Re: [FFmpeg-devel] [PATCH 2/4] lavc/mpegvideo: use H263DSP dequant function

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Rémi Denis-Courmont:
> Le lauantaina 6. heinäkuuta 2024, 18.23.00 EEST Andreas Rheinhardt a écrit :
>>>  static void dct_unquantize_h263_inter_c(MpegEncContext *s,
>>>  
>>>                                    int16_t *block, int n, int qscale)
>>>  
>>>  {
>>>
>>> -    int i, level, qmul, qadd;
>>> +    int qmul = qscale << 1;
>>> +    int qadd = (qscale - 1) | 1;
>>>
>>>      int nCoeffs;
>>>      
>>>      av_assert2(s->block_last_index[n]>=0);
>>>
>>> -    qadd = (qscale - 1) | 1;
>>> -    qmul = qscale << 1;
>>> -
>>>
>>>      nCoeffs= s->inter_scantable.raster_end[ s->block_last_index[n] ];
>>>
>>> -
>>> -    for(i=0; i<=nCoeffs; i++) {
>>> -        level = block[i];
>>> -        if (level) {
>>> -            if (level < 0) {
>>> -                level = level * qmul - qadd;
>>> -            } else {
>>> -                level = level * qmul + qadd;
>>> -            }
>>> -            block[i] = level;
>>> -        }
>>> -    }
>>> +    s->h263dsp.h263_dct_unquantize_inter(block, nCoeffs, qmul, qadd);
>>
>> This adds an indirection. I have asked you to actually benchmark this
>> code (and not only the DSP function you add), but you never did.
> 
> I already pointed out previously that this is the way this project does DSP 
> code. Certainly it would be nice to hard-code the path when there is only one 
> possible. This is often the case on Armv8 notably, and of course on platforms 
> without optimisations.
> 
> But that's a general problem way beyond the scope of this patchset. We always 
> add indirect function calls in this sort of situation, and I don't see why I 
> would have duty to benchmark it, so I am going to ignore this.

You have a duty to benchmark it because you add it where it wasn't before.

- Andreas

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