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

[Mathieu Desnoyers]

* H. Peter Anvin (hpa@xxxxxxxxx) 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)




> 	-hpa

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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