[Patch, fortran] PR32239 Use builtin_powi

[Janne Blomqvist]

On 6/27/07, Steve Kargl <sgk@troutmask.apl.washington.edu> wrote:
> On Wed, Jun 27, 2007 at 11:23:19AM +0200, Tobias Burnus wrote:
> > Tobias Burnus wrote:
> > > Ok for the trunk. I assume you have regtested it; in any case it builds
> > > and some limited testing shows no problems.
> > >
> > I bootstrapped and regtested it (x86-64/Linux). For -m32 everything is
> > ok, but for -m64 it fails for:
> >
> > gfortran.fortran-torture/execute/intrinsic_fraction_exponent.f90's
> > test_4 (third call).
> >
> > Instead of:
> >  test_4; x =  1.4012985E-45  y=  0.5000000      z=        -148
> >  test_4; y=  1.4012985E-45  |x-y|=  0.000000     |x*1e-6|=   0.000000
> > the result is:
> >  test_4; x =  1.4012985E-45  y=  0.5000000      z=        -148
> >  test_4; y=   0.000000      |x-y|= 1.4012985E-45 |x*1e-6|=   0.000000
> > and the latter fails because |x-y| > |x*1e-6|.
> >
> > The dump looks ok:
> > -      y = (y * 2.0e+0) * _gfortran_pow_r4_i4 (2.0e+0, z + -1);
> > +      y = (y * 2.0e+0) * __builtin_powif (2.0e+0, z + -1);
> >      }
> >    else
> >      {
> > -      y = (y / 2.0e+0) * _gfortran_pow_r4_i4 (2.0e+0, z + 1);
> > +      y = (y / 2.0e+0) * __builtin_powif (2.0e+0, z + 1);
> >
> > Thus the builtin_powi gives a different result; I don't see ad hoc
> > whether both results are ok or whether one implementation has a bug.
> >
>
> Looks like an issue with subnormal numbers.  If 1.4012985E-45 isn't
> the smallest subnorml number, it's awefully close.  y/2. has underflowed
> to zero.

The algorithms implemented by _gfortran_pow vs. builtin_powi
(builtin_powi in the PR) basically do the same thing, I don't think we
can say one is better than the other.

Perhaps the testcase should be changed to test something like

abs(x-y) < ulp * spacing(max(abs(x),abs(y)))

instead of the somewhat ad hoc test used now?

-- 
Janne Blomqvist