[llvm] [DomTree] Fix root attachment in runDFS() (PR #73148)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-support

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

Currently, runDFS() only sets the Parent of the DFS root if it is already in the NodeToInfo map. This works out okay 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.

---
Full diff: https://github.com/llvm/llvm-project/pull/73148.diff


1 Files Affected:

- (modified) llvm/include/llvm/Support/GenericDomTreeConstruction.h (+3-2) 


``````````diff
diff --git a/llvm/include/llvm/Support/GenericDomTreeConstruction.h b/llvm/include/llvm/Support/GenericDomTreeConstruction.h
index 6564f98ab0234b0..0a70648c9eab36e 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();
@@ -306,6 +306,7 @@ struct SemiNCAInfo {
     for (unsigned i = 2; i < NextDFSNum; ++i) {
       const NodePtr W = NumToNode[i];
       auto &WInfo = NodeToInfo[W];
+      assert(WInfo.Semi != 0);
       const unsigned SDomNum = NodeToInfo[NumToNode[WInfo.Semi]].DFSNum;
       NodePtr WIDomCandidate = WInfo.IDom;
       while (NodeToInfo[WIDomCandidate].DFSNum > SDomNum)
@@ -552,7 +553,7 @@ 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) {

``````````

</details>


https://github.com/llvm/llvm-project/pull/73148