Only the most useful options are listed here; see below for the
remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The ``overall options'' allow you to stop this
process at an intermediate stage. For example, the -c option
says not to run the linker. Then the output consists of object files
output by the assembler.
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.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are
in the following sections.
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. The first three
stages apply to an individual source file, and end by producing an
object file; linking combines all the object files (those newly
compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the library
libobjc.a to make an Objective-C program work.
file.mi
Objective-C source code which should not be preprocessed.
file.h
C header file (not to be compiled or linked).
file.cc
file.cp
file.cxx
file.cpp
file.c++
file.C
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.
file.f
file.for
file.FOR
Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fortran source code which must be preprocessed (with the traditional
preprocessor).
file.r
Fortran source code which must be preprocessed with a RATFOR
preprocessor (not included with GCC).
file.s
Assembler code.
file.S
Assembler code which must be preprocessed.
other
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:
-xlanguage
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
f77 f77-cpp-input ratfor
java
-x none
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).
-pass-exit-codes
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.
-c
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.
-S
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.
-E
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.
-ofile
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.
-v
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.
-pipe
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.
--help
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.
--target-help
Print (on the standard output) a description of target specific command
line options for each tool.
Compiling C++ Programs
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.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived
from C, such as C++ and Objective C) that the compiler accepts:
-ansi
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 "asm" and "typeof" keywords, and
predefined macros such as "unix" and "vax" 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 "inline" keyword.
The alternate keywords "__asm__", "__extension__",
"__inline__" and "__typeof__" 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 "__unix__" and "__vax__" 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 "__STRICT_ANSI__" 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 "alloca" and "ffs") are not built-in
functions with -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C. A value for this option must be provided;
possible values are
c89
iso9899:1990
ISO C89 (same as -ansi).
iso9899:199409
ISO C89 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported; see
<http://gcc.gnu.org/c99status.html> for more information. The
names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C89 plus GNU extensions (including some C99 features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
this will become the default. The name gnu9x is deprecated.
Even when this option is not specified, you can still use some of the
features of newer standards in so far as they do not conflict with
previous C standards. For example, you may use "__restrict__" even
when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same
effects as -ansi, except that features that were not in ISO C89
but are in the specified version (for example, // comments and
the "inline" keyword in ISO C99) are not disabled.
-aux-infofilename
Output to the given filename prototyped declarations for all functions
declared and/or defined in a translation unit, including those in header
files. This option is silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of
each declaration (source file and line), whether the declaration was
implicit, prototyped or unprototyped (I, N for new or
O for old, respectively, in the first character after the line
number and the colon), and whether it came from a declaration or a
definition (C or F, respectively, in the following
character). In the case of function definitions, a K&R-style list of
arguments followed by their declarations is also provided, inside
comments, after the declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a
keyword, so that code can use these words as identifiers. You can use
the keywords "__asm__", "__inline__" and "__typeof__"
instead. -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since
"asm" and "inline" are standard keywords. You may want to
use the -fno-gnu-keywords flag instead, which has the same
effect. In C99 mode (-std=c99 or -std=gnu99), this
switch only affects the "asm" and "typeof" keywords, since
"inline" is a standard keyword in ISO C99.
-fno-builtin
Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to "alloca" may become single
instructions that adjust the stack directly, and calls to "memcpy"
may become inline copy loops. The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.
In C++, -fno-builtin is always in effect. The -fbuiltin
option has no effect. Therefore, in C++, the only way to get the
optimization benefits of built-in functions is to call the function
using the __builtin_ prefix. The GNU C++ Standard Library uses
built-in functions to implement many functions (like
"std::strchr"), so that you automatically get efficient code.
-fhosted
Assert that compilation takes place in a hosted environment. This implies
-fbuiltin. A hosted environment is one in which the
entire standard library is available, and in which "main" has a return
type of "int". Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This
implies -fno-builtin. A freestanding environment
is one in which the standard library may not exist, and program startup may
not necessarily be at "main". The most obvious example is an OS kernel.
This is equivalent to -fno-hosted.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std
options for strict ISO C conformance) implies -trigraphs.
-traditional
Attempt to support some aspects of traditional C compilers.
Specifically:
*
All "extern" declarations take effect globally even if they
are written inside of a function definition. This includes implicit
declarations of functions.
*
The newer keywords "typeof", "inline", "signed", "const"
and "volatile" are not recognized. (You can still use the
alternative keywords such as "__typeof__", "__inline__", and
so on.)
*
Comparisons between pointers and integers are always allowed.
*
Integer types "unsigned short" and "unsigned char" promote
to "unsigned int".
*
Out-of-range floating point literals are not an error.
*
Certain constructs which ISO regards as a single invalid preprocessing
number, such as 0xe-0xd, are treated as expressions instead.
*
String ``constants'' are not necessarily constant; they are stored in
writable space, and identical looking constants are allocated
separately. (This is the same as the effect of
-fwritable-strings.)
*
All automatic variables not declared "register" are preserved by
"longjmp". Ordinarily, GNU C follows ISO C: automatic variables
not declared "volatile" may be clobbered.
*
The character escape sequences \x and \a evaluate as the
literal characters x and a respectively. Without
-traditional, \x is a prefix for the hexadecimal
representation of a character, and \a produces a bell.
You may wish to use -fno-builtin as well as -traditional
if your program uses names that are normally GNU C built-in functions for
other purposes of its own.
You cannot use -traditional if you include any header files that
rely on ISO C features. Some vendors are starting to ship systems with
ISO C header files and you cannot use -traditional on such
systems to compile files that include any system headers.
The -traditional option also enables -traditional-cpp,
which is described next.
-traditional-cpp
Attempt to support some aspects of traditional C preprocessors.
Specifically:
*
Comments convert to nothing at all, rather than to a space. This allows
traditional token concatenation.
*
In a preprocessing directive, the # symbol must appear as the first
character of a line.
*
Macro arguments are recognized within string constants in a macro
definition (and their values are stringified, though without additional
quote marks, when they appear in such a context). The preprocessor
always considers a string constant to end at a newline.
*
The predefined macro "__STDC__" is not defined when you use
-traditional, but "__GNUC__" is (since the GNU extensions
which "__GNUC__" indicates are not affected by
-traditional). If you need to write header files that work
differently depending on whether -traditional is in use, by
testing both of these predefined macros you can distinguish four
situations: GNU C, traditional GNU C, other ISO C compilers, and other
old C compilers. The predefined macro "__STDC_VERSION__" is also
not defined when you use -traditional.
*
The preprocessor considers a string constant to end at a newline (unless
the newline is escaped with \). (Without -traditional,
string constants can contain the newline character as typed.)
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and
third arguments. The value of such an expression is void. This option
is not supported for C++.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should
be. It is either like "unsigned char" by default or like
"signed char" by default.
Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object.
But many programs have been written to use plain "char" and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you
make such a program work with the opposite default.
The type "char" is always a distinct type from each of
"signed char" or "unsigned char", even though its behavior
is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is
the negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the
declaration does not use either "signed" or "unsigned". By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as "int" are signed types.
However, when -traditional is used, bit-fields are all unsigned
no matter what.
-fwritable-strings
Store string constants in the writable data segment and don't uniquize
them. This is for compatibility with old programs which assume they can
write into string constants. The option -traditional also has
this effect.
Writing into string constants is a very bad idea; ``constants'' should
be constant.
-fallow-single-precision
Do not promote single precision math operations to double precision,
even when compiling with -traditional.
Traditional K&R C promotes all floating point operations to double
precision, regardless of the sizes of the operands. On the
architecture for which you are compiling, single precision may be faster
than double precision. If you must use -traditional, but want
to use single precision operations when the operands are single
precision, use this option. This option has no effect when compiling
with ISO or GNU C conventions (the default).
-fshort-wchar
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.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful
for C++ programs; but you can also use most of the GNU compiler options
regardless of what language your program is in. For example, you
might compile a file "firstClass.C" like this:
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:
-fno-access-control
Turn off all access checking. This switch is mainly useful for working
around bugs in the access control code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. The current Working
Paper requires that "operator new" never return a null pointer, so
this check is normally unnecessary.
An alternative to using this option is to specify that your
"operator new" does not throw any exceptions; if you declare it
throw(), G++ will check the return value. See also new
(nothrow).
-fconserve-space
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 "main()" 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.
-fno-const-strings
Give string constants type "char *" instead of type "const
char *". By default, G++ uses type "const char *" 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 "const char *".
-fdollars-in-identifiers
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.
-fno-elide-constructors
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.
-fno-enforce-eh-specs
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.
-fexternal-templates
Cause template instantiations to obey #pragma interface and
implementation; template instances are emitted or not according
to the location of the template definition.
This option is deprecated.
-falt-external-templates
Similar to -fexternal-templates, but template instances are emitted or
not according to the place where they are first instantiated.
This option is deprecated.
-ffor-scope
-fno-for-scope
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.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__" instead.
-ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated
implicitly (i.e. by use); only emit code for explicit instantiations.
-fno-implicit-inline-templates
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.
-fno-implement-inlines
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.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as implicit
int and getting a pointer to member function via non-standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include "ffs", "alloca", "_exit",
"index", "bzero", "conjf", and other related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand",
"bitor", "compl", "not", "or" and "xor" as
synonyms as keywords.
-fno-optional-diags
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.
-fpermissive
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.
-frepo
Enable automatic template instantiation. This option also implies
-fno-implicit-templates.
-fno-rtti
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.
-fstats
Emit statistics about front-end processing at the end of the compilation.
This information is generally only useful to the G++ development team.
-ftemplate-depth-n
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.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the
"__cxa_atexit" function rather than the "atexit" function.
This option is required for fully standards-compliant handling of static
destructors, but will only work if your C library supports
"__cxa_atexit".
-fno-weak
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++.
-nostdinc++
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:
-fno-default-inline
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.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable, because all the constructors or
destructors in a class are private and the class has no friends or
public static member functions.
-Wnon-virtual-dtor (C++ only)
Warn when a class declares a non-virtual destructor that should probably
be virtual, because it looks like the class will be used polymorphically.
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does not
match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
Here the compiler will warn that the member initializers for i
and j will be rearranged to match the declaration order of the
members.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of various style guidelines from Scott Meyers'
Effective C++ books. If you use this option, you should be aware
that the standard library headers do not obey all of these guidelines;
you can use grep -v to filter out those warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared
within a template. With the advent of explicit template specification
support in G++, if the name of the friend is an unqualified-id (i.e.,
friend foo(int)), the C++ language specification demands that the
friend declare or define an ordinary, nontemplate function. (Section
14.5.3). Before G++ implemented explicit specification, unqualified-ids
could be interpreted as a particular specialization of a templatized
function. Because this non-conforming behavior is no longer the default
behavior for G++, -Wnon-template-friend allows the compiler to
check existing code for potential trouble spots, and is on by default.
This new compiler behavior can be turned off with
-Wno-non-template-friend which keeps the conformant compiler code
but disables the helpful warning.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast is used within a C++ program. The
new-style casts (static_cast, reinterpret_cast, and
const_cast) are less vulnerable to unintended effects, and much
easier to grep for.
-Woverloaded-virtual (C++ only)
Warn when a derived class function declaration may be an error in
defining a virtual function. In a derived class, the
definitions of virtual functions must match the type signature of a
virtual function declared in the base class. With this option, the
compiler warns when you define a function with the same name as a
virtual function, but with a type signature that does not match any
declarations from the base class.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member function
to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or
enumeral type to a signed type over a conversion to an unsigned type of
the same size. Previous versions of G++ would try to preserve
unsignedness, but the standard mandates the current behavior.
-Wsynth (C++ only)
Warn when G++'s synthesis behavior does not match that of cfront. For
instance:
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator =
(const A&);, while cfront will use the user-defined operator =.
Options Controlling Objective-C Dialect
This section describes the command-line options that are only meaningful
for Objective-C programs; but you can also use most of the GNU compiler
options regardless of what language your program is in. For example,
you might compile a file "some_class.m" like this:
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:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each
literal string specified with the syntax "@"..."". The default
class name is "NXConstantString".
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default
for NeXT-based systems, including Darwin and Mac OS X.
-gen-decls
Dump interface declarations for all classes seen in the source file to a
file named sourcename.decl.
-Wno-protocol
Do not warn if methods required by a protocol are not implemented
in the class adopting it.
-Wselector
Warn if a selector has multiple methods of different types defined.
Options to Control Diagnostic Messages Formatting
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.
-fmessage-length=n
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.
-fdiagnostics-show-location=once
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
behaviour.
-fdiagnostics-show-location=every-line
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.
Options to Request or Suppress Warnings
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.
These options control the amount and kinds of warnings produced by GCC:
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-pedantic
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
"__extension__". 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.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than
warnings.
-w
Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /*
comment, or whenever a Backslash-Newline appears in a // comment.
-Wformat
Check calls to "printf" and "scanf", 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 "printf",
"scanf", "strftime" and "strfmon" (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 "strfmon" 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.
-Wno-format-y2k
If -Wformat is specified, do not warn about "strftime"
formats which may yield only a two-digit year.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a
"printf" or "scanf" format function. The C standard specifies
that such arguments are ignored.
