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), g77(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
47 and the Info entries for @file{gcc}, @file{cpp}, @file{g77}, @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 Dialect Options:: Variations on Objective-C.
125 * Language Independent Options:: Controlling how diagnostics should be
127 * Warning Options:: How picky should the compiler be?
128 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
129 * Optimize Options:: How much optimization?
130 * Preprocessor Options:: Controlling header files and macro definitions.
131 Also, getting dependency information for Make.
132 * Assembler Options:: Passing options to the assembler.
133 * Link Options:: Specifying libraries and so on.
134 * Directory Options:: Where to find header files and libraries.
135 Where to find the compiler executable files.
136 * Spec Files:: How to pass switches to sub-processes.
137 * Target Options:: Running a cross-compiler, or an old version of GCC.
138 * Submodel Options:: Specifying minor hardware or convention variations,
139 such as 68010 vs 68020.
140 * Code Gen Options:: Specifying conventions for function calls, data layout
142 * Environment Variables:: Env vars that affect GCC.
143 * Precompiled Headers:: Compiling a header once, and using it many times.
144 * Running Protoize:: Automatically adding or removing function prototypes.
150 @section Option Summary
152 Here is a summary of all the options, grouped by type. Explanations are
153 in the following sections.
156 @item Overall Options
157 @xref{Overall Options,,Options Controlling the Kind of Output}.
158 @gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
159 -x @var{language} -v -### --help --target-help --version}
161 @item C Language Options
162 @xref{C Dialect Options,,Options Controlling C Dialect}.
163 @gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
164 -fno-asm -fno-builtin -fno-builtin-@var{function} @gol
165 -fhosted -ffreestanding -fms-extensions @gol
166 -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
167 -fallow-single-precision -fcond-mismatch @gol
168 -fsigned-bitfields -fsigned-char @gol
169 -funsigned-bitfields -funsigned-char}
171 @item C++ Language Options
172 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
173 @gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
174 -fconserve-space -fno-const-strings @gol
175 -fno-elide-constructors @gol
176 -fno-enforce-eh-specs @gol
177 -ffor-scope -fno-for-scope -fno-gnu-keywords @gol
178 -fno-implicit-templates @gol
179 -fno-implicit-inline-templates @gol
180 -fno-implement-inlines -fms-extensions @gol
181 -fno-nonansi-builtins -fno-operator-names @gol
182 -fno-optional-diags -fpermissive @gol
183 -frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
184 -fuse-cxa-atexit -fno-weak -nostdinc++ @gol
185 -fno-default-inline -Wabi -Wctor-dtor-privacy @gol
186 -Wnon-virtual-dtor -Wreorder @gol
187 -Weffc++ -Wno-deprecated @gol
188 -Wno-non-template-friend -Wold-style-cast @gol
189 -Woverloaded-virtual -Wno-pmf-conversions @gol
190 -Wsign-promo -Wsynth}
192 @item Objective-C Language Options
193 @xref{Objective-C Dialect Options,,Options Controlling Objective-C Dialect}.
195 -fconstant-string-class=@var{class-name} @gol
196 -fgnu-runtime -fnext-runtime @gol
197 -fno-nil-receivers @gol
198 -fobjc-exceptions @gol
199 -freplace-objc-classes @gol
202 -Wno-protocol -Wselector -Wundeclared-selector}
204 @item Language Independent Options
205 @xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
206 @gccoptlist{-fmessage-length=@var{n} @gol
207 -fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]}}
209 @item Warning Options
210 @xref{Warning Options,,Options to Request or Suppress Warnings}.
211 @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
212 -w -Wextra -Wall -Waggregate-return @gol
213 -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment @gol
214 -Wconversion -Wno-deprecated-declarations @gol
215 -Wdisabled-optimization -Wno-div-by-zero -Wendif-labels @gol
216 -Werror -Werror-implicit-function-declaration @gol
217 -Wfloat-equal -Wformat -Wformat=2 @gol
218 -Wno-format-extra-args -Wformat-nonliteral @gol
219 -Wformat-security -Wformat-y2k @gol
220 -Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
221 -Wimport -Wno-import -Winit-self -Winline @gol
222 -Wno-invalid-offsetof -Winvalid-pch @gol
223 -Wlarger-than-@var{len} -Wlong-long @gol
224 -Wmain -Wmissing-braces @gol
225 -Wmissing-format-attribute -Wmissing-noreturn @gol
226 -Wno-multichar -Wnonnull -Wpacked -Wpadded @gol
227 -Wparentheses -Wpointer-arith -Wredundant-decls @gol
228 -Wreturn-type -Wsequence-point -Wshadow @gol
229 -Wsign-compare -Wstrict-aliasing -Wstrict-aliasing=2 @gol
230 -Wswitch -Wswitch-default -Wswitch-enum @gol
231 -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
232 -Wunknown-pragmas -Wunreachable-code @gol
233 -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
234 -Wunused-value -Wunused-variable -Wwrite-strings @gol
237 @item C-only Warning Options
238 @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
239 -Wmissing-prototypes -Wnested-externs -Wold-style-definition @gol
240 -Wstrict-prototypes -Wtraditional @gol
241 -Wdeclaration-after-statement}
243 @item Debugging Options
244 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
245 @gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
246 -fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
247 -fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
248 -fdump-tree-original@r{[}-@var{n}@r{]} @gol
249 -fdump-tree-optimized@r{[}-@var{n}@r{]} @gol
250 -fdump-tree-inlined@r{[}-@var{n}@r{]} @gol
251 -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
252 -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs @gol
253 -frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
254 -ftest-coverage -ftime-report -fvar-tracking @gol
255 -g -g@var{level} -gcoff -gdwarf-2 @gol
256 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
257 -p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
258 -print-multi-directory -print-multi-lib @gol
259 -print-prog-name=@var{program} -print-search-dirs -Q @gol
262 @item Optimization Options
263 @xref{Optimize Options,,Options that Control Optimization}.
264 @gccoptlist{-falign-functions=@var{n} -falign-jumps=@var{n} @gol
265 -falign-labels=@var{n} -falign-loops=@var{n} @gol
266 -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize @gol
267 -fbranch-target-load-optimize2 -fbtr-bb-exclusive @gol
268 -fcaller-saves -fcprop-registers @gol
269 -fcse-follow-jumps -fcse-skip-blocks -fdata-sections @gol
270 -fdelayed-branch -fdelete-null-pointer-checks @gol
271 -fexpensive-optimizations -ffast-math -ffloat-store @gol
272 -fforce-addr -fforce-mem -ffunction-sections @gol
273 -fgcse -fgcse-lm -fgcse-sm -fgcse-las -floop-optimize @gol
274 -fcrossjumping -fif-conversion -fif-conversion2 @gol
275 -finline-functions -finline-limit=@var{n} -fkeep-inline-functions @gol
276 -fkeep-static-consts -fmerge-constants -fmerge-all-constants @gol
277 -fmove-all-movables -fnew-ra -fno-branch-count-reg @gol
278 -fno-default-inline -fno-defer-pop @gol
279 -fno-function-cse -fno-guess-branch-probability @gol
280 -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
281 -funsafe-math-optimizations -ffinite-math-only @gol
282 -fno-trapping-math -fno-zero-initialized-in-bss @gol
283 -fomit-frame-pointer -foptimize-register-move @gol
284 -foptimize-sibling-calls -fprefetch-loop-arrays @gol
285 -fprofile-generate -fprofile-use @gol
286 -freduce-all-givs -fregmove -frename-registers @gol
287 -freorder-blocks -freorder-functions @gol
288 -frerun-cse-after-loop -frerun-loop-opt @gol
289 -frounding-math -fschedule-insns -fschedule-insns2 @gol
290 -fno-sched-interblock -fno-sched-spec -fsched-spec-load @gol
291 -fsched-spec-load-dangerous @gol
292 -fsched-stalled-insns=@var{n} -sched-stalled-insns-dep=@var{n} @gol
293 -fsched2-use-superblocks @gol
294 -fsched2-use-traces -fsignaling-nans @gol
295 -fsingle-precision-constant @gol
296 -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps @gol
297 -funroll-all-loops -funroll-loops -fpeel-loops @gol
298 -funswitch-loops -fold-unroll-loops -fold-unroll-all-loops @gol
299 --param @var{name}=@var{value}
300 -O -O0 -O1 -O2 -O3 -Os}
302 @item Preprocessor Options
303 @xref{Preprocessor Options,,Options Controlling the Preprocessor}.
304 @gccoptlist{-A@var{question}=@var{answer} @gol
305 -A-@var{question}@r{[}=@var{answer}@r{]} @gol
306 -C -dD -dI -dM -dN @gol
307 -D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
308 -idirafter @var{dir} @gol
309 -include @var{file} -imacros @var{file} @gol
310 -iprefix @var{file} -iwithprefix @var{dir} @gol
311 -iwithprefixbefore @var{dir} -isystem @var{dir} @gol
312 -M -MM -MF -MG -MP -MQ -MT -nostdinc @gol
313 -P -fworking-directory -remap @gol
314 -trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
315 -Xpreprocessor @var{option}}
317 @item Assembler Option
318 @xref{Assembler Options,,Passing Options to the Assembler}.
319 @gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
322 @xref{Link Options,,Options for Linking}.
323 @gccoptlist{@var{object-file-name} -l@var{library} @gol
324 -nostartfiles -nodefaultlibs -nostdlib -pie @gol
325 -s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
326 -Wl,@var{option} -Xlinker @var{option} @gol
329 @item Directory Options
330 @xref{Directory Options,,Options for Directory Search}.
331 @gccoptlist{-B@var{prefix} -I@var{dir} -I- -L@var{dir} -specs=@var{file}}
334 @c I wrote this xref this way to avoid overfull hbox. -- rms
335 @xref{Target Options}.
336 @gccoptlist{-V @var{version} -b @var{machine}}
338 @item Machine Dependent Options
339 @xref{Submodel Options,,Hardware Models and Configurations}.
341 @emph{M680x0 Options}
342 @gccoptlist{-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
343 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 @gol
344 -mnobitfield -mrtd -mshort -msoft-float -mpcrel @gol
345 -malign-int -mstrict-align -msep-data -mno-sep-data @gol
346 -mshared-library-id=n -mid-shared-library -mno-id-shared-library}
348 @emph{M68hc1x Options}
349 @gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
350 -mauto-incdec -minmax -mlong-calls -mshort @gol
351 -msoft-reg-count=@var{count}}
354 @gccoptlist{-mg -mgnu -munix}
357 @gccoptlist{-mcpu=@var{cpu-type} @gol
358 -mtune=@var{cpu-type} @gol
359 -mcmodel=@var{code-model} @gol
360 -m32 -m64 -mapp-regs -mno-app-regs @gol
361 -mfaster-structs -mno-faster-structs @gol
362 -mfpu -mno-fpu -mhard-float -msoft-float @gol
363 -mhard-quad-float -msoft-quad-float @gol
364 -mimpure-text -mno-impure-text -mlittle-endian @gol
365 -mstack-bias -mno-stack-bias @gol
366 -munaligned-doubles -mno-unaligned-doubles @gol
367 -mv8plus -mno-v8plus -mvis -mno-vis}
370 @gccoptlist{-mapcs-frame -mno-apcs-frame @gol
371 -mabi=@var{name} @gol
372 -mapcs-26 -mapcs-32 @gol
373 -mapcs-stack-check -mno-apcs-stack-check @gol
374 -mapcs-float -mno-apcs-float @gol
375 -mapcs-reentrant -mno-apcs-reentrant @gol
376 -msched-prolog -mno-sched-prolog @gol
377 -mlittle-endian -mbig-endian -mwords-little-endian @gol
378 -malignment-traps -mno-alignment-traps @gol
379 -mfloat-abi=@var{name} soft-float -mhard-float -mfpe @gol
380 -mthumb-interwork -mno-thumb-interwork @gol
381 -mcpu=@var{name} -march=@var{name} -mfpu=@var{name} @gol
382 -mstructure-size-boundary=@var{n} @gol
383 -mabort-on-noreturn @gol
384 -mlong-calls -mno-long-calls @gol
385 -msingle-pic-base -mno-single-pic-base @gol
386 -mpic-register=@var{reg} @gol
387 -mnop-fun-dllimport @gol
388 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
389 -mpoke-function-name @gol
391 -mtpcs-frame -mtpcs-leaf-frame @gol
392 -mcaller-super-interworking -mcallee-super-interworking}
394 @emph{MN10300 Options}
395 @gccoptlist{-mmult-bug -mno-mult-bug @gol
396 -mam33 -mno-am33 @gol
397 -mam33-2 -mno-am33-2 @gol
400 @emph{M32R/D Options}
401 @gccoptlist{-m32r2 -m32rx -m32r @gol
403 -malign-loops -mno-align-loops @gol
404 -missue-rate=@var{number} @gol
405 -mbranch-cost=@var{number} @gol
406 -mmodel=@var{code-size-model-type} @gol
407 -msdata=@var{sdata-type} @gol
408 -mno-flush-func -mflush-func=@var{name} @gol
409 -mno-flush-trap -mflush-trap=@var{number} @gol
412 @emph{RS/6000 and PowerPC Options}
413 @gccoptlist{-mcpu=@var{cpu-type} @gol
414 -mtune=@var{cpu-type} @gol
415 -mpower -mno-power -mpower2 -mno-power2 @gol
416 -mpowerpc -mpowerpc64 -mno-powerpc @gol
417 -maltivec -mno-altivec @gol
418 -mpowerpc-gpopt -mno-powerpc-gpopt @gol
419 -mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
420 -mnew-mnemonics -mold-mnemonics @gol
421 -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
422 -m64 -m32 -mxl-call -mno-xl-call -mpe @gol
423 -malign-power -malign-natural @gol
424 -msoft-float -mhard-float -mmultiple -mno-multiple @gol
425 -mstring -mno-string -mupdate -mno-update @gol
426 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
427 -mstrict-align -mno-strict-align -mrelocatable @gol
428 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
429 -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
430 -mdynamic-no-pic @gol
431 -mprioritize-restricted-insns=@var{priority} @gol
432 -msched-costly-dep=@var{dependence_type} @gol
433 -minsert-sched-nops=@var{scheme} @gol
434 -mcall-sysv -mcall-netbsd @gol
435 -maix-struct-return -msvr4-struct-return @gol
436 -mabi=altivec -mabi=no-altivec @gol
437 -mabi=spe -mabi=no-spe @gol
438 -misel=yes -misel=no @gol
439 -mspe=yes -mspe=no @gol
440 -mfloat-gprs=yes -mfloat-gprs=no @gol
441 -mprototype -mno-prototype @gol
442 -msim -mmvme -mads -myellowknife -memb -msdata @gol
443 -msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
445 @emph{Darwin Options}
446 @gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
447 -arch_only -bind_at_load -bundle -bundle_loader @gol
448 -client_name -compatibility_version -current_version @gol
449 -dependency-file -dylib_file -dylinker_install_name @gol
450 -dynamic -dynamiclib -exported_symbols_list @gol
451 -filelist -flat_namespace -force_cpusubtype_ALL @gol
452 -force_flat_namespace -headerpad_max_install_names @gol
453 -image_base -init -install_name -keep_private_externs @gol
454 -multi_module -multiply_defined -multiply_defined_unused @gol
455 -noall_load -nofixprebinding -nomultidefs -noprebind -noseglinkedit @gol
456 -pagezero_size -prebind -prebind_all_twolevel_modules @gol
457 -private_bundle -read_only_relocs -sectalign @gol
458 -sectobjectsymbols -whyload -seg1addr @gol
459 -sectcreate -sectobjectsymbols -sectorder @gol
460 -seg_addr_table -seg_addr_table_filename -seglinkedit @gol
461 -segprot -segs_read_only_addr -segs_read_write_addr @gol
462 -single_module -static -sub_library -sub_umbrella @gol
463 -twolevel_namespace -umbrella -undefined @gol
464 -unexported_symbols_list -weak_reference_mismatches @gol
468 @gccoptlist{-EL -EB -march=@var{arch} -mtune=@var{arch} @gol
469 -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
470 -mips16 -mno-mips16 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
471 -mxgot -mno-xgot -membedded-pic -mno-embedded-pic @gol
472 -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float -msoft-float @gol
473 -msingle-float -mdouble-float -mint64 -mlong64 -mlong32 @gol
474 -G@var{num} -membedded-data -mno-embedded-data @gol
475 -muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
476 -msplit-addresses -mno-split-addresses @gol
477 -mexplicit-relocs -mno-explicit-relocs @gol
478 -mrnames -mno-rnames @gol
479 -mcheck-zero-division -mno-check-zero-division @gol
480 -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls @gol
481 -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp @gol
482 -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 @gol
483 -mfix-vr4122-bugs -mno-fix-vr4122-bugs -mfix-sb1 -mno-fix-sb1 @gol
484 -mflush-func=@var{func} -mno-flush-func @gol
485 -mbranch-likely -mno-branch-likely}
487 @emph{i386 and x86-64 Options}
488 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
489 -mfpmath=@var{unit} @gol
490 -masm=@var{dialect} -mno-fancy-math-387 @gol
491 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib @gol
492 -mno-wide-multiply -mrtd -malign-double @gol
493 -mpreferred-stack-boundary=@var{num} @gol
494 -mmmx -msse -msse2 -msse3 -m3dnow @gol
495 -mthreads -mno-align-stringops -minline-all-stringops @gol
496 -mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
497 -m96bit-long-double -mregparm=@var{num} -momit-leaf-frame-pointer @gol
498 -mno-red-zone -mno-tls-direct-seg-refs @gol
499 -mcmodel=@var{code-model} @gol
503 @gccoptlist{-march=@var{architecture-type} @gol
504 -mbig-switch -mdisable-fpregs -mdisable-indexing @gol
505 -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
506 -mjump-in-delay -mlinker-opt -mlong-calls @gol
507 -mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
508 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
509 -mno-jump-in-delay -mno-long-load-store @gol
510 -mno-portable-runtime -mno-soft-float @gol
511 -mno-space-regs -msoft-float -mpa-risc-1-0 @gol
512 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
513 -mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
514 -nolibdld -static -threads}
516 @emph{DEC Alpha Options}
517 @gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
518 -mieee -mieee-with-inexact -mieee-conformant @gol
519 -mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
520 -mtrap-precision=@var{mode} -mbuild-constants @gol
521 -mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
522 -mbwx -mmax -mfix -mcix @gol
523 -mfloat-vax -mfloat-ieee @gol
524 -mexplicit-relocs -msmall-data -mlarge-data @gol
525 -msmall-text -mlarge-text @gol
526 -mmemory-latency=@var{time}}
528 @emph{DEC Alpha/VMS Options}
529 @gccoptlist{-mvms-return-codes}
531 @emph{H8/300 Options}
532 @gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
535 @gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
536 -m4-nofpu -m4-single-only -m4-single -m4 @gol
537 -m5-64media -m5-64media-nofpu @gol
538 -m5-32media -m5-32media-nofpu @gol
539 -m5-compact -m5-compact-nofpu @gol
540 -mb -ml -mdalign -mrelax @gol
541 -mbigtable -mfmovd -mhitachi -mnomacsave @gol
542 -mieee -misize -mpadstruct -mspace @gol
543 -mprefergot -musermode}
545 @emph{System V Options}
546 @gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
549 @gccoptlist{-EB -EL @gol
550 -mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
551 -mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
553 @emph{TMS320C3x/C4x Options}
554 @gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
555 -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
556 -mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
557 -mparallel-insns -mparallel-mpy -mpreserve-float}
560 @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
561 -mprolog-function -mno-prolog-function -mspace @gol
562 -mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
563 -mapp-regs -mno-app-regs @gol
564 -mdisable-callt -mno-disable-callt @gol
570 @gccoptlist{-m32032 -m32332 -m32532 -m32081 -m32381 @gol
571 -mmult-add -mnomult-add -msoft-float -mrtd -mnortd @gol
572 -mregparam -mnoregparam -msb -mnosb @gol
573 -mbitfield -mnobitfield -mhimem -mnohimem}
576 @gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
577 -mcall-prologues -mno-tablejump -mtiny-stack}
580 @gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
581 -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
582 -m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
583 -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
584 -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
587 @gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
588 -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
589 -melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
590 -mno-base-addresses -msingle-exit -mno-single-exit}
593 @gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
594 -mvolatile-asm-stop -mb-step -mregister-names -mno-sdata @gol
595 -mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
596 -minline-float-divide-max-throughput @gol
597 -minline-int-divide-min-latency @gol
598 -minline-int-divide-max-throughput -mno-dwarf2-asm @gol
599 -mfixed-range=@var{register-range}}
601 @emph{S/390 and zSeries Options}
602 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
603 -mhard-float -msoft-float -mbackchain -mno-backchain @gol
604 -msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
605 -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mfused-madd -mno-fused-madd}
608 @gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
609 -mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
610 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
611 -mstack-align -mdata-align -mconst-align @gol
612 -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
613 -melf -maout -melinux -mlinux -sim -sim2 @gol
614 -mmul-bug-workaround -mno-mul-bug-workaround}
616 @emph{PDP-11 Options}
617 @gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
618 -mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
619 -mint16 -mno-int32 -mfloat32 -mno-float64 @gol
620 -mfloat64 -mno-float32 -mabshi -mno-abshi @gol
621 -mbranch-expensive -mbranch-cheap @gol
622 -msplit -mno-split -munix-asm -mdec-asm}
624 @emph{Xstormy16 Options}
627 @emph{Xtensa Options}
628 @gccoptlist{-mconst16 -mno-const16 @gol
629 -mfused-madd -mno-fused-madd @gol
630 -mtext-section-literals -mno-text-section-literals @gol
631 -mtarget-align -mno-target-align @gol
632 -mlongcalls -mno-longcalls}
635 @gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
636 -mhard-float -msoft-float @gol
637 -malloc-cc -mfixed-cc -mdword -mno-dword @gol
638 -mdouble -mno-double @gol
639 -mmedia -mno-media -mmuladd -mno-muladd @gol
640 -mfdpic -minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp @gol
641 -mlibrary-pic -macc-4 -macc-8 @gol
642 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
643 -mscc -mno-scc -mcond-exec -mno-cond-exec @gol
644 -mvliw-branch -mno-vliw-branch @gol
645 -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
646 -mno-nested-cond-exec -mtomcat-stats @gol
649 @item Code Generation Options
650 @xref{Code Gen Options,,Options for Code Generation Conventions}.
651 @gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
652 -ffixed-@var{reg} -fexceptions @gol
653 -fnon-call-exceptions -funwind-tables @gol
654 -fasynchronous-unwind-tables @gol
655 -finhibit-size-directive -finstrument-functions @gol
656 -fno-common -fno-ident @gol
657 -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
658 -freg-struct-return -fshared-data -fshort-enums @gol
659 -fshort-double -fshort-wchar @gol
660 -fverbose-asm -fpack-struct -fstack-check @gol
661 -fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
662 -fargument-alias -fargument-noalias @gol
663 -fargument-noalias-global -fleading-underscore @gol
664 -ftls-model=@var{model} @gol
665 -ftrapv -fwrapv -fbounds-check}
669 * Overall Options:: Controlling the kind of output:
670 an executable, object files, assembler files,
671 or preprocessed source.
672 * C Dialect Options:: Controlling the variant of C language compiled.
673 * C++ Dialect Options:: Variations on C++.
674 * Objective-C Dialect Options:: Variations on Objective-C.
675 * Language Independent Options:: Controlling how diagnostics should be
677 * Warning Options:: How picky should the compiler be?
678 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
679 * Optimize Options:: How much optimization?
680 * Preprocessor Options:: Controlling header files and macro definitions.
681 Also, getting dependency information for Make.
682 * Assembler Options:: Passing options to the assembler.
683 * Link Options:: Specifying libraries and so on.
684 * Directory Options:: Where to find header files and libraries.
685 Where to find the compiler executable files.
686 * Spec Files:: How to pass switches to sub-processes.
687 * Target Options:: Running a cross-compiler, or an old version of GCC.
690 @node Overall Options
691 @section Options Controlling the Kind of Output
693 Compilation can involve up to four stages: preprocessing, compilation
694 proper, assembly and linking, always in that order. GCC is capable of
695 preprocessing and compiling several files either into several
696 assembler input files, or into one assembler input file; then each
697 assembler input file produces an object file, and linking combines all
698 the object files (those newly compiled, and those specified as input)
699 into an executable file.
701 @cindex file name suffix
702 For any given input file, the file name suffix determines what kind of
707 C source code which must be preprocessed.
710 C source code which should not be preprocessed.
713 C++ source code which should not be preprocessed.
716 Objective-C source code. Note that you must link with the library
717 @file{libobjc.a} to make an Objective-C program work.
720 Objective-C source code which should not be preprocessed.
723 C or C++ header file to be turned into a precompiled header.
727 @itemx @var{file}.cxx
728 @itemx @var{file}.cpp
729 @itemx @var{file}.CPP
730 @itemx @var{file}.c++
732 C++ source code which must be preprocessed. Note that in @samp{.cxx},
733 the last two letters must both be literally @samp{x}. Likewise,
734 @samp{.C} refers to a literal capital C@.
738 C++ header file to be turned into a precompiled header.
741 @itemx @var{file}.for
742 @itemx @var{file}.FOR
743 Fortran source code which should not be preprocessed.
746 @itemx @var{file}.fpp
747 @itemx @var{file}.FPP
748 Fortran source code which must be preprocessed (with the traditional
752 Fortran source code which must be preprocessed with a RATFOR
753 preprocessor (not included with GCC)@.
755 @xref{Overall Options,,Options Controlling the Kind of Output, g77,
756 Using and Porting GNU Fortran}, for more details of the handling of
759 @c FIXME: Descriptions of Java file types.
766 Ada source code file which contains a library unit declaration (a
767 declaration of a package, subprogram, or generic, or a generic
768 instantiation), or a library unit renaming declaration (a package,
769 generic, or subprogram renaming declaration). Such files are also
772 @itemx @var{file}.adb
773 Ada source code file containing a library unit body (a subprogram or
774 package body). Such files are also called @dfn{bodies}.
776 @c GCC also knows about some suffixes for languages not yet included:
785 Assembler code which must be preprocessed.
788 An object file to be fed straight into linking.
789 Any file name with no recognized suffix is treated this way.
793 You can specify the input language explicitly with the @option{-x} option:
796 @item -x @var{language}
797 Specify explicitly the @var{language} for the following input files
798 (rather than letting the compiler choose a default based on the file
799 name suffix). This option applies to all following input files until
800 the next @option{-x} option. Possible values for @var{language} are:
802 c c-header cpp-output
803 c++ c++-header c++-cpp-output
804 objective-c objective-c-header objc-cpp-output
805 assembler assembler-with-cpp
807 f77 f77-cpp-input ratfor
813 Turn off any specification of a language, so that subsequent files are
814 handled according to their file name suffixes (as they are if @option{-x}
815 has not been used at all).
817 @item -pass-exit-codes
818 @opindex pass-exit-codes
819 Normally the @command{gcc} program will exit with the code of 1 if any
820 phase of the compiler returns a non-success return code. If you specify
821 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
822 numerically highest error produced by any phase that returned an error
826 If you only want some of the stages of compilation, you can use
827 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
828 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
829 @command{gcc} is to stop. Note that some combinations (for example,
830 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
835 Compile or assemble the source files, but do not link. The linking
836 stage simply is not done. The ultimate output is in the form of an
837 object file for each source file.
839 By default, the object file name for a source file is made by replacing
840 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
842 Unrecognized input files, not requiring compilation or assembly, are
847 Stop after the stage of compilation proper; do not assemble. The output
848 is in the form of an assembler code file for each non-assembler input
851 By default, the assembler file name for a source file is made by
852 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
854 Input files that don't require compilation are ignored.
858 Stop after the preprocessing stage; do not run the compiler proper. The
859 output is in the form of preprocessed source code, which is sent to the
862 Input files which don't require preprocessing are ignored.
864 @cindex output file option
867 Place output in file @var{file}. This applies regardless to whatever
868 sort of output is being produced, whether it be an executable file,
869 an object file, an assembler file or preprocessed C code.
871 If @option{-o} is not specified, the default is to put an executable file
872 in @file{a.out}, the object file for @file{@var{source}.@var{suffix}} in
873 @file{@var{source}.o}, its assembler file in @file{@var{source}.s}, and
874 all preprocessed C source on standard output.
878 Print (on standard error output) the commands executed to run the stages
879 of compilation. Also print the version number of the compiler driver
880 program and of the preprocessor and the compiler proper.
884 Like @option{-v} except the commands are not executed and all command
885 arguments are quoted. This is useful for shell scripts to capture the
886 driver-generated command lines.
890 Use pipes rather than temporary files for communication between the
891 various stages of compilation. This fails to work on some systems where
892 the assembler is unable to read from a pipe; but the GNU assembler has
897 If you are compiling multiple source files, this option tells the driver
898 to pass all the source files to the compiler at once (for those
899 languages for which the compiler can handle this). This will allow
900 intermodule analysis (IMA) to be performed by the compiler. Currently the only
901 language for which this is supported is C. If you pass source files for
902 multiple languages to the driver, using this option, the driver will invoke
903 the compiler(s) that support IMA once each, passing each compiler all the
904 source files appropriate for it. For those languages that do not support
905 IMA this option will be ignored, and the compiler will be invoked once for
906 each source file in that language. If you use this option in conjunction
907 with -save-temps, the compiler will generate multiple pre-processed files
908 (one for each source file), but only one (combined) .o or .s file.
912 Print (on the standard output) a description of the command line options
913 understood by @command{gcc}. If the @option{-v} option is also specified
914 then @option{--help} will also be passed on to the various processes
915 invoked by @command{gcc}, so that they can display the command line options
916 they accept. If the @option{-Wextra} option is also specified then command
917 line options which have no documentation associated with them will also
922 Print (on the standard output) a description of target specific command
923 line options for each tool.
927 Display the version number and copyrights of the invoked GCC.
931 @section Compiling C++ Programs
933 @cindex suffixes for C++ source
934 @cindex C++ source file suffixes
935 C++ source files conventionally use one of the suffixes @samp{.C},
936 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
937 @samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
938 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
939 files with these names and compiles them as C++ programs even if you
940 call the compiler the same way as for compiling C programs (usually
941 with the name @command{gcc}).
945 However, C++ programs often require class libraries as well as a
946 compiler that understands the C++ language---and under some
947 circumstances, you might want to compile programs or header files from
948 standard input, or otherwise without a suffix that flags them as C++
949 programs. You might also like to precompile a C header file with a
950 @samp{.h} extension to be used in C++ compilations. @command{g++} is a
951 program that calls GCC with the default language set to C++, and
952 automatically specifies linking against the C++ library. On many
953 systems, @command{g++} is also installed with the name @command{c++}.
955 @cindex invoking @command{g++}
956 When you compile C++ programs, you may specify many of the same
957 command-line options that you use for compiling programs in any
958 language; or command-line options meaningful for C and related
959 languages; or options that are meaningful only for C++ programs.
960 @xref{C Dialect Options,,Options Controlling C Dialect}, for
961 explanations of options for languages related to C@.
962 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
963 explanations of options that are meaningful only for C++ programs.
965 @node C Dialect Options
966 @section Options Controlling C Dialect
967 @cindex dialect options
968 @cindex language dialect options
969 @cindex options, dialect
971 The following options control the dialect of C (or languages derived
972 from C, such as C++ and Objective-C) that the compiler accepts:
979 In C mode, support all ISO C90 programs. In C++ mode,
980 remove GNU extensions that conflict with ISO C++.
982 This turns off certain features of GCC that are incompatible with ISO
983 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
984 such as the @code{asm} and @code{typeof} keywords, and
985 predefined macros such as @code{unix} and @code{vax} that identify the
986 type of system you are using. It also enables the undesirable and
987 rarely used ISO trigraph feature. For the C compiler,
988 it disables recognition of C++ style @samp{//} comments as well as
989 the @code{inline} keyword.
991 The alternate keywords @code{__asm__}, @code{__extension__},
992 @code{__inline__} and @code{__typeof__} continue to work despite
993 @option{-ansi}. You would not want to use them in an ISO C program, of
994 course, but it is useful to put them in header files that might be included
995 in compilations done with @option{-ansi}. Alternate predefined macros
996 such as @code{__unix__} and @code{__vax__} are also available, with or
997 without @option{-ansi}.
999 The @option{-ansi} option does not cause non-ISO programs to be
1000 rejected gratuitously. For that, @option{-pedantic} is required in
1001 addition to @option{-ansi}. @xref{Warning Options}.
1003 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1004 option is used. Some header files may notice this macro and refrain
1005 from declaring certain functions or defining certain macros that the
1006 ISO standard doesn't call for; this is to avoid interfering with any
1007 programs that might use these names for other things.
1009 Functions which would normally be built in but do not have semantics
1010 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1011 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1012 built-in functions provided by GCC}, for details of the functions
1017 Determine the language standard. This option is currently only
1018 supported when compiling C or C++. A value for this option must be
1019 provided; possible values are
1024 ISO C90 (same as @option{-ansi}).
1026 @item iso9899:199409
1027 ISO C90 as modified in amendment 1.
1033 ISO C99. Note that this standard is not yet fully supported; see
1034 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1035 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1038 Default, ISO C90 plus GNU extensions (including some C99 features).
1042 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1043 this will become the default. The name @samp{gnu9x} is deprecated.
1046 The 1998 ISO C++ standard plus amendments.
1049 The same as @option{-std=c++98} plus GNU extensions. This is the
1050 default for C++ code.
1053 Even when this option is not specified, you can still use some of the
1054 features of newer standards in so far as they do not conflict with
1055 previous C standards. For example, you may use @code{__restrict__} even
1056 when @option{-std=c99} is not specified.
1058 The @option{-std} options specifying some version of ISO C have the same
1059 effects as @option{-ansi}, except that features that were not in ISO C90
1060 but are in the specified version (for example, @samp{//} comments and
1061 the @code{inline} keyword in ISO C99) are not disabled.
1063 @xref{Standards,,Language Standards Supported by GCC}, for details of
1064 these standard versions.
1066 @item -aux-info @var{filename}
1068 Output to the given filename prototyped declarations for all functions
1069 declared and/or defined in a translation unit, including those in header
1070 files. This option is silently ignored in any language other than C@.
1072 Besides declarations, the file indicates, in comments, the origin of
1073 each declaration (source file and line), whether the declaration was
1074 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1075 @samp{O} for old, respectively, in the first character after the line
1076 number and the colon), and whether it came from a declaration or a
1077 definition (@samp{C} or @samp{F}, respectively, in the following
1078 character). In the case of function definitions, a K&R-style list of
1079 arguments followed by their declarations is also provided, inside
1080 comments, after the declaration.
1084 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1085 keyword, so that code can use these words as identifiers. You can use
1086 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1087 instead. @option{-ansi} implies @option{-fno-asm}.
1089 In C++, this switch only affects the @code{typeof} keyword, since
1090 @code{asm} and @code{inline} are standard keywords. You may want to
1091 use the @option{-fno-gnu-keywords} flag instead, which has the same
1092 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1093 switch only affects the @code{asm} and @code{typeof} keywords, since
1094 @code{inline} is a standard keyword in ISO C99.
1097 @itemx -fno-builtin-@var{function}
1098 @opindex fno-builtin
1099 @cindex built-in functions
1100 Don't recognize built-in functions that do not begin with
1101 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1102 functions provided by GCC}, for details of the functions affected,
1103 including those which are not built-in functions when @option{-ansi} or
1104 @option{-std} options for strict ISO C conformance are used because they
1105 do not have an ISO standard meaning.
1107 GCC normally generates special code to handle certain built-in functions
1108 more efficiently; for instance, calls to @code{alloca} may become single
1109 instructions that adjust the stack directly, and calls to @code{memcpy}
1110 may become inline copy loops. The resulting code is often both smaller
1111 and faster, but since the function calls no longer appear as such, you
1112 cannot set a breakpoint on those calls, nor can you change the behavior
1113 of the functions by linking with a different library.
1115 With the @option{-fno-builtin-@var{function}} option
1116 only the built-in function @var{function} is
1117 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1118 function is named this is not built-in in this version of GCC, this
1119 option is ignored. There is no corresponding
1120 @option{-fbuiltin-@var{function}} option; if you wish to enable
1121 built-in functions selectively when using @option{-fno-builtin} or
1122 @option{-ffreestanding}, you may define macros such as:
1125 #define abs(n) __builtin_abs ((n))
1126 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1131 @cindex hosted environment
1133 Assert that compilation takes place in a hosted environment. This implies
1134 @option{-fbuiltin}. A hosted environment is one in which the
1135 entire standard library is available, and in which @code{main} has a return
1136 type of @code{int}. Examples are nearly everything except a kernel.
1137 This is equivalent to @option{-fno-freestanding}.
1139 @item -ffreestanding
1140 @opindex ffreestanding
1141 @cindex hosted environment
1143 Assert that compilation takes place in a freestanding environment. This
1144 implies @option{-fno-builtin}. A freestanding environment
1145 is one in which the standard library may not exist, and program startup may
1146 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1147 This is equivalent to @option{-fno-hosted}.
1149 @xref{Standards,,Language Standards Supported by GCC}, for details of
1150 freestanding and hosted environments.
1152 @item -fms-extensions
1153 @opindex fms-extensions
1154 Accept some non-standard constructs used in Microsoft header files.
1158 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1159 options for strict ISO C conformance) implies @option{-trigraphs}.
1161 @item -no-integrated-cpp
1162 @opindex no-integrated-cpp
1163 Performs a compilation in two passes: preprocessing and compiling. This
1164 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1165 @option{-B} option. The user supplied compilation step can then add in
1166 an additional preprocessing step after normal preprocessing but before
1167 compiling. The default is to use the integrated cpp (internal cpp)
1169 The semantics of this option will change if "cc1", "cc1plus", and
1170 "cc1obj" are merged.
1172 @cindex traditional C language
1173 @cindex C language, traditional
1175 @itemx -traditional-cpp
1176 @opindex traditional-cpp
1177 @opindex traditional
1178 Formerly, these options caused GCC to attempt to emulate a pre-standard
1179 C compiler. They are now only supported with the @option{-E} switch.
1180 The preprocessor continues to support a pre-standard mode. See the GNU
1181 CPP manual for details.
1183 @item -fcond-mismatch
1184 @opindex fcond-mismatch
1185 Allow conditional expressions with mismatched types in the second and
1186 third arguments. The value of such an expression is void. This option
1187 is not supported for C++.
1189 @item -funsigned-char
1190 @opindex funsigned-char
1191 Let the type @code{char} be unsigned, like @code{unsigned char}.
1193 Each kind of machine has a default for what @code{char} should
1194 be. It is either like @code{unsigned char} by default or like
1195 @code{signed char} by default.
1197 Ideally, a portable program should always use @code{signed char} or
1198 @code{unsigned char} when it depends on the signedness of an object.
1199 But many programs have been written to use plain @code{char} and
1200 expect it to be signed, or expect it to be unsigned, depending on the
1201 machines they were written for. This option, and its inverse, let you
1202 make such a program work with the opposite default.
1204 The type @code{char} is always a distinct type from each of
1205 @code{signed char} or @code{unsigned char}, even though its behavior
1206 is always just like one of those two.
1209 @opindex fsigned-char
1210 Let the type @code{char} be signed, like @code{signed char}.
1212 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1213 the negative form of @option{-funsigned-char}. Likewise, the option
1214 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1216 @item -fsigned-bitfields
1217 @itemx -funsigned-bitfields
1218 @itemx -fno-signed-bitfields
1219 @itemx -fno-unsigned-bitfields
1220 @opindex fsigned-bitfields
1221 @opindex funsigned-bitfields
1222 @opindex fno-signed-bitfields
1223 @opindex fno-unsigned-bitfields
1224 These options control whether a bit-field is signed or unsigned, when the
1225 declaration does not use either @code{signed} or @code{unsigned}. By
1226 default, such a bit-field is signed, because this is consistent: the
1227 basic integer types such as @code{int} are signed types.
1230 @node C++ Dialect Options
1231 @section Options Controlling C++ Dialect
1233 @cindex compiler options, C++
1234 @cindex C++ options, command line
1235 @cindex options, C++
1236 This section describes the command-line options that are only meaningful
1237 for C++ programs; but you can also use most of the GNU compiler options
1238 regardless of what language your program is in. For example, you
1239 might compile a file @code{firstClass.C} like this:
1242 g++ -g -frepo -O -c firstClass.C
1246 In this example, only @option{-frepo} is an option meant
1247 only for C++ programs; you can use the other options with any
1248 language supported by GCC@.
1250 Here is a list of options that are @emph{only} for compiling C++ programs:
1254 @item -fabi-version=@var{n}
1255 @opindex fabi-version
1256 Use version @var{n} of the C++ ABI. Version 2 is the version of the
1257 C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
1258 the C++ ABI that first appeared in G++ 3.2. Version 0 will always be
1259 the version that conforms most closely to the C++ ABI specification.
1260 Therefore, the ABI obtained using version 0 will change as ABI bugs
1263 The default is version 2.
1265 @item -fno-access-control
1266 @opindex fno-access-control
1267 Turn off all access checking. This switch is mainly useful for working
1268 around bugs in the access control code.
1272 Check that the pointer returned by @code{operator new} is non-null
1273 before attempting to modify the storage allocated. This check is
1274 normally unnecessary because the C++ standard specifies that
1275 @code{operator new} will only return @code{0} if it is declared
1276 @samp{throw()}, in which case the compiler will always check the
1277 return value even without this option. In all other cases, when
1278 @code{operator new} has a non-empty exception specification, memory
1279 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1280 @samp{new (nothrow)}.
1282 @item -fconserve-space
1283 @opindex fconserve-space
1284 Put uninitialized or runtime-initialized global variables into the
1285 common segment, as C does. This saves space in the executable at the
1286 cost of not diagnosing duplicate definitions. If you compile with this
1287 flag and your program mysteriously crashes after @code{main()} has
1288 completed, you may have an object that is being destroyed twice because
1289 two definitions were merged.
1291 This option is no longer useful on most targets, now that support has
1292 been added for putting variables into BSS without making them common.
1294 @item -fno-const-strings
1295 @opindex fno-const-strings
1296 Give string constants type @code{char *} instead of type @code{const
1297 char *}. By default, G++ uses type @code{const char *} as required by
1298 the standard. Even if you use @option{-fno-const-strings}, you cannot
1299 actually modify the value of a string constant.
1301 This option might be removed in a future release of G++. For maximum
1302 portability, you should structure your code so that it works with
1303 string constants that have type @code{const char *}.
1305 @item -fno-elide-constructors
1306 @opindex fno-elide-constructors
1307 The C++ standard allows an implementation to omit creating a temporary
1308 which is only used to initialize another object of the same type.
1309 Specifying this option disables that optimization, and forces G++ to
1310 call the copy constructor in all cases.
1312 @item -fno-enforce-eh-specs
1313 @opindex fno-enforce-eh-specs
1314 Don't check for violation of exception specifications at runtime. This
1315 option violates the C++ standard, but may be useful for reducing code
1316 size in production builds, much like defining @samp{NDEBUG}. The compiler
1317 will still optimize based on the exception specifications.
1320 @itemx -fno-for-scope
1322 @opindex fno-for-scope
1323 If @option{-ffor-scope} is specified, the scope of variables declared in
1324 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1325 as specified by the C++ standard.
1326 If @option{-fno-for-scope} is specified, the scope of variables declared in
1327 a @i{for-init-statement} extends to the end of the enclosing scope,
1328 as was the case in old versions of G++, and other (traditional)
1329 implementations of C++.
1331 The default if neither flag is given to follow the standard,
1332 but to allow and give a warning for old-style code that would
1333 otherwise be invalid, or have different behavior.
1335 @item -fno-gnu-keywords
1336 @opindex fno-gnu-keywords
1337 Do not recognize @code{typeof} as a keyword, so that code can use this
1338 word as an identifier. You can use the keyword @code{__typeof__} instead.
1339 @option{-ansi} implies @option{-fno-gnu-keywords}.
1341 @item -fno-implicit-templates
1342 @opindex fno-implicit-templates
1343 Never emit code for non-inline templates which are instantiated
1344 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1345 @xref{Template Instantiation}, for more information.
1347 @item -fno-implicit-inline-templates
1348 @opindex fno-implicit-inline-templates
1349 Don't emit code for implicit instantiations of inline templates, either.
1350 The default is to handle inlines differently so that compiles with and
1351 without optimization will need the same set of explicit instantiations.
1353 @item -fno-implement-inlines
1354 @opindex fno-implement-inlines
1355 To save space, do not emit out-of-line copies of inline functions
1356 controlled by @samp{#pragma implementation}. This will cause linker
1357 errors if these functions are not inlined everywhere they are called.
1359 @item -fms-extensions
1360 @opindex fms-extensions
1361 Disable pedantic warnings about constructs used in MFC, such as implicit
1362 int and getting a pointer to member function via non-standard syntax.
1364 @item -fno-nonansi-builtins
1365 @opindex fno-nonansi-builtins
1366 Disable built-in declarations of functions that are not mandated by
1367 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1368 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1370 @item -fno-operator-names
1371 @opindex fno-operator-names
1372 Do not treat the operator name keywords @code{and}, @code{bitand},
1373 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1374 synonyms as keywords.
1376 @item -fno-optional-diags
1377 @opindex fno-optional-diags
1378 Disable diagnostics that the standard says a compiler does not need to
1379 issue. Currently, the only such diagnostic issued by G++ is the one for
1380 a name having multiple meanings within a class.
1383 @opindex fpermissive
1384 Downgrade some diagnostics about nonconformant code from errors to
1385 warnings. Thus, using @option{-fpermissive} will allow some
1386 nonconforming code to compile.
