1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2 @c 2000, 2001, 2002, 2003, 2004 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 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}}. Use of the @command{gccbug}
53 script to report bugs is recommended.
56 See the Info entry for @command{gcc}, or
57 @w{@uref{http://gcc.gnu.org/onlinedocs/gcc/Contributors.html}},
58 for contributors to GCC@.
63 @chapter GCC Command Options
64 @cindex GCC command options
65 @cindex command options
66 @cindex options, GCC command
68 @c man begin DESCRIPTION
69 When you invoke GCC, it normally does preprocessing, compilation,
70 assembly and linking. The ``overall options'' allow you to stop this
71 process at an intermediate stage. For example, the @option{-c} option
72 says not to run the linker. Then the output consists of object files
73 output by the assembler.
75 Other options are passed on to one stage of processing. Some options
76 control the preprocessor and others the compiler itself. Yet other
77 options control the assembler and linker; most of these are not
78 documented here, since you rarely need to use any of them.
80 @cindex C compilation options
81 Most of the command line options that you can use with GCC are useful
82 for C programs; when an option is only useful with another language
83 (usually C++), the explanation says so explicitly. If the description
84 for a particular option does not mention a source language, you can use
85 that option with all supported languages.
87 @cindex C++ compilation options
88 @xref{Invoking G++,,Compiling C++ Programs}, for a summary of special
89 options for compiling C++ programs.
91 @cindex grouping options
92 @cindex options, grouping
93 The @command{gcc} program accepts options and file names as operands. Many
94 options have multi-letter names; therefore multiple single-letter options
95 may @emph{not} be grouped: @option{-dr} is very different from @w{@samp{-d
98 @cindex order of options
99 @cindex options, order
100 You can mix options and other arguments. For the most part, the order
101 you use doesn't matter. Order does matter when you use several options
102 of the same kind; for example, if you specify @option{-L} more than once,
103 the directories are searched in the order specified.
105 Many options have long names starting with @samp{-f} or with
106 @samp{-W}---for example, @option{-fforce-mem},
107 @option{-fstrength-reduce}, @option{-Wformat} and so on. Most of
108 these have both positive and negative forms; the negative form of
109 @option{-ffoo} would be @option{-fno-foo}. This manual documents
110 only one of these two forms, whichever one is not the default.
114 @xref{Option Index}, for an index to GCC's options.
117 * Option Summary:: Brief list of all options, without explanations.
118 * Overall Options:: Controlling the kind of output:
119 an executable, object files, assembler files,
120 or preprocessed source.
121 * Invoking G++:: Compiling C++ programs.
122 * C Dialect Options:: Controlling the variant of C language compiled.
123 * C++ Dialect Options:: Variations on C++.
124 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
126 * Language Independent Options:: Controlling how diagnostics should be
128 * Warning Options:: How picky should the compiler be?
129 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
130 * Optimize Options:: How much optimization?
131 * Preprocessor Options:: Controlling header files and macro definitions.
132 Also, getting dependency information for Make.
133 * Assembler Options:: Passing options to the assembler.
134 * Link Options:: Specifying libraries and so on.
135 * Directory Options:: Where to find header files and libraries.
136 Where to find the compiler executable files.
137 * Spec Files:: How to pass switches to sub-processes.
138 * Target Options:: Running a cross-compiler, or an old version of GCC.
139 * Submodel Options:: Specifying minor hardware or convention variations,
140 such as 68010 vs 68020.
141 * Code Gen Options:: Specifying conventions for function calls, data layout
143 * Environment Variables:: Env vars that affect GCC.
144 * Precompiled Headers:: Compiling a header once, and using it many times.
145 * Running Protoize:: Automatically adding or removing function prototypes.
151 @section Option Summary
153 Here is a summary of all the options, grouped by type. Explanations are
154 in the following sections.
157 @item Overall Options
158 @xref{Overall Options,,Options Controlling the Kind of Output}.
159 @gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
160 -x @var{language} -v -### --help --target-help --version}
162 @item C Language Options
163 @xref{C Dialect Options,,Options Controlling C Dialect}.
164 @gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
165 -fno-asm -fno-builtin -fno-builtin-@var{function} @gol
166 -fhosted -ffreestanding -fms-extensions @gol
167 -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
168 -fallow-single-precision -fcond-mismatch @gol
169 -fsigned-bitfields -fsigned-char @gol
170 -funsigned-bitfields -funsigned-char}
172 @item C++ Language Options
173 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
174 @gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
175 -fconserve-space -fno-const-strings @gol
176 -fno-elide-constructors @gol
177 -fno-enforce-eh-specs @gol
178 -ffor-scope -fno-for-scope -fno-gnu-keywords @gol
179 -fno-implicit-templates @gol
180 -fno-implicit-inline-templates @gol
181 -fno-implement-inlines -fms-extensions @gol
182 -fno-nonansi-builtins -fno-operator-names @gol
183 -fno-optional-diags -fpermissive @gol
184 -frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
185 -fuse-cxa-atexit -fno-weak -nostdinc++ @gol
186 -fno-default-inline -fvisibility-inlines-hidden @gol
187 -Wabi -Wctor-dtor-privacy @gol
188 -Wnon-virtual-dtor -Wreorder @gol
189 -Weffc++ -Wno-deprecated @gol
190 -Wno-non-template-friend -Wold-style-cast @gol
191 -Woverloaded-virtual -Wno-pmf-conversions @gol
192 -Wsign-promo -Wsynth}
194 @item Objective-C and Objective-C++ Language Options
195 @xref{Objective-C and Objective-C++ Dialect Options,,Options Controlling
196 Objective-C and Objective-C++ Dialects}.
198 -fconstant-string-class=@var{class-name} @gol
199 -fgnu-runtime -fnext-runtime @gol
200 -fno-nil-receivers @gol
201 -fobjc-exceptions @gol
202 -freplace-objc-classes @gol
205 -Wno-protocol -Wselector -Wundeclared-selector}
207 @item Language Independent Options
208 @xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
209 @gccoptlist{-fmessage-length=@var{n} @gol
210 -fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]}}
212 @item Warning Options
213 @xref{Warning Options,,Options to Request or Suppress Warnings}.
214 @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
215 -w -Wextra -Wall -Waggregate-return @gol
216 -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment @gol
217 -Wconversion -Wno-deprecated-declarations @gol
218 -Wdisabled-optimization -Wno-div-by-zero -Wendif-labels @gol
219 -Werror -Werror-implicit-function-declaration @gol
220 -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol
221 -Wno-format-extra-args -Wformat-nonliteral @gol
222 -Wformat-security -Wformat-y2k @gol
223 -Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
224 -Wimport -Wno-import -Winit-self -Winline @gol
225 -Wno-invalid-offsetof -Winvalid-pch @gol
226 -Wlarger-than-@var{len} -Wlong-long @gol
227 -Wmain -Wmissing-braces -Wmissing-field-initializers @gol
228 -Wmissing-format-attribute -Wmissing-include-dirs @gol
229 -Wmissing-noreturn @gol
230 -Wno-multichar -Wnonnull -Wpacked -Wpadded @gol
231 -Wparentheses -Wpointer-arith -Wredundant-decls @gol
232 -Wreturn-type -Wsequence-point -Wshadow @gol
233 -Wsign-compare -Wstrict-aliasing -Wstrict-aliasing=2 @gol
234 -Wswitch -Wswitch-default -Wswitch-enum @gol
235 -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
236 -Wunknown-pragmas -Wunreachable-code @gol
237 -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
238 -Wunused-value -Wunused-variable -Wwrite-strings @gol
241 @item C-only Warning Options
242 @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
243 -Wmissing-prototypes -Wnested-externs -Wold-style-definition @gol
244 -Wstrict-prototypes -Wtraditional @gol
245 -Wdeclaration-after-statement}
247 @item Debugging Options
248 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
249 @gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
250 -fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
251 -fdump-class-hierarchy@r{[}-@var{n}@r{]} @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
267 -fdump-tree-sra@r{[}-@var{n}@r{]} @gol
268 -fdump-tree-fre@r{[}-@var{n}@r{]} @gol
269 -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
270 -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs -ftree-based-profiling @gol
271 -frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
272 -ftest-coverage -ftime-report -fvar-tracking @gol
273 -g -g@var{level} -gcoff -gdwarf-2 @gol
274 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
275 -p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
276 -print-multi-directory -print-multi-lib @gol
277 -print-prog-name=@var{program} -print-search-dirs -Q @gol
280 @item Optimization Options
281 @xref{Optimize Options,,Options that Control Optimization}.
282 @gccoptlist{-falign-functions=@var{n} -falign-jumps=@var{n} @gol
283 -falign-labels=@var{n} -falign-loops=@var{n} @gol
284 -fbounds-check -fmudflap -fmudflapth -fmudflapir @gol
285 -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize @gol
286 -fbranch-target-load-optimize2 -fbtr-bb-exclusive @gol
287 -fcaller-saves -fcprop-registers @gol
288 -fcse-follow-jumps -fcse-skip-blocks -fdata-sections @gol
289 -fdelayed-branch -fdelete-null-pointer-checks @gol
290 -fexpensive-optimizations -ffast-math -ffloat-store @gol
291 -fforce-addr -fforce-mem -ffunction-sections @gol
292 -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload @gol
293 -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 @gol
294 -finline-functions -finline-limit=@var{n} -fkeep-inline-functions @gol
295 -fkeep-static-consts -fmerge-constants -fmerge-all-constants @gol
296 -fmodulo-sched -fmove-all-movables -fnew-ra -fno-branch-count-reg @gol
297 -fno-default-inline -fno-defer-pop -floop-optimize2 -fmove-loop-invariants @gol
298 -fno-function-cse -fno-guess-branch-probability @gol
299 -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
300 -funsafe-math-optimizations -ffinite-math-only @gol
301 -fno-trapping-math -fno-zero-initialized-in-bss @gol
302 -fomit-frame-pointer -foptimize-register-move @gol
303 -foptimize-sibling-calls -fprefetch-loop-arrays @gol
304 -fprofile-generate -fprofile-use @gol
305 -freduce-all-givs -fregmove -frename-registers @gol
306 -freorder-blocks -freorder-blocks-and-partition -freorder-functions @gol
307 -frerun-cse-after-loop -frerun-loop-opt @gol
308 -frounding-math -fschedule-insns -fschedule-insns2 @gol
309 -fno-sched-interblock -fno-sched-spec -fsched-spec-load @gol
310 -fsched-spec-load-dangerous @gol
311 -fsched-stalled-insns=@var{n} -sched-stalled-insns-dep=@var{n} @gol
312 -fsched2-use-superblocks @gol
313 -fsched2-use-traces -freschedule-modulo-scheduled-loops @gol
314 -fsignaling-nans -fsingle-precision-constant @gol
315 -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps @gol
316 -funroll-all-loops -funroll-loops -fpeel-loops @gol
317 -funswitch-loops -fold-unroll-loops -fold-unroll-all-loops @gol
318 -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize @gol
320 -ftree-dominator-opts -ftree-dse -ftree-copyrename @gol
321 -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre @gol
322 --param @var{name}=@var{value}
323 -O -O0 -O1 -O2 -O3 -Os}
325 @item Preprocessor Options
326 @xref{Preprocessor Options,,Options Controlling the Preprocessor}.
327 @gccoptlist{-A@var{question}=@var{answer} @gol
328 -A-@var{question}@r{[}=@var{answer}@r{]} @gol
329 -C -dD -dI -dM -dN @gol
330 -D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
331 -idirafter @var{dir} @gol
332 -include @var{file} -imacros @var{file} @gol
333 -iprefix @var{file} -iwithprefix @var{dir} @gol
334 -iwithprefixbefore @var{dir} -isystem @var{dir} @gol
335 -M -MM -MF -MG -MP -MQ -MT -nostdinc @gol
336 -P -fworking-directory -remap @gol
337 -trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
338 -Xpreprocessor @var{option}}
340 @item Assembler Option
341 @xref{Assembler Options,,Passing Options to the Assembler}.
342 @gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
345 @xref{Link Options,,Options for Linking}.
346 @gccoptlist{@var{object-file-name} -l@var{library} @gol
347 -nostartfiles -nodefaultlibs -nostdlib -pie @gol
348 -s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
349 -Wl,@var{option} -Xlinker @var{option} @gol
352 @item Directory Options
353 @xref{Directory Options,,Options for Directory Search}.
354 @gccoptlist{-B@var{prefix} -I@var{dir} -iquote@var{dir} -L@var{dir} -specs=@var{file} -I-}
357 @c I wrote this xref this way to avoid overfull hbox. -- rms
358 @xref{Target Options}.
359 @gccoptlist{-V @var{version} -b @var{machine}}
361 @item Machine Dependent Options
362 @xref{Submodel Options,,Hardware Models and Configurations}.
363 @c This list is ordered alphanumerically by subsection name.
364 @c Try and put the significant identifier (CPU or system) first,
365 @c so users have a clue at guessing where the ones they want will be.
368 @gccoptlist{-EB -EL @gol
369 -mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
370 -mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
373 @gccoptlist{-mapcs-frame -mno-apcs-frame @gol
374 -mabi=@var{name} @gol
375 -mapcs-stack-check -mno-apcs-stack-check @gol
376 -mapcs-float -mno-apcs-float @gol
377 -mapcs-reentrant -mno-apcs-reentrant @gol
378 -msched-prolog -mno-sched-prolog @gol
379 -mlittle-endian -mbig-endian -mwords-little-endian @gol
380 -mfloat-abi=@var{name} -msoft-float -mhard-float -mfpe @gol
381 -mthumb-interwork -mno-thumb-interwork @gol
382 -mcpu=@var{name} -march=@var{name} -mfpu=@var{name} @gol
383 -mstructure-size-boundary=@var{n} @gol
384 -mabort-on-noreturn @gol
385 -mlong-calls -mno-long-calls @gol
386 -msingle-pic-base -mno-single-pic-base @gol
387 -mpic-register=@var{reg} @gol
388 -mnop-fun-dllimport @gol
389 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
390 -mpoke-function-name @gol
392 -mtpcs-frame -mtpcs-leaf-frame @gol
393 -mcaller-super-interworking -mcallee-super-interworking}
396 @gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
397 -mcall-prologues -mno-tablejump -mtiny-stack -mint8}
400 @gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
401 -mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
402 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
403 -mstack-align -mdata-align -mconst-align @gol
404 -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
405 -melf -maout -melinux -mlinux -sim -sim2 @gol
406 -mmul-bug-workaround -mno-mul-bug-workaround}
408 @emph{Darwin Options}
409 @gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
410 -arch_only -bind_at_load -bundle -bundle_loader @gol
411 -client_name -compatibility_version -current_version @gol
413 -dependency-file -dylib_file -dylinker_install_name @gol
414 -dynamic -dynamiclib -exported_symbols_list @gol
415 -filelist -flat_namespace -force_cpusubtype_ALL @gol
416 -force_flat_namespace -headerpad_max_install_names @gol
417 -image_base -init -install_name -keep_private_externs @gol
418 -multi_module -multiply_defined -multiply_defined_unused @gol
419 -noall_load -no_dead_strip_inits_and_terms @gol
420 -nofixprebinding -nomultidefs -noprebind -noseglinkedit @gol
421 -pagezero_size -prebind -prebind_all_twolevel_modules @gol
422 -private_bundle -read_only_relocs -sectalign @gol
423 -sectobjectsymbols -whyload -seg1addr @gol
424 -sectcreate -sectobjectsymbols -sectorder @gol
425 -seg_addr_table -seg_addr_table_filename -seglinkedit @gol
426 -segprot -segs_read_only_addr -segs_read_write_addr @gol
427 -single_module -static -sub_library -sub_umbrella @gol
428 -twolevel_namespace -umbrella -undefined @gol
429 -unexported_symbols_list -weak_reference_mismatches @gol
430 -whatsloaded -F -gused -gfull -mone-byte-bool}
432 @emph{DEC Alpha Options}
433 @gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
434 -mieee -mieee-with-inexact -mieee-conformant @gol
435 -mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
436 -mtrap-precision=@var{mode} -mbuild-constants @gol
437 -mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
438 -mbwx -mmax -mfix -mcix @gol
439 -mfloat-vax -mfloat-ieee @gol
440 -mexplicit-relocs -msmall-data -mlarge-data @gol
441 -msmall-text -mlarge-text @gol
442 -mmemory-latency=@var{time}}
444 @emph{DEC Alpha/VMS Options}
445 @gccoptlist{-mvms-return-codes}
448 @gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
449 -mhard-float -msoft-float @gol
450 -malloc-cc -mfixed-cc -mdword -mno-dword @gol
451 -mdouble -mno-double @gol
452 -mmedia -mno-media -mmuladd -mno-muladd @gol
453 -mfdpic -minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp @gol
454 -mlibrary-pic -macc-4 -macc-8 @gol
455 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
456 -mscc -mno-scc -mcond-exec -mno-cond-exec @gol
457 -mvliw-branch -mno-vliw-branch @gol
458 -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
459 -mno-nested-cond-exec -mtomcat-stats @gol
462 @emph{H8/300 Options}
463 @gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
466 @gccoptlist{-march=@var{architecture-type} @gol
467 -mbig-switch -mdisable-fpregs -mdisable-indexing @gol
468 -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
469 -mfixed-range=@var{register-range} @gol
470 -mjump-in-delay -mlinker-opt -mlong-calls @gol
471 -mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
472 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
473 -mno-jump-in-delay -mno-long-load-store @gol
474 -mno-portable-runtime -mno-soft-float @gol
475 -mno-space-regs -msoft-float -mpa-risc-1-0 @gol
476 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
477 -mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
478 -nolibdld -static -threads}
480 @emph{i386 and x86-64 Options}
481 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
482 -mfpmath=@var{unit} @gol
483 -masm=@var{dialect} -mno-fancy-math-387 @gol
484 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib @gol
485 -mno-wide-multiply -mrtd -malign-double @gol
486 -mpreferred-stack-boundary=@var{num} @gol
487 -mmmx -msse -msse2 -msse3 -m3dnow @gol
488 -mthreads -mno-align-stringops -minline-all-stringops @gol
489 -mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
490 -m96bit-long-double -mregparm=@var{num} -momit-leaf-frame-pointer @gol
491 -mno-red-zone -mno-tls-direct-seg-refs @gol
492 -mcmodel=@var{code-model} @gol
496 @gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
497 -mvolatile-asm-stop -mb-step -mregister-names -mno-sdata @gol
498 -mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
499 -minline-float-divide-max-throughput @gol
500 -minline-int-divide-min-latency @gol
501 -minline-int-divide-max-throughput -mno-dwarf2-asm @gol
502 -mfixed-range=@var{register-range}}
504 @emph{M32R/D Options}
505 @gccoptlist{-m32r2 -m32rx -m32r @gol
507 -malign-loops -mno-align-loops @gol
508 -missue-rate=@var{number} @gol
509 -mbranch-cost=@var{number} @gol
510 -mmodel=@var{code-size-model-type} @gol
511 -msdata=@var{sdata-type} @gol
512 -mno-flush-func -mflush-func=@var{name} @gol
513 -mno-flush-trap -mflush-trap=@var{number} @gol
516 @emph{M680x0 Options}
517 @gccoptlist{-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
518 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 @gol
519 -mnobitfield -mrtd -mshort -msoft-float -mpcrel @gol
520 -malign-int -mstrict-align -msep-data -mno-sep-data @gol
521 -mshared-library-id=n -mid-shared-library -mno-id-shared-library}
523 @emph{M68hc1x Options}
524 @gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
525 -mauto-incdec -minmax -mlong-calls -mshort @gol
526 -msoft-reg-count=@var{count}}
529 @gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
530 -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
531 -m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
532 -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
533 -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
536 @gccoptlist{-EL -EB -march=@var{arch} -mtune=@var{arch} @gol
537 -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
538 -mips16 -mno-mips16 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
539 -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 @gol
540 -mhard-float -msoft-float -msingle-float -mdouble-float @gol
541 -mint64 -mlong64 -mlong32 @gol
542 -G@var{num} -membedded-data -mno-embedded-data @gol
543 -muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
544 -msplit-addresses -mno-split-addresses @gol
545 -mexplicit-relocs -mno-explicit-relocs @gol
546 -mcheck-zero-division -mno-check-zero-division @gol
547 -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls @gol
548 -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp @gol
549 -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 @gol
550 -mfix-vr4120 -mno-fix-vr4120 -mfix-sb1 -mno-fix-sb1 @gol
551 -mflush-func=@var{func} -mno-flush-func @gol
552 -mbranch-likely -mno-branch-likely @gol
553 -mfp-exceptions -mno-fp-exceptions @gol
554 -mvr4130-align -mno-vr4130-align}
557 @gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
558 -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
559 -melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
560 -mno-base-addresses -msingle-exit -mno-single-exit}
562 @emph{MN10300 Options}
563 @gccoptlist{-mmult-bug -mno-mult-bug @gol
564 -mam33 -mno-am33 @gol
565 -mam33-2 -mno-am33-2 @gol
569 @gccoptlist{-m32032 -m32332 -m32532 -m32081 -m32381 @gol
570 -mmult-add -mnomult-add -msoft-float -mrtd -mnortd @gol
571 -mregparam -mnoregparam -msb -mnosb @gol
572 -mbitfield -mnobitfield -mhimem -mnohimem}
574 @emph{PDP-11 Options}
575 @gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
576 -mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
577 -mint16 -mno-int32 -mfloat32 -mno-float64 @gol
578 -mfloat64 -mno-float32 -mabshi -mno-abshi @gol
579 -mbranch-expensive -mbranch-cheap @gol
580 -msplit -mno-split -munix-asm -mdec-asm}
582 @emph{PowerPC Options}
583 See RS/6000 and PowerPC Options.
585 @emph{RS/6000 and PowerPC Options}
586 @gccoptlist{-mcpu=@var{cpu-type} @gol
587 -mtune=@var{cpu-type} @gol
588 -mpower -mno-power -mpower2 -mno-power2 @gol
589 -mpowerpc -mpowerpc64 -mno-powerpc @gol
590 -maltivec -mno-altivec @gol
591 -mpowerpc-gpopt -mno-powerpc-gpopt @gol
592 -mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
593 -mnew-mnemonics -mold-mnemonics @gol
594 -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
595 -m64 -m32 -mxl-call -mno-xl-call -mpe @gol
596 -malign-power -malign-natural @gol
597 -msoft-float -mhard-float -mmultiple -mno-multiple @gol
598 -mstring -mno-string -mupdate -mno-update @gol
599 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
600 -mstrict-align -mno-strict-align -mrelocatable @gol
601 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
602 -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
603 -mdynamic-no-pic @gol
604 -mprioritize-restricted-insns=@var{priority} @gol
605 -msched-costly-dep=@var{dependence_type} @gol
606 -minsert-sched-nops=@var{scheme} @gol
607 -mcall-sysv -mcall-netbsd @gol
608 -maix-struct-return -msvr4-struct-return @gol
609 -mabi=altivec -mabi=no-altivec @gol
610 -mabi=spe -mabi=no-spe @gol
611 -misel=yes -misel=no @gol
612 -mspe=yes -mspe=no @gol
613 -mfloat-gprs=yes -mfloat-gprs=no @gol
614 -mprototype -mno-prototype @gol
615 -msim -mmvme -mads -myellowknife -memb -msdata @gol
616 -msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
618 @emph{S/390 and zSeries Options}
619 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
620 -mhard-float -msoft-float -mbackchain -mno-backchain -mkernel-backchain @gol
621 -msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
622 -m64 -m31 -mdebug -mno-debug -mesa -mzarch @gol
623 -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd}
626 @gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
627 -m4-nofpu -m4-single-only -m4-single -m4 @gol
628 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al @gol
629 -m5-64media -m5-64media-nofpu @gol
630 -m5-32media -m5-32media-nofpu @gol
631 -m5-compact -m5-compact-nofpu @gol
632 -mb -ml -mdalign -mrelax @gol
633 -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave @gol
634 -mieee -misize -mpadstruct -mspace @gol
635 -mprefergot -musermode}
638 @gccoptlist{-mcpu=@var{cpu-type} @gol
639 -mtune=@var{cpu-type} @gol
640 -mcmodel=@var{code-model} @gol
641 -m32 -m64 -mapp-regs -mno-app-regs @gol
642 -mfaster-structs -mno-faster-structs @gol
643 -mfpu -mno-fpu -mhard-float -msoft-float @gol
644 -mhard-quad-float -msoft-quad-float @gol
645 -mimpure-text -mno-impure-text -mlittle-endian @gol
646 -mstack-bias -mno-stack-bias @gol
647 -munaligned-doubles -mno-unaligned-doubles @gol
648 -mv8plus -mno-v8plus -mvis -mno-vis
651 @emph{System V Options}
652 @gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
654 @emph{TMS320C3x/C4x Options}
655 @gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
656 -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
657 -mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
658 -mparallel-insns -mparallel-mpy -mpreserve-float}
661 @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
662 -mprolog-function -mno-prolog-function -mspace @gol
663 -mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
664 -mapp-regs -mno-app-regs @gol
665 -mdisable-callt -mno-disable-callt @gol
671 @gccoptlist{-mg -mgnu -munix}
673 @emph{x86-64 Options}
674 See i386 and x86-64 Options.
676 @emph{Xstormy16 Options}
679 @emph{Xtensa Options}
680 @gccoptlist{-mconst16 -mno-const16 @gol
681 -mfused-madd -mno-fused-madd @gol
682 -mtext-section-literals -mno-text-section-literals @gol
683 -mtarget-align -mno-target-align @gol
684 -mlongcalls -mno-longcalls}
686 @emph{zSeries Options}
687 See S/390 and zSeries Options.
689 @item Code Generation Options
690 @xref{Code Gen Options,,Options for Code Generation Conventions}.
691 @gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
692 -ffixed-@var{reg} -fexceptions @gol
693 -fnon-call-exceptions -funwind-tables @gol
694 -fasynchronous-unwind-tables @gol
695 -finhibit-size-directive -finstrument-functions @gol
696 -fno-common -fno-ident @gol
697 -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
698 -freg-struct-return -fshared-data -fshort-enums @gol
699 -fshort-double -fshort-wchar @gol
700 -fverbose-asm -fpack-struct -fstack-check @gol
701 -fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
702 -fargument-alias -fargument-noalias @gol
703 -fargument-noalias-global -fleading-underscore @gol
704 -ftls-model=@var{model} @gol
705 -ftrapv -fwrapv -fbounds-check @gol
710 * Overall Options:: Controlling the kind of output:
711 an executable, object files, assembler files,
712 or preprocessed source.
713 * C Dialect Options:: Controlling the variant of C language compiled.
714 * C++ Dialect Options:: Variations on C++.
715 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
717 * Language Independent Options:: Controlling how diagnostics should be
719 * Warning Options:: How picky should the compiler be?
720 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
721 * Optimize Options:: How much optimization?
722 * Preprocessor Options:: Controlling header files and macro definitions.
723 Also, getting dependency information for Make.
724 * Assembler Options:: Passing options to the assembler.
725 * Link Options:: Specifying libraries and so on.
726 * Directory Options:: Where to find header files and libraries.
727 Where to find the compiler executable files.
728 * Spec Files:: How to pass switches to sub-processes.
729 * Target Options:: Running a cross-compiler, or an old version of GCC.
732 @node Overall Options
733 @section Options Controlling the Kind of Output
735 Compilation can involve up to four stages: preprocessing, compilation
736 proper, assembly and linking, always in that order. GCC is capable of
737 preprocessing and compiling several files either into several
738 assembler input files, or into one assembler input file; then each
739 assembler input file produces an object file, and linking combines all
740 the object files (those newly compiled, and those specified as input)
741 into an executable file.
743 @cindex file name suffix
744 For any given input file, the file name suffix determines what kind of
749 C source code which must be preprocessed.
752 C source code which should not be preprocessed.
755 C++ source code which should not be preprocessed.
758 Objective-C source code. Note that you must link with the @file{libobjc}
759 library to make an Objective-C program work.
762 Objective-C source code which should not be preprocessed.
766 Objective-C++ source code. Note that you must link with the @file{libobjc}
767 library to make an Objective-C++ program work. Note that @samp{.M} refers
768 to a literal capital M@.
771 Objective-C++ source code which should not be preprocessed.
774 C, C++, Objective-C or Objective-C++ header file to be turned into a
779 @itemx @var{file}.cxx
780 @itemx @var{file}.cpp
781 @itemx @var{file}.CPP
782 @itemx @var{file}.c++
784 C++ source code which must be preprocessed. Note that in @samp{.cxx},
785 the last two letters must both be literally @samp{x}. Likewise,
786 @samp{.C} refers to a literal capital C@.
790 C++ header file to be turned into a precompiled header.
793 @itemx @var{file}.for
794 @itemx @var{file}.FOR
795 Fortran source code which should not be preprocessed.
798 @itemx @var{file}.fpp
799 @itemx @var{file}.FPP
800 Fortran source code which must be preprocessed (with the traditional
804 Fortran source code which must be preprocessed with a RATFOR
805 preprocessor (not included with GCC)@.
808 @itemx @var{file}.f95
809 Fortran 90/95 source code which should not be preprocessed.
811 @c FIXME: Descriptions of Java file types.
818 Ada source code file which contains a library unit declaration (a
819 declaration of a package, subprogram, or generic, or a generic
820 instantiation), or a library unit renaming declaration (a package,
821 generic, or subprogram renaming declaration). Such files are also
824 @itemx @var{file}.adb
825 Ada source code file containing a library unit body (a subprogram or
826 package body). Such files are also called @dfn{bodies}.
828 @c GCC also knows about some suffixes for languages not yet included:
837 Assembler code which must be preprocessed.
840 An object file to be fed straight into linking.
841 Any file name with no recognized suffix is treated this way.
845 You can specify the input language explicitly with the @option{-x} option:
848 @item -x @var{language}
849 Specify explicitly the @var{language} for the following input files
850 (rather than letting the compiler choose a default based on the file
851 name suffix). This option applies to all following input files until
852 the next @option{-x} option. Possible values for @var{language} are:
854 c c-header c-cpp-output
855 c++ c++-header c++-cpp-output
856 objective-c objective-c-header objective-c-cpp-output
857 objective-c++ objective-c++-header objective-c++-cpp-output
858 assembler assembler-with-cpp
860 f77 f77-cpp-input ratfor
867 Turn off any specification of a language, so that subsequent files are
868 handled according to their file name suffixes (as they are if @option{-x}
869 has not been used at all).
871 @item -pass-exit-codes
872 @opindex pass-exit-codes
873 Normally the @command{gcc} program will exit with the code of 1 if any
874 phase of the compiler returns a non-success return code. If you specify
875 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
876 numerically highest error produced by any phase that returned an error
880 If you only want some of the stages of compilation, you can use
881 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
882 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
883 @command{gcc} is to stop. Note that some combinations (for example,
884 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
889 Compile or assemble the source files, but do not link. The linking
890 stage simply is not done. The ultimate output is in the form of an
891 object file for each source file.
893 By default, the object file name for a source file is made by replacing
894 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
896 Unrecognized input files, not requiring compilation or assembly, are
901 Stop after the stage of compilation proper; do not assemble. The output
902 is in the form of an assembler code file for each non-assembler input
905 By default, the assembler file name for a source file is made by
906 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
908 Input files that don't require compilation are ignored.
912 Stop after the preprocessing stage; do not run the compiler proper. The
913 output is in the form of preprocessed source code, which is sent to the
916 Input files which don't require preprocessing are ignored.
918 @cindex output file option
921 Place output in file @var{file}. This applies regardless to whatever
922 sort of output is being produced, whether it be an executable file,
923 an object file, an assembler file or preprocessed C code.
925 If @option{-o} is not specified, the default is to put an executable
926 file in @file{a.out}, the object file for
927 @file{@var{source}.@var{suffix}} in @file{@var{source}.o}, its
928 assembler file in @file{@var{source}.s}, a precompiled header file in
929 @file{@var{source}.@var{suffix}.gch}, and all preprocessed C source on
934 Print (on standard error output) the commands executed to run the stages
935 of compilation. Also print the version number of the compiler driver
936 program and of the preprocessor and the compiler proper.
940 Like @option{-v} except the commands are not executed and all command
941 arguments are quoted. This is useful for shell scripts to capture the
942 driver-generated command lines.
946 Use pipes rather than temporary files for communication between the
947 various stages of compilation. This fails to work on some systems where
948 the assembler is unable to read from a pipe; but the GNU assembler has
953 If you are compiling multiple source files, this option tells the driver
954 to pass all the source files to the compiler at once (for those
955 languages for which the compiler can handle this). This will allow
956 intermodule analysis (IMA) to be performed by the compiler. Currently the only
957 language for which this is supported is C. If you pass source files for
958 multiple languages to the driver, using this option, the driver will invoke
959 the compiler(s) that support IMA once each, passing each compiler all the
960 source files appropriate for it. For those languages that do not support
961 IMA this option will be ignored, and the compiler will be invoked once for
962 each source file in that language. If you use this option in conjunction
963 with -save-temps, the compiler will generate multiple pre-processed files
964 (one for each source file), but only one (combined) .o or .s file.
968 Print (on the standard output) a description of the command line options
969 understood by @command{gcc}. If the @option{-v} option is also specified
970 then @option{--help} will also be passed on to the various processes
971 invoked by @command{gcc}, so that they can display the command line options
972 they accept. If the @option{-Wextra} option is also specified then command
973 line options which have no documentation associated with them will also
978 Print (on the standard output) a description of target specific command
979 line options for each tool.
983 Display the version number and copyrights of the invoked GCC.
987 @section Compiling C++ Programs
989 @cindex suffixes for C++ source
990 @cindex C++ source file suffixes
991 C++ source files conventionally use one of the suffixes @samp{.C},
992 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
993 @samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
994 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
995 files with these names and compiles them as C++ programs even if you
996 call the compiler the same way as for compiling C programs (usually
997 with the name @command{gcc}).
1001 However, C++ programs often require class libraries as well as a
1002 compiler that understands the C++ language---and under some
1003 circumstances, you might want to compile programs or header files from
1004 standard input, or otherwise without a suffix that flags them as C++
1005 programs. You might also like to precompile a C header file with a
1006 @samp{.h} extension to be used in C++ compilations. @command{g++} is a
1007 program that calls GCC with the default language set to C++, and
1008 automatically specifies linking against the C++ library. On many
1009 systems, @command{g++} is also installed with the name @command{c++}.
1011 @cindex invoking @command{g++}
1012 When you compile C++ programs, you may specify many of the same
1013 command-line options that you use for compiling programs in any
1014 language; or command-line options meaningful for C and related
1015 languages; or options that are meaningful only for C++ programs.
1016 @xref{C Dialect Options,,Options Controlling C Dialect}, for
1017 explanations of options for languages related to C@.
1018 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
1019 explanations of options that are meaningful only for C++ programs.
1021 @node C Dialect Options
1022 @section Options Controlling C Dialect
1023 @cindex dialect options
1024 @cindex language dialect options
1025 @cindex options, dialect
1027 The following options control the dialect of C (or languages derived
1028 from C, such as C++, Objective-C and Objective-C++) that the compiler
1032 @cindex ANSI support
1036 In C mode, support all ISO C90 programs. In C++ mode,
1037 remove GNU extensions that conflict with ISO C++.
1039 This turns off certain features of GCC that are incompatible with ISO
1040 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
1041 such as the @code{asm} and @code{typeof} keywords, and
1042 predefined macros such as @code{unix} and @code{vax} that identify the
1043 type of system you are using. It also enables the undesirable and
1044 rarely used ISO trigraph feature. For the C compiler,
1045 it disables recognition of C++ style @samp{//} comments as well as
1046 the @code{inline} keyword.
1048 The alternate keywords @code{__asm__}, @code{__extension__},
1049 @code{__inline__} and @code{__typeof__} continue to work despite
1050 @option{-ansi}. You would not want to use them in an ISO C program, of
1051 course, but it is useful to put them in header files that might be included
1052 in compilations done with @option{-ansi}. Alternate predefined macros
1053 such as @code{__unix__} and @code{__vax__} are also available, with or
1054 without @option{-ansi}.
1056 The @option{-ansi} option does not cause non-ISO programs to be
1057 rejected gratuitously. For that, @option{-pedantic} is required in
1058 addition to @option{-ansi}. @xref{Warning Options}.
1060 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1061 option is used. Some header files may notice this macro and refrain
1062 from declaring certain functions or defining certain macros that the
1063 ISO standard doesn't call for; this is to avoid interfering with any
1064 programs that might use these names for other things.
1066 Functions which would normally be built in but do not have semantics
1067 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1068 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1069 built-in functions provided by GCC}, for details of the functions
1074 Determine the language standard. This option is currently only
1075 supported when compiling C or C++. A value for this option must be
1076 provided; possible values are
1081 ISO C90 (same as @option{-ansi}).
1083 @item iso9899:199409
1084 ISO C90 as modified in amendment 1.
1090 ISO C99. Note that this standard is not yet fully supported; see
1091 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1092 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1095 Default, ISO C90 plus GNU extensions (including some C99 features).
1099 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1100 this will become the default. The name @samp{gnu9x} is deprecated.
1103 The 1998 ISO C++ standard plus amendments.
1106 The same as @option{-std=c++98} plus GNU extensions. This is the
1107 default for C++ code.
1110 Even when this option is not specified, you can still use some of the
1111 features of newer standards in so far as they do not conflict with
1112 previous C standards. For example, you may use @code{__restrict__} even
1113 when @option{-std=c99} is not specified.
1115 The @option{-std} options specifying some version of ISO C have the same
1116 effects as @option{-ansi}, except that features that were not in ISO C90
1117 but are in the specified version (for example, @samp{//} comments and
1118 the @code{inline} keyword in ISO C99) are not disabled.
1120 @xref{Standards,,Language Standards Supported by GCC}, for details of
1121 these standard versions.
1123 @item -aux-info @var{filename}
1125 Output to the given filename prototyped declarations for all functions
1126 declared and/or defined in a translation unit, including those in header
1127 files. This option is silently ignored in any language other than C@.
1129 Besides declarations, the file indicates, in comments, the origin of
1130 each declaration (source file and line), whether the declaration was
1131 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1132 @samp{O} for old, respectively, in the first character after the line
1133 number and the colon), and whether it came from a declaration or a
1134 definition (@samp{C} or @samp{F}, respectively, in the following
1135 character). In the case of function definitions, a K&R-style list of
1136 arguments followed by their declarations is also provided, inside
1137 comments, after the declaration.
1141 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1142 keyword, so that code can use these words as identifiers. You can use
1143 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1144 instead. @option{-ansi} implies @option{-fno-asm}.
1146 In C++, this switch only affects the @code{typeof} keyword, since
1147 @code{asm} and @code{inline} are standard keywords. You may want to
1148 use the @option{-fno-gnu-keywords} flag instead, which has the same
1149 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1150 switch only affects the @code{asm} and @code{typeof} keywords, since
1151 @code{inline} is a standard keyword in ISO C99.
1154 @itemx -fno-builtin-@var{function}
1155 @opindex fno-builtin
1156 @cindex built-in functions
1157 Don't recognize built-in functions that do not begin with
1158 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1159 functions provided by GCC}, for details of the functions affected,
1160 including those which are not built-in functions when @option{-ansi} or
1161 @option{-std} options for strict ISO C conformance are used because they
1162 do not have an ISO standard meaning.
1164 GCC normally generates special code to handle certain built-in functions
1165 more efficiently; for instance, calls to @code{alloca} may become single
1166 instructions that adjust the stack directly, and calls to @code{memcpy}
1167 may become inline copy loops. The resulting code is often both smaller
1168 and faster, but since the function calls no longer appear as such, you
1169 cannot set a breakpoint on those calls, nor can you change the behavior
1170 of the functions by linking with a different library.
1172 With the @option{-fno-builtin-@var{function}} option
1173 only the built-in function @var{function} is
1174 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1175 function is named this is not built-in in this version of GCC, this
1176 option is ignored. There is no corresponding
1177 @option{-fbuiltin-@var{function}} option; if you wish to enable
1178 built-in functions selectively when using @option{-fno-builtin} or
1179 @option{-ffreestanding}, you may define macros such as:
1182 #define abs(n) __builtin_abs ((n))
1183 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1188 @cindex hosted environment
1190 Assert that compilation takes place in a hosted environment. This implies
1191 @option{-fbuiltin}. A hosted environment is one in which the
1192 entire standard library is available, and in which @code{main} has a return
1193 type of @code{int}. Examples are nearly everything except a kernel.
1194 This is equivalent to @option{-fno-freestanding}.
1196 @item -ffreestanding
1197 @opindex ffreestanding
1198 @cindex hosted environment
1200 Assert that compilation takes place in a freestanding environment. This
1201 implies @option{-fno-builtin}. A freestanding environment
1202 is one in which the standard library may not exist, and program startup may
1203 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1204 This is equivalent to @option{-fno-hosted}.
1206 @xref{Standards,,Language Standards Supported by GCC}, for details of
1207 freestanding and hosted environments.
1209 @item -fms-extensions
1210 @opindex fms-extensions
1211 Accept some non-standard constructs used in Microsoft header files.
1215 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1216 options for strict ISO C conformance) implies @option{-trigraphs}.
1218 @item -no-integrated-cpp
1219 @opindex no-integrated-cpp
1220 Performs a compilation in two passes: preprocessing and compiling. This
1221 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1222 @option{-B} option. The user supplied compilation step can then add in
1223 an additional preprocessing step after normal preprocessing but before
1224 compiling. The default is to use the integrated cpp (internal cpp)
1226 The semantics of this option will change if "cc1", "cc1plus", and
1227 "cc1obj" are merged.
1229 @cindex traditional C language
1230 @cindex C language, traditional
1232 @itemx -traditional-cpp
1233 @opindex traditional-cpp
1234 @opindex traditional
1235 Formerly, these options caused GCC to attempt to emulate a pre-standard
1236 C compiler. They are now only supported with the @option{-E} switch.
1237 The preprocessor continues to support a pre-standard mode. See the GNU
1238 CPP manual for details.
1240 @item -fcond-mismatch
1241 @opindex fcond-mismatch
1242 Allow conditional expressions with mismatched types in the second and
1243 third arguments. The value of such an expression is void. This option
1244 is not supported for C++.
1246 @item -funsigned-char
1247 @opindex funsigned-char
1248 Let the type @code{char} be unsigned, like @code{unsigned char}.
1250 Each kind of machine has a default for what @code{char} should
1251 be. It is either like @code{unsigned char} by default or like
1252 @code{signed char} by default.
1254 Ideally, a portable program should always use @code{signed char} or
1255 @code{unsigned char} when it depends on the signedness of an object.
1256 But many programs have been written to use plain @code{char} and
1257 expect it to be signed, or expect it to be unsigned, depending on the
1258 machines they were written for. This option, and its inverse, let you
1259 make such a program work with the opposite default.
1261 The type @code{char} is always a distinct type from each of
1262 @code{signed char} or @code{unsigned char}, even though its behavior
1263 is always just like one of those two.
1266 @opindex fsigned-char
1267 Let the type @code{char} be signed, like @code{signed char}.
1269 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1270 the negative form of @option{-funsigned-char}. Likewise, the option
1271 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1273 @item -fsigned-bitfields
1274 @itemx -funsigned-bitfields
1275 @itemx -fno-signed-bitfields
1276 @itemx -fno-unsigned-bitfields
1277 @opindex fsigned-bitfields
1278 @opindex funsigned-bitfields
1279 @opindex fno-signed-bitfields
1280 @opindex fno-unsigned-bitfields
1281 These options control whether a bit-field is signed or unsigned, when the
1282 declaration does not use either @code{signed} or @code{unsigned}. By
1283 default, such a bit-field is signed, because this is consistent: the
1284 basic integer types such as @code{int} are signed types.
1287 @node C++ Dialect Options
1288 @section Options Controlling C++ Dialect
1290 @cindex compiler options, C++
1291 @cindex C++ options, command line
1292 @cindex options, C++
1293 This section describes the command-line options that are only meaningful
1294 for C++ programs; but you can also use most of the GNU compiler options
1295 regardless of what language your program is in. For example, you
1296 might compile a file @code{firstClass.C} like this:
1299 g++ -g -frepo -O -c firstClass.C
1303 In this example, only @option{-frepo} is an option meant
1304 only for C++ programs; you can use the other options with any
1305 language supported by GCC@.
1307 Here is a list of options that are @emph{only} for compiling C++ programs:
1311 @item -fabi-version=@var{n}
1312 @opindex fabi-version
1313 Use version @var{n} of the C++ ABI. Version 2 is the version of the
1314 C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
1315 the C++ ABI that first appeared in G++ 3.2. Version 0 will always be
1316 the version that conforms most closely to the C++ ABI specification.
1317 Therefore, the ABI obtained using version 0 will change as ABI bugs
1320 The default is version 2.
1322 @item -fno-access-control
1323 @opindex fno-access-control
1324 Turn off all access checking. This switch is mainly useful for working
1325 around bugs in the access control code.
1329 Check that the pointer returned by @code{operator new} is non-null
1330 before attempting to modify the storage allocated. This check is
1331 normally unnecessary because the C++ standard specifies that
1332 @code{operator new} will only return @code{0} if it is declared
1333 @samp{throw()}, in which case the compiler will always check the
1334 return value even without this option. In all other cases, when
1335 @code{operator new} has a non-empty exception specification, memory
1336 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1337 @samp{new (nothrow)}.
1339 @item -fconserve-space
1340 @opindex fconserve-space
1341 Put uninitialized or runtime-initialized global variables into the
1342 common segment, as C does. This saves space in the executable at the
1343 cost of not diagnosing duplicate definitions. If you compile with this
1344 flag and your program mysteriously crashes after @code{main()} has
1345 completed, you may have an object that is being destroyed twice because
1346 two definitions were merged.
1348 This option is no longer useful on most targets, now that support has
1349 been added for putting variables into BSS without making them common.
1351 @item -fno-const-strings
1352 @opindex fno-const-strings
1353 Give string constants type @code{char *} instead of type @code{const
1354 char *}. By default, G++ uses type @code{const char *} as required by
1355 the standard. Even if you use @option{-fno-const-strings}, you cannot
1356 actually modify the value of a string constant.
1358 This option might be removed in a future release of G++. For maximum
1359 portability, you should structure your code so that it works with
1360 string constants that have type @code{const char *}.
1362 @item -fno-elide-constructors
1363 @opindex fno-elide-constructors
1364 The C++ standard allows an implementation to omit creating a temporary
1365 which is only used to initialize another object of the same type.
1366 Specifying this option disables that optimization, and forces G++ to
1367 call the copy constructor in all cases.
1369 @item -fno-enforce-eh-specs
1370 @opindex fno-enforce-eh-specs
1371 Don't check for violation of exception specifications at runtime. This
1372 option violates the C++ standard, but may be useful for reducing code
1373 size in production builds, much like defining @samp{NDEBUG}. The compiler
1374 will still optimize based on the exception specifications.
1377 @itemx -fno-for-scope
1379 @opindex fno-for-scope
1380 If @option{-ffor-scope} is specified, the scope of variables declared in
1381 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1382 as specified by the C++ standard.
1383 If @option{-fno-for-scope} is specified, the scope of variables declared in
1384 a @i{for-init-statement} extends to the end of the enclosing scope,
1385 as was the case in old versions of G++, and other (traditional)
1386 implementations of C++.
1388 The default if neither flag is given to follow the standard,
1389 but to allow and give a warning for old-style code that would
1390 otherwise be invalid, or have different behavior.
1392 @item -fno-gnu-keywords
1393 @opindex fno-gnu-keywords
1394 Do not recognize @code{typeof} as a keyword, so that code can use this
1395 word as an identifier. You can use the keyword @code{__typeof__} instead.
1396 @option{-ansi} implies @option{-fno-gnu-keywords}.
1398 @item -fno-implicit-templates
1399 @opindex fno-implicit-templates
1400 Never emit code for non-inline templates which are instantiated
1401 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1402 @xref{Template Instantiation}, for more information.
1404 @item -fno-implicit-inline-templates
1405 @opindex fno-implicit-inline-templates
1406 Don't emit code for implicit instantiations of inline templates, either.
1407 The default is to handle inlines differently so that compiles with and
1408 without optimization will need the same set of explicit instantiations.
1410 @item -fno-implement-inlines
1411 @opindex fno-implement-inlines
1412 To save space, do not emit out-of-line copies of inline functions
1413 controlled by @samp{#pragma implementation}. This will cause linker
1414 errors if these functions are not inlined everywhere they are called.
1416 @item -fms-extensions
1417 @opindex fms-extensions
1418 Disable pedantic warnings about constructs used in MFC, such as implicit
1419 int and getting a pointer to member function via non-standard syntax.
1421 @item -fno-nonansi-builtins
1422 @opindex fno-nonansi-builtins
1423 Disable built-in declarations of functions that are not mandated by
1424 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1425 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1427 @item -fno-operator-names
1428 @opindex fno-operator-names
1429 Do not treat the operator name keywords @code{and}, @code{bitand},
1430 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1431 synonyms as keywords.
