[Bug tree-optimization/66873] fortran variant of outer-1.c not parallelized by autopar

["kargl at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66873

kargl at gcc dot gnu.org changed:

           What    |Removed                     |Added
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                 CC|                            |kargl at gcc dot gnu.org

--- Comment #5 from kargl at gcc dot gnu.org ---
(In reply to vries from comment #0)
> Consider this example, a fortran version of autopar/outer-1.c:
> ...
> program main
>   implicit none
>   integer, parameter         :: n = 500
>   integer, dimension (0:n-1, 0:n-1) :: x
>   integer                    :: i, j, ii, jj
> 
>   do ii = 0, n - 1
>      do jj = 0, n - 1
>         x(ii, jj) = ii + jj + 3
>      end do
>   end do
> 
>   do i = 0, n - 1
>      do j = 0, n - 1
>         if (x(i, j) .ne. i + j + 3) call abort
>      end do
>   end do
> 
> end program main
> ...
> 
> When trying to parallelize this using -O2 -ftree-parallelize-loops=2, it
> fails on the dependencies:

Does the loop ordering matter?  Fortran is a column major language,
so your nested loops are backwards.  One would normally write.

  do jj = 0, n - 1
      do ii = 0, n - 1
         x(ii, jj) = ii + jj + 3
      end do
   end do

where the first loop index varies most rapidly.