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Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.
Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually [C+]), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified.
Many options have long names starting with -f or with -W---for example, -fforce-mem, -fstrength-reduce, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual documents only one of these two forms, whichever one is not the default.
For any given input file, the file name suffix determines what kind of compilation is done: C source code which must be preprocessed. C source code which should not be preprocessed. [C+] source code which should not be preprocessed. Objective-C source code. Note that you must link with the library libobjc.a to make an Objective-C program work. Objective-C source code which should not be preprocessed. C header file (not to be compiled or linked). [C+] source code which must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C. Fortran source code which should not be preprocessed. Fortran source code which must be preprocessed (with the traditional preprocessor). Fortran source code which must be preprocessed with a RATFOR preprocessor (not included with GCC). Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs. Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies. Assembler code. Assembler code which must be preprocessed. An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the file
name suffix). This option applies to all following input files until
the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-cpp-output
objective-c objc-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
Turn off any specification of a language, so that subsequent files are
handled according to their file name suffixes (as they are if -x
has not been used at all).
Normally the gcc program will exit with the code of 1 if any
phase of the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program will instead return with
numerically highest error produced by any phase that returned an error
indication.
If you only want some of the stages of compilation, you can use
-x (or filename suffixes) to tell gcc where to start, and
one of the options -c, -S, or -E to say where
gcc is to stop. Note that some combinations (for example,
-x cpp-output -E) instruct gcc to do nothing at all.
Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by replacing
the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are
ignored.
Stop after the stage of compilation proper; do not assemble. The output
is in the form of an assembler code file for each non-assembler input
file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
Stop after the preprocessing stage; do not run the compiler proper. The
output is in the form of preprocessed source code, which is sent to the
standard output.
Input files which don't require preprocessing are ignored.
Place output in file file. This applies regardless to whatever
sort of output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C code.
Since only one output file can be specified, it does not make sense to
use -o when compiling more than one input file, unless you are
producing an executable file as output.
If -o is not specified, the default is to put an executable file
in a.out, the object file for source.suffix in
source.o, its assembler file in source.s, and
all preprocessed C source on standard output.
Print (on standard error output) the commands executed to run the stages
of compilation. Also print the version number of the compiler driver
program and of the preprocessor and the compiler proper.
Like -v except the commands are not executed and all command
arguments are quoted. This is useful for shell scripts to capture the
driver-generated command lines.
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems where
the assembler is unable to read from a pipe; but the GNU assembler has
no trouble.
Print (on the standard output) a description of the command line options
understood by gcc. If the -v option is also specified
then [--]help will also be passed on to the various processes
invoked by gcc, so that they can display the command line options
they accept. If the -W option is also specified then command
line options which have no documentation associated with them will also
be displayed.
Print (on the standard output) a description of target specific command
line options for each tool.
Display the version number and copyrights of the invoked GCC.
[C+] source files conventionally use one of the suffixes .C,
.cc, .cpp, .c++, .cp, or .cxx;
preprocessed [C+] files use the suffix .ii. GCC recognizes
files with these names and compiles them as [C+] programs even if you
call the compiler the same way as for compiling C programs (usually with
the name gcc).
However, [C+] programs often require class libraries as well as a compiler that understands the [C+] language---and under some circumstances, you might want to compile programs from standard input, or otherwise without a suffix that flags them as [C+] programs. g++ is a program that calls GCC with the default language set to [C+], and automatically specifies linking against the [C+] library. On many systems, g++ is also installed with the name c++.
When you compile [C+] programs, you may specify many of the same
command-line options that you use for compiling programs in any
language; or command-line options meaningful for C and related
languages; or options that are meaningful only for [C+] programs.
The following options control the dialect of C (or languages derived
from C, such as [C+] and Objective-C) that the compiler accepts:
In C mode, support all ISO C89 programs. In [C+] mode,
remove GNU extensions that conflict with ISO [C+].
This turns off certain features of GCC that are incompatible with ISO
C89 (when compiling C code), or of standard [C+] (when compiling [C+] code),
such as the [C`]asm[C'] and [C`]typeof[C'] keywords, and
predefined macros such as [C`]unix[C'] and [C`]vax[C'] that identify the
type of system you are using. It also enables the undesirable and
rarely used ISO trigraph feature. For the C compiler,
it disables recognition of [C+] style // comments as well as
the [C`]inline[C'] keyword.
The alternate keywords [C`]__asm__[C'], [C`]__extension__[C'],
[C`]__inline__[C'] and [C`]__typeof__[C'] continue to work despite
-ansi. You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be included
in compilations done with -ansi. Alternate predefined macros
such as [C`]__unix__[C'] and [C`]__vax__[C'] are also available, with or
without -ansi.
The -ansi option does not cause non-ISO programs to be
rejected gratuitously. For that, -pedantic is required in
addition to -ansi.
The macro [C`]__STRICT_ANSI__[C'] is predefined when the -ansi
option is used. Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ISO standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions which would normally be built in but do not have semantics
defined by ISO C (such as [C`]alloca[C'] and [C`]ffs[C']) are not built-in
functions with -ansi is used.
Determine the language standard. This option is currently only
supported when compiling C. A value for this option must be provided;
possible values are
g++ -g -frepo -O -c firstClass.C In this example, only -frepo is an option meant only for [C+] programs; you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling [C+] programs:
Turn off all access checking. This switch is mainly useful for working
around bugs in the access control code.
Check that the pointer returned by [C`]operator new[C'] is non-null
before attempting to modify the storage allocated. The current Working
Paper requires that [C`]operator new[C'] never return a null pointer, so
this check is normally unnecessary.
An alternative to using this option is to specify that your
[C`]operator new[C'] does not throw any exceptions; if you declare it
BIthrow(), G++ will check the return value. See also new
(nothrow).
Put uninitialized or runtime-initialized global variables into the
common segment, as C does. This saves space in the executable at the
cost of not diagnosing duplicate definitions. If you compile with this
flag and your program mysteriously crashes after [C`]main()[C'] has
completed, you may have an object that is being destroyed twice because
two definitions were merged.
This option is no longer useful on most targets, now that support has
been added for putting variables into BSS without making them common.
Give string constants type [C`]char *[C'] instead of type [C`]const
char *[C']. By default, G++ uses type [C`]const char *[C'] as required by
the standard. Even if you use -fno-const-strings, you cannot
actually modify the value of a string constant, unless you also use
-fwritable-strings.
This option might be removed in a future release of G++. For maximum
portability, you should structure your code so that it works with
string constants that have type [C`]const char *[C'].
Accept $ in identifiers. You can also explicitly prohibit use of
$ with the option -fno-dollars-in-identifiers. (GNU C allows
$ by default on most target systems, but there are a few exceptions.)
Traditional C allowed the character $ to form part of
identifiers. However, ISO C and [C+] forbid $ in identifiers.
The [C+] standard allows an implementation to omit creating a temporary
which is only used to initialize another object of the same type.
Specifying this option disables that optimization, and forces G++ to
call the copy constructor in all cases.
Don't check for violation of exception specifications at runtime. This
option violates the [C+] standard, but may be useful for reducing code
size in production builds, much like defining NDEBUG. The compiler
will still optimize based on the exception specifications.
Cause #pragma interface and implementation to apply to
template instantiation; template instances are emitted or not according
to the location of the template definition.
This option is deprecated.
Similar to -fexternal-templates, but template instances are
emitted or not according to the place where they are first instantiated.
This option is deprecated.
If -ffor-scope is specified, the scope of variables declared in
a for-init-statement is limited to the for loop itself,
as specified by the [C+] standard.
If -fno-for-scope is specified, the scope of variables declared in
a for-init-statement extends to the end of the enclosing scope,
as was the case in old versions of G++, and other (traditional)
implementations of [C+].
The default if neither flag is given to follow the standard,
but to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.
Do not recognize [C`]typeof[C'] as a keyword, so that code can use this
word as an identifier. You can use the keyword [C`]__typeof__[C'] instead.
-ansi implies -fno-gnu-keywords.
Never emit code for non-inline templates which are instantiated
implicitly (i.e. by use); only emit code for explicit instantiations.
Don't emit code for implicit instantiations of inline templates, either.
The default is to handle inlines differently so that compiles with and
without optimization will need the same set of explicit instantiations.
To save space, do not emit out-of-line copies of inline functions
controlled by #pragma implementation. This will cause linker
errors if these functions are not inlined everywhere they are called.
Disable pedantic warnings about constructs used in MFC, such as implicit
int and getting a pointer to member function via non-standard syntax.
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include [C`]ffs[C'], [C`]alloca[C'], [C`]_exit[C'],
[C`]index[C'], [C`]bzero[C'], [C`]conjf[C'], and other related functions.
Do not treat the operator name keywords [C`]and[C'], [C`]bitand[C'],
[C`]bitor[C'], [C`]compl[C'], [C`]not[C'], [C`]or[C'] and [C`]xor[C'] as
synonyms as keywords.
Disable diagnostics that the standard says a compiler does not need to
issue. Currently, the only such diagnostic issued by G++ is the one for
a name having multiple meanings within a class.
Downgrade messages about nonconformant code from errors to warnings. By
default, G++ effectively sets -pedantic-errors without
-pedantic; this option reverses that. This behavior and this
option are superseded by -pedantic, which works as it does for GNU C.
Enable automatic template instantiation at link time. This option also
implies -fno-implicit-templates.
Disable generation of information about every class with virtual
functions for use by the [C+] runtime type identification features
(dynamic_cast and typeid). If you don't use those parts
of the language, you can save some space by using this flag. Note that
exception handling uses the same information, but it will generate it as
needed.
Emit statistics about front-end processing at the end of the compilation.
This information is generally only useful to the G++ development team.
Set the maximum instantiation depth for template classes to n.
A limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO [C+]
conforming programs must not rely on a maximum depth greater than 17.
Register destructors for objects with static storage duration with the
[C`]__cxa_atexit[C'] function rather than the [C`]atexit[C'] function.
This option is required for fully standards-compliant handling of static
destructors, but will only work if your C library supports
[C`]__cxa_atexit[C'].
Emit special relocations for vtables and virtual function references
so that the linker can identify unused virtual functions and zero out
vtable slots that refer to them. This is most useful with
-ffunction-sections and -Wl,--gc-sections, in order to
also discard the functions themselves.
This optimization requires GNU as and GNU ld. Not all systems support
this option. -Wl,--gc-sections is ignored without -static.
Do not use weak symbol support, even if it is provided by the linker.
By default, G++ will use weak symbols if they are available. This
option exists only for testing, and should not be used by end-users;
it will result in inferior code and has no benefits. This option may
be removed in a future release of G++.
Do not search for header files in the standard directories specific to
[C+], but do still search the other standard directories. (This option
is used when building the [C+] library.)
In addition, these optimization, warning, and code generation options
have meanings only for [C+] programs:
Do not assume inline for functions defined inside a class scope.
Note that these
functions will have linkage like inline functions; they just won't be
inlined by default.
Warn when G++ generates code that is probably not compatible with the
vendor-neutral [C+] ABI. Although an effort has been made to warn about
all such cases, there are probably some cases that are not warned about,
even though G++ is generating incompatible code. There may also be
cases where warnings are emitted even though the code that is generated
will be compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be binary
compatible with code generated by other compilers.
The known incompatibilites at this point include:
The following -W... options are not affected by -Wall. Warn about violations of the following style guidelines from Scott Meyers' Effective [C+] book:
gcc -g -fgnu-runtime -O -c some_class.m In this example, only -fgnu-runtime is an option meant only for Objective-C programs; you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling Objective-C programs: Use class-name as the name of the class to instantiate for each literal string specified with the syntax [C`]@"..."[C']. The default class name is [C`]NXConstantString[C']. Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems. Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. Dump interface declarations for all classes seen in the source file to a file named sourcename.decl. Do not warn if methods required by a protocol are not implemented in the class adopting it. Warn if a selector has multiple methods of different types defined. Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the [C+] front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly. Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping will be done; each error message will appear on a single line. Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source location information; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior. Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line. Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default.
The following options control the amount and kinds of warnings produced
by GCC; for further, language-specific options also refer to
@ref{[C+] Dialect Options} and @ref{Objective-C Dialect Options}.
Check the code for syntax errors, but don't do anything beyond that.
Issue all the warnings demanded by strict ISO C and ISO [C+];
reject all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO [C+]. For ISO C, follows the
version of the ISO C standard specified by any -std option used.
Valid ISO C and ISO [C+] programs should compile properly with or without
this option (though a rare few will require -ansi or a
-std option specifying the required version of ISO C). However,
without this option, certain GNU extensions and traditional C and [C+]
features are supported as well. With this option, they are rejected.
-pedantic does not cause warning messages for use of the
alternate keywords whose names begin and end with __. Pedantic
warnings are also disabled in the expression that follows
[C`]__extension__[C']. However, only system header files should use
these escape routes; application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO
C conformance. They soon find that it does not do quite what they want:
it finds some non-ISO practices, but not all---only those for which
ISO C requires a diagnostic, and some others for which
diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in
some instances, but would require considerable additional work and would
be quite different from -pedantic. We don't have plans to
support such a feature in the near future.
Where the standard specified with -std represents a GNU
extended dialect of C, such as gnu89 or gnu99, there is a
corresponding base standard, the version of ISO C on which the GNU
extended dialect is based. Warnings from -pedantic are given
where they are required by the base standard. (It would not make sense
for such warnings to be given only for features not in the specified GNU
C dialect, since by definition the GNU dialects of C include all
features the compiler supports with the given option, and there would be
nothing to warn about.)
Like -pedantic, except that errors are produced rather than
warnings.
Inhibit all warning messages.
Inhibit warning messages about the use of #import.
Warn if an array subscript has type [C`]char[C']. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
Warn whenever a comment-start sequence /* appears in a /*
comment, or whenever a Backslash-Newline appears in a // comment.
Check calls to [C`]printf[C'] and [C`]scanf[C'], etc., to make sure that
the arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string make
sense. This includes standard functions, and others specified by format
attributes, in the [C`]printf[C'],
[C`]scanf[C'], [C`]strftime[C'] and [C`]strfmon[C'] (an X/Open extension,
not in the C standard) families.
The formats are checked against the format features supported by GNU
libc version 2.2. These include all ISO C89 and C99 features, as well
as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library's limitations. However, if -pedantic is used
with -Wformat, warnings will be given about format features not
in the selected standard version (but not for [C`]strfmon[C'] formats,
since those are not in any version of the C standard).
