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Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

From: Gabriel Dos Reis
Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
Date: 30 Dec 2006 23:55:46 +0100

address@hidden (Richard Kenner) writes:

| > Here's an example from the intprops module of gnulib.  
| These are interesting case.
| Note that all the computations are constant-folded.
| And I think this points the fact that we can "have our cake and eat it too"
| in many cases.  Everything we're seeing points to the fact that the cases
| where assuming undefined behavior for overflow will help optimizations are
| cases where few if any sane programmers would write the code depending on
| wrap semantics (e.g., loop variables).  And nearly all the cases where wrap
| semantics are expected (e.g., the above) are such that there's no reasonable
| optimization benefit in assuming they're undefined.
| Take constant folding: if we were pendantic about it, we could say, "this
| folded expression overflows and so is undefined, so let's set its value to be
| whatever constant would yield the most efficient code in that case".
| Such behavior would be standard-compliant, but as unfriendly as possible
| because it wouldn't "optimize" any real code, just break common idioms.
| I doubt anybody would suggest not implementing wrapping semantics in
| constant folding.

As I'm looking into the VRP and CHREC codes to implement the
-Wundefined warning, I came across this:

   /* Wrapper around int_const_binop.  If the operation overflows and we
      are not using wrapping arithmetic, then adjust the result to be
      -INF or +INF depending on CODE, VAL1 and VAL2.  */

   static inline tree
   vrp_int_const_binop (enum tree_code code, tree val1, tree val2)
     /* ... */
     else if (TREE_OVERFLOW (res)
              && !TREE_OVERFLOW (val1)
              && !TREE_OVERFLOW (val2))
         /* If the operation overflowed but neither VAL1 nor VAL2 are
            overflown, return -INF or +INF depending on the operation
            and the combination of signs of the operands.  */
         int sgn1 = tree_int_cst_sgn (val1);
         int sgn2 = tree_int_cst_sgn (val2);

         /* Notice that we only need to handle the restricted set of
            operations handled by extract_range_from_binary_expr.
            Among them, only multiplication, addition and subtraction
            can yield overflow without overflown operands because we
            are working with integral types only... except in the
            case VAL1 = -INF and VAL2 = -1 which overflows to +INF
            for division too.  */

         /* For multiplication, the sign of the overflow is given
            by the comparison of the signs of the operands.  */
         if ((code == MULT_EXPR && sgn1 == sgn2)
             /* For addition, the operands must be of the same sign
                to yield an overflow.  Its sign is therefore that
                of one of the operands, for example the first.  */
             || (code == PLUS_EXPR && sgn1 > 0)
             /* For subtraction, the operands must be of different
                signs to yield an overflow.  Its sign is therefore
                that of the first operand or the opposite of that
                of the second operand.  A first operand of 0 counts
                as positive here, for the corner case 0 - (-INF),
                which overflows, but must yield +INF.  */
             || (code == MINUS_EXPR && sgn1 >= 0)
             /* For division, the only case is -INF / -1 = +INF.  */
             || code == TRUNC_DIV_EXPR
             || code == FLOOR_DIV_EXPR
             || code == CEIL_DIV_EXPR
             || code == EXACT_DIV_EXPR
             || code == ROUND_DIV_EXPR)
           return TYPE_MAX_VALUE (TREE_TYPE (res));
           return TYPE_MIN_VALUE (TREE_TYPE (res));
     /* ... */

What would you suggest this function to do, based on your comments?

-- Gaby

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