Re: [FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Kacper Michajlow]

One minor nitpick about commit message. You could mention which compiler
was used to generate code for benchmark. For example Clang 3.7 replaces
pow(2,...) with exp2(...) call by itself. So you probably did use gcc.
Anyway since it is already merged I guess take my reply as a hint for next
time :)

Regards,
Kacper
17 gru 2015 5:14 PM "Ganesh Ajjanagadde"  napisał(a):

> On Tue, Dec 15, 2015 at 6:40 PM, Ganesh Ajjanagadde 
> wrote:
> > On Tue, Dec 15, 2015 at 5:25 PM, Ganesh Ajjanagadde 
> wrote:
> >> On Tue, Dec 15, 2015 at 2:23 AM, Michael Niedermayer 
> wrote:
> >>> On Wed, Dec 09, 2015 at 06:55:25PM -0500, Ganesh Ajjanagadde wrote:
> > [...]
> 
>  diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
>  index d998dba..e6023e3 100644
>  --- a/libavcodec/nellymoserenc.c
>  +++ b/libavcodec/nellymoserenc.c
>  @@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext
> *avctx)
> 
>   /* Generate overlap window */
>   ff_init_ff_sine_windows(7);
>  -for (i = 0; i < POW_TABLE_SIZE; i++)
>  -pow_table[i] = pow(2, -i / 2048.0 - 3.0 + POW_TABLE_OFFSET);
>  +pow_table[0] = 1;
>  +pow_table[1024] = M_SQRT1_2;
>  +for (i = 1; i < 513; i++) {
>  +double tmp = exp2(-i / 2048.0);
>  +pow_table[i] = tmp;
>  +pow_table[1024-i] = M_SQRT1_2 / tmp;
>  +pow_table[1024+i] = tmp * M_SQRT1_2;
>  +pow_table[2048-i] = 0.5 / tmp;
> >>>
> >>> how much overall init time is gained by this ?
> >>> that is time in ffmpeg main() from start to finish when just opening
> >>> the file with no decoding aka ./ffmpeg -i somefile
> >>
> >> Don't know, all I know is cycles are unnecessarily wasted. Will put in
> >> cycle numbers.
> >>
> >
> > Here they are:
> > proposed: 424160 decicycles in pow_table, 512 runs,  0 skips
> > exp2 only: 1262093 decicycles in pow_table, 512 runs,  0 skips
> > old: 2849085 decicycles in pow_table, 512 runs,  0 skips
> >
> > Thus old to exp2 is roughly 2.25x speedup, exp2 to proposed roughly 3x
> > speedup, net ~ 6.7x speedup.
>
> took Michael's comment as a general ack, so pushed with addition of a
> comment and cycle numbers.
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Re: [FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Kacper Michajlow]