-Wformat is included in -Wall. For more control over some
aspects of format checking, the options -Wno-format-y2k,
-Wno-format-extra-args, -Wformat-nonliteral,
-Wformat-security and -Wformat=2 are available, but are
not included in -Wall.
If -Wformat is specified, do not warn about [C`]strftime[C']
formats which may yield only a two-digit year.
If -Wformat is specified, do not warn about excess arguments to a
[C`]printf[C'] or [C`]scanf[C'] format function. The C standard specifies
that such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with $ operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to [C`]va_arg[C'] to skip the unused arguments. However,
in the case of [C`]scanf[C'] formats, this option will suppress the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
If -Wformat is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a [C`]va_list[C'].
If -Wformat is specified, also warn about uses of format
functions that represent possible security problems. At present, this
warns about calls to [C`]printf[C'] and [C`]scanf[C'] functions where the
format string is not a string literal and there are no format arguments,
as in [C`]printf (foo);[C']. This may be a security hole if the format
string came from untrusted input and contains %n. (This is
currently a subset of what -Wformat-nonliteral warns about, but
in future warnings may be added to -Wformat-security that are not
included in -Wformat-nonliteral.)
Enable -Wformat plus format checks not included in
-Wformat. Currently equivalent to -Wformat
-Wformat-nonliteral -Wformat-security.
Warn when a declaration does not specify a type.
Give a warning (or error) whenever a function is used before being
declared.
Same as -Wimplicit-int and -Wimplicit-function-declaration.
Warn if the type of main is suspicious. main should be a
function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types.
Warn if an aggregate or union initializer is not fully bracketed. In
the following example, the initializer for a is not fully
bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
Warn if parentheses are omitted in certain contexts, such
as when there is an assignment in a context where a truth value
is expected, or when operators are nested whose precedence people
often get confused about.
Also warn about constructions where there may be confusion to which
[C`]if[C'] statement an [C`]else[C'] branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every [C`]else[C'] branch belongs to the innermost possible [C`]if[C']
statement, which in this example is [C`]if (b)[C']. This is often not
what the programmer expected, as illustrated in the above example by
indentation the programmer chose. When there is the potential for this
confusion, GCC will issue a warning when this flag is specified.
To eliminate the warning, add explicit braces around the innermost
[C`]if[C'] statement so there is no way the [C`]else[C'] could belong to
the enclosing [C`]if[C']. The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
Warn about code that may have undefined semantics because of violations
of sequence point rules in the C standard.
The C standard defines the order in which expressions in a C program are
evaluated in terms of sequence points, which represent a partial
ordering between the execution of parts of the program: those executed
before the sequence point, and those executed after it. These occur
after the evaluation of a full expression (one which is not part of a
larger expression), after the evaluation of the first operand of a
[C`]&&[C'], [C`]||[C'], [C`]? :[C'] or [C`],[C'] (comma) operator, before a
function is called (but after the evaluation of its arguments and the
expression denoting the called function), and in certain other places.
Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order,
since, for example, if two functions are called within one expression
with no sequence point between them, the order in which the functions
are called is not specified. However, the standards committee have
ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the
values of objects take effect. Programs whose behavior depends on this
have undefined behavior; the C standard specifies that ``Between the
previous and next sequence point an object shall have its stored value
modified at most once by the evaluation of an expression. Furthermore,
the prior value shall be read only to determine the value to be
stored.''. If a program breaks these rules, the results on any
particular implementation are entirely unpredictable.
Examples of code with undefined behavior are [C`]a = a++;[C'], [C`]a[n]
= b[n++][C'] and [C`]a[i++] = i;[C']. Some more complicated cases are not
diagnosed by this option, and it may give an occasional false positive
result, but in general it has been found fairly effective at detecting
this sort of problem in programs.
The present implementation of this option only works for C programs. A
future implementation may also work for [C+] programs.
The C standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle cases.
Links to discussions of the problem, including proposed formal
definitions, may be found on our readings page, at
<http://gcc.gnu.org/readings.html>.
Warn whenever a function is defined with a return-type that defaults to
[C`]int[C']. Also warn about any [C`]return[C'] statement with no
return-value in a function whose return-type is not [C`]void[C'].
For [C+], a function without return type always produces a diagnostic
message, even when -Wno-return-type is specified. The only
exceptions are main and functions defined in system headers.
Warn whenever a [C`]switch[C'] statement has an index of enumeral type
and lacks a [C`]case[C'] for one or more of the named codes of that
enumeration. (The presence of a [C`]default[C'] label prevents this
warning.) [C`]case[C'] labels outside the enumeration range also
provoke warnings when this option is used.
Warn if any trigraphs are encountered that might change the meaning of
the program (trigraphs within comments are not warned about).
Warn whenever a static function is declared but not defined or a
non\-inline static function is unused.
Warn whenever a label is declared but not used.
To suppress this warning use the unused attribute.
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
Warn whenever a local variable or non-constant static variable is unused
aside from its declaration
To suppress this warning use the unused attribute.
Warn whenever a statement computes a result that is explicitly not used.
To suppress this warning cast the expression to void.
All all the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must
either specify -W -Wunused or separately specify
-Wunused-parameter.
Warn if an automatic variable is used without first being initialized or
if a variable may be clobbered by a [C`]setjmp[C'] call.
These warnings are possible only in optimizing compilation,
because they require data flow information that is computed only
when optimizing. If you don't specify -O, you simply won't
get these warnings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable that
is declared [C`]volatile[C'], or whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
Note that there may be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the warnings
are printed.
These warnings are made optional because GCC is not smart
enough to see all the reasons why the code might be correct
despite appearing to have an error. Here is one example of how
this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of [C`]y[C'] is always 1, 2 or 3, then [C`]x[C'] is
always initialized, but GCC doesn't know this. Here is
another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because [C`]save_y[C'] is used only if it is set.
This option also warns when a non-volatile automatic variable might be
changed by a call to [C`]longjmp[C']. These warnings as well are possible
only in optimizing compilation.
The compiler sees only the calls to [C`]setjmp[C']. It cannot know
where [C`]longjmp[C'] will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a warning
even when there is in fact no problem because [C`]longjmp[C'] cannot
in fact be called at the place which would cause a problem.
Some spurious warnings can be avoided if you declare all the functions
you use that never return as [C`]noreturn[C'].
Warn when the order of member initializers given in the code does not
match the order in which they must be executed. For instance:
Warn when a #pragma directive is encountered which is not understood by
GCC. If this command line option is used, warnings will even be issued
for unknown pragmas in system header files. This is not the case if
the warnings were only enabled by the -Wall command line option.
All of the above -W options combined. This enables all the
warnings about constructions that some users consider questionable, and
that are easy to avoid (or modify to prevent the warning), even in
conjunction with macros.
Warn about compile-time integer division by zero. This is default. To
inhibit the warning messages, use -Wno-div-by-zero. Floating
point division by zero is not warned about, as it can be a legitimate
way of obtaining infinities and NaNs.
Warn if a multicharacter constant ('FOOF') is used. This is
default. To inhibit the warning messages, use -Wno-multichar.
Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable code.
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the assumption
that they usually do not indicate real problems and would only make the
compiler output harder to read. Using this command line option tells
GCC to emit warnings from system headers as if they occurred in user
code. However, note that using -Wall in conjunction with this
option will not warn about unknown pragmas in system
headers---for that, -Wunknown-pragmas must also be used.
The following -W... options are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Print extra warning messages for these events:
Options of the form -fflag specify machine-independent
flags. Most flags have both positive and negative forms; the negative
form of -ffoo would be -fno-foo. In the table below,
only one of the forms is listed---the one which is not the default.
You can figure out the other form by either removing no- or
adding it.
Do not store floating point variables in registers, and inhibit other
options that might change whether a floating point value is taken from a
register or memory.
This option prevents undesirable excess precision on machines such as
the 68000 where the floating registers (of the 68881) keep more
precision than a [C`]double[C'] is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE floating
point. Use -ffloat-store for such programs, after modifying
them to store all pertinent intermediate computations into variables.
Do not make member functions inline by default merely because they are
defined inside the class scope ([C+] only). Otherwise, when you specify
-O, member functions defined inside class scope are compiled
inline by default; i.e., you don't need to add inline in front of
the member function name.
Always pop the arguments to each function call as soon as that function
returns. For machines which must pop arguments after a function call,
the compiler normally lets arguments accumulate on the stack for several
function calls and pops them all at once.
Force memory operands to be copied into registers before doing
arithmetic on them. This produces better code by making all memory
references potential common subexpressions. When they are not common
subexpressions, instruction combination should eliminate the separate
register-load. The -O2 option turns on this option.
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
-fforce-mem may.
Don't keep the frame pointer in a register for functions that
don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available
in many functions. It also makes debugging impossible on
some machines.
On some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame pointer
and nothing is saved by pretending it doesn't exist. The
machine-description macro [C`]FRAME_POINTER_REQUIRED[C'] controls
whether a target machine supports this flag.
Optimize sibling and tail recursive calls.
This option generates traps for signed overflow on addition, subtraction,
multiplication operations.
Don't pay attention to the [C`]inline[C'] keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded inline.
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.
If all calls to a given function are integrated, and the function is
declared [C`]static[C'], then the function is normally not output as
assembler code in its own right.
By default, gcc limits the size of functions that can be inlined. This flag
allows the control of this limit for functions that are explicitly marked as
inline (ie marked with the inline keyword or defined within the class
definition in c++). n is the size of functions that can be inlined in
number of pseudo instructions (not counting parameter handling). The default
value of n is 600.
Increasing this value can result in more inlined code at
the cost of compilation time and memory consumption. Decreasing usually makes
the compilation faster and less code will be inlined (which presumably
means slower programs). This option is particularly useful for programs that
use inlining heavily such as those based on recursive templates with [C+].
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way, it represents a count
of assembly instructions and as such its exact meaning might change from one
release to an another.
Even if all calls to a given function are integrated, and the function
is declared [C`]static[C'], nevertheless output a separate run-time
callable version of the function. This switch does not affect
[C`]extern inline[C'] functions.
Emit variables declared [C`]static const[C'] when optimization isn't turned
on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to
check if the variable was referenced, regardless of whether or not
optimization is turned on, use the -fno-keep-static-consts option.
Attempt to merge identical constants (string constants and floating point
constants) accross compilation units.
This option is default for optimized compilation if assembler and linker
support it. Use -fno-merge-constants to inhibit this behavior.
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating point
types. Languages like C or [C+] require each non-automatic variable to
have distinct location, so using this option will result in non-conforming
behavior.
Do not use ``decrement and branch'' instructions on a count register,
but instead generate a sequence of instructions that decrement a
register, compare it against zero, then branch based upon the result.
This option is only meaningful on architectures that support such
instructions, which include x86, PowerPC, IA-64 and S/390.
Do not put function addresses in registers; make each instruction that
calls a constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the optimizations
performed when this option is not used.
Sets -fno-math-errno, -funsafe-math-optimizations, and -fno-trapping-math.
This option causes the preprocessor macro [C`]__FAST_MATH__[C'] to be defined.
This option should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.
Do not set ERRNO after calling math functions that are executed
with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.
The default is -fmath-errno.
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.
This option should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.
The default is -fno-unsafe-math-optimizations.
Compile code assuming that floating-point operations cannot generate
user-visible traps. Setting this option may allow faster code
if one relies on ``non-stop'' IEEE arithmetic, for example.
This option should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.
The default is -ftrapping-math.
For front-ends that support it, generate additional code to check that
indices used to access arrays are within the declared range. This is
currenly only supported by the Java and Fortran 77 front-ends, where
this option defaults to true and false respectively.
The following options control specific optimizations. The -O2 option turns on all of these optimizations except -funroll-loops and -funroll-all-loops. On most machines, the -O option turns on the -fthread-jumps and -fdelayed-branch options, but specific machines may handle it differently.
You can use the following flags in the rare cases when ``fine-tuning'' of optimizations to be performed is desired.
Not all of the optimizations performed by GCC have -f options
to control them.
Perform the optimizations of loop strength reduction and
elimination of iteration variables.
Perform optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is found. If
so, the first branch is redirected to either the destination of the
second branch or a point immediately following it, depending on whether
the condition is known to be true or false.
In common subexpression elimination, scan through jump instructions
when the target of the jump is not reached by any other path. For
example, when CSE encounters an [C`]if[C'] statement with an
[C`]else[C'] clause, CSE will follow the jump when the condition
tested is false.
This is similar to -fcse-follow-jumps, but causes CSE to
follow jumps which conditionally skip over blocks. When CSE
encounters a simple [C`]if[C'] statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the
body of the [C`]if[C'].
Re-run common subexpression elimination after loop optimizations has been
performed.
Run the loop optimizer twice.
Perform a global common subexpression elimination pass.
This pass also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC
extension, you may get better runtime performance if you disable
the global common subexpression elmination pass by adding
-fno-gcse to the command line.
When -fgcse-lm is enabled, global common subexpression elimination will
attempt to move loads which are only killed by stores into themselves. This
allows a loop containing a load/store sequence to be changed to a load outside
the loop, and a copy/store within the loop.
When -fgcse-sm is enabled, A store motion pass is run after global common
subexpression elimination. This pass will attempt to move stores out of loops.
When used in conjunction with -fgcse-lm, loops containing a load/store sequence
can be changed to a load before the loop and a store after the loop.
Use global dataflow analysis to identify and eliminate useless checks
for null pointers. The compiler assumes that dereferencing a null
pointer would have halted the program. If a pointer is checked after
it has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs can
safely dereference null pointers. Use
-fno-delete-null-pointer-checks to disable this optimization
for programs which depend on that behavior.
Perform a number of minor optimizations that are relatively expensive.
Attempt to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize the amount of
register tying. This is especially helpful on machines with two-operand
instructions. GCC enables this optimization by default with -O2
or higher.
Note -fregmove and -foptimize-register-move are the same
optimization.
If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.
If supported for the target machine, attempt to reorder instructions to
eliminate execution stalls due to required data being unavailable. This
helps machines that have slow floating point or memory load instructions
by allowing other instructions to be issued until the result of the load
or floating point instruction is required.
Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done. This is
especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one cycle.
Don't schedule instructions across basic blocks. This is normally
enabled by default when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
Don't allow speculative motion of non-load instructions. This is normally
enabled by default when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
Allow speculative motion of some load instructions. This only makes
sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
Allow speculative motion of more load instructions. This only makes
sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
Place each function or data item into its own section in the output
file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name
in the output file.
