[FFmpeg-devel] [PATCH] use MUL64 in ac3dec.c

[Reimar Döffinger]

On Tue, Jan 12, 2010 at 08:12:42PM +0300, Yuriy Kaminskiy wrote:
> Index: libavcodec/ac3dec.c
> ===================================================================
> --- libavcodec/ac3dec.c.orig	2010-01-12 19:05:46.000000000 +0300
> +++ libavcodec/ac3dec.c	2010-01-12 19:18:44.000000000 +0300
> @@ -420,10 +420,9 @@ static void calc_transform_coeffs_cpl(AC
>          int band_end = bin + s->cpl_band_sizes[band];
>          for (ch = 1; ch <= s->fbw_channels; ch++) {
>              if (s->channel_in_cpl[ch]) {
> -                int64_t cpl_coord = s->cpl_coords[ch][band];
> +                int cpl_coord = s->cpl_coords[ch][band] << 4;
>                  for (bin = band_start; bin < band_end; bin++) {
> -                    s->fixed_coeffs[ch][bin] = ((int64_t)s->fixed_coeffs[CPL_CH][bin] *
> -                                                cpl_coord) >> 23;
> +                    s->fixed_coeffs[ch][bin] = MULH(s->fixed_coeffs[CPL_CH][bin]<< 5, cpl_coord);

Going by what Justin said, shifting cpl_coord by 5 and fixed_coeffs by 4
instead should be safe(r) and might be even faster on some architectures.
This variant might be slightly slower on 64 bit architectures since there
it means one shift more - I don't think it matters though.