title: What information does GCC Profile Guided Optimization (PGO) collect and which optimizations use it?
body: <p>Which information does GCC collect when I enable <code>-fprofile-generate</code> and which optimization does in fact uses the collected information (when setting the <code>-fprofile-use</code> flag) ?</p>
  
  <p>I need citations here. I've searched for a while but didn't found anything documented.</p>
  
  <p>Information regarding link-time optimization (LTO) would be a plus! =D</p>