--- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Mon Mar 24 15:30:46 2014 +0100
+++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Mon Mar 24 15:30:50 2014 +0100
@@ -1954,7 +1954,7 @@
   int n_queues = MAX2((int)ParallelGCThreads, 1);
   _task_queues = new RefToScanQueueSet(n_queues);
 
-  int n_rem_sets = HeapRegionRemSet::num_par_rem_sets();
+  uint n_rem_sets = HeapRegionRemSet::num_par_rem_sets();
   assert(n_rem_sets > 0, "Invariant.");
 
   _worker_cset_start_region = NEW_C_HEAP_ARRAY(HeapRegion*, n_queues, mtGC);

--- a/src/share/vm/gc_implementation/g1/heapRegionRemSet.cpp	Mon Mar 24 15:30:46 2014 +0100
+++ b/src/share/vm/gc_implementation/g1/heapRegionRemSet.cpp	Mon Mar 24 15:30:50 2014 +0100
@@ -358,48 +358,66 @@
          "just checking");
 }
 
-int**  OtherRegionsTable::_from_card_cache = NULL;
-uint   OtherRegionsTable::_from_card_cache_max_regions = 0;
-size_t OtherRegionsTable::_from_card_cache_mem_size = 0;
+int**  FromCardCache::_cache = NULL;
+uint   FromCardCache::_max_regions = 0;
+size_t FromCardCache::_static_mem_size = 0;
 
-void OtherRegionsTable::init_from_card_cache(uint max_regions) {
-  guarantee(_from_card_cache == NULL, "Should not call this multiple times");
-  uint n_par_rs = HeapRegionRemSet::num_par_rem_sets();
+void FromCardCache::initialize(uint n_par_rs, uint max_num_regions) {
+  guarantee(_cache == NULL, "Should not call this multiple times");
 
-  _from_card_cache_max_regions = max_regions;
-  _from_card_cache = Padded2DArray<int, mtGC>::create_unfreeable(n_par_rs,
-                                                                 _from_card_cache_max_regions,
-                                                                 &_from_card_cache_mem_size);
+  _max_regions = max_num_regions;
+  _cache = Padded2DArray<int, mtGC>::create_unfreeable(n_par_rs,
+                                                       _max_regions,
+                                                       &_static_mem_size);
 
   for (uint i = 0; i < n_par_rs; i++) {
-    for (uint j = 0; j < _from_card_cache_max_regions; j++) {
-      _from_card_cache[i][j] = -1;  // An invalid value.
+    for (uint j = 0; j < _max_regions; j++) {
+      set(i, j, InvalidCard);
     }
   }
 }
 
-void OtherRegionsTable::shrink_from_card_cache(uint new_n_regs) {
+void FromCardCache::shrink(uint new_num_regions) {
   for (uint i = 0; i < HeapRegionRemSet::num_par_rem_sets(); i++) {
-    assert(new_n_regs <= _from_card_cache_max_regions, "Must be within max.");
-    for (uint j = new_n_regs; j < _from_card_cache_max_regions; j++) {
-      _from_card_cache[i][j] = -1;  // An invalid value.
+    assert(new_num_regions <= _max_regions, "Must be within max.");
+    for (uint j = new_num_regions; j < _max_regions; j++) {
+      set(i, j, InvalidCard);
     }
   }
 }
 
 #ifndef PRODUCT
-void OtherRegionsTable::print_from_card_cache() {
+void FromCardCache::print(outputStream* out) {
   for (uint i = 0; i < HeapRegionRemSet::num_par_rem_sets(); i++) {
-    for (uint j = 0; j < _from_card_cache_max_regions; j++) {
-      gclog_or_tty->print_cr("_from_card_cache[%d][%d] = %d.",
-                    i, j, _from_card_cache[i][j]);
+    for (uint j = 0; j < _max_regions; j++) {
+      out->print_cr("_from_card_cache["UINT32_FORMAT"]["UINT32_FORMAT"] = "INT32_FORMAT".",
+                    i, j, at(i, j));
     }
   }
 }
 #endif
 
+void FromCardCache::clear(uint region_idx) {
+  uint num_par_remsets = HeapRegionRemSet::num_par_rem_sets();
+  for (uint i = 0; i < num_par_remsets; i++) {
+    set(i, region_idx, InvalidCard);
+  }
+}
+
+void OtherRegionsTable::init_from_card_cache(uint max_regions) {
+  FromCardCache::initialize(HeapRegionRemSet::num_par_rem_sets(), max_regions);
+}
+
+void OtherRegionsTable::shrink_from_card_cache(uint new_num_regions) {
+  FromCardCache::shrink(new_num_regions);
+}
+
+void OtherRegionsTable::print_from_card_cache() {
+  FromCardCache::print();
+}
+
 void OtherRegionsTable::add_reference(OopOrNarrowOopStar from, int tid) {
-  size_t cur_hrs_ind = (size_t) hr()->hrs_index();
+  uint cur_hrs_ind = hr()->hrs_index();
 
   if (G1TraceHeapRegionRememberedSet) {
     gclog_or_tty->print_cr("ORT::add_reference_work(" PTR_FORMAT "->" PTR_FORMAT ").",
@@ -412,19 +430,17 @@
   int from_card = (int)(uintptr_t(from) >> CardTableModRefBS::card_shift);
 
   if (G1TraceHeapRegionRememberedSet) {
-    gclog_or_tty->print_cr("Table for [" PTR_FORMAT "...): card %d (cache = %d)",
+    gclog_or_tty->print_cr("Table for [" PTR_FORMAT "...): card %d (cache = "INT32_FORMAT")",
                   hr()->bottom(), from_card,
-                  _from_card_cache[tid][cur_hrs_ind]);
+                  FromCardCache::at((uint)tid, cur_hrs_ind));
   }
 
