Re: [PATCH] Builtin function roundeven folding implementation

[Tejas Joshi <tejasjoshi9673@gmail.com>]

> I think you should have at least four tests of sign of zero result
> (arguments -0.5, -0.0, 0.0 and 0.5).  Probably also tests of values
> between +/- 0.5 and 0, e.g. test -0.25 and 0.25 as well.

Okay, I have made the following changes and again, the tests pass for roundeven.

void
real_roundeven (REAL_VALUE_TYPE *r, format_helper fmt,
        const REAL_VALUE_TYPE *x)
{
  if (is_halfway_below (x))
  {
    /* Special case as -0.5 rounds to -0.0 and
       similarly +0.5 rounds to +0.0.  */
    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__);
  if (__builtin_copysign (1, __builtin_roundeven (-0.0)) != -1)
    link_error (__LINE__);
  if (__builtin_copysign (-1, __builtin_roundeven (0.5)) != 1)
    link_error (__LINE__);
  if (__builtin_copysign (-1, __builtin_roundeven (0.0)) != 1)
    link_error (__LINE__);
  if (__builtin_copysign (1, __builtin_roundeven (-0.25)) != -1)
    link_error (__LINE__);
  if (__builtin_copysign (-1, __builtin_roundeven (0.25)) != 1)
    link_error (__LINE__);
  return 0;
}

Thanks,
Tejas


On Thu, 22 Aug 2019 at 22:05, Joseph Myers <joseph@codesourcery.com> wrote:
>
> On Thu, 22 Aug 2019, Tejas Joshi wrote:
>
> > > 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.
>
> I think that would be reasonable with a comment added to explain that it's
> ensuring the correct sign of a zero result.
>
> >   if (__builtin_copysign (1, __builtin_roundeven (-0.5)) != -1)
> >     link_error (__LINE__);
>
> I think you should have at least four tests of sign of zero result
> (arguments -0.5, -0.0, 0.0 and 0.5).  Probably also tests of values
> between +/- 0.5 and 0, e.g. test -0.25 and 0.25 as well.
>
> --
> Joseph S. Myers
> joseph@codesourcery.com