Skip to content

[StdLib][RFC][DNM] Add isIdentical Methods for Quick Comparisons to Dictionary and Set #82439

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom
Open
wants to merge 4 commits into
base: main
Choose a base branch
from
Open

[StdLib][RFC][DNM] Add isIdentical Methods for Quick Comparisons to Dictionary and Set #82439

wants to merge 4 commits into from

Conversation

vanvoorden
Copy link
Contributor

@vanvoorden vanvoorden commented Jun 23, 2025 •
edited
Loading

Background

swiftlang/swift-evolution#2875

We propose new isIdentical instance methods to the following concrete types for determining in constant-time if two instances must be equal by-value:

  • String
  • Substring
  • Array
  • ArraySlice
  • ContiguousArray
  • Dictionary
  • Set

Instead of “one big diff”… we can try and keep the diffs grouped together by similar functionality:

  • String, Substring
  • Array, ArraySlice, ContiguousArray
  • Dictionary, Set

Changes

Our Dictionary does not directly perform a fast path for equality checking in our == operator.1 We can actually steal an idea from Dictionary.Keys.2 If both variants are native, we compare the storage buffer directly. If both variants are Cocoa, we compare the objects directly.

extension Dictionary {
  @_alwaysEmitIntoClient
  public func isIdentical(to other: Self) -> Bool {
#if _runtime(_ObjC)
    if
      self._variant.isNative,
      other._variant.isNative,
      unsafe (self._variant.asNative._storage === other._variant.asNative._storage)
    {
      return true
    }
    if
      !self._variant.isNative,
      !other._variant.isNative,
      self._variant.asCocoa.object === other._variant.asCocoa.object
    {
      return true
    }
#else
    if unsafe (self._variant.asNative._storage === other._variant.asNative._storage) {
      return true
    }
#endif
    return false
  }
}

Our Set performs a similar check over variants for our == operator.3 We can check identity similar to Dictionary:

extension Set {
  @_alwaysEmitIntoClient
  public func isIdentical(to other: Self) -> Bool {
#if _runtime(_ObjC)
    if
      self._variant.isNative,
      other._variant.isNative,
      unsafe (self._variant.asNative._storage === other._variant.asNative._storage)
    {
      return true
    }
    if
      !self._variant.isNative,
      !other._variant.isNative,
      self._variant.asCocoa.object === other._variant.asCocoa.object
    {
      return true
    }
#else
    if unsafe (self._variant.asNative._storage === other._variant.asNative._storage) {
      return true
    }
#endif
    return false
  }
}

Test Plan

New tests were added for Dictionary and Set.

Benchmarks

New benchmarks were added for Dictionary and Set.

Footnotes

  1. https://github.com/swiftlang/swift/blob/swift-6.1.2-RELEASE/stdlib/public/core/Dictionary.swift#L1583-L1598 ↩

  2. https://github.com/swiftlang/swift/blob/swift-6.1.2-RELEASE/stdlib/public/core/Dictionary.swift#L1365-L1384 ↩

  3. https://github.com/swiftlang/swift/blob/swift-6.1.2-RELEASE/stdlib/public/core/Set.swift#L408-L424 ↩

@vanvoorden vanvoorden force-pushed the dictionary-identical branch from 18f927e to 9d866eb Compare June 23, 2025 23:52
@vanvoorden vanvoorden mentioned this pull request Jun 24, 2025
Open
@vanvoorden vanvoorden force-pushed the dictionary-identical branch from e395baa to 99ad169 Compare August 11, 2025 20:45
@vanvoorden vanvoorden marked this pull request as ready for review August 12, 2025 07:07
@vanvoorden vanvoorden requested a review from a team as a code owner August 12, 2025 07:07
@vanvoorden vanvoorden requested a review from eeckstein as a code owner August 12, 2025 20:09
@glessard
Copy link
Contributor

@swift-ci please Apple silicon benchmark

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Reviewers

@eeckstein eeckstein Awaiting requested review from eeckstein eeckstein is a code owner

Assignees
No one assigned
Labels
None yet
Projects
None yet
Milestone
No milestone
Development

Successfully merging this pull request may close these issues.
2 participants
@vanvoorden @glessard