LiveVariables.cpp revision f3fb5c5395d382ffd24ec2675237fa9bdff6266e 
1#include "clang/Analysis/Analyses/LiveVariables.h"
2#include "clang/Analysis/Analyses/PostOrderCFGView.h"
3
4#include "clang/AST/Stmt.h"
5#include "clang/Analysis/CFG.h"
6#include "clang/Analysis/AnalysisContext.h"
7#include "clang/AST/StmtVisitor.h"
8
9#include "llvm/ADT/PostOrderIterator.h"
10#include "llvm/ADT/DenseMap.h"
11
12#include <deque>
13#include <algorithm>
14#include <vector>
15
16using namespace clang;
17
18namespace {
19
20class DataflowWorklist {
21  SmallVector<const CFGBlock *, 20> worklist;
22  llvm::BitVector enqueuedBlocks;
23  PostOrderCFGView *POV;
24public:
25  DataflowWorklist(const CFG &cfg, AnalysisDeclContext &Ctx)
26    : enqueuedBlocks(cfg.getNumBlockIDs()),
27      POV(Ctx.getAnalysis<PostOrderCFGView>()) {}
28
29  void enqueueBlock(const CFGBlock *block);
30  void enqueueSuccessors(const CFGBlock *block);
31  void enqueuePredecessors(const CFGBlock *block);
32
33  const CFGBlock *dequeue();
34
35  void sortWorklist();
36};
37
38}
39
40void DataflowWorklist::enqueueBlock(const clang::CFGBlock *block) {
41  if (block && !enqueuedBlocks[block->getBlockID()]) {
42    enqueuedBlocks[block->getBlockID()] = true;
43    worklist.push_back(block);
44  }
45}
46
47void DataflowWorklist::enqueueSuccessors(const clang::CFGBlock *block) {
48  const unsigned OldWorklistSize = worklist.size();
49  for (CFGBlock::const_succ_iterator I = block->succ_begin(),
50       E = block->succ_end(); I != E; ++I) {
51    enqueueBlock(*I);
52  }
53
54  if (OldWorklistSize == 0 || OldWorklistSize == worklist.size())
55    return;
56
57  sortWorklist();
58}
59
60void DataflowWorklist::enqueuePredecessors(const clang::CFGBlock *block) {
61  const unsigned OldWorklistSize = worklist.size();
62  for (CFGBlock::const_pred_iterator I = block->pred_begin(),
63       E = block->pred_end(); I != E; ++I) {
64    enqueueBlock(*I);
65  }
66
67  if (OldWorklistSize == 0 || OldWorklistSize == worklist.size())
68    return;
69
70  sortWorklist();
71}
72
73void DataflowWorklist::sortWorklist() {
74  std::sort(worklist.begin(), worklist.end(), POV->getComparator());
75}
76
77const CFGBlock *DataflowWorklist::dequeue() {
78  if (worklist.empty())
79    return 0;
80  const CFGBlock *b = worklist.back();
81  worklist.pop_back();
82  enqueuedBlocks[b->getBlockID()] = false;
83  return b;
84}
85
86namespace {
87class LiveVariablesImpl {
88public:
89  AnalysisDeclContext &analysisContext;
90  std::vector<LiveVariables::LivenessValues> cfgBlockValues;
91  llvm::ImmutableSet<const Stmt *>::Factory SSetFact;
92  llvm::ImmutableSet<const VarDecl *>::Factory DSetFact;
93  llvm::DenseMap<const CFGBlock *, LiveVariables::LivenessValues> blocksEndToLiveness;
94  llvm::DenseMap<const CFGBlock *, LiveVariables::LivenessValues> blocksBeginToLiveness;
95  llvm::DenseMap<const Stmt *, LiveVariables::LivenessValues> stmtsToLiveness;
96  llvm::DenseMap<const DeclRefExpr *, unsigned> inAssignment;
97  const bool killAtAssign;
98
99  LiveVariables::LivenessValues
100  merge(LiveVariables::LivenessValues valsA,
101        LiveVariables::LivenessValues valsB);
102
103  LiveVariables::LivenessValues runOnBlock(const CFGBlock *block,
104                                           LiveVariables::LivenessValues val,
105                                           LiveVariables::Observer *obs = 0);
106
107  void dumpBlockLiveness(const SourceManager& M);
