1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2 @c 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
3 @c This is part of the GCC manual.
4 @c For copying conditions, see the file gcc.texi.
8 Copyright @copyright{} 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997,
9 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
11 Permission is granted to copy, distribute and/or modify this document
12 under the terms of the GNU Free Documentation License, Version 1.2 or
13 any later version published by the Free Software Foundation; with the
14 Invariant Sections being ``GNU General Public License'' and ``Funding
15 Free Software'', the Front-Cover texts being (a) (see below), and with
16 the Back-Cover Texts being (b) (see below). A copy of the license is
17 included in the gfdl(7) man page.
19 (a) The FSF's Front-Cover Text is:
23 (b) The FSF's Back-Cover Text is:
25 You have freedom to copy and modify this GNU Manual, like GNU
26 software. Copies published by the Free Software Foundation raise
27 funds for GNU development.
29 @c Set file name and title for the man page.
31 @settitle GNU project C and C++ compiler
33 gcc [@option{-c}|@option{-S}|@option{-E}] [@option{-std=}@var{standard}]
34 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
35 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
36 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
37 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
38 [@option{-f}@var{option}@dots{}] [@option{-m}@var{machine-option}@dots{}]
39 [@option{-o} @var{outfile}] @var{infile}@dots{}
41 Only the most useful options are listed here; see below for the
42 remainder. @samp{g++} accepts mostly the same options as @samp{gcc}.
45 gpl(7), gfdl(7), fsf-funding(7),
46 cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
47 and the Info entries for @file{gcc}, @file{cpp}, @file{as},
48 @file{ld}, @file{binutils} and @file{gdb}.
51 For instructions on reporting bugs, see
52 @w{@uref{http://gcc.gnu.org/bugs.html}}.
55 See the Info entry for @command{gcc}, or
56 @w{@uref{http://gcc.gnu.org/onlinedocs/gcc/Contributors.html}},
57 for contributors to GCC@.
62 @chapter GCC Command Options
63 @cindex GCC command options
64 @cindex command options
65 @cindex options, GCC command
67 @c man begin DESCRIPTION
68 When you invoke GCC, it normally does preprocessing, compilation,
69 assembly and linking. The ``overall options'' allow you to stop this
70 process at an intermediate stage. For example, the @option{-c} option
71 says not to run the linker. Then the output consists of object files
72 output by the assembler.
74 Other options are passed on to one stage of processing. Some options
75 control the preprocessor and others the compiler itself. Yet other
76 options control the assembler and linker; most of these are not
77 documented here, since you rarely need to use any of them.
79 @cindex C compilation options
80 Most of the command line options that you can use with GCC are useful
81 for C programs; when an option is only useful with another language
82 (usually C++), the explanation says so explicitly. If the description
83 for a particular option does not mention a source language, you can use
84 that option with all supported languages.
86 @cindex C++ compilation options
87 @xref{Invoking G++,,Compiling C++ Programs}, for a summary of special
88 options for compiling C++ programs.
90 @cindex grouping options
91 @cindex options, grouping
92 The @command{gcc} program accepts options and file names as operands. Many
93 options have multi-letter names; therefore multiple single-letter options
94 may @emph{not} be grouped: @option{-dr} is very different from @w{@samp{-d
97 @cindex order of options
98 @cindex options, order
99 You can mix options and other arguments. For the most part, the order
100 you use doesn't matter. Order does matter when you use several options
101 of the same kind; for example, if you specify @option{-L} more than once,
102 the directories are searched in the order specified.
104 Many options have long names starting with @samp{-f} or with
105 @samp{-W}---for example, @option{-fforce-mem},
106 @option{-fstrength-reduce}, @option{-Wformat} and so on. Most of
107 these have both positive and negative forms; the negative form of
108 @option{-ffoo} would be @option{-fno-foo}. This manual documents
109 only one of these two forms, whichever one is not the default.
113 @xref{Option Index}, for an index to GCC's options.
116 * Option Summary:: Brief list of all options, without explanations.
117 * Overall Options:: Controlling the kind of output:
118 an executable, object files, assembler files,
119 or preprocessed source.
120 * Invoking G++:: Compiling C++ programs.
121 * C Dialect Options:: Controlling the variant of C language compiled.
122 * C++ Dialect Options:: Variations on C++.
123 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
125 * Language Independent Options:: Controlling how diagnostics should be
127 * Warning Options:: How picky should the compiler be?
128 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
129 * Optimize Options:: How much optimization?
130 * Preprocessor Options:: Controlling header files and macro definitions.
131 Also, getting dependency information for Make.
132 * Assembler Options:: Passing options to the assembler.
133 * Link Options:: Specifying libraries and so on.
134 * Directory Options:: Where to find header files and libraries.
135 Where to find the compiler executable files.
136 * Spec Files:: How to pass switches to sub-processes.
137 * Target Options:: Running a cross-compiler, or an old version of GCC.
138 * Submodel Options:: Specifying minor hardware or convention variations,
139 such as 68010 vs 68020.
140 * Code Gen Options:: Specifying conventions for function calls, data layout
142 * Environment Variables:: Env vars that affect GCC.
143 * Precompiled Headers:: Compiling a header once, and using it many times.
144 * Running Protoize:: Automatically adding or removing function prototypes.
150 @section Option Summary
152 Here is a summary of all the options, grouped by type. Explanations are
153 in the following sections.
156 @item Overall Options
157 @xref{Overall Options,,Options Controlling the Kind of Output}.
158 @gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
159 -x @var{language} -v -### --help --target-help --version}
161 @item C Language Options
162 @xref{C Dialect Options,,Options Controlling C Dialect}.
163 @gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
164 -fno-asm -fno-builtin -fno-builtin-@var{function} @gol
165 -fhosted -ffreestanding -fms-extensions @gol
166 -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
167 -fallow-single-precision -fcond-mismatch @gol
168 -fsigned-bitfields -fsigned-char @gol
169 -funsigned-bitfields -funsigned-char}
171 @item C++ Language Options
172 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
173 @gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
174 -fconserve-space -fno-const-strings @gol
175 -fno-elide-constructors @gol
176 -fno-enforce-eh-specs @gol
177 -ffor-scope -fno-for-scope -fno-gnu-keywords @gol
178 -fno-implicit-templates @gol
179 -fno-implicit-inline-templates @gol
180 -fno-implement-inlines -fms-extensions @gol
181 -fno-nonansi-builtins -fno-operator-names @gol
182 -fno-optional-diags -fpermissive @gol
183 -frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
184 -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ @gol
185 -fno-default-inline -fvisibility-inlines-hidden @gol
186 -Wabi -Wctor-dtor-privacy @gol
187 -Wnon-virtual-dtor -Wreorder @gol
188 -Weffc++ -Wno-deprecated @gol
189 -Wno-non-template-friend -Wold-style-cast @gol
190 -Woverloaded-virtual -Wno-pmf-conversions @gol
191 -Wsign-promo -Wsynth}
193 @item Objective-C and Objective-C++ Language Options
194 @xref{Objective-C and Objective-C++ Dialect Options,,Options Controlling
195 Objective-C and Objective-C++ Dialects}.
197 -fconstant-string-class=@var{class-name} @gol
198 -fgnu-runtime -fnext-runtime @gol
199 -fno-nil-receivers @gol
200 -fobjc-exceptions @gol
201 -freplace-objc-classes @gol
204 -Wno-protocol -Wselector -Wundeclared-selector}
206 @item Language Independent Options
207 @xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
208 @gccoptlist{-fmessage-length=@var{n} @gol
209 -fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]}}
211 @item Warning Options
212 @xref{Warning Options,,Options to Request or Suppress Warnings}.
213 @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
214 -w -Wextra -Wall -Waggregate-return @gol
215 -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment @gol
216 -Wconversion -Wno-deprecated-declarations @gol
217 -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels @gol
218 -Werror -Werror-implicit-function-declaration @gol
219 -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol
220 -Wno-format-extra-args -Wformat-nonliteral @gol
221 -Wformat-security -Wformat-y2k @gol
222 -Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
223 -Wimport -Wno-import -Winit-self -Winline @gol
224 -Wno-invalid-offsetof -Winvalid-pch @gol
225 -Wlarger-than-@var{len} -Wlong-long @gol
226 -Wmain -Wmissing-braces -Wmissing-field-initializers @gol
227 -Wmissing-format-attribute -Wmissing-include-dirs @gol
228 -Wmissing-noreturn @gol
229 -Wno-multichar -Wnonnull -Wpacked -Wpadded @gol
230 -Wparentheses -Wpointer-arith -Wredundant-decls @gol
231 -Wreturn-type -Wsequence-point -Wshadow @gol
232 -Wsign-compare -Wstrict-aliasing -Wstrict-aliasing=2 @gol
233 -Wswitch -Wswitch-default -Wswitch-enum @gol
234 -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
235 -Wunknown-pragmas -Wunreachable-code @gol
236 -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
237 -Wunused-value -Wunused-variable -Wwrite-strings @gol
240 @item C-only Warning Options
241 @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
242 -Wmissing-prototypes -Wnested-externs -Wold-style-definition @gol
243 -Wstrict-prototypes -Wtraditional @gol
244 -Wdeclaration-after-statement -Wno-pointer-sign}
246 @item Debugging Options
247 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
248 @gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
249 -fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
250 -fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
251 -fdump-ipa-all -fdump-ipa-cgraph @gol
253 -fdump-tree-original@r{[}-@var{n}@r{]} @gol
254 -fdump-tree-optimized@r{[}-@var{n}@r{]} @gol
255 -fdump-tree-inlined@r{[}-@var{n}@r{]} @gol
256 -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias @gol
258 -fdump-tree-ssa@r{[}-@var{n}@r{]} -fdump-tree-pre@r{[}-@var{n}@r{]} @gol
259 -fdump-tree-ccp@r{[}-@var{n}@r{]} -fdump-tree-dce@r{[}-@var{n}@r{]} @gol
260 -fdump-tree-gimple@r{[}-raw@r{]} -fdump-tree-mudflap@r{[}-@var{n}@r{]} @gol
261 -fdump-tree-dom@r{[}-@var{n}@r{]} @gol
262 -fdump-tree-dse@r{[}-@var{n}@r{]} @gol
263 -fdump-tree-phiopt@r{[}-@var{n}@r{]} @gol
264 -fdump-tree-forwprop@r{[}-@var{n}@r{]} @gol
265 -fdump-tree-copyrename@r{[}-@var{n}@r{]} @gol
266 -fdump-tree-nrv -fdump-tree-vect @gol
267 -fdump-tree-sra@r{[}-@var{n}@r{]} @gol
268 -fdump-tree-fre@r{[}-@var{n}@r{]} @gol
269 -ftree-vectorizer-verbose=@var{n} @gol
270 -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
271 -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs -ftree-based-profiling @gol
272 -frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
273 -ftest-coverage -ftime-report -fvar-tracking @gol
274 -g -g@var{level} -gcoff -gdwarf-2 @gol
275 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
276 -p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
277 -print-multi-directory -print-multi-lib @gol
278 -print-prog-name=@var{program} -print-search-dirs -Q @gol
281 @item Optimization Options
282 @xref{Optimize Options,,Options that Control Optimization}.
283 @gccoptlist{-falign-functions=@var{n} -falign-jumps=@var{n} @gol
284 -falign-labels=@var{n} -falign-loops=@var{n} @gol
285 -fbounds-check -fmudflap -fmudflapth -fmudflapir @gol
286 -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize @gol
287 -fbranch-target-load-optimize2 -fbtr-bb-exclusive @gol
288 -fcaller-saves -fcprop-registers @gol
289 -fcse-follow-jumps -fcse-skip-blocks -fdata-sections @gol
290 -fdelayed-branch -fdelete-null-pointer-checks @gol
291 -fexpensive-optimizations -ffast-math -ffloat-store @gol
292 -fforce-addr -fforce-mem -ffunction-sections @gol
293 -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload @gol
294 -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 @gol
295 -finline-functions -finline-limit=@var{n} -fkeep-inline-functions @gol
296 -fkeep-static-consts -fmerge-constants -fmerge-all-constants @gol
297 -fmodulo-sched -fno-branch-count-reg @gol
298 -fno-default-inline -fno-defer-pop -floop-optimize2 -fmove-loop-invariants @gol
299 -fno-function-cse -fno-guess-branch-probability @gol
300 -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
301 -funsafe-math-optimizations -ffinite-math-only @gol
302 -fno-trapping-math -fno-zero-initialized-in-bss @gol
303 -fomit-frame-pointer -foptimize-register-move @gol
304 -foptimize-sibling-calls -fprefetch-loop-arrays @gol
305 -fprofile-generate -fprofile-use @gol
306 -fregmove -frename-registers @gol
307 -freorder-blocks -freorder-blocks-and-partition -freorder-functions @gol
308 -frerun-cse-after-loop -frerun-loop-opt @gol
309 -frounding-math -fschedule-insns -fschedule-insns2 @gol
310 -fno-sched-interblock -fno-sched-spec -fsched-spec-load @gol
311 -fsched-spec-load-dangerous @gol
312 -fsched-stalled-insns=@var{n} -sched-stalled-insns-dep=@var{n} @gol
313 -fsched2-use-superblocks @gol
314 -fsched2-use-traces -freschedule-modulo-scheduled-loops @gol
315 -fsignaling-nans -fsingle-precision-constant -fspeculative-prefetching @gol
316 -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps @gol
317 -funroll-all-loops -funroll-loops -fpeel-loops @gol
318 -fsplit-ivs-in-unroller -funswitch-loops @gol
319 -fvariable-expansion-in-unroller @gol
320 -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize @gol
321 -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts @gol
322 -ftree-dominator-opts -ftree-dse -ftree-copyrename @gol
323 -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize @gol
324 --param @var{name}=@var{value}
325 -O -O0 -O1 -O2 -O3 -Os}
327 @item Preprocessor Options
328 @xref{Preprocessor Options,,Options Controlling the Preprocessor}.
329 @gccoptlist{-A@var{question}=@var{answer} @gol
330 -A-@var{question}@r{[}=@var{answer}@r{]} @gol
331 -C -dD -dI -dM -dN @gol
332 -D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
333 -idirafter @var{dir} @gol
334 -include @var{file} -imacros @var{file} @gol
335 -iprefix @var{file} -iwithprefix @var{dir} @gol
336 -iwithprefixbefore @var{dir} -isystem @var{dir} @gol
337 -M -MM -MF -MG -MP -MQ -MT -nostdinc @gol
338 -P -fworking-directory -remap @gol
339 -trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
340 -Xpreprocessor @var{option}}
342 @item Assembler Option
343 @xref{Assembler Options,,Passing Options to the Assembler}.
344 @gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
347 @xref{Link Options,,Options for Linking}.
348 @gccoptlist{@var{object-file-name} -l@var{library} @gol
349 -nostartfiles -nodefaultlibs -nostdlib -pie @gol
350 -s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
351 -Wl,@var{option} -Xlinker @var{option} @gol
354 @item Directory Options
355 @xref{Directory Options,,Options for Directory Search}.
356 @gccoptlist{-B@var{prefix} -I@var{dir} -iquote@var{dir} -L@var{dir} -specs=@var{file} -I-}
359 @c I wrote this xref this way to avoid overfull hbox. -- rms
360 @xref{Target Options}.
361 @gccoptlist{-V @var{version} -b @var{machine}}
363 @item Machine Dependent Options
364 @xref{Submodel Options,,Hardware Models and Configurations}.
365 @c This list is ordered alphanumerically by subsection name.
366 @c Try and put the significant identifier (CPU or system) first,
367 @c so users have a clue at guessing where the ones they want will be.
370 @gccoptlist{-EB -EL @gol
371 -mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
372 -mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
375 @gccoptlist{-mapcs-frame -mno-apcs-frame @gol
376 -mabi=@var{name} @gol
377 -mapcs-stack-check -mno-apcs-stack-check @gol
378 -mapcs-float -mno-apcs-float @gol
379 -mapcs-reentrant -mno-apcs-reentrant @gol
380 -msched-prolog -mno-sched-prolog @gol
381 -mlittle-endian -mbig-endian -mwords-little-endian @gol
382 -mfloat-abi=@var{name} -msoft-float -mhard-float -mfpe @gol
383 -mthumb-interwork -mno-thumb-interwork @gol
384 -mcpu=@var{name} -march=@var{name} -mfpu=@var{name} @gol
385 -mstructure-size-boundary=@var{n} @gol
386 -mabort-on-noreturn @gol
387 -mlong-calls -mno-long-calls @gol
388 -msingle-pic-base -mno-single-pic-base @gol
389 -mpic-register=@var{reg} @gol
390 -mnop-fun-dllimport @gol
391 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
392 -mpoke-function-name @gol
394 -mtpcs-frame -mtpcs-leaf-frame @gol
395 -mcaller-super-interworking -mcallee-super-interworking}
398 @gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
399 -mcall-prologues -mno-tablejump -mtiny-stack -mint8}
402 @gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
403 -mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
404 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
405 -mstack-align -mdata-align -mconst-align @gol
406 -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
407 -melf -maout -melinux -mlinux -sim -sim2 @gol
408 -mmul-bug-workaround -mno-mul-bug-workaround}
410 @emph{Darwin Options}
411 @gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
412 -arch_only -bind_at_load -bundle -bundle_loader @gol
413 -client_name -compatibility_version -current_version @gol
415 -dependency-file -dylib_file -dylinker_install_name @gol
416 -dynamic -dynamiclib -exported_symbols_list @gol
417 -filelist -flat_namespace -force_cpusubtype_ALL @gol
418 -force_flat_namespace -headerpad_max_install_names @gol
419 -image_base -init -install_name -keep_private_externs @gol
420 -multi_module -multiply_defined -multiply_defined_unused @gol
421 -noall_load -no_dead_strip_inits_and_terms @gol
422 -nofixprebinding -nomultidefs -noprebind -noseglinkedit @gol
423 -pagezero_size -prebind -prebind_all_twolevel_modules @gol
424 -private_bundle -read_only_relocs -sectalign @gol
425 -sectobjectsymbols -whyload -seg1addr @gol
426 -sectcreate -sectobjectsymbols -sectorder @gol
427 -segaddr -segs_read_only_addr -segs_read_write_addr @gol
428 -seg_addr_table -seg_addr_table_filename -seglinkedit @gol
429 -segprot -segs_read_only_addr -segs_read_write_addr @gol
430 -single_module -static -sub_library -sub_umbrella @gol
431 -twolevel_namespace -umbrella -undefined @gol
432 -unexported_symbols_list -weak_reference_mismatches @gol
433 -whatsloaded -F -gused -gfull -mone-byte-bool}
435 @emph{DEC Alpha Options}
436 @gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
437 -mieee -mieee-with-inexact -mieee-conformant @gol
438 -mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
439 -mtrap-precision=@var{mode} -mbuild-constants @gol
440 -mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
441 -mbwx -mmax -mfix -mcix @gol
442 -mfloat-vax -mfloat-ieee @gol
443 -mexplicit-relocs -msmall-data -mlarge-data @gol
444 -msmall-text -mlarge-text @gol
445 -mmemory-latency=@var{time}}
447 @emph{DEC Alpha/VMS Options}
448 @gccoptlist{-mvms-return-codes}
451 @gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
452 -mhard-float -msoft-float @gol
453 -malloc-cc -mfixed-cc -mdword -mno-dword @gol
454 -mdouble -mno-double @gol
455 -mmedia -mno-media -mmuladd -mno-muladd @gol
456 -mfdpic -minline-plt -mgprel-ro -multilib-library-pic @gol
457 -mlinked-fp -mlong-calls -malign-labels @gol
458 -mlibrary-pic -macc-4 -macc-8 @gol
459 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
460 -mscc -mno-scc -mcond-exec -mno-cond-exec @gol
461 -mvliw-branch -mno-vliw-branch @gol
462 -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
463 -mno-nested-cond-exec -mtomcat-stats @gol
467 @emph{H8/300 Options}
468 @gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
471 @gccoptlist{-march=@var{architecture-type} @gol
472 -mbig-switch -mdisable-fpregs -mdisable-indexing @gol
473 -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
474 -mfixed-range=@var{register-range} @gol
475 -mjump-in-delay -mlinker-opt -mlong-calls @gol
476 -mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
477 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
478 -mno-jump-in-delay -mno-long-load-store @gol
479 -mno-portable-runtime -mno-soft-float @gol
480 -mno-space-regs -msoft-float -mpa-risc-1-0 @gol
481 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
482 -mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
483 -munix=@var{unix-std} -nolibdld -static -threads}
485 @emph{i386 and x86-64 Options}
486 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
487 -mfpmath=@var{unit} @gol
488 -masm=@var{dialect} -mno-fancy-math-387 @gol
489 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib @gol
490 -mno-wide-multiply -mrtd -malign-double @gol
491 -mpreferred-stack-boundary=@var{num} @gol
492 -mmmx -msse -msse2 -msse3 -m3dnow @gol
493 -mthreads -mno-align-stringops -minline-all-stringops @gol
494 -mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
495 -m96bit-long-double -mregparm=@var{num} -momit-leaf-frame-pointer @gol
496 -mno-red-zone -mno-tls-direct-seg-refs @gol
497 -mcmodel=@var{code-model} @gol
501 @gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
502 -mvolatile-asm-stop -mregister-names -mno-sdata @gol
503 -mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
504 -minline-float-divide-max-throughput @gol
505 -minline-int-divide-min-latency @gol
506 -minline-int-divide-max-throughput @gol
507 -minline-sqrt-min-latency -minline-sqrt-max-throughput @gol
508 -mno-dwarf2-asm -mearly-stop-bits @gol
509 -mfixed-range=@var{register-range} -mtls-size=@var{tls-size} @gol
510 -mtune=@var{cpu-type} -mt -pthread -milp32 -mlp64}
512 @emph{M32R/D Options}
513 @gccoptlist{-m32r2 -m32rx -m32r @gol
515 -malign-loops -mno-align-loops @gol
516 -missue-rate=@var{number} @gol
517 -mbranch-cost=@var{number} @gol
518 -mmodel=@var{code-size-model-type} @gol
519 -msdata=@var{sdata-type} @gol
520 -mno-flush-func -mflush-func=@var{name} @gol
521 -mno-flush-trap -mflush-trap=@var{number} @gol
524 @emph{M680x0 Options}
525 @gccoptlist{-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
526 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 @gol
527 -mnobitfield -mrtd -mshort -msoft-float -mpcrel @gol
528 -malign-int -mstrict-align -msep-data -mno-sep-data @gol
529 -mshared-library-id=n -mid-shared-library -mno-id-shared-library}
531 @emph{M68hc1x Options}
532 @gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
533 -mauto-incdec -minmax -mlong-calls -mshort @gol
534 -msoft-reg-count=@var{count}}
537 @gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
538 -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
539 -m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
540 -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
541 -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
544 @gccoptlist{-EL -EB -march=@var{arch} -mtune=@var{arch} @gol
545 -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
546 -mips16 -mno-mips16 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
547 -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 @gol
548 -mhard-float -msoft-float -msingle-float -mdouble-float @gol
549 -mpaired-single -mips3d @gol
550 -mint64 -mlong64 -mlong32 @gol
551 -G@var{num} -membedded-data -mno-embedded-data @gol
552 -muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
553 -msplit-addresses -mno-split-addresses @gol
554 -mexplicit-relocs -mno-explicit-relocs @gol
555 -mcheck-zero-division -mno-check-zero-division @gol
556 -mdivide-traps -mdivide-breaks @gol
557 -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls @gol
558 -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp @gol
559 -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 @gol
560 -mfix-vr4120 -mno-fix-vr4120 -mfix-sb1 -mno-fix-sb1 @gol
561 -mflush-func=@var{func} -mno-flush-func @gol
562 -mbranch-likely -mno-branch-likely @gol
563 -mfp-exceptions -mno-fp-exceptions @gol
564 -mvr4130-align -mno-vr4130-align}
567 @gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
568 -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
569 -melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
570 -mno-base-addresses -msingle-exit -mno-single-exit}
572 @emph{MN10300 Options}
573 @gccoptlist{-mmult-bug -mno-mult-bug @gol
574 -mam33 -mno-am33 @gol
575 -mam33-2 -mno-am33-2 @gol
579 @gccoptlist{-m32032 -m32332 -m32532 -m32081 -m32381 @gol
580 -mmult-add -mnomult-add -msoft-float -mrtd -mnortd @gol
581 -mregparam -mnoregparam -msb -mnosb @gol
582 -mbitfield -mnobitfield -mhimem -mnohimem}
584 @emph{PDP-11 Options}
585 @gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
586 -mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
587 -mint16 -mno-int32 -mfloat32 -mno-float64 @gol
588 -mfloat64 -mno-float32 -mabshi -mno-abshi @gol
589 -mbranch-expensive -mbranch-cheap @gol
590 -msplit -mno-split -munix-asm -mdec-asm}
592 @emph{PowerPC Options}
593 See RS/6000 and PowerPC Options.
595 @emph{RS/6000 and PowerPC Options}
596 @gccoptlist{-mcpu=@var{cpu-type} @gol
597 -mtune=@var{cpu-type} @gol
598 -mpower -mno-power -mpower2 -mno-power2 @gol
599 -mpowerpc -mpowerpc64 -mno-powerpc @gol
600 -maltivec -mno-altivec @gol
601 -mpowerpc-gpopt -mno-powerpc-gpopt @gol
602 -mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
603 -mnew-mnemonics -mold-mnemonics @gol
604 -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
605 -m64 -m32 -mxl-compat -mno-xl-compat -mpe @gol
606 -malign-power -malign-natural @gol
607 -msoft-float -mhard-float -mmultiple -mno-multiple @gol
608 -mstring -mno-string -mupdate -mno-update @gol
609 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
610 -mstrict-align -mno-strict-align -mrelocatable @gol
611 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
612 -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
613 -mdynamic-no-pic @gol
614 -mprioritize-restricted-insns=@var{priority} @gol
615 -msched-costly-dep=@var{dependence_type} @gol
616 -minsert-sched-nops=@var{scheme} @gol
617 -mcall-sysv -mcall-netbsd @gol
618 -maix-struct-return -msvr4-struct-return @gol
619 -mabi=altivec -mabi=no-altivec @gol
620 -mabi=spe -mabi=no-spe @gol
621 -misel=yes -misel=no @gol
622 -mspe=yes -mspe=no @gol
623 -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double @gol
624 -mprototype -mno-prototype @gol
625 -msim -mmvme -mads -myellowknife -memb -msdata @gol
626 -msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
628 @emph{S/390 and zSeries Options}
629 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
630 -mhard-float -msoft-float -mbackchain -mno-backchain @gol
631 -mpacked-stack -mno-packed-stack @gol
632 -msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
633 -m64 -m31 -mdebug -mno-debug -mesa -mzarch @gol
634 -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd @gol
635 -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard}
638 @gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
639 -m4-nofpu -m4-single-only -m4-single -m4 @gol
640 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al @gol
641 -m5-64media -m5-64media-nofpu @gol
642 -m5-32media -m5-32media-nofpu @gol
643 -m5-compact -m5-compact-nofpu @gol
644 -mb -ml -mdalign -mrelax @gol
645 -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave @gol
646 -mieee -misize -mpadstruct -mspace @gol
647 -mprefergot -musermode}
650 @gccoptlist{-mcpu=@var{cpu-type} @gol
651 -mtune=@var{cpu-type} @gol
652 -mcmodel=@var{code-model} @gol
653 -m32 -m64 -mapp-regs -mno-app-regs @gol
654 -mfaster-structs -mno-faster-structs @gol
655 -mfpu -mno-fpu -mhard-float -msoft-float @gol
656 -mhard-quad-float -msoft-quad-float @gol
657 -mimpure-text -mno-impure-text -mlittle-endian @gol
658 -mstack-bias -mno-stack-bias @gol
659 -munaligned-doubles -mno-unaligned-doubles @gol
660 -mv8plus -mno-v8plus -mvis -mno-vis
663 @emph{System V Options}
664 @gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
666 @emph{TMS320C3x/C4x Options}
667 @gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
668 -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
669 -mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
670 -mparallel-insns -mparallel-mpy -mpreserve-float}
673 @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
674 -mprolog-function -mno-prolog-function -mspace @gol
675 -mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
676 -mapp-regs -mno-app-regs @gol
677 -mdisable-callt -mno-disable-callt @gol
683 @gccoptlist{-mg -mgnu -munix}
685 @emph{x86-64 Options}
686 See i386 and x86-64 Options.
688 @emph{Xstormy16 Options}
691 @emph{Xtensa Options}
692 @gccoptlist{-mconst16 -mno-const16 @gol
693 -mfused-madd -mno-fused-madd @gol
694 -mtext-section-literals -mno-text-section-literals @gol
695 -mtarget-align -mno-target-align @gol
696 -mlongcalls -mno-longcalls}
698 @emph{zSeries Options}
699 See S/390 and zSeries Options.
701 @item Code Generation Options
702 @xref{Code Gen Options,,Options for Code Generation Conventions}.
703 @gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
704 -ffixed-@var{reg} -fexceptions @gol
705 -fnon-call-exceptions -funwind-tables @gol
706 -fasynchronous-unwind-tables @gol
707 -finhibit-size-directive -finstrument-functions @gol
708 -fno-common -fno-ident @gol
709 -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
710 -freg-struct-return -fshared-data -fshort-enums @gol
711 -fshort-double -fshort-wchar @gol
712 -fverbose-asm -fpack-struct[=@var{n}] -fstack-check @gol
713 -fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
714 -fargument-alias -fargument-noalias @gol
715 -fargument-noalias-global -fleading-underscore @gol
716 -ftls-model=@var{model} @gol
717 -ftrapv -fwrapv -fbounds-check @gol
722 * Overall Options:: Controlling the kind of output:
723 an executable, object files, assembler files,
724 or preprocessed source.
725 * C Dialect Options:: Controlling the variant of C language compiled.
726 * C++ Dialect Options:: Variations on C++.
727 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
729 * Language Independent Options:: Controlling how diagnostics should be
731 * Warning Options:: How picky should the compiler be?
732 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
733 * Optimize Options:: How much optimization?
734 * Preprocessor Options:: Controlling header files and macro definitions.
735 Also, getting dependency information for Make.
736 * Assembler Options:: Passing options to the assembler.
737 * Link Options:: Specifying libraries and so on.
738 * Directory Options:: Where to find header files and libraries.
739 Where to find the compiler executable files.
740 * Spec Files:: How to pass switches to sub-processes.
741 * Target Options:: Running a cross-compiler, or an old version of GCC.
744 @node Overall Options
745 @section Options Controlling the Kind of Output
747 Compilation can involve up to four stages: preprocessing, compilation
748 proper, assembly and linking, always in that order. GCC is capable of
749 preprocessing and compiling several files either into several
750 assembler input files, or into one assembler input file; then each
751 assembler input file produces an object file, and linking combines all
752 the object files (those newly compiled, and those specified as input)
753 into an executable file.
755 @cindex file name suffix
756 For any given input file, the file name suffix determines what kind of
761 C source code which must be preprocessed.
764 C source code which should not be preprocessed.
767 C++ source code which should not be preprocessed.
770 Objective-C source code. Note that you must link with the @file{libobjc}
771 library to make an Objective-C program work.
774 Objective-C source code which should not be preprocessed.
778 Objective-C++ source code. Note that you must link with the @file{libobjc}
779 library to make an Objective-C++ program work. Note that @samp{.M} refers
780 to a literal capital M@.
783 Objective-C++ source code which should not be preprocessed.
786 C, C++, Objective-C or Objective-C++ header file to be turned into a
791 @itemx @var{file}.cxx
792 @itemx @var{file}.cpp
793 @itemx @var{file}.CPP
794 @itemx @var{file}.c++
796 C++ source code which must be preprocessed. Note that in @samp{.cxx},
797 the last two letters must both be literally @samp{x}. Likewise,
798 @samp{.C} refers to a literal capital C@.
802 C++ header file to be turned into a precompiled header.
805 @itemx @var{file}.for
806 @itemx @var{file}.FOR
807 Fortran source code which should not be preprocessed.
810 @itemx @var{file}.fpp
811 @itemx @var{file}.FPP
812 Fortran source code which must be preprocessed (with the traditional
816 Fortran source code which must be preprocessed with a RATFOR
817 preprocessor (not included with GCC)@.
820 @itemx @var{file}.f95
821 Fortran 90/95 source code which should not be preprocessed.
823 @c FIXME: Descriptions of Java file types.
830 Ada source code file which contains a library unit declaration (a
831 declaration of a package, subprogram, or generic, or a generic
832 instantiation), or a library unit renaming declaration (a package,
833 generic, or subprogram renaming declaration). Such files are also
836 @itemx @var{file}.adb
837 Ada source code file containing a library unit body (a subprogram or
838 package body). Such files are also called @dfn{bodies}.
840 @c GCC also knows about some suffixes for languages not yet included:
849 Assembler code which must be preprocessed.
852 An object file to be fed straight into linking.
853 Any file name with no recognized suffix is treated this way.
857 You can specify the input language explicitly with the @option{-x} option:
860 @item -x @var{language}
861 Specify explicitly the @var{language} for the following input files
862 (rather than letting the compiler choose a default based on the file
863 name suffix). This option applies to all following input files until
864 the next @option{-x} option. Possible values for @var{language} are:
866 c c-header c-cpp-output
867 c++ c++-header c++-cpp-output
868 objective-c objective-c-header objective-c-cpp-output
869 objective-c++ objective-c++-header objective-c++-cpp-output
870 assembler assembler-with-cpp
872 f77 f77-cpp-input ratfor
879 Turn off any specification of a language, so that subsequent files are
880 handled according to their file name suffixes (as they are if @option{-x}
881 has not been used at all).
883 @item -pass-exit-codes
884 @opindex pass-exit-codes
885 Normally the @command{gcc} program will exit with the code of 1 if any
886 phase of the compiler returns a non-success return code. If you specify
887 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
888 numerically highest error produced by any phase that returned an error
892 If you only want some of the stages of compilation, you can use
893 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
894 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
895 @command{gcc} is to stop. Note that some combinations (for example,
896 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
901 Compile or assemble the source files, but do not link. The linking
902 stage simply is not done. The ultimate output is in the form of an
903 object file for each source file.
905 By default, the object file name for a source file is made by replacing
906 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
908 Unrecognized input files, not requiring compilation or assembly, are
913 Stop after the stage of compilation proper; do not assemble. The output
914 is in the form of an assembler code file for each non-assembler input
917 By default, the assembler file name for a source file is made by
918 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
920 Input files that don't require compilation are ignored.
924 Stop after the preprocessing stage; do not run the compiler proper. The
925 output is in the form of preprocessed source code, which is sent to the
928 Input files which don't require preprocessing are ignored.
930 @cindex output file option
933 Place output in file @var{file}. This applies regardless to whatever
934 sort of output is being produced, whether it be an executable file,
935 an object file, an assembler file or preprocessed C code.
937 If @option{-o} is not specified, the default is to put an executable
938 file in @file{a.out}, the object file for
939 @file{@var{source}.@var{suffix}} in @file{@var{source}.o}, its
940 assembler file in @file{@var{source}.s}, a precompiled header file in
941 @file{@var{source}.@var{suffix}.gch}, and all preprocessed C source on
946 Print (on standard error output) the commands executed to run the stages
947 of compilation. Also print the version number of the compiler driver
948 program and of the preprocessor and the compiler proper.
952 Like @option{-v} except the commands are not executed and all command
953 arguments are quoted. This is useful for shell scripts to capture the
954 driver-generated command lines.
958 Use pipes rather than temporary files for communication between the
959 various stages of compilation. This fails to work on some systems where
960 the assembler is unable to read from a pipe; but the GNU assembler has
965 If you are compiling multiple source files, this option tells the driver
966 to pass all the source files to the compiler at once (for those
967 languages for which the compiler can handle this). This will allow
968 intermodule analysis (IMA) to be performed by the compiler. Currently the only
969 language for which this is supported is C@. If you pass source files for
970 multiple languages to the driver, using this option, the driver will invoke
971 the compiler(s) that support IMA once each, passing each compiler all the
972 source files appropriate for it. For those languages that do not support
973 IMA this option will be ignored, and the compiler will be invoked once for
974 each source file in that language. If you use this option in conjunction
975 with @option{-save-temps}, the compiler will generate multiple
977 (one for each source file), but only one (combined) @file{.o} or
982 Print (on the standard output) a description of the command line options
983 understood by @command{gcc}. If the @option{-v} option is also specified
984 then @option{--help} will also be passed on to the various processes
985 invoked by @command{gcc}, so that they can display the command line options
986 they accept. If the @option{-Wextra} option is also specified then command
987 line options which have no documentation associated with them will also
992 Print (on the standard output) a description of target specific command
993 line options for each tool.
997 Display the version number and copyrights of the invoked GCC@.
1001 @section Compiling C++ Programs
1003 @cindex suffixes for C++ source
1004 @cindex C++ source file suffixes
1005 C++ source files conventionally use one of the suffixes @samp{.C},
1006 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
1007 @samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
1008 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
1009 files with these names and compiles them as C++ programs even if you
1010 call the compiler the same way as for compiling C programs (usually
1011 with the name @command{gcc}).
1015 However, C++ programs often require class libraries as well as a
1016 compiler that understands the C++ language---and under some
1017 circumstances, you might want to compile programs or header files from
1018 standard input, or otherwise without a suffix that flags them as C++
1019 programs. You might also like to precompile a C header file with a
1020 @samp{.h} extension to be used in C++ compilations. @command{g++} is a
1021 program that calls GCC with the default language set to C++, and
1022 automatically specifies linking against the C++ library. On many
1023 systems, @command{g++} is also installed with the name @command{c++}.
1025 @cindex invoking @command{g++}
1026 When you compile C++ programs, you may specify many of the same
1027 command-line options that you use for compiling programs in any
1028 language; or command-line options meaningful for C and related
1029 languages; or options that are meaningful only for C++ programs.
1030 @xref{C Dialect Options,,Options Controlling C Dialect}, for
1031 explanations of options for languages related to C@.
1032 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
1033 explanations of options that are meaningful only for C++ programs.
1035 @node C Dialect Options
1036 @section Options Controlling C Dialect
1037 @cindex dialect options
1038 @cindex language dialect options
1039 @cindex options, dialect
1041 The following options control the dialect of C (or languages derived
1042 from C, such as C++, Objective-C and Objective-C++) that the compiler
1046 @cindex ANSI support
1050 In C mode, support all ISO C90 programs. In C++ mode,
1051 remove GNU extensions that conflict with ISO C++.
1053 This turns off certain features of GCC that are incompatible with ISO
1054 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
1055 such as the @code{asm} and @code{typeof} keywords, and
1056 predefined macros such as @code{unix} and @code{vax} that identify the
1057 type of system you are using. It also enables the undesirable and
1058 rarely used ISO trigraph feature. For the C compiler,
1059 it disables recognition of C++ style @samp{//} comments as well as
1060 the @code{inline} keyword.
1062 The alternate keywords @code{__asm__}, @code{__extension__},
1063 @code{__inline__} and @code{__typeof__} continue to work despite
1064 @option{-ansi}. You would not want to use them in an ISO C program, of
1065 course, but it is useful to put them in header files that might be included
1066 in compilations done with @option{-ansi}. Alternate predefined macros
1067 such as @code{__unix__} and @code{__vax__} are also available, with or
1068 without @option{-ansi}.
1070 The @option{-ansi} option does not cause non-ISO programs to be
1071 rejected gratuitously. For that, @option{-pedantic} is required in
1072 addition to @option{-ansi}. @xref{Warning Options}.
1074 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1075 option is used. Some header files may notice this macro and refrain
1076 from declaring certain functions or defining certain macros that the
1077 ISO standard doesn't call for; this is to avoid interfering with any
1078 programs that might use these names for other things.
1080 Functions which would normally be built in but do not have semantics
1081 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1082 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1083 built-in functions provided by GCC}, for details of the functions
1088 Determine the language standard. This option is currently only
1089 supported when compiling C or C++. A value for this option must be
1090 provided; possible values are
1095 ISO C90 (same as @option{-ansi}).
1097 @item iso9899:199409
1098 ISO C90 as modified in amendment 1.
1104 ISO C99. Note that this standard is not yet fully supported; see
1105 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1106 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1109 Default, ISO C90 plus GNU extensions (including some C99 features).
1113 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1114 this will become the default. The name @samp{gnu9x} is deprecated.
1117 The 1998 ISO C++ standard plus amendments.
1120 The same as @option{-std=c++98} plus GNU extensions. This is the
1121 default for C++ code.
1124 Even when this option is not specified, you can still use some of the
1125 features of newer standards in so far as they do not conflict with
1126 previous C standards. For example, you may use @code{__restrict__} even
1127 when @option{-std=c99} is not specified.
1129 The @option{-std} options specifying some version of ISO C have the same
1130 effects as @option{-ansi}, except that features that were not in ISO C90
1131 but are in the specified version (for example, @samp{//} comments and
1132 the @code{inline} keyword in ISO C99) are not disabled.
1134 @xref{Standards,,Language Standards Supported by GCC}, for details of
1135 these standard versions.
1137 @item -aux-info @var{filename}
1139 Output to the given filename prototyped declarations for all functions
1140 declared and/or defined in a translation unit, including those in header
1141 files. This option is silently ignored in any language other than C@.
1143 Besides declarations, the file indicates, in comments, the origin of
1144 each declaration (source file and line), whether the declaration was
1145 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1146 @samp{O} for old, respectively, in the first character after the line
1147 number and the colon), and whether it came from a declaration or a
1148 definition (@samp{C} or @samp{F}, respectively, in the following
1149 character). In the case of function definitions, a K&R-style list of
1150 arguments followed by their declarations is also provided, inside
1151 comments, after the declaration.
1155 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1156 keyword, so that code can use these words as identifiers. You can use
1157 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1158 instead. @option{-ansi} implies @option{-fno-asm}.
1160 In C++, this switch only affects the @code{typeof} keyword, since
1161 @code{asm} and @code{inline} are standard keywords. You may want to
1162 use the @option{-fno-gnu-keywords} flag instead, which has the same
1163 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1164 switch only affects the @code{asm} and @code{typeof} keywords, since
1165 @code{inline} is a standard keyword in ISO C99.
1168 @itemx -fno-builtin-@var{function}
1169 @opindex fno-builtin
1170 @cindex built-in functions
1171 Don't recognize built-in functions that do not begin with
1172 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1173 functions provided by GCC}, for details of the functions affected,
1174 including those which are not built-in functions when @option{-ansi} or
1175 @option{-std} options for strict ISO C conformance are used because they
1176 do not have an ISO standard meaning.
1178 GCC normally generates special code to handle certain built-in functions
1179 more efficiently; for instance, calls to @code{alloca} may become single
1180 instructions that adjust the stack directly, and calls to @code{memcpy}
1181 may become inline copy loops. The resulting code is often both smaller
1182 and faster, but since the function calls no longer appear as such, you
1183 cannot set a breakpoint on those calls, nor can you change the behavior
1184 of the functions by linking with a different library. In addition,
1185 when a function is recognized as a built-in function, GCC may use
1186 information about that function to warn about problems with calls to
1187 that function, or to generate more efficient code, even if the
1188 resulting code still contains calls to that function. For example,
1189 warnings are given with @option{-Wformat} for bad calls to
1190 @code{printf}, when @code{printf} is built in, and @code{strlen} is
1191 known not to modify global memory.
1193 With the @option{-fno-builtin-@var{function}} option
1194 only the built-in function @var{function} is
1195 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1196 function is named this is not built-in in this version of GCC, this
1197 option is ignored. There is no corresponding
1198 @option{-fbuiltin-@var{function}} option; if you wish to enable
1199 built-in functions selectively when using @option{-fno-builtin} or
1200 @option{-ffreestanding}, you may define macros such as:
1203 #define abs(n) __builtin_abs ((n))
1204 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1209 @cindex hosted environment
1211 Assert that compilation takes place in a hosted environment. This implies
1212 @option{-fbuiltin}. A hosted environment is one in which the
1213 entire standard library is available, and in which @code{main} has a return
1214 type of @code{int}. Examples are nearly everything except a kernel.
1215 This is equivalent to @option{-fno-freestanding}.
1217 @item -ffreestanding
1218 @opindex ffreestanding
1219 @cindex hosted environment
1221 Assert that compilation takes place in a freestanding environment. This
1222 implies @option{-fno-builtin}. A freestanding environment
1223 is one in which the standard library may not exist, and program startup may
1224 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1225 This is equivalent to @option{-fno-hosted}.
1227 @xref{Standards,,Language Standards Supported by GCC}, for details of
1228 freestanding and hosted environments.
1230 @item -fms-extensions
1231 @opindex fms-extensions
1232 Accept some non-standard constructs used in Microsoft header files.
1234 Some cases of unnamed fields in structures and unions are only
1235 accepted with this option. @xref{Unnamed Fields,,Unnamed struct/union
1236 fields within structs/unions}, for details.
1240 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1241 options for strict ISO C conformance) implies @option{-trigraphs}.
1243 @item -no-integrated-cpp
1244 @opindex no-integrated-cpp
1245 Performs a compilation in two passes: preprocessing and compiling. This
1246 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1247 @option{-B} option. The user supplied compilation step can then add in
1248 an additional preprocessing step after normal preprocessing but before
1249 compiling. The default is to use the integrated cpp (internal cpp)
1251 The semantics of this option will change if "cc1", "cc1plus", and
1252 "cc1obj" are merged.
1254 @cindex traditional C language
1255 @cindex C language, traditional
1257 @itemx -traditional-cpp
1258 @opindex traditional-cpp
1259 @opindex traditional
1260 Formerly, these options caused GCC to attempt to emulate a pre-standard
1261 C compiler. They are now only supported with the @option{-E} switch.
1262 The preprocessor continues to support a pre-standard mode. See the GNU
1263 CPP manual for details.
1265 @item -fcond-mismatch
1266 @opindex fcond-mismatch
1267 Allow conditional expressions with mismatched types in the second and
1268 third arguments. The value of such an expression is void. This option
1269 is not supported for C++.
1271 @item -funsigned-char
1272 @opindex funsigned-char
1273 Let the type @code{char} be unsigned, like @code{unsigned char}.
1275 Each kind of machine has a default for what @code{char} should
1276 be. It is either like @code{unsigned char} by default or like
1277 @code{signed char} by default.
1279 Ideally, a portable program should always use @code{signed char} or
1280 @code{unsigned char} when it depends on the signedness of an object.
1281 But many programs have been written to use plain @code{char} and
1282 expect it to be signed, or expect it to be unsigned, depending on the
1283 machines they were written for. This option, and its inverse, let you
1284 make such a program work with the opposite default.
1286 The type @code{char} is always a distinct type from each of
1287 @code{signed char} or @code{unsigned char}, even though its behavior
1288 is always just like one of those two.
1291 @opindex fsigned-char
1292 Let the type @code{char} be signed, like @code{signed char}.
1294 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1295 the negative form of @option{-funsigned-char}. Likewise, the option
1296 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1298 @item -fsigned-bitfields
1299 @itemx -funsigned-bitfields
1300 @itemx -fno-signed-bitfields
1301 @itemx -fno-unsigned-bitfields
1302 @opindex fsigned-bitfields
1303 @opindex funsigned-bitfields
1304 @opindex fno-signed-bitfields
1305 @opindex fno-unsigned-bitfields
1306 These options control whether a bit-field is signed or unsigned, when the
1307 declaration does not use either @code{signed} or @code{unsigned}. By
1308 default, such a bit-field is signed, because this is consistent: the
1309 basic integer types such as @code{int} are signed types.
1312 @node C++ Dialect Options
1313 @section Options Controlling C++ Dialect
1315 @cindex compiler options, C++
1316 @cindex C++ options, command line
1317 @cindex options, C++
1318 This section describes the command-line options that are only meaningful
1319 for C++ programs; but you can also use most of the GNU compiler options
1320 regardless of what language your program is in. For example, you
1321 might compile a file @code{firstClass.C} like this:
1324 g++ -g -frepo -O -c firstClass.C
1328 In this example, only @option{-frepo} is an option meant
1329 only for C++ programs; you can use the other options with any
1330 language supported by GCC@.
1332 Here is a list of options that are @emph{only} for compiling C++ programs:
1336 @item -fabi-version=@var{n}
1337 @opindex fabi-version
1338 Use version @var{n} of the C++ ABI@. Version 2 is the version of the
1339 C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
1340 the C++ ABI that first appeared in G++ 3.2. Version 0 will always be
1341 the version that conforms most closely to the C++ ABI specification.
1342 Therefore, the ABI obtained using version 0 will change as ABI bugs
1345 The default is version 2.
1347 @item -fno-access-control
1348 @opindex fno-access-control
1349 Turn off all access checking. This switch is mainly useful for working
1350 around bugs in the access control code.
1354 Check that the pointer returned by @code{operator new} is non-null
1355 before attempting to modify the storage allocated. This check is
1356 normally unnecessary because the C++ standard specifies that
1357 @code{operator new} will only return @code{0} if it is declared
1358 @samp{throw()}, in which case the compiler will always check the
1359 return value even without this option. In all other cases, when
1360 @code{operator new} has a non-empty exception specification, memory
1361 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1362 @samp{new (nothrow)}.
1364 @item -fconserve-space
1365 @opindex fconserve-space
1366 Put uninitialized or runtime-initialized global variables into the
1367 common segment, as C does. This saves space in the executable at the
1368 cost of not diagnosing duplicate definitions. If you compile with this
1369 flag and your program mysteriously crashes after @code{main()} has
1370 completed, you may have an object that is being destroyed twice because
1371 two definitions were merged.
1373 This option is no longer useful on most targets, now that support has
1374 been added for putting variables into BSS without making them common.
1376 @item -fno-const-strings
1377 @opindex fno-const-strings
1378 Give string constants type @code{char *} instead of type @code{const
1379 char *}. By default, G++ uses type @code{const char *} as required by
1380 the standard. Even if you use @option{-fno-const-strings}, you cannot
1381 actually modify the value of a string constant.
1383 This option might be removed in a future release of G++. For maximum
1384 portability, you should structure your code so that it works with
1385 string constants that have type @code{const char *}.
1387 @item -fno-elide-constructors
1388 @opindex fno-elide-constructors
1389 The C++ standard allows an implementation to omit creating a temporary
1390 which is only used to initialize another object of the same type.
1391 Specifying this option disables that optimization, and forces G++ to
1392 call the copy constructor in all cases.
1394 @item -fno-enforce-eh-specs
1395 @opindex fno-enforce-eh-specs
1396 Don't check for violation of exception specifications at runtime. This
1397 option violates the C++ standard, but may be useful for reducing code
1398 size in production builds, much like defining @samp{NDEBUG}. The compiler
1399 will still optimize based on the exception specifications.
1402 @itemx -fno-for-scope
1404 @opindex fno-for-scope
1405 If @option{-ffor-scope} is specified, the scope of variables declared in
1406 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1407 as specified by the C++ standard.
1408 If @option{-fno-for-scope} is specified, the scope of variables declared in
1409 a @i{for-init-statement} extends to the end of the enclosing scope,
1410 as was the case in old versions of G++, and other (traditional)
1411 implementations of C++.
1413 The default if neither flag is given to follow the standard,
1414 but to allow and give a warning for old-style code that would
1415 otherwise be invalid, or have different behavior.
1417 @item -fno-gnu-keywords
1418 @opindex fno-gnu-keywords
1419 Do not recognize @code{typeof} as a keyword, so that code can use this
1420 word as an identifier. You can use the keyword @code{__typeof__} instead.
1421 @option{-ansi} implies @option{-fno-gnu-keywords}.
1423 @item -fno-implicit-templates
1424 @opindex fno-implicit-templates
1425 Never emit code for non-inline templates which are instantiated
1426 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1427 @xref{Template Instantiation}, for more information.
1429 @item -fno-implicit-inline-templates
1430 @opindex fno-implicit-inline-templates
1431 Don't emit code for implicit instantiations of inline templates, either.
1432 The default is to handle inlines differently so that compiles with and
1433 without optimization will need the same set of explicit instantiations.
1435 @item -fno-implement-inlines
1436 @opindex fno-implement-inlines
1437 To save space, do not emit out-of-line copies of inline functions
1438 controlled by @samp{#pragma implementation}. This will cause linker
1439 errors if these functions are not inlined everywhere they are called.
