RFR (M): 8224741: Optimize the scan area during the Scan Heap Roots phase

[Kim Barrett]

> On Jul 2, 2019, at 6:40 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> Updated webrevs:
> http://cr.openjdk.java.net/~tschatzl/8224741/webrev.0_to_1 (diff)
> http://cr.openjdk.java.net/~tschatzl/8224741/webrev.1 (full)

Looks good.

A couple minor comment nits, and one (highly) optional suggestion.

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src/hotspot/share/gc/g1/heapRegion.hpp 

Thanks for the "card" => "memregion" renamings.

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src/hotspot/share/gc/g1/heapRegion.hpp 
 296   // Returns the address after the last actually scanned or NULL if the area could

"the address after the last actually scanned" isn't really accurate.
More accurate might be "the next unscanned address".

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src/hotspot/share/gc/g1/heapRegion.inline.hpp
 296     // If obj is not an objArray and mr contains the start of the
 297     // obj, then this could be an imprecise mark, and we need to
 298     // process the entire object.
 299     int size = obj->oop_iterate_size(cl);
 300     return MAX2((HeapWord*)obj + size, mr.end());

Maybe mention here also that there aren't any objects after obj in the
region.

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src/hotspot/share/gc/g1/heapRegion.inline.hpp
 300     return MAX2((HeapWord*)obj + size, mr.end());

obj + size could be in a region following this region.  I think rather
than the above result, we could instead return the end of the region
containing obj + size.  But I suspect this case also rarely occurs,
and even then there's rarely going to be anything to be saved by that.

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