Editing Control-flow graph (section)

== Reachability ==
{{Main|Reachability}}

[[Reachability]] is a graph property useful in optimization.

If a subgraph is not connected from the subgraph containing the entry block, that subgraph is unreachable during any execution, and so is [[unreachable code]]; under normal conditions it can be safely removed.

If the exit block is unreachable from the entry block, an [[infinite loop]] may exist. Not all infinite loops are detectable, see [[Halting problem]]. A halting order may also exist there.

Unreachable code and infinite loops are possible even if the programmer does not explicitly code them: optimizations like [[constant propagation]] and [[constant folding]] followed by [[jump threading]] can collapse multiple basic blocks into one, cause edges to be removed from a CFG, etc., thus possibly disconnecting parts of the graph.