[llvm] [IDF] Use BitVectors indexed by DFS number for visited sets (NFC). (PR #227832)

[Florian Hahn via llvm-commits]

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@@ -144,18 +145,20 @@ void IDFCalculatorBase<NodeTy, IsPostDom>::calculate(
 
   DT.updateDFSNumbers();
 
+  // The DFS in-numbers are unique and dense in [0, number of nodes), with the
+  // root's DFS out-number being the number of nodes. Use them to index the
+  // visited sets.
+  const DomTreeNodeBase<NodeTy> *RootNode = DT.getRootNode();
+  unsigned NumNodes = RootNode ? RootNode->getDFSNumOut() : 0;
+
   SmallVector<DomTreeNodeBase<NodeTy> *, 32> Worklist;
-  SmallPtrSet<DomTreeNodeBase<NodeTy> *, 16> VisitedPQ;
-  SmallPtrSet<DomTreeNodeBase<NodeTy> *, 16> VisitedWorklist;
-  if (useLiveIn) {
-    VisitedPQ.reserve(LiveInBlocks->size());
-    VisitedWorklist.reserve(LiveInBlocks->size());
-  }
+  BitVector VisitedPQ(NumNodes);
+  BitVector VisitedWorklist(NumNodes);
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fhahn wrote:

Updated to use `SmallVector<bool, 32>`, although it does not really move things much https://llvm-compile-time-tracker.com/compare.php?from=d9cf0304e74379f3b1da8405822dcc6726f39ebb&to=becc578a7c17f5b06ff4333ad1f334ffe2e85751&stat=instructions:u

https://github.com/llvm/llvm-project/pull/227832