-Wformat-nonliteral
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 "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format
functions that represent possible security problems. At present, this
warns about calls to "printf" and "scanf" functions where the
format string is not a string literal and there are no format arguments,
as in "printf (foo);". 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.)
-Wformat=2
Enable -Wformat plus format checks not included in
-Wformat. Currently equivalent to -Wformat
-Wformat-nonliteral -Wformat-security.
-Wimplicit-int
Warn when a declaration does not specify a type.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being
declared.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
-Wmain
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.
-Wmissing-braces
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.
Warn if a multicharacter constant ('FOOF') is used. Usually they
indicate a typo in the user's code, as they have implementation-defined
values, and should not be used in portable code.
-Wparentheses
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
"if" statement an "else" branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if"
statement, which in this example is "if (b)". 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
"if" statement so there is no way the "else" could belong to
the enclosing "if". The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
-Wsequence-point
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
"&&", "||", "? :" or "," (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 "a = a++;", "a[n]
= b[n++]" and "a[i++] = i;". 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.
There is some controversy over the precise meaning of the sequence point
rules in subtle cases. Links to papers with alternative formal definitions
and other related discussions may be found on our readings page
<http://gcc.gnu.org/readings.html>.
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults to
"int". Also warn about any "return" statement with no
return-value in a function whose return-type is not "void".
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.
-Wswitch
Warn whenever a "switch" statement has an index of enumeral type
and lacks a "case" for one or more of the named codes of that
enumeration. (The presence of a "default" label prevents this
warning.) "case" labels outside the enumeration range also
provoke warnings when this option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of
the program (trigraphs within comments are not warned about).
-Wunused-function
Warn whenever a static function is declared but not defined or a
non\-inline static function is unused.
-Wunused-label
Warn whenever a label is declared but not used.
To suppress this warning use the unused attribute.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused
aside from its declaration
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used.
To suppress this warning cast the expression to void.
-Wunused
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.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or
if a variable may be clobbered by a "setjmp" 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 "volatile", 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 "y" is always 1, 2 or 3, then "x" 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 "save_y" is used only if it is set.
This option also warns when a non-volatile automatic variable might be
changed by a call to "longjmp". These warnings as well are possible
only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know
where "longjmp" 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 "longjmp" 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 "noreturn".
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does not
match the order in which they must be executed. For instance:
-Wunknown-pragmas
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.
-Wall
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.
-Wsystem-headers
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.
-W
Print extra warning messages for these events:
*
A function can return either with or without a value. (Falling
off the end of the function body is considered returning without
a value.) For example, this function would evoke such a
warning:
foo (a)
{
if (a > 0)
return a;
}
*
An expression-statement or the left-hand side of a comma expression
contains no side effects.
To suppress the warning, cast the unused expression to void.
For example, an expression such as x[i,j] will cause a warning,
but x[(void)i,j] will not.
*
An unsigned value is compared against zero with < or <=.
*
A comparison like x<=y<=z appears; this is equivalent to
(x<=y ? 1 : 0) <= z, which is a different interpretation from
that of ordinary mathematical notation.
*
Storage-class specifiers like "static" are not the first things in
a declaration. According to the C Standard, this usage is obsolescent.
*
The return type of a function has a type qualifier such as "const".
Such a type qualifier has no effect, since the value returned by a
function is not an lvalue. (But don't warn about the GNU extension of
"volatile void" return types. That extension will be warned about
if -pedantic is specified.)
*
If -Wall or -Wunused is also specified, warn about unused
arguments.
*
A comparison between signed and unsigned values could produce an
incorrect result when the signed value is converted to unsigned.
(But don't warn if -Wno-sign-compare is also specified.)
*
An aggregate has a partly bracketed initializer.
For example, the following code would evoke such a warning,
because braces are missing around the initializer for "x.h":
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
*
An aggregate has an initializer which does not initialize all members.
For example, the following code would cause such a warning, because
"x.h" would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you need
to compute (by analysing the code, or in some other way) the maximum or
likely maximum error that the computation introduces, and allow for it
when performing comparisons (and when producing output, but that's a
different problem). In particular, instead of testing for equality, you
would check to see whether the two values have ranges that overlap; and
this is done with the relational operators, so equality comparisons are
probably mistaken.
-Wtraditional (C only)
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/or problematic constructs which should be avoided.
*
Macro parameters that appear within string literals in the macro body.
In traditional C macro replacement takes place within string literals,
but does not in ISO C.
*
In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider a line to be a directive
if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C
understands but would ignore because the # does not appear as the
first character on the line. It also suggests you hide directives like
#pragma not understood by traditional C by indenting them. Some
traditional implementations would not recognise #elif, so it
suggests avoiding it altogether.
*
A function-like macro that appears without arguments.
*
The unary plus operator.
*
The U integer constant suffix, or the F or L floating point
constant suffixes. (Traditional C does support the L suffix on integer
constants.) Note, these suffixes appear in macros defined in the system
headers of most modern systems, e.g. the _MIN/_MAX macros in "<limits.h>".
Use of these macros in user code might normally lead to spurious
warnings, however gcc's integrated preprocessor has enough context to
avoid warning in these cases.
*
A function declared external in one block and then used after the end of
the block.
*
A "switch" statement has an operand of type "long".
*
A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C compilers.
*
The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only issued if
the base of the constant is ten. I.e. hexadecimal or octal values, which
typically represent bit patterns, are not warned about.
*
Usage of ISO string concatenation is detected.
*
Initialization of automatic aggregates.
*
Identifier conflicts with labels. Traditional C lacks a separate
namespace for labels.
*
Initialization of unions. If the initializer is zero, the warning is
omitted. This is done under the assumption that the zero initializer in
user code appears conditioned on e.g. "__STDC__" to avoid missing
initializer warnings and relies on default initialization to zero in the
traditional C case.
*
Conversions by prototypes between fixed/floating point values and vice
versa. The absence of these prototypes when compiling with traditional
C would cause serious problems. This is a subset of the possible
conversion warnings, for the full set use -Wconversion.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wshadow
Warn whenever a local variable shadows another local variable, parameter or
global variable or whenever a built-in function is shadowed.
-Wid-clash-len
Warn whenever two distinct identifiers match in the first len
characters. This may help you prepare a program that will compile
with certain obsolete, brain-damaged compilers.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wpointer-arith
Warn about anything that depends on the ``size of'' a function type or
of "void". GNU C assigns these types a size of 1, for
convenience in calculations with "void *" pointers and pointers
to functions.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type.
For example, warn if "int malloc()" is cast to "anything *".
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from
the target type. For example, warn if a "const char *" is cast
to an ordinary "char *".
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the
target is increased. For example, warn if a "char *" is cast to
an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that
copying the address of one into a non-"const""char *"
pointer will get a warning; when compiling C++, warn about the
deprecated conversion from string constants to "char *".
These warnings will help you find at
compile time code that can try to write into a string constant, but
only if you have been very careful about using "const" in
declarations and prototypes. Otherwise, it will just be a nuisance;
this is why we did not make -Wall request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different from what
would happen to the same argument in the absence of a prototype. This
includes conversions of fixed point to floating and vice versa, and
conversions changing the width or signedness of a fixed point argument
except when the same as the default promotion.
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the assignment
"x = -1" if "x" is unsigned. But do not warn about explicit
casts like "(unsigned) -1".
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to unsigned.
This warning is also enabled by -W; to get the other warnings
of -W without this warning, use -W -Wno-sign-compare.
-Waggregate-return
Warn if any functions that return structures or unions are defined or
called. (In languages where you can return an array, this also elicits
a warning.)
-Wstrict-prototypes (C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted without
a warning if preceded by a declaration which specifies the argument
types.)
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that fail
to be declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "noreturn".
Note these are only possible candidates, not absolute ones. Care should
be taken to manually verify functions actually do not ever return before
adding the "noreturn" attribute, otherwise subtle code generation
bugs could be introduced. You will not get a warning for "main" in
hosted C environments.
-Wmissing-format-attribute
If -Wformat is enabled, also warn about functions which might be
candidates for "format" attributes. Note these are only possible
candidates, not absolute ones. GCC will guess that "format"
attributes might be appropriate for any function that calls a function
like "vprintf" or "vscanf", but this might not always be the
case, and some functions for which "format" attributes are
appropriate may not be detected. This option has no effect unless
-Wformat is enabled (possibly by -Wall).
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar"
will be misaligned even though "struct bar" does not itself
have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpadded
Warn if padding is included in a structure, either to align an element
of the structure or to align the whole structure. Sometimes when this
happens it is possible to rearrange the fields of the structure to
reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in
cases where multiple declaration is valid and changes nothing.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at
least a whole line of source code will never be executed, because
some condition is never satisfied or because it is after a
procedure that never returns.
It is possible for this option to produce a warning even though there
are circumstances under which part of the affected line can be executed,
so care should be taken when removing apparently-unreachable code.
For instance, when a function is inlined, a warning may mean that the
line is unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging
version of a program there is often substantial code which checks
correct functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behaviour which is selectable at compile-time.
-Winline
Warn if a function can not be inlined and it was declared as inline.
-Wlong-long
Warn if long long type is used. This is default. To inhibit
the warning messages, use -Wno-long-long. Flags
-Wlong-long and -Wno-long-long are taken into account
only when -pedantic flag is used.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does
not generally indicate that there is anything wrong with your code; it
merely indicates that GCC's optimizers were unable to handle the code
effectively. Often, the problem is that your code is too big or too
complex; GCC will refuse to optimize programs when the optimization
itself is likely to take inordinate amounts of time.
-Werror
Make all warnings into errors.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging
either your program or GCC:
-g
Produce debugging information in the operating system's native format
(stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging
information.
On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but will probably make other debuggers
crash or
refuse to read the program. If you want to control for certain whether
to generate the extra information, use -gstabs+, -gstabs,
-gxcoff+, -gxcoff, -gdwarf-1+, or -gdwarf-1
(see below).
Unlike most other C compilers, GCC allows you to use -g with
-O. The shortcuts taken by optimized code may occasionally
produce surprising results: some variables you declared may not exist
at all; flow of control may briefly move where you did not expect it;
some statements may not be executed because they compute constant
results or their values were already at hand; some statements may
execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes
it reasonable to use the optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the
capability for more than one debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use the
most expressive format available (DWARF 2, stabs, or the native format
if neither of those are supported), including GDB extensions if at all
possible.
-gstabs
Produce debugging information in stabs format (if that is supported),
without GDB extensions. This is the format used by DBX on most BSD
systems. On MIPS, Alpha and System V Release 4 systems this option
produces stabs debugging output which is not understood by DBX or SDB.
On System V Release 4 systems this option requires the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB). The
use of these extensions is likely to make other debuggers crash or
refuse to read the program.
-gcoff
Produce debugging information in COFF format (if that is supported).
This is the format used by SDB on most System V systems prior to
System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is supported).
This is the format used by the DBX debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB). The
use of these extensions is likely to make other debuggers crash or
refuse to read the program, and may cause assemblers other than the GNU
assembler (GAS) to fail with an error.
-gdwarf
Produce debugging information in DWARF version 1 format (if that is
supported). This is the format used by SDB on most System V Release 4
systems.
-gdwarf+
Produce debugging information in DWARF version 1 format (if that is
supported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other debuggers
crash or refuse to read the program.
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is
supported). This is the format used by DBX on IRIX 6.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gdwarflevel
-gdwarf-2level
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces in
parts of the program that you don't plan to debug. This includes
descriptions of functions and external variables, but no information
about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions
present in the program. Some debuggers support macro expansion when
you use -g3.
-p
Generate extra code to write profile information suitable for the
analysis program "prof". You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-pg
Generate extra code to write profile information suitable for the
analysis program "gprof". You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-a
Generate extra code to write profile information for basic blocks, which will
record the number of times each basic block is executed, the basic block start
address, and the function name containing the basic block. If -g is
used, the line number and filename of the start of the basic block will also be
recorded. If not overridden by the machine description, the default action is
to append to the text file bb.out.
This data could be analyzed by a program like "tcov". Note,
however, that the format of the data is not what "tcov" expects.
Eventually GNU "gprof" should be extended to process this data.
-Q
Makes the compiler print out each function name as it is compiled, and
print some statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each
pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
-ax
Generate extra code to profile basic blocks. Your executable will
produce output that is a superset of that produced when -a is
used. Additional output is the source and target address of the basic
blocks where a jump takes place, the number of times a jump is executed,
and (optionally) the complete sequence of basic blocks being executed.
The output is appended to file bb.out.
You can examine different profiling aspects without recompilation. Your
executable will read a list of function names from file bb.in.
Profiling starts when a function on the list is entered and stops when
that invocation is exited. To exclude a function from profiling, prefix
its name with -. If a function name is not unique, you can
disambiguate it by writing it in the form
/path/filename.d:functionname. Your executable will write the
available paths and filenames in file bb.out.
Several function names have a special meaning:
__bb_jumps__
Write source, target and frequency of jumps to file bb.out.
