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

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Message-ID: <4739FCA0.4040702@zytor.com>
Date:	Tue, 13 Nov 2007 11:36:00 -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:
>> - 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".

	-hpa
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