[J3] Question: Aliasing in Parallel Procedure Instances in OpenMP Parallel Regions

[Klemm, Michael]

[Public]

Hi folks,

In the OpenMP Language Committee at discussion happened about the below piece of Fortran code.  There was some dissent in the discussion, so I'm hoping for some clarification.

In the example, someone spawns a few threads and calls a Fortran procedure, passing along a variable that's shared in the parallel region.  We understand that dummy arguments cannot alias.  But what about the "variable" dummy argument that aliases in two parallel instances of the same procedure?  It seems that the wording of the aliasing rule does not define that at all (which is not surprising, given that Fortran does not know threads).

So, the question is: Are we in UB land with the below example?  Does the definition of dummy-argument aliasing in Fortran extend to the parallel case, too?

Kind regards,
        -michael


subroutine test(variable)
  use omp_lib, only : omp_get_thread_num
  implicit none
  integer :: variable

  !$omp single
  variable = 1234
  !$omp end single

  if (variable .ne. 1234) then
    print '(A,I0,A,I0)', "ERROR: variable=", variable, ", TID=",omp_get_thread_num()
  endif
end subroutine test

program main
  implicit none
  integer :: variable2

  interface
     subroutine test(variable)
       implicit none
       integer :: variable
     end subroutine test
  end interface

  variable2 = -42
  !$omp parallel shared(variable2)
  call test(variable2)
  !$omp end parallel
end program main