code: public final long  at8( long i ) {
      long x = i - (_start>0 ? _start : 0);
      if( 0 <= x && x < _len ) return at80((int)x);
      throw new ArrayIndexOutOfBoundsException(""+_start+" <= "+i+" < "+(_start+_len));
    }
docs: Load a long value.  Floating point values are silently rounded to an
  integer.  Throws if the value is missing.
  <p>
  Loads from the 1-entry chunk cache, or misses-out.  This version uses
  absolute element numbers, but must convert them to chunk-relative indices
  - requiring a load from an aliasing local var, leading to lower quality
  JIT'd code (similar issue to using iterator objects).
  <p>
  Slightly slower than 'at0' since it range checks within a chunk.