# [racket] Style and Performance question

At Tue, 10 May 2011 10:05:35 -0400, Matthias Felleisen wrote:
>* 2. The addition in lieu of the multiplication is consistently the fastest
*>* version of the three I mentioned last night:
*>*
*>* (: the-sqrt : Real -> Real)
*>* (define (the-sqrt y)
*>* (let loop [(x (/ y 2.0))]
*>* (let [(error (- y (* x x)))]
*>* (if (< (abs error) epsilon)
*>* x
*>* (loop (+ x (/ error (+ x x))))))))
*>*
*>* Our compiler should probably implement reduction of strength optimizations
*>* based on this one experiment alone. The savings here are over 10%.
*
In the variant where you divide by 2, are you using `2' or `2.0'?
I'd expect `(/ error 2.0 x)' to be faster than `(/ error (+ x x))' in
the case that `x' and `error' are flonums, because it avoids a boxing
step. But `(/ error 2 x)' would be slower, because it mixes a fixnum
with floats.
Of course, if you want the code to go fast, either use flonum
operations or make the type `Float':
(: my-sqrt : Natural -> Float)
(define (my-sqrt y)
(let loop [(x (/ (exact->inexact y) 2.0))]
(let [(error (- y (* x x)))]
(if (< (abs error) epsilon)
x
(loop (+ x (/ error (+ x x))))))))
Then it's unlikely to matter whether you use `(/ error (+ x x))' or `(/
error 2 x)' because there's no representation mixing and flonums are
unboxed.