RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v13]

[Thomas Stuefe]

On Wed, 11 Sep 2024 14:47:07 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Revert accidental change of UCOH default
>
> src/hotspot/share/opto/machnode.cpp line 390:
> 
>> 388:     t = t->make_ptr();
>> 389:   }
>> 390:   if (t->isa_narrowklass() && CompressedKlassPointers::shift() == 0) {
> 
> Does this change have any effect? `UseCompressedClassPointers` should be implied by `t->isa_narrowklass()`.

I don't remember if this change was a reaction to an error or if I just guarded `CompressedKlassPointers::shift()` with +UseCCP because that is the prerequisite now. Probably the latter. I can remove this. Probably should assert then for +UseCCP.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1757135035