Currently, we have weird code generation in aarch64 Shenandoah barriers, e.g.:

  // Check for heap stability
  if (is_strong) {
    __ tbz(rscratch2, ShenandoahHeap::HAS_FORWARDED_BITPOS, heap_stable);
  } else {
    Label lrb;
    __ tbnz(rscratch2, ShenandoahHeap::WEAK_ROOTS_BITPOS, lrb);
    __ tbz(rscratch2, ShenandoahHeap::HAS_FORWARDED_BITPOS, heap_stable);
    __ bind(lrb);
  }

The double-test in the weak-path could be avoided by doing something like:
  __ andb(rscratch2, rscratch2, ShenandoahHeap::WEAK_ROOTS | ShenandoahHeap::HAS_FORWARDED);
  __ cbz(done);

However, the two constants together makes 17, and this can not be encoded as immediate in aarch64. Re-ordering the GC state enum such that the two constants are adjacent would likely solve this.