Tim's ilog module.

[ccan] / ccan / ilog / ilog.c

diff --git a/ccan/ilog/ilog.c b/ccan/ilog/ilog.c
new file mode 100644 (file)
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+++ b/ccan/ilog/ilog.c
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+/*(C) Timothy B. Terriberry (tterribe@xiph.org) 2001-2009 LGPL (v2 or later).*/
+#include "ilog.h"
+#include <limits.h>
+
+/*The fastest fallback strategy for platforms with fast multiplication appears
+   to be based on de Bruijn sequences~\cite{LP98}.
+  Tests confirmed this to be true even on an ARM11, where it is actually faster
+   than using the native clz instruction.
+  Define ILOG_NODEBRUIJN to use a simpler fallback on platforms where
+   multiplication or table lookups are too expensive.
+
+  @UNPUBLISHED{LP98,
+    author="Charles E. Leiserson and Harald Prokop",
+    title="Using de {Bruijn} Sequences to Index a 1 in a Computer Word",
+    month=Jun,
+    year=1998,
+    note="\url{http://supertech.csail.mit.edu/papers/debruijn.pdf}"
+  }*/
+#if !defined(ILOG_NODEBRUIJN)&& \
+ !defined(CLZ32)||!defined(CLZ64)&&LONG_MAX<9223372036854775807LL
+static const unsigned char DEBRUIJN_IDX32[32]={
+   0, 1,28, 2,29,14,24, 3,30,22,20,15,25,17, 4, 8,
+  31,27,13,23,21,19,16, 7,26,12,18, 6,11, 5,10, 9
+};
+#endif
+
+int ilog32(uint32_t _v){
+#if defined(CLZ32)
+  return (CLZ32_OFFS-CLZ32(_v))&-!!_v;
+#else
+/*On a Pentium M, this branchless version tested as the fastest version without
+   multiplications on 1,000,000,000 random 32-bit integers, edging out a
+   similar version with branches, and a 256-entry LUT version.*/
+# if defined(ILOG_NODEBRUIJN)
+  int ret;
+  int m;
+  ret=_v>0;
+  m=(_v>0xFFFFU)<<4;
+  _v>>=m;
+  ret|=m;
+  m=(_v>0xFFU)<<3;
+  _v>>=m;
+  ret|=m;
+  m=(_v>0xFU)<<2;
+  _v>>=m;
+  ret|=m;
+  m=(_v>3)<<1;
+  _v>>=m;
+  ret|=m;
+  ret+=_v>1;
+  return ret;
+/*This de Bruijn sequence version is faster if you have a fast multiplier.*/
+# else
+  int ret;
+  ret=_v>0;
+  _v|=_v>>1;
+  _v|=_v>>2;
+  _v|=_v>>4;
+  _v|=_v>>8;
+  _v|=_v>>16;
+  _v=(_v>>1)+1;
+  ret+=DEBRUIJN_IDX32[_v*0x77CB531U>>27&0x1F];
+  return ret;
+# endif
+#endif
+}
+
+int ilog64(uint64_t _v){
+#if defined(CLZ64)
+  return (CLZ64_OFFS-CLZ64(_v))&-!!_v;
+#else
+# if defined(ILOG_NODEBRUIJN)
+  uint32_t v;
+  int      ret;
+  int      m;
+  ret=_v>0;
+  m=(_v>0xFFFFFFFFU)<<5;
+  v=(uint32_t)(_v>>m);
+  ret|=m;
+  m=(v>0xFFFFU)<<4;
+  v>>=m;
+  ret|=m;
+  m=(v>0xFFU)<<3;
+  v>>=m;
+  ret|=m;
+  m=(v>0xFU)<<2;
+  v>>=m;
+  ret|=m;
+  m=(v>3)<<1;
+  v>>=m;
+  ret|=m;
+  ret+=v>1;
+  return ret;
+# else
+/*If we don't have a 64-bit word, split it into two 32-bit halves.*/
+#  if LONG_MAX<9223372036854775807LL
+  uint32_t v;
+  int      ret;
+  int      m;
+  ret=_v>0;
+  m=(_v>0xFFFFFFFFU)<<5;
+  v=(uint32_t)(_v>>m);
+  ret|=m;
+  v|=v>>1;
+  v|=v>>2;
+  v|=v>>4;
+  v|=v>>8;
+  v|=v>>16;
+  v=(v>>1)+1;
+  ret+=DEBRUIJN_IDX32[v*0x77CB531U>>27&0x1F];
+  return ret;
+/*Otherwise do it in one 64-bit operation.*/
+#  else
+  static const unsigned char DEBRUIJN_IDX64[64]={
+     0, 1, 2, 7, 3,13, 8,19, 4,25,14,28, 9,34,20,40,
+     5,17,26,38,15,46,29,48,10,31,35,54,21,50,41,57,
+    63, 6,12,18,24,27,33,39,16,37,45,47,30,53,49,56,
+    62,11,23,32,36,44,52,55,61,22,43,51,60,42,59,58
+  };
+  int ret;
+  ret=_v>0;
+  _v|=_v>>1;
+  _v|=_v>>2;
+  _v|=_v>>4;
+  _v|=_v>>8;
+  _v|=_v>>16;
+  _v|=_v>>32;
+  _v=(_v>>1)+1;
+  ret+=DEBRUIJN_IDX64[_v*0x218A392CD3D5DBF>>58&0x3F];
+  return ret;
+#  endif
+# endif
+#endif
+}