| //==- DeadStores.cpp - Check for stores to dead variables --------*- C++ -*-==// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file defines a DeadStores, a flow-sensitive checker that looks for |
| // stores to variables that are no longer live. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "clang/Checker/Checkers/LocalCheckers.h" |
| #include "clang/Analysis/Analyses/LiveVariables.h" |
| #include "clang/Analysis/Visitors/CFGRecStmtVisitor.h" |
| #include "clang/Checker/BugReporter/BugReporter.h" |
| #include "clang/Checker/PathSensitive/GRExprEngine.h" |
| #include "clang/Analysis/Visitors/CFGRecStmtDeclVisitor.h" |
| #include "clang/Basic/Diagnostic.h" |
| #include "clang/AST/ASTContext.h" |
| #include "clang/AST/ParentMap.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| |
| using namespace clang; |
| |
| namespace { |
| |
| class DeadStoreObs : public LiveVariables::ObserverTy { |
| ASTContext &Ctx; |
| BugReporter& BR; |
| ParentMap& Parents; |
| llvm::SmallPtrSet<VarDecl*, 20> Escaped; |
| |
| enum DeadStoreKind { Standard, Enclosing, DeadIncrement, DeadInit }; |
| |
| public: |
| DeadStoreObs(ASTContext &ctx, BugReporter& br, ParentMap& parents, |
| llvm::SmallPtrSet<VarDecl*, 20> &escaped) |
| : Ctx(ctx), BR(br), Parents(parents), Escaped(escaped) {} |
| |
| virtual ~DeadStoreObs() {} |
| |
| void Report(VarDecl* V, DeadStoreKind dsk, SourceLocation L, SourceRange R) { |
| if (Escaped.count(V)) |
| return; |
| |
| std::string name = V->getNameAsString(); |
| |
| const char* BugType = 0; |
| std::string msg; |
| |
| switch (dsk) { |
| default: |
| assert(false && "Impossible dead store type."); |
| |
| case DeadInit: |
| BugType = "Dead initialization"; |
| msg = "Value stored to '" + name + |
| "' during its initialization is never read"; |
| break; |
| |
| case DeadIncrement: |
| BugType = "Dead increment"; |
| case Standard: |
| if (!BugType) BugType = "Dead assignment"; |
| msg = "Value stored to '" + name + "' is never read"; |
| break; |
| |
| case Enclosing: |
| BugType = "Dead nested assignment"; |
| msg = "Although the value stored to '" + name + |
| "' is used in the enclosing expression, the value is never actually" |
| " read from '" + name + "'"; |
| break; |
| } |
| |
| BR.EmitBasicReport(BugType, "Dead store", msg, L, R); |
| } |
| |
| void CheckVarDecl(VarDecl* VD, Expr* Ex, Expr* Val, |
| DeadStoreKind dsk, |
| const LiveVariables::AnalysisDataTy& AD, |
| const LiveVariables::ValTy& Live) { |
| |
| if (!VD->hasLocalStorage()) |
| return; |
| // Reference types confuse the dead stores checker. Skip them |
| // for now. |
| if (VD->getType()->getAs<ReferenceType>()) |
| return; |
| |
| if (!Live(VD, AD) && |
| !(VD->getAttr<UnusedAttr>() || VD->getAttr<BlocksAttr>())) |
| Report(VD, dsk, Ex->getSourceRange().getBegin(), |
| Val->getSourceRange()); |
| } |
| |
| void CheckDeclRef(DeclRefExpr* DR, Expr* Val, DeadStoreKind dsk, |
| const LiveVariables::AnalysisDataTy& AD, |
| const LiveVariables::ValTy& Live) { |
| if (VarDecl* VD = dyn_cast<VarDecl>(DR->getDecl())) |
| CheckVarDecl(VD, DR, Val, dsk, AD, Live); |
| } |
| |
| bool isIncrement(VarDecl* VD, BinaryOperator* B) { |
| if (B->isCompoundAssignmentOp()) |
| return true; |
| |
| Expr* RHS = B->getRHS()->IgnoreParenCasts(); |
| BinaryOperator* BRHS = dyn_cast<BinaryOperator>(RHS); |
| |
| if (!BRHS) |
| return false; |
| |
| DeclRefExpr *DR; |
| |
| if ((DR = dyn_cast<DeclRefExpr>(BRHS->getLHS()->IgnoreParenCasts()))) |
| if (DR->getDecl() == VD) |
| return true; |
| |
| if ((DR = dyn_cast<DeclRefExpr>(BRHS->getRHS()->IgnoreParenCasts()))) |
| if (DR->getDecl() == VD) |
| return true; |
| |
| return false; |
| } |
| |
| virtual void ObserveStmt(Stmt* S, |
| const LiveVariables::AnalysisDataTy& AD, |
| const LiveVariables::ValTy& Live) { |
| |
| // Skip statements in macros. |
| if (S->getLocStart().isMacroID()) |
| return; |
| |
| if (BinaryOperator* B = dyn_cast<BinaryOperator>(S)) { |
| if (!B->isAssignmentOp()) return; // Skip non-assignments. |
| |
| if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(B->getLHS())) |
| if (VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) { |
| // Special case: check for assigning null to a pointer. |
| // This is a common form of defensive programming. |
| QualType T = VD->getType(); |
| if (T->isPointerType() || T->isObjCObjectPointerType()) { |
| if (B->getRHS()->isNullPointerConstant(Ctx, |
| Expr::NPC_ValueDependentIsNull)) |
| return; |
| } |
| |
| Expr* RHS = B->getRHS()->IgnoreParenCasts(); |
