[llvm] [CodeGen][SDAG] Remove Combiner WorklistMap (PR #92900)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: None (aengelke)

<details>
<summary>Changes</summary>

DenseMap for pointer lookup is expensive, and this is only used for deduplication and index lookup. Instead, store the worklist index in the node itself.

This brings a substantial performance improvement, see [here (compile-time-tracker)](http://llvm-compile-time-tracker.com/compare.php?from=9d70975c7a72f3fa58d2d63090b92886dbf8a32b&to=13eff8c73d11595da9c01a0bb6580fea698aa3c9&stat=instructions:u).

---
Full diff: https://github.com/llvm/llvm-project/pull/92900.diff


2 Files Affected:

- (modified) llvm/include/llvm/CodeGen/SelectionDAGNodes.h (+9) 
- (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+12-16) 


``````````diff
diff --git a/llvm/include/llvm/CodeGen/SelectionDAGNodes.h b/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
index ac94c6099d080..7adda059647cb 100644
--- a/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
+++ b/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
@@ -619,6 +619,9 @@ END_TWO_BYTE_PACK()
   /// Unique id per SDNode in the DAG.
   int NodeId = -1;
 
+  /// Index in worklist of DAGCombiner, or -1.
+  int CombinerWorklistIndex = -1;
+
   /// The values that are used by this operation.
   SDUse *OperandList = nullptr;
 
@@ -747,6 +750,12 @@ END_TWO_BYTE_PACK()
   /// Set unique node id.
   void setNodeId(int Id) { NodeId = Id; }
 
+  /// Get worklist index for DAGCombiner
+  int getCombinerWorklistIndex() const { return CombinerWorklistIndex; }
+
+  /// Set worklist index for DAGCombiner
+  void setCombinerWorklistIndex(int Index) { CombinerWorklistIndex = Index; }
+
   /// Return the node ordering.
   unsigned getIROrder() const { return IROrder; }
 
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 8607b50175359..90c940e991835 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -170,16 +170,11 @@ namespace {
     /// back and when processing we pop off of the back.
     ///
     /// The worklist will not contain duplicates but may contain null entries
-    /// due to nodes being deleted from the underlying DAG.
+    /// due to nodes being deleted from the underlying DAG. For fast lookup and
+    /// deduplication, the index of the node in this vector is stored in the
+    /// node in SDNode::CombinerWorklistIndex.
     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 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.
@@ -238,10 +233,9 @@ namespace {
       }
 
       if (N) {
-        bool GoodWorklistEntry = WorklistMap.erase(N);
-        (void)GoodWorklistEntry;
-        assert(GoodWorklistEntry &&
+        assert(N->getCombinerWorklistIndex() >= 0 &&
                "Found a worklist entry without a corresponding map entry!");
+        N->setCombinerWorklistIndex(-1);
       }
       return N;
     }
@@ -285,8 +279,10 @@ namespace {
       if (IsCandidateForPruning)
         ConsiderForPruning(N);
 
-      if (WorklistMap.insert(std::make_pair(N, Worklist.size())).second)
+      if (N->getCombinerWorklistIndex() == -1) {
+        N->setCombinerWorklistIndex(Worklist.size());
         Worklist.push_back(N);
+      }
     }
 
     /// Remove all instances of N from the worklist.
@@ -295,13 +291,13 @@ namespace {
       PruningList.remove(N);
       StoreRootCountMap.erase(N);
 
-      auto It = WorklistMap.find(N);
-      if (It == WorklistMap.end())
+      int WorklistIndex = N->getCombinerWorklistIndex();
+      if (WorklistIndex == -1)
         return; // Not in the worklist.
 
       // Null out the entry rather than erasing it to avoid a linear operation.
-      Worklist[It->second] = nullptr;
-      WorklistMap.erase(It);
+      Worklist[WorklistIndex] = nullptr;
+      N->setCombinerWorklistIndex(-1);
     }
 
     void deleteAndRecombine(SDNode *N);

``````````

</details>


https://github.com/llvm/llvm-project/pull/92900