Re: [FFmpeg-devel] [PATCH] avcodec/rv60dec: Avoid branch when decoding cbp16

[Peter Ross <pross@xvid.org>]

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On Mon, May 19, 2025 at 12:06:02AM +0200, Andreas Rheinhardt wrote:
> Patch attached.
> 
> - Andreas

> From 02724d5792348bea618c049034dc0febf24a46ac Mon Sep 17 00:00:00 2001
> From: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> Date: Sun, 18 May 2025 23:12:03 +0200
> Subject: [PATCH] avcodec/rv60dec: Avoid branch when decoding cbp16
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
>  libavcodec/rv60dec.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/libavcodec/rv60dec.c b/libavcodec/rv60dec.c
> index d704ae512c..2bbcb1d620 100644
> --- a/libavcodec/rv60dec.c
> +++ b/libavcodec/rv60dec.c
> @@ -82,7 +82,7 @@ enum {
>  };
>  
>  static const VLCElem * cbp8_vlc[7][4];
> -static const VLCElem * cbp16_vlc[7][3][4];
> +static const VLCElem * cbp16_vlc[7][4][4];
>  
>  typedef struct {
>      const VLCElem * l0[2];
> @@ -137,12 +137,12 @@ static av_cold void rv60_init_static_data(void)
>  
>      for (int i = 0; i < 7; i++)
>          for (int j = 0; j < 4; j++)
> -            cbp8_vlc[i][j] = gen_vlc(rv60_cbp8_lens[i][j], 64, &state);
> +            cbp16_vlc[i][0][j] = cbp8_vlc[i][j] = gen_vlc(rv60_cbp8_lens[i][j], 64, &state);
>  
>      for (int i = 0; i < 7; i++)
>          for (int j = 0; j < 3; j++)
>              for (int k = 0; k < 4; k++)
> -                cbp16_vlc[i][j][k] = gen_vlc(rv60_cbp16_lens[i][j][k], 64, &state);
> +                cbp16_vlc[i][j + 1][k] = gen_vlc(rv60_cbp16_lens[i][j][k], 64, &state);
>  
>      build_coeff_vlc(rv60_intra_lens, intra_coeff_vlc, 5, &state);
>      build_coeff_vlc(rv60_inter_lens, inter_coeff_vlc, 7, &state);
> @@ -1650,10 +1650,7 @@ static int decode_super_cbp(GetBitContext * gb, const VLCElem * vlc[4])
>  static int decode_cbp16(GetBitContext * gb, int subset, int qp)
>  {
>      int cb_set = rv60_qp_to_idx[qp];
> -    if (!subset)
> -        return decode_super_cbp(gb, cbp8_vlc[cb_set]);
> -    else
> -        return decode_super_cbp(gb, cbp16_vlc[cb_set][subset - 1]);
> +    return decode_super_cbp(gb, cbp16_vlc[cb_set][subset]);
>  }
>  
>  static int decode_cu_r(RV60Context * s, AVFrame * frame, ThreadContext * thread, GetBitContext * gb, int xpos, int ypos, int log_size, int qp, int sel_qp)
> -- 
> 2.45.2

Looks okay. What was the motivation for this change. Speed up; any numbers?

-- Peter
(A907 E02F A6E5 0CD2 34CD 20D2 6760 79C5 AC40 DD6B)

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