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Date: Thu, 22 Aug 2019 15:52:00 -0000
From: Joseph Myers <joseph@codesourcery.com>
To: Martin Jambor <mjambor@suse.cz>
CC: Tejas Joshi <tejasjoshi9673@gmail.com>, <gcc-patches@gcc.gnu.org>,	<hubicka@ucw.cz>
Subject: Re: [PATCH] Builtin function roundeven folding implementation
In-Reply-To: <ri6ftlte12y.fsf@suse.cz>
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On Thu, 22 Aug 2019, Martin Jambor wrote:

> +/* Round X to nearest integer, rounding halfway cases towards even.  */
> +
> +void
> +real_roundeven (REAL_VALUE_TYPE *r, format_helper fmt,
> +		const REAL_VALUE_TYPE *x)
> +{
> +  if (is_halfway_below (x))
> +  {
> +    do_add (r, x, &dconsthalf, x->sign);
> +    if (!is_even (r))
> +      do_add (r, r, &dconstm1, x->sign);

I'm concerned that this would produce +0.0 for an argument of -0.5 (via 
-0.5 - 0.5 - -1.0 producing +0.0) when it needs to produce -0.0.

Note that testcases for the sign of zero results need to check e.g. 
!!__builtin_signbit on the result, or the result of calling 
__builtin_copysign* to extract the sign of the result, since 0.0 == -0.0 
so checking with ==, while necessary, is not sufficient in that case.

-- 
Joseph S. Myers
joseph@codesourcery.com