Re: [Caml-list] OCaml Speed for Block Convolutions

[Chris Hecker <checker@d6.com>]

>maybe it would be better to just make the
>straightforward functional implementation fast:
>  let loop i total =
>    if i=n then total else
>    loop (i+1) (total + Array.unsafe_get xs i)
>  in loop 0 0.0

I was planning on asking about this after I timed it relative to a nonrecursive loop.  I noticed a while back that Markus' code for the Language Shootout (for example, http://www.bagley.org/~doug/shootout/bench/matrix/) does tail recursion rather than looping, so I was curious about whether it was superior.

However, this doesn't solve all the problems where this crops up:

let a = ref 0 in
Hashtbl.iter (fun k d -> a := !a + d) my_hash

or 

let hash_to_list h =
        let l = ref [] in
        Hashtbl.iter (fun k d -> l := d :: !l) h

or whatever.  Sometimes these can be handled by fold type functions, but not always (Hashtbl has no fold, for example).

Sometimes you need an imperative variable around, and it'd be nice if it was optimal in the easy cases.  Since function calls are so cheap these days on branch predicting CPUs, these functional map/iter/fold type things can still be high performance.

I haven't done any performance timings yet on these types of operations, though, so a huge grain of salt is attached to this email.

Chris


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