Add support for __attribute__((objc_class_stub))

[slavapestov]

Summary

This CL lands the Clang-side support for a new Objective-C runtime feature that adds support for dynamically-allocated metadata.

What follows is a basic description of this mechanism:

In Swift, the class metadata contains an Objective-C class object as a prefix, followed by a vtable, generic parameters, and field offsets. If the inheritance hierarchy of a class crosses a resilience boundary, we cannot statically emit the entire metadata. Instead, it is lazily built at runtime when first needed.

In order for such classes to be accessed from Clang, the Objective-C runtime will support a new mechanism whereby Swift can instead emit a "class stub" object. The class stub object is used by Clang in places where a class reference is normally emitted:

• Classrefs emitted when messaging the class
• Categories

A class stub contains an "isa pointer" which is a scalar value 1 followed by a function pointer which realizes the real class metadata. A reference to a class stub from a classref also has its least significant pointer set to 1.

The Objective-C runtime distinguishes a class stub from a real class using this fake "isa pointer". Instead of directly loading from a classref, Clang instead calls objc_loadClassref(); this function checks if the classref contains a class stub, and if so, it calls the function pointer and stores the result back into the classref to fast path future loads.

Note that Clang already supports a objc_runtime_visible attribute. The objc_class_stub attribute can be thought of as a generalization of objc_runtime_visible. Whereas objc_runtime_visible replaces the classref load with a runtime lookup of a class by string, the class stub mechanism is more efficient because the result of the lookup is cached. Also, objc_runtime_visible does not support categories to be defined on the class, whereas objc_class_stub does.

[dexonsmith]

+Erik and Alex, can you take a look at this?

[rjmccall]

include/clang/Basic/AttrDocs.td

1118 ↗

(On Diff #191626)

You should probably check that the user doesn't try to subclass classes annotated with this attribute, then. :)

lib/CodeGen/CGObjCMac.cpp

735 ↗

(On Diff #191626)

It should be safe to make it readnone, which could theoretically optimize something like doing a class message-send in a loop.

7274 ↗

(On Diff #191626)

This isn't for an arbitrary class ref, it's for the global class list. I'd say something like "the global class list sets the LSB to 1 on any class stubs".

7278 ↗

(On Diff #191626)

It's more typical to do this with a GEP, although the add should still work.

lib/Sema/SemaDeclAttr.cpp

7126 ↗

(On Diff #191626)

You should emit an error if !getLangOpts().ObjCRuntime.allowsClassStubs(), which should be straightforward to implement.

[aaron.ballman]

You should also add Sema tests that ensure the attribute applies to the expected AST nodes, is diagnosed appropriately when applied to something it shouldn't be applied to, accepts no args, etc. Basically, all of the semantic places we could warn on should have test coverage, as well as demonstrations of correct usage.

include/clang/Basic/Attr.td

1798 ↗

(On Diff #191626)

Formatting is a smidge off here -- should be ObjCClassStub : Attr with the extra whitespace.

1802 ↗

(On Diff #191626)

Does this attribute make sense outside of ObjC? e.g., should it require an ObjC compiland? If it should only be used in ObjC, then it should have a LangOpts field.

include/clang/Basic/AttrDocs.td

1116 ↗

(On Diff #191626)

This seems like the wrong category -- the attribute doesn't apply to functions.

1118 ↗

(On Diff #191626)

Try to keep the docs wrapped to the usual 80-col limit.

I think this could use a bit more exposition, or links to explain stuff. Based on the small amount here, I still have no idea why I would use this attribute.

[jordan_rose]

lib/CodeGen/CGObjCMac.cpp

7285 ↗

(On Diff #191626)

Is there a concern here in the non-stub case if GetClassGlobal no longer produces a ObjCTypes.ClassnfABIPtrTy? (Probably not, but thought I'd check [again].)

[slavapestov]

Thanks for the comments everyone. In the next version of the patch, I fixed most of the review feedback and also addressed an issue where super message sends were not calling objc_loadClassRef().

include/clang/Basic/Attr.td

1802 ↗

(On Diff #191626)

The other ObjC attributes, such as ObjCSubclassingRestricted, do not have a LangOpts. Do you want me to add a LangOpts field to those too? Or is it unnecessary since they're already restricted to InterfaceDecls?

include/clang/Basic/AttrDocs.td

1116 ↗

(On Diff #191626)

Would DocCatType make more sense? Would you like me to change ObjCRuntimeVisible and a handful of other miscategorized attributes too?

lib/CodeGen/CGObjCMac.cpp

7274 ↗

(On Diff #191626)

Can you explain what the difference is? My understanding is that the thing you pass to objc_loadClassref() (or load from directly) is a "classref". This is for classes you reference, not classes you define.

7285 ↗

(On Diff #191626)

You raise a good point. I brought back the assert in the next iteration of the patch.

[slavapestov]

Second revision adds the following:

• Validation of the attribute in Sema
• Sema tests for the above
• Correct behavior of super method calls of a class with the attribute
• More comprehensive CodeGen tests

[rjmccall]

lib/CodeGen/CGObjCMac.cpp

7274 ↗

(On Diff #191626)

Oh, I see what you're saying, nevermind.

[slavapestov]

I don't know what the etiquette is around here about pinging reviewers for a re-review, but this CL is ready for another look. Your feedback is much appreciated!

[aaron.ballman]

In D59628#1446626, @slavapestov wrote:

I don't know what the etiquette is around here about pinging reviewers for a re-review, but this CL is ready for another look. Your feedback is much appreciated!

Thanks for letting us know you think you've addressed all the feedback. We usually recommend pinging once a week if there's no activity on a review thread, but it's no problem to ping sooner when you have more information.

include/clang/Basic/Attr.td

1802 ↗

(On Diff #191626)

The other ObjC attributes, such as ObjCSubclassingRestricted, do not have a LangOpts. Do you want me to add a LangOpts field to those too? Or is it unnecessary since they're already restricted to InterfaceDecls?

