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Bug#905868: ffmpeg fails autopkgtest on ppc64el



Control: reassign -1 gcc-8 8.2.0-3
Control: severity -1 normal
Control: retitle -1 gcc-8: miscompiles vec_sl at -O3 with -fwrapv on ppc64el
Control: affects -1 src:ffmpeg
Control: forwarded -1 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86731
Control: tags -1 upstream

Hi,

On 10/08/18 23:54, Simon Quigley wrote:
> Package: ffmpeg
> Severity: important
> 
> Attached is the most recent autopkgtest failure log against the new
> ffmpeg release in Ubuntu.

I managed to debug this issue during DebConf and I think it is a gcc-8
regression. I filed this upstream bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86731

> I don't believe this is an Ubuntu-only issue.

The bug only happens when -O3 is passed and this only happens on Ubuntu
so this bug is Ubuntu specific. I suggest Ubuntu temporarily patches
ffmpeg to strip the -O3 option and pass -O2 instead. Alternatively you
could try rewriting the broken code (in libavcodec/ppc/fdctdsp.c) to
avoid the miscompilation.

James

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