lib/AST/ExprConstant.cpp

//===--- Expr.cpp - Expression Constant Evaluator -------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the Expr constant evaluator.
//
//===----------------------------------------------------------------------===//

#include "clang/AST/APValue.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Expr.h"
#include "clang/AST/STmtVisitor.h"
#include "clang/Basic/TargetInfo.h"
#include "llvm/Support/Compiler.h"

using namespace clang;

#define USE_NEW_EVALUATOR 0

static bool CalcFakeICEVal(const Expr* Expr,
                           llvm::APSInt& Result,
                           ASTContext& Context) {
  // Calculate the value of an expression that has a calculatable
  // value, but isn't an ICE. Currently, this only supports
  // a very narrow set of extensions, but it can be expanded if needed.
  if (const ParenExpr *PE = dyn_cast<ParenExpr>(Expr))
    return CalcFakeICEVal(PE->getSubExpr(), Result, Context);
  
  if (const CastExpr *CE = dyn_cast<CastExpr>(Expr)) {
    QualType CETy = CE->getType();
    if ((CETy->isIntegralType() && !CETy->isBooleanType()) ||
        CETy->isPointerType()) {
      if (CalcFakeICEVal(CE->getSubExpr(), Result, Context)) {
        Result.extOrTrunc(Context.getTypeSize(CETy));
        // FIXME: This assumes pointers are signed.
        Result.setIsSigned(CETy->isSignedIntegerType() ||
                           CETy->isPointerType());
        return true;
      }
    }
  }
  
  if (Expr->getType()->isIntegralType())
    return Expr->isIntegerConstantExpr(Result, Context);
  
  return false;
}

namespace {
class VISIBILITY_HIDDEN IntExprEvaluator
  : public StmtVisitor<IntExprEvaluator, APValue> {
  ASTContext &Ctx;

  IntExprEvaluator(ASTContext &ctx)
    : Ctx(ctx) {}

public:
    static bool Evaluate(const Expr* E, llvm::APSInt& Result, ASTContext &Ctx) {
    APValue Value = IntExprEvaluator(Ctx).Visit(const_cast<Expr*>(E));
    if (!Value.isSInt())
      return false;
    
    Result = Value.getSInt();
    return true;
  }
    
  //===--------------------------------------------------------------------===//
  //                            Visitor Methods
  //===--------------------------------------------------------------------===//
  APValue VisitStmt(Stmt *S) {
    // FIXME: Remove this when we support more expressions.
    printf("Unhandled statement\n");
    S->dump();  
    return APValue();
  }
  
  APValue VisitParenExpr(ParenExpr *E) { return Visit(E->getSubExpr()); }

  APValue VisitBinaryOperator(const BinaryOperator *E) {
    // The LHS of a constant expr is always evaluated and needed.
    llvm::APSInt Result(32);
    if (!Evaluate(E->getRHS(), Result, Ctx))
      return APValue(); 

    llvm::APSInt RHS(32);
    if (!Evaluate(E->getRHS(), RHS, Ctx))
      return APValue();
    
    switch (E->getOpcode()) {
    default:
      return APValue();
    case BinaryOperator::Mul:
      Result *= RHS;
      break;
    case BinaryOperator::Div:
      if (RHS == 0)
        return APValue();
     Result /= RHS;
       break;
    case BinaryOperator::Rem:
      if (RHS == 0)
        return APValue();
      Result %= RHS;
      break;
    case BinaryOperator::Add: Result += RHS; break;
    case BinaryOperator::Sub: Result -= RHS; break;
    case BinaryOperator::Shl:
      Result <<= 
        static_cast<uint32_t>(RHS.getLimitedValue(Result.getBitWidth()-1));
      break;
    case BinaryOperator::Shr:
      Result >>= 
        static_cast<uint32_t>(RHS.getLimitedValue(Result.getBitWidth()-1));
      break;
    case BinaryOperator::LT:  Result = Result < RHS; break;
    case BinaryOperator::GT:  Result = Result > RHS; break;
    case BinaryOperator::LE:  Result = Result <= RHS; break;
    case BinaryOperator::GE:  Result = Result >= RHS; break;
    case BinaryOperator::EQ:  Result = Result == RHS; break;
    case BinaryOperator::NE:  Result = Result != RHS; break;
    case BinaryOperator::And: Result &= RHS; break;
    case BinaryOperator::Xor: Result ^= RHS; break;
    case BinaryOperator::Or:  Result |= RHS; break;
      
