RFR: 8139457: Array bases are aligned at HeapWord granularity [v59]

[Albert Mingkun Yang]

On Mon, 25 Sep 2023 12:30:33 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 with a new target base due to a merge or a rebase. The pull request now contains 88 commits:
> 
>  - 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()
>  - Fix call to arrayOopDesc::header_size() in arm port
>  - Fix wrong alignment
>  - ... and 78 more: https://git.openjdk.org/jdk/compare/0f0c5b2d...8617a596

Much of my confusion is caused by the unclear definition of 'header/body' – specifically, whether the alignment gap is included as part of the header. It's a preexisting issue for instance objs, though.

Since some parts of this will be fixed in upcoming PRs, as indicated by `// TODO: This could perhaps go into initialize_body()...`, I believe it can be merged to move this forward.

A comprehensive explanation of the memory representation of instances and arrays, including the mark word, klass-pointer, optional alignment gap, and possible squeezed-into fields, would be greatly appreciated in future PRs in this area. That would help clarify the boundary between the header and body.

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PR Comment: https://git.openjdk.org/jdk/pull/11044#issuecomment-1733705839