:facepalm: my bad, I completely missed the more rational intepretation of OP's comment... I interpreted "directly representable" as "uniquely representable", a

:facepalm: my bad, I completely missed the more rational intepretation of OP's comment... I interpreted "directly representable" as "uniquely representable", all < 15 digit decimals are uniquely represented in fp64 so it is always safe to roundtrip between those decimals <-> f64, though indeed this guarantee is lost once you perform any math.