default. Specifying
7833 @option{-mno-long-calls} will restore the default behavior. Note these
7834 switches have no effect on how the compiler generates code to handle
7835 function calls via function pointers.
7839 @subsection CRIS Options
7840 @cindex CRIS Options
7842 These options are defined specifically for the CRIS ports.
7845 @item -march=@var{architecture-type}
7846 @itemx -mcpu=@var{architecture-type}
7849 Generate code for the specified architecture. The choices for
7850 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
7851 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX@.
7852 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
7855 @item -mtune=@var{architecture-type}
7857 Tune to @var{architecture-type} everything applicable about the generated
7858 code, except for the ABI and the set of available instructions. The
7859 choices for @var{architecture-type} are the same as for
7860 @option{-march=@var{architecture-type}}.
7862 @item -mmax-stack-frame=@var{n}
7863 @opindex mmax-stack-frame
7864 Warn when the stack frame of a function exceeds @var{n} bytes.
7866 @item -melinux-stacksize=@var{n}
7867 @opindex melinux-stacksize
7868 Only available with the @samp{cris-axis-aout} target. Arranges for
7869 indications in the program to the kernel loader that the stack of the
7870 program should be set to @var{n} bytes.
7876 The options @option{-metrax4} and @option{-metrax100} are synonyms for
7877 @option{-march=v3} and @option{-march=v8} respectively.
7879 @item -mmul-bug-workaround
7880 @itemx -mno-mul-bug-workaround
7881 @opindex mmul-bug-workaround
7882 @opindex mno-mul-bug-workaround
7883 Work around a bug in the @code{muls} and @code{mulu} instructions for CPU
7884 models where it applies. This option is active by default.
7888 Enable CRIS-specific verbose debug-related information in the assembly
7889 code. This option also has the effect to turn off the @samp{#NO_APP}
7890 formatted-code indicator to the assembler at the beginning of the
7895 Do not use condition-code results from previous instruction; always emit
7896 compare and test instructions before use of condition codes.
7898 @item -mno-side-effects
7899 @opindex mno-side-effects
7900 Do not emit instructions with side-effects in addressing modes other than
7904 @itemx -mno-stack-align
7906 @itemx -mno-data-align
7907 @itemx -mconst-align
7908 @itemx -mno-const-align
7909 @opindex mstack-align
7910 @opindex mno-stack-align
7911 @opindex mdata-align
7912 @opindex mno-data-align
7913 @opindex mconst-align
7914 @opindex mno-const-align
7915 These options (no-options) arranges (eliminate arrangements) for the
7916 stack-frame, individual data and constants to be aligned for the maximum
7917 single data access size for the chosen CPU model. The default is to
7918 arrange for 32-bit alignment. ABI details such as structure layout are
7919 not affected by these options.
7927 Similar to the stack- data- and const-align options above, these options
7928 arrange for stack-frame, writable data and constants to all be 32-bit,
7929 16-bit or 8-bit aligned. The default is 32-bit alignment.
7931 @item -mno-prologue-epilogue
7932 @itemx -mprologue-epilogue
7933 @opindex mno-prologue-epilogue
7934 @opindex mprologue-epilogue
7935 With @option{-mno-prologue-epilogue}, the normal function prologue and
7936 epilogue that sets up the stack-frame are omitted and no return
7937 instructions or return sequences are generated in the code. Use this
7938 option only together with visual inspection of the compiled code: no
7939 warnings or errors are generated when call-saved registers must be saved,
7940 or storage for local variable needs to be allocated.
7946 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
7947 instruction sequences that load addresses for functions from the PLT part
7948 of the GOT rather than (traditional on other architectures) calls to the
7949 PLT@. The default is @option{-mgotplt}.
7953 Legacy no-op option only recognized with the cris-axis-aout target.
7957 Legacy no-op option only recognized with the cris-axis-elf and
7958 cris-axis-linux-gnu targets.
7962 Only recognized with the cris-axis-aout target, where it selects a
7963 GNU/linux-like multilib, include files and instruction set for
7968 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
7972 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
7973 to link with input-output functions from a simulator library. Code,
7974 initialized data and zero-initialized data are allocated consecutively.
7978 Like @option{-sim}, but pass linker options to locate initialized data at
7979 0x40000000 and zero-initialized data at 0x80000000.
7983 @subsection CRX Options
7986 These options are defined specifically for the CRX ports.
7992 Enable the use of multiply-accumulate instructions. Disabled by default.
7996 Push instructions will be used to pass outgoing arguments when functions
7997 are called. Enabled by default.
8000 @node Darwin Options
8001 @subsection Darwin Options
8002 @cindex Darwin options
8004 These options are defined for all architectures running the Darwin operating
8007 FSF GCC on Darwin does not create ``fat'' object files; it will create
8008 an object file for the single architecture that it was built to
8009 target. Apple's GCC on Darwin does create ``fat'' files if multiple
8010 @option{-arch} options are used; it does so by running the compiler or
8011 linker multiple times and joining the results together with
8014 The subtype of the file created (like @samp{ppc7400} or @samp{ppc970} or
8015 @samp{i686}) is determined by the flags that specify the ISA
8016 that GCC is targetting, like @option{-mcpu} or @option{-march}. The
8017 @option{-force_cpusubtype_ALL} option can be used to override this.
8019 The Darwin tools vary in their behavior when presented with an ISA
8020 mismatch. The assembler, @file{as}, will only permit instructions to
8021 be used that are valid for the subtype of the file it is generating,
8022 so you cannot put 64-bit instructions in an @samp{ppc750} object file.
8023 The linker for shared libraries, @file{/usr/bin/libtool}, will fail
8024 and print an error if asked to create a shared library with a less
8025 restrictive subtype than its input files (for instance, trying to put
8026 a @samp{ppc970} object file in a @samp{ppc7400} library). The linker
8027 for executables, @file{ld}, will quietly give the executable the most
8028 restrictive subtype of any of its input files.
8033 Add the framework directory @var{dir} to the head of the list of
8034 directories to be searched for header files. These directories are
8035 interleaved with those specified by @option{-I} options and are
8036 scanned in a left-to-right order.
8038 A framework directory is a directory with frameworks in it. A
8039 framework is a directory with a @samp{"Headers"} and/or
8040 @samp{"PrivateHeaders"} directory contained directly in it that ends
8041 in @samp{".framework"}. The name of a framework is the name of this
8042 directory excluding the @samp{".framework"}. Headers associated with
8043 the framework are found in one of those two directories, with
8044 @samp{"Headers"} being searched first. A subframework is a framework
8045 directory that is in a framework's @samp{"Frameworks"} directory.
8046 Includes of subframework headers can only appear in a header of a
8047 framework that contains the subframework, or in a sibling subframework
8048 header. Two subframeworks are siblings if they occur in the same
8049 framework. A subframework should not have the same name as a
8050 framework, a warning will be issued if this is violated. Currently a
8051 subframework cannot have subframeworks, in the future, the mechanism
8052 may be extended to support this. The standard frameworks can be found
8053 in @samp{"/System/Library/Frameworks"} and
8054 @samp{"/Library/Frameworks"}. An example include looks like
8055 @code{#include <Framework/header.h>}, where @samp{Framework} denotes
8056 the name of the framework and header.h is found in the
8057 @samp{"PrivateHeaders"} or @samp{"Headers"} directory.
8061 Emit debugging information for symbols that are used. For STABS
8062 debugging format, this enables @option{-feliminate-unused-debug-symbols}.
8063 This is by default ON@.
8067 Emit debugging information for all symbols and types.
8069 @item -mmacosx-version-min=@var{version}
8070 The earliest version of MacOS X that this executable will run on
8071 is @var{version}. Typical values of @var{version} include @code{10.1},
8072 @code{10.2}, and @code{10.3.9}.
8074 The default for this option is to make choices that seem to be most
8079 Enable kernel development mode. The @option{-mkernel} sets
8080 @option{-static}, @option{-fno-common}, @option{-fno-cxa-atexit},
8081 @option{-fno-exceptions}, @option{-fno-non-call-exceptions},
8082 @option{-fapple-kext}, @option{-fno-weak} and @option{-fno-rtti} where
8083 applicable. This mode also sets @option{-mno-altivec},
8084 @option{-msoft-float}, @option{-fno-builtin} and
8085 @option{-mlong-branch} for PowerPC targets.
8087 @item -mone-byte-bool
8088 @opindex -mone-byte-bool
8089 Override the defaults for @samp{bool} so that @samp{sizeof(bool)==1}.
8090 By default @samp{sizeof(bool)} is @samp{4} when compiling for
8091 Darwin/PowerPC and @samp{1} when compiling for Darwin/x86, so this
8092 option has no effect on x86.
8094 @strong{Warning:} The @option{-mone-byte-bool} switch causes GCC
8095 to generate code that is not binary compatible with code generated
8096 without that switch. Using this switch may require recompiling all
8097 other modules in a program, including system libraries. Use this
8098 switch to conform to a non-default data model.
8100 @item -mfix-and-continue
8101 @itemx -ffix-and-continue
8102 @itemx -findirect-data
8103 @opindex mfix-and-continue
8104 @opindex ffix-and-continue
8105 @opindex findirect-data
8106 Generate code suitable for fast turn around development. Needed to
8107 enable gdb to dynamically load @code{.o} files into already running
8108 programs. @option{-findirect-data} and @option{-ffix-and-continue}
8109 are provided for backwards compatibility.
8113 Loads all members of static archive libraries.
8114 See man ld(1) for more information.
8116 @item -arch_errors_fatal
8117 @opindex arch_errors_fatal
8118 Cause the errors having to do with files that have the wrong architecture
8122 @opindex bind_at_load
8123 Causes the output file to be marked such that the dynamic linker will
8124 bind all undefined references when the file is loaded or launched.
8128 Produce a Mach-o bundle format file.
8129 See man ld(1) for more information.
8131 @item -bundle_loader @var{executable}
8132 @opindex bundle_loader
8133 This option specifies the @var{executable} that will be loading the build
8134 output file being linked. See man ld(1) for more information.
8137 @opindex -dynamiclib
8138 When passed this option, GCC will produce a dynamic library instead of
8139 an executable when linking, using the Darwin @file{libtool} command.
8141 @item -force_cpusubtype_ALL
8142 @opindex -force_cpusubtype_ALL
8143 This causes GCC's output file to have the @var{ALL} subtype, instead of
8144 one controlled by the @option{-mcpu} or @option{-march} option.
8146 @item -allowable_client @var{client_name}
8148 @itemx -compatibility_version
8149 @itemx -current_version
8151 @itemx -dependency-file
8153 @itemx -dylinker_install_name
8155 @itemx -exported_symbols_list
8157 @itemx -flat_namespace
8158 @itemx -force_flat_namespace
8159 @itemx -headerpad_max_install_names
8162 @itemx -install_name
8163 @itemx -keep_private_externs
8164 @itemx -multi_module
8165 @itemx -multiply_defined
8166 @itemx -multiply_defined_unused
8168 @itemx -no_dead_strip_inits_and_terms
8169 @itemx -nofixprebinding
8172 @itemx -noseglinkedit
8173 @itemx -pagezero_size
8175 @itemx -prebind_all_twolevel_modules
8176 @itemx -private_bundle
8177 @itemx -read_only_relocs
8179 @itemx -sectobjectsymbols
8183 @itemx -sectobjectsymbols
8186 @itemx -segs_read_only_addr
8187 @itemx -segs_read_write_addr
8188 @itemx -seg_addr_table
8189 @itemx -seg_addr_table_filename
8192 @itemx -segs_read_only_addr
8193 @itemx -segs_read_write_addr
8194 @itemx -single_module
8197 @itemx -sub_umbrella
8198 @itemx -twolevel_namespace
8201 @itemx -unexported_symbols_list
8202 @itemx -weak_reference_mismatches
8205 @opindex allowable_client
8206 @opindex client_name
8207 @opindex compatibility_version
8208 @opindex current_version
8210 @opindex dependency-file
8212 @opindex dylinker_install_name
8214 @opindex exported_symbols_list
8216 @opindex flat_namespace
8217 @opindex force_flat_namespace
8218 @opindex headerpad_max_install_names
8221 @opindex install_name
8222 @opindex keep_private_externs
8223 @opindex multi_module
8224 @opindex multiply_defined
8225 @opindex multiply_defined_unused
8227 @opindex no_dead_strip_inits_and_terms
8228 @opindex nofixprebinding
8229 @opindex nomultidefs
8231 @opindex noseglinkedit
8232 @opindex pagezero_size
8234 @opindex prebind_all_twolevel_modules
8235 @opindex private_bundle
8236 @opindex read_only_relocs
8238 @opindex sectobjectsymbols
8242 @opindex sectobjectsymbols
8245 @opindex segs_read_only_addr
8246 @opindex segs_read_write_addr
8247 @opindex seg_addr_table
8248 @opindex seg_addr_table_filename
8249 @opindex seglinkedit
8251 @opindex segs_read_only_addr
8252 @opindex segs_read_write_addr
8253 @opindex single_module
8255 @opindex sub_library
8256 @opindex sub_umbrella
8257 @opindex twolevel_namespace
8260 @opindex unexported_symbols_list
8261 @opindex weak_reference_mismatches
8262 @opindex whatsloaded
8264 These options are passed to the Darwin linker. The Darwin linker man page
8265 describes them in detail.
8268 @node DEC Alpha Options
8269 @subsection DEC Alpha Options
8271 These @samp{-m} options are defined for the DEC Alpha implementations:
8274 @item -mno-soft-float
8276 @opindex mno-soft-float
8277 @opindex msoft-float
8278 Use (do not use) the hardware floating-point instructions for
8279 floating-point operations. When @option{-msoft-float} is specified,
8280 functions in @file{libgcc.a} will be used to perform floating-point
8281 operations. Unless they are replaced by routines that emulate the
8282 floating-point operations, or compiled in such a way as to call such
8283 emulations routines, these routines will issue floating-point
8284 operations. If you are compiling for an Alpha without floating-point
8285 operations, you must ensure that the library is built so as not to call
8288 Note that Alpha implementations without floating-point operations are
8289 required to have floating-point registers.
8294 @opindex mno-fp-regs
8295 Generate code that uses (does not use) the floating-point register set.
8296 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
8297 register set is not used, floating point operands are passed in integer
8298 registers as if they were integers and floating-point results are passed
8299 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
8300 so any function with a floating-point argument or return value called by code
8301 compiled with @option{-mno-fp-regs} must also be compiled with that
8304 A typical use of this option is building a kernel that does not use,
8305 and hence need not save and restore, any floating-point registers.
8309 The Alpha architecture implements floating-point hardware optimized for
8310 maximum performance. It is mostly compliant with the IEEE floating
8311 point standard. However, for full compliance, software assistance is
8312 required. This option generates code fully IEEE compliant code
8313 @emph{except} that the @var{inexact-flag} is not maintained (see below).
8314 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
8315 defined during compilation. The resulting code is less efficient but is
8316 able to correctly support denormalized numbers and exceptional IEEE
8317 values such as not-a-number and plus/minus infinity. Other Alpha
8318 compilers call this option @option{-ieee_with_no_inexact}.
8320 @item -mieee-with-inexact
8321 @opindex mieee-with-inexact
8322 This is like @option{-mieee} except the generated code also maintains
8323 the IEEE @var{inexact-flag}. Turning on this option causes the
8324 generated code to implement fully-compliant IEEE math. In addition to
8325 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
8326 macro. On some Alpha implementations the resulting code may execute
8327 significantly slower than the code generated by default. Since there is
8328 very little code that depends on the @var{inexact-flag}, you should
8329 normally not specify this option. Other Alpha compilers call this
8330 option @option{-ieee_with_inexact}.
8332 @item -mfp-trap-mode=@var{trap-mode}
8333 @opindex mfp-trap-mode
8334 This option controls what floating-point related traps are enabled.
8335 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
8336 The trap mode can be set to one of four values:
8340 This is the default (normal) setting. The only traps that are enabled
8341 are the ones that cannot be disabled in software (e.g., division by zero
8345 In addition to the traps enabled by @samp{n}, underflow traps are enabled
8349 Like @samp{su}, but the instructions are marked to be safe for software
8350 completion (see Alpha architecture manual for details).
8353 Like @samp{su}, but inexact traps are enabled as well.
8356 @item -mfp-rounding-mode=@var{rounding-mode}
8357 @opindex mfp-rounding-mode
8358 Selects the IEEE rounding mode. Other Alpha compilers call this option
8359 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
8364 Normal IEEE rounding mode. Floating point numbers are rounded towards
8365 the nearest machine number or towards the even machine number in case
8369 Round towards minus infinity.
8372 Chopped rounding mode. Floating point numbers are rounded towards zero.
8375 Dynamic rounding mode. A field in the floating point control register
8376 (@var{fpcr}, see Alpha architecture reference manual) controls the
8377 rounding mode in effect. The C library initializes this register for
8378 rounding towards plus infinity. Thus, unless your program modifies the
8379 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
8382 @item -mtrap-precision=@var{trap-precision}
8383 @opindex mtrap-precision
8384 In the Alpha architecture, floating point traps are imprecise. This
8385 means without software assistance it is impossible to recover from a
8386 floating trap and program execution normally needs to be terminated.
8387 GCC can generate code that can assist operating system trap handlers
8388 in determining the exact location that caused a floating point trap.
8389 Depending on the requirements of an application, different levels of
8390 precisions can be selected:
8394 Program precision. This option is the default and means a trap handler
8395 can only identify which program caused a floating point exception.
8398 Function precision. The trap handler can determine the function that
8399 caused a floating point exception.
8402 Instruction precision. The trap handler can determine the exact
8403 instruction that caused a floating point exception.
8406 Other Alpha compilers provide the equivalent options called
8407 @option{-scope_safe} and @option{-resumption_safe}.
8409 @item -mieee-conformant
8410 @opindex mieee-conformant
8411 This option marks the generated code as IEEE conformant. You must not
8412 use this option unless you also specify @option{-mtrap-precision=i} and either
8413 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
8414 is to emit the line @samp{.eflag 48} in the function prologue of the
8415 generated assembly file. Under DEC Unix, this has the effect that
8416 IEEE-conformant math library routines will be linked in.
8418 @item -mbuild-constants
8419 @opindex mbuild-constants
8420 Normally GCC examines a 32- or 64-bit integer constant to
8421 see if it can construct it from smaller constants in two or three
8422 instructions. If it cannot, it will output the constant as a literal and
8423 generate code to load it from the data segment at runtime.
8425 Use this option to require GCC to construct @emph{all} integer constants
8426 using code, even if it takes more instructions (the maximum is six).
8428 You would typically use this option to build a shared library dynamic
8429 loader. Itself a shared library, it must relocate itself in memory
8430 before it can find the variables and constants in its own data segment.
8436 Select whether to generate code to be assembled by the vendor-supplied
8437 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
8455 Indicate whether GCC should generate code to use the optional BWX,
8456 CIX, FIX and MAX instruction sets. The default is to use the instruction
8457 sets supported by the CPU type specified via @option{-mcpu=} option or that
8458 of the CPU on which GCC was built if none was specified.
8463 @opindex mfloat-ieee
8464 Generate code that uses (does not use) VAX F and G floating point
8465 arithmetic instead of IEEE single and double precision.
8467 @item -mexplicit-relocs
8468 @itemx -mno-explicit-relocs
8469 @opindex mexplicit-relocs
8470 @opindex mno-explicit-relocs
8471 Older Alpha assemblers provided no way to generate symbol relocations
8472 except via assembler macros. Use of these macros does not allow
8473 optimal instruction scheduling. GNU binutils as of version 2.12
8474 supports a new syntax that allows the compiler to explicitly mark
8475 which relocations should apply to which instructions. This option
8476 is mostly useful for debugging, as GCC detects the capabilities of
8477 the assembler when it is built and sets the default accordingly.
8481 @opindex msmall-data
8482 @opindex mlarge-data
8483 When @option{-mexplicit-relocs} is in effect, static data is
8484 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
8485 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
8486 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
8487 16-bit relocations off of the @code{$gp} register. This limits the
8488 size of the small data area to 64KB, but allows the variables to be
8489 directly accessed via a single instruction.
8491 The default is @option{-mlarge-data}. With this option the data area
8492 is limited to just below 2GB@. Programs that require more than 2GB of
8493 data must use @code{malloc} or @code{mmap} to allocate the data in the
8494 heap instead of in the program's data segment.
8496 When generating code for shared libraries, @option{-fpic} implies
8497 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
8501 @opindex msmall-text
8502 @opindex mlarge-text
8503 When @option{-msmall-text} is used, the compiler assumes that the
8504 code of the entire program (or shared library) fits in 4MB, and is
8505 thus reachable with a branch instruction. When @option{-msmall-data}
8506 is used, the compiler can assume that all local symbols share the
8507 same @code{$gp} value, and thus reduce the number of instructions
8508 required for a function call from 4 to 1.
8510 The default is @option{-mlarge-text}.
8512 @item -mcpu=@var{cpu_type}
8514 Set the instruction set and instruction scheduling parameters for
8515 machine type @var{cpu_type}. You can specify either the @samp{EV}
8516 style name or the corresponding chip number. GCC supports scheduling
8517 parameters for the EV4, EV5 and EV6 family of processors and will
8518 choose the default values for the instruction set from the processor
8519 you specify. If you do not specify a processor type, GCC will default
8520 to the processor on which the compiler was built.
8522 Supported values for @var{cpu_type} are
8528 Schedules as an EV4 and has no instruction set extensions.
8532 Schedules as an EV5 and has no instruction set extensions.
8536 Schedules as an EV5 and supports the BWX extension.
8541 Schedules as an EV5 and supports the BWX and MAX extensions.
8545 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
8549 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
8552 @item -mtune=@var{cpu_type}
8554 Set only the instruction scheduling parameters for machine type
8555 @var{cpu_type}. The instruction set is not changed.
8557 @item -mmemory-latency=@var{time}
8558 @opindex mmemory-latency
8559 Sets the latency the scheduler should assume for typical memory
8560 references as seen by the application. This number is highly
8561 dependent on the memory access patterns used by the application
8562 and the size of the external cache on the machine.
8564 Valid options for @var{time} are
8568 A decimal number representing clock cycles.
8574 The compiler contains estimates of the number of clock cycles for
8575 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
8576 (also called Dcache, Scache, and Bcache), as well as to main memory.
8577 Note that L3 is only valid for EV5.
8582 @node DEC Alpha/VMS Options
8583 @subsection DEC Alpha/VMS Options
8585 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
8588 @item -mvms-return-codes
8589 @opindex mvms-return-codes
8590 Return VMS condition codes from main. The default is to return POSIX
8591 style condition (e.g.@ error) codes.
8595 @subsection FRV Options
8602 Only use the first 32 general purpose registers.
8607 Use all 64 general purpose registers.
8612 Use only the first 32 floating point registers.
8617 Use all 64 floating point registers
8620 @opindex mhard-float
8622 Use hardware instructions for floating point operations.
8625 @opindex msoft-float
8627 Use library routines for floating point operations.
8632 Dynamically allocate condition code registers.
8637 Do not try to dynamically allocate condition code registers, only
8638 use @code{icc0} and @code{fcc0}.
8643 Change ABI to use double word insns.
8648 Do not use double word instructions.
8653 Use floating point double instructions.
8658 Do not use floating point double instructions.
8663 Use media instructions.
8668 Do not use media instructions.
8673 Use multiply and add/subtract instructions.
8678 Do not use multiply and add/subtract instructions.
8683 Select the FDPIC ABI, that uses function descriptors to represent
8684 pointers to functions. Without any PIC/PIE-related options, it
8685 implies @option{-fPIE}. With @option{-fpic} or @option{-fpie}, it
8686 assumes GOT entries and small data are within a 12-bit range from the
8687 GOT base address; with @option{-fPIC} or @option{-fPIE}, GOT offsets
8688 are computed with 32 bits.
8691 @opindex minline-plt
8693 Enable inlining of PLT entries in function calls to functions that are
8694 not known to bind locally. It has no effect without @option{-mfdpic}.
8695 It's enabled by default if optimizing for speed and compiling for
8696 shared libraries (i.e., @option{-fPIC} or @option{-fpic}), or when an
8697 optimization option such as @option{-O3} or above is present in the
8703 Assume a large TLS segment when generating thread-local code.
8708 Do not assume a large TLS segment when generating thread-local code.
8713 Enable the use of @code{GPREL} relocations in the FDPIC ABI for data
8714 that is known to be in read-only sections. It's enabled by default,
8715 except for @option{-fpic} or @option{-fpie}: even though it may help
8716 make the global offset table smaller, it trades 1 instruction for 4.
8717 With @option{-fPIC} or @option{-fPIE}, it trades 3 instructions for 4,
8718 one of which may be shared by multiple symbols, and it avoids the need
8719 for a GOT entry for the referenced symbol, so it's more likely to be a
8720 win. If it is not, @option{-mno-gprel-ro} can be used to disable it.
