RFR: 8139457: Relax alignment of array elements [v69]

[Galder Zamarreño]

On Thu, 22 Feb 2024 16:08:33 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:
> 
>   Improve comment

src/hotspot/cpu/aarch64/c1_MacroAssembler_aarch64.hpp line 100:

> 98:   // len        : array length in number of elements
> 99:   // t          : scratch register - contents destroyed
> 100:   // header_size: size of object header in words

@rkennke Should this be updated to `base_offset_in_bytes` too?

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PR Review Comment: https://git.openjdk.org/jdk/pull/11044#discussion_r1510812835