RFR: 8324969: C2: prevent elimination of unbalanced coarsened locking regions [v5]

[Vladimir Kozlov]

On Wed, 14 Feb 2024 19:38:01 GMT, Dean Long <dlong at openjdk.org> wrote:

>> Vladimir Kozlov has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Additional changes
>
> My concern is that a group of unbalanced locks is added to coarsened_locks (they have different BoxLockNodes), but later some of those locks are removed by `remove_useless_coarsened_locks` or `remove_coarsened_lock`.  Then when `mark_unbalanced_boxes` finally runs, it incorrectly sees the group as balanced.
> 
> It is easy to check for unbalanced in `add_coarsened_locks` as the locks are added.  I think this is the most conservative approach.

@dean-long, I added verification code as I described in previous comment. Please look.
I am running tier1-5,xcopm,stress testing with it. So far did not hit new asserts.

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PR Comment: https://git.openjdk.org/jdk/pull/17697#issuecomment-1945407681