CallGraphSCCPass.cpp 25.3 KB
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711
//===- 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/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/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;

    // Get the set of call sites currently in the function.
    for (CallGraphNode::iterator I = CGN->begin(), E = CGN->end(); I != E; ) {
      // 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 (!I->first ||
          // 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(I->first) ||

          // 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 ||
          (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;

        // 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 == E;
        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)
          break;
        E = CGN->end();
        continue;
      }

      assert(!Calls.count(I->first) &&
             "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(I->first, 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 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);

    // Actually run this pass on the current SCC.
    Changed |= RunPassOnSCC(P, CurSCC, CG,
                            CallGraphUpToDate, DevirtualizedCall);

    if (Changed)
      dumpPassInfo(P, MODIFICATION_MSG, ON_CG_MSG, "");
    dumpPreservedSet(P);

    verifyPreservedAnalysis(P);
    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;
    Nodes[i] = New;
    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);
}

//===----------------------------------------------------------------------===//
// 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)