If you know the divisor x ahead of time you can pre-compute 1/x at compile time, so that your actual compiled code never does the division--only your compiler does. Your actual compiled code just does the multiplication by a pre-computed constant (and the compiled code doesn't have to know that that constant was pre-computed as the reciprocal of x).

Often, you can amortize a division or reciprocal by calculating it once and then reusing it. Frequently the divisor is dynamic, but reused locally.

For example, if you want to normalize a 3D vector you could do:

    mag = sqrt(x*x + y*y + z*z)
    x /= mag
    y /= mag
    z /= mag

That's three divisions with the same divisor. But you could instead do:

    invMag = 1.0 / sqrt(x*x + y*y + z*z)
    x *= invMag
    y *= invMag
    z *= invMag

There's still a single division (or reciprocal) done here. But you've eliminated at least the other two. (And it's even better if you have an rsqrt function.)

There’s no lookup needed if the value is constant