Question

to use cas, gcc provides some useful functions such as
__sync_bool_compare_and_swap

but we can also use asm code like cmpxchg

bool  ret;
__asm__ __volatile__(
    "lock cmpxchg16b %1;\n"
    "sete %0;\n"
    :"=m"(ret),"+m" (*(volatile pointer_t *) (addr))
    :"a" (old_value.ptr), "d" (old_value.tag), "b" (new_value.ptr), "c" (new_value.tag));
return ret;

I have grep the source code of gcc 4.6.3, and found that __sync_bool_compare_and_swap is implemented use

typedef int (__kernel_cmpxchg_t) (int oldval, int newval, int *ptr); 
#define __kernel_cmpxchg (*(__kernel_cmpxchg_t *) 0xffff0fc0)

it seems that 0xffff0fc0 is the adress of some kernel helper functions

but in gcc 4.1.2 , there is no such codes like __kernel_cmpxchg_t, and I can’t find the implementation of __sync_bool_compare_and_swap.

so what’s the difference between __sync_bool_compare_and_swap and cmpxchg?

is __sync_bool_compare_and_swap implemented by cmpxchg?

and with kernel helper function __kernel_cmpxchg_t, is it implementd by cmpxchg?

thanks!

Answer 1

I think the __kernel_cmpxchg is a fallback which Linux makes available on some architectures which don’t have native hardware support for CAS. E.g. ARMv5 or something like that.

Usually, GCC inline expands the _sync* builtins. Unless you’re really interested in GCC internals, an easier way to find out what it does is to make a simple C example and look at the ASM the compiler generates.

Consider

#include <stdbool.h>

bool my_cmpchg(int *ptr, int oldval, int newval)
{
    return __sync_bool_compare_and_swap(ptr, oldval, newval);
}

Compiling this on an x86_64 Linux machine with GCC 4.4 the following asm is generated:

my_cmpchg:
.LFB0:
    .cfi_startproc
    movl    %esi, %eax
    lock cmpxchgl   %edx, (%rdi)
    sete    %al
    ret
    .cfi_endproc