text: //===- CallGraphSCCPass.cpp - Pass that operates BU on call graph ---------===//
  //
  // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  // See https://llvm.org/LICENSE.txt for license information.
  // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  //
  //===----------------------------------------------------------------------===//
  //
  // This file implements the CallGraphSCCPass class, which is used for passes
  // which are implemented as bottom-up traversals on the call graph.  Because
  // there may be cycles in the call graph, passes of this type operate on the
  // call-graph in SCC order: that is, they process function bottom-up, except for
  // recursive functions, which they process all at once.
  //
  //===----------------------------------------------------------------------===//
  
  #include "llvm/Analysis/CallGraphSCCPass.h"
  #include "llvm/ADT/DenseMap.h"
  #include "llvm/ADT/SCCIterator.h"
  #include "llvm/ADT/Statistic.h"
  #include "llvm/Analysis/CallGraph.h"
  #include "llvm/IR/AbstractCallSite.h"
  #include "llvm/IR/Function.h"
  #include "llvm/IR/IRPrintingPasses.h"
  #include "llvm/IR/Intrinsics.h"
  #include "llvm/IR/LLVMContext.h"
  #include "llvm/IR/LegacyPassManagers.h"
  #include "llvm/IR/Module.h"
  #include "llvm/IR/OptBisect.h"
  #include "llvm/IR/PassTimingInfo.h"
  #include "llvm/IR/StructuralHash.h"
  #include "llvm/Pass.h"
  #include "llvm/Support/CommandLine.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/Timer.h"
  #include "llvm/Support/raw_ostream.h"
  #include <cassert>
  #include <string>
  #include <utility>
  #include <vector>
  
  using namespace llvm;
  
  #define DEBUG_TYPE "cgscc-passmgr"
  
  static cl::opt<unsigned>
  MaxIterations("max-cg-scc-iterations", cl::ReallyHidden, cl::init(4));
  
  STATISTIC(MaxSCCIterations, "Maximum CGSCCPassMgr iterations on one SCC");
  
  //===----------------------------------------------------------------------===//
  // CGPassManager
  //
  /// CGPassManager manages FPPassManagers and CallGraphSCCPasses.
  
  namespace {
  
  class CGPassManager : public ModulePass, public PMDataManager {
  public:
    static char ID;
  
    explicit CGPassManager() : ModulePass(ID), PMDataManager() {}
  
    /// Execute all of the passes scheduled for execution.  Keep track of
    /// whether any of the passes modifies the module, and if so, return true.
    bool runOnModule(Module &M) override;
  
    using ModulePass::doInitialization;
    using ModulePass::doFinalization;
  
    bool doInitialization(CallGraph &CG);
    bool doFinalization(CallGraph &CG);
  
    /// Pass Manager itself does not invalidate any analysis info.
    void getAnalysisUsage(AnalysisUsage &Info) const override {
      // CGPassManager walks SCC and it needs CallGraph.
      Info.addRequired<CallGraphWrapperPass>();
      Info.setPreservesAll();
    }
  
    StringRef getPassName() const override { return "CallGraph Pass Manager"; }
  
    PMDataManager *getAsPMDataManager() override { return this; }
    Pass *getAsPass() override { return this; }
  
    // Print passes managed by this manager
    void dumpPassStructure(unsigned Offset) override {
      errs().indent(Offset*2) << "Call Graph SCC Pass Manager\n";
      for (unsigned Index = 0; Index < getNumContainedPasses(); ++Index) {
        Pass *P = getContainedPass(Index);
        P->dumpPassStructure(Offset + 1);
        dumpLastUses(P, Offset+1);
      }
    }
  
    Pass *getContainedPass(unsigned N) {
      assert(N < PassVector.size() && "Pass number out of range!");
      return static_cast<Pass *>(PassVector[N]);
    }
  
    PassManagerType getPassManagerType() const override {
      return PMT_CallGraphPassManager;
    }
  
  private:
    bool RunAllPassesOnSCC(CallGraphSCC &CurSCC, CallGraph &CG,
                           bool &DevirtualizedCall);
  
    bool RunPassOnSCC(Pass *P, CallGraphSCC &CurSCC,
                      CallGraph &CG, bool &CallGraphUpToDate,
                      bool &DevirtualizedCall);
    bool RefreshCallGraph(const CallGraphSCC &CurSCC, CallGraph &CG,
                          bool IsCheckingMode);
  };
  
