CoreEngine.cpp revision 341825 
1//===- CoreEngine.cpp - Path-Sensitive Dataflow Engine --------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10//  This file defines a generic engine for intraprocedural, path-sensitive,
11//  dataflow analysis via graph reachability engine.
12//
13//===----------------------------------------------------------------------===//
14
15#include "clang/StaticAnalyzer/Core/PathSensitive/CoreEngine.h"
16#include "clang/AST/Expr.h"
17#include "clang/AST/ExprCXX.h"
18#include "clang/AST/Stmt.h"
19#include "clang/AST/StmtCXX.h"
20#include "clang/Analysis/AnalysisDeclContext.h"
21#include "clang/Analysis/CFG.h"
22#include "clang/Analysis/ProgramPoint.h"
23#include "clang/Basic/LLVM.h"
24#include "clang/StaticAnalyzer/Core/AnalyzerOptions.h"
25#include "clang/StaticAnalyzer/Core/PathSensitive/BlockCounter.h"
26#include "clang/StaticAnalyzer/Core/PathSensitive/ExplodedGraph.h"
27#include "clang/StaticAnalyzer/Core/PathSensitive/FunctionSummary.h"
28#include "clang/StaticAnalyzer/Core/PathSensitive/SubEngine.h"
29#include "clang/StaticAnalyzer/Core/PathSensitive/WorkList.h"
30#include "llvm/ADT/Optional.h"
31#include "llvm/ADT/STLExtras.h"
32#include "llvm/ADT/Statistic.h"
33#include "llvm/Support/Casting.h"
34#include "llvm/Support/ErrorHandling.h"
35#include <algorithm>
36#include <cassert>
37#include <memory>
38#include <utility>
39
40using namespace clang;
41using namespace ento;
42
43#define DEBUG_TYPE "CoreEngine"
44
45STATISTIC(NumSteps,
46            "The # of steps executed.");
47STATISTIC(NumReachedMaxSteps,
48            "The # of times we reached the max number of steps.");
49STATISTIC(NumPathsExplored,
50            "The # of paths explored by the analyzer.");
51
52//===----------------------------------------------------------------------===//
53// Core analysis engine.
54//===----------------------------------------------------------------------===//
55
56static std::unique_ptr<WorkList> generateWorkList(AnalyzerOptions &Opts) {
57  switch (Opts.getExplorationStrategy()) {
58    case AnalyzerOptions::ExplorationStrategyKind::DFS:
59      return WorkList::makeDFS();
60    case AnalyzerOptions::ExplorationStrategyKind::BFS:
61      return WorkList::makeBFS();
62    case AnalyzerOptions::ExplorationStrategyKind::BFSBlockDFSContents:
63      return WorkList::makeBFSBlockDFSContents();
64    case AnalyzerOptions::ExplorationStrategyKind::UnexploredFirst:
65      return WorkList::makeUnexploredFirst();
66    case AnalyzerOptions::ExplorationStrategyKind::UnexploredFirstQueue:
67      return WorkList::makeUnexploredFirstPriorityQueue();
68    default:
69      llvm_unreachable("Unexpected case");
70  }
71}
72
73CoreEngine::CoreEngine(SubEngine &subengine, FunctionSummariesTy *FS,
74                       AnalyzerOptions &Opts)
75    : SubEng(subengine), WList(generateWorkList(Opts)),
76      BCounterFactory(G.getAllocator()), FunctionSummaries(FS) {}
77
78/// ExecuteWorkList - Run the worklist algorithm for a maximum number of steps.
79bool CoreEngine::ExecuteWorkList(const LocationContext *L, unsigned Steps,
80                                   ProgramStateRef InitState) {
81  if (G.num_roots() == 0) { // Initialize the analysis by constructing
82    // the root if none exists.
83
84    const CFGBlock *Entry = &(L->getCFG()->getEntry());
85
86    assert(Entry->empty() && "Entry block must be empty.");
87
88    assert(Entry->succ_size() == 1 && "Entry block must have 1 successor.");
89
90    // Mark the entry block as visited.
91    FunctionSummaries->markVisitedBasicBlock(Entry->getBlockID(),
92                                             L->getDecl(),
93                                             L->getCFG()->getNumBlockIDs());
94
95    // Get the solitary successor.
96    const CFGBlock *Succ = *(Entry->succ_begin());
97
98    // Construct an edge representing the
99    // starting location in the function.
