There are always different preferences on deciding when something needs to be run again. File modification time is a reasonable default that serves many (most?)

There are always different preferences on deciding when something needs to be run again. File modification time is a reasonable default that serves many (most?) cases pretty well. Yes, file hashes might be another way. On the other hand, if I modify a comment, the file hash will change, but re-compilation is not necessary. In this case, you might need yet another way... You can always write your Makefiles to compute and depend on hashes or whatever else you might consider appropriate for your exact use case.