clang  3.7.0
DeadStoresChecker.cpp
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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"
24 #include "llvm/ADT/BitVector.h"
25 #include "llvm/ADT/SmallString.h"
26 #include "llvm/Support/SaveAndRestore.h"
27 
28 using namespace clang;
29 using namespace ento;
30 
31 namespace {
32 
33 /// A simple visitor to record what VarDecls occur in EH-handling code.
34 class EHCodeVisitor : public RecursiveASTVisitor<EHCodeVisitor> {
35 public:
36  bool inEH;
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.
67 class ReachableCode {
68  const CFG &cfg;
69  llvm::BitVector reachable;
70 public:
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 
82 void ReachableCode::computeReachableBlocks() {
83  if (!cfg.getNumBlockIDs())
84  return;
85 
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 
102 static const Expr *
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 
122 namespace {
123 class DeadStoreObs : public LiveVariables::Observer {
124  const CFG &cfg;
125  ASTContext &Ctx;
126  BugReporter& BR;
127  const CheckerBase *Checker;
129  ParentMap& Parents;
130  llvm::SmallPtrSet<const VarDecl*, 20> Escaped;
131  std::unique_ptr<ReachableCode> reachableCode;
132  const CFGBlock *currentBlock;
133  std::unique_ptr<llvm::DenseSet<const VarDecl *>> InEH;
134 
135  enum DeadStoreKind { Standard, Enclosing, DeadIncrement, DeadInit };
136 
137 public:
138  DeadStoreObs(const CFG &cfg, ASTContext &ctx, BugReporter &br,
139  const CheckerBase *checker, AnalysisDeclContext *ac,
140  ParentMap &parents,
141  llvm::SmallPtrSet<const VarDecl *, 20> &escaped)
142  : cfg(cfg), Ctx(ctx), BR(br), Checker(checker), AC(ac), Parents(parents),
143  Escaped(escaped), currentBlock(nullptr) {}
144 
145  ~DeadStoreObs() override {}
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,
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 = nullptr;
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(), Checker, BugType, "Dead store", os.str(),
205  L, R);
206  }
207 
208  void CheckVarDecl(const VarDecl *VD, const Expr *Ex, const Expr *Val,
209  DeadStoreKind dsk,
210  const LiveVariables::LivenessValues &Live) {
211 
212  if (!VD->hasLocalStorage())
213  return;
214  // Reference types confuse the dead stores checker. Skip them
215  // for now.
216  if (VD->getType()->getAs<ReferenceType>())
217  return;
218 
219  if (!isLive(Live, VD) &&
220  !(VD->hasAttr<UnusedAttr>() || VD->hasAttr<BlocksAttr>() ||
221  VD->hasAttr<ObjCPreciseLifetimeAttr>())) {
222 
223  PathDiagnosticLocation ExLoc =
224  PathDiagnosticLocation::createBegin(Ex, BR.getSourceManager(), AC);
225  Report(VD, dsk, ExLoc, Val->getSourceRange());
226  }
227  }
228 
229  void CheckDeclRef(const DeclRefExpr *DR, const Expr *Val, DeadStoreKind dsk,
230  const LiveVariables::LivenessValues& Live) {
231  if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl()))
232  CheckVarDecl(VD, DR, Val, dsk, Live);
233  }
234 
235  bool isIncrement(VarDecl *VD, const BinaryOperator* B) {
236  if (B->isCompoundAssignmentOp())
237  return true;
238 
239  const Expr *RHS = B->getRHS()->IgnoreParenCasts();
240  const BinaryOperator* BRHS = dyn_cast<BinaryOperator>(RHS);
241 
242  if (!BRHS)
243  return false;
244 
245  const DeclRefExpr *DR;
246 
247  if ((DR = dyn_cast<DeclRefExpr>(BRHS->getLHS()->IgnoreParenCasts())))
248  if (DR->getDecl() == VD)
249  return true;
250 
251  if ((DR = dyn_cast<DeclRefExpr>(BRHS->getRHS()->IgnoreParenCasts())))
252  if (DR->getDecl() == VD)
253  return true;
254 
255  return false;
256  }
257 
258  void observeStmt(const Stmt *S, const CFGBlock *block,
259  const LiveVariables::LivenessValues &Live) override {
260 
261  currentBlock = block;
262 
263  // Skip statements in macros.