1390 Enable automatic template instantiation at link time. This option also
1391 implies @option{-fno-implicit-templates}. @xref{Template
1392 Instantiation}, for more information.
1396 Disable generation of information about every class with virtual
1397 functions for use by the C++ runtime type identification features
1398 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1399 of the language, you can save some space by using this flag. Note that
1400 exception handling uses the same information, but it will generate it as
1405 Emit statistics about front-end processing at the end of the compilation.
1406 This information is generally only useful to the G++ development team.
1408 @item -ftemplate-depth-@var{n}
1409 @opindex ftemplate-depth
1410 Set the maximum instantiation depth for template classes to @var{n}.
1411 A limit on the template instantiation depth is needed to detect
1412 endless recursions during template class instantiation. ANSI/ISO C++
1413 conforming programs must not rely on a maximum depth greater than 17.
1415 @item -fuse-cxa-atexit
1416 @opindex fuse-cxa-atexit
1417 Register destructors for objects with static storage duration with the
1418 @code{__cxa_atexit} function rather than the @code{atexit} function.
1419 This option is required for fully standards-compliant handling of static
1420 destructors, but will only work if your C library supports
1421 @code{__cxa_atexit}.
1425 Do not use weak symbol support, even if it is provided by the linker.
1426 By default, G++ will use weak symbols if they are available. This
1427 option exists only for testing, and should not be used by end-users;
1428 it will result in inferior code and has no benefits. This option may
1429 be removed in a future release of G++.
1433 Do not search for header files in the standard directories specific to
1434 C++, but do still search the other standard directories. (This option
1435 is used when building the C++ library.)
1438 In addition, these optimization, warning, and code generation options
1439 have meanings only for C++ programs:
1442 @item -fno-default-inline
1443 @opindex fno-default-inline
1444 Do not assume @samp{inline} for functions defined inside a class scope.
1445 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1446 functions will have linkage like inline functions; they just won't be
1449 @item -Wabi @r{(C++ only)}
1451 Warn when G++ generates code that is probably not compatible with the
1452 vendor-neutral C++ ABI. Although an effort has been made to warn about
1453 all such cases, there are probably some cases that are not warned about,
1454 even though G++ is generating incompatible code. There may also be
1455 cases where warnings are emitted even though the code that is generated
1458 You should rewrite your code to avoid these warnings if you are
1459 concerned about the fact that code generated by G++ may not be binary
1460 compatible with code generated by other compilers.
1462 The known incompatibilities at this point include:
1467 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1468 pack data into the same byte as a base class. For example:
1471 struct A @{ virtual void f(); int f1 : 1; @};
1472 struct B : public A @{ int f2 : 1; @};
1476 In this case, G++ will place @code{B::f2} into the same byte
1477 as@code{A::f1}; other compilers will not. You can avoid this problem
1478 by explicitly padding @code{A} so that its size is a multiple of the
1479 byte size on your platform; that will cause G++ and other compilers to
1480 layout @code{B} identically.
1483 Incorrect handling of tail-padding for virtual bases. G++ does not use
1484 tail padding when laying out virtual bases. For example:
1487 struct A @{ virtual void f(); char c1; @};
1488 struct B @{ B(); char c2; @};
1489 struct C : public A, public virtual B @{@};
1493 In this case, G++ will not place @code{B} into the tail-padding for
1494 @code{A}; other compilers will. You can avoid this problem by
1495 explicitly padding @code{A} so that its size is a multiple of its
1496 alignment (ignoring virtual base classes); that will cause G++ and other
1497 compilers to layout @code{C} identically.
1500 Incorrect handling of bit-fields with declared widths greater than that
1501 of their underlying types, when the bit-fields appear in a union. For
1505 union U @{ int i : 4096; @};
1509 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1510 union too small by the number of bits in an @code{int}.
1513 Empty classes can be placed at incorrect offsets. For example:
1523 struct C : public B, public A @{@};
1527 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1528 it should be placed at offset zero. G++ mistakenly believes that the
1529 @code{A} data member of @code{B} is already at offset zero.
1532 Names of template functions whose types involve @code{typename} or
1533 template template parameters can be mangled incorrectly.
1536 template <typename Q>
1537 void f(typename Q::X) @{@}
1539 template <template <typename> class Q>
1540 void f(typename Q<int>::X) @{@}
1544 Instantiations of these templates may be mangled incorrectly.
1548 @item -Wctor-dtor-privacy @r{(C++ only)}
1549 @opindex Wctor-dtor-privacy
1550 Warn when a class seems unusable because all the constructors or
1551 destructors in that class are private, and it has neither friends nor
1552 public static member functions.
1554 @item -Wnon-virtual-dtor @r{(C++ only)}
1555 @opindex Wnon-virtual-dtor
1556 Warn when a class appears to be polymorphic, thereby requiring a virtual
1557 destructor, yet it declares a non-virtual one.
1558 This warning is enabled by @option{-Wall}.
1560 @item -Wreorder @r{(C++ only)}
1562 @cindex reordering, warning
1563 @cindex warning for reordering of member initializers
1564 Warn when the order of member initializers given in the code does not
1565 match the order in which they must be executed. For instance:
1571 A(): j (0), i (1) @{ @}
1575 The compiler will rearrange the member initializers for @samp{i}
1576 and @samp{j} to match the declaration order of the members, emitting
1577 a warning to that effect. This warning is enabled by @option{-Wall}.
1580 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
1583 @item -Weffc++ @r{(C++ only)}
1585 Warn about violations of the following style guidelines from Scott Meyers'
1586 @cite{Effective C++} book:
1590 Item 11: Define a copy constructor and an assignment operator for classes
1591 with dynamically allocated memory.
1594 Item 12: Prefer initialization to assignment in constructors.
1597 Item 14: Make destructors virtual in base classes.
1600 Item 15: Have @code{operator=} return a reference to @code{*this}.
1603 Item 23: Don't try to return a reference when you must return an object.
1607 Also warn about violations of the following style guidelines from
1608 Scott Meyers' @cite{More Effective C++} book:
1612 Item 6: Distinguish between prefix and postfix forms of increment and
1613 decrement operators.
1616 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
1620 When selecting this option, be aware that the standard library
1621 headers do not obey all of these guidelines; use @samp{grep -v}
1622 to filter out those warnings.
1624 @item -Wno-deprecated @r{(C++ only)}
1625 @opindex Wno-deprecated
1626 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
1628 @item -Wno-non-template-friend @r{(C++ only)}
1629 @opindex Wno-non-template-friend
1630 Disable warnings when non-templatized friend functions are declared
1631 within a template. Since the advent of explicit template specification
1632 support in G++, if the name of the friend is an unqualified-id (i.e.,
1633 @samp{friend foo(int)}), the C++ language specification demands that the
1634 friend declare or define an ordinary, nontemplate function. (Section
1635 14.5.3). Before G++ implemented explicit specification, unqualified-ids
1636 could be interpreted as a particular specialization of a templatized
1637 function. Because this non-conforming behavior is no longer the default
1638 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
1639 check existing code for potential trouble spots and is on by default.
1640 This new compiler behavior can be turned off with
1641 @option{-Wno-non-template-friend} which keeps the conformant compiler code
1642 but disables the helpful warning.
1644 @item -Wold-style-cast @r{(C++ only)}
1645 @opindex Wold-style-cast
1646 Warn if an old-style (C-style) cast to a non-void type is used within
1647 a C++ program. The new-style casts (@samp{static_cast},
1648 @samp{reinterpret_cast}, and @samp{const_cast}) are less vulnerable to
1649 unintended effects and much easier to search for.
1651 @item -Woverloaded-virtual @r{(C++ only)}
1652 @opindex Woverloaded-virtual
1653 @cindex overloaded virtual fn, warning
1654 @cindex warning for overloaded virtual fn
1655 Warn when a function declaration hides virtual functions from a
1656 base class. For example, in:
1663 struct B: public A @{
1668 the @code{A} class version of @code{f} is hidden in @code{B}, and code
1676 will fail to compile.
1678 @item -Wno-pmf-conversions @r{(C++ only)}
1679 @opindex Wno-pmf-conversions
1680 Disable the diagnostic for converting a bound pointer to member function
1683 @item -Wsign-promo @r{(C++ only)}
1684 @opindex Wsign-promo
1685 Warn when overload resolution chooses a promotion from unsigned or
1686 enumeral type to a signed type, over a conversion to an unsigned type of
1687 the same size. Previous versions of G++ would try to preserve
1688 unsignedness, but the standard mandates the current behavior.
1690 @item -Wsynth @r{(C++ only)}
1692 @cindex warning for synthesized methods
1693 @cindex synthesized methods, warning
1694 Warn when G++'s synthesis behavior does not match that of cfront. For
1700 A& operator = (int);
1710 In this example, G++ will synthesize a default @samp{A& operator =
1711 (const A&);}, while cfront will use the user-defined @samp{operator =}.
1714 @node Objective-C Dialect Options
1715 @section Options Controlling Objective-C Dialect
1717 @cindex compiler options, Objective-C
1718 @cindex Objective-C options, command line
1719 @cindex options, Objective-C
1720 (NOTE: This manual does not describe the Objective-C language itself. See
1721 @w{@uref{http://gcc.gnu.org/readings.html}} for references.)
1723 This section describes the command-line options that are only meaningful
1724 for Objective-C programs, but you can also use most of the GNU compiler
1725 options regardless of what language your program is in. For example,
1726 you might compile a file @code{some_class.m} like this:
1729 gcc -g -fgnu-runtime -O -c some_class.m
1733 In this example, @option{-fgnu-runtime} is an option meant only for
1734 Objective-C programs; you can use the other options with any language
1737 Here is a list of options that are @emph{only} for compiling Objective-C
1741 @item -fconstant-string-class=@var{class-name}
1742 @opindex fconstant-string-class
1743 Use @var{class-name} as the name of the class to instantiate for each
1744 literal string specified with the syntax @code{@@"@dots{}"}. The default
1745 class name is @code{NXConstantString} if the GNU runtime is being used, and
1746 @code{NSConstantString} if the NeXT runtime is being used (see below). The
1747 @option{-fconstant-cfstrings} option, if also present, will override the
1748 @option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
1749 to be laid out as constant CoreFoundation strings.
1752 @opindex fgnu-runtime
1753 Generate object code compatible with the standard GNU Objective-C
1754 runtime. This is the default for most types of systems.
1756 @item -fnext-runtime
1757 @opindex fnext-runtime
1758 Generate output compatible with the NeXT runtime. This is the default
1759 for NeXT-based systems, including Darwin and Mac OS X@. The macro
1760 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
1763 @item -fno-nil-receivers
1764 @opindex -fno-nil-receivers
1765 Assume that all Objective-C message dispatches (e.g.,
1766 @code{[receiver message:arg]}) in this translation unit ensure that the receiver
1767 is not @code{nil}. This allows for more efficient entry points in the runtime to be
1768 used. Currently, this option is only available in conjunction with
1769 the NeXT runtime on Mac OS X 10.3 and later.
1771 @item -fobjc-exceptions
1772 @opindex -fobjc-exceptions
1773 Enable syntactic support for structured exception handling in Objective-C,
1774 similar to what is offered by C++ and Java. Currently, this option is only
1775 available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
1783 @@catch (AnObjCClass *exc) @{
1790 @@catch (AnotherClass *exc) @{
1793 @@catch (id allOthers) @{
1803 The @code{@@throw} statement may appear anywhere in an Objective-C or
1804 Objective-C++ program; when used inside of a @code{@@catch} block, the
1805 @code{@@throw} may appear without an argument (as shown above), in which case
1806 the object caught by the @code{@@catch} will be rethrown.
1808 Note that only (pointers to) Objective-C objects may be thrown and
1809 caught using this scheme. When an object is thrown, it will be caught
1810 by the nearest @code{@@catch} clause capable of handling objects of that type,
1811 analogously to how @code{catch} blocks work in C++ and Java. A
1812 @code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
1813 any and all Objective-C exceptions not caught by previous @code{@@catch}
1816 The @code{@@finally} clause, if present, will be executed upon exit from the
1817 immediately preceding @code{@@try @dots{} @@catch} section. This will happen
1818 regardless of whether any exceptions are thrown, caught or rethrown
1819 inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
1820 of the @code{finally} clause in Java.
1822 There are several caveats to using the new exception mechanism:
1826 Although currently designed to be binary compatible with @code{NS_HANDLER}-style
1827 idioms provided by the @code{NSException} class, the new
1828 exceptions can only be used on Mac OS X 10.3 (Panther) and later
1829 systems, due to additional functionality needed in the (NeXT) Objective-C
1833 As mentioned above, the new exceptions do not support handling
1834 types other than Objective-C objects. Furthermore, when used from
1835 Objective-C++, the Objective-C exception model does not interoperate with C++
1836 exceptions at this time. This means you cannot @code{@@throw} an exception
1837 from Objective-C and @code{catch} it in C++, or vice versa
1838 (i.e., @code{throw @dots{} @@catch}).
1841 The @option{-fobjc-exceptions} switch also enables the use of synchronization
1842 blocks for thread-safe execution:
1845 @@synchronized (ObjCClass *guard) @{
1850 Upon entering the @code{@@synchronized} block, a thread of execution shall
1851 first check whether a lock has been placed on the corresponding @code{guard}
1852 object by another thread. If it has, the current thread shall wait until
1853 the other thread relinquishes its lock. Once @code{guard} becomes available,
1854 the current thread will place its own lock on it, execute the code contained in
1855 the @code{@@synchronized} block, and finally relinquish the lock (thereby
1856 making @code{guard} available to other threads).
1858 Unlike Java, Objective-C does not allow for entire methods to be marked
1859 @code{@@synchronized}. Note that throwing exceptions out of
1860 @code{@@synchronized} blocks is allowed, and will cause the guarding object
1861 to be unlocked properly.
1863 @item -freplace-objc-classes
1864 @opindex -freplace-objc-classes
1865 Emit a special marker instructing @command{ld(1)} not to statically link in
1866 the resulting object file, and allow @command{dyld(1)} to load it in at
1867 run time instead. This is used in conjunction with the Fix-and-Continue
1868 debugging mode, where the object file in question may be recompiled and
1869 dynamically reloaded in the course of program execution, without the need
1870 to restart the program itself. Currently, Fix-and-Continue functionality
1871 is only available in conjunction with the NeXT runtime on Mac OS X 10.3
1875 @opindex -fzero-link
1876 When compiling for the NeXT runtime, the compiler ordinarily replaces calls
1877 to @code{objc_getClass("@dots{}")} (when the name of the class is known at
1878 compile time) with static class references that get initialized at load time,
1879 which improves run-time performance. Specifying the @option{-fzero-link} flag
1880 suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
1881 to be retained. This is useful in Zero-Link debugging mode, since it allows
1882 for individual class implementations to be modified during program execution.
1886 Dump interface declarations for all classes seen in the source file to a
1887 file named @file{@var{sourcename}.decl}.
1890 @opindex Wno-protocol
1891 If a class is declared to implement a protocol, a warning is issued for
1892 every method in the protocol that is not implemented by the class. The
1893 default behavior is to issue a warning for every method not explicitly
1894 implemented in the class, even if a method implementation is inherited
1895 from the superclass. If you use the @code{-Wno-protocol} option, then
1896 methods inherited from the superclass are considered to be implemented,
1897 and no warning is issued for them.
1901 Warn if multiple methods of different types for the same selector are
1902 found during compilation. The check is performed on the list of methods
1903 in the final stage of compilation. Additionally, a check is performed
1904 for each selector appearing in a @code{@@selector(@dots{})}
1905 expression, and a corresponding method for that selector has been found
1906 during compilation. Because these checks scan the method table only at
1907 the end of compilation, these warnings are not produced if the final
1908 stage of compilation is not reached, for example because an error is
1909 found during compilation, or because the @code{-fsyntax-only} option is
1912 @item -Wundeclared-selector
1913 @opindex Wundeclared-selector
1914 Warn if a @code{@@selector(@dots{})} expression referring to an
1915 undeclared selector is found. A selector is considered undeclared if no
1916 method with that name has been declared before the
1917 @code{@@selector(@dots{})} expression, either explicitly in an
1918 @code{@@interface} or @code{@@protocol} declaration, or implicitly in
1919 an @code{@@implementation} section. This option always performs its
1920 checks as soon as a @code{@@selector(@dots{})} expression is found,
1921 while @code{-Wselector} only performs its checks in the final stage of
1922 compilation. This also enforces the coding style convention
1923 that methods and selectors must be declared before being used.
1925 @item -print-objc-runtime-info
1926 @opindex -print-objc-runtime-info
1927 Generate C header describing the largest structure that is passed by
1932 @node Language Independent Options
1933 @section Options to Control Diagnostic Messages Formatting
1934 @cindex options to control diagnostics formatting
1935 @cindex diagnostic messages
1936 @cindex message formatting
1938 Traditionally, diagnostic messages have been formatted irrespective of
1939 the output device's aspect (e.g.@: its width, @dots{}). The options described
1940 below can be used to control the diagnostic messages formatting
1941 algorithm, e.g.@: how many characters per line, how often source location
1942 information should be reported. Right now, only the C++ front end can
1943 honor these options. However it is expected, in the near future, that
1944 the remaining front ends would be able to digest them correctly.
1947 @item -fmessage-length=@var{n}
1948 @opindex fmessage-length
1949 Try to format error messages so that they fit on lines of about @var{n}
1950 characters. The default is 72 characters for @command{g++} and 0 for the rest of
1951 the front ends supported by GCC@. If @var{n} is zero, then no
1952 line-wrapping will be done; each error message will appear on a single
1955 @opindex fdiagnostics-show-location
1956 @item -fdiagnostics-show-location=once
1957 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
1958 reporter to emit @emph{once} source location information; that is, in
1959 case the message is too long to fit on a single physical line and has to
1960 be wrapped, the source location won't be emitted (as prefix) again,
1961 over and over, in subsequent continuation lines. This is the default
1964 @item -fdiagnostics-show-location=every-line
1965 Only meaningful in line-wrapping mode. Instructs the diagnostic
1966 messages reporter to emit the same source location information (as
1967 prefix) for physical lines that result from the process of breaking
1968 a message which is too long to fit on a single line.
1972 @node Warning Options
1973 @section Options to Request or Suppress Warnings
1974 @cindex options to control warnings
1975 @cindex warning messages
1976 @cindex messages, warning
1977 @cindex suppressing warnings
1979 Warnings are diagnostic messages that report constructions which
1980 are not inherently erroneous but which are risky or suggest there
1981 may have been an error.
1983 You can request many specific warnings with options beginning @samp{-W},
1984 for example @option{-Wimplicit} to request warnings on implicit
1985 declarations. Each of these specific warning options also has a
1986 negative form beginning @samp{-Wno-} to turn off warnings;
1987 for example, @option{-Wno-implicit}. This manual lists only one of the
1988 two forms, whichever is not the default.
1990 The following options control the amount and kinds of warnings produced
1991 by GCC; for further, language-specific options also refer to
1992 @ref{C++ Dialect Options} and @ref{Objective-C Dialect Options}.
1995 @cindex syntax checking
1997 @opindex fsyntax-only
1998 Check the code for syntax errors, but don't do anything beyond that.
2002 Issue all the warnings demanded by strict ISO C and ISO C++;
2003 reject all programs that use forbidden extensions, and some other
2004 programs that do not follow ISO C and ISO C++. For ISO C, follows the
2005 version of the ISO C standard specified by any @option{-std} option used.
2007 Valid ISO C and ISO C++ programs should compile properly with or without
2008 this option (though a rare few will require @option{-ansi} or a
2009 @option{-std} option specifying the required version of ISO C)@. However,
2010 without this option, certain GNU extensions and traditional C and C++
2011 features are supported as well. With this option, they are rejected.
2013 @option{-pedantic} does not cause warning messages for use of the
2014 alternate keywords whose names begin and end with @samp{__}. Pedantic
2015 warnings are also disabled in the expression that follows
2016 @code{__extension__}. However, only system header files should use
2017 these escape routes; application programs should avoid them.
2018 @xref{Alternate Keywords}.
2020 Some users try to use @option{-pedantic} to check programs for strict ISO
2021 C conformance. They soon find that it does not do quite what they want:
2022 it finds some non-ISO practices, but not all---only those for which
2023 ISO C @emph{requires} a diagnostic, and some others for which
2024 diagnostics have been added.
2026 A feature to report any failure to conform to ISO C might be useful in
2027 some instances, but would require considerable additional work and would
2028 be quite different from @option{-pedantic}. We don't have plans to
2029 support such a feature in the near future.
2031 Where the standard specified with @option{-std} represents a GNU
2032 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
2033 corresponding @dfn{base standard}, the version of ISO C on which the GNU
2034 extended dialect is based. Warnings from @option{-pedantic} are given
2035 where they are required by the base standard. (It would not make sense
2036 for such warnings to be given only for features not in the specified GNU
2037 C dialect, since by definition the GNU dialects of C include all
2038 features the compiler supports with the given option, and there would be
2039 nothing to warn about.)
2041 @item -pedantic-errors
2042 @opindex pedantic-errors
2043 Like @option{-pedantic}, except that errors are produced rather than
2048 Inhibit all warning messages.
2052 Inhibit warning messages about the use of @samp{#import}.
2054 @item -Wchar-subscripts
2055 @opindex Wchar-subscripts
2056 Warn if an array subscript has type @code{char}. This is a common cause
2057 of error, as programmers often forget that this type is signed on some
2062 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
2063 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
2067 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
2068 the arguments supplied have types appropriate to the format string
2069 specified, and that the conversions specified in the format string make
2070 sense. This includes standard functions, and others specified by format
2071 attributes (@pxref{Function Attributes}), in the @code{printf},
2072 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
2073 not in the C standard) families.
2075 The formats are checked against the format features supported by GNU
2076 libc version 2.2. These include all ISO C90 and C99 features, as well
2077 as features from the Single Unix Specification and some BSD and GNU
2078 extensions. Other library implementations may not support all these
2079 features; GCC does not support warning about features that go beyond a
2080 particular library's limitations. However, if @option{-pedantic} is used
2081 with @option{-Wformat}, warnings will be given about format features not
2082 in the selected standard version (but not for @code{strfmon} formats,
2083 since those are not in any version of the C standard). @xref{C Dialect
2084 Options,,Options Controlling C Dialect}.
2086 Since @option{-Wformat} also checks for null format arguments for
2087 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2089 @option{-Wformat} is included in @option{-Wall}. For more control over some
2090 aspects of format checking, the options @option{-Wformat-y2k},
2091 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2092 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2093 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2096 @opindex Wformat-y2k
2097 If @option{-Wformat} is specified, also warn about @code{strftime}
2098 formats which may yield only a two-digit year.
2100 @item -Wno-format-extra-args
2101 @opindex Wno-format-extra-args
2102 If @option{-Wformat} is specified, do not warn about excess arguments to a
2103 @code{printf} or @code{scanf} format function. The C standard specifies
2104 that such arguments are ignored.
2106 Where the unused arguments lie between used arguments that are
2107 specified with @samp{$} operand number specifications, normally
2108 warnings are still given, since the implementation could not know what
2109 type to pass to @code{va_arg} to skip the unused arguments. However,
2110 in the case of @code{scanf} formats, this option will suppress the
2111 warning if the unused arguments are all pointers, since the Single
2112 Unix Specification says that such unused arguments are allowed.
2114 @item -Wno-format-zero-length
2115 @opindex Wno-format-zero-length
2116 If @option{-Wformat} is specified, do not warn about zero-length formats.
2117 The C standard specifies that zero-length formats are allowed.
2119 @item -Wformat-nonliteral
2120 @opindex Wformat-nonliteral
2121 If @option{-Wformat} is specified, also warn if the format string is not a
2122 string literal and so cannot be checked, unless the format function
2123 takes its format arguments as a @code{va_list}.
2125 @item -Wformat-security
2126 @opindex Wformat-security
2127 If @option{-Wformat} is specified, also warn about uses of format
2128 functions that represent possible security problems. At present, this
2129 warns about calls to @code{printf} and @code{scanf} functions where the
2130 format string is not a string literal and there are no format arguments,
2131 as in @code{printf (foo);}. This may be a security hole if the format
2132 string came from untrusted input and contains @samp{%n}. (This is
2133 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2134 in future warnings may be added to @option{-Wformat-security} that are not
2135 included in @option{-Wformat-nonliteral}.)
2139 Enable @option{-Wformat} plus format checks not included in
2140 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2141 -Wformat-nonliteral -Wformat-security -Wformat-y2k}.
2145 Warn about passing a null pointer for arguments marked as
2146 requiring a non-null value by the @code{nonnull} function attribute.
2148 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2149 can be disabled with the @option{-Wno-nonnull} option.
2151 @item -Winit-self @r{(C, C++, and Objective-C only)}
2153 Warn about uninitialized variables which are initialized with themselves.
2154 Note this option can only be used with the @option{-Wuninitialized} option,
2155 which in turn only works with @option{-O1} and above.
2157 For example, GCC will warn about @code{i} being uninitialized in the
2158 following snippet only when @option{-Winit-self} has been specified:
2169 @item -Wimplicit-int
2170 @opindex Wimplicit-int
2171 Warn when a declaration does not specify a type.
2173 @item -Wimplicit-function-declaration
2174 @itemx -Werror-implicit-function-declaration
2175 @opindex Wimplicit-function-declaration
2176 @opindex Werror-implicit-function-declaration
2177 Give a warning (or error) whenever a function is used before being
2182 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2186 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2187 function with external linkage, returning int, taking either zero
2188 arguments, two, or three arguments of appropriate types.
2190 @item -Wmissing-braces
2191 @opindex Wmissing-braces
2192 Warn if an aggregate or union initializer is not fully bracketed. In
2193 the following example, the initializer for @samp{a} is not fully
2194 bracketed, but that for @samp{b} is fully bracketed.
2197 int a[2][2] = @{ 0, 1, 2, 3 @};
2198 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2202 @opindex Wparentheses
2203 Warn if parentheses are omitted in certain contexts, such
2204 as when there is an assignment in a context where a truth value
2205 is expected, or when operators are nested whose precedence people
2206 often get confused about.
2208 Also warn about constructions where there may be confusion to which
2209 @code{if} statement an @code{else} branch belongs. Here is an example of
2224 In C, every @code{else} branch belongs to the innermost possible @code{if}
2225 statement, which in this example is @code{if (b)}. This is often not
2226 what the programmer expected, as illustrated in the above example by
2227 indentation the programmer chose. When there is the potential for this
2228 confusion, GCC will issue a warning when this flag is specified.
2229 To eliminate the warning, add explicit braces around the innermost
2230 @code{if} statement so there is no way the @code{else} could belong to
2231 the enclosing @code{if}. The resulting code would look like this:
2247 @item -Wsequence-point
2248 @opindex Wsequence-point
2249 Warn about code that may have undefined semantics because of violations
2250 of sequence point rules in the C standard.
2252 The C standard defines the order in which expressions in a C program are
2253 evaluated in terms of @dfn{sequence points}, which represent a partial
2254 ordering between the execution of parts of the program: those executed
2255 before the sequence point, and those executed after it. These occur
2256 after the evaluation of a full expression (one which is not part of a
2257 larger expression), after the evaluation of the first operand of a
2258 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2259 function is called (but after the evaluation of its arguments and the
2260 expression denoting the called function), and in certain other places.
2261 Other than as expressed by the sequence point rules, the order of
2262 evaluation of subexpressions of an expression is not specified. All
2263 these rules describe only a partial order rather than a total order,
2264 since, for example, if two functions are called within one expression
2265 with no sequence point between them, the order in which the functions
2266 are called is not specified. However, the standards committee have
2267 ruled that function calls do not overlap.
2269 It is not specified when between sequence points modifications to the
2270 values of objects take effect. Programs whose behavior depends on this
2271 have undefined behavior; the C standard specifies that ``Between the
2272 previous and next sequence point an object shall have its stored value
2273 modified at most once by the evaluation of an expression. Furthermore,
2274 the prior value shall be read only to determine the value to be
2275 stored.''. If a program breaks these rules, the results on any
2276 particular implementation are entirely unpredictable.
2278 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2279 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2280 diagnosed by this option, and it may give an occasional false positive
2281 result, but in general it has been found fairly effective at detecting
2282 this sort of problem in programs.
2284 The present implementation of this option only works for C programs. A
2285 future implementation may also work for C++ programs.
2287 The C standard is worded confusingly, therefore there is some debate
2288 over the precise meaning of the sequence point rules in subtle cases.
2289 Links to discussions of the problem, including proposed formal
2290 definitions, may be found on our readings page, at
2291 @w{@uref{http://gcc.gnu.org/readings.html}}.
2294 @opindex Wreturn-type
2295 Warn whenever a function is defined with a return-type that defaults to
2296 @code{int}. Also warn about any @code{return} statement with no
2297 return-value in a function whose return-type is not @code{void}.
2299 For C++, a function without return type always produces a diagnostic
2300 message, even when @option{-Wno-return-type} is specified. The only
2301 exceptions are @samp{main} and functions defined in system headers.
2305 Warn whenever a @code{switch} statement has an index of enumeral type
2306 and lacks a @code{case} for one or more of the named codes of that
2307 enumeration. (The presence of a @code{default} label prevents this
2308 warning.) @code{case} labels outside the enumeration range also
2309 provoke warnings when this option is used.
2311 @item -Wswitch-default
2312 @opindex Wswitch-switch
2313 Warn whenever a @code{switch} statement does not have a @code{default}
2317 @opindex Wswitch-enum
2318 Warn whenever a @code{switch} statement has an index of enumeral type
2319 and lacks a @code{case} for one or more of the named codes of that
2320 enumeration. @code{case} labels outside the enumeration range also
2321 provoke warnings when this option is used.
2325 Warn if any trigraphs are encountered that might change the meaning of
2326 the program (trigraphs within comments are not warned about).
2328 @item -Wunused-function
2329 @opindex Wunused-function
2330 Warn whenever a static function is declared but not defined or a
2331 non\-inline static function is unused.
2333 @item -Wunused-label
2334 @opindex Wunused-label
2335 Warn whenever a label is declared but not used.
2337 To suppress this warning use the @samp{unused} attribute
2338 (@pxref{Variable Attributes}).
2340 @item -Wunused-parameter
2341 @opindex Wunused-parameter
2342 Warn whenever a function parameter is unused aside from its declaration.
2344 To suppress this warning use the @samp{unused} attribute
2345 (@pxref{Variable Attributes}).
2347 @item -Wunused-variable
2348 @opindex Wunused-variable
2349 Warn whenever a local variable or non-constant static variable is unused
2350 aside from its declaration
2352 To suppress this warning use the @samp{unused} attribute
2353 (@pxref{Variable Attributes}).
2355 @item -Wunused-value
2356 @opindex Wunused-value
2357 Warn whenever a statement computes a result that is explicitly not used.
2359 To suppress this warning cast the expression to @samp{void}.
2363 All the above @option{-Wunused} options combined.
2365 In order to get a warning about an unused function parameter, you must
2366 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
2367 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
2369 @item -Wuninitialized
2370 @opindex Wuninitialized
2371 Warn if an automatic variable is used without first being initialized or
2372 if a variable may be clobbered by a @code{setjmp} call.
2374 These warnings are possible only in optimizing compilation,
2375 because they require data flow information that is computed only
2376 when optimizing. If you don't specify @option{-O}, you simply won't
2379 If you want to warn about code which uses the uninitialized value of the
2380 variable in its own initializer, use the @option{-Winit-self} option.
2382 These warnings occur only for variables that are candidates for
2383 register allocation. Therefore, they do not occur for a variable that
2384 is declared @code{volatile}, or whose address is taken, or whose size
2385 is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
2386 structures, unions or arrays, even when they are in registers.
2388 Note that there may be no warning about a variable that is used only
2389 to compute a value that itself is never used, because such
2390 computations may be deleted by data flow analysis before the warnings
2393 These warnings are made optional because GCC is not smart
2394 enough to see all the reasons why the code might be correct
2395 despite appearing to have an error. Here is one example of how
2416 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
2417 always initialized, but GCC doesn't know this. Here is
2418 another common case:
2423 if (change_y) save_y = y, y = new_y;
2425 if (change_y) y = save_y;
2430 This has no bug because @code{save_y} is used only if it is set.
2432 @cindex @code{longjmp} warnings
2433 This option also warns when a non-volatile automatic variable might be
2434 changed by a call to @code{longjmp}. These warnings as well are possible
2435 only in optimizing compilation.
2437 The compiler sees only the calls to @code{setjmp}. It cannot know
2438 where @code{longjmp} will be called; in fact, a signal handler could
2439 call it at any point in the code. As a result, you may get a warning
2440 even when there is in fact no problem because @code{longjmp} cannot
2441 in fact be called at the place which would cause a problem.
2443 Some spurious warnings can be avoided if you declare all the functions
2444 you use that never return as @code{noreturn}. @xref{Function
2447 @item -Wunknown-pragmas
2448 @opindex Wunknown-pragmas
2449 @cindex warning for unknown pragmas
2450 @cindex unknown pragmas, warning
2451 @cindex pragmas, warning of unknown
2452 Warn when a #pragma directive is encountered which is not understood by
2453 GCC@. If this command line option is used, warnings will even be issued
2454 for unknown pragmas in system header files. This is not the case if
2455 the warnings were only enabled by the @option{-Wall} command line option.
2457 @item -Wstrict-aliasing
2458 @opindex Wstrict-aliasing
2459 This option is only active when @option{-fstrict-aliasing} is active.
2460 It warns about code which might break the strict aliasing rules that the
2461 compiler is using for optimization. The warning does not catch all
2462 cases, but does attempt to catch the more common pitfalls. It is
2463 included in @option{-Wall}.
2465 @item -Wstrict-aliasing=2
2466 @opindex Wstrict-aliasing=2
2467 This option is only active when @option{-fstrict-aliasing} is active.
2468 It warns about all code which might break the strict aliasing rules that the
2469 compiler is using for optimization. This warning catches all cases, but
2470 it will also give a warning for some ambiguous cases that are safe.
2474 All of the above @samp{-W} options combined. This enables all the
2475 warnings about constructions that some users consider questionable, and
2476 that are easy to avoid (or modify to prevent the warning), even in
2477 conjunction with macros. This also enables some language-specific
2478 warnings described in @ref{C++ Dialect Options} and
2479 @ref{Objective-C Dialect Options}.
2482 The following @option{-W@dots{}} options are not implied by @option{-Wall}.
2483 Some of them warn about constructions that users generally do not
2484 consider questionable, but which occasionally you might wish to check
2485 for; others warn about constructions that are necessary or hard to avoid
2486 in some cases, and there is no simple way to modify the code to suppress
2493 (This option used to be called @option{-W}. The older name is still
2494 supported, but the newer name is more descriptive.) Print extra warning
2495 messages for these events:
2499 A function can return either with or without a value. (Falling
2500 off the end of the function body is considered returning without
2501 a value.) For example, this function would evoke such a
2515 An expression-statement or the left-hand side of a comma expression
2516 contains no side effects.
2517 To suppress the warning, cast the unused expression to void.
2518 For example, an expression such as @samp{x[i,j]} will cause a warning,
2519 but @samp{x[(void)i,j]} will not.
2522 An unsigned value is compared against zero with @samp{<} or @samp{>=}.
2525 A comparison like @samp{x<=y<=z} appears; this is equivalent to
2526 @samp{(x<=y ? 1 : 0) <= z}, which is a different interpretation from
2527 that of ordinary mathematical notation.
2530 Storage-class specifiers like @code{static} are not the first things in
2531 a declaration. According to the C Standard, this usage is obsolescent.
2534 The return type of a function has a type qualifier such as @code{const}.
2535 Such a type qualifier has no effect, since the value returned by a
2536 function is not an lvalue. (But don't warn about the GNU extension of
2537 @code{volatile void} return types. That extension will be warned about
2538 if @option{-pedantic} is specified.)
2541 If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
2545 A comparison between signed and unsigned values could produce an
2546 incorrect result when the signed value is converted to unsigned.
2547 (But don't warn if @option{-Wno-sign-compare} is also specified.)
2550 An aggregate has an initializer which does not initialize all members.
2551 For example, the following code would cause such a warning, because
2552 @code{x.h} would be implicitly initialized to zero:
2555 struct s @{ int f, g, h; @};
2556 struct s x = @{ 3, 4 @};
2560 A function parameter is declared without a type specifier in K&R-style
2568 An empty body occurs in an @samp{if} or @samp{else} statement.
2571 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2572 @samp{>}, or @samp{>=}.
2575 A variable might be changed by @samp{longjmp} or @samp{vfork}.
2578 Any of several floating-point events that often indicate errors, such as
2579 overflow, underflow, loss of precision, etc.
2581 @item @r{(C++ only)}
2582 An enumerator and a non-enumerator both appear in a conditional expression.
2584 @item @r{(C++ only)}
2585 A non-static reference or non-static @samp{const} member appears in a
2586 class without constructors.
2588 @item @r{(C++ only)}
2589 Ambiguous virtual bases.
2591 @item @r{(C++ only)}
2592 Subscripting an array which has been declared @samp{register}.
2594 @item @r{(C++ only)}
2595 Taking the address of a variable which has been declared @samp{register}.
2597 @item @r{(C++ only)}
2598 A base class is not initialized in a derived class' copy constructor.
2601 @item -Wno-div-by-zero
2602 @opindex Wno-div-by-zero
2603 @opindex Wdiv-by-zero
2604 Do not warn about compile-time integer division by zero. Floating point
2605 division by zero is not warned about, as it can be a legitimate way of
2606 obtaining infinities and NaNs.
2608 @item -Wsystem-headers
2609 @opindex Wsystem-headers
2610 @cindex warnings from system headers
2611 @cindex system headers, warnings from
2612 Print warning messages for constructs found in system header files.
2613 Warnings from system headers are normally suppressed, on the assumption
2614 that they usually do not indicate real problems and would only make the
2615 compiler output harder to read. Using this command line option tells
2616 GCC to emit warnings from system headers as if they occurred in user
2617 code. However, note that using @option{-Wall} in conjunction with this
2618 option will @emph{not} warn about unknown pragmas in system
2619 headers---for that, @option{-Wunknown-pragmas} must also be used.
2622 @opindex Wfloat-equal
2623 Warn if floating point values are used in equality comparisons.
2625 The idea behind this is that sometimes it is convenient (for the
2626 programmer) to consider floating-point values as approximations to
2627 infinitely precise real numbers. If you are doing this, then you need
2628 to compute (by analyzing the code, or in some other way) the maximum or
2629 likely maximum error that the computation introduces, and allow for it
2630 when performing comparisons (and when producing output, but that's a
2631 different problem). In particular, instead of testing for equality, you
2632 would check to see whether the two values have ranges that overlap; and
2633 this is done with the relational operators, so equality comparisons are
2636 @item -Wtraditional @r{(C only)}
2637 @opindex Wtraditional
2638 Warn about certain constructs that behave differently in traditional and
2639 ISO C@. Also warn about ISO C constructs that have no traditional C
2640 equivalent, and/or problematic constructs which should be avoided.
2644 Macro parameters that appear within string literals in the macro body.
2645 In traditional C macro replacement takes place within string literals,
2646 but does not in ISO C@.
2649 In traditional C, some preprocessor directives did not exist.
2650 Traditional preprocessors would only consider a line to be a directive
2651 if the @samp{#} appeared in column 1 on the line. Therefore
2652 @option{-Wtraditional} warns about directives that traditional C
2653 understands but would ignore because the @samp{#} does not appear as the
2654 first character on the line. It also suggests you hide directives like
2655 @samp{#pragma} not understood by traditional C by indenting them. Some
2656 traditional implementations would not recognize @samp{#elif}, so it
2657 suggests avoiding it altogether.
2660 A function-like macro that appears without arguments.
2663 The unary plus operator.
2666 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
2667 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
2668 constants.) Note, these suffixes appear in macros defined in the system
2669 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
2670 Use of these macros in user code might normally lead to spurious
2671 warnings, however GCC's integrated preprocessor has enough context to
2672 avoid warning in these cases.
2675 A function declared external in one block and then used after the end of
2679 A @code{switch} statement has an operand of type @code{long}.
2682 A non-@code{static} function declaration follows a @code{static} one.
2683 This construct is not accepted by some traditional C compilers.
2686 The ISO type of an integer constant has a different width or
2687 signedness from its traditional type. This warning is only issued if
2688 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
2689 typically represent bit patterns, are not warned about.
2692 Usage of ISO string concatenation is detected.
2695 Initialization of automatic aggregates.
2698 Identifier conflicts with labels. Traditional C lacks a separate
2699 namespace for labels.
2702 Initialization of unions. If the initializer is zero, the warning is
2703 omitted. This is done under the assumption that the zero initializer in
2704 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
2705 initializer warnings and relies on default initialization to zero in the
2709 Conversions by prototypes between fixed/floating point values and vice
2710 versa. The absence of these prototypes when compiling with traditional
2711 C would cause serious problems. This is a subset of the possible
2712 conversion warnings, for the full set use @option{-Wconversion}.
2715 Use of ISO C style function definitions. This warning intentionally is
2716 @emph{not} issued for prototype declarations or variadic functions
2717 because these ISO C features will appear in your code when using
2718 libiberty's traditional C compatibility macros, @code{PARAMS} and
2719 @code{VPARAMS}. This warning is also bypassed for nested functions
2720 because that feature is already a GCC extension and thus not relevant to
2721 traditional C compatibility.
2724 @item -Wdeclaration-after-statement @r{(C only)}
2725 @opindex Wdeclaration-after-statement
2726 Warn when a declaration is found after a statement in a block. This
2727 construct, known from C++, was introduced with ISO C99 and is by default
2728 allowed in GCC@. It is not supported by ISO C90 and was not supported by
2729 GCC versions before GCC 3.0. @xref{Mixed Declarations}.
2733 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
2735 @item -Wendif-labels
2736 @opindex Wendif-labels
2737 Warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
2741 Warn whenever a local variable shadows another local variable, parameter or
2742 global variable or whenever a built-in function is shadowed.
2744 @item -Wlarger-than-@var{len}
2745 @opindex Wlarger-than
2746 Warn whenever an object of larger than @var{len} bytes is defined.
2748 @item -Wpointer-arith
2749 @opindex Wpointer-arith
2750 Warn about anything that depends on the ``size of'' a function type or
2751 of @code{void}. GNU C assigns these types a size of 1, for
2752 convenience in calculations with @code{void *} pointers and pointers
2755 @item -Wbad-function-cast @r{(C only)}
2756 @opindex Wbad-function-cast
2757 Warn whenever a function call is cast to a non-matching type.
2758 For example, warn if @code{int malloc()} is cast to @code{anything *}.
2762 Warn whenever a pointer is cast so as to remove a type qualifier from
2763 the target type. For example, warn if a @code{const char *} is cast
2764 to an ordinary @code{char *}.
2767 @opindex Wcast-align
2768 Warn whenever a pointer is cast such that the required alignment of the
2769 target is increased. For example, warn if a @code{char *} is cast to
2770 an @code{int *} on machines where integers can only be accessed at
2771 two- or four-byte boundaries.
2773 @item -Wwrite-strings
2774 @opindex Wwrite-strings
2775 When compiling C, give string constants the type @code{const
2776 char[@var{length}]} so that
2777 copying the address of one into a non-@code{const} @code{char *}
2778 pointer will get a warning; when compiling C++, warn about the
2779 deprecated conversion from string constants to @code{char *}.
2780 These warnings will help you find at
2781 compile time code that can try to write into a string constant, but
2782 only if you have been very careful about using @code{const} in
2783 declarations and prototypes. Otherwise, it will just be a nuisance;
2784 this is why we did not make @option{-Wall} request these warnings.
2787 @opindex Wconversion
2788 Warn if a prototype causes a type conversion that is different from what
2789 would happen to the same argument in the absence of a prototype. This
2790 includes conversions of fixed point to floating and vice versa, and
2791 conversions changing the width or signedness of a fixed point argument
2792 except when the same as the default promotion.
2794 Also, warn if a negative integer constant expression is implicitly
2795 converted to an unsigned type. For example, warn about the assignment
2796 @code{x = -1} if @code{x} is unsigned. But do not warn about explicit
2797 casts like @code{(unsigned) -1}.
2799 @item -Wsign-compare
2800 @opindex Wsign-compare
2801 @cindex warning for comparison of signed and unsigned values
2802 @cindex comparison of signed and unsigned values, warning
2803 @cindex signed and unsigned values, comparison warning
2804 Warn when a comparison between signed and unsigned values could produce
2805 an incorrect result when the signed value is converted to unsigned.
2806 This warning is also enabled by @option{-Wextra}; to get the other warnings
2807 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
2809 @item -Waggregate-return
2810 @opindex Waggregate-return
2811 Warn if any functions that return structures or unions are defined or
2812 called. (In languages where you can return an array, this also elicits
2815 @item -Wstrict-prototypes @r{(C only)}
2816 @opindex Wstrict-prototypes
2817 Warn if a function is declared or defined without specifying the
2818 argument types. (An old-style function definition is permitted without
2819 a warning if preceded by a declaration which specifies the argument
2822 @item -Wold-style-definition @r{(C only)}
2823 @opindex Wold-style-definition
2824 Warn if an old-style function definition is used. A warning is given
2825 even if there is a previous prototype.
2827 @item -Wmissing-prototypes @r{(C only)}
2828 @opindex Wmissing-prototypes
2829 Warn if a global function is defined without a previous prototype
2830 declaration. This warning is issued even if the definition itself
2831 provides a prototype. The aim is to detect global functions that fail
2832 to be declared in header files.
2834 @item -Wmissing-declarations @r{(C only)}
2835 @opindex Wmissing-declarations
2836 Warn if a global function is defined without a previous declaration.
2837 Do so even if the definition itself provides a prototype.
2838 Use this option to detect global functions that are not declared in
2841 @item -Wmissing-noreturn
2842 @opindex Wmissing-noreturn
2843 Warn about functions which might be candidates for attribute @code{noreturn}.
2844 Note these are only possible candidates, not absolute ones. Care should
2845 be taken to manually verify functions actually do not ever return before
2846 adding the @code{noreturn} attribute, otherwise subtle code generation
2847 bugs could be introduced. You will not get a warning for @code{main} in
2848 hosted C environments.
