IR: Update references to temporaries before deleting

During `MDNode::deleteTemporary()`, call `replaceAllUsesWith(nullptr)`
to update all tracking references to `nullptr`.

This fixes PR22280, where inverted destruction order between tracking
references and the temporaries themselves caused a use-after-free in
`LLParser`.

An alternative fix would be to add an assertion that there are no users,
and continue to fix inverted destruction order in clients (like
`LLParser`), but instead I decided to make getting-teardown-right easy.
(If someone disagrees let me know.)

llvm-svn: 226866

llvm/include/llvm/IR/Metadata.h

@@ -727,7 +727,8 @@ public:
 
   /// \brief Deallocate a node created by getTemporary.
   ///
-  /// The node must not have any users.
+  /// Calls \c replaceAllUsesWith(nullptr) before deleting, so any remaining
+  /// references will be reset.
   static void deleteTemporary(MDNode *N);
 
   LLVMContext &getContext() const { return Context.getContext(); }

llvm/lib/IR/Metadata.cpp

@@ -788,6 +788,7 @@ GenericDwarfNode *GenericDwarfNode::getImpl(LLVMContext &Context, unsigned Tag,
 
 void MDNode::deleteTemporary(MDNode *N) {
   assert(N->isTemporary() && "Expected temporary node");
+  N->replaceAllUsesWith(nullptr);
   N->deleteAsSubclass();
 }
 

llvm/test/Assembler/invalid-mdnode-badref.ll

@@ -0,0 +1,5 @@
+; RUN: not llvm-as < %s -disable-output 2>&1 | FileCheck %s
+!named = !{!0}
+
+; CHECK: [[@LINE+1]]:14: error: use of undefined metadata '!1'
+!0 = !{!0, !1}

llvm/unittests/IR/MetadataTest.cpp

@@ -517,6 +517,17 @@ TEST_F(MDNodeTest, replaceWithDistinct) {
   }
 }
 
+TEST_F(MDNodeTest, deleteTemporaryWithTrackingRef) {
+  TrackingMDRef Ref;
+  EXPECT_EQ(nullptr, Ref.get());
+  {
+    auto Temp = MDTuple::getTemporary(Context, None);
+    Ref.reset(Temp.get());
+    EXPECT_EQ(Temp.get(), Ref.get());
+  }
+  EXPECT_EQ(nullptr, Ref.get());
+}
+
 typedef MetadataTest MDLocationTest;
 
 TEST_F(MDLocationTest, Overflow) {