18 gru 2015 10:06 AM "Kacper Michajlow"  napisał(a):
>
> One minor nitpick about commit message. You could mention which compiler
was used to generate code for benchmark. For example Clang 3.7 replaces
pow(2,...) with exp2(...) call by itself. So you probably did use gcc.
Anyway since it is already merged I guess take my reply as a hint for next
time :)
>
> Regards,
> Kacper
>
> 17 gru 2015 5:14 PM "Ganesh Ajjanagadde"  napisał(a):
>>
>> On Tue, Dec 15, 2015 at 6:40 PM, Ganesh Ajjanagadde 
wrote:
>> > On Tue, Dec 15, 2015 at 5:25 PM, Ganesh Ajjanagadde 
wrote:
>> >> On Tue, Dec 15, 2015 at 2:23 AM, Michael Niedermayer 
wrote:
>> >>> On Wed, Dec 09, 2015 at 06:55:25PM -0500, Ganesh Ajjanagadde wrote:
>> > [...]
>> 
>>  diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
>>  index d998dba..e6023e3 100644
>>  --- a/libavcodec/nellymoserenc.c
>>  +++ b/libavcodec/nellymoserenc.c
>>  @@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext
*avctx)
>> 
>>   /* Generate overlap window */
>>   ff_init_ff_sine_windows(7);
>>  -for (i = 0; i < POW_TABLE_SIZE; i++)
>>  -pow_table[i] = pow(2, -i / 2048.0 - 3.0 +
POW_TABLE_OFFSET);
>>  +pow_table[0] = 1;
>>  +pow_table[1024] = M_SQRT1_2;
>>  +for (i = 1; i < 513; i++) {
>>  +double tmp = exp2(-i / 2048.0);
>>  +pow_table[i] = tmp;
>>  +pow_table[1024-i] = M_SQRT1_2 / tmp;
>>  +pow_table[1024+i] = tmp * M_SQRT1_2;
>>  +pow_table[2048-i] = 0.5 / tmp;
>> >>>
>> >>> how much overall init time is gained by this ?
>> >>> that is time in ffmpeg main() from start to finish when just opening
>> >>> the file with no decoding aka ./ffmpeg -i somefile
>> >>
>> >> Don't know, all I know is cycles are unnecessarily wasted. Will put in
>> >> cycle numbers.
>> >>
>> >
>> > Here they are:
>> > proposed: 424160 decicycles in pow_table, 512 runs,  0 skips
>> > exp2 only: 1262093 decicycles in pow_table, 512 runs,  0 skips
>> > old: 2849085 decicycles in pow_table, 512 runs,  0 skips
>> >
>> > Thus old to exp2 is roughly 2.25x speedup, exp2 to proposed roughly 3x
>> > speedup, net ~ 6.7x speedup.
>>
>> took Michael's comment as a general ack, so pushed with addition of a
>> comment and cycle numbers.
>> ___
>> ffmpeg-devel mailing list
>> ffmpeg-devel@ffmpeg.org
>> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel

Sorry for top post.

-Kacper
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Re: [FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Ganesh Ajjanagadde]

On Fri, Dec 18, 2015 at 1:11 AM, Kacper Michajlow  wrote:
> 18 gru 2015 10:06 AM "Kacper Michajlow"  napisał(a):
>>
>> One minor nitpick about commit message. You could mention which compiler
> was used to generate code for benchmark. For example Clang 3.7 replaces
> pow(2,...) with exp2(...) call by itself. So you probably did use gcc.
> Anyway since it is already merged I guess take my reply as a hint for next
> time :)

Thanks: yes, I have been sloppy about this.

>>
>> Regards,
>> Kacper
>>
>> 17 gru 2015 5:14 PM "Ganesh Ajjanagadde"  napisał(a):
>>>
>>> On Tue, Dec 15, 2015 at 6:40 PM, Ganesh Ajjanagadde 
> wrote:
>>> > On Tue, Dec 15, 2015 at 5:25 PM, Ganesh Ajjanagadde 
> wrote:
>>> >> On Tue, Dec 15, 2015 at 2:23 AM, Michael Niedermayer 
> wrote:
>>> >>> On Wed, Dec 09, 2015 at 06:55:25PM -0500, Ganesh Ajjanagadde wrote:
>>> > [...]
>>> 
>>>  diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
>>>  index d998dba..e6023e3 100644
>>>  --- a/libavcodec/nellymoserenc.c
>>>  +++ b/libavcodec/nellymoserenc.c
>>>  @@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext
> *avctx)
>>> 
>>>   /* Generate overlap window */
>>>   ff_init_ff_sine_windows(7);
>>>  -for (i = 0; i < POW_TABLE_SIZE; i++)
>>>  -pow_table[i] = pow(2, -i / 2048.0 - 3.0 +
> POW_TABLE_OFFSET);
>>>  +pow_table[0] = 1;
>>>  +pow_table[1024] = M_SQRT1_2;
>>>  +for (i = 1; i < 513; i++) {
>>>  +double tmp = exp2(-i / 2048.0);
>>>  +pow_table[i] = tmp;
>>>  +pow_table[1024-i] = M_SQRT1_2 / tmp;
>>>  +pow_table[1024+i] = tmp * M_SQRT1_2;
>>>  +pow_table[2048-i] = 0.5 / tmp;
>>> >>>
>>> >>> how much overall init time is gained by this ?
>>> >>> that is time in ffmpeg main() from start to finish when just opening
>>> >>> the file with no decoding aka ./ffmpeg -i somefile
>>> >>
>>> >> Don't know, all I know is cycles are unnecessarily wasted. Will put in
>>> >> cycle numbers.
>>> >>
>>> >
>>> > Here they are:
>>> > proposed: 424160 decicycles in pow_table, 512 runs,  0 skips
>>> > exp2 only: 1262093 decicycles in pow_table, 512 runs,  0 skips
>>> > old: 2849085 decicycles in pow_table, 512 runs,  0 skips
>>> >
>>> > Thus old to exp2 is roughly 2.25x speedup, exp2 to proposed roughly 3x
>>> > speedup, net ~ 6.7x speedup.
>>>
>>> took Michael's comment as a general ack, so pushed with addition of a
>>> comment and cycle numbers.
>>> ___
>>> ffmpeg-devel mailing list
>>> ffmpeg-devel@ffmpeg.org
>>> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>
> Sorry for top post.

