DeadStoresChecker.cpp revision 9ed6d8068f767819951bc4eebf6f4912087c442a 
1//==- DeadStoresChecker.cpp - Check for stores to dead variables -*- C++ -*-==//
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 DeadStores, a flow-sensitive checker that looks for
11//  stores to variables that are no longer live.
12//
13//===----------------------------------------------------------------------===//
14
15#include "ClangSACheckers.h"
16#include "clang/AST/ASTContext.h"
17#include "clang/AST/Attr.h"
18#include "clang/AST/ParentMap.h"
19#include "clang/AST/RecursiveASTVisitor.h"
20#include "clang/Analysis/Analyses/LiveVariables.h"
21#include "clang/Analysis/Visitors/CFGRecStmtDeclVisitor.h"
22#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
23#include "clang/StaticAnalyzer/Core/Checker.h"
24#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
25#include "llvm/ADT/BitVector.h"
26#include "llvm/ADT/SmallString.h"
27#include "llvm/Support/SaveAndRestore.h"
28
29using namespace clang;
30using namespace ento;
31
32namespace {
33
34/// A simple visitor to record what VarDecls occur in EH-handling code.
35class EHCodeVisitor : public RecursiveASTVisitor<EHCodeVisitor> {
36public:
37  bool inEH;
38  llvm::DenseSet<const VarDecl *> &S;
39
40  bool TraverseObjCAtFinallyStmt(ObjCAtFinallyStmt *S) {
41    SaveAndRestore<bool> inFinally(inEH, true);
42    return ::RecursiveASTVisitor<EHCodeVisitor>::TraverseObjCAtFinallyStmt(S);
43  }
44
45  bool TraverseObjCAtCatchStmt(ObjCAtCatchStmt *S) {
46    SaveAndRestore<bool> inCatch(inEH, true);
47    return ::RecursiveASTVisitor<EHCodeVisitor>::TraverseObjCAtCatchStmt(S);
48  }
49
50  bool TraverseCXXCatchStmt(CXXCatchStmt *S) {
51    SaveAndRestore<bool> inCatch(inEH, true);
52    return TraverseStmt(S->getHandlerBlock());
53  }
54
55  bool VisitDeclRefExpr(DeclRefExpr *DR) {
56    if (inEH)
57      if (const VarDecl *D = dyn_cast<VarDecl>(DR->getDecl()))
58        S.insert(D);
59    return true;
60  }
61
62  EHCodeVisitor(llvm::DenseSet<const VarDecl *> &S) :
63  inEH(false), S(S) {}
64};
65
66// FIXME: Eventually migrate into its own file, and have it managed by
67// AnalysisManager.
68class ReachableCode {
69  const CFG &cfg;
70  llvm::BitVector reachable;
71public:
72  ReachableCode(const CFG &cfg)
73    : cfg(cfg), reachable(cfg.getNumBlockIDs(), false) {}
74
75  void computeReachableBlocks();
76
77  bool isReachable(const CFGBlock *block) const {
78    return reachable[block->getBlockID()];
79  }
80};
81}
82
83void ReachableCode::computeReachableBlocks() {
84  if (!cfg.getNumBlockIDs())
85    return;
86
87  SmallVector<const CFGBlock*, 10> worklist;
88  worklist.push_back(&cfg.getEntry());
89
90  while (!worklist.empty()) {
91    const CFGBlock *block = worklist.back();
92    worklist.pop_back();
93    llvm::BitVector::reference isReachable = reachable[block->getBlockID()];
94    if (isReachable)
95      continue;
96    isReachable = true;
97    for (CFGBlock::const_succ_iterator i = block->succ_begin(),
98                                       e = block->succ_end(); i != e; ++i)
99      if (const CFGBlock *succ = *i)
100        worklist.push_back(succ);
101  }
102}
103
104static const Expr *
105LookThroughTransitiveAssignmentsAndCommaOperators(const Expr *Ex) {
106  while (Ex) {
107    const BinaryOperator *BO =
108      dyn_cast<BinaryOperator>(Ex->IgnoreParenCasts());
109    if (!BO)
110      break;
111    if (BO->getOpcode() == BO_Assign) {