__bb_hidecall__
Exclude function calls from frequency count.
__bb_showret__
Include function returns in frequency count.
__bb_trace__
Write the sequence of basic blocks executed to file bbtrace.gz.
The file will be compressed using the program gzip, which must
exist in your PATH. On systems without the popen
function, the file will be named bbtrace and will not be
compressed. Profiling for even a few seconds on these systems
will produce a very large file. Note: "__bb_hidecall__" and
"__bb_showret__" will not affect the sequence written to
bbtrace.gz.
Here's a short example using different profiling parameters
in file bb.in. Assume function "foo" consists of basic blocks
1 and 2 and is called twice from block 3 of function "main". After
the calls, block 3 transfers control to block 4 of "main".
With "__bb_trace__" and "main" contained in file bb.in,
the following sequence of blocks is written to file bbtrace.gz:
0 3 1 2 1 2 4. The return from block 2 to block 3 is not shown, because
the return is to a point inside the block and not to the top. The
block address 0 always indicates, that control is transferred
to the trace from somewhere outside the observed functions. With
-foo added to bb.in, the blocks of function
"foo" are removed from the trace, so only 0 3 4 remains.
With "__bb_jumps__" and "main" contained in file bb.in,
jump frequencies will be written to file bb.out. The
frequencies are obtained by constructing a trace of blocks
and incrementing a counter for every neighbouring pair of blocks
in the trace. The trace 0 3 1 2 1 2 4 displays the following
frequencies:
Jump from block 0x0 to block 0x3 executed 1 time(s)
Jump from block 0x3 to block 0x1 executed 1 time(s)
Jump from block 0x1 to block 0x2 executed 2 time(s)
Jump from block 0x2 to block 0x1 executed 1 time(s)
Jump from block 0x2 to block 0x4 executed 1 time(s)
With "__bb_hidecall__", control transfer due to call instructions
is removed from the trace, that is the trace is cut into three parts: 0
3 4, 0 1 2 and 0 1 2. With "__bb_showret__", control transfer due
to return instructions is added to the trace. The trace becomes: 0 3 1
2 3 1 2 3 4. Note, that this trace is not the same, as the sequence
written to bbtrace.gz. It is solely used for counting jump
frequencies.
-fprofile-arcs
Instrument arcs during compilation. For each function of your
program, GCC creates a program flow graph, then finds a spanning tree
for the graph. Only arcs that are not on the spanning tree have to be
instrumented: the compiler adds code to count the number of times that these
arcs are executed. When an arc is the only exit or only entrance to a
block, the instrumentation code can be added to the block; otherwise, a
new basic block must be created to hold the instrumentation code.
Since not every arc in the program must be instrumented, programs
compiled with this option run faster than programs compiled with
-a, which adds instrumentation code to every basic block in the
program. The tradeoff: since "gcov" does not have
execution counts for all branches, it must start with the execution
counts for the instrumented branches, and then iterate over the program
flow graph until the entire graph has been solved. Hence, "gcov"
runs a little more slowly than a program which uses information from
-a.
-fprofile-arcs also makes it possible to estimate branch
probabilities, and to calculate basic block execution counts. In
general, basic block execution counts do not give enough information to
estimate all branch probabilities. When the compiled program exits, it
saves the arc execution counts to a file called
sourcename.da. Use the compiler option
-fbranch-probabilities when recompiling, to optimize using estimated
branch probabilities.
-ftest-coverage
Create data files for the "gcov" code-coverage utility.
The data file names begin with the name of your source file:
sourcename.bb
A mapping from basic blocks to line numbers, which "gcov" uses to
associate basic block execution counts with line numbers.
sourcename.bbg
A list of all arcs in the program flow graph. This allows "gcov"
to reconstruct the program flow graph, so that it can compute all basic
block and arc execution counts from the information in the
"sourcename.da" file (this last file is the output from
-fprofile-arcs).
-dletters
Says to make debugging dumps during compilation at times specified by
letters. This is used for debugging the compiler. The file names
for most of the dumps are made by appending a pass number and a word to
the source file name (e.g. foo.c.00.rtl or foo.c.01.sibling).
Here are the possible letters for use in letters, and their meanings:
A
Annotate the assembler output with miscellaneous debugging information.
b
Dump after computing branch probabilities, to file.11.bp.
B
Dump after block reordering, to file.26.bbro.
c
Dump after instruction combination, to the file file.14.combine.
C
Dump after the first if conversion, to the file file.15.ce.
d
Dump after delayed branch scheduling, to file.29.dbr.
D
Dump all macro definitions, at the end of preprocessing, in addition to
normal output.
e
Dump after SSA optimizations, to file.05.ssa and
file.06.ussa.
E
Dump after the second if conversion, to file.24.ce2.
f
Dump after life analysis, to file.13.life.
F
Dump after purging "ADDRESSOF" codes, to file.04.addressof.
g
Dump after global register allocation, to file.19.greg.
o
Dump after post-reload CSE and other optimizations, to file.20.postreload.
G
Dump after GCSE, to file.08.gcse.
i
Dump after sibling call optimizations, to file.01.sibling.
j
Dump after the first jump optimization, to file.02.jump.
J
Dump after the last jump optimization, to file.27.jump2.
k
Dump after conversion from registers to stack, to file.29.stack.
l
Dump after local register allocation, to file.18.lreg.
L
Dump after loop optimization, to file.09.loop.
M
Dump after performing the machine dependent reorganisation pass, to
file.28.mach.
n
Dump after register renumbering, to file.23.rnreg.
N
Dump after the register move pass, to file.16.regmove.
r
Dump after RTL generation, to file.00.rtl.
R
Dump after the second instruction scheduling pass, to
file.25.sched2.
s
Dump after CSE (including the jump optimization that sometimes follows
CSE), to file.03.cse.
S
Dump after the first instruction scheduling pass, to
file.17.sched.
t
Dump after the second CSE pass (including the jump optimization that
sometimes follows CSE), to file.10.cse2.
w
Dump after the second flow pass, to file.21.flow2.
X
Dump after dead code elimination, to file.06.dce.
z
Dump after the peephole pass, to file.22.peephole2.
a
Produce all the dumps listed above.
m
Print statistics on memory usage, at the end of the run, to
standard error.
p
Annotate the assembler output with a comment indicating which
pattern and alternative was used. The length of each instruction is
also printed.
P
Dump the RTL in the assembler output as a comment before each instruction.
Also turns on -dp annotation.
v
For each of the other indicated dump files (except for
file.00.rtl), dump a representation of the control flow graph
suitable for viewing with VCG to file.pass.vcg.
x
Just generate RTL for a function instead of compiling it. Usually used
with r.
y
Dump debugging information during parsing, to standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress instruction
numbers and line number note output. This makes it more feasible to
use diff on debugging dumps for compiler invocations with different
options, in particular with and without -g.
-fdump-translation-unit (C and C++ only)
-fdump-translation-unit-number(C and C++ only)
Dump a representation of the tree structure for the entire translation
unit to a file. The file name is made by appending .tu to the
source file name. If the -number form is used, number
controls the details of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-number(C++ only)
Dump a representation of each class's hierarchy and virtual function
table layout to a file. The file name is made by appending .class
to the source file name. If the -number form is used, number
controls the details of the dump as described for the -fdump-tree
options.
-fdump-ast-switch(C++ only)
-fdump-ast-switch-number(C++ only)
Control the dumping at various stages of processing the abstract syntax
tree to a file. The file name is generated by appending a switch
specific suffix to the source file name. If the -number form is
used, number is a bit mask which controls the details of the
dump. The following bits are meaningful (these are not set symbolically,
as the primary function of these dumps is for debugging gcc itself):
bit0 (1)
Print the address of each node. Usually this is not meaningful as it
changes according to the environment and source file.
bit1 (2)
Inhibit dumping of members of a scope or body of a function, unless they
are reachable by some other path.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
-fpretend-float
When running a cross-compiler, pretend that the target machine uses the
same floating point format as the host machine. This causes incorrect
output of the actual floating constants, but the actual instruction
sequence will probably be the same as GCC would make when running on
the target machine.
-save-temps
Store the usual ``temporary'' intermediate files permanently; place them
in the current directory and name them based on the source file. Thus,
compiling foo.c with -c -save-temps would produce files
foo.i and foo.s, as well as foo.o. This creates a
preprocessed foo.i output file even though the compiler now
normally uses an integrated preprocessor.
-time
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and assembler
(plus the linker if linking is done). The output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the ``user time,'' that is time spent
executing the program itself. The second number is ``system time,''
time spent executing operating system routines on behalf of the program.
Both numbers are in seconds.
-print-file-name=library
Print the full absolute name of the library file library that
would be used when linking---and don't do anything else. With this
option, GCC does not compile or link anything; it just prints the
file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any
other switches present in the command line. This directory is supposed
to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches
that enable them. The directory name is separated from the switches by
;, and each switch starts with an @} instead of the
@samp{-, without spaces between multiple switches. This is supposed to
ease shell-processing.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs
but you do want to link with libgcc.a. You can do
Print the name of the configured installation directory and a list of
program and library directories gcc will search---and don't do anything else.
This is useful when gcc prints the error message
installation problem, cannot exec cpp0: No such file or directory.
To resolve this you either need to put cpp0 and the other compiler
components where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing '/'.
-dumpmachine
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0)---and don't do
anything else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This
is used when GCC itself is being built.)
Options That Control Optimization
These options control various sorts of optimizations:
-O
-O1
Optimize. Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.
Without -O, the compiler's goal is to reduce the cost of
compilation and to make debugging produce the expected results.
Statements are independent: if you stop the program with a breakpoint
between statements, you can then assign a new value to any variable or
change the program counter to any other statement in the function and
get exactly the results you would expect from the source code.
Without -O, the compiler only allocates variables declared
"register" in registers. The resulting compiled code is a little
worse than produced by PCC without -O.
With -O, the compiler tries to reduce code size and execution
time.
When you specify -O, the compiler turns on -fthread-jumps
and -fdefer-pop on all machines. The compiler turns on
-fdelayed-branch on machines that have delay slots, and
-fomit-frame-pointer on machines that can support debugging even
without a frame pointer. On some machines the compiler also turns
on other flags.
-O2
Optimize even more. GCC performs nearly all supported optimizations
that do not involve a space-speed tradeoff. The compiler does not
perform loop unrolling or function inlining when you specify -O2.
As compared to -O, this option increases both compilation time
and the performance of the generated code.
-O2 turns on all optional optimizations except for loop unrolling,
function inlining, and register renaming. It also turns on the
-fforce-mem option on all machines and frame pointer elimination
on machines where doing so does not interfere with debugging.
Please note the warning under -fgcse about
invoking -O2 on programs that use computed gotos.
-O3
Optimize yet more. -O3 turns on all optimizations specified by
-O2 and also turns on the -finline-functions and
-frename-registers options.
-O0
Do not optimize.
-Os
Optimize for size. -Os enables all -O2 optimizations that
do not typically increase code size. It also performs further
optimizations designed to reduce code size.
If you use multiple -O options, with or without level numbers,
the last such option is the one that is effective.
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.
-ffloat-store
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 "double" 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.
-fno-default-inline
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.
-fno-defer-pop
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.
-fforce-mem
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.
-fforce-addr
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
-fforce-mem may.
-fomit-frame-pointer
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 "FRAME_POINTER_REQUIRED" controls
whether a target machine supports this flag.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
-ftrapv
This option generates traps for signed overflow on addition, subtraction,
multiplication operations.
-fno-inline
Don't pay attention to the "inline" 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.
-finline-functions
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 "static", then the function is normally not output as
assembler code in its own right.
-finline-limit=n
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.
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the function
is declared "static", nevertheless output a separate run-time
callable version of the function. This switch does not affect
"extern inline" functions.
-fkeep-static-consts
Emit variables declared "static const" 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.
-fno-function-cse
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.
-ffast-math
This option allows GCC to violate some ISO or IEEE rules and/or
specifications in the interest of optimizing code for speed. For
example, it allows the compiler to assume arguments to the "sqrt"
function are non-negative numbers and that no floating-point values
are NaNs.
This option causes the preprocessor macro "__FAST_MATH__" 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.
-fno-math-errno
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.
The default is -fmath-errno. The -ffast-math option
sets -fno-math-errno.
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.
-fstrength-reduce
Perform the optimizations of loop strength reduction and
elimination of iteration variables.
-fthread-jumps
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.
-fcse-follow-jumps
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 "if" statement with an
"else" clause, CSE will follow the jump when the condition
tested is false.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to
follow jumps which conditionally skip over blocks. When CSE
encounters a simple "if" statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the
body of the "if".
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been
performed.
-frerun-loop-opt
Run the loop optimizer twice.
-fgcse
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.
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless null
pointer checks. Programs which rely on NULL pointer dereferences not
halting the program may not work properly with this option. Use
-fno-delete-null-pointer-checks to disable this optimizing for programs
which depend on that behavior.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
-foptimize-register-move
-fregmove
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.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.
-fschedule-insns
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.
-fschedule-insns2
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.
