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Re: NaN/Inf fix for ECPG

From: Boszormenyi Zoltan <zb(at)cybertec(dot)at>
To: Rémi Zara <remi_zara(at)mac(dot)com>
Cc: Michael Meskes <meskes(at)postgresql(dot)org>,PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>,Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: NaN/Inf fix for ECPG
Date: 2010-02-25 10:26:12
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Rémi Zara írta:
> Le 24 févr. 2010 à 18:58, Boszormenyi Zoltan a écrit :
>> Here's the attached test code. Compile it with
>> gcc -Wall -o nantest nantest.c -lm
>> and run it. It tests NAN anf INFINITY values with isinf() and isnan().
>> The expected output is:
>> ==================
>> $ ./nantest
>> computed NAN
>> 1 0
>> computed INFINITY
>> 0 1
>> ==================
>> Instead of "computed", NetBSD/x86-64 prints "defined"
>> but the test results are the same as under Linux/x86-64.
> Here it is :

First, thanks for running it.

> -bash-4.1$ gcc -Wall -o nantest nantest.c -lm
> -bash-4.1$ ./nantest 
> defined NAN
> 0 1

So: isnan((double)NAN) == false, isinf((double)NAN) == true?
No wonder this causes a little problem.

> defined INFINITY
> 0 1
> Ok. So, on NetBSD/mips (#ifdef __NetBSD__ && __mips__), isnan(NAN) is true, isnan((double)NAN) is false, and isnan((double)(0.0 / 0.0)) is true.
> Regards,
> Rémi Zara

NAN on NetBSD/x86-64 is defined as:

  extern const union __float_u __nanf;
  #define NAN      __nanf.__val

I would guess that it's similar on mips. Is is possible that
NetBSD/mips has a conversion bug?

What I don't get is that the code I used in ECPG and in this
test code is the same as in src/backend/utils/adt/float.c. E.g.:
float8in sees "NaN" -> value will be (double)NAN
float8out sees isnan(value) -> outputs "NaN" string

Can someone shed some light on why the backend
doesn't get the problem as above? :-(

As Rémi says, isnan((double)(0.0 / 0.0)) == true for him.
Michael: IIRC, IEEE754 explicit about that the (0.0/0.0) division
produces NaN. How about doing it explicitely in ECPG?

Rémi: please, run this code to confirm the above?

Best regards,
Zoltán Böszörményi

Bible has answers for everything. Proof:
"But let your communication be, Yea, yea; Nay, nay: for whatsoever is more
than these cometh of evil." (Matthew 5:37) - basics of digital technology.
"May your kingdom come" - superficial description of plate tectonics

Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH

Attachment: nantest1.c
Description: text/x-csrc (531 bytes)

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