RFR (S): CR 8004318/JEP 171 Fences intrinsics

[Doug Lea]

Footnote: A minor difference between the implementation and
the sketch I wrote up in JEP171 is that, while writing JEP171,
I hadn't completely checked whether just MemBarVolatile
without a MemBarAcquire suffices for c2 internals dealing
with loads. But I now, have, and it does. For the only
interesting case, see Block::schedule_local (in lcm.cpp)]
that does the right thing here by checking for either
kind of fence. So inline_unsafe_fence need not generate
MemBarAcquire for fullFence.


-Doug