RFR: 8354897: Support Soft/Weak Reference in AOT cache

[Erik Österlund]

On Fri, 18 Apr 2025 18:31:05 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> This PR contains 2 parts
> 
> - Upstream of Soft/Weak Reference support authored by @macarte from [the Leyden repo](https://github.com/openjdk/leyden/commit/4ca75d156519596e23abc8a312496b7c2f0e0ca5)
> - New C++ class `AOTReferenceObjSupport` and new Java method `ReferencedKeyMap::prepareForAOTCache()` developed by @iklam on the advice of @fisk from the GC team. These control the lifecycles of reference objects during the assembly phase to simplify the implementation.
> 
> One problem we faced in this PR is the handling of Reference objects that are waiting for clean up. Currently, the only cached Reference objects that require clean up are the `WeakReferenceKey`s  used by `ReferencedKeyMap` (which is used by `MethodType::internTable`):
> 
> - When the referent of a `WeakReferenceKey` K has been collected, the key will be placed on `Universe::reference_pending_list()`. It's linked to other pending references with the `Reference::discovered` field. At this point, K is still stored in the `ReferencedKeyMap`.
> - When heapShared.cpp discovered the `ReferencedKeyMap`, it will discover K, and it may also discover other pending references that are not intended for the AOT cache. As a result, we end up caching unnecessary objects. 
> 
> `ReferencedKeyMap::prepareForAOTCache()`  avoids the above problem. It goes over all entries in the table:
> 
> - If an entry has not yet been collected, we make sure it will never be collected.
> - If an entry has been collected, we remove it from the table
> 
> Therefore, by the time heapShared.cpp starts scanning the `ReferencedKeyMap`, it will never see any keys that are on the pending list, so we will not see unintended objects.
> 
> This implementation is the very first step of Reference support in the AOT cache, so we chose a simplified approach that makes no assumptions on when the pending reference list is processed. This is sufficient for the current set of references objects in the AOT cache.
> 
> In the future, we may relax the implementation to allow for other use cases.

Changes requested by eosterlund (Reviewer).

src/hotspot/share/cds/heapShared.cpp line 1408:

> 1406:     oop obj = RawAccess<>::oop_load(p);
> 1407:     if (!CompressedOops::is_null(obj)) {
> 1408:       int field_offset = pointer_delta_as_int((char*)p, cast_from_oop<char*>(_referencing_obj));

The RawAccess oop load above should probably be a HeapAccess<ON_UNKNOWN_OOP_REF>::oop_load_at. By using the unknown oop ref and supplying the field offset, we allow GCs to infer the actual reference strength in the backend when loading and apply appropriate barriers for non-strong references, while also applying appropriate load barriers for conc GCs.

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PR Review: https://git.openjdk.org/jdk/pull/24757#pullrequestreview-2792646427
PR Review Comment: https://git.openjdk.org/jdk/pull/24757#discussion_r2059322068