RFR: 8139457: Array bases are aligned at HeapWord granularity [v29]

[Thomas Stuefe]

On Thu, 2 Mar 2023 16:34:05 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:
> 
>   Rename payload_start -> payload_offset

Note: if you rebase this to current upstream, you may need to fix/switch off test/hotspot/jtreg/compiler/c2/irTests/TestVectorizationMismatchedAccess.java since it relies on array members for byte/long starting at the same alignment. There may be more vectorization tests like this.

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PR Comment: https://git.openjdk.org/jdk/pull/11044#issuecomment-1531013846