[arch-general] imagemagick 6.8.9.8-1 is slower than 6.8.9.7-1

[Lukas Fleischer]

On Fri, 17 Oct 2014 at 04:39:33, Matthew Wynn wrote:
> After upgrading to imagemagick 6.8.9.8-1, I've found it to be a lot slower than 6.8.9.7-1.  I only get this issue when downloading from the repositories or using the PKGBUILD, not when compiling using the instructions at imagemagic.org.
> 
> I've discussed the issue with the ImageMagick developers, which you can find at http://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=26369&sid=a29400f51bd723e98da6b74c0819e9b3.  Even when trying to convert a 500x500 image to 100x100, it appears that an image that is 2048x1536 is created.
> 
> I've tried this on multiple Arch machines with the same result.
>                                           

Anatol and I were able to reproduce this issue. It seems to be related
to __memcpy_avx_unaligned() in glibc which means that it only occurs on
architectures with the AVX extension.

If you have a look at the memcpy-avx-unaligned.S source code [1], you
will notice that there are several branches that copy blocks of
different sizes. Now, for some reason, 6.8.9.8-1 always (or almost
always) picks the L(less_32bytes) branch which means that only small
blocks are copied, while 6.8.9.7-1 copies larger blocks. I do not have
the time to debug this in detail but maybe you can add this information
to the upstream report?

Regards,
Lukas

[1] https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=sysdeps/x86_64/multiarch/memcpy-avx-unaligned.S;hb=HEAD