112      Ex = BO->getRHS();
113      continue;
114    }
115    if (BO->getOpcode() == BO_Comma) {
116      Ex = BO->getRHS();
117      continue;
118    }
119    break;
120  }
121  return Ex;
122}
123
124namespace {
125class DeadStoreObs : public LiveVariables::Observer {
126  const CFG &cfg;
127  ASTContext &Ctx;
128  BugReporter& BR;
129  AnalysisDeclContext* AC;
130  ParentMap& Parents;
131  llvm::SmallPtrSet<const VarDecl*, 20> Escaped;
132  OwningPtr<ReachableCode> reachableCode;
133  const CFGBlock *currentBlock;
134  OwningPtr<llvm::DenseSet<const VarDecl *> > InEH;
135
136  enum DeadStoreKind { Standard, Enclosing, DeadIncrement, DeadInit };
137
138public:
139  DeadStoreObs(const CFG &cfg, ASTContext &ctx,
140               BugReporter& br, AnalysisDeclContext* ac, ParentMap& parents,
141               llvm::SmallPtrSet<const VarDecl*, 20> &escaped)
142    : cfg(cfg), Ctx(ctx), BR(br), AC(ac), Parents(parents),
143      Escaped(escaped), currentBlock(0) {}
144
145  virtual ~DeadStoreObs() {}
146
147  bool isLive(const LiveVariables::LivenessValues &Live, const VarDecl *D) {
148    if (Live.isLive(D))
149      return true;
150    // Lazily construct the set that records which VarDecls are in
151    // EH code.
152    if (!InEH.get()) {
153      InEH.reset(new llvm::DenseSet<const VarDecl *>());
154      EHCodeVisitor V(*InEH.get());
155      V.TraverseStmt(AC->getBody());
156    }
157    // Treat all VarDecls that occur in EH code as being "always live"
158    // when considering to suppress dead stores.  Frequently stores
159    // are followed by reads in EH code, but we don't have the ability
160    // to analyze that yet.
161    return InEH->count(D);
162  }
163
164  void Report(const VarDecl *V, DeadStoreKind dsk,
165              PathDiagnosticLocation L, SourceRange R) {
166    if (Escaped.count(V))
167      return;
168
169    // Compute reachable blocks within the CFG for trivial cases
170    // where a bogus dead store can be reported because itself is unreachable.
171    if (!reachableCode.get()) {
172      reachableCode.reset(new ReachableCode(cfg));
173      reachableCode->computeReachableBlocks();
174    }
175
176    if (!reachableCode->isReachable(currentBlock))
177      return;
178
179    SmallString<64> buf;
180    llvm::raw_svector_ostream os(buf);
181    const char *BugType = 0;
182
183    switch (dsk) {
184      case DeadInit:
185        BugType = "Dead initialization";
186        os << "Value stored to '" << *V
187           << "' during its initialization is never read";
188        break;
189
190      case DeadIncrement:
191        BugType = "Dead increment";
192      case Standard:
193        if (!BugType) BugType = "Dead assignment";
194        os << "Value stored to '" << *V << "' is never read";
195        break;
196
197      case Enclosing:
198        // Don't report issues in this case, e.g.: "if (x = foo())",
199        // where 'x' is unused later.  We have yet to see a case where
200        // this is a real bug.
201        return;
202    }
203
204    BR.EmitBasicReport(AC->getDecl(), BugType, "Dead store", os.str(), L, R);
205  }
206
207  void CheckVarDecl(const VarDecl *VD, const Expr *Ex, const Expr *Val,
208                    DeadStoreKind dsk,
209                    const LiveVariables::LivenessValues &Live) {
210
211    if (!VD->hasLocalStorage())
212      return;
213    // Reference types confuse the dead stores checker.  Skip them
214    // for now.
