Re: [PATCH] [gfortran] Set omp_requires_mask for dynamic_allocators.

[Tobias Burnus <tobias@codesourcery.com>]

Hi Jakub,

On 21.02.22 18:47, Jakub Jelinek wrote:
> Where ME is involved is
> !$omp requires atomic_default_mem_order(whatever) vs.
> !$omp declare variant ...atomic_default_mem_order(whatever).

Ups, missed that case. (Also because there wasn't 'declare variant' when
implementing 'requires' in Fortran.)

Disclaimer to all of the following remarks: I do not understand context
selectors and their fineprint. Thus, my comments my be completely off:

> subroutine baz
> ...
>    interface
>      subroutine bar
>      end subroutine
>      !$omp declare variant (foo) &
>      !$omp & match (implementation={atomic_default_mem_order(seq_cst)})
>    end interface
>    call bar
> end subroutine baz

I concur that in this case, it needs to know the 'atomic_default_mem_order'
of baz. — But that seems to be not implementable using a module as

module m_foo
   !$omp requires atomic_default_mem_order(...)
contains
   subroutine foo
     ...
   end
end module m_bar
...
subroutine baz
   use m_foo, only: foo
   ...
end

seems to make the 'requires' available - such that it cannot be overridden
via a local 'require atomic_default_mem_order'. And having a 'use m_bar'
then has conflicting declarations. Similar probably with C++ modules,
unless the 'requires' does not propagate. (Does it?)

I find it odd to have only code which works when not using modules.
(Explicitly using the mem_order on 'omp atomic' still works.)


And for the other requires in context selectors, I do not really understand how they
are supposed to get used, either. If any 'unified_shared_memory' or 'dynamic_allocators'
appears (in linked-in code), it is in principle callable – the the run-time library
should then remove all devices which do not support it, possibly only keeping the
host device; for USM, it even has to be present in all compilation units. Thus, just
directly calling the dynamic_allocators/unified_shared_memory should have the same
effect at the end, shouldn't it?

Tobias

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