2850 @item -Wmissing-format-attribute
2851 @opindex Wmissing-format-attribute
2853 If @option{-Wformat} is enabled, also warn about functions which might be
2854 candidates for @code{format} attributes. Note these are only possible
2855 candidates, not absolute ones. GCC will guess that @code{format}
2856 attributes might be appropriate for any function that calls a function
2857 like @code{vprintf} or @code{vscanf}, but this might not always be the
2858 case, and some functions for which @code{format} attributes are
2859 appropriate may not be detected. This option has no effect unless
2860 @option{-Wformat} is enabled (possibly by @option{-Wall}).
2862 @item -Wno-multichar
2863 @opindex Wno-multichar
2865 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
2866 Usually they indicate a typo in the user's code, as they have
2867 implementation-defined values, and should not be used in portable code.
2869 @item -Wno-deprecated-declarations
2870 @opindex Wno-deprecated-declarations
2871 Do not warn about uses of functions, variables, and types marked as
2872 deprecated by using the @code{deprecated} attribute.
2873 (@pxref{Function Attributes}, @pxref{Variable Attributes},
2874 @pxref{Type Attributes}.)
2878 Warn if a structure is given the packed attribute, but the packed
2879 attribute has no effect on the layout or size of the structure.
2880 Such structures may be mis-aligned for little benefit. For
2881 instance, in this code, the variable @code{f.x} in @code{struct bar}
2882 will be misaligned even though @code{struct bar} does not itself
2883 have the packed attribute:
2890 @} __attribute__((packed));
2900 Warn if padding is included in a structure, either to align an element
2901 of the structure or to align the whole structure. Sometimes when this
2902 happens it is possible to rearrange the fields of the structure to
2903 reduce the padding and so make the structure smaller.
2905 @item -Wredundant-decls
2906 @opindex Wredundant-decls
2907 Warn if anything is declared more than once in the same scope, even in
2908 cases where multiple declaration is valid and changes nothing.
2910 @item -Wnested-externs @r{(C only)}
2911 @opindex Wnested-externs
2912 Warn if an @code{extern} declaration is encountered within a function.
2914 @item -Wunreachable-code
2915 @opindex Wunreachable-code
2916 Warn if the compiler detects that code will never be executed.
2918 This option is intended to warn when the compiler detects that at
2919 least a whole line of source code will never be executed, because
2920 some condition is never satisfied or because it is after a
2921 procedure that never returns.
2923 It is possible for this option to produce a warning even though there
2924 are circumstances under which part of the affected line can be executed,
2925 so care should be taken when removing apparently-unreachable code.
2927 For instance, when a function is inlined, a warning may mean that the
2928 line is unreachable in only one inlined copy of the function.
2930 This option is not made part of @option{-Wall} because in a debugging
2931 version of a program there is often substantial code which checks
2932 correct functioning of the program and is, hopefully, unreachable
2933 because the program does work. Another common use of unreachable
2934 code is to provide behavior which is selectable at compile-time.
2938 Warn if a function can not be inlined and it was declared as inline.
2939 Even with this option, the compiler will not warn about failures to
2940 inline functions declared in system headers.
2942 The compiler uses a variety of heuristics to determine whether or not
2943 to inline a function. For example, the compiler takes into account
2944 the size of the function being inlined and the the amount of inlining
2945 that has already been done in the current function. Therefore,
2946 seemingly insignificant changes in the source program can cause the
2947 warnings produced by @option{-Winline} to appear or disappear.
2949 @item -Wno-invalid-offsetof @r{(C++ only)}
2950 @opindex Wno-invalid-offsetof
2951 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
2952 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
2953 to a non-POD type is undefined. In existing C++ implementations,
2954 however, @samp{offsetof} typically gives meaningful results even when
2955 applied to certain kinds of non-POD types. (Such as a simple
2956 @samp{struct} that fails to be a POD type only by virtue of having a
2957 constructor.) This flag is for users who are aware that they are
2958 writing nonportable code and who have deliberately chosen to ignore the
2961 The restrictions on @samp{offsetof} may be relaxed in a future version
2962 of the C++ standard.
2965 @opindex Winvalid-pch
2966 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
2967 the search path but can't be used.
2971 @opindex Wno-long-long
2972 Warn if @samp{long long} type is used. This is default. To inhibit
2973 the warning messages, use @option{-Wno-long-long}. Flags
2974 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
2975 only when @option{-pedantic} flag is used.
2977 @item -Wvariadic-macros
2978 @opindex Wvariadic-macros
2979 @opindex Wno-variadic-macros
2980 Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
2981 alternate syntax when in pedantic ISO C99 mode. This is default.
2982 To inhibit the warning messages, use @option{-Wno-variadic-macros}.
2984 @item -Wdisabled-optimization
2985 @opindex Wdisabled-optimization
2986 Warn if a requested optimization pass is disabled. This warning does
2987 not generally indicate that there is anything wrong with your code; it
2988 merely indicates that GCC's optimizers were unable to handle the code
2989 effectively. Often, the problem is that your code is too big or too
2990 complex; GCC will refuse to optimize programs when the optimization
2991 itself is likely to take inordinate amounts of time.
2995 Make all warnings into errors.
2998 @node Debugging Options
2999 @section Options for Debugging Your Program or GCC
3000 @cindex options, debugging
3001 @cindex debugging information options
3003 GCC has various special options that are used for debugging
3004 either your program or GCC:
3009 Produce debugging information in the operating system's native format
3010 (stabs, COFF, XCOFF, or DWARF)@. GDB can work with this debugging
3013 On most systems that use stabs format, @option{-g} enables use of extra
3014 debugging information that only GDB can use; this extra information
3015 makes debugging work better in GDB but will probably make other debuggers
3017 refuse to read the program. If you want to control for certain whether
3018 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
3019 @option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
3021 Unlike most other C compilers, GCC allows you to use @option{-g} with
3022 @option{-O}. The shortcuts taken by optimized code may occasionally
3023 produce surprising results: some variables you declared may not exist
3024 at all; flow of control may briefly move where you did not expect it;
3025 some statements may not be executed because they compute constant
3026 results or their values were already at hand; some statements may
3027 execute in different places because they were moved out of loops.
3029 Nevertheless it proves possible to debug optimized output. This makes
3030 it reasonable to use the optimizer for programs that might have bugs.
3032 The following options are useful when GCC is generated with the
3033 capability for more than one debugging format.
3037 Produce debugging information for use by GDB@. This means to use the
3038 most expressive format available (DWARF 2, stabs, or the native format
3039 if neither of those are supported), including GDB extensions if at all
3044 Produce debugging information in stabs format (if that is supported),
3045 without GDB extensions. This is the format used by DBX on most BSD
3046 systems. On MIPS, Alpha and System V Release 4 systems this option
3047 produces stabs debugging output which is not understood by DBX or SDB@.
3048 On System V Release 4 systems this option requires the GNU assembler.
3050 @item -feliminate-unused-debug-symbols
3051 @opindex feliminate-unused-debug-symbols
3052 Produce debugging information in stabs format (if that is supported),
3053 for only symbols that are actually used.
3057 Produce debugging information in stabs format (if that is supported),
3058 using GNU extensions understood only by the GNU debugger (GDB)@. The
3059 use of these extensions is likely to make other debuggers crash or
3060 refuse to read the program.
3064 Produce debugging information in COFF format (if that is supported).
3065 This is the format used by SDB on most System V systems prior to
3070 Produce debugging information in XCOFF format (if that is supported).
3071 This is the format used by the DBX debugger on IBM RS/6000 systems.
3075 Produce debugging information in XCOFF format (if that is supported),
3076 using GNU extensions understood only by the GNU debugger (GDB)@. The
3077 use of these extensions is likely to make other debuggers crash or
3078 refuse to read the program, and may cause assemblers other than the GNU
3079 assembler (GAS) to fail with an error.
3083 Produce debugging information in DWARF version 2 format (if that is
3084 supported). This is the format used by DBX on IRIX 6.
3088 Produce debugging information in VMS debug format (if that is
3089 supported). This is the format used by DEBUG on VMS systems.
3092 @itemx -ggdb@var{level}
3093 @itemx -gstabs@var{level}
3094 @itemx -gcoff@var{level}
3095 @itemx -gxcoff@var{level}
3096 @itemx -gvms@var{level}
3097 Request debugging information and also use @var{level} to specify how
3098 much information. The default level is 2.
3100 Level 1 produces minimal information, enough for making backtraces in
3101 parts of the program that you don't plan to debug. This includes
3102 descriptions of functions and external variables, but no information
3103 about local variables and no line numbers.
3105 Level 3 includes extra information, such as all the macro definitions
3106 present in the program. Some debuggers support macro expansion when
3107 you use @option{-g3}.
3109 Note that in order to avoid confusion between DWARF1 debug level 2,
3110 and DWARF2 @option{-gdwarf-2} does not accept a concatenated debug
3111 level. Instead use an additional @option{-g@var{level}} option to
3112 change the debug level for DWARF2.
3114 @item -feliminate-dwarf2-dups
3115 @opindex feliminate-dwarf2-dups
3116 Compress DWARF2 debugging information by eliminating duplicated
3117 information about each symbol. This option only makes sense when
3118 generating DWARF2 debugging information with @option{-gdwarf-2}.
3120 @cindex @command{prof}
3123 Generate extra code to write profile information suitable for the
3124 analysis program @command{prof}. You must use this option when compiling
3125 the source files you want data about, and you must also use it when
3128 @cindex @command{gprof}
3131 Generate extra code to write profile information suitable for the
3132 analysis program @command{gprof}. You must use this option when compiling
3133 the source files you want data about, and you must also use it when
3138 Makes the compiler print out each function name as it is compiled, and
3139 print some statistics about each pass when it finishes.
3142 @opindex ftime-report
3143 Makes the compiler print some statistics about the time consumed by each
3144 pass when it finishes.
3147 @opindex fmem-report
3148 Makes the compiler print some statistics about permanent memory
3149 allocation when it finishes.
3151 @item -fprofile-arcs
3152 @opindex fprofile-arcs
3153 Add code so that program flow @dfn{arcs} are instrumented. During
3154 execution the program records how many times each branch and call is
3155 executed and how many times it is taken or returns. When the compiled
3156 program exits it saves this data to a file called
3157 @file{@var{auxname}.gcda} for each source file. The data may be used for
3158 profile-directed optimizations (@option{-fbranch-probabilities}), or for
3159 test coverage analysis (@option{-ftest-coverage}). Each object file's
3160 @var{auxname} is generated from the name of the output file, if
3161 explicitly specified and it is not the final executable, otherwise it is
3162 the basename of the source file. In both cases any suffix is removed
3163 (e.g. @file{foo.gcda} for input file @file{dir/foo.c}, or
3164 @file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
3169 Compile the source files with @option{-fprofile-arcs} plus optimization
3170 and code generation options. For test coverage analysis, use the
3171 additional @option{-ftest-coverage} option. You do not need to profile
3172 every source file in a program.
3175 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
3176 (the latter implies the former).
3179 Run the program on a representative workload to generate the arc profile
3180 information. This may be repeated any number of times. You can run
3181 concurrent instances of your program, and provided that the file system
3182 supports locking, the data files will be correctly updated. Also
3183 @code{fork} calls are detected and correctly handled (double counting
3187 For profile-directed optimizations, compile the source files again with
3188 the same optimization and code generation options plus
3189 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
3190 Control Optimization}).
3193 For test coverage analysis, use @command{gcov} to produce human readable
3194 information from the @file{.gcno} and @file{.gcda} files. Refer to the
3195 @command{gcov} documentation for further information.
3199 With @option{-fprofile-arcs}, for each function of your program GCC
3200 creates a program flow graph, then finds a spanning tree for the graph.
3201 Only arcs that are not on the spanning tree have to be instrumented: the
3202 compiler adds code to count the number of times that these arcs are
3203 executed. When an arc is the only exit or only entrance to a block, the
3204 instrumentation code can be added to the block; otherwise, a new basic
3205 block must be created to hold the instrumentation code.
3208 @item -ftest-coverage
3209 @opindex ftest-coverage
3210 Produce a notes file that the @command{gcov} code-coverage utility
3211 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
3212 show program coverage. Each source file's note file is called
3213 @file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
3214 above for a description of @var{auxname} and instructions on how to
3215 generate test coverage data. Coverage data will match the source files
3216 more closely, if you do not optimize.
3218 @item -d@var{letters}
3220 Says to make debugging dumps during compilation at times specified by
3221 @var{letters}. This is used for debugging the compiler. The file names
3222 for most of the dumps are made by appending a pass number and a word to
3223 the @var{dumpname}. @var{dumpname} is generated from the name of the
3224 output file, if explicitly specified and it is not an executable,
3225 otherwise it is the basename of the source file. In both cases any
3226 suffix is removed (e.g. @file{foo.01.rtl} or @file{foo.02.sibling}).
3227 Here are the possible letters for use in @var{letters}, and their
3233 Annotate the assembler output with miscellaneous debugging information.
3236 Dump after computing branch probabilities, to @file{@var{file}.12.bp}.
3239 Dump after block reordering, to @file{@var{file}.31.bbro}.
3242 Dump after instruction combination, to the file @file{@var{file}.20.combine}.
3245 Dump after the first if conversion, to the file @file{@var{file}.14.ce1}.
3246 Also dump after the second if conversion, to the file @file{@var{file}.21.ce2}.
3249 Dump after branch target load optimization, to to @file{@var{file}.32.btl}.
3250 Also dump after delayed branch scheduling, to @file{@var{file}.36.dbr}.
3253 Dump all macro definitions, at the end of preprocessing, in addition to
3257 Dump after the third if conversion, to @file{@var{file}.30.ce3}.
3260 Dump after control and data flow analysis, to @file{@var{file}.11.cfg}.
3261 Also dump after life analysis, to @file{@var{file}.19.life}.
3264 Dump after purging @code{ADDRESSOF} codes, to @file{@var{file}.07.addressof}.
3267 Dump after global register allocation, to @file{@var{file}.25.greg}.
3270 Dump after GCSE, to @file{@var{file}.08.gcse}.
3271 Also dump after jump bypassing and control flow optimizations, to
3272 @file{@var{file}.10.bypass}.
3275 Dump after finalization of EH handling code, to @file{@var{file}.03.eh}.
3278 Dump after sibling call optimizations, to @file{@var{file}.02.sibling}.
3281 Dump after the first jump optimization, to @file{@var{file}.04.jump}.
3284 Dump after conversion from registers to stack, to @file{@var{file}.34.stack}.
3287 Dump after local register allocation, to @file{@var{file}.24.lreg}.
3290 Dump after loop optimization passes, to @file{@var{file}.09.loop} and
3291 @file{@var{file}.16.loop2}.
3294 Dump after performing the machine dependent reorganization pass, to
3295 @file{@var{file}.35.mach}.
3298 Dump after register renumbering, to @file{@var{file}.29.rnreg}.
3301 Dump after the register move pass, to @file{@var{file}.22.regmove}.
3304 Dump after post-reload optimizations, to @file{@var{file}.26.postreload}.
3307 Dump after RTL generation, to @file{@var{file}.01.rtl}.
3310 Dump after the second scheduling pass, to @file{@var{file}.33.sched2}.
3313 Dump after CSE (including the jump optimization that sometimes follows
3314 CSE), to @file{@var{file}.06.cse}.
3317 Dump after the first scheduling pass, to @file{@var{file}.23.sched}.
3320 Dump after the second CSE pass (including the jump optimization that
3321 sometimes follows CSE), to @file{@var{file}.18.cse2}.
3324 Dump after running tracer, to @file{@var{file}.15.tracer}.
3327 Dump after null pointer elimination pass to @file{@var{file}.05.null}.
3330 Dump callgraph and unit-at-a-time optimization @file{@var{file}.00.unit}.
3333 Dump after the value profile transformations, to @file{@var{file}.13.vpt}.
3334 Also dump after variable tracking, to @file{@var{file}.35.vartrack}.
3337 Dump after the second flow pass, to @file{@var{file}.27.flow2}.
3340 Dump after the peephole pass, to @file{@var{file}.28.peephole2}.
3343 Dump after constructing the web, to @file{@var{file}.17.web}.
3346 Produce all the dumps listed above.
3349 Produce a core dump whenever an error occurs.
3352 Print statistics on memory usage, at the end of the run, to
3356 Annotate the assembler output with a comment indicating which
3357 pattern and alternative was used. The length of each instruction is
3361 Dump the RTL in the assembler output as a comment before each instruction.
3362 Also turns on @option{-dp} annotation.
3365 For each of the other indicated dump files (except for
3366 @file{@var{file}.01.rtl}), dump a representation of the control flow graph
3367 suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
3370 Just generate RTL for a function instead of compiling it. Usually used
3374 Dump debugging information during parsing, to standard error.
3377 @item -fdump-unnumbered
3378 @opindex fdump-unnumbered
3379 When doing debugging dumps (see @option{-d} option above), suppress instruction
3380 numbers and line number note output. This makes it more feasible to
3381 use diff on debugging dumps for compiler invocations with different
3382 options, in particular with and without @option{-g}.
3384 @item -fdump-translation-unit @r{(C and C++ only)}
3385 @itemx -fdump-translation-unit-@var{options} @r{(C and C++ only)}
3386 @opindex fdump-translation-unit
3387 Dump a representation of the tree structure for the entire translation
3388 unit to a file. The file name is made by appending @file{.tu} to the
3389 source file name. If the @samp{-@var{options}} form is used, @var{options}
3390 controls the details of the dump as described for the
3391 @option{-fdump-tree} options.
3393 @item -fdump-class-hierarchy @r{(C++ only)}
3394 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
3395 @opindex fdump-class-hierarchy
3396 Dump a representation of each class's hierarchy and virtual function
3397 table layout to a file. The file name is made by appending @file{.class}
3398 to the source file name. If the @samp{-@var{options}} form is used,
3399 @var{options} controls the details of the dump as described for the
3400 @option{-fdump-tree} options.
3402 @item -fdump-tree-@var{switch} @r{(C++ only)}
3403 @itemx -fdump-tree-@var{switch}-@var{options} @r{(C++ only)}
3405 Control the dumping at various stages of processing the intermediate
3406 language tree to a file. The file name is generated by appending a switch
3407 specific suffix to the source file name. If the @samp{-@var{options}}
3408 form is used, @var{options} is a list of @samp{-} separated options that
3409 control the details of the dump. Not all options are applicable to all
3410 dumps, those which are not meaningful will be ignored. The following
3411 options are available
3415 Print the address of each node. Usually this is not meaningful as it
3416 changes according to the environment and source file. Its primary use
3417 is for tying up a dump file with a debug environment.
3419 Inhibit dumping of members of a scope or body of a function merely
3420 because that scope has been reached. Only dump such items when they
3421 are directly reachable by some other path.
3423 Turn on all options.
3426 The following tree dumps are possible:
3429 Dump before any tree based optimization, to @file{@var{file}.original}.
3431 Dump after all tree based optimization, to @file{@var{file}.optimized}.
3433 Dump after function inlining, to @file{@var{file}.inlined}.
3436 @item -frandom-seed=@var{string}
3437 @opindex frandom-string
3438 This option provides a seed that GCC uses when it would otherwise use
3439 random numbers. It is used to generate certain symbol names
3440 that have to be different in every compiled file. It is also used to
3441 place unique stamps in coverage data files and the object files that
3442 produce them. You can use the @option{-frandom-seed} option to produce
3443 reproducibly identical object files.
3445 The @var{string} should be different for every file you compile.
3447 @item -fsched-verbose=@var{n}
3448 @opindex fsched-verbose
3449 On targets that use instruction scheduling, this option controls the
3450 amount of debugging output the scheduler prints. This information is
3451 written to standard error, unless @option{-dS} or @option{-dR} is
3452 specified, in which case it is output to the usual dump
3453 listing file, @file{.sched} or @file{.sched2} respectively. However
3454 for @var{n} greater than nine, the output is always printed to standard
3457 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
3458 same information as @option{-dRS}. For @var{n} greater than one, it
3459 also output basic block probabilities, detailed ready list information
3460 and unit/insn info. For @var{n} greater than two, it includes RTL
3461 at abort point, control-flow and regions info. And for @var{n} over
3462 four, @option{-fsched-verbose} also includes dependence info.
3466 Store the usual ``temporary'' intermediate files permanently; place them
3467 in the current directory and name them based on the source file. Thus,
3468 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
3469 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
3470 preprocessed @file{foo.i} output file even though the compiler now
3471 normally uses an integrated preprocessor.
3475 Report the CPU time taken by each subprocess in the compilation
3476 sequence. For C source files, this is the compiler proper and assembler
3477 (plus the linker if linking is done). The output looks like this:
3484 The first number on each line is the ``user time,'' that is time spent
3485 executing the program itself. The second number is ``system time,''
3486 time spent executing operating system routines on behalf of the program.
3487 Both numbers are in seconds.
3489 @item -fvar-tracking
3490 @opindex fvar-tracking
3491 Run variable tracking pass. It computes where variables are stored at each
3492 position in code. Better debugging information is then generated
3493 (if the debugging information format supports this information).
3495 It is enabled by default when compiling with optimization (@option{-Os},
3496 @option{-O}, @option{-O2}, ...), debugging information (@option{-g}) and
3497 the debug info format supports it.
3499 @item -print-file-name=@var{library}
3500 @opindex print-file-name
3501 Print the full absolute name of the library file @var{library} that
3502 would be used when linking---and don't do anything else. With this
3503 option, GCC does not compile or link anything; it just prints the
3506 @item -print-multi-directory
3507 @opindex print-multi-directory
3508 Print the directory name corresponding to the multilib selected by any
3509 other switches present in the command line. This directory is supposed
3510 to exist in @env{GCC_EXEC_PREFIX}.
3512 @item -print-multi-lib
3513 @opindex print-multi-lib
3514 Print the mapping from multilib directory names to compiler switches
3515 that enable them. The directory name is separated from the switches by
3516 @samp{;}, and each switch starts with an @samp{@@} instead of the
3517 @samp{-}, without spaces between multiple switches. This is supposed to
3518 ease shell-processing.
3520 @item -print-prog-name=@var{program}
3521 @opindex print-prog-name
3522 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
3524 @item -print-libgcc-file-name
3525 @opindex print-libgcc-file-name
3526 Same as @option{-print-file-name=libgcc.a}.
3528 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
3529 but you do want to link with @file{libgcc.a}. You can do
3532 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
3535 @item -print-search-dirs
3536 @opindex print-search-dirs
3537 Print the name of the configured installation directory and a list of
3538 program and library directories @command{gcc} will search---and don't do anything else.
3540 This is useful when @command{gcc} prints the error message
3541 @samp{installation problem, cannot exec cpp0: No such file or directory}.
3542 To resolve this you either need to put @file{cpp0} and the other compiler
3543 components where @command{gcc} expects to find them, or you can set the environment
3544 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
3545 Don't forget the trailing '/'.
3546 @xref{Environment Variables}.
3549 @opindex dumpmachine
3550 Print the compiler's target machine (for example,
3551 @samp{i686-pc-linux-gnu})---and don't do anything else.
3554 @opindex dumpversion
3555 Print the compiler version (for example, @samp{3.0})---and don't do
3560 Print the compiler's built-in specs---and don't do anything else. (This
3561 is used when GCC itself is being built.) @xref{Spec Files}.
3563 @item -feliminate-unused-debug-types
3564 @opindex feliminate-unused-debug-types
3565 Normally, when producing DWARF2 output, GCC will emit debugging
3566 information for all types declared in a compilation
3567 unit, regardless of whether or not they are actually used
3568 in that compilation unit. Sometimes this is useful, such as
3569 if, in the debugger, you want to cast a value to a type that is
3570 not actually used in your program (but is declared). More often,
3571 however, this results in a significant amount of wasted space.
3572 With this option, GCC will avoid producing debug symbol output
3573 for types that are nowhere used in the source file being compiled.
3576 @node Optimize Options
3577 @section Options That Control Optimization
3578 @cindex optimize options
3579 @cindex options, optimization
3581 These options control various sorts of optimizations.
3583 Without any optimization option, the compiler's goal is to reduce the
3584 cost of compilation and to make debugging produce the expected
3585 results. Statements are independent: if you stop the program with a
3586 breakpoint between statements, you can then assign a new value to any
3587 variable or change the program counter to any other statement in the
3588 function and get exactly the results you would expect from the source
3591 Turning on optimization flags makes the compiler attempt to improve
3592 the performance and/or code size at the expense of compilation time
3593 and possibly the ability to debug the program.
3595 The compiler performs optimization based on the knowledge it has of
3596 the program. Using the @option{-funit-at-a-time} flag will allow the
3597 compiler to consider information gained from later functions in the
3598 file when compiling a function. Compiling multiple files at once to a
3599 single output file (and using @option{-funit-at-a-time}) will allow
3600 the compiler to use information gained from all of the files when
3601 compiling each of them.
3603 Not all optimizations are controlled directly by a flag. Only
3604 optimizations that have a flag are listed.
3611 Optimize. Optimizing compilation takes somewhat more time, and a lot
3612 more memory for a large function.
3614 With @option{-O}, the compiler tries to reduce code size and execution
3615 time, without performing any optimizations that take a great deal of
3618 @option{-O} turns on the following optimization flags:
3619 @gccoptlist{-fdefer-pop @gol
3620 -fmerge-constants @gol
3622 -floop-optimize @gol
3623 -fif-conversion @gol
3624 -fif-conversion2 @gol
3625 -fdelayed-branch @gol
3626 -fguess-branch-probability @gol
3629 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
3630 where doing so does not interfere with debugging.
3634 Optimize even more. GCC performs nearly all supported optimizations
3635 that do not involve a space-speed tradeoff. The compiler does not
3636 perform loop unrolling or function inlining when you specify @option{-O2}.
3637 As compared to @option{-O}, this option increases both compilation time
3638 and the performance of the generated code.
3640 @option{-O2} turns on all optimization flags specified by @option{-O}. It
3641 also turns on the following optimization flags:
3642 @gccoptlist{-fforce-mem @gol
3643 -foptimize-sibling-calls @gol
3644 -fstrength-reduce @gol
3645 -fcse-follow-jumps -fcse-skip-blocks @gol
3646 -frerun-cse-after-loop -frerun-loop-opt @gol
3647 -fgcse -fgcse-lm -fgcse-sm -fgcse-las @gol
3648 -fdelete-null-pointer-checks @gol
3649 -fexpensive-optimizations @gol
3651 -fschedule-insns -fschedule-insns2 @gol
3652 -fsched-interblock -fsched-spec @gol
3655 -freorder-blocks -freorder-functions @gol
3656 -fstrict-aliasing @gol
3657 -funit-at-a-time @gol
3658 -falign-functions -falign-jumps @gol
3659 -falign-loops -falign-labels @gol
3662 Please note the warning under @option{-fgcse} about
3663 invoking @option{-O2} on programs that use computed gotos.
3667 Optimize yet more. @option{-O3} turns on all optimizations specified by
3668 @option{-O2} and also turns on the @option{-finline-functions},
3669 @option{-fweb} and @option{-frename-registers} options.
3673 Do not optimize. This is the default.
3677 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
3678 do not typically increase code size. It also performs further
3679 optimizations designed to reduce code size.
3681 @option{-Os} disables the following optimization flags:
3682 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
3683 -falign-labels -freorder-blocks -fprefetch-loop-arrays}
3685 If you use multiple @option{-O} options, with or without level numbers,
3686 the last such option is the one that is effective.
3689 Options of the form @option{-f@var{flag}} specify machine-independent
3690 flags. Most flags have both positive and negative forms; the negative
3691 form of @option{-ffoo} would be @option{-fno-foo}. In the table
3692 below, only one of the forms is listed---the one you typically will
3693 use. You can figure out the other form by either removing @samp{no-}
3696 The following options control specific optimizations. They are either
3697 activated by @option{-O} options or are related to ones that are. You
3698 can use the following flags in the rare cases when ``fine-tuning'' of
3699 optimizations to be performed is desired.
3702 @item -fno-default-inline
3703 @opindex fno-default-inline
3704 Do not make member functions inline by default merely because they are
3705 defined inside the class scope (C++ only). Otherwise, when you specify
3706 @w{@option{-O}}, member functions defined inside class scope are compiled
3707 inline by default; i.e., you don't need to add @samp{inline} in front of
3708 the member function name.
3710 @item -fno-defer-pop
3711 @opindex fno-defer-pop
3712 Always pop the arguments to each function call as soon as that function
3713 returns. For machines which must pop arguments after a function call,
3714 the compiler normally lets arguments accumulate on the stack for several
3715 function calls and pops them all at once.
3717 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3721 Force memory operands to be copied into registers before doing
3722 arithmetic on them. This produces better code by making all memory
3723 references potential common subexpressions. When they are not common
3724 subexpressions, instruction combination should eliminate the separate
3727 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3730 @opindex fforce-addr
3731 Force memory address constants to be copied into registers before
3732 doing arithmetic on them. This may produce better code just as
3733 @option{-fforce-mem} may.
3735 @item -fomit-frame-pointer
3736 @opindex fomit-frame-pointer
3737 Don't keep the frame pointer in a register for functions that
3738 don't need one. This avoids the instructions to save, set up and
3739 restore frame pointers; it also makes an extra register available
3740 in many functions. @strong{It also makes debugging impossible on
3743 On some machines, such as the VAX, this flag has no effect, because
3744 the standard calling sequence automatically handles the frame pointer
3745 and nothing is saved by pretending it doesn't exist. The
3746 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
3747 whether a target machine supports this flag. @xref{Registers,,Register
3748 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
3750 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3752 @item -foptimize-sibling-calls
3753 @opindex foptimize-sibling-calls
3754 Optimize sibling and tail recursive calls.
3756 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3760 Don't pay attention to the @code{inline} keyword. Normally this option
3761 is used to keep the compiler from expanding any functions inline.
3762 Note that if you are not optimizing, no functions can be expanded inline.
3764 @item -finline-functions
3765 @opindex finline-functions
3766 Integrate all simple functions into their callers. The compiler
3767 heuristically decides which functions are simple enough to be worth
3768 integrating in this way.
3770 If all calls to a given function are integrated, and the function is
3771 declared @code{static}, then the function is normally not output as
3772 assembler code in its own right.
3774 Enabled at level @option{-O3}.
3776 @item -finline-limit=@var{n}
3777 @opindex finline-limit
3778 By default, GCC limits the size of functions that can be inlined. This flag
3779 allows the control of this limit for functions that are explicitly marked as
3780 inline (i.e., marked with the inline keyword or defined within the class
3781 definition in c++). @var{n} is the size of functions that can be inlined in
3782 number of pseudo instructions (not counting parameter handling). The default
3783 value of @var{n} is 600.
3784 Increasing this value can result in more inlined code at
3785 the cost of compilation time and memory consumption. Decreasing usually makes
3786 the compilation faster and less code will be inlined (which presumably
3787 means slower programs). This option is particularly useful for programs that
3788 use inlining heavily such as those based on recursive templates with C++.
3790 Inlining is actually controlled by a number of parameters, which may be
3791 specified individually by using @option{--param @var{name}=@var{value}}.
3792 The @option{-finline-limit=@var{n}} option sets some of these parameters
3796 @item max-inline-insns-single
3797 is set to @var{n}/2.
3798 @item max-inline-insns-auto
3799 is set to @var{n}/2.
3800 @item min-inline-insns
3801 is set to 130 or @var{n}/4, whichever is smaller.
3802 @item max-inline-insns-rtl
3806 See below for a documentation of the individual
3807 parameters controlling inlining.
3809 @emph{Note:} pseudo instruction represents, in this particular context, an
3810 abstract measurement of function's size. In no way, it represents a count
3811 of assembly instructions and as such its exact meaning might change from one
3812 release to an another.
3814 @item -fkeep-inline-functions
3815 @opindex fkeep-inline-functions
3816 Even if all calls to a given function are integrated, and the function
3817 is declared @code{static}, nevertheless output a separate run-time
3818 callable version of the function. This switch does not affect
3819 @code{extern inline} functions.
3821 @item -fkeep-static-consts
3822 @opindex fkeep-static-consts
3823 Emit variables declared @code{static const} when optimization isn't turned
3824 on, even if the variables aren't referenced.
3826 GCC enables this option by default. If you want to force the compiler to
3827 check if the variable was referenced, regardless of whether or not
3828 optimization is turned on, use the @option{-fno-keep-static-consts} option.
3830 @item -fmerge-constants
3831 Attempt to merge identical constants (string constants and floating point
3832 constants) across compilation units.
3834 This option is the default for optimized compilation if the assembler and
3835 linker support it. Use @option{-fno-merge-constants} to inhibit this
3838 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3840 @item -fmerge-all-constants
3841 Attempt to merge identical constants and identical variables.
3843 This option implies @option{-fmerge-constants}. In addition to
3844 @option{-fmerge-constants} this considers e.g. even constant initialized
3845 arrays or initialized constant variables with integral or floating point
3846 types. Languages like C or C++ require each non-automatic variable to
3847 have distinct location, so using this option will result in non-conforming
3852 Use a graph coloring register allocator. Currently this option is meant
3853 only for testing. Users should not specify this option, since it is not
3854 yet ready for production use.
3856 @item -fno-branch-count-reg
3857 @opindex fno-branch-count-reg
3858 Do not use ``decrement and branch'' instructions on a count register,
3859 but instead generate a sequence of instructions that decrement a
3860 register, compare it against zero, then branch based upon the result.
3861 This option is only meaningful on architectures that support such
3862 instructions, which include x86, PowerPC, IA-64 and S/390.
3864 The default is @option{-fbranch-count-reg}, enabled when
3865 @option{-fstrength-reduce} is enabled.
3867 @item -fno-function-cse
3868 @opindex fno-function-cse
3869 Do not put function addresses in registers; make each instruction that
3870 calls a constant function contain the function's address explicitly.
3872 This option results in less efficient code, but some strange hacks
3873 that alter the assembler output may be confused by the optimizations
3874 performed when this option is not used.
3876 The default is @option{-ffunction-cse}
3878 @item -fno-zero-initialized-in-bss
3879 @opindex fno-zero-initialized-in-bss
3880 If the target supports a BSS section, GCC by default puts variables that
3881 are initialized to zero into BSS@. This can save space in the resulting
3884 This option turns off this behavior because some programs explicitly
3885 rely on variables going to the data section. E.g., so that the
3886 resulting executable can find the beginning of that section and/or make
3887 assumptions based on that.
3889 The default is @option{-fzero-initialized-in-bss}.
3891 @item -fstrength-reduce
3892 @opindex fstrength-reduce
3893 Perform the optimizations of loop strength reduction and
3894 elimination of iteration variables.
3896 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3898 @item -fthread-jumps
3899 @opindex fthread-jumps
3900 Perform optimizations where we check to see if a jump branches to a
3901 location where another comparison subsumed by the first is found. If
3902 so, the first branch is redirected to either the destination of the
3903 second branch or a point immediately following it, depending on whether
3904 the condition is known to be true or false.
3906 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3908 @item -fcse-follow-jumps
3909 @opindex fcse-follow-jumps
3910 In common subexpression elimination, scan through jump instructions
3911 when the target of the jump is not reached by any other path. For
3912 example, when CSE encounters an @code{if} statement with an
3913 @code{else} clause, CSE will follow the jump when the condition
3916 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3918 @item -fcse-skip-blocks
3919 @opindex fcse-skip-blocks
3920 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
3921 follow jumps which conditionally skip over blocks. When CSE
3922 encounters a simple @code{if} statement with no else clause,
3923 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
3924 body of the @code{if}.
3926 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3928 @item -frerun-cse-after-loop
3929 @opindex frerun-cse-after-loop
3930 Re-run common subexpression elimination after loop optimizations has been
3933 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3935 @item -frerun-loop-opt
3936 @opindex frerun-loop-opt
3937 Run the loop optimizer twice.
3939 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3943 Perform a global common subexpression elimination pass.
3944 This pass also performs global constant and copy propagation.
3946 @emph{Note:} When compiling a program using computed gotos, a GCC
3947 extension, you may get better runtime performance if you disable
3948 the global common subexpression elimination pass by adding
3949 @option{-fno-gcse} to the command line.
3951 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3955 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
3956 attempt to move loads which are only killed by stores into themselves. This
3957 allows a loop containing a load/store sequence to be changed to a load outside
3958 the loop, and a copy/store within the loop.
3960 Enabled by default when gcse is enabled.
3964 When @option{-fgcse-sm} is enabled, a store motion pass is run after
3965 global common subexpression elimination. This pass will attempt to move
3966 stores out of loops. When used in conjunction with @option{-fgcse-lm},
3967 loops containing a load/store sequence can be changed to a load before
3968 the loop and a store after the loop.
3970 Enabled by default when gcse is enabled.
3974 When @option{-fgcse-las} is enabled, the global common subexpression
3975 elimination pass eliminates redundant loads that come after stores to the
3976 same memory location (both partial and full redundancies).
3978 Enabled by default when gcse is enabled.
3980 @item -floop-optimize
3981 @opindex floop-optimize
3982 Perform loop optimizations: move constant expressions out of loops, simplify
3983 exit test conditions and optionally do strength-reduction and loop unrolling as
3986 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3988 @item -fcrossjumping
3989 @opindex crossjumping
3990 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
3991 resulting code may or may not perform better than without cross-jumping.
3993 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3995 @item -fif-conversion
3996 @opindex if-conversion
3997 Attempt to transform conditional jumps into branch-less equivalents. This
3998 include use of conditional moves, min, max, set flags and abs instructions, and
3999 some tricks doable by standard arithmetics. The use of conditional execution
4000 on chips where it is available is controlled by @code{if-conversion2}.
4002 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4004 @item -fif-conversion2
4005 @opindex if-conversion2
4006 Use conditional execution (where available) to transform conditional jumps into
4007 branch-less equivalents.
4009 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4011 @item -fdelete-null-pointer-checks
4012 @opindex fdelete-null-pointer-checks
4013 Use global dataflow analysis to identify and eliminate useless checks
4014 for null pointers. The compiler assumes that dereferencing a null
4015 pointer would have halted the program. If a pointer is checked after
4016 it has already been dereferenced, it cannot be null.
4018 In some environments, this assumption is not true, and programs can
4019 safely dereference null pointers. Use
4020 @option{-fno-delete-null-pointer-checks} to disable this optimization
4021 for programs which depend on that behavior.
4023 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4025 @item -fexpensive-optimizations
4026 @opindex fexpensive-optimizations
4027 Perform a number of minor optimizations that are relatively expensive.
4029 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4031 @item -foptimize-register-move
4033 @opindex foptimize-register-move
4035 Attempt to reassign register numbers in move instructions and as
4036 operands of other simple instructions in order to maximize the amount of
4037 register tying. This is especially helpful on machines with two-operand
4040 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
4043 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4045 @item -fdelayed-branch
4046 @opindex fdelayed-branch
4047 If supported for the target machine, attempt to reorder instructions
4048 to exploit instruction slots available after delayed branch
4051 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4053 @item -fschedule-insns
4054 @opindex fschedule-insns
4055 If supported for the target machine, attempt to reorder instructions to
4056 eliminate execution stalls due to required data being unavailable. This
4057 helps machines that have slow floating point or memory load instructions
4058 by allowing other instructions to be issued until the result of the load
4059 or floating point instruction is required.
4061 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4063 @item -fschedule-insns2
4064 @opindex fschedule-insns2
4065 Similar to @option{-fschedule-insns}, but requests an additional pass of
4066 instruction scheduling after register allocation has been done. This is
4067 especially useful on machines with a relatively small number of
4068 registers and where memory load instructions take more than one cycle.
4070 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4072 @item -fno-sched-interblock
4073 @opindex fno-sched-interblock
4074 Don't schedule instructions across basic blocks. This is normally
4075 enabled by default when scheduling before register allocation, i.e.@:
4076 with @option{-fschedule-insns} or at @option{-O2} or higher.
4078 @item -fno-sched-spec
4079 @opindex fno-sched-spec
4080 Don't allow speculative motion of non-load instructions. This is normally
4081 enabled by default when scheduling before register allocation, i.e.@:
4082 with @option{-fschedule-insns} or at @option{-O2} or higher.
4084 @item -fsched-spec-load
4085 @opindex fsched-spec-load
4086 Allow speculative motion of some load instructions. This only makes
4087 sense when scheduling before register allocation, i.e.@: with
4088 @option{-fschedule-insns} or at @option{-O2} or higher.
4090 @item -fsched-spec-load-dangerous
4091 @opindex fsched-spec-load-dangerous
4092 Allow speculative motion of more load instructions. This only makes
4093 sense when scheduling before register allocation, i.e.@: with
4094 @option{-fschedule-insns} or at @option{-O2} or higher.
4096 @item -fsched-stalled-insns=@var{n}
4097 @opindex fsched-stalled-insns
4098 Define how many insns (if any) can be moved prematurely from the queue
4099 of stalled insns into the ready list, during the second scheduling pass.
4101 @item -fsched-stalled-insns-dep=@var{n}
4102 @opindex fsched-stalled-insns-dep
4103 Define how many insn groups (cycles) will be examined for a dependency
4104 on a stalled insn that is candidate for premature removal from the queue
4105 of stalled insns. Has an effect only during the second scheduling pass,
4106 and only if @option{-fsched-stalled-insns} is used and its value is not zero.
4108 @item -fsched2-use-superblocks
4109 @opindex fsched2-use-superblocks
4110 When scheduling after register allocation, do use superblock scheduling
4111 algorithm. Superblock scheduling allows motion across basic block boundaries
4112 resulting on faster schedules. This option is experimental, as not all machine
4113 descriptions used by GCC model the CPU closely enough to avoid unreliable
4114 results from the algorithm.
4116 This only makes sense when scheduling after register allocation, i.e.@: with
4117 @option{-fschedule-insns2} or at @option{-O2} or higher.
4119 @item -fsched2-use-traces
4120 @opindex fsched2-use-traces
4121 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
4122 allocation and additionally perform code duplication in order to increase the
4123 size of superblocks using tracer pass. See @option{-ftracer} for details on
4126 This mode should produce faster but significantly longer programs. Also
4127 without @code{-fbranch-probabilities} the traces constructed may not match the
4128 reality and hurt the performance. This only makes
4129 sense when scheduling after register allocation, i.e.@: with
4130 @option{-fschedule-insns2} or at @option{-O2} or higher.
4132 @item -fcaller-saves
4133 @opindex fcaller-saves
4134 Enable values to be allocated in registers that will be clobbered by
4135 function calls, by emitting extra instructions to save and restore the
4136 registers around such calls. Such allocation is done only when it
4137 seems to result in better code than would otherwise be produced.
4139 This option is always enabled by default on certain machines, usually
4140 those which have no call-preserved registers to use instead.
4142 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4144 @item -fmove-all-movables
4145 @opindex fmove-all-movables
4146 Forces all invariant computations in loops to be moved
4149 @item -freduce-all-givs
4150 @opindex freduce-all-givs
4151 Forces all general-induction variables in loops to be
4154 @emph{Note:} When compiling programs written in Fortran,
4155 @option{-fmove-all-movables} and @option{-freduce-all-givs} are enabled
4156 by default when you use the optimizer.
4158 These options may generate better or worse code; results are highly
4159 dependent on the structure of loops within the source code.
4161 These two options are intended to be removed someday, once
4162 they have helped determine the efficacy of various
4163 approaches to improving loop optimizations.
4165 Please let us (@w{@email{gcc@@gcc.gnu.org}} and @w{@email{fortran@@gnu.org}})
4166 know how use of these options affects
4167 the performance of your production code.
4168 We're very interested in code that runs @emph{slower}
4169 when these options are @emph{enabled}.
4172 @itemx -fno-peephole2
4173 @opindex fno-peephole
4174 @opindex fno-peephole2
4175 Disable any machine-specific peephole optimizations. The difference
4176 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
4177 are implemented in the compiler; some targets use one, some use the
4178 other, a few use both.
4180 @option{-fpeephole} is enabled by default.
4181 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4183 @item -fno-guess-branch-probability
4184 @opindex fno-guess-branch-probability
4185 Do not guess branch probabilities using a randomized model.
4187 Sometimes GCC will opt to use a randomized model to guess branch
4188 probabilities, when none are available from either profiling feedback
4189 (@option{-fprofile-arcs}) or @samp{__builtin_expect}. This means that
4190 different runs of the compiler on the same program may produce different
4193 In a hard real-time system, people don't want different runs of the
4194 compiler to produce code that has different behavior; minimizing
4195 non-determinism is of paramount import. This switch allows users to
4196 reduce non-determinism, possibly at the expense of inferior
4199 The default is @option{-fguess-branch-probability} at levels
4200 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4202 @item -freorder-blocks
4203 @opindex freorder-blocks
4204 Reorder basic blocks in the compiled function in order to reduce number of
4205 taken branches and improve code locality.
4207 Enabled at levels @option{-O2}, @option{-O3}.
4209 @item -freorder-functions
4210 @opindex freorder-functions
4211 Reorder basic blocks in the compiled function in order to reduce number of
4212 taken branches and improve code locality. This is implemented by using special
4213 subsections @code{text.hot} for most frequently executed functions and
4214 @code{text.unlikely} for unlikely executed functions. Reordering is done by
4215 the linker so object file format must support named sections and linker must
4216 place them in a reasonable way.
4218 Also profile feedback must be available in to make this option effective. See
4219 @option{-fprofile-arcs} for details.