  } // end anonymous namespace.
  
  char CGPassManager::ID = 0;
  
  bool CGPassManager::RunPassOnSCC(Pass *P, CallGraphSCC &CurSCC,
                                   CallGraph &CG, bool &CallGraphUpToDate,
                                   bool &DevirtualizedCall) {
    bool Changed = false;
    PMDataManager *PM = P->getAsPMDataManager();
    Module &M = CG.getModule();
  
    if (!PM) {
      CallGraphSCCPass *CGSP = (CallGraphSCCPass *)P;
      if (!CallGraphUpToDate) {
        DevirtualizedCall |= RefreshCallGraph(CurSCC, CG, false);
        CallGraphUpToDate = true;
      }
  
      {
        unsigned InstrCount, SCCCount = 0;
        StringMap<std::pair<unsigned, unsigned>> FunctionToInstrCount;
        bool EmitICRemark = M.shouldEmitInstrCountChangedRemark();
        TimeRegion PassTimer(getPassTimer(CGSP));
        if (EmitICRemark)
          InstrCount = initSizeRemarkInfo(M, FunctionToInstrCount);
        Changed = CGSP->runOnSCC(CurSCC);
  
        if (EmitICRemark) {
          // FIXME: Add getInstructionCount to CallGraphSCC.
          SCCCount = M.getInstructionCount();
          // Is there a difference in the number of instructions in the module?
          if (SCCCount != InstrCount) {
            // Yep. Emit a remark and update InstrCount.
            int64_t Delta =
                static_cast<int64_t>(SCCCount) - static_cast<int64_t>(InstrCount);
            emitInstrCountChangedRemark(P, M, Delta, InstrCount,
                                        FunctionToInstrCount);
            InstrCount = SCCCount;
          }
        }
      }
  
      // After the CGSCCPass is done, when assertions are enabled, use
      // RefreshCallGraph to verify that the callgraph was correctly updated.
  #ifndef NDEBUG
      if (Changed)
        RefreshCallGraph(CurSCC, CG, true);
  #endif
  
      return Changed;
    }
  
    assert(PM->getPassManagerType() == PMT_FunctionPassManager &&
           "Invalid CGPassManager member");
    FPPassManager *FPP = (FPPassManager*)P;
  
    // Run pass P on all functions in the current SCC.
    for (CallGraphNode *CGN : CurSCC) {
      if (Function *F = CGN->getFunction()) {
        dumpPassInfo(P, EXECUTION_MSG, ON_FUNCTION_MSG, F->getName());
        {
          TimeRegion PassTimer(getPassTimer(FPP));
          Changed |= FPP->runOnFunction(*F);
        }
        F->getContext().yield();
      }
    }
  
    // The function pass(es) modified the IR, they may have clobbered the
    // callgraph.
    if (Changed && CallGraphUpToDate) {
      LLVM_DEBUG(dbgs() << "CGSCCPASSMGR: Pass Dirtied SCC: " << P->getPassName()
                        << '\n');
      CallGraphUpToDate = false;
    }
    return Changed;
  }
  
  /// Scan the functions in the specified CFG and resync the
  /// callgraph with the call sites found in it.  This is used after
  /// FunctionPasses have potentially munged the callgraph, and can be used after
  /// CallGraphSCC passes to verify that they correctly updated the callgraph.
  ///
  /// This function returns true if it devirtualized an existing function call,
  /// meaning it turned an indirect call into a direct call.  This happens when
  /// a function pass like GVN optimizes away stuff feeding the indirect call.
  /// This never happens in checking mode.
  bool CGPassManager::RefreshCallGraph(const CallGraphSCC &CurSCC, CallGraph &CG,
                                       bool CheckingMode) {
    DenseMap<Value *, CallGraphNode *> Calls;
  
    LLVM_DEBUG(dbgs() << "CGSCCPASSMGR: Refreshing SCC with " << CurSCC.size()
                      << " nodes:\n";
               for (CallGraphNode *CGN
                    : CurSCC) CGN->dump(););
  
    bool MadeChange = false;
    bool DevirtualizedCall = false;
  
    // Scan all functions in the SCC.
    unsigned FunctionNo = 0;
    for (CallGraphSCC::iterator SCCIdx = CurSCC.begin(), E = CurSCC.end();
         SCCIdx != E; ++SCCIdx, ++FunctionNo) {
      CallGraphNode *CGN = *SCCIdx;
      Function *F = CGN->getFunction();
      if (!F || F->isDeclaration()) continue;
  
      // Walk the function body looking for call sites.  Sync up the call sites in
      // CGN with those actually in the function.
  
