Sema/SemaChecking.cpp

//===--- SemaChecking.cpp - Extra Semantic Checking -----------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//  This file implements extra semantic analysis beyond what is enforced 
//  by the C type system.
//
//===----------------------------------------------------------------------===//

#include "Sema.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Decl.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprCXX.h"
#include "clang/Lex/Preprocessor.h"
#include "clang/Lex/LiteralSupport.h"
#include "clang/Basic/SourceManager.h"
#include "clang/Basic/Diagnostic.h"
#include "clang/Basic/LangOptions.h"
#include "clang/Basic/TargetInfo.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringExtras.h"
#include "SemaUtil.h"
using namespace clang;

/// CheckFunctionCall - Check a direct function call for various correctness
/// and safety properties not strictly enforced by the C type system.
bool
Sema::CheckFunctionCall(FunctionDecl *FDecl, CallExpr *TheCall) {
                        
  // Get the IdentifierInfo* for the called function.
  IdentifierInfo *FnInfo = FDecl->getIdentifier();
  
  switch (FnInfo->getBuiltinID()) {
  case Builtin::BI__builtin___CFStringMakeConstantString:
    assert(TheCall->getNumArgs() == 1 &&
           "Wrong # arguments to builtin CFStringMakeConstantString");
    return CheckBuiltinCFStringArgument(TheCall->getArg(0));
  case Builtin::BI__builtin_va_start:
    return SemaBuiltinVAStart(TheCall);
    
  case Builtin::BI__builtin_isgreater:
  case Builtin::BI__builtin_isgreaterequal:
  case Builtin::BI__builtin_isless:
  case Builtin::BI__builtin_islessequal:
  case Builtin::BI__builtin_islessgreater:
  case Builtin::BI__builtin_isunordered:
    return SemaBuiltinUnorderedCompare(TheCall);
  }
  
  // Search the KnownFunctionIDs for the identifier.
  unsigned i = 0, e = id_num_known_functions;
  for (; i != e; ++i) { if (KnownFunctionIDs[i] == FnInfo) break; }
  if (i == e) return false;
  
  // Printf checking.
  if (i <= id_vprintf) {
    // Retrieve the index of the format string parameter and determine
    // if the function is passed a va_arg argument.
    unsigned format_idx = 0;
    bool HasVAListArg = false;
    
    switch (i) {
    default: assert(false && "No format string argument index.");
    case id_printf:    format_idx = 0; break;
    case id_fprintf:   format_idx = 1; break;
    case id_sprintf:   format_idx = 1; break;
    case id_snprintf:  format_idx = 2; break;
    case id_asprintf:  format_idx = 1; break;
    case id_vsnprintf: format_idx = 2; HasVAListArg = true; break;
    case id_vasprintf: format_idx = 1; HasVAListArg = true; break;
    case id_vfprintf:  format_idx = 1; HasVAListArg = true; break;
    case id_vsprintf:  format_idx = 1; HasVAListArg = true; break;
    case id_vprintf:   format_idx = 0; HasVAListArg = true; break;
    }
    
    CheckPrintfArguments(TheCall, HasVAListArg, format_idx);       
  }
  
  return false;
}

/// CheckBuiltinCFStringArgument - Checks that the argument to the builtin
/// CFString constructor is correct
bool Sema::CheckBuiltinCFStringArgument(Expr* Arg) {
  Arg = Arg->IgnoreParenCasts();
  
  StringLiteral *Literal = dyn_cast<StringLiteral>(Arg);

  if (!Literal || Literal->isWide()) {
    Diag(Arg->getLocStart(),
         diag::err_cfstring_literal_not_string_constant,
         Arg->getSourceRange());
    return true;
  }
  
  const char *Data = Literal->getStrData();
  unsigned Length = Literal->getByteLength();
  
  for (unsigned i = 0; i < Length; ++i) {
    if (!isascii(Data[i])) {
      Diag(PP.AdvanceToTokenCharacter(Arg->getLocStart(), i + 1),
           diag::warn_cfstring_literal_contains_non_ascii_character,
           Arg->getSourceRange());
      break;
    }
    
    if (!Data[i]) {
      Diag(PP.AdvanceToTokenCharacter(Arg->getLocStart(), i + 1),
           diag::warn_cfstring_literal_contains_nul_character,
           Arg->getSourceRange());
      break;
    }
  }
  