    case BinaryOperator::Comma:
      // C99 6.6p3: "shall not contain assignment, ..., or comma operators,
      // *except* when they are contained within a subexpression that is not
      // evaluated".  Note that Assignment can never happen due to constraints
      // on the LHS subexpr, so we don't need to check it here.
      // FIXME: Need to come up with an efficient way to deal with the C99
      // rules on evaluation while still evaluating this.  Maybe a
      // "evaluated comma" out parameter?
      return APValue();
    }

    Result.setIsUnsigned(E->getType()->isUnsignedIntegerType());

    return APValue(Result);
  }

  APValue VisitUnaryOperator(const UnaryOperator *E) {
    llvm::APSInt Result(32);
    
    if (E->isOffsetOfOp())
      Result = E->evaluateOffsetOf(Ctx);
    else if (E->isSizeOfAlignOfOp()) {
      // Return the result in the right width.
      Result.zextOrTrunc(static_cast<uint32_t>(Ctx.getTypeSize(E->getType())));

      // sizeof(void) and __alignof__(void) = 1 as a gcc extension.
      if (E->getSubExpr()->getType()->isVoidType())
        Result = 1;

      // sizeof(vla) is not a constantexpr: C99 6.5.3.4p2.
      if (!E->getSubExpr()->getType()->isConstantSizeType()) {
        // FIXME: Should we attempt to evaluate this?
        return APValue();
      }

      // Get information about the size or align.
      if (E->getSubExpr()->getType()->isFunctionType()) {
        // GCC extension: sizeof(function) = 1.
        // FIXME: AlignOf shouldn't be unconditionally 4!
        Result = E->getOpcode() == UnaryOperator::AlignOf ? 4 : 1;
      } else {
        unsigned CharSize = Ctx.Target.getCharWidth();
        if (E->getOpcode() == UnaryOperator::AlignOf)
          Result = Ctx.getTypeAlign(E->getSubExpr()->getType()) / CharSize;
        else
          Result = Ctx.getTypeSize(E->getSubExpr()->getType()) / CharSize;
      }
    } else {
      // Get the operand value.  If this is sizeof/alignof, do not evalute the
      // operand.  This affects C99 6.6p3.
      if (!Evaluate(E->getSubExpr(), Result, Ctx))
        return APValue();

      switch (E->getOpcode()) {
        // Address, indirect, pre/post inc/dec, etc are not valid constant exprs.
        // See C99 6.6p3.
      default:
        return APValue();
      case UnaryOperator::Extension:
        assert(0 && "Handle UnaryOperator::Extension");
        return APValue();  
      case UnaryOperator::LNot: {
        bool Val = Result == 0;
        uint32_t typeSize = Ctx.getTypeSize(E->getType());
        Result.zextOrTrunc(typeSize);
        Result = Val;
        break;
      }
      case UnaryOperator::Plus:
        break;
      case UnaryOperator::Minus:
        Result = -Result;
        break;
      case UnaryOperator::Not:
        Result = ~Result;
        break;
      }
    }

    Result.setIsUnsigned(E->getType()->isUnsignedIntegerType());
    return APValue(Result);    
  }
};    
}
  
bool Expr::tryEvaluate(APValue& Result, ASTContext &Ctx) const
{
  llvm::APSInt sInt(1);
  
#if USE_NEW_EVALUATOR
  if (getType()->isIntegerType()) {
    if (IntExprEvaluator::Evaluate(this, sInt, Ctx)) {
      Result = APValue(sInt);
      return true;
    }
  } else
    return false;
    
#else
  if (CalcFakeICEVal(this, sInt, Ctx)) {
    Result = APValue(sInt);
    return true;
  }
#endif
  
  return false;
}