100    BlockEdge StartLoc(Entry, Succ, L);
101
102    // Set the current block counter to being empty.
103    WList->setBlockCounter(BCounterFactory.GetEmptyCounter());
104
105    if (!InitState)
106      InitState = SubEng.getInitialState(L);
107
108    bool IsNew;
109    ExplodedNode *Node = G.getNode(StartLoc, InitState, false, &IsNew);
110    assert(IsNew);
111    G.addRoot(Node);
112
113    NodeBuilderContext BuilderCtx(*this, StartLoc.getDst(), Node);
114    ExplodedNodeSet DstBegin;
115    SubEng.processBeginOfFunction(BuilderCtx, Node, DstBegin, StartLoc);
116
117    enqueue(DstBegin);
118  }
119
120  // Check if we have a steps limit
121  bool UnlimitedSteps = Steps == 0;
122  // Cap our pre-reservation in the event that the user specifies
123  // a very large number of maximum steps.
124  const unsigned PreReservationCap = 4000000;
125  if(!UnlimitedSteps)
126    G.reserve(std::min(Steps,PreReservationCap));
127
128  while (WList->hasWork()) {
129    if (!UnlimitedSteps) {
130      if (Steps == 0) {
131        NumReachedMaxSteps++;
132        break;
133      }
134      --Steps;
135    }
136
137    NumSteps++;
138
139    const WorkListUnit& WU = WList->dequeue();
140
141    // Set the current block counter.
142    WList->setBlockCounter(WU.getBlockCounter());
143
144    // Retrieve the node.
145    ExplodedNode *Node = WU.getNode();
146
147    dispatchWorkItem(Node, Node->getLocation(), WU);
148  }
149  SubEng.processEndWorklist(hasWorkRemaining());
150  return WList->hasWork();
151}
152
153void CoreEngine::dispatchWorkItem(ExplodedNode* Pred, ProgramPoint Loc,
154                                  const WorkListUnit& WU) {
155  // Dispatch on the location type.
156  switch (Loc.getKind()) {
157    case ProgramPoint::BlockEdgeKind:
158      HandleBlockEdge(Loc.castAs<BlockEdge>(), Pred);
159      break;
160
161    case ProgramPoint::BlockEntranceKind:
162      HandleBlockEntrance(Loc.castAs<BlockEntrance>(), Pred);
163      break;
164
165    case ProgramPoint::BlockExitKind:
166      assert(false && "BlockExit location never occur in forward analysis.");
167      break;
168
169    case ProgramPoint::CallEnterKind:
170      HandleCallEnter(Loc.castAs<CallEnter>(), Pred);
171      break;
172
173    case ProgramPoint::CallExitBeginKind:
174      SubEng.processCallExit(Pred);
175      break;
176
177    case ProgramPoint::EpsilonKind: {
178      assert(Pred->hasSinglePred() &&
179             "Assume epsilon has exactly one predecessor by construction");
180      ExplodedNode *PNode = Pred->getFirstPred();
181      dispatchWorkItem(Pred, PNode->getLocation(), WU);
182      break;
183    }
184    default:
185      assert(Loc.getAs<PostStmt>() ||
186             Loc.getAs<PostInitializer>() ||
187             Loc.getAs<PostImplicitCall>() ||
188             Loc.getAs<CallExitEnd>() ||
189             Loc.getAs<LoopExit>() ||
190             Loc.getAs<PostAllocatorCall>());
191      HandlePostStmt(WU.getBlock(), WU.getIndex(), Pred);
192      break;
193  }
194}
195
196bool CoreEngine::ExecuteWorkListWithInitialState(const LocationContext *L,
197                                                 unsigned Steps,
198                                                 ProgramStateRef InitState,
199                                                 ExplodedNodeSet &Dst) {
200  bool DidNotFinish = ExecuteWorkList(L, Steps, InitState);
201  for (ExplodedGraph::eop_iterator I = G.eop_begin(), E = G.eop_end(); I != E;
202       ++I) {
203    Dst.Add(*I);
204  }
205  return DidNotFinish;
206}
207
208void CoreEngine::HandleBlockEdge(const BlockEdge &L, ExplodedNode *Pred) {
209  const CFGBlock *Blk = L.getDst();
210  NodeBuilderContext BuilderCtx(*this, Blk, Pred);
211
212  // Mark this block as visited.
