RFR (S): 8227084: Add timing information for merge heap root preparation

[Kim Barrett]

> On Jul 10, 2019, at 11:36 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> Hi all,
> 
>  can I have reviews for this change that adds timing information for
> parts of the Merge Heap Roots phase as preparation for JDK-8226913.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8227084
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8227084
> Testing:
> hs-tier1-5
> 
> Thanks,
>  Thomas

Looks good.

Just one minor thing that you can change or not, as you prefer.

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src/hotspot/share/gc/g1/g1RemSet.cpp
1129       assert(merge_remset_phase == G1GCPhaseTimes::MergeRS, "Wrong merge phase");
1137       assert(merge_remset_phase == G1GCPhaseTimes::MergeRS, "Wrong merge phase");

I'm not sure these assertions carry their weight anymore.  They used
to verify consistency between two arguments, but now
merge_remset_phase is computed locally from the argument whose value
is used in the immediately enclosing if-statement.

------------------------------------------------------------------------------

An aside: We really need a helper somewhere for expressions like

   total.seconds() * 1000.0

I know that Tickspan::milliseconds() doesn't do what we want, because
it returns an integral value.