[racket] Untyped arithmetics performance

Jens Axel and Alexander have provided the answer, but in case it helps
to see what they mean, try these loops that display the intermediate
results:

 (for/fold ([v 0]) ([x (in-range 100)])
   (displayln v)
   (+ v (/ x 100)))

 (for/fold ([v 0]) ([x (in-range 100)])
   (displayln v)
   (+ v (/ x (- 100 x))))


At Sat, 24 Jan 2015 17:39:39 +0100, Daniel Kvasnička wrote:
> Hi, I'm observing the following difference in performance:
> 
> > (time (exact->inexact (for/sum ([x (in-range 10000)]) (/ x 10000))))
> cpu time: 6 real time: 6 gc time: 0
> 4999.5
> > (time (exact->inexact (for/sum ([x (in-range 10000)]) (/ x (- 10000 x)))))
> cpu time: 3624 real time: 3622 gc time: 39
> 87876.06036044382
> 
> The difference seems huge to me and I'm wondering whether it's "normal" and 
> what are the right tools to overcome it.
> If I understand it correctly, it's about contract checking and the fact that 
> there is one more math opeartion involved does not in itself lead to such 
> difference. The division operator needs to be sure that the outcome of the 
> minus op. is of the right type, is that correct?
> 
> If so -- and the increase in time cannot be considered a "bug" -- what is the 
> right way to get around that. I've employed 2 ways: Typed Racket and switching 
> the portion of code to racket/flonum. Both of these approaches resulted in big 
> perf. improvement and basically make the computation instant again. Are these 
> the right, "rackety", solutions? I've also tried to use unchecked ops but 
> Racket 6.1.1 is constantly segfaulting on me when running this (from my 
> digging I think unsafe-fl/ is what segfaults it):
> 
> (time (exact->inexact (for/sum ([x (in-range 10000)]) (unsafe-fl/ (->fl x) 
> (unsafe-fx- 10000 x)))))
> 
> Daniel
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