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
Date: Thu, 22 Aug 2019 15:52:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.21.1908221426570.27000@digraph.polyomino.org.uk> (raw)
In-Reply-To: <ri6ftlte12y.fsf@suse.cz>
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
next prev parent reply other threads:[~2019-08-22 14:33 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-28 3:40 Tejas Joshi
2019-07-29 16:49 ` Martin Jambor
2019-08-09 21:26 ` Joseph Myers
2019-08-14 7:24 ` Tejas Joshi
2019-08-21 11:52 ` Martin Jambor
2019-08-21 12:48 ` Joseph Myers
2019-08-21 19:10 ` Martin Jambor
2019-08-21 20:50 ` Joseph Myers
2019-08-22 14:40 ` Martin Jambor
2019-08-22 15:52 ` Joseph Myers [this message]
2019-08-22 21:39 ` Tejas Joshi
2019-08-23 12:21 ` Martin Jambor
2019-08-23 21:24 ` Joseph Myers
2019-08-25 20:28 ` Tejas Joshi
2020-02-12 6:20 ` [PATCH] real: Fix roundeven on inf/nan [PR93663] Jakub Jelinek
2020-02-12 20:56 ` Joseph Myers
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