1441 @item -fms-extensions
1442 @opindex fms-extensions
1443 Disable pedantic warnings about constructs used in MFC, such as implicit
1444 int and getting a pointer to member function via non-standard syntax.
1446 @item -fno-nonansi-builtins
1447 @opindex fno-nonansi-builtins
1448 Disable built-in declarations of functions that are not mandated by
1449 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1450 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1452 @item -fno-operator-names
1453 @opindex fno-operator-names
1454 Do not treat the operator name keywords @code{and}, @code{bitand},
1455 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1456 synonyms as keywords.
1458 @item -fno-optional-diags
1459 @opindex fno-optional-diags
1460 Disable diagnostics that the standard says a compiler does not need to
1461 issue. Currently, the only such diagnostic issued by G++ is the one for
1462 a name having multiple meanings within a class.
1465 @opindex fpermissive
1466 Downgrade some diagnostics about nonconformant code from errors to
1467 warnings. Thus, using @option{-fpermissive} will allow some
1468 nonconforming code to compile.
1472 Enable automatic template instantiation at link time. This option also
1473 implies @option{-fno-implicit-templates}. @xref{Template
1474 Instantiation}, for more information.
1478 Disable generation of information about every class with virtual
1479 functions for use by the C++ runtime type identification features
1480 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1481 of the language, you can save some space by using this flag. Note that
1482 exception handling uses the same information, but it will generate it as
1487 Emit statistics about front-end processing at the end of the compilation.
1488 This information is generally only useful to the G++ development team.
1490 @item -ftemplate-depth-@var{n}
1491 @opindex ftemplate-depth
1492 Set the maximum instantiation depth for template classes to @var{n}.
1493 A limit on the template instantiation depth is needed to detect
1494 endless recursions during template class instantiation. ANSI/ISO C++
1495 conforming programs must not rely on a maximum depth greater than 17.
1497 @item -fno-threadsafe-statics
1498 @opindex fno-threadsafe-statics
1499 Do not emit the extra code to use the routines specified in the C++
1500 ABI for thread-safe initialization of local statics. You can use this
1501 option to reduce code size slightly in code that doesn't need to be
1504 @item -fuse-cxa-atexit
1505 @opindex fuse-cxa-atexit
1506 Register destructors for objects with static storage duration with the
1507 @code{__cxa_atexit} function rather than the @code{atexit} function.
1508 This option is required for fully standards-compliant handling of static
1509 destructors, but will only work if your C library supports
1510 @code{__cxa_atexit}.
1512 @item -fvisibility-inlines-hidden
1513 @opindex fvisibility-inlines-hidden
1514 Causes all inlined methods to be marked with
1515 @code{__attribute__ ((visibility ("hidden")))} so that they do not
1516 appear in the export table of a DSO and do not require a PLT indirection
1517 when used within the DSO@. Enabling this option can have a dramatic effect
1518 on load and link times of a DSO as it massively reduces the size of the
1519 dynamic export table when the library makes heavy use of templates. While
1520 it can cause bloating through duplication of code within each DSO where
1521 it is used, often the wastage is less than the considerable space occupied
1522 by a long symbol name in the export table which is typical when using
1523 templates and namespaces. For even more savings, combine with the
1524 @option{-fvisibility=hidden} switch.
1528 Do not use weak symbol support, even if it is provided by the linker.
1529 By default, G++ will use weak symbols if they are available. This
1530 option exists only for testing, and should not be used by end-users;
1531 it will result in inferior code and has no benefits. This option may
1532 be removed in a future release of G++.
1536 Do not search for header files in the standard directories specific to
1537 C++, but do still search the other standard directories. (This option
1538 is used when building the C++ library.)
1541 In addition, these optimization, warning, and code generation options
1542 have meanings only for C++ programs:
1545 @item -fno-default-inline
1546 @opindex fno-default-inline
1547 Do not assume @samp{inline} for functions defined inside a class scope.
1548 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1549 functions will have linkage like inline functions; they just won't be
1552 @item -Wabi @r{(C++ only)}
1554 Warn when G++ generates code that is probably not compatible with the
1555 vendor-neutral C++ ABI@. Although an effort has been made to warn about
1556 all such cases, there are probably some cases that are not warned about,
1557 even though G++ is generating incompatible code. There may also be
1558 cases where warnings are emitted even though the code that is generated
1561 You should rewrite your code to avoid these warnings if you are
1562 concerned about the fact that code generated by G++ may not be binary
1563 compatible with code generated by other compilers.
1565 The known incompatibilities at this point include:
1570 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1571 pack data into the same byte as a base class. For example:
1574 struct A @{ virtual void f(); int f1 : 1; @};
1575 struct B : public A @{ int f2 : 1; @};
1579 In this case, G++ will place @code{B::f2} into the same byte
1580 as@code{A::f1}; other compilers will not. You can avoid this problem
1581 by explicitly padding @code{A} so that its size is a multiple of the
1582 byte size on your platform; that will cause G++ and other compilers to
1583 layout @code{B} identically.
1586 Incorrect handling of tail-padding for virtual bases. G++ does not use
1587 tail padding when laying out virtual bases. For example:
1590 struct A @{ virtual void f(); char c1; @};
1591 struct B @{ B(); char c2; @};
1592 struct C : public A, public virtual B @{@};
1596 In this case, G++ will not place @code{B} into the tail-padding for
1597 @code{A}; other compilers will. You can avoid this problem by
1598 explicitly padding @code{A} so that its size is a multiple of its
1599 alignment (ignoring virtual base classes); that will cause G++ and other
1600 compilers to layout @code{C} identically.
1603 Incorrect handling of bit-fields with declared widths greater than that
1604 of their underlying types, when the bit-fields appear in a union. For
1608 union U @{ int i : 4096; @};
1612 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1613 union too small by the number of bits in an @code{int}.
1616 Empty classes can be placed at incorrect offsets. For example:
1626 struct C : public B, public A @{@};
1630 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1631 it should be placed at offset zero. G++ mistakenly believes that the
1632 @code{A} data member of @code{B} is already at offset zero.
1635 Names of template functions whose types involve @code{typename} or
1636 template template parameters can be mangled incorrectly.
1639 template <typename Q>
1640 void f(typename Q::X) @{@}
1642 template <template <typename> class Q>
1643 void f(typename Q<int>::X) @{@}
1647 Instantiations of these templates may be mangled incorrectly.
1651 @item -Wctor-dtor-privacy @r{(C++ only)}
1652 @opindex Wctor-dtor-privacy
1653 Warn when a class seems unusable because all the constructors or
1654 destructors in that class are private, and it has neither friends nor
1655 public static member functions.
1657 @item -Wnon-virtual-dtor @r{(C++ only)}
1658 @opindex Wnon-virtual-dtor
1659 Warn when a class appears to be polymorphic, thereby requiring a virtual
1660 destructor, yet it declares a non-virtual one.
1661 This warning is enabled by @option{-Wall}.
1663 @item -Wreorder @r{(C++ only)}
1665 @cindex reordering, warning
1666 @cindex warning for reordering of member initializers
1667 Warn when the order of member initializers given in the code does not
1668 match the order in which they must be executed. For instance:
1674 A(): j (0), i (1) @{ @}
1678 The compiler will rearrange the member initializers for @samp{i}
1679 and @samp{j} to match the declaration order of the members, emitting
1680 a warning to that effect. This warning is enabled by @option{-Wall}.
1683 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
1686 @item -Weffc++ @r{(C++ only)}
1688 Warn about violations of the following style guidelines from Scott Meyers'
1689 @cite{Effective C++} book:
1693 Item 11: Define a copy constructor and an assignment operator for classes
1694 with dynamically allocated memory.
1697 Item 12: Prefer initialization to assignment in constructors.
1700 Item 14: Make destructors virtual in base classes.
1703 Item 15: Have @code{operator=} return a reference to @code{*this}.
1706 Item 23: Don't try to return a reference when you must return an object.
1710 Also warn about violations of the following style guidelines from
1711 Scott Meyers' @cite{More Effective C++} book:
1715 Item 6: Distinguish between prefix and postfix forms of increment and
1716 decrement operators.
1719 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
1723 When selecting this option, be aware that the standard library
1724 headers do not obey all of these guidelines; use @samp{grep -v}
1725 to filter out those warnings.
1727 @item -Wno-deprecated @r{(C++ only)}
1728 @opindex Wno-deprecated
1729 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
1731 @item -Wno-non-template-friend @r{(C++ only)}
1732 @opindex Wno-non-template-friend
1733 Disable warnings when non-templatized friend functions are declared
1734 within a template. Since the advent of explicit template specification
1735 support in G++, if the name of the friend is an unqualified-id (i.e.,
1736 @samp{friend foo(int)}), the C++ language specification demands that the
1737 friend declare or define an ordinary, nontemplate function. (Section
1738 14.5.3). Before G++ implemented explicit specification, unqualified-ids
1739 could be interpreted as a particular specialization of a templatized
1740 function. Because this non-conforming behavior is no longer the default
1741 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
1742 check existing code for potential trouble spots and is on by default.
1743 This new compiler behavior can be turned off with
1744 @option{-Wno-non-template-friend} which keeps the conformant compiler code
1745 but disables the helpful warning.
1747 @item -Wold-style-cast @r{(C++ only)}
1748 @opindex Wold-style-cast
1749 Warn if an old-style (C-style) cast to a non-void type is used within
1750 a C++ program. The new-style casts (@samp{static_cast},
1751 @samp{reinterpret_cast}, and @samp{const_cast}) are less vulnerable to
1752 unintended effects and much easier to search for.
1754 @item -Woverloaded-virtual @r{(C++ only)}
1755 @opindex Woverloaded-virtual
1756 @cindex overloaded virtual fn, warning
1757 @cindex warning for overloaded virtual fn
1758 Warn when a function declaration hides virtual functions from a
1759 base class. For example, in:
1766 struct B: public A @{
1771 the @code{A} class version of @code{f} is hidden in @code{B}, and code
1779 will fail to compile.
1781 @item -Wno-pmf-conversions @r{(C++ only)}
1782 @opindex Wno-pmf-conversions
1783 Disable the diagnostic for converting a bound pointer to member function
1786 @item -Wsign-promo @r{(C++ only)}
1787 @opindex Wsign-promo
1788 Warn when overload resolution chooses a promotion from unsigned or
1789 enumerated type to a signed type, over a conversion to an unsigned type of
1790 the same size. Previous versions of G++ would try to preserve
1791 unsignedness, but the standard mandates the current behavior.
1793 @item -Wsynth @r{(C++ only)}
1795 @cindex warning for synthesized methods
1796 @cindex synthesized methods, warning
1797 Warn when G++'s synthesis behavior does not match that of cfront. For
1803 A& operator = (int);
1813 In this example, G++ will synthesize a default @samp{A& operator =
1814 (const A&);}, while cfront will use the user-defined @samp{operator =}.
1817 @node Objective-C and Objective-C++ Dialect Options
1818 @section Options Controlling Objective-C and Objective-C++ Dialects
1820 @cindex compiler options, Objective-C and Objective-C++
1821 @cindex Objective-C and Objective-C++ options, command line
1822 @cindex options, Objective-C and Objective-C++
1823 (NOTE: This manual does not describe the Objective-C and Objective-C++
1824 languages themselves. See @xref{Standards,,Language Standards
1825 Supported by GCC}, for references.)
1827 This section describes the command-line options that are only meaningful
1828 for Objective-C and Objective-C++ programs, but you can also use most of
1829 the language-independent GNU compiler options.
1830 For example, you might compile a file @code{some_class.m} like this:
1833 gcc -g -fgnu-runtime -O -c some_class.m
1837 In this example, @option{-fgnu-runtime} is an option meant only for
1838 Objective-C and Objective-C++ programs; you can use the other options with
1839 any language supported by GCC@.
1841 Note that since Objective-C is an extension of the C language, Objective-C
1842 compilations may also use options specific to the C front-end (e.g.,
1843 @option{-Wtraditional}). Similarly, Objective-C++ compilations may use
1844 C++-specific options (e.g., @option{-Wabi}).
1846 Here is a list of options that are @emph{only} for compiling Objective-C
1847 and Objective-C++ programs:
1850 @item -fconstant-string-class=@var{class-name}
1851 @opindex fconstant-string-class
1852 Use @var{class-name} as the name of the class to instantiate for each
1853 literal string specified with the syntax @code{@@"@dots{}"}. The default
1854 class name is @code{NXConstantString} if the GNU runtime is being used, and
1855 @code{NSConstantString} if the NeXT runtime is being used (see below). The
1856 @option{-fconstant-cfstrings} option, if also present, will override the
1857 @option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
1858 to be laid out as constant CoreFoundation strings.
1861 @opindex fgnu-runtime
1862 Generate object code compatible with the standard GNU Objective-C
1863 runtime. This is the default for most types of systems.
1865 @item -fnext-runtime
1866 @opindex fnext-runtime
1867 Generate output compatible with the NeXT runtime. This is the default
1868 for NeXT-based systems, including Darwin and Mac OS X@. The macro
1869 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
1872 @item -fno-nil-receivers
1873 @opindex fno-nil-receivers
1874 Assume that all Objective-C message dispatches (e.g.,
1875 @code{[receiver message:arg]}) in this translation unit ensure that the receiver
1876 is not @code{nil}. This allows for more efficient entry points in the runtime
1877 to be used. Currently, this option is only available in conjunction with
1878 the NeXT runtime on Mac OS X 10.3 and later.
1880 @item -fobjc-exceptions
1881 @opindex fobjc-exceptions
1882 Enable syntactic support for structured exception handling in Objective-C,
1883 similar to what is offered by C++ and Java. Currently, this option is only
1884 available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
1892 @@catch (AnObjCClass *exc) @{
1899 @@catch (AnotherClass *exc) @{
1902 @@catch (id allOthers) @{
1912 The @code{@@throw} statement may appear anywhere in an Objective-C or
1913 Objective-C++ program; when used inside of a @code{@@catch} block, the
1914 @code{@@throw} may appear without an argument (as shown above), in which case
1915 the object caught by the @code{@@catch} will be rethrown.
1917 Note that only (pointers to) Objective-C objects may be thrown and
1918 caught using this scheme. When an object is thrown, it will be caught
1919 by the nearest @code{@@catch} clause capable of handling objects of that type,
1920 analogously to how @code{catch} blocks work in C++ and Java. A
1921 @code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
1922 any and all Objective-C exceptions not caught by previous @code{@@catch}
1925 The @code{@@finally} clause, if present, will be executed upon exit from the
1926 immediately preceding @code{@@try @dots{} @@catch} section. This will happen
1927 regardless of whether any exceptions are thrown, caught or rethrown
1928 inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
1929 of the @code{finally} clause in Java.
1931 There are several caveats to using the new exception mechanism:
1935 Although currently designed to be binary compatible with @code{NS_HANDLER}-style
1936 idioms provided by the @code{NSException} class, the new
1937 exceptions can only be used on Mac OS X 10.3 (Panther) and later
1938 systems, due to additional functionality needed in the (NeXT) Objective-C
1942 As mentioned above, the new exceptions do not support handling
1943 types other than Objective-C objects. Furthermore, when used from
1944 Objective-C++, the Objective-C exception model does not interoperate with C++
1945 exceptions at this time. This means you cannot @code{@@throw} an exception
1946 from Objective-C and @code{catch} it in C++, or vice versa
1947 (i.e., @code{throw @dots{} @@catch}).
1950 The @option{-fobjc-exceptions} switch also enables the use of synchronization
1951 blocks for thread-safe execution:
1954 @@synchronized (ObjCClass *guard) @{
1959 Upon entering the @code{@@synchronized} block, a thread of execution shall
1960 first check whether a lock has been placed on the corresponding @code{guard}
1961 object by another thread. If it has, the current thread shall wait until
1962 the other thread relinquishes its lock. Once @code{guard} becomes available,
1963 the current thread will place its own lock on it, execute the code contained in
1964 the @code{@@synchronized} block, and finally relinquish the lock (thereby
1965 making @code{guard} available to other threads).
1967 Unlike Java, Objective-C does not allow for entire methods to be marked
1968 @code{@@synchronized}. Note that throwing exceptions out of
1969 @code{@@synchronized} blocks is allowed, and will cause the guarding object
1970 to be unlocked properly.
1972 @item -freplace-objc-classes
1973 @opindex freplace-objc-classes
1974 Emit a special marker instructing @command{ld(1)} not to statically link in
1975 the resulting object file, and allow @command{dyld(1)} to load it in at
1976 run time instead. This is used in conjunction with the Fix-and-Continue
1977 debugging mode, where the object file in question may be recompiled and
1978 dynamically reloaded in the course of program execution, without the need
1979 to restart the program itself. Currently, Fix-and-Continue functionality
1980 is only available in conjunction with the NeXT runtime on Mac OS X 10.3
1985 When compiling for the NeXT runtime, the compiler ordinarily replaces calls
1986 to @code{objc_getClass("@dots{}")} (when the name of the class is known at
1987 compile time) with static class references that get initialized at load time,
1988 which improves run-time performance. Specifying the @option{-fzero-link} flag
1989 suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
1990 to be retained. This is useful in Zero-Link debugging mode, since it allows
1991 for individual class implementations to be modified during program execution.
1995 Dump interface declarations for all classes seen in the source file to a
1996 file named @file{@var{sourcename}.decl}.
1999 @opindex Wno-protocol
2000 If a class is declared to implement a protocol, a warning is issued for
2001 every method in the protocol that is not implemented by the class. The
2002 default behavior is to issue a warning for every method not explicitly
2003 implemented in the class, even if a method implementation is inherited
2004 from the superclass. If you use the @option{-Wno-protocol} option, then
2005 methods inherited from the superclass are considered to be implemented,
2006 and no warning is issued for them.
2010 Warn if multiple methods of different types for the same selector are
2011 found during compilation. The check is performed on the list of methods
2012 in the final stage of compilation. Additionally, a check is performed
2013 for each selector appearing in a @code{@@selector(@dots{})}
2014 expression, and a corresponding method for that selector has been found
2015 during compilation. Because these checks scan the method table only at
2016 the end of compilation, these warnings are not produced if the final
2017 stage of compilation is not reached, for example because an error is
2018 found during compilation, or because the @option{-fsyntax-only} option is
2021 @item -Wundeclared-selector
2022 @opindex Wundeclared-selector
2023 Warn if a @code{@@selector(@dots{})} expression referring to an
2024 undeclared selector is found. A selector is considered undeclared if no
2025 method with that name has been declared before the
2026 @code{@@selector(@dots{})} expression, either explicitly in an
2027 @code{@@interface} or @code{@@protocol} declaration, or implicitly in
2028 an @code{@@implementation} section. This option always performs its
2029 checks as soon as a @code{@@selector(@dots{})} expression is found,
2030 while @option{-Wselector} only performs its checks in the final stage of
2031 compilation. This also enforces the coding style convention
2032 that methods and selectors must be declared before being used.
2034 @item -print-objc-runtime-info
2035 @opindex print-objc-runtime-info
2036 Generate C header describing the largest structure that is passed by
2041 @node Language Independent Options
2042 @section Options to Control Diagnostic Messages Formatting
2043 @cindex options to control diagnostics formatting
2044 @cindex diagnostic messages
2045 @cindex message formatting
2047 Traditionally, diagnostic messages have been formatted irrespective of
2048 the output device's aspect (e.g.@: its width, @dots{}). The options described
2049 below can be used to control the diagnostic messages formatting
2050 algorithm, e.g.@: how many characters per line, how often source location
2051 information should be reported. Right now, only the C++ front end can
2052 honor these options. However it is expected, in the near future, that
2053 the remaining front ends would be able to digest them correctly.
2056 @item -fmessage-length=@var{n}
2057 @opindex fmessage-length
2058 Try to format error messages so that they fit on lines of about @var{n}
2059 characters. The default is 72 characters for @command{g++} and 0 for the rest of
2060 the front ends supported by GCC@. If @var{n} is zero, then no
2061 line-wrapping will be done; each error message will appear on a single
2064 @opindex fdiagnostics-show-location
2065 @item -fdiagnostics-show-location=once
2066 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
2067 reporter to emit @emph{once} source location information; that is, in
2068 case the message is too long to fit on a single physical line and has to
2069 be wrapped, the source location won't be emitted (as prefix) again,
2070 over and over, in subsequent continuation lines. This is the default
2073 @item -fdiagnostics-show-location=every-line
2074 Only meaningful in line-wrapping mode. Instructs the diagnostic
2075 messages reporter to emit the same source location information (as
2076 prefix) for physical lines that result from the process of breaking
2077 a message which is too long to fit on a single line.
2081 @node Warning Options
2082 @section Options to Request or Suppress Warnings
2083 @cindex options to control warnings
2084 @cindex warning messages
2085 @cindex messages, warning
2086 @cindex suppressing warnings
2088 Warnings are diagnostic messages that report constructions which
2089 are not inherently erroneous but which are risky or suggest there
2090 may have been an error.
2092 You can request many specific warnings with options beginning @samp{-W},
2093 for example @option{-Wimplicit} to request warnings on implicit
2094 declarations. Each of these specific warning options also has a
2095 negative form beginning @samp{-Wno-} to turn off warnings;
2096 for example, @option{-Wno-implicit}. This manual lists only one of the
2097 two forms, whichever is not the default.
2099 The following options control the amount and kinds of warnings produced
2100 by GCC; for further, language-specific options also refer to
2101 @ref{C++ Dialect Options} and @ref{Objective-C and Objective-C++ Dialect
2105 @cindex syntax checking
2107 @opindex fsyntax-only
2108 Check the code for syntax errors, but don't do anything beyond that.
2112 Issue all the warnings demanded by strict ISO C and ISO C++;
2113 reject all programs that use forbidden extensions, and some other
2114 programs that do not follow ISO C and ISO C++. For ISO C, follows the
2115 version of the ISO C standard specified by any @option{-std} option used.
2117 Valid ISO C and ISO C++ programs should compile properly with or without
2118 this option (though a rare few will require @option{-ansi} or a
2119 @option{-std} option specifying the required version of ISO C)@. However,
2120 without this option, certain GNU extensions and traditional C and C++
2121 features are supported as well. With this option, they are rejected.
2123 @option{-pedantic} does not cause warning messages for use of the
2124 alternate keywords whose names begin and end with @samp{__}. Pedantic
2125 warnings are also disabled in the expression that follows
2126 @code{__extension__}. However, only system header files should use
2127 these escape routes; application programs should avoid them.
2128 @xref{Alternate Keywords}.
2130 Some users try to use @option{-pedantic} to check programs for strict ISO
2131 C conformance. They soon find that it does not do quite what they want:
2132 it finds some non-ISO practices, but not all---only those for which
2133 ISO C @emph{requires} a diagnostic, and some others for which
2134 diagnostics have been added.
2136 A feature to report any failure to conform to ISO C might be useful in
2137 some instances, but would require considerable additional work and would
2138 be quite different from @option{-pedantic}. We don't have plans to
2139 support such a feature in the near future.
2141 Where the standard specified with @option{-std} represents a GNU
2142 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
2143 corresponding @dfn{base standard}, the version of ISO C on which the GNU
2144 extended dialect is based. Warnings from @option{-pedantic} are given
2145 where they are required by the base standard. (It would not make sense
2146 for such warnings to be given only for features not in the specified GNU
2147 C dialect, since by definition the GNU dialects of C include all
2148 features the compiler supports with the given option, and there would be
2149 nothing to warn about.)
2151 @item -pedantic-errors
2152 @opindex pedantic-errors
2153 Like @option{-pedantic}, except that errors are produced rather than
2158 Inhibit all warning messages.
2162 Inhibit warning messages about the use of @samp{#import}.
2164 @item -Wchar-subscripts
2165 @opindex Wchar-subscripts
2166 Warn if an array subscript has type @code{char}. This is a common cause
2167 of error, as programmers often forget that this type is signed on some
2169 This warning is enabled by @option{-Wall}.
2173 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
2174 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
2175 This warning is enabled by @option{-Wall}.
2177 @item -Wfatal-errors
2178 @opindex Wfatal-errors
2179 This option causes the compiler to abort compilation on the first error
2180 occurred rather than trying to keep going and printing further error
2185 @opindex ffreestanding
2186 @opindex fno-builtin
2187 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
2188 the arguments supplied have types appropriate to the format string
2189 specified, and that the conversions specified in the format string make
2190 sense. This includes standard functions, and others specified by format
2191 attributes (@pxref{Function Attributes}), in the @code{printf},
2192 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
2193 not in the C standard) families (or other target-specific families).
2194 Which functions are checked without format attributes having been
2195 specified depends on the standard version selected, and such checks of
2196 functions without the attribute specified are disabled by
2197 @option{-ffreestanding} or @option{-fno-builtin}.
2199 The formats are checked against the format features supported by GNU
2200 libc version 2.2. These include all ISO C90 and C99 features, as well
2201 as features from the Single Unix Specification and some BSD and GNU
2202 extensions. Other library implementations may not support all these
2203 features; GCC does not support warning about features that go beyond a
2204 particular library's limitations. However, if @option{-pedantic} is used
2205 with @option{-Wformat}, warnings will be given about format features not
2206 in the selected standard version (but not for @code{strfmon} formats,
2207 since those are not in any version of the C standard). @xref{C Dialect
2208 Options,,Options Controlling C Dialect}.
2210 Since @option{-Wformat} also checks for null format arguments for
2211 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2213 @option{-Wformat} is included in @option{-Wall}. For more control over some
2214 aspects of format checking, the options @option{-Wformat-y2k},
2215 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2216 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2217 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2220 @opindex Wformat-y2k
2221 If @option{-Wformat} is specified, also warn about @code{strftime}
2222 formats which may yield only a two-digit year.
2224 @item -Wno-format-extra-args
2225 @opindex Wno-format-extra-args
2226 If @option{-Wformat} is specified, do not warn about excess arguments to a
2227 @code{printf} or @code{scanf} format function. The C standard specifies
2228 that such arguments are ignored.
2230 Where the unused arguments lie between used arguments that are
2231 specified with @samp{$} operand number specifications, normally
2232 warnings are still given, since the implementation could not know what
2233 type to pass to @code{va_arg} to skip the unused arguments. However,
2234 in the case of @code{scanf} formats, this option will suppress the
2235 warning if the unused arguments are all pointers, since the Single
2236 Unix Specification says that such unused arguments are allowed.
2238 @item -Wno-format-zero-length
2239 @opindex Wno-format-zero-length
2240 If @option{-Wformat} is specified, do not warn about zero-length formats.
2241 The C standard specifies that zero-length formats are allowed.
2243 @item -Wformat-nonliteral
2244 @opindex Wformat-nonliteral
2245 If @option{-Wformat} is specified, also warn if the format string is not a
2246 string literal and so cannot be checked, unless the format function
2247 takes its format arguments as a @code{va_list}.
2249 @item -Wformat-security
2250 @opindex Wformat-security
2251 If @option{-Wformat} is specified, also warn about uses of format
2252 functions that represent possible security problems. At present, this
2253 warns about calls to @code{printf} and @code{scanf} functions where the
2254 format string is not a string literal and there are no format arguments,
2255 as in @code{printf (foo);}. This may be a security hole if the format
2256 string came from untrusted input and contains @samp{%n}. (This is
2257 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2258 in future warnings may be added to @option{-Wformat-security} that are not
2259 included in @option{-Wformat-nonliteral}.)
2263 Enable @option{-Wformat} plus format checks not included in
2264 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2265 -Wformat-nonliteral -Wformat-security -Wformat-y2k}.
2269 Warn about passing a null pointer for arguments marked as
2270 requiring a non-null value by the @code{nonnull} function attribute.
2272 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2273 can be disabled with the @option{-Wno-nonnull} option.
2275 @item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
2277 Warn about uninitialized variables which are initialized with themselves.
2278 Note this option can only be used with the @option{-Wuninitialized} option,
2279 which in turn only works with @option{-O1} and above.
2281 For example, GCC will warn about @code{i} being uninitialized in the
2282 following snippet only when @option{-Winit-self} has been specified:
2293 @item -Wimplicit-int
2294 @opindex Wimplicit-int
2295 Warn when a declaration does not specify a type.
2296 This warning is enabled by @option{-Wall}.
2298 @item -Wimplicit-function-declaration
2299 @itemx -Werror-implicit-function-declaration
2300 @opindex Wimplicit-function-declaration
2301 @opindex Werror-implicit-function-declaration
2302 Give a warning (or error) whenever a function is used before being
2303 declared. The form @option{-Wno-error-implicit-function-declaration}
2305 This warning is enabled by @option{-Wall} (as a warning, not an error).
2309 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2310 This warning is enabled by @option{-Wall}.
2314 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2315 function with external linkage, returning int, taking either zero
2316 arguments, two, or three arguments of appropriate types.
2317 This warning is enabled by @option{-Wall}.
2319 @item -Wmissing-braces
2320 @opindex Wmissing-braces
2321 Warn if an aggregate or union initializer is not fully bracketed. In
2322 the following example, the initializer for @samp{a} is not fully
2323 bracketed, but that for @samp{b} is fully bracketed.
2326 int a[2][2] = @{ 0, 1, 2, 3 @};
2327 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2330 This warning is enabled by @option{-Wall}.
2332 @item -Wmissing-include-dirs @r{(C, C++, Objective-C and Objective-C++ only)}
2333 @opindex Wmissing-include-dirs
2334 Warn if a user-supplied include directory does not exist.
2337 @opindex Wparentheses
2338 Warn if parentheses are omitted in certain contexts, such
2339 as when there is an assignment in a context where a truth value
2340 is expected, or when operators are nested whose precedence people
2341 often get confused about. Only the warning for an assignment used as
2342 a truth value is supported when compiling C++; the other warnings are
2343 only supported when compiling C@.
2345 Also warn if a comparison like @samp{x<=y<=z} appears; this is
2346 equivalent to @samp{(x<=y ? 1 : 0) <= z}, which is a different
2347 interpretation from that of ordinary mathematical notation.
2349 Also warn about constructions where there may be confusion to which
2350 @code{if} statement an @code{else} branch belongs. Here is an example of
2365 In C, every @code{else} branch belongs to the innermost possible @code{if}
2366 statement, which in this example is @code{if (b)}. This is often not
2367 what the programmer expected, as illustrated in the above example by
2368 indentation the programmer chose. When there is the potential for this
2369 confusion, GCC will issue a warning when this flag is specified.
2370 To eliminate the warning, add explicit braces around the innermost
2371 @code{if} statement so there is no way the @code{else} could belong to
2372 the enclosing @code{if}. The resulting code would look like this:
2388 This warning is enabled by @option{-Wall}.
2390 @item -Wsequence-point
2391 @opindex Wsequence-point
2392 Warn about code that may have undefined semantics because of violations
2393 of sequence point rules in the C standard.
2395 The C standard defines the order in which expressions in a C program are
2396 evaluated in terms of @dfn{sequence points}, which represent a partial
2397 ordering between the execution of parts of the program: those executed
2398 before the sequence point, and those executed after it. These occur
2399 after the evaluation of a full expression (one which is not part of a
2400 larger expression), after the evaluation of the first operand of a
2401 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2402 function is called (but after the evaluation of its arguments and the
2403 expression denoting the called function), and in certain other places.
2404 Other than as expressed by the sequence point rules, the order of
2405 evaluation of subexpressions of an expression is not specified. All
2406 these rules describe only a partial order rather than a total order,
2407 since, for example, if two functions are called within one expression
2408 with no sequence point between them, the order in which the functions
2409 are called is not specified. However, the standards committee have
2410 ruled that function calls do not overlap.
2412 It is not specified when between sequence points modifications to the
2413 values of objects take effect. Programs whose behavior depends on this
2414 have undefined behavior; the C standard specifies that ``Between the
2415 previous and next sequence point an object shall have its stored value
2416 modified at most once by the evaluation of an expression. Furthermore,
2417 the prior value shall be read only to determine the value to be
2418 stored.''. If a program breaks these rules, the results on any
2419 particular implementation are entirely unpredictable.
2421 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2422 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2423 diagnosed by this option, and it may give an occasional false positive
2424 result, but in general it has been found fairly effective at detecting
2425 this sort of problem in programs.
2427 The present implementation of this option only works for C programs. A
2428 future implementation may also work for C++ programs.
2430 The C standard is worded confusingly, therefore there is some debate
2431 over the precise meaning of the sequence point rules in subtle cases.
2432 Links to discussions of the problem, including proposed formal
2433 definitions, may be found on the GCC readings page, at
2434 @w{@uref{http://gcc.gnu.org/readings.html}}.
2436 This warning is enabled by @option{-Wall}.
2439 @opindex Wreturn-type
2440 Warn whenever a function is defined with a return-type that defaults to
2441 @code{int}. Also warn about any @code{return} statement with no
2442 return-value in a function whose return-type is not @code{void}.
2444 For C, also warn if the return type of a function has a type qualifier
2445 such as @code{const}. Such a type qualifier has no effect, since the
2446 value returned by a function is not an lvalue. ISO C prohibits
2447 qualified @code{void} return types on function definitions, so such
2448 return types always receive a warning even without this option.
2450 For C++, a function without return type always produces a diagnostic
2451 message, even when @option{-Wno-return-type} is specified. The only
2452 exceptions are @samp{main} and functions defined in system headers.
2454 This warning is enabled by @option{-Wall}.
2458 Warn whenever a @code{switch} statement has an index of enumerated type
2459 and lacks a @code{case} for one or more of the named codes of that
2460 enumeration. (The presence of a @code{default} label prevents this
2461 warning.) @code{case} labels outside the enumeration range also
2462 provoke warnings when this option is used.
2463 This warning is enabled by @option{-Wall}.
2465 @item -Wswitch-default
2466 @opindex Wswitch-switch
2467 Warn whenever a @code{switch} statement does not have a @code{default}
2471 @opindex Wswitch-enum
2472 Warn whenever a @code{switch} statement has an index of enumerated type
2473 and lacks a @code{case} for one or more of the named codes of that
2474 enumeration. @code{case} labels outside the enumeration range also
2475 provoke warnings when this option is used.
2479 Warn if any trigraphs are encountered that might change the meaning of
2480 the program (trigraphs within comments are not warned about).
2481 This warning is enabled by @option{-Wall}.
2483 @item -Wunused-function
2484 @opindex Wunused-function
2485 Warn whenever a static function is declared but not defined or a
2486 non\-inline static function is unused.
2487 This warning is enabled by @option{-Wall}.
2489 @item -Wunused-label
2490 @opindex Wunused-label
2491 Warn whenever a label is declared but not used.
2492 This warning is enabled by @option{-Wall}.
2494 To suppress this warning use the @samp{unused} attribute
2495 (@pxref{Variable Attributes}).
2497 @item -Wunused-parameter
2498 @opindex Wunused-parameter
2499 Warn whenever a function parameter is unused aside from its declaration.
2501 To suppress this warning use the @samp{unused} attribute
2502 (@pxref{Variable Attributes}).
2504 @item -Wunused-variable
2505 @opindex Wunused-variable
2506 Warn whenever a local variable or non-constant static variable is unused
2507 aside from its declaration
2508 This warning is enabled by @option{-Wall}.
2510 To suppress this warning use the @samp{unused} attribute
2511 (@pxref{Variable Attributes}).
2513 @item -Wunused-value
2514 @opindex Wunused-value
2515 Warn whenever a statement computes a result that is explicitly not used.
2516 This warning is enabled by @option{-Wall}.
2518 To suppress this warning cast the expression to @samp{void}.
2522 All the above @option{-Wunused} options combined.
2524 In order to get a warning about an unused function parameter, you must
2525 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
2526 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
2528 @item -Wuninitialized
2529 @opindex Wuninitialized
2530 Warn if an automatic variable is used without first being initialized or
2531 if a variable may be clobbered by a @code{setjmp} call.
2533 These warnings are possible only in optimizing compilation,
2534 because they require data flow information that is computed only
2535 when optimizing. If you don't specify @option{-O}, you simply won't
2538 If you want to warn about code which uses the uninitialized value of the
2539 variable in its own initializer, use the @option{-Winit-self} option.
2541 These warnings occur only for variables that are candidates for
2542 register allocation. Therefore, they do not occur for a variable that
2543 is declared @code{volatile}, or whose address is taken, or whose size
2544 is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
2545 structures, unions or arrays, even when they are in registers.
2547 Note that there may be no warning about a variable that is used only
2548 to compute a value that itself is never used, because such
2549 computations may be deleted by data flow analysis before the warnings
2552 These warnings are made optional because GCC is not smart
2553 enough to see all the reasons why the code might be correct
2554 despite appearing to have an error. Here is one example of how
2575 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
2576 always initialized, but GCC doesn't know this. Here is
2577 another common case:
2582 if (change_y) save_y = y, y = new_y;
2584 if (change_y) y = save_y;
2589 This has no bug because @code{save_y} is used only if it is set.
2591 @cindex @code{longjmp} warnings
2592 This option also warns when a non-volatile automatic variable might be
2593 changed by a call to @code{longjmp}. These warnings as well are possible
2594 only in optimizing compilation.
2596 The compiler sees only the calls to @code{setjmp}. It cannot know
2597 where @code{longjmp} will be called; in fact, a signal handler could
2598 call it at any point in the code. As a result, you may get a warning
2599 even when there is in fact no problem because @code{longjmp} cannot
2600 in fact be called at the place which would cause a problem.
2602 Some spurious warnings can be avoided if you declare all the functions
2603 you use that never return as @code{noreturn}. @xref{Function
2606 This warning is enabled by @option{-Wall}.
2608 @item -Wunknown-pragmas
2609 @opindex Wunknown-pragmas
2610 @cindex warning for unknown pragmas
2611 @cindex unknown pragmas, warning
2612 @cindex pragmas, warning of unknown
2613 Warn when a #pragma directive is encountered which is not understood by
2614 GCC@. If this command line option is used, warnings will even be issued
2615 for unknown pragmas in system header files. This is not the case if
2616 the warnings were only enabled by the @option{-Wall} command line option.
2618 @item -Wstrict-aliasing
2619 @opindex Wstrict-aliasing
2620 This option is only active when @option{-fstrict-aliasing} is active.
2621 It warns about code which might break the strict aliasing rules that the
2622 compiler is using for optimization. The warning does not catch all
2623 cases, but does attempt to catch the more common pitfalls. It is
2624 included in @option{-Wall}.
2626 @item -Wstrict-aliasing=2
2627 @opindex Wstrict-aliasing=2
2628 This option is only active when @option{-fstrict-aliasing} is active.
2629 It warns about all code which might break the strict aliasing rules that the
2630 compiler is using for optimization. This warning catches all cases, but
2631 it will also give a warning for some ambiguous cases that are safe.
2635 All of the above @samp{-W} options combined. This enables all the
2636 warnings about constructions that some users consider questionable, and
2637 that are easy to avoid (or modify to prevent the warning), even in
2638 conjunction with macros. This also enables some language-specific
2639 warnings described in @ref{C++ Dialect Options} and
2640 @ref{Objective-C and Objective-C++ Dialect Options}.
2643 The following @option{-W@dots{}} options are not implied by @option{-Wall}.
2644 Some of them warn about constructions that users generally do not
2645 consider questionable, but which occasionally you might wish to check
2646 for; others warn about constructions that are necessary or hard to avoid
2647 in some cases, and there is no simple way to modify the code to suppress
2654 (This option used to be called @option{-W}. The older name is still
2655 supported, but the newer name is more descriptive.) Print extra warning
2656 messages for these events:
2660 A function can return either with or without a value. (Falling
2661 off the end of the function body is considered returning without
2662 a value.) For example, this function would evoke such a
2676 An expression-statement or the left-hand side of a comma expression
2677 contains no side effects.
2678 To suppress the warning, cast the unused expression to void.
2679 For example, an expression such as @samp{x[i,j]} will cause a warning,
2680 but @samp{x[(void)i,j]} will not.
2683 An unsigned value is compared against zero with @samp{<} or @samp{>=}.
2686 Storage-class specifiers like @code{static} are not the first things in
2687 a declaration. According to the C Standard, this usage is obsolescent.
2690 If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
2694 A comparison between signed and unsigned values could produce an
2695 incorrect result when the signed value is converted to unsigned.
2696 (But don't warn if @option{-Wno-sign-compare} is also specified.)
2699 An aggregate has an initializer which does not initialize all members.
2700 This warning can be independently controlled by
2701 @option{-Wmissing-field-initializers}.
2704 A function parameter is declared without a type specifier in K&R-style
2712 An empty body occurs in an @samp{if} or @samp{else} statement.
2715 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2716 @samp{>}, or @samp{>=}.
2719 A variable might be changed by @samp{longjmp} or @samp{vfork}.
2722 Any of several floating-point events that often indicate errors, such as
2723 overflow, underflow, loss of precision, etc.
2725 @item @r{(C++ only)}
2726 An enumerator and a non-enumerator both appear in a conditional expression.
2728 @item @r{(C++ only)}
2729 A non-static reference or non-static @samp{const} member appears in a
2730 class without constructors.
2732 @item @r{(C++ only)}
2733 Ambiguous virtual bases.
2735 @item @r{(C++ only)}
2736 Subscripting an array which has been declared @samp{register}.
2738 @item @r{(C++ only)}
2739 Taking the address of a variable which has been declared @samp{register}.
2741 @item @r{(C++ only)}
2742 A base class is not initialized in a derived class' copy constructor.
2745 @item -Wno-div-by-zero
2746 @opindex Wno-div-by-zero
2747 @opindex Wdiv-by-zero
2748 Do not warn about compile-time integer division by zero. Floating point
2749 division by zero is not warned about, as it can be a legitimate way of
2750 obtaining infinities and NaNs.
2752 @item -Wsystem-headers
2753 @opindex Wsystem-headers
2754 @cindex warnings from system headers
2755 @cindex system headers, warnings from
2756 Print warning messages for constructs found in system header files.
2757 Warnings from system headers are normally suppressed, on the assumption
2758 that they usually do not indicate real problems and would only make the
2759 compiler output harder to read. Using this command line option tells
2760 GCC to emit warnings from system headers as if they occurred in user
2761 code. However, note that using @option{-Wall} in conjunction with this
2762 option will @emph{not} warn about unknown pragmas in system
2763 headers---for that, @option{-Wunknown-pragmas} must also be used.
2766 @opindex Wfloat-equal
2767 Warn if floating point values are used in equality comparisons.
2769 The idea behind this is that sometimes it is convenient (for the
2770 programmer) to consider floating-point values as approximations to
2771 infinitely precise real numbers. If you are doing this, then you need
2772 to compute (by analyzing the code, or in some other way) the maximum or
2773 likely maximum error that the computation introduces, and allow for it
2774 when performing comparisons (and when producing output, but that's a
2775 different problem). In particular, instead of testing for equality, you
2776 would check to see whether the two values have ranges that overlap; and
2777 this is done with the relational operators, so equality comparisons are
2780 @item -Wtraditional @r{(C only)}
2781 @opindex Wtraditional
2782 Warn about certain constructs that behave differently in traditional and
2783 ISO C@. Also warn about ISO C constructs that have no traditional C
2784 equivalent, and/or problematic constructs which should be avoided.
2788 Macro parameters that appear within string literals in the macro body.
2789 In traditional C macro replacement takes place within string literals,
2790 but does not in ISO C@.
2793 In traditional C, some preprocessor directives did not exist.
2794 Traditional preprocessors would only consider a line to be a directive
2795 if the @samp{#} appeared in column 1 on the line. Therefore
2796 @option{-Wtraditional} warns about directives that traditional C
2797 understands but would ignore because the @samp{#} does not appear as the
2798 first character on the line. It also suggests you hide directives like
2799 @samp{#pragma} not understood by traditional C by indenting them. Some
2800 traditional implementations would not recognize @samp{#elif}, so it
2801 suggests avoiding it altogether.
2804 A function-like macro that appears without arguments.
2807 The unary plus operator.
2810 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
2811 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
2812 constants.) Note, these suffixes appear in macros defined in the system
2813 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
2814 Use of these macros in user code might normally lead to spurious
2815 warnings, however GCC's integrated preprocessor has enough context to
2816 avoid warning in these cases.
2819 A function declared external in one block and then used after the end of
2823 A @code{switch} statement has an operand of type @code{long}.
2826 A non-@code{static} function declaration follows a @code{static} one.
2827 This construct is not accepted by some traditional C compilers.
2830 The ISO type of an integer constant has a different width or
2831 signedness from its traditional type. This warning is only issued if
2832 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
2833 typically represent bit patterns, are not warned about.
2836 Usage of ISO string concatenation is detected.
2839 Initialization of automatic aggregates.
2842 Identifier conflicts with labels. Traditional C lacks a separate
2843 namespace for labels.
2846 Initialization of unions. If the initializer is zero, the warning is
2847 omitted. This is done under the assumption that the zero initializer in
2848 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
2849 initializer warnings and relies on default initialization to zero in the
2853 Conversions by prototypes between fixed/floating point values and vice
2854 versa. The absence of these prototypes when compiling with traditional
2855 C would cause serious problems. This is a subset of the possible
2856 conversion warnings, for the full set use @option{-Wconversion}.
2859 Use of ISO C style function definitions. This warning intentionally is
2860 @emph{not} issued for prototype declarations or variadic functions
2861 because these ISO C features will appear in your code when using
2862 libiberty's traditional C compatibility macros, @code{PARAMS} and
2863 @code{VPARAMS}. This warning is also bypassed for nested functions
2864 because that feature is already a GCC extension and thus not relevant to
2865 traditional C compatibility.
2868 @item -Wdeclaration-after-statement @r{(C only)}
2869 @opindex Wdeclaration-after-statement
2870 Warn when a declaration is found after a statement in a block. This
2871 construct, known from C++, was introduced with ISO C99 and is by default
2872 allowed in GCC@. It is not supported by ISO C90 and was not supported by
2873 GCC versions before GCC 3.0. @xref{Mixed Declarations}.
2877 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
2879 @item -Wno-endif-labels
2880 @opindex Wno-endif-labels
2881 @opindex Wendif-labels
2882 Do not warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
2886 Warn whenever a local variable shadows another local variable, parameter or
2887 global variable or whenever a built-in function is shadowed.
2889 @item -Wlarger-than-@var{len}
2890 @opindex Wlarger-than
2891 Warn whenever an object of larger than @var{len} bytes is defined.
2893 @item -Wpointer-arith
2894 @opindex Wpointer-arith
2895 Warn about anything that depends on the ``size of'' a function type or
2896 of @code{void}. GNU C assigns these types a size of 1, for
2897 convenience in calculations with @code{void *} pointers and pointers
2900 @item -Wbad-function-cast @r{(C only)}
2901 @opindex Wbad-function-cast
2902 Warn whenever a function call is cast to a non-matching type.
2903 For example, warn if @code{int malloc()} is cast to @code{anything *}.
2907 Warn whenever a pointer is cast so as to remove a type qualifier from
2908 the target type. For example, warn if a @code{const char *} is cast
2909 to an ordinary @code{char *}.
2912 @opindex Wcast-align
2913 Warn whenever a pointer is cast such that the required alignment of the
2914 target is increased. For example, warn if a @code{char *} is cast to
2915 an @code{int *} on machines where integers can only be accessed at
2916 two- or four-byte boundaries.
2918 @item -Wwrite-strings
2919 @opindex Wwrite-strings
2920 When compiling C, give string constants the type @code{const
2921 char[@var{length}]} so that
2922 copying the address of one into a non-@code{const} @code{char *}
2923 pointer will get a warning; when compiling C++, warn about the
2924 deprecated conversion from string constants to @code{char *}.
2925 These warnings will help you find at
2926 compile time code that can try to write into a string constant, but
2927 only if you have been very careful about using @code{const} in
2928 declarations and prototypes. Otherwise, it will just be a nuisance;
2929 this is why we did not make @option{-Wall} request these warnings.
2932 @opindex Wconversion
2933 Warn if a prototype causes a type conversion that is different from what
2934 would happen to the same argument in the absence of a prototype. This
2935 includes conversions of fixed point to floating and vice versa, and
2936 conversions changing the width or signedness of a fixed point argument
2937 except when the same as the default promotion.
2939 Also, warn if a negative integer constant expression is implicitly
2940 converted to an unsigned type. For example, warn about the assignment
2941 @code{x = -1} if @code{x} is unsigned. But do not warn about explicit
2942 casts like @code{(unsigned) -1}.
2944 @item -Wsign-compare
2945 @opindex Wsign-compare
2946 @cindex warning for comparison of signed and unsigned values
2947 @cindex comparison of signed and unsigned values, warning
2948 @cindex signed and unsigned values, comparison warning
2949 Warn when a comparison between signed and unsigned values could produce
2950 an incorrect result when the signed value is converted to unsigned.
2951 This warning is also enabled by @option{-Wextra}; to get the other warnings
2952 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
2954 @item -Waggregate-return
2955 @opindex Waggregate-return
2956 Warn if any functions that return structures or unions are defined or
2957 called. (In languages where you can return an array, this also elicits
2960 @item -Wstrict-prototypes @r{(C only)}
2961 @opindex Wstrict-prototypes
2962 Warn if a function is declared or defined without specifying the
2963 argument types. (An old-style function definition is permitted without
2964 a warning if preceded by a declaration which specifies the argument
2967 @item -Wold-style-definition @r{(C only)}
2968 @opindex Wold-style-definition
2969 Warn if an old-style function definition is used. A warning is given
2970 even if there is a previous prototype.
2972 @item -Wmissing-prototypes @r{(C only)}
2973 @opindex Wmissing-prototypes
2974 Warn if a global function is defined without a previous prototype
2975 declaration. This warning is issued even if the definition itself
2976 provides a prototype. The aim is to detect global functions that fail
2977 to be declared in header files.
2979 @item -Wmissing-declarations @r{(C only)}
2980 @opindex Wmissing-declarations
2981 Warn if a global function is defined without a previous declaration.
2982 Do so even if the definition itself provides a prototype.
2983 Use this option to detect global functions that are not declared in
2986 @item -Wmissing-field-initializers
2987 @opindex Wmissing-field-initializers
2990 Warn if a structure's initializer has some fields missing. For
2991 example, the following code would cause such a warning, because
2992 @code{x.h} is implicitly zero:
2995 struct s @{ int f, g, h; @};
2996 struct s x = @{ 3, 4 @};
2999 This option does not warn about designated initializers, so the following
3000 modification would not trigger a warning:
3003 struct s @{ int f, g, h; @};
3004 struct s x = @{ .f = 3, .g = 4 @};
3007 This warning is included in @option{-Wextra}. To get other @option{-Wextra}
3008 warnings without this one, use @samp{-Wextra -Wno-missing-field-initializers}.
3010 @item -Wmissing-noreturn
3011 @opindex Wmissing-noreturn
3012 Warn about functions which might be candidates for attribute @code{noreturn}.
3013 Note these are only possible candidates, not absolute ones. Care should
3014 be taken to manually verify functions actually do not ever return before
3015 adding the @code{noreturn} attribute, otherwise subtle code generation
3016 bugs could be introduced. You will not get a warning for @code{main} in
3017 hosted C environments.
3019 @item -Wmissing-format-attribute
3020 @opindex Wmissing-format-attribute
3022 If @option{-Wformat} is enabled, also warn about functions which might be
3023 candidates for @code{format} attributes. Note these are only possible
3024 candidates, not absolute ones. GCC will guess that @code{format}
3025 attributes might be appropriate for any function that calls a function
3026 like @code{vprintf} or @code{vscanf}, but this might not always be the
3027 case, and some functions for which @code{format} attributes are
3028 appropriate may not be detected. This option has no effect unless
3029 @option{-Wformat} is enabled (possibly by @option{-Wall}).
3031 @item -Wno-multichar
3032 @opindex Wno-multichar
3034 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
3035 Usually they indicate a typo in the user's code, as they have
3036 implementation-defined values, and should not be used in portable code.
3038 @item -Wno-deprecated-declarations
3039 @opindex Wno-deprecated-declarations
3040 Do not warn about uses of functions, variables, and types marked as
3041 deprecated by using the @code{deprecated} attribute.
3042 (@pxref{Function Attributes}, @pxref{Variable Attributes},
3043 @pxref{Type Attributes}.)
3047 Warn if a structure is given the packed attribute, but the packed
3048 attribute has no effect on the layout or size of the structure.
