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// RUN: %target-run-simple-swift(%import-libdispatch)
// REQUIRES: executable_test
// REQUIRES: libdispatch
// REQUIRES: synchronization
import Synchronization
import StdlibUnittest
import Dispatch
let suite = TestSuite("LockFreeSingleConsumerStackTests")
@available(SwiftStdlib 6.0, *)
class LockFreeSingleConsumerStack<Element> {
struct Node {
let value: Element
var next: UnsafeMutablePointer<Node>?
}
typealias NodePtr = UnsafeMutablePointer<Node>
private let _last = Atomic<NodePtr?>(nil)
private let _consumerCount = Atomic<Int>(0)
private var foo = 0
deinit {
// Discard remaining nodes
while let _ = pop() {}
}
// 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.wrappingAdd(1, ordering: .acquiring).oldValue == 0,
"Multiple consumers detected")
defer { _consumerCount.wrappingSubtract(1, 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
}
}
if #available(SwiftStdlib 6.0, *) {
suite.test("basics") {
let stack = LockFreeSingleConsumerStack<Int>()
expectNil(stack.pop())
stack.push(0)
expectEqual(0, stack.pop())
stack.push(1)
stack.push(2)
stack.push(3)
stack.push(4)
expectEqual(4, stack.pop())
expectEqual(3, stack.pop())
expectEqual(2, stack.pop())
expectEqual(1, stack.pop())
expectNil(stack.pop())
}
suite.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() {
expectEqual(expected[thread], value)
expected[thread] -= 1
}
expectEqual(Array(repeating: 0, count: numThreads), expected)
}
suite.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 {
expectEqual(Array(repeating: numValues * (numValues - 1) / 2, count: numThreads), perThreadSums)
}
}
} // if #available(SwiftStdlib 6.0)
runAllTests()
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