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Subject: Re: [OMPI users] memcpy overlap in ompi_ddt_copy_content_same_ddt and glibc 2.12
From: Jed Brown (jed_at_[hidden])
Date: 2010-11-10 16:17:02


On Wed, Nov 10, 2010 at 22:08, e-mail number.cruncher <
number.cruncher_at_[hidden]> wrote:

> In short, someone from Intel submitted a glibc patch that does faster
> memcpy's on e.g. Intel i7, respects the ISO C definition, but does
> things backwards.
>

However, the commit message and mailing list, as far as I can tell, does not
explain how the implementations were benchmarked. Linus claims that his
(entirely trivial) implementation matches or beats the new one. If indeed
the performance gains claimed by Lu (2X to 4X) are real, then the old
implementation must have been truly horrible (as stated by Agner Fog in
http://sourceware.org/ml/libc-help/2008-08/msg00007.html). I'd like to see
the benchmark results demonstrating that the backward memcpy is really
faster than forward.

> I think any software that ignores the ISO warning
> "If copying takes place between objects that overlap, the behavior is
> undefined" needs fixing.
>

Absolutely, it is incorrect and should be fixed.

Jed