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 -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-options
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-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
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 -msselibm @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
782 -fvisibility -fopenmp}
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
953 Turn off any specification of a language, so that subsequent files are
954 handled according to their file name suffixes (as they are if @option{-x}
955 has not been used at all).
957 @item -pass-exit-codes
958 @opindex pass-exit-codes
959 Normally the @command{gcc} program will exit with the code of 1 if any
960 phase of the compiler returns a non-success return code. If you specify
961 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
962 numerically highest error produced by any phase that returned an error
963 indication. The C, C++, and Fortran frontends return 4, if an internal
964 compiler error is encountered.
967 If you only want some of the stages of compilation, you can use
968 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
969 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
970 @command{gcc} is to stop. Note that some combinations (for example,
971 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
976 Compile or assemble the source files, but do not link. The linking
977 stage simply is not done. The ultimate output is in the form of an
978 object file for each source file.
980 By default, the object file name for a source file is made by replacing
981 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
983 Unrecognized input files, not requiring compilation or assembly, are
988 Stop after the stage of compilation proper; do not assemble. The output
989 is in the form of an assembler code file for each non-assembler input
992 By default, the assembler file name for a source file is made by
993 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
995 Input files that don't require compilation are ignored.
999 Stop after the preprocessing stage; do not run the compiler proper. The
1000 output is in the form of preprocessed source code, which is sent to the
1003 Input files which don't require preprocessing are ignored.
1005 @cindex output file option
1008 Place output in file @var{file}. This applies regardless to whatever
1009 sort of output is being produced, whether it be an executable file,
1010 an object file, an assembler file or preprocessed C code.
1012 If @option{-o} is not specified, the default is to put an executable
1013 file in @file{a.out}, the object file for
1014 @file{@var{source}.@var{suffix}} in @file{@var{source}.o}, its
1015 assembler file in @file{@var{source}.s}, a precompiled header file in
1016 @file{@var{source}.@var{suffix}.gch}, and all preprocessed C source on
1021 Print (on standard error output) the commands executed to run the stages
1022 of compilation. Also print the version number of the compiler driver
1023 program and of the preprocessor and the compiler proper.
1027 Like @option{-v} except the commands are not executed and all command
1028 arguments are quoted. This is useful for shell scripts to capture the
1029 driver-generated command lines.
1033 Use pipes rather than temporary files for communication between the
1034 various stages of compilation. This fails to work on some systems where
1035 the assembler is unable to read from a pipe; but the GNU assembler has
1040 If you are compiling multiple source files, this option tells the driver
1041 to pass all the source files to the compiler at once (for those
1042 languages for which the compiler can handle this). This will allow
1043 intermodule analysis (IMA) to be performed by the compiler. Currently the only
1044 language for which this is supported is C@. If you pass source files for
1045 multiple languages to the driver, using this option, the driver will invoke
1046 the compiler(s) that support IMA once each, passing each compiler all the
1047 source files appropriate for it. For those languages that do not support
1048 IMA this option will be ignored, and the compiler will be invoked once for
1049 each source file in that language. If you use this option in conjunction
1050 with @option{-save-temps}, the compiler will generate multiple
1052 (one for each source file), but only one (combined) @file{.o} or
1057 Print (on the standard output) a description of the command line options
1058 understood by @command{gcc}. If the @option{-v} option is also specified
1059 then @option{--help} will also be passed on to the various processes
1060 invoked by @command{gcc}, so that they can display the command line options
1061 they accept. If the @option{-Wextra} option is also specified then command
1062 line options which have no documentation associated with them will also
1066 @opindex target-help
1067 Print (on the standard output) a description of target specific command
1068 line options for each tool.
1072 Display the version number and copyrights of the invoked GCC@.
1074 @include @value{srcdir}/../libiberty/at-file.texi
1078 @section Compiling C++ Programs
1080 @cindex suffixes for C++ source
1081 @cindex C++ source file suffixes
1082 C++ source files conventionally use one of the suffixes @samp{.C},
1083 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
1084 @samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
1085 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
1086 files with these names and compiles them as C++ programs even if you
1087 call the compiler the same way as for compiling C programs (usually
1088 with the name @command{gcc}).
1092 However, C++ programs often require class libraries as well as a
1093 compiler that understands the C++ language---and under some
1094 circumstances, you might want to compile programs or header files from
1095 standard input, or otherwise without a suffix that flags them as C++
1096 programs. You might also like to precompile a C header file with a
1097 @samp{.h} extension to be used in C++ compilations. @command{g++} is a
1098 program that calls GCC with the default language set to C++, and
1099 automatically specifies linking against the C++ library. On many
1100 systems, @command{g++} is also installed with the name @command{c++}.
1102 @cindex invoking @command{g++}
1103 When you compile C++ programs, you may specify many of the same
1104 command-line options that you use for compiling programs in any
1105 language; or command-line options meaningful for C and related
1106 languages; or options that are meaningful only for C++ programs.
1107 @xref{C Dialect Options,,Options Controlling C Dialect}, for
1108 explanations of options for languages related to C@.
1109 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
1110 explanations of options that are meaningful only for C++ programs.
1112 @node C Dialect Options
1113 @section Options Controlling C Dialect
1114 @cindex dialect options
1115 @cindex language dialect options
1116 @cindex options, dialect
1118 The following options control the dialect of C (or languages derived
1119 from C, such as C++, Objective-C and Objective-C++) that the compiler
1123 @cindex ANSI support
1127 In C mode, support all ISO C90 programs. In C++ mode,
1128 remove GNU extensions that conflict with ISO C++.
1130 This turns off certain features of GCC that are incompatible with ISO
1131 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
1132 such as the @code{asm} and @code{typeof} keywords, and
1133 predefined macros such as @code{unix} and @code{vax} that identify the
1134 type of system you are using. It also enables the undesirable and
1135 rarely used ISO trigraph feature. For the C compiler,
1136 it disables recognition of C++ style @samp{//} comments as well as
1137 the @code{inline} keyword.
1139 The alternate keywords @code{__asm__}, @code{__extension__},
1140 @code{__inline__} and @code{__typeof__} continue to work despite
1141 @option{-ansi}. You would not want to use them in an ISO C program, of
1142 course, but it is useful to put them in header files that might be included
1143 in compilations done with @option{-ansi}. Alternate predefined macros
1144 such as @code{__unix__} and @code{__vax__} are also available, with or
1145 without @option{-ansi}.
1147 The @option{-ansi} option does not cause non-ISO programs to be
1148 rejected gratuitously. For that, @option{-pedantic} is required in
1149 addition to @option{-ansi}. @xref{Warning Options}.
1151 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1152 option is used. Some header files may notice this macro and refrain
1153 from declaring certain functions or defining certain macros that the
1154 ISO standard doesn't call for; this is to avoid interfering with any
1155 programs that might use these names for other things.
1157 Functions which would normally be built in but do not have semantics
1158 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1159 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1160 built-in functions provided by GCC}, for details of the functions
1165 Determine the language standard. This option is currently only
1166 supported when compiling C or C++. A value for this option must be
1167 provided; possible values are
1172 ISO C90 (same as @option{-ansi}).
1174 @item iso9899:199409
1175 ISO C90 as modified in amendment 1.
1181 ISO C99. Note that this standard is not yet fully supported; see
1182 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1183 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1186 Default, ISO C90 plus GNU extensions (including some C99 features).
1190 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1191 this will become the default. The name @samp{gnu9x} is deprecated.
1194 The 1998 ISO C++ standard plus amendments.
1197 The same as @option{-std=c++98} plus GNU extensions. This is the
1198 default for C++ code.
1201 Even when this option is not specified, you can still use some of the
1202 features of newer standards in so far as they do not conflict with
1203 previous C standards. For example, you may use @code{__restrict__} even
1204 when @option{-std=c99} is not specified.
1206 The @option{-std} options specifying some version of ISO C have the same
1207 effects as @option{-ansi}, except that features that were not in ISO C90
1208 but are in the specified version (for example, @samp{//} comments and
1209 the @code{inline} keyword in ISO C99) are not disabled.
1211 @xref{Standards,,Language Standards Supported by GCC}, for details of
1212 these standard versions.
1214 @item -aux-info @var{filename}
1216 Output to the given filename prototyped declarations for all functions
1217 declared and/or defined in a translation unit, including those in header
1218 files. This option is silently ignored in any language other than C@.
1220 Besides declarations, the file indicates, in comments, the origin of
1221 each declaration (source file and line), whether the declaration was
1222 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1223 @samp{O} for old, respectively, in the first character after the line
1224 number and the colon), and whether it came from a declaration or a
1225 definition (@samp{C} or @samp{F}, respectively, in the following
1226 character). In the case of function definitions, a K&R-style list of
1227 arguments followed by their declarations is also provided, inside
1228 comments, after the declaration.
1232 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1233 keyword, so that code can use these words as identifiers. You can use
1234 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1235 instead. @option{-ansi} implies @option{-fno-asm}.
1237 In C++, this switch only affects the @code{typeof} keyword, since
1238 @code{asm} and @code{inline} are standard keywords. You may want to
1239 use the @option{-fno-gnu-keywords} flag instead, which has the same
1240 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1241 switch only affects the @code{asm} and @code{typeof} keywords, since
1242 @code{inline} is a standard keyword in ISO C99.
1245 @itemx -fno-builtin-@var{function}
1246 @opindex fno-builtin
1247 @cindex built-in functions
1248 Don't recognize built-in functions that do not begin with
1249 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1250 functions provided by GCC}, for details of the functions affected,
1251 including those which are not built-in functions when @option{-ansi} or
1252 @option{-std} options for strict ISO C conformance are used because they
1253 do not have an ISO standard meaning.
1255 GCC normally generates special code to handle certain built-in functions
1256 more efficiently; for instance, calls to @code{alloca} may become single
1257 instructions that adjust the stack directly, and calls to @code{memcpy}
1258 may become inline copy loops. The resulting code is often both smaller
1259 and faster, but since the function calls no longer appear as such, you
1260 cannot set a breakpoint on those calls, nor can you change the behavior
1261 of the functions by linking with a different library. In addition,
1262 when a function is recognized as a built-in function, GCC may use
1263 information about that function to warn about problems with calls to
1264 that function, or to generate more efficient code, even if the
1265 resulting code still contains calls to that function. For example,
1266 warnings are given with @option{-Wformat} for bad calls to
1267 @code{printf}, when @code{printf} is built in, and @code{strlen} is
1268 known not to modify global memory.
1270 With the @option{-fno-builtin-@var{function}} option
1271 only the built-in function @var{function} is
1272 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1273 function is named this is not built-in in this version of GCC, this
1274 option is ignored. There is no corresponding
1275 @option{-fbuiltin-@var{function}} option; if you wish to enable
1276 built-in functions selectively when using @option{-fno-builtin} or
1277 @option{-ffreestanding}, you may define macros such as:
1280 #define abs(n) __builtin_abs ((n))
1281 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1286 @cindex hosted environment
1288 Assert that compilation takes place in a hosted environment. This implies
1289 @option{-fbuiltin}. A hosted environment is one in which the
1290 entire standard library is available, and in which @code{main} has a return
1291 type of @code{int}. Examples are nearly everything except a kernel.
1292 This is equivalent to @option{-fno-freestanding}.
1294 @item -ffreestanding
1295 @opindex ffreestanding
1296 @cindex hosted environment
1298 Assert that compilation takes place in a freestanding environment. This
1299 implies @option{-fno-builtin}. A freestanding environment
1300 is one in which the standard library may not exist, and program startup may
1301 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1302 This is equivalent to @option{-fno-hosted}.
1304 @xref{Standards,,Language Standards Supported by GCC}, for details of
1305 freestanding and hosted environments.
1307 @item -fms-extensions
1308 @opindex fms-extensions
1309 Accept some non-standard constructs used in Microsoft header files.
1311 Some cases of unnamed fields in structures and unions are only
1312 accepted with this option. @xref{Unnamed Fields,,Unnamed struct/union
1313 fields within structs/unions}, for details.
1317 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1318 options for strict ISO C conformance) implies @option{-trigraphs}.
1320 @item -no-integrated-cpp
1321 @opindex no-integrated-cpp
1322 Performs a compilation in two passes: preprocessing and compiling. This
1323 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1324 @option{-B} option. The user supplied compilation step can then add in
1325 an additional preprocessing step after normal preprocessing but before
1326 compiling. The default is to use the integrated cpp (internal cpp)
1328 The semantics of this option will change if "cc1", "cc1plus", and
1329 "cc1obj" are merged.
1331 @cindex traditional C language
1332 @cindex C language, traditional
1334 @itemx -traditional-cpp
1335 @opindex traditional-cpp
1336 @opindex traditional
1337 Formerly, these options caused GCC to attempt to emulate a pre-standard
1338 C compiler. They are now only supported with the @option{-E} switch.
1339 The preprocessor continues to support a pre-standard mode. See the GNU
1340 CPP manual for details.
1342 @item -fcond-mismatch
1343 @opindex fcond-mismatch
1344 Allow conditional expressions with mismatched types in the second and
1345 third arguments. The value of such an expression is void. This option
1346 is not supported for C++.
1348 @item -funsigned-char
1349 @opindex funsigned-char
1350 Let the type @code{char} be unsigned, like @code{unsigned char}.
1352 Each kind of machine has a default for what @code{char} should
1353 be. It is either like @code{unsigned char} by default or like
1354 @code{signed char} by default.
1356 Ideally, a portable program should always use @code{signed char} or
1357 @code{unsigned char} when it depends on the signedness of an object.
1358 But many programs have been written to use plain @code{char} and
1359 expect it to be signed, or expect it to be unsigned, depending on the
1360 machines they were written for. This option, and its inverse, let you
1361 make such a program work with the opposite default.
1363 The type @code{char} is always a distinct type from each of
1364 @code{signed char} or @code{unsigned char}, even though its behavior
1365 is always just like one of those two.
1368 @opindex fsigned-char
1369 Let the type @code{char} be signed, like @code{signed char}.
1371 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1372 the negative form of @option{-funsigned-char}. Likewise, the option
1373 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1375 @item -fsigned-bitfields
1376 @itemx -funsigned-bitfields
1377 @itemx -fno-signed-bitfields
1378 @itemx -fno-unsigned-bitfields
1379 @opindex fsigned-bitfields
1380 @opindex funsigned-bitfields
1381 @opindex fno-signed-bitfields
1382 @opindex fno-unsigned-bitfields
1383 These options control whether a bit-field is signed or unsigned, when the
1384 declaration does not use either @code{signed} or @code{unsigned}. By
1385 default, such a bit-field is signed, because this is consistent: the
1386 basic integer types such as @code{int} are signed types.
1389 @node C++ Dialect Options
1390 @section Options Controlling C++ Dialect
1392 @cindex compiler options, C++
1393 @cindex C++ options, command line
1394 @cindex options, C++
1395 This section describes the command-line options that are only meaningful
1396 for C++ programs; but you can also use most of the GNU compiler options
1397 regardless of what language your program is in. For example, you
1398 might compile a file @code{firstClass.C} like this:
1401 g++ -g -frepo -O -c firstClass.C
1405 In this example, only @option{-frepo} is an option meant
1406 only for C++ programs; you can use the other options with any
1407 language supported by GCC@.
1409 Here is a list of options that are @emph{only} for compiling C++ programs:
1413 @item -fabi-version=@var{n}
1414 @opindex fabi-version
1415 Use version @var{n} of the C++ ABI@. Version 2 is the version of the
1416 C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
1417 the C++ ABI that first appeared in G++ 3.2. Version 0 will always be
1418 the version that conforms most closely to the C++ ABI specification.
1419 Therefore, the ABI obtained using version 0 will change as ABI bugs
1422 The default is version 2.
1424 @item -fno-access-control
1425 @opindex fno-access-control
1426 Turn off all access checking. This switch is mainly useful for working
1427 around bugs in the access control code.
1431 Check that the pointer returned by @code{operator new} is non-null
1432 before attempting to modify the storage allocated. This check is
1433 normally unnecessary because the C++ standard specifies that
1434 @code{operator new} will only return @code{0} if it is declared
1435 @samp{throw()}, in which case the compiler will always check the
1436 return value even without this option. In all other cases, when
1437 @code{operator new} has a non-empty exception specification, memory
1438 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1439 @samp{new (nothrow)}.
1441 @item -fconserve-space
1442 @opindex fconserve-space
1443 Put uninitialized or runtime-initialized global variables into the
1444 common segment, as C does. This saves space in the executable at the
1445 cost of not diagnosing duplicate definitions. If you compile with this
1446 flag and your program mysteriously crashes after @code{main()} has
1447 completed, you may have an object that is being destroyed twice because
1448 two definitions were merged.
1450 This option is no longer useful on most targets, now that support has
1451 been added for putting variables into BSS without making them common.
1453 @item -ffriend-injection
1454 @opindex ffriend-injection
1455 Inject friend functions into the enclosing namespace, so that they are
1456 visible outside the scope of the class in which they are declared.
1457 Friend functions were documented to work this way in the old Annotated
1458 C++ Reference Manual, and versions of G++ before 4.1 always worked
1459 that way. However, in ISO C++ a friend function which is not declared
1460 in an enclosing scope can only be found using argument dependent
1461 lookup. This option causes friends to be injected as they were in
1464 This option is for compatibility, and may be removed in a future
1467 @item -fno-elide-constructors
1468 @opindex fno-elide-constructors
1469 The C++ standard allows an implementation to omit creating a temporary
1470 which is only used to initialize another object of the same type.
1471 Specifying this option disables that optimization, and forces G++ to
1472 call the copy constructor in all cases.
1474 @item -fno-enforce-eh-specs
1475 @opindex fno-enforce-eh-specs
1476 Don't generate code to check for violation of exception specifications
1477 at runtime. This option violates the C++ standard, but may be useful
1478 for reducing code size in production builds, much like defining
1479 @samp{NDEBUG}. This does not give user code permission to throw
1480 exceptions in violation of the exception specifications; the compiler
1481 will still optimize based on the specifications, so throwing an
1482 unexpected exception will result in undefined behavior.
1485 @itemx -fno-for-scope
1487 @opindex fno-for-scope
1488 If @option{-ffor-scope} is specified, the scope of variables declared in
1489 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1490 as specified by the C++ standard.
1491 If @option{-fno-for-scope} is specified, the scope of variables declared in
1492 a @i{for-init-statement} extends to the end of the enclosing scope,
1493 as was the case in old versions of G++, and other (traditional)
1494 implementations of C++.
1496 The default if neither flag is given to follow the standard,
1497 but to allow and give a warning for old-style code that would
1498 otherwise be invalid, or have different behavior.
1500 @item -fno-gnu-keywords
1501 @opindex fno-gnu-keywords
1502 Do not recognize @code{typeof} as a keyword, so that code can use this
1503 word as an identifier. You can use the keyword @code{__typeof__} instead.
1504 @option{-ansi} implies @option{-fno-gnu-keywords}.
1506 @item -fno-implicit-templates
1507 @opindex fno-implicit-templates
1508 Never emit code for non-inline templates which are instantiated
1509 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1510 @xref{Template Instantiation}, for more information.
1512 @item -fno-implicit-inline-templates
1513 @opindex fno-implicit-inline-templates
1514 Don't emit code for implicit instantiations of inline templates, either.
1515 The default is to handle inlines differently so that compiles with and
1516 without optimization will need the same set of explicit instantiations.
1518 @item -fno-implement-inlines
1519 @opindex fno-implement-inlines
1520 To save space, do not emit out-of-line copies of inline functions
1521 controlled by @samp{#pragma implementation}. This will cause linker
1522 errors if these functions are not inlined everywhere they are called.
1524 @item -fms-extensions
1525 @opindex fms-extensions
1526 Disable pedantic warnings about constructs used in MFC, such as implicit
1527 int and getting a pointer to member function via non-standard syntax.
1529 @item -fno-nonansi-builtins
1530 @opindex fno-nonansi-builtins
1531 Disable built-in declarations of functions that are not mandated by
1532 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1533 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1535 @item -fno-operator-names
1536 @opindex fno-operator-names
1537 Do not treat the operator name keywords @code{and}, @code{bitand},
1538 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1539 synonyms as keywords.
1541 @item -fno-optional-diags
1542 @opindex fno-optional-diags
1543 Disable diagnostics that the standard says a compiler does not need to
1544 issue. Currently, the only such diagnostic issued by G++ is the one for
1545 a name having multiple meanings within a class.
1548 @opindex fpermissive
1549 Downgrade some diagnostics about nonconformant code from errors to
1550 warnings. Thus, using @option{-fpermissive} will allow some
1551 nonconforming code to compile.
1555 Enable automatic template instantiation at link time. This option also
1556 implies @option{-fno-implicit-templates}. @xref{Template
1557 Instantiation}, for more information.
1561 Disable generation of information about every class with virtual
1562 functions for use by the C++ runtime type identification features
1563 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1564 of the language, you can save some space by using this flag. Note that
1565 exception handling uses the same information, but it will generate it as
1570 Emit statistics about front-end processing at the end of the compilation.
1571 This information is generally only useful to the G++ development team.
1573 @item -ftemplate-depth-@var{n}
1574 @opindex ftemplate-depth
1575 Set the maximum instantiation depth for template classes to @var{n}.
1576 A limit on the template instantiation depth is needed to detect
1577 endless recursions during template class instantiation. ANSI/ISO C++
1578 conforming programs must not rely on a maximum depth greater than 17.
1580 @item -fno-threadsafe-statics
1581 @opindex fno-threadsafe-statics
1582 Do not emit the extra code to use the routines specified in the C++
1583 ABI for thread-safe initialization of local statics. You can use this
1584 option to reduce code size slightly in code that doesn't need to be
1587 @item -fuse-cxa-atexit
1588 @opindex fuse-cxa-atexit
1589 Register destructors for objects with static storage duration with the
1590 @code{__cxa_atexit} function rather than the @code{atexit} function.
1591 This option is required for fully standards-compliant handling of static
1592 destructors, but will only work if your C library supports
1593 @code{__cxa_atexit}.
1595 @item -fno-use-cxa-get-exception-ptr
1596 @opindex fno-use-cxa-get-exception-ptr
1597 Don't use the @code{__cxa_get_exception_ptr} runtime routine. This
1598 will cause @code{std::uncaught_exception} to be incorrect, but is necessary
1599 if the runtime routine is not available.
1601 @item -fvisibility-inlines-hidden
1602 @opindex fvisibility-inlines-hidden
1603 This switch declares that the user does not attempt to compare
1604 pointers to inline methods where the addresses of the two functions
1605 were taken in different shared objects.
1607 The effect of this is that GCC may, effectively, mark inline methods with
1608 @code{__attribute__ ((visibility ("hidden")))} so that they do not
1609 appear in the export table of a DSO and do not require a PLT indirection
1610 when used within the DSO@. Enabling this option can have a dramatic effect
1611 on load and link times of a DSO as it massively reduces the size of the
1612 dynamic export table when the library makes heavy use of templates.
1614 The behaviour of this switch is not quite the same as marking the
1615 methods as hidden directly. Normally if there is a class with default
1616 visibility which has a hidden method, the effect of this is that the
1617 method must be defined in only one shared object. This switch does
1618 not have this restriction.
1620 You may mark a method as having a visibility explicitly to negate the
1621 effect of the switch for that method. For example, if you do want to
1622 compare pointers to a particular inline method, you might mark it as
1623 having default visibility.
1627 Do not use weak symbol support, even if it is provided by the linker.
1628 By default, G++ will use weak symbols if they are available. This
1629 option exists only for testing, and should not be used by end-users;
1630 it will result in inferior code and has no benefits. This option may
1631 be removed in a future release of G++.
1635 Do not search for header files in the standard directories specific to
1636 C++, but do still search the other standard directories. (This option
1637 is used when building the C++ library.)
1640 In addition, these optimization, warning, and code generation options
1641 have meanings only for C++ programs:
1644 @item -fno-default-inline
1645 @opindex fno-default-inline
1646 Do not assume @samp{inline} for functions defined inside a class scope.
1647 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1648 functions will have linkage like inline functions; they just won't be
1651 @item -Wabi @r{(C++ only)}
1653 Warn when G++ generates code that is probably not compatible with the
1654 vendor-neutral C++ ABI@. Although an effort has been made to warn about
1655 all such cases, there are probably some cases that are not warned about,
1656 even though G++ is generating incompatible code. There may also be
1657 cases where warnings are emitted even though the code that is generated
1660 You should rewrite your code to avoid these warnings if you are
1661 concerned about the fact that code generated by G++ may not be binary
1662 compatible with code generated by other compilers.
1664 The known incompatibilities at this point include:
1669 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1670 pack data into the same byte as a base class. For example:
1673 struct A @{ virtual void f(); int f1 : 1; @};
1674 struct B : public A @{ int f2 : 1; @};
1678 In this case, G++ will place @code{B::f2} into the same byte
1679 as@code{A::f1}; other compilers will not. You can avoid this problem
1680 by explicitly padding @code{A} so that its size is a multiple of the
1681 byte size on your platform; that will cause G++ and other compilers to
1682 layout @code{B} identically.
1685 Incorrect handling of tail-padding for virtual bases. G++ does not use
1686 tail padding when laying out virtual bases. For example:
1689 struct A @{ virtual void f(); char c1; @};
1690 struct B @{ B(); char c2; @};
1691 struct C : public A, public virtual B @{@};
1695 In this case, G++ will not place @code{B} into the tail-padding for
1696 @code{A}; other compilers will. You can avoid this problem by
1697 explicitly padding @code{A} so that its size is a multiple of its
1698 alignment (ignoring virtual base classes); that will cause G++ and other
1699 compilers to layout @code{C} identically.
1702 Incorrect handling of bit-fields with declared widths greater than that
1703 of their underlying types, when the bit-fields appear in a union. For
1707 union U @{ int i : 4096; @};
1711 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1712 union too small by the number of bits in an @code{int}.
1715 Empty classes can be placed at incorrect offsets. For example:
1725 struct C : public B, public A @{@};
1729 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1730 it should be placed at offset zero. G++ mistakenly believes that the
1731 @code{A} data member of @code{B} is already at offset zero.
1734 Names of template functions whose types involve @code{typename} or
1735 template template parameters can be mangled incorrectly.
1738 template <typename Q>
1739 void f(typename Q::X) @{@}
1741 template <template <typename> class Q>
1742 void f(typename Q<int>::X) @{@}
1746 Instantiations of these templates may be mangled incorrectly.
1750 @item -Wctor-dtor-privacy @r{(C++ only)}
1751 @opindex Wctor-dtor-privacy
1752 Warn when a class seems unusable because all the constructors or
1753 destructors in that class are private, and it has neither friends nor
1754 public static member functions.
1756 @item -Wnon-virtual-dtor @r{(C++ only)}
1757 @opindex Wnon-virtual-dtor
1758 Warn when a class appears to be polymorphic, thereby requiring a virtual
1759 destructor, yet it declares a non-virtual one. This warning is also
1760 enabled if -Weffc++ is specified.
1762 @item -Wreorder @r{(C++ only)}
1764 @cindex reordering, warning
1765 @cindex warning for reordering of member initializers
1766 Warn when the order of member initializers given in the code does not
1767 match the order in which they must be executed. For instance:
1773 A(): j (0), i (1) @{ @}
1777 The compiler will rearrange the member initializers for @samp{i}
1778 and @samp{j} to match the declaration order of the members, emitting
1779 a warning to that effect. This warning is enabled by @option{-Wall}.
1782 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
1785 @item -Weffc++ @r{(C++ only)}
1787 Warn about violations of the following style guidelines from Scott Meyers'
1788 @cite{Effective C++} book:
1792 Item 11: Define a copy constructor and an assignment operator for classes
1793 with dynamically allocated memory.
1796 Item 12: Prefer initialization to assignment in constructors.
1799 Item 14: Make destructors virtual in base classes.
1802 Item 15: Have @code{operator=} return a reference to @code{*this}.
1805 Item 23: Don't try to return a reference when you must return an object.
1809 Also warn about violations of the following style guidelines from
1810 Scott Meyers' @cite{More Effective C++} book:
1814 Item 6: Distinguish between prefix and postfix forms of increment and
1815 decrement operators.
1818 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
1822 When selecting this option, be aware that the standard library
1823 headers do not obey all of these guidelines; use @samp{grep -v}
1824 to filter out those warnings.
1826 @item -Wno-deprecated @r{(C++ only)}
1827 @opindex Wno-deprecated
1828 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
1830 @item -Wstrict-null-sentinel @r{(C++ only)}
1831 @opindex Wstrict-null-sentinel
1832 Warn also about the use of an uncasted @code{NULL} as sentinel. When
1833 compiling only with GCC this is a valid sentinel, as @code{NULL} is defined
1834 to @code{__null}. Although it is a null pointer constant not a null pointer,
1835 it is guaranteed to of the same size as a pointer. But this use is
1836 not portable across different compilers.
1838 @item -Wno-non-template-friend @r{(C++ only)}
1839 @opindex Wno-non-template-friend
1840 Disable warnings when non-templatized friend functions are declared
1841 within a template. Since the advent of explicit template specification
1842 support in G++, if the name of the friend is an unqualified-id (i.e.,
1843 @samp{friend foo(int)}), the C++ language specification demands that the
1844 friend declare or define an ordinary, nontemplate function. (Section
1845 14.5.3). Before G++ implemented explicit specification, unqualified-ids
1846 could be interpreted as a particular specialization of a templatized
1847 function. Because this non-conforming behavior is no longer the default
1848 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
1849 check existing code for potential trouble spots and is on by default.
1850 This new compiler behavior can be turned off with
1851 @option{-Wno-non-template-friend} which keeps the conformant compiler code
1852 but disables the helpful warning.
1854 @item -Wold-style-cast @r{(C++ only)}
1855 @opindex Wold-style-cast
1856 Warn if an old-style (C-style) cast to a non-void type is used within
1857 a C++ program. The new-style casts (@samp{dynamic_cast},
1858 @samp{static_cast}, @samp{reinterpret_cast}, and @samp{const_cast}) are
1859 less vulnerable to unintended effects and much easier to search for.
1861 @item -Woverloaded-virtual @r{(C++ only)}
1862 @opindex Woverloaded-virtual
1863 @cindex overloaded virtual fn, warning
1864 @cindex warning for overloaded virtual fn
1865 Warn when a function declaration hides virtual functions from a
1866 base class. For example, in:
1873 struct B: public A @{
1878 the @code{A} class version of @code{f} is hidden in @code{B}, and code
1886 will fail to compile.
1888 @item -Wno-pmf-conversions @r{(C++ only)}
1889 @opindex Wno-pmf-conversions
1890 Disable the diagnostic for converting a bound pointer to member function
1893 @item -Wsign-promo @r{(C++ only)}
1894 @opindex Wsign-promo
1895 Warn when overload resolution chooses a promotion from unsigned or
1896 enumerated type to a signed type, over a conversion to an unsigned type of
1897 the same size. Previous versions of G++ would try to preserve
1898 unsignedness, but the standard mandates the current behavior.
1903 A& operator = (int);
1913 In this example, G++ will synthesize a default @samp{A& operator =
1914 (const A&);}, while cfront will use the user-defined @samp{operator =}.
1917 @node Objective-C and Objective-C++ Dialect Options
1918 @section Options Controlling Objective-C and Objective-C++ Dialects
1920 @cindex compiler options, Objective-C and Objective-C++
1921 @cindex Objective-C and Objective-C++ options, command line
1922 @cindex options, Objective-C and Objective-C++
1923 (NOTE: This manual does not describe the Objective-C and Objective-C++
1924 languages themselves. See @xref{Standards,,Language Standards
1925 Supported by GCC}, for references.)
1927 This section describes the command-line options that are only meaningful
1928 for Objective-C and Objective-C++ programs, but you can also use most of
1929 the language-independent GNU compiler options.
1930 For example, you might compile a file @code{some_class.m} like this:
1933 gcc -g -fgnu-runtime -O -c some_class.m
1937 In this example, @option{-fgnu-runtime} is an option meant only for
1938 Objective-C and Objective-C++ programs; you can use the other options with
1939 any language supported by GCC@.
1941 Note that since Objective-C is an extension of the C language, Objective-C
1942 compilations may also use options specific to the C front-end (e.g.,
1943 @option{-Wtraditional}). Similarly, Objective-C++ compilations may use
1944 C++-specific options (e.g., @option{-Wabi}).
1946 Here is a list of options that are @emph{only} for compiling Objective-C
1947 and Objective-C++ programs:
1950 @item -fconstant-string-class=@var{class-name}
1951 @opindex fconstant-string-class
1952 Use @var{class-name} as the name of the class to instantiate for each
1953 literal string specified with the syntax @code{@@"@dots{}"}. The default
1954 class name is @code{NXConstantString} if the GNU runtime is being used, and
1955 @code{NSConstantString} if the NeXT runtime is being used (see below). The
1956 @option{-fconstant-cfstrings} option, if also present, will override the
1957 @option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
1958 to be laid out as constant CoreFoundation strings.
1961 @opindex fgnu-runtime
1962 Generate object code compatible with the standard GNU Objective-C
1963 runtime. This is the default for most types of systems.
1965 @item -fnext-runtime
1966 @opindex fnext-runtime
1967 Generate output compatible with the NeXT runtime. This is the default
1968 for NeXT-based systems, including Darwin and Mac OS X@. The macro
1969 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
1972 @item -fno-nil-receivers
1973 @opindex fno-nil-receivers
1974 Assume that all Objective-C message dispatches (e.g.,
1975 @code{[receiver message:arg]}) in this translation unit ensure that the receiver
1976 is not @code{nil}. This allows for more efficient entry points in the runtime
1977 to be used. Currently, this option is only available in conjunction with
1978 the NeXT runtime on Mac OS X 10.3 and later.
1980 @item -fobjc-call-cxx-cdtors
1981 @opindex fobjc-call-cxx-cdtors
1982 For each Objective-C class, check if any of its instance variables is a
1983 C++ object with a non-trivial default constructor. If so, synthesize a
1984 special @code{- (id) .cxx_construct} instance method that will run
1985 non-trivial default constructors on any such instance variables, in order,
1986 and then return @code{self}. Similarly, check if any instance variable
1987 is a C++ object with a non-trivial destructor, and if so, synthesize a
1988 special @code{- (void) .cxx_destruct} method that will run
1989 all such default destructors, in reverse order.
1991 The @code{- (id) .cxx_construct} and/or @code{- (void) .cxx_destruct} methods
1992 thusly generated will only operate on instance variables declared in the
1993 current Objective-C class, and not those inherited from superclasses. It
1994 is the responsibility of the Objective-C runtime to invoke all such methods
1995 in an object's inheritance hierarchy. The @code{- (id) .cxx_construct} methods
1996 will be invoked by the runtime immediately after a new object
1997 instance is allocated; the @code{- (void) .cxx_destruct} methods will
1998 be invoked immediately before the runtime deallocates an object instance.
2000 As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
2001 support for invoking the @code{- (id) .cxx_construct} and
2002 @code{- (void) .cxx_destruct} methods.
2004 @item -fobjc-direct-dispatch
2005 @opindex fobjc-direct-dispatch
2006 Allow fast jumps to the message dispatcher. On Darwin this is
2007 accomplished via the comm page.
2009 @item -fobjc-exceptions
2010 @opindex fobjc-exceptions
2011 Enable syntactic support for structured exception handling in Objective-C,
2012 similar to what is offered by C++ and Java. This option is
2013 unavailable in conjunction with the NeXT runtime on Mac OS X 10.2 and
2022 @@catch (AnObjCClass *exc) @{
2029 @@catch (AnotherClass *exc) @{
2032 @@catch (id allOthers) @{
2042 The @code{@@throw} statement may appear anywhere in an Objective-C or
2043 Objective-C++ program; when used inside of a @code{@@catch} block, the
2044 @code{@@throw} may appear without an argument (as shown above), in which case
2045 the object caught by the @code{@@catch} will be rethrown.
2047 Note that only (pointers to) Objective-C objects may be thrown and
2048 caught using this scheme. When an object is thrown, it will be caught
2049 by the nearest @code{@@catch} clause capable of handling objects of that type,
2050 analogously to how @code{catch} blocks work in C++ and Java. A
2051 @code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
2052 any and all Objective-C exceptions not caught by previous @code{@@catch}
2055 The @code{@@finally} clause, if present, will be executed upon exit from the
2056 immediately preceding @code{@@try @dots{} @@catch} section. This will happen
2057 regardless of whether any exceptions are thrown, caught or rethrown
2058 inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
2059 of the @code{finally} clause in Java.
2061 There are several caveats to using the new exception mechanism:
2065 Although currently designed to be binary compatible with @code{NS_HANDLER}-style
2066 idioms provided by the @code{NSException} class, the new
2067 exceptions can only be used on Mac OS X 10.3 (Panther) and later
2068 systems, due to additional functionality needed in the (NeXT) Objective-C
2072 As mentioned above, the new exceptions do not support handling
2073 types other than Objective-C objects. Furthermore, when used from
2074 Objective-C++, the Objective-C exception model does not interoperate with C++
2075 exceptions at this time. This means you cannot @code{@@throw} an exception
2076 from Objective-C and @code{catch} it in C++, or vice versa
2077 (i.e., @code{throw @dots{} @@catch}).
2080 The @option{-fobjc-exceptions} switch also enables the use of synchronization
2081 blocks for thread-safe execution:
2084 @@synchronized (ObjCClass *guard) @{
2089 Upon entering the @code{@@synchronized} block, a thread of execution shall
2090 first check whether a lock has been placed on the corresponding @code{guard}
2091 object by another thread. If it has, the current thread shall wait until
2092 the other thread relinquishes its lock. Once @code{guard} becomes available,
2093 the current thread will place its own lock on it, execute the code contained in
2094 the @code{@@synchronized} block, and finally relinquish the lock (thereby
2095 making @code{guard} available to other threads).
2097 Unlike Java, Objective-C does not allow for entire methods to be marked
2098 @code{@@synchronized}. Note that throwing exceptions out of
2099 @code{@@synchronized} blocks is allowed, and will cause the guarding object
2100 to be unlocked properly.
2104 Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
2106 @item -freplace-objc-classes
2107 @opindex freplace-objc-classes
2108 Emit a special marker instructing @command{ld(1)} not to statically link in
2109 the resulting object file, and allow @command{dyld(1)} to load it in at
2110 run time instead. This is used in conjunction with the Fix-and-Continue
2111 debugging mode, where the object file in question may be recompiled and
2112 dynamically reloaded in the course of program execution, without the need
2113 to restart the program itself. Currently, Fix-and-Continue functionality
2114 is only available in conjunction with the NeXT runtime on Mac OS X 10.3
2119 When compiling for the NeXT runtime, the compiler ordinarily replaces calls
2120 to @code{objc_getClass("@dots{}")} (when the name of the class is known at
2121 compile time) with static class references that get initialized at load time,
2122 which improves run-time performance. Specifying the @option{-fzero-link} flag
2123 suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
2124 to be retained. This is useful in Zero-Link debugging mode, since it allows
2125 for individual class implementations to be modified during program execution.
2129 Dump interface declarations for all classes seen in the source file to a
2130 file named @file{@var{sourcename}.decl}.
2132 @item -Wassign-intercept
2133 @opindex Wassign-intercept
2134 Warn whenever an Objective-C assignment is being intercepted by the
2138 @opindex Wno-protocol
2139 If a class is declared to implement a protocol, a warning is issued for
2140 every method in the protocol that is not implemented by the class. The
2141 default behavior is to issue a warning for every method not explicitly
2142 implemented in the class, even if a method implementation is inherited
2143 from the superclass. If you use the @option{-Wno-protocol} option, then
2144 methods inherited from the superclass are considered to be implemented,
2145 and no warning is issued for them.
2149 Warn if multiple methods of different types for the same selector are
2150 found during compilation. The check is performed on the list of methods
2151 in the final stage of compilation. Additionally, a check is performed
2152 for each selector appearing in a @code{@@selector(@dots{})}
2153 expression, and a corresponding method for that selector has been found
2154 during compilation. Because these checks scan the method table only at
2155 the end of compilation, these warnings are not produced if the final
2156 stage of compilation is not reached, for example because an error is
2157 found during compilation, or because the @option{-fsyntax-only} option is
2160 @item -Wstrict-selector-match
2161 @opindex Wstrict-selector-match
2162 Warn if multiple methods with differing argument and/or return types are
2163 found for a given selector when attempting to send a message using this
2164 selector to a receiver of type @code{id} or @code{Class}. When this flag
2165 is off (which is the default behavior), the compiler will omit such warnings
2166 if any differences found are confined to types which share the same size
2169 @item -Wundeclared-selector
2170 @opindex Wundeclared-selector
2171 Warn if a @code{@@selector(@dots{})} expression referring to an
2172 undeclared selector is found. A selector is considered undeclared if no
2173 method with that name has been declared before the
2174 @code{@@selector(@dots{})} expression, either explicitly in an
2175 @code{@@interface} or @code{@@protocol} declaration, or implicitly in
2176 an @code{@@implementation} section. This option always performs its
2177 checks as soon as a @code{@@selector(@dots{})} expression is found,
2178 while @option{-Wselector} only performs its checks in the final stage of
2179 compilation. This also enforces the coding style convention
2180 that methods and selectors must be declared before being used.
2182 @item -print-objc-runtime-info
2183 @opindex print-objc-runtime-info
2184 Generate C header describing the largest structure that is passed by
2189 @node Language Independent Options
2190 @section Options to Control Diagnostic Messages Formatting
2191 @cindex options to control diagnostics formatting
2192 @cindex diagnostic messages
2193 @cindex message formatting
2195 Traditionally, diagnostic messages have been formatted irrespective of
2196 the output device's aspect (e.g.@: its width, @dots{}). The options described
2197 below can be used to control the diagnostic messages formatting
2198 algorithm, e.g.@: how many characters per line, how often source location
2199 information should be reported. Right now, only the C++ front end can
2200 honor these options. However it is expected, in the near future, that
2201 the remaining front ends would be able to digest them correctly.
2204 @item -fmessage-length=@var{n}
2205 @opindex fmessage-length
2206 Try to format error messages so that they fit on lines of about @var{n}
2207 characters. The default is 72 characters for @command{g++} and 0 for the rest of
2208 the front ends supported by GCC@. If @var{n} is zero, then no
2209 line-wrapping will be done; each error message will appear on a single
2212 @opindex fdiagnostics-show-location
2213 @item -fdiagnostics-show-location=once
2214 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
2215 reporter to emit @emph{once} source location information; that is, in
2216 case the message is too long to fit on a single physical line and has to
2217 be wrapped, the source location won't be emitted (as prefix) again,
2218 over and over, in subsequent continuation lines. This is the default
2221 @item -fdiagnostics-show-location=every-line
2222 Only meaningful in line-wrapping mode. Instructs the diagnostic
2223 messages reporter to emit the same source location information (as
2224 prefix) for physical lines that result from the process of breaking
2225 a message which is too long to fit on a single line.
2227 @item -fdiagnostics-show-options
2228 @opindex fdiagnostics-show-options
2229 This option instructs the diagnostic machinery to add text to each
2230 diagnostic emitted, which indicates which command line option directly
2231 controls that diagnostic, when such an option is known to the
2232 diagnostic machinery.
2236 @node Warning Options
2237 @section Options to Request or Suppress Warnings
2238 @cindex options to control warnings
2239 @cindex warning messages
2240 @cindex messages, warning
2241 @cindex suppressing warnings
2243 Warnings are diagnostic messages that report constructions which
2244 are not inherently erroneous but which are risky or suggest there
2245 may have been an error.
2247 You can request many specific warnings with options beginning @samp{-W},
2248 for example @option{-Wimplicit} to request warnings on implicit
2249 declarations. Each of these specific warning options also has a
2250 negative form beginning @samp{-Wno-} to turn off warnings;
2251 for example, @option{-Wno-implicit}. This manual lists only one of the
2252 two forms, whichever is not the default.
2254 The following options control the amount and kinds of warnings produced
2255 by GCC; for further, language-specific options also refer to
2256 @ref{C++ Dialect Options} and @ref{Objective-C and Objective-C++ Dialect
2260 @cindex syntax checking
2262 @opindex fsyntax-only
2263 Check the code for syntax errors, but don't do anything beyond that.
2267 Issue all the warnings demanded by strict ISO C and ISO C++;
2268 reject all programs that use forbidden extensions, and some other
2269 programs that do not follow ISO C and ISO C++. For ISO C, follows the
2270 version of the ISO C standard specified by any @option{-std} option used.
2272 Valid ISO C and ISO C++ programs should compile properly with or without
2273 this option (though a rare few will require @option{-ansi} or a
2274 @option{-std} option specifying the required version of ISO C)@. However,
2275 without this option, certain GNU extensions and traditional C and C++
2276 features are supported as well. With this option, they are rejected.
2278 @option{-pedantic} does not cause warning messages for use of the
2279 alternate keywords whose names begin and end with @samp{__}. Pedantic
2280 warnings are also disabled in the expression that follows
2281 @code{__extension__}. However, only system header files should use
2282 these escape routes; application programs should avoid them.
2283 @xref{Alternate Keywords}.
2285 Some users try to use @option{-pedantic} to check programs for strict ISO
2286 C conformance. They soon find that it does not do quite what they want:
2287 it finds some non-ISO practices, but not all---only those for which
2288 ISO C @emph{requires} a diagnostic, and some others for which
2289 diagnostics have been added.
2291 A feature to report any failure to conform to ISO C might be useful in
2292 some instances, but would require considerable additional work and would
2293 be quite different from @option{-pedantic}. We don't have plans to
2294 support such a feature in the near future.
2296 Where the standard specified with @option{-std} represents a GNU
2297 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
2298 corresponding @dfn{base standard}, the version of ISO C on which the GNU
2299 extended dialect is based. Warnings from @option{-pedantic} are given
2300 where they are required by the base standard. (It would not make sense
2301 for such warnings to be given only for features not in the specified GNU
2302 C dialect, since by definition the GNU dialects of C include all
2303 features the compiler supports with the given option, and there would be
2304 nothing to warn about.)
2306 @item -pedantic-errors
2307 @opindex pedantic-errors
2308 Like @option{-pedantic}, except that errors are produced rather than
2313 Inhibit all warning messages.
2317 Inhibit warning messages about the use of @samp{#import}.
2319 @item -Wchar-subscripts
2320 @opindex Wchar-subscripts
2321 Warn if an array subscript has type @code{char}. This is a common cause
2322 of error, as programmers often forget that this type is signed on some
2324 This warning is enabled by @option{-Wall}.
2328 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
2329 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
2330 This warning is enabled by @option{-Wall}.
2332 @item -Wfatal-errors
2333 @opindex Wfatal-errors
2334 This option causes the compiler to abort compilation on the first error
2335 occurred rather than trying to keep going and printing further error
2340 @opindex ffreestanding
2341 @opindex fno-builtin
2342 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
2343 the arguments supplied have types appropriate to the format string
2344 specified, and that the conversions specified in the format string make
2345 sense. This includes standard functions, and others specified by format
2346 attributes (@pxref{Function Attributes}), in the @code{printf},
2347 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
2348 not in the C standard) families (or other target-specific families).
2349 Which functions are checked without format attributes having been
2350 specified depends on the standard version selected, and such checks of
2351 functions without the attribute specified are disabled by
2352 @option{-ffreestanding} or @option{-fno-builtin}.
2354 The formats are checked against the format features supported by GNU
2355 libc version 2.2. These include all ISO C90 and C99 features, as well
2356 as features from the Single Unix Specification and some BSD and GNU
2357 extensions. Other library implementations may not support all these
2358 features; GCC does not support warning about features that go beyond a
2359 particular library's limitations. However, if @option{-pedantic} is used
2360 with @option{-Wformat}, warnings will be given about format features not
2361 in the selected standard version (but not for @code{strfmon} formats,
2362 since those are not in any version of the C standard). @xref{C Dialect
2363 Options,,Options Controlling C Dialect}.
2365 Since @option{-Wformat} also checks for null format arguments for
2366 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2368 @option{-Wformat} is included in @option{-Wall}. For more control over some
2369 aspects of format checking, the options @option{-Wformat-y2k},
2370 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2371 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2372 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2375 @opindex Wformat-y2k
2376 If @option{-Wformat} is specified, also warn about @code{strftime}
2377 formats which may yield only a two-digit year.
2379 @item -Wno-format-extra-args
2380 @opindex Wno-format-extra-args
2381 If @option{-Wformat} is specified, do not warn about excess arguments to a
2382 @code{printf} or @code{scanf} format function. The C standard specifies
2383 that such arguments are ignored.
2385 Where the unused arguments lie between used arguments that are
2386 specified with @samp{$} operand number specifications, normally
2387 warnings are still given, since the implementation could not know what
2388 type to pass to @code{va_arg} to skip the unused arguments. However,
2389 in the case of @code{scanf} formats, this option will suppress the
2390 warning if the unused arguments are all pointers, since the Single
2391 Unix Specification says that such unused arguments are allowed.
2393 @item -Wno-format-zero-length
2394 @opindex Wno-format-zero-length
2395 If @option{-Wformat} is specified, do not warn about zero-length formats.
2396 The C standard specifies that zero-length formats are allowed.
2398 @item -Wformat-nonliteral
2399 @opindex Wformat-nonliteral
2400 If @option{-Wformat} is specified, also warn if the format string is not a
2401 string literal and so cannot be checked, unless the format function
2402 takes its format arguments as a @code{va_list}.
2404 @item -Wformat-security
2405 @opindex Wformat-security
2406 If @option{-Wformat} is specified, also warn about uses of format
2407 functions that represent possible security problems. At present, this
2408 warns about calls to @code{printf} and @code{scanf} functions where the
2409 format string is not a string literal and there are no format arguments,
2410 as in @code{printf (foo);}. This may be a security hole if the format
2411 string came from untrusted input and contains @samp{%n}. (This is
2412 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2413 in future warnings may be added to @option{-Wformat-security} that are not
2414 included in @option{-Wformat-nonliteral}.)
2418 Enable @option{-Wformat} plus format checks not included in
2419 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2420 -Wformat-nonliteral -Wformat-security -Wformat-y2k}.
2424 Warn about passing a null pointer for arguments marked as
2425 requiring a non-null value by the @code{nonnull} function attribute.
2427 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2428 can be disabled with the @option{-Wno-nonnull} option.
2430 @item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
2432 Warn about uninitialized variables which are initialized with themselves.
2433 Note this option can only be used with the @option{-Wuninitialized} option,
2434 which in turn only works with @option{-O1} and above.
2436 For example, GCC will warn about @code{i} being uninitialized in the
2437 following snippet only when @option{-Winit-self} has been specified:
2448 @item -Wimplicit-int
2449 @opindex Wimplicit-int
2450 Warn when a declaration does not specify a type.
2451 This warning is enabled by @option{-Wall}.
2453 @item -Wimplicit-function-declaration
2454 @itemx -Werror-implicit-function-declaration
2455 @opindex Wimplicit-function-declaration
2456 @opindex Werror-implicit-function-declaration
2457 Give a warning (or error) whenever a function is used before being
2458 declared. The form @option{-Wno-error-implicit-function-declaration}
2460 This warning is enabled by @option{-Wall} (as a warning, not an error).
2464 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2465 This warning is enabled by @option{-Wall}.
2469 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2470 function with external linkage, returning int, taking either zero
2471 arguments, two, or three arguments of appropriate types.
2472 This warning is enabled by @option{-Wall}.
2474 @item -Wmissing-braces
2475 @opindex Wmissing-braces
2476 Warn if an aggregate or union initializer is not fully bracketed. In
2477 the following example, the initializer for @samp{a} is not fully
2478 bracketed, but that for @samp{b} is fully bracketed.
2481 int a[2][2] = @{ 0, 1, 2, 3 @};
2482 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2485 This warning is enabled by @option{-Wall}.
2487 @item -Wmissing-include-dirs @r{(C, C++, Objective-C and Objective-C++ only)}
2488 @opindex Wmissing-include-dirs
2489 Warn if a user-supplied include directory does not exist.
2492 @opindex Wparentheses
2493 Warn if parentheses are omitted in certain contexts, such
2494 as when there is an assignment in a context where a truth value
2495 is expected, or when operators are nested whose precedence people
2496 often get confused about. Only the warning for an assignment used as
2497 a truth value is supported when compiling C++; the other warnings are
2498 only supported when compiling C@.
2500 Also warn if a comparison like @samp{x<=y<=z} appears; this is
2501 equivalent to @samp{(x<=y ? 1 : 0) <= z}, which is a different
2502 interpretation from that of ordinary mathematical notation.
2504 Also warn about constructions where there may be confusion to which
2505 @code{if} statement an @code{else} branch belongs. Here is an example of
2520 In C, every @code{else} branch belongs to the innermost possible @code{if}
2521 statement, which in this example is @code{if (b)}. This is often not
2522 what the programmer expected, as illustrated in the above example by
2523 indentation the programmer chose. When there is the potential for this
2524 confusion, GCC will issue a warning when this flag is specified.
2525 To eliminate the warning, add explicit braces around the innermost
2526 @code{if} statement so there is no way the @code{else} could belong to
2527 the enclosing @code{if}. The resulting code would look like this:
2543 This warning is enabled by @option{-Wall}.
2545 @item -Wsequence-point
2546 @opindex Wsequence-point
2547 Warn about code that may have undefined semantics because of violations
2548 of sequence point rules in the C and C++ standards.
2550 The C and C++ standards defines the order in which expressions in a C/C++
2551 program are evaluated in terms of @dfn{sequence points}, which represent
2552 a partial ordering between the execution of parts of the program: those
2553 executed before the sequence point, and those executed after it. These
2554 occur after the evaluation of a full expression (one which is not part
2555 of a larger expression), after the evaluation of the first operand of a
2556 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2557 function is called (but after the evaluation of its arguments and the
2558 expression denoting the called function), and in certain other places.
