The projected savings are on 64-bit only: 8 bytes savings per hotspot Method instance.

When allocating a Method class, also allocate the ConstMethod in a consecutive block in metaspace. This guarantees the distance from the Method to its _constMethod is never larger than 32-bits.
Also, change Method::_method_counters to a 2-level index with a 16bit top and 16bit bottom.

class Method {
private:
  jint _const_method_offset;
  ConstMethod* constMethod() const { return (ConstMethod*)(address(this) + _const_method_offset); }
  ushort _method_counter_block;
  ushort _method_counter_index;

  MethodCounters* method_counters() const {
    if (_method_counters_block == 0xffff) {
        return NULL;
    }
    return &(_blocks[_method_counters_block][_method_counter_index]);
  }

  static MethodCounters** _blocks;

   ...
}

Note: in a CDS image, the Method and ConstMethod are allocated from two different regions (RW and RO), so they cannot be allocated consecutively. However, we can make sure that the (SharedReadWriteSize + SharedReadOnlySize) is less than 4GB, so _const_method_offset can still fit in 32 bits.