fold-const.c

gcc库的原代码,对编程有很大帮助.

  return fold (build (code, sizetype, arg0, arg1));
}

/* Given T, a tree representing type conversion of ARG1, a constant,
   return a constant tree representing the result of conversion.  */

static tree
fold_convert (t, arg1)
     register tree t;
     register tree arg1;
{
  register tree type = TREE_TYPE (t);
  int overflow = 0;

  if (TREE_CODE (type) == POINTER_TYPE || INTEGRAL_TYPE_P (type))
    {
      if (TREE_CODE (arg1) == INTEGER_CST)
	{
	  /* If we would build a constant wider than GCC supports,
	     leave the conversion unfolded.  */
	  if (TYPE_PRECISION (type) > 2 * HOST_BITS_PER_WIDE_INT)
	    return t;

	  /* Given an integer constant, make new constant with new type,
	     appropriately sign-extended or truncated.  */
	  t = build_int_2 (TREE_INT_CST_LOW (arg1),
			   TREE_INT_CST_HIGH (arg1));
	  TREE_TYPE (t) = type;
	  /* Indicate an overflow if (1) ARG1 already overflowed,
	     or (2) force_fit_type indicates an overflow.
	     Tell force_fit_type that an overflow has already occurred
	     if ARG1 is a too-large unsigned value and T is signed.  */
	  TREE_OVERFLOW (t)
	    = (TREE_OVERFLOW (arg1)
	       | force_fit_type (t,
				 (TREE_INT_CST_HIGH (arg1) < 0
				  & (TREE_UNSIGNED (type)
				     < TREE_UNSIGNED (TREE_TYPE (arg1))))));
	  TREE_CONSTANT_OVERFLOW (t)
	    = TREE_OVERFLOW (t) | TREE_CONSTANT_OVERFLOW (arg1);
	}
#if !defined (REAL_IS_NOT_DOUBLE) || defined (REAL_ARITHMETIC)
      else if (TREE_CODE (arg1) == REAL_CST)
	{
	  /* Don't initialize these, use assignments.
	     Initialized local aggregates don't work on old compilers.  */
	  REAL_VALUE_TYPE x;
	  REAL_VALUE_TYPE l;
	  REAL_VALUE_TYPE u;

	  x = TREE_REAL_CST (arg1);
	  l = real_value_from_int_cst (TYPE_MIN_VALUE (type));
	  u = real_value_from_int_cst (TYPE_MAX_VALUE (type));
	  /* See if X will be in range after truncation towards 0.
	     To compensate for truncation, move the bounds away from 0,
	     but reject if X exactly equals the adjusted bounds.  */
#ifdef REAL_ARITHMETIC
	  REAL_ARITHMETIC (l, MINUS_EXPR, l, dconst1);
	  REAL_ARITHMETIC (u, PLUS_EXPR, u, dconst1);
#else
	  l--;
	  u++;
#endif
	  /* If X is a NaN, use zero instead and show we have an overflow.
	     Otherwise, range check.  */
	  if (REAL_VALUE_ISNAN (x))
	    overflow = 1, x = dconst0;
	  else if (! (REAL_VALUES_LESS (l, x) && REAL_VALUES_LESS (x, u)))
	    overflow = 1;

#ifndef REAL_ARITHMETIC
	  {
	    HOST_WIDE_INT low, high;
	    HOST_WIDE_INT half_word
	      = (HOST_WIDE_INT) 1 << (HOST_BITS_PER_WIDE_INT / 2);

	    if (x < 0)
	      x = -x;