264  if (S->getLocStart().isMacroID())
265  return;
266 
267  // Only cover dead stores from regular assignments. ++/-- dead stores
268  // have never flagged a real bug.
269  if (const BinaryOperator* B = dyn_cast<BinaryOperator>(S)) {
270  if (!B->isAssignmentOp()) return; // Skip non-assignments.
271 
272  if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(B->getLHS()))
273  if (VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
274  // Special case: check for assigning null to a pointer.
275  // This is a common form of defensive programming.
276  const Expr *RHS =
278  RHS = RHS->IgnoreParenCasts();
279 
280  QualType T = VD->getType();
281  if (T->isPointerType() || T->isObjCObjectPointerType()) {
282  if (RHS->isNullPointerConstant(Ctx, Expr::NPC_ValueDependentIsNull))
283  return;
284  }
285 
286  // Special case: self-assignments. These are often used to shut up
287  // "unused variable" compiler warnings.
288  if (const DeclRefExpr *RhsDR = dyn_cast<DeclRefExpr>(RHS))
289  if (VD == dyn_cast<VarDecl>(RhsDR->getDecl()))
290  return;
291 
292  // Otherwise, issue a warning.
293  DeadStoreKind dsk = Parents.isConsumedExpr(B)
294  ? Enclosing
295  : (isIncrement(VD,B) ? DeadIncrement : Standard);
296 
297  CheckVarDecl(VD, DR, B->getRHS(), dsk, Live);
298  }
299  }
300  else if (const UnaryOperator* U = dyn_cast<UnaryOperator>(S)) {
301  if (!U->isIncrementOp() || U->isPrefix())
302  return;
303 
304  const Stmt *parent = Parents.getParentIgnoreParenCasts(U);
305  if (!parent || !isa<ReturnStmt>(parent))
306  return;
307 
308  const Expr *Ex = U->getSubExpr()->IgnoreParenCasts();
309 
310  if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(Ex))
311  CheckDeclRef(DR, U, DeadIncrement, Live);
312  }
313  else if (const DeclStmt *DS = dyn_cast<DeclStmt>(S))
314  // Iterate through the decls. Warn if any initializers are complex
315  // expressions that are not live (never used).
316  for (const auto *DI : DS->decls()) {
317  const auto *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;
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' or 'objc_precise_lifetime'.
345  if (!isLive(Live, V) &&
346  !V->hasAttr<UnusedAttr>() &&
347  !V->hasAttr<ObjCPreciseLifetimeAttr>()) {
348  // Special case: check for initializations with constants.
349  //
350  // e.g. : int x = 0;
351  //
352  // If x is EVER assigned a new value later, don't issue
353  // a warning. This is because such initialization can be
354  // due to defensive programming.
355  if (E->isEvaluatable(Ctx))
356  return;
357 
358  if (const DeclRefExpr *DRE =
359  dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
360  if (const VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl())) {
361  // Special case: check for initialization from constant
362  // variables.
363  //
364  // e.g. extern const int MyConstant;
365  // int x = MyConstant;
366  //
367  if (VD->hasGlobalStorage() &&
368  VD->getType().isConstQualified())
369  return;
370  // Special case: check for initialization from scalar
371  // parameters. This is often a form of defensive
372  // programming. Non-scalars are still an error since
373  // because it more likely represents an actual algorithmic
374  // bug.
375  if (isa<ParmVarDecl>(VD) && VD->getType()->isScalarType())
376  return;
377  }
378 
380  PathDiagnosticLocation::create(V, BR.getSourceManager());
381  Report(V, DeadInit, Loc, E->getSourceRange());
382  }
383  }
384  }
385  }
386  }
387 };
388 
389 } // end anonymous namespace
390 
391 //===----------------------------------------------------------------------===//
392 // Driver function to invoke the Dead-Stores checker on a CFG.
393 //===----------------------------------------------------------------------===//
394 
395 namespace {
396 class FindEscaped {
397 public:
398  llvm::SmallPtrSet<const VarDecl*, 20> Escaped;
399 
400  void operator()(const Stmt *S) {
401  // Check for '&'. Any VarDecl whose address has been taken we treat as
402  // escaped.
403  // FIXME: What about references?
