[jdk21u-dev] RFR: 8341715: PPC64: ObjectMonitor::_owner should be reset unconditionally in nmethod unlocking

[Richard Reingruber]

Backport of https://github.com/openjdk/jdk/commit/f9208fadde8141e18a025ddb6ce28423861ba391

The change removes instructions that check if an ObjectMonitor is owned by the current thread before unlocking it in a c1/c2 compiled nmethod.

The check is imprecise. It can fail if the lock was inflated into an ObjectMonitor by another thread.
It is also redundant since only bytecode with symmetric locking operations is compiled.
See JBS-Issue for details.

Note that there existed a special diagnostic locking mode to do the same check also on x86. Purpose was to protect against missbehaving JNI code. It got removed though with [JDK-8210381](https://bugs.openjdk.org/browse/JDK-8210381) in jdk12. On ppc64le it was not removed as the check was done in default mode as well (unclear why).

The fix didn't apply because in jdk21 there is still just one method to handle LM_LEGACY and LM_LIGHTWEIGHT and LM_LIGHTWEIGHT requires the owner check to handle ANONYMOUS owner. Also RTM is still supported in jdk21.

I'd consider the risk low. Aside from the assertions, the change is very small and it just brings ppc64 in line with how unlocking is done on other platforms.

The fix passed our CI testing: JTReg tests: tier1-4 of hotspot and jdk. All of Langtools and jaxp. JCK, SPECjvm2008, SPECjbb2015, Renaissance Suite, and SAP specific tests.
Testing was done with fastdebug builds on the main platforms and also on Linux/PPC64le and AIX.

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Commit messages:
 - Adapt for RTM and LM_LIGHTWEIGHT
 - 8341715: PPC64: ObjectMonitor::_owner should be reset unconditionally in nmethod unlocking

Changes: https://git.openjdk.org/jdk21u-dev/pull/1191/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1191&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341715
  Stats: 37 lines in 1 file changed: 31 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1191.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1191/head:pull/1191

PR: https://git.openjdk.org/jdk21u-dev/pull/1191