-ffunction-sections
-fdata-sections
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 "gprof" on all systems if you
specify this option and you may have problems with debugging if
you specify both this option and -g.
-fcaller-saves
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.
-funroll-loops
Perform the optimization of loop unrolling. This is only done for loops
whose number of iterations can be determined at compile time or run time.
-funroll-loops implies both -fstrength-reduce and
-frerun-cse-after-loop.
-funroll-all-loops
Perform the optimization of loop unrolling. This is done for all loops
and usually makes programs run more slowly. -funroll-all-loops
implies -fstrength-reduce as well as -frerun-cse-after-loop.
-fmove-all-movables
Forces all invariant computations in loops to be moved
outside the loop.
-freduce-all-givs
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.
-fno-peephole
-fno-peephole2
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.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on
guessing the path a branch might take.
-fno-guess-branch-probability
Sometimes gcc will opt to guess branch probabilities when none are
available from either profile directed feedback (-fprofile-arcs)
or __builtin_expect. 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.
-fstrict-aliasing
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 "unsigned int" can alias an "int", but not a
"void*" or a "double". 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;
}
-falign-functions
-falign-functions=n
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.
-falign-labels
-falign-labels=n
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.
-falign-loops
-falign-loops=n
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.
-falign-jumps
-falign-jumps=n
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.
-fssa
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.
-fdce
Perform dead-code elimination in SSA form. Requires -fssa. Like
-fssa, this is an experimental feature.
-fsingle-precision-constant
Treat floating point constant as single precision constant instead of
implicitly converting it to double precision constant.
-frename-registers
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''.
--paramname=value
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:
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an
instruction to fill a delay slot. If more than this arbitrary number of
instructions is searched, the time savings from filling the delay slot
will be minimal so stop searching. Increasing values mean more
aggressive optimization, making the compile time increase with probably
small improvement in executable run time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to
consider when searching for a block with valid live register
information. Increasing this arbitrarily chosen value means more
aggressive optimization, increasing the compile time. This parameter
should be removed when the delay slot code is rewritten to maintain the
control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that will be allocated in
order to perform the global common subexpression elimination
optimization. If more memory than specified is required, the
optimization will not be done.
max-pending-list-length
The maximum number of pending dependencies scheduling will allow
before flushing the current state and starting over. Large functions
with few branches or calls can create excessively large lists which
needlessly consume memory and resources.
max-inline-insns
If an function contains more than this many instructions, it
will not be inlined. This option is precisely equivalent to
-finline-limit.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source
file before actual compilation.
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.
-includefile
Process file as input before processing the regular input file.
In effect, the contents of file are compiled first. Any -D
and -U options on the command line are always processed before
-includefile, regardless of the order in which they are
written. All the -include and -imacros options are
processed in the order in which they are written.
-imacrosfile
Process file as input, discarding the resulting output, before
processing the regular input file. Because the output generated from
file is discarded, the only effect of -imacrosfile
is to make the macros defined in file available for use in the
main input. All the -include and -imacros options are
processed in the order in which they are written.
-idirafterdir
Add the directory dir to the second include path. The directories
on the second include path are searched when a header file is not found
in any of the directories in the main include path (the one that
-I adds to).
-iprefixprefix
Specify prefix as the prefix for subsequent -iwithprefix
options.
-iwithprefixdir
Add a directory to the second include path. The directory's name is
made by concatenating prefix and dir, where prefix was
specified previously with -iprefix. If you have not specified a
prefix yet, the directory containing the installed passes of the
compiler is used as the default.
-iwithprefixbeforedir
Add a directory to the main include path. The directory's name is made
by concatenating prefix and dir, as in the case of
-iwithprefix.
-isystemdir
Add a directory to the beginning of the second include path, marking it
as a system directory, so that it gets the same special treatment as
is applied to the standard system directories.
-nostdinc
Do not search the standard system directories for header files. Only
the directories you have specified with -I options (and the
current directory, if appropriate) are searched.
By using both -nostdinc and -I-, you can limit the include-file
search path to only those directories you specify explicitly.
-remap
When searching for a header file in a directory, remap file names if a
file named header.gcc exists in that directory. This can be used
to work around limitations of file systems with file name restrictions.
The header.gcc file should contain a series of lines with two
tokens on each line: the first token is the name to map, and the second
token is the actual name to use.
-undef
Do not predefine any nonstandard macros. (Including architecture flags).
-E
Run only the C preprocessor. Preprocess all the C source files
specified and output the results to standard output or to the
specified output file.
-C
Tell the preprocessor not to discard comments. Used with the
-E option.
-P
Tell the preprocessor not to generate #line directives.
Used with the -E option.
-M
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. Unless overridden explicitly, 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.
-M implies -E.
-MM
Like -M, but mention only the files included with #include
"file". System header files included with #include
<file> are omitted.
-MD
Like -M but the dependency information is written to a file
rather than stdout. "gcc" will use the same file name and
directory as the object file, but with the suffix .d instead.
This is in addition to compiling the main file as specified----MD
does not inhibit ordinary compilation the way -M does,
unless you also specify -MG.
With Mach, you can use the utility "md" to merge multiple
dependency files into a single dependency file suitable for using with
the make command.
-MMD
Like -MD except mention only user header files, not system
-header files.
-MFfile
When used with -M or -MM, specifies a file to write the
dependencies to. This allows the preprocessor to write the preprocessed
file to stdout normally. If no -MF switch is given, CPP sends
the rules to stdout and suppresses normal preprocessed output.
Another way to specify output of a "make" rule is by setting
the environment variable DEPENDENCIES_OUTPUT.
-MG
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.
-MP
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:-
/tmp/test.o: /tmp/test.c /tmp/test.h
/tmp/test.h:
-MQtarget
-MTtarget
By default CPP uses the main file name, including any path, and appends
the object suffix, normally ``.o'', to it to obtain the name of the
target for dependency generation. With -MT you can specify a
target yourself, overriding the default one.
If you want multiple targets, you can specify them as a single argument
to -MT, or use multiple -MT options.
The targets you specify are output in the order they appear on the
command line. -MQ is identical to -MT, except that the
target name is quoted for Make, but with -MT it isn't. For
example, -MT '$(objpfx)foo.o' gives
$(objpfx)foo.o: /tmp/foo.c
but -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: /tmp/foo.c
The default target is automatically quoted, as if it were given with
-MQ.
-H
Print the name of each header file used, in addition to other normal
activities.
-Aquestion(answer)
Assert the answer answer for question, in case it is tested
with a preprocessing conditional such as #if
#question(answer). -A- disables the standard
assertions that normally describe the target machine.
-Dmacro
Define macro macro with the string 1 as its definition.
-Dmacro=defn
Define macro macro as defn. All instances of -D on
the command line are processed before any -U options.
Any -D and -U options on the command line are processed in
order, and always before -imacrosfile, regardless of the
order in which they are written.
-Umacro
Undefine macro macro. -U options are evaluated after all
-D options, but before any -include and -imacros
options.
Any -D and -U options on the command line are processed in
order, and always before -imacrosfile, regardless of the
order in which they are written.
-dM
Tell the preprocessor to output only a list of the macro definitions
that are in effect at the end of preprocessing. Used with the -E
option.
-dD
Tell the preprocessing to pass all macro definitions into the output, in
their proper sequence in the rest of the output.
-dN
Like -dD except that the macro arguments and contents are omitted.
Only #definename is included in the output.
-dI
Output #include directives in addition to the result of
preprocessing.
-fpreprocessed
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion, trigraph
conversion, escaped newline splicing, and processing of most directives.
In this mode the integrated preprocessor is little more than a tokenizer
for the front ends.
-fpreprocessed is implicit if the input file has one of the
extensions i, ii or mi indicating it has already
been preprocessed.
-trigraphs
Process ISO standard trigraph sequences. These are three-character
sequences, all starting with ??, that are defined by ISO C to
stand for single characters. For example, ??/ stands for
\, so '??/n' is a character constant for a newline. By
default, GCC ignores trigraphs, but in standard-conforming modes it
converts them. See the -std and -ansi options.
The nine trigraph sequences are
??(
@expansion{} [
??)
@expansion{} ]
??<
@expansion{} {
??>
@expansion{} }
??=
@expansion{} #
??/
@expansion{} \
??'
@expansion{} ^
??!
@expansion{} |
??-
@expansion{} ~
Trigraph support is not popular, so many compilers do not implement it
properly. Portable code should not rely on trigraphs being either
converted or ignored.
-Wp,option
Pass option as an option to the preprocessor. If option
contains commas, it is split into multiple options at the commas.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option
contains commas, it is split into multiple options at the commas.
Options for Linking
These options come into play when the compiler links object files into
an executable output file. They are meaningless if the compiler is
not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as input
to the linker.
-c
-S
-E
If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.
-llibrary
Search the library named library when linking.
It makes a difference where in the command you write this option; the
linker searches processes libraries and object files in the order they
are specified. Thus, foo.o -lz bar.o searches library z
after file foo.o but before bar.o. If bar.o refers
to functions in z, those functions may not be loaded.
The linker searches a standard list of directories for the library,
which is actually a file named liblibrary.a. The linker
then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories
plus any that you specify with -L.
Normally the files found this way are library files---archive files
whose members are object files. The linker handles an archive file by
scanning through it for members which define symbols that have so far
been referenced but not defined. But if the file that is found is an
ordinary object file, it is linked in the usual fashion. The only
difference between using an -l option and specifying a file name
is that -l surrounds library with lib and .a
and searches several directories.
-lobjc
You need this special case of the -l option in order to
link an Objective C program.
-nostartfiles
Do not use the standard system startup files when linking.
The standard system libraries are used normally, unless -nostdlib
or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking.
Only the libraries you specify will be passed to the linker.
The standard startup files are used normally, unless -nostartfiles
is used. The compiler may generate calls to memcmp, memset, and memcpy
for System V (and ISO C) environments or to bcopy and bzero for
BSD environments. These entries are usually resolved by entries in
libc. These entry points should be supplied through some other
mechanism when this option is specified.
-nostdlib
Do not use the standard system startup files or libraries when linking.
No startup files and only the libraries you specify will be passed to
the linker. The compiler may generate calls to memcmp, memset, and memcpy
for System V (and ISO C) environments or to bcopy and bzero for
BSD environments. These entries are usually resolved by entries in
libc. These entry points should be supplied through some other
mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal subroutines
that GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
In most cases, you need libgcc.a even when you want to avoid
other standard libraries. In other words, when you specify -nostdlib
or -nodefaultlibs you should usually specify -lgcc as well.
This ensures that you have no unresolved references to internal GCC
library subroutines. (For example, __main, used to ensure C++
constructors will be called.)
-s
Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this prevents linking with the shared
libraries. On other systems, this option has no effect.
-shared
Produce a shared object which can then be linked with other objects to
form an executable. Not all systems support this option. For predictable
results, you must also specify the same set of options that were used to
generate code (-fpic, -fPIC, or model suboptions)
when you specify this option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options
force the use of either the shared or static version respectively.
If no shared version of libgcc was built when the compiler was
configured, these options have no effect.
There are several situations in which an application should use the
shared libgcc instead of the static version. The most common
of these is when the application wishes to throw and catch exceptions
across different shared libraries. In that case, each of the libraries
as well as the application itself should use the shared libgcc.
Therefore, whenever you specify the -shared option, the GCC
driver automatically adds -shared-libgcc, unless you explicitly
specify -static-libgcc. The G++ driver automatically adds
-shared-libgcc when you build a main executable as well because
for C++ programs that is typically the right thing to do.
(Exception-handling will not work reliably otherwise.)
However, when linking a main executable written in C, you must
explicitly say -shared-libgcc if you want to use the shared
libgcc.
-symbolic
Bind references to global symbols when building a shared object. Warn
about any unresolved references (unless overridden by the link editor
option -Xlinker -z -Xlinker defs). Only a few systems support
this option.
-Xlinkeroption
Pass option as an option to the linker. You can use this to
supply system-specific linker options which GCC does not know how to
recognize.
If you want to pass an option that takes an argument, you must use
-Xlinker twice, once for the option and once for the argument.
For example, to pass -assert definitions, you must write
-Xlinker -assert -Xlinker definitions. It does not work to write
-Xlinker ``-assert definitions'', because this passes the entire
string as a single argument, which is not what the linker expects.
-Wl,option
Pass option as an option to the linker. If option contains
commas, it is split into multiple options at the commas.
-usymbol
Pretend the symbol symbol is undefined, to force linking of
library modules to define it. You can use -u multiple times with
different symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for
libraries and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be
searched for header files. This can be used to override a system header
file, substituting your own version, since these directories are
searched before the system header file directories. However, you should
not use this option to add directories that contain vendor-supplied
system header files (use -isystem for that). If you use more than
one -I option, the directories are scanned in left-to-right
order; the standard system directories come after.
-I-
Any directories you specify with -I options before the -I-
option are searched only for the case of #include "file";
they are not searched for #include <file>.
If additional directories are specified with -I options after
the -I-, these directories are searched for all #include
directives. (Ordinarily all-I directories are used
this way.)
