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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift Atomics open source project
//
// Copyright (c) 2020 - 2023 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
import XCTest
import Atomics
import Dispatch
class LockFreeSingleConsumerStack<Element> {
struct Node {
let value: Element
var next: UnsafeMutablePointer<Node>?
}
typealias NodePtr = UnsafeMutablePointer<Node>
private var _last = UnsafeAtomic<NodePtr?>.create(nil)
private var _consumerCount = UnsafeAtomic<Int>.create(0)
private var foo = 0
deinit {
// Discard remaining nodes
while let _ = pop() {}
_last.destroy()
_consumerCount.destroy()
}
// Push the given element to the top of the stack.
// It is okay to concurrently call this in an arbitrary number of threads.
func push(_ value: Element) {
let new = NodePtr.allocate(capacity: 1)
new.initialize(to: Node(value: value, next: nil))
var done = false
var current = _last.load(ordering: .relaxed)
while !done {
new.pointee.next = current
(done, current) = _last.compareExchange(
expected: current,
desired: new,
ordering: .releasing)
}
}
// Pop and return the topmost element from the stack.
// This method does not support multiple overlapping concurrent calls.
func pop() -> Element? {
precondition(
_consumerCount.loadThenWrappingIncrement(ordering: .acquiring) == 0,
"Multiple consumers detected")
defer { _consumerCount.wrappingDecrement(ordering: .releasing) }
var done = false
var current = _last.load(ordering: .acquiring)
while let c = current {
(done, current) = _last.compareExchange(
expected: c,
desired: c.pointee.next,
ordering: .acquiring)
if done {
let result = c.move()
c.deallocate()
return result.value
}
}
return nil
}
}
class LockFreeSingleConsumerStackTests: XCTestCase {
func test_Basics() {
let stack = LockFreeSingleConsumerStack<Int>()
XCTAssertNil(stack.pop())
stack.push(0)
XCTAssertEqual(0, stack.pop())
stack.push(1)
stack.push(2)
stack.push(3)
stack.push(4)
XCTAssertEqual(4, stack.pop())
XCTAssertEqual(3, stack.pop())
XCTAssertEqual(2, stack.pop())
XCTAssertEqual(1, stack.pop())
XCTAssertNil(stack.pop())
}
func test_ConcurrentPushes() {
let stack = LockFreeSingleConsumerStack<(thread: Int, value: Int)>()
let numThreads = 100
let numValues = 10_000
DispatchQueue.concurrentPerform(iterations: numThreads) { thread in
for value in 1 ... numValues {
stack.push((thread: thread, value: value))
}
}
var expected: [Int] = Array(repeating: numValues, count: numThreads)
while let (thread, value) = stack.pop() {
XCTAssertEqual(expected[thread], value)
expected[thread] -= 1
}
XCTAssertEqual(Array(repeating: 0, count: numThreads), expected)
}
func test_ConcurrentPushesAndPops() {
let stack = LockFreeSingleConsumerStack<(thread: Int, value: Int)>()
let numThreads = 100
let numValues = 10_000
var perThreadSums: [Int] = Array(repeating: 0, count: numThreads)
let consumerQueue = DispatchQueue(label: "org.swift.background")
consumerQueue.async {
var count = 0
while count < numThreads * numValues {
// Note: busy wait
if let (thread, value) = stack.pop() {
perThreadSums[thread] += value
count += 1
}
}
}
DispatchQueue.concurrentPerform(iterations: numThreads + 1) { thread in
if thread < numThreads {
// Producers
for value in 0 ..< numValues {
stack.push((thread: thread, value: value))
}
}
}
consumerQueue.sync {
XCTAssertEqual(Array(repeating: numValues * (numValues - 1) / 2, count: numThreads), perThreadSums)
}
}
#if MANUAL_TEST_DISCOVERY
public static var allTests = [
("test_Basics", test_Basics),
("test_ConcurrentPushes", test_ConcurrentPushes),
("test_ConcurrentPushesAndPops", test_ConcurrentPushesAndPops),
]
#endif
}
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