+/*
+ * Copyright 2013 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