213  const LocationContext *LC = Pred->getLocationContext();
214  FunctionSummaries->markVisitedBasicBlock(Blk->getBlockID(),
215                                           LC->getDecl(),
216                                           LC->getCFG()->getNumBlockIDs());
217
218  // Check if we are entering the EXIT block.
219  if (Blk == &(L.getLocationContext()->getCFG()->getExit())) {
220    assert(L.getLocationContext()->getCFG()->getExit().empty() &&
221           "EXIT block cannot contain Stmts.");
222
223    // Get return statement..
224    const ReturnStmt *RS = nullptr;
225    if (!L.getSrc()->empty()) {
226      if (Optional<CFGStmt> LastStmt = L.getSrc()->back().getAs<CFGStmt>()) {
227        RS = dyn_cast<ReturnStmt>(LastStmt->getStmt());
228      }
229    }
230
231    // Process the final state transition.
232    SubEng.processEndOfFunction(BuilderCtx, Pred, RS);
233
234    // This path is done. Don't enqueue any more nodes.
235    return;
236  }
237
238  // Call into the SubEngine to process entering the CFGBlock.
239  ExplodedNodeSet dstNodes;
240  BlockEntrance BE(Blk, Pred->getLocationContext());
241  NodeBuilderWithSinks nodeBuilder(Pred, dstNodes, BuilderCtx, BE);
242  SubEng.processCFGBlockEntrance(L, nodeBuilder, Pred);
243
244  // Auto-generate a node.
245  if (!nodeBuilder.hasGeneratedNodes()) {
246    nodeBuilder.generateNode(Pred->State, Pred);
247  }
248
249  // Enqueue nodes onto the worklist.
250  enqueue(dstNodes);
251}
252
253void CoreEngine::HandleBlockEntrance(const BlockEntrance &L,
254                                       ExplodedNode *Pred) {
255  // Increment the block counter.
256  const LocationContext *LC = Pred->getLocationContext();
257  unsigned BlockId = L.getBlock()->getBlockID();
258  BlockCounter Counter = WList->getBlockCounter();
259  Counter = BCounterFactory.IncrementCount(Counter, LC->getStackFrame(),
260                                           BlockId);
261  WList->setBlockCounter(Counter);
262
263  // Process the entrance of the block.
264  if (Optional<CFGElement> E = L.getFirstElement()) {
265    NodeBuilderContext Ctx(*this, L.getBlock(), Pred);
266    SubEng.processCFGElement(*E, Pred, 0, &Ctx);
267  }
268  else
269    HandleBlockExit(L.getBlock(), Pred);
270}
271
272void CoreEngine::HandleBlockExit(const CFGBlock * B, ExplodedNode *Pred) {
273  if (const Stmt *Term = B->getTerminator()) {
274    switch (Term->getStmtClass()) {
275      default:
276        llvm_unreachable("Analysis for this terminator not implemented.");
277
278      case Stmt::CXXBindTemporaryExprClass:
279        HandleCleanupTemporaryBranch(
280            cast<CXXBindTemporaryExpr>(B->getTerminator().getStmt()), B, Pred);
281        return;
282
283      // Model static initializers.
284      case Stmt::DeclStmtClass:
285        HandleStaticInit(cast<DeclStmt>(Term), B, Pred);
286        return;
287
288      case Stmt::BinaryOperatorClass: // '&&' and '||'
289        HandleBranch(cast<BinaryOperator>(Term)->getLHS(), Term, B, Pred);
290        return;
291
292      case Stmt::BinaryConditionalOperatorClass:
293      case Stmt::ConditionalOperatorClass:
294        HandleBranch(cast<AbstractConditionalOperator>(Term)->getCond(),
295                     Term, B, Pred);
296        return;
297
298        // FIXME: Use constant-folding in CFG construction to simplify this
299        // case.
300
301      case Stmt::ChooseExprClass:
302        HandleBranch(cast<ChooseExpr>(Term)->getCond(), Term, B, Pred);
303        return;
304
305      case Stmt::CXXTryStmtClass:
306        // Generate a node for each of the successors.
307        // Our logic for EH analysis can certainly be improved.