Use these options on systems where the linker can perform optimizations
to improve locality of reference in the instruction space. HPPA
processors running HP-UX and Sparc processors running Solaris 2 have
linkers with such optimizations. Other systems using the ELF object format
as well as AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing
so. When you specify these options, the assembler and linker will
create larger object and executable files and will also be slower.
You will not be able to use [C`]gprof[C'] on all systems if you
specify this option and you may have problems with debugging if
you specify both this option and -g.
Enable values to be allocated in registers that will be clobbered by
function calls, by emitting extra instructions to save and restore the
registers around such calls. Such allocation is done only when it
seems to result in better code than would otherwise be produced.
This option is always enabled by default on certain machines, usually
those which have no call-preserved registers to use instead.
For all machines, optimization level 2 and higher enables this flag by
default.
Unroll loops whose number of iterations can be determined at compile
time or upon entry to the loop. -funroll-loops implies both
-fstrength-reduce and -frerun-cse-after-loop. This
option makes code larger, and may or may not make it run faster.
Unroll all loops, even if their number of iterations is uncertain when
the loop is entered. This usually makes programs run more slowly.
-funroll-all-loops implies the same options as
-funroll-loops,
If supported by the target machine, generate instructions to prefetch
memory to improve the performance of loops that access large arrays.
Forces all invariant computations in loops to be moved
outside the loop.
Forces all general-induction variables in loops to be
strength-reduced.
Note: When compiling programs written in Fortran,
-fmove-all-movables and -freduce-all-givs are enabled
by default when you use the optimizer.
These options may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
These two options are intended to be removed someday, once
they have helped determine the efficacy of various
approaches to improving loop optimizations.
Please let us (<gcc@gcc.gnu.org> and <fortran@gnu.org>)
know how use of these options affects
the performance of your production code.
We're very interested in code that runs slower
when these options are enabled.
Disable any machine-specific peephole optimizations. The difference
between -fno-peephole and -fno-peephole2 is in how they
are implemented in the compiler; some targets use one, some use the
other, a few use both.
After running a program compiled with -fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on
the number of times each branch was taken. When the program
compiled with -fprofile-arcs exits it saves arc execution
counts to a file called sourcename.da for each source
file The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code
and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_EXEC_COUNT
note on the first instruction of each basic block, and a
REG_BR_PROB note on each JUMP_INSN and CALL_INSN.
These can be used to improve optimization. Currently, they are only
used in one place: in reorg.c, instead of guessing which path a
branch is mostly to take, the REG_BR_PROB values are used to
exactly determine which path is taken more often.
Do not guess branch probabilities using a randomized model.
Sometimes gcc will opt to use a randomized model to guess branch
probabilities, when none are available from either profiling feedback
(-fprofile-arcs) or __builtin_expect. This means that
different runs of the compiler on the same program may produce different
object code.
In a hard real-time system, people don't want different runs of the
compiler to produce code that has different behavior; minimizing
non-determinism is of paramount import. This switch allows users to
reduce non-determinism, possibly at the expense of inferior
optimization.
Allows the compiler to assume the strictest aliasing rules applicable to
the language being compiled. For C (and [C+]), this activates
optimizations based on the type of expressions. In particular, an
object of one type is assumed never to reside at the same address as an
object of a different type, unless the types are almost the same. For
example, an [C`]unsigned int[C'] can alias an [C`]int[C'], but not a
[C`]void*[C'] or a [C`]double[C']. A character type may alias any other
type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most
recently written to (called ``type-punning'') is common. Even with
-fstrict-aliasing, type-punning is allowed, provided the memory
is accessed through the union type. So, the code above will work as
expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias analysis
should define a function that computes, given an [C`]tree[C']
node, an alias set for the node. Nodes in different alias sets are not
allowed to alias. For an example, see the C front-end function
[C`]c_get_alias_set[C'].
Align the start of functions to the next power-of-two greater than
n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next 32-byte
boundary, but -falign-functions=24 would align to the next
32-byte boundary only if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are
equivalent and mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two;
in that case, it is rounded up.
If n is not specified, use a machine-dependent default.
Align all branch targets to a power-of-two boundary, skipping up to
n bytes like -falign-functions. This option can easily
make code slower, because it must insert dummy operations for when the
branch target is reached in the usual flow of the code.
If -falign-loops or -falign-jumps are applicable and
are greater than this value, then their values are used instead.
If n is not specified, use a machine-dependent default which is
very likely to be 1, meaning no alignment.
Align loops to a power-of-two boundary, skipping up to n bytes
like -falign-functions. The hope is that the loop will be
executed many times, which will make up for any execution of the dummy
operations.
If n is not specified, use a machine-dependent default.
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping, skipping up to n
bytes like -falign-functions. In this case, no dummy operations
need be executed.
If n is not specified, use a machine-dependent default.
Perform optimizations in static single assignment form. Each function's
flow graph is translated into SSA form, optimizations are performed, and
the flow graph is translated back from SSA form. Users should not
specify this option, since it is not yet ready for production use.
Perform Sparse Conditional Constant Propagation in SSA form. Requires
-fssa. Like -fssa, this is an experimental feature.
Perform aggressive dead-code elimination in SSA form. Requires -fssa.
Like -fssa, this is an experimental feature.
Treat floating point constant as single precision constant instead of
implicitly converting it to double precision constant.
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This optimization
will most benefit processors with lots of registers. It can, however,
make debugging impossible, since variables will no longer stay in
a ``home register''.
After register allocation and post-register allocation instruction splitting,
we perform a copy-propagation pass to try to reduce scheduling dependencies
and occasionally eliminate the copy.
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC will not inline functions
that contain more that a certain number of instructions. You can
control some of these constants on the command-line using the
[--]param option.
In each case, the value is an integer. The allowable choices for
name are given in the following table:
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation.
You can use -Wp,option to bypass the compiler driver
and pass option directly through to the preprocessor. If
option contains commas, it is split into multiple options at the
commas. However, many options are modified, translated or interpreted
by the compiler driver before being passed to the preprocessor, and
-Wp forcibly bypasses this phase. The preprocessor's direct
interface is undocumented and subject to change, so whenever possible
you should avoid using -Wp and let the driver handle the
options instead.
Predefine name as a macro, with definition [C`]1[C'].
Predefine name as a macro, with definition definition.
There are no restrictions on the contents of definition, but if
you are invoking the preprocessor from a shell or shell-like program you
may need to use the shell's quoting syntax to protect characters such as
spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write
its argument list with surrounding parentheses before the equals sign
(if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they
are given on the command line. All -imacros file and
-include file options are processed after all
-D and -U options.
Cancel any previous definition of name, either built in or
provided with a -D option.
Do not predefine any system-specific macros. The common predefined
macros remain defined.
Add the directory dir to the list of directories to be searched
for header files.
Directories named by -I are searched before the standard
system include directories.
It is dangerous to specify a standard system include directory in an
-I option. This defeats the special treatment of system
headers
. It can also defeat the repairs to buggy system headers which GCC
makes when it is installed.
Write output to file. This is the same as specifying file
as the second non-option argument to cpp. gcc has a
different interpretation of a second non-option argument, so you must
use -o to specify the output file.
Turns on all optional warnings which are desirable for normal code. At
present this is -Wcomment and -Wtrigraphs. Note that
many of the preprocessor's warnings are on by default and have no
options to control them.
Warn whenever a comment-start sequence /* appears in a /*
comment, or whenever a backslash-newline appears in a // comment.
(Both forms have the same effect.)
Warn if any trigraphs are encountered. This option used to take effect
only if -trigraphs was also specified, but now works
independently. Warnings are not given for trigraphs within comments, as
they do not affect the meaning of the program.
Warn about certain constructs that behave differently in traditional and
ISO C. Also warn about ISO C constructs that have no traditional C
equivalent, and problematic constructs which should be avoided.
Warn the first time #import is used.
Warn whenever an identifier which is not a macro is encountered in an
#if directive, outside of defined. Such identifiers are
replaced with zero.
Make all warnings into hard errors. Source code which triggers warnings
will be rejected.
Issue warnings for code in system headers. These are normally unhelpful
in finding bugs in your own code, therefore suppressed. If you are
responsible for the system library, you may want to see them.
Suppress all warnings, including those which GNU CPP issues by default.
Issue all the mandatory diagnostics listed in the C standard. Some of
them are left out by default, since they trigger frequently on harmless
code.
Issue all the mandatory diagnostics, and make all mandatory diagnostics
into errors. This includes mandatory diagnostics that GCC issues
without -pedantic but treats as warnings.
Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main
source file. The preprocessor outputs one make rule containing
the object file name for that source file, a colon, and the names of all
the included files, including those coming from -include or
-imacros command line options.
Unless specified explicitly (with -MT or -MQ), the
object file name consists of the basename of the source file with any
suffix replaced with object file suffix. If there are many included
files then the rule is split into several lines using \-newline.
The rule has no commands.
This option does not suppress the preprocessor's debug output, such as
-dM. To avoid mixing such debug output with the dependency
rules you should explicitly specify the dependency output file with
-MF, or use an environment variable like
DEPENDENCIES_OUTPUT. Debug output
will still be sent to the regular output stream as normal.
Passing -M to the driver implies -E.
Like -M but do not mention header files that are found in
system header directories, nor header files that are included,
directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an
#include directive does not in itself determine whether that
header will appear in -MM dependency output. This is a
slight change in semantics from GCC versions 3.0 and earlier.
@anchor{-MF}
When used with -M or -MM, specifies a
file to write the dependencies to. If no -MF switch is given
the preprocessor sends the rules to the same place it would have sent
preprocessed output.
When used with the driver options -MD or -MMD,
-MF overrides the default dependency output file.
When used with -M or -MM, -MG says to treat missing
header files as generated files and assume they live in the same
directory as the source file. It suppresses preprocessed output, as a
missing header file is ordinarily an error.
This feature is used in automatic updating of makefiles.
This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, including any path,
deletes any file suffix such as .c, and appends the platform's
usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a single
argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with
-MQ.
-MD is equivalent to -M -MF file, except that
-E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its
argument but with a suffix of .d, otherwise it take the
basename of the input file and applies a .d suffix.
If -MD is used in conjunction with -E, any
-o switch is understood to specify the dependency output file
(but @pxref{-MF}), but if used without -E, each -o
is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate
a dependency output file as a side-effect of the compilation process.
Like -MD except mention only user header files, not system
-header files.
Specify the source language: C, [C+], Objective-C, or assembly. This has
nothing to do with standards conformance or extensions; it merely
selects which base syntax to expect. If you give none of these options,
cpp will deduce the language from the extension of the source file:
.c, .cc, .m, or .S. Some other common
extensions for [C+] and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the most
generic mode.
Note: Previous versions of cpp accepted a -lang option
which selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the -l
option.
Specify the standard to which the code should conform. Currently cpp
only knows about the standards for C; other language standards will be
added in the future.
standard
may be one of:
In addition, older and newer versions of GCC can be installed side
by side. One of them (probably the newest) will be the default, but
you may sometimes wish to use another.
The argument machine specifies the target machine for compilation.
This is useful when you have installed GCC as a cross-compiler.
The value to use for machine is the same as was specified as the
machine type when configuring GCC as a cross-compiler. For
example, if a cross-compiler was configured with configure
i386v, meaning to compile for an 80386 running System V, then you
would specify -b i386v to run that cross compiler.
When you do not specify -b, it normally means to compile for
the same type of machine that you are using.
The argument version specifies which version of GCC to run.
This is useful when multiple versions are installed. For example,
version might be 2.0, meaning to run GCC version 2.0.
The default version, when you do not specify -V, is the last
version of GCC that you installed.
The -b and -V options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GCC, for a given target machine, is normally kept in the directory /usr/local/lib/gcc-lib/machine/version.
Thus, sites can customize the effect of -b or -V either by changing the names of these directories or adding alternate names (or symbolic links). If in directory /usr/local/lib/gcc-lib/ the file 80386 is a link to the file i386v, then -b 80386 becomes an alias for -b i386v.
In one respect, the -b or -V do not completely change to a different compiler: the top-level driver program gcc that you originally invoked continues to run and invoke the other executables (preprocessor, compiler per se, assembler and linker) that do the real work. However, since no real work is done in the driver program, it usually does not matter that the driver program in use is not the one for the specified target. It is common for the interface to the other executables to change incompatibly between compiler versions, so unless the version specified is very close to that of the driver (for example, -V 3.0 with a driver program from GCC version 3.0.1), use of -V may not work; for example, using -V 2.95.2 will not work with a driver program from GCC 3.0.
The only way that the driver program depends on the target machine is in the parsing and handling of special machine-specific options. However, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine, and sufficiently similar compiler versions.
The driver program executable does control one significant thing, however: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for different targets or versions, under different names.
For example, if the driver for version 2.0 is installed as ogcc and that for version 2.1 is installed as gcc, then the command gcc will use version 2.1 by default, while ogcc will use 2.0 by default. However, you can choose either version with either command with the -V option. Earlier we discussed the standard option -b which chooses among different installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386.
In addition, each of these target machine types can have its own special options, starting with -m, to choose among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.
These options are defined by the macro [C`]TARGET_SWITCHES[C'] in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults.
M680x0 Options
These are the -m options defined for the 68000 series. The default
values for these options depends on which style of 68000 was selected when
the compiler was configured; the defaults for the most common choices are
given below.
Generate output for a 68000. This is the default
when the compiler is configured for 68000-based systems.
Use this option for microcontrollers with a 68000 or EC000 core,
including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
Generate output for a 68020. This is the default
when the compiler is configured for 68020-based systems.
Generate output containing 68881 instructions for floating point.
This is the default for most 68020 systems unless [--]nfp was
specified when the compiler was configured.
Generate output for a 68030. This is the default when the compiler is
configured for 68030-based systems.
Generate output for a 68040. This is the default when the compiler is
configured for 68040-based systems.
This option inhibits the use of 68881/68882 instructions that have to be
emulated by software on the 68040. Use this option if your 68040 does not
have code to emulate those instructions.
Generate output for a 68060. This is the default when the compiler is
configured for 68060-based systems.
This option inhibits the use of 68020 and 68881/68882 instructions that
have to be emulated by software on the 68060. Use this option if your 68060
does not have code to emulate those instructions.
Generate output for a CPU32. This is the default
when the compiler is configured for CPU32-based systems.
Use this option for microcontrollers with a
CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
68336, 68340, 68341, 68349 and 68360.
Generate output for a 520X ``coldfire'' family cpu. This is the default
when the compiler is configured for 520X-based systems.
