[CodeGen][ObjC] Emit the retainRV marker as a module flag instead of
named metadata.

This fixes a bug where ARC contract wasn't inserting the retainRV
marker when LTO was enabled, which caused objects returned from a
function to be auto-released.

rdar://problem/49464214

Differential Revision: https://reviews.llvm.org/D60302

llvm-svn: 358048
diff --git a/clang/lib/CodeGen/CGObjC.cpp b/clang/lib/CodeGen/CGObjC.cpp
index 69ced58..d5906cf 100644
--- a/clang/lib/CodeGen/CGObjC.cpp
+++ b/clang/lib/CodeGen/CGObjC.cpp
@@ -2161,14 +2161,10 @@
     // with this marker yet, so leave a breadcrumb for the ARC
     // optimizer to pick up.
     } else {
-      llvm::NamedMDNode *metadata =
-        CGF.CGM.getModule().getOrInsertNamedMetadata(
-                            "clang.arc.retainAutoreleasedReturnValueMarker");
-      assert(metadata->getNumOperands() <= 1);
-      if (metadata->getNumOperands() == 0) {
-        auto &ctx = CGF.getLLVMContext();
-        metadata->addOperand(llvm::MDNode::get(ctx,
-                                     llvm::MDString::get(ctx, assembly)));
+      const char *markerKey = "clang.arc.retainAutoreleasedReturnValueMarker";
+      if (!CGF.CGM.getModule().getModuleFlag(markerKey)) {
+        auto *str = llvm::MDString::get(CGF.getLLVMContext(), assembly);
+        CGF.CGM.getModule().addModuleFlag(llvm::Module::Error, markerKey, str);
       }
     }
   }