[FFmpeg-devel] [PATCH] lavc/lpc: exploit even symmetry of window function

[Ganesh Ajjanagadde]

On Wed, Mar 9, 2016 at 1:32 PM, Reimar Döffinger
<Reimar.Doeffinger at gmx.de> wrote:
> On Wed, Mar 09, 2016 at 01:13:58PM +0100, Michael Niedermayer wrote:
>> On Tue, Mar 08, 2016 at 10:16:50PM -0500, Ganesh Ajjanagadde wrote:
>> > Yields 2x improvement in function performance, and boosts aac encoding
>> > speed by ~ 4% overall. Sample benchmark (Haswell+GCC under -march=native):
>> > after:
>> > ffmpeg -i sin.flac -acodec aac -y sin_new.aac  5.22s user 0.03s system 105% cpu 4.970 total
>> >
>> > before:
>> > ffmpeg -i sin.flac -acodec aac -y sin_new.aac  5.40s user 0.05s system 105% cpu 5.162 total
>> >
>> > Big shame that len-1 is -1 mod 4; 0 mod 4 would have yielded a further 2x through
>> > additional symmetry. Of course, one could approximate with the 0 mod 4 variant,
>> > error would essentially be ~ 1/len in the worst case.
>> >
>> > Signed-off-by: Ganesh Ajjanagadde <gajjanag at gmail.com>
>> > ---
>> >  libavcodec/lpc.c | 3 ++-
>> >  1 file changed, 2 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/libavcodec/lpc.c b/libavcodec/lpc.c
>> > index 3839119..052aeaa 100644
>> > --- a/libavcodec/lpc.c
>> > +++ b/libavcodec/lpc.c
>> > @@ -176,9 +176,10 @@ double ff_lpc_calc_ref_coefs_f(LPCContext *s, const float *samples, int len,
>> >      const double a = 0.5f, b = 1.0f - a;
>> >
>> >      /* Apply windowing */
>> > -    for (i = 0; i < len; i++) {
>> > +    for (i = 0; i <= len / 2; i++) {
>>
>> >          double weight = a - b*cos((2*M_PI*i)/(len - 1));
>>
>> the windows should not be recalcuated for each frame. But either
>> calculated once during init or calculated at first use if that makes
>> sense
>>
>> actually i suspect there should be close to 0 calls to libc math
>> functions after init in the encoder, or is there some use of these
>> functions in aac that cannot be replaced by a simple table?
>
> I don't understand the code, but I assumed that len changes
> regularly here, which is the main problem.
> I never actually checked if there is maybe only a small
> number of common values at least though.

So I did check this; len does change quite regularly. On the other
hand, it does not change too drastically over time, so a cache
approach could work at a significant code complexity increase for
minimal gain.

In fact, I personally have no idea why it is absolutely critical that
a Hamming window is used, see 0cfdaf45c4 where Welch was used
previously. At a first glance I think one can find a window that works
and avoids libm, e.g Parzen or other polynomial windows.
Rostislav, any thoughts?

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