RFR: 8348853: Fold layout helper check for objects implementing non-array interfaces

[Tobias Hartmann]

On Wed, 26 Mar 2025 09:16:17 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

> If `TypeInstKlassPtr` represents an array type, it has to be `java.lang.Object`. From contraposition, if it is not `java.lang.Object`, we can conclude it is not an array, and we can skip some array checks, for instance.
> 
> In this PR, we improve this deduction with an interface base reasoning: arrays implements only Cloneable and Serializable, so if a type implements anything else, it cannot be an array.
> 
> This change partially reverts the changes from [JDK-8348631](https://bugs.openjdk.org/browse/JDK-8348631) (#23331) (in `LibraryCallKit::generate_array_guard_common`) and the test still passes.
> 
> The way interfaces are check might be done differently. The current situation is a balance between visibility (not to leak too much things explicitly private), having not overly general methods for one use-case and avoiding too concrete (and brittle) interfaces.
> 
> Tested with tier1..3, hs-precheckin-comp and hs-comp-stress
> 
> Thanks,
> Marc

Right, I was hoping that there would be some other suitable users of `GraphKit::get_layout_helper` that would now be folded but all current uses either trap or don't handle both arrays and non-arrays (and therefore wouldn't fold). So I agree, adding an IR framework test does not make sense. The existing test is sufficient.

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PR Comment: https://git.openjdk.org/jdk/pull/24245#issuecomment-2762234661