Experience with 4839641 has shown us that JVM_Clone uses word-at-a-time
copying. If an object contains volatile long or volatile double fields,
then this copying may produce a clone that isn't a valid copy of the
original. In particular, if another thread is modifying a volatile
long or double field while the clone is being made, the clone may see
one word of the field from before the modification and one word from
after the modification, resulting in a value that was never in the
field. That would violate the specification of Object.clone.