Re: [patch 5/8] Immediate Values - x86 Optimization

H. Peter Anvin
2007-11-13T20:00:34Z

Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@zytor.com) wrote:
>> Mathieu Desnoyers wrote:
>>>> - Use "=g" constraint for char immediate value inline assembly.
>>>>
>>>> "=g" is the same as "=rmi" which is inherently bogus. In your actual
>>>> code you use "=r", the correct constraint is "=q".
>>> q
>>> Any register accessible as rl. In 32-bit mode, a, b, c, and d; in
>>> 64-bit mode, any integer register. I am worried that "=q" might exclude
>>> the si and di registers in 32-bit mode.
>>> What exactly is wrong with "=r" ?
>> For "char" (8-bit) values, sp/bp/si/di are illegal in 32-bit mode.
>>
>> Hence "=q".
>>
>
> Ah! yep, I see, so we say:
>
> 1 byte : "=q"
> 2 bytes : "=r"
> 4 bytes : "=r"
> 8 bytes : "=r"
>
> ? (si and di appear to be legal for 2 and 4 bytes in 32-bit mode)
>

That's right.

-hpa