RFR: Increase card offset mask [v2]

[William Kemper]

On Tue, 9 May 2023 19:52:12 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
>> 
>>  - Merge branch 'openjdk:master' into increase-card-offset-mask
>>  - Re-problem-list failing pipeline tests
>>  - Re-problem-list failing GHA tests
>>  - Increase card offset mask to cover 32-bit words and/or 1k cards
>
> src/hotspot/share/gc/shenandoah/shenandoahScanRemembered.hpp line 394:
> 
>> 392:   // frequently than last byte.  This is true when number of clean cards is greater than number of dirty cards.
>> 393:   static const uint16_t ObjectStartsInCardRegion = 0x80;
>> 394:   static const uint16_t FirstStartBits           = 0x3f;
> 
> Nice catch.  BTW, the cross_map structure is no longer needed and neither is the last field, and the comment that makes mention of the "last" field can be updated.  We really only need one byte to represent crossing information of each card in the remembered set.
> 
> I suggest integrating this as is, and doing further refactoring as a future TODO.

Hmm, we are definitely using the `last` field in the `cross_map` struct. I'd have to study the code to understand how we could remove it.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/275#discussion_r1189117348