Add a private implementation of a String initializer with access to uninitialized storage (https://github.com/apple/swift-evolution/pull/1022) and use it to speed up uppercased() and lowercased()

benchmark/single-source/NSStringConversion.swift

@@ -30,13 +30,41 @@ public let NSStringConversion = [
                 runFunction: run_NSMutableStringConversion,
                 tags: [.validation, .api, .String, .bridging],
                 setUpFunction: { test = NSMutableString(cString: "test", encoding: String.Encoding.ascii.rawValue)! }),
+  BenchmarkInfo(name: "NSStringConversion.Medium",
+                runFunction: run_NSStringConversion_medium,
+                tags: [.validation, .api, .String, .bridging],
+                setUpFunction: { test = NSString(cString: "aaaaaaaaaaaaaaa", encoding: String.Encoding.ascii.rawValue)! } ),
   BenchmarkInfo(name: "NSStringConversion.Long",
                 runFunction: run_NSStringConversion_long,
                 tags: [.validation, .api, .String, .bridging],
                 setUpFunction: { test = NSString(cString: "The quick brown fox jumps over the lazy dog", encoding: String.Encoding.ascii.rawValue)! } ),
   BenchmarkInfo(name: "NSStringConversion.LongUTF8",
                 runFunction: run_NSStringConversion_longNonASCII,
                 tags: [.validation, .api, .String, .bridging],
+                setUpFunction: { test = NSString(cString: "Thë qüick bröwn föx jumps over the lazy dög", encoding: String.Encoding.utf8.rawValue)! } ),
+  BenchmarkInfo(name: "NSStringConversion.Rebridge",
+                runFunction: run_NSStringConversion_rebridge,
+                tags: [.validation, .api, .String, .bridging],
+                setUpFunction: { test = NSString(cString: "test", encoding: String.Encoding.ascii.rawValue)! }),
+  BenchmarkInfo(name: "NSStringConversion.Rebridge.UTF8",
+                runFunction: run_NSStringConversion_nonASCII_rebridge,
+                tags: [.validation, .api, .String, .bridging],
+                setUpFunction: { test = NSString(cString: "tëst", encoding: String.Encoding.utf8.rawValue)! }),
+  BenchmarkInfo(name: "NSStringConversion.Rebridge.Mutable",
+                runFunction: run_NSMutableStringConversion_rebridge,
+                tags: [.validation, .api, .String, .bridging],
+                setUpFunction: { test = NSMutableString(cString: "test", encoding: String.Encoding.ascii.rawValue)! }),
+  BenchmarkInfo(name: "NSStringConversion.Rebridge.Medium",
+                runFunction: run_NSStringConversion_medium_rebridge,
+                tags: [.validation, .api, .String, .bridging],
+                setUpFunction: { test = NSString(cString: "aaaaaaaaaaaaaaa", encoding: String.Encoding.ascii.rawValue)! } ),
+  BenchmarkInfo(name: "NSStringConversion.Rebridge.Long",
+                runFunction: run_NSStringConversion_long_rebridge,
+                tags: [.validation, .api, .String, .bridging],
+                setUpFunction: { test = NSString(cString: "The quick brown fox jumps over the lazy dog", encoding: String.Encoding.ascii.rawValue)! } ),
+  BenchmarkInfo(name: "NSStringConversion.Rebridge.LongUTF8",
+                runFunction: run_NSStringConversion_longNonASCII_rebridge,
+                tags: [.validation, .api, .String, .bridging],
                 setUpFunction: { test = NSString(cString: "Thë qüick bröwn föx jumps over the lazy dög", encoding: String.Encoding.utf8.rawValue)! })]
 
 public func run_NSStringConversion(_ N: Int) {
@@ -63,6 +91,10 @@ public func run_NSMutableStringConversion(_ N: Int) {
   innerLoop(test, N)
 }
 
+public func run_NSStringConversion_medium(_ N: Int) {
+  innerLoop(test, N, 1000)
+}
+
 public func run_NSStringConversion_long(_ N: Int) {
   innerLoop(test, N, 1000)
 }
@@ -71,4 +103,36 @@ public func run_NSStringConversion_longNonASCII(_ N: Int) {
   innerLoop(test, N, 300)
 }
 