  return false;
}

/// SemaBuiltinVAStart - Check the arguments to __builtin_va_start for validity.
/// Emit an error and return true on failure, return false on success.
bool Sema::SemaBuiltinVAStart(CallExpr *TheCall) {
  Expr *Fn = TheCall->getCallee();
  if (TheCall->getNumArgs() > 2) {
    Diag(TheCall->getArg(2)->getLocStart(), 
         diag::err_typecheck_call_too_many_args, Fn->getSourceRange(),
         SourceRange(TheCall->getArg(2)->getLocStart(), 
                     (*(TheCall->arg_end()-1))->getLocEnd()));
    return true;
  }
  
  // Determine whether the current function is variadic or not.
  bool isVariadic;
  if (CurFunctionDecl)
    isVariadic =
      cast<FunctionTypeProto>(CurFunctionDecl->getType())->isVariadic();
  else
    isVariadic = CurMethodDecl->isVariadic();
  
  if (!isVariadic) {
    Diag(Fn->getLocStart(), diag::err_va_start_used_in_non_variadic_function);
    return true;
  }
  
  // Verify that the second argument to the builtin is the last argument of the
  // current function or method.
  bool SecondArgIsLastNamedArgument = false;
  const Expr *Arg = TheCall->getArg(1)->IgnoreParenCasts();
  
  if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(Arg)) {
    if (const ParmVarDecl *PV = dyn_cast<ParmVarDecl>(DR->getDecl())) {
      // FIXME: This isn't correct for methods (results in bogus warning).
      // Get the last formal in the current function.
      const ParmVarDecl *LastArg;
      if (CurFunctionDecl)
        LastArg = *(CurFunctionDecl->param_end()-1);
      else
        LastArg = *(CurMethodDecl->param_end()-1);
      SecondArgIsLastNamedArgument = PV == LastArg;
    }
  }
  
  if (!SecondArgIsLastNamedArgument)
    Diag(TheCall->getArg(1)->getLocStart(), 
         diag::warn_second_parameter_of_va_start_not_last_named_argument);
  return false;
}  

/// SemaBuiltinUnorderedCompare - Handle functions like __builtin_isgreater and
/// friends.  This is declared to take (...), so we have to check everything.
bool Sema::SemaBuiltinUnorderedCompare(CallExpr *TheCall) {
  if (TheCall->getNumArgs() < 2)
    return Diag(TheCall->getLocEnd(), diag::err_typecheck_call_too_few_args);
  if (TheCall->getNumArgs() > 2)
    return Diag(TheCall->getArg(2)->getLocStart(), 
                diag::err_typecheck_call_too_many_args,
                SourceRange(TheCall->getArg(2)->getLocStart(),
                            (*(TheCall->arg_end()-1))->getLocEnd()));
  
  Expr *OrigArg0 = TheCall->getArg(0);
  Expr *OrigArg1 = TheCall->getArg(1);
  
  // Do standard promotions between the two arguments, returning their common
  // type.
  QualType Res = UsualArithmeticConversions(OrigArg0, OrigArg1, false);
  
  // If the common type isn't a real floating type, then the arguments were
  // invalid for this operation.
  if (!Res->isRealFloatingType())
    return Diag(OrigArg0->getLocStart(), 
                diag::err_typecheck_call_invalid_ordered_compare,
                OrigArg0->getType().getAsString(),
                OrigArg1->getType().getAsString(),
                SourceRange(OrigArg0->getLocStart(), OrigArg1->getLocEnd()));
  
