DeadStoresChecker.cpp revision 5f9e272e632e951b1efe824cd16acb4d96077930
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/StaticAnalyzer/Core/Checker.h" 17#include "clang/Analysis/Analyses/LiveVariables.h" 18#include "clang/Analysis/Visitors/CFGRecStmtVisitor.h" 19#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h" 20#include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h" 21#include "clang/Analysis/Visitors/CFGRecStmtDeclVisitor.h" 22#include "clang/Basic/Diagnostic.h" 23#include "clang/AST/ASTContext.h" 24#include "clang/AST/ParentMap.h" 25#include "llvm/ADT/SmallPtrSet.h" 26 27using namespace clang; 28using namespace ento; 29 30namespace { 31 32// FIXME: Eventually migrate into its own file, and have it managed by 33// AnalysisManager. 34class ReachableCode { 35 const CFG &cfg; 36 llvm::BitVector reachable; 37public: 38 ReachableCode(const CFG &cfg) 39 : cfg(cfg), reachable(cfg.getNumBlockIDs(), false) {} 40 41 void computeReachableBlocks(); 42 43 bool isReachable(const CFGBlock *block) const { 44 return reachable[block->getBlockID()]; 45 } 46}; 47} 48 49void ReachableCode::computeReachableBlocks() { 50 if (!cfg.getNumBlockIDs()) 51 return; 52 53 SmallVector<const CFGBlock*, 10> worklist; 54 worklist.push_back(&cfg.getEntry()); 55 56 while (!worklist.empty()) { 57 const CFGBlock *block = worklist.back(); 58 worklist.pop_back(); 59 llvm::BitVector::reference isReachable = reachable[block->getBlockID()]; 60 if (isReachable) 61 continue; 62 isReachable = true; 63 for (CFGBlock::const_succ_iterator i = block->succ_begin(), 64 e = block->succ_end(); i != e; ++i) 65 if (const CFGBlock *succ = *i) 66 worklist.push_back(succ); 67 } 68} 69 70namespace { 71class DeadStoreObs : public LiveVariables::ObserverTy { 72 const CFG &cfg; 73 ASTContext &Ctx; 74 BugReporter& BR; 75 ParentMap& Parents; 76 llvm::SmallPtrSet<VarDecl*, 20> Escaped; 77 llvm::OwningPtr<ReachableCode> reachableCode; 78 const CFGBlock *currentBlock; 79 80 enum DeadStoreKind { Standard, Enclosing, DeadIncrement, DeadInit }; 81 82public: 83 DeadStoreObs(const CFG &cfg, ASTContext &ctx, 84 BugReporter& br, ParentMap& parents, 85 llvm::SmallPtrSet<VarDecl*, 20> &escaped) 86 : cfg(cfg), Ctx(ctx), BR(br), Parents(parents), 87 Escaped(escaped), currentBlock(0) {} 88 89 virtual ~DeadStoreObs() {} 90 91 void Report(VarDecl* V, DeadStoreKind dsk, SourceLocation L, SourceRange R) { 92 if (Escaped.count(V)) 93 return; 94 95 // Compute reachable blocks within the CFG for trivial cases 96 // where a bogus dead store can be reported because itself is unreachable. 97 if (!reachableCode.get()) { 98 reachableCode.reset(new ReachableCode(cfg)); 99 reachableCode->computeReachableBlocks(); 100 } 101 102 if (!reachableCode->isReachable(currentBlock)) 103 return; 104 105 const std::string &name = V->getNameAsString(); 106 107 const char* BugType = 0; 108 std::string msg; 109 110 switch (dsk) { 111 default: 112 assert(false && "Impossible dead store type."); 113 114 case DeadInit: 115 BugType = "Dead initialization"; 116 msg = "Value stored to '" + name + 117 "' during its initialization is never read"; 118 break; 119 120 case DeadIncrement: 121 BugType = "Dead increment"; 122 case Standard: 123 if (!BugType) BugType = "Dead assignment"; 124 msg = "Value stored to '" + name + "' is never read"; 125 break; 126 127 case Enclosing: 128 // Don't report issues in this case, e.g.: "if (x = foo())", 129 // where 'x' is unused later. We have yet to see a case where 130 // this is a real bug. 131 return; 132 } 133 134 BR.EmitBasicReport(BugType, "Dead store", msg, L, R); 135 } 136 137 void CheckVarDecl(VarDecl* VD, Expr* Ex, Expr* Val, 138 DeadStoreKind dsk, 139 const LiveVariables::AnalysisDataTy& AD, 140 const LiveVariables::ValTy& Live) { 141 142 if (!VD->hasLocalStorage()) 143 return; 144 // Reference types confuse the dead stores checker. Skip them 145 // for now. 146 if (VD->getType()->getAs<ReferenceType>()) 147 return; 148 149 if (!Live(VD, AD) && 150 !