+fileprivate func innerRebridge(_ str: NSString, _ N: Int, _ scale: Int = 5000) {
+  for _ in 1...N * scale {
+    let bridged = identity(str) as String
+    blackHole(bridged)
+    blackHole(bridged as NSString)
+  }
+}
+
+public func run_NSStringConversion_rebridge(_ N: Int) {
+  innerRebridge(test, N, 2500)
+}
+
+public func run_NSStringConversion_nonASCII_rebridge(_ N: Int) {
+  innerRebridge(test, N, 2500)
+}
+
+public func run_NSMutableStringConversion_rebridge(_ N: Int) {
+  innerRebridge(test, N)
+}
+
+public func run_NSStringConversion_medium_rebridge(_ N: Int) {
+  innerRebridge(test, N, 1000)
+}
+
+public func run_NSStringConversion_long_rebridge(_ N: Int) {
+  innerRebridge(test, N, 1000)
+}
+
+public func run_NSStringConversion_longNonASCII_rebridge(_ N: Int) {
+  innerRebridge(test, N, 300)
+}
+
 #endif

stdlib/public/core/SmallString.swift

@@ -218,6 +218,10 @@ extension _SmallString {
       return try f(UnsafeMutableBufferPointer(
         start: ptr, count: _SmallString.capacity))
     }
+    if len == 0 {
+      self = _SmallString()
+      return
+    }
     _internalInvariant(len <= _SmallString.capacity)
 
     let (leading, trailing) = self.zeroTerminatedRawCodeUnits
@@ -260,6 +264,19 @@ extension _SmallString {
 
     self.init(leading: leading, trailing: trailing, count: count)
   }
+
+  @inline(__always)
+  internal init(
+    initializingUTF8With initializer: (
+      _ buffer: UnsafeMutableBufferPointer<UInt8>
+    ) throws -> Int
+  ) rethrows {
+    self.init()
+    try self.withMutableCapacity {
+      return try initializer($0)
+    }
+    self._invariantCheck()
+  }
 
   @usableFromInline // @testable
   internal init?(_ base: _SmallString, appending other: _SmallString) {
@@ -292,7 +309,7 @@ extension _SmallString {
     self.init()
     self.withMutableCapacity {
       let len = _bridgeTagged(cocoa, intoUTF8: $0)
-      _internalInvariant(len != nil && len! < _SmallString.capacity,
+      _internalInvariant(len != nil && len! <= _SmallString.capacity,
         "Internal invariant violated: large tagged NSStrings")
       return len._unsafelyUnwrappedUnchecked
     }

stdlib/public/core/String.swift

@@ -427,6 +427,77 @@ extension String {
     }
     return
   }
+
+  /// Creates a new String with the specified capacity in UTF-8 code units then
+  /// calls the given closure with a buffer covering the String's uninitialized
+  /// memory.
+  ///
+  /// The closure should return the number of initialized code units,
+  /// or 0 if it couldn't initialize the buffer (for example if the
+  /// requested capacity was too small).
+  ///
+  /// This method replaces ill-formed UTF-8 sequences with the Unicode
+  /// replacement character (`"\u{FFFD}"`); This may require resizing
+  /// the buffer beyond its original capacity.
+  ///
+  /// The following examples use this initializer with the contents of two
+  /// different `UInt8` arrays---the first with well-formed UTF-8 code unit
+  /// sequences and the second with an ill-formed sequence at the end.
+  ///
+  ///     let validUTF8: [UInt8] = [67, 97, 102, -61, -87, 0]
+  ///     let s = String(uninitializedCapacity: validUTF8.count,
+  ///                    initializingUTF8With: { ptr in
+  ///         ptr.initializeFrom(validUTF8)
+  ///         return validUTF8.count
+  ///     })
+  ///     // Prints "Café"
+  ///
+  ///     let invalidUTF8: [UInt8] = [67, 97, 102, -61, 0]
+  ///     let s = String(uninitializedCapacity: invalidUTF8.count,
+  ///                    initializingUTF8With: { ptr in
+  ///         ptr.initializeFrom(invalidUTF8)
+  ///         return invalidUTF8.count
+  ///     })
+  ///     // Prints "Caf�"
+  ///
+  ///     let s = String(uninitializedCapacity: invalidUTF8.count,
+  ///                    initializingUTF8With: { ptr in
+  ///         ptr.initializeFrom(invalidUTF8)
+  ///         return 0
+  ///     })
+  ///     // Prints ""
+  ///
+  /// - Parameters:
+  ///   - capacity: The number of UTF-8 code units worth of memory to allocate
+  ///       for the String.
+  ///   - initializer: A closure that initializes elements and sets the count of
+  ///       the new String
+  ///     - Parameters:
+  ///       - buffer: A buffer covering uninitialized memory with room for the
+  ///           specified number of UTF-8 code units.
+  @inline(__always)
+  internal init(
+    uninitializedCapacity capacity: Int,
+    initializingUTF8With initializer: (
+      _ buffer: UnsafeMutableBufferPointer<UInt8>
+    ) throws -> Int
+  ) rethrows {
+    if _fastPath(capacity <= _SmallString.capacity) {
+      let smol = try _SmallString(initializingUTF8With: initializer)
+      // Fast case where we fit in a _SmallString and don't need UTF8 validation
+      if _fastPath(smol.isASCII) {
+        self = String(_StringGuts(smol))
+      } else {
+        //We succeeded in making a _SmallString, but may need to repair UTF8
+        self = smol.withUTF8 { String._fromUTF8Repairing($0).result }
+      }
+      return
+    }
+
+    self = try String._fromLargeUTF8Repairing(
+      uninitializedCapacity: capacity,
+      initializingWith: initializer)
+  }
 
