[lworld] Integrated: 8332406: [lworld] C2: Turn flat in array property into proper lattice in type system to address remaining issues

[Christian Hagedorn]

On Mon, 8 Dec 2025 08:13:34 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

> This patch turns the boolean flag `_flat_in_array` into a new `FlatInArray` enum to properly define a (Boolean) lattice. This mainly allows us to cleanly represent "maybe flat in array" and "not flat in array". The dedicated top element is the dual of "maybe flat in array".
> 
> To simplify the review and to better understand what the changes are, I added code comments directly in the PR.
> 
> Here is a high-level overview of the changes:
> - `FlatInArray` enum to define a lattice for `flat_in_array`:
> https://github.com/openjdk/valhalla/blob/6b1930c7b9a359223a998d0086a1326a9d7905e7/src/hotspot/share/opto/type.hpp#L1182-L1199
> - Updated `meet` operations accordingly. Needed to be careful when doing the meet above the centerline.
> - Added single `compute_flat_in_array()` method to properly compute the `flat_in_array` property from the information we have when we need to set flat in array newly (this was no properly done before).
> - In `flatten_alias_type()`, we flatten to "maybe flat in array".
> 
> #### Testing:
> - t1-4 + valhalla-comp-stress
> 
> Thanks for your feedback,
> Christian

This pull request has now been integrated.

Changeset: a826fbd0
Author:    Christian Hagedorn <chagedorn at openjdk.org>
URL:       https://git.openjdk.org/valhalla/commit/a826fbd06ab41c483f59c06fed1757fae8c3a183
Stats:     530 lines in 11 files changed: 288 ins; 141 del; 101 mod

8332406: [lworld] C2: Turn flat in array property into proper lattice in type system to address remaining issues

Reviewed-by: thartmann

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PR: https://git.openjdk.org/valhalla/pull/1774