RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v8]

[Roman Kennke]

On Tue, 10 Sep 2024 09:28:41 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> With compact headers, this value should only be used in C2, and not really as an actual offset. An earlier version of the change had the value in src/hotspot/share/opto/type.hpp instead, and only an assert(!UCOH) in oopDesc::klass_offset_in_bytes(). I think this would be a better solution overall, because it prevents accidental (and wrong) usage of the klass_offset in the runtime. Back then it has been rejected by somebody (don't remember), because it made the C2 diff a little messier, so I kept it like it is now. I would prefer to reinstate it, though.
>
>> (Fwiw, the method is also used during Universe initialization).
> 
> Yes, but only in the -UCOH branch.

We will deal with it in a follow-up. https://bugs.openjdk.org/browse/JDK-8340453

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1791605009