Index: include/clang/Basic/Attr.td
===================================================================
--- include/clang/Basic/Attr.td
+++ include/clang/Basic/Attr.td
@@ -1795,6 +1795,12 @@
   let Documentation = [ObjCRuntimeVisibleDocs];
 }
 
+def ObjCClassStub: Attr {
+  let Spellings = [Clang<"objc_class_stub">];
+  let Subjects = SubjectList<[ObjCInterface], ErrorDiag>;
+  let Documentation = [ObjCClassStubDocs];
+}
+
 def ObjCBoxable : Attr {
   let Spellings = [Clang<"objc_boxable">];
   let Subjects = SubjectList<[Record], ErrorDiag>;
Index: include/clang/Basic/AttrDocs.td
===================================================================
--- include/clang/Basic/AttrDocs.td
+++ include/clang/Basic/AttrDocs.td
@@ -1112,6 +1112,13 @@
     }];
 }
 
+def ObjCClassStubDocs : Documentation {
+    let Category = DocCatFunction;
+    let Content = [{
+This attribute specifies that the Objective-C class to which it applies has dynamically-allocated metadata. Classes annotated with this attribute cannot be subclassed.
+    }];
+}
+
 def ObjCBoxableDocs : Documentation {
     let Category = DocCatFunction;
     let Content = [{
Index: lib/CodeGen/CGObjCMac.cpp
===================================================================
--- lib/CodeGen/CGObjCMac.cpp
+++ lib/CodeGen/CGObjCMac.cpp
@@ -726,6 +726,25 @@
                                      "objc_begin_catch");
   }
 
+  /// Class objc_loadClassref (void *)
+  ///
+  /// Loads from a classref. For Objective-C stub classes, this invokes the
+  /// initialization callback stored inside the stub. For all other classes
+  /// this simply dereferences the pointer.
+  llvm::Constant *getLoadClassrefFn() const {
+    // FIXME: Other attributes?
+
+    // Add the non-lazy-bind attribute, since objc_loadClassref is likely to
+    // be called a lot.
+    llvm::Type *params[] = { Int8PtrPtrTy };
+    return CGM.CreateRuntimeFunction(
+        llvm::FunctionType::get(ClassnfABIPtrTy, params, false),
+        "objc_loadClassref",
+        llvm::AttributeList::get(CGM.getLLVMContext(),
+                                 llvm::AttributeList::FunctionIndex,
+                                 llvm::Attribute::NonLazyBind));
+  }
+
   llvm::StructType *EHTypeTy;
   llvm::Type *EHTypePtrTy;
 
@@ -7247,12 +7266,23 @@
     llvm::Constant *ClassGV;
     if (ID) {
       ClassGV = GetClassGlobal(ID, /*metaclass*/ false, NotForDefinition);
+
+      if (ID->hasAttr<ObjCClassStubAttr>()) {
+        ClassGV = llvm::ConstantExpr::getPtrToInt(ClassGV, CGM.IntPtrTy);
+
+        // Classrefs pointing at Objective-C stub classes have the least
+        // significant bit set to 1.
+        auto *Tag = llvm::ConstantInt::get(CGM.IntPtrTy, 1);
+        ClassGV = llvm::ConstantExpr::getAdd(ClassGV, Tag);
+        ClassGV = llvm::ConstantExpr::getIntToPtr(ClassGV,
+                                                  ObjCTypes.Int8PtrTy);
+      }
     } else {
       ClassGV = GetClassGlobal((getClassSymbolPrefix() + II->getName()).str(),
                                NotForDefinition);
     }
 
-    Entry = new llvm::GlobalVariable(CGM.getModule(), ObjCTypes.ClassnfABIPtrTy,
+    Entry = new llvm::GlobalVariable(CGM.getModule(), ClassGV->getType(),
                                      false, llvm::GlobalValue::PrivateLinkage,
                                      ClassGV, "OBJC_CLASSLIST_REFERENCES_$_");
     Entry->setAlignment(Align.getQuantity());
@@ -7260,6 +7290,14 @@
                                      "regular,no_dead_strip"));
     CGM.addCompilerUsedGlobal(Entry);
   }
+
+  if (ID->hasAttr<ObjCClassStubAttr>()) {
+    // Classrefs pointing at Objective-C stub classes must be loaded by calling
+    // a special runtime function.
+    return CGF.EmitNounwindRuntimeCall(
+      ObjCTypes.getLoadClassrefFn(), Entry, "load_classref_result");
+  }
+
   return CGF.Builder.CreateAlignedLoad(Entry, Align);
 }
 
Index: lib/Sema/SemaDeclAttr.cpp
===================================================================
--- lib/Sema/SemaDeclAttr.cpp
+++ lib/Sema/SemaDeclAttr.cpp
@@ -7123,6 +7123,9 @@
   case ParsedAttr::AT_ObjCSubclassingRestricted:
     handleSimpleAttribute<ObjCSubclassingRestrictedAttr>(S, D, AL);
     break;
+  case ParsedAttr::AT_ObjCClassStub:
+    handleSimpleAttribute<ObjCClassStubAttr>(S, D, AL);
+    break;
   case ParsedAttr::AT_ObjCCompleteDefinition:
     handleSimpleAttribute<ObjCCompleteDefinitionAttr>(S, D, AL);
     break;
Index: test/CodeGenObjC/class-stubs.m
===================================================================
--- /dev/null
+++ test/CodeGenObjC/class-stubs.m
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -Wno-objc-root-class -emit-llvm -o - %s | \
+// RUN: FileCheck %s
+
+__attribute__((objc_class_stub))
+@interface A
++ (void) classMethod;
+@end
+
+int main() {
+  [A classMethod];
+}
+
+// CHECK-LABEL: @"OBJC_CLASSLIST_REFERENCES_$_" = private global i8* inttoptr (i64 add (i64 ptrtoint (%struct._class_t* @"OBJC_CLASS_$_A" to i64), i64 1) to i8*), section "__DATA,__objc_classrefs,regular,no_dead_strip", align 8
+
+
+// CHECK-LABEL: define i32 @main()
+// CHECK-NEXT: entry:
+// CHECK-NEXT:   [[CLASS:%.*]] = call %struct._class_t* @objc_loadClassref(i8** @"OBJC_CLASSLIST_REFERENCES_$_")
+// CHECK-NEXT:   [[SELECTOR:%.*]] = load i8*, i8** @OBJC_SELECTOR_REFERENCES_
+// CHECK-NEXT:   [[RECEIVER:%.*]] = bitcast %struct._class_t* [[CLASS]] to i8*
+// CHECK-NEXT:   call void bitcast (i8* (i8*, i8*, ...)* @objc_msgSend to void (i8*, i8*)*)(i8* [[RECEIVER]], i8* [[SELECTOR]])
+// CHECK-NEXT:   ret i32 0
+