Re: [Caml-list] optimizing numerical code

[Alain Frisch <alain@frisch.fr>]

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.)


-- Alain