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.
