RFR: 8324969: assert(false) failed: Non-balanced monitor enter/exit! [v4]

[Vladimir Kozlov]

On Wed, 14 Feb 2024 00:49:19 GMT, Dean Long <dlong at openjdk.org> wrote:

>>> Doesn't this end up marking even the simplest lock coarsening cases as unbalanced?
>> 
>> Yes, the simplest lock coarsening makes locking regions as unbalanced.
>> See second code example in PR's description. Coarsening eliminates Unlock in regions 1 and Lock in region 2. Both regions become unbalanced.
>> 
>>> Is there a reason the above check can't be done as part of coarsened_locks_consistent()?
>> 
>> `Unbalanced` in coarsened_locks_consistent() is different from `Unbalanced` in `mark_coarsened_boxes()`
>> Or do you mean to recompile without lock coarsening when regions are unbalanced? But that will disable all coarsening, even valid one (for example, when no outer locking in my examples).
>
>> > Is there a reason the above check can't be done as part of coarsened_locks_consistent()?
>> 
>> `Unbalanced` in coarsened_locks_consistent() is different from `Unbalanced` in `mark_coarsened_boxes()` Or do you mean to recompile without lock coarsening when regions are unbalanced? But that will disable all coarsening, even valid one (for example, when no outer locking in my examples).
> 
> If we are simply making sure that all nodes in a group have the same BoxLock, then that check could be done earlier, in coarsened_locks_consistent(), or even as the nodes are added to the list.  But I guess it's possible to get a premature answer is nodes are removed from the list.

I don't want to mix these two methods. They do different things:
 - `coarsened_locks_consistent()` checks that graph is correct otherwise bailout compilation
 - `mark_coarsened_boxes()` marks BoxLock nodes to prevent further locks optimizations

It could be done but it will complicate code in `coarsened_locks_consistent()` and you would still need 2 iterations:
 - first, you need to check that all `alock` on `locks_list` are marked as `coarsened` and also set flag if they reference different Boxes
 - only then you do second round over them to mark all referenced Box as unbalanced

And if we bailout from compilation Boxes marking will be useless work.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17697#discussion_r1488804952