4221 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4223 @item -fstrict-aliasing
4224 @opindex fstrict-aliasing
4225 Allows the compiler to assume the strictest aliasing rules applicable to
4226 the language being compiled. For C (and C++), this activates
4227 optimizations based on the type of expressions. In particular, an
4228 object of one type is assumed never to reside at the same address as an
4229 object of a different type, unless the types are almost the same. For
4230 example, an @code{unsigned int} can alias an @code{int}, but not a
4231 @code{void*} or a @code{double}. A character type may alias any other
4234 Pay special attention to code like this:
4247 The practice of reading from a different union member than the one most
4248 recently written to (called ``type-punning'') is common. Even with
4249 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
4250 is accessed through the union type. So, the code above will work as
4251 expected. However, this code might not:
4262 Every language that wishes to perform language-specific alias analysis
4263 should define a function that computes, given an @code{tree}
4264 node, an alias set for the node. Nodes in different alias sets are not
4265 allowed to alias. For an example, see the C front-end function
4266 @code{c_get_alias_set}.
4268 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4270 @item -falign-functions
4271 @itemx -falign-functions=@var{n}
4272 @opindex falign-functions
4273 Align the start of functions to the next power-of-two greater than
4274 @var{n}, skipping up to @var{n} bytes. For instance,
4275 @option{-falign-functions=32} aligns functions to the next 32-byte
4276 boundary, but @option{-falign-functions=24} would align to the next
4277 32-byte boundary only if this can be done by skipping 23 bytes or less.
4279 @option{-fno-align-functions} and @option{-falign-functions=1} are
4280 equivalent and mean that functions will not be aligned.
4282 Some assemblers only support this flag when @var{n} is a power of two;
4283 in that case, it is rounded up.
4285 If @var{n} is not specified or is zero, use a machine-dependent default.
4287 Enabled at levels @option{-O2}, @option{-O3}.
4289 @item -falign-labels
4290 @itemx -falign-labels=@var{n}
4291 @opindex falign-labels
4292 Align all branch targets to a power-of-two boundary, skipping up to
4293 @var{n} bytes like @option{-falign-functions}. This option can easily
4294 make code slower, because it must insert dummy operations for when the
4295 branch target is reached in the usual flow of the code.
4297 @option{-fno-align-labels} and @option{-falign-labels=1} are
4298 equivalent and mean that labels will not be aligned.
4300 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
4301 are greater than this value, then their values are used instead.
4303 If @var{n} is not specified or is zero, use a machine-dependent default
4304 which is very likely to be @samp{1}, meaning no alignment.
4306 Enabled at levels @option{-O2}, @option{-O3}.
4309 @itemx -falign-loops=@var{n}
4310 @opindex falign-loops
4311 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
4312 like @option{-falign-functions}. The hope is that the loop will be
4313 executed many times, which will make up for any execution of the dummy
4316 @option{-fno-align-loops} and @option{-falign-loops=1} are
4317 equivalent and mean that loops will not be aligned.
4319 If @var{n} is not specified or is zero, use a machine-dependent default.
4321 Enabled at levels @option{-O2}, @option{-O3}.
4324 @itemx -falign-jumps=@var{n}
4325 @opindex falign-jumps
4326 Align branch targets to a power-of-two boundary, for branch targets
4327 where the targets can only be reached by jumping, skipping up to @var{n}
4328 bytes like @option{-falign-functions}. In this case, no dummy operations
4331 @option{-fno-align-jumps} and @option{-falign-jumps=1} are
4332 equivalent and mean that loops will not be aligned.
4334 If @var{n} is not specified or is zero, use a machine-dependent default.
4336 Enabled at levels @option{-O2}, @option{-O3}.
4338 @item -frename-registers
4339 @opindex frename-registers
4340 Attempt to avoid false dependencies in scheduled code by making use
4341 of registers left over after register allocation. This optimization
4342 will most benefit processors with lots of registers. It can, however,
4343 make debugging impossible, since variables will no longer stay in
4344 a ``home register''.
4346 Enabled at levels @option{-O3}.
4350 Constructs webs as commonly used for register allocation purposes and assign
4351 each web individual pseudo register. This allows our register allocation pass
4352 to operate on pseudos directly, but also strengthens several other optimization
4353 passes, such as CSE, loop optimizer and trivial dead code remover. It can,
4354 however, make debugging impossible, since variables will no longer stay in a
4357 Enabled at levels @option{-O3}.
4359 @item -fno-cprop-registers
4360 @opindex fno-cprop-registers
4361 After register allocation and post-register allocation instruction splitting,
4362 we perform a copy-propagation pass to try to reduce scheduling dependencies
4363 and occasionally eliminate the copy.
4365 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4367 @item -fprofile-generate
4368 @opindex fprofile-generate
4369 Enable options usually used for instrumenting application to produce profile usefull
4370 for later recompilation profile feedback based optimization.
4372 The following options are enabled: @code{-fprofile-arcs}, @code{-fprofile-values}, @code{-fvpt}
4375 @opindex fprofile-use
4376 Enable profile feedback directed optimizations, and optimizations
4377 generally profitable only with profile feedback available.
4379 The following options are enabled: @code{-fbranch-probabilities},
4380 @code{-fvpt}, @code{-funroll-loops}, @code{-fpeel-loops}, @code{-ftracer}.
4384 The following options control compiler behavior regarding floating
4385 point arithmetic. These options trade off between speed and
4386 correctness. All must be specifically enabled.
4390 @opindex ffloat-store
4391 Do not store floating point variables in registers, and inhibit other
4392 options that might change whether a floating point value is taken from a
4395 @cindex floating point precision
4396 This option prevents undesirable excess precision on machines such as
4397 the 68000 where the floating registers (of the 68881) keep more
4398 precision than a @code{double} is supposed to have. Similarly for the
4399 x86 architecture. For most programs, the excess precision does only
4400 good, but a few programs rely on the precise definition of IEEE floating
4401 point. Use @option{-ffloat-store} for such programs, after modifying
4402 them to store all pertinent intermediate computations into variables.
4406 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations}, @*
4407 @option{-fno-trapping-math}, @option{-ffinite-math-only},
4408 @option{-fno-rounding-math} and @option{-fno-signaling-nans}.
4410 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
4412 This option should never be turned on by any @option{-O} option since
4413 it can result in incorrect output for programs which depend on
4414 an exact implementation of IEEE or ISO rules/specifications for
4417 @item -fno-math-errno
4418 @opindex fno-math-errno
4419 Do not set ERRNO after calling math functions that are executed
4420 with a single instruction, e.g., sqrt. A program that relies on
4421 IEEE exceptions for math error handling may want to use this flag
4422 for speed while maintaining IEEE arithmetic compatibility.
4424 This option should never be turned on by any @option{-O} option since
4425 it can result in incorrect output for programs which depend on
4426 an exact implementation of IEEE or ISO rules/specifications for
4429 The default is @option{-fmath-errno}.
4431 @item -funsafe-math-optimizations
4432 @opindex funsafe-math-optimizations
4433 Allow optimizations for floating-point arithmetic that (a) assume
4434 that arguments and results are valid and (b) may violate IEEE or
4435 ANSI standards. When used at link-time, it may include libraries
4436 or startup files that change the default FPU control word or other
4437 similar optimizations.
4439 This option should never be turned on by any @option{-O} option since
4440 it can result in incorrect output for programs which depend on
4441 an exact implementation of IEEE or ISO rules/specifications for
4444 The default is @option{-fno-unsafe-math-optimizations}.
4446 @item -ffinite-math-only
4447 @opindex ffinite-math-only
4448 Allow optimizations for floating-point arithmetic that assume
4449 that arguments and results are not NaNs or +-Infs.
4451 This option should never be turned on by any @option{-O} option since
4452 it can result in incorrect output for programs which depend on
4453 an exact implementation of IEEE or ISO rules/specifications.
4455 The default is @option{-fno-finite-math-only}.
4457 @item -fno-trapping-math
4458 @opindex fno-trapping-math
4459 Compile code assuming that floating-point operations cannot generate
4460 user-visible traps. These traps include division by zero, overflow,
4461 underflow, inexact result and invalid operation. This option implies
4462 @option{-fno-signaling-nans}. Setting this option may allow faster
4463 code if one relies on ``non-stop'' IEEE arithmetic, for example.
4465 This option should never be turned on by any @option{-O} option since
4466 it can result in incorrect output for programs which depend on
4467 an exact implementation of IEEE or ISO rules/specifications for
4470 The default is @option{-ftrapping-math}.
4472 @item -frounding-math
4473 @opindex frounding-math
4474 Disable transformations and optimizations that assume default floating
4475 point rounding behavior. This is round-to-zero for all floating point
4476 to integer conversions, and round-to-nearest for all other arithmetic
4477 truncations. This option should be specified for programs that change
4478 the FP rounding mode dynamically, or that may be executed with a
4479 non-default rounding mode. This option disables constant folding of
4480 floating point expressions at compile-time (which may be affected by
4481 rounding mode) and arithmetic transformations that are unsafe in the
4482 presence of sign-dependent rounding modes.
4484 The default is @option{-fno-rounding-math}.
4486 This option is experimental and does not currently guarantee to
4487 disable all GCC optimizations that are affected by rounding mode.
4488 Future versions of GCC may provide finer control of this setting
4489 using C99's @code{FENV_ACCESS} pragma. This command line option
4490 will be used to specify the default state for @code{FENV_ACCESS}.
4492 @item -fsignaling-nans
4493 @opindex fsignaling-nans
4494 Compile code assuming that IEEE signaling NaNs may generate user-visible
4495 traps during floating-point operations. Setting this option disables
4496 optimizations that may change the number of exceptions visible with
4497 signaling NaNs. This option implies @option{-ftrapping-math}.
4499 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
4502 The default is @option{-fno-signaling-nans}.
4504 This option is experimental and does not currently guarantee to
4505 disable all GCC optimizations that affect signaling NaN behavior.
4507 @item -fsingle-precision-constant
4508 @opindex fsingle-precision-constant
4509 Treat floating point constant as single precision constant instead of
4510 implicitly converting it to double precision constant.
4515 The following options control optimizations that may improve
4516 performance, but are not enabled by any @option{-O} options. This
4517 section includes experimental options that may produce broken code.
4520 @item -fbranch-probabilities
4521 @opindex fbranch-probabilities
4522 After running a program compiled with @option{-fprofile-arcs}
4523 (@pxref{Debugging Options,, Options for Debugging Your Program or
4524 @command{gcc}}), you can compile it a second time using
4525 @option{-fbranch-probabilities}, to improve optimizations based on
4526 the number of times each branch was taken. When the program
4527 compiled with @option{-fprofile-arcs} exits it saves arc execution
4528 counts to a file called @file{@var{sourcename}.gcda} for each source
4529 file The information in this data file is very dependent on the
4530 structure of the generated code, so you must use the same source code
4531 and the same optimization options for both compilations.
4533 With @option{-fbranch-probabilities}, GCC puts a
4534 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
4535 These can be used to improve optimization. Currently, they are only
4536 used in one place: in @file{reorg.c}, instead of guessing which path a
4537 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
4538 exactly determine which path is taken more often.
4540 @item -fprofile-values
4541 @opindex fprofile-values
4542 If combined with @option{-fprofile-arcs}, it adds code so that some
4543 data about values of expressions in the program is gathered.
4545 With @option{-fbranch-probabilities}, it reads back the data gathered
4546 from profiling values of expressions and adds @samp{REG_VALUE_PROFILE}
4547 notes to instructions for their later usage in optimizations.
4549 Enabled with @option{-profile-generate} and @option{-profile-use}.
4553 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
4554 a code to gather information about values of expressions.
4556 With @option{-fbranch-probabilities}, it reads back the data gathered
4557 and actually performs the optimizations based on them.
4558 Currently the optimizations include specialization of division operation
4559 using the knowledge about the value of the denominator.
4561 Enabled with @option{-profile-generate} and @option{-profile-use}.
4565 Use a graph coloring register allocator. Currently this option is meant
4566 for testing, so we are interested to hear about miscompilations with
4571 Perform tail duplication to enlarge superblock size. This transformation
4572 simplifies the control flow of the function allowing other optimizations to do
4575 Enabled with @option{-profile-use}.
4577 @item -funit-at-a-time
4578 @opindex funit-at-a-time
4579 Parse the whole compilation unit before starting to produce code.
4580 This allows some extra optimizations to take place but consumes more
4583 Enabled at levels @option{-O2}, @option{-O3}.
4585 @item -funroll-loops
4586 @opindex funroll-loops
4587 Unroll loops whose number of iterations can be determined at compile time or
4588 upon entry to the loop. @option{-funroll-loops} implies
4589 @option{-frerun-cse-after-loop}. It also turns on complete loop peeling
4590 (i.e. complete removal of loops with small constant number of iterations).
4591 This option makes code larger, and may or may not make it run faster.
4593 Enabled with @option{-profile-use}.
4595 @item -funroll-all-loops
4596 @opindex funroll-all-loops
4597 Unroll all loops, even if their number of iterations is uncertain when
4598 the loop is entered. This usually makes programs run more slowly.
4599 @option{-funroll-all-loops} implies the same options as
4600 @option{-funroll-loops}.
4603 @opindex fpeel-loops
4604 Peels the loops for that there is enough information that they do not
4605 roll much (from profile feedback). It also turns on complete loop peeling
4606 (i.e. complete removal of loops with small constant number of iterations).
4608 Enabled with @option{-profile-use}.
4610 @item -funswitch-loops
4611 @opindex funswitch-loops
4612 Move branches with loop invariant conditions out of the loop, with duplicates
4613 of the loop on both branches (modified according to result of the condition).
4615 @item -fold-unroll-loops
4616 @opindex fold-unroll-loops
4617 Unroll loops whose number of iterations can be determined at compile
4618 time or upon entry to the loop, using the old loop unroller whose loop
4619 recognition is based on notes from frontend. @option{-fold-unroll-loops} implies
4620 both @option{-fstrength-reduce} and @option{-frerun-cse-after-loop}. This
4621 option makes code larger, and may or may not make it run faster.
4623 @item -fold-unroll-all-loops
4624 @opindex fold-unroll-all-loops
4625 Unroll all loops, even if their number of iterations is uncertain when
4626 the loop is entered. This is done using the old loop unroller whose loop
4627 recognition is based on notes from frontend. This usually makes programs run more slowly.
4628 @option{-fold-unroll-all-loops} implies the same options as
4629 @option{-fold-unroll-loops}.
4631 @item -fprefetch-loop-arrays
4632 @opindex fprefetch-loop-arrays
4633 If supported by the target machine, generate instructions to prefetch
4634 memory to improve the performance of loops that access large arrays.
4636 Disabled at level @option{-Os}.
4638 @item -ffunction-sections
4639 @itemx -fdata-sections
4640 @opindex ffunction-sections
4641 @opindex fdata-sections
4642 Place each function or data item into its own section in the output
4643 file if the target supports arbitrary sections. The name of the
4644 function or the name of the data item determines the section's name
4647 Use these options on systems where the linker can perform optimizations
4648 to improve locality of reference in the instruction space. Most systems
4649 using the ELF object format and SPARC processors running Solaris 2 have
4650 linkers with such optimizations. AIX may have these optimizations in
4653 Only use these options when there are significant benefits from doing
4654 so. When you specify these options, the assembler and linker will
4655 create larger object and executable files and will also be slower.
4656 You will not be able to use @code{gprof} on all systems if you
4657 specify this option and you may have problems with debugging if
4658 you specify both this option and @option{-g}.
4660 @item -fbranch-target-load-optimize
4661 @opindex fbranch-target-load-optimize
4662 Perform branch target register load optimization before prologue / epilogue
4664 The use of target registers can typically be exposed only during reload,
4665 thus hoisting loads out of loops and doing inter-block scheduling needs
4666 a separate optimization pass.
4668 @item -fbranch-target-load-optimize2
4669 @opindex fbranch-target-load-optimize2
4670 Perform branch target register load optimization after prologue / epilogue
4673 @item -fbtr-bb-exclusive
4674 @opindex fbtr-bb-exclusive
4675 When performing branch target register load optimization, don't reuse
4676 branch target registers in within any basic block.
4678 @item --param @var{name}=@var{value}
4680 In some places, GCC uses various constants to control the amount of
4681 optimization that is done. For example, GCC will not inline functions
4682 that contain more that a certain number of instructions. You can
4683 control some of these constants on the command-line using the
4684 @option{--param} option.
4686 The names of specific parameters, and the meaning of the values, are
4687 tied to the internals of the compiler, and are subject to change
4688 without notice in future releases.
4690 In each case, the @var{value} is an integer. The allowable choices for
4691 @var{name} are given in the following table:
4694 @item max-crossjump-edges
4695 The maximum number of incoming edges to consider for crossjumping.
4696 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
4697 the number of edges incoming to each block. Increasing values mean
4698 more aggressive optimization, making the compile time increase with
4699 probably small improvement in executable size.
4701 @item max-delay-slot-insn-search
4702 The maximum number of instructions to consider when looking for an
4703 instruction to fill a delay slot. If more than this arbitrary number of
4704 instructions is searched, the time savings from filling the delay slot
4705 will be minimal so stop searching. Increasing values mean more
4706 aggressive optimization, making the compile time increase with probably
4707 small improvement in executable run time.
4709 @item max-delay-slot-live-search
4710 When trying to fill delay slots, the maximum number of instructions to
4711 consider when searching for a block with valid live register
4712 information. Increasing this arbitrarily chosen value means more
4713 aggressive optimization, increasing the compile time. This parameter
4714 should be removed when the delay slot code is rewritten to maintain the
4717 @item max-gcse-memory
4718 The approximate maximum amount of memory that will be allocated in
4719 order to perform the global common subexpression elimination
4720 optimization. If more memory than specified is required, the
4721 optimization will not be done.
4723 @item max-gcse-passes
4724 The maximum number of passes of GCSE to run. The default is 1.
4726 @item max-pending-list-length
4727 The maximum number of pending dependencies scheduling will allow
4728 before flushing the current state and starting over. Large functions
4729 with few branches or calls can create excessively large lists which
4730 needlessly consume memory and resources.
4732 @item max-inline-insns-single
4733 Several parameters control the tree inliner used in gcc.
4734 This number sets the maximum number of instructions (counted in GCC's
4735 internal representation) in a single function that the tree inliner
4736 will consider for inlining. This only affects functions declared
4737 inline and methods implemented in a class declaration (C++).
4738 The default value is 500.
4740 @item max-inline-insns-auto
4741 When you use @option{-finline-functions} (included in @option{-O3}),
4742 a lot of functions that would otherwise not be considered for inlining
4743 by the compiler will be investigated. To those functions, a different
4744 (more restrictive) limit compared to functions declared inline can
4746 The default value is 120.
4748 @item large-function-insns
4749 The limit specifying really large functions. For functions greater than this
4750 limit inlining is constrained by @option{--param large-function-growth}.
4751 This parameter is useful primarily to avoid extreme compilation time caused by non-linear
4752 algorithms used by the backend.
4753 This parameter is ignored when @option{-funit-at-a-time} is not used.
4754 The default value is 3000.
4756 @item large-function-growth
4757 Specifies maximal growth of large function caused by inlining in percents.
4758 This parameter is ignored when @option{-funit-at-a-time} is not used.
4759 The default value is 200.
4761 @item inline-unit-growth
4762 Specifies maximal overall growth of the compilation unit caused by inlining.
4763 This parameter is ignored when @option{-funit-at-a-time} is not used.
4764 The default value is 150.
4766 @item max-inline-insns-rtl
4767 For languages that use the RTL inliner (this happens at a later stage
4768 than tree inlining), you can set the maximum allowable size (counted
4769 in RTL instructions) for the RTL inliner with this parameter.
4770 The default value is 600.
4772 @item max-unrolled-insns
4773 The maximum number of instructions that a loop should have if that loop
4774 is unrolled, and if the loop is unrolled, it determines how many times
4775 the loop code is unrolled.
4777 @item max-average-unrolled-insns
4778 The maximum number of instructions biased by probabilities of their execution
4779 that a loop should have if that loop is unrolled, and if the loop is unrolled,
4780 it determines how many times the loop code is unrolled.
4782 @item max-unroll-times
4783 The maximum number of unrollings of a single loop.
4785 @item max-peeled-insns
4786 The maximum number of instructions that a loop should have if that loop
4787 is peeled, and if the loop is peeled, it determines how many times
4788 the loop code is peeled.
4790 @item max-peel-times
4791 The maximum number of peelings of a single loop.
4793 @item max-completely-peeled-insns
4794 The maximum number of insns of a completely peeled loop.
4796 @item max-completely-peel-times
4797 The maximum number of iterations of a loop to be suitable for complete peeling.
4799 @item max-unswitch-insns
4800 The maximum number of insns of an unswitched loop.
4802 @item max-unswitch-level
4803 The maximum number of branches unswitched in a single loop.
4805 @item hot-bb-count-fraction
4806 Select fraction of the maximal count of repetitions of basic block in program
4807 given basic block needs to have to be considered hot.
4809 @item hot-bb-frequency-fraction
4810 Select fraction of the maximal frequency of executions of basic block in
4811 function given basic block needs to have to be considered hot
4813 @item tracer-dynamic-coverage
4814 @itemx tracer-dynamic-coverage-feedback
4816 This value is used to limit superblock formation once the given percentage of
4817 executed instructions is covered. This limits unnecessary code size
4820 The @option{tracer-dynamic-coverage-feedback} is used only when profile
4821 feedback is available. The real profiles (as opposed to statically estimated
4822 ones) are much less balanced allowing the threshold to be larger value.
4824 @item tracer-max-code-growth
4825 Stop tail duplication once code growth has reached given percentage. This is
4826 rather hokey argument, as most of the duplicates will be eliminated later in
4827 cross jumping, so it may be set to much higher values than is the desired code
4830 @item tracer-min-branch-ratio
4832 Stop reverse growth when the reverse probability of best edge is less than this
4833 threshold (in percent).
4835 @item tracer-min-branch-ratio
4836 @itemx tracer-min-branch-ratio-feedback
4838 Stop forward growth if the best edge do have probability lower than this
4841 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
4842 compilation for profile feedback and one for compilation without. The value
4843 for compilation with profile feedback needs to be more conservative (higher) in
4844 order to make tracer effective.
4846 @item max-cse-path-length
4848 Maximum number of basic blocks on path that cse considers. The default is 10.
4850 @item ggc-min-expand
4852 GCC uses a garbage collector to manage its own memory allocation. This
4853 parameter specifies the minimum percentage by which the garbage
4854 collector's heap should be allowed to expand between collections.
4855 Tuning this may improve compilation speed; it has no effect on code
4858 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
4859 RAM >= 1GB. If @code{getrlimit} is available, the notion of "RAM" is
4860 the smallest of actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If
4861 GCC is not able to calculate RAM on a particular platform, the lower
4862 bound of 30% is used. Setting this parameter and
4863 @option{ggc-min-heapsize} to zero causes a full collection to occur at
4864 every opportunity. This is extremely slow, but can be useful for
4867 @item ggc-min-heapsize
4869 Minimum size of the garbage collector's heap before it begins bothering
4870 to collect garbage. The first collection occurs after the heap expands
4871 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
4872 tuning this may improve compilation speed, and has no effect on code
4875 The default is RAM/8, with a lower bound of 4096 (four megabytes) and an
4876 upper bound of 131072 (128 megabytes). If @code{getrlimit} is
4877 available, the notion of "RAM" is the smallest of actual RAM,
4878 RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate
4879 RAM on a particular platform, the lower bound is used. Setting this
4880 parameter very large effectively disables garbage collection. Setting
4881 this parameter and @option{ggc-min-expand} to zero causes a full
4882 collection to occur at every opportunity.
4884 @item max-reload-search-insns
4885 The maximum number of instruction reload should look backward for equivalent
4886 register. Increasing values mean more aggressive optimization, making the
4887 compile time increase with probably slightly better performance. The default
4890 @item max-cselib-memory-location
4891 The maximum number of memory locations cselib should take into acount.
4892 Increasing values mean more aggressive optimization, making the compile time
4893 increase with probably slightly better performance. The default value is 500.
4895 @item reorder-blocks-duplicate
4896 @itemx reorder-blocks-duplicate-feedback
4898 Used by basic block reordering pass to decide whether to use unconditional
4899 branch or duplicate the code on its destination. Code is duplicated when its
4900 estimated size is smaller than this value multiplied by the estimated size of
4901 unconditional jump in the hot spots of the program.
4903 The @option{reorder-block-duplicate-feedback} is used only when profile
4904 feedback is available and may be set to higher values than
4905 @option{reorder-block-duplicate} since information about the hot spots is more
4908 @item max-sched-region-blocks
4909 The maximum number of blocks in a region to be considered for
4910 interblock scheduling. The default value is 10.
4912 @item max-sched-region-insns
4913 The maximum number of insns in a region to be considered for
4914 interblock scheduling. The default value is 100.
4918 @node Preprocessor Options
4919 @section Options Controlling the Preprocessor
4920 @cindex preprocessor options
4921 @cindex options, preprocessor
4923 These options control the C preprocessor, which is run on each C source
4924 file before actual compilation.
4926 If you use the @option{-E} option, nothing is done except preprocessing.
4927 Some of these options make sense only together with @option{-E} because
4928 they cause the preprocessor output to be unsuitable for actual
4933 You can use @option{-Wp,@var{option}} to bypass the compiler driver
4934 and pass @var{option} directly through to the preprocessor. If
4935 @var{option} contains commas, it is split into multiple options at the
4936 commas. However, many options are modified, translated or interpreted
4937 by the compiler driver before being passed to the preprocessor, and
4938 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
4939 interface is undocumented and subject to change, so whenever possible
4940 you should avoid using @option{-Wp} and let the driver handle the
4943 @item -Xpreprocessor @var{option}
4944 @opindex preprocessor
4945 Pass @var{option} as an option to the preprocessor. You can use this to
4946 supply system-specific preprocessor options which GCC does not know how to
4949 If you want to pass an option that takes an argument, you must use
4950 @option{-Xpreprocessor} twice, once for the option and once for the argument.
4953 @include cppopts.texi
4955 @node Assembler Options
4956 @section Passing Options to the Assembler
4958 @c prevent bad page break with this line
4959 You can pass options to the assembler.
4962 @item -Wa,@var{option}
4964 Pass @var{option} as an option to the assembler. If @var{option}
4965 contains commas, it is split into multiple options at the commas.
4967 @item -Xassembler @var{option}
4969 Pass @var{option} as an option to the assembler. You can use this to
4970 supply system-specific assembler options which GCC does not know how to
4973 If you want to pass an option that takes an argument, you must use
4974 @option{-Xassembler} twice, once for the option and once for the argument.
4979 @section Options for Linking
4980 @cindex link options
4981 @cindex options, linking
4983 These options come into play when the compiler links object files into
4984 an executable output file. They are meaningless if the compiler is
4985 not doing a link step.
4989 @item @var{object-file-name}
4990 A file name that does not end in a special recognized suffix is
4991 considered to name an object file or library. (Object files are
4992 distinguished from libraries by the linker according to the file
4993 contents.) If linking is done, these object files are used as input
5002 If any of these options is used, then the linker is not run, and
5003 object file names should not be used as arguments. @xref{Overall
5007 @item -l@var{library}
5008 @itemx -l @var{library}
5010 Search the library named @var{library} when linking. (The second
5011 alternative with the library as a separate argument is only for
5012 POSIX compliance and is not recommended.)
5014 It makes a difference where in the command you write this option; the
5015 linker searches and processes libraries and object files in the order they
5016 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
5017 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
5018 to functions in @samp{z}, those functions may not be loaded.
5020 The linker searches a standard list of directories for the library,
5021 which is actually a file named @file{lib@var{library}.a}. The linker
5022 then uses this file as if it had been specified precisely by name.
5024 The directories searched include several standard system directories
5025 plus any that you specify with @option{-L}.
5027 Normally the files found this way are library files---archive files
5028 whose members are object files. The linker handles an archive file by
5029 scanning through it for members which define symbols that have so far
5030 been referenced but not defined. But if the file that is found is an
5031 ordinary object file, it is linked in the usual fashion. The only
5032 difference between using an @option{-l} option and specifying a file name
5033 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
5034 and searches several directories.
5038 You need this special case of the @option{-l} option in order to
5039 link an Objective-C program.
5042 @opindex nostartfiles
5043 Do not use the standard system startup files when linking.
5044 The standard system libraries are used normally, unless @option{-nostdlib}
5045 or @option{-nodefaultlibs} is used.
5047 @item -nodefaultlibs
5048 @opindex nodefaultlibs
5049 Do not use the standard system libraries when linking.
5050 Only the libraries you specify will be passed to the linker.
5051 The standard startup files are used normally, unless @option{-nostartfiles}
5052 is used. The compiler may generate calls to memcmp, memset, and memcpy
5053 for System V (and ISO C) environments or to bcopy and bzero for
5054 BSD environments. These entries are usually resolved by entries in
5055 libc. These entry points should be supplied through some other
5056 mechanism when this option is specified.
5060 Do not use the standard system startup files or libraries when linking.
5061 No startup files and only the libraries you specify will be passed to
5062 the linker. The compiler may generate calls to memcmp, memset, and memcpy
5063 for System V (and ISO C) environments or to bcopy and bzero for
5064 BSD environments. These entries are usually resolved by entries in
5065 libc. These entry points should be supplied through some other
5066 mechanism when this option is specified.
5068 @cindex @option{-lgcc}, use with @option{-nostdlib}
5069 @cindex @option{-nostdlib} and unresolved references
5070 @cindex unresolved references and @option{-nostdlib}
5071 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
5072 @cindex @option{-nodefaultlibs} and unresolved references
5073 @cindex unresolved references and @option{-nodefaultlibs}
5074 One of the standard libraries bypassed by @option{-nostdlib} and
5075 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
5076 that GCC uses to overcome shortcomings of particular machines, or special
5077 needs for some languages.
5078 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
5079 Collection (GCC) Internals},
5080 for more discussion of @file{libgcc.a}.)
5081 In most cases, you need @file{libgcc.a} even when you want to avoid
5082 other standard libraries. In other words, when you specify @option{-nostdlib}
5083 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
5084 This ensures that you have no unresolved references to internal GCC
5085 library subroutines. (For example, @samp{__main}, used to ensure C++
5086 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
5087 GNU Compiler Collection (GCC) Internals}.)
5091 Produce a position independent executable on targets which support it.
5092 For predictable results, you must also specify the same set of options
5093 that were used to generate code (@option{-fpie}, @option{-fPIE},
5094 or model suboptions) when you specify this option.
5098 Remove all symbol table and relocation information from the executable.
5102 On systems that support dynamic linking, this prevents linking with the shared
5103 libraries. On other systems, this option has no effect.
5107 Produce a shared object which can then be linked with other objects to
5108 form an executable. Not all systems support this option. For predictable
5109 results, you must also specify the same set of options that were used to
5110 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
5111 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
5112 needs to build supplementary stub code for constructors to work. On
5113 multi-libbed systems, @samp{gcc -shared} must select the correct support
5114 libraries to link against. Failing to supply the correct flags may lead
5115 to subtle defects. Supplying them in cases where they are not necessary
5118 @item -shared-libgcc
5119 @itemx -static-libgcc
5120 @opindex shared-libgcc
5121 @opindex static-libgcc
5122 On systems that provide @file{libgcc} as a shared library, these options
5123 force the use of either the shared or static version respectively.
5124 If no shared version of @file{libgcc} was built when the compiler was
5125 configured, these options have no effect.
5127 There are several situations in which an application should use the
5128 shared @file{libgcc} instead of the static version. The most common
5129 of these is when the application wishes to throw and catch exceptions
5130 across different shared libraries. In that case, each of the libraries
5131 as well as the application itself should use the shared @file{libgcc}.
5133 Therefore, the G++ and GCJ drivers automatically add
5134 @option{-shared-libgcc} whenever you build a shared library or a main
5135 executable, because C++ and Java programs typically use exceptions, so
5136 this is the right thing to do.
5138 If, instead, you use the GCC driver to create shared libraries, you may
5139 find that they will not always be linked with the shared @file{libgcc}.
5140 If GCC finds, at its configuration time, that you have a GNU linker that
5141 does not support option @option{--eh-frame-hdr}, it will link the shared
5142 version of @file{libgcc} into shared libraries by default. Otherwise,
5143 it will take advantage of the linker and optimize away the linking with
5144 the shared version of @file{libgcc}, linking with the static version of
5145 libgcc by default. This allows exceptions to propagate through such
5146 shared libraries, without incurring relocation costs at library load
5149 However, if a library or main executable is supposed to throw or catch
5150 exceptions, you must link it using the G++ or GCJ driver, as appropriate
5151 for the languages used in the program, or using the option
5152 @option{-shared-libgcc}, such that it is linked with the shared
5157 Bind references to global symbols when building a shared object. Warn
5158 about any unresolved references (unless overridden by the link editor
5159 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
5162 @item -Xlinker @var{option}
5164 Pass @var{option} as an option to the linker. You can use this to
5165 supply system-specific linker options which GCC does not know how to
5168 If you want to pass an option that takes an argument, you must use
5169 @option{-Xlinker} twice, once for the option and once for the argument.
5170 For example, to pass @option{-assert definitions}, you must write
5171 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
5172 @option{-Xlinker "-assert definitions"}, because this passes the entire
5173 string as a single argument, which is not what the linker expects.
5175 @item -Wl,@var{option}
5177 Pass @var{option} as an option to the linker. If @var{option} contains
5178 commas, it is split into multiple options at the commas.
5180 @item -u @var{symbol}
5182 Pretend the symbol @var{symbol} is undefined, to force linking of
5183 library modules to define it. You can use @option{-u} multiple times with
5184 different symbols to force loading of additional library modules.
5187 @node Directory Options
5188 @section Options for Directory Search
5189 @cindex directory options
5190 @cindex options, directory search
5193 These options specify directories to search for header files, for
5194 libraries and for parts of the compiler:
5199 Add the directory @var{dir} to the head of the list of directories to be
5200 searched for header files. This can be used to override a system header
5201 file, substituting your own version, since these directories are
5202 searched before the system header file directories. However, you should
5203 not use this option to add directories that contain vendor-supplied
5204 system header files (use @option{-isystem} for that). If you use more than
5205 one @option{-I} option, the directories are scanned in left-to-right
5206 order; the standard system directories come after.
5208 If a standard system include directory, or a directory specified with
5209 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
5210 option will be ignored. The directory will still be searched but as a
5211 system directory at its normal position in the system include chain.
5212 This is to ensure that GCC's procedure to fix buggy system headers and
5213 the ordering for the include_next directive are not inadvertently changed.
5214 If you really need to change the search order for system directories,
5215 use the @option{-nostdinc} and/or @option{-isystem} options.
5219 Any directories you specify with @option{-I} options before the @option{-I-}
5220 option are searched only for the case of @samp{#include "@var{file}"};
5221 they are not searched for @samp{#include <@var{file}>}.
5223 If additional directories are specified with @option{-I} options after
5224 the @option{-I-}, these directories are searched for all @samp{#include}
5225 directives. (Ordinarily @emph{all} @option{-I} directories are used
5228 In addition, the @option{-I-} option inhibits the use of the current
5229 directory (where the current input file came from) as the first search
5230 directory for @samp{#include "@var{file}"}. There is no way to
5231 override this effect of @option{-I-}. With @option{-I.} you can specify
5232 searching the directory which was current when the compiler was
5233 invoked. That is not exactly the same as what the preprocessor does
5234 by default, but it is often satisfactory.
5236 @option{-I-} does not inhibit the use of the standard system directories
5237 for header files. Thus, @option{-I-} and @option{-nostdinc} are
5242 Add directory @var{dir} to the list of directories to be searched
5245 @item -B@var{prefix}
5247 This option specifies where to find the executables, libraries,
5248 include files, and data files of the compiler itself.
5250 The compiler driver program runs one or more of the subprograms
5251 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
5252 @var{prefix} as a prefix for each program it tries to run, both with and
5253 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
5255 For each subprogram to be run, the compiler driver first tries the
5256 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
5257 was not specified, the driver tries two standard prefixes, which are
5258 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}. If neither of
5259 those results in a file name that is found, the unmodified program
5260 name is searched for using the directories specified in your
5261 @env{PATH} environment variable.
5263 The compiler will check to see if the path provided by the @option{-B}
5264 refers to a directory, and if necessary it will add a directory
5265 separator character at the end of the path.
5267 @option{-B} prefixes that effectively specify directory names also apply
5268 to libraries in the linker, because the compiler translates these
5269 options into @option{-L} options for the linker. They also apply to
5270 includes files in the preprocessor, because the compiler translates these
5271 options into @option{-isystem} options for the preprocessor. In this case,
5272 the compiler appends @samp{include} to the prefix.
5274 The run-time support file @file{libgcc.a} can also be searched for using
5275 the @option{-B} prefix, if needed. If it is not found there, the two
5276 standard prefixes above are tried, and that is all. The file is left
5277 out of the link if it is not found by those means.
5279 Another way to specify a prefix much like the @option{-B} prefix is to use
5280 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
5283 As a special kludge, if the path provided by @option{-B} is
5284 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
5285 9, then it will be replaced by @file{[dir/]include}. This is to help
5286 with boot-strapping the compiler.
5288 @item -specs=@var{file}
5290 Process @var{file} after the compiler reads in the standard @file{specs}
5291 file, in order to override the defaults that the @file{gcc} driver
5292 program uses when determining what switches to pass to @file{cc1},
5293 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
5294 @option{-specs=@var{file}} can be specified on the command line, and they
5295 are processed in order, from left to right.
5301 @section Specifying subprocesses and the switches to pass to them
5304 @command{gcc} is a driver program. It performs its job by invoking a
5305 sequence of other programs to do the work of compiling, assembling and
5306 linking. GCC interprets its command-line parameters and uses these to
5307 deduce which programs it should invoke, and which command-line options
5308 it ought to place on their command lines. This behavior is controlled
5309 by @dfn{spec strings}. In most cases there is one spec string for each
5310 program that GCC can invoke, but a few programs have multiple spec
5311 strings to control their behavior. The spec strings built into GCC can
5312 be overridden by using the @option{-specs=} command-line switch to specify
5315 @dfn{Spec files} are plaintext files that are used to construct spec
5316 strings. They consist of a sequence of directives separated by blank
5317 lines. The type of directive is determined by the first non-whitespace
5318 character on the line and it can be one of the following:
5321 @item %@var{command}
5322 Issues a @var{command} to the spec file processor. The commands that can
5326 @item %include <@var{file}>
5328 Search for @var{file} and insert its text at the current point in the
5331 @item %include_noerr <@var{file}>
5332 @cindex %include_noerr
5333 Just like @samp{%include}, but do not generate an error message if the include
5334 file cannot be found.
5336 @item %rename @var{old_name} @var{new_name}
5338 Rename the spec string @var{old_name} to @var{new_name}.
5342 @item *[@var{spec_name}]:
5343 This tells the compiler to create, override or delete the named spec
5344 string. All lines after this directive up to the next directive or
5345 blank line are considered to be the text for the spec string. If this
5346 results in an empty string then the spec will be deleted. (Or, if the
5347 spec did not exist, then nothing will happened.) Otherwise, if the spec
5348 does not currently exist a new spec will be created. If the spec does
5349 exist then its contents will be overridden by the text of this
5350 directive, unless the first character of that text is the @samp{+}
5351 character, in which case the text will be appended to the spec.
5353 @item [@var{suffix}]:
5354 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
5355 and up to the next directive or blank line are considered to make up the
5356 spec string for the indicated suffix. When the compiler encounters an
5357 input file with the named suffix, it will processes the spec string in
5358 order to work out how to compile that file. For example:
5365 This says that any input file whose name ends in @samp{.ZZ} should be
5366 passed to the program @samp{z-compile}, which should be invoked with the
5367 command-line switch @option{-input} and with the result of performing the
5368 @samp{%i} substitution. (See below.)
5370 As an alternative to providing a spec string, the text that follows a
5371 suffix directive can be one of the following:
5374 @item @@@var{language}
5375 This says that the suffix is an alias for a known @var{language}. This is
5376 similar to using the @option{-x} command-line switch to GCC to specify a
5377 language explicitly. For example:
5384 Says that .ZZ files are, in fact, C++ source files.
5387 This causes an error messages saying:
5390 @var{name} compiler not installed on this system.
5394 GCC already has an extensive list of suffixes built into it.
5395 This directive will add an entry to the end of the list of suffixes, but
5396 since the list is searched from the end backwards, it is effectively
5397 possible to override earlier entries using this technique.
5401 GCC has the following spec strings built into it. Spec files can
5402 override these strings or create their own. Note that individual
5403 targets can also add their own spec strings to this list.
5406 asm Options to pass to the assembler
5407 asm_final Options to pass to the assembler post-processor
5408 cpp Options to pass to the C preprocessor
5409 cc1 Options to pass to the C compiler
5410 cc1plus Options to pass to the C++ compiler
5411 endfile Object files to include at the end of the link
5412 link Options to pass to the linker
5413 lib Libraries to include on the command line to the linker
5414 libgcc Decides which GCC support library to pass to the linker
5415 linker Sets the name of the linker
5416 predefines Defines to be passed to the C preprocessor
5417 signed_char Defines to pass to CPP to say whether @code{char} is signed
5419 startfile Object files to include at the start of the link
5422 Here is a small example of a spec file:
5428 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
5431 This example renames the spec called @samp{lib} to @samp{old_lib} and
5432 then overrides the previous definition of @samp{lib} with a new one.
5433 The new definition adds in some extra command-line options before
5434 including the text of the old definition.
5436 @dfn{Spec strings} are a list of command-line options to be passed to their
5437 corresponding program. In addition, the spec strings can contain
5438 @samp{%}-prefixed sequences to substitute variable text or to
5439 conditionally insert text into the command line. Using these constructs
5440 it is possible to generate quite complex command lines.
5442 Here is a table of all defined @samp{%}-sequences for spec
5443 strings. Note that spaces are not generated automatically around the
5444 results of expanding these sequences. Therefore you can concatenate them
5445 together or combine them with constant text in a single argument.
5449 Substitute one @samp{%} into the program name or argument.
5452 Substitute the name of the input file being processed.
5455 Substitute the basename of the input file being processed.
5456 This is the substring up to (and not including) the last period
5457 and not including the directory.
5460 This is the same as @samp{%b}, but include the file suffix (text after
5464 Marks the argument containing or following the @samp{%d} as a
5465 temporary file name, so that that file will be deleted if GCC exits
5466 successfully. Unlike @samp{%g}, this contributes no text to the
5469 @item %g@var{suffix}
5470 Substitute a file name that has suffix @var{suffix} and is chosen
5471 once per compilation, and mark the argument in the same way as
5472 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
5473 name is now chosen in a way that is hard to predict even when previously
5474 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
5475 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
5476 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
5477 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
5478 was simply substituted with a file name chosen once per compilation,
5479 without regard to any appended suffix (which was therefore treated
5480 just like ordinary text), making such attacks more likely to succeed.
5482 @item %u@var{suffix}
5483 Like @samp{%g}, but generates a new temporary file name even if
5484 @samp{%u@var{suffix}} was already seen.
5486 @item %U@var{suffix}
5487 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
5488 new one if there is no such last file name. In the absence of any
5489 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
5490 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
5491 would involve the generation of two distinct file names, one
5492 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
5493 simply substituted with a file name chosen for the previous @samp{%u},
5494 without regard to any appended suffix.
5496 @item %j@var{suffix}
5497 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
5498 writable, and if save-temps is off; otherwise, substitute the name
5499 of a temporary file, just like @samp{%u}. This temporary file is not
5500 meant for communication between processes, but rather as a junk
5503 @item %|@var{suffix}
5504 @itemx %m@var{suffix}
5505 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
5506 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
5507 all. These are the two most common ways to instruct a program that it
5508 should read from standard input or write to standard output. If you
5509 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
5510 construct: see for example @file{f/lang-specs.h}.
5512 @item %.@var{SUFFIX}
5513 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
5514 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
5515 terminated by the next space or %.
5518 Marks the argument containing or following the @samp{%w} as the
5519 designated output file of this compilation. This puts the argument
5520 into the sequence of arguments that @samp{%o} will substitute later.
5523 Substitutes the names of all the output files, with spaces
5524 automatically placed around them. You should write spaces
5525 around the @samp{%o} as well or the results are undefined.
5526 @samp{%o} is for use in the specs for running the linker.
5527 Input files whose names have no recognized suffix are not compiled
5528 at all, but they are included among the output files, so they will
5532 Substitutes the suffix for object files. Note that this is
5533 handled specially when it immediately follows @samp{%g, %u, or %U},
5534 because of the need for those to form complete file names. The
5535 handling is such that @samp{%O} is treated exactly as if it had already
5536 been substituted, except that @samp{%g, %u, and %U} do not currently
5537 support additional @var{suffix} characters following @samp{%O} as they would
5538 following, for example, @samp{.o}.
5541 Substitutes the standard macro predefinitions for the
5542 current target machine. Use this when running @code{cpp}.
5545 Like @samp{%p}, but puts @samp{__} before and after the name of each
5546 predefined macro, except for macros that start with @samp{__} or with
5547 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
5551 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
5552 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}), and
5553 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
5557 Current argument is the name of a library or startup file of some sort.
5558 Search for that file in a standard list of directories and substitute
5559 the full name found.
5562 Print @var{str} as an error message. @var{str} is terminated by a newline.
5563 Use this when inconsistent options are detected.
5566 Substitute the contents of spec string @var{name} at this point.
5569 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
5571 @item %x@{@var{option}@}
5572 Accumulate an option for @samp{%X}.
5575 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
5579 Output the accumulated assembler options specified by @option{-Wa}.
5582 Output the accumulated preprocessor options specified by @option{-Wp}.
5585 Process the @code{asm} spec. This is used to compute the
5586 switches to be passed to the assembler.
5589 Process the @code{asm_final} spec. This is a spec string for
5590 passing switches to an assembler post-processor, if such a program is
5594 Process the @code{link} spec. This is the spec for computing the
5595 command line passed to the linker. Typically it will make use of the
5596 @samp{%L %G %S %D and %E} sequences.
