[PATCH] D130717: [SCCIterator] Fix an issue in scc_member_iterator sorting
Hongtao Yu via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Thu Jul 28 10:49:30 PDT 2022
hoy created this revision.
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Members in an scc are supposed to be sorted in a top-down or topological order based on edge weights. Previously this is achived by building a MST out of the SCC and enforcing an BFS walk on the MST. A BFS on a tree does give a top-down topological order, however, the MST built here isn't really a tree. This is becuase of a trick done to avoid expansive detection of a cycle on a directed graph when an edge is added. When the MST is built, its edges are considered undirected. But in reality they are directed, thus a BST walk doesn't necessarily give a topological order. I'm tweaking the BFS walk slightly to yield a topological order.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D130717
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
@@ -23,6 +23,7 @@
#define LLVM_ADT_SCCITERATOR_H
#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/GraphTraits.h"
#include "llvm/ADT/iterator.h"
#include <cassert>
@@ -258,7 +259,7 @@
struct NodeInfo {
NodeInfo *Group = this;
uint32_t Rank = 0;
- bool Visited = true;
+ DenseSet<const EdgeType *> IncomingMSTEdges;
};
// Find the root group of the node and compress the path from node to the
@@ -340,21 +341,20 @@
MSTEdges.insert(Edge);
}
- // Do BFS on MST, starting from nodes that have no incoming edge. These nodes
- // are "roots" of the MST forest. This ensures that nodes are visited before
- // their decsendents are, thus ensures hot edges are processed before cold
- // edges, based on how MST is computed.
+ // Run Kahn's algorithm on MST to compute a topological traversal order.
+ // Starting from nodes that have no incoming edge. These nodes are "roots" of
+ // the MST forest. This ensures that nodes are visited before their
+ // decsendents are, thus ensures hot edges are processed before cold edges,
+ // based on how MST is computed.
for (const auto *Edge : MSTEdges)
- NodeInfoMap[Edge->Target].Visited = false;
+ NodeInfoMap[Edge->Target].IncomingMSTEdges.insert(Edge);
std::queue<NodeType *> Queue;
// 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) {
+ if (NodeInfoMap[Edge->Source].IncomingMSTEdges.empty())
Queue.push(Edge->Source);
- NodeInfoMap[Edge->Source].Visited = false;
- }
}
while (!Queue.empty()) {
@@ -362,10 +362,10 @@
Queue.pop();
Nodes.push_back(Node);
for (auto &Edge : Node->Edges) {
- if (MSTEdges.count(&Edge) && !NodeInfoMap[Edge.Target].Visited) {
- NodeInfoMap[Edge.Target].Visited = true;
+ NodeInfoMap[Edge.Target].IncomingMSTEdges.erase(&Edge);
+ if (MSTEdges.count(&Edge) &&
+ NodeInfoMap[Edge.Target].IncomingMSTEdges.empty())
Queue.push(Edge.Target);
- }
}
}
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