API review of VarHandles

Doug Lea dl at cs.oswego.edu
Fri Jan 22 13:23:53 UTC 2016


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





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