https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82895
--- Comment #5 from Harald Anlauf <anlauf at gmx dot de> ---
(In reply to Arjen Markus from comment #4)
> It was precisely this sort of issues that led me to write this small sample
> program. And I realised this morning, that indeed the host variables need to
> be avoided.
The OpenMP standard says that host or use associated variables are
shared. For q, this means that different threads would access this
variable, i.e. you get a data race.
> But does:
> - loop_body accesses the host-associated, non-threadprivate variables q,
> x, and id, and you get data races
>
> mean that it is utterly unsafe to access even the shared variables (x and id
> in this case)? That would defy the intentions of using a(n internal) routine
> altogether. I need to have access to the entire array and the pattern of
> iterations avoids data collisions.
For your code and for your iteration pattern (different threads will use
different j ranges), you could declare q in the main as threadprivate,
(now gfortran-7 complains that it should be save'd), and you should pass
the private variables i and j to your worker subroutine.
Try the following (checked with valgrind):
! chkloop_internal.f90 --
! I want to know:
! - OpenMP-enabled outer loops with a small number of iterations (but a
large body with long loops)
! - Combination with internal routines
!
program chkloop_internal
use omp_lib
implicit none
integer, parameter :: noelems = 100000
real, dimension(:), allocatable :: x, y, z
integer, dimension(:), allocatable :: id
integer :: i, j, noperlayer, nolayers
!$omp threadprivate(q)
real, save :: q
allocate( x(noelems), y(noelems), z(noelems), id(noelems) )
id = 0
call random_number( x )
call random_number( y )
call random_number( z )
nolayers = 10
noperlayer = noelems / nolayers
!$omp parallel private(i, j)
!$omp do schedule(dynamic)
do i = 1,nolayers
do j = noperlayer * (i - 1) + 1, noperlayer * i
call loop_body (i,j)
enddo
enddo
!$omp end do
!$omp end parallel
do i = 1,omp_get_max_threads()
write(*,*) i, count( id == i-1 )
enddo
contains
subroutine loop_body (i,j)
integer :: i, j
! real :: q ! use local variables if possible
q = z(j)
x(j) = x(j) + q * y(j)
id(j) = omp_get_thread_num()
write(80+id(j),*) i,j,x(j),y(j),q
end subroutine loop_body
end program chkloop_internal
