RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v26]

[Scott Gibbons]

On Thu, 26 Sep 2024 16:15:39 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>>> Does this look correct to you? Or better to do it as a follow-up?
>> 
>> I do not feel confident enough to review this part. If you want to include https://github.com/rkennke/jdk/commit/097c2afa04397773e514552dfb942aa889bfa2c1 in this changeset, I would prefer that the original author of JDK-8320448 or at least someone from Intel reviews it, otherwise I think it is fine to leave it as a follow-up enhancement.
>
> @sviswa7 or @asgibbons WDYT about including https://github.com/rkennke/jdk/commit/097c2afa04397773e514552dfb942aa889bfa2c1 as part of compact object headers implementation? Otherwise we would have to disable indexOf intrinsic when running with compact headers, because of the assumption that array headers are >= 16 bytes, which is no longer true with compact headers.

@rkennke I reviewed [rkennke@ 097c2af](https://github.com/rkennke/jdk/commit/097c2afa04397773e514552dfb942aa889bfa2c1) and the code looks good to me.  I would prefer this approach instead of not generating the IndexOf intrinsic.

Should the controlling `if` be conditioned on `UseCompactObjectHeaders` instead of `arrayOopDesc::base_offset_in_bytes`?  I can see benefits to either - which provides more clarity?  I like the assert as it makes the intention clear (thanks!).

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1777485078