Fwd: Incorrect conversion of unnormalized IEEE doubles

[<kynn@panix.com>]

In our lab we use PARI via the Perl module Math::Pari.  Recently I
discovered a bug in the way the Math::Pari::PARI function converts
unnormalized IEEE doubles into PARI reals (see bug report below, and
also at the listed URL).

The author of Math::Pari (Ilya Zakharevich) is of the opinion that the
bug is in the underlying PARI library (see correspondence below).

I was not able to find mention of this bug at
http://pari.math.u-bordeaux.fr/Bugs/, but on the other hand I was not
able to determine what versions numbers correspond to the
"pari-stable" and "pari" options in that page, so it is possible that
the version I am using, which is quite old (2.1.3), is not covered in
these bug reports and that the bug I'm referring to was already
fixed in "pari-stable".

Anyway, if any of you is able to reproduce the bug in the PARI
versions corresponding to the "pari-stable" or "pari" options, please
let me know, and I'll send in a formal bug report.

You can find more details about my machine's architecture, OS, etc.,
at http://rt.cpan.org/NoAuth/Bug.html?id=14891.

Regards,

KJ

======================================================================

Subject: Re: [cpan #14891] Incorrect conversion of unnormalized IEEE doubles
From: "Ilya Zakharevich via RT" <comment-Math-Pari@rt.cpan.org>
Reply-To: comment-Math-Pari@rt.cpan.org
RT-Ticket: cpan #14891
To: kynn@panix.com
Date: Tue,  4 Oct 2005 21:58:07 -0400 (EDT)


Full context and any attached attachments can be found at:
<URL: http://rt.cpan.org/NoAuth/Bug.html?id=14891 >

On Mon, Oct 03, 2005 at 12:19:11PM -0400, kynn@panix.com via RT wrote:
> Math::Pari produces wrong results when it converts unnormalized IEEE
> doubles to PARI reals.  For example
> 
>   print POSIX::DBL_MIN()/2
>   1.1125369292536e-308
> 
> ...while
> 
>   print Math::Pari::PARI( POSIX::DBL_MIN()/2 )
>   1.668805393880401037E-308
> 
> ...which is actually POSIX::DBL_MIN*(3/4).  This represents a relative
> error of 50%, which is far from negligible.
> 
> It appears that Math::Pari::PARI is converting numbers of the form
> 
>   POSIX::DBL_MIN()/2**$n
> 
> (where $n is a positive integer) into numbers of the form
> 
>   POSIX::DBL_MIN()*((2**$n)-1)/(2**(-$n-1))

Thanks for a great bug report?  Could you also produce a great bug
fix? ;-)

The problem is (IIRC) that Math::Pari uses a PARI API which
(apparently) is very rarely used otherwise.  So I would think it is a
GP/PARI bug exposed by Math::Pari...

Thanks,
Ilya
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