Control Flow Graph - Loop Management

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.

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_pre and M_loop. The contents of M and back edges are moved to M_loop, the rest of the edges are moved to point into M_pre, and a new edge from M_pre to M_loop is inserted (so that M_pre is the immediate dominator of M_loop). In the beginning, M_pre would be empty, but passes like loop-invariant code motion could populate it. M_pre is called the loop pre-header, and M_loop would be the loop header.