In addition, the -I- option inhibits the use of the current
directory (where the current input file came from) as the first search
directory for #include "file". There is no way to
override this effect of -I-. With -I. you can specify
searching the directory which was current when the compiler was
invoked. That is not exactly the same as what the preprocessor does
by default, but it is often satisfactory.
-I- does not inhibit the use of the standard system directories
for header files. Thus, -I- and -nostdinc are
independent.
-Ldir
Add directory dir to the list of directories to be searched
for -l.
-Bprefix
This option specifies where to find the executables, libraries,
include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
cpp, cc1, as and ld. It tries
prefix as a prefix for each program it tries to run, both with and
without machine/version/.
For each subprogram to be run, the compiler driver first tries the
-B prefix, if any. If that name is not found, or if -B
was not specified, the driver tries two standard prefixes, which are
/usr/lib/gcc/ and /usr/local/lib/gcc-lib/. If neither of
those results in a file name that is found, the unmodified program
name is searched for using the directories specified in your
PATH environment variable.
-B prefixes that effectively specify directory names also apply
to libraries in the linker, because the compiler translates these
options into -L options for the linker. They also apply to
includes files in the preprocessor, because the compiler translates these
options into -isystem options for the preprocessor. In this case,
the compiler appends include to the prefix.
The run-time support file libgcc.a can also be searched for using
the -B prefix, if needed. If it is not found there, the two
standard prefixes above are tried, and that is all. The file is left
out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use
the environment variable GCC_EXEC_PREFIX.
-specs=file
Process file after the compiler reads in the standard specs
file, in order to override the defaults that the gcc driver
program uses when determining what switches to pass to cc1,
cc1plus, as, ld, etc. More than one
-specs=file can be specified on the command line, and they
are processed in order, from left to right.
Specifying Target Machine and Compiler Version
By default, GCC compiles code for the same type of machine that you
are using. However, it can also be installed as a cross-compiler, to
compile for some other type of machine. In fact, several different
configurations of GCC, for different target machines, can be
installed side by side. Then you specify which one to use with the
-b option.
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.
-bmachine
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.
-Vversion
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.
Hardware Models and Configurations
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.
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.
-m68000
-mc68000
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.
-m68020
-mc68020
Generate output for a 68020. This is the default
when the compiler is configured for 68020-based systems.
-m68881
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.
-m68030
Generate output for a 68030. This is the default when the compiler is
configured for 68030-based systems.
-m68040
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.
-m68060
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.
-mcpu32
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.
-m5200
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.
-m68020-40
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.
-m68020-60
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.
-mfpa
Generate output containing Sun FPA instructions for floating point.
-msoft-float
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.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
-mnobitfield
Do not use the bit-field instructions. The -m68000, -mcpu32
and -m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies
-mbitfield. This is the default if you use a configuration
designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the "rtd"
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 "printf");
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 "rtd" instruction is supported by the 68010, 68020, 68030,
68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long",
"float", "double", and "long double" 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.
-mpcrel
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.
-mno-strict-align
-mstrict-align
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.
-m6811
-m68hc11
Generate output for a 68HC11. This is the default
when the compiler is configured for 68HC11-based systems.
-m6812
-m68hc12
Generate output for a 68HC12. This is the default
when the compiler is configured for 68HC12-based systems.
-mauto-incdec
Enable the use of 68HC12 pre and post auto-increment and auto-decrement
addressing modes.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
-msoft-reg-count=count
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:
-munix
Do not output certain jump instructions ("aobleq" and so on)
that the Unix assembler for the Vax cannot handle across long
ranges.
-mgnu
Do output those jump instructions, on the assumption that you
will assemble with the GNU assembler.
-mg
Output code for g-format floating point numbers instead of d-format.
SPARC Options
These -m switches are supported on the SPARC:
-mno-app-regs
-mapp-regs
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.
-mfpu
-mhard-float
Generate output containing floating point instructions. This is the
default.
-mno-fpu
-msoft-float
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.
-mhard-quad-float
Generate output containing quad-word (long double) floating point
instructions.
-msoft-quad-float
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.
-mno-epilogue
-mepilogue
With -mepilogue (the default), the compiler always emits code for
function exit at the end of each function. Any function exit in
the middle of the function (such as a return statement in C) will
generate a jump to the exit code at the end of the function.
With -mno-epilogue, the compiler tries to emit exit code inline
at every function exit.
-mno-flat
-mflat
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.
-mno-unaligned-doubles
-munaligned-doubles
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.
-mno-faster-structs
-mfaster-structs
With -mfaster-structs, the compiler assumes that structures
should have 8 byte alignment. This enables the use of pairs of
"ldd" and "std" instructions for copies in structure
assignment, in place of twice as many "ld" and "st" 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.
-mv8
-msparclite
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 ("ffs") 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.
-mcypress
-msupersparc
These two options select the processor for which the code is optimised.
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.
-mcpu=cpu_type
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.
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 are used for
-mtune=cpu_type, though the only useful values are those that
select a particular cpu implementation: cypress, supersparc,
hypersparc, f930, f934, sparclite86x,
tsc701, ultrasparc.
These -m switches are supported in addition to the above
on the SPARCLET processor.
-mlittle-endian
Generate code for a processor running in little-endian mode.
-mlive-g0
Treat register %g0 as a normal register.
GCC will continue to clobber it as necessary but will not assume
it always reads as 0.
-mbroken-saverestore
Generate code that does not use non-trivial forms of the "save" and
"restore" instructions. Early versions of the SPARCLET processor do
not correctly handle "save" and "restore" instructions used with
arguments. They correctly handle them used without arguments. A "save"
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.
-mlittle-endian
Generate code for a processor running in little-endian mode.
-m32
-m64
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.
-mcmodel=medlow
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.
-mcmodel=medmid
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.
-mcmodel=medany
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.
-mcmodel=embmedany
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.
-mstack-bias
-mno-stack-bias
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:
-mc1
Generate output for C1. The code will run on any Convex machine.
The preprocessor symbol "__convex__c1__" is defined.
-mc2
Generate output for C2. Uses instructions not available on C1.
Scheduling and other optimizations are chosen for max performance on C2.
The preprocessor symbol "__convex_c2__" is defined.
-mc32
Generate output for C32xx. Uses instructions not available on C1.
Scheduling and other optimizations are chosen for max performance on C32.
The preprocessor symbol "__convex_c32__" is defined.
-mc34
Generate output for C34xx. Uses instructions not available on C1.
Scheduling and other optimizations are chosen for max performance on C34.
The preprocessor symbol "__convex_c34__" is defined.
-mc38
Generate output for C38xx. Uses instructions not available on C1.
Scheduling and other optimizations are chosen for max performance on C38.
The preprocessor symbol "__convex_c38__" is defined.
-margcount
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.
-mnoargcount
Omit the argument count word. This is the default.
-mvolatile-cache
Allow volatile references to be cached. This is the default.
-mvolatile-nocache
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.
-mlong32
Type long is 32 bits, the same as type int. This is the default.
-mlong64
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:
-mdw
Generate code that assumes the "DW" bit is set, i.e., that byte and
halfword operations are directly supported by the hardware. This is the
default.
-mndw
Generate code that assumes the "DW" bit is not set.
-mbw
Generate code that assumes the system supports byte and halfword write
operations. This is the default.
-mnbw
Generate code that assumes the systems does not support byte and
halfword write operations. -mnbw implies -mndw.
-msmall
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 "call" instruction to be used instead
of a "const", "consth", "calli" sequence.
-mnormal
Use the normal memory model: Generate "call" instructions only when
calling functions in the same file and "calli" 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.
-mlarge
Always use "calli" instructions. Specify this option if you expect
a single file to compile into more than 256 KB of code.
-m29050
Generate code for the Am29050.
-m29000
Generate code for the Am29000. This is the default.
-mkernel-registers
Generate references to registers "gr64-gr95" instead of to
registers "gr96-gr127". 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.
-muser-registers
Use the normal set of global registers, "gr96-gr127". This is the
default.
-mstack-check
-mno-stack-check
Insert (or do not insert) a call to "__msp_check" after each stack
adjustment. This is often used for kernel code.
-mstorem-bug
-mno-storem-bug
-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
-mreuse-arg-regs
-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.
-mno-impure-text
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to
not pass -assert pure-text to the linker when linking a shared object.
-msoft-float
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.
-mno-multm
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:
-mapcs-frame
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.
-mapcs
This is a synonym for -mapcs-frame.
-mapcs-26
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.
-mapcs-32
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.
-mthumb-interwork
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.
-mno-sched-prolog
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.
-mhard-float
Generate output containing floating point instructions. This is the
default.
-msoft-float
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.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is
the default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default is
to compile code for a little-endian processor.
-mwords-little-endian
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.
-malignment-traps
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.
-mno-alignment-traps
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.
-mshort-load-bytes
-mno-short-load-words
These are deprecated aliases for -malignment-traps.
-mno-short-load-bytes
-mshort-load-words
This are deprecated aliases for -mno-alignment-traps.
-mbsd
This option only applies to RISC iX. Emulate the native BSD-mode
compiler. This is the default if -ansi is not specified.
-mxopen
This option only applies to RISC iX. Emulate the native X/Open-mode
compiler.
-mno-symrename
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.
-mcpu=name
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.
-mtune=name
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.
-march=name
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.
-mfpe=number
-mfp=number
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.
-mstructure-size-boundary=n
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.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a
"noreturn" function. It will be executed if the function tries to
return.
-mlong-calls
-mno-long-calls
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 behaviour, 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.
-mnop-fun-dllimport
Disable support for the "dllimport" attribute.
-msingle-pic-base
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 initialising this register with an appropriate value
before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. The default is R10
unless stack-checking is enabled, when R9 is used.
-mpoke-function-name
Write the name of each function into the text section, directly
preceding the function prologue. The generated code is similar to this:
When performing a stack backtrace, code can inspect the value of
"pc" stored at "fp + 0". If the trace function then looks at
location "pc - 12" and the top 8 bits are set, then we know that
there is a function name embedded immediately preceding this location
and has length "((pc[-3]) & 0xff000000)".
-mthumb
Generate code for the 16-bit Thumb instruction set. The default is to
use the 32-bit ARM instruction set.
-mtpcs-frame
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.
-mtpcs-leaf-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.
-mcallee-super-interworking
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.
-mcaller-super-interworking
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:
-mrelax
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:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300
processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the
MN10300 processors.
-mam33
Generate code which uses features specific to the AM33 processor.
-mno-am33
Do not generate code which uses features specific to the AM33 processor. This
is the default.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
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:
-mcode-model=small
Assume all objects live in the lower 16MB of memory (so that their addresses
can be loaded with the "ld24" instruction), and assume all subroutines
are reachable with the "bl" instruction.
This is the default.
The addressability of a particular object can be set with the
"model" attribute.
-mcode-model=medium
Assume objects may be anywhere in the 32-bit address space (the compiler
will generate "seth/add3" instructions to load their addresses), and
assume all subroutines are reachable with the "bl" instruction.
-mcode-model=large
Assume objects may be anywhere in the 32-bit address space (the compiler
will generate "seth/add3" instructions to load their addresses), and
assume subroutines may not be reachable with the "bl" instruction
(the compiler will generate the much slower "seth/add3/jl"
instruction sequence).
-msdata=none
Disable use of the small data area. Variables will be put into
one of .data, bss, or .rodata (unless the
"section" 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
"section" attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not
generate special code to reference them.
-msdata=use
Put small global and static data in the small data area, and generate
special instructions to reference them.
-Gnum
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 -Gnum 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:
-m88000
Generate code that works well on both the m88100 and the
m88110.
-m88100
Generate code that works best for the m88100, but that also
runs on the m88110.
-m88110
Generate code that works best for the m88110, and may not run
on the m88100.
-mbig-pic
Obsolete option to be removed from the next revision.
Use -fPIC.
-midentify-revision
Include an "ident" directive in the assembler output recording the
source file name, compiler name and version, timestamp, and compilation
flags used.
-mno-underscores
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.
-mocs-debug-info
-mno-ocs-debug-info
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.
-mocs-frame-position
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.
-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.
-moptimize-arg-area
-mno-optimize-arg-area
Control how function arguments are stored in stack frames.
-moptimize-arg-area saves space by optimizing them, but this
conflicts with the 88open specifications. The opposite alternative,
-mno-optimize-arg-area, agrees with 88open standards. By default
GCC does not optimize the argument area.
-mshort-data-num
Generate smaller data references by making them relative to "r0",
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.
-mserialize-volatile
-mno-serialize-volatile
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.
-msvr4
-msvr3
Turn on (-msvr4) or off (-msvr3) compiler extensions
related to System V release 4 (SVr4). This controls the following:
1.
Which variant of the assembler syntax to emit.
2.
-msvr4 makes the C preprocessor recognize #pragma weak
that is used on System V release 4.
3.
-msvr4 makes GCC issue additional declaration directives used in
SVr4.
-msvr4 is the default for the m88k-motorola-sysv4 and
m88k-dg-dgux m88k configurations. -msvr3 is the default for all
other m88k configurations.
-mversion-03.00
This option is obsolete, and is ignored.