8722 @item -multilib-library-pic
8723 @opindex multilib-library-pic
8725 Link with the (library, not FD) pic libraries. It's implied by
8726 @option{-mlibrary-pic}, as well as by @option{-fPIC} and
8727 @option{-fpic} without @option{-mfdpic}. You should never have to use
8733 Follow the EABI requirement of always creating a frame pointer whenever
8734 a stack frame is allocated. This option is enabled by default and can
8735 be disabled with @option{-mno-linked-fp}.
8738 @opindex mlong-calls
8740 Use indirect addressing to call functions outside the current
8741 compilation unit. This allows the functions to be placed anywhere
8742 within the 32-bit address space.
8744 @item -malign-labels
8745 @opindex malign-labels
8747 Try to align labels to an 8-byte boundary by inserting nops into the
8748 previous packet. This option only has an effect when VLIW packing
8749 is enabled. It doesn't create new packets; it merely adds nops to
8753 @opindex mlibrary-pic
8755 Generate position-independent EABI code.
8760 Use only the first four media accumulator registers.
8765 Use all eight media accumulator registers.
8770 Pack VLIW instructions.
8775 Do not pack VLIW instructions.
8780 Do not mark ABI switches in e_flags.
8785 Enable the use of conditional-move instructions (default).
8787 This switch is mainly for debugging the compiler and will likely be removed
8788 in a future version.
8790 @item -mno-cond-move
8791 @opindex mno-cond-move
8793 Disable the use of conditional-move instructions.
8795 This switch is mainly for debugging the compiler and will likely be removed
8796 in a future version.
8801 Enable the use of conditional set instructions (default).
8803 This switch is mainly for debugging the compiler and will likely be removed
8804 in a future version.
8809 Disable the use of conditional set instructions.
8811 This switch is mainly for debugging the compiler and will likely be removed
8812 in a future version.
8817 Enable the use of conditional execution (default).
8819 This switch is mainly for debugging the compiler and will likely be removed
8820 in a future version.
8822 @item -mno-cond-exec
8823 @opindex mno-cond-exec
8825 Disable the use of conditional execution.
8827 This switch is mainly for debugging the compiler and will likely be removed
8828 in a future version.
8831 @opindex mvliw-branch
8833 Run a pass to pack branches into VLIW instructions (default).
8835 This switch is mainly for debugging the compiler and will likely be removed
8836 in a future version.
8838 @item -mno-vliw-branch
8839 @opindex mno-vliw-branch
8841 Do not run a pass to pack branches into VLIW instructions.
8843 This switch is mainly for debugging the compiler and will likely be removed
8844 in a future version.
8846 @item -mmulti-cond-exec
8847 @opindex mmulti-cond-exec
8849 Enable optimization of @code{&&} and @code{||} in conditional execution
8852 This switch is mainly for debugging the compiler and will likely be removed
8853 in a future version.
8855 @item -mno-multi-cond-exec
8856 @opindex mno-multi-cond-exec
8858 Disable optimization of @code{&&} and @code{||} in conditional execution.
8860 This switch is mainly for debugging the compiler and will likely be removed
8861 in a future version.
8863 @item -mnested-cond-exec
8864 @opindex mnested-cond-exec
8866 Enable nested conditional execution optimizations (default).
8868 This switch is mainly for debugging the compiler and will likely be removed
8869 in a future version.
8871 @item -mno-nested-cond-exec
8872 @opindex mno-nested-cond-exec
8874 Disable nested conditional execution optimizations.
8876 This switch is mainly for debugging the compiler and will likely be removed
8877 in a future version.
8879 @item -moptimize-membar
8880 @opindex moptimize-membar
8882 This switch removes redundant @code{membar} instructions from the
8883 compiler generated code. It is enabled by default.
8885 @item -mno-optimize-membar
8886 @opindex mno-optimize-membar
8888 This switch disables the automatic removal of redundant @code{membar}
8889 instructions from the generated code.
8891 @item -mtomcat-stats
8892 @opindex mtomcat-stats
8894 Cause gas to print out tomcat statistics.
8896 @item -mcpu=@var{cpu}
8899 Select the processor type for which to generate code. Possible values are
8900 @samp{frv}, @samp{fr550}, @samp{tomcat}, @samp{fr500}, @samp{fr450},
8901 @samp{fr405}, @samp{fr400}, @samp{fr300} and @samp{simple}.
8905 @node GNU/Linux Options
8906 @subsection GNU/Linux Options
8908 These @samp{-m} options are defined for GNU/Linux targets:
8913 Use the GNU C library instead of uClibc. This is the default except
8914 on @samp{*-*-linux-*uclibc*} targets.
8918 Use uClibc instead of the GNU C library. This is the default on
8919 @samp{*-*-linux-*uclibc*} targets.
8922 @node H8/300 Options
8923 @subsection H8/300 Options
8925 These @samp{-m} options are defined for the H8/300 implementations:
8930 Shorten some address references at link time, when possible; uses the
8931 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
8932 ld, Using ld}, for a fuller description.
8936 Generate code for the H8/300H@.
8940 Generate code for the H8S@.
8944 Generate code for the H8S and H8/300H in the normal mode. This switch
8945 must be used either with @option{-mh} or @option{-ms}.
8949 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
8953 Make @code{int} data 32 bits by default.
8957 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
8958 The default for the H8/300H and H8S is to align longs and floats on 4
8960 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
8961 This option has no effect on the H8/300.
8965 @subsection HPPA Options
8966 @cindex HPPA Options
8968 These @samp{-m} options are defined for the HPPA family of computers:
8971 @item -march=@var{architecture-type}
8973 Generate code for the specified architecture. The choices for
8974 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
8975 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
8976 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
8977 architecture option for your machine. Code compiled for lower numbered
8978 architectures will run on higher numbered architectures, but not the
8982 @itemx -mpa-risc-1-1
8983 @itemx -mpa-risc-2-0
8984 @opindex mpa-risc-1-0
8985 @opindex mpa-risc-1-1
8986 @opindex mpa-risc-2-0
8987 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
8990 @opindex mbig-switch
8991 Generate code suitable for big switch tables. Use this option only if
8992 the assembler/linker complain about out of range branches within a switch
8995 @item -mjump-in-delay
8996 @opindex mjump-in-delay
8997 Fill delay slots of function calls with unconditional jump instructions
8998 by modifying the return pointer for the function call to be the target
8999 of the conditional jump.
9001 @item -mdisable-fpregs
9002 @opindex mdisable-fpregs
9003 Prevent floating point registers from being used in any manner. This is
9004 necessary for compiling kernels which perform lazy context switching of
9005 floating point registers. If you use this option and attempt to perform
9006 floating point operations, the compiler will abort.
9008 @item -mdisable-indexing
9009 @opindex mdisable-indexing
9010 Prevent the compiler from using indexing address modes. This avoids some
9011 rather obscure problems when compiling MIG generated code under MACH@.
9013 @item -mno-space-regs
9014 @opindex mno-space-regs
9015 Generate code that assumes the target has no space registers. This allows
9016 GCC to generate faster indirect calls and use unscaled index address modes.
9018 Such code is suitable for level 0 PA systems and kernels.
9020 @item -mfast-indirect-calls
9021 @opindex mfast-indirect-calls
9022 Generate code that assumes calls never cross space boundaries. This
9023 allows GCC to emit code which performs faster indirect calls.
9025 This option will not work in the presence of shared libraries or nested
9028 @item -mfixed-range=@var{register-range}
9029 @opindex mfixed-range
9030 Generate code treating the given register range as fixed registers.
9031 A fixed register is one that the register allocator can not use. This is
9032 useful when compiling kernel code. A register range is specified as
9033 two registers separated by a dash. Multiple register ranges can be
9034 specified separated by a comma.
9036 @item -mlong-load-store
9037 @opindex mlong-load-store
9038 Generate 3-instruction load and store sequences as sometimes required by
9039 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
9042 @item -mportable-runtime
9043 @opindex mportable-runtime
9044 Use the portable calling conventions proposed by HP for ELF systems.
9048 Enable the use of assembler directives only GAS understands.
9050 @item -mschedule=@var{cpu-type}
9052 Schedule code according to the constraints for the machine type
9053 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
9054 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
9055 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
9056 proper scheduling option for your machine. The default scheduling is
9060 @opindex mlinker-opt
9061 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
9062 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
9063 linkers in which they give bogus error messages when linking some programs.
9066 @opindex msoft-float
9067 Generate output containing library calls for floating point.
9068 @strong{Warning:} the requisite libraries are not available for all HPPA
9069 targets. Normally the facilities of the machine's usual C compiler are
9070 used, but this cannot be done directly in cross-compilation. You must make
9071 your own arrangements to provide suitable library functions for
9072 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
9073 does provide software floating point support.
9075 @option{-msoft-float} changes the calling convention in the output file;
9076 therefore, it is only useful if you compile @emph{all} of a program with
9077 this option. In particular, you need to compile @file{libgcc.a}, the
9078 library that comes with GCC, with @option{-msoft-float} in order for
9083 Generate the predefine, @code{_SIO}, for server IO@. The default is
9084 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
9085 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO@. These
9086 options are available under HP-UX and HI-UX@.
9090 Use GNU ld specific options. This passes @option{-shared} to ld when
9091 building a shared library. It is the default when GCC is configured,
9092 explicitly or implicitly, with the GNU linker. This option does not
9093 have any affect on which ld is called, it only changes what parameters
9094 are passed to that ld. The ld that is called is determined by the
9095 @option{--with-ld} configure option, GCC's program search path, and
9096 finally by the user's @env{PATH}. The linker used by GCC can be printed
9097 using @samp{which `gcc -print-prog-name=ld`}. This option is only available
9098 on the 64 bit HP-UX GCC, i.e. configured with @samp{hppa*64*-*-hpux*}.
9102 Use HP ld specific options. This passes @option{-b} to ld when building
9103 a shared library and passes @option{+Accept TypeMismatch} to ld on all
9104 links. It is the default when GCC is configured, explicitly or
9105 implicitly, with the HP linker. This option does not have any affect on
9106 which ld is called, it only changes what parameters are passed to that
9107 ld. The ld that is called is determined by the @option{--with-ld}
9108 configure option, GCC's program search path, and finally by the user's
9109 @env{PATH}. The linker used by GCC can be printed using @samp{which
9110 `gcc -print-prog-name=ld`}. This option is only available on the 64 bit
9111 HP-UX GCC, i.e. configured with @samp{hppa*64*-*-hpux*}.
9114 @opindex mno-long-calls
9115 Generate code that uses long call sequences. This ensures that a call
9116 is always able to reach linker generated stubs. The default is to generate
9117 long calls only when the distance from the call site to the beginning
9118 of the function or translation unit, as the case may be, exceeds a
9119 predefined limit set by the branch type being used. The limits for
9120 normal calls are 7,600,000 and 240,000 bytes, respectively for the
9121 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
9124 Distances are measured from the beginning of functions when using the
9125 @option{-ffunction-sections} option, or when using the @option{-mgas}
9126 and @option{-mno-portable-runtime} options together under HP-UX with
9129 It is normally not desirable to use this option as it will degrade
9130 performance. However, it may be useful in large applications,
9131 particularly when partial linking is used to build the application.
9133 The types of long calls used depends on the capabilities of the
9134 assembler and linker, and the type of code being generated. The
9135 impact on systems that support long absolute calls, and long pic
9136 symbol-difference or pc-relative calls should be relatively small.
9137 However, an indirect call is used on 32-bit ELF systems in pic code
9138 and it is quite long.
9140 @item -munix=@var{unix-std}
9142 Generate compiler predefines and select a startfile for the specified
9143 UNIX standard. The choices for @var{unix-std} are @samp{93}, @samp{95}
9144 and @samp{98}. @samp{93} is supported on all HP-UX versions. @samp{95}
9145 is available on HP-UX 10.10 and later. @samp{98} is available on HP-UX
9146 11.11 and later. The default values are @samp{93} for HP-UX 10.00,
9147 @samp{95} for HP-UX 10.10 though to 11.00, and @samp{98} for HP-UX 11.11
9150 @option{-munix=93} provides the same predefines as GCC 3.3 and 3.4.
9151 @option{-munix=95} provides additional predefines for @code{XOPEN_UNIX}
9152 and @code{_XOPEN_SOURCE_EXTENDED}, and the startfile @file{unix95.o}.
9153 @option{-munix=98} provides additional predefines for @code{_XOPEN_UNIX},
9154 @code{_XOPEN_SOURCE_EXTENDED}, @code{_INCLUDE__STDC_A1_SOURCE} and
9155 @code{_INCLUDE_XOPEN_SOURCE_500}, and the startfile @file{unix98.o}.
9157 It is @emph{important} to note that this option changes the interfaces
9158 for various library routines. It also affects the operational behavior
9159 of the C library. Thus, @emph{extreme} care is needed in using this
9162 Library code that is intended to operate with more than one UNIX
9163 standard must test, set and restore the variable @var{__xpg4_extended_mask}
9164 as appropriate. Most GNU software doesn't provide this capability.
9168 Suppress the generation of link options to search libdld.sl when the
9169 @option{-static} option is specified on HP-UX 10 and later.
9173 The HP-UX implementation of setlocale in libc has a dependency on
9174 libdld.sl. There isn't an archive version of libdld.sl. Thus,
9175 when the @option{-static} option is specified, special link options
9176 are needed to resolve this dependency.
9178 On HP-UX 10 and later, the GCC driver adds the necessary options to
9179 link with libdld.sl when the @option{-static} option is specified.
9180 This causes the resulting binary to be dynamic. On the 64-bit port,
9181 the linkers generate dynamic binaries by default in any case. The
9182 @option{-nolibdld} option can be used to prevent the GCC driver from
9183 adding these link options.
9187 Add support for multithreading with the @dfn{dce thread} library
9188 under HP-UX@. This option sets flags for both the preprocessor and
9192 @node i386 and x86-64 Options
9193 @subsection Intel 386 and AMD x86-64 Options
9194 @cindex i386 Options
9195 @cindex x86-64 Options
9196 @cindex Intel 386 Options
9197 @cindex AMD x86-64 Options
9199 These @samp{-m} options are defined for the i386 and x86-64 family of
9203 @item -mtune=@var{cpu-type}
9205 Tune to @var{cpu-type} everything applicable about the generated code, except
9206 for the ABI and the set of available instructions. The choices for
9210 Produce code optimized for the most common IA32/AMD64/EM64T processors.
9211 If you know the CPU on which your code will run, then you should use
9212 the corresponding @option{-mtune} option instead of
9213 @option{-mtune=generic}. But, if you do not know exactly what CPU users
9214 of your application will have, then you should use this option.
9216 As new processors are deployed in the marketplace, the behavior of this
9217 option will change. Therefore, if you upgrade to a newer version of
9218 GCC, the code generated option will change to reflect the processors
9219 that were most common when that version of GCC was released.
9221 There is no @option{-march=generic} option because @option{-march}
9222 indicates the instruction set the compiler can use, and there is no
9223 generic instruction set applicable to all processors. In contrast,
9224 @option{-mtune} indicates the processor (or, in this case, collection of
9225 processors) for which the code is optimized.
9227 This selects the CPU to tune for at compilation time by determining
9228 the processor type of the compiling machine. Using @option{-mtune=native}
9229 will produce code optimized for the local machine under the constraints
9230 of the selected instruction set. Using @option{-march=native} will
9231 enable all instruction subsets supported by the local machine (hence
9232 the result might not run on different machines).
9234 Original Intel's i386 CPU@.
9236 Intel's i486 CPU@. (No scheduling is implemented for this chip.)
9238 Intel Pentium CPU with no MMX support.
9240 Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
9242 Intel PentiumPro CPU@.
9244 Same as @code{generic}, but when used as @code{march} option, PentiumPro
9245 instruction set will be used, so the code will run on all i686 family chips.
9247 Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
9248 @item pentium3, pentium3m
9249 Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set
9252 Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set
9253 support. Used by Centrino notebooks.
9254 @item pentium4, pentium4m
9255 Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
9257 Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction
9260 Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
9261 SSE2 and SSE3 instruction set support.
9263 AMD K6 CPU with MMX instruction set support.
9265 Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
9266 @item athlon, athlon-tbird
9267 AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions
9269 @item athlon-4, athlon-xp, athlon-mp
9270 Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE
9271 instruction set support.
9272 @item k8, opteron, athlon64, athlon-fx
9273 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets
9274 MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
9276 IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction
9279 IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW!
9280 instruction set support.
9282 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is
9283 implemented for this chip.)
9285 Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is
9286 implemented for this chip.)
9289 While picking a specific @var{cpu-type} will schedule things appropriately
9290 for that particular chip, the compiler will not generate any code that
9291 does not run on the i386 without the @option{-march=@var{cpu-type}} option
9294 @item -march=@var{cpu-type}
9296 Generate instructions for the machine type @var{cpu-type}. The choices
9297 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
9298 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
9300 @item -mcpu=@var{cpu-type}
9302 A deprecated synonym for @option{-mtune}.
9311 @opindex mpentiumpro
9312 These options are synonyms for @option{-mtune=i386}, @option{-mtune=i486},
9313 @option{-mtune=pentium}, and @option{-mtune=pentiumpro} respectively.
9314 These synonyms are deprecated.
9316 @item -mfpmath=@var{unit}
9318 Generate floating point arithmetics for selected unit @var{unit}. The choices
9323 Use the standard 387 floating point coprocessor present majority of chips and
9324 emulated otherwise. Code compiled with this option will run almost everywhere.
9325 The temporary results are computed in 80bit precision instead of precision
9326 specified by the type resulting in slightly different results compared to most
9327 of other chips. See @option{-ffloat-store} for more detailed description.
9329 This is the default choice for i386 compiler.
9332 Use scalar floating point instructions present in the SSE instruction set.
9333 This instruction set is supported by Pentium3 and newer chips, in the AMD line
9334 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
9335 instruction set supports only single precision arithmetics, thus the double and
9336 extended precision arithmetics is still done using 387. Later version, present
9337 only in Pentium4 and the future AMD x86-64 chips supports double precision
9340 For the i386 compiler, you need to use @option{-march=@var{cpu-type}}, @option{-msse}
9341 or @option{-msse2} switches to enable SSE extensions and make this option
9342 effective. For the x86-64 compiler, these extensions are enabled by default.
9344 The resulting code should be considerably faster in the majority of cases and avoid
9345 the numerical instability problems of 387 code, but may break some existing
9346 code that expects temporaries to be 80bit.
9348 This is the default choice for the x86-64 compiler.
9351 Attempt to utilize both instruction sets at once. This effectively double the
9352 amount of available registers and on chips with separate execution units for
9353 387 and SSE the execution resources too. Use this option with care, as it is
9354 still experimental, because the GCC register allocator does not model separate
9355 functional units well resulting in instable performance.
9358 @item -masm=@var{dialect}
9359 @opindex masm=@var{dialect}
9360 Output asm instructions using selected @var{dialect}. Supported
9361 choices are @samp{intel} or @samp{att} (the default one). Darwin does
9362 not support @samp{intel}.
9367 @opindex mno-ieee-fp
9368 Control whether or not the compiler uses IEEE floating point
9369 comparisons. These handle correctly the case where the result of a
9370 comparison is unordered.
9373 @opindex msoft-float
9374 Generate output containing library calls for floating point.
9375 @strong{Warning:} the requisite libraries are not part of GCC@.
9376 Normally the facilities of the machine's usual C compiler are used, but
9377 this can't be done directly in cross-compilation. You must make your
9378 own arrangements to provide suitable library functions for
9381 On machines where a function returns floating point results in the 80387
9382 register stack, some floating point opcodes may be emitted even if
9383 @option{-msoft-float} is used.
9385 @item -mno-fp-ret-in-387
9386 @opindex mno-fp-ret-in-387
9387 Do not use the FPU registers for return values of functions.
9389 The usual calling convention has functions return values of types
9390 @code{float} and @code{double} in an FPU register, even if there
9391 is no FPU@. The idea is that the operating system should emulate
9394 The option @option{-mno-fp-ret-in-387} causes such values to be returned
9395 in ordinary CPU registers instead.
9397 @item -mno-fancy-math-387
9398 @opindex mno-fancy-math-387
9399 Some 387 emulators do not support the @code{sin}, @code{cos} and
9400 @code{sqrt} instructions for the 387. Specify this option to avoid
9401 generating those instructions. This option is the default on FreeBSD,
9402 OpenBSD and NetBSD@. This option is overridden when @option{-march}
9403 indicates that the target cpu will always have an FPU and so the
9404 instruction will not need emulation. As of revision 2.6.1, these
9405 instructions are not generated unless you also use the
9406 @option{-funsafe-math-optimizations} switch.
9408 @item -malign-double
9409 @itemx -mno-align-double
9410 @opindex malign-double
9411 @opindex mno-align-double
9412 Control whether GCC aligns @code{double}, @code{long double}, and
9413 @code{long long} variables on a two word boundary or a one word
9414 boundary. Aligning @code{double} variables on a two word boundary will
9415 produce code that runs somewhat faster on a @samp{Pentium} at the
9416 expense of more memory.
9418 On x86-64, @option{-malign-double} is enabled by default.
9420 @strong{Warning:} if you use the @option{-malign-double} switch,
9421 structures containing the above types will be aligned differently than
9422 the published application binary interface specifications for the 386
9423 and will not be binary compatible with structures in code compiled
9424 without that switch.
9426 @item -m96bit-long-double
9427 @itemx -m128bit-long-double
9428 @opindex m96bit-long-double
9429 @opindex m128bit-long-double
9430 These switches control the size of @code{long double} type. The i386
9431 application binary interface specifies the size to be 96 bits,
9432 so @option{-m96bit-long-double} is the default in 32 bit mode.
9434 Modern architectures (Pentium and newer) would prefer @code{long double}
9435 to be aligned to an 8 or 16 byte boundary. In arrays or structures
9436 conforming to the ABI, this would not be possible. So specifying a
9437 @option{-m128bit-long-double} will align @code{long double}
9438 to a 16 byte boundary by padding the @code{long double} with an additional
9441 In the x86-64 compiler, @option{-m128bit-long-double} is the default choice as
9442 its ABI specifies that @code{long double} is to be aligned on 16 byte boundary.
9444 Notice that neither of these options enable any extra precision over the x87
9445 standard of 80 bits for a @code{long double}.
9447 @strong{Warning:} if you override the default value for your target ABI, the
9448 structures and arrays containing @code{long double} variables will change
9449 their size as well as function calling convention for function taking
9450 @code{long double} will be modified. Hence they will not be binary
9451 compatible with arrays or structures in code compiled without that switch.
9453 @item -mmlarge-data-threshold=@var{number}
9454 @opindex mlarge-data-threshold=@var{number}
9455 When @option{-mcmodel=medium} is specified, the data greater than
9456 @var{threshold} are placed in large data section. This value must be the
9457 same across all object linked into the binary and defaults to 65535.
9460 @itemx -mno-svr3-shlib
9461 @opindex msvr3-shlib
9462 @opindex mno-svr3-shlib
9463 Control whether GCC places uninitialized local variables into the
9464 @code{bss} or @code{data} segments. @option{-msvr3-shlib} places them
9465 into @code{bss}. These options are meaningful only on System V Release 3.
9469 Use a different function-calling convention, in which functions that
9470 take a fixed number of arguments return with the @code{ret} @var{num}
9471 instruction, which pops their arguments while returning. This saves one
9472 instruction in the caller since there is no need to pop the arguments
9475 You can specify that an individual function is called with this calling
9476 sequence with the function attribute @samp{stdcall}. You can also
9477 override the @option{-mrtd} option by using the function attribute
9478 @samp{cdecl}. @xref{Function Attributes}.
9480 @strong{Warning:} this calling convention is incompatible with the one
9481 normally used on Unix, so you cannot use it if you need to call
9482 libraries compiled with the Unix compiler.
9484 Also, you must provide function prototypes for all functions that
9485 take variable numbers of arguments (including @code{printf});
9486 otherwise incorrect code will be generated for calls to those
9489 In addition, seriously incorrect code will result if you call a
9490 function with too many arguments. (Normally, extra arguments are
9491 harmlessly ignored.)
9493 @item -mregparm=@var{num}
9495 Control how many registers are used to pass integer arguments. By
9496 default, no registers are used to pass arguments, and at most 3
9497 registers can be used. You can control this behavior for a specific
9498 function by using the function attribute @samp{regparm}.
9499 @xref{Function Attributes}.
9501 @strong{Warning:} if you use this switch, and
9502 @var{num} is nonzero, then you must build all modules with the same
9503 value, including any libraries. This includes the system libraries and
9507 @opindex msseregparm
9508 Use SSE register passing conventions for float and double arguments
9509 and return values. You can control this behavior for a specific
9510 function by using the function attribute @samp{sseregparm}.
9511 @xref{Function Attributes}.
9513 @strong{Warning:} if you use this switch then you must build all
9514 modules with the same value, including any libraries. This includes
9515 the system libraries and startup modules.