2559 Other than as expressed by the sequence point rules, the order of
2560 evaluation of subexpressions of an expression is not specified. All
2561 these rules describe only a partial order rather than a total order,
2562 since, for example, if two functions are called within one expression
2563 with no sequence point between them, the order in which the functions
2564 are called is not specified. However, the standards committee have
2565 ruled that function calls do not overlap.
2567 It is not specified when between sequence points modifications to the
2568 values of objects take effect. Programs whose behavior depends on this
2569 have undefined behavior; the C and C++ standards specify that ``Between
2570 the previous and next sequence point an object shall have its stored
2571 value modified at most once by the evaluation of an expression.
2572 Furthermore, the prior value shall be read only to determine the value
2573 to be stored.''. If a program breaks these rules, the results on any
2574 particular implementation are entirely unpredictable.
2576 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2577 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2578 diagnosed by this option, and it may give an occasional false positive
2579 result, but in general it has been found fairly effective at detecting
2580 this sort of problem in programs.
2582 The standard is worded confusingly, therefore there is some debate
2583 over the precise meaning of the sequence point rules in subtle cases.
2584 Links to discussions of the problem, including proposed formal
2585 definitions, may be found on the GCC readings page, at
2586 @w{@uref{http://gcc.gnu.org/readings.html}}.
2588 This warning is enabled by @option{-Wall} for C and C++.
2591 @opindex Wreturn-type
2592 Warn whenever a function is defined with a return-type that defaults to
2593 @code{int}. Also warn about any @code{return} statement with no
2594 return-value in a function whose return-type is not @code{void}.
2596 For C, also warn if the return type of a function has a type qualifier
2597 such as @code{const}. Such a type qualifier has no effect, since the
2598 value returned by a function is not an lvalue. ISO C prohibits
2599 qualified @code{void} return types on function definitions, so such
2600 return types always receive a warning even without this option.
2602 For C++, a function without return type always produces a diagnostic
2603 message, even when @option{-Wno-return-type} is specified. The only
2604 exceptions are @samp{main} and functions defined in system headers.
2606 This warning is enabled by @option{-Wall}.
2610 Warn whenever a @code{switch} statement has an index of enumerated type
2611 and lacks a @code{case} for one or more of the named codes of that
2612 enumeration. (The presence of a @code{default} label prevents this
2613 warning.) @code{case} labels outside the enumeration range also
2614 provoke warnings when this option is used.
2615 This warning is enabled by @option{-Wall}.
2617 @item -Wswitch-default
2618 @opindex Wswitch-switch
2619 Warn whenever a @code{switch} statement does not have a @code{default}
2623 @opindex Wswitch-enum
2624 Warn whenever a @code{switch} statement has an index of enumerated type
2625 and lacks a @code{case} for one or more of the named codes of that
2626 enumeration. @code{case} labels outside the enumeration range also
2627 provoke warnings when this option is used.
2631 Warn if any trigraphs are encountered that might change the meaning of
2632 the program (trigraphs within comments are not warned about).
2633 This warning is enabled by @option{-Wall}.
2635 @item -Wunused-function
2636 @opindex Wunused-function
2637 Warn whenever a static function is declared but not defined or a
2638 non-inline static function is unused.
2639 This warning is enabled by @option{-Wall}.
2641 @item -Wunused-label
2642 @opindex Wunused-label
2643 Warn whenever a label is declared but not used.
2644 This warning is enabled by @option{-Wall}.
2646 To suppress this warning use the @samp{unused} attribute
2647 (@pxref{Variable Attributes}).
2649 @item -Wunused-parameter
2650 @opindex Wunused-parameter
2651 Warn whenever a function parameter is unused aside from its declaration.
2653 To suppress this warning use the @samp{unused} attribute
2654 (@pxref{Variable Attributes}).
2656 @item -Wunused-variable
2657 @opindex Wunused-variable
2658 Warn whenever a local variable or non-constant static variable is unused
2659 aside from its declaration
2660 This warning is enabled by @option{-Wall}.
2662 To suppress this warning use the @samp{unused} attribute
2663 (@pxref{Variable Attributes}).
2665 @item -Wunused-value
2666 @opindex Wunused-value
2667 Warn whenever a statement computes a result that is explicitly not used.
2668 This warning is enabled by @option{-Wall}.
2670 To suppress this warning cast the expression to @samp{void}.
2674 All the above @option{-Wunused} options combined.
2676 In order to get a warning about an unused function parameter, you must
2677 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
2678 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
2680 @item -Wuninitialized
2681 @opindex Wuninitialized
2682 Warn if an automatic variable is used without first being initialized or
2683 if a variable may be clobbered by a @code{setjmp} call.
2685 These warnings are possible only in optimizing compilation,
2686 because they require data flow information that is computed only
2687 when optimizing. If you do not specify @option{-O}, you will not get
2688 these warnings. Instead, GCC will issue a warning about @option{-Wuninitialized}
2689 requiring @option{-O}.
2691 If you want to warn about code which uses the uninitialized value of the
2692 variable in its own initializer, use the @option{-Winit-self} option.
2694 These warnings occur for individual uninitialized or clobbered
2695 elements of structure, union or array variables as well as for
2696 variables which are uninitialized or clobbered as a whole. They do
2697 not occur for variables or elements declared @code{volatile}. Because
2698 these warnings depend on optimization, the exact variables or elements
2699 for which there are warnings will depend on the precise optimization
2700 options and version of GCC used.
2702 Note that there may be no warning about a variable that is used only
2703 to compute a value that itself is never used, because such
2704 computations may be deleted by data flow analysis before the warnings
2707 These warnings are made optional because GCC is not smart
2708 enough to see all the reasons why the code might be correct
2709 despite appearing to have an error. Here is one example of how
2730 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
2731 always initialized, but GCC doesn't know this. Here is
2732 another common case:
2737 if (change_y) save_y = y, y = new_y;
2739 if (change_y) y = save_y;
2744 This has no bug because @code{save_y} is used only if it is set.
2746 @cindex @code{longjmp} warnings
2747 This option also warns when a non-volatile automatic variable might be
2748 changed by a call to @code{longjmp}. These warnings as well are possible
2749 only in optimizing compilation.
2751 The compiler sees only the calls to @code{setjmp}. It cannot know
2752 where @code{longjmp} will be called; in fact, a signal handler could
2753 call it at any point in the code. As a result, you may get a warning
2754 even when there is in fact no problem because @code{longjmp} cannot
2755 in fact be called at the place which would cause a problem.
2757 Some spurious warnings can be avoided if you declare all the functions
2758 you use that never return as @code{noreturn}. @xref{Function
2761 This warning is enabled by @option{-Wall}.
2763 @item -Wunknown-pragmas
2764 @opindex Wunknown-pragmas
2765 @cindex warning for unknown pragmas
2766 @cindex unknown pragmas, warning
2767 @cindex pragmas, warning of unknown
2768 Warn when a #pragma directive is encountered which is not understood by
2769 GCC@. If this command line option is used, warnings will even be issued
2770 for unknown pragmas in system header files. This is not the case if
2771 the warnings were only enabled by the @option{-Wall} command line option.
2774 @opindex Wno-pragmas
2776 Do not warn about misuses of pragmas, such as incorrect parameters,
2777 invalid syntax, or conflicts between pragmas. See also
2778 @samp{-Wunknown-pragmas}.
2780 @item -Wstrict-aliasing
2781 @opindex Wstrict-aliasing
2782 This option is only active when @option{-fstrict-aliasing} is active.
2783 It warns about code which might break the strict aliasing rules that the
2784 compiler is using for optimization. The warning does not catch all
2785 cases, but does attempt to catch the more common pitfalls. It is
2786 included in @option{-Wall}.
2788 @item -Wstrict-aliasing=2
2789 @opindex Wstrict-aliasing=2
2790 This option is only active when @option{-fstrict-aliasing} is active.
2791 It warns about code which might break the strict aliasing rules that the
2792 compiler is using for optimization. This warning catches more cases than
2793 @option{-Wstrict-aliasing}, but it will also give a warning for some ambiguous
2794 cases that are safe.
2798 All of the above @samp{-W} options combined. This enables all the
2799 warnings about constructions that some users consider questionable, and
2800 that are easy to avoid (or modify to prevent the warning), even in
2801 conjunction with macros. This also enables some language-specific
2802 warnings described in @ref{C++ Dialect Options} and
2803 @ref{Objective-C and Objective-C++ Dialect Options}.
2806 The following @option{-W@dots{}} options are not implied by @option{-Wall}.
2807 Some of them warn about constructions that users generally do not
2808 consider questionable, but which occasionally you might wish to check
2809 for; others warn about constructions that are necessary or hard to avoid
2810 in some cases, and there is no simple way to modify the code to suppress
2817 (This option used to be called @option{-W}. The older name is still
2818 supported, but the newer name is more descriptive.) Print extra warning
2819 messages for these events:
2823 A function can return either with or without a value. (Falling
2824 off the end of the function body is considered returning without
2825 a value.) For example, this function would evoke such a
2839 An expression-statement or the left-hand side of a comma expression
2840 contains no side effects.
2841 To suppress the warning, cast the unused expression to void.
2842 For example, an expression such as @samp{x[i,j]} will cause a warning,
2843 but @samp{x[(void)i,j]} will not.
2846 An unsigned value is compared against zero with @samp{<} or @samp{>=}.
2849 Storage-class specifiers like @code{static} are not the first things in
2850 a declaration. According to the C Standard, this usage is obsolescent.
2853 If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
2857 A comparison between signed and unsigned values could produce an
2858 incorrect result when the signed value is converted to unsigned.
2859 (But don't warn if @option{-Wno-sign-compare} is also specified.)
2862 An aggregate has an initializer which does not initialize all members.
2863 This warning can be independently controlled by
2864 @option{-Wmissing-field-initializers}.
2867 A function parameter is declared without a type specifier in K&R-style
2875 An empty body occurs in an @samp{if} or @samp{else} statement.
2878 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2879 @samp{>}, or @samp{>=}.
2882 A variable might be changed by @samp{longjmp} or @samp{vfork}.
2885 Any of several floating-point events that often indicate errors, such as
2886 overflow, underflow, loss of precision, etc.
2888 @item @r{(C++ only)}
2889 An enumerator and a non-enumerator both appear in a conditional expression.
2891 @item @r{(C++ only)}
2892 A non-static reference or non-static @samp{const} member appears in a
2893 class without constructors.
2895 @item @r{(C++ only)}
2896 Ambiguous virtual bases.
2898 @item @r{(C++ only)}
2899 Subscripting an array which has been declared @samp{register}.
2901 @item @r{(C++ only)}
2902 Taking the address of a variable which has been declared @samp{register}.
2904 @item @r{(C++ only)}
2905 A base class is not initialized in a derived class' copy constructor.
2908 @item -Wno-div-by-zero
2909 @opindex Wno-div-by-zero
2910 @opindex Wdiv-by-zero
2911 Do not warn about compile-time integer division by zero. Floating point
2912 division by zero is not warned about, as it can be a legitimate way of
2913 obtaining infinities and NaNs.
2915 @item -Wsystem-headers
2916 @opindex Wsystem-headers
2917 @cindex warnings from system headers
2918 @cindex system headers, warnings from
2919 Print warning messages for constructs found in system header files.
2920 Warnings from system headers are normally suppressed, on the assumption
2921 that they usually do not indicate real problems and would only make the
2922 compiler output harder to read. Using this command line option tells
2923 GCC to emit warnings from system headers as if they occurred in user
2924 code. However, note that using @option{-Wall} in conjunction with this
2925 option will @emph{not} warn about unknown pragmas in system
2926 headers---for that, @option{-Wunknown-pragmas} must also be used.
2929 @opindex Wfloat-equal
2930 Warn if floating point values are used in equality comparisons.
2932 The idea behind this is that sometimes it is convenient (for the
2933 programmer) to consider floating-point values as approximations to
2934 infinitely precise real numbers. If you are doing this, then you need
2935 to compute (by analyzing the code, or in some other way) the maximum or
2936 likely maximum error that the computation introduces, and allow for it
2937 when performing comparisons (and when producing output, but that's a
2938 different problem). In particular, instead of testing for equality, you
2939 would check to see whether the two values have ranges that overlap; and
2940 this is done with the relational operators, so equality comparisons are
2943 @item -Wtraditional @r{(C only)}
2944 @opindex Wtraditional
2945 Warn about certain constructs that behave differently in traditional and
2946 ISO C@. Also warn about ISO C constructs that have no traditional C
2947 equivalent, and/or problematic constructs which should be avoided.
2951 Macro parameters that appear within string literals in the macro body.
2952 In traditional C macro replacement takes place within string literals,
2953 but does not in ISO C@.
2956 In traditional C, some preprocessor directives did not exist.
2957 Traditional preprocessors would only consider a line to be a directive
2958 if the @samp{#} appeared in column 1 on the line. Therefore
2959 @option{-Wtraditional} warns about directives that traditional C
2960 understands but would ignore because the @samp{#} does not appear as the
2961 first character on the line. It also suggests you hide directives like
2962 @samp{#pragma} not understood by traditional C by indenting them. Some
2963 traditional implementations would not recognize @samp{#elif}, so it
2964 suggests avoiding it altogether.
2967 A function-like macro that appears without arguments.
2970 The unary plus operator.
2973 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
2974 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
2975 constants.) Note, these suffixes appear in macros defined in the system
2976 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
2977 Use of these macros in user code might normally lead to spurious
2978 warnings, however GCC's integrated preprocessor has enough context to
2979 avoid warning in these cases.
2982 A function declared external in one block and then used after the end of
2986 A @code{switch} statement has an operand of type @code{long}.
2989 A non-@code{static} function declaration follows a @code{static} one.
2990 This construct is not accepted by some traditional C compilers.
2993 The ISO type of an integer constant has a different width or
2994 signedness from its traditional type. This warning is only issued if
2995 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
2996 typically represent bit patterns, are not warned about.
2999 Usage of ISO string concatenation is detected.
3002 Initialization of automatic aggregates.
3005 Identifier conflicts with labels. Traditional C lacks a separate
3006 namespace for labels.
3009 Initialization of unions. If the initializer is zero, the warning is
3010 omitted. This is done under the assumption that the zero initializer in
3011 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
3012 initializer warnings and relies on default initialization to zero in the
3016 Conversions by prototypes between fixed/floating point values and vice
3017 versa. The absence of these prototypes when compiling with traditional
3018 C would cause serious problems. This is a subset of the possible
3019 conversion warnings, for the full set use @option{-Wconversion}.
3022 Use of ISO C style function definitions. This warning intentionally is
3023 @emph{not} issued for prototype declarations or variadic functions
3024 because these ISO C features will appear in your code when using
3025 libiberty's traditional C compatibility macros, @code{PARAMS} and
3026 @code{VPARAMS}. This warning is also bypassed for nested functions
3027 because that feature is already a GCC extension and thus not relevant to
3028 traditional C compatibility.
3031 @item -Wdeclaration-after-statement @r{(C only)}
3032 @opindex Wdeclaration-after-statement
3033 Warn when a declaration is found after a statement in a block. This
3034 construct, known from C++, was introduced with ISO C99 and is by default
3035 allowed in GCC@. It is not supported by ISO C90 and was not supported by
3036 GCC versions before GCC 3.0. @xref{Mixed Declarations}.
3040 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
3042 @item -Wno-endif-labels
3043 @opindex Wno-endif-labels
3044 @opindex Wendif-labels
3045 Do not warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
3049 Warn whenever a local variable shadows another local variable, parameter or
3050 global variable or whenever a built-in function is shadowed.
3052 @item -Wlarger-than-@var{len}
3053 @opindex Wlarger-than
3054 Warn whenever an object of larger than @var{len} bytes is defined.
3056 @item -Wunsafe-loop-optimizations
3057 @opindex Wunsafe-loop-optimizations
3058 Warn if the loop cannot be optimized because the compiler could not
3059 assume anything on the bounds of the loop indices. With
3060 @option{-funsafe-loop-optimizations} warn if the compiler made
3063 @item -Wpointer-arith
3064 @opindex Wpointer-arith
3065 Warn about anything that depends on the ``size of'' a function type or
3066 of @code{void}. GNU C assigns these types a size of 1, for
3067 convenience in calculations with @code{void *} pointers and pointers
3070 @item -Wbad-function-cast @r{(C only)}
3071 @opindex Wbad-function-cast
3072 Warn whenever a function call is cast to a non-matching type.
3073 For example, warn if @code{int malloc()} is cast to @code{anything *}.
3076 Warn about ISO C constructs that are outside of the common subset of
3077 ISO C and ISO C++, e.g.@: request for implicit conversion from
3078 @code{void *} to a pointer to non-@code{void} type.
3082 Warn whenever a pointer is cast so as to remove a type qualifier from
3083 the target type. For example, warn if a @code{const char *} is cast
3084 to an ordinary @code{char *}.
3087 @opindex Wcast-align
3088 Warn whenever a pointer is cast such that the required alignment of the
3089 target is increased. For example, warn if a @code{char *} is cast to
3090 an @code{int *} on machines where integers can only be accessed at
3091 two- or four-byte boundaries.
3093 @item -Wwrite-strings
3094 @opindex Wwrite-strings
3095 When compiling C, give string constants the type @code{const
3096 char[@var{length}]} so that
3097 copying the address of one into a non-@code{const} @code{char *}
3098 pointer will get a warning; when compiling C++, warn about the
3099 deprecated conversion from string literals to @code{char *}. This
3100 warning, by default, is enabled for C++ programs.
3101 These warnings will help you find at
3102 compile time code that can try to write into a string constant, but
3103 only if you have been very careful about using @code{const} in
3104 declarations and prototypes. Otherwise, it will just be a nuisance;
3105 this is why we did not make @option{-Wall} request these warnings.
3108 @opindex Wconversion
3109 Warn if a prototype causes a type conversion that is different from what
3110 would happen to the same argument in the absence of a prototype. This
3111 includes conversions of fixed point to floating and vice versa, and
3112 conversions changing the width or signedness of a fixed point argument
3113 except when the same as the default promotion.
3115 Also, warn if a negative integer constant expression is implicitly
3116 converted to an unsigned type. For example, warn about the assignment
3117 @code{x = -1} if @code{x} is unsigned. But do not warn about explicit
3118 casts like @code{(unsigned) -1}.
3120 @item -Wsign-compare
3121 @opindex Wsign-compare
3122 @cindex warning for comparison of signed and unsigned values
3123 @cindex comparison of signed and unsigned values, warning
3124 @cindex signed and unsigned values, comparison warning
3125 Warn when a comparison between signed and unsigned values could produce
3126 an incorrect result when the signed value is converted to unsigned.
3127 This warning is also enabled by @option{-Wextra}; to get the other warnings
3128 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
3130 @item -Waggregate-return
3131 @opindex Waggregate-return
3132 Warn if any functions that return structures or unions are defined or
3133 called. (In languages where you can return an array, this also elicits
3137 @opindex Walways-true
3138 Warn about comparisons which are always true such as testing if
3139 unsigned values are greater than or equal to zero. This warning is
3140 enabled by @option{-Wall}.
3142 @item -Wno-attributes
3143 @opindex Wno-attributes
3144 @opindex Wattributes
3145 Do not warn if an unexpected @code{__attribute__} is used, such as
3146 unrecognized attributes, function attributes applied to variables,
3147 etc. This will not stop errors for incorrect use of supported
3150 @item -Wstrict-prototypes @r{(C only)}
3151 @opindex Wstrict-prototypes
3152 Warn if a function is declared or defined without specifying the
3153 argument types. (An old-style function definition is permitted without
3154 a warning if preceded by a declaration which specifies the argument
3157 @item -Wold-style-definition @r{(C only)}
3158 @opindex Wold-style-definition
3159 Warn if an old-style function definition is used. A warning is given
3160 even if there is a previous prototype.
3162 @item -Wmissing-prototypes @r{(C only)}
3163 @opindex Wmissing-prototypes
3164 Warn if a global function is defined without a previous prototype
3165 declaration. This warning is issued even if the definition itself
3166 provides a prototype. The aim is to detect global functions that fail
3167 to be declared in header files.
3169 @item -Wmissing-declarations @r{(C only)}
3170 @opindex Wmissing-declarations
3171 Warn if a global function is defined without a previous declaration.
3172 Do so even if the definition itself provides a prototype.
3173 Use this option to detect global functions that are not declared in
3176 @item -Wmissing-field-initializers
3177 @opindex Wmissing-field-initializers
3180 Warn if a structure's initializer has some fields missing. For
3181 example, the following code would cause such a warning, because
3182 @code{x.h} is implicitly zero:
3185 struct s @{ int f, g, h; @};
3186 struct s x = @{ 3, 4 @};
3189 This option does not warn about designated initializers, so the following
3190 modification would not trigger a warning:
3193 struct s @{ int f, g, h; @};
3194 struct s x = @{ .f = 3, .g = 4 @};
3197 This warning is included in @option{-Wextra}. To get other @option{-Wextra}
3198 warnings without this one, use @samp{-Wextra -Wno-missing-field-initializers}.
3200 @item -Wmissing-noreturn
3201 @opindex Wmissing-noreturn
3202 Warn about functions which might be candidates for attribute @code{noreturn}.
3203 Note these are only possible candidates, not absolute ones. Care should
3204 be taken to manually verify functions actually do not ever return before
3205 adding the @code{noreturn} attribute, otherwise subtle code generation
3206 bugs could be introduced. You will not get a warning for @code{main} in
3207 hosted C environments.
3209 @item -Wmissing-format-attribute
3210 @opindex Wmissing-format-attribute
3212 Warn about function pointers which might be candidates for @code{format}
3213 attributes. Note these are only possible candidates, not absolute ones.
3214 GCC will guess that function pointers with @code{format} attributes that
3215 are used in assignment, initialization, parameter passing or return
3216 statements should have a corresponding @code{format} attribute in the
3217 resulting type. I.e.@: the left-hand side of the assignment or
3218 initialization, the type of the parameter variable, or the return type
3219 of the containing function respectively should also have a @code{format}
3220 attribute to avoid the warning.
3222 GCC will also warn about function definitions which might be
3223 candidates for @code{format} attributes. Again, these are only
3224 possible candidates. GCC will guess that @code{format} attributes
3225 might be appropriate for any function that calls a function like
3226 @code{vprintf} or @code{vscanf}, but this might not always be the
3227 case, and some functions for which @code{format} attributes are
3228 appropriate may not be detected.
3230 @item -Wno-multichar
3231 @opindex Wno-multichar
3233 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
3234 Usually they indicate a typo in the user's code, as they have
3235 implementation-defined values, and should not be used in portable code.
3237 @item -Wnormalized=<none|id|nfc|nfkc>
3238 @opindex Wnormalized
3241 @cindex character set, input normalization
3242 In ISO C and ISO C++, two identifiers are different if they are
3243 different sequences of characters. However, sometimes when characters
3244 outside the basic ASCII character set are used, you can have two
3245 different character sequences that look the same. To avoid confusion,
3246 the ISO 10646 standard sets out some @dfn{normalization rules} which
3247 when applied ensure that two sequences that look the same are turned into
3248 the same sequence. GCC can warn you if you are using identifiers which
3249 have not been normalized; this option controls that warning.
3251 There are four levels of warning that GCC supports. The default is
3252 @option{-Wnormalized=nfc}, which warns about any identifier which is
3253 not in the ISO 10646 ``C'' normalized form, @dfn{NFC}. NFC is the
3254 recommended form for most uses.
3256 Unfortunately, there are some characters which ISO C and ISO C++ allow
3257 in identifiers that when turned into NFC aren't allowable as
3258 identifiers. That is, there's no way to use these symbols in portable
3259 ISO C or C++ and have all your identifiers in NFC.
3260 @option{-Wnormalized=id} suppresses the warning for these characters.
3261 It is hoped that future versions of the standards involved will correct
3262 this, which is why this option is not the default.
3264 You can switch the warning off for all characters by writing
3265 @option{-Wnormalized=none}. You would only want to do this if you
3266 were using some other normalization scheme (like ``D''), because
3267 otherwise you can easily create bugs that are literally impossible to see.
3269 Some characters in ISO 10646 have distinct meanings but look identical
3270 in some fonts or display methodologies, especially once formatting has
3271 been applied. For instance @code{\u207F}, ``SUPERSCRIPT LATIN SMALL
3272 LETTER N'', will display just like a regular @code{n} which has been
3273 placed in a superscript. ISO 10646 defines the @dfn{NFKC}
3274 normalisation scheme to convert all these into a standard form as
3275 well, and GCC will warn if your code is not in NFKC if you use
3276 @option{-Wnormalized=nfkc}. This warning is comparable to warning
3277 about every identifier that contains the letter O because it might be
3278 confused with the digit 0, and so is not the default, but may be
3279 useful as a local coding convention if the programming environment is
3280 unable to be fixed to display these characters distinctly.
3282 @item -Wno-deprecated-declarations
3283 @opindex Wno-deprecated-declarations
3284 Do not warn about uses of functions, variables, and types marked as
3285 deprecated by using the @code{deprecated} attribute.
3286 (@pxref{Function Attributes}, @pxref{Variable Attributes},
3287 @pxref{Type Attributes}.)
3290 @opindex Wno-overflow
3291 Do not warn about compile-time overflow in constant expressions.
3295 Warn if a structure is given the packed attribute, but the packed
3296 attribute has no effect on the layout or size of the structure.
3297 Such structures may be mis-aligned for little benefit. For
3298 instance, in this code, the variable @code{f.x} in @code{struct bar}
3299 will be misaligned even though @code{struct bar} does not itself
3300 have the packed attribute:
3307 @} __attribute__((packed));
3317 Warn if padding is included in a structure, either to align an element
3318 of the structure or to align the whole structure. Sometimes when this
3319 happens it is possible to rearrange the fields of the structure to
3320 reduce the padding and so make the structure smaller.
3322 @item -Wredundant-decls
3323 @opindex Wredundant-decls
3324 Warn if anything is declared more than once in the same scope, even in
3325 cases where multiple declaration is valid and changes nothing.
3327 @item -Wnested-externs @r{(C only)}
3328 @opindex Wnested-externs
3329 Warn if an @code{extern} declaration is encountered within a function.
3331 @item -Wunreachable-code
3332 @opindex Wunreachable-code
3333 Warn if the compiler detects that code will never be executed.
3335 This option is intended to warn when the compiler detects that at
3336 least a whole line of source code will never be executed, because
3337 some condition is never satisfied or because it is after a
3338 procedure that never returns.
3340 It is possible for this option to produce a warning even though there
3341 are circumstances under which part of the affected line can be executed,
3342 so care should be taken when removing apparently-unreachable code.
3344 For instance, when a function is inlined, a warning may mean that the
3345 line is unreachable in only one inlined copy of the function.
3347 This option is not made part of @option{-Wall} because in a debugging
3348 version of a program there is often substantial code which checks
3349 correct functioning of the program and is, hopefully, unreachable
3350 because the program does work. Another common use of unreachable
3351 code is to provide behavior which is selectable at compile-time.
3355 Warn if a function can not be inlined and it was declared as inline.
3356 Even with this option, the compiler will not warn about failures to
3357 inline functions declared in system headers.
3359 The compiler uses a variety of heuristics to determine whether or not
3360 to inline a function. For example, the compiler takes into account
3361 the size of the function being inlined and the amount of inlining
3362 that has already been done in the current function. Therefore,
3363 seemingly insignificant changes in the source program can cause the
3364 warnings produced by @option{-Winline} to appear or disappear.
3366 @item -Wno-invalid-offsetof @r{(C++ only)}
3367 @opindex Wno-invalid-offsetof
3368 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
3369 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
3370 to a non-POD type is undefined. In existing C++ implementations,
3371 however, @samp{offsetof} typically gives meaningful results even when
3372 applied to certain kinds of non-POD types. (Such as a simple
3373 @samp{struct} that fails to be a POD type only by virtue of having a
3374 constructor.) This flag is for users who are aware that they are
3375 writing nonportable code and who have deliberately chosen to ignore the
3378 The restrictions on @samp{offsetof} may be relaxed in a future version
3379 of the C++ standard.
3381 @item -Wno-int-to-pointer-cast @r{(C only)}
3382 @opindex Wno-int-to-pointer-cast
3383 Suppress warnings from casts to pointer type of an integer of a
3386 @item -Wno-pointer-to-int-cast @r{(C only)}
3387 @opindex Wno-pointer-to-int-cast
3388 Suppress warnings from casts from a pointer to an integer type of a
3392 @opindex Winvalid-pch
3393 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
3394 the search path but can't be used.
3398 @opindex Wno-long-long
3399 Warn if @samp{long long} type is used. This is default. To inhibit
3400 the warning messages, use @option{-Wno-long-long}. Flags
3401 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
3402 only when @option{-pedantic} flag is used.
3404 @item -Wvariadic-macros
3405 @opindex Wvariadic-macros
3406 @opindex Wno-variadic-macros
3407 Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
3408 alternate syntax when in pedantic ISO C99 mode. This is default.
3409 To inhibit the warning messages, use @option{-Wno-variadic-macros}.
3411 @item -Wvolatile-register-var
3412 @opindex Wvolatile-register-var
3413 @opindex Wno-volatile-register-var
3414 Warn if a register variable is declared volatile. The volatile
3415 modifier does not inhibit all optimizations that may eliminate reads
3416 and/or writes to register variables.
3418 @item -Wdisabled-optimization
3419 @opindex Wdisabled-optimization
3420 Warn if a requested optimization pass is disabled. This warning does
3421 not generally indicate that there is anything wrong with your code; it
3422 merely indicates that GCC's optimizers were unable to handle the code
3423 effectively. Often, the problem is that your code is too big or too
3424 complex; GCC will refuse to optimize programs when the optimization
3425 itself is likely to take inordinate amounts of time.
3427 @item -Wpointer-sign
3428 @opindex Wpointer-sign
3429 @opindex Wno-pointer-sign
3430 Warn for pointer argument passing or assignment with different signedness.
3431 This option is only supported for C and Objective-C@. It is implied by
3432 @option{-Wall} and by @option{-pedantic}, which can be disabled with
3433 @option{-Wno-pointer-sign}.
3437 Make all warnings into errors.
3441 Make the specified warning into an errors. The specifier for a
3442 warning is appended, for example @option{-Werror=switch} turns the
3443 warnings controlled by @option{-Wswitch} into errors. This switch
3444 takes a negative form, to be used to negate @option{-Werror} for
3445 specific warnings, for example @option{-Wno-error=switch} makes
3446 @option{-Wswitch} warnings not be errors, even when @option{-Werror}
3447 is in effect. You can use the @option{-fdiagnostics-show-option}
3448 option to have each controllable warning amended with the option which
3449 controls it, to determine what to use with this option.
3451 Note that specifying @option{-Werror=}@var{foo} automatically implies
3452 @option{-W}@var{foo}. However, @option{-Wno-error=}@var{foo} does not
3455 @item -Wstack-protector
3456 @opindex Wstack-protector
3457 This option is only active when @option{-fstack-protector} is active. It
3458 warns about functions that will not be protected against stack smashing.
3460 @item -Wstring-literal-comparison
3461 @opindex Wstring-literal-comparison
3462 Warn about suspicious comparisons to string literal constants. In C,
3463 direct comparisons against the memory address of a string literal, such
3464 as @code{if (x == "abc")}, typically indicate a programmer error, and
3465 even when intentional, result in unspecified behavior and are not portable.
3466 Usually these warnings alert that the programmer intended to use
3467 @code{strcmp}. This warning is enabled by @option{-Wall}.
3469 @item -Woverlength-strings
3470 @opindex Woverlength-strings
3471 Warn about string constants which are longer than the ``minimum
3472 maximum'' length specified in the C standard. Modern compilers
3473 generally allow string constants which are much longer than the
3474 standard's minimum limit, but very portable programs should avoid
3475 using longer strings.
3477 The limit applies @emph{after} string constant concatenation, and does
3478 not count the trailing NUL@. In C89, the limit was 509 characters; in
3479 C99, it was raised to 4095. C++98 does not specify a normative
3480 minimum maximum, so we do not diagnose overlength strings in C++@.
3482 This option is implied by @option{-pedantic}, and can be disabled with
3483 @option{-Wno-overlength-strings}.
3486 @node Debugging Options
3487 @section Options for Debugging Your Program or GCC
3488 @cindex options, debugging
3489 @cindex debugging information options
3491 GCC has various special options that are used for debugging
3492 either your program or GCC:
3497 Produce debugging information in the operating system's native format
3498 (stabs, COFF, XCOFF, or DWARF 2)@. GDB can work with this debugging
3501 On most systems that use stabs format, @option{-g} enables use of extra
3502 debugging information that only GDB can use; this extra information
3503 makes debugging work better in GDB but will probably make other debuggers
3505 refuse to read the program. If you want to control for certain whether
3506 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
3507 @option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
3509 GCC allows you to use @option{-g} with
3510 @option{-O}. The shortcuts taken by optimized code may occasionally
3511 produce surprising results: some variables you declared may not exist
3512 at all; flow of control may briefly move where you did not expect it;
3513 some statements may not be executed because they compute constant
3514 results or their values were already at hand; some statements may
3515 execute in different places because they were moved out of loops.
3517 Nevertheless it proves possible to debug optimized output. This makes
3518 it reasonable to use the optimizer for programs that might have bugs.
3520 The following options are useful when GCC is generated with the
3521 capability for more than one debugging format.
3525 Produce debugging information for use by GDB@. This means to use the
3526 most expressive format available (DWARF 2, stabs, or the native format
3527 if neither of those are supported), including GDB extensions if at all
3532 Produce debugging information in stabs format (if that is supported),
3533 without GDB extensions. This is the format used by DBX on most BSD
3534 systems. On MIPS, Alpha and System V Release 4 systems this option
3535 produces stabs debugging output which is not understood by DBX or SDB@.
3536 On System V Release 4 systems this option requires the GNU assembler.
3538 @item -feliminate-unused-debug-symbols
3539 @opindex feliminate-unused-debug-symbols
3540 Produce debugging information in stabs format (if that is supported),
3541 for only symbols that are actually used.
3543 @item -femit-class-debug-always
3544 Instead of emitting debugging information for a C++ class in only one
3545 object file, emit it in all object files using the class. This option
3546 should be used only with debuggers that are unable to handle the way GCC
3547 normally emits debugging information for classes because using this
3548 option will increase the size of debugging information by as much as a
3553 Produce debugging information in stabs format (if that is supported),
3554 using GNU extensions understood only by the GNU debugger (GDB)@. The
3555 use of these extensions is likely to make other debuggers crash or
3556 refuse to read the program.
3560 Produce debugging information in COFF format (if that is supported).
3561 This is the format used by SDB on most System V systems prior to
3566 Produce debugging information in XCOFF format (if that is supported).
3567 This is the format used by the DBX debugger on IBM RS/6000 systems.
3571 Produce debugging information in XCOFF format (if that is supported),
3572 using GNU extensions understood only by the GNU debugger (GDB)@. The
3573 use of these extensions is likely to make other debuggers crash or
3574 refuse to read the program, and may cause assemblers other than the GNU
3575 assembler (GAS) to fail with an error.
3579 Produce debugging information in DWARF version 2 format (if that is
3580 supported). This is the format used by DBX on IRIX 6. With this
3581 option, GCC uses features of DWARF version 3 when they are useful;
3582 version 3 is upward compatible with version 2, but may still cause
3583 problems for older debuggers.
3587 Produce debugging information in VMS debug format (if that is
3588 supported). This is the format used by DEBUG on VMS systems.
3591 @itemx -ggdb@var{level}
3592 @itemx -gstabs@var{level}
3593 @itemx -gcoff@var{level}
3594 @itemx -gxcoff@var{level}
3595 @itemx -gvms@var{level}
3596 Request debugging information and also use @var{level} to specify how
3597 much information. The default level is 2.
3599 Level 1 produces minimal information, enough for making backtraces in
3600 parts of the program that you don't plan to debug. This includes
3601 descriptions of functions and external variables, but no information
3602 about local variables and no line numbers.
3604 Level 3 includes extra information, such as all the macro definitions
3605 present in the program. Some debuggers support macro expansion when
3606 you use @option{-g3}.
3608 @option{-gdwarf-2} does not accept a concatenated debug level, because
3609 GCC used to support an option @option{-gdwarf} that meant to generate
3610 debug information in version 1 of the DWARF format (which is very
3611 different from version 2), and it would have been too confusing. That
3612 debug format is long obsolete, but the option cannot be changed now.
3613 Instead use an additional @option{-g@var{level}} option to change the
3614 debug level for DWARF2.
3616 @item -feliminate-dwarf2-dups
3617 @opindex feliminate-dwarf2-dups
3618 Compress DWARF2 debugging information by eliminating duplicated
3619 information about each symbol. This option only makes sense when
3620 generating DWARF2 debugging information with @option{-gdwarf-2}.
3622 @cindex @command{prof}
3625 Generate extra code to write profile information suitable for the
3626 analysis program @command{prof}. You must use this option when compiling
3627 the source files you want data about, and you must also use it when
3630 @cindex @command{gprof}
3633 Generate extra code to write profile information suitable for the
3634 analysis program @command{gprof}. You must use this option when compiling
3635 the source files you want data about, and you must also use it when
3640 Makes the compiler print out each function name as it is compiled, and
3641 print some statistics about each pass when it finishes.
3644 @opindex ftime-report
3645 Makes the compiler print some statistics about the time consumed by each
3646 pass when it finishes.
3649 @opindex fmem-report
3650 Makes the compiler print some statistics about permanent memory
3651 allocation when it finishes.
3653 @item -fprofile-arcs
3654 @opindex fprofile-arcs
3655 Add code so that program flow @dfn{arcs} are instrumented. During
3656 execution the program records how many times each branch and call is
3657 executed and how many times it is taken or returns. When the compiled
3658 program exits it saves this data to a file called
3659 @file{@var{auxname}.gcda} for each source file. The data may be used for
3660 profile-directed optimizations (@option{-fbranch-probabilities}), or for
3661 test coverage analysis (@option{-ftest-coverage}). Each object file's
3662 @var{auxname} is generated from the name of the output file, if
3663 explicitly specified and it is not the final executable, otherwise it is
3664 the basename of the source file. In both cases any suffix is removed
3665 (e.g.@: @file{foo.gcda} for input file @file{dir/foo.c}, or
3666 @file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
3667 @xref{Cross-profiling}.
3669 @cindex @command{gcov}
3673 This option is used to compile and link code instrumented for coverage
3674 analysis. The option is a synonym for @option{-fprofile-arcs}
3675 @option{-ftest-coverage} (when compiling) and @option{-lgcov} (when
3676 linking). See the documentation for those options for more details.
3681 Compile the source files with @option{-fprofile-arcs} plus optimization
3682 and code generation options. For test coverage analysis, use the
3683 additional @option{-ftest-coverage} option. You do not need to profile
3684 every source file in a program.
3687 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
3688 (the latter implies the former).
3691 Run the program on a representative workload to generate the arc profile
3692 information. This may be repeated any number of times. You can run
3693 concurrent instances of your program, and provided that the file system
3694 supports locking, the data files will be correctly updated. Also
3695 @code{fork} calls are detected and correctly handled (double counting
3699 For profile-directed optimizations, compile the source files again with
3700 the same optimization and code generation options plus
3701 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
3702 Control Optimization}).
3705 For test coverage analysis, use @command{gcov} to produce human readable
3706 information from the @file{.gcno} and @file{.gcda} files. Refer to the
3707 @command{gcov} documentation for further information.
3711 With @option{-fprofile-arcs}, for each function of your program GCC
3712 creates a program flow graph, then finds a spanning tree for the graph.
3713 Only arcs that are not on the spanning tree have to be instrumented: the
3714 compiler adds code to count the number of times that these arcs are
3715 executed. When an arc is the only exit or only entrance to a block, the
3716 instrumentation code can be added to the block; otherwise, a new basic
3717 block must be created to hold the instrumentation code.
3720 @item -ftest-coverage
3721 @opindex ftest-coverage
3722 Produce a notes file that the @command{gcov} code-coverage utility
3723 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
3724 show program coverage. Each source file's note file is called
3725 @file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
3726 above for a description of @var{auxname} and instructions on how to
3727 generate test coverage data. Coverage data will match the source files
3728 more closely, if you do not optimize.
3730 @item -d@var{letters}
3731 @item -fdump-rtl-@var{pass}
3733 Says to make debugging dumps during compilation at times specified by
3734 @var{letters}. This is used for debugging the RTL-based passes of the
3735 compiler. The file names for most of the dumps are made by appending a
3736 pass number and a word to the @var{dumpname}. @var{dumpname} is generated
3737 from the name of the output file, if explicitly specified and it is not
3738 an executable, otherwise it is the basename of the source file.
3740 Most debug dumps can be enabled either passing a letter to the @option{-d}
3741 option, or with a long @option{-fdump-rtl} switch; here are the possible
3742 letters for use in @var{letters} and @var{pass}, and their meanings:
3747 Annotate the assembler output with miscellaneous debugging information.
3750 @itemx -fdump-rtl-bbro
3752 @opindex fdump-rtl-bbro
3753 Dump after block reordering, to @file{@var{file}.148r.bbro}.
3756 @itemx -fdump-rtl-combine
3758 @opindex fdump-rtl-combine
3759 Dump after instruction combination, to the file @file{@var{file}.129r.combine}.
3762 @itemx -fdump-rtl-ce1
3763 @itemx -fdump-rtl-ce2
3765 @opindex fdump-rtl-ce1
3766 @opindex fdump-rtl-ce2
3767 @option{-dC} and @option{-fdump-rtl-ce1} enable dumping after the
3768 first if conversion, to the file @file{@var{file}.117r.ce1}. @option{-dC}
3769 and @option{-fdump-rtl-ce2} enable dumping after the second if
3770 conversion, to the file @file{@var{file}.130r.ce2}.
3773 @itemx -fdump-rtl-btl
3774 @itemx -fdump-rtl-dbr
3776 @opindex fdump-rtl-btl
3777 @opindex fdump-rtl-dbr
3778 @option{-dd} and @option{-fdump-rtl-btl} enable dumping after branch
3779 target load optimization, to @file{@var{file}.31.btl}. @option{-dd}
3780 and @option{-fdump-rtl-dbr} enable dumping after delayed branch
3781 scheduling, to @file{@var{file}.36.dbr}.
3785 Dump all macro definitions, at the end of preprocessing, in addition to
3789 @itemx -fdump-rtl-ce3
3791 @opindex fdump-rtl-ce3
3792 Dump after the third if conversion, to @file{@var{file}.146r.ce3}.
3795 @itemx -fdump-rtl-cfg
3796 @itemx -fdump-rtl-life
3798 @opindex fdump-rtl-cfg
3799 @opindex fdump-rtl-life
3800 @option{-df} and @option{-fdump-rtl-cfg} enable dumping after control
3801 and data flow analysis, to @file{@var{file}.116r.cfg}. @option{-df}
3802 and @option{-fdump-rtl-cfg} enable dumping dump after life analysis,
3803 to @file{@var{file}.128r.life1} and @file{@var{file}.135r.life2}.
3806 @itemx -fdump-rtl-greg
3808 @opindex fdump-rtl-greg
3809 Dump after global register allocation, to @file{@var{file}.139r.greg}.
3812 @itemx -fdump-rtl-gcse
3813 @itemx -fdump-rtl-bypass
3815 @opindex fdump-rtl-gcse
3816 @opindex fdump-rtl-bypass
3817 @option{-dG} and @option{-fdump-rtl-gcse} enable dumping after GCSE, to
3818 @file{@var{file}.114r.gcse}. @option{-dG} and @option{-fdump-rtl-bypass}
3819 enable dumping after jump bypassing and control flow optimizations, to
3820 @file{@var{file}.115r.bypass}.
3823 @itemx -fdump-rtl-eh
3825 @opindex fdump-rtl-eh
3826 Dump after finalization of EH handling code, to @file{@var{file}.02.eh}.
3829 @itemx -fdump-rtl-sibling
3831 @opindex fdump-rtl-sibling
3832 Dump after sibling call optimizations, to @file{@var{file}.106r.sibling}.
3835 @itemx -fdump-rtl-jump
3837 @opindex fdump-rtl-jump
3838 Dump after the first jump optimization, to @file{@var{file}.112r.jump}.
3841 @itemx -fdump-rtl-stack
3843 @opindex fdump-rtl-stack
3844 Dump after conversion from registers to stack, to @file{@var{file}.152r.stack}.
3847 @itemx -fdump-rtl-lreg
3849 @opindex fdump-rtl-lreg
3850 Dump after local register allocation, to @file{@var{file}.138r.lreg}.
3853 @itemx -fdump-rtl-loop2
3855 @opindex fdump-rtl-loop2
3856 @option{-dL} and @option{-fdump-rtl-loop2} enable dumping after the
3857 loop optimization pass, to @file{@var{file}.119r.loop2},
3858 @file{@var{file}.120r.loop2_init},
3859 @file{@var{file}.121r.loop2_invariant}, and
3860 @file{@var{file}.125r.loop2_done}.
3863 @itemx -fdump-rtl-sms
3865 @opindex fdump-rtl-sms
3866 Dump after modulo scheduling, to @file{@var{file}.136r.sms}.
3869 @itemx -fdump-rtl-mach
3871 @opindex fdump-rtl-mach
3872 Dump after performing the machine dependent reorganization pass, to
3873 @file{@var{file}.155r.mach}.
3876 @itemx -fdump-rtl-rnreg
3878 @opindex fdump-rtl-rnreg
3879 Dump after register renumbering, to @file{@var{file}.147r.rnreg}.
3882 @itemx -fdump-rtl-regmove
3884 @opindex fdump-rtl-regmove
3885 Dump after the register move pass, to @file{@var{file}.132r.regmove}.
3888 @itemx -fdump-rtl-postreload
3890 @opindex fdump-rtl-postreload
3891 Dump after post-reload optimizations, to @file{@var{file}.24.postreload}.
3894 @itemx -fdump-rtl-expand
3896 @opindex fdump-rtl-expand
3897 Dump after RTL generation, to @file{@var{file}.104r.expand}.
3900 @itemx -fdump-rtl-sched2
3902 @opindex fdump-rtl-sched2
3903 Dump after the second scheduling pass, to @file{@var{file}.150r.sched2}.
3906 @itemx -fdump-rtl-cse
3908 @opindex fdump-rtl-cse
3909 Dump after CSE (including the jump optimization that sometimes follows
3910 CSE), to @file{@var{file}.113r.cse}.
3913 @itemx -fdump-rtl-sched
3915 @opindex fdump-rtl-sched
3916 Dump after the first scheduling pass, to @file{@var{file}.21.sched}.
3919 @itemx -fdump-rtl-cse2
3921 @opindex fdump-rtl-cse2
3922 Dump after the second CSE pass (including the jump optimization that
3923 sometimes follows CSE), to @file{@var{file}.127r.cse2}.
3926 @itemx -fdump-rtl-tracer
3928 @opindex fdump-rtl-tracer
3929 Dump after running tracer, to @file{@var{file}.118r.tracer}.
3932 @itemx -fdump-rtl-vpt
3933 @itemx -fdump-rtl-vartrack
3935 @opindex fdump-rtl-vpt
3936 @opindex fdump-rtl-vartrack
3937 @option{-dV} and @option{-fdump-rtl-vpt} enable dumping after the value
3938 profile transformations, to @file{@var{file}.10.vpt}. @option{-dV}
3939 and @option{-fdump-rtl-vartrack} enable dumping after variable tracking,
3940 to @file{@var{file}.154r.vartrack}.
3943 @itemx -fdump-rtl-flow2
3945 @opindex fdump-rtl-flow2
3946 Dump after the second flow pass, to @file{@var{file}.142r.flow2}.
3949 @itemx -fdump-rtl-peephole2
3951 @opindex fdump-rtl-peephole2
3952 Dump after the peephole pass, to @file{@var{file}.145r.peephole2}.
3955 @itemx -fdump-rtl-web
3957 @opindex fdump-rtl-web
3958 Dump after live range splitting, to @file{@var{file}.126r.web}.
3961 @itemx -fdump-rtl-all
3963 @opindex fdump-rtl-all
3964 Produce all the dumps listed above.
3968 Produce a core dump whenever an error occurs.
3972 Print statistics on memory usage, at the end of the run, to
3977 Annotate the assembler output with a comment indicating which
3978 pattern and alternative was used. The length of each instruction is
3983 Dump the RTL in the assembler output as a comment before each instruction.
3984 Also turns on @option{-dp} annotation.
3988 For each of the other indicated dump files (either with @option{-d} or
3989 @option{-fdump-rtl-@var{pass}}), dump a representation of the control flow
3990 graph suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
3994 Just generate RTL for a function instead of compiling it. Usually used
3995 with @samp{r} (@option{-fdump-rtl-expand}).
3999 Dump debugging information during parsing, to standard error.
4002 @item -fdump-unnumbered
4003 @opindex fdump-unnumbered
4004 When doing debugging dumps (see @option{-d} option above), suppress instruction
4005 numbers and line number note output. This makes it more feasible to
4006 use diff on debugging dumps for compiler invocations with different
4007 options, in particular with and without @option{-g}.
4009 @item -fdump-translation-unit @r{(C++ only)}
4010 @itemx -fdump-translation-unit-@var{options} @r{(C++ only)}
4011 @opindex fdump-translation-unit
4012 Dump a representation of the tree structure for the entire translation
4013 unit to a file. The file name is made by appending @file{.tu} to the
4014 source file name. If the @samp{-@var{options}} form is used, @var{options}
4015 controls the details of the dump as described for the
4016 @option{-fdump-tree} options.
4018 @item -fdump-class-hierarchy @r{(C++ only)}
4019 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
4020 @opindex fdump-class-hierarchy
4021 Dump a representation of each class's hierarchy and virtual function
4022 table layout to a file. The file name is made by appending @file{.class}
4023 to the source file name. If the @samp{-@var{options}} form is used,
4024 @var{options} controls the details of the dump as described for the
4025 @option{-fdump-tree} options.
4027 @item -fdump-ipa-@var{switch}
4029 Control the dumping at various stages of inter-procedural analysis
4030 language tree to a file. The file name is generated by appending a switch
4031 specific suffix to the source file name. The following dumps are possible:
4035 Enables all inter-procedural analysis dumps; currently the only produced
4036 dump is the @samp{cgraph} dump.
4039 Dumps information about call-graph optimization, unused function removal,
4040 and inlining decisions.
4043 @item -fdump-tree-@var{switch}
4044 @itemx -fdump-tree-@var{switch}-@var{options}
4046 Control the dumping at various stages of processing the intermediate
4047 language tree to a file. The file name is generated by appending a switch
4048 specific suffix to the source file name. If the @samp{-@var{options}}
4049 form is used, @var{options} is a list of @samp{-} separated options that
4050 control the details of the dump. Not all options are applicable to all
4051 dumps, those which are not meaningful will be ignored. The following
4052 options are available
4056 Print the address of each node. Usually this is not meaningful as it
4057 changes according to the environment and source file. Its primary use
4058 is for tying up a dump file with a debug environment.
4060 Inhibit dumping of members of a scope or body of a function merely
4061 because that scope has been reached. Only dump such items when they
4062 are directly reachable by some other path. When dumping pretty-printed
4063 trees, this option inhibits dumping the bodies of control structures.
4065 Print a raw representation of the tree. By default, trees are
4066 pretty-printed into a C-like representation.
4068 Enable more detailed dumps (not honored by every dump option).
4070 Enable dumping various statistics about the pass (not honored by every dump
4073 Enable showing basic block boundaries (disabled in raw dumps).
4075 Enable showing virtual operands for every statement.
4077 Enable showing line numbers for statements.
4079 Enable showing the unique ID (@code{DECL_UID}) for each variable.
4081 Turn on all options, except @option{raw}, @option{slim} and @option{lineno}.
4084 The following tree dumps are possible:
4088 Dump before any tree based optimization, to @file{@var{file}.original}.
4091 Dump after all tree based optimization, to @file{@var{file}.optimized}.
4094 Dump after function inlining, to @file{@var{file}.inlined}.
4097 @opindex fdump-tree-gimple
4098 Dump each function before and after the gimplification pass to a file. The
4099 file name is made by appending @file{.gimple} to the source file name.
4102 @opindex fdump-tree-cfg
4103 Dump the control flow graph of each function to a file. The file name is
4104 made by appending @file{.cfg} to the source file name.
4107 @opindex fdump-tree-vcg
4108 Dump the control flow graph of each function to a file in VCG format. The
4109 file name is made by appending @file{.vcg} to the source file name. Note
4110 that if the file contains more than one function, the generated file cannot
4111 be used directly by VCG@. You will need to cut and paste each function's
4112 graph into its own separate file first.
4115 @opindex fdump-tree-ch
4116 Dump each function after copying loop headers. The file name is made by
4117 appending @file{.ch} to the source file name.
4120 @opindex fdump-tree-ssa
4121 Dump SSA related information to a file. The file name is made by appending
4122 @file{.ssa} to the source file name.
4125 @opindex fdump-tree-salias
4126 Dump structure aliasing variable information to a file. This file name
4127 is made by appending @file{.salias} to the source file name.
4130 @opindex fdump-tree-alias
4131 Dump aliasing information for each function. The file name is made by
4132 appending @file{.alias} to the source file name.
4135 @opindex fdump-tree-ccp
4136 Dump each function after CCP@. The file name is made by appending
4137 @file{.ccp} to the source file name.
4140 @opindex fdump-tree-storeccp
4141 Dump each function after STORE-CCP. The file name is made by appending
4142 @file{.storeccp} to the source file name.
4145 @opindex fdump-tree-pre
4146 Dump trees after partial redundancy elimination. The file name is made
4147 by appending @file{.pre} to the source file name.
4150 @opindex fdump-tree-fre
4151 Dump trees after full redundancy elimination. The file name is made
4152 by appending @file{.fre} to the source file name.
4155 @opindex fdump-tree-copyprop
4156 Dump trees after copy propagation. The file name is made
4157 by appending @file{.copyprop} to the source file name.
