Martin Sustrik wrote:
Hi all,
I've encountered a problem with gcc inline assembly.
Following code, when optimised with -O2 gives following machine code:
__asm__ volatile (
"lock; cmpxchgl %1, %3\n\t"
"jz 1f\n\t"
"mov %2, %%eax\n\t"
"lock; xchgl %%eax, %3\n\t"
"1:\n\t"
: "=a" (oldval)
: "r" (thenval_), "r" (elseval_), "m" (value), "0" (0)
: "memory", "cc");
4031a0: 31 c0 xor %eax,%eax
4031a2: f0 0f b1 55 40 lock cmpxchg %edx,0x40(%rbp)
4031a7: 74 06 je 4031af
4031a9: 89 c0 mov %eax,%eax
4031ab: f0 87 45 40 lock xchg %eax,0x40(%rbp)
Note that %2 maps to %%eax (mov %2, %%eax --> mov %eax,%eax). This
shouldn't happen given that cmpxchg modifies the value of %%eax.
Any idea whether this should be considered a bug and reported as such?
Look for "earlyclobber" in the section Constraint Modifier Characters
Andrew.
