[Bug fortran/63938] New: OpenMP atomic update does not protect access to automatic array

["paul.k.romano at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63938

            Bug ID: 63938
           Summary: OpenMP atomic update does not protect access to
                    automatic array
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: paul.k.romano at gmail dot com

It appears that performing !$omp atomic update on an automatic array does not
protect access to the array. As an example, consider the following program:

program atomic

  integer :: i, x(1)

  x(1) = 0

!$omp parallel do
  do i = 1, 1000
    !$omp atomic
    x(1) = x(1) + 1
  end do
!$omp end parallel do

  print *, x

end program atomic

Compiling and running this program should result in the value 1000. Instead, it
results in an unpredictable value for x, indicating there is a race condition.
If you change x to a scalar or an allocatable array, it works fine and reports
the value 1000. Equivalent code in C does not produce a race condition. The
code gives the correct value when compiled with the Intel fortran compiler.

Information on my build of gcc:
Target: x86_64-unknown-linux-gnu
Configured with: ./configure --disable-multilib
Thread model: posix