[llvm] [DominanceFrontier] Support multiple root nodes for post-dom (PR #181257)

[Andrei Elovikov via llvm-commits]

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@@ -74,14 +74,16 @@ void DominanceFrontierBase<BlockT, IsPostDom>::dump() const {
 #endif
 
 template <class BlockT, bool IsPostDom>
-void DominanceFrontierBase<BlockT, IsPostDom>::calculate(
-    const DomTreeT &DT, const DomTreeNodeT *Node) {
-  BlockT *BB = Node->getBlock();
+void DominanceFrontierBase<BlockT, IsPostDom>::calculate(const DomTreeT &DT) {
+  // NOTE: RootNode might be virtual for `IsPostDom == true`.
+  const DomTreeNodeT *RootNode = DT.getRootNode();
+  BlockT *BB = RootNode->getBlock();
 
   std::vector<DFCalculateWorkObject<BlockT>> workList;
   SmallPtrSet<BlockT *, 32> visited;
 
-  workList.push_back(DFCalculateWorkObject<BlockT>(BB, nullptr, Node, nullptr));
+  workList.push_back(
+      DFCalculateWorkObject<BlockT>(BB, nullptr, RootNode, nullptr));
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eas wrote:

I prefer having the virtual root, because that means the algorithm itself remains the same. If we start initializing the worklist differently then we kinda need to prove the math is still correct (however simple that may be).

https://github.com/llvm/llvm-project/pull/181257