RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v19]

[Emanuel Peter]

On Mon, 18 Nov 2024 14:35:41 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>>> @rkennke Ok, fair enough. As far as I know, we at Oracle do not super care about strict alignment `AlignVector`. But maybe other people care, and have to make that tradeoff between vectorization and small object headers.
>> 
>> BTW, this problem is not specific to UseCompactObjectHeaders - I think the same problem would happen with -UseCompressedClassPointers. With uncompressed class-pointers, byte[] would start at offset 20, while long[] start at offset 24. But nobody cares about -UCCP I think.
>> 
>> What is the failure mode, though? When running with -UCOH and +AlignVector, would it crash or misbehave? Or would it (silently?) not vectorize? I think we could live with the latter, but not with the former.
>
>> @rkennke It just will (silently) not vectorize, thus running slower but still correct.
> 
> Ok, I think we can live with that for now.
> 
> As said elsewhere, we are currently working on 4-byte-headers, which would make that problem go away.
> 
> The tests need fixing, though.

@rkennke
> As said elsewhere, we are currently working on 4-byte-headers, which would make that problem go away.

Ah. So we would eventually have not a `12-byte` but `8-byte` offset from base to payload? Would that happen in all cases? And could that happen before `UseCompactObjectHeaders` leaves experimental status? Because it is going to be a little annoying to adjust all vectorization tests for the special case of `UseCompactObjectHeaders + AlignVector`. Though I can surely do it.

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PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2483242899