Editing Control-flow graph (section)

==Loop management==

A ''loop header'' (sometimes called the ''entry point'' of the loop) is a dominator that is the target of a loop-forming back edge. The loop header dominates all blocks in the loop body.  A block may be a loop header for more than one loop.  A loop may have multiple entry points, in which case it has no "loop header".

Suppose block M is a dominator with several incoming edges, some of them being back edges (so M is a loop header). It is advantageous to several optimization passes to break M up into two blocks M<sub>pre</sub> and M<sub>loop</sub>. The contents of M and back edges are moved to M<sub>loop</sub>, the rest of the edges are moved to point into M<sub>pre</sub>, and a new edge from M<sub>pre</sub> to M<sub>loop</sub> is inserted (so that M<sub>pre</sub> is the immediate dominator of M<sub>loop</sub>). In the beginning, M<sub>pre</sub> would be empty, but passes like [[loop-invariant code motion]] could populate it. M<sub>pre</sub> is called the ''loop pre-header'', and M<sub>loop</sub> would be the loop header.