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

[Ganesh Ajjanagadde]

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.

>
> [...]
>
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> I do not agree with what you have to say, but I'll defend to the death your
> right to say it. -- Voltaire
>
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