Don't feel obligated, but if you wanted to post a follow-up patch that adds this to the other ObjC attribute, I think it would be a good change to make. It's not strictly required, but I think the diagnostics are a bit more clear when we restrict the language options.

include/clang/Basic/AttrDocs.td

1116 ↗

(On Diff #191626)

Oye, it looks like we have a lot of inconsistent categories currently. It's not really a type attribute, it's a declaration attribute, but we don't have such a notion yet. Go ahead and leave this as DocCatType for now; I'll see about cleaning this up in a follow-up.

include/clang/Basic/ObjCRuntime.h

437–443 ↗

(On Diff #192378)

I'd prefer if you grouped these cases and did it like:

switch (getKind()) {
case FragileMacOSX:
case GCC:
case GNUstep:
case ObjFW:
  return false;
case MacOSX:
case iOS:
case WatchOS:
  return true;
}

lib/CodeGen/CGObjCMac.cpp

7269 ↗

(On Diff #192378)

Please don't use auto here, the type is not spelled out in the initialization.

7347 ↗

(On Diff #192378)

Don't use auto here either.

lib/Sema/SemaDeclObjC.cpp

4097 ↗

(On Diff #192378)

Elide braces.

4131–4133 ↗

(On Diff #192378)

This should be done in Attr.td. You'll need to add a new LangOpt subclass (around line 298 or so are examples), and then add it to the LangOpts array when defining the attribute. Then you can remove this code as well as the new diagnostic.

4134 ↗

(On Diff #192378)

Elide braces.

test/CodeGenObjC/class-stubs.m

2 ↗

(On Diff #192378)

Hmm, this looks suspicious -- I think it's more clear to just bump this to the previous line rather than wonder if the RUN: command will confuse things.

[rjmccall]

lib/Sema/SemaDeclObjC.cpp

4131–4133 ↗

(On Diff #192378)

I don't think there's a way to run arbitrary code in a LangOpt right now, but it should be relatively straightforward to generalize ClangAttrEmitter to handle this. Just add an optional Code property to LangOpt that's expressed in terms of an assumed variable LangOpts, so that Attr.td could have a line like:

def ObjCClassStubsAllowed : LangOpt<"ObjCClassStubsAllowed", "LangOpts.ObjCRuntime.allowsClassStubs()">;

ClangAttrEmitter would take that expression, parenthesize it, and use it where it currently expands to "LangOpts." + Name. It should be possible to remove the Negated field in favor of this.

I guess that's probably worth doing vs. just having some hard-coded logic.

[aaron.ballman]

include/clang/Basic/Attr.td

293 ↗

(On Diff #193407)

I think the type here should be code instead of string since the user is passing in code snippets, no?

lib/Sema/SemaDeclObjC.cpp

4129–4130 ↗

(On Diff #193407)

Combine these if statements?

[aaron.ballman]

include/clang/Basic/AttrDocs.td

1116 ↗

(On Diff #191626)

I added DocCatDecl in r357585, which I think would be the correct category to use here.

[rjmccall]

include/clang/Basic/Attr.td

290 ↗

(On Diff #193407)

I think there's a grand total of one use of negated, so you might as well rewrite it to use customCode; see below.

306 ↗

(On Diff #193407)

This is not an appropriate name for this LangOpt. How about ObjCAllowsClassStubs?

include/clang/Basic/AttrDocs.td

1100 ↗

(On Diff #193407)

We should add something here to make it clear what the evolution story with this attribute is. Presumably you can't add it to an existing class without breaking ABI. Are there circumstances under which we're willing to guarantee that it can be removed (under a sufficiently-recent deployment target)?

utils/TableGen/ClangAttrEmitter.cpp

3442 ↗

(On Diff #193407)

I think the contract with CustomCode ought to be that LangOpts will be bound as an unqualified name, so that the custom code can be an arbitrary expression in terms of that. That will allow e.g. compound and negated LangOpts to be defined, like LangOpts.OpenCL && LangOpts.CPlusPlus. So you need to generate auto &LangOpts = S.LangOpts; at the top of the function, at least when custom code is present; and you should probably add parens around the code just to be safe.

Alternatively, you could make the contract that CustomCode should contain %0 or some other string that'll be expanded to an expression for the language options, but that sounds pretty complex compared to just binding LangOpts in the context.

[slavapestov]

Updated patch to address review feedback.

[rjmccall]

Thanks! Down to minor stuff now.

clang/include/clang/Basic/Attr.td

293

Please add a comment here explaining how to use CustomCode.

Can we just remove Negated and define COnly as !LangOpts.CPlusPlus?

[aaron.ballman]

clang/include/clang/Basic/Attr.td

292

If this is code, should it be using a code type rather than a string type?

clang/include/clang/Basic/AttrDocs.td

1101

Swift generated -> Swift-generated

clang/lib/CodeGen/CGObjCMac.cpp

736

Please spell the type out explicitly rather than use auto here.

7325

Can you add a message to this assertion so that when it triggers, users get something more helpful to report?

7366

Can you spell out the type here, since you're changing the initialization already?

7410–7411

I'm not opposed to this change, but it seems unrelated to the patch. Feel free to commit separately as an NFC.

[rjmccall]

LGTM, thanks! You should wait for Aaron's approval, too.

[aaron.ballman]

LGTM!

clang/include/clang/Basic/Attr.td

297

This compiles? I would have assumed that it would have to be def COnly : LangOpt<"COnly", [{!LangOpts.CPlusPlus}]>;

(If it compiles as-is, that's fine, I just wasn't aware that you could use string syntax for code blocks.)