3049 Such structures may be mis-aligned for little benefit. For
3050 instance, in this code, the variable @code{f.x} in @code{struct bar}
3051 will be misaligned even though @code{struct bar} does not itself
3052 have the packed attribute:
3059 @} __attribute__((packed));
3069 Warn if padding is included in a structure, either to align an element
3070 of the structure or to align the whole structure. Sometimes when this
3071 happens it is possible to rearrange the fields of the structure to
3072 reduce the padding and so make the structure smaller.
3074 @item -Wredundant-decls
3075 @opindex Wredundant-decls
3076 Warn if anything is declared more than once in the same scope, even in
3077 cases where multiple declaration is valid and changes nothing.
3079 @item -Wnested-externs @r{(C only)}
3080 @opindex Wnested-externs
3081 Warn if an @code{extern} declaration is encountered within a function.
3083 @item -Wunreachable-code
3084 @opindex Wunreachable-code
3085 Warn if the compiler detects that code will never be executed.
3087 This option is intended to warn when the compiler detects that at
3088 least a whole line of source code will never be executed, because
3089 some condition is never satisfied or because it is after a
3090 procedure that never returns.
3092 It is possible for this option to produce a warning even though there
3093 are circumstances under which part of the affected line can be executed,
3094 so care should be taken when removing apparently-unreachable code.
3096 For instance, when a function is inlined, a warning may mean that the
3097 line is unreachable in only one inlined copy of the function.
3099 This option is not made part of @option{-Wall} because in a debugging
3100 version of a program there is often substantial code which checks
3101 correct functioning of the program and is, hopefully, unreachable
3102 because the program does work. Another common use of unreachable
3103 code is to provide behavior which is selectable at compile-time.
3107 Warn if a function can not be inlined and it was declared as inline.
3108 Even with this option, the compiler will not warn about failures to
3109 inline functions declared in system headers.
3111 The compiler uses a variety of heuristics to determine whether or not
3112 to inline a function. For example, the compiler takes into account
3113 the size of the function being inlined and the the amount of inlining
3114 that has already been done in the current function. Therefore,
3115 seemingly insignificant changes in the source program can cause the
3116 warnings produced by @option{-Winline} to appear or disappear.
3118 @item -Wno-invalid-offsetof @r{(C++ only)}
3119 @opindex Wno-invalid-offsetof
3120 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
3121 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
3122 to a non-POD type is undefined. In existing C++ implementations,
3123 however, @samp{offsetof} typically gives meaningful results even when
3124 applied to certain kinds of non-POD types. (Such as a simple
3125 @samp{struct} that fails to be a POD type only by virtue of having a
3126 constructor.) This flag is for users who are aware that they are
3127 writing nonportable code and who have deliberately chosen to ignore the
3130 The restrictions on @samp{offsetof} may be relaxed in a future version
3131 of the C++ standard.
3134 @opindex Winvalid-pch
3135 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
3136 the search path but can't be used.
3140 @opindex Wno-long-long
3141 Warn if @samp{long long} type is used. This is default. To inhibit
3142 the warning messages, use @option{-Wno-long-long}. Flags
3143 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
3144 only when @option{-pedantic} flag is used.
3146 @item -Wvariadic-macros
3147 @opindex Wvariadic-macros
3148 @opindex Wno-variadic-macros
3149 Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
3150 alternate syntax when in pedantic ISO C99 mode. This is default.
3151 To inhibit the warning messages, use @option{-Wno-variadic-macros}.
3153 @item -Wdisabled-optimization
3154 @opindex Wdisabled-optimization
3155 Warn if a requested optimization pass is disabled. This warning does
3156 not generally indicate that there is anything wrong with your code; it
3157 merely indicates that GCC's optimizers were unable to handle the code
3158 effectively. Often, the problem is that your code is too big or too
3159 complex; GCC will refuse to optimize programs when the optimization
3160 itself is likely to take inordinate amounts of time.
3162 @item -Wno-pointer-sign
3163 @opindex Wno-pointer-sign
3164 Don't warn for pointer argument passing or assignment with different signedness.
3165 Only useful in the negative form since this warning is enabled by default.
3166 This option is only supported for C and Objective-C@.
3170 Make all warnings into errors.
3173 @node Debugging Options
3174 @section Options for Debugging Your Program or GCC
3175 @cindex options, debugging
3176 @cindex debugging information options
3178 GCC has various special options that are used for debugging
3179 either your program or GCC:
3184 Produce debugging information in the operating system's native format
3185 (stabs, COFF, XCOFF, or DWARF 2)@. GDB can work with this debugging
3188 On most systems that use stabs format, @option{-g} enables use of extra
3189 debugging information that only GDB can use; this extra information
3190 makes debugging work better in GDB but will probably make other debuggers
3192 refuse to read the program. If you want to control for certain whether
3193 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
3194 @option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
3196 GCC allows you to use @option{-g} with
3197 @option{-O}. The shortcuts taken by optimized code may occasionally
3198 produce surprising results: some variables you declared may not exist
3199 at all; flow of control may briefly move where you did not expect it;
3200 some statements may not be executed because they compute constant
3201 results or their values were already at hand; some statements may
3202 execute in different places because they were moved out of loops.
3204 Nevertheless it proves possible to debug optimized output. This makes
3205 it reasonable to use the optimizer for programs that might have bugs.
3207 The following options are useful when GCC is generated with the
3208 capability for more than one debugging format.
3212 Produce debugging information for use by GDB@. This means to use the
3213 most expressive format available (DWARF 2, stabs, or the native format
3214 if neither of those are supported), including GDB extensions if at all
3219 Produce debugging information in stabs format (if that is supported),
3220 without GDB extensions. This is the format used by DBX on most BSD
3221 systems. On MIPS, Alpha and System V Release 4 systems this option
3222 produces stabs debugging output which is not understood by DBX or SDB@.
3223 On System V Release 4 systems this option requires the GNU assembler.
3225 @item -feliminate-unused-debug-symbols
3226 @opindex feliminate-unused-debug-symbols
3227 Produce debugging information in stabs format (if that is supported),
3228 for only symbols that are actually used.
3232 Produce debugging information in stabs format (if that is supported),
3233 using GNU extensions understood only by the GNU debugger (GDB)@. The
3234 use of these extensions is likely to make other debuggers crash or
3235 refuse to read the program.
3239 Produce debugging information in COFF format (if that is supported).
3240 This is the format used by SDB on most System V systems prior to
3245 Produce debugging information in XCOFF format (if that is supported).
3246 This is the format used by the DBX debugger on IBM RS/6000 systems.
3250 Produce debugging information in XCOFF format (if that is supported),
3251 using GNU extensions understood only by the GNU debugger (GDB)@. The
3252 use of these extensions is likely to make other debuggers crash or
3253 refuse to read the program, and may cause assemblers other than the GNU
3254 assembler (GAS) to fail with an error.
3258 Produce debugging information in DWARF version 2 format (if that is
3259 supported). This is the format used by DBX on IRIX 6. With this
3260 option, GCC uses features of DWARF version 3 when they are useful;
3261 version 3 is upward compatible with version 2, but may still cause
3262 problems for older debuggers.
3266 Produce debugging information in VMS debug format (if that is
3267 supported). This is the format used by DEBUG on VMS systems.
3270 @itemx -ggdb@var{level}
3271 @itemx -gstabs@var{level}
3272 @itemx -gcoff@var{level}
3273 @itemx -gxcoff@var{level}
3274 @itemx -gvms@var{level}
3275 Request debugging information and also use @var{level} to specify how
3276 much information. The default level is 2.
3278 Level 1 produces minimal information, enough for making backtraces in
3279 parts of the program that you don't plan to debug. This includes
3280 descriptions of functions and external variables, but no information
3281 about local variables and no line numbers.
3283 Level 3 includes extra information, such as all the macro definitions
3284 present in the program. Some debuggers support macro expansion when
3285 you use @option{-g3}.
3287 @option{-gdwarf-2} does not accept a concatenated debug level, because
3288 GCC used to support an option @option{-gdwarf} that meant to generate
3289 debug information in version 1 of the DWARF format (which is very
3290 different from version 2), and it would have been too confusing. That
3291 debug format is long obsolete, but the option cannot be changed now.
3292 Instead use an additional @option{-g@var{level}} option to change the
3293 debug level for DWARF2.
3295 @item -feliminate-dwarf2-dups
3296 @opindex feliminate-dwarf2-dups
3297 Compress DWARF2 debugging information by eliminating duplicated
3298 information about each symbol. This option only makes sense when
3299 generating DWARF2 debugging information with @option{-gdwarf-2}.
3301 @cindex @command{prof}
3304 Generate extra code to write profile information suitable for the
3305 analysis program @command{prof}. You must use this option when compiling
3306 the source files you want data about, and you must also use it when
3309 @cindex @command{gprof}
3312 Generate extra code to write profile information suitable for the
3313 analysis program @command{gprof}. You must use this option when compiling
3314 the source files you want data about, and you must also use it when
3319 Makes the compiler print out each function name as it is compiled, and
3320 print some statistics about each pass when it finishes.
3323 @opindex ftime-report
3324 Makes the compiler print some statistics about the time consumed by each
3325 pass when it finishes.
3328 @opindex fmem-report
3329 Makes the compiler print some statistics about permanent memory
3330 allocation when it finishes.
3332 @item -fprofile-arcs
3333 @opindex fprofile-arcs
3334 Add code so that program flow @dfn{arcs} are instrumented. During
3335 execution the program records how many times each branch and call is
3336 executed and how many times it is taken or returns. When the compiled
3337 program exits it saves this data to a file called
3338 @file{@var{auxname}.gcda} for each source file. The data may be used for
3339 profile-directed optimizations (@option{-fbranch-probabilities}), or for
3340 test coverage analysis (@option{-ftest-coverage}). Each object file's
3341 @var{auxname} is generated from the name of the output file, if
3342 explicitly specified and it is not the final executable, otherwise it is
3343 the basename of the source file. In both cases any suffix is removed
3344 (e.g.@: @file{foo.gcda} for input file @file{dir/foo.c}, or
3345 @file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
3350 Compile the source files with @option{-fprofile-arcs} plus optimization
3351 and code generation options. For test coverage analysis, use the
3352 additional @option{-ftest-coverage} option. You do not need to profile
3353 every source file in a program.
3356 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
3357 (the latter implies the former).
3360 Run the program on a representative workload to generate the arc profile
3361 information. This may be repeated any number of times. You can run
3362 concurrent instances of your program, and provided that the file system
3363 supports locking, the data files will be correctly updated. Also
3364 @code{fork} calls are detected and correctly handled (double counting
3368 For profile-directed optimizations, compile the source files again with
3369 the same optimization and code generation options plus
3370 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
3371 Control Optimization}).
3374 For test coverage analysis, use @command{gcov} to produce human readable
3375 information from the @file{.gcno} and @file{.gcda} files. Refer to the
3376 @command{gcov} documentation for further information.
3380 With @option{-fprofile-arcs}, for each function of your program GCC
3381 creates a program flow graph, then finds a spanning tree for the graph.
3382 Only arcs that are not on the spanning tree have to be instrumented: the
3383 compiler adds code to count the number of times that these arcs are
3384 executed. When an arc is the only exit or only entrance to a block, the
3385 instrumentation code can be added to the block; otherwise, a new basic
3386 block must be created to hold the instrumentation code.
3388 @item -ftree-based-profiling
3389 @opindex ftree-based-profiling
3390 This option is used in addition to @option{-fprofile-arcs} or
3391 @option{-fbranch-probabilities} to control whether those optimizations
3392 are performed on a tree-based or rtl-based internal representation.
3393 If you use this option when compiling with @option{-fprofile-arcs},
3394 you must also use it when compiling later with @option{-fbranch-probabilities}.
3395 Currently the tree-based optimization is in an early stage of
3396 development, and this option is recommended only for those people
3397 working on improving it.
3400 @item -ftest-coverage
3401 @opindex ftest-coverage
3402 Produce a notes file that the @command{gcov} code-coverage utility
3403 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
3404 show program coverage. Each source file's note file is called
3405 @file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
3406 above for a description of @var{auxname} and instructions on how to
3407 generate test coverage data. Coverage data will match the source files
3408 more closely, if you do not optimize.
3410 @item -d@var{letters}
3411 @item -fdump-rtl-@var{pass}
3413 Says to make debugging dumps during compilation at times specified by
3414 @var{letters}. This is used for debugging the RTL-based passes of the
3415 compiler. The file names for most of the dumps are made by appending a
3416 pass number and a word to the @var{dumpname}. @var{dumpname} is generated
3417 from the name of the output file, if explicitly specified and it is not
3418 an executable, otherwise it is the basename of the source file.
3420 Most debug dumps can be enabled either passing a letter to the @option{-d}
3421 option, or with a long @option{-fdump-rtl} switch; here are the possible
3422 letters for use in @var{letters} and @var{pass}, and their meanings:
3427 Annotate the assembler output with miscellaneous debugging information.
3430 @itemx -fdump-rtl-bp
3432 @opindex fdump-rtl-bp
3433 Dump after computing branch probabilities, to @file{@var{file}.09.bp}.
3436 @itemx -fdump-rtl-bbro
3438 @opindex fdump-rtl-bbro
3439 Dump after block reordering, to @file{@var{file}.30.bbro}.
3442 @itemx -fdump-rtl-combine
3444 @opindex fdump-rtl-combine
3445 Dump after instruction combination, to the file @file{@var{file}.17.combine}.
3448 @itemx -fdump-rtl-ce1
3449 @itemx -fdump-rtl-ce2
3451 @opindex fdump-rtl-ce1
3452 @opindex fdump-rtl-ce2
3453 @option{-dC} and @option{-fdump-rtl-ce1} enable dumping after the
3454 first if conversion, to the file @file{@var{file}.11.ce1}. @option{-dC}
3455 and @option{-fdump-rtl-ce2} enable dumping after the second if
3456 conversion, to the file @file{@var{file}.18.ce2}.
3459 @itemx -fdump-rtl-btl
3460 @itemx -fdump-rtl-dbr
3462 @opindex fdump-rtl-btl
3463 @opindex fdump-rtl-dbr
3464 @option{-dd} and @option{-fdump-rtl-btl} enable dumping after branch
3465 target load optimization, to to @file{@var{file}.31.btl}. @option{-dd}
3466 and @option{-fdump-rtl-dbr} enable dumping after delayed branch
3467 scheduling, to @file{@var{file}.36.dbr}.
3471 Dump all macro definitions, at the end of preprocessing, in addition to
3475 @itemx -fdump-rtl-ce3
3477 @opindex fdump-rtl-ce3
3478 Dump after the third if conversion, to @file{@var{file}.28.ce3}.
3481 @itemx -fdump-rtl-cfg
3482 @itemx -fdump-rtl-life
3484 @opindex fdump-rtl-cfg
3485 @opindex fdump-rtl-life
3486 @option{-df} and @option{-fdump-rtl-cfg} enable dumping after control
3487 and data flow analysis, to @file{@var{file}.08.cfg}. @option{-df}
3488 and @option{-fdump-rtl-cfg} enable dumping dump after life analysis,
3489 to @file{@var{file}.16.life}.
3492 @itemx -fdump-rtl-greg
3494 @opindex fdump-rtl-greg
3495 Dump after global register allocation, to @file{@var{file}.23.greg}.
3498 @itemx -fdump-rtl-gcse
3499 @itemx -fdump-rtl-bypass
3501 @opindex fdump-rtl-gcse
3502 @opindex fdump-rtl-bypass
3503 @option{-dG} and @option{-fdump-rtl-gcse} enable dumping after GCSE, to
3504 @file{@var{file}.05.gcse}. @option{-dG} and @option{-fdump-rtl-bypass}
3505 enable dumping after jump bypassing and control flow optimizations, to
3506 @file{@var{file}.07.bypass}.
3509 @itemx -fdump-rtl-eh
3511 @opindex fdump-rtl-eh
3512 Dump after finalization of EH handling code, to @file{@var{file}.02.eh}.
3515 @itemx -fdump-rtl-sibling
3517 @opindex fdump-rtl-sibling
3518 Dump after sibling call optimizations, to @file{@var{file}.01.sibling}.
3521 @itemx -fdump-rtl-jump
3523 @opindex fdump-rtl-jump
3524 Dump after the first jump optimization, to @file{@var{file}.03.jump}.
3527 @itemx -fdump-rtl-stack
3529 @opindex fdump-rtl-stack
3530 Dump after conversion from registers to stack, to @file{@var{file}.33.stack}.
3533 @itemx -fdump-rtl-lreg
3535 @opindex fdump-rtl-lreg
3536 Dump after local register allocation, to @file{@var{file}.22.lreg}.
3539 @itemx -fdump-rtl-loop
3540 @itemx -fdump-rtl-loop2
3542 @opindex fdump-rtl-loop
3543 @opindex fdump-rtl-loop2
3544 @option{-dL} and @option{-fdump-rtl-loop} enable dumping after the first
3545 loop optimization pass, to @file{@var{file}.06.loop}. @option{-dL} and
3546 @option{-fdump-rtl-loop2} enable dumping after the second pass, to
3547 @file{@var{file}.13.loop2}.
3550 @itemx -fdump-rtl-sms
3552 @opindex fdump-rtl-sms
3553 Dump after modulo scheduling, to @file{@var{file}.20.sms}.
3556 @itemx -fdump-rtl-mach
3558 @opindex fdump-rtl-mach
3559 Dump after performing the machine dependent reorganization pass, to
3560 @file{@var{file}.35.mach}.
3563 @itemx -fdump-rtl-rnreg
3565 @opindex fdump-rtl-rnreg
3566 Dump after register renumbering, to @file{@var{file}.29.rnreg}.
3569 @itemx -fdump-rtl-regmove
3571 @opindex fdump-rtl-regmove
3572 Dump after the register move pass, to @file{@var{file}.19.regmove}.
3575 @itemx -fdump-rtl-postreload
3577 @opindex fdump-rtl-postreload
3578 Dump after post-reload optimizations, to @file{@var{file}.24.postreload}.
3581 @itemx -fdump-rtl-expand
3583 @opindex fdump-rtl-expand
3584 Dump after RTL generation, to @file{@var{file}.00.expand}.
3587 @itemx -fdump-rtl-sched2
3589 @opindex fdump-rtl-sched2
3590 Dump after the second scheduling pass, to @file{@var{file}.32.sched2}.
3593 @itemx -fdump-rtl-cse
3595 @opindex fdump-rtl-cse
3596 Dump after CSE (including the jump optimization that sometimes follows
3597 CSE), to @file{@var{file}.04.cse}.
3600 @itemx -fdump-rtl-sched
3602 @opindex fdump-rtl-sched
3603 Dump after the first scheduling pass, to @file{@var{file}.21.sched}.
3606 @itemx -fdump-rtl-cse2
3608 @opindex fdump-rtl-cse2
3609 Dump after the second CSE pass (including the jump optimization that
3610 sometimes follows CSE), to @file{@var{file}.15.cse2}.
3613 @itemx -fdump-rtl-tracer
3615 @opindex fdump-rtl-tracer
3616 Dump after running tracer, to @file{@var{file}.12.tracer}.
3619 @itemx -fdump-rtl-vpt
3620 @itemx -fdump-rtl-vartrack
3622 @opindex fdump-rtl-vpt
3623 @opindex fdump-rtl-vartrack
3624 @option{-dV} and @option{-fdump-rtl-vpt} enable dumping after the value
3625 profile transformations, to @file{@var{file}.10.vpt}. @option{-dV}
3626 and @option{-fdump-rtl-vartrack} enable dumping after variable tracking,
3627 to @file{@var{file}.34.vartrack}.
3630 @itemx -fdump-rtl-flow2
3632 @opindex fdump-rtl-flow2
3633 Dump after the second flow pass, to @file{@var{file}.26.flow2}.
3636 @itemx -fdump-rtl-peephole2
3638 @opindex fdump-rtl-peephole2
3639 Dump after the peephole pass, to @file{@var{file}.27.peephole2}.
3642 @itemx -fdump-rtl-web
3644 @opindex fdump-rtl-web
3645 Dump after live range splitting, to @file{@var{file}.14.web}.
3648 @itemx -fdump-rtl-all
3650 @opindex fdump-rtl-all
3651 Produce all the dumps listed above.
3655 Produce a core dump whenever an error occurs.
3659 Print statistics on memory usage, at the end of the run, to
3664 Annotate the assembler output with a comment indicating which
3665 pattern and alternative was used. The length of each instruction is
3670 Dump the RTL in the assembler output as a comment before each instruction.
3671 Also turns on @option{-dp} annotation.
3675 For each of the other indicated dump files (either with @option{-d} or
3676 @option{-fdump-rtl-@var{pass}}), dump a representation of the control flow
3677 graph suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
3681 Just generate RTL for a function instead of compiling it. Usually used
3682 with @samp{r} (@option{-fdump-rtl-expand}).
3686 Dump debugging information during parsing, to standard error.
3689 @item -fdump-unnumbered
3690 @opindex fdump-unnumbered
3691 When doing debugging dumps (see @option{-d} option above), suppress instruction
3692 numbers and line number note output. This makes it more feasible to
3693 use diff on debugging dumps for compiler invocations with different
3694 options, in particular with and without @option{-g}.
3696 @item -fdump-translation-unit @r{(C and C++ only)}
3697 @itemx -fdump-translation-unit-@var{options} @r{(C and C++ only)}
3698 @opindex fdump-translation-unit
3699 Dump a representation of the tree structure for the entire translation
3700 unit to a file. The file name is made by appending @file{.tu} to the
3701 source file name. If the @samp{-@var{options}} form is used, @var{options}
3702 controls the details of the dump as described for the
3703 @option{-fdump-tree} options.
3705 @item -fdump-class-hierarchy @r{(C++ only)}
3706 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
3707 @opindex fdump-class-hierarchy
3708 Dump a representation of each class's hierarchy and virtual function
3709 table layout to a file. The file name is made by appending @file{.class}
3710 to the source file name. If the @samp{-@var{options}} form is used,
3711 @var{options} controls the details of the dump as described for the
3712 @option{-fdump-tree} options.
3714 @item -fdump-ipa-@var{switch}
3716 Control the dumping at various stages of inter-procedural analysis
3717 language tree to a file. The file name is generated by appending a switch
3718 specific suffix to the source file name. The following dumps are possible:
3722 Enables all inter-procedural analysis dumps; currently the only produced
3723 dump is the @samp{cgraph} dump.
3726 Dumps information about call-graph optimization, unused function removal,
3727 and inlining decisions.
3730 @item -fdump-tree-@var{switch} @r{(C and C++ only)}
3731 @itemx -fdump-tree-@var{switch}-@var{options} @r{(C and C++ only)}
3733 Control the dumping at various stages of processing the intermediate
3734 language tree to a file. The file name is generated by appending a switch
3735 specific suffix to the source file name. If the @samp{-@var{options}}
3736 form is used, @var{options} is a list of @samp{-} separated options that
3737 control the details of the dump. Not all options are applicable to all
3738 dumps, those which are not meaningful will be ignored. The following
3739 options are available
3743 Print the address of each node. Usually this is not meaningful as it
3744 changes according to the environment and source file. Its primary use
3745 is for tying up a dump file with a debug environment.
3747 Inhibit dumping of members of a scope or body of a function merely
3748 because that scope has been reached. Only dump such items when they
3749 are directly reachable by some other path. When dumping pretty-printed
3750 trees, this option inhibits dumping the bodies of control structures.
3752 Print a raw representation of the tree. By default, trees are
3753 pretty-printed into a C-like representation.
3755 Enable more detailed dumps (not honored by every dump option).
3757 Enable dumping various statistics about the pass (not honored by every dump
3760 Enable showing basic block boundaries (disabled in raw dumps).
3762 Enable showing virtual operands for every statement.
3764 Enable showing line numbers for statements.
3766 Enable showing the unique ID (@code{DECL_UID}) for each variable.
3768 Turn on all options, except @option{raw}, @option{slim} and @option{lineno}.
3771 The following tree dumps are possible:
3775 Dump before any tree based optimization, to @file{@var{file}.original}.
3778 Dump after all tree based optimization, to @file{@var{file}.optimized}.
3781 Dump after function inlining, to @file{@var{file}.inlined}.
3784 @opindex fdump-tree-gimple
3785 Dump each function before and after the gimplification pass to a file. The
3786 file name is made by appending @file{.gimple} to the source file name.
3789 @opindex fdump-tree-cfg
3790 Dump the control flow graph of each function to a file. The file name is
3791 made by appending @file{.cfg} to the source file name.
3794 @opindex fdump-tree-vcg
3795 Dump the control flow graph of each function to a file in VCG format. The
3796 file name is made by appending @file{.vcg} to the source file name. Note
3797 that if the file contains more than one function, the generated file cannot
3798 be used directly by VCG@. You will need to cut and paste each function's
3799 graph into its own separate file first.
3802 @opindex fdump-tree-ch
3803 Dump each function after copying loop headers. The file name is made by
3804 appending @file{.ch} to the source file name.
3807 @opindex fdump-tree-ssa
3808 Dump SSA related information to a file. The file name is made by appending
3809 @file{.ssa} to the source file name.
3812 @opindex fdump-tree-alias
3813 Dump aliasing information for each function. The file name is made by
3814 appending @file{.alias} to the source file name.
3817 @opindex fdump-tree-ccp
3818 Dump each function after CCP@. The file name is made by appending
3819 @file{.ccp} to the source file name.
3822 @opindex fdump-tree-pre
3823 Dump trees after partial redundancy elimination. The file name is made
3824 by appending @file{.pre} to the source file name.
3827 @opindex fdump-tree-fre
3828 Dump trees after full redundancy elimination. The file name is made
3829 by appending @file{.fre} to the source file name.
3832 @opindex fdump-tree-dce
3833 Dump each function after dead code elimination. The file name is made by
3834 appending @file{.dce} to the source file name.
3837 @opindex fdump-tree-mudflap
3838 Dump each function after adding mudflap instrumentation. The file name is
3839 made by appending @file{.mudflap} to the source file name.
3842 @opindex fdump-tree-sra
3843 Dump each function after performing scalar replacement of aggregates. The
3844 file name is made by appending @file{.sra} to the source file name.
3847 @opindex fdump-tree-dom
3848 Dump each function after applying dominator tree optimizations. The file
3849 name is made by appending @file{.dom} to the source file name.
3852 @opindex fdump-tree-dse
3853 Dump each function after applying dead store elimination. The file
3854 name is made by appending @file{.dse} to the source file name.
3857 @opindex fdump-tree-phiopt
3858 Dump each function after optimizing PHI nodes into straightline code. The file
3859 name is made by appending @file{.phiopt} to the source file name.
3862 @opindex fdump-tree-forwprop
3863 Dump each function after forward propagating single use variables. The file
3864 name is made by appending @file{.forwprop} to the source file name.
3867 @opindex fdump-tree-copyrename
3868 Dump each function after applying the copy rename optimization. The file
3869 name is made by appending @file{.copyrename} to the source file name.
3872 @opindex fdump-tree-nrv
3873 Dump each function after applying the named return value optimization on
3874 generic trees. The file name is made by appending @file{.nrv} to the source
3878 @opindex fdump-tree-vect
3879 Dump each function after applying vectorization of loops. The file name is
3880 made by appending @file{.vect} to the source file name.
3883 @opindex fdump-tree-all
3884 Enable all the available tree dumps with the flags provided in this option.
3887 @item -ftree-vectorizer-verbose=@var{n}
3888 @opindex ftree-vectorizer-verbose
3889 This option controls the amount of debugging output the vectorizer prints.
3890 This information is written to standard error, unless @option{-fdump-tree-all}
3891 or @option{-fdump-tree-vect} is specified, in which case it is output to the
3892 usual dump listing file, @file{.vect}.
3894 @item -frandom-seed=@var{string}
3895 @opindex frandom-string
3896 This option provides a seed that GCC uses when it would otherwise use
3897 random numbers. It is used to generate certain symbol names
3898 that have to be different in every compiled file. It is also used to
3899 place unique stamps in coverage data files and the object files that
3900 produce them. You can use the @option{-frandom-seed} option to produce
3901 reproducibly identical object files.
3903 The @var{string} should be different for every file you compile.
3905 @item -fsched-verbose=@var{n}
3906 @opindex fsched-verbose
3907 On targets that use instruction scheduling, this option controls the
3908 amount of debugging output the scheduler prints. This information is
3909 written to standard error, unless @option{-dS} or @option{-dR} is
3910 specified, in which case it is output to the usual dump
3911 listing file, @file{.sched} or @file{.sched2} respectively. However
3912 for @var{n} greater than nine, the output is always printed to standard
3915 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
3916 same information as @option{-dRS}. For @var{n} greater than one, it
3917 also output basic block probabilities, detailed ready list information
3918 and unit/insn info. For @var{n} greater than two, it includes RTL
3919 at abort point, control-flow and regions info. And for @var{n} over
3920 four, @option{-fsched-verbose} also includes dependence info.
3924 Store the usual ``temporary'' intermediate files permanently; place them
3925 in the current directory and name them based on the source file. Thus,
3926 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
3927 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
3928 preprocessed @file{foo.i} output file even though the compiler now
3929 normally uses an integrated preprocessor.
3931 When used in combination with the @option{-x} command line option,
3932 @option{-save-temps} is sensible enough to avoid over writing an
3933 input source file with the same extension as an intermediate file.
3934 The corresponding intermediate file may be obtained by renaming the
3935 source file before using @option{-save-temps}.
3939 Report the CPU time taken by each subprocess in the compilation
3940 sequence. For C source files, this is the compiler proper and assembler
3941 (plus the linker if linking is done). The output looks like this:
3948 The first number on each line is the ``user time'', that is time spent
3949 executing the program itself. The second number is ``system time'',
3950 time spent executing operating system routines on behalf of the program.
3951 Both numbers are in seconds.
3953 @item -fvar-tracking
3954 @opindex fvar-tracking
3955 Run variable tracking pass. It computes where variables are stored at each
3956 position in code. Better debugging information is then generated
3957 (if the debugging information format supports this information).
3959 It is enabled by default when compiling with optimization (@option{-Os},
3960 @option{-O}, @option{-O2}, ...), debugging information (@option{-g}) and
3961 the debug info format supports it.
3963 @item -print-file-name=@var{library}
3964 @opindex print-file-name
3965 Print the full absolute name of the library file @var{library} that
3966 would be used when linking---and don't do anything else. With this
3967 option, GCC does not compile or link anything; it just prints the
3970 @item -print-multi-directory
3971 @opindex print-multi-directory
3972 Print the directory name corresponding to the multilib selected by any
3973 other switches present in the command line. This directory is supposed
3974 to exist in @env{GCC_EXEC_PREFIX}.
3976 @item -print-multi-lib
3977 @opindex print-multi-lib
3978 Print the mapping from multilib directory names to compiler switches
3979 that enable them. The directory name is separated from the switches by
3980 @samp{;}, and each switch starts with an @samp{@@} instead of the
3981 @samp{-}, without spaces between multiple switches. This is supposed to
3982 ease shell-processing.
3984 @item -print-prog-name=@var{program}
3985 @opindex print-prog-name
3986 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
3988 @item -print-libgcc-file-name
3989 @opindex print-libgcc-file-name
3990 Same as @option{-print-file-name=libgcc.a}.
3992 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
3993 but you do want to link with @file{libgcc.a}. You can do
3996 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
3999 @item -print-search-dirs
4000 @opindex print-search-dirs
4001 Print the name of the configured installation directory and a list of
4002 program and library directories @command{gcc} will search---and don't do anything else.
4004 This is useful when @command{gcc} prints the error message
4005 @samp{installation problem, cannot exec cpp0: No such file or directory}.
4006 To resolve this you either need to put @file{cpp0} and the other compiler
4007 components where @command{gcc} expects to find them, or you can set the environment
4008 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
4009 Don't forget the trailing @samp{/}.
4010 @xref{Environment Variables}.
4013 @opindex dumpmachine
4014 Print the compiler's target machine (for example,
4015 @samp{i686-pc-linux-gnu})---and don't do anything else.
4018 @opindex dumpversion
4019 Print the compiler version (for example, @samp{3.0})---and don't do
4024 Print the compiler's built-in specs---and don't do anything else. (This
4025 is used when GCC itself is being built.) @xref{Spec Files}.
4027 @item -feliminate-unused-debug-types
4028 @opindex feliminate-unused-debug-types
4029 Normally, when producing DWARF2 output, GCC will emit debugging
4030 information for all types declared in a compilation
4031 unit, regardless of whether or not they are actually used
4032 in that compilation unit. Sometimes this is useful, such as
4033 if, in the debugger, you want to cast a value to a type that is
4034 not actually used in your program (but is declared). More often,
4035 however, this results in a significant amount of wasted space.
4036 With this option, GCC will avoid producing debug symbol output
4037 for types that are nowhere used in the source file being compiled.
4040 @node Optimize Options
4041 @section Options That Control Optimization
4042 @cindex optimize options
4043 @cindex options, optimization
4045 These options control various sorts of optimizations.
4047 Without any optimization option, the compiler's goal is to reduce the
4048 cost of compilation and to make debugging produce the expected
4049 results. Statements are independent: if you stop the program with a
4050 breakpoint between statements, you can then assign a new value to any
4051 variable or change the program counter to any other statement in the
4052 function and get exactly the results you would expect from the source
4055 Turning on optimization flags makes the compiler attempt to improve
4056 the performance and/or code size at the expense of compilation time
4057 and possibly the ability to debug the program.
4059 The compiler performs optimization based on the knowledge it has of
4060 the program. Optimization levels @option{-O2} and above, in
4061 particular, enable @emph{unit-at-a-time} mode, which allows the
4062 compiler to consider information gained from later functions in
4063 the file when compiling a function. Compiling multiple files at
4064 once to a single output file in @emph{unit-at-a-time} mode allows
4065 the compiler to use information gained from all of the files when
4066 compiling each of them.
4068 Not all optimizations are controlled directly by a flag. Only
4069 optimizations that have a flag are listed.
4076 Optimize. Optimizing compilation takes somewhat more time, and a lot
4077 more memory for a large function.
4079 With @option{-O}, the compiler tries to reduce code size and execution
4080 time, without performing any optimizations that take a great deal of
4083 @option{-O} turns on the following optimization flags:
4084 @gccoptlist{-fdefer-pop @gol
4085 -fdelayed-branch @gol
4086 -fguess-branch-probability @gol
4087 -fcprop-registers @gol
4088 -floop-optimize @gol
4089 -fif-conversion @gol
4090 -fif-conversion2 @gol
4096 -ftree-live_range_split @gol
4098 -ftree-copyrename @gol
4103 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
4104 where doing so does not interfere with debugging.
4108 Optimize even more. GCC performs nearly all supported optimizations
4109 that do not involve a space-speed tradeoff. The compiler does not
4110 perform loop unrolling or function inlining when you specify @option{-O2}.
4111 As compared to @option{-O}, this option increases both compilation time
4112 and the performance of the generated code.
4114 @option{-O2} turns on all optimization flags specified by @option{-O}. It
4115 also turns on the following optimization flags:
4116 @gccoptlist{-fthread-jumps @gol
4118 -foptimize-sibling-calls @gol
4119 -fcse-follow-jumps -fcse-skip-blocks @gol
4120 -fgcse -fgcse-lm @gol
4121 -fexpensive-optimizations @gol
4122 -fstrength-reduce @gol
4123 -frerun-cse-after-loop -frerun-loop-opt @gol
4127 -fschedule-insns -fschedule-insns2 @gol
4128 -fsched-interblock -fsched-spec @gol
4130 -fstrict-aliasing @gol
4131 -fdelete-null-pointer-checks @gol
4132 -freorder-blocks -freorder-functions @gol
4133 -funit-at-a-time @gol
4134 -falign-functions -falign-jumps @gol
4135 -falign-loops -falign-labels @gol
4138 Please note the warning under @option{-fgcse} about
4139 invoking @option{-O2} on programs that use computed gotos.
4143 Optimize yet more. @option{-O3} turns on all optimizations specified by
4144 @option{-O2} and also turns on the @option{-finline-functions},
4145 @option{-funswitch-loops} and @option{-fgcse-after-reload} options.
4149 Do not optimize. This is the default.
4153 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
4154 do not typically increase code size. It also performs further
4155 optimizations designed to reduce code size.
4157 @option{-Os} disables the following optimization flags:
4158 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
4159 -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays}
4161 If you use multiple @option{-O} options, with or without level numbers,
4162 the last such option is the one that is effective.
4165 Options of the form @option{-f@var{flag}} specify machine-independent
4166 flags. Most flags have both positive and negative forms; the negative
4167 form of @option{-ffoo} would be @option{-fno-foo}. In the table
4168 below, only one of the forms is listed---the one you typically will
4169 use. You can figure out the other form by either removing @samp{no-}
4172 The following options control specific optimizations. They are either
4173 activated by @option{-O} options or are related to ones that are. You
4174 can use the following flags in the rare cases when ``fine-tuning'' of
4175 optimizations to be performed is desired.
4178 @item -fno-default-inline
4179 @opindex fno-default-inline
4180 Do not make member functions inline by default merely because they are
4181 defined inside the class scope (C++ only). Otherwise, when you specify
4182 @w{@option{-O}}, member functions defined inside class scope are compiled
4183 inline by default; i.e., you don't need to add @samp{inline} in front of
4184 the member function name.
4186 @item -fno-defer-pop
4187 @opindex fno-defer-pop
4188 Always pop the arguments to each function call as soon as that function
4189 returns. For machines which must pop arguments after a function call,
4190 the compiler normally lets arguments accumulate on the stack for several
4191 function calls and pops them all at once.
4193 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4197 Force memory operands to be copied into registers before doing
4198 arithmetic on them. This produces better code by making all memory
4199 references potential common subexpressions. When they are not common
4200 subexpressions, instruction combination should eliminate the separate
4203 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4206 @opindex fforce-addr
4207 Force memory address constants to be copied into registers before
4208 doing arithmetic on them. This may produce better code just as
4209 @option{-fforce-mem} may.
4211 @item -fomit-frame-pointer
4212 @opindex fomit-frame-pointer
4213 Don't keep the frame pointer in a register for functions that
4214 don't need one. This avoids the instructions to save, set up and
4215 restore frame pointers; it also makes an extra register available
4216 in many functions. @strong{It also makes debugging impossible on
4219 On some machines, such as the VAX, this flag has no effect, because
4220 the standard calling sequence automatically handles the frame pointer
4221 and nothing is saved by pretending it doesn't exist. The
4222 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
4223 whether a target machine supports this flag. @xref{Registers,,Register
4224 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
4226 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4228 @item -foptimize-sibling-calls
4229 @opindex foptimize-sibling-calls
4230 Optimize sibling and tail recursive calls.
4232 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4236 Don't pay attention to the @code{inline} keyword. Normally this option
4237 is used to keep the compiler from expanding any functions inline.
4238 Note that if you are not optimizing, no functions can be expanded inline.
4240 @item -finline-functions
4241 @opindex finline-functions
4242 Integrate all simple functions into their callers. The compiler
4243 heuristically decides which functions are simple enough to be worth
4244 integrating in this way.
4246 If all calls to a given function are integrated, and the function is
4247 declared @code{static}, then the function is normally not output as
4248 assembler code in its own right.
4250 Enabled at level @option{-O3}.
4252 @item -finline-limit=@var{n}
4253 @opindex finline-limit
4254 By default, GCC limits the size of functions that can be inlined. This flag
4255 allows the control of this limit for functions that are explicitly marked as
4256 inline (i.e., marked with the inline keyword or defined within the class
4257 definition in c++). @var{n} is the size of functions that can be inlined in
4258 number of pseudo instructions (not counting parameter handling). The default
4259 value of @var{n} is 600.
4260 Increasing this value can result in more inlined code at
4261 the cost of compilation time and memory consumption. Decreasing usually makes
4262 the compilation faster and less code will be inlined (which presumably
4263 means slower programs). This option is particularly useful for programs that
4264 use inlining heavily such as those based on recursive templates with C++.
4266 Inlining is actually controlled by a number of parameters, which may be
4267 specified individually by using @option{--param @var{name}=@var{value}}.
4268 The @option{-finline-limit=@var{n}} option sets some of these parameters
4272 @item max-inline-insns-single
4273 is set to @var{n}/2.
4274 @item max-inline-insns-auto
4275 is set to @var{n}/2.
4276 @item min-inline-insns
4277 is set to 130 or @var{n}/4, whichever is smaller.
4278 @item max-inline-insns-rtl
4282 See below for a documentation of the individual
4283 parameters controlling inlining.
4285 @emph{Note:} pseudo instruction represents, in this particular context, an
4286 abstract measurement of function's size. In no way, it represents a count
4287 of assembly instructions and as such its exact meaning might change from one
4288 release to an another.
4290 @item -fkeep-inline-functions
4291 @opindex fkeep-inline-functions
4292 In C, emit @code{static} functions that are declared @code{inline}
4293 into the object file, even if the function has been inlined into all
4294 of its callers. This switch does not affect functions using the
4295 @code{extern inline} extension in GNU C@. In C++, emit any and all
4296 inline functions into the object file.
4298 @item -fkeep-static-consts
4299 @opindex fkeep-static-consts
4300 Emit variables declared @code{static const} when optimization isn't turned
4301 on, even if the variables aren't referenced.
4303 GCC enables this option by default. If you want to force the compiler to
4304 check if the variable was referenced, regardless of whether or not
4305 optimization is turned on, use the @option{-fno-keep-static-consts} option.
4307 @item -fmerge-constants
4308 Attempt to merge identical constants (string constants and floating point
4309 constants) across compilation units.
4311 This option is the default for optimized compilation if the assembler and
4312 linker support it. Use @option{-fno-merge-constants} to inhibit this
4315 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4317 @item -fmerge-all-constants
4318 Attempt to merge identical constants and identical variables.
4320 This option implies @option{-fmerge-constants}. In addition to
4321 @option{-fmerge-constants} this considers e.g.@: even constant initialized
4322 arrays or initialized constant variables with integral or floating point
4323 types. Languages like C or C++ require each non-automatic variable to
4324 have distinct location, so using this option will result in non-conforming
4327 @item -fmodulo-sched
4328 @opindex fmodulo-sched
4329 Perform swing modulo scheduling immediately before the first scheduling
4330 pass. This pass looks at innermost loops and reorders their
4331 instructions by overlapping different iterations.
4333 @item -fno-branch-count-reg
4334 @opindex fno-branch-count-reg
4335 Do not use ``decrement and branch'' instructions on a count register,
4336 but instead generate a sequence of instructions that decrement a
4337 register, compare it against zero, then branch based upon the result.
4338 This option is only meaningful on architectures that support such
4339 instructions, which include x86, PowerPC, IA-64 and S/390.
4341 The default is @option{-fbranch-count-reg}, enabled when
4342 @option{-fstrength-reduce} is enabled.
4344 @item -fno-function-cse
4345 @opindex fno-function-cse
4346 Do not put function addresses in registers; make each instruction that
4347 calls a constant function contain the function's address explicitly.
4349 This option results in less efficient code, but some strange hacks
4350 that alter the assembler output may be confused by the optimizations
4351 performed when this option is not used.
4353 The default is @option{-ffunction-cse}
4355 @item -fno-zero-initialized-in-bss
4356 @opindex fno-zero-initialized-in-bss
4357 If the target supports a BSS section, GCC by default puts variables that
4358 are initialized to zero into BSS@. This can save space in the resulting
4361 This option turns off this behavior because some programs explicitly
4362 rely on variables going to the data section. E.g., so that the
4363 resulting executable can find the beginning of that section and/or make
4364 assumptions based on that.
4366 The default is @option{-fzero-initialized-in-bss}.
4368 @item -fbounds-check
4369 @opindex fbounds-check
4370 For front-ends that support it, generate additional code to check that
4371 indices used to access arrays are within the declared range. This is
4372 currently only supported by the Java and Fortran front-ends, where
4373 this option defaults to true and false respectively.
4375 @item -fmudflap -fmudflapth -fmudflapir
4379 @cindex bounds checking
4381 For front-ends that support it (C and C++), instrument all risky
4382 pointer/array dereferencing operations, some standard library
4383 string/heap functions, and some other associated constructs with
4384 range/validity tests. Modules so instrumented should be immune to
4385 buffer overflows, invalid heap use, and some other classes of C/C++
4386 programming errors. The instrumentation relies on a separate runtime
4387 library (@file{libmudflap}), which will be linked into a program if
4388 @option{-fmudflap} is given at link time. Run-time behavior of the
4389 instrumented program is controlled by the @env{MUDFLAP_OPTIONS}
4390 environment variable. See @code{env MUDFLAP_OPTIONS=-help a.out}
4393 Use @option{-fmudflapth} instead of @option{-fmudflap} to compile and to
4394 link if your program is multi-threaded. Use @option{-fmudflapir}, in
4395 addition to @option{-fmudflap} or @option{-fmudflapth}, if
4396 instrumentation should ignore pointer reads. This produces less
4397 instrumentation (and therefore faster execution) and still provides
4398 some protection against outright memory corrupting writes, but allows
4399 erroneously read data to propagate within a program.
4401 @item -fstrength-reduce
4402 @opindex fstrength-reduce
4403 Perform the optimizations of loop strength reduction and
4404 elimination of iteration variables.
4406 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4408 @item -fthread-jumps
4409 @opindex fthread-jumps
4410 Perform optimizations where we check to see if a jump branches to a
4411 location where another comparison subsumed by the first is found. If
4412 so, the first branch is redirected to either the destination of the
4413 second branch or a point immediately following it, depending on whether
4414 the condition is known to be true or false.
4416 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4418 @item -fcse-follow-jumps
4419 @opindex fcse-follow-jumps
4420 In common subexpression elimination, scan through jump instructions
4421 when the target of the jump is not reached by any other path. For
4422 example, when CSE encounters an @code{if} statement with an
4423 @code{else} clause, CSE will follow the jump when the condition
4426 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4428 @item -fcse-skip-blocks
4429 @opindex fcse-skip-blocks
4430 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
4431 follow jumps which conditionally skip over blocks. When CSE
4432 encounters a simple @code{if} statement with no else clause,
4433 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
4434 body of the @code{if}.
4436 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4438 @item -frerun-cse-after-loop
4439 @opindex frerun-cse-after-loop
4440 Re-run common subexpression elimination after loop optimizations has been
4443 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4445 @item -frerun-loop-opt
4446 @opindex frerun-loop-opt
4447 Run the loop optimizer twice.
4449 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4453 Perform a global common subexpression elimination pass.
4454 This pass also performs global constant and copy propagation.
4456 @emph{Note:} When compiling a program using computed gotos, a GCC
4457 extension, you may get better runtime performance if you disable
4458 the global common subexpression elimination pass by adding
4459 @option{-fno-gcse} to the command line.
4461 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4465 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
4466 attempt to move loads which are only killed by stores into themselves. This
4467 allows a loop containing a load/store sequence to be changed to a load outside
4468 the loop, and a copy/store within the loop.
4470 Enabled by default when gcse is enabled.
4474 When @option{-fgcse-sm} is enabled, a store motion pass is run after
4475 global common subexpression elimination. This pass will attempt to move
4476 stores out of loops. When used in conjunction with @option{-fgcse-lm},
4477 loops containing a load/store sequence can be changed to a load before
4478 the loop and a store after the loop.
4480 Not enabled at any optimization level.
4484 When @option{-fgcse-las} is enabled, the global common subexpression
4485 elimination pass eliminates redundant loads that come after stores to the
4486 same memory location (both partial and full redundancies).
4488 Not enabled at any optimization level.
4490 @item -fgcse-after-reload
4491 @opindex fgcse-after-reload
4492 When @option{-fgcse-after-reload} is enabled, a redundant load elimination
4493 pass is performed after reload. The purpose of this pass is to cleanup
4496 @item -floop-optimize
4497 @opindex floop-optimize
4498 Perform loop optimizations: move constant expressions out of loops, simplify
4499 exit test conditions and optionally do strength-reduction as well.
4501 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4503 @item -floop-optimize2
4504 @opindex floop-optimize2
4505 Perform loop optimizations using the new loop optimizer. The optimizations
4506 (loop unrolling, peeling and unswitching, loop invariant motion) are enabled
4509 @item -fcrossjumping
4510 @opindex crossjumping
4511 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
4512 resulting code may or may not perform better than without cross-jumping.
4514 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4516 @item -fif-conversion
4517 @opindex if-conversion
4518 Attempt to transform conditional jumps into branch-less equivalents. This
4519 include use of conditional moves, min, max, set flags and abs instructions, and
4520 some tricks doable by standard arithmetics. The use of conditional execution
4521 on chips where it is available is controlled by @code{if-conversion2}.
4523 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4525 @item -fif-conversion2
4526 @opindex if-conversion2
4527 Use conditional execution (where available) to transform conditional jumps into
4528 branch-less equivalents.
4530 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4532 @item -fdelete-null-pointer-checks
4533 @opindex fdelete-null-pointer-checks
4534 Use global dataflow analysis to identify and eliminate useless checks
4535 for null pointers. The compiler assumes that dereferencing a null
4536 pointer would have halted the program. If a pointer is checked after
4537 it has already been dereferenced, it cannot be null.
4539 In some environments, this assumption is not true, and programs can
4540 safely dereference null pointers. Use
4541 @option{-fno-delete-null-pointer-checks} to disable this optimization
4542 for programs which depend on that behavior.
4544 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4546 @item -fexpensive-optimizations
4547 @opindex fexpensive-optimizations
4548 Perform a number of minor optimizations that are relatively expensive.
4550 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4552 @item -foptimize-register-move
4554 @opindex foptimize-register-move
4556 Attempt to reassign register numbers in move instructions and as
4557 operands of other simple instructions in order to maximize the amount of
4558 register tying. This is especially helpful on machines with two-operand
4561 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
4564 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4566 @item -fdelayed-branch
4567 @opindex fdelayed-branch
4568 If supported for the target machine, attempt to reorder instructions
4569 to exploit instruction slots available after delayed branch
4572 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4574 @item -fschedule-insns
4575 @opindex fschedule-insns
4576 If supported for the target machine, attempt to reorder instructions to
4577 eliminate execution stalls due to required data being unavailable. This
4578 helps machines that have slow floating point or memory load instructions
4579 by allowing other instructions to be issued until the result of the load
4580 or floating point instruction is required.
4582 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4584 @item -fschedule-insns2
4585 @opindex fschedule-insns2
4586 Similar to @option{-fschedule-insns}, but requests an additional pass of
4587 instruction scheduling after register allocation has been done. This is
4588 especially useful on machines with a relatively small number of
4589 registers and where memory load instructions take more than one cycle.
4591 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4593 @item -fno-sched-interblock
4594 @opindex fno-sched-interblock
4595 Don't schedule instructions across basic blocks. This is normally
4596 enabled by default when scheduling before register allocation, i.e.@:
4597 with @option{-fschedule-insns} or at @option{-O2} or higher.
4599 @item -fno-sched-spec
4600 @opindex fno-sched-spec
4601 Don't allow speculative motion of non-load instructions. This is normally
4602 enabled by default when scheduling before register allocation, i.e.@:
4603 with @option{-fschedule-insns} or at @option{-O2} or higher.
4605 @item -fsched-spec-load
4606 @opindex fsched-spec-load
4607 Allow speculative motion of some load instructions. This only makes
4608 sense when scheduling before register allocation, i.e.@: with
4609 @option{-fschedule-insns} or at @option{-O2} or higher.
4611 @item -fsched-spec-load-dangerous
4612 @opindex fsched-spec-load-dangerous
4613 Allow speculative motion of more load instructions. This only makes
4614 sense when scheduling before register allocation, i.e.@: with
4615 @option{-fschedule-insns} or at @option{-O2} or higher.
4617 @item -fsched-stalled-insns=@var{n}
4618 @opindex fsched-stalled-insns
4619 Define how many insns (if any) can be moved prematurely from the queue
4620 of stalled insns into the ready list, during the second scheduling pass.
4622 @item -fsched-stalled-insns-dep=@var{n}
4623 @opindex fsched-stalled-insns-dep
4624 Define how many insn groups (cycles) will be examined for a dependency
4625 on a stalled insn that is candidate for premature removal from the queue
4626 of stalled insns. Has an effect only during the second scheduling pass,
4627 and only if @option{-fsched-stalled-insns} is used and its value is not zero.