1433 @item -fno-optional-diags
1434 @opindex fno-optional-diags
1435 Disable diagnostics that the standard says a compiler does not need to
1436 issue. Currently, the only such diagnostic issued by G++ is the one for
1437 a name having multiple meanings within a class.
1440 @opindex fpermissive
1441 Downgrade some diagnostics about nonconformant code from errors to
1442 warnings. Thus, using @option{-fpermissive} will allow some
1443 nonconforming code to compile.
1447 Enable automatic template instantiation at link time. This option also
1448 implies @option{-fno-implicit-templates}. @xref{Template
1449 Instantiation}, for more information.
1453 Disable generation of information about every class with virtual
1454 functions for use by the C++ runtime type identification features
1455 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1456 of the language, you can save some space by using this flag. Note that
1457 exception handling uses the same information, but it will generate it as
1462 Emit statistics about front-end processing at the end of the compilation.
1463 This information is generally only useful to the G++ development team.
1465 @item -ftemplate-depth-@var{n}
1466 @opindex ftemplate-depth
1467 Set the maximum instantiation depth for template classes to @var{n}.
1468 A limit on the template instantiation depth is needed to detect
1469 endless recursions during template class instantiation. ANSI/ISO C++
1470 conforming programs must not rely on a maximum depth greater than 17.
1472 @item -fuse-cxa-atexit
1473 @opindex fuse-cxa-atexit
1474 Register destructors for objects with static storage duration with the
1475 @code{__cxa_atexit} function rather than the @code{atexit} function.
1476 This option is required for fully standards-compliant handling of static
1477 destructors, but will only work if your C library supports
1478 @code{__cxa_atexit}.
1480 @item -fvisibility-inlines-hidden
1481 @opindex fvisibility-inlines-hidden
1482 Causes all inlined methods to be marked with
1483 @code{__attribute__ ((visibility ("hidden")))} so that they do not
1484 appear in the export table of a DSO and do not require a PLT indirection
1485 when used within the DSO. Enabling this option can have a dramatic effect
1486 on load and link times of a DSO as it massively reduces the size of the
1487 dynamic export table when the library makes heavy use of templates. While
1488 it can cause bloating through duplication of code within each DSO where
1489 it is used, often the wastage is less than the considerable space occupied
1490 by a long symbol name in the export table which is typical when using
1491 templates and namespaces. For even more savings, combine with the
1492 @code{-fvisibility=hidden} switch.
1496 Do not use weak symbol support, even if it is provided by the linker.
1497 By default, G++ will use weak symbols if they are available. This
1498 option exists only for testing, and should not be used by end-users;
1499 it will result in inferior code and has no benefits. This option may
1500 be removed in a future release of G++.
1504 Do not search for header files in the standard directories specific to
1505 C++, but do still search the other standard directories. (This option
1506 is used when building the C++ library.)
1509 In addition, these optimization, warning, and code generation options
1510 have meanings only for C++ programs:
1513 @item -fno-default-inline
1514 @opindex fno-default-inline
1515 Do not assume @samp{inline} for functions defined inside a class scope.
1516 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1517 functions will have linkage like inline functions; they just won't be
1520 @item -Wabi @r{(C++ only)}
1522 Warn when G++ generates code that is probably not compatible with the
1523 vendor-neutral C++ ABI. Although an effort has been made to warn about
1524 all such cases, there are probably some cases that are not warned about,
1525 even though G++ is generating incompatible code. There may also be
1526 cases where warnings are emitted even though the code that is generated
1529 You should rewrite your code to avoid these warnings if you are
1530 concerned about the fact that code generated by G++ may not be binary
1531 compatible with code generated by other compilers.
1533 The known incompatibilities at this point include:
1538 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1539 pack data into the same byte as a base class. For example:
1542 struct A @{ virtual void f(); int f1 : 1; @};
1543 struct B : public A @{ int f2 : 1; @};
1547 In this case, G++ will place @code{B::f2} into the same byte
1548 as@code{A::f1}; other compilers will not. You can avoid this problem
1549 by explicitly padding @code{A} so that its size is a multiple of the
1550 byte size on your platform; that will cause G++ and other compilers to
1551 layout @code{B} identically.
1554 Incorrect handling of tail-padding for virtual bases. G++ does not use
1555 tail padding when laying out virtual bases. For example:
1558 struct A @{ virtual void f(); char c1; @};
1559 struct B @{ B(); char c2; @};
1560 struct C : public A, public virtual B @{@};
1564 In this case, G++ will not place @code{B} into the tail-padding for
1565 @code{A}; other compilers will. You can avoid this problem by
1566 explicitly padding @code{A} so that its size is a multiple of its
1567 alignment (ignoring virtual base classes); that will cause G++ and other
1568 compilers to layout @code{C} identically.
1571 Incorrect handling of bit-fields with declared widths greater than that
1572 of their underlying types, when the bit-fields appear in a union. For
1576 union U @{ int i : 4096; @};
1580 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1581 union too small by the number of bits in an @code{int}.
1584 Empty classes can be placed at incorrect offsets. For example:
1594 struct C : public B, public A @{@};
1598 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1599 it should be placed at offset zero. G++ mistakenly believes that the
1600 @code{A} data member of @code{B} is already at offset zero.
1603 Names of template functions whose types involve @code{typename} or
1604 template template parameters can be mangled incorrectly.
1607 template <typename Q>
1608 void f(typename Q::X) @{@}
1610 template <template <typename> class Q>
1611 void f(typename Q<int>::X) @{@}
1615 Instantiations of these templates may be mangled incorrectly.
1619 @item -Wctor-dtor-privacy @r{(C++ only)}
1620 @opindex Wctor-dtor-privacy
1621 Warn when a class seems unusable because all the constructors or
1622 destructors in that class are private, and it has neither friends nor
1623 public static member functions.
1625 @item -Wnon-virtual-dtor @r{(C++ only)}
1626 @opindex Wnon-virtual-dtor
1627 Warn when a class appears to be polymorphic, thereby requiring a virtual
1628 destructor, yet it declares a non-virtual one.
1629 This warning is enabled by @option{-Wall}.
1631 @item -Wreorder @r{(C++ only)}
1633 @cindex reordering, warning
1634 @cindex warning for reordering of member initializers
1635 Warn when the order of member initializers given in the code does not
1636 match the order in which they must be executed. For instance:
1642 A(): j (0), i (1) @{ @}
1646 The compiler will rearrange the member initializers for @samp{i}
1647 and @samp{j} to match the declaration order of the members, emitting
1648 a warning to that effect. This warning is enabled by @option{-Wall}.
1651 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
1654 @item -Weffc++ @r{(C++ only)}
1656 Warn about violations of the following style guidelines from Scott Meyers'
1657 @cite{Effective C++} book:
1661 Item 11: Define a copy constructor and an assignment operator for classes
1662 with dynamically allocated memory.
1665 Item 12: Prefer initialization to assignment in constructors.
1668 Item 14: Make destructors virtual in base classes.
1671 Item 15: Have @code{operator=} return a reference to @code{*this}.
1674 Item 23: Don't try to return a reference when you must return an object.
1678 Also warn about violations of the following style guidelines from
1679 Scott Meyers' @cite{More Effective C++} book:
1683 Item 6: Distinguish between prefix and postfix forms of increment and
1684 decrement operators.
1687 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
1691 When selecting this option, be aware that the standard library
1692 headers do not obey all of these guidelines; use @samp{grep -v}
1693 to filter out those warnings.
1695 @item -Wno-deprecated @r{(C++ only)}
1696 @opindex Wno-deprecated
1697 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
1699 @item -Wno-non-template-friend @r{(C++ only)}
1700 @opindex Wno-non-template-friend
1701 Disable warnings when non-templatized friend functions are declared
1702 within a template. Since the advent of explicit template specification
1703 support in G++, if the name of the friend is an unqualified-id (i.e.,
1704 @samp{friend foo(int)}), the C++ language specification demands that the
1705 friend declare or define an ordinary, nontemplate function. (Section
1706 14.5.3). Before G++ implemented explicit specification, unqualified-ids
1707 could be interpreted as a particular specialization of a templatized
1708 function. Because this non-conforming behavior is no longer the default
1709 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
1710 check existing code for potential trouble spots and is on by default.
1711 This new compiler behavior can be turned off with
1712 @option{-Wno-non-template-friend} which keeps the conformant compiler code
1713 but disables the helpful warning.
1715 @item -Wold-style-cast @r{(C++ only)}
1716 @opindex Wold-style-cast
1717 Warn if an old-style (C-style) cast to a non-void type is used within
1718 a C++ program. The new-style casts (@samp{static_cast},
1719 @samp{reinterpret_cast}, and @samp{const_cast}) are less vulnerable to
1720 unintended effects and much easier to search for.
1722 @item -Woverloaded-virtual @r{(C++ only)}
1723 @opindex Woverloaded-virtual
1724 @cindex overloaded virtual fn, warning
1725 @cindex warning for overloaded virtual fn
1726 Warn when a function declaration hides virtual functions from a
1727 base class. For example, in:
1734 struct B: public A @{
1739 the @code{A} class version of @code{f} is hidden in @code{B}, and code
1747 will fail to compile.
1749 @item -Wno-pmf-conversions @r{(C++ only)}
1750 @opindex Wno-pmf-conversions
1751 Disable the diagnostic for converting a bound pointer to member function
1754 @item -Wsign-promo @r{(C++ only)}
1755 @opindex Wsign-promo
1756 Warn when overload resolution chooses a promotion from unsigned or
1757 enumerated type to a signed type, over a conversion to an unsigned type of
1758 the same size. Previous versions of G++ would try to preserve
1759 unsignedness, but the standard mandates the current behavior.
1761 @item -Wsynth @r{(C++ only)}
1763 @cindex warning for synthesized methods
1764 @cindex synthesized methods, warning
1765 Warn when G++'s synthesis behavior does not match that of cfront. For
1771 A& operator = (int);
1781 In this example, G++ will synthesize a default @samp{A& operator =
1782 (const A&);}, while cfront will use the user-defined @samp{operator =}.
1785 @node Objective-C and Objective-C++ Dialect Options
1786 @section Options Controlling Objective-C and Objective-C++ Dialects
1788 @cindex compiler options, Objective-C and Objective-C++
1789 @cindex Objective-C and Objective-C++ options, command line
1790 @cindex options, Objective-C and Objective-C++
1791 (NOTE: This manual does not describe the Objective-C and Objective-C++
1792 languages themselves. See @xref{Standards,,Language Standards
1793 Supported by GCC}, for references.)
1795 This section describes the command-line options that are only meaningful
1796 for Objective-C and Objective-C++ programs, but you can also use most of
1797 the language-independent GNU compiler options.
1798 For example, you might compile a file @code{some_class.m} like this:
1801 gcc -g -fgnu-runtime -O -c some_class.m
1805 In this example, @option{-fgnu-runtime} is an option meant only for
1806 Objective-C and Objective-C++ programs; you can use the other options with
1807 any language supported by GCC@.
1809 Note that since Objective-C is an extension of the C language, Objective-C
1810 compilations may also use options specific to the C front-end (e.g.,
1811 @option{-Wtraditional}). Similarly, Objective-C++ compilations may use
1812 C++-specific options (e.g., @option{-Wabi}).
1814 Here is a list of options that are @emph{only} for compiling Objective-C
1815 and Objective-C++ programs:
1818 @item -fconstant-string-class=@var{class-name}
1819 @opindex fconstant-string-class
1820 Use @var{class-name} as the name of the class to instantiate for each
1821 literal string specified with the syntax @code{@@"@dots{}"}. The default
1822 class name is @code{NXConstantString} if the GNU runtime is being used, and
1823 @code{NSConstantString} if the NeXT runtime is being used (see below). The
1824 @option{-fconstant-cfstrings} option, if also present, will override the
1825 @option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
1826 to be laid out as constant CoreFoundation strings.
1829 @opindex fgnu-runtime
1830 Generate object code compatible with the standard GNU Objective-C
1831 runtime. This is the default for most types of systems.
1833 @item -fnext-runtime
1834 @opindex fnext-runtime
1835 Generate output compatible with the NeXT runtime. This is the default
1836 for NeXT-based systems, including Darwin and Mac OS X@. The macro
1837 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
1840 @item -fno-nil-receivers
1841 @opindex fno-nil-receivers
1842 Assume that all Objective-C message dispatches (e.g.,
1843 @code{[receiver message:arg]}) in this translation unit ensure that the receiver
1844 is not @code{nil}. This allows for more efficient entry points in the runtime
1845 to be used. Currently, this option is only available in conjunction with
1846 the NeXT runtime on Mac OS X 10.3 and later.
1848 @item -fobjc-exceptions
1849 @opindex fobjc-exceptions
1850 Enable syntactic support for structured exception handling in Objective-C,
1851 similar to what is offered by C++ and Java. Currently, this option is only
1852 available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
1860 @@catch (AnObjCClass *exc) @{
1867 @@catch (AnotherClass *exc) @{
1870 @@catch (id allOthers) @{
1880 The @code{@@throw} statement may appear anywhere in an Objective-C or
1881 Objective-C++ program; when used inside of a @code{@@catch} block, the
1882 @code{@@throw} may appear without an argument (as shown above), in which case
1883 the object caught by the @code{@@catch} will be rethrown.
1885 Note that only (pointers to) Objective-C objects may be thrown and
1886 caught using this scheme. When an object is thrown, it will be caught
1887 by the nearest @code{@@catch} clause capable of handling objects of that type,
1888 analogously to how @code{catch} blocks work in C++ and Java. A
1889 @code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
1890 any and all Objective-C exceptions not caught by previous @code{@@catch}
1893 The @code{@@finally} clause, if present, will be executed upon exit from the
1894 immediately preceding @code{@@try @dots{} @@catch} section. This will happen
1895 regardless of whether any exceptions are thrown, caught or rethrown
1896 inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
1897 of the @code{finally} clause in Java.
1899 There are several caveats to using the new exception mechanism:
1903 Although currently designed to be binary compatible with @code{NS_HANDLER}-style
1904 idioms provided by the @code{NSException} class, the new
1905 exceptions can only be used on Mac OS X 10.3 (Panther) and later
1906 systems, due to additional functionality needed in the (NeXT) Objective-C
1910 As mentioned above, the new exceptions do not support handling
1911 types other than Objective-C objects. Furthermore, when used from
1912 Objective-C++, the Objective-C exception model does not interoperate with C++
1913 exceptions at this time. This means you cannot @code{@@throw} an exception
1914 from Objective-C and @code{catch} it in C++, or vice versa
1915 (i.e., @code{throw @dots{} @@catch}).
1918 The @option{-fobjc-exceptions} switch also enables the use of synchronization
1919 blocks for thread-safe execution:
1922 @@synchronized (ObjCClass *guard) @{
1927 Upon entering the @code{@@synchronized} block, a thread of execution shall
1928 first check whether a lock has been placed on the corresponding @code{guard}
1929 object by another thread. If it has, the current thread shall wait until
1930 the other thread relinquishes its lock. Once @code{guard} becomes available,
1931 the current thread will place its own lock on it, execute the code contained in
1932 the @code{@@synchronized} block, and finally relinquish the lock (thereby
1933 making @code{guard} available to other threads).
1935 Unlike Java, Objective-C does not allow for entire methods to be marked
1936 @code{@@synchronized}. Note that throwing exceptions out of
1937 @code{@@synchronized} blocks is allowed, and will cause the guarding object
1938 to be unlocked properly.
1940 @item -freplace-objc-classes
1941 @opindex freplace-objc-classes
1942 Emit a special marker instructing @command{ld(1)} not to statically link in
1943 the resulting object file, and allow @command{dyld(1)} to load it in at
1944 run time instead. This is used in conjunction with the Fix-and-Continue
1945 debugging mode, where the object file in question may be recompiled and
1946 dynamically reloaded in the course of program execution, without the need
1947 to restart the program itself. Currently, Fix-and-Continue functionality
1948 is only available in conjunction with the NeXT runtime on Mac OS X 10.3
1953 When compiling for the NeXT runtime, the compiler ordinarily replaces calls
1954 to @code{objc_getClass("@dots{}")} (when the name of the class is known at
1955 compile time) with static class references that get initialized at load time,
1956 which improves run-time performance. Specifying the @option{-fzero-link} flag
1957 suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
1958 to be retained. This is useful in Zero-Link debugging mode, since it allows
1959 for individual class implementations to be modified during program execution.
1963 Dump interface declarations for all classes seen in the source file to a
1964 file named @file{@var{sourcename}.decl}.
1967 @opindex Wno-protocol
1968 If a class is declared to implement a protocol, a warning is issued for
1969 every method in the protocol that is not implemented by the class. The
1970 default behavior is to issue a warning for every method not explicitly
1971 implemented in the class, even if a method implementation is inherited
1972 from the superclass. If you use the @code{-Wno-protocol} option, then
1973 methods inherited from the superclass are considered to be implemented,
1974 and no warning is issued for them.
1978 Warn if multiple methods of different types for the same selector are
1979 found during compilation. The check is performed on the list of methods
1980 in the final stage of compilation. Additionally, a check is performed
1981 for each selector appearing in a @code{@@selector(@dots{})}
1982 expression, and a corresponding method for that selector has been found
1983 during compilation. Because these checks scan the method table only at
1984 the end of compilation, these warnings are not produced if the final
1985 stage of compilation is not reached, for example because an error is
1986 found during compilation, or because the @code{-fsyntax-only} option is
1989 @item -Wundeclared-selector
1990 @opindex Wundeclared-selector
1991 Warn if a @code{@@selector(@dots{})} expression referring to an
1992 undeclared selector is found. A selector is considered undeclared if no
1993 method with that name has been declared before the
1994 @code{@@selector(@dots{})} expression, either explicitly in an
1995 @code{@@interface} or @code{@@protocol} declaration, or implicitly in
1996 an @code{@@implementation} section. This option always performs its
1997 checks as soon as a @code{@@selector(@dots{})} expression is found,
1998 while @code{-Wselector} only performs its checks in the final stage of
1999 compilation. This also enforces the coding style convention
2000 that methods and selectors must be declared before being used.
2002 @item -print-objc-runtime-info
2003 @opindex print-objc-runtime-info
2004 Generate C header describing the largest structure that is passed by
2009 @node Language Independent Options
2010 @section Options to Control Diagnostic Messages Formatting
2011 @cindex options to control diagnostics formatting
2012 @cindex diagnostic messages
2013 @cindex message formatting
2015 Traditionally, diagnostic messages have been formatted irrespective of
2016 the output device's aspect (e.g.@: its width, @dots{}). The options described
2017 below can be used to control the diagnostic messages formatting
2018 algorithm, e.g.@: how many characters per line, how often source location
2019 information should be reported. Right now, only the C++ front end can
2020 honor these options. However it is expected, in the near future, that
2021 the remaining front ends would be able to digest them correctly.
2024 @item -fmessage-length=@var{n}
2025 @opindex fmessage-length
2026 Try to format error messages so that they fit on lines of about @var{n}
2027 characters. The default is 72 characters for @command{g++} and 0 for the rest of
2028 the front ends supported by GCC@. If @var{n} is zero, then no
2029 line-wrapping will be done; each error message will appear on a single
2032 @opindex fdiagnostics-show-location
2033 @item -fdiagnostics-show-location=once
2034 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
2035 reporter to emit @emph{once} source location information; that is, in
2036 case the message is too long to fit on a single physical line and has to
2037 be wrapped, the source location won't be emitted (as prefix) again,
2038 over and over, in subsequent continuation lines. This is the default
2041 @item -fdiagnostics-show-location=every-line
2042 Only meaningful in line-wrapping mode. Instructs the diagnostic
2043 messages reporter to emit the same source location information (as
2044 prefix) for physical lines that result from the process of breaking
2045 a message which is too long to fit on a single line.
2049 @node Warning Options
2050 @section Options to Request or Suppress Warnings
2051 @cindex options to control warnings
2052 @cindex warning messages
2053 @cindex messages, warning
2054 @cindex suppressing warnings
2056 Warnings are diagnostic messages that report constructions which
2057 are not inherently erroneous but which are risky or suggest there
2058 may have been an error.
2060 You can request many specific warnings with options beginning @samp{-W},
2061 for example @option{-Wimplicit} to request warnings on implicit
2062 declarations. Each of these specific warning options also has a
2063 negative form beginning @samp{-Wno-} to turn off warnings;
2064 for example, @option{-Wno-implicit}. This manual lists only one of the
2065 two forms, whichever is not the default.
2067 The following options control the amount and kinds of warnings produced
2068 by GCC; for further, language-specific options also refer to
2069 @ref{C++ Dialect Options} and @ref{Objective-C and Objective-C++ Dialect
2073 @cindex syntax checking
2075 @opindex fsyntax-only
2076 Check the code for syntax errors, but don't do anything beyond that.
2080 Issue all the warnings demanded by strict ISO C and ISO C++;
2081 reject all programs that use forbidden extensions, and some other
2082 programs that do not follow ISO C and ISO C++. For ISO C, follows the
2083 version of the ISO C standard specified by any @option{-std} option used.
2085 Valid ISO C and ISO C++ programs should compile properly with or without
2086 this option (though a rare few will require @option{-ansi} or a
2087 @option{-std} option specifying the required version of ISO C)@. However,
2088 without this option, certain GNU extensions and traditional C and C++
2089 features are supported as well. With this option, they are rejected.
2091 @option{-pedantic} does not cause warning messages for use of the
2092 alternate keywords whose names begin and end with @samp{__}. Pedantic
2093 warnings are also disabled in the expression that follows
2094 @code{__extension__}. However, only system header files should use
2095 these escape routes; application programs should avoid them.
2096 @xref{Alternate Keywords}.
2098 Some users try to use @option{-pedantic} to check programs for strict ISO
2099 C conformance. They soon find that it does not do quite what they want:
2100 it finds some non-ISO practices, but not all---only those for which
2101 ISO C @emph{requires} a diagnostic, and some others for which
2102 diagnostics have been added.
2104 A feature to report any failure to conform to ISO C might be useful in
2105 some instances, but would require considerable additional work and would
2106 be quite different from @option{-pedantic}. We don't have plans to
2107 support such a feature in the near future.
2109 Where the standard specified with @option{-std} represents a GNU
2110 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
2111 corresponding @dfn{base standard}, the version of ISO C on which the GNU
2112 extended dialect is based. Warnings from @option{-pedantic} are given
2113 where they are required by the base standard. (It would not make sense
2114 for such warnings to be given only for features not in the specified GNU
2115 C dialect, since by definition the GNU dialects of C include all
2116 features the compiler supports with the given option, and there would be
2117 nothing to warn about.)
2119 @item -pedantic-errors
2120 @opindex pedantic-errors
2121 Like @option{-pedantic}, except that errors are produced rather than
2126 Inhibit all warning messages.
2130 Inhibit warning messages about the use of @samp{#import}.
2132 @item -Wchar-subscripts
2133 @opindex Wchar-subscripts
2134 Warn if an array subscript has type @code{char}. This is a common cause
2135 of error, as programmers often forget that this type is signed on some
2140 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
2141 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
2143 @item -Wfatal-errors
2144 @opindex Wfatal-errors
2145 This option causes the compiler to abort compilation on the first error
2146 occurred rather than trying to keep going and printing further error
2151 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
2152 the arguments supplied have types appropriate to the format string
2153 specified, and that the conversions specified in the format string make
2154 sense. This includes standard functions, and others specified by format
2155 attributes (@pxref{Function Attributes}), in the @code{printf},
2156 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
2157 not in the C standard) families (or other target-specific families).
2159 The formats are checked against the format features supported by GNU
2160 libc version 2.2. These include all ISO C90 and C99 features, as well
2161 as features from the Single Unix Specification and some BSD and GNU
2162 extensions. Other library implementations may not support all these
2163 features; GCC does not support warning about features that go beyond a
2164 particular library's limitations. However, if @option{-pedantic} is used
2165 with @option{-Wformat}, warnings will be given about format features not
2166 in the selected standard version (but not for @code{strfmon} formats,
2167 since those are not in any version of the C standard). @xref{C Dialect
2168 Options,,Options Controlling C Dialect}.
2170 Since @option{-Wformat} also checks for null format arguments for
2171 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2173 @option{-Wformat} is included in @option{-Wall}. For more control over some
2174 aspects of format checking, the options @option{-Wformat-y2k},
2175 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2176 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2177 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2180 @opindex Wformat-y2k
2181 If @option{-Wformat} is specified, also warn about @code{strftime}
2182 formats which may yield only a two-digit year.
2184 @item -Wno-format-extra-args
2185 @opindex Wno-format-extra-args
2186 If @option{-Wformat} is specified, do not warn about excess arguments to a
2187 @code{printf} or @code{scanf} format function. The C standard specifies
2188 that such arguments are ignored.
2190 Where the unused arguments lie between used arguments that are
2191 specified with @samp{$} operand number specifications, normally
2192 warnings are still given, since the implementation could not know what
2193 type to pass to @code{va_arg} to skip the unused arguments. However,
2194 in the case of @code{scanf} formats, this option will suppress the
2195 warning if the unused arguments are all pointers, since the Single
2196 Unix Specification says that such unused arguments are allowed.
2198 @item -Wno-format-zero-length
2199 @opindex Wno-format-zero-length
2200 If @option{-Wformat} is specified, do not warn about zero-length formats.
2201 The C standard specifies that zero-length formats are allowed.
2203 @item -Wformat-nonliteral
2204 @opindex Wformat-nonliteral
2205 If @option{-Wformat} is specified, also warn if the format string is not a
2206 string literal and so cannot be checked, unless the format function
2207 takes its format arguments as a @code{va_list}.
2209 @item -Wformat-security
2210 @opindex Wformat-security
2211 If @option{-Wformat} is specified, also warn about uses of format
2212 functions that represent possible security problems. At present, this
2213 warns about calls to @code{printf} and @code{scanf} functions where the
2214 format string is not a string literal and there are no format arguments,
2215 as in @code{printf (foo);}. This may be a security hole if the format
2216 string came from untrusted input and contains @samp{%n}. (This is
2217 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2218 in future warnings may be added to @option{-Wformat-security} that are not
2219 included in @option{-Wformat-nonliteral}.)
2223 Enable @option{-Wformat} plus format checks not included in
2224 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2225 -Wformat-nonliteral -Wformat-security -Wformat-y2k}.
2229 Warn about passing a null pointer for arguments marked as
2230 requiring a non-null value by the @code{nonnull} function attribute.
2232 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2233 can be disabled with the @option{-Wno-nonnull} option.
2235 @item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
2237 Warn about uninitialized variables which are initialized with themselves.
2238 Note this option can only be used with the @option{-Wuninitialized} option,
2239 which in turn only works with @option{-O1} and above.
2241 For example, GCC will warn about @code{i} being uninitialized in the
2242 following snippet only when @option{-Winit-self} has been specified:
2253 @item -Wimplicit-int
2254 @opindex Wimplicit-int
2255 Warn when a declaration does not specify a type.
2257 @item -Wimplicit-function-declaration
2258 @itemx -Werror-implicit-function-declaration
2259 @opindex Wimplicit-function-declaration
2260 @opindex Werror-implicit-function-declaration
2261 Give a warning (or error) whenever a function is used before being
2266 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2270 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2271 function with external linkage, returning int, taking either zero
2272 arguments, two, or three arguments of appropriate types.
2274 @item -Wmissing-braces
2275 @opindex Wmissing-braces
2276 Warn if an aggregate or union initializer is not fully bracketed. In
2277 the following example, the initializer for @samp{a} is not fully
2278 bracketed, but that for @samp{b} is fully bracketed.
2281 int a[2][2] = @{ 0, 1, 2, 3 @};
2282 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2285 @item -Wmissing-include-dirs @r{(C, C++, Objective-C and Objective-C++ only)}
2286 @opindex Wmissing-include-dirs
2287 Warn if a user-supplied include directory does not exist.
2290 @opindex Wparentheses
2291 Warn if parentheses are omitted in certain contexts, such
2292 as when there is an assignment in a context where a truth value
2293 is expected, or when operators are nested whose precedence people
2294 often get confused about. Only the warning for an assignment used as
2295 a truth value is supported when compiling C++; the other warnings are
2296 only supported when compiling C@.
2298 Also warn if a comparison like @samp{x<=y<=z} appears; this is
2299 equivalent to @samp{(x<=y ? 1 : 0) <= z}, which is a different
2300 interpretation from that of ordinary mathematical notation.
2302 Also warn about constructions where there may be confusion to which
2303 @code{if} statement an @code{else} branch belongs. Here is an example of
2318 In C, every @code{else} branch belongs to the innermost possible @code{if}
2319 statement, which in this example is @code{if (b)}. This is often not
2320 what the programmer expected, as illustrated in the above example by
2321 indentation the programmer chose. When there is the potential for this
2322 confusion, GCC will issue a warning when this flag is specified.
2323 To eliminate the warning, add explicit braces around the innermost
2324 @code{if} statement so there is no way the @code{else} could belong to
2325 the enclosing @code{if}. The resulting code would look like this:
2341 @item -Wsequence-point
2342 @opindex Wsequence-point
2343 Warn about code that may have undefined semantics because of violations
2344 of sequence point rules in the C standard.
2346 The C standard defines the order in which expressions in a C program are
2347 evaluated in terms of @dfn{sequence points}, which represent a partial
2348 ordering between the execution of parts of the program: those executed
2349 before the sequence point, and those executed after it. These occur
2350 after the evaluation of a full expression (one which is not part of a
2351 larger expression), after the evaluation of the first operand of a
2352 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2353 function is called (but after the evaluation of its arguments and the
2354 expression denoting the called function), and in certain other places.
2355 Other than as expressed by the sequence point rules, the order of
2356 evaluation of subexpressions of an expression is not specified. All
2357 these rules describe only a partial order rather than a total order,
2358 since, for example, if two functions are called within one expression
2359 with no sequence point between them, the order in which the functions
2360 are called is not specified. However, the standards committee have
2361 ruled that function calls do not overlap.
2363 It is not specified when between sequence points modifications to the
2364 values of objects take effect. Programs whose behavior depends on this
2365 have undefined behavior; the C standard specifies that ``Between the
2366 previous and next sequence point an object shall have its stored value
2367 modified at most once by the evaluation of an expression. Furthermore,
2368 the prior value shall be read only to determine the value to be
2369 stored.''. If a program breaks these rules, the results on any
2370 particular implementation are entirely unpredictable.
2372 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2373 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2374 diagnosed by this option, and it may give an occasional false positive
2375 result, but in general it has been found fairly effective at detecting
2376 this sort of problem in programs.
2378 The present implementation of this option only works for C programs. A
2379 future implementation may also work for C++ programs.
2381 The C standard is worded confusingly, therefore there is some debate
2382 over the precise meaning of the sequence point rules in subtle cases.
2383 Links to discussions of the problem, including proposed formal
2384 definitions, may be found on the GCC readings page, at
2385 @w{@uref{http://gcc.gnu.org/readings.html}}.
2388 @opindex Wreturn-type
2389 Warn whenever a function is defined with a return-type that defaults to
2390 @code{int}. Also warn about any @code{return} statement with no
2391 return-value in a function whose return-type is not @code{void}.
2393 For C, also warn if the return type of a function has a type qualifier
2394 such as @code{const}. Such a type qualifier has no effect, since the
2395 value returned by a function is not an lvalue. ISO C prohibits
2396 qualified @code{void} return types on function definitions, so such
2397 return types always receive a warning even without this option.
2399 For C++, a function without return type always produces a diagnostic
2400 message, even when @option{-Wno-return-type} is specified. The only
2401 exceptions are @samp{main} and functions defined in system headers.
2405 Warn whenever a @code{switch} statement has an index of enumerated type
2406 and lacks a @code{case} for one or more of the named codes of that
2407 enumeration. (The presence of a @code{default} label prevents this
2408 warning.) @code{case} labels outside the enumeration range also
2409 provoke warnings when this option is used.
2411 @item -Wswitch-default
2412 @opindex Wswitch-switch
2413 Warn whenever a @code{switch} statement does not have a @code{default}
2417 @opindex Wswitch-enum
2418 Warn whenever a @code{switch} statement has an index of enumerated type
2419 and lacks a @code{case} for one or more of the named codes of that
2420 enumeration. @code{case} labels outside the enumeration range also
2421 provoke warnings when this option is used.
2425 Warn if any trigraphs are encountered that might change the meaning of
2426 the program (trigraphs within comments are not warned about).
2428 @item -Wunused-function
2429 @opindex Wunused-function
2430 Warn whenever a static function is declared but not defined or a
2431 non\-inline static function is unused.
2433 @item -Wunused-label
2434 @opindex Wunused-label
2435 Warn whenever a label is declared but not used.
2437 To suppress this warning use the @samp{unused} attribute
2438 (@pxref{Variable Attributes}).
2440 @item -Wunused-parameter
2441 @opindex Wunused-parameter
2442 Warn whenever a function parameter is unused aside from its declaration.
2444 To suppress this warning use the @samp{unused} attribute
2445 (@pxref{Variable Attributes}).
2447 @item -Wunused-variable
2448 @opindex Wunused-variable
2449 Warn whenever a local variable or non-constant static variable is unused
2450 aside from its declaration
2452 To suppress this warning use the @samp{unused} attribute
2453 (@pxref{Variable Attributes}).
2455 @item -Wunused-value
2456 @opindex Wunused-value
2457 Warn whenever a statement computes a result that is explicitly not used.
2459 To suppress this warning cast the expression to @samp{void}.
2463 All the above @option{-Wunused} options combined.
2465 In order to get a warning about an unused function parameter, you must
2466 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
2467 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
2469 @item -Wuninitialized
2470 @opindex Wuninitialized
2471 Warn if an automatic variable is used without first being initialized or
2472 if a variable may be clobbered by a @code{setjmp} call.
2474 These warnings are possible only in optimizing compilation,
2475 because they require data flow information that is computed only
2476 when optimizing. If you don't specify @option{-O}, you simply won't
2479 If you want to warn about code which uses the uninitialized value of the
2480 variable in its own initializer, use the @option{-Winit-self} option.
2482 These warnings occur only for variables that are candidates for
2483 register allocation. Therefore, they do not occur for a variable that
2484 is declared @code{volatile}, or whose address is taken, or whose size
2485 is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
2486 structures, unions or arrays, even when they are in registers.
2488 Note that there may be no warning about a variable that is used only
2489 to compute a value that itself is never used, because such
2490 computations may be deleted by data flow analysis before the warnings
2493 These warnings are made optional because GCC is not smart
2494 enough to see all the reasons why the code might be correct
2495 despite appearing to have an error. Here is one example of how
2516 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
2517 always initialized, but GCC doesn't know this. Here is
2518 another common case:
2523 if (change_y) save_y = y, y = new_y;
2525 if (change_y) y = save_y;
2530 This has no bug because @code{save_y} is used only if it is set.
2532 @cindex @code{longjmp} warnings
2533 This option also warns when a non-volatile automatic variable might be
2534 changed by a call to @code{longjmp}. These warnings as well are possible
2535 only in optimizing compilation.
2537 The compiler sees only the calls to @code{setjmp}. It cannot know
2538 where @code{longjmp} will be called; in fact, a signal handler could
2539 call it at any point in the code. As a result, you may get a warning
2540 even when there is in fact no problem because @code{longjmp} cannot
2541 in fact be called at the place which would cause a problem.
2543 Some spurious warnings can be avoided if you declare all the functions
2544 you use that never return as @code{noreturn}. @xref{Function
2547 @item -Wunknown-pragmas
2548 @opindex Wunknown-pragmas
2549 @cindex warning for unknown pragmas
2550 @cindex unknown pragmas, warning
2551 @cindex pragmas, warning of unknown
2552 Warn when a #pragma directive is encountered which is not understood by
2553 GCC@. If this command line option is used, warnings will even be issued
2554 for unknown pragmas in system header files. This is not the case if
2555 the warnings were only enabled by the @option{-Wall} command line option.
2557 @item -Wstrict-aliasing
2558 @opindex Wstrict-aliasing
2559 This option is only active when @option{-fstrict-aliasing} is active.
2560 It warns about code which might break the strict aliasing rules that the
2561 compiler is using for optimization. The warning does not catch all
2562 cases, but does attempt to catch the more common pitfalls. It is
2563 included in @option{-Wall}.
2565 @item -Wstrict-aliasing=2
2566 @opindex Wstrict-aliasing=2
2567 This option is only active when @option{-fstrict-aliasing} is active.
2568 It warns about all code which might break the strict aliasing rules that the
2569 compiler is using for optimization. This warning catches all cases, but
2570 it will also give a warning for some ambiguous cases that are safe.
2574 All of the above @samp{-W} options combined. This enables all the
2575 warnings about constructions that some users consider questionable, and
2576 that are easy to avoid (or modify to prevent the warning), even in
2577 conjunction with macros. This also enables some language-specific
2578 warnings described in @ref{C++ Dialect Options} and
2579 @ref{Objective-C and Objective-C++ Dialect Options}.
2582 The following @option{-W@dots{}} options are not implied by @option{-Wall}.
2583 Some of them warn about constructions that users generally do not
2584 consider questionable, but which occasionally you might wish to check
2585 for; others warn about constructions that are necessary or hard to avoid
2586 in some cases, and there is no simple way to modify the code to suppress
2593 (This option used to be called @option{-W}. The older name is still
2594 supported, but the newer name is more descriptive.) Print extra warning
2595 messages for these events:
2599 A function can return either with or without a value. (Falling
2600 off the end of the function body is considered returning without
2601 a value.) For example, this function would evoke such a
2615 An expression-statement or the left-hand side of a comma expression
2616 contains no side effects.
2617 To suppress the warning, cast the unused expression to void.
2618 For example, an expression such as @samp{x[i,j]} will cause a warning,
2619 but @samp{x[(void)i,j]} will not.
2622 An unsigned value is compared against zero with @samp{<} or @samp{>=}.
2625 Storage-class specifiers like @code{static} are not the first things in
2626 a declaration. According to the C Standard, this usage is obsolescent.
2629 If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
2633 A comparison between signed and unsigned values could produce an
2634 incorrect result when the signed value is converted to unsigned.
2635 (But don't warn if @option{-Wno-sign-compare} is also specified.)
2638 An aggregate has an initializer which does not initialize all members.
2639 This warning can be independently controlled by
2640 @option{-Wmissing-field-initializers}.
2643 A function parameter is declared without a type specifier in K&R-style
2651 An empty body occurs in an @samp{if} or @samp{else} statement.
2654 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2655 @samp{>}, or @samp{>=}.
2658 A variable might be changed by @samp{longjmp} or @samp{vfork}.
2661 Any of several floating-point events that often indicate errors, such as
2662 overflow, underflow, loss of precision, etc.
2664 @item @r{(C++ only)}
2665 An enumerator and a non-enumerator both appear in a conditional expression.
2667 @item @r{(C++ only)}
2668 A non-static reference or non-static @samp{const} member appears in a
2669 class without constructors.
2671 @item @r{(C++ only)}
2672 Ambiguous virtual bases.
2674 @item @r{(C++ only)}
2675 Subscripting an array which has been declared @samp{register}.
2677 @item @r{(C++ only)}
2678 Taking the address of a variable which has been declared @samp{register}.
2680 @item @r{(C++ only)}
2681 A base class is not initialized in a derived class' copy constructor.
2684 @item -Wno-div-by-zero
2685 @opindex Wno-div-by-zero
2686 @opindex Wdiv-by-zero
2687 Do not warn about compile-time integer division by zero. Floating point
2688 division by zero is not warned about, as it can be a legitimate way of
2689 obtaining infinities and NaNs.
2691 @item -Wsystem-headers
2692 @opindex Wsystem-headers
2693 @cindex warnings from system headers
2694 @cindex system headers, warnings from
2695 Print warning messages for constructs found in system header files.
2696 Warnings from system headers are normally suppressed, on the assumption
2697 that they usually do not indicate real problems and would only make the
2698 compiler output harder to read. Using this command line option tells
2699 GCC to emit warnings from system headers as if they occurred in user
2700 code. However, note that using @option{-Wall} in conjunction with this
2701 option will @emph{not} warn about unknown pragmas in system
2702 headers---for that, @option{-Wunknown-pragmas} must also be used.
2705 @opindex Wfloat-equal
2706 Warn if floating point values are used in equality comparisons.
2708 The idea behind this is that sometimes it is convenient (for the
2709 programmer) to consider floating-point values as approximations to
2710 infinitely precise real numbers. If you are doing this, then you need
2711 to compute (by analyzing the code, or in some other way) the maximum or
2712 likely maximum error that the computation introduces, and allow for it
2713 when performing comparisons (and when producing output, but that's a
2714 different problem). In particular, instead of testing for equality, you
2715 would check to see whether the two values have ranges that overlap; and
2716 this is done with the relational operators, so equality comparisons are
2719 @item -Wtraditional @r{(C only)}
2720 @opindex Wtraditional
2721 Warn about certain constructs that behave differently in traditional and
2722 ISO C@. Also warn about ISO C constructs that have no traditional C
2723 equivalent, and/or problematic constructs which should be avoided.
2727 Macro parameters that appear within string literals in the macro body.
2728 In traditional C macro replacement takes place within string literals,
2729 but does not in ISO C@.
2732 In traditional C, some preprocessor directives did not exist.
2733 Traditional preprocessors would only consider a line to be a directive
2734 if the @samp{#} appeared in column 1 on the line. Therefore
2735 @option{-Wtraditional} warns about directives that traditional C
2736 understands but would ignore because the @samp{#} does not appear as the
2737 first character on the line. It also suggests you hide directives like
2738 @samp{#pragma} not understood by traditional C by indenting them. Some
2739 traditional implementations would not recognize @samp{#elif}, so it
2740 suggests avoiding it altogether.
2743 A function-like macro that appears without arguments.
2746 The unary plus operator.
2749 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
2750 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
2751 constants.) Note, these suffixes appear in macros defined in the system
2752 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
2753 Use of these macros in user code might normally lead to spurious
2754 warnings, however GCC's integrated preprocessor has enough context to
2755 avoid warning in these cases.
2758 A function declared external in one block and then used after the end of
2762 A @code{switch} statement has an operand of type @code{long}.
2765 A non-@code{static} function declaration follows a @code{static} one.
2766 This construct is not accepted by some traditional C compilers.
2769 The ISO type of an integer constant has a different width or
2770 signedness from its traditional type. This warning is only issued if
2771 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
2772 typically represent bit patterns, are not warned about.
2775 Usage of ISO string concatenation is detected.
2778 Initialization of automatic aggregates.
2781 Identifier conflicts with labels. Traditional C lacks a separate
2782 namespace for labels.
2785 Initialization of unions. If the initializer is zero, the warning is
2786 omitted. This is done under the assumption that the zero initializer in
2787 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
2788 initializer warnings and relies on default initialization to zero in the
2792 Conversions by prototypes between fixed/floating point values and vice
2793 versa. The absence of these prototypes when compiling with traditional
2794 C would cause serious problems. This is a subset of the possible
2795 conversion warnings, for the full set use @option{-Wconversion}.
2798 Use of ISO C style function definitions. This warning intentionally is
2799 @emph{not} issued for prototype declarations or variadic functions
2800 because these ISO C features will appear in your code when using
2801 libiberty's traditional C compatibility macros, @code{PARAMS} and
2802 @code{VPARAMS}. This warning is also bypassed for nested functions
2803 because that feature is already a GCC extension and thus not relevant to
2804 traditional C compatibility.
2807 @item -Wdeclaration-after-statement @r{(C only)}
2808 @opindex Wdeclaration-after-statement
2809 Warn when a declaration is found after a statement in a block. This
2810 construct, known from C++, was introduced with ISO C99 and is by default
2811 allowed in GCC@. It is not supported by ISO C90 and was not supported by
2812 GCC versions before GCC 3.0. @xref{Mixed Declarations}.
2816 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
2818 @item -Wendif-labels
2819 @opindex Wendif-labels
2820 Warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
2824 Warn whenever a local variable shadows another local variable, parameter or
2825 global variable or whenever a built-in function is shadowed.
2827 @item -Wlarger-than-@var{len}
2828 @opindex Wlarger-than
2829 Warn whenever an object of larger than @var{len} bytes is defined.
2831 @item -Wpointer-arith
2832 @opindex Wpointer-arith
2833 Warn about anything that depends on the ``size of'' a function type or
2834 of @code{void}. GNU C assigns these types a size of 1, for
2835 convenience in calculations with @code{void *} pointers and pointers
2838 @item -Wbad-function-cast @r{(C only)}
2839 @opindex Wbad-function-cast
2840 Warn whenever a function call is cast to a non-matching type.
2841 For example, warn if @code{int malloc()} is cast to @code{anything *}.
2845 Warn whenever a pointer is cast so as to remove a type qualifier from
2846 the target type. For example, warn if a @code{const char *} is cast
2847 to an ordinary @code{char *}.
2850 @opindex Wcast-align
2851 Warn whenever a pointer is cast such that the required alignment of the
2852 target is increased. For example, warn if a @code{char *} is cast to
2853 an @code{int *} on machines where integers can only be accessed at
2854 two- or four-byte boundaries.
2856 @item -Wwrite-strings
2857 @opindex Wwrite-strings
2858 When compiling C, give string constants the type @code{const
2859 char[@var{length}]} so that
2860 copying the address of one into a non-@code{const} @code{char *}
2861 pointer will get a warning; when compiling C++, warn about the
2862 deprecated conversion from string constants to @code{char *}.
2863 These warnings will help you find at
2864 compile time code that can try to write into a string constant, but
2865 only if you have been very careful about using @code{const} in
2866 declarations and prototypes. Otherwise, it will just be a nuisance;
2867 this is why we did not make @option{-Wall} request these warnings.
2870 @opindex Wconversion
2871 Warn if a prototype causes a type conversion that is different from what
2872 would happen to the same argument in the absence of a prototype. This
2873 includes conversions of fixed point to floating and vice versa, and
2874 conversions changing the width or signedness of a fixed point argument
2875 except when the same as the default promotion.
2877 Also, warn if a negative integer constant expression is implicitly
2878 converted to an unsigned type. For example, warn about the assignment
2879 @code{x = -1} if @code{x} is unsigned. But do not warn about explicit
2880 casts like @code{(unsigned) -1}.
2882 @item -Wsign-compare
2883 @opindex Wsign-compare
2884 @cindex warning for comparison of signed and unsigned values
2885 @cindex comparison of signed and unsigned values, warning
2886 @cindex signed and unsigned values, comparison warning
2887 Warn when a comparison between signed and unsigned values could produce
2888 an incorrect result when the signed value is converted to unsigned.
2889 This warning is also enabled by @option{-Wextra}; to get the other warnings
2890 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
2892 @item -Waggregate-return
2893 @opindex Waggregate-return
2894 Warn if any functions that return structures or unions are defined or
2895 called. (In languages where you can return an array, this also elicits
2898 @item -Wstrict-prototypes @r{(C only)}
2899 @opindex Wstrict-prototypes
2900 Warn if a function is declared or defined without specifying the
2901 argument types. (An old-style function definition is permitted without
2902 a warning if preceded by a declaration which specifies the argument
2905 @item -Wold-style-definition @r{(C only)}
2906 @opindex Wold-style-definition
2907 Warn if an old-style function definition is used. A warning is given
2908 even if there is a previous prototype.
2910 @item -Wmissing-prototypes @r{(C only)}
2911 @opindex Wmissing-prototypes
2912 Warn if a global function is defined without a previous prototype
2913 declaration. This warning is issued even if the definition itself
2914 provides a prototype. The aim is to detect global functions that fail
2915 to be declared in header files.
2917 @item -Wmissing-declarations @r{(C only)}
2918 @opindex Wmissing-declarations
2919 Warn if a global function is defined without a previous declaration.
2920 Do so even if the definition itself provides a prototype.
2921 Use this option to detect global functions that are not declared in
2924 @item -Wmissing-field-initializers
2925 @opindex Wmissing-field-initializers
2928 Warn if a structure's initializer has some fields missing. For
2929 example, the following code would cause such a warning, because
2930 @code{x.h} is implicitly zero:
2933 struct s @{ int f, g, h; @};
2934 struct s x = @{ 3, 4 @};
2937 This option does not warn about designated initializers, so the following
2938 modification would not trigger a warning:
2941 struct s @{ int f, g, h; @};
2942 struct s x = @{ .f = 3, .g = 4 @};
2945 This warning is included in @option{-Wextra}. To get other @option{-Wextra}
2946 warnings without this one, use @samp{-Wextra -Wno-missing-field-initializers}.
2948 @item -Wmissing-noreturn
2949 @opindex Wmissing-noreturn
2950 Warn about functions which might be candidates for attribute @code{noreturn}.
2951 Note these are only possible candidates, not absolute ones. Care should
2952 be taken to manually verify functions actually do not ever return before
2953 adding the @code{noreturn} attribute, otherwise subtle code generation
2954 bugs could be introduced. You will not get a warning for @code{main} in
2955 hosted C environments.
2957 @item -Wmissing-format-attribute
2958 @opindex Wmissing-format-attribute
2960 If @option{-Wformat} is enabled, also warn about functions which might be
2961 candidates for @code{format} attributes. Note these are only possible
2962 candidates, not absolute ones. GCC will guess that @code{format}
2963 attributes might be appropriate for any function that calls a function
2964 like @code{vprintf} or @code{vscanf}, but this might not always be the
2965 case, and some functions for which @code{format} attributes are
2966 appropriate may not be detected. This option has no effect unless
2967 @option{-Wformat} is enabled (possibly by @option{-Wall}).
