RFR: 8139457: Array bases are aligned at HeapWord granularity [v31]

[Stefan Karlsson]

On Sun, 7 May 2023 18:14:13 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 incrementally with one additional commit since the last revision:
> 
>   Simplify aarch64 code

Hi Roman, right now I need to focus on getting Generational ZGC upstreamed. I hope others can help out and look at these changes.

I'd also like ask you to wait with the integration of this patch until after we have integrated Generational ZGC. I know that there were some array chunking changes that will conflict with our code. If all goes well we'll integrate the Generational ZGC code in a few days.

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PR Comment: https://git.openjdk.org/jdk/pull/11044#issuecomment-1538920362