[Bug fortran/79247] New: Race condition on OpenMP reduction variable

[paul.k.romano at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79247

            Bug ID: 79247
           Summary: Race condition on OpenMP reduction variable
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: paul.k.romano at gmail dot com
  Target Milestone: ---

The following program results in a race condition on x:

program main
  integer :: i, x = 0
!$omp parallel do reduction(+:x)
  do i = 1, 100000
    call accumulate()
  end do
!$omp end parallel do
  print *, x
contains
  subroutine accumulate()
    x = x + 1
  end subroutine accumulate
end program main

As far as I can tell, this is permitted by the OpenMP standard. If one
explicitly passes x to the subroutine, the race condition disappears. It's also
worth noting that with Intel Fortran (17.0.1), no race condition occurs and the
correct value of 100000 is printed.

I am using gcc 6.2.0 from the system packages on Ubuntu 16.10.