[9] RFR (S): 8141420: Compiler runtime entries don't hold Klass* from being GCed

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8141420/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8141420

Though compiler runtime entries use raw Klass*, they don't ensure the 
classes can't be unloaded. It causes rare crashes when Full GC and class 
unloading happens when freshly loaded class is being constructed and the 
only live reference to it is the Klass* passed into the runtime call.

There are KlassHandles/instanceKlassHandles, but they don't do anything 
after PermGen was removed.

The fix is to add mirror handles to keep classes alive across safepoints 
during the runtime calls. FTR handles aren't needed for primitive 
arrays.

I chose the conservative fix, since I plan to backport it into 8u. Filed 
JDK-8141420 [1] to refactor the code to use mirrors instead. It should 
simplify the logic to track class liveness.

No regression test provided, since I wasn't able to write one w/o 
instrumenting the JVM.

Testing: manual (instrumented build which triggers class unloading from 
runtime entries), JPRT.

Thanks!

Best regards,
Vladimir Ivanov

[1] https://bugs.openjdk.java.net/browse/JDK-8141420