[Caml-list] NaN reresentations

[Caml-list] NaN reresentations

[Andreas Rossberg]

Does the Ocaml implementation make guarantees about the stable representation of floats? In particular, if I use Int64.float_of_bits to create a particular NaN representation, am I guaranteed that its bit pattern is maintained no matter where the value is stored or passed?

We are currently in the process of implementing a reference interpreter for a little low-level language, and that tries to be as accurate as possible about float representations.

Thanks,
/Andreas

Re: [Caml-list] NaN reresentations

[Daniel Bünzli]

Le mercredi, 2 septembre 2015 à 18:56, Andreas Rossberg a écrit :

> Does the Ocaml implementation make guarantees about the stable representation of floats? In particular, if I use Int64.float_of_bits to create a particular NaN representation, am I guaranteed that its bit pattern is maintained no matter where the value is stored or passed?

I don't remember exactly but I think there may be issues with marshaling. A quick search with in the bt leads to:  

http://caml.inria.fr/mantis/view.php?id=5038

But there may be other to look at.  

Best,

Daniel

Re: [Caml-list] NaN reresentations

[Xavier Leroy]

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2015-09-02 10:56 GMT-07:00 Andreas Rossberg <rossberg@mpi-sws.org>:

> Does the Ocaml implementation make guarantees about the stable
> representation of floats? In particular, if I use Int64.float_of_bits to
> create a particular NaN representation, am I guaranteed that its bit
> pattern is maintained no matter where the value is stored or passed?
>

It depends on the underlying hardware.  For instance, with x86-32 bits,
some FP moves go through the x87 FP stack, undergoing a double -> extended
-> double conversion.   These conversions turn signaling NaNs into quiet
NaNs, and I'm not sure they preserve the other bits of the NaN payload.

On other platforms, esp. x86-64 bits, I'm pretty confident that NaN bits
are preserved by copying and parameter passing.


>
> We are currently in the process of implementing a reference interpreter
> for a little low-level language, and that tries to be as accurate as
> possible about float representations.
>

One possibility would be to represent your floats as int64 values (= their
bit-level representation), and convert only when you operate over them, e.g.

let fp_add x y = Int64.bits_of_float (Int64.float_of_bits x +.
Int64.float_of_bits y)

Best,

- Xavier

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Re: [Caml-list] NaN reresentations

[Andreas Rossberg]

On Sep 2, 2015, at 21:14 , Xavier Leroy <Xavier.Leroy@inria.fr> wrote:

> 2015-09-02 10:56 GMT-07:00 Andreas Rossberg <rossberg@mpi-sws.org>:
> > Does the Ocaml implementation make guarantees about the stable representation of floats? In particular, if I use Int64.float_of_bits to create a particular NaN representation, am I guaranteed that its bit pattern is maintained no matter where the value is stored or passed?
> 
> It depends on the underlying hardware.  For instance, with x86-32 bits, some FP moves go through the x87 FP stack, undergoing a double -> extended -> double conversion.   These conversions turn signaling NaNs into quiet NaNs, and I'm not sure they preserve the other bits of the NaN payload.
> 
> On other platforms, esp. x86-64 bits, I'm pretty confident that NaN bits are preserved by copying and parameter passing.

I see. Okay, in that case it seems safer not to rely on it, for the sake of portability.

> > We are currently in the process of implementing a reference interpreter for a little low-level language, and that tries to be as accurate as possible about float representations.
> 
> One possibility would be to represent your floats as int64 values (= their bit-level representation), and convert only when you operate over them, e.g.
> 
> let fp_add x y = Int64.bits_of_float (Int64.float_of_bits x +. Int64.float_of_bits y)

Right, that is the approach we are currently discussing. but I was wondering whether it is necessary.

Thanks!

/Andreas