> Are you saying C compilers are O(n)?

OP asked for sources that substantiated a technical claim. No assertion was given.

Unless you have a source that either supports or refutes the claim, trying to deflect the question does nothing to address the problem.

I very much expect O(n) compilation of stuff like array declarations, enums, CPP macro definitions and so forth. What else would it be?

They may well be, provided one stays away from bad patterns such as humongous complex functions?