IEEE 754 requires that -0.0 and 0.0 compare equal.

> Does x*0 evaluate to 0.0

No, but it will compare equal, unless x is either infinite or a NaN.

> Does x+0 evaluate to x? Maybe!

Yes, unless x is either infinite or a NaN.

No, if x = -0.0 then x+0 = 0.0 and x*0 = -0.0. This violates fundamental tenets of arithmetic, surprises most, and makes many compiler optimizations impossible.