  return false;
}


/// CheckPrintfArguments - Check calls to printf (and similar functions) for
/// correct use of format strings.  
///
///  HasVAListArg - A predicate indicating whether the printf-like
///    function is passed an explicit va_arg argument (e.g., vprintf)
///
///  format_idx - The index into Args for the format string.
///
/// Improper format strings to functions in the printf family can be
/// the source of bizarre bugs and very serious security holes.  A
/// good source of information is available in the following paper
/// (which includes additional references):
///
///  FormatGuard: Automatic Protection From printf Format String
///  Vulnerabilities, Proceedings of the 10th USENIX Security Symposium, 2001.
///
/// Functionality implemented:
///
///  We can statically check the following properties for string
///  literal format strings for non v.*printf functions (where the
///  arguments are passed directly):
//
///  (1) Are the number of format conversions equal to the number of
///      data arguments?
///
///  (2) Does each format conversion correctly match the type of the
///      corresponding data argument?  (TODO)
///
/// Moreover, for all printf functions we can:
///
///  (3) Check for a missing format string (when not caught by type checking).
///
///  (4) Check for no-operation flags; e.g. using "#" with format
///      conversion 'c'  (TODO)
///
///  (5) Check the use of '%n', a major source of security holes.
///
///  (6) Check for malformed format conversions that don't specify anything.
///
///  (7) Check for empty format strings.  e.g: printf("");
///
///  (8) Check that the format string is a wide literal.
///
/// All of these checks can be done by parsing the format string.
///
/// For now, we ONLY do (1), (3), (5), (6), (7), and (8).
void
Sema::CheckPrintfArguments(CallExpr *TheCall, bool HasVAListArg, 
                           unsigned format_idx) {
  Expr *Fn = TheCall->getCallee();

  // CHECK: printf-like function is called with no format string.  
  if (format_idx >= TheCall->getNumArgs()) {
    Diag(TheCall->getRParenLoc(), diag::warn_printf_missing_format_string, 
         Fn->getSourceRange());
    return;
  }
  
  Expr *OrigFormatExpr = TheCall->getArg(format_idx)->IgnoreParenCasts();
  
  // CHECK: format string is not a string literal.
  // 
  // Dynamically generated format strings are difficult to
  // automatically vet at compile time.  Requiring that format strings
  // are string literals: (1) permits the checking of format strings by
  // the compiler and thereby (2) can practically remove the source of
  // many format string exploits.
  StringLiteral *FExpr = dyn_cast<StringLiteral>(OrigFormatExpr);
  if (FExpr == NULL) {
    // For vprintf* functions (i.e., HasVAListArg==true), we add a
    // special check to see if the format string is a function parameter
    // of the function calling the printf function.  If the function
    // has an attribute indicating it is a printf-like function, then we
    // should suppress warnings concerning non-literals being used in a call
    // to a vprintf function.  For example:
    //
    // void
    // logmessage(char const *fmt __attribute__ (format (printf, 1, 2)), ...) {
    //      va_list ap;
    //      va_start(ap, fmt);
    //      vprintf(fmt, ap);  // Do NOT emit a warning about "fmt".
    //      ...
    //
    //
    //  FIXME: We don't have full attribute support yet, so just check to see
    //    if the argument is a DeclRefExpr that references a parameter.  We'll
    //    add proper support for checking the attribute later.
    if (HasVAListArg)
      if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(OrigFormatExpr))
        if (isa<ParmVarDecl>(DR->getDecl()))
          return;
    
    Diag(TheCall->getArg(format_idx)->getLocStart(), 
         diag::warn_printf_not_string_constant, Fn->getSourceRange());
    return;
  }

  // CHECK: is the format string a wide literal?
  if (FExpr->isWide()) {
    Diag(FExpr->getLocStart(),
         diag::warn_printf_format_string_is_wide_literal, Fn->getSourceRange());
    return;
  }

  // Str - The format string.  NOTE: this is NOT null-terminated!
  const char * const Str = FExpr->getStrData();

  // CHECK: empty format string?
  const unsigned StrLen = FExpr->getByteLength();
  
  if (StrLen == 0) {
    Diag(FExpr->getLocStart(), diag::warn_printf_empty_format_string,
         Fn->getSourceRange());
    return;
  }

  // We process the format string using a binary state machine.  The
  // current state is stored in CurrentState.
  enum {
    state_OrdChr,
    state_Conversion
  } CurrentState = state_OrdChr;
  
  // numConversions - The number of conversions seen so far.  This is
  //  incremented as we traverse the format string.
  unsigned numConversions = 0;

  // numDataArgs - The number of data arguments after the format
  //  string.  This can only be determined for non vprintf-like
  //  functions.  For those functions, this value is 1 (the sole
  //  va_arg argument).
  unsigned numDataArgs = TheCall->getNumArgs()-(format_idx+1);

  // Inspect the format string.
  unsigned StrIdx = 0;
  
