chromium/third_party/libvpx/source/libvpx/vpx_ports/bitops.h

/*
 *  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#ifndef VPX_VPX_PORTS_BITOPS_H_
#define VPX_VPX_PORTS_BITOPS_H_

#include <assert.h>

#ifdef _MSC_VER
#if defined(_M_X64) || defined(_M_IX86)
#include <intrin.h>
#define USE_MSC_INTRINSICS
#endif
#endif

#ifdef __cplusplus
extern "C" {
#endif

// These versions of get_lsb() and get_msb() are only valid when n != 0
// because all of the optimized versions are undefined when n == 0:
// https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

// use GNU builtins where available.
#if defined(__GNUC__) && \
    ((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
static INLINE int get_lsb(unsigned int n) { … }

static INLINE int get_msb(unsigned int n) { … }
#elif defined(USE_MSC_INTRINSICS)
#pragma intrinsic(_BitScanForward)
#pragma intrinsic(_BitScanReverse)

static INLINE int get_lsb(unsigned int n) {
  unsigned long first_set_bit;  // NOLINT(runtime/int)
  _BitScanForward(&first_set_bit, n);
  return first_set_bit;
}

static INLINE int get_msb(unsigned int n) {
  unsigned long first_set_bit;
  assert(n != 0);
  _BitScanReverse(&first_set_bit, n);
  return first_set_bit;
}
#undef USE_MSC_INTRINSICS
#else
static INLINE int get_lsb(unsigned int n) {
  int i;
  assert(n != 0);
  for (i = 0; i < 32 && !(n & 1); ++i) n >>= 1;
  return i;
}

// Returns (int)floor(log2(n)). n must be > 0.
static INLINE int get_msb(unsigned int n) {
  int log = 0;
  unsigned int value = n;
  int i;

  assert(n != 0);

  for (i = 4; i >= 0; --i) {
    const int shift = (1 << i);
    const unsigned int x = value >> shift;
    if (x != 0) {
      value = x;
      log += shift;
    }
  }
  return log;
}
#endif

#ifdef __cplusplus
}  // extern "C"
#endif

#endif  // VPX_VPX_PORTS_BITOPS_H_