Index: include/llvm/Transforms/Utils/ValueMapper.h
===================================================================
--- include/llvm/Transforms/Utils/ValueMapper.h
+++ include/llvm/Transforms/Utils/ValueMapper.h
@@ -69,6 +69,11 @@
     /// Instruct the remapper to move distinct metadata instead of duplicating
     /// it when there are module-level changes.
     RF_MoveDistinctMDs = 4,
+
+    /// RF_NullMapMissingGlobalValues - Any global values not in value map are
+    /// mapped to null instead of mapping to self. Illegal if
+    /// RF_IgnoreMissingEntries is also set.
+    RF_NullMapMissingGlobalValues = 8,
   };
 
   static inline RemapFlags operator|(RemapFlags LHS, RemapFlags RHS) {
Index: lib/Linker/LinkModules.cpp
===================================================================
--- lib/Linker/LinkModules.cpp
+++ lib/Linker/LinkModules.cpp
@@ -1628,8 +1628,9 @@
     NamedMDNode *DestNMD = DstM->getOrInsertNamedMetadata(NMD.getName());
     // Add Src elements into Dest node.
     for (const MDNode *op : NMD.operands())
-      DestNMD->addOperand(MapMetadata(op, ValueMap, RF_MoveDistinctMDs,
-                                      &TypeMap, &ValMaterializer));
+      DestNMD->addOperand(MapMetadata(
+          op, ValueMap, RF_MoveDistinctMDs | RF_NullMapMissingGlobalValues,
+          &TypeMap, &ValMaterializer));
   }
 }
 
Index: lib/Transforms/Utils/ValueMapper.cpp
===================================================================
--- lib/Transforms/Utils/ValueMapper.cpp
+++ lib/Transforms/Utils/ValueMapper.cpp
@@ -42,9 +42,14 @@
 
   // Global values do not need to be seeded into the VM if they
   // are using the identity mapping.
-  if (isa<GlobalValue>(V))
+  if (isa<GlobalValue>(V)) {
+    if (Flags & RF_NullMapMissingGlobalValues) {
+      assert(!(Flags & RF_IgnoreMissingEntries));
+      return nullptr;
+    }
     return VM[V] = const_cast<Value*>(V);
-  
+  }
+
   if (const InlineAsm *IA = dyn_cast<InlineAsm>(V)) {
     // Inline asm may need *type* remapping.
     FunctionType *NewTy = IA->getFunctionType();
@@ -74,7 +79,8 @@
     // correct.  For now, just match behaviour from before the metadata/value
     // split.
     //
-    //    assert(MappedMD && "Referenced metadata value not in value map");
+    //    assert((MappedMD || (Flags & RF_NullMapMissingGlobalValues)) &&
+    //           "Referenced metadata value not in value map");
     return VM[V] = MetadataAsValue::get(V->getContext(), MappedMD);
   }
 
@@ -184,7 +190,8 @@
   // correct.  For now, just match behaviour from before the metadata/value
   // split.
   //
-  //    llvm_unreachable("Referenced metadata not in value map!");
+  //    assert((Flags & RF_NullMapMissingGlobalValues) &&
+  //           "Referenced metadata not in value map!");
   return nullptr;
 }
 
@@ -305,7 +312,8 @@
     // correct.  For now, just match behaviour from before the metadata/value
     // split.
     //
-    //    assert(MappedV && "Referenced metadata not in value map!");
+    //    assert((MappedV || (Flags & RF_NullMapMissingGlobalValues)) &&
+    //           "Referenced metadata not in value map!");
     if (MappedV)
       return mapToMetadata(VM, MD, ValueAsMetadata::get(MappedV));
     return nullptr;