[slavapestov]

clang/include/clang/Basic/Attr.td

297

It appears that both work! Going with your suggested version.

clang/include/clang/Basic/Attr.td

 
 class SubjectList<list<AttrSubject> subjects, SubjectDiag diag = WarnDiag,
                   string customDiag = ""> {
   list<AttrSubject> Subjects = subjects;
   SubjectDiag Diag = diag;
   string CustomDiag = customDiag;
 }
 
-class LangOpt<string name, bit negated = 0> {
+class LangOpt<string name, string customCode = ""> {
   string Name = name;
-  bit Negated = negated;
+
+  // A custom predicate, written as an expression evaluated in a context with
+  // "LangOpts" bound.
+  string CustomCode = customCode;
 }
 def MicrosoftExt : LangOpt<"MicrosoftExt">;
 def Borland : LangOpt<"Borland">;
 def CUDA : LangOpt<"CUDA">;
-def COnly : LangOpt<"CPlusPlus", 1>;
+def COnly : LangOpt<"COnly", "!LangOpts.CPlusPlus">;
 def CPlusPlus : LangOpt<"CPlusPlus">;
 def OpenCL : LangOpt<"OpenCL">;
 def RenderScript : LangOpt<"RenderScript">;
 def ObjC : LangOpt<"ObjC">;
 def BlocksSupported : LangOpt<"Blocks">;
 def ObjCAutoRefCount : LangOpt<"ObjCAutoRefCount">;
+def ObjCNonFragileRuntime : LangOpt<"ObjCNonFragileRuntime",
+                                    "LangOpts.ObjCRuntime.allowsClassStubs()">;
 
 // Defines targets for target-specific attributes. Empty lists are unchecked.
 class TargetSpec {
   // Specifies Architectures for which the target applies, based off the
   // ArchType enumeration in Triple.h.
   list<string> Arches = [];
   // Specifies Operating Systems for which the target applies, based off the
   // OSType enumeration in Triple.h

 }
 
 def ObjCRuntimeVisible : Attr {
   let Spellings = [Clang<"objc_runtime_visible">];
   let Subjects = SubjectList<[ObjCInterface], ErrorDiag>;
   let Documentation = [ObjCRuntimeVisibleDocs];
 }
 
+def ObjCClassStub : Attr {
+  let Spellings = [Clang<"objc_class_stub">];
+  let Subjects = SubjectList<[ObjCInterface], ErrorDiag>;
+  let Documentation = [ObjCClassStubDocs];
+  let LangOpts = [ObjCNonFragileRuntime];
+}
+
 def ObjCBoxable : Attr {
   let Spellings = [Clang<"objc_boxable">];
   let Subjects = SubjectList<[Record], ErrorDiag>;
   let Documentation = [ObjCBoxableDocs];
 }
 
 def OptimizeNone : InheritableAttr {
   let Spellings = [Clang<"optnone">];

clang/include/clang/Basic/AttrDocs.td

def ObjCRuntimeVisibleDocs : Documentation {
     let Content = [{
 This attribute specifies that the Objective-C class to which it applies is
 visible to the Objective-C runtime but not to the linker. Classes annotated
 with this attribute cannot be subclassed and cannot have categories defined for
 them.
     }];
 }
 
+def ObjCClassStubDocs : Documentation {
+    let Category = DocCatType;
+    let Content = [{
+This attribute specifies that the Objective-C class to which it applies is
+instantiated at runtime.
+
+Unlike ``__attribute__((objc_runtime_visible))``, a class having this attribute
+still has a "class stub" that is visible to the linker. This allows categories
+to be defined. Static message sends with the class as a receiver use a special
+access pattern to ensure the class is lazily instantiated from the class stub.
+
+Classes annotated with this attribute cannot be subclassed and cannot have
+implementations defined for them. This attribute is intended for use in
+Swift generated headers for classes defined in Swift.
+
+Adding or removing this attribute to a class is an ABI-breaking change.
+    }];
+}
+
 def ObjCBoxableDocs : Documentation {
     let Category = DocCatDecl;
     let Content = [{
 Structs and unions marked with the ``objc_boxable`` attribute can be used
 with the Objective-C boxed expression syntax, ``@(...)``.
 
 **Usage**: ``__attribute__((objc_boxable))``. This attribute
 can only be placed on a declaration of a trivially-copyable struct or union:

clang/include/clang/Basic/DiagnosticSemaKinds.td

def note_receiver_is_id : Note<
   "receiver is treated with 'id' type for purpose of method lookup">;
 def note_suppressed_class_declare : Note<
   "class with specified objc_requires_property_definitions attribute is declared here">;
 def err_objc_root_class_subclass : Error<
   "objc_root_class attribute may only be specified on a root class declaration">;
 def err_restricted_superclass_mismatch : Error<
   "cannot subclass a class that was declared with the "
   "'objc_subclassing_restricted' attribute">;
+def err_class_stub_subclassing_mismatch : Error<
+  "'objc_class_stub' attribute cannot be specified on a class that does not "
+  "have the 'objc_subclassing_restricted' attribute">;
+def err_implementation_of_class_stub : Error<
+  "cannot declare implementation of a class declared with the "
+  "'objc_class_stub' attribute">;
 def warn_objc_root_class_missing : Warning<
   "class %0 defined without specifying a base class">,
   InGroup<ObjCRootClass>;
 def err_objc_runtime_visible_category : Error<
   "cannot implement a category for class %0 that is only visible via the "
   "Objective-C runtime">;
 def err_objc_runtime_visible_subclass : Error<
   "cannot implement subclass %0 of a superclass %1 that is only visible via the "

clang/include/clang/Basic/ObjCRuntime.h

case MacOSX:
       return getVersion() >= VersionTuple(10, 11);
     case iOS:
       return getVersion() >= VersionTuple(9);
     case WatchOS:
       return getVersion() >= VersionTuple(2);
     }
   }
 
+  /// Returns true if this Objective-C runtime supports Objective-C class
+  /// stubs.
+  bool allowsClassStubs() const {
+    switch (getKind()) {
+    case FragileMacOSX:
+    case GCC:
+    case GNUstep:
+    case ObjFW:
+      return false;
+    case MacOSX:
+    case iOS:
+    case WatchOS:
+      return true;
+    }
+  }
+
   /// Try to parse an Objective-C runtime specification from the given
   /// string.
   ///
   /// \return true on error.
   bool tryParse(StringRef input);
 
   std::string getAsString() const;
 

clang/lib/CodeGen/CGObjCMac.cpp

public:
 
