[LazyCallGraph] Handle spurious ref edges when deleting a dead function

Spurious ref edges are ref edges that still exist in the call graph even
though the corresponding IR reference no longer exists. This can cause
issues when deleting a dead function which has a spurious ref edge
pointed at it because currently we expect the dead function's RefSCC to
be trivial.

In the case that the dead function's RefSCC is not trivial, remove all
ref edges from other nodes in the RefSCC to it.

Removing a ref edge can result in splitting RefSCCs. There's actually no
reason to revisit those RefSCCs because currently we only run passes on
SCCs, and we've already added all SCCs in the RefSCC to the worklist.
(as opposed to removing the ref edge in
updateCGAndAnalysisManagerForPass() which can modify the call graph of
SCCs we have not visited yet). We also don't expect that RefSCC
refinement will allow us to glean any more information for optimization
use. Also, doing so would drastically increase the complexity of
LazyCallGraph::removeDeadFunction(), requiring us to return a list of
invalidated RefSCCs and new RefSCCs to add to the worklist.

Fixes #56503

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D133907

1 files changed, 28 insertions, 8 deletions

diff --git a/llvm/lib/Analysis/LazyCallGraph.cpp b/llvm/lib/Analysis/LazyCallGraph.cpp
index 51459ee..a28e8f8 100644
--- a/llvm/lib/Analysis/LazyCallGraph.cpp
+++ b/llvm/lib/Analysis/LazyCallGraph.cpp
@@ -1508,10 +1508,6 @@ void LazyCallGraph::removeDeadFunction(Function &F) {
     return;
 
   Node &N = *NI->second;
-  NodeMap.erase(NI);
-
-  // Remove this from the entry edges if present.
-  EntryEdges.removeEdgeInternal(N);
 
   // Cannot remove a function which has yet to be visited in the DFS walk, so
   // if we have a node at all then we must have an SCC and RefSCC.
@@ -1519,14 +1515,38 @@ void LazyCallGraph::removeDeadFunction(Function &F) {
   assert(CI != SCCMap.end() &&
          "Tried to remove a node without an SCC after DFS walk started!");
   SCC &C = *CI->second;
+  RefSCC *RC = &C.getOuterRefSCC();
+
+  // In extremely rare cases, we can delete a dead function which is still in a
+  // non-trivial RefSCC. This can happen due to spurious ref edges sticking
+  // around after an IR function reference is removed.
+  if (RC->size() != 1) {
+    SmallVector<Node *, 0> NodesInRC;
+    for (SCC &OtherC : *RC) {
+      for (Node &OtherN : OtherC)
+        NodesInRC.push_back(&OtherN);
+    }
+    for (Node *OtherN : NodesInRC) {
+      if ((*OtherN)->lookup(N)) {
+        auto NewRefSCCs =
+            RC->removeInternalRefEdge(*OtherN, ArrayRef<Node *>(&N));
+        // If we've split into multiple RefSCCs, RC is now invalid and the
+        // RefSCC containing C will be different.
+        if (!NewRefSCCs.empty())
+          RC = &C.getOuterRefSCC();
+      }
+    }
+  }
+
+  NodeMap.erase(NI);
+  EntryEdges.removeEdgeInternal(N);
   SCCMap.erase(CI);
-  RefSCC &RC = C.getOuterRefSCC();
 
   // This node must be the only member of its SCC as it has no callers, and
   // that SCC must be the only member of a RefSCC as it has no references.
   // Validate these properties first.
   assert(C.size() == 1 && "Dead functions must be in a singular SCC");
-  assert(RC.size() == 1 && "Dead functions must be in a singular RefSCC");
+  assert(RC->size() == 1 && "Dead functions must be in a singular RefSCC");
 
   // Finally clear out all the data structures from the node down through the
   // components. postorder_ref_scc_iterator will skip empty RefSCCs, so no need
@@ -1535,8 +1555,8 @@ void LazyCallGraph::removeDeadFunction(Function &F) {
   N.G = nullptr;
   N.F = nullptr;
   C.clear();
-  RC.clear();
-  RC.G = nullptr;
+  RC->clear();
+  RC->G = nullptr;
 
   // Nothing to delete as all the objects are allocated in stable bump pointer
   // allocators.