RFR: 8139457: Relax alignment of array elements [v59]

[Roman Kennke]

On Mon, 25 Sep 2023 13:23:48 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> 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.

> I know @albertnetymk already touched on this but some thoughts on the unclear boundaries between the header and the data. My feeling is that the most pragmatic solution would be to have the header initialization always initialize up to the word aligned (up) `header_size_in_bytes`. (Similarly to how it is done for the `instanceOop` where the klass gap gets initialized with the header, even if it may be data.) And have the body initialization do the rest (word aligned to word aligned clear).

Like what I did for x86 in latest commit? If you agree that should be the way to go, then I'll do the same for aarch64.

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PR Comment: https://git.openjdk.org/jdk/pull/11044#issuecomment-1959283824