4159 @item store_copyprop
4160 @opindex fdump-tree-store_copyprop
4161 Dump trees after store copy-propagation. The file name is made
4162 by appending @file{.store_copyprop} to the source file name.
4165 @opindex fdump-tree-dce
4166 Dump each function after dead code elimination. The file name is made by
4167 appending @file{.dce} to the source file name.
4170 @opindex fdump-tree-mudflap
4171 Dump each function after adding mudflap instrumentation. The file name is
4172 made by appending @file{.mudflap} to the source file name.
4175 @opindex fdump-tree-sra
4176 Dump each function after performing scalar replacement of aggregates. The
4177 file name is made by appending @file{.sra} to the source file name.
4180 @opindex fdump-tree-sink
4181 Dump each function after performing code sinking. The file name is made
4182 by appending @file{.sink} to the source file name.
4185 @opindex fdump-tree-dom
4186 Dump each function after applying dominator tree optimizations. The file
4187 name is made by appending @file{.dom} to the source file name.
4190 @opindex fdump-tree-dse
4191 Dump each function after applying dead store elimination. The file
4192 name is made by appending @file{.dse} to the source file name.
4195 @opindex fdump-tree-phiopt
4196 Dump each function after optimizing PHI nodes into straightline code. The file
4197 name is made by appending @file{.phiopt} to the source file name.
4200 @opindex fdump-tree-forwprop
4201 Dump each function after forward propagating single use variables. The file
4202 name is made by appending @file{.forwprop} to the source file name.
4205 @opindex fdump-tree-copyrename
4206 Dump each function after applying the copy rename optimization. The file
4207 name is made by appending @file{.copyrename} to the source file name.
4210 @opindex fdump-tree-nrv
4211 Dump each function after applying the named return value optimization on
4212 generic trees. The file name is made by appending @file{.nrv} to the source
4216 @opindex fdump-tree-vect
4217 Dump each function after applying vectorization of loops. The file name is
4218 made by appending @file{.vect} to the source file name.
4221 @opindex fdump-tree-vrp
4222 Dump each function after Value Range Propagation (VRP). The file name
4223 is made by appending @file{.vrp} to the source file name.
4226 @opindex fdump-tree-all
4227 Enable all the available tree dumps with the flags provided in this option.
4230 @item -ftree-vectorizer-verbose=@var{n}
4231 @opindex ftree-vectorizer-verbose
4232 This option controls the amount of debugging output the vectorizer prints.
4233 This information is written to standard error, unless
4234 @option{-fdump-tree-all} or @option{-fdump-tree-vect} is specified,
4235 in which case it is output to the usual dump listing file, @file{.vect}.
4236 For @var{n}=0 no diagnostic information is reported.
4237 If @var{n}=1 the vectorizer reports each loop that got vectorized,
4238 and the total number of loops that got vectorized.
4239 If @var{n}=2 the vectorizer also reports non-vectorized loops that passed
4240 the first analysis phase (vect_analyze_loop_form) - i.e. countable,
4241 inner-most, single-bb, single-entry/exit loops. This is the same verbosity
4242 level that @option{-fdump-tree-vect-stats} uses.
4243 Higher verbosity levels mean either more information dumped for each
4244 reported loop, or same amount of information reported for more loops:
4245 If @var{n}=3, alignment related information is added to the reports.
4246 If @var{n}=4, data-references related information (e.g. memory dependences,
4247 memory access-patterns) is added to the reports.
4248 If @var{n}=5, the vectorizer reports also non-vectorized inner-most loops
4249 that did not pass the first analysis phase (i.e. may not be countable, or
4250 may have complicated control-flow).
4251 If @var{n}=6, the vectorizer reports also non-vectorized nested loops.
4252 For @var{n}=7, all the information the vectorizer generates during its
4253 analysis and transformation is reported. This is the same verbosity level
4254 that @option{-fdump-tree-vect-details} uses.
4256 @item -frandom-seed=@var{string}
4257 @opindex frandom-string
4258 This option provides a seed that GCC uses when it would otherwise use
4259 random numbers. It is used to generate certain symbol names
4260 that have to be different in every compiled file. It is also used to
4261 place unique stamps in coverage data files and the object files that
4262 produce them. You can use the @option{-frandom-seed} option to produce
4263 reproducibly identical object files.
4265 The @var{string} should be different for every file you compile.
4267 @item -fsched-verbose=@var{n}
4268 @opindex fsched-verbose
4269 On targets that use instruction scheduling, this option controls the
4270 amount of debugging output the scheduler prints. This information is
4271 written to standard error, unless @option{-dS} or @option{-dR} is
4272 specified, in which case it is output to the usual dump
4273 listing file, @file{.sched} or @file{.sched2} respectively. However
4274 for @var{n} greater than nine, the output is always printed to standard
4277 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
4278 same information as @option{-dRS}. For @var{n} greater than one, it
4279 also output basic block probabilities, detailed ready list information
4280 and unit/insn info. For @var{n} greater than two, it includes RTL
4281 at abort point, control-flow and regions info. And for @var{n} over
4282 four, @option{-fsched-verbose} also includes dependence info.
4286 Store the usual ``temporary'' intermediate files permanently; place them
4287 in the current directory and name them based on the source file. Thus,
4288 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
4289 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
4290 preprocessed @file{foo.i} output file even though the compiler now
4291 normally uses an integrated preprocessor.
4293 When used in combination with the @option{-x} command line option,
4294 @option{-save-temps} is sensible enough to avoid over writing an
4295 input source file with the same extension as an intermediate file.
4296 The corresponding intermediate file may be obtained by renaming the
4297 source file before using @option{-save-temps}.
4301 Report the CPU time taken by each subprocess in the compilation
4302 sequence. For C source files, this is the compiler proper and assembler
4303 (plus the linker if linking is done). The output looks like this:
4310 The first number on each line is the ``user time'', that is time spent
4311 executing the program itself. The second number is ``system time'',
4312 time spent executing operating system routines on behalf of the program.
4313 Both numbers are in seconds.
4315 @item -fvar-tracking
4316 @opindex fvar-tracking
4317 Run variable tracking pass. It computes where variables are stored at each
4318 position in code. Better debugging information is then generated
4319 (if the debugging information format supports this information).
4321 It is enabled by default when compiling with optimization (@option{-Os},
4322 @option{-O}, @option{-O2}, ...), debugging information (@option{-g}) and
4323 the debug info format supports it.
4325 @item -print-file-name=@var{library}
4326 @opindex print-file-name
4327 Print the full absolute name of the library file @var{library} that
4328 would be used when linking---and don't do anything else. With this
4329 option, GCC does not compile or link anything; it just prints the
4332 @item -print-multi-directory
4333 @opindex print-multi-directory
4334 Print the directory name corresponding to the multilib selected by any
4335 other switches present in the command line. This directory is supposed
4336 to exist in @env{GCC_EXEC_PREFIX}.
4338 @item -print-multi-lib
4339 @opindex print-multi-lib
4340 Print the mapping from multilib directory names to compiler switches
4341 that enable them. The directory name is separated from the switches by
4342 @samp{;}, and each switch starts with an @samp{@@} instead of the
4343 @samp{-}, without spaces between multiple switches. This is supposed to
4344 ease shell-processing.
4346 @item -print-prog-name=@var{program}
4347 @opindex print-prog-name
4348 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
4350 @item -print-libgcc-file-name
4351 @opindex print-libgcc-file-name
4352 Same as @option{-print-file-name=libgcc.a}.
4354 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
4355 but you do want to link with @file{libgcc.a}. You can do
4358 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
4361 @item -print-search-dirs
4362 @opindex print-search-dirs
4363 Print the name of the configured installation directory and a list of
4364 program and library directories @command{gcc} will search---and don't do anything else.
4366 This is useful when @command{gcc} prints the error message
4367 @samp{installation problem, cannot exec cpp0: No such file or directory}.
4368 To resolve this you either need to put @file{cpp0} and the other compiler
4369 components where @command{gcc} expects to find them, or you can set the environment
4370 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
4371 Don't forget the trailing @samp{/}.
4372 @xref{Environment Variables}.
4375 @opindex dumpmachine
4376 Print the compiler's target machine (for example,
4377 @samp{i686-pc-linux-gnu})---and don't do anything else.
4380 @opindex dumpversion
4381 Print the compiler version (for example, @samp{3.0})---and don't do
4386 Print the compiler's built-in specs---and don't do anything else. (This
4387 is used when GCC itself is being built.) @xref{Spec Files}.
4389 @item -feliminate-unused-debug-types
4390 @opindex feliminate-unused-debug-types
4391 Normally, when producing DWARF2 output, GCC will emit debugging
4392 information for all types declared in a compilation
4393 unit, regardless of whether or not they are actually used
4394 in that compilation unit. Sometimes this is useful, such as
4395 if, in the debugger, you want to cast a value to a type that is
4396 not actually used in your program (but is declared). More often,
4397 however, this results in a significant amount of wasted space.
4398 With this option, GCC will avoid producing debug symbol output
4399 for types that are nowhere used in the source file being compiled.
4402 @node Optimize Options
4403 @section Options That Control Optimization
4404 @cindex optimize options
4405 @cindex options, optimization
4407 These options control various sorts of optimizations.
4409 Without any optimization option, the compiler's goal is to reduce the
4410 cost of compilation and to make debugging produce the expected
4411 results. Statements are independent: if you stop the program with a
4412 breakpoint between statements, you can then assign a new value to any
4413 variable or change the program counter to any other statement in the
4414 function and get exactly the results you would expect from the source
4417 Turning on optimization flags makes the compiler attempt to improve
4418 the performance and/or code size at the expense of compilation time
4419 and possibly the ability to debug the program.
4421 The compiler performs optimization based on the knowledge it has of
4422 the program. Optimization levels @option{-O2} and above, in
4423 particular, enable @emph{unit-at-a-time} mode, which allows the
4424 compiler to consider information gained from later functions in
4425 the file when compiling a function. Compiling multiple files at
4426 once to a single output file in @emph{unit-at-a-time} mode allows
4427 the compiler to use information gained from all of the files when
4428 compiling each of them.
4430 Not all optimizations are controlled directly by a flag. Only
4431 optimizations that have a flag are listed.
4438 Optimize. Optimizing compilation takes somewhat more time, and a lot
4439 more memory for a large function.
4441 With @option{-O}, the compiler tries to reduce code size and execution
4442 time, without performing any optimizations that take a great deal of
4445 @option{-O} turns on the following optimization flags:
4446 @gccoptlist{-fdefer-pop @gol
4447 -fdelayed-branch @gol
4448 -fguess-branch-probability @gol
4449 -fcprop-registers @gol
4450 -fif-conversion @gol
4451 -fif-conversion2 @gol
4454 -ftree-dominator-opts @gol
4459 -ftree-copyrename @gol
4464 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
4465 where doing so does not interfere with debugging.
4469 Optimize even more. GCC performs nearly all supported optimizations
4470 that do not involve a space-speed tradeoff. The compiler does not
4471 perform loop unrolling or function inlining when you specify @option{-O2}.
4472 As compared to @option{-O}, this option increases both compilation time
4473 and the performance of the generated code.
4475 @option{-O2} turns on all optimization flags specified by @option{-O}. It
4476 also turns on the following optimization flags:
4477 @gccoptlist{-fthread-jumps @gol
4479 -foptimize-sibling-calls @gol
4480 -fcse-follow-jumps -fcse-skip-blocks @gol
4481 -fgcse -fgcse-lm @gol
4482 -fexpensive-optimizations @gol
4483 -frerun-cse-after-loop @gol
4486 -fschedule-insns -fschedule-insns2 @gol
4487 -fsched-interblock -fsched-spec @gol
4489 -fstrict-aliasing @gol
4490 -fdelete-null-pointer-checks @gol
4491 -freorder-blocks -freorder-functions @gol
4492 -funit-at-a-time @gol
4493 -falign-functions -falign-jumps @gol
4494 -falign-loops -falign-labels @gol
4498 Please note the warning under @option{-fgcse} about
4499 invoking @option{-O2} on programs that use computed gotos.
4503 Optimize yet more. @option{-O3} turns on all optimizations specified by
4504 @option{-O2} and also turns on the @option{-finline-functions},
4505 @option{-funswitch-loops} and @option{-fgcse-after-reload} options.
4509 Do not optimize. This is the default.
4513 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
4514 do not typically increase code size. It also performs further
4515 optimizations designed to reduce code size.
4517 @option{-Os} disables the following optimization flags:
4518 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
4519 -falign-labels -freorder-blocks -freorder-blocks-and-partition @gol
4520 -fprefetch-loop-arrays -ftree-vect-loop-version}
4522 If you use multiple @option{-O} options, with or without level numbers,
4523 the last such option is the one that is effective.
4526 Options of the form @option{-f@var{flag}} specify machine-independent
4527 flags. Most flags have both positive and negative forms; the negative
4528 form of @option{-ffoo} would be @option{-fno-foo}. In the table
4529 below, only one of the forms is listed---the one you typically will
4530 use. You can figure out the other form by either removing @samp{no-}
4533 The following options control specific optimizations. They are either
4534 activated by @option{-O} options or are related to ones that are. You
4535 can use the following flags in the rare cases when ``fine-tuning'' of
4536 optimizations to be performed is desired.
4539 @item -fno-default-inline
4540 @opindex fno-default-inline
4541 Do not make member functions inline by default merely because they are
4542 defined inside the class scope (C++ only). Otherwise, when you specify
4543 @w{@option{-O}}, member functions defined inside class scope are compiled
4544 inline by default; i.e., you don't need to add @samp{inline} in front of
4545 the member function name.
4547 @item -fno-defer-pop
4548 @opindex fno-defer-pop
4549 Always pop the arguments to each function call as soon as that function
4550 returns. For machines which must pop arguments after a function call,
4551 the compiler normally lets arguments accumulate on the stack for several
4552 function calls and pops them all at once.
4554 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4558 Force memory operands to be copied into registers before doing
4559 arithmetic on them. This produces better code by making all memory
4560 references potential common subexpressions. When they are not common
4561 subexpressions, instruction combination should eliminate the separate
4562 register-load. This option is now a nop and will be removed in 4.2.
4565 @opindex fforce-addr
4566 Force memory address constants to be copied into registers before
4567 doing arithmetic on them.
4569 @item -fomit-frame-pointer
4570 @opindex fomit-frame-pointer
4571 Don't keep the frame pointer in a register for functions that
4572 don't need one. This avoids the instructions to save, set up and
4573 restore frame pointers; it also makes an extra register available
4574 in many functions. @strong{It also makes debugging impossible on
4577 On some machines, such as the VAX, this flag has no effect, because
4578 the standard calling sequence automatically handles the frame pointer
4579 and nothing is saved by pretending it doesn't exist. The
4580 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
4581 whether a target machine supports this flag. @xref{Registers,,Register
4582 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
4584 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4586 @item -foptimize-sibling-calls
4587 @opindex foptimize-sibling-calls
4588 Optimize sibling and tail recursive calls.
4590 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4594 Don't pay attention to the @code{inline} keyword. Normally this option
4595 is used to keep the compiler from expanding any functions inline.
4596 Note that if you are not optimizing, no functions can be expanded inline.
4598 @item -finline-functions
4599 @opindex finline-functions
4600 Integrate all simple functions into their callers. The compiler
4601 heuristically decides which functions are simple enough to be worth
4602 integrating in this way.
4604 If all calls to a given function are integrated, and the function is
4605 declared @code{static}, then the function is normally not output as
4606 assembler code in its own right.
4608 Enabled at level @option{-O3}.
4610 @item -finline-functions-called-once
4611 @opindex finline-functions-called-once
4612 Consider all @code{static} functions called once for inlining into their
4613 caller even if they are not marked @code{inline}. If a call to a given
4614 function is integrated, then the function is not output as assembler code
4617 Enabled if @option{-funit-at-a-time} is enabled.
4619 @item -fearly-inlining
4620 @opindex fearly-inlining
4621 Inline functions marked by @code{always_inline} and functions whose body seems
4622 smaller than the function call overhead early before doing
4623 @option{-fprofile-generate} instrumentation and real inlining pass. Doing so
4624 makes profiling significantly cheaper and usually inlining faster on programs
4625 having large chains of nested wrapper functions.
4629 @item -finline-limit=@var{n}
4630 @opindex finline-limit
4631 By default, GCC limits the size of functions that can be inlined. This flag
4632 allows the control of this limit for functions that are explicitly marked as
4633 inline (i.e., marked with the inline keyword or defined within the class
4634 definition in c++). @var{n} is the size of functions that can be inlined in
4635 number of pseudo instructions (not counting parameter handling). The default
4636 value of @var{n} is 600.
4637 Increasing this value can result in more inlined code at
4638 the cost of compilation time and memory consumption. Decreasing usually makes
4639 the compilation faster and less code will be inlined (which presumably
4640 means slower programs). This option is particularly useful for programs that
4641 use inlining heavily such as those based on recursive templates with C++.
4643 Inlining is actually controlled by a number of parameters, which may be
4644 specified individually by using @option{--param @var{name}=@var{value}}.
4645 The @option{-finline-limit=@var{n}} option sets some of these parameters
4649 @item max-inline-insns-single
4650 is set to @var{n}/2.
4651 @item max-inline-insns-auto
4652 is set to @var{n}/2.
4653 @item min-inline-insns
4654 is set to 130 or @var{n}/4, whichever is smaller.
4655 @item max-inline-insns-rtl
4659 See below for a documentation of the individual
4660 parameters controlling inlining.
4662 @emph{Note:} pseudo instruction represents, in this particular context, an
4663 abstract measurement of function's size. In no way does it represent a count
4664 of assembly instructions and as such its exact meaning might change from one
4665 release to an another.
4667 @item -fkeep-inline-functions
4668 @opindex fkeep-inline-functions
4669 In C, emit @code{static} functions that are declared @code{inline}
4670 into the object file, even if the function has been inlined into all
4671 of its callers. This switch does not affect functions using the
4672 @code{extern inline} extension in GNU C@. In C++, emit any and all
4673 inline functions into the object file.
4675 @item -fkeep-static-consts
4676 @opindex fkeep-static-consts
4677 Emit variables declared @code{static const} when optimization isn't turned
4678 on, even if the variables aren't referenced.
4680 GCC enables this option by default. If you want to force the compiler to
4681 check if the variable was referenced, regardless of whether or not
4682 optimization is turned on, use the @option{-fno-keep-static-consts} option.
4684 @item -fmerge-constants
4685 Attempt to merge identical constants (string constants and floating point
4686 constants) across compilation units.
4688 This option is the default for optimized compilation if the assembler and
4689 linker support it. Use @option{-fno-merge-constants} to inhibit this
4692 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4694 @item -fmerge-all-constants
4695 Attempt to merge identical constants and identical variables.
4697 This option implies @option{-fmerge-constants}. In addition to
4698 @option{-fmerge-constants} this considers e.g.@: even constant initialized
4699 arrays or initialized constant variables with integral or floating point
4700 types. Languages like C or C++ require each non-automatic variable to
4701 have distinct location, so using this option will result in non-conforming
4704 @item -fmodulo-sched
4705 @opindex fmodulo-sched
4706 Perform swing modulo scheduling immediately before the first scheduling
4707 pass. This pass looks at innermost loops and reorders their
4708 instructions by overlapping different iterations.
4710 @item -fno-branch-count-reg
4711 @opindex fno-branch-count-reg
4712 Do not use ``decrement and branch'' instructions on a count register,
4713 but instead generate a sequence of instructions that decrement a
4714 register, compare it against zero, then branch based upon the result.
4715 This option is only meaningful on architectures that support such
4716 instructions, which include x86, PowerPC, IA-64 and S/390.
4718 The default is @option{-fbranch-count-reg}.
4720 @item -fno-function-cse
4721 @opindex fno-function-cse
4722 Do not put function addresses in registers; make each instruction that
4723 calls a constant function contain the function's address explicitly.
4725 This option results in less efficient code, but some strange hacks
4726 that alter the assembler output may be confused by the optimizations
4727 performed when this option is not used.
4729 The default is @option{-ffunction-cse}
4731 @item -fno-zero-initialized-in-bss
4732 @opindex fno-zero-initialized-in-bss
4733 If the target supports a BSS section, GCC by default puts variables that
4734 are initialized to zero into BSS@. This can save space in the resulting
4737 This option turns off this behavior because some programs explicitly
4738 rely on variables going to the data section. E.g., so that the
4739 resulting executable can find the beginning of that section and/or make
4740 assumptions based on that.
4742 The default is @option{-fzero-initialized-in-bss}.
4744 @item -fbounds-check
4745 @opindex fbounds-check
4746 For front-ends that support it, generate additional code to check that
4747 indices used to access arrays are within the declared range. This is
4748 currently only supported by the Java and Fortran front-ends, where
4749 this option defaults to true and false respectively.
4751 @item -fmudflap -fmudflapth -fmudflapir
4755 @cindex bounds checking
4757 For front-ends that support it (C and C++), instrument all risky
4758 pointer/array dereferencing operations, some standard library
4759 string/heap functions, and some other associated constructs with
4760 range/validity tests. Modules so instrumented should be immune to
4761 buffer overflows, invalid heap use, and some other classes of C/C++
4762 programming errors. The instrumentation relies on a separate runtime
4763 library (@file{libmudflap}), which will be linked into a program if
4764 @option{-fmudflap} is given at link time. Run-time behavior of the
4765 instrumented program is controlled by the @env{MUDFLAP_OPTIONS}
4766 environment variable. See @code{env MUDFLAP_OPTIONS=-help a.out}
4769 Use @option{-fmudflapth} instead of @option{-fmudflap} to compile and to
4770 link if your program is multi-threaded. Use @option{-fmudflapir}, in
4771 addition to @option{-fmudflap} or @option{-fmudflapth}, if
4772 instrumentation should ignore pointer reads. This produces less
4773 instrumentation (and therefore faster execution) and still provides
4774 some protection against outright memory corrupting writes, but allows
4775 erroneously read data to propagate within a program.
4777 @item -fthread-jumps
4778 @opindex fthread-jumps
4779 Perform optimizations where we check to see if a jump branches to a
4780 location where another comparison subsumed by the first is found. If
4781 so, the first branch is redirected to either the destination of the
4782 second branch or a point immediately following it, depending on whether
4783 the condition is known to be true or false.
4785 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4787 @item -fcse-follow-jumps
4788 @opindex fcse-follow-jumps
4789 In common subexpression elimination, scan through jump instructions
4790 when the target of the jump is not reached by any other path. For
4791 example, when CSE encounters an @code{if} statement with an
4792 @code{else} clause, CSE will follow the jump when the condition
4795 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4797 @item -fcse-skip-blocks
4798 @opindex fcse-skip-blocks
4799 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
4800 follow jumps which conditionally skip over blocks. When CSE
4801 encounters a simple @code{if} statement with no else clause,
4802 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
4803 body of the @code{if}.
4805 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4807 @item -frerun-cse-after-loop
4808 @opindex frerun-cse-after-loop
4809 Re-run common subexpression elimination after loop optimizations has been
4812 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4816 Perform a global common subexpression elimination pass.
4817 This pass also performs global constant and copy propagation.
4819 @emph{Note:} When compiling a program using computed gotos, a GCC
4820 extension, you may get better runtime performance if you disable
4821 the global common subexpression elimination pass by adding
4822 @option{-fno-gcse} to the command line.
4824 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4828 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
4829 attempt to move loads which are only killed by stores into themselves. This
4830 allows a loop containing a load/store sequence to be changed to a load outside
4831 the loop, and a copy/store within the loop.
4833 Enabled by default when gcse is enabled.
4837 When @option{-fgcse-sm} is enabled, a store motion pass is run after
4838 global common subexpression elimination. This pass will attempt to move
4839 stores out of loops. When used in conjunction with @option{-fgcse-lm},
4840 loops containing a load/store sequence can be changed to a load before
4841 the loop and a store after the loop.
4843 Not enabled at any optimization level.
4847 When @option{-fgcse-las} is enabled, the global common subexpression
4848 elimination pass eliminates redundant loads that come after stores to the
4849 same memory location (both partial and full redundancies).
4851 Not enabled at any optimization level.
4853 @item -fgcse-after-reload
4854 @opindex fgcse-after-reload
4855 When @option{-fgcse-after-reload} is enabled, a redundant load elimination
4856 pass is performed after reload. The purpose of this pass is to cleanup
4859 @item -funsafe-loop-optimizations
4860 @opindex funsafe-loop-optimizations
4861 If given, the loop optimizer will assume that loop indices do not
4862 overflow, and that the loops with nontrivial exit condition are not
4863 infinite. This enables a wider range of loop optimizations even if
4864 the loop optimizer itself cannot prove that these assumptions are valid.
4865 Using @option{-Wunsafe-loop-optimizations}, the compiler will warn you
4866 if it finds this kind of loop.
4868 @item -fcrossjumping
4869 @opindex crossjumping
4870 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
4871 resulting code may or may not perform better than without cross-jumping.
4873 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4875 @item -fif-conversion
4876 @opindex if-conversion
4877 Attempt to transform conditional jumps into branch-less equivalents. This
4878 include use of conditional moves, min, max, set flags and abs instructions, and
4879 some tricks doable by standard arithmetics. The use of conditional execution
4880 on chips where it is available is controlled by @code{if-conversion2}.
4882 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4884 @item -fif-conversion2
4885 @opindex if-conversion2
4886 Use conditional execution (where available) to transform conditional jumps into
4887 branch-less equivalents.
4889 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4891 @item -fdelete-null-pointer-checks
4892 @opindex fdelete-null-pointer-checks
4893 Use global dataflow analysis to identify and eliminate useless checks
4894 for null pointers. The compiler assumes that dereferencing a null
4895 pointer would have halted the program. If a pointer is checked after
4896 it has already been dereferenced, it cannot be null.
4898 In some environments, this assumption is not true, and programs can
4899 safely dereference null pointers. Use
4900 @option{-fno-delete-null-pointer-checks} to disable this optimization
4901 for programs which depend on that behavior.
4903 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4905 @item -fexpensive-optimizations
4906 @opindex fexpensive-optimizations
4907 Perform a number of minor optimizations that are relatively expensive.
4909 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4911 @item -foptimize-register-move
4913 @opindex foptimize-register-move
4915 Attempt to reassign register numbers in move instructions and as
4916 operands of other simple instructions in order to maximize the amount of
4917 register tying. This is especially helpful on machines with two-operand
4920 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
4923 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4925 @item -fdelayed-branch
4926 @opindex fdelayed-branch
4927 If supported for the target machine, attempt to reorder instructions
4928 to exploit instruction slots available after delayed branch
4931 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4933 @item -fschedule-insns
4934 @opindex fschedule-insns
4935 If supported for the target machine, attempt to reorder instructions to
4936 eliminate execution stalls due to required data being unavailable. This
4937 helps machines that have slow floating point or memory load instructions
4938 by allowing other instructions to be issued until the result of the load
4939 or floating point instruction is required.
4941 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4943 @item -fschedule-insns2
4944 @opindex fschedule-insns2
4945 Similar to @option{-fschedule-insns}, but requests an additional pass of
4946 instruction scheduling after register allocation has been done. This is
4947 especially useful on machines with a relatively small number of
4948 registers and where memory load instructions take more than one cycle.
4950 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4952 @item -fno-sched-interblock
4953 @opindex fno-sched-interblock
4954 Don't schedule instructions across basic blocks. This is normally
4955 enabled by default when scheduling before register allocation, i.e.@:
4956 with @option{-fschedule-insns} or at @option{-O2} or higher.
4958 @item -fno-sched-spec
4959 @opindex fno-sched-spec
4960 Don't allow speculative motion of non-load instructions. This is normally
4961 enabled by default when scheduling before register allocation, i.e.@:
4962 with @option{-fschedule-insns} or at @option{-O2} or higher.
4964 @item -fsched-spec-load
4965 @opindex fsched-spec-load
4966 Allow speculative motion of some load instructions. This only makes
4967 sense when scheduling before register allocation, i.e.@: with
4968 @option{-fschedule-insns} or at @option{-O2} or higher.
4970 @item -fsched-spec-load-dangerous
4971 @opindex fsched-spec-load-dangerous
4972 Allow speculative motion of more load instructions. This only makes
4973 sense when scheduling before register allocation, i.e.@: with
4974 @option{-fschedule-insns} or at @option{-O2} or higher.
4976 @item -fsched-stalled-insns=@var{n}
4977 @opindex fsched-stalled-insns
4978 Define how many insns (if any) can be moved prematurely from the queue
4979 of stalled insns into the ready list, during the second scheduling pass.
4981 @item -fsched-stalled-insns-dep=@var{n}
4982 @opindex fsched-stalled-insns-dep
4983 Define how many insn groups (cycles) will be examined for a dependency
4984 on a stalled insn that is candidate for premature removal from the queue
4985 of stalled insns. Has an effect only during the second scheduling pass,
4986 and only if @option{-fsched-stalled-insns} is used and its value is not zero.
4988 @item -fsched2-use-superblocks
4989 @opindex fsched2-use-superblocks
4990 When scheduling after register allocation, do use superblock scheduling
4991 algorithm. Superblock scheduling allows motion across basic block boundaries
4992 resulting on faster schedules. This option is experimental, as not all machine
4993 descriptions used by GCC model the CPU closely enough to avoid unreliable
4994 results from the algorithm.
4996 This only makes sense when scheduling after register allocation, i.e.@: with
4997 @option{-fschedule-insns2} or at @option{-O2} or higher.
4999 @item -fsched2-use-traces
5000 @opindex fsched2-use-traces
5001 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
5002 allocation and additionally perform code duplication in order to increase the
5003 size of superblocks using tracer pass. See @option{-ftracer} for details on
5006 This mode should produce faster but significantly longer programs. Also
5007 without @option{-fbranch-probabilities} the traces constructed may not
5008 match the reality and hurt the performance. This only makes
5009 sense when scheduling after register allocation, i.e.@: with
5010 @option{-fschedule-insns2} or at @option{-O2} or higher.
5014 Eliminates redundant extension instructions and move the non redundant
5015 ones to optimal placement using LCM.
5017 @item -freschedule-modulo-scheduled-loops
5018 @opindex fscheduling-in-modulo-scheduled-loops
5019 The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled
5020 we may want to prevent the later scheduling passes from changing its schedule, we use this
5021 option to control that.
5023 @item -fcaller-saves
5024 @opindex fcaller-saves
5025 Enable values to be allocated in registers that will be clobbered by
5026 function calls, by emitting extra instructions to save and restore the
5027 registers around such calls. Such allocation is done only when it
5028 seems to result in better code than would otherwise be produced.
5030 This option is always enabled by default on certain machines, usually
5031 those which have no call-preserved registers to use instead.
5033 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5036 Perform Partial Redundancy Elimination (PRE) on trees. This flag is
5037 enabled by default at @option{-O2} and @option{-O3}.
5040 Perform Full Redundancy Elimination (FRE) on trees. The difference
5041 between FRE and PRE is that FRE only considers expressions
5042 that are computed on all paths leading to the redundant computation.
5043 This analysis faster than PRE, though it exposes fewer redundancies.
5044 This flag is enabled by default at @option{-O} and higher.
5046 @item -ftree-copy-prop
5047 Perform copy propagation on trees. This pass eliminates unnecessary
5048 copy operations. This flag is enabled by default at @option{-O} and
5051 @item -ftree-store-copy-prop
5052 Perform copy propagation of memory loads and stores. This pass
5053 eliminates unnecessary copy operations in memory references
5054 (structures, global variables, arrays, etc). This flag is enabled by
5055 default at @option{-O2} and higher.
5058 Perform structural alias analysis on trees. This flag
5059 is enabled by default at @option{-O} and higher.
5062 Perform interprocedural pointer analysis.
5065 Perform forward store motion on trees. This flag is
5066 enabled by default at @option{-O} and higher.
5069 Perform sparse conditional constant propagation (CCP) on trees. This
5070 pass only operates on local scalar variables and is enabled by default
5071 at @option{-O} and higher.
5073 @item -ftree-store-ccp
5074 Perform sparse conditional constant propagation (CCP) on trees. This
5075 pass operates on both local scalar variables and memory stores and
5076 loads (global variables, structures, arrays, etc). This flag is
5077 enabled by default at @option{-O2} and higher.
5080 Perform dead code elimination (DCE) on trees. This flag is enabled by
5081 default at @option{-O} and higher.
5083 @item -ftree-dominator-opts
5084 Perform a variety of simple scalar cleanups (constant/copy
5085 propagation, redundancy elimination, range propagation and expression
5086 simplification) based on a dominator tree traversal. This also
5087 performs jump threading (to reduce jumps to jumps). This flag is
5088 enabled by default at @option{-O} and higher.
5091 Perform loop header copying on trees. This is beneficial since it increases
5092 effectiveness of code motion optimizations. It also saves one jump. This flag
5093 is enabled by default at @option{-O} and higher. It is not enabled
5094 for @option{-Os}, since it usually increases code size.
5096 @item -ftree-loop-optimize
5097 Perform loop optimizations on trees. This flag is enabled by default
5098 at @option{-O} and higher.
5100 @item -ftree-loop-linear
5101 Perform linear loop transformations on tree. This flag can improve cache
5102 performance and allow further loop optimizations to take place.
5104 @item -ftree-loop-im
5105 Perform loop invariant motion on trees. This pass moves only invariants that
5106 would be hard to handle at RTL level (function calls, operations that expand to
5107 nontrivial sequences of insns). With @option{-funswitch-loops} it also moves
5108 operands of conditions that are invariant out of the loop, so that we can use
5109 just trivial invariantness analysis in loop unswitching. The pass also includes
5112 @item -ftree-loop-ivcanon
5113 Create a canonical counter for number of iterations in the loop for that
5114 determining number of iterations requires complicated analysis. Later
5115 optimizations then may determine the number easily. Useful especially
5116 in connection with unrolling.
5119 Perform induction variable optimizations (strength reduction, induction
5120 variable merging and induction variable elimination) on trees.
5123 Perform scalar replacement of aggregates. This pass replaces structure
5124 references with scalars to prevent committing structures to memory too
5125 early. This flag is enabled by default at @option{-O} and higher.
5127 @item -ftree-copyrename
5128 Perform copy renaming on trees. This pass attempts to rename compiler
5129 temporaries to other variables at copy locations, usually resulting in
5130 variable names which more closely resemble the original variables. This flag
5131 is enabled by default at @option{-O} and higher.
5134 Perform temporary expression replacement during the SSA->normal phase. Single
5135 use/single def temporaries are replaced at their use location with their
5136 defining expression. This results in non-GIMPLE code, but gives the expanders
5137 much more complex trees to work on resulting in better RTL generation. This is
5138 enabled by default at @option{-O} and higher.
5141 Perform live range splitting during the SSA->normal phase. Distinct live
5142 ranges of a variable are split into unique variables, allowing for better
5143 optimization later. This is enabled by default at @option{-O} and higher.
5145 @item -ftree-vectorize
5146 Perform loop vectorization on trees.
5148 @item -ftree-vect-loop-version
5149 @opindex ftree-vect-loop-version
5150 Perform loop versioning when doing loop vectorization on trees. When a loop
5151 appears to be vectorizable except that data alignment or data dependence cannot
5152 be determined at compile time then vectorized and non-vectorized versions of
5153 the loop are generated along with runtime checks for alignment or dependence
5154 to control which version is executed. This option is enabled by default
5155 except at level @option{-Os} where it is disabled.
5158 Perform Value Range Propagation on trees. This is similar to the
5159 constant propagation pass, but instead of values, ranges of values are
5160 propagated. This allows the optimizers to remove unnecessary range
5161 checks like array bound checks and null pointer checks. This is
5162 enabled by default at @option{-O2} and higher. Null pointer check
5163 elimination is only done if @option{-fdelete-null-pointer-checks} is
5168 Perform tail duplication to enlarge superblock size. This transformation
5169 simplifies the control flow of the function allowing other optimizations to do
5172 @item -funroll-loops
5173 @opindex funroll-loops
5174 Unroll loops whose number of iterations can be determined at compile
5175 time or upon entry to the loop. @option{-funroll-loops} implies
5176 @option{-frerun-cse-after-loop}. This option makes code larger,
5177 and may or may not make it run faster.
5179 @item -funroll-all-loops
5180 @opindex funroll-all-loops
5181 Unroll all loops, even if their number of iterations is uncertain when
5182 the loop is entered. This usually makes programs run more slowly.
5183 @option{-funroll-all-loops} implies the same options as
5184 @option{-funroll-loops},
5186 @item -fsplit-ivs-in-unroller
5187 @opindex -fsplit-ivs-in-unroller
5188 Enables expressing of values of induction variables in later iterations
5189 of the unrolled loop using the value in the first iteration. This breaks
5190 long dependency chains, thus improving efficiency of the scheduling passes.
5192 Combination of @option{-fweb} and CSE is often sufficient to obtain the
5193 same effect. However in cases the loop body is more complicated than
5194 a single basic block, this is not reliable. It also does not work at all
5195 on some of the architectures due to restrictions in the CSE pass.
5197 This optimization is enabled by default.
5199 @item -fvariable-expansion-in-unroller
5200 @opindex -fvariable-expansion-in-unroller
5201 With this option, the compiler will create multiple copies of some
5202 local variables when unrolling a loop which can result in superior code.
5204 @item -fprefetch-loop-arrays
5205 @opindex fprefetch-loop-arrays
5206 If supported by the target machine, generate instructions to prefetch
5207 memory to improve the performance of loops that access large arrays.
5209 This option may generate better or worse code; results are highly
5210 dependent on the structure of loops within the source code.
5212 Disabled at level @option{-Os}.
5215 @itemx -fno-peephole2
5216 @opindex fno-peephole
5217 @opindex fno-peephole2
5218 Disable any machine-specific peephole optimizations. The difference
5219 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
5220 are implemented in the compiler; some targets use one, some use the
5221 other, a few use both.
5223 @option{-fpeephole} is enabled by default.
5224 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5226 @item -fno-guess-branch-probability
5227 @opindex fno-guess-branch-probability
5228 Do not guess branch probabilities using heuristics.
5230 GCC will use heuristics to guess branch probabilities if they are
5231 not provided by profiling feedback (@option{-fprofile-arcs}). These
5232 heuristics are based on the control flow graph. If some branch probabilities
5233 are specified by @samp{__builtin_expect}, then the heuristics will be
5234 used to guess branch probabilities for the rest of the control flow graph,
5235 taking the @samp{__builtin_expect} info into account. The interactions
5236 between the heuristics and @samp{__builtin_expect} can be complex, and in
5237 some cases, it may be useful to disable the heuristics so that the effects
5238 of @samp{__builtin_expect} are easier to understand.
5240 The default is @option{-fguess-branch-probability} at levels
5241 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5243 @item -freorder-blocks
5244 @opindex freorder-blocks
5245 Reorder basic blocks in the compiled function in order to reduce number of
5246 taken branches and improve code locality.
5248 Enabled at levels @option{-O2}, @option{-O3}.
5250 @item -freorder-blocks-and-partition
5251 @opindex freorder-blocks-and-partition
5252 In addition to reordering basic blocks in the compiled function, in order
5253 to reduce number of taken branches, partitions hot and cold basic blocks
5254 into separate sections of the assembly and .o files, to improve
5255 paging and cache locality performance.
5257 This optimization is automatically turned off in the presence of
5258 exception handling, for linkonce sections, for functions with a user-defined
5259 section attribute and on any architecture that does not support named
5262 @item -freorder-functions
5263 @opindex freorder-functions
5264 Reorder functions in the object file in order to
5265 improve code locality. This is implemented by using special
5266 subsections @code{.text.hot} for most frequently executed functions and
5267 @code{.text.unlikely} for unlikely executed functions. Reordering is done by
5268 the linker so object file format must support named sections and linker must
5269 place them in a reasonable way.
5271 Also profile feedback must be available in to make this option effective. See
5272 @option{-fprofile-arcs} for details.
5274 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5276 @item -fstrict-aliasing
5277 @opindex fstrict-aliasing
5278 Allows the compiler to assume the strictest aliasing rules applicable to
5279 the language being compiled. For C (and C++), this activates
5280 optimizations based on the type of expressions. In particular, an
5281 object of one type is assumed never to reside at the same address as an
5282 object of a different type, unless the types are almost the same. For
5283 example, an @code{unsigned int} can alias an @code{int}, but not a
5284 @code{void*} or a @code{double}. A character type may alias any other
5287 Pay special attention to code like this:
5300 The practice of reading from a different union member than the one most
5301 recently written to (called ``type-punning'') is common. Even with
5302 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
5303 is accessed through the union type. So, the code above will work as
5304 expected. However, this code might not:
5315 Every language that wishes to perform language-specific alias analysis
5316 should define a function that computes, given an @code{tree}
5317 node, an alias set for the node. Nodes in different alias sets are not
5318 allowed to alias. For an example, see the C front-end function
5319 @code{c_get_alias_set}.
5321 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5323 @item -falign-functions
5324 @itemx -falign-functions=@var{n}
5325 @opindex falign-functions
5326 Align the start of functions to the next power-of-two greater than
5327 @var{n}, skipping up to @var{n} bytes. For instance,
5328 @option{-falign-functions=32} aligns functions to the next 32-byte
5329 boundary, but @option{-falign-functions=24} would align to the next
5330 32-byte boundary only if this can be done by skipping 23 bytes or less.
5332 @option{-fno-align-functions} and @option{-falign-functions=1} are
5333 equivalent and mean that functions will not be aligned.
5335 Some assemblers only support this flag when @var{n} is a power of two;
5336 in that case, it is rounded up.
5338 If @var{n} is not specified or is zero, use a machine-dependent default.
5340 Enabled at levels @option{-O2}, @option{-O3}.
5342 @item -falign-labels
5343 @itemx -falign-labels=@var{n}
5344 @opindex falign-labels
5345 Align all branch targets to a power-of-two boundary, skipping up to
5346 @var{n} bytes like @option{-falign-functions}. This option can easily
5347 make code slower, because it must insert dummy operations for when the
5348 branch target is reached in the usual flow of the code.
5350 @option{-fno-align-labels} and @option{-falign-labels=1} are
5351 equivalent and mean that labels will not be aligned.
5353 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
5354 are greater than this value, then their values are used instead.
5356 If @var{n} is not specified or is zero, use a machine-dependent default
5357 which is very likely to be @samp{1}, meaning no alignment.
5359 Enabled at levels @option{-O2}, @option{-O3}.
5362 @itemx -falign-loops=@var{n}
5363 @opindex falign-loops
5364 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
5365 like @option{-falign-functions}. The hope is that the loop will be
5366 executed many times, which will make up for any execution of the dummy
5369 @option{-fno-align-loops} and @option{-falign-loops=1} are
5370 equivalent and mean that loops will not be aligned.
5372 If @var{n} is not specified or is zero, use a machine-dependent default.
5374 Enabled at levels @option{-O2}, @option{-O3}.
5377 @itemx -falign-jumps=@var{n}
5378 @opindex falign-jumps
5379 Align branch targets to a power-of-two boundary, for branch targets
5380 where the targets can only be reached by jumping, skipping up to @var{n}
5381 bytes like @option{-falign-functions}. In this case, no dummy operations
5384 @option{-fno-align-jumps} and @option{-falign-jumps=1} are
5385 equivalent and mean that loops will not be aligned.
5387 If @var{n} is not specified or is zero, use a machine-dependent default.
5389 Enabled at levels @option{-O2}, @option{-O3}.
5391 @item -funit-at-a-time
5392 @opindex funit-at-a-time
5393 Parse the whole compilation unit before starting to produce code.
5394 This allows some extra optimizations to take place but consumes
5395 more memory (in general). There are some compatibility issues
5396 with @emph{unit-at-a-time} mode:
5399 enabling @emph{unit-at-a-time} mode may change the order
5400 in which functions, variables, and top-level @code{asm} statements
5401 are emitted, and will likely break code relying on some particular
5402 ordering. The majority of such top-level @code{asm} statements,
5403 though, can be replaced by @code{section} attributes. The
5404 @option{fno-toplevel-reorder} option may be used to keep the ordering
5405 used in the input file, at the cost of some optimizations.
5408 @emph{unit-at-a-time} mode removes unreferenced static variables
5409 and functions. This may result in undefined references
5410 when an @code{asm} statement refers directly to variables or functions
5411 that are otherwise unused. In that case either the variable/function
5412 shall be listed as an operand of the @code{asm} statement operand or,
5413 in the case of top-level @code{asm} statements the attribute @code{used}
5414 shall be used on the declaration.
5417 Static functions now can use non-standard passing conventions that
5418 may break @code{asm} statements calling functions directly. Again,
5419 attribute @code{used} will prevent this behavior.
5422 As a temporary workaround, @option{-fno-unit-at-a-time} can be used,
5423 but this scheme may not be supported by future releases of GCC@.
5425 Enabled at levels @option{-O2}, @option{-O3}.
5427 @item -fno-toplevel-reorder
5428 Do not reorder top-level functions, variables, and @code{asm}
5429 statements. Output them in the same order that they appear in the
5430 input file. When this option is used, unreferenced static variables
5431 will not be removed. This option is intended to support existing code
5432 which relies on a particular ordering. For new code, it is better to
5437 Constructs webs as commonly used for register allocation purposes and assign
5438 each web individual pseudo register. This allows the register allocation pass
5439 to operate on pseudos directly, but also strengthens several other optimization
5440 passes, such as CSE, loop optimizer and trivial dead code remover. It can,
5441 however, make debugging impossible, since variables will no longer stay in a
5444 Enabled by default with @option{-funroll-loops}.
5446 @item -fwhole-program
5447 @opindex fwhole-program
5448 Assume that the current compilation unit represents whole program being
5449 compiled. All public functions and variables with the exception of @code{main}
5450 and those merged by attribute @code{externally_visible} become static functions
5451 and in a affect gets more aggressively optimized by interprocedural optimizers.
5452 While this option is equivalent to proper use of @code{static} keyword for
5453 programs consisting of single file, in combination with option
5454 @option{--combine} this flag can be used to compile most of smaller scale C
5455 programs since the functions and variables become local for the whole combined
5456 compilation unit, not for the single source file itself.
5459 @item -fno-cprop-registers
5460 @opindex fno-cprop-registers
5461 After register allocation and post-register allocation instruction splitting,
5462 we perform a copy-propagation pass to try to reduce scheduling dependencies
5463 and occasionally eliminate the copy.
5465 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5467 @item -fprofile-generate
5468 @opindex fprofile-generate
5470 Enable options usually used for instrumenting application to produce
5471 profile useful for later recompilation with profile feedback based
5472 optimization. You must use @option{-fprofile-generate} both when
5473 compiling and when linking your program.
5475 The following options are enabled: @code{-fprofile-arcs}, @code{-fprofile-values}, @code{-fvpt}.
5478 @opindex fprofile-use
5479 Enable profile feedback directed optimizations, and optimizations
5480 generally profitable only with profile feedback available.
5482 The following options are enabled: @code{-fbranch-probabilities}, @code{-fvpt},
5483 @code{-funroll-loops}, @code{-fpeel-loops}, @code{-ftracer}
5487 The following options control compiler behavior regarding floating
5488 point arithmetic. These options trade off between speed and
5489 correctness. All must be specifically enabled.
5493 @opindex ffloat-store
5494 Do not store floating point variables in registers, and inhibit other
5495 options that might change whether a floating point value is taken from a
5498 @cindex floating point precision
5499 This option prevents undesirable excess precision on machines such as
5500 the 68000 where the floating registers (of the 68881) keep more
5501 precision than a @code{double} is supposed to have. Similarly for the
5502 x86 architecture. For most programs, the excess precision does only
5503 good, but a few programs rely on the precise definition of IEEE floating
5504 point. Use @option{-ffloat-store} for such programs, after modifying
5505 them to store all pertinent intermediate computations into variables.
5509 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations}, @*
5510 @option{-fno-trapping-math}, @option{-ffinite-math-only},
5511 @option{-fno-rounding-math}, @option{-fno-signaling-nans}
5512 and @option{fcx-limited-range}.
5514 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
5516 This option should never be turned on by any @option{-O} option since
5517 it can result in incorrect output for programs which depend on
5518 an exact implementation of IEEE or ISO rules/specifications for
5521 @item -fno-math-errno
5522 @opindex fno-math-errno
5523 Do not set ERRNO after calling math functions that are executed
5524 with a single instruction, e.g., sqrt. A program that relies on
5525 IEEE exceptions for math error handling may want to use this flag
5526 for speed while maintaining IEEE arithmetic compatibility.
5528 This option should never be turned on by any @option{-O} option since
5529 it can result in incorrect output for programs which depend on
5530 an exact implementation of IEEE or ISO rules/specifications for
5533 The default is @option{-fmath-errno}.
5535 On Darwin systems, the math library never sets @code{errno}. There is therefore
5536 no reason for the compiler to consider the possibility that it might,
5537 and @option{-fno-math-errno} is the default.
5539 @item -funsafe-math-optimizations
5540 @opindex funsafe-math-optimizations
5541 Allow optimizations for floating-point arithmetic that (a) assume
5542 that arguments and results are valid and (b) may violate IEEE or
5543 ANSI standards. When used at link-time, it may include libraries
5544 or startup files that change the default FPU control word or other
5545 similar optimizations.
5547 This option should never be turned on by any @option{-O} option since
5548 it can result in incorrect output for programs which depend on
5549 an exact implementation of IEEE or ISO rules/specifications for
5552 The default is @option{-fno-unsafe-math-optimizations}.
5554 @item -ffinite-math-only
5555 @opindex ffinite-math-only
5556 Allow optimizations for floating-point arithmetic that assume
5557 that arguments and results are not NaNs or +-Infs.
5559 This option should never be turned on by any @option{-O} option since
5560 it can result in incorrect output for programs which depend on
5561 an exact implementation of IEEE or ISO rules/specifications.
5563 The default is @option{-fno-finite-math-only}.
5565 @item -fno-trapping-math
5566 @opindex fno-trapping-math
5567 Compile code assuming that floating-point operations cannot generate
5568 user-visible traps. These traps include division by zero, overflow,
5569 underflow, inexact result and invalid operation. This option implies
5570 @option{-fno-signaling-nans}. Setting this option may allow faster
5571 code if one relies on ``non-stop'' IEEE arithmetic, for example.
5573 This option should never be turned on by any @option{-O} option since
5574 it can result in incorrect output for programs which depend on
5575 an exact implementation of IEEE or ISO rules/specifications for
5578 The default is @option{-ftrapping-math}.
5580 @item -frounding-math
5581 @opindex frounding-math
5582 Disable transformations and optimizations that assume default floating
5583 point rounding behavior. This is round-to-zero for all floating point
5584 to integer conversions, and round-to-nearest for all other arithmetic
5585 truncations. This option should be specified for programs that change
5586 the FP rounding mode dynamically, or that may be executed with a
5587 non-default rounding mode. This option disables constant folding of
5588 floating point expressions at compile-time (which may be affected by
5589 rounding mode) and arithmetic transformations that are unsafe in the
5590 presence of sign-dependent rounding modes.
5592 The default is @option{-fno-rounding-math}.
5594 This option is experimental and does not currently guarantee to
5595 disable all GCC optimizations that are affected by rounding mode.
5596 Future versions of GCC may provide finer control of this setting
5597 using C99's @code{FENV_ACCESS} pragma. This command line option
5598 will be used to specify the default state for @code{FENV_ACCESS}.
5600 @item -frtl-abstract-sequences
5601 @opindex frtl-abstract-sequences
5602 It is a size optimization method. This option is to find identical
5603 sequences of code, which can be turned into pseudo-procedures and
5604 then replace all occurrences with calls to the newly created
5605 subroutine. It is kind of an opposite of @option{-finline-functions}.
5606 This optimization runs at RTL level.
5608 @item -fsignaling-nans
5609 @opindex fsignaling-nans
5610 Compile code assuming that IEEE signaling NaNs may generate user-visible
5611 traps during floating-point operations. Setting this option disables
5612 optimizations that may change the number of exceptions visible with
5613 signaling NaNs. This option implies @option{-ftrapping-math}.
5615 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
5618 The default is @option{-fno-signaling-nans}.
5620 This option is experimental and does not currently guarantee to
5621 disable all GCC optimizations that affect signaling NaN behavior.
5623 @item -fsingle-precision-constant
5624 @opindex fsingle-precision-constant
5625 Treat floating point constant as single precision constant instead of
5626 implicitly converting it to double precision constant.
5628 @item -fcx-limited-range
5629 @itemx -fno-cx-limited-range
5630 @opindex fcx-limited-range
5631 @opindex fno-cx-limited-range
5632 When enabled, this option states that a range reduction step is not
5633 needed when performing complex division. The default is
5634 @option{-fno-cx-limited-range}, but is enabled by @option{-ffast-math}.
5636 This option controls the default setting of the ISO C99
5637 @code{CX_LIMITED_RANGE} pragma. Nevertheless, the option applies to
5642 The following options control optimizations that may improve
5643 performance, but are not enabled by any @option{-O} options. This
5644 section includes experimental options that may produce broken code.
5647 @item -fbranch-probabilities
5648 @opindex fbranch-probabilities
5649 After running a program compiled with @option{-fprofile-arcs}
5650 (@pxref{Debugging Options,, Options for Debugging Your Program or
5651 @command{gcc}}), you can compile it a second time using
5652 @option{-fbranch-probabilities}, to improve optimizations based on
5653 the number of times each branch was taken. When the program
5654 compiled with @option{-fprofile-arcs} exits it saves arc execution
5655 counts to a file called @file{@var{sourcename}.gcda} for each source
5656 file The information in this data file is very dependent on the
5657 structure of the generated code, so you must use the same source code
5658 and the same optimization options for both compilations.
5660 With @option{-fbranch-probabilities}, GCC puts a
5661 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
5662 These can be used to improve optimization. Currently, they are only
5663 used in one place: in @file{reorg.c}, instead of guessing which path a
5664 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
5665 exactly determine which path is taken more often.
