[Objective-C] Introduce objc_runtime_visible attribute.

The objc_runtime_visible attribute deals with an odd corner case where
a particular Objective-C class is known to the Objective-C runtime
(and, therefore, accessible by name) but its symbol has been hidden
for some reason. For such classes, teach CodeGen to use
objc_lookUpClass to retrieve the Class object, rather than referencing
the class symbol directly.

Classes annotated with objc_runtime_visible have two major limitations
that fall out from places where Objective-C metadata needs to refer to
the class (or metaclass) symbol directly:

* One cannot implement a subclass of an objc_runtime_visible class.
* One cannot implement a category on an objc_runtime_visible class.

Implements rdar://problem/25494092.

llvm-svn: 265201

clang/include/clang/Basic/Attr.td

@@ -1241,6 +1241,12 @@ def ObjCRuntimeName : Attr {
   let Documentation = [ObjCRuntimeNameDocs];
 }
 
+def ObjCRuntimeVisible : Attr {
+  let Spellings = [GNU<"objc_runtime_visible">];
+  let Subjects = SubjectList<[ObjCInterface], ErrorDiag>;
+  let Documentation = [ObjCRuntimeVisibleDocs];
+}
+
 def ObjCBoxable : Attr {
   let Spellings = [GNU<"objc_boxable">];
   let Subjects = SubjectList<[Record], ErrorDiag, "ExpectedStructOrUnion">;

clang/include/clang/Basic/AttrDocs.td

@@ -612,6 +612,13 @@ can only be placed before an @protocol or @interface declaration:
     }];
 }
 
+def ObjCRuntimeVisibleDocs : Documentation {
+    let Category = DocCatFunction;
+    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 ObjCBoxableDocs : Documentation {
     let Category = DocCatFunction;
     let Content = [{

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -715,6 +715,12 @@ def err_objc_root_class_subclass : Error<
 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 "
+  "Objective-C runtime">;
 def note_objc_needs_superclass : Note<
   "add a super class to fix this problem">;
 def warn_dup_category_def : Warning<

clang/lib/CodeGen/CGObjCMac.cpp

@@ -349,6 +349,20 @@ public:
     return CGM.CreateRuntimeFunction(FTy, "objc_enumerationMutation");
   }
 
+  llvm::Constant *getLookUpClassFn() {
+    CodeGen::CodeGenTypes &Types = CGM.getTypes();
+    ASTContext &Ctx = CGM.getContext();
+    // Class objc_lookUpClass (const char *)
+    SmallVector<CanQualType,1> Params;
+    Params.push_back(
+      Ctx.getCanonicalType(Ctx.getPointerType(Ctx.CharTy.withConst())));
+    llvm::FunctionType *FTy =
+        Types.GetFunctionType(Types.arrangeBuiltinFunctionDeclaration(
+                                Ctx.getCanonicalType(Ctx.getObjCClassType()),
+                                Params));
+    return CGM.CreateRuntimeFunction(FTy, "objc_lookUpClass");
+  }
+
   /// GcReadWeakFn -- LLVM objc_read_weak (id *src) function.
   llvm::Constant *getGcReadWeakFn() {
     // id objc_read_weak (id *)
@@ -981,6 +995,12 @@ protected:
   /// defined. The return value has type ProtocolPtrTy.
   llvm::Constant *GetProtocolRef(const ObjCProtocolDecl *PD);
 
+  /// Return a reference to the given Class using runtime calls rather than
+  /// by a symbol reference.
+  llvm::Value *EmitClassRefViaRuntime(CodeGenFunction &CGF,
+                                      const ObjCInterfaceDecl *ID,
+                                      ObjCCommonTypesHelper &ObjCTypes);
+
 public:
   /// CreateMetadataVar - Create a global variable with internal
   /// linkage for use by the Objective-C runtime.
@@ -2673,6 +2693,25 @@ llvm::Constant *CGObjCCommonMac::GetProtocolRef(const ObjCProtocolDecl *PD) {
   return GetOrEmitProtocolRef(PD);
 }
 
