Try using a faster class check

stdlib/public/core/Builtin.swift

@@ -323,6 +323,11 @@ internal func _branchHint(_ actual: Bool, expected: Bool) -> Bool {
 @usableFromInline
 @_silgen_name("_swift_objcClassUsesNativeSwiftReferenceCounting")
 internal func _usesNativeSwiftReferenceCounting(_ theClass: AnyClass) -> Bool
+
+/// Returns the class of a non-tagged-pointer Objective-C object
+@_effects(readonly)
+@_silgen_name("_swift_classOfObjCHeapObject")
+internal func _swift_classOfObjCHeapObject(_ object: AnyObject) -> AnyClass
 #else
 @inlinable
 @inline(__always)

stdlib/public/core/StringBridge.swift

@@ -125,11 +125,6 @@ private var kCFStringEncodingUTF8 : _swift_shims_CFStringEncoding {
   @inline(__always) get { return 0x8000100 }
 }
 
-@_effects(readonly)
-private func _unsafeAddressOfCocoaStringClass(_ str: _CocoaString) -> UInt {
-  return _swift_stdlib_unsafeAddressOfClass(str)
-}
-
 internal enum _KnownCocoaString {
   case storage
   case shared
@@ -148,7 +143,7 @@ internal enum _KnownCocoaString {
     }
 #endif
 
-    switch _unsafeAddressOfCocoaStringClass(str) {
+    switch unsafeBitCast(_swift_classOfObjCHeapObject(str), to: UInt.self) {
     case unsafeBitCast(__StringStorage.self, to: UInt.self):
       self = .storage
     case unsafeBitCast(__SharedStringStorage.self, to: UInt.self):

stdlib/public/runtime/SwiftObject.mm

@@ -177,6 +177,11 @@ static Class _swift_getObjCClassOfAllocated(const void *object) {
     class_getInstanceSize(cls), mask));
 }
 
+SWIFT_CC(swift) SWIFT_RUNTIME_STDLIB_API
+Class _swift_classOfObjCHeapObject(OpaqueValue *value) {
+  return _swift_getObjCClassOfAllocated(value);
+}
+
 SWIFT_CC(swift) SWIFT_RUNTIME_STDLIB_API
 NSString *swift_stdlib_getDescription(OpaqueValue *value,
                                       const Metadata *type);