[flang-commits] [flang] [flang][OpenMP] Diagnose privatized array section reductions (PR #215997)

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MattPD wrote:

You're right. Under [OpenMP 5.0, section 2.19.5.1](https://www.openmp.org/spec-html/5.0/openmpsu107.html#x140-5810002.19.5.1), the access to `a(1)` gives my original reproducer unspecified behavior. Thanks for catching it.

The replacement below uses a user-defined reduction. Its `taskloop` body accesses only `a(2:3)`, the section named in the reduction clause:

```fortran
subroutine s(a)
  integer :: a(4), i
  !$omp declare reduction(myred : integer : omp_out = omp_out + omp_in) &
  !$omp& initializer(omp_priv = 1)
  !$omp taskloop reduction(myred : a(2:3))
  do i = 1, 1
    a(2:3) = a(2:3) + i
  end do
end
```

With delayed privatization disabled, Flang emits a four-element array reduction for `myred : a(2:3)`. The initializer fills all four elements with 1. The combiner iterates from 1 through 4.

The specification requires a separate reduction for each element of an array section. Only `a(2)` and `a(3)` should participate. The generated HLFIR also combines `a(1)` and `a(4)`.

The incorrect lowering therefore remains on that path for a program with defined behavior.

https://github.com/llvm/llvm-project/pull/215997