[lworld] RFR: 8228361: [lworld] Optimize the substitutability check in C2 [v2]

[Tobias Hartmann]

On Thu, 18 Dec 2025 12:16:48 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> C2 will now remove the slow call to `ValueObjectMethods::isSubstitutable(Alt)` whenever it's able to determine the layout of one of the operands. It will then emit code to directly compare the fields.
>> 
>> This patch also contains an intrinsic for `_getFieldMap` that will be used by the new core-libs implementation of the substitutability check ([JDK-8370450](https://bugs.openjdk.org/browse/JDK-8370450)) that's used by the interpreter / C1 and as a slow path in C2.
>> 
>> When browsing code, I marked a few rough edges in unrelated code for follow-up cleanups with the corresponding bug numbers.
>> 
>> Testing: tier1-tier6 + valhalla-comp-stress
>> 
>> Thanks,
>> Tobias
>
> Tobias Hartmann has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 21 commits:
> 
>  - Merge branch 'lworld' into JDK-8228361
>  - More refactoring, better comments
>  - More comments, removing workaround
>  - Merge
>  - v5
>  - Refactoring v4
>  - Refactoring v3
>  - Refactoring, more checks, new test
>  - Merge branch 'lworld' into JDK-8228362
>  - All tests pass
>  - ... and 11 more: https://git.openjdk.org/valhalla/compare/3c41c2aa...4f8556c7

Thanks a lot for the review, Christian! I replied to all of your comments and adjusted the code accordingly.

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PR Comment: https://git.openjdk.org/valhalla/pull/1823#issuecomment-3670341995