On Mon, 6 Jul 2026 08:04:20 GMT, Marc Chevalier <[email protected]> wrote:
> Nothing is flat! Well... sometimes, things are. But not arrays! Ok, arrays > can be flat... but you can never be sure an array is flat from its unrefined > type (and current JVM flags). Indeed, the "new" > `ValueClass::newReferenceArray`: this allows to bypass > `ObjArrayKlass::array_layout_selection(Klass*, ArrayProperties)` and get an > array of references from any value class, regardless of the available > layouts, thus fooling `InlineKlass::is_always_flat_in_array()`. In general, > we can ensure the flatness of an array in only few situations: if we see the > allocation, or after having tested the flatness with a runtime check. Thus > `InlineKlass::is_always_flat_in_array()` doesn't really make sense... And > consequently, it is not correct for > `TypePtr::compute_flat_in_array(ciInstanceKlass* instance_klass, bool > is_exact)` to ever return `Flat`. Yet, we still need the constructor `Flat` > in `FlatInArray`. For instance, it is used in > `TypeInstPtr::cast_to_flat_in_array()` which is indirectly used (through `load_from_unknown_flat_array`) in the "flat array" branch of `Parse::array_load(BasicType bt)`: > > https://github.com/openjdk/valhalla/blob/a918cc9f5f8dcc35f4f73ed19b439a033fb39883/src/hotspot/share/opto/parse2.cpp#L121-L143 > > But here, casting to flat is fine since we are under a runtime check that > enforces that the array is indeed flat. > > Fortunately, I don't think the current usage could have lead to an error: we > only use it to compare types: if one is sure to be flat but not the other > one, the types can't be the same. It's fine, even if the "sure to be flat" > criterion is buggy. Would work with a bit anything. Like "if a type starts > with the letter 'A', but the other by the letter 'B', they can't be the > same". If we had any tracking of that, it would allow to (very weirdly) prove > type inequality even with imprecise types... > > Thanks, > Marc > > --------- > - [x] I confirm that I make this contribution in accordance with the [OpenJDK > Interim AI Policy](https://openjdk.org/legal/ai). This pull request has now been integrated. Changeset: 52cfb4e1 Author: Marc Chevalier <[email protected]> URL: https://git.openjdk.org/valhalla/commit/52cfb4e199a64f929666adf3f67759a155e9512e Stats: 24 lines in 5 files changed: 0 ins; 24 del; 0 mod 8387405: [lworld] is_always_flat_in_array should be always false Reviewed-by: thartmann ------------- PR: https://git.openjdk.org/valhalla/pull/2629
