RFR(L) 8227745: Enable Escape Analysis for Better Performance in the Presence of JVMTI Agents

[Robbin Ehn]

Hi Richard, as mentioned it would be better if you could do this with
handshakes, instead of using _suspend_flag (since they are going away).
But I can't think of a way doing it without blocking safepoints, so we need to
add some more features in handshakes first.
When possible I hope you are willing to move this code to handshakes instead.

You could stop one thread with, e.g.:
class EscapeBarrierSuspendHandshake : public HandshakeClosure {
   Semaphore _is_waiting;
   Semaphore _wait;
   bool _started;
  public:
   EscapeBarrierSuspendHandshake() : HandshakeClosure("EscapeBarrierSuspend"), 
_wait(0), _started(false) { }
   void do_thread(Thread* th) {
     _is_waiting.signal();
     _wait.wait();
     Atomic::store(&_started, true);
   }
   void wait_until_eb_stopped() { _is_waiting.wait(); }
   void start_thread() {
     _wait.signal();
     while(!Atomic::load(&_started)) {
       os::naked_yield();
     }
   }
};

But it would block safepoints.

Thanks, Robbin

On 12/10/19 10:45 PM, Reingruber, Richard wrote:
> Hi,
> 
> I would like to get reviews please for
> 
> http://cr.openjdk.java.net/~rrich/webrevs/2019/8227745/webrev.3/
> 
> Corresponding RFE:
> https://bugs.openjdk.java.net/browse/JDK-8227745
> 
> Fixes also https://bugs.openjdk.java.net/browse/JDK-8233915
> And potentially https://bugs.openjdk.java.net/browse/JDK-8214584 [1]
> 
> Vladimir Kozlov kindly put webrev.3 through tier1-8 testing without issues (thanks!). In addition the
> change is being tested at SAP since I posted the first RFR some months ago.
> 
> The intention of this enhancement is to benefit performance wise from escape analysis even if JVMTI
> agents request capabilities that allow them to access local variable values. E.g. if you start-up
> with -agentlib:jdwp=transport=dt_socket,server=y,suspend=n, then escape analysis is disabled right
> from the beginning, well before a debugger attaches -- if ever one should do so. With the
> enhancement, escape analysis will remain enabled until and after a debugger attaches. EA based
> optimizations are reverted just before an agent acquires the reference to an object. In the JBS item
> you'll find more details.
> 
> Thanks,
> Richard.
> 
> [1] Experimental fix for JDK-8214584 based on JDK-8227745
>      http://cr.openjdk.java.net/~rrich/webrevs/2019/8214584/experiment_v1.patch
>