[FFmpeg-devel] [PATCH 12/12] WMAPRO: use some type punning in decode_coeffs()

[Reimar Döffinger]

On Sun, Sep 27, 2009 at 11:49:28AM +0100, Mans Rullgard wrote:
> ---
>  libavcodec/wmaprodec.c |   35 ++++++++++++++++++++++-------------
>  1 files changed, 22 insertions(+), 13 deletions(-)
> 
> diff --git a/libavcodec/wmaprodec.c b/libavcodec/wmaprodec.c
> index e483733..2598ff2 100644
> --- a/libavcodec/wmaprodec.c
> +++ b/libavcodec/wmaprodec.c
> @@ -766,6 +766,12 @@ static int decode_channel_transform(WMAProDecodeCtx* s)
>   */
>  static int decode_coeffs(WMAProDecodeCtx *s, int c)
>  {
> +    static const int fval_tab[16] = {
> +        0x00000000, 0x3f800000, 0x40000000, 0x40400000,
> +        0x40800000, 0x40a00000, 0x40c00000, 0x40e00000,
> +        0x41000000, 0x41100000, 0x41200000, 0x41300000,
> +        0x41400000, 0x41500000, 0x41600000, 0x41700000,
> +    };
>      int vlctable;
>      VLC* vlc;
>      WMAProChannelCtx* ci = &s->channel[c];
> @@ -801,29 +807,32 @@ static int decode_coeffs(WMAProDecodeCtx *s, int c)
>              for (i = 0; i < 4; i += 2) {
>                  idx = get_vlc2(&s->gb, vec2_vlc.table, VLCBITS, VEC2MAXDEPTH);
>                  if (idx == HUFF_VEC2_SIZE - 1) {
> -                    vals[i] = get_vlc2(&s->gb, vec1_vlc.table, VLCBITS, VEC1MAXDEPTH);
> -                    if (vals[i] == HUFF_VEC1_SIZE - 1)
> -                        vals[i] += ff_wma_get_large_val(&s->gb);
> -                    vals[i+1] = get_vlc2(&s->gb, vec1_vlc.table, VLCBITS, VEC1MAXDEPTH);
> -                    if (vals[i+1] == HUFF_VEC1_SIZE - 1)
> -                        vals[i+1] += ff_wma_get_large_val(&s->gb);
> +                    int v0, v1;
> +                    v0 = get_vlc2(&s->gb, vec1_vlc.table, VLCBITS, VEC1MAXDEPTH);
> +                    if (v0 == HUFF_VEC1_SIZE - 1)
> +                        v0 += ff_wma_get_large_val(&s->gb);
> +                    v1 = get_vlc2(&s->gb, vec1_vlc.table, VLCBITS, VEC1MAXDEPTH);
> +                    if (v1 == HUFF_VEC1_SIZE - 1)
> +                        v1 += ff_wma_get_large_val(&s->gb);
> +                    ((float*)vals)[i  ] = v0;
> +                    ((float*)vals)[i+1] = v1;
>                  } else {
> -                    vals[i]   = symbol_to_vec2[idx] >> 4;
> -                    vals[i+1] = symbol_to_vec2[idx] & 0xF;
> +                    vals[i]   = fval_tab[symbol_to_vec2[idx] >> 4 ];
> +                    vals[i+1] = fval_tab[symbol_to_vec2[idx] & 0xF];
>                  }
>              }
>          } else {
> -            vals[0] =  symbol_to_vec4[idx] >> 12;
> -            vals[1] = (symbol_to_vec4[idx] >> 8) & 0xF;
> -            vals[2] = (symbol_to_vec4[idx] >> 4) & 0xF;
> -            vals[3] =  symbol_to_vec4[idx]       & 0xF;
> +            vals[0] = fval_tab[ symbol_to_vec4[idx] >> 12      ];
> +            vals[1] = fval_tab[(symbol_to_vec4[idx] >> 8) & 0xF];
> +            vals[2] = fval_tab[(symbol_to_vec4[idx] >> 4) & 0xF];
> +            vals[3] = fval_tab[ symbol_to_vec4[idx]       & 0xF];
>          }
>  
>          /** decode sign */
>          for (i = 0; i < 4; i++) {
>              if (vals[i]) {
>                  int sign = get_bits1(&s->gb) - 1;
> -                ci->coeffs[cur_coeff] = (vals[i] ^ sign) - sign;
> +                *(uint32_t*)&ci->coeffs[cur_coeff] = vals[i] ^ sign<<31;
>                  num_zeros = 0;

Hmm... Have you tried how much of the speedup is just from using the
table?
I.e. making vals float, fval_tab float,
ci->coeffs[cur_coeff] = get_bits1(&s->gb) ? vals[i] : -vals[i];
?
Would be a bit less ugly and avoid aliasing issues, though since as you
confirmed in the other patch gcc is really stupid it might not make
anything faster...