[stdlib] Implement Atomics module #30553

lorentey · 2020-03-21T06:05:56Z

This is the implementation that goes with the Low-Level Atomic Operations pitch. For details, see there!

In this pitch, we are gathering feedback about the idea of adding a limited set of low-level atomic operations to the Standard Library, including native spellings for C++-style acquire-release memory orderings. Our goal is to enable intrepid library authors to start building synchronization constructs directly in Swift.

As a quick taste, we want to make atomics look like this:

import Atomics
import Dispatch

let counter = UnsafeAtomic<Int>.create(initialValue: 0)

DispatchQueue.concurrentPerform(iterations: 10) { _ in
  for _ in 0 ..< 1_000_000 {
    counter.wrappingIncrement(ordering: .relaxed)
  }
}
print(counter.load(ordering: .relaxed))
counter.destroy()

(This is a followup to the atomic experiments in #27229.)

Fixes #51640
rdar://17171396

lorentey · 2020-03-21T06:09:32Z

@swift-ci test

stdlib/public/Atomics/AtomicMemoryOrderings.swift

stdlib/public/Atomics/CMakeLists.txt

lorentey · 2020-03-24T08:20:38Z

@swift-ci test

lorentey · 2020-03-28T02:13:52Z

@swift-ci test

lorentey · 2020-03-28T04:18:26Z

@swift-ci test

lorentey · 2020-03-28T04:18:38Z

@swift-ci test Windows platform

stdlib/public/Atomics/AtomicIntegers.swift.gyb

stdlib/public/Atomics/AtomicRepresentable.swift

stdlib/public/Atomics/AtomicPointers.swift.gyb

stdlib/public/Atomics/AtomicProtocols.swift

…calls

…& storage mgmt - _PrimitiveAtomic is a new hidden protocol implementing the core atomic operations. - _PrimitiveAtomicOptional refines it adding nullability. - AtomicProtocol now consists of a _PrimitiveAtomic-constrained _AtomicStorage, plus conversion methods — all underscored. - Pointer types have opaque atomic storage. - User-visible atomic storage type is UnsafeAtomic.Storage — an opaque value type with a convenience initializer and a dispose() method.

…tack

freak4pc · 2020-04-10T19:48:30Z

This looks great, thanks for working on it! Looking forward to seeing this added, and help avoid many hacks non-official implementations.

I do wonder though if there are any plans for Atomicity in a type-safe level, as opposed to this UnsafeAtomic implementation. Not necessarily in this implementation, but generally speaking ?

Thanks !

lorentey · 2020-04-10T19:52:11Z

Yes -- the current implementation was specifically designed to allow easy transition to the eventual memory-safe constructs later. The preliminary proposal text has a section explaining what's missing.

lorentey · 2020-04-10T19:56:00Z

@swift-ci test

swift-ci · 2020-04-10T20:38:28Z

Build failed
Swift Test Linux Platform
Git Sha - 01a0184

benrimmington · 2020-04-13T17:46:19Z

stdlib/public/Atomics/AtomicOptional.swift

+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+public protocol _PrimitiveAtomicOptional: _PrimitiveAtomic {
+  var _isNil: Bool { get }
+  static var _nilValue: Self { get }
+}
+
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+extension Optional: AtomicProtocol
+where Wrapped: AtomicProtocol, Wrapped._AtomicStorage: _PrimitiveAtomicOptional
+{
+  public typealias _AtomicStorage = Wrapped._AtomicStorage


Could you use Equatable & ExpressibleByNilLiteral storage to eliminate the _PrimitiveAtomicOptional protocol?

return _AtomicStorage._nilValue ➡️ return nil

!storage._isNil ➡️ storage != nil

Suggested change

@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)

public protocol _PrimitiveAtomicOptional: _PrimitiveAtomic {

var _isNil: Bool { get }

static var _nilValue: Self { get }

}

@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)

extension Optional: AtomicProtocol

where Wrapped: AtomicProtocol, Wrapped._AtomicStorage: _PrimitiveAtomicOptional

{

public typealias _AtomicStorage = Wrapped._AtomicStorage

@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)

extension Optional: AtomicProtocol where

Wrapped: AtomicProtocol,

Wrapped._AtomicStorage: Equatable & ExpressibleByNilLiteral

{

public typealias _AtomicStorage = Wrapped._AtomicStorage

Yes, but that doesn't look like a particularly great idea to me. I prefer the explicit un-cleverness of a dedicated protocol here.

