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Message-ID: <4DD4D3C3.9050603@frisch.fr>
Date: Thu, 19 May 2011 10:24:35 +0200
From: Alain Frisch <alain@frisch.fr>
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To: Joel Reymont <joelr1@gmail.com>
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Subject: Re: [Caml-list] optimizing numerical code

On 05/18/2011 08:50 PM, Alain Frisch wrote:
> On 5/18/2011 8:35 PM, Joel Reymont wrote:
>> Consider the following two functions that I'm trying to optimize.
>>
>> Why is there an allocation for every iteration of the loop in
>> divergence1 but not in divergence2?
>
> First, note that the compiler does indeed unbox the reference cell acc
> as a local variable. It turns out that the float contained in the
> reference is not itself unboxed. This is due to the heuristic used by
> ocamlopt to unbox floats. Currently, a float variable is unboxed only if
> all the places where its value is used are unboxing contexts. A way to
> force unboxing in divergence1 is to replace (!acc) with (!acc +. 0) at
> the end. Without this change, the compilers can find a use of !acc as a
> boxed float (because this is what the function needs to return) and so
> it decides not to unbox acc at all.
>
>
> See also:
> http://caml.inria.fr/mantis/view.php?id=5204
>
> (The more_unboxing branch in the OCaml SVN implements a different
> unboxing strategy.)

Actually, even in this branch, acc would not be unboxed. The new 
heuristic unboxes float variables unless they are both needed in boxed 
form AND mutated, which is the case for acc in your function.

-- Alain