LiveVariables.cpp revision 51e6c0d7596325c33919a589a308c3c1ff07baed 
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  if (const OpaqueValueExpr *OVE = dyn_cast<OpaqueValueExpr>(S))
236    return OVE->getSourceExpr()->IgnoreParens();
237  if (const Expr *E = dyn_cast<Expr>(S))
238    return E->IgnoreParens();
239  return S;
240}
241
242static void AddLiveStmt(llvm::ImmutableSet<const Stmt *> &Set,
243                        llvm::ImmutableSet<const Stmt *>::Factory &F,
244                        const Stmt *S) {
245  Set = F.add(Set, LookThroughStmt(S));
246}
247
248void TransferFunctions::Visit(Stmt *S) {
249  if (observer)
250    observer->observeStmt(S, currentBlock, val);
251
252  StmtVisitor<TransferFunctions>::Visit(S);
253
254  if (isa<Expr>(S)) {
255    val.liveStmts = LV.SSetFact.remove(val.liveStmts, S);
256  }
257
258  // Mark all children expressions live.
259
260  switch (S->getStmtClass()) {
261    default:
262      break;
263    case Stmt::StmtExprClass: {
264      // For statement expressions, look through the compound statement.
265      S = cast<StmtExpr>(S)->getSubStmt();
266      break;
267    }
268    case Stmt::CXXMemberCallExprClass: {
269      // Include the implicit "this" pointer as being live.
270      CXXMemberCallExpr *CE = cast<CXXMemberCallExpr>(S);
271      if (Expr *ImplicitObj = CE->getImplicitObjectArgument()) {
272        AddLiveStmt(val.liveStmts, LV.SSetFact, ImplicitObj);
273      }
274      break;
275    }
276    case Stmt::DeclStmtClass: {
277      const DeclStmt *DS = cast<DeclStmt>(S);
278      if (const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl())) {
279        for (const VariableArrayType* VA = FindVA(VD->getType());
280             VA != 0; VA = FindVA(VA->getElementType())) {
281          AddLiveStmt(val.liveStmts, LV.SSetFact, VA->getSizeExpr());
282        }
283      }
284      break;
285    }
286    // FIXME: These cases eventually shouldn't be needed.
287    case Stmt::ExprWithCleanupsClass: {
288      S = cast<ExprWithCleanups>(S)->getSubExpr();
289      break;
290    }
291    case Stmt::CXXBindTemporaryExprClass: {
292      S = cast<CXXBindTemporaryExpr>(S)->getSubExpr();
293      break;
294    }
295    case Stmt::UnaryExprOrTypeTraitExprClass: {
296      // No need to unconditionally visit subexpressions.
297      return;
298    }
299  }
300
301  for (Stmt::child_iterator it = S->child_begin(), ei = S->child_end();
302       it != ei; ++it) {
303    if (Stmt *child = *it)
304      AddLiveStmt(val.liveStmts, LV.SSetFact, child);
305  }
306}
307
308void TransferFunctions::VisitBinaryOperator(BinaryOperator *B) {
309  if (B->isAssignmentOp()) {
310    if (!LV.killAtAssign)
311      return;
312
313    // Assigning to a variable?
314    Expr *LHS = B->getLHS()->IgnoreParens();
315
316    if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(LHS))
317      if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
318        // Assignments to references don't kill the ref's address
319        if (VD->getType()->isReferenceType())
320          return;
321
322        if (!isAlwaysAlive(VD)) {
323          // The variable is now dead.
324          val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
325        }
326
327        if (observer)
328          observer->observerKill(DR);
329      }
330  }
331}
332
333void TransferFunctions::VisitBlockExpr(BlockExpr *BE) {
334  AnalysisDeclContext::referenced_decls_iterator I, E;
335  llvm::tie(I, E) =
336    LV.analysisContext.getReferencedBlockVars(BE->getBlockDecl());
337  for ( ; I != E ; ++I) {
338    const VarDecl *VD = *I;
339    if (isAlwaysAlive(VD))
340      continue;
341    val.liveDecls = LV.DSetFact.add(val.liveDecls, VD);
342  }
343}
344
345void TransferFunctions::VisitDeclRefExpr(DeclRefExpr *DR) {
346  if (const VarDecl *D = dyn_cast<VarDecl>(DR->getDecl()))
347    if (!isAlwaysAlive(D) && LV.inAssignment.find(DR) == LV.inAssignment.end())
348      val.liveDecls = LV.DSetFact.add(val.liveDecls, D);
349}
350
351void TransferFunctions::VisitDeclStmt(DeclStmt *DS) {
352  for (DeclStmt::decl_iterator DI=DS->decl_begin(), DE = DS->decl_end();
353       DI != DE; ++DI)
354    if (VarDecl *VD = dyn_cast<VarDecl>(*DI)) {
355      if (!isAlwaysAlive(VD))
356        val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
357    }
358}
359
360void TransferFunctions::VisitObjCForCollectionStmt(ObjCForCollectionStmt *OS) {
361  // Kill the iteration variable.