2969 @item -Wno-multichar
2970 @opindex Wno-multichar
2972 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
2973 Usually they indicate a typo in the user's code, as they have
2974 implementation-defined values, and should not be used in portable code.
2976 @item -Wno-deprecated-declarations
2977 @opindex Wno-deprecated-declarations
2978 Do not warn about uses of functions, variables, and types marked as
2979 deprecated by using the @code{deprecated} attribute.
2980 (@pxref{Function Attributes}, @pxref{Variable Attributes},
2981 @pxref{Type Attributes}.)
2985 Warn if a structure is given the packed attribute, but the packed
2986 attribute has no effect on the layout or size of the structure.
2987 Such structures may be mis-aligned for little benefit. For
2988 instance, in this code, the variable @code{f.x} in @code{struct bar}
2989 will be misaligned even though @code{struct bar} does not itself
2990 have the packed attribute:
2997 @} __attribute__((packed));
3007 Warn if padding is included in a structure, either to align an element
3008 of the structure or to align the whole structure. Sometimes when this
3009 happens it is possible to rearrange the fields of the structure to
3010 reduce the padding and so make the structure smaller.
3012 @item -Wredundant-decls
3013 @opindex Wredundant-decls
3014 Warn if anything is declared more than once in the same scope, even in
3015 cases where multiple declaration is valid and changes nothing.
3017 @item -Wnested-externs @r{(C only)}
3018 @opindex Wnested-externs
3019 Warn if an @code{extern} declaration is encountered within a function.
3021 @item -Wunreachable-code
3022 @opindex Wunreachable-code
3023 Warn if the compiler detects that code will never be executed.
3025 This option is intended to warn when the compiler detects that at
3026 least a whole line of source code will never be executed, because
3027 some condition is never satisfied or because it is after a
3028 procedure that never returns.
3030 It is possible for this option to produce a warning even though there
3031 are circumstances under which part of the affected line can be executed,
3032 so care should be taken when removing apparently-unreachable code.
3034 For instance, when a function is inlined, a warning may mean that the
3035 line is unreachable in only one inlined copy of the function.
3037 This option is not made part of @option{-Wall} because in a debugging
3038 version of a program there is often substantial code which checks
3039 correct functioning of the program and is, hopefully, unreachable
3040 because the program does work. Another common use of unreachable
3041 code is to provide behavior which is selectable at compile-time.
3045 Warn if a function can not be inlined and it was declared as inline.
3046 Even with this option, the compiler will not warn about failures to
3047 inline functions declared in system headers.
3049 The compiler uses a variety of heuristics to determine whether or not
3050 to inline a function. For example, the compiler takes into account
3051 the size of the function being inlined and the the amount of inlining
3052 that has already been done in the current function. Therefore,
3053 seemingly insignificant changes in the source program can cause the
3054 warnings produced by @option{-Winline} to appear or disappear.
3056 @item -Wno-invalid-offsetof @r{(C++ only)}
3057 @opindex Wno-invalid-offsetof
3058 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
3059 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
3060 to a non-POD type is undefined. In existing C++ implementations,
3061 however, @samp{offsetof} typically gives meaningful results even when
3062 applied to certain kinds of non-POD types. (Such as a simple
3063 @samp{struct} that fails to be a POD type only by virtue of having a
3064 constructor.) This flag is for users who are aware that they are
3065 writing nonportable code and who have deliberately chosen to ignore the
3068 The restrictions on @samp{offsetof} may be relaxed in a future version
3069 of the C++ standard.
3072 @opindex Winvalid-pch
3073 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
3074 the search path but can't be used.
3078 @opindex Wno-long-long
3079 Warn if @samp{long long} type is used. This is default. To inhibit
3080 the warning messages, use @option{-Wno-long-long}. Flags
3081 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
3082 only when @option{-pedantic} flag is used.
3084 @item -Wvariadic-macros
3085 @opindex Wvariadic-macros
3086 @opindex Wno-variadic-macros
3087 Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
3088 alternate syntax when in pedantic ISO C99 mode. This is default.
3089 To inhibit the warning messages, use @option{-Wno-variadic-macros}.
3091 @item -Wdisabled-optimization
3092 @opindex Wdisabled-optimization
3093 Warn if a requested optimization pass is disabled. This warning does
3094 not generally indicate that there is anything wrong with your code; it
3095 merely indicates that GCC's optimizers were unable to handle the code
3096 effectively. Often, the problem is that your code is too big or too
3097 complex; GCC will refuse to optimize programs when the optimization
3098 itself is likely to take inordinate amounts of time.
3102 Make all warnings into errors.
3105 @node Debugging Options
3106 @section Options for Debugging Your Program or GCC
3107 @cindex options, debugging
3108 @cindex debugging information options
3110 GCC has various special options that are used for debugging
3111 either your program or GCC:
3116 Produce debugging information in the operating system's native format
3117 (stabs, COFF, XCOFF, or DWARF)@. GDB can work with this debugging
3120 On most systems that use stabs format, @option{-g} enables use of extra
3121 debugging information that only GDB can use; this extra information
3122 makes debugging work better in GDB but will probably make other debuggers
3124 refuse to read the program. If you want to control for certain whether
3125 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
3126 @option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
3128 Unlike most other C compilers, GCC allows you to use @option{-g} with
3129 @option{-O}. The shortcuts taken by optimized code may occasionally
3130 produce surprising results: some variables you declared may not exist
3131 at all; flow of control may briefly move where you did not expect it;
3132 some statements may not be executed because they compute constant
3133 results or their values were already at hand; some statements may
3134 execute in different places because they were moved out of loops.
3136 Nevertheless it proves possible to debug optimized output. This makes
3137 it reasonable to use the optimizer for programs that might have bugs.
3139 The following options are useful when GCC is generated with the
3140 capability for more than one debugging format.
3144 Produce debugging information for use by GDB@. This means to use the
3145 most expressive format available (DWARF 2, stabs, or the native format
3146 if neither of those are supported), including GDB extensions if at all
3151 Produce debugging information in stabs format (if that is supported),
3152 without GDB extensions. This is the format used by DBX on most BSD
3153 systems. On MIPS, Alpha and System V Release 4 systems this option
3154 produces stabs debugging output which is not understood by DBX or SDB@.
3155 On System V Release 4 systems this option requires the GNU assembler.
3157 @item -feliminate-unused-debug-symbols
3158 @opindex feliminate-unused-debug-symbols
3159 Produce debugging information in stabs format (if that is supported),
3160 for only symbols that are actually used.
3164 Produce debugging information in stabs format (if that is supported),
3165 using GNU extensions understood only by the GNU debugger (GDB)@. The
3166 use of these extensions is likely to make other debuggers crash or
3167 refuse to read the program.
3171 Produce debugging information in COFF format (if that is supported).
3172 This is the format used by SDB on most System V systems prior to
3177 Produce debugging information in XCOFF format (if that is supported).
3178 This is the format used by the DBX debugger on IBM RS/6000 systems.
3182 Produce debugging information in XCOFF format (if that is supported),
3183 using GNU extensions understood only by the GNU debugger (GDB)@. The
3184 use of these extensions is likely to make other debuggers crash or
3185 refuse to read the program, and may cause assemblers other than the GNU
3186 assembler (GAS) to fail with an error.
3190 Produce debugging information in DWARF version 2 format (if that is
3191 supported). This is the format used by DBX on IRIX 6.
3195 Produce debugging information in VMS debug format (if that is
3196 supported). This is the format used by DEBUG on VMS systems.
3199 @itemx -ggdb@var{level}
3200 @itemx -gstabs@var{level}
3201 @itemx -gcoff@var{level}
3202 @itemx -gxcoff@var{level}
3203 @itemx -gvms@var{level}
3204 Request debugging information and also use @var{level} to specify how
3205 much information. The default level is 2.
3207 Level 1 produces minimal information, enough for making backtraces in
3208 parts of the program that you don't plan to debug. This includes
3209 descriptions of functions and external variables, but no information
3210 about local variables and no line numbers.
3212 Level 3 includes extra information, such as all the macro definitions
3213 present in the program. Some debuggers support macro expansion when
3214 you use @option{-g3}.
3216 Note that in order to avoid confusion between DWARF1 debug level 2,
3217 and DWARF2 @option{-gdwarf-2} does not accept a concatenated debug
3218 level. Instead use an additional @option{-g@var{level}} option to
3219 change the debug level for DWARF2.
3221 @item -feliminate-dwarf2-dups
3222 @opindex feliminate-dwarf2-dups
3223 Compress DWARF2 debugging information by eliminating duplicated
3224 information about each symbol. This option only makes sense when
3225 generating DWARF2 debugging information with @option{-gdwarf-2}.
3227 @cindex @command{prof}
3230 Generate extra code to write profile information suitable for the
3231 analysis program @command{prof}. You must use this option when compiling
3232 the source files you want data about, and you must also use it when
3235 @cindex @command{gprof}
3238 Generate extra code to write profile information suitable for the
3239 analysis program @command{gprof}. You must use this option when compiling
3240 the source files you want data about, and you must also use it when
3245 Makes the compiler print out each function name as it is compiled, and
3246 print some statistics about each pass when it finishes.
3249 @opindex ftime-report
3250 Makes the compiler print some statistics about the time consumed by each
3251 pass when it finishes.
3254 @opindex fmem-report
3255 Makes the compiler print some statistics about permanent memory
3256 allocation when it finishes.
3258 @item -fprofile-arcs
3259 @opindex fprofile-arcs
3260 Add code so that program flow @dfn{arcs} are instrumented. During
3261 execution the program records how many times each branch and call is
3262 executed and how many times it is taken or returns. When the compiled
3263 program exits it saves this data to a file called
3264 @file{@var{auxname}.gcda} for each source file. The data may be used for
3265 profile-directed optimizations (@option{-fbranch-probabilities}), or for
3266 test coverage analysis (@option{-ftest-coverage}). Each object file's
3267 @var{auxname} is generated from the name of the output file, if
3268 explicitly specified and it is not the final executable, otherwise it is
3269 the basename of the source file. In both cases any suffix is removed
3270 (e.g. @file{foo.gcda} for input file @file{dir/foo.c}, or
3271 @file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
3276 Compile the source files with @option{-fprofile-arcs} plus optimization
3277 and code generation options. For test coverage analysis, use the
3278 additional @option{-ftest-coverage} option. You do not need to profile
3279 every source file in a program.
3282 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
3283 (the latter implies the former).
3286 Run the program on a representative workload to generate the arc profile
3287 information. This may be repeated any number of times. You can run
3288 concurrent instances of your program, and provided that the file system
3289 supports locking, the data files will be correctly updated. Also
3290 @code{fork} calls are detected and correctly handled (double counting
3294 For profile-directed optimizations, compile the source files again with
3295 the same optimization and code generation options plus
3296 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
3297 Control Optimization}).
3300 For test coverage analysis, use @command{gcov} to produce human readable
3301 information from the @file{.gcno} and @file{.gcda} files. Refer to the
3302 @command{gcov} documentation for further information.
3306 With @option{-fprofile-arcs}, for each function of your program GCC
3307 creates a program flow graph, then finds a spanning tree for the graph.
3308 Only arcs that are not on the spanning tree have to be instrumented: the
3309 compiler adds code to count the number of times that these arcs are
3310 executed. When an arc is the only exit or only entrance to a block, the
3311 instrumentation code can be added to the block; otherwise, a new basic
3312 block must be created to hold the instrumentation code.
3314 @item -ftree-based-profiling
3315 @opindex ftree-based-profiling
3316 This option is used in addition to @option{-fprofile-arcs} or
3317 @option{-fbranch-probabilities} to control whether those optimizations
3318 are performed on a tree-based or rtl-based internal representation.
3319 If you use this option when compiling with @option{-fprofile-arcs},
3320 you must also use it when compiling later with @option{-fbranch-probabilities}.
3321 Currently the tree-based optimization is in an early stage of
3322 development, and this option is recommended only for those people
3323 working on improving it.
3326 @item -ftest-coverage
3327 @opindex ftest-coverage
3328 Produce a notes file that the @command{gcov} code-coverage utility
3329 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
3330 show program coverage. Each source file's note file is called
3331 @file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
3332 above for a description of @var{auxname} and instructions on how to
3333 generate test coverage data. Coverage data will match the source files
3334 more closely, if you do not optimize.
3336 @item -d@var{letters}
3338 Says to make debugging dumps during compilation at times specified by
3339 @var{letters}. This is used for debugging the compiler. The file names
3340 for most of the dumps are made by appending a pass number and a word to
3341 the @var{dumpname}. @var{dumpname} is generated from the name of the
3342 output file, if explicitly specified and it is not an executable,
3343 otherwise it is the basename of the source file. In both cases any
3344 suffix is removed (e.g. @file{foo.01.rtl} or @file{foo.02.sibling}).
3345 Here are the possible letters for use in @var{letters}, and their
3351 Annotate the assembler output with miscellaneous debugging information.
3354 Dump after computing branch probabilities, to @file{@var{file}.12.bp}.
3357 Dump after block reordering, to @file{@var{file}.32.bbro}.
3360 Dump after instruction combination, to the file @file{@var{file}.20.combine}.
3363 Dump after the first if conversion, to the file @file{@var{file}.14.ce1}.
3364 Also dump after the second if conversion, to the file @file{@var{file}.21.ce2}.
3367 Dump after branch target load optimization, to to @file{@var{file}.33.btl}.
3368 Also dump after delayed branch scheduling, to @file{@var{file}.37.dbr}.
3371 Dump all macro definitions, at the end of preprocessing, in addition to
3375 Dump after the third if conversion, to @file{@var{file}.31.ce3}.
3378 Dump after control and data flow analysis, to @file{@var{file}.11.cfg}.
3379 Also dump after life analysis, to @file{@var{file}.19.life}.
3382 Dump after global register allocation, to @file{@var{file}.26.greg}.
3385 Dump after GCSE, to @file{@var{file}.08.gcse}.
3386 Also dump after jump bypassing and control flow optimizations, to
3387 @file{@var{file}.10.bypass}.
3390 Dump after finalization of EH handling code, to @file{@var{file}.03.eh}.
3393 Dump after sibling call optimizations, to @file{@var{file}.02.sibling}.
3396 Dump after the first jump optimization, to @file{@var{file}.04.jump}.
3399 Dump after conversion from registers to stack, to @file{@var{file}.35.stack}.
3402 Dump after local register allocation, to @file{@var{file}.25.lreg}.
3405 Dump after loop optimization passes, to @file{@var{file}.09.loop} and
3406 @file{@var{file}.16.loop2}.
3409 Dump after modulo scheduling, to @file{@var{file}.23.sms}.
3412 Dump after performing the machine dependent reorganization pass, to
3413 @file{@var{file}.36.mach}.
3416 Dump after register renumbering, to @file{@var{file}.30.rnreg}.
3419 Dump after the register move pass, to @file{@var{file}.22.regmove}.
3422 Dump after post-reload optimizations, to @file{@var{file}.27.postreload}.
3425 Dump after RTL generation, to @file{@var{file}.01.rtl}.
3428 Dump after the second scheduling pass, to @file{@var{file}.34.sched2}.
3431 Dump after CSE (including the jump optimization that sometimes follows
3432 CSE), to @file{@var{file}.06.cse}.
3435 Dump after the first scheduling pass, to @file{@var{file}.24.sched}.
3438 Dump after the second CSE pass (including the jump optimization that
3439 sometimes follows CSE), to @file{@var{file}.18.cse2}.
3442 Dump after running tracer, to @file{@var{file}.15.tracer}.
3445 Dump after null pointer elimination pass to @file{@var{file}.05.null}.
3448 Dump callgraph and unit-at-a-time optimization @file{@var{file}.00.unit}.
3451 Dump after the value profile transformations, to @file{@var{file}.13.vpt}.
3452 Also dump after variable tracking, to @file{@var{file}.35.vartrack}.
3455 Dump after the second flow pass, to @file{@var{file}.28.flow2}.
3458 Dump after the peephole pass, to @file{@var{file}.29.peephole2}.
3461 Dump after constructing the web, to @file{@var{file}.17.web}.
3464 Produce all the dumps listed above.
3467 Produce a core dump whenever an error occurs.
3470 Print statistics on memory usage, at the end of the run, to
3474 Annotate the assembler output with a comment indicating which
3475 pattern and alternative was used. The length of each instruction is
3479 Dump the RTL in the assembler output as a comment before each instruction.
3480 Also turns on @option{-dp} annotation.
3483 For each of the other indicated dump files (except for
3484 @file{@var{file}.01.rtl}), dump a representation of the control flow graph
3485 suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
3488 Just generate RTL for a function instead of compiling it. Usually used
3492 Dump debugging information during parsing, to standard error.
3495 @item -fdump-unnumbered
3496 @opindex fdump-unnumbered
3497 When doing debugging dumps (see @option{-d} option above), suppress instruction
3498 numbers and line number note output. This makes it more feasible to
3499 use diff on debugging dumps for compiler invocations with different
3500 options, in particular with and without @option{-g}.
3502 @item -fdump-translation-unit @r{(C and C++ only)}
3503 @itemx -fdump-translation-unit-@var{options} @r{(C and C++ only)}
3504 @opindex fdump-translation-unit
3505 Dump a representation of the tree structure for the entire translation
3506 unit to a file. The file name is made by appending @file{.tu} to the
3507 source file name. If the @samp{-@var{options}} form is used, @var{options}
3508 controls the details of the dump as described for the
3509 @option{-fdump-tree} options.
3511 @item -fdump-class-hierarchy @r{(C++ only)}
3512 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
3513 @opindex fdump-class-hierarchy
3514 Dump a representation of each class's hierarchy and virtual function
3515 table layout to a file. The file name is made by appending @file{.class}
3516 to the source file name. If the @samp{-@var{options}} form is used,
3517 @var{options} controls the details of the dump as described for the
3518 @option{-fdump-tree} options.
3520 @item -fdump-tree-@var{switch} @r{(C and C++ only)}
3521 @itemx -fdump-tree-@var{switch}-@var{options} @r{(C and C++ only)}
3523 Control the dumping at various stages of processing the intermediate
3524 language tree to a file. The file name is generated by appending a switch
3525 specific suffix to the source file name. If the @samp{-@var{options}}
3526 form is used, @var{options} is a list of @samp{-} separated options that
3527 control the details of the dump. Not all options are applicable to all
3528 dumps, those which are not meaningful will be ignored. The following
3529 options are available
3533 Print the address of each node. Usually this is not meaningful as it
3534 changes according to the environment and source file. Its primary use
3535 is for tying up a dump file with a debug environment.
3537 Inhibit dumping of members of a scope or body of a function merely
3538 because that scope has been reached. Only dump such items when they
3539 are directly reachable by some other path. When dumping pretty-printed
3540 trees, this option inhibits dumping the bodies of control structures.
3542 Print a raw representation of the tree. By default, trees are
3543 pretty-printed into a C-like representation.
3545 Enable more detailed dumps (not honored by every dump option).
3547 Enable dumping various statistics about the pass (not honored by every dump
3550 Enable showing basic block boundaries (disabled in raw dumps).
3552 Enable showing virtual operands for every statement.
3554 Enable showing line numbers for statements.
3556 Enable showing the unique ID (@code{DECL_UID}) for each variable.
3558 Turn on all options, except @option{raw}, @option{slim} and @option{lineno}.
3561 The following tree dumps are possible:
3565 Dump before any tree based optimization, to @file{@var{file}.original}.
3568 Dump after all tree based optimization, to @file{@var{file}.optimized}.
3571 Dump after function inlining, to @file{@var{file}.inlined}.
3574 @opindex fdump-tree-gimple
3575 Dump each function before and after the gimplification pass to a file. The
3576 file name is made by appending @file{.gimple} to the source file name.
3579 @opindex fdump-tree-cfg
3580 Dump the control flow graph of each function to a file. The file name is
3581 made by appending @file{.cfg} to the source file name.
3584 @opindex fdump-tree-vcg
3585 Dump the control flow graph of each function to a file in VCG format. The
3586 file name is made by appending @file{.vcg} to the source file name. Note
3587 that if the file contains more than one function, the generated file cannot
3588 be used directly by VCG. You will need to cut and paste each function's
3589 graph into its own separate file first.
3592 @opindex fdump-tree-ch
3593 Dump each function after copying loop headers. The file name is made by
3594 appending @file{.ch} to the source file name.
3597 @opindex fdump-tree-ssa
3598 Dump SSA related information to a file. The file name is made by appending
3599 @file{.ssa} to the source file name.
3602 @opindex fdump-tree-alias
3603 Dump aliasing information for each function. The file name is made by
3604 appending @file{.alias} to the source file name.
3607 @opindex fdump-tree-ccp
3608 Dump each function after CCP. The file name is made by appending
3609 @file{.ccp} to the source file name.
3612 @opindex fdump-tree-pre
3613 Dump trees after partial redundancy elimination. The file name is made
3614 by appending @file{.pre} to the source file name.
3617 @opindex fdump-tree-fre
3618 Dump trees after full redundancy elimination. The file name is made
3619 by appending @file{.fre} to the source file name.
3622 @opindex fdump-tree-dce
3623 Dump each function after dead code elimination. The file name is made by
3624 appending @file{.dce} to the source file name.
3627 @opindex fdump-tree-mudflap
3628 Dump each function after adding mudflap instrumentation. The file name is
3629 made by appending @file{.mudflap} to the source file name.
3632 @opindex fdump-tree-sra
3633 Dump each function after performing scalar replacement of aggregates. The
3634 file name is made by appending @file{.sra} to the source file name.
3637 @opindex fdump-tree-dom
3638 Dump each function after applying dominator tree optimizations. The file
3639 name is made by appending @file{.dom} to the source file name.
3642 @opindex fdump-tree-dse
3643 Dump each function after applying dead store elimination. The file
3644 name is made by appending @file{.dse} to the source file name.
3647 @opindex fdump-tree-phiopt
3648 Dump each function after optimizing PHI nodes into straightline code. The file
3649 name is made by appending @file{.phiopt} to the source file name.
3652 @opindex fdump-tree-forwprop
3653 Dump each function after forward propagating single use variables. The file
3654 name is made by appending @file{.forwprop} to the source file name.
3657 @opindex fdump-tree-copyrename
3658 Dump each function after applying the copy rename optimization. The file
3659 name is made by appending @file{.copyrename} to the source file name.
3662 @opindex fdump-tree-nrv
3663 Dump each function after applying the named return value optimization on
3664 generic trees. The file name is made by appending @file{.nrv} to the source
3668 @opindex fdump-tree-all
3669 Enable all the available tree dumps with the flags provided in this option.
3672 @item -frandom-seed=@var{string}
3673 @opindex frandom-string
3674 This option provides a seed that GCC uses when it would otherwise use
3675 random numbers. It is used to generate certain symbol names
3676 that have to be different in every compiled file. It is also used to
3677 place unique stamps in coverage data files and the object files that
3678 produce them. You can use the @option{-frandom-seed} option to produce
3679 reproducibly identical object files.
3681 The @var{string} should be different for every file you compile.
3683 @item -fsched-verbose=@var{n}
3684 @opindex fsched-verbose
3685 On targets that use instruction scheduling, this option controls the
3686 amount of debugging output the scheduler prints. This information is
3687 written to standard error, unless @option{-dS} or @option{-dR} is
3688 specified, in which case it is output to the usual dump
3689 listing file, @file{.sched} or @file{.sched2} respectively. However
3690 for @var{n} greater than nine, the output is always printed to standard
3693 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
3694 same information as @option{-dRS}. For @var{n} greater than one, it
3695 also output basic block probabilities, detailed ready list information
3696 and unit/insn info. For @var{n} greater than two, it includes RTL
3697 at abort point, control-flow and regions info. And for @var{n} over
3698 four, @option{-fsched-verbose} also includes dependence info.
3702 Store the usual ``temporary'' intermediate files permanently; place them
3703 in the current directory and name them based on the source file. Thus,
3704 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
3705 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
3706 preprocessed @file{foo.i} output file even though the compiler now
3707 normally uses an integrated preprocessor.
3711 Report the CPU time taken by each subprocess in the compilation
3712 sequence. For C source files, this is the compiler proper and assembler
3713 (plus the linker if linking is done). The output looks like this:
3720 The first number on each line is the ``user time,'' that is time spent
3721 executing the program itself. The second number is ``system time,''
3722 time spent executing operating system routines on behalf of the program.
3723 Both numbers are in seconds.
3725 @item -fvar-tracking
3726 @opindex fvar-tracking
3727 Run variable tracking pass. It computes where variables are stored at each
3728 position in code. Better debugging information is then generated
3729 (if the debugging information format supports this information).
3731 It is enabled by default when compiling with optimization (@option{-Os},
3732 @option{-O}, @option{-O2}, ...), debugging information (@option{-g}) and
3733 the debug info format supports it.
3735 @item -print-file-name=@var{library}
3736 @opindex print-file-name
3737 Print the full absolute name of the library file @var{library} that
3738 would be used when linking---and don't do anything else. With this
3739 option, GCC does not compile or link anything; it just prints the
3742 @item -print-multi-directory
3743 @opindex print-multi-directory
3744 Print the directory name corresponding to the multilib selected by any
3745 other switches present in the command line. This directory is supposed
3746 to exist in @env{GCC_EXEC_PREFIX}.
3748 @item -print-multi-lib
3749 @opindex print-multi-lib
3750 Print the mapping from multilib directory names to compiler switches
3751 that enable them. The directory name is separated from the switches by
3752 @samp{;}, and each switch starts with an @samp{@@} instead of the
3753 @samp{-}, without spaces between multiple switches. This is supposed to
3754 ease shell-processing.
3756 @item -print-prog-name=@var{program}
3757 @opindex print-prog-name
3758 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
3760 @item -print-libgcc-file-name
3761 @opindex print-libgcc-file-name
3762 Same as @option{-print-file-name=libgcc.a}.
3764 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
3765 but you do want to link with @file{libgcc.a}. You can do
3768 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
3771 @item -print-search-dirs
3772 @opindex print-search-dirs
3773 Print the name of the configured installation directory and a list of
3774 program and library directories @command{gcc} will search---and don't do anything else.
3776 This is useful when @command{gcc} prints the error message
3777 @samp{installation problem, cannot exec cpp0: No such file or directory}.
3778 To resolve this you either need to put @file{cpp0} and the other compiler
3779 components where @command{gcc} expects to find them, or you can set the environment
3780 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
3781 Don't forget the trailing '/'.
3782 @xref{Environment Variables}.
3785 @opindex dumpmachine
3786 Print the compiler's target machine (for example,
3787 @samp{i686-pc-linux-gnu})---and don't do anything else.
3790 @opindex dumpversion
3791 Print the compiler version (for example, @samp{3.0})---and don't do
3796 Print the compiler's built-in specs---and don't do anything else. (This
3797 is used when GCC itself is being built.) @xref{Spec Files}.
3799 @item -feliminate-unused-debug-types
3800 @opindex feliminate-unused-debug-types
3801 Normally, when producing DWARF2 output, GCC will emit debugging
3802 information for all types declared in a compilation
3803 unit, regardless of whether or not they are actually used
3804 in that compilation unit. Sometimes this is useful, such as
3805 if, in the debugger, you want to cast a value to a type that is
3806 not actually used in your program (but is declared). More often,
3807 however, this results in a significant amount of wasted space.
3808 With this option, GCC will avoid producing debug symbol output
3809 for types that are nowhere used in the source file being compiled.
3812 @node Optimize Options
3813 @section Options That Control Optimization
3814 @cindex optimize options
3815 @cindex options, optimization
3817 These options control various sorts of optimizations.
3819 Without any optimization option, the compiler's goal is to reduce the
3820 cost of compilation and to make debugging produce the expected
3821 results. Statements are independent: if you stop the program with a
3822 breakpoint between statements, you can then assign a new value to any
3823 variable or change the program counter to any other statement in the
3824 function and get exactly the results you would expect from the source
3827 Turning on optimization flags makes the compiler attempt to improve
3828 the performance and/or code size at the expense of compilation time
3829 and possibly the ability to debug the program.
3831 The compiler performs optimization based on the knowledge it has of
3832 the program. Optimization levels @option{-O2} and above, in
3833 particular, enable @emph{unit-at-a-time} mode, which allows the
3834 compiler to consider information gained from later functions in
3835 the file when compiling a function. Compiling multiple files at
3836 once to a single output file in @emph{unit-at-a-time} mode allows
3837 the compiler to use information gained from all of the files when
3838 compiling each of them.
3840 Not all optimizations are controlled directly by a flag. Only
3841 optimizations that have a flag are listed.
3848 Optimize. Optimizing compilation takes somewhat more time, and a lot
3849 more memory for a large function.
3851 With @option{-O}, the compiler tries to reduce code size and execution
3852 time, without performing any optimizations that take a great deal of
3855 @option{-O} turns on the following optimization flags:
3856 @gccoptlist{-fdefer-pop @gol
3857 -fmerge-constants @gol
3859 -floop-optimize @gol
3860 -fif-conversion @gol
3861 -fif-conversion2 @gol
3862 -fdelayed-branch @gol
3863 -fguess-branch-probability @gol
3866 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
3867 where doing so does not interfere with debugging.
3871 Optimize even more. GCC performs nearly all supported optimizations
3872 that do not involve a space-speed tradeoff. The compiler does not
3873 perform loop unrolling or function inlining when you specify @option{-O2}.
3874 As compared to @option{-O}, this option increases both compilation time
3875 and the performance of the generated code.
3877 @option{-O2} turns on all optimization flags specified by @option{-O}. It
3878 also turns on the following optimization flags:
3879 @gccoptlist{-fforce-mem @gol
3880 -foptimize-sibling-calls @gol
3881 -fstrength-reduce @gol
3882 -fcse-follow-jumps -fcse-skip-blocks @gol
3883 -frerun-cse-after-loop -frerun-loop-opt @gol
3884 -fgcse -fgcse-lm -fgcse-sm -fgcse-las @gol
3885 -fdelete-null-pointer-checks @gol
3886 -fexpensive-optimizations @gol
3888 -fschedule-insns -fschedule-insns2 @gol
3889 -fsched-interblock -fsched-spec @gol
3892 -freorder-blocks -freorder-functions @gol
3893 -fstrict-aliasing @gol
3894 -funit-at-a-time @gol
3895 -falign-functions -falign-jumps @gol
3896 -falign-loops -falign-labels @gol
3899 Please note the warning under @option{-fgcse} about
3900 invoking @option{-O2} on programs that use computed gotos.
3904 Optimize yet more. @option{-O3} turns on all optimizations specified by
3905 @option{-O2} and also turns on the @option{-finline-functions},
3906 @option{-fweb} and @option{-fgcse-after-reload} options.
3910 Do not optimize. This is the default.
3914 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
3915 do not typically increase code size. It also performs further
3916 optimizations designed to reduce code size.
3918 @option{-Os} disables the following optimization flags:
3919 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
3920 -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays}
3922 If you use multiple @option{-O} options, with or without level numbers,
3923 the last such option is the one that is effective.
3926 Options of the form @option{-f@var{flag}} specify machine-independent
3927 flags. Most flags have both positive and negative forms; the negative
3928 form of @option{-ffoo} would be @option{-fno-foo}. In the table
3929 below, only one of the forms is listed---the one you typically will
3930 use. You can figure out the other form by either removing @samp{no-}
3933 The following options control specific optimizations. They are either
3934 activated by @option{-O} options or are related to ones that are. You
3935 can use the following flags in the rare cases when ``fine-tuning'' of
3936 optimizations to be performed is desired.
3939 @item -fno-default-inline
3940 @opindex fno-default-inline
3941 Do not make member functions inline by default merely because they are
3942 defined inside the class scope (C++ only). Otherwise, when you specify
3943 @w{@option{-O}}, member functions defined inside class scope are compiled
3944 inline by default; i.e., you don't need to add @samp{inline} in front of
3945 the member function name.
3947 @item -fno-defer-pop
3948 @opindex fno-defer-pop
3949 Always pop the arguments to each function call as soon as that function
3950 returns. For machines which must pop arguments after a function call,
3951 the compiler normally lets arguments accumulate on the stack for several
3952 function calls and pops them all at once.
3954 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3958 Force memory operands to be copied into registers before doing
3959 arithmetic on them. This produces better code by making all memory
3960 references potential common subexpressions. When they are not common
3961 subexpressions, instruction combination should eliminate the separate
3964 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3967 @opindex fforce-addr
3968 Force memory address constants to be copied into registers before
3969 doing arithmetic on them. This may produce better code just as
3970 @option{-fforce-mem} may.
3972 @item -fomit-frame-pointer
3973 @opindex fomit-frame-pointer
3974 Don't keep the frame pointer in a register for functions that
3975 don't need one. This avoids the instructions to save, set up and
3976 restore frame pointers; it also makes an extra register available
3977 in many functions. @strong{It also makes debugging impossible on
3980 On some machines, such as the VAX, this flag has no effect, because
3981 the standard calling sequence automatically handles the frame pointer
3982 and nothing is saved by pretending it doesn't exist. The
3983 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
3984 whether a target machine supports this flag. @xref{Registers,,Register
3985 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
3987 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3989 @item -foptimize-sibling-calls
3990 @opindex foptimize-sibling-calls
3991 Optimize sibling and tail recursive calls.
3993 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3997 Don't pay attention to the @code{inline} keyword. Normally this option
3998 is used to keep the compiler from expanding any functions inline.
3999 Note that if you are not optimizing, no functions can be expanded inline.
4001 @item -finline-functions
4002 @opindex finline-functions
4003 Integrate all simple functions into their callers. The compiler
4004 heuristically decides which functions are simple enough to be worth
4005 integrating in this way.
4007 If all calls to a given function are integrated, and the function is
4008 declared @code{static}, then the function is normally not output as
4009 assembler code in its own right.
4011 Enabled at level @option{-O3}.
4013 @item -finline-limit=@var{n}
4014 @opindex finline-limit
4015 By default, GCC limits the size of functions that can be inlined. This flag
4016 allows the control of this limit for functions that are explicitly marked as
4017 inline (i.e., marked with the inline keyword or defined within the class
4018 definition in c++). @var{n} is the size of functions that can be inlined in
4019 number of pseudo instructions (not counting parameter handling). The default
4020 value of @var{n} is 600.
4021 Increasing this value can result in more inlined code at
4022 the cost of compilation time and memory consumption. Decreasing usually makes
4023 the compilation faster and less code will be inlined (which presumably
4024 means slower programs). This option is particularly useful for programs that
4025 use inlining heavily such as those based on recursive templates with C++.
4027 Inlining is actually controlled by a number of parameters, which may be
4028 specified individually by using @option{--param @var{name}=@var{value}}.
4029 The @option{-finline-limit=@var{n}} option sets some of these parameters
4033 @item max-inline-insns-single
4034 is set to @var{n}/2.
4035 @item max-inline-insns-auto
4036 is set to @var{n}/2.
4037 @item min-inline-insns
4038 is set to 130 or @var{n}/4, whichever is smaller.
4039 @item max-inline-insns-rtl
4043 See below for a documentation of the individual
4044 parameters controlling inlining.
4046 @emph{Note:} pseudo instruction represents, in this particular context, an
4047 abstract measurement of function's size. In no way, it represents a count
4048 of assembly instructions and as such its exact meaning might change from one
4049 release to an another.
4051 @item -fkeep-inline-functions
4052 @opindex fkeep-inline-functions
4053 Even if all calls to a given function are integrated, and the function
4054 is declared @code{static}, nevertheless output a separate run-time
4055 callable version of the function. This switch does not affect
4056 @code{extern inline} functions.
4058 @item -fkeep-static-consts
4059 @opindex fkeep-static-consts
4060 Emit variables declared @code{static const} when optimization isn't turned
4061 on, even if the variables aren't referenced.
4063 GCC enables this option by default. If you want to force the compiler to
4064 check if the variable was referenced, regardless of whether or not
4065 optimization is turned on, use the @option{-fno-keep-static-consts} option.
4067 @item -fmerge-constants
4068 Attempt to merge identical constants (string constants and floating point
4069 constants) across compilation units.
4071 This option is the default for optimized compilation if the assembler and
4072 linker support it. Use @option{-fno-merge-constants} to inhibit this
4075 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4077 @item -fmerge-all-constants
4078 Attempt to merge identical constants and identical variables.
4080 This option implies @option{-fmerge-constants}. In addition to
4081 @option{-fmerge-constants} this considers e.g. even constant initialized
4082 arrays or initialized constant variables with integral or floating point
4083 types. Languages like C or C++ require each non-automatic variable to
4084 have distinct location, so using this option will result in non-conforming
4087 @item -fmodulo-sched
4088 @opindex fmodulo-sched
4089 Perform swing modulo scheduling immediately before the first scheduling
4090 pass. This pass looks at innermost loops and reorders their
4091 instructions by overlapping different iterations.
4095 Use a graph coloring register allocator. Currently this option is meant
4096 only for testing. Users should not specify this option, since it is not
4097 yet ready for production use.
4099 @item -fno-branch-count-reg
4100 @opindex fno-branch-count-reg
4101 Do not use ``decrement and branch'' instructions on a count register,
4102 but instead generate a sequence of instructions that decrement a
4103 register, compare it against zero, then branch based upon the result.
4104 This option is only meaningful on architectures that support such
4105 instructions, which include x86, PowerPC, IA-64 and S/390.
4107 The default is @option{-fbranch-count-reg}, enabled when
4108 @option{-fstrength-reduce} is enabled.
4110 @item -fno-function-cse
4111 @opindex fno-function-cse
4112 Do not put function addresses in registers; make each instruction that
4113 calls a constant function contain the function's address explicitly.
4115 This option results in less efficient code, but some strange hacks
4116 that alter the assembler output may be confused by the optimizations
4117 performed when this option is not used.
4119 The default is @option{-ffunction-cse}
4121 @item -fno-zero-initialized-in-bss
4122 @opindex fno-zero-initialized-in-bss
4123 If the target supports a BSS section, GCC by default puts variables that
4124 are initialized to zero into BSS@. This can save space in the resulting
4127 This option turns off this behavior because some programs explicitly
4128 rely on variables going to the data section. E.g., so that the
4129 resulting executable can find the beginning of that section and/or make
4130 assumptions based on that.
4132 The default is @option{-fzero-initialized-in-bss}.
4134 @item -fbounds-check
4135 @opindex fbounds-check
4136 For front-ends that support it, generate additional code to check that
4137 indices used to access arrays are within the declared range. This is
4138 currently only supported by the Java and Fortran front-ends, where
4139 this option defaults to true and false respectively.
4141 @item -fmudflap -fmudflapth -fmudflapir
4145 @cindex bounds checking
4147 For front-ends that support it (C and C++), instrument all risky
4148 pointer/array dereferencing operations, some standard library
4149 string/heap functions, and some other associated constructs with
4150 range/validity tests. Modules so instrumented should be immune to
4151 buffer overflows, invalid heap use, and some other classes of C/C++
4152 programming errors. The instrumentation relies on a separate runtime
4153 library (@file{libmudflap}), which will be linked into a program if
4154 @option{-fmudflap} is given at link time. Run-time behavior of the
4155 instrumented program is controlled by the @env{MUDFLAP_OPTIONS}
4156 environment variable. See @code{env MUDFLAP_OPTIONS=-help a.out}
4159 Use @option{-fmudflapth} instead of @option{-fmudflap} to compile and to
4160 link if your program is multi-threaded. Use @option{-fmudflapir}, in
4161 addition to @option{-fmudflap} or @option{-fmudflapth}, if
4162 instrumentation should ignore pointer reads. This produces less
4163 instrumentation (and therefore faster execution) and still provides
4164 some protection against outright memory corrupting writes, but allows
4165 erroneously read data to propagate within a program.
4167 @item -fstrength-reduce
4168 @opindex fstrength-reduce
4169 Perform the optimizations of loop strength reduction and
4170 elimination of iteration variables.
4172 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4174 @item -fthread-jumps
4175 @opindex fthread-jumps
4176 Perform optimizations where we check to see if a jump branches to a
4177 location where another comparison subsumed by the first is found. If
4178 so, the first branch is redirected to either the destination of the
4179 second branch or a point immediately following it, depending on whether
4180 the condition is known to be true or false.
4182 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4184 @item -fcse-follow-jumps
4185 @opindex fcse-follow-jumps
4186 In common subexpression elimination, scan through jump instructions
4187 when the target of the jump is not reached by any other path. For
4188 example, when CSE encounters an @code{if} statement with an
4189 @code{else} clause, CSE will follow the jump when the condition
4192 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4194 @item -fcse-skip-blocks
4195 @opindex fcse-skip-blocks
4196 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
4197 follow jumps which conditionally skip over blocks. When CSE
4198 encounters a simple @code{if} statement with no else clause,
4199 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
4200 body of the @code{if}.
4202 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4204 @item -frerun-cse-after-loop
4205 @opindex frerun-cse-after-loop
4206 Re-run common subexpression elimination after loop optimizations has been
4209 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4211 @item -frerun-loop-opt
4212 @opindex frerun-loop-opt
4213 Run the loop optimizer twice.
4215 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4219 Perform a global common subexpression elimination pass.
4220 This pass also performs global constant and copy propagation.
4222 @emph{Note:} When compiling a program using computed gotos, a GCC
4223 extension, you may get better runtime performance if you disable
4224 the global common subexpression elimination pass by adding
4225 @option{-fno-gcse} to the command line.
4227 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4231 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
4232 attempt to move loads which are only killed by stores into themselves. This
4233 allows a loop containing a load/store sequence to be changed to a load outside
4234 the loop, and a copy/store within the loop.
4236 Enabled by default when gcse is enabled.
4240 When @option{-fgcse-sm} is enabled, a store motion pass is run after
4241 global common subexpression elimination. This pass will attempt to move
4242 stores out of loops. When used in conjunction with @option{-fgcse-lm},
4243 loops containing a load/store sequence can be changed to a load before
4244 the loop and a store after the loop.
4246 Enabled by default when gcse is enabled.
4250 When @option{-fgcse-las} is enabled, the global common subexpression
4251 elimination pass eliminates redundant loads that come after stores to the
4252 same memory location (both partial and full redundancies).
4254 Enabled by default when gcse is enabled.
4256 @item -fgcse-after-reload
4257 @opindex fgcse-after-reload
4258 When @option{-fgcse-after-reload} is enabled, a redundant load elimination
4259 pass is performed after reload. The purpose of this pass is to cleanup
4262 @item -floop-optimize
4263 @opindex floop-optimize
4264 Perform loop optimizations: move constant expressions out of loops, simplify
4265 exit test conditions and optionally do strength-reduction and loop unrolling as
4268 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4270 @item -floop-optimize2
4271 @opindex floop-optimize2
4272 Perform loop optimizations using the new loop optimizer. The optimizations
4273 (loop unrolling, peeling and unswitching, loop invariant motion) are enabled
4276 @item -fcrossjumping
4277 @opindex crossjumping
4278 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
4279 resulting code may or may not perform better than without cross-jumping.
4281 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4283 @item -fif-conversion
4284 @opindex if-conversion
4285 Attempt to transform conditional jumps into branch-less equivalents. This
4286 include use of conditional moves, min, max, set flags and abs instructions, and
4287 some tricks doable by standard arithmetics. The use of conditional execution
4288 on chips where it is available is controlled by @code{if-conversion2}.
4290 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4292 @item -fif-conversion2
4293 @opindex if-conversion2
4294 Use conditional execution (where available) to transform conditional jumps into
4295 branch-less equivalents.
4297 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4299 @item -fdelete-null-pointer-checks
4300 @opindex fdelete-null-pointer-checks
4301 Use global dataflow analysis to identify and eliminate useless checks
4302 for null pointers. The compiler assumes that dereferencing a null
4303 pointer would have halted the program. If a pointer is checked after
4304 it has already been dereferenced, it cannot be null.
4306 In some environments, this assumption is not true, and programs can
4307 safely dereference null pointers. Use
4308 @option{-fno-delete-null-pointer-checks} to disable this optimization
4309 for programs which depend on that behavior.
4311 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4313 @item -fexpensive-optimizations
4314 @opindex fexpensive-optimizations
4315 Perform a number of minor optimizations that are relatively expensive.
4317 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4319 @item -foptimize-register-move
4321 @opindex foptimize-register-move
4323 Attempt to reassign register numbers in move instructions and as
4324 operands of other simple instructions in order to maximize the amount of
4325 register tying. This is especially helpful on machines with two-operand
4328 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
4331 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4333 @item -fdelayed-branch
4334 @opindex fdelayed-branch
4335 If supported for the target machine, attempt to reorder instructions
4336 to exploit instruction slots available after delayed branch
4339 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4341 @item -fschedule-insns
4342 @opindex fschedule-insns
4343 If supported for the target machine, attempt to reorder instructions to
4344 eliminate execution stalls due to required data being unavailable. This
4345 helps machines that have slow floating point or memory load instructions
4346 by allowing other instructions to be issued until the result of the load
4347 or floating point instruction is required.
4349 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4351 @item -fschedule-insns2
4352 @opindex fschedule-insns2
4353 Similar to @option{-fschedule-insns}, but requests an additional pass of
4354 instruction scheduling after register allocation has been done. This is
4355 especially useful on machines with a relatively small number of
4356 registers and where memory load instructions take more than one cycle.
4358 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4360 @item -fno-sched-interblock
4361 @opindex fno-sched-interblock
4362 Don't schedule instructions across basic blocks. This is normally
4363 enabled by default when scheduling before register allocation, i.e.@:
4364 with @option{-fschedule-insns} or at @option{-O2} or higher.
4366 @item -fno-sched-spec
4367 @opindex fno-sched-spec
4368 Don't allow speculative motion of non-load instructions. This is normally
4369 enabled by default when scheduling before register allocation, i.e.@:
4370 with @option{-fschedule-insns} or at @option{-O2} or higher.
4372 @item -fsched-spec-load
4373 @opindex fsched-spec-load
4374 Allow speculative motion of some load instructions. This only makes
4375 sense when scheduling before register allocation, i.e.@: with
4376 @option{-fschedule-insns} or at @option{-O2} or higher.
4378 @item -fsched-spec-load-dangerous
4379 @opindex fsched-spec-load-dangerous
4380 Allow speculative motion of more load instructions. This only makes
4381 sense when scheduling before register allocation, i.e.@: with
4382 @option{-fschedule-insns} or at @option{-O2} or higher.
4384 @item -fsched-stalled-insns=@var{n}
4385 @opindex fsched-stalled-insns
4386 Define how many insns (if any) can be moved prematurely from the queue
4387 of stalled insns into the ready list, during the second scheduling pass.
4389 @item -fsched-stalled-insns-dep=@var{n}
4390 @opindex fsched-stalled-insns-dep
4391 Define how many insn groups (cycles) will be examined for a dependency
4392 on a stalled insn that is candidate for premature removal from the queue
4393 of stalled insns. Has an effect only during the second scheduling pass,
4394 and only if @option{-fsched-stalled-insns} is used and its value is not zero.
4396 @item -fsched2-use-superblocks
4397 @opindex fsched2-use-superblocks
4398 When scheduling after register allocation, do use superblock scheduling
4399 algorithm. Superblock scheduling allows motion across basic block boundaries
4400 resulting on faster schedules. This option is experimental, as not all machine
4401 descriptions used by GCC model the CPU closely enough to avoid unreliable
4402 results from the algorithm.
4404 This only makes sense when scheduling after register allocation, i.e.@: with
4405 @option{-fschedule-insns2} or at @option{-O2} or higher.
4407 @item -fsched2-use-traces
4408 @opindex fsched2-use-traces
4409 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
4410 allocation and additionally perform code duplication in order to increase the
4411 size of superblocks using tracer pass. See @option{-ftracer} for details on
4414 This mode should produce faster but significantly longer programs. Also
4415 without @code{-fbranch-probabilities} the traces constructed may not match the
4416 reality and hurt the performance. This only makes
4417 sense when scheduling after register allocation, i.e.@: with
4418 @option{-fschedule-insns2} or at @option{-O2} or higher.
4420 @item -freschedule-modulo-scheduled-loops
4421 @opindex fscheduling-in-modulo-scheduled-loops
4422 The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled
4423 we may want to prevent the later scheduling passes from changing its schedule, we use this
4424 option to control that.
4426 @item -fcaller-saves
4427 @opindex fcaller-saves
4428 Enable values to be allocated in registers that will be clobbered by
4429 function calls, by emitting extra instructions to save and restore the
4430 registers around such calls. Such allocation is done only when it
4431 seems to result in better code than would otherwise be produced.
4433 This option is always enabled by default on certain machines, usually
4434 those which have no call-preserved registers to use instead.
4436 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4439 Perform Partial Redundancy Elimination (PRE) on trees. This flag is
4440 enabled by default at -O and higher.
4443 Perform Full Redundancy Elimination (FRE) on trees. The difference
4444 between FRE and PRE is that FRE only considers expressions
4445 that are computed on all paths leading to the redundant computation.
