[PATCH] D120670: [SCCIterator] Fix a non-determinism in scc_member_iterator

[Hongtao Yu via Phabricator via llvm-commits]

hoy created this revision.
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Prevoiusly we initialzed a queue with MST roots based on NodeInfoMap which is an unordered map. This could cause a non-determinism. I'm fixing this by initializing the queue based on SortedEdges.

This should be a no-diff change in terms of code quality. However this helps debugging.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D120670

Files:
  llvm/include/llvm/ADT/SCCIterator.h


Index: llvm/include/llvm/ADT/SCCIterator.h
===================================================================
--- llvm/include/llvm/ADT/SCCIterator.h
+++ llvm/include/llvm/ADT/SCCIterator.h
@@ -348,9 +348,14 @@
     NodeInfoMap[Edge->Target].Visited = false;
 
   std::queue<NodeType *> Queue;
-  for (auto &Node : NodeInfoMap)
-    if (Node.second.Visited)
-      Queue.push(Node.first);
+  // Initialze the queue with MST roots. Note that walking through SortedEdges
+  // instead of NodeInfoMap ensures an ordered deterministic push.
+  for (auto *Edge : SortedEdges) {
+    if (NodeInfoMap[Edge->Source].Visited) {
+      Queue.push(Edge->Source);
+      NodeInfoMap[Edge->Source].Visited = false;
+    }
+  }
 
   while (!Queue.empty()) {
     auto *Node = Queue.front();


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