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

[Roman Kennke]

On Fri, 10 Nov 2023 12:18:29 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.

This is great! I've done something like that in my original LW-locking PR, but then ripped it out to keep it simpler. Never got around to re-do it. Changes look good to me!

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Marked as reviewed by rkennke (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16603#pullrequestreview-1724683360