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_PORTS_BITOPS_H_
  13 #define AOM_PORTS_BITOPS_H_
  14
  15 #include <assert.h>
  16
  17 #include "aom_ports/msvc.h"
  18
  19 #ifdef _MSC_VER
  20 #if defined(_M_X64) || defined(_M_IX86)
  21 #include <intrin.h>
  22 #define USE_MSC_INTRINSICS
  23 #endif
  24 #endif
  25
  26 #ifdef __cplusplus
  27 extern "C" {
  28 #endif
  29
  30 // These versions of get_msb() are only valid when n != 0 because all
  31 // of the optimized versions are undefined when n == 0:
  32 // https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
  33
  34 // use GNU builtins where available.
  35 #if defined(__GNUC__) && \
  36     ((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
  37 static INLINE int get_msb(unsigned int n) {
  38   assert(n != 0);
  39   return 31 ^ __builtin_clz(n);
  40 }
  41 #elif defined(USE_MSC_INTRINSICS)
  42 #pragma intrinsic(_BitScanReverse)
  43
  44 static INLINE int get_msb(unsigned int n) {
  45   unsigned long first_set_bit;
  46   assert(n != 0);
  47   _BitScanReverse(&first_set_bit, n);
  48   return first_set_bit;
  49 }
  50 #undef USE_MSC_INTRINSICS
  51 #else
  52 // Returns (int)floor(log2(n)). n must be > 0.
  53 static INLINE int get_msb(unsigned int n) {
  54   int log = 0;
  55   unsigned int value = n;
  56   int i;
  57
  58   assert(n != 0);
  59
  60   for (i = 4; i >= 0; --i) {
  61     const int shift = (1 << i);
  62     const unsigned int x = value >> shift;
  63     if (x != 0) {
  64       value = x;
  65       log += shift;
  66     }
  67   }
  68   return log;
  69 }
  70 #endif
  71
  72 #ifdef __cplusplus
  73 }  // extern "C"
  74 #endif
  75
  76 #endif  // AOM_PORTS_BITOPS_H_