9517 @item -mstackrealign
9518 @opindex mstackrealign
9519 Realign the stack at entry. On the Intel x86, the
9520 @option{-mstackrealign} option will generate an alternate prologue and
9521 epilogue that realigns the runtime stack. This supports mixing legacy
9522 codes that keep a 4-byte aligned stack with modern codes that keep a
9523 16-byte stack for SSE compatibility. The alternate prologue and
9524 epilogue are slower and bigger than the regular ones, and the
9525 alternate prologue requires an extra scratch register; this lowers the
9526 number of registers available if used in conjunction with the
9527 @code{regparm} attribute. The @option{-mstackrealign} option is
9528 incompatible with the nested function prologue; this is considered a
9529 hard error. See also the attribute @code{force_align_arg_pointer},
9530 applicable to individual functions.
9532 @item -mpreferred-stack-boundary=@var{num}
9533 @opindex mpreferred-stack-boundary
9534 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
9535 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
9536 the default is 4 (16 bytes or 128 bits).
9538 On Pentium and PentiumPro, @code{double} and @code{long double} values
9539 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
9540 suffer significant run time performance penalties. On Pentium III, the
9541 Streaming SIMD Extension (SSE) data type @code{__m128} may not work
9542 properly if it is not 16 byte aligned.
9544 To ensure proper alignment of this values on the stack, the stack boundary
9545 must be as aligned as that required by any value stored on the stack.
9546 Further, every function must be generated such that it keeps the stack
9547 aligned. Thus calling a function compiled with a higher preferred
9548 stack boundary from a function compiled with a lower preferred stack
9549 boundary will most likely misalign the stack. It is recommended that
9550 libraries that use callbacks always use the default setting.
9552 This extra alignment does consume extra stack space, and generally
9553 increases code size. Code that is sensitive to stack space usage, such
9554 as embedded systems and operating system kernels, may want to reduce the
9555 preferred alignment to @option{-mpreferred-stack-boundary=2}.
9573 These switches enable or disable the use of instructions in the MMX,
9574 SSE, SSE2 or 3DNow! extended instruction sets. These extensions are
9575 also available as built-in functions: see @ref{X86 Built-in Functions},
9576 for details of the functions enabled and disabled by these switches.
9578 To have SSE/SSE2 instructions generated automatically from floating-point
9579 code (as opposed to 387 instructions), see @option{-mfpmath=sse}.
9581 These options will enable GCC to use these extended instructions in
9582 generated code, even without @option{-mfpmath=sse}. Applications which
9583 perform runtime CPU detection must compile separate files for each
9584 supported architecture, using the appropriate flags. In particular,
9585 the file containing the CPU detection code should be compiled without
9589 @itemx -mno-push-args
9591 @opindex mno-push-args
9592 Use PUSH operations to store outgoing parameters. This method is shorter
9593 and usually equally fast as method using SUB/MOV operations and is enabled
9594 by default. In some cases disabling it may improve performance because of
9595 improved scheduling and reduced dependencies.
9597 @item -maccumulate-outgoing-args
9598 @opindex maccumulate-outgoing-args
9599 If enabled, the maximum amount of space required for outgoing arguments will be
9600 computed in the function prologue. This is faster on most modern CPUs
9601 because of reduced dependencies, improved scheduling and reduced stack usage
9602 when preferred stack boundary is not equal to 2. The drawback is a notable
9603 increase in code size. This switch implies @option{-mno-push-args}.
9607 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
9608 on thread-safe exception handling must compile and link all code with the
9609 @option{-mthreads} option. When compiling, @option{-mthreads} defines
9610 @option{-D_MT}; when linking, it links in a special thread helper library
9611 @option{-lmingwthrd} which cleans up per thread exception handling data.
9613 @item -mno-align-stringops
9614 @opindex mno-align-stringops
9615 Do not align destination of inlined string operations. This switch reduces
9616 code size and improves performance in case the destination is already aligned,
9617 but GCC doesn't know about it.
9619 @item -minline-all-stringops
9620 @opindex minline-all-stringops
9621 By default GCC inlines string operations only when destination is known to be
9622 aligned at least to 4 byte boundary. This enables more inlining, increase code
9623 size, but may improve performance of code that depends on fast memcpy, strlen
9624 and memset for short lengths.
9626 @item -momit-leaf-frame-pointer
9627 @opindex momit-leaf-frame-pointer
9628 Don't keep the frame pointer in a register for leaf functions. This
9629 avoids the instructions to save, set up and restore frame pointers and
9630 makes an extra register available in leaf functions. The option
9631 @option{-fomit-frame-pointer} removes the frame pointer for all functions
9632 which might make debugging harder.
9634 @item -mtls-direct-seg-refs
9635 @itemx -mno-tls-direct-seg-refs
9636 @opindex mtls-direct-seg-refs
9637 Controls whether TLS variables may be accessed with offsets from the
9638 TLS segment register (@code{%gs} for 32-bit, @code{%fs} for 64-bit),
9639 or whether the thread base pointer must be added. Whether or not this
9640 is legal depends on the operating system, and whether it maps the
9641 segment to cover the entire TLS area.
9643 For systems that use GNU libc, the default is on.
9646 These @samp{-m} switches are supported in addition to the above
9647 on AMD x86-64 processors in 64-bit environments.
9654 Generate code for a 32-bit or 64-bit environment.
9655 The 32-bit environment sets int, long and pointer to 32 bits and
9656 generates code that runs on any i386 system.
9657 The 64-bit environment sets int to 32 bits and long and pointer
9658 to 64 bits and generates code for AMD's x86-64 architecture.
9661 @opindex no-red-zone
9662 Do not use a so called red zone for x86-64 code. The red zone is mandated
9663 by the x86-64 ABI, it is a 128-byte area beyond the location of the
9664 stack pointer that will not be modified by signal or interrupt handlers
9665 and therefore can be used for temporary data without adjusting the stack
9666 pointer. The flag @option{-mno-red-zone} disables this red zone.
9668 @item -mcmodel=small
9669 @opindex mcmodel=small
9670 Generate code for the small code model: the program and its symbols must
9671 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
9672 Programs can be statically or dynamically linked. This is the default
9675 @item -mcmodel=kernel
9676 @opindex mcmodel=kernel
9677 Generate code for the kernel code model. The kernel runs in the
9678 negative 2 GB of the address space.
9679 This model has to be used for Linux kernel code.
9681 @item -mcmodel=medium
9682 @opindex mcmodel=medium
9683 Generate code for the medium model: The program is linked in the lower 2
9684 GB of the address space but symbols can be located anywhere in the
9685 address space. Programs can be statically or dynamically linked, but
9686 building of shared libraries are not supported with the medium model.
9688 @item -mcmodel=large
9689 @opindex mcmodel=large
9690 Generate code for the large model: This model makes no assumptions
9691 about addresses and sizes of sections. Currently GCC does not implement
9696 @subsection IA-64 Options
9697 @cindex IA-64 Options
9699 These are the @samp{-m} options defined for the Intel IA-64 architecture.
9703 @opindex mbig-endian
9704 Generate code for a big endian target. This is the default for HP-UX@.
9706 @item -mlittle-endian
9707 @opindex mlittle-endian
9708 Generate code for a little endian target. This is the default for AIX5
9715 Generate (or don't) code for the GNU assembler. This is the default.
9716 @c Also, this is the default if the configure option @option{--with-gnu-as}
9723 Generate (or don't) code for the GNU linker. This is the default.
9724 @c Also, this is the default if the configure option @option{--with-gnu-ld}
9729 Generate code that does not use a global pointer register. The result
9730 is not position independent code, and violates the IA-64 ABI@.
9732 @item -mvolatile-asm-stop
9733 @itemx -mno-volatile-asm-stop
9734 @opindex mvolatile-asm-stop
9735 @opindex mno-volatile-asm-stop
9736 Generate (or don't) a stop bit immediately before and after volatile asm
9739 @item -mregister-names
9740 @itemx -mno-register-names
9741 @opindex mregister-names
9742 @opindex mno-register-names
9743 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
9744 the stacked registers. This may make assembler output more readable.
9750 Disable (or enable) optimizations that use the small data section. This may
9751 be useful for working around optimizer bugs.
9754 @opindex mconstant-gp
9755 Generate code that uses a single constant global pointer value. This is
9756 useful when compiling kernel code.
9760 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
9761 This is useful when compiling firmware code.
9763 @item -minline-float-divide-min-latency
9764 @opindex minline-float-divide-min-latency
9765 Generate code for inline divides of floating point values
9766 using the minimum latency algorithm.
9768 @item -minline-float-divide-max-throughput
9769 @opindex minline-float-divide-max-throughput
9770 Generate code for inline divides of floating point values
9771 using the maximum throughput algorithm.
9773 @item -minline-int-divide-min-latency
9774 @opindex minline-int-divide-min-latency
9775 Generate code for inline divides of integer values
9776 using the minimum latency algorithm.
9778 @item -minline-int-divide-max-throughput
9779 @opindex minline-int-divide-max-throughput
9780 Generate code for inline divides of integer values
9781 using the maximum throughput algorithm.
9783 @item -minline-sqrt-min-latency
9784 @opindex minline-sqrt-min-latency
9785 Generate code for inline square roots
9786 using the minimum latency algorithm.
9788 @item -minline-sqrt-max-throughput
9789 @opindex minline-sqrt-max-throughput
9790 Generate code for inline square roots
9791 using the maximum throughput algorithm.
9793 @item -mno-dwarf2-asm
9795 @opindex mno-dwarf2-asm
9796 @opindex mdwarf2-asm
9797 Don't (or do) generate assembler code for the DWARF2 line number debugging
9798 info. This may be useful when not using the GNU assembler.
9800 @item -mearly-stop-bits
9801 @itemx -mno-early-stop-bits
9802 @opindex mearly-stop-bits
9803 @opindex mno-early-stop-bits
9804 Allow stop bits to be placed earlier than immediately preceding the
9805 instruction that triggered the stop bit. This can improve instruction
9806 scheduling, but does not always do so.
9808 @item -mfixed-range=@var{register-range}
9809 @opindex mfixed-range
9810 Generate code treating the given register range as fixed registers.
9811 A fixed register is one that the register allocator can not use. This is
9812 useful when compiling kernel code. A register range is specified as
9813 two registers separated by a dash. Multiple register ranges can be
9814 specified separated by a comma.
9816 @item -mtls-size=@var{tls-size}
9818 Specify bit size of immediate TLS offsets. Valid values are 14, 22, and
9821 @item -mtune=@var{cpu-type}
9823 Tune the instruction scheduling for a particular CPU, Valid values are
9824 itanium, itanium1, merced, itanium2, and mckinley.
9830 Add support for multithreading using the POSIX threads library. This
9831 option sets flags for both the preprocessor and linker. It does
9832 not affect the thread safety of object code produced by the compiler or
9833 that of libraries supplied with it. These are HP-UX specific flags.
9839 Generate code for a 32-bit or 64-bit environment.
9840 The 32-bit environment sets int, long and pointer to 32 bits.
9841 The 64-bit environment sets int to 32 bits and long and pointer
9842 to 64 bits. These are HP-UX specific flags.
9844 @item -mno-sched-br-data-spec
9845 @itemx -msched-br-data-spec
9846 @opindex -mno-sched-br-data-spec
9847 @opindex -msched-br-data-spec
9848 (Dis/En)able data speculative scheduling before reload.
9849 This will result in generation of the ld.a instructions and
9850 the corresponding check instructions (ld.c / chk.a).
9851 The default is 'disable'.
9853 @item -msched-ar-data-spec
9854 @itemx -mno-sched-ar-data-spec
9855 @opindex -msched-ar-data-spec
9856 @opindex -mno-sched-ar-data-spec
9857 (En/Dis)able data speculative scheduling after reload.
9858 This will result in generation of the ld.a instructions and
9859 the corresponding check instructions (ld.c / chk.a).
9860 The default is 'enable'.
9862 @item -mno-sched-control-spec
9863 @itemx -msched-control-spec
9864 @opindex -mno-sched-control-spec
9865 @opindex -msched-control-spec
9866 (Dis/En)able control speculative scheduling. This feature is
9867 available only during region scheduling (i.e. before reload).
9868 This will result in generation of the ld.s instructions and
9869 the corresponding check instructions chk.s .
9870 The default is 'disable'.
9872 @item -msched-br-in-data-spec
9873 @itemx -mno-sched-br-in-data-spec
9874 @opindex -msched-br-in-data-spec
9875 @opindex -mno-sched-br-in-data-spec
9876 (En/Dis)able speculative scheduling of the instructions that
9877 are dependent on the data speculative loads before reload.
9878 This is effective only with @option{-msched-br-data-spec} enabled.
9879 The default is 'enable'.
9881 @item -msched-ar-in-data-spec
9882 @itemx -mno-sched-ar-in-data-spec
9883 @opindex -msched-ar-in-data-spec
9884 @opindex -mno-sched-ar-in-data-spec
9885 (En/Dis)able speculative scheduling of the instructions that
9886 are dependent on the data speculative loads after reload.
9887 This is effective only with @option{-msched-ar-data-spec} enabled.
9888 The default is 'enable'.
9890 @item -msched-in-control-spec
9891 @itemx -mno-sched-in-control-spec
9892 @opindex -msched-in-control-spec
9893 @opindex -mno-sched-in-control-spec
9894 (En/Dis)able speculative scheduling of the instructions that
9895 are dependent on the control speculative loads.
9896 This is effective only with @option{-msched-control-spec} enabled.
9897 The default is 'enable'.
9900 @itemx -mno-sched-ldc
9901 @opindex -msched-ldc
9902 @opindex -mno-sched-ldc
9903 (En/Dis)able use of simple data speculation checks ld.c .
9904 If disabled, only chk.a instructions will be emitted to check
9905 data speculative loads.
9906 The default is 'enable'.
9908 @item -mno-sched-control-ldc
9909 @itemx -msched-control-ldc
9910 @opindex -mno-sched-control-ldc
9911 @opindex -msched-control-ldc
9912 (Dis/En)able use of ld.c instructions to check control speculative loads.
9913 If enabled, in case of control speculative load with no speculatively
9914 scheduled dependent instructions this load will be emitted as ld.sa and
9915 ld.c will be used to check it.
9916 The default is 'disable'.
9918 @item -mno-sched-spec-verbose
9919 @itemx -msched-spec-verbose
9920 @opindex -mno-sched-spec-verbose
9921 @opindex -msched-spec-verbose
9922 (Dis/En)able printing of the information about speculative motions.
9924 @item -mno-sched-prefer-non-data-spec-insns
9925 @itemx -msched-prefer-non-data-spec-insns
9926 @opindex -mno-sched-prefer-non-data-spec-insns
9927 @opindex -msched-prefer-non-data-spec-insns
9928 If enabled, data speculative instructions will be chosen for schedule
9929 only if there are no other choices at the moment. This will make
9930 the use of the data speculation much more conservative.
9931 The default is 'disable'.
9933 @item -mno-sched-prefer-non-control-spec-insns
9934 @itemx -msched-prefer-non-control-spec-insns
9935 @opindex -mno-sched-prefer-non-control-spec-insns
9936 @opindex -msched-prefer-non-control-spec-insns
9937 If enabled, control speculative instructions will be chosen for schedule
9938 only if there are no other choices at the moment. This will make
9939 the use of the control speculation much more conservative.
9940 The default is 'disable'.
9942 @item -mno-sched-count-spec-in-critical-path
9943 @itemx -msched-count-spec-in-critical-path
9944 @opindex -mno-sched-count-spec-in-critical-path
9945 @opindex -msched-count-spec-in-critical-path
9946 If enabled, speculative dependencies will be considered during
9947 computation of the instructions priorities. This will make the use of the
9948 speculation a bit more conservative.
9949 The default is 'disable'.
9954 @subsection M32C Options
9955 @cindex M32C options
9958 @item -mcpu=@var{name}
9960 Select the CPU for which code is generated. @var{name} may be one of
9961 @samp{r8c} for the R8C/Tiny series, @samp{m16c} for the M16C (up to
9962 /60) series, @samp{m32cm} for the M16C/80 series, or @samp{m32c} for
9967 Specifies that the program will be run on the simulator. This causes
9968 an alternate runtime library to be linked in which supports, for
9969 example, file I/O. You must not use this option when generating
9970 programs that will run on real hardware; you must provide your own
9971 runtime library for whatever I/O functions are needed.
9973 @item -memregs=@var{number}
9975 Specifies the number of memory-based pseudo-registers GCC will use
9976 during code generation. These pseudo-registers will be used like real
9977 registers, so there is a tradeoff between GCC's ability to fit the
9978 code into available registers, and the performance penalty of using
9979 memory instead of registers. Note that all modules in a program must
9980 be compiled with the same value for this option. Because of that, you
9981 must not use this option with the default runtime libraries gcc
9986 @node M32R/D Options
9987 @subsection M32R/D Options
9988 @cindex M32R/D options
9990 These @option{-m} options are defined for Renesas M32R/D architectures:
9995 Generate code for the M32R/2@.
9999 Generate code for the M32R/X@.
10003 Generate code for the M32R@. This is the default.
10005 @item -mmodel=small
10006 @opindex mmodel=small
10007 Assume all objects live in the lower 16MB of memory (so that their addresses
10008 can be loaded with the @code{ld24} instruction), and assume all subroutines
10009 are reachable with the @code{bl} instruction.
10010 This is the default.
10012 The addressability of a particular object can be set with the
10013 @code{model} attribute.
10015 @item -mmodel=medium
10016 @opindex mmodel=medium
10017 Assume objects may be anywhere in the 32-bit address space (the compiler
10018 will generate @code{seth/add3} instructions to load their addresses), and
10019 assume all subroutines are reachable with the @code{bl} instruction.
10021 @item -mmodel=large
10022 @opindex mmodel=large
10023 Assume objects may be anywhere in the 32-bit address space (the compiler
10024 will generate @code{seth/add3} instructions to load their addresses), and
10025 assume subroutines may not be reachable with the @code{bl} instruction
10026 (the compiler will generate the much slower @code{seth/add3/jl}
10027 instruction sequence).
10030 @opindex msdata=none
10031 Disable use of the small data area. Variables will be put into
10032 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
10033 @code{section} attribute has been specified).
10034 This is the default.
10036 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
10037 Objects may be explicitly put in the small data area with the
10038 @code{section} attribute using one of these sections.
10040 @item -msdata=sdata
10041 @opindex msdata=sdata
10042 Put small global and static data in the small data area, but do not
10043 generate special code to reference them.
10046 @opindex msdata=use
10047 Put small global and static data in the small data area, and generate
10048 special instructions to reference them.
10052 @cindex smaller data references
10053 Put global and static objects less than or equal to @var{num} bytes
10054 into the small data or bss sections instead of the normal data or bss
10055 sections. The default value of @var{num} is 8.
10056 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
10057 for this option to have any effect.
10059 All modules should be compiled with the same @option{-G @var{num}} value.
10060 Compiling with different values of @var{num} may or may not work; if it
10061 doesn't the linker will give an error message---incorrect code will not be
10066 Makes the M32R specific code in the compiler display some statistics
10067 that might help in debugging programs.
10069 @item -malign-loops
10070 @opindex malign-loops
10071 Align all loops to a 32-byte boundary.
10073 @item -mno-align-loops
10074 @opindex mno-align-loops
10075 Do not enforce a 32-byte alignment for loops. This is the default.
10077 @item -missue-rate=@var{number}
10078 @opindex missue-rate=@var{number}
10079 Issue @var{number} instructions per cycle. @var{number} can only be 1
10082 @item -mbranch-cost=@var{number}
10083 @opindex mbranch-cost=@var{number}
10084 @var{number} can only be 1 or 2. If it is 1 then branches will be
10085 preferred over conditional code, if it is 2, then the opposite will
10088 @item -mflush-trap=@var{number}
10089 @opindex mflush-trap=@var{number}
10090 Specifies the trap number to use to flush the cache. The default is
10091 12. Valid numbers are between 0 and 15 inclusive.
10093 @item -mno-flush-trap
10094 @opindex mno-flush-trap
10095 Specifies that the cache cannot be flushed by using a trap.
10097 @item -mflush-func=@var{name}
10098 @opindex mflush-func=@var{name}
10099 Specifies the name of the operating system function to call to flush
10100 the cache. The default is @emph{_flush_cache}, but a function call
10101 will only be used if a trap is not available.
10103 @item -mno-flush-func
10104 @opindex mno-flush-func
10105 Indicates that there is no OS function for flushing the cache.
10109 @node M680x0 Options
10110 @subsection M680x0 Options
10111 @cindex M680x0 options
10113 These are the @samp{-m} options defined for the 68000 series. The default
10114 values for these options depends on which style of 68000 was selected when
10115 the compiler was configured; the defaults for the most common choices are
10123 Generate output for a 68000. This is the default
10124 when the compiler is configured for 68000-based systems.
10126 Use this option for microcontrollers with a 68000 or EC000 core,
10127 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
10133 Generate output for a 68020. This is the default
10134 when the compiler is configured for 68020-based systems.
10138 Generate output containing 68881 instructions for floating point.
10139 This is the default for most 68020 systems unless @option{--nfp} was
10140 specified when the compiler was configured.
10144 Generate output for a 68030. This is the default when the compiler is
10145 configured for 68030-based systems.
10149 Generate output for a 68040. This is the default when the compiler is
10150 configured for 68040-based systems.
10152 This option inhibits the use of 68881/68882 instructions that have to be
10153 emulated by software on the 68040. Use this option if your 68040 does not
10154 have code to emulate those instructions.
10158 Generate output for a 68060. This is the default when the compiler is
10159 configured for 68060-based systems.
10161 This option inhibits the use of 68020 and 68881/68882 instructions that
10162 have to be emulated by software on the 68060. Use this option if your 68060
10163 does not have code to emulate those instructions.
10167 Generate output for a CPU32. This is the default
10168 when the compiler is configured for CPU32-based systems.
10170 Use this option for microcontrollers with a
10171 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
10172 68336, 68340, 68341, 68349 and 68360.
10176 Generate output for a 520X ``coldfire'' family cpu. This is the default
10177 when the compiler is configured for 520X-based systems.
10179 Use this option for microcontroller with a 5200 core, including
10180 the MCF5202, MCF5203, MCF5204 and MCF5202.
10184 Generate output for a ColdFire V4e family cpu (e.g.@: 547x/548x).
10185 This includes use of hardware floating point instructions.
10189 Generate output for a 68040, without using any of the new instructions.
10190 This results in code which can run relatively efficiently on either a
10191 68020/68881 or a 68030 or a 68040. The generated code does use the
10192 68881 instructions that are emulated on the 68040.
10196 Generate output for a 68060, without using any of the new instructions.
10197 This results in code which can run relatively efficiently on either a
10198 68020/68881 or a 68030 or a 68040. The generated code does use the
10199 68881 instructions that are emulated on the 68060.
10202 @opindex msoft-float
10203 Generate output containing library calls for floating point.
10204 @strong{Warning:} the requisite libraries are not available for all m68k
10205 targets. Normally the facilities of the machine's usual C compiler are
10206 used, but this can't be done directly in cross-compilation. You must
10207 make your own arrangements to provide suitable library functions for
10208 cross-compilation. The embedded targets @samp{m68k-*-aout} and
10209 @samp{m68k-*-coff} do provide software floating point support.
10213 Consider type @code{int} to be 16 bits wide, like @code{short int}.
10214 Additionally, parameters passed on the stack are also aligned to a
10215 16-bit boundary even on targets whose API mandates promotion to 32-bit.
10218 @opindex mnobitfield
10219 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
10220 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
10224 Do use the bit-field instructions. The @option{-m68020} option implies
10225 @option{-mbitfield}. This is the default if you use a configuration
10226 designed for a 68020.
10230 Use a different function-calling convention, in which functions
10231 that take a fixed number of arguments return with the @code{rtd}
10232 instruction, which pops their arguments while returning. This
10233 saves one instruction in the caller since there is no need to pop
10234 the arguments there.
10236 This calling convention is incompatible with the one normally
10237 used on Unix, so you cannot use it if you need to call libraries
10238 compiled with the Unix compiler.
10240 Also, you must provide function prototypes for all functions that
10241 take variable numbers of arguments (including @code{printf});
10242 otherwise incorrect code will be generated for calls to those
10245 In addition, seriously incorrect code will result if you call a
10246 function with too many arguments. (Normally, extra arguments are
10247 harmlessly ignored.)
10249 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
10250 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
10253 @itemx -mno-align-int
10254 @opindex malign-int
10255 @opindex mno-align-int
10256 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
10257 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
10258 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
10259 Aligning variables on 32-bit boundaries produces code that runs somewhat
10260 faster on processors with 32-bit busses at the expense of more memory.
10262 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
10263 align structures containing the above types differently than
10264 most published application binary interface specifications for the m68k.
