Add the `objc_class_stub` attribute.

Swift requires certain classes to be not just initialized lazily on first
use, but actually allocated lazily using information that is only available
at runtime.  This is incompatible with ObjC class initialization, or at least
not efficiently compatible, because there is no meaningful class symbol
that can be put in a class-ref variable at load time.  This leaves ObjC
code unable to access such classes, which is undesirable.

objc_class_stub says that class references should be resolved by calling
a new ObjC runtime function with a pointer to a new "class stub" structure.
Non-ObjC compilers (like Swift) can simply emit this structure when ObjC
interop is required for a class that cannot be statically allocated,
then apply this attribute to the `@interface` in the generated ObjC header
for the class.

This attribute can be thought of as a generalization of the existing
`objc_runtime_visible` attribute which permits more efficient class
resolution as well as supporting the additon of categories to the class.
Subclassing these classes from ObjC is currently not allowed.

Patch by Slava Pestov!

llvm-svn: 362054

3 files changed, 100 insertions, 13 deletions

diff --git a/clang/lib/CodeGen/CGObjCMac.cpp b/clang/lib/CodeGen/CGObjCMac.cpp
index 79b00f21411..4d4e54d214d 100644
--- a/clang/lib/CodeGen/CGObjCMac.cpp
+++ b/clang/lib/CodeGen/CGObjCMac.cpp
@@ -721,6 +721,33 @@ public:
                                      "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 };
+    llvm::FunctionCallee 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;
 
@@ -877,6 +904,9 @@ protected:
   /// 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;
 
@@ -1464,6 +1494,12 @@ private:
                                  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,
@@ -1933,7 +1969,7 @@ llvm::Constant *CGObjCNonFragileABIMac::getNSConstantStringClassRef() {
   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());
@@ -6069,6 +6105,9 @@ void CGObjCNonFragileABIMac::FinishNonFragileABIModule() {
   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"));
@@ -6560,7 +6599,10 @@ void CGObjCNonFragileABIMac::GenerateCategory(const ObjCCategoryImplDecl *OCD) {
   llvm::GlobalVariable *GCATV =
       finishAndCreateGlobal(values, ExtCatName.str(), CGM);
   CGM.addCompilerUsedGlobal(GCATV);
-  DefinedCategories.push_back(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))
@@ -7236,33 +7278,68 @@ CGObjCNonFragileABIMac::GetClassGlobal(StringRef Name,
   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 &&
+             "classref was emitted with the wrong type?");
     }
 
     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->setSection(SectionName);
+    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,
@@ -7284,22 +7361,22 @@ llvm::Value *CGObjCNonFragileABIMac::EmitNSAutoreleasePoolClassRef(
 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);
+    llvm::Constant *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
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 03b38bf2158..84f00dbaa2e 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -6993,6 +6993,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   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;
diff --git a/clang/lib/Sema/SemaDeclObjC.cpp b/clang/lib/Sema/SemaDeclObjC.cpp
index 5ff1f9e3408..21d9b8c3226 100644
--- a/clang/lib/Sema/SemaDeclObjC.cpp
+++ b/clang/lib/Sema/SemaDeclObjC.cpp
@@ -4061,6 +4061,9 @@ Decl *Sema::ActOnAtEnd(Scope *S, SourceRange AtEnd, ArrayRef<Decl *> allMethods,
         }
       }
 
+      if (IDecl->hasAttr<ObjCClassStubAttr>())
+        Diag(IC->getLocation(), diag::err_implementation_of_class_stub);
+
       if (LangOpts.ObjCRuntime.isNonFragile()) {
         while (IDecl->getSuperClass()) {
           DiagnoseDuplicateIvars(IDecl, IDecl->getSuperClass());
@@ -4089,6 +4092,10 @@ Decl *Sema::ActOnAtEnd(Scope *S, SourceRange AtEnd, ArrayRef<Decl *> allMethods,
         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) {