112 lines
3.1 KiB
C++
112 lines
3.1 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <atomic>
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#include <type_traits>
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#include <folly/Traits.h>
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namespace folly {
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namespace detail {
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template <typename T>
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struct atomic_ref_base {
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static_assert(sizeof(T) == sizeof(std::atomic<T>), "size mismatch");
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static_assert(alignof(T) == alignof(std::atomic<T>), "alignment mismatch");
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static_assert(is_trivially_copyable_v<T>, "value not trivially-copyable");
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explicit atomic_ref_base(T& ref) : ref_(ref) {}
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atomic_ref_base(atomic_ref_base const&) = default;
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void store(T desired, std::memory_order order = std::memory_order_seq_cst)
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const noexcept {
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return atomic().store(desired, order);
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}
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T load(std::memory_order order = std::memory_order_seq_cst) const noexcept {
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return atomic().load(order);
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}
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std::atomic<T>& atomic() const noexcept {
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return reinterpret_cast<std::atomic<T>&>(ref_); // ub dragons be here
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}
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T& ref_;
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};
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template <typename T>
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struct atomic_ref_integral_base : atomic_ref_base<T> {
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using atomic_ref_base<T>::atomic_ref_base;
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using atomic_ref_base<T>::atomic;
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T fetch_add(T arg, std::memory_order order = std::memory_order_seq_cst) const
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noexcept {
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return atomic().fetch_add(arg, order);
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}
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T fetch_sub(T arg, std::memory_order order = std::memory_order_seq_cst) const
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noexcept {
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return atomic().fetch_sub(arg, order);
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}
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};
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template <typename T>
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using atomic_ref_select = conditional_t<
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std::is_integral<T>::value,
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atomic_ref_integral_base<T>,
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atomic_ref_base<T>>;
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} // namespace detail
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// atomic_ref
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//
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// A very partial backport of std::atomic_ref from C++20, limited for now to
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// the common operations on counters for now. May become a complete backport
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// in the future.
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//
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// Relies on the assumption that `T&` is reinterpretable as `std::atomic<T>&`.
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// And that the required alignment for that reinterpretation is `alignof(T)`.
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// When that is not the case, *kaboom*.
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//
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// mimic: std::atomic_ref, C++20
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template <typename T>
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class atomic_ref : public detail::atomic_ref_select<T> {
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private:
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using base = detail::atomic_ref_select<T>;
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public:
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using base::base;
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};
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struct make_atomic_ref_t {
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template <
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typename T,
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std::enable_if_t<
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is_trivially_copyable_v<T> && sizeof(T) == sizeof(std::atomic<T>) &&
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alignof(T) == alignof(std::atomic<T>),
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int> = 0>
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atomic_ref<T> operator()(T& ref) const {
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return atomic_ref<T>{ref};
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}
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};
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FOLLY_INLINE_VARIABLE constexpr make_atomic_ref_t make_atomic_ref;
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} // namespace folly
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