The meaning of nubBy when applied to functions other than equivalence relations changed around the time you created compiler/cmm/Debug.hs. This makes it extra tricky to figure out exactly what that nubBy is expected to do. Do you think you could explain? Furthermore, it would be helpful to know what (if anything) is known about the relationships between the source spans in the the argument to the nubBy. This code is proving to be a bit of a hot spot in #11095 and the better we understand what's going on the better our chances of coming up with good solutions. Thanks, David Feuer