   llvm::FunctionCallee getObjCBeginCatchFn() {
     llvm::Type *params[] = { Int8PtrTy };
     return CGM.CreateRuntimeFunction(llvm::FunctionType::get(Int8PtrTy,
                                                              params, false),
                                      "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::FunctionCallee getLoadClassrefFn() const {
+    // Add the non-lazy-bind attribute, since objc_loadClassref is likely to
+    // be called a lot.
+    //
+    // Also it is safe to make it readnone, since we never load or store the
+    // classref except by calling this function.
+    llvm::Type *params[] = { Int8PtrPtrTy };
+    auto F = CGM.CreateRuntimeFunction(
+        llvm::FunctionType::get(ClassnfABIPtrTy, params, false),
+        "objc_loadClassref",
+        llvm::AttributeList::get(CGM.getLLVMContext(),
+                                 llvm::AttributeList::FunctionIndex,
+                                 {llvm::Attribute::NonLazyBind,
+                                  llvm::Attribute::ReadNone,
+                                  llvm::Attribute::NoUnwind}));
+    if (!CGM.getTriple().isOSBinFormatCOFF())
+      cast<llvm::Function>(F.getCallee())->setLinkage(
+        llvm::Function::ExternalWeakLinkage);
+
+    return F;
+  }
+
   llvm::StructType *EHTypeTy;
   llvm::Type *EHTypePtrTy;
 
   ObjCNonFragileABITypesHelper(CodeGen::CodeGenModule &cgm);
 };
 
 enum class ObjCLabelType {
   ClassName,
protected:
   SmallVector<const ObjCInterfaceDecl*, 16> ImplementedClasses;
 
   /// DefinedNonLazyClasses - List of defined "non-lazy" classes.
   SmallVector<llvm::GlobalValue*, 16> DefinedNonLazyClasses;
 
   /// DefinedCategories - List of defined categories.
   SmallVector<llvm::GlobalValue*, 16> DefinedCategories;
 
+  /// DefinedStubCategories - List of defined categories on class stubs.
+  SmallVector<llvm::GlobalValue*, 16> DefinedStubCategories;
+
   /// DefinedNonLazyCategories - List of defined "non-lazy" categories.
   SmallVector<llvm::GlobalValue*, 16> DefinedNonLazyCategories;
 
   /// Cached reference to the class for constant strings. This value has type
   /// int * but is actually an Obj-C class pointer.
   llvm::WeakTrackingVH ConstantStringClassRef;
 
   /// The LLVM type corresponding to NSConstantString.
private:
   /// class of the given name.
   llvm::Constant *GetClassGlobal(StringRef Name,
                                  ForDefinition_t IsForDefinition,
                                  bool Weak = false, bool DLLImport = false);
   llvm::Constant *GetClassGlobal(const ObjCInterfaceDecl *ID,
                                  bool isMetaclass,
                                  ForDefinition_t isForDefinition);
 
+  llvm::Constant *GetClassGlobalForClassRef(const ObjCInterfaceDecl *ID);
+
+  llvm::Value *EmitLoadOfClassRef(CodeGenFunction &CGF,
+                                  const ObjCInterfaceDecl *ID,
+                                  llvm::GlobalVariable *Entry);
+
   /// EmitClassRef - Return a Value*, of type ObjCTypes.ClassPtrTy,
   /// for the given class reference.
   llvm::Value *EmitClassRef(CodeGenFunction &CGF,
                             const ObjCInterfaceDecl *ID);
 
   llvm::Value *EmitClassRefFromId(CodeGenFunction &CGF,
                                   IdentifierInfo *II,
                                   const ObjCInterfaceDecl *ID);

 llvm::Constant *CGObjCNonFragileABIMac::getNSConstantStringClassRef() {
   if (llvm::Value *V = ConstantStringClassRef)
     return cast<llvm::Constant>(V);
 
   auto &StringClass = CGM.getLangOpts().ObjCConstantStringClass;
   std::string str =
     StringClass.empty() ? "OBJC_CLASS_$_NSConstantString"
                         : "OBJC_CLASS_$_" + StringClass;
-  auto GV = GetClassGlobal(str, NotForDefinition);
+  llvm::Constant *GV = GetClassGlobal(str, NotForDefinition);
 
   // Make sure the result is of the correct type.
   auto V = llvm::ConstantExpr::getBitCast(GV, CGM.IntTy->getPointerTo());
 
   ConstantStringClassRef = V;
   return V;
 }
 
AddModuleClassList(DefinedNonLazyClasses, "OBJC_LABEL_NONLAZY_CLASS_$",
                      GetSectionName("__objc_nlclslist",
                                     "regular,no_dead_strip"));
 
   // Build list of all implemented category addresses in array
   // L_OBJC_LABEL_CATEGORY_$.
   AddModuleClassList(DefinedCategories, "OBJC_LABEL_CATEGORY_$",
                      GetSectionName("__objc_catlist",
                                     "regular,no_dead_strip"));
+  AddModuleClassList(DefinedStubCategories, "OBJC_LABEL_STUB_CATEGORY_$",
+                     GetSectionName("__objc_catlist2",
+                                    "regular,no_dead_strip"));
   AddModuleClassList(DefinedNonLazyCategories, "OBJC_LABEL_NONLAZY_CATEGORY_$",
                      GetSectionName("__objc_nlcatlist",
                                     "regular,no_dead_strip"));
 
   EmitImageInfo();
 }
 
 /// isVTableDispatchedSelector - Returns true if SEL is not in the list of
void CGObjCNonFragileABIMac::GenerateCategory(const ObjCCategoryImplDecl *OCD) {
   }
 
   unsigned Size = CGM.getDataLayout().getTypeAllocSize(ObjCTypes.CategorynfABITy);
   values.addInt(ObjCTypes.IntTy, Size);
 
   llvm::GlobalVariable *GCATV =
       finishAndCreateGlobal(values, ExtCatName.str(), CGM);
   CGM.addCompilerUsedGlobal(GCATV);
+  if (Interface->hasAttr<ObjCClassStubAttr>())
+    DefinedStubCategories.push_back(GCATV);
+  else
     DefinedCategories.push_back(GCATV);
 
