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are considered 'live'. This hack isn't needed anymore because we have a separation in the path-sensitive analyzer between variable names and bindings; the analyzer can continue to reason about the storage of a variable after its name is no longer directly referenced. Now the live variables analysis literally means "is this name live". Along this line, update the dead stores checker to explicitly look for variables whose values have escaped. llvm-svn: 68504
260 lines
8.9 KiB
C++
260 lines
8.9 KiB
C++
//==- DeadStores.cpp - Check for stores to dead variables --------*- C++ -*-==//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file defines a DeadStores, a flow-sensitive checker that looks for
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// stores to variables that are no longer live.
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//
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//===----------------------------------------------------------------------===//
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#include "clang/Analysis/LocalCheckers.h"
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#include "clang/Analysis/Analyses/LiveVariables.h"
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#include "clang/Analysis/Visitors/CFGRecStmtVisitor.h"
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#include "clang/Analysis/PathSensitive/BugReporter.h"
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#include "clang/Analysis/PathSensitive/GRExprEngine.h"
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#include "clang/Analysis/Visitors/CFGRecStmtDeclVisitor.h"
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#include "clang/Basic/Diagnostic.h"
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#include "clang/AST/ASTContext.h"
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#include "clang/AST/ParentMap.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/Support/Compiler.h"
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using namespace clang;
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namespace {
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class VISIBILITY_HIDDEN DeadStoreObs : public LiveVariables::ObserverTy {
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ASTContext &Ctx;
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BugReporter& BR;
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ParentMap& Parents;
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llvm::SmallPtrSet<VarDecl*, 20> Escaped;
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enum DeadStoreKind { Standard, Enclosing, DeadIncrement, DeadInit };
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public:
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DeadStoreObs(ASTContext &ctx, BugReporter& br, ParentMap& parents,
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llvm::SmallPtrSet<VarDecl*, 20> &escaped)
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: Ctx(ctx), BR(br), Parents(parents), Escaped(escaped) {}
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virtual ~DeadStoreObs() {}
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void Report(VarDecl* V, DeadStoreKind dsk, SourceLocation L, SourceRange R) {
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if (Escaped.count(V))
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return;
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std::string name = V->getNameAsString();
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const char* BugType = 0;
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std::string msg;
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switch (dsk) {
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default:
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assert(false && "Impossible dead store type.");
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case DeadInit:
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BugType = "Dead initialization";
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msg = "Value stored to '" + name +
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"' during its initialization is never read";
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break;
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case DeadIncrement:
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BugType = "Dead increment";
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case Standard:
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if (!BugType) BugType = "Dead assignment";
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msg = "Value stored to '" + name + "' is never read";
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break;
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case Enclosing:
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BugType = "Dead nested assignment";
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msg = "Although the value stored to '" + name +
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"' is used in the enclosing expression, the value is never actually"
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" read from '" + name + "'";
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break;
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}
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BR.EmitBasicReport(BugType, "Dead store", msg.c_str(), L, R);
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}
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void CheckVarDecl(VarDecl* VD, Expr* Ex, Expr* Val,
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DeadStoreKind dsk,
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const LiveVariables::AnalysisDataTy& AD,
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const LiveVariables::ValTy& Live) {
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if (VD->hasLocalStorage() && !Live(VD, AD) && !VD->getAttr<UnusedAttr>())
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Report(VD, dsk, Ex->getSourceRange().getBegin(),
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Val->getSourceRange());
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}
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void CheckDeclRef(DeclRefExpr* DR, Expr* Val, DeadStoreKind dsk,
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const LiveVariables::AnalysisDataTy& AD,
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const LiveVariables::ValTy& Live) {
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if (VarDecl* VD = dyn_cast<VarDecl>(DR->getDecl()))
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CheckVarDecl(VD, DR, Val, dsk, AD, Live);
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}
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bool isIncrement(VarDecl* VD, BinaryOperator* B) {
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if (B->isCompoundAssignmentOp())
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return true;
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Expr* RHS = B->getRHS()->IgnoreParenCasts();
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BinaryOperator* BRHS = dyn_cast<BinaryOperator>(RHS);
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if (!BRHS)
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return false;
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DeclRefExpr *DR;
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if ((DR = dyn_cast<DeclRefExpr>(BRHS->getLHS()->IgnoreParenCasts())))
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if (DR->getDecl() == VD)
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return true;
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if ((DR = dyn_cast<DeclRefExpr>(BRHS->getRHS()->IgnoreParenCasts())))
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if (DR->getDecl() == VD)
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return true;
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return false;
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}
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virtual void ObserveStmt(Stmt* S,
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const LiveVariables::AnalysisDataTy& AD,
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const LiveVariables::ValTy& Live) {
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// Skip statements in macros.
