[bhk]

Whether or not something is "undefined behavior" has nothing to do with the hardware. The C specification says what is specified, what is "unspecified", what is "implementation defined", and what is "undefined".

If something is "undefined" according to C, you can't rely on what the hardware does, because the hardware might not even get a chance to do anything. The compiler may completely elide sections of your program -- and they do in practice (for example, bounds checks).

Actually, hardware always does something reasonable for add instructions (throw exception or overflow or saturate). It's additions in C that can have unreasonable results.