[PATCH] D58327: [Dominators] Simplify and optimize path compression used in eval()

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: kuhar, sanjoy, asbirlea.
Herald added subscribers: llvm-commits, jdoerfert, kristina.
Herald added a project: LLVM.

- NodeToInfo[*] have been allocated so the addresses are stable. We can store them instead of NodePtr to save NumToNode lookups.
- Nodes are traversed twice. Using `Visited` to check the traversal number is expensive and obscure. Just split the two traversals explicitly.
- The check `VInInfo.DFSNum < LastLinked` is redundant as it is implied by `VInInfo->Parent < LastLinked`
- VLabelInfo PLabelInfo are used to save a NodeToInfo lookup in the second traversal.


Repository:
  rL LLVM

https://reviews.llvm.org/D58327

Files:
  include/llvm/Support/GenericDomTreeConstruction.h


Index: include/llvm/Support/GenericDomTreeConstruction.h
===================================================================
--- include/llvm/Support/GenericDomTreeConstruction.h
+++ include/llvm/Support/GenericDomTreeConstruction.h
@@ -254,42 +254,31 @@
     return LastNum;
   }
 
-  NodePtr eval(NodePtr VIn, unsigned LastLinked) {
-    auto &VInInfo = NodeToInfo[VIn];
-    if (VInInfo.DFSNum < LastLinked)
-      return VIn;
-
-    SmallVector<NodePtr, 32> Work;
-    SmallPtrSet<NodePtr, 32> Visited;
-
-    if (VInInfo.Parent >= LastLinked)
-      Work.push_back(VIn);
-
-    while (!Work.empty()) {
-      NodePtr V = Work.back();
-      auto &VInfo = NodeToInfo[V];
-      NodePtr VAncestor = NumToNode[VInfo.Parent];
-
-      // Process Ancestor first
-      if (Visited.insert(VAncestor).second && VInfo.Parent >= LastLinked) {
-        Work.push_back(VAncestor);
-        continue;
-      }
-      Work.pop_back();
-
-      // Update VInfo based on Ancestor info
-      if (VInfo.Parent < LastLinked)
-        continue;
-
-      auto &VAInfo = NodeToInfo[VAncestor];
-      NodePtr VAncestorLabel = VAInfo.Label;
-      NodePtr VLabel = VInfo.Label;
-      if (NodeToInfo[VAncestorLabel].Semi < NodeToInfo[VLabel].Semi)
-        VInfo.Label = VAncestorLabel;
-      VInfo.Parent = VAInfo.Parent;
+  NodePtr eval(NodePtr V, unsigned LastLinked) {
+    InfoRec *VInfo = &NodeToInfo[V];
+    if (VInfo->Parent < LastLinked)
+      return VInfo->Label;
+
+    SmallVector<InfoRec *, 32> Stack;
+    while (VInfo->Parent >= LastLinked) {
+      Stack.push_back(VInfo);
+      VInfo = &NodeToInfo[NumToNode[VInfo->Parent]];
     }
 
-    return VInInfo.Label;
+    // Path compression.
+    const InfoRec *PInfo = VInfo;
+    const InfoRec *PLabelInfo = &NodeToInfo[PInfo->Label];
+    do {
+      VInfo = Stack.pop_back_val();
+      VInfo->Parent = PInfo->Parent;
+      const InfoRec *VLabelInfo = &NodeToInfo[VInfo->Label];
+      if (PLabelInfo->Semi < VLabelInfo->Semi)
+        VInfo->Label = PInfo->Label;
+      else
+        PLabelInfo = VLabelInfo;
+      PInfo = VInfo;
+    } while (!Stack.empty());
+    return VInfo->Label;
   }
 
   // This function requires DFS to be run before calling it.


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