RFR: 8341091: CDS: Segmented roots array misses roots [v2]

[Andrew Dinn]

On Wed, 2 Oct 2024 16:05:10 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> There are a few bugs in [JDK-8338912](https://bugs.openjdk.org/browse/JDK-8338912) that we discovered in Leyden that uses multiple segments: a) root_index in ArchiveHeapWriter::copy_roots_to_buffer is initialized to zero for every segment, so all segments have the same set of roots; b) HeapShared::get_root and friends compute segment and internal indices modulo _byte_ size, while they should instead do it modulo _element_ size.
>> 
>> This does not affect current mainline directly, since we run with a single segment the overwhelming majority of the time.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `runtime/cds`
>>  - [x] Linux x86_64 server fastdebug, `runtime/cds` + `-XX:-UseCompressedOops`
>>  - [x] Transplanted the patch to Leyden repo and verified it passes `runtime/cds`
>>  - [x]  Linux x86_64 server fastdebug, `all`
>
> Aleksey Shipilev 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 three additional commits since the last revision:
> 
>  - Tighthen the comment
>  - Merge branch 'master' into JDK-8341091-cds-segmented-roots-bugs
>  - Fix

Good!

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Marked as reviewed by adinn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21221#pullrequestreview-2343412652