clang  8.0.0svn
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,
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 = 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  LLVM_FALLTHROUGH;
193  case Standard:
194  if (!BugType) BugType = "Dead assignment";
195  os << "Value stored to '" << *V << "' is never read";
196  break;
197 
198  case Enclosing:
199  // Don't report issues in this case, e.g.: "if (x = foo())",
200  // where 'x' is unused later. We have yet to see a case where
201  // this is a real bug.
202  return;
203  }
204 
205  BR.EmitBasicReport(AC->getDecl(), Checker, BugType, "Dead store", os.str(),
206  L, R);
207  }
208 
209  void CheckVarDecl(const VarDecl *VD, const Expr *Ex, const Expr *Val,
210  DeadStoreKind dsk,
211  const LiveVariables::LivenessValues &Live) {
212 
213  if (!VD->hasLocalStorage())
214  return;
215  // Reference types confuse the dead stores checker. Skip them
216  // for now.
217  if (VD->getType()->getAs<ReferenceType>())
218  return;
219 
220  if (!isLive(Live, VD) &&
221  !(VD->hasAttr<UnusedAttr>() || VD->hasAttr<BlocksAttr>() ||
222  VD->hasAttr<ObjCPreciseLifetimeAttr>())) {
223 
224  PathDiagnosticLocation ExLoc =
225  PathDiagnosticLocation::createBegin(Ex, BR.getSourceManager(), AC);
226  Report(VD, dsk, ExLoc, Val->getSourceRange());
227  }
228  }
229 
230  void CheckDeclRef(const DeclRefExpr *DR, const Expr *Val, DeadStoreKind dsk,
231  const LiveVariables::LivenessValues& Live) {
232  if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl()))
233  CheckVarDecl(VD, DR, Val, dsk, Live);
234  }
235 
236  bool isIncrement(VarDecl *VD, const BinaryOperator* B) {
237  if (B->isCompoundAssignmentOp())
238  return true;
239 
240  const Expr *RHS = B->getRHS()->IgnoreParenCasts();
241  const BinaryOperator* BRHS = dyn_cast<BinaryOperator>(RHS);
242 
243  if (!BRHS)
244  return false;
245 
246  const DeclRefExpr *DR;
247 
248  if ((DR = dyn_cast<DeclRefExpr>(BRHS->getLHS()->IgnoreParenCasts())))
249  if (DR->getDecl() == VD)
250  return true;
251 
252  if ((DR = dyn_cast<DeclRefExpr>(BRHS->getRHS()->IgnoreParenCasts())))
253  if (DR->getDecl() == VD)
254  return true;
255 
256  return false;
257  }
258 
259  void observeStmt(const Stmt *S, const CFGBlock *block,
260  const LiveVariables::LivenessValues &Live) override {
261 
262  currentBlock = block;
263 
264  // Skip statements in macros.
265  if (S->getBeginLoc().isMacroID())
266  return;
267 
268  // Only cover dead stores from regular assignments. ++/-- dead stores
269  // have never flagged a real bug.
270  if (const BinaryOperator* B = dyn_cast<BinaryOperator>(S)) {
271  if (!B->isAssignmentOp()) return; // Skip non-assignments.
272 
273  if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(B->getLHS()))
274  if (VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
275  // Special case: check for assigning null to a pointer.
276  // This is a common form of defensive programming.
277  const Expr *RHS =
279  RHS = RHS->IgnoreParenCasts();
280 
281  QualType T = VD->getType();
282  if (T.isVolatileQualified())
283  return;
284  if (T->isPointerType() || T->isObjCObjectPointerType()) {
285  if (RHS->isNullPointerConstant(Ctx, Expr::NPC_ValueDependentIsNull))
286  return;
287  }
288 
289  // Special case: self-assignments. These are often used to shut up
290  // "unused variable" compiler warnings.
291  if (const DeclRefExpr *RhsDR = dyn_cast<DeclRefExpr>(RHS))
292  if (VD == dyn_cast<VarDecl>(RhsDR->getDecl()))
293  return;
294 
295  // Otherwise, issue a warning.
296  DeadStoreKind dsk = Parents.isConsumedExpr(B)
297  ? Enclosing
298  : (isIncrement(VD,B) ? DeadIncrement : Standard);
299 
300  CheckVarDecl(VD, DR, B->getRHS(), dsk, Live);
301  }
302  }
303  else if (const UnaryOperator* U = dyn_cast<UnaryOperator>(S)) {
304  if (!U->isIncrementOp() || U->isPrefix())
305  return;
306 
307  const Stmt *parent = Parents.getParentIgnoreParenCasts(U);
308  if (!parent || !isa<ReturnStmt>(parent))
309  return;
310 
311  const Expr *Ex = U->getSubExpr()->IgnoreParenCasts();
312 
313  if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(Ex))
314  CheckDeclRef(DR, U, DeadIncrement, Live);
315  }
316  else if (const DeclStmt *DS = dyn_cast<DeclStmt>(S))
317  // Iterate through the decls. Warn if any initializers are complex
318  // expressions that are not live (never used).
319  for (const auto *DI : DS->decls()) {
320  const auto *V = dyn_cast<VarDecl>(DI);
321 
322  if (!V)
323  continue;
324 
325  if (V->hasLocalStorage()) {
326  // Reference types confuse the dead stores checker. Skip them
327  // for now.
328  if (V->getType()->getAs<ReferenceType>())
329  return;
330 
331  if (const Expr *E = V->getInit()) {
332  while (const FullExpr *FE = dyn_cast<FullExpr>(E))
333  E = FE->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 
381  PathDiagnosticLocation Loc =
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);
468  AnalysisDeclContext *AC = mgr.getAnalysisDeclContext(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.
Represents a function declaration or definition.