  // LastConversionIdx - Index within the format string where we last saw
  //  a '%' character that starts a new format conversion.
  unsigned LastConversionIdx = 0;
  
  for (; StrIdx < StrLen; ++StrIdx) {
    
    // Is the number of detected conversion conversions greater than
    // the number of matching data arguments?  If so, stop.
    if (!HasVAListArg && numConversions > numDataArgs) break;
    
    // Handle "\0"
    if (Str[StrIdx] == '\0') {
      // The string returned by getStrData() is not null-terminated,
      // so the presence of a null character is likely an error.
      Diag(PP.AdvanceToTokenCharacter(FExpr->getLocStart(), StrIdx+1),
           diag::warn_printf_format_string_contains_null_char,
           Fn->getSourceRange());
      return;
    }
    
    // Ordinary characters (not processing a format conversion).
    if (CurrentState == state_OrdChr) {
      if (Str[StrIdx] == '%') {
        CurrentState = state_Conversion;
        LastConversionIdx = StrIdx;
      }
      continue;
    }

    // Seen '%'.  Now processing a format conversion.
    switch (Str[StrIdx]) {
    // Handle dynamic precision or width specifier.     
    case '*': {
      ++numConversions;
      
      if (!HasVAListArg && numConversions > numDataArgs) {
        SourceLocation Loc = FExpr->getLocStart();
        Loc = PP.AdvanceToTokenCharacter(Loc, StrIdx+1);

        if (Str[StrIdx-1] == '.')
          Diag(Loc, diag::warn_printf_asterisk_precision_missing_arg,
               Fn->getSourceRange());
        else
          Diag(Loc, diag::warn_printf_asterisk_width_missing_arg,
               Fn->getSourceRange());
        
        // Don't do any more checking.  We'll just emit spurious errors.
        return;
      }
      
      // Perform type checking on width/precision specifier.
      Expr *E = TheCall->getArg(format_idx+numConversions);
      if (const BuiltinType *BT = E->getType()->getAsBuiltinType())
        if (BT->getKind() == BuiltinType::Int)
          break;

      SourceLocation Loc =
        PP.AdvanceToTokenCharacter(FExpr->getLocStart(), StrIdx+1);
      
      if (Str[StrIdx-1] == '.')
        Diag(Loc, diag::warn_printf_asterisk_precision_wrong_type,
             E->getType().getAsString(), E->getSourceRange());
      else
        Diag(Loc, diag::warn_printf_asterisk_width_wrong_type,
             E->getType().getAsString(), E->getSourceRange());
      
      break;
    }
      
    // Characters which can terminate a format conversion
    // (e.g. "%d").  Characters that specify length modifiers or
    // other flags are handled by the default case below.
    //
    // FIXME: additional checks will go into the following cases.                
    case 'i':
    case 'd':
    case 'o': 
    case 'u': 
    case 'x':
    case 'X':
    case 'D':
    case 'O':
    case 'U':
    case 'e':
    case 'E':
    case 'f':
    case 'F':
    case 'g':
    case 'G':
    case 'a':
    case 'A':
    case 'c':
    case 'C':
    case 'S':
    case 's':
    case 'p': 
      ++numConversions;
      CurrentState = state_OrdChr;
      break;

    // CHECK: Are we using "%n"?  Issue a warning.
    case 'n': {
      ++numConversions;
      CurrentState = state_OrdChr;
      SourceLocation Loc = PP.AdvanceToTokenCharacter(FExpr->getLocStart(),
                                                      LastConversionIdx+1);
                                   
      Diag(Loc, diag::warn_printf_write_back, Fn->getSourceRange());
      break;
    }
                  
    // Handle "%%"
    case '%':
      // Sanity check: Was the first "%" character the previous one?
      // If not, we will assume that we have a malformed format
      // conversion, and that the current "%" character is the start
      // of a new conversion.
      if (StrIdx - LastConversionIdx == 1)
        CurrentState = state_OrdChr; 
      else {
        // Issue a warning: invalid format conversion.
        SourceLocation Loc = PP.AdvanceToTokenCharacter(FExpr->getLocStart(),
                                                        LastConversionIdx+1);
            
        Diag(Loc, diag::warn_printf_invalid_conversion, 
             std::string(Str+LastConversionIdx, Str+StrIdx),
             Fn->getSourceRange());
             
