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

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Date:	Wed, 9 Apr 2008 20:42:19 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
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

* H. Peter Anvin (hpa@...or.com) wrote:
> 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

Ok. Sorry, it's been a few months since we looked at this. So the =q
opcode lets the compiler choose instructions with or without REX prefix.
We can allow this because 

- We don't need the opcode length in the stop_machine_run() version
- we support variable length opcode in the nmi-safe version

Am I remembering correctly now ?

Mathieu

-- 
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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