	    high = (HOST_WIDE_INT) (x / half_word / half_word);
	    x -= (REAL_VALUE_TYPE) high * half_word * half_word;
	    if (x >= (REAL_VALUE_TYPE) half_word * half_word / 2)
	      {
		low = x - (REAL_VALUE_TYPE) half_word * half_word / 2;
		low |= (HOST_WIDE_INT) -1 << (HOST_BITS_PER_WIDE_INT - 1);
	      }
	    else
	      low = (HOST_WIDE_INT) x;
	    if (TREE_REAL_CST (arg1) < 0)
	      neg_double (low, high, &low, &high);
	    t = build_int_2 (low, high);
	  }
#else
	  {
	    HOST_WIDE_INT low, high;
	    REAL_VALUE_TO_INT (&low, &high, x);
	    t = build_int_2 (low, high);
	  }
#endif
	  TREE_TYPE (t) = type;
	  TREE_OVERFLOW (t)
	    = TREE_OVERFLOW (arg1) | force_fit_type (t, overflow);
	  TREE_CONSTANT_OVERFLOW (t)
	    = TREE_OVERFLOW (t) | TREE_CONSTANT_OVERFLOW (arg1);
	}
#endif /* not REAL_IS_NOT_DOUBLE, or REAL_ARITHMETIC */
      TREE_TYPE (t) = type;
    }
  else if (TREE_CODE (type) == REAL_TYPE)
    {
#if !defined (REAL_IS_NOT_DOUBLE) || defined (REAL_ARITHMETIC)
      if (TREE_CODE (arg1) == INTEGER_CST)
	return build_real_from_int_cst (type, arg1);
#endif /* not REAL_IS_NOT_DOUBLE, or REAL_ARITHMETIC */
      if (TREE_CODE (arg1) == REAL_CST)
	{
	  if (REAL_VALUE_ISNAN (TREE_REAL_CST (arg1)))
	    return arg1;
	  else if (setjmp (float_error))
	    {
	      overflow = 1;
	      t = copy_node (arg1);
	      goto got_it;
	    }
	  set_float_handler (float_error);

	  t = build_real (type, real_value_truncate (TYPE_MODE (type),
						     TREE_REAL_CST (arg1)));
	  set_float_handler (NULL_PTR);

	got_it:
	  TREE_OVERFLOW (t)
	    = TREE_OVERFLOW (arg1) | force_fit_type (t, overflow);
	  TREE_CONSTANT_OVERFLOW (t)
	    = TREE_OVERFLOW (t) | TREE_CONSTANT_OVERFLOW (arg1);
	  return t;
	}
    }
  TREE_CONSTANT (t) = 1;
  return t;
}

/* Return an expr equal to X but certainly not valid as an lvalue.
   Also make sure it is not valid as an null pointer constant.  */

tree
non_lvalue (x)
     tree x;
{
  tree result;

  /* These things are certainly not lvalues.  */
  if (TREE_CODE (x) == NON_LVALUE_EXPR
      || TREE_CODE (x) == INTEGER_CST
      || TREE_CODE (x) == REAL_CST
      || TREE_CODE (x) == STRING_CST
      || TREE_CODE (x) == ADDR_EXPR)
    {
      if (TREE_CODE (x) == INTEGER_CST && integer_zerop (x))
	{
	  /* Use NOP_EXPR instead of NON_LVALUE_EXPR
	     so convert_for_assignment won't strip it.
	     This is so this 0 won't be treated as a null pointer constant.  */
	  result = build1 (NOP_EXPR, TREE_TYPE (x), x);
	  TREE_CONSTANT (result) = TREE_CONSTANT (x);
	  return result;
	}
      return x;
    }

  result = build1 (NON_LVALUE_EXPR, TREE_TYPE (x), x);
  TREE_CONSTANT (result) = TREE_CONSTANT (x);
  return result;
}

/* Nonzero means lvalues are limited to those valid in pedantic ANSI C.
   Zero means allow extended lvalues.  */

int pedantic_lvalues;

/* When pedantic, return an expr equal to X but certainly not valid as a
   pedantic lvalue.  Otherwise, return X.  */

tree
pedantic_non_lvalue (x)
     tree x;
{
  if (pedantic_lvalues)
    return non_lvalue (x);
  else
    return x;
}

/* Given a tree comparison code, return the code that is the logical inverse
   of the given code.  It is not safe to do this for floating-point
   comparisons, except for NE_EXPR and EQ_EXPR.  */

static enum tree_code
invert_tree_comparison (code)
     enum tree_code code;
{
  switch (code)
    {
    case EQ_EXPR:
      return NE_EXPR;
    case NE_EXPR:
      return EQ_EXPR;
    case GT_EXPR:
      return LE_EXPR;
    case GE_EXPR:
      return LT_EXPR;
    case LT_EXPR:
      return GE_EXPR;
    case LE_EXPR:
      return GT_EXPR;
    default:
      abort ();
    }
}

