LiveVariables.cpp revision 16fdb0488caf3960eda2dadc9b2fb87dd160bba9 
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 OpaqueValueExpr *OVE = dyn_cast<OpaqueValueExpr>(S)) {
239      S = OVE->getSourceExpr();
240      continue;
241    }
242    break;
243  }
244  return S;
245}
246
247static void AddLiveStmt(llvm::ImmutableSet<const Stmt *> &Set,
248                        llvm::ImmutableSet<const Stmt *>::Factory &F,
249                        const Stmt *S) {
250  Set = F.add(Set, LookThroughStmt(S));
251}
252
253void TransferFunctions::Visit(Stmt *S) {
254  if (observer)
255    observer->observeStmt(S, currentBlock, val);
256
257  StmtVisitor<TransferFunctions>::Visit(S);
258
259  if (isa<Expr>(S)) {
260    val.liveStmts = LV.SSetFact.remove(val.liveStmts, S);
261  }
262
263  // Mark all children expressions live.
264
265  switch (S->getStmtClass()) {
266    default:
267      break;
268    case Stmt::StmtExprClass: {
269      // For statement expressions, look through the compound statement.
270      S = cast<StmtExpr>(S)->getSubStmt();
271      break;
272    }
273    case Stmt::CXXMemberCallExprClass: {
274      // Include the implicit "this" pointer as being live.
275      CXXMemberCallExpr *CE = cast<CXXMemberCallExpr>(S);
276      if (Expr *ImplicitObj = CE->getImplicitObjectArgument()) {
277        AddLiveStmt(val.liveStmts, LV.SSetFact, ImplicitObj);
278      }
279      break;
280    }
281    case Stmt::DeclStmtClass: {
282      const DeclStmt *DS = cast<DeclStmt>(S);
283      if (const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl())) {
284        for (const VariableArrayType* VA = FindVA(VD->getType());
285             VA != 0; VA = FindVA(VA->getElementType())) {
286          AddLiveStmt(val.liveStmts, LV.SSetFact, VA->getSizeExpr());
287        }
288      }
289      break;
290    }
291    // FIXME: These cases eventually shouldn't be needed.
292    case Stmt::ExprWithCleanupsClass: {
293      S = cast<ExprWithCleanups>(S)->getSubExpr();
294      break;
295    }
296    case Stmt::CXXBindTemporaryExprClass: {
297      S = cast<CXXBindTemporaryExpr>(S)->getSubExpr();
298      break;
299    }
300    case Stmt::UnaryExprOrTypeTraitExprClass: {
301      // No need to unconditionally visit subexpressions.
302      return;
303    }
304  }
305
306  for (Stmt::child_iterator it = S->child_begin(), ei = S->child_end();
307       it != ei; ++it) {
308    if (Stmt *child = *it)
309      AddLiveStmt(val.liveStmts, LV.SSetFact, child);
310  }
311}
312
313void TransferFunctions::VisitBinaryOperator(BinaryOperator *B) {
314  if (B->isAssignmentOp()) {
315    if (!LV.killAtAssign)
316      return;
317
318    // Assigning to a variable?
319    Expr *LHS = B->getLHS()->IgnoreParens();
320
321    if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(LHS))
322      if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
323        // Assignments to references don't kill the ref's address
324        if (VD->getType()->isReferenceType())
325          return;
326
327        if (!isAlwaysAlive(VD)) {
328          // The variable is now dead.
329          val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
330        }
331
332        if (observer)
333          observer->observerKill(DR);
334      }
335  }
336}
337
338void TransferFunctions::VisitBlockExpr(BlockExpr *BE) {
339  AnalysisDeclContext::referenced_decls_iterator I, E;
340  llvm::tie(I, E) =
341    LV.analysisContext.getReferencedBlockVars(BE->getBlockDecl());
342  for ( ; I != E ; ++I) {
343    const VarDecl *VD = *I;
344    if (isAlwaysAlive(VD))
345      continue;
346    val.liveDecls = LV.DSetFact.add(val.liveDecls, VD);
347  }
348}
349
350void TransferFunctions::VisitDeclRefExpr(DeclRefExpr *DR) {
351  if (const VarDecl *D = dyn_cast<VarDecl>(DR->getDecl()))
352    if (!isAlwaysAlive(D) && LV.inAssignment.find(DR) == LV.inAssignment.end())
353      val.liveDecls = LV.DSetFact.add(val.liveDecls, D);
354}
355
356void TransferFunctions::VisitDeclStmt(DeclStmt *DS) {
357  for (DeclStmt::decl_iterator DI=DS->decl_begin(), DE = DS->decl_end();
358       DI != DE; ++DI)
359    if (VarDecl *VD = dyn_cast<VarDecl>(*DI)) {
360      if (!isAlwaysAlive(VD))
361        val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
362    }
363}
364
365void TransferFunctions::VisitObjCForCollectionStmt(ObjCForCollectionStmt *OS) {
366  // Kill the iteration variable.
367  DeclRefExpr *DR = 0;
368  const VarDecl *VD = 0;
369
370  Stmt *element = OS->getElement();
371  if (DeclStmt *DS = dyn_cast<DeclStmt>(element)) {
372    VD = cast<VarDecl>(DS->getSingleDecl());
373  }
374  else if ((DR = dyn_cast<DeclRefExpr>(cast<Expr>(element)->IgnoreParens()))) {
375    VD = cast<VarDecl>(DR->getDecl());
376  }
377
378  if (VD) {
379    val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
380    if (observer && DR)
381      observer->observerKill(DR);
382  }
383}
384
385void TransferFunctions::
386VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *UE)
387{
388  // While sizeof(var) doesn't technically extend the liveness of 'var', it
389  // does extent the liveness of metadata if 'var' is a VariableArrayType.
390  // We handle that special case here.
391  if (UE->getKind() != UETT_SizeOf || UE->isArgumentType())
392    return;
393
394  const Expr *subEx = UE->getArgumentExpr();
395  if (subEx->getType()->isVariableArrayType()) {
396    assert(subEx->isLValue());
397    val.liveStmts = LV.SSetFact.add(val.liveStmts, subEx->IgnoreParens());
398  }
399}
400
401void TransferFunctions::VisitUnaryOperator(UnaryOperator *UO) {
402  // Treat ++/-- as a kill.
403  // Note we don't actually have to do anything if we don't have an observer,
404  // since a ++/-- acts as both a kill and a "use".
405  if (!observer)
406    return;
407
408  switch (UO->getOpcode()) {
409  default:
410    return;
411  case UO_PostInc:
412  case UO_PostDec:
413  case UO_PreInc:
414  case UO_PreDec:
415    break;
416  }
417
418  if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(UO->getSubExpr()->IgnoreParens()))
419    if (isa<VarDecl>(DR->getDecl())) {
420      // Treat ++/-- as a kill.
421      observer->observerKill(DR);
422    }
423}
424
425LiveVariables::LivenessValues
426LiveVariablesImpl::runOnBlock(const CFGBlock *block,
427                              LiveVariables::LivenessValues val,
428                              LiveVariables::Observer *obs) {
429
430  TransferFunctions TF(*this, val, obs, block);
431
432  // Visit the terminator (if any).
433  if (const Stmt *term = block->getTerminator())
434    TF.Visit(const_cast<Stmt*>(term));
435
436  // Apply the transfer function for all Stmts in the block.
437  for (CFGBlock::const_reverse_iterator it = block->rbegin(),
438       ei = block->rend(); it != ei; ++it) {
439    const CFGElement &elem = *it;
440    if (!isa<CFGStmt>(elem))
441      continue;
442
443    const Stmt *S = cast<CFGStmt>(elem).getStmt();
444    TF.Visit(const_cast<Stmt*>(S));
445    stmtsToLiveness[S] = val;
446  }
447  return val;
448}
449
450void LiveVariables::runOnAllBlocks(LiveVariables::Observer &obs) {
451  const CFG *cfg = getImpl(impl).analysisContext.getCFG();
452  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it)
453    getImpl(impl).runOnBlock(*it, getImpl(impl).blocksEndToLiveness[*it], &obs);
454}
455
456LiveVariables::LiveVariables(void *im) : impl(im) {}
457
458LiveVariables::~LiveVariables() {
459  delete (LiveVariablesImpl*) impl;
460}
461
462LiveVariables *
463LiveVariables::computeLiveness(AnalysisDeclContext &AC,
464                                 bool killAtAssign) {
465
466  // No CFG?  Bail out.
467  CFG *cfg = AC.getCFG();
468  if (!cfg)
469    return 0;
470
471  LiveVariablesImpl *LV = new LiveVariablesImpl(AC, killAtAssign);
472
473  // Construct the dataflow worklist.  Enqueue the exit block as the
474  // start of the analysis.
475  DataflowWorklist worklist(*cfg, AC);
476  llvm::BitVector everAnalyzedBlock(cfg->getNumBlockIDs());
477
478  // FIXME: we should enqueue using post order.
