Re: Patching the GCC build system to build MPICH and OpenCoarrays

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Apr 4, 2018 at 8:24 AM, Damian Rouson
<damian@sourceryinstitute.org> wrote:
> On April 3, 2018 at 1:36:37 AM, Richard Biener (richard.guenther@gmail.com)
> wrote:
>
>
> You probably only want a new target_module for the MPI library. Note
> it's name has to match that of the directory containing the sources
> which
> as far as I see is 'mpich', not 'libmpi'.
>
> Thanks!  I’ll ask Daniel to respond if he has further questions.
>
> May I ask again why mpi/opencoarrays should be built together with
> gfortran rather than separately? Does gfortran or any of its current
> target libraries
> link against them?
>
> Yes, gfortran links to OpenCoarrays via the -fcoarray=lib argument.

In that case user programs compiled with -fcoarray=lib are but gfortran
or libgfortran itself is not linked against OpenCoarrays?

> OpenCoarrays provides the only parallel implementations of gfortran’s
> Coarray ABI (http://bit.ly/Coarray-ABI). Any Fortran program that uses the
> large set of parallel features in Fortran 2008 and Fortran 2018 must have
> OpenCoarrays installed in order for the code to execute in parallel.

I see.  So if we consider OpenCoarrays part of the gfortran runtime then
it makes sense to build it in-tree...

> We have found MPICH to be the most robust option and it has the only
> released support for some of the more advanced requirements such as the
> Fortran 2018 fault-tolerance features.  However, there will be configure
> options for using MPI implementations other than MPICH.

... but building an mpi library in-tree might not?

I'm still lacking an idea of what it takes to enable coarrays with gfortran
since install.texi doesn't talk about this at all, neither in the prerequesites
section nor in a fortran/coarray specific section.

In fact the only thing I find is in invoke.texi which says

@item -fcoarray=@var{<keyword>}
@opindex @code{fcoarray}
...
@item @samp{lib}
Library-based coarray parallelization; a suitable GNU Fortran coarray
library needs to be linked.
@end table

which suggests linking to the coarray library doesn't happen automatically
but the user is supposed to link a suitable library?

I'd love to "enable" coarray support for openSUSE but as said I have a hard
time assessing what I'd need to do here.

Thanks,
Richard.

>
> Damian