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weird behavior with built-in ignore function (a bug?)
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Date: -- (:)
From: Peng Zang <peng.zang@g...>
Subject: Re: [Caml-list] weird behavior with built-in ignore function (a bug?)
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Thanks for the link to that thread and the examples, it was very helpful.

Peng


On Sunday 25 November 2007 01:05:49 am Julien Moutinho wrote:
> On Sat, Nov 24, 2007 at 01:48:18PM -0500, Peng Zang wrote:
> > Ahhh... it's good to know this is a bug and that it has been fixed.  I
> > think for the interim I will simply define my own ignore function.  It
> > also bothers me that even in the fixed version ignore does not equal
> > itself.
>
> Have a glance at this little thread:
>  
> http://caml.inria.fr/pub/ml-archives/caml-list/2005/04/838a1399a42491c4db9a
>eaa684458a6b.en.html
>
> > I also don't see why ignore is even defined externally.
> > Doesn't the compiler optimize ignore calls away?
>
> Well, in bytecomp/translcore.ml in the primitives_table
> "%ignore" is associated with Pignore
>
> and in asmcomp/cmmgen.ml:
>   | Pignore ->
>
>       return_unit(remove_unit (transl arg))
>
> So I guess Pervasives.ignore is defined like that
> to be able to segregate it at compile time to avoid
> a function call.
>
> % cat t.ml
> ignore (Random.bits ())
>
> % ocamlopt -dcmm t.ml
> [...]
> (function camlT__entry ()
>  (app{random.ml:157,14-32} "camlRandom__bits_89" (load (+a "camlRandom"
> 52)) unit)
>  1a)
> [...]
>
>
> % cat tt.ml
> let ignore _ = ()
> in ignore (Random.bits ())
>
> % ocamlopt -dcmm tt.ml
> [...]
> (data int 2295 "camlTt__1": addr "camlTt__ignore_58" int 3)
> (function camlTt__ignore_58 (x/59: addr) 1a)
>
> (function camlTt__entry ()
>  (let ignore/58 "camlTt__1"
>    (app{random.ml:157,14-32} "camlRandom__bits_89"
>      (load (+a "camlRandom" 52)) unit))
>  1a)
> [...]
>
>
> BTW that's the use of return_unit which disables tail-rec optimization:
> % ocaml
>         Objective Caml version 3.10.1+dev2 (2007-11-20)
>
> # let rec f () = ignore (f ());;
> val f : unit -> unit = <fun>
> # f ();;
> Stack overflow during evaluation (looping recursion?).
>
> # let rec f () = f ();;
> val f : unit -> 'a = <fun>
> # f ();;
> Interrupted.
>
> # let rec f () = let _ = f () in ();;
> val f : unit -> unit = <fun>
> # f ();;
> Stack overflow during evaluation (looping recursion?).
>
> # let rec f () : unit = Obj.magic (f ());;
> val f : unit -> unit = <fun>
> # f ();;
> Interrupted.
>
> Regards,
>   Julien.
>
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