RFR: 8252294: Remove OopsInGenClosure usage from younger_refs_iterate

[Kim Barrett]

> On Aug 25, 2020, at 4:12 AM, Stefan Karlsson <stefan.karlsson at oracle.com> wrote:
> 
> Hi all,
> 
> Please review this patch to remove the usage of OopsInGenClosure from younger_refs_iterate and related functions.
> 
> https://cr.openjdk.java.net/~stefank/8252294/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8252294

Looks good.  Thanks for the detailed overview.

I don't need a new webrev for the comment deletion below.

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src/hotspot/share/gc/serial/serialHeap.cpp
 105   // When collection is parallel, all threads get to cooperate to do
 106   // old generation scanning.

This comment no longer makes any sense.

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src/hotspot/share/gc/serial/serialHeap.cpp
 101   if (_process_strong_tasks->try_claim_task(GCH_PS_younger_gens)) {
...
 112   _process_strong_tasks->all_tasks_completed(scope->n_threads());

The use of parallel task support no longer makes sense here. I'm
guessing this is a cleanup you want to save for later?

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