Re: [Valgrind-users] Documentation suggestion

[Nicholas N. <nj...@cs...>]

On Thu, 5 May 2005, Jeremy Fitzhardinge wrote:

>> Maybe it's worth giving more of an explanation here.  I've noticed while floating around on the internet
>> that people thing valgrind is just being pedantic when giving this warning and target < source.  There are
>> two problems:
>>
>> (1) if copying is done from largest address to smallest address.  I don't know of any memcpy that does this.
>>
>
> The some optimisation guide (AMD? Via?) recommends it for some cases.
>
>> (2) if memcpy zeroes out the target before copying.  This has been shown to improve the performance of memcpy
>> on some intel architectures, due to cache effects.  Of course it is fatal if there is any overlap between
>> source and target.  Most memcpy's don't do this kind of trick, but it's worth keeping in mind.
>>
>
> I think it's common on the PPC, because the dcbz instruction zeros a
> cache line in preparation for the destination write (this is based on 5
> year old PPC experience, so I don't know what's best on a modern
> implementation).

I've updated the 3.0 docs for this.

N