[CodeGen] Handle extern references to OBJC_CLASS_$_*

[erik.pilkington]

Summary

Some ObjC users declare a extern variable named OBJC_CLASS_$_Foo, then use it's address as a Class. I.e., one could define isInstanceOfF:

BOOL isInstanceOfF(id c) {
  extern void OBJC_CLASS_$_F;
  return [c class] == (Class)&OBJC_CLASS_$_F;
}

This leads to asserts in clang CodeGen if there is an @implementation of F in the same TU as an instance of this pattern, because CodeGen assumes that a variable named OBJC_CLASS_$_* has the right type. This patch fixes the problem by RAUWing the old (incorrectly typed) global with a new global, then removing the old global.

I know almost nothing about Objective-C runtime stuff, so take this patch with a grain of salt!

Fixes rdar://45077269

[jfb]

Overall this seems fine, but I'm no ObjC expert either so I'll defer to @rjmccall

clang/lib/CodeGen/CGObjCMac.cpp

7193

Add /* isConstant=*/false

[ahatanak]

LGTM

clang/lib/CodeGen/CGObjCMac.cpp

 llvm::Constant *
 CGObjCNonFragileABIMac::GetClassGlobal(StringRef Name,
                                        ForDefinition_t IsForDefinition,
                                        bool Weak, bool DLLImport) {
   llvm::GlobalValue::LinkageTypes L =
       Weak ? llvm::GlobalValue::ExternalWeakLinkage
            : llvm::GlobalValue::ExternalLinkage;
 
-
-
   llvm::GlobalVariable *GV = CGM.getModule().getGlobalVariable(Name);
-  if (!GV) {
-    GV = new llvm::GlobalVariable(CGM.getModule(), ObjCTypes.ClassnfABITy,
-                                  false, L, nullptr, Name);
+  if (!GV || GV->getType() != ObjCTypes.ClassnfABITy->getPointerTo()) {
+    auto *NewGV = new llvm::GlobalVariable(ObjCTypes.ClassnfABITy, false, L,
+                                           nullptr, Name);
 
     if (DLLImport)
-      GV->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
+      NewGV->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
+
+    if (GV) {
+      GV->replaceAllUsesWith(
+          llvm::ConstantExpr::getBitCast(NewGV, GV->getType()));
+      GV->eraseFromParent();
+    }
+    GV = NewGV;
+    CGM.getModule().getGlobalList().push_back(GV);
   }
 
   assert(GV->getLinkage() == L);
   return GV;
 }
 
 llvm::Value *
 CGObjCNonFragileABIMac::EmitClassRefFromId(CodeGenFunction &CGF,

clang/test/CodeGenObjC/extern-void-class-decl.m

+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.14.0 %s -emit-llvm -o - | FileCheck %s
+
+// rdar://45077269
+
+extern void OBJC_CLASS_$_f;
+Class c = (Class)&OBJC_CLASS_$_f;
+
+@implementation f @end
+
+// Check that we override the initializer for c, and that OBJC_CLASS_$_f gets
+// the right definition.
+
+// CHECK: @c = global i8* bitcast (%struct._class_t* @"OBJC_CLASS_$_f" to i8*)
+// CHECK: @"OBJC_CLASS_$_f" = global %struct._class_t