DeadStoresChecker.cpp revision 263508 
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/StaticAnalyzer/Core/BugReporter/BugReporter.h"
22#include "clang/StaticAnalyzer/Core/Checker.h"
23#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
24#include "llvm/ADT/BitVector.h"
25#include "llvm/ADT/SmallString.h"
26#include "llvm/Support/SaveAndRestore.h"
27
28using namespace clang;
29using namespace ento;
30
31namespace {
32
33/// A simple visitor to record what VarDecls occur in EH-handling code.
34class EHCodeVisitor : public RecursiveASTVisitor<EHCodeVisitor> {
35public:
36  bool inEH;
37  llvm::DenseSet<const VarDecl *> &S;
38
39  bool TraverseObjCAtFinallyStmt(ObjCAtFinallyStmt *S) {
40    SaveAndRestore<bool> inFinally(inEH, true);
41    return ::RecursiveASTVisitor<EHCodeVisitor>::TraverseObjCAtFinallyStmt(S);
42  }
43
44  bool TraverseObjCAtCatchStmt(ObjCAtCatchStmt *S) {
45    SaveAndRestore<bool> inCatch(inEH, true);
46    return ::RecursiveASTVisitor<EHCodeVisitor>::TraverseObjCAtCatchStmt(S);
47  }
48
49  bool TraverseCXXCatchStmt(CXXCatchStmt *S) {
50    SaveAndRestore<bool> inCatch(inEH, true);
51    return TraverseStmt(S->getHandlerBlock());
52  }
53
54  bool VisitDeclRefExpr(DeclRefExpr *DR) {
55    if (inEH)
56      if (const VarDecl *D = dyn_cast<VarDecl>(DR->getDecl()))
57        S.insert(D);
58    return true;
59  }
60
61  EHCodeVisitor(llvm::DenseSet<const VarDecl *> &S) :
62  inEH(false), S(S) {}
63};
64
65// FIXME: Eventually migrate into its own file, and have it managed by
66// AnalysisManager.
67class ReachableCode {
68  const CFG &cfg;
69  llvm::BitVector reachable;
70public:
71  ReachableCode(const CFG &cfg)
72    : cfg(cfg), reachable(cfg.getNumBlockIDs(), false) {}
73
74  void computeReachableBlocks();
75
76  bool isReachable(const CFGBlock *block) const {
77    return reachable[block->getBlockID()];
78  }
79};
80}
81
82void ReachableCode::computeReachableBlocks() {
83  if (!cfg.getNumBlockIDs())
84    return;
85
86  SmallVector<const CFGBlock*, 10> worklist;
87  worklist.push_back(&cfg.getEntry());
88
89  while (!worklist.empty()) {
90    const CFGBlock *block = worklist.pop_back_val();
91    llvm::BitVector::reference isReachable = reachable[block->getBlockID()];
92    if (isReachable)
93      continue;
94    isReachable = true;
95    for (CFGBlock::const_succ_iterator i = block->succ_begin(),
96                                       e = block->succ_end(); i != e; ++i)
97      if (const CFGBlock *succ = *i)
98        worklist.push_back(succ);
99  }
100}
101
102static const Expr *
103LookThroughTransitiveAssignmentsAndCommaOperators(const Expr *Ex) {
104  while (Ex) {
105    const BinaryOperator *BO =
106      dyn_cast<BinaryOperator>(Ex->IgnoreParenCasts());
107    if (!BO)
108      break;
109    if (BO->getOpcode() == BO_Assign) {
110      Ex = BO->getRHS();
111      continue;
112    }
113    if (BO->getOpcode() == BO_Comma) {
114      Ex = BO->getRHS();
115      continue;
116    }
117    break;
118  }
119  return Ex;
120}
121
122namespace {
123class DeadStoreObs : public LiveVariables::Observer {
124  const CFG &cfg;
125  ASTContext &Ctx;
126  BugReporter& BR;
127  AnalysisDeclContext* AC;
128  ParentMap& Parents;
129  llvm::SmallPtrSet<const VarDecl*, 20> Escaped;
130  OwningPtr<ReachableCode> reachableCode;
131  const CFGBlock *currentBlock;
132  OwningPtr<llvm::DenseSet<const VarDecl *> > InEH;
133
134  enum DeadStoreKind { Standard, Enclosing, DeadIncrement, DeadInit };
135
136public:
137  DeadStoreObs(const CFG &cfg, ASTContext &ctx,
138               BugReporter& br, AnalysisDeclContext* ac, ParentMap& parents,
139               llvm::SmallPtrSet<const VarDecl*, 20> &escaped)
140    : cfg(cfg), Ctx(ctx), BR(br), AC(ac), Parents(parents),
141      Escaped(escaped), currentBlock(0) {}
142
143  virtual ~DeadStoreObs() {}
144
145  bool isLive(const LiveVariables::LivenessValues &Live, const VarDecl *D) {
146    if (Live.isLive(D))
147      return true;
148    // Lazily construct the set that records which VarDecls are in
149    // EH code.
