src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

rev 59366 : 8245464: Shenandoah: allocate collection set bitmap at lower addresses
Reviewed-by: XXX

*** 290,302 ****
    if (!region_storage.special()) {
      os::commit_memory_or_exit(region_storage.base(), region_storage_size, region_page_size, false,
                                "Cannot commit region memory");
    }
  
    _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());
  
      for (size_t i = 0; i < _num_regions; i++) {
--- 290,324 ----
    if (!region_storage.special()) {
      os::commit_memory_or_exit(region_storage.base(), region_storage_size, region_page_size, false,
                                "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);
  
    {
      ShenandoahHeapLocker locker(lock());
  
      for (size_t i = 0; i < _num_regions; i++) {