On Wed, Nov 13, 2024 at 3:21 PM Toon Moene <t...@moene.org> wrote:
>
> On 11/13/24 15:12, Richard Biener wrote:
>
> > On Wed, Nov 13, 2024 at 3:05 PM Thomas Koenig <tkoe...@netcologne.de> wrote:
> >>
> >> Hello world,
> >>
> >> J3, the US Fortran standards committee, has passed
> >> https://j3-fortran.org/doc/year/24/24-179.txt
> >> which states (with a bit of an overabundance of
> >> clarity) that, in Fortran, it is possible special-case
> >> complex multiplication when one of the numbers is known
> >> to have a zero component, for example when promoting
> >> a real to complex for complex multiplication. For
> >> example, multiplying a complex variable b with a real
> >> variable a can be done with c%re = b%re * a, c%im = b%im * a,
> >> without considering NaNs and infinities. Apparently, other
> >> Fortran compilers do this.
> >>
> >> They also stated that ISO/IEC 60559:2020 (aka IEEE 754) does
> >> not specify complex arithmetic (I wouldn't know, because it is a
> >> paywalled standard).
> >>
> >> How do we want to deal with this? Do we want to implement this
> >> (it's an obvious speed advantage)? Should it be the default?
> >> Do we want to include this in -fcx-fortran-rules?
> >
> > The middle-end complex lowering pass does this already, irrespective
> > of NaNs, same for some degenerate cases with division.
>
> Are you sure ?
>
> For this code:
>
> $ cat complex.f90
> complex function p(c, r)
> complex, intent(in) :: c
> real, intent(in) :: r
> p = c * r
> end
>
> I definitely see a difference between
>
> $ gfortran -O2 -S complex.f90
>
> and
>
> $ gfortran -O2 -ffast-math -S complex.f90
Ah. This is because of
static int
some_nonzerop (tree t)
{
int zerop = false;
/* Operations with real or imaginary part of a complex number zero
cannot be treated the same as operations with a real or imaginary
operand if we care about the signs of zeros in the result. */
if (TREE_CODE (t) == REAL_CST && !flag_signed_zeros)
zerop = real_identical (&TREE_REAL_CST (t), &dconst0);
so the -ffast-math result can be obtained with just -fno-signed-zeros (I assumed
fortran doesn't have signed zeros?)
Richard.
> --
> Toon Moene - e-mail: t...@moene.org - phone: +31 346 214290
> Saturnushof 14, 3738 XG Maartensdijk, The Netherlands
