Re: [FFmpeg-devel] [PATCHv4 2/4] lavc/mpegvideo: use H263DSP dequant function

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

Rémi Denis-Courmont:
> 
> 
> Le 11 juin 2024 13:37:57 GMT+03:00, Andreas Rheinhardt <andreas.rheinhardt@outlook.com> a écrit :
>> Rémi Denis-Courmont:
>>> ---
>>>  libavcodec/mpegvideo.c | 40 +++++++++-------------------------------
>>>  1 file changed, 9 insertions(+), 31 deletions(-)
>>>
>>> diff --git a/libavcodec/mpegvideo.c b/libavcodec/mpegvideo.c
>>> index 7af823b8bd..810888fc47 100644
>>> --- a/libavcodec/mpegvideo.c
>>> +++ b/libavcodec/mpegvideo.c
>>> @@ -201,13 +201,11 @@ static void dct_unquantize_mpeg2_inter_c(MpegEncContext *s,
>>>  static void dct_unquantize_h263_intra_c(MpegEncContext *s,
>>>                                    int16_t *block, int n, int qscale)
>>>  {
>>> -    int i, level, qmul, qadd;
>>> -    int nCoeffs;
>>> +    int qmul = qscale << 1;
>>> +    int qadd, nCoeffs;
>>>  
>>>      av_assert2(s->block_last_index[n]>=0 || s->h263_aic);
>>>  
>>> -    qmul = qscale << 1;
>>> -
>>>      if (!s->h263_aic) {
>>>          block[0] *= n < 4 ? s->y_dc_scale : s->c_dc_scale;
>>>          qadd = (qscale - 1) | 1;
>>> @@ -219,43 +217,20 @@ static void dct_unquantize_h263_intra_c(MpegEncContext *s,
>>>      else
>>>          nCoeffs= s->intra_scantable.raster_end[ s->block_last_index[n] ];
>>>  
>>> -    for(i=1; 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_intra(block, nCoeffs, qmul, qadd);
>>>  }
>>>  
>>>  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);
>>
>> What is the cost of these function calls?
> 
> The same as any other indirect function tail call. What do you want me to say?! Changing how FFmpeg handles assembler optimisations, or removing the upper level of indirection is outside the scope of this MR (and not very realistic).
> 
> For that matter, there are hundreds of indirect calls that could be optimised out on some platforms, notably almost all Armv8 NEON functions and, on x86-64, all MMX or SSE2 exclusives. I don't get nitpicking this one.

1. Given that we always build the C versions of dsp functions, the
latter claim is wrong. (It is only the functions without assembly
versions (for a given platform) for which indirect calls could be avoided.)
2. I am not specifically asking about the cost of the indirect call, but
of the call. The callee will only be called from one place in this
scenario (presuming (for the C versions) that the compiler inlines
h263_dct_unquantize_inter_c into h263_dct_unquantize_intra_c), so if it
were not for arch-specific optimized versions we would inline it in its
caller (just as it is now). The loop does not look like it benefits a
lot from vectorizing, so I am really wondering whether this optimization
is even a win when benchmarking the real thing (not only the loop).

- Andreas

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