RFR: Increase card offset mask [v2]

[Y. Srinivas Ramakrishna]

On Wed, 10 May 2023 15:19:28 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> 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.
>
> You are right that we are still "maintaining" the last field and there are many references to it in the code.  I am thinking that with Ramki's changes to remembered set scanning, it is no longer used in scanning, but I may be remembering this incorrectly.
> 
> I agree this is not a subtle change and would require some "study" to make sure it is done properly, if at all.

@kdnilsen : `object_starts` uses the xmap info, which is still used in card scanning in the `block_start` method. The use of a traditional BOT was deferred to future work when I redid the RS scanning earlier this year. There's an open ticket for that work/investigation.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/275#discussion_r1190384053