RFR(XS): make shared variables volatile

[Andrew Haley]

On 22/09/16 16:20, Zhengyu Gu wrote:
> On 09/22/2016 11:04 AM, Andrew Haley wrote:
> 
>> On 21/09/16 15:47, Aleksey Shipilev wrote:
>>> On 09/21/2016 04:40 PM, Zhengyu Gu wrote:
>>>> http://cr.openjdk.java.net/~zgu/volatile/webrev.01/
>>> +1
>>>
>>> I wonder if this means we should update/poll these volatile variables
>>> with Atomic::* too, at least for the benefit of weak arches like AArch64.
>> Always.  volatile is never sufficient for variables accessed racily by
>> multiple threads, and in C++11 it's undefined behaviour.  I remember
>> finding and fixing a couple of these when I did the AArch64 Shenandoah
>> port.
> 
> My intention here is simply preventing compiler from caching the shared variables, ex.
> 
> ShenandoahHeapRegion::object_iterate_interruptible()
>    ...
>    while (p < top() && !(allow_cancel && heap->cancelled_concgc()))
>    ...
> 
> 
> Setting _cancelled_concgc is followed by a full fence, I have yet checked the others.

There is no point preventing the compiler from caching the shared variables
unless you also prevent the CPU from doing the same thing.

Andrew.