Andrew Haley wrote:
Please don't top-post.
Martin Sustrik wrote:
Andrew,
Thanks for prompt response. However, I am not sure how to use the
earlyclobber modifier - this is actually the first gcc inline asembly
code I've ever written.
Do you mean that I should add '&' sign to every operand in input
operand list, or should I add the sign when using the operands or what?
Use "=&a" on the first operand.
Ok, I've modified the code this way:
__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");
However, the generated code is exactly the same as before.
The problem, AFAICS, is that cmpxchg modifies eax although eax is not
an explicit operand of the instruction. Therefore optimiser has no
idea that it is modified and acts as if it was unchanged :(
Martin
Andrew Haley wrote:
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.
