[jmm-dev] Optimizing external actions in the JMM

[Nitsan Wakart]

Given:
"...
where f is some external action that the compiler understands.  If the
compiler knows `f` always returns 42 and has no other effect, can it
optimize ThreadA to
...
thereby introducing a OOTA-like value of 42 into the system?"
Why is this OOTA?
The thing is you define:
"external action that... always returns 42 and has no other effect"
Which according to:
"An external action is an action that may be observable outside of an execution,
and has a result based on an environment external to the execution."
(from https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.2)
is not an external action.

To make a function an "external action" we need to satisfy:
1. "may be observable outside of an execution" AND
2. "has a result based on an environment external to the execution"

The concerns you raise around off-heap memory handling boil down to:
- Unsafe.put(long address,*)/Unsafe.put(null, long address,*): don't fullfil 2
- Unsafe.get*(long address)/Unsafe.get*(null, long address): don't fullfil 1
The volatile accesses to offheap are not covered by JMM AFAIK, but are relied
upon by many to mean the same as their heap counter parts.