Re: [Caml-list] The verdict on "%identity"

["Török Edwin" <edwin+ml-ocaml@etorok.net>]

On 11/19/2012 07:49 PM, Dario Teixeira wrote:
> Hi,
> 
> 
> I've found conflicting information regarding the use of "%identity",
> which I hope to see clarified.
> 
> Let's consider a typical example where a module defines an abstract
> type t and provides (de)serialisation functions of_string/to_string.
> Moreover, the actual implementation of t uses a string, and the
> (de)serialisation functions are just identities:
> 
>   module Foo:
>   sig
>         type t
> 
>         val of_string: string -> t
>         val to_string: t -> string
>   end =
>   struct
>         type t = string
> 
>         let of_string x = x
>         let to_string x = x
>   end
> 
> 
> In practice, it's not unusual for such code to be implemented using
> the compiler's "%identity" builtin, all in the name of performance:
> 
>   module Foo:
>   sig
>         type t

Wouldn't 'type t = private string' help the compiler optimize this?

> 
>         external of_string: string -> t = "%identity"
>         external to_string: t -> string = "%identity"
>   end =
>   struct
>         type t = string
> 
>         external of_string: string -> t = "%identity"
>         external to_string: t -> string = "%identity"
>   end
> 
> 
> I realise that the use of "%identity" is dangerous.  This is, after all,
> how Obj.magic is defined.  Moreover, it uglifies interface definitions
> and makes a ridicule of the abstraction.  However, on the assumption that
> ocamlopt won't otherwise optimise away the no-op across module boundaries,
> the use of "%identity" may well be justified for performance reasons.
> 
> With all the above in mind, I have two questions:
> 
> 1) Is the assumption correct that today's ocamlopt won't optimise no-ops
>    across module boundaries? (I know that ocamlopt does not generally engage
>    in MLton-style whole programme optimisation, but is this also true for
>    low-hanging fruit such as the first example above?)
> 
> 2) Consider the code below.  For which modules can one expect of_string calls
>    to be optimised across module boundaries?
> 
>   module type SIG1 = sig type t val of_string: string -> t end
>   module type SIG2 = sig type t external of_string: string -> t = "%identity" end
> 
>   module Impl1 = struct type t = string let of_string x = x end
>   module Impl2 = struct type t = string external of_string: string -> t = "%identity" end
> 
>   module A: SIG1 = Impl1
>   module B: SIG1 = Impl2
>   module C: SIG2 = Impl1
>   module D: SIG2 = Impl2
> 
> Thank you in advance for your time!
> Best regards,
> Dario Teixeira
> 
>