RFR: 8139457: Relax alignment of array elements [v64]

[Roman Kennke]

On Thu, 15 Feb 2024 11:46:39 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> See [JDK-8139457](https://bugs.openjdk.org/browse/JDK-8139457) for details.
>> 
>> Basically, when running with -XX:-UseCompressedClassPointers, arrays will have a gap between the length field and the first array element, because array elements will only start at word-aligned offsets. This is not necessary for smaller-than-word elements.
>> 
>> Also, while it is not very important now, it will become very important with Lilliput, which eliminates the Klass field and would always put the length field at offset 8, and leave a gap between offset 12 and 16.
>> 
>> Testing:
>>  - [x] runtime/FieldLayout/ArrayBaseOffsets.java (x86_64, x86_32, aarch64, arm, riscv, s390)
>>  - [x] bootcycle (x86_64, x86_32, aarch64, arm, riscv, s390)
>>  - [x] tier1 (x86_64, x86_32, aarch64, riscv)
>>  - [x] tier2 (x86_64, aarch64, riscv)
>>  - [x] tier3 (x86_64, riscv)
>
> Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 93 commits:
> 
>  - Merge branch 'master' into JDK-8139457
>  - Merge remote-tracking branch 'upstream/master' into JDK-8139457
>  - Update copyright headers
>  - Merge branch 'master' into JDK-8139457
>  - Fix ARM build
>  - Merge remote-tracking branch 'upstream/master' into JDK-8139457
>  - Various cleanups
>  - RISC changes
>  - Move gap init into allocate_header() (x86)
>  - Fix gtest failure on x86
>  - ... and 83 more: https://git.openjdk.org/jdk/compare/a0e5e16a...d0ec1fa2

FWIW, I just ran tier1 and tier2 with this change and -CCP with no regressions. tier2 fails a number of tests with and without the change because it can not handle -CCP, but that is not specific to this change.

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PR Comment: https://git.openjdk.org/jdk/pull/11044#issuecomment-1947382603