[FFmpeg-devel] [PATCH 1/2] lavc/pcm_tablegen: slight speedup of table generation

[Michael Niedermayer]

On Sun, Jan 03, 2016 at 09:11:28PM -0800, Ganesh Ajjanagadde wrote:
> On Sun, Jan 3, 2016 at 7:32 PM, Michael Niedermayer
> <michael at niedermayer.cc> wrote:
> > On Mon, Jan 04, 2016 at 04:04:02AM +0100, Michael Niedermayer wrote:
> >> On Wed, Dec 30, 2015 at 08:34:55PM -0800, Ganesh Ajjanagadde wrote:
> >> > This gets rid of some branches to speed up table generation slightly
> >> > (impact higher on mulaw than alaw). Tables are identical to before,
> >> > tested with FATE.
> >> >
> >> > Sample benchmark (Haswell, GNU/Linux+gcc):
> >> > old:
> >> >  313494 decicycles in build_alaw_table,    4094 runs,      2 skips
> >> >  315959 decicycles in build_alaw_table,    8190 runs,      2 skips
> >> >
> >> >  323599 decicycles in build_ulaw_table,    4095 runs,      1 skips
> >> >  318849 decicycles in build_ulaw_table,    8188 runs,      4 skips
> >> >
> >> > new:
> >> >  261902 decicycles in build_alaw_table,    4096 runs,      0 skips
> >> >  266519 decicycles in build_alaw_table,    8192 runs,      0 skips
> >> >
> >> >  209657 decicycles in build_ulaw_table,    4096 runs,      0 skips
> >> >  232656 decicycles in build_ulaw_table,    8192 runs,      0 skips
> >> >
> >> > Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> >> > ---
> >> >  libavcodec/pcm_tablegen.h | 24 ++++++++++++------------
> >> >  1 file changed, 12 insertions(+), 12 deletions(-)
> >> >
> >> > diff --git a/libavcodec/pcm_tablegen.h b/libavcodec/pcm_tablegen.h
> >> > index 1387210..7269977 100644
> >> > --- a/libavcodec/pcm_tablegen.h
> >> > +++ b/libavcodec/pcm_tablegen.h
> >> > @@ -87,21 +87,21 @@ static av_cold void build_xlaw_table(uint8_t *linear_to_xlaw,
> >> >  {
> >> >      int i, j, v, v1, v2;
> >> >
> >> > -    j = 0;
> >> > -    for(i=0;i<128;i++) {
> >> > -        if (i != 127) {
> >> > -            v1 = xlaw2linear(i ^ mask);
> >> > -            v2 = xlaw2linear((i + 1) ^ mask);
> >> > -            v = (v1 + v2 + 4) >> 3;
> >> > -        } else {
> >> > -            v = 8192;
> >> > -        }
> >> > -        for(;j<v;j++) {
> >> > +    j = 1;
> >> > +    linear_to_xlaw[8192] = mask;
> >> > +    for(i=0;i<127;i++) {
> >> > +        v1 = xlaw2linear(i ^ mask);
> >> > +        v2 = xlaw2linear((i + 1) ^ mask);
> >> > +        v = (v1 + v2 + 4) >> 3;
> >> > +        for(;j<v;j+=1) {
> >> > +            linear_to_xlaw[8192 - j] = (i ^ (mask ^ 0x80));
> >> >              linear_to_xlaw[8192 + j] = (i ^ mask);
> >> > -            if (j > 0)
> >> > -                linear_to_xlaw[8192 - j] = (i ^ (mask ^ 0x80));
> >> >          }
> >> >      }
> >> > +    for(;j<8192;j++) {
> >> > +        linear_to_xlaw[8192 - j] = (127 ^ (mask ^ 0x80));
> >> > +        linear_to_xlaw[8192 + j] = (127 ^ mask);
> >> > +    }
> >> >      linear_to_xlaw[0] = linear_to_xlaw[1];
> >>
> >> i think you can make the tables 8 times smaller
> >
> > forget this, i should have checked the whole table or looked when i
> > am awake ...
> 
> ha ha. By the way, both changes are needed to get this level of
> speedup, with only the j change which you acked, the speedup is much
> smaller. But then also note that the other parts of the patch also
> increase the binary size more.

hmm, ok if its needed to get the speedup then LGTM

thanks

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Concerning the gods, I have no means of knowing whether they exist or not
or of what sort they may be, because of the obscurity of the subject, and
the brevity of human life -- Protagoras
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