5667 @item -fprofile-values
5668 @opindex fprofile-values
5669 If combined with @option{-fprofile-arcs}, it adds code so that some
5670 data about values of expressions in the program is gathered.
5672 With @option{-fbranch-probabilities}, it reads back the data gathered
5673 from profiling values of expressions and adds @samp{REG_VALUE_PROFILE}
5674 notes to instructions for their later usage in optimizations.
5676 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
5680 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
5681 a code to gather information about values of expressions.
5683 With @option{-fbranch-probabilities}, it reads back the data gathered
5684 and actually performs the optimizations based on them.
5685 Currently the optimizations include specialization of division operation
5686 using the knowledge about the value of the denominator.
5688 @item -frename-registers
5689 @opindex frename-registers
5690 Attempt to avoid false dependencies in scheduled code by making use
5691 of registers left over after register allocation. This optimization
5692 will most benefit processors with lots of registers. Depending on the
5693 debug information format adopted by the target, however, it can
5694 make debugging impossible, since variables will no longer stay in
5695 a ``home register''.
5697 Enabled by default with @option{-funroll-loops}.
5701 Perform tail duplication to enlarge superblock size. This transformation
5702 simplifies the control flow of the function allowing other optimizations to do
5705 Enabled with @option{-fprofile-use}.
5707 @item -funroll-loops
5708 @opindex funroll-loops
5709 Unroll loops whose number of iterations can be determined at compile time or
5710 upon entry to the loop. @option{-funroll-loops} implies
5711 @option{-frerun-cse-after-loop}, @option{-fweb} and @option{-frename-registers}.
5712 It also turns on complete loop peeling (i.e.@: complete removal of loops with
5713 small constant number of iterations). This option makes code larger, and may
5714 or may not make it run faster.
5716 Enabled with @option{-fprofile-use}.
5718 @item -funroll-all-loops
5719 @opindex funroll-all-loops
5720 Unroll all loops, even if their number of iterations is uncertain when
5721 the loop is entered. This usually makes programs run more slowly.
5722 @option{-funroll-all-loops} implies the same options as
5723 @option{-funroll-loops}.
5726 @opindex fpeel-loops
5727 Peels the loops for that there is enough information that they do not
5728 roll much (from profile feedback). It also turns on complete loop peeling
5729 (i.e.@: complete removal of loops with small constant number of iterations).
5731 Enabled with @option{-fprofile-use}.
5733 @item -fmove-loop-invariants
5734 @opindex fmove-loop-invariants
5735 Enables the loop invariant motion pass in the RTL loop optimizer. Enabled
5736 at level @option{-O1}
5738 @item -funswitch-loops
5739 @opindex funswitch-loops
5740 Move branches with loop invariant conditions out of the loop, with duplicates
5741 of the loop on both branches (modified according to result of the condition).
5743 @item -ffunction-sections
5744 @itemx -fdata-sections
5745 @opindex ffunction-sections
5746 @opindex fdata-sections
5747 Place each function or data item into its own section in the output
5748 file if the target supports arbitrary sections. The name of the
5749 function or the name of the data item determines the section's name
5752 Use these options on systems where the linker can perform optimizations
5753 to improve locality of reference in the instruction space. Most systems
5754 using the ELF object format and SPARC processors running Solaris 2 have
5755 linkers with such optimizations. AIX may have these optimizations in
5758 Only use these options when there are significant benefits from doing
5759 so. When you specify these options, the assembler and linker will
5760 create larger object and executable files and will also be slower.
5761 You will not be able to use @code{gprof} on all systems if you
5762 specify this option and you may have problems with debugging if
5763 you specify both this option and @option{-g}.
5765 @item -fbranch-target-load-optimize
5766 @opindex fbranch-target-load-optimize
5767 Perform branch target register load optimization before prologue / epilogue
5769 The use of target registers can typically be exposed only during reload,
5770 thus hoisting loads out of loops and doing inter-block scheduling needs
5771 a separate optimization pass.
5773 @item -fbranch-target-load-optimize2
5774 @opindex fbranch-target-load-optimize2
5775 Perform branch target register load optimization after prologue / epilogue
5778 @item -fbtr-bb-exclusive
5779 @opindex fbtr-bb-exclusive
5780 When performing branch target register load optimization, don't reuse
5781 branch target registers in within any basic block.
5783 @item -fstack-protector
5784 Emit extra code to check for buffer overflows, such as stack smashing
5785 attacks. This is done by adding a guard variable to functions with
5786 vulnerable objects. This includes functions that call alloca, and
5787 functions with buffers larger than 8 bytes. The guards are initialized
5788 when a function is entered and then checked when the function exits.
5789 If a guard check fails, an error message is printed and the program exits.
5791 @item -fstack-protector-all
5792 Like @option{-fstack-protector} except that all functions are protected.
5794 @item -fsection-anchors
5795 @opindex fsection-anchors
5796 Try to reduce the number of symbolic address calculations by using
5797 shared ``anchor'' symbols to address nearby objects. This transformation
5798 can help to reduce the number of GOT entries and GOT accesses on some
5801 For example, the implementation of the following function @code{foo}:
5805 int foo (void) @{ return a + b + c; @}
5808 would usually calculate the addresses of all three variables, but if you
5809 compile it with @option{-fsection-anchors}, it will access the variables
5810 from a common anchor point instead. The effect is similar to the
5811 following pseudocode (which isn't valid C):
5816 register int *xr = &x;
5817 return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
5821 Not all targets support this option.
5823 @item --param @var{name}=@var{value}
5825 In some places, GCC uses various constants to control the amount of
5826 optimization that is done. For example, GCC will not inline functions
5827 that contain more that a certain number of instructions. You can
5828 control some of these constants on the command-line using the
5829 @option{--param} option.
5831 The names of specific parameters, and the meaning of the values, are
5832 tied to the internals of the compiler, and are subject to change
5833 without notice in future releases.
5835 In each case, the @var{value} is an integer. The allowable choices for
5836 @var{name} are given in the following table:
5839 @item salias-max-implicit-fields
5840 The maximum number of fields in a variable without direct
5841 structure accesses for which structure aliasing will consider trying
5842 to track each field. The default is 5
5844 @item salias-max-array-elements
5845 The maximum number of elements an array can have and its elements
5846 still be tracked individually by structure aliasing. The default is 4
5848 @item sra-max-structure-size
5849 The maximum structure size, in bytes, at which the scalar replacement
5850 of aggregates (SRA) optimization will perform block copies. The
5851 default value, 0, implies that GCC will select the most appropriate
5854 @item sra-field-structure-ratio
5855 The threshold ratio (as a percentage) between instantiated fields and
5856 the complete structure size. We say that if the ratio of the number
5857 of bytes in instantiated fields to the number of bytes in the complete
5858 structure exceeds this parameter, then block copies are not used. The
5861 @item max-crossjump-edges
5862 The maximum number of incoming edges to consider for crossjumping.
5863 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
5864 the number of edges incoming to each block. Increasing values mean
5865 more aggressive optimization, making the compile time increase with
5866 probably small improvement in executable size.
5868 @item min-crossjump-insns
5869 The minimum number of instructions which must be matched at the end
5870 of two blocks before crossjumping will be performed on them. This
5871 value is ignored in the case where all instructions in the block being
5872 crossjumped from are matched. The default value is 5.
5874 @item max-grow-copy-bb-insns
5875 The maximum code size expansion factor when copying basic blocks
5876 instead of jumping. The expansion is relative to a jump instruction.
5877 The default value is 8.
5879 @item max-goto-duplication-insns
5880 The maximum number of instructions to duplicate to a block that jumps
5881 to a computed goto. To avoid @math{O(N^2)} behavior in a number of
5882 passes, GCC factors computed gotos early in the compilation process,
5883 and unfactors them as late as possible. Only computed jumps at the
5884 end of a basic blocks with no more than max-goto-duplication-insns are
5885 unfactored. The default value is 8.
5887 @item max-delay-slot-insn-search
5888 The maximum number of instructions to consider when looking for an
5889 instruction to fill a delay slot. If more than this arbitrary number of
5890 instructions is searched, the time savings from filling the delay slot
5891 will be minimal so stop searching. Increasing values mean more
5892 aggressive optimization, making the compile time increase with probably
5893 small improvement in executable run time.
5895 @item max-delay-slot-live-search
5896 When trying to fill delay slots, the maximum number of instructions to
5897 consider when searching for a block with valid live register
5898 information. Increasing this arbitrarily chosen value means more
5899 aggressive optimization, increasing the compile time. This parameter
5900 should be removed when the delay slot code is rewritten to maintain the
5903 @item max-gcse-memory
5904 The approximate maximum amount of memory that will be allocated in
5905 order to perform the global common subexpression elimination
5906 optimization. If more memory than specified is required, the
5907 optimization will not be done.
5909 @item max-gcse-passes
5910 The maximum number of passes of GCSE to run. The default is 1.
5912 @item max-pending-list-length
5913 The maximum number of pending dependencies scheduling will allow
5914 before flushing the current state and starting over. Large functions
5915 with few branches or calls can create excessively large lists which
5916 needlessly consume memory and resources.
5918 @item max-inline-insns-single
5919 Several parameters control the tree inliner used in gcc.
5920 This number sets the maximum number of instructions (counted in GCC's
5921 internal representation) in a single function that the tree inliner
5922 will consider for inlining. This only affects functions declared
5923 inline and methods implemented in a class declaration (C++).
5924 The default value is 450.
5926 @item max-inline-insns-auto
5927 When you use @option{-finline-functions} (included in @option{-O3}),
5928 a lot of functions that would otherwise not be considered for inlining
5929 by the compiler will be investigated. To those functions, a different
5930 (more restrictive) limit compared to functions declared inline can
5932 The default value is 90.
5934 @item large-function-insns
5935 The limit specifying really large functions. For functions larger than this
5936 limit after inlining inlining is constrained by
5937 @option{--param large-function-growth}. This parameter is useful primarily
5938 to avoid extreme compilation time caused by non-linear algorithms used by the
5940 This parameter is ignored when @option{-funit-at-a-time} is not used.
5941 The default value is 2700.
5943 @item large-function-growth
5944 Specifies maximal growth of large function caused by inlining in percents.
5945 This parameter is ignored when @option{-funit-at-a-time} is not used.
5946 The default value is 100 which limits large function growth to 2.0 times
5949 @item large-unit-insns
5950 The limit specifying large translation unit. Growth caused by inlining of
5951 units larger than this limit is limited by @option{--param inline-unit-growth}.
5952 For small units this might be too tight (consider unit consisting of function A
5953 that is inline and B that just calls A three time. If B is small relative to
5954 A, the growth of unit is 300\% and yet such inlining is very sane. For very
5955 large units consisting of small inlininable functions however the overall unit
5956 growth limit is needed to avoid exponential explosion of code size. Thus for
5957 smaller units, the size is increased to @option{--param large-unit-insns}
5958 before applying @option{--param inline-unit-growth}. The default is 10000
5960 @item inline-unit-growth
5961 Specifies maximal overall growth of the compilation unit caused by inlining.
5962 This parameter is ignored when @option{-funit-at-a-time} is not used.
5963 The default value is 50 which limits unit growth to 1.5 times the original
5966 @item max-inline-insns-recursive
5967 @itemx max-inline-insns-recursive-auto
5968 Specifies maximum number of instructions out-of-line copy of self recursive inline
5969 function can grow into by performing recursive inlining.
5971 For functions declared inline @option{--param max-inline-insns-recursive} is
5972 taken into acount. For function not declared inline, recursive inlining
5973 happens only when @option{-finline-functions} (included in @option{-O3}) is
5974 enabled and @option{--param max-inline-insns-recursive-auto} is used. The
5975 default value is 450.
5977 @item max-inline-recursive-depth
5978 @itemx max-inline-recursive-depth-auto
5979 Specifies maximum recursion depth used by the recursive inlining.
5981 For functions declared inline @option{--param max-inline-recursive-depth} is
5982 taken into acount. For function not declared inline, recursive inlining
5983 happens only when @option{-finline-functions} (included in @option{-O3}) is
5984 enabled and @option{--param max-inline-recursive-depth-auto} is used. The
5985 default value is 450.
5987 @item min-inline-recursive-probability
5988 Recursive inlining is profitable only for function having deep recursion
5989 in average and can hurt for function having little recursion depth by
5990 increasing the prologue size or complexity of function body to other
5993 When profile feedback is available (see @option{-fprofile-generate}) the actual
5994 recursion depth can be guessed from probability that function will recurse via
5995 given call expression. This parameter limits inlining only to call expression
5996 whose probability exceeds given threshold (in percents). The default value is
5999 @item inline-call-cost
6000 Specify cost of call instruction relative to simple arithmetics operations
6001 (having cost of 1). Increasing this cost disqualifies inlining of non-leaf
6002 functions and at the same time increases size of leaf function that is believed to
6003 reduce function size by being inlined. In effect it increases amount of
6004 inlining for code having large abstraction penalty (many functions that just
6005 pass the arguments to other functions) and decrease inlining for code with low
6006 abstraction penalty. The default value is 16.
6008 @item max-unrolled-insns
6009 The maximum number of instructions that a loop should have if that loop
6010 is unrolled, and if the loop is unrolled, it determines how many times
6011 the loop code is unrolled.
6013 @item max-average-unrolled-insns
6014 The maximum number of instructions biased by probabilities of their execution
6015 that a loop should have if that loop is unrolled, and if the loop is unrolled,
6016 it determines how many times the loop code is unrolled.
6018 @item max-unroll-times
6019 The maximum number of unrollings of a single loop.
6021 @item max-peeled-insns
6022 The maximum number of instructions that a loop should have if that loop
6023 is peeled, and if the loop is peeled, it determines how many times
6024 the loop code is peeled.
6026 @item max-peel-times
6027 The maximum number of peelings of a single loop.
6029 @item max-completely-peeled-insns
6030 The maximum number of insns of a completely peeled loop.
6032 @item max-completely-peel-times
6033 The maximum number of iterations of a loop to be suitable for complete peeling.
6035 @item max-unswitch-insns
6036 The maximum number of insns of an unswitched loop.
6038 @item max-unswitch-level
6039 The maximum number of branches unswitched in a single loop.
6042 The minimum cost of an expensive expression in the loop invariant motion.
6044 @item iv-consider-all-candidates-bound
6045 Bound on number of candidates for induction variables below that
6046 all candidates are considered for each use in induction variable
6047 optimizations. Only the most relevant candidates are considered
6048 if there are more candidates, to avoid quadratic time complexity.
6050 @item iv-max-considered-uses
6051 The induction variable optimizations give up on loops that contain more
6052 induction variable uses.
6054 @item iv-always-prune-cand-set-bound
6055 If number of candidates in the set is smaller than this value,
6056 we always try to remove unnecessary ivs from the set during its
6057 optimization when a new iv is added to the set.
6059 @item scev-max-expr-size
6060 Bound on size of expressions used in the scalar evolutions analyzer.
6061 Large expressions slow the analyzer.
6063 @item vect-max-version-checks
6064 The maximum number of runtime checks that can be performed when doing
6065 loop versioning in the vectorizer. See option ftree-vect-loop-version
6066 for more information.
6068 @item max-iterations-to-track
6070 The maximum number of iterations of a loop the brute force algorithm
6071 for analysis of # of iterations of the loop tries to evaluate.
6073 @item hot-bb-count-fraction
6074 Select fraction of the maximal count of repetitions of basic block in program
6075 given basic block needs to have to be considered hot.
6077 @item hot-bb-frequency-fraction
6078 Select fraction of the maximal frequency of executions of basic block in
6079 function given basic block needs to have to be considered hot
6081 @item max-predicted-iterations
6082 The maximum number of loop iterations we predict statically. This is useful
6083 in cases where function contain single loop with known bound and other loop
6084 with unknown. We predict the known number of iterations correctly, while
6085 the unknown number of iterations average to roughly 10. This means that the
6086 loop without bounds would appear artificially cold relative to the other one.
6088 @item tracer-dynamic-coverage
6089 @itemx tracer-dynamic-coverage-feedback
6091 This value is used to limit superblock formation once the given percentage of
6092 executed instructions is covered. This limits unnecessary code size
6095 The @option{tracer-dynamic-coverage-feedback} is used only when profile
6096 feedback is available. The real profiles (as opposed to statically estimated
6097 ones) are much less balanced allowing the threshold to be larger value.
6099 @item tracer-max-code-growth
6100 Stop tail duplication once code growth has reached given percentage. This is
6101 rather hokey argument, as most of the duplicates will be eliminated later in
6102 cross jumping, so it may be set to much higher values than is the desired code
6105 @item tracer-min-branch-ratio
6107 Stop reverse growth when the reverse probability of best edge is less than this
6108 threshold (in percent).
6110 @item tracer-min-branch-ratio
6111 @itemx tracer-min-branch-ratio-feedback
6113 Stop forward growth if the best edge do have probability lower than this
6116 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
6117 compilation for profile feedback and one for compilation without. The value
6118 for compilation with profile feedback needs to be more conservative (higher) in
6119 order to make tracer effective.
6121 @item max-cse-path-length
6123 Maximum number of basic blocks on path that cse considers. The default is 10.
6126 The maximum instructions CSE process before flushing. The default is 1000.
6128 @item global-var-threshold
6130 Counts the number of function calls (@var{n}) and the number of
6131 call-clobbered variables (@var{v}). If @var{n}x@var{v} is larger than this limit, a
6132 single artificial variable will be created to represent all the
6133 call-clobbered variables at function call sites. This artificial
6134 variable will then be made to alias every call-clobbered variable.
6135 (done as @code{int * size_t} on the host machine; beware overflow).
6137 @item max-aliased-vops
6139 Maximum number of virtual operands allowed to represent aliases
6140 before triggering the alias grouping heuristic. Alias grouping
6141 reduces compile times and memory consumption needed for aliasing at
6142 the expense of precision loss in alias information.
6144 @item ggc-min-expand
6146 GCC uses a garbage collector to manage its own memory allocation. This
6147 parameter specifies the minimum percentage by which the garbage
6148 collector's heap should be allowed to expand between collections.
6149 Tuning this may improve compilation speed; it has no effect on code
6152 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
6153 RAM >= 1GB@. If @code{getrlimit} is available, the notion of "RAM" is
6154 the smallest of actual RAM and @code{RLIMIT_DATA} or @code{RLIMIT_AS}. If
6155 GCC is not able to calculate RAM on a particular platform, the lower
6156 bound of 30% is used. Setting this parameter and
6157 @option{ggc-min-heapsize} to zero causes a full collection to occur at
6158 every opportunity. This is extremely slow, but can be useful for
6161 @item ggc-min-heapsize
6163 Minimum size of the garbage collector's heap before it begins bothering
6164 to collect garbage. The first collection occurs after the heap expands
6165 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
6166 tuning this may improve compilation speed, and has no effect on code
6169 The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which
6170 tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
6171 with a lower bound of 4096 (four megabytes) and an upper bound of
6172 131072 (128 megabytes). If GCC is not able to calculate RAM on a
6173 particular platform, the lower bound is used. Setting this parameter
6174 very large effectively disables garbage collection. Setting this
6175 parameter and @option{ggc-min-expand} to zero causes a full collection
6176 to occur at every opportunity.
6178 @item max-reload-search-insns
6179 The maximum number of instruction reload should look backward for equivalent
6180 register. Increasing values mean more aggressive optimization, making the
6181 compile time increase with probably slightly better performance. The default
6184 @item max-cselib-memory-location
6185 The maximum number of memory locations cselib should take into acount.
6186 Increasing values mean more aggressive optimization, making the compile time
6187 increase with probably slightly better performance. The default value is 500.
6189 @item max-flow-memory-location
6190 Similar as @option{max-cselib-memory-location} but for dataflow liveness.
6191 The default value is 100.
6193 @item reorder-blocks-duplicate
6194 @itemx reorder-blocks-duplicate-feedback
6196 Used by basic block reordering pass to decide whether to use unconditional
6197 branch or duplicate the code on its destination. Code is duplicated when its
6198 estimated size is smaller than this value multiplied by the estimated size of
6199 unconditional jump in the hot spots of the program.
6201 The @option{reorder-block-duplicate-feedback} is used only when profile
6202 feedback is available and may be set to higher values than
6203 @option{reorder-block-duplicate} since information about the hot spots is more
6206 @item max-sched-ready-insns
6207 The maximum number of instructions ready to be issued the scheduler should
6208 consider at any given time during the first scheduling pass. Increasing
6209 values mean more thorough searches, making the compilation time increase
6210 with probably little benefit. The default value is 100.
6212 @item max-sched-region-blocks
6213 The maximum number of blocks in a region to be considered for
6214 interblock scheduling. The default value is 10.
6216 @item max-sched-region-insns
6217 The maximum number of insns in a region to be considered for
6218 interblock scheduling. The default value is 100.
6221 The minimum probability (in percents) of reaching a source block
6222 for interblock speculative scheduling. The default value is 40.
6224 @item max-sched-extend-regions-iters
6225 The maximum number of iterations through CFG to extend regions.
6226 0 - disable region extension,
6227 N - do at most N iterations.
6228 The default value is 2.
6230 @item max-sched-insn-conflict-delay
6231 The maximum conflict delay for an insn to be considered for speculative motion.
6232 The default value is 3.
6234 @item sched-spec-prob-cutoff
6235 The minimal probability of speculation success (in percents), so that
6236 speculative insn will be scheduled.
6237 The default value is 40.
6239 @item max-last-value-rtl
6241 The maximum size measured as number of RTLs that can be recorded in an expression
6242 in combiner for a pseudo register as last known value of that register. The default
6245 @item integer-share-limit
6246 Small integer constants can use a shared data structure, reducing the
6247 compiler's memory usage and increasing its speed. This sets the maximum
6248 value of a shared integer constant's. The default value is 256.
6250 @item min-virtual-mappings
6251 Specifies the minimum number of virtual mappings in the incremental
6252 SSA updater that should be registered to trigger the virtual mappings
6253 heuristic defined by virtual-mappings-ratio. The default value is
6256 @item virtual-mappings-ratio
6257 If the number of virtual mappings is virtual-mappings-ratio bigger
6258 than the number of virtual symbols to be updated, then the incremental
6259 SSA updater switches to a full update for those symbols. The default
6262 @item ssp-buffer-size
6263 The minimum size of buffers (i.e. arrays) that will receive stack smashing
6264 protection when @option{-fstack-protection} is used.
6266 @item max-jump-thread-duplication-stmts
6267 Maximum number of statements allowed in a block that needs to be
6268 duplicated when threading jumps.
6270 @item max-fields-for-field-sensitive
6271 Maximum number of fields in a structure we will treat in
6272 a field sensitive manner during pointer analysis.
6277 @node Preprocessor Options
6278 @section Options Controlling the Preprocessor
6279 @cindex preprocessor options
6280 @cindex options, preprocessor
6282 These options control the C preprocessor, which is run on each C source
6283 file before actual compilation.
6285 If you use the @option{-E} option, nothing is done except preprocessing.
6286 Some of these options make sense only together with @option{-E} because
6287 they cause the preprocessor output to be unsuitable for actual
6292 You can use @option{-Wp,@var{option}} to bypass the compiler driver
6293 and pass @var{option} directly through to the preprocessor. If
6294 @var{option} contains commas, it is split into multiple options at the
6295 commas. However, many options are modified, translated or interpreted
6296 by the compiler driver before being passed to the preprocessor, and
6297 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
6298 interface is undocumented and subject to change, so whenever possible
6299 you should avoid using @option{-Wp} and let the driver handle the
6302 @item -Xpreprocessor @var{option}
6303 @opindex preprocessor
6304 Pass @var{option} as an option to the preprocessor. You can use this to
6305 supply system-specific preprocessor options which GCC does not know how to
6308 If you want to pass an option that takes an argument, you must use
6309 @option{-Xpreprocessor} twice, once for the option and once for the argument.
6312 @include cppopts.texi
6314 @node Assembler Options
6315 @section Passing Options to the Assembler
6317 @c prevent bad page break with this line
6318 You can pass options to the assembler.
6321 @item -Wa,@var{option}
6323 Pass @var{option} as an option to the assembler. If @var{option}
6324 contains commas, it is split into multiple options at the commas.
6326 @item -Xassembler @var{option}
6328 Pass @var{option} as an option to the assembler. You can use this to
6329 supply system-specific assembler options which GCC does not know how to
6332 If you want to pass an option that takes an argument, you must use
6333 @option{-Xassembler} twice, once for the option and once for the argument.
6338 @section Options for Linking
6339 @cindex link options
6340 @cindex options, linking
6342 These options come into play when the compiler links object files into
6343 an executable output file. They are meaningless if the compiler is
6344 not doing a link step.
6348 @item @var{object-file-name}
6349 A file name that does not end in a special recognized suffix is
6350 considered to name an object file or library. (Object files are
6351 distinguished from libraries by the linker according to the file
6352 contents.) If linking is done, these object files are used as input
6361 If any of these options is used, then the linker is not run, and
6362 object file names should not be used as arguments. @xref{Overall
6366 @item -l@var{library}
6367 @itemx -l @var{library}
6369 Search the library named @var{library} when linking. (The second
6370 alternative with the library as a separate argument is only for
6371 POSIX compliance and is not recommended.)
6373 It makes a difference where in the command you write this option; the
6374 linker searches and processes libraries and object files in the order they
6375 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
6376 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
6377 to functions in @samp{z}, those functions may not be loaded.
6379 The linker searches a standard list of directories for the library,
6380 which is actually a file named @file{lib@var{library}.a}. The linker
6381 then uses this file as if it had been specified precisely by name.
6383 The directories searched include several standard system directories
6384 plus any that you specify with @option{-L}.
6386 Normally the files found this way are library files---archive files
6387 whose members are object files. The linker handles an archive file by
6388 scanning through it for members which define symbols that have so far
6389 been referenced but not defined. But if the file that is found is an
6390 ordinary object file, it is linked in the usual fashion. The only
6391 difference between using an @option{-l} option and specifying a file name
6392 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
6393 and searches several directories.
6397 You need this special case of the @option{-l} option in order to
6398 link an Objective-C or Objective-C++ program.
6401 @opindex nostartfiles
6402 Do not use the standard system startup files when linking.
6403 The standard system libraries are used normally, unless @option{-nostdlib}
6404 or @option{-nodefaultlibs} is used.
6406 @item -nodefaultlibs
6407 @opindex nodefaultlibs
6408 Do not use the standard system libraries when linking.
6409 Only the libraries you specify will be passed to the linker.
6410 The standard startup files are used normally, unless @option{-nostartfiles}
6411 is used. The compiler may generate calls to @code{memcmp},
6412 @code{memset}, @code{memcpy} and @code{memmove}.
6413 These entries are usually resolved by entries in
6414 libc. These entry points should be supplied through some other
6415 mechanism when this option is specified.
6419 Do not use the standard system startup files or libraries when linking.
6420 No startup files and only the libraries you specify will be passed to
6421 the linker. The compiler may generate calls to @code{memcmp}, @code{memset},
6422 @code{memcpy} and @code{memmove}.
6423 These entries are usually resolved by entries in
6424 libc. These entry points should be supplied through some other
6425 mechanism when this option is specified.
6427 @cindex @option{-lgcc}, use with @option{-nostdlib}
6428 @cindex @option{-nostdlib} and unresolved references
6429 @cindex unresolved references and @option{-nostdlib}
6430 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
6431 @cindex @option{-nodefaultlibs} and unresolved references
6432 @cindex unresolved references and @option{-nodefaultlibs}
6433 One of the standard libraries bypassed by @option{-nostdlib} and
6434 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
6435 that GCC uses to overcome shortcomings of particular machines, or special
6436 needs for some languages.
6437 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
6438 Collection (GCC) Internals},
6439 for more discussion of @file{libgcc.a}.)
6440 In most cases, you need @file{libgcc.a} even when you want to avoid
6441 other standard libraries. In other words, when you specify @option{-nostdlib}
6442 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
6443 This ensures that you have no unresolved references to internal GCC
6444 library subroutines. (For example, @samp{__main}, used to ensure C++
6445 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
6446 GNU Compiler Collection (GCC) Internals}.)
6450 Produce a position independent executable on targets which support it.
6451 For predictable results, you must also specify the same set of options
6452 that were used to generate code (@option{-fpie}, @option{-fPIE},
6453 or model suboptions) when you specify this option.
6457 Pass the flag @option{-export-dynamic} to the ELF linker, on targets
6458 that support it. This instructs the linker to add all symbols, not
6459 only used ones, to the dynamic symbol table. This option is needed
6460 for some uses of @code{dlopen} or to allow obtaining backtraces
6461 from within a program.
6465 Remove all symbol table and relocation information from the executable.
6469 On systems that support dynamic linking, this prevents linking with the shared
6470 libraries. On other systems, this option has no effect.
6474 Produce a shared object which can then be linked with other objects to
6475 form an executable. Not all systems support this option. For predictable
6476 results, you must also specify the same set of options that were used to
6477 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
6478 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
6479 needs to build supplementary stub code for constructors to work. On
6480 multi-libbed systems, @samp{gcc -shared} must select the correct support
6481 libraries to link against. Failing to supply the correct flags may lead
6482 to subtle defects. Supplying them in cases where they are not necessary
6485 @item -shared-libgcc
6486 @itemx -static-libgcc
6487 @opindex shared-libgcc
6488 @opindex static-libgcc
6489 On systems that provide @file{libgcc} as a shared library, these options
6490 force the use of either the shared or static version respectively.
6491 If no shared version of @file{libgcc} was built when the compiler was
6492 configured, these options have no effect.
6494 There are several situations in which an application should use the
6495 shared @file{libgcc} instead of the static version. The most common
6496 of these is when the application wishes to throw and catch exceptions
6497 across different shared libraries. In that case, each of the libraries
6498 as well as the application itself should use the shared @file{libgcc}.
6500 Therefore, the G++ and GCJ drivers automatically add
6501 @option{-shared-libgcc} whenever you build a shared library or a main
6502 executable, because C++ and Java programs typically use exceptions, so
6503 this is the right thing to do.
6505 If, instead, you use the GCC driver to create shared libraries, you may
6506 find that they will not always be linked with the shared @file{libgcc}.
6507 If GCC finds, at its configuration time, that you have a non-GNU linker
6508 or a GNU linker that does not support option @option{--eh-frame-hdr},
6509 it will link the shared version of @file{libgcc} into shared libraries
6510 by default. Otherwise, it will take advantage of the linker and optimize
6511 away the linking with the shared version of @file{libgcc}, linking with
6512 the static version of libgcc by default. This allows exceptions to
6513 propagate through such shared libraries, without incurring relocation
6514 costs at library load time.
6516 However, if a library or main executable is supposed to throw or catch
6517 exceptions, you must link it using the G++ or GCJ driver, as appropriate
6518 for the languages used in the program, or using the option
6519 @option{-shared-libgcc}, such that it is linked with the shared
6524 Bind references to global symbols when building a shared object. Warn
6525 about any unresolved references (unless overridden by the link editor
6526 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
6529 @item -Xlinker @var{option}
6531 Pass @var{option} as an option to the linker. You can use this to
6532 supply system-specific linker options which GCC does not know how to
6535 If you want to pass an option that takes an argument, you must use
6536 @option{-Xlinker} twice, once for the option and once for the argument.
6537 For example, to pass @option{-assert definitions}, you must write
6538 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
6539 @option{-Xlinker "-assert definitions"}, because this passes the entire
6540 string as a single argument, which is not what the linker expects.
6542 @item -Wl,@var{option}
6544 Pass @var{option} as an option to the linker. If @var{option} contains
6545 commas, it is split into multiple options at the commas.
6547 @item -u @var{symbol}
6549 Pretend the symbol @var{symbol} is undefined, to force linking of
6550 library modules to define it. You can use @option{-u} multiple times with
6551 different symbols to force loading of additional library modules.
6554 @node Directory Options
6555 @section Options for Directory Search
6556 @cindex directory options
6557 @cindex options, directory search
6560 These options specify directories to search for header files, for
6561 libraries and for parts of the compiler:
6566 Add the directory @var{dir} to the head of the list of directories to be
6567 searched for header files. This can be used to override a system header
6568 file, substituting your own version, since these directories are
6569 searched before the system header file directories. However, you should
6570 not use this option to add directories that contain vendor-supplied
6571 system header files (use @option{-isystem} for that). If you use more than
6572 one @option{-I} option, the directories are scanned in left-to-right
6573 order; the standard system directories come after.
6575 If a standard system include directory, or a directory specified with
6576 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
6577 option will be ignored. The directory will still be searched but as a
6578 system directory at its normal position in the system include chain.
6579 This is to ensure that GCC's procedure to fix buggy system headers and
6580 the ordering for the include_next directive are not inadvertently changed.
6581 If you really need to change the search order for system directories,
6582 use the @option{-nostdinc} and/or @option{-isystem} options.
6584 @item -iquote@var{dir}
6586 Add the directory @var{dir} to the head of the list of directories to
6587 be searched for header files only for the case of @samp{#include
6588 "@var{file}"}; they are not searched for @samp{#include <@var{file}>},
6589 otherwise just like @option{-I}.
6593 Add directory @var{dir} to the list of directories to be searched
6596 @item -B@var{prefix}
6598 This option specifies where to find the executables, libraries,
6599 include files, and data files of the compiler itself.
6601 The compiler driver program runs one or more of the subprograms
6602 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
6603 @var{prefix} as a prefix for each program it tries to run, both with and
6604 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
6606 For each subprogram to be run, the compiler driver first tries the
6607 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
6608 was not specified, the driver tries two standard prefixes, which are
6609 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}. If neither of
6610 those results in a file name that is found, the unmodified program
6611 name is searched for using the directories specified in your
6612 @env{PATH} environment variable.
6614 The compiler will check to see if the path provided by the @option{-B}
6615 refers to a directory, and if necessary it will add a directory
6616 separator character at the end of the path.
6618 @option{-B} prefixes that effectively specify directory names also apply
6619 to libraries in the linker, because the compiler translates these
6620 options into @option{-L} options for the linker. They also apply to
6621 includes files in the preprocessor, because the compiler translates these
6622 options into @option{-isystem} options for the preprocessor. In this case,
6623 the compiler appends @samp{include} to the prefix.
6625 The run-time support file @file{libgcc.a} can also be searched for using
6626 the @option{-B} prefix, if needed. If it is not found there, the two
6627 standard prefixes above are tried, and that is all. The file is left
6628 out of the link if it is not found by those means.
6630 Another way to specify a prefix much like the @option{-B} prefix is to use
6631 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
6634 As a special kludge, if the path provided by @option{-B} is
6635 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
6636 9, then it will be replaced by @file{[dir/]include}. This is to help
6637 with boot-strapping the compiler.
6639 @item -specs=@var{file}
6641 Process @var{file} after the compiler reads in the standard @file{specs}
6642 file, in order to override the defaults that the @file{gcc} driver
6643 program uses when determining what switches to pass to @file{cc1},
6644 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
6645 @option{-specs=@var{file}} can be specified on the command line, and they
6646 are processed in order, from left to right.
6648 @item --sysroot=@var{dir}
6650 Use @var{dir} as the logical root directory for headers and libraries.
6651 For example, if the compiler would normally search for headers in
6652 @file{/usr/include} and libraries in @file{/usr/lib}, it will instead
6653 search @file{@var{dir}/usr/include} and @file{@var{dir}/usr/lib}.
6655 If you use both this option and the @option{-isysroot} option, then
6656 the @option{--sysroot} option will apply to libraries, but the
6657 @option{-isysroot} option will apply to header files.
6659 The GNU linker (beginning with version 2.16) has the necessary support
6660 for this option. If your linker does not support this option, the
6661 header file aspect of @option{--sysroot} will still work, but the
6662 library aspect will not.
6666 This option has been deprecated. Please use @option{-iquote} instead for
6667 @option{-I} directories before the @option{-I-} and remove the @option{-I-}.
6668 Any directories you specify with @option{-I} options before the @option{-I-}
6669 option are searched only for the case of @samp{#include "@var{file}"};
6670 they are not searched for @samp{#include <@var{file}>}.
6672 If additional directories are specified with @option{-I} options after
6673 the @option{-I-}, these directories are searched for all @samp{#include}
6674 directives. (Ordinarily @emph{all} @option{-I} directories are used
6677 In addition, the @option{-I-} option inhibits the use of the current
6678 directory (where the current input file came from) as the first search
6679 directory for @samp{#include "@var{file}"}. There is no way to
6680 override this effect of @option{-I-}. With @option{-I.} you can specify
6681 searching the directory which was current when the compiler was
6682 invoked. That is not exactly the same as what the preprocessor does
6683 by default, but it is often satisfactory.
6685 @option{-I-} does not inhibit the use of the standard system directories
6686 for header files. Thus, @option{-I-} and @option{-nostdinc} are
6693 @section Specifying subprocesses and the switches to pass to them
6696 @command{gcc} is a driver program. It performs its job by invoking a
6697 sequence of other programs to do the work of compiling, assembling and
6698 linking. GCC interprets its command-line parameters and uses these to
6699 deduce which programs it should invoke, and which command-line options
6700 it ought to place on their command lines. This behavior is controlled
6701 by @dfn{spec strings}. In most cases there is one spec string for each
6702 program that GCC can invoke, but a few programs have multiple spec
6703 strings to control their behavior. The spec strings built into GCC can
6704 be overridden by using the @option{-specs=} command-line switch to specify
6707 @dfn{Spec files} are plaintext files that are used to construct spec
6708 strings. They consist of a sequence of directives separated by blank
6709 lines. The type of directive is determined by the first non-whitespace
6710 character on the line and it can be one of the following:
6713 @item %@var{command}
6714 Issues a @var{command} to the spec file processor. The commands that can
6718 @item %include <@var{file}>
6720 Search for @var{file} and insert its text at the current point in the
6723 @item %include_noerr <@var{file}>
6724 @cindex %include_noerr
6725 Just like @samp{%include}, but do not generate an error message if the include
6726 file cannot be found.
6728 @item %rename @var{old_name} @var{new_name}
6730 Rename the spec string @var{old_name} to @var{new_name}.
6734 @item *[@var{spec_name}]:
6735 This tells the compiler to create, override or delete the named spec
6736 string. All lines after this directive up to the next directive or
6737 blank line are considered to be the text for the spec string. If this
6738 results in an empty string then the spec will be deleted. (Or, if the
6739 spec did not exist, then nothing will happened.) Otherwise, if the spec
6740 does not currently exist a new spec will be created. If the spec does
6741 exist then its contents will be overridden by the text of this
6742 directive, unless the first character of that text is the @samp{+}
6743 character, in which case the text will be appended to the spec.
6745 @item [@var{suffix}]:
6746 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
6747 and up to the next directive or blank line are considered to make up the
6748 spec string for the indicated suffix. When the compiler encounters an
6749 input file with the named suffix, it will processes the spec string in
6750 order to work out how to compile that file. For example:
6757 This says that any input file whose name ends in @samp{.ZZ} should be
6758 passed to the program @samp{z-compile}, which should be invoked with the
6759 command-line switch @option{-input} and with the result of performing the
6760 @samp{%i} substitution. (See below.)
6762 As an alternative to providing a spec string, the text that follows a
6763 suffix directive can be one of the following:
6766 @item @@@var{language}
6767 This says that the suffix is an alias for a known @var{language}. This is
6768 similar to using the @option{-x} command-line switch to GCC to specify a
6769 language explicitly. For example:
6776 Says that .ZZ files are, in fact, C++ source files.
6779 This causes an error messages saying:
6782 @var{name} compiler not installed on this system.
6786 GCC already has an extensive list of suffixes built into it.
6787 This directive will add an entry to the end of the list of suffixes, but
6788 since the list is searched from the end backwards, it is effectively
6789 possible to override earlier entries using this technique.
6793 GCC has the following spec strings built into it. Spec files can
6794 override these strings or create their own. Note that individual
6795 targets can also add their own spec strings to this list.
6798 asm Options to pass to the assembler
6799 asm_final Options to pass to the assembler post-processor
6800 cpp Options to pass to the C preprocessor
6801 cc1 Options to pass to the C compiler
6802 cc1plus Options to pass to the C++ compiler
6803 endfile Object files to include at the end of the link
6804 link Options to pass to the linker
6805 lib Libraries to include on the command line to the linker
6806 libgcc Decides which GCC support library to pass to the linker
6807 linker Sets the name of the linker
6808 predefines Defines to be passed to the C preprocessor
6809 signed_char Defines to pass to CPP to say whether @code{char} is signed
6811 startfile Object files to include at the start of the link
6814 Here is a small example of a spec file:
6820 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
6823 This example renames the spec called @samp{lib} to @samp{old_lib} and
6824 then overrides the previous definition of @samp{lib} with a new one.
6825 The new definition adds in some extra command-line options before
6826 including the text of the old definition.
6828 @dfn{Spec strings} are a list of command-line options to be passed to their
6829 corresponding program. In addition, the spec strings can contain
6830 @samp{%}-prefixed sequences to substitute variable text or to
6831 conditionally insert text into the command line. Using these constructs
6832 it is possible to generate quite complex command lines.
6834 Here is a table of all defined @samp{%}-sequences for spec
6835 strings. Note that spaces are not generated automatically around the
6836 results of expanding these sequences. Therefore you can concatenate them
6837 together or combine them with constant text in a single argument.
6841 Substitute one @samp{%} into the program name or argument.
6844 Substitute the name of the input file being processed.
6847 Substitute the basename of the input file being processed.
6848 This is the substring up to (and not including) the last period
6849 and not including the directory.
6852 This is the same as @samp{%b}, but include the file suffix (text after
6856 Marks the argument containing or following the @samp{%d} as a
6857 temporary file name, so that that file will be deleted if GCC exits
6858 successfully. Unlike @samp{%g}, this contributes no text to the
6861 @item %g@var{suffix}
6862 Substitute a file name that has suffix @var{suffix} and is chosen
6863 once per compilation, and mark the argument in the same way as
6864 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
6865 name is now chosen in a way that is hard to predict even when previously
6866 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
6867 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
6868 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
6869 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
6870 was simply substituted with a file name chosen once per compilation,
6871 without regard to any appended suffix (which was therefore treated
6872 just like ordinary text), making such attacks more likely to succeed.
6874 @item %u@var{suffix}
6875 Like @samp{%g}, but generates a new temporary file name even if
6876 @samp{%u@var{suffix}} was already seen.
6878 @item %U@var{suffix}
6879 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
6880 new one if there is no such last file name. In the absence of any
6881 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
6882 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
6883 would involve the generation of two distinct file names, one
6884 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
6885 simply substituted with a file name chosen for the previous @samp{%u},
6886 without regard to any appended suffix.
6888 @item %j@var{suffix}
6889 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
6890 writable, and if save-temps is off; otherwise, substitute the name
6891 of a temporary file, just like @samp{%u}. This temporary file is not
6892 meant for communication between processes, but rather as a junk
6895 @item %|@var{suffix}
6896 @itemx %m@var{suffix}
6897 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
6898 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
6899 all. These are the two most common ways to instruct a program that it
6900 should read from standard input or write to standard output. If you
6901 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
6902 construct: see for example @file{f/lang-specs.h}.
6904 @item %.@var{SUFFIX}
6905 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
6906 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
6907 terminated by the next space or %.
6910 Marks the argument containing or following the @samp{%w} as the
6911 designated output file of this compilation. This puts the argument
6912 into the sequence of arguments that @samp{%o} will substitute later.
6915 Substitutes the names of all the output files, with spaces
6916 automatically placed around them. You should write spaces
6917 around the @samp{%o} as well or the results are undefined.
6918 @samp{%o} is for use in the specs for running the linker.
6919 Input files whose names have no recognized suffix are not compiled
6920 at all, but they are included among the output files, so they will
6924 Substitutes the suffix for object files. Note that this is
6925 handled specially when it immediately follows @samp{%g, %u, or %U},
6926 because of the need for those to form complete file names. The
6927 handling is such that @samp{%O} is treated exactly as if it had already
6928 been substituted, except that @samp{%g, %u, and %U} do not currently
6929 support additional @var{suffix} characters following @samp{%O} as they would
6930 following, for example, @samp{.o}.
6933 Substitutes the standard macro predefinitions for the
6934 current target machine. Use this when running @code{cpp}.
6937 Like @samp{%p}, but puts @samp{__} before and after the name of each
6938 predefined macro, except for macros that start with @samp{__} or with
6939 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
6943 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
6944 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}),
6945 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
6946 and @option{-imultilib} as necessary.
6949 Current argument is the name of a library or startup file of some sort.
6950 Search for that file in a standard list of directories and substitute
6951 the full name found.
6954 Print @var{str} as an error message. @var{str} is terminated by a newline.
6955 Use this when inconsistent options are detected.
6958 Substitute the contents of spec string @var{name} at this point.
6961 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
6963 @item %x@{@var{option}@}
6964 Accumulate an option for @samp{%X}.
6967 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
6971 Output the accumulated assembler options specified by @option{-Wa}.
6974 Output the accumulated preprocessor options specified by @option{-Wp}.
6977 Process the @code{asm} spec. This is used to compute the
6978 switches to be passed to the assembler.
6981 Process the @code{asm_final} spec. This is a spec string for
6982 passing switches to an assembler post-processor, if such a program is
6986 Process the @code{link} spec. This is the spec for computing the
6987 command line passed to the linker. Typically it will make use of the
6988 @samp{%L %G %S %D and %E} sequences.
6991 Dump out a @option{-L} option for each directory that GCC believes might
6992 contain startup files. If the target supports multilibs then the
6993 current multilib directory will be prepended to each of these paths.
6996 Process the @code{lib} spec. This is a spec string for deciding which
6997 libraries should be included on the command line to the linker.
7000 Process the @code{libgcc} spec. This is a spec string for deciding
7001 which GCC support library should be included on the command line to the linker.
7004 Process the @code{startfile} spec. This is a spec for deciding which
7005 object files should be the first ones passed to the linker. Typically
7006 this might be a file named @file{crt0.o}.
7009 Process the @code{endfile} spec. This is a spec string that specifies
7010 the last object files that will be passed to the linker.
7013 Process the @code{cpp} spec. This is used to construct the arguments
7014 to be passed to the C preprocessor.
7017 Process the @code{cc1} spec. This is used to construct the options to be
7018 passed to the actual C compiler (@samp{cc1}).
7021 Process the @code{cc1plus} spec. This is used to construct the options to be
7022 passed to the actual C++ compiler (@samp{cc1plus}).
7025 Substitute the variable part of a matched option. See below.
7026 Note that each comma in the substituted string is replaced by
7030 Remove all occurrences of @code{-S} from the command line. Note---this
7031 command is position dependent. @samp{%} commands in the spec string
7032 before this one will see @code{-S}, @samp{%} commands in the spec string
7033 after this one will not.
7035 @item %:@var{function}(@var{args})
7036 Call the named function @var{function}, passing it @var{args}.
7037 @var{args} is first processed as a nested spec string, then split
7038 into an argument vector in the usual fashion. The function returns
7039 a string which is processed as if it had appeared literally as part
7040 of the current spec.
7042 The following built-in spec functions are provided:
7045 @item @code{if-exists}
7046 The @code{if-exists} spec function takes one argument, an absolute
7047 pathname to a file. If the file exists, @code{if-exists} returns the
7048 pathname. Here is a small example of its usage:
7052 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
7055 @item @code{if-exists-else}
7056 The @code{if-exists-else} spec function is similar to the @code{if-exists}
7057 spec function, except that it takes two arguments. The first argument is
7058 an absolute pathname to a file. If the file exists, @code{if-exists-else}
7059 returns the pathname. If it does not exist, it returns the second argument.
7060 This way, @code{if-exists-else} can be used to select one file or another,
7061 based on the existence of the first. Here is a small example of its usage:
7065 crt0%O%s %:if-exists(crti%O%s) \
7066 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
7069 @item @code{replace-outfile}
7070 The @code{replace-outfile} spec function takes two arguments. It looks for the
7071 first argument in the outfiles array and replaces it with the second argument. Here
7072 is a small example of its usage:
7075 %@{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)@}
7081 Substitutes the @code{-S} switch, if that switch was given to GCC@.
7082 If that switch was not specified, this substitutes nothing. Note that
7083 the leading dash is omitted when specifying this option, and it is
7084 automatically inserted if the substitution is performed. Thus the spec
7085 string @samp{%@{foo@}} would match the command-line option @option{-foo}
7086 and would output the command line option @option{-foo}.
7088 @item %W@{@code{S}@}
7089 Like %@{@code{S}@} but mark last argument supplied within as a file to be
7092 @item %@{@code{S}*@}
7093 Substitutes all the switches specified to GCC whose names start
7094 with @code{-S}, but which also take an argument. This is used for
7095 switches like @option{-o}, @option{-D}, @option{-I}, etc.
7096 GCC considers @option{-o foo} as being
7097 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
7098 text, including the space. Thus two arguments would be generated.
7100 @item %@{@code{S}*&@code{T}*@}
7101 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
7102 (the order of @code{S} and @code{T} in the spec is not significant).
7103 There can be any number of ampersand-separated variables; for each the
7104 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
7106 @item %@{@code{S}:@code{X}@}
7107 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
7109 @item %@{!@code{S}:@code{X}@}
7110 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
7112 @item %@{@code{S}*:@code{X}@}
7113 Substitutes @code{X} if one or more switches whose names start with
7114 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
7115 once, no matter how many such switches appeared. However, if @code{%*}
7116 appears somewhere in @code{X}, then @code{X} will be substituted once
7117 for each matching switch, with the @code{%*} replaced by the part of
7118 that switch that matched the @code{*}.
7120 @item %@{.@code{S}:@code{X}@}
7121 Substitutes @code{X}, if processing a file with suffix @code{S}.
7123 @item %@{!.@code{S}:@code{X}@}
7124 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
7126 @item %@{@code{S}|@code{P}:@code{X}@}
7127 Substitutes @code{X} if either @code{-S} or @code{-P} was given to GCC@.
7128 This may be combined with @samp{!}, @samp{.}, and @code{*} sequences as well,
7129 although they have a stronger binding than the @samp{|}. If @code{%*}
7130 appears in @code{X}, all of the alternatives must be starred, and only
7131 the first matching alternative is substituted.
7133 For example, a spec string like this:
7136 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
7139 will output the following command-line options from the following input
7140 command-line options:
7145 -d fred.c -foo -baz -boggle
7146 -d jim.d -bar -baz -boggle
7149 @item %@{S:X; T:Y; :D@}
7151 If @code{S} was given to GCC, substitutes @code{X}; else if @code{T} was
7152 given to GCC, substitutes @code{Y}; else substitutes @code{D}. There can
7153 be as many clauses as you need. This may be combined with @code{.},
7154 @code{!}, @code{|}, and @code{*} as needed.
7159 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
7160 construct may contain other nested @samp{%} constructs or spaces, or
7161 even newlines. They are processed as usual, as described above.
7162 Trailing white space in @code{X} is ignored. White space may also
7163 appear anywhere on the left side of the colon in these constructs,
7164 except between @code{.} or @code{*} and the corresponding word.
7166 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
7167 handled specifically in these constructs. If another value of
7168 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
7169 @option{-W} switch is found later in the command line, the earlier
7170 switch value is ignored, except with @{@code{S}*@} where @code{S} is
7171 just one letter, which passes all matching options.
7173 The character @samp{|} at the beginning of the predicate text is used to
7174 indicate that a command should be piped to the following command, but
7175 only if @option{-pipe} is specified.
7177 It is built into GCC which switches take arguments and which do not.
7178 (You might think it would be useful to generalize this to allow each
7179 compiler's spec to say which switches take arguments. But this cannot
7180 be done in a consistent fashion. GCC cannot even decide which input
7181 files have been specified without knowing which switches take arguments,
7182 and it must know which input files to compile in order to tell which
7185 GCC also knows implicitly that arguments starting in @option{-l} are to be
7186 treated as compiler output files, and passed to the linker in their
7187 proper position among the other output files.
7189 @c man begin OPTIONS
7191 @node Target Options
7192 @section Specifying Target Machine and Compiler Version
7193 @cindex target options
7194 @cindex cross compiling
7195 @cindex specifying machine version
7196 @cindex specifying compiler version and target machine
7197 @cindex compiler version, specifying
7198 @cindex target machine, specifying
7200 The usual way to run GCC is to run the executable called @file{gcc}, or
7201 @file{<machine>-gcc} when cross-compiling, or
7202 @file{<machine>-gcc-<version>} to run a version other than the one that
7203 was installed last. Sometimes this is inconvenient, so GCC provides
7204 options that will switch to another cross-compiler or version.
7207 @item -b @var{machine}
7209 The argument @var{machine} specifies the target machine for compilation.
7211 The value to use for @var{machine} is the same as was specified as the
7212 machine type when configuring GCC as a cross-compiler. For
7213 example, if a cross-compiler was configured with @samp{configure
7214 arm-elf}, meaning to compile for an arm processor with elf binaries,
7215 then you would specify @option{-b arm-elf} to run that cross compiler.
7216 Because there are other options beginning with @option{-b}, the
7217 configuration must contain a hyphen.
7219 @item -V @var{version}
7221 The argument @var{version} specifies which version of GCC to run.
7222 This is useful when multiple versions are installed. For example,
7223 @var{version} might be @samp{4.0}, meaning to run GCC version 4.0.
7226 The @option{-V} and @option{-b} options work by running the
7227 @file{<machine>-gcc-<version>} executable, so there's no real reason to
7228 use them if you can just run that directly.
7230 @node Submodel Options
7231 @section Hardware Models and Configurations
7232 @cindex submodel options
7233 @cindex specifying hardware config
7234 @cindex hardware models and configurations, specifying
7235 @cindex machine dependent options
7237 Earlier we discussed the standard option @option{-b} which chooses among
7238 different installed compilers for completely different target
7239 machines, such as VAX vs.@: 68000 vs.@: 80386.
