Pre-Header

• Many optimizations require code movement from inside a loop to just before it.
• Pre-header is a new empty block just before header.
  • All edges from outside to header go to pre-header
  • A new edge goes from pre-header to header

Loops With Common Headers

• Two loops with different headers are either disjoint or nested
• Two loops with the same header
  • Not clear if they are nested
  • Or are one loop
  • Can not be decided without looking at the source code