1 files changed, 167 insertions, 0 deletions
diff --git a/src/common/bounded_threadsafe_queue.h b/src/common/bounded_threadsafe_queue.h
new file mode 100644
index 000000000..7e465549b
--- /dev/null
+++ b/src/common/bounded_threadsafe_queue.h
@@ -0,0 +1,167 @@
+// SPDX-FileCopyrightText: Copyright (c) 2020 Erik Rigtorp <erik@rigtorp.se>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <atomic>
+#include <bit>
+#include <condition_variable>
+#include <memory>
+#include <mutex>
+#include <new>
+#include <stop_token>
+#include <type_traits>
+#include <utility>
+
+namespace Common {
+
+#if defined(__cpp_lib_hardware_interference_size)
+constexpr size_t hardware_interference_size = std::hardware_destructive_interference_size;
+#else
+constexpr size_t hardware_interference_size = 64;
+#endif
+
+#ifdef _MSC_VER
+#pragma warning(push)
+#pragma warning(disable : 4324)
+#endif
+
+template <typename T, size_t capacity = 0x400>
+class MPSCQueue {
+public:
+    explicit MPSCQueue() : allocator{std::allocator<Slot<T>>()} {
+        // Allocate one extra slot to prevent false sharing on the last slot
+        slots = allocator.allocate(capacity + 1);
+        // Allocators are not required to honor alignment for over-aligned types
+        // (see http://eel.is/c++draft/allocator.requirements#10) so we verify
+        // alignment here
+        if (reinterpret_cast<uintptr_t>(slots) % alignof(Slot<T>) != 0) {
+            allocator.deallocate(slots, capacity + 1);
+            throw std::bad_alloc();
+        }
+        for (size_t i = 0; i < capacity; ++i) {
+            std::construct_at(&slots[i]);
+        }
+        static_assert(std::has_single_bit(capacity), "capacity must be an integer power of 2");
+        static_assert(alignof(Slot<T>) == hardware_interference_size,
+                      "Slot must be aligned to cache line boundary to prevent false sharing");
+        static_assert(sizeof(Slot<T>) % hardware_interference_size == 0,
+                      "Slot size must be a multiple of cache line size to prevent "
+                      "false sharing between adjacent slots");
+        static_assert(sizeof(MPSCQueue) % hardware_interference_size == 0,
+                      "Queue size must be a multiple of cache line size to "
+                      "prevent false sharing between adjacent queues");
+    }
+
+    ~MPSCQueue() noexcept {
+        for (size_t i = 0; i < capacity; ++i) {
+            std::destroy_at(&slots[i]);
+        }
+        allocator.deallocate(slots, capacity + 1);
+    }
+
+    // The queue must be both non-copyable and non-movable
+    MPSCQueue(const MPSCQueue&) = delete;
+    MPSCQueue& operator=(const MPSCQueue&) = delete;
+
+    MPSCQueue(MPSCQueue&&) = delete;
+    MPSCQueue& operator=(MPSCQueue&&) = delete;
+
+    void Push(const T& v) noexcept {
+        static_assert(std::is_nothrow_copy_constructible_v<T>,
+                      "T must be nothrow copy constructible");
+        emplace(v);
+    }
+
+    template <typename P, typename = std::enable_if_t<std::is_nothrow_constructible_v<T, P&&>>>
+    void Push(P&& v) noexcept {
+        emplace(std::forward<P>(v));
+    }
+
+    void Pop(T& v, std::stop_token stop) noexcept {
+        auto const tail = tail_.fetch_add(1);
+        auto& slot = slots[idx(tail)];
+        if (!slot.turn.test()) {
+            std::unique_lock lock{cv_mutex};
+            cv.wait(lock, stop, [&slot] { return slot.turn.test(); });
+        }
+        v = slot.move();
+        slot.destroy();
+        slot.turn.clear();
+        slot.turn.notify_one();
+    }
+
+private:
+    template <typename U = T>
+    struct Slot {
+        ~Slot() noexcept {
+            if (turn.test()) {
+                destroy();
+            }
+        }
+
+        template <typename... Args>
+        void construct(Args&&... args) noexcept {
+            static_assert(std::is_nothrow_constructible_v<U, Args&&...>,
+                          "T must be nothrow constructible with Args&&...");
+            std::construct_at(reinterpret_cast<U*>(&storage), std::forward<Args>(args)...);
+        }
+
+        void destroy() noexcept {
+            static_assert(std::is_nothrow_destructible_v<U>, "T must be nothrow destructible");
+            std::destroy_at(reinterpret_cast<U*>(&storage));
+        }
+
+        U&& move() noexcept {
+            return reinterpret_cast<U&&>(storage);
+        }
+
+        // Align to avoid false sharing between adjacent slots
+        alignas(hardware_interference_size) std::atomic_flag turn{};
+        struct aligned_store {
+            struct type {
+                alignas(U) unsigned char data[sizeof(U)];
+            };
+        };
+        typename aligned_store::type storage;
+    };
+
+    template <typename... Args>
+    void emplace(Args&&... args) noexcept {
+        static_assert(std::is_nothrow_constructible_v<T, Args&&...>,
+                      "T must be nothrow constructible with Args&&...");
+        auto const head = head_.fetch_add(1);
+        auto& slot = slots[idx(head)];
+        slot.turn.wait(true);
+        slot.construct(std::forward<Args>(args)...);
+        slot.turn.test_and_set();
+        cv.notify_one();
+    }
+
+    constexpr size_t idx(size_t i) const noexcept {
+        return i & mask;
+    }
+
+    static constexpr size_t mask = capacity - 1;
+
+    // Align to avoid false sharing between head_ and tail_
+    alignas(hardware_interference_size) std::atomic<size_t> head_{0};
+    alignas(hardware_interference_size) std::atomic<size_t> tail_{0};
+
+    std::mutex cv_mutex;
+    std::condition_variable_any cv;
+
+    Slot<T>* slots;
+    [[no_unique_address]] std::allocator<Slot<T>> allocator;
+
+    static_assert(std::is_nothrow_copy_assignable_v<T> || std::is_nothrow_move_assignable_v<T>,
+                  "T must be nothrow copy or move assignable");
+
+    static_assert(std::is_nothrow_destructible_v<T>, "T must be nothrow destructible");
+};
+
+#ifdef _MSC_VER
+#pragma warning(pop)
+#endif
+
+} // namespace Common