108
109  LiveVariablesImpl(AnalysisDeclContext &ac, bool KillAtAssign)
110    : analysisContext(ac),
111      SSetFact(false), // Do not canonicalize ImmutableSets by default.
112      DSetFact(false), // This is a *major* performance win.
113      killAtAssign(KillAtAssign) {}
114};
115}
116
117static LiveVariablesImpl &getImpl(void *x) {
118  return *((LiveVariablesImpl *) x);
119}
120
121//===----------------------------------------------------------------------===//
122// Operations and queries on LivenessValues.
123//===----------------------------------------------------------------------===//
124
125bool LiveVariables::LivenessValues::isLive(const Stmt *S) const {
126  return liveStmts.contains(S);
127}
128
129bool LiveVariables::LivenessValues::isLive(const VarDecl *D) const {
130  return liveDecls.contains(D);
131}
132
133namespace {
134  template <typename SET>
135  SET mergeSets(SET A, SET B) {
136    if (A.isEmpty())
137      return B;
138
139    for (typename SET::iterator it = B.begin(), ei = B.end(); it != ei; ++it) {
140      A = A.add(*it);
141    }
142    return A;
143  }
144}
145
146LiveVariables::LivenessValues
147LiveVariablesImpl::merge(LiveVariables::LivenessValues valsA,
148                         LiveVariables::LivenessValues valsB) {
149
150  llvm::ImmutableSetRef<const Stmt *>
151    SSetRefA(valsA.liveStmts.getRootWithoutRetain(), SSetFact.getTreeFactory()),
152    SSetRefB(valsB.liveStmts.getRootWithoutRetain(), SSetFact.getTreeFactory());
153
154
155  llvm::ImmutableSetRef<const VarDecl *>
156    DSetRefA(valsA.liveDecls.getRootWithoutRetain(), DSetFact.getTreeFactory()),
157    DSetRefB(valsB.liveDecls.getRootWithoutRetain(), DSetFact.getTreeFactory());
158
159
160  SSetRefA = mergeSets(SSetRefA, SSetRefB);
161  DSetRefA = mergeSets(DSetRefA, DSetRefB);
162
163  // asImmutableSet() canonicalizes the tree, allowing us to do an easy
164  // comparison afterwards.
165  return LiveVariables::LivenessValues(SSetRefA.asImmutableSet(),
166                                       DSetRefA.asImmutableSet());
167}
168
169bool LiveVariables::LivenessValues::equals(const LivenessValues &V) const {
170  return liveStmts == V.liveStmts && liveDecls == V.liveDecls;
171}
172
173//===----------------------------------------------------------------------===//
174// Query methods.
175//===----------------------------------------------------------------------===//
176
177static bool isAlwaysAlive(const VarDecl *D) {
178  return D->hasGlobalStorage();
179}
180
181bool LiveVariables::isLive(const CFGBlock *B, const VarDecl *D) {
182  return isAlwaysAlive(D) || getImpl(impl).blocksEndToLiveness[B].isLive(D);
183}
184
185bool LiveVariables::isLive(const Stmt *S, const VarDecl *D) {
186  return isAlwaysAlive(D) || getImpl(impl).stmtsToLiveness[S].isLive(D);
187}
188
189bool LiveVariables::isLive(const Stmt *Loc, const Stmt *S) {
190  return getImpl(impl).stmtsToLiveness[Loc].isLive(S);
191}
192
193//===----------------------------------------------------------------------===//
194// Dataflow computation.
