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

[Reimar Döffinger]

Hello,
a real 64x64 multiply is really slow on 32 bit processors.
Patch below speeds up AC3 decoding on an Atom by 5 % overall
(fastest out of five runs: from 3.183 to 3.024 for my sample).

Index: libavcodec/ac3dec.c
===================================================================
--- libavcodec/ac3dec.c	(revision 21153)
+++ libavcodec/ac3dec.c	(working copy)
@@ -420,10 +420,9 @@
         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];
                 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] = MUL64(s->fixed_coeffs[CPL_CH][bin], cpl_coord) >> 23;
                 }
                 if (ch == 2 && s->phase_flags[band]) {
                     for (bin = band_start; bin < band_end; bin++)


Even faster (though possibly wrong, even though I hear no issues with
my samples) should be the variant with MULH, though I could not really
measure a difference:
Index: libavcodec/ac3dec.c
===================================================================
--- libavcodec/ac3dec.c	(revision 21153)
+++ libavcodec/ac3dec.c	(working copy)
@@ -420,10 +420,9 @@
         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] << 9;
                 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], cpl_coord);
                 }
                 if (ch == 2 && s->phase_flags[band]) {
                     for (bin = band_start; bin < band_end; bin++)