Re: [Caml-list] OCaml optimization

[Mark Shinwell <mshinwell@janestreet.com>]

The forthcoming version of OCaml (4.03) with the flambda optimization
passes can compile the example "sum0" to code that does not allocate.
(The particular optimization that enables that probably won't be
enabled by default in 4.03, though.)

Mark

On 29 November 2015 at 18:16, Mark Hayden <markghayden@yahoo.com> wrote:
> Below are two ways to write array summation in OCaml, sum0 and sum1.  Both
> of them use recursive functions.  ’sum0' allocates a closure on each call
> but I think is more natural.  ‘sum1’ does do any allocation, but is awkward
> to write.  Is there a compiler flag or other way to get the OCaml native
> compiler to avoid closure allocation in functions in the form of ‘sum0’?
>
> best, Mark
>
>
> Running each function 1000 times show sum0 is allocating 6 words on each
> call.
>
> % ocamlopt -o loop0 -S -inline 10 -unsafe loop0.ml
> ./loop0
> sum0:6029
> sum1:23
>
>
>
> open Printf ;;
>
> (* Allocates
>  *)
> let sum0 a =
>   let len = Array.length a in
>   let rec loop ofs sum =
>     if ofs < len then (
>       loop (succ ofs) (sum + a.(ofs))
>     ) else (
>       sum
>     )
>   in
>   loop 0 0
> ;;
>
>
> let rec sum1_help a len ofs sum =
>   if ofs < len then (
>     sum1_help a len (succ ofs) (sum + a.(ofs))
>   ) else (
>     sum
>   )
> ;;
>
> (* Does not allocate
>  *)
> let sum1 a =
>   let len = Array.length a in
>   sum1_help a len 0 0
> ;;
>
>
> let test which sum =
>   let a = Array.make 123 123 in
>
>   let minor0 = (Gc.stat ()).Gc.minor_words in
>
>   for ofs = 0 to 1000 do
>     ignore (sum a)
>   done ;
>
>   let minor1 = (Gc.stat ()).Gc.minor_words in
>
>   printf "%s:%.0f\n" which (minor1 -. minor0) ;
> ;;
>
> let _ =
>   test "sum0" sum0 ;
>   test "sum1" sum1 ;
> ;;
>
>
>
>
>
>