There is also this:

nil has a specific meaning in Swift—the absence of a value. Only the Optional type conforms to ExpressibleByNilLiteral. ExpressibleByNilLiteral conformance for types that use nil for other purposes is discouraged.

There is also this:

nil has a specific meaning in Swift—the absence of a value. Only the Optional type conforms to ExpressibleByNilLiteral.

And the _OptionalNilComparisonType!

I do think it'd be fair to say _PrimitiveAtomicOptional inherits from ExpressibleByNilLiteral though. That would still simplify the implementation.

The storage types aren't currently ExpressibleByNilLiteral, though, and I think I'd like to keep them that way.

I am ambivalent towards adding Equatable conformance to atomic storage. (For the same reason I keep fiddling with the FixedWidthInteger conformance on _PrimitiveAtomicInteger. I dislike adding too much structure to the storage types -- these should be treated as opaque boxes.)

benrimmington · 2020-04-13T17:56:29Z

stdlib/public/Atomics/UnsafeAtomic.swift.gyb

+  @frozen
+  public struct Storage {
+    @usableFromInline
+    internal var _value: Value._AtomicStorage
+
+    @inlinable @inline(__always)
+    internal init(_value: __owned Value._AtomicStorage) {
+      self._value = _value
+    }
+
+    @inlinable @inline(__always)
+    public init(_ value: __owned Value) {
+      self._value = Value._prepareAtomicStorage(for: value)
+    }
+
+    @inlinable @inline(__always)
+    @discardableResult
+    public mutating func dispose() -> Value {
+      return Value._disposeAtomicStorage(&_value)
+    }
+  }


Could you use a typealias to eliminate the UnsafeAtomic.Storage structure?

the computed var _ptr and its assumingMemoryBound(to:) wouldn't be needed.

possibly rename _AtomicStorage ➡️ AtomicStorage (since integers already have nested SIMDStorage types).

Suggested change

@frozen

public struct Storage {

@usableFromInline

internal var _value: Value._AtomicStorage

@inlinable @inline(__always)

internal init(_value: __owned Value._AtomicStorage) {

self._value = _value

}

@inlinable @inline(__always)

public init(_ value: __owned Value) {

self._value = Value._prepareAtomicStorage(for: value)

}

@inlinable @inline(__always)

@discardableResult

public mutating func dispose() -> Value {

return Value._disposeAtomicStorage(&_value)

}

}

public typealias Storage = Value._AtomicStorage

Again, this is technically possible, but I prefer to have a dedicated, public wrapper so that particular choices for _AtomicStorage won't get hardwired into (ostensibly) portable codebases.

UnsafeAtomic.Storage has the additional benefit that we can attach API to it (init(_:), dispose()) that would be inappropriate on _AtomicStorage, esp. in the case where atomic types use Self as their atomic storage.

benrimmington · 2020-04-13T18:02:44Z

stdlib/public/Atomics/UnsafeAtomicLazyReference.swift

+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+@frozen
+public struct UnsafeAtomicLazyReference<Instance: AnyObject> {
+  public typealias Value = Instance?


It might be clearer to remove this typealias, and use Instance? directly.

(Instance and Instance? are already used in the store/load methods.)

Value is part of the new unsafe API pattern for high-performance synchronization primitives. This is pure 100% ceremony, but it helps establishing / highlighting patterns for such types.

stdlib/public/Atomics/UnsafeAtomic.swift.gyb

…ge variants

lorentey · 2020-04-13T20:00:46Z

@swift-ci test

lorentey · 2020-04-13T21:40:58Z

@swift-ci test

shahmishal · 2020-10-01T06:56:21Z

Please update the base branch to main by Oct 5th otherwise the pull request will be closed automatically.