4629 @item -fsched2-use-superblocks
4630 @opindex fsched2-use-superblocks
4631 When scheduling after register allocation, do use superblock scheduling
4632 algorithm. Superblock scheduling allows motion across basic block boundaries
4633 resulting on faster schedules. This option is experimental, as not all machine
4634 descriptions used by GCC model the CPU closely enough to avoid unreliable
4635 results from the algorithm.
4637 This only makes sense when scheduling after register allocation, i.e.@: with
4638 @option{-fschedule-insns2} or at @option{-O2} or higher.
4640 @item -fsched2-use-traces
4641 @opindex fsched2-use-traces
4642 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
4643 allocation and additionally perform code duplication in order to increase the
4644 size of superblocks using tracer pass. See @option{-ftracer} for details on
4647 This mode should produce faster but significantly longer programs. Also
4648 without @option{-fbranch-probabilities} the traces constructed may not
4649 match the reality and hurt the performance. This only makes
4650 sense when scheduling after register allocation, i.e.@: with
4651 @option{-fschedule-insns2} or at @option{-O2} or higher.
4653 @item -freschedule-modulo-scheduled-loops
4654 @opindex fscheduling-in-modulo-scheduled-loops
4655 The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled
4656 we may want to prevent the later scheduling passes from changing its schedule, we use this
4657 option to control that.
4659 @item -fcaller-saves
4660 @opindex fcaller-saves
4661 Enable values to be allocated in registers that will be clobbered by
4662 function calls, by emitting extra instructions to save and restore the
4663 registers around such calls. Such allocation is done only when it
4664 seems to result in better code than would otherwise be produced.
4666 This option is always enabled by default on certain machines, usually
4667 those which have no call-preserved registers to use instead.
4669 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4672 Perform Partial Redundancy Elimination (PRE) on trees. This flag is
4673 enabled by default at @option{-O2} and @option{-O3}.
4676 Perform Full Redundancy Elimination (FRE) on trees. The difference
4677 between FRE and PRE is that FRE only considers expressions
4678 that are computed on all paths leading to the redundant computation.
4679 This analysis faster than PRE, though it exposes fewer redundancies.
4680 This flag is enabled by default at @option{-O} and higher.
4683 Perform sparse conditional constant propagation (CCP) on trees. This flag
4684 is enabled by default at @option{-O} and higher.
4687 Perform dead code elimination (DCE) on trees. This flag is enabled by
4688 default at @option{-O} and higher.
4690 @item -ftree-dominator-opts
4691 Perform dead code elimination (DCE) on trees. This flag is enabled by
4692 default at @option{-O} and higher.
4695 Perform loop header copying on trees. This is beneficial since it increases
4696 effectiveness of code motion optimizations. It also saves one jump. This flag
4697 is enabled by default at @option{-O} and higher. It is not enabled
4698 for @option{-Os}, since it usually increases code size.
4700 @item -ftree-loop-optimize
4701 Perform loop optimizations on trees. This flag is enabled by default
4702 at @option{-O} and higher.
4704 @item -ftree-loop-linear
4705 Perform linear loop transformations on tree. This flag can improve cache
4706 performance and allow further loop optimizations to take place.
4709 Perform loop invariant motion on trees. This pass moves only invartiants that
4710 would be hard to handle on rtl level (function calls, operations that expand to
4711 nontrivial sequences of insns). With @option{-funswitch-loops} it also moves
4712 operands of conditions that are invariant out of the loop, so that we can use
4713 just trivial invariantness analysis in loop unswitching. The pass also includes
4717 Create a canonical counter for number of iterations in the loop for that
4718 determining number of iterations requires complicated analysis. Later
4719 optimizations then may determine the number easily. Useful especially
4720 in connection with unrolling.
4723 Perform induction variable optimizations (strength reduction, induction
4724 variable merging and induction variable elimination) on trees.
4727 Perform scalar replacement of aggregates. This pass replaces structure
4728 references with scalars to prevent committing structures to memory too
4729 early. This flag is enabled by default at @option{-O} and higher.
4731 @item -ftree-copyrename
4732 Perform copy renaming on trees. This pass attempts to rename compiler
4733 temporaries to other variables at copy locations, usually resulting in
4734 variable names which more closely resemble the original variables. This flag
4735 is enabled by default at @option{-O} and higher.
4738 Perform temporary expression replacement during the SSA->normal phase. Single
4739 use/single def temporaries are replaced at their use location with their
4740 defining expression. This results in non-GIMPLE code, but gives the expanders
4741 much more complex trees to work on resulting in better RTL generation. This is
4742 enabled by default at @option{-O} and higher.
4745 Perform live range splitting during the SSA->normal phase. Distinct live
4746 ranges of a variable are split into unique variables, allowing for better
4747 optimization later. This is enabled by default at @option{-O} and higher.
4749 @item -ftree-vectorize
4750 Perform loop vectorization on trees.
4754 Perform tail duplication to enlarge superblock size. This transformation
4755 simplifies the control flow of the function allowing other optimizations to do
4758 @item -funroll-loops
4759 @opindex funroll-loops
4760 Unroll loops whose number of iterations can be determined at compile
4761 time or upon entry to the loop. @option{-funroll-loops} implies both
4762 @option{-fstrength-reduce} and @option{-frerun-cse-after-loop}. This
4763 option makes code larger, and may or may not make it run faster.
4765 @item -funroll-all-loops
4766 @opindex funroll-all-loops
4767 Unroll all loops, even if their number of iterations is uncertain when
4768 the loop is entered. This usually makes programs run more slowly.
4769 @option{-funroll-all-loops} implies the same options as
4770 @option{-funroll-loops},
4772 @item -fsplit-ivs-in-unroller
4773 @opindex -fsplit-ivs-in-unroller
4774 Enables expressing of values of induction variables in later iterations
4775 of the unrolled loop using the value in the first iteration. This breaks
4776 long dependency chains, thus improving efficiency of the scheduling passes
4777 (for best results, @option{-fweb} should be used as well).
4779 Combination of @option{-fweb} and CSE is often sufficient to obtain the
4780 same effect. However in cases the loop body is more complicated than
4781 a single basic block, this is not reliable. It also does not work at all
4782 on some of the architectures due to restrictions in the CSE pass.
4784 This optimization is enabled by default.
4786 @item -fvariable-expansion-in-unroller
4787 @opindex -fvariable-expansion-in-unroller
4788 With this option, the compiler will create multiple copies of some
4789 local variables when unrolling a loop which can result in superior code.
4791 @item -fprefetch-loop-arrays
4792 @opindex fprefetch-loop-arrays
4793 If supported by the target machine, generate instructions to prefetch
4794 memory to improve the performance of loops that access large arrays.
4796 These options may generate better or worse code; results are highly
4797 dependent on the structure of loops within the source code.
4800 @itemx -fno-peephole2
4801 @opindex fno-peephole
4802 @opindex fno-peephole2
4803 Disable any machine-specific peephole optimizations. The difference
4804 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
4805 are implemented in the compiler; some targets use one, some use the
4806 other, a few use both.
4808 @option{-fpeephole} is enabled by default.
4809 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4811 @item -fno-guess-branch-probability
4812 @opindex fno-guess-branch-probability
4813 Do not guess branch probabilities using heuristics.
4815 GCC will use heuristics to guess branch probabilities if they are
4816 not provided by profiling feedback (@option{-fprofile-arcs}). These
4817 heuristics are based on the control flow graph. If some branch probabilities
4818 are specified by @samp{__builtin_expect}, then the heuristics will be
4819 used to guess branch probabilities for the rest of the control flow graph,
4820 taking the @samp{__builtin_expect} info into account. The interactions
4821 between the heuristics and @samp{__builtin_expect} can be complex, and in
4822 some cases, it may be useful to disable the heuristics so that the effects
4823 of @samp{__builtin_expect} are easier to understand.
4825 The default is @option{-fguess-branch-probability} at levels
4826 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4828 @item -freorder-blocks
4829 @opindex freorder-blocks
4830 Reorder basic blocks in the compiled function in order to reduce number of
4831 taken branches and improve code locality.
4833 Enabled at levels @option{-O2}, @option{-O3}.
4835 @item -freorder-blocks-and-partition
4836 @opindex freorder-blocks-and-partition
4837 In addition to reordering basic blocks in the compiled function, in order
4838 to reduce number of taken branches, partitions hot and cold basic blocks
4839 into separate sections of the assembly and .o files, to improve
4840 paging and cache locality performance.
4842 This optimization is automatically turned off in the presence of
4843 exception handling, for linkonce sections, for functions with a user-defined
4844 section attribute and on any architecture that does not support named
4847 @item -freorder-functions
4848 @opindex freorder-functions
4849 Reorder functions in the object file in order to
4850 improve code locality. This is implemented by using special
4851 subsections @code{.text.hot} for most frequently executed functions and
4852 @code{.text.unlikely} for unlikely executed functions. Reordering is done by
4853 the linker so object file format must support named sections and linker must
4854 place them in a reasonable way.
4856 Also profile feedback must be available in to make this option effective. See
4857 @option{-fprofile-arcs} for details.
4859 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4861 @item -fstrict-aliasing
4862 @opindex fstrict-aliasing
4863 Allows the compiler to assume the strictest aliasing rules applicable to
4864 the language being compiled. For C (and C++), this activates
4865 optimizations based on the type of expressions. In particular, an
4866 object of one type is assumed never to reside at the same address as an
4867 object of a different type, unless the types are almost the same. For
4868 example, an @code{unsigned int} can alias an @code{int}, but not a
4869 @code{void*} or a @code{double}. A character type may alias any other
4872 Pay special attention to code like this:
4885 The practice of reading from a different union member than the one most
4886 recently written to (called ``type-punning'') is common. Even with
4887 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
4888 is accessed through the union type. So, the code above will work as
4889 expected. However, this code might not:
4900 Every language that wishes to perform language-specific alias analysis
4901 should define a function that computes, given an @code{tree}
4902 node, an alias set for the node. Nodes in different alias sets are not
4903 allowed to alias. For an example, see the C front-end function
4904 @code{c_get_alias_set}.
4906 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4908 @item -falign-functions
4909 @itemx -falign-functions=@var{n}
4910 @opindex falign-functions
4911 Align the start of functions to the next power-of-two greater than
4912 @var{n}, skipping up to @var{n} bytes. For instance,
4913 @option{-falign-functions=32} aligns functions to the next 32-byte
4914 boundary, but @option{-falign-functions=24} would align to the next
4915 32-byte boundary only if this can be done by skipping 23 bytes or less.
4917 @option{-fno-align-functions} and @option{-falign-functions=1} are
4918 equivalent and mean that functions will not be aligned.
4920 Some assemblers only support this flag when @var{n} is a power of two;
4921 in that case, it is rounded up.
4923 If @var{n} is not specified or is zero, use a machine-dependent default.
4925 Enabled at levels @option{-O2}, @option{-O3}.
4927 @item -falign-labels
4928 @itemx -falign-labels=@var{n}
4929 @opindex falign-labels
4930 Align all branch targets to a power-of-two boundary, skipping up to
4931 @var{n} bytes like @option{-falign-functions}. This option can easily
4932 make code slower, because it must insert dummy operations for when the
4933 branch target is reached in the usual flow of the code.
4935 @option{-fno-align-labels} and @option{-falign-labels=1} are
4936 equivalent and mean that labels will not be aligned.
4938 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
4939 are greater than this value, then their values are used instead.
4941 If @var{n} is not specified or is zero, use a machine-dependent default
4942 which is very likely to be @samp{1}, meaning no alignment.
4944 Enabled at levels @option{-O2}, @option{-O3}.
4947 @itemx -falign-loops=@var{n}
4948 @opindex falign-loops
4949 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
4950 like @option{-falign-functions}. The hope is that the loop will be
4951 executed many times, which will make up for any execution of the dummy
4954 @option{-fno-align-loops} and @option{-falign-loops=1} are
4955 equivalent and mean that loops will not be aligned.
4957 If @var{n} is not specified or is zero, use a machine-dependent default.
4959 Enabled at levels @option{-O2}, @option{-O3}.
4962 @itemx -falign-jumps=@var{n}
4963 @opindex falign-jumps
4964 Align branch targets to a power-of-two boundary, for branch targets
4965 where the targets can only be reached by jumping, skipping up to @var{n}
4966 bytes like @option{-falign-functions}. In this case, no dummy operations
4969 @option{-fno-align-jumps} and @option{-falign-jumps=1} are
4970 equivalent and mean that loops will not be aligned.
4972 If @var{n} is not specified or is zero, use a machine-dependent default.
4974 Enabled at levels @option{-O2}, @option{-O3}.
4976 @item -funit-at-a-time
4977 @opindex funit-at-a-time
4978 Parse the whole compilation unit before starting to produce code.
4979 This allows some extra optimizations to take place but consumes
4980 more memory (in general). There are some compatibility issues
4981 with @emph{unit-at-at-time} mode:
4984 enabling @emph{unit-at-a-time} mode may change the order
4985 in which functions, variables, and top-level @code{asm} statements
4986 are emitted, and will likely break code relying on some particular
4987 ordering. The majority of such top-level @code{asm} statements,
4988 though, can be replaced by @code{section} attributes.
4991 @emph{unit-at-a-time} mode removes unreferenced static variables
4992 and functions are removed. This may result in undefined references
4993 when an @code{asm} statement refers directly to variables or functions
4994 that are otherwise unused. In that case either the variable/function
4995 shall be listed as an operand of the @code{asm} statement operand or,
4996 in the case of top-level @code{asm} statements the attribute @code{used}
4997 shall be used on the declaration.
5000 Static functions now can use non-standard passing conventions that
5001 may break @code{asm} statements calling functions directly. Again,
5002 attribute @code{used} will prevent this behavior.
5005 As a temporary workaround, @option{-fno-unit-at-a-time} can be used,
5006 but this scheme may not be supported by future releases of GCC@.
5008 Enabled at levels @option{-O2}, @option{-O3}.
5012 Constructs webs as commonly used for register allocation purposes and assign
5013 each web individual pseudo register. This allows the register allocation pass
5014 to operate on pseudos directly, but also strengthens several other optimization
5015 passes, such as CSE, loop optimizer and trivial dead code remover. It can,
5016 however, make debugging impossible, since variables will no longer stay in a
5019 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os},
5020 on targets where the default format for debugging information supports
5023 @item -fno-cprop-registers
5024 @opindex fno-cprop-registers
5025 After register allocation and post-register allocation instruction splitting,
5026 we perform a copy-propagation pass to try to reduce scheduling dependencies
5027 and occasionally eliminate the copy.
5029 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5031 @item -fprofile-generate
5032 @opindex fprofile-generate
5034 Enable options usually used for instrumenting application to produce
5035 profile useful for later recompilation with profile feedback based
5036 optimization. You must use @option{-fprofile-generate} both when
5037 compiling and when linking your program.
5039 The following options are enabled: @code{-fprofile-arcs}, @code{-fprofile-values}, @code{-fvpt}.
5042 @opindex fprofile-use
5043 Enable profile feedback directed optimizations, and optimizations
5044 generally profitable only with profile feedback available.
5046 The following options are enabled: @code{-fbranch-probabilities},
5047 @code{-fvpt}, @code{-funroll-loops}, @code{-fpeel-loops}, @code{-ftracer}.
5051 The following options control compiler behavior regarding floating
5052 point arithmetic. These options trade off between speed and
5053 correctness. All must be specifically enabled.
5057 @opindex ffloat-store
5058 Do not store floating point variables in registers, and inhibit other
5059 options that might change whether a floating point value is taken from a
5062 @cindex floating point precision
5063 This option prevents undesirable excess precision on machines such as
5064 the 68000 where the floating registers (of the 68881) keep more
5065 precision than a @code{double} is supposed to have. Similarly for the
5066 x86 architecture. For most programs, the excess precision does only
5067 good, but a few programs rely on the precise definition of IEEE floating
5068 point. Use @option{-ffloat-store} for such programs, after modifying
5069 them to store all pertinent intermediate computations into variables.
5073 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations}, @*
5074 @option{-fno-trapping-math}, @option{-ffinite-math-only},
5075 @option{-fno-rounding-math} and @option{-fno-signaling-nans}.
5077 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
5079 This option should never be turned on by any @option{-O} option since
5080 it can result in incorrect output for programs which depend on
5081 an exact implementation of IEEE or ISO rules/specifications for
5084 @item -fno-math-errno
5085 @opindex fno-math-errno
5086 Do not set ERRNO after calling math functions that are executed
5087 with a single instruction, e.g., sqrt. A program that relies on
5088 IEEE exceptions for math error handling may want to use this flag
5089 for speed while maintaining IEEE arithmetic compatibility.
5091 This option should never be turned on by any @option{-O} option since
5092 it can result in incorrect output for programs which depend on
5093 an exact implementation of IEEE or ISO rules/specifications for
5096 The default is @option{-fmath-errno}.
5098 @item -funsafe-math-optimizations
5099 @opindex funsafe-math-optimizations
5100 Allow optimizations for floating-point arithmetic that (a) assume
5101 that arguments and results are valid and (b) may violate IEEE or
5102 ANSI standards. When used at link-time, it may include libraries
5103 or startup files that change the default FPU control word or other
5104 similar optimizations.
5106 This option should never be turned on by any @option{-O} option since
5107 it can result in incorrect output for programs which depend on
5108 an exact implementation of IEEE or ISO rules/specifications for
5111 The default is @option{-fno-unsafe-math-optimizations}.
5113 @item -ffinite-math-only
5114 @opindex ffinite-math-only
5115 Allow optimizations for floating-point arithmetic that assume
5116 that arguments and results are not NaNs or +-Infs.
5118 This option should never be turned on by any @option{-O} option since
5119 it can result in incorrect output for programs which depend on
5120 an exact implementation of IEEE or ISO rules/specifications.
5122 The default is @option{-fno-finite-math-only}.
5124 @item -fno-trapping-math
5125 @opindex fno-trapping-math
5126 Compile code assuming that floating-point operations cannot generate
5127 user-visible traps. These traps include division by zero, overflow,
5128 underflow, inexact result and invalid operation. This option implies
5129 @option{-fno-signaling-nans}. Setting this option may allow faster
5130 code if one relies on ``non-stop'' IEEE arithmetic, for example.
5132 This option should never be turned on by any @option{-O} option since
5133 it can result in incorrect output for programs which depend on
5134 an exact implementation of IEEE or ISO rules/specifications for
5137 The default is @option{-ftrapping-math}.
5139 @item -frounding-math
5140 @opindex frounding-math
5141 Disable transformations and optimizations that assume default floating
5142 point rounding behavior. This is round-to-zero for all floating point
5143 to integer conversions, and round-to-nearest for all other arithmetic
5144 truncations. This option should be specified for programs that change
5145 the FP rounding mode dynamically, or that may be executed with a
5146 non-default rounding mode. This option disables constant folding of
5147 floating point expressions at compile-time (which may be affected by
5148 rounding mode) and arithmetic transformations that are unsafe in the
5149 presence of sign-dependent rounding modes.
5151 The default is @option{-fno-rounding-math}.
5153 This option is experimental and does not currently guarantee to
5154 disable all GCC optimizations that are affected by rounding mode.
5155 Future versions of GCC may provide finer control of this setting
5156 using C99's @code{FENV_ACCESS} pragma. This command line option
5157 will be used to specify the default state for @code{FENV_ACCESS}.
5159 @item -fsignaling-nans
5160 @opindex fsignaling-nans
5161 Compile code assuming that IEEE signaling NaNs may generate user-visible
5162 traps during floating-point operations. Setting this option disables
5163 optimizations that may change the number of exceptions visible with
5164 signaling NaNs. This option implies @option{-ftrapping-math}.
5166 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
5169 The default is @option{-fno-signaling-nans}.
5171 This option is experimental and does not currently guarantee to
5172 disable all GCC optimizations that affect signaling NaN behavior.
5174 @item -fsingle-precision-constant
5175 @opindex fsingle-precision-constant
5176 Treat floating point constant as single precision constant instead of
5177 implicitly converting it to double precision constant.
5182 The following options control optimizations that may improve
5183 performance, but are not enabled by any @option{-O} options. This
5184 section includes experimental options that may produce broken code.
5187 @item -fbranch-probabilities
5188 @opindex fbranch-probabilities
5189 After running a program compiled with @option{-fprofile-arcs}
5190 (@pxref{Debugging Options,, Options for Debugging Your Program or
5191 @command{gcc}}), you can compile it a second time using
5192 @option{-fbranch-probabilities}, to improve optimizations based on
5193 the number of times each branch was taken. When the program
5194 compiled with @option{-fprofile-arcs} exits it saves arc execution
5195 counts to a file called @file{@var{sourcename}.gcda} for each source
5196 file The information in this data file is very dependent on the
5197 structure of the generated code, so you must use the same source code
5198 and the same optimization options for both compilations.
5200 With @option{-fbranch-probabilities}, GCC puts a
5201 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
5202 These can be used to improve optimization. Currently, they are only
5203 used in one place: in @file{reorg.c}, instead of guessing which path a
5204 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
5205 exactly determine which path is taken more often.
5207 @item -fprofile-values
5208 @opindex fprofile-values
5209 If combined with @option{-fprofile-arcs}, it adds code so that some
5210 data about values of expressions in the program is gathered.
5212 With @option{-fbranch-probabilities}, it reads back the data gathered
5213 from profiling values of expressions and adds @samp{REG_VALUE_PROFILE}
5214 notes to instructions for their later usage in optimizations.
5216 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
5220 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
5221 a code to gather information about values of expressions.
5223 With @option{-fbranch-probabilities}, it reads back the data gathered
5224 and actually performs the optimizations based on them.
5225 Currently the optimizations include specialization of division operation
5226 using the knowledge about the value of the denominator.
5228 @item -fspeculative-prefetching
5229 @opindex fspeculative-prefetching
5230 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
5231 a code to gather information about addresses of memory references in the
5234 With @option{-fbranch-probabilities}, it reads back the data gathered
5235 and issues prefetch instructions according to them. In addition to the opportunities
5236 noticed by @option{-fprefetch-loop-arrays}, it also notices more complicated
5237 memory access patterns---for example accesses to the data stored in linked
5238 list whose elements are usually allocated sequentially.
5240 In order to prevent issuing double prefetches, usage of
5241 @option{-fspeculative-prefetching} implies @option{-fno-prefetch-loop-arrays}.
5243 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
5245 @item -frename-registers
5246 @opindex frename-registers
5247 Attempt to avoid false dependencies in scheduled code by making use
5248 of registers left over after register allocation. This optimization
5249 will most benefit processors with lots of registers. Depending on the
5250 debug information format adopted by the target, however, it can
5251 make debugging impossible, since variables will no longer stay in
5252 a ``home register''.
5254 Not enabled by default at any level because it has known bugs.
5258 Perform tail duplication to enlarge superblock size. This transformation
5259 simplifies the control flow of the function allowing other optimizations to do
5262 Enabled with @option{-fprofile-use}.
5264 @item -funroll-loops
5265 @opindex funroll-loops
5266 Unroll loops whose number of iterations can be determined at compile time or
5267 upon entry to the loop. @option{-funroll-loops} implies
5268 @option{-frerun-cse-after-loop}. It also turns on complete loop peeling
5269 (i.e.@: complete removal of loops with small constant number of iterations).
5270 This option makes code larger, and may or may not make it run faster.
5272 Enabled with @option{-fprofile-use}.
5274 @item -funroll-all-loops
5275 @opindex funroll-all-loops
5276 Unroll all loops, even if their number of iterations is uncertain when
5277 the loop is entered. This usually makes programs run more slowly.
5278 @option{-funroll-all-loops} implies the same options as
5279 @option{-funroll-loops}.
5282 @opindex fpeel-loops
5283 Peels the loops for that there is enough information that they do not
5284 roll much (from profile feedback). It also turns on complete loop peeling
5285 (i.e.@: complete removal of loops with small constant number of iterations).
5287 Enabled with @option{-fprofile-use}.
5289 @item -fmove-loop-invariants
5290 @opindex fmove-loop-invariants
5291 Enables the loop invariant motion pass in the new loop optimizer. Enabled
5292 at level @option{-O1}
5294 @item -funswitch-loops
5295 @opindex funswitch-loops
5296 Move branches with loop invariant conditions out of the loop, with duplicates
5297 of the loop on both branches (modified according to result of the condition).
5299 @item -fprefetch-loop-arrays
5300 @opindex fprefetch-loop-arrays
5301 If supported by the target machine, generate instructions to prefetch
5302 memory to improve the performance of loops that access large arrays.
5304 Disabled at level @option{-Os}.
5306 @item -ffunction-sections
5307 @itemx -fdata-sections
5308 @opindex ffunction-sections
5309 @opindex fdata-sections
5310 Place each function or data item into its own section in the output
5311 file if the target supports arbitrary sections. The name of the
5312 function or the name of the data item determines the section's name
5315 Use these options on systems where the linker can perform optimizations
5316 to improve locality of reference in the instruction space. Most systems
5317 using the ELF object format and SPARC processors running Solaris 2 have
5318 linkers with such optimizations. AIX may have these optimizations in
5321 Only use these options when there are significant benefits from doing
5322 so. When you specify these options, the assembler and linker will
5323 create larger object and executable files and will also be slower.
5324 You will not be able to use @code{gprof} on all systems if you
5325 specify this option and you may have problems with debugging if
5326 you specify both this option and @option{-g}.
5328 @item -fbranch-target-load-optimize
5329 @opindex fbranch-target-load-optimize
5330 Perform branch target register load optimization before prologue / epilogue
5332 The use of target registers can typically be exposed only during reload,
5333 thus hoisting loads out of loops and doing inter-block scheduling needs
5334 a separate optimization pass.
5336 @item -fbranch-target-load-optimize2
5337 @opindex fbranch-target-load-optimize2
5338 Perform branch target register load optimization after prologue / epilogue
5341 @item -fbtr-bb-exclusive
5342 @opindex fbtr-bb-exclusive
5343 When performing branch target register load optimization, don't reuse
5344 branch target registers in within any basic block.
5346 @item --param @var{name}=@var{value}
5348 In some places, GCC uses various constants to control the amount of
5349 optimization that is done. For example, GCC will not inline functions
5350 that contain more that a certain number of instructions. You can
5351 control some of these constants on the command-line using the
5352 @option{--param} option.
5354 The names of specific parameters, and the meaning of the values, are
5355 tied to the internals of the compiler, and are subject to change
5356 without notice in future releases.
5358 In each case, the @var{value} is an integer. The allowable choices for
5359 @var{name} are given in the following table:
5362 @item sra-max-structure-size
5363 The maximum structure size, in bytes, at which the scalar replacement
5364 of aggregates (SRA) optimization will perform block copies. The
5365 default value, 0, implies that GCC will select the most appropriate
5368 @item sra-field-structure-ratio
5369 The threshold ratio (as a percentage) between instantiated fields and
5370 the complete structure size. We say that if the ratio of the number
5371 of bytes in instantiated fields to the number of bytes in the complete
5372 structure exceeds this parameter, then block copies are not used. The
5375 @item max-crossjump-edges
5376 The maximum number of incoming edges to consider for crossjumping.
5377 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
5378 the number of edges incoming to each block. Increasing values mean
5379 more aggressive optimization, making the compile time increase with
5380 probably small improvement in executable size.
5382 @item min-crossjump-insns
5383 The minimum number of instructions which must be matched at the end
5384 of two blocks before crossjumping will be performed on them. This
5385 value is ignored in the case where all instructions in the block being
5386 crossjumped from are matched. The default value is 5.
5388 @item max-goto-duplication-insns
5389 The maximum number of instructions to duplicate to a block that jumps
5390 to a computed goto. To avoid @math{O(N^2)} behavior in a number of
5391 passes, GCC factors computed gotos early in the compilation process,
5392 and unfactors them as late as possible. Only computed jumps at the
5393 end of a basic blocks with no more than max-goto-duplication-insns are
5394 unfactored. The default value is 8.
5396 @item max-delay-slot-insn-search
5397 The maximum number of instructions to consider when looking for an
5398 instruction to fill a delay slot. If more than this arbitrary number of
5399 instructions is searched, the time savings from filling the delay slot
5400 will be minimal so stop searching. Increasing values mean more
5401 aggressive optimization, making the compile time increase with probably
5402 small improvement in executable run time.
5404 @item max-delay-slot-live-search
5405 When trying to fill delay slots, the maximum number of instructions to
5406 consider when searching for a block with valid live register
5407 information. Increasing this arbitrarily chosen value means more
5408 aggressive optimization, increasing the compile time. This parameter
5409 should be removed when the delay slot code is rewritten to maintain the
5412 @item max-gcse-memory
5413 The approximate maximum amount of memory that will be allocated in
5414 order to perform the global common subexpression elimination
5415 optimization. If more memory than specified is required, the
5416 optimization will not be done.
5418 @item max-gcse-passes
5419 The maximum number of passes of GCSE to run. The default is 1.
5421 @item max-pending-list-length
5422 The maximum number of pending dependencies scheduling will allow
5423 before flushing the current state and starting over. Large functions
5424 with few branches or calls can create excessively large lists which
5425 needlessly consume memory and resources.
5427 @item max-inline-insns-single
5428 Several parameters control the tree inliner used in gcc.
5429 This number sets the maximum number of instructions (counted in GCC's
5430 internal representation) in a single function that the tree inliner
5431 will consider for inlining. This only affects functions declared
5432 inline and methods implemented in a class declaration (C++).
5433 The default value is 500.
5435 @item max-inline-insns-auto
5436 When you use @option{-finline-functions} (included in @option{-O3}),
5437 a lot of functions that would otherwise not be considered for inlining
5438 by the compiler will be investigated. To those functions, a different
5439 (more restrictive) limit compared to functions declared inline can
5441 The default value is 120.
5443 @item large-function-insns
5444 The limit specifying really large functions. For functions larger than this
5445 limit after inlining inlining is constrained by
5446 @option{--param large-function-growth}. This parameter is useful primarily
5447 to avoid extreme compilation time caused by non-linear algorithms used by the
5449 This parameter is ignored when @option{-funit-at-a-time} is not used.
5450 The default value is 3000.
5452 @item large-function-growth
5453 Specifies maximal growth of large function caused by inlining in percents.
5454 This parameter is ignored when @option{-funit-at-a-time} is not used.
5455 The default value is 100 which limits large function growth to 2.0 times
5458 @item inline-unit-growth
5459 Specifies maximal overall growth of the compilation unit caused by inlining.
5460 This parameter is ignored when @option{-funit-at-a-time} is not used.
5461 The default value is 50 which limits unit growth to 1.5 times the original
5464 @item max-inline-insns-recursive
5465 @itemx max-inline-insns-recursive-auto
5466 Specifies maximum number of instructions out-of-line copy of self recursive inline
5467 function can grow into by performing recursive inlining.
5469 For functions declared inline @option{--param max-inline-insns-recursive} is
5470 taken into acount. For function not declared inline, recursive inlining
5471 happens only when @option{-finline-functions} (included in @option{-O3}) is
5472 enabled and @option{--param max-inline-insns-recursive-auto} is used. The
5473 default value is 500.
5475 @item max-inline-recursive-depth
5476 @itemx max-inline-recursive-depth-auto
5477 Specifies maximum recursion depth used by the recursive inlining.
5479 For functions declared inline @option{--param max-inline-recursive-depth} is
5480 taken into acount. For function not declared inline, recursive inlining
5481 happens only when @option{-finline-functions} (included in @option{-O3}) is
5482 enabled and @option{--param max-inline-recursive-depth-auto} is used. The
5483 default value is 500.
5485 @item max-unrolled-insns
5486 The maximum number of instructions that a loop should have if that loop
5487 is unrolled, and if the loop is unrolled, it determines how many times
5488 the loop code is unrolled.
5490 @item max-average-unrolled-insns
5491 The maximum number of instructions biased by probabilities of their execution
5492 that a loop should have if that loop is unrolled, and if the loop is unrolled,
5493 it determines how many times the loop code is unrolled.
5495 @item max-unroll-times
5496 The maximum number of unrollings of a single loop.
5498 @item max-peeled-insns
5499 The maximum number of instructions that a loop should have if that loop
5500 is peeled, and if the loop is peeled, it determines how many times
5501 the loop code is peeled.
5503 @item max-peel-times
5504 The maximum number of peelings of a single loop.
5506 @item max-completely-peeled-insns
5507 The maximum number of insns of a completely peeled loop.
5509 @item max-completely-peel-times
5510 The maximum number of iterations of a loop to be suitable for complete peeling.
5512 @item max-unswitch-insns
5513 The maximum number of insns of an unswitched loop.
5515 @item max-unswitch-level
5516 The maximum number of branches unswitched in a single loop.
5519 The minimum cost of an expensive expression in the loop invariant motion.
5521 @item iv-consider-all-candidates-bound
5522 Bound on number of candidates for induction variables below that
5523 all candidates are considered for each use in induction variable
5524 optimizations. Only the most relevant candidates are considered
5525 if there are more candidates, to avoid quadratic time complexity.
5527 @item iv-max-considered-uses
5528 The induction variable optimizations give up on loops that contain more
5529 induction variable uses.
5531 @item iv-always-prune-cand-set-bound
5532 If number of candidates in the set is smaller than this value,
5533 we always try to remove unnecessary ivs from the set during its
5534 optimization when a new iv is added to the set.
5536 @item max-iterations-to-track
5538 The maximum number of iterations of a loop the brute force algorithm
5539 for analysis of # of iterations of the loop tries to evaluate.
5541 @item hot-bb-count-fraction
5542 Select fraction of the maximal count of repetitions of basic block in program
5543 given basic block needs to have to be considered hot.
5545 @item hot-bb-frequency-fraction
5546 Select fraction of the maximal frequency of executions of basic block in
5547 function given basic block needs to have to be considered hot
5549 @item tracer-dynamic-coverage
5550 @itemx tracer-dynamic-coverage-feedback
5552 This value is used to limit superblock formation once the given percentage of
5553 executed instructions is covered. This limits unnecessary code size
5556 The @option{tracer-dynamic-coverage-feedback} is used only when profile
5557 feedback is available. The real profiles (as opposed to statically estimated
5558 ones) are much less balanced allowing the threshold to be larger value.
5560 @item tracer-max-code-growth
5561 Stop tail duplication once code growth has reached given percentage. This is
5562 rather hokey argument, as most of the duplicates will be eliminated later in
5563 cross jumping, so it may be set to much higher values than is the desired code
5566 @item tracer-min-branch-ratio
5568 Stop reverse growth when the reverse probability of best edge is less than this
5569 threshold (in percent).
5571 @item tracer-min-branch-ratio
5572 @itemx tracer-min-branch-ratio-feedback
5574 Stop forward growth if the best edge do have probability lower than this
5577 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
5578 compilation for profile feedback and one for compilation without. The value
5579 for compilation with profile feedback needs to be more conservative (higher) in
5580 order to make tracer effective.
5582 @item max-cse-path-length
5584 Maximum number of basic blocks on path that cse considers. The default is 10.
5586 @item global-var-threshold
5588 Counts the number of function calls (@var{n}) and the number of
5589 call-clobbered variables (@var{v}). If @var{n}x@var{v} is larger than this limit, a
5590 single artificial variable will be created to represent all the
5591 call-clobbered variables at function call sites. This artificial
5592 variable will then be made to alias every call-clobbered variable.
5593 (done as @code{int * size_t} on the host machine; beware overflow).
5595 @item max-aliased-vops
5597 Maximum number of virtual operands allowed to represent aliases
5598 before triggering the alias grouping heuristic. Alias grouping
5599 reduces compile times and memory consumption needed for aliasing at
5600 the expense of precision loss in alias information.
5602 @item ggc-min-expand
5604 GCC uses a garbage collector to manage its own memory allocation. This
5605 parameter specifies the minimum percentage by which the garbage
5606 collector's heap should be allowed to expand between collections.
5607 Tuning this may improve compilation speed; it has no effect on code
5610 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
5611 RAM >= 1GB@. If @code{getrlimit} is available, the notion of "RAM" is
5612 the smallest of actual RAM and @code{RLIMIT_DATA} or @code{RLIMIT_AS}. If
5613 GCC is not able to calculate RAM on a particular platform, the lower
5614 bound of 30% is used. Setting this parameter and
5615 @option{ggc-min-heapsize} to zero causes a full collection to occur at
5616 every opportunity. This is extremely slow, but can be useful for
5619 @item ggc-min-heapsize
5621 Minimum size of the garbage collector's heap before it begins bothering
5622 to collect garbage. The first collection occurs after the heap expands
5623 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
5624 tuning this may improve compilation speed, and has no effect on code
5627 The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which
5628 tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
5629 with a lower bound of 4096 (four megabytes) and an upper bound of
5630 131072 (128 megabytes). If GCC is not able to calculate RAM on a
5631 particular platform, the lower bound is used. Setting this parameter
5632 very large effectively disables garbage collection. Setting this
5633 parameter and @option{ggc-min-expand} to zero causes a full collection
5634 to occur at every opportunity.
5636 @item max-reload-search-insns
5637 The maximum number of instruction reload should look backward for equivalent
5638 register. Increasing values mean more aggressive optimization, making the
5639 compile time increase with probably slightly better performance. The default
5642 @item max-cselib-memory-location
5643 The maximum number of memory locations cselib should take into acount.
5644 Increasing values mean more aggressive optimization, making the compile time
5645 increase with probably slightly better performance. The default value is 500.
5647 @item reorder-blocks-duplicate
5648 @itemx reorder-blocks-duplicate-feedback
5650 Used by basic block reordering pass to decide whether to use unconditional
5651 branch or duplicate the code on its destination. Code is duplicated when its
5652 estimated size is smaller than this value multiplied by the estimated size of
5653 unconditional jump in the hot spots of the program.
5655 The @option{reorder-block-duplicate-feedback} is used only when profile
5656 feedback is available and may be set to higher values than
5657 @option{reorder-block-duplicate} since information about the hot spots is more
5660 @item max-sched-region-blocks
5661 The maximum number of blocks in a region to be considered for
5662 interblock scheduling. The default value is 10.
5664 @item max-sched-region-insns
5665 The maximum number of insns in a region to be considered for
5666 interblock scheduling. The default value is 100.
5668 @item max-last-value-rtl
5670 The maximum size measured as number of RTLs that can be recorded in an expression
5671 in combiner for a pseudo register as last known value of that register. The default
5674 @item integer-share-limit
5675 Small integer constants can use a shared data structure, reducing the
5676 compiler's memory usage and increasing its speed. This sets the maximum
5677 value of a shared integer constant's. The default value is 256.
5682 @node Preprocessor Options
5683 @section Options Controlling the Preprocessor
5684 @cindex preprocessor options
5685 @cindex options, preprocessor
5687 These options control the C preprocessor, which is run on each C source
5688 file before actual compilation.
5690 If you use the @option{-E} option, nothing is done except preprocessing.
5691 Some of these options make sense only together with @option{-E} because
5692 they cause the preprocessor output to be unsuitable for actual
5697 You can use @option{-Wp,@var{option}} to bypass the compiler driver
5698 and pass @var{option} directly through to the preprocessor. If
5699 @var{option} contains commas, it is split into multiple options at the
5700 commas. However, many options are modified, translated or interpreted
5701 by the compiler driver before being passed to the preprocessor, and
5702 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
5703 interface is undocumented and subject to change, so whenever possible
5704 you should avoid using @option{-Wp} and let the driver handle the
5707 @item -Xpreprocessor @var{option}
5708 @opindex preprocessor
5709 Pass @var{option} as an option to the preprocessor. You can use this to
5710 supply system-specific preprocessor options which GCC does not know how to
5713 If you want to pass an option that takes an argument, you must use
5714 @option{-Xpreprocessor} twice, once for the option and once for the argument.
5717 @include cppopts.texi
5719 @node Assembler Options
5720 @section Passing Options to the Assembler
5722 @c prevent bad page break with this line
5723 You can pass options to the assembler.
5726 @item -Wa,@var{option}
5728 Pass @var{option} as an option to the assembler. If @var{option}
5729 contains commas, it is split into multiple options at the commas.
5731 @item -Xassembler @var{option}
5733 Pass @var{option} as an option to the assembler. You can use this to
5734 supply system-specific assembler options which GCC does not know how to
5737 If you want to pass an option that takes an argument, you must use
5738 @option{-Xassembler} twice, once for the option and once for the argument.
5743 @section Options for Linking
5744 @cindex link options
5745 @cindex options, linking
5747 These options come into play when the compiler links object files into
5748 an executable output file. They are meaningless if the compiler is
5749 not doing a link step.
5753 @item @var{object-file-name}
5754 A file name that does not end in a special recognized suffix is
5755 considered to name an object file or library. (Object files are
5756 distinguished from libraries by the linker according to the file
5757 contents.) If linking is done, these object files are used as input
5766 If any of these options is used, then the linker is not run, and
5767 object file names should not be used as arguments. @xref{Overall
5771 @item -l@var{library}
5772 @itemx -l @var{library}
5774 Search the library named @var{library} when linking. (The second
5775 alternative with the library as a separate argument is only for
5776 POSIX compliance and is not recommended.)
5778 It makes a difference where in the command you write this option; the
5779 linker searches and processes libraries and object files in the order they
5780 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
5781 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
5782 to functions in @samp{z}, those functions may not be loaded.
5784 The linker searches a standard list of directories for the library,
5785 which is actually a file named @file{lib@var{library}.a}. The linker
5786 then uses this file as if it had been specified precisely by name.
5788 The directories searched include several standard system directories
5789 plus any that you specify with @option{-L}.
5791 Normally the files found this way are library files---archive files
5792 whose members are object files. The linker handles an archive file by
5793 scanning through it for members which define symbols that have so far
5794 been referenced but not defined. But if the file that is found is an
5795 ordinary object file, it is linked in the usual fashion. The only
5796 difference between using an @option{-l} option and specifying a file name
5797 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
5798 and searches several directories.
5802 You need this special case of the @option{-l} option in order to
5803 link an Objective-C or Objective-C++ program.
5806 @opindex nostartfiles
5807 Do not use the standard system startup files when linking.
5808 The standard system libraries are used normally, unless @option{-nostdlib}
5809 or @option{-nodefaultlibs} is used.
5811 @item -nodefaultlibs
5812 @opindex nodefaultlibs
5813 Do not use the standard system libraries when linking.
5814 Only the libraries you specify will be passed to the linker.
5815 The standard startup files are used normally, unless @option{-nostartfiles}
5816 is used. The compiler may generate calls to @code{memcmp},
5817 @code{memset}, @code{memcpy} and @code{memmove}.
5818 These entries are usually resolved by entries in
5819 libc. These entry points should be supplied through some other
5820 mechanism when this option is specified.
5824 Do not use the standard system startup files or libraries when linking.
5825 No startup files and only the libraries you specify will be passed to
5826 the linker. The compiler may generate calls to @code{memcmp}, @code{memset},
5827 @code{memcpy} and @code{memmove}.
5828 These entries are usually resolved by entries in
5829 libc. These entry points should be supplied through some other
5830 mechanism when this option is specified.
5832 @cindex @option{-lgcc}, use with @option{-nostdlib}
5833 @cindex @option{-nostdlib} and unresolved references
5834 @cindex unresolved references and @option{-nostdlib}
5835 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
5836 @cindex @option{-nodefaultlibs} and unresolved references
5837 @cindex unresolved references and @option{-nodefaultlibs}
5838 One of the standard libraries bypassed by @option{-nostdlib} and
5839 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
5840 that GCC uses to overcome shortcomings of particular machines, or special
5841 needs for some languages.
5842 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
5843 Collection (GCC) Internals},
5844 for more discussion of @file{libgcc.a}.)
5845 In most cases, you need @file{libgcc.a} even when you want to avoid
5846 other standard libraries. In other words, when you specify @option{-nostdlib}
5847 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
5848 This ensures that you have no unresolved references to internal GCC
5849 library subroutines. (For example, @samp{__main}, used to ensure C++
5850 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
5851 GNU Compiler Collection (GCC) Internals}.)
5855 Produce a position independent executable on targets which support it.
5856 For predictable results, you must also specify the same set of options
5857 that were used to generate code (@option{-fpie}, @option{-fPIE},
5858 or model suboptions) when you specify this option.
5862 Remove all symbol table and relocation information from the executable.
5866 On systems that support dynamic linking, this prevents linking with the shared
5867 libraries. On other systems, this option has no effect.
5871 Produce a shared object which can then be linked with other objects to
5872 form an executable. Not all systems support this option. For predictable
5873 results, you must also specify the same set of options that were used to
5874 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
5875 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
5876 needs to build supplementary stub code for constructors to work. On
5877 multi-libbed systems, @samp{gcc -shared} must select the correct support
5878 libraries to link against. Failing to supply the correct flags may lead
5879 to subtle defects. Supplying them in cases where they are not necessary
5882 @item -shared-libgcc
5883 @itemx -static-libgcc
5884 @opindex shared-libgcc
5885 @opindex static-libgcc
5886 On systems that provide @file{libgcc} as a shared library, these options
5887 force the use of either the shared or static version respectively.
5888 If no shared version of @file{libgcc} was built when the compiler was
5889 configured, these options have no effect.
5891 There are several situations in which an application should use the
5892 shared @file{libgcc} instead of the static version. The most common
5893 of these is when the application wishes to throw and catch exceptions
5894 across different shared libraries. In that case, each of the libraries
5895 as well as the application itself should use the shared @file{libgcc}.
5897 Therefore, the G++ and GCJ drivers automatically add
5898 @option{-shared-libgcc} whenever you build a shared library or a main
5899 executable, because C++ and Java programs typically use exceptions, so
5900 this is the right thing to do.
5902 If, instead, you use the GCC driver to create shared libraries, you may
5903 find that they will not always be linked with the shared @file{libgcc}.
5904 If GCC finds, at its configuration time, that you have a non-GNU linker
5905 or a GNU linker that does not support option @option{--eh-frame-hdr},
5906 it will link the shared version of @file{libgcc} into shared libraries
5907 by default. Otherwise, it will take advantage of the linker and optimize
5908 away the linking with the shared version of @file{libgcc}, linking with
5909 the static version of libgcc by default. This allows exceptions to
5910 propagate through such shared libraries, without incurring relocation
5911 costs at library load time.
5913 However, if a library or main executable is supposed to throw or catch
5914 exceptions, you must link it using the G++ or GCJ driver, as appropriate
5915 for the languages used in the program, or using the option
5916 @option{-shared-libgcc}, such that it is linked with the shared
5921 Bind references to global symbols when building a shared object. Warn
5922 about any unresolved references (unless overridden by the link editor
5923 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
5926 @item -Xlinker @var{option}
5928 Pass @var{option} as an option to the linker. You can use this to
5929 supply system-specific linker options which GCC does not know how to
5932 If you want to pass an option that takes an argument, you must use
5933 @option{-Xlinker} twice, once for the option and once for the argument.
5934 For example, to pass @option{-assert definitions}, you must write
5935 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
5936 @option{-Xlinker "-assert definitions"}, because this passes the entire
5937 string as a single argument, which is not what the linker expects.
5939 @item -Wl,@var{option}
5941 Pass @var{option} as an option to the linker. If @var{option} contains
5942 commas, it is split into multiple options at the commas.
5944 @item -u @var{symbol}
5946 Pretend the symbol @var{symbol} is undefined, to force linking of
5947 library modules to define it. You can use @option{-u} multiple times with
5948 different symbols to force loading of additional library modules.
5951 @node Directory Options
5952 @section Options for Directory Search
5953 @cindex directory options
5954 @cindex options, directory search
5957 These options specify directories to search for header files, for
5958 libraries and for parts of the compiler:
5963 Add the directory @var{dir} to the head of the list of directories to be
5964 searched for header files. This can be used to override a system header
5965 file, substituting your own version, since these directories are
5966 searched before the system header file directories. However, you should
5967 not use this option to add directories that contain vendor-supplied
5968 system header files (use @option{-isystem} for that). If you use more than
5969 one @option{-I} option, the directories are scanned in left-to-right
5970 order; the standard system directories come after.
5972 If a standard system include directory, or a directory specified with
5973 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
5974 option will be ignored. The directory will still be searched but as a
5975 system directory at its normal position in the system include chain.
5976 This is to ensure that GCC's procedure to fix buggy system headers and
5977 the ordering for the include_next directive are not inadvertently changed.
5978 If you really need to change the search order for system directories,
5979 use the @option{-nostdinc} and/or @option{-isystem} options.
5981 @item -iquote@var{dir}
5983 Add the directory @var{dir} to the head of the list of directories to
5984 be searched for header files only for the case of @samp{#include
5985 "@var{file}"}; they are not searched for @samp{#include <@var{file}>},
5986 otherwise just like @option{-I}.
5990 Add directory @var{dir} to the list of directories to be searched
5993 @item -B@var{prefix}
5995 This option specifies where to find the executables, libraries,
5996 include files, and data files of the compiler itself.
5998 The compiler driver program runs one or more of the subprograms
5999 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
6000 @var{prefix} as a prefix for each program it tries to run, both with and
6001 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
6003 For each subprogram to be run, the compiler driver first tries the
6004 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
6005 was not specified, the driver tries two standard prefixes, which are
6006 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}. If neither of
6007 those results in a file name that is found, the unmodified program
6008 name is searched for using the directories specified in your
6009 @env{PATH} environment variable.
6011 The compiler will check to see if the path provided by the @option{-B}
6012 refers to a directory, and if necessary it will add a directory
6013 separator character at the end of the path.
6015 @option{-B} prefixes that effectively specify directory names also apply
6016 to libraries in the linker, because the compiler translates these
6017 options into @option{-L} options for the linker. They also apply to
6018 includes files in the preprocessor, because the compiler translates these
6019 options into @option{-isystem} options for the preprocessor. In this case,
6020 the compiler appends @samp{include} to the prefix.
6022 The run-time support file @file{libgcc.a} can also be searched for using
6023 the @option{-B} prefix, if needed. If it is not found there, the two
6024 standard prefixes above are tried, and that is all. The file is left
6025 out of the link if it is not found by those means.
6027 Another way to specify a prefix much like the @option{-B} prefix is to use
6028 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
6031 As a special kludge, if the path provided by @option{-B} is
6032 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
6033 9, then it will be replaced by @file{[dir/]include}. This is to help
6034 with boot-strapping the compiler.
6036 @item -specs=@var{file}
6038 Process @var{file} after the compiler reads in the standard @file{specs}
6039 file, in order to override the defaults that the @file{gcc} driver
6040 program uses when determining what switches to pass to @file{cc1},
6041 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
6042 @option{-specs=@var{file}} can be specified on the command line, and they
6043 are processed in order, from left to right.
6047 This option has been deprecated. Please use @option{-iquote} instead for
6048 @option{-I} directories before the @option{-I-} and remove the @option{-I-}.
6049 Any directories you specify with @option{-I} options before the @option{-I-}
6050 option are searched only for the case of @samp{#include "@var{file}"};
6051 they are not searched for @samp{#include <@var{file}>}.
6053 If additional directories are specified with @option{-I} options after
6054 the @option{-I-}, these directories are searched for all @samp{#include}
6055 directives. (Ordinarily @emph{all} @option{-I} directories are used
6058 In addition, the @option{-I-} option inhibits the use of the current
6059 directory (where the current input file came from) as the first search
6060 directory for @samp{#include "@var{file}"}. There is no way to
6061 override this effect of @option{-I-}. With @option{-I.} you can specify
6062 searching the directory which was current when the compiler was
6063 invoked. That is not exactly the same as what the preprocessor does
6064 by default, but it is often satisfactory.
6066 @option{-I-} does not inhibit the use of the standard system directories
6067 for header files. Thus, @option{-I-} and @option{-nostdinc} are
6074 @section Specifying subprocesses and the switches to pass to them
6077 @command{gcc} is a driver program. It performs its job by invoking a
6078 sequence of other programs to do the work of compiling, assembling and
6079 linking. GCC interprets its command-line parameters and uses these to
6080 deduce which programs it should invoke, and which command-line options
6081 it ought to place on their command lines. This behavior is controlled
6082 by @dfn{spec strings}. In most cases there is one spec string for each
6083 program that GCC can invoke, but a few programs have multiple spec
6084 strings to control their behavior. The spec strings built into GCC can
6085 be overridden by using the @option{-specs=} command-line switch to specify
6088 @dfn{Spec files} are plaintext files that are used to construct spec
6089 strings. They consist of a sequence of directives separated by blank
6090 lines. The type of directive is determined by the first non-whitespace
6091 character on the line and it can be one of the following:
6094 @item %@var{command}
6095 Issues a @var{command} to the spec file processor. The commands that can
6099 @item %include <@var{file}>
6101 Search for @var{file} and insert its text at the current point in the
6104 @item %include_noerr <@var{file}>
6105 @cindex %include_noerr
6106 Just like @samp{%include}, but do not generate an error message if the include
6107 file cannot be found.