4446 This analysis faster than PRE, though it exposes fewer redundancies.
4447 This flag is enabled by default at -O and higher.
4450 Perform sparse conditional constant propagation (CCP) on trees. This flag
4451 is enabled by default at -O and higher.
4454 Perform dead code elimination (DCE) on trees. This flag is enabled by
4455 default at -O and higher.
4457 @item -ftree-dominator-opts
4458 Perform dead code elimination (DCE) on trees. This flag is enabled by
4459 default at -O and higher.
4462 Perform loop header copying on trees. This is beneficial since it increases
4463 effectivity of code motion optimizations. It also saves one jump. This flag
4464 is enabled by default at -O and higher. It is not enabled for -Os, since it
4465 usually increases code size.
4467 @item -ftree-loop-optimize
4468 Perform loop optimizations on trees. This flag is enabled by default at -O
4472 Perform loop invariant motion on trees. This pass moves only invartiants that
4473 would be hard to handle on rtl level (function calls, operations that expand to
4474 nontrivial sequences of insns). With @option{-funswitch-loops} it also moves
4475 operands of conditions that are invariant out of the loop, so that we can use
4476 just trivial invariantness analysis in loop unswitching. The pass also includes
4480 Perform scalar replacement of aggregates. This pass replaces structure
4481 references with scalars to prevent committing structures to memory too
4482 early. This flag is enabled by default at -O and higher.
4484 @item -ftree-copyrename
4485 Perform copy renaming on trees. This pass attempts to rename compiler
4486 temporaries to other variables at copy locations, usually resulting in
4487 variable names which more closely resemble the original variables. This flag
4488 is enabled by default at -O and higher.
4491 Perform temporary expression replacement during the SSA->normal phase. Single
4492 use/single def temporaries are replaced at their use location with their
4493 defining expression. This results in non-GIMPLE code, but gives the expanders
4494 much more complex trees to work on resulting in better RTL generation. This is
4495 enabled by default at -O and higher.
4498 Perform live range splitting during the SSA->normal phase. Distinct live
4499 ranges of a variable are split into unique variables, allowing for better
4500 optimization later. This is enabled by default at -O and higher.
4504 Perform tail duplication to enlarge superblock size. This transformation
4505 simplifies the control flow of the function allowing other optimizations to do
4508 @item -funroll-loops
4509 @opindex funroll-loops
4510 Unroll loops whose number of iterations can be determined at compile
4511 time or upon entry to the loop. @option{-funroll-loops} implies both
4512 @option{-fstrength-reduce} and @option{-frerun-cse-after-loop}. This
4513 option makes code larger, and may or may not make it run faster.
4515 @item -funroll-all-loops
4516 @opindex funroll-all-loops
4517 Unroll all loops, even if their number of iterations is uncertain when
4518 the loop is entered. This usually makes programs run more slowly.
4519 @option{-funroll-all-loops} implies the same options as
4520 @option{-funroll-loops},
4522 @item -fprefetch-loop-arrays
4523 @opindex fprefetch-loop-arrays
4524 If supported by the target machine, generate instructions to prefetch
4525 memory to improve the performance of loops that access large arrays.
4527 @item -fmove-all-movables
4528 @opindex fmove-all-movables
4529 Forces all invariant computations in loops to be moved
4532 @item -freduce-all-givs
4533 @opindex freduce-all-givs
4534 Forces all general-induction variables in loops to be
4537 @emph{Note:} When compiling programs written in Fortran,
4538 @option{-fmove-all-movables} and @option{-freduce-all-givs} are enabled
4539 by default when you use the optimizer.
4541 These options may generate better or worse code; results are highly
4542 dependent on the structure of loops within the source code.
4544 These two options are intended to be removed someday, once
4545 they have helped determine the efficacy of various
4546 approaches to improving loop optimizations.
4548 Please contact @w{@email{gcc@@gcc.gnu.org}}, and describe how use of
4549 these options affects the performance of your production code.
4550 Examples of code that runs @emph{slower} when these options are
4551 @emph{enabled} are very valuable.
4554 @itemx -fno-peephole2
4555 @opindex fno-peephole
4556 @opindex fno-peephole2
4557 Disable any machine-specific peephole optimizations. The difference
4558 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
4559 are implemented in the compiler; some targets use one, some use the
4560 other, a few use both.
4562 @option{-fpeephole} is enabled by default.
4563 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4565 @item -fno-guess-branch-probability
4566 @opindex fno-guess-branch-probability
4567 Do not guess branch probabilities using a randomized model.
4569 Sometimes GCC will opt to use a randomized model to guess branch
4570 probabilities, when none are available from either profiling feedback
4571 (@option{-fprofile-arcs}) or @samp{__builtin_expect}. This means that
4572 different runs of the compiler on the same program may produce different
4575 In a hard real-time system, people don't want different runs of the
4576 compiler to produce code that has different behavior; minimizing
4577 non-determinism is of paramount import. This switch allows users to
4578 reduce non-determinism, possibly at the expense of inferior
4581 The default is @option{-fguess-branch-probability} at levels
4582 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4584 @item -freorder-blocks
4585 @opindex freorder-blocks
4586 Reorder basic blocks in the compiled function in order to reduce number of
4587 taken branches and improve code locality.
4589 Enabled at levels @option{-O2}, @option{-O3}.
4591 @item -freorder-blocks-and-partition
4592 @opindex freorder-blocks-and-partition
4593 In addition to reordering basic blocks in the compiled function, in order
4594 to reduce number of taken branches, partitions hot and cold basic blocks
4595 into separate sections of the assembly and .o files, to improve
4596 paging and cache locality performance.
4598 @item -freorder-functions
4599 @opindex freorder-functions
4600 Reorder basic blocks in the compiled function in order to reduce number of
4601 taken branches and improve code locality. This is implemented by using special
4602 subsections @code{.text.hot} for most frequently executed functions and
4603 @code{.text.unlikely} for unlikely executed functions. Reordering is done by
4604 the linker so object file format must support named sections and linker must
4605 place them in a reasonable way.
4607 Also profile feedback must be available in to make this option effective. See
4608 @option{-fprofile-arcs} for details.
4610 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4612 @item -fstrict-aliasing
4613 @opindex fstrict-aliasing
4614 Allows the compiler to assume the strictest aliasing rules applicable to
4615 the language being compiled. For C (and C++), this activates
4616 optimizations based on the type of expressions. In particular, an
4617 object of one type is assumed never to reside at the same address as an
4618 object of a different type, unless the types are almost the same. For
4619 example, an @code{unsigned int} can alias an @code{int}, but not a
4620 @code{void*} or a @code{double}. A character type may alias any other
4623 Pay special attention to code like this:
4636 The practice of reading from a different union member than the one most
4637 recently written to (called ``type-punning'') is common. Even with
4638 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
4639 is accessed through the union type. So, the code above will work as
4640 expected. However, this code might not:
4651 Every language that wishes to perform language-specific alias analysis
4652 should define a function that computes, given an @code{tree}
4653 node, an alias set for the node. Nodes in different alias sets are not
4654 allowed to alias. For an example, see the C front-end function
4655 @code{c_get_alias_set}.
4657 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4659 @item -falign-functions
4660 @itemx -falign-functions=@var{n}
4661 @opindex falign-functions
4662 Align the start of functions to the next power-of-two greater than
4663 @var{n}, skipping up to @var{n} bytes. For instance,
4664 @option{-falign-functions=32} aligns functions to the next 32-byte
4665 boundary, but @option{-falign-functions=24} would align to the next
4666 32-byte boundary only if this can be done by skipping 23 bytes or less.
4668 @option{-fno-align-functions} and @option{-falign-functions=1} are
4669 equivalent and mean that functions will not be aligned.
4671 Some assemblers only support this flag when @var{n} is a power of two;
4672 in that case, it is rounded up.
4674 If @var{n} is not specified or is zero, use a machine-dependent default.
4676 Enabled at levels @option{-O2}, @option{-O3}.
4678 @item -falign-labels
4679 @itemx -falign-labels=@var{n}
4680 @opindex falign-labels
4681 Align all branch targets to a power-of-two boundary, skipping up to
4682 @var{n} bytes like @option{-falign-functions}. This option can easily
4683 make code slower, because it must insert dummy operations for when the
4684 branch target is reached in the usual flow of the code.
4686 @option{-fno-align-labels} and @option{-falign-labels=1} are
4687 equivalent and mean that labels will not be aligned.
4689 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
4690 are greater than this value, then their values are used instead.
4692 If @var{n} is not specified or is zero, use a machine-dependent default
4693 which is very likely to be @samp{1}, meaning no alignment.
4695 Enabled at levels @option{-O2}, @option{-O3}.
4698 @itemx -falign-loops=@var{n}
4699 @opindex falign-loops
4700 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
4701 like @option{-falign-functions}. The hope is that the loop will be
4702 executed many times, which will make up for any execution of the dummy
4705 @option{-fno-align-loops} and @option{-falign-loops=1} are
4706 equivalent and mean that loops will not be aligned.
4708 If @var{n} is not specified or is zero, use a machine-dependent default.
4710 Enabled at levels @option{-O2}, @option{-O3}.
4713 @itemx -falign-jumps=@var{n}
4714 @opindex falign-jumps
4715 Align branch targets to a power-of-two boundary, for branch targets
4716 where the targets can only be reached by jumping, skipping up to @var{n}
4717 bytes like @option{-falign-functions}. In this case, no dummy operations
4720 @option{-fno-align-jumps} and @option{-falign-jumps=1} are
4721 equivalent and mean that loops will not be aligned.
4723 If @var{n} is not specified or is zero, use a machine-dependent default.
4725 Enabled at levels @option{-O2}, @option{-O3}.
4727 @item -funit-at-a-time
4728 @opindex funit-at-a-time
4729 Parse the whole compilation unit before starting to produce code.
4730 This allows some extra optimizations to take place but consumes
4731 more memory (in general). There are some compatibility issues
4732 with @emph{unit-at-at-time} mode:
4735 enabling @emph{unit-at-a-time} mode may change the order
4736 in which functions, variables, and top-level @code{asm} statements
4737 are emitted, and will likely break code relying on some particular
4738 ordering. The majority of such top-level @code{asm} statements,
4739 though, can be replaced by @code{section} attributes.
4742 @emph{unit-at-a-time} mode removes unreferenced static variables
4743 and functions are removed. This may result in undefined references
4744 when an @code{asm} statement refers directly to variables or functions
4745 that are otherwise unused. In that case either the variable/function
4746 shall be listed as an operand of the @code{asm} statement operand or,
4747 in the case of top-level @code{asm} statements the attribute @code{used}
4748 shall be used on the declaration.
4751 Static functions now can use non-standard passing conventions that
4752 may break @code{asm} statements calling functions directly. Again,
4753 attribute @code{used} will prevent this behavior.
4756 As a temporary workaround, @option{-fno-unit-at-a-time} can be used,
4757 but this scheme may not be supported by future releases of GCC.
4759 Enabled at levels @option{-O2}, @option{-O3}.
4763 Constructs webs as commonly used for register allocation purposes and assign
4764 each web individual pseudo register. This allows the register allocation pass
4765 to operate on pseudos directly, but also strengthens several other optimization
4766 passes, such as CSE, loop optimizer and trivial dead code remover. It can,
4767 however, make debugging impossible, since variables will no longer stay in a
4770 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os},
4771 on targets where the default format for debugging information supports
4774 @item -fno-cprop-registers
4775 @opindex fno-cprop-registers
4776 After register allocation and post-register allocation instruction splitting,
4777 we perform a copy-propagation pass to try to reduce scheduling dependencies
4778 and occasionally eliminate the copy.
4780 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4782 @item -fprofile-generate
4783 @opindex fprofile-generate
4785 Enable options usually used for instrumenting application to produce
4786 profile useful for later recompilation with profile feedback based
4787 optimization. You must use @code{-fprofile-generate} both when
4788 compiling and when linking your program.
4790 The following options are enabled: @code{-fprofile-arcs}, @code{-fprofile-values}, @code{-fvpt}.
4793 @opindex fprofile-use
4794 Enable profile feedback directed optimizations, and optimizations
4795 generally profitable only with profile feedback available.
4797 The following options are enabled: @code{-fbranch-probabilities},
4798 @code{-fvpt}, @code{-funroll-loops}, @code{-fpeel-loops}, @code{-ftracer}.
4802 The following options control compiler behavior regarding floating
4803 point arithmetic. These options trade off between speed and
4804 correctness. All must be specifically enabled.
4808 @opindex ffloat-store
4809 Do not store floating point variables in registers, and inhibit other
4810 options that might change whether a floating point value is taken from a
4813 @cindex floating point precision
4814 This option prevents undesirable excess precision on machines such as
4815 the 68000 where the floating registers (of the 68881) keep more
4816 precision than a @code{double} is supposed to have. Similarly for the
4817 x86 architecture. For most programs, the excess precision does only
4818 good, but a few programs rely on the precise definition of IEEE floating
4819 point. Use @option{-ffloat-store} for such programs, after modifying
4820 them to store all pertinent intermediate computations into variables.
4824 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations}, @*
4825 @option{-fno-trapping-math}, @option{-ffinite-math-only},
4826 @option{-fno-rounding-math} and @option{-fno-signaling-nans}.
4828 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
4830 This option should never be turned on by any @option{-O} option since
4831 it can result in incorrect output for programs which depend on
4832 an exact implementation of IEEE or ISO rules/specifications for
4835 @item -fno-math-errno
4836 @opindex fno-math-errno
4837 Do not set ERRNO after calling math functions that are executed
4838 with a single instruction, e.g., sqrt. A program that relies on
4839 IEEE exceptions for math error handling may want to use this flag
4840 for speed while maintaining IEEE arithmetic compatibility.
4842 This option should never be turned on by any @option{-O} option since
4843 it can result in incorrect output for programs which depend on
4844 an exact implementation of IEEE or ISO rules/specifications for
4847 The default is @option{-fmath-errno}.
4849 @item -funsafe-math-optimizations
4850 @opindex funsafe-math-optimizations
4851 Allow optimizations for floating-point arithmetic that (a) assume
4852 that arguments and results are valid and (b) may violate IEEE or
4853 ANSI standards. When used at link-time, it may include libraries
4854 or startup files that change the default FPU control word or other
4855 similar optimizations.
4857 This option should never be turned on by any @option{-O} option since
4858 it can result in incorrect output for programs which depend on
4859 an exact implementation of IEEE or ISO rules/specifications for
4862 The default is @option{-fno-unsafe-math-optimizations}.
4864 @item -ffinite-math-only
4865 @opindex ffinite-math-only
4866 Allow optimizations for floating-point arithmetic that assume
4867 that arguments and results are not NaNs or +-Infs.
4869 This option should never be turned on by any @option{-O} option since
4870 it can result in incorrect output for programs which depend on
4871 an exact implementation of IEEE or ISO rules/specifications.
4873 The default is @option{-fno-finite-math-only}.
4875 @item -fno-trapping-math
4876 @opindex fno-trapping-math
4877 Compile code assuming that floating-point operations cannot generate
4878 user-visible traps. These traps include division by zero, overflow,
4879 underflow, inexact result and invalid operation. This option implies
4880 @option{-fno-signaling-nans}. Setting this option may allow faster
4881 code if one relies on ``non-stop'' IEEE arithmetic, for example.
4883 This option should never be turned on by any @option{-O} option since
4884 it can result in incorrect output for programs which depend on
4885 an exact implementation of IEEE or ISO rules/specifications for
4888 The default is @option{-ftrapping-math}.
4890 @item -frounding-math
4891 @opindex frounding-math
4892 Disable transformations and optimizations that assume default floating
4893 point rounding behavior. This is round-to-zero for all floating point
4894 to integer conversions, and round-to-nearest for all other arithmetic
4895 truncations. This option should be specified for programs that change
4896 the FP rounding mode dynamically, or that may be executed with a
4897 non-default rounding mode. This option disables constant folding of
4898 floating point expressions at compile-time (which may be affected by
4899 rounding mode) and arithmetic transformations that are unsafe in the
4900 presence of sign-dependent rounding modes.
4902 The default is @option{-fno-rounding-math}.
4904 This option is experimental and does not currently guarantee to
4905 disable all GCC optimizations that are affected by rounding mode.
4906 Future versions of GCC may provide finer control of this setting
4907 using C99's @code{FENV_ACCESS} pragma. This command line option
4908 will be used to specify the default state for @code{FENV_ACCESS}.
4910 @item -fsignaling-nans
4911 @opindex fsignaling-nans
4912 Compile code assuming that IEEE signaling NaNs may generate user-visible
4913 traps during floating-point operations. Setting this option disables
4914 optimizations that may change the number of exceptions visible with
4915 signaling NaNs. This option implies @option{-ftrapping-math}.
4917 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
4920 The default is @option{-fno-signaling-nans}.
4922 This option is experimental and does not currently guarantee to
4923 disable all GCC optimizations that affect signaling NaN behavior.
4925 @item -fsingle-precision-constant
4926 @opindex fsingle-precision-constant
4927 Treat floating point constant as single precision constant instead of
4928 implicitly converting it to double precision constant.
4933 The following options control optimizations that may improve
4934 performance, but are not enabled by any @option{-O} options. This
4935 section includes experimental options that may produce broken code.
4938 @item -fbranch-probabilities
4939 @opindex fbranch-probabilities
4940 After running a program compiled with @option{-fprofile-arcs}
4941 (@pxref{Debugging Options,, Options for Debugging Your Program or
4942 @command{gcc}}), you can compile it a second time using
4943 @option{-fbranch-probabilities}, to improve optimizations based on
4944 the number of times each branch was taken. When the program
4945 compiled with @option{-fprofile-arcs} exits it saves arc execution
4946 counts to a file called @file{@var{sourcename}.gcda} for each source
4947 file The information in this data file is very dependent on the
4948 structure of the generated code, so you must use the same source code
4949 and the same optimization options for both compilations.
4951 With @option{-fbranch-probabilities}, GCC puts a
4952 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
4953 These can be used to improve optimization. Currently, they are only
4954 used in one place: in @file{reorg.c}, instead of guessing which path a
4955 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
4956 exactly determine which path is taken more often.
4958 @item -fprofile-values
4959 @opindex fprofile-values
4960 If combined with @option{-fprofile-arcs}, it adds code so that some
4961 data about values of expressions in the program is gathered.
4963 With @option{-fbranch-probabilities}, it reads back the data gathered
4964 from profiling values of expressions and adds @samp{REG_VALUE_PROFILE}
4965 notes to instructions for their later usage in optimizations.
4967 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
4971 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
4972 a code to gather information about values of expressions.
4974 With @option{-fbranch-probabilities}, it reads back the data gathered
4975 and actually performs the optimizations based on them.
4976 Currently the optimizations include specialization of division operation
4977 using the knowledge about the value of the denominator.
4979 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
4981 @item -frename-registers
4982 @opindex frename-registers
4983 Attempt to avoid false dependencies in scheduled code by making use
4984 of registers left over after register allocation. This optimization
4985 will most benefit processors with lots of registers. Depending on the
4986 debug information format adopted by the target, however, it can
4987 make debugging impossible, since variables will no longer stay in
4988 a ``home register''.
4990 Not enabled by default at any level because it has known bugs.
4994 Use a graph coloring register allocator. Currently this option is meant
4995 for testing, so we are interested to hear about miscompilations with
5000 Perform tail duplication to enlarge superblock size. This transformation
5001 simplifies the control flow of the function allowing other optimizations to do
5004 Enabled with @option{-fprofile-use}.
5006 @item -funroll-loops
5007 @opindex funroll-loops
5008 Unroll loops whose number of iterations can be determined at compile time or
5009 upon entry to the loop. @option{-funroll-loops} implies
5010 @option{-frerun-cse-after-loop}. It also turns on complete loop peeling
5011 (i.e. complete removal of loops with small constant number of iterations).
5012 This option makes code larger, and may or may not make it run faster.
5014 Enabled with @option{-fprofile-use}.
5016 @item -funroll-all-loops
5017 @opindex funroll-all-loops
5018 Unroll all loops, even if their number of iterations is uncertain when
5019 the loop is entered. This usually makes programs run more slowly.
5020 @option{-funroll-all-loops} implies the same options as
5021 @option{-funroll-loops}.
5024 @opindex fpeel-loops
5025 Peels the loops for that there is enough information that they do not
5026 roll much (from profile feedback). It also turns on complete loop peeling
5027 (i.e. complete removal of loops with small constant number of iterations).
5029 Enabled with @option{-fprofile-use}.
5031 @item -fmove-loop-invariants
5032 @opindex fmove-loop-invariants
5033 Enables the loop invariant motion pass in the new loop optimizer. Enabled
5034 at level @option{-O1}
5036 @item -funswitch-loops
5037 @opindex funswitch-loops
5038 Move branches with loop invariant conditions out of the loop, with duplicates
5039 of the loop on both branches (modified according to result of the condition).
5041 @item -fold-unroll-loops
5042 @opindex fold-unroll-loops
5043 Unroll loops whose number of iterations can be determined at compile
5044 time or upon entry to the loop, using the old loop unroller whose loop
5045 recognition is based on notes from frontend. @option{-fold-unroll-loops} implies
5046 both @option{-fstrength-reduce} and @option{-frerun-cse-after-loop}. This
5047 option makes code larger, and may or may not make it run faster.
5049 @item -fold-unroll-all-loops
5050 @opindex fold-unroll-all-loops
5051 Unroll all loops, even if their number of iterations is uncertain when
5052 the loop is entered. This is done using the old loop unroller whose loop
5053 recognition is based on notes from frontend. This usually makes programs run more slowly.
5054 @option{-fold-unroll-all-loops} implies the same options as
5055 @option{-fold-unroll-loops}.
5057 @item -fprefetch-loop-arrays
5058 @opindex fprefetch-loop-arrays
5059 If supported by the target machine, generate instructions to prefetch
5060 memory to improve the performance of loops that access large arrays.
5062 Disabled at level @option{-Os}.
5064 @item -ffunction-sections
5065 @itemx -fdata-sections
5066 @opindex ffunction-sections
5067 @opindex fdata-sections
5068 Place each function or data item into its own section in the output
5069 file if the target supports arbitrary sections. The name of the
5070 function or the name of the data item determines the section's name
5073 Use these options on systems where the linker can perform optimizations
5074 to improve locality of reference in the instruction space. Most systems
5075 using the ELF object format and SPARC processors running Solaris 2 have
5076 linkers with such optimizations. AIX may have these optimizations in
5079 Only use these options when there are significant benefits from doing
5080 so. When you specify these options, the assembler and linker will
5081 create larger object and executable files and will also be slower.
5082 You will not be able to use @code{gprof} on all systems if you
5083 specify this option and you may have problems with debugging if
5084 you specify both this option and @option{-g}.
5086 @item -fbranch-target-load-optimize
5087 @opindex fbranch-target-load-optimize
5088 Perform branch target register load optimization before prologue / epilogue
5090 The use of target registers can typically be exposed only during reload,
5091 thus hoisting loads out of loops and doing inter-block scheduling needs
5092 a separate optimization pass.
5094 @item -fbranch-target-load-optimize2
5095 @opindex fbranch-target-load-optimize2
5096 Perform branch target register load optimization after prologue / epilogue
5099 @item -fbtr-bb-exclusive
5100 @opindex fbtr-bb-exclusive
5101 When performing branch target register load optimization, don't reuse
5102 branch target registers in within any basic block.
5104 @item --param @var{name}=@var{value}
5106 In some places, GCC uses various constants to control the amount of
5107 optimization that is done. For example, GCC will not inline functions
5108 that contain more that a certain number of instructions. You can
5109 control some of these constants on the command-line using the
5110 @option{--param} option.
5112 The names of specific parameters, and the meaning of the values, are
5113 tied to the internals of the compiler, and are subject to change
5114 without notice in future releases.
5116 In each case, the @var{value} is an integer. The allowable choices for
5117 @var{name} are given in the following table:
5120 @item max-crossjump-edges
5121 The maximum number of incoming edges to consider for crossjumping.
5122 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
5123 the number of edges incoming to each block. Increasing values mean
5124 more aggressive optimization, making the compile time increase with
5125 probably small improvement in executable size.
5127 @item min-crossjump-insns
5128 The minimum number of instructions which must be matched at the end
5129 of two blocks before crossjumping will be performed on them. This
5130 value is ignored in the case where all instructions in the block being
5131 crossjumped from are matched. The default value is 5.
5133 @item max-delay-slot-insn-search
5134 The maximum number of instructions to consider when looking for an
5135 instruction to fill a delay slot. If more than this arbitrary number of
5136 instructions is searched, the time savings from filling the delay slot
5137 will be minimal so stop searching. Increasing values mean more
5138 aggressive optimization, making the compile time increase with probably
5139 small improvement in executable run time.
5141 @item max-delay-slot-live-search
5142 When trying to fill delay slots, the maximum number of instructions to
5143 consider when searching for a block with valid live register
5144 information. Increasing this arbitrarily chosen value means more
5145 aggressive optimization, increasing the compile time. This parameter
5146 should be removed when the delay slot code is rewritten to maintain the
5149 @item max-gcse-memory
5150 The approximate maximum amount of memory that will be allocated in
5151 order to perform the global common subexpression elimination
5152 optimization. If more memory than specified is required, the
5153 optimization will not be done.
5155 @item max-gcse-passes
5156 The maximum number of passes of GCSE to run. The default is 1.
5158 @item max-pending-list-length
5159 The maximum number of pending dependencies scheduling will allow
5160 before flushing the current state and starting over. Large functions
5161 with few branches or calls can create excessively large lists which
5162 needlessly consume memory and resources.
5164 @item max-inline-insns-single
5165 Several parameters control the tree inliner used in gcc.
5166 This number sets the maximum number of instructions (counted in GCC's
5167 internal representation) in a single function that the tree inliner
5168 will consider for inlining. This only affects functions declared
5169 inline and methods implemented in a class declaration (C++).
5170 The default value is 500.
5172 @item max-inline-insns-auto
5173 When you use @option{-finline-functions} (included in @option{-O3}),
5174 a lot of functions that would otherwise not be considered for inlining
5175 by the compiler will be investigated. To those functions, a different
5176 (more restrictive) limit compared to functions declared inline can
5178 The default value is 120.
5180 @item large-function-insns
5181 The limit specifying really large functions. For functions greater than this
5182 limit inlining is constrained by @option{--param large-function-growth}.
5183 This parameter is useful primarily to avoid extreme compilation time caused by non-linear
5184 algorithms used by the backend.
5185 This parameter is ignored when @option{-funit-at-a-time} is not used.
5186 The default value is 3000.
5188 @item large-function-growth
5189 Specifies maximal growth of large function caused by inlining in percents.
5190 This parameter is ignored when @option{-funit-at-a-time} is not used.
5191 The default value is 200.
5193 @item inline-unit-growth
5194 Specifies maximal overall growth of the compilation unit caused by inlining.
5195 This parameter is ignored when @option{-funit-at-a-time} is not used.
5196 The default value is 150.
5198 @item max-inline-insns-recursive
5199 @itemx max-inline-insns-recursive-auto
5200 Specifies maximum number of instructions out-of-line copy of self recursive inline
5201 function can grow into by performing recursive inlining.
5203 For functions declared inline @option{--param max-inline-insns-recursive} is
5204 taken into acount. For function not declared inline, recursive inlining
5205 happens only when @option{-finline-functions} (included in @option{-O3}) is
5206 enabled and @option{--param max-inline-insns-recursive-auto} is used. The
5207 default value is 500.
5209 @item max-inline-recursive-depth
5210 @itemx max-inline-recursive-depth-auto
5211 Specifies maximum recursion depth used by the recursive inlining.
5213 For functions declared inline @option{--param max-inline-recursive-depth} is
5214 taken into acount. For function not declared inline, recursive inlining
5215 happens only when @option{-finline-functions} (included in @option{-O3}) is
5216 enabled and @option{--param max-inline-recursive-depth-auto} is used. The
5217 default value is 500.
5219 @item max-inline-insns-rtl
5220 For languages that use the RTL inliner (this happens at a later stage
5221 than tree inlining), you can set the maximum allowable size (counted
5222 in RTL instructions) for the RTL inliner with this parameter.
5223 The default value is 600.
5225 @item max-unrolled-insns
5226 The maximum number of instructions that a loop should have if that loop
5227 is unrolled, and if the loop is unrolled, it determines how many times
5228 the loop code is unrolled.
5230 @item max-average-unrolled-insns
5231 The maximum number of instructions biased by probabilities of their execution
5232 that a loop should have if that loop is unrolled, and if the loop is unrolled,
5233 it determines how many times the loop code is unrolled.
5235 @item max-unroll-times
5236 The maximum number of unrollings of a single loop.
5238 @item max-peeled-insns
5239 The maximum number of instructions that a loop should have if that loop
5240 is peeled, and if the loop is peeled, it determines how many times
5241 the loop code is peeled.
5243 @item max-peel-times
5244 The maximum number of peelings of a single loop.
5246 @item max-completely-peeled-insns
5247 The maximum number of insns of a completely peeled loop.
5249 @item max-completely-peel-times
5250 The maximum number of iterations of a loop to be suitable for complete peeling.
5252 @item max-unswitch-insns
5253 The maximum number of insns of an unswitched loop.
5255 @item max-unswitch-level
5256 The maximum number of branches unswitched in a single loop.
5259 The minimum cost of an expensive expression in the loop invariant motion.
5261 @item max-iterations-to-track
5263 The maximum number of iterations of a loop the brute force algorithm
5264 for analysis of # of iterations of the loop tries to evaluate.
5266 @item hot-bb-count-fraction
5267 Select fraction of the maximal count of repetitions of basic block in program
5268 given basic block needs to have to be considered hot.
5270 @item hot-bb-frequency-fraction
5271 Select fraction of the maximal frequency of executions of basic block in
5272 function given basic block needs to have to be considered hot
5274 @item tracer-dynamic-coverage
5275 @itemx tracer-dynamic-coverage-feedback
5277 This value is used to limit superblock formation once the given percentage of
5278 executed instructions is covered. This limits unnecessary code size
5281 The @option{tracer-dynamic-coverage-feedback} is used only when profile
5282 feedback is available. The real profiles (as opposed to statically estimated
5283 ones) are much less balanced allowing the threshold to be larger value.
5285 @item tracer-max-code-growth
5286 Stop tail duplication once code growth has reached given percentage. This is
5287 rather hokey argument, as most of the duplicates will be eliminated later in
5288 cross jumping, so it may be set to much higher values than is the desired code
5291 @item tracer-min-branch-ratio
5293 Stop reverse growth when the reverse probability of best edge is less than this
5294 threshold (in percent).
5296 @item tracer-min-branch-ratio
5297 @itemx tracer-min-branch-ratio-feedback
5299 Stop forward growth if the best edge do have probability lower than this
5302 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
5303 compilation for profile feedback and one for compilation without. The value
5304 for compilation with profile feedback needs to be more conservative (higher) in
5305 order to make tracer effective.
5307 @item max-cse-path-length
5309 Maximum number of basic blocks on path that cse considers. The default is 10.
5311 @item global-var-threshold
5313 Counts the number of function calls (N) and the number of
5314 call-clobbered variables (V). If NxV is larger than this limit, a
5315 single artificial variable will be created to represent all the
5316 call-clobbered variables at function call sites. This artificial
5317 variable will then be made to alias every call-clobbered variable.
5318 (done as int * size_t on the host machine; beware overflow).
5320 @item max-aliased-vops
5322 Maxiumum number of virtual operands allowed to represent aliases
5323 before triggering the alias grouping heuristic. Alias grouping
5324 reduces compile times and memory consumption needed for aliasing at
5325 the expense of precision loss in alias information.
5327 @item ggc-min-expand
5329 GCC uses a garbage collector to manage its own memory allocation. This
5330 parameter specifies the minimum percentage by which the garbage
5331 collector's heap should be allowed to expand between collections.
5332 Tuning this may improve compilation speed; it has no effect on code
5335 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
5336 RAM >= 1GB. If @code{getrlimit} is available, the notion of "RAM" is
5337 the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If
5338 GCC is not able to calculate RAM on a particular platform, the lower
5339 bound of 30% is used. Setting this parameter and
5340 @option{ggc-min-heapsize} to zero causes a full collection to occur at
5341 every opportunity. This is extremely slow, but can be useful for
5344 @item ggc-min-heapsize
5346 Minimum size of the garbage collector's heap before it begins bothering
5347 to collect garbage. The first collection occurs after the heap expands
5348 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
5349 tuning this may improve compilation speed, and has no effect on code
5352 The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which
5353 tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
5354 with a lower bound of 4096 (four megabytes) and an upper bound of
5355 131072 (128 megabytes). If GCC is not able to calculate RAM on a
5356 particular platform, the lower bound is used. Setting this parameter
5357 very large effectively disables garbage collection. Setting this
5358 parameter and @option{ggc-min-expand} to zero causes a full collection
5359 to occur at every opportunity.
5361 @item max-reload-search-insns
5362 The maximum number of instruction reload should look backward for equivalent
5363 register. Increasing values mean more aggressive optimization, making the
5364 compile time increase with probably slightly better performance. The default
5367 @item max-cselib-memory-location
5368 The maximum number of memory locations cselib should take into acount.
5369 Increasing values mean more aggressive optimization, making the compile time
5370 increase with probably slightly better performance. The default value is 500.
5372 @item reorder-blocks-duplicate
5373 @itemx reorder-blocks-duplicate-feedback
5375 Used by basic block reordering pass to decide whether to use unconditional
5376 branch or duplicate the code on its destination. Code is duplicated when its
5377 estimated size is smaller than this value multiplied by the estimated size of
5378 unconditional jump in the hot spots of the program.
5380 The @option{reorder-block-duplicate-feedback} is used only when profile
5381 feedback is available and may be set to higher values than
5382 @option{reorder-block-duplicate} since information about the hot spots is more
5385 @item max-sched-region-blocks
5386 The maximum number of blocks in a region to be considered for
5387 interblock scheduling. The default value is 10.
5389 @item max-sched-region-insns
5390 The maximum number of insns in a region to be considered for
5391 interblock scheduling. The default value is 100.
5395 @node Preprocessor Options
5396 @section Options Controlling the Preprocessor
5397 @cindex preprocessor options
5398 @cindex options, preprocessor
5400 These options control the C preprocessor, which is run on each C source
5401 file before actual compilation.
5403 If you use the @option{-E} option, nothing is done except preprocessing.
5404 Some of these options make sense only together with @option{-E} because
5405 they cause the preprocessor output to be unsuitable for actual
5410 You can use @option{-Wp,@var{option}} to bypass the compiler driver
5411 and pass @var{option} directly through to the preprocessor. If
5412 @var{option} contains commas, it is split into multiple options at the
5413 commas. However, many options are modified, translated or interpreted
5414 by the compiler driver before being passed to the preprocessor, and
5415 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
5416 interface is undocumented and subject to change, so whenever possible
5417 you should avoid using @option{-Wp} and let the driver handle the
5420 @item -Xpreprocessor @var{option}
5421 @opindex preprocessor
5422 Pass @var{option} as an option to the preprocessor. You can use this to
5423 supply system-specific preprocessor options which GCC does not know how to
5426 If you want to pass an option that takes an argument, you must use
5427 @option{-Xpreprocessor} twice, once for the option and once for the argument.
5430 @include cppopts.texi
5432 @node Assembler Options
5433 @section Passing Options to the Assembler
5435 @c prevent bad page break with this line
5436 You can pass options to the assembler.
5439 @item -Wa,@var{option}
5441 Pass @var{option} as an option to the assembler. If @var{option}
5442 contains commas, it is split into multiple options at the commas.
5444 @item -Xassembler @var{option}
5446 Pass @var{option} as an option to the assembler. You can use this to
5447 supply system-specific assembler options which GCC does not know how to
5450 If you want to pass an option that takes an argument, you must use
5451 @option{-Xassembler} twice, once for the option and once for the argument.
5456 @section Options for Linking
5457 @cindex link options
5458 @cindex options, linking
5460 These options come into play when the compiler links object files into
5461 an executable output file. They are meaningless if the compiler is
5462 not doing a link step.
5466 @item @var{object-file-name}
5467 A file name that does not end in a special recognized suffix is
5468 considered to name an object file or library. (Object files are
5469 distinguished from libraries by the linker according to the file
5470 contents.) If linking is done, these object files are used as input
5479 If any of these options is used, then the linker is not run, and
5480 object file names should not be used as arguments. @xref{Overall
5484 @item -l@var{library}
5485 @itemx -l @var{library}
5487 Search the library named @var{library} when linking. (The second
5488 alternative with the library as a separate argument is only for
5489 POSIX compliance and is not recommended.)
5491 It makes a difference where in the command you write this option; the
5492 linker searches and processes libraries and object files in the order they
5493 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
5494 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
5495 to functions in @samp{z}, those functions may not be loaded.
5497 The linker searches a standard list of directories for the library,
5498 which is actually a file named @file{lib@var{library}.a}. The linker
5499 then uses this file as if it had been specified precisely by name.
5501 The directories searched include several standard system directories
5502 plus any that you specify with @option{-L}.
5504 Normally the files found this way are library files---archive files
5505 whose members are object files. The linker handles an archive file by
5506 scanning through it for members which define symbols that have so far
5507 been referenced but not defined. But if the file that is found is an
5508 ordinary object file, it is linked in the usual fashion. The only
5509 difference between using an @option{-l} option and specifying a file name
5510 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
5511 and searches several directories.
5515 You need this special case of the @option{-l} option in order to
5516 link an Objective-C or Objective-C++ program.
5519 @opindex nostartfiles
5520 Do not use the standard system startup files when linking.
5521 The standard system libraries are used normally, unless @option{-nostdlib}
5522 or @option{-nodefaultlibs} is used.
5524 @item -nodefaultlibs
5525 @opindex nodefaultlibs
5526 Do not use the standard system libraries when linking.
5527 Only the libraries you specify will be passed to the linker.
5528 The standard startup files are used normally, unless @option{-nostartfiles}
5529 is used. The compiler may generate calls to @code{memcmp},
5530 @code{memset}, @code{memcpy} and @code{memmove}.
5531 These entries are usually resolved by entries in
5532 libc. These entry points should be supplied through some other
5533 mechanism when this option is specified.
5537 Do not use the standard system startup files or libraries when linking.
5538 No startup files and only the libraries you specify will be passed to
5539 the linker. The compiler may generate calls to @code{memcmp}, @code{memset},
5540 @code{memcpy} and @code{memmove}.
5541 These entries are usually resolved by entries in
5542 libc. These entry points should be supplied through some other
5543 mechanism when this option is specified.
5545 @cindex @option{-lgcc}, use with @option{-nostdlib}
5546 @cindex @option{-nostdlib} and unresolved references
5547 @cindex unresolved references and @option{-nostdlib}
5548 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
5549 @cindex @option{-nodefaultlibs} and unresolved references
5550 @cindex unresolved references and @option{-nodefaultlibs}
5551 One of the standard libraries bypassed by @option{-nostdlib} and
5552 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
5553 that GCC uses to overcome shortcomings of particular machines, or special
5554 needs for some languages.
5555 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
5556 Collection (GCC) Internals},
5557 for more discussion of @file{libgcc.a}.)
5558 In most cases, you need @file{libgcc.a} even when you want to avoid
5559 other standard libraries. In other words, when you specify @option{-nostdlib}
5560 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
5561 This ensures that you have no unresolved references to internal GCC
5562 library subroutines. (For example, @samp{__main}, used to ensure C++
5563 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
5564 GNU Compiler Collection (GCC) Internals}.)
5568 Produce a position independent executable on targets which support it.
5569 For predictable results, you must also specify the same set of options
5570 that were used to generate code (@option{-fpie}, @option{-fPIE},
5571 or model suboptions) when you specify this option.
5575 Remove all symbol table and relocation information from the executable.
5579 On systems that support dynamic linking, this prevents linking with the shared
5580 libraries. On other systems, this option has no effect.
5584 Produce a shared object which can then be linked with other objects to
5585 form an executable. Not all systems support this option. For predictable
5586 results, you must also specify the same set of options that were used to
5587 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
5588 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
5589 needs to build supplementary stub code for constructors to work. On
5590 multi-libbed systems, @samp{gcc -shared} must select the correct support
5591 libraries to link against. Failing to supply the correct flags may lead
5592 to subtle defects. Supplying them in cases where they are not necessary
5595 @item -shared-libgcc
5596 @itemx -static-libgcc
5597 @opindex shared-libgcc
5598 @opindex static-libgcc
5599 On systems that provide @file{libgcc} as a shared library, these options
5600 force the use of either the shared or static version respectively.
5601 If no shared version of @file{libgcc} was built when the compiler was
5602 configured, these options have no effect.
5604 There are several situations in which an application should use the
5605 shared @file{libgcc} instead of the static version. The most common
5606 of these is when the application wishes to throw and catch exceptions
5607 across different shared libraries. In that case, each of the libraries
5608 as well as the application itself should use the shared @file{libgcc}.
5610 Therefore, the G++ and GCJ drivers automatically add
5611 @option{-shared-libgcc} whenever you build a shared library or a main
5612 executable, because C++ and Java programs typically use exceptions, so
5613 this is the right thing to do.
5615 If, instead, you use the GCC driver to create shared libraries, you may
5616 find that they will not always be linked with the shared @file{libgcc}.
5617 If GCC finds, at its configuration time, that you have a non-GNU linker
5618 or a GNU linker that does not support option @option{--eh-frame-hdr},
5619 it will link the shared version of @file{libgcc} into shared libraries
5620 by default. Otherwise, it will take advantage of the linker and optimize
5621 away the linking with the shared version of @file{libgcc}, linking with
5622 the static version of libgcc by default. This allows exceptions to
5623 propagate through such shared libraries, without incurring relocation
5624 costs at library load time.
5626 However, if a library or main executable is supposed to throw or catch
5627 exceptions, you must link it using the G++ or GCJ driver, as appropriate
5628 for the languages used in the program, or using the option
5629 @option{-shared-libgcc}, such that it is linked with the shared
5634 Bind references to global symbols when building a shared object. Warn
5635 about any unresolved references (unless overridden by the link editor
5636 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
5639 @item -Xlinker @var{option}
5641 Pass @var{option} as an option to the linker. You can use this to
5642 supply system-specific linker options which GCC does not know how to
5645 If you want to pass an option that takes an argument, you must use
5646 @option{-Xlinker} twice, once for the option and once for the argument.
5647 For example, to pass @option{-assert definitions}, you must write
5648 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
5649 @option{-Xlinker "-assert definitions"}, because this passes the entire
5650 string as a single argument, which is not what the linker expects.
5652 @item -Wl,@var{option}
5654 Pass @var{option} as an option to the linker. If @var{option} contains
5655 commas, it is split into multiple options at the commas.
5657 @item -u @var{symbol}
5659 Pretend the symbol @var{symbol} is undefined, to force linking of
5660 library modules to define it. You can use @option{-u} multiple times with
5661 different symbols to force loading of additional library modules.
5664 @node Directory Options
5665 @section Options for Directory Search
5666 @cindex directory options
5667 @cindex options, directory search
5670 These options specify directories to search for header files, for
5671 libraries and for parts of the compiler:
5676 Add the directory @var{dir} to the head of the list of directories to be
5677 searched for header files. This can be used to override a system header
5678 file, substituting your own version, since these directories are
5679 searched before the system header file directories. However, you should
5680 not use this option to add directories that contain vendor-supplied
5681 system header files (use @option{-isystem} for that). If you use more than
5682 one @option{-I} option, the directories are scanned in left-to-right
5683 order; the standard system directories come after.
5685 If a standard system include directory, or a directory specified with
5686 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
5687 option will be ignored. The directory will still be searched but as a
5688 system directory at its normal position in the system include chain.
5689 This is to ensure that GCC's procedure to fix buggy system headers and
5690 the ordering for the include_next directive are not inadvertently changed.
5691 If you really need to change the search order for system directories,
5692 use the @option{-nostdinc} and/or @option{-isystem} options.
5694 @item -iquote@var{dir}
5696 Add the directory @var{dir} to the head of the list of directories to
5697 be searched for header files only for the case of @samp{#include
5698 "@var{file}"}; they are not searched for @samp{#include <@var{file}>},
5699 otherwise just like @option{-I}.
5703 Add directory @var{dir} to the list of directories to be searched
5706 @item -B@var{prefix}
5708 This option specifies where to find the executables, libraries,
5709 include files, and data files of the compiler itself.
5711 The compiler driver program runs one or more of the subprograms
5712 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
5713 @var{prefix} as a prefix for each program it tries to run, both with and
5714 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
5716 For each subprogram to be run, the compiler driver first tries the
5717 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
5718 was not specified, the driver tries two standard prefixes, which are
5719 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}. If neither of
5720 those results in a file name that is found, the unmodified program
5721 name is searched for using the directories specified in your
5722 @env{PATH} environment variable.
5724 The compiler will check to see if the path provided by the @option{-B}
5725 refers to a directory, and if necessary it will add a directory
5726 separator character at the end of the path.
5728 @option{-B} prefixes that effectively specify directory names also apply
5729 to libraries in the linker, because the compiler translates these
5730 options into @option{-L} options for the linker. They also apply to
5731 includes files in the preprocessor, because the compiler translates these
5732 options into @option{-isystem} options for the preprocessor. In this case,
5733 the compiler appends @samp{include} to the prefix.
5735 The run-time support file @file{libgcc.a} can also be searched for using
5736 the @option{-B} prefix, if needed. If it is not found there, the two
5737 standard prefixes above are tried, and that is all. The file is left
5738 out of the link if it is not found by those means.
5740 Another way to specify a prefix much like the @option{-B} prefix is to use
5741 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
5744 As a special kludge, if the path provided by @option{-B} is
5745 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
5746 9, then it will be replaced by @file{[dir/]include}. This is to help
5747 with boot-strapping the compiler.
5749 @item -specs=@var{file}
5751 Process @var{file} after the compiler reads in the standard @file{specs}
5752 file, in order to override the defaults that the @file{gcc} driver
5753 program uses when determining what switches to pass to @file{cc1},
5754 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
5755 @option{-specs=@var{file}} can be specified on the command line, and they
5756 are processed in order, from left to right.
5760 This option has been deprecated. Please use @option{-iquote} instead for
5761 @option{-I} directories before the @option{-I-} and remove the @option{-I-}.
5762 Any directories you specify with @option{-I} options before the @option{-I-}
5763 option are searched only for the case of @samp{#include "@var{file}"};
5764 they are not searched for @samp{#include <@var{file}>}.
5766 If additional directories are specified with @option{-I} options after
5767 the @option{-I-}, these directories are searched for all @samp{#include}
5768 directives. (Ordinarily @emph{all} @option{-I} directories are used
5771 In addition, the @option{-I-} option inhibits the use of the current
5772 directory (where the current input file came from) as the first search
5773 directory for @samp{#include "@var{file}"}. There is no way to
5774 override this effect of @option{-I-}. With @option{-I.} you can specify
5775 searching the directory which was current when the compiler was
5776 invoked. That is not exactly the same as what the preprocessor does
5777 by default, but it is often satisfactory.
5779 @option{-I-} does not inhibit the use of the standard system directories
5780 for header files. Thus, @option{-I-} and @option{-nostdinc} are
5787 @section Specifying subprocesses and the switches to pass to them
5790 @command{gcc} is a driver program. It performs its job by invoking a
5791 sequence of other programs to do the work of compiling, assembling and
5792 linking. GCC interprets its command-line parameters and uses these to
5793 deduce which programs it should invoke, and which command-line options
5794 it ought to place on their command lines. This behavior is controlled
5795 by @dfn{spec strings}. In most cases there is one spec string for each
5796 program that GCC can invoke, but a few programs have multiple spec
5797 strings to control their behavior. The spec strings built into GCC can
5798 be overridden by using the @option{-specs=} command-line switch to specify
5801 @dfn{Spec files} are plaintext files that are used to construct spec
5802 strings. They consist of a sequence of directives separated by blank
5803 lines. The type of directive is determined by the first non-whitespace
5804 character on the line and it can be one of the following:
5807 @item %@var{command}
5808 Issues a @var{command} to the spec file processor. The commands that can
5812 @item %include <@var{file}>
5814 Search for @var{file} and insert its text at the current point in the
5817 @item %include_noerr <@var{file}>
5818 @cindex %include_noerr
5819 Just like @samp{%include}, but do not generate an error message if the include
5820 file cannot be found.
5822 @item %rename @var{old_name} @var{new_name}
5824 Rename the spec string @var{old_name} to @var{new_name}.
5828 @item *[@var{spec_name}]:
5829 This tells the compiler to create, override or delete the named spec
5830 string. All lines after this directive up to the next directive or
5831 blank line are considered to be the text for the spec string. If this
5832 results in an empty string then the spec will be deleted. (Or, if the
5833 spec did not exist, then nothing will happened.) Otherwise, if the spec
5834 does not currently exist a new spec will be created. If the spec does
5835 exist then its contents will be overridden by the text of this
5836 directive, unless the first character of that text is the @samp{+}
5837 character, in which case the text will be appended to the spec.
