RFR (S/M): 8225478: Make G1CMRootRegions independent of HeapRegions

[Kim Barrett]

> On Jun 7, 2019, at 12:52 PM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> […]
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8225478
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8225478/webrev/
> Testing:
> hs-tier1-3 (Sangheon already pushed 8220080 through hs-tier1-8).
> 
> 
> [0] https://bugs.openjdk.java.net/browse/JDK-8220089
> [1] http://cr.openjdk.java.net/~sangheki/8220089/webrev.4/
> [2] 
> http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2019-May/025774.html

Generally looks good; just a few minor issues and suggestions.

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src/hotspot/share/gc/g1/g1ConcurrentMark.hpp
 271   MemRegion* claim_next();

Perhaps the return type should be const MemRegion* ?

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src/hotspot/share/gc/g1/g1ConcurrentMark.hpp
 315   G1CMRootMemRegions         _root_regions;

Member is no longer aligned with all the other members.

(Not that I like that kind of formatting, but it looks odd to have the
one outlier now.)

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src/hotspot/share/gc/g1/g1ConcurrentMark.hpp
 559   // Scan a single root MemRegion which holds from nTAMS to top and mark everything reachable from it.

Maybe drop "which holds from nTAMS to top".  The intent is that will
no longer be true because of allocation from survivor regions changing
top.  And nTAMS and top are not part of a MemRegion.

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src/hotspot/share/gc/g1/g1ConcurrentMark.cpp
 275   _root_regions = NULL;

Unnecessary.

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src/hotspot/share/gc/g1/g1ConcurrentMark.cpp
 286   assert(start != NULL && end != NULL && start <= end, "Start (" PTR_FORMAT ") should be equal to or greater "

Error message is backward.

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