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

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Message-ID: <20071113192445.GA1463@Krystal>
Date:	Tue, 13 Nov 2007 14:24:45 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
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

* 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".
>

Hi Peter,

Yup, =g wasn't what I was looking for at all, the header comment is
bogus.

From
http://gcc.gnu.org/onlinedocs/gcc/Simple-Constraints.html#Simple-Constraints

`r'
    A register operand is allowed provided that it is in a general register.

From
http://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints
Intel 386   config/i386/constraints.md

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" ?


> 	-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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