Re: Help with fortran pointer ans OpenACC

[Tobias Burnus <tobias@codesourcery.com>]

Hi,

On 23.08.23 12:19, Patrick Begou via Fortran wrote:
> For several days I have some trouble with OpenACC offloading and
> fortran pointers. I'm testing with a very small peace of code to
> investigate but I do not progress for several days and I need your help.
>
> Could someone give me advices or a small explanation on what I have
> not understood there ?

First, for debugging, using -fdump-tree-original -fdump-tree-gimple
-fdump-tree-omplower and looking at the file <prefix-><filename>.* might
help - grep for '#pragma'; this shows the internal representation
(incomplete) in a C like output. That's how I spotted the issue below.

* * *

For gfortran, the issue is:

         !$acc parallel loop collapse(2) default(present) if(runongpu)

It works if you add a "present(current)" → see also newly filed
https://gcc.gnu.org/PR111116

gfortran seems to follow - but not completely - the spec:

Quoting from OpenACC 3.2: "2.6.2 Variables with Implicitly Determined
Data Attributes":

"A scalar variable will be treated as if it appears either:

* In a copy clause if the compute construct is a kernels construct.
* In a firstprivate clause otherwise.

Note: Any default(none) clause visible at the compute construct applies
to both aggregate and scalar variables. However, any default(present)
clause visible at the compute construct applies only to aggregate
variables."

However, the glossary defines:

"Aggregate variables – a variable of any non-scalar datatype, including
array or composite variables.
In Fortran, this includes any variable with allocatable or pointer
attribute and character variables."

And, as you have a (scalar) Fortran pointer, the following should have
applied instead:

"An aggregate variable will be treated as if it appears either:
• In a present clause if there is a default(present) clause visible at
the compute construct.
• In a copy clause otherwise."

* * *

Thus: It looks as if your program is valid, adding 'present(current)'
will fix it for gfortran (and Cray?), 'default(...)' and implicit
mapping is confusing, and there is now a gfortran PR to track this
issue: https://gcc.gnu.org/PR111116

Tobias

-----------------
Siemens Electronic Design Automation GmbH; Anschrift: Arnulfstraße 201, 80634 München; Gesellschaft mit beschränkter Haftung; Geschäftsführer: Thomas Heurung, Frank Thürauf; Sitz der Gesellschaft: München; Registergericht München, HRB 106955