fold-const.c

早期freebsd实现

      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 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 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 (TREE_CODE (TREE_TYPE (TREE_OPERAND (arg, 0))) == REAL_TYPE
	  && 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_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)))
	abort ();
      return build (EQ_EXPR, type, arg, convert (type, integer_zero_node));
    }

  abort ();
}

/* 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 (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 (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);
      rbitpos -= rnbitpos;
      if (rnbitsize == rbitsize)
	return 0;
    }

#if BYTES_BIG_ENDIAN
  lbitpos = lnbitsize - lbitsize - lbitpos;
#endif

  /* Make the mask to be used against the extracted field.  */
  mask = convert (unsigned_type, build_int_2 (~0, ~0));
  mask = const_binop (LSHIFT_EXPR, mask, size_int (lnbitsize - lbitsize));
  mask = const_binop (RSHIFT_EXPR, mask,
		      size_int (lnbitsize - lbitsize - lbitpos));

  if (! const_p)
    /* If not comparing with constant, just rework the comparison
       and return.  */
    return build (code, compare_type,
		  build (BIT_AND_EXPR, unsigned_type,
			 make_bit_field_ref (linner, unsigned_type,
					     lnbitsize, lnbitpos, 1),
			 mask),
		  build (BIT_AND_EXPR, unsigned_type,
			 make_bit_field_ref (rinner, unsigned_type,
					     rnbitsize, rnbitpos, 1),
			 mask));

  /* Otherwise, we are handling the constant case. See if the constant is too
     big for the field.  Warn and return a tree of for 0 (false) if so.  We do
     this not only for its own sake, but to avoid having to test for this
     error case below.  If we didn't, we might generate wrong code.

     For unsigned fields, the constant shifted right by the field length should
     be all zero.  For signed fields, the high-order bits should agree with 
     the sign bit.  */

  if (lunsignedp)
    {
      if (! integer_zerop (const_binop (RSHIFT_EXPR,
					convert (unsigned_type, rhs),
					size_int (lbitsize))))
	{
	  warning ("comparison is always %s due to width of bitfield",
		   code == NE_EXPR ? "one" : "zero");
	  return convert (compare_type,
			  (code == NE_EXPR
			   ? integer_one_node : integer_zero_node));
	}
    }
  else
    {
      tree tem = const_binop (RSHIFT_EXPR, convert (signed_type, rhs),
			      size_int (lbitsize - 1));
      if (! integer_zerop (tem) && ! integer_all_onesp (tem))
	{
	  warning ("comparison is always %s due to width of bitfield",
		   code == NE_EXPR ? "one" : "zero");
	  return convert (compare_type,
			  (code == NE_EXPR
			   ? integer_one_node : integer_zero_node));
	}
    }

  /* Single-bit compares should always be against zero.  */
  if (lbitsize == 1 && ! integer_zerop (rhs))
    {
      code = code == EQ_EXPR ? NE_EXPR : EQ_EXPR;
      rhs = convert (type, integer_zero_node);
    }

  /* Make a new bitfield reference, shift the constant over the
     appropriate number of bits and mask it with the computed mask
     (in case this was a signed field).  If we changed it, make a new one.  */
  lhs = make_bit_field_ref (linner, unsigned_type, lnbitsize, lnbitpos, 1);

  rhs = fold (const_binop (BIT_AND_EXPR,
			   const_binop (LSHIFT_EXPR,
					convert (unsigned_type, rhs),
					size_int (lbitpos)),
			   mask));

  return build (code, compare_type,
		build (BIT_AND_EXPR, unsigned_type, lhs, mask),
		rhs);
}

/* Subroutine for fold_truthop: decode a field reference.

   If EXP is a comparison reference, we return the innermost reference.

   *PBITSIZE is set to the number of bits in the reference, *PBITPOS is
   set to the starting bit number.

   If the innermost field can be completely contained in a mode-sized
   unit, *PMODE is set to that mode.  Otherwise, it is set to VOIDmode.

   *PVOLATILEP is set to 1 if the any expression encountered is volatile;
   otherwise it is not changed.

   *PUNSIGNEDP is set to the signedness of the field.

   *PMASK is set to the mask used.  This is either contained in a
   BIT_AND_EXPR or derived from the width of the field.

   Return 0 if this is not a component reference or is one that we can't
   do anything with.  */

static tree
decode_field_reference (exp, pbitsize, pbitpos, pmode, punsignedp,
			pvolatilep, pmask)
     tree exp;
     int *pbitsize, *pbitpos;
     enum machine_mode *pmode;
     int *punsignedp, *pvolatilep;
     tree *pmask;
{
  tree mask = 0;
  tree inner;
  tree offset;

  STRIP_NOPS (exp);

  if (TREE_CODE (exp) == BIT_AND_EXPR)
    {
      mask = TREE_OPERAND (exp, 1);
      exp = TREE_OPERAND (exp, 0);
      STRIP_NOPS (exp); STRIP_NOPS (mask);
      if (TREE_CODE (mask) != INTEGER_CST)
	return 0;
    }

  if (TREE_CODE (exp) != COMPONENT_REF && TREE_CODE (exp) != ARRAY_REF
      && TREE_CODE (exp) != BIT_FIELD_REF)
    return 0;

  inner = get_inner_reference (exp, pbitsize, pbitpos, &offset, pmode,
			       punsignedp, pvolatilep);
  if (*pbitsize < 0 || offset != 0)
    return 0;
  
  if (mask == 0)
    {
      tree unsigned_type = type_for_size (*pbitsize, 1);
      int precision = TYPE_PRECISION (unsigned_type);

      mask = convert (unsigned_type, build_int_2 (~0, ~0));
      mask = const_binop (LSHIFT_EXPR, mask, size_int (precision - *pbitsize));
      mask = const_binop (RSHIFT_EXPR, mask, size_int (precision - *pbitsize));
    }

  *pmask = mask;
  return inner;
}

/* Return non-