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 generation of frame unwind
information for all functions. This can produce significant data size
overhead, although it does not affect execution.
If you do not specify this option, it is enabled by
default for languages like C++ which normally require exception handling,
and disabled for languages like C that do not normally require it.
However, when compiling C code that needs to interoperate properly with
exception handlers written in C++, you may need to enable this option.
You may also wish to disable this option is you are compiling older C++
programs that don't use exception handling.
-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
Allocate even uninitialized global variables in the bss 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. The
compiler cannot understand what the asm statement will do, and
therefore cannot generate the appropriate code, so it is rejected.
However, the function attribute no_check_memory_usage will
disable memory checking within a function, and asm statements can
be put inside such functions. Inline expansion of a non-checked
function within a checked function is permitted; the inline function's
memory accesses won't be checked, but the rest will.
If you move your asm statements to non-checked inline functions,
but 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.
-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.