RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v42]

[Roman Kennke]

On Tue, 15 Oct 2024 22:09:54 GMT, Volodymyr Paprotski <duke at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix aarch64.ad
>
> src/hotspot/cpu/x86/c2_stubGenerator_x86_64_string.cpp line 1659:
> 
>> 1657:   Label L_moreThan8, L_moreThan16, L_moreThan24, L_adjustHaystack;
>> 1658: 
>> 1659:   assert(arrayOopDesc::base_offset_in_bytes(isU ? T_CHAR : T_BYTE) >= 8,
> 
> If we had to also optimize for header-size 16, it might be possible to remove one jump here. Looks correct for either size.

Yeah. The old code optimized for header-size >= 16. But given that compact headers will soon become the default, I don't think it's worth optimizing for the old header layout.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1802665723