[NFC][DAGCombiner]: Only consider nodes with no uses for pruning when forming initial worklist

[0xdc03]

Summary

When the worklist is initially being formed, there is no need to
consider all nodes for pruning. This is because the first time calling
getNextWorklistEntry will only clear those nodes which have no uses,
with their operands being added to the worklist. However, when the worklist is
created for the first time all nodes are added anyways, so this operation
actually ends up adding no nodes.

This patch adds a parameter IsCandidateForPruning to AddToWorklist with a
default value of true to avoid having to update every call site.

[0xdc03]

• Removed unnecessary braces

[nikic]

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

181

I'd move this comment next to the initial population of the worklist.

1781

[0xdc03]

• Address reviewer comment

[0xdc03]

• Address missed reviewer comment

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

181

Done!

1781

Done!

[nikic]

LGTM

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

class DAGCombiner {
     SmallVector<SDNode *, 64> Worklist;
 
     /// Mapping from an SDNode to its position on the worklist.
     ///
     /// This is used to find and remove nodes from the worklist (by nulling
     /// them) when they are deleted from the underlying DAG. It relies on
     /// stable indices of nodes within the worklist.
     DenseMap<SDNode *, unsigned> WorklistMap;
-    /// This records all nodes attempted to add to the worklist since we
+
+    /// This records all nodes attempted to be added to the worklist since we
     /// considered a new worklist entry. As we keep do not add duplicate nodes
     /// in the worklist, this is different from the tail of the worklist.
     SmallSetVector<SDNode *, 32> PruningList;
 
     /// Set of nodes which have been combined (at least once).
     ///
     /// This is used to allow us to reliably add any operands of a DAG node
     /// which have not yet been combined to the worklist.
     SmallPtrSet<SDNode *, 32> CombinedNodes;
 
     /// Map from candidate StoreNode to the pair of RootNode and count.
     /// The count is used to track how many times we have seen the StoreNode
     /// with the same RootNode bail out in dependence check. If we have seen
     /// the bail out for the same pair many times over a limit, we won't
     /// consider the StoreNode with the same RootNode as store merging
public:
 
     void ConsiderForPruning(SDNode *N) {
       // Mark this for potential pruning.
       PruningList.insert(N);
     }
 
     /// Add to the worklist making sure its instance is at the back (next to be
     /// processed.)
-    void AddToWorklist(SDNode *N) {
+    void AddToWorklist(SDNode *N, bool IsCandidateForPruning = true) {
       assert(N->getOpcode() != ISD::DELETED_NODE &&
              "Deleted Node added to Worklist");
 
       // Skip handle nodes as they can't usefully be combined and confuse the
       // zero-use deletion strategy.
       if (N->getOpcode() == ISD::HANDLENODE)
         return;
 
+      if (IsCandidateForPruning)
         ConsiderForPruning(N);
 
       if (WorklistMap.insert(std::make_pair(N, Worklist.size())).second)
         Worklist.push_back(N);
     }
 
     /// Remove all instances of N from the worklist.
     void removeFromWorklist(SDNode *N) {
       CombinedNodes.erase(N);
void DAGCombiner::Run(CombineLevel AtLevel) {
   Level = AtLevel;
   LegalDAG = Level >= AfterLegalizeDAG;
   LegalOperations = Level >= AfterLegalizeVectorOps;
   LegalTypes = Level >= AfterLegalizeTypes;
 
   WorklistInserter AddNodes(*this);
 
   // Add all the dag nodes to the worklist.
+  //
+  // Note: All nodes are not added to PruningList here, this is because the only
+  // nodes which can be deleted are those which have no uses and all other nodes
+  // which would otherwise be added to the worklist by the first call to
+  // getNextWorklistEntry are already present in it.
   for (SDNode &Node : DAG.allnodes())
-    AddToWorklist(&Node);
+    AddToWorklist(&Node, /* IsCandidateForPruning */ Node.use_empty());
 
   // Create a dummy node (which is not added to allnodes), that adds a reference
   // to the root node, preventing it from being deleted, and tracking any
   // changes of the root.
   HandleSDNode Dummy(DAG.getRoot());
 
   // While we have a valid worklist entry node, try to combine it.
   while (SDNode *N = getNextWorklistEntry()) {