literalt bv_utilst::overflow_add(
const bvt &op0, const bvt &op1, representationt rep)
{
if(rep==SIGNED)
{
// An overflow occurs if the signs of the two operands are the same
// and the sign of the sum is the opposite.
literalt old_sign=op0[op0.size()-1];
literalt sign_the_same=prop.lequal(op0[op0.size()-1], op1[op1.size()-1]);
bvt result=add(op0, op1);
return prop.land(sign_the_same, prop.lxor(result[result.size()-1], old_sign));
}
else if(rep==UNSIGNED)
{
// overflow is simply carry-out
return carry_out(op0, op1, const_literal(false));
}
else
assert(false);
}
literalt bv_utilst::overflow_sub(
const bvt &op0, const bvt &op1, representationt rep)
{
if(rep==SIGNED)
{
// We special-case x-INT_MIN, which is >=0 if
// x is negative, always representable, and
// thus not an overflow.
literalt op1_is_int_min=is_int_min(op1);
literalt op0_is_negative=op0[op0.size()-1];
return
prop.lselect(op1_is_int_min,
!op0_is_negative,
overflow_add(op0, negate(op1), SIGNED));
}
else if(rep==UNSIGNED)
{
// overflow is simply _negated_ carry-out
return !carry_out(op0, inverted(op1), const_literal(true));
}
else
assert(false);
}