362  DeclRefExpr *DR = 0;
363  const VarDecl *VD = 0;
364
365  Stmt *element = OS->getElement();
366  if (DeclStmt *DS = dyn_cast<DeclStmt>(element)) {
367    VD = cast<VarDecl>(DS->getSingleDecl());
368  }
369  else if ((DR = dyn_cast<DeclRefExpr>(cast<Expr>(element)->IgnoreParens()))) {
370    VD = cast<VarDecl>(DR->getDecl());
371  }
372
373  if (VD) {
374    val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
375    if (observer && DR)
376      observer->observerKill(DR);
377  }
378}
379
380void TransferFunctions::
381VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *UE)
382{
383  // While sizeof(var) doesn't technically extend the liveness of 'var', it
384  // does extent the liveness of metadata if 'var' is a VariableArrayType.
385  // We handle that special case here.
386  if (UE->getKind() != UETT_SizeOf || UE->isArgumentType())
387    return;
388
389  const Expr *subEx = UE->getArgumentExpr();
390  if (subEx->getType()->isVariableArrayType()) {
391    assert(subEx->isLValue());
392    val.liveStmts = LV.SSetFact.add(val.liveStmts, subEx->IgnoreParens());
393  }
394}
395
396void TransferFunctions::VisitUnaryOperator(UnaryOperator *UO) {
397  // Treat ++/-- as a kill.
398  // Note we don't actually have to do anything if we don't have an observer,
399  // since a ++/-- acts as both a kill and a "use".
400  if (!observer)
401    return;
402
403  switch (UO->getOpcode()) {
404  default:
405    return;
406  case UO_PostInc:
407  case UO_PostDec:
408  case UO_PreInc:
409  case UO_PreDec:
410    break;
411  }
412
413  if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(UO->getSubExpr()->IgnoreParens()))
414    if (isa<VarDecl>(DR->getDecl())) {
415      // Treat ++/-- as a kill.
416      observer->observerKill(DR);
417    }
418}
419
420LiveVariables::LivenessValues
421LiveVariablesImpl::runOnBlock(const CFGBlock *block,
422                              LiveVariables::LivenessValues val,
423                              LiveVariables::Observer *obs) {
424
425  TransferFunctions TF(*this, val, obs, block);
426
427  // Visit the terminator (if any).
428  if (const Stmt *term = block->getTerminator())
429    TF.Visit(const_cast<Stmt*>(term));
430
431  // Apply the transfer function for all Stmts in the block.
432  for (CFGBlock::const_reverse_iterator it = block->rbegin(),
433       ei = block->rend(); it != ei; ++it) {
434    const CFGElement &elem = *it;
435    if (!isa<CFGStmt>(elem))
436      continue;
437
438    const Stmt *S = cast<CFGStmt>(elem).getStmt();
439    TF.Visit(const_cast<Stmt*>(S));
440    stmtsToLiveness[S] = val;
441  }
442  return val;
443}
444
445void LiveVariables::runOnAllBlocks(LiveVariables::Observer &obs) {
446  const CFG *cfg = getImpl(impl).analysisContext.getCFG();
447  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it)
448    getImpl(impl).runOnBlock(*it, getImpl(impl).blocksEndToLiveness[*it], &obs);
449}
450
451LiveVariables::LiveVariables(void *im) : impl(im) {}
452
453LiveVariables::~LiveVariables() {
454  delete (LiveVariablesImpl*) impl;
455}
456
457LiveVariables *
458LiveVariables::computeLiveness(AnalysisDeclContext &AC,
459                                 bool killAtAssign) {
460
461  // No CFG?  Bail out.
462  CFG *cfg = AC.getCFG();
463  if (!cfg)
464    return 0;
465
466  LiveVariablesImpl *LV = new LiveVariablesImpl(AC, killAtAssign);
467
468  // Construct the dataflow worklist.  Enqueue the exit block as the
469  // start of the analysis.
470  DataflowWorklist worklist(*cfg, AC);
471  llvm::BitVector everAnalyzedBlock(cfg->getNumBlockIDs());
472
473  // FIXME: we should enqueue using post order.
