RFR: 8139457: Array bases are aligned at HeapWord granularity [v61]

[Jorn Vernee]

On Tue, 24 Oct 2023 10:35:29 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 90 commits:
> 
>  - 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
>  - Merge remote-tracking branch 'upstream/master' into JDK-8139457
>  - Fix comments
>  - Fix inconsistencies in argument naming C1_MacroAssembler::allocate_array()
>  - ... and 80 more: https://git.openjdk.org/jdk/compare/9bfa0829...7eaca124

Just to check: object headers of arrays are still aligned to 8-bytes, and only the elements alignment of e.g. a `byte[]` is 4? If that's the case, than someone can always get back to 8-byte alignment simply by adding 4 bytes of offset when doing the access. Probably not great to rely on that though.

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PR Comment: https://git.openjdk.org/jdk/pull/11044#issuecomment-1782650313