No problem.

>
> -Kacper
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Re: [FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Ganesh Ajjanagadde]

On Tue, Dec 15, 2015 at 6:40 PM, Ganesh Ajjanagadde  wrote:
> On Tue, Dec 15, 2015 at 5:25 PM, Ganesh Ajjanagadde  wrote:
>> On Tue, Dec 15, 2015 at 2:23 AM, Michael Niedermayer  
>> wrote:
>>> On Wed, Dec 09, 2015 at 06:55:25PM -0500, Ganesh Ajjanagadde wrote:
> [...]

 diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
 index d998dba..e6023e3 100644
 --- a/libavcodec/nellymoserenc.c
 +++ b/libavcodec/nellymoserenc.c
 @@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext *avctx)

  /* Generate overlap window */
  ff_init_ff_sine_windows(7);
 -for (i = 0; i < POW_TABLE_SIZE; i++)
 -pow_table[i] = pow(2, -i / 2048.0 - 3.0 + POW_TABLE_OFFSET);
 +pow_table[0] = 1;
 +pow_table[1024] = M_SQRT1_2;
 +for (i = 1; i < 513; i++) {
 +double tmp = exp2(-i / 2048.0);
 +pow_table[i] = tmp;
 +pow_table[1024-i] = M_SQRT1_2 / tmp;
 +pow_table[1024+i] = tmp * M_SQRT1_2;
 +pow_table[2048-i] = 0.5 / tmp;
>>>
>>> how much overall init time is gained by this ?
>>> that is time in ffmpeg main() from start to finish when just opening
>>> the file with no decoding aka ./ffmpeg -i somefile
>>
>> Don't know, all I know is cycles are unnecessarily wasted. Will put in
>> cycle numbers.
>>
>
> Here they are:
> proposed: 424160 decicycles in pow_table, 512 runs,  0 skips
> exp2 only: 1262093 decicycles in pow_table, 512 runs,  0 skips
> old: 2849085 decicycles in pow_table, 512 runs,  0 skips
>
> Thus old to exp2 is roughly 2.25x speedup, exp2 to proposed roughly 3x
> speedup, net ~ 6.7x speedup.

took Michael's comment as a general ack, so pushed with addition of a
comment and cycle numbers.
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Re: [FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Ganesh Ajjanagadde]

On Tue, Dec 15, 2015 at 2:23 AM, Michael Niedermayer  wrote:
> On Wed, Dec 09, 2015 at 06:55:25PM -0500, Ganesh Ajjanagadde wrote:
>> exp2 suffices here. Some trivial (~ 4x) speedup is done in addition here by
>> reusing results.
>>
>
>> This is bit-identical to the old values.
>
> That may be true with a specific compiler and version but
> C makes no such guarantee unless iam missing something

Indeed, fpu behavior is largely uncontrolled. What I meant was that
under reasonable settings, there is no accuracy loss (tested with
clang and gcc). Basically, a priori it is not obvious that this change
is ok, but I tested and the values don't change.

For that matter, this is a rather minor point - many pow
implementations are anyway broken wrt precise < 1 ulp or better yet <
0.5 ulp rounding.

