--- a/src/hotspot/share/gc/g1/g1CollectedHeap.hpp	Wed Nov 20 14:16:29 2019 +0100
+++ b/src/hotspot/share/gc/g1/g1CollectedHeap.hpp	Thu Nov 21 11:42:54 2019 +0100
@@ -214,6 +214,10 @@
 
   void set_used(size_t bytes);
 
+  // Number of bytes used in all regions during GC. Typically changed when
+  // retiring a GC alloc region.
+  size_t _bytes_used_during_gc;
+
   // Class that handles archive allocation ranges.
   G1ArchiveAllocator* _archive_allocator;