10268 Use the pc-relative addressing mode of the 68000 directly, instead of
10269 using a global offset table. At present, this option implies @option{-fpic},
10270 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
10271 not presently supported with @option{-mpcrel}, though this could be supported for
10272 68020 and higher processors.
10274 @item -mno-strict-align
10275 @itemx -mstrict-align
10276 @opindex mno-strict-align
10277 @opindex mstrict-align
10278 Do not (do) assume that unaligned memory references will be handled by
10282 Generate code that allows the data segment to be located in a different
10283 area of memory from the text segment. This allows for execute in place in
10284 an environment without virtual memory management. This option implies
10287 @item -mno-sep-data
10288 Generate code that assumes that the data segment follows the text segment.
10289 This is the default.
10291 @item -mid-shared-library
10292 Generate code that supports shared libraries via the library ID method.
10293 This allows for execute in place and shared libraries in an environment
10294 without virtual memory management. This option implies @option{-fPIC}.
10296 @item -mno-id-shared-library
10297 Generate code that doesn't assume ID based shared libraries are being used.
10298 This is the default.
10300 @item -mshared-library-id=n
10301 Specified the identification number of the ID based shared library being
10302 compiled. Specifying a value of 0 will generate more compact code, specifying
10303 other values will force the allocation of that number to the current
10304 library but is no more space or time efficient than omitting this option.
10308 @node M68hc1x Options
10309 @subsection M68hc1x Options
10310 @cindex M68hc1x options
10312 These are the @samp{-m} options defined for the 68hc11 and 68hc12
10313 microcontrollers. The default values for these options depends on
10314 which style of microcontroller was selected when the compiler was configured;
10315 the defaults for the most common choices are given below.
10322 Generate output for a 68HC11. This is the default
10323 when the compiler is configured for 68HC11-based systems.
10329 Generate output for a 68HC12. This is the default
10330 when the compiler is configured for 68HC12-based systems.
10336 Generate output for a 68HCS12.
10338 @item -mauto-incdec
10339 @opindex mauto-incdec
10340 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
10347 Enable the use of 68HC12 min and max instructions.
10350 @itemx -mno-long-calls
10351 @opindex mlong-calls
10352 @opindex mno-long-calls
10353 Treat all calls as being far away (near). If calls are assumed to be
10354 far away, the compiler will use the @code{call} instruction to
10355 call a function and the @code{rtc} instruction for returning.
10359 Consider type @code{int} to be 16 bits wide, like @code{short int}.
10361 @item -msoft-reg-count=@var{count}
10362 @opindex msoft-reg-count
10363 Specify the number of pseudo-soft registers which are used for the
10364 code generation. The maximum number is 32. Using more pseudo-soft
10365 register may or may not result in better code depending on the program.
10366 The default is 4 for 68HC11 and 2 for 68HC12.
10370 @node MCore Options
10371 @subsection MCore Options
10372 @cindex MCore options
10374 These are the @samp{-m} options defined for the Motorola M*Core
10380 @itemx -mno-hardlit
10382 @opindex mno-hardlit
10383 Inline constants into the code stream if it can be done in two
10384 instructions or less.
10390 Use the divide instruction. (Enabled by default).
10392 @item -mrelax-immediate
10393 @itemx -mno-relax-immediate
10394 @opindex mrelax-immediate
10395 @opindex mno-relax-immediate
10396 Allow arbitrary sized immediates in bit operations.
10398 @item -mwide-bitfields
10399 @itemx -mno-wide-bitfields
10400 @opindex mwide-bitfields
10401 @opindex mno-wide-bitfields
10402 Always treat bit-fields as int-sized.
10404 @item -m4byte-functions
10405 @itemx -mno-4byte-functions
10406 @opindex m4byte-functions
10407 @opindex mno-4byte-functions
10408 Force all functions to be aligned to a four byte boundary.
10410 @item -mcallgraph-data
10411 @itemx -mno-callgraph-data
10412 @opindex mcallgraph-data
10413 @opindex mno-callgraph-data
10414 Emit callgraph information.
10417 @itemx -mno-slow-bytes
10418 @opindex mslow-bytes
10419 @opindex mno-slow-bytes
10420 Prefer word access when reading byte quantities.
10422 @item -mlittle-endian
10423 @itemx -mbig-endian
10424 @opindex mlittle-endian
10425 @opindex mbig-endian
10426 Generate code for a little endian target.
10432 Generate code for the 210 processor.
10436 @subsection MIPS Options
10437 @cindex MIPS options
10443 Generate big-endian code.
10447 Generate little-endian code. This is the default for @samp{mips*el-*-*}
10450 @item -march=@var{arch}
10452 Generate code that will run on @var{arch}, which can be the name of a
10453 generic MIPS ISA, or the name of a particular processor.
10455 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
10456 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
10457 The processor names are:
10458 @samp{4kc}, @samp{4km}, @samp{4kp},
10459 @samp{5kc}, @samp{5kf},
10461 @samp{24k}, @samp{24kc}, @samp{24kf}, @samp{24kx},
10464 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
10465 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000},
10466 @samp{rm7000}, @samp{rm9000},
10469 @samp{vr4100}, @samp{vr4111}, @samp{vr4120}, @samp{vr4130}, @samp{vr4300},
10470 @samp{vr5000}, @samp{vr5400} and @samp{vr5500}.
10471 The special value @samp{from-abi} selects the
10472 most compatible architecture for the selected ABI (that is,
10473 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
10475 In processor names, a final @samp{000} can be abbreviated as @samp{k}
10476 (for example, @samp{-march=r2k}). Prefixes are optional, and
10477 @samp{vr} may be written @samp{r}.
10479 GCC defines two macros based on the value of this option. The first
10480 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
10481 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
10482 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
10483 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
10484 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
10486 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
10487 above. In other words, it will have the full prefix and will not
10488 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
10489 the macro names the resolved architecture (either @samp{"mips1"} or
10490 @samp{"mips3"}). It names the default architecture when no
10491 @option{-march} option is given.
10493 @item -mtune=@var{arch}
10495 Optimize for @var{arch}. Among other things, this option controls
10496 the way instructions are scheduled, and the perceived cost of arithmetic
10497 operations. The list of @var{arch} values is the same as for
10500 When this option is not used, GCC will optimize for the processor
10501 specified by @option{-march}. By using @option{-march} and
10502 @option{-mtune} together, it is possible to generate code that will
10503 run on a family of processors, but optimize the code for one
10504 particular member of that family.
10506 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
10507 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
10508 @samp{-march} ones described above.
10512 Equivalent to @samp{-march=mips1}.
10516 Equivalent to @samp{-march=mips2}.
10520 Equivalent to @samp{-march=mips3}.
10524 Equivalent to @samp{-march=mips4}.
10528 Equivalent to @samp{-march=mips32}.
10532 Equivalent to @samp{-march=mips32r2}.
10536 Equivalent to @samp{-march=mips64}.
10541 @opindex mno-mips16
10542 Generate (do not generate) MIPS16 code. If GCC is targetting a
10543 MIPS32 or MIPS64 architecture, it will make use of the MIPS16e ASE@.
10555 Generate code for the given ABI@.
10557 Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
10558 generates 64-bit code when you select a 64-bit architecture, but you
10559 can use @option{-mgp32} to get 32-bit code instead.
10561 For information about the O64 ABI, see
10562 @w{@uref{http://gcc.gnu.org/projects/mipso64-abi.html}}.
10565 @itemx -mno-abicalls
10567 @opindex mno-abicalls
10568 Generate (do not generate) code that is suitable for SVR4-style
10569 dynamic objects. @option{-mabicalls} is the default for SVR4-based
10574 Generate (do not generate) code that is fully position-independent,
10575 and that can therefore be linked into shared libraries. This option
10576 only affects @option{-mabicalls}.
10578 All @option{-mabicalls} code has traditionally been position-independent,
10579 regardless of options like @option{-fPIC} and @option{-fpic}. However,
10580 as an extension, the GNU toolchain allows executables to use absolute
10581 accesses for locally-binding symbols. It can also use shorter GP
10582 initialization sequences and generate direct calls to locally-defined
10583 functions. This mode is selected by @option{-mno-shared}.
10585 @option{-mno-shared} depends on binutils 2.16 or higher and generates
10586 objects that can only be linked by the GNU linker. However, the option
10587 does not affect the ABI of the final executable; it only affects the ABI
10588 of relocatable objects. Using @option{-mno-shared} will generally make
10589 executables both smaller and quicker.
10591 @option{-mshared} is the default.
10597 Lift (do not lift) the usual restrictions on the size of the global
10600 GCC normally uses a single instruction to load values from the GOT@.
10601 While this is relatively efficient, it will only work if the GOT
10602 is smaller than about 64k. Anything larger will cause the linker
10603 to report an error such as:
10605 @cindex relocation truncated to fit (MIPS)
10607 relocation truncated to fit: R_MIPS_GOT16 foobar
10610 If this happens, you should recompile your code with @option{-mxgot}.
10611 It should then work with very large GOTs, although it will also be
10612 less efficient, since it will take three instructions to fetch the
10613 value of a global symbol.
10615 Note that some linkers can create multiple GOTs. If you have such a
10616 linker, you should only need to use @option{-mxgot} when a single object
10617 file accesses more than 64k's worth of GOT entries. Very few do.
10619 These options have no effect unless GCC is generating position
10624 Assume that general-purpose registers are 32 bits wide.
10628 Assume that general-purpose registers are 64 bits wide.
10632 Assume that floating-point registers are 32 bits wide.
10636 Assume that floating-point registers are 64 bits wide.
10639 @opindex mhard-float
10640 Use floating-point coprocessor instructions.
10643 @opindex msoft-float
10644 Do not use floating-point coprocessor instructions. Implement
10645 floating-point calculations using library calls instead.
10647 @item -msingle-float
10648 @opindex msingle-float
10649 Assume that the floating-point coprocessor only supports single-precision
10652 @itemx -mdouble-float
10653 @opindex mdouble-float
10654 Assume that the floating-point coprocessor supports double-precision
10655 operations. This is the default.
10661 Use (do not use) the MIPS DSP ASE. @xref{MIPS DSP Built-in Functions}.
10663 @itemx -mpaired-single
10664 @itemx -mno-paired-single
10665 @opindex mpaired-single
10666 @opindex mno-paired-single
10667 Use (do not use) paired-single floating-point instructions.
10668 @xref{MIPS Paired-Single Support}. This option can only be used
10669 when generating 64-bit code and requires hardware floating-point
10670 support to be enabled.
10675 @opindex mno-mips3d
10676 Use (do not use) the MIPS-3D ASE@. @xref{MIPS-3D Built-in Functions}.
10677 The option @option{-mips3d} implies @option{-mpaired-single}.
10681 Force @code{long} types to be 64 bits wide. See @option{-mlong32} for
10682 an explanation of the default and the way that the pointer size is
10687 Force @code{long}, @code{int}, and pointer types to be 32 bits wide.
10689 The default size of @code{int}s, @code{long}s and pointers depends on
10690 the ABI@. All the supported ABIs use 32-bit @code{int}s. The n64 ABI
10691 uses 64-bit @code{long}s, as does the 64-bit EABI; the others use
10692 32-bit @code{long}s. Pointers are the same size as @code{long}s,
10693 or the same size as integer registers, whichever is smaller.
10699 Assume (do not assume) that all symbols have 32-bit values, regardless
10700 of the selected ABI@. This option is useful in combination with
10701 @option{-mabi=64} and @option{-mno-abicalls} because it allows GCC
10702 to generate shorter and faster references to symbolic addresses.
10706 @cindex smaller data references (MIPS)
10707 @cindex gp-relative references (MIPS)
10708 Put global and static items less than or equal to @var{num} bytes into
10709 the small data or bss section instead of the normal data or bss section.
10710 This allows the data to be accessed using a single instruction.
10712 All modules should be compiled with the same @option{-G @var{num}}
10715 @item -membedded-data
10716 @itemx -mno-embedded-data
10717 @opindex membedded-data
10718 @opindex mno-embedded-data
10719 Allocate variables to the read-only data section first if possible, then
10720 next in the small data section if possible, otherwise in data. This gives
10721 slightly slower code than the default, but reduces the amount of RAM required
10722 when executing, and thus may be preferred for some embedded systems.
10724 @item -muninit-const-in-rodata
10725 @itemx -mno-uninit-const-in-rodata
10726 @opindex muninit-const-in-rodata
10727 @opindex mno-uninit-const-in-rodata
10728 Put uninitialized @code{const} variables in the read-only data section.
10729 This option is only meaningful in conjunction with @option{-membedded-data}.
10731 @item -msplit-addresses
10732 @itemx -mno-split-addresses
10733 @opindex msplit-addresses
10734 @opindex mno-split-addresses
10735 Enable (disable) use of the @code{%hi()} and @code{%lo()} assembler
10736 relocation operators. This option has been superseded by
10737 @option{-mexplicit-relocs} but is retained for backwards compatibility.
10739 @item -mexplicit-relocs
10740 @itemx -mno-explicit-relocs
10741 @opindex mexplicit-relocs
10742 @opindex mno-explicit-relocs
10743 Use (do not use) assembler relocation operators when dealing with symbolic
10744 addresses. The alternative, selected by @option{-mno-explicit-relocs},
10745 is to use assembler macros instead.
10747 @option{-mexplicit-relocs} is the default if GCC was configured
10748 to use an assembler that supports relocation operators.
10750 @item -mcheck-zero-division
10751 @itemx -mno-check-zero-division
10752 @opindex mcheck-zero-division
10753 @opindex mno-check-zero-division
10754 Trap (do not trap) on integer division by zero. The default is
10755 @option{-mcheck-zero-division}.
10757 @item -mdivide-traps
10758 @itemx -mdivide-breaks
10759 @opindex mdivide-traps
10760 @opindex mdivide-breaks
10761 MIPS systems check for division by zero by generating either a
10762 conditional trap or a break instruction. Using traps results in
10763 smaller code, but is only supported on MIPS II and later. Also, some
10764 versions of the Linux kernel have a bug that prevents trap from
10765 generating the proper signal (@code{SIGFPE}). Use @option{-mdivide-traps} to
10766 allow conditional traps on architectures that support them and
10767 @option{-mdivide-breaks} to force the use of breaks.
10769 The default is usually @option{-mdivide-traps}, but this can be
10770 overridden at configure time using @option{--with-divide=breaks}.
10771 Divide-by-zero checks can be completely disabled using
10772 @option{-mno-check-zero-division}.
10777 @opindex mno-memcpy
10778 Force (do not force) the use of @code{memcpy()} for non-trivial block
10779 moves. The default is @option{-mno-memcpy}, which allows GCC to inline
10780 most constant-sized copies.
10783 @itemx -mno-long-calls
10784 @opindex mlong-calls
10785 @opindex mno-long-calls
10786 Disable (do not disable) use of the @code{jal} instruction. Calling
10787 functions using @code{jal} is more efficient but requires the caller
10788 and callee to be in the same 256 megabyte segment.
10790 This option has no effect on abicalls code. The default is
10791 @option{-mno-long-calls}.
10797 Enable (disable) use of the @code{mad}, @code{madu} and @code{mul}
10798 instructions, as provided by the R4650 ISA@.
10801 @itemx -mno-fused-madd
10802 @opindex mfused-madd
10803 @opindex mno-fused-madd
10804 Enable (disable) use of the floating point multiply-accumulate
10805 instructions, when they are available. The default is
10806 @option{-mfused-madd}.
10808 When multiply-accumulate instructions are used, the intermediate
10809 product is calculated to infinite precision and is not subject to
10810 the FCSR Flush to Zero bit. This may be undesirable in some
10815 Tell the MIPS assembler to not run its preprocessor over user
10816 assembler files (with a @samp{.s} suffix) when assembling them.
10819 @itemx -mno-fix-r4000
10820 @opindex mfix-r4000
10821 @opindex mno-fix-r4000
10822 Work around certain R4000 CPU errata:
10825 A double-word or a variable shift may give an incorrect result if executed
10826 immediately after starting an integer division.
10828 A double-word or a variable shift may give an incorrect result if executed
10829 while an integer multiplication is in progress.
10831 An integer division may give an incorrect result if started in a delay slot
10832 of a taken branch or a jump.
10836 @itemx -mno-fix-r4400
10837 @opindex mfix-r4400
10838 @opindex mno-fix-r4400
10839 Work around certain R4400 CPU errata:
10842 A double-word or a variable shift may give an incorrect result if executed
10843 immediately after starting an integer division.
10847 @itemx -mno-fix-vr4120
10848 @opindex mfix-vr4120
10849 Work around certain VR4120 errata:
10852 @code{dmultu} does not always produce the correct result.
10854 @code{div} and @code{ddiv} do not always produce the correct result if one
10855 of the operands is negative.
10857 The workarounds for the division errata rely on special functions in
10858 @file{libgcc.a}. At present, these functions are only provided by
10859 the @code{mips64vr*-elf} configurations.
10861 Other VR4120 errata require a nop to be inserted between certain pairs of
10862 instructions. These errata are handled by the assembler, not by GCC itself.
10865 @opindex mfix-vr4130
10866 Work around the VR4130 @code{mflo}/@code{mfhi} errata. The
10867 workarounds are implemented by the assembler rather than by GCC,
10868 although GCC will avoid using @code{mflo} and @code{mfhi} if the
10869 VR4130 @code{macc}, @code{macchi}, @code{dmacc} and @code{dmacchi}
10870 instructions are available instead.
10873 @itemx -mno-fix-sb1
10875 Work around certain SB-1 CPU core errata.
10876 (This flag currently works around the SB-1 revision 2
10877 ``F1'' and ``F2'' floating point errata.)
10879 @item -mflush-func=@var{func}
10880 @itemx -mno-flush-func
10881 @opindex mflush-func
10882 Specifies the function to call to flush the I and D caches, or to not
10883 call any such function. If called, the function must take the same
10884 arguments as the common @code{_flush_func()}, that is, the address of the
10885 memory range for which the cache is being flushed, the size of the
10886 memory range, and the number 3 (to flush both caches). The default
10887 depends on the target GCC was configured for, but commonly is either
10888 @samp{_flush_func} or @samp{__cpu_flush}.
10890 @item -mbranch-likely
10891 @itemx -mno-branch-likely
10892 @opindex mbranch-likely
10893 @opindex mno-branch-likely
10894 Enable or disable use of Branch Likely instructions, regardless of the
10895 default for the selected architecture. By default, Branch Likely
10896 instructions may be generated if they are supported by the selected
10897 architecture. An exception is for the MIPS32 and MIPS64 architectures
10898 and processors which implement those architectures; for those, Branch
10899 Likely instructions will not be generated by default because the MIPS32
10900 and MIPS64 architectures specifically deprecate their use.
10902 @item -mfp-exceptions
10903 @itemx -mno-fp-exceptions
10904 @opindex mfp-exceptions
10905 Specifies whether FP exceptions are enabled. This affects how we schedule
10906 FP instructions for some processors. The default is that FP exceptions are
10909 For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
10910 64-bit code, then we can use both FP pipes. Otherwise, we can only use one
10913 @item -mvr4130-align
10914 @itemx -mno-vr4130-align
10915 @opindex mvr4130-align
10916 The VR4130 pipeline is two-way superscalar, but can only issue two
10917 instructions together if the first one is 8-byte aligned. When this
10918 option is enabled, GCC will align pairs of instructions that it
10919 thinks should execute in parallel.
10921 This option only has an effect when optimizing for the VR4130.
10922 It normally makes code faster, but at the expense of making it bigger.
10923 It is enabled by default at optimization level @option{-O3}.
10927 @subsection MMIX Options
10928 @cindex MMIX Options
10930 These options are defined for the MMIX:
10934 @itemx -mno-libfuncs
10936 @opindex mno-libfuncs
10937 Specify that intrinsic library functions are being compiled, passing all
10938 values in registers, no matter the size.
10941 @itemx -mno-epsilon
10943 @opindex mno-epsilon
10944 Generate floating-point comparison instructions that compare with respect
10945 to the @code{rE} epsilon register.
10947 @item -mabi=mmixware
10949 @opindex mabi-mmixware
10951 Generate code that passes function parameters and return values that (in
10952 the called function) are seen as registers @code{$0} and up, as opposed to
10953 the GNU ABI which uses global registers @code{$231} and up.
10955 @item -mzero-extend
10956 @itemx -mno-zero-extend
10957 @opindex mzero-extend
10958 @opindex mno-zero-extend
10959 When reading data from memory in sizes shorter than 64 bits, use (do not
10960 use) zero-extending load instructions by default, rather than
10961 sign-extending ones.
10964 @itemx -mno-knuthdiv
10966 @opindex mno-knuthdiv
10967 Make the result of a division yielding a remainder have the same sign as
10968 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
10969 remainder follows the sign of the dividend. Both methods are
10970 arithmetically valid, the latter being almost exclusively used.
10972 @item -mtoplevel-symbols
10973 @itemx -mno-toplevel-symbols
10974 @opindex mtoplevel-symbols
10975 @opindex mno-toplevel-symbols
10976 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
10977 code can be used with the @code{PREFIX} assembly directive.
10981 Generate an executable in the ELF format, rather than the default
10982 @samp{mmo} format used by the @command{mmix} simulator.
10984 @item -mbranch-predict
10985 @itemx -mno-branch-predict
10986 @opindex mbranch-predict
10987 @opindex mno-branch-predict
10988 Use (do not use) the probable-branch instructions, when static branch
10989 prediction indicates a probable branch.
10991 @item -mbase-addresses
10992 @itemx -mno-base-addresses
10993 @opindex mbase-addresses
10994 @opindex mno-base-addresses
10995 Generate (do not generate) code that uses @emph{base addresses}. Using a
10996 base address automatically generates a request (handled by the assembler
10997 and the linker) for a constant to be set up in a global register. The
10998 register is used for one or more base address requests within the range 0
10999 to 255 from the value held in the register. The generally leads to short
11000 and fast code, but the number of different data items that can be
11001 addressed is limited. This means that a program that uses lots of static
11002 data may require @option{-mno-base-addresses}.
11004 @item -msingle-exit
11005 @itemx -mno-single-exit
11006 @opindex msingle-exit
11007 @opindex mno-single-exit
11008 Force (do not force) generated code to have a single exit point in each
11012 @node MN10300 Options
11013 @subsection MN10300 Options
11014 @cindex MN10300 options
11016 These @option{-m} options are defined for Matsushita MN10300 architectures:
11021 Generate code to avoid bugs in the multiply instructions for the MN10300
11022 processors. This is the default.
11024 @item -mno-mult-bug
11025 @opindex mno-mult-bug
11026 Do not generate code to avoid bugs in the multiply instructions for the
11027 MN10300 processors.
11031 Generate code which uses features specific to the AM33 processor.
11035 Do not generate code which uses features specific to the AM33 processor. This
11038 @item -mreturn-pointer-on-d0
11039 @opindex mreturn-pointer-on-d0
11040 When generating a function which returns a pointer, return the pointer
11041 in both @code{a0} and @code{d0}. Otherwise, the pointer is returned
11042 only in a0, and attempts to call such functions without a prototype
11043 would result in errors. Note that this option is on by default; use
11044 @option{-mno-return-pointer-on-d0} to disable it.
11048 Do not link in the C run-time initialization object file.
11052 Indicate to the linker that it should perform a relaxation optimization pass
11053 to shorten branches, calls and absolute memory addresses. This option only
11054 has an effect when used on the command line for the final link step.
11056 This option makes symbolic debugging impossible.
11060 @subsection MT Options
11063 These @option{-m} options are defined for Morpho MT architectures:
11067 @item -march=@var{cpu-type}
11069 Generate code that will run on @var{cpu-type}, which is the name of a system
11070 representing a certain processor type. Possible values for
11071 @var{cpu-type} are @samp{ms1-64-001}, @samp{ms1-16-002},
11072 @samp{ms1-16-003} and @samp{ms2}.
11074 When this option is not used, the default is @option{-march=ms1-16-002}.
11078 Use byte loads and stores when generating code.
11082 Do not use byte loads and stores when generating code.
11086 Use simulator runtime
11090 Do not link in the C run-time initialization object file
11091 @file{crti.o}. Other run-time initialization and termination files
11092 such as @file{startup.o} and @file{exit.o} are still included on the
11093 linker command line.
11097 @node PDP-11 Options
11098 @subsection PDP-11 Options
11099 @cindex PDP-11 Options
11101 These options are defined for the PDP-11:
11106 Use hardware FPP floating point. This is the default. (FIS floating
11107 point on the PDP-11/40 is not supported.)
