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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