      // Keep track of the number of direct and indirect calls that were
      // invalidated and removed.
      unsigned NumDirectRemoved = 0, NumIndirectRemoved = 0;
  
      CallGraphNode::iterator CGNEnd = CGN->end();
  
      auto RemoveAndCheckForDone = [&](CallGraphNode::iterator I) {
        // Just remove the edge from the set of callees, keep track of whether
        // I points to the last element of the vector.
        bool WasLast = I + 1 == CGNEnd;
        CGN->removeCallEdge(I);
  
        // If I pointed to the last element of the vector, we have to bail out:
        // iterator checking rejects comparisons of the resultant pointer with
        // end.
        if (WasLast)
          return true;
  
        CGNEnd = CGN->end();
        return false;
      };
  
      // Get the set of call sites currently in the function.
      for (CallGraphNode::iterator I = CGN->begin(); I != CGNEnd;) {
        // Delete "reference" call records that do not have call instruction. We
        // reinsert them as needed later. However, keep them in checking mode.
        if (!I->first) {
          if (CheckingMode) {
            ++I;
            continue;
          }
          if (RemoveAndCheckForDone(I))
            break;
          continue;
        }
  
        // If this call site is null, then the function pass deleted the call
        // entirely and the WeakTrackingVH nulled it out.
        auto *Call = dyn_cast_or_null<CallBase>(*I->first);
        if (!Call ||
            // If we've already seen this call site, then the FunctionPass RAUW'd
            // one call with another, which resulted in two "uses" in the edge
            // list of the same call.
            Calls.count(Call) ||
  
            // If the call edge is not from a call or invoke, or it is a
            // instrinsic call, then the function pass RAUW'd a call with
            // another value. This can happen when constant folding happens
            // of well known functions etc.
            (Call->getCalledFunction() &&
             Call->getCalledFunction()->isIntrinsic() &&
             Intrinsic::isLeaf(Call->getCalledFunction()->getIntrinsicID()))) {
          assert(!CheckingMode &&
                 "CallGraphSCCPass did not update the CallGraph correctly!");
  
          // If this was an indirect call site, count it.
          if (!I->second->getFunction())
            ++NumIndirectRemoved;
          else
            ++NumDirectRemoved;
  
          if (RemoveAndCheckForDone(I))
            break;
          continue;
        }
  
        assert(!Calls.count(Call) && "Call site occurs in node multiple times");
  
        if (Call) {
          Function *Callee = Call->getCalledFunction();
          // Ignore intrinsics because they're not really function calls.
          if (!Callee || !(Callee->isIntrinsic()))
            Calls.insert(std::make_pair(Call, I->second));
        }
        ++I;
      }
  
      // Loop over all of the instructions in the function, getting the callsites.
      // Keep track of the number of direct/indirect calls added.
      unsigned NumDirectAdded = 0, NumIndirectAdded = 0;
  
      for (BasicBlock &BB : *F)
        for (Instruction &I : BB) {
          auto *Call = dyn_cast<CallBase>(&I);
          if (!Call)
            continue;
          Function *Callee = Call->getCalledFunction();
          if (Callee && Callee->isIntrinsic())
            continue;
  
          // If we are not in checking mode, insert potential callback calls as
          // references. This is not a requirement but helps to iterate over the
          // functions in the right order.
          if (!CheckingMode) {
            forEachCallbackFunction(*Call, [&](Function *CB) {
              CGN->addCalledFunction(nullptr, CG.getOrInsertFunction(CB));
            });
          }
  
