[FFmpeg-devel] [PATCH 00/41] Stop including superseded functions for x64

[Andreas Rheinhardt]

Andreas Rheinhardt:
> x64 requires MMX, MMXEXT, SSE and SSE2; yet there is no shortage
> of code like the following:
> 
>     if (EXTERNAL_MMX(cpu_flags)) {
>         c->ssd_int8_vs_int16 = ff_ssd_int8_vs_int16_mmx;
>     }
>     if (EXTERNAL_SSE2(cpu_flags)) {
>         c->ssd_int8_vs_int16 = ff_ssd_int8_vs_int16_sse2;
>     }
> 
> Given that SSE2 is always present on x64, the only way
> for the mmx version to be chosen in the above example
> is if SSE2 has been disabled either at compile-time
> or at runtime, i.e. it is never used unless one shoots
> oneself in the foot.
> This patchset therefore disables such functions for x64
> by #if'ing them away; x86 has not been affected. This
> saves about 140KB.
> 
> (Another way to handle this would be to remove every function
> that would be overridden if one had a processor capable of
> MMX, MMXEXT, SSE and SSE2. x86 processors not fulfilling
> this requirement (which are truely ancient nowadays)
> would still work, but would be slower, i.e. they would be treated
> as second-class citizens. This would have the advantage of
> avoiding #ifs and would lighten x86 binaries of code that is
> not used at all by the overwhelming majority of users.
> I'll update this patchset if it is preferred to do it that way.)
> 

I have now implemented this other way mentioned above (i.e. removing
stuff that is overridden if SSE2 is available altogether also for
x86-32); the result can be seen here:
https://github.com/mkver/FFmpeg/commits/mmx2
I prefer this to the old version because of the reduced complexity which
dwarfs the potential to slow down some ancient systems a bit (if these
ancient systems use an up-to-date FFmpeg which is quite unlikely).
Furthermore, some of the MMX scale functions that are removed are
buggy/not bixexact. See
https://github.com/mkver/FFmpeg/commit/c5513ad962100040601b5eba0042692a740ac50a
(or shall I post these patches?)
Is anyone against this removal?

- Andreas