Merge pull request swiftlang#38677 from glessard/sr14491-v1

[stdlib] fast path for UMBP.initialize<C>(from: C) when C is a Slice

Author: glessard

stdlib/public/core/Sequence.swift

@@ -1140,6 +1140,13 @@ extension Sequence {
   @inlinable
   public __consuming func _copyContents(
     initializing buffer: UnsafeMutableBufferPointer<Element>
+  ) -> (Iterator,UnsafeMutableBufferPointer<Element>.Index) {
+    return _copySequenceContents(initializing: buffer)
+  }
+
+  @_alwaysEmitIntoClient
+  internal __consuming func _copySequenceContents(
+    initializing buffer: UnsafeMutableBufferPointer<Element>
   ) -> (Iterator,UnsafeMutableBufferPointer<Element>.Index) {
     var it = self.makeIterator()
     guard var ptr = buffer.baseAddress else { return (it,buffer.startIndex) }

stdlib/public/core/Slice.swift

@@ -229,6 +229,22 @@ extension Slice: Collection {
   }
 }
 
+extension Slice {
+  @_alwaysEmitIntoClient
+  public __consuming func _copyContents(
+      initializing buffer: UnsafeMutableBufferPointer<Element>
+  ) -> (Iterator, UnsafeMutableBufferPointer<Element>.Index) {
+    if let (_, copied) = self.withContiguousStorageIfAvailable({
+      $0._copyContents(initializing: buffer)
+    }) {
+      let position = index(startIndex, offsetBy: copied)
+      return (Iterator(_elements: self, _position: position), copied)
+    }
+
+    return _copySequenceContents(initializing: buffer)
+  }
+}
+
 extension Slice: BidirectionalCollection where Base: BidirectionalCollection {
   @inlinable // generic-performance
   public func index(before i: Index) -> Index {

validation-test/stdlib/UnsafeBufferPointer.swift.gyb

@@ -361,6 +361,22 @@ UnsafeMutableBufferPointerTestSuite.test("withContiguous(Mutable)StorageIfAvaila
   expectEqual(result1, result2)
 }
 
+UnsafeMutableBufferPointerTestSuite.test("initialize(from: Slice)") {
+  let o = 91
+  let c = 1_000
+  let a = Slice(base: Array(0..<c), bounds: o..<c)
+
+  let buffer = UnsafeMutableBufferPointer<Int>.allocate(capacity: c-o)
+  defer { buffer.deallocate() }
+
+  var (iterator, copied) = buffer.initialize(from: a)
+  expectEqual(iterator.next(), nil)
+  expectEqual(copied, c-o)
+
+  let t = buffer.indices.randomElement()!
+  expectEqual(a[t+o], buffer[t])
+}
+
 UnsafeMutableBufferPointerTestSuite.test("Slice.withContiguousStorageIfAvailable") {
   guard #available(macOS 10.16, iOS 14.0, watchOS 7.0, tvOS 14.0, *) else {
     return