          // If this call site already existed in the callgraph, just verify it
          // matches up to expectations and remove it from Calls.
          DenseMap<Value *, CallGraphNode *>::iterator ExistingIt =
              Calls.find(Call);
          if (ExistingIt != Calls.end()) {
            CallGraphNode *ExistingNode = ExistingIt->second;
  
            // Remove from Calls since we have now seen it.
            Calls.erase(ExistingIt);
  
            // Verify that the callee is right.
            if (ExistingNode->getFunction() == Call->getCalledFunction())
              continue;
  
            // If we are in checking mode, we are not allowed to actually mutate
            // the callgraph.  If this is a case where we can infer that the
            // callgraph is less precise than it could be (e.g. an indirect call
            // site could be turned direct), don't reject it in checking mode, and
            // don't tweak it to be more precise.
            if (CheckingMode && Call->getCalledFunction() &&
                ExistingNode->getFunction() == nullptr)
              continue;
  
            assert(!CheckingMode &&
                   "CallGraphSCCPass did not update the CallGraph correctly!");
  
            // If not, we either went from a direct call to indirect, indirect to
            // direct, or direct to different direct.
            CallGraphNode *CalleeNode;
            if (Function *Callee = Call->getCalledFunction()) {
              CalleeNode = CG.getOrInsertFunction(Callee);
              // Keep track of whether we turned an indirect call into a direct
              // one.
              if (!ExistingNode->getFunction()) {
                DevirtualizedCall = true;
                LLVM_DEBUG(dbgs() << "  CGSCCPASSMGR: Devirtualized call to '"
                                  << Callee->getName() << "'\n");
              }
            } else {
              CalleeNode = CG.getCallsExternalNode();
            }
  
            // Update the edge target in CGN.
            CGN->replaceCallEdge(*Call, *Call, CalleeNode);
            MadeChange = true;
            continue;
          }
  
          assert(!CheckingMode &&
                 "CallGraphSCCPass did not update the CallGraph correctly!");
  
          // If the call site didn't exist in the CGN yet, add it.
          CallGraphNode *CalleeNode;
          if (Function *Callee = Call->getCalledFunction()) {
            CalleeNode = CG.getOrInsertFunction(Callee);
            ++NumDirectAdded;
          } else {
            CalleeNode = CG.getCallsExternalNode();
            ++NumIndirectAdded;
          }
  
          CGN->addCalledFunction(Call, CalleeNode);
          MadeChange = true;
        }
  
      // We scanned the old callgraph node, removing invalidated call sites and
      // then added back newly found call sites.  One thing that can happen is
      // that an old indirect call site was deleted and replaced with a new direct
      // call.  In this case, we have devirtualized a call, and CGSCCPM would like
      // to iteratively optimize the new code.  Unfortunately, we don't really
      // have a great way to detect when this happens.  As an approximation, we
      // just look at whether the number of indirect calls is reduced and the
      // number of direct calls is increased.  There are tons of ways to fool this
      // (e.g. DCE'ing an indirect call and duplicating an unrelated block with a
      // direct call) but this is close enough.
      if (NumIndirectRemoved > NumIndirectAdded &&
          NumDirectRemoved < NumDirectAdded)
        DevirtualizedCall = true;
  
      // After scanning this function, if we still have entries in callsites, then
      // they are dangling pointers.  WeakTrackingVH should save us for this, so
      // abort if
      // this happens.
      assert(Calls.empty() && "Dangling pointers found in call sites map");
  
      // Periodically do an explicit clear to remove tombstones when processing
      // large scc's.
      if ((FunctionNo & 15) == 15)
        Calls.clear();
    }
  
    LLVM_DEBUG(if (MadeChange) {
      dbgs() << "CGSCCPASSMGR: Refreshed SCC is now:\n";
      for (CallGraphNode *CGN : CurSCC)
        CGN->dump();
      if (DevirtualizedCall)
        dbgs() << "CGSCCPASSMGR: Refresh devirtualized a call!\n";
    } else {
      dbgs() << "CGSCCPASSMGR: SCC Refresh didn't change call graph.\n";
    });
    (void)MadeChange;
  
    return DevirtualizedCall;
  }
  
  /// Execute the body of the entire pass manager on the specified SCC.
  /// This keeps track of whether a function pass devirtualizes
  /// any calls and returns it in DevirtualizedCall.
  bool CGPassManager::RunAllPassesOnSCC(CallGraphSCC &CurSCC, CallGraph &CG,
                                        bool &DevirtualizedCall) {
    bool Changed = false;
  
