azerothcore-wotlk/src/common/Threading/MPSCQueue.h

/*
 * This file is part of the AzerothCore Project. See AUTHORS file for Copyright information
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Affero General Public License as published by the
 * Free Software Foundation; either version 3 of the License, or (at your
 * option) any later version.
 *
 * 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 GNU Affero General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef MPSCQueue_h__
#define MPSCQueue_h__

#include <atomic>

namespace Acore::Impl
{
    // C++ implementation of Dmitry Vyukov's lock free MPSC queue
    // http://www.1024cores.net/home/lock-free-algorithms/queues/non-intrusive-mpsc-node-based-queue
    template<typename T>
    class MPSCQueueNonIntrusive
    {
    public:
        MPSCQueueNonIntrusive() : _head(new Node()), _tail(_head.load(std::memory_order_relaxed))
        {
            Node* front = _head.load(std::memory_order_relaxed);
            front->Next.store(nullptr, std::memory_order_relaxed);
        }

        ~MPSCQueueNonIntrusive()
        {
            T* output;
            while (Dequeue(output))
                delete output;

            Node* front = _head.load(std::memory_order_relaxed);
            delete front;
        }

        void Enqueue(T* input)
        {
            Node* node = new Node(input);
            Node* prevHead = _head.exchange(node, std::memory_order_acq_rel);
            prevHead->Next.store(node, std::memory_order_release);
        }

        bool Dequeue(T*& result)
        {
            Node* tail = _tail.load(std::memory_order_relaxed);
            Node* next = tail->Next.load(std::memory_order_acquire);
            if (!next)
                return false;

            result = next->Data;
            _tail.store(next, std::memory_order_release);
            delete tail;
            return true;
        }

    private:
        struct Node
        {
            Node() = default;
            explicit Node(T* data) : Data(data)
            {
                Next.store(nullptr, std::memory_order_relaxed);
            }

            T* Data;
            std::atomic<Node*> Next;
        };

        std::atomic<Node*> _head;
        std::atomic<Node*> _tail;

        MPSCQueueNonIntrusive(MPSCQueueNonIntrusive const&) = delete;
        MPSCQueueNonIntrusive& operator=(MPSCQueueNonIntrusive const&) = delete;
    };

    // C++ implementation of Dmitry Vyukov's lock free MPSC queue
    // http://www.1024cores.net/home/lock-free-algorithms/queues/intrusive-mpsc-node-based-queue
    template<typename T, std::atomic<T*> T::* IntrusiveLink>
    class MPSCQueueIntrusive
    {
    public:
        MPSCQueueIntrusive() : _dummyPtr(reinterpret_cast<T*>(std::addressof(_dummy))), _head(_dummyPtr), _tail(_dummyPtr)
        {
            // _dummy is constructed from aligned_storage and is intentionally left uninitialized (it might not be default constructible)
            // so we init only its IntrusiveLink here
            std::atomic<T*>* dummyNext = new (&(_dummyPtr->*IntrusiveLink)) std::atomic<T*>();
            dummyNext->store(nullptr, std::memory_order_relaxed);
        }

        ~MPSCQueueIntrusive()
        {
            T* output;
            while (Dequeue(output))
                delete output;
        }

        void Enqueue(T* input)
        {
            (input->*IntrusiveLink).store(nullptr, std::memory_order_release);
            T* prevHead = _head.exchange(input, std::memory_order_acq_rel);
            (prevHead->*IntrusiveLink).store(input, std::memory_order_release);
        }

        bool Dequeue(T*& result)
        {
            T* tail = _tail.load(std::memory_order_relaxed);
            T* next = (tail->*IntrusiveLink).load(std::memory_order_acquire);
            if (tail == _dummyPtr)
            {
                if (!next)
                    return false;

                _tail.store(next, std::memory_order_release);
                tail = next;
                next = (next->*IntrusiveLink).load(std::memory_order_acquire);
            }

            if (next)
            {
                _tail.store(next, std::memory_order_release);
                result = tail;
                return true;
            }

            T* head = _head.load(std::memory_order_acquire);
            if (tail != head)
                return false;

            Enqueue(_dummyPtr);
            next = (tail->*IntrusiveLink).load(std::memory_order_acquire);
            if (next)
            {
                _tail.store(next, std::memory_order_release);
                result = tail;
                return true;
            }
            return false;
        }

    private:
        std::aligned_storage_t<sizeof(T), alignof(T)> _dummy;
        T* _dummyPtr;
        std::atomic<T*> _head;
        std::atomic<T*> _tail;

        MPSCQueueIntrusive(MPSCQueueIntrusive const&) = delete;
        MPSCQueueIntrusive& operator=(MPSCQueueIntrusive const&) = delete;
    };
}

template<typename T, std::atomic<T*> T::* IntrusiveLink = nullptr>
using MPSCQueue = std::conditional_t<IntrusiveLink != nullptr, Acore::Impl::MPSCQueueIntrusive<T, IntrusiveLink>, Acore::Impl::MPSCQueueNonIntrusive<T>>;

#endif // MPSCQueue_h__