llvm-project/clang/lib/Analysis/CheckDeadStores.cpp

260 lines
8.9 KiB
C++

//==- 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/Analysis/LocalCheckers.h"
#include "clang/Analysis/Analyses/LiveVariables.h"
#include "clang/Analysis/Visitors/CFGRecStmtVisitor.h"
#include "clang/Analysis/PathSensitive/BugReporter.h"
#include "clang/Analysis/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"
#include "llvm/Support/Compiler.h"
using namespace clang;
namespace {
class VISIBILITY_HIDDEN 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.c_str(), L, R);
}
void CheckVarDecl(VarDecl* VD, Expr* Ex, Expr* Val,
DeadStoreKind dsk,
const LiveVariables::AnalysisDataTy& AD,
const LiveVariables::ValTy& Live) {
if (VD->hasLocalStorage() && !Live(VD, AD) && !VD->getAttr<UnusedAttr>(Ctx))
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())) {
Expr* RHS = B->getRHS()->IgnoreParenCasts();
// Special case: check for assigning null to a pointer.
// This is a common form of defensive programming.
if (VD->getType()->isPointerType()) {
if (IntegerLiteral* L = dyn_cast<IntegerLiteral>(RHS))
// FIXME: Probably should have an Expr::isNullPointerConstant.
if (L->getValue() == 0)
return;
}
// 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())
if (Expr* E = V->getInit()) {
// 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>(Ctx) == 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;
// Special case: check for initializations from constant
// variables.
//
// e.g. extern const int MyConstant;
// int x = MyConstant;
//
if (DeclRefExpr *DRE=dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
if (VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl()))
if (VD->hasGlobalStorage() &&
VD->getType().isConstQualified()) return;
Report(V, DeadInit, V->getLocation(), E->getSourceRange());
}
}
}
}
};
} // end anonymous namespace
//===----------------------------------------------------------------------===//
// Driver function to invoke the Dead-Stores checker on a CFG.
//===----------------------------------------------------------------------===//
namespace {
class VISIBILITY_HIDDEN 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(LiveVariables& L, BugReporter& BR) {
FindEscaped FS(BR.getCFG());
FS.getCFG().VisitBlockStmts(FS);
DeadStoreObs A(BR.getContext(), BR, BR.getParentMap(), FS.Escaped);
L.runOnAllBlocks(*BR.getCFG(), &A);
}