6109 @item %rename @var{old_name} @var{new_name}
6111 Rename the spec string @var{old_name} to @var{new_name}.
6115 @item *[@var{spec_name}]:
6116 This tells the compiler to create, override or delete the named spec
6117 string. All lines after this directive up to the next directive or
6118 blank line are considered to be the text for the spec string. If this
6119 results in an empty string then the spec will be deleted. (Or, if the
6120 spec did not exist, then nothing will happened.) Otherwise, if the spec
6121 does not currently exist a new spec will be created. If the spec does
6122 exist then its contents will be overridden by the text of this
6123 directive, unless the first character of that text is the @samp{+}
6124 character, in which case the text will be appended to the spec.
6126 @item [@var{suffix}]:
6127 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
6128 and up to the next directive or blank line are considered to make up the
6129 spec string for the indicated suffix. When the compiler encounters an
6130 input file with the named suffix, it will processes the spec string in
6131 order to work out how to compile that file. For example:
6138 This says that any input file whose name ends in @samp{.ZZ} should be
6139 passed to the program @samp{z-compile}, which should be invoked with the
6140 command-line switch @option{-input} and with the result of performing the
6141 @samp{%i} substitution. (See below.)
6143 As an alternative to providing a spec string, the text that follows a
6144 suffix directive can be one of the following:
6147 @item @@@var{language}
6148 This says that the suffix is an alias for a known @var{language}. This is
6149 similar to using the @option{-x} command-line switch to GCC to specify a
6150 language explicitly. For example:
6157 Says that .ZZ files are, in fact, C++ source files.
6160 This causes an error messages saying:
6163 @var{name} compiler not installed on this system.
6167 GCC already has an extensive list of suffixes built into it.
6168 This directive will add an entry to the end of the list of suffixes, but
6169 since the list is searched from the end backwards, it is effectively
6170 possible to override earlier entries using this technique.
6174 GCC has the following spec strings built into it. Spec files can
6175 override these strings or create their own. Note that individual
6176 targets can also add their own spec strings to this list.
6179 asm Options to pass to the assembler
6180 asm_final Options to pass to the assembler post-processor
6181 cpp Options to pass to the C preprocessor
6182 cc1 Options to pass to the C compiler
6183 cc1plus Options to pass to the C++ compiler
6184 endfile Object files to include at the end of the link
6185 link Options to pass to the linker
6186 lib Libraries to include on the command line to the linker
6187 libgcc Decides which GCC support library to pass to the linker
6188 linker Sets the name of the linker
6189 predefines Defines to be passed to the C preprocessor
6190 signed_char Defines to pass to CPP to say whether @code{char} is signed
6192 startfile Object files to include at the start of the link
6195 Here is a small example of a spec file:
6201 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
6204 This example renames the spec called @samp{lib} to @samp{old_lib} and
6205 then overrides the previous definition of @samp{lib} with a new one.
6206 The new definition adds in some extra command-line options before
6207 including the text of the old definition.
6209 @dfn{Spec strings} are a list of command-line options to be passed to their
6210 corresponding program. In addition, the spec strings can contain
6211 @samp{%}-prefixed sequences to substitute variable text or to
6212 conditionally insert text into the command line. Using these constructs
6213 it is possible to generate quite complex command lines.
6215 Here is a table of all defined @samp{%}-sequences for spec
6216 strings. Note that spaces are not generated automatically around the
6217 results of expanding these sequences. Therefore you can concatenate them
6218 together or combine them with constant text in a single argument.
6222 Substitute one @samp{%} into the program name or argument.
6225 Substitute the name of the input file being processed.
6228 Substitute the basename of the input file being processed.
6229 This is the substring up to (and not including) the last period
6230 and not including the directory.
6233 This is the same as @samp{%b}, but include the file suffix (text after
6237 Marks the argument containing or following the @samp{%d} as a
6238 temporary file name, so that that file will be deleted if GCC exits
6239 successfully. Unlike @samp{%g}, this contributes no text to the
6242 @item %g@var{suffix}
6243 Substitute a file name that has suffix @var{suffix} and is chosen
6244 once per compilation, and mark the argument in the same way as
6245 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
6246 name is now chosen in a way that is hard to predict even when previously
6247 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
6248 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
6249 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
6250 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
6251 was simply substituted with a file name chosen once per compilation,
6252 without regard to any appended suffix (which was therefore treated
6253 just like ordinary text), making such attacks more likely to succeed.
6255 @item %u@var{suffix}
6256 Like @samp{%g}, but generates a new temporary file name even if
6257 @samp{%u@var{suffix}} was already seen.
6259 @item %U@var{suffix}
6260 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
6261 new one if there is no such last file name. In the absence of any
6262 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
6263 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
6264 would involve the generation of two distinct file names, one
6265 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
6266 simply substituted with a file name chosen for the previous @samp{%u},
6267 without regard to any appended suffix.
6269 @item %j@var{suffix}
6270 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
6271 writable, and if save-temps is off; otherwise, substitute the name
6272 of a temporary file, just like @samp{%u}. This temporary file is not
6273 meant for communication between processes, but rather as a junk
6276 @item %|@var{suffix}
6277 @itemx %m@var{suffix}
6278 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
6279 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
6280 all. These are the two most common ways to instruct a program that it
6281 should read from standard input or write to standard output. If you
6282 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
6283 construct: see for example @file{f/lang-specs.h}.
6285 @item %.@var{SUFFIX}
6286 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
6287 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
6288 terminated by the next space or %.
6291 Marks the argument containing or following the @samp{%w} as the
6292 designated output file of this compilation. This puts the argument
6293 into the sequence of arguments that @samp{%o} will substitute later.
6296 Substitutes the names of all the output files, with spaces
6297 automatically placed around them. You should write spaces
6298 around the @samp{%o} as well or the results are undefined.
6299 @samp{%o} is for use in the specs for running the linker.
6300 Input files whose names have no recognized suffix are not compiled
6301 at all, but they are included among the output files, so they will
6305 Substitutes the suffix for object files. Note that this is
6306 handled specially when it immediately follows @samp{%g, %u, or %U},
6307 because of the need for those to form complete file names. The
6308 handling is such that @samp{%O} is treated exactly as if it had already
6309 been substituted, except that @samp{%g, %u, and %U} do not currently
6310 support additional @var{suffix} characters following @samp{%O} as they would
6311 following, for example, @samp{.o}.
6314 Substitutes the standard macro predefinitions for the
6315 current target machine. Use this when running @code{cpp}.
6318 Like @samp{%p}, but puts @samp{__} before and after the name of each
6319 predefined macro, except for macros that start with @samp{__} or with
6320 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
6324 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
6325 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}), and
6326 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
6330 Current argument is the name of a library or startup file of some sort.
6331 Search for that file in a standard list of directories and substitute
6332 the full name found.
6335 Print @var{str} as an error message. @var{str} is terminated by a newline.
6336 Use this when inconsistent options are detected.
6339 Substitute the contents of spec string @var{name} at this point.
6342 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
6344 @item %x@{@var{option}@}
6345 Accumulate an option for @samp{%X}.
6348 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
6352 Output the accumulated assembler options specified by @option{-Wa}.
6355 Output the accumulated preprocessor options specified by @option{-Wp}.
6358 Process the @code{asm} spec. This is used to compute the
6359 switches to be passed to the assembler.
6362 Process the @code{asm_final} spec. This is a spec string for
6363 passing switches to an assembler post-processor, if such a program is
6367 Process the @code{link} spec. This is the spec for computing the
6368 command line passed to the linker. Typically it will make use of the
6369 @samp{%L %G %S %D and %E} sequences.
6372 Dump out a @option{-L} option for each directory that GCC believes might
6373 contain startup files. If the target supports multilibs then the
6374 current multilib directory will be prepended to each of these paths.
6377 Process the @code{lib} spec. This is a spec string for deciding which
6378 libraries should be included on the command line to the linker.
6381 Process the @code{libgcc} spec. This is a spec string for deciding
6382 which GCC support library should be included on the command line to the linker.
6385 Process the @code{startfile} spec. This is a spec for deciding which
6386 object files should be the first ones passed to the linker. Typically
6387 this might be a file named @file{crt0.o}.
6390 Process the @code{endfile} spec. This is a spec string that specifies
6391 the last object files that will be passed to the linker.
6394 Process the @code{cpp} spec. This is used to construct the arguments
6395 to be passed to the C preprocessor.
6398 Process the @code{cc1} spec. This is used to construct the options to be
6399 passed to the actual C compiler (@samp{cc1}).
6402 Process the @code{cc1plus} spec. This is used to construct the options to be
6403 passed to the actual C++ compiler (@samp{cc1plus}).
6406 Substitute the variable part of a matched option. See below.
6407 Note that each comma in the substituted string is replaced by
6411 Remove all occurrences of @code{-S} from the command line. Note---this
6412 command is position dependent. @samp{%} commands in the spec string
6413 before this one will see @code{-S}, @samp{%} commands in the spec string
6414 after this one will not.
6416 @item %:@var{function}(@var{args})
6417 Call the named function @var{function}, passing it @var{args}.
6418 @var{args} is first processed as a nested spec string, then split
6419 into an argument vector in the usual fashion. The function returns
6420 a string which is processed as if it had appeared literally as part
6421 of the current spec.
6423 The following built-in spec functions are provided:
6426 @item @code{if-exists}
6427 The @code{if-exists} spec function takes one argument, an absolute
6428 pathname to a file. If the file exists, @code{if-exists} returns the
6429 pathname. Here is a small example of its usage:
6433 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
6436 @item @code{if-exists-else}
6437 The @code{if-exists-else} spec function is similar to the @code{if-exists}
6438 spec function, except that it takes two arguments. The first argument is
6439 an absolute pathname to a file. If the file exists, @code{if-exists-else}
6440 returns the pathname. If it does not exist, it returns the second argument.
6441 This way, @code{if-exists-else} can be used to select one file or another,
6442 based on the existence of the first. Here is a small example of its usage:
6446 crt0%O%s %:if-exists(crti%O%s) \
6447 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
6450 @item @code{replace-outfile}
6451 The @code{replace-outfile} spec function takes two arguments. It looks for the
6452 first argument in the outfiles array and replaces it with the second argument. Here
6453 is a small example of its usage:
6456 %@{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)@}
6462 Substitutes the @code{-S} switch, if that switch was given to GCC@.
6463 If that switch was not specified, this substitutes nothing. Note that
6464 the leading dash is omitted when specifying this option, and it is
6465 automatically inserted if the substitution is performed. Thus the spec
6466 string @samp{%@{foo@}} would match the command-line option @option{-foo}
6467 and would output the command line option @option{-foo}.
6469 @item %W@{@code{S}@}
6470 Like %@{@code{S}@} but mark last argument supplied within as a file to be
6473 @item %@{@code{S}*@}
6474 Substitutes all the switches specified to GCC whose names start
6475 with @code{-S}, but which also take an argument. This is used for
6476 switches like @option{-o}, @option{-D}, @option{-I}, etc.
6477 GCC considers @option{-o foo} as being
6478 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
6479 text, including the space. Thus two arguments would be generated.
6481 @item %@{@code{S}*&@code{T}*@}
6482 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
6483 (the order of @code{S} and @code{T} in the spec is not significant).
6484 There can be any number of ampersand-separated variables; for each the
6485 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
6487 @item %@{@code{S}:@code{X}@}
6488 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
6490 @item %@{!@code{S}:@code{X}@}
6491 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
6493 @item %@{@code{S}*:@code{X}@}
6494 Substitutes @code{X} if one or more switches whose names start with
6495 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
6496 once, no matter how many such switches appeared. However, if @code{%*}
6497 appears somewhere in @code{X}, then @code{X} will be substituted once
6498 for each matching switch, with the @code{%*} replaced by the part of
6499 that switch that matched the @code{*}.
6501 @item %@{.@code{S}:@code{X}@}
6502 Substitutes @code{X}, if processing a file with suffix @code{S}.
6504 @item %@{!.@code{S}:@code{X}@}
6505 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
6507 @item %@{@code{S}|@code{P}:@code{X}@}
6508 Substitutes @code{X} if either @code{-S} or @code{-P} was given to GCC@.
6509 This may be combined with @samp{!}, @samp{.}, and @code{*} sequences as well,
6510 although they have a stronger binding than the @samp{|}. If @code{%*}
6511 appears in @code{X}, all of the alternatives must be starred, and only
6512 the first matching alternative is substituted.
6514 For example, a spec string like this:
6517 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
6520 will output the following command-line options from the following input
6521 command-line options:
6526 -d fred.c -foo -baz -boggle
6527 -d jim.d -bar -baz -boggle
6530 @item %@{S:X; T:Y; :D@}
6532 If @code{S} was given to GCC, substitutes @code{X}; else if @code{T} was
6533 given to GCC, substitutes @code{Y}; else substitutes @code{D}. There can
6534 be as many clauses as you need. This may be combined with @code{.},
6535 @code{!}, @code{|}, and @code{*} as needed.
6540 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
6541 construct may contain other nested @samp{%} constructs or spaces, or
6542 even newlines. They are processed as usual, as described above.
6543 Trailing white space in @code{X} is ignored. White space may also
6544 appear anywhere on the left side of the colon in these constructs,
6545 except between @code{.} or @code{*} and the corresponding word.
6547 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
6548 handled specifically in these constructs. If another value of
6549 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
6550 @option{-W} switch is found later in the command line, the earlier
6551 switch value is ignored, except with @{@code{S}*@} where @code{S} is
6552 just one letter, which passes all matching options.
6554 The character @samp{|} at the beginning of the predicate text is used to
6555 indicate that a command should be piped to the following command, but
6556 only if @option{-pipe} is specified.
6558 It is built into GCC which switches take arguments and which do not.
6559 (You might think it would be useful to generalize this to allow each
6560 compiler's spec to say which switches take arguments. But this cannot
6561 be done in a consistent fashion. GCC cannot even decide which input
6562 files have been specified without knowing which switches take arguments,
6563 and it must know which input files to compile in order to tell which
6566 GCC also knows implicitly that arguments starting in @option{-l} are to be
6567 treated as compiler output files, and passed to the linker in their
6568 proper position among the other output files.
6570 @c man begin OPTIONS
6572 @node Target Options
6573 @section Specifying Target Machine and Compiler Version
6574 @cindex target options
6575 @cindex cross compiling
6576 @cindex specifying machine version
6577 @cindex specifying compiler version and target machine
6578 @cindex compiler version, specifying
6579 @cindex target machine, specifying
6581 The usual way to run GCC is to run the executable called @file{gcc}, or
6582 @file{<machine>-gcc} when cross-compiling, or
6583 @file{<machine>-gcc-<version>} to run a version other than the one that
6584 was installed last. Sometimes this is inconvenient, so GCC provides
6585 options that will switch to another cross-compiler or version.
6588 @item -b @var{machine}
6590 The argument @var{machine} specifies the target machine for compilation.
6592 The value to use for @var{machine} is the same as was specified as the
6593 machine type when configuring GCC as a cross-compiler. For
6594 example, if a cross-compiler was configured with @samp{configure
6595 i386v}, meaning to compile for an 80386 running System V, then you
6596 would specify @option{-b i386v} to run that cross compiler.
6598 @item -V @var{version}
6600 The argument @var{version} specifies which version of GCC to run.
6601 This is useful when multiple versions are installed. For example,
6602 @var{version} might be @samp{2.0}, meaning to run GCC version 2.0.
6605 The @option{-V} and @option{-b} options work by running the
6606 @file{<machine>-gcc-<version>} executable, so there's no real reason to
6607 use them if you can just run that directly.
6609 @node Submodel Options
6610 @section Hardware Models and Configurations
6611 @cindex submodel options
6612 @cindex specifying hardware config
6613 @cindex hardware models and configurations, specifying
6614 @cindex machine dependent options
6616 Earlier we discussed the standard option @option{-b} which chooses among
6617 different installed compilers for completely different target
6618 machines, such as VAX vs.@: 68000 vs.@: 80386.
6620 In addition, each of these target machine types can have its own
6621 special options, starting with @samp{-m}, to choose among various
6622 hardware models or configurations---for example, 68010 vs 68020,
6623 floating coprocessor or none. A single installed version of the
6624 compiler can compile for any model or configuration, according to the
6627 Some configurations of the compiler also support additional special
6628 options, usually for compatibility with other compilers on the same
6631 These options are defined by the macro @code{TARGET_SWITCHES} in the
6632 machine description. The default for the options is also defined by
6633 that macro, which enables you to change the defaults.
6635 @c This list is ordered alphanumerically by subsection name.
6636 @c It should be the same order and spelling as these options are listed
6637 @c in Machine Dependent Options
6645 * DEC Alpha Options::
6646 * DEC Alpha/VMS Options::
6650 * i386 and x86-64 Options::
6662 * RS/6000 and PowerPC Options::
6663 * S/390 and zSeries Options::
6666 * System V Options::
6667 * TMS320C3x/C4x Options::
6671 * Xstormy16 Options::
6677 @subsection ARC Options
6680 These options are defined for ARC implementations:
6685 Compile code for little endian mode. This is the default.
6689 Compile code for big endian mode.
6692 @opindex mmangle-cpu
6693 Prepend the name of the cpu to all public symbol names.
6694 In multiple-processor systems, there are many ARC variants with different
6695 instruction and register set characteristics. This flag prevents code
6696 compiled for one cpu to be linked with code compiled for another.
6697 No facility exists for handling variants that are ``almost identical''.
6698 This is an all or nothing option.
6700 @item -mcpu=@var{cpu}
6702 Compile code for ARC variant @var{cpu}.
6703 Which variants are supported depend on the configuration.
6704 All variants support @option{-mcpu=base}, this is the default.
6706 @item -mtext=@var{text-section}
6707 @itemx -mdata=@var{data-section}
6708 @itemx -mrodata=@var{readonly-data-section}
6712 Put functions, data, and readonly data in @var{text-section},
6713 @var{data-section}, and @var{readonly-data-section} respectively
6714 by default. This can be overridden with the @code{section} attribute.
6715 @xref{Variable Attributes}.
6720 @subsection ARM Options
6723 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
6727 @item -mabi=@var{name}
6729 Generate code for the specified ABI@. Permissible values are: @samp{apcs-gnu},
6730 @samp{atpcs}, @samp{aapcs} and @samp{iwmmxt}.
6733 @opindex mapcs-frame
6734 Generate a stack frame that is compliant with the ARM Procedure Call
6735 Standard for all functions, even if this is not strictly necessary for
6736 correct execution of the code. Specifying @option{-fomit-frame-pointer}
6737 with this option will cause the stack frames not to be generated for
6738 leaf functions. The default is @option{-mno-apcs-frame}.
6742 This is a synonym for @option{-mapcs-frame}.
6745 @c not currently implemented
6746 @item -mapcs-stack-check
6747 @opindex mapcs-stack-check
6748 Generate code to check the amount of stack space available upon entry to
6749 every function (that actually uses some stack space). If there is
6750 insufficient space available then either the function
6751 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
6752 called, depending upon the amount of stack space required. The run time
6753 system is required to provide these functions. The default is
6754 @option{-mno-apcs-stack-check}, since this produces smaller code.
6756 @c not currently implemented
6758 @opindex mapcs-float
6759 Pass floating point arguments using the float point registers. This is
6760 one of the variants of the APCS@. This option is recommended if the
6761 target hardware has a floating point unit or if a lot of floating point
6762 arithmetic is going to be performed by the code. The default is
6763 @option{-mno-apcs-float}, since integer only code is slightly increased in
6764 size if @option{-mapcs-float} is used.
6766 @c not currently implemented
6767 @item -mapcs-reentrant
6768 @opindex mapcs-reentrant
6769 Generate reentrant, position independent code. The default is
6770 @option{-mno-apcs-reentrant}.
6773 @item -mthumb-interwork
6774 @opindex mthumb-interwork
6775 Generate code which supports calling between the ARM and Thumb
6776 instruction sets. Without this option the two instruction sets cannot
6777 be reliably used inside one program. The default is
6778 @option{-mno-thumb-interwork}, since slightly larger code is generated
6779 when @option{-mthumb-interwork} is specified.
6781 @item -mno-sched-prolog
6782 @opindex mno-sched-prolog
6783 Prevent the reordering of instructions in the function prolog, or the
6784 merging of those instruction with the instructions in the function's
6785 body. This means that all functions will start with a recognizable set
6786 of instructions (or in fact one of a choice from a small set of
6787 different function prologues), and this information can be used to
6788 locate the start if functions inside an executable piece of code. The
6789 default is @option{-msched-prolog}.
6792 @opindex mhard-float
6793 Generate output containing floating point instructions. This is the
6797 @opindex msoft-float
6798 Generate output containing library calls for floating point.
6799 @strong{Warning:} the requisite libraries are not available for all ARM
6800 targets. Normally the facilities of the machine's usual C compiler are
6801 used, but this cannot be done directly in cross-compilation. You must make
6802 your own arrangements to provide suitable library functions for
6805 @option{-msoft-float} changes the calling convention in the output file;
6806 therefore, it is only useful if you compile @emph{all} of a program with
6807 this option. In particular, you need to compile @file{libgcc.a}, the
6808 library that comes with GCC, with @option{-msoft-float} in order for
6811 @item -mfloat-abi=@var{name}
6813 Specifies which ABI to use for floating point values. Permissible values
6814 are: @samp{soft}, @samp{softfp} and @samp{hard}.
6816 @samp{soft} and @samp{hard} are equivalent to @option{-msoft-float}
6817 and @option{-mhard-float} respectively. @samp{softfp} allows the generation
6818 of floating point instructions, but still uses the soft-float calling
6821 @item -mlittle-endian
6822 @opindex mlittle-endian
6823 Generate code for a processor running in little-endian mode. This is
6824 the default for all standard configurations.
6827 @opindex mbig-endian
6828 Generate code for a processor running in big-endian mode; the default is
6829 to compile code for a little-endian processor.
6831 @item -mwords-little-endian
6832 @opindex mwords-little-endian
6833 This option only applies when generating code for big-endian processors.
6834 Generate code for a little-endian word order but a big-endian byte
6835 order. That is, a byte order of the form @samp{32107654}. Note: this
6836 option should only be used if you require compatibility with code for
6837 big-endian ARM processors generated by versions of the compiler prior to
6840 @item -mcpu=@var{name}
6842 This specifies the name of the target ARM processor. GCC uses this name
6843 to determine what kind of instructions it can emit when generating
6844 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
6845 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
6846 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
6847 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
6848 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
6849 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm7tdmi-s},
6850 @samp{arm8}, @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
6851 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
6852 @samp{arm920t}, @samp{arm922t}, @samp{arm946e-s}, @samp{arm966e-s},
6853 @samp{arm968e-s}, @samp{arm926ej-s}, @samp{arm940t}, @samp{arm9tdmi},
6854 @samp{arm10tdmi}, @samp{arm1020t}, @samp{arm1026ej-s},
6855 @samp{arm10e}, @samp{arm1020e}, @samp{arm1022e},
6856 @samp{arm1136j-s}, @samp{arm1136jf-s}, @samp{mpcore}, @samp{mpcorenovfp},
6857 @samp{arm1176jz-s}, @samp{arm1176jzf-s}, @samp{xscale}, @samp{iwmmxt},
6860 @itemx -mtune=@var{name}
6862 This option is very similar to the @option{-mcpu=} option, except that
6863 instead of specifying the actual target processor type, and hence
6864 restricting which instructions can be used, it specifies that GCC should
6865 tune the performance of the code as if the target were of the type
6866 specified in this option, but still choosing the instructions that it
6867 will generate based on the cpu specified by a @option{-mcpu=} option.
6868 For some ARM implementations better performance can be obtained by using
6871 @item -march=@var{name}
6873 This specifies the name of the target ARM architecture. GCC uses this
6874 name to determine what kind of instructions it can emit when generating
6875 assembly code. This option can be used in conjunction with or instead
6876 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
6877 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
6878 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{armv6}, @samp{armv6j},
6879 @samp{iwmmxt}, @samp{ep9312}.
6881 @item -mfpu=@var{name}
6882 @itemx -mfpe=@var{number}
6883 @itemx -mfp=@var{number}
6887 This specifies what floating point hardware (or hardware emulation) is
6888 available on the target. Permissible names are: @samp{fpa}, @samp{fpe2},
6889 @samp{fpe3}, @samp{maverick}, @samp{vfp}. @option{-mfp} and @option{-mfpe}
6890 are synonyms for @option{-mfpu}=@samp{fpe}@var{number}, for compatibility
6891 with older versions of GCC@.
6893 If @option{-msoft-float} is specified this specifies the format of
6894 floating point values.
6896 @item -mstructure-size-boundary=@var{n}
6897 @opindex mstructure-size-boundary
6898 The size of all structures and unions will be rounded up to a multiple
6899 of the number of bits set by this option. Permissible values are 8, 32
6900 and 64. The default value varies for different toolchains. For the COFF
6901 targeted toolchain the default value is 8. A value of 64 is only allowed
6902 if the underlying ABI supports it.
6904 Specifying the larger number can produce faster, more efficient code, but
6905 can also increase the size of the program. Different values are potentially
6906 incompatible. Code compiled with one value cannot necessarily expect to
6907 work with code or libraries compiled with another value, if they exchange
6908 information using structures or unions.
6910 @item -mabort-on-noreturn
6911 @opindex mabort-on-noreturn
6912 Generate a call to the function @code{abort} at the end of a
6913 @code{noreturn} function. It will be executed if the function tries to
6917 @itemx -mno-long-calls
6918 @opindex mlong-calls
6919 @opindex mno-long-calls
6920 Tells the compiler to perform function calls by first loading the
6921 address of the function into a register and then performing a subroutine
6922 call on this register. This switch is needed if the target function
6923 will lie outside of the 64 megabyte addressing range of the offset based
6924 version of subroutine call instruction.
6926 Even if this switch is enabled, not all function calls will be turned
6927 into long calls. The heuristic is that static functions, functions
6928 which have the @samp{short-call} attribute, functions that are inside
6929 the scope of a @samp{#pragma no_long_calls} directive and functions whose
6930 definitions have already been compiled within the current compilation
6931 unit, will not be turned into long calls. The exception to this rule is
6932 that weak function definitions, functions with the @samp{long-call}
6933 attribute or the @samp{section} attribute, and functions that are within
6934 the scope of a @samp{#pragma long_calls} directive, will always be
6935 turned into long calls.
6937 This feature is not enabled by default. Specifying
6938 @option{-mno-long-calls} will restore the default behavior, as will
6939 placing the function calls within the scope of a @samp{#pragma
6940 long_calls_off} directive. Note these switches have no effect on how
6941 the compiler generates code to handle function calls via function
6944 @item -mnop-fun-dllimport
6945 @opindex mnop-fun-dllimport
6946 Disable support for the @code{dllimport} attribute.
6948 @item -msingle-pic-base
6949 @opindex msingle-pic-base
6950 Treat the register used for PIC addressing as read-only, rather than
6951 loading it in the prologue for each function. The run-time system is
6952 responsible for initializing this register with an appropriate value
6953 before execution begins.
6955 @item -mpic-register=@var{reg}
6956 @opindex mpic-register
6957 Specify the register to be used for PIC addressing. The default is R10
6958 unless stack-checking is enabled, when R9 is used.
6960 @item -mcirrus-fix-invalid-insns
6961 @opindex mcirrus-fix-invalid-insns
6962 @opindex mno-cirrus-fix-invalid-insns
6963 Insert NOPs into the instruction stream to in order to work around
6964 problems with invalid Maverick instruction combinations. This option
6965 is only valid if the @option{-mcpu=ep9312} option has been used to
6966 enable generation of instructions for the Cirrus Maverick floating
6967 point co-processor. This option is not enabled by default, since the
6968 problem is only present in older Maverick implementations. The default
6969 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
6972 @item -mpoke-function-name
6973 @opindex mpoke-function-name
6974 Write the name of each function into the text section, directly
6975 preceding the function prologue. The generated code is similar to this:
6979 .ascii "arm_poke_function_name", 0
6982 .word 0xff000000 + (t1 - t0)
6983 arm_poke_function_name
6985 stmfd sp!, @{fp, ip, lr, pc@}
6989 When performing a stack backtrace, code can inspect the value of
6990 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
6991 location @code{pc - 12} and the top 8 bits are set, then we know that
6992 there is a function name embedded immediately preceding this location
6993 and has length @code{((pc[-3]) & 0xff000000)}.
6997 Generate code for the 16-bit Thumb instruction set. The default is to
6998 use the 32-bit ARM instruction set.
7001 @opindex mtpcs-frame
7002 Generate a stack frame that is compliant with the Thumb Procedure Call
7003 Standard for all non-leaf functions. (A leaf function is one that does
7004 not call any other functions.) The default is @option{-mno-tpcs-frame}.
7006 @item -mtpcs-leaf-frame
7007 @opindex mtpcs-leaf-frame
7008 Generate a stack frame that is compliant with the Thumb Procedure Call
7009 Standard for all leaf functions. (A leaf function is one that does
7010 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
7012 @item -mcallee-super-interworking
7013 @opindex mcallee-super-interworking
7014 Gives all externally visible functions in the file being compiled an ARM
7015 instruction set header which switches to Thumb mode before executing the
7016 rest of the function. This allows these functions to be called from
7017 non-interworking code.
7019 @item -mcaller-super-interworking
7020 @opindex mcaller-super-interworking
7021 Allows calls via function pointers (including virtual functions) to
7022 execute correctly regardless of whether the target code has been
7023 compiled for interworking or not. There is a small overhead in the cost
7024 of executing a function pointer if this option is enabled.
7029 @subsection AVR Options
7032 These options are defined for AVR implementations:
7035 @item -mmcu=@var{mcu}
7037 Specify ATMEL AVR instruction set or MCU type.
7039 Instruction set avr1 is for the minimal AVR core, not supported by the C
7040 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
7041 attiny11, attiny12, attiny15, attiny28).
7043 Instruction set avr2 (default) is for the classic AVR core with up to
7044 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
7045 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
7046 at90c8534, at90s8535).
7048 Instruction set avr3 is for the classic AVR core with up to 128K program
7049 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
7051 Instruction set avr4 is for the enhanced AVR core with up to 8K program
7052 memory space (MCU types: atmega8, atmega83, atmega85).
7054 Instruction set avr5 is for the enhanced AVR core with up to 128K program
7055 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
7056 atmega64, atmega128, at43usb355, at94k).
7060 Output instruction sizes to the asm file.
7062 @item -minit-stack=@var{N}
7063 @opindex minit-stack
7064 Specify the initial stack address, which may be a symbol or numeric value,
7065 @samp{__stack} is the default.
7067 @item -mno-interrupts
7068 @opindex mno-interrupts
7069 Generated code is not compatible with hardware interrupts.
7070 Code size will be smaller.
7072 @item -mcall-prologues
7073 @opindex mcall-prologues
7074 Functions prologues/epilogues expanded as call to appropriate
7075 subroutines. Code size will be smaller.
7077 @item -mno-tablejump
7078 @opindex mno-tablejump
7079 Do not generate tablejump insns which sometimes increase code size.
7082 @opindex mtiny-stack
7083 Change only the low 8 bits of the stack pointer.
7087 Assume int to be 8 bit integer. This affects the sizes of all types: A
7088 char will be 1 byte, an int will be 1 byte, an long will be 2 bytes
7089 and long long will be 4 bytes. Please note that this option does not
7090 comply to the C standards, but it will provide you with smaller code
7095 @subsection CRIS Options
7096 @cindex CRIS Options
7098 These options are defined specifically for the CRIS ports.
7101 @item -march=@var{architecture-type}
7102 @itemx -mcpu=@var{architecture-type}
7105 Generate code for the specified architecture. The choices for
7106 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
7107 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX@.
7108 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
7111 @item -mtune=@var{architecture-type}
7113 Tune to @var{architecture-type} everything applicable about the generated
7114 code, except for the ABI and the set of available instructions. The
7115 choices for @var{architecture-type} are the same as for
7116 @option{-march=@var{architecture-type}}.
7118 @item -mmax-stack-frame=@var{n}
7119 @opindex mmax-stack-frame
7120 Warn when the stack frame of a function exceeds @var{n} bytes.
7122 @item -melinux-stacksize=@var{n}
7123 @opindex melinux-stacksize
7124 Only available with the @samp{cris-axis-aout} target. Arranges for
7125 indications in the program to the kernel loader that the stack of the
7126 program should be set to @var{n} bytes.
7132 The options @option{-metrax4} and @option{-metrax100} are synonyms for
7133 @option{-march=v3} and @option{-march=v8} respectively.
7135 @item -mmul-bug-workaround
7136 @itemx -mno-mul-bug-workaround
7137 @opindex mmul-bug-workaround
7138 @opindex mno-mul-bug-workaround
7139 Work around a bug in the @code{muls} and @code{mulu} instructions for CPU
7140 models where it applies. This option is active by default.
7144 Enable CRIS-specific verbose debug-related information in the assembly
7145 code. This option also has the effect to turn off the @samp{#NO_APP}
7146 formatted-code indicator to the assembler at the beginning of the
7151 Do not use condition-code results from previous instruction; always emit
7152 compare and test instructions before use of condition codes.
7154 @item -mno-side-effects
7155 @opindex mno-side-effects
7156 Do not emit instructions with side-effects in addressing modes other than
7160 @itemx -mno-stack-align
7162 @itemx -mno-data-align
7163 @itemx -mconst-align
7164 @itemx -mno-const-align
7165 @opindex mstack-align
7166 @opindex mno-stack-align
7167 @opindex mdata-align
7168 @opindex mno-data-align
7169 @opindex mconst-align
7170 @opindex mno-const-align
7171 These options (no-options) arranges (eliminate arrangements) for the
7172 stack-frame, individual data and constants to be aligned for the maximum
7173 single data access size for the chosen CPU model. The default is to
7174 arrange for 32-bit alignment. ABI details such as structure layout are
7175 not affected by these options.
7183 Similar to the stack- data- and const-align options above, these options
7184 arrange for stack-frame, writable data and constants to all be 32-bit,
7185 16-bit or 8-bit aligned. The default is 32-bit alignment.
7187 @item -mno-prologue-epilogue
7188 @itemx -mprologue-epilogue
7189 @opindex mno-prologue-epilogue
7190 @opindex mprologue-epilogue
7191 With @option{-mno-prologue-epilogue}, the normal function prologue and
7192 epilogue that sets up the stack-frame are omitted and no return
7193 instructions or return sequences are generated in the code. Use this
7194 option only together with visual inspection of the compiled code: no
7195 warnings or errors are generated when call-saved registers must be saved,
7196 or storage for local variable needs to be allocated.
7202 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
7203 instruction sequences that load addresses for functions from the PLT part
7204 of the GOT rather than (traditional on other architectures) calls to the
7205 PLT@. The default is @option{-mgotplt}.
7209 Legacy no-op option only recognized with the cris-axis-aout target.
7213 Legacy no-op option only recognized with the cris-axis-elf and
7214 cris-axis-linux-gnu targets.
7218 Only recognized with the cris-axis-aout target, where it selects a
7219 GNU/linux-like multilib, include files and instruction set for
7224 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
7228 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
7229 to link with input-output functions from a simulator library. Code,
7230 initialized data and zero-initialized data are allocated consecutively.
7234 Like @option{-sim}, but pass linker options to locate initialized data at
7235 0x40000000 and zero-initialized data at 0x80000000.
7238 @node Darwin Options
7239 @subsection Darwin Options
7240 @cindex Darwin options
7242 These options are defined for all architectures running the Darwin operating
7245 FSF GCC on Darwin does not create ``fat'' object files; it will create
7246 an object file for the single architecture that it was built to
7247 target. Apple's GCC on Darwin does create ``fat'' files if multiple
7248 @option{-arch} options are used; it does so by running the compiler or
7249 linker multiple times and joining the results together with
7252 The subtype of the file created (like @samp{ppc7400} or @samp{ppc970} or
7253 @samp{i686}) is determined by the flags that specify the ISA
7254 that GCC is targetting, like @option{-mcpu} or @option{-march}. The
7255 @option{-force_cpusubtype_ALL} option can be used to override this.
7257 The Darwin tools vary in their behaviour when presented with an ISA
7258 mismatch. The assembler, @file{as}, will only permit instructions to
7259 be used that are valid for the subtype of the file it is generating,
7260 so you cannot put 64-bit instructions in an @samp{ppc750} object file.
7261 The linker for shared libraries, @file{/usr/bin/libtool}, will fail
7262 and print an error if asked to create a shared library with a less
7263 restrictive subtype than its input files (for instance, trying to put
7264 a @samp{ppc970} object file in a @samp{ppc7400} library). The linker
7265 for executables, @file{ld}, will quietly give the executable the most
7266 restrictive subtype of any of its input files.
7271 Add the framework directory @var{dir} to the head of the list of
7272 directories to be searched for header files. These directories are
7273 interleaved with those specified by @option{-I} options and are
7274 scanned in a left-to-right order.
7276 A framework directory is a directory with frameworks in it. A
7277 framework is a directory with a @samp{"Headers"} and/or
7278 @samp{"PrivateHeaders"} directory contained directly in it that ends
7279 in @samp{".framework"}. The name of a framework is the name of this
7280 directory excluding the @samp{".framework"}. Headers associated with
7281 the framework are found in one of those two directories, with
7282 @samp{"Headers"} being searched first. A subframework is a framework
7283 directory that is in a framework's @samp{"Frameworks"} directory.
7284 Includes of subframework headers can only appear in a header of a
7285 framework that contains the subframework, or in a sibling subframework
7286 header. Two subframeworks are siblings if they occur in the same
7287 framework. A subframework should not have the same name as a
7288 framework, a warning will be issued if this is violated. Currently a
7289 subframework cannot have subframeworks, in the future, the mechanism
7290 may be extended to support this. The standard frameworks can be found
7291 in @samp{"/System/Library/Frameworks"} and
7292 @samp{"/Library/Frameworks"}. An example include looks like
7293 @code{#include <Framework/header.h>}, where @samp{Framework} denotes
7294 the name of the framework and header.h is found in the
7295 @samp{"PrivateHeaders"} or @samp{"Headers"} directory.
7299 Emit debugging information for symbols that are used. For STABS
7300 debugging format, this enables @option{-feliminate-unused-debug-symbols}.
7301 This is by default ON@.
7305 Emit debugging information for all symbols and types.
7307 @item -mone-byte-bool
7308 @opindex -mone-byte-bool
7309 Override the defaults for @samp{bool} so that @samp{sizeof(bool)==1}.
7310 By default @samp{sizeof(bool)} is @samp{4} when compiling for
7311 Darwin/PowerPC and @samp{1} when compiling for Darwin/x86, so this
7312 option has no effect on x86.
7314 @strong{Warning:} The @option{-mone-byte-bool} switch causes GCC
7315 to generate code that is not binary compatible with code generated
7316 without that switch. Using this switch may require recompiling all
7317 other modules in a program, including system libraries. Use this
7318 switch to conform to a non-default data model.
7320 @item -mfix-and-continue
7321 @itemx -ffix-and-continue
7322 @itemx -findirect-data
7323 @opindex mfix-and-continue
7324 @opindex ffix-and-continue
7325 @opindex findirect-data
7326 Generate code suitable for fast turn around development. Needed to
7327 enable gdb to dynamically load @code{.o} files into already running
7328 programs. @option{-findirect-data} and @option{-ffix-and-continue}
7329 are provided for backwards compatibility.
7333 Loads all members of static archive libraries.
7334 See man ld(1) for more information.
7336 @item -arch_errors_fatal
7337 @opindex arch_errors_fatal
7338 Cause the errors having to do with files that have the wrong architecture
7342 @opindex bind_at_load
7343 Causes the output file to be marked such that the dynamic linker will
7344 bind all undefined references when the file is loaded or launched.
7348 Produce a Mach-o bundle format file.
7349 See man ld(1) for more information.
7351 @item -bundle_loader @var{executable}
7352 @opindex bundle_loader
7353 This option specifies the @var{executable} that will be loading the build
7354 output file being linked. See man ld(1) for more information.
7357 @opindex -dynamiclib
7358 When passed this option, GCC will produce a dynamic library instead of
7359 an executable when linking, using the Darwin @file{libtool} command.
7361 @item -force_cpusubtype_ALL
7362 @opindex -force_cpusubtype_ALL
7363 This causes GCC's output file to have the @var{ALL} subtype, instead of
7364 one controlled by the @option{-mcpu} or @option{-march} option.
7366 @item -allowable_client @var{client_name}
7368 @itemx -compatibility_version
7369 @itemx -current_version
7371 @itemx -dependency-file
7373 @itemx -dylinker_install_name
7375 @itemx -exported_symbols_list
7377 @itemx -flat_namespace
7378 @itemx -force_flat_namespace
7379 @itemx -headerpad_max_install_names
7382 @itemx -install_name
7383 @itemx -keep_private_externs
7384 @itemx -multi_module
7385 @itemx -multiply_defined
7386 @itemx -multiply_defined_unused
7388 @itemx -no_dead_strip_inits_and_terms
7389 @itemx -nofixprebinding
7392 @itemx -noseglinkedit
7393 @itemx -pagezero_size
7395 @itemx -prebind_all_twolevel_modules
7396 @itemx -private_bundle
7397 @itemx -read_only_relocs
7399 @itemx -sectobjectsymbols
7403 @itemx -sectobjectsymbols
7406 @itemx -segs_read_only_addr
7407 @itemx -segs_read_write_addr
7408 @itemx -seg_addr_table
7409 @itemx -seg_addr_table_filename
7412 @itemx -segs_read_only_addr
7413 @itemx -segs_read_write_addr
7414 @itemx -single_module
7417 @itemx -sub_umbrella
7418 @itemx -twolevel_namespace
7421 @itemx -unexported_symbols_list
7422 @itemx -weak_reference_mismatches
7425 @opindex allowable_client
7426 @opindex client_name
7427 @opindex compatibility_version
7428 @opindex current_version
7430 @opindex dependency-file
7432 @opindex dylinker_install_name
7434 @opindex exported_symbols_list
7436 @opindex flat_namespace
7437 @opindex force_flat_namespace
7438 @opindex headerpad_max_install_names
7441 @opindex install_name
7442 @opindex keep_private_externs
7443 @opindex multi_module
7444 @opindex multiply_defined
7445 @opindex multiply_defined_unused
7447 @opindex no_dead_strip_inits_and_terms
7448 @opindex nofixprebinding
7449 @opindex nomultidefs
7451 @opindex noseglinkedit
7452 @opindex pagezero_size
7454 @opindex prebind_all_twolevel_modules
7455 @opindex private_bundle
7456 @opindex read_only_relocs
7458 @opindex sectobjectsymbols
7462 @opindex sectobjectsymbols
7465 @opindex segs_read_only_addr
7466 @opindex segs_read_write_addr
7467 @opindex seg_addr_table
7468 @opindex seg_addr_table_filename
7469 @opindex seglinkedit
7471 @opindex segs_read_only_addr
7472 @opindex segs_read_write_addr
7473 @opindex single_module
7475 @opindex sub_library
7476 @opindex sub_umbrella
7477 @opindex twolevel_namespace
7480 @opindex unexported_symbols_list
7481 @opindex weak_reference_mismatches
7482 @opindex whatsloaded
7484 These options are passed to the Darwin linker. The Darwin linker man page
7485 describes them in detail.
7488 @node DEC Alpha Options
7489 @subsection DEC Alpha Options
7491 These @samp{-m} options are defined for the DEC Alpha implementations:
7494 @item -mno-soft-float
7496 @opindex mno-soft-float
7497 @opindex msoft-float
7498 Use (do not use) the hardware floating-point instructions for
7499 floating-point operations. When @option{-msoft-float} is specified,
7500 functions in @file{libgcc.a} will be used to perform floating-point
7501 operations. Unless they are replaced by routines that emulate the
7502 floating-point operations, or compiled in such a way as to call such
7503 emulations routines, these routines will issue floating-point
7504 operations. If you are compiling for an Alpha without floating-point
7505 operations, you must ensure that the library is built so as not to call
7508 Note that Alpha implementations without floating-point operations are
7509 required to have floating-point registers.
7514 @opindex mno-fp-regs
7515 Generate code that uses (does not use) the floating-point register set.
7516 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
7517 register set is not used, floating point operands are passed in integer
7518 registers as if they were integers and floating-point results are passed
7519 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
7520 so any function with a floating-point argument or return value called by code
7521 compiled with @option{-mno-fp-regs} must also be compiled with that
7524 A typical use of this option is building a kernel that does not use,
7525 and hence need not save and restore, any floating-point registers.
7529 The Alpha architecture implements floating-point hardware optimized for
7530 maximum performance. It is mostly compliant with the IEEE floating
7531 point standard. However, for full compliance, software assistance is
7532 required. This option generates code fully IEEE compliant code
7533 @emph{except} that the @var{inexact-flag} is not maintained (see below).
7534 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
7535 defined during compilation. The resulting code is less efficient but is
7536 able to correctly support denormalized numbers and exceptional IEEE
7537 values such as not-a-number and plus/minus infinity. Other Alpha
7538 compilers call this option @option{-ieee_with_no_inexact}.
7540 @item -mieee-with-inexact
7541 @opindex mieee-with-inexact
7542 This is like @option{-mieee} except the generated code also maintains
7543 the IEEE @var{inexact-flag}. Turning on this option causes the
7544 generated code to implement fully-compliant IEEE math. In addition to
7545 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
7546 macro. On some Alpha implementations the resulting code may execute
7547 significantly slower than the code generated by default. Since there is
7548 very little code that depends on the @var{inexact-flag}, you should
7549 normally not specify this option. Other Alpha compilers call this
7550 option @option{-ieee_with_inexact}.
7552 @item -mfp-trap-mode=@var{trap-mode}
7553 @opindex mfp-trap-mode
7554 This option controls what floating-point related traps are enabled.
7555 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
7556 The trap mode can be set to one of four values:
7560 This is the default (normal) setting. The only traps that are enabled
7561 are the ones that cannot be disabled in software (e.g., division by zero
7565 In addition to the traps enabled by @samp{n}, underflow traps are enabled
7569 Like @samp{su}, but the instructions are marked to be safe for software
7570 completion (see Alpha architecture manual for details).
7573 Like @samp{su}, but inexact traps are enabled as well.
7576 @item -mfp-rounding-mode=@var{rounding-mode}
7577 @opindex mfp-rounding-mode
7578 Selects the IEEE rounding mode. Other Alpha compilers call this option
7579 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
7584 Normal IEEE rounding mode. Floating point numbers are rounded towards
7585 the nearest machine number or towards the even machine number in case
7589 Round towards minus infinity.
7592 Chopped rounding mode. Floating point numbers are rounded towards zero.
7595 Dynamic rounding mode. A field in the floating point control register
7596 (@var{fpcr}, see Alpha architecture reference manual) controls the
7597 rounding mode in effect. The C library initializes this register for
7598 rounding towards plus infinity. Thus, unless your program modifies the
7599 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
7602 @item -mtrap-precision=@var{trap-precision}
7603 @opindex mtrap-precision
7604 In the Alpha architecture, floating point traps are imprecise. This
7605 means without software assistance it is impossible to recover from a
7606 floating trap and program execution normally needs to be terminated.
7607 GCC can generate code that can assist operating system trap handlers
7608 in determining the exact location that caused a floating point trap.
7609 Depending on the requirements of an application, different levels of
7610 precisions can be selected:
7614 Program precision. This option is the default and means a trap handler
7615 can only identify which program caused a floating point exception.
7618 Function precision. The trap handler can determine the function that
7619 caused a floating point exception.
7622 Instruction precision. The trap handler can determine the exact
7623 instruction that caused a floating point exception.
7626 Other Alpha compilers provide the equivalent options called
7627 @option{-scope_safe} and @option{-resumption_safe}.
7629 @item -mieee-conformant
7630 @opindex mieee-conformant
7631 This option marks the generated code as IEEE conformant. You must not
7632 use this option unless you also specify @option{-mtrap-precision=i} and either
7633 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
7634 is to emit the line @samp{.eflag 48} in the function prologue of the
7635 generated assembly file. Under DEC Unix, this has the effect that
7636 IEEE-conformant math library routines will be linked in.
7638 @item -mbuild-constants
7639 @opindex mbuild-constants
7640 Normally GCC examines a 32- or 64-bit integer constant to
7641 see if it can construct it from smaller constants in two or three
7642 instructions. If it cannot, it will output the constant as a literal and
7643 generate code to load it from the data segment at runtime.
7645 Use this option to require GCC to construct @emph{all} integer constants
7646 using code, even if it takes more instructions (the maximum is six).
7648 You would typically use this option to build a shared library dynamic
7649 loader. Itself a shared library, it must relocate itself in memory
7650 before it can find the variables and constants in its own data segment.
7656 Select whether to generate code to be assembled by the vendor-supplied
7657 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
7675 Indicate whether GCC should generate code to use the optional BWX,
7676 CIX, FIX and MAX instruction sets. The default is to use the instruction
7677 sets supported by the CPU type specified via @option{-mcpu=} option or that
7678 of the CPU on which GCC was built if none was specified.
7683 @opindex mfloat-ieee
7684 Generate code that uses (does not use) VAX F and G floating point
7685 arithmetic instead of IEEE single and double precision.
7687 @item -mexplicit-relocs
7688 @itemx -mno-explicit-relocs
7689 @opindex mexplicit-relocs
7690 @opindex mno-explicit-relocs
7691 Older Alpha assemblers provided no way to generate symbol relocations
7692 except via assembler macros. Use of these macros does not allow
7693 optimal instruction scheduling. GNU binutils as of version 2.12
7694 supports a new syntax that allows the compiler to explicitly mark
7695 which relocations should apply to which instructions. This option
7696 is mostly useful for debugging, as GCC detects the capabilities of
7697 the assembler when it is built and sets the default accordingly.
7701 @opindex msmall-data
7702 @opindex mlarge-data
7703 When @option{-mexplicit-relocs} is in effect, static data is
7704 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
7705 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
7706 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
7707 16-bit relocations off of the @code{$gp} register. This limits the
7708 size of the small data area to 64KB, but allows the variables to be
7709 directly accessed via a single instruction.
7711 The default is @option{-mlarge-data}. With this option the data area
7712 is limited to just below 2GB@. Programs that require more than 2GB of
7713 data must use @code{malloc} or @code{mmap} to allocate the data in the
7714 heap instead of in the program's data segment.
7716 When generating code for shared libraries, @option{-fpic} implies
7717 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
7721 @opindex msmall-text
7722 @opindex mlarge-text
7723 When @option{-msmall-text} is used, the compiler assumes that the
7724 code of the entire program (or shared library) fits in 4MB, and is
7725 thus reachable with a branch instruction. When @option{-msmall-data}
7726 is used, the compiler can assume that all local symbols share the
7727 same @code{$gp} value, and thus reduce the number of instructions
7728 required for a function call from 4 to 1.
7730 The default is @option{-mlarge-text}.
7732 @item -mcpu=@var{cpu_type}
7734 Set the instruction set and instruction scheduling parameters for
7735 machine type @var{cpu_type}. You can specify either the @samp{EV}
7736 style name or the corresponding chip number. GCC supports scheduling
7737 parameters for the EV4, EV5 and EV6 family of processors and will
7738 choose the default values for the instruction set from the processor
7739 you specify. If you do not specify a processor type, GCC will default
7740 to the processor on which the compiler was built.
7742 Supported values for @var{cpu_type} are
7748 Schedules as an EV4 and has no instruction set extensions.
7752 Schedules as an EV5 and has no instruction set extensions.
7756 Schedules as an EV5 and supports the BWX extension.
7761 Schedules as an EV5 and supports the BWX and MAX extensions.
7765 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
7769 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
7772 @item -mtune=@var{cpu_type}
7774 Set only the instruction scheduling parameters for machine type
7775 @var{cpu_type}. The instruction set is not changed.
