co-arrays and libgfortran

[Steve Kargl]

On Fri, Dec 18, 2020 at 07:52:52AM -0800, Steve Kargl wrote:
> 
> I don't know if memory barriers are needed. As random_init.f90 is
> Fortran, the psuedocode might be
> 
>    if (hidden < 2) then  
>       ! get seeds on image 1
>       call random_seed()
>       call random_seed(get=seeds)
>    else
>       call co_broadcast(seeds,1)
>    end if
> 

Well, one possible solution would have conv_intrinsic_random_init()
convert random_init(repeatable, image_distinct) into 

_gfortran_random_init(repeatable, image_distinct, hidden, sub)

where hidden is the value of this_image() as before, and sub is the
name (or pointer?) to a subroutine.  For -fcoarrays=none or single,
sub could be a stub that is never called.  For -fcoarrays=lib, sub
would be whatever gfor_fndecl_co_broadcast is associated (opencoarray
is _gfortran_caf_co_broadcast).  The Fortran code in random_init.f90
would then be

    if (hidden < 2) then
       ! get seeds on image 1
       call random_seed()
       call random_seed(get=seeds)
    else
       call sub(seeds,1)
    end if

A second possibility is to use a weak reference.  libgfortran
can contain a weak reference to _gfortran_caf_co_broadcast
when linked with -fcoarrays=none or single the weak reference
will never be called.  With  -fcoarrays=lib, the linker should
resolve to the version provided by opencoarrays.

Not sure how this would play out with shared memory coarray.

-- 
Steve