[9] RFR (S): 8153540: C2 intrinsic for Unsafe.allocateInstance doesn't properly filter out array classes

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8153540/webrev.00/hotspot/
http://cr.openjdk.java.net/~vlivanov/8153540/webrev.00/jdk/

https://bugs.openjdk.java.net/browse/JDK-8153540

Unsafe.allocateInstance intrinsic can instantiate arrays, but the 
allocation logic is broken.

The proposed fix is to perform necessary checks in Java code before 
calling the intrinsic.

I did some performance measurements [1] and reflection (non-constant 
class) case (non-constant class) regressed ~5-10% due to new guards added.

I also experimented with a hotspot-only fix [2], but it looks uglier. 
So, if you consider the regression in reflective case non-critical, I'd 
prefer to go with JDK checks.

Testing: regression test, JPRT, RBT (pit-hs-comp; in progress), 
microbenchmarks.

Thanks!

Best regards,
Vladimir Ivanov

[1] http://cr.openjdk.java.net/~vlivanov/8153540/AllocInstance.java

Baseline:
   AllocInstance.testConstant    avgt   25  3.736 ± 0.054  ns/op
   AllocInstance.testReflective  avgt   25  5.880 ± 0.080  ns/op

JDK fix:
   AllocInstance.testConstant    avgt   25  3.959 ± 0.205  ns/op
   AllocInstance.testReflective  avgt   25  6.274 ± 0.180  ns/op

[2] http://cr.openjdk.java.net/~vlivanov/8153540/webrev.slow_path

   AllocInstance.testConstant avgt 25 3.957 ± 0.159 ns/op
   AllocInstance.testReflective avgt 25 5.901 ± 0.057 ns/op