RFR #2 (M) 8148146: Integrate new internal Unsafe entry points, and basic intrinsic support for VarHandles

[Andrew Haley]

I'm writing the AArch64 intrinsics for the weak entry points, and
the generated code for volatile compare&swap looks wrong to me.

I'm seeing this:

 	membar_release
 	cmpxchg
 	membar_acquire

... but this is not enough for volatile.  It is enough for
AcquireRelease, but for Volatile It needs a trailing membar_volatile
to make the store visible to observers in other threads.  Looking at
LibraryCallKit::inline_unsafe_access, it seems to me that the same
code is generated for both Release and Volatile:

  switch (access_kind) {
    case Relaxed:
    case Release:
      break; // do nothing
    case Acquire:
    case Volatile:
      insert_mem_bar(Op_MemBarVolatile);
      // !support_IRIW_for_not_multiple_copy_atomic_cpu handled in platform code
      break;
    default:
      ShouldNotReachHere();
  }

But it really shouldn't be: a volatile store should be followed by
MemBarVolatile, like so:

  switch (access_kind) {
    case Relaxed:
    case Release:
      break; // do nothing
    case Acquire:
      insert_mem_bar(Op_MemBarAcquire);
      break;
    case Volatile:
      insert_mem_bar(Op_MemBarVolatile);
      break;
    default:
      ShouldNotReachHere();
  }

... and this does indeed generate the correct code.

There is a problem, though: this is going to generate suboptimal code
for x86:

       lock cmpxchg
       lock addl $0x0,-0x40(%rsp)

So I suppose we're going to have to kludge around this in the AArch64
back end.

Andrew.