IR: Drop metadata references more aggressively during teardown

Sometimes teardown happens before the debug info graph is complete
(e.g., when clang throws an error).  In that case, `MDNode`s will still
have RAUW, so deleting constants that the `MDNode`s point at will be
relatively expensive -- it'll cause re-uniquing all up the chain (what
I've been referring to as "teardown madness").

So, drop references *before* deleting constants.  We need to drop a few
more references now: the metadata side of the metadata/value bridges
needs to be dropped off the cliff along with the rest of it (previously,
the bridges were cleaned before we did anything with the `MDNode`s).

There's no real functionality change here -- state before and after
`LLVMContextImpl::~LLVMContextImpl()` is unchanged -- so no testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226044 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Metadata.h

@@ -115,6 +115,9 @@ class MetadataAsValue : public Value {
   MetadataAsValue(Type *Ty, Metadata *MD);
   ~MetadataAsValue();
 
+  /// \brief Drop use of metadata (during teardown).
+  void dropUse() { MD = nullptr; }
+
 public:
   static MetadataAsValue *get(LLVMContext &Context, Metadata *MD);
   static MetadataAsValue *getIfExists(LLVMContext &Context, Metadata *MD);
@@ -185,6 +188,11 @@ class ValueAsMetadata : public Metadata, ReplaceableMetadataImpl {
 
   Value *V;
 
+  /// \brief Drop users without RAUW (during teardown).
+  void dropUsers() {
+    ReplaceableMetadataImpl::resolveAllUses(/* ResolveUsers */ false);
+  }
+
 protected:
   ValueAsMetadata(unsigned ID, Value *V)
       : Metadata(ID), V(V) {

lib/IR/LLVMContextImpl.cpp

@@ -72,7 +72,30 @@ LLVMContextImpl::~LLVMContextImpl() {
   // the container. Avoid iterators during this operation:
   while (!OwnedModules.empty())
     delete *OwnedModules.begin();
-  
+
+  // Drop references for MDNodes.  Do this before Values get deleted to avoid
+  // unnecessary RAUW when nodes are still unresolved.
+  for (auto *I : DistinctMDNodes)
+    I->dropAllReferences();
+  for (auto *I : MDTuples)
+    I->dropAllReferences();
+  for (auto *I : MDLocations)
+    I->dropAllReferences();
+
+  // Also drop references that come from the Value bridges.
+  for (auto &Pair : ValuesAsMetadata)
+    Pair.second->dropUsers();
+  for (auto &Pair : MetadataAsValues)
+    Pair.second->dropUse();
+
+  // Destroy MDNodes.
+  for (UniquableMDNode *I : DistinctMDNodes)
+    I->deleteAsSubclass();
+  for (MDTuple *I : MDTuples)
+    delete I;
+  for (MDLocation *I : MDLocations)
+    delete I;
+
   // Free the constants.  This is important to do here to ensure that they are
   // freed before the LeakDetector is torn down.
   std::for_each(ExprConstants.map_begin(), ExprConstants.map_end(),
@@ -135,21 +158,6 @@ LLVMContextImpl::~LLVMContextImpl() {
   for (auto &Pair : ValuesAsMetadata)
     delete Pair.second;
 
-  // Destroy MDNodes.
-  for (auto *I : DistinctMDNodes)
-    I->dropAllReferences();
-  for (auto *I : MDTuples)
-    I->dropAllReferences();
-  for (auto *I : MDLocations)
-    I->dropAllReferences();
-
-  for (UniquableMDNode *I : DistinctMDNodes)
-    I->deleteAsSubclass();
-  for (MDTuple *I : MDTuples)
-    delete I;
-  for (MDLocation *I : MDLocations)
-    delete I;
-
   // Destroy MDStrings.
   MDStringCache.clear();
 }