215    if (VD->getType()->getAs<ReferenceType>())
216      return;
217
218    if (!isLive(Live, VD) &&
219        !(VD->getAttr<UnusedAttr>() || VD->getAttr<BlocksAttr>())) {
220
221      PathDiagnosticLocation ExLoc =
222        PathDiagnosticLocation::createBegin(Ex, BR.getSourceManager(), AC);
223      Report(VD, dsk, ExLoc, Val->getSourceRange());
224    }
225  }
226
227  void CheckDeclRef(const DeclRefExpr *DR, const Expr *Val, DeadStoreKind dsk,
228                    const LiveVariables::LivenessValues& Live) {
229    if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl()))
230      CheckVarDecl(VD, DR, Val, dsk, Live);
231  }
232
233  bool isIncrement(VarDecl *VD, const BinaryOperator* B) {
234    if (B->isCompoundAssignmentOp())
235      return true;
236
237    const Expr *RHS = B->getRHS()->IgnoreParenCasts();
238    const BinaryOperator* BRHS = dyn_cast<BinaryOperator>(RHS);
239
240    if (!BRHS)
241      return false;
242
243    const DeclRefExpr *DR;
244
245    if ((DR = dyn_cast<DeclRefExpr>(BRHS->getLHS()->IgnoreParenCasts())))
246      if (DR->getDecl() == VD)
247        return true;
248
249    if ((DR = dyn_cast<DeclRefExpr>(BRHS->getRHS()->IgnoreParenCasts())))
250      if (DR->getDecl() == VD)
251        return true;
252
253    return false;
254  }
255
256  virtual void observeStmt(const Stmt *S, const CFGBlock *block,
257                           const LiveVariables::LivenessValues &Live) {
258
259    currentBlock = block;
260
261    // Skip statements in macros.
262    if (S->getLocStart().isMacroID())
263      return;
264
265    // Only cover dead stores from regular assignments.  ++/-- dead stores
266    // have never flagged a real bug.
267    if (const BinaryOperator* B = dyn_cast<BinaryOperator>(S)) {
268      if (!B->isAssignmentOp()) return; // Skip non-assignments.
269
270      if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(B->getLHS()))
271        if (VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
272          // Special case: check for assigning null to a pointer.
273          //  This is a common form of defensive programming.
274          const Expr *RHS =
275            LookThroughTransitiveAssignmentsAndCommaOperators(B->getRHS());
276          RHS = RHS->IgnoreParenCasts();
277
278          QualType T = VD->getType();
279          if (T->isPointerType() || T->isObjCObjectPointerType()) {
280            if (RHS->isNullPointerConstant(Ctx, Expr::NPC_ValueDependentIsNull))
281              return;
282          }
283
284          // Special case: self-assignments.  These are often used to shut up
285          //  "unused variable" compiler warnings.
286          if (const DeclRefExpr *RhsDR = dyn_cast<DeclRefExpr>(RHS))
287            if (VD == dyn_cast<VarDecl>(RhsDR->getDecl()))
288              return;
289
290          // Otherwise, issue a warning.
291          DeadStoreKind dsk = Parents.isConsumedExpr(B)
292                              ? Enclosing
293                              : (isIncrement(VD,B) ? DeadIncrement : Standard);
294
295          CheckVarDecl(VD, DR, B->getRHS(), dsk, Live);
296        }
297    }
298    else if (const UnaryOperator* U = dyn_cast<UnaryOperator>(S)) {
299      if (!U->isIncrementOp() || U->isPrefix())
300        return;
301
302      const Stmt *parent = Parents.getParentIgnoreParenCasts(U);
303      if (!parent || !isa<ReturnStmt>(parent))
304        return;
305
306      const Expr *Ex = U->getSubExpr()->IgnoreParenCasts();
307
308      if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(Ex))
309        CheckDeclRef(DR, U, DeadIncrement, Live);
310    }
311    else if (const DeclStmt *DS = dyn_cast<DeclStmt>(S))
312      // Iterate through the decls.  Warn if any initializers are complex
313      // expressions that are not live (never used).
314      for (DeclStmt::const_decl_iterator DI=DS->decl_begin(), DE=DS->decl_end();
315           DI != DE; ++DI) {
316
317        VarDecl *V = dyn_cast<VarDecl>(*DI);
318
319        if (!V)
320          continue;
321
322        if (V->hasLocalStorage()) {
323          // Reference types confuse the dead stores checker.  Skip them
324          // for now.
325          if (V->getType()->getAs<ReferenceType>())
326            return;
327
328          if (const Expr *E = V->getInit()) {
329            while (const ExprWithCleanups *exprClean =
330                    dyn_cast<ExprWithCleanups>(E))
331              E = exprClean->getSubExpr();
332
333            // Look through transitive assignments, e.g.:
334            // int x = y = 0;
335            E = LookThroughTransitiveAssignmentsAndCommaOperators(E);
336
337            // Don't warn on C++ objects (yet) until we can show that their
338            // constructors/destructors don't have side effects.
