[FFmpeg-devel] [RFC] slightly "simplify" ff_ac3_bit_alloc_calc_psd

[Reimar Döffinger]

Hello,
this is one instruction less in the inner loop due to a "clever" use of
lea by the compiler instead of needing a cmov.
Might help more on machines without cmov, because on Intel Atom the
difference is quite unstable even with START_TIMER/STOP_TIMER.
Somewhat representative values (sorry, the numbers I got were
all next to useless):
new:
30846 dezicycles in calc_psd, 65501 runs, 35 skips
old:
31299 dezicycles in calc_psd, 65496 runs, 40 skips

Index: libavcodec/ac3.c
===================================================================
--- libavcodec/ac3.c	(revision 21191)
+++ libavcodec/ac3.c	(working copy)
@@ -112,9 +112,10 @@
         int v = psd[bin++];
         int band_end = FFMIN(band_start_tab[band+1], end);
         for (; bin < band_end; bin++) {
+            int max = FFMAX(v, psd[bin]);
             /* logadd */
-            int adr = FFMIN(FFABS(v - psd[bin]) >> 1, 255);
-            v = FFMAX(v, psd[bin]) + ff_ac3_log_add_tab[adr];
+            int adr = FFMIN(max - ((v + psd[bin] + 1) >> 1), 255);
+            v = max + ff_ac3_log_add_tab[adr];
         }
         band_psd[band++] = v;
     } while (end > band_start_tab[band]);