6862661: G1: _gc_alloc_region_counts is not updated properly after 6604422

Summary: Implementation of RFE 6604422 (G1: re-use half-promoted regions) introduced incorrect _gc_alloc_region_counts updates which effectively disabled survivor spaces.
Reviewed-by: johnc, jmasa, tonyp

src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

@@ -2981,6 +2981,7 @@ void G1CollectedHeap::get_gc_alloc_regions() {
 
   for (int ap = 0; ap < GCAllocPurposeCount; ++ap) {
     assert(_gc_alloc_regions[ap] == NULL, "invariant");
+    assert(_gc_alloc_region_counts[ap] == 0, "invariant");
 
     // Create new GC alloc regions.
     HeapRegion* alloc_region = _retained_gc_alloc_regions[ap];
@@ -3009,6 +3010,9 @@ void G1CollectedHeap::get_gc_alloc_regions() {
     if (alloc_region == NULL) {
       // we will get a new GC alloc region
       alloc_region = newAllocRegionWithExpansion(ap, 0);
+    } else {
+      // the region was retained from the last collection
+      ++_gc_alloc_region_counts[ap];
     }
 
     if (alloc_region != NULL) {
@@ -3047,11 +3051,11 @@ void G1CollectedHeap::release_gc_alloc_regions(bool totally) {
   for (int ap = 0; ap < GCAllocPurposeCount; ++ap) {
     HeapRegion* r = _gc_alloc_regions[ap];
     _retained_gc_alloc_regions[ap] = NULL;
+    _gc_alloc_region_counts[ap] = 0;
 
     if (r != NULL) {
       // we retain nothing on _gc_alloc_regions between GCs
       set_gc_alloc_region(ap, NULL);
-      _gc_alloc_region_counts[ap] = 0;
 
       if (r->is_empty()) {
         // we didn't actually allocate anything in it; let's just put