[ObjC] Check entire chain of superclasses to see if class layout is statically known (#81335)

As of now, we only check if a class directly inherits from NSObject to
determine if said class 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.

Fixes: #81369

1 files changed, 14 insertions, 6 deletions

diff --git a/clang/lib/CodeGen/CGObjCMac.cpp b/clang/lib/CodeGen/CGObjCMac.cpp
index 27d77e9..e815e09 100644
--- a/clang/lib/CodeGen/CGObjCMac.cpp
+++ b/clang/lib/CodeGen/CGObjCMac.cpp
@@ -1593,12 +1593,20 @@ private:
   }
 
   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.
+    for (; ID; ID = ID->getSuperClass()) {
+      // The layout of base class NSObject
+      // is guaranteed to be statically known
+      if (ID->getIdentifier()->getName() == "NSObject")
+        return true;
+
+      // If we cannot see the @implementation of a class,
+      // we cannot statically know the class layout.
+      if (!ID->getImplementation())
+        return false;
+    }
+    return false;
   }
 
 public: