LiveVariables.cpp revision 5112fc48495fa705cabe7ec455f9a6e73ec9ecc7 
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 ParenExpr *ParenE = dyn_cast<ParenExpr>(S))
237      S = ParenE->getSubExpr();
238    else if (const OpaqueValueExpr *OVE = dyn_cast<OpaqueValueExpr>(S))
239      S = OVE->getSourceExpr();
240    else
241      break;
242  }
243  return S;
244}
245
246static void AddLiveStmt(llvm::ImmutableSet<const Stmt *> &Set,
247                        llvm::ImmutableSet<const Stmt *>::Factory &F,
248                        const Stmt *S) {
249  Set = F.add(Set, LookThroughStmt(S));
250}
251
252void TransferFunctions::Visit(Stmt *S) {
253  if (observer)
254    observer->observeStmt(S, currentBlock, val);
255
256  StmtVisitor<TransferFunctions>::Visit(S);
257
258  if (isa<Expr>(S)) {
259    val.liveStmts = LV.SSetFact.remove(val.liveStmts, S);
260  }
261
262  // Mark all children expressions live.
263
264  switch (S->getStmtClass()) {
265    default:
266      break;
267    case Stmt::StmtExprClass: {
268      // For statement expressions, look through the compound statement.
269      S = cast<StmtExpr>(S)->getSubStmt();
270      break;
271    }
272    case Stmt::CXXMemberCallExprClass: {
273      // Include the implicit "this" pointer as being live.
274      CXXMemberCallExpr *CE = cast<CXXMemberCallExpr>(S);
275      if (Expr *ImplicitObj = CE->getImplicitObjectArgument()) {
276        AddLiveStmt(val.liveStmts, LV.SSetFact, ImplicitObj);
277      }
278      break;
279    }
280    case Stmt::DeclStmtClass: {
281      const DeclStmt *DS = cast<DeclStmt>(S);
282      if (const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl())) {
283        for (const VariableArrayType* VA = FindVA(VD->getType());
284             VA != 0; VA = FindVA(VA->getElementType())) {
285          AddLiveStmt(val.liveStmts, LV.SSetFact, VA->getSizeExpr());
286        }
287      }
288      break;
289    }
290    // FIXME: These cases eventually shouldn't be needed.
291    case Stmt::ExprWithCleanupsClass: {
292      S = cast<ExprWithCleanups>(S)->getSubExpr();
293      break;
294    }
295    case Stmt::CXXBindTemporaryExprClass: {
296      S = cast<CXXBindTemporaryExpr>(S)->getSubExpr();
297      break;
298    }
299    case Stmt::UnaryExprOrTypeTraitExprClass: {
300      // No need to unconditionally visit subexpressions.
301      return;
302    }
303  }
304
305  for (Stmt::child_iterator it = S->child_begin(), ei = S->child_end();
306       it != ei; ++it) {
307    if (Stmt *child = *it)
308      AddLiveStmt(val.liveStmts, LV.SSetFact, child);
309  }
310}
311
312void TransferFunctions::VisitBinaryOperator(BinaryOperator *B) {
313  if (B->isAssignmentOp()) {
314    if (!LV.killAtAssign)
315      return;
316
317    // Assigning to a variable?
318    Expr *LHS = B->getLHS()->IgnoreParens();
319
320    if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(LHS))
321      if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
322        // Assignments to references don't kill the ref's address
323        if (VD->getType()->isReferenceType())
324          return;
325
326        if (!isAlwaysAlive(VD)) {
327          // The variable is now dead.
328          val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
329        }
330
331        if (observer)
332          observer->observerKill(DR);
333      }
334  }
335}
336
337void TransferFunctions::VisitBlockExpr(BlockExpr *BE) {
338  AnalysisDeclContext::referenced_decls_iterator I, E;
339  llvm::tie(I, E) =
340    LV.analysisContext.getReferencedBlockVars(BE->getBlockDecl());
341  for ( ; I != E ; ++I) {
342    const VarDecl *VD = *I;
343    if (isAlwaysAlive(VD))
344      continue;
345    val.liveDecls = LV.DSetFact.add(val.liveDecls, VD);
346  }
347}
348
349void TransferFunctions::VisitDeclRefExpr(DeclRefExpr *DR) {
350  if (const VarDecl *D = dyn_cast<VarDecl>(DR->getDecl()))
351    if (!isAlwaysAlive(D) && LV.inAssignment.find(DR) == LV.inAssignment.end())
352      val.liveDecls = LV.DSetFact.add(val.liveDecls, D);
353}
354
355void TransferFunctions::VisitDeclStmt(DeclStmt *DS) {
356  for (DeclStmt::decl_iterator DI=DS->decl_begin(), DE = DS->decl_end();
357       DI != DE; ++DI)
358    if (VarDecl *VD = dyn_cast<VarDecl>(*DI)) {
359      if (!isAlwaysAlive(VD))
360        val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
361    }
362}
363
364void TransferFunctions::VisitObjCForCollectionStmt(ObjCForCollectionStmt *OS) {
365  // Kill the iteration variable.
366  DeclRefExpr *DR = 0;
367  const VarDecl *VD = 0;
368
369  Stmt *element = OS->getElement();
370  if (DeclStmt *DS = dyn_cast<DeclStmt>(element)) {
371    VD = cast<VarDecl>(DS->getSingleDecl());
372  }
373  else if ((DR = dyn_cast<DeclRefExpr>(cast<Expr>(element)->IgnoreParens()))) {
374    VD = cast<VarDecl>(DR->getDecl());
375  }
376
377  if (VD) {
378    val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
379    if (observer && DR)
380      observer->observerKill(DR);
381  }
382}
383
384void TransferFunctions::
385VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *UE)
386{
387  // While sizeof(var) doesn't technically extend the liveness of 'var', it
388  // does extent the liveness of metadata if 'var' is a VariableArrayType.
389  // We handle that special case here.
390  if (UE->getKind() != UETT_SizeOf || UE->isArgumentType())
391    return;
392
393  const Expr *subEx = UE->getArgumentExpr();
394  if (subEx->getType()->isVariableArrayType()) {
395    assert(subEx->isLValue());
396    val.liveStmts = LV.SSetFact.add(val.liveStmts, subEx->IgnoreParens());
397  }
398}
399
400void TransferFunctions::VisitUnaryOperator(UnaryOperator *UO) {
401  // Treat ++/-- as a kill.
402  // Note we don't actually have to do anything if we don't have an observer,
403  // since a ++/-- acts as both a kill and a "use".
404  if (!observer)
405    return;
406
407  switch (UO->getOpcode()) {
408  default:
409    return;
410  case UO_PostInc:
411  case UO_PostDec:
412  case UO_PreInc:
413  case UO_PreDec:
414    break;
415  }
416
417  if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(UO->getSubExpr()->IgnoreParens()))
418    if (isa<VarDecl>(DR->getDecl())) {
419      // Treat ++/-- as a kill.
420      observer->observerKill(DR);
421    }
422}
423
424LiveVariables::LivenessValues
425LiveVariablesImpl::runOnBlock(const CFGBlock *block,
426                              LiveVariables::LivenessValues val,
427                              LiveVariables::Observer *obs) {
428
429  TransferFunctions TF(*this, val, obs, block);
430
431  // Visit the terminator (if any).
432  if (const Stmt *term = block->getTerminator())
433    TF.Visit(const_cast<Stmt*>(term));
434
435  // Apply the transfer function for all Stmts in the block.
436  for (CFGBlock::const_reverse_iterator it = block->rbegin(),
437       ei = block->rend(); it != ei; ++it) {
438    const CFGElement &elem = *it;
439    if (!isa<CFGStmt>(elem))
440      continue;
441
442    const Stmt *S = cast<CFGStmt>(elem).getStmt();
443    TF.Visit(const_cast<Stmt*>(S));
444    stmtsToLiveness[S] = val;
445  }
446  return val;
447}
448
449void LiveVariables::runOnAllBlocks(LiveVariables::Observer &obs) {
450  const CFG *cfg = getImpl(impl).analysisContext.getCFG();
451  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it)
452    getImpl(impl).runOnBlock(*it, getImpl(impl).blocksEndToLiveness[*it], &obs);
453}
454
455LiveVariables::LiveVariables(void *im) : impl(im) {}
456
457LiveVariables::~LiveVariables() {
458  delete (LiveVariablesImpl*) impl;
459}
460
461LiveVariables *
462LiveVariables::computeLiveness(AnalysisDeclContext &AC,
463                                 bool killAtAssign) {
464
465  // No CFG?  Bail out.
466  CFG *cfg = AC.getCFG();
467  if (!cfg)
468    return 0;
469
470  LiveVariablesImpl *LV = new LiveVariablesImpl(AC, killAtAssign);
471
472  // Construct the dataflow worklist.  Enqueue the exit block as the
473  // start of the analysis.
474  DataflowWorklist worklist(*cfg, AC);
475  llvm::BitVector everAnalyzedBlock(cfg->getNumBlockIDs());
476
477  // FIXME: we should enqueue using post order.