308        for (CFGBlock::const_succ_iterator it = B->succ_begin(),
309             et = B->succ_end(); it != et; ++it) {
310          if (const CFGBlock *succ = *it) {
311            generateNode(BlockEdge(B, succ, Pred->getLocationContext()),
312                         Pred->State, Pred);
313          }
314        }
315        return;
316
317      case Stmt::DoStmtClass:
318        HandleBranch(cast<DoStmt>(Term)->getCond(), Term, B, Pred);
319        return;
320
321      case Stmt::CXXForRangeStmtClass:
322        HandleBranch(cast<CXXForRangeStmt>(Term)->getCond(), Term, B, Pred);
323        return;
324
325      case Stmt::ForStmtClass:
326        HandleBranch(cast<ForStmt>(Term)->getCond(), Term, B, Pred);
327        return;
328
329      case Stmt::ContinueStmtClass:
330      case Stmt::BreakStmtClass:
331      case Stmt::GotoStmtClass:
332        break;
333
334      case Stmt::IfStmtClass:
335        HandleBranch(cast<IfStmt>(Term)->getCond(), Term, B, Pred);
336        return;
337
338      case Stmt::IndirectGotoStmtClass: {
339        // Only 1 successor: the indirect goto dispatch block.
340        assert(B->succ_size() == 1);
341
342        IndirectGotoNodeBuilder
343           builder(Pred, B, cast<IndirectGotoStmt>(Term)->getTarget(),
344                   *(B->succ_begin()), this);
345
346        SubEng.processIndirectGoto(builder);
347        return;
348      }
349
350      case Stmt::ObjCForCollectionStmtClass:
351        // In the case of ObjCForCollectionStmt, it appears twice in a CFG:
352        //
353        //  (1) inside a basic block, which represents the binding of the
354        //      'element' variable to a value.
355        //  (2) in a terminator, which represents the branch.
356        //
357        // For (1), subengines will bind a value (i.e., 0 or 1) indicating
358        // whether or not collection contains any more elements.  We cannot
359        // just test to see if the element is nil because a container can
360        // contain nil elements.
361        HandleBranch(Term, Term, B, Pred);
362        return;
363
364      case Stmt::SwitchStmtClass: {
365        SwitchNodeBuilder builder(Pred, B, cast<SwitchStmt>(Term)->getCond(),
366                                    this);
367
368        SubEng.processSwitch(builder);
369        return;
370      }
371
372      case Stmt::WhileStmtClass:
373        HandleBranch(cast<WhileStmt>(Term)->getCond(), Term, B, Pred);
374        return;
375    }
376  }
377
378  assert(B->succ_size() == 1 &&
379         "Blocks with no terminator should have at most 1 successor.");
380
381  generateNode(BlockEdge(B, *(B->succ_begin()), Pred->getLocationContext()),
382               Pred->State, Pred);
383}
384
385void CoreEngine::HandleCallEnter(const CallEnter &CE, ExplodedNode *Pred) {
386  NodeBuilderContext BuilderCtx(*this, CE.getEntry(), Pred);
387  SubEng.processCallEnter(BuilderCtx, CE, Pred);
388}
389
390void CoreEngine::HandleBranch(const Stmt *Cond, const Stmt *Term,
391                                const CFGBlock * B, ExplodedNode *Pred) {
392  assert(B->succ_size() == 2);
393  NodeBuilderContext Ctx(*this, B, Pred);
394  ExplodedNodeSet Dst;
395  SubEng.processBranch(Cond, Term, Ctx, Pred, Dst,
396                       *(B->succ_begin()), *(B->succ_begin()+1));
397  // Enqueue the new frontier onto the worklist.
398  enqueue(Dst);
399}
400
401void CoreEngine::HandleCleanupTemporaryBranch(const CXXBindTemporaryExpr *BTE,
402                                              const CFGBlock *B,
403                                              ExplodedNode *Pred) {
404  assert(B->succ_size() == 2);
405  NodeBuilderContext Ctx(*this, B, Pred);
406  ExplodedNodeSet Dst;
407  SubEng.processCleanupTemporaryBranch(BTE, Ctx, Pred, Dst, *(B->succ_begin()),
408                                       *(B->succ_begin() + 1));
409  // Enqueue the new frontier onto the worklist.
