RFR: 8322743: assert(held_monitor_count() == jni_monitor_count()) failed

[Vladimir Ivanov]

On Wed, 17 Jan 2024 20:16:49 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Corner case with a local (not escaped) object used for synchronization. C2 Escape Analysis thinks that it can eliminate locks for it. In most cases it is true but not in this case.
>> 
>> 
>>         for (int i = 0; i < 2; ++i) {
>>             Object o = new Object();
>>             synchronized (o) { // monitorenter
>>                 // Trigger OSR compilation
>>                 for (int j = 0; j < 100_000; ++j) {
>> 
>> The test has nested loop which trigger OSR compilation. The locked object comes from Interpreter into compiled OSR code. During parsing C2 creates an other non escaped object and correctly merge both together (with Phi node) so that non escaped object is not scalar replaceable. Because it does not globally escapes EA still removes locks for it and, as result, also for merged locked object from Interpreter which is the bug.
>> 
>> The fix is to check that synchronized block does not have any associated escaped objects when EA decides if locks can be eliminated.
>> 
>> Added regression test prepared by @TobiHartmann. Tested tier1-5, xcomp and stress.
>> Performance testing show no difference.
>
> Thank you, @TobiHartmann, for review. I addressed your comments.

@vnkozlov sorry, I still have a hard time reasoning about the correctness of the proposed fix.

It's not clear to me what "synchronized block does not have any associated escaped objects" means in practice and how it relates to the original problem. When does the situation with a single `BoxLock` shared between multiple `AbstractLock`s bug distinct `obj_node()` inputs occur? Does it only happen for matched `Lock`/`Unlock` node pairs?

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PR Comment: https://git.openjdk.org/jdk/pull/17331#issuecomment-1897554128