The BOT for G1 uses vm_page_size() to align the space required for the BOT offset table. This is wrong on Windows, as the reservations need to be aligned according to os::vm_allocation_granularity.

During development I observed crashes on Windows due to that.

The suggested fix is 

static size_t compute_size(size_t mem_region_words) {
  size_t number_of_slots = (mem_region_words / N_words);
-  return ReservedSpace::page_align_size_up(number_of_slots);
+  return ReservedSpace::allocation_align_size(number_of_slots);
}