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

[Martin Sustrik <sustrik@imatix.com>]

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?

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.