Use this option for microcontroller with a 5200 core, including
the MCF5202, MCF5203, MCF5204 and MCF5202.
Generate output for a 68040, without using any of the new instructions.
This results in code which can run relatively efficiently on either a
68020/68881 or a 68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68040.
Generate output for a 68060, without using any of the new instructions.
This results in code which can run relatively efficiently on either a
68020/68881 or a 68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68060.
Generate output containing Sun FPA instructions for floating point.
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all m68k
targets. Normally the facilities of the machine's usual C compiler are
used, but this can't be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions for
cross-compilation. The embedded targets m68k-*-aout and
m68k-*-coff do provide software floating point support.
Consider type [C`]int[C'] to be 16 bits wide, like [C`]short int[C'].
Do not use the bit-field instructions. The -m68000, -mcpu32
and -m5200 options imply -mnobitfield.
Do use the bit-field instructions. The -m68020 option implies
-mbitfield. This is the default if you use a configuration
designed for a 68020.
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the [C`]rtd[C']
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including [C`]printf[C']);
otherwise incorrect code will be generated for calls to those
functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
The [C`]rtd[C'] instruction is supported by the 68010, 68020, 68030,
68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
Control whether GCC aligns [C`]int[C'], [C`]long[C'], [C`]long long[C'],
[C`]float[C'], [C`]double[C'], and [C`]long double[C'] variables on a 32-bit
boundary (-malign-int) or a 16-bit boundary (-mno-align-int).
Aligning variables on 32-bit boundaries produces code that runs somewhat
faster on processors with 32-bit busses at the expense of more memory.
Warning: if you use the -malign-int switch, GCC will
align structures containing the above types differently than
most published application binary interface specifications for the m68k.
Use the pc-relative addressing mode of the 68000 directly, instead of
using a global offset table. At present, this option implies -fpic,
allowing at most a 16-bit offset for pc-relative addressing. -fPIC is
not presently supported with -mpcrel, though this could be supported for
68020 and higher processors.
Do not (do) assume that unaligned memory references will be handled by
the system.
M68hc1x Options
These are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The default values for these options depends on which style of microcontroller was selected when the compiler was configured; the defaults for the most common choices are given below. Generate output for a 68HC11. This is the default when the compiler is configured for 68HC11-based systems. Generate output for a 68HC12. This is the default when the compiler is configured for 68HC12-based systems. Enable the use of 68HC12 pre and post auto-increment and auto-decrement addressing modes. Consider type [C`]int[C'] to be 16 bits wide, like [C`]short int[C']. Specify the number of pseudo-soft registers which are used for the code generation. The maximum number is 32. Using more pseudo-soft register may or may not result in better code depending on the program. The default is 4 for 68HC11 and 2 for 68HC12.
VAX Options
These -m options are defined for the VAX: Do not output certain jump instructions ([C`]aobleq[C'] and so on) that the Unix assembler for the VAX cannot handle across long ranges. Do output those jump instructions, on the assumption that you will assemble with the GNU assembler. Output code for g-format floating point numbers instead of d-format.
SPARC Options
These -m switches are supported on the SPARC:
Specify -mapp-regs to generate output using the global registers
2 through 4, which the SPARC SVR4 ABI reserves for applications. This
is the default.
To be fully SVR4 ABI compliant at the cost of some performance loss,
specify -mno-app-regs. You should compile libraries and system
software with this option.
Generate output containing floating point instructions. This is the
default.
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all SPARC
targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions for
cross-compilation. The embedded targets sparc-*-aout and
sparclite-*-* do provide software floating point support.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for
this to work.
Generate output containing quad-word (long double) floating point
instructions.
Generate output containing library calls for quad-word (long double)
floating point instructions. The functions called are those specified
in the SPARC ABI. This is the default.
As of this writing, there are no sparc implementations that have hardware
support for the quad-word floating point instructions. They all invoke
a trap handler for one of these instructions, and then the trap handler
emulates the effect of the instruction. Because of the trap handler overhead,
this is much slower than calling the ABI library routines. Thus the
-msoft-quad-float option is the default.
With -mflat, the compiler does not generate save/restore instructions
and will use a ``flat'' or single register window calling convention.
This model uses %i7 as the frame pointer and is compatible with the normal
register window model. Code from either may be intermixed.
The local registers and the input registers (0[--]5) are still treated as
``call saved'' registers and will be saved on the stack as necessary.
With -mno-flat (the default), the compiler emits save/restore
instructions (except for leaf functions) and is the normal mode of operation.
Assume that doubles have 8 byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8 byte
alignment only if they are contained in another type, or if they have an
absolute address. Otherwise, it assumes they have 4 byte alignment.
Specifying this option avoids some rare compatibility problems with code
generated by other compilers. It is not the default because it results
in a performance loss, especially for floating point code.
With -mfaster-structs, the compiler assumes that structures
should have 8 byte alignment. This enables the use of pairs of
[C`]ldd[C'] and [C`]std[C'] instructions for copies in structure
assignment, in place of twice as many [C`]ld[C'] and [C`]st[C'] pairs.
However, the use of this changed alignment directly violates the Sparc
ABI. Thus, it's intended only for use on targets where the developer
acknowledges that their resulting code will not be directly in line with
the rules of the ABI.
These two options select variations on the SPARC architecture.
By default (unless specifically configured for the Fujitsu SPARClite),
GCC generates code for the v7 variant of the SPARC architecture.
-mv8 will give you SPARC v8 code. The only difference from v7
code is that the compiler emits the integer multiply and integer
divide instructions which exist in SPARC v8 but not in SPARC v7.
-msparclite will give you SPARClite code. This adds the integer
multiply, integer divide step and scan ([C`]ffs[C']) instructions which
exist in SPARClite but not in SPARC v7.
These options are deprecated and will be deleted in a future GCC release.
They have been replaced with -mcpu=xxx.
These two options select the processor for which the code is optimized.
With -mcypress (the default), the compiler optimizes code for the
Cypress CY7C602 chip, as used in the SparcStation/SparcServer 3xx series.
This is also appropriate for the older SparcStation 1, 2, IPX etc.
With -msupersparc the compiler optimizes code for the SuperSparc cpu, as
used in the SparcStation 10, 1000 and 2000 series. This flag also enables use
of the full SPARC v8 instruction set.
These options are deprecated and will be deleted in a future GCC release.
They have been replaced with -mcpu=xxx.
Set the instruction set, register set, and instruction scheduling parameters
for machine type cpu_type. Supported values for cpu_type are
v7, cypress, v8, supersparc, sparclite,
hypersparc, sparclite86x, f930, f934,
sparclet, tsc701, v9, and ultrasparc.
Default instruction scheduling parameters are used for values that select
an architecture and not an implementation. These are v7, v8,
sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported
implementations.
v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set that the
option -mcpu=cpu_type would.
The same values for -mcpu=cpu_type can be used for
-mtune=cpu_type, but the only useful values are those
that select a particular cpu implementation. Those are cypress,
supersparc, hypersparc, f930, f934,
sparclite86x, tsc701, and ultrasparc.
These -m switches are supported in addition to the above on the SPARCLET processor. Generate code for a processor running in little-endian mode. Treat register [C`]%g0[C'] as a normal register. GCC will continue to clobber it as necessary but will not assume it always reads as 0. Generate code that does not use non-trivial forms of the [C`]save[C'] and [C`]restore[C'] instructions. Early versions of the SPARCLET processor do not correctly handle [C`]save[C'] and [C`]restore[C'] instructions used with arguments. They correctly handle them used without arguments. A [C`]save[C'] instruction used without arguments increments the current window pointer but does not allocate a new stack frame. It is assumed that the window overflow trap handler will properly handle this case as will interrupt handlers.
These -m switches are supported in addition to the above on SPARC V9 processors in 64-bit environments. Generate code for a processor running in little-endian mode. Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. Generate code for the Medium/Low code model: the program must be linked in the low 32 bits of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. Generate code for the Medium/Middle code model: the program must be linked in the low 44 bits of the address space, the text segment must be less than 2G bytes, and data segment must be within 2G of the text segment. Pointers are 64 bits. Generate code for the Medium/Anywhere code model: the program may be linked anywhere in the address space, the text segment must be less than 2G bytes, and data segment must be within 2G of the text segment. Pointers are 64 bits. Generate code for the Medium/Anywhere code model for embedded systems: assume a 32-bit text and a 32-bit data segment, both starting anywhere (determined at link time). Register %g4 points to the base of the data segment. Pointers are still 64 bits. Programs are statically linked, PIC is not supported. With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. Otherwise, assume no such offset is present.
Convex Options
These -m options are defined for Convex: Generate output for C1. The code will run on any Convex machine. The preprocessor symbol [C`]__convex__c1__[C'] is defined. Generate output for C2. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C2. The preprocessor symbol [C`]__convex_c2__[C'] is defined. Generate output for C32xx. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C32. The preprocessor symbol [C`]__convex_c32__[C'] is defined. Generate output for C34xx. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C34. The preprocessor symbol [C`]__convex_c34__[C'] is defined. Generate output for C38xx. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C38. The preprocessor symbol [C`]__convex_c38__[C'] is defined. Generate code which puts an argument count in the word preceding each argument list. This is compatible with regular CC, and a few programs may need the argument count word. GDB and other source-level debuggers do not need it; this info is in the symbol table. Omit the argument count word. This is the default. Allow volatile references to be cached. This is the default. Volatile references bypass the data cache, going all the way to memory. This is only needed for multi-processor code that does not use standard synchronization instructions. Making non-volatile references to volatile locations will not necessarily work. Type long is 32 bits, the same as type int. This is the default. Type long is 64 bits, the same as type long long. This option is useless, because no library support exists for it.
AMD29K Options
These -m options are defined for the AMD Am29000:
Generate code that assumes the [C`]DW[C'] bit is set, i.e., that byte and
halfword operations are directly supported by the hardware. This is the
default.
Generate code that assumes the [C`]DW[C'] bit is not set.
Generate code that assumes the system supports byte and halfword write
operations. This is the default.
Generate code that assumes the systems does not support byte and
halfword write operations. -mnbw implies -mndw.
Use a small memory model that assumes that all function addresses are
either within a single 256 KB segment or at an absolute address of less
than 256k. This allows the [C`]call[C'] instruction to be used instead
of a [C`]const[C'], [C`]consth[C'], [C`]calli[C'] sequence.
Use the normal memory model: Generate [C`]call[C'] instructions only when
calling functions in the same file and [C`]calli[C'] instructions
otherwise. This works if each file occupies less than 256 KB but allows
the entire executable to be larger than 256 KB. This is the default.
Always use [C`]calli[C'] instructions. Specify this option if you expect
a single file to compile into more than 256 KB of code.
Generate code for the Am29050.
Generate code for the Am29000. This is the default.
Generate references to registers [C`]gr64-gr95[C'] instead of to
registers [C`]gr96-gr127[C']. This option can be used when compiling
kernel code that wants a set of global registers disjoint from that used
by user-mode code.
Note that when this option is used, register names in -f flags
must use the normal, user-mode, names.
Use the normal set of global registers, [C`]gr96-gr127[C']. This is the
default.
Insert (or do not insert) a call to [C`]__msp_check[C'] after each stack
adjustment. This is often used for kernel code.
-mstorem-bug handles 29k processors which cannot handle the
separation of a mtsrim insn and a storem instruction (most 29000 chips
to date, but not the 29050).
-mno-reuse-arg-regs tells the compiler to only use incoming argument
registers for copying out arguments. This helps detect calling a function
with fewer arguments than it was declared with.
-mimpure-text, used in addition to -shared, tells the compiler to
not pass -assert pure-text to the linker when linking a shared object.
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC.
Normally the facilities of the machine's usual C compiler are used, but
this can't be done directly in cross-compilation. You must make your
own arrangements to provide suitable library functions for
cross-compilation.
Do not generate multm or multmu instructions. This is useful for some embedded
systems which do not have trap handlers for these instructions.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM)
architectures:
Generate a stack frame that is compliant with the ARM Procedure Call
Standard for all functions, even if this is not strictly necessary for
correct execution of the code. Specifying -fomit-frame-pointer
with this option will cause the stack frames not to be generated for
leaf functions. The default is -mno-apcs-frame.
This is a synonym for -mapcs-frame.
Generate code for a processor running with a 26-bit program counter,
and conforming to the function calling standards for the APCS 26-bit
option. This option replaces the -m2 and -m3 options
of previous releases of the compiler.
Generate code for a processor running with a 32-bit program counter,
and conforming to the function calling standards for the APCS 32-bit
option. This option replaces the -m6 option of previous releases
of the compiler.
Generate code which supports calling between the ARM and Thumb
instruction sets. Without this option the two instruction sets cannot
be reliably used inside one program. The default is
-mno-thumb-interwork, since slightly larger code is generated
when -mthumb-interwork is specified.
Prevent the reordering of instructions in the function prolog, or the
merging of those instruction with the instructions in the function's
body. This means that all functions will start with a recognizable set
of instructions (or in fact one of a choice from a small set of
different function prologues), and this information can be used to
locate the start if functions inside an executable piece of code. The
default is -msched-prolog.
Generate output containing floating point instructions. This is the
default.
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all ARM
targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions for
cross-compilation.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for
this to work.
Generate code for a processor running in little-endian mode. This is
the default for all standard configurations.
Generate code for a processor running in big-endian mode; the default is
to compile code for a little-endian processor.
This option only applies when generating code for big-endian processors.
Generate code for a little-endian word order but a big-endian byte
order. That is, a byte order of the form 32107654. Note: this
option should only be used if you require compatibility with code for
big-endian ARM processors generated by versions of the compiler prior to
2.8.
Generate code that will not trap if the MMU has alignment traps enabled.
On ARM architectures prior to ARMv4, there were no instructions to
access half-word objects stored in memory. However, when reading from
memory a feature of the ARM architecture allows a word load to be used,
even if the address is unaligned, and the processor core will rotate the
data as it is being loaded. This option tells the compiler that such
misaligned accesses will cause a MMU trap and that it should instead
synthesise the access as a series of byte accesses. The compiler can
still use word accesses to load half-word data if it knows that the
address is aligned to a word boundary.
This option is ignored when compiling for ARM architecture 4 or later,
since these processors have instructions to directly access half-word
objects in memory.
Generate code that assumes that the MMU will not trap unaligned
accesses. This produces better code when the target instruction set
does not have half-word memory operations (i.e. implementations prior to
ARMv4).
