RFR: 8276098: Do precise BOT updates in G1 evacuation phase [v2]

[Stefan Johansson]

On Mon, 8 Nov 2021 13:40:11 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Stefan Johansson has updated the pull request incrementally with four additional commits since the last revision:
>> 
>>  - Move BOT info into PLAB
>>  - Predicate for needs_bot_update
>>  - Rename allocate_no_bot_updates
>>  - Handle waste when filling in dummy objects
>
> Lgtm. Some minor naming/method location suggestions.

Me and @tschatzl have discussed this offline and the above changes are the outcome of that. I will redo testing and if anything doesn't look good I'll update the PR.

To summarize the changes a bit:
* The new G1 PLAB is changed to hold the region and threshold information and is only used for old generation PLABs.
* BOT updates for PLAB and region waste is handled by making sure any filler objects get updates if in old regions.
* No need to filter direct allocations when updating BOT for PLAB allocation since a direct allocation will never alter the PLAB `_top`.

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PR: https://git.openjdk.java.net/jdk/pull/6166