Speedup for NSString -getCharacters:range:

Author: Catfish-Man

Foundation.xcodeproj/xcshareddata/xcschemes/SwiftFoundation.xcscheme

@@ -20,6 +20,20 @@
                ReferencedContainer = "container:Foundation.xcodeproj">
             </BuildableReference>
          </BuildActionEntry>
+         <BuildActionEntry
+            buildForTesting = "YES"
+            buildForRunning = "YES"
+            buildForProfiling = "YES"
+            buildForArchiving = "YES"
+            buildForAnalyzing = "YES">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "5B5D86DA1BBC74AD00234F36"
+               BuildableName = "SwiftXCTest.framework"
+               BlueprintName = "SwiftXCTest"
+               ReferencedContainer = "container:../swift-corelibs-xctest/XCTest.xcodeproj">
+            </BuildableReference>
+         </BuildActionEntry>
       </BuildActionEntries>
    </BuildAction>
    <TestAction
@@ -29,6 +43,15 @@
       shouldUseLaunchSchemeArgsEnv = "YES">
       <Testables>
       </Testables>
+      <MacroExpansion>
+         <BuildableReference
+            BuildableIdentifier = "primary"
+            BlueprintIdentifier = "5B5D885C1BBC938800234F36"
+            BuildableName = "SwiftFoundation.framework"
+            BlueprintName = "SwiftFoundation"
+            ReferencedContainer = "container:Foundation.xcodeproj">
+         </BuildableReference>
+      </MacroExpansion>
       <AdditionalOptions>
       </AdditionalOptions>
    </TestAction>

Foundation/NSString.swift

@@ -343,8 +343,18 @@ open class NSString : NSObject, NSCopying, NSMutableCopying, NSSecureCoding, NSC
 
 extension NSString {
     public func getCharacters(_ buffer: UnsafeMutablePointer<unichar>, range: NSRange) {
-        for idx in 0..<range.length {
-            buffer[idx] = character(at: idx + range.location)
+        if type(of: self) == NSString.self || type(of: self) == NSMutableString.self {
+            let utf16 = _storage.utf16
+            var idx = utf16.index(utf16.startIndex, offsetBy: range.location)
+            for offset in 0..<range.length {
+                buffer[offset] = utf16[idx]
+                idx = utf16.index(after: idx)
+            }
+        } else {
+            //TODO: fast paths for _fastContents != nil / _fastCString != nil
+            for idx in 0..<range.length {
+                buffer[idx] = character(at: idx + range.location)
+            }
         }
     }
 
@@ -368,7 +378,7 @@ extension NSString {
         if type(of: self) == NSString.self || type(of: self) == NSMutableString.self {
             let start = _storage.utf16.startIndex
             let min = _storage.utf16.index(start, offsetBy: range.location)
-            let max = _storage.utf16.index(start, offsetBy: range.location + range.length)
+            let max = _storage.utf16.index(min, offsetBy: range.length)
             return String(decoding: _storage.utf16[min..<max], as: UTF16.self)
         } else {
             let buff = UnsafeMutablePointer<unichar>.allocate(capacity: range.length)