--- old/src/hotspot/share/gc/g1/g1HeapSizingPolicy.cpp	2020-02-19 11:43:12.811599757 +0100
+++ new/src/hotspot/share/gc/g1/g1HeapSizingPolicy.cpp	2020-02-19 11:43:12.370585979 +0100
@@ -26,6 +26,7 @@
 #include "gc/g1/g1CollectedHeap.hpp"
 #include "gc/g1/g1HeapSizingPolicy.hpp"
 #include "gc/g1/g1Analytics.hpp"
+#include "gc/g1/g1Policy.hpp"
 #include "logging/log.hpp"
 #include "runtime/globals.hpp"
 #include "utilities/debug.hpp"
@@ -50,7 +51,7 @@
   _pauses_since_start = 0;
 }
 
-size_t G1HeapSizingPolicy::expansion_amount() {
+size_t G1HeapSizingPolicy::expansion_amount_after_young_collection() {
   double recent_gc_overhead = _analytics->recent_avg_pause_time_ratio() * 100.0;
   double last_gc_overhead = _analytics->last_pause_time_ratio() * 100.0;
   assert(GCTimeRatio > 0,
@@ -160,3 +161,36 @@
 
   return expand_bytes;
 }
+
+size_t G1HeapSizingPolicy::target_heap_capacity(size_t used_bytes, uintx free_ratio) const {
+  const double free_percentage = (double) free_ratio / 100.0;
+  const double used_percentage = 1.0 - free_percentage;
+
+  // We have to be careful here as these two calculations can overflow
+  // 32-bit size_t's.
+  double used_bytes_d = (double) used_bytes;
+  double desired_capacity_d = used_bytes_d / used_percentage;
+  // Let's make sure that they are both under the max heap size, which
+  // by default will make it fit into a size_t.
+  double desired_capacity_upper_bound = (double) MaxHeapSize;
+  desired_capacity_d = MIN2(desired_capacity_d, desired_capacity_upper_bound);
+  // We can now safely turn it into size_t's.
+  return (size_t) desired_capacity_d;
+}
+
+size_t G1HeapSizingPolicy::shrink_amount_at_last_mixed_gc(size_t desired_bytes_after_concurrent_mark) {
+  size_t shrink_bytes = 0;
+  const size_t capacity_after_gc = _g1h->capacity();
+  const size_t used_after_gc = capacity_after_gc - _g1h->unused_committed_regions_in_bytes();
+  size_t maximum_desired_capacity = target_heap_capacity(used_after_gc, MaxHeapFreeRatio);
+  // soft_max_capacity can be smaller
+  maximum_desired_capacity = MIN2(maximum_desired_capacity, _g1h->soft_max_capacity());
+  // Make sure not less than _minimum_desired_bytes_after_last_cm
+  maximum_desired_capacity = MAX2(maximum_desired_capacity, desired_bytes_after_concurrent_mark);
+
+  if (capacity_after_gc > maximum_desired_capacity) {
+    shrink_bytes = capacity_after_gc - maximum_desired_capacity;
+  }
+
+  return shrink_bytes;
+}