11110 @opindex msoft-float
11111 Do not use hardware floating point.
11115 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
11119 Return floating-point results in memory. This is the default.
11123 Generate code for a PDP-11/40.
11127 Generate code for a PDP-11/45. This is the default.
11131 Generate code for a PDP-11/10.
11133 @item -mbcopy-builtin
11134 @opindex bcopy-builtin
11135 Use inline @code{movmemhi} patterns for copying memory. This is the
11140 Do not use inline @code{movmemhi} patterns for copying memory.
11146 Use 16-bit @code{int}. This is the default.
11152 Use 32-bit @code{int}.
11155 @itemx -mno-float32
11157 @opindex mno-float32
11158 Use 64-bit @code{float}. This is the default.
11161 @itemx -mno-float64
11163 @opindex mno-float64
11164 Use 32-bit @code{float}.
11168 Use @code{abshi2} pattern. This is the default.
11172 Do not use @code{abshi2} pattern.
11174 @item -mbranch-expensive
11175 @opindex mbranch-expensive
11176 Pretend that branches are expensive. This is for experimenting with
11177 code generation only.
11179 @item -mbranch-cheap
11180 @opindex mbranch-cheap
11181 Do not pretend that branches are expensive. This is the default.
11185 Generate code for a system with split I&D@.
11189 Generate code for a system without split I&D@. This is the default.
11193 Use Unix assembler syntax. This is the default when configured for
11194 @samp{pdp11-*-bsd}.
11198 Use DEC assembler syntax. This is the default when configured for any
11199 PDP-11 target other than @samp{pdp11-*-bsd}.
11202 @node PowerPC Options
11203 @subsection PowerPC Options
11204 @cindex PowerPC options
11206 These are listed under @xref{RS/6000 and PowerPC Options}.
11208 @node RS/6000 and PowerPC Options
11209 @subsection IBM RS/6000 and PowerPC Options
11210 @cindex RS/6000 and PowerPC Options
11211 @cindex IBM RS/6000 and PowerPC Options
11213 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
11220 @itemx -mno-powerpc
11221 @itemx -mpowerpc-gpopt
11222 @itemx -mno-powerpc-gpopt
11223 @itemx -mpowerpc-gfxopt
11224 @itemx -mno-powerpc-gfxopt
11226 @itemx -mno-powerpc64
11230 @itemx -mno-popcntb
11236 @opindex mno-power2
11238 @opindex mno-powerpc
11239 @opindex mpowerpc-gpopt
11240 @opindex mno-powerpc-gpopt
11241 @opindex mpowerpc-gfxopt
11242 @opindex mno-powerpc-gfxopt
11243 @opindex mpowerpc64
11244 @opindex mno-powerpc64
11248 @opindex mno-popcntb
11251 GCC supports two related instruction set architectures for the
11252 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
11253 instructions supported by the @samp{rios} chip set used in the original
11254 RS/6000 systems and the @dfn{PowerPC} instruction set is the
11255 architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and
11256 the IBM 4xx, 6xx, and follow-on microprocessors.
11258 Neither architecture is a subset of the other. However there is a
11259 large common subset of instructions supported by both. An MQ
11260 register is included in processors supporting the POWER architecture.
11262 You use these options to specify which instructions are available on the
11263 processor you are using. The default value of these options is
11264 determined when configuring GCC@. Specifying the
11265 @option{-mcpu=@var{cpu_type}} overrides the specification of these
11266 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
11267 rather than the options listed above.
11269 The @option{-mpower} option allows GCC to generate instructions that
11270 are found only in the POWER architecture and to use the MQ register.
11271 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
11272 to generate instructions that are present in the POWER2 architecture but
11273 not the original POWER architecture.
11275 The @option{-mpowerpc} option allows GCC to generate instructions that
11276 are found only in the 32-bit subset of the PowerPC architecture.
11277 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
11278 GCC to use the optional PowerPC architecture instructions in the
11279 General Purpose group, including floating-point square root. Specifying
11280 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
11281 use the optional PowerPC architecture instructions in the Graphics
11282 group, including floating-point select.
11284 The @option{-mmfcrf} option allows GCC to generate the move from
11285 condition register field instruction implemented on the POWER4
11286 processor and other processors that support the PowerPC V2.01
11288 The @option{-mpopcntb} option allows GCC to generate the popcount and
11289 double precision FP reciprocal estimate instruction implemented on the
11290 POWER5 processor and other processors that support the PowerPC V2.02
11292 The @option{-mfprnd} option allows GCC to generate the FP round to
11293 integer instructions implemented on the POWER5+ processor and other
11294 processors that support the PowerPC V2.03 architecture.
11296 The @option{-mpowerpc64} option allows GCC to generate the additional
11297 64-bit instructions that are found in the full PowerPC64 architecture
11298 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
11299 @option{-mno-powerpc64}.
11301 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
11302 will use only the instructions in the common subset of both
11303 architectures plus some special AIX common-mode calls, and will not use
11304 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
11305 permits GCC to use any instruction from either architecture and to
11306 allow use of the MQ register; specify this for the Motorola MPC601.
11308 @item -mnew-mnemonics
11309 @itemx -mold-mnemonics
11310 @opindex mnew-mnemonics
11311 @opindex mold-mnemonics
11312 Select which mnemonics to use in the generated assembler code. With
11313 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
11314 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
11315 assembler mnemonics defined for the POWER architecture. Instructions
11316 defined in only one architecture have only one mnemonic; GCC uses that
11317 mnemonic irrespective of which of these options is specified.
11319 GCC defaults to the mnemonics appropriate for the architecture in
11320 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
11321 value of these option. Unless you are building a cross-compiler, you
11322 should normally not specify either @option{-mnew-mnemonics} or
11323 @option{-mold-mnemonics}, but should instead accept the default.
11325 @item -mcpu=@var{cpu_type}
11327 Set architecture type, register usage, choice of mnemonics, and
11328 instruction scheduling parameters for machine type @var{cpu_type}.
11329 Supported values for @var{cpu_type} are @samp{401}, @samp{403},
11330 @samp{405}, @samp{405fp}, @samp{440}, @samp{440fp}, @samp{505},
11331 @samp{601}, @samp{602}, @samp{603}, @samp{603e}, @samp{604},
11332 @samp{604e}, @samp{620}, @samp{630}, @samp{740}, @samp{7400},
11333 @samp{7450}, @samp{750}, @samp{801}, @samp{821}, @samp{823},
11334 @samp{860}, @samp{970}, @samp{8540}, @samp{ec603e}, @samp{G3},
11335 @samp{G4}, @samp{G5}, @samp{power}, @samp{power2}, @samp{power3},
11336 @samp{power4}, @samp{power5}, @samp{power5+}, @samp{power6},
11337 @samp{common}, @samp{powerpc}, @samp{powerpc64},
11338 @samp{rios}, @samp{rios1}, @samp{rios2}, @samp{rsc}, and @samp{rs64}.
11340 @option{-mcpu=common} selects a completely generic processor. Code
11341 generated under this option will run on any POWER or PowerPC processor.
11342 GCC will use only the instructions in the common subset of both
11343 architectures, and will not use the MQ register. GCC assumes a generic
11344 processor model for scheduling purposes.
11346 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
11347 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
11348 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
11349 types, with an appropriate, generic processor model assumed for
11350 scheduling purposes.
11352 The other options specify a specific processor. Code generated under
11353 those options will run best on that processor, and may not run at all on
11356 The @option{-mcpu} options automatically enable or disable the
11357 following options: @option{-maltivec}, @option{-mfprnd},
11358 @option{-mhard-float}, @option{-mmfcrf}, @option{-mmultiple},
11359 @option{-mnew-mnemonics}, @option{-mpopcntb}, @option{-mpower},
11360 @option{-mpower2}, @option{-mpowerpc64}, @option{-mpowerpc-gpopt},
11361 @option{-mpowerpc-gfxopt}, @option{-mstring}, @option{-mmulhw}, @option{-mdlmzb}.
11362 The particular options
11363 set for any particular CPU will vary between compiler versions,
11364 depending on what setting seems to produce optimal code for that CPU;
11365 it doesn't necessarily reflect the actual hardware's capabilities. If
11366 you wish to set an individual option to a particular value, you may
11367 specify it after the @option{-mcpu} option, like @samp{-mcpu=970
11370 On AIX, the @option{-maltivec} and @option{-mpowerpc64} options are
11371 not enabled or disabled by the @option{-mcpu} option at present because
11372 AIX does not have full support for these options. You may still
11373 enable or disable them individually if you're sure it'll work in your
11376 @item -mtune=@var{cpu_type}
11378 Set the instruction scheduling parameters for machine type
11379 @var{cpu_type}, but do not set the architecture type, register usage, or
11380 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
11381 values for @var{cpu_type} are used for @option{-mtune} as for
11382 @option{-mcpu}. If both are specified, the code generated will use the
11383 architecture, registers, and mnemonics set by @option{-mcpu}, but the
11384 scheduling parameters set by @option{-mtune}.
11390 Generate code to compute division as reciprocal estimate and iterative
11391 refinement, creating opportunities for increased throughput. This
11392 feature requires: optional PowerPC Graphics instruction set for single
11393 precision and FRE instruction for double precision, assuming divides
11394 cannot generate user-visible traps, and the domain values not include
11395 Infinities, denormals or zero denominator.
11398 @itemx -mno-altivec
11400 @opindex mno-altivec
11401 Generate code that uses (does not use) AltiVec instructions, and also
11402 enable the use of built-in functions that allow more direct access to
11403 the AltiVec instruction set. You may also need to set
11404 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
11410 @opindex mno-vrsave
11411 Generate VRSAVE instructions when generating AltiVec code.
11414 @opindex msecure-plt
11415 Generate code that allows ld and ld.so to build executables and shared
11416 libraries with non-exec .plt and .got sections. This is a PowerPC
11417 32-bit SYSV ABI option.
11421 Generate code that uses a BSS .plt section that ld.so fills in, and
11422 requires .plt and .got sections that are both writable and executable.
11423 This is a PowerPC 32-bit SYSV ABI option.
11429 This switch enables or disables the generation of ISEL instructions.
11431 @item -misel=@var{yes/no}
11432 This switch has been deprecated. Use @option{-misel} and
11433 @option{-mno-isel} instead.
11439 This switch enables or disables the generation of SPE simd
11442 @item -mspe=@var{yes/no}
11443 This option has been deprecated. Use @option{-mspe} and
11444 @option{-mno-spe} instead.
11446 @item -mfloat-gprs=@var{yes/single/double/no}
11447 @itemx -mfloat-gprs
11448 @opindex mfloat-gprs
11449 This switch enables or disables the generation of floating point
11450 operations on the general purpose registers for architectures that
11453 The argument @var{yes} or @var{single} enables the use of
11454 single-precision floating point operations.
11456 The argument @var{double} enables the use of single and
11457 double-precision floating point operations.
11459 The argument @var{no} disables floating point operations on the
11460 general purpose registers.
11462 This option is currently only available on the MPC854x.
11468 Generate code for 32-bit or 64-bit environments of Darwin and SVR4
11469 targets (including GNU/Linux). The 32-bit environment sets int, long
11470 and pointer to 32 bits and generates code that runs on any PowerPC
11471 variant. The 64-bit environment sets int to 32 bits and long and
11472 pointer to 64 bits, and generates code for PowerPC64, as for
11473 @option{-mpowerpc64}.
11476 @itemx -mno-fp-in-toc
11477 @itemx -mno-sum-in-toc
11478 @itemx -mminimal-toc
11480 @opindex mno-fp-in-toc
11481 @opindex mno-sum-in-toc
11482 @opindex mminimal-toc
11483 Modify generation of the TOC (Table Of Contents), which is created for
11484 every executable file. The @option{-mfull-toc} option is selected by
11485 default. In that case, GCC will allocate at least one TOC entry for
11486 each unique non-automatic variable reference in your program. GCC
11487 will also place floating-point constants in the TOC@. However, only
11488 16,384 entries are available in the TOC@.
11490 If you receive a linker error message that saying you have overflowed
11491 the available TOC space, you can reduce the amount of TOC space used
11492 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
11493 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
11494 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
11495 generate code to calculate the sum of an address and a constant at
11496 run-time instead of putting that sum into the TOC@. You may specify one
11497 or both of these options. Each causes GCC to produce very slightly
11498 slower and larger code at the expense of conserving TOC space.
11500 If you still run out of space in the TOC even when you specify both of
11501 these options, specify @option{-mminimal-toc} instead. This option causes
11502 GCC to make only one TOC entry for every file. When you specify this
11503 option, GCC will produce code that is slower and larger but which
11504 uses extremely little TOC space. You may wish to use this option
11505 only on files that contain less frequently executed code.
11511 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
11512 @code{long} type, and the infrastructure needed to support them.
11513 Specifying @option{-maix64} implies @option{-mpowerpc64} and
11514 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
11515 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
11518 @itemx -mno-xl-compat
11519 @opindex mxl-compat
11520 @opindex mno-xl-compat
11521 Produce code that conforms more closely to IBM XL compiler semantics
11522 when using AIX-compatible ABI. Pass floating-point arguments to
11523 prototyped functions beyond the register save area (RSA) on the stack
11524 in addition to argument FPRs. Do not assume that most significant
11525 double in 128-bit long double value is properly rounded when comparing
11526 values and converting to double. Use XL symbol names for long double
11529 The AIX calling convention was extended but not initially documented to
11530 handle an obscure K&R C case of calling a function that takes the
11531 address of its arguments with fewer arguments than declared. IBM XL
11532 compilers access floating point arguments which do not fit in the
11533 RSA from the stack when a subroutine is compiled without
11534 optimization. Because always storing floating-point arguments on the
11535 stack is inefficient and rarely needed, this option is not enabled by
11536 default and only is necessary when calling subroutines compiled by IBM
11537 XL compilers without optimization.
11541 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
11542 application written to use message passing with special startup code to
11543 enable the application to run. The system must have PE installed in the
11544 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
11545 must be overridden with the @option{-specs=} option to specify the
11546 appropriate directory location. The Parallel Environment does not
11547 support threads, so the @option{-mpe} option and the @option{-pthread}
11548 option are incompatible.
11550 @item -malign-natural
11551 @itemx -malign-power
11552 @opindex malign-natural
11553 @opindex malign-power
11554 On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
11555 @option{-malign-natural} overrides the ABI-defined alignment of larger
11556 types, such as floating-point doubles, on their natural size-based boundary.
11557 The option @option{-malign-power} instructs GCC to follow the ABI-specified
11558 alignment rules. GCC defaults to the standard alignment defined in the ABI@.
11560 On 64-bit Darwin, natural alignment is the default, and @option{-malign-power}
11564 @itemx -mhard-float
11565 @opindex msoft-float
11566 @opindex mhard-float
11567 Generate code that does not use (uses) the floating-point register set.
11568 Software floating point emulation is provided if you use the
11569 @option{-msoft-float} option, and pass the option to GCC when linking.
11572 @itemx -mno-multiple
11574 @opindex mno-multiple
11575 Generate code that uses (does not use) the load multiple word
11576 instructions and the store multiple word instructions. These
11577 instructions are generated by default on POWER systems, and not
11578 generated on PowerPC systems. Do not use @option{-mmultiple} on little
11579 endian PowerPC systems, since those instructions do not work when the
11580 processor is in little endian mode. The exceptions are PPC740 and
11581 PPC750 which permit the instructions usage in little endian mode.
11586 @opindex mno-string
11587 Generate code that uses (does not use) the load string instructions
11588 and the store string word instructions to save multiple registers and
11589 do small block moves. These instructions are generated by default on
11590 POWER systems, and not generated on PowerPC systems. Do not use
11591 @option{-mstring} on little endian PowerPC systems, since those
11592 instructions do not work when the processor is in little endian mode.
11593 The exceptions are PPC740 and PPC750 which permit the instructions
11594 usage in little endian mode.
11599 @opindex mno-update
11600 Generate code that uses (does not use) the load or store instructions
11601 that update the base register to the address of the calculated memory
11602 location. These instructions are generated by default. If you use
11603 @option{-mno-update}, there is a small window between the time that the
11604 stack pointer is updated and the address of the previous frame is
11605 stored, which means code that walks the stack frame across interrupts or
11606 signals may get corrupted data.
11609 @itemx -mno-fused-madd
11610 @opindex mfused-madd
11611 @opindex mno-fused-madd
11612 Generate code that uses (does not use) the floating point multiply and
11613 accumulate instructions. These instructions are generated by default if
11614 hardware floating is used.
11620 Generate code that uses (does not use) the half-word multiply and
11621 multiply-accumulate instructions on the IBM 405 and 440 processors.
11622 These instructions are generated by default when targetting those
11629 Generate code that uses (does not use) the string-search @samp{dlmzb}
11630 instruction on the IBM 405 and 440 processors. This instruction is
11631 generated by default when targetting those processors.
11633 @item -mno-bit-align
11635 @opindex mno-bit-align
11636 @opindex mbit-align
11637 On System V.4 and embedded PowerPC systems do not (do) force structures
11638 and unions that contain bit-fields to be aligned to the base type of the
11641 For example, by default a structure containing nothing but 8
11642 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
11643 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
11644 the structure would be aligned to a 1 byte boundary and be one byte in
11647 @item -mno-strict-align
11648 @itemx -mstrict-align
11649 @opindex mno-strict-align
11650 @opindex mstrict-align
11651 On System V.4 and embedded PowerPC systems do not (do) assume that
11652 unaligned memory references will be handled by the system.
11654 @item -mrelocatable
11655 @itemx -mno-relocatable
11656 @opindex mrelocatable
11657 @opindex mno-relocatable
11658 On embedded PowerPC systems generate code that allows (does not allow)
11659 the program to be relocated to a different address at runtime. If you
11660 use @option{-mrelocatable} on any module, all objects linked together must
11661 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
11663 @item -mrelocatable-lib
11664 @itemx -mno-relocatable-lib
11665 @opindex mrelocatable-lib
11666 @opindex mno-relocatable-lib
11667 On embedded PowerPC systems generate code that allows (does not allow)
11668 the program to be relocated to a different address at runtime. Modules
11669 compiled with @option{-mrelocatable-lib} can be linked with either modules
11670 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
11671 with modules compiled with the @option{-mrelocatable} options.
11677 On System V.4 and embedded PowerPC systems do not (do) assume that
11678 register 2 contains a pointer to a global area pointing to the addresses
11679 used in the program.
11682 @itemx -mlittle-endian
11684 @opindex mlittle-endian
11685 On System V.4 and embedded PowerPC systems compile code for the
11686 processor in little endian mode. The @option{-mlittle-endian} option is
11687 the same as @option{-mlittle}.
11690 @itemx -mbig-endian
11692 @opindex mbig-endian
11693 On System V.4 and embedded PowerPC systems compile code for the
11694 processor in big endian mode. The @option{-mbig-endian} option is
11695 the same as @option{-mbig}.
11697 @item -mdynamic-no-pic
11698 @opindex mdynamic-no-pic
11699 On Darwin and Mac OS X systems, compile code so that it is not
11700 relocatable, but that its external references are relocatable. The
11701 resulting code is suitable for applications, but not shared
11704 @item -mprioritize-restricted-insns=@var{priority}
11705 @opindex mprioritize-restricted-insns
11706 This option controls the priority that is assigned to
11707 dispatch-slot restricted instructions during the second scheduling
11708 pass. The argument @var{priority} takes the value @var{0/1/2} to assign
11709 @var{no/highest/second-highest} priority to dispatch slot restricted
11712 @item -msched-costly-dep=@var{dependence_type}
11713 @opindex msched-costly-dep
11714 This option controls which dependences are considered costly
11715 by the target during instruction scheduling. The argument
11716 @var{dependence_type} takes one of the following values:
11717 @var{no}: no dependence is costly,
11718 @var{all}: all dependences are costly,
11719 @var{true_store_to_load}: a true dependence from store to load is costly,
11720 @var{store_to_load}: any dependence from store to load is costly,
11721 @var{number}: any dependence which latency >= @var{number} is costly.
11723 @item -minsert-sched-nops=@var{scheme}
11724 @opindex minsert-sched-nops
11725 This option controls which nop insertion scheme will be used during
11726 the second scheduling pass. The argument @var{scheme} takes one of the
11728 @var{no}: Don't insert nops.
11729 @var{pad}: Pad with nops any dispatch group which has vacant issue slots,
11730 according to the scheduler's grouping.
11731 @var{regroup_exact}: Insert nops to force costly dependent insns into
11732 separate groups. Insert exactly as many nops as needed to force an insn
11733 to a new group, according to the estimated processor grouping.
11734 @var{number}: Insert nops to force costly dependent insns into
11735 separate groups. Insert @var{number} nops to force an insn to a new group.
11738 @opindex mcall-sysv
11739 On System V.4 and embedded PowerPC systems compile code using calling
11740 conventions that adheres to the March 1995 draft of the System V
11741 Application Binary Interface, PowerPC processor supplement. This is the
11742 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
11744 @item -mcall-sysv-eabi
11745 @opindex mcall-sysv-eabi
11746 Specify both @option{-mcall-sysv} and @option{-meabi} options.
11748 @item -mcall-sysv-noeabi
11749 @opindex mcall-sysv-noeabi
11750 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
11752 @item -mcall-solaris
11753 @opindex mcall-solaris
11754 On System V.4 and embedded PowerPC systems compile code for the Solaris
11758 @opindex mcall-linux
11759 On System V.4 and embedded PowerPC systems compile code for the
11760 Linux-based GNU system.
11764 On System V.4 and embedded PowerPC systems compile code for the
11765 Hurd-based GNU system.
11767 @item -mcall-netbsd
11768 @opindex mcall-netbsd
11769 On System V.4 and embedded PowerPC systems compile code for the
11770 NetBSD operating system.
11772 @item -maix-struct-return
11773 @opindex maix-struct-return
11774 Return all structures in memory (as specified by the AIX ABI)@.
11776 @item -msvr4-struct-return
11777 @opindex msvr4-struct-return
11778 Return structures smaller than 8 bytes in registers (as specified by the
11781 @item -mabi=@var{abi-type}
11783 Extend the current ABI with a particular extension, or remove such extension.
11784 Valid values are @var{altivec}, @var{no-altivec}, @var{spe},
11785 @var{no-spe}, @var{ibmlongdouble}, @var{ieeelongdouble}@.
11789 Extend the current ABI with SPE ABI extensions. This does not change
11790 the default ABI, instead it adds the SPE ABI extensions to the current
11794 @opindex mabi=no-spe
11795 Disable Booke SPE ABI extensions for the current ABI@.
11797 @item -mabi=ibmlongdouble
11798 @opindex mabi=ibmlongdouble
11799 Change the current ABI to use IBM extended precision long double.
11800 This is a PowerPC 32-bit SYSV ABI option.
11802 @item -mabi=ieeelongdouble
11803 @opindex mabi=ieeelongdouble
11804 Change the current ABI to use IEEE extended precision long double.
11805 This is a PowerPC 32-bit Linux ABI option.
11808 @itemx -mno-prototype
11809 @opindex mprototype
11810 @opindex mno-prototype
11811 On System V.4 and embedded PowerPC systems assume that all calls to
11812 variable argument functions are properly prototyped. Otherwise, the
11813 compiler must insert an instruction before every non prototyped call to
11814 set or clear bit 6 of the condition code register (@var{CR}) to
11815 indicate whether floating point values were passed in the floating point
11816 registers in case the function takes a variable arguments. With
11817 @option{-mprototype}, only calls to prototyped variable argument functions
11818 will set or clear the bit.
11822 On embedded PowerPC systems, assume that the startup module is called
11823 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
11824 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
11829 On embedded PowerPC systems, assume that the startup module is called
11830 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
11835 On embedded PowerPC systems, assume that the startup module is called
11836 @file{crt0.o} and the standard C libraries are @file{libads.a} and
11839 @item -myellowknife
11840 @opindex myellowknife
11841 On embedded PowerPC systems, assume that the startup module is called
11842 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
11847 On System V.4 and embedded PowerPC systems, specify that you are
11848 compiling for a VxWorks system.
11852 Specify that you are compiling for the WindISS simulation environment.
11856 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
11857 header to indicate that @samp{eabi} extended relocations are used.
11863 On System V.4 and embedded PowerPC systems do (do not) adhere to the
11864 Embedded Applications Binary Interface (eabi) which is a set of
11865 modifications to the System V.4 specifications. Selecting @option{-meabi}
11866 means that the stack is aligned to an 8 byte boundary, a function
11867 @code{__eabi} is called to from @code{main} to set up the eabi
11868 environment, and the @option{-msdata} option can use both @code{r2} and
11869 @code{r13} to point to two separate small data areas. Selecting
11870 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
11871 do not call an initialization function from @code{main}, and the
11872 @option{-msdata} option will only use @code{r13} to point to a single
11873 small data area. The @option{-meabi} option is on by default if you
11874 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
11877 @opindex msdata=eabi
11878 On System V.4 and embedded PowerPC systems, put small initialized
11879 @code{const} global and static data in the @samp{.sdata2} section, which
11880 is pointed to by register @code{r2}. Put small initialized
11881 non-@code{const} global and static data in the @samp{.sdata} section,
11882 which is pointed to by register @code{r13}. Put small uninitialized
11883 global and static data in the @samp{.sbss} section, which is adjacent to
11884 the @samp{.sdata} section. The @option{-msdata=eabi} option is
11885 incompatible with the @option{-mrelocatable} option. The
11886 @option{-msdata=eabi} option also sets the @option{-memb} option.