How to change the base branch: (Link)
More detail about the branch update: (Link)

lorentey · 2020-10-02T22:09:00Z

Closing -- Atomics are published as a standalone package for now: https://github.com/apple/swift-atomics

[stdlib] Add an Atomics module for low-level atomic operations

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lorentey added the swift evolution pending discussion Flag → feature: A feature that has a Swift evolution proposal currently in review label Mar 21, 2020

benrimmington reviewed Mar 21, 2020

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stdlib/public/Atomics/CMakeLists.txt Outdated Show resolved Hide resolved

lorentey added 5 commits March 23, 2020 11:54

[Atomics][nfc] Fix file headers

694bbb7

[Atomics][nfc] Prefer Self to spelling out the type name

3f4b6fb

[Atomics] Add default value of 1 for increment/decrement

84eb34f

[Atomics] Add @_nonEphemeral attribute to pointer-based initializers

9b05757

[Atomics] Make .sequentiallyConsistent the default ordering

f4006a6

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lorentey added 2 commits March 25, 2020 14:06

Merge branch 'master' into atomics2

63f041e

[Atomics] Introduce AtomicProtocol

a32ef12

lorentey added 3 commits March 27, 2020 20:13

[Atomics] Add AtomicRepresentable

6c4a639

[Atomics] Switch back to no default ordering

9576528

[Atomics] UnsafeAtomicLazyReference.initialize(to:) ⟹ .storeIfNil(_:)

f123869

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xwu reviewed Mar 28, 2020

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belkadan reviewed Mar 28, 2020

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stdlib/public/Atomics/AtomicPointers.swift.gyb Outdated Show resolved Hide resolved

stdlib/public/Atomics/AtomicProtocols.swift Outdated Show resolved Hide resolved

lorentey added 6 commits March 30, 2020 16:19

[Atomics] Put address argument after operands in primitive atomics

40d349b

[Atomics] Add support for immutable pointer types

0a263f6

[Atomics] Simplify gyb

8eda0c8

[Atomics][test][NFC] Move atomics tests to separate subdirectory

eb061c7

[Atomics][test] Verify that atomic ops get emitted as direct builtin …

6f685a4

…calls

[Atomics] Minor edits, code reorganization

4f1e412

lorentey added 3 commits April 9, 2020 20:25

[Atomics] Small API adjustments

4b31896

[Atomics][test] Add a test implementing a lock-free single-consumer s…

01a0184

…tack

benrimmington reviewed Apr 13, 2020

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lorentey added 3 commits April 13, 2020 12:29

[Atomics] ordering: ⟹ successOrdering: in two-ordering compare-exchan…

a032ff4

…ge variants

[Atomics] storeIfNil ⟹ storeIfNilThenLoad

5f43d00

[test] LockFreeSingleConsumerStack requires Dispatch

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lorentey added 2 commits April 13, 2020 14:38

[Atomics] Allow integer types to have a custom atomic representation

640e196

[test][Atomics] Fix Dispatch import in LockFreeSingleConsumerStack test

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glessard mentioned this pull request Apr 13, 2020

Revision 0.4.0 glessard/SwiftCompatibleAtomics#3

Merged

lorentey added 2 commits April 15, 2020 14:45

Merge branch 'master' into atomics2

0e2274c

[Atomics] AtomicProtocol ⟹ AtomicValue

e13f1fc

lorentey force-pushed the atomics2 branch from cb2c510 to e13f1fc Compare April 20, 2020 20:01

[Atomics] Be more assertive about inlining storage/value conversions

ad30578

lorentey closed this Oct 2, 2020

lorentey mentioned this pull request Apr 10, 2020

[SR-9144] Low-level atomics in Swift #51640

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AnthonyLatsis removed the swift evolution pending discussion Flag → feature: A feature that has a Swift evolution proposal currently in review label Oct 25, 2023

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[SR-9144] Low-level atomics in Swift #51640

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[stdlib] Implement Atomics module #30553

[stdlib] Implement Atomics module #30553

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[stdlib] Implement Atomics module #30553

[stdlib] Implement Atomics module #30553

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