flag_final.h

// The minimum atomic size we believe to generate lock free code, i.e. all
// trivially copyable types not bigger this size generate lock free code.
static constexpr int kMinLockFreeAtomicSize = 8;

// The same as kMinLockFreeAtomicSize but maximum atomic size. As double words
// might use two registers, we want to dispatch the logic for them.
#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
static constexpr int kMaxLockFreeAtomicSize = 16;
#else
static constexpr int kMaxLockFreeAtomicSize = 8;
#endif

// We can use atomic in cases when it fits in the register, trivially copyable
// in order to make memcpy operations.
template <typename T>
struct IsAtomicFlagTypeTrait {
  static constexpr bool value =
      (sizeof(T) <= kMaxLockFreeAtomicSize &&
       type_traits_internal::is_trivially_copyable<T>::value);
};

// Clang does not always produce cmpxchg16b instruction when alignment of a 16
// bytes type is not 16.
struct alignas(16) FlagsInternalTwoWordsType {
  int64_t first;
  int64_t second;
};

constexpr bool operator==(const FlagsInternalTwoWordsType& that,
                          const FlagsInternalTwoWordsType& other) {
  return that.first == other.first && that.second == other.second;
}
constexpr bool operator!=(const FlagsInternalTwoWordsType& that,
                          const FlagsInternalTwoWordsType& other) {
  return !(that == other);
}

constexpr int64_t SmallAtomicInit() { return 0xababababababababll; }

template <typename T, typename S = void>
struct BestAtomicType {
  using type = int64_t;
  static constexpr int64_t AtomicInit() { return SmallAtomicInit(); }
};

template <typename T>
struct BestAtomicType<
    T, typename std::enable_if<(kMinLockFreeAtomicSize < sizeof(T) &&
                                sizeof(T) <= kMaxLockFreeAtomicSize),
                               void>::type> {
  using type = FlagsInternalTwoWordsType;
  static constexpr FlagsInternalTwoWordsType AtomicInit() {
    return {SmallAtomicInit(), SmallAtomicInit()};
  }
};

template <typename T>
class Flag {
...
  // For some types, a copy of the current value is kept in an atomically
  // accessible field.
  union Atomics {
    // Using small atomic for small types.
    std::atomic<int64_t> small_atomic;
    template <typename T,
              typename K = typename std::enable_if<
                  (sizeof(T) <= kMinLockFreeAtomicSize), void>::type>
    int64_t load() const {
      return small_atomic.load(std::memory_order_acquire);
    }

#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
    // Using big atomics for big types.
    std::atomic<FlagsInternalTwoWordsType> big_atomic;
    template <typename T, typename K = typename std::enable_if<
                              (kMinLockFreeAtomicSize < sizeof(T) &&
                               sizeof(T) <= kMaxLockFreeAtomicSize),
                              void>::type>
    FlagsInternalTwoWordsType load() const {
      return big_atomic.load(std::memory_order_acquire);
    }
    constexpr Atomics()
        : big_atomic{FlagsInternalTwoWordsType{SmallAtomicInit(),
                                               SmallAtomicInit()}} {}
#else
    constexpr Atomics() : small_atomic{SmallAtomicInit()} {}
#endif
  };
  Atomics atomics_{};
  ...
};