11889 @opindex msdata=sysv
11890 On System V.4 and embedded PowerPC systems, put small global and static
11891 data in the @samp{.sdata} section, which is pointed to by register
11892 @code{r13}. Put small uninitialized global and static data in the
11893 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
11894 The @option{-msdata=sysv} option is incompatible with the
11895 @option{-mrelocatable} option.
11897 @item -msdata=default
11899 @opindex msdata=default
11901 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
11902 compile code the same as @option{-msdata=eabi}, otherwise compile code the
11903 same as @option{-msdata=sysv}.
11906 @opindex msdata-data
11907 On System V.4 and embedded PowerPC systems, put small global
11908 data in the @samp{.sdata} section. Put small uninitialized global
11909 data in the @samp{.sbss} section. Do not use register @code{r13}
11910 to address small data however. This is the default behavior unless
11911 other @option{-msdata} options are used.
11915 @opindex msdata=none
11917 On embedded PowerPC systems, put all initialized global and static data
11918 in the @samp{.data} section, and all uninitialized data in the
11919 @samp{.bss} section.
11923 @cindex smaller data references (PowerPC)
11924 @cindex .sdata/.sdata2 references (PowerPC)
11925 On embedded PowerPC systems, put global and static items less than or
11926 equal to @var{num} bytes into the small data or bss sections instead of
11927 the normal data or bss section. By default, @var{num} is 8. The
11928 @option{-G @var{num}} switch is also passed to the linker.
11929 All modules should be compiled with the same @option{-G @var{num}} value.
11932 @itemx -mno-regnames
11934 @opindex mno-regnames
11935 On System V.4 and embedded PowerPC systems do (do not) emit register
11936 names in the assembly language output using symbolic forms.
11939 @itemx -mno-longcall
11941 @opindex mno-longcall
11942 By default assume that all calls are far away so that a longer more
11943 expensive calling sequence is required. This is required for calls
11944 further than 32 megabytes (33,554,432 bytes) from the current location.
11945 A short call will be generated if the compiler knows
11946 the call cannot be that far away. This setting can be overridden by
11947 the @code{shortcall} function attribute, or by @code{#pragma
11950 Some linkers are capable of detecting out-of-range calls and generating
11951 glue code on the fly. On these systems, long calls are unnecessary and
11952 generate slower code. As of this writing, the AIX linker can do this,
11953 as can the GNU linker for PowerPC/64. It is planned to add this feature
11954 to the GNU linker for 32-bit PowerPC systems as well.
11956 On Darwin/PPC systems, @code{#pragma longcall} will generate ``jbsr
11957 callee, L42'', plus a ``branch island'' (glue code). The two target
11958 addresses represent the callee and the ``branch island''. The
11959 Darwin/PPC linker will prefer the first address and generate a ``bl
11960 callee'' if the PPC ``bl'' instruction will reach the callee directly;
11961 otherwise, the linker will generate ``bl L42'' to call the ``branch
11962 island''. The ``branch island'' is appended to the body of the
11963 calling function; it computes the full 32-bit address of the callee
11966 On Mach-O (Darwin) systems, this option directs the compiler emit to
11967 the glue for every direct call, and the Darwin linker decides whether
11968 to use or discard it.
11970 In the future, we may cause GCC to ignore all longcall specifications
11971 when the linker is known to generate glue.
11975 Adds support for multithreading with the @dfn{pthreads} library.
11976 This option sets flags for both the preprocessor and linker.
11980 @node S/390 and zSeries Options
11981 @subsection S/390 and zSeries Options
11982 @cindex S/390 and zSeries Options
11984 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
11988 @itemx -msoft-float
11989 @opindex mhard-float
11990 @opindex msoft-float
11991 Use (do not use) the hardware floating-point instructions and registers
11992 for floating-point operations. When @option{-msoft-float} is specified,
11993 functions in @file{libgcc.a} will be used to perform floating-point
11994 operations. When @option{-mhard-float} is specified, the compiler
11995 generates IEEE floating-point instructions. This is the default.
11997 @item -mlong-double-64
11998 @itemx -mlong-double-128
11999 @opindex mlong-double-64
12000 @opindex mlong-double-128
12001 These switches control the size of @code{long double} type. A size
12002 of 64bit makes the @code{long double} type equivalent to the @code{double}
12003 type. This is the default.
12006 @itemx -mno-backchain
12007 @opindex mbackchain
12008 @opindex mno-backchain
12009 Store (do not store) the address of the caller's frame as backchain pointer
12010 into the callee's stack frame.
12011 A backchain may be needed to allow debugging using tools that do not understand
12012 DWARF-2 call frame information.
12013 When @option{-mno-packed-stack} is in effect, the backchain pointer is stored
12014 at the bottom of the stack frame; when @option{-mpacked-stack} is in effect,
12015 the backchain is placed into the topmost word of the 96/160 byte register
12018 In general, code compiled with @option{-mbackchain} is call-compatible with
12019 code compiled with @option{-mmo-backchain}; however, use of the backchain
12020 for debugging purposes usually requires that the whole binary is built with
12021 @option{-mbackchain}. Note that the combination of @option{-mbackchain},
12022 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
12023 to build a linux kernel use @option{-msoft-float}.
12025 The default is to not maintain the backchain.
12027 @item -mpacked-stack
12028 @item -mno-packed-stack
12029 @opindex mpacked-stack
12030 @opindex mno-packed-stack
12031 Use (do not use) the packed stack layout. When @option{-mno-packed-stack} is
12032 specified, the compiler uses the all fields of the 96/160 byte register save
12033 area only for their default purpose; unused fields still take up stack space.
12034 When @option{-mpacked-stack} is specified, register save slots are densely
12035 packed at the top of the register save area; unused space is reused for other
12036 purposes, allowing for more efficient use of the available stack space.
12037 However, when @option{-mbackchain} is also in effect, the topmost word of
12038 the save area is always used to store the backchain, and the return address
12039 register is always saved two words below the backchain.
12041 As long as the stack frame backchain is not used, code generated with
12042 @option{-mpacked-stack} is call-compatible with code generated with
12043 @option{-mno-packed-stack}. Note that some non-FSF releases of GCC 2.95 for
12044 S/390 or zSeries generated code that uses the stack frame backchain at run
12045 time, not just for debugging purposes. Such code is not call-compatible
12046 with code compiled with @option{-mpacked-stack}. Also, note that the
12047 combination of @option{-mbackchain},
12048 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
12049 to build a linux kernel use @option{-msoft-float}.
12051 The default is to not use the packed stack layout.
12054 @itemx -mno-small-exec
12055 @opindex msmall-exec
12056 @opindex mno-small-exec
12057 Generate (or do not generate) code using the @code{bras} instruction
12058 to do subroutine calls.
12059 This only works reliably if the total executable size does not
12060 exceed 64k. The default is to use the @code{basr} instruction instead,
12061 which does not have this limitation.
12067 When @option{-m31} is specified, generate code compliant to the
12068 GNU/Linux for S/390 ABI@. When @option{-m64} is specified, generate
12069 code compliant to the GNU/Linux for zSeries ABI@. This allows GCC in
12070 particular to generate 64-bit instructions. For the @samp{s390}
12071 targets, the default is @option{-m31}, while the @samp{s390x}
12072 targets default to @option{-m64}.
12078 When @option{-mzarch} is specified, generate code using the
12079 instructions available on z/Architecture.
12080 When @option{-mesa} is specified, generate code using the
12081 instructions available on ESA/390. Note that @option{-mesa} is
12082 not possible with @option{-m64}.
12083 When generating code compliant to the GNU/Linux for S/390 ABI,
12084 the default is @option{-mesa}. When generating code compliant
12085 to the GNU/Linux for zSeries ABI, the default is @option{-mzarch}.
12091 Generate (or do not generate) code using the @code{mvcle} instruction
12092 to perform block moves. When @option{-mno-mvcle} is specified,
12093 use a @code{mvc} loop instead. This is the default unless optimizing for
12100 Print (or do not print) additional debug information when compiling.
12101 The default is to not print debug information.
12103 @item -march=@var{cpu-type}
12105 Generate code that will run on @var{cpu-type}, which is the name of a system
12106 representing a certain processor type. Possible values for
12107 @var{cpu-type} are @samp{g5}, @samp{g6}, @samp{z900}, and @samp{z990}.
12108 When generating code using the instructions available on z/Architecture,
12109 the default is @option{-march=z900}. Otherwise, the default is
12110 @option{-march=g5}.
12112 @item -mtune=@var{cpu-type}
12114 Tune to @var{cpu-type} everything applicable about the generated code,
12115 except for the ABI and the set of available instructions.
12116 The list of @var{cpu-type} values is the same as for @option{-march}.
12117 The default is the value used for @option{-march}.
12120 @itemx -mno-tpf-trace
12121 @opindex mtpf-trace
12122 @opindex mno-tpf-trace
12123 Generate code that adds (does not add) in TPF OS specific branches to trace
12124 routines in the operating system. This option is off by default, even
12125 when compiling for the TPF OS@.
12128 @itemx -mno-fused-madd
12129 @opindex mfused-madd
12130 @opindex mno-fused-madd
12131 Generate code that uses (does not use) the floating point multiply and
12132 accumulate instructions. These instructions are generated by default if
12133 hardware floating point is used.
12135 @item -mwarn-framesize=@var{framesize}
12136 @opindex mwarn-framesize
12137 Emit a warning if the current function exceeds the given frame size. Because
12138 this is a compile time check it doesn't need to be a real problem when the program
12139 runs. It is intended to identify functions which most probably cause
12140 a stack overflow. It is useful to be used in an environment with limited stack
12141 size e.g.@: the linux kernel.
12143 @item -mwarn-dynamicstack
12144 @opindex mwarn-dynamicstack
12145 Emit a warning if the function calls alloca or uses dynamically
12146 sized arrays. This is generally a bad idea with a limited stack size.
12148 @item -mstack-guard=@var{stack-guard}
12149 @item -mstack-size=@var{stack-size}
12150 @opindex mstack-guard
12151 @opindex mstack-size
12152 These arguments always have to be used in conjunction. If they are present the s390
12153 back end emits additional instructions in the function prologue which trigger a trap
12154 if the stack size is @var{stack-guard} bytes above the @var{stack-size}
12155 (remember that the stack on s390 grows downward). These options are intended to
12156 be used to help debugging stack overflow problems. The additionally emitted code
12157 causes only little overhead and hence can also be used in production like systems
12158 without greater performance degradation. The given values have to be exact
12159 powers of 2 and @var{stack-size} has to be greater than @var{stack-guard} without
12161 In order to be efficient the extra code makes the assumption that the stack starts
12162 at an address aligned to the value given by @var{stack-size}.
12166 @subsection SH Options
12168 These @samp{-m} options are defined for the SH implementations:
12173 Generate code for the SH1.
12177 Generate code for the SH2.
12180 Generate code for the SH2e.
12184 Generate code for the SH3.
12188 Generate code for the SH3e.
12192 Generate code for the SH4 without a floating-point unit.
12194 @item -m4-single-only
12195 @opindex m4-single-only
12196 Generate code for the SH4 with a floating-point unit that only
12197 supports single-precision arithmetic.
12201 Generate code for the SH4 assuming the floating-point unit is in
12202 single-precision mode by default.
12206 Generate code for the SH4.
12210 Generate code for the SH4al-dsp, or for a SH4a in such a way that the
12211 floating-point unit is not used.
12213 @item -m4a-single-only
12214 @opindex m4a-single-only
12215 Generate code for the SH4a, in such a way that no double-precision
12216 floating point operations are used.
12219 @opindex m4a-single
12220 Generate code for the SH4a assuming the floating-point unit is in
12221 single-precision mode by default.
12225 Generate code for the SH4a.
12229 Same as @option{-m4a-nofpu}, except that it implicitly passes
12230 @option{-dsp} to the assembler. GCC doesn't generate any DSP
12231 instructions at the moment.
12235 Compile code for the processor in big endian mode.
12239 Compile code for the processor in little endian mode.
12243 Align doubles at 64-bit boundaries. Note that this changes the calling
12244 conventions, and thus some functions from the standard C library will
12245 not work unless you recompile it first with @option{-mdalign}.
12249 Shorten some address references at link time, when possible; uses the
12250 linker option @option{-relax}.
12254 Use 32-bit offsets in @code{switch} tables. The default is to use
12259 Enable the use of the instruction @code{fmovd}.
12263 Comply with the calling conventions defined by Renesas.
12267 Comply with the calling conventions defined by Renesas.
12271 Comply with the calling conventions defined for GCC before the Renesas
12272 conventions were available. This option is the default for all
12273 targets of the SH toolchain except for @samp{sh-symbianelf}.
12276 @opindex mnomacsave
12277 Mark the @code{MAC} register as call-clobbered, even if
12278 @option{-mhitachi} is given.
12282 Increase IEEE-compliance of floating-point code.
12283 At the moment, this is equivalent to @option{-fno-finite-math-only}.
12284 When generating 16 bit SH opcodes, getting IEEE-conforming results for
12285 comparisons of NANs / infinities incurs extra overhead in every
12286 floating point comparison, therefore the default is set to
12287 @option{-ffinite-math-only}.
12291 Dump instruction size and location in the assembly code.
12294 @opindex mpadstruct
12295 This option is deprecated. It pads structures to multiple of 4 bytes,
12296 which is incompatible with the SH ABI@.
12300 Optimize for space instead of speed. Implied by @option{-Os}.
12303 @opindex mprefergot
12304 When generating position-independent code, emit function calls using
12305 the Global Offset Table instead of the Procedure Linkage Table.
12309 Generate a library function call to invalidate instruction cache
12310 entries, after fixing up a trampoline. This library function call
12311 doesn't assume it can write to the whole memory address space. This
12312 is the default when the target is @code{sh-*-linux*}.
12314 @item -multcost=@var{number}
12315 @opindex multcost=@var{number}
12316 Set the cost to assume for a multiply insn.
12318 @item -mdiv=@var{strategy}
12319 @opindex mdiv=@var{strategy}
12320 Set the division strategy to use for SHmedia code. @var{strategy} must be
12321 one of: call, call2, fp, inv, inv:minlat, inv20u, inv20l, inv:call,
12322 inv:call2, inv:fp .
12323 "fp" performs the operation in floating point. This has a very high latency,
12324 but needs only a few instructions, so it might be a good choice if
12325 your code has enough easily exploitable ILP to allow the compiler to
12326 schedule the floating point instructions together with other instructions.
12327 Division by zero causes a floating point exception.
12328 "inv" uses integer operations to calculate the inverse of the divisor,
12329 and then multiplies the dividend with the inverse. This strategy allows
12330 cse and hoisting of the inverse calculation. Division by zero calculates
12331 an unspecified result, but does not trap.
12332 "inv:minlat" is a variant of "inv" where if no cse / hoisting opportunities
12333 have been found, or if the entire operation has been hoisted to the same
12334 place, the last stages of the inverse calculation are intertwined with the
12335 final multiply to reduce the overall latency, at the expense of using a few
12336 more instructions, and thus offering fewer scheduling opportunities with
12338 "call" calls a library function that usually implements the inv:minlat
12340 This gives high code density for m5-*media-nofpu compilations.
12341 "call2" uses a different entry point of the same library function, where it
12342 assumes that a pointer to a lookup table has already been set up, which
12343 exposes the pointer load to cse / code hoisting optimizations.
12344 "inv:call", "inv:call2" and "inv:fp" all use the "inv" algorithm for initial
12345 code generation, but if the code stays unoptimized, revert to the "call",
12346 "call2", or "fp" strategies, respectively. Note that the
12347 potentially-trapping side effect of division by zero is carried by a
12348 separate instruction, so it is possible that all the integer instructions
12349 are hoisted out, but the marker for the side effect stays where it is.
12350 A recombination to fp operations or a call is not possible in that case.
12351 "inv20u" and "inv20l" are variants of the "inv:minlat" strategy. In the case
12352 that the inverse calculation was nor separated from the multiply, they speed
12353 up division where the dividend fits into 20 bits (plus sign where applicable),
12354 by inserting a test to skip a number of operations in this case; this test
12355 slows down the case of larger dividends. inv20u assumes the case of a such
12356 a small dividend to be unlikely, and inv20l assumes it to be likely.
12358 @item -mdivsi3_libfunc=@var{name}
12359 @opindex mdivsi3_libfunc=@var{name}
12360 Set the name of the library function used for 32 bit signed division to
12361 @var{name}. This only affect the name used in the call and inv:call
12362 division strategies, and the compiler will still expect the same
12363 sets of input/output/clobbered registers as if this option was not present.
12365 @item -madjust-unroll
12366 @opindex madjust-unroll
12367 Throttle unrolling to avoid thrashing target registers.
12368 This option only has an effect if the gcc code base supports the
12369 TARGET_ADJUST_UNROLL_MAX target hook.
12371 @item -mindexed-addressing
12372 @opindex mindexed-addressing
12373 Enable the use of the indexed addressing mode for SHmedia32/SHcompact.
12374 This is only safe if the hardware and/or OS implement 32 bit wrap-around
12375 semantics for the indexed addressing mode. The architecture allows the
12376 implementation of processors with 64 bit MMU, which the OS could use to
12377 get 32 bit addressing, but since no current hardware implementation supports
12378 this or any other way to make the indexed addressing mode safe to use in
12379 the 32 bit ABI, the default is -mno-indexed-addressing.
12381 @item -mgettrcost=@var{number}
12382 @opindex mgettrcost=@var{number}
12383 Set the cost assumed for the gettr instruction to @var{number}.
12384 The default is 2 if @option{-mpt-fixed} is in effect, 100 otherwise.
12388 Assume pt* instructions won't trap. This will generally generate better
12389 scheduled code, but is unsafe on current hardware. The current architecture
12390 definition says that ptabs and ptrel trap when the target anded with 3 is 3.
12391 This has the unintentional effect of making it unsafe to schedule ptabs /
12392 ptrel before a branch, or hoist it out of a loop. For example,
12393 __do_global_ctors, a part of libgcc that runs constructors at program
12394 startup, calls functions in a list which is delimited by -1. With the
12395 -mpt-fixed option, the ptabs will be done before testing against -1.
12396 That means that all the constructors will be run a bit quicker, but when
12397 the loop comes to the end of the list, the program crashes because ptabs
12398 loads -1 into a target register. Since this option is unsafe for any
12399 hardware implementing the current architecture specification, the default
12400 is -mno-pt-fixed. Unless the user specifies a specific cost with
12401 @option{-mgettrcost}, -mno-pt-fixed also implies @option{-mgettrcost=100};
12402 this deters register allocation using target registers for storing
12405 @item -minvalid-symbols
12406 @opindex minvalid-symbols
12407 Assume symbols might be invalid. Ordinary function symbols generated by
12408 the compiler will always be valid to load with movi/shori/ptabs or
12409 movi/shori/ptrel, but with assembler and/or linker tricks it is possible
12410 to generate symbols that will cause ptabs / ptrel to trap.
12411 This option is only meaningful when @option{-mno-pt-fixed} is in effect.
12412 It will then prevent cross-basic-block cse, hoisting and most scheduling
12413 of symbol loads. The default is @option{-mno-invalid-symbols}.
12416 @node SPARC Options
12417 @subsection SPARC Options
12418 @cindex SPARC options
12420 These @samp{-m} options are supported on the SPARC:
12423 @item -mno-app-regs
12425 @opindex mno-app-regs
12427 Specify @option{-mapp-regs} to generate output using the global registers
12428 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
12431 To be fully SVR4 ABI compliant at the cost of some performance loss,
12432 specify @option{-mno-app-regs}. You should compile libraries and system
12433 software with this option.
12436 @itemx -mhard-float
12438 @opindex mhard-float
12439 Generate output containing floating point instructions. This is the
12443 @itemx -msoft-float
12445 @opindex msoft-float
12446 Generate output containing library calls for floating point.
12447 @strong{Warning:} the requisite libraries are not available for all SPARC
12448 targets. Normally the facilities of the machine's usual C compiler are
12449 used, but this cannot be done directly in cross-compilation. You must make
12450 your own arrangements to provide suitable library functions for
12451 cross-compilation. The embedded targets @samp{sparc-*-aout} and
12452 @samp{sparclite-*-*} do provide software floating point support.
12454 @option{-msoft-float} changes the calling convention in the output file;
12455 therefore, it is only useful if you compile @emph{all} of a program with
12456 this option. In particular, you need to compile @file{libgcc.a}, the
12457 library that comes with GCC, with @option{-msoft-float} in order for
12460 @item -mhard-quad-float
12461 @opindex mhard-quad-float
12462 Generate output containing quad-word (long double) floating point
12465 @item -msoft-quad-float
12466 @opindex msoft-quad-float
12467 Generate output containing library calls for quad-word (long double)
12468 floating point instructions. The functions called are those specified
12469 in the SPARC ABI@. This is the default.
12471 As of this writing, there are no SPARC implementations that have hardware
12472 support for the quad-word floating point instructions. They all invoke
12473 a trap handler for one of these instructions, and then the trap handler
12474 emulates the effect of the instruction. Because of the trap handler overhead,
12475 this is much slower than calling the ABI library routines. Thus the
12476 @option{-msoft-quad-float} option is the default.
12478 @item -mno-unaligned-doubles
12479 @itemx -munaligned-doubles
12480 @opindex mno-unaligned-doubles
12481 @opindex munaligned-doubles
12482 Assume that doubles have 8 byte alignment. This is the default.
12484 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
12485 alignment only if they are contained in another type, or if they have an
12486 absolute address. Otherwise, it assumes they have 4 byte alignment.
12487 Specifying this option avoids some rare compatibility problems with code
12488 generated by other compilers. It is not the default because it results
12489 in a performance loss, especially for floating point code.
12491 @item -mno-faster-structs
12492 @itemx -mfaster-structs
12493 @opindex mno-faster-structs
12494 @opindex mfaster-structs
12495 With @option{-mfaster-structs}, the compiler assumes that structures
12496 should have 8 byte alignment. This enables the use of pairs of
12497 @code{ldd} and @code{std} instructions for copies in structure
12498 assignment, in place of twice as many @code{ld} and @code{st} pairs.
12499 However, the use of this changed alignment directly violates the SPARC
12500 ABI@. Thus, it's intended only for use on targets where the developer
12501 acknowledges that their resulting code will not be directly in line with
12502 the rules of the ABI@.
12504 @item -mimpure-text
12505 @opindex mimpure-text
12506 @option{-mimpure-text}, used in addition to @option{-shared}, tells
12507 the compiler to not pass @option{-z text} to the linker when linking a
12508 shared object. Using this option, you can link position-dependent
12509 code into a shared object.
12511 @option{-mimpure-text} suppresses the ``relocations remain against
12512 allocatable but non-writable sections'' linker error message.
12513 However, the necessary relocations will trigger copy-on-write, and the
12514 shared object is not actually shared across processes. Instead of
12515 using @option{-mimpure-text}, you should compile all source code with
12516 @option{-fpic} or @option{-fPIC}.
12518 This option is only available on SunOS and Solaris.
12520 @item -mcpu=@var{cpu_type}
12522 Set the instruction set, register set, and instruction scheduling parameters
12523 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
12524 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
12525 @samp{f930}, @samp{f934}, @samp{hypersparc}, @samp{sparclite86x},
12526 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc},
12527 @samp{ultrasparc3}, and @samp{niagara}.
12529 Default instruction scheduling parameters are used for values that select
12530 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
12531 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
12533 Here is a list of each supported architecture and their supported
12538 v8: supersparc, hypersparc
12539 sparclite: f930, f934, sparclite86x
12541 v9: ultrasparc, ultrasparc3, niagara
12544 By default (unless configured otherwise), GCC generates code for the V7
12545 variant of the SPARC architecture. With @option{-mcpu=cypress}, the compiler
12546 additionally optimizes it for the Cypress CY7C602 chip, as used in the
12547 SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
12548 SPARCStation 1, 2, IPX etc.
12550 With @option{-mcpu=v8}, GCC generates code for the V8 variant of the SPARC
12551 architecture. The only difference from V7 code is that the compiler emits
12552 the integer multiply and integer divide instructions which exist in SPARC-V8
12553 but not in SPARC-V7. With @option{-mcpu=supersparc}, the compiler additionally
12554 optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
12557 With @option{-mcpu=sparclite}, GCC generates code for the SPARClite variant of
12558 the SPARC architecture. This adds the integer multiply, integer divide step
12559 and scan (@code{ffs}) instructions which exist in SPARClite but not in SPARC-V7.