    // Keep track of whether the callgraph is known to be up-to-date or not.
    // The CGSSC pass manager runs two types of passes:
    // CallGraphSCC Passes and other random function passes.  Because other
    // random function passes are not CallGraph aware, they may clobber the
    // call graph by introducing new calls or deleting other ones.  This flag
    // is set to false when we run a function pass so that we know to clean up
    // the callgraph when we need to run a CGSCCPass again.
    bool CallGraphUpToDate = true;
  
    // Run all passes on current SCC.
    for (unsigned PassNo = 0, e = getNumContainedPasses();
         PassNo != e; ++PassNo) {
      Pass *P = getContainedPass(PassNo);
  
      // If we're in -debug-pass=Executions mode, construct the SCC node list,
      // otherwise avoid constructing this string as it is expensive.
      if (isPassDebuggingExecutionsOrMore()) {
        std::string Functions;
    #ifndef NDEBUG
        raw_string_ostream OS(Functions);
        for (CallGraphSCC::iterator I = CurSCC.begin(), E = CurSCC.end();
             I != E; ++I) {
          if (I != CurSCC.begin()) OS << ", ";
          (*I)->print(OS);
        }
        OS.flush();
    #endif
        dumpPassInfo(P, EXECUTION_MSG, ON_CG_MSG, Functions);
      }
      dumpRequiredSet(P);
  
      initializeAnalysisImpl(P);
  
  #ifdef EXPENSIVE_CHECKS
      uint64_t RefHash = StructuralHash(CG.getModule());
  #endif
  
      // Actually run this pass on the current SCC.
      bool LocalChanged =
          RunPassOnSCC(P, CurSCC, CG, CallGraphUpToDate, DevirtualizedCall);
  
      Changed |= LocalChanged;
  
  #ifdef EXPENSIVE_CHECKS
      if (!LocalChanged && (RefHash != StructuralHash(CG.getModule()))) {
        llvm::errs() << "Pass modifies its input and doesn't report it: "
                     << P->getPassName() << "\n";
        llvm_unreachable("Pass modifies its input and doesn't report it");
      }
  #endif
      if (LocalChanged)
        dumpPassInfo(P, MODIFICATION_MSG, ON_CG_MSG, "");
      dumpPreservedSet(P);
  
      verifyPreservedAnalysis(P);
      if (LocalChanged)
        removeNotPreservedAnalysis(P);
      recordAvailableAnalysis(P);
      removeDeadPasses(P, "", ON_CG_MSG);
    }
  
    // If the callgraph was left out of date (because the last pass run was a
    // functionpass), refresh it before we move on to the next SCC.
    if (!CallGraphUpToDate)
      DevirtualizedCall |= RefreshCallGraph(CurSCC, CG, false);
    return Changed;
  }
  
  /// Execute all of the passes scheduled for execution.  Keep track of
  /// whether any of the passes modifies the module, and if so, return true.
  bool CGPassManager::runOnModule(Module &M) {
    CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
    bool Changed = doInitialization(CG);
  
    // Walk the callgraph in bottom-up SCC order.
    scc_iterator<CallGraph*> CGI = scc_begin(&CG);
  
    CallGraphSCC CurSCC(CG, &CGI);
    while (!CGI.isAtEnd()) {
      // Copy the current SCC and increment past it so that the pass can hack
      // on the SCC if it wants to without invalidating our iterator.
      const std::vector<CallGraphNode *> &NodeVec = *CGI;
      CurSCC.initialize(NodeVec);
      ++CGI;
  
