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

[Andreas Cadhalpun]

On 24.11.2016 18:29, Michael Niedermayer wrote:
> On Thu, Nov 24, 2016 at 01:15:54AM +0100, Andreas Cadhalpun wrote:
>> 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
> 
> hmm
> does increasing the limits for SoftFloats exponent solve this ?

Yes, patch is attached.

> If that works and doesnt break anything it would be good to have
> SoftFloat cover the range float has

I'm not entirely sure about not breaking anything. For example the output of
the softfloat test changes, but for some reason that test is not enabled anyway.

Best regards,
Andreas
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