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Date: Fri, 15 Feb 2008 11:44:00 -0000
From: Andrew Haley <aph@redhat.com>
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To: Martin Sustrik <sustrik@imatix.com>
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Subject: Re: Should it be reported as a bug? (-O2 and cmpxchg instruction)
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Martin Sustrik wrote:
> 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)

Ah, maybe I told you wrong: I see that Opernad 0 is also an input.  Sorry.

>                : "memory", "cc");
> 
> However, the generated code is exactly the same as before.

I certainly don't get that.  Please post a complete test case, with
assembly output.  We also need to know exactly your version of gcc.

Andrew.