RFR: 8233061: ZGC: Enforce memory ordering in segmented bit maps

[Erik Österlund]

Hi,

In ZGC, bitmaps are lazily cleared in a segmented fashion. In this 
scheme, liveness is determined by looking at a counter, a segment bit 
map and finally the flat bit map structure. The accesses for the various 
stages need to be ordered properly. This patch sprinkles some 
OrderAccess calls to enforce this ordering.

Out of curiosity, I disassembled libjvm.so with and without this patch 
to see if the reordering has bitten us in practice on x86_64. 
Fortunately, according to my analysis, it has not; we seem to have been 
lucky. But there is a lot of machine code, so I could have missed 
something. However, given that we now have an AArch64 port which is 
definitely affected by this problem, and compilers really are free to do 
whatever they want to in the future, it seems in order to enforce this 
explicitly.

This patch depends on https://bugs.openjdk.java.net/browse/JDK-8233073 
which exposes some memory ordering aware getters on BitMap. I did not 
want to just wrap the existing API in ZGC, so I split that out to a 
separate RFE.

CR:
http://cr.openjdk.java.net/~eosterlund/8233061/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8233061

Thanks,
/Erik