-mno-check-zero-division
-mcheck-zero-division
Do, or don't, generate code to guarantee that integer division by
zero will be detected. By default, detection is guaranteed.
Some models of the MC88100 processor fail to trap upon integer
division by zero under certain conditions. By default, when
compiling code that might be run on such a processor, GCC
generates code that explicitly checks for zero-valued divisors
and traps with exception number 503 when one is detected. Use of
mno-check-zero-division suppresses such checking for code
generated to run on an MC88100 processor.
GCC assumes that the MC88110 processor correctly detects all
instances of integer division by zero. When -m88110 is
specified, both -mcheck-zero-division and
-mno-check-zero-division are ignored, and no explicit checks for
zero-valued divisors are generated.
-muse-div-instruction
Use the div instruction for signed integer division on the
MC88100 processor. By default, the div instruction is not used.
On the MC88100 processor the signed integer division instruction
div) traps to the operating system on a negative operand. The
operating system transparently completes the operation, but at a
large cost in execution time. By default, when compiling code
that might be run on an MC88100 processor, GCC emulates signed
integer division using the unsigned integer division instruction
divu), thereby avoiding the large penalty of a trap to the
operating system. Such emulation has its own, smaller, execution
cost in both time and space. To the extent that your code's
important signed integer division operations are performed on two
nonnegative operands, it may be desirable to use the div
instruction directly.
On the MC88110 processor the div instruction (also known as the
divs instruction) processes negative operands without trapping to
the operating system. When -m88110 is specified,
-muse-div-instruction is ignored, and the div instruction is used
for signed integer division.
Note that the result of dividing "INT_MIN" by -1 is undefined. In
particular, the behavior of such a division with and without
-muse-div-instruction may differ.
-mtrap-large-shift
-mhandle-large-shift
Include code to detect bit-shifts of more than 31 bits; respectively,
trap such shifts or emit code to handle them properly. By default GCC
makes no special provision for large bit shifts.
-mwarn-passed-structs
Warn when a function passes a struct as an argument or result.
Structure-passing conventions have changed during the evolution of the C
language, and are often the source of portability problems. By default,
GCC issues no such warning.
IBMRS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
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.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code.
-mnew-mnemonics requests output that uses the assembler mnemonics
defined for the PowerPC architecture, while -mold-mnemonics
requests 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.
-mcpu=cpu_type
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, 750, power, power2,
powerpc, 403, 505, 801, 821,
823, and 860 and common. -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.
Specifying any of the following options:
-mcpu=rios1, -mcpu=rios2, -mcpu=rsc,
-mcpu=power, or -mcpu=power2
enables the -mpower option and disables the -mpowerpc option;
-mcpu=601 enables both the -mpower and -mpowerpc options.
All of -mcpu=rs64a, -mcpu=602, -mcpu=603,
-mcpu=603e, -mcpu=604, -mcpu=620, -mcpu=630,
-mcpu=740, and -mcpu=750
enable the -mpowerpc option and disable the -mpower option.
Exactly similarly, all of -mcpu=403,
-mcpu=505, -mcpu=821, -mcpu=860 and -mcpu=powerpc
enable the -mpowerpc option and disable the -mpower option.
-mcpu=common disables both the
-mpower and -mpowerpc options.
AIX versions 4 or greater selects -mcpu=common by default, so
that code will operate on all members of the RS/6000 POWER and PowerPC
families. In that case, 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. GCC assumes a generic
processor model for scheduling purposes.
Specifying any of the options -mcpu=rios1, -mcpu=rios2,
-mcpu=rsc, -mcpu=power, or -mcpu=power2 also
disables the new-mnemonics option. Specifying -mcpu=601,
-mcpu=602, -mcpu=603, -mcpu=603e, -mcpu=604,
-mcpu=620, -mcpu=630, -mcpu=403, -mcpu=505,
-mcpu=821, -mcpu=860 or -mcpu=powerpc also enables
the new-mnemonics option.
Specifying -mcpu=403, -mcpu=821, or -mcpu=860 also
enables the -msoft-float option.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type, register usage,
choice of mnemonics like -mcpu=cpu_type would. The same
values for cpu_type are used for -mtune=cpu_type as
for -mcpu=cpu_type. The -mtune=cpu_type
option overrides the -mcpu=cpu_type option in terms of
instruction scheduling parameters.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for
every executable file. The -mfull-toc option is selected by
default. In that case, GCC will allocate at least one TOC entry for
each unique non-automatic variable reference in your program. GCC
will also place floating-point constants in the TOC. However, only
16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed
the available TOC space, you can reduce the amount of TOC space used
with the -mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point
constants in the TOC and -mno-sum-in-toc forces GCC to
generate code to calculate the sum of an address and a constant at
run-time instead of putting that sum into the TOC. You may specify one
or both of these options. Each causes GCC to produce very slightly
slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of
these options, specify -mminimal-toc instead. This option causes
GCC to make only one TOC entry for every file. When you specify this
option, GCC will produce code that is slower and larger but which
uses extremely little TOC space. You may wish to use this option
only on files that contain less frequently executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
"long" type, and the infrastructure needed to support them.
Specifying -maix64 implies -mpowerpc64 and
-mpowerpc, while -maix32 disables the 64-bit ABI and
implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-call
-mno-xl-call
On AIX, pass floating-point arguments to prototyped functions beyond the
register save area (RSA) on the stack in addition to argument FPRs. The
AIX calling convention was extended but not initially documented to
handle an obscure K&R C case of calling a function that takes the
address of its arguments with fewer arguments than declared. AIX XL
compilers access floating point arguments which do not fit in the
RSA from the stack when a subroutine is compiled without
optimization. Because always storing floating-point arguments on the
stack is inefficient and rarely needed, this option is not enabled by
default and only is necessary when calling subroutines compiled by AIX
XL compilers without optimization.
-mthreads
Support AIX Threads. Link an application written to use
pthreads with special libraries and startup code to enable the
application to run.
-mpe
Support IBMRS/6000SPParallel Environment (PE). Link an
application written to use message passing with special startup code to
enable the application to run. The system must have PE installed in the
standard location (/usr/lpp/ppe.poe/), or the specs file
must be overridden with the -specs= option to specify the
appropriate directory location. The Parallel Environment does not
support threads, so the -mpe option and the -mthreads
option are incompatible.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set.
Software floating point emulation is provided if you use the
-msoft-float option, and pass the option to GCC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use -mmultiple on little
endian PowerPC systems, since those instructions do not work when the
processor is in little endian mode. The exceptions are PPC740 and
PPC750 which permit the instructions usage in little endian mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions
and the store string word instructions to save multiple registers and
do small block moves. These instructions are generated by default on
POWER systems, and not generated on PowerPC systems. Do not use
-mstring on little endian PowerPC systems, since those
instructions do not work when the processor is in little endian mode.
The exceptions are PPC740 and PPC750 which permit the instructions
usage in little endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions
that update the base register to the address of the calculated memory
location. These instructions are generated by default. If you use
-mno-update, there is a small window between the time that the
stack pointer is updated and the address of the previous frame is
stored, which means code that walks the stack frame across interrupts or
signals may get corrupted data.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and
accumulate instructions. These instructions are generated by default if
hardware floating is used.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures
and unions that contain bit-fields to be aligned to the base type of the
bit-field.
For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 would be aligned to a 4 byte
boundary and have a size of 4 bytes. By using -mno-bit-align,
the structure would be aligned to a 1 byte boundary and be one byte in
size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that
unaligned memory references will be handled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not allow)
the program to be relocated to a different address at runtime. If you
use -mrelocatable on any module, all objects linked together must
be compiled with -mrelocatable or -mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not allow)
the program to be relocated to a different address at runtime. Modules
compiled with -mrelocatable-lib can be linked with either modules
compiled without -mrelocatable and -mrelocatable-lib or
with modules compiled with the -mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that
register 2 contains a pointer to a global area pointing to the addresses
used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in little endian mode. The -mlittle-endian option is
the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in big endian mode. The -mbig-endian option is
the same as -mbig.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling
conventions that adheres to the March 1995 draft of the System V
Application Binary Interface, PowerPC processor supplement. This is the
default unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aix
On System V.4 and embedded PowerPC systems compile code using calling
conventions that are similar to those used on AIX. This is the
default if you configured GCC using powerpc-*-eabiaix.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the Solaris
operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the
NetBSD operating system.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to
variable argument functions are properly prototyped. Otherwise, the
compiler must insert an instruction before every non prototyped call to
set or clear bit 6 of the condition code register (CR) to
indicate whether floating point values were passed in the floating point
registers in case the function takes a variable arguments. With
-mprototype, only calls to prototyped variable argument functions
will set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called
sim-crt0.o and that the standard C libraries are libsim.a and
libc.a. This is the default for powerpc-*-eabisim.
configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called
crt0.o and the standard C libraries are libmvme.a and
libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called
crt0.o and the standard C libraries are libads.a and
libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called
crt0.o and the standard C libraries are libyk.a and
libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are
compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags
header to indicate that eabi extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the
Embedded Applications Binary Interface (eabi) which is a set of
modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8 byte boundary, a function
"__eabi" is called to from "main" to set up the eabi
environment, and the -msdata option can use both "r2" and
"r13" to point to two separate small data areas. Selecting
-mno-eabi means that the stack is aligned to a 16 byte boundary,
do not call an initialization function from "main", and the
-msdata option will only use "r13" to point to a single
small data area. The -meabi option is on by default if you
configured GCC using one of the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized
"const" global and static data in the .sdata2 section, which
is pointed to by register "r2". Put small initialized
non-"const" global and static data in the .sdata section,
which is pointed to by register "r13". Put small uninitialized
global and static data in the .sbss section, which is adjacent to
the .sdata section. The -msdata=eabi option is
incompatible with the -mrelocatable option. The
-msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static
data in the .sdata section, which is pointed to by register
"r13". Put small uninitialized global and static data in the
.sbss section, which is adjacent to the .sdata section.
The -msdata=sysv option is incompatible with the
-mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used,
compile code the same as -msdata=eabi, otherwise compile code the
same as -msdata=sysv.
-msdata-data
On System V.4 and embedded PowerPC systems, put small global and static
data in the .sdata section. Put small uninitialized global and
static data in the .sbss section. Do not use register "r13"
to address small data however. This is the default behavior unless
other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data
in the .data section, and all uninitialized data in the
.bss section.
-Gnum
On embedded PowerPC systems, 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. By default, num is 8. The
-Gnum switch is also passed to the linker.
All modules should be compiled with the same -Gnum value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register
names in the assembly language output using symbolic forms.
IBMRT Options
These -m options are defined for the IBM RT PC:
-min-line-mul
Use an in-line code sequence for integer multiplies. This is the
default.
-mcall-lib-mul
Call "lmul$$" for integer multiples.
-mfull-fp-blocks
Generate full-size floating point data blocks, including the minimum
amount of scratch space recommended by IBM. This is the default.
-mminimum-fp-blocks
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.
-mfp-arg-in-fpregs
Use a calling sequence incompatible with the IBM calling convention in
which floating point arguments are passed in floating point registers.
Note that "varargs.h" and "stdarg.h" will not work with
floating point operands if this option is specified.
-mfp-arg-in-gregs
Use the normal calling convention for floating point arguments. This is
the default.
-mhc-struct-return
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).
-mnohc-struct-return
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:
-mcpu=cpu-type
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.
-mips1
Issue instructions from level 1 of the MIPS ISA. This is the default.
r3000 is the default cpu-type at this ISA level.
-mips2
Issue instructions from level 2 of the MIPS ISA (branch likely, square
root instructions). r6000 is the default cpu-type at this
ISA level.
-mips3
Issue instructions from level 3 of the MIPS ISA (64-bit instructions).
r4000 is the default cpu-type at this ISA level.
-mips4
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.
-mfp32
Assume that 32 32-bit floating point registers are available. This is
the default.
-mfp64
Assume that 32 64-bit floating point registers are available. This is
the default when the -mips3 option is used.
-mgp32
Assume that 32 32-bit general purpose registers are available. This is
the default.
-mgp64
Assume that 32 64-bit general purpose registers are available. This is
the default when the -mips3 option is used.
-mint64
Force int and long types to be 64 bits wide. See -mlong32 for an
explanation of the default, and the width of pointers.
-mlong64
Force long types to be 64 bits wide. See -mlong32 for an
explanation of the default, and the width of pointers.
-mlong32
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).
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
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.
-mmips-as
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.
-mgas
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.
-msplit-addresses
-mno-split-addresses
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.
-mrnames
-mno-rnames
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.
-mgpopt
-mno-gpopt
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.
-mstats
-mno-stats
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.).
-mmemcpy
-mno-memcpy
The -mmemcpy switch makes all block moves call the appropriate
string function (memcpy or bcopy) instead of possibly
generating inline code.
-mmips-tfile
-mno-mips-tfile
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.
-msoft-float
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.
-mhard-float
Generate output containing floating point instructions. This is the
default if you use the unmodified sources.
-mabicalls
-mno-abicalls
Emit (or do not emit) the pseudo operations .abicalls,
.cpload, and .cprestore that some System V.4 ports use for
position independent code.