(VD->getAttr<UnusedAttr>() || VD->getAttr<BlocksAttr>())) 151 Report(VD, dsk, Ex->getSourceRange().getBegin(), 152 Val->getSourceRange()); 153 } 154 155 void CheckDeclRef(DeclRefExpr* DR, Expr* Val, DeadStoreKind dsk, 156 const LiveVariables::AnalysisDataTy& AD, 157 const LiveVariables::ValTy& Live) { 158 if (VarDecl* VD = dyn_cast<VarDecl>(DR->getDecl())) 159 CheckVarDecl(VD, DR, Val, dsk, AD, Live); 160 } 161 162 bool isIncrement(VarDecl* VD, BinaryOperator* B) { 163 if (B->isCompoundAssignmentOp()) 164 return true; 165 166 Expr* RHS = B->getRHS()->IgnoreParenCasts(); 167 BinaryOperator* BRHS = dyn_cast<BinaryOperator>(RHS); 168 169 if (!BRHS) 170 return false; 171 172 DeclRefExpr *DR; 173 174 if ((DR = dyn_cast<DeclRefExpr>(BRHS->getLHS()->IgnoreParenCasts()))) 175 if (DR->getDecl() == VD) 176 return true; 177 178 if ((DR = dyn_cast<DeclRefExpr>(BRHS->getRHS()->IgnoreParenCasts()))) 179 if (DR->getDecl() == VD) 180 return true; 181 182 return false; 183 } 184 185 virtual void ObserveStmt(Stmt* S, const CFGBlock *block, 186 const LiveVariables::AnalysisDataTy& AD, 187 const LiveVariables::ValTy& Live) { 188 189 currentBlock = block; 190 191 // Skip statements in macros. 192 if (S->getLocStart().isMacroID()) 193 return; 194 195 // Only cover dead stores from regular assignments. ++/-- dead stores 196 // have never flagged a real bug. 197 if (BinaryOperator* B = dyn_cast<BinaryOperator>(S)) { 198 if (!B->isAssignmentOp()) return; // Skip non-assignments. 199 200 if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(B->getLHS())) 201 if (VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) { 202 // Special case: check for assigning null to a pointer. 203 // This is a common form of defensive programming. 204 QualType T = VD->getType(); 205 if (T->isPointerType() || T->isObjCObjectPointerType()) { 206 if (B->getRHS()->isNullPointerConstant(Ctx, 207 Expr::NPC_ValueDependentIsNull)) 208 return; 209 } 210 211 Expr* RHS = B->getRHS()->IgnoreParenCasts(); 212 // Special case: self-assignments. These are often used to shut up 213 // "unused variable" compiler warnings. 214 if (DeclRefExpr* RhsDR = dyn_cast<DeclRefExpr>(RHS)) 215 if (VD == dyn_cast<VarDecl>(RhsDR->getDecl())) 216 return; 217 218 // Otherwise, issue a warning. 219 DeadStoreKind dsk = Parents.isConsumedExpr(B) 220 ? Enclosing 221 : (isIncrement(VD,B) ? DeadIncrement : Standard); 222 223 CheckVarDecl(VD, DR, B->getRHS(), dsk, AD, Live); 224 } 225 } 226 else if (UnaryOperator* U = dyn_cast<UnaryOperator>(S)) { 227 if (!U->isIncrementOp() || U->isPrefix()) 228 return; 229 230 Stmt *parent = Parents.getParentIgnoreParenCasts(U); 231 if (!parent || !isa<ReturnStmt>(parent)) 232 return; 233 234 Expr *Ex = U->getSubExpr()->IgnoreParenCasts(); 235 236 if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(Ex)) 237 CheckDeclRef(DR, U, DeadIncrement, AD, Live); 238 } 239 else if (DeclStmt* DS = dyn_cast<DeclStmt>(S)) 240 // Iterate through the decls. Warn if any initializers are complex 241 // expressions that are not live (never used). 