On Fri, Mar 24, 2017 at 06:23:42PM +0100, Peter Zijlstra wrote:
> I tried a few variants, but nothing really made it better.
>
> Find the tiny.c file below; I'm using:
>
> gcc (Debian 6.3.0-5) 6.3.0 20170124
>
> it has both an inline and an stmt-expr try_cmpxchg variant to play with;
> the 'expected' output is at the bottom (same as above left).
>
> Note that clang doesn't compile this stuff due to missing features.
>
static inline bool try_cmpxchg2(unsigned int *ptr, unsigned int *val, unsigned
int new)
{
unsigned int old = *val;
bool success;
asm volatile goto("lock cmpxchgl %[new], %[ptr]; "
"jnz %[label]"
: /* no output */
: [ptr] "+m" (*ptr),
[old] "+a" (old)
[new] "r" (new)
: "memory"
: label);
return 1;
label:
*val = old;
return 0;
}
generates better code than the @cc output with extra if (!success), but
not as good as the original.
