https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109008
--- Comment #19 from Richard Biener <rguenth at gcc dot gnu.org> --- I still think we should avoid iteration. Looking at plus we have x = y + a which is actually x = y + a +- 0.5ulp (y + a) (0.5ulp of the y + a result), so we can compute a as a = x - y +- 0.5ulp (y + a) note 0.5ulp (y + a) isn't 0.5ulp (x), not exactly at least, so we have to approximate it and a conservative bound is 1ulp (x) here. We may also have to make sure to round toward -Inf for the lower bound of the result range and +Inf of the upper bound of the result range I think. For multiplication it's x = y * a +- 0.5ulp (y * a) a = (x +- 0.5ulp (y * a)) / y so it's again similar. So what we can indeed do is widen the LHS range by 0.5 ulp and since we cannot represent that and it might be imprecise if 1 ulp _after_ the rounding is smaller than 1 ulp _before_ the rounding operation we simply use 1 ulp widening of the LHS?
