[flang-commits] [flang] [Flang][OpenMP] DEFAULT(NONE) error checking on implicit references (PR #182214)

[Leandro Lupori via flang-commits]

luporl wrote:

@sscalpone In this case it seems to me that the test is incorrect.

I agree that `i1` is predetermined as lastprivate for the `parallel do simd` construct, according to OpenMP 6.0 spec.
However, `i1` data-sharing attribute is undefined for the `task` construct, and lastprivate causes an implicit reference to `i1` in the `task` construct, according to section 7.1.1:

Specifying a variable in a copyprivate clause or a data-sharing attribute clause other than the
private clause on a nested construct causes an implicit reference to the variable in the enclosing
construct.

Gfortran seems to agree with this interpretation and emits an error:
```
ftest.f90:4:42:

    4 |                     !$omp parallel do simd
      |                                          ^
Error: 'i1' not specified in enclosing 'task'
ftest.f90:3:44:

    3 |                     !$omp task default(none)
      |                                            ^
note: enclosing 'task'
```

Ifx doesn't complain about this, but it also limits the scope of predetermined variables to the affected construct, as in the example below.

```f90
    implicit none
    integer :: i = 1

    !$omp parallel
    !$omp task
        print *, "task i (before parallel do):", i
        i = 2
        !$omp parallel do
            do i = 1, 5
            end do
        print *, "task i (after parallel do):", i
    !$omp end task
    !$omp end parallel
    print *, "program i:", i
end
```

The output is:
```
 task i (before parallel do):           1
 task i (before parallel do):           2
 task i (after parallel do):           2
 task i (after parallel do):           2
 program i:           2
```

`i` is shared in the first `parallel` construct and in `task`, before `parallel do`.

I'm not an OpenMP expert either, so it would be good to hear other opinions on this.

https://github.com/llvm/llvm-project/pull/182214