The important thing isn't the flag (piece of CPU state) but the function which calculates it from the two inputs and result. That is a pure Boolean function: do these three values indicate signed overflow or unsigned carry?

  int a = b + c; // safe version, like ADD on Motorola 68K.

  if (add_overflow_int(a, b, c)) ... macro/function determines overflow

The compiler could optimize that into accessing a flag, if the three inputs were just involved in an addition. In the general case, it has to emit the logic to test the bits of a, b, c.