Optimise av_log2 with clz when available

10% faster flac decoding on x86 and ARM.

Originally committed as revision 21217 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/common.h

@@ -117,6 +117,10 @@
 #endif
 #endif
 
+#ifdef HAVE_AV_CONFIG_H
+#   include "intmath.h"
+#endif
+
 //rounded division & shift
 #define RSHIFT(a,b) ((a) > 0 ? ((a) + ((1<<(b))>>1))>>(b) : ((a) + ((1<<(b))>>1)-1)>>(b))
 /* assume b>0 */
@@ -138,6 +142,7 @@ extern const uint8_t ff_log2_tab[256];
 
 extern const uint8_t av_reverse[256];
 
+#ifndef av_log2
 static inline av_const int av_log2(unsigned int v)
 {
     int n = 0;
@@ -153,7 +158,9 @@ static inline av_const int av_log2(unsigned int v)
 
     return n;
 }
+#endif
 
+#ifndef av_log2_16bit
 static inline av_const int av_log2_16bit(unsigned int v)
 {
     int n = 0;
@@ -165,6 +172,7 @@ static inline av_const int av_log2_16bit(unsigned int v)
 
     return n;
 }
+#endif
 
 /**
  * Clips a signed integer value into the amin-amax range.

libavutil/intmath.h

+/*
+ * Copyright (c) 2010 Mans Rullgard <mans@mansr.com>
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVUTIL_INTMATH_H
+#define AVUTIL_INTMATH_H
+
+#include "config.h"
+#include "common.h"
+
+#if HAVE_FAST_CLZ && AV_GCC_VERSION_AT_LEAST(3,4)
+
+#ifndef av_log2
+
+#define av_log2(x) (31 - __builtin_clz((x)|1))
+
+#ifndef av_log2_16bit
+#define av_log2_16bit av_log2
+#endif
+
+#endif /* av_log2 */
+
+#endif /* AV_GCC_VERSION_AT_LEAST(3,4) */
+
+#endif /* AVUTIL_INTMATH_H */