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From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/104131] ICE in gfc_conv_array_ref, at
 fortran/trans-array.c:3810
Date: Wed, 25 Oct 2023 07:07:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104131

--- Comment #8 from Tobias Burnus <burnus at gcc dot gnu.org> ---
(In reply to anlauf from comment #7)
> (In reply to Tobias Burnus from comment #6)
> > !$omp task detach (x)
> >       !$omp end task
> >=20
> > This seems to be valid. OpenMP mostly only rejects coindexed variables =
like:
>=20
> Alright, I did not expect this, and no other compiler I have access to
> accepts it.

On the technical side, a coarray is just a normal variable that is also
remotely accessible; I think it is usually just 'malloc'ed but in principle=
 the
coarray library could allocate it also in special memory.
There might be some issues when used inside target regions, i.e. on a device
with shared memory (page migrations etc.) but, otherwise, there is no real
reason to disallow it.

Of course, everything related to memory allocation or coindexed access will=
 not
work (in general - at least some special cases will fail).

* * *

In terms of the OpenMP specification, there is a section "Normative
References";

* in OpenMP 5.0, it has: "Fortran 2008 ... their use may result in unspecif=
ied
behavior ... Coarrays"

* in OpenMP 5.1, there are no exceptions for 2008 and Fortran 2018 with a n=
ew
list of unspecified behavior was added.

Thus, coarrays were never explicitly rejected but in 5.0 their use might re=
sult
in unspecified behavior, i.e. an implementation could reject it.
Since 5.1, they can be used where deemed to be okay.

(Disclaimer: What might be okay for most implementation can be unimplementa=
ble
in another; additionally, some corner cases might have been overlooked whic=
h do
cause problems.)

* * *

My quotes in comment 6 were from OpenMP 5.2, the latest release. TR11 (pre-=
6.0)
is the latest spec, semi-solid but breaking changes are permitted until the
final 6.0 (to fix oversights).=