--- a/src/hotspot/share/gc/g1/g1CollectedHeap.cpp	Tue Jun 11 17:20:51 2019 -0700
+++ b/src/hotspot/share/gc/g1/g1CollectedHeap.cpp	Wed Jun 12 10:34:29 2019 +0200
@@ -4640,7 +4640,7 @@
 
   bool const during_im = collector_state()->in_initial_mark_gc();
   if (during_im && allocated_bytes > 0) {
-    _cm->root_regions()->add(alloc_region);
+    _cm->root_regions()->add(alloc_region->next_top_at_mark_start(), alloc_region->top());
   }
   _hr_printer.retire(alloc_region);
 }

--- a/src/hotspot/share/gc/g1/g1ConcurrentMark.cpp	Tue Jun 11 17:20:51 2019 -0700
+++ b/src/hotspot/share/gc/g1/g1ConcurrentMark.cpp	Wed Jun 12 10:34:29 2019 +0200
@@ -257,30 +257,38 @@
   _free_list = NULL;
 }
 
-G1CMRootRegions::G1CMRootRegions(uint const max_regions) :
-  _root_regions(NEW_C_HEAP_ARRAY(HeapRegion*, max_regions, mtGC)),
-  _max_regions(max_regions),
-  _num_root_regions(0),
-  _claimed_root_regions(0),
-  _scan_in_progress(false),
-  _should_abort(false) { }
-
-G1CMRootRegions::~G1CMRootRegions() {
-  FREE_C_HEAP_ARRAY(HeapRegion*, _max_regions);
+G1CMRootMemRegions::G1CMRootMemRegions(uint const max_regions) :
+    _root_regions(NULL),
+    _max_regions(max_regions),
+    _num_root_regions(0),
+    _claimed_root_regions(0),
+    _scan_in_progress(false),
+    _should_abort(false) {
+  _root_regions = new MemRegion[_max_regions];
+  if (_root_regions == NULL) {
+    vm_exit_during_initialization("Could not allocate root MemRegion set.");
+  }
 }
 
-void G1CMRootRegions::reset() {
+G1CMRootMemRegions::~G1CMRootMemRegions() {
+  delete[] _root_regions;
+}
+
+void G1CMRootMemRegions::reset() {
   _num_root_regions = 0;
 }
 
-void G1CMRootRegions::add(HeapRegion* hr) {
+void G1CMRootMemRegions::add(HeapWord* start, HeapWord* end) {
   assert_at_safepoint();
   size_t idx = Atomic::add((size_t)1, &_num_root_regions) - 1;
-  assert(idx < _max_regions, "Trying to add more root regions than there is space " SIZE_FORMAT, _max_regions);
-  _root_regions[idx] = hr;
+  assert(idx < _max_regions, "Trying to add more root MemRegions than there is space " SIZE_FORMAT, _max_regions);
+  assert(start != NULL && end != NULL && start <= end, "Start (" PTR_FORMAT ") should be less or equal to "
+         "end (" PTR_FORMAT ")", p2i(start), p2i(end));
+  _root_regions[idx].set_start(start);
+  _root_regions[idx].set_end(end);
 }
 
-void G1CMRootRegions::prepare_for_scan() {
+void G1CMRootMemRegions::prepare_for_scan() {
   assert(!scan_in_progress(), "pre-condition");
 
   _scan_in_progress = _num_root_regions > 0;
@@ -289,7 +297,7 @@
   _should_abort = false;
 }
 
-HeapRegion* G1CMRootRegions::claim_next() {
+const MemRegion* G1CMRootMemRegions::claim_next() {
   if (_should_abort) {
     // If someone has set the should_abort flag, we return NULL to
     // force the caller to bail out of their loop.
@@ -302,26 +310,26 @@
 
   size_t claimed_index = Atomic::add((size_t)1, &_claimed_root_regions) - 1;
   if (claimed_index < _num_root_regions) {
-    return _root_regions[claimed_index];
+    return &_root_regions[claimed_index];
   }
   return NULL;
 }
 