7777 @item -mmemory-latency=@var{time}
7778 @opindex mmemory-latency
7779 Sets the latency the scheduler should assume for typical memory
7780 references as seen by the application. This number is highly
7781 dependent on the memory access patterns used by the application
7782 and the size of the external cache on the machine.
7784 Valid options for @var{time} are
7788 A decimal number representing clock cycles.
7794 The compiler contains estimates of the number of clock cycles for
7795 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
7796 (also called Dcache, Scache, and Bcache), as well as to main memory.
7797 Note that L3 is only valid for EV5.
7802 @node DEC Alpha/VMS Options
7803 @subsection DEC Alpha/VMS Options
7805 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
7808 @item -mvms-return-codes
7809 @opindex mvms-return-codes
7810 Return VMS condition codes from main. The default is to return POSIX
7811 style condition (e.g.@ error) codes.
7815 @subsection FRV Options
7822 Only use the first 32 general purpose registers.
7827 Use all 64 general purpose registers.
7832 Use only the first 32 floating point registers.
7837 Use all 64 floating point registers
7840 @opindex mhard-float
7842 Use hardware instructions for floating point operations.
7845 @opindex msoft-float
7847 Use library routines for floating point operations.
7852 Dynamically allocate condition code registers.
7857 Do not try to dynamically allocate condition code registers, only
7858 use @code{icc0} and @code{fcc0}.
7863 Change ABI to use double word insns.
7868 Do not use double word instructions.
7873 Use floating point double instructions.
7878 Do not use floating point double instructions.
7883 Use media instructions.
7888 Do not use media instructions.
7893 Use multiply and add/subtract instructions.
7898 Do not use multiply and add/subtract instructions.
7903 Select the FDPIC ABI, that uses function descriptors to represent
7904 pointers to functions. Without any PIC/PIE-related options, it
7905 implies @option{-fPIE}. With @option{-fpic} or @option{-fpie}, it
7906 assumes GOT entries and small data are within a 12-bit range from the
7907 GOT base address; with @option{-fPIC} or @option{-fPIE}, GOT offsets
7908 are computed with 32 bits.
7911 @opindex minline-plt
7913 Enable inlining of PLT entries in function calls to functions that are
7914 not known to bind locally. It has no effect without @option{-mfdpic}.
7915 It's enabled by default if optimizing for speed and compiling for
7916 shared libraries (i.e., @option{-fPIC} or @option{-fpic}), or when an
7917 optimization option such as @option{-O3} or above is present in the
7923 Assume a large TLS segment when generating thread-local code.
7928 Do not assume a large TLS segment when generating thread-local code.
7933 Enable the use of @code{GPREL} relocations in the FDPIC ABI for data
7934 that is known to be in read-only sections. It's enabled by default,
7935 except for @option{-fpic} or @option{-fpie}: even though it may help
7936 make the global offset table smaller, it trades 1 instruction for 4.
7937 With @option{-fPIC} or @option{-fPIE}, it trades 3 instructions for 4,
7938 one of which may be shared by multiple symbols, and it avoids the need
7939 for a GOT entry for the referenced symbol, so it's more likely to be a
7940 win. If it is not, @option{-mno-gprel-ro} can be used to disable it.
7942 @item -multilib-library-pic
7943 @opindex multilib-library-pic
7945 Link with the (library, not FD) pic libraries. It's implied by
7946 @option{-mlibrary-pic}, as well as by @option{-fPIC} and
7947 @option{-fpic} without @option{-mfdpic}. You should never have to use
7953 Follow the EABI requirement of always creating a frame pointer whenever
7954 a stack frame is allocated. This option is enabled by default and can
7955 be disabled with @option{-mno-linked-fp}.
7958 @opindex mlong-calls
7960 Use indirect addressing to call functions outside the current
7961 compilation unit. This allows the functions to be placed anywhere
7962 within the 32-bit address space.
7964 @item -malign-labels
7965 @opindex malign-labels
7967 Try to align labels to an 8-byte boundary by inserting nops into the
7968 previous packet. This option only has an effect when VLIW packing
7969 is enabled. It doesn't create new packets; it merely adds nops to
7973 @opindex mlibrary-pic
7975 Generate position-independent EABI code.
7980 Use only the first four media accumulator registers.
7985 Use all eight media accumulator registers.
7990 Pack VLIW instructions.
7995 Do not pack VLIW instructions.
8000 Do not mark ABI switches in e_flags.
8005 Enable the use of conditional-move instructions (default).
8007 This switch is mainly for debugging the compiler and will likely be removed
8008 in a future version.
8010 @item -mno-cond-move
8011 @opindex mno-cond-move
8013 Disable the use of conditional-move instructions.
8015 This switch is mainly for debugging the compiler and will likely be removed
8016 in a future version.
8021 Enable the use of conditional set instructions (default).
8023 This switch is mainly for debugging the compiler and will likely be removed
8024 in a future version.
8029 Disable the use of conditional set instructions.
8031 This switch is mainly for debugging the compiler and will likely be removed
8032 in a future version.
8037 Enable the use of conditional execution (default).
8039 This switch is mainly for debugging the compiler and will likely be removed
8040 in a future version.
8042 @item -mno-cond-exec
8043 @opindex mno-cond-exec
8045 Disable the use of conditional execution.
8047 This switch is mainly for debugging the compiler and will likely be removed
8048 in a future version.
8051 @opindex mvliw-branch
8053 Run a pass to pack branches into VLIW instructions (default).
8055 This switch is mainly for debugging the compiler and will likely be removed
8056 in a future version.
8058 @item -mno-vliw-branch
8059 @opindex mno-vliw-branch
8061 Do not run a pass to pack branches into VLIW instructions.
8063 This switch is mainly for debugging the compiler and will likely be removed
8064 in a future version.
8066 @item -mmulti-cond-exec
8067 @opindex mmulti-cond-exec
8069 Enable optimization of @code{&&} and @code{||} in conditional execution
8072 This switch is mainly for debugging the compiler and will likely be removed
8073 in a future version.
8075 @item -mno-multi-cond-exec
8076 @opindex mno-multi-cond-exec
8078 Disable optimization of @code{&&} and @code{||} in conditional execution.
8080 This switch is mainly for debugging the compiler and will likely be removed
8081 in a future version.
8083 @item -mnested-cond-exec
8084 @opindex mnested-cond-exec
8086 Enable nested conditional execution optimizations (default).
8088 This switch is mainly for debugging the compiler and will likely be removed
8089 in a future version.
8091 @item -mno-nested-cond-exec
8092 @opindex mno-nested-cond-exec
8094 Disable nested conditional execution optimizations.
8096 This switch is mainly for debugging the compiler and will likely be removed
8097 in a future version.
8099 @item -mtomcat-stats
8100 @opindex mtomcat-stats
8102 Cause gas to print out tomcat statistics.
8104 @item -mcpu=@var{cpu}
8107 Select the processor type for which to generate code. Possible values are
8108 @samp{frv}, @samp{fr550}, @samp{tomcat}, @samp{fr500}, @samp{fr450},
8109 @samp{fr405}, @samp{fr400}, @samp{fr300} and @samp{simple}.
8113 @node H8/300 Options
8114 @subsection H8/300 Options
8116 These @samp{-m} options are defined for the H8/300 implementations:
8121 Shorten some address references at link time, when possible; uses the
8122 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
8123 ld, Using ld}, for a fuller description.
8127 Generate code for the H8/300H@.
8131 Generate code for the H8S@.
8135 Generate code for the H8S and H8/300H in the normal mode. This switch
8136 must be used either with @option{-mh} or @option{-ms}.
8140 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
8144 Make @code{int} data 32 bits by default.
8148 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
8149 The default for the H8/300H and H8S is to align longs and floats on 4
8151 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
8152 This option has no effect on the H8/300.
8156 @subsection HPPA Options
8157 @cindex HPPA Options
8159 These @samp{-m} options are defined for the HPPA family of computers:
8162 @item -march=@var{architecture-type}
8164 Generate code for the specified architecture. The choices for
8165 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
8166 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
8167 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
8168 architecture option for your machine. Code compiled for lower numbered
8169 architectures will run on higher numbered architectures, but not the
8173 @itemx -mpa-risc-1-1
8174 @itemx -mpa-risc-2-0
8175 @opindex mpa-risc-1-0
8176 @opindex mpa-risc-1-1
8177 @opindex mpa-risc-2-0
8178 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
8181 @opindex mbig-switch
8182 Generate code suitable for big switch tables. Use this option only if
8183 the assembler/linker complain about out of range branches within a switch
8186 @item -mjump-in-delay
8187 @opindex mjump-in-delay
8188 Fill delay slots of function calls with unconditional jump instructions
8189 by modifying the return pointer for the function call to be the target
8190 of the conditional jump.
8192 @item -mdisable-fpregs
8193 @opindex mdisable-fpregs
8194 Prevent floating point registers from being used in any manner. This is
8195 necessary for compiling kernels which perform lazy context switching of
8196 floating point registers. If you use this option and attempt to perform
8197 floating point operations, the compiler will abort.
8199 @item -mdisable-indexing
8200 @opindex mdisable-indexing
8201 Prevent the compiler from using indexing address modes. This avoids some
8202 rather obscure problems when compiling MIG generated code under MACH@.
8204 @item -mno-space-regs
8205 @opindex mno-space-regs
8206 Generate code that assumes the target has no space registers. This allows
8207 GCC to generate faster indirect calls and use unscaled index address modes.
8209 Such code is suitable for level 0 PA systems and kernels.
8211 @item -mfast-indirect-calls
8212 @opindex mfast-indirect-calls
8213 Generate code that assumes calls never cross space boundaries. This
8214 allows GCC to emit code which performs faster indirect calls.
8216 This option will not work in the presence of shared libraries or nested
8219 @item -mfixed-range=@var{register-range}
8220 @opindex mfixed-range
8221 Generate code treating the given register range as fixed registers.
8222 A fixed register is one that the register allocator can not use. This is
8223 useful when compiling kernel code. A register range is specified as
8224 two registers separated by a dash. Multiple register ranges can be
8225 specified separated by a comma.
8227 @item -mlong-load-store
8228 @opindex mlong-load-store
8229 Generate 3-instruction load and store sequences as sometimes required by
8230 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
8233 @item -mportable-runtime
8234 @opindex mportable-runtime
8235 Use the portable calling conventions proposed by HP for ELF systems.
8239 Enable the use of assembler directives only GAS understands.
8241 @item -mschedule=@var{cpu-type}
8243 Schedule code according to the constraints for the machine type
8244 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
8245 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
8246 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
8247 proper scheduling option for your machine. The default scheduling is
8251 @opindex mlinker-opt
8252 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
8253 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
8254 linkers in which they give bogus error messages when linking some programs.
8257 @opindex msoft-float
8258 Generate output containing library calls for floating point.
8259 @strong{Warning:} the requisite libraries are not available for all HPPA
8260 targets. Normally the facilities of the machine's usual C compiler are
8261 used, but this cannot be done directly in cross-compilation. You must make
8262 your own arrangements to provide suitable library functions for
8263 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
8264 does provide software floating point support.
8266 @option{-msoft-float} changes the calling convention in the output file;
8267 therefore, it is only useful if you compile @emph{all} of a program with
8268 this option. In particular, you need to compile @file{libgcc.a}, the
8269 library that comes with GCC, with @option{-msoft-float} in order for
8274 Generate the predefine, @code{_SIO}, for server IO@. The default is
8275 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
8276 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO@. These
8277 options are available under HP-UX and HI-UX@.
8281 Use GNU ld specific options. This passes @option{-shared} to ld when
8282 building a shared library. It is the default when GCC is configured,
8283 explicitly or implicitly, with the GNU linker. This option does not
8284 have any affect on which ld is called, it only changes what parameters
8285 are passed to that ld. The ld that is called is determined by the
8286 @option{--with-ld} configure option, GCC's program search path, and
8287 finally by the user's @env{PATH}. The linker used by GCC can be printed
8288 using @samp{which `gcc -print-prog-name=ld`}.
8292 Use HP ld specific options. This passes @option{-b} to ld when building
8293 a shared library and passes @option{+Accept TypeMismatch} to ld on all
8294 links. It is the default when GCC is configured, explicitly or
8295 implicitly, with the HP linker. This option does not have any affect on
8296 which ld is called, it only changes what parameters are passed to that
8297 ld. The ld that is called is determined by the @option{--with-ld}
8298 configure option, GCC's program search path, and finally by the user's
8299 @env{PATH}. The linker used by GCC can be printed using @samp{which
8300 `gcc -print-prog-name=ld`}.
8303 @opindex mno-long-calls
8304 Generate code that uses long call sequences. This ensures that a call
8305 is always able to reach linker generated stubs. The default is to generate
8306 long calls only when the distance from the call site to the beginning
8307 of the function or translation unit, as the case may be, exceeds a
8308 predefined limit set by the branch type being used. The limits for
8309 normal calls are 7,600,000 and 240,000 bytes, respectively for the
8310 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
8313 Distances are measured from the beginning of functions when using the
8314 @option{-ffunction-sections} option, or when using the @option{-mgas}
8315 and @option{-mno-portable-runtime} options together under HP-UX with
8318 It is normally not desirable to use this option as it will degrade
8319 performance. However, it may be useful in large applications,
8320 particularly when partial linking is used to build the application.
8322 The types of long calls used depends on the capabilities of the
8323 assembler and linker, and the type of code being generated. The
8324 impact on systems that support long absolute calls, and long pic
8325 symbol-difference or pc-relative calls should be relatively small.
8326 However, an indirect call is used on 32-bit ELF systems in pic code
8327 and it is quite long.
8329 @item -munix=@var{unix-std}
8331 Generate compiler predefines and select a startfile for the specified
8332 UNIX standard. The choices for @var{unix-std} are @samp{93}, @samp{95}
8333 and @samp{98}. @samp{93} is supported on all HP-UX versions. @samp{95}
8334 is available on HP-UX 10.10 and later. @samp{98} is available on HP-UX
8335 11.11 and later. The default values are @samp{93} for HP-UX 10.00,
8336 @samp{95} for HP-UX 10.10 though to 11.00, and @samp{98} for HP-UX 11.11
8339 @option{-munix=93} provides the same predefines as GCC 3.3 and 3.4.
8340 @option{-munix=95} provides additional predefines for @code{XOPEN_UNIX}
8341 and @code{_XOPEN_SOURCE_EXTENDED}, and the startfile @file{unix95.o}.
8342 @option{-munix=98} provides additional predefines for @code{_XOPEN_UNIX},
8343 @code{_XOPEN_SOURCE_EXTENDED}, @code{_INCLUDE__STDC_A1_SOURCE} and
8344 @code{_INCLUDE_XOPEN_SOURCE_500}, and the startfile @file{unix98.o}.
8346 It is @emph{important} to note that this option changes the interfaces
8347 for various library routines. It also affects the operational behavior
8348 of the C library. Thus, @emph{extreme} care is needed in using this
8351 Library code that is intended to operate with more than one UNIX
8352 standard must test, set and restore the variable @var{__xpg4_extended_mask}
8353 as appropriate. Most GNU software doesn't provide this capability.
8357 Suppress the generation of link options to search libdld.sl when the
8358 @option{-static} option is specified on HP-UX 10 and later.
8362 The HP-UX implementation of setlocale in libc has a dependency on
8363 libdld.sl. There isn't an archive version of libdld.sl. Thus,
8364 when the @option{-static} option is specified, special link options
8365 are needed to resolve this dependency.
8367 On HP-UX 10 and later, the GCC driver adds the necessary options to
8368 link with libdld.sl when the @option{-static} option is specified.
8369 This causes the resulting binary to be dynamic. On the 64-bit port,
8370 the linkers generate dynamic binaries by default in any case. The
8371 @option{-nolibdld} option can be used to prevent the GCC driver from
8372 adding these link options.
8376 Add support for multithreading with the @dfn{dce thread} library
8377 under HP-UX@. This option sets flags for both the preprocessor and
8381 @node i386 and x86-64 Options
8382 @subsection Intel 386 and AMD x86-64 Options
8383 @cindex i386 Options
8384 @cindex x86-64 Options
8385 @cindex Intel 386 Options
8386 @cindex AMD x86-64 Options
8388 These @samp{-m} options are defined for the i386 and x86-64 family of
8392 @item -mtune=@var{cpu-type}
8394 Tune to @var{cpu-type} everything applicable about the generated code, except
8395 for the ABI and the set of available instructions. The choices for
8399 Original Intel's i386 CPU@.
8401 Intel's i486 CPU@. (No scheduling is implemented for this chip.)
8403 Intel Pentium CPU with no MMX support.
8405 Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
8406 @item i686, pentiumpro
8407 Intel PentiumPro CPU@.
8409 Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
8410 @item pentium3, pentium3m
8411 Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set
8414 Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set
8415 support. Used by Centrino notebooks.
8416 @item pentium4, pentium4m
8417 Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
8419 Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction
8422 Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
8423 SSE2 and SSE3 instruction set support.
8425 AMD K6 CPU with MMX instruction set support.
8427 Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
8428 @item athlon, athlon-tbird
8429 AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions
8431 @item athlon-4, athlon-xp, athlon-mp
8432 Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE
8433 instruction set support.
8434 @item k8, opteron, athlon64, athlon-fx
8435 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets
8436 MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
8438 IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction
8441 IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW!
8442 instruction set support.
8444 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is
8445 implemented for this chip.)
8447 Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is
8448 implemented for this chip.)
8451 While picking a specific @var{cpu-type} will schedule things appropriately
8452 for that particular chip, the compiler will not generate any code that
8453 does not run on the i386 without the @option{-march=@var{cpu-type}} option
8456 @item -march=@var{cpu-type}
8458 Generate instructions for the machine type @var{cpu-type}. The choices
8459 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
8460 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
8462 @item -mcpu=@var{cpu-type}
8464 A deprecated synonym for @option{-mtune}.
8473 @opindex mpentiumpro
8474 These options are synonyms for @option{-mtune=i386}, @option{-mtune=i486},
8475 @option{-mtune=pentium}, and @option{-mtune=pentiumpro} respectively.
8476 These synonyms are deprecated.
8478 @item -mfpmath=@var{unit}
8480 Generate floating point arithmetics for selected unit @var{unit}. The choices
8485 Use the standard 387 floating point coprocessor present majority of chips and
8486 emulated otherwise. Code compiled with this option will run almost everywhere.
8487 The temporary results are computed in 80bit precision instead of precision
8488 specified by the type resulting in slightly different results compared to most
8489 of other chips. See @option{-ffloat-store} for more detailed description.
8491 This is the default choice for i386 compiler.
8494 Use scalar floating point instructions present in the SSE instruction set.
8495 This instruction set is supported by Pentium3 and newer chips, in the AMD line
8496 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
8497 instruction set supports only single precision arithmetics, thus the double and
8498 extended precision arithmetics is still done using 387. Later version, present
8499 only in Pentium4 and the future AMD x86-64 chips supports double precision
8502 For the i386 compiler, you need to use @option{-march=@var{cpu-type}}, @option{-msse}
8503 or @option{-msse2} switches to enable SSE extensions and make this option
8504 effective. For the x86-64 compiler, these extensions are enabled by default.
8506 The resulting code should be considerably faster in the majority of cases and avoid
8507 the numerical instability problems of 387 code, but may break some existing
8508 code that expects temporaries to be 80bit.
8510 This is the default choice for the x86-64 compiler.
8513 Attempt to utilize both instruction sets at once. This effectively double the
8514 amount of available registers and on chips with separate execution units for
8515 387 and SSE the execution resources too. Use this option with care, as it is
8516 still experimental, because the GCC register allocator does not model separate
8517 functional units well resulting in instable performance.
8520 @item -masm=@var{dialect}
8521 @opindex masm=@var{dialect}
8522 Output asm instructions using selected @var{dialect}. Supported choices are
8523 @samp{intel} or @samp{att} (the default one).
8528 @opindex mno-ieee-fp
8529 Control whether or not the compiler uses IEEE floating point
8530 comparisons. These handle correctly the case where the result of a
8531 comparison is unordered.
8534 @opindex msoft-float
8535 Generate output containing library calls for floating point.
8536 @strong{Warning:} the requisite libraries are not part of GCC@.
8537 Normally the facilities of the machine's usual C compiler are used, but
8538 this can't be done directly in cross-compilation. You must make your
8539 own arrangements to provide suitable library functions for
8542 On machines where a function returns floating point results in the 80387
8543 register stack, some floating point opcodes may be emitted even if
8544 @option{-msoft-float} is used.
8546 @item -mno-fp-ret-in-387
8547 @opindex mno-fp-ret-in-387
8548 Do not use the FPU registers for return values of functions.
8550 The usual calling convention has functions return values of types
8551 @code{float} and @code{double} in an FPU register, even if there
8552 is no FPU@. The idea is that the operating system should emulate
8555 The option @option{-mno-fp-ret-in-387} causes such values to be returned
8556 in ordinary CPU registers instead.
8558 @item -mno-fancy-math-387
8559 @opindex mno-fancy-math-387
8560 Some 387 emulators do not support the @code{sin}, @code{cos} and
8561 @code{sqrt} instructions for the 387. Specify this option to avoid
8562 generating those instructions. This option is the default on FreeBSD,
8563 OpenBSD and NetBSD@. This option is overridden when @option{-march}
8564 indicates that the target cpu will always have an FPU and so the
8565 instruction will not need emulation. As of revision 2.6.1, these
8566 instructions are not generated unless you also use the
8567 @option{-funsafe-math-optimizations} switch.
8569 @item -malign-double
8570 @itemx -mno-align-double
8571 @opindex malign-double
8572 @opindex mno-align-double
8573 Control whether GCC aligns @code{double}, @code{long double}, and
8574 @code{long long} variables on a two word boundary or a one word
8575 boundary. Aligning @code{double} variables on a two word boundary will
8576 produce code that runs somewhat faster on a @samp{Pentium} at the
8577 expense of more memory.
8579 @strong{Warning:} if you use the @option{-malign-double} switch,
8580 structures containing the above types will be aligned differently than
8581 the published application binary interface specifications for the 386
8582 and will not be binary compatible with structures in code compiled
8583 without that switch.
8585 @item -m96bit-long-double
8586 @itemx -m128bit-long-double
8587 @opindex m96bit-long-double
8588 @opindex m128bit-long-double
8589 These switches control the size of @code{long double} type. The i386
8590 application binary interface specifies the size to be 96 bits,
8591 so @option{-m96bit-long-double} is the default in 32 bit mode.
8593 Modern architectures (Pentium and newer) would prefer @code{long double}
8594 to be aligned to an 8 or 16 byte boundary. In arrays or structures
8595 conforming to the ABI, this would not be possible. So specifying a
8596 @option{-m128bit-long-double} will align @code{long double}
8597 to a 16 byte boundary by padding the @code{long double} with an additional
8600 In the x86-64 compiler, @option{-m128bit-long-double} is the default choice as
8601 its ABI specifies that @code{long double} is to be aligned on 16 byte boundary.
8603 Notice that neither of these options enable any extra precision over the x87
8604 standard of 80 bits for a @code{long double}.
8606 @strong{Warning:} if you override the default value for your target ABI, the
8607 structures and arrays containing @code{long double} variables will change
8608 their size as well as function calling convention for function taking
8609 @code{long double} will be modified. Hence they will not be binary
8610 compatible with arrays or structures in code compiled without that switch.
8614 @itemx -mno-svr3-shlib
8615 @opindex msvr3-shlib
8616 @opindex mno-svr3-shlib
8617 Control whether GCC places uninitialized local variables into the
8618 @code{bss} or @code{data} segments. @option{-msvr3-shlib} places them
8619 into @code{bss}. These options are meaningful only on System V Release 3.
8623 Use a different function-calling convention, in which functions that
8624 take a fixed number of arguments return with the @code{ret} @var{num}
8625 instruction, which pops their arguments while returning. This saves one
8626 instruction in the caller since there is no need to pop the arguments
8629 You can specify that an individual function is called with this calling
8630 sequence with the function attribute @samp{stdcall}. You can also
8631 override the @option{-mrtd} option by using the function attribute
8632 @samp{cdecl}. @xref{Function Attributes}.
8634 @strong{Warning:} this calling convention is incompatible with the one
8635 normally used on Unix, so you cannot use it if you need to call
8636 libraries compiled with the Unix compiler.
8638 Also, you must provide function prototypes for all functions that
8639 take variable numbers of arguments (including @code{printf});
8640 otherwise incorrect code will be generated for calls to those
8643 In addition, seriously incorrect code will result if you call a
8644 function with too many arguments. (Normally, extra arguments are
8645 harmlessly ignored.)
8647 @item -mregparm=@var{num}
8649 Control how many registers are used to pass integer arguments. By
8650 default, no registers are used to pass arguments, and at most 3
8651 registers can be used. You can control this behavior for a specific
8652 function by using the function attribute @samp{regparm}.
8653 @xref{Function Attributes}.
8655 @strong{Warning:} if you use this switch, and
8656 @var{num} is nonzero, then you must build all modules with the same
8657 value, including any libraries. This includes the system libraries and
8660 @item -mpreferred-stack-boundary=@var{num}
8661 @opindex mpreferred-stack-boundary
8662 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
8663 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
8664 the default is 4 (16 bytes or 128 bits), except when optimizing for code
8665 size (@option{-Os}), in which case the default is the minimum correct
8666 alignment (4 bytes for x86, and 8 bytes for x86-64).
8668 On Pentium and PentiumPro, @code{double} and @code{long double} values
8669 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
8670 suffer significant run time performance penalties. On Pentium III, the
8671 Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
8672 penalties if it is not 16 byte aligned.
8674 To ensure proper alignment of this values on the stack, the stack boundary
8675 must be as aligned as that required by any value stored on the stack.
8676 Further, every function must be generated such that it keeps the stack
8677 aligned. Thus calling a function compiled with a higher preferred
8678 stack boundary from a function compiled with a lower preferred stack
8679 boundary will most likely misalign the stack. It is recommended that
8680 libraries that use callbacks always use the default setting.
8682 This extra alignment does consume extra stack space, and generally
8683 increases code size. Code that is sensitive to stack space usage, such
8684 as embedded systems and operating system kernels, may want to reduce the
8685 preferred alignment to @option{-mpreferred-stack-boundary=2}.
8703 These switches enable or disable the use of built-in functions that allow
8704 direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow extensions of the
8707 @xref{X86 Built-in Functions}, for details of the functions enabled
8708 and disabled by these switches.
8710 To have SSE/SSE2 instructions generated automatically from floating-point
8711 code, see @option{-mfpmath=sse}.
8714 @itemx -mno-push-args
8716 @opindex mno-push-args
8717 Use PUSH operations to store outgoing parameters. This method is shorter
8718 and usually equally fast as method using SUB/MOV operations and is enabled
8719 by default. In some cases disabling it may improve performance because of
8720 improved scheduling and reduced dependencies.
8722 @item -maccumulate-outgoing-args
8723 @opindex maccumulate-outgoing-args
8724 If enabled, the maximum amount of space required for outgoing arguments will be
8725 computed in the function prologue. This is faster on most modern CPUs
8726 because of reduced dependencies, improved scheduling and reduced stack usage
8727 when preferred stack boundary is not equal to 2. The drawback is a notable
8728 increase in code size. This switch implies @option{-mno-push-args}.
8732 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
8733 on thread-safe exception handling must compile and link all code with the
8734 @option{-mthreads} option. When compiling, @option{-mthreads} defines
8735 @option{-D_MT}; when linking, it links in a special thread helper library
8736 @option{-lmingwthrd} which cleans up per thread exception handling data.
8738 @item -mno-align-stringops
8739 @opindex mno-align-stringops
8740 Do not align destination of inlined string operations. This switch reduces
8741 code size and improves performance in case the destination is already aligned,
8742 but GCC doesn't know about it.
8744 @item -minline-all-stringops
8745 @opindex minline-all-stringops
8746 By default GCC inlines string operations only when destination is known to be
8747 aligned at least to 4 byte boundary. This enables more inlining, increase code
8748 size, but may improve performance of code that depends on fast memcpy, strlen
8749 and memset for short lengths.
8751 @item -momit-leaf-frame-pointer
8752 @opindex momit-leaf-frame-pointer
8753 Don't keep the frame pointer in a register for leaf functions. This
8754 avoids the instructions to save, set up and restore frame pointers and
8755 makes an extra register available in leaf functions. The option
8756 @option{-fomit-frame-pointer} removes the frame pointer for all functions
8757 which might make debugging harder.
8759 @item -mtls-direct-seg-refs
8760 @itemx -mno-tls-direct-seg-refs
8761 @opindex mtls-direct-seg-refs
8762 Controls whether TLS variables may be accessed with offsets from the
8763 TLS segment register (@code{%gs} for 32-bit, @code{%fs} for 64-bit),
8764 or whether the thread base pointer must be added. Whether or not this
8765 is legal depends on the operating system, and whether it maps the
8766 segment to cover the entire TLS area.
8768 For systems that use GNU libc, the default is on.
8771 These @samp{-m} switches are supported in addition to the above
8772 on AMD x86-64 processors in 64-bit environments.
8779 Generate code for a 32-bit or 64-bit environment.
8780 The 32-bit environment sets int, long and pointer to 32 bits and
8781 generates code that runs on any i386 system.
8782 The 64-bit environment sets int to 32 bits and long and pointer
8783 to 64 bits and generates code for AMD's x86-64 architecture.
8786 @opindex no-red-zone
8787 Do not use a so called red zone for x86-64 code. The red zone is mandated
8788 by the x86-64 ABI, it is a 128-byte area beyond the location of the
8789 stack pointer that will not be modified by signal or interrupt handlers
8790 and therefore can be used for temporary data without adjusting the stack
8791 pointer. The flag @option{-mno-red-zone} disables this red zone.
8793 @item -mcmodel=small
8794 @opindex mcmodel=small
8795 Generate code for the small code model: the program and its symbols must
8796 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
8797 Programs can be statically or dynamically linked. This is the default
8800 @item -mcmodel=kernel
8801 @opindex mcmodel=kernel
8802 Generate code for the kernel code model. The kernel runs in the
8803 negative 2 GB of the address space.
8804 This model has to be used for Linux kernel code.
8806 @item -mcmodel=medium
8807 @opindex mcmodel=medium
8808 Generate code for the medium model: The program is linked in the lower 2
8809 GB of the address space but symbols can be located anywhere in the
8810 address space. Programs can be statically or dynamically linked, but
8811 building of shared libraries are not supported with the medium model.
8813 @item -mcmodel=large
8814 @opindex mcmodel=large
8815 Generate code for the large model: This model makes no assumptions
8816 about addresses and sizes of sections. Currently GCC does not implement
8821 @subsection IA-64 Options
8822 @cindex IA-64 Options
8824 These are the @samp{-m} options defined for the Intel IA-64 architecture.
8828 @opindex mbig-endian
8829 Generate code for a big endian target. This is the default for HP-UX@.
8831 @item -mlittle-endian
8832 @opindex mlittle-endian
8833 Generate code for a little endian target. This is the default for AIX5
8840 Generate (or don't) code for the GNU assembler. This is the default.
8841 @c Also, this is the default if the configure option @option{--with-gnu-as}
8848 Generate (or don't) code for the GNU linker. This is the default.
8849 @c Also, this is the default if the configure option @option{--with-gnu-ld}
8854 Generate code that does not use a global pointer register. The result
8855 is not position independent code, and violates the IA-64 ABI@.
8857 @item -mvolatile-asm-stop
8858 @itemx -mno-volatile-asm-stop
8859 @opindex mvolatile-asm-stop
8860 @opindex mno-volatile-asm-stop
8861 Generate (or don't) a stop bit immediately before and after volatile asm
8864 @item -mregister-names
8865 @itemx -mno-register-names
8866 @opindex mregister-names
8867 @opindex mno-register-names
8868 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
8869 the stacked registers. This may make assembler output more readable.
8875 Disable (or enable) optimizations that use the small data section. This may
8876 be useful for working around optimizer bugs.
8879 @opindex mconstant-gp
8880 Generate code that uses a single constant global pointer value. This is
8881 useful when compiling kernel code.
8885 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
8886 This is useful when compiling firmware code.
8888 @item -minline-float-divide-min-latency
8889 @opindex minline-float-divide-min-latency
8890 Generate code for inline divides of floating point values
8891 using the minimum latency algorithm.
8893 @item -minline-float-divide-max-throughput
8894 @opindex minline-float-divide-max-throughput
8895 Generate code for inline divides of floating point values
8896 using the maximum throughput algorithm.
8898 @item -minline-int-divide-min-latency
8899 @opindex minline-int-divide-min-latency
8900 Generate code for inline divides of integer values
8901 using the minimum latency algorithm.
8903 @item -minline-int-divide-max-throughput
8904 @opindex minline-int-divide-max-throughput
8905 Generate code for inline divides of integer values
8906 using the maximum throughput algorithm.
8908 @item -minline-sqrt-min-latency
8909 @opindex minline-sqrt-min-latency
8910 Generate code for inline square roots
8911 using the minimum latency algorithm.
8913 @item -minline-sqrt-max-throughput
8914 @opindex minline-sqrt-max-throughput
8915 Generate code for inline square roots
8916 using the maximum throughput algorithm.
8918 @item -mno-dwarf2-asm
8920 @opindex mno-dwarf2-asm
8921 @opindex mdwarf2-asm
8922 Don't (or do) generate assembler code for the DWARF2 line number debugging
8923 info. This may be useful when not using the GNU assembler.
8925 @item -mearly-stop-bits
8926 @itemx -mno-early-stop-bits
8927 @opindex mearly-stop-bits
8928 @opindex mno-early-stop-bits
8929 Allow stop bits to be placed earlier than immediately preceding the
8930 instruction that triggered the stop bit. This can improve instruction
8931 scheduling, but does not always do so.
8933 @item -mfixed-range=@var{register-range}
8934 @opindex mfixed-range
8935 Generate code treating the given register range as fixed registers.
8936 A fixed register is one that the register allocator can not use. This is
8937 useful when compiling kernel code. A register range is specified as
8938 two registers separated by a dash. Multiple register ranges can be
8939 specified separated by a comma.
8941 @item -mtls-size=@var{tls-size}
8943 Specify bit size of immediate TLS offsets. Valid values are 14, 22, and
8946 @item -mtune-arch=@var{cpu-type}
8948 Tune the instruction scheduling for a particular CPU, Valid values are
8949 itanium, itanium1, merced, itanium2, and mckinley.
8955 Add support for multithreading using the POSIX threads library. This
8956 option sets flags for both the preprocessor and linker. It does
8957 not affect the thread safety of object code produced by the compiler or
8958 that of libraries supplied with it. These are HP-UX specific flags.
8964 Generate code for a 32-bit or 64-bit environment.
8965 The 32-bit environment sets int, long and pointer to 32 bits.
8966 The 64-bit environment sets int to 32 bits and long and pointer
8967 to 64 bits. These are HP-UX specific flags.
8971 @node M32R/D Options
8972 @subsection M32R/D Options
8973 @cindex M32R/D options
8975 These @option{-m} options are defined for Renesas M32R/D architectures:
8980 Generate code for the M32R/2@.
8984 Generate code for the M32R/X@.
8988 Generate code for the M32R@. This is the default.
8991 @opindex mmodel=small
8992 Assume all objects live in the lower 16MB of memory (so that their addresses
8993 can be loaded with the @code{ld24} instruction), and assume all subroutines
8994 are reachable with the @code{bl} instruction.
8995 This is the default.
8997 The addressability of a particular object can be set with the
8998 @code{model} attribute.
9000 @item -mmodel=medium
9001 @opindex mmodel=medium
9002 Assume objects may be anywhere in the 32-bit address space (the compiler
9003 will generate @code{seth/add3} instructions to load their addresses), and
9004 assume all subroutines are reachable with the @code{bl} instruction.
9007 @opindex mmodel=large
9008 Assume objects may be anywhere in the 32-bit address space (the compiler
9009 will generate @code{seth/add3} instructions to load their addresses), and
9010 assume subroutines may not be reachable with the @code{bl} instruction
9011 (the compiler will generate the much slower @code{seth/add3/jl}
9012 instruction sequence).
9015 @opindex msdata=none
9016 Disable use of the small data area. Variables will be put into
9017 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
9018 @code{section} attribute has been specified).
9019 This is the default.
9021 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
9022 Objects may be explicitly put in the small data area with the
9023 @code{section} attribute using one of these sections.
9026 @opindex msdata=sdata
9027 Put small global and static data in the small data area, but do not
9028 generate special code to reference them.
9032 Put small global and static data in the small data area, and generate
9033 special instructions to reference them.
9037 @cindex smaller data references
9038 Put global and static objects less than or equal to @var{num} bytes
9039 into the small data or bss sections instead of the normal data or bss
9040 sections. The default value of @var{num} is 8.
9041 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
9042 for this option to have any effect.
9044 All modules should be compiled with the same @option{-G @var{num}} value.
9045 Compiling with different values of @var{num} may or may not work; if it
9046 doesn't the linker will give an error message---incorrect code will not be
9051 Makes the M32R specific code in the compiler display some statistics
9052 that might help in debugging programs.
9055 @opindex malign-loops
9056 Align all loops to a 32-byte boundary.
9058 @item -mno-align-loops
9059 @opindex mno-align-loops
9060 Do not enforce a 32-byte alignment for loops. This is the default.
9062 @item -missue-rate=@var{number}
9063 @opindex missue-rate=@var{number}
9064 Issue @var{number} instructions per cycle. @var{number} can only be 1
9067 @item -mbranch-cost=@var{number}
9068 @opindex mbranch-cost=@var{number}
9069 @var{number} can only be 1 or 2. If it is 1 then branches will be
9070 preferred over conditional code, if it is 2, then the opposite will
9073 @item -mflush-trap=@var{number}
9074 @opindex mflush-trap=@var{number}
9075 Specifies the trap number to use to flush the cache. The default is
9076 12. Valid numbers are between 0 and 15 inclusive.
9078 @item -mno-flush-trap
9079 @opindex mno-flush-trap
9080 Specifies that the cache cannot be flushed by using a trap.
9082 @item -mflush-func=@var{name}
9083 @opindex mflush-func=@var{name}
9084 Specifies the name of the operating system function to call to flush
9085 the cache. The default is @emph{_flush_cache}, but a function call
9086 will only be used if a trap is not available.
9088 @item -mno-flush-func
9089 @opindex mno-flush-func
9090 Indicates that there is no OS function for flushing the cache.
9094 @node M680x0 Options
9095 @subsection M680x0 Options
9096 @cindex M680x0 options
9098 These are the @samp{-m} options defined for the 68000 series. The default
9099 values for these options depends on which style of 68000 was selected when
9100 the compiler was configured; the defaults for the most common choices are
9108 Generate output for a 68000. This is the default
9109 when the compiler is configured for 68000-based systems.
9111 Use this option for microcontrollers with a 68000 or EC000 core,
9112 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
9118 Generate output for a 68020. This is the default
9119 when the compiler is configured for 68020-based systems.
9123 Generate output containing 68881 instructions for floating point.
9124 This is the default for most 68020 systems unless @option{--nfp} was
9125 specified when the compiler was configured.
9129 Generate output for a 68030. This is the default when the compiler is
9130 configured for 68030-based systems.
9134 Generate output for a 68040. This is the default when the compiler is
9135 configured for 68040-based systems.
9137 This option inhibits the use of 68881/68882 instructions that have to be
9138 emulated by software on the 68040. Use this option if your 68040 does not
9139 have code to emulate those instructions.
9143 Generate output for a 68060. This is the default when the compiler is
9144 configured for 68060-based systems.
9146 This option inhibits the use of 68020 and 68881/68882 instructions that
9147 have to be emulated by software on the 68060. Use this option if your 68060
9148 does not have code to emulate those instructions.
9152 Generate output for a CPU32. This is the default
9153 when the compiler is configured for CPU32-based systems.
9155 Use this option for microcontrollers with a
9156 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
9157 68336, 68340, 68341, 68349 and 68360.
9161 Generate output for a 520X ``coldfire'' family cpu. This is the default
9162 when the compiler is configured for 520X-based systems.
9164 Use this option for microcontroller with a 5200 core, including
9165 the MCF5202, MCF5203, MCF5204 and MCF5202.
9170 Generate output for a 68040, without using any of the new instructions.
9171 This results in code which can run relatively efficiently on either a
9172 68020/68881 or a 68030 or a 68040. The generated code does use the
9173 68881 instructions that are emulated on the 68040.
9177 Generate output for a 68060, without using any of the new instructions.
9178 This results in code which can run relatively efficiently on either a
9179 68020/68881 or a 68030 or a 68040. The generated code does use the
9180 68881 instructions that are emulated on the 68060.
9183 @opindex msoft-float
9184 Generate output containing library calls for floating point.
9185 @strong{Warning:} the requisite libraries are not available for all m68k
9186 targets. Normally the facilities of the machine's usual C compiler are
9187 used, but this can't be done directly in cross-compilation. You must
9188 make your own arrangements to provide suitable library functions for
9189 cross-compilation. The embedded targets @samp{m68k-*-aout} and
9190 @samp{m68k-*-coff} do provide software floating point support.
9194 Consider type @code{int} to be 16 bits wide, like @code{short int}.
9195 Additionally, parameters passed on the stack are also aligned to a
9196 16-bit boundary even on targets whose API mandates promotion to 32-bit.
9199 @opindex mnobitfield
9200 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
9201 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
9205 Do use the bit-field instructions. The @option{-m68020} option implies
9206 @option{-mbitfield}. This is the default if you use a configuration
9207 designed for a 68020.
9211 Use a different function-calling convention, in which functions
9212 that take a fixed number of arguments return with the @code{rtd}
9213 instruction, which pops their arguments while returning. This
9214 saves one instruction in the caller since there is no need to pop
9215 the arguments there.
9217 This calling convention is incompatible with the one normally
9218 used on Unix, so you cannot use it if you need to call libraries
9219 compiled with the Unix compiler.
9221 Also, you must provide function prototypes for all functions that
9222 take variable numbers of arguments (including @code{printf});
9223 otherwise incorrect code will be generated for calls to those
9226 In addition, seriously incorrect code will result if you call a
9227 function with too many arguments. (Normally, extra arguments are
9228 harmlessly ignored.)
9230 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
9231 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
9234 @itemx -mno-align-int
9236 @opindex mno-align-int
9237 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
9238 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
9239 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
9240 Aligning variables on 32-bit boundaries produces code that runs somewhat
9241 faster on processors with 32-bit busses at the expense of more memory.
9243 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
9244 align structures containing the above types differently than
9245 most published application binary interface specifications for the m68k.
9249 Use the pc-relative addressing mode of the 68000 directly, instead of
9250 using a global offset table. At present, this option implies @option{-fpic},
9251 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
9252 not presently supported with @option{-mpcrel}, though this could be supported for
9253 68020 and higher processors.
9255 @item -mno-strict-align
9256 @itemx -mstrict-align
9257 @opindex mno-strict-align
9258 @opindex mstrict-align
9259 Do not (do) assume that unaligned memory references will be handled by
9263 Generate code that allows the data segment to be located in a different
9264 area of memory from the text segment. This allows for execute in place in
9265 an environment without virtual memory management. This option implies
9269 Generate code that assumes that the data segment follows the text segment.
9270 This is the default.
9272 @item -mid-shared-library
9273 Generate code that supports shared libraries via the library ID method.
9274 This allows for execute in place and shared libraries in an environment
9275 without virtual memory management. This option implies @option{-fPIC}.
9277 @item -mno-id-shared-library
9278 Generate code that doesn't assume ID based shared libraries are being used.
9279 This is the default.
9281 @item -mshared-library-id=n
9282 Specified the identification number of the ID based shared library being
9283 compiled. Specifying a value of 0 will generate more compact code, specifying
9284 other values will force the allocation of that number to the current
9285 library but is no more space or time efficient than omitting this option.
9289 @node M68hc1x Options
9290 @subsection M68hc1x Options
9291 @cindex M68hc1x options
9293 These are the @samp{-m} options defined for the 68hc11 and 68hc12
9294 microcontrollers. The default values for these options depends on
9295 which style of microcontroller was selected when the compiler was configured;
9296 the defaults for the most common choices are given below.
9303 Generate output for a 68HC11. This is the default
9304 when the compiler is configured for 68HC11-based systems.
9310 Generate output for a 68HC12. This is the default
9311 when the compiler is configured for 68HC12-based systems.
9317 Generate output for a 68HCS12.
9320 @opindex mauto-incdec
9321 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
9328 Enable the use of 68HC12 min and max instructions.
9331 @itemx -mno-long-calls
9332 @opindex mlong-calls
9333 @opindex mno-long-calls
9334 Treat all calls as being far away (near). If calls are assumed to be
9335 far away, the compiler will use the @code{call} instruction to
9336 call a function and the @code{rtc} instruction for returning.
9340 Consider type @code{int} to be 16 bits wide, like @code{short int}.
9342 @item -msoft-reg-count=@var{count}
9343 @opindex msoft-reg-count
9344 Specify the number of pseudo-soft registers which are used for the
9345 code generation. The maximum number is 32. Using more pseudo-soft
9346 register may or may not result in better code depending on the program.
9347 The default is 4 for 68HC11 and 2 for 68HC12.
9352 @subsection MCore Options
9353 @cindex MCore options
9355 These are the @samp{-m} options defined for the Motorola M*Core
9363 @opindex mno-hardlit
9364 Inline constants into the code stream if it can be done in two
9365 instructions or less.
9371 Use the divide instruction. (Enabled by default).
9373 @item -mrelax-immediate
9374 @itemx -mno-relax-immediate
9375 @opindex mrelax-immediate
9376 @opindex mno-relax-immediate
9377 Allow arbitrary sized immediates in bit operations.
9379 @item -mwide-bitfields
9380 @itemx -mno-wide-bitfields
9381 @opindex mwide-bitfields
9382 @opindex mno-wide-bitfields
9383 Always treat bit-fields as int-sized.
9385 @item -m4byte-functions
9386 @itemx -mno-4byte-functions
9387 @opindex m4byte-functions
9388 @opindex mno-4byte-functions
9389 Force all functions to be aligned to a four byte boundary.
9391 @item -mcallgraph-data
9392 @itemx -mno-callgraph-data
9393 @opindex mcallgraph-data
9394 @opindex mno-callgraph-data
9395 Emit callgraph information.
9398 @itemx -mno-slow-bytes
9399 @opindex mslow-bytes
9400 @opindex mno-slow-bytes
9401 Prefer word access when reading byte quantities.
9403 @item -mlittle-endian
9405 @opindex mlittle-endian
9406 @opindex mbig-endian
9407 Generate code for a little endian target.
9413 Generate code for the 210 processor.
9417 @subsection MIPS Options
9418 @cindex MIPS options
9424 Generate big-endian code.
9428 Generate little-endian code. This is the default for @samp{mips*el-*-*}
9431 @item -march=@var{arch}
9433 Generate code that will run on @var{arch}, which can be the name of a
9434 generic MIPS ISA, or the name of a particular processor.
9436 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
9437 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
9438 The processor names are:
9439 @samp{4kc}, @samp{4kp}, @samp{5kc}, @samp{20kc},
9441 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
9442 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000}, @samp{rm7000},
9446 @samp{vr4100}, @samp{vr4111}, @samp{vr4120}, @samp{vr4130}, @samp{vr4300},
9447 @samp{vr5000}, @samp{vr5400} and @samp{vr5500}.
9448 The special value @samp{from-abi} selects the
9449 most compatible architecture for the selected ABI (that is,
9450 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
9452 In processor names, a final @samp{000} can be abbreviated as @samp{k}
9453 (for example, @samp{-march=r2k}). Prefixes are optional, and
9454 @samp{vr} may be written @samp{r}.
9456 GCC defines two macros based on the value of this option. The first
9457 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
9458 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
9459 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
9460 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
9461 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
9463 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
9464 above. In other words, it will have the full prefix and will not
9465 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
9466 the macro names the resolved architecture (either @samp{"mips1"} or
9467 @samp{"mips3"}). It names the default architecture when no
9468 @option{-march} option is given.
9470 @item -mtune=@var{arch}
9472 Optimize for @var{arch}. Among other things, this option controls
9473 the way instructions are scheduled, and the perceived cost of arithmetic
9474 operations. The list of @var{arch} values is the same as for
9477 When this option is not used, GCC will optimize for the processor
9478 specified by @option{-march}. By using @option{-march} and
9479 @option{-mtune} together, it is possible to generate code that will
9480 run on a family of processors, but optimize the code for one
9481 particular member of that family.
9483 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
9484 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
9485 @samp{-march} ones described above.
9489 Equivalent to @samp{-march=mips1}.
9493 Equivalent to @samp{-march=mips2}.
9497 Equivalent to @samp{-march=mips3}.
9501 Equivalent to @samp{-march=mips4}.
9505 Equivalent to @samp{-march=mips32}.
9509 Equivalent to @samp{-march=mips32r2}.
9513 Equivalent to @samp{-march=mips64}.
9519 Use (do not use) the MIPS16 ISA@.
9531 Generate code for the given ABI@.
9533 Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
9534 generates 64-bit code when you select a 64-bit architecture, but you
9535 can use @option{-mgp32} to get 32-bit code instead.
9537 For information about the O64 ABI, see
9538 @w{@uref{http://gcc.gnu.org/projects/mipso64-abi.html}}.
9541 @itemx -mno-abicalls
9543 @opindex mno-abicalls
9544 Generate (do not generate) SVR4-style position-independent code.
9545 @option{-mabicalls} is the default for SVR4-based systems.
9551 Lift (do not lift) the usual restrictions on the size of the global
9554 GCC normally uses a single instruction to load values from the GOT@.
9555 While this is relatively efficient, it will only work if the GOT
9556 is smaller than about 64k. Anything larger will cause the linker
9557 to report an error such as:
9559 @cindex relocation truncated to fit (MIPS)
9561 relocation truncated to fit: R_MIPS_GOT16 foobar
9564 If this happens, you should recompile your code with @option{-mxgot}.
9565 It should then work with very large GOTs, although it will also be
9566 less efficient, since it will take three instructions to fetch the
9567 value of a global symbol.
9569 Note that some linkers can create multiple GOTs. If you have such a
9570 linker, you should only need to use @option{-mxgot} when a single object
9571 file accesses more than 64k's worth of GOT entries. Very few do.
9573 These options have no effect unless GCC is generating position
9578 Assume that general-purpose registers are 32 bits wide.
9582 Assume that general-purpose registers are 64 bits wide.
9586 Assume that floating-point registers are 32 bits wide.
9590 Assume that floating-point registers are 64 bits wide.
9593 @opindex mhard-float
9594 Use floating-point coprocessor instructions.
9597 @opindex msoft-float
9598 Do not use floating-point coprocessor instructions. Implement
9599 floating-point calculations using library calls instead.
9601 @item -msingle-float
9602 @opindex msingle-float
9603 Assume that the floating-point coprocessor only supports single-precision
9606 @itemx -mdouble-float
9607 @opindex mdouble-float
9608 Assume that the floating-point coprocessor supports double-precision
9609 operations. This is the default.
9611 @itemx -mpaired-single
9612 @itemx -mno-paired-single
9613 @opindex mpaired-single
9614 @opindex mno-paired-single
9615 Use (do not use) paired-single floating-point instructions.
9616 @xref{MIPS Paired-Single Support}. This option can only be used
9617 when generating 64-bit code and requires hardware floating-point
9618 support to be enabled.
9624 Use (do not use) the MIPS-3D ASE@. @xref{MIPS-3D Built-in Functions}.
9625 The option @option{-mips3d} implies @option{-mpaired-single}.
9629 Force @code{int} and @code{long} types to be 64 bits wide. See
9630 @option{-mlong32} for an explanation of the default and the way
9631 that the pointer size is determined.
9633 This option has been deprecated and will be removed in a future release.
9637 Force @code{long} types to be 64 bits wide. See @option{-mlong32} for
9638 an explanation of the default and the way that the pointer size is
9643 Force @code{long}, @code{int}, and pointer types to be 32 bits wide.
9645 The default size of @code{int}s, @code{long}s and pointers depends on
9646 the ABI@. All the supported ABIs use 32-bit @code{int}s. The n64 ABI
9647 uses 64-bit @code{long}s, as does the 64-bit EABI; the others use
9648 32-bit @code{long}s. Pointers are the same size as @code{long}s,
9649 or the same size as integer registers, whichever is smaller.
