RFR: 8364111: InstanceMirrorKlass iterators should handle CDS and hidden classes consistently

[Thomas Stuefe]

On Fri, 25 Jul 2025 12:01:00 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> See the bug for more investigation. I think the root cause is already fixed in one of the `InstanceMirrorKlass` iterators, but not in the other one. Additionally, only one iterator handles the hidden classes. Also, there is a stale comment about CMS that apparently prevents us from asserting `is_primitive`, which is not relevant anymore.
> 
> So this PR commons out the metadata handling on both iterators, which fixes the GenShen+CDS bug, likely fixes hidden classes bug somewhere, and prevents the omission like this from happening again.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, Generational Shenandoah + CDS bugs are not reproducing now
>  - [x] Linux x86_64 server fastdebug, `tier1`
>  - [ ] Linux x86_64 server fastdebug, `all`

src/hotspot/share/oops/instanceMirrorKlass.inline.hpp line 69:

> 67:     // Klass is null means this has been a mirror for a primitive type
> 68:     // that we do not need to follow as they are always strong roots.
> 69:     assert(java_lang_Class::is_primitive(obj), "Sanity");

Question, since this assert is new:

When we create a mirror (`java_lang_Class::allocate_mirror`) during normal - non-shared - class loading, is there not a small time window where the mirror exists on the heap, but the Klass is still NULL? Could concurrent heap walkers not observe that?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26477#discussion_r2231435613