   /// Calls the given closure with a pointer to the contents of the string,
   /// represented as a null-terminated sequence of code units.
@@ -715,13 +786,12 @@ extension String {
   public func lowercased() -> String {
     if _fastPath(_guts.isFastASCII) {
       return _guts.withFastUTF8 { utf8 in
-        // TODO(String performance): We can directly call appendInPlace
-        var result = String()
-        result.reserveCapacity(utf8.count)
-        for u8 in utf8 {
-          result._guts.append(String(Unicode.Scalar(_lowercaseASCII(u8)))._guts)
+        return String(uninitializedCapacity: utf8.count) { buffer in
+          for i in 0 ..< utf8.count {
+            buffer[i] = _lowercaseASCII(utf8[i])
+          }
+          return utf8.count
         }
-        return result
       }
     }
 
@@ -776,13 +846,12 @@ extension String {
   public func uppercased() -> String {
     if _fastPath(_guts.isFastASCII) {
       return _guts.withFastUTF8 { utf8 in
-        // TODO(String performance): code-unit appendInPlace on guts
-        var result = String()
-        result.reserveCapacity(utf8.count)
-        for u8 in utf8 {
-          result._guts.append(String(Unicode.Scalar(_uppercaseASCII(u8)))._guts)
+        return String(uninitializedCapacity: utf8.count) { buffer in
+          for i in 0 ..< utf8.count {
+            buffer[i] = _uppercaseASCII(utf8[i])
+          }
+          return utf8.count
         }
-        return result
       }
     }
 

stdlib/public/core/StringCreate.swift

@@ -79,13 +79,36 @@ extension String {
   ) -> (result: String, repairsMade: Bool) {
     switch validateUTF8(input) {
     case .success(let extraInfo):
-        return (String._uncheckedFromUTF8(
-          input, asciiPreScanResult: extraInfo.isASCII
-        ), false)
+      return (String._uncheckedFromUTF8(
+        input, asciiPreScanResult: extraInfo.isASCII
+      ), false)
     case .error(let initialRange):
         return (repairUTF8(input, firstKnownBrokenRange: initialRange), true)
     }
   }
+
+  internal static func _fromLargeUTF8Repairing(
+    uninitializedCapacity capacity: Int,
+    initializingWith initializer: (
+      _ buffer: UnsafeMutableBufferPointer<UInt8>
+    ) throws -> Int
+  ) rethrows -> String {
+    let result = try __StringStorage.create(
+      uninitializedCapacity: capacity,
+      initializingUncheckedUTF8With: initializer)
+
+    switch validateUTF8(result.codeUnits) {
+    case .success(let info):
+      result._updateCountAndFlags(
+        newCount: result.count,
+        newIsASCII: info.isASCII
+      )
+      return result.asString
+    case .error(let initialRange):
+      //This could be optimized to use excess tail capacity
+      return repairUTF8(result.codeUnits, firstKnownBrokenRange: initialRange)
+    }
+  }
 
