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

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Message-ID: <20080409190816.GB30202@Krystal>
Date:	Wed, 9 Apr 2008 15:08:16 -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:
>> 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.

That's required for the NMI-safe version of the immediate values, which
uses a breakpoint, but not for this version based on stop_machine_run().
However, to minimize the amount of changes between the two versions, I
left the =q constraint, which is more restrictive. Is it worth it to use
=r instead ? It will typically let the compiler use a wider range of
registers on x86_64.

Thanks,

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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