195//===----------------------------------------------------------------------===//
196
197namespace {
198class TransferFunctions : public StmtVisitor<TransferFunctions> {
199  LiveVariablesImpl &LV;
200  LiveVariables::LivenessValues &val;
201  LiveVariables::Observer *observer;
202  const CFGBlock *currentBlock;
203public:
204  TransferFunctions(LiveVariablesImpl &im,
205                    LiveVariables::LivenessValues &Val,
206                    LiveVariables::Observer *Observer,
207                    const CFGBlock *CurrentBlock)
208  : LV(im), val(Val), observer(Observer), currentBlock(CurrentBlock) {}
209
210  void VisitBinaryOperator(BinaryOperator *BO);
211  void VisitBlockExpr(BlockExpr *BE);
212  void VisitDeclRefExpr(DeclRefExpr *DR);
213  void VisitDeclStmt(DeclStmt *DS);
214  void VisitObjCForCollectionStmt(ObjCForCollectionStmt *OS);
215  void VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *UE);
216  void VisitUnaryOperator(UnaryOperator *UO);
217  void Visit(Stmt *S);
218};
219}
220
221static const VariableArrayType *FindVA(QualType Ty) {
222  const Type *ty = Ty.getTypePtr();
223  while (const ArrayType *VT = dyn_cast<ArrayType>(ty)) {
224    if (const VariableArrayType *VAT = dyn_cast<VariableArrayType>(VT))
225      if (VAT->getSizeExpr())
226        return VAT;
227
228    ty = VT->getElementType().getTypePtr();
229  }
230
231  return 0;
232}
233
234static const Stmt *LookThroughStmt(const Stmt *S) {
235  while (S) {
236    if (const Expr *Ex = dyn_cast<Expr>(S))
237      S = Ex->IgnoreParens();
238    if (const ExprWithCleanups *EWC = dyn_cast<ExprWithCleanups>(S)) {
239      S = EWC->getSubExpr();
240      continue;
241    }
242    if (const OpaqueValueExpr *OVE = dyn_cast<OpaqueValueExpr>(S)) {
243      S = OVE->getSourceExpr();
244      continue;
245    }
246    break;
247  }
248  return S;
249}
250
251static void AddLiveStmt(llvm::ImmutableSet<const Stmt *> &Set,
252                        llvm::ImmutableSet<const Stmt *>::Factory &F,
253                        const Stmt *S) {
254  Set = F.add(Set, LookThroughStmt(S));
255}
256
257void TransferFunctions::Visit(Stmt *S) {
258  if (observer)
259    observer->observeStmt(S, currentBlock, val);
260
261  StmtVisitor<TransferFunctions>::Visit(S);
262
263  if (isa<Expr>(S)) {
264    val.liveStmts = LV.SSetFact.remove(val.liveStmts, S);
265  }
266
267  // Mark all children expressions live.
268
269  switch (S->getStmtClass()) {
270    default:
271      break;
272    case Stmt::StmtExprClass: {
273      // For statement expressions, look through the compound statement.
274      S = cast<StmtExpr>(S)->getSubStmt();
275      break;
276    }
277    case Stmt::CXXMemberCallExprClass: {
278      // Include the implicit "this" pointer as being live.
279      CXXMemberCallExpr *CE = cast<CXXMemberCallExpr>(S);
280      if (Expr *ImplicitObj = CE->getImplicitObjectArgument()) {
281        AddLiveStmt(val.liveStmts, LV.SSetFact, ImplicitObj);
282      }
283      break;
284    }
285    case Stmt::DeclStmtClass: {
286      const DeclStmt *DS = cast<DeclStmt>(S);
287      if (const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl())) {
288        for (const VariableArrayType* VA = FindVA(VD->getType());
289             VA != 0; VA = FindVA(VA->getElementType())) {
290          AddLiveStmt(val.liveStmts, LV.SSetFact, VA->getSizeExpr());
291        }
292      }
293      break;
294    }
295    case Stmt::PseudoObjectExprClass: {
296      // A pseudo-object operation only directly consumes its result
297      // expression.
