A new way to bridge constant NSStrings (#74881)

Author: Catfish-Man

stdlib/public/core/StringBridge.swift

@@ -386,6 +386,16 @@ private func _NSStringUTF8Pointer(_ str: _StringSelectorHolder) -> UnsafePointer
   return _NSStringASCIIPointer(str)
 }
 
+@_effects(readonly)
+internal func _getNSCFConstantStringContentsPointer(
+  _ cocoa: AnyObject
+) -> UnsafePointer<UInt8> {
+  return unsafeBitCast(
+    cocoa,
+    to: UnsafePointer<_swift_shims_builtin_CFString>.self
+  ).pointee.str
+}
+
 @_effects(readonly) // @opaque
 private func _withCocoaASCIIPointer<R>(
   _ str: _CocoaString,
@@ -635,6 +645,31 @@ internal func _bridgeCocoaString(_ cocoaString: _CocoaString) -> _StringGuts {
 }
 
 extension String {
+  @available(SwiftStdlib 6.1, *)
+  @_spi(Foundation)
+  public init<Encoding: Unicode.Encoding>(
+    _immortalCocoaString: AnyObject,
+    count: Int,
+    encoding: Encoding.Type
+  ) {
+    if encoding == Unicode.ASCII.self || encoding == Unicode.UTF8.self {
+      self._guts = _StringGuts(
+        constantCocoa: _immortalCocoaString,
+        providesFastUTF8: true,
+        isASCII: encoding == Unicode.ASCII.self,
+        length: count)
+    } else {
+      _precondition(encoding == Unicode.UTF16.self)
+      // Only need the very last bit of _bridgeCocoaString here,
+      // since we know the fast paths don't apply
+      self._guts = _StringGuts(
+        cocoa: _immortalCocoaString,
+        providesFastUTF8: false,
+        isASCII: false,
+        length: count)
+    }
+  }
+  
   @_spi(Foundation)
   public init(_cocoaString: AnyObject) {
     self._guts = _bridgeCocoaString(_cocoaString)
@@ -696,7 +731,7 @@ extension String {
         _internalInvariant(!copy._guts.isSmall)
         return copy._bridgeToObjectiveCImpl()
     }
-    if _guts._object.isImmortal {
+    if _guts._object.isImmortal && !_guts._object.largeFastIsConstantCocoa {
       // TODO: We'd rather emit a valid ObjC object statically than create a
       // shared string class instance.
       let gutsCountAndFlags = _guts._object._countAndFlags

stdlib/public/core/StringGuts.swift

@@ -63,6 +63,21 @@ extension _StringGuts {
   internal init(_ storage: __SharedStringStorage) {
     self.init(_StringObject(storage))
   }
+  
+#if !$Embedded
+internal init(
+  constantCocoa cocoa: AnyObject,
+  providesFastUTF8: Bool,
+  isASCII: Bool,
+  length: Int
+) {
+  self.init(_StringObject(
+    constantCocoa: cocoa,
+    providesFastUTF8: providesFastUTF8,
+    isASCII: isASCII,
+    length: length))
+}
+#endif
 
   #if !$Embedded
   internal init(

stdlib/public/core/StringObject.swift

@@ -28,6 +28,8 @@
   │ Immortal, Small     ║  1  │ASCII│  1  │  0  │
   ├─────────────────────╫─────┼─────┼─────┼─────┤
   │ Immortal, Large     ║  1  │  0  │  0  │  0  │
+  ├─────────────────────╫─────┼─────┼─────┼─────┤
+  │ Immortal, Bridged   ║  1  │  1  │  0  │  0  │
   ╞═════════════════════╬═════╪═════╪═════╪═════╡
   │ Native              ║  0  │  0  │  0  │  0  │
   ├─────────────────────╫─────┼─────┼─────┼─────┤
@@ -436,6 +438,10 @@ extension _StringObject.Nibbles {
   internal static func largeCocoa(providesFastUTF8: Bool) -> UInt64 {
     return providesFastUTF8 ? 0x4000_0000_0000_0000 : 0x5000_0000_0000_0000
   }
+  
+  internal static func largeFastImmortalCocoa() -> UInt64 {
+    0xC000_0000_0000_0000
+  }
 }
 
 extension _StringObject {
@@ -549,6 +555,15 @@ extension _StringObject {
     return (discriminatedObjectRawBits & 0x4000_0000_0000_0000) != 0
 #endif
   }
+  
+  @inline(__always)
+  internal var largeFastIsConstantCocoa: Bool {
+#if os(Android) && arch(arm64)
+    false
+#else
+    (discriminatedObjectRawBits & 0xF000_0000_0000_0000) == 0xC000_0000_0000_0000
+#endif
+  }
 