        // This conversion is broken.  Advance to the next format
        // conversion.
        LastConversionIdx = StrIdx;
        ++numConversions;
      }
      break;
              
    default:
      // This case catches all other characters: flags, widths, etc.
      // We should eventually process those as well.
      break;
    }
  }

  if (CurrentState == state_Conversion) {
    // Issue a warning: invalid format conversion.
    SourceLocation Loc = PP.AdvanceToTokenCharacter(FExpr->getLocStart(),
                                                    LastConversionIdx+1);
    
    Diag(Loc, diag::warn_printf_invalid_conversion,
         std::string(Str+LastConversionIdx,
                     Str+std::min(LastConversionIdx+2, StrLen)),
         Fn->getSourceRange());
    return;
  }
  
  if (!HasVAListArg) {
    // CHECK: Does the number of format conversions exceed the number
    //        of data arguments?
    if (numConversions > numDataArgs) {
      SourceLocation Loc = PP.AdvanceToTokenCharacter(FExpr->getLocStart(),
                                                      LastConversionIdx);
                                   
      Diag(Loc, diag::warn_printf_insufficient_data_args,
           Fn->getSourceRange());
    }
    // CHECK: Does the number of data arguments exceed the number of
    //        format conversions in the format string?
    else if (numConversions < numDataArgs)
      Diag(TheCall->getArg(format_idx+numConversions+1)->getLocStart(),
           diag::warn_printf_too_many_data_args, Fn->getSourceRange());
  }
}

//===--- CHECK: Return Address of Stack Variable --------------------------===//

static DeclRefExpr* EvalVal(Expr *E);
static DeclRefExpr* EvalAddr(Expr* E);

/// CheckReturnStackAddr - Check if a return statement returns the address
///   of a stack variable.
void
Sema::CheckReturnStackAddr(Expr *RetValExp, QualType lhsType,
                           SourceLocation ReturnLoc) {
   
  // Perform checking for returned stack addresses.
  if (lhsType->isPointerType()) {
    if (DeclRefExpr *DR = EvalAddr(RetValExp))
      Diag(DR->getLocStart(), diag::warn_ret_stack_addr,
           DR->getDecl()->getIdentifier()->getName(),
           RetValExp->getSourceRange());
  }
  // Perform checking for stack values returned by reference.
  else if (lhsType->isReferenceType()) {
    // Check for an implicit cast to a reference.
    if (ImplicitCastExpr *I = dyn_cast<ImplicitCastExpr>(RetValExp))
      if (DeclRefExpr *DR = EvalVal(I->getSubExpr()))
        Diag(DR->getLocStart(), diag::warn_ret_stack_ref,
             DR->getDecl()->getIdentifier()->getName(),
             RetValExp->getSourceRange());
  }
}

/// EvalAddr - EvalAddr and EvalVal are mutually recursive functions that
///  check if the expression in a return statement evaluates to an address
///  to a location on the stack.  The recursion is used to traverse the
///  AST of the return expression, with recursion backtracking when we
///  encounter a subexpression that (1) clearly does not lead to the address
///  of a stack variable or (2) is something we cannot determine leads to
///  the address of a stack variable based on such local checking.
///
///  EvalAddr processes expressions that are pointers that are used as
///  references (and not L-values).  EvalVal handles all other values.
///  At the base case of the recursion is a check for a DeclRefExpr* in 
///  the refers to a stack variable.
///
///  This implementation handles:
///
///   * pointer-to-pointer casts
///   * implicit conversions from array references to pointers
///   * taking the address of fields
///   * arbitrary interplay between "&" and "*" operators
///   * pointer arithmetic from an address of a stack variable
///   * taking the address of an array element where the array is on the stack
static DeclRefExpr* EvalAddr(Expr *E) {
  // We should only be called for evaluating pointer expressions.
  assert((E->getType()->isPointerType() || 
          E->getType()->isObjCQualifiedIdType()) &&
         "EvalAddr only works on pointers");
    
