[8] RFR: Cherry-pick JDK-8231201: hs_err should print coalesced safepoint operations in Events section

[Roman Kennke]

Ok, that makes sense. Do it!

Thanks,
Roman

> Looking at hs_errs from Alexander, I suspect there are more VM operations happening than meets the
> eye. See for example:
> 
> Event: 749.332 Executing VM operation: FindDeadlocks
> Event: 749.333 Pause Init Update Refs
> Event: 751.083 Pause Init Update Refs done
> Event: 751.083 Executing VM operation: FindDeadlocks done
> 
> Where's the "Executing VM operation: ShenandoahInitUpdateRefs"? The answer is here:
>   https://bugs.openjdk.java.net/browse/JDK-8231201
> 
> I requested the jdk8u backport here:
>   https://mail.openjdk.java.net/pipermail/jdk8u-dev/2019-November/010552.html
> 
> ...but it would be a while before it lands in sh/jdk8. So, let's cherry-pick it:
> 
> diff -r e8582ad276a2 src/share/vm/runtime/vmThread.cpp
> --- a/src/share/vm/runtime/vmThread.cpp Mon Nov 04 12:25:21 2019 +0100
> +++ b/src/share/vm/runtime/vmThread.cpp Mon Nov 04 19:23:12 2019 +0100
> @@ -505,10 +505,11 @@
>          // the queue until there are none left
>          do {
>            _cur_vm_operation = safepoint_ops;
>            if (_cur_vm_operation != NULL) {
>              do {
> +              EventMark em("Executing coalesced VM operation: %s", _cur_vm_operation->name());
>                // evaluate_operation deletes the op object so we have
>                // to grab the next op now
>                VM_Operation* next = _cur_vm_operation->next();
>                _vm_queue->set_drain_list(next);
>                evaluate_operation(_cur_vm_operation);
> 
> Testing: hotspot_gc_shenandoah
>