   // Determine if this category is also "non-lazy".
   if (ImplementationIsNonLazy(OCD))
     DefinedNonLazyCategories.push_back(GCATV);
   // method definition entries must be clear for next implementation.
   MethodDefinitions.clear();
 }
 
if (!GV || GV->getType() != ObjCTypes.ClassnfABITy->getPointerTo()) {
     GV = NewGV;
     CGM.getModule().getGlobalList().push_back(GV);
   }
 
   assert(GV->getLinkage() == L);
   return GV;
 }
 
+llvm::Constant *
+CGObjCNonFragileABIMac::GetClassGlobalForClassRef(const ObjCInterfaceDecl *ID) {
+  llvm::Constant *ClassGV = GetClassGlobal(ID, /*metaclass*/ false,
+                                           NotForDefinition);
+
+  if (!ID->hasAttr<ObjCClassStubAttr>())
+    return ClassGV;
+
+  ClassGV = llvm::ConstantExpr::getPointerCast(ClassGV, ObjCTypes.Int8PtrTy);
+
+  // Stub classes are pointer-aligned. Classrefs pointing at stub classes
+  // must set the least significant bit set to 1.
+  auto *Idx = llvm::ConstantInt::get(CGM.Int32Ty, 1);
+  return llvm::ConstantExpr::getGetElementPtr(CGM.Int8Ty, ClassGV, Idx);
+}
+
+llvm::Value *
+CGObjCNonFragileABIMac::EmitLoadOfClassRef(CodeGenFunction &CGF,
+                                           const ObjCInterfaceDecl *ID,
+                                           llvm::GlobalVariable *Entry) {
+  if (ID && ID->hasAttr<ObjCClassStubAttr>()) {
+    // Classrefs pointing at Objective-C stub classes must be loaded by calling
+    // a special runtime function.
+    return CGF.EmitRuntimeCall(
+      ObjCTypes.getLoadClassrefFn(), Entry, "load_classref_result");
+  }
+
+  CharUnits Align = CGF.getPointerAlign();
+  return CGF.Builder.CreateAlignedLoad(Entry, Align);
+}
+
 llvm::Value *
 CGObjCNonFragileABIMac::EmitClassRefFromId(CodeGenFunction &CGF,
                                            IdentifierInfo *II,
                                            const ObjCInterfaceDecl *ID) {
-  CharUnits Align = CGF.getPointerAlign();
   llvm::GlobalVariable *&Entry = ClassReferences[II];
 
   if (!Entry) {
     llvm::Constant *ClassGV;
     if (ID) {
-      ClassGV = GetClassGlobal(ID, /*metaclass*/ false, NotForDefinition);
+      ClassGV = GetClassGlobalForClassRef(ID);
     } else {
       ClassGV = GetClassGlobal((getClassSymbolPrefix() + II->getName()).str(),
                                NotForDefinition);
+      assert(ClassGV->getType() == ObjCTypes.ClassnfABIPtrTy);
     }
 
     std::string SectionName =
         GetSectionName("__objc_classrefs", "regular,no_dead_strip");
     Entry = new llvm::GlobalVariable(
-        CGM.getModule(), ObjCTypes.ClassnfABIPtrTy, false,
+        CGM.getModule(), ClassGV->getType(), false,
         getLinkageTypeForObjCMetadata(CGM, SectionName), ClassGV,
         "OBJC_CLASSLIST_REFERENCES_$_");
-    Entry->setAlignment(Align.getQuantity());
+    Entry->setAlignment(CGF.getPointerAlign().getQuantity());
+    if (!ID || !ID->hasAttr<ObjCClassStubAttr>())
       Entry->setSection(SectionName);
+
     CGM.addCompilerUsedGlobal(Entry);
   }
-  return CGF.Builder.CreateAlignedLoad(Entry, Align);
+
+  return EmitLoadOfClassRef(CGF, ID, Entry);
 }
 
 llvm::Value *CGObjCNonFragileABIMac::EmitClassRef(CodeGenFunction &CGF,
                                                   const ObjCInterfaceDecl *ID) {
   // If the class has the objc_runtime_visible attribute, we need to
   // use the Objective-C runtime to get the class.
   if (ID->hasAttr<ObjCRuntimeVisibleAttr>())
     return EmitClassRefViaRuntime(CGF, ID, ObjCTypes);
 
   return EmitClassRefFromId(CGF, ID->getIdentifier(), ID);
 }
 
 llvm::Value *CGObjCNonFragileABIMac::EmitNSAutoreleasePoolClassRef(
                                                     CodeGenFunction &CGF) {
   IdentifierInfo *II = &CGM.getContext().Idents.get("NSAutoreleasePool");
   return EmitClassRefFromId(CGF, II, nullptr);
 }
 
 llvm::Value *
 CGObjCNonFragileABIMac::EmitSuperClassRef(CodeGenFunction &CGF,
                                           const ObjCInterfaceDecl *ID) {
-  CharUnits Align = CGF.getPointerAlign();
   llvm::GlobalVariable *&Entry = SuperClassReferences[ID->getIdentifier()];
 
   if (!Entry) {
-    auto ClassGV = GetClassGlobal(ID, /*metaclass*/ false, NotForDefinition);
+    auto ClassGV = GetClassGlobalForClassRef(ID);
     std::string SectionName =
         GetSectionName("__objc_superrefs", "regular,no_dead_strip");
     Entry = new llvm::GlobalVariable(
-        CGM.getModule(), ObjCTypes.ClassnfABIPtrTy, false,
+        CGM.getModule(), ClassGV->getType(), false,
         getLinkageTypeForObjCMetadata(CGM, SectionName), ClassGV,
         "OBJC_CLASSLIST_SUP_REFS_$_");
-    Entry->setAlignment(Align.getQuantity());
+    Entry->setAlignment(CGF.getPointerAlign().getQuantity());
     Entry->setSection(SectionName);
     CGM.addCompilerUsedGlobal(Entry);
   }
-  return CGF.Builder.CreateAlignedLoad(Entry, Align);
+
+  return EmitLoadOfClassRef(CGF, ID, Entry);
 }
 
 /// EmitMetaClassRef - Return a Value * of the address of _class_t
 /// meta-data
 ///
 llvm::Value *CGObjCNonFragileABIMac::EmitMetaClassRef(CodeGenFunction &CGF,
                                                       const ObjCInterfaceDecl *ID,
                                                       bool Weak) {
llvm::Value *CGObjCNonFragileABIMac::EmitMetaClassRef(CodeGenFunction &CGF,
   return CGF.Builder.CreateAlignedLoad(Entry, Align);
 }
 