Note that you cannot use this option to access unaligned word objects,
since the processor will only fetch one 32-bit aligned object from
memory.
The default setting for most targets is -mno-alignment-traps, since
this produces better code when there are no half-word memory
instructions available.
These are deprecated aliases for -malignment-traps.
This are deprecated aliases for -mno-alignment-traps.
This option only applies to RISC iX. Emulate the native BSD-mode
compiler. This is the default if -ansi is not specified.
This option only applies to RISC iX. Emulate the native X/Open-mode
compiler.
This option only applies to RISC iX. Do not run the assembler
post-processor, symrename, after code has been assembled.
Normally it is necessary to modify some of the standard symbols in
preparation for linking with the RISC iX C library; this option
suppresses this pass. The post-processor is never run when the
compiler is built for cross-compilation.
This specifies the name of the target ARM processor. GCC uses this name
to determine what kind of instructions it can emit when generating
assembly code. Permissible names are: arm2, arm250,
arm3, arm6, arm60, arm600, arm610,
arm620, arm7, arm7m, arm7d, arm7dm,
arm7di, arm7dmi, arm70, arm700,
arm700i, arm710, arm710c, arm7100,
arm7500, arm7500fe, arm7tdmi, arm8,
strongarm, strongarm110, strongarm1100,
arm8, arm810, arm9, arm9e, arm920,
arm920t, arm940t, arm9tdmi, arm10tdmi,
arm1020t, xscale.
This option is very similar to the -mcpu= option, except that
instead of specifying the actual target processor type, and hence
restricting which instructions can be used, it specifies that GCC should
tune the performance of the code as if the target were of the type
specified in this option, but still choosing the instructions that it
will generate based on the cpu specified by a -mcpu= option.
For some ARM implementations better performance can be obtained by using
this option.
This specifies the name of the target ARM architecture. GCC uses this
name to determine what kind of instructions it can emit when generating
assembly code. This option can be used in conjunction with or instead
of the -mcpu= option. Permissible names are: armv2,
armv2a, armv3, armv3m, armv4, armv4t,
armv5, armv5t, armv5te.
This specifies the version of the floating point emulation available on
the target. Permissible values are 2 and 3. -mfp= is a synonym
for -mfpe=, for compatibility with older versions of GCC.
The size of all structures and unions will be rounded up to a multiple
of the number of bits set by this option. Permissible values are 8 and
32. The default value varies for different toolchains. For the COFF
targeted toolchain the default value is 8. Specifying the larger number
can produce faster, more efficient code, but can also increase the size
of the program. The two values are potentially incompatible. Code
compiled with one value cannot necessarily expect to work with code or
libraries compiled with the other value, if they exchange information
using structures or unions.
Generate a call to the function [C`]abort[C'] at the end of a
[C`]noreturn[C'] function. It will be executed if the function tries to
return.
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a subroutine
call on this register. This switch is needed if the target function
will lie outside of the 64 megabyte addressing range of the offset based
version of subroutine call instruction.
Even if this switch is enabled, not all function calls will be turned
into long calls. The heuristic is that static functions, functions
which have the short-call attribute, functions that are inside
the scope of a #pragma no_long_calls directive and functions whose
definitions have already been compiled within the current compilation
unit, will not be turned into long calls. The exception to this rule is
that weak function definitions, functions with the long-call
attribute or the section attribute, and functions that are within
the scope of a #pragma long_calls directive, will always be
turned into long calls.
This feature is not enabled by default. Specifying
-mno-long-calls will restore the default behavior, as will
placing the function calls within the scope of a #pragma
long_calls_off directive. Note these switches have no effect on how
the compiler generates code to handle function calls via function
pointers.
Disable support for the [C`]dllimport[C'] attribute.
Treat the register used for PIC addressing as read-only, rather than
loading it in the prologue for each function. The run-time system is
responsible for initializing this register with an appropriate value
before execution begins.
Specify the register to be used for PIC addressing. The default is R10
unless stack-checking is enabled, when R9 is used.
Write the name of each function into the text section, directly
preceding the function prologue. The generated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of
[C`]pc[C'] stored at [C`]fp + 0[C']. If the trace function then looks at
location [C`]pc - 12[C'] and the top 8 bits are set, then we know that
there is a function name embedded immediately preceding this location
and has length [C`]((pc[-3]) & 0xff000000)[C'].
Generate code for the 16-bit Thumb instruction set. The default is to
use the 32-bit ARM instruction set.
Generate a stack frame that is compliant with the Thumb Procedure Call
Standard for all non-leaf functions. (A leaf function is one that does
not call any other functions.) The default is -mno-tpcs-frame.
Generate a stack frame that is compliant with the Thumb Procedure Call
Standard for all leaf functions. (A leaf function is one that does
not call any other functions.) The default is -mno-apcs-leaf-frame.
Gives all externally visible functions in the file being compiled an ARM
instruction set header which switches to Thumb mode before executing the
rest of the function. This allows these functions to be called from
non-interworking code.
Allows calls via function pointers (including virtual functions) to
execute correctly regardless of whether the target code has been
compiled for interworking or not. There is a small overhead in the cost
of executing a function pointer if this option is enabled.
MN10200 Options
These -m options are defined for Matsushita MN10200 architectures:
Indicate to the linker that it should perform a relaxation optimization pass
to shorten branches, calls and absolute memory addresses. This option only
has an effect when used on the command line for the final link step.
This option makes symbolic debugging impossible.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
Generate code to avoid bugs in the multiply instructions for the MN10300
processors. This is the default.
Do not generate code to avoid bugs in the multiply instructions for the
MN10300 processors.
Generate code which uses features specific to the AM33 processor.
Do not generate code which uses features specific to the AM33 processor. This
is the default.
Do not link in the C run-time initialization object file.
Indicate to the linker that it should perform a relaxation optimization pass
to shorten branches, calls and absolute memory addresses. This option only
has an effect when used on the command line for the final link step.
This option makes symbolic debugging impossible.
M32R/D Options
These -m options are defined for Mitsubishi M32R/D architectures:
Generate code for the M32R/X.
Generate code for the M32R. This is the default.
Assume all objects live in the lower 16MB of memory (so that their addresses
can be loaded with the [C`]ld24[C'] instruction), and assume all subroutines
are reachable with the [C`]bl[C'] instruction.
This is the default.
The addressability of a particular object can be set with the
[C`]model[C'] attribute.
Assume objects may be anywhere in the 32-bit address space (the compiler
will generate [C`]seth/add3[C'] instructions to load their addresses), and
assume all subroutines are reachable with the [C`]bl[C'] instruction.
Assume objects may be anywhere in the 32-bit address space (the compiler
will generate [C`]seth/add3[C'] instructions to load their addresses), and
assume subroutines may not be reachable with the [C`]bl[C'] instruction
(the compiler will generate the much slower [C`]seth/add3/jl[C']
instruction sequence).
Disable use of the small data area. Variables will be put into
one of .data, bss, or .rodata (unless the
[C`]section[C'] attribute has been specified).
This is the default.
The small data area consists of sections .sdata and .sbss.
Objects may be explicitly put in the small data area with the
[C`]section[C'] attribute using one of these sections.
Put small global and static data in the small data area, but do not
generate special code to reference them.
Put small global and static data in the small data area, and generate
special instructions to reference them.
Put global and static objects less than or equal to num bytes
into the small data or bss sections instead of the normal data or bss
sections. The default value of num is 8.
The -msdata option must be set to one of sdata or use
for this option to have any effect.
All modules should be compiled with the same -G num value.
Compiling with different values of num may or may not work; if it
doesn't the linker will give an error message---incorrect code will not be
generated.
M88K Options
These -m options are defined for Motorola 88k architectures:
Generate code that works well on both the m88100 and the
m88110.
Generate code that works best for the m88100, but that also
runs on the m88110.
Generate code that works best for the m88110, and may not run
on the m88100.
Obsolete option to be removed from the next revision.
Use -fPIC.
Include an [C`]ident[C'] directive in the assembler output recording the
source file name, compiler name and version, timestamp, and compilation
flags used.
In assembler output, emit symbol names without adding an underscore
character at the beginning of each name. The default is to use an
underscore as prefix on each name.
Include (or omit) additional debugging information (about registers used
in each stack frame) as specified in the 88open Object Compatibility
Standard, ``OCS''. This extra information allows debugging of code that
has had the frame pointer eliminated. The default for DG/UX, SVr4, and
Delta 88 SVr3.2 is to include this information; other 88k configurations
omit this information by default.
When emitting COFF debugging information for automatic variables and
parameters stored on the stack, use the offset from the canonical frame
address, which is the stack pointer (register 31) on entry to the
function. The DG/UX, SVr4, Delta88 SVr3.2, and BCS configurations use
-mocs-frame-position; other 88k configurations have the default
-mno-ocs-frame-position.
When emitting COFF debugging information for automatic variables and
parameters stored on the stack, use the offset from the frame pointer
register (register 30). When this option is in effect, the frame
pointer is not eliminated when debugging information is selected by the
-g switch.
Save space by reorganizing the stack frame. This option generates code
that does not agree with the 88open specifications, but uses less
memory.
Do not reorganize the stack frame to save space. This is the default.
The generated conforms to the specification, but uses more memory.
Generate smaller data references by making them relative to [C`]r0[C'],
which allows loading a value using a single instruction (rather than the
usual two). You control which data references are affected by
specifying num with this option. For example, if you specify
-mshort-data-512, then the data references affected are those
involving displacements of less than 512 bytes.
-mshort-data-num is not effective for num greater
than 64k.
Do, or don't, generate code to guarantee sequential consistency
of volatile memory references. By default, consistency is
guaranteed.
The order of memory references made by the MC88110 processor does
not always match the order of the instructions requesting those
references. In particular, a load instruction may execute before
a preceding store instruction. Such reordering violates
sequential consistency of volatile memory references, when there
are multiple processors. When consistency must be guaranteed,
GCC generates special instructions, as needed, to force
execution in the proper order.
The MC88100 processor does not reorder memory references and so
always provides sequential consistency. However, by default, GCC
generates the special instructions to guarantee consistency
even when you use -m88100, so that the code may be run on an
MC88110 processor. If you intend to run your code only on the
MC88100 processor, you may use -mno-serialize-volatile.
The extra code generated to guarantee consistency may affect the
performance of your application. If you know that you can safely
forgo this guarantee, you may use -mno-serialize-volatile.
Turn on (-msvr4) or off (-msvr3) compiler extensions
related to System V release 4 (SVr4). This controls the following:
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
GCC supports two related instruction set architectures for the
RS/6000 and PowerPC. The POWER instruction set are those
instructions supported by the rios chip set used in the original
RS/6000 systems and the PowerPC instruction set is the
architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and
the IBM 4xx microprocessors.
Neither architecture is a subset of the other. However there is a
large common subset of instructions supported by both. An MQ
register is included in processors supporting the POWER architecture.
You use these options to specify which instructions are available on the
processor you are using. The default value of these options is
determined when configuring GCC. Specifying the
-mcpu=cpu_type overrides the specification of these
options. We recommend you use the -mcpu=cpu_type option
rather than the options listed above.
The -mpower option allows GCC to generate instructions that
are found only in the POWER architecture and to use the MQ register.
Specifying -mpower2 implies -power and also allows GCC
to generate instructions that are present in the POWER2 architecture but
not the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that
are found only in the 32-bit subset of the PowerPC architecture.
Specifying -mpowerpc-gpopt implies -mpowerpc and also allows
GCC to use the optional PowerPC architecture instructions in the
General Purpose group, including floating-point square root. Specifying
-mpowerpc-gfxopt implies -mpowerpc and also allows GCC to
use the optional PowerPC architecture instructions in the Graphics
group, including floating-point select.
The -mpowerpc64 option allows GCC to generate the additional
64-bit instructions that are found in the full PowerPC64 architecture
and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC
will use only the instructions in the common subset of both
architectures plus some special AIX common-mode calls, and will not use
the MQ register. Specifying both -mpower and -mpowerpc
permits GCC to use any instruction from either architecture and to
allow use of the MQ register; specify this for the Motorola MPC601.
Select which mnemonics to use in the generated assembler code. With
-mnew-mnemonics, GCC uses the assembler mnemonics defined for
the PowerPC architecture. With -mold-mnemonics it uses the
assembler mnemonics defined for the POWER architecture. Instructions
defined in only one architecture have only one mnemonic; GCC uses that
mnemonic irrespective of which of these options is specified.
GCC defaults to the mnemonics appropriate for the architecture in
use. Specifying -mcpu=cpu_type sometimes overrides the
value of these option. Unless you are building a cross-compiler, you
should normally not specify either -mnew-mnemonics or
-mold-mnemonics, but should instead accept the default.
Set architecture type, register usage, choice of mnemonics, and
instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are rios, rios1,
rsc, rios2, rs64a, 601, 602,
603, 603e, 604, 604e, 620,
630, 740, 7400, 7450, 750,
power, power2, powerpc, 403, 505,
801, 821, 823, and 860 and common.
-mcpu=common selects a completely generic processor. Code
generated under this option will run on any POWER or PowerPC processor.
GCC will use only the instructions in the common subset of both
architectures, and will not use the MQ register. GCC assumes a generic
processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and
-mcpu=powerpc64 specify generic POWER, POWER2, pure 32-bit
PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
types, with an appropriate, generic processor model assumed for
scheduling purposes.
The other options specify a specific processor. Code generated under
those options will run best on that processor, and may not run at all on
others.
The -mcpu options automatically enable or disable other
-m options as follows:
IBM RT Options
These -m options are defined for the IBM RT PC: Use an in-line code sequence for integer multiplies. This is the default. Call [C`]lmul$$[C'] for integer multiples. Generate full-size floating point data blocks, including the minimum amount of scratch space recommended by IBM. This is the default. Do not include extra scratch space in floating point data blocks. This results in smaller code, but slower execution, since scratch space must be allocated dynamically. Use a calling sequence incompatible with the IBM calling convention in which floating point arguments are passed in floating point registers. Note that [C`]varargs.h[C'] and [C`]stdarg.h[C'] will not work with floating point operands if this option is specified. Use the normal calling convention for floating point arguments. This is the default. Return structures of more than one word in memory, rather than in a register. This provides compatibility with the MetaWare HighC (hc) compiler. Use the option -fpcc-struct-return for compatibility with the Portable C Compiler (pcc). Return some structures of more than one word in registers, when convenient. This is the default. For compatibility with the IBM-supplied compilers, use the option -fpcc-struct-return or the option -mhc-struct-return.
