"Nonportable" is a significant element of this definition. A programmer who intends to compile their C program for one particular processor family might reasonably expect to write code which makes use of the very-much-defined behavior found on that architecture: integer overflow, for example. A C compiler which does the naively obvious thing in this situation would be a useful tool, and many C compilers in the past used to behave this way. Modern C compilers which assume that the programmer will never intentionally write non-portable code are.... less helpful.