7241 In addition, each of these target machine types can have its own
7242 special options, starting with @samp{-m}, to choose among various
7243 hardware models or configurations---for example, 68010 vs 68020,
7244 floating coprocessor or none. A single installed version of the
7245 compiler can compile for any model or configuration, according to the
7248 Some configurations of the compiler also support additional special
7249 options, usually for compatibility with other compilers on the same
7252 @c This list is ordered alphanumerically by subsection name.
7253 @c It should be the same order and spelling as these options are listed
7254 @c in Machine Dependent Options
7260 * Blackfin Options::
7264 * DEC Alpha Options::
7265 * DEC Alpha/VMS Options::
7267 * GNU/Linux Options::
7270 * i386 and x86-64 Options::
7283 * RS/6000 and PowerPC Options::
7284 * S/390 and zSeries Options::
7287 * System V Options::
7288 * TMS320C3x/C4x Options::
7292 * Xstormy16 Options::
7298 @subsection ARC Options
7301 These options are defined for ARC implementations:
7306 Compile code for little endian mode. This is the default.
7310 Compile code for big endian mode.
7313 @opindex mmangle-cpu
7314 Prepend the name of the cpu to all public symbol names.
7315 In multiple-processor systems, there are many ARC variants with different
7316 instruction and register set characteristics. This flag prevents code
7317 compiled for one cpu to be linked with code compiled for another.
7318 No facility exists for handling variants that are ``almost identical''.
7319 This is an all or nothing option.
7321 @item -mcpu=@var{cpu}
7323 Compile code for ARC variant @var{cpu}.
7324 Which variants are supported depend on the configuration.
7325 All variants support @option{-mcpu=base}, this is the default.
7327 @item -mtext=@var{text-section}
7328 @itemx -mdata=@var{data-section}
7329 @itemx -mrodata=@var{readonly-data-section}
7333 Put functions, data, and readonly data in @var{text-section},
7334 @var{data-section}, and @var{readonly-data-section} respectively
7335 by default. This can be overridden with the @code{section} attribute.
7336 @xref{Variable Attributes}.
7341 @subsection ARM Options
7344 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
7348 @item -mabi=@var{name}
7350 Generate code for the specified ABI@. Permissible values are: @samp{apcs-gnu},
7351 @samp{atpcs}, @samp{aapcs}, @samp{aapcs-linux} and @samp{iwmmxt}.
7354 @opindex mapcs-frame
7355 Generate a stack frame that is compliant with the ARM Procedure Call
7356 Standard for all functions, even if this is not strictly necessary for
7357 correct execution of the code. Specifying @option{-fomit-frame-pointer}
7358 with this option will cause the stack frames not to be generated for
7359 leaf functions. The default is @option{-mno-apcs-frame}.
7363 This is a synonym for @option{-mapcs-frame}.
7366 @c not currently implemented
7367 @item -mapcs-stack-check
7368 @opindex mapcs-stack-check
7369 Generate code to check the amount of stack space available upon entry to
7370 every function (that actually uses some stack space). If there is
7371 insufficient space available then either the function
7372 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
7373 called, depending upon the amount of stack space required. The run time
7374 system is required to provide these functions. The default is
7375 @option{-mno-apcs-stack-check}, since this produces smaller code.
7377 @c not currently implemented
7379 @opindex mapcs-float
7380 Pass floating point arguments using the float point registers. This is
7381 one of the variants of the APCS@. This option is recommended if the
7382 target hardware has a floating point unit or if a lot of floating point
7383 arithmetic is going to be performed by the code. The default is
7384 @option{-mno-apcs-float}, since integer only code is slightly increased in
7385 size if @option{-mapcs-float} is used.
7387 @c not currently implemented
7388 @item -mapcs-reentrant
7389 @opindex mapcs-reentrant
7390 Generate reentrant, position independent code. The default is
7391 @option{-mno-apcs-reentrant}.
7394 @item -mthumb-interwork
7395 @opindex mthumb-interwork
7396 Generate code which supports calling between the ARM and Thumb
7397 instruction sets. Without this option the two instruction sets cannot
7398 be reliably used inside one program. The default is
7399 @option{-mno-thumb-interwork}, since slightly larger code is generated
7400 when @option{-mthumb-interwork} is specified.
7402 @item -mno-sched-prolog
7403 @opindex mno-sched-prolog
7404 Prevent the reordering of instructions in the function prolog, or the
7405 merging of those instruction with the instructions in the function's
7406 body. This means that all functions will start with a recognizable set
7407 of instructions (or in fact one of a choice from a small set of
7408 different function prologues), and this information can be used to
7409 locate the start if functions inside an executable piece of code. The
7410 default is @option{-msched-prolog}.
7413 @opindex mhard-float
7414 Generate output containing floating point instructions. This is the
7418 @opindex msoft-float
7419 Generate output containing library calls for floating point.
7420 @strong{Warning:} the requisite libraries are not available for all ARM
7421 targets. Normally the facilities of the machine's usual C compiler are
7422 used, but this cannot be done directly in cross-compilation. You must make
7423 your own arrangements to provide suitable library functions for
7426 @option{-msoft-float} changes the calling convention in the output file;
7427 therefore, it is only useful if you compile @emph{all} of a program with
7428 this option. In particular, you need to compile @file{libgcc.a}, the
7429 library that comes with GCC, with @option{-msoft-float} in order for
7432 @item -mfloat-abi=@var{name}
7434 Specifies which ABI to use for floating point values. Permissible values
7435 are: @samp{soft}, @samp{softfp} and @samp{hard}.
7437 @samp{soft} and @samp{hard} are equivalent to @option{-msoft-float}
7438 and @option{-mhard-float} respectively. @samp{softfp} allows the generation
7439 of floating point instructions, but still uses the soft-float calling
7442 @item -mlittle-endian
7443 @opindex mlittle-endian
7444 Generate code for a processor running in little-endian mode. This is
7445 the default for all standard configurations.
7448 @opindex mbig-endian
7449 Generate code for a processor running in big-endian mode; the default is
7450 to compile code for a little-endian processor.
7452 @item -mwords-little-endian
7453 @opindex mwords-little-endian
7454 This option only applies when generating code for big-endian processors.
7455 Generate code for a little-endian word order but a big-endian byte
7456 order. That is, a byte order of the form @samp{32107654}. Note: this
7457 option should only be used if you require compatibility with code for
7458 big-endian ARM processors generated by versions of the compiler prior to
7461 @item -mcpu=@var{name}
7463 This specifies the name of the target ARM processor. GCC uses this name
7464 to determine what kind of instructions it can emit when generating
7465 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
7466 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
7467 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
7468 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
7469 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
7470 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm7tdmi-s},
7471 @samp{arm8}, @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
7472 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
7473 @samp{arm920t}, @samp{arm922t}, @samp{arm946e-s}, @samp{arm966e-s},
7474 @samp{arm968e-s}, @samp{arm926ej-s}, @samp{arm940t}, @samp{arm9tdmi},
7475 @samp{arm10tdmi}, @samp{arm1020t}, @samp{arm1026ej-s},
7476 @samp{arm10e}, @samp{arm1020e}, @samp{arm1022e},
7477 @samp{arm1136j-s}, @samp{arm1136jf-s}, @samp{mpcore}, @samp{mpcorenovfp},
7478 @samp{arm1176jz-s}, @samp{arm1176jzf-s}, @samp{xscale}, @samp{iwmmxt},
7481 @itemx -mtune=@var{name}
7483 This option is very similar to the @option{-mcpu=} option, except that
7484 instead of specifying the actual target processor type, and hence
7485 restricting which instructions can be used, it specifies that GCC should
7486 tune the performance of the code as if the target were of the type
7487 specified in this option, but still choosing the instructions that it
7488 will generate based on the cpu specified by a @option{-mcpu=} option.
7489 For some ARM implementations better performance can be obtained by using
7492 @item -march=@var{name}
7494 This specifies the name of the target ARM architecture. GCC uses this
7495 name to determine what kind of instructions it can emit when generating
7496 assembly code. This option can be used in conjunction with or instead
7497 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
7498 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
7499 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{armv6}, @samp{armv6j},
7500 @samp{iwmmxt}, @samp{ep9312}.
7502 @item -mfpu=@var{name}
7503 @itemx -mfpe=@var{number}
7504 @itemx -mfp=@var{number}
7508 This specifies what floating point hardware (or hardware emulation) is
7509 available on the target. Permissible names are: @samp{fpa}, @samp{fpe2},
7510 @samp{fpe3}, @samp{maverick}, @samp{vfp}. @option{-mfp} and @option{-mfpe}
7511 are synonyms for @option{-mfpu}=@samp{fpe}@var{number}, for compatibility
7512 with older versions of GCC@.
7514 If @option{-msoft-float} is specified this specifies the format of
7515 floating point values.
7517 @item -mstructure-size-boundary=@var{n}
7518 @opindex mstructure-size-boundary
7519 The size of all structures and unions will be rounded up to a multiple
7520 of the number of bits set by this option. Permissible values are 8, 32
7521 and 64. The default value varies for different toolchains. For the COFF
7522 targeted toolchain the default value is 8. A value of 64 is only allowed
7523 if the underlying ABI supports it.
7525 Specifying the larger number can produce faster, more efficient code, but
7526 can also increase the size of the program. Different values are potentially
7527 incompatible. Code compiled with one value cannot necessarily expect to
7528 work with code or libraries compiled with another value, if they exchange
7529 information using structures or unions.
7531 @item -mabort-on-noreturn
7532 @opindex mabort-on-noreturn
7533 Generate a call to the function @code{abort} at the end of a
7534 @code{noreturn} function. It will be executed if the function tries to
7538 @itemx -mno-long-calls
7539 @opindex mlong-calls
7540 @opindex mno-long-calls
7541 Tells the compiler to perform function calls by first loading the
7542 address of the function into a register and then performing a subroutine
7543 call on this register. This switch is needed if the target function
7544 will lie outside of the 64 megabyte addressing range of the offset based
7545 version of subroutine call instruction.
7547 Even if this switch is enabled, not all function calls will be turned
7548 into long calls. The heuristic is that static functions, functions
7549 which have the @samp{short-call} attribute, functions that are inside
7550 the scope of a @samp{#pragma no_long_calls} directive and functions whose
7551 definitions have already been compiled within the current compilation
7552 unit, will not be turned into long calls. The exception to this rule is
7553 that weak function definitions, functions with the @samp{long-call}
7554 attribute or the @samp{section} attribute, and functions that are within
7555 the scope of a @samp{#pragma long_calls} directive, will always be
7556 turned into long calls.
7558 This feature is not enabled by default. Specifying
7559 @option{-mno-long-calls} will restore the default behavior, as will
7560 placing the function calls within the scope of a @samp{#pragma
7561 long_calls_off} directive. Note these switches have no effect on how
7562 the compiler generates code to handle function calls via function
7565 @item -mnop-fun-dllimport
7566 @opindex mnop-fun-dllimport
7567 Disable support for the @code{dllimport} attribute.
7569 @item -msingle-pic-base
7570 @opindex msingle-pic-base
7571 Treat the register used for PIC addressing as read-only, rather than
7572 loading it in the prologue for each function. The run-time system is
7573 responsible for initializing this register with an appropriate value
7574 before execution begins.
7576 @item -mpic-register=@var{reg}
7577 @opindex mpic-register
7578 Specify the register to be used for PIC addressing. The default is R10
7579 unless stack-checking is enabled, when R9 is used.
7581 @item -mcirrus-fix-invalid-insns
7582 @opindex mcirrus-fix-invalid-insns
7583 @opindex mno-cirrus-fix-invalid-insns
7584 Insert NOPs into the instruction stream to in order to work around
7585 problems with invalid Maverick instruction combinations. This option
7586 is only valid if the @option{-mcpu=ep9312} option has been used to
7587 enable generation of instructions for the Cirrus Maverick floating
7588 point co-processor. This option is not enabled by default, since the
7589 problem is only present in older Maverick implementations. The default
7590 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
7593 @item -mpoke-function-name
7594 @opindex mpoke-function-name
7595 Write the name of each function into the text section, directly
7596 preceding the function prologue. The generated code is similar to this:
7600 .ascii "arm_poke_function_name", 0
7603 .word 0xff000000 + (t1 - t0)
7604 arm_poke_function_name
7606 stmfd sp!, @{fp, ip, lr, pc@}
7610 When performing a stack backtrace, code can inspect the value of
7611 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
7612 location @code{pc - 12} and the top 8 bits are set, then we know that
7613 there is a function name embedded immediately preceding this location
7614 and has length @code{((pc[-3]) & 0xff000000)}.
7618 Generate code for the 16-bit Thumb instruction set. The default is to
7619 use the 32-bit ARM instruction set.
7622 @opindex mtpcs-frame
7623 Generate a stack frame that is compliant with the Thumb Procedure Call
7624 Standard for all non-leaf functions. (A leaf function is one that does
7625 not call any other functions.) The default is @option{-mno-tpcs-frame}.
7627 @item -mtpcs-leaf-frame
7628 @opindex mtpcs-leaf-frame
7629 Generate a stack frame that is compliant with the Thumb Procedure Call
7630 Standard for all leaf functions. (A leaf function is one that does
7631 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
7633 @item -mcallee-super-interworking
7634 @opindex mcallee-super-interworking
7635 Gives all externally visible functions in the file being compiled an ARM
7636 instruction set header which switches to Thumb mode before executing the
7637 rest of the function. This allows these functions to be called from
7638 non-interworking code.
7640 @item -mcaller-super-interworking
7641 @opindex mcaller-super-interworking
7642 Allows calls via function pointers (including virtual functions) to
7643 execute correctly regardless of whether the target code has been
7644 compiled for interworking or not. There is a small overhead in the cost
7645 of executing a function pointer if this option is enabled.
7647 @item -mtp=@var{name}
7649 Specify the access model for the thread local storage pointer. The valid
7650 models are @option{soft}, which generates calls to @code{__aeabi_read_tp},
7651 @option{cp15}, which fetches the thread pointer from @code{cp15} directly
7652 (supported in the arm6k architecture), and @option{auto}, which uses the
7653 best available method for the selected processor. The default setting is
7659 @subsection AVR Options
7662 These options are defined for AVR implementations:
7665 @item -mmcu=@var{mcu}
7667 Specify ATMEL AVR instruction set or MCU type.
7669 Instruction set avr1 is for the minimal AVR core, not supported by the C
7670 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
7671 attiny11, attiny12, attiny15, attiny28).
7673 Instruction set avr2 (default) is for the classic AVR core with up to
7674 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
7675 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
7676 at90c8534, at90s8535).
7678 Instruction set avr3 is for the classic AVR core with up to 128K program
7679 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
7681 Instruction set avr4 is for the enhanced AVR core with up to 8K program
7682 memory space (MCU types: atmega8, atmega83, atmega85).
7684 Instruction set avr5 is for the enhanced AVR core with up to 128K program
7685 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
7686 atmega64, atmega128, at43usb355, at94k).
7690 Output instruction sizes to the asm file.
7692 @item -minit-stack=@var{N}
7693 @opindex minit-stack
7694 Specify the initial stack address, which may be a symbol or numeric value,
7695 @samp{__stack} is the default.
7697 @item -mno-interrupts
7698 @opindex mno-interrupts
7699 Generated code is not compatible with hardware interrupts.
7700 Code size will be smaller.
7702 @item -mcall-prologues
7703 @opindex mcall-prologues
7704 Functions prologues/epilogues expanded as call to appropriate
7705 subroutines. Code size will be smaller.
7707 @item -mno-tablejump
7708 @opindex mno-tablejump
7709 Do not generate tablejump insns which sometimes increase code size.
7712 @opindex mtiny-stack
7713 Change only the low 8 bits of the stack pointer.
7717 Assume int to be 8 bit integer. This affects the sizes of all types: A
7718 char will be 1 byte, an int will be 1 byte, an long will be 2 bytes
7719 and long long will be 4 bytes. Please note that this option does not
7720 comply to the C standards, but it will provide you with smaller code
7724 @node Blackfin Options
7725 @subsection Blackfin Options
7726 @cindex Blackfin Options
7729 @item -momit-leaf-frame-pointer
7730 @opindex momit-leaf-frame-pointer
7731 Don't keep the frame pointer in a register for leaf functions. This
7732 avoids the instructions to save, set up and restore frame pointers and
7733 makes an extra register available in leaf functions. The option
7734 @option{-fomit-frame-pointer} removes the frame pointer for all functions
7735 which might make debugging harder.
7737 @item -mspecld-anomaly
7738 @opindex mspecld-anomaly
7739 When enabled, the compiler will ensure that the generated code does not
7740 contain speculative loads after jump instructions. This option is enabled
7743 @item -mno-specld-anomaly
7744 @opindex mno-specld-anomaly
7745 Don't generate extra code to prevent speculative loads from occurring.
7747 @item -mcsync-anomaly
7748 @opindex mcsync-anomaly
7749 When enabled, the compiler will ensure that the generated code does not
7750 contain CSYNC or SSYNC instructions too soon after conditional branches.
7751 This option is enabled by default.
7753 @item -mno-csync-anomaly
7754 @opindex mno-csync-anomaly
7755 Don't generate extra code to prevent CSYNC or SSYNC instructions from
7756 occurring too soon after a conditional branch.
7760 When enabled, the compiler is free to take advantage of the knowledge that
7761 the entire program fits into the low 64k of memory.
7764 @opindex mno-low-64k
7765 Assume that the program is arbitrarily large. This is the default.
7767 @item -mid-shared-library
7768 @opindex mid-shared-library
7769 Generate code that supports shared libraries via the library ID method.
7770 This allows for execute in place and shared libraries in an environment
7771 without virtual memory management. This option implies @option{-fPIC}.
7773 @item -mno-id-shared-library
7774 @opindex mno-id-shared-library
7775 Generate code that doesn't assume ID based shared libraries are being used.
7776 This is the default.
7778 @item -mshared-library-id=n
7779 @opindex mshared-library-id
7780 Specified the identification number of the ID based shared library being
7781 compiled. Specifying a value of 0 will generate more compact code, specifying
7782 other values will force the allocation of that number to the current
7783 library but is no more space or time efficient than omitting this option.
7786 @itemx -mno-long-calls
7787 @opindex mlong-calls
7788 @opindex mno-long-calls
7789 Tells the compiler to perform function calls by first loading the
7790 address of the function into a register and then performing a subroutine
7791 call on this register. This switch is needed if the target function
7792 will lie outside of the 24 bit addressing range of the offset based
7793 version of subroutine call instruction.
7795 This feature is not enabled by default. Specifying
7796 @option{-mno-long-calls} will restore the default behavior. Note these
7797 switches have no effect on how the compiler generates code to handle
7798 function calls via function pointers.
7802 @subsection CRIS Options
7803 @cindex CRIS Options
7805 These options are defined specifically for the CRIS ports.
7808 @item -march=@var{architecture-type}
7809 @itemx -mcpu=@var{architecture-type}
7812 Generate code for the specified architecture. The choices for
7813 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
7814 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX@.
7815 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
7818 @item -mtune=@var{architecture-type}
7820 Tune to @var{architecture-type} everything applicable about the generated
7821 code, except for the ABI and the set of available instructions. The
7822 choices for @var{architecture-type} are the same as for
7823 @option{-march=@var{architecture-type}}.
7825 @item -mmax-stack-frame=@var{n}
7826 @opindex mmax-stack-frame
7827 Warn when the stack frame of a function exceeds @var{n} bytes.
7829 @item -melinux-stacksize=@var{n}
7830 @opindex melinux-stacksize
7831 Only available with the @samp{cris-axis-aout} target. Arranges for
7832 indications in the program to the kernel loader that the stack of the
7833 program should be set to @var{n} bytes.
7839 The options @option{-metrax4} and @option{-metrax100} are synonyms for
7840 @option{-march=v3} and @option{-march=v8} respectively.
7842 @item -mmul-bug-workaround
7843 @itemx -mno-mul-bug-workaround
7844 @opindex mmul-bug-workaround
7845 @opindex mno-mul-bug-workaround
7846 Work around a bug in the @code{muls} and @code{mulu} instructions for CPU
7847 models where it applies. This option is active by default.
7851 Enable CRIS-specific verbose debug-related information in the assembly
7852 code. This option also has the effect to turn off the @samp{#NO_APP}
7853 formatted-code indicator to the assembler at the beginning of the
7858 Do not use condition-code results from previous instruction; always emit
7859 compare and test instructions before use of condition codes.
7861 @item -mno-side-effects
7862 @opindex mno-side-effects
7863 Do not emit instructions with side-effects in addressing modes other than
7867 @itemx -mno-stack-align
7869 @itemx -mno-data-align
7870 @itemx -mconst-align
7871 @itemx -mno-const-align
7872 @opindex mstack-align
7873 @opindex mno-stack-align
7874 @opindex mdata-align
7875 @opindex mno-data-align
7876 @opindex mconst-align
7877 @opindex mno-const-align
7878 These options (no-options) arranges (eliminate arrangements) for the
7879 stack-frame, individual data and constants to be aligned for the maximum
7880 single data access size for the chosen CPU model. The default is to
7881 arrange for 32-bit alignment. ABI details such as structure layout are
7882 not affected by these options.
7890 Similar to the stack- data- and const-align options above, these options
7891 arrange for stack-frame, writable data and constants to all be 32-bit,
7892 16-bit or 8-bit aligned. The default is 32-bit alignment.
7894 @item -mno-prologue-epilogue
7895 @itemx -mprologue-epilogue
7896 @opindex mno-prologue-epilogue
7897 @opindex mprologue-epilogue
7898 With @option{-mno-prologue-epilogue}, the normal function prologue and
7899 epilogue that sets up the stack-frame are omitted and no return
7900 instructions or return sequences are generated in the code. Use this
7901 option only together with visual inspection of the compiled code: no
7902 warnings or errors are generated when call-saved registers must be saved,
7903 or storage for local variable needs to be allocated.
7909 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
7910 instruction sequences that load addresses for functions from the PLT part
7911 of the GOT rather than (traditional on other architectures) calls to the
7912 PLT@. The default is @option{-mgotplt}.
7916 Legacy no-op option only recognized with the cris-axis-aout target.
7920 Legacy no-op option only recognized with the cris-axis-elf and
7921 cris-axis-linux-gnu targets.
7925 Only recognized with the cris-axis-aout target, where it selects a
7926 GNU/linux-like multilib, include files and instruction set for
7931 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
7935 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
7936 to link with input-output functions from a simulator library. Code,
7937 initialized data and zero-initialized data are allocated consecutively.
7941 Like @option{-sim}, but pass linker options to locate initialized data at
7942 0x40000000 and zero-initialized data at 0x80000000.
7946 @subsection CRX Options
7949 These options are defined specifically for the CRX ports.
7955 Enable the use of multiply-accumulate instructions. Disabled by default.
7959 Push instructions will be used to pass outgoing arguments when functions
7960 are called. Enabled by default.
7963 @node Darwin Options
7964 @subsection Darwin Options
7965 @cindex Darwin options
7967 These options are defined for all architectures running the Darwin operating
7970 FSF GCC on Darwin does not create ``fat'' object files; it will create
7971 an object file for the single architecture that it was built to
7972 target. Apple's GCC on Darwin does create ``fat'' files if multiple
7973 @option{-arch} options are used; it does so by running the compiler or
7974 linker multiple times and joining the results together with
7977 The subtype of the file created (like @samp{ppc7400} or @samp{ppc970} or
7978 @samp{i686}) is determined by the flags that specify the ISA
7979 that GCC is targetting, like @option{-mcpu} or @option{-march}. The
7980 @option{-force_cpusubtype_ALL} option can be used to override this.
7982 The Darwin tools vary in their behavior when presented with an ISA
7983 mismatch. The assembler, @file{as}, will only permit instructions to
7984 be used that are valid for the subtype of the file it is generating,
7985 so you cannot put 64-bit instructions in an @samp{ppc750} object file.
7986 The linker for shared libraries, @file{/usr/bin/libtool}, will fail
7987 and print an error if asked to create a shared library with a less
7988 restrictive subtype than its input files (for instance, trying to put
7989 a @samp{ppc970} object file in a @samp{ppc7400} library). The linker
7990 for executables, @file{ld}, will quietly give the executable the most
7991 restrictive subtype of any of its input files.
7996 Add the framework directory @var{dir} to the head of the list of
7997 directories to be searched for header files. These directories are
7998 interleaved with those specified by @option{-I} options and are
7999 scanned in a left-to-right order.
8001 A framework directory is a directory with frameworks in it. A
8002 framework is a directory with a @samp{"Headers"} and/or
8003 @samp{"PrivateHeaders"} directory contained directly in it that ends
8004 in @samp{".framework"}. The name of a framework is the name of this
8005 directory excluding the @samp{".framework"}. Headers associated with
8006 the framework are found in one of those two directories, with
8007 @samp{"Headers"} being searched first. A subframework is a framework
8008 directory that is in a framework's @samp{"Frameworks"} directory.
8009 Includes of subframework headers can only appear in a header of a
8010 framework that contains the subframework, or in a sibling subframework
8011 header. Two subframeworks are siblings if they occur in the same
8012 framework. A subframework should not have the same name as a
8013 framework, a warning will be issued if this is violated. Currently a
8014 subframework cannot have subframeworks, in the future, the mechanism
8015 may be extended to support this. The standard frameworks can be found
8016 in @samp{"/System/Library/Frameworks"} and
8017 @samp{"/Library/Frameworks"}. An example include looks like
8018 @code{#include <Framework/header.h>}, where @samp{Framework} denotes
8019 the name of the framework and header.h is found in the
8020 @samp{"PrivateHeaders"} or @samp{"Headers"} directory.
8024 Emit debugging information for symbols that are used. For STABS
8025 debugging format, this enables @option{-feliminate-unused-debug-symbols}.
8026 This is by default ON@.
8030 Emit debugging information for all symbols and types.
8032 @item -mmacosx-version-min=@var{version}
8033 The earliest version of MacOS X that this executable will run on
8034 is @var{version}. Typical values of @var{version} include @code{10.1},
8035 @code{10.2}, and @code{10.3.9}.
8037 The default for this option is to make choices that seem to be most
8040 @item -mone-byte-bool
8041 @opindex -mone-byte-bool
8042 Override the defaults for @samp{bool} so that @samp{sizeof(bool)==1}.
8043 By default @samp{sizeof(bool)} is @samp{4} when compiling for
8044 Darwin/PowerPC and @samp{1} when compiling for Darwin/x86, so this
8045 option has no effect on x86.
8047 @strong{Warning:} The @option{-mone-byte-bool} switch causes GCC
8048 to generate code that is not binary compatible with code generated
8049 without that switch. Using this switch may require recompiling all
8050 other modules in a program, including system libraries. Use this
8051 switch to conform to a non-default data model.
8053 @item -mfix-and-continue
8054 @itemx -ffix-and-continue
8055 @itemx -findirect-data
8056 @opindex mfix-and-continue
8057 @opindex ffix-and-continue
8058 @opindex findirect-data
8059 Generate code suitable for fast turn around development. Needed to
8060 enable gdb to dynamically load @code{.o} files into already running
8061 programs. @option{-findirect-data} and @option{-ffix-and-continue}
8062 are provided for backwards compatibility.
8066 Loads all members of static archive libraries.
8067 See man ld(1) for more information.
8069 @item -arch_errors_fatal
8070 @opindex arch_errors_fatal
8071 Cause the errors having to do with files that have the wrong architecture
8075 @opindex bind_at_load
8076 Causes the output file to be marked such that the dynamic linker will
8077 bind all undefined references when the file is loaded or launched.
8081 Produce a Mach-o bundle format file.
8082 See man ld(1) for more information.
8084 @item -bundle_loader @var{executable}
8085 @opindex bundle_loader
8086 This option specifies the @var{executable} that will be loading the build
8087 output file being linked. See man ld(1) for more information.
8090 @opindex -dynamiclib
8091 When passed this option, GCC will produce a dynamic library instead of
8092 an executable when linking, using the Darwin @file{libtool} command.
8094 @item -force_cpusubtype_ALL
8095 @opindex -force_cpusubtype_ALL
8096 This causes GCC's output file to have the @var{ALL} subtype, instead of
8097 one controlled by the @option{-mcpu} or @option{-march} option.
8099 @item -allowable_client @var{client_name}
8101 @itemx -compatibility_version
8102 @itemx -current_version
8104 @itemx -dependency-file
8106 @itemx -dylinker_install_name
8108 @itemx -exported_symbols_list
8110 @itemx -flat_namespace
8111 @itemx -force_flat_namespace
8112 @itemx -headerpad_max_install_names
8115 @itemx -install_name
8116 @itemx -keep_private_externs
8117 @itemx -multi_module
8118 @itemx -multiply_defined
8119 @itemx -multiply_defined_unused
8121 @itemx -no_dead_strip_inits_and_terms
8122 @itemx -nofixprebinding
8125 @itemx -noseglinkedit
8126 @itemx -pagezero_size
8128 @itemx -prebind_all_twolevel_modules
8129 @itemx -private_bundle
8130 @itemx -read_only_relocs
8132 @itemx -sectobjectsymbols
8136 @itemx -sectobjectsymbols
8139 @itemx -segs_read_only_addr
8140 @itemx -segs_read_write_addr
8141 @itemx -seg_addr_table
8142 @itemx -seg_addr_table_filename
8145 @itemx -segs_read_only_addr
8146 @itemx -segs_read_write_addr
8147 @itemx -single_module
8150 @itemx -sub_umbrella
8151 @itemx -twolevel_namespace
8154 @itemx -unexported_symbols_list
8155 @itemx -weak_reference_mismatches
8158 @opindex allowable_client
8159 @opindex client_name
8160 @opindex compatibility_version
8161 @opindex current_version
8163 @opindex dependency-file
8165 @opindex dylinker_install_name
8167 @opindex exported_symbols_list
8169 @opindex flat_namespace
8170 @opindex force_flat_namespace
8171 @opindex headerpad_max_install_names
8174 @opindex install_name
8175 @opindex keep_private_externs
8176 @opindex multi_module
8177 @opindex multiply_defined
8178 @opindex multiply_defined_unused
8180 @opindex no_dead_strip_inits_and_terms
8181 @opindex nofixprebinding
8182 @opindex nomultidefs
8184 @opindex noseglinkedit
8185 @opindex pagezero_size
8187 @opindex prebind_all_twolevel_modules
8188 @opindex private_bundle
8189 @opindex read_only_relocs
8191 @opindex sectobjectsymbols
8195 @opindex sectobjectsymbols
8198 @opindex segs_read_only_addr
8199 @opindex segs_read_write_addr
8200 @opindex seg_addr_table
8201 @opindex seg_addr_table_filename
8202 @opindex seglinkedit
8204 @opindex segs_read_only_addr
8205 @opindex segs_read_write_addr
8206 @opindex single_module
8208 @opindex sub_library
8209 @opindex sub_umbrella
8210 @opindex twolevel_namespace
8213 @opindex unexported_symbols_list
8214 @opindex weak_reference_mismatches
8215 @opindex whatsloaded
8217 These options are passed to the Darwin linker. The Darwin linker man page
8218 describes them in detail.
8221 @node DEC Alpha Options
8222 @subsection DEC Alpha Options
8224 These @samp{-m} options are defined for the DEC Alpha implementations:
8227 @item -mno-soft-float
8229 @opindex mno-soft-float
8230 @opindex msoft-float
8231 Use (do not use) the hardware floating-point instructions for
8232 floating-point operations. When @option{-msoft-float} is specified,
8233 functions in @file{libgcc.a} will be used to perform floating-point
8234 operations. Unless they are replaced by routines that emulate the
8235 floating-point operations, or compiled in such a way as to call such
8236 emulations routines, these routines will issue floating-point
8237 operations. If you are compiling for an Alpha without floating-point
8238 operations, you must ensure that the library is built so as not to call
8241 Note that Alpha implementations without floating-point operations are
8242 required to have floating-point registers.
8247 @opindex mno-fp-regs
8248 Generate code that uses (does not use) the floating-point register set.
8249 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
8250 register set is not used, floating point operands are passed in integer
8251 registers as if they were integers and floating-point results are passed
8252 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
8253 so any function with a floating-point argument or return value called by code
8254 compiled with @option{-mno-fp-regs} must also be compiled with that
8257 A typical use of this option is building a kernel that does not use,
8258 and hence need not save and restore, any floating-point registers.
8262 The Alpha architecture implements floating-point hardware optimized for
8263 maximum performance. It is mostly compliant with the IEEE floating
8264 point standard. However, for full compliance, software assistance is
8265 required. This option generates code fully IEEE compliant code
8266 @emph{except} that the @var{inexact-flag} is not maintained (see below).
8267 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
8268 defined during compilation. The resulting code is less efficient but is
8269 able to correctly support denormalized numbers and exceptional IEEE
8270 values such as not-a-number and plus/minus infinity. Other Alpha
8271 compilers call this option @option{-ieee_with_no_inexact}.
8273 @item -mieee-with-inexact
8274 @opindex mieee-with-inexact
8275 This is like @option{-mieee} except the generated code also maintains
8276 the IEEE @var{inexact-flag}. Turning on this option causes the
8277 generated code to implement fully-compliant IEEE math. In addition to
8278 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
8279 macro. On some Alpha implementations the resulting code may execute
8280 significantly slower than the code generated by default. Since there is
8281 very little code that depends on the @var{inexact-flag}, you should
8282 normally not specify this option. Other Alpha compilers call this
8283 option @option{-ieee_with_inexact}.
8285 @item -mfp-trap-mode=@var{trap-mode}
8286 @opindex mfp-trap-mode
8287 This option controls what floating-point related traps are enabled.
8288 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
8289 The trap mode can be set to one of four values:
8293 This is the default (normal) setting. The only traps that are enabled
8294 are the ones that cannot be disabled in software (e.g., division by zero
8298 In addition to the traps enabled by @samp{n}, underflow traps are enabled
8302 Like @samp{su}, but the instructions are marked to be safe for software
8303 completion (see Alpha architecture manual for details).
8306 Like @samp{su}, but inexact traps are enabled as well.
8309 @item -mfp-rounding-mode=@var{rounding-mode}
8310 @opindex mfp-rounding-mode
8311 Selects the IEEE rounding mode. Other Alpha compilers call this option
8312 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
8317 Normal IEEE rounding mode. Floating point numbers are rounded towards
8318 the nearest machine number or towards the even machine number in case
8322 Round towards minus infinity.
8325 Chopped rounding mode. Floating point numbers are rounded towards zero.
8328 Dynamic rounding mode. A field in the floating point control register
8329 (@var{fpcr}, see Alpha architecture reference manual) controls the
8330 rounding mode in effect. The C library initializes this register for
8331 rounding towards plus infinity. Thus, unless your program modifies the
8332 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
8335 @item -mtrap-precision=@var{trap-precision}
8336 @opindex mtrap-precision
8337 In the Alpha architecture, floating point traps are imprecise. This
8338 means without software assistance it is impossible to recover from a
8339 floating trap and program execution normally needs to be terminated.
8340 GCC can generate code that can assist operating system trap handlers
8341 in determining the exact location that caused a floating point trap.
8342 Depending on the requirements of an application, different levels of
8343 precisions can be selected:
8347 Program precision. This option is the default and means a trap handler
8348 can only identify which program caused a floating point exception.
8351 Function precision. The trap handler can determine the function that
8352 caused a floating point exception.
8355 Instruction precision. The trap handler can determine the exact
8356 instruction that caused a floating point exception.
8359 Other Alpha compilers provide the equivalent options called
8360 @option{-scope_safe} and @option{-resumption_safe}.
8362 @item -mieee-conformant
8363 @opindex mieee-conformant
8364 This option marks the generated code as IEEE conformant. You must not
8365 use this option unless you also specify @option{-mtrap-precision=i} and either
8366 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
8367 is to emit the line @samp{.eflag 48} in the function prologue of the
8368 generated assembly file. Under DEC Unix, this has the effect that
8369 IEEE-conformant math library routines will be linked in.
8371 @item -mbuild-constants
8372 @opindex mbuild-constants
8373 Normally GCC examines a 32- or 64-bit integer constant to
8374 see if it can construct it from smaller constants in two or three
8375 instructions. If it cannot, it will output the constant as a literal and
8376 generate code to load it from the data segment at runtime.
8378 Use this option to require GCC to construct @emph{all} integer constants
8379 using code, even if it takes more instructions (the maximum is six).
8381 You would typically use this option to build a shared library dynamic
8382 loader. Itself a shared library, it must relocate itself in memory
8383 before it can find the variables and constants in its own data segment.
8389 Select whether to generate code to be assembled by the vendor-supplied
8390 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
8408 Indicate whether GCC should generate code to use the optional BWX,
8409 CIX, FIX and MAX instruction sets. The default is to use the instruction
8410 sets supported by the CPU type specified via @option{-mcpu=} option or that
8411 of the CPU on which GCC was built if none was specified.
8416 @opindex mfloat-ieee
8417 Generate code that uses (does not use) VAX F and G floating point
8418 arithmetic instead of IEEE single and double precision.
8420 @item -mexplicit-relocs
8421 @itemx -mno-explicit-relocs
8422 @opindex mexplicit-relocs
8423 @opindex mno-explicit-relocs
8424 Older Alpha assemblers provided no way to generate symbol relocations
8425 except via assembler macros. Use of these macros does not allow
8426 optimal instruction scheduling. GNU binutils as of version 2.12
8427 supports a new syntax that allows the compiler to explicitly mark
8428 which relocations should apply to which instructions. This option
8429 is mostly useful for debugging, as GCC detects the capabilities of
8430 the assembler when it is built and sets the default accordingly.
8434 @opindex msmall-data
8435 @opindex mlarge-data
8436 When @option{-mexplicit-relocs} is in effect, static data is
8437 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
8438 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
8439 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
8440 16-bit relocations off of the @code{$gp} register. This limits the
8441 size of the small data area to 64KB, but allows the variables to be
8442 directly accessed via a single instruction.
8444 The default is @option{-mlarge-data}. With this option the data area
8445 is limited to just below 2GB@. Programs that require more than 2GB of
8446 data must use @code{malloc} or @code{mmap} to allocate the data in the
8447 heap instead of in the program's data segment.
8449 When generating code for shared libraries, @option{-fpic} implies
8450 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
8454 @opindex msmall-text
8455 @opindex mlarge-text
8456 When @option{-msmall-text} is used, the compiler assumes that the
8457 code of the entire program (or shared library) fits in 4MB, and is
8458 thus reachable with a branch instruction. When @option{-msmall-data}
8459 is used, the compiler can assume that all local symbols share the
8460 same @code{$gp} value, and thus reduce the number of instructions
8461 required for a function call from 4 to 1.
8463 The default is @option{-mlarge-text}.
8465 @item -mcpu=@var{cpu_type}
8467 Set the instruction set and instruction scheduling parameters for
8468 machine type @var{cpu_type}. You can specify either the @samp{EV}
8469 style name or the corresponding chip number. GCC supports scheduling
8470 parameters for the EV4, EV5 and EV6 family of processors and will
8471 choose the default values for the instruction set from the processor
8472 you specify. If you do not specify a processor type, GCC will default
8473 to the processor on which the compiler was built.
8475 Supported values for @var{cpu_type} are
8481 Schedules as an EV4 and has no instruction set extensions.
8485 Schedules as an EV5 and has no instruction set extensions.
8489 Schedules as an EV5 and supports the BWX extension.
8494 Schedules as an EV5 and supports the BWX and MAX extensions.
8498 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
8502 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
8505 @item -mtune=@var{cpu_type}
8507 Set only the instruction scheduling parameters for machine type
8508 @var{cpu_type}. The instruction set is not changed.
8510 @item -mmemory-latency=@var{time}
8511 @opindex mmemory-latency
8512 Sets the latency the scheduler should assume for typical memory
8513 references as seen by the application. This number is highly
8514 dependent on the memory access patterns used by the application
8515 and the size of the external cache on the machine.
8517 Valid options for @var{time} are
8521 A decimal number representing clock cycles.
8527 The compiler contains estimates of the number of clock cycles for
8528 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
8529 (also called Dcache, Scache, and Bcache), as well as to main memory.
8530 Note that L3 is only valid for EV5.
8535 @node DEC Alpha/VMS Options
8536 @subsection DEC Alpha/VMS Options
8538 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
8541 @item -mvms-return-codes
8542 @opindex mvms-return-codes
8543 Return VMS condition codes from main. The default is to return POSIX
8544 style condition (e.g.@ error) codes.
8548 @subsection FRV Options
8555 Only use the first 32 general purpose registers.
8560 Use all 64 general purpose registers.
8565 Use only the first 32 floating point registers.
8570 Use all 64 floating point registers
8573 @opindex mhard-float
8575 Use hardware instructions for floating point operations.
8578 @opindex msoft-float
8580 Use library routines for floating point operations.
8585 Dynamically allocate condition code registers.
8590 Do not try to dynamically allocate condition code registers, only
8591 use @code{icc0} and @code{fcc0}.
8596 Change ABI to use double word insns.
8601 Do not use double word instructions.
8606 Use floating point double instructions.
8611 Do not use floating point double instructions.
8616 Use media instructions.
8621 Do not use media instructions.
8626 Use multiply and add/subtract instructions.
8631 Do not use multiply and add/subtract instructions.
8636 Select the FDPIC ABI, that uses function descriptors to represent
8637 pointers to functions. Without any PIC/PIE-related options, it
8638 implies @option{-fPIE}. With @option{-fpic} or @option{-fpie}, it
8639 assumes GOT entries and small data are within a 12-bit range from the
8640 GOT base address; with @option{-fPIC} or @option{-fPIE}, GOT offsets
8641 are computed with 32 bits.
8644 @opindex minline-plt
8646 Enable inlining of PLT entries in function calls to functions that are
8647 not known to bind locally. It has no effect without @option{-mfdpic}.
8648 It's enabled by default if optimizing for speed and compiling for
8649 shared libraries (i.e., @option{-fPIC} or @option{-fpic}), or when an
8650 optimization option such as @option{-O3} or above is present in the
8656 Assume a large TLS segment when generating thread-local code.
8661 Do not assume a large TLS segment when generating thread-local code.
8666 Enable the use of @code{GPREL} relocations in the FDPIC ABI for data
8667 that is known to be in read-only sections. It's enabled by default,
8668 except for @option{-fpic} or @option{-fpie}: even though it may help
8669 make the global offset table smaller, it trades 1 instruction for 4.
8670 With @option{-fPIC} or @option{-fPIE}, it trades 3 instructions for 4,
8671 one of which may be shared by multiple symbols, and it avoids the need
8672 for a GOT entry for the referenced symbol, so it's more likely to be a
8673 win. If it is not, @option{-mno-gprel-ro} can be used to disable it.
8675 @item -multilib-library-pic
8676 @opindex multilib-library-pic
8678 Link with the (library, not FD) pic libraries. It's implied by
8679 @option{-mlibrary-pic}, as well as by @option{-fPIC} and
8680 @option{-fpic} without @option{-mfdpic}. You should never have to use
8686 Follow the EABI requirement of always creating a frame pointer whenever
8687 a stack frame is allocated. This option is enabled by default and can
8688 be disabled with @option{-mno-linked-fp}.
8691 @opindex mlong-calls
8693 Use indirect addressing to call functions outside the current
8694 compilation unit. This allows the functions to be placed anywhere
8695 within the 32-bit address space.
8697 @item -malign-labels
8698 @opindex malign-labels
8700 Try to align labels to an 8-byte boundary by inserting nops into the
8701 previous packet. This option only has an effect when VLIW packing
8702 is enabled. It doesn't create new packets; it merely adds nops to
8706 @opindex mlibrary-pic
8708 Generate position-independent EABI code.
8713 Use only the first four media accumulator registers.
8718 Use all eight media accumulator registers.
8723 Pack VLIW instructions.
8728 Do not pack VLIW instructions.
8733 Do not mark ABI switches in e_flags.
8738 Enable the use of conditional-move instructions (default).
8740 This switch is mainly for debugging the compiler and will likely be removed
8741 in a future version.
8743 @item -mno-cond-move
8744 @opindex mno-cond-move
8746 Disable the use of conditional-move instructions.
8748 This switch is mainly for debugging the compiler and will likely be removed
8749 in a future version.
8754 Enable the use of conditional set instructions (default).
8756 This switch is mainly for debugging the compiler and will likely be removed
8757 in a future version.
8762 Disable the use of conditional set instructions.
8764 This switch is mainly for debugging the compiler and will likely be removed
8765 in a future version.
8770 Enable the use of conditional execution (default).
8772 This switch is mainly for debugging the compiler and will likely be removed
8773 in a future version.
8775 @item -mno-cond-exec
8776 @opindex mno-cond-exec
8778 Disable the use of conditional execution.
8780 This switch is mainly for debugging the compiler and will likely be removed
8781 in a future version.
8784 @opindex mvliw-branch
8786 Run a pass to pack branches into VLIW instructions (default).
8788 This switch is mainly for debugging the compiler and will likely be removed
8789 in a future version.
8791 @item -mno-vliw-branch
8792 @opindex mno-vliw-branch
8794 Do not run a pass to pack branches into VLIW instructions.
8796 This switch is mainly for debugging the compiler and will likely be removed
8797 in a future version.
8799 @item -mmulti-cond-exec
8800 @opindex mmulti-cond-exec
8802 Enable optimization of @code{&&} and @code{||} in conditional execution
8805 This switch is mainly for debugging the compiler and will likely be removed
8806 in a future version.
8808 @item -mno-multi-cond-exec
8809 @opindex mno-multi-cond-exec
8811 Disable optimization of @code{&&} and @code{||} in conditional execution.
8813 This switch is mainly for debugging the compiler and will likely be removed
8814 in a future version.
8816 @item -mnested-cond-exec
8817 @opindex mnested-cond-exec
8819 Enable nested conditional execution optimizations (default).
8821 This switch is mainly for debugging the compiler and will likely be removed
8822 in a future version.
8824 @item -mno-nested-cond-exec
8825 @opindex mno-nested-cond-exec
8827 Disable nested conditional execution optimizations.
8829 This switch is mainly for debugging the compiler and will likely be removed
8830 in a future version.
8832 @item -moptimize-membar
8833 @opindex moptimize-membar
8835 This switch removes redundant @code{membar} instructions from the
8836 compiler generated code. It is enabled by default.
8838 @item -mno-optimize-membar
8839 @opindex mno-optimize-membar
8841 This switch disables the automatic removal of redundant @code{membar}
8842 instructions from the generated code.
8844 @item -mtomcat-stats
8845 @opindex mtomcat-stats
8847 Cause gas to print out tomcat statistics.
8849 @item -mcpu=@var{cpu}
8852 Select the processor type for which to generate code. Possible values are
8853 @samp{frv}, @samp{fr550}, @samp{tomcat}, @samp{fr500}, @samp{fr450},
8854 @samp{fr405}, @samp{fr400}, @samp{fr300} and @samp{simple}.
8858 @node GNU/Linux Options
8859 @subsection GNU/Linux Options
8861 These @samp{-m} options are defined for GNU/Linux targets:
8866 Use the GNU C library instead of uClibc. This is the default except
8867 on @samp{*-*-linux-*uclibc*} targets.
8871 Use uClibc instead of the GNU C library. This is the default on
8872 @samp{*-*-linux-*uclibc*} targets.
8875 @node H8/300 Options
8876 @subsection H8/300 Options
8878 These @samp{-m} options are defined for the H8/300 implementations:
8883 Shorten some address references at link time, when possible; uses the
8884 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
8885 ld, Using ld}, for a fuller description.
8889 Generate code for the H8/300H@.
8893 Generate code for the H8S@.
8897 Generate code for the H8S and H8/300H in the normal mode. This switch
8898 must be used either with @option{-mh} or @option{-ms}.
8902 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
8906 Make @code{int} data 32 bits by default.
8910 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
8911 The default for the H8/300H and H8S is to align longs and floats on 4
8913 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
8914 This option has no effect on the H8/300.
8918 @subsection HPPA Options
8919 @cindex HPPA Options
8921 These @samp{-m} options are defined for the HPPA family of computers:
8924 @item -march=@var{architecture-type}
8926 Generate code for the specified architecture. The choices for
8927 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
8928 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
8929 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
8930 architecture option for your machine. Code compiled for lower numbered
8931 architectures will run on higher numbered architectures, but not the
8935 @itemx -mpa-risc-1-1
8936 @itemx -mpa-risc-2-0
8937 @opindex mpa-risc-1-0
8938 @opindex mpa-risc-1-1
8939 @opindex mpa-risc-2-0
8940 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
8943 @opindex mbig-switch
8944 Generate code suitable for big switch tables. Use this option only if
8945 the assembler/linker complain about out of range branches within a switch
8948 @item -mjump-in-delay
8949 @opindex mjump-in-delay
8950 Fill delay slots of function calls with unconditional jump instructions
8951 by modifying the return pointer for the function call to be the target
8952 of the conditional jump.
8954 @item -mdisable-fpregs
8955 @opindex mdisable-fpregs
8956 Prevent floating point registers from being used in any manner. This is
8957 necessary for compiling kernels which perform lazy context switching of
8958 floating point registers. If you use this option and attempt to perform
8959 floating point operations, the compiler will abort.
8961 @item -mdisable-indexing
8962 @opindex mdisable-indexing
8963 Prevent the compiler from using indexing address modes. This avoids some
8964 rather obscure problems when compiling MIG generated code under MACH@.
8966 @item -mno-space-regs
8967 @opindex mno-space-regs
8968 Generate code that assumes the target has no space registers. This allows
8969 GCC to generate faster indirect calls and use unscaled index address modes.
8971 Such code is suitable for level 0 PA systems and kernels.
8973 @item -mfast-indirect-calls
8974 @opindex mfast-indirect-calls
8975 Generate code that assumes calls never cross space boundaries. This
8976 allows GCC to emit code which performs faster indirect calls.
8978 This option will not work in the presence of shared libraries or nested
8981 @item -mfixed-range=@var{register-range}
8982 @opindex mfixed-range
8983 Generate code treating the given register range as fixed registers.
8984 A fixed register is one that the register allocator can not use. This is
8985 useful when compiling kernel code. A register range is specified as
8986 two registers separated by a dash. Multiple register ranges can be
8987 specified separated by a comma.
8989 @item -mlong-load-store
8990 @opindex mlong-load-store
8991 Generate 3-instruction load and store sequences as sometimes required by
8992 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
8995 @item -mportable-runtime
8996 @opindex mportable-runtime
8997 Use the portable calling conventions proposed by HP for ELF systems.
9001 Enable the use of assembler directives only GAS understands.
9003 @item -mschedule=@var{cpu-type}
9005 Schedule code according to the constraints for the machine type
9006 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
9007 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
9008 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
9009 proper scheduling option for your machine. The default scheduling is
9013 @opindex mlinker-opt
9014 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
9015 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
9016 linkers in which they give bogus error messages when linking some programs.
9019 @opindex msoft-float
9020 Generate output containing library calls for floating point.
9021 @strong{Warning:} the requisite libraries are not available for all HPPA
9022 targets. Normally the facilities of the machine's usual C compiler are
9023 used, but this cannot be done directly in cross-compilation. You must make
9024 your own arrangements to provide suitable library functions for
9025 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
9026 does provide software floating point support.
9028 @option{-msoft-float} changes the calling convention in the output file;
9029 therefore, it is only useful if you compile @emph{all} of a program with
9030 this option. In particular, you need to compile @file{libgcc.a}, the
9031 library that comes with GCC, with @option{-msoft-float} in order for
9036 Generate the predefine, @code{_SIO}, for server IO@. The default is
9037 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
9038 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO@. These
9039 options are available under HP-UX and HI-UX@.
9043 Use GNU ld specific options. This passes @option{-shared} to ld when
9044 building a shared library. It is the default when GCC is configured,
9045 explicitly or implicitly, with the GNU linker. This option does not
9046 have any affect on which ld is called, it only changes what parameters
9047 are passed to that ld. The ld that is called is determined by the
9048 @option{--with-ld} configure option, GCC's program search path, and
9049 finally by the user's @env{PATH}. The linker used by GCC can be printed
9050 using @samp{which `gcc -print-prog-name=ld`}. This option is only available
9051 on the 64 bit HP-UX GCC, i.e. configured with @samp{hppa*64*-*-hpux*}.
9055 Use HP ld specific options. This passes @option{-b} to ld when building
9056 a shared library and passes @option{+Accept TypeMismatch} to ld on all
9057 links. It is the default when GCC is configured, explicitly or
9058 implicitly, with the HP linker. This option does not have any affect on
9059 which ld is called, it only changes what parameters are passed to that
9060 ld. The ld that is called is determined by the @option{--with-ld}
9061 configure option, GCC's program search path, and finally by the user's
9062 @env{PATH}. The linker used by GCC can be printed using @samp{which
9063 `gcc -print-prog-name=ld`}. This option is only available on the 64 bit
9064 HP-UX GCC, i.e. configured with @samp{hppa*64*-*-hpux*}.
9067 @opindex mno-long-calls
9068 Generate code that uses long call sequences. This ensures that a call
9069 is always able to reach linker generated stubs. The default is to generate
9070 long calls only when the distance from the call site to the beginning
9071 of the function or translation unit, as the case may be, exceeds a
9072 predefined limit set by the branch type being used. The limits for
9073 normal calls are 7,600,000 and 240,000 bytes, respectively for the
9074 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
9077 Distances are measured from the beginning of functions when using the
9078 @option{-ffunction-sections} option, or when using the @option{-mgas}
9079 and @option{-mno-portable-runtime} options together under HP-UX with
9082 It is normally not desirable to use this option as it will degrade
9083 performance. However, it may be useful in large applications,
9084 particularly when partial linking is used to build the application.
9086 The types of long calls used depends on the capabilities of the
9087 assembler and linker, and the type of code being generated. The
9088 impact on systems that support long absolute calls, and long pic
9089 symbol-difference or pc-relative calls should be relatively small.
9090 However, an indirect call is used on 32-bit ELF systems in pic code
9091 and it is quite long.
