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

[Martin Sustrik <sustrik@imatix.com>]

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.
>>
>