IR: Don't drop MDNode uniquing on null operands

Now that `LLVMContextImpl` can call `MDNode::dropAllReferences()` to
prevent teardown madness, stop dropping uniquing just because an operand
drops to null.

Part of PR21532.

llvm-svn: 225223

2 files changed, 22 insertions, 7 deletions

diff --git a/llvm/lib/IR/Metadata.cpp b/llvm/lib/IR/Metadata.cpp
index bed2b6bd422..1a651573733 100644
--- a/llvm/lib/IR/Metadata.cpp
+++ b/llvm/lib/IR/Metadata.cpp
@@ -515,13 +515,8 @@ void GenericMDNode::handleChangedOperand(void *Ref, Metadata *New) {
   Metadata *Old = getOperand(Op);
   setOperand(Op, New);
 
-  // Drop uniquing for self-reference cycles or if an operand drops to null.
-  //
-  // FIXME: Stop dropping uniquing when an operand drops to null.  The original
-  // motivation was to prevent madness during teardown of LLVMContextImpl, but
-  // dropAllReferences() fixes that problem in a better way.  (It's just here
-  // now for better staging of semantic changes.)
-  if (New == this || !New) {
+  // Drop uniquing for self-reference cycles.
+  if (New == this) {
     storeDistinctInContext();
     setHash(0);
     if (!isResolved())
diff --git a/llvm/unittests/IR/MetadataTest.cpp b/llvm/unittests/IR/MetadataTest.cpp
index ccdda1c356a..ef071b38a7b 100644
--- a/llvm/unittests/IR/MetadataTest.cpp
+++ b/llvm/unittests/IR/MetadataTest.cpp
@@ -202,6 +202,26 @@ TEST_F(MDNodeTest, Print) {
   EXPECT_EQ(Expected, Actual);
 }
 
+TEST_F(MDNodeTest, NullOperand) {
+  // metadata !{}
+  MDNode *Empty = MDNode::get(Context, None);
+
+  // metadata !{metadata !{}}
+  Metadata *Ops[] = {Empty};
+  MDNode *N = MDNode::get(Context, Ops);
+  ASSERT_EQ(Empty, N->getOperand(0));
+
+  // metadata !{metadata !{}} => metadata !{null}
+  N->replaceOperandWith(0, nullptr);
+  ASSERT_EQ(nullptr, N->getOperand(0));
+
+  // metadata !{null}
+  Ops[0] = nullptr;
+  MDNode *NullOp = MDNode::get(Context, Ops);
+  ASSERT_EQ(nullptr, NullOp->getOperand(0));
+  EXPECT_EQ(N, NullOp);
+}
+
 typedef MetadataTest MetadataAsValueTest;
 
 TEST_F(MetadataAsValueTest, MDNode) {