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if (S->getLocStart().isMacroID())
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return;
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if (BinaryOperator* B = dyn_cast<BinaryOperator>(S)) {
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if (!B->isAssignmentOp()) return; // Skip non-assignments.
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if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(B->getLHS()))
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if (VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
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Expr* RHS = B->getRHS()->IgnoreParenCasts();
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// Special case: check for assigning null to a pointer.
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// This is a common form of defensive programming.
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if (VD->getType()->isPointerType()) {
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if (IntegerLiteral* L = dyn_cast<IntegerLiteral>(RHS))
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// FIXME: Probably should have an Expr::isNullPointerConstant.
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if (L->getValue() == 0)
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return;
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}
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// Special case: self-assignments. These are often used to shut up
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// "unused variable" compiler warnings.
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if (DeclRefExpr* RhsDR = dyn_cast<DeclRefExpr>(RHS))
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if (VD == dyn_cast<VarDecl>(RhsDR->getDecl()))
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return;
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// Otherwise, issue a warning.
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DeadStoreKind dsk = Parents.isConsumedExpr(B)
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? Enclosing
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: (isIncrement(VD,B) ? DeadIncrement : Standard);
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CheckVarDecl(VD, DR, B->getRHS(), dsk, AD, Live);
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}
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}
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else if (UnaryOperator* U = dyn_cast<UnaryOperator>(S)) {
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if (!U->isIncrementOp())
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return;
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// Handle: ++x within a subexpression. The solution is not warn
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// about preincrements to dead variables when the preincrement occurs
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// as a subexpression. This can lead to false negatives, e.g. "(++x);"
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// A generalized dead code checker should find such issues.
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if (U->isPrefix() && Parents.isConsumedExpr(U))
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return;
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Expr *Ex = U->getSubExpr()->IgnoreParenCasts();
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if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(Ex))
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CheckDeclRef(DR, U, DeadIncrement, AD, Live);
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}
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else if (DeclStmt* DS = dyn_cast<DeclStmt>(S))
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// Iterate through the decls. Warn if any initializers are complex
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// expressions that are not live (never used).
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for (DeclStmt::decl_iterator DI=DS->decl_begin(), DE=DS->decl_end();
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DI != DE; ++DI) {
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VarDecl* V = dyn_cast<VarDecl>(*DI);
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if (!V)
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continue;
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if (V->hasLocalStorage())
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if (Expr* E = V->getInit()) {
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// A dead initialization is a variable that is dead after it
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// is initialized. We don't flag warnings for those variables
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// marked 'unused'.
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if (!Live(V, AD) && V->getAttr<UnusedAttr>() == 0) {
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// Special case: check for initializations with constants.
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//
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// e.g. : int x = 0;
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//
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// If x is EVER assigned a new value later, don't issue
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// a warning. This is because such initialization can be
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// due to defensive programming.
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if (E->isConstantInitializer(Ctx))
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return;
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// Special case: check for initializations from constant
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// variables.
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//
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// e.g. extern const int MyConstant;
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// int x = MyConstant;
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//
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if (DeclRefExpr *DRE=dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
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if (VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl()))
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if (VD->hasGlobalStorage() &&
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VD->getType().isConstQualified()) return;
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Report(V, DeadInit, V->getLocation(), E->getSourceRange());
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}
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}
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}
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}
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};
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} // end anonymous namespace
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//===----------------------------------------------------------------------===//
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// Driver function to invoke the Dead-Stores checker on a CFG.
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//===----------------------------------------------------------------------===//
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namespace {
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class VISIBILITY_HIDDEN FindEscaped : public CFGRecStmtDeclVisitor<FindEscaped>{
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CFG *cfg;
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public:
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FindEscaped(CFG *c) : cfg(c) {}
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CFG& getCFG() { return *cfg; }
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llvm::SmallPtrSet<VarDecl*, 20> Escaped;
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void VisitUnaryOperator(UnaryOperator* U) {
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// Check for '&'. Any VarDecl whose value has its address-taken we
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// treat as escaped.
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Expr* E = U->getSubExpr()->IgnoreParenCasts();
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if (U->getOpcode() == UnaryOperator::AddrOf)
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if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(E))
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if (VarDecl* VD = dyn_cast<VarDecl>(DR->getDecl())) {
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Escaped.insert(VD);
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return;
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}
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Visit(E);
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}
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};
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} // end anonymous namespace
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void clang::CheckDeadStores(LiveVariables& L, BugReporter& BR) {
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FindEscaped FS(BR.getCFG());
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FS.getCFG().VisitBlockStmts(FS);
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DeadStoreObs A(BR.getContext(), BR, BR.getParentMap(), FS.Escaped);
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L.runOnAllBlocks(*BR.getCFG(), &A);
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}
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