+llvm::Value *CGObjCCommonMac::EmitClassRefViaRuntime(
+               CodeGenFunction &CGF,
+               const ObjCInterfaceDecl *ID,
+               ObjCCommonTypesHelper &ObjCTypes) {
+  llvm::Constant *lookUpClassFn = ObjCTypes.getLookUpClassFn();
+
+  llvm::Value *className =
+      CGF.CGM.GetAddrOfConstantCString(ID->getObjCRuntimeNameAsString())
+        .getPointer();
+  ASTContext &ctx = CGF.CGM.getContext();
+  className =
+      CGF.Builder.CreateBitCast(className,
+                                CGF.ConvertType(
+                                  ctx.getPointerType(ctx.CharTy.withConst())));
+  llvm::CallInst *call = CGF.Builder.CreateCall(lookUpClassFn, className);
+  call->setDoesNotThrow();
+  return call;
+}
+
 /*
 // Objective-C 1.0 extensions
 struct _objc_protocol {
@@ -4633,6 +4672,11 @@ llvm::Value *CGObjCMac::EmitClassRefFromId(CodeGenFunction &CGF,
 
 llvm::Value *CGObjCMac::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());
 }
 
@@ -6874,6 +6918,11 @@ llvm::Value *CGObjCNonFragileABIMac::EmitClassRefFromId(CodeGenFunction &CGF,
 
 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->isWeakImported(), ID);
 }
 

clang/lib/Sema/SemaDeclAttr.cpp

@@ -5548,6 +5548,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case AttributeList::AT_ObjCRuntimeName:
     handleObjCRuntimeName(S, D, Attr);
     break;
+   case AttributeList::AT_ObjCRuntimeVisible:
+    handleSimpleAttribute<ObjCRuntimeVisibleAttr>(S, D, Attr);
+    break;
   case AttributeList::AT_ObjCBoxable:
     handleObjCBoxable(S, D, Attr);
     break;

clang/lib/Sema/SemaDeclObjC.cpp

@@ -1831,6 +1831,13 @@ Decl *Sema::ActOnStartCategoryImplementation(
   if (IDecl)
     DiagnoseUseOfDecl(IDecl, ClassLoc);
 
+  // If the interface has the objc_runtime_visible attribute, we
+  // cannot implement a category for it.
+  if (IDecl && IDecl->hasAttr<ObjCRuntimeVisibleAttr>()) {
+    Diag(ClassLoc, diag::err_objc_runtime_visible_category)
+      << IDecl->getDeclName();
+  }
+
   /// Check that CatName, category name, is not used in another implementation.
   if (CatIDecl) {
     if (CatIDecl->getImplementation()) {
@@ -1968,6 +1975,16 @@ Decl *Sema::ActOnStartClassImplementation(
                                         dyn_cast<NamedDecl>(IDecl), 
                                         IMPDecl->getLocation(), 1);
   }
+
+  // If the superclass has the objc_runtime_visible attribute, we
+  // cannot implement a subclass of it.
+  if (IDecl->getSuperClass() &&
+      IDecl->getSuperClass()->hasAttr<ObjCRuntimeVisibleAttr>()) {
+    Diag(ClassLoc, diag::err_objc_runtime_visible_subclass)
+      << IDecl->getDeclName()
+      << IDecl->getSuperClass()->getDeclName();
+  }
+
   return ActOnObjCContainerStartDefinition(IMPDecl);
 }
 

clang/test/CodeGenObjC/attr-objc-runtime-visible.m

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -fobjc-runtime=macosx-10.9.0 -emit-llvm %s -o - | FileCheck %s
+
+// RUN: %clang_cc1 -triple i386-apple-darwin -fobjc-runtime=macosx-fragile-10.9.0 -emit-llvm %s -o - | FileCheck %s
+
+@interface Root
++(Class)class;
+@end
+
+__attribute__((objc_runtime_visible))
+__attribute__((objc_runtime_name("MyRuntimeVisibleClass")))
+@interface A : Root
+@end
+
+// CHECK: [[CLASSNAME:@.*]] = private unnamed_addr constant [22 x i8] c"MyRuntimeVisibleClass
+// CHECK: define i8* @getClass() #0 {
+Class getClass(void) {
+  // CHECK: call i8* @objc_lookUpClass(i8* getelementptr inbounds ([22 x i8], [22 x i8]* [[CLASSNAME]], i32 0, i32 0)) #2
+  return [A class];
+}

clang/test/SemaObjC/attr-objc-runtime-visible.m

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -verify -fsyntax-only  %s
+
+__attribute__((objc_runtime_visible))
+@interface A
+@end
+
+@interface A(X)
+@end
+
+@implementation A(X) // expected-error{{cannot implement a category for class 'A' that is only visible via the Objective-C runtime}}
+@end
+
+@interface B : A
+@end
+
+@implementation B // expected-error{{cannot implement subclass 'B' of a superclass 'A' that is only visible via the Objective-C runtime}}
+@end
+
+