9093 @item -munix=@var{unix-std}
9095 Generate compiler predefines and select a startfile for the specified
9096 UNIX standard. The choices for @var{unix-std} are @samp{93}, @samp{95}
9097 and @samp{98}. @samp{93} is supported on all HP-UX versions. @samp{95}
9098 is available on HP-UX 10.10 and later. @samp{98} is available on HP-UX
9099 11.11 and later. The default values are @samp{93} for HP-UX 10.00,
9100 @samp{95} for HP-UX 10.10 though to 11.00, and @samp{98} for HP-UX 11.11
9103 @option{-munix=93} provides the same predefines as GCC 3.3 and 3.4.
9104 @option{-munix=95} provides additional predefines for @code{XOPEN_UNIX}
9105 and @code{_XOPEN_SOURCE_EXTENDED}, and the startfile @file{unix95.o}.
9106 @option{-munix=98} provides additional predefines for @code{_XOPEN_UNIX},
9107 @code{_XOPEN_SOURCE_EXTENDED}, @code{_INCLUDE__STDC_A1_SOURCE} and
9108 @code{_INCLUDE_XOPEN_SOURCE_500}, and the startfile @file{unix98.o}.
9110 It is @emph{important} to note that this option changes the interfaces
9111 for various library routines. It also affects the operational behavior
9112 of the C library. Thus, @emph{extreme} care is needed in using this
9115 Library code that is intended to operate with more than one UNIX
9116 standard must test, set and restore the variable @var{__xpg4_extended_mask}
9117 as appropriate. Most GNU software doesn't provide this capability.
9121 Suppress the generation of link options to search libdld.sl when the
9122 @option{-static} option is specified on HP-UX 10 and later.
9126 The HP-UX implementation of setlocale in libc has a dependency on
9127 libdld.sl. There isn't an archive version of libdld.sl. Thus,
9128 when the @option{-static} option is specified, special link options
9129 are needed to resolve this dependency.
9131 On HP-UX 10 and later, the GCC driver adds the necessary options to
9132 link with libdld.sl when the @option{-static} option is specified.
9133 This causes the resulting binary to be dynamic. On the 64-bit port,
9134 the linkers generate dynamic binaries by default in any case. The
9135 @option{-nolibdld} option can be used to prevent the GCC driver from
9136 adding these link options.
9140 Add support for multithreading with the @dfn{dce thread} library
9141 under HP-UX@. This option sets flags for both the preprocessor and
9145 @node i386 and x86-64 Options
9146 @subsection Intel 386 and AMD x86-64 Options
9147 @cindex i386 Options
9148 @cindex x86-64 Options
9149 @cindex Intel 386 Options
9150 @cindex AMD x86-64 Options
9152 These @samp{-m} options are defined for the i386 and x86-64 family of
9156 @item -mtune=@var{cpu-type}
9158 Tune to @var{cpu-type} everything applicable about the generated code, except
9159 for the ABI and the set of available instructions. The choices for
9163 Produce code optimized for the most common IA32/AMD64/EM64T processors.
9164 If you know the CPU on which your code will run, then you should use
9165 the corresponding @option{-mtune} option instead of
9166 @option{-mtune=generic}. But, if you do not know exactly what CPU users
9167 of your application will have, then you should use this option.
9169 As new processors are deployed in the marketplace, the behavior of this
9170 option will change. Therefore, if you upgrade to a newer version of
9171 GCC, the code generated option will change to reflect the processors
9172 that were most common when that version of GCC was released.
9174 There is no @option{-march=generic} option because @option{-march}
9175 indicates the instruction set the compiler can use, and there is no
9176 generic instruction set applicable to all processors. In contrast,
9177 @option{-mtune} indicates the processor (or, in this case, collection of
9178 processors) for which the code is optimized.
9180 This selects the CPU to tune for at compilation time by determining
9181 the processor type of the compiling machine. Using @option{-mtune=native}
9182 will produce code optimized for the local machine under the constraints
9183 of the selected instruction set. Using @option{-march=native} will
9184 enable all instruction subsets supported by the local machine (hence
9185 the result might not run on different machines).
9187 Original Intel's i386 CPU@.
9189 Intel's i486 CPU@. (No scheduling is implemented for this chip.)
9191 Intel Pentium CPU with no MMX support.
9193 Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
9195 Intel PentiumPro CPU@.
9197 Same as @code{generic}, but when used as @code{march} option, PentiumPro
9198 instruction set will be used, so the code will run on all i686 family chips.
9200 Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
9201 @item pentium3, pentium3m
9202 Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set
9205 Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set
9206 support. Used by Centrino notebooks.
9207 @item pentium4, pentium4m
9208 Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
9210 Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction
9213 Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
9214 SSE2 and SSE3 instruction set support.
9216 AMD K6 CPU with MMX instruction set support.
9218 Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
9219 @item athlon, athlon-tbird
9220 AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions
9222 @item athlon-4, athlon-xp, athlon-mp
9223 Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE
9224 instruction set support.
9225 @item k8, opteron, athlon64, athlon-fx
9226 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets
9227 MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
9229 IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction
9232 IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW!
9233 instruction set support.
9235 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is
9236 implemented for this chip.)
9238 Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is
9239 implemented for this chip.)
9242 While picking a specific @var{cpu-type} will schedule things appropriately
9243 for that particular chip, the compiler will not generate any code that
9244 does not run on the i386 without the @option{-march=@var{cpu-type}} option
9247 @item -march=@var{cpu-type}
9249 Generate instructions for the machine type @var{cpu-type}. The choices
9250 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
9251 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
9253 @item -mcpu=@var{cpu-type}
9255 A deprecated synonym for @option{-mtune}.
9264 @opindex mpentiumpro
9265 These options are synonyms for @option{-mtune=i386}, @option{-mtune=i486},
9266 @option{-mtune=pentium}, and @option{-mtune=pentiumpro} respectively.
9267 These synonyms are deprecated.
9269 @item -mfpmath=@var{unit}
9271 Generate floating point arithmetics for selected unit @var{unit}. The choices
9276 Use the standard 387 floating point coprocessor present majority of chips and
9277 emulated otherwise. Code compiled with this option will run almost everywhere.
9278 The temporary results are computed in 80bit precision instead of precision
9279 specified by the type resulting in slightly different results compared to most
9280 of other chips. See @option{-ffloat-store} for more detailed description.
9282 This is the default choice for i386 compiler.
9285 Use scalar floating point instructions present in the SSE instruction set.
9286 This instruction set is supported by Pentium3 and newer chips, in the AMD line
9287 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
9288 instruction set supports only single precision arithmetics, thus the double and
9289 extended precision arithmetics is still done using 387. Later version, present
9290 only in Pentium4 and the future AMD x86-64 chips supports double precision
9293 For the i386 compiler, you need to use @option{-march=@var{cpu-type}}, @option{-msse}
9294 or @option{-msse2} switches to enable SSE extensions and make this option
9295 effective. For the x86-64 compiler, these extensions are enabled by default.
9297 The resulting code should be considerably faster in the majority of cases and avoid
9298 the numerical instability problems of 387 code, but may break some existing
9299 code that expects temporaries to be 80bit.
9301 This is the default choice for the x86-64 compiler.
9304 Attempt to utilize both instruction sets at once. This effectively double the
9305 amount of available registers and on chips with separate execution units for
9306 387 and SSE the execution resources too. Use this option with care, as it is
9307 still experimental, because the GCC register allocator does not model separate
9308 functional units well resulting in instable performance.
9311 @item -masm=@var{dialect}
9312 @opindex masm=@var{dialect}
9313 Output asm instructions using selected @var{dialect}. Supported
9314 choices are @samp{intel} or @samp{att} (the default one). Darwin does
9315 not support @samp{intel}.
9320 @opindex mno-ieee-fp
9321 Control whether or not the compiler uses IEEE floating point
9322 comparisons. These handle correctly the case where the result of a
9323 comparison is unordered.
9326 @opindex msoft-float
9327 Generate output containing library calls for floating point.
9328 @strong{Warning:} the requisite libraries are not part of GCC@.
9329 Normally the facilities of the machine's usual C compiler are used, but
9330 this can't be done directly in cross-compilation. You must make your
9331 own arrangements to provide suitable library functions for
9334 On machines where a function returns floating point results in the 80387
9335 register stack, some floating point opcodes may be emitted even if
9336 @option{-msoft-float} is used.
9338 @item -mno-fp-ret-in-387
9339 @opindex mno-fp-ret-in-387
9340 Do not use the FPU registers for return values of functions.
9342 The usual calling convention has functions return values of types
9343 @code{float} and @code{double} in an FPU register, even if there
9344 is no FPU@. The idea is that the operating system should emulate
9347 The option @option{-mno-fp-ret-in-387} causes such values to be returned
9348 in ordinary CPU registers instead.
9350 @item -mno-fancy-math-387
9351 @opindex mno-fancy-math-387
9352 Some 387 emulators do not support the @code{sin}, @code{cos} and
9353 @code{sqrt} instructions for the 387. Specify this option to avoid
9354 generating those instructions. This option is the default on FreeBSD,
9355 OpenBSD and NetBSD@. This option is overridden when @option{-march}
9356 indicates that the target cpu will always have an FPU and so the
9357 instruction will not need emulation. As of revision 2.6.1, these
9358 instructions are not generated unless you also use the
9359 @option{-funsafe-math-optimizations} switch.
9361 @item -malign-double
9362 @itemx -mno-align-double
9363 @opindex malign-double
9364 @opindex mno-align-double
9365 Control whether GCC aligns @code{double}, @code{long double}, and
9366 @code{long long} variables on a two word boundary or a one word
9367 boundary. Aligning @code{double} variables on a two word boundary will
9368 produce code that runs somewhat faster on a @samp{Pentium} at the
9369 expense of more memory.
9371 @strong{Warning:} if you use the @option{-malign-double} switch,
9372 structures containing the above types will be aligned differently than
9373 the published application binary interface specifications for the 386
9374 and will not be binary compatible with structures in code compiled
9375 without that switch.
9377 @item -m96bit-long-double
9378 @itemx -m128bit-long-double
9379 @opindex m96bit-long-double
9380 @opindex m128bit-long-double
9381 These switches control the size of @code{long double} type. The i386
9382 application binary interface specifies the size to be 96 bits,
9383 so @option{-m96bit-long-double} is the default in 32 bit mode.
9385 Modern architectures (Pentium and newer) would prefer @code{long double}
9386 to be aligned to an 8 or 16 byte boundary. In arrays or structures
9387 conforming to the ABI, this would not be possible. So specifying a
9388 @option{-m128bit-long-double} will align @code{long double}
9389 to a 16 byte boundary by padding the @code{long double} with an additional
9392 In the x86-64 compiler, @option{-m128bit-long-double} is the default choice as
9393 its ABI specifies that @code{long double} is to be aligned on 16 byte boundary.
9395 Notice that neither of these options enable any extra precision over the x87
9396 standard of 80 bits for a @code{long double}.
9398 @strong{Warning:} if you override the default value for your target ABI, the
9399 structures and arrays containing @code{long double} variables will change
9400 their size as well as function calling convention for function taking
9401 @code{long double} will be modified. Hence they will not be binary
9402 compatible with arrays or structures in code compiled without that switch.
9404 @item -mmlarge-data-threshold=@var{number}
9405 @opindex mlarge-data-threshold=@var{number}
9406 When @option{-mcmodel=medium} is specified, the data greater than
9407 @var{threshold} are placed in large data section. This value must be the
9408 same across all object linked into the binary and defaults to 65535.
9411 @itemx -mno-svr3-shlib
9412 @opindex msvr3-shlib
9413 @opindex mno-svr3-shlib
9414 Control whether GCC places uninitialized local variables into the
9415 @code{bss} or @code{data} segments. @option{-msvr3-shlib} places them
9416 into @code{bss}. These options are meaningful only on System V Release 3.
9420 Use a different function-calling convention, in which functions that
9421 take a fixed number of arguments return with the @code{ret} @var{num}
9422 instruction, which pops their arguments while returning. This saves one
9423 instruction in the caller since there is no need to pop the arguments
9426 You can specify that an individual function is called with this calling
9427 sequence with the function attribute @samp{stdcall}. You can also
9428 override the @option{-mrtd} option by using the function attribute
9429 @samp{cdecl}. @xref{Function Attributes}.
9431 @strong{Warning:} this calling convention is incompatible with the one
9432 normally used on Unix, so you cannot use it if you need to call
9433 libraries compiled with the Unix compiler.
9435 Also, you must provide function prototypes for all functions that
9436 take variable numbers of arguments (including @code{printf});
9437 otherwise incorrect code will be generated for calls to those
9440 In addition, seriously incorrect code will result if you call a
9441 function with too many arguments. (Normally, extra arguments are
9442 harmlessly ignored.)
9444 @item -mregparm=@var{num}
9446 Control how many registers are used to pass integer arguments. By
9447 default, no registers are used to pass arguments, and at most 3
9448 registers can be used. You can control this behavior for a specific
9449 function by using the function attribute @samp{regparm}.
9450 @xref{Function Attributes}.
9452 @strong{Warning:} if you use this switch, and
9453 @var{num} is nonzero, then you must build all modules with the same
9454 value, including any libraries. This includes the system libraries and
9458 @opindex msseregparm
9459 Use SSE register passing conventions for float and double arguments
9460 and return values. You can control this behavior for a specific
9461 function by using the function attribute @samp{sseregparm}.
9462 @xref{Function Attributes}.
9464 @strong{Warning:} if you use this switch then you must build all
9465 modules with the same value, including any libraries. This includes
9466 the system libraries and startup modules.
9468 @item -mstackrealign
9469 @opindex mstackrealign
9470 Realign the stack at entry. On the Intel x86, the
9471 @option{-mstackrealign} option will generate an alternate prologue and
9472 epilogue that realigns the runtime stack. This supports mixing legacy
9473 codes that keep a 4-byte aligned stack with modern codes that keep a
9474 16-byte stack for SSE compatibility. The alternate prologue and
9475 epilogue are slower and bigger than the regular ones, and the
9476 alternate prologue requires an extra scratch register; this lowers the
9477 number of registers available if used in conjunction with the
9478 @code{regparm} attribute. The @option{-mstackrealign} option is
9479 incompatible with the nested function prologue; this is considered a
9480 hard error. See also the attribute @code{force_align_arg_pointer},
9481 applicable to individual functions.
9483 @item -mpreferred-stack-boundary=@var{num}
9484 @opindex mpreferred-stack-boundary
9485 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
9486 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
9487 the default is 4 (16 bytes or 128 bits), except when optimizing for code
9488 size (@option{-Os}), in which case the default is the minimum correct
9489 alignment (4 bytes for x86, and 8 bytes for x86-64).
9491 On Pentium and PentiumPro, @code{double} and @code{long double} values
9492 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
9493 suffer significant run time performance penalties. On Pentium III, the
9494 Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
9495 penalties if it is not 16 byte aligned.
9497 To ensure proper alignment of this values on the stack, the stack boundary
9498 must be as aligned as that required by any value stored on the stack.
9499 Further, every function must be generated such that it keeps the stack
9500 aligned. Thus calling a function compiled with a higher preferred
9501 stack boundary from a function compiled with a lower preferred stack
9502 boundary will most likely misalign the stack. It is recommended that
9503 libraries that use callbacks always use the default setting.
9505 This extra alignment does consume extra stack space, and generally
9506 increases code size. Code that is sensitive to stack space usage, such
9507 as embedded systems and operating system kernels, may want to reduce the
9508 preferred alignment to @option{-mpreferred-stack-boundary=2}.
9526 These switches enable or disable the use of instructions in the MMX,
9527 SSE, SSE2 or 3DNow! extended instruction sets. These extensions are
9528 also available as built-in functions: see @ref{X86 Built-in Functions},
9529 for details of the functions enabled and disabled by these switches.
9531 To have SSE/SSE2 instructions generated automatically from floating-point
9532 code (as opposed to 387 instructions), see @option{-mfpmath=sse}.
9534 These options will enable GCC to use these extended instructions in
9535 generated code, even without @option{-mfpmath=sse}. Applications which
9536 perform runtime CPU detection must compile separate files for each
9537 supported architecture, using the appropriate flags. In particular,
9538 the file containing the CPU detection code should be compiled without
9543 Use special versions of certain libm routines that come with an SSE
9544 ABI and an SSE implementation. Useful together with @option{-mfpmath=sse}
9545 to avoid moving values between SSE registers and the x87 FP stack.
9548 @itemx -mno-push-args
9550 @opindex mno-push-args
9551 Use PUSH operations to store outgoing parameters. This method is shorter
9552 and usually equally fast as method using SUB/MOV operations and is enabled
9553 by default. In some cases disabling it may improve performance because of
9554 improved scheduling and reduced dependencies.
9556 @item -maccumulate-outgoing-args
9557 @opindex maccumulate-outgoing-args
9558 If enabled, the maximum amount of space required for outgoing arguments will be
9559 computed in the function prologue. This is faster on most modern CPUs
9560 because of reduced dependencies, improved scheduling and reduced stack usage
9561 when preferred stack boundary is not equal to 2. The drawback is a notable
9562 increase in code size. This switch implies @option{-mno-push-args}.
9566 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
9567 on thread-safe exception handling must compile and link all code with the
9568 @option{-mthreads} option. When compiling, @option{-mthreads} defines
9569 @option{-D_MT}; when linking, it links in a special thread helper library
9570 @option{-lmingwthrd} which cleans up per thread exception handling data.
9572 @item -mno-align-stringops
9573 @opindex mno-align-stringops
9574 Do not align destination of inlined string operations. This switch reduces
9575 code size and improves performance in case the destination is already aligned,
9576 but GCC doesn't know about it.
9578 @item -minline-all-stringops
9579 @opindex minline-all-stringops
9580 By default GCC inlines string operations only when destination is known to be
9581 aligned at least to 4 byte boundary. This enables more inlining, increase code
9582 size, but may improve performance of code that depends on fast memcpy, strlen
9583 and memset for short lengths.
9585 @item -momit-leaf-frame-pointer
9586 @opindex momit-leaf-frame-pointer
9587 Don't keep the frame pointer in a register for leaf functions. This
9588 avoids the instructions to save, set up and restore frame pointers and
9589 makes an extra register available in leaf functions. The option
9590 @option{-fomit-frame-pointer} removes the frame pointer for all functions
9591 which might make debugging harder.
9593 @item -mtls-direct-seg-refs
9594 @itemx -mno-tls-direct-seg-refs
9595 @opindex mtls-direct-seg-refs
9596 Controls whether TLS variables may be accessed with offsets from the
9597 TLS segment register (@code{%gs} for 32-bit, @code{%fs} for 64-bit),
9598 or whether the thread base pointer must be added. Whether or not this
9599 is legal depends on the operating system, and whether it maps the
9600 segment to cover the entire TLS area.
9602 For systems that use GNU libc, the default is on.
9605 These @samp{-m} switches are supported in addition to the above
9606 on AMD x86-64 processors in 64-bit environments.
9613 Generate code for a 32-bit or 64-bit environment.
9614 The 32-bit environment sets int, long and pointer to 32 bits and
9615 generates code that runs on any i386 system.
9616 The 64-bit environment sets int to 32 bits and long and pointer
9617 to 64 bits and generates code for AMD's x86-64 architecture.
9620 @opindex no-red-zone
9621 Do not use a so called red zone for x86-64 code. The red zone is mandated
9622 by the x86-64 ABI, it is a 128-byte area beyond the location of the
9623 stack pointer that will not be modified by signal or interrupt handlers
9624 and therefore can be used for temporary data without adjusting the stack
9625 pointer. The flag @option{-mno-red-zone} disables this red zone.
9627 @item -mcmodel=small
9628 @opindex mcmodel=small
9629 Generate code for the small code model: the program and its symbols must
9630 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
9631 Programs can be statically or dynamically linked. This is the default
9634 @item -mcmodel=kernel
9635 @opindex mcmodel=kernel
9636 Generate code for the kernel code model. The kernel runs in the
9637 negative 2 GB of the address space.
9638 This model has to be used for Linux kernel code.
9640 @item -mcmodel=medium
9641 @opindex mcmodel=medium
9642 Generate code for the medium model: The program is linked in the lower 2
9643 GB of the address space but symbols can be located anywhere in the
9644 address space. Programs can be statically or dynamically linked, but
9645 building of shared libraries are not supported with the medium model.
9647 @item -mcmodel=large
9648 @opindex mcmodel=large
9649 Generate code for the large model: This model makes no assumptions
9650 about addresses and sizes of sections. Currently GCC does not implement
9655 @subsection IA-64 Options
9656 @cindex IA-64 Options
9658 These are the @samp{-m} options defined for the Intel IA-64 architecture.
9662 @opindex mbig-endian
9663 Generate code for a big endian target. This is the default for HP-UX@.
9665 @item -mlittle-endian
9666 @opindex mlittle-endian
9667 Generate code for a little endian target. This is the default for AIX5
9674 Generate (or don't) code for the GNU assembler. This is the default.
9675 @c Also, this is the default if the configure option @option{--with-gnu-as}
9682 Generate (or don't) code for the GNU linker. This is the default.
9683 @c Also, this is the default if the configure option @option{--with-gnu-ld}
9688 Generate code that does not use a global pointer register. The result
9689 is not position independent code, and violates the IA-64 ABI@.
9691 @item -mvolatile-asm-stop
9692 @itemx -mno-volatile-asm-stop
9693 @opindex mvolatile-asm-stop
9694 @opindex mno-volatile-asm-stop
9695 Generate (or don't) a stop bit immediately before and after volatile asm
9698 @item -mregister-names
9699 @itemx -mno-register-names
9700 @opindex mregister-names
9701 @opindex mno-register-names
9702 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
9703 the stacked registers. This may make assembler output more readable.
9709 Disable (or enable) optimizations that use the small data section. This may
9710 be useful for working around optimizer bugs.
9713 @opindex mconstant-gp
9714 Generate code that uses a single constant global pointer value. This is
9715 useful when compiling kernel code.
9719 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
9720 This is useful when compiling firmware code.
9722 @item -minline-float-divide-min-latency
9723 @opindex minline-float-divide-min-latency
9724 Generate code for inline divides of floating point values
9725 using the minimum latency algorithm.
9727 @item -minline-float-divide-max-throughput
9728 @opindex minline-float-divide-max-throughput
9729 Generate code for inline divides of floating point values
9730 using the maximum throughput algorithm.
9732 @item -minline-int-divide-min-latency
9733 @opindex minline-int-divide-min-latency
9734 Generate code for inline divides of integer values
9735 using the minimum latency algorithm.
9737 @item -minline-int-divide-max-throughput
9738 @opindex minline-int-divide-max-throughput
9739 Generate code for inline divides of integer values
9740 using the maximum throughput algorithm.
9742 @item -minline-sqrt-min-latency
9743 @opindex minline-sqrt-min-latency
9744 Generate code for inline square roots
9745 using the minimum latency algorithm.
9747 @item -minline-sqrt-max-throughput
9748 @opindex minline-sqrt-max-throughput
9749 Generate code for inline square roots
9750 using the maximum throughput algorithm.
9752 @item -mno-dwarf2-asm
9754 @opindex mno-dwarf2-asm
9755 @opindex mdwarf2-asm
9756 Don't (or do) generate assembler code for the DWARF2 line number debugging
9757 info. This may be useful when not using the GNU assembler.
9759 @item -mearly-stop-bits
9760 @itemx -mno-early-stop-bits
9761 @opindex mearly-stop-bits
9762 @opindex mno-early-stop-bits
9763 Allow stop bits to be placed earlier than immediately preceding the
9764 instruction that triggered the stop bit. This can improve instruction
9765 scheduling, but does not always do so.
9767 @item -mfixed-range=@var{register-range}
9768 @opindex mfixed-range
9769 Generate code treating the given register range as fixed registers.
9770 A fixed register is one that the register allocator can not use. This is
9771 useful when compiling kernel code. A register range is specified as
9772 two registers separated by a dash. Multiple register ranges can be
9773 specified separated by a comma.
9775 @item -mtls-size=@var{tls-size}
9777 Specify bit size of immediate TLS offsets. Valid values are 14, 22, and
9780 @item -mtune=@var{cpu-type}
9782 Tune the instruction scheduling for a particular CPU, Valid values are
9783 itanium, itanium1, merced, itanium2, and mckinley.
9789 Add support for multithreading using the POSIX threads library. This
9790 option sets flags for both the preprocessor and linker. It does
9791 not affect the thread safety of object code produced by the compiler or
9792 that of libraries supplied with it. These are HP-UX specific flags.
9798 Generate code for a 32-bit or 64-bit environment.
9799 The 32-bit environment sets int, long and pointer to 32 bits.
9800 The 64-bit environment sets int to 32 bits and long and pointer
9801 to 64 bits. These are HP-UX specific flags.
9803 @item -mno-sched-br-data-spec
9804 @itemx -msched-br-data-spec
9805 @opindex -mno-sched-br-data-spec
9806 @opindex -msched-br-data-spec
9807 (Dis/En)able data speculative scheduling before reload.
9808 This will result in generation of the ld.a instructions and
9809 the corresponding check instructions (ld.c / chk.a).
9810 The default is 'disable'.
9812 @item -msched-ar-data-spec
9813 @itemx -mno-sched-ar-data-spec
9814 @opindex -msched-ar-data-spec
9815 @opindex -mno-sched-ar-data-spec
9816 (En/Dis)able data speculative scheduling after reload.
9817 This will result in generation of the ld.a instructions and
9818 the corresponding check instructions (ld.c / chk.a).
9819 The default is 'enable'.
9821 @item -mno-sched-control-spec
9822 @itemx -msched-control-spec
9823 @opindex -mno-sched-control-spec
9824 @opindex -msched-control-spec
9825 (Dis/En)able control speculative scheduling. This feature is
9826 available only during region scheduling (i.e. before reload).
9827 This will result in generation of the ld.s instructions and
9828 the corresponding check instructions chk.s .
9829 The default is 'disable'.
9831 @item -msched-br-in-data-spec
9832 @itemx -mno-sched-br-in-data-spec
9833 @opindex -msched-br-in-data-spec
9834 @opindex -mno-sched-br-in-data-spec
9835 (En/Dis)able speculative scheduling of the instructions that
9836 are dependent on the data speculative loads before reload.
9837 This is effective only with @option{-msched-br-data-spec} enabled.
9838 The default is 'enable'.
9840 @item -msched-ar-in-data-spec
9841 @itemx -mno-sched-ar-in-data-spec
9842 @opindex -msched-ar-in-data-spec
9843 @opindex -mno-sched-ar-in-data-spec
9844 (En/Dis)able speculative scheduling of the instructions that
9845 are dependent on the data speculative loads after reload.
9846 This is effective only with @option{-msched-ar-data-spec} enabled.
9847 The default is 'enable'.
9849 @item -msched-in-control-spec
9850 @itemx -mno-sched-in-control-spec
9851 @opindex -msched-in-control-spec
9852 @opindex -mno-sched-in-control-spec
9853 (En/Dis)able speculative scheduling of the instructions that
9854 are dependent on the control speculative loads.
9855 This is effective only with @option{-msched-control-spec} enabled.
9856 The default is 'enable'.
9859 @itemx -mno-sched-ldc
9860 @opindex -msched-ldc
9861 @opindex -mno-sched-ldc
9862 (En/Dis)able use of simple data speculation checks ld.c .
9863 If disabled, only chk.a instructions will be emitted to check
9864 data speculative loads.
9865 The default is 'enable'.
9867 @item -mno-sched-control-ldc
9868 @itemx -msched-control-ldc
9869 @opindex -mno-sched-control-ldc
9870 @opindex -msched-control-ldc
9871 (Dis/En)able use of ld.c instructions to check control speculative loads.
9872 If enabled, in case of control speculative load with no speculatively
9873 scheduled dependent instructions this load will be emitted as ld.sa and
9874 ld.c will be used to check it.
9875 The default is 'disable'.
9877 @item -mno-sched-spec-verbose
9878 @itemx -msched-spec-verbose
9879 @opindex -mno-sched-spec-verbose
9880 @opindex -msched-spec-verbose
9881 (Dis/En)able printing of the information about speculative motions.
9883 @item -mno-sched-prefer-non-data-spec-insns
9884 @itemx -msched-prefer-non-data-spec-insns
9885 @opindex -mno-sched-prefer-non-data-spec-insns
9886 @opindex -msched-prefer-non-data-spec-insns
9887 If enabled, data speculative instructions will be chosen for schedule
9888 only if there are no other choices at the moment. This will make
9889 the use of the data speculation much more conservative.
9890 The default is 'disable'.
9892 @item -mno-sched-prefer-non-control-spec-insns
9893 @itemx -msched-prefer-non-control-spec-insns
9894 @opindex -mno-sched-prefer-non-control-spec-insns
9895 @opindex -msched-prefer-non-control-spec-insns
9896 If enabled, control speculative instructions will be chosen for schedule
9897 only if there are no other choices at the moment. This will make
9898 the use of the control speculation much more conservative.
9899 The default is 'disable'.
9901 @item -mno-sched-count-spec-in-critical-path
9902 @itemx -msched-count-spec-in-critical-path
9903 @opindex -mno-sched-count-spec-in-critical-path
9904 @opindex -msched-count-spec-in-critical-path
9905 If enabled, speculative dependencies will be considered during
9906 computation of the instructions priorities. This will make the use of the
9907 speculation a bit more conservative.
9908 The default is 'disable'.
9913 @subsection M32C Options
9914 @cindex M32C options
9917 @item -mcpu=@var{name}
9919 Select the CPU for which code is generated. @var{name} may be one of
9920 @samp{r8c} for the R8C/Tiny series, @samp{m16c} for the M16C (up to
9921 /60) series, @samp{m32cm} for the M16C/80 series, or @samp{m32c} for
9926 Specifies that the program will be run on the simulator. This causes
9927 an alternate runtime library to be linked in which supports, for
9928 example, file I/O. You must not use this option when generating
9929 programs that will run on real hardware; you must provide your own
9930 runtime library for whatever I/O functions are needed.
9932 @item -memregs=@var{number}
9934 Specifies the number of memory-based pseudo-registers GCC will use
9935 during code generation. These pseudo-registers will be used like real
9936 registers, so there is a tradeoff between GCC's ability to fit the
9937 code into available registers, and the performance penalty of using
9938 memory instead of registers. Note that all modules in a program must
9939 be compiled with the same value for this option. Because of that, you
9940 must not use this option with the default runtime libraries gcc
9945 @node M32R/D Options
9946 @subsection M32R/D Options
9947 @cindex M32R/D options
9949 These @option{-m} options are defined for Renesas M32R/D architectures:
9954 Generate code for the M32R/2@.
9958 Generate code for the M32R/X@.
9962 Generate code for the M32R@. This is the default.
9965 @opindex mmodel=small
9966 Assume all objects live in the lower 16MB of memory (so that their addresses
9967 can be loaded with the @code{ld24} instruction), and assume all subroutines
9968 are reachable with the @code{bl} instruction.
9969 This is the default.
9971 The addressability of a particular object can be set with the
9972 @code{model} attribute.
9974 @item -mmodel=medium
9975 @opindex mmodel=medium
9976 Assume objects may be anywhere in the 32-bit address space (the compiler
9977 will generate @code{seth/add3} instructions to load their addresses), and
9978 assume all subroutines are reachable with the @code{bl} instruction.
9981 @opindex mmodel=large
9982 Assume objects may be anywhere in the 32-bit address space (the compiler
9983 will generate @code{seth/add3} instructions to load their addresses), and
9984 assume subroutines may not be reachable with the @code{bl} instruction
9985 (the compiler will generate the much slower @code{seth/add3/jl}
9986 instruction sequence).
9989 @opindex msdata=none
9990 Disable use of the small data area. Variables will be put into
9991 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
9992 @code{section} attribute has been specified).
9993 This is the default.
9995 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
9996 Objects may be explicitly put in the small data area with the
9997 @code{section} attribute using one of these sections.
10000 @opindex msdata=sdata
10001 Put small global and static data in the small data area, but do not
10002 generate special code to reference them.
10005 @opindex msdata=use
10006 Put small global and static data in the small data area, and generate
10007 special instructions to reference them.
10011 @cindex smaller data references
10012 Put global and static objects less than or equal to @var{num} bytes
10013 into the small data or bss sections instead of the normal data or bss
10014 sections. The default value of @var{num} is 8.
10015 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
10016 for this option to have any effect.
10018 All modules should be compiled with the same @option{-G @var{num}} value.
10019 Compiling with different values of @var{num} may or may not work; if it
10020 doesn't the linker will give an error message---incorrect code will not be
10025 Makes the M32R specific code in the compiler display some statistics
10026 that might help in debugging programs.
10028 @item -malign-loops
10029 @opindex malign-loops
10030 Align all loops to a 32-byte boundary.
10032 @item -mno-align-loops
10033 @opindex mno-align-loops
10034 Do not enforce a 32-byte alignment for loops. This is the default.
10036 @item -missue-rate=@var{number}
10037 @opindex missue-rate=@var{number}
10038 Issue @var{number} instructions per cycle. @var{number} can only be 1
10041 @item -mbranch-cost=@var{number}
10042 @opindex mbranch-cost=@var{number}
10043 @var{number} can only be 1 or 2. If it is 1 then branches will be
10044 preferred over conditional code, if it is 2, then the opposite will
10047 @item -mflush-trap=@var{number}
10048 @opindex mflush-trap=@var{number}
10049 Specifies the trap number to use to flush the cache. The default is
10050 12. Valid numbers are between 0 and 15 inclusive.
10052 @item -mno-flush-trap
10053 @opindex mno-flush-trap
10054 Specifies that the cache cannot be flushed by using a trap.
10056 @item -mflush-func=@var{name}
10057 @opindex mflush-func=@var{name}
10058 Specifies the name of the operating system function to call to flush
10059 the cache. The default is @emph{_flush_cache}, but a function call
10060 will only be used if a trap is not available.
10062 @item -mno-flush-func
10063 @opindex mno-flush-func
10064 Indicates that there is no OS function for flushing the cache.
10068 @node M680x0 Options
10069 @subsection M680x0 Options
10070 @cindex M680x0 options
10072 These are the @samp{-m} options defined for the 68000 series. The default
10073 values for these options depends on which style of 68000 was selected when
10074 the compiler was configured; the defaults for the most common choices are
10082 Generate output for a 68000. This is the default
10083 when the compiler is configured for 68000-based systems.
10085 Use this option for microcontrollers with a 68000 or EC000 core,
10086 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
10092 Generate output for a 68020. This is the default
10093 when the compiler is configured for 68020-based systems.
10097 Generate output containing 68881 instructions for floating point.
10098 This is the default for most 68020 systems unless @option{--nfp} was
10099 specified when the compiler was configured.
10103 Generate output for a 68030. This is the default when the compiler is
10104 configured for 68030-based systems.
10108 Generate output for a 68040. This is the default when the compiler is
10109 configured for 68040-based systems.
10111 This option inhibits the use of 68881/68882 instructions that have to be
10112 emulated by software on the 68040. Use this option if your 68040 does not
10113 have code to emulate those instructions.
10117 Generate output for a 68060. This is the default when the compiler is
10118 configured for 68060-based systems.
10120 This option inhibits the use of 68020 and 68881/68882 instructions that
10121 have to be emulated by software on the 68060. Use this option if your 68060
10122 does not have code to emulate those instructions.
10126 Generate output for a CPU32. This is the default
10127 when the compiler is configured for CPU32-based systems.
10129 Use this option for microcontrollers with a
10130 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
10131 68336, 68340, 68341, 68349 and 68360.
10135 Generate output for a 520X ``coldfire'' family cpu. This is the default
10136 when the compiler is configured for 520X-based systems.
10138 Use this option for microcontroller with a 5200 core, including
10139 the MCF5202, MCF5203, MCF5204 and MCF5202.
10143 Generate output for a ColdFire V4e family cpu (e.g.@: 547x/548x).
10144 This includes use of hardware floating point instructions.
10148 Generate output for a 68040, without using any of the new instructions.
10149 This results in code which can run relatively efficiently on either a
10150 68020/68881 or a 68030 or a 68040. The generated code does use the
10151 68881 instructions that are emulated on the 68040.
10155 Generate output for a 68060, without using any of the new instructions.
10156 This results in code which can run relatively efficiently on either a
10157 68020/68881 or a 68030 or a 68040. The generated code does use the
10158 68881 instructions that are emulated on the 68060.
10161 @opindex msoft-float
10162 Generate output containing library calls for floating point.
10163 @strong{Warning:} the requisite libraries are not available for all m68k
10164 targets. Normally the facilities of the machine's usual C compiler are
10165 used, but this can't be done directly in cross-compilation. You must
10166 make your own arrangements to provide suitable library functions for
10167 cross-compilation. The embedded targets @samp{m68k-*-aout} and
10168 @samp{m68k-*-coff} do provide software floating point support.
10172 Consider type @code{int} to be 16 bits wide, like @code{short int}.
10173 Additionally, parameters passed on the stack are also aligned to a
10174 16-bit boundary even on targets whose API mandates promotion to 32-bit.
10177 @opindex mnobitfield
10178 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
10179 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
10183 Do use the bit-field instructions. The @option{-m68020} option implies
10184 @option{-mbitfield}. This is the default if you use a configuration
10185 designed for a 68020.
10189 Use a different function-calling convention, in which functions
10190 that take a fixed number of arguments return with the @code{rtd}
10191 instruction, which pops their arguments while returning. This
10192 saves one instruction in the caller since there is no need to pop
10193 the arguments there.
10195 This calling convention is incompatible with the one normally
10196 used on Unix, so you cannot use it if you need to call libraries
10197 compiled with the Unix compiler.
10199 Also, you must provide function prototypes for all functions that
10200 take variable numbers of arguments (including @code{printf});
10201 otherwise incorrect code will be generated for calls to those
10204 In addition, seriously incorrect code will result if you call a
10205 function with too many arguments. (Normally, extra arguments are
10206 harmlessly ignored.)
10208 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
10209 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
10212 @itemx -mno-align-int
10213 @opindex malign-int
10214 @opindex mno-align-int
10215 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
10216 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
10217 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
10218 Aligning variables on 32-bit boundaries produces code that runs somewhat
10219 faster on processors with 32-bit busses at the expense of more memory.
10221 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
10222 align structures containing the above types differently than
10223 most published application binary interface specifications for the m68k.
10227 Use the pc-relative addressing mode of the 68000 directly, instead of
10228 using a global offset table. At present, this option implies @option{-fpic},
10229 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
10230 not presently supported with @option{-mpcrel}, though this could be supported for
10231 68020 and higher processors.
10233 @item -mno-strict-align
10234 @itemx -mstrict-align
10235 @opindex mno-strict-align
10236 @opindex mstrict-align
10237 Do not (do) assume that unaligned memory references will be handled by
10241 Generate code that allows the data segment to be located in a different
10242 area of memory from the text segment. This allows for execute in place in
10243 an environment without virtual memory management. This option implies
10246 @item -mno-sep-data
10247 Generate code that assumes that the data segment follows the text segment.
10248 This is the default.
10250 @item -mid-shared-library
10251 Generate code that supports shared libraries via the library ID method.
10252 This allows for execute in place and shared libraries in an environment
10253 without virtual memory management. This option implies @option{-fPIC}.
10255 @item -mno-id-shared-library
10256 Generate code that doesn't assume ID based shared libraries are being used.
10257 This is the default.
10259 @item -mshared-library-id=n
10260 Specified the identification number of the ID based shared library being
10261 compiled. Specifying a value of 0 will generate more compact code, specifying
10262 other values will force the allocation of that number to the current
10263 library but is no more space or time efficient than omitting this option.
10267 @node M68hc1x Options
10268 @subsection M68hc1x Options
10269 @cindex M68hc1x options
10271 These are the @samp{-m} options defined for the 68hc11 and 68hc12
10272 microcontrollers. The default values for these options depends on
10273 which style of microcontroller was selected when the compiler was configured;
10274 the defaults for the most common choices are given below.
10281 Generate output for a 68HC11. This is the default
10282 when the compiler is configured for 68HC11-based systems.
10288 Generate output for a 68HC12. This is the default
10289 when the compiler is configured for 68HC12-based systems.
10295 Generate output for a 68HCS12.
10297 @item -mauto-incdec
10298 @opindex mauto-incdec
10299 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
10306 Enable the use of 68HC12 min and max instructions.
10309 @itemx -mno-long-calls
10310 @opindex mlong-calls
10311 @opindex mno-long-calls
10312 Treat all calls as being far away (near). If calls are assumed to be
10313 far away, the compiler will use the @code{call} instruction to
10314 call a function and the @code{rtc} instruction for returning.
10318 Consider type @code{int} to be 16 bits wide, like @code{short int}.
10320 @item -msoft-reg-count=@var{count}
10321 @opindex msoft-reg-count
10322 Specify the number of pseudo-soft registers which are used for the
10323 code generation. The maximum number is 32. Using more pseudo-soft
10324 register may or may not result in better code depending on the program.
10325 The default is 4 for 68HC11 and 2 for 68HC12.
10329 @node MCore Options
10330 @subsection MCore Options
10331 @cindex MCore options
10333 These are the @samp{-m} options defined for the Motorola M*Core
10339 @itemx -mno-hardlit
10341 @opindex mno-hardlit
10342 Inline constants into the code stream if it can be done in two
10343 instructions or less.
10349 Use the divide instruction. (Enabled by default).
10351 @item -mrelax-immediate
10352 @itemx -mno-relax-immediate
10353 @opindex mrelax-immediate
10354 @opindex mno-relax-immediate
10355 Allow arbitrary sized immediates in bit operations.
10357 @item -mwide-bitfields
10358 @itemx -mno-wide-bitfields
10359 @opindex mwide-bitfields
10360 @opindex mno-wide-bitfields
10361 Always treat bit-fields as int-sized.
10363 @item -m4byte-functions
10364 @itemx -mno-4byte-functions
10365 @opindex m4byte-functions
10366 @opindex mno-4byte-functions
10367 Force all functions to be aligned to a four byte boundary.
10369 @item -mcallgraph-data
10370 @itemx -mno-callgraph-data
10371 @opindex mcallgraph-data
10372 @opindex mno-callgraph-data
10373 Emit callgraph information.
10376 @itemx -mno-slow-bytes
10377 @opindex mslow-bytes
10378 @opindex mno-slow-bytes
10379 Prefer word access when reading byte quantities.
10381 @item -mlittle-endian
10382 @itemx -mbig-endian
10383 @opindex mlittle-endian
10384 @opindex mbig-endian
10385 Generate code for a little endian target.
10391 Generate code for the 210 processor.
10395 @subsection MIPS Options
10396 @cindex MIPS options
10402 Generate big-endian code.
10406 Generate little-endian code. This is the default for @samp{mips*el-*-*}
10409 @item -march=@var{arch}
10411 Generate code that will run on @var{arch}, which can be the name of a
10412 generic MIPS ISA, or the name of a particular processor.
10414 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
10415 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
10416 The processor names are:
10417 @samp{4kc}, @samp{4km}, @samp{4kp},
10418 @samp{5kc}, @samp{5kf},
10420 @samp{24k}, @samp{24kc}, @samp{24kf}, @samp{24kx},
10423 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
10424 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000},
10425 @samp{rm7000}, @samp{rm9000},
10428 @samp{vr4100}, @samp{vr4111}, @samp{vr4120}, @samp{vr4130}, @samp{vr4300},
10429 @samp{vr5000}, @samp{vr5400} and @samp{vr5500}.
10430 The special value @samp{from-abi} selects the
10431 most compatible architecture for the selected ABI (that is,
10432 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
10434 In processor names, a final @samp{000} can be abbreviated as @samp{k}
10435 (for example, @samp{-march=r2k}). Prefixes are optional, and
10436 @samp{vr} may be written @samp{r}.
10438 GCC defines two macros based on the value of this option. The first
10439 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
10440 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
10441 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
10442 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
10443 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
10445 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
10446 above. In other words, it will have the full prefix and will not
10447 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
10448 the macro names the resolved architecture (either @samp{"mips1"} or
10449 @samp{"mips3"}). It names the default architecture when no
10450 @option{-march} option is given.
10452 @item -mtune=@var{arch}
10454 Optimize for @var{arch}. Among other things, this option controls
10455 the way instructions are scheduled, and the perceived cost of arithmetic
10456 operations. The list of @var{arch} values is the same as for
10459 When this option is not used, GCC will optimize for the processor
10460 specified by @option{-march}. By using @option{-march} and
10461 @option{-mtune} together, it is possible to generate code that will
10462 run on a family of processors, but optimize the code for one
10463 particular member of that family.
10465 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
10466 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
10467 @samp{-march} ones described above.
10471 Equivalent to @samp{-march=mips1}.
10475 Equivalent to @samp{-march=mips2}.
10479 Equivalent to @samp{-march=mips3}.
10483 Equivalent to @samp{-march=mips4}.
10487 Equivalent to @samp{-march=mips32}.
10491 Equivalent to @samp{-march=mips32r2}.
10495 Equivalent to @samp{-march=mips64}.
10500 @opindex mno-mips16
10501 Generate (do not generate) MIPS16 code. If GCC is targetting a
10502 MIPS32 or MIPS64 architecture, it will make use of the MIPS16e ASE@.
10514 Generate code for the given ABI@.
10516 Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
10517 generates 64-bit code when you select a 64-bit architecture, but you
10518 can use @option{-mgp32} to get 32-bit code instead.
10520 For information about the O64 ABI, see
10521 @w{@uref{http://gcc.gnu.org/projects/mipso64-abi.html}}.
10524 @itemx -mno-abicalls
10526 @opindex mno-abicalls
10527 Generate (do not generate) code that is suitable for SVR4-style
10528 dynamic objects. @option{-mabicalls} is the default for SVR4-based
10533 Generate (do not generate) code that is fully position-independent,
10534 and that can therefore be linked into shared libraries. This option
10535 only affects @option{-mabicalls}.
10537 All @option{-mabicalls} code has traditionally been position-independent,
10538 regardless of options like @option{-fPIC} and @option{-fpic}. However,
10539 as an extension, the GNU toolchain allows executables to use absolute
10540 accesses for locally-binding symbols. It can also use shorter GP
10541 initialization sequences and generate direct calls to locally-defined
10542 functions. This mode is selected by @option{-mno-shared}.
10544 @option{-mno-shared} depends on binutils 2.16 or higher and generates
10545 objects that can only be linked by the GNU linker. However, the option
10546 does not affect the ABI of the final executable; it only affects the ABI
10547 of relocatable objects. Using @option{-mno-shared} will generally make
10548 executables both smaller and quicker.
10550 @option{-mshared} is the default.
10556 Lift (do not lift) the usual restrictions on the size of the global
10559 GCC normally uses a single instruction to load values from the GOT@.
10560 While this is relatively efficient, it will only work if the GOT
10561 is smaller than about 64k. Anything larger will cause the linker
10562 to report an error such as:
10564 @cindex relocation truncated to fit (MIPS)
10566 relocation truncated to fit: R_MIPS_GOT16 foobar
10569 If this happens, you should recompile your code with @option{-mxgot}.
10570 It should then work with very large GOTs, although it will also be
10571 less efficient, since it will take three instructions to fetch the
10572 value of a global symbol.
10574 Note that some linkers can create multiple GOTs. If you have such a
10575 linker, you should only need to use @option{-mxgot} when a single object
10576 file accesses more than 64k's worth of GOT entries. Very few do.
10578 These options have no effect unless GCC is generating position
10583 Assume that general-purpose registers are 32 bits wide.
10587 Assume that general-purpose registers are 64 bits wide.
10591 Assume that floating-point registers are 32 bits wide.
10595 Assume that floating-point registers are 64 bits wide.
10598 @opindex mhard-float
10599 Use floating-point coprocessor instructions.
10602 @opindex msoft-float
10603 Do not use floating-point coprocessor instructions. Implement
10604 floating-point calculations using library calls instead.
10606 @item -msingle-float
10607 @opindex msingle-float
10608 Assume that the floating-point coprocessor only supports single-precision
10611 @itemx -mdouble-float
10612 @opindex mdouble-float
10613 Assume that the floating-point coprocessor supports double-precision
10614 operations. This is the default.
10620 Use (do not use) the MIPS DSP ASE. @xref{MIPS DSP Built-in Functions}.
10622 @itemx -mpaired-single
10623 @itemx -mno-paired-single
10624 @opindex mpaired-single
10625 @opindex mno-paired-single
10626 Use (do not use) paired-single floating-point instructions.
10627 @xref{MIPS Paired-Single Support}. This option can only be used
10628 when generating 64-bit code and requires hardware floating-point
10629 support to be enabled.
10634 @opindex mno-mips3d
10635 Use (do not use) the MIPS-3D ASE@. @xref{MIPS-3D Built-in Functions}.
10636 The option @option{-mips3d} implies @option{-mpaired-single}.
10640 Force @code{long} types to be 64 bits wide. See @option{-mlong32} for
10641 an explanation of the default and the way that the pointer size is
10646 Force @code{long}, @code{int}, and pointer types to be 32 bits wide.
10648 The default size of @code{int}s, @code{long}s and pointers depends on
10649 the ABI@. All the supported ABIs use 32-bit @code{int}s. The n64 ABI
10650 uses 64-bit @code{long}s, as does the 64-bit EABI; the others use
10651 32-bit @code{long}s. Pointers are the same size as @code{long}s,
10652 or the same size as integer registers, whichever is smaller.
10658 Assume (do not assume) that all symbols have 32-bit values, regardless
10659 of the selected ABI@. This option is useful in combination with
10660 @option{-mabi=64} and @option{-mno-abicalls} because it allows GCC
10661 to generate shorter and faster references to symbolic addresses.
10665 @cindex smaller data references (MIPS)
10666 @cindex gp-relative references (MIPS)
10667 Put global and static items less than or equal to @var{num} bytes into
10668 the small data or bss section instead of the normal data or bss section.
10669 This allows the data to be accessed using a single instruction.
10671 All modules should be compiled with the same @option{-G @var{num}}
10674 @item -membedded-data
10675 @itemx -mno-embedded-data
10676 @opindex membedded-data
10677 @opindex mno-embedded-data
10678 Allocate variables to the read-only data section first if possible, then
10679 next in the small data section if possible, otherwise in data. This gives
10680 slightly slower code than the default, but reduces the amount of RAM required
10681 when executing, and thus may be preferred for some embedded systems.
10683 @item -muninit-const-in-rodata
10684 @itemx -mno-uninit-const-in-rodata
10685 @opindex muninit-const-in-rodata
10686 @opindex mno-uninit-const-in-rodata
10687 Put uninitialized @code{const} variables in the read-only data section.
10688 This option is only meaningful in conjunction with @option{-membedded-data}.
10690 @item -msplit-addresses
10691 @itemx -mno-split-addresses
10692 @opindex msplit-addresses
10693 @opindex mno-split-addresses
10694 Enable (disable) use of the @code{%hi()} and @code{%lo()} assembler
10695 relocation operators. This option has been superseded by
10696 @option{-mexplicit-relocs} but is retained for backwards compatibility.
10698 @item -mexplicit-relocs
10699 @itemx -mno-explicit-relocs
10700 @opindex mexplicit-relocs
10701 @opindex mno-explicit-relocs
10702 Use (do not use) assembler relocation operators when dealing with symbolic
10703 addresses. The alternative, selected by @option{-mno-explicit-relocs},
10704 is to use assembler macros instead.
10706 @option{-mexplicit-relocs} is the default if GCC was configured
10707 to use an assembler that supports relocation operators.
10709 @item -mcheck-zero-division
10710 @itemx -mno-check-zero-division
10711 @opindex mcheck-zero-division
10712 @opindex mno-check-zero-division
10713 Trap (do not trap) on integer division by zero. The default is
10714 @option{-mcheck-zero-division}.
10716 @item -mdivide-traps
10717 @itemx -mdivide-breaks
10718 @opindex mdivide-traps
10719 @opindex mdivide-breaks
10720 MIPS systems check for division by zero by generating either a
10721 conditional trap or a break instruction. Using traps results in
10722 smaller code, but is only supported on MIPS II and later. Also, some
10723 versions of the Linux kernel have a bug that prevents trap from
10724 generating the proper signal (@code{SIGFPE}). Use @option{-mdivide-traps} to
10725 allow conditional traps on architectures that support them and
10726 @option{-mdivide-breaks} to force the use of breaks.
10728 The default is usually @option{-mdivide-traps}, but this can be
10729 overridden at configure time using @option{--with-divide=breaks}.
10730 Divide-by-zero checks can be completely disabled using
10731 @option{-mno-check-zero-division}.
10736 @opindex mno-memcpy
10737 Force (do not force) the use of @code{memcpy()} for non-trivial block
10738 moves. The default is @option{-mno-memcpy}, which allows GCC to inline
10739 most constant-sized copies.
10742 @itemx -mno-long-calls
10743 @opindex mlong-calls
10744 @opindex mno-long-calls
10745 Disable (do not disable) use of the @code{jal} instruction. Calling
10746 functions using @code{jal} is more efficient but requires the caller
10747 and callee to be in the same 256 megabyte segment.
10749 This option has no effect on abicalls code. The default is
10750 @option{-mno-long-calls}.
10756 Enable (disable) use of the @code{mad}, @code{madu} and @code{mul}
10757 instructions, as provided by the R4650 ISA@.
10760 @itemx -mno-fused-madd
10761 @opindex mfused-madd
10762 @opindex mno-fused-madd
10763 Enable (disable) use of the floating point multiply-accumulate
10764 instructions, when they are available. The default is
10765 @option{-mfused-madd}.
10767 When multiply-accumulate instructions are used, the intermediate
10768 product is calculated to infinite precision and is not subject to
10769 the FCSR Flush to Zero bit. This may be undesirable in some
10774 Tell the MIPS assembler to not run its preprocessor over user
10775 assembler files (with a @samp{.s} suffix) when assembling them.