 /// GetClass - Return a reference to the class for the given interface
 /// decl.
 llvm::Value *CGObjCNonFragileABIMac::GetClass(CodeGenFunction &CGF,
                                               const ObjCInterfaceDecl *ID) {
   if (ID->isWeakImported()) {
-    auto ClassGV = GetClassGlobal(ID, /*metaclass*/ false, NotForDefinition);
+    llvm::Constant *ClassGV = GetClassGlobal(ID, /*metaclass*/ false,
+                                             NotForDefinition);
     (void)ClassGV;
     assert(!isa<llvm::GlobalVariable>(ClassGV) ||
            cast<llvm::GlobalVariable>(ClassGV)->hasExternalWeakLinkage());
   }
 
   return EmitClassRef(CGF, ID);
 }
 

clang/lib/Sema/SemaDeclAttr.cpp

case ParsedAttr::AT_ObjCRootClass:
     handleSimpleAttribute<ObjCRootClassAttr>(S, D, AL);
     break;
   case ParsedAttr::AT_ObjCNonLazyClass:
     handleSimpleAttribute<ObjCNonLazyClassAttr>(S, D, AL);
     break;
   case ParsedAttr::AT_ObjCSubclassingRestricted:
     handleSimpleAttribute<ObjCSubclassingRestrictedAttr>(S, D, AL);
     break;
+  case ParsedAttr::AT_ObjCClassStub:
+    handleSimpleAttribute<ObjCClassStubAttr>(S, D, AL);
+    break;
   case ParsedAttr::AT_ObjCExplicitProtocolImpl:
     handleObjCSuppresProtocolAttr(S, D, AL);
     break;
   case ParsedAttr::AT_ObjCRequiresPropertyDefs:
     handleSimpleAttribute<ObjCRequiresPropertyDefsAttr>(S, D, AL);
     break;
   case ParsedAttr::AT_Unused:
     handleUnusedAttr(S, D, AL);

clang/lib/Sema/SemaDeclObjC.cpp

if (ObjCInterfaceDecl* IDecl = IC->getClassInterface()) {
         // to check if we can subclass in the implementation as well.
         if (IDecl->hasAttr<ObjCSubclassingRestrictedAttr>() &&
             Super->hasAttr<ObjCSubclassingRestrictedAttr>()) {
           Diag(IC->getLocation(), diag::err_restricted_superclass_mismatch);
           Diag(Super->getLocation(), diag::note_class_declared);
         }
       }
 
+      if (IDecl->hasAttr<ObjCClassStubAttr>())
+        Diag(IC->getLocation(), diag::err_implementation_of_class_stub);
+
       if (LangOpts.ObjCRuntime.isNonFragile()) {
         while (IDecl->getSuperClass()) {
           DiagnoseDuplicateIvars(IDecl, IDecl->getSuperClass());
           IDecl = IDecl->getSuperClass();
         }
       }
     }
     SetIvarInitializers(IC);
Decl *Sema::ActOnAtEnd(Scope *S, SourceRange AtEnd, ArrayRef<Decl *> allMethods,
   } else if (const auto *IntfDecl = dyn_cast<ObjCInterfaceDecl>(ClassDecl)) {
     if (const ObjCInterfaceDecl *Super = IntfDecl->getSuperClass()) {
       if (!IntfDecl->hasAttr<ObjCSubclassingRestrictedAttr>() &&
           Super->hasAttr<ObjCSubclassingRestrictedAttr>()) {
         Diag(IntfDecl->getLocation(), diag::err_restricted_superclass_mismatch);
         Diag(Super->getLocation(), diag::note_class_declared);
       }
     }
+
+    if (IntfDecl->hasAttr<ObjCClassStubAttr>() &&
+        !IntfDecl->hasAttr<ObjCSubclassingRestrictedAttr>())
+      Diag(IntfDecl->getLocation(), diag::err_class_stub_subclassing_mismatch);
   }
   DiagnoseVariableSizedIvars(*this, OCD);
   if (isInterfaceDeclKind) {
     // Reject invalid vardecls.
     for (unsigned i = 0, e = allTUVars.size(); i != e; i++) {
       DeclGroupRef DG = allTUVars[i].get();
       for (DeclGroupRef::iterator I = DG.begin(), E = DG.end(); I != E; ++I)
         if (VarDecl *VDecl = dyn_cast<VarDecl>(*I)) {

