RFR: 8139457: Array bases are aligned at HeapWord granularity [v61]

[Jorn Vernee]

On Tue, 24 Oct 2023 10:35:29 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 with a new target base due to a merge or a rebase. The pull request now contains 90 commits:
> 
>  - Merge branch 'master' into JDK-8139457
>  - Fix ARM build
>  - Merge remote-tracking branch 'upstream/master' into JDK-8139457
>  - Various cleanups
>  - RISC changes
>  - Move gap init into allocate_header() (x86)
>  - Fix gtest failure on x86
>  - Merge remote-tracking branch 'upstream/master' into JDK-8139457
>  - Fix comments
>  - Fix inconsistencies in argument naming C1_MacroAssembler::allocate_array()
>  - ... and 80 more: https://git.openjdk.org/jdk/compare/9bfa0829...7eaca124

Also, I think we can handle the issues with `ByteBuffer::alignedSlice`, `ByteBuffer::alignmentOffset`, `MethodHandles::byteArrayViewVarHandle` and `MethodHandles::byteBufferViewVarHandle` separately, ahead of this PR.

There is already a bug in the spec of these methods, as they try to give alignment guarantees for heap memory that the VM spec does not support.

I've filed a couple of JBS issues for these: https://bugs.openjdk.org/browse/JDK-8318967 & https://bugs.openjdk.org/browse/JDK-8318966

-------------

PR Comment: https://git.openjdk.org/jdk/pull/11044#issuecomment-1782681427