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

[David Holmes]

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.

The patch seems reasonable from a backporting perspective. Though it does beg the question as to why `do_monitor_enter` does not need the same fix. I suspect this is a very old flag and the code has bit-rotted somewhat. A question for the compiler folk: does `GenerateSynchronizationCode` still have any use or should it be scrapped?

Thanks

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PR Review: https://git.openjdk.org/jdk/pull/26108#pullrequestreview-2981633438