clang/test/CodeGenObjC/class-stubs.m

+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -Wno-objc-root-class -emit-llvm -o - %s | FileCheck %s
+
+// -- classref for the message send in main()
+//
+// The class is declared with objc_class_stub, so LSB of the class pointer
+// must be set to 1.
+//
+// CHECK-LABEL: @"OBJC_CLASSLIST_REFERENCES_$_" = internal global i8* getelementptr (i8, i8* bitcast (%struct._class_t* @"OBJC_CLASS_$_Base" to i8*), i32 1), align 8
+
+// -- classref for the super message send in anotherClassMethod()
+//
+// Metaclasses do not use the "stub" mechanism and are referenced statically.
+//
+// CHECK-LABEL: @"OBJC_CLASSLIST_SUP_REFS_$_" = internal global %struct._class_t* @"OBJC_METACLASS_$_Derived", section "__DATA,__objc_superrefs,regular,no_dead_strip", align 8
+
+// -- classref for the super message send in anotherInstanceMethod()
+//
+// The class is declared with objc_class_stub, so LSB of the class pointer
+// must be set to 1.
+//
+// CHECK-LABEL: @"OBJC_CLASSLIST_SUP_REFS_$_.1" = internal global i8* getelementptr (i8, i8* bitcast (%struct._class_t* @"OBJC_CLASS_$_Derived" to i8*), i32 1), section "__DATA,__objc_superrefs,regular,no_dead_strip", align 8
+
+// -- category list for class stubs goes in __objc_catlist2.
+//
+// CHECK-LABEL: @"OBJC_LABEL_STUB_CATEGORY_$" = internal global [1 x i8*] [i8* bitcast (%struct._category_t* @"_OBJC_$_CATEGORY_Derived_$_MyCategory" to i8*)], section "__DATA,__objc_catlist2,regular,no_dead_strip", align 8
+
+__attribute__((objc_class_stub))
+__attribute__((objc_subclassing_restricted))
+@interface Base
++ (void) classMethod;
+- (void) instanceMethod;
+@end
+
+__attribute__((objc_class_stub))
+__attribute__((objc_subclassing_restricted))
+@interface Derived : Base
+@end
+
+int main() {
+  [Base classMethod];
+}
+// CHECK-LABEL: define i32 @main()
+// CHECK-NEXT: entry:
+// CHECK-NEXT:   [[CLASS:%.*]] = call %struct._class_t* @objc_loadClassref(i8** @"OBJC_CLASSLIST_REFERENCES_$_")
+// 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
+
+// CHECK-LABEL: declare extern_weak %struct._class_t* @objc_loadClassref(i8**)
+// CHECK-SAME: [[ATTRLIST:#.*]]
+
+@implementation Derived (MyCategory)
+
++ (void) anotherClassMethod {
+  [super classMethod];
+}
+// CHECK-LABEL: define internal void @"\01+[Derived(MyCategory) anotherClassMethod]"(i8* %self, i8* %_cmd) #0 {
+// CHECK-NEXT: entry:
+// CHECK:        [[SUPER:%.*]] = alloca %struct._objc_super, align 8
+// CHECK:        [[METACLASS_REF:%.*]] = load %struct._class_t*, %struct._class_t** @"OBJC_CLASSLIST_SUP_REFS_$_", align 8
+// CHECK:        [[CAST_METACLASS_REF:%.*]] = bitcast %struct._class_t* [[METACLASS_REF]] to i8*
+// CHECK:        [[DEST:%.*]] = getelementptr inbounds %struct._objc_super, %struct._objc_super* [[SUPER]], i32 0, i32 1
+// CHECK:        store i8* [[CAST_METACLASS_REF]], i8** [[DEST]], align 8
+// CHECK:        call void bitcast (i8* (%struct._objc_super*, i8*, ...)* @objc_msgSendSuper2 to void (%struct._objc_super*, i8*)*)(%struct._objc_super* [[SUPER]], i8* {{%.*}})
+// CHECK:        ret void
+
+- (void) anotherInstanceMethod {
+  [super instanceMethod];
+}
+// CHECK-LABEL: define internal void @"\01-[Derived(MyCategory) anotherInstanceMethod]"(%0* %self, i8* %_cmd) #0 {
+// CHECK-NEXT: entry:
+// CHECK:        [[SUPER:%.*]] = alloca %struct._objc_super, align 8
+// CHECK:        [[CLASS_REF:%.*]] = call %struct._class_t* @objc_loadClassref(i8** @"OBJC_CLASSLIST_SUP_REFS_$_.1")
+// CHECK:        [[CAST_CLASS_REF:%.*]] = bitcast %struct._class_t* [[CLASS_REF]] to i8*
+// CHECK:        [[DEST:%.*]] = getelementptr inbounds %struct._objc_super, %struct._objc_super* [[SUPER]], i32 0, i32 1
+// CHECK:        store i8* [[CAST_CLASS_REF]], i8** [[DEST]], align 8
+// CHECK:        call void bitcast (i8* (%struct._objc_super*, i8*, ...)* @objc_msgSendSuper2 to void (%struct._objc_super*, i8*)*)(%struct._objc_super* [[SUPER]], i8* {{%.*}})
+// CHECK:        ret void
+
+@end
+
+// -- calls to objc_loadClassRef() are readnone
+// CHECK: attributes [[ATTRLIST]] = { nounwind nonlazybind readnone }

clang/test/Misc/pragma-attribute-supported-attributes-list.test

 // CHECK-NEXT: OSReturnsNotRetained (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property, SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: OSReturnsRetained (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property, SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: OSReturnsRetainedOnNonZero (SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: OSReturnsRetainedOnZero (SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: ObjCBoxable (SubjectMatchRule_record)
 // CHECK-NEXT: ObjCBridge (SubjectMatchRule_record, SubjectMatchRule_type_alias)
 // CHECK-NEXT: ObjCBridgeMutable (SubjectMatchRule_record)
 // CHECK-NEXT: ObjCBridgeRelated (SubjectMatchRule_record)
+// CHECK-NEXT: ObjCClassStub (SubjectMatchRule_objc_interface)
 // CHECK-NEXT: ObjCDesignatedInitializer (SubjectMatchRule_objc_method)
 // CHECK-NEXT: ObjCException (SubjectMatchRule_objc_interface)
 // CHECK-NEXT: ObjCExplicitProtocolImpl (SubjectMatchRule_objc_protocol)
 // CHECK-NEXT: ObjCExternallyRetained (SubjectMatchRule_variable_not_is_parameter, SubjectMatchRule_function, SubjectMatchRule_block, SubjectMatchRule_objc_method)
 // CHECK-NEXT: ObjCMethodFamily (SubjectMatchRule_objc_method)
 // CHECK-NEXT: ObjCNonLazyClass (SubjectMatchRule_objc_interface, SubjectMatchRule_objc_implementation)
 // CHECK-NEXT: ObjCPreciseLifetime (SubjectMatchRule_variable)
 // CHECK-NEXT: ObjCRequiresPropertyDefs (SubjectMatchRule_objc_interface)

clang/test/SemaObjC/class-stub-attr-unsupported.m

+// RUN: %clang -target i386-apple-darwin -fsyntax-only -Xclang -verify %s
+// RUN: %clang -target i386-apple-darwin -x objective-c++ -fsyntax-only -Xclang -verify %s
+
+@interface NSObject
+@end
+
+__attribute__((objc_class_stub)) // expected-warning {{'objc_class_stub' attribute ignored}}
+__attribute__((objc_subclassing_restricted))
+@interface StubClass : NSObject
+@end