-mlong-calls
-mno-long-calls
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.
-mhalf-pic
-mno-half-pic
Put pointers to extern references into the data section and load them
up, rather than put the references in the text section.
-membedded-pic
-mno-embedded-pic
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.
-membedded-data
-mno-embedded-data
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.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
When used together with -membedded-data, it will always store uninitialized
const variables in the read-only data section.
-msingle-float
-mdouble-float
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.
-mmad
-mno-mad
Permit use of the mad, madu and mul instructions,
as on the r4650 chip.
-m4650
Turns on -msingle-float, -mmad, and, at least for now,
-mcpu=r4650.
-mips16
-mno-mips16
Enable 16-bit instructions.
-mentry
Use the entry and exit pseudo ops. This option can only be used with
-mips16.
-EL
Compile code for the processor in little endian mode.
The requisite libraries are assumed to exist.
-EB
Compile code for the processor in big endian mode.
The requisite libraries are assumed to exist.
-Gnum
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
-Gnum switch is also passed to the assembler and linker.
All modules should be compiled with the same -Gnum
value.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a .s suffix) when assembling them.
-mfix7000
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.
-no-crt0
Do not include the default crt0.
Intel 386 Options
These -m options are defined for the i386 family of computers:
-mcpu=cpu-type
Assume the defaults for the machine type cpu-type when scheduling
instructions. The choices for cpu-type are i386,
i486, i586, i686, pentium,
pentiumpro, k6, and athlon
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 is the AMD chip as
opposed to the Intel ones.
-march=cpu-type
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.
-m386
-m486
-mpentium
-mpentiumpro
Synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pentium, and -mcpu=pentiumpro
respectively. These synonyms are deprecated.
-mintel-syntax
Emit assembly using Intel syntax opcodes instead of AT&T syntax.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point
comparisons. These handle correctly the case where the result of a
comparison is unordered.
-msoft-float
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.
On machines where a function returns floating point results in the 80387
register stack, some floating point opcodes may be emitted even if
-msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types
"float" and "double" in an FPU register, even if there
is no FPU. The idea is that the operating system should emulate
an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned
in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and
"sqrt" instructions for the 387. Specify this option to avoid
generating those instructions. This option is the default on FreeBSD.
As of revision 2.6.1, these instructions are not generated unless you
also use the -ffast-math switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and
"long long" variables on a two word boundary or a one word
boundary. Aligning "double" variables on a two word boundary will
produce code that runs somewhat faster on a Pentium at the
expense of more memory.
-m128bit-long-double
Control the size of "long double" type. i386 application binary interface
specify the size to be 12 bytes, while modern architectures (Pentium and newer)
prefer "long double" aligned to 8 or 16 byte boundary. This is
impossible to reach with 12 byte long doubles in the array accesses.
Warning: if you use the -m128bit-long-double switch, the
structures and arrays containing "long double" will change their size as
well as function calling convention for function taking "long double"
will be modified.
-m96bit-long-double
Set the size of "long double" to 96 bits as required by the i386
application binary interface. This is the default.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized locals into "bss" or
"data". -msvr3-shlib places these locals into "bss".
These options are meaningful only on System V Release 3.
-mrtd
Use a different function-calling convention, in which functions that
take a fixed number of arguments return with the "ret"num
instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments
there.
You can specify that an individual function is called with this calling
sequence with the function attribute stdcall. You can also
override the -mrtd option by using the function attribute
cdecl.
Warning: 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 "printf");
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.)
-mreg-alloc=regs
Control the default allocation order of integer registers. The
string regs is a series of letters specifying a register. The
supported letters are: "a" allocate EAX; "b" allocate EBX;
"c" allocate ECX; "d" allocate EDX; "S" allocate ESI;
"D" allocate EDI; "B" allocate EBP.
This option is deprecated and will not be supported by future releases
of gcc.
-mregparm=num
Control how many registers are used to pass integer arguments. By
default, no registers are used to pass arguments, and at most 3
registers can be used. You can control this behavior for a specific
function by using the function attribute regparm.
Warning: if you use this switch, and
num is nonzero, then you must build all modules with the same
value, including any libraries. This includes the system libraries and
startup modules.
-malign-loops=num
Align loops to a 2 raised to a num byte boundary. If
-malign-loops is not specified, the default is 2 unless
gas 2.8 (or later) is being used in which case the default is
to align the loop on a 16 byte boundary if it is less than 8
bytes away.
-malign-jumps=num
Align instructions that are only jumped to to a 2 raised to a num
byte boundary. If -malign-jumps is not specified, the default is
2 if optimizing for a 386, and 4 if optimizing for a 486 unless
gas 2.8 (or later) is being used in which case the default is
to align the instruction on a 16 byte boundary if it is less
than 8 bytes away.
-malign-functions=num
Align the start of functions to a 2 raised to num byte boundary.
If -malign-functions is not specified, the default is 2 if optimizing
for a 386, and 4 if optimizing for a 486.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num
byte boundary. If -mpreferred-stack-boundary is not specified,
the default is 4 (16 bytes or 128 bits).
The stack is required to be aligned on a 4 byte boundary. On Pentium
and PentiumPro, "double" and "long double" values should be
aligned to an 8 byte boundary (see -malign-double) or suffer
significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" suffers similar
penalties if it is not 16 byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary
must be as aligned as that required by any value stored on the stack.
Further, every function must be generated such that it keeps the stack
aligned. Thus calling a function compiled with a higher preferred
stack boundary from a function compiled with a lower preferred stack
boundary will most likely misalign the stack. It is recommended that
libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space. Code that is sensitive
to stack space usage, such as embedded systems and operating system kernels,
may want to reduce the preferred alignment to
-mpreferred-stack-boundary=2.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter
and usually equally fast as method using SUB/MOV operations and is enabled
by default. In some cases disabling it may improve performance because of
improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments will be
computed in the function prologue. This is faster on most modern CPUs
because of reduced dependencies, improved scheduling and reduced stack usage
when preferred stack boundary is not equal to 2. The drawback is a notable
increase in code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on Mingw32. Code that relies
on thread-safe exception handling must compile and link all code with the
-mthreads option. When compiling, -mthreads defines
-D_MT; when linking, it links in a special thread helper library
-lmingwthrd which cleans up per thread exception handling data.
-mno-align-stringops
Do not align destination of inlined string operations. This switch reduces
code size and improves performance in case the destination is already aligned,
but gcc don't know about it.
-minline-all-stringops
By default GCC inlines string operations only when destination is known to be
aligned at least to 4 byte boundary. This enables more inlining, increase code
size, but may improve performance of code that depends on fast memcpy, strlen
and memset for short lengths.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This
avoids the instructions to save, set up and restore frame pointers and
makes an extra register available in leaf functions. The option
-fomit-frame-pointer removes the frame pointer for all functions
which might make debugging harder.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
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.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-mbig-switch
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.
-mjump-in-delay
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.
-mdisable-fpregs
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.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some
rather obscure problems when compiling MIG generated code under MACH.
-mno-space-regs
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.
-mfast-indirect-calls
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.
-mlong-load-store
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.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type
cpu-type. The choices for cpu-type are 7007100, 7100LC, 7200, and 8000. Refer to
/usr/lib/sched.models on an HP-UX system to determine the
proper scheduling option for your machine.
-mlinker-opt
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.
-msoft-float
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:
-mcpu-type
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.
-mnumerics
-msoft-float
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.
-mleaf-procedures
-mno-leaf-procedures
Do (or do not) attempt to alter leaf procedures to be callable with the
"bal" instruction as well as "call". This will result in more
efficient code for explicit calls when the "bal" 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.
-mtail-call
-mno-tail-call
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.
-mcomplex-addr
-mno-complex-addr
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.
-mcode-align
-mno-code-align
Align code to 8-byte boundaries for faster fetching (or don't bother).
Currently turned on by default for C-series implementations only.
-mic-compat
-mic2.0-compat
-mic3.0-compat
Enable compatibility with iC960 v2.0 or v3.0.
-masm-compat
-mintel-asm
Enable compatibility with the iC960 assembler.
-mstrict-align
-mno-strict-align
Do not permit (do permit) unaligned accesses.
-mold-align
Enable structure-alignment compatibility with Intel's gcc release version
1.3 (based on gcc 1.37). This option implies -mstrict-align.
-mlong-double-64
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:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in libgcc1.c 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.
-mfp-reg
-mno-fp-regs
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 $0 instead of $f0. 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.
-mieee
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 CPP macro "_IEEE_FP" is defined
during compilation. The option is a shorthand for: -D_IEEE_FP
-mfp-trap-mode=su -mtrap-precision=i -mieee-conformant. 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.
-mieee-with-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. The option is a shorthand
for -D_IEEE_FP -D_IEEE_FP_INEXACT plus the three following:
-mieee-conformant,
-mfp-trap-mode=sui,
and -mtrap-precision=i.
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.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled.
Other Alpha compilers call this option -fptmtrap-mode.
The trap mode can be set to one of four values:
n
This is the default (normal) setting. The only traps that are enabled
are the ones that cannot be disabled in software (e.g., division by zero
trap).
u
In addition to the traps enabled by n, underflow traps are enabled
as well.
su
Like su, but the instructions are marked to be safe for software
completion (see Alpha architecture manual for details).
sui
Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option
-fprmrounding-mode. The rounding-mode can be one
of:
n
Normal IEEE rounding mode. Floating point numbers are rounded towards
the nearest machine number or towards the even machine number in case
of a tie.
m
Round towards minus infinity.
c
Chopped rounding mode. Floating point numbers are rounded towards zero.
d
Dynamic rounding mode. A field in the floating point control register
(fpcr, see Alpha architecture reference manual) controls the
rounding mode in effect. The C library initializes this register for
rounding towards plus infinity. Thus, unless your program modifies the
fpcr, d corresponds to round towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating point traps are imprecise. This
means without software assistance it is impossible to recover from a
floating trap and program execution normally needs to be terminated.
GCC can generate code that can assist operating system trap handlers
in determining the exact location that caused a floating point trap.
Depending on the requirements of an application, different levels of
precisions can be selected:
p
Program precision. This option is the default and means a trap handler
can only identify which program caused a floating point exception.
f
Function precision. The trap handler can determine the function that
caused a floating point exception.
i
Instruction precision. The trap handler can determine the exact
instruction that caused a floating point exception.
Other Alpha compilers provide the equivalent options called
-scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must not
use this option unless you also specify -mtrap-precision=i and either
-mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect
is to emit the line .eflag 48 in the function prologue of the
generated assembly file. Under DEC Unix, this has the effect that
IEEE-conformant math library routines will be linked in.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to
see if it can construct it from smaller constants in two or three
instructions. If it cannot, it will output the constant as a literal and
generate code to load it from the data segment at runtime.
Use this option to require GCC to construct all integer constants
using code, even if it takes more instructions (the maximum is six).
You would typically use this option to build a shared library dynamic
loader. Itself a shared library, it must relocate itself in memory
before it can find the variables and constants in its own data segment.
-malpha-as
-mgas
Select whether to generate code to be assembled by the vendor-supplied
assembler (-malpha-as) or by the GNU assembler -mgas.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX,
CIX, and MAX instruction sets. The default is to use the instruction sets
supported by the CPU type specified via -mcpu= option or that
of the CPU on which GCC was built if none was specified.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. You can specify either the
EV style name or the corresponding chip number. GCC
supports scheduling parameters for the EV4 and EV5 family of processors
and will choose the default values for the instruction set from
the processor you specify. If you do not specify a processor type,
GCC will default to the processor on which the compiler was built.
Supported values for cpu_type are
ev4
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV5 (until Digital releases the scheduling parameters
for the EV6) and supports the BWX, CIX, and MAX extensions.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory
references as seen by the application. This number is highly
dependent on the memory access patterns used by the application
and the size of the external cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for
``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
(also called Dcache, Scache, and Bcache), as well as to main memory.
Note that L3 is only valid for EV5.
Clipper Options
These -m options are defined for the Clipper implementations:
-mc300
Produce code for a C300 Clipper processor. This is the default.
-mc400
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:
-mrelax
Shorten some address references at link time, when possible; uses the
linker option -relax.
-mh
Generate code for the H8/300H.
-ms
Generate code for the H8/S.
-ms2600
Generate code for the H8/S2600. This switch must be used with -ms.
-mint32
Make "int" data 32 bits by default.
-malign-300
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:
-m1
Generate code for the SH1.
-m2
Generate code for the SH2.
-m3
Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only
supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in
single-precision mode by default.
-m4
Generate code for the SH4.
-mb
Compile code for the processor in big endian mode.
-ml
Compile code for the processor in little endian mode.
-mdalign
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.
-mrelax
Shorten some address references at link time, when possible; uses the
linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use
16-bit offsets.
-mfmovd
Enable the use of the instruction "fmovd".
-mhitachi
Comply with the calling conventions defined by Hitachi.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if
-mhitachi is given.