9653 @cindex smaller data references (MIPS)
9654 @cindex gp-relative references (MIPS)
9655 Put global and static items less than or equal to @var{num} bytes into
9656 the small data or bss section instead of the normal data or bss section.
9657 This allows the data to be accessed using a single instruction.
9659 All modules should be compiled with the same @option{-G @var{num}}
9662 @item -membedded-data
9663 @itemx -mno-embedded-data
9664 @opindex membedded-data
9665 @opindex mno-embedded-data
9666 Allocate variables to the read-only data section first if possible, then
9667 next in the small data section if possible, otherwise in data. This gives
9668 slightly slower code than the default, but reduces the amount of RAM required
9669 when executing, and thus may be preferred for some embedded systems.
9671 @item -muninit-const-in-rodata
9672 @itemx -mno-uninit-const-in-rodata
9673 @opindex muninit-const-in-rodata
9674 @opindex mno-uninit-const-in-rodata
9675 Put uninitialized @code{const} variables in the read-only data section.
9676 This option is only meaningful in conjunction with @option{-membedded-data}.
9678 @item -msplit-addresses
9679 @itemx -mno-split-addresses
9680 @opindex msplit-addresses
9681 @opindex mno-split-addresses
9682 Enable (disable) use of the @code{%hi()} and @code{%lo()} assembler
9683 relocation operators. This option has been superceded by
9684 @option{-mexplicit-relocs} but is retained for backwards compatibility.
9686 @item -mexplicit-relocs
9687 @itemx -mno-explicit-relocs
9688 @opindex mexplicit-relocs
9689 @opindex mno-explicit-relocs
9690 Use (do not use) assembler relocation operators when dealing with symbolic
9691 addresses. The alternative, selected by @option{-mno-explicit-relocs},
9692 is to use assembler macros instead.
9694 @option{-mexplicit-relocs} is the default if GCC was configured
9695 to use an assembler that supports relocation operators.
9697 @item -mcheck-zero-division
9698 @itemx -mno-check-zero-division
9699 @opindex mcheck-zero-division
9700 @opindex mno-check-zero-division
9701 Trap (do not trap) on integer division by zero. The default is
9702 @option{-mcheck-zero-division}.
9704 @item -mdivide-traps
9705 @itemx -mdivide-breaks
9706 @opindex mdivide-traps
9707 @opindex mdivide-breaks
9708 MIPS systems check for division by zero by generating either a
9709 conditional trap or a break instruction. Using traps results in
9710 smaller code, but is only supported on MIPS II and later. Also, some
9711 versions of the Linux kernel have a bug that prevents trap from
9712 generating the proper signal (@code{SIGFPE}). Use @option{-mdivide-traps} to
9713 allow conditional traps on architectures that support them and
9714 @option{-mdivide-breaks} to force the use of breaks.
9716 The default is usually @option{-mdivide-traps}, but this can be
9717 overridden at configure time using @option{--with-divide=breaks}.
9718 Divide-by-zero checks can be completely disabled using
9719 @option{-mno-check-zero-division}.
9725 Force (do not force) the use of @code{memcpy()} for non-trivial block
9726 moves. The default is @option{-mno-memcpy}, which allows GCC to inline
9727 most constant-sized copies.
9730 @itemx -mno-long-calls
9731 @opindex mlong-calls
9732 @opindex mno-long-calls
9733 Disable (do not disable) use of the @code{jal} instruction. Calling
9734 functions using @code{jal} is more efficient but requires the caller
9735 and callee to be in the same 256 megabyte segment.
9737 This option has no effect on abicalls code. The default is
9738 @option{-mno-long-calls}.
9744 Enable (disable) use of the @code{mad}, @code{madu} and @code{mul}
9745 instructions, as provided by the R4650 ISA@.
9748 @itemx -mno-fused-madd
9749 @opindex mfused-madd
9750 @opindex mno-fused-madd
9751 Enable (disable) use of the floating point multiply-accumulate
9752 instructions, when they are available. The default is
9753 @option{-mfused-madd}.
9755 When multiply-accumulate instructions are used, the intermediate
9756 product is calculated to infinite precision and is not subject to
9757 the FCSR Flush to Zero bit. This may be undesirable in some
9762 Tell the MIPS assembler to not run its preprocessor over user
9763 assembler files (with a @samp{.s} suffix) when assembling them.
9766 @itemx -mno-fix-r4000
9768 @opindex mno-fix-r4000
9769 Work around certain R4000 CPU errata:
9772 A double-word or a variable shift may give an incorrect result if executed
9773 immediately after starting an integer division.
9775 A double-word or a variable shift may give an incorrect result if executed
9776 while an integer multiplication is in progress.
9778 An integer division may give an incorrect result if started in a delay slot
9779 of a taken branch or a jump.
9783 @itemx -mno-fix-r4400
9785 @opindex mno-fix-r4400
9786 Work around certain R4400 CPU errata:
9789 A double-word or a variable shift may give an incorrect result if executed
9790 immediately after starting an integer division.
9794 @itemx -mno-fix-vr4120
9795 @opindex mfix-vr4120
9796 Work around certain VR4120 errata:
9799 @code{dmultu} does not always produce the correct result.
9801 @code{div} and @code{ddiv} do not always produce the correct result if one
9802 of the operands is negative.
9804 The workarounds for the division errata rely on special functions in
9805 @file{libgcc.a}. At present, these functions are only provided by
9806 the @code{mips64vr*-elf} configurations.
9808 Other VR4120 errata require a nop to be inserted between certain pairs of
9809 instructions. These errata are handled by the assembler, not by GCC itself.
9814 Work around certain SB-1 CPU core errata.
9815 (This flag currently works around the SB-1 revision 2
9816 ``F1'' and ``F2'' floating point errata.)
9818 @item -mflush-func=@var{func}
9819 @itemx -mno-flush-func
9820 @opindex mflush-func
9821 Specifies the function to call to flush the I and D caches, or to not
9822 call any such function. If called, the function must take the same
9823 arguments as the common @code{_flush_func()}, that is, the address of the
9824 memory range for which the cache is being flushed, the size of the
9825 memory range, and the number 3 (to flush both caches). The default
9826 depends on the target GCC was configured for, but commonly is either
9827 @samp{_flush_func} or @samp{__cpu_flush}.
9829 @item -mbranch-likely
9830 @itemx -mno-branch-likely
9831 @opindex mbranch-likely
9832 @opindex mno-branch-likely
9833 Enable or disable use of Branch Likely instructions, regardless of the
9834 default for the selected architecture. By default, Branch Likely
9835 instructions may be generated if they are supported by the selected
9836 architecture. An exception is for the MIPS32 and MIPS64 architectures
9837 and processors which implement those architectures; for those, Branch
9838 Likely instructions will not be generated by default because the MIPS32
9839 and MIPS64 architectures specifically deprecate their use.
9841 @item -mfp-exceptions
9842 @itemx -mno-fp-exceptions
9843 @opindex mfp-exceptions
9844 Specifies whether FP exceptions are enabled. This affects how we schedule
9845 FP instructions for some processors. The default is that FP exceptions are
9848 For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
9849 64-bit code, then we can use both FP pipes. Otherwise, we can only use one
9852 @item -mvr4130-align
9853 @itemx -mno-vr4130-align
9854 @opindex mvr4130-align
9855 The VR4130 pipeline is two-way superscalar, but can only issue two
9856 instructions together if the first one is 8-byte aligned. When this
9857 option is enabled, GCC will align pairs of instructions that it
9858 thinks should execute in parallel.
9860 This option only has an effect when optimizing for the VR4130.
9861 It normally makes code faster, but at the expense of making it bigger.
9862 It is enabled by default at optimization level @option{-O3}.
9866 @subsection MMIX Options
9867 @cindex MMIX Options
9869 These options are defined for the MMIX:
9873 @itemx -mno-libfuncs
9875 @opindex mno-libfuncs
9876 Specify that intrinsic library functions are being compiled, passing all
9877 values in registers, no matter the size.
9882 @opindex mno-epsilon
9883 Generate floating-point comparison instructions that compare with respect
9884 to the @code{rE} epsilon register.
9886 @item -mabi=mmixware
9888 @opindex mabi-mmixware
9890 Generate code that passes function parameters and return values that (in
9891 the called function) are seen as registers @code{$0} and up, as opposed to
9892 the GNU ABI which uses global registers @code{$231} and up.
9895 @itemx -mno-zero-extend
9896 @opindex mzero-extend
9897 @opindex mno-zero-extend
9898 When reading data from memory in sizes shorter than 64 bits, use (do not
9899 use) zero-extending load instructions by default, rather than
9900 sign-extending ones.
9903 @itemx -mno-knuthdiv
9905 @opindex mno-knuthdiv
9906 Make the result of a division yielding a remainder have the same sign as
9907 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
9908 remainder follows the sign of the dividend. Both methods are
9909 arithmetically valid, the latter being almost exclusively used.
9911 @item -mtoplevel-symbols
9912 @itemx -mno-toplevel-symbols
9913 @opindex mtoplevel-symbols
9914 @opindex mno-toplevel-symbols
9915 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
9916 code can be used with the @code{PREFIX} assembly directive.
9920 Generate an executable in the ELF format, rather than the default
9921 @samp{mmo} format used by the @command{mmix} simulator.
9923 @item -mbranch-predict
9924 @itemx -mno-branch-predict
9925 @opindex mbranch-predict
9926 @opindex mno-branch-predict
9927 Use (do not use) the probable-branch instructions, when static branch
9928 prediction indicates a probable branch.
9930 @item -mbase-addresses
9931 @itemx -mno-base-addresses
9932 @opindex mbase-addresses
9933 @opindex mno-base-addresses
9934 Generate (do not generate) code that uses @emph{base addresses}. Using a
9935 base address automatically generates a request (handled by the assembler
9936 and the linker) for a constant to be set up in a global register. The
9937 register is used for one or more base address requests within the range 0
9938 to 255 from the value held in the register. The generally leads to short
9939 and fast code, but the number of different data items that can be
9940 addressed is limited. This means that a program that uses lots of static
9941 data may require @option{-mno-base-addresses}.
9944 @itemx -mno-single-exit
9945 @opindex msingle-exit
9946 @opindex mno-single-exit
9947 Force (do not force) generated code to have a single exit point in each
9951 @node MN10300 Options
9952 @subsection MN10300 Options
9953 @cindex MN10300 options
9955 These @option{-m} options are defined for Matsushita MN10300 architectures:
9960 Generate code to avoid bugs in the multiply instructions for the MN10300
9961 processors. This is the default.
9964 @opindex mno-mult-bug
9965 Do not generate code to avoid bugs in the multiply instructions for the
9970 Generate code which uses features specific to the AM33 processor.
9974 Do not generate code which uses features specific to the AM33 processor. This
9979 Do not link in the C run-time initialization object file.
9983 Indicate to the linker that it should perform a relaxation optimization pass
9984 to shorten branches, calls and absolute memory addresses. This option only
9985 has an effect when used on the command line for the final link step.
9987 This option makes symbolic debugging impossible.
9991 @subsection NS32K Options
9992 @cindex NS32K options
9994 These are the @samp{-m} options defined for the 32000 series. The default
9995 values for these options depends on which style of 32000 was selected when
9996 the compiler was configured; the defaults for the most common choices are
10004 Generate output for a 32032. This is the default
10005 when the compiler is configured for 32032 and 32016 based systems.
10011 Generate output for a 32332. This is the default
10012 when the compiler is configured for 32332-based systems.
10018 Generate output for a 32532. This is the default
10019 when the compiler is configured for 32532-based systems.
10023 Generate output containing 32081 instructions for floating point.
10024 This is the default for all systems.
10028 Generate output containing 32381 instructions for floating point. This
10029 also implies @option{-m32081}. The 32381 is only compatible with the 32332
10030 and 32532 cpus. This is the default for the pc532-netbsd configuration.
10033 @opindex mmulti-add
10034 Try and generate multiply-add floating point instructions @code{polyF}
10035 and @code{dotF}. This option is only available if the @option{-m32381}
10036 option is in effect. Using these instructions requires changes to
10037 register allocation which generally has a negative impact on
10038 performance. This option should only be enabled when compiling code
10039 particularly likely to make heavy use of multiply-add instructions.
10041 @item -mnomulti-add
10042 @opindex mnomulti-add
10043 Do not try and generate multiply-add floating point instructions
10044 @code{polyF} and @code{dotF}. This is the default on all platforms.
10047 @opindex msoft-float
10048 Generate output containing library calls for floating point.
10049 @strong{Warning:} the requisite libraries may not be available.
10051 @item -mieee-compare
10052 @itemx -mno-ieee-compare
10053 @opindex mieee-compare
10054 @opindex mno-ieee-compare
10055 Control whether or not the compiler uses IEEE floating point
10056 comparisons. These handle correctly the case where the result of a
10057 comparison is unordered.
10058 @strong{Warning:} the requisite kernel support may not be available.
10061 @opindex mnobitfield
10062 Do not use the bit-field instructions. On some machines it is faster to
10063 use shifting and masking operations. This is the default for the pc532.
10067 Do use the bit-field instructions. This is the default for all platforms
10072 Use a different function-calling convention, in which functions
10073 that take a fixed number of arguments return pop their
10074 arguments on return with the @code{ret} instruction.
10076 This calling convention is incompatible with the one normally
10077 used on Unix, so you cannot use it if you need to call libraries
10078 compiled with the Unix compiler.
10080 Also, you must provide function prototypes for all functions that
10081 take variable numbers of arguments (including @code{printf});
10082 otherwise incorrect code will be generated for calls to those
10085 In addition, seriously incorrect code will result if you call a
10086 function with too many arguments. (Normally, extra arguments are
10087 harmlessly ignored.)
10089 This option takes its name from the 680x0 @code{rtd} instruction.
10094 Use a different function-calling convention where the first two arguments
10095 are passed in registers.
10097 This calling convention is incompatible with the one normally
10098 used on Unix, so you cannot use it if you need to call libraries
10099 compiled with the Unix compiler.
10102 @opindex mnoregparam
10103 Do not pass any arguments in registers. This is the default for all
10108 It is OK to use the sb as an index register which is always loaded with
10109 zero. This is the default for the pc532-netbsd target.
10113 The sb register is not available for use or has not been initialized to
10114 zero by the run time system. This is the default for all targets except
10115 the pc532-netbsd. It is also implied whenever @option{-mhimem} or
10116 @option{-fpic} is set.
10120 Many ns32000 series addressing modes use displacements of up to 512MB@.
10121 If an address is above 512MB then displacements from zero can not be used.
10122 This option causes code to be generated which can be loaded above 512MB@.
10123 This may be useful for operating systems or ROM code.
10127 Assume code will be loaded in the first 512MB of virtual address space.
10128 This is the default for all platforms.
10132 @node PDP-11 Options
10133 @subsection PDP-11 Options
10134 @cindex PDP-11 Options
10136 These options are defined for the PDP-11:
10141 Use hardware FPP floating point. This is the default. (FIS floating
10142 point on the PDP-11/40 is not supported.)
10145 @opindex msoft-float
10146 Do not use hardware floating point.
10150 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
10154 Return floating-point results in memory. This is the default.
10158 Generate code for a PDP-11/40.
10162 Generate code for a PDP-11/45. This is the default.
10166 Generate code for a PDP-11/10.
10168 @item -mbcopy-builtin
10169 @opindex bcopy-builtin
10170 Use inline @code{movmemhi} patterns for copying memory. This is the
10175 Do not use inline @code{movmemhi} patterns for copying memory.
10181 Use 16-bit @code{int}. This is the default.
10187 Use 32-bit @code{int}.
10190 @itemx -mno-float32
10192 @opindex mno-float32
10193 Use 64-bit @code{float}. This is the default.
10196 @itemx -mno-float64
10198 @opindex mno-float64
10199 Use 32-bit @code{float}.
10203 Use @code{abshi2} pattern. This is the default.
10207 Do not use @code{abshi2} pattern.
10209 @item -mbranch-expensive
10210 @opindex mbranch-expensive
10211 Pretend that branches are expensive. This is for experimenting with
10212 code generation only.
10214 @item -mbranch-cheap
10215 @opindex mbranch-cheap
10216 Do not pretend that branches are expensive. This is the default.
10220 Generate code for a system with split I&D@.
10224 Generate code for a system without split I&D@. This is the default.
10228 Use Unix assembler syntax. This is the default when configured for
10229 @samp{pdp11-*-bsd}.
10233 Use DEC assembler syntax. This is the default when configured for any
10234 PDP-11 target other than @samp{pdp11-*-bsd}.
10237 @node PowerPC Options
10238 @subsection PowerPC Options
10239 @cindex PowerPC options
10241 These are listed under @xref{RS/6000 and PowerPC Options}.
10243 @node RS/6000 and PowerPC Options
10244 @subsection IBM RS/6000 and PowerPC Options
10245 @cindex RS/6000 and PowerPC Options
10246 @cindex IBM RS/6000 and PowerPC Options
10248 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
10255 @itemx -mno-powerpc
10256 @itemx -mpowerpc-gpopt
10257 @itemx -mno-powerpc-gpopt
10258 @itemx -mpowerpc-gfxopt
10259 @itemx -mno-powerpc-gfxopt
10261 @itemx -mno-powerpc64
10265 @opindex mno-power2
10267 @opindex mno-powerpc
10268 @opindex mpowerpc-gpopt
10269 @opindex mno-powerpc-gpopt
10270 @opindex mpowerpc-gfxopt
10271 @opindex mno-powerpc-gfxopt
10272 @opindex mpowerpc64
10273 @opindex mno-powerpc64
10274 GCC supports two related instruction set architectures for the
10275 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
10276 instructions supported by the @samp{rios} chip set used in the original
10277 RS/6000 systems and the @dfn{PowerPC} instruction set is the
10278 architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and
10279 the IBM 4xx microprocessors.
10281 Neither architecture is a subset of the other. However there is a
10282 large common subset of instructions supported by both. An MQ
10283 register is included in processors supporting the POWER architecture.
10285 You use these options to specify which instructions are available on the
10286 processor you are using. The default value of these options is
10287 determined when configuring GCC@. Specifying the
10288 @option{-mcpu=@var{cpu_type}} overrides the specification of these
10289 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
10290 rather than the options listed above.
10292 The @option{-mpower} option allows GCC to generate instructions that
10293 are found only in the POWER architecture and to use the MQ register.
10294 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
10295 to generate instructions that are present in the POWER2 architecture but
10296 not the original POWER architecture.
10298 The @option{-mpowerpc} option allows GCC to generate instructions that
10299 are found only in the 32-bit subset of the PowerPC architecture.
10300 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
10301 GCC to use the optional PowerPC architecture instructions in the
10302 General Purpose group, including floating-point square root. Specifying
10303 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
10304 use the optional PowerPC architecture instructions in the Graphics
10305 group, including floating-point select.
10307 The @option{-mpowerpc64} option allows GCC to generate the additional
10308 64-bit instructions that are found in the full PowerPC64 architecture
10309 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
10310 @option{-mno-powerpc64}.
10312 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
10313 will use only the instructions in the common subset of both
10314 architectures plus some special AIX common-mode calls, and will not use
10315 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
10316 permits GCC to use any instruction from either architecture and to
10317 allow use of the MQ register; specify this for the Motorola MPC601.
10319 @item -mnew-mnemonics
10320 @itemx -mold-mnemonics
10321 @opindex mnew-mnemonics
10322 @opindex mold-mnemonics
10323 Select which mnemonics to use in the generated assembler code. With
10324 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
10325 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
10326 assembler mnemonics defined for the POWER architecture. Instructions
10327 defined in only one architecture have only one mnemonic; GCC uses that
10328 mnemonic irrespective of which of these options is specified.
10330 GCC defaults to the mnemonics appropriate for the architecture in
10331 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
10332 value of these option. Unless you are building a cross-compiler, you
10333 should normally not specify either @option{-mnew-mnemonics} or
10334 @option{-mold-mnemonics}, but should instead accept the default.
10336 @item -mcpu=@var{cpu_type}
10338 Set architecture type, register usage, choice of mnemonics, and
10339 instruction scheduling parameters for machine type @var{cpu_type}.
10340 Supported values for @var{cpu_type} are @samp{401}, @samp{403},
10341 @samp{405}, @samp{405fp}, @samp{440}, @samp{440fp}, @samp{505},
10342 @samp{601}, @samp{602}, @samp{603}, @samp{603e}, @samp{604},
10343 @samp{604e}, @samp{620}, @samp{630}, @samp{740}, @samp{7400},
10344 @samp{7450}, @samp{750}, @samp{801}, @samp{821}, @samp{823},
10345 @samp{860}, @samp{970}, @samp{common}, @samp{ec603e}, @samp{G3},
10346 @samp{G4}, @samp{G5}, @samp{power}, @samp{power2}, @samp{power3},
10347 @samp{power4}, @samp{power5}, @samp{powerpc}, @samp{powerpc64},
10348 @samp{rios}, @samp{rios1}, @samp{rios2}, @samp{rsc}, and @samp{rs64a}.
10350 @option{-mcpu=common} selects a completely generic processor. Code
10351 generated under this option will run on any POWER or PowerPC processor.
10352 GCC will use only the instructions in the common subset of both
10353 architectures, and will not use the MQ register. GCC assumes a generic
10354 processor model for scheduling purposes.
10356 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
10357 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
10358 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
10359 types, with an appropriate, generic processor model assumed for
10360 scheduling purposes.
10362 The other options specify a specific processor. Code generated under
10363 those options will run best on that processor, and may not run at all on
10366 The @option{-mcpu} options automatically enable or disable the
10367 following options: @option{-maltivec}, @option{-mhard-float},
10368 @option{-mmfcrf}, @option{-mmultiple}, @option{-mnew-mnemonics},
10369 @option{-mpower}, @option{-mpower2}, @option{-mpowerpc64},
10370 @option{-mpowerpc-gpopt}, @option{-mpowerpc-gfxopt},
10371 @option{-mstring}. The particular options set for any particular CPU
10372 will vary between compiler versions, depending on what setting seems
10373 to produce optimal code for that CPU; it doesn't necessarily reflect
10374 the actual hardware's capabilities. If you wish to set an individual
10375 option to a particular value, you may specify it after the
10376 @option{-mcpu} option, like @samp{-mcpu=970 -mno-altivec}.
10378 On AIX, the @option{-maltivec} and @option{-mpowerpc64} options are
10379 not enabled or disabled by the @option{-mcpu} option at present, since
10380 AIX does not have full support for these options. You may still
10381 enable or disable them individually if you're sure it'll work in your
10384 @item -mtune=@var{cpu_type}
10386 Set the instruction scheduling parameters for machine type
10387 @var{cpu_type}, but do not set the architecture type, register usage, or
10388 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
10389 values for @var{cpu_type} are used for @option{-mtune} as for
10390 @option{-mcpu}. If both are specified, the code generated will use the
10391 architecture, registers, and mnemonics set by @option{-mcpu}, but the
10392 scheduling parameters set by @option{-mtune}.
10395 @itemx -mno-altivec
10397 @opindex mno-altivec
10398 Generate code that uses (does not use) AltiVec instructions, and also
10399 enable the use of built-in functions that allow more direct access to
10400 the AltiVec instruction set. You may also need to set
10401 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
10406 Extend the current ABI with SPE ABI extensions. This does not change
10407 the default ABI, instead it adds the SPE ABI extensions to the current
10411 @opindex mabi=no-spe
10412 Disable Booke SPE ABI extensions for the current ABI@.
10414 @item -misel=@var{yes/no}
10417 This switch enables or disables the generation of ISEL instructions.
10419 @item -mspe=@var{yes/no}
10422 This switch enables or disables the generation of SPE simd
10425 @item -mfloat-gprs=@var{yes/single/double/no}
10426 @itemx -mfloat-gprs
10427 @opindex mfloat-gprs
10428 This switch enables or disables the generation of floating point
10429 operations on the general purpose registers for architectures that
10432 The argument @var{yes} or @var{single} enables the use of
10433 single-precision floating point operations.
10435 The argument @var{double} enables the use of single and
10436 double-precision floating point operations.
10438 The argument @var{no} disables floating point operations on the
10439 general purpose registers.
10441 This option is currently only available on the MPC854x.
10447 Generate code for 32-bit or 64-bit environments of Darwin and SVR4
10448 targets (including GNU/Linux). The 32-bit environment sets int, long
10449 and pointer to 32 bits and generates code that runs on any PowerPC
10450 variant. The 64-bit environment sets int to 32 bits and long and
10451 pointer to 64 bits, and generates code for PowerPC64, as for
10452 @option{-mpowerpc64}.
10455 @itemx -mno-fp-in-toc
10456 @itemx -mno-sum-in-toc
10457 @itemx -mminimal-toc
10459 @opindex mno-fp-in-toc
10460 @opindex mno-sum-in-toc
10461 @opindex mminimal-toc
10462 Modify generation of the TOC (Table Of Contents), which is created for
10463 every executable file. The @option{-mfull-toc} option is selected by
10464 default. In that case, GCC will allocate at least one TOC entry for
10465 each unique non-automatic variable reference in your program. GCC
10466 will also place floating-point constants in the TOC@. However, only
10467 16,384 entries are available in the TOC@.
10469 If you receive a linker error message that saying you have overflowed
10470 the available TOC space, you can reduce the amount of TOC space used
10471 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
10472 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
10473 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
10474 generate code to calculate the sum of an address and a constant at
10475 run-time instead of putting that sum into the TOC@. You may specify one
10476 or both of these options. Each causes GCC to produce very slightly
10477 slower and larger code at the expense of conserving TOC space.
10479 If you still run out of space in the TOC even when you specify both of
10480 these options, specify @option{-mminimal-toc} instead. This option causes
10481 GCC to make only one TOC entry for every file. When you specify this
10482 option, GCC will produce code that is slower and larger but which
10483 uses extremely little TOC space. You may wish to use this option
10484 only on files that contain less frequently executed code.
10490 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
10491 @code{long} type, and the infrastructure needed to support them.
10492 Specifying @option{-maix64} implies @option{-mpowerpc64} and
10493 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
10494 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
10497 @itemx -mno-xl-compat
10498 @opindex mxl-compat
10499 @opindex mno-xl-compat
10500 Produce code that conforms more closely to IBM XLC semantics when using
10501 AIX-compatible ABI. Pass floating-point arguments to prototyped
10502 functions beyond the register save area (RSA) on the stack in addition
10503 to argument FPRs. Do not assume that most significant double in 128
10504 bit long double value is properly rounded when comparing values.
10506 The AIX calling convention was extended but not initially documented to
10507 handle an obscure K&R C case of calling a function that takes the
10508 address of its arguments with fewer arguments than declared. AIX XL
10509 compilers access floating point arguments which do not fit in the
10510 RSA from the stack when a subroutine is compiled without
10511 optimization. Because always storing floating-point arguments on the
10512 stack is inefficient and rarely needed, this option is not enabled by
10513 default and only is necessary when calling subroutines compiled by AIX
10514 XL compilers without optimization.
10518 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
10519 application written to use message passing with special startup code to
10520 enable the application to run. The system must have PE installed in the
10521 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
10522 must be overridden with the @option{-specs=} option to specify the
10523 appropriate directory location. The Parallel Environment does not
10524 support threads, so the @option{-mpe} option and the @option{-pthread}
10525 option are incompatible.
10527 @item -malign-natural
10528 @itemx -malign-power
10529 @opindex malign-natural
10530 @opindex malign-power
10531 On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
10532 @option{-malign-natural} overrides the ABI-defined alignment of larger
10533 types, such as floating-point doubles, on their natural size-based boundary.
10534 The option @option{-malign-power} instructs GCC to follow the ABI-specified
10535 alignment rules. GCC defaults to the standard alignment defined in the ABI@.
10537 On 64-bit Darwin, natural alignment is the default, and @option{-malign-power}
10541 @itemx -mhard-float
10542 @opindex msoft-float
10543 @opindex mhard-float
10544 Generate code that does not use (uses) the floating-point register set.
10545 Software floating point emulation is provided if you use the
10546 @option{-msoft-float} option, and pass the option to GCC when linking.
10549 @itemx -mno-multiple
10551 @opindex mno-multiple
10552 Generate code that uses (does not use) the load multiple word
10553 instructions and the store multiple word instructions. These
10554 instructions are generated by default on POWER systems, and not
10555 generated on PowerPC systems. Do not use @option{-mmultiple} on little
10556 endian PowerPC systems, since those instructions do not work when the
10557 processor is in little endian mode. The exceptions are PPC740 and
10558 PPC750 which permit the instructions usage in little endian mode.
10563 @opindex mno-string
10564 Generate code that uses (does not use) the load string instructions
10565 and the store string word instructions to save multiple registers and
10566 do small block moves. These instructions are generated by default on
10567 POWER systems, and not generated on PowerPC systems. Do not use
10568 @option{-mstring} on little endian PowerPC systems, since those
10569 instructions do not work when the processor is in little endian mode.
10570 The exceptions are PPC740 and PPC750 which permit the instructions
10571 usage in little endian mode.
10576 @opindex mno-update
10577 Generate code that uses (does not use) the load or store instructions
10578 that update the base register to the address of the calculated memory
10579 location. These instructions are generated by default. If you use
10580 @option{-mno-update}, there is a small window between the time that the
10581 stack pointer is updated and the address of the previous frame is
10582 stored, which means code that walks the stack frame across interrupts or
10583 signals may get corrupted data.
10586 @itemx -mno-fused-madd
10587 @opindex mfused-madd
10588 @opindex mno-fused-madd
10589 Generate code that uses (does not use) the floating point multiply and
10590 accumulate instructions. These instructions are generated by default if
10591 hardware floating is used.
10593 @item -mno-bit-align
10595 @opindex mno-bit-align
10596 @opindex mbit-align
10597 On System V.4 and embedded PowerPC systems do not (do) force structures
10598 and unions that contain bit-fields to be aligned to the base type of the
10601 For example, by default a structure containing nothing but 8
10602 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
10603 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
10604 the structure would be aligned to a 1 byte boundary and be one byte in
10607 @item -mno-strict-align
10608 @itemx -mstrict-align
10609 @opindex mno-strict-align
10610 @opindex mstrict-align
10611 On System V.4 and embedded PowerPC systems do not (do) assume that
10612 unaligned memory references will be handled by the system.
10614 @item -mrelocatable
10615 @itemx -mno-relocatable
10616 @opindex mrelocatable
10617 @opindex mno-relocatable
10618 On embedded PowerPC systems generate code that allows (does not allow)
10619 the program to be relocated to a different address at runtime. If you
10620 use @option{-mrelocatable} on any module, all objects linked together must
10621 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
10623 @item -mrelocatable-lib
10624 @itemx -mno-relocatable-lib
10625 @opindex mrelocatable-lib
10626 @opindex mno-relocatable-lib
10627 On embedded PowerPC systems generate code that allows (does not allow)
10628 the program to be relocated to a different address at runtime. Modules
10629 compiled with @option{-mrelocatable-lib} can be linked with either modules
10630 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
10631 with modules compiled with the @option{-mrelocatable} options.
10637 On System V.4 and embedded PowerPC systems do not (do) assume that
10638 register 2 contains a pointer to a global area pointing to the addresses
10639 used in the program.
10642 @itemx -mlittle-endian
10644 @opindex mlittle-endian
10645 On System V.4 and embedded PowerPC systems compile code for the
10646 processor in little endian mode. The @option{-mlittle-endian} option is
10647 the same as @option{-mlittle}.
10650 @itemx -mbig-endian
10652 @opindex mbig-endian
10653 On System V.4 and embedded PowerPC systems compile code for the
10654 processor in big endian mode. The @option{-mbig-endian} option is
10655 the same as @option{-mbig}.
10657 @item -mdynamic-no-pic
10658 @opindex mdynamic-no-pic
10659 On Darwin and Mac OS X systems, compile code so that it is not
10660 relocatable, but that its external references are relocatable. The
10661 resulting code is suitable for applications, but not shared
10664 @item -mprioritize-restricted-insns=@var{priority}
10665 @opindex mprioritize-restricted-insns
10666 This option controls the priority that is assigned to
10667 dispatch-slot restricted instructions during the second scheduling
10668 pass. The argument @var{priority} takes the value @var{0/1/2} to assign
10669 @var{no/highest/second-highest} priority to dispatch slot restricted
10672 @item -msched-costly-dep=@var{dependence_type}
10673 @opindex msched-costly-dep
10674 This option controls which dependences are considered costly
10675 by the target during instruction scheduling. The argument
10676 @var{dependence_type} takes one of the following values:
10677 @var{no}: no dependence is costly,
10678 @var{all}: all dependences are costly,
10679 @var{true_store_to_load}: a true dependence from store to load is costly,
10680 @var{store_to_load}: any dependence from store to load is costly,
10681 @var{number}: any dependence which latency >= @var{number} is costly.
10683 @item -minsert-sched-nops=@var{scheme}
10684 @opindex minsert-sched-nops
10685 This option controls which nop insertion scheme will be used during
10686 the second scheduling pass. The argument @var{scheme} takes one of the
10688 @var{no}: Don't insert nops.
10689 @var{pad}: Pad with nops any dispatch group which has vacant issue slots,
10690 according to the scheduler's grouping.
10691 @var{regroup_exact}: Insert nops to force costly dependent insns into
10692 separate groups. Insert exactly as many nops as needed to force an insn
10693 to a new group, according to the estimated processor grouping.
10694 @var{number}: Insert nops to force costly dependent insns into
10695 separate groups. Insert @var{number} nops to force an insn to a new group.
10698 @opindex mcall-sysv
10699 On System V.4 and embedded PowerPC systems compile code using calling
10700 conventions that adheres to the March 1995 draft of the System V
10701 Application Binary Interface, PowerPC processor supplement. This is the
10702 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
10704 @item -mcall-sysv-eabi
10705 @opindex mcall-sysv-eabi
10706 Specify both @option{-mcall-sysv} and @option{-meabi} options.
10708 @item -mcall-sysv-noeabi
10709 @opindex mcall-sysv-noeabi
10710 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
10712 @item -mcall-solaris
10713 @opindex mcall-solaris
10714 On System V.4 and embedded PowerPC systems compile code for the Solaris
10718 @opindex mcall-linux
10719 On System V.4 and embedded PowerPC systems compile code for the
10720 Linux-based GNU system.
10724 On System V.4 and embedded PowerPC systems compile code for the
10725 Hurd-based GNU system.
10727 @item -mcall-netbsd
10728 @opindex mcall-netbsd
10729 On System V.4 and embedded PowerPC systems compile code for the
10730 NetBSD operating system.
10732 @item -maix-struct-return
10733 @opindex maix-struct-return
10734 Return all structures in memory (as specified by the AIX ABI)@.
10736 @item -msvr4-struct-return
10737 @opindex msvr4-struct-return
10738 Return structures smaller than 8 bytes in registers (as specified by the
10741 @item -mabi=altivec
10742 @opindex mabi=altivec
10743 Extend the current ABI with AltiVec ABI extensions. This does not
10744 change the default ABI, instead it adds the AltiVec ABI extensions to
10747 @item -mabi=no-altivec
10748 @opindex mabi=no-altivec
10749 Disable AltiVec ABI extensions for the current ABI@.
10752 @itemx -mno-prototype
10753 @opindex mprototype
10754 @opindex mno-prototype
10755 On System V.4 and embedded PowerPC systems assume that all calls to
10756 variable argument functions are properly prototyped. Otherwise, the
10757 compiler must insert an instruction before every non prototyped call to
10758 set or clear bit 6 of the condition code register (@var{CR}) to
10759 indicate whether floating point values were passed in the floating point
10760 registers in case the function takes a variable arguments. With
10761 @option{-mprototype}, only calls to prototyped variable argument functions
10762 will set or clear the bit.
10766 On embedded PowerPC systems, assume that the startup module is called
10767 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
10768 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
10773 On embedded PowerPC systems, assume that the startup module is called
10774 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
10779 On embedded PowerPC systems, assume that the startup module is called
10780 @file{crt0.o} and the standard C libraries are @file{libads.a} and
10783 @item -myellowknife
10784 @opindex myellowknife
10785 On embedded PowerPC systems, assume that the startup module is called
10786 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
10791 On System V.4 and embedded PowerPC systems, specify that you are
10792 compiling for a VxWorks system.
10796 Specify that you are compiling for the WindISS simulation environment.
10800 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
10801 header to indicate that @samp{eabi} extended relocations are used.
10807 On System V.4 and embedded PowerPC systems do (do not) adhere to the
10808 Embedded Applications Binary Interface (eabi) which is a set of
10809 modifications to the System V.4 specifications. Selecting @option{-meabi}
10810 means that the stack is aligned to an 8 byte boundary, a function
10811 @code{__eabi} is called to from @code{main} to set up the eabi
10812 environment, and the @option{-msdata} option can use both @code{r2} and
10813 @code{r13} to point to two separate small data areas. Selecting
10814 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
10815 do not call an initialization function from @code{main}, and the
10816 @option{-msdata} option will only use @code{r13} to point to a single
10817 small data area. The @option{-meabi} option is on by default if you
10818 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
10821 @opindex msdata=eabi
10822 On System V.4 and embedded PowerPC systems, put small initialized
10823 @code{const} global and static data in the @samp{.sdata2} section, which
10824 is pointed to by register @code{r2}. Put small initialized
10825 non-@code{const} global and static data in the @samp{.sdata} section,
10826 which is pointed to by register @code{r13}. Put small uninitialized
10827 global and static data in the @samp{.sbss} section, which is adjacent to
10828 the @samp{.sdata} section. The @option{-msdata=eabi} option is
10829 incompatible with the @option{-mrelocatable} option. The
10830 @option{-msdata=eabi} option also sets the @option{-memb} option.
10833 @opindex msdata=sysv
10834 On System V.4 and embedded PowerPC systems, put small global and static
10835 data in the @samp{.sdata} section, which is pointed to by register
10836 @code{r13}. Put small uninitialized global and static data in the
10837 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
10838 The @option{-msdata=sysv} option is incompatible with the
10839 @option{-mrelocatable} option.
10841 @item -msdata=default
10843 @opindex msdata=default
10845 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
10846 compile code the same as @option{-msdata=eabi}, otherwise compile code the
10847 same as @option{-msdata=sysv}.
10850 @opindex msdata-data
10851 On System V.4 and embedded PowerPC systems, put small global and static
10852 data in the @samp{.sdata} section. Put small uninitialized global and
10853 static data in the @samp{.sbss} section. Do not use register @code{r13}
10854 to address small data however. This is the default behavior unless
10855 other @option{-msdata} options are used.
10859 @opindex msdata=none
10861 On embedded PowerPC systems, put all initialized global and static data
10862 in the @samp{.data} section, and all uninitialized data in the
10863 @samp{.bss} section.
10867 @cindex smaller data references (PowerPC)
10868 @cindex .sdata/.sdata2 references (PowerPC)
10869 On embedded PowerPC systems, put global and static items less than or
10870 equal to @var{num} bytes into the small data or bss sections instead of
10871 the normal data or bss section. By default, @var{num} is 8. The
10872 @option{-G @var{num}} switch is also passed to the linker.
10873 All modules should be compiled with the same @option{-G @var{num}} value.
10876 @itemx -mno-regnames
10878 @opindex mno-regnames
10879 On System V.4 and embedded PowerPC systems do (do not) emit register
10880 names in the assembly language output using symbolic forms.
10883 @itemx -mno-longcall
10885 @opindex mno-longcall
10886 Default to making all function calls indirectly, using a register, so
10887 that functions which reside further than 32 megabytes (33,554,432
10888 bytes) from the current location can be called. This setting can be
10889 overridden by the @code{shortcall} function attribute, or by
10890 @code{#pragma longcall(0)}.
10892 Some linkers are capable of detecting out-of-range calls and generating
10893 glue code on the fly. On these systems, long calls are unnecessary and
10894 generate slower code. As of this writing, the AIX linker can do this,
10895 as can the GNU linker for PowerPC/64. It is planned to add this feature
10896 to the GNU linker for 32-bit PowerPC systems as well.
10898 On Darwin/PPC systems, @code{#pragma longcall} will generate ``jbsr
10899 callee, L42'', plus a ``branch island'' (glue code). The two target
10900 addresses represent the callee and the ``branch island''. The
10901 Darwin/PPC linker will prefer the first address and generate a ``bl
10902 callee'' if the PPC ``bl'' instruction will reach the callee directly;
10903 otherwise, the linker will generate ``bl L42'' to call the ``branch
10904 island''. The ``branch island'' is appended to the body of the
10905 calling function; it computes the full 32-bit address of the callee
10908 On Mach-O (Darwin) systems, this option directs the compiler emit to
10909 the glue for every direct call, and the Darwin linker decides whether
10910 to use or discard it.
10912 In the future, we may cause GCC to ignore all longcall specifications
10913 when the linker is known to generate glue.
10917 Adds support for multithreading with the @dfn{pthreads} library.
10918 This option sets flags for both the preprocessor and linker.
10922 @node S/390 and zSeries Options
10923 @subsection S/390 and zSeries Options
10924 @cindex S/390 and zSeries Options
10926 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
10930 @itemx -msoft-float
10931 @opindex mhard-float
10932 @opindex msoft-float
10933 Use (do not use) the hardware floating-point instructions and registers
10934 for floating-point operations. When @option{-msoft-float} is specified,
10935 functions in @file{libgcc.a} will be used to perform floating-point
10936 operations. When @option{-mhard-float} is specified, the compiler
10937 generates IEEE floating-point instructions. This is the default.
10940 @itemx -mno-backchain
10941 @opindex mbackchain
10942 @opindex mno-backchain
10943 Store (do not store) the address of the caller's frame as backchain pointer
10944 into the callee's stack frame.
10945 A backchain may be needed to allow debugging using tools that do not understand
10946 DWARF-2 call frame information.
10947 When @option{-mno-packed-stack} is in effect, the backchain pointer is stored
10948 at the bottom of the stack frame; when @option{-mpacked-stack} is in effect,
10949 the backchain is placed into the topmost word of the 96/160 byte register
10952 In general, code compiled with @option{-mbackchain} is call-compatible with
10953 code compiled with @option{-mmo-backchain}; however, use of the backchain
10954 for debugging purposes usually requires that the whole binary is built with
10955 @option{-mbackchain}. Note that the combination of @option{-mbackchain},
10956 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
10957 to build a linux kernel use @option{-msoft-float}.
10959 The default is to not maintain the backchain.
10961 @item -mpacked-stack
10962 @item -mno-packed-stack
10963 @opindex mpacked-stack
10964 @opindex mno-packed-stack
10965 Use (do not use) the packed stack layout. When @option{-mno-packed-stack} is
10966 specified, the compiler uses the all fields of the 96/160 byte register save
10967 area only for their default purpose; unused fields still take up stack space.
10968 When @option{-mpacked-stack} is specified, register save slots are densely
10969 packed at the top of the register save area; unused space is reused for other
10970 purposes, allowing for more efficient use of the available stack space.
10971 However, when @option{-mbackchain} is also in effect, the topmost word of
10972 the save area is always used to store the backchain, and the return address
10973 register is always saved two words below the backchain.
10975 As long as the stack frame backchain is not used, code generated with
10976 @option{-mpacked-stack} is call-compatible with code generated with
10977 @option{-mno-packed-stack}. Note that some non-FSF releases of GCC 2.95 for
10978 S/390 or zSeries generated code that uses the stack frame backchain at run
10979 time, not just for debugging purposes. Such code is not call-compatible
10980 with code compiled with @option{-mpacked-stack}. Also, note that the
10981 combination of @option{-mbackchain},
10982 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
10983 to build a linux kernel use @option{-msoft-float}.
10985 The default is to not use the packed stack layout.
10988 @itemx -mno-small-exec
10989 @opindex msmall-exec
10990 @opindex mno-small-exec
10991 Generate (or do not generate) code using the @code{bras} instruction
10992 to do subroutine calls.
10993 This only works reliably if the total executable size does not
10994 exceed 64k. The default is to use the @code{basr} instruction instead,
10995 which does not have this limitation.
11001 When @option{-m31} is specified, generate code compliant to the
11002 GNU/Linux for S/390 ABI@. When @option{-m64} is specified, generate
11003 code compliant to the GNU/Linux for zSeries ABI@. This allows GCC in
11004 particular to generate 64-bit instructions. For the @samp{s390}
11005 targets, the default is @option{-m31}, while the @samp{s390x}
11006 targets default to @option{-m64}.
11012 When @option{-mzarch} is specified, generate code using the
11013 instructions available on z/Architecture.
11014 When @option{-mesa} is specified, generate code using the
11015 instructions available on ESA/390. Note that @option{-mesa} is
11016 not possible with @option{-m64}.
11017 When generating code compliant to the GNU/Linux for S/390 ABI,
11018 the default is @option{-mesa}. When generating code compliant
11019 to the GNU/Linux for zSeries ABI, the default is @option{-mzarch}.
11025 Generate (or do not generate) code using the @code{mvcle} instruction
11026 to perform block moves. When @option{-mno-mvcle} is specified,
11027 use a @code{mvc} loop instead. This is the default.
11033 Print (or do not print) additional debug information when compiling.
11034 The default is to not print debug information.
11036 @item -march=@var{cpu-type}
11038 Generate code that will run on @var{cpu-type}, which is the name of a system
11039 representing a certain processor type. Possible values for
11040 @var{cpu-type} are @samp{g5}, @samp{g6}, @samp{z900}, and @samp{z990}.
11041 When generating code using the instructions available on z/Architecture,
11042 the default is @option{-march=z900}. Otherwise, the default is
11043 @option{-march=g5}.
11045 @item -mtune=@var{cpu-type}
11047 Tune to @var{cpu-type} everything applicable about the generated code,
11048 except for the ABI and the set of available instructions.
11049 The list of @var{cpu-type} values is the same as for @option{-march}.
11050 The default is the value used for @option{-march}.
11053 @itemx -mno-tpf-trace
11054 @opindex mtpf-trace
11055 @opindex mno-tpf-trace
11056 Generate code that adds (does not add) in TPF OS specific branches to trace
11057 routines in the operating system. This option is off by default, even
11058 when compiling for the TPF OS@.
11061 @itemx -mno-fused-madd
11062 @opindex mfused-madd
11063 @opindex mno-fused-madd
11064 Generate code that uses (does not use) the floating point multiply and
11065 accumulate instructions. These instructions are generated by default if
11066 hardware floating point is used.
11068 @item -mwarn-framesize=@var{framesize}
11069 @opindex mwarn-framesize
11070 Emit a warning if the current function exceeds the given frame size. Because
11071 this is a compile time check it doesn't need to be a real problem when the program
11072 runs. It is intended to identify functions which most probably cause
11073 a stack overflow. It is useful to be used in an environment with limited stack
11074 size e.g.@: the linux kernel.
11076 @item -mwarn-dynamicstack
11077 @opindex mwarn-dynamicstack
11078 Emit a warning if the function calls alloca or uses dynamically
11079 sized arrays. This is generally a bad idea with a limited stack size.
11081 @item -mstack-guard=@var{stack-guard}
11082 @item -mstack-size=@var{stack-size}
11083 @opindex mstack-guard
11084 @opindex mstack-size
11085 These arguments always have to be used in conjunction. If they are present the s390
11086 back end emits additional instructions in the function prologue which trigger a trap
11087 if the stack size is @var{stack-guard} bytes above the @var{stack-size}
11088 (remember that the stack on s390 grows downward). These options are intended to
11089 be used to help debugging stack overflow problems. The additionally emitted code
11090 cause only little overhead and hence can also be used in production like systems
11091 without greater performance degradation. The given values have to be exact
11092 powers of 2 and @var{stack-size} has to be greater than @var{stack-guard}.
11093 In order to be efficient the extra code makes the assumption that the stack starts
11094 at an address aligned to the value given by @var{stack-size}.
11098 @subsection SH Options
11100 These @samp{-m} options are defined for the SH implementations:
11105 Generate code for the SH1.
11109 Generate code for the SH2.
11112 Generate code for the SH2e.
11116 Generate code for the SH3.
11120 Generate code for the SH3e.
11124 Generate code for the SH4 without a floating-point unit.
11126 @item -m4-single-only
11127 @opindex m4-single-only
11128 Generate code for the SH4 with a floating-point unit that only
11129 supports single-precision arithmetic.
11133 Generate code for the SH4 assuming the floating-point unit is in
11134 single-precision mode by default.
11138 Generate code for the SH4.
11142 Generate code for the SH4al-dsp, or for a SH4a in such a way that the
11143 floating-point unit is not used.
11145 @item -m4a-single-only
11146 @opindex m4a-single-only
11147 Generate code for the SH4a, in such a way that no double-precision
11148 floating point operations are used.
11151 @opindex m4a-single
11152 Generate code for the SH4a assuming the floating-point unit is in
11153 single-precision mode by default.
11157 Generate code for the SH4a.
11161 Same as @option{-m4a-nofpu}, except that it implicitly passes
11162 @option{-dsp} to the assembler. GCC doesn't generate any DSP
11163 instructions at the moment.
11167 Compile code for the processor in big endian mode.
11171 Compile code for the processor in little endian mode.
11175 Align doubles at 64-bit boundaries. Note that this changes the calling
11176 conventions, and thus some functions from the standard C library will
11177 not work unless you recompile it first with @option{-mdalign}.
11181 Shorten some address references at link time, when possible; uses the
11182 linker option @option{-relax}.
11186 Use 32-bit offsets in @code{switch} tables. The default is to use
11191 Enable the use of the instruction @code{fmovd}.
11195 Comply with the calling conventions defined by Renesas.
11199 Comply with the calling conventions defined by Renesas.
11203 Comply with the calling conventions defined for GCC before the Renesas
11204 conventions were available. This option is the default for all
11205 targets of the SH toolchain except for @samp{sh-symbianelf}.
11208 @opindex mnomacsave
11209 Mark the @code{MAC} register as call-clobbered, even if
11210 @option{-mhitachi} is given.
11214 Increase IEEE-compliance of floating-point code.
11218 Dump instruction size and location in the assembly code.
11221 @opindex mpadstruct
11222 This option is deprecated. It pads structures to multiple of 4 bytes,
11223 which is incompatible with the SH ABI@.
11227 Optimize for space instead of speed. Implied by @option{-Os}.
11230 @opindex mprefergot
11231 When generating position-independent code, emit function calls using
11232 the Global Offset Table instead of the Procedure Linkage Table.
11236 Generate a library function call to invalidate instruction cache
11237 entries, after fixing up a trampoline. This library function call
11238 doesn't assume it can write to the whole memory address space. This
11239 is the default when the target is @code{sh-*-linux*}.
11242 @node SPARC Options
11243 @subsection SPARC Options
11244 @cindex SPARC options
11246 These @samp{-m} options are supported on the SPARC:
11249 @item -mno-app-regs
11251 @opindex mno-app-regs
11253 Specify @option{-mapp-regs} to generate output using the global registers
11254 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
11257 To be fully SVR4 ABI compliant at the cost of some performance loss,
11258 specify @option{-mno-app-regs}. You should compile libraries and system
11259 software with this option.
11262 @itemx -mhard-float
11264 @opindex mhard-float
11265 Generate output containing floating point instructions. This is the
11269 @itemx -msoft-float
11271 @opindex msoft-float
11272 Generate output containing library calls for floating point.
11273 @strong{Warning:} the requisite libraries are not available for all SPARC
11274 targets. Normally the facilities of the machine's usual C compiler are
11275 used, but this cannot be done directly in cross-compilation. You must make
11276 your own arrangements to provide suitable library functions for
11277 cross-compilation. The embedded targets @samp{sparc-*-aout} and
11278 @samp{sparclite-*-*} do provide software floating point support.
11280 @option{-msoft-float} changes the calling convention in the output file;
11281 therefore, it is only useful if you compile @emph{all} of a program with
11282 this option. In particular, you need to compile @file{libgcc.a}, the
11283 library that comes with GCC, with @option{-msoft-float} in order for
11286 @item -mhard-quad-float
11287 @opindex mhard-quad-float
11288 Generate output containing quad-word (long double) floating point
11291 @item -msoft-quad-float
11292 @opindex msoft-quad-float
11293 Generate output containing library calls for quad-word (long double)
11294 floating point instructions. The functions called are those specified
11295 in the SPARC ABI@. This is the default.
