linux-kernel - Re: [RFC] Documentation about unaligned memory access

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Date:	Sat, 24 Nov 2007 14:34:41 +0100
From:	Pierre Ossman <drzeus-list@...eus.cx>
To:	Daniel Drake <dsd@...too.org>
Cc:	linux-kernel@...r.kernel.org, davem@...emloft.net,
	kune@...ne-taler.de, johannes@...solutions.net
Subject: Re: [RFC] Documentation about unaligned memory access

On Fri, 23 Nov 2007 00:15:53 +0000 (GMT)
Daniel Drake <dsd@...too.org> wrote:

> Being spoilt by the luxuries of i386/x86_64 I've never really had a good
> grasp on unaligned memory access problems on other architectures and decided
> it was time to figure it out. As a result I've written this documentation
> which I plan to submit for inclusion as
> Documentation/unaligned_memory_access.txt
> 
> Before I do so, any comments on the following?
> 

A very nice, and much needed document. I think you should include one thing though:

memcpy() is _only_ safe when one of the pointers is char* or void*. If it is anything more complex than that, gcc will assume alignment and optimise based on that. E.g. memcpy() of two long:s generates the same assembly as doing an assignment.

(Technically it is no different for char* and void*, but since they have byte alignment, gcc can't really do anything creative.)

Rgds
-- 
     -- Pierre Ossman

  Linux kernel, MMC maintainer        http://www.kernel.org
  PulseAudio, core developer          http://pulseaudio.org
  rdesktop, core developer          http://www.rdesktop.org
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