>
>
>>
>> Signed-off-by: Ganesh Ajjanagadde 
>> ---
>>  libavcodec/nellymoserenc.c | 11 +--
>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
>> index d998dba..e6023e3 100644
>> --- a/libavcodec/nellymoserenc.c
>> +++ b/libavcodec/nellymoserenc.c
>> @@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext *avctx)
>>
>>  /* Generate overlap window */
>>  ff_init_ff_sine_windows(7);
>> -for (i = 0; i < POW_TABLE_SIZE; i++)
>> -pow_table[i] = pow(2, -i / 2048.0 - 3.0 + POW_TABLE_OFFSET);
>> +pow_table[0] = 1;
>> +pow_table[1024] = M_SQRT1_2;
>> +for (i = 1; i < 513; i++) {
>> +double tmp = exp2(-i / 2048.0);
>> +pow_table[i] = tmp;
>> +pow_table[1024-i] = M_SQRT1_2 / tmp;
>> +pow_table[1024+i] = tmp * M_SQRT1_2;
>> +pow_table[2048-i] = 0.5 / tmp;
>
> how much overall init time is gained by this ?
> that is time in ffmpeg main() from start to finish when just opening
> the file with no decoding aka ./ffmpeg -i somefile

Don't know, all I know is cycles are unnecessarily wasted. Will put in
cycle numbers.

> ?
>
> iam asking as this makes the intend of the loop harder to
> see/understand
> if this change is done then please add a comment explaining what the
> code does or maybe leave the unoptimized code in a comment

Comment will be added, sorry.

Note that the trick may be employed to a finer resolution, ie by
storing exp2(3/4) etc and utilizing in simple multiplications and
divisions. There will come a point where the overhead of the binary
size increase will dominate. I chose this as this is reasonably
readable and improves speed at minimal binary size impact.

>
> either way replacing pow -> exp2 LGTM
>
> [...]
> --
> Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope
>
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Re: [FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Ganesh Ajjanagadde]

On Tue, Dec 15, 2015 at 5:25 PM, Ganesh Ajjanagadde  wrote:
> On Tue, Dec 15, 2015 at 2:23 AM, Michael Niedermayer  wrote:
>> On Wed, Dec 09, 2015 at 06:55:25PM -0500, Ganesh Ajjanagadde wrote:
[...]
>>>
>>> diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
>>> index d998dba..e6023e3 100644
>>> --- a/libavcodec/nellymoserenc.c
>>> +++ b/libavcodec/nellymoserenc.c
>>> @@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext *avctx)
>>>
>>>  /* Generate overlap window */
>>>  ff_init_ff_sine_windows(7);
>>> -for (i = 0; i < POW_TABLE_SIZE; i++)
>>> -pow_table[i] = pow(2, -i / 2048.0 - 3.0 + POW_TABLE_OFFSET);
>>> +pow_table[0] = 1;
>>> +pow_table[1024] = M_SQRT1_2;
>>> +for (i = 1; i < 513; i++) {
>>> +double tmp = exp2(-i / 2048.0);
>>> +pow_table[i] = tmp;
>>> +pow_table[1024-i] = M_SQRT1_2 / tmp;
>>> +pow_table[1024+i] = tmp * M_SQRT1_2;
>>> +pow_table[2048-i] = 0.5 / tmp;
>>
>> how much overall init time is gained by this ?
>> that is time in ffmpeg main() from start to finish when just opening
>> the file with no decoding aka ./ffmpeg -i somefile
>
> Don't know, all I know is cycles are unnecessarily wasted. Will put in
> cycle numbers.
>

Here they are:
proposed: 424160 decicycles in pow_table, 512 runs,  0 skips
exp2 only: 1262093 decicycles in pow_table, 512 runs,  0 skips
old: 2849085 decicycles in pow_table, 512 runs,  0 skips

Thus old to exp2 is roughly 2.25x speedup, exp2 to proposed roughly 3x
speedup, net ~ 6.7x speedup.
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Re: [FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Ganesh Ajjanagadde]

