AOT caching of WeakReference

[Erik Osterlund]

I would probably rather ensure that the CDS code that copies object payload into the archived heap, does not copy discovered fields, hence leaving them blank (null). As for next fields, finding non-null values would typically imply that the referent has just been cleared by the GC, and the reference just hasn’t been processed yet by its registered queue, to perform some cleanup. This could mean we are about to snapshot state that hasn’t been cleaned up yet and hence contains garbage state you probably don’t want to linger around to the next run. Perhaps it would be better to only allow snapshotted WRs to be registered with the NULL queue, and treat the next fields the same as discovered fields: don’t copy them to the archived heap.

/Erik

> On 15 Nov 2024, at 05:12, John Rose <john.r.rose at oracle.com> wrote:
> 
> Right. Is there any way to deterministically normalize or clear those links? Maybe we could unlink them at dump and relink at startup. 
> 
>> On Nov 14, 2024, at 2:52 PM, Erik Osterlund <erik.osterlund at oracle.com> wrote:
>> 
>> Hi Ioi,
>> 
>> You can also get caught with a WR with discovered fields and next fields populated. They are mutated by the GC. They can drag in random unrelated stuff into your object graph that you might not want to stuff into the archive.
>> 
>> /Erik
>> 
>>> On 14 Nov 2024, at 22:48, ioi.lam at oracle.com wrote:
>>> 
>>> Here's my  experiment with storing weak references in the AOT cache
>>> 
>>> https://github.com/iklam/jdk/blob/8341587-example-of-weakref-problem-in-aot-cache/AOT-Cache-And-WeakRef.md
>>> 
>>> As discussed in the Leyden meeting today, there seem to be at least two issues
>>> 
>>> - In premain, CDS keeps every archived heap object alive (look for UsePermanentHeapObjects in the code). I plan to get rid of this shortly. UsePermanentHeapObjects is intended for the AOT compiler to access heap objects, but it should be implemented differently (only remember objects that are required by AOT code, which should include only Strings and class mirrors).
>>> 
>>> - WeakReference.queue is not pointing to the production-time value of ReferenceQueue.NULL
>>> 
>>> 
>>> Thanks
>>> 
>>> - Ioi
>>>