5839 @item [@var{suffix}]:
5840 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
5841 and up to the next directive or blank line are considered to make up the
5842 spec string for the indicated suffix. When the compiler encounters an
5843 input file with the named suffix, it will processes the spec string in
5844 order to work out how to compile that file. For example:
5851 This says that any input file whose name ends in @samp{.ZZ} should be
5852 passed to the program @samp{z-compile}, which should be invoked with the
5853 command-line switch @option{-input} and with the result of performing the
5854 @samp{%i} substitution. (See below.)
5856 As an alternative to providing a spec string, the text that follows a
5857 suffix directive can be one of the following:
5860 @item @@@var{language}
5861 This says that the suffix is an alias for a known @var{language}. This is
5862 similar to using the @option{-x} command-line switch to GCC to specify a
5863 language explicitly. For example:
5870 Says that .ZZ files are, in fact, C++ source files.
5873 This causes an error messages saying:
5876 @var{name} compiler not installed on this system.
5880 GCC already has an extensive list of suffixes built into it.
5881 This directive will add an entry to the end of the list of suffixes, but
5882 since the list is searched from the end backwards, it is effectively
5883 possible to override earlier entries using this technique.
5887 GCC has the following spec strings built into it. Spec files can
5888 override these strings or create their own. Note that individual
5889 targets can also add their own spec strings to this list.
5892 asm Options to pass to the assembler
5893 asm_final Options to pass to the assembler post-processor
5894 cpp Options to pass to the C preprocessor
5895 cc1 Options to pass to the C compiler
5896 cc1plus Options to pass to the C++ compiler
5897 endfile Object files to include at the end of the link
5898 link Options to pass to the linker
5899 lib Libraries to include on the command line to the linker
5900 libgcc Decides which GCC support library to pass to the linker
5901 linker Sets the name of the linker
5902 predefines Defines to be passed to the C preprocessor
5903 signed_char Defines to pass to CPP to say whether @code{char} is signed
5905 startfile Object files to include at the start of the link
5908 Here is a small example of a spec file:
5914 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
5917 This example renames the spec called @samp{lib} to @samp{old_lib} and
5918 then overrides the previous definition of @samp{lib} with a new one.
5919 The new definition adds in some extra command-line options before
5920 including the text of the old definition.
5922 @dfn{Spec strings} are a list of command-line options to be passed to their
5923 corresponding program. In addition, the spec strings can contain
5924 @samp{%}-prefixed sequences to substitute variable text or to
5925 conditionally insert text into the command line. Using these constructs
5926 it is possible to generate quite complex command lines.
5928 Here is a table of all defined @samp{%}-sequences for spec
5929 strings. Note that spaces are not generated automatically around the
5930 results of expanding these sequences. Therefore you can concatenate them
5931 together or combine them with constant text in a single argument.
5935 Substitute one @samp{%} into the program name or argument.
5938 Substitute the name of the input file being processed.
5941 Substitute the basename of the input file being processed.
5942 This is the substring up to (and not including) the last period
5943 and not including the directory.
5946 This is the same as @samp{%b}, but include the file suffix (text after
5950 Marks the argument containing or following the @samp{%d} as a
5951 temporary file name, so that that file will be deleted if GCC exits
5952 successfully. Unlike @samp{%g}, this contributes no text to the
5955 @item %g@var{suffix}
5956 Substitute a file name that has suffix @var{suffix} and is chosen
5957 once per compilation, and mark the argument in the same way as
5958 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
5959 name is now chosen in a way that is hard to predict even when previously
5960 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
5961 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
5962 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
5963 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
5964 was simply substituted with a file name chosen once per compilation,
5965 without regard to any appended suffix (which was therefore treated
5966 just like ordinary text), making such attacks more likely to succeed.
5968 @item %u@var{suffix}
5969 Like @samp{%g}, but generates a new temporary file name even if
5970 @samp{%u@var{suffix}} was already seen.
5972 @item %U@var{suffix}
5973 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
5974 new one if there is no such last file name. In the absence of any
5975 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
5976 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
5977 would involve the generation of two distinct file names, one
5978 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
5979 simply substituted with a file name chosen for the previous @samp{%u},
5980 without regard to any appended suffix.
5982 @item %j@var{suffix}
5983 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
5984 writable, and if save-temps is off; otherwise, substitute the name
5985 of a temporary file, just like @samp{%u}. This temporary file is not
5986 meant for communication between processes, but rather as a junk
5989 @item %|@var{suffix}
5990 @itemx %m@var{suffix}
5991 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
5992 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
5993 all. These are the two most common ways to instruct a program that it
5994 should read from standard input or write to standard output. If you
5995 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
5996 construct: see for example @file{f/lang-specs.h}.
5998 @item %.@var{SUFFIX}
5999 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
6000 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
6001 terminated by the next space or %.
6004 Marks the argument containing or following the @samp{%w} as the
6005 designated output file of this compilation. This puts the argument
6006 into the sequence of arguments that @samp{%o} will substitute later.
6009 Substitutes the names of all the output files, with spaces
6010 automatically placed around them. You should write spaces
6011 around the @samp{%o} as well or the results are undefined.
6012 @samp{%o} is for use in the specs for running the linker.
6013 Input files whose names have no recognized suffix are not compiled
6014 at all, but they are included among the output files, so they will
6018 Substitutes the suffix for object files. Note that this is
6019 handled specially when it immediately follows @samp{%g, %u, or %U},
6020 because of the need for those to form complete file names. The
6021 handling is such that @samp{%O} is treated exactly as if it had already
6022 been substituted, except that @samp{%g, %u, and %U} do not currently
6023 support additional @var{suffix} characters following @samp{%O} as they would
6024 following, for example, @samp{.o}.
6027 Substitutes the standard macro predefinitions for the
6028 current target machine. Use this when running @code{cpp}.
6031 Like @samp{%p}, but puts @samp{__} before and after the name of each
6032 predefined macro, except for macros that start with @samp{__} or with
6033 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
6037 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
6038 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}), and
6039 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
6043 Current argument is the name of a library or startup file of some sort.
6044 Search for that file in a standard list of directories and substitute
6045 the full name found.
6048 Print @var{str} as an error message. @var{str} is terminated by a newline.
6049 Use this when inconsistent options are detected.
6052 Substitute the contents of spec string @var{name} at this point.
6055 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
6057 @item %x@{@var{option}@}
6058 Accumulate an option for @samp{%X}.
6061 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
6065 Output the accumulated assembler options specified by @option{-Wa}.
6068 Output the accumulated preprocessor options specified by @option{-Wp}.
6071 Process the @code{asm} spec. This is used to compute the
6072 switches to be passed to the assembler.
6075 Process the @code{asm_final} spec. This is a spec string for
6076 passing switches to an assembler post-processor, if such a program is
6080 Process the @code{link} spec. This is the spec for computing the
6081 command line passed to the linker. Typically it will make use of the
6082 @samp{%L %G %S %D and %E} sequences.
6085 Dump out a @option{-L} option for each directory that GCC believes might
6086 contain startup files. If the target supports multilibs then the
6087 current multilib directory will be prepended to each of these paths.
6090 Output the multilib directory with directory separators replaced with
6091 @samp{_}. If multilib directories are not set, or the multilib directory is
6092 @file{.} then this option emits nothing.
6095 Process the @code{lib} spec. This is a spec string for deciding which
6096 libraries should be included on the command line to the linker.
6099 Process the @code{libgcc} spec. This is a spec string for deciding
6100 which GCC support library should be included on the command line to the linker.
6103 Process the @code{startfile} spec. This is a spec for deciding which
6104 object files should be the first ones passed to the linker. Typically
6105 this might be a file named @file{crt0.o}.
6108 Process the @code{endfile} spec. This is a spec string that specifies
6109 the last object files that will be passed to the linker.
6112 Process the @code{cpp} spec. This is used to construct the arguments
6113 to be passed to the C preprocessor.
6116 Process the @code{cc1} spec. This is used to construct the options to be
6117 passed to the actual C compiler (@samp{cc1}).
6120 Process the @code{cc1plus} spec. This is used to construct the options to be
6121 passed to the actual C++ compiler (@samp{cc1plus}).
6124 Substitute the variable part of a matched option. See below.
6125 Note that each comma in the substituted string is replaced by
6129 Remove all occurrences of @code{-S} from the command line. Note---this
6130 command is position dependent. @samp{%} commands in the spec string
6131 before this one will see @code{-S}, @samp{%} commands in the spec string
6132 after this one will not.
6134 @item %:@var{function}(@var{args})
6135 Call the named function @var{function}, passing it @var{args}.
6136 @var{args} is first processed as a nested spec string, then split
6137 into an argument vector in the usual fashion. The function returns
6138 a string which is processed as if it had appeared literally as part
6139 of the current spec.
6141 The following built-in spec functions are provided:
6144 @item @code{if-exists}
6145 The @code{if-exists} spec function takes one argument, an absolute
6146 pathname to a file. If the file exists, @code{if-exists} returns the
6147 pathname. Here is a small example of its usage:
6151 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
6154 @item @code{if-exists-else}
6155 The @code{if-exists-else} spec function is similar to the @code{if-exists}
6156 spec function, except that it takes two arguments. The first argument is
6157 an absolute pathname to a file. If the file exists, @code{if-exists-else}
6158 returns the pathname. If it does not exist, it returns the second argument.
6159 This way, @code{if-exists-else} can be used to select one file or another,
6160 based on the existence of the first. Here is a small example of its usage:
6164 crt0%O%s %:if-exists(crti%O%s) \
6165 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
6170 Substitutes the @code{-S} switch, if that switch was given to GCC@.
6171 If that switch was not specified, this substitutes nothing. Note that
6172 the leading dash is omitted when specifying this option, and it is
6173 automatically inserted if the substitution is performed. Thus the spec
6174 string @samp{%@{foo@}} would match the command-line option @option{-foo}
6175 and would output the command line option @option{-foo}.
6177 @item %W@{@code{S}@}
6178 Like %@{@code{S}@} but mark last argument supplied within as a file to be
6181 @item %@{@code{S}*@}
6182 Substitutes all the switches specified to GCC whose names start
6183 with @code{-S}, but which also take an argument. This is used for
6184 switches like @option{-o}, @option{-D}, @option{-I}, etc.
6185 GCC considers @option{-o foo} as being
6186 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
6187 text, including the space. Thus two arguments would be generated.
6189 @item %@{@code{S}*&@code{T}*@}
6190 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
6191 (the order of @code{S} and @code{T} in the spec is not significant).
6192 There can be any number of ampersand-separated variables; for each the
6193 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
6195 @item %@{@code{S}:@code{X}@}
6196 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
6198 @item %@{!@code{S}:@code{X}@}
6199 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
6201 @item %@{@code{S}*:@code{X}@}
6202 Substitutes @code{X} if one or more switches whose names start with
6203 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
6204 once, no matter how many such switches appeared. However, if @code{%*}
6205 appears somewhere in @code{X}, then @code{X} will be substituted once
6206 for each matching switch, with the @code{%*} replaced by the part of
6207 that switch that matched the @code{*}.
6209 @item %@{.@code{S}:@code{X}@}
6210 Substitutes @code{X}, if processing a file with suffix @code{S}.
6212 @item %@{!.@code{S}:@code{X}@}
6213 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
6215 @item %@{@code{S}|@code{P}:@code{X}@}
6216 Substitutes @code{X} if either @code{-S} or @code{-P} was given to GCC@.
6217 This may be combined with @samp{!}, @samp{.}, and @code{*} sequences as well,
6218 although they have a stronger binding than the @samp{|}. If @code{%*}
6219 appears in @code{X}, all of the alternatives must be starred, and only
6220 the first matching alternative is substituted.
6222 For example, a spec string like this:
6225 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
6228 will output the following command-line options from the following input
6229 command-line options:
6234 -d fred.c -foo -baz -boggle
6235 -d jim.d -bar -baz -boggle
6238 @item %@{S:X; T:Y; :D@}
6240 If @code{S} was given to GCC, substitutes @code{X}; else if @code{T} was
6241 given to GCC, substitutes @code{Y}; else substitutes @code{D}. There can
6242 be as many clauses as you need. This may be combined with @code{.},
6243 @code{!}, @code{|}, and @code{*} as needed.
6248 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
6249 construct may contain other nested @samp{%} constructs or spaces, or
6250 even newlines. They are processed as usual, as described above.
6251 Trailing white space in @code{X} is ignored. White space may also
6252 appear anywhere on the left side of the colon in these constructs,
6253 except between @code{.} or @code{*} and the corresponding word.
6255 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
6256 handled specifically in these constructs. If another value of
6257 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
6258 @option{-W} switch is found later in the command line, the earlier
6259 switch value is ignored, except with @{@code{S}*@} where @code{S} is
6260 just one letter, which passes all matching options.
6262 The character @samp{|} at the beginning of the predicate text is used to
6263 indicate that a command should be piped to the following command, but
6264 only if @option{-pipe} is specified.
6266 It is built into GCC which switches take arguments and which do not.
6267 (You might think it would be useful to generalize this to allow each
6268 compiler's spec to say which switches take arguments. But this cannot
6269 be done in a consistent fashion. GCC cannot even decide which input
6270 files have been specified without knowing which switches take arguments,
6271 and it must know which input files to compile in order to tell which
6274 GCC also knows implicitly that arguments starting in @option{-l} are to be
6275 treated as compiler output files, and passed to the linker in their
6276 proper position among the other output files.
6278 @c man begin OPTIONS
6280 @node Target Options
6281 @section Specifying Target Machine and Compiler Version
6282 @cindex target options
6283 @cindex cross compiling
6284 @cindex specifying machine version
6285 @cindex specifying compiler version and target machine
6286 @cindex compiler version, specifying
6287 @cindex target machine, specifying
6289 The usual way to run GCC is to run the executable called @file{gcc}, or
6290 @file{<machine>-gcc} when cross-compiling, or
6291 @file{<machine>-gcc-<version>} to run a version other than the one that
6292 was installed last. Sometimes this is inconvenient, so GCC provides
6293 options that will switch to another cross-compiler or version.
6296 @item -b @var{machine}
6298 The argument @var{machine} specifies the target machine for compilation.
6300 The value to use for @var{machine} is the same as was specified as the
6301 machine type when configuring GCC as a cross-compiler. For
6302 example, if a cross-compiler was configured with @samp{configure
6303 i386v}, meaning to compile for an 80386 running System V, then you
6304 would specify @option{-b i386v} to run that cross compiler.
6306 @item -V @var{version}
6308 The argument @var{version} specifies which version of GCC to run.
6309 This is useful when multiple versions are installed. For example,
6310 @var{version} might be @samp{2.0}, meaning to run GCC version 2.0.
6313 The @option{-V} and @option{-b} options work by running the
6314 @file{<machine>-gcc-<version>} executable, so there's no real reason to
6315 use them if you can just run that directly.
6317 @node Submodel Options
6318 @section Hardware Models and Configurations
6319 @cindex submodel options
6320 @cindex specifying hardware config
6321 @cindex hardware models and configurations, specifying
6322 @cindex machine dependent options
6324 Earlier we discussed the standard option @option{-b} which chooses among
6325 different installed compilers for completely different target
6326 machines, such as VAX vs.@: 68000 vs.@: 80386.
6328 In addition, each of these target machine types can have its own
6329 special options, starting with @samp{-m}, to choose among various
6330 hardware models or configurations---for example, 68010 vs 68020,
6331 floating coprocessor or none. A single installed version of the
6332 compiler can compile for any model or configuration, according to the
6335 Some configurations of the compiler also support additional special
6336 options, usually for compatibility with other compilers on the same
6339 These options are defined by the macro @code{TARGET_SWITCHES} in the
6340 machine description. The default for the options is also defined by
6341 that macro, which enables you to change the defaults.
6343 @c This list is ordered alphanumerically by subsection name.
6344 @c It should be the same order and spelling as these options are listed
6345 @c in Machine Dependent Options
6353 * DEC Alpha Options::
6354 * DEC Alpha/VMS Options::
6358 * i386 and x86-64 Options::
6370 * RS/6000 and PowerPC Options::
6371 * S/390 and zSeries Options::
6374 * System V Options::
6375 * TMS320C3x/C4x Options::
6379 * Xstormy16 Options::
6385 @subsection ARC Options
6388 These options are defined for ARC implementations:
6393 Compile code for little endian mode. This is the default.
6397 Compile code for big endian mode.
6400 @opindex mmangle-cpu
6401 Prepend the name of the cpu to all public symbol names.
6402 In multiple-processor systems, there are many ARC variants with different
6403 instruction and register set characteristics. This flag prevents code
6404 compiled for one cpu to be linked with code compiled for another.
6405 No facility exists for handling variants that are ``almost identical''.
6406 This is an all or nothing option.
6408 @item -mcpu=@var{cpu}
6410 Compile code for ARC variant @var{cpu}.
6411 Which variants are supported depend on the configuration.
6412 All variants support @option{-mcpu=base}, this is the default.
6414 @item -mtext=@var{text-section}
6415 @itemx -mdata=@var{data-section}
6416 @itemx -mrodata=@var{readonly-data-section}
6420 Put functions, data, and readonly data in @var{text-section},
6421 @var{data-section}, and @var{readonly-data-section} respectively
6422 by default. This can be overridden with the @code{section} attribute.
6423 @xref{Variable Attributes}.
6428 @subsection ARM Options
6431 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
6435 @item -mabi=@var{name}
6437 Generate code for the specified ABI. Permissible values are: @samp{apcs-gnu},
6438 @samp{atpcs}, @samp{aapcs} and @samp{iwmmxt}.
6441 @opindex mapcs-frame
6442 Generate a stack frame that is compliant with the ARM Procedure Call
6443 Standard for all functions, even if this is not strictly necessary for
6444 correct execution of the code. Specifying @option{-fomit-frame-pointer}
6445 with this option will cause the stack frames not to be generated for
6446 leaf functions. The default is @option{-mno-apcs-frame}.
6450 This is a synonym for @option{-mapcs-frame}.
6453 @c not currently implemented
6454 @item -mapcs-stack-check
6455 @opindex mapcs-stack-check
6456 Generate code to check the amount of stack space available upon entry to
6457 every function (that actually uses some stack space). If there is
6458 insufficient space available then either the function
6459 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
6460 called, depending upon the amount of stack space required. The run time
6461 system is required to provide these functions. The default is
6462 @option{-mno-apcs-stack-check}, since this produces smaller code.
6464 @c not currently implemented
6466 @opindex mapcs-float
6467 Pass floating point arguments using the float point registers. This is
6468 one of the variants of the APCS@. This option is recommended if the
6469 target hardware has a floating point unit or if a lot of floating point
6470 arithmetic is going to be performed by the code. The default is
6471 @option{-mno-apcs-float}, since integer only code is slightly increased in
6472 size if @option{-mapcs-float} is used.
6474 @c not currently implemented
6475 @item -mapcs-reentrant
6476 @opindex mapcs-reentrant
6477 Generate reentrant, position independent code. The default is
6478 @option{-mno-apcs-reentrant}.
6481 @item -mthumb-interwork
6482 @opindex mthumb-interwork
6483 Generate code which supports calling between the ARM and Thumb
6484 instruction sets. Without this option the two instruction sets cannot
6485 be reliably used inside one program. The default is
6486 @option{-mno-thumb-interwork}, since slightly larger code is generated
6487 when @option{-mthumb-interwork} is specified.
6489 @item -mno-sched-prolog
6490 @opindex mno-sched-prolog
6491 Prevent the reordering of instructions in the function prolog, or the
6492 merging of those instruction with the instructions in the function's
6493 body. This means that all functions will start with a recognizable set
6494 of instructions (or in fact one of a choice from a small set of
6495 different function prologues), and this information can be used to
6496 locate the start if functions inside an executable piece of code. The
6497 default is @option{-msched-prolog}.
6500 @opindex mhard-float
6501 Generate output containing floating point instructions. This is the
6505 @opindex msoft-float
6506 Generate output containing library calls for floating point.
6507 @strong{Warning:} the requisite libraries are not available for all ARM
6508 targets. Normally the facilities of the machine's usual C compiler are
6509 used, but this cannot be done directly in cross-compilation. You must make
6510 your own arrangements to provide suitable library functions for
6513 @option{-msoft-float} changes the calling convention in the output file;
6514 therefore, it is only useful if you compile @emph{all} of a program with
6515 this option. In particular, you need to compile @file{libgcc.a}, the
6516 library that comes with GCC, with @option{-msoft-float} in order for
6519 @item -mfloat-abi=@var{name}
6521 Specifies which ABI to use for floating point values. Permissible values
6522 are: @samp{soft}, @samp{softfp} and @samp{hard}.
6524 @samp{soft} and @samp{hard} are equivalent to @option{-msoft-float}
6525 and @option{-mhard-float} respectively. @samp{softfp} allows the generation
6526 of floating point instructions, but still uses the soft-float calling
6529 @item -mlittle-endian
6530 @opindex mlittle-endian
6531 Generate code for a processor running in little-endian mode. This is
6532 the default for all standard configurations.
6535 @opindex mbig-endian
6536 Generate code for a processor running in big-endian mode; the default is
6537 to compile code for a little-endian processor.
6539 @item -mwords-little-endian
6540 @opindex mwords-little-endian
6541 This option only applies when generating code for big-endian processors.
6542 Generate code for a little-endian word order but a big-endian byte
6543 order. That is, a byte order of the form @samp{32107654}. Note: this
6544 option should only be used if you require compatibility with code for
6545 big-endian ARM processors generated by versions of the compiler prior to
6548 @item -mcpu=@var{name}
6550 This specifies the name of the target ARM processor. GCC uses this name
6551 to determine what kind of instructions it can emit when generating
6552 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
6553 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
6554 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
6555 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
6556 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
6557 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm8},
6558 @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
6559 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
6560 @samp{arm920t}, @samp{arm922t}, @samp{arm946es}, @samp{arm966es},
6561 @samp{arm968es}, @samp{arm926ejs}, @samp{arm940t}, @samp{arm9tdmi},
6562 @samp{arm10tdmi}, @samp{arm1020t}, @samp{arm1026ejs},
6563 @samp{arm10e}, @samp{arm1020e}, @samp{arm1022e},
6564 @samp{arm1136js}, @samp{arm1136jfs} ,@samp{xscale}, @samp{iwmmxt},
6567 @itemx -mtune=@var{name}
6569 This option is very similar to the @option{-mcpu=} option, except that
6570 instead of specifying the actual target processor type, and hence
6571 restricting which instructions can be used, it specifies that GCC should
6572 tune the performance of the code as if the target were of the type
6573 specified in this option, but still choosing the instructions that it
6574 will generate based on the cpu specified by a @option{-mcpu=} option.
6575 For some ARM implementations better performance can be obtained by using
6578 @item -march=@var{name}
6580 This specifies the name of the target ARM architecture. GCC uses this
6581 name to determine what kind of instructions it can emit when generating
6582 assembly code. This option can be used in conjunction with or instead
6583 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
6584 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
6585 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{armv6}, @samp{armv6j},
6586 @samp{iwmmxt}, @samp{ep9312}.
6588 @item -mfpu=@var{name}
6589 @itemx -mfpe=@var{number}
6590 @itemx -mfp=@var{number}
6594 This specifies what floating point hardware (or hardware emulation) is
6595 available on the target. Permissible names are: @samp{fpa}, @samp{fpe2},
6596 @samp{fpe3}, @samp{maverick}, @samp{vfp}. @option{-mfp} and @option{-mfpe}
6597 are synonyms for @option{-mfpu}=@samp{fpe}@var{number}, for compatibility
6598 with older versions of GCC@.
6600 If @option{-msoft-float} is specified this specifies the format of
6601 floating point values.
6603 @item -mstructure-size-boundary=@var{n}
6604 @opindex mstructure-size-boundary
6605 The size of all structures and unions will be rounded up to a multiple
6606 of the number of bits set by this option. Permissible values are 8, 32
6607 and 64. The default value varies for different toolchains. For the COFF
6608 targeted toolchain the default value is 8. A value of 64 is only allowed
6609 if the underlying ABI supports it.
6611 Specifying the larger number can produce faster, more efficient code, but
6612 can also increase the size of the program. Different values are potentially
6613 incompatible. Code compiled with one value cannot necessarily expect to
6614 work with code or libraries compiled with another value, if they exchange
6615 information using structures or unions.
6617 @item -mabort-on-noreturn
6618 @opindex mabort-on-noreturn
6619 Generate a call to the function @code{abort} at the end of a
6620 @code{noreturn} function. It will be executed if the function tries to
6624 @itemx -mno-long-calls
6625 @opindex mlong-calls
6626 @opindex mno-long-calls
6627 Tells the compiler to perform function calls by first loading the
6628 address of the function into a register and then performing a subroutine
6629 call on this register. This switch is needed if the target function
6630 will lie outside of the 64 megabyte addressing range of the offset based
6631 version of subroutine call instruction.
6633 Even if this switch is enabled, not all function calls will be turned
6634 into long calls. The heuristic is that static functions, functions
6635 which have the @samp{short-call} attribute, functions that are inside
6636 the scope of a @samp{#pragma no_long_calls} directive and functions whose
6637 definitions have already been compiled within the current compilation
6638 unit, will not be turned into long calls. The exception to this rule is
6639 that weak function definitions, functions with the @samp{long-call}
6640 attribute or the @samp{section} attribute, and functions that are within
6641 the scope of a @samp{#pragma long_calls} directive, will always be
6642 turned into long calls.
6644 This feature is not enabled by default. Specifying
6645 @option{-mno-long-calls} will restore the default behavior, as will
6646 placing the function calls within the scope of a @samp{#pragma
6647 long_calls_off} directive. Note these switches have no effect on how
6648 the compiler generates code to handle function calls via function
6651 @item -mnop-fun-dllimport
6652 @opindex mnop-fun-dllimport
6653 Disable support for the @code{dllimport} attribute.
6655 @item -msingle-pic-base
6656 @opindex msingle-pic-base
6657 Treat the register used for PIC addressing as read-only, rather than
6658 loading it in the prologue for each function. The run-time system is
6659 responsible for initializing this register with an appropriate value
6660 before execution begins.
6662 @item -mpic-register=@var{reg}
6663 @opindex mpic-register
6664 Specify the register to be used for PIC addressing. The default is R10
6665 unless stack-checking is enabled, when R9 is used.
6667 @item -mcirrus-fix-invalid-insns
6668 @opindex mcirrus-fix-invalid-insns
6669 @opindex mno-cirrus-fix-invalid-insns
6670 Insert NOPs into the instruction stream to in order to work around
6671 problems with invalid Maverick instruction combinations. This option
6672 is only valid if the @option{-mcpu=ep9312} option has been used to
6673 enable generation of instructions for the Cirrus Maverick floating
6674 point co-processor. This option is not enabled by default, since the
6675 problem is only present in older Maverick implementations. The default
6676 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
6679 @item -mpoke-function-name
6680 @opindex mpoke-function-name
6681 Write the name of each function into the text section, directly
6682 preceding the function prologue. The generated code is similar to this:
6686 .ascii "arm_poke_function_name", 0
6689 .word 0xff000000 + (t1 - t0)
6690 arm_poke_function_name
6692 stmfd sp!, @{fp, ip, lr, pc@}
6696 When performing a stack backtrace, code can inspect the value of
6697 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
6698 location @code{pc - 12} and the top 8 bits are set, then we know that
6699 there is a function name embedded immediately preceding this location
6700 and has length @code{((pc[-3]) & 0xff000000)}.
6704 Generate code for the 16-bit Thumb instruction set. The default is to
6705 use the 32-bit ARM instruction set.
6708 @opindex mtpcs-frame
6709 Generate a stack frame that is compliant with the Thumb Procedure Call
6710 Standard for all non-leaf functions. (A leaf function is one that does
6711 not call any other functions.) The default is @option{-mno-tpcs-frame}.
6713 @item -mtpcs-leaf-frame
6714 @opindex mtpcs-leaf-frame
6715 Generate a stack frame that is compliant with the Thumb Procedure Call
6716 Standard for all leaf functions. (A leaf function is one that does
6717 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
6719 @item -mcallee-super-interworking
6720 @opindex mcallee-super-interworking
6721 Gives all externally visible functions in the file being compiled an ARM
6722 instruction set header which switches to Thumb mode before executing the
6723 rest of the function. This allows these functions to be called from
6724 non-interworking code.
6726 @item -mcaller-super-interworking
6727 @opindex mcaller-super-interworking
6728 Allows calls via function pointers (including virtual functions) to
6729 execute correctly regardless of whether the target code has been
6730 compiled for interworking or not. There is a small overhead in the cost
6731 of executing a function pointer if this option is enabled.
6736 @subsection AVR Options
6739 These options are defined for AVR implementations:
6742 @item -mmcu=@var{mcu}
6744 Specify ATMEL AVR instruction set or MCU type.
6746 Instruction set avr1 is for the minimal AVR core, not supported by the C
6747 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
6748 attiny11, attiny12, attiny15, attiny28).
6750 Instruction set avr2 (default) is for the classic AVR core with up to
6751 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
6752 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
6753 at90c8534, at90s8535).
6755 Instruction set avr3 is for the classic AVR core with up to 128K program
6756 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
6758 Instruction set avr4 is for the enhanced AVR core with up to 8K program
6759 memory space (MCU types: atmega8, atmega83, atmega85).
6761 Instruction set avr5 is for the enhanced AVR core with up to 128K program
6762 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
6763 atmega64, atmega128, at43usb355, at94k).
6767 Output instruction sizes to the asm file.
6769 @item -minit-stack=@var{N}
6770 @opindex minit-stack
6771 Specify the initial stack address, which may be a symbol or numeric value,
6772 @samp{__stack} is the default.
6774 @item -mno-interrupts
6775 @opindex mno-interrupts
6776 Generated code is not compatible with hardware interrupts.
6777 Code size will be smaller.
6779 @item -mcall-prologues
6780 @opindex mcall-prologues
6781 Functions prologues/epilogues expanded as call to appropriate
6782 subroutines. Code size will be smaller.
6784 @item -mno-tablejump
6785 @opindex mno-tablejump
6786 Do not generate tablejump insns which sometimes increase code size.
6789 @opindex mtiny-stack
6790 Change only the low 8 bits of the stack pointer.
6794 Assume int to be 8 bit integer. This affects the sizes of all types: A
6795 char will be 1 byte, an int will be 1 byte, an long will be 2 bytes
6796 and long long will be 4 bytes. Please note that this option does not
6797 comply to the C standards, but it will provide you with smaller code
6802 @subsection CRIS Options
6803 @cindex CRIS Options
6805 These options are defined specifically for the CRIS ports.
6808 @item -march=@var{architecture-type}
6809 @itemx -mcpu=@var{architecture-type}
6812 Generate code for the specified architecture. The choices for
6813 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
6814 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX.
6815 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
6818 @item -mtune=@var{architecture-type}
6820 Tune to @var{architecture-type} everything applicable about the generated
6821 code, except for the ABI and the set of available instructions. The
6822 choices for @var{architecture-type} are the same as for
6823 @option{-march=@var{architecture-type}}.
6825 @item -mmax-stack-frame=@var{n}
6826 @opindex mmax-stack-frame
6827 Warn when the stack frame of a function exceeds @var{n} bytes.
6829 @item -melinux-stacksize=@var{n}
6830 @opindex melinux-stacksize
6831 Only available with the @samp{cris-axis-aout} target. Arranges for
6832 indications in the program to the kernel loader that the stack of the
6833 program should be set to @var{n} bytes.
6839 The options @option{-metrax4} and @option{-metrax100} are synonyms for
6840 @option{-march=v3} and @option{-march=v8} respectively.
6842 @item -mmul-bug-workaround
6843 @itemx -mno-mul-bug-workaround
6844 @opindex mmul-bug-workaround
6845 @opindex mno-mul-bug-workaround
6846 Work around a bug in the @code{muls} and @code{mulu} instructions for CPU
6847 models where it applies. This option is active by default.
6851 Enable CRIS-specific verbose debug-related information in the assembly
6852 code. This option also has the effect to turn off the @samp{#NO_APP}
6853 formatted-code indicator to the assembler at the beginning of the
6858 Do not use condition-code results from previous instruction; always emit
6859 compare and test instructions before use of condition codes.
6861 @item -mno-side-effects
6862 @opindex mno-side-effects
6863 Do not emit instructions with side-effects in addressing modes other than
6867 @itemx -mno-stack-align
6869 @itemx -mno-data-align
6870 @itemx -mconst-align
6871 @itemx -mno-const-align
6872 @opindex mstack-align
6873 @opindex mno-stack-align
6874 @opindex mdata-align
6875 @opindex mno-data-align
6876 @opindex mconst-align
6877 @opindex mno-const-align
6878 These options (no-options) arranges (eliminate arrangements) for the
6879 stack-frame, individual data and constants to be aligned for the maximum
6880 single data access size for the chosen CPU model. The default is to
6881 arrange for 32-bit alignment. ABI details such as structure layout are
6882 not affected by these options.
6890 Similar to the stack- data- and const-align options above, these options
6891 arrange for stack-frame, writable data and constants to all be 32-bit,
6892 16-bit or 8-bit aligned. The default is 32-bit alignment.
6894 @item -mno-prologue-epilogue
6895 @itemx -mprologue-epilogue
6896 @opindex mno-prologue-epilogue
6897 @opindex mprologue-epilogue
6898 With @option{-mno-prologue-epilogue}, the normal function prologue and
6899 epilogue that sets up the stack-frame are omitted and no return
6900 instructions or return sequences are generated in the code. Use this
6901 option only together with visual inspection of the compiled code: no
6902 warnings or errors are generated when call-saved registers must be saved,
6903 or storage for local variable needs to be allocated.
6909 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
6910 instruction sequences that load addresses for functions from the PLT part
6911 of the GOT rather than (traditional on other architectures) calls to the
6912 PLT. The default is @option{-mgotplt}.
6916 Legacy no-op option only recognized with the cris-axis-aout target.
6920 Legacy no-op option only recognized with the cris-axis-elf and
6921 cris-axis-linux-gnu targets.
6925 Only recognized with the cris-axis-aout target, where it selects a
6926 GNU/linux-like multilib, include files and instruction set for
6931 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
6935 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
6936 to link with input-output functions from a simulator library. Code,
6937 initialized data and zero-initialized data are allocated consecutively.
6941 Like @option{-sim}, but pass linker options to locate initialized data at
6942 0x40000000 and zero-initialized data at 0x80000000.
6945 @node Darwin Options
6946 @subsection Darwin Options
6947 @cindex Darwin options
6949 These options are defined for all architectures running the Darwin operating
6950 system. They are useful for compatibility with other Mac OS compilers.
6955 Add the framework directory @var{dir} to the head of the list of
6956 directories to be searched for header files. These directories are
6957 interleaved with those specified by @option{-I} options and are
6958 scanned in a left-to-right order.
6960 A framework directory is a directory with frameworks in it. A
6961 framework is a directory with a @samp{"Headers"} and/or
6962 @samp{"PrivateHeaders"} directory contained directly in it that ends
6963 in @samp{".framework"}. The name of a framework is the name of this
6964 directory excluding the @samp{".framework"}. Headers associated with
6965 the framework are found in one of those two directories, with
6966 @samp{"Headers"} being searched first. A subframework is a framework
6967 directory that is in a framework's @samp{"Frameworks"} directory.
6968 Includes of subframework headers can only appear in a header of a
6969 framework that contains the subframework, or in a sibling subframework
6970 header. Two subframeworks are siblings if they occur in the same
6971 framework. A subframework should not have the same name as a
6972 framework, a warning will be issued if this is violated. Currently a
6973 subframework cannot have subframeworks, in the future, the mechanism
6974 may be extended to support this. The standard frameworks can be found
6975 in @samp{"/System/Library/Frameworks"} and
6976 @samp{"/Library/Frameworks"}. An example include looks like
6977 @code{#include <Framework/header.h>}, where @samp{Framework} denotes
6978 the name of the framework and header.h is found in the
6979 @samp{"PrivateHeaders"} or @samp{"Headers"} directory.
6983 Emit debugging information for symbols that are used. For STABS
6984 debugging format, this enables @option{-feliminate-unused-debug-symbols}.
6985 This is by default ON.
6989 Emit debugging information for all symbols and types.
6991 @item -mone-byte-bool
6992 @opindex -mone-byte-bool
6993 Override the defaults for @samp{bool} so that @samp{sizeof(bool)==1}.
6994 By default @samp{sizeof(bool)} is @samp{4} when compiling for
6995 Darwin/PowerPC and @samp{1} when compiling for Darwin/x86, so this
6996 option has no effect on x86.
6998 @strong{Warning:} The @option{-mone-byte-bool} switch causes GCC
6999 to generate code that is not binary compatible with code generated
7000 without that switch. Using this switch may require recompiling all
7001 other modules in a program, including system libraries. Use this
7002 switch to conform to a non-default data model.
7006 Loads all members of static archive libraries.
7007 See man ld(1) for more information.
7009 @item -arch_errors_fatal
7010 @opindex arch_errors_fatal
7011 Cause the errors having to do with files that have the wrong architecture
7015 @opindex bind_at_load
7016 Causes the output file to be marked such that the dynamic linker will
7017 bind all undefined references when the file is loaded or launched.
7021 Produce a Mach-o bundle format file.
7022 See man ld(1) for more information.
7024 @item -bundle_loader @var{executable}
7025 @opindex bundle_loader
7026 This specifies the @var{executable} that will be loading the build
7027 output file being linked. See man ld(1) for more information.
7029 @item -allowable_client @var{client_name}
7033 @itemx -compatibility_version
7034 @itemx -current_version
7036 @itemx -dependency-file
7038 @itemx -dylinker_install_name
7041 @itemx -exported_symbols_list
7043 @itemx -flat_namespace
7044 @itemx -force_cpusubtype_ALL
7045 @itemx -force_flat_namespace
7046 @itemx -headerpad_max_install_names
7049 @itemx -install_name
7050 @itemx -keep_private_externs
7051 @itemx -multi_module
7052 @itemx -multiply_defined
7053 @itemx -multiply_defined_unused
7055 @itemx -no_dead_strip_inits_and_terms
7056 @itemx -nofixprebinding
7059 @itemx -noseglinkedit
7060 @itemx -pagezero_size
7062 @itemx -prebind_all_twolevel_modules
7063 @itemx -private_bundle
7064 @itemx -read_only_relocs
7066 @itemx -sectobjectsymbols
7070 @itemx -sectobjectsymbols
7072 @itemx -seg_addr_table
7073 @itemx -seg_addr_table_filename
7076 @itemx -segs_read_only_addr
7077 @itemx -segs_read_write_addr
7078 @itemx -single_module
7081 @itemx -sub_umbrella
7082 @itemx -twolevel_namespace
7085 @itemx -unexported_symbols_list
7086 @itemx -weak_reference_mismatches
7089 @opindex allowable_client
7091 @opindex client_name
7092 @opindex compatibility_version
7093 @opindex current_version
7095 @opindex dependency-file
7097 @opindex dylinker_install_name
7100 @opindex exported_symbols_list
7102 @opindex flat_namespace
7103 @opindex force_cpusubtype_ALL
7104 @opindex force_flat_namespace
7105 @opindex headerpad_max_install_names
7108 @opindex install_name
7109 @opindex keep_private_externs
7110 @opindex multi_module
7111 @opindex multiply_defined
7112 @opindex multiply_defined_unused
7114 @opindex no_dead_strip_inits_and_terms
7115 @opindex nofixprebinding
7116 @opindex nomultidefs
7118 @opindex noseglinkedit
7119 @opindex pagezero_size
7121 @opindex prebind_all_twolevel_modules
7122 @opindex private_bundle
7123 @opindex read_only_relocs
7125 @opindex sectobjectsymbols
7129 @opindex sectobjectsymbols
7131 @opindex seg_addr_table
7132 @opindex seg_addr_table_filename
7133 @opindex seglinkedit
7135 @opindex segs_read_only_addr
7136 @opindex segs_read_write_addr
7137 @opindex single_module
7139 @opindex sub_library
7140 @opindex sub_umbrella
7141 @opindex twolevel_namespace
7144 @opindex unexported_symbols_list
7145 @opindex weak_reference_mismatches
7146 @opindex whatsloaded
7148 These options are available for Darwin linker. Darwin linker man page
7149 describes them in detail.
7152 @node DEC Alpha Options
7153 @subsection DEC Alpha Options
7155 These @samp{-m} options are defined for the DEC Alpha implementations:
7158 @item -mno-soft-float
7160 @opindex mno-soft-float
7161 @opindex msoft-float
7162 Use (do not use) the hardware floating-point instructions for
7163 floating-point operations. When @option{-msoft-float} is specified,
7164 functions in @file{libgcc.a} will be used to perform floating-point
7165 operations. Unless they are replaced by routines that emulate the
7166 floating-point operations, or compiled in such a way as to call such
7167 emulations routines, these routines will issue floating-point
7168 operations. If you are compiling for an Alpha without floating-point
7169 operations, you must ensure that the library is built so as not to call
7172 Note that Alpha implementations without floating-point operations are
7173 required to have floating-point registers.
7178 @opindex mno-fp-regs
7179 Generate code that uses (does not use) the floating-point register set.
7180 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
7181 register set is not used, floating point operands are passed in integer
7182 registers as if they were integers and floating-point results are passed
7183 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
7184 so any function with a floating-point argument or return value called by code
7185 compiled with @option{-mno-fp-regs} must also be compiled with that
7188 A typical use of this option is building a kernel that does not use,
7189 and hence need not save and restore, any floating-point registers.
7193 The Alpha architecture implements floating-point hardware optimized for
7194 maximum performance. It is mostly compliant with the IEEE floating
7195 point standard. However, for full compliance, software assistance is
7196 required. This option generates code fully IEEE compliant code
7197 @emph{except} that the @var{inexact-flag} is not maintained (see below).
7198 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
7199 defined during compilation. The resulting code is less efficient but is
7200 able to correctly support denormalized numbers and exceptional IEEE
7201 values such as not-a-number and plus/minus infinity. Other Alpha
7202 compilers call this option @option{-ieee_with_no_inexact}.
7204 @item -mieee-with-inexact
7205 @opindex mieee-with-inexact
7206 This is like @option{-mieee} except the generated code also maintains
7207 the IEEE @var{inexact-flag}. Turning on this option causes the
7208 generated code to implement fully-compliant IEEE math. In addition to
7209 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
7210 macro. On some Alpha implementations the resulting code may execute
7211 significantly slower than the code generated by default. Since there is
7212 very little code that depends on the @var{inexact-flag}, you should
7213 normally not specify this option. Other Alpha compilers call this
7214 option @option{-ieee_with_inexact}.
7216 @item -mfp-trap-mode=@var{trap-mode}
7217 @opindex mfp-trap-mode
7218 This option controls what floating-point related traps are enabled.
7219 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
7220 The trap mode can be set to one of four values:
7224 This is the default (normal) setting. The only traps that are enabled
7225 are the ones that cannot be disabled in software (e.g., division by zero
7229 In addition to the traps enabled by @samp{n}, underflow traps are enabled
7233 Like @samp{su}, but the instructions are marked to be safe for software
7234 completion (see Alpha architecture manual for details).
7237 Like @samp{su}, but inexact traps are enabled as well.
7240 @item -mfp-rounding-mode=@var{rounding-mode}
7241 @opindex mfp-rounding-mode
7242 Selects the IEEE rounding mode. Other Alpha compilers call this option
7243 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
7248 Normal IEEE rounding mode. Floating point numbers are rounded towards
7249 the nearest machine number or towards the even machine number in case
7253 Round towards minus infinity.
7256 Chopped rounding mode. Floating point numbers are rounded towards zero.
7259 Dynamic rounding mode. A field in the floating point control register
7260 (@var{fpcr}, see Alpha architecture reference manual) controls the
7261 rounding mode in effect. The C library initializes this register for
7262 rounding towards plus infinity. Thus, unless your program modifies the
7263 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
7266 @item -mtrap-precision=@var{trap-precision}
7267 @opindex mtrap-precision
7268 In the Alpha architecture, floating point traps are imprecise. This
7269 means without software assistance it is impossible to recover from a
7270 floating trap and program execution normally needs to be terminated.
7271 GCC can generate code that can assist operating system trap handlers
7272 in determining the exact location that caused a floating point trap.
7273 Depending on the requirements of an application, different levels of
7274 precisions can be selected:
7278 Program precision. This option is the default and means a trap handler
7279 can only identify which program caused a floating point exception.
7282 Function precision. The trap handler can determine the function that
7283 caused a floating point exception.
7286 Instruction precision. The trap handler can determine the exact
7287 instruction that caused a floating point exception.
7290 Other Alpha compilers provide the equivalent options called
7291 @option{-scope_safe} and @option{-resumption_safe}.
7293 @item -mieee-conformant
7294 @opindex mieee-conformant
7295 This option marks the generated code as IEEE conformant. You must not
7296 use this option unless you also specify @option{-mtrap-precision=i} and either
7297 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
7298 is to emit the line @samp{.eflag 48} in the function prologue of the
7299 generated assembly file. Under DEC Unix, this has the effect that
7300 IEEE-conformant math library routines will be linked in.
7302 @item -mbuild-constants
7303 @opindex mbuild-constants
7304 Normally GCC examines a 32- or 64-bit integer constant to
7305 see if it can construct it from smaller constants in two or three
7306 instructions. If it cannot, it will output the constant as a literal and
7307 generate code to load it from the data segment at runtime.
7309 Use this option to require GCC to construct @emph{all} integer constants
7310 using code, even if it takes more instructions (the maximum is six).
7312 You would typically use this option to build a shared library dynamic
7313 loader. Itself a shared library, it must relocate itself in memory
7314 before it can find the variables and constants in its own data segment.
7320 Select whether to generate code to be assembled by the vendor-supplied
7321 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
7339 Indicate whether GCC should generate code to use the optional BWX,
7340 CIX, FIX and MAX instruction sets. The default is to use the instruction
7341 sets supported by the CPU type specified via @option{-mcpu=} option or that
7342 of the CPU on which GCC was built if none was specified.
7347 @opindex mfloat-ieee
7348 Generate code that uses (does not use) VAX F and G floating point
7349 arithmetic instead of IEEE single and double precision.
7351 @item -mexplicit-relocs
7352 @itemx -mno-explicit-relocs
7353 @opindex mexplicit-relocs
7354 @opindex mno-explicit-relocs
7355 Older Alpha assemblers provided no way to generate symbol relocations
7356 except via assembler macros. Use of these macros does not allow
7357 optimal instruction scheduling. GNU binutils as of version 2.12
7358 supports a new syntax that allows the compiler to explicitly mark
7359 which relocations should apply to which instructions. This option
7360 is mostly useful for debugging, as GCC detects the capabilities of
7361 the assembler when it is built and sets the default accordingly.
7365 @opindex msmall-data
7366 @opindex mlarge-data
7367 When @option{-mexplicit-relocs} is in effect, static data is
7368 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
7369 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
7370 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
7371 16-bit relocations off of the @code{$gp} register. This limits the
7372 size of the small data area to 64KB, but allows the variables to be
7373 directly accessed via a single instruction.
7375 The default is @option{-mlarge-data}. With this option the data area
7376 is limited to just below 2GB. Programs that require more than 2GB of
7377 data must use @code{malloc} or @code{mmap} to allocate the data in the
7378 heap instead of in the program's data segment.
7380 When generating code for shared libraries, @option{-fpic} implies
7381 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
7385 @opindex msmall-text
7386 @opindex mlarge-text
7387 When @option{-msmall-text} is used, the compiler assumes that the
7388 code of the entire program (or shared library) fits in 4MB, and is
7389 thus reachable with a branch instruction. When @option{-msmall-data}
7390 is used, the compiler can assume that all local symbols share the
7391 same @code{$gp} value, and thus reduce the number of instructions
7392 required for a function call from 4 to 1.
7394 The default is @option{-mlarge-text}.
7396 @item -mcpu=@var{cpu_type}
7398 Set the instruction set and instruction scheduling parameters for
7399 machine type @var{cpu_type}. You can specify either the @samp{EV}
7400 style name or the corresponding chip number. GCC supports scheduling
7401 parameters for the EV4, EV5 and EV6 family of processors and will
7402 choose the default values for the instruction set from the processor
7403 you specify. If you do not specify a processor type, GCC will default
7404 to the processor on which the compiler was built.
7406 Supported values for @var{cpu_type} are
7412 Schedules as an EV4 and has no instruction set extensions.
7416 Schedules as an EV5 and has no instruction set extensions.
7420 Schedules as an EV5 and supports the BWX extension.
7425 Schedules as an EV5 and supports the BWX and MAX extensions.
7429 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
7433 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
7436 @item -mtune=@var{cpu_type}
7438 Set only the instruction scheduling parameters for machine type
7439 @var{cpu_type}. The instruction set is not changed.
7441 @item -mmemory-latency=@var{time}
7442 @opindex mmemory-latency
7443 Sets the latency the scheduler should assume for typical memory
7444 references as seen by the application. This number is highly
7445 dependent on the memory access patterns used by the application
7446 and the size of the external cache on the machine.
7448 Valid options for @var{time} are
7452 A decimal number representing clock cycles.
7458 The compiler contains estimates of the number of clock cycles for
7459 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
7460 (also called Dcache, Scache, and Bcache), as well as to main memory.
