cpu - What has a better performance: multiplication or division?

Question

Which version is faster ? x * 0.5 or x / 2

Ive had a course at the university called computer systems some time ago. From back then i remember that multiplying two values can be achieved with comparably "simple" logical gates but division is not a "native" operation and requires a sum register that is in a loop increased by the divisor and compared to the dividend.

Now i have to optimise an algorithm with a lot of divisions. Unfortunately its not just dividing by two so binary shifting is no option. Will it make a difference to change all divisions to multiplications ?

update:

I have changed my code and didnt notice any difference. You're probably right about compiler optimisations. Since all the answers were great ive upvoted them all. I chose rahul's answer because of the great link.

Answer 1

Usually division is a lot more expensive than multiplication, but a smart compiler will often convert division by a compile-time constant to a multiplication anyway. If your compiler is not smart enough though, or if there are floating point accuracy issues, then you can always do the optimisation explicitly, e.g. change:

 float x = y / 2.5f;

to:

 const float k = 1.0f / 2.5f;

 ...

 float x = y * k;

Note that this is most likely a case of premature optimisation - you should only do this kind of thing if you have profiled your code and positively identified division as being a performance bottlneck.