474  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it) {
475    const CFGBlock *block = *it;
476    worklist.enqueueBlock(block);
477
478    // FIXME: Scan for DeclRefExprs using in the LHS of an assignment.
479    // We need to do this because we lack context in the reverse analysis
480    // to determine if a DeclRefExpr appears in such a context, and thus
481    // doesn't constitute a "use".
482    if (killAtAssign)
483      for (CFGBlock::const_iterator bi = block->begin(), be = block->end();
484           bi != be; ++bi) {
485        if (const CFGStmt *cs = bi->getAs<CFGStmt>()) {
486          if (const BinaryOperator *BO = dyn_cast<BinaryOperator>(cs->getStmt())) {
487            if (BO->getOpcode() == BO_Assign) {
488              if (const DeclRefExpr *DR =
489                    dyn_cast<DeclRefExpr>(BO->getLHS()->IgnoreParens())) {
490                LV->inAssignment[DR] = 1;
491              }
492            }
493          }
494        }
495      }
496  }
497
498  worklist.sortWorklist();
499
500  while (const CFGBlock *block = worklist.dequeue()) {
501    // Determine if the block's end value has changed.  If not, we
502    // have nothing left to do for this block.
503    LivenessValues &prevVal = LV->blocksEndToLiveness[block];
504
505    // Merge the values of all successor blocks.
506    LivenessValues val;
507    for (CFGBlock::const_succ_iterator it = block->succ_begin(),
508                                       ei = block->succ_end(); it != ei; ++it) {
509      if (const CFGBlock *succ = *it) {
510        val = LV->merge(val, LV->blocksBeginToLiveness[succ]);
511      }
512    }
513
514    if (!everAnalyzedBlock[block->getBlockID()])
515      everAnalyzedBlock[block->getBlockID()] = true;
516    else if (prevVal.equals(val))
517      continue;
518
519    prevVal = val;
520
521    // Update the dataflow value for the start of this block.
522    LV->blocksBeginToLiveness[block] = LV->runOnBlock(block, val);
523
524    // Enqueue the value to the predecessors.
525    worklist.enqueuePredecessors(block);
526  }
527
528  return new LiveVariables(LV);
529}
530
531static bool compare_entries(const CFGBlock *A, const CFGBlock *B) {
532  return A->getBlockID() < B->getBlockID();
533}
534
535static bool compare_vd_entries(const Decl *A, const Decl *B) {
536  SourceLocation ALoc = A->getLocStart();
537  SourceLocation BLoc = B->getLocStart();
538  return ALoc.getRawEncoding() < BLoc.getRawEncoding();
539}
540
541void LiveVariables::dumpBlockLiveness(const SourceManager &M) {
542  getImpl(impl).dumpBlockLiveness(M);
543}
544
545void LiveVariablesImpl::dumpBlockLiveness(const SourceManager &M) {
546  std::vector<const CFGBlock *> vec;
547  for (llvm::DenseMap<const CFGBlock *, LiveVariables::LivenessValues>::iterator
548       it = blocksEndToLiveness.begin(), ei = blocksEndToLiveness.end();
549       it != ei; ++it) {
550    vec.push_back(it->first);
551  }
552  std::sort(vec.begin(), vec.end(), compare_entries);
553
554  std::vector<const VarDecl*> declVec;
555
556  for (std::vector<const CFGBlock *>::iterator
557        it = vec.begin(), ei = vec.end(); it != ei; ++it) {
558    llvm::errs() << "\n[ B" << (*it)->getBlockID()
559                 << " (live variables at block exit) ]\n";
560
561    LiveVariables::LivenessValues vals = blocksEndToLiveness[*it];
562    declVec.clear();
563
564    for (llvm::ImmutableSet<const VarDecl *>::iterator si =
565          vals.liveDecls.begin(),
566          se = vals.liveDecls.end(); si != se; ++si) {
567      declVec.push_back(*si);
568    }
569
570    std::sort(declVec.begin(), declVec.end(), compare_vd_entries);
571
572    for (std::vector<const VarDecl*>::iterator di = declVec.begin(),
573         de = declVec.end(); di != de; ++di) {
574      llvm::errs() << " " << (*di)->getDeclName().getAsString()
575                   << " <";
576      (*di)->getLocation().dump(M);
577      llvm::errs() << ">\n";
578    }
579  }
580  llvm::errs() << "\n";
581}
582
583const void *LiveVariables::getTag() { static int x; return &x; }
584const void *RelaxedLiveVariables::getTag() { static int x; return &x; }
585