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Date: Thu, 22 Jul 2004 19:10:48 -0500 (CDT)
From: Brian Hurt <bhurt@spnz.org>
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To: John Prevost <j.prevost@gmail.com>
cc: Ocaml Mailing List <caml-list@inria.fr>
Subject: Re: [Caml-list] Interesting optimization
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On Thu, 22 Jul 2004, John Prevost wrote:

> let update'' c =
>   let rec loop c' a =
>     if a >= 0
>       then loop (c' land 0xff) (succ a)
>       else c'
>   in
>   c := (loop !c 0)


let update''' c =
    let rec loop c i =
        let c = c land 0xff in
        if (i < max_int) then
            loop c (i+1)
        else
            c
    in
    loop c 0
;;

Haven't tried timing it.  But the core code on the x86 becomes:
Temp__loop_59:
.L101:
        andl    $511, %eax
        movl    $2147483647, %ecx
        cmpl    %ecx, %ebx
        jge     .L100
        addl    $2, %ebx
        jmp     .L101
        .align  16

I'm slightly disappointed that Ocaml didn't just
	cmpl	$2147483647, %ebx
and not use ecx at all.  Not that this is that big a problem.  The only 
other possible optimizations would be to recognize that the entire loop is 
pointless, and the function can be replaced by:
let update c = c land 0xff;;

I feel comfortable leaving that particular optimization in the hands of 
the programmer, however.

-- 
"Usenet is like a herd of performing elephants with diarrhea -- massive,
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                                - Gene Spafford 
Brian

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