[ObjC] Recursively check superclasses to see if any inherit from NSObject

If an NSObject subclass has fixed offsets, then so must the subclasses of that subclass!

Author: AZero13

clang/lib/CodeGen/CGObjCMac.cpp

@@ -1593,12 +1593,19 @@ class CGObjCNonFragileABIMac : public CGObjCCommonMac {
   }
 
   bool isClassLayoutKnownStatically(const ObjCInterfaceDecl *ID) {
+    if (!ID)
+      return false;
+
+    if (ID->getName() == "NSObject")
+      return true;
+
     // NSObject is a fixed size. If we can see the @implementation of a class
-    // which inherits from NSObject then we know that all it's offsets also must
-    // be fixed. FIXME: Can we do this if see a chain of super classes with
-    // implementations leading to NSObject?
-    return ID->getImplementation() && ID->getSuperClass() &&
-           ID->getSuperClass()->getName() == "NSObject";
+    // which inherits from NSObject, then we know that all its offsets must
+    // be fixed.
+
+    // Check recursively for all intermediate superclasses.
+    return ID->getImplementation() &&
+           isClassLayoutKnownStatically(ID->getSuperClass());
   }
 
 public:

clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m

@@ -1,7 +1,7 @@
 // RUN: %clang_cc1 -triple x86_64-apple-macosx10.14.0 -emit-llvm %s -o - | FileCheck %s
 
 // CHECK: @"OBJC_IVAR_$_StaticLayout.static_layout_ivar" = hidden constant i64 20
-// CHECK: @"OBJC_IVAR_$_StaticLayoutSubClass.static_layout_ivar2" = hidden global i64 24
+// CHECK: @"OBJC_IVAR_$_StaticLayoutSubClass.static_layout_ivar2" = hidden constant i64 24
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -29,7 +29,7 @@ @implementation StaticLayoutSubClass {
 }
 -(void)meth2 {
   static_layout_ivar2 = 0;
-  // CHECK: load i64, ptr @"OBJC_IVAR_$_StaticLayoutSubClass
+  // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_StaticLayoutSubClass
 }
 @end