Index: lib/IR/Metadata.cpp
===================================================================
--- lib/IR/Metadata.cpp
+++ lib/IR/Metadata.cpp
@@ -675,8 +675,8 @@
   Metadata *Old = getOperand(Op);
   setOperand(Op, New);
 
-  // Drop uniquing for self-reference cycles.
-  if (New == this) {
+  // Drop uniquing for self-reference cycles and deleted constants.
+  if (New == this || (!New && Old && isa<ConstantAsMetadata>(Old))) {
     if (!isResolved())
       resolve();
     storeDistinctInContext();
Index: test/Linker/Inputs/metadata-with-global-value-operand.ll
===================================================================
--- /dev/null
+++ test/Linker/Inputs/metadata-with-global-value-operand.ll
@@ -0,0 +1,3 @@
+!named.null = !{!0}
+
+!0 = !{null}
Index: test/Linker/metadata-with-global-value-operand.ll
===================================================================
--- /dev/null
+++ test/Linker/metadata-with-global-value-operand.ll
@@ -0,0 +1,14 @@
+; RUN: llvm-link -S -o - %s %S/Inputs/metadata-with-global-value-operand.ll | FileCheck %s
+; This test confirms that the !{null} from the second module doesn't get mapped
+; onto the abandoned !{i1* @var} node from this module.
+
+; CHECK: @var = global
+@var = global i1 false
+
+; CHECK: !named.vars = !{!0}
+; CHECK: !named.null = !{!1}
+!named.vars = !{!0}
+
+; CHECK: !0 = !{i1* @var}
+; CHECK: !1 = !{null}
+!0 = !{i1* @var}
Index: test/Transforms/GlobalOpt/metadata.ll
===================================================================
--- test/Transforms/GlobalOpt/metadata.ll
+++ test/Transforms/GlobalOpt/metadata.ll
@@ -28,5 +28,5 @@
 ; CHECK: !named = !{![[NULL:[0-9]+]]}
 
 !0 = !{i8*** @G}
-; CHECK-DAG: ![[NULL]] = !{null}
+; CHECK-DAG: ![[NULL]] = distinct !{null}
 ; CHECK-DAG: ![[EMPTY]] = !{}
Index: unittests/IR/MetadataTest.cpp
===================================================================
--- unittests/IR/MetadataTest.cpp
+++ unittests/IR/MetadataTest.cpp
@@ -449,6 +449,40 @@
   EXPECT_FALSE(Wrapped1->isDistinct());
 }
 
+TEST_F(MDNodeTest, UniquedOnDeletedOperand) {
+  // temp !{}
+  TempMDTuple T = MDTuple::getTemporary(Context, None);
+
+  // !{temp !{}}
+  Metadata *Ops[] = {T.get()};
+  MDTuple *N = MDTuple::get(Context, Ops);
+
+  // !{temp !{}} => !{null}
+  T.reset();
+  ASSERT_TRUE(N->isUniqued());
+  Metadata *NullOps[] = {nullptr};
+  ASSERT_EQ(N, MDTuple::get(Context, NullOps));
+}
+
+TEST_F(MDNodeTest, DistinctOnDeletedValueOperand) {
+  // i1* @GV
+  Type *Ty = Type::getInt1PtrTy(Context);
+  std::unique_ptr<GlobalVariable> GV(
+      new GlobalVariable(Ty, false, GlobalValue::ExternalLinkage));
+  ConstantAsMetadata *Op = ConstantAsMetadata::get(GV.get());
+
+  // !{i1* @GV}
+  Metadata *Ops[] = {Op};
+  MDTuple *N = MDTuple::get(Context, Ops);
+
+  // !{i1* @GV} => !{null}
+  GV.reset();
+  ASSERT_TRUE(N->isDistinct());
+  ASSERT_EQ(nullptr, N->getOperand(0));
+  Metadata *NullOps[] = {nullptr};
+  ASSERT_NE(N, MDTuple::get(Context, NullOps));
+}
+
 TEST_F(MDNodeTest, getDistinct) {
   // !{}
   MDNode *Empty = MDNode::get(Context, None);
@@ -669,7 +703,7 @@
   EXPECT_TRUE(N->isResolved());
 }
 
-TEST_F(MDNodeTest, replaceWithUniquedChangingOperand) {
+TEST_F(MDNodeTest, replaceWithUniquedDeletedOperand) {
   // i1* @GV
   Type *Ty = Type::getInt1PtrTy(Context);
   std::unique_ptr<GlobalVariable> GV(
@@ -686,8 +720,33 @@
 
   // !{i1* @GV} => !{null}
   GV.reset();
-  ASSERT_TRUE(N->isUniqued());
+  ASSERT_TRUE(N->isDistinct());
+  ASSERT_EQ(nullptr, N->getOperand(0));
   Metadata *NullOps[] = {nullptr};
+  ASSERT_NE(N, MDTuple::get(Context, NullOps));
+}
+
+TEST_F(MDNodeTest, replaceWithUniquedChangedOperand) {
+  // i1* @GV
+  Type *Ty = Type::getInt1PtrTy(Context);
+  std::unique_ptr<GlobalVariable> GV(
+      new GlobalVariable(Ty, false, GlobalValue::ExternalLinkage));
+  ConstantAsMetadata *Op = ConstantAsMetadata::get(GV.get());
+
+  // temp !{i1* @GV}
+  Metadata *Ops[] = {Op};
+  MDTuple *N = MDTuple::getTemporary(Context, Ops).release();
+
+  // temp !{i1* @GV} => !{i1* @GV}
+  ASSERT_EQ(N, MDNode::replaceWithUniqued(TempMDTuple(N)));
+  ASSERT_TRUE(N->isUniqued());
+
+  // !{i1* @GV} => !{i1* @GV2}
+  std::unique_ptr<GlobalVariable> GV2(
+      new GlobalVariable(Ty, false, GlobalValue::ExternalLinkage));
+  GV->replaceAllUsesWith(GV2.get());
+  ASSERT_TRUE(N->isUniqued());
+  Metadata *NullOps[] = {ConstantAsMetadata::get(GV2.get())};
   ASSERT_EQ(N, MDTuple::get(Context, NullOps));
 }