5599 Dump out a @option{-L} option for each directory that GCC believes might
5600 contain startup files. If the target supports multilibs then the
5601 current multilib directory will be prepended to each of these paths.
5604 Output the multilib directory with directory separators replaced with
5605 @samp{_}. If multilib directories are not set, or the multilib directory is
5606 @file{.} then this option emits nothing.
5609 Process the @code{lib} spec. This is a spec string for deciding which
5610 libraries should be included on the command line to the linker.
5613 Process the @code{libgcc} spec. This is a spec string for deciding
5614 which GCC support library should be included on the command line to the linker.
5617 Process the @code{startfile} spec. This is a spec for deciding which
5618 object files should be the first ones passed to the linker. Typically
5619 this might be a file named @file{crt0.o}.
5622 Process the @code{endfile} spec. This is a spec string that specifies
5623 the last object files that will be passed to the linker.
5626 Process the @code{cpp} spec. This is used to construct the arguments
5627 to be passed to the C preprocessor.
5630 Process the @code{signed_char} spec. This is intended to be used
5631 to tell cpp whether a char is signed. It typically has the definition:
5633 %@{funsigned-char:-D__CHAR_UNSIGNED__@}
5637 Process the @code{cc1} spec. This is used to construct the options to be
5638 passed to the actual C compiler (@samp{cc1}).
5641 Process the @code{cc1plus} spec. This is used to construct the options to be
5642 passed to the actual C++ compiler (@samp{cc1plus}).
5645 Substitute the variable part of a matched option. See below.
5646 Note that each comma in the substituted string is replaced by
5650 Remove all occurrences of @code{-S} from the command line. Note---this
5651 command is position dependent. @samp{%} commands in the spec string
5652 before this one will see @code{-S}, @samp{%} commands in the spec string
5653 after this one will not.
5655 @item %:@var{function}(@var{args})
5656 Call the named function @var{function}, passing it @var{args}.
5657 @var{args} is first processed as a nested spec string, then split
5658 into an argument vector in the usual fashion. The function returns
5659 a string which is processed as if it had appeared literally as part
5660 of the current spec.
5662 The following built-in spec functions are provided:
5665 @item @code{if-exists}
5666 The @code{if-exists} spec function takes one argument, an absolute
5667 pathname to a file. If the file exists, @code{if-exists} returns the
5668 pathname. Here is a small example of its usage:
5672 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
5675 @item @code{if-exists-else}
5676 The @code{if-exists-else} spec function is similar to the @code{if-exists}
5677 spec function, except that it takes two arguments. The first argument is
5678 an absolute pathname to a file. If the file exists, @code{if-exists-else}
5679 returns the pathname. If it does not exist, it returns the second argument.
5680 This way, @code{if-exists-else} can be used to select one file or another,
5681 based on the existence of the first. Here is a small example of its usage:
5685 crt0%O%s %:if-exists(crti%O%s) \
5686 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
5691 Substitutes the @code{-S} switch, if that switch was given to GCC@.
5692 If that switch was not specified, this substitutes nothing. Note that
5693 the leading dash is omitted when specifying this option, and it is
5694 automatically inserted if the substitution is performed. Thus the spec
5695 string @samp{%@{foo@}} would match the command-line option @option{-foo}
5696 and would output the command line option @option{-foo}.
5698 @item %W@{@code{S}@}
5699 Like %@{@code{S}@} but mark last argument supplied within as a file to be
5702 @item %@{@code{S}*@}
5703 Substitutes all the switches specified to GCC whose names start
5704 with @code{-S}, but which also take an argument. This is used for
5705 switches like @option{-o}, @option{-D}, @option{-I}, etc.
5706 GCC considers @option{-o foo} as being
5707 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
5708 text, including the space. Thus two arguments would be generated.
5710 @item %@{@code{S}*&@code{T}*@}
5711 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
5712 (the order of @code{S} and @code{T} in the spec is not significant).
5713 There can be any number of ampersand-separated variables; for each the
5714 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
5716 @item %@{@code{S}:@code{X}@}
5717 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
5719 @item %@{!@code{S}:@code{X}@}
5720 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
5722 @item %@{@code{S}*:@code{X}@}
5723 Substitutes @code{X} if one or more switches whose names start with
5724 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
5725 once, no matter how many such switches appeared. However, if @code{%*}
5726 appears somewhere in @code{X}, then @code{X} will be substituted once
5727 for each matching switch, with the @code{%*} replaced by the part of
5728 that switch that matched the @code{*}.
5730 @item %@{.@code{S}:@code{X}@}
5731 Substitutes @code{X}, if processing a file with suffix @code{S}.
5733 @item %@{!.@code{S}:@code{X}@}
5734 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
5736 @item %@{@code{S}|@code{P}:@code{X}@}
5737 Substitutes @code{X} if either @code{-S} or @code{-P} was given to GCC@.
5738 This may be combined with @samp{!}, @samp{.}, and @code{*} sequences as well,
5739 although they have a stronger binding than the @samp{|}. If @code{%*}
5740 appears in @code{X}, all of the alternatives must be starred, and only
5741 the first matching alternative is substituted.
5743 For example, a spec string like this:
5746 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
5749 will output the following command-line options from the following input
5750 command-line options:
5755 -d fred.c -foo -baz -boggle
5756 -d jim.d -bar -baz -boggle
5759 @item %@{S:X; T:Y; :D@}
5761 If @code{S} was given to GCC, substitutes @code{X}; else if @code{T} was
5762 given to GCC, substitutes @code{Y}; else substitutes @code{D}. There can
5763 be as many clauses as you need. This may be combined with @code{.},
5764 @code{!}, @code{|}, and @code{*} as needed.
5769 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
5770 construct may contain other nested @samp{%} constructs or spaces, or
5771 even newlines. They are processed as usual, as described above.
5772 Trailing white space in @code{X} is ignored. White space may also
5773 appear anywhere on the left side of the colon in these constructs,
5774 except between @code{.} or @code{*} and the corresponding word.
5776 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
5777 handled specifically in these constructs. If another value of
5778 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
5779 @option{-W} switch is found later in the command line, the earlier
5780 switch value is ignored, except with @{@code{S}*@} where @code{S} is
5781 just one letter, which passes all matching options.
5783 The character @samp{|} at the beginning of the predicate text is used to
5784 indicate that a command should be piped to the following command, but
5785 only if @option{-pipe} is specified.
5787 It is built into GCC which switches take arguments and which do not.
5788 (You might think it would be useful to generalize this to allow each
5789 compiler's spec to say which switches take arguments. But this cannot
5790 be done in a consistent fashion. GCC cannot even decide which input
5791 files have been specified without knowing which switches take arguments,
5792 and it must know which input files to compile in order to tell which
5795 GCC also knows implicitly that arguments starting in @option{-l} are to be
5796 treated as compiler output files, and passed to the linker in their
5797 proper position among the other output files.
5799 @c man begin OPTIONS
5801 @node Target Options
5802 @section Specifying Target Machine and Compiler Version
5803 @cindex target options
5804 @cindex cross compiling
5805 @cindex specifying machine version
5806 @cindex specifying compiler version and target machine
5807 @cindex compiler version, specifying
5808 @cindex target machine, specifying
5810 The usual way to run GCC is to run the executable called @file{gcc}, or
5811 @file{<machine>-gcc} when cross-compiling, or
5812 @file{<machine>-gcc-<version>} to run a version other than the one that
5813 was installed last. Sometimes this is inconvenient, so GCC provides
5814 options that will switch to another cross-compiler or version.
5817 @item -b @var{machine}
5819 The argument @var{machine} specifies the target machine for compilation.
5821 The value to use for @var{machine} is the same as was specified as the
5822 machine type when configuring GCC as a cross-compiler. For
5823 example, if a cross-compiler was configured with @samp{configure
5824 i386v}, meaning to compile for an 80386 running System V, then you
5825 would specify @option{-b i386v} to run that cross compiler.
5827 @item -V @var{version}
5829 The argument @var{version} specifies which version of GCC to run.
5830 This is useful when multiple versions are installed. For example,
5831 @var{version} might be @samp{2.0}, meaning to run GCC version 2.0.
5834 The @option{-V} and @option{-b} options work by running the
5835 @file{<machine>-gcc-<version>} executable, so there's no real reason to
5836 use them if you can just run that directly.
5838 @node Submodel Options
5839 @section Hardware Models and Configurations
5840 @cindex submodel options
5841 @cindex specifying hardware config
5842 @cindex hardware models and configurations, specifying
5843 @cindex machine dependent options
5845 Earlier we discussed the standard option @option{-b} which chooses among
5846 different installed compilers for completely different target
5847 machines, such as VAX vs.@: 68000 vs.@: 80386.
5849 In addition, each of these target machine types can have its own
5850 special options, starting with @samp{-m}, to choose among various
5851 hardware models or configurations---for example, 68010 vs 68020,
5852 floating coprocessor or none. A single installed version of the
5853 compiler can compile for any model or configuration, according to the
5856 Some configurations of the compiler also support additional special
5857 options, usually for compatibility with other compilers on the same
5860 These options are defined by the macro @code{TARGET_SWITCHES} in the
5861 machine description. The default for the options is also defined by
5862 that macro, which enables you to change the defaults.
5872 * RS/6000 and PowerPC Options::
5875 * i386 and x86-64 Options::
5877 * DEC Alpha Options::
5878 * DEC Alpha/VMS Options::
5881 * System V Options::
5882 * TMS320C3x/C4x Options::
5889 * S/390 and zSeries Options::
5893 * Xstormy16 Options::
5898 @node M680x0 Options
5899 @subsection M680x0 Options
5900 @cindex M680x0 options
5902 These are the @samp{-m} options defined for the 68000 series. The default
5903 values for these options depends on which style of 68000 was selected when
5904 the compiler was configured; the defaults for the most common choices are
5912 Generate output for a 68000. This is the default
5913 when the compiler is configured for 68000-based systems.
5915 Use this option for microcontrollers with a 68000 or EC000 core,
5916 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
5922 Generate output for a 68020. This is the default
5923 when the compiler is configured for 68020-based systems.
5927 Generate output containing 68881 instructions for floating point.
5928 This is the default for most 68020 systems unless @option{--nfp} was
5929 specified when the compiler was configured.
5933 Generate output for a 68030. This is the default when the compiler is
5934 configured for 68030-based systems.
5938 Generate output for a 68040. This is the default when the compiler is
5939 configured for 68040-based systems.
5941 This option inhibits the use of 68881/68882 instructions that have to be
5942 emulated by software on the 68040. Use this option if your 68040 does not
5943 have code to emulate those instructions.
5947 Generate output for a 68060. This is the default when the compiler is
5948 configured for 68060-based systems.
5950 This option inhibits the use of 68020 and 68881/68882 instructions that
5951 have to be emulated by software on the 68060. Use this option if your 68060
5952 does not have code to emulate those instructions.
5956 Generate output for a CPU32. This is the default
5957 when the compiler is configured for CPU32-based systems.
5959 Use this option for microcontrollers with a
5960 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
5961 68336, 68340, 68341, 68349 and 68360.
5965 Generate output for a 520X ``coldfire'' family cpu. This is the default
5966 when the compiler is configured for 520X-based systems.
5968 Use this option for microcontroller with a 5200 core, including
5969 the MCF5202, MCF5203, MCF5204 and MCF5202.
5974 Generate output for a 68040, without using any of the new instructions.
5975 This results in code which can run relatively efficiently on either a
5976 68020/68881 or a 68030 or a 68040. The generated code does use the
5977 68881 instructions that are emulated on the 68040.
5981 Generate output for a 68060, without using any of the new instructions.
5982 This results in code which can run relatively efficiently on either a
5983 68020/68881 or a 68030 or a 68040. The generated code does use the
5984 68881 instructions that are emulated on the 68060.
5987 @opindex msoft-float
5988 Generate output containing library calls for floating point.
5989 @strong{Warning:} the requisite libraries are not available for all m68k
5990 targets. Normally the facilities of the machine's usual C compiler are
5991 used, but this can't be done directly in cross-compilation. You must
5992 make your own arrangements to provide suitable library functions for
5993 cross-compilation. The embedded targets @samp{m68k-*-aout} and
5994 @samp{m68k-*-coff} do provide software floating point support.
5998 Consider type @code{int} to be 16 bits wide, like @code{short int}.
6001 @opindex mnobitfield
6002 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
6003 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
6007 Do use the bit-field instructions. The @option{-m68020} option implies
6008 @option{-mbitfield}. This is the default if you use a configuration
6009 designed for a 68020.
6013 Use a different function-calling convention, in which functions
6014 that take a fixed number of arguments return with the @code{rtd}
6015 instruction, which pops their arguments while returning. This
6016 saves one instruction in the caller since there is no need to pop
6017 the arguments there.
6019 This calling convention is incompatible with the one normally
6020 used on Unix, so you cannot use it if you need to call libraries
6021 compiled with the Unix compiler.
6023 Also, you must provide function prototypes for all functions that
6024 take variable numbers of arguments (including @code{printf});
6025 otherwise incorrect code will be generated for calls to those
6028 In addition, seriously incorrect code will result if you call a
6029 function with too many arguments. (Normally, extra arguments are
6030 harmlessly ignored.)
6032 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
6033 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
6036 @itemx -mno-align-int
6038 @opindex mno-align-int
6039 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
6040 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
6041 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
6042 Aligning variables on 32-bit boundaries produces code that runs somewhat
6043 faster on processors with 32-bit busses at the expense of more memory.
6045 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
6046 align structures containing the above types differently than
6047 most published application binary interface specifications for the m68k.
6051 Use the pc-relative addressing mode of the 68000 directly, instead of
6052 using a global offset table. At present, this option implies @option{-fpic},
6053 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
6054 not presently supported with @option{-mpcrel}, though this could be supported for
6055 68020 and higher processors.
6057 @item -mno-strict-align
6058 @itemx -mstrict-align
6059 @opindex mno-strict-align
6060 @opindex mstrict-align
6061 Do not (do) assume that unaligned memory references will be handled by
6065 Generate code that allows the data segment to be located in a different
6066 area of memory from the text segment. This allows for execute in place in
6067 an environment without virtual memory management. This option implies -fPIC.
6070 Generate code that assumes that the data segment follows the text segment.
6071 This is the default.
6073 @item -mid-shared-library
6074 Generate code that supports shared libraries via the library ID method.
6075 This allows for execute in place and shared libraries in an environment
6076 without virtual memory management. This option implies -fPIC.
6078 @item -mno-id-shared-library
6079 Generate code that doesn't assume ID based shared libraries are being used.
6080 This is the default.
6082 @item -mshared-library-id=n
6083 Specified the identification number of the ID based shared library being
6084 compiled. Specifying a value of 0 will generate more compact code, specifying
6085 other values will force the allocation of that number to the current
6086 library but is no more space or time efficient than omitting this option.
6090 @node M68hc1x Options
6091 @subsection M68hc1x Options
6092 @cindex M68hc1x options
6094 These are the @samp{-m} options defined for the 68hc11 and 68hc12
6095 microcontrollers. The default values for these options depends on
6096 which style of microcontroller was selected when the compiler was configured;
6097 the defaults for the most common choices are given below.
6104 Generate output for a 68HC11. This is the default
6105 when the compiler is configured for 68HC11-based systems.
6111 Generate output for a 68HC12. This is the default
6112 when the compiler is configured for 68HC12-based systems.
6118 Generate output for a 68HCS12.
6121 @opindex mauto-incdec
6122 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
6129 Enable the use of 68HC12 min and max instructions.
6132 @itemx -mno-long-calls
6133 @opindex mlong-calls
6134 @opindex mno-long-calls
6135 Treat all calls as being far away (near). If calls are assumed to be
6136 far away, the compiler will use the @code{call} instruction to
6137 call a function and the @code{rtc} instruction for returning.
6141 Consider type @code{int} to be 16 bits wide, like @code{short int}.
6143 @item -msoft-reg-count=@var{count}
6144 @opindex msoft-reg-count
6145 Specify the number of pseudo-soft registers which are used for the
6146 code generation. The maximum number is 32. Using more pseudo-soft
6147 register may or may not result in better code depending on the program.
6148 The default is 4 for 68HC11 and 2 for 68HC12.
6153 @subsection VAX Options
6156 These @samp{-m} options are defined for the VAX:
6161 Do not output certain jump instructions (@code{aobleq} and so on)
6162 that the Unix assembler for the VAX cannot handle across long
6167 Do output those jump instructions, on the assumption that you
6168 will assemble with the GNU assembler.
6172 Output code for g-format floating point numbers instead of d-format.
6176 @subsection SPARC Options
6177 @cindex SPARC options
6179 These @samp{-m} options are supported on the SPARC:
6184 @opindex mno-app-regs
6186 Specify @option{-mapp-regs} to generate output using the global registers
6187 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
6190 To be fully SVR4 ABI compliant at the cost of some performance loss,
6191 specify @option{-mno-app-regs}. You should compile libraries and system
6192 software with this option.
6197 @opindex mhard-float
6198 Generate output containing floating point instructions. This is the
6204 @opindex msoft-float
6205 Generate output containing library calls for floating point.
6206 @strong{Warning:} the requisite libraries are not available for all SPARC
6207 targets. Normally the facilities of the machine's usual C compiler are
6208 used, but this cannot be done directly in cross-compilation. You must make
6209 your own arrangements to provide suitable library functions for
6210 cross-compilation. The embedded targets @samp{sparc-*-aout} and
6211 @samp{sparclite-*-*} do provide software floating point support.
6213 @option{-msoft-float} changes the calling convention in the output file;
6214 therefore, it is only useful if you compile @emph{all} of a program with
6215 this option. In particular, you need to compile @file{libgcc.a}, the
6216 library that comes with GCC, with @option{-msoft-float} in order for
6219 @item -mhard-quad-float
6220 @opindex mhard-quad-float
6221 Generate output containing quad-word (long double) floating point
6224 @item -msoft-quad-float
6225 @opindex msoft-quad-float
6226 Generate output containing library calls for quad-word (long double)
6227 floating point instructions. The functions called are those specified
6228 in the SPARC ABI@. This is the default.
6230 As of this writing, there are no SPARC implementations that have hardware
6231 support for the quad-word floating point instructions. They all invoke
6232 a trap handler for one of these instructions, and then the trap handler
6233 emulates the effect of the instruction. Because of the trap handler overhead,
6234 this is much slower than calling the ABI library routines. Thus the
6235 @option{-msoft-quad-float} option is the default.
6237 @item -mno-unaligned-doubles
6238 @itemx -munaligned-doubles
6239 @opindex mno-unaligned-doubles
6240 @opindex munaligned-doubles
6241 Assume that doubles have 8 byte alignment. This is the default.
6243 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
6244 alignment only if they are contained in another type, or if they have an
6245 absolute address. Otherwise, it assumes they have 4 byte alignment.
6246 Specifying this option avoids some rare compatibility problems with code
6247 generated by other compilers. It is not the default because it results
6248 in a performance loss, especially for floating point code.
6250 @item -mno-faster-structs
6251 @itemx -mfaster-structs
6252 @opindex mno-faster-structs
6253 @opindex mfaster-structs
6254 With @option{-mfaster-structs}, the compiler assumes that structures
6255 should have 8 byte alignment. This enables the use of pairs of
6256 @code{ldd} and @code{std} instructions for copies in structure
6257 assignment, in place of twice as many @code{ld} and @code{st} pairs.
6258 However, the use of this changed alignment directly violates the SPARC
6259 ABI@. Thus, it's intended only for use on targets where the developer
6260 acknowledges that their resulting code will not be directly in line with
6261 the rules of the ABI@.
6264 @opindex mimpure-text
6265 @option{-mimpure-text}, used in addition to @option{-shared}, tells
6266 the compiler to not pass @option{-z text} to the linker when linking a
6267 shared object. Using this option, you can link position-dependent
6268 code into a shared object.
6270 @option{-mimpure-text} suppresses the ``relocations remain against
6271 allocatable but non-writable sections'' linker error message.
6272 However, the necessary relocations will trigger copy-on-write, and the
6273 shared object is not actually shared across processes. Instead of
6274 using @option{-mimpure-text}, you should compile all source code with
6275 @option{-fpic} or @option{-fPIC}.
6277 This option is only available on SunOS and Solaris.
6279 @item -mcpu=@var{cpu_type}
6281 Set the instruction set, register set, and instruction scheduling parameters
6282 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
6283 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
6284 @samp{f930}, @samp{f934}, @samp{hypersparc}, @samp{sparclite86x},
6285 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc}, and
6288 Default instruction scheduling parameters are used for values that select
6289 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
6290 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
6292 Here is a list of each supported architecture and their supported
6297 v8: supersparc, hypersparc
6298 sparclite: f930, f934, sparclite86x
6300 v9: ultrasparc, ultrasparc3
6303 By default (unless configured otherwise), GCC generates code for the V7
6304 variant of the SPARC architecture. With @option{-mcpu=cypress}, the compiler
6305 additionally optimizes it for the Cypress CY7C602 chip, as used in the
6306 SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
6307 SPARCStation 1, 2, IPX etc.
6309 With @option{-mcpu=v8}, GCC generates code for the V8 variant of the SPARC
6310 architecture. The only difference from V7 code is that the compiler emits
6311 the integer multiply and integer divide instructions which exist in SPARC-V8
6312 but not in SPARC-V7. With @option{-mcpu=supersparc}, the compiler additionally
6313 optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
6316 With @option{-mcpu=sparclite}, GCC generates code for the SPARClite variant of
6317 the SPARC architecture. This adds the integer multiply, integer divide step
6318 and scan (@code{ffs}) instructions which exist in SPARClite but not in SPARC-V7.
6319 With @option{-mcpu=f930}, the compiler additionally optimizes it for the
6320 Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With
6321 @option{-mcpu=f934}, the compiler additionally optimizes it for the Fujitsu
6322 MB86934 chip, which is the more recent SPARClite with FPU.
6324 With @option{-mcpu=sparclet}, GCC generates code for the SPARClet variant of
6325 the SPARC architecture. This adds the integer multiply, multiply/accumulate,
6326 integer divide step and scan (@code{ffs}) instructions which exist in SPARClet
6327 but not in SPARC-V7. With @option{-mcpu=tsc701}, the compiler additionally
6328 optimizes it for the TEMIC SPARClet chip.
6330 With @option{-mcpu=v9}, GCC generates code for the V9 variant of the SPARC
6331 architecture. This adds 64-bit integer and floating-point move instructions,
6332 3 additional floating-point condition code registers and conditional move
6333 instructions. With @option{-mcpu=ultrasparc}, the compiler additionally
6334 optimizes it for the Sun UltraSPARC I/II chips. With
6335 @option{-mcpu=ultrasparc3}, the compiler additionally optimizes it for the
6336 Sun UltraSPARC III chip.
6338 @item -mtune=@var{cpu_type}
6340 Set the instruction scheduling parameters for machine type
6341 @var{cpu_type}, but do not set the instruction set or register set that the
6342 option @option{-mcpu=@var{cpu_type}} would.
6344 The same values for @option{-mcpu=@var{cpu_type}} can be used for
6345 @option{-mtune=@var{cpu_type}}, but the only useful values are those
6346 that select a particular cpu implementation. Those are @samp{cypress},
6347 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
6348 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc}, and
6354 @opindex -mno-v8plus
6355 With @option{-mv8plus}, GCC generates code for the SPARC-V8+ ABI. The
6356 difference from the V8 ABI is that the global and out registers are
6357 considered 64-bit wide. This is enabled by default on Solaris in 32-bit
6358 mode for all SPARC-V9 processors.
6364 With @option{-mvis}, GCC generates code that takes advantage of the UltraSPARC
6365 Visual Instruction Set extensions. The default is @option{-mno-vis}.
6368 These @samp{-m} options are supported in addition to the above
6369 on SPARC-V9 processors in 64-bit environments:
6372 @item -mlittle-endian
6373 @opindex mlittle-endian
6374 Generate code for a processor running in little-endian mode. It is only
6375 available for a few configurations and most notably not on Solaris.
6381 Generate code for a 32-bit or 64-bit environment.
6382 The 32-bit environment sets int, long and pointer to 32 bits.
6383 The 64-bit environment sets int to 32 bits and long and pointer
6386 @item -mcmodel=medlow
6387 @opindex mcmodel=medlow
6388 Generate code for the Medium/Low code model: 64-bit addresses, programs
6389 must be linked in the low 32 bits of memory. Programs can be statically
6390 or dynamically linked.
6392 @item -mcmodel=medmid
6393 @opindex mcmodel=medmid
6394 Generate code for the Medium/Middle code model: 64-bit addresses, programs
6395 must be linked in the low 44 bits of memory, the text and data segments must
6396 be less than 2GB in size and the data segment must be located within 2GB of
6399 @item -mcmodel=medany
6400 @opindex mcmodel=medany
6401 Generate code for the Medium/Anywhere code model: 64-bit addresses, programs
6402 may be linked anywhere in memory, the text and data segments must be less
6403 than 2GB in size and the data segment must be located within 2GB of the
6406 @item -mcmodel=embmedany
6407 @opindex mcmodel=embmedany
6408 Generate code for the Medium/Anywhere code model for embedded systems:
6409 64-bit addresses, the text and data segments must be less than 2GB in
6410 size, both starting anywhere in memory (determined at link time). The
6411 global register %g4 points to the base of the data segment. Programs
6412 are statically linked and PIC is not supported.
6415 @itemx -mno-stack-bias
6416 @opindex mstack-bias
6417 @opindex mno-stack-bias
6418 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
6419 frame pointer if present, are offset by @minus{}2047 which must be added back
6420 when making stack frame references. This is the default in 64-bit mode.
6421 Otherwise, assume no such offset is present.
6425 @subsection ARM Options
6428 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
6432 @item -mabi=@var{name}
6434 Generate code for the specified ABI. Permissible values are: @samp{apcs-gnu},
6435 @samp{atpcs}, @samp{aapcs} and @samp{iwmmxt}.
6438 @opindex mapcs-frame
6439 Generate a stack frame that is compliant with the ARM Procedure Call
6440 Standard for all functions, even if this is not strictly necessary for
6441 correct execution of the code. Specifying @option{-fomit-frame-pointer}
6442 with this option will cause the stack frames not to be generated for
6443 leaf functions. The default is @option{-mno-apcs-frame}.
6447 This is a synonym for @option{-mapcs-frame}.
6451 Generate code for a processor running with a 26-bit program counter,
6452 and conforming to the function calling standards for the APCS 26-bit
6453 option. This option replaces the @option{-m2} and @option{-m3} options
6454 of previous releases of the compiler.
6458 Generate code for a processor running with a 32-bit program counter,
6459 and conforming to the function calling standards for the APCS 32-bit
6460 option. This option replaces the @option{-m6} option of previous releases
6464 @c not currently implemented
6465 @item -mapcs-stack-check
6466 @opindex mapcs-stack-check
6467 Generate code to check the amount of stack space available upon entry to
6468 every function (that actually uses some stack space). If there is
6469 insufficient space available then either the function
6470 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
6471 called, depending upon the amount of stack space required. The run time
6472 system is required to provide these functions. The default is
6473 @option{-mno-apcs-stack-check}, since this produces smaller code.
6475 @c not currently implemented
6477 @opindex mapcs-float
6478 Pass floating point arguments using the float point registers. This is
6479 one of the variants of the APCS@. This option is recommended if the
6480 target hardware has a floating point unit or if a lot of floating point
6481 arithmetic is going to be performed by the code. The default is
6482 @option{-mno-apcs-float}, since integer only code is slightly increased in
6483 size if @option{-mapcs-float} is used.
6485 @c not currently implemented
6486 @item -mapcs-reentrant
6487 @opindex mapcs-reentrant
6488 Generate reentrant, position independent code. The default is
6489 @option{-mno-apcs-reentrant}.
6492 @item -mthumb-interwork
6493 @opindex mthumb-interwork
6494 Generate code which supports calling between the ARM and Thumb
6495 instruction sets. Without this option the two instruction sets cannot
6496 be reliably used inside one program. The default is
6497 @option{-mno-thumb-interwork}, since slightly larger code is generated
6498 when @option{-mthumb-interwork} is specified.
6500 @item -mno-sched-prolog
6501 @opindex mno-sched-prolog
6502 Prevent the reordering of instructions in the function prolog, or the
6503 merging of those instruction with the instructions in the function's
6504 body. This means that all functions will start with a recognizable set
6505 of instructions (or in fact one of a choice from a small set of
6506 different function prologues), and this information can be used to
6507 locate the start if functions inside an executable piece of code. The
6508 default is @option{-msched-prolog}.
6511 @opindex mhard-float
6512 Generate output containing floating point instructions. This is the
6516 @opindex msoft-float
6517 Generate output containing library calls for floating point.
6518 @strong{Warning:} the requisite libraries are not available for all ARM
6519 targets. Normally the facilities of the machine's usual C compiler are
6520 used, but this cannot be done directly in cross-compilation. You must make
6521 your own arrangements to provide suitable library functions for
6524 @option{-msoft-float} changes the calling convention in the output file;
6525 therefore, it is only useful if you compile @emph{all} of a program with
6526 this option. In particular, you need to compile @file{libgcc.a}, the
6527 library that comes with GCC, with @option{-msoft-float} in order for
6530 @item -mfloat-abi=@var{name}
6532 Specifies which ABI to use for floating point values. Permissible values
6533 are: @samp{soft}, @samp{softfp} and @samp{hard}.
6535 @samp{soft} and @samp{hard} are equivalent to @option{-msoft-float}
6536 and @option{-mhard-float} respectively. @samp{softfp} allows the generation
6537 of floating point instructions, but still uses the soft-float calling
6540 @item -mlittle-endian
6541 @opindex mlittle-endian
6542 Generate code for a processor running in little-endian mode. This is
6543 the default for all standard configurations.
6546 @opindex mbig-endian
6547 Generate code for a processor running in big-endian mode; the default is
6548 to compile code for a little-endian processor.
6550 @item -mwords-little-endian
6551 @opindex mwords-little-endian
6552 This option only applies when generating code for big-endian processors.
6553 Generate code for a little-endian word order but a big-endian byte
6554 order. That is, a byte order of the form @samp{32107654}. Note: this
6555 option should only be used if you require compatibility with code for
6556 big-endian ARM processors generated by versions of the compiler prior to
6559 @item -malignment-traps
6560 @opindex malignment-traps
6561 Generate code that will not trap if the MMU has alignment traps enabled.
6562 On ARM architectures prior to ARMv4, there were no instructions to
6563 access half-word objects stored in memory. However, when reading from
6564 memory a feature of the ARM architecture allows a word load to be used,
6565 even if the address is unaligned, and the processor core will rotate the
6566 data as it is being loaded. This option tells the compiler that such
6567 misaligned accesses will cause a MMU trap and that it should instead
6568 synthesize the access as a series of byte accesses. The compiler can
6569 still use word accesses to load half-word data if it knows that the
6570 address is aligned to a word boundary.
6572 This option is ignored when compiling for ARM architecture 4 or later,
6573 since these processors have instructions to directly access half-word
6576 @item -mno-alignment-traps
6577 @opindex mno-alignment-traps
6578 Generate code that assumes that the MMU will not trap unaligned
6579 accesses. This produces better code when the target instruction set
6580 does not have half-word memory operations (i.e.@: implementations prior to
6583 Note that you cannot use this option to access unaligned word objects,
6584 since the processor will only fetch one 32-bit aligned object from
6587 The default setting for most targets is @option{-mno-alignment-traps}, since
6588 this produces better code when there are no half-word memory
6589 instructions available.
6591 @item -mshort-load-bytes
6592 @itemx -mno-short-load-words
6593 @opindex mshort-load-bytes
6594 @opindex mno-short-load-words
6595 These are deprecated aliases for @option{-malignment-traps}.
6597 @item -mno-short-load-bytes
6598 @itemx -mshort-load-words
6599 @opindex mno-short-load-bytes
6600 @opindex mshort-load-words
6601 This are deprecated aliases for @option{-mno-alignment-traps}.
6603 @item -mcpu=@var{name}
6605 This specifies the name of the target ARM processor. GCC uses this name
6606 to determine what kind of instructions it can emit when generating
6607 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
6608 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
6609 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
6610 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
6611 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
6612 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm8},
6613 @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
6614 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
6615 @samp{arm920t}, @samp{arm926ejs}, @samp{arm940t}, @samp{arm9tdmi},
6616 @samp{arm10tdmi}, @samp{arm1020t}, @samp{arm1026ejs},
6617 @samp{arm1136js}, @samp{arm1136jfs} ,@samp{xscale}, @samp{iwmmxt},
6620 @itemx -mtune=@var{name}
6622 This option is very similar to the @option{-mcpu=} option, except that
6623 instead of specifying the actual target processor type, and hence
6624 restricting which instructions can be used, it specifies that GCC should
6625 tune the performance of the code as if the target were of the type
6626 specified in this option, but still choosing the instructions that it
6627 will generate based on the cpu specified by a @option{-mcpu=} option.
6628 For some ARM implementations better performance can be obtained by using
6631 @item -march=@var{name}
6633 This specifies the name of the target ARM architecture. GCC uses this
6634 name to determine what kind of instructions it can emit when generating
6635 assembly code. This option can be used in conjunction with or instead
6636 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
6637 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
6638 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{armv6}, @samp{armv6j},
6639 @samp{iwmmxt}, @samp{ep9312}.
6641 @item -mfpu=@var{name}
6642 @itemx -mfpe=@var{number}
6643 @itemx -mfp=@var{number}
6647 This specifies what floating point hardware (or hardware emulation) is
6648 available on the target. Permissible names are: @samp{fpa}, @samp{fpe2},
6649 @samp{fpe3}, @samp{maverick}, @samp{vfp}. @option{-mfp} and @option{-mfpe}
6650 are synonyms for @option{-mfpu}=@samp{fpe}@var{number}, for compatibility
6651 with older versions of GCC@.
6653 If @option{-msoft-float} is specified this specifies the format of
6654 floating point values.
6656 @item -mstructure-size-boundary=@var{n}
6657 @opindex mstructure-size-boundary
6658 The size of all structures and unions will be rounded up to a multiple
6659 of the number of bits set by this option. Permissible values are 8, 32
6660 and 64. The default value varies for different toolchains. For the COFF
6661 targeted toolchain the default value is 8. A value of 64 is only allowed
6662 if the underlying ABI supports it.
6664 Specifying the larger number can produce faster, more efficient code, but
6665 can also increase the size of the program. Different values are potentially
6666 incompatible. Code compiled with one value cannot necessarily expect to
6667 work with code or libraries compiled with another value, if they exchange
6668 information using structures or unions.
6670 @item -mabort-on-noreturn
6671 @opindex mabort-on-noreturn
6672 Generate a call to the function @code{abort} at the end of a
6673 @code{noreturn} function. It will be executed if the function tries to
6677 @itemx -mno-long-calls
6678 @opindex mlong-calls
6679 @opindex mno-long-calls
6680 Tells the compiler to perform function calls by first loading the
6681 address of the function into a register and then performing a subroutine
6682 call on this register. This switch is needed if the target function
6683 will lie outside of the 64 megabyte addressing range of the offset based
6684 version of subroutine call instruction.
6686 Even if this switch is enabled, not all function calls will be turned
6687 into long calls. The heuristic is that static functions, functions
6688 which have the @samp{short-call} attribute, functions that are inside
6689 the scope of a @samp{#pragma no_long_calls} directive and functions whose
6690 definitions have already been compiled within the current compilation
6691 unit, will not be turned into long calls. The exception to this rule is
6692 that weak function definitions, functions with the @samp{long-call}
6693 attribute or the @samp{section} attribute, and functions that are within
6694 the scope of a @samp{#pragma long_calls} directive, will always be
6695 turned into long calls.
6697 This feature is not enabled by default. Specifying
6698 @option{-mno-long-calls} will restore the default behavior, as will
6699 placing the function calls within the scope of a @samp{#pragma
6700 long_calls_off} directive. Note these switches have no effect on how
6701 the compiler generates code to handle function calls via function
6704 @item -mnop-fun-dllimport
6705 @opindex mnop-fun-dllimport
6706 Disable support for the @code{dllimport} attribute.
6708 @item -msingle-pic-base
6709 @opindex msingle-pic-base
6710 Treat the register used for PIC addressing as read-only, rather than
6711 loading it in the prologue for each function. The run-time system is
6712 responsible for initializing this register with an appropriate value
6713 before execution begins.
6715 @item -mpic-register=@var{reg}
6716 @opindex mpic-register
6717 Specify the register to be used for PIC addressing. The default is R10
6718 unless stack-checking is enabled, when R9 is used.
6720 @item -mcirrus-fix-invalid-insns
6721 @opindex mcirrus-fix-invalid-insns
6722 @opindex mno-cirrus-fix-invalid-insns
6723 Insert NOPs into the instruction stream to in order to work around
6724 problems with invalid Maverick instruction combinations. This option
6725 is only valid if the @option{-mcpu=ep9312} option has been used to
6726 enable generation of instructions for the Cirrus Maverick floating
6727 point co-processor. This option is not enabled by default, since the
6728 problem is only present in older Maverick implementations. The default
6729 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
6732 @item -mpoke-function-name
6733 @opindex mpoke-function-name
6734 Write the name of each function into the text section, directly
6735 preceding the function prologue. The generated code is similar to this:
6739 .ascii "arm_poke_function_name", 0
6742 .word 0xff000000 + (t1 - t0)
6743 arm_poke_function_name
6745 stmfd sp!, @{fp, ip, lr, pc@}
6749 When performing a stack backtrace, code can inspect the value of
6750 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
6751 location @code{pc - 12} and the top 8 bits are set, then we know that
6752 there is a function name embedded immediately preceding this location
6753 and has length @code{((pc[-3]) & 0xff000000)}.
6757 Generate code for the 16-bit Thumb instruction set. The default is to
6758 use the 32-bit ARM instruction set.
6761 @opindex mtpcs-frame
6762 Generate a stack frame that is compliant with the Thumb Procedure Call
6763 Standard for all non-leaf functions. (A leaf function is one that does
6764 not call any other functions.) The default is @option{-mno-tpcs-frame}.
6766 @item -mtpcs-leaf-frame
6767 @opindex mtpcs-leaf-frame
6768 Generate a stack frame that is compliant with the Thumb Procedure Call
6769 Standard for all leaf functions. (A leaf function is one that does
6770 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
6772 @item -mcallee-super-interworking
6773 @opindex mcallee-super-interworking
6774 Gives all externally visible functions in the file being compiled an ARM
6775 instruction set header which switches to Thumb mode before executing the
6776 rest of the function. This allows these functions to be called from
6777 non-interworking code.
6779 @item -mcaller-super-interworking
6780 @opindex mcaller-super-interworking
6781 Allows calls via function pointers (including virtual functions) to
6782 execute correctly regardless of whether the target code has been
6783 compiled for interworking or not. There is a small overhead in the cost
6784 of executing a function pointer if this option is enabled.
6788 @node MN10300 Options
6789 @subsection MN10300 Options
6790 @cindex MN10300 options
6792 These @option{-m} options are defined for Matsushita MN10300 architectures:
6797 Generate code to avoid bugs in the multiply instructions for the MN10300
6798 processors. This is the default.
6801 @opindex mno-mult-bug
6802 Do not generate code to avoid bugs in the multiply instructions for the
6807 Generate code which uses features specific to the AM33 processor.
6811 Do not generate code which uses features specific to the AM33 processor. This
6816 Do not link in the C run-time initialization object file.
6820 Indicate to the linker that it should perform a relaxation optimization pass
6821 to shorten branches, calls and absolute memory addresses. This option only
6822 has an effect when used on the command line for the final link step.
6824 This option makes symbolic debugging impossible.
6828 @node M32R/D Options
6829 @subsection M32R/D Options
6830 @cindex M32R/D options
6832 These @option{-m} options are defined for Renesas M32R/D architectures:
6837 Generate code for the M32R/2@.
6841 Generate code for the M32R/X@.
6845 Generate code for the M32R@. This is the default.
6848 @opindex mmodel=small
6849 Assume all objects live in the lower 16MB of memory (so that their addresses
6850 can be loaded with the @code{ld24} instruction), and assume all subroutines
6851 are reachable with the @code{bl} instruction.
6852 This is the default.
6854 The addressability of a particular object can be set with the
6855 @code{model} attribute.
6857 @item -mmodel=medium
6858 @opindex mmodel=medium
6859 Assume objects may be anywhere in the 32-bit address space (the compiler
6860 will generate @code{seth/add3} instructions to load their addresses), and
6861 assume all subroutines are reachable with the @code{bl} instruction.
6864 @opindex mmodel=large
6865 Assume objects may be anywhere in the 32-bit address space (the compiler
6866 will generate @code{seth/add3} instructions to load their addresses), and
6867 assume subroutines may not be reachable with the @code{bl} instruction
6868 (the compiler will generate the much slower @code{seth/add3/jl}
6869 instruction sequence).
6872 @opindex msdata=none
6873 Disable use of the small data area. Variables will be put into
6874 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
6875 @code{section} attribute has been specified).
6876 This is the default.
6878 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
6879 Objects may be explicitly put in the small data area with the
6880 @code{section} attribute using one of these sections.
6883 @opindex msdata=sdata
6884 Put small global and static data in the small data area, but do not
6885 generate special code to reference them.
6889 Put small global and static data in the small data area, and generate
6890 special instructions to reference them.
6894 @cindex smaller data references
6895 Put global and static objects less than or equal to @var{num} bytes
6896 into the small data or bss sections instead of the normal data or bss
6897 sections. The default value of @var{num} is 8.
6898 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
6899 for this option to have any effect.
6901 All modules should be compiled with the same @option{-G @var{num}} value.
6902 Compiling with different values of @var{num} may or may not work; if it
6903 doesn't the linker will give an error message---incorrect code will not be
6908 Makes the M32R specific code in the compiler display some statistics
6909 that might help in debugging programs.
6912 @opindex malign-loops
6913 Align all loops to a 32-byte boundary.
6915 @item -mno-align-loops
6916 @opindex mno-align-loops
6917 Do not enforce a 32-byte alignment for loops. This is the default.
6919 @item -missue-rate=@var{number}
6920 @opindex missue-rate=@var{number}
6921 Issue @var{number} instructions per cycle. @var{number} can only be 1
6924 @item -mbranch-cost=@var{number}
6925 @opindex mbranch-cost=@var{number}
6926 @var{number} can only be 1 or 2. If it is 1 then branches will be
6927 preferred over conditional code, if it is 2, then the opposite will
6930 @item -mflush-trap=@var{number}
6931 @opindex mflush-trap=@var{number}
6932 Specifies the trap number to use to flush the cache. The default is
6933 12. Valid numbers are between 0 and 15 inclusive.
6935 @item -mno-flush-trap
6936 @opindex mno-flush-trap
6937 Specifies that the cache cannot be flushed by using a trap.
6939 @item -mflush-func=@var{name}
6940 @opindex mflush-func=@var{name}
6941 Specifies the name of the operating system function to call to flush
6942 the cache. The default is @emph{_flush_cache}, but a function call
6943 will only be used if a trap is not available.
6945 @item -mno-flush-func
6946 @opindex mno-flush-func
6947 Indicates that there is no OS function for flushing the cache.
6951 @node RS/6000 and PowerPC Options
6952 @subsection IBM RS/6000 and PowerPC Options
6953 @cindex RS/6000 and PowerPC Options
6954 @cindex IBM RS/6000 and PowerPC Options
6956 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
6964 @itemx -mpowerpc-gpopt
6965 @itemx -mno-powerpc-gpopt
6966 @itemx -mpowerpc-gfxopt
6967 @itemx -mno-powerpc-gfxopt
6969 @itemx -mno-powerpc64
6975 @opindex mno-powerpc
6976 @opindex mpowerpc-gpopt
6977 @opindex mno-powerpc-gpopt
6978 @opindex mpowerpc-gfxopt
6979 @opindex mno-powerpc-gfxopt
6981 @opindex mno-powerpc64
6982 GCC supports two related instruction set architectures for the
6983 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
6984 instructions supported by the @samp{rios} chip set used in the original
6985 RS/6000 systems and the @dfn{PowerPC} instruction set is the
6986 architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and
6987 the IBM 4xx microprocessors.
6989 Neither architecture is a subset of the other. However there is a
6990 large common subset of instructions supported by both. An MQ
6991 register is included in processors supporting the POWER architecture.
6993 You use these options to specify which instructions are available on the
6994 processor you are using. The default value of these options is
6995 determined when configuring GCC@. Specifying the
6996 @option{-mcpu=@var{cpu_type}} overrides the specification of these
6997 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
6998 rather than the options listed above.
7000 The @option{-mpower} option allows GCC to generate instructions that
7001 are found only in the POWER architecture and to use the MQ register.
7002 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
7003 to generate instructions that are present in the POWER2 architecture but
7004 not the original POWER architecture.
7006 The @option{-mpowerpc} option allows GCC to generate instructions that
7007 are found only in the 32-bit subset of the PowerPC architecture.
7008 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
7009 GCC to use the optional PowerPC architecture instructions in the
7010 General Purpose group, including floating-point square root. Specifying
7011 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
7012 use the optional PowerPC architecture instructions in the Graphics
7013 group, including floating-point select.
7015 The @option{-mpowerpc64} option allows GCC to generate the additional
7016 64-bit instructions that are found in the full PowerPC64 architecture
7017 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
7018 @option{-mno-powerpc64}.
7020 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
7021 will use only the instructions in the common subset of both
7022 architectures plus some special AIX common-mode calls, and will not use
7023 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
7024 permits GCC to use any instruction from either architecture and to
7025 allow use of the MQ register; specify this for the Motorola MPC601.
