commit 4983aec494119048ccbdd19d237f92ebb24c5d62
author Tom Eccles <tom.eccles@arm.com> Mon Apr 14 17:25:59 2025 +0100
committer GitHub <noreply@github.com> Mon Apr 14 17:25:59 2025 +0100
tree 2dcada1b3eebac4b49cdf0cae78e285508057217
parent fef5b6f9530c52d623f4729900855e626d5ee722

[flang][OpenMP][HLFIR] Support vector subscripted array sections for DEPEND (#133892)

The OpenMP runtime needs the base address of the array section to
identify the dependency.

If we just put the vector subscript through the usual HLFIR expression
lowering, that would generate a new contiguous array representing the
values of the elements in the array which was sectioned. We cannot use
addresses from this array because these addresses would not match
dependencies on the original array. For example

```
integer :: array(1024)
integer :: indices(2)

indices(1) = 1
indices(2) = 100

!$omp task depend(out: array(1:512))
!$omp end task

!$omp task depend(in: array(indices))
!$omp end task
```

This requires taking the lowering path previously only used for ordered
assignments to get the address of the elements in the original array
which were indexed. This is done using `hlfir.elemental_addr`. e.g.
```
array(indices) = 2
```

`hlfir.elemental_addr` is awkward to use because it (by design) doesn't
return something like `!hlfir.expr<>` (like `hlfir.elemental`) and so it
can't have a generic lowering: each place it is used has to carefully
inline the contents of the operation and extract the needed address.

For this reason, `hlfir.elemental_addr` is not allowed outside of these
ordered assignments. In this commit I ignore this restriction so that I
can use `hlfir.elemental_addr` to lower the OpenMP DEPEND clause (this
works because the operation is inlined and removed before the verifier
runs).

One alternative solution would have been to provide my own more limited
re-implementation of `HlfirDesignatorBuilder` which skipped
`hlfir::elemental_addr`, instead inlining its body directly at the
current insertion point applying indices only for the first element.
This would have been difficult to maintain because designation in
Fortran is complex.