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Optimization: use write index cache in pop #45

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34 changes: 10 additions & 24 deletions include/rigtorp/SPSCQueue.h
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Original file line number Diff line number Diff line change
Expand Up @@ -22,12 +22,12 @@ SOFTWARE.

#pragma once

#include <algorithm>
#include <atomic>
#include <cassert>
#include <cstddef>
#include <memory> // std::allocator
#include <new> // std::hardware_destructive_interference_size
#include <stdexcept>
#include <type_traits> // std::enable_if, std::is_*_constructible

#ifdef __has_cpp_attribute
Expand Down Expand Up @@ -58,15 +58,8 @@ template <typename T, typename Allocator = std::allocator<T>> class SPSCQueue {
explicit SPSCQueue(const size_t capacity,
const Allocator &allocator = Allocator())
: capacity_(capacity), allocator_(allocator) {
// The queue needs at least one element
if (capacity_ < 1) {
capacity_ = 1;
}
capacity_++; // Needs one slack element
// Prevent overflowing size_t
if (capacity_ > SIZE_MAX - 2 * kPadding) {
capacity_ = SIZE_MAX - 2 * kPadding;
}
// The queue needs at least one element plus one slack
capacity_ = std::min(std::max(capacity_ + 1, 2ul), SIZE_MAX - 2 * kPadding);

#if defined(__cpp_if_constexpr) && defined(__cpp_lib_void_t)
if constexpr (has_allocate_at_least<Allocator>::value) {
Expand Down Expand Up @@ -107,10 +100,7 @@ template <typename T, typename Allocator = std::allocator<T>> class SPSCQueue {
static_assert(std::is_constructible<T, Args &&...>::value,
"T must be constructible with Args&&...");
auto const writeIdx = writeIdx_.load(std::memory_order_relaxed);
auto nextWriteIdx = writeIdx + 1;
if (nextWriteIdx == capacity_) {
nextWriteIdx = 0;
}
auto nextWriteIdx = (writeIdx + 1) % capacity_;
while (nextWriteIdx == readIdxCache_) {
readIdxCache_ = readIdx_.load(std::memory_order_acquire);
}
Expand All @@ -124,10 +114,7 @@ template <typename T, typename Allocator = std::allocator<T>> class SPSCQueue {
static_assert(std::is_constructible<T, Args &&...>::value,
"T must be constructible with Args&&...");
auto const writeIdx = writeIdx_.load(std::memory_order_relaxed);
auto nextWriteIdx = writeIdx + 1;
if (nextWriteIdx == capacity_) {
nextWriteIdx = 0;
}
auto nextWriteIdx = (writeIdx + 1) % capacity_;
if (nextWriteIdx == readIdxCache_) {
readIdxCache_ = readIdx_.load(std::memory_order_acquire);
if (nextWriteIdx == readIdxCache_) {
Expand Down Expand Up @@ -180,13 +167,12 @@ template <typename T, typename Allocator = std::allocator<T>> class SPSCQueue {
static_assert(std::is_nothrow_destructible<T>::value,
"T must be nothrow destructible");
auto const readIdx = readIdx_.load(std::memory_order_relaxed);
assert(writeIdx_.load(std::memory_order_acquire) != readIdx &&
"Can only call pop() after front() has returned a non-nullptr");
slots_[readIdx + kPadding].~T();
auto nextReadIdx = readIdx + 1;
if (nextReadIdx == capacity_) {
nextReadIdx = 0;
if (readIdx == writeIdxCache_) {
writeIdxCache_ = writeIdx_.load(std::memory_order_acquire);
}
assert(writeIdxCache_ != readIdx && "Can only call pop() after front() has returned a non-nullptr");
slots_[readIdx + kPadding].~T();
auto nextReadIdx = (readIdx + 1) % capacity_;
readIdx_.store(nextReadIdx, std::memory_order_release);
}

Expand Down