[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

diff --git a/llvm/lib/Transforms/Utils/CallGraphUpdater.cpp b/llvm/lib/Transforms/Utils/CallGraphUpdater.cpp
index 4de7124..00c6007 100644 (file)
--- 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();

diff --git a/llvm/unittests/IR/LegacyPassManagerTest.cpp b/llvm/unittests/IR/LegacyPassManagerTest.cpp
index b716d65..2fa9746 100644 (file)
--- a/llvm/unittests/IR/LegacyPassManagerTest.cpp
+++ b/llvm/unittests/IR/LegacyPassManagerTest.cpp
@@ -560,6 +560,21 @@ namespace llvm {
       return mod;
     }
 
+    /// Split a simple function which contains only a call and a return into two
+    /// such that the first calls the second and the second whoever was called
+    /// initially.
+    Function *splitSimpleFunction(Function &F) {
+      LLVMContext &Context = F.getContext();
+      Function *SF = Function::Create(F.getFunctionType(), F.getLinkage(),
+                                      F.getName() + "b", F.getParent());
+      F.setName(F.getName() + "a");
+      BasicBlock *Entry = BasicBlock::Create(Context, "entry", SF, nullptr);
+      CallInst &CI = cast<CallInst>(F.getEntryBlock().front());
+      CI.clone()->insertBefore(ReturnInst::Create(Context, Entry));
+      CI.setCalledFunction(SF);
+      return SF;
+    }
+
     struct CGModifierPass : public CGPass {
       unsigned NumSCCs = 0;
       unsigned NumFns = 0;
@@ -582,7 +597,8 @@ namespace llvm {
         Function *F = N->getFunction();
         Module *M = F->getParent();
         Function *Test1F = M->getFunction("test1");
-        Function *Test2F = M->getFunction("test2");
+        Function *Test2aF = M->getFunction("test2a");
+        Function *Test2bF = M->getFunction("test2b");
         Function *Test3F = M->getFunction("test3");
         auto InSCC = [&](Function *Fn) {
           return llvm::any_of(SCMM, [Fn](CallGraphNode *CGN) {
@@ -590,18 +606,19 @@ namespace llvm {
           });
         };
 
-        if (!Test1F || !Test2F || !Test3F || !InSCC(Test1F) || !InSCC(Test2F) ||
-            !InSCC(Test3F))
+        if (!Test1F || !Test2aF || !Test2bF || !Test3F || !InSCC(Test1F) ||
+            !InSCC(Test2aF) || !InSCC(Test2bF) || !InSCC(Test3F))
           return SetupWorked = false;
 
         CallInst *CI = dyn_cast<CallInst>(&Test1F->getEntryBlock().front());
-        if (!CI || CI->getCalledFunction() != Test2F)
+        if (!CI || CI->getCalledFunction() != Test2aF)
           return SetupWorked = false;
 
         CI->setCalledFunction(Test3F);
 
         CGU.initialize(const_cast<CallGraph &>(SCMM.getCallGraph()), SCMM);
-        CGU.removeFunction(*Test2F);
+        CGU.removeFunction(*Test2aF);
+        CGU.removeFunction(*Test2bF);
         CGU.reanalyzeFunction(*Test1F);
         return true;
       }
@@ -610,20 +627,24 @@ namespace llvm {
     };
 
     TEST(PassManager, CallGraphUpdater0) {
-      // SCC#1: test1->test2->test3->test1
+      // SCC#1: test1->test2a->test2b->test3->test1
       // SCC#2: test4
       // SCC#3: indirect call node
 
       LLVMContext Context;
       std::unique_ptr<Module> M(makeLLVMModule(Context));
       ASSERT_EQ(M->getFunctionList().size(), 4U);
+      Function *F = M->getFunction("test2");
+      Function *SF = splitSimpleFunction(*F);
+      CallInst::Create(F, "", &SF->getEntryBlock());
+      ASSERT_EQ(M->getFunctionList().size(), 5U);
       CGModifierPass *P = new CGModifierPass();
       legacy::PassManager Passes;
       Passes.add(P);
       Passes.run(*M);
       ASSERT_TRUE(P->SetupWorked);
       ASSERT_EQ(P->NumSCCs, 3U);
-      ASSERT_EQ(P->NumFns, 4U);
+      ASSERT_EQ(P->NumFns, 5U);
       ASSERT_EQ(M->getFunctionList().size(), 3U);
     }
   }