[ObjC] Check entire chain of superclasses to see if class has fixed offsets

As of now, we only check if a class directly inherits from NSObject to determine if said lass has fixed offsets and can therefore "opt-out" from the non-fragile ABI for ivars.

However, if an NSObject subclass has fixed offsets, then so must the subclasses of that subclass, so this allows us to optimize instances of subclasses of subclasses that inherit from NSObject and so on.

To determine this, we need to find that the compiler can see the implementation of each intermediate class, as that means it is statically linked.

Perhaps there is another way to figure this out at link time as well, but that is for another time and another PR.

Fixes: #81369

Author: AZero13

clang/lib/CodeGen/CGObjCMac.cpp

@@ -1593,12 +1593,22 @@ class CGObjCNonFragileABIMac : public CGObjCCommonMac {
   }
 
   bool isClassLayoutKnownStatically(const ObjCInterfaceDecl *ID) {
-    // 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";
+    // Test a class by checking its superclasses up to its base class if it has
+    // one
+    while (ID) {
+      // The base class NSObject is a fixed size
+      if (ID->getName() == "NSObject")
+        return true;
+
+      // If we cannot see the @implementation of a class, we cannot assume fixed
+      // offsets
+      if (!ID->getImplementation())
+        return false;
+
+      // Test superclass
+      ID = ID->getSuperClass();
+    }
+    return false;
   }
 
 public:

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

@@ -7,7 +7,7 @@
 // CHECK: @"OBJC_IVAR_$_AnotherClass.privateId" = constant i64 24
 // CHECK: @"OBJC_IVAR_$_AnotherClass.anotherPrivateId" = hidden constant i64 32
 // CHECK: @"OBJC_IVAR_$_SuperClass.superClassIvar" = constant i64 20
-// CHECK: @"OBJC_IVAR_$_SubClass.subClassIvar" = global i64 24
+// CHECK: @"OBJC_IVAR_$_SubClass.subClassIvar" = constant i64 24
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -83,7 +83,7 @@ @interface SubClass : SuperClass
 
 @implementation SubClass
 - (void)exampleMethod {
-    // CHECK: load i64, ptr @"OBJC_IVAR_$SuperClass
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$SuperClass
     superClassIvar = 100; // Access superclass ivar
     subClassIvar = 3.14; // Access subclass ivar
 }