src/v8/src/base/bits.cc

// Copyright 2014 the V8 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.

#include "src/base/bits.h"

#include <limits>

#include "src/base/logging.h"

namespace v8 {
namespace base {
namespace bits {

uint32_t RoundUpToPowerOfTwo32(uint32_t value) {
  DCHECK_LE(value, uint32_t{1} << 31);
  if (value) --value;
// Use computation based on leading zeros if we have compiler support for that.
#if V8_HAS_BUILTIN_CLZ || V8_CC_MSVC
  return 1u << (32 - CountLeadingZeros(value));
#else
  value |= value >> 1;
  value |= value >> 2;
  value |= value >> 4;
  value |= value >> 8;
  value |= value >> 16;
  return value + 1;
#endif
}

uint64_t RoundUpToPowerOfTwo64(uint64_t value) {
  DCHECK_LE(value, uint64_t{1} << 63);
  if (value) --value;
// Use computation based on leading zeros if we have compiler support for that.
#if V8_HAS_BUILTIN_CLZ
  return uint64_t{1} << (64 - CountLeadingZeros(value));
#else
  value |= value >> 1;
  value |= value >> 2;
  value |= value >> 4;
  value |= value >> 8;
  value |= value >> 16;
  value |= value >> 32;
  return value + 1;
#endif
}


int32_t SignedMulHigh32(int32_t lhs, int32_t rhs) {
  int64_t const value = static_cast<int64_t>(lhs) * static_cast<int64_t>(rhs);
  return bit_cast<int32_t, uint32_t>(bit_cast<uint64_t>(value) >> 32u);
}


int32_t SignedMulHighAndAdd32(int32_t lhs, int32_t rhs, int32_t acc) {
  return bit_cast<int32_t>(bit_cast<uint32_t>(acc) +
                           bit_cast<uint32_t>(SignedMulHigh32(lhs, rhs)));
}


int32_t SignedDiv32(int32_t lhs, int32_t rhs) {
  if (rhs == 0) return 0;
  if (rhs == -1) return lhs == std::numeric_limits<int32_t>::min() ? lhs : -lhs;
  return lhs / rhs;
}


int32_t SignedMod32(int32_t lhs, int32_t rhs) {
  if (rhs == 0 || rhs == -1) return 0;
  return lhs % rhs;
}

int64_t SignedSaturatedAdd64(int64_t lhs, int64_t rhs) {
  using limits = std::numeric_limits<int64_t>;
  // Underflow if {lhs + rhs < min}. In that case, return {min}.
  if (rhs < 0 && lhs < limits::min() - rhs) return limits::min();
  // Overflow if {lhs + rhs > max}. In that case, return {max}.
  if (rhs >= 0 && lhs > limits::max() - rhs) return limits::max();
  return lhs + rhs;
}

int64_t SignedSaturatedSub64(int64_t lhs, int64_t rhs) {
  using limits = std::numeric_limits<int64_t>;
  // Underflow if {lhs - rhs < min}. In that case, return {min}.
  if (rhs > 0 && lhs < limits::min() + rhs) return limits::min();
  // Overflow if {lhs - rhs > max}. In that case, return {max}.
  if (rhs <= 0 && lhs > limits::max() + rhs) return limits::max();
  return lhs - rhs;
}

bool SignedMulOverflow32(int32_t lhs, int32_t rhs, int32_t* val) {
  // Compute the result as {int64_t}, then check for overflow.
  int64_t result = int64_t{lhs} * int64_t{rhs};
  *val = static_cast<int32_t>(result);
  using limits = std::numeric_limits<int32_t>;
  return result < limits::min() || result > limits::max();
}

}  // namespace bits
}  // namespace base
}  // namespace v8