API review of VarHandles

[Vitaly Davidovich]

Hi Doug,

I somehow missed your reply and just now noticed it after Martin's last
email quoted it -- thanks for elaborating.

The new name is required because opaque is only "like" C++
> memory_order_relaxed.
> See the jmm-dev list discussions last year for details, but annoyingly,
> Java normal variables and C++ memory_order_relaxed are not quite the
> same, and neither are the same as C-volatile/opaque.


AFAICT, difference between java normal load/store and memory_order_relaxed
is the latter guaranteeing atomicity.  If I'm understanding you correctly,
"opaque" is *just* compiler barrier and does not guarantee atomicity? If
that's so, what is/are the use case(s) for opaque? Andrew mentioned
preventing roach motel movement of loads/stores into the motel.  Is there
something more than that?

Assuming my above statement is correct, it would seem like having
memory_order_relaxed would be more useful than simply "no code motion
around this load/store", or if not more useful, just useful on its own;
there's plenty of code that doesn't care about ordering but would not
tolerate torn reads/stores.


On Fri, Jan 22, 2016 at 8:23 AM, Doug Lea <dl at cs.oswego.edu> wrote:

> On 01/22/2016 04:51 AM, Andrew Haley wrote:
>
>> On 22/01/16 00:01, Vitaly Davidovich wrote:
>>
>>> I think the get/setOpaque methods need a bit more explanation ("opaque"
>>> is
>>> an odd naming choice, IMO).  Specifically, it says the operations are
>>> done
>>> in program order but have no effect on inter-thread ordering/visibility.
>>> Is this spec verbiage for a full compiler-only fence?
>>>
>>
>
> Basically yes. It would be the same as C "volatile" (not C++/C11
> "atomic"), if C volatile had a memory model spec. But in any case
> we can't use "volatile" for this in Java because it already means
> something else.
>
>
>> It's like C++ memory_order_relaxed, I guessed.  I understand that but
>> not "opaque".
>>
>>
> The new name is required because opaque is only "like" C++
> memory_order_relaxed.
> See the jmm-dev list discussions last year for details, but annoyingly,
> Java normal variables and C++ memory_order_relaxed are not quite the
> same, and neither are the same as C-volatile/opaque.
>
> -Doug
>
>
>
>