7027 @item -mnew-mnemonics
7028 @itemx -mold-mnemonics
7029 @opindex mnew-mnemonics
7030 @opindex mold-mnemonics
7031 Select which mnemonics to use in the generated assembler code. With
7032 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
7033 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
7034 assembler mnemonics defined for the POWER architecture. Instructions
7035 defined in only one architecture have only one mnemonic; GCC uses that
7036 mnemonic irrespective of which of these options is specified.
7038 GCC defaults to the mnemonics appropriate for the architecture in
7039 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
7040 value of these option. Unless you are building a cross-compiler, you
7041 should normally not specify either @option{-mnew-mnemonics} or
7042 @option{-mold-mnemonics}, but should instead accept the default.
7044 @item -mcpu=@var{cpu_type}
7046 Set architecture type, register usage, choice of mnemonics, and
7047 instruction scheduling parameters for machine type @var{cpu_type}.
7048 Supported values for @var{cpu_type} are @samp{401}, @samp{403},
7049 @samp{405}, @samp{405fp}, @samp{440}, @samp{440fp}, @samp{505},
7050 @samp{601}, @samp{602}, @samp{603}, @samp{603e}, @samp{604},
7051 @samp{604e}, @samp{620}, @samp{630}, @samp{740}, @samp{7400},
7052 @samp{7450}, @samp{750}, @samp{801}, @samp{821}, @samp{823},
7053 @samp{860}, @samp{970}, @samp{common}, @samp{ec603e}, @samp{G3},
7054 @samp{G4}, @samp{G5}, @samp{power}, @samp{power2}, @samp{power3},
7055 @samp{power4}, @samp{power5}, @samp{powerpc}, @samp{powerpc64},
7056 @samp{rios}, @samp{rios1}, @samp{rios2}, @samp{rsc}, and @samp{rs64a}.
7058 @option{-mcpu=common} selects a completely generic processor. Code
7059 generated under this option will run on any POWER or PowerPC processor.
7060 GCC will use only the instructions in the common subset of both
7061 architectures, and will not use the MQ register. GCC assumes a generic
7062 processor model for scheduling purposes.
7064 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
7065 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
7066 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
7067 types, with an appropriate, generic processor model assumed for
7068 scheduling purposes.
7070 The other options specify a specific processor. Code generated under
7071 those options will run best on that processor, and may not run at all on
7074 The @option{-mcpu} options automatically enable or disable the
7075 following options: @option{-maltivec}, @option{-mhard-float},
7076 @option{-mmfcrf}, @option{-mmultiple}, @option{-mnew-mnemonics},
7077 @option{-mpower}, @option{-mpower2}, @option{-mpowerpc64},
7078 @option{-mpowerpc-gpopt}, @option{-mpowerpc-gfxopt},
7079 @option{-mstring}. The particular options set for any particular CPU
7080 will vary between compiler versions, depending on what setting seems
7081 to produce optimal code for that CPU; it doesn't necessarily reflect
7082 the actual hardware's capabilities. If you wish to set an individual
7083 option to a particular value, you may specify it after the
7084 @option{-mcpu} option, like @samp{-mcpu=970 -mno-altivec}.
7086 On AIX, the @option{-maltivec} and @option{-mpowerpc64} options are
7087 not enabled or disabled by the @option{-mcpu} option at present, since
7088 AIX does not have full support for these options. You may still
7089 enable or disable them individually if you're sure it'll work in your
7092 @item -mtune=@var{cpu_type}
7094 Set the instruction scheduling parameters for machine type
7095 @var{cpu_type}, but do not set the architecture type, register usage, or
7096 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
7097 values for @var{cpu_type} are used for @option{-mtune} as for
7098 @option{-mcpu}. If both are specified, the code generated will use the
7099 architecture, registers, and mnemonics set by @option{-mcpu}, but the
7100 scheduling parameters set by @option{-mtune}.
7105 @opindex mno-altivec
7106 These switches enable or disable the use of built-in functions that
7107 allow access to the AltiVec instruction set. You may also need to set
7108 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
7113 Extend the current ABI with SPE ABI extensions. This does not change
7114 the default ABI, instead it adds the SPE ABI extensions to the current
7118 @opindex mabi=no-spe
7119 Disable Booke SPE ABI extensions for the current ABI.
7121 @item -misel=@var{yes/no}
7124 This switch enables or disables the generation of ISEL instructions.
7126 @item -mspe=@var{yes/no}
7129 This switch enables or disables the generation of SPE simd
7132 @item -mfloat-gprs=@var{yes/no}
7134 @opindex mfloat-gprs
7135 This switch enables or disables the generation of floating point
7136 operations on the general purpose registers for architectures that
7137 support it. This option is currently only available on the MPC8540.
7140 @itemx -mno-fp-in-toc
7141 @itemx -mno-sum-in-toc
7142 @itemx -mminimal-toc
7144 @opindex mno-fp-in-toc
7145 @opindex mno-sum-in-toc
7146 @opindex mminimal-toc
7147 Modify generation of the TOC (Table Of Contents), which is created for
7148 every executable file. The @option{-mfull-toc} option is selected by
7149 default. In that case, GCC will allocate at least one TOC entry for
7150 each unique non-automatic variable reference in your program. GCC
7151 will also place floating-point constants in the TOC@. However, only
7152 16,384 entries are available in the TOC@.
7154 If you receive a linker error message that saying you have overflowed
7155 the available TOC space, you can reduce the amount of TOC space used
7156 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
7157 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
7158 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
7159 generate code to calculate the sum of an address and a constant at
7160 run-time instead of putting that sum into the TOC@. You may specify one
7161 or both of these options. Each causes GCC to produce very slightly
7162 slower and larger code at the expense of conserving TOC space.
7164 If you still run out of space in the TOC even when you specify both of
7165 these options, specify @option{-mminimal-toc} instead. This option causes
7166 GCC to make only one TOC entry for every file. When you specify this
7167 option, GCC will produce code that is slower and larger but which
7168 uses extremely little TOC space. You may wish to use this option
7169 only on files that contain less frequently executed code.
7175 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
7176 @code{long} type, and the infrastructure needed to support them.
7177 Specifying @option{-maix64} implies @option{-mpowerpc64} and
7178 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
7179 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
7184 @opindex mno-xl-call
7185 On AIX, pass floating-point arguments to prototyped functions beyond the
7186 register save area (RSA) on the stack in addition to argument FPRs. The
7187 AIX calling convention was extended but not initially documented to
7188 handle an obscure K&R C case of calling a function that takes the
7189 address of its arguments with fewer arguments than declared. AIX XL
7190 compilers access floating point arguments which do not fit in the
7191 RSA from the stack when a subroutine is compiled without
7192 optimization. Because always storing floating-point arguments on the
7193 stack is inefficient and rarely needed, this option is not enabled by
7194 default and only is necessary when calling subroutines compiled by AIX
7195 XL compilers without optimization.
7199 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
7200 application written to use message passing with special startup code to
7201 enable the application to run. The system must have PE installed in the
7202 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
7203 must be overridden with the @option{-specs=} option to specify the
7204 appropriate directory location. The Parallel Environment does not
7205 support threads, so the @option{-mpe} option and the @option{-pthread}
7206 option are incompatible.
7208 @item -malign-natural
7209 @itemx -malign-power
7210 @opindex malign-natural
7211 @opindex malign-power
7212 On AIX, Darwin, and 64-bit PowerPC GNU/Linux, the option
7213 @option{-malign-natural} overrides the ABI-defined alignment of larger
7214 types, such as floating-point doubles, on their natural size-based boundary.
7215 The option @option{-malign-power} instructs GCC to follow the ABI-specified
7216 alignment rules. GCC defaults to the standard alignment defined in the ABI.
7220 @opindex msoft-float
7221 @opindex mhard-float
7222 Generate code that does not use (uses) the floating-point register set.
7223 Software floating point emulation is provided if you use the
7224 @option{-msoft-float} option, and pass the option to GCC when linking.
7227 @itemx -mno-multiple
7229 @opindex mno-multiple
7230 Generate code that uses (does not use) the load multiple word
7231 instructions and the store multiple word instructions. These
7232 instructions are generated by default on POWER systems, and not
7233 generated on PowerPC systems. Do not use @option{-mmultiple} on little
7234 endian PowerPC systems, since those instructions do not work when the
7235 processor is in little endian mode. The exceptions are PPC740 and
7236 PPC750 which permit the instructions usage in little endian mode.
7242 Generate code that uses (does not use) the load string instructions
7243 and the store string word instructions to save multiple registers and
7244 do small block moves. These instructions are generated by default on
7245 POWER systems, and not generated on PowerPC systems. Do not use
7246 @option{-mstring} on little endian PowerPC systems, since those
7247 instructions do not work when the processor is in little endian mode.
7248 The exceptions are PPC740 and PPC750 which permit the instructions
7249 usage in little endian mode.
7255 Generate code that uses (does not use) the load or store instructions
7256 that update the base register to the address of the calculated memory
7257 location. These instructions are generated by default. If you use
7258 @option{-mno-update}, there is a small window between the time that the
7259 stack pointer is updated and the address of the previous frame is
7260 stored, which means code that walks the stack frame across interrupts or
7261 signals may get corrupted data.
7264 @itemx -mno-fused-madd
7265 @opindex mfused-madd
7266 @opindex mno-fused-madd
7267 Generate code that uses (does not use) the floating point multiply and
7268 accumulate instructions. These instructions are generated by default if
7269 hardware floating is used.
7271 @item -mno-bit-align
7273 @opindex mno-bit-align
7275 On System V.4 and embedded PowerPC systems do not (do) force structures
7276 and unions that contain bit-fields to be aligned to the base type of the
7279 For example, by default a structure containing nothing but 8
7280 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
7281 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
7282 the structure would be aligned to a 1 byte boundary and be one byte in
7285 @item -mno-strict-align
7286 @itemx -mstrict-align
7287 @opindex mno-strict-align
7288 @opindex mstrict-align
7289 On System V.4 and embedded PowerPC systems do not (do) assume that
7290 unaligned memory references will be handled by the system.
7293 @itemx -mno-relocatable
7294 @opindex mrelocatable
7295 @opindex mno-relocatable
7296 On embedded PowerPC systems generate code that allows (does not allow)
7297 the program to be relocated to a different address at runtime. If you
7298 use @option{-mrelocatable} on any module, all objects linked together must
7299 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
7301 @item -mrelocatable-lib
7302 @itemx -mno-relocatable-lib
7303 @opindex mrelocatable-lib
7304 @opindex mno-relocatable-lib
7305 On embedded PowerPC systems generate code that allows (does not allow)
7306 the program to be relocated to a different address at runtime. Modules
7307 compiled with @option{-mrelocatable-lib} can be linked with either modules
7308 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
7309 with modules compiled with the @option{-mrelocatable} options.
7315 On System V.4 and embedded PowerPC systems do not (do) assume that
7316 register 2 contains a pointer to a global area pointing to the addresses
7317 used in the program.
7320 @itemx -mlittle-endian
7322 @opindex mlittle-endian
7323 On System V.4 and embedded PowerPC systems compile code for the
7324 processor in little endian mode. The @option{-mlittle-endian} option is
7325 the same as @option{-mlittle}.
7330 @opindex mbig-endian
7331 On System V.4 and embedded PowerPC systems compile code for the
7332 processor in big endian mode. The @option{-mbig-endian} option is
7333 the same as @option{-mbig}.
7335 @item -mdynamic-no-pic
7336 @opindex mdynamic-no-pic
7337 On Darwin and Mac OS X systems, compile code so that it is not
7338 relocatable, but that its external references are relocatable. The
7339 resulting code is suitable for applications, but not shared
7342 @item -mprioritize-restricted-insns=@var{priority}
7343 @opindex mprioritize-restricted-insns
7344 This option controls the priority that is assigned to
7345 dispatch-slot restricted instructions during the second scheduling
7346 pass. The argument @var{priority} takes the value @var{0/1/2} to assign
7347 @var{no/highest/second-highest} priority to dispatch slot restricted
7350 @item -msched-costly-dep=@var{dependence_type}
7351 @opindex msched-costly-dep
7352 This option controls which dependences are considered costly
7353 by the target during instruction scheduling. The argument
7354 @var{dependence_type} takes one of the following values:
7355 @var{no}: no dependence is costly,
7356 @var{all}: all dependences are costly,
7357 @var{true_store_to_load}: a true dependence from store to load is costly,
7358 @var{store_to_load}: any dependence from store to load is costly,
7359 @var{number}: any dependence which latency >= @var{number} is costly.
7361 @item -minsert-sched-nops=@var{scheme}
7362 @opindex minsert-sched-nops
7363 This option controls which nop insertion scheme will be used during
7364 the second scheduling pass. The argument @var{scheme} takes one of the
7366 @var{no}: Don't insert nops.
7367 @var{pad}: Pad with nops any dispatch group which has vacant issue slots,
7368 according to the scheduler's grouping.
7369 @var{regroup_exact}: Insert nops to force costly dependent insns into
7370 separate groups. Insert exactly as many nops as needed to force an insn
7371 to a new group, according to the estimated processor grouping.
7372 @var{number}: Insert nops to force costly dependent insns into
7373 separate groups. Insert @var{number} nops to force an insn to a new group.
7377 On System V.4 and embedded PowerPC systems compile code using calling
7378 conventions that adheres to the March 1995 draft of the System V
7379 Application Binary Interface, PowerPC processor supplement. This is the
7380 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
7382 @item -mcall-sysv-eabi
7383 @opindex mcall-sysv-eabi
7384 Specify both @option{-mcall-sysv} and @option{-meabi} options.
7386 @item -mcall-sysv-noeabi
7387 @opindex mcall-sysv-noeabi
7388 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
7390 @item -mcall-solaris
7391 @opindex mcall-solaris
7392 On System V.4 and embedded PowerPC systems compile code for the Solaris
7396 @opindex mcall-linux
7397 On System V.4 and embedded PowerPC systems compile code for the
7398 Linux-based GNU system.
7402 On System V.4 and embedded PowerPC systems compile code for the
7403 Hurd-based GNU system.
7406 @opindex mcall-netbsd
7407 On System V.4 and embedded PowerPC systems compile code for the
7408 NetBSD operating system.
7410 @item -maix-struct-return
7411 @opindex maix-struct-return
7412 Return all structures in memory (as specified by the AIX ABI)@.
7414 @item -msvr4-struct-return
7415 @opindex msvr4-struct-return
7416 Return structures smaller than 8 bytes in registers (as specified by the
7420 @opindex mabi=altivec
7421 Extend the current ABI with AltiVec ABI extensions. This does not
7422 change the default ABI, instead it adds the AltiVec ABI extensions to
7425 @item -mabi=no-altivec
7426 @opindex mabi=no-altivec
7427 Disable AltiVec ABI extensions for the current ABI.
7430 @itemx -mno-prototype
7432 @opindex mno-prototype
7433 On System V.4 and embedded PowerPC systems assume that all calls to
7434 variable argument functions are properly prototyped. Otherwise, the
7435 compiler must insert an instruction before every non prototyped call to
7436 set or clear bit 6 of the condition code register (@var{CR}) to
7437 indicate whether floating point values were passed in the floating point
7438 registers in case the function takes a variable arguments. With
7439 @option{-mprototype}, only calls to prototyped variable argument functions
7440 will set or clear the bit.
7444 On embedded PowerPC systems, assume that the startup module is called
7445 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
7446 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
7451 On embedded PowerPC systems, assume that the startup module is called
7452 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
7457 On embedded PowerPC systems, assume that the startup module is called
7458 @file{crt0.o} and the standard C libraries are @file{libads.a} and
7462 @opindex myellowknife
7463 On embedded PowerPC systems, assume that the startup module is called
7464 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
7469 On System V.4 and embedded PowerPC systems, specify that you are
7470 compiling for a VxWorks system.
7474 Specify that you are compiling for the WindISS simulation environment.
7478 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
7479 header to indicate that @samp{eabi} extended relocations are used.
7485 On System V.4 and embedded PowerPC systems do (do not) adhere to the
7486 Embedded Applications Binary Interface (eabi) which is a set of
7487 modifications to the System V.4 specifications. Selecting @option{-meabi}
7488 means that the stack is aligned to an 8 byte boundary, a function
7489 @code{__eabi} is called to from @code{main} to set up the eabi
7490 environment, and the @option{-msdata} option can use both @code{r2} and
7491 @code{r13} to point to two separate small data areas. Selecting
7492 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
7493 do not call an initialization function from @code{main}, and the
7494 @option{-msdata} option will only use @code{r13} to point to a single
7495 small data area. The @option{-meabi} option is on by default if you
7496 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
7499 @opindex msdata=eabi
7500 On System V.4 and embedded PowerPC systems, put small initialized
7501 @code{const} global and static data in the @samp{.sdata2} section, which
7502 is pointed to by register @code{r2}. Put small initialized
7503 non-@code{const} global and static data in the @samp{.sdata} section,
7504 which is pointed to by register @code{r13}. Put small uninitialized
7505 global and static data in the @samp{.sbss} section, which is adjacent to
7506 the @samp{.sdata} section. The @option{-msdata=eabi} option is
7507 incompatible with the @option{-mrelocatable} option. The
7508 @option{-msdata=eabi} option also sets the @option{-memb} option.
7511 @opindex msdata=sysv
7512 On System V.4 and embedded PowerPC systems, put small global and static
7513 data in the @samp{.sdata} section, which is pointed to by register
7514 @code{r13}. Put small uninitialized global and static data in the
7515 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
7516 The @option{-msdata=sysv} option is incompatible with the
7517 @option{-mrelocatable} option.
7519 @item -msdata=default
7521 @opindex msdata=default
7523 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
7524 compile code the same as @option{-msdata=eabi}, otherwise compile code the
7525 same as @option{-msdata=sysv}.
7528 @opindex msdata-data
7529 On System V.4 and embedded PowerPC systems, put small global and static
7530 data in the @samp{.sdata} section. Put small uninitialized global and
7531 static data in the @samp{.sbss} section. Do not use register @code{r13}
7532 to address small data however. This is the default behavior unless
7533 other @option{-msdata} options are used.
7537 @opindex msdata=none
7539 On embedded PowerPC systems, put all initialized global and static data
7540 in the @samp{.data} section, and all uninitialized data in the
7541 @samp{.bss} section.
7545 @cindex smaller data references (PowerPC)
7546 @cindex .sdata/.sdata2 references (PowerPC)
7547 On embedded PowerPC systems, put global and static items less than or
7548 equal to @var{num} bytes into the small data or bss sections instead of
7549 the normal data or bss section. By default, @var{num} is 8. The
7550 @option{-G @var{num}} switch is also passed to the linker.
7551 All modules should be compiled with the same @option{-G @var{num}} value.
7554 @itemx -mno-regnames
7556 @opindex mno-regnames
7557 On System V.4 and embedded PowerPC systems do (do not) emit register
7558 names in the assembly language output using symbolic forms.
7561 @itemx -mno-longcall
7563 @opindex mno-longcall
7564 Default to making all function calls indirectly, using a register, so
7565 that functions which reside further than 32 megabytes (33,554,432
7566 bytes) from the current location can be called. This setting can be
7567 overridden by the @code{shortcall} function attribute, or by
7568 @code{#pragma longcall(0)}.
7570 Some linkers are capable of detecting out-of-range calls and generating
7571 glue code on the fly. On these systems, long calls are unnecessary and
7572 generate slower code. As of this writing, the AIX linker can do this,
7573 as can the GNU linker for PowerPC/64. It is planned to add this feature
7574 to the GNU linker for 32-bit PowerPC systems as well.
7576 On Darwin/PPC systems, @code{#pragma longcall} will generate ``jbsr
7577 callee, L42'', plus a ``branch island'' (glue code). The two target
7578 addresses represent the callee and the ``branch island.'' The
7579 Darwin/PPC linker will prefer the first address and generate a ``bl
7580 callee'' if the PPC ``bl'' instruction will reach the callee directly;
7581 otherwise, the linker will generate ``bl L42'' to call the ``branch
7582 island.'' The ``branch island'' is appended to the body of the
7583 calling function; it computes the full 32-bit address of the callee
7586 On Mach-O (Darwin) systems, this option directs the compiler emit to
7587 the glue for every direct call, and the Darwin linker decides whether
7588 to use or discard it.
7590 In the future, we may cause GCC to ignore all longcall specifications
7591 when the linker is known to generate glue.
7595 Adds support for multithreading with the @dfn{pthreads} library.
7596 This option sets flags for both the preprocessor and linker.
7600 @node Darwin Options
7601 @subsection Darwin Options
7602 @cindex Darwin options
7604 These options are defined for all architectures running the Darwin operating
7605 system. They are useful for compatibility with other Mac OS compilers.
7610 Loads all members of static archive libraries.
7611 See man ld(1) for more information.
7613 @item -arch_errors_fatal
7614 @opindex arch_errors_fatal
7615 Cause the errors having to do with files that have the wrong architecture
7619 @opindex bind_at_load
7620 Causes the output file to be marked such that the dynamic linker will
7621 bind all undefined references when the file is loaded or launched.
7625 Produce a Mach-o bundle format file.
7626 See man ld(1) for more information.
7628 @item -bundle_loader @var{executable}
7629 @opindex bundle_loader
7630 This specifies the @var{executable} that will be loading the build
7631 output file being linked. See man ld(1) for more information.
7633 @item -allowable_client @var{client_name}
7637 @itemx -compatibility_version
7638 @itemx -current_version
7639 @itemx -dependency-file
7641 @itemx -dylinker_install_name
7644 @itemx -exported_symbols_list
7646 @itemx -flat_namespace
7647 @itemx -force_cpusubtype_ALL
7648 @itemx -force_flat_namespace
7649 @itemx -headerpad_max_install_names
7652 @itemx -install_name
7653 @itemx -keep_private_externs
7654 @itemx -multi_module
7655 @itemx -multiply_defined
7656 @itemx -multiply_defined_unused
7658 @itemx -nofixprebinding
7661 @itemx -noseglinkedit
7662 @itemx -pagezero_size
7664 @itemx -prebind_all_twolevel_modules
7665 @itemx -private_bundle
7666 @itemx -read_only_relocs
7668 @itemx -sectobjectsymbols
7672 @itemx -sectobjectsymbols
7674 @itemx -seg_addr_table
7675 @itemx -seg_addr_table_filename
7678 @itemx -segs_read_only_addr
7679 @itemx -segs_read_write_addr
7680 @itemx -single_module
7683 @itemx -sub_umbrella
7684 @itemx -twolevel_namespace
7687 @itemx -unexported_symbols_list
7688 @itemx -weak_reference_mismatches
7691 @opindex allowable_client
7693 @opindex client_name
7694 @opindex compatibility_version
7695 @opindex current_version
7696 @opindex dependency-file
7698 @opindex dylinker_install_name
7701 @opindex exported_symbols_list
7703 @opindex flat_namespace
7704 @opindex force_cpusubtype_ALL
7705 @opindex force_flat_namespace
7706 @opindex headerpad_max_install_names
7709 @opindex install_name
7710 @opindex keep_private_externs
7711 @opindex multi_module
7712 @opindex multiply_defined
7713 @opindex multiply_defined_unused
7715 @opindex nofixprebinding
7716 @opindex nomultidefs
7718 @opindex noseglinkedit
7719 @opindex pagezero_size
7721 @opindex prebind_all_twolevel_modules
7722 @opindex private_bundle
7723 @opindex read_only_relocs
7725 @opindex sectobjectsymbols
7729 @opindex sectobjectsymbols
7731 @opindex seg_addr_table
7732 @opindex seg_addr_table_filename
7733 @opindex seglinkedit
7735 @opindex segs_read_only_addr
7736 @opindex segs_read_write_addr
7737 @opindex single_module
7739 @opindex sub_library
7740 @opindex sub_umbrella
7741 @opindex twolevel_namespace
7744 @opindex unexported_symbols_list
7745 @opindex weak_reference_mismatches
7746 @opindex whatsloaded
7748 These options are available for Darwin linker. Darwin linker man page
7749 describes them in detail.
7754 @subsection MIPS Options
7755 @cindex MIPS options
7761 Generate big-endian code.
7765 Generate little-endian code. This is the default for @samp{mips*el-*-*}
7768 @item -march=@var{arch}
7770 Generate code that will run on @var{arch}, which can be the name of a
7771 generic MIPS ISA, or the name of a particular processor.
7773 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
7774 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
7775 The processor names are:
7776 @samp{4kc}, @samp{4kp}, @samp{5kc}, @samp{20kc},
7778 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
7779 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000}, @samp{rm7000},
7783 @samp{vr4100}, @samp{vr4111}, @samp{vr4120}, @samp{vr4130}, @samp{vr4300},
7784 @samp{vr5000}, @samp{vr5400} and @samp{vr5500}.
7785 The special value @samp{from-abi} selects the
7786 most compatible architecture for the selected ABI (that is,
7787 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
7789 In processor names, a final @samp{000} can be abbreviated as @samp{k}
7790 (for example, @samp{-march=r2k}). Prefixes are optional, and
7791 @samp{vr} may be written @samp{r}.
7793 GCC defines two macros based on the value of this option. The first
7794 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
7795 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
7796 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
7797 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
7798 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
7800 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
7801 above. In other words, it will have the full prefix and will not
7802 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
7803 the macro names the resolved architecture (either @samp{"mips1"} or
7804 @samp{"mips3"}). It names the default architecture when no
7805 @option{-march} option is given.
7807 @item -mtune=@var{arch}
7809 Optimize for @var{arch}. Among other things, this option controls
7810 the way instructions are scheduled, and the perceived cost of arithmetic
7811 operations. The list of @var{arch} values is the same as for
7814 When this option is not used, GCC will optimize for the processor
7815 specified by @option{-march}. By using @option{-march} and
7816 @option{-mtune} together, it is possible to generate code that will
7817 run on a family of processors, but optimize the code for one
7818 particular member of that family.
7820 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
7821 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
7822 @samp{-march} ones described above.
7826 Equivalent to @samp{-march=mips1}.
7830 Equivalent to @samp{-march=mips2}.
7834 Equivalent to @samp{-march=mips3}.
7838 Equivalent to @samp{-march=mips4}.
7842 Equivalent to @samp{-march=mips32}.
7846 Equivalent to @samp{-march=mips32r2}.
7850 Equivalent to @samp{-march=mips64}.
7856 Use (do not use) the MIPS16 ISA.
7868 Generate code for the given ABI@.
7870 Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
7871 generates 64-bit code when you select a 64-bit architecture, but you
7872 can use @option{-mgp32} to get 32-bit code instead.
7874 For information about the O64 ABI, see
7875 @w{@uref{http://gcc.gnu.org/projects/mipso64-abi.html}}.
7878 @itemx -mno-abicalls
7880 @opindex mno-abicalls
7881 Generate (do not generate) SVR4-style position-independent code.
7882 @option{-mabicalls} is the default for SVR4-based systems.
7888 Lift (do not lift) the usual restrictions on the size of the global
7891 GCC normally uses a single instruction to load values from the GOT.
7892 While this is relatively efficient, it will only work if the GOT
7893 is smaller than about 64k. Anything larger will cause the linker
7894 to report an error such as:
7896 @cindex relocation truncated to fit (MIPS)
7898 relocation truncated to fit: R_MIPS_GOT16 foobar
7901 If this happens, you should recompile your code with @option{-mxgot}.
7902 It should then work with very large GOTs, although it will also be
7903 less efficient, since it will take three instructions to fetch the
7904 value of a global symbol.
7906 Note that some linkers can create multiple GOTs. If you have such a
7907 linker, you should only need to use @option{-mxgot} when a single object
7908 file accesses more than 64k's worth of GOT entries. Very few do.
7910 These options have no effect unless GCC is generating position
7913 @item -membedded-pic
7914 @itemx -mno-embedded-pic
7915 @opindex membedded-pic
7916 @opindex mno-embedded-pic
7917 Generate (do not generate) position-independent code suitable for some
7918 embedded systems. All calls are made using PC relative addresses, and
7919 all data is addressed using the $gp register. No more than 65536
7920 bytes of global data may be used. This requires GNU as and GNU ld,
7921 which do most of the work.
7925 Assume that general-purpose registers are 32 bits wide.
7929 Assume that general-purpose registers are 64 bits wide.
7933 Assume that floating-point registers are 32 bits wide.
7937 Assume that floating-point registers are 64 bits wide.
7940 @opindex mhard-float
7941 Use floating-point coprocessor instructions.
7944 @opindex msoft-float
7945 Do not use floating-point coprocessor instructions. Implement
7946 floating-point calculations using library calls instead.
7948 @item -msingle-float
7949 @opindex msingle-float
7950 Assume that the floating-point coprocessor only supports single-precision
7953 @itemx -mdouble-float
7954 @opindex mdouble-float
7955 Assume that the floating-point coprocessor supports double-precision
7956 operations. This is the default.
7960 Force @code{int} and @code{long} types to be 64 bits wide. See
7961 @option{-mlong32} for an explanation of the default and the way
7962 that the pointer size is determined.
7966 Force @code{long} types to be 64 bits wide. See @option{-mlong32} for
7967 an explanation of the default and the way that the pointer size is
7972 Force @code{long}, @code{int}, and pointer types to be 32 bits wide.
7974 The default size of @code{int}s, @code{long}s and pointers depends on
7975 the ABI@. All the supported ABIs use 32-bit @code{int}s. The n64 ABI
7976 uses 64-bit @code{long}s, as does the 64-bit EABI; the others use
7977 32-bit @code{long}s. Pointers are the same size as @code{long}s,
7978 or the same size as integer registers, whichever is smaller.
7982 @cindex smaller data references (MIPS)
7983 @cindex gp-relative references (MIPS)
7984 Put global and static items less than or equal to @var{num} bytes into
7985 the small data or bss section instead of the normal data or bss section.
7986 This allows the data to be accessed using a single instruction.
7988 All modules should be compiled with the same @option{-G @var{num}}
7991 @item -membedded-data
7992 @itemx -mno-embedded-data
7993 @opindex membedded-data
7994 @opindex mno-embedded-data
7995 Allocate variables to the read-only data section first if possible, then
7996 next in the small data section if possible, otherwise in data. This gives
7997 slightly slower code than the default, but reduces the amount of RAM required
7998 when executing, and thus may be preferred for some embedded systems.
8000 @item -muninit-const-in-rodata
8001 @itemx -mno-uninit-const-in-rodata
8002 @opindex muninit-const-in-rodata
8003 @opindex mno-uninit-const-in-rodata
8004 Put uninitialized @code{const} variables in the read-only data section.
8005 This option is only meaningful in conjunction with @option{-membedded-data}.
8007 @item -msplit-addresses
8008 @itemx -mno-split-addresses
8009 @opindex msplit-addresses
8010 @opindex mno-split-addresses
8011 Enable (disable) use of the @code{%hi()} and @code{%lo()} assembler
8012 relocation operators. This option has been superceded by
8013 @option{-mexplicit-relocs} but is retained for backwards compatibility.
8015 @item -mexplicit-relocs
8016 @itemx -mno-explicit-relocs
8017 @opindex mexplicit-relocs
8018 @opindex mno-explicit-relocs
8019 Use (do not use) assembler relocation operators when dealing with symbolic
8020 addresses. The alternative, selected by @option{-mno-explicit-relocs},
8021 is to use assembler macros instead.
8023 @option{-mexplicit-relocs} is usually the default if GCC was configured
8024 to use an assembler that supports relocation operators. However, the
8025 combination of @option{-mabicalls} and @option{-fno-unit-at-a-time}
8026 implies @option{-mno-explicit-relocs} unless explicitly overridden.
8027 This is because, when generating abicalls, the choice of relocation
8028 depends on whether a symbol is local or global. In some rare cases,
8029 GCC will not be able to decide this until the whole compilation unit
8036 Generate (do not generate) code that refers to registers using their
8037 software names. The default is @option{-mno-rnames}, which tells GCC
8038 to use hardware names like @samp{$4} instead of software names like
8039 @samp{a0}. The only assembler known to support @option{-rnames} is
8040 the Algorithmics assembler.
8042 @item -mcheck-zero-division
8043 @itemx -mno-check-zero-division
8044 @opindex mcheck-zero-division
8045 @opindex mno-check-zero-division
8046 Trap (do not trap) on integer division by zero. The default is
8047 @option{-mcheck-zero-division}.
8053 Force (do not force) the use of @code{memcpy()} for non-trivial block
8054 moves. The default is @option{-mno-memcpy}, which allows GCC to inline
8055 most constant-sized copies.
8058 @itemx -mno-long-calls
8059 @opindex mlong-calls
8060 @opindex mno-long-calls
8061 Disable (do not disable) use of the @code{jal} instruction. Calling
8062 functions using @code{jal} is more efficient but requires the caller
8063 and callee to be in the same 256 megabyte segment.
8065 This option has no effect on abicalls code. The default is
8066 @option{-mno-long-calls}.
8072 Enable (disable) use of the @code{mad}, @code{madu} and @code{mul}
8073 instructions, as provided by the R4650 ISA.
8076 @itemx -mno-fused-madd
8077 @opindex mfused-madd
8078 @opindex mno-fused-madd
8079 Enable (disable) use of the floating point multiply-accumulate
8080 instructions, when they are available. The default is
8081 @option{-mfused-madd}.
8083 When multiply-accumulate instructions are used, the intermediate
8084 product is calculated to infinite precision and is not subject to
8085 the FCSR Flush to Zero bit. This may be undesirable in some
8090 Tell the MIPS assembler to not run its preprocessor over user
8091 assembler files (with a @samp{.s} suffix) when assembling them.
8094 @itemx -mno-fix-r4000
8096 @opindex mno-fix-r4000
8097 Work around certain R4000 CPU errata:
8100 A double-word or a variable shift may give an incorrect result if executed
8101 immediately after starting an integer division.
8103 A double-word or a variable shift may give an incorrect result if executed
8104 while an integer multiplication is in progress.
8106 An integer division may give an incorrect result if started in a delay slot
8107 of a taken branch or a jump.
8111 @itemx -mno-fix-r4400
8113 @opindex mno-fix-r4400
8114 Work around certain R4400 CPU errata:
8117 A double-word or a variable shift may give an incorrect result if executed
8118 immediately after starting an integer division.
8121 @item -mfix-vr4122-bugs
8122 @itemx -mno-fix-vr4122-bugs
8123 @opindex mfix-vr4122-bugs
8124 Work around certain VR4122 errata:
8127 @code{dmultu} does not always produce the correct result.
8129 @code{div} and @code{ddiv} do not always produce the correct result if one
8130 of the operands is negative.
8132 The workarounds for the division errata rely on special functions in
8133 @file{libgcc.a}. At present, these functions are only provided by
8134 the @code{mips64vr*-elf} configurations.
8136 Other VR4122 errata require a nop to be inserted between certain pairs of
8137 instructions. These errata are handled by the assembler, not by GCC itself.
8142 Work around certain SB-1 CPU core errata.
8143 (This flag currently works around the SB-1 revision 2
8144 ``F1'' and ``F2'' floating point errata.)
8146 @item -mflush-func=@var{func}
8147 @itemx -mno-flush-func
8148 @opindex mflush-func
8149 Specifies the function to call to flush the I and D caches, or to not
8150 call any such function. If called, the function must take the same
8151 arguments as the common @code{_flush_func()}, that is, the address of the
8152 memory range for which the cache is being flushed, the size of the
8153 memory range, and the number 3 (to flush both caches). The default
8154 depends on the target GCC was configured for, but commonly is either
8155 @samp{_flush_func} or @samp{__cpu_flush}.
8157 @item -mbranch-likely
8158 @itemx -mno-branch-likely
8159 @opindex mbranch-likely
8160 @opindex mno-branch-likely
8161 Enable or disable use of Branch Likely instructions, regardless of the
8162 default for the selected architecture. By default, Branch Likely
8163 instructions may be generated if they are supported by the selected
8164 architecture. An exception is for the MIPS32 and MIPS64 architectures
8165 and processors which implement those architectures; for those, Branch
8166 Likely instructions will not be generated by default because the MIPS32
8167 and MIPS64 architectures specifically deprecate their use.
8170 @node i386 and x86-64 Options
8171 @subsection Intel 386 and AMD x86-64 Options
8172 @cindex i386 Options
8173 @cindex x86-64 Options
8174 @cindex Intel 386 Options
8175 @cindex AMD x86-64 Options
8177 These @samp{-m} options are defined for the i386 and x86-64 family of
8181 @item -mtune=@var{cpu-type}
8183 Tune to @var{cpu-type} everything applicable about the generated code, except
8184 for the ABI and the set of available instructions. The choices for
8188 Original Intel's i386 CPU.
8190 Intel's i486 CPU. (No scheduling is implemented for this chip.)
8192 Intel Pentium CPU with no MMX support.
8194 Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
8195 @item i686, pentiumpro
8196 Intel PentiumPro CPU.
8198 Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
8199 @item pentium3, pentium3m
8200 Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set
8203 Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set
8204 support. Used by Centrino notebooks.
8205 @item pentium4, pentium4m
8206 Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
8208 Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction
8211 Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
8212 SSE2 and SSE3 instruction set support.
8214 AMD K6 CPU with MMX instruction set support.
8216 Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
8217 @item athlon, athlon-tbird
8218 AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions
8220 @item athlon-4, athlon-xp, athlon-mp
8221 Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE
8222 instruction set support.
8223 @item k8, opteron, athlon64, athlon-fx
8224 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets
8225 MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
8227 IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction
8230 IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW!
8231 instruction set support.
8233 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is
8234 implemented for this chip.)
8236 Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is
8237 implemented for this chip.)
8240 While picking a specific @var{cpu-type} will schedule things appropriately
8241 for that particular chip, the compiler will not generate any code that
8242 does not run on the i386 without the @option{-march=@var{cpu-type}} option
8245 @item -march=@var{cpu-type}
8247 Generate instructions for the machine type @var{cpu-type}. The choices
8248 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
8249 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
8251 @item -mcpu=@var{cpu-type}
8253 A deprecated synonym for @option{-mtune}.
8262 @opindex mpentiumpro
8263 These options are synonyms for @option{-mtune=i386}, @option{-mtune=i486},
8264 @option{-mtune=pentium}, and @option{-mtune=pentiumpro} respectively.
8265 These synonyms are deprecated.
8267 @item -mfpmath=@var{unit}
8269 Generate floating point arithmetics for selected unit @var{unit}. The choices
8274 Use the standard 387 floating point coprocessor present majority of chips and
8275 emulated otherwise. Code compiled with this option will run almost everywhere.
8276 The temporary results are computed in 80bit precision instead of precision
8277 specified by the type resulting in slightly different results compared to most
8278 of other chips. See @option{-ffloat-store} for more detailed description.
8280 This is the default choice for i386 compiler.
8283 Use scalar floating point instructions present in the SSE instruction set.
8284 This instruction set is supported by Pentium3 and newer chips, in the AMD line
8285 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
8286 instruction set supports only single precision arithmetics, thus the double and
8287 extended precision arithmetics is still done using 387. Later version, present
8288 only in Pentium4 and the future AMD x86-64 chips supports double precision
8291 For i387 you need to use @option{-march=@var{cpu-type}}, @option{-msse} or
8292 @option{-msse2} switches to enable SSE extensions and make this option
8293 effective. For x86-64 compiler, these extensions are enabled by default.
8295 The resulting code should be considerably faster in the majority of cases and avoid
8296 the numerical instability problems of 387 code, but may break some existing
8297 code that expects temporaries to be 80bit.
8299 This is the default choice for the x86-64 compiler.
8302 Attempt to utilize both instruction sets at once. This effectively double the
8303 amount of available registers and on chips with separate execution units for
8304 387 and SSE the execution resources too. Use this option with care, as it is
8305 still experimental, because the GCC register allocator does not model separate
8306 functional units well resulting in instable performance.
8309 @item -masm=@var{dialect}
8310 @opindex masm=@var{dialect}
8311 Output asm instructions using selected @var{dialect}. Supported choices are
8312 @samp{intel} or @samp{att} (the default one).
8317 @opindex mno-ieee-fp
8318 Control whether or not the compiler uses IEEE floating point
8319 comparisons. These handle correctly the case where the result of a
8320 comparison is unordered.
8323 @opindex msoft-float
8324 Generate output containing library calls for floating point.
8325 @strong{Warning:} the requisite libraries are not part of GCC@.
8326 Normally the facilities of the machine's usual C compiler are used, but
8327 this can't be done directly in cross-compilation. You must make your
8328 own arrangements to provide suitable library functions for
8331 On machines where a function returns floating point results in the 80387
8332 register stack, some floating point opcodes may be emitted even if
8333 @option{-msoft-float} is used.
8335 @item -mno-fp-ret-in-387
8336 @opindex mno-fp-ret-in-387
8337 Do not use the FPU registers for return values of functions.
8339 The usual calling convention has functions return values of types
8340 @code{float} and @code{double} in an FPU register, even if there
8341 is no FPU@. The idea is that the operating system should emulate
8344 The option @option{-mno-fp-ret-in-387} causes such values to be returned
8345 in ordinary CPU registers instead.
8347 @item -mno-fancy-math-387
8348 @opindex mno-fancy-math-387
8349 Some 387 emulators do not support the @code{sin}, @code{cos} and
8350 @code{sqrt} instructions for the 387. Specify this option to avoid
8351 generating those instructions. This option is the default on FreeBSD,
8352 OpenBSD and NetBSD@. This option is overridden when @option{-march}
8353 indicates that the target cpu will always have an FPU and so the
8354 instruction will not need emulation. As of revision 2.6.1, these
8355 instructions are not generated unless you also use the
8356 @option{-funsafe-math-optimizations} switch.
8358 @item -malign-double
8359 @itemx -mno-align-double
8360 @opindex malign-double
8361 @opindex mno-align-double
8362 Control whether GCC aligns @code{double}, @code{long double}, and
8363 @code{long long} variables on a two word boundary or a one word
8364 boundary. Aligning @code{double} variables on a two word boundary will
8365 produce code that runs somewhat faster on a @samp{Pentium} at the
8366 expense of more memory.
8368 @strong{Warning:} if you use the @option{-malign-double} switch,
8369 structures containing the above types will be aligned differently than
8370 the published application binary interface specifications for the 386
8371 and will not be binary compatible with structures in code compiled
8372 without that switch.
8374 @item -m96bit-long-double
8375 @itemx -m128bit-long-double
8376 @opindex m96bit-long-double
8377 @opindex m128bit-long-double
8378 These switches control the size of @code{long double} type. The i386
8379 application binary interface specifies the size to be 96 bits,
8380 so @option{-m96bit-long-double} is the default in 32 bit mode.
8382 Modern architectures (Pentium and newer) would prefer @code{long double}
8383 to be aligned to an 8 or 16 byte boundary. In arrays or structures
8384 conforming to the ABI, this would not be possible. So specifying a
8385 @option{-m128bit-long-double} will align @code{long double}
8386 to a 16 byte boundary by padding the @code{long double} with an additional
8389 In the x86-64 compiler, @option{-m128bit-long-double} is the default choice as
8390 its ABI specifies that @code{long double} is to be aligned on 16 byte boundary.
8392 Notice that neither of these options enable any extra precision over the x87
8393 standard of 80 bits for a @code{long double}.
8395 @strong{Warning:} if you override the default value for your target ABI, the
8396 structures and arrays containing @code{long double} variables will change
8397 their size as well as function calling convention for function taking
8398 @code{long double} will be modified. Hence they will not be binary
8399 compatible with arrays or structures in code compiled without that switch.
8403 @itemx -mno-svr3-shlib
8404 @opindex msvr3-shlib
8405 @opindex mno-svr3-shlib
8406 Control whether GCC places uninitialized local variables into the
8407 @code{bss} or @code{data} segments. @option{-msvr3-shlib} places them
8408 into @code{bss}. These options are meaningful only on System V Release 3.
8412 Use a different function-calling convention, in which functions that
8413 take a fixed number of arguments return with the @code{ret} @var{num}
8414 instruction, which pops their arguments while returning. This saves one
8415 instruction in the caller since there is no need to pop the arguments
8418 You can specify that an individual function is called with this calling
8419 sequence with the function attribute @samp{stdcall}. You can also
8420 override the @option{-mrtd} option by using the function attribute
8421 @samp{cdecl}. @xref{Function Attributes}.
8423 @strong{Warning:} this calling convention is incompatible with the one
8424 normally used on Unix, so you cannot use it if you need to call
8425 libraries compiled with the Unix compiler.
8427 Also, you must provide function prototypes for all functions that
8428 take variable numbers of arguments (including @code{printf});
8429 otherwise incorrect code will be generated for calls to those
8432 In addition, seriously incorrect code will result if you call a
8433 function with too many arguments. (Normally, extra arguments are
8434 harmlessly ignored.)
8436 @item -mregparm=@var{num}
8438 Control how many registers are used to pass integer arguments. By
8439 default, no registers are used to pass arguments, and at most 3
8440 registers can be used. You can control this behavior for a specific
8441 function by using the function attribute @samp{regparm}.
8442 @xref{Function Attributes}.
8444 @strong{Warning:} if you use this switch, and
8445 @var{num} is nonzero, then you must build all modules with the same
8446 value, including any libraries. This includes the system libraries and
8449 @item -mpreferred-stack-boundary=@var{num}
8450 @opindex mpreferred-stack-boundary
8451 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
8452 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
8453 the default is 4 (16 bytes or 128 bits), except when optimizing for code
8454 size (@option{-Os}), in which case the default is the minimum correct
8455 alignment (4 bytes for x86, and 8 bytes for x86-64).
8457 On Pentium and PentiumPro, @code{double} and @code{long double} values
8458 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
8459 suffer significant run time performance penalties. On Pentium III, the
8460 Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
8461 penalties if it is not 16 byte aligned.
8463 To ensure proper alignment of this values on the stack, the stack boundary
8464 must be as aligned as that required by any value stored on the stack.
8465 Further, every function must be generated such that it keeps the stack
8466 aligned. Thus calling a function compiled with a higher preferred
8467 stack boundary from a function compiled with a lower preferred stack
8468 boundary will most likely misalign the stack. It is recommended that
8469 libraries that use callbacks always use the default setting.
