--- old/src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.cpp	2020-05-20 15:10:46.537806467 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.cpp	2020-05-20 15:10:46.233804357 +0200
@@ -33,10 +33,10 @@
 #include "services/memTracker.hpp"
 #include "utilities/copy.hpp"
 
-ShenandoahCollectionSet::ShenandoahCollectionSet(ShenandoahHeap* heap, char* heap_base, size_t size) :
+ShenandoahCollectionSet::ShenandoahCollectionSet(ShenandoahHeap* heap, ReservedSpace space, char* heap_base) :
   _map_size(heap->num_regions()),
   _region_size_bytes_shift(ShenandoahHeapRegion::region_size_bytes_shift()),
-  _map_space(align_up(((uintx)heap_base + size) >> _region_size_bytes_shift, os::vm_allocation_granularity())),
+  _map_space(space),
   _cset_map(_map_space.base() + ((uintx)heap_base >> _region_size_bytes_shift)),
   _biased_cset_map(_map_space.base()),
   _heap(heap),
--- old/src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.hpp	2020-05-20 15:10:47.305811788 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.hpp	2020-05-20 15:10:46.997809657 +0200
@@ -51,7 +51,7 @@
   shenandoah_padding(1);
 
 public:
-  ShenandoahCollectionSet(ShenandoahHeap* heap, char* heap_base, size_t size);
+  ShenandoahCollectionSet(ShenandoahHeap* heap, ReservedSpace space, char* heap_base);
 
   // Add region to collection set
   void add_region(ShenandoahHeapRegion* r);
--- old/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp	2020-05-20 15:10:48.081817159 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp	2020-05-20 15:10:47.769815001 +0200
@@ -292,9 +292,31 @@
                               "Cannot commit region memory");
   }
 
+  // Try to fit the collection set bitmap at lower addresses. This optimizes code generation for cset checks.
+  // Go up until a sensible limit and try to reserve the space there.
+  // If not successful, bite a bullet and allocate at whatever address.
+  {
+    int cset_align = MAX2(os::vm_page_size(), os::vm_allocation_granularity());
+    uintx cset_size = align_up(((uintx) sh_rs.base() + sh_rs.size()) >> ShenandoahHeapRegion::region_size_bytes_shift(), cset_align);
+    uintptr_t min = round_up_power_of_2(cset_align);
+    uintptr_t max = (1u << 31u);
+    for (uintptr_t cset_base = min; cset_base <= max; cset_base <<= 1u) {
+      ReservedSpace cset_rs(cset_size, os::vm_page_size(), false, (char*) cset_base);
+      if (cset_rs.is_reserved()) {
+        assert((char*)cset_base == cset_rs.base(), "Addresses should agree: " PTR_FORMAT ", " PTR_FORMAT, cset_base, p2i(cset_rs.base()));
+        _collection_set = new ShenandoahCollectionSet(this, cset_rs, sh_rs.base());
+        break;
+      }
+    }
+
+    if (_collection_set == NULL) {
+      ReservedSpace cset_rs(cset_size, os::vm_page_size(), false);
+      _collection_set = new ShenandoahCollectionSet(this, cset_rs, sh_rs.base());
+    }
+  }
+
   _regions = NEW_C_HEAP_ARRAY(ShenandoahHeapRegion*, _num_regions, mtGC);
   _free_set = new ShenandoahFreeSet(this, _num_regions);
-  _collection_set = new ShenandoahCollectionSet(this, sh_rs.base(), sh_rs.size());
 
   {
     ShenandoahHeapLocker locker(lock());