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

[Roman Kennke]

On Thu, 26 Sep 2024 15:59:50 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> Ok, this is indeed relevant and helpful. This could segfault if we happen to read from the very first object on the heap. I can solve this by allowing to copy only 8 bytes onto the stack: https://github.com/rkennke/jdk/commit/097c2afa04397773e514552dfb942aa889bfa2c1
>> 
>> Does this look correct to you? Or better to do it as a follow-up?
>> (It passes a couple of indexOf tests, will run tier1-4 on it).
>
>> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1777396409