fold-const.c

gcc库的原代码,对编程有很大帮助.

      class = '1';
      *save_p = 1;
    }
#endif

  switch (class)
    {
    case '1':
      return twoval_comparison_p (TREE_OPERAND (arg, 0), cval1, cval2, save_p);

    case '2':
      return (twoval_comparison_p (TREE_OPERAND (arg, 0), cval1, cval2, save_p)
	      && twoval_comparison_p (TREE_OPERAND (arg, 1),
				      cval1, cval2, save_p));

    case 'c':
      return 1;

    case 'e':
      if (code == COND_EXPR)
	return (twoval_comparison_p (TREE_OPERAND (arg, 0),
				     cval1, cval2, save_p)
		&& twoval_comparison_p (TREE_OPERAND (arg, 1),
					cval1, cval2, save_p)
		&& twoval_comparison_p (TREE_OPERAND (arg, 2),
					cval1, cval2, save_p));
      return 0;
	  
    case '<':
      /* First see if we can handle the first operand, then the second.  For
	 the second operand, we know *CVAL1 can't be zero.  It must be that
	 one side of the comparison is each of the values; test for the
	 case where this isn't true by failing if the two operands
	 are the same.  */

      if (operand_equal_p (TREE_OPERAND (arg, 0),
			   TREE_OPERAND (arg, 1), 0))
	return 0;

      if (*cval1 == 0)
	*cval1 = TREE_OPERAND (arg, 0);
      else if (operand_equal_p (*cval1, TREE_OPERAND (arg, 0), 0))
	;
      else if (*cval2 == 0)
	*cval2 = TREE_OPERAND (arg, 0);
      else if (operand_equal_p (*cval2, TREE_OPERAND (arg, 0), 0))
	;
      else
	return 0;

      if (operand_equal_p (*cval1, TREE_OPERAND (arg, 1), 0))
	;
      else if (*cval2 == 0)
	*cval2 = TREE_OPERAND (arg, 1);
      else if (operand_equal_p (*cval2, TREE_OPERAND (arg, 1), 0))
	;
      else
	return 0;

      return 1;
    }

  return 0;
}

/* ARG is a tree that is known to contain just arithmetic operations and
   comparisons.  Evaluate the operations in the tree substituting NEW0 for
   any occurrence of OLD0 as an operand of a comparison and likewise for
   NEW1 and OLD1.  */

static tree
eval_subst (arg, old0, new0, old1, new1)
     tree arg;
     tree old0, new0, old1, new1;
{
  tree type = TREE_TYPE (arg);
  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))
    class = '2';

  switch (class)
    {
    case '1':
      return fold (build1 (code, type,
			   eval_subst (TREE_OPERAND (arg, 0),
				       old0, new0, old1, new1)));

    case '2':
      return fold (build (code, type,
			  eval_subst (TREE_OPERAND (arg, 0),
				      old0, new0, old1, new1),
			  eval_subst (TREE_OPERAND (arg, 1),
				      old0, new0, old1, new1)));

    case 'e':
      switch (code)
	{
	case SAVE_EXPR:
	  return eval_subst (TREE_OPERAND (arg, 0), old0, new0, old1, new1);

	case COMPOUND_EXPR:
	  return eval_subst (TREE_OPERAND (arg, 1), old0, new0, old1, new1);

	case COND_EXPR:
	  return fold (build (code, type,
			      eval_subst (TREE_OPERAND (arg, 0),
					  old0, new0, old1, new1),
			      eval_subst (TREE_OPERAND (arg, 1),
					  old0, new0, old1, new1),
			      eval_subst (TREE_OPERAND (arg, 2),
					  old0, new0, old1, new1)));
	}

    case '<':
      {
	tree arg0 = TREE_OPERAND (arg, 0);
	tree arg1 = TREE_OPERAND (arg, 1);

	/* We need to check both for exact equality and tree equality.  The
	   former will be true if the operand has a side-effect.  In that
	   case, we know the operand occurred exactly once.  */

	if (arg0 == old0 || operand_equal_p (arg0, old0, 0))
	  arg0 = new0;
	else if (arg0 == old1 || operand_equal_p (arg0, old1, 0))
	  arg0 = new1;

	if (arg1 == old0 || operand_equal_p (arg1, old0, 0))
	  arg1 = new0;
	else if (arg1 == old1 || operand_equal_p (arg1, old1, 0))
	  arg1 = new1;

	return fold (build (code, type, arg0, arg1));
      }
    }

  return arg;
}

/* Return a tree for the case when the result of an expression is RESULT
   converted to TYPE and OMITTED was previously an operand of the expression
   but is now not needed (e.g., we folded OMITTED * 0).