150    if (!InEH.get()) {
151      InEH.reset(new llvm::DenseSet<const VarDecl *>());
152      EHCodeVisitor V(*InEH.get());
153      V.TraverseStmt(AC->getBody());
154    }
155    // Treat all VarDecls that occur in EH code as being "always live"
156    // when considering to suppress dead stores.  Frequently stores
157    // are followed by reads in EH code, but we don't have the ability
158    // to analyze that yet.
159    return InEH->count(D);
160  }
161
162  void Report(const VarDecl *V, DeadStoreKind dsk,
163              PathDiagnosticLocation L, SourceRange R) {
164    if (Escaped.count(V))
165      return;
166
167    // Compute reachable blocks within the CFG for trivial cases
168    // where a bogus dead store can be reported because itself is unreachable.
169    if (!reachableCode.get()) {
170      reachableCode.reset(new ReachableCode(cfg));
171      reachableCode->computeReachableBlocks();
172    }
173
174    if (!reachableCode->isReachable(currentBlock))
175      return;
176
177    SmallString<64> buf;
178    llvm::raw_svector_ostream os(buf);
179    const char *BugType = 0;
180
181    switch (dsk) {
182      case DeadInit:
183        BugType = "Dead initialization";
184        os << "Value stored to '" << *V
185           << "' during its initialization is never read";
186        break;
187
188      case DeadIncrement:
189        BugType = "Dead increment";
190      case Standard:
191        if (!BugType) BugType = "Dead assignment";
192        os << "Value stored to '" << *V << "' is never read";
193        break;
194
195      case Enclosing:
196        // Don't report issues in this case, e.g.: "if (x = foo())",
197        // where 'x' is unused later.  We have yet to see a case where
198        // this is a real bug.
199        return;
200    }
201
202    BR.EmitBasicReport(AC->getDecl(), BugType, "Dead store", os.str(), L, R);
203  }
204
205  void CheckVarDecl(const VarDecl *VD, const Expr *Ex, const Expr *Val,
206                    DeadStoreKind dsk,
207                    const LiveVariables::LivenessValues &Live) {
208
209    if (!VD->hasLocalStorage())
210      return;
211    // Reference types confuse the dead stores checker.  Skip them
212    // for now.