298      Expr *child = cast<PseudoObjectExpr>(S)->getResultExpr();
299      if (!child) return;
300      if (OpaqueValueExpr *OV = dyn_cast<OpaqueValueExpr>(child))
301        child = OV->getSourceExpr();
302      child = child->IgnoreParens();
303      val.liveStmts = LV.SSetFact.add(val.liveStmts, child);
304      return;
305    }
306
307    // FIXME: These cases eventually shouldn't be needed.
308    case Stmt::ExprWithCleanupsClass: {
309      S = cast<ExprWithCleanups>(S)->getSubExpr();
310      break;
311    }
312    case Stmt::CXXBindTemporaryExprClass: {
313      S = cast<CXXBindTemporaryExpr>(S)->getSubExpr();
314      break;
315    }
316    case Stmt::UnaryExprOrTypeTraitExprClass: {
317      // No need to unconditionally visit subexpressions.
318      return;
319    }
320  }
321
322  for (Stmt::child_iterator it = S->child_begin(), ei = S->child_end();
323       it != ei; ++it) {
324    if (Stmt *child = *it)
325      AddLiveStmt(val.liveStmts, LV.SSetFact, child);
326  }
327}
328
329void TransferFunctions::VisitBinaryOperator(BinaryOperator *B) {
330  if (B->isAssignmentOp()) {
331    if (!LV.killAtAssign)
332      return;
333
334    // Assigning to a variable?
335    Expr *LHS = B->getLHS()->IgnoreParens();
336
337    if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(LHS))
338      if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
339        // Assignments to references don't kill the ref's address
340        if (VD->getType()->isReferenceType())
341          return;
342
343        if (!isAlwaysAlive(VD)) {
344          // The variable is now dead.
345          val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
346        }
347
348        if (observer)
349          observer->observerKill(DR);
350      }
351  }
352}
353
354void TransferFunctions::VisitBlockExpr(BlockExpr *BE) {
355  AnalysisDeclContext::referenced_decls_iterator I, E;
356  llvm::tie(I, E) =
357    LV.analysisContext.getReferencedBlockVars(BE->getBlockDecl());
358  for ( ; I != E ; ++I) {
359    const VarDecl *VD = *I;
360    if (isAlwaysAlive(VD))
361      continue;
362    val.liveDecls = LV.DSetFact.add(val.liveDecls, VD);
363  }
364}
365
366void TransferFunctions::VisitDeclRefExpr(DeclRefExpr *DR) {
367  if (const VarDecl *D = dyn_cast<VarDecl>(DR->getDecl()))
368    if (!isAlwaysAlive(D) && LV.inAssignment.find(DR) == LV.inAssignment.end())
369      val.liveDecls = LV.DSetFact.add(val.liveDecls, D);
370}
371
372void TransferFunctions::VisitDeclStmt(DeclStmt *DS) {
373  for (DeclStmt::decl_iterator DI=DS->decl_begin(), DE = DS->decl_end();
374       DI != DE; ++DI)
375    if (VarDecl *VD = dyn_cast<VarDecl>(*DI)) {
376      if (!isAlwaysAlive(VD))
377        val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
378    }
379}
380
381void TransferFunctions::VisitObjCForCollectionStmt(ObjCForCollectionStmt *OS) {
382  // Kill the iteration variable.
383  DeclRefExpr *DR = 0;
384  const VarDecl *VD = 0;
385
386  Stmt *element = OS->getElement();
387  if (DeclStmt *DS = dyn_cast<DeclStmt>(element)) {
388    VD = cast<VarDecl>(DS->getSingleDecl());
389  }
390  else if ((DR = dyn_cast<DeclRefExpr>(cast<Expr>(element)->IgnoreParens()))) {
391    VD = cast<VarDecl>(DR->getDecl());
392  }
393
394  if (VD) {
395    val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
396    if (observer && DR)
397      observer->observerKill(DR);
398  }
399}
400
401void TransferFunctions::
402VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *UE)
403{
404  // While sizeof(var) doesn't technically extend the liveness of 'var', it
405  // does extent the liveness of metadata if 'var' is a VariableArrayType.
