LockFreeFifoDyn does not require power of 2 sized buffer

Author: danngreen

util/lockfree_fifo_spsc_dyn.hh

@@ -1,5 +1,4 @@
 #pragma once
-#include "util/math.hh"
 #include <atomic>
 #include <optional>
 #include <vector>
@@ -9,18 +8,13 @@
 // For cheaply copied types
 // Will not overwrite -- put() returns false if buffer is full, and get() returns nullopt if buffer is empty.
 // Provide the buffer size in the constructor.
-// max_size_ MUST be a power of 2. If it's not, the next-highest power of 2 will be used for the size.
 //
 // Edge-case: max_size cannot be more than half the largest integer representable by size_t
 template<class T>
 class LockFreeFifoSpscDyn {
 public:
-	size_t max_size_;
-	size_t SIZE_MASK;
-
 	LockFreeFifoSpscDyn(size_t max_size)
-		: max_size_{MathTools::next_power_of_2(max_size)}
-		, SIZE_MASK{max_size_ - 1}
+		: max_size_{max_size}
 		, head_{0}
 		, tail_{0}
 		, buf_(max_size_) {
@@ -30,19 +24,15 @@ public:
 	// Useful, for example, if get() and put() happen at the same rates
 	// and you want to provide a fixed delay
 	LockFreeFifoSpscDyn(size_t max_size, size_t head)
-		: max_size_{MathTools::next_power_of_2(max_size)}
-		, SIZE_MASK{max_size_ - 1}
+		: max_size_{max_size}
 		, head_{head}
 		, tail_{0}
 		, buf_(max_size_) {
 	}
 
 	// Resets and resizes the vector. All contents are lost.
-	// max_size must be a power of 2 or else the next highest power of 2 will be used.
 	// If max_size does not change, do nothing.
-	// Edge-case: max_size cannot be more than half the largest integer representable by size_t
 	void resize(size_t new_max_size) {
-		new_max_size = MathTools::next_power_of_2(new_max_size);
 		if (new_max_size != max_size_) {
 			reset();
 
@@ -54,7 +44,6 @@ public:
 			buf_.resize(new_max_size);
 
 			max_size_ = new_max_size;
-			SIZE_MASK = max_size_ - 1;
 		}
 	}
 
@@ -67,7 +56,7 @@ public:
 		if ((tmp_head - tail_.load(std::memory_order_acquire)) == max_size_)
 			return false;
 
-		buf_[tmp_head & SIZE_MASK] = item;
+		buf_[tmp_head % max_size_] = item;
 		tmp_head++;
 		std::atomic_signal_fence(std::memory_order_release);
 		head_.store(tmp_head, std::memory_order_release);
@@ -101,7 +90,7 @@ public:
 			return std::nullopt;
 		}
 
-		auto item = buf_[tmp_tail & SIZE_MASK];
+		auto item = buf_[tmp_tail % max_size_];
 		tmp_tail++;
 		std::atomic_signal_fence(std::memory_order_release);
 		tail_.store(tmp_tail, std::memory_order_release);
@@ -115,7 +104,7 @@ public:
 			return false;
 		}
 
-		t = std::move(buf_[tmp_tail & SIZE_MASK]);
+		t = std::move(buf_[tmp_tail % max_size_]);
 		tmp_tail++;
 		std::atomic_signal_fence(std::memory_order_release);
 		tail_.store(tmp_tail, std::memory_order_release);
@@ -168,6 +157,7 @@ public:
 	}
 
 private:
+	size_t max_size_;
 	std::atomic<size_t> head_ = 0;
 	std::atomic<size_t> tail_ = 0;
 	std::vector<T> buf_;