RFR: 8279008: G1: Calculate BOT threshold on-the-fly during Object Copy phase [v3]

[Albert Mingkun Yang]

On Thu, 20 Jan 2022 16:09:24 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Simple change of unifying BOT logic for PLAB and direct allocation during GC.
>> 
>> Test: tier1-6; no difference observed while running BigRamTester, SPECjbb2015
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

As discussed with Stefan, I will keep the term "card boundary" in this PR and remove `G1BlockOffsetTablePart::_next_offset_threshold` in a new PR, since "threshold" is too abstract and less precise.

Thanks for the review.

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PR: https://git.openjdk.java.net/jdk/pull/6916