   If OMITTED has side effects, we must evaluate it.  Otherwise, just do
   the conversion of RESULT to TYPE.  */

static tree
omit_one_operand (type, result, omitted)
     tree type, result, omitted;
{
  tree t = convert (type, result);

  if (TREE_SIDE_EFFECTS (omitted))
    return build (COMPOUND_EXPR, type, omitted, t);

  return non_lvalue (t);
}

/* Similar, but call pedantic_non_lvalue instead of non_lvalue.  */

static tree
pedantic_omit_one_operand (type, result, omitted)
     tree type, result, omitted;
{
  tree t = convert (type, result);

  if (TREE_SIDE_EFFECTS (omitted))
    return build (COMPOUND_EXPR, type, omitted, t);

  return pedantic_non_lvalue (t);
}



/* Return a simplified tree node for the truth-negation of ARG.  This
   never alters ARG itself.  We assume that ARG is an operation that
   returns a truth value (0 or 1).  */

tree
invert_truthvalue (arg)
     tree arg;
{
  tree type = TREE_TYPE (arg);
  enum tree_code code = TREE_CODE (arg);

  if (code == ERROR_MARK)
    return arg;

  /* If this is a comparison, we can simply invert it, except for
     floating-point non-equality comparisons, in which case we just
     enclose a TRUTH_NOT_EXPR around what we have.  */

  if (TREE_CODE_CLASS (code) == '<')
    {
      if (FLOAT_TYPE_P (TREE_TYPE (TREE_OPERAND (arg, 0)))
	  && code != NE_EXPR && code != EQ_EXPR)
	return build1 (TRUTH_NOT_EXPR, type, arg);
      else
	return build (invert_tree_comparison (code), type,
		      TREE_OPERAND (arg, 0), TREE_OPERAND (arg, 1));
    }

  switch (code)
    {
    case INTEGER_CST:
      return convert (type, build_int_2 (TREE_INT_CST_LOW (arg) == 0
					 && TREE_INT_CST_HIGH (arg) == 0, 0));

    case TRUTH_AND_EXPR:
      return build (TRUTH_OR_EXPR, type,
		    invert_truthvalue (TREE_OPERAND (arg, 0)),
		    invert_truthvalue (TREE_OPERAND (arg, 1)));

    case TRUTH_OR_EXPR:
      return build (TRUTH_AND_EXPR, type,
		    invert_truthvalue (TREE_OPERAND (arg, 0)),
		    invert_truthvalue (TREE_OPERAND (arg, 1)));

    case TRUTH_XOR_EXPR:
      /* Here we can invert either operand.  We invert the first operand
	 unless the second operand is a TRUTH_NOT_EXPR in which case our
	 result is the XOR of the first operand with the inside of the
	 negation of the second operand.  */

      if (TREE_CODE (TREE_OPERAND (arg, 1)) == TRUTH_NOT_EXPR)
	return build (TRUTH_XOR_EXPR, type, TREE_OPERAND (arg, 0),
		      TREE_OPERAND (TREE_OPERAND (arg, 1), 0));
      else
	return build (TRUTH_XOR_EXPR, type,
		      invert_truthvalue (TREE_OPERAND (arg, 0)),
		      TREE_OPERAND (arg, 1));

    case TRUTH_ANDIF_EXPR:
      return build (TRUTH_ORIF_EXPR, type,
		    invert_truthvalue (TREE_OPERAND (arg, 0)),
		    invert_truthvalue (TREE_OPERAND (arg, 1)));