      // At the top level, we run all the passes in this pass manager on the
      // functions in this SCC.  However, we support iterative compilation in the
      // case where a function pass devirtualizes a call to a function.  For
      // example, it is very common for a function pass (often GVN or instcombine)
      // to eliminate the addressing that feeds into a call.  With that improved
      // information, we would like the call to be an inline candidate, infer
      // mod-ref information etc.
      //
      // Because of this, we allow iteration up to a specified iteration count.
      // This only happens in the case of a devirtualized call, so we only burn
      // compile time in the case that we're making progress.  We also have a hard
      // iteration count limit in case there is crazy code.
      unsigned Iteration = 0;
      bool DevirtualizedCall = false;
      do {
        LLVM_DEBUG(if (Iteration) dbgs()
                   << "  SCCPASSMGR: Re-visiting SCC, iteration #" << Iteration
                   << '\n');
        DevirtualizedCall = false;
        Changed |= RunAllPassesOnSCC(CurSCC, CG, DevirtualizedCall);
      } while (Iteration++ < MaxIterations && DevirtualizedCall);
  
      if (DevirtualizedCall)
        LLVM_DEBUG(dbgs() << "  CGSCCPASSMGR: Stopped iteration after "
                          << Iteration
                          << " times, due to -max-cg-scc-iterations\n");
  
      MaxSCCIterations.updateMax(Iteration);
    }
    Changed |= doFinalization(CG);
    return Changed;
  }
  
  /// Initialize CG
  bool CGPassManager::doInitialization(CallGraph &CG) {
    bool Changed = false;
    for (unsigned i = 0, e = getNumContainedPasses(); i != e; ++i) {
      if (PMDataManager *PM = getContainedPass(i)->getAsPMDataManager()) {
        assert(PM->getPassManagerType() == PMT_FunctionPassManager &&
               "Invalid CGPassManager member");
        Changed |= ((FPPassManager*)PM)->doInitialization(CG.getModule());
      } else {
        Changed |= ((CallGraphSCCPass*)getContainedPass(i))->doInitialization(CG);
      }
    }
    return Changed;
  }
  
  /// Finalize CG
  bool CGPassManager::doFinalization(CallGraph &CG) {
    bool Changed = false;
    for (unsigned i = 0, e = getNumContainedPasses(); i != e; ++i) {
      if (PMDataManager *PM = getContainedPass(i)->getAsPMDataManager()) {
        assert(PM->getPassManagerType() == PMT_FunctionPassManager &&
               "Invalid CGPassManager member");
        Changed |= ((FPPassManager*)PM)->doFinalization(CG.getModule());
      } else {
        Changed |= ((CallGraphSCCPass*)getContainedPass(i))->doFinalization(CG);
      }
    }
    return Changed;
  }
  
  //===----------------------------------------------------------------------===//
  // CallGraphSCC Implementation
  //===----------------------------------------------------------------------===//
  
  /// This informs the SCC and the pass manager that the specified
  /// Old node has been deleted, and New is to be used in its place.
  void CallGraphSCC::ReplaceNode(CallGraphNode *Old, CallGraphNode *New) {
    assert(Old != New && "Should not replace node with self");
    for (unsigned i = 0; ; ++i) {
      assert(i != Nodes.size() && "Node not in SCC");
      if (Nodes[i] != Old) continue;
      if (New)
        Nodes[i] = New;
      else
        Nodes.erase(Nodes.begin() + i);
      break;
    }
  
    // Update the active scc_iterator so that it doesn't contain dangling
    // pointers to the old CallGraphNode.
    scc_iterator<CallGraph*> *CGI = (scc_iterator<CallGraph*>*)Context;
    CGI->ReplaceNode(Old, New);
  }
  
  void CallGraphSCC::DeleteNode(CallGraphNode *Old) {
    ReplaceNode(Old, /*New=*/nullptr);
  }
  
  //===----------------------------------------------------------------------===//
  // CallGraphSCCPass Implementation
  //===----------------------------------------------------------------------===//
  
  /// Assign pass manager to manage this pass.
  void CallGraphSCCPass::assignPassManager(PMStack &PMS,
                                           PassManagerType PreferredType) {
    // Find CGPassManager
    while (!PMS.empty() &&
           PMS.top()->getPassManagerType() > PMT_CallGraphPassManager)
      PMS.pop();
  
    assert(!PMS.empty() && "Unable to handle Call Graph Pass");
    CGPassManager *CGP;
  
    if (PMS.top()->getPassManagerType() == PMT_CallGraphPassManager)
      CGP = (CGPassManager*)PMS.top();
    else {
      // Create new Call Graph SCC Pass Manager if it does not exist.
      assert(!PMS.empty() && "Unable to create Call Graph Pass Manager");
      PMDataManager *PMD = PMS.top();
  
