[FFmpeg-devel] lurking bugs in the mmx-related assembler code (?)

Michael Niedermayer michael at niedermayer.cc
Sun Oct 2 03:25:30 EEST 2016

On Sun, Oct 02, 2016 at 01:18:29AM +0200, Henrik Gramner wrote:
> Ensuring that emms is issued before every single libc function call is
> likely problematic.

maybe, maybe not, iam not sure but
calling emms between init/de/reinint and optimized inner loops should
be doable, we generally shouldnt be doing malloc in highly optimized
loops as factorizing allocation out and reusing buffers is likely
a better choice

beyond malloc() what else is there ?

string functions ?
these are in fact not unlikely to use SIMD of some sort
same for memcpy/move()

also there are callbacks like av_log() we should document any
requirements that apply to them or ensure no such requirements exist

exact backtraces of where issues occur would be interesting to better
understand how much code is affected by this

> What if we simply document the requirement that C standard library
> functions are assumed to not modify the x87 FPU state unless
> specifically designated to handle floating-point numbers?

thats like saying that we require undefined behavior to be defined
in a specific way. We can do this but thats like saying we support
only a subset of POSIX platforms and that subset could shrink at
any time if implementations change

I think our first choice should be to comply to specs where its
needed in practice and doable.
when its not useful in practice and a total unreadable mess if done
i would tend suggest to ignore specs. 

If all else fails we could add a emms call behind #if in
av_malloc() and detect affected libcs but thats IMHO a ugly hack
but better than declaring "non support"

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Republics decline into democracies and democracies degenerate into
despotisms. -- Aristotle
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