Definition: Decl.h:1739
A (possibly-)qualified type.
Definition: Type.h:642
Stmt * getBody() const
Get the body of the Declaration.
succ_iterator succ_begin()
Definition: CFG.h:751
Stmt - This represents one statement.
Definition: Stmt.h:66
unsigned getBlockID() const
Definition: CFG.h:856
Decl - This represents one declaration (or definition), e.g.
Definition: DeclBase.h:87
Stmt * getHandlerBlock() const
Definition: StmtCXX.h:52
Opcode getOpcode() const
Definition: Expr.h:3283
Describes the capture of a variable or of this, or of a C++1y init-capture.
Definition: LambdaCapture.h:26
bool isConsumedExpr(Expr *E) const
Definition: ParentMap.cpp:160
Represents a variable declaration or definition.
Definition: Decl.h:812
const T * getAs() const
Member-template getAs<specific type>&#39;.
Definition: Type.h:6716
static bool isAssignmentOp(Opcode Opc)
Definition: Expr.h:3371
CXXRecordDecl * getLambdaClass() const
Retrieve the class that corresponds to the lambda.
Definition: ExprCXX.cpp:1015
Holds long-lived AST nodes (such as types and decls) that can be referred to throughout the semantic ...
Definition: ASTContext.h:154
FullExpr - Represents a "full-expression" node.
Definition: Expr.h:877
SourceLocation getBeginLoc() const LLVM_READONLY
Definition: Stmt.cpp:288
Represents a member of a struct/union/class.
Definition: Decl.h:2588
AnalysisDeclContext contains the context data for the function or method under analysis.
bool isReferenceType() const
Definition: Type.h:6294
Represents Objective-C&#39;s @catch statement.
Definition: StmtObjC.h:74
AdjacentBlocks::const_iterator const_succ_iterator
Definition: CFG.h:727
A builtin binary operation expression such as "x + y" or "x <= y".
Definition: Expr.h:3248
bool isVolatileQualified() const
Determine whether this type is volatile-qualified.
Definition: Type.h:6128
Expr * IgnoreParenCasts() LLVM_READONLY
IgnoreParenCasts - Ignore parentheses and casts.
Definition: Expr.cpp:2557
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:1610
bool isScalarType() const
Definition: Type.h:6615
A class that does preorder or postorder depth-first traversal on the entire Clang AST and visits each...
bool hasAttr() const
Definition: DeclBase.h:531
Represents a single basic block in a source-level CFG.
Definition: CFG.h:552
This represents one expression.
Definition: Expr.h:106
Represents a source-level, intra-procedural CFG that represents the control-flow of a Stmt...
Definition: CFG.h:1003
bool hasLocalStorage() const
Returns true if a variable with function scope is a non-static local variable.
Definition: Decl.h:1035
Specifies that a value-dependent expression of integral or dependent type should be considered a null...
Definition: Expr.h:715
UnaryOperator - This represents the unary-expression&#39;s (except sizeof and alignof), the postinc/postdec operators from postfix-expression, and various extensions.
Definition: Expr.h:1907
ValueDecl * getDecl()
Definition: Expr.h:1125
bool isConstQualified() const
Determine whether this type is const-qualified.
Definition: Type.h:6117
static PathDiagnosticLocation createBegin(const Decl *D, const SourceManager &SM)
Create a location for the beginning of the declaration.
#define false
Definition: stdbool.h:33
Expr * getSubExpr() const
Definition: Expr.h:1937
DeclStmt - Adaptor class for mixing declarations with statements and expressions. ...
Definition: Stmt.h:926
bool hasGlobalStorage() const
Returns true for all variables that do not have local storage.
Definition: Decl.h:1077
const Decl * getDecl() const
bool isObjCObjectPointerType() const
Definition: Type.h:6379
SourceRange getSourceRange() const override LLVM_READONLY
Source range that this declaration covers.
Definition: Decl.cpp:1958
succ_iterator succ_end()
Definition: CFG.h:752
Expr * getLHS() const
Definition: Expr.h:3288
Dataflow Directional Tag Classes.
void VisitBlockStmts(CALLBACK &O) const
Definition: CFG.h:1155
static const Expr * LookThroughTransitiveAssignmentsAndCommaOperators(const Expr *Ex)
bool isLive(const Stmt *S) const
const Expr * getInit() const
Definition: Decl.h:1219
Opcode getOpcode() const
Definition: Expr.h:1932
Base for LValueReferenceType and RValueReferenceType.
Definition: Type.h:2679
Represents Objective-C&#39;s @finally statement.
Definition: StmtObjC.h:120
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:1394
Represents a C++ struct/union/class.
Definition: DeclCXX.h:300
static bool isCompoundAssignmentOp(Opcode Opc)
Definition: Expr.h:3376
CXXCatchStmt - This represents a C++ catch block.
Definition: StmtCXX.h:29
capture_range captures() const
Retrieve this lambda&#39;s captures.
Definition: ExprCXX.cpp:985
SourceRange getSourceRange() const LLVM_READONLY
SourceLocation tokens are not useful in isolation - they are low level value objects created/interpre...
Definition: Stmt.cpp:276
Stmt * getParentIgnoreParenCasts(Stmt *) const
Definition: ParentMap.cpp:134
A reference to a declared variable, function, enum, etc.
Definition: Expr.h:1042
Expr * getRHS() const
Definition: Expr.h:3290
bool isPointerType() const
Definition: Type.h:6282
QualType getType() const
Definition: Decl.h:647
A trivial tuple used to represent a source range.