-  if (from_card == _from_card_cache[tid][cur_hrs_ind]) {
+  if (FromCardCache::contains_or_replace((uint)tid, cur_hrs_ind, from_card)) {
     if (G1TraceHeapRegionRememberedSet) {
       gclog_or_tty->print_cr("  from-card cache hit.");
     }
     assert(contains_reference(from), "We just added it!");
     return;
-  } else {
-    _from_card_cache[tid][cur_hrs_ind] = from_card;
   }
 
   // Note that this may be a continued H region.
@@ -722,7 +738,7 @@
 }
 
 size_t OtherRegionsTable::static_mem_size() {
-  return _from_card_cache_mem_size;
+  return FromCardCache::static_mem_size();
 }
 
 size_t OtherRegionsTable::fl_mem_size() {
@@ -730,11 +746,7 @@
 }
 
 void OtherRegionsTable::clear_fcc() {
-  uint hrs_idx = hr()->hrs_index();
-  uint num_par_remsets = HeapRegionRemSet::num_par_rem_sets();
-  for (uint i = 0; i < num_par_remsets; i++) {
-    _from_card_cache[i][hrs_idx] = -1;
-  }
+  FromCardCache::clear(hr()->hrs_index());
 }
 
 void OtherRegionsTable::clear() {
@@ -768,13 +780,13 @@
   // Check to see if any of the fcc entries come from here.
   uint hr_ind = hr()->hrs_index();
   for (uint tid = 0; tid < HeapRegionRemSet::num_par_rem_sets(); tid++) {
-    int fcc_ent = _from_card_cache[tid][hr_ind];
-    if (fcc_ent != -1) {
+    int fcc_ent = FromCardCache::at(tid, hr_ind);
+    if (fcc_ent != FromCardCache::InvalidCard) {
       HeapWord* card_addr = (HeapWord*)
         (uintptr_t(fcc_ent) << CardTableModRefBS::card_shift);
       if (hr()->is_in_reserved(card_addr)) {
         // Clear the from card cache.
-        _from_card_cache[tid][hr_ind] = -1;
+        FromCardCache::set(tid, hr_ind, FromCardCache::InvalidCard);
       }
     }
   }
@@ -830,8 +842,6 @@
            "Must be in range.");
     return _sparse_table.contains_card(hr_ind, card_index);
   }
-
-
 }
 
 void
@@ -932,7 +942,6 @@
   _other_regions.scrub(ctbs, region_bm, card_bm);
 }
 
-
 // Code roots support
 
 void HeapRegionRemSet::add_strong_code_root(nmethod* nm) {

--- a/src/share/vm/gc_implementation/g1/heapRegionRemSet.hpp	Mon Mar 24 15:30:46 2014 +0100
+++ b/src/share/vm/gc_implementation/g1/heapRegionRemSet.hpp	Mon Mar 24 15:30:50 2014 +0100
@@ -45,6 +45,54 @@
 class HRRSCleanupTask : public SparsePRTCleanupTask {
 };
 
+// The FromCardCache remembers the most recently processed card on the heap on
+// a per-region and per-thread basis.
+class FromCardCache : public AllStatic {
+ private:
+  // Array of card indices. Indexed by thread X and heap region to minimize
+  // thread contention.
+  static int** _cache;
+  static uint _max_regions;
+  static size_t _static_mem_size;
+
+ public:
+  enum {
+    InvalidCard = -1 // Card value of an invalid card, i.e. a card index not otherwise used.
+  };
+
+  static void clear(uint region_idx);
+
+  // Returns true if the given card is in the cache at the given location, or
+  // replaces the card at that location and returns false.
+  static bool contains_or_replace(uint worker_id, uint region_idx, int card) {
+    int card_in_cache = at(worker_id, region_idx);
+    if (card_in_cache == card) {
+      return true;
+    } else {
+      set(worker_id, region_idx, card);
+      return false;
+    }
+  }
+
+  static int at(uint worker_id, uint region_idx) {
+    return _cache[worker_id][region_idx];
+  }
+
+  static void set(uint worker_id, uint region_idx, int val) {
+    _cache[worker_id][region_idx] = val;
+  }
+
+  static void initialize(uint n_par_rs, uint max_num_regions);
+
+  static void shrink(uint new_num_regions);
+
+  static void print(outputStream* out = gclog_or_tty) PRODUCT_RETURN;
+
+  static size_t static_mem_size() {
+    return _static_mem_size;
+  }
+};
+
 // The "_coarse_map" is a bitmap with one bit for each region, where set
 // bits indicate that the corresponding region may contain some pointer
 // into the owning region.
@@ -119,11 +167,6 @@
   // false.
   bool del_single_region_table(size_t ind, HeapRegion* hr);
 
-  // Indexed by thread X heap region, to minimize thread contention.
-  static int** _from_card_cache;
-  static uint _from_card_cache_max_regions;
-  static size_t _from_card_cache_mem_size;
-
   // link/add the given fine grain remembered set into the "all" list
   void link_to_all(PerRegionTable * prt);
   // unlink/remove the given fine grain remembered set into the "all" list
@@ -174,7 +217,7 @@
 
   // Declares that only regions i s.t. 0 <= i < new_n_regs are in use.
   // Make sure any entries for higher regions are invalid.
-  static void shrink_from_card_cache(uint new_n_regs);
+  static void shrink_from_card_cache(uint new_num_regions);
 
   static void print_from_card_cache();
 };