undefined behaviour

If a programer wants to write non portable code, by using such vendor specific extensions, he can. Such constructs indeed exists between Linux and GCC, but not in this case.

If no such extension exists and some code triggers an undefined behavior, then is just a bug in this code. It can be because of a false assumption of the programmer about what the standard says (so the programmer thinks a program is portable and correct on some aspect while it is indeed not) or because of a bug (seems to be the case here). In NO way this is a bug of the compiler. In NO way this is an indication the compiler is bad. There are more people than you suspect that value compilers that produces fast binaries, thanks of optimisations completely allowed by the standard.

As long as the compiler does not pretend to support undefined behaviors it indeed does not support, this is reasonable. This is indeed the very way to use standardized technology : do not rely on non standard features unless you are concious this is non portable and sure the way you use it is supported by the implementation.

And about NULL pointer dereferencing, the unreasonable behavior is indeed to think it can be dereferenced under special circonstances. There is no good reason to do that. The very purpose of the NULL pointer is to designate that something does NOT point to a valid object. ("If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compater unequal to a pointer to any object or function".) The standard even says that NULL is an _invalid_ value for dereferencing a pointer.

As for "A C compiler should wrap around numbers on a two's complement system", this is not in the standard, and this is indeed NOT what a lot of people think it should do when the alternative is an optimisation. This clearly can't be portable as C is not limited to 2 complement computers, and this is not needed even on 2 complement computers as you can cast to (unsigned) and then back to (int) if you really do want 2-complement behavior on such computers, with the additionnal advantage that such construct triggers future readers of the code into thinking there can be an overflow there the original programmer has thought about.