[10] RFR(M): 8176506: C2: loop unswitching and unsafe accesses cause crash

[Roland Westrelin]

> A lot of people need off-heap ByteBuffers to be really fast.  This is
> important for Lucene, big data, etc.  So we must be careful.

For off heap accesses, with this change, I expect we should be back to
where we were before https://bugs.openjdk.java.net/browse/JDK-8176513
was fixed. From that thread:

http://mail.openjdk.java.net/pipermail/hotspot-dev/2017-March/026169.html

as I understand, the lucene folks were ok with the level of
optimizations before JDK-8176513 was fixed.

Actually, profiling would tell us if an access is always on heap, always
off heap or mixed and I suppose we could drop membars and be less
conservative for always off heap accesses. Mixed accesses are the hard
ones.

Roland.