[FFmpeg-devel] Pipeline: H.264 speed improvements

Måns Rullgård mans
Sun Dec 28 12:29:21 CET 2008

Vitor Sessak <vitor1001 at gmail.com> writes:

> Michael Niedermayer wrote:
>> On Tue, Dec 23, 2008 at 08:41:00PM +0000, M?ns Rullg?rd wrote:
>>> Michael Niedermayer <michaelni at gmx.at> writes:
>>>> On Tue, Dec 23, 2008 at 04:08:26AM -0500, Jason Garrett-Glaser wrote:
>>>>> I've put together a list of all the possible speed improvements I can
>>>>> see, including both some obvious ones and non-obvious ones.  If you're
>>>>> interested in implementing anything here, say so to make sure your
>>>>> work isn't duplicated by Michael or I.  Also feel free to discuss some
>>>>> of the more nutty ideas, like the VLC table, or tell me that I'm wrong
>>>>> about something.
>>>>> Non-assembly stuff:
>>>> [...]
>>>>> av_log2 is unnecessarily powerful for use in h264.c.  All signed
>>>>> golomb values in H.264 fit in 16-bit, and all unsigned golomb values
>>>>> other than headers fit in 8-bit.  Thus all ordinary unsigned golomb
>>>>> code reads can literally be put in a 256-byte VLC table and replaced
>>>>> with a single array lookup.
>>>> it may be that all ue golomb coded values are <256 outside the headers,
>>>> though even this seems wrong for mb_skip_run the way i understand the spec.
>>>> But a value of 255 corresponds to a 15bit long vlc code.
>>>> a 256 (or 128) entry LUT limits one to values 0-15 512 (or 1024) to 0-31
>>>> Now there are surely a few left that are that small but thats far from
>>>> all non header values.
>>> av_log2() can be trivially implemented on most CPUs using a count
>>> leading zeros instruction.  That should be even faster than a table.
>>> On ARM this instruction takes one cycle.
>> patch & benchmark are welcome, but note, i dont think av_log2() is
>> used much in h264.c

It is used in golomb.h, which used by h264.c.  I can't say what uses
it, I only know it showed up in a profile run I did with inlining

I did a quick benchmark with a clz implementation, which quite
remarkably came out slower, apparently due to gcc inlining changing.
Randomly forcing inline/noinline on functions in h264.c easily causes
overall performance to fluctuate 3% or so.

> Please benchmark also ALAC decoding, it seems av_log2() is much more 
> speed critical there.

Can you suggest a good sample to benchmark?

M?ns Rullg?rd
mans at mansr.com

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