479  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it) {
480    const CFGBlock *block = *it;
481    worklist.enqueueBlock(block);
482
483    // FIXME: Scan for DeclRefExprs using in the LHS of an assignment.
484    // We need to do this because we lack context in the reverse analysis
485    // to determine if a DeclRefExpr appears in such a context, and thus
486    // doesn't constitute a "use".
487    if (killAtAssign)
488      for (CFGBlock::const_iterator bi = block->begin(), be = block->end();
489           bi != be; ++bi) {
490        if (const CFGStmt *cs = bi->getAs<CFGStmt>()) {
491          if (const BinaryOperator *BO = dyn_cast<BinaryOperator>(cs->getStmt())) {
492            if (BO->getOpcode() == BO_Assign) {
493              if (const DeclRefExpr *DR =
494                    dyn_cast<DeclRefExpr>(BO->getLHS()->IgnoreParens())) {
495                LV->inAssignment[DR] = 1;
496              }
497            }
498          }
499        }
500      }
501  }
502
503  worklist.sortWorklist();
504
505  while (const CFGBlock *block = worklist.dequeue()) {
506    // Determine if the block's end value has changed.  If not, we
507    // have nothing left to do for this block.
508    LivenessValues &prevVal = LV->blocksEndToLiveness[block];
509
510    // Merge the values of all successor blocks.
511    LivenessValues val;
512    for (CFGBlock::const_succ_iterator it = block->succ_begin(),
513                                       ei = block->succ_end(); it != ei; ++it) {
514      if (const CFGBlock *succ = *it) {
515        val = LV->merge(val, LV->blocksBeginToLiveness[succ]);
516      }
517    }
518
519    if (!everAnalyzedBlock[block->getBlockID()])
520      everAnalyzedBlock[block->getBlockID()] = true;
521    else if (prevVal.equals(val))
522      continue;
523
524    prevVal = val;
525
526    // Update the dataflow value for the start of this block.
527    LV->blocksBeginToLiveness[block] = LV->runOnBlock(block, val);
528
529    // Enqueue the value to the predecessors.
530    worklist.enqueuePredecessors(block);
531  }
532
533  return new LiveVariables(LV);
534}
535
536static bool compare_entries(const CFGBlock *A, const CFGBlock *B) {
537  return A->getBlockID() < B->getBlockID();
538}
539
540static bool compare_vd_entries(const Decl *A, const Decl *B) {
541  SourceLocation ALoc = A->getLocStart();
542  SourceLocation BLoc = B->getLocStart();
543  return ALoc.getRawEncoding() < BLoc.getRawEncoding();
544}
545
546void LiveVariables::dumpBlockLiveness(const SourceManager &M) {
547  getImpl(impl).dumpBlockLiveness(M);
548}
549
550void LiveVariablesImpl::dumpBlockLiveness(const SourceManager &M) {
551  std::vector<const CFGBlock *> vec;
552  for (llvm::DenseMap<const CFGBlock *, LiveVariables::LivenessValues>::iterator
553       it = blocksEndToLiveness.begin(), ei = blocksEndToLiveness.end();
554       it != ei; ++it) {
555    vec.push_back(it->first);
556  }
557  std::sort(vec.begin(), vec.end(), compare_entries);
558
559  std::vector<const VarDecl*> declVec;
560
561  for (std::vector<const CFGBlock *>::iterator
562        it = vec.begin(), ei = vec.end(); it != ei; ++it) {
563    llvm::errs() << "\n[ B" << (*it)->getBlockID()
564                 << " (live variables at block exit) ]\n";
565
566    LiveVariables::LivenessValues vals = blocksEndToLiveness[*it];
567    declVec.clear();
568
569    for (llvm::ImmutableSet<const VarDecl *>::iterator si =
570          vals.liveDecls.begin(),
571          se = vals.liveDecls.end(); si != se; ++si) {
572      declVec.push_back(*si);
573    }
574
575    std::sort(declVec.begin(), declVec.end(), compare_vd_entries);
576
577    for (std::vector<const VarDecl*>::iterator di = declVec.begin(),
578         de = declVec.end(); di != de; ++di) {
579      llvm::errs() << " " << (*di)->getDeclName().getAsString()
580                   << " <";
581      (*di)->getLocation().dump(M);
582      llvm::errs() << ">\n";
583    }
584  }
585  llvm::errs() << "\n";
586}
587
588const void *LiveVariables::getTag() { static int x; return &x; }
589const void *RelaxedLiveVariables::getTag() { static int x; return &x; }
590