[jmm-dev] VarHandle.safepoint() methods

[Andrew Haley]

On 05/01/17 15:40, Doug Lea wrote:
> On 01/05/2017 09:09 AM, Andrew Haley wrote:
>>
>>   if (VarHandles.checkPointed()) // returns true if safepoint
>>      VarHandles.fullFence();
>>
>> It'd certainly be easier to implement, and it naturally matches
>> exactly what a safepoint does.
>>
> 
> It matches it so well that it makes sense to specify the
> method to perform fullFence after safepoint execution.
> 
> You'd still need epoch-returning form to deal with
> safepoints occurring in nested calls in loops.
> 
>    e = VH.safePointEpoch();
>    for (...) {
>      use(x.get());
>      e = VH.safePointCheck(e);
>    }

I can't see how this might work in the MappedByteBuffer case.
The user is only going to be writing things like

  for (int i = 0; i < n; i ++) {
    sum += buf.get(i);
  }

I'm not proposing any change to the MappedByteBuffer API.  What
would insert the call to safePointEpoch() ?

> The difference between implicit and explicit safepoint checks is
> that in the implicit ones, some variables can be restored
> to those saved on safepoint entry, but not so in the explicit
> case -- they must be freshly read.

In the case of a real safepoint, nothing is saved around a safepoint
call.  What actually happens is that an interrupt occurs, as a
consequence of which live registers are pushed onto the stack,
safepoint code is run, registers are restored, and we continue.

Andrew.