Re: Should it be reported as a bug? (-O2 and cmpxchg instruction)

[Andrew Haley <aph@redhat.com>]

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.

> 
> 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.
>