[llvm] 0cdacd5 - [DomTree] Fix root attachment in runDFS() (#73148)

[via llvm-commits]

Author: Nikita Popov
Date: 2023-12-04T09:24:13+01:00
New Revision: 0cdacd5f492991362bfc8e252673aafdb9651322

URL: https://github.com/llvm/llvm-project/commit/0cdacd5f492991362bfc8e252673aafdb9651322
DIFF: https://github.com/llvm/llvm-project/commit/0cdacd5f492991362bfc8e252673aafdb9651322.diff

LOG: [DomTree] Fix root attachment in runDFS() (#73148)

Currently, runDFS() only sets the Parent of the DFS root if it is
already in the NodeToInfo map. This doesn't matter if we're running DFS
on the DT root, which doesn't have a parent anyway. However, when
running on PDT roots, this means we end up keeping the parent at 0,
rather than setting it to 1 for the virtual PDT root. Because the
virtual root (nullptr) has the same value as the dummy value in
NumToNode (nullptr) this happens to work out by accident right now.

I believe we should always be setting the parent in runDFS(), and adjust
AttachToNum in doFullDFSWalk() to be 1 (the virtual root) for PDTs.

Added: 
    

Modified: 
    llvm/include/llvm/Support/GenericDomTreeConstruction.h

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/Support/GenericDomTreeConstruction.h b/llvm/include/llvm/Support/GenericDomTreeConstruction.h
index 48a59491e486d..29b5f9159c68e 100644
--- a/llvm/include/llvm/Support/GenericDomTreeConstruction.h
+++ b/llvm/include/llvm/Support/GenericDomTreeConstruction.h
@@ -181,7 +181,7 @@ struct SemiNCAInfo {
                   const NodeOrderMap *SuccOrder = nullptr) {
     assert(V);
     SmallVector<NodePtr, 64> WorkList = {V};
-    if (NodeToInfo.count(V) != 0) NodeToInfo[V].Parent = AttachToNum;
+    NodeToInfo[V].Parent = AttachToNum;
 
     while (!WorkList.empty()) {
       const NodePtr BB = WorkList.pop_back_val();
@@ -299,7 +299,8 @@ struct SemiNCAInfo {
     // during path compression in Eval.
     for (unsigned i = 2; i < NextDFSNum; ++i) {
       auto &WInfo = *NumToInfo[i];
-      const unsigned SDomNum = NodeToInfo[NumToNode[WInfo.Semi]].DFSNum;
+      assert(WInfo.Semi != 0);
+      const unsigned SDomNum = NumToInfo[WInfo.Semi]->DFSNum;
       NodePtr WIDomCandidate = WInfo.IDom;
       while (true) {
         auto &WIDomCandidateInfo = NodeToInfo.find(WIDomCandidate)->second;
@@ -548,7 +549,7 @@ struct SemiNCAInfo {
 
     addVirtualRoot();
     unsigned Num = 1;
-    for (const NodePtr Root : DT.Roots) Num = runDFS(Root, Num, DC, 0);
+    for (const NodePtr Root : DT.Roots) Num = runDFS(Root, Num, DC, 1);
   }
 
   static void CalculateFromScratch(DomTreeT &DT, BatchUpdatePtr BUI) {