text: // 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.
  
  #ifndef V8_BASE_BITS_H_
  #define V8_BASE_BITS_H_
  
  #include <stdint.h>
  #include <type_traits>
  
  #include "src/base/base-export.h"
  #include "src/base/macros.h"
  #if V8_CC_MSVC
  #include <intrin.h>
  #endif
  #if V8_OS_WIN32
  #include "src/base/win32-headers.h"
  #endif
  
  namespace v8 {
  namespace base {
  namespace bits {
  
  // CountPopulation(value) returns the number of bits set in |value|.
  template <typename T>
  constexpr inline
      typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 8,
                              unsigned>::type
      CountPopulation(T value) {
    STATIC_ASSERT(sizeof(T) <= 8);
  #if V8_HAS_BUILTIN_POPCOUNT
    return sizeof(T) == 8 ? __builtin_popcountll(static_cast<uint64_t>(value))
                          : __builtin_popcount(static_cast<uint32_t>(value));
  #else
    constexpr uint64_t mask[] = {0x5555555555555555, 0x3333333333333333,
                                 0x0f0f0f0f0f0f0f0f};
    // Start with 1 bit wide buckets of [0,1].
    value = ((value >> 1) & mask[0]) + (value & mask[0]);
    // Having 2 bit wide buckets of [0,2] now.
    value = ((value >> 2) & mask[1]) + (value & mask[1]);
    // Having 4 bit wide buckets of [0,4] now.
    value = (value >> 4) + value;
    // Having 4 bit wide buckets of [0,8] now.
    if (sizeof(T) > 1)
      value = ((value >> (sizeof(T) > 1 ? 8 : 0)) & mask[2]) + (value & mask[2]);
    // Having 8 bit wide buckets of [0,16] now.
    if (sizeof(T) > 2) value = (value >> (sizeof(T) > 2 ? 16 : 0)) + value;
    // Having 8 bit wide buckets of [0,32] now.
    if (sizeof(T) > 4) value = (value >> (sizeof(T) > 4 ? 32 : 0)) + value;
    // Having 8 bit wide buckets of [0,64] now.
    return static_cast<unsigned>(value & 0xff);
  #endif
  }
  
  // ReverseBits(value) returns |value| in reverse bit order.
  template <typename T>
  T ReverseBits(T value) {
    STATIC_ASSERT((sizeof(value) == 1) || (sizeof(value) == 2) ||
                  (sizeof(value) == 4) || (sizeof(value) == 8));
    T result = 0;
    for (unsigned i = 0; i < (sizeof(value) * 8); i++) {
      result = (result << 1) | (value & 1);
      value >>= 1;
    }
    return result;
  }
  
  // CountLeadingZeros(value) returns the number of zero bits following the most
  // significant 1 bit in |value| if |value| is non-zero, otherwise it returns
  // {sizeof(T) * 8}.
  template <typename T, unsigned bits = sizeof(T) * 8>
  inline constexpr
      typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 8,
                              unsigned>::type
      CountLeadingZeros(T value) {
    static_assert(bits > 0, "invalid instantiation");
  #if V8_HAS_BUILTIN_CLZ
    return value == 0
               ? bits
               : bits == 64
                     ? __builtin_clzll(static_cast<uint64_t>(value))
                     : __builtin_clz(static_cast<uint32_t>(value)) - (32 - bits);
  #else
    // Binary search algorithm taken from "Hacker's Delight" (by Henry S. Warren,
    // Jr.), figures 5-11 and 5-12.
    if (bits == 1) return static_cast<unsigned>(value) ^ 1;
    T upper_half = value >> (bits / 2);
    T next_value = upper_half != 0 ? upper_half : value;
    unsigned add = upper_half != 0 ? 0 : bits / 2;
    constexpr unsigned next_bits = bits == 1 ? 1 : bits / 2;
    return CountLeadingZeros<T, next_bits>(next_value) + add;
  #endif
  }
  
  inline constexpr unsigned CountLeadingZeros32(uint32_t value) {
    return CountLeadingZeros(value);
  }
  inline constexpr unsigned CountLeadingZeros64(uint64_t value) {
    return CountLeadingZeros(value);
  }
  
  // CountTrailingZeros(value) returns the number of zero bits preceding the
  // least significant 1 bit in |value| if |value| is non-zero, otherwise it
  // returns {sizeof(T) * 8}.
  template <typename T, unsigned bits = sizeof(T) * 8>
  inline constexpr
      typename std::enable_if<std::is_integral<T>::value && sizeof(T) <= 8,
                              unsigned>::type
      CountTrailingZeros(T value) {
  #if V8_HAS_BUILTIN_CTZ
    return value == 0 ? bits
                      : bits == 64 ? __builtin_ctzll(static_cast<uint64_t>(value))
                                   : __builtin_ctz(static_cast<uint32_t>(value));
  #else
    // Fall back to popcount (see "Hacker's Delight" by Henry S. Warren, Jr.),
    // chapter 5-4. On x64, since is faster than counting in a loop and faster
    // than doing binary search.
    using U = typename std::make_unsigned<T>::type;
    U u = value;
    return CountPopulation(static_cast<U>(~u & (u - 1u)));
  #endif
  }
  
  inline constexpr unsigned CountTrailingZeros32(uint32_t value) {
    return CountTrailingZeros(value);
  }
  inline constexpr unsigned CountTrailingZeros64(uint64_t value) {
    return CountTrailingZeros(value);
  }
  
