Efficient algorithm for graph canonization for directed acyclic graphs?
According to Wikipedia (which references lecture notes of Zemlyachenko, Korneenko and Tyshkevic), isomorphism of directed acyclic graphs is GI-complete. Therefore any polynomial time canonicalization algorithm for directed acyclic graphs would result in a polynomial time algorithm for graph isomorphism. Since no such algorithm is known, one concludes that no polynomial time canonicalization algorithm for directed acyclic graphs is known, and it is conjectured that none exists.