   @usableFromInline
   internal static func _uncheckedFromUTF8(

stdlib/public/core/StringStorage.swift

@@ -398,6 +398,34 @@ extension __StringStorage {
     return __StringStorage.create(
       realCodeUnitCapacity: realCapacity, countAndFlags: countAndFlags)
   }
+
+  // The caller is expected to check UTF8 validity and ASCII-ness and update
+  // the resulting StringStorage accordingly
+  internal static func create(
+    uninitializedCapacity capacity: Int,
+    initializingUncheckedUTF8With initializer: (
+      _ buffer: UnsafeMutableBufferPointer<UInt8>
+    ) throws -> Int
+  ) rethrows -> __StringStorage {
+    let storage = __StringStorage.create(
+      capacity: capacity,
+      countAndFlags: CountAndFlags(mortalCount: 0, isASCII: false)
+    )
+    let buffer = UnsafeMutableBufferPointer(start: storage.mutableStart,
+                                            count: capacity)
+    let count = try initializer(buffer)
+
+    let countAndFlags = CountAndFlags(mortalCount: count, isASCII: false)
+    #if arch(i386) || arch(arm)
+    storage._count = countAndFlags.count
+    storage._flags = countAndFlags.flags
+    #else
+    storage._countAndFlags = countAndFlags
+    #endif
+
+    storage.terminator.pointee = 0 // nul-terminated
+    return storage
+  }
 
   @_effects(releasenone)
   internal static func create(
@@ -453,7 +481,7 @@ extension __StringStorage {
   }
 
   @inline(__always)
-  private var codeUnits: UnsafeBufferPointer<UInt8> {
+  internal var codeUnits: UnsafeBufferPointer<UInt8> {
     return UnsafeBufferPointer(start: start, count: count)
   }
 
@@ -518,7 +546,7 @@ extension __StringStorage {
 extension __StringStorage {
   // Perform common post-RRC adjustments and invariant enforcement.
   @_effects(releasenone)
-  private func _postRRCAdjust(newCount: Int, newIsASCII: Bool) {
+  internal func _updateCountAndFlags(newCount: Int, newIsASCII: Bool) {
     let countAndFlags = CountAndFlags(
       mortalCount: newCount, isASCII: newIsASCII)
 #if arch(i386) || arch(arm)
@@ -540,7 +568,7 @@ extension __StringStorage {
     appendedCount: Int, appendedIsASCII isASCII: Bool
   ) {
     let oldTerminator = self.terminator
-    _postRRCAdjust(
+    _updateCountAndFlags(
       newCount: self.count + appendedCount, newIsASCII: self.isASCII && isASCII)
     _internalInvariant(oldTerminator + appendedCount == self.terminator)
   }
@@ -570,7 +598,7 @@ extension __StringStorage {
   }
 
   internal func clear() {
-    _postRRCAdjust(newCount: 0, newIsASCII: true)
+    _updateCountAndFlags(newCount: 0, newIsASCII: true)
   }
 }
 
@@ -585,7 +613,7 @@ extension __StringStorage {
     let tailCount = mutableEnd - upperPtr
     lowerPtr.moveInitialize(from: upperPtr, count: tailCount)
 
-    _postRRCAdjust(
+    _updateCountAndFlags(
       newCount: self.count &- (upper &- lower), newIsASCII: self.isASCII)
   }
 
@@ -622,7 +650,7 @@ extension __StringStorage {
       count: replCount)
 
     let isASCII = self.isASCII && _allASCII(replacement)
-    _postRRCAdjust(newCount: lower + replCount + tailCount, newIsASCII: isASCII)
+    _updateCountAndFlags(newCount: lower + replCount + tailCount, newIsASCII: isASCII)
   }
 
 
@@ -651,7 +679,7 @@ extension __StringStorage {
     }
     _internalInvariant(srcCount == replCount)
 
-    _postRRCAdjust(
+    _updateCountAndFlags(
       newCount: lower + replCount + tailCount, newIsASCII: isASCII)
   }
 }