RFR(M) JDK-8014555: G1: Memory ordering problem with Conc refinement and card marking

[Roland Westrelin]

> I'd like someone from the compiler (and runtime? the interpreter uses macroAssembler_*, right?) teams to at least look at the changes to:
> macroAssembler_*.cpp
> c1_Runtime_*.cpp
> graphKit.cpp

That looks good to me.
Maybe a comment that says you're using Op_MemBarVolatile because you need a StoreLoad.

Roland.