[5.0] String performance and code size improvements

stdlib/public/core/StringComparison.swift

@@ -18,9 +18,31 @@ internal func _stringCompare(
   _ lhs: _StringGuts, _ rhs: _StringGuts, expecting: _StringComparisonResult
 ) -> Bool {
   if lhs.rawBits == rhs.rawBits { return expecting == .equal }
+  return _stringCompareWithSmolCheck(lhs, rhs, expecting: expecting)
+}
+
+@usableFromInline
+@_effects(readonly)
+internal func _stringCompareWithSmolCheck(
+  _ lhs: _StringGuts, _ rhs: _StringGuts, expecting: _StringComparisonResult
+) -> Bool {
+  // ASCII small-string fast-path:
+  if lhs.isSmallASCII && rhs.isSmallASCII {
+    let lhsRaw = lhs.asSmall._storage
+    let rhsRaw = rhs.asSmall._storage
+
+    if lhsRaw.0 != rhsRaw.0 {
+      return _lexicographicalCompare(
+        lhsRaw.0.byteSwapped, rhsRaw.0.byteSwapped, expecting: expecting)
+    }
+    return _lexicographicalCompare(
+      lhsRaw.1.byteSwapped, rhsRaw.1.byteSwapped, expecting: expecting)
+  }
+
   return _stringCompareInternal(lhs, rhs, expecting: expecting)
 }
 
+@inline(never) // Keep `_stringCompareWithSmolCheck` fast-path fast
 @usableFromInline
 @_effects(readonly)
 internal func _stringCompareInternal(

stdlib/public/core/StringCreate.swift

@@ -169,5 +169,16 @@ extension String {
   ) -> String {
     return String._fromCodeUnits(utf16, encoding: UTF16.self, repair: true)!.0
   }
+
+  @usableFromInline
+  internal static func _fromSubstring(
+    _ substring: __shared Substring
+  ) -> String {
+    if substring._offsetRange == substring._wholeString._offsetRange {
+      return substring._wholeString
+    }
+
+    return substring._withUTF8 { return String._uncheckedFromUTF8($0) }
+  }
 }
 

stdlib/public/core/StringGuts.swift

@@ -90,6 +90,10 @@ extension _StringGuts {
     @inline(__always) get { return _object.isSmall }
   }
 
+  internal var isSmallASCII: Bool {
+    @inline(__always) get { return _object.isSmall && _object.smallIsASCII }
+  }
+
   @inlinable
   internal var asSmall: _SmallString {
     @inline(__always) get { return _SmallString(_object) }

stdlib/public/core/Substring.swift

@@ -20,7 +20,7 @@ extension String {
   /// - Complexity: O(*n*), where *n* is the length of `substring`.
   @inlinable
   public init(_ substring: __shared Substring) {
-    self = substring._withUTF8 { return String._uncheckedFromUTF8($0) }
+    self = String._fromSubstring(substring)
   }
 }