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

Mathieu Desnoyers
2007-11-13T19:46:07Z

* 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)

> -hpa