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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <atomic>
#include <cstring>
#include <type_traits>
#include "mongo/base/static_assert.h"
#include "mongo/stdx/type_traits.h"
namespace mongo {
namespace atomic_word_detail {
enum class Category { kBasic, kArithmetic };
template <typename T>
constexpr Category getCategory() {
if (std::is_integral<T>() && !std::is_same<T, bool>())
return Category::kArithmetic;
return Category::kBasic;
}
template <typename T, Category = getCategory<T>()>
class Base;
/**
* Implementation of the AtomicWord interface in terms of the C++11 Atomics.
* Defines the operations provided by a non-incrementable AtomicWord.
* All operations have sequentially consistent semantics unless otherwise noted.
*/
template <typename T>
class Base<T, Category::kBasic> {
public:
/**
* Underlying value type.
*/
using WordType = T;
/**
* Construct a new word, default-initialized.
*/
constexpr Base() : _value() {}
/**
* Construct a new word with the given initial value.
*/
explicit constexpr Base(WordType v) : _value(v) {}
/**
* Gets the current value of this AtomicWord.
*/
WordType load() const {
return _value.load();
}
/**
* Gets the current value of this AtomicWord.
*
* Has relaxed semantics.
*/
WordType loadRelaxed() const {
return _value.load(std::memory_order_relaxed);
}
/**
* Sets the value of this AtomicWord to "newValue".
*/
void store(WordType newValue) {
_value.store(newValue);
}
/**
* Atomically swaps the current value of this with "newValue".
*
* Returns the old value.
*/
WordType swap(WordType newValue) {
return _value.exchange(newValue);
}
/**
* Atomic compare and swap.
*
* If this value equals "expected", sets this to "newValue".
* Always returns the original of this.
*/
WordType compareAndSwap(WordType expected, WordType newValue) {
// NOTE: Subtle: compare_exchange mutates its first argument.
_value.compare_exchange_strong(expected, newValue);
return expected;
}
protected:
// MONGO_STATIC_ASSERT(std::atomic<WordType>::is_always_lockfree); // TODO C++17
std::atomic<WordType> _value; // NOLINT
};
/**
* Has the basic operations, plus some arithmetic operations.
*/
template <typename T>
class Base<T, Category::kArithmetic> : public Base<T, Category::kBasic> {
private:
using Parent = Base<T, Category::kBasic>;
using Parent::_value;
public:
using WordType = typename Parent::WordType;
using Parent::Parent;
/**
* Get the current value of this, add "increment" and store it, atomically.
*
* Returns the value of this before incrementing.
*/
WordType fetchAndAdd(WordType increment) {
return _value.fetch_add(increment);
}
/**
* Like "fetchAndAdd", but with relaxed memory order. Appropriate where relative
* order of operations doesn't matter. A stat counter, for example.
*/
WordType fetchAndAddRelaxed(WordType increment) {
return _value.fetch_add(increment, std::memory_order_relaxed);
}
/**
* Get the current value of this, subtract "decrement" and store it, atomically.
* Returns the value of this before decrementing.
*/
WordType fetchAndSubtract(WordType decrement) {
return _value.fetch_sub(decrement);
}
/**
* Get the current value of this, add "increment" and store it, atomically.
* Returns the value of this after incrementing.
*/
WordType addAndFetch(WordType increment) {
return fetchAndAdd(increment) + increment;
}
/**
* Get the current value of this, subtract "decrement" and store it, atomically.
* Returns the value of this after decrementing.
*/
WordType subtractAndFetch(WordType decrement) {
return fetchAndSubtract(decrement) - decrement;
}
};
} // namespace atomic_word_detail
/**
* Instantiations of AtomicWord must be trivially copyable.
*/
template <typename T>
class AtomicWord : public atomic_word_detail::Base<T> {
public:
MONGO_STATIC_ASSERT(!std::is_integral<T>::value ||
sizeof(T) == sizeof(atomic_word_detail::Base<T>));
using atomic_word_detail::Base<T>::Base;
};
} // namespace mongo
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