code: static final int nextSecondarySeed() {
          int r;
          Thread t = Thread.currentThread();
          if ((r = U.getInt(t, SECONDARY)) != 0) {
              r ^= r << 13;   // xorshift
              r ^= r >>> 17;
              r ^= r << 5;
          }
          else if ((r = mix32(seeder.getAndAdd(SEEDER_INCREMENT))) == 0)
              r = 1; // avoid zero
          U.putInt(t, SECONDARY, r);
          return r;
      }
docs: Returns the pseudo-randomly initialized or updated secondary seed.