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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
//
//===----------------------------------------------------------------------===//
/// A reference type holding a lazily initializable atomic
/// strong reference, with automatic memory management.
///
/// These values can be set (initialized) exactly once, but read many
/// times.
@_fixed_layout
public class ManagedAtomicLazyReference<Instance: AnyObject> {
/// The value logically stored in an atomic lazy reference value.
public typealias Value = Instance?
@usableFromInline
internal typealias _Rep = Optional<Unmanaged<Instance>>.AtomicRepresentation
/// The atomic representation of the value stored inside.
///
/// Warning: This ivar must only ever be accessed via `_ptr` after
/// its initialization.
@usableFromInline
internal let _storage: _Rep
/// Initializes a new managed atomic lazy reference with a nil value.
@inlinable
public init() {
_storage = _Rep(nil)
}
deinit {
if let unmanaged = _ptr.pointee.dispose() {
unmanaged.release()
}
}
@_alwaysEmitIntoClient @inline(__always)
internal var _ptr: UnsafeMutablePointer<_Rep> {
_getUnsafePointerToStoredProperties(self).assumingMemoryBound(to: _Rep.self)
}
}
extension ManagedAtomicLazyReference: @unchecked Sendable
where Instance: Sendable {}
extension ManagedAtomicLazyReference {
/// Atomically initializes this reference if its current value is nil, then
/// returns the initialized value. If this reference is already initialized,
/// then `storeIfNilThenLoad(_:)` discards its supplied argument and returns
/// the current value without updating it.
///
/// The following example demonstrates how this can be used to implement a
/// thread-safe lazily initialized reference:
///
/// ```
/// class Image {
/// var _histogram: UnsafeAtomicLazyReference<Histogram> = .init()
///
/// // This is safe to call concurrently from multiple threads.
/// var atomicLazyHistogram: Histogram {
/// if let histogram = _histogram.load() { return histogram }
/// // Note that code here may run concurrently on
/// // multiple threads, but only one of them will get to
/// // succeed setting the reference.
/// let histogram = ...
/// return _histogram.storeIfNilThenLoad(histogram)
/// }
/// ```
///
/// This operation uses acquiring-and-releasing memory ordering.
public func storeIfNilThenLoad(_ desired: __owned Instance) -> Instance {
let desiredUnmanaged = Unmanaged.passRetained(desired)
let (exchanged, current) = _Rep.atomicCompareExchange(
expected: nil,
desired: desiredUnmanaged,
at: _ptr,
ordering: .acquiringAndReleasing)
if !exchanged {
// The reference has already been initialized. Balance the retain that
// we performed on `desired`.
desiredUnmanaged.release()
return current!.takeUnretainedValue()
}
return desiredUnmanaged.takeUnretainedValue()
}
/// Atomically loads and returns the current value of this reference.
///
/// The load operation is performed with the memory ordering
/// `AtomicLoadOrdering.acquiring`.
public func load() -> Instance? {
let value = _Rep.atomicLoad(at: _ptr, ordering: .acquiring)
return value?.takeUnretainedValue()
}
}
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