clang  3.9.0
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.isVolatileQualified())
282  return;
283  if (T->isPointerType() || T->isObjCObjectPointerType()) {
284  if (RHS->isNullPointerConstant(Ctx, Expr::NPC_ValueDependentIsNull))
285  return;
286  }
287 
288  // Special case: self-assignments. These are often used to shut up
289  // "unused variable" compiler warnings.
290  if (const DeclRefExpr *RhsDR = dyn_cast<DeclRefExpr>(RHS))
291  if (VD == dyn_cast<VarDecl>(RhsDR->getDecl()))
292  return;
293 
294  // Otherwise, issue a warning.
295  DeadStoreKind dsk = Parents.isConsumedExpr(B)
296  ? Enclosing
297  : (isIncrement(VD,B) ? DeadIncrement : Standard);
298 
299  CheckVarDecl(VD, DR, B->getRHS(), dsk, Live);
300  }
301  }
302  else if (const UnaryOperator* U = dyn_cast<UnaryOperator>(S)) {
303  if (!U->isIncrementOp() || U->isPrefix())
304  return;
305 
306  const Stmt *parent = Parents.getParentIgnoreParenCasts(U);
307  if (!parent || !isa<ReturnStmt>(parent))
308  return;
309 
310  const Expr *Ex = U->getSubExpr()->IgnoreParenCasts();
311 
312  if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(Ex))
313  CheckDeclRef(DR, U, DeadIncrement, Live);
314  }
315  else if (const DeclStmt *DS = dyn_cast<DeclStmt>(S))
316  // Iterate through the decls. Warn if any initializers are complex
317  // expressions that are not live (never used).
318  for (const auto *DI : DS->decls()) {
319  const auto *V = dyn_cast<VarDecl>(DI);
320 
321  if (!V)
322  continue;
323 
324  if (V->hasLocalStorage()) {
325  // Reference types confuse the dead stores checker. Skip them
326  // for now.
327  if (V->getType()->getAs<ReferenceType>())
328  return;
329 
330  if (const Expr *E = V->getInit()) {
331  while (const ExprWithCleanups *exprClean =
332  dyn_cast<ExprWithCleanups>(E))
333  E = exprClean->getSubExpr();
334 
335  // Look through transitive assignments, e.g.:
336  // int x = y = 0;
338 
339  // Don't warn on C++ objects (yet) until we can show that their
340  // constructors/destructors don't have side effects.
341  if (isa<CXXConstructExpr>(E))
342  return;
343 
344  // A dead initialization is a variable that is dead after it
345  // is initialized. We don't flag warnings for those variables
346  // marked 'unused' or 'objc_precise_lifetime'.
347  if (!isLive(Live, V) &&
348  !V->hasAttr<UnusedAttr>() &&
349  !V->hasAttr<ObjCPreciseLifetimeAttr>()) {
350  // Special case: check for initializations with constants.
351  //
352  // e.g. : int x = 0;
353  //
354  // If x is EVER assigned a new value later, don't issue
355  // a warning. This is because such initialization can be
356  // due to defensive programming.
357  if (E->isEvaluatable(Ctx))
358  return;
359 
360  if (const DeclRefExpr *DRE =
361  dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
362  if (const VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl())) {
363  // Special case: check for initialization from constant
364  // variables.
365  //
366  // e.g. extern const int MyConstant;
367  // int x = MyConstant;
368  //
369  if (VD->hasGlobalStorage() &&
370  VD->getType().isConstQualified())
371  return;
372  // Special case: check for initialization from scalar
373  // parameters. This is often a form of defensive
374  // programming. Non-scalars are still an error since
375  // because it more likely represents an actual algorithmic
376  // bug.
377  if (isa<ParmVarDecl>(VD) && VD->getType()->isScalarType())
378  return;
379  }
380 
382  PathDiagnosticLocation::create(V, BR.getSourceManager());
383  Report(V, DeadInit, Loc, E->getSourceRange());
384  }
385  }
386  }
387  }
388  }
389 };
390 
391 } // end anonymous namespace
392 
393 //===----------------------------------------------------------------------===//
394 // Driver function to invoke the Dead-Stores checker on a CFG.
