[Class modifiers] VM implementation

[itsjustkevin]

No description provided.

[itsjustkevin]

@aam is there work that needs to be done on this?

[alexmarkov]

@itsjustkevin It looks like your question should be addressed to me and not @aam .

I don't expect any work in the VM related to the correctness of the class modifiers feature. However, we may consider using new class modifiers to do some minor optimizations (for example, we can try to reduce number of guarded classes in JIT mode when classes are declared final).

[itsjustkevin]

This is the second time I've done this @alexmarkov, won't make the mistake again.

With your comment above, maybe this is more appropriate in the beta 3 milestone than beta 2, or something we call for now and consider post launch.

[alexmarkov]

Sounds reasonable. Changed milestone to Dart 3 stable.

[kallentu]

I prematurely closed #51287 for sealed classes.
May want to sync it to the same status as class modifiers VM implementation if needed.

[alexmarkov]

As specified in the current versions of the feature specification proposals (https://github.com/dart-lang/language/blob/master/accepted/future-releases/class-modifiers/feature-specification.md and https://github.com/dart-lang/language/blob/master/accepted/future-releases/sealed-types/feature-specification.md), sealed and final modifiers are not very useful as they allow subtyping within library and even "re-opening" of the subtype hierarchy. In order to make them useful for open-world implementations (such as VM/JIT and maybe DDC), we should probably calculate effectively final flag on classes (meaning they cannot have subtypes), both from sealed and final, and perform optimizations based on that flag. Note that closed-world implementations (such as AOT, dart2js and dart2wasm) would not benefit from those modifiers at all as they have full knowledge of the class hierarchy already.

So I think it's okay to keep just one issue open.

[alexmarkov]

Filed a separate issue about "effectively final" flag: #51765. It's a minor optimization and can be done after Dart 3.0 release.

We don't have anything else planned for class modifiers and sealed, so closing this issue.