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A back edge in a flow graph is an edge

  • A.from a node to one of its dominators
  • B.from a node to any node earlier in textual order
  • C.leaving the procedure and returning to its caller
  • D.connecting two nodes that have no common dominator

Answer: A

Correct answer: from a node to one of its dominators

A dominator of a node is a node through which every path to it must pass, so an edge back to a dominator closes a cycle in which the target is guaranteed to precede the source. This is what identifies a natural loop, and finding back edges is therefore the first step in every loop transformation.

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