395 //===----------------------------------------------------------------------===//
396 
397 namespace {
398 class FindEscaped {
399 public:
400  llvm::SmallPtrSet<const VarDecl*, 20> Escaped;
401 
402  void operator()(const Stmt *S) {
403  // Check for '&'. Any VarDecl whose address has been taken we treat as
404  // escaped.
405  // FIXME: What about references?
406  if (auto *LE = dyn_cast<LambdaExpr>(S)) {
407  findLambdaReferenceCaptures(LE);
408  return;
409  }
410 
411  const UnaryOperator *U = dyn_cast<UnaryOperator>(S);
412  if (!U)
413  return;
414  if (U->getOpcode() != UO_AddrOf)
415  return;
416 
417  const Expr *E = U->getSubExpr()->IgnoreParenCasts();
418  if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E))
419  if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl()))
420  Escaped.insert(VD);
421  }
422 
423  // Treat local variables captured by reference in C++ lambdas as escaped.
424  void findLambdaReferenceCaptures(const LambdaExpr *LE) {
425  const CXXRecordDecl *LambdaClass = LE->getLambdaClass();
426  llvm::DenseMap<const VarDecl *, FieldDecl *> CaptureFields;
427  FieldDecl *ThisCaptureField;
428  LambdaClass->getCaptureFields(CaptureFields, ThisCaptureField);
429 
430  for (const LambdaCapture &C : LE->captures()) {
431  if (!C.capturesVariable())
432  continue;
433 
434  VarDecl *VD = C.getCapturedVar();
435  const FieldDecl *FD = CaptureFields[VD];
436  if (!FD)
437  continue;
438 
439  // If the capture field is a reference type, it is capture-by-reference.
440  if (FD->getType()->isReferenceType())
441  Escaped.insert(VD);
442  }
443  }
444 };
445 } // end anonymous namespace
446 
447 
448 //===----------------------------------------------------------------------===//
449 // DeadStoresChecker
450 //===----------------------------------------------------------------------===//
451 
452 namespace {
453 class DeadStoresChecker : public Checker<check::ASTCodeBody> {
454 public:
455  void checkASTCodeBody(const Decl *D, AnalysisManager& mgr,
456  BugReporter &BR) const {
457 
458  // Don't do anything for template instantiations.
459  // Proving that code in a template instantiation is "dead"
460  // means proving that it is dead in all instantiations.
461  // This same problem exists with -Wunreachable-code.
462  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
463  if (FD->isTemplateInstantiation())
464  return;
465 
466  if (LiveVariables *L = mgr.getAnalysis<LiveVariables>(D)) {
467  CFG &cfg = *mgr.getCFG(D);
469  ParentMap &pmap = mgr.getParentMap(D);
470  FindEscaped FS;
471  cfg.VisitBlockStmts(FS);
472  DeadStoreObs A(cfg, BR.getContext(), BR, this, AC, pmap, FS.Escaped);
473  L->runOnAllBlocks(A);
474  }
475  }
476 };
477 }
478 
479 void ento::registerDeadStoresChecker(CheckerManager &mgr) {
480  mgr.registerChecker<DeadStoresChecker>();
481 }
Defines the clang::ASTContext interface.
FunctionDecl - An instance of this class is created to represent a function declaration or definition...
Definition: Decl.h:1561
A (possibly-)qualified type.
Definition: Type.h:598
succ_iterator succ_begin()
Definition: CFG.h:541
capture_range captures() const
Retrieve this lambda's captures.
Definition: ExprCXX.cpp:952
Describes the capture of a variable or of this, or of a C++1y init-capture.
Definition: LambdaCapture.h:26
VarDecl - An instance of this class is created to represent a variable declaration or definition...
Definition: Decl.h:768
bool hasGlobalStorage() const
Returns true for all variables that do not have local storage.
Definition: Decl.h:998
Represents an expression – generally a full-expression – that introduces cleanups to be run at the en...
Definition: ExprCXX.h:2936
static bool isAssignmentOp(Opcode Opc)
Definition: Expr.h:3022
bool isScalarType() const
Definition: Type.h:5715
Holds long-lived AST nodes (such as types and decls) that can be referred to throughout the semantic ...
