linux-kernel - Re: [patch 13/17] Immediate Values

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Message-ID: <47FD4442.3050000@zytor.com>
Date:	Wed, 09 Apr 2008 15:33:38 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@....de>,
	Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Adrian Bunk <bunk@...sta.de>,
	Alexey Dobriyan <adobriyan@...il.com>, akpm@...l.org
Subject: Re: [patch 13/17] Immediate Values - x86 Optimization

Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@...or.com) wrote:
>> Mathieu Desnoyers wrote:
>>> Ok, so the most flexible solution that I see, that should fit for both
>>> x86 and x86_64 would be :
>>> 1 byte  :       "=q" : "a", "b", "c", or "d" register for the i386.  For
>>>                        x86-64 it is equivalent to "r" class (for 8-bit
>>>                        instructions that do not use upper halves).
>>> 2, 4, 8 bytes : "=r" : A register operand is allowed provided that it is 
>>> in a
>>>                        general register.
>> Any reason to keep carrying this completely misleading comment chunk still?
>>
>> 	-hpa
> 
> This comment explains why I use the =q constraint for the 1 bytes
> immediate value. It makes sure we use an instruction with 1-byte opcode,
> without REX.R prefix, on x86_64.

No, it doesn't.  That would be "=Q".

	-hpa
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