Index: clang/lib/CodeGen/CGObjCMac.cpp
===================================================================
--- clang/lib/CodeGen/CGObjCMac.cpp
+++ clang/lib/CodeGen/CGObjCMac.cpp
@@ -6702,6 +6702,15 @@
       IvarOffsetGV->setVisibility(llvm::GlobalValue::DefaultVisibility);
   }
 
+  // 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, so we can set this as a constant global.
+  // FIXME: Can we do this if see a chain of super classes with implementations
+  // leading to NSObject?
+  if (ID->getImplementation() && ID->getSuperClass() &&
+      ID->getSuperClass()->getName() == "NSObject")
+    IvarOffsetGV->setConstant(true);
+
   if (CGM.getTriple().isOSBinFormatMachO())
     IvarOffsetGV->setSection("__DATA, __objc_ivar");
   return IvarOffsetGV;
Index: clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
===================================================================
--- /dev/null
+++ clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.14.0 -emit-llvm %s -o - | FileCheck %s --dump-input-on-failure
+
+@interface NSObject {
+  int these, will, never, change, ever;
+}
+@end
+
+@interface Sub : NSObject
+@end
+
+@implementation Sub {
+  int sub_ivar;
+}
+@end
+
+// CHECK: @"OBJC_IVAR_$_Sub.sub_ivar" = hidden constant i64 20
+
+@interface NotNSObject {
+  int these, might, change;
+}
+@end
+
+@interface Sub2 : NotNSObject
+@end
+
+@implementation Sub2 {
+  int sub2_ivar;
+}
+@end
+
+// CHECK: @"OBJC_IVAR_$_Sub2.sub2_ivar" = hidden global i64 12
+