[CallGraphUpdater] Properly remove strongly connected components (oldPM)

Summary:
The old code did eliminate references from and to functions that were
about to be deleted only just before we deleted them. This can cause
references from other functions that are supposed to be deleted to still
exist, depending on the order. If the functions form a strongly
connected component the problem manifests regardless of the order in
which we try to actually delete the functions.

This patch introduces a two step deletion. First we remove all
references and then we delete the function. Note that this only affects
the old call graph. There should not be any functional changes if no old
style call graph was given.

To test this we delete two strongly connected functions instead of one
in an existing test.

Reviewers: hfinkel

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77975

1 files changed, 45 insertions, 38 deletions

diff --git a/llvm/lib/Transforms/Utils/CallGraphUpdater.cpp b/llvm/lib/Transforms/Utils/CallGraphUpdater.cpp
index 4de7124..00c6007 100644
--- a/llvm/lib/Transforms/Utils/CallGraphUpdater.cpp
+++ b/llvm/lib/Transforms/Utils/CallGraphUpdater.cpp
@@ -26,49 +26,56 @@ bool CallGraphUpdater::finalize() {
                          DeadFunctionsInComdats.end());
   }
 
-  for (Function *DeadFn : DeadFunctions) {
-    DeadFn->removeDeadConstantUsers();
-
-    if (CG) {
-      CallGraphNode *OldCGN = CG->getOrInsertFunction(DeadFn);
-      CG->getExternalCallingNode()->removeAnyCallEdgeTo(OldCGN);
-      OldCGN->removeAllCalledFunctions();
+  if (CG) {
+    // First remove all references, e.g., outgoing via called functions. This is
+    // necessary as we can delete functions that have circular references.
+    for (Function *DeadFn : DeadFunctions) {
+      DeadFn->removeDeadConstantUsers();
+      CallGraphNode *DeadCGN = (*CG)[DeadFn];
+      DeadCGN->removeAllCalledFunctions();
+      CG->getExternalCallingNode()->removeAnyCallEdgeTo(DeadCGN);
       DeadFn->replaceAllUsesWith(UndefValue::get(DeadFn->getType()));
-
-      assert(OldCGN->getNumReferences() == 0);
-
-      delete CG->removeFunctionFromModule(OldCGN);
-      continue;
     }
 
-    // The old style call graph (CG) has a value handle we do not want to
-    // replace with undef so we do this here.
-    DeadFn->replaceAllUsesWith(UndefValue::get(DeadFn->getType()));
-
-    if (LCG && !ReplacedFunctions.count(DeadFn)) {
-      // Taken mostly from the inliner:
-      LazyCallGraph::Node &N = LCG->get(*DeadFn);
-      auto *DeadSCC = LCG->lookupSCC(N);
-      assert(DeadSCC && DeadSCC->size() == 1 &&
-             &DeadSCC->begin()->getFunction() == DeadFn);
-      auto &DeadRC = DeadSCC->getOuterRefSCC();
-
-      FunctionAnalysisManager &FAM =
-          AM->getResult<FunctionAnalysisManagerCGSCCProxy>(*DeadSCC, *LCG)
-              .getManager();
-
-      FAM.clear(*DeadFn, DeadFn->getName());
-      AM->clear(*DeadSCC, DeadSCC->getName());
-      LCG->removeDeadFunction(*DeadFn);
-
-      // Mark the relevant parts of the call graph as invalid so we don't visit
-      // them.
-      UR->InvalidatedSCCs.insert(DeadSCC);
-      UR->InvalidatedRefSCCs.insert(&DeadRC);
+    // Then remove the node and function from the module.
+    for (Function *DeadFn : DeadFunctions) {
+      CallGraphNode *DeadCGN = CG->getOrInsertFunction(DeadFn);
+      assert(DeadCGN->getNumReferences() == 0 &&
+             "References should have been handled by now");
+      delete CG->removeFunctionFromModule(DeadCGN);
     }
+  } else {
+    // This is the code path for the new lazy call graph and for the case were
+    // no call graph was provided.
+    for (Function *DeadFn : DeadFunctions) {
+      DeadFn->removeDeadConstantUsers();
+      DeadFn->replaceAllUsesWith(UndefValue::get(DeadFn->getType()));
 
-    // The function is now really dead and de-attached from everything.
-    DeadFn->eraseFromParent();
+      if (LCG && !ReplacedFunctions.count(DeadFn)) {
+        // Taken mostly from the inliner:
+        LazyCallGraph::Node &N = LCG->get(*DeadFn);
+        auto *DeadSCC = LCG->lookupSCC(N);
+        assert(DeadSCC && DeadSCC->size() == 1 &&
+               &DeadSCC->begin()->getFunction() == DeadFn);
+        auto &DeadRC = DeadSCC->getOuterRefSCC();
+
+        FunctionAnalysisManager &FAM =
+            AM->getResult<FunctionAnalysisManagerCGSCCProxy>(*DeadSCC, *LCG)
+                .getManager();
+
+        FAM.clear(*DeadFn, DeadFn->getName());
+        AM->clear(*DeadSCC, DeadSCC->getName());
+        LCG->removeDeadFunction(*DeadFn);
+
+        // Mark the relevant parts of the call graph as invalid so we don't
+        // visit them.
+        UR->InvalidatedSCCs.insert(DeadSCC);
+        UR->InvalidatedRefSCCs.insert(&DeadRC);
+      }
+
+      // The function is now really dead and de-attached from everything.
+      DeadFn->eraseFromParent();
+    }
   }
 
   bool Changed = !DeadFunctions.empty();