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