IN OVERFLOW

Do not assign the result of the difference to a signed variable and
checking whether it is negative afterwards because this limits the max diff
to 2G on 32 bit systems. E.g. "signed = 3.5G - 1G" would be negative and the
code would assume that 3.5G < 1G. Instead compare the two variables directly
and assign to unsigned only if we know that the result of the subtraction
will be positive.

Discussed with:	Jimmy and Sunny (via IRC)