RFR: Increase card offset mask [v2]

[Y. Srinivas Ramakrishna]

On Tue, 9 May 2023 20:43:26 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> On 32-bit platforms, a 512 byte card covers 128 words. On 64-bit platforms, a 1024 byte card (the maximum allowed) also covers 128 words. This change increases the size of the mask we use when encoding object start offsets to handle these situations.
>
> 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 401:

> 399:   // If we're setting first_start, assume the card has an object.
> 400:   inline void set_first_start(size_t card_index, uint8_t value) {
> 401:     assert(value < FirstStartBits, "Offset into card would be truncated");

`value <= FirstStartBits` ?

Can/should replace (or augment, if you prefer) with a static assert using the maximum offset computed using card size and oop size?

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/275#discussion_r1190389207