404  const UnaryOperator *U = dyn_cast<UnaryOperator>(S);
405  if (!U)
406  return;
407  if (U->getOpcode() != UO_AddrOf)
408  return;
409 
410  const Expr *E = U->getSubExpr()->IgnoreParenCasts();
411  if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E))
412  if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl()))
413  Escaped.insert(VD);
414  }
415 };
416 } // end anonymous namespace
417 
418 
419 //===----------------------------------------------------------------------===//
420 // DeadStoresChecker
421 //===----------------------------------------------------------------------===//
422 
423 namespace {
424 class DeadStoresChecker : public Checker<check::ASTCodeBody> {
425 public:
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);
440  ParentMap &pmap = mgr.getParentMap(D);
441  FindEscaped FS;
442  cfg.VisitBlockStmts(FS);
443  DeadStoreObs A(cfg, BR.getContext(), BR, this, AC, pmap, FS.Escaped);
444  L->runOnAllBlocks(A);
445  }
446  }
447 };
448 }
449 
450 void ento::registerDeadStoresChecker(CheckerManager &mgr) {
451  mgr.registerChecker<DeadStoresChecker>();
452 }
Defines the clang::ASTContext interface.
succ_iterator succ_begin()
Definition: CFG.h:542
bool hasGlobalStorage() const
Returns true for all variables that do not have local storage.
Definition: Decl.h:922
static bool isAssignmentOp(Opcode Opc)
Definition: Expr.h:3040
bool isScalarType() const
Definition: Type.h:5461
bool hasAttr() const
Definition: DeclBase.h:487
Holds long-lived AST nodes (such as types and decls) that can be referred to throughout the semantic ...
Definition: ASTContext.h:89
Expr * getLHS() const
Definition: Expr.h:2964
Represents Objective-C's @catch statement.
Definition: StmtObjC.h:74
ASTContext & getContext()
Definition: BugReporter.h:446
Stmt * getHandlerBlock() const
Definition: StmtCXX.h:52
A builtin binary operation expression such as "x + y" or "x <= y".
Definition: Expr.h:2918
Expr * IgnoreParenCasts() LLVM_READONLY
Definition: Expr.cpp:2439
static PathDiagnosticLocation create(const Decl *D, const SourceManager &SM)
Create a location corresponding to the given declaration.
A class that does preorder depth-first traversal on the entire Clang AST and visits each node...
QualType getType() const
Definition: Decl.h:538
AnalysisDeclContext * getAnalysisDeclContext(const Decl *D)
void VisitBlockStmts(CALLBACK &O) const
Definition: CFG.h:917
Specifies that a value-dependent expression of integral or dependent type should be considered a null...
Definition: Expr.h:662
Expr * getSubExpr() const
Definition: Expr.h:1699
unsigned getBlockID() const
Definition: CFG.h:639
CFG * getCFG(Decl const *D)
ValueDecl * getDecl()
Definition: Expr.h:994
static PathDiagnosticLocation createBegin(const Decl *D, const SourceManager &SM)
Create a location for the beginning of the declaration.
#define false
Definition: stdbool.h:33
CHECKER * registerChecker()
Used to register checkers.
ParentMap & getParentMap(Decl const *D)
succ_iterator succ_end()
Definition: CFG.h:543
AdjacentBlocks::const_iterator const_succ_iterator
Definition: CFG.h:528
Opcode getOpcode() const
Definition: Expr.h:1696
static const Expr * LookThroughTransitiveAssignmentsAndCommaOperators(const Expr *Ex)
const T * getAs() const
Definition: Type.h:5555
bool isLive(const Stmt *S) const
Represents Objective-C's @finally statement.
Definition: StmtObjC.h:120
static bool isCompoundAssignmentOp(Opcode Opc)
Definition: Expr.h:3045
bool isObjCObjectPointerType() const
Definition: Type.h:5304
Opcode getOpcode() const
Definition: Expr.h:2961
Expr * getRHS() const
Definition: Expr.h:2966
A reference to a declared variable, function, enum, etc. [C99 6.5.1p2].
Definition: Expr.h:899
A trivial tuple used to represent a source range.
bool isConstQualified() const
Determine whether this type is const-qualified.
Definition: Type.h:5075
T * getAnalysis(Decl const *D)
bool hasLocalStorage() const
Definition: Decl.h:887
bool isPointerType() const
Definition: Type.h:5232