  // Our "symbolic interpreter" is just a dispatch off the currently
  // viewed AST node.  We then recursively traverse the AST by calling
  // EvalAddr and EvalVal appropriately.
  switch (E->getStmtClass()) {
  case Stmt::ParenExprClass:
    // Ignore parentheses.
    return EvalAddr(cast<ParenExpr>(E)->getSubExpr());

  case Stmt::UnaryOperatorClass: {
    // The only unary operator that make sense to handle here
    // is AddrOf.  All others don't make sense as pointers.
    UnaryOperator *U = cast<UnaryOperator>(E);
    
    if (U->getOpcode() == UnaryOperator::AddrOf)
      return EvalVal(U->getSubExpr());
    else
      return NULL;
  }
  
  case Stmt::BinaryOperatorClass: {
    // Handle pointer arithmetic.  All other binary operators are not valid
    // in this context.
    BinaryOperator *B = cast<BinaryOperator>(E);
    BinaryOperator::Opcode op = B->getOpcode();
      
    if (op != BinaryOperator::Add && op != BinaryOperator::Sub)
      return NULL;
      
    Expr *Base = B->getLHS();

    // Determine which argument is the real pointer base.  It could be
    // the RHS argument instead of the LHS.
    if (!Base->getType()->isPointerType()) Base = B->getRHS();
      
    assert (Base->getType()->isPointerType());
    return EvalAddr(Base);
  }
    
  // For conditional operators we need to see if either the LHS or RHS are
  // valid DeclRefExpr*s.  If one of them is valid, we return it.
  case Stmt::ConditionalOperatorClass: {
    ConditionalOperator *C = cast<ConditionalOperator>(E);
    
    // Handle the GNU extension for missing LHS.
    if (Expr *lhsExpr = C->getLHS())
      if (DeclRefExpr* LHS = EvalAddr(lhsExpr))
        return LHS;

     return EvalAddr(C->getRHS());
  }
    
  // For implicit casts, we need to handle conversions from arrays to
  // pointer values, and implicit pointer-to-pointer conversions.
  case Stmt::ImplicitCastExprClass: {
    ImplicitCastExpr *IE = cast<ImplicitCastExpr>(E);
    Expr* SubExpr = IE->getSubExpr();
    
    if (SubExpr->getType()->isPointerType() ||
        SubExpr->getType()->isObjCQualifiedIdType())
      return EvalAddr(SubExpr);
    else
      return EvalVal(SubExpr);
  }

  // For casts, we handle pointer-to-pointer conversions (which
  // is essentially a no-op from our mini-interpreter's standpoint).
  // For other casts we abort.
  case Stmt::CastExprClass: {
    CastExpr *C = cast<CastExpr>(E);
    Expr *SubExpr = C->getSubExpr();
    
    if (SubExpr->getType()->isPointerType())
      return EvalAddr(SubExpr);
    else
      return NULL;
  }
    
  // C++ casts.  For dynamic casts, static casts, and const casts, we
  // are always converting from a pointer-to-pointer, so we just blow
  // through the cast.  In the case the dynamic cast doesn't fail
  // (and return NULL), we take the conservative route and report cases
  // where we return the address of a stack variable.  For Reinterpre
  case Stmt::CXXCastExprClass: {
    CXXCastExpr *C = cast<CXXCastExpr>(E);
    
    if (C->getOpcode() == CXXCastExpr::ReinterpretCast) {
      Expr *S = C->getSubExpr();
      if (S->getType()->isPointerType())
        return EvalAddr(S);
      else
        return NULL;
    }
    else
      return EvalAddr(C->getSubExpr());
  }
    
  // Everything else: we simply don't reason about them.
  default:
    return NULL;
  }
}
  

///  EvalVal - This function is complements EvalAddr in the mutual recursion.
///   See the comments for EvalAddr for more details.
static DeclRefExpr* EvalVal(Expr *E) {
  
  // We should only be called for evaluating non-pointer expressions, or
  // expressions with a pointer type that are not used as references but instead
  // are l-values (e.g., DeclRefExpr with a pointer type).
    