MIPS Options
These -m options are defined for the MIPS family of computers:
Assume the defaults for the machine type cpu-type when generating
instructions. The choices for cpu-type are r2000, r3000,
r3900, r4000, r4100, r4300, r4400,
r4600, r4650, r5000, r6000, r8000,
and orion. Additionally, the r2000, r3000,
r4000, r5000, and r6000 can be abbreviated as
r2k (or r2K), r3k, etc.
Assume the defaults for the machine type cpu-type when scheduling
instructions. The choices for cpu-type are r2000, r3000,
r3900, r4000, r4100, r4300, r4400,
r4600, r4650, r5000, r6000, r8000,
and orion. Additionally, the r2000, r3000,
r4000, r5000, and r6000 can be abbreviated as
r2k (or r2K), r3k, etc. While picking a specific
cpu-type will schedule things appropriately for that particular
chip, the compiler will not generate any code that does not meet level 1
of the MIPS ISA (instruction set architecture) without a -mipsX
or -mabi switch being used.
This is identical to specifying both -march and -mtune.
Issue instructions from level 1 of the MIPS ISA. This is the default.
r3000 is the default cpu-type at this ISA level.
Issue instructions from level 2 of the MIPS ISA (branch likely, square
root instructions). r6000 is the default cpu-type at this
ISA level.
Issue instructions from level 3 of the MIPS ISA (64-bit instructions).
r4000 is the default cpu-type at this ISA level.
Issue instructions from level 4 of the MIPS ISA (conditional move,
prefetch, enhanced FPU instructions). r8000 is the default
cpu-type at this ISA level.
Assume that 32 32-bit floating point registers are available. This is
the default.
Assume that 32 64-bit floating point registers are available. This is
the default when the -mips3 option is used.
Generate code that uses (does not use) the floating point multiply and
accumulate instructions, when they are available. These instructions
are generated by default if they are available, but this may be
undesirable if the extra precision causes problems or on certain chips
in the mode where denormals are rounded to zero where denormals
generated by multiply and accumulate instructions cause exceptions
anyway.
Assume that 32 32-bit general purpose registers are available. This is
the default.
Assume that 32 64-bit general purpose registers are available. This is
the default when the -mips3 option is used.
Force int and long types to be 64 bits wide. See -mlong32 for an
explanation of the default, and the width of pointers.
Force long types to be 64 bits wide. See -mlong32 for an
explanation of the default, and the width of pointers.
Force long, int, and pointer types to be 32 bits wide.
If none of -mlong32, -mlong64, or -mint64 are set,
the size of ints, longs, and pointers depends on the ABI and ISA chosen.
For -mabi=32, and -mabi=n32, ints and longs are 32 bits
wide. For -mabi=64, ints are 32 bits, and longs are 64 bits wide.
For -mabi=eabi and either -mips1 or -mips2, ints
and longs are 32 bits wide. For -mabi=eabi and higher ISAs, ints
are 32 bits, and longs are 64 bits wide. The width of pointer types is
the smaller of the width of longs or the width of general purpose
registers (which in turn depends on the ISA).
Generate code for the indicated ABI. The default instruction level is
-mips1 for 32, -mips3 for n32, and
-mips4 otherwise. Conversely, with -mips1 or
-mips2, the default ABI is 32; otherwise, the default ABI
is 64.
Generate code for the MIPS assembler, and invoke mips-tfile to
add normal debug information. This is the default for all
platforms except for the OSF/1 reference platform, using the OSF/rose
object format. If the either of the -gstabs or -gstabs+
switches are used, the mips-tfile program will encapsulate the
stabs within MIPS ECOFF.
Generate code for the GNU assembler. This is the default on the OSF/1
reference platform, using the OSF/rose object format. Also, this is
the default if the configure option [--]with-gnu-as is used.
Generate code to load the high and low parts of address constants separately.
This allows GCC to optimize away redundant loads of the high order
bits of addresses. This optimization requires GNU as and GNU ld.
This optimization is enabled by default for some embedded targets where
GNU as and GNU ld are standard.
The -mrnames switch says to output code using the MIPS software
names for the registers, instead of the hardware names (ie, a0
instead of $4). The only known assembler that supports this option
is the Algorithmics assembler.
The -mgpopt switch says to write all of the data declarations
before the instructions in the text section, this allows the MIPS
assembler to generate one word memory references instead of using two
words for short global or static data items. This is on by default if
optimization is selected.
For each non-inline function processed, the -mstats switch
causes the compiler to emit one line to the standard error file to
print statistics about the program (number of registers saved, stack
size, etc.).
The -mmemcpy switch makes all block moves call the appropriate
string function (memcpy or bcopy) instead of possibly
generating inline code.
The -mno-mips-tfile switch causes the compiler not
postprocess the object file with the mips-tfile program,
after the MIPS assembler has generated it to add debug support. If
mips-tfile is not run, then no local variables will be
available to the debugger. In addition, stage2 and
stage3 objects will have the temporary file names passed to the
assembler embedded in the object file, which means the objects will
not compare the same. The -mno-mips-tfile switch should only
be used when there are bugs in the mips-tfile program that
prevents compilation.
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC.
Normally the facilities of the machine's usual C compiler are used, but
this can't be done directly in cross-compilation. You must make your
own arrangements to provide suitable library functions for
cross-compilation.
Generate output containing floating point instructions. This is the
default if you use the unmodified sources.
Emit (or do not emit) the pseudo operations .abicalls,
.cpload, and .cprestore that some System V.4 ports use for
position independent code.
Do all calls with the JALR instruction, which requires
loading up a function's address into a register before the call.
You need to use this switch, if you call outside of the current
512 megabyte segment to functions that are not through pointers.
Put pointers to extern references into the data section and load them
up, rather than put the references in the text section.
Generate PIC code suitable for some embedded systems. All calls are
made using PC relative address, and all data is addressed using the $gp
register. No more than 65536 bytes of global data may be used. This
requires GNU as and GNU ld which do most of the work. This currently
only works on targets which use ECOFF; it does not work with ELF.
Allocate variables to the read-only data section first if possible, then
next in the small data section if possible, otherwise in data. This gives
slightly slower code than the default, but reduces the amount of RAM required
when executing, and thus may be preferred for some embedded systems.
When used together with -membedded-data, it will always store uninitialized
const variables in the read-only data section.
The -msingle-float switch tells gcc to assume that the floating
point coprocessor only supports single precision operations, as on the
r4650 chip. The -mdouble-float switch permits gcc to use
double precision operations. This is the default.
Permit use of the mad, madu and mul instructions,
as on the r4650 chip.
Turns on -msingle-float, -mmad, and, at least for now,
-mcpu=r4650.
Enable 16-bit instructions.
Use the entry and exit pseudo ops. This option can only be used with
-mips16.
Compile code for the processor in little endian mode.
The requisite libraries are assumed to exist.
Compile code for the processor in big endian mode.
The requisite libraries are assumed to exist.
Put global and static items less than or equal to num bytes into
the small data or bss sections instead of the normal data or bss
section. This allows the assembler to emit one word memory reference
instructions based on the global pointer (gp or $28),
instead of the normal two words used. By default, num is 8 when
the MIPS assembler is used, and 0 when the GNU assembler is used. The
-G num switch is also passed to the assembler and linker.
All modules should be compiled with the same -G num
value.
Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a .s suffix) when assembling them.
Pass an option to gas which will cause nops to be inserted if
the read of the destination register of an mfhi or mflo instruction
occurs in the following two instructions.
Do not include the default crt0.
Specifies the function to call to flush the I and D caches, or to not
call any such function. If called, the function must take the same
arguments as the common [C`]_flush_func()[C'], that is, the address of the
memory range for which the cache is being flushed, the size of the
memory range, and the number 3 (to flush both caches). The default
depends on the target gcc was configured for, but commonly is either
_flush_func or __cpu_flush.
These options are defined by the macro [C`]TARGET_SWITCHES[C'] in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of
computers:
Tune to cpu-type everything applicable about the generated code, except
for the ABI and the set of available instructions. The choices for
cpu-type are i386, i486, i586, i686,
pentium, pentium-mmx, pentiumpro, pentium2,
pentium3, pentium4, k6, k6-2, k6-3,
athlon, athlon-tbird, athlon-4, athlon-xp
and athlon-mp.
While picking a specific cpu-type will schedule things appropriately
for that particular chip, the compiler will not generate any code that
does not run on the i386 without the -march=cpu-type option
being used. i586 is equivalent to pentium and i686
is equivalent to pentiumpro. k6 and athlon are the
AMD chips as opposed to the Intel ones.
Generate instructions for the machine type cpu-type. The choices
for cpu-type are the same as for -mcpu. Moreover,
specifying -march=cpu-type implies -mcpu=cpu-type.
These options are synonyms for -mcpu=i386, -mcpu=i486,
-mcpu=pentium, and -mcpu=pentiumpro respectively.
These synonyms are deprecated.
generate floating point arithmetics for selected unit unit. the choices
for unit are:
These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments. Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any i386 system. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture. Do not use a so called red zone for x86-64 code. The red zone is mandated by the x86-64 ABI, it is a 128-byte area beyond the location of the stack pointer that will not be modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone. Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model. Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code. Generate code for the medium model: The program is linked in the lower 2 GB of the address space but symbols can be located anywhere in the address space. Programs can be statically or dynamically linked, but building of shared libraries are not supported with the medium model. Generate code for the large model: This model makes no assumptions about addresses and sizes of sections. Currently GCC does not implement this model.
HPPA Options
These -m options are defined for the HPPA family of computers:
Generate code for the specified architecture. The choices for
architecture-type are 1.0 for PA 1.0, 1.1 for PA
1.1, and 2.0 for PA 2.0 processors. Refer to
/usr/lib/sched.models on an HP-UX system to determine the proper
architecture option for your machine. Code compiled for lower numbered
architectures will run on higher numbered architectures, but not the
other way around.
PA 2.0 support currently requires gas snapshot 19990413 or later. The
next release of binutils (current is 2.9.1) will probably contain PA 2.0
support.
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
Generate code suitable for big switch tables. Use this option only if
the assembler/linker complain about out of range branches within a switch
table.
Fill delay slots of function calls with unconditional jump instructions
by modifying the return pointer for the function call to be the target
of the conditional jump.
Prevent floating point registers from being used in any manner. This is
necessary for compiling kernels which perform lazy context switching of
floating point registers. If you use this option and attempt to perform
floating point operations, the compiler will abort.
Prevent the compiler from using indexing address modes. This avoids some
rather obscure problems when compiling MIG generated code under MACH.
Generate code that assumes the target has no space registers. This allows
GCC to generate faster indirect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
Generate code that assumes calls never cross space boundaries. This
allows GCC to emit code which performs faster indirect calls.
This option will not work in the presence of shared libraries or nested
functions.
Generate 3-instruction load and store sequences as sometimes required by
the HP-UX 10 linker. This is equivalent to the +k option to
the HP compilers.
Use the portable calling conventions proposed by HP for ELF systems.
Enable the use of assembler directives only GAS understands.
Schedule code according to the constraints for the machine type
cpu-type. The choices for cpu-type are 700
7100, 7100LC, 7200, and 8000. Refer to
/usr/lib/sched.models on an HP-UX system to determine the
proper scheduling option for your machine.
Enable the optimization pass in the HPUX linker. Note this makes symbolic
debugging impossible. It also triggers a bug in the HPUX 8 and HPUX 9 linkers
in which they give bogus error messages when linking some programs.
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all HPPA
targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions for
cross-compilation. The embedded target hppa1.1-*-pro
does provide software floating point support.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for
this to work.
Intel 960 Options
These -m options are defined for the Intel 960 implementations: Assume the defaults for the machine type cpu-type for some of the other options, including instruction scheduling, floating point support, and addressing modes. The choices for cpu-type are ka, kb, mc, ca, cf, sa, and sb. The default is kb. The -mnumerics option indicates that the processor does support floating-point instructions. The -msoft-float option indicates that floating-point support should not be assumed. Do (or do not) attempt to alter leaf procedures to be callable with the [C`]bal[C'] instruction as well as [C`]call[C']. This will result in more efficient code for explicit calls when the [C`]bal[C'] instruction can be substituted by the assembler or linker, but less efficient code in other cases, such as calls via function pointers, or using a linker that doesn't support this optimization. Do (or do not) make additional attempts (beyond those of the machine-independent portions of the compiler) to optimize tail-recursive calls into branches. You may not want to do this because the detection of cases where this is not valid is not totally complete. The default is -mno-tail-call. Assume (or do not assume) that the use of a complex addressing mode is a win on this implementation of the i960. Complex addressing modes may not be worthwhile on the K-series, but they definitely are on the C-series. The default is currently -mcomplex-addr for all processors except the CB and CC. Align code to 8-byte boundaries for faster fetching (or don't bother). Currently turned on by default for C-series implementations only. Enable compatibility with iC960 v2.0 or v3.0. Enable compatibility with the iC960 assembler. Do not permit (do permit) unaligned accesses. Enable structure-alignment compatibility with Intel's gcc release version 1.3 (based on gcc 1.37). This option implies -mstrict-align. Implement type long double as 64-bit floating point numbers. Without the option long double is implemented by 80-bit floating point numbers. The only reason we have it because there is no 128-bit long double support in fp-bit.c yet. So it is only useful for people using soft-float targets. Otherwise, we should recommend against use of it.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in libgcc.a will be used to perform floating-point
operations. Unless they are replaced by routines that emulate the
floating-point operations, or compiled in such a way as to call such
emulations routines, these routines will issue floating-point
operations. If you are compiling for an Alpha without floating-point
operations, you must ensure that the library is built so as not to call
them.
Note that Alpha implementations without floating-point operations are
required to have floating-point registers.
Generate code that uses (does not use) the floating-point register set.
-mno-fp-regs implies -msoft-float. If the floating-point
register set is not used, floating point operands are passed in integer
registers as if they were integers and floating-point results are passed
in [C`]$0[C'] instead of [C`]$f0[C']. This is a non-standard calling sequence,
so any function with a floating-point argument or return value called by code
compiled with -mno-fp-regs must also be compiled with that
option.
A typical use of this option is building a kernel that does not use,
and hence need not save and restore, any floating-point registers.
The Alpha architecture implements floating-point hardware optimized for
maximum performance. It is mostly compliant with the IEEE floating
point standard. However, for full compliance, software assistance is
required. This option generates code fully IEEE compliant code
except that the inexact-flag is not maintained (see below).
