Re: [RFC] Documentation about unaligned memory access

[Pierre Ossman <drzeus-list@drzeus.cx>]

On Fri, 23 Nov 2007 00:15:53 +0000 (GMT)
Daniel Drake <dsd@gentoo.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
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     -- Pierre Ossman

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