faux-folly/folly/AtomicStruct.h

/*
 * Copyright 2015 Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef FOLLY_ATOMIC_STRUCT_H_
#define FOLLY_ATOMIC_STRUCT_H_

#include <atomic>
#include <type_traits>
#include <folly/Traits.h>
#include <string.h>
#include <stdint.h>

namespace folly {

namespace detail {
template <int N> struct AtomicStructIntPick {};
}

/// AtomicStruct<T> work like C++ atomics, but can be used on any POD
/// type <= 8 bytes.
template <
    typename T,
    template<typename> class Atom = std::atomic,
    typename Raw = typename detail::AtomicStructIntPick<sizeof(T)>::type>
class AtomicStruct {
  static_assert(alignof(T) <= alignof(Raw),
      "target type can't have stricter alignment than matching int");
  static_assert(sizeof(T) <= sizeof(Raw),
      "underlying type isn't big enough");
  static_assert(std::is_trivial<T>::value ||
                folly::IsTriviallyCopyable<T>::value,
      "target type must be trivially copyable");

  Atom<Raw> data;

  static Raw encode(T v) noexcept {
    // we expect the compiler to optimize away the memcpy, but without
    // it we would violate strict aliasing rules
    Raw d = 0;
    memcpy(&d, &v, sizeof(T));
    return d;
  }

  static T decode(Raw d) noexcept {
    T v;
    memcpy(&v, &d, sizeof(T));
    return v;
  }

 public:
  AtomicStruct() = default;
  ~AtomicStruct() = default;
  AtomicStruct(AtomicStruct<T> const &) = delete;
  AtomicStruct<T>& operator= (AtomicStruct<T> const &) = delete;

  constexpr /* implicit */ AtomicStruct(T v) noexcept : data(encode(v)) {}

  bool is_lock_free() const noexcept {
    return data.is_lock_free();
  }

  bool compare_exchange_strong(
          T& v0, T v1,
          std::memory_order mo = std::memory_order_seq_cst) noexcept {
    Raw d0 = encode(v0);
    bool rv = data.compare_exchange_strong(d0, encode(v1), mo);
    if (!rv) {
      v0 = decode(d0);
    }
    return rv;
  }

  bool compare_exchange_weak(
          T& v0, T v1,
          std::memory_order mo = std::memory_order_seq_cst) noexcept {
    Raw d0 = encode(v0);
    bool rv = data.compare_exchange_weak(d0, encode(v1), mo);
    if (!rv) {
      v0 = decode(d0);
    }
    return rv;
  }

  T exchange(T v, std::memory_order mo = std::memory_order_seq_cst) noexcept {
    return decode(data.exchange(encode(v), mo));
  }

  /* implicit */ operator T () const noexcept {
    return decode(data);
  }

  T load(std::memory_order mo = std::memory_order_seq_cst) const noexcept {
    return decode(data.load(mo));
  }

  T operator= (T v) noexcept {
    return decode(data = encode(v));
  }

  void store(T v, std::memory_order mo = std::memory_order_seq_cst) noexcept {
    data.store(encode(v), mo);
  }

  // std::atomic also provides volatile versions of all of the access
  // methods.  These are callable on volatile objects, and also can
  // theoretically have different implementations than their non-volatile
  // counterpart.  If someone wants them here they can easily be added
  // by duplicating the above code and the corresponding unit tests.
};

namespace detail {

template <> struct AtomicStructIntPick<1> { typedef uint8_t type; };
template <> struct AtomicStructIntPick<2> { typedef uint16_t type; };
template <> struct AtomicStructIntPick<3> { typedef uint32_t type; };
template <> struct AtomicStructIntPick<4> { typedef uint32_t type; };
template <> struct AtomicStructIntPick<5> { typedef uint64_t type; };
template <> struct AtomicStructIntPick<6> { typedef uint64_t type; };
template <> struct AtomicStructIntPick<7> { typedef uint64_t type; };
template <> struct AtomicStructIntPick<8> { typedef uint64_t type; };

} // namespace detail

} // namespace folly

#endif