[Patch][Fortran] OpenACC – permit common blocks in some clauses

[Thomas Schwinge]

Hi Tobias!

On 2019-11-26T15:02:34+0100, Tobias Burnus <tobias@codesourcery.com> wrote:
> I now played also around common blocks with "!$acc declare 
> device_resident (/block/)". [See attached test-case diff.]

If you'd like to, please commit that, to document the status quo.  (I
have not reviewed.)


There are several issues with the OpenACC 'declare' implementation, so
that one generally needs to be re-visited as some point.  Basically
everything from the front ends handling, to middle end handling, to nvptx
back end handling (supposedly?; see <https://gcc.gnu.org/PR81689>), to
libgomp handling.  So, you're adding here some more.  ;-)

> Observations:
>
> * !$acc declare has to come after the declaration of the common block. 
> In terms of the spec, it just needs to be in the declaration section, 
> i.e. it could also be before. – Seems as if one needs to split parsing 
> and resolving clauses.

Good find -- purely a Fortran front end issue, as I understand.  Please
file a GCC PR, unless there is a reason (implementation complexity?) to
be more "strict" ("referenced variable/common block needs to be lexically
in scope", or something like that?), and the OpenACC specification should
be changed instead?

> * If I just use '!$acc parallel', the used variables are copied in 
> according to OpenMP 4.0 semantics, i.e. without a defaultmap clause (of 
> OpenMP 4.5+; not yet in gfortran), scalars are firstprivate and arrays 
> are map(fromto:). – Does this behaviour match the spec or should this 
> automatically mapped to, e.g., no_create as the 'device_resident' is 
> known? [Side remark: the module file does contain 
> "OACC_DECLARE_DEVICE_RESIDENT".]

Not sure at this point.

> * If I explicitly use '!$acc parallel present(/block/)' that fails 
> because present() does not permit common blocks.
> (OpenACC 2.7, p36, l.1054: "For all clauses except deviceptr and 
> present, the list argument may include a Fortran common block name 
> enclosed within slashes").

Do you understand the rationale behind that restriction, by the way?  I'm
not sure I do.  Is it because we don't know/can't be sure that *all* of
the common block has been mapped (per the rules set elsewhere)?  That
would make sense in context of this:

> I could use no_create

... which basically means 'present' but don't complain if not actually
present.

> but that's not yet 
> supported.

But will be soon.  :-)


Grüße
 Thomas


> --- a/libgomp/testsuite/libgomp.oacc-fortran/declare-5.f90
> +++ b/libgomp/testsuite/libgomp.oacc-fortran/declare-5.f90
> @@ -1,29 +1,106 @@
>  ! { dg-do run }
>  
>  module vars
>    implicit none
>    real b
> - !$acc declare device_resident (b)
> +  !$acc declare device_resident (b)
> +
> +  integer :: x, y, z
> +  common /block/ x, y, z
> +  !$acc declare device_resident (/block/)
>  end module vars
>  
> +subroutine set()
> +  use openacc
> +  implicit none
> +  integer :: a(5), b(1), c, vals(7)
> +  common /another/ a, b, c
> +  !$acc declare device_resident (/another/)
> +  if (.not. acc_is_present (a)) stop 10
> +  if (.not. acc_is_present (b)) stop 11
> +  if (.not. acc_is_present (c)) stop 12
> +
> +  vals = 99
> +  !$acc parallel copyout(vals) present(a, b, c) ! OK
> +                                                ! but w/o 'present', 'c' is firstprivate and a+b are 'map(fromto:'
> +                                                ! additionally, OpenACC 2.7 does not permit present(/another/)
> +                                                ! and no_create is not yet in the trunk (but submitted)
> +    a = [11,12,13,14,15]
> +    b = 16
> +    c = 47
> +    vals(1:5) = a
> +    vals(6:6) = b
> +    vals(7) = c
> +  !$acc end parallel
> +
> +  if (.not. acc_is_present (a)) stop 13
> +  if (.not. acc_is_present (b)) stop 14
> +  if (.not. acc_is_present (c)) stop 15
> +
> +  if (any (vals /= [11,12,13,14,15,16,47])) stop 16
> +end subroutine set
> +
> +subroutine check()
> +  use openacc
> +  implicit none
> +  integer :: g, h(3), i(3)
> +  common /another/ g, h, i
> +  integer :: val(7)
> +  !$acc declare device_resident (/another/)
> +  if (.not. acc_is_present (g)) stop 20
> +  if (.not. acc_is_present (h)) stop 21
> +  if (.not. acc_is_present (i)) stop 22
> +
> +  val = 99
> +  !$acc parallel copyout(val) present(g, h, i)
> +    val(5:7) = i
> +    val(1) = g
> +    val(2:4) = h
> +  !$acc end parallel
> +
> +  if (.not. acc_is_present (g)) stop 23
> +  if (.not. acc_is_present (h)) stop 24
> +  if (.not. acc_is_present (i)) stop 25
> +
> +
> +  !print *, val
> +  if (any (val /= [11,12,13,14,15,16,47])) stop 26
> +end subroutine check
> +
> +
>  program test
>    use vars
>    use openacc
>    implicit none
>    real a
> +  integer :: k
>  
> -  if (acc_is_present (b) .neqv. .true.) STOP 1
> +  call set()
> +  call check()
> +
> +  if (.not. acc_is_present (b)) stop 1
> +  if (.not. acc_is_present (x)) stop 2
> +  if (.not. acc_is_present (y)) stop 3
> +  if (.not. acc_is_present (z)) stop 4
>  
>    a = 2.0
> +  k = 42
>  
> -  !$acc parallel copy (a)
> +  !$acc parallel copy (a, k)
>      b = a
>      a = 1.0
>      a = a + b
> +    x = k
> +    y = 7*k - 2*x
> +    z = 3*y
> +    k = k - z + y
>     !$acc end parallel
>  
> -  if (acc_is_present (b) .neqv. .true.) STOP 2
> -
> -  if (a .ne. 3.0) STOP 3
> +  if (.not. acc_is_present (b)) stop 5
> +  if (.not. acc_is_present (x)) stop 6
> +  if (.not. acc_is_present (y)) stop 7
> +  if (.not. acc_is_present (z)) stop 8
>  
> +  if (a /= 3.0) stop 3
> +  if (k /= -378) stop 3
>  end program test
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