linux-kernel - Re: [patch 5/8] Immediate Values

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Date:	Tue, 13 Nov 2007 11:56:45 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@....de>, Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [patch 5/8] Immediate Values - x86 Optimization

Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@...or.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
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