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

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

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@vanvoorden vanvoorden commented Jun 23, 2025 •
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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
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@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
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@swift-ci please Apple silicon benchmark

@vanvoorden vanvoorden mentioned this pull request Aug 22, 2025
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@vanvoorden vanvoorden force-pushed the dictionary-identical branch 2 times, most recently from adbdeb8 to e73320c Compare August 26, 2025 01:03
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@vanvoorden @glessard