RFR: 8324969: C2: prevent elimination of unbalanced coarsened locking regions [v7]

[Vladimir Kozlov]

On Mon, 26 Feb 2024 20:00:46 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Could the box be `nested` at this point? If so, the assert inside `set_local` could trigger, right?
>
>> Could the box be nested at this point? If so, the assert inside set_local could trigger, right?
> 
> No, it can't be nested.
> Check for nesting and its elimination is done in one place from which it does not escape [macro.cpp#L2047](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/macro.cpp#L2047)
> (Nested state is set only when `is_nested_lock_region()` returns true in the check).
> And it will be overwritten immediately by `box_node->set_eliminated()`.

Also BoxLock can't be Local when we check for nesting because of `(!alock->is_non_esc_obj())` check above.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17697#discussion_r1503253522