Implementation of Safepoint events

[zhengyu123]

Add implementation of Safepoint event that provides safepoint latency information.

e.g.

jdk.SafepointBegin {
  startTime = 15:43:26.488
  duration = 4.315 us
  safepointId = 2
  totalThreadCount = 5
  jniCriticalThreadCount = 0
  eventThread = "main" (javaThreadId = 1)
}

jdk.SafepointEnd {
  startTime = 15:43:26.490
  duration = 0.318 us
  safepointId = 2
  eventThread = "main" (javaThreadId = 1)
}

[christianhaeubl]

Thanks again for contributing, I added a small number of comments.

[christianhaeubl]

Seems that this pattern will be pretty common.
I think it makes sense to move the HasJfrSupport.get() check directly into JfrTicks.elapsedTicks() (JfrTicks is a helper class anyways). Then, it is possible to unconditionally call JfrTicks.elapsedTicks(), regardless if the JFR support is present in the image or not.

[zhengyu123]

Okay.

[christianhaeubl]

Shouldn't this count also include the current thread? This code is always executed by the VM operation thread but the VM operation thread is a normal Java thread in Native Image. We support two different execution modes (see VMOperationControl.useDedicatedVMOperationThread()):
a. Without a dedicated VM operation thread, any normal Java thread can be temporarily promoted to the VM operation thread.
b. If a dedicated VM operation thread should be used, then a dedicated Java thread is started early on and only that thread may execute VM operations.

[zhengyu123]

I believe it means how many threads were brought to the safepoint. In hotspot, it does not count vmThread.

[christianhaeubl]

HotSpot reports the number of Java threads, so I think we should the same.
However, in our case that includes the VM operation thread because it is a Java thread as well. This is a major difference to HotSpot anyways and affects all JFR areas so I think it is best to consistently report the VM operation thread as a normal Java thread (e.g., the VM operation thread can allocate Java heap memory, so it will also show up in all allocation-related JFR events).

[zhengyu123]

Okay, fixed

[christianhaeubl]

Thanks, this PR will also be merged to Graal master in the next couple days.