    case TRUTH_ORIF_EXPR:
      return build (TRUTH_ANDIF_EXPR, type,
		    invert_truthvalue (TREE_OPERAND (arg, 0)),
		    invert_truthvalue (TREE_OPERAND (arg, 1)));

    case TRUTH_NOT_EXPR:
      return TREE_OPERAND (arg, 0);

    case COND_EXPR:
      return build (COND_EXPR, type, TREE_OPERAND (arg, 0),
		    invert_truthvalue (TREE_OPERAND (arg, 1)),
		    invert_truthvalue (TREE_OPERAND (arg, 2)));

    case COMPOUND_EXPR:
      return build (COMPOUND_EXPR, type, TREE_OPERAND (arg, 0),
		    invert_truthvalue (TREE_OPERAND (arg, 1)));

    case NON_LVALUE_EXPR:
      return invert_truthvalue (TREE_OPERAND (arg, 0));

    case NOP_EXPR:
    case CONVERT_EXPR:
    case FLOAT_EXPR:
      return build1 (TREE_CODE (arg), type,
		     invert_truthvalue (TREE_OPERAND (arg, 0)));

    case BIT_AND_EXPR:
      if (!integer_onep (TREE_OPERAND (arg, 1)))
	break;
      return build (EQ_EXPR, type, arg, convert (type, integer_zero_node));

    case SAVE_EXPR:
      return build1 (TRUTH_NOT_EXPR, type, arg);

    case CLEANUP_POINT_EXPR:
      return build1 (CLEANUP_POINT_EXPR, type,
		     invert_truthvalue (TREE_OPERAND (arg, 0)));
    }
  if (TREE_CODE (TREE_TYPE (arg)) != BOOLEAN_TYPE)
    abort ();
  return build1 (TRUTH_NOT_EXPR, type, arg);
}

/* Given a bit-wise operation CODE applied to ARG0 and ARG1, see if both
   operands are another bit-wise operation with a common input.  If so,
   distribute the bit operations to save an operation and possibly two if
   constants are involved.  For example, convert
   	(A | B) & (A | C) into A | (B & C)
   Further simplification will occur if B and C are constants.

   If this optimization cannot be done, 0 will be returned.  */

static tree
distribute_bit_expr (code, type, arg0, arg1)
     enum tree_code code;
     tree type;
     tree arg0, arg1;
{
  tree common;
  tree left, right;

  if (TREE_CODE (arg0) != TREE_CODE (arg1)
      || TREE_CODE (arg0) == code
      || (TREE_CODE (arg0) != BIT_AND_EXPR
	  && TREE_CODE (arg0) != BIT_IOR_EXPR))
    return 0;

  if (operand_equal_p (TREE_OPERAND (arg0, 0), TREE_OPERAND (arg1, 0), 0))
    {
      common = TREE_OPERAND (arg0, 0);
      left = TREE_OPERAND (arg0, 1);
      right = TREE_OPERAND (arg1, 1);
    }
  else if (operand_equal_p (TREE_OPERAND (arg0, 0), TREE_OPERAND (arg1, 1), 0))
    {
      common = TREE_OPERAND (arg0, 0);
      left = TREE_OPERAND (arg0, 1);
      right = TREE_OPERAND (arg1, 0);
    }
  else if (operand_equal_p (TREE_OPERAND (arg0, 1), TREE_OPERAND (arg1, 0), 0))
    {
      common = TREE_OPERAND (arg0, 1);
      left = TREE_OPERAND (arg0, 0);
      right = TREE_OPERAND (arg1, 1);
    }
  else if (operand_equal_p (TREE_OPERAND (arg0, 1), TREE_OPERAND (arg1, 1), 0))
    {
      common = TREE_OPERAND (arg0, 1);
      left = TREE_OPERAND (arg0, 0);
      right = TREE_OPERAND (arg1, 0);
    }
  else
    return 0;

  return fold (build (TREE_CODE (arg0), type, common,
		      fold (build (code, type, left, right))));
}

/* Return a BIT_FIELD_REF of type TYPE to refer to BITSIZE bits of INNER
   starting at BITPOS.  The field is unsigned if UNSIGNEDP is non-zero.  */

static tree
make_bit_field_ref (inner, type, bitsize, bitpos, unsignedp)
     tree inner;
     tree type;
     int bitsize, bitpos;
     int unsignedp;
{
  tree result = build (BIT_FIELD_REF, type, inner,
		       size_int (bitsize), size_int (bitpos));

  TREE_UNSIGNED (result) = unsignedp;

  return result;
}

/* Optimize a bit-field compare.

