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

[Richard Biener <rguenther@suse.de>]

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,
Richard.

> Thanks,
> Kyrill
> 
> 
> 2015-07-20  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
> 
>     PR middle-end/66915
>     * match.pd (A - B -> A + (-B)): Don't allow folding
>     when type if a fixed-point type.
> 

-- 
Richard Biener <rguenther@suse.de>
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