RFR: 8319773: Avoid inflating monitors when installing hash codes for LM_LIGHTWEIGHT [v3]

[Coleen Phillimore]

On Tue, 14 Nov 2023 13:34:49 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 three additional commits since the last revision:
> 
>  - Merge remote-tracking branch 'upstream_jdk/pr/16602' into JDK-8319773
>  - Simplify test.
>  - 8319773: Avoid inflating monitors when installing hash codes for LM_LIGHTWEIGHT

src/hotspot/share/runtime/synchronizer.cpp line 966:

> 964:       // Fall thru so we only have one place that installs the hash in
> 965:       // the ObjectMonitor.
> 966:     } else if (LockingMode == LM_LIGHTWEIGHT && mark.is_fast_locked() && is_lock_owned(current, obj)) {

You can delete the is_lock_owned() function just above this with this patch.

Edit: nope, never mind, I just found another caller. edit2: but you could move it down to before 'inflate'

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PR Review Comment: https://git.openjdk.org/jdk/pull/16603#discussion_r1394514252