  // Our "symbolic interpreter" is just a dispatch off the currently
  // viewed AST node.  We then recursively traverse the AST by calling
  // EvalAddr and EvalVal appropriately.
  switch (E->getStmtClass()) {
  case Stmt::DeclRefExprClass: {
    // DeclRefExpr: the base case.  When we hit a DeclRefExpr we are looking
    //  at code that refers to a variable's name.  We check if it has local
    //  storage within the function, and if so, return the expression.
    DeclRefExpr *DR = cast<DeclRefExpr>(E);
      
    if (VarDecl *V = dyn_cast<VarDecl>(DR->getDecl()))
      if(V->hasLocalStorage()) return DR;
      
    return NULL;
  }
        
  case Stmt::ParenExprClass:
    // Ignore parentheses.
    return EvalVal(cast<ParenExpr>(E)->getSubExpr());
  
  case Stmt::UnaryOperatorClass: {
    // The only unary operator that make sense to handle here
    // is Deref.  All others don't resolve to a "name."  This includes
    // handling all sorts of rvalues passed to a unary operator.
    UnaryOperator *U = cast<UnaryOperator>(E);
              
    if (U->getOpcode() == UnaryOperator::Deref)
      return EvalAddr(U->getSubExpr());

    return NULL;
  }
  
  case Stmt::ArraySubscriptExprClass: {
    // Array subscripts are potential references to data on the stack.  We
    // retrieve the DeclRefExpr* for the array variable if it indeed
    // has local storage.
    return EvalAddr(cast<ArraySubscriptExpr>(E)->getBase());
  }
    
  case Stmt::ConditionalOperatorClass: {
    // For conditional operators we need to see if either the LHS or RHS are
    // non-NULL DeclRefExpr's.  If one is non-NULL, we return it.
    ConditionalOperator *C = cast<ConditionalOperator>(E);

    // Handle the GNU extension for missing LHS.
    if (Expr *lhsExpr = C->getLHS())
      if (DeclRefExpr *LHS = EvalVal(lhsExpr))
        return LHS;

    return EvalVal(C->getRHS());
  }
  
  // Accesses to members are potential references to data on the stack.
  case Stmt::MemberExprClass: {
    MemberExpr *M = cast<MemberExpr>(E);
      
    // Check for indirect access.  We only want direct field accesses.
    if (!M->isArrow())
      return EvalVal(M->getBase());
    else
      return NULL;
  }
    
  // Everything else: we simply don't reason about them.
  default:
    return NULL;
  }
}

//===--- CHECK: Floating-Point comparisons (-Wfloat-equal) ---------------===//

/// Check for comparisons of floating point operands using != and ==.
/// Issue a warning if these are no self-comparisons, as they are not likely
/// to do what the programmer intended.
void Sema::CheckFloatComparison(SourceLocation loc, Expr* lex, Expr *rex) {
  bool EmitWarning = true;
  
  Expr* LeftExprSansParen = lex->IgnoreParens();
  Expr* RightExprSansParen = rex->IgnoreParens();

  // Special case: check for x == x (which is OK).
  // Do not emit warnings for such cases.
  if (DeclRefExpr* DRL = dyn_cast<DeclRefExpr>(LeftExprSansParen))
    if (DeclRefExpr* DRR = dyn_cast<DeclRefExpr>(RightExprSansParen))
      if (DRL->getDecl() == DRR->getDecl())
        EmitWarning = false;
  
  
  // Special case: check for comparisons against literals that can be exactly
  //  represented by APFloat.  In such cases, do not emit a warning.  This
  //  is a heuristic: often comparison against such literals are used to
  //  detect if a value in a variable has not changed.  This clearly can
  //  lead to false negatives.
  if (EmitWarning) {
    if (FloatingLiteral* FLL = dyn_cast<FloatingLiteral>(LeftExprSansParen)) {
      if (FLL->isExact())
        EmitWarning = false;
    }
    else
      if (FloatingLiteral* FLR = dyn_cast<FloatingLiteral>(RightExprSansParen)){
        if (FLR->isExact())
          EmitWarning = false;
    }
  }
  
  // Check for comparisons with builtin types.
  if (EmitWarning)           
    if (CallExpr* CL = dyn_cast<CallExpr>(LeftExprSansParen))
      if (isCallBuiltin(CL))
        EmitWarning = false;
  
  if (EmitWarning)            
    if (CallExpr* CR = dyn_cast<CallExpr>(RightExprSansParen))
      if (isCallBuiltin(CR))
        EmitWarning = false;
  
  // Emit the diagnostic.
  if (EmitWarning)
    Diag(loc, diag::warn_floatingpoint_eq,
         lex->getSourceRange(),rex->getSourceRange());
}