On 11/13/24 15:40, Richard Biener wrote:
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?)
Since the Fortran 95 Standard it does (in the current Standard: 7.4.3.2
Real type):
The real type includes a zero value. Processors that distinguish between
positive and negative zeros shall treat them as mathematically equivalent
• in all intrinsic relational operations, and
• as actual arguments to intrinsic procedures other than those for which
it is explicitly specified that negative zero is distinguished.
[Note that "processor" in Fortran standardese means everything
(combined) from the compiler down to the actual hardware].
So we have to comb through the Standard to see where bullet 2 applies ...
Kind regards,
--
Toon Moene - e-mail: t...@moene.org - phone: +31 346 214290
Saturnushof 14, 3738 XG Maartensdijk, The Netherlands