410  enqueue(Dst);
411}
412
413void CoreEngine::HandleStaticInit(const DeclStmt *DS, const CFGBlock *B,
414                                  ExplodedNode *Pred) {
415  assert(B->succ_size() == 2);
416  NodeBuilderContext Ctx(*this, B, Pred);
417  ExplodedNodeSet Dst;
418  SubEng.processStaticInitializer(DS, Ctx, Pred, Dst,
419                                  *(B->succ_begin()), *(B->succ_begin()+1));
420  // Enqueue the new frontier onto the worklist.
421  enqueue(Dst);
422}
423
424void CoreEngine::HandlePostStmt(const CFGBlock *B, unsigned StmtIdx,
425                                ExplodedNode *Pred) {
426  assert(B);
427  assert(!B->empty());
428
429  if (StmtIdx == B->size())
430    HandleBlockExit(B, Pred);
431  else {
432    NodeBuilderContext Ctx(*this, B, Pred);
433    SubEng.processCFGElement((*B)[StmtIdx], Pred, StmtIdx, &Ctx);
434  }
435}
436
437/// generateNode - Utility method to generate nodes, hook up successors,
438///  and add nodes to the worklist.
439void CoreEngine::generateNode(const ProgramPoint &Loc,
440                              ProgramStateRef State,
441                              ExplodedNode *Pred) {
442  bool IsNew;
443  ExplodedNode *Node = G.getNode(Loc, State, false, &IsNew);
444
445  if (Pred)
446    Node->addPredecessor(Pred, G); // Link 'Node' with its predecessor.
447  else {
448    assert(IsNew);
449    G.addRoot(Node); // 'Node' has no predecessor.  Make it a root.
450  }
451
452  // Only add 'Node' to the worklist if it was freshly generated.
453  if (IsNew) WList->enqueue(Node);
454}
455
456void CoreEngine::enqueueStmtNode(ExplodedNode *N,
457                                 const CFGBlock *Block, unsigned Idx) {
458  assert(Block);
459  assert(!N->isSink());
460
461  // Check if this node entered a callee.
462  if (N->getLocation().getAs<CallEnter>()) {
463    // Still use the index of the CallExpr. It's needed to create the callee
464    // StackFrameContext.
465    WList->enqueue(N, Block, Idx);
466    return;
467  }
468
469  // Do not create extra nodes. Move to the next CFG element.
470  if (N->getLocation().getAs<PostInitializer>() ||
471      N->getLocation().getAs<PostImplicitCall>()||
472      N->getLocation().getAs<LoopExit>()) {
473    WList->enqueue(N, Block, Idx+1);
474    return;
475  }
476
477  if (N->getLocation().getAs<EpsilonPoint>()) {
478    WList->enqueue(N, Block, Idx);
479    return;
480  }
481
482  if ((*Block)[Idx].getKind() == CFGElement::NewAllocator) {
483    WList->enqueue(N, Block, Idx+1);
484    return;
485  }
486
487  // At this point, we know we're processing a normal statement.
488  CFGStmt CS = (*Block)[Idx].castAs<CFGStmt>();
489  PostStmt Loc(CS.getStmt(), N->getLocationContext());
490
491  if (Loc == N->getLocation().withTag(nullptr)) {
492    // Note: 'N' should be a fresh node because otherwise it shouldn't be
493    // a member of Deferred.
494    WList->enqueue(N, Block, Idx+1);
495    return;
496  }
497
498  bool IsNew;
499  ExplodedNode *Succ = G.getNode(Loc, N->getState(), false, &IsNew);
500  Succ->addPredecessor(N, G);
501
502  if (IsNew)
503    WList->enqueue(Succ, Block, Idx+1);
504}
505
506ExplodedNode *CoreEngine::generateCallExitBeginNode(ExplodedNode *N,
507                                                    const ReturnStmt *RS) {
508  // Create a CallExitBegin node and enqueue it.
509  const auto *LocCtx = cast<StackFrameContext>(N->getLocationContext());
510
511  // Use the callee location context.
512  CallExitBegin Loc(LocCtx, RS);
513
514  bool isNew;
515  ExplodedNode *Node = G.getNode(Loc, N->getState(), false, &isNew);
516  Node->addPredecessor(N, G);
517  return isNew ? Node : nullptr;
518}
519
520void CoreEngine::enqueue(ExplodedNodeSet &Set) {
521  for (const auto I : Set)
522    WList->enqueue(I);
523}
524
525void CoreEngine::enqueue(ExplodedNodeSet &Set,
526                         const CFGBlock *Block, unsigned Idx) {
527  for (const auto I : Set)
528    enqueueStmtNode(I, Block, Idx);
529}
530
531void CoreEngine::enqueueEndOfFunction(ExplodedNodeSet &Set, const ReturnStmt *RS) {
532  for (auto I : Set) {
533    // If we are in an inlined call, generate CallExitBegin node.
