Cheap algorithm to find measure of angle between vectors

Question

Finding the angle between two vectors is not hard using the cosine rule. However, because I am programming for a platform with very limited resources, I would like to avoid

Answer 1

if you need to compute the square root, then consider using the invsqrt hack.

acos((x1*x2 + y1*y2) * invsqrt((x1*x1+y1*y1)*(x2*x2+y2*y2)));