242 for (DeclStmt::decl_iterator DI=DS->decl_begin(), DE=DS->decl_end(); 243 DI != DE; ++DI) { 244 245 VarDecl* V = dyn_cast<VarDecl>(*DI); 246 247 if (!V) 248 continue; 249 250 if (V->hasLocalStorage()) { 251 // Reference types confuse the dead stores checker. Skip them 252 // for now. 253 if (V->getType()->getAs<ReferenceType>()) 254 return; 255 256 if (Expr* E = V->getInit()) { 257 while (ExprWithCleanups *exprClean = dyn_cast<ExprWithCleanups>(E)) 258 E = exprClean->getSubExpr(); 259 260 // Don't warn on C++ objects (yet) until we can show that their 261 // constructors/destructors don't have side effects. 262 if (isa<CXXConstructExpr>(E)) 263 return; 264 265 // A dead initialization is a variable that is dead after it 266 // is initialized. We don't flag warnings for those variables 267 // marked 'unused'. 268 if (!Live(V, AD) && V->getAttr<UnusedAttr>() == 0) { 269 // Special case: check for initializations with constants. 270 // 271 // e.g. : int x = 0; 272 // 273 // If x is EVER assigned a new value later, don't issue 274 // a warning. This is because such initialization can be 275 // due to defensive programming. 276 if (E->isConstantInitializer(Ctx, false)) 277 return; 278 279 if (DeclRefExpr *DRE=dyn_cast<DeclRefExpr>(E->IgnoreParenCasts())) 280 if (VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl())) { 281 // Special case: check for initialization from constant 282 // variables. 283 // 284 // e.g. extern const int MyConstant; 285 // int x = MyConstant; 286 // 287 if (VD->hasGlobalStorage() && 288 VD->getType().isConstQualified()) 289 return; 290 // Special case: check for initialization from scalar 291 // parameters. This is often a form of defensive 292 // programming. Non-scalars are still an error since 293 // because it more likely represents an actual algorithmic 294 // bug. 295 if (isa<ParmVarDecl>(VD) && VD->getType()->isScalarType()) 296 return; 297 } 298 299 Report(V, DeadInit, V->getLocation(), E->getSourceRange()); 300 } 301 } 302 } 303 } 304 } 305}; 306 307} // end anonymous namespace 308 309//===----------------------------------------------------------------------===// 310// Driver function to invoke the Dead-Stores checker on a CFG. 311//===----------------------------------------------------------------------===// 312 313namespace { 314class FindEscaped : public CFGRecStmtDeclVisitor<FindEscaped>{ 315 CFG *cfg; 316public: 317 FindEscaped(CFG *c) : cfg(c) {} 318 319 CFG& getCFG() { return *cfg; } 320 321 llvm::SmallPtrSet<VarDecl*, 20> Escaped; 322 323 void VisitUnaryOperator(UnaryOperator* U) { 324 // Check for '&'. Any VarDecl whose value has its address-taken we 325 // treat as escaped. 326 Expr* E = U->getSubExpr()->IgnoreParenCasts(); 327 if (U->getOpcode() == UO_AddrOf) 328 if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(E)) 329 if (VarDecl* VD = dyn_cast<VarDecl>(DR->getDecl())) { 330 Escaped.insert(VD); 331 return; 332 } 333 Visit(E); 334 } 335}; 336} // end anonymous namespace 337 338 339//===----------------------------------------------------------------------===// 340// DeadStoresChecker 341//===----------------------------------------------------------------------===// 342 343namespace { 344class DeadStoresChecker : public Checker<check::ASTCodeBody> { 345public: 346 void checkASTCodeBody(const Decl *D, AnalysisManager& mgr, 347 BugReporter &BR) const { 348 if (LiveVariables *L = mgr.getLiveVariables(D)) { 349 CFG &cfg = *mgr.getCFG(D); 350 ParentMap &pmap = mgr.getParentMap(D); 351 FindEscaped FS(&cfg); 352 FS.getCFG().VisitBlockStmts(FS); 353 DeadStoreObs A(cfg, BR.getContext(), BR, pmap, FS.Escaped); 354 L->runOnAllBlocks(cfg, &A); 355 } 356 } 357}; 358} 359 360void ento::registerDeadStoresChecker(CheckerManager &mgr) { 361 mgr.registerChecker<DeadStoresChecker>(); 362} 363