early-access version 1255

src/common/threadsafe_queue.h

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+// Copyright 2010 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+// a simple lockless thread-safe,
+// single reader, single writer queue
+
+#include <atomic>
+#include <condition_variable>
+#include <cstddef>
+#include <mutex>
+#include <utility>
+
+namespace Common {
+template <typename T>
+class SPSCQueue {
+public:
+    SPSCQueue() {
+        write_ptr = read_ptr = new ElementPtr();
+    }
+    ~SPSCQueue() {
+        // this will empty out the whole queue
+        delete read_ptr;
+    }
+
+    [[nodiscard]] std::size_t Size() const {
+        return size.load();
+    }
+
+    [[nodiscard]] bool Empty() const {
+        return Size() == 0;
+    }
+
+    [[nodiscard]] T& Front() const {
+        return read_ptr->current;
+    }
+
+    template <typename Arg>
+    void Push(Arg&& t) {
+        // create the element, add it to the queue
+        write_ptr->current = std::forward<Arg>(t);
+        // set the next pointer to a new element ptr
+        // then advance the write pointer
+        ElementPtr* new_ptr = new ElementPtr();
+        write_ptr->next.store(new_ptr, std::memory_order_release);
+        write_ptr = new_ptr;
+
+        const size_t previous_size{size++};
+
+        // Acquire the mutex and then immediately release it as a fence.
+        // TODO(bunnei): This can be replaced with C++20 waitable atomics when properly supported.
+        // See discussion on https://github.com/yuzu-emu/yuzu/pull/3173 for details.
+        if (previous_size == 0) {
+            std::lock_guard lock{cv_mutex};
+        }
+        cv.notify_one();
+    }
+
+    void Pop() {
+        --size;
+
+        ElementPtr* tmpptr = read_ptr;
+        // advance the read pointer
+        read_ptr = tmpptr->next.load();
+        // set the next element to nullptr to stop the recursive deletion
+        tmpptr->next.store(nullptr);
+        delete tmpptr; // this also deletes the element
+    }
+
+    bool Pop(T& t) {
+        if (Empty())
+            return false;
+
+        --size;
+
+        ElementPtr* tmpptr = read_ptr;
+        read_ptr = tmpptr->next.load(std::memory_order_acquire);
+        t = std::move(tmpptr->current);
+        tmpptr->next.store(nullptr);
+        delete tmpptr;
+        return true;
+    }
+
+    T PopWait() {
+        if (Empty()) {
+            std::unique_lock lock{cv_mutex};
+            cv.wait(lock, [this]() { return !Empty(); });
+        }
+        T t;
+        Pop(t);
+        return t;
+    }
+
+    // not thread-safe
+    void Clear() {
+        size.store(0);
+        delete read_ptr;
+        write_ptr = read_ptr = new ElementPtr();
+    }
+
+private:
+    // stores a pointer to element
+    // and a pointer to the next ElementPtr
+    class ElementPtr {
+    public:
+        ElementPtr() {}
+        ~ElementPtr() {
+            ElementPtr* next_ptr = next.load();
+
+            if (next_ptr)
+                delete next_ptr;
+        }
+
+        T current;
+        std::atomic<ElementPtr*> next{nullptr};
+    };
+
+    ElementPtr* write_ptr;
+    ElementPtr* read_ptr;
+    std::atomic_size_t size{0};
+    std::mutex cv_mutex;
+    std::condition_variable cv;
+};
+
+// a simple thread-safe,
+// single reader, multiple writer queue
+
+template <typename T>
+class MPSCQueue {
+public:
+    [[nodiscard]] std::size_t Size() const {
+        return spsc_queue.Size();
+    }
+
+    [[nodiscard]] bool Empty() const {
+        return spsc_queue.Empty();
+    }
+
+    [[nodiscard]] T& Front() const {
+        return spsc_queue.Front();
+    }
+
+    template <typename Arg>
+    void Push(Arg&& t) {
+        std::lock_guard lock{write_lock};
+        spsc_queue.Push(t);
+    }
+
+    void Pop() {
+        return spsc_queue.Pop();
+    }
+
+    bool Pop(T& t) {
+        return spsc_queue.Pop(t);
+    }
+
+    T PopWait() {
+        return spsc_queue.PopWait();
+    }
+
+    // not thread-safe
+    void Clear() {
+        spsc_queue.Clear();
+    }
+
+private:
+    SPSCQueue<T> spsc_queue;
+    std::mutex write_lock;
+};
+} // namespace Common