On Wed, Dec 9, 2015 at 6:55 PM, Ganesh Ajjanagadde
 wrote:
> exp2 suffices here. Some trivial (~ 4x) speedup is done in addition here by
> reusing results.
>
> This is bit-identical to the old values.
>
> Signed-off-by: Ganesh Ajjanagadde 
> ---
>  libavcodec/nellymoserenc.c | 11 +--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
> index d998dba..e6023e3 100644
> --- a/libavcodec/nellymoserenc.c
> +++ b/libavcodec/nellymoserenc.c
> @@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext *avctx)
>
>  /* Generate overlap window */
>  ff_init_ff_sine_windows(7);
> -for (i = 0; i < POW_TABLE_SIZE; i++)
> -pow_table[i] = pow(2, -i / 2048.0 - 3.0 + POW_TABLE_OFFSET);
> +pow_table[0] = 1;
> +pow_table[1024] = M_SQRT1_2;
> +for (i = 1; i < 513; i++) {
> +double tmp = exp2(-i / 2048.0);
> +pow_table[i] = tmp;
> +pow_table[1024-i] = M_SQRT1_2 / tmp;
> +pow_table[1024+i] = tmp * M_SQRT1_2;
> +pow_table[2048-i] = 0.5 / tmp;
> +}
>
>  if (s->avctx->trellis) {
>  s->opt  = av_malloc(NELLY_BANDS * OPT_SIZE * sizeof(float  ));
> --
> 2.6.3
>

going in tomorrow in the absence of comments since it lacks a maintainer.
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Re: [FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Michael Niedermayer]

On Wed, Dec 09, 2015 at 06:55:25PM -0500, Ganesh Ajjanagadde wrote:
> exp2 suffices here. Some trivial (~ 4x) speedup is done in addition here by
> reusing results.
> 

> This is bit-identical to the old values.

That may be true with a specific compiler and version but
C makes no such guarantee unless iam missing something 


> 
> Signed-off-by: Ganesh Ajjanagadde 
> ---
>  libavcodec/nellymoserenc.c | 11 +--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
> index d998dba..e6023e3 100644
> --- a/libavcodec/nellymoserenc.c
> +++ b/libavcodec/nellymoserenc.c
> @@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext *avctx)
>  
>  /* Generate overlap window */
>  ff_init_ff_sine_windows(7);
> -for (i = 0; i < POW_TABLE_SIZE; i++)
> -pow_table[i] = pow(2, -i / 2048.0 - 3.0 + POW_TABLE_OFFSET);
> +pow_table[0] = 1;
> +pow_table[1024] = M_SQRT1_2;
> +for (i = 1; i < 513; i++) {
> +double tmp = exp2(-i / 2048.0);
> +pow_table[i] = tmp;
> +pow_table[1024-i] = M_SQRT1_2 / tmp;
> +pow_table[1024+i] = tmp * M_SQRT1_2;
> +pow_table[2048-i] = 0.5 / tmp;

how much overall init time is gained by this ?
that is time in ffmpeg main() from start to finish when just opening
the file with no decoding aka ./ffmpeg -i somefile
?

iam asking as this makes the intend of the loop harder to
see/understand
if this change is done then please add a comment explaining what the
code does or maybe leave the unoptimized code in a comment

either way replacing pow -> exp2 LGTM

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

Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope


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[FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Ganesh Ajjanagadde]

exp2 suffices here. Some trivial (~ 4x) speedup is done in addition here by
reusing results.

This is bit-identical to the old values.

Signed-off-by: Ganesh Ajjanagadde 
---
 libavcodec/nellymoserenc.c | 11 +--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
index d998dba..e6023e3 100644
--- a/libavcodec/nellymoserenc.c
+++ b/libavcodec/nellymoserenc.c
@@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext *avctx)
 
 /* Generate overlap window */
 ff_init_ff_sine_windows(7);
-for (i = 0; i < POW_TABLE_SIZE; i++)
-pow_table[i] = pow(2, -i / 2048.0 - 3.0 + POW_TABLE_OFFSET);
+pow_table[0] = 1;
+pow_table[1024] = M_SQRT1_2;
+for (i = 1; i < 513; i++) {
+double tmp = exp2(-i / 2048.0);
+pow_table[i] = tmp;
+pow_table[1024-i] = M_SQRT1_2 / tmp;
+pow_table[1024+i] = tmp * M_SQRT1_2;
+pow_table[2048-i] = 0.5 / tmp;
+}
 
 if (s->avctx->trellis) {
 s->opt  = av_malloc(NELLY_BANDS * OPT_SIZE * sizeof(float  ));
-- 
2.6.3

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