linux-kernel - Re: [patch 4/7] Immediate Values

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Date:	Wed, 19 Sep 2007 09:12:08 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@....de>,
	Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [patch 4/7] Immediate Values - i386 Optimization

Mathieu Desnoyers wrote:
> 
> Ok, let's have a good look at what we want:
> 
> 1 - get a pointer to the beginning of the immediate value within the
>     instruction.
> 2 - make sure that the immediate value, within the instruction, is
>     written to atomically wrt all CPUs, even on older architectures
>     where non aligned writes are not atomic.
> 

I think you'll find that even on modern architectures cross-cacheline
writes aren't atomic.

Anyway, disregard the bit I mentioned about one byte longer; I've been
staring myself too blind at instruction variants lately (lots of
arguments about how to generate various instructions using NASM syntax).

> 
> * 4 bytes
> B8 + rd         MOV r32, imm32   (1 byte opcode)
> C7 /0           MOV r/m32, imm32 (2 bytes opcode)
> (the 2 bytes opcode can be a problem)
> 

If gas generates the C7 opcodes by default, then that's a bug, nothing less.

	-hpa
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