1/*
2 *  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
3 *
4 *  Use of this source code is governed by a BSD-style license
5 *  that can be found in the LICENSE file in the root of the source
6 *  tree. An additional intellectual property rights grant can be found
7 *  in the file PATENTS.  All contributing project authors may
8 *  be found in the AUTHORS file in the root of the source tree.
9 */
10
11#ifndef VPX_PORTS_BITOPS_H_
12#define VPX_PORTS_BITOPS_H_
13
14#include <assert.h>
15
16#include "vpx_ports/msvc.h"
17
18#ifdef _MSC_VER
19# include <math.h>  // the ceil() definition must precede intrin.h
20# if _MSC_VER > 1310 && (defined(_M_X64) || defined(_M_IX86))
21#  include <intrin.h>
22#  define USE_MSC_INTRINSICS
23# endif
24#endif
25
26#ifdef __cplusplus
27extern "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)
37static 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
44static 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.
53static 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  // VPX_PORTS_BITOPS_H_
77