RFR: 8261838: Shenandoah: reconsider heap region iterators memory ordering

[Roman Kennke]

On Tue, 16 Feb 2021 19:13:03 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> We use CASes to distributed workers between regions. Hotspot's default for atomic operations is memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64.
> 
> This seems to be excessive for region distribution code, and "relaxed" is enough, since we don't piggyback memory ordering on these.
> 
> This also calls for some refactoring in the code itself.
> 
> Additional testing:
>  - [x] `hotspot_gc_shenandoah`
>  - [ ] Ad-hoc performance runs

Looks good!

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2593