534    if (I->getLocationContext()->getParent()) {
535      I = generateCallExitBeginNode(I, RS);
536      if (I)
537        WList->enqueue(I);
538    } else {
539      // TODO: We should run remove dead bindings here.
540      G.addEndOfPath(I);
541      NumPathsExplored++;
542    }
543  }
544}
545
546void NodeBuilder::anchor() {}
547
548ExplodedNode* NodeBuilder::generateNodeImpl(const ProgramPoint &Loc,
549                                            ProgramStateRef State,
550                                            ExplodedNode *FromN,
551                                            bool MarkAsSink) {
552  HasGeneratedNodes = true;
553  bool IsNew;
554  ExplodedNode *N = C.Eng.G.getNode(Loc, State, MarkAsSink, &IsNew);
555  N->addPredecessor(FromN, C.Eng.G);
556  Frontier.erase(FromN);
557
558  if (!IsNew)
559    return nullptr;
560
561  if (!MarkAsSink)
562    Frontier.Add(N);
563
564  return N;
565}
566
567void NodeBuilderWithSinks::anchor() {}
568
569StmtNodeBuilder::~StmtNodeBuilder() {
570  if (EnclosingBldr)
571    for (const auto I : Frontier)
572      EnclosingBldr->addNodes(I);
573}
574
575void BranchNodeBuilder::anchor() {}
576
577ExplodedNode *BranchNodeBuilder::generateNode(ProgramStateRef State,
578                                              bool branch,
579                                              ExplodedNode *NodePred) {
580  // If the branch has been marked infeasible we should not generate a node.
581  if (!isFeasible(branch))
582    return nullptr;
583
584  ProgramPoint Loc = BlockEdge(C.Block, branch ? DstT:DstF,
585                               NodePred->getLocationContext());
586  ExplodedNode *Succ = generateNodeImpl(Loc, State, NodePred);
587  return Succ;
588}
589
590ExplodedNode*
591IndirectGotoNodeBuilder::generateNode(const iterator &I,
592                                      ProgramStateRef St,
593                                      bool IsSink) {
594  bool IsNew;
595  ExplodedNode *Succ =
596      Eng.G.getNode(BlockEdge(Src, I.getBlock(), Pred->getLocationContext()),
597                    St, IsSink, &IsNew);
598  Succ->addPredecessor(Pred, Eng.G);
599
600  if (!IsNew)
601    return nullptr;
602
603  if (!IsSink)
604    Eng.WList->enqueue(Succ);
605
606  return Succ;
607}
608
609ExplodedNode*
610SwitchNodeBuilder::generateCaseStmtNode(const iterator &I,
611                                        ProgramStateRef St) {
612  bool IsNew;
613  ExplodedNode *Succ =
614      Eng.G.getNode(BlockEdge(Src, I.getBlock(), Pred->getLocationContext()),
615                    St, false, &IsNew);
616  Succ->addPredecessor(Pred, Eng.G);
617  if (!IsNew)
618    return nullptr;
619
620  Eng.WList->enqueue(Succ);
621  return Succ;
622}
623
624ExplodedNode*
625SwitchNodeBuilder::generateDefaultCaseNode(ProgramStateRef St,
626                                           bool IsSink) {
627  // Get the block for the default case.
628  assert(Src->succ_rbegin() != Src->succ_rend());
629  CFGBlock *DefaultBlock = *Src->succ_rbegin();
630
631  // Sanity check for default blocks that are unreachable and not caught
632  // by earlier stages.
633  if (!DefaultBlock)
634    return nullptr;
635
636  bool IsNew;
637  ExplodedNode *Succ =
638      Eng.G.getNode(BlockEdge(Src, DefaultBlock, Pred->getLocationContext()),
639                    St, IsSink, &IsNew);
640  Succ->addPredecessor(Pred, Eng.G);
641
642  if (!IsNew)
643    return nullptr;
644
645  if (!IsSink)
646    Eng.WList->enqueue(Succ);
647
648  return Succ;
649}
650