-mieee
Increase IEEE-compliance of floating-point code.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes,
which is incompatible with the SH ABI.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot
When generating position-independent code, emit function calls using
the Global Offset Table instead of the Procedure Linkage Table.
-musermode
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 "sh-*-linux*".
Options for System V
These additional options are available on System V Release 4 for
compatibility with other compilers on those systems:
-G
Create a shared object.
It is recommended that -symbolic or -shared be used instead.
-Qy
Identify the versions of each tool used by the compiler, in a
".ident" assembler directive in the output.
-Qn
Refrain from adding ".ident" directives to the output file (this is
the default).
-YP,dirs
Search the directories dirs, and no others, for libraries
specified with -l.
-Ym,dir
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:
-mcpu=cpu_type
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.
-mbig-memory
-mbig
-msmall-memory
-msmall
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.
-mbk
-mno-bk
Allow (disallow) allocation of general integer operands into the block
count register BK.
-mdb
-mno-db
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.
-mdp-isr-reload
-mparanoid
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.
-mmpyi
-mno-mpyi
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.
-mfast-fix
-mno-fast-fix
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.
-mrptb
-mno-rptb
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.
-mrpts=count
-mno-rpts
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.
-mloop-unsigned
-mno-loop-unsigned
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.
-mti
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.
-mregparm
-mmemparm
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.
-mparallel-insns
-mno-parallel-insns
Allow the generation of parallel instructions. This is enabled by
default with -O2.
-mparallel-mpy
-mno-parallel-mpy
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:
-mlong-calls
-mno-long-calls
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.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index
pointer 4 or more times to copy pointer into the "ep" register, and
use the shorter "sld" and "sst" instructions. The -mep
option is on by default if you optimize.
-mno-prolog-function
-mprolog-function
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.
-mspace
Try to make the code as small as possible. At present, this just turns
on the -mep and -mprolog-function options.
-mtda=n
Put static or global variables whose size is n bytes or less into
the tiny data area that register "ep" points to. The tiny data
area can hold up to 256 bytes in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes or less into
the small data area that register "gp" points to. The small data
area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into
the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mbig-switch
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:
-EL
Compile code for little endian mode. This is the default.
-EB
Compile code for big endian mode.
-mmangle-cpu
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.
-mcpu=cpu
Compile code for ARC variant cpu.
Which variants are supported depend on the configuration.
All variants support -mcpu=base, this is the default.
-mtext=text-section
-mdata=data-section
-mrodata=readonly-data-section
Put functions, data, and readonly data in text-section,
data-section, and readonly-data-section respectively
by default. This can be overridden with the "section" 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.
-m32032
-m32032
Generate output for a 32032. This is the default
when the compiler is configured for 32032 and 32016 based systems.
-m32332
-m32332
Generate output for a 32332. This is the default
when the compiler is configured for 32332-based systems.
-m32532
-m32532
Generate output for a 32532. This is the default
when the compiler is configured for 32532-based systems.
-m32081
Generate output containing 32081 instructions for floating point.
This is the default for all systems.
-m32381
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.
-mmulti-add
Try and generate multiply-add floating point instructions "polyF"
and "dotF". 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.
-mnomulti-add
Do not try and generate multiply-add floating point instructions
"polyF" and "dotF". This is the default on all platforms.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries may not be available.
-mnobitfield
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.
-mbitfield
Do use the bit-field instructions. This is the default for all platforms
except the pc532.
-mrtd
Use a different function-calling convention, in which functions
that take a fixed number of arguments return pop their
arguments on return with the "ret" 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 "printf");
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 "rtd" instruction.
-mregparam
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.
-mnoregparam
Do not pass any arguments in registers. This is the default for all
targets.
-msb
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.
-mnosb
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.
-mhimem
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.
-mnohimem
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:
-mmcu=mcu
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).
Instruction set avr4 is for the enhanced AVR core with up to 8K program
memory space (MCU types: atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with up to 128K program
memory space (MCU types: atmega161, atmega163, atmega32, at94k).
-msize
Output instruction sizes to the asm file.
-minit-stack=N
Specify the initial stack address, which may be a symbol or numeric value,
__stack is the default.
-mno-interrupts
Generated code is not compatible with hardware interrupts.
Code size will be smaller.
-mcall-prologues
Functions prologues/epilogues expanded as call to appropriate
subroutines. Code size will be smaller.
-mno-tablejump
Do not generate tablejump insns which sometimes increase code size.
-mtiny-stack
Change only the low 8 bits of the stack pointer.
MCore Options
These are the -m options defined for the Motorola M*Core
processors.
-mhardlit
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two
instructions or less.
-mdiv
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary sized immediates in bit operations.
-mwide-bitfields
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as int-sized.
-m4byte-functions
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a four byte boundary.
-mcallgraph-data
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mlittle-endian
-mbig-endian
Generate code for a little endian target.
-m210
-m210
-m340
Generate code for the 210 processor.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big endian target. This is the default for HPUX.
-mlittle-endian
Generate code for a little endian target. This is the default for AIX5
and Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The result
is not position independent code, and violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after volatile asm
statements.
-mb-step
Generate code that works around Itanium B step errata.
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for
the stacked registers. This may make assembler output more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may
be useful for working around optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This is
useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp.
This is useful when compiling firmware code.
-minline-divide-min-latency
Generate code for inline divides using the minimum latency algorithm.
-minline-divide-max-throughput
Generate code for inline divides using the maximum throughput algorithm.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF2 line number debugging
info. This may be useful when not using the GNU assembler.
-mfixed-range=register-range
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:
-mextmem
Link the .text, .data, .bss, .strings,
.rodata, .rodata1, .data1 sections into external
memory, which starts at location 0x80000000.
-mextmemory
Same as the -mextmem switch.
-monchip
Link the .text section into onchip text memory, which starts at
location 0x0. Also link .data, .bss,
.strings, .rodata, .rodata1, .data1 sections
into onchip data memory, which starts at location 0x20000000.
-mno-asm-optimize
-masm-optimize
Disable (enable) passing -O to the assembler when optimizing.
The assembler uses the -O option to automatically parallelize
adjacent short instructions where possible.
-mbranch-cost=n
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.
-mcond-exec=n
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.
-mhard-float
-msoft-float
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.
-mbackchain
-mno-backchain
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.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction
to do subroutine calls.
This only works reliably if the total executable size does not
exceed 64k. The default is to use the "basr" instruction instead,
which does not have this limitation.
-m64
-m31
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.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction
to perform block moves. When -mno-mvcle is specifed,
use a "mvc" loop instead. This is the default.
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling.
The default is to not print debug information.
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.
-mbig-endian
-mlittle-endian
Specify big-endian or little-endian byte ordering for the target Xtensa
processor.
-mdensity
-mno-density
Enable or disable use of the optional Xtensa code density instructions.
-mmac16
-mno-mac16
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.
-mmul16
-mno-mul16
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.
-mmul32
-mno-mul32
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.
-mnsa
-mno-nsa
Enable or disable use of the optional normalization shift amount
("NSA") instructions to implement the built-in "ffs" function.
-mminmax
-mno-minmax
Enable or disable use of the optional minimum and maximum value
instructions.
-msext
-mno-sext
Enable or disable use of the optional sign extend ("SEXT")
instruction.
-mbooleans
-mno-booleans
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).
-mhard-float
-msoft-float
Enable or disable use of the floating-point option. When enabled, GCC
generates floating-point instructions for 32-bit "float"
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 "double" operations are always
emulated with calls to library functions.
-mfused-madd
-mno-fused-madd
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.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions before
"volatile" memory references to guarantee sequential consistency.
The default is -mserialize-volatile. Use
-mno-serialize-volatile to omit the "MEMW" instructions.
-mtext-section-literals
-mno-text-section-literals
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.
-mtarget-align
-mno-target-align
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 "LOOP", which the
assembler will always align, either by widening density instructions or
by inserting no-op instructions.
-mlongcalls
-mno-longcalls
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 "CALL"
instruction into an "L32R" followed by a "CALLX" 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.
Options for Code Generation Conventions
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.
-fexceptions
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.
-fnon-call-exceptions
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 "SIGALRM".
-funwind-tables
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.
-fpcc-struct-return
Return ``short'' "struct" and "union" 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.
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.
-freg-struct-return
Use the convention that "struct" and "union" values are
returned in registers when possible. This is more efficient for small
structures than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor its contrary
-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.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the
declared range of possible values. Specifically, the "enum" type
will be equivalent to the smallest integer type which has enough room.
-fshort-double
Use the same size for "double" as for "float".
-fshared-data
Requests that the data and non-"const" 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.
-fno-common
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 "extern") 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.
-fno-ident
Ignore the #ident directive.
-fno-gnu-linker
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.
-finhibit-size-directive
Don't output a ".size" 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.
-fverbose-asm
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.
-fvolatile
Consider all memory references through pointers to be volatile.
-fvolatile-global
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.
-fvolatile-static
Consider all memory references to static data to be volatile.
-fpic
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.
-fPIC
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.
-ffixed-reg
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 "REGISTER_NAMES"
macro in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
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.
-fcall-saved-reg
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.
-fpack-struct
Pack all structure members together without holes. Usually you would
not want to use this option, since it makes the code suboptimal, and
the offsets of structure members won't agree with system libraries.
-fcheck-memory-usage
Generate extra code to check each memory access. GCC will generate
code that is suitable for a detector of bad memory accesses such as
Checker.
Normally, you should compile all, or none, of your code with this option.
If you do mix code compiled with and without this option,
you must ensure that all code that has side effects
and that is called by code compiled with this option
is, itself, compiled with this option.
If you do not, you might get erroneous messages from the detector.
If you use functions from a library that have side-effects (such as
"read"), you might not be able to recompile the library and
specify this option. In that case, you can enable the
-fprefix-function-name option, which requests GCC to encapsulate
your code and make other functions look as if they were compiled with
-fcheck-memory-usage. This is done by calling ``stubs'',
which are provided by the detector. If you cannot find or build
stubs for every function you call, you might have to specify
-fcheck-memory-usage without -fprefix-function-name.
If you specify this option, you can not use the "asm" or
"__asm__" keywords in functions with memory checking enabled. GCC
cannot understand what the "asm" statement may do, and therefore
cannot generate the appropriate code, so it will reject it. However, if
you specify the function attribute "no_check_memory_usage", GCC will disable memory checking within a
function; you may use "asm" statements inside such functions. You
may have an inline expansion of a non-checked function within a checked
function; in that case GCC will not generate checks for the inlined
function's memory accesses.
If you move your "asm" statements to non-checked inline functions
and they do access memory, you can add calls to the support code in your
inline function, to indicate any reads, writes, or copies being done.
These calls would be similar to those done in the stubs described above.
-fprefix-function-name
Request GCC to add a prefix to the symbols generated for function names.
GCC adds a prefix to the names of functions defined as well as
functions called. Code compiled with this option and code compiled
without the option can't be linked together, unless stubs are used.
If you compile the following code with -fprefix-function-name
extern void bar (int);
void
foo (int a)
{
return bar (a + 5);
}
GCC will compile the code as if it was written:
extern void prefix_bar (int);
void
prefix_foo (int a)
{
return prefix_bar (a + 5);
}
This option is designed to be used with -fcheck-memory-usage.
-finstrument-functions
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,
"__builtin_return_address" does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
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 "no_instrument_function", 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).
-fstack-check
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.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
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 address 0x80000000 and grows
downwards you can use the flags
-fstack-limit-symbol=__stack_limit
-Wl,--defsym,__stack_limit=0x7ffe0000 which will enforce a stack
limit of 128K.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
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.
-fleading-underscore
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.
Be warned that you should know what you are doing when invoking this
option, and that not all targets provide complete support for it.
ENVIRONMENT
This section describes several environment variables that affect how GCC
operates. Some of them work by specifying directories or prefixes to use
when searching for various kinds of files. Some are used to specify other
aspects of the compilation environment.
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.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
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.
TMPDIR
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.
GCC_EXEC_PREFIX
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 "prefix" 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.
COMPILER_PATH
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.
LIBRARY_PATH
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).
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
These environment variables pertain to particular languages. Each
variable's value is a colon-separated list of directories, much like
PATH. When GCC searches for header files, it tries the
directories listed in the variable for the language you are using, after
the directories specified with -I but before the standard header
file directories.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies
for Make based on the header files processed by the compiler. This
output looks much like the output from the -M option, but it goes to a separate file, and is
in addition to the usual results of compilation.
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
filetarget, in which case the rules are written to
file file using target as the target name.
LANG
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:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the
compiler will use mblen and mbtowc as defined by the default locale to
recognize and translate multibyte characters.
BUGS
For instructions on reporting bugs, see
<http://gcc.gnu.org/bugs.html>. Use of the gccbug
script to report bugs is recommended.
FOOTNOTES
1.
On some systems, gcc -shared
needs to build supplementary stub code for constructors to work. On
multi-libbed systems, gcc -shared must select the correct support
libraries to link against. Failing to supply the correct flags may lead
to subtle defects. Supplying them in cases where they are not necessary
is innocuous.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'' and ``Funding
Free Software'', the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below). A copy of the license is
included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.