> 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 <[email protected]> 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
> [email protected]
