vpx/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_PORTS_BITOPS_H_
#define VPX_PORTS_BITOPS_H_

#include <assert.h>

#include "vpx_ports/msvc.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_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_msb(unsigned int n) {
  assert(n != 0);
  return 31 ^ __builtin_clz(n);
}
#elif defined(USE_MSC_INTRINSICS)
#pragma intrinsic(_BitScanReverse)

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
// 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_PORTS_BITOPS_H_