[jmm-dev] jdk9 APIs

[Doug Lea]

On 08/12/2015 05:02 AM, Aleksey Shipilev wrote:
> On 08/10/2015 09:22 PM, Doug Lea wrote:

>>      boolean compareAndSet(Object owner, T cmp, T val);
>>      boolean compareAndSetAcquire(Object owner, T cmp, T val);
>>      boolean compareAndSetRelease(Object owner, T cmp, T val);
>
> Another, perhaps, "can of worms" question about CAS-es that was lurking
> in my mind: which <T>-s are we nominally handling?

The answers need to be a little different for Java and C++.
In C++, you can only apply to "atomic" types.

Under the  current VarHandle scheme, you can attach to
anything at compile time, but methods may bail out
throwing an UnsupportedOperationException at run-time.

This will always occur for example when applying getAndAdd
to a VarHandle of any Object type.

This was apparently the tradeoff most acceptable to all concerned.
It means that in addition to memory-model-related specs,
the methods will need to carry warnings that they might
just throw UOE; hopefully enumerating common cases
like CAS for sub-word types and unaligned memory.

>
> Therefore, the question seems to be the hardware ability to make subword
> CASes.

My understanding (and preference) is that trying to CAS with
VarHandles to bytes and other small types just throws
UOE and should be spec'ed to do so.

-Doug