7461 Note that L3 is only valid for EV5.
7466 @node DEC Alpha/VMS Options
7467 @subsection DEC Alpha/VMS Options
7469 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
7472 @item -mvms-return-codes
7473 @opindex mvms-return-codes
7474 Return VMS condition codes from main. The default is to return POSIX
7475 style condition (e.g.@ error) codes.
7479 @subsection FRV Options
7486 Only use the first 32 general purpose registers.
7491 Use all 64 general purpose registers.
7496 Use only the first 32 floating point registers.
7501 Use all 64 floating point registers
7504 @opindex mhard-float
7506 Use hardware instructions for floating point operations.
7509 @opindex msoft-float
7511 Use library routines for floating point operations.
7516 Dynamically allocate condition code registers.
7521 Do not try to dynamically allocate condition code registers, only
7522 use @code{icc0} and @code{fcc0}.
7527 Change ABI to use double word insns.
7532 Do not use double word instructions.
7537 Use floating point double instructions.
7542 Do not use floating point double instructions.
7547 Use media instructions.
7552 Do not use media instructions.
7557 Use multiply and add/subtract instructions.
7562 Do not use multiply and add/subtract instructions.
7565 @opindex mlibrary-pic
7567 Generate position-independent EABI code.
7572 Use only the first four media accumulator registers.
7577 Use all eight media accumulator registers.
7582 Pack VLIW instructions.
7587 Do not pack VLIW instructions.
7592 Do not mark ABI switches in e_flags.
7597 Enable the use of conditional-move instructions (default).
7599 This switch is mainly for debugging the compiler and will likely be removed
7600 in a future version.
7602 @item -mno-cond-move
7603 @opindex mno-cond-move
7605 Disable the use of conditional-move instructions.
7607 This switch is mainly for debugging the compiler and will likely be removed
7608 in a future version.
7613 Enable the use of conditional set instructions (default).
7615 This switch is mainly for debugging the compiler and will likely be removed
7616 in a future version.
7621 Disable the use of conditional set instructions.
7623 This switch is mainly for debugging the compiler and will likely be removed
7624 in a future version.
7629 Enable the use of conditional execution (default).
7631 This switch is mainly for debugging the compiler and will likely be removed
7632 in a future version.
7634 @item -mno-cond-exec
7635 @opindex mno-cond-exec
7637 Disable the use of conditional execution.
7639 This switch is mainly for debugging the compiler and will likely be removed
7640 in a future version.
7643 @opindex mvliw-branch
7645 Run a pass to pack branches into VLIW instructions (default).
7647 This switch is mainly for debugging the compiler and will likely be removed
7648 in a future version.
7650 @item -mno-vliw-branch
7651 @opindex mno-vliw-branch
7653 Do not run a pass to pack branches into VLIW instructions.
7655 This switch is mainly for debugging the compiler and will likely be removed
7656 in a future version.
7658 @item -mmulti-cond-exec
7659 @opindex mmulti-cond-exec
7661 Enable optimization of @code{&&} and @code{||} in conditional execution
7664 This switch is mainly for debugging the compiler and will likely be removed
7665 in a future version.
7667 @item -mno-multi-cond-exec
7668 @opindex mno-multi-cond-exec
7670 Disable optimization of @code{&&} and @code{||} in conditional execution.
7672 This switch is mainly for debugging the compiler and will likely be removed
7673 in a future version.
7675 @item -mnested-cond-exec
7676 @opindex mnested-cond-exec
7678 Enable nested conditional execution optimizations (default).
7680 This switch is mainly for debugging the compiler and will likely be removed
7681 in a future version.
7683 @item -mno-nested-cond-exec
7684 @opindex mno-nested-cond-exec
7686 Disable nested conditional execution optimizations.
7688 This switch is mainly for debugging the compiler and will likely be removed
7689 in a future version.
7691 @item -mtomcat-stats
7692 @opindex mtomcat-stats
7694 Cause gas to print out tomcat statistics.
7696 @item -mcpu=@var{cpu}
7699 Select the processor type for which to generate code. Possible values are
7700 @samp{simple}, @samp{tomcat}, @samp{fr500}, @samp{fr400}, @samp{fr300},
7705 @node H8/300 Options
7706 @subsection H8/300 Options
7708 These @samp{-m} options are defined for the H8/300 implementations:
7713 Shorten some address references at link time, when possible; uses the
7714 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
7715 ld, Using ld}, for a fuller description.
7719 Generate code for the H8/300H@.
7723 Generate code for the H8S@.
7727 Generate code for the H8S and H8/300H in the normal mode. This switch
7728 must be used either with -mh or -ms.
7732 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
7736 Make @code{int} data 32 bits by default.
7740 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
7741 The default for the H8/300H and H8S is to align longs and floats on 4
7743 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
7744 This option has no effect on the H8/300.
7748 @subsection HPPA Options
7749 @cindex HPPA Options
7751 These @samp{-m} options are defined for the HPPA family of computers:
7754 @item -march=@var{architecture-type}
7756 Generate code for the specified architecture. The choices for
7757 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
7758 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
7759 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
7760 architecture option for your machine. Code compiled for lower numbered
7761 architectures will run on higher numbered architectures, but not the
7764 PA 2.0 support currently requires gas snapshot 19990413 or later. The
7765 next release of binutils (current is 2.9.1) will probably contain PA 2.0
7769 @itemx -mpa-risc-1-1
7770 @itemx -mpa-risc-2-0
7771 @opindex mpa-risc-1-0
7772 @opindex mpa-risc-1-1
7773 @opindex mpa-risc-2-0
7774 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
7777 @opindex mbig-switch
7778 Generate code suitable for big switch tables. Use this option only if
7779 the assembler/linker complain about out of range branches within a switch
7782 @item -mjump-in-delay
7783 @opindex mjump-in-delay
7784 Fill delay slots of function calls with unconditional jump instructions
7785 by modifying the return pointer for the function call to be the target
7786 of the conditional jump.
7788 @item -mdisable-fpregs
7789 @opindex mdisable-fpregs
7790 Prevent floating point registers from being used in any manner. This is
7791 necessary for compiling kernels which perform lazy context switching of
7792 floating point registers. If you use this option and attempt to perform
7793 floating point operations, the compiler will abort.
7795 @item -mdisable-indexing
7796 @opindex mdisable-indexing
7797 Prevent the compiler from using indexing address modes. This avoids some
7798 rather obscure problems when compiling MIG generated code under MACH@.
7800 @item -mno-space-regs
7801 @opindex mno-space-regs
7802 Generate code that assumes the target has no space registers. This allows
7803 GCC to generate faster indirect calls and use unscaled index address modes.
7805 Such code is suitable for level 0 PA systems and kernels.
7807 @item -mfast-indirect-calls
7808 @opindex mfast-indirect-calls
7809 Generate code that assumes calls never cross space boundaries. This
7810 allows GCC to emit code which performs faster indirect calls.
7812 This option will not work in the presence of shared libraries or nested
7815 @item -mfixed-range=@var{register-range}
7816 @opindex mfixed-range
7817 Generate code treating the given register range as fixed registers.
7818 A fixed register is one that the register allocator can not use. This is
7819 useful when compiling kernel code. A register range is specified as
7820 two registers separated by a dash. Multiple register ranges can be
7821 specified separated by a comma.
7823 @item -mlong-load-store
7824 @opindex mlong-load-store
7825 Generate 3-instruction load and store sequences as sometimes required by
7826 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
7829 @item -mportable-runtime
7830 @opindex mportable-runtime
7831 Use the portable calling conventions proposed by HP for ELF systems.
7835 Enable the use of assembler directives only GAS understands.
7837 @item -mschedule=@var{cpu-type}
7839 Schedule code according to the constraints for the machine type
7840 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
7841 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
7842 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
7843 proper scheduling option for your machine. The default scheduling is
7847 @opindex mlinker-opt
7848 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
7849 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
7850 linkers in which they give bogus error messages when linking some programs.
7853 @opindex msoft-float
7854 Generate output containing library calls for floating point.
7855 @strong{Warning:} the requisite libraries are not available for all HPPA
7856 targets. Normally the facilities of the machine's usual C compiler are
7857 used, but this cannot be done directly in cross-compilation. You must make
7858 your own arrangements to provide suitable library functions for
7859 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
7860 does provide software floating point support.
7862 @option{-msoft-float} changes the calling convention in the output file;
7863 therefore, it is only useful if you compile @emph{all} of a program with
7864 this option. In particular, you need to compile @file{libgcc.a}, the
7865 library that comes with GCC, with @option{-msoft-float} in order for
7870 Generate the predefine, @code{_SIO}, for server IO. The default is
7871 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
7872 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO. These
7873 options are available under HP-UX and HI-UX.
7877 Use GNU ld specific options. This passes @option{-shared} to ld when
7878 building a shared library. It is the default when GCC is configured,
7879 explicitly or implicitly, with the GNU linker. This option does not
7880 have any affect on which ld is called, it only changes what parameters
7881 are passed to that ld. The ld that is called is determined by the
7882 @option{--with-ld} configure option, GCC's program search path, and
7883 finally by the user's @env{PATH}. The linker used by GCC can be printed
7884 using @samp{which `gcc -print-prog-name=ld`}.
7888 Use HP ld specific options. This passes @option{-b} to ld when building
7889 a shared library and passes @option{+Accept TypeMismatch} to ld on all
7890 links. It is the default when GCC is configured, explicitly or
7891 implicitly, with the HP linker. This option does not have any affect on
7892 which ld is called, it only changes what parameters are passed to that
7893 ld. The ld that is called is determined by the @option{--with-ld}
7894 configure option, GCC's program search path, and finally by the user's
7895 @env{PATH}. The linker used by GCC can be printed using @samp{which
7896 `gcc -print-prog-name=ld`}.
7901 Select the FDPIC ABI, that uses function descriptors to represent
7902 pointers to functions. Without any PIC/PIE-related options, it
7903 implies @option{-fPIE}. With @option{-fpic} or @option{-fpie}, it
7904 assumes GOT entries and small data are within a 12-bit range from the
7905 GOT base address; with @option{-fPIC} or @option{-fPIE}, GOT offsets
7906 are computed with 32 bits.
7909 @opindex minline-plt
7911 Enable inlining of PLT entries in function calls to functions that are
7912 not known to bind locally. It has no effect without @option{-mfdpic}.
7913 It's enabled by default if optimizing for speed and compiling for
7914 shared libraries (i.e., @option{-fPIC} or @option{-fpic}), or when an
7915 optimization option such as @option{-O3} or above is present in the
7921 Enable the use of @code{GPREL} relocations in the FDPIC ABI for data
7922 that is known to be in read-only sections. It's enabled by default,
7923 except for @option{-fpic} or @option{-fpie}: even though it may help
7924 make the global offset table smaller, it trades 1 instruction for 4.
7925 With @option{-fPIC} or @option{-fPIE}, it trades 3 instructions for 4,
7926 one of which may be shared by multiple symbols, and it avoids the need
7927 for a GOT entry for the referenced symbol, so it's more likely to be a
7928 win. If it is not, @option{-mno-gprel-ro} can be used to disable it.
7930 @item -multilib-library-pic
7931 @opindex multilib-library-pic
7933 Link with the (library, not FD) pic libraries. It's implied by
7934 @option{-mlibrary-pic}, as well as by @option{-fPIC} and
7935 @option{-fpic} without @option{-mfdpic}. You should never have to use
7941 Follow the EABI requirement of always creating a frame pointer whenever
7942 a stack frame is allocated. This option is enabled by default and can
7943 be disabled with @option{-mno-linked-fp}.
7946 @opindex mno-long-calls
7947 Generate code that uses long call sequences. This ensures that a call
7948 is always able to reach linker generated stubs. The default is to generate
7949 long calls only when the distance from the call site to the beginning
7950 of the function or translation unit, as the case may be, exceeds a
7951 predefined limit set by the branch type being used. The limits for
7952 normal calls are 7,600,000 and 240,000 bytes, respectively for the
7953 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
7956 Distances are measured from the beginning of functions when using the
7957 @option{-ffunction-sections} option, or when using the @option{-mgas}
7958 and @option{-mno-portable-runtime} options together under HP-UX with
7961 It is normally not desirable to use this option as it will degrade
7962 performance. However, it may be useful in large applications,
7963 particularly when partial linking is used to build the application.
7965 The types of long calls used depends on the capabilities of the
7966 assembler and linker, and the type of code being generated. The
7967 impact on systems that support long absolute calls, and long pic
7968 symbol-difference or pc-relative calls should be relatively small.
7969 However, an indirect call is used on 32-bit ELF systems in pic code
7970 and it is quite long.
7974 Suppress the generation of link options to search libdld.sl when the
7975 @option{-static} option is specified on HP-UX 10 and later.
7979 The HP-UX implementation of setlocale in libc has a dependency on
7980 libdld.sl. There isn't an archive version of libdld.sl. Thus,
7981 when the @option{-static} option is specified, special link options
7982 are needed to resolve this dependency.
7984 On HP-UX 10 and later, the GCC driver adds the necessary options to
7985 link with libdld.sl when the @option{-static} option is specified.
7986 This causes the resulting binary to be dynamic. On the 64-bit port,
7987 the linkers generate dynamic binaries by default in any case. The
7988 @option{-nolibdld} option can be used to prevent the GCC driver from
7989 adding these link options.
7993 Add support for multithreading with the @dfn{dce thread} library
7994 under HP-UX. This option sets flags for both the preprocessor and
7998 @node i386 and x86-64 Options
7999 @subsection Intel 386 and AMD x86-64 Options
8000 @cindex i386 Options
8001 @cindex x86-64 Options
8002 @cindex Intel 386 Options
8003 @cindex AMD x86-64 Options
8005 These @samp{-m} options are defined for the i386 and x86-64 family of
8009 @item -mtune=@var{cpu-type}
8011 Tune to @var{cpu-type} everything applicable about the generated code, except
8012 for the ABI and the set of available instructions. The choices for
8016 Original Intel's i386 CPU.
8018 Intel's i486 CPU. (No scheduling is implemented for this chip.)
8020 Intel Pentium CPU with no MMX support.
8022 Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
8023 @item i686, pentiumpro
8024 Intel PentiumPro CPU.
8026 Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
8027 @item pentium3, pentium3m
8028 Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set
8031 Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set
8032 support. Used by Centrino notebooks.
8033 @item pentium4, pentium4m
8034 Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
8036 Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction
8039 Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
8040 SSE2 and SSE3 instruction set support.
8042 AMD K6 CPU with MMX instruction set support.
8044 Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
8045 @item athlon, athlon-tbird
8046 AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions
8048 @item athlon-4, athlon-xp, athlon-mp
8049 Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE
8050 instruction set support.
8051 @item k8, opteron, athlon64, athlon-fx
8052 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets
8053 MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
8055 IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction
8058 IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW!
8059 instruction set support.
8061 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is
8062 implemented for this chip.)
8064 Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is
8065 implemented for this chip.)
8068 While picking a specific @var{cpu-type} will schedule things appropriately
8069 for that particular chip, the compiler will not generate any code that
8070 does not run on the i386 without the @option{-march=@var{cpu-type}} option
8073 @item -march=@var{cpu-type}
8075 Generate instructions for the machine type @var{cpu-type}. The choices
8076 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
8077 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
8079 @item -mcpu=@var{cpu-type}
8081 A deprecated synonym for @option{-mtune}.
8090 @opindex mpentiumpro
8091 These options are synonyms for @option{-mtune=i386}, @option{-mtune=i486},
8092 @option{-mtune=pentium}, and @option{-mtune=pentiumpro} respectively.
8093 These synonyms are deprecated.
8095 @item -mfpmath=@var{unit}
8097 Generate floating point arithmetics for selected unit @var{unit}. The choices
8102 Use the standard 387 floating point coprocessor present majority of chips and
8103 emulated otherwise. Code compiled with this option will run almost everywhere.
8104 The temporary results are computed in 80bit precision instead of precision
8105 specified by the type resulting in slightly different results compared to most
8106 of other chips. See @option{-ffloat-store} for more detailed description.
8108 This is the default choice for i386 compiler.
8111 Use scalar floating point instructions present in the SSE instruction set.
8112 This instruction set is supported by Pentium3 and newer chips, in the AMD line
8113 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
8114 instruction set supports only single precision arithmetics, thus the double and
8115 extended precision arithmetics is still done using 387. Later version, present
8116 only in Pentium4 and the future AMD x86-64 chips supports double precision
8119 For i387 you need to use @option{-march=@var{cpu-type}}, @option{-msse} or
8120 @option{-msse2} switches to enable SSE extensions and make this option
8121 effective. For x86-64 compiler, these extensions are enabled by default.
8123 The resulting code should be considerably faster in the majority of cases and avoid
8124 the numerical instability problems of 387 code, but may break some existing
8125 code that expects temporaries to be 80bit.
8127 This is the default choice for the x86-64 compiler.
8130 Attempt to utilize both instruction sets at once. This effectively double the
8131 amount of available registers and on chips with separate execution units for
8132 387 and SSE the execution resources too. Use this option with care, as it is
8133 still experimental, because the GCC register allocator does not model separate
8134 functional units well resulting in instable performance.
8137 @item -masm=@var{dialect}
8138 @opindex masm=@var{dialect}
8139 Output asm instructions using selected @var{dialect}. Supported choices are
8140 @samp{intel} or @samp{att} (the default one).
8145 @opindex mno-ieee-fp
8146 Control whether or not the compiler uses IEEE floating point
8147 comparisons. These handle correctly the case where the result of a
8148 comparison is unordered.
8151 @opindex msoft-float
8152 Generate output containing library calls for floating point.
8153 @strong{Warning:} the requisite libraries are not part of GCC@.
8154 Normally the facilities of the machine's usual C compiler are used, but
8155 this can't be done directly in cross-compilation. You must make your
8156 own arrangements to provide suitable library functions for
8159 On machines where a function returns floating point results in the 80387
8160 register stack, some floating point opcodes may be emitted even if
8161 @option{-msoft-float} is used.
8163 @item -mno-fp-ret-in-387
8164 @opindex mno-fp-ret-in-387
8165 Do not use the FPU registers for return values of functions.
8167 The usual calling convention has functions return values of types
8168 @code{float} and @code{double} in an FPU register, even if there
8169 is no FPU@. The idea is that the operating system should emulate
8172 The option @option{-mno-fp-ret-in-387} causes such values to be returned
8173 in ordinary CPU registers instead.
8175 @item -mno-fancy-math-387
8176 @opindex mno-fancy-math-387
8177 Some 387 emulators do not support the @code{sin}, @code{cos} and
8178 @code{sqrt} instructions for the 387. Specify this option to avoid
8179 generating those instructions. This option is the default on FreeBSD,
8180 OpenBSD and NetBSD@. This option is overridden when @option{-march}
8181 indicates that the target cpu will always have an FPU and so the
8182 instruction will not need emulation. As of revision 2.6.1, these
8183 instructions are not generated unless you also use the
8184 @option{-funsafe-math-optimizations} switch.
8186 @item -malign-double
8187 @itemx -mno-align-double
8188 @opindex malign-double
8189 @opindex mno-align-double
8190 Control whether GCC aligns @code{double}, @code{long double}, and
8191 @code{long long} variables on a two word boundary or a one word
8192 boundary. Aligning @code{double} variables on a two word boundary will
8193 produce code that runs somewhat faster on a @samp{Pentium} at the
8194 expense of more memory.
8196 @strong{Warning:} if you use the @option{-malign-double} switch,
8197 structures containing the above types will be aligned differently than
8198 the published application binary interface specifications for the 386
8199 and will not be binary compatible with structures in code compiled
8200 without that switch.
8202 @item -m96bit-long-double
8203 @itemx -m128bit-long-double
8204 @opindex m96bit-long-double
8205 @opindex m128bit-long-double
8206 These switches control the size of @code{long double} type. The i386
8207 application binary interface specifies the size to be 96 bits,
8208 so @option{-m96bit-long-double} is the default in 32 bit mode.
8210 Modern architectures (Pentium and newer) would prefer @code{long double}
8211 to be aligned to an 8 or 16 byte boundary. In arrays or structures
8212 conforming to the ABI, this would not be possible. So specifying a
8213 @option{-m128bit-long-double} will align @code{long double}
8214 to a 16 byte boundary by padding the @code{long double} with an additional
8217 In the x86-64 compiler, @option{-m128bit-long-double} is the default choice as
8218 its ABI specifies that @code{long double} is to be aligned on 16 byte boundary.
8220 Notice that neither of these options enable any extra precision over the x87
8221 standard of 80 bits for a @code{long double}.
8223 @strong{Warning:} if you override the default value for your target ABI, the
8224 structures and arrays containing @code{long double} variables will change
8225 their size as well as function calling convention for function taking
8226 @code{long double} will be modified. Hence they will not be binary
8227 compatible with arrays or structures in code compiled without that switch.
8231 @itemx -mno-svr3-shlib
8232 @opindex msvr3-shlib
8233 @opindex mno-svr3-shlib
8234 Control whether GCC places uninitialized local variables into the
8235 @code{bss} or @code{data} segments. @option{-msvr3-shlib} places them
8236 into @code{bss}. These options are meaningful only on System V Release 3.
8240 Use a different function-calling convention, in which functions that
8241 take a fixed number of arguments return with the @code{ret} @var{num}
8242 instruction, which pops their arguments while returning. This saves one
8243 instruction in the caller since there is no need to pop the arguments
8246 You can specify that an individual function is called with this calling
8247 sequence with the function attribute @samp{stdcall}. You can also
8248 override the @option{-mrtd} option by using the function attribute
8249 @samp{cdecl}. @xref{Function Attributes}.
8251 @strong{Warning:} this calling convention is incompatible with the one
8252 normally used on Unix, so you cannot use it if you need to call
8253 libraries compiled with the Unix compiler.
8255 Also, you must provide function prototypes for all functions that
8256 take variable numbers of arguments (including @code{printf});
8257 otherwise incorrect code will be generated for calls to those
8260 In addition, seriously incorrect code will result if you call a
8261 function with too many arguments. (Normally, extra arguments are
8262 harmlessly ignored.)
8264 @item -mregparm=@var{num}
8266 Control how many registers are used to pass integer arguments. By
8267 default, no registers are used to pass arguments, and at most 3
8268 registers can be used. You can control this behavior for a specific
8269 function by using the function attribute @samp{regparm}.
8270 @xref{Function Attributes}.
8272 @strong{Warning:} if you use this switch, and
8273 @var{num} is nonzero, then you must build all modules with the same
8274 value, including any libraries. This includes the system libraries and
8277 @item -mpreferred-stack-boundary=@var{num}
8278 @opindex mpreferred-stack-boundary
8279 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
8280 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
8281 the default is 4 (16 bytes or 128 bits), except when optimizing for code
8282 size (@option{-Os}), in which case the default is the minimum correct
8283 alignment (4 bytes for x86, and 8 bytes for x86-64).
8285 On Pentium and PentiumPro, @code{double} and @code{long double} values
8286 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
8287 suffer significant run time performance penalties. On Pentium III, the
8288 Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
8289 penalties if it is not 16 byte aligned.
8291 To ensure proper alignment of this values on the stack, the stack boundary
8292 must be as aligned as that required by any value stored on the stack.
8293 Further, every function must be generated such that it keeps the stack
8294 aligned. Thus calling a function compiled with a higher preferred
8295 stack boundary from a function compiled with a lower preferred stack
8296 boundary will most likely misalign the stack. It is recommended that
8297 libraries that use callbacks always use the default setting.
8299 This extra alignment does consume extra stack space, and generally
8300 increases code size. Code that is sensitive to stack space usage, such
8301 as embedded systems and operating system kernels, may want to reduce the
8302 preferred alignment to @option{-mpreferred-stack-boundary=2}.
8320 These switches enable or disable the use of built-in functions that allow
8321 direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow extensions of the
8324 @xref{X86 Built-in Functions}, for details of the functions enabled
8325 and disabled by these switches.
8327 To have SSE/SSE2 instructions generated automatically from floating-point
8328 code, see @option{-mfpmath=sse}.
8331 @itemx -mno-push-args
8333 @opindex mno-push-args
8334 Use PUSH operations to store outgoing parameters. This method is shorter
8335 and usually equally fast as method using SUB/MOV operations and is enabled
8336 by default. In some cases disabling it may improve performance because of
8337 improved scheduling and reduced dependencies.
8339 @item -maccumulate-outgoing-args
8340 @opindex maccumulate-outgoing-args
8341 If enabled, the maximum amount of space required for outgoing arguments will be
8342 computed in the function prologue. This is faster on most modern CPUs
8343 because of reduced dependencies, improved scheduling and reduced stack usage
8344 when preferred stack boundary is not equal to 2. The drawback is a notable
8345 increase in code size. This switch implies @option{-mno-push-args}.
8349 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
8350 on thread-safe exception handling must compile and link all code with the
8351 @option{-mthreads} option. When compiling, @option{-mthreads} defines
8352 @option{-D_MT}; when linking, it links in a special thread helper library
8353 @option{-lmingwthrd} which cleans up per thread exception handling data.
8355 @item -mno-align-stringops
8356 @opindex mno-align-stringops
8357 Do not align destination of inlined string operations. This switch reduces
8358 code size and improves performance in case the destination is already aligned,
8359 but GCC doesn't know about it.
8361 @item -minline-all-stringops
8362 @opindex minline-all-stringops
8363 By default GCC inlines string operations only when destination is known to be
8364 aligned at least to 4 byte boundary. This enables more inlining, increase code
8365 size, but may improve performance of code that depends on fast memcpy, strlen
8366 and memset for short lengths.
8368 @item -momit-leaf-frame-pointer
8369 @opindex momit-leaf-frame-pointer
8370 Don't keep the frame pointer in a register for leaf functions. This
8371 avoids the instructions to save, set up and restore frame pointers and
8372 makes an extra register available in leaf functions. The option
8373 @option{-fomit-frame-pointer} removes the frame pointer for all functions
8374 which might make debugging harder.
8376 @item -mtls-direct-seg-refs
8377 @itemx -mno-tls-direct-seg-refs
8378 @opindex mtls-direct-seg-refs
8379 Controls whether TLS variables may be accessed with offsets from the
8380 TLS segment register (@code{%gs} for 32-bit, @code{%fs} for 64-bit),
8381 or whether the thread base pointer must be added. Whether or not this
8382 is legal depends on the operating system, and whether it maps the
8383 segment to cover the entire TLS area.
8385 For systems that use GNU libc, the default is on.
8388 These @samp{-m} switches are supported in addition to the above
8389 on AMD x86-64 processors in 64-bit environments.
8396 Generate code for a 32-bit or 64-bit environment.
8397 The 32-bit environment sets int, long and pointer to 32 bits and
8398 generates code that runs on any i386 system.
8399 The 64-bit environment sets int to 32 bits and long and pointer
8400 to 64 bits and generates code for AMD's x86-64 architecture.
8403 @opindex no-red-zone
8404 Do not use a so called red zone for x86-64 code. The red zone is mandated
8405 by the x86-64 ABI, it is a 128-byte area beyond the location of the
8406 stack pointer that will not be modified by signal or interrupt handlers
8407 and therefore can be used for temporary data without adjusting the stack
8408 pointer. The flag @option{-mno-red-zone} disables this red zone.
8410 @item -mcmodel=small
8411 @opindex mcmodel=small
8412 Generate code for the small code model: the program and its symbols must
8413 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
8414 Programs can be statically or dynamically linked. This is the default
8417 @item -mcmodel=kernel
8418 @opindex mcmodel=kernel
8419 Generate code for the kernel code model. The kernel runs in the
8420 negative 2 GB of the address space.
8421 This model has to be used for Linux kernel code.
8423 @item -mcmodel=medium
8424 @opindex mcmodel=medium
8425 Generate code for the medium model: The program is linked in the lower 2
8426 GB of the address space but symbols can be located anywhere in the
8427 address space. Programs can be statically or dynamically linked, but
8428 building of shared libraries are not supported with the medium model.
8430 @item -mcmodel=large
8431 @opindex mcmodel=large
8432 Generate code for the large model: This model makes no assumptions
8433 about addresses and sizes of sections. Currently GCC does not implement
8438 @subsection IA-64 Options
8439 @cindex IA-64 Options
8441 These are the @samp{-m} options defined for the Intel IA-64 architecture.
8445 @opindex mbig-endian
8446 Generate code for a big endian target. This is the default for HP-UX@.
8448 @item -mlittle-endian
8449 @opindex mlittle-endian
8450 Generate code for a little endian target. This is the default for AIX5
8457 Generate (or don't) code for the GNU assembler. This is the default.
8458 @c Also, this is the default if the configure option @option{--with-gnu-as}
8465 Generate (or don't) code for the GNU linker. This is the default.
8466 @c Also, this is the default if the configure option @option{--with-gnu-ld}
8471 Generate code that does not use a global pointer register. The result
8472 is not position independent code, and violates the IA-64 ABI@.
8474 @item -mvolatile-asm-stop
8475 @itemx -mno-volatile-asm-stop
8476 @opindex mvolatile-asm-stop
8477 @opindex mno-volatile-asm-stop
8478 Generate (or don't) a stop bit immediately before and after volatile asm
8483 Generate code that works around Itanium B step errata.
8485 @item -mregister-names
8486 @itemx -mno-register-names
8487 @opindex mregister-names
8488 @opindex mno-register-names
8489 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
8490 the stacked registers. This may make assembler output more readable.
8496 Disable (or enable) optimizations that use the small data section. This may
8497 be useful for working around optimizer bugs.
8500 @opindex mconstant-gp
8501 Generate code that uses a single constant global pointer value. This is
8502 useful when compiling kernel code.
8506 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
8507 This is useful when compiling firmware code.
8509 @item -minline-float-divide-min-latency
8510 @opindex minline-float-divide-min-latency
8511 Generate code for inline divides of floating point values
8512 using the minimum latency algorithm.
8514 @item -minline-float-divide-max-throughput
8515 @opindex minline-float-divide-max-throughput
8516 Generate code for inline divides of floating point values
8517 using the maximum throughput algorithm.
8519 @item -minline-int-divide-min-latency
8520 @opindex minline-int-divide-min-latency
8521 Generate code for inline divides of integer values
8522 using the minimum latency algorithm.
8524 @item -minline-int-divide-max-throughput
8525 @opindex minline-int-divide-max-throughput
8526 Generate code for inline divides of integer values
8527 using the maximum throughput algorithm.
8529 @item -mno-dwarf2-asm
8531 @opindex mno-dwarf2-asm
8532 @opindex mdwarf2-asm
8533 Don't (or do) generate assembler code for the DWARF2 line number debugging
8534 info. This may be useful when not using the GNU assembler.
8536 @item -mfixed-range=@var{register-range}
8537 @opindex mfixed-range
8538 Generate code treating the given register range as fixed registers.
8539 A fixed register is one that the register allocator can not use. This is
8540 useful when compiling kernel code. A register range is specified as
8541 two registers separated by a dash. Multiple register ranges can be
8542 specified separated by a comma.
8544 @item -mearly-stop-bits
8545 @itemx -mno-early-stop-bits
8546 @opindex mearly-stop-bits
8547 @opindex mno-early-stop-bits
8548 Allow stop bits to be placed earlier than immediately preceding the
8549 instruction that triggered the stop bit. This can improve instruction
8550 scheduling, but does not always do so.
8553 @node M32R/D Options
8554 @subsection M32R/D Options
8555 @cindex M32R/D options
8557 These @option{-m} options are defined for Renesas M32R/D architectures:
8562 Generate code for the M32R/2@.
8566 Generate code for the M32R/X@.
8570 Generate code for the M32R@. This is the default.
8573 @opindex mmodel=small
8574 Assume all objects live in the lower 16MB of memory (so that their addresses
8575 can be loaded with the @code{ld24} instruction), and assume all subroutines
8576 are reachable with the @code{bl} instruction.
8577 This is the default.
8579 The addressability of a particular object can be set with the
8580 @code{model} attribute.
8582 @item -mmodel=medium
8583 @opindex mmodel=medium
8584 Assume objects may be anywhere in the 32-bit address space (the compiler
8585 will generate @code{seth/add3} instructions to load their addresses), and
8586 assume all subroutines are reachable with the @code{bl} instruction.
8589 @opindex mmodel=large
8590 Assume objects may be anywhere in the 32-bit address space (the compiler
8591 will generate @code{seth/add3} instructions to load their addresses), and
8592 assume subroutines may not be reachable with the @code{bl} instruction
8593 (the compiler will generate the much slower @code{seth/add3/jl}
8594 instruction sequence).
8597 @opindex msdata=none
8598 Disable use of the small data area. Variables will be put into
8599 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
8600 @code{section} attribute has been specified).
8601 This is the default.
8603 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
8604 Objects may be explicitly put in the small data area with the
8605 @code{section} attribute using one of these sections.
8608 @opindex msdata=sdata
8609 Put small global and static data in the small data area, but do not
8610 generate special code to reference them.
8614 Put small global and static data in the small data area, and generate
8615 special instructions to reference them.
8619 @cindex smaller data references
8620 Put global and static objects less than or equal to @var{num} bytes
8621 into the small data or bss sections instead of the normal data or bss
8622 sections. The default value of @var{num} is 8.
8623 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
8624 for this option to have any effect.
8626 All modules should be compiled with the same @option{-G @var{num}} value.
8627 Compiling with different values of @var{num} may or may not work; if it
8628 doesn't the linker will give an error message---incorrect code will not be
8633 Makes the M32R specific code in the compiler display some statistics
8634 that might help in debugging programs.
8637 @opindex malign-loops
8638 Align all loops to a 32-byte boundary.
8640 @item -mno-align-loops
8641 @opindex mno-align-loops
8642 Do not enforce a 32-byte alignment for loops. This is the default.
8644 @item -missue-rate=@var{number}
8645 @opindex missue-rate=@var{number}
8646 Issue @var{number} instructions per cycle. @var{number} can only be 1
8649 @item -mbranch-cost=@var{number}
8650 @opindex mbranch-cost=@var{number}
8651 @var{number} can only be 1 or 2. If it is 1 then branches will be
8652 preferred over conditional code, if it is 2, then the opposite will
8655 @item -mflush-trap=@var{number}
8656 @opindex mflush-trap=@var{number}
8657 Specifies the trap number to use to flush the cache. The default is
8658 12. Valid numbers are between 0 and 15 inclusive.
8660 @item -mno-flush-trap
8661 @opindex mno-flush-trap
8662 Specifies that the cache cannot be flushed by using a trap.
8664 @item -mflush-func=@var{name}
8665 @opindex mflush-func=@var{name}
8666 Specifies the name of the operating system function to call to flush
8667 the cache. The default is @emph{_flush_cache}, but a function call
8668 will only be used if a trap is not available.
8670 @item -mno-flush-func
8671 @opindex mno-flush-func
8672 Indicates that there is no OS function for flushing the cache.
8676 @node M680x0 Options
8677 @subsection M680x0 Options
8678 @cindex M680x0 options
8680 These are the @samp{-m} options defined for the 68000 series. The default
8681 values for these options depends on which style of 68000 was selected when
8682 the compiler was configured; the defaults for the most common choices are
8690 Generate output for a 68000. This is the default
8691 when the compiler is configured for 68000-based systems.
8693 Use this option for microcontrollers with a 68000 or EC000 core,
8694 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
8700 Generate output for a 68020. This is the default
8701 when the compiler is configured for 68020-based systems.
8705 Generate output containing 68881 instructions for floating point.
8706 This is the default for most 68020 systems unless @option{--nfp} was
8707 specified when the compiler was configured.
8711 Generate output for a 68030. This is the default when the compiler is
8712 configured for 68030-based systems.
8716 Generate output for a 68040. This is the default when the compiler is
8717 configured for 68040-based systems.
8719 This option inhibits the use of 68881/68882 instructions that have to be
8720 emulated by software on the 68040. Use this option if your 68040 does not
8721 have code to emulate those instructions.
8725 Generate output for a 68060. This is the default when the compiler is
8726 configured for 68060-based systems.
8728 This option inhibits the use of 68020 and 68881/68882 instructions that
8729 have to be emulated by software on the 68060. Use this option if your 68060
8730 does not have code to emulate those instructions.
8734 Generate output for a CPU32. This is the default
8735 when the compiler is configured for CPU32-based systems.
8737 Use this option for microcontrollers with a
8738 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
8739 68336, 68340, 68341, 68349 and 68360.
8743 Generate output for a 520X ``coldfire'' family cpu. This is the default
8744 when the compiler is configured for 520X-based systems.
8746 Use this option for microcontroller with a 5200 core, including
8747 the MCF5202, MCF5203, MCF5204 and MCF5202.
8752 Generate output for a 68040, without using any of the new instructions.
8753 This results in code which can run relatively efficiently on either a
8754 68020/68881 or a 68030 or a 68040. The generated code does use the
8755 68881 instructions that are emulated on the 68040.
8759 Generate output for a 68060, without using any of the new instructions.
8760 This results in code which can run relatively efficiently on either a
8761 68020/68881 or a 68030 or a 68040. The generated code does use the
8762 68881 instructions that are emulated on the 68060.
8765 @opindex msoft-float
8766 Generate output containing library calls for floating point.
8767 @strong{Warning:} the requisite libraries are not available for all m68k
8768 targets. Normally the facilities of the machine's usual C compiler are
8769 used, but this can't be done directly in cross-compilation. You must
8770 make your own arrangements to provide suitable library functions for
8771 cross-compilation. The embedded targets @samp{m68k-*-aout} and
8772 @samp{m68k-*-coff} do provide software floating point support.
8776 Consider type @code{int} to be 16 bits wide, like @code{short int}.
8777 Additionally, parameters passed on the stack are also aligned to a
8778 16-bit boundary even on targets whose API mandates promotion to 32-bit.
8781 @opindex mnobitfield
8782 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
8783 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
8787 Do use the bit-field instructions. The @option{-m68020} option implies
8788 @option{-mbitfield}. This is the default if you use a configuration
8789 designed for a 68020.
8793 Use a different function-calling convention, in which functions
8794 that take a fixed number of arguments return with the @code{rtd}
8795 instruction, which pops their arguments while returning. This
8796 saves one instruction in the caller since there is no need to pop
8797 the arguments there.
8799 This calling convention is incompatible with the one normally
8800 used on Unix, so you cannot use it if you need to call libraries
8801 compiled with the Unix compiler.
8803 Also, you must provide function prototypes for all functions that
8804 take variable numbers of arguments (including @code{printf});
8805 otherwise incorrect code will be generated for calls to those
8808 In addition, seriously incorrect code will result if you call a
8809 function with too many arguments. (Normally, extra arguments are
8810 harmlessly ignored.)
8812 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
8813 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
8816 @itemx -mno-align-int
8818 @opindex mno-align-int
8819 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
8820 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
8821 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
8822 Aligning variables on 32-bit boundaries produces code that runs somewhat
8823 faster on processors with 32-bit busses at the expense of more memory.
8825 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
8826 align structures containing the above types differently than
8827 most published application binary interface specifications for the m68k.
8831 Use the pc-relative addressing mode of the 68000 directly, instead of
8832 using a global offset table. At present, this option implies @option{-fpic},
8833 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
8834 not presently supported with @option{-mpcrel}, though this could be supported for
8835 68020 and higher processors.
8837 @item -mno-strict-align
8838 @itemx -mstrict-align
8839 @opindex mno-strict-align
8840 @opindex mstrict-align
8841 Do not (do) assume that unaligned memory references will be handled by
8845 Generate code that allows the data segment to be located in a different
8846 area of memory from the text segment. This allows for execute in place in
8847 an environment without virtual memory management. This option implies -fPIC.
8850 Generate code that assumes that the data segment follows the text segment.
8851 This is the default.
8853 @item -mid-shared-library
8854 Generate code that supports shared libraries via the library ID method.
8855 This allows for execute in place and shared libraries in an environment
8856 without virtual memory management. This option implies -fPIC.
8858 @item -mno-id-shared-library
8859 Generate code that doesn't assume ID based shared libraries are being used.
8860 This is the default.
8862 @item -mshared-library-id=n
8863 Specified the identification number of the ID based shared library being
8864 compiled. Specifying a value of 0 will generate more compact code, specifying
8865 other values will force the allocation of that number to the current
8866 library but is no more space or time efficient than omitting this option.
8870 @node M68hc1x Options
8871 @subsection M68hc1x Options
8872 @cindex M68hc1x options
8874 These are the @samp{-m} options defined for the 68hc11 and 68hc12
8875 microcontrollers. The default values for these options depends on
8876 which style of microcontroller was selected when the compiler was configured;
8877 the defaults for the most common choices are given below.
8884 Generate output for a 68HC11. This is the default
8885 when the compiler is configured for 68HC11-based systems.
8891 Generate output for a 68HC12. This is the default
8892 when the compiler is configured for 68HC12-based systems.
8898 Generate output for a 68HCS12.
8901 @opindex mauto-incdec
8902 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
8909 Enable the use of 68HC12 min and max instructions.
8912 @itemx -mno-long-calls
8913 @opindex mlong-calls
8914 @opindex mno-long-calls
8915 Treat all calls as being far away (near). If calls are assumed to be
8916 far away, the compiler will use the @code{call} instruction to
8917 call a function and the @code{rtc} instruction for returning.
8921 Consider type @code{int} to be 16 bits wide, like @code{short int}.
8923 @item -msoft-reg-count=@var{count}
8924 @opindex msoft-reg-count
8925 Specify the number of pseudo-soft registers which are used for the
8926 code generation. The maximum number is 32. Using more pseudo-soft
8927 register may or may not result in better code depending on the program.
8928 The default is 4 for 68HC11 and 2 for 68HC12.
8933 @subsection MCore Options
8934 @cindex MCore options
8936 These are the @samp{-m} options defined for the Motorola M*Core
8944 @opindex mno-hardlit
8945 Inline constants into the code stream if it can be done in two
8946 instructions or less.
8952 Use the divide instruction. (Enabled by default).
8954 @item -mrelax-immediate
8955 @itemx -mno-relax-immediate
8956 @opindex mrelax-immediate
8957 @opindex mno-relax-immediate
8958 Allow arbitrary sized immediates in bit operations.
8960 @item -mwide-bitfields
8961 @itemx -mno-wide-bitfields
8962 @opindex mwide-bitfields
8963 @opindex mno-wide-bitfields
8964 Always treat bit-fields as int-sized.
8966 @item -m4byte-functions
8967 @itemx -mno-4byte-functions
8968 @opindex m4byte-functions
8969 @opindex mno-4byte-functions
8970 Force all functions to be aligned to a four byte boundary.
8972 @item -mcallgraph-data
8973 @itemx -mno-callgraph-data
8974 @opindex mcallgraph-data
8975 @opindex mno-callgraph-data
8976 Emit callgraph information.
8979 @itemx -mno-slow-bytes
8980 @opindex mslow-bytes
8981 @opindex mno-slow-bytes
8982 Prefer word access when reading byte quantities.
8984 @item -mlittle-endian
8986 @opindex mlittle-endian
8987 @opindex mbig-endian
8988 Generate code for a little endian target.
8994 Generate code for the 210 processor.
8998 @subsection MIPS Options
8999 @cindex MIPS options
9005 Generate big-endian code.
9009 Generate little-endian code. This is the default for @samp{mips*el-*-*}
9012 @item -march=@var{arch}
9014 Generate code that will run on @var{arch}, which can be the name of a
9015 generic MIPS ISA, or the name of a particular processor.
9017 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
9018 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
9019 The processor names are:
9020 @samp{4kc}, @samp{4kp}, @samp{5kc}, @samp{20kc},
9022 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
9023 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000}, @samp{rm7000},
9027 @samp{vr4100}, @samp{vr4111}, @samp{vr4120}, @samp{vr4130}, @samp{vr4300},
9028 @samp{vr5000}, @samp{vr5400} and @samp{vr5500}.
9029 The special value @samp{from-abi} selects the
9030 most compatible architecture for the selected ABI (that is,
9031 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
9033 In processor names, a final @samp{000} can be abbreviated as @samp{k}
9034 (for example, @samp{-march=r2k}). Prefixes are optional, and
9035 @samp{vr} may be written @samp{r}.
9037 GCC defines two macros based on the value of this option. The first
9038 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
9039 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
9040 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
9041 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
9042 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
9044 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
9045 above. In other words, it will have the full prefix and will not
9046 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
9047 the macro names the resolved architecture (either @samp{"mips1"} or
9048 @samp{"mips3"}). It names the default architecture when no
9049 @option{-march} option is given.
9051 @item -mtune=@var{arch}
9053 Optimize for @var{arch}. Among other things, this option controls
9054 the way instructions are scheduled, and the perceived cost of arithmetic
9055 operations. The list of @var{arch} values is the same as for
9058 When this option is not used, GCC will optimize for the processor
9059 specified by @option{-march}. By using @option{-march} and
9060 @option{-mtune} together, it is possible to generate code that will
9061 run on a family of processors, but optimize the code for one
9062 particular member of that family.
9064 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
9065 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
9066 @samp{-march} ones described above.
9070 Equivalent to @samp{-march=mips1}.
9074 Equivalent to @samp{-march=mips2}.
9078 Equivalent to @samp{-march=mips3}.
9082 Equivalent to @samp{-march=mips4}.
9086 Equivalent to @samp{-march=mips32}.
9090 Equivalent to @samp{-march=mips32r2}.
9094 Equivalent to @samp{-march=mips64}.
9100 Use (do not use) the MIPS16 ISA.
9112 Generate code for the given ABI@.
9114 Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
9115 generates 64-bit code when you select a 64-bit architecture, but you
9116 can use @option{-mgp32} to get 32-bit code instead.
9118 For information about the O64 ABI, see
9119 @w{@uref{http://gcc.gnu.org/projects/mipso64-abi.html}}.
9122 @itemx -mno-abicalls
9124 @opindex mno-abicalls
9125 Generate (do not generate) SVR4-style position-independent code.
9126 @option{-mabicalls} is the default for SVR4-based systems.
9132 Lift (do not lift) the usual restrictions on the size of the global
9135 GCC normally uses a single instruction to load values from the GOT.
9136 While this is relatively efficient, it will only work if the GOT
9137 is smaller than about 64k. Anything larger will cause the linker
9138 to report an error such as:
9140 @cindex relocation truncated to fit (MIPS)
9142 relocation truncated to fit: R_MIPS_GOT16 foobar
9145 If this happens, you should recompile your code with @option{-mxgot}.
9146 It should then work with very large GOTs, although it will also be
9147 less efficient, since it will take three instructions to fetch the
9148 value of a global symbol.
9150 Note that some linkers can create multiple GOTs. If you have such a
9151 linker, you should only need to use @option{-mxgot} when a single object
9152 file accesses more than 64k's worth of GOT entries. Very few do.
9154 These options have no effect unless GCC is generating position
9159 Assume that general-purpose registers are 32 bits wide.
9163 Assume that general-purpose registers are 64 bits wide.
9167 Assume that floating-point registers are 32 bits wide.
9171 Assume that floating-point registers are 64 bits wide.
9174 @opindex mhard-float
9175 Use floating-point coprocessor instructions.
9178 @opindex msoft-float
9179 Do not use floating-point coprocessor instructions. Implement
9180 floating-point calculations using library calls instead.
9182 @item -msingle-float
9183 @opindex msingle-float
9184 Assume that the floating-point coprocessor only supports single-precision
9187 @itemx -mdouble-float
9188 @opindex mdouble-float
9189 Assume that the floating-point coprocessor supports double-precision
9190 operations. This is the default.
9194 Force @code{int} and @code{long} types to be 64 bits wide. See
9195 @option{-mlong32} for an explanation of the default and the way
9196 that the pointer size is determined.
9200 Force @code{long} types to be 64 bits wide. See @option{-mlong32} for
9201 an explanation of the default and the way that the pointer size is
9206 Force @code{long}, @code{int}, and pointer types to be 32 bits wide.
9208 The default size of @code{int}s, @code{long}s and pointers depends on
9209 the ABI@. All the supported ABIs use 32-bit @code{int}s. The n64 ABI
9210 uses 64-bit @code{long}s, as does the 64-bit EABI; the others use
9211 32-bit @code{long}s. Pointers are the same size as @code{long}s,
9212 or the same size as integer registers, whichever is smaller.
9216 @cindex smaller data references (MIPS)
9217 @cindex gp-relative references (MIPS)
9218 Put global and static items less than or equal to @var{num} bytes into
9219 the small data or bss section instead of the normal data or bss section.
9220 This allows the data to be accessed using a single instruction.
9222 All modules should be compiled with the same @option{-G @var{num}}
9225 @item -membedded-data
9226 @itemx -mno-embedded-data
9227 @opindex membedded-data
9228 @opindex mno-embedded-data
9229 Allocate variables to the read-only data section first if possible, then
9230 next in the small data section if possible, otherwise in data. This gives
9231 slightly slower code than the default, but reduces the amount of RAM required
9232 when executing, and thus may be preferred for some embedded systems.