      // [1] Create new Call Graph Pass Manager
      CGP = new CGPassManager();
  
      // [2] Set up new manager's top level manager
      PMTopLevelManager *TPM = PMD->getTopLevelManager();
      TPM->addIndirectPassManager(CGP);
  
      // [3] Assign manager to manage this new manager. This may create
      // and push new managers into PMS
      Pass *P = CGP;
      TPM->schedulePass(P);
  
      // [4] Push new manager into PMS
      PMS.push(CGP);
    }
  
    CGP->add(this);
  }
  
  /// For this class, we declare that we require and preserve the call graph.
  /// If the derived class implements this method, it should
  /// always explicitly call the implementation here.
  void CallGraphSCCPass::getAnalysisUsage(AnalysisUsage &AU) const {
    AU.addRequired<CallGraphWrapperPass>();
    AU.addPreserved<CallGraphWrapperPass>();
  }
  
  //===----------------------------------------------------------------------===//
  // PrintCallGraphPass Implementation
  //===----------------------------------------------------------------------===//
  
  namespace {
  
    /// PrintCallGraphPass - Print a Module corresponding to a call graph.
    ///
    class PrintCallGraphPass : public CallGraphSCCPass {
      std::string Banner;
      raw_ostream &OS;       // raw_ostream to print on.
  
    public:
      static char ID;
  
      PrintCallGraphPass(const std::string &B, raw_ostream &OS)
        : CallGraphSCCPass(ID), Banner(B), OS(OS) {}
  
      void getAnalysisUsage(AnalysisUsage &AU) const override {
        AU.setPreservesAll();
      }
  
      bool runOnSCC(CallGraphSCC &SCC) override {
        bool BannerPrinted = false;
        auto PrintBannerOnce = [&]() {
          if (BannerPrinted)
            return;
          OS << Banner;
          BannerPrinted = true;
        };
  
        bool NeedModule = llvm::forcePrintModuleIR();
        if (isFunctionInPrintList("*") && NeedModule) {
          PrintBannerOnce();
          OS << "\n";
          SCC.getCallGraph().getModule().print(OS, nullptr);
          return false;
        }
        bool FoundFunction = false;
        for (CallGraphNode *CGN : SCC) {
          if (Function *F = CGN->getFunction()) {
            if (!F->isDeclaration() && isFunctionInPrintList(F->getName())) {
              FoundFunction = true;
              if (!NeedModule) {
                PrintBannerOnce();
                F->print(OS);
              }
            }
          } else if (isFunctionInPrintList("*")) {
            PrintBannerOnce();
            OS << "\nPrinting <null> Function\n";
          }
        }
        if (NeedModule && FoundFunction) {
          PrintBannerOnce();
          OS << "\n";
          SCC.getCallGraph().getModule().print(OS, nullptr);
        }
        return false;
      }
  
      StringRef getPassName() const override { return "Print CallGraph IR"; }
    };
  
  } // end anonymous namespace.
  
  char PrintCallGraphPass::ID = 0;
  
  Pass *CallGraphSCCPass::createPrinterPass(raw_ostream &OS,
                                            const std::string &Banner) const {
    return new PrintCallGraphPass(Banner, OS);
  }
  
  static std::string getDescription(const CallGraphSCC &SCC) {
    std::string Desc = "SCC (";
    bool First = true;
    for (CallGraphNode *CGN : SCC) {
      if (First)
        First = false;
      else
        Desc += ", ";
      Function *F = CGN->getFunction();
      if (F)
        Desc += F->getName();
      else
        Desc += "<<null function>>";
    }
    Desc += ")";
    return Desc;
  }
  
  bool CallGraphSCCPass::skipSCC(CallGraphSCC &SCC) const {
    OptPassGate &Gate =
        SCC.getCallGraph().getModule().getContext().getOptPassGate();
    return Gate.isEnabled() && !Gate.shouldRunPass(this, getDescription(SCC));
  }
  
  char DummyCGSCCPass::ID = 0;
  
  INITIALIZE_PASS(DummyCGSCCPass, "DummyCGSCCPass", "DummyCGSCCPass", false,
                  false)
  