   // Whether this string is in one of our fastest representations:
   // small or tail-allocated (i.e. mortal/immortal native)
@@ -952,6 +967,11 @@ extension _StringObject {
   internal func getSharedUTF8Start() -> UnsafePointer<UInt8> {
     _internalInvariant(largeFastIsShared)
 #if _runtime(_ObjC)
+    if largeFastIsConstantCocoa {
+      return withCocoaObject {
+        _getNSCFConstantStringContentsPointer($0)
+      }
+    }
     if largeIsCocoa {
       return withCocoaObject {
         stableCocoaUTF8Pointer($0)._unsafelyUnwrappedUnchecked
@@ -1075,7 +1095,9 @@ extension _StringObject {
 #if $Embedded
     fatalError("unreachable in embedded Swift")
 #elseif _pointerBitWidth(_64)
-    _internalInvariant(largeIsCocoa && !isImmortal)
+    _internalInvariant(
+      (largeIsCocoa && !isImmortal) || largeFastIsConstantCocoa
+    )
     let unmanaged = Unmanaged<AnyObject>.fromOpaque(largeAddress)
     return unmanaged._withUnsafeGuaranteedRef { body($0) }
 #elseif _pointerBitWidth(_32)
@@ -1168,7 +1190,7 @@ extension _StringObject {
   internal var hasObjCBridgeableObject: Bool {
     @_effects(releasenone) get {
       // Currently, all mortal objects can zero-cost bridge
-      return !self.isImmortal
+      return !self.isImmortal || self.largeFastIsConstantCocoa
     }
   }
 
@@ -1291,6 +1313,33 @@ extension _StringObject {
       countAndFlags: countAndFlags)
 #else
 #error("Unknown platform")
+#endif
+  }
+  
+  @_unavailableInEmbedded
+  internal init(
+    constantCocoa cocoa: AnyObject,
+    providesFastUTF8: Bool,
+    isASCII: Bool,
+    length: Int
+  ) {
+    let countAndFlags = CountAndFlags(sharedCount: length, isASCII: isASCII)
+    let discriminator = Nibbles.largeFastImmortalCocoa()
+#if $Embedded
+    fatalError("unreachable in embedded Swift")
+#elseif _pointerBitWidth(_64)
+    self.init(
+      object: cocoa, discriminator: discriminator, countAndFlags: countAndFlags)
+    _internalInvariant(self.largeAddressBits == Builtin.reinterpretCast(cocoa))
+    _internalInvariant(self.providesFastUTF8 == providesFastUTF8)
+    _internalInvariant(self.largeCount == length)
+#elseif _pointerBitWidth(_32)
+    self.init(
+      variant: .bridged(cocoa),
+      discriminator: discriminator,
+      countAndFlags: countAndFlags)
+#else
+#error("Unknown platform")
 #endif
   }
 }
@@ -1357,7 +1406,7 @@ extension _StringObject {
           _internalInvariant(nativeStorage.count == self.count)
         }
       }
-      if largeIsCocoa {
+      if largeFastIsConstantCocoa || largeIsCocoa {
         _internalInvariant(hasObjCBridgeableObject)
         _internalInvariant(!isSmall)
         _internalInvariant(!_countAndFlags.isNativelyStored)

test/abi/macOS/arm64/stdlib.swift

@@ -573,4 +573,7 @@ Added: _swift_updatePureObjCClassMetadata
 Added: _swift_bincompat_useLegacyNonCrashingExecutorChecks
 
 // Add add SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE
-Added: _concurrencyIsCurrentExecutorLegacyModeOverride
+Added: _concurrencyIsCurrentExecutorLegacyModeOverride
+
+//String.init<Encoding: Unicode.Encoding>(_immortalCocoaString: AnyObject, count: Int, encoding: Encoding.Type)
+Added: _$sSS20_immortalCocoaString5count8encodingSSyXl_Sixmtcs16_UnicodeEncodingRzlufC

test/abi/macOS/x86_64/stdlib.swift

@@ -573,4 +573,7 @@ Added: _swift_updatePureObjCClassMetadata
 Added: _swift_bincompat_useLegacyNonCrashingExecutorChecks
 
 // Add add SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE
-Added: _concurrencyIsCurrentExecutorLegacyModeOverride
+Added: _concurrencyIsCurrentExecutorLegacyModeOverride
+
+//String.init<Encoding: Unicode.Encoding>(_immortalCocoaString: AnyObject, count: Int, encoding: Encoding.Type)
+Added: _$sSS20_immortalCocoaString5count8encodingSSyXl_Sixmtcs16_UnicodeEncodingRzlufC

test/stdlib/StringBridge.swift

@@ -56,6 +56,27 @@ StringBridgeTests.test("Tagged NSString") {
 #endif // not 32bit
 }
 
+StringBridgeTests.test("Constant NSString New SPI") {
+  if #available(SwiftStdlib 6.1, *) {
+    //21 characters long so avoids _SmallString
+    let constantString:NSString = CFRunLoopMode.commonModes.rawValue as NSString
+    let regularBridged = constantString as String
+    let count = regularBridged.count
+    let bridged = String(
+      _immortalCocoaString: constantString,
+      count: count,
+      encoding: Unicode.ASCII.self
+    )
+    let reverseBridged = bridged as NSString
+    expectEqual(constantString, reverseBridged)
+    expectEqual(
+      ObjectIdentifier(constantString),
+      ObjectIdentifier(reverseBridged)
+    )
+    expectEqual(bridged, regularBridged)
+  }
+}
+
 StringBridgeTests.test("Bridging") {
   // Test bridging retains small string form
   func bridge(_ small: _SmallString) -> String {