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

Michael Niedermayer michaelni
Tue Dec 23 22:24:09 CET 2008

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

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you really think that XML is the answer, then you definitly missunderstood
the question -- Attila Kinali
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