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

[Chen Liang]

On Thu, 27 Nov 2025 16:16:05 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.

This uses another one of the 16-bit instanceKlassFlags, which requires runtime engineers to agree. Need compiler review to check if such IR tests are the best way to ensure constant folding for core library classes.

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PR Comment: https://git.openjdk.org/jdk/pull/28540#issuecomment-3586679853