RFR: JDK-8305711: Arm: C2 always enters slowpath for monitorexit

[Thomas Stuefe]

A small bug in the C2 implementation of monitorexit for thin locks causes us to always enter the slow path.

This seems to be a day zero bug of the arm port, since JEP 297: "Unified arm32/arm64 Port". It has a significant effect on locking performance, but its effect had been hidden until JDK 15 by biased locking. Biased locking removal made the bug appearant.

With this patch, @rkennke's artificial microbenchmark that does nothing but uncontended locking improves greatly (see https://github.com/rkennke/fastlockbench):


Benchmark                      (backoff)  Mode  Cnt      Score   Error  Units
FastLockingBenchmark.testSync          0  avgt    2    110.600          ns/op
FastLockingBenchmark.testSync          1  avgt    2    105.725          ns/op
FastLockingBenchmark.testSync          2  avgt    2    122.780          ns/op
FastLockingBenchmark.testSync          4  avgt    2    125.133          ns/op
FastLockingBenchmark.testSync          8  avgt    2    151.915          ns/op
FastLockingBenchmark.testSync         16  avgt    2    206.458          ns/op
FastLockingBenchmark.testSync         32  avgt    2    313.980          ns/op
FastLockingBenchmark.testSync         64  avgt    2    522.206          ns/op


New:

Benchmark                      (backoff)  Mode  Cnt      Score   Error  Units
FastLockingBenchmark.testSync          0  avgt    2     60.102          ns/op
FastLockingBenchmark.testSync          1  avgt    2     61.667          ns/op
FastLockingBenchmark.testSync          2  avgt    2     74.950          ns/op
FastLockingBenchmark.testSync          4  avgt    2     85.480          ns/op
FastLockingBenchmark.testSync          8  avgt    2    115.019          ns/op
FastLockingBenchmark.testSync         16  avgt    2    178.046          ns/op
FastLockingBenchmark.testSync         32  avgt    2    273.376          ns/op
FastLockingBenchmark.testSync         64  avgt    2    500.287          ns/op


Please note that Arm remains broken since JDK-8301995; I based and tested this patch on the parent of that change.

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

Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/13376/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13376&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305711
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13376.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13376/head:pull/13376

PR: https://git.openjdk.org/jdk/pull/13376