[FFmpeg-devel] [PATCH 1/4] lavc/h263dsp: add DCT dequantisation functions

[Rémi Denis-Courmont]

Le lauantaina 6. heinäkuuta 2024, 19.19.35 EEST Andreas Rheinhardt a écrit :
> Rémi Denis-Courmont:
> > Le lauantaina 6. heinäkuuta 2024, 18.17.37 EEST Andreas Rheinhardt a écrit 
:
> >> Rémi Denis-Courmont:
> >>> Note that optimised implementations of these functions will be taken
> >>> into actual use only if MpegEncContext.dct_unquantize_h263_{inter,intra}
> >>> are *not* overloaded by existing optimisations.
> >>> ---
> >>> 
> >>>  libavcodec/h263dsp.c | 25 +++++++++++++++++++++++++
> >>>  libavcodec/h263dsp.h |  4 ++++
> >>>  2 files changed, 29 insertions(+)
> >>> 
> >>> diff --git a/libavcodec/h263dsp.c b/libavcodec/h263dsp.c
> >>> index 6a13353499..dc146bf821 100644
> >>> --- a/libavcodec/h263dsp.c
> >>> +++ b/libavcodec/h263dsp.c
> >>> @@ -19,10 +19,33 @@
> >>> 
> >>>  #include <stdint.h>
> >>>  
> >>>  #include "libavutil/attributes.h"
> >>> 
> >>> +#include "libavutil/avassert.h"
> >>> 
> >>>  #include "libavutil/common.h"
> >>>  #include "config.h"
> >>>  #include "h263dsp.h"
> >>> 
> >>> +static void h263_dct_unquantize_inter_c(int16_t *block, ptrdiff_t len,
> >>> +                                        int qmul, int qadd)
> >>> +{
> >>> +    for (ptrdiff_t i = 0; i <= len; i++) {
> >> 
> >> In the current code, the unquantizing inter does not need an initial
> >> check (the compiler can optimize the for-loop to a do-while-like loop).
> >> This will be lost here.
> > 
> > If this not the consequence of your own actions...
> > 
> > You asked:
> > - to use a signed type there, and
> > - to avoid the separate initial branch in the intra case.
> > 
> > And that is exactly how this ends up. You can't have it both ways.
> 
> One can have it both ways: Use a do-while loop.

That is only possible if the otherwise identical intra/inter code are 
duplicated, or if putting back the initial check in the intra case. The very 
check which you asked to remove.

So no, you can't have it both ways. I don't really care either way, but the 
check for intra needs to be *somewhere*.

-- 
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