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32 bit integers
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Date: -- (:)
From: Xavier Leroy <Xavier.Leroy@i...>
Subject: Re: 32 bit integers
> several people have been requesting features to be included in a
> forthcoming O'Caml/O'Labl merge, so I thought I'd add a humble plea for
> some support for untagged (presumably boxed) integers. I'm looking
> mostly at camlidl, the documentation for which says: "Since the Caml int
> type has one bit of tag, the conversion from IDL types int and long
> loses the most significant bit on 32-bit platforms". I don't think
> that's terribly satisfactory.

We all agree on that.  The "int" type will almost certainly remain
unboxed and tagged (i.e. with one fewer bits than the platform's
native integers) in the near future, but additional (boxed, untagged)
integer types are certainly important for foreign-function interface.

See for instance J.-C. Filliatre's "Int32" library,
http://www.lri.fr/~filliatr/ftp/ocaml/int32/
(although I object to the name of that library -- on an Alpha, it
implements 64-bit integers...)

> If there isn't going to be a 32-bit type,
> can anyone suggest a workaround (eg can I convince camlidl to map all
> ints to floats)?

I'm not sure you really want to do it for all ints.  But you can do it
on a function-per-function basis in at least two ways.  Consider a C
function

        int myfunc(int x)

where the two "int" need to be represented as floats in Caml.

The first way to do this is to cheat in the .idl file given to Camlidl:
just declare myfunc as

        double myfunc(double x)

and make sure that the generated C stub code knowns the real prototype
of "myfunc", either by including a header file or by putting an extra
prototype:

        quote(C, "#include <myfunc.h>")
or
        quote(C, "extern int myfunc(int x);")

Then, the stub code generated by Camlidl will treat the argument and
the result of "myfunc" as doubles, but the C compiler will insert the
required int<->double coercions in the actual call to "myfunc" in the
stub code.

The second, more general way is to define (using typedef in the .idl
file) a C type int32 synonymous for int, but represented in Caml as float:

  quote(C, "
    static value int32_c2ml(int * input)
    { return copy_double((double) (*input)); }
    static void int32_ml2c(value input, int * output)
    { *output = (int) Double_val(input); }
  ")

  typedef [mltype("float"),c2ml(int32_c2ml),ml2c(int32_ml2c)] int int32;

  int32 myfunc(int32 x);

This approach is more general in that you can use other Caml types
than float, e.g. Filliatre's Int32.t type.  Of course, you have to
provide manually the correct conversion functions (c2ml and ml2c).

Hope this helps,

- Xavier Leroy