aom_ports/bitops.h

   1 /*
   2  * Copyright (c) 2016, Alliance for Open Media. All rights reserved
   3  *
   4  * This source code is subject to the terms of the BSD 2 Clause License and
   5  * the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
   6  * was not distributed with this source code in the LICENSE file, you can
   7  * obtain it at www.aomedia.org/license/software. If the Alliance for Open
   8  * Media Patent License 1.0 was not distributed with this source code in the
   9  * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
  10  */
  11
  12 #ifndef AOM_AOM_PORTS_BITOPS_H_
  13 #define AOM_AOM_PORTS_BITOPS_H_
  14
  15 #include <assert.h>
  16
  17 #include "aom_ports/msvc.h"
  18 #include "config/aom_config.h"
  19
  20 #ifdef _MSC_VER
  21 #if defined(_M_X64) || defined(_M_IX86)
  22 #include <intrin.h>
  23 #define USE_MSC_INTRINSICS
  24 #endif
  25 #endif
  26
  27 #ifdef __cplusplus
  28 extern "C" {
  29 #endif
  30
  31 // get_msb:
  32 // Returns (int)floor(log2(n)). n must be > 0.
  33 // These versions of get_msb() are only valid when n != 0 because all
  34 // of the optimized versions are undefined when n == 0:
  35 // https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
  36
  37 // use GNU builtins where available.
  38 #if defined(__GNUC__) && \
  39     ((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
  40 static INLINE int get_msb(unsigned int n) {
  41   assert(n != 0);
  42   return 31 ^ __builtin_clz(n);
  43 }
  44 #elif defined(USE_MSC_INTRINSICS)
  45 #pragma intrinsic(_BitScanReverse)
  46
  47 static INLINE int get_msb(unsigned int n) {
  48   unsigned long first_set_bit;
  49   assert(n != 0);
  50   _BitScanReverse(&first_set_bit, n);
  51   return first_set_bit;
  52 }
  53 #undef USE_MSC_INTRINSICS
  54 #else
  55 static INLINE int get_msb(unsigned int n) {
  56   int log = 0;
  57   unsigned int value = n;
  58   int i;
  59
  60   assert(n != 0);
  61
  62   for (i = 4; i >= 0; --i) {
  63     const int shift = (1 << i);
  64     const unsigned int x = value >> shift;
  65     if (x != 0) {
  66       value = x;
  67       log += shift;
  68     }
  69   }
  70   return log;
  71 }
  72 #endif
  73
  74 #ifdef __cplusplus
  75 }  // extern "C"
  76 #endif
  77
  78 #endif  // AOM_AOM_PORTS_BITOPS_H_