missing memory barrier in acmp with C2

[Roland Westrelin]

> Yes, I get that, in this particular case, it's OK.  No argument.  But
> the additional cost of loadload|loadstore is close to zero (and may
> actually be zero) on many architectures.  Except in some extraordinary
> cases we don't need to apply such finicky reasoning.  And if we do, we
> may get it wrong, and we only have to get it wrong once to suffer some
> major pain.  So let's not go there.

What about the memory barrier in the shenandoah write barrier:

  Address evacuation_in_progress = Address(rthread, in_bytes(JavaThread::evacuation_in_progress_offset()));
  ldrb(rscratch1, evacuation_in_progress);
  membar(Assembler::LoadLoad);

  // The read-barrier.
  ldr(dst, Address(dst, BrooksPointer::byte_offset()));

Should it be loadload|loadstore?

The reason I'm asking, is that to expand the barrier as c2 IR, I had to
add support for a loadload memory barrier in the IR. If MemBarAcquire
can be used instead then I can remove all the shared changes for the
loadload memory barrier from my patch.

Roland.