RFR: 8319773: Avoid inflating monitors when installing hash codes for LM_LIGHTWEIGHT [v12]

[Axel Boldt-Christmas]

On Wed, 10 Jan 2024 15:34:34 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> LM_LIGHTWEIGHT only uses the lock bits for its locking. This leaves the hashCode bits free when a monitor is not inflated. So instead of inflating when installing the hashCode on a fast locked object it can simply use the hashCode bits in the markWord.
>> 
>> The mark word transitions Unlocked (0b01) <=> Locked (0b00) are done by retrying the CAS if it fails due to non-lock bit changes. 
>> The mark word transitions Monitor (0b10) <=> Locked/Unlocked (0b0X) are the same as before, inflation already handles hash codes. This change does not interact with the mark word if it is in a Monitor (0b10) state, so the strong CAS which is used for deflation are still valid, and will not fail to any other reason than the cooperative race to help transition the mark word during deflation.
>> 
>> This is dependent on JDK-8319778 simply because JDK-8319797 is dependent on both this and JDK-8319778.
>
> Axel Boldt-Christmas has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 18 additional commits since the last revision:
> 
>  - Merge remote-tracking branch 'upstream_jdk/master' into JDK-8319773
>  - Merge remote-tracking branch 'upstream_jdk/master' into JDK-8319773
>  - Merge remote-tracking branch 'upstream_jdk/master' into JDK-8319773
>  - Merge remote-tracking branch 'upstream_jdk/pr/16602' into JDK-8319773
>  - Merge remote-tracking branch 'upstream_jdk/master' into JDK-8319778
>  - Fix copy paste typo.
>  - Update src/hotspot/share/opto/library_call.cpp
>    
>    Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>
>  - Add retry CAS comment
>  - Use is_neutral over is_unlocked
>  - Merge remote-tracking branch 'upstream_jdk/pr/16602' into JDK-8319773
>  - ... and 8 more: https://git.openjdk.org/jdk/compare/dfa488c1...a83ad377

Ran tier1-5 all Oracle platforms with both LM_LEGACY and LM_LIGHTWEIGHT after the merge. 
Will go ahead and integrate this. I'll create an RFE for the arm32 hashcode optimisation as well as for the re-inflate race.

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PR Comment: https://git.openjdk.org/jdk/pull/16603#issuecomment-1889370923