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

[Axel Boldt-Christmas]

On Mon, 13 Nov 2023 00:51:26 GMT, David Holmes <dholmes 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.
>
> src/hotspot/share/runtime/synchronizer.cpp line 582:
> 
>> 580:           // It can only have installed an anonymously locked monitor at this point.
>> 581:           // Fetch that monitor, set owner correctly to this thread, and
>> 582:           // exit it (allowing waiting threads to enter).
> 
> I don't understand why the anonymous owner case is no longer being checked. ??

The condition does now check for a successful CAS, not the unsuccessful one. If it was successful then there is no monitor, thus no anonymous owner.

If the CAS failed and the mark word is no longer fast locked. It must be inflated. So we fallthrough down to the inflated case.

`ObjectSynchronizer::inflate` correctly handles fixing the owner.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16603#discussion_r1390700789