Definition: ASTContext.h:92
bool isReferenceType() const
Definition: Type.h:5491
FieldDecl - An instance of this class is created by Sema::ActOnField to represent a member of a struc...
Definition: Decl.h:2293
AnalysisDeclContext contains the context data for the function or method under analysis.
Expr * getLHS() const
Definition: Expr.h:2943
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:2897
Expr * IgnoreParenCasts() LLVM_READONLY
IgnoreParenCasts - Ignore parentheses and casts.
Definition: Expr.cpp:2326
static PathDiagnosticLocation create(const Decl *D, const SourceManager &SM)
Create a location corresponding to the given declaration.
A C++ lambda expression, which produces a function object (of unspecified type) that can be invoked l...
Definition: ExprCXX.h:1503
A class that does preordor or postorder depth-first traversal on the entire Clang AST and visits each...
QualType getType() const
Definition: Decl.h:599
AnalysisDeclContext * getAnalysisDeclContext(const Decl *D)
CFGBlock - Represents a single basic block in a source-level CFG.
Definition: CFG.h:353
Expr - This represents one expression.
Definition: Expr.h:105
void VisitBlockStmts(CALLBACK &O) const
Definition: CFG.h:916
CFG - Represents a source-level, intra-procedural CFG that represents the control-flow of a Stmt...
Definition: CFG.h:721
Specifies that a value-dependent expression of integral or dependent type should be considered a null...
Definition: Expr.h:692
Expr * getSubExpr() const
Definition: Expr.h:1695
unsigned getBlockID() const
Definition: CFG.h:638
UnaryOperator - This represents the unary-expression's (except sizeof and alignof), the postinc/postdec operators from postfix-expression, and various extensions.
Definition: Expr.h:1668
CFG * getCFG(Decl const *D)
ValueDecl * getDecl()
Definition: Expr.h:1017
BugReporter is a utility class for generating PathDiagnostics for analysis.
Definition: BugReporter.h:388
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.
DeclStmt - Adaptor class for mixing declarations with statements and expressions. ...
Definition: Stmt.h:443
ParentMap & getParentMap(Decl const *D)
void getCaptureFields(llvm::DenseMap< const VarDecl *, FieldDecl * > &Captures, FieldDecl *&ThisCapture) const
For a closure type, retrieve the mapping from captured variables and this to the non-static data memb...
Definition: DeclCXX.cpp:1083
succ_iterator succ_end()
Definition: CFG.h:542
bool isVolatileQualified() const
Determine whether this type is volatile-qualified.
Definition: Type.h:5329
AdjacentBlocks::const_iterator const_succ_iterator
Definition: CFG.h:527
Opcode getOpcode() const
Definition: Expr.h:1692
static const Expr * LookThroughTransitiveAssignmentsAndCommaOperators(const Expr *Ex)
detail::InMemoryDirectory::const_iterator E
const T * getAs() const
Member-template getAs<specific type>'.
Definition: Type.h:5818
Base for LValueReferenceType and RValueReferenceType.
Definition: Type.h:2319
bool isLive(const Stmt *S) const
Represents Objective-C's @finally statement.
Definition: StmtObjC.h:120
Represents a C++ struct/union/class.
Definition: DeclCXX.h:263
static bool isCompoundAssignmentOp(Opcode Opc)
Definition: Expr.h:3027
bool isObjCObjectPointerType() const
Definition: Type.h:5554
Opcode getOpcode() const
Definition: Expr.h:2940
CXXCatchStmt - This represents a C++ catch block.
Definition: StmtCXX.h:29
Expr * getRHS() const
Definition: Expr.h:2945
A reference to a declared variable, function, enum, etc.
Definition: Expr.h:932
A trivial tuple used to represent a source range.
bool isConstQualified() const
Determine whether this type is const-qualified.
Definition: Type.h:5318
T * getAnalysis(Decl const *D)
CXXRecordDecl * getLambdaClass() const
Retrieve the class that corresponds to the lambda.
Definition: ExprCXX.cpp:995
bool hasLocalStorage() const
hasLocalStorage - Returns true if a variable with function scope is a non-static local variable...
Definition: Decl.h:963
bool isPointerType() const
Definition: Type.h:5482