Re: [PATCH][match.pd] PR middle-end/66915 Restrict A - B -> A + (-B) to non-fixed-point types

[Kyrill Tkachov]

On 21/07/15 08:24, Richard Biener wrote:

On Mon, 20 Jul 2015, Kyrill Tkachov wrote:

Hi all,

This patch fixes the PR in question which is a miscompilation of
gcc.dg/fixed-point/unary.c on arm.
It just restricts the A - B -> A + (-B) transformation when the type is
fixed-point.

This fixes the testcase for me.
Is this the right approach?

Bootstrap and test on arm and x86 running.

Ok if testing is clean?

Ok, but I think the fold-const.c code has the same issue, no:

       /* A - B -> A + (-B) if B is easily negatable.  */
       if (negate_expr_p (arg1)
           && !TYPE_OVERFLOW_SANITIZED (type)
           && ((FLOAT_TYPE_P (type)
                /* Avoid this transformation if B is a positive REAL_CST.
*/
                && (TREE_CODE (arg1) != REAL_CST
                    ||  REAL_VALUE_NEGATIVE (TREE_REAL_CST (arg1))))
               || INTEGRAL_TYPE_P (type)))
         return fold_build2_loc (loc, PLUS_EXPR, type,
                             fold_convert_loc (loc, type, arg0),
                             fold_convert_loc (loc, type,
                                               negate_expr (arg1)));

ah, no.  The above only applies to float-type and integral-types.

Thus yes, your patch is ok.  Can you double-check the other pattern,

/* -(A + B) -> (-B) - A.  */
(simplify
  (negate (plus:c @0 negate_expr_p@1))
  (if (!HONOR_SIGN_DEPENDENT_ROUNDING (element_mode (type))
       && !HONOR_SIGNED_ZEROS (element_mode (type)))
   (minus (negate @1) @0)))

?

Thanks, committed with r226028.
I can add (FLOAT_TYPE_P (type) || INTEGRAL_TYPE_P (type)) to the condition.
That would more closely mirror the original logic, right?
That passes x86_64 bootstrap and aarch64 testing looks ok.

Thanks,
Richard.

Thanks,
Kyrill


2015-07-20  Kyrylo Tkachov  <kyrylo.tkac...@arm.com>

     PR middle-end/66915
     * match.pd (A - B -> A + (-B)): Don't allow folding
     when type if a fixed-point type.