RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v28]

[Roberto Castañeda Lozano]

On Thu, 12 Sep 2024 15:42:59 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> 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.

@tstuefe @rkennke what do you think about this suggestion? If there is a known case where `t->isa_narrowklass() && !UseCompressedClassPointers` holds, it should be investigated because it might be a symptom of a larger problem. If there is no such a case, I think the explicit `UseCompressedClassPointers` test should be removed to avoid confusion.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1778724120