Java compare and swap semantics and performance

What is the semantics of compare and swap in Java? Namely, does the compare and swap method of an AtomicInteger just guarantee ordered access between different threads to the particular memory location of the atomic integer instance, or does it guarantee ordered access to all the locations in memory, i.e. it acts as if it were a volatile (a memory fence).

From the docs:

• weakCompareAndSet atomically reads and conditionally writes a variable but does not create any happens-before orderings, so provides no guarantees with respect to previous or subsequent reads and writes of any variables other than the target of the weakCompareAndSet.
• compareAndSet and all other read-and-update operations such as getAndIncrement have the memory effects of both reading and writing volatile variables.

It's apparent from the API documentation that compareAndSet acts as if it were a volatile variable. However, weakCompareAndSet is supposed to just change its specific memory location. Thus, if that memory location is exclusive to the cache of a single processor, weakCompareAndSet is supposed to be much faster than the regular compareAndSet.

I'm asking this because I've benchmarked the following methods by running threadnum different threads, varying threadnum from 1 to 8, and having totalwork=1e9 (the code is written in Scala, a statically compiled JVM language, but both its meaning and bytecode translation are isomorphic to that of Java in this case - this short snippets should be clear):

val atomic_cnt = new AtomicInteger(0)
val atomic_tlocal_cnt = new java.lang.ThreadLocal[AtomicInteger] {
  override def initialValue