[FFmpeg-devel] [PATCH 2/2] lavc/aacsbr: sbr_dequant optimization

[Andreas Cadhalpun]

On 18.12.2015 16:52, Ganesh Ajjanagadde wrote:
> On Tue, Dec 15, 2015 at 9:53 PM, Ganesh Ajjanagadde
> <gajjanagadde at gmail.com> wrote:
>> This uses ff_exp2fi to get a speedup (~ 6x).
>>
>> sample benchmark (Haswell, GNU/Linux):
>> old:
>>   19102 decicycles in sbr_dequant,    1023 runs,      1 skips
>>   19002 decicycles in sbr_dequant,    2045 runs,      3 skips
>>   17638 decicycles in sbr_dequant,    4093 runs,      3 skips
>>   15825 decicycles in sbr_dequant,    8189 runs,      3 skips
>>   16404 decicycles in sbr_dequant,   16379 runs,      5 skips
>>
>> new:
>>    3063 decicycles in sbr_dequant,    1024 runs,      0 skips
>>    3049 decicycles in sbr_dequant,    2048 runs,      0 skips
>>    2968 decicycles in sbr_dequant,    4096 runs,      0 skips
>>    2818 decicycles in sbr_dequant,    8191 runs,      1 skips
>>    2853 decicycles in sbr_dequant,   16383 runs,      1 skips
>>
>> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>> ---
>>  libavcodec/aacsbr.c | 34 ++++++++++++++++++++++------------
>>  1 file changed, 22 insertions(+), 12 deletions(-)
>>
>> diff --git a/libavcodec/aacsbr.c b/libavcodec/aacsbr.c
>> index d1e3a91..15956e3 100644
>> --- a/libavcodec/aacsbr.c
>> +++ b/libavcodec/aacsbr.c
>> @@ -33,6 +33,7 @@
>>  #include "aacsbrdata.h"
>>  #include "aacsbr_tablegen.h"
>>  #include "fft.h"
>> +#include "internal.h"
>>  #include "aacps.h"
>>  #include "sbrdsp.h"
>>  #include "libavutil/internal.h"
>> @@ -73,15 +74,22 @@ static void sbr_dequant(SpectralBandReplication *sbr, int id_aac)
>>  {
>>      int k, e;
>>      int ch;
>> -
>> +    static const double exp2_tab[2] = {1, M_SQRT2};
>>      if (id_aac == TYPE_CPE && sbr->bs_coupling) {
>> -        float alpha      = sbr->data[0].bs_amp_res ?  1.0f :  0.5f;
>> -        float pan_offset = sbr->data[0].bs_amp_res ? 12.0f : 24.0f;
>> +        int pan_offset = sbr->data[0].bs_amp_res ? 12 : 24;
>>          for (e = 1; e <= sbr->data[0].bs_num_env; e++) {
>>              for (k = 0; k < sbr->n[sbr->data[0].bs_freq_res[e]]; k++) {
>> -                float temp1 = exp2f(sbr->data[0].env_facs_q[e][k] * alpha + 7.0f);
>> -                float temp2 = exp2f((pan_offset - sbr->data[1].env_facs_q[e][k]) * alpha);
>> -                float fac;
>> +                float temp1, temp2, fac;
>> +                if (sbr->data[0].bs_amp_res) {
>> +                    temp1 = ff_exp2fi(sbr->data[0].env_facs_q[e][k] + 7);
>> +                    temp2 = ff_exp2fi(pan_offset - sbr->data[1].env_facs_q[e][k]);
>> +                }
>> +                else {
>> +                    temp1 = ff_exp2fi((sbr->data[0].env_facs_q[e][k]>>1) + 7) *
>> +                            exp2_tab[sbr->data[0].env_facs_q[e][k] & 1];
>> +                    temp2 = ff_exp2fi((pan_offset - sbr->data[1].env_facs_q[e][k])>>1) *
>> +                            exp2_tab[(pan_offset - sbr->data[1].env_facs_q[e][k]) & 1];
>> +                }
>>                  if (temp1 > 1E20) {
>>                      av_log(NULL, AV_LOG_ERROR, "envelope scalefactor overflow in dequant\n");
>>                      temp1 = 1;
>> @@ -93,8 +101,8 @@ static void sbr_dequant(SpectralBandReplication *sbr, int id_aac)
>>          }
>>          for (e = 1; e <= sbr->data[0].bs_num_noise; e++) {
>>              for (k = 0; k < sbr->n_q; k++) {
>> -                float temp1 = exp2f(NOISE_FLOOR_OFFSET - sbr->data[0].noise_facs_q[e][k] + 1);
>> -                float temp2 = exp2f(12 - sbr->data[1].noise_facs_q[e][k]);
>> +                float temp1 = ff_exp2fi(NOISE_FLOOR_OFFSET - sbr->data[0].noise_facs_q[e][k] + 1);
>> +                float temp2 = ff_exp2fi(12 - sbr->data[1].noise_facs_q[e][k]);
>>                  float fac;
>>                  av_assert0(temp1 <= 1E20);
>>                  fac = temp1 / (1.0f + temp2);
>> @@ -104,11 +112,13 @@ static void sbr_dequant(SpectralBandReplication *sbr, int id_aac)
>>          }
>>      } else { // SCE or one non-coupled CPE
>>          for (ch = 0; ch < (id_aac == TYPE_CPE) + 1; ch++) {
>> -            float alpha = sbr->data[ch].bs_amp_res ? 1.0f : 0.5f;
>>              for (e = 1; e <= sbr->data[ch].bs_num_env; e++)
>>                  for (k = 0; k < sbr->n[sbr->data[ch].bs_freq_res[e]]; k++){
>> -                    sbr->data[ch].env_facs[e][k] =
>> -                        exp2f(alpha * sbr->data[ch].env_facs_q[e][k] + 6.0f);
>> +                    if (sbr->data[ch].bs_amp_res)
>> +                        sbr->data[ch].env_facs[e][k] = ff_exp2fi(sbr->data[ch].env_facs_q[e][k] + 6);
>> +                    else
>> +                        sbr->data[ch].env_facs[e][k] = ff_exp2fi((sbr->data[ch].env_facs_q[e][k]>>1) + 6)
>> +                                                       * exp2_tab[sbr->data[ch].env_facs_q[e][k] & 1];
>>                      if (sbr->data[ch].env_facs[e][k] > 1E20) {
>>                          av_log(NULL, AV_LOG_ERROR, "envelope scalefactor overflow in dequant\n");
>>                          sbr->data[ch].env_facs[e][k] = 1;
>> @@ -118,7 +128,7 @@ static void sbr_dequant(SpectralBandReplication *sbr, int id_aac)
>>              for (e = 1; e <= sbr->data[ch].bs_num_noise; e++)
>>                  for (k = 0; k < sbr->n_q; k++)
>>                      sbr->data[ch].noise_facs[e][k] =
>> -                        exp2f(NOISE_FLOOR_OFFSET - sbr->data[ch].noise_facs_q[e][k]);
>> +                        ff_exp2fi(NOISE_FLOOR_OFFSET - sbr->data[ch].noise_facs_q[e][k]);
>>          }
>>      }
>>  }
>> --
>> 2.6.4
>>
> 
> ping for aac maintainers: Rostislav? thanks.

I'm not an aac maintainer, but the change looks correct and the speedup nice, so this
patch should be OK.

One caveat is that NOISE_FLOOR_OFFSET is defined as FIXR(6.0f) and is thus a float here.
Since this patch makes that unnecessary, it's probably a good idea to change that
to simply 6.

Best regards,
Andreas