RFR: 8272579: G1: remove unnecesary null check in G1ParScanThreadStateSet::flush

[Stefan Johansson]

On Wed, 18 Aug 2021 13:01:00 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> src/hotspot/share/gc/g1/g1ParScanThreadState.cpp line 561:
>> 
>>> 559:   for (uint worker_id = 0; worker_id < _n_workers; ++worker_id) {
>>> 560:     G1ParScanThreadState* pss = _states[worker_id];
>>> 561:     assert(pss != nullptr, "must be initialized");
>> 
>> Is it guaranteed that all workers are initialized? As you state in the bug, they are lazily initialized, so I wonder if there are use cases where a worker would not get initialized during the collection. Something like, very little work and a lot of workers. 
>> 
>> If there is a guarantee here, does it hold below in `record_unused_optional_region()` as well? We are using the same construct with `continue`there.
>
> In `G1CollectedHeap::evacuate_initial_collection_set`:
> 
> 
> G1ParScanThreadStateSet per_thread_states(this,
>                                           &rdcqs,
>                                           workers()->active_workers(), <---- setting up states for workers
>                                           collection_set()->young_region_length(),
>                                           collection_set()->optional_region_length());
> ...
> evacuate_initial_collection_set(&per_thread_states, may_do_optional_evacuation);
> \
>  -- task_time = run_task_timed(&g1_par_task); <--- launching all workers
> 
> Therefore, I think the assertion is safe.
> 
>> does it hold below in record_unused_optional_region() as well?
> 
> I think so.

In that case I would change that one as well, but it please run some more testing to ensure it is safe.

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PR: https://git.openjdk.java.net/jdk/pull/5145