RFR: 8139457: Array bases are aligned at HeapWord granularity

[Roman Kennke]

On Tue, 8 Nov 2022 20:18:09 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)

> With my proposed fix, I don't think you need the following change made in file: src/hotspot/cpu/riscv/macroAssembler_riscv.cpp
> 
> Could you please remove this change from this PR? I am running some tests on my linux-riscv64 platform.

Ok, I reverted that part. Could you test that? Also, if you're running any of the tests menioned in the PR, can you let me know and I'll update the test matrix.

Thanks,
Roman

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PR: https://git.openjdk.org/jdk/pull/11044