lib/ccan/ilog/ilog.c

   1 /*(C) Timothy B. Terriberry (tterribe@xiph.org) 2001-2009 LGPL (v2 or later).
   2  * See LICENSE file for details. */
   3 #include "ilog.h"
   4 #include <limits.h>
   5
   6 /*The fastest fallback strategy for platforms with fast multiplication appears
   7    to be based on de Bruijn sequences~\cite{LP98}.
   8   Tests confirmed this to be true even on an ARM11, where it is actually faster
   9    than using the native clz instruction.
  10   Define ILOG_NODEBRUIJN to use a simpler fallback on platforms where
  11    multiplication or table lookups are too expensive.
  12
  13   @UNPUBLISHED{LP98,
  14     author="Charles E. Leiserson and Harald Prokop",
  15     title="Using de {Bruijn} Sequences to Index a 1 in a Computer Word",
  16     month=Jun,
  17     year=1998,
  18     note="\url{http://supertech.csail.mit.edu/papers/debruijn.pdf}"
  19   }*/
  20 static UNNEEDED const unsigned char DEBRUIJN_IDX32[32]={
  21    0, 1,28, 2,29,14,24, 3,30,22,20,15,25,17, 4, 8,
  22   31,27,13,23,21,19,16, 7,26,12,18, 6,11, 5,10, 9
  23 };
  24
  25 /* We always compile these in, in case someone takes address of function. */
  26 #undef ilog32_nz
  27 #undef ilog32
  28 #undef ilog64_nz
  29 #undef ilog64
  30
  31 int ilog32(uint32_t _v){
  32 /*On a Pentium M, this branchless version tested as the fastest version without
  33    multiplications on 1,000,000,000 random 32-bit integers, edging out a
  34    similar version with branches, and a 256-entry LUT version.*/
  35 # if defined(ILOG_NODEBRUIJN)
  36   int ret;
  37   int m;
  38   ret=_v>0;
  39   m=(_v>0xFFFFU)<<4;
  40   _v>>=m;
  41   ret|=m;
  42   m=(_v>0xFFU)<<3;
  43   _v>>=m;
  44   ret|=m;
  45   m=(_v>0xFU)<<2;
  46   _v>>=m;
  47   ret|=m;
  48   m=(_v>3)<<1;
  49   _v>>=m;
  50   ret|=m;
  51   ret+=_v>1;
  52   return ret;
  53 /*This de Bruijn sequence version is faster if you have a fast multiplier.*/
  54 # else
  55   int ret;
  56   ret=_v>0;
  57   _v|=_v>>1;
  58   _v|=_v>>2;
  59   _v|=_v>>4;
  60   _v|=_v>>8;
  61   _v|=_v>>16;
  62   _v=(_v>>1)+1;
  63   ret+=DEBRUIJN_IDX32[_v*0x77CB531U>>27&0x1F];
  64   return ret;
  65 # endif
  66 }
  67
  68 int ilog32_nz(uint32_t _v)
  69 {
  70   return ilog32(_v);
  71 }
  72
  73 int ilog64(uint64_t _v){
  74 # if defined(ILOG_NODEBRUIJN)
  75   uint32_t v;
  76   int      ret;
  77   int      m;
  78   ret=_v>0;
  79   m=(_v>0xFFFFFFFFU)<<5;
  80   v=(uint32_t)(_v>>m);
  81   ret|=m;
  82   m=(v>0xFFFFU)<<4;
  83   v>>=m;
  84   ret|=m;
  85   m=(v>0xFFU)<<3;
  86   v>>=m;
  87   ret|=m;
  88   m=(v>0xFU)<<2;
  89   v>>=m;
  90   ret|=m;
  91   m=(v>3)<<1;
  92   v>>=m;
  93   ret|=m;
  94   ret+=v>1;
  95   return ret;
  96 # else
  97 /*If we don't have a 64-bit word, split it into two 32-bit halves.*/
  98 #  if LONG_MAX<9223372036854775807LL
  99   uint32_t v;
 100   int      ret;
 101   int      m;
 102   ret=_v>0;
 103   m=(_v>0xFFFFFFFFU)<<5;
 104   v=(uint32_t)(_v>>m);
 105   ret|=m;
 106   v|=v>>1;
 107   v|=v>>2;
 108   v|=v>>4;
 109   v|=v>>8;
 110   v|=v>>16;
 111   v=(v>>1)+1;
 112   ret+=DEBRUIJN_IDX32[v*0x77CB531U>>27&0x1F];
 113   return ret;
 114 /*Otherwise do it in one 64-bit operation.*/
 115 #  else
 116   static const unsigned char DEBRUIJN_IDX64[64]={
 117      0, 1, 2, 7, 3,13, 8,19, 4,25,14,28, 9,34,20,40,
 118      5,17,26,38,15,46,29,48,10,31,35,54,21,50,41,57,
 119     63, 6,12,18,24,27,33,39,16,37,45,47,30,53,49,56,
 120     62,11,23,32,36,44,52,55,61,22,43,51,60,42,59,58
 121   };
 122   int ret;
 123   ret=_v>0;
 124   _v|=_v>>1;
 125   _v|=_v>>2;
 126   _v|=_v>>4;
 127   _v|=_v>>8;
 128   _v|=_v>>16;
 129   _v|=_v>>32;
 130   _v=(_v>>1)+1;
 131   ret+=DEBRUIJN_IDX64[_v*0x218A392CD3D5DBF>>58&0x3F];
 132   return ret;
 133 #  endif
 134 # endif
 135 }
 136
 137 int ilog64_nz(uint64_t _v)
 138 {
 139   return ilog64(_v);
 140 }