diff --git a/src/coreclr/gc/gc.cpp b/src/coreclr/gc/gc.cpp
index c34bc7b2fbd5d4..9871e7bbd37f36 100644
--- a/src/coreclr/gc/gc.cpp
+++ b/src/coreclr/gc/gc.cpp
@@ -2297,6 +2297,7 @@ bool        affinity_config_specified_p = false;
 #ifdef USE_REGIONS
 region_allocator global_region_allocator;
 uint8_t*(*initial_regions)[total_generation_count][2] = nullptr;
+size_t      gc_heap::region_count = 0;
 #endif //USE_REGIONS
 
 #ifdef BACKGROUND_GC
@@ -3512,12 +3513,6 @@ size_t get_basic_region_index_for_address (uint8_t* address)
     return (basic_region_index - ((size_t)g_gc_lowest_address >> gc_heap::min_segment_size_shr));
 }
 
-inline
-size_t get_total_region_count()
-{
-    return (get_basic_region_index_for_address (g_gc_highest_address) + 1);
-}
-
 // Go from a random address to its region info. The random address could be 
 // in one of the basic regions of a larger region so we need to check for that.
 inline
@@ -9741,7 +9736,6 @@ size_t gc_heap::sort_mark_list()
 
 #ifdef USE_REGIONS
     // first set the pieces for all regions to empty
-    size_t region_count = get_total_region_count();
     assert (g_mark_list_piece_size >= region_count);
     for (size_t region_index = 0; region_index < region_count; region_index++)
     {
@@ -21138,7 +21132,6 @@ size_t gc_heap::get_promoted_bytes()
     }
 
     dprintf (3, ("h%d getting surv", heap_number));
-    size_t region_count = get_total_region_count();
     size_t promoted = 0;
     for (size_t i = 0; i < region_count; i++)
     {
@@ -23934,6 +23927,7 @@ void gc_heap::mark_phase (int condemned_gen_number, BOOL mark_only_p)
 
 #ifdef USE_REGIONS
         special_sweep_p = false;
+        region_count = global_region_allocator.get_used_region_count();
         grow_mark_list_piece();
 #endif //USE_REGIONS
 
@@ -24002,7 +23996,7 @@ void gc_heap::mark_phase (int condemned_gen_number, BOOL mark_only_p)
 #endif //MULTIPLE_HEAPS
             survived_per_region = (size_t*)&g_mark_list_piece[heap_number * 2 * g_mark_list_piece_size];
             old_card_survived_per_region = (size_t*)&survived_per_region[g_mark_list_piece_size];
-            size_t region_info_to_clear = get_total_region_count() * sizeof (size_t);
+            size_t region_info_to_clear = region_count * sizeof (size_t);
             memset (survived_per_region, 0, region_info_to_clear);
             memset (old_card_survived_per_region, 0, region_info_to_clear);
         }
@@ -26213,7 +26207,6 @@ void gc_heap::process_remaining_regions (int current_plan_gen_num, generation* c
 
 void gc_heap::grow_mark_list_piece()
 {
-    size_t region_count = get_total_region_count();
     if (g_mark_list_piece_size < region_count)
     {
         delete[] g_mark_list_piece;
@@ -26238,11 +26231,10 @@ void gc_heap::save_current_survived()
 {
     if (!survived_per_region) return;
 
-    size_t region_info_to_copy = get_total_region_count() * sizeof (size_t);
+    size_t region_info_to_copy = region_count * sizeof (size_t);
     memcpy (old_card_survived_per_region, survived_per_region, region_info_to_copy);
 
 #ifdef _DEBUG
-    size_t region_count = get_total_region_count();
     for (size_t region_index = 0; region_index < region_count; region_index++)
     {
         if (survived_per_region[region_index] != 0)
@@ -26259,7 +26251,6 @@ void gc_heap::update_old_card_survived()
 {
     if (!survived_per_region) return;
 
-    size_t region_count = get_total_region_count();
     for (size_t region_index = 0; region_index < region_count; region_index++)
     {
         old_card_survived_per_region[region_index] = survived_per_region[region_index] - 
diff --git a/src/coreclr/gc/gcpriv.h b/src/coreclr/gc/gcpriv.h
index c6d7dd668868e4..4b3ff012cf11bd 100644
--- a/src/coreclr/gc/gcpriv.h
+++ b/src/coreclr/gc/gcpriv.h
@@ -3585,6 +3585,8 @@ class gc_heap
     size_t* survived_per_region;
     PER_HEAP
     size_t* old_card_survived_per_region;
+    PER_HEAP_ISOLATED
+    size_t region_count;
 #endif //USE_REGIONS
 
 #define max_oom_history_count 4
@@ -5694,6 +5696,14 @@ class region_allocator
     size_t get_free() { return (total_free_units * region_alignment) ; }
     size_t get_region_alignment () { return region_alignment; }
     size_t get_large_region_alignment () { return large_region_alignment; }
+    size_t get_used_region_count()
+    {
+        // currently we don't allocate anything from the right -
+        // once we do, we need a more sophisticated way to iterate
+        // through the used regions
+        assert (region_map_right_start == region_map_right_end);
+        return (region_map_left_end - region_map_left_start);
+    }
 };
 #endif //USE_REGIONS