11297 As of this writing, there are no SPARC implementations that have hardware
11298 support for the quad-word floating point instructions. They all invoke
11299 a trap handler for one of these instructions, and then the trap handler
11300 emulates the effect of the instruction. Because of the trap handler overhead,
11301 this is much slower than calling the ABI library routines. Thus the
11302 @option{-msoft-quad-float} option is the default.
11304 @item -mno-unaligned-doubles
11305 @itemx -munaligned-doubles
11306 @opindex mno-unaligned-doubles
11307 @opindex munaligned-doubles
11308 Assume that doubles have 8 byte alignment. This is the default.
11310 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
11311 alignment only if they are contained in another type, or if they have an
11312 absolute address. Otherwise, it assumes they have 4 byte alignment.
11313 Specifying this option avoids some rare compatibility problems with code
11314 generated by other compilers. It is not the default because it results
11315 in a performance loss, especially for floating point code.
11317 @item -mno-faster-structs
11318 @itemx -mfaster-structs
11319 @opindex mno-faster-structs
11320 @opindex mfaster-structs
11321 With @option{-mfaster-structs}, the compiler assumes that structures
11322 should have 8 byte alignment. This enables the use of pairs of
11323 @code{ldd} and @code{std} instructions for copies in structure
11324 assignment, in place of twice as many @code{ld} and @code{st} pairs.
11325 However, the use of this changed alignment directly violates the SPARC
11326 ABI@. Thus, it's intended only for use on targets where the developer
11327 acknowledges that their resulting code will not be directly in line with
11328 the rules of the ABI@.
11330 @item -mimpure-text
11331 @opindex mimpure-text
11332 @option{-mimpure-text}, used in addition to @option{-shared}, tells
11333 the compiler to not pass @option{-z text} to the linker when linking a
11334 shared object. Using this option, you can link position-dependent
11335 code into a shared object.
11337 @option{-mimpure-text} suppresses the ``relocations remain against
11338 allocatable but non-writable sections'' linker error message.
11339 However, the necessary relocations will trigger copy-on-write, and the
11340 shared object is not actually shared across processes. Instead of
11341 using @option{-mimpure-text}, you should compile all source code with
11342 @option{-fpic} or @option{-fPIC}.
11344 This option is only available on SunOS and Solaris.
11346 @item -mcpu=@var{cpu_type}
11348 Set the instruction set, register set, and instruction scheduling parameters
11349 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
11350 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
11351 @samp{f930}, @samp{f934}, @samp{hypersparc}, @samp{sparclite86x},
11352 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc}, and
11353 @samp{ultrasparc3}.
11355 Default instruction scheduling parameters are used for values that select
11356 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
11357 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
11359 Here is a list of each supported architecture and their supported
11364 v8: supersparc, hypersparc
11365 sparclite: f930, f934, sparclite86x
11367 v9: ultrasparc, ultrasparc3
11370 By default (unless configured otherwise), GCC generates code for the V7
11371 variant of the SPARC architecture. With @option{-mcpu=cypress}, the compiler
11372 additionally optimizes it for the Cypress CY7C602 chip, as used in the
11373 SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
11374 SPARCStation 1, 2, IPX etc.
11376 With @option{-mcpu=v8}, GCC generates code for the V8 variant of the SPARC
11377 architecture. The only difference from V7 code is that the compiler emits
11378 the integer multiply and integer divide instructions which exist in SPARC-V8
11379 but not in SPARC-V7. With @option{-mcpu=supersparc}, the compiler additionally
11380 optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
11383 With @option{-mcpu=sparclite}, GCC generates code for the SPARClite variant of
11384 the SPARC architecture. This adds the integer multiply, integer divide step
11385 and scan (@code{ffs}) instructions which exist in SPARClite but not in SPARC-V7.
11386 With @option{-mcpu=f930}, the compiler additionally optimizes it for the
11387 Fujitsu MB86930 chip, which is the original SPARClite, with no FPU@. With
11388 @option{-mcpu=f934}, the compiler additionally optimizes it for the Fujitsu
11389 MB86934 chip, which is the more recent SPARClite with FPU@.
11391 With @option{-mcpu=sparclet}, GCC generates code for the SPARClet variant of
11392 the SPARC architecture. This adds the integer multiply, multiply/accumulate,
11393 integer divide step and scan (@code{ffs}) instructions which exist in SPARClet
11394 but not in SPARC-V7. With @option{-mcpu=tsc701}, the compiler additionally
11395 optimizes it for the TEMIC SPARClet chip.
11397 With @option{-mcpu=v9}, GCC generates code for the V9 variant of the SPARC
11398 architecture. This adds 64-bit integer and floating-point move instructions,
11399 3 additional floating-point condition code registers and conditional move
11400 instructions. With @option{-mcpu=ultrasparc}, the compiler additionally
11401 optimizes it for the Sun UltraSPARC I/II chips. With
11402 @option{-mcpu=ultrasparc3}, the compiler additionally optimizes it for the
11403 Sun UltraSPARC III chip.
11405 @item -mtune=@var{cpu_type}
11407 Set the instruction scheduling parameters for machine type
11408 @var{cpu_type}, but do not set the instruction set or register set that the
11409 option @option{-mcpu=@var{cpu_type}} would.
11411 The same values for @option{-mcpu=@var{cpu_type}} can be used for
11412 @option{-mtune=@var{cpu_type}}, but the only useful values are those
11413 that select a particular cpu implementation. Those are @samp{cypress},
11414 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
11415 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc}, and
11416 @samp{ultrasparc3}.
11421 @opindex mno-v8plus
11422 With @option{-mv8plus}, GCC generates code for the SPARC-V8+ ABI@. The
11423 difference from the V8 ABI is that the global and out registers are
11424 considered 64-bit wide. This is enabled by default on Solaris in 32-bit
11425 mode for all SPARC-V9 processors.
11431 With @option{-mvis}, GCC generates code that takes advantage of the UltraSPARC
11432 Visual Instruction Set extensions. The default is @option{-mno-vis}.
11435 These @samp{-m} options are supported in addition to the above
11436 on SPARC-V9 processors in 64-bit environments:
11439 @item -mlittle-endian
11440 @opindex mlittle-endian
11441 Generate code for a processor running in little-endian mode. It is only
11442 available for a few configurations and most notably not on Solaris.
11448 Generate code for a 32-bit or 64-bit environment.
11449 The 32-bit environment sets int, long and pointer to 32 bits.
11450 The 64-bit environment sets int to 32 bits and long and pointer
11453 @item -mcmodel=medlow
11454 @opindex mcmodel=medlow
11455 Generate code for the Medium/Low code model: 64-bit addresses, programs
11456 must be linked in the low 32 bits of memory. Programs can be statically
11457 or dynamically linked.
11459 @item -mcmodel=medmid
11460 @opindex mcmodel=medmid
11461 Generate code for the Medium/Middle code model: 64-bit addresses, programs
11462 must be linked in the low 44 bits of memory, the text and data segments must
11463 be less than 2GB in size and the data segment must be located within 2GB of
11466 @item -mcmodel=medany
11467 @opindex mcmodel=medany
11468 Generate code for the Medium/Anywhere code model: 64-bit addresses, programs
11469 may be linked anywhere in memory, the text and data segments must be less
11470 than 2GB in size and the data segment must be located within 2GB of the
11473 @item -mcmodel=embmedany
11474 @opindex mcmodel=embmedany
11475 Generate code for the Medium/Anywhere code model for embedded systems:
11476 64-bit addresses, the text and data segments must be less than 2GB in
11477 size, both starting anywhere in memory (determined at link time). The
11478 global register %g4 points to the base of the data segment. Programs
11479 are statically linked and PIC is not supported.
11482 @itemx -mno-stack-bias
11483 @opindex mstack-bias
11484 @opindex mno-stack-bias
11485 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
11486 frame pointer if present, are offset by @minus{}2047 which must be added back
11487 when making stack frame references. This is the default in 64-bit mode.
11488 Otherwise, assume no such offset is present.
11491 These switches are supported in addition to the above on Solaris:
11496 Add support for multithreading using the Solaris threads library. This
11497 option sets flags for both the preprocessor and linker. This option does
11498 not affect the thread safety of object code produced by the compiler or
11499 that of libraries supplied with it.
11503 Add support for multithreading using the POSIX threads library. This
11504 option sets flags for both the preprocessor and linker. This option does
11505 not affect the thread safety of object code produced by the compiler or
11506 that of libraries supplied with it.
11509 @node System V Options
11510 @subsection Options for System V
11512 These additional options are available on System V Release 4 for
11513 compatibility with other compilers on those systems:
11518 Create a shared object.
11519 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
11523 Identify the versions of each tool used by the compiler, in a
11524 @code{.ident} assembler directive in the output.
11528 Refrain from adding @code{.ident} directives to the output file (this is
11531 @item -YP,@var{dirs}
11533 Search the directories @var{dirs}, and no others, for libraries
11534 specified with @option{-l}.
11536 @item -Ym,@var{dir}
11538 Look in the directory @var{dir} to find the M4 preprocessor.
11539 The assembler uses this option.
11540 @c This is supposed to go with a -Yd for predefined M4 macro files, but
11541 @c the generic assembler that comes with Solaris takes just -Ym.
11544 @node TMS320C3x/C4x Options
11545 @subsection TMS320C3x/C4x Options
11546 @cindex TMS320C3x/C4x Options
11548 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
11552 @item -mcpu=@var{cpu_type}
11554 Set the instruction set, register set, and instruction scheduling
11555 parameters for machine type @var{cpu_type}. Supported values for
11556 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
11557 @samp{c44}. The default is @samp{c40} to generate code for the
11562 @itemx -msmall-memory
11564 @opindex mbig-memory
11566 @opindex msmall-memory
11568 Generates code for the big or small memory model. The small memory
11569 model assumed that all data fits into one 64K word page. At run-time
11570 the data page (DP) register must be set to point to the 64K page
11571 containing the .bss and .data program sections. The big memory model is
11572 the default and requires reloading of the DP register for every direct
11579 Allow (disallow) allocation of general integer operands into the block
11580 count register BK@.
11586 Enable (disable) generation of code using decrement and branch,
11587 DBcond(D), instructions. This is enabled by default for the C4x. To be
11588 on the safe side, this is disabled for the C3x, since the maximum
11589 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
11590 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
11591 that it can utilize the decrement and branch instruction, but will give
11592 up if there is more than one memory reference in the loop. Thus a loop
11593 where the loop counter is decremented can generate slightly more
11594 efficient code, in cases where the RPTB instruction cannot be utilized.
11596 @item -mdp-isr-reload
11598 @opindex mdp-isr-reload
11600 Force the DP register to be saved on entry to an interrupt service
11601 routine (ISR), reloaded to point to the data section, and restored on
11602 exit from the ISR@. This should not be required unless someone has
11603 violated the small memory model by modifying the DP register, say within
11610 For the C3x use the 24-bit MPYI instruction for integer multiplies
11611 instead of a library call to guarantee 32-bit results. Note that if one
11612 of the operands is a constant, then the multiplication will be performed
11613 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
11614 then squaring operations are performed inline instead of a library call.
11617 @itemx -mno-fast-fix
11619 @opindex mno-fast-fix
11620 The C3x/C4x FIX instruction to convert a floating point value to an
11621 integer value chooses the nearest integer less than or equal to the
11622 floating point value rather than to the nearest integer. Thus if the
11623 floating point number is negative, the result will be incorrectly
11624 truncated an additional code is necessary to detect and correct this
11625 case. This option can be used to disable generation of the additional
11626 code required to correct the result.
11632 Enable (disable) generation of repeat block sequences using the RPTB
11633 instruction for zero overhead looping. The RPTB construct is only used
11634 for innermost loops that do not call functions or jump across the loop
11635 boundaries. There is no advantage having nested RPTB loops due to the
11636 overhead required to save and restore the RC, RS, and RE registers.
11637 This is enabled by default with @option{-O2}.
11639 @item -mrpts=@var{count}
11643 Enable (disable) the use of the single instruction repeat instruction
11644 RPTS@. If a repeat block contains a single instruction, and the loop
11645 count can be guaranteed to be less than the value @var{count}, GCC will
11646 emit a RPTS instruction instead of a RPTB@. If no value is specified,
11647 then a RPTS will be emitted even if the loop count cannot be determined
11648 at compile time. Note that the repeated instruction following RPTS does
11649 not have to be reloaded from memory each iteration, thus freeing up the
11650 CPU buses for operands. However, since interrupts are blocked by this
11651 instruction, it is disabled by default.
11653 @item -mloop-unsigned
11654 @itemx -mno-loop-unsigned
11655 @opindex mloop-unsigned
11656 @opindex mno-loop-unsigned
11657 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
11658 is @math{2^{31} + 1} since these instructions test if the iteration count is
11659 negative to terminate the loop. If the iteration count is unsigned
11660 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
11661 exceeded. This switch allows an unsigned iteration count.
11665 Try to emit an assembler syntax that the TI assembler (asm30) is happy
11666 with. This also enforces compatibility with the API employed by the TI
11667 C3x C compiler. For example, long doubles are passed as structures
11668 rather than in floating point registers.
11674 Generate code that uses registers (stack) for passing arguments to functions.
11675 By default, arguments are passed in registers where possible rather
11676 than by pushing arguments on to the stack.
11678 @item -mparallel-insns
11679 @itemx -mno-parallel-insns
11680 @opindex mparallel-insns
11681 @opindex mno-parallel-insns
11682 Allow the generation of parallel instructions. This is enabled by
11683 default with @option{-O2}.
11685 @item -mparallel-mpy
11686 @itemx -mno-parallel-mpy
11687 @opindex mparallel-mpy
11688 @opindex mno-parallel-mpy
11689 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
11690 provided @option{-mparallel-insns} is also specified. These instructions have
11691 tight register constraints which can pessimize the code generation
11692 of large functions.
11697 @subsection V850 Options
11698 @cindex V850 Options
11700 These @samp{-m} options are defined for V850 implementations:
11704 @itemx -mno-long-calls
11705 @opindex mlong-calls
11706 @opindex mno-long-calls
11707 Treat all calls as being far away (near). If calls are assumed to be
11708 far away, the compiler will always load the functions address up into a
11709 register, and call indirect through the pointer.
11715 Do not optimize (do optimize) basic blocks that use the same index
11716 pointer 4 or more times to copy pointer into the @code{ep} register, and
11717 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
11718 option is on by default if you optimize.
11720 @item -mno-prolog-function
11721 @itemx -mprolog-function
11722 @opindex mno-prolog-function
11723 @opindex mprolog-function
11724 Do not use (do use) external functions to save and restore registers
11725 at the prologue and epilogue of a function. The external functions
11726 are slower, but use less code space if more than one function saves
11727 the same number of registers. The @option{-mprolog-function} option
11728 is on by default if you optimize.
11732 Try to make the code as small as possible. At present, this just turns
11733 on the @option{-mep} and @option{-mprolog-function} options.
11735 @item -mtda=@var{n}
11737 Put static or global variables whose size is @var{n} bytes or less into
11738 the tiny data area that register @code{ep} points to. The tiny data
11739 area can hold up to 256 bytes in total (128 bytes for byte references).
11741 @item -msda=@var{n}
11743 Put static or global variables whose size is @var{n} bytes or less into
11744 the small data area that register @code{gp} points to. The small data
11745 area can hold up to 64 kilobytes.
11747 @item -mzda=@var{n}
11749 Put static or global variables whose size is @var{n} bytes or less into
11750 the first 32 kilobytes of memory.
11754 Specify that the target processor is the V850.
11757 @opindex mbig-switch
11758 Generate code suitable for big switch tables. Use this option only if
11759 the assembler/linker complain about out of range branches within a switch
11764 This option will cause r2 and r5 to be used in the code generated by
11765 the compiler. This setting is the default.
11767 @item -mno-app-regs
11768 @opindex mno-app-regs
11769 This option will cause r2 and r5 to be treated as fixed registers.
11773 Specify that the target processor is the V850E1. The preprocessor
11774 constants @samp{__v850e1__} and @samp{__v850e__} will be defined if
11775 this option is used.
11779 Specify that the target processor is the V850E@. The preprocessor
11780 constant @samp{__v850e__} will be defined if this option is used.
11782 If neither @option{-mv850} nor @option{-mv850e} nor @option{-mv850e1}
11783 are defined then a default target processor will be chosen and the
11784 relevant @samp{__v850*__} preprocessor constant will be defined.
11786 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
11787 defined, regardless of which processor variant is the target.
11789 @item -mdisable-callt
11790 @opindex mdisable-callt
11791 This option will suppress generation of the CALLT instruction for the
11792 v850e and v850e1 flavors of the v850 architecture. The default is
11793 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
11798 @subsection VAX Options
11799 @cindex VAX options
11801 These @samp{-m} options are defined for the VAX:
11806 Do not output certain jump instructions (@code{aobleq} and so on)
11807 that the Unix assembler for the VAX cannot handle across long
11812 Do output those jump instructions, on the assumption that you
11813 will assemble with the GNU assembler.
11817 Output code for g-format floating point numbers instead of d-format.
11820 @node x86-64 Options
11821 @subsection x86-64 Options
11822 @cindex x86-64 options
11824 These are listed under @xref{i386 and x86-64 Options}.
11826 @node Xstormy16 Options
11827 @subsection Xstormy16 Options
11828 @cindex Xstormy16 Options
11830 These options are defined for Xstormy16:
11835 Choose startup files and linker script suitable for the simulator.
11838 @node Xtensa Options
11839 @subsection Xtensa Options
11840 @cindex Xtensa Options
11842 These options are supported for Xtensa targets:
11846 @itemx -mno-const16
11848 @opindex mno-const16
11849 Enable or disable use of @code{CONST16} instructions for loading
11850 constant values. The @code{CONST16} instruction is currently not a
11851 standard option from Tensilica. When enabled, @code{CONST16}
11852 instructions are always used in place of the standard @code{L32R}
11853 instructions. The use of @code{CONST16} is enabled by default only if
11854 the @code{L32R} instruction is not available.
11857 @itemx -mno-fused-madd
11858 @opindex mfused-madd
11859 @opindex mno-fused-madd
11860 Enable or disable use of fused multiply/add and multiply/subtract
11861 instructions in the floating-point option. This has no effect if the
11862 floating-point option is not also enabled. Disabling fused multiply/add
11863 and multiply/subtract instructions forces the compiler to use separate
11864 instructions for the multiply and add/subtract operations. This may be
11865 desirable in some cases where strict IEEE 754-compliant results are
11866 required: the fused multiply add/subtract instructions do not round the
11867 intermediate result, thereby producing results with @emph{more} bits of
11868 precision than specified by the IEEE standard. Disabling fused multiply
11869 add/subtract instructions also ensures that the program output is not
11870 sensitive to the compiler's ability to combine multiply and add/subtract
11873 @item -mtext-section-literals
11874 @itemx -mno-text-section-literals
11875 @opindex mtext-section-literals
11876 @opindex mno-text-section-literals
11877 Control the treatment of literal pools. The default is
11878 @option{-mno-text-section-literals}, which places literals in a separate
11879 section in the output file. This allows the literal pool to be placed
11880 in a data RAM/ROM, and it also allows the linker to combine literal
11881 pools from separate object files to remove redundant literals and
11882 improve code size. With @option{-mtext-section-literals}, the literals
11883 are interspersed in the text section in order to keep them as close as
11884 possible to their references. This may be necessary for large assembly
11887 @item -mtarget-align
11888 @itemx -mno-target-align
11889 @opindex mtarget-align
11890 @opindex mno-target-align
11891 When this option is enabled, GCC instructs the assembler to
11892 automatically align instructions to reduce branch penalties at the
11893 expense of some code density. The assembler attempts to widen density
11894 instructions to align branch targets and the instructions following call
11895 instructions. If there are not enough preceding safe density
11896 instructions to align a target, no widening will be performed. The
11897 default is @option{-mtarget-align}. These options do not affect the
11898 treatment of auto-aligned instructions like @code{LOOP}, which the
11899 assembler will always align, either by widening density instructions or
11900 by inserting no-op instructions.
11903 @itemx -mno-longcalls
11904 @opindex mlongcalls
11905 @opindex mno-longcalls
11906 When this option is enabled, GCC instructs the assembler to translate
11907 direct calls to indirect calls unless it can determine that the target
11908 of a direct call is in the range allowed by the call instruction. This
11909 translation typically occurs for calls to functions in other source
11910 files. Specifically, the assembler translates a direct @code{CALL}
11911 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
11912 The default is @option{-mno-longcalls}. This option should be used in
11913 programs where the call target can potentially be out of range. This
11914 option is implemented in the assembler, not the compiler, so the
11915 assembly code generated by GCC will still show direct call
11916 instructions---look at the disassembled object code to see the actual
11917 instructions. Note that the assembler will use an indirect call for
11918 every cross-file call, not just those that really will be out of range.
11921 @node zSeries Options
11922 @subsection zSeries Options
11923 @cindex zSeries options
11925 These are listed under @xref{S/390 and zSeries Options}.
11927 @node Code Gen Options
11928 @section Options for Code Generation Conventions
11929 @cindex code generation conventions
11930 @cindex options, code generation
11931 @cindex run-time options
11933 These machine-independent options control the interface conventions
11934 used in code generation.
11936 Most of them have both positive and negative forms; the negative form
11937 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
11938 one of the forms is listed---the one which is not the default. You
11939 can figure out the other form by either removing @samp{no-} or adding
11943 @item -fbounds-check
11944 @opindex fbounds-check
11945 For front-ends that support it, generate additional code to check that
11946 indices used to access arrays are within the declared range. This is
11947 currently only supported by the Java and Fortran 77 front-ends, where
11948 this option defaults to true and false respectively.
11952 This option generates traps for signed overflow on addition, subtraction,
11953 multiplication operations.
11957 This option instructs the compiler to assume that signed arithmetic
11958 overflow of addition, subtraction and multiplication wraps around
11959 using twos-complement representation. This flag enables some optimizations
11960 and disables other. This option is enabled by default for the Java
11961 front-end, as required by the Java language specification.
11964 @opindex fexceptions
11965 Enable exception handling. Generates extra code needed to propagate
11966 exceptions. For some targets, this implies GCC will generate frame
11967 unwind information for all functions, which can produce significant data
11968 size overhead, although it does not affect execution. If you do not
11969 specify this option, GCC will enable it by default for languages like
11970 C++ which normally require exception handling, and disable it for
11971 languages like C that do not normally require it. However, you may need
11972 to enable this option when compiling C code that needs to interoperate
11973 properly with exception handlers written in C++. You may also wish to
11974 disable this option if you are compiling older C++ programs that don't
11975 use exception handling.
11977 @item -fnon-call-exceptions
11978 @opindex fnon-call-exceptions
11979 Generate code that allows trapping instructions to throw exceptions.
11980 Note that this requires platform-specific runtime support that does
11981 not exist everywhere. Moreover, it only allows @emph{trapping}
11982 instructions to throw exceptions, i.e.@: memory references or floating
11983 point instructions. It does not allow exceptions to be thrown from
11984 arbitrary signal handlers such as @code{SIGALRM}.
11986 @item -funwind-tables
11987 @opindex funwind-tables
11988 Similar to @option{-fexceptions}, except that it will just generate any needed
11989 static data, but will not affect the generated code in any other way.
11990 You will normally not enable this option; instead, a language processor
11991 that needs this handling would enable it on your behalf.
11993 @item -fasynchronous-unwind-tables
11994 @opindex fasynchronous-unwind-tables
11995 Generate unwind table in dwarf2 format, if supported by target machine. The
11996 table is exact at each instruction boundary, so it can be used for stack
11997 unwinding from asynchronous events (such as debugger or garbage collector).
11999 @item -fpcc-struct-return
12000 @opindex fpcc-struct-return
12001 Return ``short'' @code{struct} and @code{union} values in memory like
12002 longer ones, rather than in registers. This convention is less
12003 efficient, but it has the advantage of allowing intercallability between
12004 GCC-compiled files and files compiled with other compilers, particularly
12005 the Portable C Compiler (pcc).
12007 The precise convention for returning structures in memory depends
12008 on the target configuration macros.
12010 Short structures and unions are those whose size and alignment match
12011 that of some integer type.
12013 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
12014 switch is not binary compatible with code compiled with the
12015 @option{-freg-struct-return} switch.
12016 Use it to conform to a non-default application binary interface.
12018 @item -freg-struct-return
12019 @opindex freg-struct-return
12020 Return @code{struct} and @code{union} values in registers when possible.
12021 This is more efficient for small structures than
12022 @option{-fpcc-struct-return}.
12024 If you specify neither @option{-fpcc-struct-return} nor
12025 @option{-freg-struct-return}, GCC defaults to whichever convention is
12026 standard for the target. If there is no standard convention, GCC
12027 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
12028 the principal compiler. In those cases, we can choose the standard, and
12029 we chose the more efficient register return alternative.
12031 @strong{Warning:} code compiled with the @option{-freg-struct-return}
12032 switch is not binary compatible with code compiled with the
12033 @option{-fpcc-struct-return} switch.
12034 Use it to conform to a non-default application binary interface.
12036 @item -fshort-enums
12037 @opindex fshort-enums
12038 Allocate to an @code{enum} type only as many bytes as it needs for the
12039 declared range of possible values. Specifically, the @code{enum} type
12040 will be equivalent to the smallest integer type which has enough room.
12042 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
12043 code that is not binary compatible with code generated without that switch.
12044 Use it to conform to a non-default application binary interface.
12046 @item -fshort-double
12047 @opindex fshort-double
12048 Use the same size for @code{double} as for @code{float}.
12050 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
12051 code that is not binary compatible with code generated without that switch.
12052 Use it to conform to a non-default application binary interface.
12054 @item -fshort-wchar
12055 @opindex fshort-wchar
12056 Override the underlying type for @samp{wchar_t} to be @samp{short
12057 unsigned int} instead of the default for the target. This option is
12058 useful for building programs to run under WINE@.
12060 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
12061 code that is not binary compatible with code generated without that switch.
12062 Use it to conform to a non-default application binary interface.
12064 @item -fshared-data
12065 @opindex fshared-data
12066 Requests that the data and non-@code{const} variables of this
12067 compilation be shared data rather than private data. The distinction
12068 makes sense only on certain operating systems, where shared data is
12069 shared between processes running the same program, while private data
12070 exists in one copy per process.
12073 @opindex fno-common
12074 In C, allocate even uninitialized global variables in the data section of the
12075 object file, rather than generating them as common blocks. This has the
12076 effect that if the same variable is declared (without @code{extern}) in
12077 two different compilations, you will get an error when you link them.
12078 The only reason this might be useful is if you wish to verify that the
12079 program will work on other systems which always work this way.
12083 Ignore the @samp{#ident} directive.
12085 @item -finhibit-size-directive
12086 @opindex finhibit-size-directive
12087 Don't output a @code{.size} assembler directive, or anything else that
12088 would cause trouble if the function is split in the middle, and the
12089 two halves are placed at locations far apart in memory. This option is
12090 used when compiling @file{crtstuff.c}; you should not need to use it
12093 @item -fverbose-asm
12094 @opindex fverbose-asm
12095 Put extra commentary information in the generated assembly code to
12096 make it more readable. This option is generally only of use to those
12097 who actually need to read the generated assembly code (perhaps while
12098 debugging the compiler itself).
12100 @option{-fno-verbose-asm}, the default, causes the
12101 extra information to be omitted and is useful when comparing two assembler
12106 @cindex global offset table
12108 Generate position-independent code (PIC) suitable for use in a shared
12109 library, if supported for the target machine. Such code accesses all
12110 constant addresses through a global offset table (GOT)@. The dynamic
12111 loader resolves the GOT entries when the program starts (the dynamic
12112 loader is not part of GCC; it is part of the operating system). If
12113 the GOT size for the linked executable exceeds a machine-specific
12114 maximum size, you get an error message from the linker indicating that
12115 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
12116 instead. (These maximums are 8k on the SPARC and 32k
12117 on the m68k and RS/6000. The 386 has no such limit.)
12119 Position-independent code requires special support, and therefore works
12120 only on certain machines. For the 386, GCC supports PIC for System V
12121 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
12122 position-independent.
12126 If supported for the target machine, emit position-independent code,
12127 suitable for dynamic linking and avoiding any limit on the size of the
12128 global offset table. This option makes a difference on the m68k,
12129 PowerPC and SPARC@.
12131 Position-independent code requires special support, and therefore works
12132 only on certain machines.
12138 These options are similar to @option{-fpic} and @option{-fPIC}, but
12139 generated position independent code can be only linked into executables.
12140 Usually these options are used when @option{-pie} GCC option will be
12141 used during linking.
12143 @item -ffixed-@var{reg}
12145 Treat the register named @var{reg} as a fixed register; generated code
12146 should never refer to it (except perhaps as a stack pointer, frame
12147 pointer or in some other fixed role).
12149 @var{reg} must be the name of a register. The register names accepted
12150 are machine-specific and are defined in the @code{REGISTER_NAMES}
12151 macro in the machine description macro file.
12153 This flag does not have a negative form, because it specifies a
12156 @item -fcall-used-@var{reg}
12157 @opindex fcall-used
12158 Treat the register named @var{reg} as an allocable register that is
12159 clobbered by function calls. It may be allocated for temporaries or
12160 variables that do not live across a call. Functions compiled this way
12161 will not save and restore the register @var{reg}.
12163 It is an error to used this flag with the frame pointer or stack pointer.
12164 Use of this flag for other registers that have fixed pervasive roles in
12165 the machine's execution model will produce disastrous results.
12167 This flag does not have a negative form, because it specifies a
12170 @item -fcall-saved-@var{reg}
12171 @opindex fcall-saved
12172 Treat the register named @var{reg} as an allocable register saved by
12173 functions. It may be allocated even for temporaries or variables that
12174 live across a call. Functions compiled this way will save and restore
12175 the register @var{reg} if they use it.
12177 It is an error to used this flag with the frame pointer or stack pointer.
12178 Use of this flag for other registers that have fixed pervasive roles in
12179 the machine's execution model will produce disastrous results.
12181 A different sort of disaster will result from the use of this flag for
12182 a register in which function values may be returned.
12184 This flag does not have a negative form, because it specifies a
12187 @item -fpack-struct[=@var{n}]
12188 @opindex fpack-struct
12189 Without a value specified, pack all structure members together without
12190 holes. When a value is specified (which must be a small power of two), pack
12191 structure members according to this value, representing the maximum
12192 alignment (that is, objects with default alignment requirements larger than
12193 this will be output potentially unaligned at the next fitting location.
12195 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
12196 code that is not binary compatible with code generated without that switch.
12197 Additionally, it makes the code suboptimal.
12198 Use it to conform to a non-default application binary interface.
12200 @item -finstrument-functions
12201 @opindex finstrument-functions
12202 Generate instrumentation calls for entry and exit to functions. Just
12203 after function entry and just before function exit, the following
12204 profiling functions will be called with the address of the current
12205 function and its call site. (On some platforms,
12206 @code{__builtin_return_address} does not work beyond the current
12207 function, so the call site information may not be available to the
12208 profiling functions otherwise.)
12211 void __cyg_profile_func_enter (void *this_fn,
12213 void __cyg_profile_func_exit (void *this_fn,
12217 The first argument is the address of the start of the current function,
12218 which may be looked up exactly in the symbol table.
12220 This instrumentation is also done for functions expanded inline in other
12221 functions. The profiling calls will indicate where, conceptually, the
12222 inline function is entered and exited. This means that addressable
12223 versions of such functions must be available. If all your uses of a
12224 function are expanded inline, this may mean an additional expansion of
12225 code size. If you use @samp{extern inline} in your C code, an
12226 addressable version of such functions must be provided. (This is
12227 normally the case anyways, but if you get lucky and the optimizer always
12228 expands the functions inline, you might have gotten away without
12229 providing static copies.)
12231 A function may be given the attribute @code{no_instrument_function}, in
12232 which case this instrumentation will not be done. This can be used, for
12233 example, for the profiling functions listed above, high-priority
12234 interrupt routines, and any functions from which the profiling functions
12235 cannot safely be called (perhaps signal handlers, if the profiling
12236 routines generate output or allocate memory).
12238 @item -fstack-check
12239 @opindex fstack-check
12240 Generate code to verify that you do not go beyond the boundary of the
12241 stack. You should specify this flag if you are running in an
12242 environment with multiple threads, but only rarely need to specify it in
12243 a single-threaded environment since stack overflow is automatically
12244 detected on nearly all systems if there is only one stack.
12246 Note that this switch does not actually cause checking to be done; the
12247 operating system must do that. The switch causes generation of code
12248 to ensure that the operating system sees the stack being extended.
12250 @item -fstack-limit-register=@var{reg}
12251 @itemx -fstack-limit-symbol=@var{sym}
12252 @itemx -fno-stack-limit
12253 @opindex fstack-limit-register
12254 @opindex fstack-limit-symbol
12255 @opindex fno-stack-limit
12256 Generate code to ensure that the stack does not grow beyond a certain value,
12257 either the value of a register or the address of a symbol. If the stack
12258 would grow beyond the value, a signal is raised. For most targets,
12259 the signal is raised before the stack overruns the boundary, so
12260 it is possible to catch the signal without taking special precautions.
12262 For instance, if the stack starts at absolute address @samp{0x80000000}
12263 and grows downwards, you can use the flags
12264 @option{-fstack-limit-symbol=__stack_limit} and
12265 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
12266 of 128KB@. Note that this may only work with the GNU linker.
12268 @cindex aliasing of parameters
12269 @cindex parameters, aliased
12270 @item -fargument-alias
12271 @itemx -fargument-noalias
12272 @itemx -fargument-noalias-global
12273 @opindex fargument-alias
12274 @opindex fargument-noalias
12275 @opindex fargument-noalias-global
12276 Specify the possible relationships among parameters and between
12277 parameters and global data.
12279 @option{-fargument-alias} specifies that arguments (parameters) may
12280 alias each other and may alias global storage.@*
12281 @option{-fargument-noalias} specifies that arguments do not alias
12282 each other, but may alias global storage.@*
12283 @option{-fargument-noalias-global} specifies that arguments do not
12284 alias each other and do not alias global storage.
12286 Each language will automatically use whatever option is required by
12287 the language standard. You should not need to use these options yourself.
12289 @item -fleading-underscore
12290 @opindex fleading-underscore
12291 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
12292 change the way C symbols are represented in the object file. One use
12293 is to help link with legacy assembly code.
12295 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
12296 generate code that is not binary compatible with code generated without that
12297 switch. Use it to conform to a non-default application binary interface.
12298 Not all targets provide complete support for this switch.
12300 @item -ftls-model=@var{model}
12301 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
12302 The @var{model} argument should be one of @code{global-dynamic},
12303 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
12305 The default without @option{-fpic} is @code{initial-exec}; with
12306 @option{-fpic} the default is @code{global-dynamic}.
12308 @item -fvisibility=@var{default|internal|hidden|protected}
12309 @opindex fvisibility
12310 Set the default ELF image symbol visibility to the specified option---all
12311 symbols will be marked with this unless overridden within the code.
12312 Using this feature can very substantially improve linking and
12313 load times of shared object libraries, produce more optimized
12314 code, provide near-perfect API export and prevent symbol clashes.
12315 It is @strong{strongly} recommended that you use this in any shared objects
12318 Despite the nomenclature, @code{default} always means public ie;
12319 available to be linked against from outside the shared object.
12320 @code{protected} and @code{internal} are pretty useless in real-world
12321 usage so the only other commonly used option will be @code{hidden}.
12322 The default if @option{-fvisibility} isn't specified is
12323 @code{default}, i.e., make every
12324 symbol public---this causes the same behavior as previous versions of
12327 A good explanation of the benefits offered by ensuring ELF
12328 symbols have the correct visibility is given by ``How To Write
12329 Shared Libraries'' by Ulrich Drepper (which can be found at
12330 @w{@uref{http://people.redhat.com/~drepper/}})---however a superior
12331 solution made possible by this option to marking things hidden when
12332 the default is public is to make the default hidden and mark things
12333 public. This is the norm with DLL's on Windows and with @option{-fvisibility=hidden}
12334 and @code{__attribute__ ((visibility("default")))} instead of
12335 @code{__declspec(dllexport)} you get almost identical semantics with
12336 identical syntax. This is a great boon to those working with
12337 cross-platform projects.
12339 For those adding visibility support to existing code, you may find
12340 @samp{#pragma GCC visibility} of use. This works by you enclosing
12341 the declarations you wish to set visibility for with (for example)
12342 @samp{#pragma GCC visibility push(hidden)} and
12343 @samp{#pragma GCC visibility pop}. These can be nested up to sixteen
12344 times. Bear in mind that symbol visibility should be viewed @strong{as
12345 part of the API interface contract} and thus all new code should
12346 always specify visibility when it is not the default ie; declarations
12347 only for use within the local DSO should @strong{always} be marked explicitly
12348 as hidden as so to avoid PLT indirection overheads---making this
12349 abundantly clear also aids readability and self-documentation of the code.
12350 Note that due to ISO C++ specification requirements, operator new and
12351 operator delete must always be of default visibility.
12353 An overview of these techniques, their benefits and how to use them
12354 is at @w{@uref{http://www.nedprod.com/programs/gccvisibility.html}}.
12360 @node Environment Variables
12361 @section Environment Variables Affecting GCC
12362 @cindex environment variables
12364 @c man begin ENVIRONMENT
12365 This section describes several environment variables that affect how GCC
12366 operates. Some of them work by specifying directories or prefixes to use
12367 when searching for various kinds of files. Some are used to specify other
12368 aspects of the compilation environment.
12370 Note that you can also specify places to search using options such as
12371 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
12372 take precedence over places specified using environment variables, which
12373 in turn take precedence over those specified by the configuration of GCC@.
12374 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
12375 GNU Compiler Collection (GCC) Internals}.
12380 @c @itemx LC_COLLATE
12382 @c @itemx LC_MONETARY
12383 @c @itemx LC_NUMERIC
12388 @c @findex LC_COLLATE
12389 @findex LC_MESSAGES
12390 @c @findex LC_MONETARY
12391 @c @findex LC_NUMERIC
12395 These environment variables control the way that GCC uses
12396 localization information that allow GCC to work with different
12397 national conventions. GCC inspects the locale categories
12398 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
12399 so. These locale categories can be set to any value supported by your
12400 installation. A typical value is @samp{en_GB.UTF-8} for English in the United
12401 Kingdom encoded in UTF-8.
12403 The @env{LC_CTYPE} environment variable specifies character
12404 classification. GCC uses it to determine the character boundaries in
12405 a string; this is needed for some multibyte encodings that contain quote
12406 and escape characters that would otherwise be interpreted as a string
12409 The @env{LC_MESSAGES} environment variable specifies the language to
12410 use in diagnostic messages.
12412 If the @env{LC_ALL} environment variable is set, it overrides the value
12413 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
12414 and @env{LC_MESSAGES} default to the value of the @env{LANG}
12415 environment variable. If none of these variables are set, GCC
12416 defaults to traditional C English behavior.
12420 If @env{TMPDIR} is set, it specifies the directory to use for temporary
12421 files. GCC uses temporary files to hold the output of one stage of
12422 compilation which is to be used as input to the next stage: for example,
12423 the output of the preprocessor, which is the input to the compiler
12426 @item GCC_EXEC_PREFIX
12427 @findex GCC_EXEC_PREFIX
12428 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
12429 names of the subprograms executed by the compiler. No slash is added
12430 when this prefix is combined with the name of a subprogram, but you can
12431 specify a prefix that ends with a slash if you wish.
12433 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
12434 an appropriate prefix to use based on the pathname it was invoked with.
12436 If GCC cannot find the subprogram using the specified prefix, it
12437 tries looking in the usual places for the subprogram.
12439 The default value of @env{GCC_EXEC_PREFIX} is
12440 @file{@var{prefix}/lib/gcc/} where @var{prefix} is the value
12441 of @code{prefix} when you ran the @file{configure} script.
12443 Other prefixes specified with @option{-B} take precedence over this prefix.
12445 This prefix is also used for finding files such as @file{crt0.o} that are
12448 In addition, the prefix is used in an unusual way in finding the
12449 directories to search for header files. For each of the standard
12450 directories whose name normally begins with @samp{/usr/local/lib/gcc}
12451 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
12452 replacing that beginning with the specified prefix to produce an
12453 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
12454 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
12455 These alternate directories are searched first; the standard directories
12458 @item COMPILER_PATH
12459 @findex COMPILER_PATH
12460 The value of @env{COMPILER_PATH} is a colon-separated list of
12461 directories, much like @env{PATH}. GCC tries the directories thus
12462 specified when searching for subprograms, if it can't find the
12463 subprograms using @env{GCC_EXEC_PREFIX}.
12466 @findex LIBRARY_PATH
12467 The value of @env{LIBRARY_PATH} is a colon-separated list of
12468 directories, much like @env{PATH}. When configured as a native compiler,
12469 GCC tries the directories thus specified when searching for special
12470 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
12471 using GCC also uses these directories when searching for ordinary
12472 libraries for the @option{-l} option (but directories specified with
12473 @option{-L} come first).
12477 @cindex locale definition
12478 This variable is used to pass locale information to the compiler. One way in
12479 which this information is used is to determine the character set to be used
12480 when character literals, string literals and comments are parsed in C and C++.
12481 When the compiler is configured to allow multibyte characters,
12482 the following values for @env{LANG} are recognized:
12486 Recognize JIS characters.
12488 Recognize SJIS characters.
12490 Recognize EUCJP characters.
12493 If @env{LANG} is not defined, or if it has some other value, then the
12494 compiler will use mblen and mbtowc as defined by the default locale to
12495 recognize and translate multibyte characters.
12499 Some additional environments variables affect the behavior of the
12502 @include cppenv.texi
12506 @node Precompiled Headers
12507 @section Using Precompiled Headers
12508 @cindex precompiled headers
12509 @cindex speed of compilation
12511 Often large projects have many header files that are included in every
12512 source file. The time the compiler takes to process these header files
12513 over and over again can account for nearly all of the time required to
12514 build the project. To make builds faster, GCC allows users to
12515 `precompile' a header file; then, if builds can use the precompiled
12516 header file they will be much faster.
12518 @strong{Caution:} There are a few known situations where GCC will
12519 crash when trying to use a precompiled header. If you have trouble
12520 with a precompiled header, you should remove the precompiled header
12521 and compile without it. In addition, please use GCC's on-line
12522 defect-tracking system to report any problems you encounter with
12523 precompiled headers. @xref{Bugs}.
12525 To create a precompiled header file, simply compile it as you would any
12526 other file, if necessary using the @option{-x} option to make the driver
12527 treat it as a C or C++ header file. You will probably want to use a
12528 tool like @command{make} to keep the precompiled header up-to-date when
12529 the headers it contains change.
12531 A precompiled header file will be searched for when @code{#include} is
12532 seen in the compilation. As it searches for the included file
12533 (@pxref{Search Path,,Search Path,cpp,The C Preprocessor}) the
12534 compiler looks for a precompiled header in each directory just before it
12535 looks for the include file in that directory. The name searched for is
12536 the name specified in the @code{#include} with @samp{.gch} appended. If
12537 the precompiled header file can't be used, it is ignored.
12539 For instance, if you have @code{#include "all.h"}, and you have
12540 @file{all.h.gch} in the same directory as @file{all.h}, then the
12541 precompiled header file will be used if possible, and the original
12542 header will be used otherwise.
12544 Alternatively, you might decide to put the precompiled header file in a
12545 directory and use @option{-I} to ensure that directory is searched
12546 before (or instead of) the directory containing the original header.
12547 Then, if you want to check that the precompiled header file is always
12548 used, you can put a file of the same name as the original header in this
12549 directory containing an @code{#error} command.
12551 This also works with @option{-include}. So yet another way to use
12552 precompiled headers, good for projects not designed with precompiled
12553 header files in mind, is to simply take most of the header files used by
12554 a project, include them from another header file, precompile that header
12555 file, and @option{-include} the precompiled header. If the header files
12556 have guards against multiple inclusion, they will be skipped because
12557 they've already been included (in the precompiled header).
12559 If you need to precompile the same header file for different
12560 languages, targets, or compiler options, you can instead make a
12561 @emph{directory} named like @file{all.h.gch}, and put each precompiled
12562 header in the directory, perhaps using @option{-o}. It doesn't matter
12563 what you call the files in the directory, every precompiled header in
12564 the directory will be considered. The first precompiled header
12565 encountered in the directory that is valid for this compilation will
12566 be used; they're searched in no particular order.
12568 There are many other possibilities, limited only by your imagination,
12569 good sense, and the constraints of your build system.
12571 A precompiled header file can be used only when these conditions apply:
12575 Only one precompiled header can be used in a particular compilation.
12578 A precompiled header can't be used once the first C token is seen. You
12579 can have preprocessor directives before a precompiled header; you can
12580 even include a precompiled header from inside another header, so long as
12581 there are no C tokens before the @code{#include}.
12584 The precompiled header file must be produced for the same language as
12585 the current compilation. You can't use a C precompiled header for a C++
12589 The precompiled header file must be produced by the same compiler
12590 version and configuration as the current compilation is using.
12591 The easiest way to guarantee this is to use the same compiler binary
12592 for creating and using precompiled headers.
12595 Any macros defined before the precompiled header is included must
12596 either be defined in the same way as when the precompiled header was
12597 generated, or must not affect the precompiled header, which usually
12598 means that the they don't appear in the precompiled header at all.
12600 The @option{-D} option is one way to define a macro before a
12601 precompiled header is included; using a @code{#define} can also do it.
12602 There are also some options that define macros implicitly, like
12603 @option{-O} and @option{-Wdeprecated}; the same rule applies to macros
12606 @item If debugging information is output when using the precompiled
12607 header, using @option{-g} or similar, the same kind of debugging information
12608 must have been output when building the precompiled header. However,
12609 a precompiled header built using @option{-g} can be used in a compilation
12610 when no debugging information is being output.
12612 @item The same @option{-m} options must generally be used when building
12613 and using the precompiled header. @xref{Submodel Options},
12614 for any cases where this rule is relaxed.
12616 @item Each of the following options must be the same when building and using
12617 the precompiled header:
12619 @gccoptlist{-fexceptions -funit-at-a-time}
12622 Some other command-line options starting with @option{-f},
12623 @option{-p}, or @option{-O} must be defined in the same way as when
12624 the precompiled header was generated. At present, it's not clear
12625 which options are safe to change and which are not; the safest choice
12626 is to use exactly the same options when generating and using the
12627 precompiled header. The following are known to be safe:
12629 @gccoptlist{-fpreprocessed -pedantic-errors}
12633 For all of these except the last, the compiler will automatically
12634 ignore the precompiled header if the conditions aren't met. If you
12635 find an option combination that doesn't work and doesn't cause the
12636 precompiled header to be ignored, please consider filing a bug report,
12639 If you do use differing options when generating and using the
12640 precompiled header, the actual behavior will be a mixture of the
12641 behavior for the options. For instance, if you use @option{-g} to
12642 generate the precompiled header but not when using it, you may or may
12643 not get debugging information for routines in the precompiled header.
12645 @node Running Protoize
12646 @section Running Protoize
12648 The program @code{protoize} is an optional part of GCC@. You can use
12649 it to add prototypes to a program, thus converting the program to ISO
12650 C in one respect. The companion program @code{unprotoize} does the
12651 reverse: it removes argument types from any prototypes that are found.
12653 When you run these programs, you must specify a set of source files as
12654 command line arguments. The conversion programs start out by compiling
12655 these files to see what functions they define. The information gathered
12656 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
12658 After scanning comes actual conversion. The specified files are all
12659 eligible to be converted; any files they include (whether sources or
12660 just headers) are eligible as well.
12662 But not all the eligible files are converted. By default,
12663 @code{protoize} and @code{unprotoize} convert only source and header
12664 files in the current directory. You can specify additional directories
12665 whose files should be converted with the @option{-d @var{directory}}
12666 option. You can also specify particular files to exclude with the
12667 @option{-x @var{file}} option. A file is converted if it is eligible, its
12668 directory name matches one of the specified directory names, and its
12669 name within the directory has not been excluded.
12671 Basic conversion with @code{protoize} consists of rewriting most
12672 function definitions and function declarations to specify the types of
12673 the arguments. The only ones not rewritten are those for varargs
12676 @code{protoize} optionally inserts prototype declarations at the
12677 beginning of the source file, to make them available for any calls that
12678 precede the function's definition. Or it can insert prototype
12679 declarations with block scope in the blocks where undeclared functions
12682 Basic conversion with @code{unprotoize} consists of rewriting most
12683 function declarations to remove any argument types, and rewriting
12684 function definitions to the old-style pre-ISO form.
12686 Both conversion programs print a warning for any function declaration or
12687 definition that they can't convert. You can suppress these warnings
12690 The output from @code{protoize} or @code{unprotoize} replaces the
12691 original source file. The original file is renamed to a name ending
12692 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
12693 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
12694 for DOS) file already exists, then the source file is simply discarded.
12696 @code{protoize} and @code{unprotoize} both depend on GCC itself to
12697 scan the program and collect information about the functions it uses.
12698 So neither of these programs will work until GCC is installed.
12700 Here is a table of the options you can use with @code{protoize} and
12701 @code{unprotoize}. Each option works with both programs unless
12705 @item -B @var{directory}
12706 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
12707 usual directory (normally @file{/usr/local/lib}). This file contains
12708 prototype information about standard system functions. This option
12709 applies only to @code{protoize}.
12711 @item -c @var{compilation-options}
12712 Use @var{compilation-options} as the options when running @command{gcc} to
12713 produce the @samp{.X} files. The special option @option{-aux-info} is
12714 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
12716 Note that the compilation options must be given as a single argument to
12717 @code{protoize} or @code{unprotoize}. If you want to specify several
12718 @command{gcc} options, you must quote the entire set of compilation options
12719 to make them a single word in the shell.
12721 There are certain @command{gcc} arguments that you cannot use, because they
12722 would produce the wrong kind of output. These include @option{-g},
12723 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
12724 the @var{compilation-options}, they are ignored.
12727 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
12728 systems) instead of @samp{.c}. This is convenient if you are converting
12729 a C program to C++. This option applies only to @code{protoize}.
12732 Add explicit global declarations. This means inserting explicit
12733 declarations at the beginning of each source file for each function
12734 that is called in the file and was not declared. These declarations
12735 precede the first function definition that contains a call to an
12736 undeclared function. This option applies only to @code{protoize}.
12738 @item -i @var{string}
12739 Indent old-style parameter declarations with the string @var{string}.
12740 This option applies only to @code{protoize}.
12742 @code{unprotoize} converts prototyped function definitions to old-style
12743 function definitions, where the arguments are declared between the
12744 argument list and the initial @samp{@{}. By default, @code{unprotoize}
12745 uses five spaces as the indentation. If you want to indent with just
12746 one space instead, use @option{-i " "}.
12749 Keep the @samp{.X} files. Normally, they are deleted after conversion
12753 Add explicit local declarations. @code{protoize} with @option{-l} inserts
12754 a prototype declaration for each function in each block which calls the
12755 function without any declaration. This option applies only to
12759 Make no real changes. This mode just prints information about the conversions
12760 that would have been done without @option{-n}.
12763 Make no @samp{.save} files. The original files are simply deleted.
12764 Use this option with caution.
12766 @item -p @var{program}
12767 Use the program @var{program} as the compiler. Normally, the name
12768 @file{gcc} is used.
12771 Work quietly. Most warnings are suppressed.
12774 Print the version number, just like @option{-v} for @command{gcc}.
12777 If you need special compiler options to compile one of your program's
12778 source files, then you should generate that file's @samp{.X} file
12779 specially, by running @command{gcc} on that source file with the
12780 appropriate options and the option @option{-aux-info}. Then run
12781 @code{protoize} on the entire set of files. @code{protoize} will use
12782 the existing @samp{.X} file because it is newer than the source file.
12786 gcc -Dfoo=bar file1.c -aux-info file1.X
12791 You need to include the special files along with the rest in the
12792 @code{protoize} command, even though their @samp{.X} files already
12793 exist, because otherwise they won't get converted.
12795 @xref{Protoize Caveats}, for more information on how to use
12796 @code{protoize} successfully.