/* Similar, but return the comparison that results if the operands are
   swapped.  This is safe for floating-point.  */

static enum tree_code
swap_tree_comparison (code)
     enum tree_code code;
{
  switch (code)
    {
    case EQ_EXPR:
    case NE_EXPR:
      return code;
    case GT_EXPR:
      return LT_EXPR;
    case GE_EXPR:
      return LE_EXPR;
    case LT_EXPR:
      return GT_EXPR;
    case LE_EXPR:
      return GE_EXPR;
    default:
      abort ();
    }
}

/* Return nonzero if CODE is a tree code that represents a truth value.  */

static int
truth_value_p (code)
     enum tree_code code;
{
  return (TREE_CODE_CLASS (code) == '<'
	  || code == TRUTH_AND_EXPR || code == TRUTH_ANDIF_EXPR
	  || code == TRUTH_OR_EXPR || code == TRUTH_ORIF_EXPR
	  || code == TRUTH_XOR_EXPR || code == TRUTH_NOT_EXPR);
}

/* Return nonzero if two operands are necessarily equal.
   If ONLY_CONST is non-zero, only return non-zero for constants.
   This function tests whether the operands are indistinguishable;
   it does not test whether they are equal using C's == operation.
   The distinction is important for IEEE floating point, because
   (1) -0.0 and 0.0 are distinguishable, but -0.0==0.0, and
   (2) two NaNs may be indistinguishable, but NaN!=NaN.  */

int
operand_equal_p (arg0, arg1, only_const)
     tree arg0, arg1;
     int only_const;
{
  /* If both types don't have the same signedness, then we can't consider
     them equal.  We must check this before the STRIP_NOPS calls
     because they may change the signedness of the arguments.  */
  if (TREE_UNSIGNED (TREE_TYPE (arg0)) != TREE_UNSIGNED (TREE_TYPE (arg1)))
    return 0;

  STRIP_NOPS (arg0);
  STRIP_NOPS (arg1);

  /* If ARG0 and ARG1 are the same SAVE_EXPR, they are necessarily equal.
     We don't care about side effects in that case because the SAVE_EXPR
     takes care of that for us.  */
  if (TREE_CODE (arg0) == SAVE_EXPR && arg0 == arg1)
    return ! only_const;

  if (TREE_SIDE_EFFECTS (arg0) || TREE_SIDE_EFFECTS (arg1))
    return 0;

  if (TREE_CODE (arg0) == TREE_CODE (arg1)
      && TREE_CODE (arg0) == ADDR_EXPR
      && TREE_OPERAND (arg0, 0) == TREE_OPERAND (arg1, 0))
    return 1;

  if (TREE_CODE (arg0) == TREE_CODE (arg1)
      && TREE_CODE (arg0) == INTEGER_CST
      && TREE_INT_CST_LOW (arg0) == TREE_INT_CST_LOW (arg1)
      && TREE_INT_CST_HIGH (arg0) == TREE_INT_CST_HIGH (arg1))
    return 1;

  /* Detect when real constants are equal.  */
  if (TREE_CODE (arg0) == TREE_CODE (arg1)
      && TREE_CODE (arg0) == REAL_CST)
    return !bcmp ((char *) &TREE_REAL_CST (arg0),
		  (char *) &TREE_REAL_CST (arg1),
		  sizeof (REAL_VALUE_TYPE));

  if (only_const)
    return 0;

  if (arg0 == arg1)
    return 1;

  if (TREE_CODE (arg0) != TREE_CODE (arg1))
    return 0;
  /* This is needed for conversions and for COMPONENT_REF.
     Might as well play it safe and always test this.  */
  if (TYPE_MODE (TREE_TYPE (arg0)) != TYPE_MODE (TREE_TYPE (arg1)))
    return 0;

  switch (TREE_CODE_CLASS (TREE_CODE (arg0)))
    {
    case '1':
      /* Two conversions are equal only if signedness and modes match.  */
      if ((TREE_CODE (arg0) == NOP_EXPR || TREE_CODE (arg0) == CONVERT_EXPR)
	  && (TREE_UNSIGNED (TREE_TYPE (arg0))
	      != TREE_UNSIGNED (TREE_TYPE (arg1))))
	return 0;

      return operand_equal_p (TREE_OPERAND (arg0, 0),
			      TREE_OPERAND (arg1, 0), 0);

    case '<':
    case '2':
      return (operand_equal_p (TREE_OPERAND (arg0, 0),
			       TREE_OPERAND (arg1, 0), 0)
	      && operand_equal_p (TREE_OPERAND (arg0, 1),
				  TREE_OPERAND (arg1, 1), 0));

