[Mlir-commits] [mlir] [mlir][EmitC] Model lvalues as a type in EmitC (PR #91475)

[Marius Brehler]

https://github.com/marbre approved this pull request.

Thanks @simon-camp for getting the work done, thanks @aniragil and @mgehre-amd for reviewing and for the valuable discussions. This really pushes the dialect forward!

There is one thing we should agree on before this is merged. As the patch introduces breaking changes, @simon-camp and I discussed offline that this requires a PSA. An non-breaking intermediate solution would be to *relax* the restriction on `emitc.variable` and to allow `EmitCType` as a result. This would mean to replace
```mlir
let results = (outs Res<AnyTypeOf<[EmitC_ArrayType, EmitC_LValueType]>, "",
```
with
```mlir
let results = (outs Res<AnyTypeOf<[EmitCType, EmitC_ArrayType, EmitC_LValueType]>, "",
```
while we ignore the changes regarding the memory effects. Furthermore, the modifications to `emitc.apply` would need to be rolled-back and instead an `emitc.dereference` (https://en.cppreference.com/w/c/language/operator_member_access) / `emitc.indirection` (https://en.cppreference.com/w/cpp/language/operator_member_access) and `emitc.address_of` should be introduced (which we probably want to do in a follow up anyway). With this, the `emitc.variable` as well as `emitc.apply` op could still be used as before but users can adapt to the new behavior before the result type of the `emitc.variable` op gets restricted and the `emitc.apply` op is removed. However, adopting to the behavior shouldn't be too hard, thus we tend to keep the effort low and just go with a PSA and before merging this in **x?** weeks. WDYT?

https://github.com/llvm/llvm-project/pull/91475