RFR: 8139457: Array bases are aligned at HeapWord granularity

[Roman Kennke]

On Thu, 17 Nov 2022 13:37:56 GMT, Thomas Stuefe <stuefe 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)
>
> src/hotspot/share/oops/arrayOop.hpp line 77:
> 
>> 75:       return !UseCompressedOops;
>> 76:     }
>> 77: #endif
> 
> I'm confused why this is not needed today?

> 

Today this is only used by typeArrayOops, afaict.

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