RFR (S): 8198909: [Graal] compiler/codecache/stress/UnexpectedDeoptimizationTest.java crashed with SIGSEGV

[Tom Rodriguez]

The JVMCI API may read Klass* and java.lang.Class instances from 
locations which G1 would consider to be weakly referenced.  This can 
result in HotSpotResolvedObjectTypeImpl instances with references to 
Classes that have been unloaded.  In the this crash, JVMCI was reading a 
Klass* from the profile in an MDO and building a wrapper around it.  The 
MDO reference is weak and was the only remaining reference to the type 
so it could be dropped resulting in an eventual crash.

I've added an explicit G1 enqueue before we call out to create the 
wrapper object but is there a more recommended way of doing this?  Dean 
had pointed out the oddly named InstanceKlass::holder_phantom which is 
used by the CI.  Should I be using that?  The G1 barrier is only really 
need when reading from non-Java heap memory but since the get_jvmci_type 
method is the main entry point for this logic it safest to always 
perform it in this path.

https://bugs.openjdk.java.net/browse/JDK-8198909
http://cr.openjdk.java.net/~never/8198909/webrev