RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v9]

[Coleen Phillimore]

On Wed, 18 Sep 2024 12:54:34 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> src/hotspot/share/oops/markWord.inline.hpp line 90:
>> 
>>> 88:   ShouldNotReachHere();
>>> 89:   return markWord();
>>> 90: #endif
>> 
>> Is the ifdef _LP64 necessary, since UseCompactObjectHeaders should always be false for 32 bits?
>
> Kindof. The problem is that klass_shift is larger than 31, and shifting with it would thus be UB and generate a compiler warning. I opted to simply not compile any of that code in 32bit builds. We could also define klass_shift differently on 32bit.
> Long-term (maybe with Lilliput2/4-byte-headers?) it would be nice to consolidate the header layout between 32 and 64 bit builds and not make any distinction anywhere. E.g. define markWord (or objectHeader?) in a single way, and use that to extract all the relevant stuff. It's not totally unlikely that we deprecate 32-bit builds before that can happen, though.

Ok.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1769069007