[DAG] Set up infrastructure to avoid smart constructor-based dangling nodes

Summary:
Various SelectionDAG non-combine operations (e.g. the getNode smart
constructor and legalization) may leave dangling nodes by applying
optimizations without fully pruning unused result values. This results
in nodes that are never added to the worklist and therefore can not be
pruned.

Add a node inserter for the combiner to make sure such nodes have the
chance of being pruned. This allows a number of additional peephole
optimizations.

Reviewers: efriedma, RKSimon, craig.topper, jyknight

Reviewed By: jyknight

Subscribers: msearles, jyknight, sdardis, nemanjai, javed.absar, hiraditya, jrtc27, atanasyan, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58068

llvm-svn: 357279

llvm/include/llvm/CodeGen/SelectionDAG.h

@@ -297,6 +297,9 @@ public:
 
     /// The node N that was updated.
     virtual void NodeUpdated(SDNode *N);
+
+    /// The node N that was inserted.
+    virtual void NodeInserted(SDNode *N);
   };
 
   struct DAGNodeDeletedListener : public DAGUpdateListener {

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -647,6 +647,17 @@ public:
   }
 };
 
+class WorklistInserter : public SelectionDAG::DAGUpdateListener {
+  DAGCombiner &DC;
+
+public:
+  explicit WorklistInserter(DAGCombiner &dc)
+      : SelectionDAG::DAGUpdateListener(dc.getDAG()), DC(dc) {}
+
+  // This should eventually be pruning.
+  void NodeInserted(SDNode *N) override { }
+};
+
 } // end anonymous namespace
 
 //===----------------------------------------------------------------------===//
@@ -1399,6 +1410,8 @@ void DAGCombiner::Run(CombineLevel AtLevel) {
   LegalOperations = Level >= AfterLegalizeVectorOps;
   LegalTypes = Level >= AfterLegalizeTypes;
 
+  WorklistInserter AddNodes(*this);
+
   // Add all the dag nodes to the worklist.
   for (SDNode &Node : DAG.allnodes())
     AddToWorklist(&Node);

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -85,6 +85,7 @@ static SDVTList makeVTList(const EVT *VTs, unsigned NumVTs) {
 // Default null implementations of the callbacks.
 void SelectionDAG::DAGUpdateListener::NodeDeleted(SDNode*, SDNode*) {}
 void SelectionDAG::DAGUpdateListener::NodeUpdated(SDNode*) {}
+void SelectionDAG::DAGUpdateListener::NodeInserted(SDNode *) {}
 
 void SelectionDAG::DAGNodeDeletedListener::anchor() {}
 
@@ -840,6 +841,8 @@ void SelectionDAG::InsertNode(SDNode *N) {
   N->PersistentId = NextPersistentId++;
   VerifySDNode(N);
 #endif
+  for (DAGUpdateListener *DUL = UpdateListeners; DUL; DUL = DUL->Next)
+    DUL->NodeInserted(N);
 }
 
 /// RemoveNodeFromCSEMaps - Take the specified node out of the CSE map that