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

[Alan Bateman]

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.

src/hotspot/share/ci/ciField.cpp line 220:

> 218:     return false;
> 219:   // Explicit opt-in from system classes
> 220:   if (holder->trust_final_fields())

This is definitely nicer than listing specific classes. It would be nicer again once we can make this exceptions go away.

src/java.base/share/classes/jdk/internal/vm/annotation/TrustFinalFields.java line 61:

> 59: /// fields in classes specified by this annotation.
> 60: ///
> 61: /// This annotation is only recognized on privileged code and is ignored elsewhere.

"privileged code" hints of protection domains, permissions or security manager. Some of the annotations are limited to classes defined by the boot loader, is it the case here too?

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PR Review Comment: https://git.openjdk.org/jdk/pull/28540#discussion_r2569767299
PR Review Comment: https://git.openjdk.org/jdk/pull/28540#discussion_r2569764340