[Bug fortran/98913] New: Create-temporary difference coarray/noncoarray – invalid code due to missing temporary

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98913

            Bug ID: 98913
           Summary: Create-temporary difference coarray/noncoarray –
                    invalid code due to missing temporary
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
                CC: jdelia at intec dot unl.edu.ar, tkoenig at gcc dot gnu.org
  Target Milestone: ---

Reported at https://gcc.gnu.org/pipermail/fortran/2021-January/055657.html

The testcase there uses
  real    (kind=idp), allocatable :: AA (:,:)[:]
  real    (kind=idp), allocatable :: BB (:,:)

For identical assignment, a temporary is generated for the
(this_image()-access) coarray variable but not for the local variable:

   37 |     AA (k1:nn,j) = AA (k1:nn,j) - UU (k1:nn) * AA (k,j)
      |                   1
Warning: Creating array temporary at (1) [-Warray-temporaries]


On the compiler side, I think the behaviour differs due to:
gfc_conv_resolve_dependencies():

          if (flag_realloc_lhs
              && dest_expr != ss_expr
              && gfc_is_reallocatable_lhs (dest_expr)
              && ss_expr->rank)
            nDepend = gfc_check_dependency (dest_expr, ss_expr, true);

 * * *

Looking at

  do  j = 1, nn
    AA (k1:nn,j) = AA (k1:nn,j) - UU (k1:nn) * AA (k,j)
  end do

I think a temporary is needed for "A(k,j)", i.e.
  do  j = 1, nn
    tmp = AA (k,j)
    do ii = k1, nn
      AA (ii,j) = AA (ii,j) - UU (ii) * tmp
    end do
  end do

without that temporary, k might be any value within k1:nn!

In this example, k = 1; k1 = 2 – thus, there is no dependency.

 * * *

For (b), I see (for "BB(k,j)"):

D.3991 = (*(real(kind=8)[0:] * restrict) bb.data)[(bb.offset +
(integer(kind=8)) j * bb.dim[1].stride) + (integer(kind=8)) k];

...
            D.3986 = (real(kind=8)[0:] * restrict) bb.data;
            D.3978 = (real(kind=8)[0:] * restrict) bb.data;

              while (1)
...
                  (*D.3986)[(S.0 + D.3994) + D.3997] = (*D.3978)[S.0 + D.3996]
- (*D.3982)[(S.0 + D.3993) + D.3983] * D.3991;

Namely:
  b(k,j) is accessed.

I am quite certain that "k = 1; k1 = k + 1" is not used for this analysis →
wrong code.

If it is, it should also apply to coarrays.