RFR: 8344665: Refactor PartialArrayState allocation for reuse [v2]

[Kim Barrett]

On Fri, 29 Nov 2024 19:17:16 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> What about using a union/struct instead of all that manual masking and shifting?
>> 
>> I agree that encoding them in this way should give a rationale for that.
>
> I tried using some combination of union/struct a couple of different ways, but
> the the result seemd significantly worse than what I published. But some
> simplifications since those attempts, plus an idea for a different way of
> structuring things, and I've now got something union/struct-based that seems
> better.
> 
> It could be slightly further improved in syntax if I was willing to drop the
> CounterState::_cf member name and make it an anonymous struct. That's a C11
> feature that is provided as a C++ extension by all of our supported compilers.
> 
> You might not want to look at the difference between the two commits very
> much, but instead look at the updated complete change.

I also expanded commentary for `PartialArrayStateManager::_counters`.  Maybe that will help?

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PR Review Comment: https://git.openjdk.org/jdk/pull/22287#discussion_r1864216818