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

[vanvoorden]

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

TODO

Benchmarks

TODO

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 ↩