-uint G1CMRootRegions::num_root_regions() const {
+uint G1CMRootMemRegions::num_root_regions() const {
   return (uint)_num_root_regions;
 }
 
-void G1CMRootRegions::notify_scan_done() {
+void G1CMRootMemRegions::notify_scan_done() {
   MutexLocker x(RootRegionScan_lock, Mutex::_no_safepoint_check_flag);
   _scan_in_progress = false;
   RootRegionScan_lock->notify_all();
 }
 
-void G1CMRootRegions::cancel_scan() {
+void G1CMRootMemRegions::cancel_scan() {
   notify_scan_done();
 }
 
-void G1CMRootRegions::scan_finished() {
+void G1CMRootMemRegions::scan_finished() {
   assert(scan_in_progress(), "pre-condition");
 
   if (!_should_abort) {
@@ -333,7 +341,7 @@
   notify_scan_done();
 }
 
-bool G1CMRootRegions::wait_until_scan_finished() {
+bool G1CMRootMemRegions::wait_until_scan_finished() {
   if (!scan_in_progress()) {
     return false;
   }
@@ -875,14 +883,21 @@
   return result;
 }
 
-void G1ConcurrentMark::scan_root_region(HeapRegion* hr, uint worker_id) {
-  assert(hr->is_old() || (hr->is_survivor() && hr->next_top_at_mark_start() == hr->bottom()),
-         "Root regions must be old or survivor but region %u is %s", hr->hrm_index(), hr->get_type_str());
+void G1ConcurrentMark::scan_root_region(const MemRegion* region, uint worker_id) {
+#ifdef ASSERT
+  HeapWord* last = region->last();
+  HeapRegion* hr = _g1h->heap_region_containing(last);
+  assert(hr->is_old() || hr->next_top_at_mark_start() == hr->bottom(),
+         "Root regions must be old or survivor/eden but region %u is %s", hr->hrm_index(), hr->get_type_str());
+  assert(hr->next_top_at_mark_start() == region->start(),
+         "MemRegion start should be equal to nTAMS");
+#endif
+
   G1RootRegionScanClosure cl(_g1h, this, worker_id);
 
   const uintx interval = PrefetchScanIntervalInBytes;
-  HeapWord* curr = hr->next_top_at_mark_start();
-  const HeapWord* end = hr->top();
+  HeapWord* curr = region->start();
+  const HeapWord* end = region->end();
   while (curr < end) {
     Prefetch::read(curr, interval);
     oop obj = oop(curr);
@@ -902,11 +917,11 @@
     assert(Thread::current()->is_ConcurrentGC_thread(),
            "this should only be done by a conc GC thread");
 
-    G1CMRootRegions* root_regions = _cm->root_regions();
-    HeapRegion* hr = root_regions->claim_next();
-    while (hr != NULL) {
-      _cm->scan_root_region(hr, worker_id);
-      hr = root_regions->claim_next();
+    G1CMRootMemRegions* root_regions = _cm->root_regions();
+    const MemRegion* region = root_regions->claim_next();
+    while (region != NULL) {
+      _cm->scan_root_region(region, worker_id);
+      region = root_regions->claim_next();
     }
   }
 };

--- a/src/hotspot/share/gc/g1/g1ConcurrentMark.hpp	Tue Jun 11 17:20:51 2019 -0700
+++ b/src/hotspot/share/gc/g1/g1ConcurrentMark.hpp	Wed Jun 12 10:34:29 2019 +0200
@@ -222,18 +222,20 @@
   template<typename Fn> void iterate(Fn fn) const PRODUCT_RETURN;
 };
 
