Control: forcemerge 870622 -1

Hi James,

On Sat, Aug 05, 2017 at 02:40:12PM -0400, James Cowgill wrote:

> > The latest release of ffmpeg enables NEON support by default when building
> > on armhf; however, NEON support is not a standard part of the ARMv7 ABI, and
> > Debian supports running armhf on chips that do not implement NEON.

> > Using NEON based on runtime detection of support for it is fine, but the
> > existing ffmpeg implementation doesn't appear to do this, instead using NEON
> > based on build-time configuration with no fallback.

> Are you sure this is true? I tried running the failing test on abel.d.o
> (which AFAIK does not have NEON) and harris (which does). The test only
> caused ffmpeg to crash on harris, which seems to suggest that the
> runtime NEON detection is working properly.

Nope, not sure at all, I only know what I saw from code inspection where I
failed to find where the code falls back on non-NEON systems.

I see you're right that abel does not have NEON (it's not exposed in
/proc/cpuinfo feature flags for the CPU), so if ffmpeg runs without crashing
there, I guess that's pretty clear evidence that there is a runtime fallback
that I overlooked.

> These are the commands to reproduce the autopkgtest fail if you want to
> try it:

> ffmpeg -f lavfi -i testsrc=s=32x32:d=0.1 -strict -2 -c:v libx264rgb -f avi 
> libx264rgb.avi -y -hide_banner -nostdin
> ffmpeg -strict -2 -i libx264rgb.avi -t 1 -c:v rawvideo -c:a pcm_s32le -f nut 
> /dev/null -y -hide_banner -nostdin

> > This issue was noticed in Ubuntu only because the autopkgtests for ffmpeg
> > and x264 triggered an unaligned access in the NEON code, which is *also* not
> > a portable assumption on armhf; however, if the NEON code had not had any
> > unaligned access, the fact that NEON was used would have gone unnoticed on
> > Ubuntu infrastructure.
> > 
> >   http://autopkgtest.ubuntu.com/packages/f/ffmpeg/artful/armhf
> >   http://autopkgtest.ubuntu.com/packages/x/x264/artful/armhf
> > 
> > (And if upstream does fix their code to support runtime detection of NEON
> > support, then there will be a different bug for us to worry about fixing!)
> 
> This is #870622 BTW. If possible, I would much rather fix these bugs
> without having to disable all the NEON optimizations.

So, marking this bug as a duplicate of the real bug (the unaligned trap
problem due to binutils).

Thanks,
-- 
Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                    http://www.debian.org/
slanga...@ubuntu.com                                     vor...@debian.org

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