213    if (VD->getType()->getAs<ReferenceType>())
214      return;
215
216    if (!isLive(Live, VD) &&
217        !(VD->getAttr<UnusedAttr>() || VD->getAttr<BlocksAttr>())) {
218
219      PathDiagnosticLocation ExLoc =
220        PathDiagnosticLocation::createBegin(Ex, BR.getSourceManager(), AC);
221      Report(VD, dsk, ExLoc, Val->getSourceRange());
222    }
223  }
224
225  void CheckDeclRef(const DeclRefExpr *DR, const Expr *Val, DeadStoreKind dsk,
226                    const LiveVariables::LivenessValues& Live) {
227    if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl()))
228      CheckVarDecl(VD, DR, Val, dsk, Live);
229  }
230
231  bool isIncrement(VarDecl *VD, const BinaryOperator* B) {
232    if (B->isCompoundAssignmentOp())
233      return true;
234
235    const Expr *RHS = B->getRHS()->IgnoreParenCasts();
236    const BinaryOperator* BRHS = dyn_cast<BinaryOperator>(RHS);
237
238    if (!BRHS)
239      return false;
240
241    const DeclRefExpr *DR;
242
243    if ((DR = dyn_cast<DeclRefExpr>(BRHS->getLHS()->IgnoreParenCasts())))
244      if (DR->getDecl() == VD)
245        return true;
246
247    if ((DR = dyn_cast<DeclRefExpr>(BRHS->getRHS()->IgnoreParenCasts())))
248      if (DR->getDecl() == VD)
249        return true;
250
251    return false;
252  }
253
254  virtual void observeStmt(const Stmt *S, const CFGBlock *block,
255                           const LiveVariables::LivenessValues &Live) {
256
257    currentBlock = block;
258
259    // Skip statements in macros.
260    if (S->getLocStart().isMacroID())
261      return;
262
263    // Only cover dead stores from regular assignments.  ++/-- dead stores
264    // have never flagged a real bug.
265    if (const BinaryOperator* B = dyn_cast<BinaryOperator>(S)) {
266      if (!B->isAssignmentOp()) return; // Skip non-assignments.
267
268      if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(B->getLHS()))
269        if (VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
270          // Special case: check for assigning null to a pointer.
271          //  This is a common form of defensive programming.
272          const Expr *RHS =
273            LookThroughTransitiveAssignmentsAndCommaOperators(B->getRHS());
274          RHS = RHS->IgnoreParenCasts();
275
276          QualType T = VD->getType();
277          if (T->isPointerType() || T->isObjCObjectPointerType()) {
278            if (RHS->isNullPointerConstant(Ctx, Expr::NPC_ValueDependentIsNull))
279              return;
280          }
281
282          // Special case: self-assignments.  These are often used to shut up
283          //  "unused variable" compiler warnings.
284          if (const DeclRefExpr *RhsDR = dyn_cast<DeclRefExpr>(RHS))
285            if (VD == dyn_cast<VarDecl>(RhsDR->getDecl()))
286              return;
287
288          // Otherwise, issue a warning.
289          DeadStoreKind dsk = Parents.isConsumedExpr(B)
290                              ? Enclosing
291                              : (isIncrement(VD,B) ? DeadIncrement : Standard);
292
293          CheckVarDecl(VD, DR, B->getRHS(), dsk, Live);
294        }
295    }
296    else if (const UnaryOperator* U = dyn_cast<UnaryOperator>(S)) {
297      if (!U->isIncrementOp() || U->isPrefix())
298        return;
299
300      const Stmt *parent = Parents.getParentIgnoreParenCasts(U);
301      if (!parent || !isa<ReturnStmt>(parent))
302        return;
303
304      const Expr *Ex = U->getSubExpr()->IgnoreParenCasts();
305
306      if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(Ex))
307        CheckDeclRef(DR, U, DeadIncrement, Live);
308    }
309    else if (const DeclStmt *DS = dyn_cast<DeclStmt>(S))
310      // Iterate through the decls.  Warn if any initializers are complex
311      // expressions that are not live (never used).
312      for (DeclStmt::const_decl_iterator DI=DS->decl_begin(), DE=DS->decl_end();
313           DI != DE; ++DI) {
314
315        VarDecl *V = dyn_cast<VarDecl>(*DI);
316
317        if (!V)
318          continue;
319
320        if (V->hasLocalStorage()) {
321          // Reference types confuse the dead stores checker.  Skip them
322          // for now.
323          if (V->getType()->getAs<ReferenceType>())
324            return;
325
326          if (const Expr *E = V->getInit()) {
327            while (const ExprWithCleanups *exprClean =
328                    dyn_cast<ExprWithCleanups>(E))
329              E = exprClean->getSubExpr();
330
331            // Look through transitive assignments, e.g.:
332            // int x = y = 0;
333            E = LookThroughTransitiveAssignmentsAndCommaOperators(E);
334
335            // Don't warn on C++ objects (yet) until we can show that their
336            // constructors/destructors don't have side effects.
