RFR (XS) 8161280 - assert failed: reference count underflow for symbol

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8161280
http://cr.openjdk.java.net/~iklam/jdk9/8161280-symbol-refcount-underflow.v01/

Summary:

The test was loading a lot of JCK classes into the same VM. Many of the 
JCK classes refer to "javasoft/sqe/javatest/Status", so the refcount (a 
signed short integer) of this Symbol would run up and past 0x7fff.

The assert was caused by a race condition: the refcount started with a 
large (16-bit) positive value such as 0x7fff, one thread is decrementing 
and several other threads are incrementing. The refcount will end up 
being 0x8000 or slightly higher (limited to the number of concurrent 
threads that are running within a small window of several instructions 
in the decrementing thread, so most likely it will be 0x800?).

As a result, the decrementing thread found that the refecount is 
negative after the operation, and thought that an underflow had happened.

The fix is to ignore any value that may appear in the [0x8000 - 0xbfff] 
range and do not flag these as underflows (since they are most likely 
overflows -- overflows are already handled by making the Symbol permanent).

Thanks
- Ioi