  // Returns true iff |value| is a power of 2.
  template <typename T,
            typename = typename std::enable_if<std::is_integral<T>::value ||
                                               std::is_enum<T>::value>::type>
  constexpr inline bool IsPowerOfTwo(T value) {
    return value > 0 && (value & (value - 1)) == 0;
  }
  
  // Identical to {CountTrailingZeros}, but only works for powers of 2.
  template <typename T,
            typename = typename std::enable_if<std::is_integral<T>::value>::type>
  inline constexpr int WhichPowerOfTwo(T value) {
  #if V8_HAS_CXX14_CONSTEXPR
    DCHECK(IsPowerOfTwo(value));
  #endif
  #if V8_HAS_BUILTIN_CTZ
    STATIC_ASSERT(sizeof(T) <= 8);
    return sizeof(T) == 8 ? __builtin_ctzll(static_cast<uint64_t>(value))
                          : __builtin_ctz(static_cast<uint32_t>(value));
  #else
    // Fall back to popcount (see "Hacker's Delight" by Henry S. Warren, Jr.),
    // chapter 5-4. On x64, since is faster than counting in a loop and faster
    // than doing binary search.
    using U = typename std::make_unsigned<T>::type;
    U u = value;
    return CountPopulation(static_cast<U>(u - 1));
  #endif
  }
  
  // RoundUpToPowerOfTwo32(value) returns the smallest power of two which is
  // greater than or equal to |value|. If you pass in a |value| that is already a
  // power of two, it is returned as is. |value| must be less than or equal to
  // 0x80000000u. Uses computation based on leading zeros if we have compiler
  // support for that. Falls back to the implementation from "Hacker's Delight" by
  // Henry S. Warren, Jr., figure 3-3, page 48, where the function is called clp2.
  V8_BASE_EXPORT uint32_t RoundUpToPowerOfTwo32(uint32_t value);
  // Same for 64 bit integers. |value| must be <= 2^63
  V8_BASE_EXPORT uint64_t RoundUpToPowerOfTwo64(uint64_t value);
  // Same for size_t integers.
  inline size_t RoundUpToPowerOfTwo(size_t value) {
    if (sizeof(size_t) == sizeof(uint64_t)) {
      return RoundUpToPowerOfTwo64(value);
    } else {
      return RoundUpToPowerOfTwo32(value);
    }
  }
  
  // RoundDownToPowerOfTwo32(value) returns the greatest power of two which is
  // less than or equal to |value|. If you pass in a |value| that is already a
  // power of two, it is returned as is.
  inline uint32_t RoundDownToPowerOfTwo32(uint32_t value) {
    if (value > 0x80000000u) return 0x80000000u;
    uint32_t result = RoundUpToPowerOfTwo32(value);
    if (result > value) result >>= 1;
    return result;
  }
  
  
  // Precondition: 0 <= shift < 32
  inline constexpr uint32_t RotateRight32(uint32_t value, uint32_t shift) {
    return (value >> shift) | (value << ((32 - shift) & 31));
  }
  
  // Precondition: 0 <= shift < 32
  inline constexpr uint32_t RotateLeft32(uint32_t value, uint32_t shift) {
    return (value << shift) | (value >> ((32 - shift) & 31));
  }
  
  // Precondition: 0 <= shift < 64
  inline constexpr uint64_t RotateRight64(uint64_t value, uint64_t shift) {
    return (value >> shift) | (value << ((64 - shift) & 63));
  }
  
  // Precondition: 0 <= shift < 64
  inline constexpr uint64_t RotateLeft64(uint64_t value, uint64_t shift) {
    return (value << shift) | (value >> ((64 - shift) & 63));
  }
  
  // SignedAddOverflow32(lhs,rhs,val) performs a signed summation of |lhs| and
  // |rhs| and stores the result into the variable pointed to by |val| and
  // returns true if the signed summation resulted in an overflow.
  inline bool SignedAddOverflow32(int32_t lhs, int32_t rhs, int32_t* val) {
  #if V8_HAS_BUILTIN_SADD_OVERFLOW
    return __builtin_sadd_overflow(lhs, rhs, val);
  #else
    uint32_t res = static_cast<uint32_t>(lhs) + static_cast<uint32_t>(rhs);
    *val = bit_cast<int32_t>(res);
    return ((res ^ lhs) & (res ^ rhs) & (1U << 31)) != 0;
  #endif
  }
  
  
  // SignedSubOverflow32(lhs,rhs,val) performs a signed subtraction of |lhs| and
  // |rhs| and stores the result into the variable pointed to by |val| and
  // returns true if the signed subtraction resulted in an overflow.
  inline bool SignedSubOverflow32(int32_t lhs, int32_t rhs, int32_t* val) {
  #if V8_HAS_BUILTIN_SSUB_OVERFLOW
    return __builtin_ssub_overflow(lhs, rhs, val);
  #else
    uint32_t res = static_cast<uint32_t>(lhs) - static_cast<uint32_t>(rhs);
    *val = bit_cast<int32_t>(res);
    return ((res ^ lhs) & (res ^ ~rhs) & (1U << 31)) != 0;
  #endif
  }
  
