[14] RFR 8230765: Implement nmethod barrier for x86_32 platforms

[Zhengyu Gu]

Hi all,

Please review this implementation of nmethod barrier for x86_32 platforms.

x86_32 implementation mirrors x86_64's. The only difference is where it 
reads nmethod disarmed value.

Unlike 64-bits, 32-bits platform does not have a dedicated register for 
current thread. So that it is cheaper to read disarmed value from global 
location than from per-thread GC data.

Currently, only Shenandoah GC uses the implementation for its concurrent 
class unloading. This implementation, along with Shenandoah concurrent 
class unloading, has been baked in shenandoah/jdk repo for some time 
now,  they are ready for integration.


Bug: https://bugs.openjdk.java.net/browse/JDK-8230765
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8230765/webrev.01/


Test:
   hotspot_gc with x86_64 and x86_32 JVM on Linux
   Submit test.

Thanks,

-Zhengyu