[Tobba_]

On that subject, I'm curious whether there is any CPU out there that sets the overflow flag incorrectly when computing (-1) - n when n is the most negative number (which negates to itself, so implementing subtraction by simply negating the RHS and adding will screw up the flags).

[Vogtinator]

The ARM ARM documents sub(left, right, no carry) as add(left, ~right, carry set), which is also the most straightforward implementation if you have to account for carry anyway.