These function attributes are supported by the x86 back end:
cdeclcdecl attribute causes the compiler to
assume that the calling function pops off the stack space used to
pass arguments. This is
useful to override the effects of the -mrtd switch.
fastcallfastcall attribute causes the compiler to
pass the first argument (if of integral type) in the register ECX and
the second argument (if of integral type) in the register EDX. Subsequent
and other typed arguments are passed on the stack. The called function
pops the arguments off the stack. If the number of arguments is variable all
arguments are pushed on the stack.
thiscallthiscall attribute causes the compiler to
pass the first argument (if of integral type) in the register ECX.
Subsequent and other typed arguments are passed on the stack. The called
function pops the arguments off the stack.
If the number of arguments is variable all arguments are pushed on the
stack.
The thiscall attribute is intended for C++ non-static member functions.
As a GCC extension, this calling convention can be used for C functions
and for static member methods.
ms_abisysv_abims_abi attribute tells the compiler to use the Microsoft ABI,
while the sysv_abi attribute tells the compiler to use the ABI
used on GNU/Linux and other systems. The default is to use the Microsoft ABI
when targeting Windows. On all other systems, the default is the x86/AMD ABI.
Note, the ms_abi attribute for Microsoft Windows 64-bit targets currently
requires the -maccumulate-outgoing-args option.
callee_pop_aggregate_return (number)The default x86-32 ABI assumes that the callee pops the
stack for hidden pointer. However, on x86-32 Microsoft Windows targets,
the compiler assumes that the
caller pops the stack for hidden pointer.
ms_hook_prologuenakedasm statements can safely be included in naked functions
(see Basic Asm). While using extended asm or a mixture of
basic asm and C code may appear to work, they cannot be
depended upon to work reliably and are not supported.
regparm (number)regparm attribute causes the compiler to
pass arguments number one to number if they are of integral type
in registers EAX, EDX, and ECX instead of on the stack. Functions that
take a variable number of arguments continue to be passed all of their
arguments on the stack.
Beware that on some ELF systems this attribute is unsuitable for
global functions in shared libraries with lazy binding (which is the
default). Lazy binding sends the first call via resolving code in
the loader, which might assume EAX, EDX and ECX can be clobbered, as
per the standard calling conventions. Solaris 8 is affected by this.
Systems with the GNU C Library version 2.1 or higher
and FreeBSD are believed to be
safe since the loaders there save EAX, EDX and ECX. (Lazy binding can be
disabled with the linker or the loader if desired, to avoid the
problem.)
sseregparmsseregparm attribute
causes the compiler to pass up to 3 floating-point arguments in
SSE registers instead of on the stack. Functions that take a
variable number of arguments continue to pass all of their
floating-point arguments on the stack.
force_align_arg_pointerforce_align_arg_pointer attribute may be
applied to individual function definitions, generating an alternate
prologue and epilogue that realigns the run-time stack if necessary.
This supports mixing legacy codes that run with a 4-byte aligned stack
with modern codes that keep a 16-byte stack for SSE compatibility.
stdcallstdcall attribute causes the compiler to
assume that the called function pops off the stack space used to
pass arguments, unless it takes a variable number of arguments.
no_caller_saved_registersno_caller_saved_registers attribute.
interruptIRET instruction, instead of the
RET instruction, is used to return from interrupt handlers. All
registers, except for the EFLAGS register which is restored by the
IRET instruction, are preserved by the compiler. Since GCC
doesn't preserve MPX, SSE, MMX nor x87 states, the GCC option
-mgeneral-regs-only should be used to compile interrupt and
exception handlers.
Any interruptible-without-stack-switch code must be compiled with -mno-red-zone since interrupt handlers can and will, because of the hardware design, touch the red zone.
An interrupt handler must be declared with a mandatory pointer argument:
struct interrupt_frame;
__attribute__ ((interrupt))
void
f (struct interrupt_frame *frame)
{
}
and you must define struct interrupt_frame as described in the
processor's manual.
Exception handlers differ from interrupt handlers because the system
pushes an error code on the stack. An exception handler declaration is
similar to that for an interrupt handler, but with a different mandatory
function signature. The compiler arranges to pop the error code off the
stack before the IRET instruction.
#ifdef __x86_64__
typedef unsigned long long int uword_t;
#else
typedef unsigned int uword_t;
#endif
struct interrupt_frame;
__attribute__ ((interrupt))
void
f (struct interrupt_frame *frame, uword_t error_code)
{
...
}
Exception handlers should only be used for exceptions that push an error
code; you should use an interrupt handler in other cases. The system
will crash if the wrong kind of handler is used.
target (options)On the x86, the following options are allowed:
sin, cos, and
sqrt instructions on the 387 floating-point unit.
target("fpmath=sse,387") option as
target("fpmath=sse+387") because the comma would separate
different options.
indirect_branch attribute causes the compiler
to convert indirect call and jump with choice. ‘keep’
keeps indirect call and jump unmodified. ‘thunk’ converts indirect
call and jump to call and return thunk. ‘thunk-inline’ converts
indirect call and jump to inlined call and return thunk.
‘thunk-extern’ converts indirect call and jump to external call
and return thunk provided in a separate object file.
function_return attribute causes the compiler
to convert function return with choice. ‘keep’ keeps function
return unmodified. ‘thunk’ converts function return to call and
return thunk. ‘thunk-inline’ converts function return to inlined
call and return thunk. ‘thunk-extern’ converts function return to
external call and return thunk provided in a separate object file.
nocf_check attribute on a function is used to inform the
compiler that the function's prologue should not be instrumented when
compiled with the -fcf-protection=branch option. The
compiler assumes that the function's address is a valid target for a
control-flow transfer.
The nocf_check attribute on a type of pointer to function is
used to inform the compiler that a call through the pointer should
not be instrumented when compiled with the
-fcf-protection=branch option. The compiler assumes
that the function's address from the pointer is a valid target for
a control-flow transfer. A direct function call through a function
name is assumed to be a safe call thus direct calls are not
instrumented by the compiler.
The nocf_check attribute is applied to an object's type.
In case of assignment of a function address or a function pointer to
another pointer, the attribute is not carried over from the right-hand
object's type; the type of left-hand object stays unchanged. The
compiler checks for nocf_check attribute mismatch and reports
a warning in case of mismatch.
{
int foo (void) __attribute__(nocf_check);
void (*foo1)(void) __attribute__(nocf_check);
void (*foo2)(void);
/* foo's address is assumed to be valid. */
int
foo (void)
/* This call site is not checked for control-flow
validity. */
(*foo1)();
/* A warning is issued about attribute mismatch. */
foo1 = foo2;
/* This call site is still not checked. */
(*foo1)();
/* This call site is checked. */
(*foo2)();
/* A warning is issued about attribute mismatch. */
foo2 = foo1;
/* This call site is still checked. */
(*foo2)();
return 0;
}
On the x86, the inliner does not inline a
function that has different target options than the caller, unless the
callee has a subset of the target options of the caller. For example
a function declared with target("sse3") can inline a function
with target("sse2"), since -msse3 implies -msse2.