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

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Date:	Wed, 19 Sep 2007 10:30:32 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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

H. Peter Anvin wrote:
> 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.
>   

Cross-cache-line, sure.  But what about just not sizeof aligned?  If its
enough to avoid cross-cache-line, then that's simpler.

Which is something I was going to comment on: Mathieu, you try to align
the constant itself, but you don't prevent the instruction overall from
crossing a cache line.  Given how delicate all this stuff is, it seems
like a good idea to do that.


>> * 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.
>   

Well, in this case, it might be preferred if it brings the constant into
alignment without explicit padding :)

    J
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