From: Tejas Joshi <tejasjoshi9673@gmail.com>
To: gcc@gcc.gnu.org
Cc: Martin Jambor <mjambor@suse.cz>, hubicka@ucw.cz, joseph@codesourcery.com
Subject: Re: [PATCH] Builtin function roundeven folding implementation
Date: Thu, 22 Aug 2019 16:28:00 -0000 [thread overview]
Message-ID: <CACMrGjCUbFja_j_gUquQuAO=HKam9j-R5+-cjde+_eWsfDs+XA@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1908221426570.27000@digraph.polyomino.org.uk>
> 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.
Would the following overhaul be acceptable as the condition is
specialized for -0.5 and +0.5 only. This seems to solve the problem. I
did test the roundeven tests and it passes the tests.
void
real_roundeven (REAL_VALUE_TYPE *r, format_helper fmt,
const REAL_VALUE_TYPE *x)
{
if (is_halfway_below (x))
{
if (REAL_EXP (x) == 0)
{
*r = *x;
clear_significand_below (r, SIGNIFICAND_BITS);
}
else
{
do_add (r, x, &dconsthalf, x->sign);
if (!is_even (r))
do_add (r, r, &dconstm1, x->sign);
}
if (fmt)
real_convert (r, fmt, r);
}
else
real_round (r, fmt, x);
}
tests:
/* { dg-do link } */
extern int link_error (int);
#define TEST(FN, VALUE, RESULT) \
if (__builtin_##FN (VALUE) != RESULT) link_error (__LINE__);
int
main (void)
{
TEST(roundeven, 0, 0);
TEST(roundeven, 0.5, 0);
TEST(roundeven, -0.5, 0);
TEST(roundeven, 6, 6);
TEST(roundeven, -8, -8);
TEST(roundeven, 2.5, 2);
TEST(roundeven, 3.5, 4);
TEST(roundeven, -1.5, -2);
TEST(roundeven, 3.499, 3);
TEST(roundeven, 3.501, 4);
if (__builtin_copysign (1, __builtin_roundeven (-0.5)) != -1)
link_error (__LINE__);
return 0;
}
Thanks,
Tejas
On Thu, 22 Aug 2019 at 20:03, Joseph Myers <joseph@codesourcery.com> wrote:
>
> 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 parent reply other threads:[~2019-08-22 16:28 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CACMrGjCu54zN+Smc7+3JF8MBZV76HXwaP-vk1HFTQGhRc=Toww@mail.gmail.com>
[not found] ` <alpine.DEB.2.21.1908092021090.31221@digraph.polyomino.org.uk>
[not found] ` <CACMrGjCPNr+Hjz--CL6ocV5y3GqGp13fpQCXtXJT4qeGQWk86A@mail.gmail.com>
[not found] ` <ri636hug2zr.fsf@suse.cz>
[not found] ` <alpine.DEB.2.21.1908211156360.6172@digraph.polyomino.org.uk>
[not found] ` <ri6pnkye7ll.fsf@suse.cz>
[not found] ` <alpine.DEB.2.21.1908211906340.25161@digraph.polyomino.org.uk>
[not found] ` <ri6ftlte12y.fsf@suse.cz>
[not found] ` <alpine.DEB.2.21.1908221426570.27000@digraph.polyomino.org.uk>
2019-08-22 16:28 ` Tejas Joshi [this message]
2019-08-22 16:35 ` Joseph Myers
2019-08-22 16:56 ` Tejas Joshi
2019-08-22 17:00 ` Joseph Myers
[not found] ` <CACMrGjA3UyUYcShruM-tiT=rqBiSLX9cmSVmpX0G6SfOq_r4EQ@mail.gmail.com>
[not found] ` <alpine.DEB.2.21.1908232023080.19272@digraph.polyomino.org.uk>
2019-08-25 8:25 ` Tejas Joshi
2019-08-25 9:38 ` Martin Jambor
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