Re: [Caml-list] obj magic performance and other questions

[Damien Doligez <damien.doligez@inria.fr>]

On 2011-03-15, at 17:05, Nicolas Ojeda Bar wrote:

> let rec2_b x =
> match x.children with
> | Rec2 r -> r.b
> | _ -> assert false
> 
> This is not very efficient. I can use Obj.magic to access that same field as
> follows:
> 
> let rec2_b x =
> Obj.obj (Obj.field (Obj.field (Obj.repr x.children 0) 0))
> 
> My question is: is this use of Obj safe? and is this compiled to (two) simple
> array accesses? Or is Obj.obj doing something behind the scenes?


Not safe.

Your first version of rec2_b is safe: it throws an exception when the "downcast"
fails.  With your second version, anything can happen if the argument is not a
rec2 (program crash, wrong result, silent failure, strange side effects, correct
result, whatever).

Also, as others have noted, the Obj version might actually do more tests than the
proper one (I can't be bothered to check exactly).

-- Damien