12560 With @option{-mcpu=f930}, the compiler additionally optimizes it for the
12561 Fujitsu MB86930 chip, which is the original SPARClite, with no FPU@. With
12562 @option{-mcpu=f934}, the compiler additionally optimizes it for the Fujitsu
12563 MB86934 chip, which is the more recent SPARClite with FPU@.
12565 With @option{-mcpu=sparclet}, GCC generates code for the SPARClet variant of
12566 the SPARC architecture. This adds the integer multiply, multiply/accumulate,
12567 integer divide step and scan (@code{ffs}) instructions which exist in SPARClet
12568 but not in SPARC-V7. With @option{-mcpu=tsc701}, the compiler additionally
12569 optimizes it for the TEMIC SPARClet chip.
12571 With @option{-mcpu=v9}, GCC generates code for the V9 variant of the SPARC
12572 architecture. This adds 64-bit integer and floating-point move instructions,
12573 3 additional floating-point condition code registers and conditional move
12574 instructions. With @option{-mcpu=ultrasparc}, the compiler additionally
12575 optimizes it for the Sun UltraSPARC I/II/IIi chips. With
12576 @option{-mcpu=ultrasparc3}, the compiler additionally optimizes it for the
12577 Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
12578 @option{-mcpu=niagara}, the compiler additionally optimizes it for
12579 Sun UltraSPARC T1 chips.
12581 @item -mtune=@var{cpu_type}
12583 Set the instruction scheduling parameters for machine type
12584 @var{cpu_type}, but do not set the instruction set or register set that the
12585 option @option{-mcpu=@var{cpu_type}} would.
12587 The same values for @option{-mcpu=@var{cpu_type}} can be used for
12588 @option{-mtune=@var{cpu_type}}, but the only useful values are those
12589 that select a particular cpu implementation. Those are @samp{cypress},
12590 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
12591 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc},
12592 @samp{ultrasparc3}, and @samp{niagara}.
12597 @opindex mno-v8plus
12598 With @option{-mv8plus}, GCC generates code for the SPARC-V8+ ABI@. The
12599 difference from the V8 ABI is that the global and out registers are
12600 considered 64-bit wide. This is enabled by default on Solaris in 32-bit
12601 mode for all SPARC-V9 processors.
12607 With @option{-mvis}, GCC generates code that takes advantage of the UltraSPARC
12608 Visual Instruction Set extensions. The default is @option{-mno-vis}.
12611 These @samp{-m} options are supported in addition to the above
12612 on SPARC-V9 processors in 64-bit environments:
12615 @item -mlittle-endian
12616 @opindex mlittle-endian
12617 Generate code for a processor running in little-endian mode. It is only
12618 available for a few configurations and most notably not on Solaris and Linux.
12624 Generate code for a 32-bit or 64-bit environment.
12625 The 32-bit environment sets int, long and pointer to 32 bits.
12626 The 64-bit environment sets int to 32 bits and long and pointer
12629 @item -mcmodel=medlow
12630 @opindex mcmodel=medlow
12631 Generate code for the Medium/Low code model: 64-bit addresses, programs
12632 must be linked in the low 32 bits of memory. Programs can be statically
12633 or dynamically linked.
12635 @item -mcmodel=medmid
12636 @opindex mcmodel=medmid
12637 Generate code for the Medium/Middle code model: 64-bit addresses, programs
12638 must be linked in the low 44 bits of memory, the text and data segments must
12639 be less than 2GB in size and the data segment must be located within 2GB of
12642 @item -mcmodel=medany
12643 @opindex mcmodel=medany
12644 Generate code for the Medium/Anywhere code model: 64-bit addresses, programs
12645 may be linked anywhere in memory, the text and data segments must be less
12646 than 2GB in size and the data segment must be located within 2GB of the
12649 @item -mcmodel=embmedany
12650 @opindex mcmodel=embmedany
12651 Generate code for the Medium/Anywhere code model for embedded systems:
12652 64-bit addresses, the text and data segments must be less than 2GB in
12653 size, both starting anywhere in memory (determined at link time). The
12654 global register %g4 points to the base of the data segment. Programs
12655 are statically linked and PIC is not supported.
12658 @itemx -mno-stack-bias
12659 @opindex mstack-bias
12660 @opindex mno-stack-bias
12661 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
12662 frame pointer if present, are offset by @minus{}2047 which must be added back
12663 when making stack frame references. This is the default in 64-bit mode.
12664 Otherwise, assume no such offset is present.
12667 These switches are supported in addition to the above on Solaris:
12672 Add support for multithreading using the Solaris threads library. This
12673 option sets flags for both the preprocessor and linker. This option does
12674 not affect the thread safety of object code produced by the compiler or
12675 that of libraries supplied with it.
12679 Add support for multithreading using the POSIX threads library. This
12680 option sets flags for both the preprocessor and linker. This option does
12681 not affect the thread safety of object code produced by the compiler or
12682 that of libraries supplied with it.
12686 This is a synonym for @option{-pthreads}.
12689 @node System V Options
12690 @subsection Options for System V
12692 These additional options are available on System V Release 4 for
12693 compatibility with other compilers on those systems:
12698 Create a shared object.
12699 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
12703 Identify the versions of each tool used by the compiler, in a
12704 @code{.ident} assembler directive in the output.
12708 Refrain from adding @code{.ident} directives to the output file (this is
12711 @item -YP,@var{dirs}
12713 Search the directories @var{dirs}, and no others, for libraries
12714 specified with @option{-l}.
12716 @item -Ym,@var{dir}
12718 Look in the directory @var{dir} to find the M4 preprocessor.
12719 The assembler uses this option.
12720 @c This is supposed to go with a -Yd for predefined M4 macro files, but
12721 @c the generic assembler that comes with Solaris takes just -Ym.
12724 @node TMS320C3x/C4x Options
12725 @subsection TMS320C3x/C4x Options
12726 @cindex TMS320C3x/C4x Options
12728 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
12732 @item -mcpu=@var{cpu_type}
12734 Set the instruction set, register set, and instruction scheduling
12735 parameters for machine type @var{cpu_type}. Supported values for
12736 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
12737 @samp{c44}. The default is @samp{c40} to generate code for the
12742 @itemx -msmall-memory
12744 @opindex mbig-memory
12746 @opindex msmall-memory
12748 Generates code for the big or small memory model. The small memory
12749 model assumed that all data fits into one 64K word page. At run-time
12750 the data page (DP) register must be set to point to the 64K page
12751 containing the .bss and .data program sections. The big memory model is
12752 the default and requires reloading of the DP register for every direct
12759 Allow (disallow) allocation of general integer operands into the block
12760 count register BK@.
12766 Enable (disable) generation of code using decrement and branch,
12767 DBcond(D), instructions. This is enabled by default for the C4x. To be
12768 on the safe side, this is disabled for the C3x, since the maximum
12769 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
12770 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
12771 that it can utilize the decrement and branch instruction, but will give
12772 up if there is more than one memory reference in the loop. Thus a loop
12773 where the loop counter is decremented can generate slightly more
12774 efficient code, in cases where the RPTB instruction cannot be utilized.
12776 @item -mdp-isr-reload
12778 @opindex mdp-isr-reload
12780 Force the DP register to be saved on entry to an interrupt service
12781 routine (ISR), reloaded to point to the data section, and restored on
12782 exit from the ISR@. This should not be required unless someone has
12783 violated the small memory model by modifying the DP register, say within
12790 For the C3x use the 24-bit MPYI instruction for integer multiplies
12791 instead of a library call to guarantee 32-bit results. Note that if one
12792 of the operands is a constant, then the multiplication will be performed
12793 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
12794 then squaring operations are performed inline instead of a library call.
12797 @itemx -mno-fast-fix
12799 @opindex mno-fast-fix
12800 The C3x/C4x FIX instruction to convert a floating point value to an
12801 integer value chooses the nearest integer less than or equal to the
12802 floating point value rather than to the nearest integer. Thus if the
12803 floating point number is negative, the result will be incorrectly
12804 truncated an additional code is necessary to detect and correct this
12805 case. This option can be used to disable generation of the additional
12806 code required to correct the result.
12812 Enable (disable) generation of repeat block sequences using the RPTB
12813 instruction for zero overhead looping. The RPTB construct is only used
12814 for innermost loops that do not call functions or jump across the loop
12815 boundaries. There is no advantage having nested RPTB loops due to the
12816 overhead required to save and restore the RC, RS, and RE registers.
12817 This is enabled by default with @option{-O2}.
12819 @item -mrpts=@var{count}
12823 Enable (disable) the use of the single instruction repeat instruction
12824 RPTS@. If a repeat block contains a single instruction, and the loop
12825 count can be guaranteed to be less than the value @var{count}, GCC will
12826 emit a RPTS instruction instead of a RPTB@. If no value is specified,
12827 then a RPTS will be emitted even if the loop count cannot be determined
12828 at compile time. Note that the repeated instruction following RPTS does
12829 not have to be reloaded from memory each iteration, thus freeing up the
12830 CPU buses for operands. However, since interrupts are blocked by this
12831 instruction, it is disabled by default.
12833 @item -mloop-unsigned
12834 @itemx -mno-loop-unsigned
12835 @opindex mloop-unsigned
12836 @opindex mno-loop-unsigned
12837 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
12838 is @math{2^{31} + 1} since these instructions test if the iteration count is
12839 negative to terminate the loop. If the iteration count is unsigned
12840 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
12841 exceeded. This switch allows an unsigned iteration count.
12845 Try to emit an assembler syntax that the TI assembler (asm30) is happy
12846 with. This also enforces compatibility with the API employed by the TI
12847 C3x C compiler. For example, long doubles are passed as structures
12848 rather than in floating point registers.
12854 Generate code that uses registers (stack) for passing arguments to functions.
12855 By default, arguments are passed in registers where possible rather
12856 than by pushing arguments on to the stack.
12858 @item -mparallel-insns
12859 @itemx -mno-parallel-insns
12860 @opindex mparallel-insns
12861 @opindex mno-parallel-insns
12862 Allow the generation of parallel instructions. This is enabled by
12863 default with @option{-O2}.
12865 @item -mparallel-mpy
12866 @itemx -mno-parallel-mpy
12867 @opindex mparallel-mpy
12868 @opindex mno-parallel-mpy
12869 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
12870 provided @option{-mparallel-insns} is also specified. These instructions have
12871 tight register constraints which can pessimize the code generation
12872 of large functions.
12877 @subsection V850 Options
12878 @cindex V850 Options
12880 These @samp{-m} options are defined for V850 implementations:
12884 @itemx -mno-long-calls
12885 @opindex mlong-calls
12886 @opindex mno-long-calls
12887 Treat all calls as being far away (near). If calls are assumed to be
12888 far away, the compiler will always load the functions address up into a
12889 register, and call indirect through the pointer.
12895 Do not optimize (do optimize) basic blocks that use the same index
12896 pointer 4 or more times to copy pointer into the @code{ep} register, and
12897 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
12898 option is on by default if you optimize.
12900 @item -mno-prolog-function
12901 @itemx -mprolog-function
12902 @opindex mno-prolog-function
12903 @opindex mprolog-function
12904 Do not use (do use) external functions to save and restore registers
12905 at the prologue and epilogue of a function. The external functions
12906 are slower, but use less code space if more than one function saves
12907 the same number of registers. The @option{-mprolog-function} option
12908 is on by default if you optimize.
12912 Try to make the code as small as possible. At present, this just turns
12913 on the @option{-mep} and @option{-mprolog-function} options.
12915 @item -mtda=@var{n}
12917 Put static or global variables whose size is @var{n} bytes or less into
12918 the tiny data area that register @code{ep} points to. The tiny data
12919 area can hold up to 256 bytes in total (128 bytes for byte references).
12921 @item -msda=@var{n}
12923 Put static or global variables whose size is @var{n} bytes or less into
12924 the small data area that register @code{gp} points to. The small data
12925 area can hold up to 64 kilobytes.
12927 @item -mzda=@var{n}
12929 Put static or global variables whose size is @var{n} bytes or less into
12930 the first 32 kilobytes of memory.
12934 Specify that the target processor is the V850.
12937 @opindex mbig-switch
12938 Generate code suitable for big switch tables. Use this option only if
12939 the assembler/linker complain about out of range branches within a switch
12944 This option will cause r2 and r5 to be used in the code generated by
12945 the compiler. This setting is the default.
12947 @item -mno-app-regs
12948 @opindex mno-app-regs
12949 This option will cause r2 and r5 to be treated as fixed registers.
12953 Specify that the target processor is the V850E1. The preprocessor
12954 constants @samp{__v850e1__} and @samp{__v850e__} will be defined if
12955 this option is used.
12959 Specify that the target processor is the V850E@. The preprocessor
12960 constant @samp{__v850e__} will be defined if this option is used.
12962 If neither @option{-mv850} nor @option{-mv850e} nor @option{-mv850e1}
12963 are defined then a default target processor will be chosen and the
12964 relevant @samp{__v850*__} preprocessor constant will be defined.
12966 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
12967 defined, regardless of which processor variant is the target.
12969 @item -mdisable-callt
12970 @opindex mdisable-callt
12971 This option will suppress generation of the CALLT instruction for the
12972 v850e and v850e1 flavors of the v850 architecture. The default is
12973 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
12978 @subsection VAX Options
12979 @cindex VAX options
12981 These @samp{-m} options are defined for the VAX:
12986 Do not output certain jump instructions (@code{aobleq} and so on)
12987 that the Unix assembler for the VAX cannot handle across long
12992 Do output those jump instructions, on the assumption that you
12993 will assemble with the GNU assembler.
12997 Output code for g-format floating point numbers instead of d-format.
13000 @node x86-64 Options
13001 @subsection x86-64 Options
13002 @cindex x86-64 options
13004 These are listed under @xref{i386 and x86-64 Options}.
13006 @node Xstormy16 Options
13007 @subsection Xstormy16 Options
13008 @cindex Xstormy16 Options
13010 These options are defined for Xstormy16:
13015 Choose startup files and linker script suitable for the simulator.
13018 @node Xtensa Options
13019 @subsection Xtensa Options
13020 @cindex Xtensa Options
13022 These options are supported for Xtensa targets:
13026 @itemx -mno-const16
13028 @opindex mno-const16
13029 Enable or disable use of @code{CONST16} instructions for loading
13030 constant values. The @code{CONST16} instruction is currently not a
13031 standard option from Tensilica. When enabled, @code{CONST16}
13032 instructions are always used in place of the standard @code{L32R}
13033 instructions. The use of @code{CONST16} is enabled by default only if
13034 the @code{L32R} instruction is not available.
13037 @itemx -mno-fused-madd
13038 @opindex mfused-madd
13039 @opindex mno-fused-madd
13040 Enable or disable use of fused multiply/add and multiply/subtract
13041 instructions in the floating-point option. This has no effect if the
13042 floating-point option is not also enabled. Disabling fused multiply/add
13043 and multiply/subtract instructions forces the compiler to use separate
13044 instructions for the multiply and add/subtract operations. This may be
13045 desirable in some cases where strict IEEE 754-compliant results are
13046 required: the fused multiply add/subtract instructions do not round the
13047 intermediate result, thereby producing results with @emph{more} bits of
13048 precision than specified by the IEEE standard. Disabling fused multiply
13049 add/subtract instructions also ensures that the program output is not
13050 sensitive to the compiler's ability to combine multiply and add/subtract
13053 @item -mtext-section-literals
13054 @itemx -mno-text-section-literals
13055 @opindex mtext-section-literals
13056 @opindex mno-text-section-literals
13057 Control the treatment of literal pools. The default is
13058 @option{-mno-text-section-literals}, which places literals in a separate
13059 section in the output file. This allows the literal pool to be placed
13060 in a data RAM/ROM, and it also allows the linker to combine literal
13061 pools from separate object files to remove redundant literals and
13062 improve code size. With @option{-mtext-section-literals}, the literals
13063 are interspersed in the text section in order to keep them as close as
13064 possible to their references. This may be necessary for large assembly
13067 @item -mtarget-align
13068 @itemx -mno-target-align
13069 @opindex mtarget-align
13070 @opindex mno-target-align
13071 When this option is enabled, GCC instructs the assembler to
13072 automatically align instructions to reduce branch penalties at the
13073 expense of some code density. The assembler attempts to widen density
13074 instructions to align branch targets and the instructions following call
13075 instructions. If there are not enough preceding safe density
13076 instructions to align a target, no widening will be performed. The
13077 default is @option{-mtarget-align}. These options do not affect the
13078 treatment of auto-aligned instructions like @code{LOOP}, which the
13079 assembler will always align, either by widening density instructions or
13080 by inserting no-op instructions.
13083 @itemx -mno-longcalls
13084 @opindex mlongcalls
13085 @opindex mno-longcalls
13086 When this option is enabled, GCC instructs the assembler to translate
13087 direct calls to indirect calls unless it can determine that the target
13088 of a direct call is in the range allowed by the call instruction. This
13089 translation typically occurs for calls to functions in other source
13090 files. Specifically, the assembler translates a direct @code{CALL}
13091 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
13092 The default is @option{-mno-longcalls}. This option should be used in
13093 programs where the call target can potentially be out of range. This
13094 option is implemented in the assembler, not the compiler, so the
13095 assembly code generated by GCC will still show direct call
13096 instructions---look at the disassembled object code to see the actual
13097 instructions. Note that the assembler will use an indirect call for
13098 every cross-file call, not just those that really will be out of range.
13101 @node zSeries Options
13102 @subsection zSeries Options
13103 @cindex zSeries options
13105 These are listed under @xref{S/390 and zSeries Options}.
13107 @node Code Gen Options
13108 @section Options for Code Generation Conventions
13109 @cindex code generation conventions
13110 @cindex options, code generation
13111 @cindex run-time options
13113 These machine-independent options control the interface conventions
13114 used in code generation.
13116 Most of them have both positive and negative forms; the negative form
13117 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
13118 one of the forms is listed---the one which is not the default. You
13119 can figure out the other form by either removing @samp{no-} or adding
13123 @item -fbounds-check
13124 @opindex fbounds-check
13125 For front-ends that support it, generate additional code to check that
13126 indices used to access arrays are within the declared range. This is
13127 currently only supported by the Java and Fortran front-ends, where
13128 this option defaults to true and false respectively.
13132 This option generates traps for signed overflow on addition, subtraction,
13133 multiplication operations.
13137 This option instructs the compiler to assume that signed arithmetic
13138 overflow of addition, subtraction and multiplication wraps around
13139 using twos-complement representation. This flag enables some optimizations
13140 and disables others. This option is enabled by default for the Java
13141 front-end, as required by the Java language specification.
13144 @opindex fexceptions
13145 Enable exception handling. Generates extra code needed to propagate
13146 exceptions. For some targets, this implies GCC will generate frame
13147 unwind information for all functions, which can produce significant data
13148 size overhead, although it does not affect execution. If you do not
13149 specify this option, GCC will enable it by default for languages like
13150 C++ which normally require exception handling, and disable it for
13151 languages like C that do not normally require it. However, you may need
13152 to enable this option when compiling C code that needs to interoperate
13153 properly with exception handlers written in C++. You may also wish to
13154 disable this option if you are compiling older C++ programs that don't
13155 use exception handling.
13157 @item -fnon-call-exceptions
13158 @opindex fnon-call-exceptions
13159 Generate code that allows trapping instructions to throw exceptions.
13160 Note that this requires platform-specific runtime support that does
13161 not exist everywhere. Moreover, it only allows @emph{trapping}
13162 instructions to throw exceptions, i.e.@: memory references or floating
13163 point instructions. It does not allow exceptions to be thrown from
13164 arbitrary signal handlers such as @code{SIGALRM}.
13166 @item -funwind-tables
13167 @opindex funwind-tables
13168 Similar to @option{-fexceptions}, except that it will just generate any needed
13169 static data, but will not affect the generated code in any other way.
13170 You will normally not enable this option; instead, a language processor
13171 that needs this handling would enable it on your behalf.
13173 @item -fasynchronous-unwind-tables
13174 @opindex fasynchronous-unwind-tables
13175 Generate unwind table in dwarf2 format, if supported by target machine. The
13176 table is exact at each instruction boundary, so it can be used for stack
13177 unwinding from asynchronous events (such as debugger or garbage collector).
13179 @item -fpcc-struct-return
13180 @opindex fpcc-struct-return
13181 Return ``short'' @code{struct} and @code{union} values in memory like
13182 longer ones, rather than in registers. This convention is less
13183 efficient, but it has the advantage of allowing intercallability between
13184 GCC-compiled files and files compiled with other compilers, particularly
13185 the Portable C Compiler (pcc).
13187 The precise convention for returning structures in memory depends
13188 on the target configuration macros.
13190 Short structures and unions are those whose size and alignment match
13191 that of some integer type.
13193 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
13194 switch is not binary compatible with code compiled with the
13195 @option{-freg-struct-return} switch.
13196 Use it to conform to a non-default application binary interface.
13198 @item -freg-struct-return
13199 @opindex freg-struct-return
13200 Return @code{struct} and @code{union} values in registers when possible.
13201 This is more efficient for small structures than
13202 @option{-fpcc-struct-return}.
13204 If you specify neither @option{-fpcc-struct-return} nor
13205 @option{-freg-struct-return}, GCC defaults to whichever convention is
13206 standard for the target. If there is no standard convention, GCC
13207 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
13208 the principal compiler. In those cases, we can choose the standard, and
13209 we chose the more efficient register return alternative.
13211 @strong{Warning:} code compiled with the @option{-freg-struct-return}
13212 switch is not binary compatible with code compiled with the
13213 @option{-fpcc-struct-return} switch.
13214 Use it to conform to a non-default application binary interface.
13216 @item -fshort-enums
13217 @opindex fshort-enums
13218 Allocate to an @code{enum} type only as many bytes as it needs for the
13219 declared range of possible values. Specifically, the @code{enum} type
13220 will be equivalent to the smallest integer type which has enough room.
13222 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
13223 code that is not binary compatible with code generated without that switch.
13224 Use it to conform to a non-default application binary interface.
13226 @item -fshort-double
13227 @opindex fshort-double
13228 Use the same size for @code{double} as for @code{float}.
13230 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
13231 code that is not binary compatible with code generated without that switch.
13232 Use it to conform to a non-default application binary interface.
13234 @item -fshort-wchar
13235 @opindex fshort-wchar
13236 Override the underlying type for @samp{wchar_t} to be @samp{short
13237 unsigned int} instead of the default for the target. This option is
13238 useful for building programs to run under WINE@.
13240 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
13241 code that is not binary compatible with code generated without that switch.
13242 Use it to conform to a non-default application binary interface.
13245 @opindex fno-common
13246 In C, allocate even uninitialized global variables in the data section of the
13247 object file, rather than generating them as common blocks. This has the
13248 effect that if the same variable is declared (without @code{extern}) in
13249 two different compilations, you will get an error when you link them.
13250 The only reason this might be useful is if you wish to verify that the
13251 program will work on other systems which always work this way.
13255 Ignore the @samp{#ident} directive.
13257 @item -finhibit-size-directive
13258 @opindex finhibit-size-directive
13259 Don't output a @code{.size} assembler directive, or anything else that
13260 would cause trouble if the function is split in the middle, and the
13261 two halves are placed at locations far apart in memory. This option is
13262 used when compiling @file{crtstuff.c}; you should not need to use it
13265 @item -fverbose-asm
13266 @opindex fverbose-asm
13267 Put extra commentary information in the generated assembly code to
13268 make it more readable. This option is generally only of use to those
13269 who actually need to read the generated assembly code (perhaps while
13270 debugging the compiler itself).
13272 @option{-fno-verbose-asm}, the default, causes the
13273 extra information to be omitted and is useful when comparing two assembler
13278 @cindex global offset table
13280 Generate position-independent code (PIC) suitable for use in a shared
13281 library, if supported for the target machine. Such code accesses all
13282 constant addresses through a global offset table (GOT)@. The dynamic
13283 loader resolves the GOT entries when the program starts (the dynamic
13284 loader is not part of GCC; it is part of the operating system). If
13285 the GOT size for the linked executable exceeds a machine-specific
13286 maximum size, you get an error message from the linker indicating that
13287 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
13288 instead. (These maximums are 8k on the SPARC and 32k
13289 on the m68k and RS/6000. The 386 has no such limit.)
13291 Position-independent code requires special support, and therefore works
13292 only on certain machines. For the 386, GCC supports PIC for System V
13293 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
13294 position-independent.
13296 When this flag is set, the macros @code{__pic__} and @code{__PIC__}
13301 If supported for the target machine, emit position-independent code,
13302 suitable for dynamic linking and avoiding any limit on the size of the
13303 global offset table. This option makes a difference on the m68k,
13304 PowerPC and SPARC@.
13306 Position-independent code requires special support, and therefore works
13307 only on certain machines.
13309 When this flag is set, the macros @code{__pic__} and @code{__PIC__}
13316 These options are similar to @option{-fpic} and @option{-fPIC}, but
13317 generated position independent code can be only linked into executables.
13318 Usually these options are used when @option{-pie} GCC option will be
13319 used during linking.
