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ade67ca899
* Create Swift libSyntax API This patch is an initial implementation of the Swift libSyntax API. It aims to provide all features of the C++ API but exposed to Swift. It currently resides in SwiftExperimental and will likely exist in a molten state for a while. * Only build SwiftSyntax on macOS
64 lines
2.6 KiB
Swift
64 lines
2.6 KiB
Swift
//===----------- AtomicCache.swift - Atomically Initialized Cache ---------===//
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//
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// This source file is part of the Swift.org open source project
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//
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// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
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// Licensed under Apache License v2.0 with Runtime Library Exception
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//
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// See https://swift.org/LICENSE.txt for license information
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// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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//
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//===----------------------------------------------------------------------===//
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import Foundation
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/// AtomicCache is a wrapper class around an uninitialized value.
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/// It takes a closure that it will use to create the value atomically. The
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/// value is guaranteed to be set exactly one time, but the provided closure
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/// may be called multiple times by threads racing to initialize the value.
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/// Do not rely on the closure being called only one time.
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class AtomicCache<Value: AnyObject> {
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/// The cached pointer that will be filled in the first time `value` is
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/// accessed.
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private var _cachedValue: AnyObject?
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/// The value inside this cache. If the value has not been initialized when
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/// this value is requested, then the closure will be called and its resulting
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/// value will be atomically compare-exchanged into the cache.
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/// If multiple threads access the value before initialization, they will all
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/// end up returning the correct, initialized value.
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/// - Parameter create: The closure that will return the fully realized value
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/// inside the cache.
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func value(_ create: () -> Value) -> Value {
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return withUnsafeMutablePointer(to: &_cachedValue) { ptr in
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// Perform an atomic load -- if we get a value, then return it.
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if let _cached = _stdlib_atomicLoadARCRef(object: ptr) {
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return _cached as! Value
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}
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// Otherwise, create the value...
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let value = create()
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// ...and attempt to initialize the pointer with that value.
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if _stdlib_atomicInitializeARCRef(object: ptr, desired: value) {
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// If we won the race, just return the value we made.
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return value
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}
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// Otherwise, perform _another_ load to get the up-to-date value,
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// and let the one we just made die.
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return _stdlib_atomicLoadARCRef(object: ptr) as! Value
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}
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}
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/// Unsafely attempts to load the value and cast it to the appropriate
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/// type.
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/// - note: Only for use in the debugger!
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@available(*, deprecated, message: "Only for use in the debugger.")
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var unsafeValue: Value? {
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return withUnsafeMutablePointer(to: &_cachedValue) {
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return _stdlib_atomicLoadARCRef(object: $0) as? Value
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}
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}
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}
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