RFR: 8358568: C2 compilation hits "must have a monitor" assert with -XX:-GenerateSynchronizationCode

[Dean Long]

On Thu, 3 Jul 2025 01:59:55 GMT, hanguanqiang <duke at openjdk.org> wrote:

> This PR fixes JDK-8358568, a JVM crash triggered when running with -XX:-GenerateSynchronizationCode
> 
> Problem：
> When synchronization code generation is disabled by -XX:-GenerateSynchronizationCode, the compiler’s do_monitor_exit() method still tries to access monitor objects without checking if any monitors exist.This causes an assertion failure and JVM crash.
> 
> Root Cause：
> Parse::do_monitor_exit() calls shared_unlock() using monitor info unconditionally,but with GenerateSynchronizationCode disabled, no monitor info is available, leading to invalid access.
> 
> Fix
> Add a check in do_monitor_exit() to skip monitor unlocking if GenerateSynchronizationCode is false, avoiding invalid monitor access and preventing the crash.

I don't see the point of trying to support this flag.  Can we just get rid of it?  I don't think it is ever tested, because testing would surely crash unless the JVM ran as single-threaded somehow, which it doesn't.  Maybe at some point this flag was useful for getting a new port limping along, but I think stubbing sync code would work just as well.

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

PR Comment: https://git.openjdk.org/jdk/pull/26108#issuecomment-3030292214