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

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Message-ID: <47FD4424.20907@zytor.com>
Date:	Wed, 09 Apr 2008 15:33:08 -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 (updated)

Mathieu Desnoyers wrote:
> 
> Hrm, since even the nmi-safe version supports REX-prefixed instructions,
> there is no need for an =q constraint on single-byte immediate values
> anymore. (thanks to your "discard" section used in the nmi-safe version)
> 

WRONG!

Using =r for single-byte values is incorrect for 32-bit code -- that 
would permit %spl, %bpl, %sil, %dil which are illegal in 32-bit mode. 
That is not the incorrect bit, it's the description that is confused.

	-hpa
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