RFR: 8272609: Add string deduplication support to SerialGC [v10]

[Albert Mingkun Yang]

On Fri, 20 Aug 2021 16:02:53 GMT, Denghui Dong <ddong at openjdk.org> wrote:

>> Hi team,
>> 
>> Please help review this change to add string deduplication support to SerialGC.
>> 
>> Thanks,
>> Denghui
>
> Denghui Dong has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix build error

src/hotspot/share/gc/serial/defNewGeneration.cpp line 606:

> 604:   assert(heap->no_allocs_since_save_marks(), "save marks have not been newly set.");
> 605: 
> 606:   _string_dedup_requests.flush();

I wonder if placing `_string_dedup_requests.flush();` before weak roots processing (`WeakProcessor::weak_oops_do`) makes more senses. The documentation says `flush` should be called when marking is done; for Serial young GC, it's equivalent to when evacuation is done, which is right after ref-processing.

src/hotspot/share/gc/serial/genMarkSweep.cpp line 115:

> 113:   deallocate_stacks();
> 114: 
> 115:   MarkSweep::_string_dedup_requests->flush();

Similarly, `flush` should be called inside `mark_sweep_phase1`, right after ref-processing.

src/hotspot/share/gc/serial/markSweep.cpp line 219:

> 217:   MarkSweep::_gc_timer = new (ResourceObj::C_HEAP, mtGC) STWGCTimer();
> 218:   MarkSweep::_gc_tracer = new (ResourceObj::C_HEAP, mtGC) SerialOldTracer();
> 219:   MarkSweep::_string_dedup_requests = new StringDedup::Requests();

It's not clear to me why `_string_dedup_requests` is a pointer. Is it possible to define `static StringDedup::Requests _string_dedup_requests;` directly? That way, `Requests` doesn't need to use the inheritance.

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PR: https://git.openjdk.java.net/jdk/pull/5153