If this option is turned on, the preprocessor macro [C`]_IEEE_FP[C'] is
defined during compilation. The resulting code is less efficient but is
able to correctly support denormalized numbers and exceptional IEEE
values such as not-a-number and plus/minus infinity. Other Alpha
compilers call this option -ieee_with_no_inexact.
This is like -mieee except the generated code also maintains
the IEEE inexact-flag. Turning on this option causes the
generated code to implement fully-compliant IEEE math. In addition to
[C`]_IEEE_FP[C'], [C`]_IEEE_FP_EXACT[C'] is defined as a preprocessor
macro. On some Alpha implementations the resulting code may execute
significantly slower than the code generated by default. Since there is
very little code that depends on the inexact-flag, you should
normally not specify this option. Other Alpha compilers call this
option -ieee_with_inexact.
This option controls what floating-point related traps are enabled.
Other Alpha compilers call this option -fptm trap-mode.
The trap mode can be set to one of four values:
DEC Alpha/VMS Options
These -m options are defined for the DEC Alpha/VMS implementations: Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error) codes.
Clipper Options
These -m options are defined for the Clipper implementations: Produce code for a C300 Clipper processor. This is the default. Produce code for a C400 Clipper processor, i.e. use floating point registers f8[--]f15.
H8/300 Options
These -m options are defined for the H8/300 implementations: Shorten some address references at link time, when possible; uses the linker option -relax. Generate code for the H8/300H. Generate code for the H8/S. Generate code for the H8/S2600. This switch must be used with -ms. Make [C`]int[C'] data 32 bits by default. On the H8/300H and H8/S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8/S is to align longs and floats on 4 byte boundaries. -malign-300 causes them to be aligned on 2 byte boundaries. This option has no effect on the H8/300.
SH Options
These -m options are defined for the SH implementations: Generate code for the SH1. Generate code for the SH2. Generate code for the SH3. Generate code for the SH3e. Generate code for the SH4 without a floating-point unit. Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic. Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default. Generate code for the SH4. Compile code for the processor in big endian mode. Compile code for the processor in little endian mode. Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library will not work unless you recompile it first with -mdalign. Shorten some address references at link time, when possible; uses the linker option -relax. Use 32-bit offsets in [C`]switch[C'] tables. The default is to use 16-bit offsets. Enable the use of the instruction [C`]fmovd[C']. Comply with the calling conventions defined by Hitachi. Mark the [C`]MAC[C'] register as call-clobbered, even if -mhitachi is given. Increase IEEE-compliance of floating-point code. Dump instruction size and location in the assembly code. This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI. Optimize for space instead of speed. Implied by -Os. When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table. Generate a library function call to invalidate instruction cache entries, after fixing up a trampoline. This library function call doesn't assume it can write to the whole memory address space. This is the default when the target is [C`]sh-*-linux*[C'].
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those systems: Create a shared object. It is recommended that -symbolic or -shared be used instead. Identify the versions of each tool used by the compiler, in a [C`].ident[C'] assembler directive in the output. Refrain from adding [C`].ident[C'] directives to the output file (this is the default). Search the directories dirs, and no others, for libraries specified with -l. Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
TMS320C3x/C4x Options
These -m options are defined for TMS320C3x/C4x implementations: Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are c30, c31, c32, c40, and c44. The default is c40 to generate code for the TMS320C40. Generates code for the big or small memory model. The small memory model assumed that all data fits into one 64K word page. At run-time the data page (DP) register must be set to point to the 64K page containing the .bss and .data program sections. The big memory model is the default and requires reloading of the DP register for every direct memory access. Allow (disallow) allocation of general integer operands into the block count register BK. Enable (disable) generation of code using decrement and branch, DBcond(D), instructions. This is enabled by default for the C4x. To be on the safe side, this is disabled for the C3x, since the maximum iteration count on the C3x is 2^{23 + 1} (but who iterates loops more than 2^{23} times on the C3x?). Note that GCC will try to reverse a loop so that it can utilise the decrement and branch instruction, but will give up if there is more than one memory reference in the loop. Thus a loop where the loop counter is decremented can generate slightly more efficient code, in cases where the RPTB instruction cannot be utilised. Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the data section, and restored on exit from the ISR. This should not be required unless someone has violated the small memory model by modifying the DP register, say within an object library. For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee 32-bit results. Note that if one of the operands is a constant, then the multiplication will be performed using shifts and adds. If the -mmpyi option is not specified for the C3x, then squaring operations are performed inline instead of a library call. The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest integer less than or equal to the floating point value rather than to the nearest integer. Thus if the floating point number is negative, the result will be incorrectly truncated an additional code is necessary to detect and correct this case. This option can be used to disable generation of the additional code required to correct the result. Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead looping. The RPTB construct is only used for innermost loops that do not call functions or jump across the loop boundaries. There is no advantage having nested RPTB loops due to the overhead required to save and restore the RC, RS, and RE registers. This is enabled by default with -O2. Enable (disable) the use of the single instruction repeat instruction RPTS. If a repeat block contains a single instruction, and the loop count can be guaranteed to be less than the value count, GCC will emit a RPTS instruction instead of a RPTB. If no value is specified, then a RPTS will be emitted even if the loop count cannot be determined at compile time. Note that the repeated instruction following RPTS does not have to be reloaded from memory each iteration, thus freeing up the CPU buses for operands. However, since interrupts are blocked by this instruction, it is disabled by default. The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^{31 + 1} since these instructions test if the iteration count is negative to terminate the loop. If the iteration count is unsigned there is a possibility than the 2^{31 + 1} maximum iteration count may be exceeded. This switch allows an unsigned iteration count. Try to emit an assembler syntax that the TI assembler (asm30) is happy with. This also enforces compatibility with the API employed by the TI C3x C compiler. For example, long doubles are passed as structures rather than in floating point registers. Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are passed in registers where possible rather than by pushing arguments on to the stack. Allow the generation of parallel instructions. This is enabled by default with -O2. Allow the generation of MPY||ADD and MPY||SUB parallel instructions, provided -mparallel-insns is also specified. These instructions have tight register constraints which can pessimize the code generation of large functions.
V850 Options
These -m options are defined for V850 implementations: Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will always load the functions address up into a register, and call indirect through the pointer. Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the [C`]ep[C'] register, and use the shorter [C`]sld[C'] and [C`]sst[C'] instructions. The -mep option is on by default if you optimize. Do not use (do use) external functions to save and restore registers at the prolog and epilog of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The -mprolog-function option is on by default if you optimize. Try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options. Put static or global variables whose size is n bytes or less into the tiny data area that register [C`]ep[C'] points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references). Put static or global variables whose size is n bytes or less into the small data area that register [C`]gp[C'] points to. The small data area can hold up to 64 kilobytes. Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory. Specify that the target processor is the V850. Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.
ARC Options
These options are defined for ARC implementations: Compile code for little endian mode. This is the default. Compile code for big endian mode. Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are many ARC variants with different instruction and register set characteristics. This flag prevents code compiled for one cpu to be linked with code compiled for another. No facility exists for handling variants that are ``almost identical''. This is an all or nothing option. Compile code for ARC variant cpu. Which variants are supported depend on the configuration. All variants support -mcpu=base, this is the default. Put functions, data, and readonly data in text-section, data-section, and readonly-data-section respectively by default. This can be overridden with the [C`]section[C'] attribute.
NS32K Options
These are the -m options defined for the 32000 series. The default
values for these options depends on which style of 32000 was selected when
the compiler was configured; the defaults for the most common choices are
given below.
Generate output for a 32032. This is the default
when the compiler is configured for 32032 and 32016 based systems.
Generate output for a 32332. This is the default
when the compiler is configured for 32332-based systems.
Generate output for a 32532. This is the default
when the compiler is configured for 32532-based systems.
Generate output containing 32081 instructions for floating point.
This is the default for all systems.
Generate output containing 32381 instructions for floating point. This
also implies -m32081. The 32381 is only compatible with the 32332
and 32532 cpus. This is the default for the pc532-netbsd configuration.
Try and generate multiply-add floating point instructions [C`]polyF[C']
and [C`]dotF[C']. This option is only available if the -m32381
option is in effect. Using these instructions requires changes to
register allocation which generally has a negative impact on
performance. This option should only be enabled when compiling code
particularly likely to make heavy use of multiply-add instructions.
Do not try and generate multiply-add floating point instructions
[C`]polyF[C'] and [C`]dotF[C']. This is the default on all platforms.
Generate output containing library calls for floating point.
Warning: the requisite libraries may not be available.
Do not use the bit-field instructions. On some machines it is faster to
use shifting and masking operations. This is the default for the pc532.
Do use the bit-field instructions. This is the default for all platforms
except the pc532.
Use a different function-calling convention, in which functions
that take a fixed number of arguments return pop their
arguments on return with the [C`]ret[C'] instruction.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including [C`]printf[C']);
otherwise incorrect code will be generated for calls to those
functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
This option takes its name from the 680x0 [C`]rtd[C'] instruction.
Use a different function-calling convention where the first two arguments
are passed in registers.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Do not pass any arguments in registers. This is the default for all
targets.
It is OK to use the sb as an index register which is always loaded with
zero. This is the default for the pc532-netbsd target.
The sb register is not available for use or has not been initialized to
zero by the run time system. This is the default for all targets except
the pc532-netbsd. It is also implied whenever -mhimem or
-fpic is set.
Many ns32000 series addressing modes use displacements of up to 512MB.
If an address is above 512MB then displacements from zero can not be used.
This option causes code to be generated which can be loaded above 512MB.
This may be useful for operating systems or ROM code.
Assume code will be loaded in the first 512MB of virtual address space.
This is the default for all platforms.
AVR Options
These options are defined for AVR implementations:
Specify ATMEL AVR instruction set or MCU type.
Instruction set avr1 is for the minimal AVR core, not supported by the C
compiler, only for assembler programs (MCU types: at90s1200, attiny10,
attiny11, attiny12, attiny15, attiny28).
Instruction set avr2 (default) is for the classic AVR core with up to
8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
at90c8534, at90s8535).
Instruction set avr3 is for the classic AVR core with up to 128K program
memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
Instruction set avr4 is for the enhanced AVR core with up to 8K program
memory space (MCU types: atmega8, atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with up to 128K program
memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
atmega64, atmega128, at43usb355, at94k).
Output instruction sizes to the asm file.
Specify the initial stack address, which may be a symbol or numeric value,
__stack is the default.
Generated code is not compatible with hardware interrupts.
Code size will be smaller.
Functions prologues/epilogues expanded as call to appropriate
subroutines. Code size will be smaller.
Do not generate tablejump insns which sometimes increase code size.
Change only the low 8 bits of the stack pointer.
MCore Options
These are the -m options defined for the Motorola M*Core processors. Inline constants into the code stream if it can be done in two instructions or less. Use the divide instruction. (Enabled by default). Allow arbitrary sized immediates in bit operations. Always treat bit-fields as int-sized. Force all functions to be aligned to a four byte boundary. Emit callgraph information. Prefer word access when reading byte quantities. Generate code for a little endian target. Generate code for the 210 processor.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture. Generate code for a big endian target. This is the default for HPUX. Generate code for a little endian target. This is the default for AIX5 and Linux. Generate (or don't) code for the GNU assembler. This is the default. Generate (or don't) code for the GNU linker. This is the default. Generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI. Generate (or don't) a stop bit immediately before and after volatile asm statements. Generate code that works around Itanium B step errata. Generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable. Disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs. Generate code that uses a single constant global pointer value. This is useful when compiling kernel code. Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code. Generate code for inline divides using the minimum latency algorithm. Generate code for inline divides using the maximum throughput algorithm. Don't (or do) generate assembler code for the DWARF2 line number debugging info. This may be useful when not using the GNU assembler. Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator can not use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
D30V Options
These -m options are defined for D30V implementations: Link the .text, .data, .bss, .strings, .rodata, .rodata1, .data1 sections into external memory, which starts at location [C`]0x80000000[C']. Same as the -mextmem switch. Link the .text section into onchip text memory, which starts at location [C`]0x0[C']. Also link .data, .bss, .strings, .rodata, .rodata1, .data1 sections into onchip data memory, which starts at location [C`]0x20000000[C']. Disable (enable) passing -O to the assembler when optimizing. The assembler uses the -O option to automatically parallelize adjacent short instructions where possible. Increase the internal costs of branches to n. Higher costs means that the compiler will issue more instructions to avoid doing a branch. The default is 2. Specify the maximum number of conditionally executed instructions that replace a branch. The default is 4.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture. Use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default. Generate (or do not generate) code which maintains an explicit backchain within the stack frame that points to the caller's frame. This is currently needed to allow debugging. The default is to generate the backchain. Generate (or do not generate) code using the [C`]bras[C'] instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use the [C`]basr[C'] instruction instead, which does not have this limitation. When -m31 is specified, generate code compliant to the Linux for S/390 ABI. When -m64 is specified, generate code compliant to the Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64. Generate (or do not generate) code using the [C`]mvcle[C'] instruction to perform block moves. When -mno-mvcle is specifed, use a [C`]mvc[C'] loop instead. This is the default. Print (or do not print) additional debug information when compiling. The default is to not print debug information.
CRIS Options
These options are defined specifically for the CRIS ports. Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10. Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type. Warn when the stack frame of a function exceeds n bytes. Only available with the cris-axis-aout target. Arranges for indications in the program to the kernel loader that the stack of the program should be set to n bytes. The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively. Enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect to turn off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file. Do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes. Do not emit instructions with side-effects in addressing modes other than post-increment. These options (no-options) arranges (eliminate arrangements) for the stack-frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options. Similar to the stack- data- and const-align options above, these options arrange for stack-frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment. With -mno-prologue-epilogue, the normal function prologue and epilogue that sets up the stack-frame are omitted and no return instructions or return sequences are generated in the code. Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variable needs to be allocated. With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The default is -mgotplt. Legacy no-op option only recognized with the cris-axis-aout target. Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets. Only recognized with the cris-axis-aout target, where it selects a GNU/linux-like multilib, include files and instruction set for -march=v8. Legacy no-op option only recognized with the cris-axis-linux-gnu target. This option, recognized for the cris-axis-aout and cris-axis-elf arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively. Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.
MMIX Options
These options are defined for the MMIX: Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size. Generate floating-point comparison instructions that compare with respect to the [C`]rE[C'] epsilon register. Generate code that passes function parameters and return values that (in the called function) are seen as registers [C`]$0[C'] and up, as opposed to the GNU ABI which uses global registers [C`]$231[C'] and up. When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones. Make the result of a division yielding a remainder have the same sign as the divisor. With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used. Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the [C`]PREFIX[C'] assembly directive. Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator. Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch. Generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. The generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require -mno-base-addresses.
