[Caml-list] OCaml not automatically specialising a polymorphic function

[Caml-list] OCaml not automatically specialising a polymorphic function

[Richard Jones]

- max3.ml ------------------------------------------------------------
let max a b =
  if a > b then a else b
in

print_int (max 2 3);;
----------------------------------------------------------------------

Looking at the assembler, OCaml doesn't work out that "max" has type
max : int -> int -> int, and so it generates very inefficient
code. This is a bit surprising because (I think) max can't be called
from outside the module, and the one place where it is called
specifies the type.

So I'm guessing here that OCaml doesn't really optimize across
functions?

Rich.

-- 
Richard Jones. http://www.annexia.org/ http://freshmeat.net/users/rwmj
Merjis Ltd. http://www.merjis.com/ - all your business data are belong to you.
"I wish more software used text based configuration files!"
 -- A Windows NT user, quoted on Slashdot.

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Re: [Caml-list] OCaml not automatically specialising a polymorphic function

[Xavier Leroy]

> - max3.ml ------------------------------------------------------------
> let max a b =
>   if a > b then a else b
> in
> 
> print_int (max 2 3);;
> ----------------------------------------------------------------------
> 
> Looking at the assembler, OCaml doesn't work out that "max" has type
> max : int -> int -> int

Yes, because it has type 'a -> 'a -> 'a according to the ML typing rules...
But if you help OCaml with a type constraint, you'll get the more
efficient code that you expect:

  let max (a:int) (b:int) = ...

> , and so it generates very inefficient
> code. This is a bit surprising because (I think) max can't be called
> from outside the module, and the one place where it is called
> specifies the type.
> So I'm guessing here that OCaml doesn't really optimize across
> functions?

You're correct that OCaml doesn't do type specialization for
user-defined functions (only for some predefined operations like ">"). 
It does perform some inter-function optimizations such as function
inlining and known call optimizations.

- Xavier Leroy

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