String-dedup test failures

[Zhengyu Gu]

Thought about overnight, it makes sense to treat it as weak root and 
deal with it the same way as other weak roots.

-Zhengyu

On 11/16/2017 04:34 AM, Aleksey Shipilev wrote:
> On 11/16/2017 10:25 AM, Roman Kennke wrote:
>> I think we need to clean up the string dedup table *after marking*. If a byte[] is not reachable
>> after marking, we need to clean all references to it, because *it may become trashed* during evac.
>> If we want to keep it alive, then we need to mark it. Evac'ing an unmarked byte[] after marking is
>> calling for troubles, because we probably access a pointer that is about to be invalidated.
> 
> Yes, that's what I was suggesting:
> 
> It seems that we want two separate modes: regular, and cleanup. Much like we do with "Parallel
> Cleanup" during Final Mark. In regular mode, we mark all StrDedup entries always, maybe even
> concurrently during mark? In cleanup mode, we don't mark StrDedup as roots, but at Final Mark we
> purge all stale entries from there. Then if any String cannot "revive" the stale unmarked entry, and
> have to insert new, reachable one. I think the mechanism above has the nice symmetry with class
> unloading, etc., which makes it easier to reason about. Also makes cleanup during pause optional.
> 
> -Aleksey
>