PDP-11 Options
These options are defined for the PDP-11: Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Do not use hardware floating point. Return floating-point results in ac0 (fr0 in Unix assembler syntax). Return floating-point results in memory. This is the default. Generate code for a PDP-11/40. Generate code for a PDP-11/45. This is the default. Generate code for a PDP-11/10. Use inline [C`]movstrhi[C'] patterns for copying memory. This is the default. Do not use inline [C`]movstrhi[C'] patterns for copying memory. Use 16-bit [C`]int[C']. This is the default. Use 32-bit [C`]int[C']. Use 64-bit [C`]float[C']. This is the default. Use 32-bit [C`]float[C']. Use [C`]abshi2[C'] pattern. This is the default. Do not use [C`]abshi2[C'] pattern. Pretend that branches are expensive. This is for experimenting with code generation only. Do not pretend that branches are expensive. This is the default. Generate code for a system with split I&D. Generate code for a system without split I&D. This is the default. Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd. Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than pdp11-*-bsd.
Xstormy16 Options
These options are defined for Xstormy16: Choose startup files and linker script suitable for the simulator.
Xtensa Options
The Xtensa architecture is designed to support many different configurations. The compiler's default options can be set to match a particular Xtensa configuration by copying a configuration file into the GCC sources when building GCC. The options below may be used to override the default options. Specify big-endian or little-endian byte ordering for the target Xtensa processor. Enable or disable use of the optional Xtensa code density instructions. Enable or disable use of the Xtensa MAC16 option. When enabled, GCC will generate MAC16 instructions from standard C code, with the limitation that it will use neither the MR register file nor any instruction that operates on the MR registers. When this option is disabled, GCC will translate 16-bit multiply/accumulate operations to a combination of core instructions and library calls, depending on whether any other multiplier options are enabled. Enable or disable use of the 16-bit integer multiplier option. When enabled, the compiler will generate 16-bit multiply instructions for multiplications of 16 bits or smaller in standard C code. When this option is disabled, the compiler will either use 32-bit multiply or MAC16 instructions if they are available or generate library calls to perform the multiply operations using shifts and adds. Enable or disable use of the 32-bit integer multiplier option. When enabled, the compiler will generate 32-bit multiply instructions for multiplications of 32 bits or smaller in standard C code. When this option is disabled, the compiler will generate library calls to perform the multiply operations using either shifts and adds or 16-bit multiply instructions if they are available. Enable or disable use of the optional normalization shift amount ([C`]NSA[C']) instructions to implement the built-in [C`]ffs[C'] function. Enable or disable use of the optional minimum and maximum value instructions. Enable or disable use of the optional sign extend ([C`]SEXT[C']) instruction. Enable or disable support for the boolean register file used by Xtensa coprocessors. This is not typically useful by itself but may be required for other options that make use of the boolean registers (e.g., the floating-point option). Enable or disable use of the floating-point option. When enabled, GCC generates floating-point instructions for 32-bit [C`]float[C'] operations. When this option is disabled, GCC generates library calls to emulate 32-bit floating-point operations using integer instructions. Regardless of this option, 64-bit [C`]double[C'] operations are always emulated with calls to library functions. Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations. When this option is enabled, GCC inserts [C`]MEMW[C'] instructions before [C`]volatile[C'] memory references to guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the [C`]MEMW[C'] instructions. Control the treatment of literal pools. The default is -mno-text-section-literals, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With -mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files. When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening will be performed. The default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions like [C`]LOOP[C'], which the assembler will always align, either by widening density instructions or by inserting no-op instructions. When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a direct [C`]CALL[C'] instruction into an [C`]L32R[C'] followed by a [C`]CALLX[C'] instruction. The default is -mno-longcalls. This option should be used in programs where the call target can potentially be out of range. This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC will still show direct call instructions---look at the disassembled object code to see the actual instructions. Note that the assembler will use an indirect call for every cross-file call, not just those that really will be out of range. These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form
of -ffoo would be -fno-foo. In the table below, only
one of the forms is listed---the one which is not the default. You
can figure out the other form by either removing no- or adding
it.
Enable exception handling. Generates extra code needed to propagate
exceptions. For some targets, this implies GCC will generate frame
unwind information for all functions, which can produce significant data
size overhead, although it does not affect execution. If you do not
specify this option, GCC will enable it by default for languages like
[C+] which normally require exception handling, and disable it for
languages like C that do not normally require it. However, you may need
to enable this option when compiling C code that needs to interoperate
properly with exception handlers written in [C+]. You may also wish to
disable this option if you are compiling older [C+] programs that don't
use exception handling.
Generate code that allows trapping instructions to throw exceptions.
Note that this requires platform-specific runtime support that does
not exist everywhere. Moreover, it only allows trapping
instructions to throw exceptions, i.e. memory references or floating
point instructions. It does not allow exceptions to be thrown from
arbitrary signal handlers such as [C`]SIGALRM[C'].
Similar to -fexceptions, except that it will just generate any needed
static data, but will not affect the generated code in any other way.
You will normally not enable this option; instead, a language processor
that needs this handling would enable it on your behalf.
Generate unwind table in dwarf2 format, if supported by target machine. The
table is exact at each instruction boundary, so it can be used for stack
unwinding from asynchronous events (such as debugger or garbage collector).
Return ``short'' [C`]struct[C'] and [C`]union[C'] values in memory like
longer ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing intercallability between
GCC-compiled files and files compiled with other compilers, particularly
the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends
on the target configuration macros.
Short structures and unions are those whose size and alignment match
that of some integer type.
Warning: code compiled with the -fpcc-struct-return
switch is not binary compatible with code compiled with the
-freg-struct-return switch.
Use it to conform to a non-default application binary interface.
Return [C`]struct[C'] and [C`]union[C'] values in registers when possible.
This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor
-freg-struct-return, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC
defaults to -fpcc-struct-return, except on targets where GCC is
the principal compiler. In those cases, we can choose the standard, and
we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return
switch is not binary compatible with code compiled with the
-fpcc-struct-return switch.
Use it to conform to a non-default application binary interface.
Allocate to an [C`]enum[C'] type only as many bytes as it needs for the
declared range of possible values. Specifically, the [C`]enum[C'] type
will be equivalent to the smallest integer type which has enough room.
Warning: the -fshort-enums switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
Use the same size for [C`]double[C'] as for [C`]float[C'].
Warning: the -fshort-double switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
Override the underlying type for wchar_t to be short
unsigned int instead of the default for the target. This option is
useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
Requests that the data and non-[C`]const[C'] variables of this
compilation be shared data rather than private data. The distinction
makes sense only on certain operating systems, where shared data is
shared between processes running the same program, while private data
exists in one copy per process.
In C, allocate even uninitialized global variables in the data section of the
object file, rather than generating them as common blocks. This has the
effect that if the same variable is declared (without [C`]extern[C']) in
two different compilations, you will get an error when you link them.
The only reason this might be useful is if you wish to verify that the
program will work on other systems which always work this way.
Ignore the #ident directive.
Do not output global initializations (such as [C+] constructors and
destructors) in the form used by the GNU linker (on systems where the GNU
linker is the standard method of handling them). Use this option when
you want to use a non-GNU linker, which also requires using the
collect2 program to make sure the system linker includes
constructors and destructors. (collect2 is included in the GCC
distribution.) For systems which must use collect2, the
compiler driver gcc is configured to do this automatically.
Don't output a [C`].size[C'] assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This option is
used when compiling crtstuff.c; you should not need to use it
for anything else.
Put extra commentary information in the generated assembly code to
make it more readable. This option is generally only of use to those
who actually need to read the generated assembly code (perhaps while
debugging the compiler itself).
-fno-verbose-asm, the default, causes the
extra information to be omitted and is useful when comparing two assembler
files.
Consider all memory references through pointers to be volatile.
Consider all memory references to extern and global data items to
be volatile. GCC does not consider static data items to be volatile
because of this switch.
Consider all memory references to static data to be volatile.
Generate position-independent code (PIC) suitable for use in a shared
library, if supported for the target machine. Such code accesses all
constant addresses through a global offset table (GOT). The dynamic
loader resolves the GOT entries when the program starts (the dynamic
loader is not part of GCC; it is part of the operating system). If
the GOT size for the linked executable exceeds a machine-specific
maximum size, you get an error message from the linker indicating that
-fpic does not work; in that case, recompile with -fPIC
instead. (These maximums are 16k on the m88k, 8k on the Sparc, and 32k
on the m68k and RS/6000. The 386 has no such limit.)
Position-independent code requires special support, and therefore works
only on certain machines. For the 386, GCC supports PIC for System V
but not for the Sun 386i. Code generated for the IBM RS/6000 is always
position-independent.
If supported for the target machine, emit position-independent code,
suitable for dynamic linking and avoiding any limit on the size of the
global offset table. This option makes a difference on the m68k, m88k,
and the Sparc.
Position-independent code requires special support, and therefore works
only on certain machines.
Treat the register named reg as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer, frame
pointer or in some other fixed role).
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the [C`]REGISTER_NAMES[C']
macro in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for temporaries or
variables that do not live across a call. Functions compiled this way
will not save and restore the register reg.
It is an error to used this flag with the frame pointer or stack pointer.
Use of this flag for other registers that have fixed pervasive roles in
the machine's execution model will produce disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or variables that
live across a call. Functions compiled this way will save and restore
the register reg if they use it.
It is an error to used this flag with the frame pointer or stack pointer.
Use of this flag for other registers that have fixed pervasive roles in
the machine's execution model will produce disastrous results.
A different sort of disaster will result from the use of this flag for
a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
Pack all structure members together without holes.
Warning: the -fpack-struct switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimial.
Use it to conform to a non-default application binary interface.
Generate instrumentation calls for entry and exit to functions. Just
after function entry and just before function exit, the following
profiling functions will be called with the address of the current
function and its call site. (On some platforms,
[C`]__builtin_return_address[C'] does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function,
which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other
functions. The profiling calls will indicate where, conceptually, the
inline function is entered and exited. This means that addressable
versions of such functions must be available. If all your uses of a
function are expanded inline, this may mean an additional expansion of
code size. If you use extern inline in your C code, an
addressable version of such functions must be provided. (This is
normally the case anyways, but if you get lucky and the optimizer always
expands the functions inline, you might have gotten away without
providing static copies.)
A function may be given the attribute [C`]no_instrument_function[C'], in
which case this instrumentation will not be done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).
Generate code to verify that you do not go beyond the boundary of the
stack. You should specify this flag if you are running in an
environment with multiple threads, but only rarely need to specify it in
a single-threaded environment since stack overflow is automatically
detected on nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the
operating system must do that. The switch causes generation of code
to ensure that the operating system sees the stack being extended.
Generate code to ensure that the stack does not grow beyond a certain value,
either the value of a register or the address of a symbol. If the stack
would grow beyond the value, a signal is raised. For most targets,
the signal is raised before the stack overruns the boundary, so
it is possible to catch the signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000
and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit
of 128KB. Note that this may only work with the GNU linker.
Specify the possible relationships among parameters and between
parameters and global data.
-fargument-alias specifies that arguments (parameters) may
alias each other and may alias global storage.-fargument-noalias specifies that arguments do not alias
each other, but may alias global storage.-fargument-noalias-global specifies that arguments do not
alias each other and do not alias global storage.
Each language will automatically use whatever option is required by
the language standard. You should not need to use these options yourself.
This option and its counterpart, -fno-leading-underscore, forcibly
change the way C symbols are represented in the object file. One use
is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to
generate code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary interface.
Not all targets provide complete support for this switch.
Note that you can also specify places to search using options such as
-B, -I and -L. These
take precedence over places specified using environment variables, which
in turn take precedence over those specified by the configuration of GCC.
These environment variables control the way that GCC uses
localization information that allow GCC to work with different
national conventions. GCC inspects the locale categories
LC_CTYPE and LC_MESSAGES if it has been configured to do
so. These locale categories can be set to any value supported by your
installation. A typical value is en_UK for English in the United
Kingdom.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character boundaries in
a string; this is needed for some multibyte encodings that contain quote
and escape characters that would otherwise be interpreted as a string
end or escape.
The LC_MESSAGES environment variable specifies the language to
use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE
and LC_MESSAGES default to the value of the LANG
environment variable. If none of these variables are set, GCC
defaults to traditional C English behavior.
If TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for example,
the output of the preprocessor, which is the input to the compiler
proper.
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is added
when this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out
an appropriate prefix to use based on the pathname it was invoked with.
If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is
prefix/lib/gcc-lib/ where prefix is the value
of [C`]prefix[C'] when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are
used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with /usr/local/lib/gcc-lib
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with -Bfoo/, GCC will search
foo/bar where it would normally search /usr/local/lib/bar.
These alternate directories are searched first; the standard directories
come next.
The value of COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using GCC_EXEC_PREFIX.
The value of LIBRARY_PATH is a colon-separated list of
directories, much like PATH. When configured as a native compiler,
GCC tries the directories thus specified when searching for special
linker files, if it can't find them using GCC_EXEC_PREFIX. Linking
using GCC also uses these directories when searching for ordinary
libraries for the -l option (but directories specified with
-L come first).
This variable is used to pass locale information to the compiler. One way in
which this information is used is to determine the character set to be used
when character literals, string literals and comments are parsed in C and [C+].
When the compiler is configured to allow multibyte characters,
the following values for LANG are recognized:
Some additional environments variables affect the behavior of the
preprocessor.
Each variable's value is a list of directories separated by a special
character, much like PATH, in which to look for header files.
The special character, [C`]PATH_SEPARATOR[C'], is target-dependent and
determined at GCC build time. For Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if
specified with -I, but after any paths given with -I
options on the command line. The environment variable is used
regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the
particular language indicated. Each specifies a list of directories
to be searched as if specified with -isystem, but after any
paths given with -isystem options on the command line.
@anchor{DEPENDENCIES_OUTPUT}
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files processed
by the compiler. System header files are ignored in the dependency
output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
file target, in which case the rules are written to
file file using target as the target name.
In other words, this environment variable is equivalent to combining
the options -MM and -MF,
with an optional -MT switch too.
This variable is the same as the environment variable
DEPENDENCIES_OUTPUT, except that
system header files are not ignored, so it implies -M rather
than -MM. However, the dependence on the main input file is
omitted.
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