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

[Roman Kennke]

It looks good to me, thanks!

Roman


> 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
>