[fir] Add fir.array_access op

The `array_access` provides a reference to a single element from an array
value. This is *not* a view in the immutable array, otherwise it couldn't
be stored to. It can be see as a logical copy of the element and its
position in the array. This reference can be written to and modified without
changing the original array.

The `array_access` operation is used to fetch the memory reference of an
element in an array value.

```fortran
      real :: a(n,m)
      ...
      ... a ...
      ... a(r,s+1) ...
```

One can use `fir.array_access` to recover the implied memory reference to
the element `a(i,j)` in an array expression `a` as shown above. It can also
be used to recover the reference element `a(r,s+1)` in the second
expression.

```mlir
      %s = fir.shape %n, %m : (index, index) -> !fir.shape<2>
      // load the entire array 'a'
      %v = fir.array_load %a(%s) : (!fir.ref<!fir.array<?x?xf32>>, !fir.shape<2>) -> !fir.array<?x?xf32>
      // fetch the value of one of the array value's elements
      %1 = fir.array_access %v, %i, %j : (!fir.array<?x?xf32>, index, index) -> !fir.ref<f32>
```

More information about `array_access` and other array operations can be
found in flang/docs/FIRArrayOperations.md.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: kiranchandramohan, schweitz

Differential Revision: https://reviews.llvm.org/D112445

Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>

0 files changed, 0 insertions, 0 deletions