339            if (isa<CXXConstructExpr>(E))
340              return;
341
342            // A dead initialization is a variable that is dead after it
343            // is initialized.  We don't flag warnings for those variables
344            // marked 'unused'.
345            if (!isLive(Live, V) && V->getAttr<UnusedAttr>() == 0) {
346              // Special case: check for initializations with constants.
347              //
348              //  e.g. : int x = 0;
349              //
350              // If x is EVER assigned a new value later, don't issue
351              // a warning.  This is because such initialization can be
352              // due to defensive programming.
353              if (E->isEvaluatable(Ctx))
354                return;
355
356              if (const DeclRefExpr *DRE =
357                  dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
358                if (const VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl())) {
359                  // Special case: check for initialization from constant
360                  //  variables.
361                  //
362                  //  e.g. extern const int MyConstant;
363                  //       int x = MyConstant;
364                  //
365                  if (VD->hasGlobalStorage() &&
366                      VD->getType().isConstQualified())
367                    return;
368                  // Special case: check for initialization from scalar
369                  //  parameters.  This is often a form of defensive
370                  //  programming.  Non-scalars are still an error since
371                  //  because it more likely represents an actual algorithmic
372                  //  bug.
373                  if (isa<ParmVarDecl>(VD) && VD->getType()->isScalarType())
374                    return;
375                }
376
377              PathDiagnosticLocation Loc =
378                PathDiagnosticLocation::create(V, BR.getSourceManager());
379              Report(V, DeadInit, Loc, E->getSourceRange());
380            }
381          }
382        }
383      }
384  }
385};
386
387} // end anonymous namespace
388
389//===----------------------------------------------------------------------===//
390// Driver function to invoke the Dead-Stores checker on a CFG.
391//===----------------------------------------------------------------------===//
392
393namespace {
394class FindEscaped : public CFGRecStmtDeclVisitor<FindEscaped>{
395  CFG *cfg;
396public:
397  FindEscaped(CFG *c) : cfg(c) {}
398
399  CFG& getCFG() { return *cfg; }
400
401  llvm::SmallPtrSet<const VarDecl*, 20> Escaped;
402
403  void VisitUnaryOperator(UnaryOperator* U) {
404    // Check for '&'.  Any VarDecl whose value has its address-taken we
405    // treat as escaped.
406    Expr *E = U->getSubExpr()->IgnoreParenCasts();
407    if (U->getOpcode() == UO_AddrOf)
408      if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E))
409        if (VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
410          Escaped.insert(VD);
411          return;
412        }
413    Visit(E);
414  }
415};
416} // end anonymous namespace
417
418
419//===----------------------------------------------------------------------===//
420// DeadStoresChecker
421//===----------------------------------------------------------------------===//
422
423namespace {
424class DeadStoresChecker : public Checker<check::ASTCodeBody> {
425public:
426  void checkASTCodeBody(const Decl *D, AnalysisManager& mgr,
427                        BugReporter &BR) const {
428
429    // Don't do anything for template instantiations.
430    // Proving that code in a template instantiation is "dead"
431    // means proving that it is dead in all instantiations.
432    // This same problem exists with -Wunreachable-code.
433    if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
434      if (FD->isTemplateInstantiation())
435        return;
436
437    if (LiveVariables *L = mgr.getAnalysis<LiveVariables>(D)) {
438      CFG &cfg = *mgr.getCFG(D);
439      AnalysisDeclContext *AC = mgr.getAnalysisDeclContext(D);
440      ParentMap &pmap = mgr.getParentMap(D);
441      FindEscaped FS(&cfg);
442      FS.getCFG().VisitBlockStmts(FS);
443      DeadStoreObs A(cfg, BR.getContext(), BR, AC, pmap, FS.Escaped);
444      L->runOnAllBlocks(A);
445    }
446  }
447};
448}
449
450void ento::registerDeadStoresChecker(CheckerManager &mgr) {
451  mgr.registerChecker<DeadStoresChecker>();
452}
453