9234 @item -muninit-const-in-rodata
9235 @itemx -mno-uninit-const-in-rodata
9236 @opindex muninit-const-in-rodata
9237 @opindex mno-uninit-const-in-rodata
9238 Put uninitialized @code{const} variables in the read-only data section.
9239 This option is only meaningful in conjunction with @option{-membedded-data}.
9241 @item -msplit-addresses
9242 @itemx -mno-split-addresses
9243 @opindex msplit-addresses
9244 @opindex mno-split-addresses
9245 Enable (disable) use of the @code{%hi()} and @code{%lo()} assembler
9246 relocation operators. This option has been superceded by
9247 @option{-mexplicit-relocs} but is retained for backwards compatibility.
9249 @item -mexplicit-relocs
9250 @itemx -mno-explicit-relocs
9251 @opindex mexplicit-relocs
9252 @opindex mno-explicit-relocs
9253 Use (do not use) assembler relocation operators when dealing with symbolic
9254 addresses. The alternative, selected by @option{-mno-explicit-relocs},
9255 is to use assembler macros instead.
9257 @option{-mexplicit-relocs} is the default if GCC was configured
9258 to use an assembler that supports relocation operators.
9260 @item -mcheck-zero-division
9261 @itemx -mno-check-zero-division
9262 @opindex mcheck-zero-division
9263 @opindex mno-check-zero-division
9264 Trap (do not trap) on integer division by zero. The default is
9265 @option{-mcheck-zero-division}.
9271 Force (do not force) the use of @code{memcpy()} for non-trivial block
9272 moves. The default is @option{-mno-memcpy}, which allows GCC to inline
9273 most constant-sized copies.
9276 @itemx -mno-long-calls
9277 @opindex mlong-calls
9278 @opindex mno-long-calls
9279 Disable (do not disable) use of the @code{jal} instruction. Calling
9280 functions using @code{jal} is more efficient but requires the caller
9281 and callee to be in the same 256 megabyte segment.
9283 This option has no effect on abicalls code. The default is
9284 @option{-mno-long-calls}.
9290 Enable (disable) use of the @code{mad}, @code{madu} and @code{mul}
9291 instructions, as provided by the R4650 ISA.
9294 @itemx -mno-fused-madd
9295 @opindex mfused-madd
9296 @opindex mno-fused-madd
9297 Enable (disable) use of the floating point multiply-accumulate
9298 instructions, when they are available. The default is
9299 @option{-mfused-madd}.
9301 When multiply-accumulate instructions are used, the intermediate
9302 product is calculated to infinite precision and is not subject to
9303 the FCSR Flush to Zero bit. This may be undesirable in some
9308 Tell the MIPS assembler to not run its preprocessor over user
9309 assembler files (with a @samp{.s} suffix) when assembling them.
9312 @itemx -mno-fix-r4000
9314 @opindex mno-fix-r4000
9315 Work around certain R4000 CPU errata:
9318 A double-word or a variable shift may give an incorrect result if executed
9319 immediately after starting an integer division.
9321 A double-word or a variable shift may give an incorrect result if executed
9322 while an integer multiplication is in progress.
9324 An integer division may give an incorrect result if started in a delay slot
9325 of a taken branch or a jump.
9329 @itemx -mno-fix-r4400
9331 @opindex mno-fix-r4400
9332 Work around certain R4400 CPU errata:
9335 A double-word or a variable shift may give an incorrect result if executed
9336 immediately after starting an integer division.
9340 @itemx -mno-fix-vr4120
9341 @opindex mfix-vr4120
9342 Work around certain VR4120 errata:
9345 @code{dmultu} does not always produce the correct result.
9347 @code{div} and @code{ddiv} do not always produce the correct result if one
9348 of the operands is negative.
9350 The workarounds for the division errata rely on special functions in
9351 @file{libgcc.a}. At present, these functions are only provided by
9352 the @code{mips64vr*-elf} configurations.
9354 Other VR4120 errata require a nop to be inserted between certain pairs of
9355 instructions. These errata are handled by the assembler, not by GCC itself.
9360 Work around certain SB-1 CPU core errata.
9361 (This flag currently works around the SB-1 revision 2
9362 ``F1'' and ``F2'' floating point errata.)
9364 @item -mflush-func=@var{func}
9365 @itemx -mno-flush-func
9366 @opindex mflush-func
9367 Specifies the function to call to flush the I and D caches, or to not
9368 call any such function. If called, the function must take the same
9369 arguments as the common @code{_flush_func()}, that is, the address of the
9370 memory range for which the cache is being flushed, the size of the
9371 memory range, and the number 3 (to flush both caches). The default
9372 depends on the target GCC was configured for, but commonly is either
9373 @samp{_flush_func} or @samp{__cpu_flush}.
9375 @item -mbranch-likely
9376 @itemx -mno-branch-likely
9377 @opindex mbranch-likely
9378 @opindex mno-branch-likely
9379 Enable or disable use of Branch Likely instructions, regardless of the
9380 default for the selected architecture. By default, Branch Likely
9381 instructions may be generated if they are supported by the selected
9382 architecture. An exception is for the MIPS32 and MIPS64 architectures
9383 and processors which implement those architectures; for those, Branch
9384 Likely instructions will not be generated by default because the MIPS32
9385 and MIPS64 architectures specifically deprecate their use.
9387 @item -mfp-exceptions
9388 @itemx -mno-fp-exceptions
9389 @opindex mfp-exceptions
9390 Specifies whether FP exceptions are enabled. This affects how we schedule
9391 FP instructions for some processors. The default is that FP exceptions are
9394 For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
9395 64-bit code, then we can use both FP pipes. Otherwise, we can only use one
9398 @item -mvr4130-align
9399 @itemx -mno-vr4130-align
9400 @opindex mvr4130-align
9401 The VR4130 pipeline is two-way superscalar, but can only issue two
9402 instructions together if the first one is 8-byte aligned. When this
9403 option is enabled, GCC will align pairs of instructions that it
9404 thinks should execute in parallel.
9406 This option only has an effect when optimizing for the VR4130.
9407 It normally makes code faster, but at the expense of making it bigger.
9408 It is enabled by default at optimization level @option{-O3}.
9412 @subsection MMIX Options
9413 @cindex MMIX Options
9415 These options are defined for the MMIX:
9419 @itemx -mno-libfuncs
9421 @opindex mno-libfuncs
9422 Specify that intrinsic library functions are being compiled, passing all
9423 values in registers, no matter the size.
9428 @opindex mno-epsilon
9429 Generate floating-point comparison instructions that compare with respect
9430 to the @code{rE} epsilon register.
9432 @item -mabi=mmixware
9434 @opindex mabi-mmixware
9436 Generate code that passes function parameters and return values that (in
9437 the called function) are seen as registers @code{$0} and up, as opposed to
9438 the GNU ABI which uses global registers @code{$231} and up.
9441 @itemx -mno-zero-extend
9442 @opindex mzero-extend
9443 @opindex mno-zero-extend
9444 When reading data from memory in sizes shorter than 64 bits, use (do not
9445 use) zero-extending load instructions by default, rather than
9446 sign-extending ones.
9449 @itemx -mno-knuthdiv
9451 @opindex mno-knuthdiv
9452 Make the result of a division yielding a remainder have the same sign as
9453 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
9454 remainder follows the sign of the dividend. Both methods are
9455 arithmetically valid, the latter being almost exclusively used.
9457 @item -mtoplevel-symbols
9458 @itemx -mno-toplevel-symbols
9459 @opindex mtoplevel-symbols
9460 @opindex mno-toplevel-symbols
9461 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
9462 code can be used with the @code{PREFIX} assembly directive.
9466 Generate an executable in the ELF format, rather than the default
9467 @samp{mmo} format used by the @command{mmix} simulator.
9469 @item -mbranch-predict
9470 @itemx -mno-branch-predict
9471 @opindex mbranch-predict
9472 @opindex mno-branch-predict
9473 Use (do not use) the probable-branch instructions, when static branch
9474 prediction indicates a probable branch.
9476 @item -mbase-addresses
9477 @itemx -mno-base-addresses
9478 @opindex mbase-addresses
9479 @opindex mno-base-addresses
9480 Generate (do not generate) code that uses @emph{base addresses}. Using a
9481 base address automatically generates a request (handled by the assembler
9482 and the linker) for a constant to be set up in a global register. The
9483 register is used for one or more base address requests within the range 0
9484 to 255 from the value held in the register. The generally leads to short
9485 and fast code, but the number of different data items that can be
9486 addressed is limited. This means that a program that uses lots of static
9487 data may require @option{-mno-base-addresses}.
9490 @itemx -mno-single-exit
9491 @opindex msingle-exit
9492 @opindex mno-single-exit
9493 Force (do not force) generated code to have a single exit point in each
9497 @node MN10300 Options
9498 @subsection MN10300 Options
9499 @cindex MN10300 options
9501 These @option{-m} options are defined for Matsushita MN10300 architectures:
9506 Generate code to avoid bugs in the multiply instructions for the MN10300
9507 processors. This is the default.
9510 @opindex mno-mult-bug
9511 Do not generate code to avoid bugs in the multiply instructions for the
9516 Generate code which uses features specific to the AM33 processor.
9520 Do not generate code which uses features specific to the AM33 processor. This
9525 Do not link in the C run-time initialization object file.
9529 Indicate to the linker that it should perform a relaxation optimization pass
9530 to shorten branches, calls and absolute memory addresses. This option only
9531 has an effect when used on the command line for the final link step.
9533 This option makes symbolic debugging impossible.
9537 @subsection NS32K Options
9538 @cindex NS32K options
9540 These are the @samp{-m} options defined for the 32000 series. The default
9541 values for these options depends on which style of 32000 was selected when
9542 the compiler was configured; the defaults for the most common choices are
9550 Generate output for a 32032. This is the default
9551 when the compiler is configured for 32032 and 32016 based systems.
9557 Generate output for a 32332. This is the default
9558 when the compiler is configured for 32332-based systems.
9564 Generate output for a 32532. This is the default
9565 when the compiler is configured for 32532-based systems.
9569 Generate output containing 32081 instructions for floating point.
9570 This is the default for all systems.
9574 Generate output containing 32381 instructions for floating point. This
9575 also implies @option{-m32081}. The 32381 is only compatible with the 32332
9576 and 32532 cpus. This is the default for the pc532-netbsd configuration.
9580 Try and generate multiply-add floating point instructions @code{polyF}
9581 and @code{dotF}. This option is only available if the @option{-m32381}
9582 option is in effect. Using these instructions requires changes to
9583 register allocation which generally has a negative impact on
9584 performance. This option should only be enabled when compiling code
9585 particularly likely to make heavy use of multiply-add instructions.
9588 @opindex mnomulti-add
9589 Do not try and generate multiply-add floating point instructions
9590 @code{polyF} and @code{dotF}. This is the default on all platforms.
9593 @opindex msoft-float
9594 Generate output containing library calls for floating point.
9595 @strong{Warning:} the requisite libraries may not be available.
9597 @item -mieee-compare
9598 @itemx -mno-ieee-compare
9599 @opindex mieee-compare
9600 @opindex mno-ieee-compare
9601 Control whether or not the compiler uses IEEE floating point
9602 comparisons. These handle correctly the case where the result of a
9603 comparison is unordered.
9604 @strong{Warning:} the requisite kernel support may not be available.
9607 @opindex mnobitfield
9608 Do not use the bit-field instructions. On some machines it is faster to
9609 use shifting and masking operations. This is the default for the pc532.
9613 Do use the bit-field instructions. This is the default for all platforms
9618 Use a different function-calling convention, in which functions
9619 that take a fixed number of arguments return pop their
9620 arguments on return with the @code{ret} instruction.
9622 This calling convention is incompatible with the one normally
9623 used on Unix, so you cannot use it if you need to call libraries
9624 compiled with the Unix compiler.
9626 Also, you must provide function prototypes for all functions that
9627 take variable numbers of arguments (including @code{printf});
9628 otherwise incorrect code will be generated for calls to those
9631 In addition, seriously incorrect code will result if you call a
9632 function with too many arguments. (Normally, extra arguments are
9633 harmlessly ignored.)
9635 This option takes its name from the 680x0 @code{rtd} instruction.
9640 Use a different function-calling convention where the first two arguments
9641 are passed in registers.
9643 This calling convention is incompatible with the one normally
9644 used on Unix, so you cannot use it if you need to call libraries
9645 compiled with the Unix compiler.
9648 @opindex mnoregparam
9649 Do not pass any arguments in registers. This is the default for all
9654 It is OK to use the sb as an index register which is always loaded with
9655 zero. This is the default for the pc532-netbsd target.
9659 The sb register is not available for use or has not been initialized to
9660 zero by the run time system. This is the default for all targets except
9661 the pc532-netbsd. It is also implied whenever @option{-mhimem} or
9662 @option{-fpic} is set.
9666 Many ns32000 series addressing modes use displacements of up to 512MB@.
9667 If an address is above 512MB then displacements from zero can not be used.
9668 This option causes code to be generated which can be loaded above 512MB@.
9669 This may be useful for operating systems or ROM code.
9673 Assume code will be loaded in the first 512MB of virtual address space.
9674 This is the default for all platforms.
9678 @node PDP-11 Options
9679 @subsection PDP-11 Options
9680 @cindex PDP-11 Options
9682 These options are defined for the PDP-11:
9687 Use hardware FPP floating point. This is the default. (FIS floating
9688 point on the PDP-11/40 is not supported.)
9691 @opindex msoft-float
9692 Do not use hardware floating point.
9696 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
9700 Return floating-point results in memory. This is the default.
9704 Generate code for a PDP-11/40.
9708 Generate code for a PDP-11/45. This is the default.
9712 Generate code for a PDP-11/10.
9714 @item -mbcopy-builtin
9715 @opindex bcopy-builtin
9716 Use inline @code{movmemhi} patterns for copying memory. This is the
9721 Do not use inline @code{movmemhi} patterns for copying memory.
9727 Use 16-bit @code{int}. This is the default.
9733 Use 32-bit @code{int}.
9738 @opindex mno-float32
9739 Use 64-bit @code{float}. This is the default.
9744 @opindex mno-float64
9745 Use 32-bit @code{float}.
9749 Use @code{abshi2} pattern. This is the default.
9753 Do not use @code{abshi2} pattern.
9755 @item -mbranch-expensive
9756 @opindex mbranch-expensive
9757 Pretend that branches are expensive. This is for experimenting with
9758 code generation only.
9760 @item -mbranch-cheap
9761 @opindex mbranch-cheap
9762 Do not pretend that branches are expensive. This is the default.
9766 Generate code for a system with split I&D.
9770 Generate code for a system without split I&D. This is the default.
9774 Use Unix assembler syntax. This is the default when configured for
9779 Use DEC assembler syntax. This is the default when configured for any
9780 PDP-11 target other than @samp{pdp11-*-bsd}.
9783 @node PowerPC Options
9784 @subsection PowerPC Options
9785 @cindex PowerPC options
9787 These are listed under @xref{RS/6000 and PowerPC Options}.
9789 @node RS/6000 and PowerPC Options
9790 @subsection IBM RS/6000 and PowerPC Options
9791 @cindex RS/6000 and PowerPC Options
9792 @cindex IBM RS/6000 and PowerPC Options
9794 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
9802 @itemx -mpowerpc-gpopt
9803 @itemx -mno-powerpc-gpopt
9804 @itemx -mpowerpc-gfxopt
9805 @itemx -mno-powerpc-gfxopt
9807 @itemx -mno-powerpc64
9813 @opindex mno-powerpc
9814 @opindex mpowerpc-gpopt
9815 @opindex mno-powerpc-gpopt
9816 @opindex mpowerpc-gfxopt
9817 @opindex mno-powerpc-gfxopt
9819 @opindex mno-powerpc64
9820 GCC supports two related instruction set architectures for the
9821 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
9822 instructions supported by the @samp{rios} chip set used in the original
9823 RS/6000 systems and the @dfn{PowerPC} instruction set is the
9824 architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and
9825 the IBM 4xx microprocessors.
9827 Neither architecture is a subset of the other. However there is a
9828 large common subset of instructions supported by both. An MQ
9829 register is included in processors supporting the POWER architecture.
9831 You use these options to specify which instructions are available on the
9832 processor you are using. The default value of these options is
9833 determined when configuring GCC@. Specifying the
9834 @option{-mcpu=@var{cpu_type}} overrides the specification of these
9835 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
9836 rather than the options listed above.
9838 The @option{-mpower} option allows GCC to generate instructions that
9839 are found only in the POWER architecture and to use the MQ register.
9840 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
9841 to generate instructions that are present in the POWER2 architecture but
9842 not the original POWER architecture.
9844 The @option{-mpowerpc} option allows GCC to generate instructions that
9845 are found only in the 32-bit subset of the PowerPC architecture.
9846 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
9847 GCC to use the optional PowerPC architecture instructions in the
9848 General Purpose group, including floating-point square root. Specifying
9849 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
9850 use the optional PowerPC architecture instructions in the Graphics
9851 group, including floating-point select.
9853 The @option{-mpowerpc64} option allows GCC to generate the additional
9854 64-bit instructions that are found in the full PowerPC64 architecture
9855 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
9856 @option{-mno-powerpc64}.
9858 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
9859 will use only the instructions in the common subset of both
9860 architectures plus some special AIX common-mode calls, and will not use
9861 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
9862 permits GCC to use any instruction from either architecture and to
9863 allow use of the MQ register; specify this for the Motorola MPC601.
9865 @item -mnew-mnemonics
9866 @itemx -mold-mnemonics
9867 @opindex mnew-mnemonics
9868 @opindex mold-mnemonics
9869 Select which mnemonics to use in the generated assembler code. With
9870 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
9871 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
9872 assembler mnemonics defined for the POWER architecture. Instructions
9873 defined in only one architecture have only one mnemonic; GCC uses that
9874 mnemonic irrespective of which of these options is specified.
9876 GCC defaults to the mnemonics appropriate for the architecture in
9877 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
9878 value of these option. Unless you are building a cross-compiler, you
9879 should normally not specify either @option{-mnew-mnemonics} or
9880 @option{-mold-mnemonics}, but should instead accept the default.
9882 @item -mcpu=@var{cpu_type}
9884 Set architecture type, register usage, choice of mnemonics, and
9885 instruction scheduling parameters for machine type @var{cpu_type}.
9886 Supported values for @var{cpu_type} are @samp{401}, @samp{403},
9887 @samp{405}, @samp{405fp}, @samp{440}, @samp{440fp}, @samp{505},
9888 @samp{601}, @samp{602}, @samp{603}, @samp{603e}, @samp{604},
9889 @samp{604e}, @samp{620}, @samp{630}, @samp{740}, @samp{7400},
9890 @samp{7450}, @samp{750}, @samp{801}, @samp{821}, @samp{823},
9891 @samp{860}, @samp{970}, @samp{common}, @samp{ec603e}, @samp{G3},
9892 @samp{G4}, @samp{G5}, @samp{power}, @samp{power2}, @samp{power3},
9893 @samp{power4}, @samp{power5}, @samp{powerpc}, @samp{powerpc64},
9894 @samp{rios}, @samp{rios1}, @samp{rios2}, @samp{rsc}, and @samp{rs64a}.
9896 @option{-mcpu=common} selects a completely generic processor. Code
9897 generated under this option will run on any POWER or PowerPC processor.
9898 GCC will use only the instructions in the common subset of both
9899 architectures, and will not use the MQ register. GCC assumes a generic
9900 processor model for scheduling purposes.
9902 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
9903 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
9904 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
9905 types, with an appropriate, generic processor model assumed for
9906 scheduling purposes.
9908 The other options specify a specific processor. Code generated under
9909 those options will run best on that processor, and may not run at all on
9912 The @option{-mcpu} options automatically enable or disable the
9913 following options: @option{-maltivec}, @option{-mhard-float},
9914 @option{-mmfcrf}, @option{-mmultiple}, @option{-mnew-mnemonics},
9915 @option{-mpower}, @option{-mpower2}, @option{-mpowerpc64},
9916 @option{-mpowerpc-gpopt}, @option{-mpowerpc-gfxopt},
9917 @option{-mstring}. The particular options set for any particular CPU
9918 will vary between compiler versions, depending on what setting seems
9919 to produce optimal code for that CPU; it doesn't necessarily reflect
9920 the actual hardware's capabilities. If you wish to set an individual
9921 option to a particular value, you may specify it after the
9922 @option{-mcpu} option, like @samp{-mcpu=970 -mno-altivec}.
9924 On AIX, the @option{-maltivec} and @option{-mpowerpc64} options are
9925 not enabled or disabled by the @option{-mcpu} option at present, since
9926 AIX does not have full support for these options. You may still
9927 enable or disable them individually if you're sure it'll work in your
9930 @item -mtune=@var{cpu_type}
9932 Set the instruction scheduling parameters for machine type
9933 @var{cpu_type}, but do not set the architecture type, register usage, or
9934 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
9935 values for @var{cpu_type} are used for @option{-mtune} as for
9936 @option{-mcpu}. If both are specified, the code generated will use the
9937 architecture, registers, and mnemonics set by @option{-mcpu}, but the
9938 scheduling parameters set by @option{-mtune}.
9943 @opindex mno-altivec
9944 These switches enable or disable the use of built-in functions that
9945 allow access to the AltiVec instruction set. You may also need to set
9946 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
9951 Extend the current ABI with SPE ABI extensions. This does not change
9952 the default ABI, instead it adds the SPE ABI extensions to the current
9956 @opindex mabi=no-spe
9957 Disable Booke SPE ABI extensions for the current ABI.
9959 @item -misel=@var{yes/no}
9962 This switch enables or disables the generation of ISEL instructions.
9964 @item -mspe=@var{yes/no}
9967 This switch enables or disables the generation of SPE simd
9970 @item -mfloat-gprs=@var{yes/no}
9972 @opindex mfloat-gprs
9973 This switch enables or disables the generation of floating point
9974 operations on the general purpose registers for architectures that
9975 support it. This option is currently only available on the MPC8540.
9978 @itemx -mno-fp-in-toc
9979 @itemx -mno-sum-in-toc
9980 @itemx -mminimal-toc
9982 @opindex mno-fp-in-toc
9983 @opindex mno-sum-in-toc
9984 @opindex mminimal-toc
9985 Modify generation of the TOC (Table Of Contents), which is created for
9986 every executable file. The @option{-mfull-toc} option is selected by
9987 default. In that case, GCC will allocate at least one TOC entry for
9988 each unique non-automatic variable reference in your program. GCC
9989 will also place floating-point constants in the TOC@. However, only
9990 16,384 entries are available in the TOC@.
9992 If you receive a linker error message that saying you have overflowed
9993 the available TOC space, you can reduce the amount of TOC space used
9994 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
9995 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
9996 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
9997 generate code to calculate the sum of an address and a constant at
9998 run-time instead of putting that sum into the TOC@. You may specify one
9999 or both of these options. Each causes GCC to produce very slightly
10000 slower and larger code at the expense of conserving TOC space.
10002 If you still run out of space in the TOC even when you specify both of
10003 these options, specify @option{-mminimal-toc} instead. This option causes
10004 GCC to make only one TOC entry for every file. When you specify this
10005 option, GCC will produce code that is slower and larger but which
10006 uses extremely little TOC space. You may wish to use this option
10007 only on files that contain less frequently executed code.
10013 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
10014 @code{long} type, and the infrastructure needed to support them.
10015 Specifying @option{-maix64} implies @option{-mpowerpc64} and
10016 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
10017 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
10020 @itemx -mno-xl-call
10022 @opindex mno-xl-call
10023 On AIX, pass floating-point arguments to prototyped functions beyond the
10024 register save area (RSA) on the stack in addition to argument FPRs. The
10025 AIX calling convention was extended but not initially documented to
10026 handle an obscure K&R C case of calling a function that takes the
10027 address of its arguments with fewer arguments than declared. AIX XL
10028 compilers access floating point arguments which do not fit in the
10029 RSA from the stack when a subroutine is compiled without
10030 optimization. Because always storing floating-point arguments on the
10031 stack is inefficient and rarely needed, this option is not enabled by
10032 default and only is necessary when calling subroutines compiled by AIX
10033 XL compilers without optimization.
10037 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
10038 application written to use message passing with special startup code to
10039 enable the application to run. The system must have PE installed in the
10040 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
10041 must be overridden with the @option{-specs=} option to specify the
10042 appropriate directory location. The Parallel Environment does not
10043 support threads, so the @option{-mpe} option and the @option{-pthread}
10044 option are incompatible.
10046 @item -malign-natural
10047 @itemx -malign-power
10048 @opindex malign-natural
10049 @opindex malign-power
10050 On AIX, Darwin, and 64-bit PowerPC GNU/Linux, the option
10051 @option{-malign-natural} overrides the ABI-defined alignment of larger
10052 types, such as floating-point doubles, on their natural size-based boundary.
10053 The option @option{-malign-power} instructs GCC to follow the ABI-specified
10054 alignment rules. GCC defaults to the standard alignment defined in the ABI.
10057 @itemx -mhard-float
10058 @opindex msoft-float
10059 @opindex mhard-float
10060 Generate code that does not use (uses) the floating-point register set.
10061 Software floating point emulation is provided if you use the
10062 @option{-msoft-float} option, and pass the option to GCC when linking.
10065 @itemx -mno-multiple
10067 @opindex mno-multiple
10068 Generate code that uses (does not use) the load multiple word
10069 instructions and the store multiple word instructions. These
10070 instructions are generated by default on POWER systems, and not
10071 generated on PowerPC systems. Do not use @option{-mmultiple} on little
10072 endian PowerPC systems, since those instructions do not work when the
10073 processor is in little endian mode. The exceptions are PPC740 and
10074 PPC750 which permit the instructions usage in little endian mode.
10079 @opindex mno-string
10080 Generate code that uses (does not use) the load string instructions
10081 and the store string word instructions to save multiple registers and
10082 do small block moves. These instructions are generated by default on
10083 POWER systems, and not generated on PowerPC systems. Do not use
10084 @option{-mstring} on little endian PowerPC systems, since those
10085 instructions do not work when the processor is in little endian mode.
10086 The exceptions are PPC740 and PPC750 which permit the instructions
10087 usage in little endian mode.
10092 @opindex mno-update
10093 Generate code that uses (does not use) the load or store instructions
10094 that update the base register to the address of the calculated memory
10095 location. These instructions are generated by default. If you use
10096 @option{-mno-update}, there is a small window between the time that the
10097 stack pointer is updated and the address of the previous frame is
10098 stored, which means code that walks the stack frame across interrupts or
10099 signals may get corrupted data.
10102 @itemx -mno-fused-madd
10103 @opindex mfused-madd
10104 @opindex mno-fused-madd
10105 Generate code that uses (does not use) the floating point multiply and
10106 accumulate instructions. These instructions are generated by default if
10107 hardware floating is used.
10109 @item -mno-bit-align
10111 @opindex mno-bit-align
10112 @opindex mbit-align
10113 On System V.4 and embedded PowerPC systems do not (do) force structures
10114 and unions that contain bit-fields to be aligned to the base type of the
10117 For example, by default a structure containing nothing but 8
10118 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
10119 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
10120 the structure would be aligned to a 1 byte boundary and be one byte in
10123 @item -mno-strict-align
10124 @itemx -mstrict-align
10125 @opindex mno-strict-align
10126 @opindex mstrict-align
10127 On System V.4 and embedded PowerPC systems do not (do) assume that
10128 unaligned memory references will be handled by the system.
10130 @item -mrelocatable
10131 @itemx -mno-relocatable
10132 @opindex mrelocatable
10133 @opindex mno-relocatable
10134 On embedded PowerPC systems generate code that allows (does not allow)
10135 the program to be relocated to a different address at runtime. If you
10136 use @option{-mrelocatable} on any module, all objects linked together must
10137 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
10139 @item -mrelocatable-lib
10140 @itemx -mno-relocatable-lib
10141 @opindex mrelocatable-lib
10142 @opindex mno-relocatable-lib
10143 On embedded PowerPC systems generate code that allows (does not allow)
10144 the program to be relocated to a different address at runtime. Modules
10145 compiled with @option{-mrelocatable-lib} can be linked with either modules
10146 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
10147 with modules compiled with the @option{-mrelocatable} options.
10153 On System V.4 and embedded PowerPC systems do not (do) assume that
10154 register 2 contains a pointer to a global area pointing to the addresses
10155 used in the program.
10158 @itemx -mlittle-endian
10160 @opindex mlittle-endian
10161 On System V.4 and embedded PowerPC systems compile code for the
10162 processor in little endian mode. The @option{-mlittle-endian} option is
10163 the same as @option{-mlittle}.
10166 @itemx -mbig-endian
10168 @opindex mbig-endian
10169 On System V.4 and embedded PowerPC systems compile code for the
10170 processor in big endian mode. The @option{-mbig-endian} option is
10171 the same as @option{-mbig}.
10173 @item -mdynamic-no-pic
10174 @opindex mdynamic-no-pic
10175 On Darwin and Mac OS X systems, compile code so that it is not
10176 relocatable, but that its external references are relocatable. The
10177 resulting code is suitable for applications, but not shared
10180 @item -mprioritize-restricted-insns=@var{priority}
10181 @opindex mprioritize-restricted-insns
10182 This option controls the priority that is assigned to
10183 dispatch-slot restricted instructions during the second scheduling
10184 pass. The argument @var{priority} takes the value @var{0/1/2} to assign
10185 @var{no/highest/second-highest} priority to dispatch slot restricted
10188 @item -msched-costly-dep=@var{dependence_type}
10189 @opindex msched-costly-dep
10190 This option controls which dependences are considered costly
10191 by the target during instruction scheduling. The argument
10192 @var{dependence_type} takes one of the following values:
10193 @var{no}: no dependence is costly,
10194 @var{all}: all dependences are costly,
10195 @var{true_store_to_load}: a true dependence from store to load is costly,
10196 @var{store_to_load}: any dependence from store to load is costly,
10197 @var{number}: any dependence which latency >= @var{number} is costly.
10199 @item -minsert-sched-nops=@var{scheme}
10200 @opindex minsert-sched-nops
10201 This option controls which nop insertion scheme will be used during
10202 the second scheduling pass. The argument @var{scheme} takes one of the
10204 @var{no}: Don't insert nops.
10205 @var{pad}: Pad with nops any dispatch group which has vacant issue slots,
10206 according to the scheduler's grouping.
10207 @var{regroup_exact}: Insert nops to force costly dependent insns into
10208 separate groups. Insert exactly as many nops as needed to force an insn
10209 to a new group, according to the estimated processor grouping.
10210 @var{number}: Insert nops to force costly dependent insns into
10211 separate groups. Insert @var{number} nops to force an insn to a new group.
10214 @opindex mcall-sysv
10215 On System V.4 and embedded PowerPC systems compile code using calling
10216 conventions that adheres to the March 1995 draft of the System V
10217 Application Binary Interface, PowerPC processor supplement. This is the
10218 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
10220 @item -mcall-sysv-eabi
10221 @opindex mcall-sysv-eabi
10222 Specify both @option{-mcall-sysv} and @option{-meabi} options.
10224 @item -mcall-sysv-noeabi
10225 @opindex mcall-sysv-noeabi
10226 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
10228 @item -mcall-solaris
10229 @opindex mcall-solaris
10230 On System V.4 and embedded PowerPC systems compile code for the Solaris
10234 @opindex mcall-linux
10235 On System V.4 and embedded PowerPC systems compile code for the
10236 Linux-based GNU system.
10240 On System V.4 and embedded PowerPC systems compile code for the
10241 Hurd-based GNU system.
10243 @item -mcall-netbsd
10244 @opindex mcall-netbsd
10245 On System V.4 and embedded PowerPC systems compile code for the
10246 NetBSD operating system.
10248 @item -maix-struct-return
10249 @opindex maix-struct-return
10250 Return all structures in memory (as specified by the AIX ABI)@.
10252 @item -msvr4-struct-return
10253 @opindex msvr4-struct-return
10254 Return structures smaller than 8 bytes in registers (as specified by the
10257 @item -mabi=altivec
10258 @opindex mabi=altivec
10259 Extend the current ABI with AltiVec ABI extensions. This does not
10260 change the default ABI, instead it adds the AltiVec ABI extensions to
10263 @item -mabi=no-altivec
10264 @opindex mabi=no-altivec
10265 Disable AltiVec ABI extensions for the current ABI.
10268 @itemx -mno-prototype
10269 @opindex mprototype
10270 @opindex mno-prototype
10271 On System V.4 and embedded PowerPC systems assume that all calls to
10272 variable argument functions are properly prototyped. Otherwise, the
10273 compiler must insert an instruction before every non prototyped call to
10274 set or clear bit 6 of the condition code register (@var{CR}) to
10275 indicate whether floating point values were passed in the floating point
10276 registers in case the function takes a variable arguments. With
10277 @option{-mprototype}, only calls to prototyped variable argument functions
10278 will set or clear the bit.
10282 On embedded PowerPC systems, assume that the startup module is called
10283 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
10284 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
10289 On embedded PowerPC systems, assume that the startup module is called
10290 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
10295 On embedded PowerPC systems, assume that the startup module is called
10296 @file{crt0.o} and the standard C libraries are @file{libads.a} and
10299 @item -myellowknife
10300 @opindex myellowknife
10301 On embedded PowerPC systems, assume that the startup module is called
10302 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
10307 On System V.4 and embedded PowerPC systems, specify that you are
10308 compiling for a VxWorks system.
10312 Specify that you are compiling for the WindISS simulation environment.
10316 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
10317 header to indicate that @samp{eabi} extended relocations are used.
10323 On System V.4 and embedded PowerPC systems do (do not) adhere to the
10324 Embedded Applications Binary Interface (eabi) which is a set of
10325 modifications to the System V.4 specifications. Selecting @option{-meabi}
10326 means that the stack is aligned to an 8 byte boundary, a function
10327 @code{__eabi} is called to from @code{main} to set up the eabi
10328 environment, and the @option{-msdata} option can use both @code{r2} and
10329 @code{r13} to point to two separate small data areas. Selecting
10330 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
10331 do not call an initialization function from @code{main}, and the
10332 @option{-msdata} option will only use @code{r13} to point to a single
10333 small data area. The @option{-meabi} option is on by default if you
10334 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
10337 @opindex msdata=eabi
10338 On System V.4 and embedded PowerPC systems, put small initialized
10339 @code{const} global and static data in the @samp{.sdata2} section, which
10340 is pointed to by register @code{r2}. Put small initialized
10341 non-@code{const} global and static data in the @samp{.sdata} section,
10342 which is pointed to by register @code{r13}. Put small uninitialized
10343 global and static data in the @samp{.sbss} section, which is adjacent to
10344 the @samp{.sdata} section. The @option{-msdata=eabi} option is
10345 incompatible with the @option{-mrelocatable} option. The
10346 @option{-msdata=eabi} option also sets the @option{-memb} option.
10349 @opindex msdata=sysv
10350 On System V.4 and embedded PowerPC systems, put small global and static
10351 data in the @samp{.sdata} section, which is pointed to by register
10352 @code{r13}. Put small uninitialized global and static data in the
10353 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
10354 The @option{-msdata=sysv} option is incompatible with the
10355 @option{-mrelocatable} option.
10357 @item -msdata=default
10359 @opindex msdata=default
10361 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
10362 compile code the same as @option{-msdata=eabi}, otherwise compile code the
10363 same as @option{-msdata=sysv}.
10366 @opindex msdata-data
10367 On System V.4 and embedded PowerPC systems, put small global and static
10368 data in the @samp{.sdata} section. Put small uninitialized global and
10369 static data in the @samp{.sbss} section. Do not use register @code{r13}
10370 to address small data however. This is the default behavior unless
10371 other @option{-msdata} options are used.
10375 @opindex msdata=none
10377 On embedded PowerPC systems, put all initialized global and static data
10378 in the @samp{.data} section, and all uninitialized data in the
10379 @samp{.bss} section.
10383 @cindex smaller data references (PowerPC)
10384 @cindex .sdata/.sdata2 references (PowerPC)
10385 On embedded PowerPC systems, put global and static items less than or
10386 equal to @var{num} bytes into the small data or bss sections instead of
10387 the normal data or bss section. By default, @var{num} is 8. The
10388 @option{-G @var{num}} switch is also passed to the linker.
10389 All modules should be compiled with the same @option{-G @var{num}} value.
10392 @itemx -mno-regnames
10394 @opindex mno-regnames
10395 On System V.4 and embedded PowerPC systems do (do not) emit register
10396 names in the assembly language output using symbolic forms.
10399 @itemx -mno-longcall
10401 @opindex mno-longcall
10402 Default to making all function calls indirectly, using a register, so
10403 that functions which reside further than 32 megabytes (33,554,432
10404 bytes) from the current location can be called. This setting can be
10405 overridden by the @code{shortcall} function attribute, or by
10406 @code{#pragma longcall(0)}.
10408 Some linkers are capable of detecting out-of-range calls and generating
10409 glue code on the fly. On these systems, long calls are unnecessary and
10410 generate slower code. As of this writing, the AIX linker can do this,
10411 as can the GNU linker for PowerPC/64. It is planned to add this feature
10412 to the GNU linker for 32-bit PowerPC systems as well.
10414 On Darwin/PPC systems, @code{#pragma longcall} will generate ``jbsr
10415 callee, L42'', plus a ``branch island'' (glue code). The two target
10416 addresses represent the callee and the ``branch island.'' The
10417 Darwin/PPC linker will prefer the first address and generate a ``bl
10418 callee'' if the PPC ``bl'' instruction will reach the callee directly;
10419 otherwise, the linker will generate ``bl L42'' to call the ``branch
10420 island.'' The ``branch island'' is appended to the body of the
10421 calling function; it computes the full 32-bit address of the callee
10424 On Mach-O (Darwin) systems, this option directs the compiler emit to
10425 the glue for every direct call, and the Darwin linker decides whether
10426 to use or discard it.
10428 In the future, we may cause GCC to ignore all longcall specifications
10429 when the linker is known to generate glue.
10433 Adds support for multithreading with the @dfn{pthreads} library.
10434 This option sets flags for both the preprocessor and linker.
10438 @node S/390 and zSeries Options
10439 @subsection S/390 and zSeries Options
10440 @cindex S/390 and zSeries Options
10442 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
10446 @itemx -msoft-float
10447 @opindex mhard-float
10448 @opindex msoft-float
10449 Use (do not use) the hardware floating-point instructions and registers
10450 for floating-point operations. When @option{-msoft-float} is specified,
10451 functions in @file{libgcc.a} will be used to perform floating-point
10452 operations. When @option{-mhard-float} is specified, the compiler
10453 generates IEEE floating-point instructions. This is the default.
10456 @itemx -mno-backchain
10457 @itemx -mkernel-backchain
10458 @opindex mbackchain
10459 @opindex mno-backchain
10460 @opindex mkernel-backchain
10461 In order to provide a backchain the address of the caller's frame
10462 is stored within the callee's stack frame.
10463 A backchain may be needed to allow debugging using tools that do not understand
10464 DWARF-2 call frame information.
10465 For @option{-mno-backchain} no backchain is maintained at all which is the
10467 If one of the other options is present the backchain pointer is placed either
10468 on top of the stack frame (@option{-mkernel-backchain}) or on
10469 the bottom (@option{-mbackchain}).
10470 Beside the different backchain location @option{-mkernel-backchain}
10471 also changes stack frame layout breaking the ABI. This option
10472 is intended to be used for code which internally needs a backchain but has
10473 to get by with a limited stack size e.g. the linux kernel.
10474 Internal unwinding code not using DWARF-2 info has to be able to locate the
10475 return address of a function. That will be eased be the fact that
10476 the return address of a function is placed two words below the backchain
10480 @itemx -mno-small-exec
10481 @opindex msmall-exec
10482 @opindex mno-small-exec
10483 Generate (or do not generate) code using the @code{bras} instruction
10484 to do subroutine calls.
10485 This only works reliably if the total executable size does not
10486 exceed 64k. The default is to use the @code{basr} instruction instead,
10487 which does not have this limitation.
10493 When @option{-m31} is specified, generate code compliant to the
10494 GNU/Linux for S/390 ABI@. When @option{-m64} is specified, generate
10495 code compliant to the GNU/Linux for zSeries ABI@. This allows GCC in
10496 particular to generate 64-bit instructions. For the @samp{s390}
10497 targets, the default is @option{-m31}, while the @samp{s390x}
10498 targets default to @option{-m64}.
10504 When @option{-mzarch} is specified, generate code using the
10505 instructions available on z/Architecture.
10506 When @option{-mesa} is specified, generate code using the
10507 instructions available on ESA/390. Note that @option{-mesa} is
10508 not possible with @option{-m64}.
10509 When generating code compliant to the GNU/Linux for S/390 ABI,
10510 the default is @option{-mesa}. When generating code compliant
10511 to the GNU/Linux for zSeries ABI, the default is @option{-mzarch}.
10517 Generate (or do not generate) code using the @code{mvcle} instruction
10518 to perform block moves. When @option{-mno-mvcle} is specified,
10519 use a @code{mvc} loop instead. This is the default.
10525 Print (or do not print) additional debug information when compiling.
10526 The default is to not print debug information.
10528 @item -march=@var{cpu-type}
10530 Generate code that will run on @var{cpu-type}, which is the name of a system
10531 representing a certain processor type. Possible values for
10532 @var{cpu-type} are @samp{g5}, @samp{g6}, @samp{z900}, and @samp{z990}.
10533 When generating code using the instructions available on z/Architecture,
10534 the default is @option{-march=z900}. Otherwise, the default is
10535 @option{-march=g5}.
10537 @item -mtune=@var{cpu-type}
10539 Tune to @var{cpu-type} everything applicable about the generated code,
10540 except for the ABI and the set of available instructions.
10541 The list of @var{cpu-type} values is the same as for @option{-march}.
10542 The default is the value used for @option{-march}.
10545 @itemx -mno-tpf-trace
10546 @opindex mtpf-trace
10547 @opindex mno-tpf-trace
10548 Generate code that adds (does not add) in TPF OS specific branches to trace
10549 routines in the operating system. This option is off by default, even
10550 when compiling for the TPF OS.
10553 @itemx -mno-fused-madd
10554 @opindex mfused-madd
10555 @opindex mno-fused-madd
10556 Generate code that uses (does not use) the floating point multiply and
10557 accumulate instructions. These instructions are generated by default if
10558 hardware floating point is used.
10562 @subsection SH Options
10564 These @samp{-m} options are defined for the SH implementations:
10569 Generate code for the SH1.
10573 Generate code for the SH2.
10576 Generate code for the SH2e.
10580 Generate code for the SH3.
10584 Generate code for the SH3e.
10588 Generate code for the SH4 without a floating-point unit.
10590 @item -m4-single-only
10591 @opindex m4-single-only
10592 Generate code for the SH4 with a floating-point unit that only
10593 supports single-precision arithmetic.
10597 Generate code for the SH4 assuming the floating-point unit is in
10598 single-precision mode by default.
10602 Generate code for the SH4.
10606 Generate code for the SH4al-dsp, or for a SH4a in such a way that the
10607 floating-point unit is not used.
10609 @item -m4a-single-only
10610 @opindex m4a-single-only
10611 Generate code for the SH4a, in such a way that no double-precision
10612 floating point operations are used.
10615 @opindex m4a-single
10616 Generate code for the SH4a assuming the floating-point unit is in
10617 single-precision mode by default.
10621 Generate code for the SH4a.
10625 Same as @option{-m4a-nofpu}, except that it implicitly passes
10626 @option{-dsp} to the assembler. GCC doesn't generate any DSP
10627 instructions at the moment.
10631 Compile code for the processor in big endian mode.
10635 Compile code for the processor in little endian mode.
10639 Align doubles at 64-bit boundaries. Note that this changes the calling
10640 conventions, and thus some functions from the standard C library will
10641 not work unless you recompile it first with @option{-mdalign}.
10645 Shorten some address references at link time, when possible; uses the
10646 linker option @option{-relax}.
10650 Use 32-bit offsets in @code{switch} tables. The default is to use
10655 Enable the use of the instruction @code{fmovd}.
10659 Comply with the calling conventions defined by Renesas.
10663 Comply with the calling conventions defined by Renesas.
10667 Comply with the calling conventions defined for GCC before the Renesas
10668 conventions were available. This option is the default for all
10669 targets of the SH toolchain except for @samp{sh-symbianelf}.
10672 @opindex mnomacsave
10673 Mark the @code{MAC} register as call-clobbered, even if
10674 @option{-mhitachi} is given.
10678 Increase IEEE-compliance of floating-point code.
10682 Dump instruction size and location in the assembly code.
10685 @opindex mpadstruct
10686 This option is deprecated. It pads structures to multiple of 4 bytes,
10687 which is incompatible with the SH ABI@.
10691 Optimize for space instead of speed. Implied by @option{-Os}.
10694 @opindex mprefergot
10695 When generating position-independent code, emit function calls using
10696 the Global Offset Table instead of the Procedure Linkage Table.
10700 Generate a library function call to invalidate instruction cache
10701 entries, after fixing up a trampoline. This library function call
10702 doesn't assume it can write to the whole memory address space. This
10703 is the default when the target is @code{sh-*-linux*}.
10706 @node SPARC Options
10707 @subsection SPARC Options
10708 @cindex SPARC options
10710 These @samp{-m} options are supported on the SPARC:
10713 @item -mno-app-regs
10715 @opindex mno-app-regs
10717 Specify @option{-mapp-regs} to generate output using the global registers
10718 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
10721 To be fully SVR4 ABI compliant at the cost of some performance loss,
10722 specify @option{-mno-app-regs}. You should compile libraries and system
10723 software with this option.
10726 @itemx -mhard-float
10728 @opindex mhard-float
10729 Generate output containing floating point instructions. This is the
10733 @itemx -msoft-float
10735 @opindex msoft-float
10736 Generate output containing library calls for floating point.
10737 @strong{Warning:} the requisite libraries are not available for all SPARC
10738 targets. Normally the facilities of the machine's usual C compiler are
10739 used, but this cannot be done directly in cross-compilation. You must make
10740 your own arrangements to provide suitable library functions for
10741 cross-compilation. The embedded targets @samp{sparc-*-aout} and
10742 @samp{sparclite-*-*} do provide software floating point support.
10744 @option{-msoft-float} changes the calling convention in the output file;
10745 therefore, it is only useful if you compile @emph{all} of a program with
10746 this option. In particular, you need to compile @file{libgcc.a}, the
10747 library that comes with GCC, with @option{-msoft-float} in order for
10750 @item -mhard-quad-float
10751 @opindex mhard-quad-float
10752 Generate output containing quad-word (long double) floating point
10755 @item -msoft-quad-float
10756 @opindex msoft-quad-float
10757 Generate output containing library calls for quad-word (long double)
10758 floating point instructions. The functions called are those specified
10759 in the SPARC ABI@. This is the default.
10761 As of this writing, there are no SPARC implementations that have hardware
10762 support for the quad-word floating point instructions. They all invoke
10763 a trap handler for one of these instructions, and then the trap handler
10764 emulates the effect of the instruction. Because of the trap handler overhead,
10765 this is much slower than calling the ABI library routines. Thus the
10766 @option{-msoft-quad-float} option is the default.
10768 @item -mno-unaligned-doubles
10769 @itemx -munaligned-doubles
10770 @opindex mno-unaligned-doubles
10771 @opindex munaligned-doubles
10772 Assume that doubles have 8 byte alignment. This is the default.
10774 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
10775 alignment only if they are contained in another type, or if they have an
10776 absolute address. Otherwise, it assumes they have 4 byte alignment.
10777 Specifying this option avoids some rare compatibility problems with code
10778 generated by other compilers. It is not the default because it results
10779 in a performance loss, especially for floating point code.
10781 @item -mno-faster-structs
10782 @itemx -mfaster-structs
10783 @opindex mno-faster-structs
10784 @opindex mfaster-structs
10785 With @option{-mfaster-structs}, the compiler assumes that structures
10786 should have 8 byte alignment. This enables the use of pairs of
10787 @code{ldd} and @code{std} instructions for copies in structure
10788 assignment, in place of twice as many @code{ld} and @code{st} pairs.
10789 However, the use of this changed alignment directly violates the SPARC
10790 ABI@. Thus, it's intended only for use on targets where the developer
10791 acknowledges that their resulting code will not be directly in line with
10792 the rules of the ABI@.
10794 @item -mimpure-text
10795 @opindex mimpure-text
10796 @option{-mimpure-text}, used in addition to @option{-shared}, tells
10797 the compiler to not pass @option{-z text} to the linker when linking a
10798 shared object. Using this option, you can link position-dependent
10799 code into a shared object.
10801 @option{-mimpure-text} suppresses the ``relocations remain against
10802 allocatable but non-writable sections'' linker error message.
10803 However, the necessary relocations will trigger copy-on-write, and the
10804 shared object is not actually shared across processes. Instead of
10805 using @option{-mimpure-text}, you should compile all source code with
10806 @option{-fpic} or @option{-fPIC}.
10808 This option is only available on SunOS and Solaris.
10810 @item -mcpu=@var{cpu_type}
10812 Set the instruction set, register set, and instruction scheduling parameters
10813 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
10814 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
10815 @samp{f930}, @samp{f934}, @samp{hypersparc}, @samp{sparclite86x},
10816 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc}, and
10817 @samp{ultrasparc3}.
