[FFmpeg-devel] [PATCH] AAC decoder

[Robert Swain]

2008/5/23 Robert Swain <robert.swain at gmail.com>:
> 2008/5/23 Robert Swain <robert.swain at gmail.com>:
>> 2008/4/2 Michael Niedermayer <michaelni at gmx.at>:
>>> On Tue, Apr 01, 2008 at 04:56:48PM +0200, Andreas ?man wrote:
>>>> Andreas ?man wrote:
>>
>> [...]
>>
>>>> +static inline float ivquant(AACContext * ac, int a) {
>>>> +    static const float sign[2] = { -1., 1. };
>>>> +    int tmp = (a>>31);
>>>> +    int abs_a = (a^tmp)-tmp;
>>>> +    if (abs_a < sizeof(ac->ivquant_tab)/sizeof(ac->ivquant_tab[0]))
>>>> +        return sign[tmp+1] * ac->ivquant_tab[abs_a];
>>>
>>> What is the point of the sign splitout? it seems that it would be simpler
>>> to have that in teh table as well
>>
>> Kostya is in favour of removing the ivquant_tab table because it
>> caches only a small number of possible values and its general impact
>> on decoding speedup is not obvious.
>>
>> Attached is a patch that removes the ivquant_tab table and simplifies
>> and moves the ivquant() functionality into the calling loop and
>> removes the ivquant() function altogether as it isn't really needed to
>> wrap pow().
>
> Oops! sign * pow(abs(a), 4./3) != pow(a, 4./3) . Fixed patch attached
> with bit magic returned.
>
> I'll do some benchmarks too, just for good measure.

Actually, I won't. abs(a) is normally <16 but can be up to 4351 (if I
understood the escape sequence decoding).

http://article.gmane.org/gmane.comp.video.ffmpeg.soc/2002/match=ivquant

Andreas just did another benchmark and this has a large impact on decoding time.

I'll look at merging the sign into the table as originally suggested.
I suspect the table size can be reduced to 16 (vs 256) with little to
no impact on speed. Ignore the patch for the moment. Sorry for the
noise.

Rob