    case 'r':
      switch (TREE_CODE (arg0))
	{
	case INDIRECT_REF:
	  return operand_equal_p (TREE_OPERAND (arg0, 0),
				  TREE_OPERAND (arg1, 0), 0);

	case COMPONENT_REF:
	case ARRAY_REF:
	  return (operand_equal_p (TREE_OPERAND (arg0, 0),
				   TREE_OPERAND (arg1, 0), 0)
		  && operand_equal_p (TREE_OPERAND (arg0, 1),
				      TREE_OPERAND (arg1, 1), 0));

	case BIT_FIELD_REF:
	  return (operand_equal_p (TREE_OPERAND (arg0, 0),
				   TREE_OPERAND (arg1, 0), 0)
		  && operand_equal_p (TREE_OPERAND (arg0, 1),
				      TREE_OPERAND (arg1, 1), 0)
		  && operand_equal_p (TREE_OPERAND (arg0, 2),
				      TREE_OPERAND (arg1, 2), 0));
	}
      break;
    }

  return 0;
}

/* Similar to operand_equal_p, but see if ARG0 might have been made by
   shorten_compare from ARG1 when ARG1 was being compared with OTHER. 

   When in doubt, return 0.  */

static int 
operand_equal_for_comparison_p (arg0, arg1, other)
     tree arg0, arg1;
     tree other;
{
  int unsignedp1, unsignedpo;
  tree primarg1, primother;
  unsigned correct_width;

  if (operand_equal_p (arg0, arg1, 0))
    return 1;

  if (! INTEGRAL_TYPE_P (TREE_TYPE (arg0))
      || ! INTEGRAL_TYPE_P (TREE_TYPE (arg1)))
    return 0;

  /* Duplicate what shorten_compare does to ARG1 and see if that gives the
     actual comparison operand, ARG0.

     First throw away any conversions to wider types
     already present in the operands.  */

  primarg1 = get_narrower (arg1, &unsignedp1);
  primother = get_narrower (other, &unsignedpo);

  correct_width = TYPE_PRECISION (TREE_TYPE (arg1));
  if (unsignedp1 == unsignedpo
      && TYPE_PRECISION (TREE_TYPE (primarg1)) < correct_width
      && TYPE_PRECISION (TREE_TYPE (primother)) < correct_width)
    {
      tree type = TREE_TYPE (arg0);

      /* Make sure shorter operand is extended the right way
	 to match the longer operand.  */
      primarg1 = convert (signed_or_unsigned_type (unsignedp1,
						  TREE_TYPE (primarg1)),
			 primarg1);

      if (operand_equal_p (arg0, convert (type, primarg1), 0))
	return 1;
    }

  return 0;
}

/* See if ARG is an expression that is either a comparison or is performing
   arithmetic on comparisons.  The comparisons must only be comparing
   two different values, which will be stored in *CVAL1 and *CVAL2; if
   they are non-zero it means that some operands have already been found.
   No variables may be used anywhere else in the expression except in the
   comparisons.  If SAVE_P is true it means we removed a SAVE_EXPR around
   the expression and save_expr needs to be called with CVAL1 and CVAL2.

   If this is true, return 1.  Otherwise, return zero.  */

static int
twoval_comparison_p (arg, cval1, cval2, save_p)
     tree arg;
     tree *cval1, *cval2;
     int *save_p;
{
  enum tree_code code = TREE_CODE (arg);
  char class = TREE_CODE_CLASS (code);

  /* We can handle some of the 'e' cases here.  */
  if (class == 'e' && code == TRUTH_NOT_EXPR)
    class = '1';
  else if (class == 'e'
	   && (code == TRUTH_ANDIF_EXPR || code == TRUTH_ORIF_EXPR
	       || code == COMPOUND_EXPR))
    class = '2';

  /* ??? Disable this since the SAVE_EXPR might already be in use outside
     the expression.  There may be no way to make this work, but it needs
     to be looked at again for 2.6.  */
#if 0
  else if (class == 'e' && code == SAVE_EXPR && SAVE_EXPR_RTL (arg) == 0)
    {
      /* If we've already found a CVAL1 or CVAL2, this expression is
	 two complex to handle.  */
      if (*cval1 || *cval2)
	return 0;