10819 Default instruction scheduling parameters are used for values that select
10820 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
10821 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
10823 Here is a list of each supported architecture and their supported
10828 v8: supersparc, hypersparc
10829 sparclite: f930, f934, sparclite86x
10831 v9: ultrasparc, ultrasparc3
10834 By default (unless configured otherwise), GCC generates code for the V7
10835 variant of the SPARC architecture. With @option{-mcpu=cypress}, the compiler
10836 additionally optimizes it for the Cypress CY7C602 chip, as used in the
10837 SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
10838 SPARCStation 1, 2, IPX etc.
10840 With @option{-mcpu=v8}, GCC generates code for the V8 variant of the SPARC
10841 architecture. The only difference from V7 code is that the compiler emits
10842 the integer multiply and integer divide instructions which exist in SPARC-V8
10843 but not in SPARC-V7. With @option{-mcpu=supersparc}, the compiler additionally
10844 optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
10847 With @option{-mcpu=sparclite}, GCC generates code for the SPARClite variant of
10848 the SPARC architecture. This adds the integer multiply, integer divide step
10849 and scan (@code{ffs}) instructions which exist in SPARClite but not in SPARC-V7.
10850 With @option{-mcpu=f930}, the compiler additionally optimizes it for the
10851 Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With
10852 @option{-mcpu=f934}, the compiler additionally optimizes it for the Fujitsu
10853 MB86934 chip, which is the more recent SPARClite with FPU.
10855 With @option{-mcpu=sparclet}, GCC generates code for the SPARClet variant of
10856 the SPARC architecture. This adds the integer multiply, multiply/accumulate,
10857 integer divide step and scan (@code{ffs}) instructions which exist in SPARClet
10858 but not in SPARC-V7. With @option{-mcpu=tsc701}, the compiler additionally
10859 optimizes it for the TEMIC SPARClet chip.
10861 With @option{-mcpu=v9}, GCC generates code for the V9 variant of the SPARC
10862 architecture. This adds 64-bit integer and floating-point move instructions,
10863 3 additional floating-point condition code registers and conditional move
10864 instructions. With @option{-mcpu=ultrasparc}, the compiler additionally
10865 optimizes it for the Sun UltraSPARC I/II chips. With
10866 @option{-mcpu=ultrasparc3}, the compiler additionally optimizes it for the
10867 Sun UltraSPARC III chip.
10869 @item -mtune=@var{cpu_type}
10871 Set the instruction scheduling parameters for machine type
10872 @var{cpu_type}, but do not set the instruction set or register set that the
10873 option @option{-mcpu=@var{cpu_type}} would.
10875 The same values for @option{-mcpu=@var{cpu_type}} can be used for
10876 @option{-mtune=@var{cpu_type}}, but the only useful values are those
10877 that select a particular cpu implementation. Those are @samp{cypress},
10878 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
10879 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc}, and
10880 @samp{ultrasparc3}.
10885 @opindex mno-v8plus
10886 With @option{-mv8plus}, GCC generates code for the SPARC-V8+ ABI. The
10887 difference from the V8 ABI is that the global and out registers are
10888 considered 64-bit wide. This is enabled by default on Solaris in 32-bit
10889 mode for all SPARC-V9 processors.
10895 With @option{-mvis}, GCC generates code that takes advantage of the UltraSPARC
10896 Visual Instruction Set extensions. The default is @option{-mno-vis}.
10899 These @samp{-m} options are supported in addition to the above
10900 on SPARC-V9 processors in 64-bit environments:
10903 @item -mlittle-endian
10904 @opindex mlittle-endian
10905 Generate code for a processor running in little-endian mode. It is only
10906 available for a few configurations and most notably not on Solaris.
10912 Generate code for a 32-bit or 64-bit environment.
10913 The 32-bit environment sets int, long and pointer to 32 bits.
10914 The 64-bit environment sets int to 32 bits and long and pointer
10917 @item -mcmodel=medlow
10918 @opindex mcmodel=medlow
10919 Generate code for the Medium/Low code model: 64-bit addresses, programs
10920 must be linked in the low 32 bits of memory. Programs can be statically
10921 or dynamically linked.
10923 @item -mcmodel=medmid
10924 @opindex mcmodel=medmid
10925 Generate code for the Medium/Middle code model: 64-bit addresses, programs
10926 must be linked in the low 44 bits of memory, the text and data segments must
10927 be less than 2GB in size and the data segment must be located within 2GB of
10930 @item -mcmodel=medany
10931 @opindex mcmodel=medany
10932 Generate code for the Medium/Anywhere code model: 64-bit addresses, programs
10933 may be linked anywhere in memory, the text and data segments must be less
10934 than 2GB in size and the data segment must be located within 2GB of the
10937 @item -mcmodel=embmedany
10938 @opindex mcmodel=embmedany
10939 Generate code for the Medium/Anywhere code model for embedded systems:
10940 64-bit addresses, the text and data segments must be less than 2GB in
10941 size, both starting anywhere in memory (determined at link time). The
10942 global register %g4 points to the base of the data segment. Programs
10943 are statically linked and PIC is not supported.
10946 @itemx -mno-stack-bias
10947 @opindex mstack-bias
10948 @opindex mno-stack-bias
10949 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
10950 frame pointer if present, are offset by @minus{}2047 which must be added back
10951 when making stack frame references. This is the default in 64-bit mode.
10952 Otherwise, assume no such offset is present.
10955 These switches are supported in addition to the above on Solaris:
10960 Add support for multithreading using the Solaris threads library. This
10961 option sets flags for both the preprocessor and linker. This option does
10962 not affect the thread safety of object code produced by the compiler or
10963 that of libraries supplied with it.
10967 Add support for multithreading using the POSIX threads library. This
10968 option sets flags for both the preprocessor and linker. This option does
10969 not affect the thread safety of object code produced by the compiler or
10970 that of libraries supplied with it.
10973 @node System V Options
10974 @subsection Options for System V
10976 These additional options are available on System V Release 4 for
10977 compatibility with other compilers on those systems:
10982 Create a shared object.
10983 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
10987 Identify the versions of each tool used by the compiler, in a
10988 @code{.ident} assembler directive in the output.
10992 Refrain from adding @code{.ident} directives to the output file (this is
10995 @item -YP,@var{dirs}
10997 Search the directories @var{dirs}, and no others, for libraries
10998 specified with @option{-l}.
11000 @item -Ym,@var{dir}
11002 Look in the directory @var{dir} to find the M4 preprocessor.
11003 The assembler uses this option.
11004 @c This is supposed to go with a -Yd for predefined M4 macro files, but
11005 @c the generic assembler that comes with Solaris takes just -Ym.
11008 @node TMS320C3x/C4x Options
11009 @subsection TMS320C3x/C4x Options
11010 @cindex TMS320C3x/C4x Options
11012 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
11016 @item -mcpu=@var{cpu_type}
11018 Set the instruction set, register set, and instruction scheduling
11019 parameters for machine type @var{cpu_type}. Supported values for
11020 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
11021 @samp{c44}. The default is @samp{c40} to generate code for the
11026 @itemx -msmall-memory
11028 @opindex mbig-memory
11030 @opindex msmall-memory
11032 Generates code for the big or small memory model. The small memory
11033 model assumed that all data fits into one 64K word page. At run-time
11034 the data page (DP) register must be set to point to the 64K page
11035 containing the .bss and .data program sections. The big memory model is
11036 the default and requires reloading of the DP register for every direct
11043 Allow (disallow) allocation of general integer operands into the block
11044 count register BK@.
11050 Enable (disable) generation of code using decrement and branch,
11051 DBcond(D), instructions. This is enabled by default for the C4x. To be
11052 on the safe side, this is disabled for the C3x, since the maximum
11053 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
11054 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
11055 that it can utilize the decrement and branch instruction, but will give
11056 up if there is more than one memory reference in the loop. Thus a loop
11057 where the loop counter is decremented can generate slightly more
11058 efficient code, in cases where the RPTB instruction cannot be utilized.
11060 @item -mdp-isr-reload
11062 @opindex mdp-isr-reload
11064 Force the DP register to be saved on entry to an interrupt service
11065 routine (ISR), reloaded to point to the data section, and restored on
11066 exit from the ISR@. This should not be required unless someone has
11067 violated the small memory model by modifying the DP register, say within
11074 For the C3x use the 24-bit MPYI instruction for integer multiplies
11075 instead of a library call to guarantee 32-bit results. Note that if one
11076 of the operands is a constant, then the multiplication will be performed
11077 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
11078 then squaring operations are performed inline instead of a library call.
11081 @itemx -mno-fast-fix
11083 @opindex mno-fast-fix
11084 The C3x/C4x FIX instruction to convert a floating point value to an
11085 integer value chooses the nearest integer less than or equal to the
11086 floating point value rather than to the nearest integer. Thus if the
11087 floating point number is negative, the result will be incorrectly
11088 truncated an additional code is necessary to detect and correct this
11089 case. This option can be used to disable generation of the additional
11090 code required to correct the result.
11096 Enable (disable) generation of repeat block sequences using the RPTB
11097 instruction for zero overhead looping. The RPTB construct is only used
11098 for innermost loops that do not call functions or jump across the loop
11099 boundaries. There is no advantage having nested RPTB loops due to the
11100 overhead required to save and restore the RC, RS, and RE registers.
11101 This is enabled by default with @option{-O2}.
11103 @item -mrpts=@var{count}
11107 Enable (disable) the use of the single instruction repeat instruction
11108 RPTS@. If a repeat block contains a single instruction, and the loop
11109 count can be guaranteed to be less than the value @var{count}, GCC will
11110 emit a RPTS instruction instead of a RPTB@. If no value is specified,
11111 then a RPTS will be emitted even if the loop count cannot be determined
11112 at compile time. Note that the repeated instruction following RPTS does
11113 not have to be reloaded from memory each iteration, thus freeing up the
11114 CPU buses for operands. However, since interrupts are blocked by this
11115 instruction, it is disabled by default.
11117 @item -mloop-unsigned
11118 @itemx -mno-loop-unsigned
11119 @opindex mloop-unsigned
11120 @opindex mno-loop-unsigned
11121 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
11122 is @math{2^{31} + 1} since these instructions test if the iteration count is
11123 negative to terminate the loop. If the iteration count is unsigned
11124 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
11125 exceeded. This switch allows an unsigned iteration count.
11129 Try to emit an assembler syntax that the TI assembler (asm30) is happy
11130 with. This also enforces compatibility with the API employed by the TI
11131 C3x C compiler. For example, long doubles are passed as structures
11132 rather than in floating point registers.
11138 Generate code that uses registers (stack) for passing arguments to functions.
11139 By default, arguments are passed in registers where possible rather
11140 than by pushing arguments on to the stack.
11142 @item -mparallel-insns
11143 @itemx -mno-parallel-insns
11144 @opindex mparallel-insns
11145 @opindex mno-parallel-insns
11146 Allow the generation of parallel instructions. This is enabled by
11147 default with @option{-O2}.
11149 @item -mparallel-mpy
11150 @itemx -mno-parallel-mpy
11151 @opindex mparallel-mpy
11152 @opindex mno-parallel-mpy
11153 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
11154 provided @option{-mparallel-insns} is also specified. These instructions have
11155 tight register constraints which can pessimize the code generation
11156 of large functions.
11161 @subsection V850 Options
11162 @cindex V850 Options
11164 These @samp{-m} options are defined for V850 implementations:
11168 @itemx -mno-long-calls
11169 @opindex mlong-calls
11170 @opindex mno-long-calls
11171 Treat all calls as being far away (near). If calls are assumed to be
11172 far away, the compiler will always load the functions address up into a
11173 register, and call indirect through the pointer.
11179 Do not optimize (do optimize) basic blocks that use the same index
11180 pointer 4 or more times to copy pointer into the @code{ep} register, and
11181 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
11182 option is on by default if you optimize.
11184 @item -mno-prolog-function
11185 @itemx -mprolog-function
11186 @opindex mno-prolog-function
11187 @opindex mprolog-function
11188 Do not use (do use) external functions to save and restore registers
11189 at the prologue and epilogue of a function. The external functions
11190 are slower, but use less code space if more than one function saves
11191 the same number of registers. The @option{-mprolog-function} option
11192 is on by default if you optimize.
11196 Try to make the code as small as possible. At present, this just turns
11197 on the @option{-mep} and @option{-mprolog-function} options.
11199 @item -mtda=@var{n}
11201 Put static or global variables whose size is @var{n} bytes or less into
11202 the tiny data area that register @code{ep} points to. The tiny data
11203 area can hold up to 256 bytes in total (128 bytes for byte references).
11205 @item -msda=@var{n}
11207 Put static or global variables whose size is @var{n} bytes or less into
11208 the small data area that register @code{gp} points to. The small data
11209 area can hold up to 64 kilobytes.
11211 @item -mzda=@var{n}
11213 Put static or global variables whose size is @var{n} bytes or less into
11214 the first 32 kilobytes of memory.
11218 Specify that the target processor is the V850.
11221 @opindex mbig-switch
11222 Generate code suitable for big switch tables. Use this option only if
11223 the assembler/linker complain about out of range branches within a switch
11228 This option will cause r2 and r5 to be used in the code generated by
11229 the compiler. This setting is the default.
11231 @item -mno-app-regs
11232 @opindex mno-app-regs
11233 This option will cause r2 and r5 to be treated as fixed registers.
11237 Specify that the target processor is the V850E1. The preprocessor
11238 constants @samp{__v850e1__} and @samp{__v850e__} will be defined if
11239 this option is used.
11243 Specify that the target processor is the V850E. The preprocessor
11244 constant @samp{__v850e__} will be defined if this option is used.
11246 If neither @option{-mv850} nor @option{-mv850e} nor @option{-mv850e1}
11247 are defined then a default target processor will be chosen and the
11248 relevant @samp{__v850*__} preprocessor constant will be defined.
11250 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
11251 defined, regardless of which processor variant is the target.
11253 @item -mdisable-callt
11254 @opindex mdisable-callt
11255 This option will suppress generation of the CALLT instruction for the
11256 v850e and v850e1 flavors of the v850 architecture. The default is
11257 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
11262 @subsection VAX Options
11263 @cindex VAX options
11265 These @samp{-m} options are defined for the VAX:
11270 Do not output certain jump instructions (@code{aobleq} and so on)
11271 that the Unix assembler for the VAX cannot handle across long
11276 Do output those jump instructions, on the assumption that you
11277 will assemble with the GNU assembler.
11281 Output code for g-format floating point numbers instead of d-format.
11284 @node x86-64 Options
11285 @subsection x86-64 Options
11286 @cindex x86-64 options
11288 These are listed under @xref{i386 and x86-64 Options}.
11290 @node Xstormy16 Options
11291 @subsection Xstormy16 Options
11292 @cindex Xstormy16 Options
11294 These options are defined for Xstormy16:
11299 Choose startup files and linker script suitable for the simulator.
11302 @node Xtensa Options
11303 @subsection Xtensa Options
11304 @cindex Xtensa Options
11306 These options are supported for Xtensa targets:
11310 @itemx -mno-const16
11312 @opindex mno-const16
11313 Enable or disable use of @code{CONST16} instructions for loading
11314 constant values. The @code{CONST16} instruction is currently not a
11315 standard option from Tensilica. When enabled, @code{CONST16}
11316 instructions are always used in place of the standard @code{L32R}
11317 instructions. The use of @code{CONST16} is enabled by default only if
11318 the @code{L32R} instruction is not available.
11321 @itemx -mno-fused-madd
11322 @opindex mfused-madd
11323 @opindex mno-fused-madd
11324 Enable or disable use of fused multiply/add and multiply/subtract
11325 instructions in the floating-point option. This has no effect if the
11326 floating-point option is not also enabled. Disabling fused multiply/add
11327 and multiply/subtract instructions forces the compiler to use separate
11328 instructions for the multiply and add/subtract operations. This may be
11329 desirable in some cases where strict IEEE 754-compliant results are
11330 required: the fused multiply add/subtract instructions do not round the
11331 intermediate result, thereby producing results with @emph{more} bits of
11332 precision than specified by the IEEE standard. Disabling fused multiply
11333 add/subtract instructions also ensures that the program output is not
11334 sensitive to the compiler's ability to combine multiply and add/subtract
11337 @item -mtext-section-literals
11338 @itemx -mno-text-section-literals
11339 @opindex mtext-section-literals
11340 @opindex mno-text-section-literals
11341 Control the treatment of literal pools. The default is
11342 @option{-mno-text-section-literals}, which places literals in a separate
11343 section in the output file. This allows the literal pool to be placed
11344 in a data RAM/ROM, and it also allows the linker to combine literal
11345 pools from separate object files to remove redundant literals and
11346 improve code size. With @option{-mtext-section-literals}, the literals
11347 are interspersed in the text section in order to keep them as close as
11348 possible to their references. This may be necessary for large assembly
11351 @item -mtarget-align
11352 @itemx -mno-target-align
11353 @opindex mtarget-align
11354 @opindex mno-target-align
11355 When this option is enabled, GCC instructs the assembler to
11356 automatically align instructions to reduce branch penalties at the
11357 expense of some code density. The assembler attempts to widen density
11358 instructions to align branch targets and the instructions following call
11359 instructions. If there are not enough preceding safe density
11360 instructions to align a target, no widening will be performed. The
11361 default is @option{-mtarget-align}. These options do not affect the
11362 treatment of auto-aligned instructions like @code{LOOP}, which the
11363 assembler will always align, either by widening density instructions or
11364 by inserting no-op instructions.
11367 @itemx -mno-longcalls
11368 @opindex mlongcalls
11369 @opindex mno-longcalls
11370 When this option is enabled, GCC instructs the assembler to translate
11371 direct calls to indirect calls unless it can determine that the target
11372 of a direct call is in the range allowed by the call instruction. This
11373 translation typically occurs for calls to functions in other source
11374 files. Specifically, the assembler translates a direct @code{CALL}
11375 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
11376 The default is @option{-mno-longcalls}. This option should be used in
11377 programs where the call target can potentially be out of range. This
11378 option is implemented in the assembler, not the compiler, so the
11379 assembly code generated by GCC will still show direct call
11380 instructions---look at the disassembled object code to see the actual
11381 instructions. Note that the assembler will use an indirect call for
11382 every cross-file call, not just those that really will be out of range.
11385 @node zSeries Options
11386 @subsection zSeries Options
11387 @cindex zSeries options
11389 These are listed under @xref{S/390 and zSeries Options}.
11391 @node Code Gen Options
11392 @section Options for Code Generation Conventions
11393 @cindex code generation conventions
11394 @cindex options, code generation
11395 @cindex run-time options
11397 These machine-independent options control the interface conventions
11398 used in code generation.
11400 Most of them have both positive and negative forms; the negative form
11401 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
11402 one of the forms is listed---the one which is not the default. You
11403 can figure out the other form by either removing @samp{no-} or adding
11407 @item -fbounds-check
11408 @opindex fbounds-check
11409 For front-ends that support it, generate additional code to check that
11410 indices used to access arrays are within the declared range. This is
11411 currently only supported by the Java and Fortran 77 front-ends, where
11412 this option defaults to true and false respectively.
11416 This option generates traps for signed overflow on addition, subtraction,
11417 multiplication operations.
11421 This option instructs the compiler to assume that signed arithmetic
11422 overflow of addition, subtraction and multiplication wraps around
11423 using twos-complement representation. This flag enables some optimizations
11424 and disables other. This option is enabled by default for the Java
11425 front-end, as required by the Java language specification.
11428 @opindex fexceptions
11429 Enable exception handling. Generates extra code needed to propagate
11430 exceptions. For some targets, this implies GCC will generate frame
11431 unwind information for all functions, which can produce significant data
11432 size overhead, although it does not affect execution. If you do not
11433 specify this option, GCC will enable it by default for languages like
11434 C++ which normally require exception handling, and disable it for
11435 languages like C that do not normally require it. However, you may need
11436 to enable this option when compiling C code that needs to interoperate
11437 properly with exception handlers written in C++. You may also wish to
11438 disable this option if you are compiling older C++ programs that don't
11439 use exception handling.
11441 @item -fnon-call-exceptions
11442 @opindex fnon-call-exceptions
11443 Generate code that allows trapping instructions to throw exceptions.
11444 Note that this requires platform-specific runtime support that does
11445 not exist everywhere. Moreover, it only allows @emph{trapping}
11446 instructions to throw exceptions, i.e.@: memory references or floating
11447 point instructions. It does not allow exceptions to be thrown from
11448 arbitrary signal handlers such as @code{SIGALRM}.
11450 @item -funwind-tables
11451 @opindex funwind-tables
11452 Similar to @option{-fexceptions}, except that it will just generate any needed
11453 static data, but will not affect the generated code in any other way.
11454 You will normally not enable this option; instead, a language processor
11455 that needs this handling would enable it on your behalf.
11457 @item -fasynchronous-unwind-tables
11458 @opindex fasynchronous-unwind-tables
11459 Generate unwind table in dwarf2 format, if supported by target machine. The
11460 table is exact at each instruction boundary, so it can be used for stack
11461 unwinding from asynchronous events (such as debugger or garbage collector).
11463 @item -fpcc-struct-return
11464 @opindex fpcc-struct-return
11465 Return ``short'' @code{struct} and @code{union} values in memory like
11466 longer ones, rather than in registers. This convention is less
11467 efficient, but it has the advantage of allowing intercallability between
11468 GCC-compiled files and files compiled with other compilers, particularly
11469 the Portable C Compiler (pcc).
11471 The precise convention for returning structures in memory depends
11472 on the target configuration macros.
11474 Short structures and unions are those whose size and alignment match
11475 that of some integer type.
11477 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
11478 switch is not binary compatible with code compiled with the
11479 @option{-freg-struct-return} switch.
11480 Use it to conform to a non-default application binary interface.
11482 @item -freg-struct-return
11483 @opindex freg-struct-return
11484 Return @code{struct} and @code{union} values in registers when possible.
11485 This is more efficient for small structures than
11486 @option{-fpcc-struct-return}.
11488 If you specify neither @option{-fpcc-struct-return} nor
11489 @option{-freg-struct-return}, GCC defaults to whichever convention is
11490 standard for the target. If there is no standard convention, GCC
11491 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
11492 the principal compiler. In those cases, we can choose the standard, and
11493 we chose the more efficient register return alternative.
11495 @strong{Warning:} code compiled with the @option{-freg-struct-return}
11496 switch is not binary compatible with code compiled with the
11497 @option{-fpcc-struct-return} switch.
11498 Use it to conform to a non-default application binary interface.
11500 @item -fshort-enums
11501 @opindex fshort-enums
11502 Allocate to an @code{enum} type only as many bytes as it needs for the
11503 declared range of possible values. Specifically, the @code{enum} type
11504 will be equivalent to the smallest integer type which has enough room.
11506 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
11507 code that is not binary compatible with code generated without that switch.
11508 Use it to conform to a non-default application binary interface.
11510 @item -fshort-double
11511 @opindex fshort-double
11512 Use the same size for @code{double} as for @code{float}.
11514 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
11515 code that is not binary compatible with code generated without that switch.
11516 Use it to conform to a non-default application binary interface.
11518 @item -fshort-wchar
11519 @opindex fshort-wchar
11520 Override the underlying type for @samp{wchar_t} to be @samp{short
11521 unsigned int} instead of the default for the target. This option is
11522 useful for building programs to run under WINE@.
11524 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
11525 code that is not binary compatible with code generated without that switch.
11526 Use it to conform to a non-default application binary interface.
11528 @item -fshared-data
11529 @opindex fshared-data
11530 Requests that the data and non-@code{const} variables of this
11531 compilation be shared data rather than private data. The distinction
11532 makes sense only on certain operating systems, where shared data is
11533 shared between processes running the same program, while private data
11534 exists in one copy per process.
11537 @opindex fno-common
11538 In C, allocate even uninitialized global variables in the data section of the
11539 object file, rather than generating them as common blocks. This has the
11540 effect that if the same variable is declared (without @code{extern}) in
11541 two different compilations, you will get an error when you link them.
11542 The only reason this might be useful is if you wish to verify that the
11543 program will work on other systems which always work this way.
11547 Ignore the @samp{#ident} directive.
11549 @item -finhibit-size-directive
11550 @opindex finhibit-size-directive
11551 Don't output a @code{.size} assembler directive, or anything else that
11552 would cause trouble if the function is split in the middle, and the
11553 two halves are placed at locations far apart in memory. This option is
11554 used when compiling @file{crtstuff.c}; you should not need to use it
11557 @item -fverbose-asm
11558 @opindex fverbose-asm
11559 Put extra commentary information in the generated assembly code to
11560 make it more readable. This option is generally only of use to those
11561 who actually need to read the generated assembly code (perhaps while
11562 debugging the compiler itself).
11564 @option{-fno-verbose-asm}, the default, causes the
11565 extra information to be omitted and is useful when comparing two assembler
11570 @cindex global offset table
11572 Generate position-independent code (PIC) suitable for use in a shared
11573 library, if supported for the target machine. Such code accesses all
11574 constant addresses through a global offset table (GOT)@. The dynamic
11575 loader resolves the GOT entries when the program starts (the dynamic
11576 loader is not part of GCC; it is part of the operating system). If
11577 the GOT size for the linked executable exceeds a machine-specific
11578 maximum size, you get an error message from the linker indicating that
11579 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
11580 instead. (These maximums are 8k on the SPARC and 32k
11581 on the m68k and RS/6000. The 386 has no such limit.)
11583 Position-independent code requires special support, and therefore works
11584 only on certain machines. For the 386, GCC supports PIC for System V
11585 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
11586 position-independent.
11590 If supported for the target machine, emit position-independent code,
11591 suitable for dynamic linking and avoiding any limit on the size of the
11592 global offset table. This option makes a difference on the m68k
11595 Position-independent code requires special support, and therefore works
11596 only on certain machines.
11602 These options are similar to @option{-fpic} and @option{-fPIC}, but
11603 generated position independent code can be only linked into executables.
11604 Usually these options are used when @option{-pie} GCC option will be
11605 used during linking.
11607 @item -ffixed-@var{reg}
11609 Treat the register named @var{reg} as a fixed register; generated code
11610 should never refer to it (except perhaps as a stack pointer, frame
11611 pointer or in some other fixed role).
11613 @var{reg} must be the name of a register. The register names accepted
11614 are machine-specific and are defined in the @code{REGISTER_NAMES}
11615 macro in the machine description macro file.
11617 This flag does not have a negative form, because it specifies a
11620 @item -fcall-used-@var{reg}
11621 @opindex fcall-used
11622 Treat the register named @var{reg} as an allocable register that is
11623 clobbered by function calls. It may be allocated for temporaries or
11624 variables that do not live across a call. Functions compiled this way
11625 will not save and restore the register @var{reg}.
11627 It is an error to used this flag with the frame pointer or stack pointer.
11628 Use of this flag for other registers that have fixed pervasive roles in
11629 the machine's execution model will produce disastrous results.
11631 This flag does not have a negative form, because it specifies a
11634 @item -fcall-saved-@var{reg}
11635 @opindex fcall-saved
11636 Treat the register named @var{reg} as an allocable register saved by
11637 functions. It may be allocated even for temporaries or variables that
11638 live across a call. Functions compiled this way will save and restore
11639 the register @var{reg} if they use it.
11641 It is an error to used this flag with the frame pointer or stack pointer.
11642 Use of this flag for other registers that have fixed pervasive roles in
11643 the machine's execution model will produce disastrous results.
11645 A different sort of disaster will result from the use of this flag for
11646 a register in which function values may be returned.
11648 This flag does not have a negative form, because it specifies a
11651 @item -fpack-struct
11652 @opindex fpack-struct
11653 Pack all structure members together without holes.
11655 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
11656 code that is not binary compatible with code generated without that switch.
11657 Additionally, it makes the code suboptimal.
11658 Use it to conform to a non-default application binary interface.
11660 @item -finstrument-functions
11661 @opindex finstrument-functions
11662 Generate instrumentation calls for entry and exit to functions. Just
11663 after function entry and just before function exit, the following
11664 profiling functions will be called with the address of the current
11665 function and its call site. (On some platforms,
11666 @code{__builtin_return_address} does not work beyond the current
11667 function, so the call site information may not be available to the
11668 profiling functions otherwise.)
11671 void __cyg_profile_func_enter (void *this_fn,
11673 void __cyg_profile_func_exit (void *this_fn,
11677 The first argument is the address of the start of the current function,
11678 which may be looked up exactly in the symbol table.
11680 This instrumentation is also done for functions expanded inline in other
11681 functions. The profiling calls will indicate where, conceptually, the
11682 inline function is entered and exited. This means that addressable
11683 versions of such functions must be available. If all your uses of a
11684 function are expanded inline, this may mean an additional expansion of
11685 code size. If you use @samp{extern inline} in your C code, an
11686 addressable version of such functions must be provided. (This is
11687 normally the case anyways, but if you get lucky and the optimizer always
11688 expands the functions inline, you might have gotten away without
11689 providing static copies.)
11691 A function may be given the attribute @code{no_instrument_function}, in
11692 which case this instrumentation will not be done. This can be used, for
11693 example, for the profiling functions listed above, high-priority
11694 interrupt routines, and any functions from which the profiling functions
11695 cannot safely be called (perhaps signal handlers, if the profiling
11696 routines generate output or allocate memory).
11698 @item -fstack-check
11699 @opindex fstack-check
11700 Generate code to verify that you do not go beyond the boundary of the
11701 stack. You should specify this flag if you are running in an
11702 environment with multiple threads, but only rarely need to specify it in
11703 a single-threaded environment since stack overflow is automatically
11704 detected on nearly all systems if there is only one stack.
11706 Note that this switch does not actually cause checking to be done; the
11707 operating system must do that. The switch causes generation of code
11708 to ensure that the operating system sees the stack being extended.
11710 @item -fstack-limit-register=@var{reg}
11711 @itemx -fstack-limit-symbol=@var{sym}
11712 @itemx -fno-stack-limit
11713 @opindex fstack-limit-register
11714 @opindex fstack-limit-symbol
11715 @opindex fno-stack-limit
11716 Generate code to ensure that the stack does not grow beyond a certain value,
11717 either the value of a register or the address of a symbol. If the stack
11718 would grow beyond the value, a signal is raised. For most targets,
11719 the signal is raised before the stack overruns the boundary, so
11720 it is possible to catch the signal without taking special precautions.
11722 For instance, if the stack starts at absolute address @samp{0x80000000}
11723 and grows downwards, you can use the flags
11724 @option{-fstack-limit-symbol=__stack_limit} and
11725 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
11726 of 128KB@. Note that this may only work with the GNU linker.
11728 @cindex aliasing of parameters
11729 @cindex parameters, aliased
11730 @item -fargument-alias
11731 @itemx -fargument-noalias
11732 @itemx -fargument-noalias-global
11733 @opindex fargument-alias
11734 @opindex fargument-noalias
11735 @opindex fargument-noalias-global
11736 Specify the possible relationships among parameters and between
11737 parameters and global data.
11739 @option{-fargument-alias} specifies that arguments (parameters) may
11740 alias each other and may alias global storage.@*
11741 @option{-fargument-noalias} specifies that arguments do not alias
11742 each other, but may alias global storage.@*
11743 @option{-fargument-noalias-global} specifies that arguments do not
11744 alias each other and do not alias global storage.
11746 Each language will automatically use whatever option is required by
11747 the language standard. You should not need to use these options yourself.
11749 @item -fleading-underscore
11750 @opindex fleading-underscore
11751 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
11752 change the way C symbols are represented in the object file. One use
11753 is to help link with legacy assembly code.
11755 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
11756 generate code that is not binary compatible with code generated without that
11757 switch. Use it to conform to a non-default application binary interface.
11758 Not all targets provide complete support for this switch.
11760 @item -ftls-model=@var{model}
11761 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
11762 The @var{model} argument should be one of @code{global-dynamic},
11763 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
11765 The default without @option{-fpic} is @code{initial-exec}; with
11766 @option{-fpic} the default is @code{global-dynamic}.
11768 @item -fvisibility=@var{default|internal|hidden|protected}
11769 @opindex fvisibility
11770 Set the default ELF image symbol visibility to the specified option - all
11771 symbols will be marked with this unless overrided within the code.
11772 Using this feature can very substantially improve linking and
11773 load times of shared object libraries, produce more optimised
11774 code, provide near-perfect API export and prevent symbol clashes.
11775 It is @strong{strongly} recommended that you use this in any shared objects
11778 Despite the nomenclature, @code{default} always means public ie;
11779 available to be linked against from outside the shared object.
11780 @code{protected} and @code{internal} are pretty useless in real-world
11781 usage so the only other commonly used option will be @code{hidden}.
11782 The default if -fvisibility isn't specified is @code{default} ie; make every
11783 symbol public - this causes the same behaviour as previous versions of
11786 A good explanation of the benefits offered by ensuring ELF
11787 symbols have the correct visibility is given by ``How To Write
11788 Shared Libraries'' by Ulrich Drepper (which can be found at
11789 @w{@uref{http://people.redhat.com/~drepper/}}) - however a superior
11790 solution made possible by this option to marking things hidden when
11791 the default is public is to make the default hidden and mark things
11792 public. This is the norm with DLL's on Windows and with @option{-fvisibility=hidden}
11793 and @code{__attribute__ ((visibility("default")))} instead of
11794 @code{__declspec(dllexport)} you get almost identical semantics with
11795 identical syntax. This is a great boon to those working with
11796 cross-platform projects.
11798 For those adding visibility support to existing code, you may find
11799 @samp{#pragma GCC visibility} of use. This works by you enclosing
11800 the declarations you wish to set visibility for with (for example)
11801 @samp{#pragma GCC visibility push(hidden)} and
11802 @samp{#pragma GCC visibility pop}. These can be nested up to sixteen
11803 times. Bear in mind that symbol visibility should be viewed @strong{as
11804 part of the API interface contract} and thus all new code should
11805 always specify visibility when it is not the default ie; declarations
11806 only for use within the local DSO should @strong{always} be marked explicitly
11807 as hidden as so to avoid PLT indirection overheads - making this
11808 abundantly clear also aids readability and self-documentation of the code.
11809 Note that due to ISO C++ specification requirements, operator new and
11810 operator delete must always be of default visibility.
11812 An overview of these techniques, their benefits and how to use them
11813 is at @w{@uref{http://www.nedprod.com/programs/gccvisibility.html}}.
11819 @node Environment Variables
11820 @section Environment Variables Affecting GCC
11821 @cindex environment variables
11823 @c man begin ENVIRONMENT
11824 This section describes several environment variables that affect how GCC
11825 operates. Some of them work by specifying directories or prefixes to use
11826 when searching for various kinds of files. Some are used to specify other
11827 aspects of the compilation environment.
11829 Note that you can also specify places to search using options such as
11830 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
11831 take precedence over places specified using environment variables, which
11832 in turn take precedence over those specified by the configuration of GCC@.
11833 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
11834 GNU Compiler Collection (GCC) Internals}.
11839 @c @itemx LC_COLLATE
11841 @c @itemx LC_MONETARY
11842 @c @itemx LC_NUMERIC
11847 @c @findex LC_COLLATE
11848 @findex LC_MESSAGES
11849 @c @findex LC_MONETARY
11850 @c @findex LC_NUMERIC
11854 These environment variables control the way that GCC uses
11855 localization information that allow GCC to work with different
11856 national conventions. GCC inspects the locale categories
11857 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
11858 so. These locale categories can be set to any value supported by your
11859 installation. A typical value is @samp{en_UK} for English in the United
11862 The @env{LC_CTYPE} environment variable specifies character
11863 classification. GCC uses it to determine the character boundaries in
11864 a string; this is needed for some multibyte encodings that contain quote
11865 and escape characters that would otherwise be interpreted as a string
11868 The @env{LC_MESSAGES} environment variable specifies the language to
11869 use in diagnostic messages.
11871 If the @env{LC_ALL} environment variable is set, it overrides the value
11872 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
11873 and @env{LC_MESSAGES} default to the value of the @env{LANG}
11874 environment variable. If none of these variables are set, GCC
11875 defaults to traditional C English behavior.
11879 If @env{TMPDIR} is set, it specifies the directory to use for temporary
11880 files. GCC uses temporary files to hold the output of one stage of
11881 compilation which is to be used as input to the next stage: for example,
11882 the output of the preprocessor, which is the input to the compiler
11885 @item GCC_EXEC_PREFIX
11886 @findex GCC_EXEC_PREFIX
11887 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
11888 names of the subprograms executed by the compiler. No slash is added
11889 when this prefix is combined with the name of a subprogram, but you can
11890 specify a prefix that ends with a slash if you wish.
11892 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
11893 an appropriate prefix to use based on the pathname it was invoked with.
11895 If GCC cannot find the subprogram using the specified prefix, it
11896 tries looking in the usual places for the subprogram.
11898 The default value of @env{GCC_EXEC_PREFIX} is
11899 @file{@var{prefix}/lib/gcc/} where @var{prefix} is the value
11900 of @code{prefix} when you ran the @file{configure} script.
11902 Other prefixes specified with @option{-B} take precedence over this prefix.
11904 This prefix is also used for finding files such as @file{crt0.o} that are
11907 In addition, the prefix is used in an unusual way in finding the
11908 directories to search for header files. For each of the standard
11909 directories whose name normally begins with @samp{/usr/local/lib/gcc}
11910 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
11911 replacing that beginning with the specified prefix to produce an
11912 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
11913 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
11914 These alternate directories are searched first; the standard directories
11917 @item COMPILER_PATH
11918 @findex COMPILER_PATH
11919 The value of @env{COMPILER_PATH} is a colon-separated list of
11920 directories, much like @env{PATH}. GCC tries the directories thus
11921 specified when searching for subprograms, if it can't find the
11922 subprograms using @env{GCC_EXEC_PREFIX}.
11925 @findex LIBRARY_PATH
11926 The value of @env{LIBRARY_PATH} is a colon-separated list of
11927 directories, much like @env{PATH}. When configured as a native compiler,
11928 GCC tries the directories thus specified when searching for special
11929 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
11930 using GCC also uses these directories when searching for ordinary
11931 libraries for the @option{-l} option (but directories specified with
11932 @option{-L} come first).
11936 @cindex locale definition
11937 This variable is used to pass locale information to the compiler. One way in
11938 which this information is used is to determine the character set to be used
11939 when character literals, string literals and comments are parsed in C and C++.
11940 When the compiler is configured to allow multibyte characters,
11941 the following values for @env{LANG} are recognized:
11945 Recognize JIS characters.
11947 Recognize SJIS characters.
11949 Recognize EUCJP characters.
11952 If @env{LANG} is not defined, or if it has some other value, then the
11953 compiler will use mblen and mbtowc as defined by the default locale to
11954 recognize and translate multibyte characters.
11958 Some additional environments variables affect the behavior of the
11961 @include cppenv.texi
11965 @node Precompiled Headers
11966 @section Using Precompiled Headers
11967 @cindex precompiled headers
11968 @cindex speed of compilation
11970 Often large projects have many header files that are included in every
11971 source file. The time the compiler takes to process these header files
11972 over and over again can account for nearly all of the time required to
11973 build the project. To make builds faster, GCC allows users to
11974 `precompile' a header file; then, if builds can use the precompiled
11975 header file they will be much faster.
11977 @strong{Caution:} There are a few known situations where GCC will
11978 crash when trying to use a precompiled header. If you have trouble
11979 with a precompiled header, you should remove the precompiled header
11980 and compile without it. In addition, please use GCC's on-line
11981 defect-tracking system to report any problems you encounter with
11982 precompiled headers. @xref{Bugs}.
11984 To create a precompiled header file, simply compile it as you would any
11985 other file, if necessary using the @option{-x} option to make the driver
11986 treat it as a C or C++ header file. You will probably want to use a
11987 tool like @command{make} to keep the precompiled header up-to-date when
11988 the headers it contains change.
11990 A precompiled header file will be searched for when @code{#include} is
11991 seen in the compilation. As it searches for the included file
11992 (@pxref{Search Path,,Search Path,cpp,The C Preprocessor}) the
11993 compiler looks for a precompiled header in each directory just before it
11994 looks for the include file in that directory. The name searched for is
11995 the name specified in the @code{#include} with @samp{.gch} appended. If
11996 the precompiled header file can't be used, it is ignored.
11998 For instance, if you have @code{#include "all.h"}, and you have
11999 @file{all.h.gch} in the same directory as @file{all.h}, then the
12000 precompiled header file will be used if possible, and the original
12001 header will be used otherwise.
12003 Alternatively, you might decide to put the precompiled header file in a
12004 directory and use @option{-I} to ensure that directory is searched
12005 before (or instead of) the directory containing the original header.
12006 Then, if you want to check that the precompiled header file is always
12007 used, you can put a file of the same name as the original header in this
12008 directory containing an @code{#error} command.
12010 This also works with @option{-include}. So yet another way to use
12011 precompiled headers, good for projects not designed with precompiled
12012 header files in mind, is to simply take most of the header files used by
12013 a project, include them from another header file, precompile that header
12014 file, and @option{-include} the precompiled header. If the header files
12015 have guards against multiple inclusion, they will be skipped because
12016 they've already been included (in the precompiled header).
12018 If you need to precompile the same header file for different
12019 languages, targets, or compiler options, you can instead make a
12020 @emph{directory} named like @file{all.h.gch}, and put each precompiled
12021 header in the directory, perhaps using @option{-o}. It doesn't matter
12022 what you call the files in the directory, every precompiled header in
12023 the directory will be considered. The first precompiled header
12024 encountered in the directory that is valid for this compilation will
12025 be used; they're searched in no particular order.
12027 There are many other possibilities, limited only by your imagination,
12028 good sense, and the constraints of your build system.
12030 A precompiled header file can be used only when these conditions apply:
12034 Only one precompiled header can be used in a particular compilation.
12037 A precompiled header can't be used once the first C token is seen. You
12038 can have preprocessor directives before a precompiled header; you can
12039 even include a precompiled header from inside another header, so long as
12040 there are no C tokens before the @code{#include}.
12043 The precompiled header file must be produced for the same language as
12044 the current compilation. You can't use a C precompiled header for a C++
12048 The precompiled header file must be produced by the same compiler
12049 version and configuration as the current compilation is using.
12050 The easiest way to guarantee this is to use the same compiler binary
12051 for creating and using precompiled headers.
12054 Any macros defined before the precompiled header is included must
12055 either be defined in the same way as when the precompiled header was
12056 generated, or must not affect the precompiled header, which usually
12057 means that the they don't appear in the precompiled header at all.
12059 The @option{-D} option is one way to define a macro before a
12060 precompiled header is included; using a @code{#define} can also do it.
12061 There are also some options that define macros implicitly, like
12062 @option{-O} and @option{-Wdeprecated}; the same rule applies to macros
12065 @item If debugging information is output when using the precompiled
12066 header, using @option{-g} or similar, the same kind of debugging information
12067 must have been output when building the precompiled header. However,
12068 a precompiled header built using @option{-g} can be used in a compilation
12069 when no debugging information is being output.
12071 @item The same @option{-m} options must generally be used when building
12072 and using the precompiled header. @xref{Submodel Options},
12073 for any cases where this rule is relaxed.
12075 @item Each of the following options must be the same when building and using
12076 the precompiled header:
12078 @gccoptlist{-fexceptions -funit-at-a-time}
12081 Some other command-line options starting with @option{-f},
12082 @option{-p}, or @option{-O} must be defined in the same way as when
12083 the precompiled header was generated. At present, it's not clear
12084 which options are safe to change and which are not; the safest choice
12085 is to use exactly the same options when generating and using the
12086 precompiled header. The following are known to be safe:
12088 @gccoptlist{-fpreprocessed -pedantic-errors}
12092 For all of these except the last, the compiler will automatically
12093 ignore the precompiled header if the conditions aren't met. If you
12094 find an option combination that doesn't work and doesn't cause the
12095 precompiled header to be ignored, please consider filing a bug report,
12098 If you do use differing options when generating and using the
12099 precompiled header, the actual behaviour will be a mixture of the
12100 behaviour for the options. For instance, if you use @option{-g} to
12101 generate the precompiled header but not when using it, you may or may
12102 not get debugging information for routines in the precompiled header.
12104 @node Running Protoize
12105 @section Running Protoize
12107 The program @code{protoize} is an optional part of GCC@. You can use
12108 it to add prototypes to a program, thus converting the program to ISO
12109 C in one respect. The companion program @code{unprotoize} does the
12110 reverse: it removes argument types from any prototypes that are found.
12112 When you run these programs, you must specify a set of source files as
12113 command line arguments. The conversion programs start out by compiling
12114 these files to see what functions they define. The information gathered
12115 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
12117 After scanning comes actual conversion. The specified files are all
12118 eligible to be converted; any files they include (whether sources or
12119 just headers) are eligible as well.
12121 But not all the eligible files are converted. By default,
12122 @code{protoize} and @code{unprotoize} convert only source and header
12123 files in the current directory. You can specify additional directories
12124 whose files should be converted with the @option{-d @var{directory}}
12125 option. You can also specify particular files to exclude with the
12126 @option{-x @var{file}} option. A file is converted if it is eligible, its
12127 directory name matches one of the specified directory names, and its
12128 name within the directory has not been excluded.
12130 Basic conversion with @code{protoize} consists of rewriting most
12131 function definitions and function declarations to specify the types of
12132 the arguments. The only ones not rewritten are those for varargs
12135 @code{protoize} optionally inserts prototype declarations at the
12136 beginning of the source file, to make them available for any calls that
12137 precede the function's definition. Or it can insert prototype
12138 declarations with block scope in the blocks where undeclared functions
12141 Basic conversion with @code{unprotoize} consists of rewriting most
12142 function declarations to remove any argument types, and rewriting
12143 function definitions to the old-style pre-ISO form.
12145 Both conversion programs print a warning for any function declaration or
12146 definition that they can't convert. You can suppress these warnings
12149 The output from @code{protoize} or @code{unprotoize} replaces the
12150 original source file. The original file is renamed to a name ending
12151 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
12152 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
12153 for DOS) file already exists, then the source file is simply discarded.
12155 @code{protoize} and @code{unprotoize} both depend on GCC itself to
12156 scan the program and collect information about the functions it uses.
12157 So neither of these programs will work until GCC is installed.
12159 Here is a table of the options you can use with @code{protoize} and
12160 @code{unprotoize}. Each option works with both programs unless
12164 @item -B @var{directory}
12165 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
12166 usual directory (normally @file{/usr/local/lib}). This file contains
12167 prototype information about standard system functions. This option
12168 applies only to @code{protoize}.
12170 @item -c @var{compilation-options}
12171 Use @var{compilation-options} as the options when running @command{gcc} to
12172 produce the @samp{.X} files. The special option @option{-aux-info} is
12173 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
12175 Note that the compilation options must be given as a single argument to
12176 @code{protoize} or @code{unprotoize}. If you want to specify several
12177 @command{gcc} options, you must quote the entire set of compilation options
12178 to make them a single word in the shell.
12180 There are certain @command{gcc} arguments that you cannot use, because they
12181 would produce the wrong kind of output. These include @option{-g},
12182 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
12183 the @var{compilation-options}, they are ignored.
12186 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
12187 systems) instead of @samp{.c}. This is convenient if you are converting
12188 a C program to C++. This option applies only to @code{protoize}.
12191 Add explicit global declarations. This means inserting explicit
12192 declarations at the beginning of each source file for each function
12193 that is called in the file and was not declared. These declarations
12194 precede the first function definition that contains a call to an
12195 undeclared function. This option applies only to @code{protoize}.
12197 @item -i @var{string}
12198 Indent old-style parameter declarations with the string @var{string}.
12199 This option applies only to @code{protoize}.
12201 @code{unprotoize} converts prototyped function definitions to old-style
12202 function definitions, where the arguments are declared between the
12203 argument list and the initial @samp{@{}. By default, @code{unprotoize}
12204 uses five spaces as the indentation. If you want to indent with just
12205 one space instead, use @option{-i " "}.
12208 Keep the @samp{.X} files. Normally, they are deleted after conversion
12212 Add explicit local declarations. @code{protoize} with @option{-l} inserts
12213 a prototype declaration for each function in each block which calls the
12214 function without any declaration. This option applies only to
12218 Make no real changes. This mode just prints information about the conversions
12219 that would have been done without @option{-n}.
12222 Make no @samp{.save} files. The original files are simply deleted.
12223 Use this option with caution.
12225 @item -p @var{program}
12226 Use the program @var{program} as the compiler. Normally, the name
12227 @file{gcc} is used.
12230 Work quietly. Most warnings are suppressed.
12233 Print the version number, just like @option{-v} for @command{gcc}.
12236 If you need special compiler options to compile one of your program's
12237 source files, then you should generate that file's @samp{.X} file
12238 specially, by running @command{gcc} on that source file with the
12239 appropriate options and the option @option{-aux-info}. Then run
12240 @code{protoize} on the entire set of files. @code{protoize} will use
12241 the existing @samp{.X} file because it is newer than the source file.
12245 gcc -Dfoo=bar file1.c -aux-info file1.X
12250 You need to include the special files along with the rest in the
12251 @code{protoize} command, even though their @samp{.X} files already
12252 exist, because otherwise they won't get converted.
12254 @xref{Protoize Caveats}, for more information on how to use
12255 @code{protoize} successfully.