Hi Harald,
Am 27. Juli 2025 22:25:40 schrieb Harald Anlauf <anl...@gmx.de>:
<Snipp>
Harald, are you still unconvinced? Do we need to discuss the behavior of
the testcase test_teams_1? or something else?
I will not make any specific suggestions about the actual implementation
of coarrays in gfortran.
Let me phrase the following requirements:
(a) Assuming that the following code is standard-conforming:
program p
implicit none
integer :: img, data
integer :: cnt[*] = 0
integer, allocatable :: res(:)[:]
img = this_image()
data = img * img + 10 ! Something different on each image
allocate(res(num_images())[*], source=-1)
select case (img)
case (1)
res(img )[1] = data
case (2)
res(this_image ())[1] = data
case (3)
res(get_val_pure ())[1] = data
case default
res(get_val_impure())[1] = data
end select
sync all
if (this_image() == 1) print *, res
contains
pure integer function get_val_pure() result (get_val)
get_val = img
end function
impure integer function get_val_impure() result (get_val)
get_val = img
cnt = cnt + 1 ! Count invocations of get_val
end function
end program
Would it work after the fix? (It does work with ifx and NAG).
Yes, I see no issue why it should not work after the fix. But please note,
that my latest fix is to evaluate *every* function on the calling site!
There is no check for pure or elemental or any such thing anymore.
I have serious doubts about the pure/elemental checks in the patch.
I got rid of them. Makes the patch far more easy!
Regarding the comparison between teams and coarrays and MPI concepts, I
think it's difficult to map between them because fortran doesn't define
any communication, it just makes the coarrays world-accessible between
images without any more details. With MPI on the contrary communication
is explicit. So there are probably many MPI concepts that just don't
exist with coarrays.
(b) I would assume that coarrays can be fully implemented using MPI as the
underlying layer for the communication also across distributed nodes. Is
this true, or am I missing anything?
No, that is true. One can use MPI_get and MPI_put, the one-sided
communication methods for res = caf[...] and caf[...] = something,
respectively.
Or you can use MPI_receive and MPI_send, but then the other image has to
have some way of knowing what to send and where to receive to. This is what
is implemented in OpenCoarrays now. This is what the accessors are for.
MPI has a concept of communicators, and if I understand Andre right,
caf has "accessors".
Ah, here is where my explanation was flawed. MPI's comminucators correspond
to Fortran's teams. The caf accessors are used in the execution the
coindexed expressions. Caf accessors know nothing about teams, they execute
the assignment or retrieval of data on the remote image. In your example
above there would be a caf accessor for each res(...) = data expression.
Each one doing something like:
res(add_data.img) = data
where both add_data and data are transferred to the remote image.
In your example above teams do not play a role. Everything is running on
the initial_team which corresponds to MPI_COMM_WORLD.
If they correspond in some way, can someone
write the equivalent of test_teams_1 as a valid MPI program? It is not
obvious to me (this may be my fault).
Well, that will take me some time, but it will be essentially something
like this (pseudo code; doing this on a mobile; it keeps correcting my
spelling):
Program test_teams_mpi
Integer, allocatable :: caf(3,3)
Type(MPI_Comm) :: row_comm, col_comm
! Here MPI_Win is needed and it will clutter the code even more. So I just
left it out. In your mind just associate a MPI_Win to caf, row and cell,
please.
Call MPI_Alloc_mem_shared(caf, sizeof(caf))
Caf = reshape(3,3, (/(I, I = 1, 9)/))
Call MPI_create_communicator(row_comm, mod(this_image(), 3) +1, MPI_COMM_WOLRD)
Associate(me => mod(this_image(), 3) +1)
Associate (row => caf (me, :), team_no => int(this_image() / 3) +1)
Call MPI_create_commincator(col_comm, me, row_comm) ! Well, not really
needed, just for demonstration.
Associate (cell => row(me), team_no => me)
Cell = team_no
If (me /= 1) then
Call MPI_Put(row(me), cell, 1, row_comm)
Endif
End associate
If (me /= 1) then
Call MPI_put(caf(team_no, :), row, 1, MPI_CAF_COMM_WORLD)
End if
End associate
End associate
End program
Does this give you an idea, Harald? If not, I will debug the program and
give you the working example, but as said it will take me some time. The
above is just written down w/o ever having seen a compiler.
I am happy to leave the discussion to you, so if you, Andre and Mikael,
come to a conclusion that resolves the above, just do it.
Well, that is very kind of you, but Mikael is funded from the same funding,
that I was paid of. We agreed that while we are paid from the same source,
we will not Okay each others work to prevent any smells.
So, I am positive that you now got what I am trying to do and I am
convinced that when you look at the latest version of my patch, attached to
this mail:
https://gcc.gnu.org/pipermail/fortran/2025-July/062530.html
You'll see, that you convinced me of my wrongs a long time ago. Please take
a look, because that patch is correcting what you criticized from the
beginning.
Regards,
Andre
