--- old/src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.cpp	2020-05-21 12:06:06.035975204 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.cpp	2020-05-21 12:06:05.567975366 +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	2020-05-21 12:06:07.463974709 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.hpp	2020-05-21 12:06:07.023974861 +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	2020-05-21 12:06:08.883974216 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp	2020-05-21 12:06:08.447974367 +0200
@@ -292,9 +292,35 @@
                               "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 (subject to encoding constraints) and try to reserve the space there.
+  // If not successful, bite a bullet and allocate at whatever address.
+  {
+    size_t cset_align = MAX2<size_t>(os::vm_page_size(), os::vm_allocation_granularity());
+    size_t cset_size = align_up(((size_t) 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 << 30u);
+
+    for (uintptr_t addr = min; addr <= max; addr <<= 1u) {
+      char* req_addr = (char*)addr;
+      assert(is_aligned(req_addr, cset_align), "Should be aligned");
+      ReservedSpace cset_rs(cset_size, os::vm_page_size(), false, req_addr);
+      if (cset_rs.is_reserved()) {
+        assert(cset_rs.base() == req_addr, "Allocated where requested: " PTR_FORMAT ", " PTR_FORMAT, p2i(cset_rs.base()), addr);
+        _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());