[FFmpeg-devel] [PATCH 2/3] lavc: add a framework to fix alignment problems.

[Ronald S. Bultje]

Hi,

On Wed, May 10, 2017 at 5:02 PM, Michael Niedermayer <michael at niedermayer.cc
> wrote:

> On Wed, May 10, 2017 at 10:28:22PM +0200, Nicolas George wrote:
> > Le primidi 21 floréal, an CCXXV, Michael Niedermayer a écrit :
> > > fixed, and now your patchset seems to cause a 1% slowdown of the
> > > example above
> >
> > Thanks. The slowdown is to be expected, since this filter used
> > non-aligned frames which are now realigned. I guess you can get back the
> > previous speed by setting "avctx->alignment = 0" in snow_encoder.init().
> >
>
> > The default in this patchset is 5 to fix all the alignment problems, at
> > the cost of some slowdown in corner cases such as this one. I am not
> > qualified to decide the best way of setting an optimal value to get
> > speed without crashes. My opinion would be:
> >
> > - have avctx->alignment default to 0;
> >
> > - have all codecs with explicit assembly set avctx->alignment to the
> >   required value;
> >
> > - have all codecs that use generic DSP code init it through a function
> >   that sets the alignment accordingly.
>
> hmm
> run fate with intentionally misaligned frames, and make a list of
> all codecs that dont crash and work fine.


That's generally arch-specific. Codecs using AVX2 need 32byte, SSE2/Neon
only need 16, MMX need none (although 8 would probably be slightly faster).

Ronald