DeadStores.cpp

//==- DeadStores.cpp - Check for stores to dead variables --------*- C++ -*-==//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Ted Kremenek and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//  This files defines a DeadStores, a flow-sensitive checker that looks for
//  stores to variables that are no longer live.
//
//===----------------------------------------------------------------------===//

#include "clang/AST/Expr.h"
#include "clang/Analysis/LocalCheckers.h"
#include "clang/Analysis/LiveVariables.h"
#include "clang/AST/CFG.h"
#include "clang/Basic/Diagnostic.h"
#include "clang/AST/ASTContext.h"

using namespace clang;

namespace {

class DeadStoreObserver : public LiveVariablesObserver {
  ASTContext &Ctx;
  Diagnostic &Diags;
public:
  DeadStoreObserver(ASTContext &ctx, Diagnostic &diags)
    : Ctx(ctx), Diags(diags) {
  }
    
  virtual ~DeadStoreObserver() {}

  virtual void ObserveStmt(Stmt* S, LiveVariables& L, llvm::BitVector& Live) {                                 
    if (BinaryOperator* B = dyn_cast<BinaryOperator>(S)) {    
      // Is this an assignment?
      if (!B->isAssignmentOp())
        return;
      
      // Is this an assignment to a variable?
      if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(B->getLHS()))
        // Is the variable live?
        if (!L.isLive(Live,cast<VarDecl>(DR->getDecl()))) {
          SourceRange R = B->getRHS()->getSourceRange();
          Diags.Report(DR->getSourceRange().Begin(), diag::warn_dead_store,
                       0, 0, &R, 1);
                                                                        
        }
    }
    else if(DeclStmt* DS = dyn_cast<DeclStmt>(S)) {
      // Iterate through the decls.  Warn if any of them (which have
      // initializers) are not live.
      for (VarDecl* V = cast<VarDecl>(DS->getDecl()); V != NULL ; 
                    V = cast_or_null<VarDecl>(V->getNextDeclarator()))
        if (Expr* E = V->getInit())
          if (!L.isLive(Live,V))
            // 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->isConstantExpr(Ctx,NULL) || 
                L.getVarInfo(V).Kills.size() == 0) {
              // Flag a warning.
              SourceRange R = E->getSourceRange();
              Diags.Report(V->getLocation(), diag::warn_dead_store, 0, 0,
                           &R,1);
            }
    }
  }
};
  
} // end anonymous namespace

namespace clang {

void CheckDeadStores(CFG& cfg, LiveVariables& L,
                     ASTContext &Ctx, Diagnostic &Diags) {
  DeadStoreObserver A(Ctx, Diags);
  
  for (CFG::iterator I = cfg.begin(), E = cfg.end(); I != E; ++I)
    L.runOnBlock(&(*I),&A);
}

void CheckDeadStores(CFG& cfg, ASTContext &Ctx, Diagnostic &Diags) {
  LiveVariables L;
  L.runOnCFG(cfg);
  CheckDeadStores(cfg,L, Ctx, Diags);
}

} // end namespace clang