Let the compiler squeeze all it can out of the letter of the standard.
Let the programmer have the compiler abide by the spirit of the language.
Right. And what is the spirit of C? I contend that it is this: "Conformant code flies faster than the lightning, and non-conformant code makes demons fly out of your nose, because if you have code that exhibits any of the 193 (or whatever) C99 undefined behaviors plus however many later standards have added, IT'S YOUR FAULT."