title: What does __sync_synchronize do?
body: <p>I saw an <a href="https://stackoverflow.com/questions/890847/uint64-utc-time">answer to a question regarding timing</a> which used __sync_synchronize().  </p>
  
  <ul>
  <li>What does this function do?</li>
  <li>And when is it necessary to be used?</li>
  </ul>