13321 @item -fno-jump-tables
13322 @opindex fno-jump-tables
13323 Do not use jump tables for switch statements even where it would be
13324 more efficient than other code generation strategies. This option is
13325 of use in conjunction with @option{-fpic} or @option{-fPIC} for
13326 building code which forms part of a dynamic linker and cannot
13327 reference the address of a jump table. On some targets, jump tables
13328 do not require a GOT and this option is not needed.
13330 @item -ffixed-@var{reg}
13332 Treat the register named @var{reg} as a fixed register; generated code
13333 should never refer to it (except perhaps as a stack pointer, frame
13334 pointer or in some other fixed role).
13336 @var{reg} must be the name of a register. The register names accepted
13337 are machine-specific and are defined in the @code{REGISTER_NAMES}
13338 macro in the machine description macro file.
13340 This flag does not have a negative form, because it specifies a
13343 @item -fcall-used-@var{reg}
13344 @opindex fcall-used
13345 Treat the register named @var{reg} as an allocable register that is
13346 clobbered by function calls. It may be allocated for temporaries or
13347 variables that do not live across a call. Functions compiled this way
13348 will not save and restore the register @var{reg}.
13350 It is an error to used this flag with the frame pointer or stack pointer.
13351 Use of this flag for other registers that have fixed pervasive roles in
13352 the machine's execution model will produce disastrous results.
13354 This flag does not have a negative form, because it specifies a
13357 @item -fcall-saved-@var{reg}
13358 @opindex fcall-saved
13359 Treat the register named @var{reg} as an allocable register saved by
13360 functions. It may be allocated even for temporaries or variables that
13361 live across a call. Functions compiled this way will save and restore
13362 the register @var{reg} if they use it.
13364 It is an error to used this flag with the frame pointer or stack pointer.
13365 Use of this flag for other registers that have fixed pervasive roles in
13366 the machine's execution model will produce disastrous results.
13368 A different sort of disaster will result from the use of this flag for
13369 a register in which function values may be returned.
13371 This flag does not have a negative form, because it specifies a
13374 @item -fpack-struct[=@var{n}]
13375 @opindex fpack-struct
13376 Without a value specified, pack all structure members together without
13377 holes. When a value is specified (which must be a small power of two), pack
13378 structure members according to this value, representing the maximum
13379 alignment (that is, objects with default alignment requirements larger than
13380 this will be output potentially unaligned at the next fitting location.
13382 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
13383 code that is not binary compatible with code generated without that switch.
13384 Additionally, it makes the code suboptimal.
13385 Use it to conform to a non-default application binary interface.
13387 @item -finstrument-functions
13388 @opindex finstrument-functions
13389 Generate instrumentation calls for entry and exit to functions. Just
13390 after function entry and just before function exit, the following
13391 profiling functions will be called with the address of the current
13392 function and its call site. (On some platforms,
13393 @code{__builtin_return_address} does not work beyond the current
13394 function, so the call site information may not be available to the
13395 profiling functions otherwise.)
13398 void __cyg_profile_func_enter (void *this_fn,
13400 void __cyg_profile_func_exit (void *this_fn,
13404 The first argument is the address of the start of the current function,
13405 which may be looked up exactly in the symbol table.
13407 This instrumentation is also done for functions expanded inline in other
13408 functions. The profiling calls will indicate where, conceptually, the
13409 inline function is entered and exited. This means that addressable
13410 versions of such functions must be available. If all your uses of a
13411 function are expanded inline, this may mean an additional expansion of
13412 code size. If you use @samp{extern inline} in your C code, an
13413 addressable version of such functions must be provided. (This is
13414 normally the case anyways, but if you get lucky and the optimizer always
13415 expands the functions inline, you might have gotten away without
13416 providing static copies.)
13418 A function may be given the attribute @code{no_instrument_function}, in
13419 which case this instrumentation will not be done. This can be used, for
13420 example, for the profiling functions listed above, high-priority
13421 interrupt routines, and any functions from which the profiling functions
13422 cannot safely be called (perhaps signal handlers, if the profiling
13423 routines generate output or allocate memory).
13425 @item -fstack-check
13426 @opindex fstack-check
13427 Generate code to verify that you do not go beyond the boundary of the
13428 stack. You should specify this flag if you are running in an
13429 environment with multiple threads, but only rarely need to specify it in
13430 a single-threaded environment since stack overflow is automatically
13431 detected on nearly all systems if there is only one stack.
13433 Note that this switch does not actually cause checking to be done; the
13434 operating system must do that. The switch causes generation of code
13435 to ensure that the operating system sees the stack being extended.
13437 @item -fstack-limit-register=@var{reg}
13438 @itemx -fstack-limit-symbol=@var{sym}
13439 @itemx -fno-stack-limit
13440 @opindex fstack-limit-register
13441 @opindex fstack-limit-symbol
13442 @opindex fno-stack-limit
13443 Generate code to ensure that the stack does not grow beyond a certain value,
13444 either the value of a register or the address of a symbol. If the stack
13445 would grow beyond the value, a signal is raised. For most targets,
13446 the signal is raised before the stack overruns the boundary, so
13447 it is possible to catch the signal without taking special precautions.
13449 For instance, if the stack starts at absolute address @samp{0x80000000}
13450 and grows downwards, you can use the flags
13451 @option{-fstack-limit-symbol=__stack_limit} and
13452 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
13453 of 128KB@. Note that this may only work with the GNU linker.
13455 @cindex aliasing of parameters
13456 @cindex parameters, aliased
13457 @item -fargument-alias
13458 @itemx -fargument-noalias
13459 @itemx -fargument-noalias-global
13460 @itemx -fargument-noalias-anything
13461 @opindex fargument-alias
13462 @opindex fargument-noalias
13463 @opindex fargument-noalias-global
13464 @opindex fargument-noalias-anything
13465 Specify the possible relationships among parameters and between
13466 parameters and global data.
13468 @option{-fargument-alias} specifies that arguments (parameters) may
13469 alias each other and may alias global storage.@*
13470 @option{-fargument-noalias} specifies that arguments do not alias
13471 each other, but may alias global storage.@*
13472 @option{-fargument-noalias-global} specifies that arguments do not
13473 alias each other and do not alias global storage.
13474 @option{-fargument-noalias-anything} specifies that arguments do not
13475 alias any other storage.
13477 Each language will automatically use whatever option is required by
13478 the language standard. You should not need to use these options yourself.
13480 @item -fleading-underscore
13481 @opindex fleading-underscore
13482 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
13483 change the way C symbols are represented in the object file. One use
13484 is to help link with legacy assembly code.
13486 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
13487 generate code that is not binary compatible with code generated without that
13488 switch. Use it to conform to a non-default application binary interface.
13489 Not all targets provide complete support for this switch.
13491 @item -ftls-model=@var{model}
13492 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
13493 The @var{model} argument should be one of @code{global-dynamic},
13494 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
13496 The default without @option{-fpic} is @code{initial-exec}; with
13497 @option{-fpic} the default is @code{global-dynamic}.
13499 @item -fvisibility=@var{default|internal|hidden|protected}
13500 @opindex fvisibility
13501 Set the default ELF image symbol visibility to the specified option---all
13502 symbols will be marked with this unless overridden within the code.
13503 Using this feature can very substantially improve linking and
13504 load times of shared object libraries, produce more optimized
13505 code, provide near-perfect API export and prevent symbol clashes.
13506 It is @strong{strongly} recommended that you use this in any shared objects
13509 Despite the nomenclature, @code{default} always means public ie;
13510 available to be linked against from outside the shared object.
13511 @code{protected} and @code{internal} are pretty useless in real-world
13512 usage so the only other commonly used option will be @code{hidden}.
13513 The default if @option{-fvisibility} isn't specified is
13514 @code{default}, i.e., make every
13515 symbol public---this causes the same behavior as previous versions of
13518 A good explanation of the benefits offered by ensuring ELF
13519 symbols have the correct visibility is given by ``How To Write
13520 Shared Libraries'' by Ulrich Drepper (which can be found at
13521 @w{@uref{http://people.redhat.com/~drepper/}})---however a superior
13522 solution made possible by this option to marking things hidden when
13523 the default is public is to make the default hidden and mark things
13524 public. This is the norm with DLL's on Windows and with @option{-fvisibility=hidden}
13525 and @code{__attribute__ ((visibility("default")))} instead of
13526 @code{__declspec(dllexport)} you get almost identical semantics with
13527 identical syntax. This is a great boon to those working with
13528 cross-platform projects.
13530 For those adding visibility support to existing code, you may find
13531 @samp{#pragma GCC visibility} of use. This works by you enclosing
13532 the declarations you wish to set visibility for with (for example)
13533 @samp{#pragma GCC visibility push(hidden)} and
13534 @samp{#pragma GCC visibility pop}.
13535 Bear in mind that symbol visibility should be viewed @strong{as
13536 part of the API interface contract} and thus all new code should
13537 always specify visibility when it is not the default ie; declarations
13538 only for use within the local DSO should @strong{always} be marked explicitly
13539 as hidden as so to avoid PLT indirection overheads---making this
13540 abundantly clear also aids readability and self-documentation of the code.
13541 Note that due to ISO C++ specification requirements, operator new and
13542 operator delete must always be of default visibility.
13544 Be aware that headers from outside your project, in particular system
13545 headers and headers from any other library you use, may not be
13546 expecting to be compiled with visibility other than the default. You
13547 may need to explicitly say @samp{#pragma GCC visibility push(default)}
13548 before including any such headers.
13550 @samp{extern} declarations are not affected by @samp{-fvisibility}, so
13551 a lot of code can be recompiled with @samp{-fvisibility=hidden} with
13552 no modifications. However, this means that calls to @samp{extern}
13553 functions with no explicit visibility will use the PLT, so it is more
13554 effective to use @samp{__attribute ((visibility))} and/or
13555 @samp{#pragma GCC visibility} to tell the compiler which @samp{extern}
13556 declarations should be treated as hidden.
13558 Note that @samp{-fvisibility} does affect C++ vague linkage
13559 entities. This means that, for instance, an exception class that will
13560 be thrown between DSOs must be explicitly marked with default
13561 visibility so that the @samp{type_info} nodes will be unified between
13564 An overview of these techniques, their benefits and how to use them
13565 is at @w{@uref{http://gcc.gnu.org/wiki/Visibility}}.
13571 @node Environment Variables
13572 @section Environment Variables Affecting GCC
13573 @cindex environment variables
13575 @c man begin ENVIRONMENT
13576 This section describes several environment variables that affect how GCC
13577 operates. Some of them work by specifying directories or prefixes to use
13578 when searching for various kinds of files. Some are used to specify other
13579 aspects of the compilation environment.
13581 Note that you can also specify places to search using options such as
13582 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
13583 take precedence over places specified using environment variables, which
13584 in turn take precedence over those specified by the configuration of GCC@.
13585 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
13586 GNU Compiler Collection (GCC) Internals}.
13591 @c @itemx LC_COLLATE
13593 @c @itemx LC_MONETARY
13594 @c @itemx LC_NUMERIC
13599 @c @findex LC_COLLATE
13600 @findex LC_MESSAGES
13601 @c @findex LC_MONETARY
13602 @c @findex LC_NUMERIC
13606 These environment variables control the way that GCC uses
13607 localization information that allow GCC to work with different
13608 national conventions. GCC inspects the locale categories
13609 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
13610 so. These locale categories can be set to any value supported by your
13611 installation. A typical value is @samp{en_GB.UTF-8} for English in the United
13612 Kingdom encoded in UTF-8.
13614 The @env{LC_CTYPE} environment variable specifies character
13615 classification. GCC uses it to determine the character boundaries in
13616 a string; this is needed for some multibyte encodings that contain quote
13617 and escape characters that would otherwise be interpreted as a string
13620 The @env{LC_MESSAGES} environment variable specifies the language to
13621 use in diagnostic messages.
13623 If the @env{LC_ALL} environment variable is set, it overrides the value
13624 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
13625 and @env{LC_MESSAGES} default to the value of the @env{LANG}
13626 environment variable. If none of these variables are set, GCC
13627 defaults to traditional C English behavior.
13631 If @env{TMPDIR} is set, it specifies the directory to use for temporary
13632 files. GCC uses temporary files to hold the output of one stage of
13633 compilation which is to be used as input to the next stage: for example,
13634 the output of the preprocessor, which is the input to the compiler
13637 @item GCC_EXEC_PREFIX
13638 @findex GCC_EXEC_PREFIX
13639 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
13640 names of the subprograms executed by the compiler. No slash is added
13641 when this prefix is combined with the name of a subprogram, but you can
13642 specify a prefix that ends with a slash if you wish.
13644 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
13645 an appropriate prefix to use based on the pathname it was invoked with.
13647 If GCC cannot find the subprogram using the specified prefix, it
13648 tries looking in the usual places for the subprogram.
13650 The default value of @env{GCC_EXEC_PREFIX} is
13651 @file{@var{prefix}/lib/gcc/} where @var{prefix} is the value
13652 of @code{prefix} when you ran the @file{configure} script.
13654 Other prefixes specified with @option{-B} take precedence over this prefix.
13656 This prefix is also used for finding files such as @file{crt0.o} that are
13659 In addition, the prefix is used in an unusual way in finding the
13660 directories to search for header files. For each of the standard
13661 directories whose name normally begins with @samp{/usr/local/lib/gcc}
13662 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
13663 replacing that beginning with the specified prefix to produce an
13664 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
13665 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
13666 These alternate directories are searched first; the standard directories
13669 @item COMPILER_PATH
13670 @findex COMPILER_PATH
13671 The value of @env{COMPILER_PATH} is a colon-separated list of
13672 directories, much like @env{PATH}. GCC tries the directories thus
13673 specified when searching for subprograms, if it can't find the
13674 subprograms using @env{GCC_EXEC_PREFIX}.
13677 @findex LIBRARY_PATH
13678 The value of @env{LIBRARY_PATH} is a colon-separated list of
13679 directories, much like @env{PATH}. When configured as a native compiler,
13680 GCC tries the directories thus specified when searching for special
13681 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
13682 using GCC also uses these directories when searching for ordinary
13683 libraries for the @option{-l} option (but directories specified with
13684 @option{-L} come first).
13688 @cindex locale definition
13689 This variable is used to pass locale information to the compiler. One way in
13690 which this information is used is to determine the character set to be used
13691 when character literals, string literals and comments are parsed in C and C++.
13692 When the compiler is configured to allow multibyte characters,
13693 the following values for @env{LANG} are recognized:
13697 Recognize JIS characters.
13699 Recognize SJIS characters.
13701 Recognize EUCJP characters.
13704 If @env{LANG} is not defined, or if it has some other value, then the
13705 compiler will use mblen and mbtowc as defined by the default locale to
13706 recognize and translate multibyte characters.
13710 Some additional environments variables affect the behavior of the
13713 @include cppenv.texi
13717 @node Precompiled Headers
13718 @section Using Precompiled Headers
13719 @cindex precompiled headers
13720 @cindex speed of compilation
13722 Often large projects have many header files that are included in every
13723 source file. The time the compiler takes to process these header files
13724 over and over again can account for nearly all of the time required to
13725 build the project. To make builds faster, GCC allows users to
13726 `precompile' a header file; then, if builds can use the precompiled
13727 header file they will be much faster.
13729 To create a precompiled header file, simply compile it as you would any
13730 other file, if necessary using the @option{-x} option to make the driver
13731 treat it as a C or C++ header file. You will probably want to use a
13732 tool like @command{make} to keep the precompiled header up-to-date when
13733 the headers it contains change.
13735 A precompiled header file will be searched for when @code{#include} is
13736 seen in the compilation. As it searches for the included file
13737 (@pxref{Search Path,,Search Path,cpp,The C Preprocessor}) the
13738 compiler looks for a precompiled header in each directory just before it
13739 looks for the include file in that directory. The name searched for is
13740 the name specified in the @code{#include} with @samp{.gch} appended. If
13741 the precompiled header file can't be used, it is ignored.
13743 For instance, if you have @code{#include "all.h"}, and you have
13744 @file{all.h.gch} in the same directory as @file{all.h}, then the
13745 precompiled header file will be used if possible, and the original
13746 header will be used otherwise.
13748 Alternatively, you might decide to put the precompiled header file in a
13749 directory and use @option{-I} to ensure that directory is searched
13750 before (or instead of) the directory containing the original header.
13751 Then, if you want to check that the precompiled header file is always
13752 used, you can put a file of the same name as the original header in this
13753 directory containing an @code{#error} command.
13755 This also works with @option{-include}. So yet another way to use
13756 precompiled headers, good for projects not designed with precompiled
13757 header files in mind, is to simply take most of the header files used by
13758 a project, include them from another header file, precompile that header
13759 file, and @option{-include} the precompiled header. If the header files
13760 have guards against multiple inclusion, they will be skipped because
13761 they've already been included (in the precompiled header).
13763 If you need to precompile the same header file for different
13764 languages, targets, or compiler options, you can instead make a
13765 @emph{directory} named like @file{all.h.gch}, and put each precompiled
13766 header in the directory, perhaps using @option{-o}. It doesn't matter
13767 what you call the files in the directory, every precompiled header in
13768 the directory will be considered. The first precompiled header
13769 encountered in the directory that is valid for this compilation will
13770 be used; they're searched in no particular order.
13772 There are many other possibilities, limited only by your imagination,
13773 good sense, and the constraints of your build system.
13775 A precompiled header file can be used only when these conditions apply:
13779 Only one precompiled header can be used in a particular compilation.
13782 A precompiled header can't be used once the first C token is seen. You
13783 can have preprocessor directives before a precompiled header; you can
13784 even include a precompiled header from inside another header, so long as
13785 there are no C tokens before the @code{#include}.
13788 The precompiled header file must be produced for the same language as
13789 the current compilation. You can't use a C precompiled header for a C++
13793 The precompiled header file must have been produced by the same compiler
13794 binary as the current compilation is using.
13797 Any macros defined before the precompiled header is included must
13798 either be defined in the same way as when the precompiled header was
13799 generated, or must not affect the precompiled header, which usually
13800 means that they don't appear in the precompiled header at all.
13802 The @option{-D} option is one way to define a macro before a
13803 precompiled header is included; using a @code{#define} can also do it.
13804 There are also some options that define macros implicitly, like
13805 @option{-O} and @option{-Wdeprecated}; the same rule applies to macros
13808 @item If debugging information is output when using the precompiled
13809 header, using @option{-g} or similar, the same kind of debugging information
13810 must have been output when building the precompiled header. However,
13811 a precompiled header built using @option{-g} can be used in a compilation
13812 when no debugging information is being output.
13814 @item The same @option{-m} options must generally be used when building
13815 and using the precompiled header. @xref{Submodel Options},
13816 for any cases where this rule is relaxed.
13818 @item Each of the following options must be the same when building and using
13819 the precompiled header:
13821 @gccoptlist{-fexceptions -funit-at-a-time}
13824 Some other command-line options starting with @option{-f},
13825 @option{-p}, or @option{-O} must be defined in the same way as when
13826 the precompiled header was generated. At present, it's not clear
13827 which options are safe to change and which are not; the safest choice
13828 is to use exactly the same options when generating and using the
13829 precompiled header. The following are known to be safe:
13831 @gccoptlist{-fmessage-length= -fpreprocessed
13832 -fsched-interblock -fsched-spec -fsched-spec-load -fsched-spec-load-dangerous
13833 -fsched-verbose=<number> -fschedule-insns -fvisibility=
13838 For all of these except the last, the compiler will automatically
13839 ignore the precompiled header if the conditions aren't met. If you
13840 find an option combination that doesn't work and doesn't cause the
13841 precompiled header to be ignored, please consider filing a bug report,
13844 If you do use differing options when generating and using the
13845 precompiled header, the actual behavior will be a mixture of the
13846 behavior for the options. For instance, if you use @option{-g} to
13847 generate the precompiled header but not when using it, you may or may
13848 not get debugging information for routines in the precompiled header.
13850 @node Running Protoize
13851 @section Running Protoize
13853 The program @code{protoize} is an optional part of GCC@. You can use
13854 it to add prototypes to a program, thus converting the program to ISO
13855 C in one respect. The companion program @code{unprotoize} does the
13856 reverse: it removes argument types from any prototypes that are found.
13858 When you run these programs, you must specify a set of source files as
13859 command line arguments. The conversion programs start out by compiling
13860 these files to see what functions they define. The information gathered
13861 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
13863 After scanning comes actual conversion. The specified files are all
13864 eligible to be converted; any files they include (whether sources or
13865 just headers) are eligible as well.
13867 But not all the eligible files are converted. By default,
13868 @code{protoize} and @code{unprotoize} convert only source and header
13869 files in the current directory. You can specify additional directories
13870 whose files should be converted with the @option{-d @var{directory}}
13871 option. You can also specify particular files to exclude with the
13872 @option{-x @var{file}} option. A file is converted if it is eligible, its
13873 directory name matches one of the specified directory names, and its
13874 name within the directory has not been excluded.
13876 Basic conversion with @code{protoize} consists of rewriting most
13877 function definitions and function declarations to specify the types of
13878 the arguments. The only ones not rewritten are those for varargs
13881 @code{protoize} optionally inserts prototype declarations at the
13882 beginning of the source file, to make them available for any calls that
13883 precede the function's definition. Or it can insert prototype
13884 declarations with block scope in the blocks where undeclared functions
13887 Basic conversion with @code{unprotoize} consists of rewriting most
13888 function declarations to remove any argument types, and rewriting
13889 function definitions to the old-style pre-ISO form.
13891 Both conversion programs print a warning for any function declaration or
13892 definition that they can't convert. You can suppress these warnings
13895 The output from @code{protoize} or @code{unprotoize} replaces the
13896 original source file. The original file is renamed to a name ending
13897 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
13898 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
13899 for DOS) file already exists, then the source file is simply discarded.
13901 @code{protoize} and @code{unprotoize} both depend on GCC itself to
13902 scan the program and collect information about the functions it uses.
13903 So neither of these programs will work until GCC is installed.
13905 Here is a table of the options you can use with @code{protoize} and
13906 @code{unprotoize}. Each option works with both programs unless
13910 @item -B @var{directory}
13911 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
13912 usual directory (normally @file{/usr/local/lib}). This file contains
13913 prototype information about standard system functions. This option
13914 applies only to @code{protoize}.
13916 @item -c @var{compilation-options}
13917 Use @var{compilation-options} as the options when running @command{gcc} to
13918 produce the @samp{.X} files. The special option @option{-aux-info} is
13919 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
13921 Note that the compilation options must be given as a single argument to
13922 @code{protoize} or @code{unprotoize}. If you want to specify several
13923 @command{gcc} options, you must quote the entire set of compilation options
13924 to make them a single word in the shell.
13926 There are certain @command{gcc} arguments that you cannot use, because they
13927 would produce the wrong kind of output. These include @option{-g},
13928 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
13929 the @var{compilation-options}, they are ignored.
13932 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
13933 systems) instead of @samp{.c}. This is convenient if you are converting
13934 a C program to C++. This option applies only to @code{protoize}.
13937 Add explicit global declarations. This means inserting explicit
13938 declarations at the beginning of each source file for each function
13939 that is called in the file and was not declared. These declarations
13940 precede the first function definition that contains a call to an
13941 undeclared function. This option applies only to @code{protoize}.
13943 @item -i @var{string}
13944 Indent old-style parameter declarations with the string @var{string}.
13945 This option applies only to @code{protoize}.
13947 @code{unprotoize} converts prototyped function definitions to old-style
13948 function definitions, where the arguments are declared between the
13949 argument list and the initial @samp{@{}. By default, @code{unprotoize}
13950 uses five spaces as the indentation. If you want to indent with just
13951 one space instead, use @option{-i " "}.
13954 Keep the @samp{.X} files. Normally, they are deleted after conversion
13958 Add explicit local declarations. @code{protoize} with @option{-l} inserts
13959 a prototype declaration for each function in each block which calls the
13960 function without any declaration. This option applies only to
13964 Make no real changes. This mode just prints information about the conversions
13965 that would have been done without @option{-n}.
13968 Make no @samp{.save} files. The original files are simply deleted.
13969 Use this option with caution.
13971 @item -p @var{program}
13972 Use the program @var{program} as the compiler. Normally, the name
13973 @file{gcc} is used.
13976 Work quietly. Most warnings are suppressed.
13979 Print the version number, just like @option{-v} for @command{gcc}.
13982 If you need special compiler options to compile one of your program's
13983 source files, then you should generate that file's @samp{.X} file
13984 specially, by running @command{gcc} on that source file with the
13985 appropriate options and the option @option{-aux-info}. Then run
13986 @code{protoize} on the entire set of files. @code{protoize} will use
13987 the existing @samp{.X} file because it is newer than the source file.
13991 gcc -Dfoo=bar file1.c -aux-info file1.X
13996 You need to include the special files along with the rest in the
13997 @code{protoize} command, even though their @samp{.X} files already
13998 exist, because otherwise they won't get converted.
14000 @xref{Protoize Caveats}, for more information on how to use
14001 @code{protoize} successfully.