406  // We handle that special case here.
407  if (UE->getKind() != UETT_SizeOf || UE->isArgumentType())
408    return;
409
410  const Expr *subEx = UE->getArgumentExpr();
411  if (subEx->getType()->isVariableArrayType()) {
412    assert(subEx->isLValue());
413    val.liveStmts = LV.SSetFact.add(val.liveStmts, subEx->IgnoreParens());
414  }
415}
416
417void TransferFunctions::VisitUnaryOperator(UnaryOperator *UO) {
418  // Treat ++/-- as a kill.
419  // Note we don't actually have to do anything if we don't have an observer,
420  // since a ++/-- acts as both a kill and a "use".
421  if (!observer)
422    return;
423
424  switch (UO->getOpcode()) {
425  default:
426    return;
427  case UO_PostInc:
428  case UO_PostDec:
429  case UO_PreInc:
430  case UO_PreDec:
431    break;
432  }
433
434  if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(UO->getSubExpr()->IgnoreParens()))
435    if (isa<VarDecl>(DR->getDecl())) {
436      // Treat ++/-- as a kill.
437      observer->observerKill(DR);
438    }
439}
440
441LiveVariables::LivenessValues
442LiveVariablesImpl::runOnBlock(const CFGBlock *block,
443                              LiveVariables::LivenessValues val,
444                              LiveVariables::Observer *obs) {
445
446  TransferFunctions TF(*this, val, obs, block);
447
448  // Visit the terminator (if any).
449  if (const Stmt *term = block->getTerminator())
450    TF.Visit(const_cast<Stmt*>(term));
451
452  // Apply the transfer function for all Stmts in the block.
453  for (CFGBlock::const_reverse_iterator it = block->rbegin(),
454       ei = block->rend(); it != ei; ++it) {
455    const CFGElement &elem = *it;
456    if (!isa<CFGStmt>(elem))
457      continue;
458
459    const Stmt *S = cast<CFGStmt>(elem).getStmt();
460    TF.Visit(const_cast<Stmt*>(S));
461    stmtsToLiveness[S] = val;
462  }
463  return val;
464}
465
466void LiveVariables::runOnAllBlocks(LiveVariables::Observer &obs) {
467  const CFG *cfg = getImpl(impl).analysisContext.getCFG();
468  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it)
469    getImpl(impl).runOnBlock(*it, getImpl(impl).blocksEndToLiveness[*it], &obs);
470}
471
472LiveVariables::LiveVariables(void *im) : impl(im) {}
473
474LiveVariables::~LiveVariables() {
475  delete (LiveVariablesImpl*) impl;
476}
477
478LiveVariables *
479LiveVariables::computeLiveness(AnalysisDeclContext &AC,
480                                 bool killAtAssign) {
481
482  // No CFG?  Bail out.
483  CFG *cfg = AC.getCFG();
484  if (!cfg)
485    return 0;
486
487  LiveVariablesImpl *LV = new LiveVariablesImpl(AC, killAtAssign);
488
489  // Construct the dataflow worklist.  Enqueue the exit block as the
490  // start of the analysis.
491  DataflowWorklist worklist(*cfg, AC);
492  llvm::BitVector everAnalyzedBlock(cfg->getNumBlockIDs());
493
494  // FIXME: we should enqueue using post order.
495  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it) {
496    const CFGBlock *block = *it;
497    worklist.enqueueBlock(block);
498
499    // FIXME: Scan for DeclRefExprs using in the LHS of an assignment.
500    // We need to do this because we lack context in the reverse analysis
501    // to determine if a DeclRefExpr appears in such a context, and thus
502    // doesn't constitute a "use".
