Re: [RFC] Documentation about unaligned memory access

[Luciano Rocha]

On Sat, Nov 24, 2007 at 02:34:41PM +0100, Pierre Ossman wrote:
> On Fri, 23 Nov 2007 00:15:53 +0000 (GMT)
> Daniel Drake <dsd@xxxxxxxxxx> 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.

Dumb memcpy (while (len--) { *d++ = *s++ }) will have alignment problems
in any case. Intelligent ones, like the one provided in glibc, first copy
bytes till output is aligned (C file) *or* size is a multiple (i686 asm file)
of word size, and then it copies word-by-word.

Linux's x86_64 memcpy does the opposite, copies 64bit words, and then
copies the last bytes.

So, in effect, as long as no packed structures are used, memcpy should
be safer on *int, etc., than *char, as the compiler ensures
word-alignment.

-- 
lfr
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