8471 This extra alignment does consume extra stack space, and generally
8472 increases code size. Code that is sensitive to stack space usage, such
8473 as embedded systems and operating system kernels, may want to reduce the
8474 preferred alignment to @option{-mpreferred-stack-boundary=2}.
8492 These switches enable or disable the use of built-in functions that allow
8493 direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow extensions of the
8496 @xref{X86 Built-in Functions}, for details of the functions enabled
8497 and disabled by these switches.
8499 To have SSE/SSE2 instructions generated automatically from floating-point
8500 code, see @option{-mfpmath=sse}.
8503 @itemx -mno-push-args
8505 @opindex mno-push-args
8506 Use PUSH operations to store outgoing parameters. This method is shorter
8507 and usually equally fast as method using SUB/MOV operations and is enabled
8508 by default. In some cases disabling it may improve performance because of
8509 improved scheduling and reduced dependencies.
8511 @item -maccumulate-outgoing-args
8512 @opindex maccumulate-outgoing-args
8513 If enabled, the maximum amount of space required for outgoing arguments will be
8514 computed in the function prologue. This is faster on most modern CPUs
8515 because of reduced dependencies, improved scheduling and reduced stack usage
8516 when preferred stack boundary is not equal to 2. The drawback is a notable
8517 increase in code size. This switch implies @option{-mno-push-args}.
8521 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
8522 on thread-safe exception handling must compile and link all code with the
8523 @option{-mthreads} option. When compiling, @option{-mthreads} defines
8524 @option{-D_MT}; when linking, it links in a special thread helper library
8525 @option{-lmingwthrd} which cleans up per thread exception handling data.
8527 @item -mno-align-stringops
8528 @opindex mno-align-stringops
8529 Do not align destination of inlined string operations. This switch reduces
8530 code size and improves performance in case the destination is already aligned,
8531 but GCC doesn't know about it.
8533 @item -minline-all-stringops
8534 @opindex minline-all-stringops
8535 By default GCC inlines string operations only when destination is known to be
8536 aligned at least to 4 byte boundary. This enables more inlining, increase code
8537 size, but may improve performance of code that depends on fast memcpy, strlen
8538 and memset for short lengths.
8540 @item -momit-leaf-frame-pointer
8541 @opindex momit-leaf-frame-pointer
8542 Don't keep the frame pointer in a register for leaf functions. This
8543 avoids the instructions to save, set up and restore frame pointers and
8544 makes an extra register available in leaf functions. The option
8545 @option{-fomit-frame-pointer} removes the frame pointer for all functions
8546 which might make debugging harder.
8548 @item -mtls-direct-seg-refs
8549 @itemx -mno-tls-direct-seg-refs
8550 @opindex mtls-direct-seg-refs
8551 Controls whether TLS variables may be accessed with offsets from the
8552 TLS segment register (@code{%gs} for 32-bit, @code{%fs} for 64-bit),
8553 or whether the thread base pointer must be added. Whether or not this
8554 is legal depends on the operating system, and whether it maps the
8555 segment to cover the entire TLS area.
8557 For systems that use GNU libc, the default is on.
8560 These @samp{-m} switches are supported in addition to the above
8561 on AMD x86-64 processors in 64-bit environments.
8568 Generate code for a 32-bit or 64-bit environment.
8569 The 32-bit environment sets int, long and pointer to 32 bits and
8570 generates code that runs on any i386 system.
8571 The 64-bit environment sets int to 32 bits and long and pointer
8572 to 64 bits and generates code for AMD's x86-64 architecture.
8575 @opindex no-red-zone
8576 Do not use a so called red zone for x86-64 code. The red zone is mandated
8577 by the x86-64 ABI, it is a 128-byte area beyond the location of the
8578 stack pointer that will not be modified by signal or interrupt handlers
8579 and therefore can be used for temporary data without adjusting the stack
8580 pointer. The flag @option{-mno-red-zone} disables this red zone.
8582 @item -mcmodel=small
8583 @opindex mcmodel=small
8584 Generate code for the small code model: the program and its symbols must
8585 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
8586 Programs can be statically or dynamically linked. This is the default
8589 @item -mcmodel=kernel
8590 @opindex mcmodel=kernel
8591 Generate code for the kernel code model. The kernel runs in the
8592 negative 2 GB of the address space.
8593 This model has to be used for Linux kernel code.
8595 @item -mcmodel=medium
8596 @opindex mcmodel=medium
8597 Generate code for the medium model: The program is linked in the lower 2
8598 GB of the address space but symbols can be located anywhere in the
8599 address space. Programs can be statically or dynamically linked, but
8600 building of shared libraries are not supported with the medium model.
8602 @item -mcmodel=large
8603 @opindex mcmodel=large
8604 Generate code for the large model: This model makes no assumptions
8605 about addresses and sizes of sections. Currently GCC does not implement
8610 @subsection HPPA Options
8611 @cindex HPPA Options
8613 These @samp{-m} options are defined for the HPPA family of computers:
8616 @item -march=@var{architecture-type}
8618 Generate code for the specified architecture. The choices for
8619 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
8620 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
8621 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
8622 architecture option for your machine. Code compiled for lower numbered
8623 architectures will run on higher numbered architectures, but not the
8626 PA 2.0 support currently requires gas snapshot 19990413 or later. The
8627 next release of binutils (current is 2.9.1) will probably contain PA 2.0
8631 @itemx -mpa-risc-1-1
8632 @itemx -mpa-risc-2-0
8633 @opindex mpa-risc-1-0
8634 @opindex mpa-risc-1-1
8635 @opindex mpa-risc-2-0
8636 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
8639 @opindex mbig-switch
8640 Generate code suitable for big switch tables. Use this option only if
8641 the assembler/linker complain about out of range branches within a switch
8644 @item -mjump-in-delay
8645 @opindex mjump-in-delay
8646 Fill delay slots of function calls with unconditional jump instructions
8647 by modifying the return pointer for the function call to be the target
8648 of the conditional jump.
8650 @item -mdisable-fpregs
8651 @opindex mdisable-fpregs
8652 Prevent floating point registers from being used in any manner. This is
8653 necessary for compiling kernels which perform lazy context switching of
8654 floating point registers. If you use this option and attempt to perform
8655 floating point operations, the compiler will abort.
8657 @item -mdisable-indexing
8658 @opindex mdisable-indexing
8659 Prevent the compiler from using indexing address modes. This avoids some
8660 rather obscure problems when compiling MIG generated code under MACH@.
8662 @item -mno-space-regs
8663 @opindex mno-space-regs
8664 Generate code that assumes the target has no space registers. This allows
8665 GCC to generate faster indirect calls and use unscaled index address modes.
8667 Such code is suitable for level 0 PA systems and kernels.
8669 @item -mfast-indirect-calls
8670 @opindex mfast-indirect-calls
8671 Generate code that assumes calls never cross space boundaries. This
8672 allows GCC to emit code which performs faster indirect calls.
8674 This option will not work in the presence of shared libraries or nested
8677 @item -mlong-load-store
8678 @opindex mlong-load-store
8679 Generate 3-instruction load and store sequences as sometimes required by
8680 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
8683 @item -mportable-runtime
8684 @opindex mportable-runtime
8685 Use the portable calling conventions proposed by HP for ELF systems.
8689 Enable the use of assembler directives only GAS understands.
8691 @item -mschedule=@var{cpu-type}
8693 Schedule code according to the constraints for the machine type
8694 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
8695 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
8696 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
8697 proper scheduling option for your machine. The default scheduling is
8701 @opindex mlinker-opt
8702 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
8703 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
8704 linkers in which they give bogus error messages when linking some programs.
8707 @opindex msoft-float
8708 Generate output containing library calls for floating point.
8709 @strong{Warning:} the requisite libraries are not available for all HPPA
8710 targets. Normally the facilities of the machine's usual C compiler are
8711 used, but this cannot be done directly in cross-compilation. You must make
8712 your own arrangements to provide suitable library functions for
8713 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
8714 does provide software floating point support.
8716 @option{-msoft-float} changes the calling convention in the output file;
8717 therefore, it is only useful if you compile @emph{all} of a program with
8718 this option. In particular, you need to compile @file{libgcc.a}, the
8719 library that comes with GCC, with @option{-msoft-float} in order for
8724 Generate the predefine, @code{_SIO}, for server IO. The default is
8725 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
8726 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO. These
8727 options are available under HP-UX and HI-UX.
8731 Use GNU ld specific options. This passes @option{-shared} to ld when
8732 building a shared library. It is the default when GCC is configured,
8733 explicitly or implicitly, with the GNU linker. This option does not
8734 have any affect on which ld is called, it only changes what parameters
8735 are passed to that ld. The ld that is called is determined by the
8736 @option{--with-ld} configure option, GCC's program search path, and
8737 finally by the user's @env{PATH}. The linker used by GCC can be printed
8738 using @samp{which `gcc -print-prog-name=ld`}.
8742 Use HP ld specific options. This passes @option{-b} to ld when building
8743 a shared library and passes @option{+Accept TypeMismatch} to ld on all
8744 links. It is the default when GCC is configured, explicitly or
8745 implicitly, with the HP linker. This option does not have any affect on
8746 which ld is called, it only changes what parameters are passed to that
8747 ld. The ld that is called is determined by the @option{--with-ld}
8748 configure option, GCC's program search path, and finally by the user's
8749 @env{PATH}. The linker used by GCC can be printed using @samp{which
8750 `gcc -print-prog-name=ld`}.
8755 Select the FDPIC ABI, that uses function descriptors to represent
8756 pointers to functions. Without any PIC/PIE-related options, it
8757 implies @option{-fPIE}. With @option{-fpic} or @option{-fpie}, it
8758 assumes GOT entries and small data are within a 12-bit range from the
8759 GOT base address; with @option{-fPIC} or @option{-fPIE}, GOT offsets
8760 are computed with 32 bits.
8763 @opindex minline-plt
8765 Enable inlining of PLT entries in function calls to functions that are
8766 not known to bind locally. It has no effect without @option{-mfdpic}.
8767 It's enabled by default if optimizing for speed and compiling for
8768 shared libraries (i.e., @option{-fPIC} or @option{-fpic}), or when an
8769 optimization option such as @option{-O3} or above is present in the
8775 Enable the use of @code{GPREL} relocations in the FDPIC ABI for data
8776 that is known to be in read-only sections. It's enabled by default,
8777 except for @option{-fpic} or @option{-fpie}: even though it may help
8778 make the global offset table smaller, it trades 1 instruction for 4.
8779 With @option{-fPIC} or @option{-fPIE}, it trades 3 instructions for 4,
8780 one of which may be shared by multiple symbols, and it avoids the need
8781 for a GOT entry for the referenced symbol, so it's more likely to be a
8782 win. If it is not, @option{-mno-gprel-ro} can be used to disable it.
8784 @item -multilib-library-pic
8785 @opindex multilib-library-pic
8787 Link with the (library, not FD) pic libraries. It's implied by
8788 @option{-mlibrary-pic}, as well as by @option{-fPIC} and
8789 @option{-fpic} without @option{-mfdpic}. You should never have to use
8795 Follow the EABI requirement of always creating a frame pointer whenever
8796 a stack frame is allocated. This option is enabled by default and can
8797 be disabled with @option{-mno-linked-fp}.
8800 @opindex mno-long-calls
8801 Generate code that uses long call sequences. This ensures that a call
8802 is always able to reach linker generated stubs. The default is to generate
8803 long calls only when the distance from the call site to the beginning
8804 of the function or translation unit, as the case may be, exceeds a
8805 predefined limit set by the branch type being used. The limits for
8806 normal calls are 7,600,000 and 240,000 bytes, respectively for the
8807 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
8810 Distances are measured from the beginning of functions when using the
8811 @option{-ffunction-sections} option, or when using the @option{-mgas}
8812 and @option{-mno-portable-runtime} options together under HP-UX with
8815 It is normally not desirable to use this option as it will degrade
8816 performance. However, it may be useful in large applications,
8817 particularly when partial linking is used to build the application.
8819 The types of long calls used depends on the capabilities of the
8820 assembler and linker, and the type of code being generated. The
8821 impact on systems that support long absolute calls, and long pic
8822 symbol-difference or pc-relative calls should be relatively small.
8823 However, an indirect call is used on 32-bit ELF systems in pic code
8824 and it is quite long.
8828 Suppress the generation of link options to search libdld.sl when the
8829 @option{-static} option is specified on HP-UX 10 and later.
8833 The HP-UX implementation of setlocale in libc has a dependency on
8834 libdld.sl. There isn't an archive version of libdld.sl. Thus,
8835 when the @option{-static} option is specified, special link options
8836 are needed to resolve this dependency.
8838 On HP-UX 10 and later, the GCC driver adds the necessary options to
8839 link with libdld.sl when the @option{-static} option is specified.
8840 This causes the resulting binary to be dynamic. On the 64-bit port,
8841 the linkers generate dynamic binaries by default in any case. The
8842 @option{-nolibdld} option can be used to prevent the GCC driver from
8843 adding these link options.
8847 Add support for multithreading with the @dfn{dce thread} library
8848 under HP-UX. This option sets flags for both the preprocessor and
8852 @node DEC Alpha Options
8853 @subsection DEC Alpha Options
8855 These @samp{-m} options are defined for the DEC Alpha implementations:
8858 @item -mno-soft-float
8860 @opindex mno-soft-float
8861 @opindex msoft-float
8862 Use (do not use) the hardware floating-point instructions for
8863 floating-point operations. When @option{-msoft-float} is specified,
8864 functions in @file{libgcc.a} will be used to perform floating-point
8865 operations. Unless they are replaced by routines that emulate the
8866 floating-point operations, or compiled in such a way as to call such
8867 emulations routines, these routines will issue floating-point
8868 operations. If you are compiling for an Alpha without floating-point
8869 operations, you must ensure that the library is built so as not to call
8872 Note that Alpha implementations without floating-point operations are
8873 required to have floating-point registers.
8878 @opindex mno-fp-regs
8879 Generate code that uses (does not use) the floating-point register set.
8880 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
8881 register set is not used, floating point operands are passed in integer
8882 registers as if they were integers and floating-point results are passed
8883 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
8884 so any function with a floating-point argument or return value called by code
8885 compiled with @option{-mno-fp-regs} must also be compiled with that
8888 A typical use of this option is building a kernel that does not use,
8889 and hence need not save and restore, any floating-point registers.
8893 The Alpha architecture implements floating-point hardware optimized for
8894 maximum performance. It is mostly compliant with the IEEE floating
8895 point standard. However, for full compliance, software assistance is
8896 required. This option generates code fully IEEE compliant code
8897 @emph{except} that the @var{inexact-flag} is not maintained (see below).
8898 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
8899 defined during compilation. The resulting code is less efficient but is
8900 able to correctly support denormalized numbers and exceptional IEEE
8901 values such as not-a-number and plus/minus infinity. Other Alpha
8902 compilers call this option @option{-ieee_with_no_inexact}.
8904 @item -mieee-with-inexact
8905 @opindex mieee-with-inexact
8906 This is like @option{-mieee} except the generated code also maintains
8907 the IEEE @var{inexact-flag}. Turning on this option causes the
8908 generated code to implement fully-compliant IEEE math. In addition to
8909 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
8910 macro. On some Alpha implementations the resulting code may execute
8911 significantly slower than the code generated by default. Since there is
8912 very little code that depends on the @var{inexact-flag}, you should
8913 normally not specify this option. Other Alpha compilers call this
8914 option @option{-ieee_with_inexact}.
8916 @item -mfp-trap-mode=@var{trap-mode}
8917 @opindex mfp-trap-mode
8918 This option controls what floating-point related traps are enabled.
8919 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
8920 The trap mode can be set to one of four values:
8924 This is the default (normal) setting. The only traps that are enabled
8925 are the ones that cannot be disabled in software (e.g., division by zero
8929 In addition to the traps enabled by @samp{n}, underflow traps are enabled
8933 Like @samp{su}, but the instructions are marked to be safe for software
8934 completion (see Alpha architecture manual for details).
8937 Like @samp{su}, but inexact traps are enabled as well.
8940 @item -mfp-rounding-mode=@var{rounding-mode}
8941 @opindex mfp-rounding-mode
8942 Selects the IEEE rounding mode. Other Alpha compilers call this option
8943 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
8948 Normal IEEE rounding mode. Floating point numbers are rounded towards
8949 the nearest machine number or towards the even machine number in case
8953 Round towards minus infinity.
8956 Chopped rounding mode. Floating point numbers are rounded towards zero.
8959 Dynamic rounding mode. A field in the floating point control register
8960 (@var{fpcr}, see Alpha architecture reference manual) controls the
8961 rounding mode in effect. The C library initializes this register for
8962 rounding towards plus infinity. Thus, unless your program modifies the
8963 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
8966 @item -mtrap-precision=@var{trap-precision}
8967 @opindex mtrap-precision
8968 In the Alpha architecture, floating point traps are imprecise. This
8969 means without software assistance it is impossible to recover from a
8970 floating trap and program execution normally needs to be terminated.
8971 GCC can generate code that can assist operating system trap handlers
8972 in determining the exact location that caused a floating point trap.
8973 Depending on the requirements of an application, different levels of
8974 precisions can be selected:
8978 Program precision. This option is the default and means a trap handler
8979 can only identify which program caused a floating point exception.
8982 Function precision. The trap handler can determine the function that
8983 caused a floating point exception.
8986 Instruction precision. The trap handler can determine the exact
8987 instruction that caused a floating point exception.
8990 Other Alpha compilers provide the equivalent options called
8991 @option{-scope_safe} and @option{-resumption_safe}.
8993 @item -mieee-conformant
8994 @opindex mieee-conformant
8995 This option marks the generated code as IEEE conformant. You must not
8996 use this option unless you also specify @option{-mtrap-precision=i} and either
8997 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
8998 is to emit the line @samp{.eflag 48} in the function prologue of the
8999 generated assembly file. Under DEC Unix, this has the effect that
9000 IEEE-conformant math library routines will be linked in.
9002 @item -mbuild-constants
9003 @opindex mbuild-constants
9004 Normally GCC examines a 32- or 64-bit integer constant to
9005 see if it can construct it from smaller constants in two or three
9006 instructions. If it cannot, it will output the constant as a literal and
9007 generate code to load it from the data segment at runtime.
9009 Use this option to require GCC to construct @emph{all} integer constants
9010 using code, even if it takes more instructions (the maximum is six).
9012 You would typically use this option to build a shared library dynamic
9013 loader. Itself a shared library, it must relocate itself in memory
9014 before it can find the variables and constants in its own data segment.
9020 Select whether to generate code to be assembled by the vendor-supplied
9021 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
9039 Indicate whether GCC should generate code to use the optional BWX,
9040 CIX, FIX and MAX instruction sets. The default is to use the instruction
9041 sets supported by the CPU type specified via @option{-mcpu=} option or that
9042 of the CPU on which GCC was built if none was specified.
9047 @opindex mfloat-ieee
9048 Generate code that uses (does not use) VAX F and G floating point
9049 arithmetic instead of IEEE single and double precision.
9051 @item -mexplicit-relocs
9052 @itemx -mno-explicit-relocs
9053 @opindex mexplicit-relocs
9054 @opindex mno-explicit-relocs
9055 Older Alpha assemblers provided no way to generate symbol relocations
9056 except via assembler macros. Use of these macros does not allow
9057 optimal instruction scheduling. GNU binutils as of version 2.12
9058 supports a new syntax that allows the compiler to explicitly mark
9059 which relocations should apply to which instructions. This option
9060 is mostly useful for debugging, as GCC detects the capabilities of
9061 the assembler when it is built and sets the default accordingly.
9065 @opindex msmall-data
9066 @opindex mlarge-data
9067 When @option{-mexplicit-relocs} is in effect, static data is
9068 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
9069 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
9070 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
9071 16-bit relocations off of the @code{$gp} register. This limits the
9072 size of the small data area to 64KB, but allows the variables to be
9073 directly accessed via a single instruction.
9075 The default is @option{-mlarge-data}. With this option the data area
9076 is limited to just below 2GB. Programs that require more than 2GB of
9077 data must use @code{malloc} or @code{mmap} to allocate the data in the
9078 heap instead of in the program's data segment.
9080 When generating code for shared libraries, @option{-fpic} implies
9081 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
9085 @opindex msmall-text
9086 @opindex mlarge-text
9087 When @option{-msmall-text} is used, the compiler assumes that the
9088 code of the entire program (or shared library) fits in 4MB, and is
9089 thus reachable with a branch instruction. When @option{-msmall-data}
9090 is used, the compiler can assume that all local symbols share the
9091 same @code{$gp} value, and thus reduce the number of instructions
9092 required for a function call from 4 to 1.
9094 The default is @option{-mlarge-text}.
9096 @item -mcpu=@var{cpu_type}
9098 Set the instruction set and instruction scheduling parameters for
9099 machine type @var{cpu_type}. You can specify either the @samp{EV}
9100 style name or the corresponding chip number. GCC supports scheduling
9101 parameters for the EV4, EV5 and EV6 family of processors and will
9102 choose the default values for the instruction set from the processor
9103 you specify. If you do not specify a processor type, GCC will default
9104 to the processor on which the compiler was built.
9106 Supported values for @var{cpu_type} are
9112 Schedules as an EV4 and has no instruction set extensions.
9116 Schedules as an EV5 and has no instruction set extensions.
9120 Schedules as an EV5 and supports the BWX extension.
9125 Schedules as an EV5 and supports the BWX and MAX extensions.
9129 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
9133 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
9136 @item -mtune=@var{cpu_type}
9138 Set only the instruction scheduling parameters for machine type
9139 @var{cpu_type}. The instruction set is not changed.
9141 @item -mmemory-latency=@var{time}
9142 @opindex mmemory-latency
9143 Sets the latency the scheduler should assume for typical memory
9144 references as seen by the application. This number is highly
9145 dependent on the memory access patterns used by the application
9146 and the size of the external cache on the machine.
9148 Valid options for @var{time} are
9152 A decimal number representing clock cycles.
9158 The compiler contains estimates of the number of clock cycles for
9159 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
9160 (also called Dcache, Scache, and Bcache), as well as to main memory.
9161 Note that L3 is only valid for EV5.
9166 @node DEC Alpha/VMS Options
9167 @subsection DEC Alpha/VMS Options
9169 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
9172 @item -mvms-return-codes
9173 @opindex mvms-return-codes
9174 Return VMS condition codes from main. The default is to return POSIX
9175 style condition (e.g.@ error) codes.
9178 @node H8/300 Options
9179 @subsection H8/300 Options
9181 These @samp{-m} options are defined for the H8/300 implementations:
9186 Shorten some address references at link time, when possible; uses the
9187 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
9188 ld, Using ld}, for a fuller description.
9192 Generate code for the H8/300H@.
9196 Generate code for the H8S@.
9200 Generate code for the H8S and H8/300H in the normal mode. This switch
9201 must be used either with -mh or -ms.
9205 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
9209 Make @code{int} data 32 bits by default.
9213 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
9214 The default for the H8/300H and H8S is to align longs and floats on 4
9216 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
9217 This option has no effect on the H8/300.
9221 @subsection SH Options
9223 These @samp{-m} options are defined for the SH implementations:
9228 Generate code for the SH1.
9232 Generate code for the SH2.
9235 Generate code for the SH2e.
9239 Generate code for the SH3.
9243 Generate code for the SH3e.
9247 Generate code for the SH4 without a floating-point unit.
9249 @item -m4-single-only
9250 @opindex m4-single-only
9251 Generate code for the SH4 with a floating-point unit that only
9252 supports single-precision arithmetic.
9256 Generate code for the SH4 assuming the floating-point unit is in
9257 single-precision mode by default.
9261 Generate code for the SH4.
9265 Compile code for the processor in big endian mode.
9269 Compile code for the processor in little endian mode.
9273 Align doubles at 64-bit boundaries. Note that this changes the calling
9274 conventions, and thus some functions from the standard C library will
9275 not work unless you recompile it first with @option{-mdalign}.
9279 Shorten some address references at link time, when possible; uses the
9280 linker option @option{-relax}.
9284 Use 32-bit offsets in @code{switch} tables. The default is to use
9289 Enable the use of the instruction @code{fmovd}.
9293 Comply with the calling conventions defined by Renesas.
9297 Mark the @code{MAC} register as call-clobbered, even if
9298 @option{-mhitachi} is given.
9302 Increase IEEE-compliance of floating-point code.
9306 Dump instruction size and location in the assembly code.
9310 This option is deprecated. It pads structures to multiple of 4 bytes,
9311 which is incompatible with the SH ABI@.
9315 Optimize for space instead of speed. Implied by @option{-Os}.
9319 When generating position-independent code, emit function calls using
9320 the Global Offset Table instead of the Procedure Linkage Table.
9324 Generate a library function call to invalidate instruction cache
9325 entries, after fixing up a trampoline. This library function call
9326 doesn't assume it can write to the whole memory address space. This
9327 is the default when the target is @code{sh-*-linux*}.
9330 @node System V Options
9331 @subsection Options for System V
9333 These additional options are available on System V Release 4 for
9334 compatibility with other compilers on those systems:
9339 Create a shared object.
9340 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
9344 Identify the versions of each tool used by the compiler, in a
9345 @code{.ident} assembler directive in the output.
9349 Refrain from adding @code{.ident} directives to the output file (this is
9352 @item -YP,@var{dirs}
9354 Search the directories @var{dirs}, and no others, for libraries
9355 specified with @option{-l}.
9359 Look in the directory @var{dir} to find the M4 preprocessor.
9360 The assembler uses this option.
9361 @c This is supposed to go with a -Yd for predefined M4 macro files, but
9362 @c the generic assembler that comes with Solaris takes just -Ym.
9365 @node TMS320C3x/C4x Options
9366 @subsection TMS320C3x/C4x Options
9367 @cindex TMS320C3x/C4x Options
9369 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
9373 @item -mcpu=@var{cpu_type}
9375 Set the instruction set, register set, and instruction scheduling
9376 parameters for machine type @var{cpu_type}. Supported values for
9377 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
9378 @samp{c44}. The default is @samp{c40} to generate code for the
9383 @itemx -msmall-memory
9385 @opindex mbig-memory
9387 @opindex msmall-memory
9389 Generates code for the big or small memory model. The small memory
9390 model assumed that all data fits into one 64K word page. At run-time
9391 the data page (DP) register must be set to point to the 64K page
9392 containing the .bss and .data program sections. The big memory model is
9393 the default and requires reloading of the DP register for every direct
9400 Allow (disallow) allocation of general integer operands into the block
9407 Enable (disable) generation of code using decrement and branch,
9408 DBcond(D), instructions. This is enabled by default for the C4x. To be
9409 on the safe side, this is disabled for the C3x, since the maximum
9410 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
9411 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
9412 that it can utilize the decrement and branch instruction, but will give
9413 up if there is more than one memory reference in the loop. Thus a loop
9414 where the loop counter is decremented can generate slightly more
9415 efficient code, in cases where the RPTB instruction cannot be utilized.
9417 @item -mdp-isr-reload
9419 @opindex mdp-isr-reload
9421 Force the DP register to be saved on entry to an interrupt service
9422 routine (ISR), reloaded to point to the data section, and restored on
9423 exit from the ISR@. This should not be required unless someone has
9424 violated the small memory model by modifying the DP register, say within
9431 For the C3x use the 24-bit MPYI instruction for integer multiplies
9432 instead of a library call to guarantee 32-bit results. Note that if one
9433 of the operands is a constant, then the multiplication will be performed
9434 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
9435 then squaring operations are performed inline instead of a library call.
9438 @itemx -mno-fast-fix
9440 @opindex mno-fast-fix
9441 The C3x/C4x FIX instruction to convert a floating point value to an
9442 integer value chooses the nearest integer less than or equal to the
9443 floating point value rather than to the nearest integer. Thus if the
9444 floating point number is negative, the result will be incorrectly
9445 truncated an additional code is necessary to detect and correct this
9446 case. This option can be used to disable generation of the additional
9447 code required to correct the result.
9453 Enable (disable) generation of repeat block sequences using the RPTB
9454 instruction for zero overhead looping. The RPTB construct is only used
9455 for innermost loops that do not call functions or jump across the loop
9456 boundaries. There is no advantage having nested RPTB loops due to the
9457 overhead required to save and restore the RC, RS, and RE registers.
9458 This is enabled by default with @option{-O2}.
9460 @item -mrpts=@var{count}
9464 Enable (disable) the use of the single instruction repeat instruction
9465 RPTS@. If a repeat block contains a single instruction, and the loop
9466 count can be guaranteed to be less than the value @var{count}, GCC will
9467 emit a RPTS instruction instead of a RPTB@. If no value is specified,
9468 then a RPTS will be emitted even if the loop count cannot be determined
9469 at compile time. Note that the repeated instruction following RPTS does
9470 not have to be reloaded from memory each iteration, thus freeing up the
9471 CPU buses for operands. However, since interrupts are blocked by this
9472 instruction, it is disabled by default.
9474 @item -mloop-unsigned
9475 @itemx -mno-loop-unsigned
9476 @opindex mloop-unsigned
9477 @opindex mno-loop-unsigned
9478 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
9479 is @math{2^{31} + 1} since these instructions test if the iteration count is
9480 negative to terminate the loop. If the iteration count is unsigned
9481 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
9482 exceeded. This switch allows an unsigned iteration count.
9486 Try to emit an assembler syntax that the TI assembler (asm30) is happy
9487 with. This also enforces compatibility with the API employed by the TI
9488 C3x C compiler. For example, long doubles are passed as structures
9489 rather than in floating point registers.
9495 Generate code that uses registers (stack) for passing arguments to functions.
9496 By default, arguments are passed in registers where possible rather
9497 than by pushing arguments on to the stack.
9499 @item -mparallel-insns
9500 @itemx -mno-parallel-insns
9501 @opindex mparallel-insns
9502 @opindex mno-parallel-insns
9503 Allow the generation of parallel instructions. This is enabled by
9504 default with @option{-O2}.
9506 @item -mparallel-mpy
9507 @itemx -mno-parallel-mpy
9508 @opindex mparallel-mpy
9509 @opindex mno-parallel-mpy
9510 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
9511 provided @option{-mparallel-insns} is also specified. These instructions have
9512 tight register constraints which can pessimize the code generation
9518 @subsection V850 Options
9519 @cindex V850 Options
9521 These @samp{-m} options are defined for V850 implementations:
9525 @itemx -mno-long-calls
9526 @opindex mlong-calls
9527 @opindex mno-long-calls
9528 Treat all calls as being far away (near). If calls are assumed to be
9529 far away, the compiler will always load the functions address up into a
9530 register, and call indirect through the pointer.
9536 Do not optimize (do optimize) basic blocks that use the same index
9537 pointer 4 or more times to copy pointer into the @code{ep} register, and
9538 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
9539 option is on by default if you optimize.
9541 @item -mno-prolog-function
9542 @itemx -mprolog-function
9543 @opindex mno-prolog-function
9544 @opindex mprolog-function
9545 Do not use (do use) external functions to save and restore registers
9546 at the prologue and epilogue of a function. The external functions
9547 are slower, but use less code space if more than one function saves
9548 the same number of registers. The @option{-mprolog-function} option
9549 is on by default if you optimize.
9553 Try to make the code as small as possible. At present, this just turns
9554 on the @option{-mep} and @option{-mprolog-function} options.
9558 Put static or global variables whose size is @var{n} bytes or less into
9559 the tiny data area that register @code{ep} points to. The tiny data
9560 area can hold up to 256 bytes in total (128 bytes for byte references).
9564 Put static or global variables whose size is @var{n} bytes or less into
9565 the small data area that register @code{gp} points to. The small data
9566 area can hold up to 64 kilobytes.
9570 Put static or global variables whose size is @var{n} bytes or less into
9571 the first 32 kilobytes of memory.
9575 Specify that the target processor is the V850.
9578 @opindex mbig-switch
9579 Generate code suitable for big switch tables. Use this option only if
9580 the assembler/linker complain about out of range branches within a switch
9585 This option will cause r2 and r5 to be used in the code generated by
9586 the compiler. This setting is the default.
9589 @opindex mno-app-regs
9590 This option will cause r2 and r5 to be treated as fixed registers.
9594 Specify that the target processor is the V850E1. The preprocessor
9595 constants @samp{__v850e1__} and @samp{__v850e__} will be defined if
9596 this option is used.
9600 Specify that the target processor is the V850E. The preprocessor
9601 constant @samp{__v850e__} will be defined if this option is used.
9603 If neither @option{-mv850} nor @option{-mv850e} nor @option{-mv850e1}
9604 are defined then a default target processor will be chosen and the
9605 relevant @samp{__v850*__} preprocessor constant will be defined.
9607 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
9608 defined, regardless of which processor variant is the target.
9610 @item -mdisable-callt
9611 @opindex mdisable-callt
9612 This option will suppress generation of the CALLT instruction for the
9613 v850e and v850e1 flavors of the v850 architecture. The default is
9614 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
9619 @subsection ARC Options
9622 These options are defined for ARC implementations:
9627 Compile code for little endian mode. This is the default.
9631 Compile code for big endian mode.
9634 @opindex mmangle-cpu
9635 Prepend the name of the cpu to all public symbol names.
9636 In multiple-processor systems, there are many ARC variants with different
9637 instruction and register set characteristics. This flag prevents code
9638 compiled for one cpu to be linked with code compiled for another.
9639 No facility exists for handling variants that are ``almost identical''.
9640 This is an all or nothing option.
9642 @item -mcpu=@var{cpu}
9644 Compile code for ARC variant @var{cpu}.
9645 Which variants are supported depend on the configuration.
9646 All variants support @option{-mcpu=base}, this is the default.
9648 @item -mtext=@var{text-section}
9649 @itemx -mdata=@var{data-section}
9650 @itemx -mrodata=@var{readonly-data-section}
9654 Put functions, data, and readonly data in @var{text-section},
9655 @var{data-section}, and @var{readonly-data-section} respectively
9656 by default. This can be overridden with the @code{section} attribute.
9657 @xref{Variable Attributes}.
9662 @subsection NS32K Options
9663 @cindex NS32K options
9665 These are the @samp{-m} options defined for the 32000 series. The default
9666 values for these options depends on which style of 32000 was selected when
9667 the compiler was configured; the defaults for the most common choices are
9675 Generate output for a 32032. This is the default
9676 when the compiler is configured for 32032 and 32016 based systems.
9682 Generate output for a 32332. This is the default
9683 when the compiler is configured for 32332-based systems.
9689 Generate output for a 32532. This is the default
9690 when the compiler is configured for 32532-based systems.
9694 Generate output containing 32081 instructions for floating point.
9695 This is the default for all systems.
9699 Generate output containing 32381 instructions for floating point. This
9700 also implies @option{-m32081}. The 32381 is only compatible with the 32332
9701 and 32532 cpus. This is the default for the pc532-netbsd configuration.
9705 Try and generate multiply-add floating point instructions @code{polyF}
9706 and @code{dotF}. This option is only available if the @option{-m32381}
9707 option is in effect. Using these instructions requires changes to
9708 register allocation which generally has a negative impact on
9709 performance. This option should only be enabled when compiling code
9710 particularly likely to make heavy use of multiply-add instructions.
9713 @opindex mnomulti-add
9714 Do not try and generate multiply-add floating point instructions
9715 @code{polyF} and @code{dotF}. This is the default on all platforms.
9718 @opindex msoft-float
9719 Generate output containing library calls for floating point.
9720 @strong{Warning:} the requisite libraries may not be available.
9722 @item -mieee-compare
9723 @itemx -mno-ieee-compare
9724 @opindex mieee-compare
9725 @opindex mno-ieee-compare
9726 Control whether or not the compiler uses IEEE floating point
9727 comparisons. These handle correctly the case where the result of a
9728 comparison is unordered.
9729 @strong{Warning:} the requisite kernel support may not be available.
9732 @opindex mnobitfield
9733 Do not use the bit-field instructions. On some machines it is faster to
9734 use shifting and masking operations. This is the default for the pc532.
9738 Do use the bit-field instructions. This is the default for all platforms
9743 Use a different function-calling convention, in which functions
9744 that take a fixed number of arguments return pop their
9745 arguments on return with the @code{ret} instruction.
9747 This calling convention is incompatible with the one normally
9748 used on Unix, so you cannot use it if you need to call libraries
9749 compiled with the Unix compiler.
9751 Also, you must provide function prototypes for all functions that
9752 take variable numbers of arguments (including @code{printf});
9753 otherwise incorrect code will be generated for calls to those
9756 In addition, seriously incorrect code will result if you call a
9757 function with too many arguments. (Normally, extra arguments are
9758 harmlessly ignored.)
9760 This option takes its name from the 680x0 @code{rtd} instruction.
9765 Use a different function-calling convention where the first two arguments
9766 are passed in registers.
9768 This calling convention is incompatible with the one normally
9769 used on Unix, so you cannot use it if you need to call libraries
9770 compiled with the Unix compiler.
9773 @opindex mnoregparam
9774 Do not pass any arguments in registers. This is the default for all
9779 It is OK to use the sb as an index register which is always loaded with
9780 zero. This is the default for the pc532-netbsd target.
9784 The sb register is not available for use or has not been initialized to
9785 zero by the run time system. This is the default for all targets except
9786 the pc532-netbsd. It is also implied whenever @option{-mhimem} or
9787 @option{-fpic} is set.
9791 Many ns32000 series addressing modes use displacements of up to 512MB@.
9792 If an address is above 512MB then displacements from zero can not be used.
9793 This option causes code to be generated which can be loaded above 512MB@.
9794 This may be useful for operating systems or ROM code.
9798 Assume code will be loaded in the first 512MB of virtual address space.
9799 This is the default for all platforms.
9805 @subsection AVR Options
9808 These options are defined for AVR implementations:
9811 @item -mmcu=@var{mcu}
9813 Specify ATMEL AVR instruction set or MCU type.
9815 Instruction set avr1 is for the minimal AVR core, not supported by the C
9816 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
9817 attiny11, attiny12, attiny15, attiny28).
9819 Instruction set avr2 (default) is for the classic AVR core with up to
9820 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
9821 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
9822 at90c8534, at90s8535).
9824 Instruction set avr3 is for the classic AVR core with up to 128K program
9825 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
9827 Instruction set avr4 is for the enhanced AVR core with up to 8K program
9828 memory space (MCU types: atmega8, atmega83, atmega85).
9830 Instruction set avr5 is for the enhanced AVR core with up to 128K program
9831 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
9832 atmega64, atmega128, at43usb355, at94k).
9836 Output instruction sizes to the asm file.
9838 @item -minit-stack=@var{N}
9839 @opindex minit-stack
9840 Specify the initial stack address, which may be a symbol or numeric value,
9841 @samp{__stack} is the default.
9843 @item -mno-interrupts
9844 @opindex mno-interrupts
9845 Generated code is not compatible with hardware interrupts.
9846 Code size will be smaller.
9848 @item -mcall-prologues
9849 @opindex mcall-prologues
9850 Functions prologues/epilogues expanded as call to appropriate
9851 subroutines. Code size will be smaller.
9853 @item -mno-tablejump
9854 @opindex mno-tablejump
9855 Do not generate tablejump insns which sometimes increase code size.
9858 @opindex mtiny-stack
9859 Change only the low 8 bits of the stack pointer.
9863 @subsection MCore Options
9864 @cindex MCore options
9866 These are the @samp{-m} options defined for the Motorola M*Core
9874 @opindex mno-hardlit
9875 Inline constants into the code stream if it can be done in two
9876 instructions or less.
9882 Use the divide instruction. (Enabled by default).
9884 @item -mrelax-immediate
9885 @itemx -mno-relax-immediate
9886 @opindex mrelax-immediate
9887 @opindex mno-relax-immediate
9888 Allow arbitrary sized immediates in bit operations.
9890 @item -mwide-bitfields
9891 @itemx -mno-wide-bitfields
9892 @opindex mwide-bitfields
9893 @opindex mno-wide-bitfields
9894 Always treat bit-fields as int-sized.
9896 @item -m4byte-functions
9897 @itemx -mno-4byte-functions
9898 @opindex m4byte-functions
9899 @opindex mno-4byte-functions
9900 Force all functions to be aligned to a four byte boundary.
9902 @item -mcallgraph-data
9903 @itemx -mno-callgraph-data
9904 @opindex mcallgraph-data
9905 @opindex mno-callgraph-data
9906 Emit callgraph information.
9909 @itemx -mno-slow-bytes
9910 @opindex mslow-bytes
9911 @opindex mno-slow-bytes
9912 Prefer word access when reading byte quantities.
9914 @item -mlittle-endian
9916 @opindex mlittle-endian
9917 @opindex mbig-endian
9918 Generate code for a little endian target.
9924 Generate code for the 210 processor.
9928 @subsection IA-64 Options
9929 @cindex IA-64 Options
9931 These are the @samp{-m} options defined for the Intel IA-64 architecture.
9935 @opindex mbig-endian
9936 Generate code for a big endian target. This is the default for HP-UX@.
9938 @item -mlittle-endian
9939 @opindex mlittle-endian
9940 Generate code for a little endian target. This is the default for AIX5
9947 Generate (or don't) code for the GNU assembler. This is the default.
9948 @c Also, this is the default if the configure option @option{--with-gnu-as}
9955 Generate (or don't) code for the GNU linker. This is the default.
9956 @c Also, this is the default if the configure option @option{--with-gnu-ld}
9961 Generate code that does not use a global pointer register. The result
9962 is not position independent code, and violates the IA-64 ABI@.
9964 @item -mvolatile-asm-stop
9965 @itemx -mno-volatile-asm-stop
9966 @opindex mvolatile-asm-stop
9967 @opindex mno-volatile-asm-stop
9968 Generate (or don't) a stop bit immediately before and after volatile asm
9973 Generate code that works around Itanium B step errata.
9975 @item -mregister-names
9976 @itemx -mno-register-names
9977 @opindex mregister-names
9978 @opindex mno-register-names
9979 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
9980 the stacked registers. This may make assembler output more readable.
9986 Disable (or enable) optimizations that use the small data section. This may
9987 be useful for working around optimizer bugs.
9990 @opindex mconstant-gp
9991 Generate code that uses a single constant global pointer value. This is
9992 useful when compiling kernel code.
9996 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
9997 This is useful when compiling firmware code.
9999 @item -minline-float-divide-min-latency
10000 @opindex minline-float-divide-min-latency
10001 Generate code for inline divides of floating point values
10002 using the minimum latency algorithm.
10004 @item -minline-float-divide-max-throughput
10005 @opindex minline-float-divide-max-throughput
10006 Generate code for inline divides of floating point values
10007 using the maximum throughput algorithm.
10009 @item -minline-int-divide-min-latency
10010 @opindex minline-int-divide-min-latency
10011 Generate code for inline divides of integer values
10012 using the minimum latency algorithm.
10014 @item -minline-int-divide-max-throughput
10015 @opindex minline-int-divide-max-throughput
10016 Generate code for inline divides of integer values
10017 using the maximum throughput algorithm.
10019 @item -mno-dwarf2-asm
10020 @itemx -mdwarf2-asm
10021 @opindex mno-dwarf2-asm
10022 @opindex mdwarf2-asm
10023 Don't (or do) generate assembler code for the DWARF2 line number debugging
10024 info. This may be useful when not using the GNU assembler.
10026 @item -mfixed-range=@var{register-range}
10027 @opindex mfixed-range
10028 Generate code treating the given register range as fixed registers.
10029 A fixed register is one that the register allocator can not use. This is
10030 useful when compiling kernel code. A register range is specified as
10031 two registers separated by a dash. Multiple register ranges can be
10032 specified separated by a comma.
10034 @item -mearly-stop-bits
10035 @itemx -mno-early-stop-bits
10036 @opindex mearly-stop-bits
10037 @opindex mno-early-stop-bits
10038 Allow stop bits to be placed earlier than immediately preceding the
10039 instruction that triggered the stop bit. This can improve instruction
10040 scheduling, but does not always do so.
10043 @node S/390 and zSeries Options
10044 @subsection S/390 and zSeries Options
10045 @cindex S/390 and zSeries Options
10047 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
10051 @itemx -msoft-float
10052 @opindex mhard-float
10053 @opindex msoft-float
10054 Use (do not use) the hardware floating-point instructions and registers
10055 for floating-point operations. When @option{-msoft-float} is specified,
10056 functions in @file{libgcc.a} will be used to perform floating-point
10057 operations. When @option{-mhard-float} is specified, the compiler
10058 generates IEEE floating-point instructions. This is the default.
10061 @itemx -mno-backchain
10062 @opindex mbackchain
10063 @opindex mno-backchain
10064 Generate (or do not generate) code which maintains an explicit
10065 backchain within the stack frame that points to the caller's frame.
10066 This may be needed to allow debugging using tools that do not understand
10067 DWARF-2 call frame information. The default is not to generate the
10071 @itemx -mno-small-exec
10072 @opindex msmall-exec
10073 @opindex mno-small-exec
10074 Generate (or do not generate) code using the @code{bras} instruction
10075 to do subroutine calls.
10076 This only works reliably if the total executable size does not
10077 exceed 64k. The default is to use the @code{basr} instruction instead,
10078 which does not have this limitation.
10084 When @option{-m31} is specified, generate code compliant to the
10085 GNU/Linux for S/390 ABI@. When @option{-m64} is specified, generate
10086 code compliant to the GNU/Linux for zSeries ABI@. This allows GCC in
10087 particular to generate 64-bit instructions. For the @samp{s390}
10088 targets, the default is @option{-m31}, while the @samp{s390x}
10089 targets default to @option{-m64}.
10095 When @option{-mzarch} is specified, generate code using the
10096 instructions available on z/Architecture.
10097 When @option{-mesa} is specified, generate code using the
10098 instructions available on ESA/390. Note that @option{-mesa} is
10099 not possible with @option{-m64}.
