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

[Thomas Stuefe]

On Thu, 12 Sep 2024 10:17:47 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/lcm.cpp line 272:
> 
>> 270:         const TypePtr* tptr;
>> 271:         if ((UseCompressedOops || UseCompressedClassPointers) &&
>> 272:             (CompressedOops::shift() == 0 || CompressedKlassPointers::shift() == 0)) {
> 
> Could you explain this change? It seems like it may affect C2's implicit null check analysis even for `-XX:-UseCompactObjectHeaders`. In particular, for the following configurations, the changed condition evaluates to true before the change and false after it, regardless of whether `UseCompactObjectHeaders` is enabled:
> 
> (!UseCompressedOops,  UseCompressedClassPointers, CompressedKlassPointers::shift() != 0)
> ( UseCompressedOops, !UseCompressedClassPointers, CompressedOops::shift() != 0)

Hi @robcasloz 

The `CompressedKlassPointers` utility class is not usable anymore with `-UseCompressedClassPointers`. One change is that if `UseCompressedClassPointers` is off, `CompressedKlassPointers` stays uninitialized. And that makes more sense then to rely on the static initialization values of `CompressedOops::_shift`.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1757126946