[String] Speed up ASCII checking.

Perform ASCII checking using pointer-width strides, making sure to
align properly.

Author: milseman

stdlib/public/core/StringCreate.swift

@@ -15,13 +15,33 @@
 internal func _allASCII(_ input: UnsafeBufferPointer<UInt8>) -> Bool {
   // NOTE: Avoiding for-in syntax to avoid bounds checks
   //
-  // TODO(String performance): Vectorize and/or incorporate into validity
-  // checking, perhaps both.
+  // TODO(String performance): SIMD-ize
   //
   let ptr = input.baseAddress._unsafelyUnwrappedUnchecked
   var i = 0
-  while i < input.count {
-    guard ptr[i] <= 0x7F else { return false }
+
+  let count = input.count
+  let stride = MemoryLayout<UInt>.stride
+  let address = Int(bitPattern: ptr)
+
+  let wordASCIIMask = UInt(truncatingIfNeeded: 0x8080_8080_8080_8080 as UInt64)
+  let byteASCIIMask = UInt8(truncatingIfNeeded: wordASCIIMask)
+
+  while (address &+ i) % stride != 0 && i < count {
+    guard ptr[i] & byteASCIIMask == 0 else { return false }
+    i &+= 1
+  }
+
+  while (i &+ stride) <= count {
+    let word: UInt = UnsafePointer(
+      bitPattern: address &+ i
+    )._unsafelyUnwrappedUnchecked.pointee
+    guard word & wordASCIIMask == 0 else { return false }
+    i &+= stride
+  }
+
+  while i < count {
+    guard ptr[i] & byteASCIIMask == 0 else { return false }
     i &+= 1
   }
   return true

stdlib/public/core/StringUTF8Validation.swift

@@ -36,6 +36,10 @@ extension UTF8ValidationResult: Equatable {}
 private struct UTF8ValidationError: Error {}
 
 internal func validateUTF8(_ buf: UnsafeBufferPointer<UInt8>) -> UTF8ValidationResult {
+  if _allASCII(buf) {
+    return .success(UTF8ExtraInfo(isASCII: true))
+  }
+
   var iter = buf.makeIterator()
   var lastValidIndex = buf.startIndex