Bug#871565: gcc-7: ppc64el: miscompiles ffmpeg's scalarproduct_int16_vsx at -O1

[James Cowgill]

Control: reassign -1 gcc-7 7.1.0-13
Control: severity -1 important
Control: retitle -1 gcc-7: ppc64el: miscompiles ffmpeg's scalarproduct_int16_vsx at -O1
Control: forwarded -1 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81833
Control: affects -1 src:ffmpeg

Hi,

On 09/08/17 06:47, Adrian Bunk wrote:
> Source: ffmpeg
> Version: 7:3.3.3-2
> Severity: serious
> 
> https://buildd.debian.org/status/fetch.php?pkg=ffmpeg&arch=ppc64el&ver=7%3A3.3.3-2&stamp=1502249633&raw=0
[...]
> TEST    checkasm-audiodsp
> /<<PKGBUILDDIR>>/tests/fate-run.sh fate-checkasm-audiodsp "" "" "/<<PKGBUILDDIR>>/debian/standard" 'run tests/checkasm/checkasm --test=audiodsp' '' '/dev/null' '' '1' '' '' '' '' '' '' '' ''
>  /<<PKGBUILDDIR>>/debian/standard/ffmpeg -nostdin -nostats -cpuflags all -threads 1 -idct simple -flags +bitexact -sws_flags +accurate_rnd+bitexact -fflags +bitexact -f image2 -vcodec pgmyuv -hwaccel none -threads 1 -thread_type frame+slice -i /<<PKGBUILDDIR>>/debian/standard/tests/vsynth1/%02d.pgm -flags +bitexact -sws_flags +accurate_rnd+bitexact -fflags +bitexact -threads 1 -idct simple -dct fastint -vf format=gbrp14be,vflip= -vcodec rawvideo -frames:v 5 -pix_fmt gbrp14be -frames:v 1 -f nut md5:
>  /<<PKGBUILDDIR>>/debian/standard/tests/checkasm/checkasm --test=audiodsp
> Test checkasm-audiodsp failed. Look at tests/data/fate/checkasm-audiodsp.err for details.
> checkasm: using random seed 3484844225
> ALTIVEC:
>    audiodsp.scalarproduct_int16_altivec (audiodsp.c:81)
>  - audiodsp.audiodsp [FAILED]
> VSX:
>    audiodsp.scalarproduct_int16_vsx (audiodsp.c:81)
>  - audiodsp.audiodsp [FAILED]
> checkasm: 2 of 2 tests have failed
> /<<PKGBUILDDIR>>/tests/Makefile:219: recipe for target 'fate-checkasm-audiodsp' failed
> make[2]: *** [fate-checkasm-audiodsp] Error 1

I've debugged this a bit and it definitely looks like GCC 7 has
miscompiled some of the VSX routines in FFmpeg. I've filed an upstream
bug againt GCC, but I'll probably just disable these optimizations on
ppc64el until it's fixed.

Thanks,
James

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