Message-ID: <46F14A58.60904@zytor.com>
Date: Wed, 19 Sep 2007 09:12:08 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: Jeremy Fitzhardinge <jeremy@goop.org>, akpm@linux-foundation.org,
       linux-kernel@vger.kernel.org, Andi Kleen <ak@muc.de>,
       Chuck Ebbert <cebbert@redhat.com>,
       Christoph Hellwig <hch@infradead.org>
Subject: Re: [patch 4/7] Immediate Values - i386 Optimization
References: <20070918210747.828804366@polymtl.ca> <20070918210853.588573678@polymtl.ca> <46F04856.3010808@goop.org> <46F04D53.6040903@zytor.com> <46F050E8.5020206@goop.org> <20070919130122.GA21750@Krystal>
In-Reply-To: <20070919130122.GA21750@Krystal>
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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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