337            if (isa<CXXConstructExpr>(E))
338              return;
339
340            // A dead initialization is a variable that is dead after it
341            // is initialized.  We don't flag warnings for those variables
342            // marked 'unused'.
343            if (!isLive(Live, V) && V->getAttr<UnusedAttr>() == 0) {
344              // Special case: check for initializations with constants.
345              //
346              //  e.g. : int x = 0;
347              //
348              // If x is EVER assigned a new value later, don't issue
349              // a warning.  This is because such initialization can be
350              // due to defensive programming.
351              if (E->isEvaluatable(Ctx))
352                return;
353
354              if (const DeclRefExpr *DRE =
355                  dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
356                if (const VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl())) {
357                  // Special case: check for initialization from constant
358                  //  variables.
359                  //
360                  //  e.g. extern const int MyConstant;
361                  //       int x = MyConstant;
362                  //
363                  if (VD->hasGlobalStorage() &&
364                      VD->getType().isConstQualified())
365                    return;
366                  // Special case: check for initialization from scalar
367                  //  parameters.  This is often a form of defensive
368                  //  programming.  Non-scalars are still an error since
369                  //  because it more likely represents an actual algorithmic
370                  //  bug.
371                  if (isa<ParmVarDecl>(VD) && VD->getType()->isScalarType())
372                    return;
373                }
374
375              PathDiagnosticLocation Loc =
376                PathDiagnosticLocation::create(V, BR.getSourceManager());
377              Report(V, DeadInit, Loc, E->getSourceRange());
378            }
379          }
380        }
381      }
382  }
383};
384
385} // end anonymous namespace
386
387//===----------------------------------------------------------------------===//
388// Driver function to invoke the Dead-Stores checker on a CFG.
389//===----------------------------------------------------------------------===//
390
391namespace {
392class FindEscaped {
393public:
394  llvm::SmallPtrSet<const VarDecl*, 20> Escaped;
395
396  void operator()(const Stmt *S) {
397    // Check for '&'. Any VarDecl whose address has been taken we treat as
398    // escaped.
399    // FIXME: What about references?
400    const UnaryOperator *U = dyn_cast<UnaryOperator>(S);
401    if (!U)
402      return;
403    if (U->getOpcode() != UO_AddrOf)
404      return;
405
406    const Expr *E = U->getSubExpr()->IgnoreParenCasts();
407    if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E))
408      if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl()))
409        Escaped.insert(VD);
410  }
411};
412} // end anonymous namespace
413
414
415//===----------------------------------------------------------------------===//
416// DeadStoresChecker
417//===----------------------------------------------------------------------===//
418
419namespace {
420class DeadStoresChecker : public Checker<check::ASTCodeBody> {
421public:
422  void checkASTCodeBody(const Decl *D, AnalysisManager& mgr,
423                        BugReporter &BR) const {
424
425    // Don't do anything for template instantiations.
426    // Proving that code in a template instantiation is "dead"
427    // means proving that it is dead in all instantiations.
428    // This same problem exists with -Wunreachable-code.
429    if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
430      if (FD->isTemplateInstantiation())
431        return;
432
433    if (LiveVariables *L = mgr.getAnalysis<LiveVariables>(D)) {
434      CFG &cfg = *mgr.getCFG(D);
435      AnalysisDeclContext *AC = mgr.getAnalysisDeclContext(D);
436      ParentMap &pmap = mgr.getParentMap(D);
437      FindEscaped FS;
438      cfg.VisitBlockStmts(FS);
439      DeadStoreObs A(cfg, BR.getContext(), BR, AC, pmap, FS.Escaped);
440      L->runOnAllBlocks(A);
441    }
442  }
443};
444}
445
446void ento::registerDeadStoresChecker(CheckerManager &mgr) {
447  mgr.registerChecker<DeadStoresChecker>();
448}
449