RFR: 8372696: Allow boot classes to explicitly opt-in for final field trusting [v4]

[Chen Liang]

On Mon, 1 Dec 2025 18:27:34 GMT, Chen Liang <liach at openjdk.org> wrote:

>> Currently, the hotspot compiler (as in ciField) trusts final fields in hidden classes, record classes, and selected jdk packages. Some classes in the JDK wish to be trusted, but they cannot apply package-wide opt-in due to other legacy classes in the package, such as java.util.
>> 
>> They currently can use `@Stable` as a workaround, but this is fragile because a stable final field may hold a trusted null, zero, or false value, which is currently treated as non-constant by ciField.
>> 
>> We should add an annotation to opt-in for a whole class, mainly for legacy packages. This would benefit greatly some of our classes already using a lot of Stable, such as java.util.Optional, whose empty instance is now constant-foldable, as demonstrated in a new IR test.
>> 
>> Paging @minborg who requested Optional folding for review.
>> 
>> I think we can remove redundant Stable in a few other java.util classes after this patch is integrated. I plan to do that in subsequent patches.
>
> Chen Liang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   bracket styles

This PR currently does not interact with JEP 500. However, as specified in `Field.set`, the result of setting a final field may be ignored, as Alan [commented](https://github.com/openjdk/jdk/pull/28540#discussion_r2573494589). So I don't think we need to rescind the current trusting even if users enable mutations.

In addition, @DanHeidinga I made the same fault as you when I first saw `--enable-final-field-mutation=` - this actually represents the callers, instead of the target, of `Field.set`. The target of mutation is specified via `--add-opens`, if the target field is not public.

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PR Comment: https://git.openjdk.org/jdk/pull/28540#issuecomment-3598676839