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// 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
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");
};
} // namespace Common
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