  // SignedMulOverflow32(lhs,rhs,val) performs a signed multiplication of |lhs|
  // and |rhs| and stores the result into the variable pointed to by |val| and
  // returns true if the signed multiplication resulted in an overflow.
  V8_BASE_EXPORT bool SignedMulOverflow32(int32_t lhs, int32_t rhs, int32_t* val);
  
  // SignedAddOverflow64(lhs,rhs,val) performs a signed summation of |lhs| and
  // |rhs| and stores the result into the variable pointed to by |val| and
  // returns true if the signed summation resulted in an overflow.
  inline bool SignedAddOverflow64(int64_t lhs, int64_t rhs, int64_t* val) {
    uint64_t res = static_cast<uint64_t>(lhs) + static_cast<uint64_t>(rhs);
    *val = bit_cast<int64_t>(res);
    return ((res ^ lhs) & (res ^ rhs) & (1ULL << 63)) != 0;
  }
  
  
  // SignedSubOverflow64(lhs,rhs,val) performs a signed subtraction of |lhs| and
  // |rhs| and stores the result into the variable pointed to by |val| and
  // returns true if the signed subtraction resulted in an overflow.
  inline bool SignedSubOverflow64(int64_t lhs, int64_t rhs, int64_t* val) {
    uint64_t res = static_cast<uint64_t>(lhs) - static_cast<uint64_t>(rhs);
    *val = bit_cast<int64_t>(res);
    return ((res ^ lhs) & (res ^ ~rhs) & (1ULL << 63)) != 0;
  }
  
  // SignedMulHigh32(lhs, rhs) multiplies two signed 32-bit values |lhs| and
  // |rhs|, extracts the most significant 32 bits of the result, and returns
  // those.
  V8_BASE_EXPORT int32_t SignedMulHigh32(int32_t lhs, int32_t rhs);
  
  // SignedMulHighAndAdd32(lhs, rhs, acc) multiplies two signed 32-bit values
  // |lhs| and |rhs|, extracts the most significant 32 bits of the result, and
  // adds the accumulate value |acc|.
  V8_BASE_EXPORT int32_t SignedMulHighAndAdd32(int32_t lhs, int32_t rhs,
                                               int32_t acc);
  
  // SignedDiv32(lhs, rhs) divides |lhs| by |rhs| and returns the quotient
  // truncated to int32. If |rhs| is zero, then zero is returned. If |lhs|
  // is minint and |rhs| is -1, it returns minint.
  V8_BASE_EXPORT int32_t SignedDiv32(int32_t lhs, int32_t rhs);
  
  // SignedMod32(lhs, rhs) divides |lhs| by |rhs| and returns the remainder
  // truncated to int32. If either |rhs| is zero or |lhs| is minint and |rhs|
  // is -1, it returns zero.
  V8_BASE_EXPORT int32_t SignedMod32(int32_t lhs, int32_t rhs);
  
  // UnsignedAddOverflow32(lhs,rhs,val) performs an unsigned summation of |lhs|
  // and |rhs| and stores the result into the variable pointed to by |val| and
  // returns true if the unsigned summation resulted in an overflow.
  inline bool UnsignedAddOverflow32(uint32_t lhs, uint32_t rhs, uint32_t* val) {
  #if V8_HAS_BUILTIN_SADD_OVERFLOW
    return __builtin_uadd_overflow(lhs, rhs, val);
  #else
    *val = lhs + rhs;
    return *val < (lhs | rhs);
  #endif
  }
  
  
  // UnsignedDiv32(lhs, rhs) divides |lhs| by |rhs| and returns the quotient
  // truncated to uint32. If |rhs| is zero, then zero is returned.
  inline uint32_t UnsignedDiv32(uint32_t lhs, uint32_t rhs) {
    return rhs ? lhs / rhs : 0u;
  }
  
  
  // UnsignedMod32(lhs, rhs) divides |lhs| by |rhs| and returns the remainder
  // truncated to uint32. If |rhs| is zero, then zero is returned.
  inline uint32_t UnsignedMod32(uint32_t lhs, uint32_t rhs) {
    return rhs ? lhs % rhs : 0u;
  }
  
  
  // SignedSaturatedAdd64(lhs, rhs) adds |lhs| and |rhs|,
  // checks and returns the result.
  V8_BASE_EXPORT int64_t SignedSaturatedAdd64(int64_t lhs, int64_t rhs);
  
  // SignedSaturatedSub64(lhs, rhs) subtracts |lhs| by |rhs|,
  // checks and returns the result.
  V8_BASE_EXPORT int64_t SignedSaturatedSub64(int64_t lhs, int64_t rhs);
  
  }  // namespace bits
  }  // namespace base
  }  // namespace v8
  
  #endif  // V8_BASE_BITS_H_
  