-// Root Regions are regions that contain objects from nTAMS to top. These are roots
-// for marking, i.e. their referenced objects must be kept alive to maintain the
+// Root MemRegions are memory areas that contain objects which references are
+// roots wrt to the marking. They must be scanned before marking to maintain the
 // SATB invariant.
-// We could scan and mark them through during the initial-mark pause, but for
+// Typically they contain the areas from nTAMS to top of the regions.
+// We could scan and mark through these objects during the initial-mark pause, but for
 // pause time reasons we move this work to the concurrent phase.
 // We need to complete this procedure before the next GC because it might determine
 // that some of these "root objects" are dead, potentially dropping some required
 // references.
-// Root regions comprise of the complete contents of survivor regions, and any
-// objects copied into old gen during GC.
-class G1CMRootRegions {
-  HeapRegion** _root_regions;
+// Root MemRegions comprise of the contents of survivor regions at the end
+// of the GC, and any objects copied into the old gen during GC.
+class G1CMRootMemRegions {
+  // The set of root MemRegions.
+  MemRegion*   _root_regions;
   size_t const _max_regions;
 
   volatile size_t _num_root_regions; // Actual number of root regions.
@@ -246,13 +248,13 @@
   void notify_scan_done();
 
 public:
-  G1CMRootRegions(uint const max_regions);
-  ~G1CMRootRegions();
+  G1CMRootMemRegions(uint const max_regions);
+  ~G1CMRootMemRegions();
 
   // Reset the data structure to allow addition of new root regions.
   void reset();
 
-  void add(HeapRegion* hr);
+  void add(HeapWord* start, HeapWord* end);
 
   // Reset the claiming / scanning of the root regions.
   void prepare_for_scan();
@@ -264,9 +266,9 @@
   // false otherwise.
   bool scan_in_progress() { return _scan_in_progress; }
 
-  // Claim the next root region to scan atomically, or return NULL if
+  // Claim the next root MemRegion to scan atomically, or return NULL if
   // all have been claimed.
-  HeapRegion* claim_next();
+  const MemRegion* claim_next();
 
   // The number of root regions to scan.
   uint num_root_regions() const;
@@ -310,7 +312,7 @@
   MemRegion const         _heap;
 
   // Root region tracking and claiming
-  G1CMRootRegions         _root_regions;
+  G1CMRootMemRegions         _root_regions;
 
   // For grey objects
   G1CMMarkStack           _global_mark_stack; // Grey objects behind global finger
@@ -501,7 +503,7 @@
   size_t partial_mark_stack_size_target() const { return _global_mark_stack.capacity() / 3; }
   bool mark_stack_empty() const                 { return _global_mark_stack.is_empty(); }
 
-  G1CMRootRegions* root_regions() { return &_root_regions; }
+  G1CMRootMemRegions* root_regions() { return &_root_regions; }
 
   void concurrent_cycle_start();
   // Abandon current marking iteration due to a Full GC.
@@ -554,8 +556,8 @@
   // them.
   void scan_root_regions();
 
-  // Scan a single root region from nTAMS to top and mark everything reachable from it.
-  void scan_root_region(HeapRegion* hr, uint worker_id);
+  // Scan a single root MemRegion to mark everything reachable from it.
+  void scan_root_region(const MemRegion* region, uint worker_id);
 
   // Do concurrent phase of marking, to a tentative transitive closure.
   void mark_from_roots();

--- a/src/hotspot/share/memory/memRegion.hpp	Tue Jun 11 17:20:51 2019 -0700
+++ b/src/hotspot/share/memory/memRegion.hpp	Wed Jun 12 10:34:29 2019 +0200
@@ -32,13 +32,13 @@
 // A very simple data structure representing a contigous region
 // region of address space.
 
-// Note that MemRegions are passed by value, not by reference.
+// Note that MemRegions are typically passed by value, not by reference.
 // The intent is that they remain very small and contain no
 // objects. The copy constructor and destructor must be trivial,
 // to support optimization for pass-by-value.
-// These should never be allocated in heap but we do
-// create MemRegions (in CardTableBarrierSet) in heap so operator
-// new and operator new [] added for this special case.
+// These should almost never be allocated in heap but we do
+// create MemRegions (in CardTable and G1CMRootMemRegions) on the heap so operator
+// new and operator new [] were added for these special cases.
 
 class MemRegion {
   friend class VMStructs;