503    if (killAtAssign)
504      for (CFGBlock::const_iterator bi = block->begin(), be = block->end();
505           bi != be; ++bi) {
506        if (const CFGStmt *cs = bi->getAs<CFGStmt>()) {
507          if (const BinaryOperator *BO = dyn_cast<BinaryOperator>(cs->getStmt())) {
508            if (BO->getOpcode() == BO_Assign) {
509              if (const DeclRefExpr *DR =
510                    dyn_cast<DeclRefExpr>(BO->getLHS()->IgnoreParens())) {
511                LV->inAssignment[DR] = 1;
512              }
513            }
514          }
515        }
516      }
517  }
518
519  worklist.sortWorklist();
520
521  while (const CFGBlock *block = worklist.dequeue()) {
522    // Determine if the block's end value has changed.  If not, we
523    // have nothing left to do for this block.
524    LivenessValues &prevVal = LV->blocksEndToLiveness[block];
525
526    // Merge the values of all successor blocks.
527    LivenessValues val;
528    for (CFGBlock::const_succ_iterator it = block->succ_begin(),
529                                       ei = block->succ_end(); it != ei; ++it) {
530      if (const CFGBlock *succ = *it) {
531        val = LV->merge(val, LV->blocksBeginToLiveness[succ]);
532      }
533    }
534
535    if (!everAnalyzedBlock[block->getBlockID()])
536      everAnalyzedBlock[block->getBlockID()] = true;
537    else if (prevVal.equals(val))
538      continue;
539
540    prevVal = val;
541
542    // Update the dataflow value for the start of this block.
543    LV->blocksBeginToLiveness[block] = LV->runOnBlock(block, val);
544
545    // Enqueue the value to the predecessors.
546    worklist.enqueuePredecessors(block);
547  }
548
549  return new LiveVariables(LV);
550}
551
552static bool compare_entries(const CFGBlock *A, const CFGBlock *B) {
553  return A->getBlockID() < B->getBlockID();
554}
555
556static bool compare_vd_entries(const Decl *A, const Decl *B) {
557  SourceLocation ALoc = A->getLocStart();
558  SourceLocation BLoc = B->getLocStart();
559  return ALoc.getRawEncoding() < BLoc.getRawEncoding();
560}
561
562void LiveVariables::dumpBlockLiveness(const SourceManager &M) {
563  getImpl(impl).dumpBlockLiveness(M);
564}
565
566void LiveVariablesImpl::dumpBlockLiveness(const SourceManager &M) {
567  std::vector<const CFGBlock *> vec;
568  for (llvm::DenseMap<const CFGBlock *, LiveVariables::LivenessValues>::iterator
569       it = blocksEndToLiveness.begin(), ei = blocksEndToLiveness.end();
570       it != ei; ++it) {
571    vec.push_back(it->first);
572  }
573  std::sort(vec.begin(), vec.end(), compare_entries);
574
575  std::vector<const VarDecl*> declVec;
576
577  for (std::vector<const CFGBlock *>::iterator
578        it = vec.begin(), ei = vec.end(); it != ei; ++it) {
579    llvm::errs() << "\n[ B" << (*it)->getBlockID()
580                 << " (live variables at block exit) ]\n";
581
582    LiveVariables::LivenessValues vals = blocksEndToLiveness[*it];
583    declVec.clear();
584
585    for (llvm::ImmutableSet<const VarDecl *>::iterator si =
586          vals.liveDecls.begin(),
587          se = vals.liveDecls.end(); si != se; ++si) {
588      declVec.push_back(*si);
589    }
590
591    std::sort(declVec.begin(), declVec.end(), compare_vd_entries);
592
593    for (std::vector<const VarDecl*>::iterator di = declVec.begin(),
594         de = declVec.end(); di != de; ++di) {
595      llvm::errs() << " " << (*di)->getDeclName().getAsString()
596                   << " <";
597      (*di)->getLocation().dump(M);
598      llvm::errs() << ">\n";
599    }
600  }
601  llvm::errs() << "\n";
602}
603
604const void *LiveVariables::getTag() { static int x; return &x; }
605const void *RelaxedLiveVariables::getTag() { static int x; return &x; }
606