[FFmpeg-devel] [PATCH] aacsbr_fixed: prevent sbr gain from being zero

[Andreas Cadhalpun]

On 23.11.2016 03:22, Michael Niedermayer wrote:
> On Sun, Nov 13, 2016 at 09:29:11PM +0100, Andreas Cadhalpun wrote:
>> It causes division by zero crashes.
>>
>> Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
>> ---
>>  libavcodec/aacsbr_fixed.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/libavcodec/aacsbr_fixed.c b/libavcodec/aacsbr_fixed.c
>> index b26314a..d2a7027 100644
>> --- a/libavcodec/aacsbr_fixed.c
>> +++ b/libavcodec/aacsbr_fixed.c
>> @@ -429,6 +429,10 @@ static void sbr_gain_calc(AACContext *ac, SpectralBandReplication *sbr,
>>                                                  av_add_sf(FLOAT_1, sbr->e_curr[e][m]),
>>                                                  av_add_sf(FLOAT_1, sbr->q_mapped[e][m]))));
>>                  }
>> +                if (sbr->gain[e][m].mant == 0) {
>> +                    sbr->gain[e][m] = FLOAT_1;
>> +                    sbr->gain[e][m].exp = MIN_EXP;
>> +                }
> 
> Why is this not not needed for the float code ?

Because float can represent smaller absolute values than normalized SoftFloat.

> is there a shortcomming in the SoftFloat code ?

In a way, because the SoftFloat gets normalized, e.g.:
 * float: 2.16840434e-19 * 2.77555756e-17 / 10337.5293 = 5.82201677e-40
 * SoftFloat: av_div_sf(av_mul_sf({mant = 536870912, exp = -61}, {mant = 536870912, exp = -54}),
                        {mant = 677439305, exp = 14}) = {mant = 850858499, exp = -130} = FLOAT_0

Best regards,
Andreas