   There are two cases:  First is a compare against a constant and the
   second is a comparison of two items where the fields are at the same
   bit position relative to the start of a chunk (byte, halfword, word)
   large enough to contain it.  In these cases we can avoid the shift
   implicit in bitfield extractions.

   For constants, we emit a compare of the shifted constant with the
   BIT_AND_EXPR of a mask and a byte, halfword, or word of the operand being
   compared.  For two fields at the same position, we do the ANDs with the
   similar mask and compare the result of the ANDs.

   CODE is the comparison code, known to be either NE_EXPR or EQ_EXPR.
   COMPARE_TYPE is the type of the comparison, and LHS and RHS
   are the left and right operands of the comparison, respectively.

   If the optimization described above can be done, we return the resulting
   tree.  Otherwise we return zero.  */

static tree
optimize_bit_field_compare (code, compare_type, lhs, rhs)
     enum tree_code code;
     tree compare_type;
     tree lhs, rhs;
{
  int lbitpos, lbitsize, rbitpos, rbitsize;
  int lnbitpos, lnbitsize, rnbitpos, rnbitsize;
  tree type = TREE_TYPE (lhs);
  tree signed_type, unsigned_type;
  int const_p = TREE_CODE (rhs) == INTEGER_CST;
  enum machine_mode lmode, rmode, lnmode, rnmode;
  int lunsignedp, runsignedp;
  int lvolatilep = 0, rvolatilep = 0;
  tree linner, rinner;
  tree mask;
  tree offset;

  /* Get all the information about the extractions being done.  If the bit size
     if the same as the size of the underlying object, we aren't doing an
     extraction at all and so can do nothing.  */
  linner = get_inner_reference (lhs, &lbitsize, &lbitpos, &offset, &lmode,
				&lunsignedp, &lvolatilep);
  if (linner == lhs || lbitsize == GET_MODE_BITSIZE (lmode) || lbitsize < 0
      || offset != 0)
    return 0;

 if (!const_p)
   {
     /* If this is not a constant, we can only do something if bit positions,
	sizes, and signedness are the same.   */
     rinner = get_inner_reference (rhs, &rbitsize, &rbitpos, &offset,
				   &rmode, &runsignedp, &rvolatilep);

     if (rinner == rhs || lbitpos != rbitpos || lbitsize != rbitsize
	 || lunsignedp != runsignedp || offset != 0)
       return 0;
   }

  /* See if we can find a mode to refer to this field.  We should be able to,
     but fail if we can't.  */
  lnmode = get_best_mode (lbitsize, lbitpos,
			  TYPE_ALIGN (TREE_TYPE (linner)), word_mode,
			  lvolatilep);
  if (lnmode == VOIDmode)
    return 0;

  /* Set signed and unsigned types of the precision of this mode for the
     shifts below.  */
  signed_type = type_for_mode (lnmode, 0);
  unsigned_type = type_for_mode (lnmode, 1);

  if (! const_p)
    {
      rnmode = get_best_mode (rbitsize, rbitpos, 
			      TYPE_ALIGN (TREE_TYPE (rinner)), word_mode,
			      rvolatilep);
      if (rnmode == VOIDmode)
	return 0;
    }
    
  /* Compute the bit position and size for the new reference and our offset
     within it. If the new reference is the same size as the original, we
     won't optimize anything, so return zero.  */
  lnbitsize = GET_MODE_BITSIZE (lnmode);
  lnbitpos = lbitpos & ~ (lnbitsize - 1);
  lbitpos -= lnbitpos;
  if (lnbitsize == lbitsize)
    return 0;

  if (! const_p)
    {
      rnbitsize = GET_MODE_BITSIZE (rnmode);
      rnbitpos = rbitpos & ~ (rnbitsize - 1);
      r