RFR: 8319797: Recursive lightweight locking: Runtime implementation [v9]

[Daniel D. Daugherty]

On Mon, 4 Dec 2023 12:35:56 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Implements the runtime part of JDK-8319796.
>> The different CPU implementations are/will be created as dependent pull requests.
>> 
>> This enhancement proposes introducing the ability for LM_LIGHTWEIGHT to handle consecutive recursive monitor enter. Limiting the implementation to only consecutive monitor enters allows for more efficient emitted code which only needs to look at the two top most entires on the lock stack to determine what to do in a monitor exit.
>> 
>> A high level overview:
>>   * Locking is still performed on the mark word
>>     * Unlocked (0b01) <=> Locked (0b00)
>>   * Monitor enter on Obj with mark word Unlocked (0b01) is the same
>>     * Transition Obj's mark word Unlocked (0b01) => Locked (0b00)
>>     * Push Obj onto the lock stack
>>     * Success
>>   * Monitor enter on Obj with mark word Locked (0b00) will check the top entry on the lock stack
>>     * If top entry is Obj
>>       * Push Obj on the lock stack
>>       * Success
>>     * If top entry is not Obj
>>       * Inflate and call ObjectMonitor::enter
>>   * Monitor exit on Obj with mark word Locked (0b00) will check the two top entries on the lock stack
>>     * If just the top entry is Obj
>>       * Transition Obj's mark word Locked (0b00) => Unlocked (0b01)
>>       * Pop the entry
>>       * Success
>>     * If both entries are Obj
>>       * Pop the top entry
>>       * Success
>>     * Any other case only occurs for unstructured locking, then just inflate and call ObjectMonitor::exit
>>   * If the monitor has been inflated for object Obj which is owned by the current thread
>>     * All corresponding entries for Obj is removed from the lock stack
>>     * The monitor recursions is set to the number of removed entries - 1
>>     * The owner is changed from anonymous to the thread
>>     * The regular ObjectMonitor::action is called.
>
> Axel Boldt-Christmas has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:
> 
>  - Merge remote-tracking branch 'upstream_jdk/pr/16603' into JDK-8319797
>  - Avoid copy from and to the same location
>  - Fix typo
>  - Update unstructured unlock comment
>  - Fix bad indent after merge
>  - Remove whitespace
>  - Merge remote-tracking branch 'upstream_jdk/pr/16603' into JDK-8319797
>  - Merge remote-tracking branch 'upstream_jdk/pr/16603' into JDK-8319797
>  - Merge remote-tracking branch 'upstream_jdk/pr/16603' into JDK-8319797
>  - Fix nit
>  - ... and 2 more: https://git.openjdk.org/jdk/compare/1b907f90...56b04f58

I re-reviewed the v08/2023.12.04 version.

The last merge was on 2023.12.04 so I'll review again after this PR is merged
with newer jdk/jdk baseline bits.

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

Marked as reviewed by dcubed (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16606#pullrequestreview-1812132931
PR Comment: https://git.openjdk.org/jdk/pull/16606#issuecomment-1883908327