# Nested reductions & compiled backends cleanup
###### tags: `functional cycle 12`

In the last cycle many improvements have been implemented to enable the development of the unstructured FVM dycore. This includes support for unstructured using the C++ backend and support for nested reductions. These changes were never merged and live on a seperate [feature branch](https://github.com/GridTools/gt4py/pull/965) in an unclean state.

## Goals

- Enable all currently disabled tests for the GTFN backend (merged into functional branch).
- Allow API tuple arguments, fields with origin and index_fields (gt::positional) when using the GTFN backend (merged into functional branch).
- Fix nested reductions (keep as a separate feature branch, as the approach needs to be revisited).

## Steps

- Till: Factor out support for unstructured into a seperate PR and merge it into functional.
- Till: Factor out nested reductions and put them into a seperate PR that will live in a feature branch until the design decisions made there are approved. Afterwards cleanup the branch (not meaningful before decision).
- Implement support for
- API tuple arguments
- Fields with origin
- index_fields

## Appetite

Full cycle.

## What is already there

What is already there:

1. Proof of concept implementation that drastically reduces the complexity of the lowering from FOAST to ITIR.
2. Chained remaps `field(Offset1[0])(Offset2[0])`
3. Fix for nested reductions using new `ignore_shift`, `translate_shift` builtins in ITIR. Requires 1.
4. Several improvements to ITIR `apply_common_transforms` pass
- Non-recursive option to `InlineLambdas` pass
- Non-opcount-preserving option to force inline lift calls in `InlineLambdas` pass
- Simplification of `InlineLifts` pass
- `RemoveShiftsTransformer` and `PropagateShiftTransformer` passes to remove newly introduced `ignore_shift` and `translate_shift` builtins (such that the lowering continues to work with the C++ backend).
- Improvements to `UnrollReduce` pass that supports nested reductions. Requires change `reduce` signature to have an offset label argument over which we reduce.
5. Support for unstructured shifts in the C++ backend (by passing neighbor tables to the generated C++ code).
6. Full support for neighbor fields as program arguments in the C++ backend (requires *3.*).
7. Caching for compiled fencils.