10778 @itemx -mno-fix-r4000
10779 @opindex mfix-r4000
10780 @opindex mno-fix-r4000
10781 Work around certain R4000 CPU errata:
10784 A double-word or a variable shift may give an incorrect result if executed
10785 immediately after starting an integer division.
10787 A double-word or a variable shift may give an incorrect result if executed
10788 while an integer multiplication is in progress.
10790 An integer division may give an incorrect result if started in a delay slot
10791 of a taken branch or a jump.
10795 @itemx -mno-fix-r4400
10796 @opindex mfix-r4400
10797 @opindex mno-fix-r4400
10798 Work around certain R4400 CPU errata:
10801 A double-word or a variable shift may give an incorrect result if executed
10802 immediately after starting an integer division.
10806 @itemx -mno-fix-vr4120
10807 @opindex mfix-vr4120
10808 Work around certain VR4120 errata:
10811 @code{dmultu} does not always produce the correct result.
10813 @code{div} and @code{ddiv} do not always produce the correct result if one
10814 of the operands is negative.
10816 The workarounds for the division errata rely on special functions in
10817 @file{libgcc.a}. At present, these functions are only provided by
10818 the @code{mips64vr*-elf} configurations.
10820 Other VR4120 errata require a nop to be inserted between certain pairs of
10821 instructions. These errata are handled by the assembler, not by GCC itself.
10824 @opindex mfix-vr4130
10825 Work around the VR4130 @code{mflo}/@code{mfhi} errata. The
10826 workarounds are implemented by the assembler rather than by GCC,
10827 although GCC will avoid using @code{mflo} and @code{mfhi} if the
10828 VR4130 @code{macc}, @code{macchi}, @code{dmacc} and @code{dmacchi}
10829 instructions are available instead.
10832 @itemx -mno-fix-sb1
10834 Work around certain SB-1 CPU core errata.
10835 (This flag currently works around the SB-1 revision 2
10836 ``F1'' and ``F2'' floating point errata.)
10838 @item -mflush-func=@var{func}
10839 @itemx -mno-flush-func
10840 @opindex mflush-func
10841 Specifies the function to call to flush the I and D caches, or to not
10842 call any such function. If called, the function must take the same
10843 arguments as the common @code{_flush_func()}, that is, the address of the
10844 memory range for which the cache is being flushed, the size of the
10845 memory range, and the number 3 (to flush both caches). The default
10846 depends on the target GCC was configured for, but commonly is either
10847 @samp{_flush_func} or @samp{__cpu_flush}.
10849 @item -mbranch-likely
10850 @itemx -mno-branch-likely
10851 @opindex mbranch-likely
10852 @opindex mno-branch-likely
10853 Enable or disable use of Branch Likely instructions, regardless of the
10854 default for the selected architecture. By default, Branch Likely
10855 instructions may be generated if they are supported by the selected
10856 architecture. An exception is for the MIPS32 and MIPS64 architectures
10857 and processors which implement those architectures; for those, Branch
10858 Likely instructions will not be generated by default because the MIPS32
10859 and MIPS64 architectures specifically deprecate their use.
10861 @item -mfp-exceptions
10862 @itemx -mno-fp-exceptions
10863 @opindex mfp-exceptions
10864 Specifies whether FP exceptions are enabled. This affects how we schedule
10865 FP instructions for some processors. The default is that FP exceptions are
10868 For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
10869 64-bit code, then we can use both FP pipes. Otherwise, we can only use one
10872 @item -mvr4130-align
10873 @itemx -mno-vr4130-align
10874 @opindex mvr4130-align
10875 The VR4130 pipeline is two-way superscalar, but can only issue two
10876 instructions together if the first one is 8-byte aligned. When this
10877 option is enabled, GCC will align pairs of instructions that it
10878 thinks should execute in parallel.
10880 This option only has an effect when optimizing for the VR4130.
10881 It normally makes code faster, but at the expense of making it bigger.
10882 It is enabled by default at optimization level @option{-O3}.
10886 @subsection MMIX Options
10887 @cindex MMIX Options
10889 These options are defined for the MMIX:
10893 @itemx -mno-libfuncs
10895 @opindex mno-libfuncs
10896 Specify that intrinsic library functions are being compiled, passing all
10897 values in registers, no matter the size.
10900 @itemx -mno-epsilon
10902 @opindex mno-epsilon
10903 Generate floating-point comparison instructions that compare with respect
10904 to the @code{rE} epsilon register.
10906 @item -mabi=mmixware
10908 @opindex mabi-mmixware
10910 Generate code that passes function parameters and return values that (in
10911 the called function) are seen as registers @code{$0} and up, as opposed to
10912 the GNU ABI which uses global registers @code{$231} and up.
10914 @item -mzero-extend
10915 @itemx -mno-zero-extend
10916 @opindex mzero-extend
10917 @opindex mno-zero-extend
10918 When reading data from memory in sizes shorter than 64 bits, use (do not
10919 use) zero-extending load instructions by default, rather than
10920 sign-extending ones.
10923 @itemx -mno-knuthdiv
10925 @opindex mno-knuthdiv
10926 Make the result of a division yielding a remainder have the same sign as
10927 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
10928 remainder follows the sign of the dividend. Both methods are
10929 arithmetically valid, the latter being almost exclusively used.
10931 @item -mtoplevel-symbols
10932 @itemx -mno-toplevel-symbols
10933 @opindex mtoplevel-symbols
10934 @opindex mno-toplevel-symbols
10935 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
10936 code can be used with the @code{PREFIX} assembly directive.
10940 Generate an executable in the ELF format, rather than the default
10941 @samp{mmo} format used by the @command{mmix} simulator.
10943 @item -mbranch-predict
10944 @itemx -mno-branch-predict
10945 @opindex mbranch-predict
10946 @opindex mno-branch-predict
10947 Use (do not use) the probable-branch instructions, when static branch
10948 prediction indicates a probable branch.
10950 @item -mbase-addresses
10951 @itemx -mno-base-addresses
10952 @opindex mbase-addresses
10953 @opindex mno-base-addresses
10954 Generate (do not generate) code that uses @emph{base addresses}. Using a
10955 base address automatically generates a request (handled by the assembler
10956 and the linker) for a constant to be set up in a global register. The
10957 register is used for one or more base address requests within the range 0
10958 to 255 from the value held in the register. The generally leads to short
10959 and fast code, but the number of different data items that can be
10960 addressed is limited. This means that a program that uses lots of static
10961 data may require @option{-mno-base-addresses}.
10963 @item -msingle-exit
10964 @itemx -mno-single-exit
10965 @opindex msingle-exit
10966 @opindex mno-single-exit
10967 Force (do not force) generated code to have a single exit point in each
10971 @node MN10300 Options
10972 @subsection MN10300 Options
10973 @cindex MN10300 options
10975 These @option{-m} options are defined for Matsushita MN10300 architectures:
10980 Generate code to avoid bugs in the multiply instructions for the MN10300
10981 processors. This is the default.
10983 @item -mno-mult-bug
10984 @opindex mno-mult-bug
10985 Do not generate code to avoid bugs in the multiply instructions for the
10986 MN10300 processors.
10990 Generate code which uses features specific to the AM33 processor.
10994 Do not generate code which uses features specific to the AM33 processor. This
10997 @item -mreturn-pointer-on-d0
10998 @opindex mreturn-pointer-on-d0
10999 When generating a function which returns a pointer, return the pointer
11000 in both @code{a0} and @code{d0}. Otherwise, the pointer is returned
11001 only in a0, and attempts to call such functions without a prototype
11002 would result in errors. Note that this option is on by default; use
11003 @option{-mno-return-pointer-on-d0} to disable it.
11007 Do not link in the C run-time initialization object file.
11011 Indicate to the linker that it should perform a relaxation optimization pass
11012 to shorten branches, calls and absolute memory addresses. This option only
11013 has an effect when used on the command line for the final link step.
11015 This option makes symbolic debugging impossible.
11019 @subsection MT Options
11022 These @option{-m} options are defined for Morpho MT architectures:
11026 @item -march=@var{cpu-type}
11028 Generate code that will run on @var{cpu-type}, which is the name of a system
11029 representing a certain processor type. Possible values for
11030 @var{cpu-type} are @samp{ms1-64-001}, @samp{ms1-16-002},
11031 @samp{ms1-16-003} and @samp{ms2}.
11033 When this option is not used, the default is @option{-march=ms1-16-002}.
11037 Use byte loads and stores when generating code.
11041 Do not use byte loads and stores when generating code.
11045 Use simulator runtime
11049 Do not link in the C run-time initialization object file
11050 @file{crti.o}. Other run-time initialization and termination files
11051 such as @file{startup.o} and @file{exit.o} are still included on the
11052 linker command line.
11056 @node PDP-11 Options
11057 @subsection PDP-11 Options
11058 @cindex PDP-11 Options
11060 These options are defined for the PDP-11:
11065 Use hardware FPP floating point. This is the default. (FIS floating
11066 point on the PDP-11/40 is not supported.)
11069 @opindex msoft-float
11070 Do not use hardware floating point.
11074 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
11078 Return floating-point results in memory. This is the default.
11082 Generate code for a PDP-11/40.
11086 Generate code for a PDP-11/45. This is the default.
11090 Generate code for a PDP-11/10.
11092 @item -mbcopy-builtin
11093 @opindex bcopy-builtin
11094 Use inline @code{movmemhi} patterns for copying memory. This is the
11099 Do not use inline @code{movmemhi} patterns for copying memory.
11105 Use 16-bit @code{int}. This is the default.
11111 Use 32-bit @code{int}.
11114 @itemx -mno-float32
11116 @opindex mno-float32
11117 Use 64-bit @code{float}. This is the default.
11120 @itemx -mno-float64
11122 @opindex mno-float64
11123 Use 32-bit @code{float}.
11127 Use @code{abshi2} pattern. This is the default.
11131 Do not use @code{abshi2} pattern.
11133 @item -mbranch-expensive
11134 @opindex mbranch-expensive
11135 Pretend that branches are expensive. This is for experimenting with
11136 code generation only.
11138 @item -mbranch-cheap
11139 @opindex mbranch-cheap
11140 Do not pretend that branches are expensive. This is the default.
11144 Generate code for a system with split I&D@.
11148 Generate code for a system without split I&D@. This is the default.
11152 Use Unix assembler syntax. This is the default when configured for
11153 @samp{pdp11-*-bsd}.
11157 Use DEC assembler syntax. This is the default when configured for any
11158 PDP-11 target other than @samp{pdp11-*-bsd}.
11161 @node PowerPC Options
11162 @subsection PowerPC Options
11163 @cindex PowerPC options
11165 These are listed under @xref{RS/6000 and PowerPC Options}.
11167 @node RS/6000 and PowerPC Options
11168 @subsection IBM RS/6000 and PowerPC Options
11169 @cindex RS/6000 and PowerPC Options
11170 @cindex IBM RS/6000 and PowerPC Options
11172 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
11179 @itemx -mno-powerpc
11180 @itemx -mpowerpc-gpopt
11181 @itemx -mno-powerpc-gpopt
11182 @itemx -mpowerpc-gfxopt
11183 @itemx -mno-powerpc-gfxopt
11185 @itemx -mno-powerpc64
11189 @itemx -mno-popcntb
11195 @opindex mno-power2
11197 @opindex mno-powerpc
11198 @opindex mpowerpc-gpopt
11199 @opindex mno-powerpc-gpopt
11200 @opindex mpowerpc-gfxopt
11201 @opindex mno-powerpc-gfxopt
11202 @opindex mpowerpc64
11203 @opindex mno-powerpc64
11207 @opindex mno-popcntb
11210 GCC supports two related instruction set architectures for the
11211 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
11212 instructions supported by the @samp{rios} chip set used in the original
11213 RS/6000 systems and the @dfn{PowerPC} instruction set is the
11214 architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and
11215 the IBM 4xx, 6xx, and follow-on microprocessors.
11217 Neither architecture is a subset of the other. However there is a
11218 large common subset of instructions supported by both. An MQ
11219 register is included in processors supporting the POWER architecture.
11221 You use these options to specify which instructions are available on the
11222 processor you are using. The default value of these options is
11223 determined when configuring GCC@. Specifying the
11224 @option{-mcpu=@var{cpu_type}} overrides the specification of these
11225 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
11226 rather than the options listed above.
11228 The @option{-mpower} option allows GCC to generate instructions that
11229 are found only in the POWER architecture and to use the MQ register.
11230 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
11231 to generate instructions that are present in the POWER2 architecture but
11232 not the original POWER architecture.
11234 The @option{-mpowerpc} option allows GCC to generate instructions that
11235 are found only in the 32-bit subset of the PowerPC architecture.
11236 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
11237 GCC to use the optional PowerPC architecture instructions in the
11238 General Purpose group, including floating-point square root. Specifying
11239 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
11240 use the optional PowerPC architecture instructions in the Graphics
11241 group, including floating-point select.
11243 The @option{-mmfcrf} option allows GCC to generate the move from
11244 condition register field instruction implemented on the POWER4
11245 processor and other processors that support the PowerPC V2.01
11247 The @option{-mpopcntb} option allows GCC to generate the popcount and
11248 double precision FP reciprocal estimate instruction implemented on the
11249 POWER5 processor and other processors that support the PowerPC V2.02
11251 The @option{-mfprnd} option allows GCC to generate the FP round to
11252 integer instructions implemented on the POWER5+ processor and other
11253 processors that support the PowerPC V2.03 architecture.
11255 The @option{-mpowerpc64} option allows GCC to generate the additional
11256 64-bit instructions that are found in the full PowerPC64 architecture
11257 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
11258 @option{-mno-powerpc64}.
11260 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
11261 will use only the instructions in the common subset of both
11262 architectures plus some special AIX common-mode calls, and will not use
11263 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
11264 permits GCC to use any instruction from either architecture and to
11265 allow use of the MQ register; specify this for the Motorola MPC601.
11267 @item -mnew-mnemonics
11268 @itemx -mold-mnemonics
11269 @opindex mnew-mnemonics
11270 @opindex mold-mnemonics
11271 Select which mnemonics to use in the generated assembler code. With
11272 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
11273 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
11274 assembler mnemonics defined for the POWER architecture. Instructions
11275 defined in only one architecture have only one mnemonic; GCC uses that
11276 mnemonic irrespective of which of these options is specified.
11278 GCC defaults to the mnemonics appropriate for the architecture in
11279 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
11280 value of these option. Unless you are building a cross-compiler, you
11281 should normally not specify either @option{-mnew-mnemonics} or
11282 @option{-mold-mnemonics}, but should instead accept the default.
11284 @item -mcpu=@var{cpu_type}
11286 Set architecture type, register usage, choice of mnemonics, and
11287 instruction scheduling parameters for machine type @var{cpu_type}.
11288 Supported values for @var{cpu_type} are @samp{401}, @samp{403},
11289 @samp{405}, @samp{405fp}, @samp{440}, @samp{440fp}, @samp{505},
11290 @samp{601}, @samp{602}, @samp{603}, @samp{603e}, @samp{604},
11291 @samp{604e}, @samp{620}, @samp{630}, @samp{740}, @samp{7400},
11292 @samp{7450}, @samp{750}, @samp{801}, @samp{821}, @samp{823},
11293 @samp{860}, @samp{970}, @samp{8540}, @samp{ec603e}, @samp{G3},
11294 @samp{G4}, @samp{G5}, @samp{power}, @samp{power2}, @samp{power3},
11295 @samp{power4}, @samp{power5}, @samp{power5+},
11296 @samp{common}, @samp{powerpc}, @samp{powerpc64},
11297 @samp{rios}, @samp{rios1}, @samp{rios2}, @samp{rsc}, and @samp{rs64}.
11299 @option{-mcpu=common} selects a completely generic processor. Code
11300 generated under this option will run on any POWER or PowerPC processor.
11301 GCC will use only the instructions in the common subset of both
11302 architectures, and will not use the MQ register. GCC assumes a generic
11303 processor model for scheduling purposes.
11305 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
11306 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
11307 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
11308 types, with an appropriate, generic processor model assumed for
11309 scheduling purposes.
11311 The other options specify a specific processor. Code generated under
11312 those options will run best on that processor, and may not run at all on
11315 The @option{-mcpu} options automatically enable or disable the
11316 following options: @option{-maltivec}, @option{-mfprnd},
11317 @option{-mhard-float}, @option{-mmfcrf}, @option{-mmultiple},
11318 @option{-mnew-mnemonics}, @option{-mpopcntb}, @option{-mpower},
11319 @option{-mpower2}, @option{-mpowerpc64}, @option{-mpowerpc-gpopt},
11320 @option{-mpowerpc-gfxopt}, @option{-mstring}, @option{-mmulhw}, @option{dlmzb}.
11321 The particular options
11322 set for any particular CPU will vary between compiler versions,
11323 depending on what setting seems to produce optimal code for that CPU;
11324 it doesn't necessarily reflect the actual hardware's capabilities. If
11325 you wish to set an individual option to a particular value, you may
11326 specify it after the @option{-mcpu} option, like @samp{-mcpu=970
11329 On AIX, the @option{-maltivec} and @option{-mpowerpc64} options are
11330 not enabled or disabled by the @option{-mcpu} option at present because
11331 AIX does not have full support for these options. You may still
11332 enable or disable them individually if you're sure it'll work in your
11335 @item -mtune=@var{cpu_type}
11337 Set the instruction scheduling parameters for machine type
11338 @var{cpu_type}, but do not set the architecture type, register usage, or
11339 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
11340 values for @var{cpu_type} are used for @option{-mtune} as for
11341 @option{-mcpu}. If both are specified, the code generated will use the
11342 architecture, registers, and mnemonics set by @option{-mcpu}, but the
11343 scheduling parameters set by @option{-mtune}.
11349 Generate code to compute division as reciprocal estimate and iterative
11350 refinement, creating opportunities for increased throughput. This
11351 feature requires: optional PowerPC Graphics instruction set for single
11352 precision and FRE instruction for double precision, assuming divides
11353 cannot generate user-visible traps, and the domain values not include
11354 Infinities, denormals or zero denominator.
11357 @itemx -mno-altivec
11359 @opindex mno-altivec
11360 Generate code that uses (does not use) AltiVec instructions, and also
11361 enable the use of built-in functions that allow more direct access to
11362 the AltiVec instruction set. You may also need to set
11363 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
11369 @opindex mno-vrsave
11370 Generate VRSAVE instructions when generating AltiVec code.
11373 @opindex msecure-plt
11374 Generate code that allows ld and ld.so to build executables and shared
11375 libraries with non-exec .plt and .got sections. This is a PowerPC
11376 32-bit SYSV ABI option.
11380 Generate code that uses a BSS .plt section that ld.so fills in, and
11381 requires .plt and .got sections that are both writable and executable.
11382 This is a PowerPC 32-bit SYSV ABI option.
11388 This switch enables or disables the generation of ISEL instructions.
11390 @item -misel=@var{yes/no}
11391 This switch has been deprecated. Use @option{-misel} and
11392 @option{-mno-isel} instead.
11398 This switch enables or disables the generation of SPE simd
11401 @item -mspe=@var{yes/no}
11402 This option has been deprecated. Use @option{-mspe} and
11403 @option{-mno-spe} instead.
11405 @item -mfloat-gprs=@var{yes/single/double/no}
11406 @itemx -mfloat-gprs
11407 @opindex mfloat-gprs
11408 This switch enables or disables the generation of floating point
11409 operations on the general purpose registers for architectures that
11412 The argument @var{yes} or @var{single} enables the use of
11413 single-precision floating point operations.
11415 The argument @var{double} enables the use of single and
11416 double-precision floating point operations.
11418 The argument @var{no} disables floating point operations on the
11419 general purpose registers.
11421 This option is currently only available on the MPC854x.
11427 Generate code for 32-bit or 64-bit environments of Darwin and SVR4
11428 targets (including GNU/Linux). The 32-bit environment sets int, long
11429 and pointer to 32 bits and generates code that runs on any PowerPC
11430 variant. The 64-bit environment sets int to 32 bits and long and
11431 pointer to 64 bits, and generates code for PowerPC64, as for
11432 @option{-mpowerpc64}.
11435 @itemx -mno-fp-in-toc
11436 @itemx -mno-sum-in-toc
11437 @itemx -mminimal-toc
11439 @opindex mno-fp-in-toc
11440 @opindex mno-sum-in-toc
11441 @opindex mminimal-toc
11442 Modify generation of the TOC (Table Of Contents), which is created for
11443 every executable file. The @option{-mfull-toc} option is selected by
11444 default. In that case, GCC will allocate at least one TOC entry for
11445 each unique non-automatic variable reference in your program. GCC
11446 will also place floating-point constants in the TOC@. However, only
11447 16,384 entries are available in the TOC@.
11449 If you receive a linker error message that saying you have overflowed
11450 the available TOC space, you can reduce the amount of TOC space used
11451 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
11452 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
11453 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
11454 generate code to calculate the sum of an address and a constant at
11455 run-time instead of putting that sum into the TOC@. You may specify one
11456 or both of these options. Each causes GCC to produce very slightly
11457 slower and larger code at the expense of conserving TOC space.
11459 If you still run out of space in the TOC even when you specify both of
11460 these options, specify @option{-mminimal-toc} instead. This option causes
11461 GCC to make only one TOC entry for every file. When you specify this
11462 option, GCC will produce code that is slower and larger but which
11463 uses extremely little TOC space. You may wish to use this option
11464 only on files that contain less frequently executed code.
11470 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
11471 @code{long} type, and the infrastructure needed to support them.
11472 Specifying @option{-maix64} implies @option{-mpowerpc64} and
11473 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
11474 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
11477 @itemx -mno-xl-compat
11478 @opindex mxl-compat
11479 @opindex mno-xl-compat
11480 Produce code that conforms more closely to IBM XL compiler semantics
11481 when using AIX-compatible ABI. Pass floating-point arguments to
11482 prototyped functions beyond the register save area (RSA) on the stack
11483 in addition to argument FPRs. Do not assume that most significant
11484 double in 128-bit long double value is properly rounded when comparing
11485 values and converting to double. Use XL symbol names for long double
11488 The AIX calling convention was extended but not initially documented to
11489 handle an obscure K&R C case of calling a function that takes the
11490 address of its arguments with fewer arguments than declared. IBM XL
11491 compilers access floating point arguments which do not fit in the
11492 RSA from the stack when a subroutine is compiled without
11493 optimization. Because always storing floating-point arguments on the
11494 stack is inefficient and rarely needed, this option is not enabled by
11495 default and only is necessary when calling subroutines compiled by IBM
11496 XL compilers without optimization.
11500 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
11501 application written to use message passing with special startup code to
11502 enable the application to run. The system must have PE installed in the
11503 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
11504 must be overridden with the @option{-specs=} option to specify the
11505 appropriate directory location. The Parallel Environment does not
11506 support threads, so the @option{-mpe} option and the @option{-pthread}
11507 option are incompatible.
11509 @item -malign-natural
11510 @itemx -malign-power
11511 @opindex malign-natural
11512 @opindex malign-power
11513 On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
11514 @option{-malign-natural} overrides the ABI-defined alignment of larger
11515 types, such as floating-point doubles, on their natural size-based boundary.
11516 The option @option{-malign-power} instructs GCC to follow the ABI-specified
11517 alignment rules. GCC defaults to the standard alignment defined in the ABI@.
11519 On 64-bit Darwin, natural alignment is the default, and @option{-malign-power}
11523 @itemx -mhard-float
11524 @opindex msoft-float
11525 @opindex mhard-float
11526 Generate code that does not use (uses) the floating-point register set.
11527 Software floating point emulation is provided if you use the
11528 @option{-msoft-float} option, and pass the option to GCC when linking.
11531 @itemx -mno-multiple
11533 @opindex mno-multiple
11534 Generate code that uses (does not use) the load multiple word
11535 instructions and the store multiple word instructions. These
11536 instructions are generated by default on POWER systems, and not
11537 generated on PowerPC systems. Do not use @option{-mmultiple} on little
11538 endian PowerPC systems, since those instructions do not work when the
11539 processor is in little endian mode. The exceptions are PPC740 and
11540 PPC750 which permit the instructions usage in little endian mode.
11545 @opindex mno-string
11546 Generate code that uses (does not use) the load string instructions
11547 and the store string word instructions to save multiple registers and
11548 do small block moves. These instructions are generated by default on
11549 POWER systems, and not generated on PowerPC systems. Do not use
11550 @option{-mstring} on little endian PowerPC systems, since those
11551 instructions do not work when the processor is in little endian mode.
11552 The exceptions are PPC740 and PPC750 which permit the instructions
11553 usage in little endian mode.
11558 @opindex mno-update
11559 Generate code that uses (does not use) the load or store instructions
11560 that update the base register to the address of the calculated memory
11561 location. These instructions are generated by default. If you use
11562 @option{-mno-update}, there is a small window between the time that the
11563 stack pointer is updated and the address of the previous frame is
11564 stored, which means code that walks the stack frame across interrupts or
11565 signals may get corrupted data.
11568 @itemx -mno-fused-madd
11569 @opindex mfused-madd
11570 @opindex mno-fused-madd
11571 Generate code that uses (does not use) the floating point multiply and
11572 accumulate instructions. These instructions are generated by default if
11573 hardware floating is used.
11579 Generate code that uses (does not use) the half-word multiply and
11580 multiply-accumulate instructions on the IBM 405 and 440 processors.
11581 These instructions are generated by default when targetting those
11588 Generate code that uses (does not use) the string-search @samp{dlmzb}
11589 instruction on the IBM 405 and 440 processors. This instruction is
11590 generated by default when targetting those processors.
11592 @item -mno-bit-align
11594 @opindex mno-bit-align
11595 @opindex mbit-align
11596 On System V.4 and embedded PowerPC systems do not (do) force structures
11597 and unions that contain bit-fields to be aligned to the base type of the
11600 For example, by default a structure containing nothing but 8
11601 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
11602 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
11603 the structure would be aligned to a 1 byte boundary and be one byte in
11606 @item -mno-strict-align
11607 @itemx -mstrict-align
11608 @opindex mno-strict-align
11609 @opindex mstrict-align
11610 On System V.4 and embedded PowerPC systems do not (do) assume that
11611 unaligned memory references will be handled by the system.
11613 @item -mrelocatable
11614 @itemx -mno-relocatable
11615 @opindex mrelocatable
11616 @opindex mno-relocatable
11617 On embedded PowerPC systems generate code that allows (does not allow)
11618 the program to be relocated to a different address at runtime. If you
11619 use @option{-mrelocatable} on any module, all objects linked together must
11620 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
11622 @item -mrelocatable-lib
11623 @itemx -mno-relocatable-lib
11624 @opindex mrelocatable-lib
11625 @opindex mno-relocatable-lib
11626 On embedded PowerPC systems generate code that allows (does not allow)
11627 the program to be relocated to a different address at runtime. Modules
11628 compiled with @option{-mrelocatable-lib} can be linked with either modules
11629 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
11630 with modules compiled with the @option{-mrelocatable} options.
11636 On System V.4 and embedded PowerPC systems do not (do) assume that
11637 register 2 contains a pointer to a global area pointing to the addresses
11638 used in the program.
11641 @itemx -mlittle-endian
11643 @opindex mlittle-endian
11644 On System V.4 and embedded PowerPC systems compile code for the
11645 processor in little endian mode. The @option{-mlittle-endian} option is
11646 the same as @option{-mlittle}.
11649 @itemx -mbig-endian
11651 @opindex mbig-endian
11652 On System V.4 and embedded PowerPC systems compile code for the
11653 processor in big endian mode. The @option{-mbig-endian} option is
11654 the same as @option{-mbig}.
11656 @item -mdynamic-no-pic
11657 @opindex mdynamic-no-pic
11658 On Darwin and Mac OS X systems, compile code so that it is not
11659 relocatable, but that its external references are relocatable. The
11660 resulting code is suitable for applications, but not shared
11663 @item -mprioritize-restricted-insns=@var{priority}
11664 @opindex mprioritize-restricted-insns
11665 This option controls the priority that is assigned to
11666 dispatch-slot restricted instructions during the second scheduling
11667 pass. The argument @var{priority} takes the value @var{0/1/2} to assign
11668 @var{no/highest/second-highest} priority to dispatch slot restricted
11671 @item -msched-costly-dep=@var{dependence_type}
11672 @opindex msched-costly-dep
11673 This option controls which dependences are considered costly
11674 by the target during instruction scheduling. The argument
11675 @var{dependence_type} takes one of the following values:
11676 @var{no}: no dependence is costly,
11677 @var{all}: all dependences are costly,
11678 @var{true_store_to_load}: a true dependence from store to load is costly,
11679 @var{store_to_load}: any dependence from store to load is costly,
11680 @var{number}: any dependence which latency >= @var{number} is costly.
11682 @item -minsert-sched-nops=@var{scheme}
11683 @opindex minsert-sched-nops
11684 This option controls which nop insertion scheme will be used during
11685 the second scheduling pass. The argument @var{scheme} takes one of the
11687 @var{no}: Don't insert nops.
11688 @var{pad}: Pad with nops any dispatch group which has vacant issue slots,
11689 according to the scheduler's grouping.
11690 @var{regroup_exact}: Insert nops to force costly dependent insns into
11691 separate groups. Insert exactly as many nops as needed to force an insn
11692 to a new group, according to the estimated processor grouping.
11693 @var{number}: Insert nops to force costly dependent insns into
11694 separate groups. Insert @var{number} nops to force an insn to a new group.
11697 @opindex mcall-sysv
11698 On System V.4 and embedded PowerPC systems compile code using calling
11699 conventions that adheres to the March 1995 draft of the System V
11700 Application Binary Interface, PowerPC processor supplement. This is the
11701 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
11703 @item -mcall-sysv-eabi
11704 @opindex mcall-sysv-eabi
11705 Specify both @option{-mcall-sysv} and @option{-meabi} options.
11707 @item -mcall-sysv-noeabi
11708 @opindex mcall-sysv-noeabi
11709 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
11711 @item -mcall-solaris
11712 @opindex mcall-solaris
11713 On System V.4 and embedded PowerPC systems compile code for the Solaris
11717 @opindex mcall-linux
11718 On System V.4 and embedded PowerPC systems compile code for the
11719 Linux-based GNU system.
11723 On System V.4 and embedded PowerPC systems compile code for the
11724 Hurd-based GNU system.
11726 @item -mcall-netbsd
11727 @opindex mcall-netbsd
11728 On System V.4 and embedded PowerPC systems compile code for the
11729 NetBSD operating system.
11731 @item -maix-struct-return
11732 @opindex maix-struct-return
11733 Return all structures in memory (as specified by the AIX ABI)@.
11735 @item -msvr4-struct-return
11736 @opindex msvr4-struct-return
11737 Return structures smaller than 8 bytes in registers (as specified by the
11740 @item -mabi=@var{abi-type}
11742 Extend the current ABI with a particular extension, or remove such extension.
11743 Valid values are @var{altivec}, @var{no-altivec}, @var{spe},
11744 @var{no-spe}, @var{ibmlongdouble}, @var{ieeelongdouble}@.
11748 Extend the current ABI with SPE ABI extensions. This does not change
11749 the default ABI, instead it adds the SPE ABI extensions to the current
11753 @opindex mabi=no-spe
11754 Disable Booke SPE ABI extensions for the current ABI@.
11756 @item -mabi=ibmlongdouble
11757 @opindex mabi=ibmlongdouble
11758 Change the current ABI to use IBM extended precision long double.
11759 This is a PowerPC 32-bit SYSV ABI option.
11761 @item -mabi=ieeelongdouble
11762 @opindex mabi=ieeelongdouble
11763 Change the current ABI to use IEEE extended precision long double.
11764 This is a PowerPC 32-bit Linux ABI option.
11767 @itemx -mno-prototype
11768 @opindex mprototype
11769 @opindex mno-prototype
11770 On System V.4 and embedded PowerPC systems assume that all calls to
11771 variable argument functions are properly prototyped. Otherwise, the
11772 compiler must insert an instruction before every non prototyped call to
11773 set or clear bit 6 of the condition code register (@var{CR}) to
11774 indicate whether floating point values were passed in the floating point
11775 registers in case the function takes a variable arguments. With
11776 @option{-mprototype}, only calls to prototyped variable argument functions
11777 will set or clear the bit.
11781 On embedded PowerPC systems, assume that the startup module is called
11782 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
11783 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
11788 On embedded PowerPC systems, assume that the startup module is called
11789 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
11794 On embedded PowerPC systems, assume that the startup module is called
11795 @file{crt0.o} and the standard C libraries are @file{libads.a} and
11798 @item -myellowknife
11799 @opindex myellowknife
11800 On embedded PowerPC systems, assume that the startup module is called
11801 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
11806 On System V.4 and embedded PowerPC systems, specify that you are
11807 compiling for a VxWorks system.
11811 Specify that you are compiling for the WindISS simulation environment.
11815 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
11816 header to indicate that @samp{eabi} extended relocations are used.
11822 On System V.4 and embedded PowerPC systems do (do not) adhere to the
11823 Embedded Applications Binary Interface (eabi) which is a set of
11824 modifications to the System V.4 specifications. Selecting @option{-meabi}
11825 means that the stack is aligned to an 8 byte boundary, a function
11826 @code{__eabi} is called to from @code{main} to set up the eabi
11827 environment, and the @option{-msdata} option can use both @code{r2} and
11828 @code{r13} to point to two separate small data areas. Selecting
11829 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
11830 do not call an initialization function from @code{main}, and the
11831 @option{-msdata} option will only use @code{r13} to point to a single
11832 small data area. The @option{-meabi} option is on by default if you
11833 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
11836 @opindex msdata=eabi
11837 On System V.4 and embedded PowerPC systems, put small initialized
11838 @code{const} global and static data in the @samp{.sdata2} section, which
11839 is pointed to by register @code{r2}. Put small initialized
11840 non-@code{const} global and static data in the @samp{.sdata} section,
11841 which is pointed to by register @code{r13}. Put small uninitialized
11842 global and static data in the @samp{.sbss} section, which is adjacent to
11843 the @samp{.sdata} section. The @option{-msdata=eabi} option is
11844 incompatible with the @option{-mrelocatable} option. The
11845 @option{-msdata=eabi} option also sets the @option{-memb} option.
11848 @opindex msdata=sysv
11849 On System V.4 and embedded PowerPC systems, put small global and static
11850 data in the @samp{.sdata} section, which is pointed to by register
11851 @code{r13}. Put small uninitialized global and static data in the
11852 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
11853 The @option{-msdata=sysv} option is incompatible with the
11854 @option{-mrelocatable} option.
11856 @item -msdata=default
11858 @opindex msdata=default
11860 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
11861 compile code the same as @option{-msdata=eabi}, otherwise compile code the
11862 same as @option{-msdata=sysv}.
11865 @opindex msdata-data
11866 On System V.4 and embedded PowerPC systems, put small global
11867 data in the @samp{.sdata} section. Put small uninitialized global
11868 data in the @samp{.sbss} section. Do not use register @code{r13}
11869 to address small data however. This is the default behavior unless
11870 other @option{-msdata} options are used.
11874 @opindex msdata=none
11876 On embedded PowerPC systems, put all initialized global and static data
11877 in the @samp{.data} section, and all uninitialized data in the
11878 @samp{.bss} section.
11882 @cindex smaller data references (PowerPC)
11883 @cindex .sdata/.sdata2 references (PowerPC)
11884 On embedded PowerPC systems, put global and static items less than or
11885 equal to @var{num} bytes into the small data or bss sections instead of
11886 the normal data or bss section. By default, @var{num} is 8. The
11887 @option{-G @var{num}} switch is also passed to the linker.
11888 All modules should be compiled with the same @option{-G @var{num}} value.
11891 @itemx -mno-regnames
11893 @opindex mno-regnames
11894 On System V.4 and embedded PowerPC systems do (do not) emit register
11895 names in the assembly language output using symbolic forms.
11898 @itemx -mno-longcall
11900 @opindex mno-longcall
11901 Default to making all function calls indirectly, using a register, so
11902 that functions which reside further than 32 megabytes (33,554,432
11903 bytes) from the current location can be called. This setting can be
11904 overridden by the @code{shortcall} function attribute, or by
11905 @code{#pragma longcall(0)}.
11907 Some linkers are capable of detecting out-of-range calls and generating
11908 glue code on the fly. On these systems, long calls are unnecessary and
11909 generate slower code. As of this writing, the AIX linker can do this,
11910 as can the GNU linker for PowerPC/64. It is planned to add this feature
11911 to the GNU linker for 32-bit PowerPC systems as well.
11913 On Darwin/PPC systems, @code{#pragma longcall} will generate ``jbsr
11914 callee, L42'', plus a ``branch island'' (glue code). The two target
11915 addresses represent the callee and the ``branch island''. The
11916 Darwin/PPC linker will prefer the first address and generate a ``bl
11917 callee'' if the PPC ``bl'' instruction will reach the callee directly;
11918 otherwise, the linker will generate ``bl L42'' to call the ``branch
11919 island''. The ``branch island'' is appended to the body of the
11920 calling function; it computes the full 32-bit address of the callee
11923 On Mach-O (Darwin) systems, this option directs the compiler emit to
11924 the glue for every direct call, and the Darwin linker decides whether
11925 to use or discard it.
11927 In the future, we may cause GCC to ignore all longcall specifications
11928 when the linker is known to generate glue.
11932 Adds support for multithreading with the @dfn{pthreads} library.
11933 This option sets flags for both the preprocessor and linker.
11937 @node S/390 and zSeries Options
11938 @subsection S/390 and zSeries Options
11939 @cindex S/390 and zSeries Options
11941 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
11945 @itemx -msoft-float
11946 @opindex mhard-float
11947 @opindex msoft-float
11948 Use (do not use) the hardware floating-point instructions and registers
11949 for floating-point operations. When @option{-msoft-float} is specified,
11950 functions in @file{libgcc.a} will be used to perform floating-point
11951 operations. When @option{-mhard-float} is specified, the compiler
11952 generates IEEE floating-point instructions. This is the default.
11954 @item -mlong-double-64
11955 @itemx -mlong-double-128
11956 @opindex mlong-double-64
11957 @opindex mlong-double-128
11958 These switches control the size of @code{long double} type. A size
11959 of 64bit makes the @code{long double} type equivalent to the @code{double}
11960 type. This is the default.
11963 @itemx -mno-backchain
11964 @opindex mbackchain
11965 @opindex mno-backchain
11966 Store (do not store) the address of the caller's frame as backchain pointer
11967 into the callee's stack frame.
11968 A backchain may be needed to allow debugging using tools that do not understand
11969 DWARF-2 call frame information.
11970 When @option{-mno-packed-stack} is in effect, the backchain pointer is stored
11971 at the bottom of the stack frame; when @option{-mpacked-stack} is in effect,
11972 the backchain is placed into the topmost word of the 96/160 byte register
11975 In general, code compiled with @option{-mbackchain} is call-compatible with
11976 code compiled with @option{-mmo-backchain}; however, use of the backchain
11977 for debugging purposes usually requires that the whole binary is built with
11978 @option{-mbackchain}. Note that the combination of @option{-mbackchain},
11979 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
11980 to build a linux kernel use @option{-msoft-float}.
11982 The default is to not maintain the backchain.
11984 @item -mpacked-stack
11985 @item -mno-packed-stack
11986 @opindex mpacked-stack
11987 @opindex mno-packed-stack
11988 Use (do not use) the packed stack layout. When @option{-mno-packed-stack} is
11989 specified, the compiler uses the all fields of the 96/160 byte register save
11990 area only for their default purpose; unused fields still take up stack space.
11991 When @option{-mpacked-stack} is specified, register save slots are densely
11992 packed at the top of the register save area; unused space is reused for other
11993 purposes, allowing for more efficient use of the available stack space.
11994 However, when @option{-mbackchain} is also in effect, the topmost word of
11995 the save area is always used to store the backchain, and the return address
11996 register is always saved two words below the backchain.
11998 As long as the stack frame backchain is not used, code generated with
11999 @option{-mpacked-stack} is call-compatible with code generated with
12000 @option{-mno-packed-stack}. Note that some non-FSF releases of GCC 2.95 for
12001 S/390 or zSeries generated code that uses the stack frame backchain at run
12002 time, not just for debugging purposes. Such code is not call-compatible
12003 with code compiled with @option{-mpacked-stack}. Also, note that the
12004 combination of @option{-mbackchain},
12005 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
12006 to build a linux kernel use @option{-msoft-float}.
12008 The default is to not use the packed stack layout.
12011 @itemx -mno-small-exec
12012 @opindex msmall-exec
12013 @opindex mno-small-exec
12014 Generate (or do not generate) code using the @code{bras} instruction
12015 to do subroutine calls.
12016 This only works reliably if the total executable size does not
12017 exceed 64k. The default is to use the @code{basr} instruction instead,
12018 which does not have this limitation.
12024 When @option{-m31} is specified, generate code compliant to the
12025 GNU/Linux for S/390 ABI@. When @option{-m64} is specified, generate
12026 code compliant to the GNU/Linux for zSeries ABI@. This allows GCC in
12027 particular to generate 64-bit instructions. For the @samp{s390}
12028 targets, the default is @option{-m31}, while the @samp{s390x}
12029 targets default to @option{-m64}.
12035 When @option{-mzarch} is specified, generate code using the
12036 instructions available on z/Architecture.
12037 When @option{-mesa} is specified, generate code using the
12038 instructions available on ESA/390. Note that @option{-mesa} is
12039 not possible with @option{-m64}.
12040 When generating code compliant to the GNU/Linux for S/390 ABI,
12041 the default is @option{-mesa}. When generating code compliant
12042 to the GNU/Linux for zSeries ABI, the default is @option{-mzarch}.
12048 Generate (or do not generate) code using the @code{mvcle} instruction
12049 to perform block moves. When @option{-mno-mvcle} is specified,
12050 use a @code{mvc} loop instead. This is the default unless optimizing for
12057 Print (or do not print) additional debug information when compiling.
12058 The default is to not print debug information.
12060 @item -march=@var{cpu-type}
12062 Generate code that will run on @var{cpu-type}, which is the name of a system
12063 representing a certain processor type. Possible values for
12064 @var{cpu-type} are @samp{g5}, @samp{g6}, @samp{z900}, and @samp{z990}.
12065 When generating code using the instructions available on z/Architecture,
12066 the default is @option{-march=z900}. Otherwise, the default is
12067 @option{-march=g5}.
12069 @item -mtune=@var{cpu-type}
12071 Tune to @var{cpu-type} everything applicable about the generated code,
12072 except for the ABI and the set of available instructions.
12073 The list of @var{cpu-type} values is the same as for @option{-march}.
12074 The default is the value used for @option{-march}.
12077 @itemx -mno-tpf-trace
12078 @opindex mtpf-trace
12079 @opindex mno-tpf-trace
12080 Generate code that adds (does not add) in TPF OS specific branches to trace
12081 routines in the operating system. This option is off by default, even
12082 when compiling for the TPF OS@.
12085 @itemx -mno-fused-madd
12086 @opindex mfused-madd
12087 @opindex mno-fused-madd
12088 Generate code that uses (does not use) the floating point multiply and
12089 accumulate instructions. These instructions are generated by default if
12090 hardware floating point is used.
12092 @item -mwarn-framesize=@var{framesize}
12093 @opindex mwarn-framesize
12094 Emit a warning if the current function exceeds the given frame size. Because
12095 this is a compile time check it doesn't need to be a real problem when the program
12096 runs. It is intended to identify functions which most probably cause
12097 a stack overflow. It is useful to be used in an environment with limited stack
12098 size e.g.@: the linux kernel.
12100 @item -mwarn-dynamicstack
12101 @opindex mwarn-dynamicstack
12102 Emit a warning if the function calls alloca or uses dynamically
12103 sized arrays. This is generally a bad idea with a limited stack size.
12105 @item -mstack-guard=@var{stack-guard}
12106 @item -mstack-size=@var{stack-size}
12107 @opindex mstack-guard
12108 @opindex mstack-size
12109 These arguments always have to be used in conjunction. If they are present the s390
12110 back end emits additional instructions in the function prologue which trigger a trap
12111 if the stack size is @var{stack-guard} bytes above the @var{stack-size}
12112 (remember that the stack on s390 grows downward). These options are intended to
12113 be used to help debugging stack overflow problems. The additionally emitted code
12114 causes only little overhead and hence can also be used in production like systems
12115 without greater performance degradation. The given values have to be exact
12116 powers of 2 and @var{stack-size} has to be greater than @var{stack-guard} without
12118 In order to be efficient the extra code makes the assumption that the stack starts
12119 at an address aligned to the value given by @var{stack-size}.
12123 @subsection SH Options
12125 These @samp{-m} options are defined for the SH implementations:
12130 Generate code for the SH1.
12134 Generate code for the SH2.
12137 Generate code for the SH2e.
12141 Generate code for the SH3.
12145 Generate code for the SH3e.
12149 Generate code for the SH4 without a floating-point unit.
12151 @item -m4-single-only
12152 @opindex m4-single-only
12153 Generate code for the SH4 with a floating-point unit that only
12154 supports single-precision arithmetic.
12158 Generate code for the SH4 assuming the floating-point unit is in
12159 single-precision mode by default.
12163 Generate code for the SH4.
12167 Generate code for the SH4al-dsp, or for a SH4a in such a way that the
12168 floating-point unit is not used.
12170 @item -m4a-single-only
12171 @opindex m4a-single-only
12172 Generate code for the SH4a, in such a way that no double-precision
12173 floating point operations are used.
12176 @opindex m4a-single
12177 Generate code for the SH4a assuming the floating-point unit is in
12178 single-precision mode by default.
12182 Generate code for the SH4a.
12186 Same as @option{-m4a-nofpu}, except that it implicitly passes
12187 @option{-dsp} to the assembler. GCC doesn't generate any DSP
12188 instructions at the moment.
12192 Compile code for the processor in big endian mode.
12196 Compile code for the processor in little endian mode.
12200 Align doubles at 64-bit boundaries. Note that this changes the calling
12201 conventions, and thus some functions from the standard C library will
12202 not work unless you recompile it first with @option{-mdalign}.
12206 Shorten some address references at link time, when possible; uses the
12207 linker option @option{-relax}.
12211 Use 32-bit offsets in @code{switch} tables. The default is to use
12216 Enable the use of the instruction @code{fmovd}.
12220 Comply with the calling conventions defined by Renesas.
12224 Comply with the calling conventions defined by Renesas.
12228 Comply with the calling conventions defined for GCC before the Renesas
12229 conventions were available. This option is the default for all
12230 targets of the SH toolchain except for @samp{sh-symbianelf}.
12233 @opindex mnomacsave
12234 Mark the @code{MAC} register as call-clobbered, even if
12235 @option{-mhitachi} is given.
12239 Increase IEEE-compliance of floating-point code.
12240 At the moment, this is equivalent to @option{-fno-finite-math-only}.
12241 When generating 16 bit SH opcodes, getting IEEE-conforming results for
12242 comparisons of NANs / infinities incurs extra overhead in every
12243 floating point comparison, therefore the default is set to
12244 @option{-ffinite-math-only}.
12248 Dump instruction size and location in the assembly code.
12251 @opindex mpadstruct
12252 This option is deprecated. It pads structures to multiple of 4 bytes,
12253 which is incompatible with the SH ABI@.
12257 Optimize for space instead of speed. Implied by @option{-Os}.
12260 @opindex mprefergot
12261 When generating position-independent code, emit function calls using
12262 the Global Offset Table instead of the Procedure Linkage Table.
12266 Generate a library function call to invalidate instruction cache
12267 entries, after fixing up a trampoline. This library function call
12268 doesn't assume it can write to the whole memory address space. This
12269 is the default when the target is @code{sh-*-linux*}.
12271 @item -multcost=@var{number}
12272 @opindex multcost=@var{number}
12273 Set the cost to assume for a multiply insn.
12275 @item -mdiv=@var{strategy}
12276 @opindex mdiv=@var{strategy}
12277 Set the division strategy to use for SHmedia code. @var{strategy} must be
12278 one of: call, call2, fp, inv, inv:minlat, inv20u, inv20l, inv:call,
12279 inv:call2, inv:fp .
12280 "fp" performs the operation in floating point. This has a very high latency,
12281 but needs only a few instructions, so it might be a good choice if
12282 your code has enough easily exploitable ILP to allow the compiler to
12283 schedule the floating point instructions together with other instructions.
12284 Division by zero causes a floating point exception.
12285 "inv" uses integer operations to calculate the inverse of the divisor,
12286 and then multiplies the dividend with the inverse. This strategy allows
12287 cse and hoisting of the inverse calculation. Division by zero calculates
12288 an unspecified result, but does not trap.
12289 "inv:minlat" is a variant of "inv" where if no cse / hoisting opportunities
12290 have been found, or if the entire operation has been hoisted to the same
12291 place, the last stages of the inverse calculation are intertwined with the
12292 final multiply to reduce the overall latency, at the expense of using a few
12293 more instructions, and thus offering fewer scheduling opportunities with
12295 "call" calls a library function that usually implements the inv:minlat
12297 This gives high code density for m5-*media-nofpu compilations.
12298 "call2" uses a different entry point of the same library function, where it
12299 assumes that a pointer to a lookup table has already been set up, which
12300 exposes the pointer load to cse / code hoisting optimizations.
12301 "inv:call", "inv:call2" and "inv:fp" all use the "inv" algorithm for initial
12302 code generation, but if the code stays unoptimized, revert to the "call",
12303 "call2", or "fp" strategies, respectively. Note that the
12304 potentially-trapping side effect of division by zero is carried by a
12305 separate instruction, so it is possible that all the integer instructions
12306 are hoisted out, but the marker for the side effect stays where it is.
12307 A recombination to fp operations or a call is not possible in that case.
12308 "inv20u" and "inv20l" are variants of the "inv:minlat" strategy. In the case
12309 that the inverse calculation was nor separated from the multiply, they speed
12310 up division where the dividend fits into 20 bits (plus sign where applicable),
12311 by inserting a test to skip a number of operations in this case; this test
12312 slows down the case of larger dividends. inv20u assumes the case of a such
12313 a small dividend to be unlikely, and inv20l assumes it to be likely.
12315 @item -mdivsi3_libfunc=@var{name}
12316 @opindex mdivsi3_libfunc=@var{name}
12317 Set the name of the library function used for 32 bit signed division to
12318 @var{name}. This only affect the name used in the call and inv:call
12319 division strategies, and the compiler will still expect the same
12320 sets of input/output/clobbered registers as if this option was not present.
12322 @item -madjust-unroll
12323 @opindex madjust-unroll
12324 Throttle unrolling to avoid thrashing target registers.
12325 This option only has an effect if the gcc code base supports the
12326 TARGET_ADJUST_UNROLL_MAX target hook.
12328 @item -mindexed-addressing
12329 @opindex mindexed-addressing
12330 Enable the use of the indexed addressing mode for SHmedia32/SHcompact.
12331 This is only safe if the hardware and/or OS implement 32 bit wrap-around
12332 semantics for the indexed addressing mode. The architecture allows the
12333 implementation of processors with 64 bit MMU, which the OS could use to
12334 get 32 bit addressing, but since no current hardware implementation supports
12335 this or any other way to make the indexed addressing mode safe to use in
12336 the 32 bit ABI, the default is -mno-indexed-addressing.
12338 @item -mgettrcost=@var{number}
12339 @opindex mgettrcost=@var{number}
12340 Set the cost assumed for the gettr instruction to @var{number}.
12341 The default is 2 if @option{-mpt-fixed} is in effect, 100 otherwise.
12345 Assume pt* instructions won't trap. This will generally generate better
12346 scheduled code, but is unsafe on current hardware. The current architecture
12347 definition says that ptabs and ptrel trap when the target anded with 3 is 3.
12348 This has the unintentional effect of making it unsafe to schedule ptabs /
12349 ptrel before a branch, or hoist it out of a loop. For example,
12350 __do_global_ctors, a part of libgcc that runs constructors at program
12351 startup, calls functions in a list which is delimited by -1. With the
12352 -mpt-fixed option, the ptabs will be done before testing against -1.
12353 That means that all the constructors will be run a bit quicker, but when
12354 the loop comes to the end of the list, the program crashes because ptabs
12355 loads -1 into a target register. Since this option is unsafe for any
12356 hardware implementing the current architecture specification, the default
12357 is -mno-pt-fixed. Unless the user specifies a specific cost with
12358 @option{-mgettrcost}, -mno-pt-fixed also implies @option{-mgettrcost=100};
12359 this deters register allocation using target registers for storing
12362 @item -minvalid-symbols
12363 @opindex minvalid-symbols
12364 Assume symbols might be invalid. Ordinary function symbols generated by
12365 the compiler will always be valid to load with movi/shori/ptabs or
12366 movi/shori/ptrel, but with assembler and/or linker tricks it is possible
12367 to generate symbols that will cause ptabs / ptrel to trap.
12368 This option is only meaningful when @option{-mno-pt-fixed} is in effect.
12369 It will then prevent cross-basic-block cse, hoisting and most scheduling
12370 of symbol loads. The default is @option{-mno-invalid-symbols}.
12373 @node SPARC Options
12374 @subsection SPARC Options
12375 @cindex SPARC options
12377 These @samp{-m} options are supported on the SPARC:
12380 @item -mno-app-regs
12382 @opindex mno-app-regs
12384 Specify @option{-mapp-regs} to generate output using the global registers
12385 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
12388 To be fully SVR4 ABI compliant at the cost of some performance loss,
12389 specify @option{-mno-app-regs}. You should compile libraries and system
12390 software with this option.
12393 @itemx -mhard-float
12395 @opindex mhard-float
12396 Generate output containing floating point instructions. This is the
12400 @itemx -msoft-float
12402 @opindex msoft-float
12403 Generate output containing library calls for floating point.
12404 @strong{Warning:} the requisite libraries are not available for all SPARC
12405 targets. Normally the facilities of the machine's usual C compiler are
12406 used, but this cannot be done directly in cross-compilation. You must make
12407 your own arrangements to provide suitable library functions for
12408 cross-compilation. The embedded targets @samp{sparc-*-aout} and
12409 @samp{sparclite-*-*} do provide software floating point support.
