[FFmpeg-devel] [PATCH] mpeg2: fix block_last_index when mismatch control modifies last coeff

[Måns Rullgård]

Jason Garrett-Glaser <darkshikari at gmail.com> writes:

> On Mon, Jun 21, 2010 at 3:23 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> I dont understand your problem, nor can i relate your rant to the
>> existing code The code as is, is fast, simple and there are no
>> special cases in any idct currently You seem to start your
>> argumentation on the need of a change.
>
> We are trying to make things a lot faster.  You are stopping us
> because we may need to make something very marginally slower in order
> to get the large speed boost.  You're stopping us from adding a larger
> engine to a racecar because the bigger exhaust pipe would slightly
> increase drag.
>
> I'm trying to merge parts of a local changeset I have that makes the
> FLV decoder 30-40% faster overall (many parts may apply to mpegvideo
> overall).  Some of these parts are unmergable, but others are quite
> mergable.  And you're stopping us from merging them because we might
> strip 2 clocks off one function to improve another by 150.

The corrected block_last_index for mpeg2 will have _no effect at all_
immediately.  When the idct is optimised, it will be faster when a
shortcut is possible, unchanged otherwise.  It is true that without
lots of extra work, mpeg2 will not get the full benefit of such
optimisations.  However, without the fix, mpeg2 decoding will break
with the optimised idct.  Fixing it will allow the idct optimisation
to be done without complications.  If someone wants to squeeze more
speed out of mpeg2, that can be done later.

-- 
M?ns Rullg?rd
mans at mansr.com