10100 When generating code compliant to the GNU/Linux for S/390 ABI,
10101 the default is @option{-mesa}. When generating code compliant
10102 to the GNU/Linux for zSeries ABI, the default is @option{-mzarch}.
10108 Generate (or do not generate) code using the @code{mvcle} instruction
10109 to perform block moves. When @option{-mno-mvcle} is specified,
10110 use a @code{mvc} loop instead. This is the default.
10116 Print (or do not print) additional debug information when compiling.
10117 The default is to not print debug information.
10119 @item -march=@var{cpu-type}
10121 Generate code that will run on @var{cpu-type}, which is the name of a system
10122 representing a certain processor type. Possible values for
10123 @var{cpu-type} are @samp{g5}, @samp{g6}, @samp{z900}, and @samp{z990}.
10124 When generating code using the instructions available on z/Architecture,
10125 the default is @option{-march=z900}. Otherwise, the default is
10126 @option{-march=g5}.
10128 @item -mtune=@var{cpu-type}
10130 Tune to @var{cpu-type} everything applicable about the generated code,
10131 except for the ABI and the set of available instructions.
10132 The list of @var{cpu-type} values is the same as for @option{-march}.
10133 The default is the value used for @option{-march}.
10136 @itemx -mno-fused-madd
10137 @opindex mfused-madd
10138 @opindex mno-fused-madd
10139 Generate code that uses (does not use) the floating point multiply and
10140 accumulate instructions. These instructions are generated by default if
10141 hardware floating point is used.
10145 @subsection CRIS Options
10146 @cindex CRIS Options
10148 These options are defined specifically for the CRIS ports.
10151 @item -march=@var{architecture-type}
10152 @itemx -mcpu=@var{architecture-type}
10155 Generate code for the specified architecture. The choices for
10156 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
10157 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX.
10158 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
10161 @item -mtune=@var{architecture-type}
10163 Tune to @var{architecture-type} everything applicable about the generated
10164 code, except for the ABI and the set of available instructions. The
10165 choices for @var{architecture-type} are the same as for
10166 @option{-march=@var{architecture-type}}.
10168 @item -mmax-stack-frame=@var{n}
10169 @opindex mmax-stack-frame
10170 Warn when the stack frame of a function exceeds @var{n} bytes.
10172 @item -melinux-stacksize=@var{n}
10173 @opindex melinux-stacksize
10174 Only available with the @samp{cris-axis-aout} target. Arranges for
10175 indications in the program to the kernel loader that the stack of the
10176 program should be set to @var{n} bytes.
10182 The options @option{-metrax4} and @option{-metrax100} are synonyms for
10183 @option{-march=v3} and @option{-march=v8} respectively.
10185 @item -mmul-bug-workaround
10186 @itemx -mno-mul-bug-workaround
10187 @opindex mmul-bug-workaround
10188 @opindex mno-mul-bug-workaround
10189 Work around a bug in the @code{muls} and @code{mulu} instructions for CPU
10190 models where it applies. This option is active by default.
10194 Enable CRIS-specific verbose debug-related information in the assembly
10195 code. This option also has the effect to turn off the @samp{#NO_APP}
10196 formatted-code indicator to the assembler at the beginning of the
10201 Do not use condition-code results from previous instruction; always emit
10202 compare and test instructions before use of condition codes.
10204 @item -mno-side-effects
10205 @opindex mno-side-effects
10206 Do not emit instructions with side-effects in addressing modes other than
10209 @item -mstack-align
10210 @itemx -mno-stack-align
10211 @itemx -mdata-align
10212 @itemx -mno-data-align
10213 @itemx -mconst-align
10214 @itemx -mno-const-align
10215 @opindex mstack-align
10216 @opindex mno-stack-align
10217 @opindex mdata-align
10218 @opindex mno-data-align
10219 @opindex mconst-align
10220 @opindex mno-const-align
10221 These options (no-options) arranges (eliminate arrangements) for the
10222 stack-frame, individual data and constants to be aligned for the maximum
10223 single data access size for the chosen CPU model. The default is to
10224 arrange for 32-bit alignment. ABI details such as structure layout are
10225 not affected by these options.
10233 Similar to the stack- data- and const-align options above, these options
10234 arrange for stack-frame, writable data and constants to all be 32-bit,
10235 16-bit or 8-bit aligned. The default is 32-bit alignment.
10237 @item -mno-prologue-epilogue
10238 @itemx -mprologue-epilogue
10239 @opindex mno-prologue-epilogue
10240 @opindex mprologue-epilogue
10241 With @option{-mno-prologue-epilogue}, the normal function prologue and
10242 epilogue that sets up the stack-frame are omitted and no return
10243 instructions or return sequences are generated in the code. Use this
10244 option only together with visual inspection of the compiled code: no
10245 warnings or errors are generated when call-saved registers must be saved,
10246 or storage for local variable needs to be allocated.
10250 @opindex mno-gotplt
10252 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
10253 instruction sequences that load addresses for functions from the PLT part
10254 of the GOT rather than (traditional on other architectures) calls to the
10255 PLT. The default is @option{-mgotplt}.
10259 Legacy no-op option only recognized with the cris-axis-aout target.
10263 Legacy no-op option only recognized with the cris-axis-elf and
10264 cris-axis-linux-gnu targets.
10268 Only recognized with the cris-axis-aout target, where it selects a
10269 GNU/linux-like multilib, include files and instruction set for
10270 @option{-march=v8}.
10274 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
10278 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
10279 to link with input-output functions from a simulator library. Code,
10280 initialized data and zero-initialized data are allocated consecutively.
10284 Like @option{-sim}, but pass linker options to locate initialized data at
10285 0x40000000 and zero-initialized data at 0x80000000.
10289 @subsection MMIX Options
10290 @cindex MMIX Options
10292 These options are defined for the MMIX:
10296 @itemx -mno-libfuncs
10298 @opindex mno-libfuncs
10299 Specify that intrinsic library functions are being compiled, passing all
10300 values in registers, no matter the size.
10303 @itemx -mno-epsilon
10305 @opindex mno-epsilon
10306 Generate floating-point comparison instructions that compare with respect
10307 to the @code{rE} epsilon register.
10309 @item -mabi=mmixware
10311 @opindex mabi-mmixware
10313 Generate code that passes function parameters and return values that (in
10314 the called function) are seen as registers @code{$0} and up, as opposed to
10315 the GNU ABI which uses global registers @code{$231} and up.
10317 @item -mzero-extend
10318 @itemx -mno-zero-extend
10319 @opindex mzero-extend
10320 @opindex mno-zero-extend
10321 When reading data from memory in sizes shorter than 64 bits, use (do not
10322 use) zero-extending load instructions by default, rather than
10323 sign-extending ones.
10326 @itemx -mno-knuthdiv
10328 @opindex mno-knuthdiv
10329 Make the result of a division yielding a remainder have the same sign as
10330 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
10331 remainder follows the sign of the dividend. Both methods are
10332 arithmetically valid, the latter being almost exclusively used.
10334 @item -mtoplevel-symbols
10335 @itemx -mno-toplevel-symbols
10336 @opindex mtoplevel-symbols
10337 @opindex mno-toplevel-symbols
10338 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
10339 code can be used with the @code{PREFIX} assembly directive.
10343 Generate an executable in the ELF format, rather than the default
10344 @samp{mmo} format used by the @command{mmix} simulator.
10346 @item -mbranch-predict
10347 @itemx -mno-branch-predict
10348 @opindex mbranch-predict
10349 @opindex mno-branch-predict
10350 Use (do not use) the probable-branch instructions, when static branch
10351 prediction indicates a probable branch.
10353 @item -mbase-addresses
10354 @itemx -mno-base-addresses
10355 @opindex mbase-addresses
10356 @opindex mno-base-addresses
10357 Generate (do not generate) code that uses @emph{base addresses}. Using a
10358 base address automatically generates a request (handled by the assembler
10359 and the linker) for a constant to be set up in a global register. The
10360 register is used for one or more base address requests within the range 0
10361 to 255 from the value held in the register. The generally leads to short
10362 and fast code, but the number of different data items that can be
10363 addressed is limited. This means that a program that uses lots of static
10364 data may require @option{-mno-base-addresses}.
10366 @item -msingle-exit
10367 @itemx -mno-single-exit
10368 @opindex msingle-exit
10369 @opindex mno-single-exit
10370 Force (do not force) generated code to have a single exit point in each
10374 @node PDP-11 Options
10375 @subsection PDP-11 Options
10376 @cindex PDP-11 Options
10378 These options are defined for the PDP-11:
10383 Use hardware FPP floating point. This is the default. (FIS floating
10384 point on the PDP-11/40 is not supported.)
10387 @opindex msoft-float
10388 Do not use hardware floating point.
10392 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
10396 Return floating-point results in memory. This is the default.
10400 Generate code for a PDP-11/40.
10404 Generate code for a PDP-11/45. This is the default.
10408 Generate code for a PDP-11/10.
10410 @item -mbcopy-builtin
10411 @opindex bcopy-builtin
10412 Use inline @code{movstrhi} patterns for copying memory. This is the
10417 Do not use inline @code{movstrhi} patterns for copying memory.
10423 Use 16-bit @code{int}. This is the default.
10429 Use 32-bit @code{int}.
10432 @itemx -mno-float32
10434 @opindex mno-float32
10435 Use 64-bit @code{float}. This is the default.
10438 @itemx -mno-float64
10440 @opindex mno-float64
10441 Use 32-bit @code{float}.
10445 Use @code{abshi2} pattern. This is the default.
10449 Do not use @code{abshi2} pattern.
10451 @item -mbranch-expensive
10452 @opindex mbranch-expensive
10453 Pretend that branches are expensive. This is for experimenting with
10454 code generation only.
10456 @item -mbranch-cheap
10457 @opindex mbranch-cheap
10458 Do not pretend that branches are expensive. This is the default.
10462 Generate code for a system with split I&D.
10466 Generate code for a system without split I&D. This is the default.
10470 Use Unix assembler syntax. This is the default when configured for
10471 @samp{pdp11-*-bsd}.
10475 Use DEC assembler syntax. This is the default when configured for any
10476 PDP-11 target other than @samp{pdp11-*-bsd}.
10479 @node Xstormy16 Options
10480 @subsection Xstormy16 Options
10481 @cindex Xstormy16 Options
10483 These options are defined for Xstormy16:
10488 Choose startup files and linker script suitable for the simulator.
10492 @subsection FRV Options
10493 @cindex FRV Options
10499 Only use the first 32 general purpose registers.
10504 Use all 64 general purpose registers.
10509 Use only the first 32 floating point registers.
10514 Use all 64 floating point registers
10517 @opindex mhard-float
10519 Use hardware instructions for floating point operations.
10522 @opindex msoft-float
10524 Use library routines for floating point operations.
10529 Dynamically allocate condition code registers.
10534 Do not try to dynamically allocate condition code registers, only
10535 use @code{icc0} and @code{fcc0}.
10540 Change ABI to use double word insns.
10545 Do not use double word instructions.
10550 Use floating point double instructions.
10553 @opindex mno-double
10555 Do not use floating point double instructions.
10560 Use media instructions.
10565 Do not use media instructions.
10570 Use multiply and add/subtract instructions.
10573 @opindex mno-muladd
10575 Do not use multiply and add/subtract instructions.
10577 @item -mlibrary-pic
10578 @opindex mlibrary-pic
10580 Generate position-independent EABI code.
10585 Use only the first four media accumulator registers.
10590 Use all eight media accumulator registers.
10595 Pack VLIW instructions.
10600 Do not pack VLIW instructions.
10603 @opindex mno-eflags
10605 Do not mark ABI switches in e_flags.
10608 @opindex mcond-move
10610 Enable the use of conditional-move instructions (default).
10612 This switch is mainly for debugging the compiler and will likely be removed
10613 in a future version.
10615 @item -mno-cond-move
10616 @opindex mno-cond-move
10618 Disable the use of conditional-move instructions.
10620 This switch is mainly for debugging the compiler and will likely be removed
10621 in a future version.
10626 Enable the use of conditional set instructions (default).
10628 This switch is mainly for debugging the compiler and will likely be removed
10629 in a future version.
10634 Disable the use of conditional set instructions.
10636 This switch is mainly for debugging the compiler and will likely be removed
10637 in a future version.
10640 @opindex mcond-exec
10642 Enable the use of conditional execution (default).
10644 This switch is mainly for debugging the compiler and will likely be removed
10645 in a future version.
10647 @item -mno-cond-exec
10648 @opindex mno-cond-exec
10650 Disable the use of conditional execution.
10652 This switch is mainly for debugging the compiler and will likely be removed
10653 in a future version.
10655 @item -mvliw-branch
10656 @opindex mvliw-branch
10658 Run a pass to pack branches into VLIW instructions (default).
10660 This switch is mainly for debugging the compiler and will likely be removed
10661 in a future version.
10663 @item -mno-vliw-branch
10664 @opindex mno-vliw-branch
10666 Do not run a pass to pack branches into VLIW instructions.
10668 This switch is mainly for debugging the compiler and will likely be removed
10669 in a future version.
10671 @item -mmulti-cond-exec
10672 @opindex mmulti-cond-exec
10674 Enable optimization of @code{&&} and @code{||} in conditional execution
10677 This switch is mainly for debugging the compiler and will likely be removed
10678 in a future version.
10680 @item -mno-multi-cond-exec
10681 @opindex mno-multi-cond-exec
10683 Disable optimization of @code{&&} and @code{||} in conditional execution.
10685 This switch is mainly for debugging the compiler and will likely be removed
10686 in a future version.
10688 @item -mnested-cond-exec
10689 @opindex mnested-cond-exec
10691 Enable nested conditional execution optimizations (default).
10693 This switch is mainly for debugging the compiler and will likely be removed
10694 in a future version.
10696 @item -mno-nested-cond-exec
10697 @opindex mno-nested-cond-exec
10699 Disable nested conditional execution optimizations.
10701 This switch is mainly for debugging the compiler and will likely be removed
10702 in a future version.
10704 @item -mtomcat-stats
10705 @opindex mtomcat-stats
10707 Cause gas to print out tomcat statistics.
10709 @item -mcpu=@var{cpu}
10712 Select the processor type for which to generate code. Possible values are
10713 @samp{simple}, @samp{tomcat}, @samp{fr500}, @samp{fr400}, @samp{fr300},
10718 @node Xtensa Options
10719 @subsection Xtensa Options
10720 @cindex Xtensa Options
10722 These options are supported for Xtensa targets:
10726 @itemx -mno-const16
10728 @opindex mno-const16
10729 Enable or disable use of @code{CONST16} instructions for loading
10730 constant values. The @code{CONST16} instruction is currently not a
10731 standard option from Tensilica. When enabled, @code{CONST16}
10732 instructions are always used in place of the standard @code{L32R}
10733 instructions. The use of @code{CONST16} is enabled by default only if
10734 the @code{L32R} instruction is not available.
10737 @itemx -mno-fused-madd
10738 @opindex mfused-madd
10739 @opindex mno-fused-madd
10740 Enable or disable use of fused multiply/add and multiply/subtract
10741 instructions in the floating-point option. This has no effect if the
10742 floating-point option is not also enabled. Disabling fused multiply/add
10743 and multiply/subtract instructions forces the compiler to use separate
10744 instructions for the multiply and add/subtract operations. This may be
10745 desirable in some cases where strict IEEE 754-compliant results are
10746 required: the fused multiply add/subtract instructions do not round the
10747 intermediate result, thereby producing results with @emph{more} bits of
10748 precision than specified by the IEEE standard. Disabling fused multiply
10749 add/subtract instructions also ensures that the program output is not
10750 sensitive to the compiler's ability to combine multiply and add/subtract
10753 @item -mtext-section-literals
10754 @itemx -mno-text-section-literals
10755 @opindex mtext-section-literals
10756 @opindex mno-text-section-literals
10757 Control the treatment of literal pools. The default is
10758 @option{-mno-text-section-literals}, which places literals in a separate
10759 section in the output file. This allows the literal pool to be placed
10760 in a data RAM/ROM, and it also allows the linker to combine literal
10761 pools from separate object files to remove redundant literals and
10762 improve code size. With @option{-mtext-section-literals}, the literals
10763 are interspersed in the text section in order to keep them as close as
10764 possible to their references. This may be necessary for large assembly
10767 @item -mtarget-align
10768 @itemx -mno-target-align
10769 @opindex mtarget-align
10770 @opindex mno-target-align
10771 When this option is enabled, GCC instructs the assembler to
10772 automatically align instructions to reduce branch penalties at the
10773 expense of some code density. The assembler attempts to widen density
10774 instructions to align branch targets and the instructions following call
10775 instructions. If there are not enough preceding safe density
10776 instructions to align a target, no widening will be performed. The
10777 default is @option{-mtarget-align}. These options do not affect the
10778 treatment of auto-aligned instructions like @code{LOOP}, which the
10779 assembler will always align, either by widening density instructions or
10780 by inserting no-op instructions.
10783 @itemx -mno-longcalls
10784 @opindex mlongcalls
10785 @opindex mno-longcalls
10786 When this option is enabled, GCC instructs the assembler to translate
10787 direct calls to indirect calls unless it can determine that the target
10788 of a direct call is in the range allowed by the call instruction. This
10789 translation typically occurs for calls to functions in other source
10790 files. Specifically, the assembler translates a direct @code{CALL}
10791 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
10792 The default is @option{-mno-longcalls}. This option should be used in
10793 programs where the call target can potentially be out of range. This
10794 option is implemented in the assembler, not the compiler, so the
10795 assembly code generated by GCC will still show direct call
10796 instructions---look at the disassembled object code to see the actual
10797 instructions. Note that the assembler will use an indirect call for
10798 every cross-file call, not just those that really will be out of range.
10801 @node Code Gen Options
10802 @section Options for Code Generation Conventions
10803 @cindex code generation conventions
10804 @cindex options, code generation
10805 @cindex run-time options
10807 These machine-independent options control the interface conventions
10808 used in code generation.
10810 Most of them have both positive and negative forms; the negative form
10811 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
10812 one of the forms is listed---the one which is not the default. You
10813 can figure out the other form by either removing @samp{no-} or adding
10817 @item -fbounds-check
10818 @opindex fbounds-check
10819 For front-ends that support it, generate additional code to check that
10820 indices used to access arrays are within the declared range. This is
10821 currently only supported by the Java and Fortran 77 front-ends, where
10822 this option defaults to true and false respectively.
10826 This option generates traps for signed overflow on addition, subtraction,
10827 multiplication operations.
10831 This option instructs the compiler to assume that signed arithmetic
10832 overflow of addition, subtraction and multiplication wraps around
10833 using twos-complement representation. This flag enables some optimizations
10834 and disables other. This option is enabled by default for the Java
10835 front-end, as required by the Java language specification.
10838 @opindex fexceptions
10839 Enable exception handling. Generates extra code needed to propagate
10840 exceptions. For some targets, this implies GCC will generate frame
10841 unwind information for all functions, which can produce significant data
10842 size overhead, although it does not affect execution. If you do not
10843 specify this option, GCC will enable it by default for languages like
10844 C++ which normally require exception handling, and disable it for
10845 languages like C that do not normally require it. However, you may need
10846 to enable this option when compiling C code that needs to interoperate
10847 properly with exception handlers written in C++. You may also wish to
10848 disable this option if you are compiling older C++ programs that don't
10849 use exception handling.
10851 @item -fnon-call-exceptions
10852 @opindex fnon-call-exceptions
10853 Generate code that allows trapping instructions to throw exceptions.
10854 Note that this requires platform-specific runtime support that does
10855 not exist everywhere. Moreover, it only allows @emph{trapping}
10856 instructions to throw exceptions, i.e.@: memory references or floating
10857 point instructions. It does not allow exceptions to be thrown from
10858 arbitrary signal handlers such as @code{SIGALRM}.
10860 @item -funwind-tables
10861 @opindex funwind-tables
10862 Similar to @option{-fexceptions}, except that it will just generate any needed
10863 static data, but will not affect the generated code in any other way.
10864 You will normally not enable this option; instead, a language processor
10865 that needs this handling would enable it on your behalf.
10867 @item -fasynchronous-unwind-tables
10868 @opindex funwind-tables
10869 Generate unwind table in dwarf2 format, if supported by target machine. The
10870 table is exact at each instruction boundary, so it can be used for stack
10871 unwinding from asynchronous events (such as debugger or garbage collector).
10873 @item -fpcc-struct-return
10874 @opindex fpcc-struct-return
10875 Return ``short'' @code{struct} and @code{union} values in memory like
10876 longer ones, rather than in registers. This convention is less
10877 efficient, but it has the advantage of allowing intercallability between
10878 GCC-compiled files and files compiled with other compilers, particularly
10879 the Portable C Compiler (pcc).
10881 The precise convention for returning structures in memory depends
10882 on the target configuration macros.
10884 Short structures and unions are those whose size and alignment match
10885 that of some integer type.
10887 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
10888 switch is not binary compatible with code compiled with the
10889 @option{-freg-struct-return} switch.
10890 Use it to conform to a non-default application binary interface.
10892 @item -freg-struct-return
10893 @opindex freg-struct-return
10894 Return @code{struct} and @code{union} values in registers when possible.
10895 This is more efficient for small structures than
10896 @option{-fpcc-struct-return}.
10898 If you specify neither @option{-fpcc-struct-return} nor
10899 @option{-freg-struct-return}, GCC defaults to whichever convention is
10900 standard for the target. If there is no standard convention, GCC
10901 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
10902 the principal compiler. In those cases, we can choose the standard, and
10903 we chose the more efficient register return alternative.
10905 @strong{Warning:} code compiled with the @option{-freg-struct-return}
10906 switch is not binary compatible with code compiled with the
10907 @option{-fpcc-struct-return} switch.
10908 Use it to conform to a non-default application binary interface.
10910 @item -fshort-enums
10911 @opindex fshort-enums
10912 Allocate to an @code{enum} type only as many bytes as it needs for the
10913 declared range of possible values. Specifically, the @code{enum} type
10914 will be equivalent to the smallest integer type which has enough room.
10916 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
10917 code that is not binary compatible with code generated without that switch.
10918 Use it to conform to a non-default application binary interface.
10920 @item -fshort-double
10921 @opindex fshort-double
10922 Use the same size for @code{double} as for @code{float}.
10924 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
10925 code that is not binary compatible with code generated without that switch.
10926 Use it to conform to a non-default application binary interface.
10928 @item -fshort-wchar
10929 @opindex fshort-wchar
10930 Override the underlying type for @samp{wchar_t} to be @samp{short
10931 unsigned int} instead of the default for the target. This option is
10932 useful for building programs to run under WINE@.
10934 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
10935 code that is not binary compatible with code generated without that switch.
10936 Use it to conform to a non-default application binary interface.
10938 @item -fshared-data
10939 @opindex fshared-data
10940 Requests that the data and non-@code{const} variables of this
10941 compilation be shared data rather than private data. The distinction
10942 makes sense only on certain operating systems, where shared data is
10943 shared between processes running the same program, while private data
10944 exists in one copy per process.
10947 @opindex fno-common
10948 In C, allocate even uninitialized global variables in the data section of the
10949 object file, rather than generating them as common blocks. This has the
10950 effect that if the same variable is declared (without @code{extern}) in
10951 two different compilations, you will get an error when you link them.
10952 The only reason this might be useful is if you wish to verify that the
10953 program will work on other systems which always work this way.
10957 Ignore the @samp{#ident} directive.
10959 @item -finhibit-size-directive
10960 @opindex finhibit-size-directive
10961 Don't output a @code{.size} assembler directive, or anything else that
10962 would cause trouble if the function is split in the middle, and the
10963 two halves are placed at locations far apart in memory. This option is
10964 used when compiling @file{crtstuff.c}; you should not need to use it
10967 @item -fverbose-asm
10968 @opindex fverbose-asm
10969 Put extra commentary information in the generated assembly code to
10970 make it more readable. This option is generally only of use to those
10971 who actually need to read the generated assembly code (perhaps while
10972 debugging the compiler itself).
10974 @option{-fno-verbose-asm}, the default, causes the
10975 extra information to be omitted and is useful when comparing two assembler
10980 @cindex global offset table
10982 Generate position-independent code (PIC) suitable for use in a shared
10983 library, if supported for the target machine. Such code accesses all
10984 constant addresses through a global offset table (GOT)@. The dynamic
10985 loader resolves the GOT entries when the program starts (the dynamic
10986 loader is not part of GCC; it is part of the operating system). If
10987 the GOT size for the linked executable exceeds a machine-specific
10988 maximum size, you get an error message from the linker indicating that
10989 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
10990 instead. (These maximums are 8k on the SPARC and 32k
10991 on the m68k and RS/6000. The 386 has no such limit.)
10993 Position-independent code requires special support, and therefore works
10994 only on certain machines. For the 386, GCC supports PIC for System V
10995 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
10996 position-independent.
11000 If supported for the target machine, emit position-independent code,
11001 suitable for dynamic linking and avoiding any limit on the size of the
11002 global offset table. This option makes a difference on the m68k
11005 Position-independent code requires special support, and therefore works
11006 only on certain machines.
11012 These options are similar to @option{-fpic} and @option{-fPIC}, but
11013 generated position independent code can be only linked into executables.
11014 Usually these options are used when @option{-pie} GCC option will be
11015 used during linking.
11017 @item -ffixed-@var{reg}
11019 Treat the register named @var{reg} as a fixed register; generated code
11020 should never refer to it (except perhaps as a stack pointer, frame
11021 pointer or in some other fixed role).
11023 @var{reg} must be the name of a register. The register names accepted
11024 are machine-specific and are defined in the @code{REGISTER_NAMES}
11025 macro in the machine description macro file.
11027 This flag does not have a negative form, because it specifies a
11030 @item -fcall-used-@var{reg}
11031 @opindex fcall-used
11032 Treat the register named @var{reg} as an allocable register that is
11033 clobbered by function calls. It may be allocated for temporaries or
11034 variables that do not live across a call. Functions compiled this way
11035 will not save and restore the register @var{reg}.
11037 It is an error to used this flag with the frame pointer or stack pointer.
11038 Use of this flag for other registers that have fixed pervasive roles in
11039 the machine's execution model will produce disastrous results.
11041 This flag does not have a negative form, because it specifies a
11044 @item -fcall-saved-@var{reg}
11045 @opindex fcall-saved
11046 Treat the register named @var{reg} as an allocable register saved by
11047 functions. It may be allocated even for temporaries or variables that
11048 live across a call. Functions compiled this way will save and restore
11049 the register @var{reg} if they use it.
11051 It is an error to used this flag with the frame pointer or stack pointer.
11052 Use of this flag for other registers that have fixed pervasive roles in
11053 the machine's execution model will produce disastrous results.
11055 A different sort of disaster will result from the use of this flag for
11056 a register in which function values may be returned.
11058 This flag does not have a negative form, because it specifies a
11061 @item -fpack-struct
11062 @opindex fpack-struct
11063 Pack all structure members together without holes.
11065 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
11066 code that is not binary compatible with code generated without that switch.
11067 Additionally, it makes the code suboptimal.
11068 Use it to conform to a non-default application binary interface.
11070 @item -finstrument-functions
11071 @opindex finstrument-functions
11072 Generate instrumentation calls for entry and exit to functions. Just
11073 after function entry and just before function exit, the following
11074 profiling functions will be called with the address of the current
11075 function and its call site. (On some platforms,
11076 @code{__builtin_return_address} does not work beyond the current
11077 function, so the call site information may not be available to the
11078 profiling functions otherwise.)
11081 void __cyg_profile_func_enter (void *this_fn,
11083 void __cyg_profile_func_exit (void *this_fn,
11087 The first argument is the address of the start of the current function,
11088 which may be looked up exactly in the symbol table.
11090 This instrumentation is also done for functions expanded inline in other
11091 functions. The profiling calls will indicate where, conceptually, the
11092 inline function is entered and exited. This means that addressable
11093 versions of such functions must be available. If all your uses of a
11094 function are expanded inline, this may mean an additional expansion of
11095 code size. If you use @samp{extern inline} in your C code, an
11096 addressable version of such functions must be provided. (This is
11097 normally the case anyways, but if you get lucky and the optimizer always
11098 expands the functions inline, you might have gotten away without
11099 providing static copies.)
11101 A function may be given the attribute @code{no_instrument_function}, in
11102 which case this instrumentation will not be done. This can be used, for
11103 example, for the profiling functions listed above, high-priority
11104 interrupt routines, and any functions from which the profiling functions
11105 cannot safely be called (perhaps signal handlers, if the profiling
11106 routines generate output or allocate memory).
11108 @item -fstack-check
11109 @opindex fstack-check
11110 Generate code to verify that you do not go beyond the boundary of the
11111 stack. You should specify this flag if you are running in an
11112 environment with multiple threads, but only rarely need to specify it in
11113 a single-threaded environment since stack overflow is automatically
11114 detected on nearly all systems if there is only one stack.
11116 Note that this switch does not actually cause checking to be done; the
11117 operating system must do that. The switch causes generation of code
11118 to ensure that the operating system sees the stack being extended.
11120 @item -fstack-limit-register=@var{reg}
11121 @itemx -fstack-limit-symbol=@var{sym}
11122 @itemx -fno-stack-limit
11123 @opindex fstack-limit-register
11124 @opindex fstack-limit-symbol
11125 @opindex fno-stack-limit
11126 Generate code to ensure that the stack does not grow beyond a certain value,
11127 either the value of a register or the address of a symbol. If the stack
11128 would grow beyond the value, a signal is raised. For most targets,
11129 the signal is raised before the stack overruns the boundary, so
11130 it is possible to catch the signal without taking special precautions.
11132 For instance, if the stack starts at absolute address @samp{0x80000000}
11133 and grows downwards, you can use the flags
11134 @option{-fstack-limit-symbol=__stack_limit} and
11135 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
11136 of 128KB@. Note that this may only work with the GNU linker.
11138 @cindex aliasing of parameters
11139 @cindex parameters, aliased
11140 @item -fargument-alias
11141 @itemx -fargument-noalias
11142 @itemx -fargument-noalias-global
11143 @opindex fargument-alias
11144 @opindex fargument-noalias
11145 @opindex fargument-noalias-global
11146 Specify the possible relationships among parameters and between
11147 parameters and global data.
11149 @option{-fargument-alias} specifies that arguments (parameters) may
11150 alias each other and may alias global storage.@*
11151 @option{-fargument-noalias} specifies that arguments do not alias
11152 each other, but may alias global storage.@*
11153 @option{-fargument-noalias-global} specifies that arguments do not
11154 alias each other and do not alias global storage.
11156 Each language will automatically use whatever option is required by
11157 the language standard. You should not need to use these options yourself.
11159 @item -fleading-underscore
11160 @opindex fleading-underscore
11161 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
11162 change the way C symbols are represented in the object file. One use
11163 is to help link with legacy assembly code.
11165 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
11166 generate code that is not binary compatible with code generated without that
11167 switch. Use it to conform to a non-default application binary interface.
11168 Not all targets provide complete support for this switch.
11170 @item -ftls-model=@var{model}
11171 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
11172 The @var{model} argument should be one of @code{global-dynamic},
11173 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
11175 The default without @option{-fpic} is @code{initial-exec}; with
11176 @option{-fpic} the default is @code{global-dynamic}.
11181 @node Environment Variables
11182 @section Environment Variables Affecting GCC
11183 @cindex environment variables
11185 @c man begin ENVIRONMENT
11186 This section describes several environment variables that affect how GCC
11187 operates. Some of them work by specifying directories or prefixes to use
11188 when searching for various kinds of files. Some are used to specify other
11189 aspects of the compilation environment.
11191 Note that you can also specify places to search using options such as
11192 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
11193 take precedence over places specified using environment variables, which
11194 in turn take precedence over those specified by the configuration of GCC@.
11195 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
11196 GNU Compiler Collection (GCC) Internals}.
11201 @c @itemx LC_COLLATE
11203 @c @itemx LC_MONETARY
11204 @c @itemx LC_NUMERIC
11209 @c @findex LC_COLLATE
11210 @findex LC_MESSAGES
11211 @c @findex LC_MONETARY
11212 @c @findex LC_NUMERIC
11216 These environment variables control the way that GCC uses
11217 localization information that allow GCC to work with different
11218 national conventions. GCC inspects the locale categories
11219 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
11220 so. These locale categories can be set to any value supported by your
11221 installation. A typical value is @samp{en_UK} for English in the United
11224 The @env{LC_CTYPE} environment variable specifies character
11225 classification. GCC uses it to determine the character boundaries in
11226 a string; this is needed for some multibyte encodings that contain quote
11227 and escape characters that would otherwise be interpreted as a string
11230 The @env{LC_MESSAGES} environment variable specifies the language to
11231 use in diagnostic messages.
11233 If the @env{LC_ALL} environment variable is set, it overrides the value
11234 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
11235 and @env{LC_MESSAGES} default to the value of the @env{LANG}
11236 environment variable. If none of these variables are set, GCC
11237 defaults to traditional C English behavior.
11241 If @env{TMPDIR} is set, it specifies the directory to use for temporary
11242 files. GCC uses temporary files to hold the output of one stage of
11243 compilation which is to be used as input to the next stage: for example,
11244 the output of the preprocessor, which is the input to the compiler
11247 @item GCC_EXEC_PREFIX
11248 @findex GCC_EXEC_PREFIX
11249 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
11250 names of the subprograms executed by the compiler. No slash is added
11251 when this prefix is combined with the name of a subprogram, but you can
11252 specify a prefix that ends with a slash if you wish.
11254 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
11255 an appropriate prefix to use based on the pathname it was invoked with.
11257 If GCC cannot find the subprogram using the specified prefix, it
11258 tries looking in the usual places for the subprogram.
11260 The default value of @env{GCC_EXEC_PREFIX} is
11261 @file{@var{prefix}/lib/gcc/} where @var{prefix} is the value
11262 of @code{prefix} when you ran the @file{configure} script.
11264 Other prefixes specified with @option{-B} take precedence over this prefix.
11266 This prefix is also used for finding files such as @file{crt0.o} that are
11269 In addition, the prefix is used in an unusual way in finding the
11270 directories to search for header files. For each of the standard
11271 directories whose name normally begins with @samp{/usr/local/lib/gcc}
11272 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
11273 replacing that beginning with the specified prefix to produce an
11274 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
11275 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
11276 These alternate directories are searched first; the standard directories
11279 @item COMPILER_PATH
11280 @findex COMPILER_PATH
11281 The value of @env{COMPILER_PATH} is a colon-separated list of
11282 directories, much like @env{PATH}. GCC tries the directories thus
11283 specified when searching for subprograms, if it can't find the
11284 subprograms using @env{GCC_EXEC_PREFIX}.
11287 @findex LIBRARY_PATH
11288 The value of @env{LIBRARY_PATH} is a colon-separated list of
11289 directories, much like @env{PATH}. When configured as a native compiler,
11290 GCC tries the directories thus specified when searching for special
11291 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
11292 using GCC also uses these directories when searching for ordinary
11293 libraries for the @option{-l} option (but directories specified with
11294 @option{-L} come first).
11298 @cindex locale definition
11299 This variable is used to pass locale information to the compiler. One way in
11300 which this information is used is to determine the character set to be used
11301 when character literals, string literals and comments are parsed in C and C++.
11302 When the compiler is configured to allow multibyte characters,
11303 the following values for @env{LANG} are recognized:
11307 Recognize JIS characters.
11309 Recognize SJIS characters.
11311 Recognize EUCJP characters.
11314 If @env{LANG} is not defined, or if it has some other value, then the
11315 compiler will use mblen and mbtowc as defined by the default locale to
11316 recognize and translate multibyte characters.
11320 Some additional environments variables affect the behavior of the
11323 @include cppenv.texi
11327 @node Precompiled Headers
11328 @section Using Precompiled Headers
11329 @cindex precompiled headers
11330 @cindex speed of compilation
11332 Often large projects have many header files that are included in every
11333 source file. The time the compiler takes to process these header files
11334 over and over again can account for nearly all of the time required to
11335 build the project. To make builds faster, GCC allows users to
11336 `precompile' a header file; then, if builds can use the precompiled
11337 header file they will be much faster.
11339 @strong{Caution:} There are a few known situations where GCC will
11340 crash when trying to use a precompiled header. If you have trouble
11341 with a precompiled header, you should remove the precompiled header
11342 and compile without it. In addition, please use GCC's on-line
11343 defect-tracking system to report any problems you encounter with
11344 precompiled headers. @xref{Bugs}.
11346 To create a precompiled header file, simply compile it as you would any
11347 other file, if necessary using the @option{-x} option to make the driver
11348 treat it as a C or C++ header file. You will probably want to use a
11349 tool like @command{make} to keep the precompiled header up-to-date when
11350 the headers it contains change.
11352 A precompiled header file will be searched for when @code{#include} is
11353 seen in the compilation. As it searches for the included file
11354 (@pxref{Search Path,,Search Path,cpp,The C Preprocessor}) the
11355 compiler looks for a precompiled header in each directory just before it
11356 looks for the include file in that directory. The name searched for is
11357 the name specified in the @code{#include} with @samp{.gch} appended. If
11358 the precompiled header file can't be used, it is ignored.
11360 For instance, if you have @code{#include "all.h"}, and you have
11361 @file{all.h.gch} in the same directory as @file{all.h}, then the
11362 precompiled header file will be used if possible, and the original
11363 header will be used otherwise.
11365 Alternatively, you might decide to put the precompiled header file in a
11366 directory and use @option{-I} to ensure that directory is searched
11367 before (or instead of) the directory containing the original header.
11368 Then, if you want to check that the precompiled header file is always
11369 used, you can put a file of the same name as the original header in this
11370 directory containing an @code{#error} command.
11372 This also works with @option{-include}. So yet another way to use
11373 precompiled headers, good for projects not designed with precompiled
11374 header files in mind, is to simply take most of the header files used by
11375 a project, include them from another header file, precompile that header
11376 file, and @option{-include} the precompiled header. If the header files
11377 have guards against multiple inclusion, they will be skipped because
11378 they've already been included (in the precompiled header).
11380 If you need to precompile the same header file for different
11381 languages, targets, or compiler options, you can instead make a
11382 @emph{directory} named like @file{all.h.gch}, and put each precompiled
11383 header in the directory, perhaps using @option{-o}. It doesn't matter
11384 what you call the files in the directory, every precompiled header in
11385 the directory will be considered. The first precompiled header
11386 encountered in the directory that is valid for this compilation will
11387 be used; they're searched in no particular order.
11389 There are many other possibilities, limited only by your imagination,
11390 good sense, and the constraints of your build system.
11392 A precompiled header file can be used only when these conditions apply:
11396 Only one precompiled header can be used in a particular compilation.
11399 A precompiled header can't be used once the first C token is seen. You
11400 can have preprocessor directives before a precompiled header; you can
11401 even include a precompiled header from inside another header, so long as
11402 there are no C tokens before the @code{#include}.
11405 The precompiled header file must be produced for the same language as
11406 the current compilation. You can't use a C precompiled header for a C++
11410 The precompiled header file must be produced by the same compiler
11411 version and configuration as the current compilation is using.
11412 The easiest way to guarantee this is to use the same compiler binary
11413 for creating and using precompiled headers.
11416 Any macros defined before the precompiled header is included must
11417 either be defined in the same way as when the precompiled header was
11418 generated, or must not affect the precompiled header, which usually
11419 means that the they don't appear in the precompiled header at all.
11421 The @option{-D} option is one way to define a macro before a
11422 precompiled header is included; using a @code{#define} can also do it.
11423 There are also some options that define macros implicitly, like
11424 @option{-O} and @option{-Wdeprecated}; the same rule applies to macros
11427 @item If debugging information is output when using the precompiled
11428 header, using @option{-g} or similar, the same kind of debugging information
11429 must have been output when building the precompiled header. However,
11430 a precompiled header built using @option{-g} can be used in a compilation
11431 when no debugging information is being output.
11433 @item The same @option{-m} options must generally be used when building
11434 and using the precompiled header. @xref{Submodel Options},
11435 for any cases where this rule is relaxed.
11437 @item Each of the following options must be the same when building and using
11438 the precompiled header:
11440 @gccoptlist{-fexceptions -funit-at-a-time}
11443 Some other command-line options starting with @option{-f},
11444 @option{-p}, or @option{-O} must be defined in the same way as when
11445 the precompiled header was generated. At present, it's not clear
11446 which options are safe to change and which are not; the safest choice
11447 is to use exactly the same options when generating and using the
11448 precompiled header. The following are known to be safe:
11450 @gccoptlist{-pedantic-errors}
11454 For all of these except the last, the compiler will automatically
11455 ignore the precompiled header if the conditions aren't met. If you
11456 find an option combination that doesn't work and doesn't cause the
11457 precompiled header to be ignored, please consider filing a bug report,
11460 @node Running Protoize
11461 @section Running Protoize
11463 The program @code{protoize} is an optional part of GCC@. You can use
11464 it to add prototypes to a program, thus converting the program to ISO
11465 C in one respect. The companion program @code{unprotoize} does the
11466 reverse: it removes argument types from any prototypes that are found.
11468 When you run these programs, you must specify a set of source files as
11469 command line arguments. The conversion programs start out by compiling
11470 these files to see what functions they define. The information gathered
11471 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
11473 After scanning comes actual conversion. The specified files are all
11474 eligible to be converted; any files they include (whether sources or
11475 just headers) are eligible as well.
11477 But not all the eligible files are converted. By default,
11478 @code{protoize} and @code{unprotoize} convert only source and header
11479 files in the current directory. You can specify additional directories
11480 whose files should be converted with the @option{-d @var{directory}}
11481 option. You can also specify particular files to exclude with the
11482 @option{-x @var{file}} option. A file is converted if it is eligible, its
11483 directory name matches one of the specified directory names, and its
11484 name within the directory has not been excluded.
11486 Basic conversion with @code{protoize} consists of rewriting most
11487 function definitions and function declarations to specify the types of
11488 the arguments. The only ones not rewritten are those for varargs
11491 @code{protoize} optionally inserts prototype declarations at the
11492 beginning of the source file, to make them available for any calls that
11493 precede the function's definition. Or it can insert prototype
11494 declarations with block scope in the blocks where undeclared functions
11497 Basic conversion with @code{unprotoize} consists of rewriting most
11498 function declarations to remove any argument types, and rewriting
11499 function definitions to the old-style pre-ISO form.
11501 Both conversion programs print a warning for any function declaration or
11502 definition that they can't convert. You can suppress these warnings
11505 The output from @code{protoize} or @code{unprotoize} replaces the
11506 original source file. The original file is renamed to a name ending
11507 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
11508 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
11509 for DOS) file already exists, then the source file is simply discarded.
11511 @code{protoize} and @code{unprotoize} both depend on GCC itself to
11512 scan the program and collect information about the functions it uses.
11513 So neither of these programs will work until GCC is installed.
11515 Here is a table of the options you can use with @code{protoize} and
11516 @code{unprotoize}. Each option works with both programs unless
11520 @item -B @var{directory}
11521 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
11522 usual directory (normally @file{/usr/local/lib}). This file contains
11523 prototype information about standard system functions. This option
11524 applies only to @code{protoize}.
11526 @item -c @var{compilation-options}
11527 Use @var{compilation-options} as the options when running @command{gcc} to
11528 produce the @samp{.X} files. The special option @option{-aux-info} is
11529 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
11531 Note that the compilation options must be given as a single argument to
11532 @code{protoize} or @code{unprotoize}. If you want to specify several
11533 @command{gcc} options, you must quote the entire set of compilation options
11534 to make them a single word in the shell.
11536 There are certain @command{gcc} arguments that you cannot use, because they
11537 would produce the wrong kind of output. These include @option{-g},
11538 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
11539 the @var{compilation-options}, they are ignored.
11542 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
11543 systems) instead of @samp{.c}. This is convenient if you are converting
11544 a C program to C++. This option applies only to @code{protoize}.
11547 Add explicit global declarations. This means inserting explicit
11548 declarations at the beginning of each source file for each function
11549 that is called in the file and was not declared. These declarations
11550 precede the first function definition that contains a call to an
11551 undeclared function. This option applies only to @code{protoize}.
11553 @item -i @var{string}
11554 Indent old-style parameter declarations with the string @var{string}.
11555 This option applies only to @code{protoize}.
11557 @code{unprotoize} converts prototyped function definitions to old-style
11558 function definitions, where the arguments are declared between the
11559 argument list and the initial @samp{@{}. By default, @code{unprotoize}
11560 uses five spaces as the indentation. If you want to indent with just
11561 one space instead, use @option{-i " "}.
11564 Keep the @samp{.X} files. Normally, they are deleted after conversion
11568 Add explicit local declarations. @code{protoize} with @option{-l} inserts
11569 a prototype declaration for each function in each block which calls the
11570 function without any declaration. This option applies only to
11574 Make no real changes. This mode just prints information about the conversions
11575 that would have been done without @option{-n}.
11578 Make no @samp{.save} files. The original files are simply deleted.
11579 Use this option with caution.
11581 @item -p @var{program}
11582 Use the program @var{program} as the compiler. Normally, the name
11583 @file{gcc} is used.
11586 Work quietly. Most warnings are suppressed.
11589 Print the version number, just like @option{-v} for @command{gcc}.
11592 If you need special compiler options to compile one of your program's
11593 source files, then you should generate that file's @samp{.X} file
11594 specially, by running @command{gcc} on that source file with the
11595 appropriate options and the option @option{-aux-info}. Then run
11596 @code{protoize} on the entire set of files. @code{protoize} will use
11597 the existing @samp{.X} file because it is newer than the source file.
11601 gcc -Dfoo=bar file1.c -aux-info file1.X
11606 You need to include the special files along with the rest in the
11607 @code{protoize} command, even though their @samp{.X} files already
11608 exist, because otherwise they won't get converted.
11610 @xref{Protoize Caveats}, for more information on how to use
11611 @code{protoize} successfully.