12411 @option{-msoft-float} changes the calling convention in the output file;
12412 therefore, it is only useful if you compile @emph{all} of a program with
12413 this option. In particular, you need to compile @file{libgcc.a}, the
12414 library that comes with GCC, with @option{-msoft-float} in order for
12417 @item -mhard-quad-float
12418 @opindex mhard-quad-float
12419 Generate output containing quad-word (long double) floating point
12422 @item -msoft-quad-float
12423 @opindex msoft-quad-float
12424 Generate output containing library calls for quad-word (long double)
12425 floating point instructions. The functions called are those specified
12426 in the SPARC ABI@. This is the default.
12428 As of this writing, there are no SPARC implementations that have hardware
12429 support for the quad-word floating point instructions. They all invoke
12430 a trap handler for one of these instructions, and then the trap handler
12431 emulates the effect of the instruction. Because of the trap handler overhead,
12432 this is much slower than calling the ABI library routines. Thus the
12433 @option{-msoft-quad-float} option is the default.
12435 @item -mno-unaligned-doubles
12436 @itemx -munaligned-doubles
12437 @opindex mno-unaligned-doubles
12438 @opindex munaligned-doubles
12439 Assume that doubles have 8 byte alignment. This is the default.
12441 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
12442 alignment only if they are contained in another type, or if they have an
12443 absolute address. Otherwise, it assumes they have 4 byte alignment.
12444 Specifying this option avoids some rare compatibility problems with code
12445 generated by other compilers. It is not the default because it results
12446 in a performance loss, especially for floating point code.
12448 @item -mno-faster-structs
12449 @itemx -mfaster-structs
12450 @opindex mno-faster-structs
12451 @opindex mfaster-structs
12452 With @option{-mfaster-structs}, the compiler assumes that structures
12453 should have 8 byte alignment. This enables the use of pairs of
12454 @code{ldd} and @code{std} instructions for copies in structure
12455 assignment, in place of twice as many @code{ld} and @code{st} pairs.
12456 However, the use of this changed alignment directly violates the SPARC
12457 ABI@. Thus, it's intended only for use on targets where the developer
12458 acknowledges that their resulting code will not be directly in line with
12459 the rules of the ABI@.
12461 @item -mimpure-text
12462 @opindex mimpure-text
12463 @option{-mimpure-text}, used in addition to @option{-shared}, tells
12464 the compiler to not pass @option{-z text} to the linker when linking a
12465 shared object. Using this option, you can link position-dependent
12466 code into a shared object.
12468 @option{-mimpure-text} suppresses the ``relocations remain against
12469 allocatable but non-writable sections'' linker error message.
12470 However, the necessary relocations will trigger copy-on-write, and the
12471 shared object is not actually shared across processes. Instead of
12472 using @option{-mimpure-text}, you should compile all source code with
12473 @option{-fpic} or @option{-fPIC}.
12475 This option is only available on SunOS and Solaris.
12477 @item -mcpu=@var{cpu_type}
12479 Set the instruction set, register set, and instruction scheduling parameters
12480 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
12481 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
12482 @samp{f930}, @samp{f934}, @samp{hypersparc}, @samp{sparclite86x},
12483 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc},
12484 @samp{ultrasparc3}, and @samp{niagara}.
12486 Default instruction scheduling parameters are used for values that select
12487 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
12488 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
12490 Here is a list of each supported architecture and their supported
12495 v8: supersparc, hypersparc
12496 sparclite: f930, f934, sparclite86x
12498 v9: ultrasparc, ultrasparc3, niagara
12501 By default (unless configured otherwise), GCC generates code for the V7
12502 variant of the SPARC architecture. With @option{-mcpu=cypress}, the compiler
12503 additionally optimizes it for the Cypress CY7C602 chip, as used in the
12504 SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
12505 SPARCStation 1, 2, IPX etc.
12507 With @option{-mcpu=v8}, GCC generates code for the V8 variant of the SPARC
12508 architecture. The only difference from V7 code is that the compiler emits
12509 the integer multiply and integer divide instructions which exist in SPARC-V8
12510 but not in SPARC-V7. With @option{-mcpu=supersparc}, the compiler additionally
12511 optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
12514 With @option{-mcpu=sparclite}, GCC generates code for the SPARClite variant of
12515 the SPARC architecture. This adds the integer multiply, integer divide step
12516 and scan (@code{ffs}) instructions which exist in SPARClite but not in SPARC-V7.
12517 With @option{-mcpu=f930}, the compiler additionally optimizes it for the
12518 Fujitsu MB86930 chip, which is the original SPARClite, with no FPU@. With
12519 @option{-mcpu=f934}, the compiler additionally optimizes it for the Fujitsu
12520 MB86934 chip, which is the more recent SPARClite with FPU@.
12522 With @option{-mcpu=sparclet}, GCC generates code for the SPARClet variant of
12523 the SPARC architecture. This adds the integer multiply, multiply/accumulate,
12524 integer divide step and scan (@code{ffs}) instructions which exist in SPARClet
12525 but not in SPARC-V7. With @option{-mcpu=tsc701}, the compiler additionally
12526 optimizes it for the TEMIC SPARClet chip.
12528 With @option{-mcpu=v9}, GCC generates code for the V9 variant of the SPARC
12529 architecture. This adds 64-bit integer and floating-point move instructions,
12530 3 additional floating-point condition code registers and conditional move
12531 instructions. With @option{-mcpu=ultrasparc}, the compiler additionally
12532 optimizes it for the Sun UltraSPARC I/II/IIi chips. With
12533 @option{-mcpu=ultrasparc3}, the compiler additionally optimizes it for the
12534 Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
12535 @option{-mcpu=niagara}, the compiler additionally optimizes it for
12536 Sun UltraSPARC T1 chips.
12538 @item -mtune=@var{cpu_type}
12540 Set the instruction scheduling parameters for machine type
12541 @var{cpu_type}, but do not set the instruction set or register set that the
12542 option @option{-mcpu=@var{cpu_type}} would.
12544 The same values for @option{-mcpu=@var{cpu_type}} can be used for
12545 @option{-mtune=@var{cpu_type}}, but the only useful values are those
12546 that select a particular cpu implementation. Those are @samp{cypress},
12547 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
12548 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc},
12549 @samp{ultrasparc3}, and @samp{niagara}.
12554 @opindex mno-v8plus
12555 With @option{-mv8plus}, GCC generates code for the SPARC-V8+ ABI@. The
12556 difference from the V8 ABI is that the global and out registers are
12557 considered 64-bit wide. This is enabled by default on Solaris in 32-bit
12558 mode for all SPARC-V9 processors.
12564 With @option{-mvis}, GCC generates code that takes advantage of the UltraSPARC
12565 Visual Instruction Set extensions. The default is @option{-mno-vis}.
12568 These @samp{-m} options are supported in addition to the above
12569 on SPARC-V9 processors in 64-bit environments:
12572 @item -mlittle-endian
12573 @opindex mlittle-endian
12574 Generate code for a processor running in little-endian mode. It is only
12575 available for a few configurations and most notably not on Solaris and Linux.
12581 Generate code for a 32-bit or 64-bit environment.
12582 The 32-bit environment sets int, long and pointer to 32 bits.
12583 The 64-bit environment sets int to 32 bits and long and pointer
12586 @item -mcmodel=medlow
12587 @opindex mcmodel=medlow
12588 Generate code for the Medium/Low code model: 64-bit addresses, programs
12589 must be linked in the low 32 bits of memory. Programs can be statically
12590 or dynamically linked.
12592 @item -mcmodel=medmid
12593 @opindex mcmodel=medmid
12594 Generate code for the Medium/Middle code model: 64-bit addresses, programs
12595 must be linked in the low 44 bits of memory, the text and data segments must
12596 be less than 2GB in size and the data segment must be located within 2GB of
12599 @item -mcmodel=medany
12600 @opindex mcmodel=medany
12601 Generate code for the Medium/Anywhere code model: 64-bit addresses, programs
12602 may be linked anywhere in memory, the text and data segments must be less
12603 than 2GB in size and the data segment must be located within 2GB of the
12606 @item -mcmodel=embmedany
12607 @opindex mcmodel=embmedany
12608 Generate code for the Medium/Anywhere code model for embedded systems:
12609 64-bit addresses, the text and data segments must be less than 2GB in
12610 size, both starting anywhere in memory (determined at link time). The
12611 global register %g4 points to the base of the data segment. Programs
12612 are statically linked and PIC is not supported.
12615 @itemx -mno-stack-bias
12616 @opindex mstack-bias
12617 @opindex mno-stack-bias
12618 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
12619 frame pointer if present, are offset by @minus{}2047 which must be added back
12620 when making stack frame references. This is the default in 64-bit mode.
12621 Otherwise, assume no such offset is present.
12624 These switches are supported in addition to the above on Solaris:
12629 Add support for multithreading using the Solaris threads library. This
12630 option sets flags for both the preprocessor and linker. This option does
12631 not affect the thread safety of object code produced by the compiler or
12632 that of libraries supplied with it.
12636 Add support for multithreading using the POSIX threads library. This
12637 option sets flags for both the preprocessor and linker. This option does
12638 not affect the thread safety of object code produced by the compiler or
12639 that of libraries supplied with it.
12643 This is a synonym for @option{-pthreads}.
12646 @node System V Options
12647 @subsection Options for System V
12649 These additional options are available on System V Release 4 for
12650 compatibility with other compilers on those systems:
12655 Create a shared object.
12656 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
12660 Identify the versions of each tool used by the compiler, in a
12661 @code{.ident} assembler directive in the output.
12665 Refrain from adding @code{.ident} directives to the output file (this is
12668 @item -YP,@var{dirs}
12670 Search the directories @var{dirs}, and no others, for libraries
12671 specified with @option{-l}.
12673 @item -Ym,@var{dir}
12675 Look in the directory @var{dir} to find the M4 preprocessor.
12676 The assembler uses this option.
12677 @c This is supposed to go with a -Yd for predefined M4 macro files, but
12678 @c the generic assembler that comes with Solaris takes just -Ym.
12681 @node TMS320C3x/C4x Options
12682 @subsection TMS320C3x/C4x Options
12683 @cindex TMS320C3x/C4x Options
12685 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
12689 @item -mcpu=@var{cpu_type}
12691 Set the instruction set, register set, and instruction scheduling
12692 parameters for machine type @var{cpu_type}. Supported values for
12693 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
12694 @samp{c44}. The default is @samp{c40} to generate code for the
12699 @itemx -msmall-memory
12701 @opindex mbig-memory
12703 @opindex msmall-memory
12705 Generates code for the big or small memory model. The small memory
12706 model assumed that all data fits into one 64K word page. At run-time
12707 the data page (DP) register must be set to point to the 64K page
12708 containing the .bss and .data program sections. The big memory model is
12709 the default and requires reloading of the DP register for every direct
12716 Allow (disallow) allocation of general integer operands into the block
12717 count register BK@.
12723 Enable (disable) generation of code using decrement and branch,
12724 DBcond(D), instructions. This is enabled by default for the C4x. To be
12725 on the safe side, this is disabled for the C3x, since the maximum
12726 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
12727 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
12728 that it can utilize the decrement and branch instruction, but will give
12729 up if there is more than one memory reference in the loop. Thus a loop
12730 where the loop counter is decremented can generate slightly more
12731 efficient code, in cases where the RPTB instruction cannot be utilized.
12733 @item -mdp-isr-reload
12735 @opindex mdp-isr-reload
12737 Force the DP register to be saved on entry to an interrupt service
12738 routine (ISR), reloaded to point to the data section, and restored on
12739 exit from the ISR@. This should not be required unless someone has
12740 violated the small memory model by modifying the DP register, say within
12747 For the C3x use the 24-bit MPYI instruction for integer multiplies
12748 instead of a library call to guarantee 32-bit results. Note that if one
12749 of the operands is a constant, then the multiplication will be performed
12750 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
12751 then squaring operations are performed inline instead of a library call.
12754 @itemx -mno-fast-fix
12756 @opindex mno-fast-fix
12757 The C3x/C4x FIX instruction to convert a floating point value to an
12758 integer value chooses the nearest integer less than or equal to the
12759 floating point value rather than to the nearest integer. Thus if the
12760 floating point number is negative, the result will be incorrectly
12761 truncated an additional code is necessary to detect and correct this
12762 case. This option can be used to disable generation of the additional
12763 code required to correct the result.
12769 Enable (disable) generation of repeat block sequences using the RPTB
12770 instruction for zero overhead looping. The RPTB construct is only used
12771 for innermost loops that do not call functions or jump across the loop
12772 boundaries. There is no advantage having nested RPTB loops due to the
12773 overhead required to save and restore the RC, RS, and RE registers.
12774 This is enabled by default with @option{-O2}.
12776 @item -mrpts=@var{count}
12780 Enable (disable) the use of the single instruction repeat instruction
12781 RPTS@. If a repeat block contains a single instruction, and the loop
12782 count can be guaranteed to be less than the value @var{count}, GCC will
12783 emit a RPTS instruction instead of a RPTB@. If no value is specified,
12784 then a RPTS will be emitted even if the loop count cannot be determined
12785 at compile time. Note that the repeated instruction following RPTS does
12786 not have to be reloaded from memory each iteration, thus freeing up the
12787 CPU buses for operands. However, since interrupts are blocked by this
12788 instruction, it is disabled by default.
12790 @item -mloop-unsigned
12791 @itemx -mno-loop-unsigned
12792 @opindex mloop-unsigned
12793 @opindex mno-loop-unsigned
12794 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
12795 is @math{2^{31} + 1} since these instructions test if the iteration count is
12796 negative to terminate the loop. If the iteration count is unsigned
12797 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
12798 exceeded. This switch allows an unsigned iteration count.
12802 Try to emit an assembler syntax that the TI assembler (asm30) is happy
12803 with. This also enforces compatibility with the API employed by the TI
12804 C3x C compiler. For example, long doubles are passed as structures
12805 rather than in floating point registers.
12811 Generate code that uses registers (stack) for passing arguments to functions.
12812 By default, arguments are passed in registers where possible rather
12813 than by pushing arguments on to the stack.
12815 @item -mparallel-insns
12816 @itemx -mno-parallel-insns
12817 @opindex mparallel-insns
12818 @opindex mno-parallel-insns
12819 Allow the generation of parallel instructions. This is enabled by
12820 default with @option{-O2}.
12822 @item -mparallel-mpy
12823 @itemx -mno-parallel-mpy
12824 @opindex mparallel-mpy
12825 @opindex mno-parallel-mpy
12826 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
12827 provided @option{-mparallel-insns} is also specified. These instructions have
12828 tight register constraints which can pessimize the code generation
12829 of large functions.
12834 @subsection V850 Options
12835 @cindex V850 Options
12837 These @samp{-m} options are defined for V850 implementations:
12841 @itemx -mno-long-calls
12842 @opindex mlong-calls
12843 @opindex mno-long-calls
12844 Treat all calls as being far away (near). If calls are assumed to be
12845 far away, the compiler will always load the functions address up into a
12846 register, and call indirect through the pointer.
12852 Do not optimize (do optimize) basic blocks that use the same index
12853 pointer 4 or more times to copy pointer into the @code{ep} register, and
12854 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
12855 option is on by default if you optimize.
12857 @item -mno-prolog-function
12858 @itemx -mprolog-function
12859 @opindex mno-prolog-function
12860 @opindex mprolog-function
12861 Do not use (do use) external functions to save and restore registers
12862 at the prologue and epilogue of a function. The external functions
12863 are slower, but use less code space if more than one function saves
12864 the same number of registers. The @option{-mprolog-function} option
12865 is on by default if you optimize.
12869 Try to make the code as small as possible. At present, this just turns
12870 on the @option{-mep} and @option{-mprolog-function} options.
12872 @item -mtda=@var{n}
12874 Put static or global variables whose size is @var{n} bytes or less into
12875 the tiny data area that register @code{ep} points to. The tiny data
12876 area can hold up to 256 bytes in total (128 bytes for byte references).
12878 @item -msda=@var{n}
12880 Put static or global variables whose size is @var{n} bytes or less into
12881 the small data area that register @code{gp} points to. The small data
12882 area can hold up to 64 kilobytes.
12884 @item -mzda=@var{n}
12886 Put static or global variables whose size is @var{n} bytes or less into
12887 the first 32 kilobytes of memory.
12891 Specify that the target processor is the V850.
12894 @opindex mbig-switch
12895 Generate code suitable for big switch tables. Use this option only if
12896 the assembler/linker complain about out of range branches within a switch
12901 This option will cause r2 and r5 to be used in the code generated by
12902 the compiler. This setting is the default.
12904 @item -mno-app-regs
12905 @opindex mno-app-regs
12906 This option will cause r2 and r5 to be treated as fixed registers.
12910 Specify that the target processor is the V850E1. The preprocessor
12911 constants @samp{__v850e1__} and @samp{__v850e__} will be defined if
12912 this option is used.
12916 Specify that the target processor is the V850E@. The preprocessor
12917 constant @samp{__v850e__} will be defined if this option is used.
12919 If neither @option{-mv850} nor @option{-mv850e} nor @option{-mv850e1}
12920 are defined then a default target processor will be chosen and the
12921 relevant @samp{__v850*__} preprocessor constant will be defined.
12923 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
12924 defined, regardless of which processor variant is the target.
12926 @item -mdisable-callt
12927 @opindex mdisable-callt
12928 This option will suppress generation of the CALLT instruction for the
12929 v850e and v850e1 flavors of the v850 architecture. The default is
12930 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
12935 @subsection VAX Options
12936 @cindex VAX options
12938 These @samp{-m} options are defined for the VAX:
12943 Do not output certain jump instructions (@code{aobleq} and so on)
12944 that the Unix assembler for the VAX cannot handle across long
12949 Do output those jump instructions, on the assumption that you
12950 will assemble with the GNU assembler.
12954 Output code for g-format floating point numbers instead of d-format.
12957 @node x86-64 Options
12958 @subsection x86-64 Options
12959 @cindex x86-64 options
12961 These are listed under @xref{i386 and x86-64 Options}.
12963 @node Xstormy16 Options
12964 @subsection Xstormy16 Options
12965 @cindex Xstormy16 Options
12967 These options are defined for Xstormy16:
12972 Choose startup files and linker script suitable for the simulator.
12975 @node Xtensa Options
12976 @subsection Xtensa Options
12977 @cindex Xtensa Options
12979 These options are supported for Xtensa targets:
12983 @itemx -mno-const16
12985 @opindex mno-const16
12986 Enable or disable use of @code{CONST16} instructions for loading
12987 constant values. The @code{CONST16} instruction is currently not a
12988 standard option from Tensilica. When enabled, @code{CONST16}
12989 instructions are always used in place of the standard @code{L32R}
12990 instructions. The use of @code{CONST16} is enabled by default only if
12991 the @code{L32R} instruction is not available.
12994 @itemx -mno-fused-madd
12995 @opindex mfused-madd
12996 @opindex mno-fused-madd
12997 Enable or disable use of fused multiply/add and multiply/subtract
12998 instructions in the floating-point option. This has no effect if the
12999 floating-point option is not also enabled. Disabling fused multiply/add
13000 and multiply/subtract instructions forces the compiler to use separate
13001 instructions for the multiply and add/subtract operations. This may be
13002 desirable in some cases where strict IEEE 754-compliant results are
13003 required: the fused multiply add/subtract instructions do not round the
13004 intermediate result, thereby producing results with @emph{more} bits of
13005 precision than specified by the IEEE standard. Disabling fused multiply
13006 add/subtract instructions also ensures that the program output is not
13007 sensitive to the compiler's ability to combine multiply and add/subtract
13010 @item -mtext-section-literals
13011 @itemx -mno-text-section-literals
13012 @opindex mtext-section-literals
13013 @opindex mno-text-section-literals
13014 Control the treatment of literal pools. The default is
13015 @option{-mno-text-section-literals}, which places literals in a separate
13016 section in the output file. This allows the literal pool to be placed
13017 in a data RAM/ROM, and it also allows the linker to combine literal
13018 pools from separate object files to remove redundant literals and
13019 improve code size. With @option{-mtext-section-literals}, the literals
13020 are interspersed in the text section in order to keep them as close as
13021 possible to their references. This may be necessary for large assembly
13024 @item -mtarget-align
13025 @itemx -mno-target-align
13026 @opindex mtarget-align
13027 @opindex mno-target-align
13028 When this option is enabled, GCC instructs the assembler to
13029 automatically align instructions to reduce branch penalties at the
13030 expense of some code density. The assembler attempts to widen density
13031 instructions to align branch targets and the instructions following call
13032 instructions. If there are not enough preceding safe density
13033 instructions to align a target, no widening will be performed. The
13034 default is @option{-mtarget-align}. These options do not affect the
13035 treatment of auto-aligned instructions like @code{LOOP}, which the
13036 assembler will always align, either by widening density instructions or
13037 by inserting no-op instructions.
13040 @itemx -mno-longcalls
13041 @opindex mlongcalls
13042 @opindex mno-longcalls
13043 When this option is enabled, GCC instructs the assembler to translate
13044 direct calls to indirect calls unless it can determine that the target
13045 of a direct call is in the range allowed by the call instruction. This
13046 translation typically occurs for calls to functions in other source
13047 files. Specifically, the assembler translates a direct @code{CALL}
13048 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
13049 The default is @option{-mno-longcalls}. This option should be used in
13050 programs where the call target can potentially be out of range. This
13051 option is implemented in the assembler, not the compiler, so the
13052 assembly code generated by GCC will still show direct call
13053 instructions---look at the disassembled object code to see the actual
13054 instructions. Note that the assembler will use an indirect call for
13055 every cross-file call, not just those that really will be out of range.
13058 @node zSeries Options
13059 @subsection zSeries Options
13060 @cindex zSeries options
13062 These are listed under @xref{S/390 and zSeries Options}.
13064 @node Code Gen Options
13065 @section Options for Code Generation Conventions
13066 @cindex code generation conventions
13067 @cindex options, code generation
13068 @cindex run-time options
13070 These machine-independent options control the interface conventions
13071 used in code generation.
13073 Most of them have both positive and negative forms; the negative form
13074 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
13075 one of the forms is listed---the one which is not the default. You
13076 can figure out the other form by either removing @samp{no-} or adding
13080 @item -fbounds-check
13081 @opindex fbounds-check
13082 For front-ends that support it, generate additional code to check that
13083 indices used to access arrays are within the declared range. This is
13084 currently only supported by the Java and Fortran 77 front-ends, where
13085 this option defaults to true and false respectively.
13089 This option generates traps for signed overflow on addition, subtraction,
13090 multiplication operations.
13094 This option instructs the compiler to assume that signed arithmetic
13095 overflow of addition, subtraction and multiplication wraps around
13096 using twos-complement representation. This flag enables some optimizations
13097 and disables others. This option is enabled by default for the Java
13098 front-end, as required by the Java language specification.
13101 @opindex fexceptions
13102 Enable exception handling. Generates extra code needed to propagate
13103 exceptions. For some targets, this implies GCC will generate frame
13104 unwind information for all functions, which can produce significant data
13105 size overhead, although it does not affect execution. If you do not
13106 specify this option, GCC will enable it by default for languages like
13107 C++ which normally require exception handling, and disable it for
13108 languages like C that do not normally require it. However, you may need
13109 to enable this option when compiling C code that needs to interoperate
13110 properly with exception handlers written in C++. You may also wish to
13111 disable this option if you are compiling older C++ programs that don't
13112 use exception handling.
13114 @item -fnon-call-exceptions
13115 @opindex fnon-call-exceptions
13116 Generate code that allows trapping instructions to throw exceptions.
13117 Note that this requires platform-specific runtime support that does
13118 not exist everywhere. Moreover, it only allows @emph{trapping}
13119 instructions to throw exceptions, i.e.@: memory references or floating
13120 point instructions. It does not allow exceptions to be thrown from
13121 arbitrary signal handlers such as @code{SIGALRM}.
13123 @item -funwind-tables
13124 @opindex funwind-tables
13125 Similar to @option{-fexceptions}, except that it will just generate any needed
13126 static data, but will not affect the generated code in any other way.
13127 You will normally not enable this option; instead, a language processor
13128 that needs this handling would enable it on your behalf.
13130 @item -fasynchronous-unwind-tables
13131 @opindex fasynchronous-unwind-tables
13132 Generate unwind table in dwarf2 format, if supported by target machine. The
13133 table is exact at each instruction boundary, so it can be used for stack
13134 unwinding from asynchronous events (such as debugger or garbage collector).
13136 @item -fpcc-struct-return
13137 @opindex fpcc-struct-return
13138 Return ``short'' @code{struct} and @code{union} values in memory like
13139 longer ones, rather than in registers. This convention is less
13140 efficient, but it has the advantage of allowing intercallability between
13141 GCC-compiled files and files compiled with other compilers, particularly
13142 the Portable C Compiler (pcc).
13144 The precise convention for returning structures in memory depends
13145 on the target configuration macros.
13147 Short structures and unions are those whose size and alignment match
13148 that of some integer type.
13150 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
13151 switch is not binary compatible with code compiled with the
13152 @option{-freg-struct-return} switch.
13153 Use it to conform to a non-default application binary interface.
13155 @item -freg-struct-return
13156 @opindex freg-struct-return
13157 Return @code{struct} and @code{union} values in registers when possible.
13158 This is more efficient for small structures than
13159 @option{-fpcc-struct-return}.
13161 If you specify neither @option{-fpcc-struct-return} nor
13162 @option{-freg-struct-return}, GCC defaults to whichever convention is
13163 standard for the target. If there is no standard convention, GCC
13164 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
13165 the principal compiler. In those cases, we can choose the standard, and
13166 we chose the more efficient register return alternative.
13168 @strong{Warning:} code compiled with the @option{-freg-struct-return}
13169 switch is not binary compatible with code compiled with the
13170 @option{-fpcc-struct-return} switch.
13171 Use it to conform to a non-default application binary interface.
13173 @item -fshort-enums
13174 @opindex fshort-enums
13175 Allocate to an @code{enum} type only as many bytes as it needs for the
13176 declared range of possible values. Specifically, the @code{enum} type
13177 will be equivalent to the smallest integer type which has enough room.
13179 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
13180 code that is not binary compatible with code generated without that switch.
13181 Use it to conform to a non-default application binary interface.
13183 @item -fshort-double
13184 @opindex fshort-double
13185 Use the same size for @code{double} as for @code{float}.
13187 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
13188 code that is not binary compatible with code generated without that switch.
13189 Use it to conform to a non-default application binary interface.
13191 @item -fshort-wchar
13192 @opindex fshort-wchar
13193 Override the underlying type for @samp{wchar_t} to be @samp{short
13194 unsigned int} instead of the default for the target. This option is
13195 useful for building programs to run under WINE@.
13197 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
13198 code that is not binary compatible with code generated without that switch.
13199 Use it to conform to a non-default application binary interface.
13202 @opindex fno-common
13203 In C, allocate even uninitialized global variables in the data section of the
13204 object file, rather than generating them as common blocks. This has the
13205 effect that if the same variable is declared (without @code{extern}) in
13206 two different compilations, you will get an error when you link them.
13207 The only reason this might be useful is if you wish to verify that the
13208 program will work on other systems which always work this way.
13212 Ignore the @samp{#ident} directive.
13214 @item -finhibit-size-directive
13215 @opindex finhibit-size-directive
13216 Don't output a @code{.size} assembler directive, or anything else that
13217 would cause trouble if the function is split in the middle, and the
13218 two halves are placed at locations far apart in memory. This option is
13219 used when compiling @file{crtstuff.c}; you should not need to use it
13222 @item -fverbose-asm
13223 @opindex fverbose-asm
13224 Put extra commentary information in the generated assembly code to
13225 make it more readable. This option is generally only of use to those
13226 who actually need to read the generated assembly code (perhaps while
13227 debugging the compiler itself).
13229 @option{-fno-verbose-asm}, the default, causes the
13230 extra information to be omitted and is useful when comparing two assembler
13235 @cindex global offset table
13237 Generate position-independent code (PIC) suitable for use in a shared
13238 library, if supported for the target machine. Such code accesses all
13239 constant addresses through a global offset table (GOT)@. The dynamic
13240 loader resolves the GOT entries when the program starts (the dynamic
13241 loader is not part of GCC; it is part of the operating system). If
13242 the GOT size for the linked executable exceeds a machine-specific
13243 maximum size, you get an error message from the linker indicating that
13244 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
13245 instead. (These maximums are 8k on the SPARC and 32k
13246 on the m68k and RS/6000. The 386 has no such limit.)
13248 Position-independent code requires special support, and therefore works
13249 only on certain machines. For the 386, GCC supports PIC for System V
13250 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
13251 position-independent.
13253 When this flag is set, the macros @code{__pic__} and @code{__PIC__}
13258 If supported for the target machine, emit position-independent code,
13259 suitable for dynamic linking and avoiding any limit on the size of the
13260 global offset table. This option makes a difference on the m68k,
13261 PowerPC and SPARC@.
13263 Position-independent code requires special support, and therefore works
13264 only on certain machines.
13266 When this flag is set, the macros @code{__pic__} and @code{__PIC__}
13273 These options are similar to @option{-fpic} and @option{-fPIC}, but
13274 generated position independent code can be only linked into executables.
13275 Usually these options are used when @option{-pie} GCC option will be
13276 used during linking.
13278 @item -fno-jump-tables
13279 @opindex fno-jump-tables
13280 Do not use jump tables for switch statements even where it would be
13281 more efficient than other code generation strategies. This option is
13282 of use in conjunction with @option{-fpic} or @option{-fPIC} for
13283 building code which forms part of a dynamic linker and cannot
13284 reference the address of a jump table. On some targets, jump tables
13285 do not require a GOT and this option is not needed.
13287 @item -ffixed-@var{reg}
13289 Treat the register named @var{reg} as a fixed register; generated code
13290 should never refer to it (except perhaps as a stack pointer, frame
13291 pointer or in some other fixed role).
13293 @var{reg} must be the name of a register. The register names accepted
13294 are machine-specific and are defined in the @code{REGISTER_NAMES}
13295 macro in the machine description macro file.
13297 This flag does not have a negative form, because it specifies a
13300 @item -fcall-used-@var{reg}
13301 @opindex fcall-used
13302 Treat the register named @var{reg} as an allocable register that is
13303 clobbered by function calls. It may be allocated for temporaries or
13304 variables that do not live across a call. Functions compiled this way
13305 will not save and restore the register @var{reg}.
13307 It is an error to used this flag with the frame pointer or stack pointer.
13308 Use of this flag for other registers that have fixed pervasive roles in
13309 the machine's execution model will produce disastrous results.
13311 This flag does not have a negative form, because it specifies a
13314 @item -fcall-saved-@var{reg}
13315 @opindex fcall-saved
13316 Treat the register named @var{reg} as an allocable register saved by
13317 functions. It may be allocated even for temporaries or variables that
13318 live across a call. Functions compiled this way will save and restore
13319 the register @var{reg} if they use it.
13321 It is an error to used this flag with the frame pointer or stack pointer.
13322 Use of this flag for other registers that have fixed pervasive roles in
13323 the machine's execution model will produce disastrous results.
13325 A different sort of disaster will result from the use of this flag for
13326 a register in which function values may be returned.
13328 This flag does not have a negative form, because it specifies a
13331 @item -fpack-struct[=@var{n}]
13332 @opindex fpack-struct
13333 Without a value specified, pack all structure members together without
13334 holes. When a value is specified (which must be a small power of two), pack
13335 structure members according to this value, representing the maximum
13336 alignment (that is, objects with default alignment requirements larger than
13337 this will be output potentially unaligned at the next fitting location.
13339 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
13340 code that is not binary compatible with code generated without that switch.
13341 Additionally, it makes the code suboptimal.
13342 Use it to conform to a non-default application binary interface.
13344 @item -finstrument-functions
13345 @opindex finstrument-functions
13346 Generate instrumentation calls for entry and exit to functions. Just
13347 after function entry and just before function exit, the following
13348 profiling functions will be called with the address of the current
13349 function and its call site. (On some platforms,
13350 @code{__builtin_return_address} does not work beyond the current
13351 function, so the call site information may not be available to the
13352 profiling functions otherwise.)
13355 void __cyg_profile_func_enter (void *this_fn,
13357 void __cyg_profile_func_exit (void *this_fn,
13361 The first argument is the address of the start of the current function,
13362 which may be looked up exactly in the symbol table.
13364 This instrumentation is also done for functions expanded inline in other
13365 functions. The profiling calls will indicate where, conceptually, the
13366 inline function is entered and exited. This means that addressable
13367 versions of such functions must be available. If all your uses of a
13368 function are expanded inline, this may mean an additional expansion of
13369 code size. If you use @samp{extern inline} in your C code, an
13370 addressable version of such functions must be provided. (This is
13371 normally the case anyways, but if you get lucky and the optimizer always
13372 expands the functions inline, you might have gotten away without
13373 providing static copies.)
13375 A function may be given the attribute @code{no_instrument_function}, in
13376 which case this instrumentation will not be done. This can be used, for
13377 example, for the profiling functions listed above, high-priority
13378 interrupt routines, and any functions from which the profiling functions
13379 cannot safely be called (perhaps signal handlers, if the profiling
13380 routines generate output or allocate memory).
13382 @item -fstack-check
13383 @opindex fstack-check
13384 Generate code to verify that you do not go beyond the boundary of the
13385 stack. You should specify this flag if you are running in an
13386 environment with multiple threads, but only rarely need to specify it in
13387 a single-threaded environment since stack overflow is automatically
13388 detected on nearly all systems if there is only one stack.
13390 Note that this switch does not actually cause checking to be done; the
13391 operating system must do that. The switch causes generation of code
13392 to ensure that the operating system sees the stack being extended.
13394 @item -fstack-limit-register=@var{reg}
13395 @itemx -fstack-limit-symbol=@var{sym}
13396 @itemx -fno-stack-limit
13397 @opindex fstack-limit-register
13398 @opindex fstack-limit-symbol
13399 @opindex fno-stack-limit
13400 Generate code to ensure that the stack does not grow beyond a certain value,
13401 either the value of a register or the address of a symbol. If the stack
13402 would grow beyond the value, a signal is raised. For most targets,
13403 the signal is raised before the stack overruns the boundary, so
13404 it is possible to catch the signal without taking special precautions.
13406 For instance, if the stack starts at absolute address @samp{0x80000000}
13407 and grows downwards, you can use the flags
13408 @option{-fstack-limit-symbol=__stack_limit} and
13409 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
13410 of 128KB@. Note that this may only work with the GNU linker.
13412 @cindex aliasing of parameters
13413 @cindex parameters, aliased
13414 @item -fargument-alias
13415 @itemx -fargument-noalias
13416 @itemx -fargument-noalias-global
13417 @itemx -fargument-noalias-anything
13418 @opindex fargument-alias
13419 @opindex fargument-noalias
13420 @opindex fargument-noalias-global
13421 @opindex fargument-noalias-anything
13422 Specify the possible relationships among parameters and between
13423 parameters and global data.
13425 @option{-fargument-alias} specifies that arguments (parameters) may
13426 alias each other and may alias global storage.@*
13427 @option{-fargument-noalias} specifies that arguments do not alias
13428 each other, but may alias global storage.@*
13429 @option{-fargument-noalias-global} specifies that arguments do not
13430 alias each other and do not alias global storage.
13431 @option{-fargument-noalias-anything} specifies that arguments do not
13432 alias any other storage.
13434 Each language will automatically use whatever option is required by
13435 the language standard. You should not need to use these options yourself.
13437 @item -fleading-underscore
13438 @opindex fleading-underscore
13439 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
13440 change the way C symbols are represented in the object file. One use
13441 is to help link with legacy assembly code.
13443 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
13444 generate code that is not binary compatible with code generated without that
13445 switch. Use it to conform to a non-default application binary interface.
13446 Not all targets provide complete support for this switch.
13448 @item -ftls-model=@var{model}
13449 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
13450 The @var{model} argument should be one of @code{global-dynamic},
13451 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
13453 The default without @option{-fpic} is @code{initial-exec}; with
13454 @option{-fpic} the default is @code{global-dynamic}.
13456 @item -fvisibility=@var{default|internal|hidden|protected}
13457 @opindex fvisibility
13458 Set the default ELF image symbol visibility to the specified option---all
13459 symbols will be marked with this unless overridden within the code.
13460 Using this feature can very substantially improve linking and
13461 load times of shared object libraries, produce more optimized
13462 code, provide near-perfect API export and prevent symbol clashes.
13463 It is @strong{strongly} recommended that you use this in any shared objects
13466 Despite the nomenclature, @code{default} always means public ie;
13467 available to be linked against from outside the shared object.
13468 @code{protected} and @code{internal} are pretty useless in real-world
13469 usage so the only other commonly used option will be @code{hidden}.
13470 The default if @option{-fvisibility} isn't specified is
13471 @code{default}, i.e., make every
13472 symbol public---this causes the same behavior as previous versions of
13475 A good explanation of the benefits offered by ensuring ELF
13476 symbols have the correct visibility is given by ``How To Write
13477 Shared Libraries'' by Ulrich Drepper (which can be found at
13478 @w{@uref{http://people.redhat.com/~drepper/}})---however a superior
13479 solution made possible by this option to marking things hidden when
13480 the default is public is to make the default hidden and mark things
13481 public. This is the norm with DLL's on Windows and with @option{-fvisibility=hidden}
13482 and @code{__attribute__ ((visibility("default")))} instead of
13483 @code{__declspec(dllexport)} you get almost identical semantics with
13484 identical syntax. This is a great boon to those working with
13485 cross-platform projects.
13487 For those adding visibility support to existing code, you may find
13488 @samp{#pragma GCC visibility} of use. This works by you enclosing
13489 the declarations you wish to set visibility for with (for example)
13490 @samp{#pragma GCC visibility push(hidden)} and
13491 @samp{#pragma GCC visibility pop}.
13492 Bear in mind that symbol visibility should be viewed @strong{as
13493 part of the API interface contract} and thus all new code should
13494 always specify visibility when it is not the default ie; declarations
13495 only for use within the local DSO should @strong{always} be marked explicitly
13496 as hidden as so to avoid PLT indirection overheads---making this
13497 abundantly clear also aids readability and self-documentation of the code.
13498 Note that due to ISO C++ specification requirements, operator new and
13499 operator delete must always be of default visibility.
13501 Be aware that headers from outside your project, in particular system
13502 headers and headers from any other library you use, may not be
13503 expecting to be compiled with visibility other than the default. You
13504 may need to explicitly say @samp{#pragma GCC visibility push(default)}
13505 before including any such headers.
13507 An overview of these techniques, their benefits and how to use them
13508 is at @w{@uref{http://gcc.gnu.org/wiki/Visibility}}.
13512 @cindex openmp parallel
13513 Enable handling of OpenMP directives @code{#pragma omp} in C/C++ and
13514 @code{!$omp} in Fortran. When @option{-fopenmp} is specified, the
13515 compiler generates parallel code according to the OpenMP Application
13516 Program Interface v2.5 @w{@uref{http://www.openmp.org/}}.
13522 @node Environment Variables
13523 @section Environment Variables Affecting GCC
13524 @cindex environment variables
13526 @c man begin ENVIRONMENT
13527 This section describes several environment variables that affect how GCC
13528 operates. Some of them work by specifying directories or prefixes to use
13529 when searching for various kinds of files. Some are used to specify other
13530 aspects of the compilation environment.
13532 Note that you can also specify places to search using options such as
13533 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
13534 take precedence over places specified using environment variables, which
13535 in turn take precedence over those specified by the configuration of GCC@.
13536 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
13537 GNU Compiler Collection (GCC) Internals}.
13542 @c @itemx LC_COLLATE
13544 @c @itemx LC_MONETARY
13545 @c @itemx LC_NUMERIC
13550 @c @findex LC_COLLATE
13551 @findex LC_MESSAGES
13552 @c @findex LC_MONETARY
13553 @c @findex LC_NUMERIC
13557 These environment variables control the way that GCC uses
13558 localization information that allow GCC to work with different
13559 national conventions. GCC inspects the locale categories
13560 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
13561 so. These locale categories can be set to any value supported by your
13562 installation. A typical value is @samp{en_GB.UTF-8} for English in the United
13563 Kingdom encoded in UTF-8.
13565 The @env{LC_CTYPE} environment variable specifies character
13566 classification. GCC uses it to determine the character boundaries in
13567 a string; this is needed for some multibyte encodings that contain quote
13568 and escape characters that would otherwise be interpreted as a string
13571 The @env{LC_MESSAGES} environment variable specifies the language to
13572 use in diagnostic messages.
13574 If the @env{LC_ALL} environment variable is set, it overrides the value
13575 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
13576 and @env{LC_MESSAGES} default to the value of the @env{LANG}
13577 environment variable. If none of these variables are set, GCC
13578 defaults to traditional C English behavior.
13582 If @env{TMPDIR} is set, it specifies the directory to use for temporary
13583 files. GCC uses temporary files to hold the output of one stage of
13584 compilation which is to be used as input to the next stage: for example,
13585 the output of the preprocessor, which is the input to the compiler
13588 @item GCC_EXEC_PREFIX
13589 @findex GCC_EXEC_PREFIX
13590 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
13591 names of the subprograms executed by the compiler. No slash is added
13592 when this prefix is combined with the name of a subprogram, but you can
13593 specify a prefix that ends with a slash if you wish.
13595 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
13596 an appropriate prefix to use based on the pathname it was invoked with.
13598 If GCC cannot find the subprogram using the specified prefix, it
13599 tries looking in the usual places for the subprogram.
13601 The default value of @env{GCC_EXEC_PREFIX} is
13602 @file{@var{prefix}/lib/gcc/} where @var{prefix} is the value
13603 of @code{prefix} when you ran the @file{configure} script.
13605 Other prefixes specified with @option{-B} take precedence over this prefix.
13607 This prefix is also used for finding files such as @file{crt0.o} that are
13610 In addition, the prefix is used in an unusual way in finding the
13611 directories to search for header files. For each of the standard
13612 directories whose name normally begins with @samp{/usr/local/lib/gcc}
13613 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
13614 replacing that beginning with the specified prefix to produce an
13615 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
13616 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
13617 These alternate directories are searched first; the standard directories
13620 @item COMPILER_PATH
13621 @findex COMPILER_PATH
13622 The value of @env{COMPILER_PATH} is a colon-separated list of
13623 directories, much like @env{PATH}. GCC tries the directories thus
13624 specified when searching for subprograms, if it can't find the
13625 subprograms using @env{GCC_EXEC_PREFIX}.
13628 @findex LIBRARY_PATH
13629 The value of @env{LIBRARY_PATH} is a colon-separated list of
13630 directories, much like @env{PATH}. When configured as a native compiler,
13631 GCC tries the directories thus specified when searching for special
13632 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
13633 using GCC also uses these directories when searching for ordinary
13634 libraries for the @option{-l} option (but directories specified with
13635 @option{-L} come first).
13639 @cindex locale definition
13640 This variable is used to pass locale information to the compiler. One way in
13641 which this information is used is to determine the character set to be used
13642 when character literals, string literals and comments are parsed in C and C++.
13643 When the compiler is configured to allow multibyte characters,
13644 the following values for @env{LANG} are recognized:
13648 Recognize JIS characters.
13650 Recognize SJIS characters.
13652 Recognize EUCJP characters.
13655 If @env{LANG} is not defined, or if it has some other value, then the
13656 compiler will use mblen and mbtowc as defined by the default locale to
13657 recognize and translate multibyte characters.
13661 Some additional environments variables affect the behavior of the
13664 @include cppenv.texi
13668 @node Precompiled Headers
13669 @section Using Precompiled Headers
13670 @cindex precompiled headers
13671 @cindex speed of compilation
13673 Often large projects have many header files that are included in every
13674 source file. The time the compiler takes to process these header files
13675 over and over again can account for nearly all of the time required to
13676 build the project. To make builds faster, GCC allows users to
13677 `precompile' a header file; then, if builds can use the precompiled
13678 header file they will be much faster.
13680 To create a precompiled header file, simply compile it as you would any
13681 other file, if necessary using the @option{-x} option to make the driver
13682 treat it as a C or C++ header file. You will probably want to use a
13683 tool like @command{make} to keep the precompiled header up-to-date when
13684 the headers it contains change.
13686 A precompiled header file will be searched for when @code{#include} is
13687 seen in the compilation. As it searches for the included file
13688 (@pxref{Search Path,,Search Path,cpp,The C Preprocessor}) the
13689 compiler looks for a precompiled header in each directory just before it
13690 looks for the include file in that directory. The name searched for is
13691 the name specified in the @code{#include} with @samp{.gch} appended. If
13692 the precompiled header file can't be used, it is ignored.
13694 For instance, if you have @code{#include "all.h"}, and you have
13695 @file{all.h.gch} in the same directory as @file{all.h}, then the
13696 precompiled header file will be used if possible, and the original
13697 header will be used otherwise.
13699 Alternatively, you might decide to put the precompiled header file in a
13700 directory and use @option{-I} to ensure that directory is searched
13701 before (or instead of) the directory containing the original header.
13702 Then, if you want to check that the precompiled header file is always
13703 used, you can put a file of the same name as the original header in this
13704 directory containing an @code{#error} command.
13706 This also works with @option{-include}. So yet another way to use
13707 precompiled headers, good for projects not designed with precompiled
13708 header files in mind, is to simply take most of the header files used by
13709 a project, include them from another header file, precompile that header
13710 file, and @option{-include} the precompiled header. If the header files
13711 have guards against multiple inclusion, they will be skipped because
13712 they've already been included (in the precompiled header).
13714 If you need to precompile the same header file for different
13715 languages, targets, or compiler options, you can instead make a
13716 @emph{directory} named like @file{all.h.gch}, and put each precompiled
13717 header in the directory, perhaps using @option{-o}. It doesn't matter
13718 what you call the files in the directory, every precompiled header in
13719 the directory will be considered. The first precompiled header
13720 encountered in the directory that is valid for this compilation will
13721 be used; they're searched in no particular order.
13723 There are many other possibilities, limited only by your imagination,
13724 good sense, and the constraints of your build system.
13726 A precompiled header file can be used only when these conditions apply:
13730 Only one precompiled header can be used in a particular compilation.
13733 A precompiled header can't be used once the first C token is seen. You
13734 can have preprocessor directives before a precompiled header; you can
13735 even include a precompiled header from inside another header, so long as
13736 there are no C tokens before the @code{#include}.
13739 The precompiled header file must be produced for the same language as
13740 the current compilation. You can't use a C precompiled header for a C++
13744 The precompiled header file must have been produced by the same compiler
13745 binary as the current compilation is using.
13748 Any macros defined before the precompiled header is included must
13749 either be defined in the same way as when the precompiled header was
13750 generated, or must not affect the precompiled header, which usually
13751 means that they don't appear in the precompiled header at all.
13753 The @option{-D} option is one way to define a macro before a
13754 precompiled header is included; using a @code{#define} can also do it.
13755 There are also some options that define macros implicitly, like
13756 @option{-O} and @option{-Wdeprecated}; the same rule applies to macros
13759 @item If debugging information is output when using the precompiled
13760 header, using @option{-g} or similar, the same kind of debugging information
13761 must have been output when building the precompiled header. However,
13762 a precompiled header built using @option{-g} can be used in a compilation
13763 when no debugging information is being output.
13765 @item The same @option{-m} options must generally be used when building
13766 and using the precompiled header. @xref{Submodel Options},
13767 for any cases where this rule is relaxed.
13769 @item Each of the following options must be the same when building and using
13770 the precompiled header:
13772 @gccoptlist{-fexceptions -funit-at-a-time}
13775 Some other command-line options starting with @option{-f},
13776 @option{-p}, or @option{-O} must be defined in the same way as when
13777 the precompiled header was generated. At present, it's not clear
13778 which options are safe to change and which are not; the safest choice
13779 is to use exactly the same options when generating and using the
13780 precompiled header. The following are known to be safe:
13782 @gccoptlist{-fmessage-length= -fpreprocessed
13783 -fsched-interblock -fsched-spec -fsched-spec-load -fsched-spec-load-dangerous
13784 -fsched-verbose=<number> -fschedule-insns -fvisibility=
13789 For all of these except the last, the compiler will automatically
13790 ignore the precompiled header if the conditions aren't met. If you
13791 find an option combination that doesn't work and doesn't cause the
13792 precompiled header to be ignored, please consider filing a bug report,
13795 If you do use differing options when generating and using the
13796 precompiled header, the actual behavior will be a mixture of the
13797 behavior for the options. For instance, if you use @option{-g} to
13798 generate the precompiled header but not when using it, you may or may
13799 not get debugging information for routines in the precompiled header.
13801 @node Running Protoize
13802 @section Running Protoize
13804 The program @code{protoize} is an optional part of GCC@. You can use
13805 it to add prototypes to a program, thus converting the program to ISO
13806 C in one respect. The companion program @code{unprotoize} does the
13807 reverse: it removes argument types from any prototypes that are found.
13809 When you run these programs, you must specify a set of source files as
13810 command line arguments. The conversion programs start out by compiling
13811 these files to see what functions they define. The information gathered
13812 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
13814 After scanning comes actual conversion. The specified files are all
13815 eligible to be converted; any files they include (whether sources or
13816 just headers) are eligible as well.
13818 But not all the eligible files are converted. By default,
13819 @code{protoize} and @code{unprotoize} convert only source and header
13820 files in the current directory. You can specify additional directories
13821 whose files should be converted with the @option{-d @var{directory}}
13822 option. You can also specify particular files to exclude with the
13823 @option{-x @var{file}} option. A file is converted if it is eligible, its
13824 directory name matches one of the specified directory names, and its
13825 name within the directory has not been excluded.
13827 Basic conversion with @code{protoize} consists of rewriting most
13828 function definitions and function declarations to specify the types of
13829 the arguments. The only ones not rewritten are those for varargs
13832 @code{protoize} optionally inserts prototype declarations at the
13833 beginning of the source file, to make them available for any calls that
13834 precede the function's definition. Or it can insert prototype
13835 declarations with block scope in the blocks where undeclared functions
13838 Basic conversion with @code{unprotoize} consists of rewriting most
13839 function declarations to remove any argument types, and rewriting
13840 function definitions to the old-style pre-ISO form.
13842 Both conversion programs print a warning for any function declaration or
13843 definition that they can't convert. You can suppress these warnings
13846 The output from @code{protoize} or @code{unprotoize} replaces the
13847 original source file. The original file is renamed to a name ending
13848 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
13849 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
13850 for DOS) file already exists, then the source file is simply discarded.
13852 @code{protoize} and @code{unprotoize} both depend on GCC itself to
13853 scan the program and collect information about the functions it uses.
13854 So neither of these programs will work until GCC is installed.
13856 Here is a table of the options you can use with @code{protoize} and
13857 @code{unprotoize}. Each option works with both programs unless
13861 @item -B @var{directory}
13862 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
13863 usual directory (normally @file{/usr/local/lib}). This file contains
13864 prototype information about standard system functions. This option
13865 applies only to @code{protoize}.
13867 @item -c @var{compilation-options}
13868 Use @var{compilation-options} as the options when running @command{gcc} to
13869 produce the @samp{.X} files. The special option @option{-aux-info} is
13870 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
13872 Note that the compilation options must be given as a single argument to
13873 @code{protoize} or @code{unprotoize}. If you want to specify several
13874 @command{gcc} options, you must quote the entire set of compilation options
13875 to make them a single word in the shell.
13877 There are certain @command{gcc} arguments that you cannot use, because they
13878 would produce the wrong kind of output. These include @option{-g},
13879 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
13880 the @var{compilation-options}, they are ignored.
13883 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
13884 systems) instead of @samp{.c}. This is convenient if you are converting
13885 a C program to C++. This option applies only to @code{protoize}.
13888 Add explicit global declarations. This means inserting explicit
13889 declarations at the beginning of each source file for each function
13890 that is called in the file and was not declared. These declarations
13891 precede the first function definition that contains a call to an
13892 undeclared function. This option applies only to @code{protoize}.
13894 @item -i @var{string}
13895 Indent old-style parameter declarations with the string @var{string}.
13896 This option applies only to @code{protoize}.
13898 @code{unprotoize} converts prototyped function definitions to old-style
13899 function definitions, where the arguments are declared between the
13900 argument list and the initial @samp{@{}. By default, @code{unprotoize}
13901 uses five spaces as the indentation. If you want to indent with just
13902 one space instead, use @option{-i " "}.
13905 Keep the @samp{.X} files. Normally, they are deleted after conversion
13909 Add explicit local declarations. @code{protoize} with @option{-l} inserts
13910 a prototype declaration for each function in each block which calls the
13911 function without any declaration. This option applies only to
13915 Make no real changes. This mode just prints information about the conversions
13916 that would have been done without @option{-n}.
13919 Make no @samp{.save} files. The original files are simply deleted.
13920 Use this option with caution.
13922 @item -p @var{program}
13923 Use the program @var{program} as the compiler. Normally, the name
13924 @file{gcc} is used.
13927 Work quietly. Most warnings are suppressed.
13930 Print the version number, just like @option{-v} for @command{gcc}.
13933 If you need special compiler options to compile one of your program's
13934 source files, then you should generate that file's @samp{.X} file
13935 specially, by running @command{gcc} on that source file with the
13936 appropriate options and the option @option{-aux-info}. Then run
13937 @code{protoize} on the entire set of files. @code{protoize} will use
13938 the existing @samp{.X} file because it is newer than the source file.
13942 gcc -Dfoo=bar file1.c -aux-info file1.X
13947 You need to include the special files along with the rest in the
13948 @code{protoize} command, even though their @samp{.X} files already
13949 exist, because otherwise they won't get converted.
13951 @xref{Protoize Caveats}, for more information on how to use
13952 @code{protoize} successfully.