If it's implemented in the preprocessor - e.g. expanding to a comma delimited char array - then the performance justification for adding it to the compiler is l

If it's implemented in the preprocessor - e.g. expanding to a comma delimited char array - then the performance justification for adding it to the compiler is lost. However you might be using "preprocessor directive" in the language semantic phase sense, not in the "preprocessor does it" sense.