[Runtime] Support lazy ObjC realization of Swift classes.

When the ObjC runtime indicates that it supports lazy realization, avoid forcing realization of classes while setting them up. This saves time and memory for classes that never touch the parts of the ObjC runtime that trigger realization

rdar://136102084

Author: mikeash

stdlib/public/runtime/SwiftObject.mm

@@ -1377,12 +1377,25 @@ id swift_dynamicCastObjCProtocolConditional(id object,
   return object;
 }
 
+// Check whether the current ObjC runtime supports lazy realization. If it does,
+// then we can avoid forcing realization of classes before we use them.
+static bool objcSupportsLazyRealization() {
+#if OBJC_SUPPORTSLAZYREALIZATION_DEFINED
+  return SWIFT_LAZY_CONSTANT(_objc_supportsLazyRealization());
+#else
+  return false;
+#endif
+}
+
 void swift::swift_instantiateObjCClass(const ClassMetadata *_c) {
   static const objc_image_info ImageInfo = {0, 0};
 
-  // Ensure the superclass is realized.
   Class c = class_const_cast(_c);
-  [class_getSuperclass(c) class];
+
+  if (!objcSupportsLazyRealization()) {
+    // Ensure the superclass is realized.
+    [class_getSuperclass(c) class];
+  }
 
   // Register the class.
   Class registered = objc_readClassPair(c, &ImageInfo);
@@ -1392,14 +1405,16 @@ id swift_dynamicCastObjCProtocolConditional(id object,
 }
 
 Class swift::swift_getInitializedObjCClass(Class c) {
-  // Used when we have class metadata and we want to ensure a class has been
-  // initialized by the Objective-C runtime. We need to do this because the
-  // class "c" might be valid metadata, but it hasn't been initialized yet.
-  // Send a message that's likely not to be overridden to minimize potential
-  // side effects. Ignore the return value in case it is overridden to
-  // return something different. See
-  // https://github.com/apple/swift/issues/52863 for an example.
-  [c self];
+  if (!objcSupportsLazyRealization()) {
+    // Used when we have class metadata and we want to ensure a class has been
+    // initialized by the Objective-C runtime. We need to do this because the
+    // class "c" might be valid metadata, but it hasn't been initialized yet.
+    // Send a message that's likely not to be overridden to minimize potential
+    // side effects. Ignore the return value in case it is overridden to
+    // return something different. See
+    // https://github.com/apple/swift/issues/52863 for an example.
+    [c self];
+  }
   return c;
 }