| // Special case: self-assignments. These are often used to shut up |
| // "unused variable" compiler warnings. |
| if (DeclRefExpr* RhsDR = dyn_cast<DeclRefExpr>(RHS)) |
| if (VD == dyn_cast<VarDecl>(RhsDR->getDecl())) |
| return; |
| |
| // Otherwise, issue a warning. |
| DeadStoreKind dsk = Parents.isConsumedExpr(B) |
| ? Enclosing |
| : (isIncrement(VD,B) ? DeadIncrement : Standard); |
| |
| CheckVarDecl(VD, DR, B->getRHS(), dsk, AD, Live); |
| } |
| } |
| else if (UnaryOperator* U = dyn_cast<UnaryOperator>(S)) { |
| if (!U->isIncrementOp()) |
| return; |
| |
| // Handle: ++x within a subexpression. The solution is not warn |
| // about preincrements to dead variables when the preincrement occurs |
| // as a subexpression. This can lead to false negatives, e.g. "(++x);" |
| // A generalized dead code checker should find such issues. |
| if (U->isPrefix() && Parents.isConsumedExpr(U)) |
| return; |
| |
| Expr *Ex = U->getSubExpr()->IgnoreParenCasts(); |
| |
| if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(Ex)) |
| CheckDeclRef(DR, U, DeadIncrement, AD, Live); |
| } |
| else if (DeclStmt* DS = dyn_cast<DeclStmt>(S)) |
| // Iterate through the decls. Warn if any initializers are complex |
| // expressions that are not live (never used). |
| for (DeclStmt::decl_iterator DI=DS->decl_begin(), DE=DS->decl_end(); |
| DI != DE; ++DI) { |
| |
| VarDecl* V = dyn_cast<VarDecl>(*DI); |
| |
| if (!V) |
| continue; |
| |
| if (V->hasLocalStorage()) { |
| // Reference types confuse the dead stores checker. Skip them |
| // for now. |
| if (V->getType()->getAs<ReferenceType>()) |
| return; |
| |
| if (Expr* E = V->getInit()) { |
| // Don't warn on C++ objects (yet) until we can show that their |
| // constructors/destructors don't have side effects. |
| if (isa<CXXConstructExpr>(E)) |
| return; |
| |
| if (isa<CXXExprWithTemporaries>(E)) |
| return; |
| |
| // A dead initialization is a variable that is dead after it |
| // is initialized. We don't flag warnings for those variables |
| // marked 'unused'. |
| if (!Live(V, AD) && V->getAttr<UnusedAttr>() == 0) { |
| // Special case: check for initializations with constants. |
| // |
| // e.g. : int x = 0; |
| // |
| // If x is EVER assigned a new value later, don't issue |
| // a warning. This is because such initialization can be |
| // due to defensive programming. |
| if (E->isConstantInitializer(Ctx)) |
| return; |
| |
| if (DeclRefExpr *DRE=dyn_cast<DeclRefExpr>(E->IgnoreParenCasts())) |
| if (VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl())) { |
| // Special case: check for initialization from constant |
| // variables. |
| // |
| // e.g. extern const int MyConstant; |
| // int x = MyConstant; |
| // |
| if (VD->hasGlobalStorage() && |
| VD->getType().isConstQualified()) |
| return; |
| // Special case: check for initialization from scalar |
| // parameters. This is often a form of defensive |
| // programming. Non-scalars are still an error since |
| // because it more likely represents an actual algorithmic |
| // bug. |
| if (isa<ParmVarDecl>(VD) && VD->getType()->isScalarType()) |
| return; |
| } |
| |
| Report(V, DeadInit, V->getLocation(), E->getSourceRange()); |
| } |
| } |
| } |
| } |
| } |
| }; |
| |
| } // end anonymous namespace |
| |
| //===----------------------------------------------------------------------===// |
| // Driver function to invoke the Dead-Stores checker on a CFG. |
| //===----------------------------------------------------------------------===// |
| |
| namespace { |
| class FindEscaped : public CFGRecStmtDeclVisitor<FindEscaped>{ |
| CFG *cfg; |
| public: |
| FindEscaped(CFG *c) : cfg(c) {} |
| |
| CFG& getCFG() { return *cfg; } |
| |
| llvm::SmallPtrSet<VarDecl*, 20> Escaped; |
| |
| void VisitUnaryOperator(UnaryOperator* U) { |
| // Check for '&'. Any VarDecl whose value has its address-taken we |
| // treat as escaped. |
| Expr* E = U->getSubExpr()->IgnoreParenCasts(); |
| if (U->getOpcode() == UnaryOperator::AddrOf) |
| if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(E)) |
| if (VarDecl* VD = dyn_cast<VarDecl>(DR->getDecl())) { |
| Escaped.insert(VD); |
| return; |
| } |
| Visit(E); |
| } |
| }; |
| } // end anonymous namespace |
| |
| |
| void clang::CheckDeadStores(CFG &cfg, LiveVariables &L, ParentMap &pmap, |
| BugReporter& BR) { |
| FindEscaped FS(&cfg); |
| FS.getCFG().VisitBlockStmts(FS); |
| DeadStoreObs A(BR.getContext(), BR, pmap, FS.Escaped); |
| L.runOnAllBlocks(cfg, &A); |
| } |