478  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it) {
479    const CFGBlock *block = *it;
480    worklist.enqueueBlock(block);
481
482    // FIXME: Scan for DeclRefExprs using in the LHS of an assignment.
483    // We need to do this because we lack context in the reverse analysis
484    // to determine if a DeclRefExpr appears in such a context, and thus
485    // doesn't constitute a "use".
486    if (killAtAssign)
487      for (CFGBlock::const_iterator bi = block->begin(), be = block->end();
488           bi != be; ++bi) {
489        if (const CFGStmt *cs = bi->getAs<CFGStmt>()) {
490          if (const BinaryOperator *BO = dyn_cast<BinaryOperator>(cs->getStmt())) {
491            if (BO->getOpcode() == BO_Assign) {
492              if (const DeclRefExpr *DR =
493                    dyn_cast<DeclRefExpr>(BO->getLHS()->IgnoreParens())) {
494                LV->inAssignment[DR] = 1;
495              }
496            }
497          }
498        }
499      }
500  }
501
502  worklist.sortWorklist();
503
504  while (const CFGBlock *block = worklist.dequeue()) {
505    // Determine if the block's end value has changed.  If not, we
506    // have nothing left to do for this block.
507    LivenessValues &prevVal = LV->blocksEndToLiveness[block];
508
509    // Merge the values of all successor blocks.
510    LivenessValues val;
511    for (CFGBlock::const_succ_iterator it = block->succ_begin(),
512                                       ei = block->succ_end(); it != ei; ++it) {
513      if (const CFGBlock *succ = *it) {
514        val = LV->merge(val, LV->blocksBeginToLiveness[succ]);
515      }
516    }
517
518    if (!everAnalyzedBlock[block->getBlockID()])
519      everAnalyzedBlock[block->getBlockID()] = true;
520    else if (prevVal.equals(val))
521      continue;
522
523    prevVal = val;
524
525    // Update the dataflow value for the start of this block.
526    LV->blocksBeginToLiveness[block] = LV->runOnBlock(block, val);
527
528    // Enqueue the value to the predecessors.
529    worklist.enqueuePredecessors(block);
530  }
531
532  return new LiveVariables(LV);
533}
534
535static bool compare_entries(const CFGBlock *A, const CFGBlock *B) {
536  return A->getBlockID() < B->getBlockID();
537}
538
539static bool compare_vd_entries(const Decl *A, const Decl *B) {
540  SourceLocation ALoc = A->getLocStart();
541  SourceLocation BLoc = B->getLocStart();
542  return ALoc.getRawEncoding() < BLoc.getRawEncoding();
543}
544
545void LiveVariables::dumpBlockLiveness(const SourceManager &M) {
546  getImpl(impl).dumpBlockLiveness(M);
547}
548
549void LiveVariablesImpl::dumpBlockLiveness(const SourceManager &M) {
550  std::vector<const CFGBlock *> vec;
551  for (llvm::DenseMap<const CFGBlock *, LiveVariables::LivenessValues>::iterator
552       it = blocksEndToLiveness.begin(), ei = blocksEndToLiveness.end();
553       it != ei; ++it) {
554    vec.push_back(it->first);
555  }
556  std::sort(vec.begin(), vec.end(), compare_entries);
557
558  std::vector<const VarDecl*> declVec;
559
560  for (std::vector<const CFGBlock *>::iterator
561        it = vec.begin(), ei = vec.end(); it != ei; ++it) {
562    llvm::errs() << "\n[ B" << (*it)->getBlockID()
563                 << " (live variables at block exit) ]\n";
564
565    LiveVariables::LivenessValues vals = blocksEndToLiveness[*it];
566    declVec.clear();
567
568    for (llvm::ImmutableSet<const VarDecl *>::iterator si =
569          vals.liveDecls.begin(),
570          se = vals.liveDecls.end(); si != se; ++si) {
571      declVec.push_back(*si);
572    }
573
574    std::sort(declVec.begin(), declVec.end(), compare_vd_entries);
575
576    for (std::vector<const VarDecl*>::iterator di = declVec.begin(),
577         de = declVec.end(); di != de; ++di) {
578      llvm::errs() << " " << (*di)->getDeclName().getAsString()
579                   << " <";
580      (*di)->getLocation().dump(M);
581      llvm::errs() << ">\n";
582    }
583  }
584  llvm::errs() << "\n";
585}
586
587const void *LiveVariables::getTag() { static int x; return &x; }
588const void *RelaxedLiveVariables::getTag() { static int x; return &x; }
589