clang/test/SemaObjC/class-stub-attr.m

+// RUN: %clang -target x86_64-apple-darwin -fsyntax-only -Xclang -verify %s
+// RUN: %clang -target x86_64-apple-darwin -x objective-c++ -fsyntax-only -Xclang -verify %s
+
+@interface NSObject
+@end
+
+__attribute__((objc_class_stub))
+@interface MissingSubclassingRestrictedAttribute : NSObject // expected-error {{'objc_class_stub' attribute cannot be specified on a class that does not have the 'objc_subclassing_restricted' attribute}}
+@end
+
+__attribute__((objc_class_stub))
+__attribute__((objc_subclassing_restricted))
+@interface ValidClassStubAttribute : NSObject
+@end
+
+@implementation ValidClassStubAttribute // expected-error {{cannot declare implementation of a class declared with the 'objc_class_stub' attribute}}
+@end
+
+@implementation ValidClassStubAttribute (MyCategory)
+@end
+
+__attribute__((objc_class_stub(123))) // expected-error {{'objc_class_stub' attribute takes no arguments}}
+@interface InvalidClassStubAttribute : NSObject
+@end
+
+__attribute__((objc_class_stub)) // expected-error {{'objc_class_stub' attribute only applies to Objective-C interfaces}}
+int cannotHaveObjCClassStubAttribute() {}

clang/utils/TableGen/ClangAttrEmitter.cpp

if (Subjects.empty())
     return false;
   for (const auto *Subject : Subjects) {
     if (SubjectsToRules.find(Subject) == SubjectsToRules.end())
       return false;
   }
   return true;
 }
 
+static std::string GenerateTestExpression(ArrayRef<Record *> LangOpts) {
+  std::string Test;
+
+  for (auto *E : LangOpts) {
+    if (!Test.empty())
+      Test += " || ";
+
+    const StringRef Code = E->getValueAsString("CustomCode");
+    if (!Code.empty()) {
+      Test += "(";
+      Test += Code;
+      Test += ")";
+    } else {
+      Test += "LangOpts.";
+      Test += E->getValueAsString("Name");
+    }
+  }
+
+  if (Test.empty())
+    return "true";
+
+  return Test;
+}
+
 std::string
 PragmaClangAttributeSupport::generateStrictConformsTo(const Record &Attr,
                                                       raw_ostream &OS) {
   if (!isAttributedSupported(Attr))
     return "nullptr";
   // Generate a function that constructs a set of matching rules that describe
   // to which declarations the attribute should apply to.
   std::string FnName = "matchRulesFor" + Attr.getName().str();
for (const auto *Subject : Subjects) {
     auto It = SubjectsToRules.find(Subject);
     assert(It != SubjectsToRules.end() &&
            "This attribute is unsupported by #pragma clang attribute");
     for (const auto &Rule : It->getSecond().getAggregateRuleSet()) {
       // The rule might be language specific, so only subtract it from the given
       // rules if the specific language options are specified.
       std::vector<Record *> LangOpts = Rule.getLangOpts();
       OS << "  MatchRules.push_back(std::make_pair(" << Rule.getEnumValue()
-         << ", /*IsSupported=*/";
-      if (!LangOpts.empty()) {
-        for (auto I = LangOpts.begin(), E = LangOpts.end(); I != E; ++I) {
-          const StringRef Part = (*I)->getValueAsString("Name");
-          if ((*I)->getValueAsBit("Negated"))
-            OS << "!";
-          OS << "LangOpts." << Part;
-          if (I + 1 != E)
-            OS << " || ";
-        }
-      } else
-        OS << "true";
-      OS << "));\n";
+         << ", /*IsSupported=*/" << GenerateTestExpression(LangOpts)
+         << "));\n";
     }
   }
   OS << "}\n\n";
   return FnName;
 }
 
 void PragmaClangAttributeSupport::generateParsingHelpers(raw_ostream &OS) {
   // Generate routines that check the names of sub-rules.

 static std::string GenerateLangOptRequirements(const Record &R,
                                                raw_ostream &OS) {
   // If the attribute has an empty or unset list of language requirements,
   // return the default handler.
   std::vector<Record *> LangOpts = R.getValueAsListOfDefs("LangOpts");
   if (LangOpts.empty())
     return "defaultDiagnoseLangOpts";
 
-  // Generate the test condition, as well as a unique function name for the
-  // diagnostic test. The list of options should usually be short (one or two
-  // options), and the uniqueness isn't strictly necessary (it is just for
-  // codegen efficiency).
-  std::string FnName = "check", Test;
-  for (auto I = LangOpts.begin(), E = LangOpts.end(); I != E; ++I) {
-    const StringRef Part = (*I)->getValueAsString("Name");
-    if ((*I)->getValueAsBit("Negated")) {
-      FnName += "Not";
-      Test += "!";
-    }
-    Test += "S.LangOpts.";
-    Test +=  Part;
-    if (I + 1 != E)
-      Test += " || ";
-    FnName += Part;
-  }
+  // Generate a unique function name for the diagnostic test. The list of
+  // options should usually be short (one or two options), and the
+  // uniqueness isn't strictly necessary (it is just for codegen efficiency).
+  std::string FnName = "check";
+  for (auto I = LangOpts.begin(), E = LangOpts.end(); I != E; ++I)
+    FnName += (*I)->getValueAsString("Name");
   FnName += "LangOpts";
 
   // If this code has already been generated, simply return the previous
   // instance of it.
   static std::set<std::string> CustomLangOptsSet;
   auto I = CustomLangOptsSet.find(FnName);
   if (I != CustomLangOptsSet.end())
     return *I;
 
   OS << "static bool " << FnName << "(Sema &S, const ParsedAttr &Attr) {\n";
-  OS << "  if (" << Test << ")\n";
+  OS << "  auto &LangOpts = S.LangOpts;\n";
+  OS << "  if (" << GenerateTestExpression(LangOpts) << ")\n";
   OS << "    return true;\n\n";
   OS << "  S.Diag(Attr.getLoc(), diag::warn_attribute_ignored) ";
   OS << "<< Attr.getName();\n";
   OS << "  return false;\n";
   OS << "}\n\n";
 
   CustomLangOptsSet.insert(FnName);
   return FnName;