Bug#210400: marked as done (libc6: generic log() is no longer exactly rounded (regression))
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Date: Thu, 11 Sep 2003 09:54:44 +0200
From: Vincent Lefevre <vincent@vinc17.org>
To: Debian Bug Tracking System <submit@bugs.debian.org>
Subject: libc6: generic log() is no longer exactly rounded (regression)
Message-ID: <20030911075444.GA25610@ay.nerim.net>
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Package: libc6
Version: 2.3.2-5
Severity: normal
(Filled as a normal bug since it is a regression.)
With libc6 2.3.1-16, the generic log() function (as used on the PowerPC)
seemed to be exactly rounded. This is no longer the case with 2.3.2-5
(see the test below, which worked with libc6 2.3.1-16).
I couldn't see anything about this problem in the changelog except:
2002-10-25 Roland McGrath <roland@redhat.com>
* math/math.h (M_LOG2El): Correct the value.
From Stephen L Moshier <steve@moshier.net>.
but I don't know if this change is the cause of the regression.
File testlog.c:
------------------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
void out (double x, char *s)
{
int exp;
long long m;
m = frexp (x, &exp) * 9007199254740992LL;
printf ("%s = %-23.17g (mant: %lld/2^53, exp: %d)\n", s, x, m, exp);
}
int main (int argc, char *argv[])
{
double x, y;
if (argc != 2)
{
fprintf (stderr, "Usage: testlog <double>\n");
exit (1);
}
x = atof (argv[1]);
y = log (x);
out (x, " x");
out (y, "log(x)");
return 0;
}
------------------------------------------------------------------------
$ ./testlog 1.000000046697654
x = 1.000000046697654 (mant: 4503599837678033/2^53, exp: 1)
log(x) = 4.6697652875585992e-08 (mant: 7056749784587155/2^53, exp: -24)
Maple gives:
> Digits := 40;
Digits := 40
> evalf(log(4503599837678033/2^52)*2^(77));
16
0.7056749784587155500000005883462666272941 10
The result is closer to 7056749784587156/2^53 than 7056749784587155/2^53.
So, log(x) should have returned a number equal to 7056749784587156/2^53.
-- System Information:
Debian Release: testing/unstable
Architecture: powerpc
Kernel: Linux ay 2.4.18-newpmac #1 Mon Jun 9 03:10:07 UTC 2003 ppc
Locale: LANG=C, LC_CTYPE=en_US.ISO8859-1
Versions of packages libc6 depends on:
ii libdb1-compat 2.1.3-7 The Berkeley database routines [gl
-- no debconf information
---------------------------------------
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Date: Mon, 12 Dec 2005 17:56:24 +0100
From: Vincent Lefevre <vincent@vinc17.org>
To: 210400-done@bugs.debian.org
Subject: Re: libc6: generic log() is no longer exactly rounded (regression)
Message-ID: <20051212165624.GB9953@dixsept.loria.fr>
References: <20030911075444.GA25610@ay.nerim.net>
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On 2003-09-11 09:54:44 +0200, Vincent Lefevre wrote:
> Package: libc6
> Version: 2.3.2-5
> Severity: normal
>
> (Filled as a normal bug since it is a regression.)
>
> With libc6 2.3.1-16, the generic log() function (as used on the PowerPC)
> seemed to be exactly rounded. This is no longer the case with 2.3.2-5
> (see the test below, which worked with libc6 2.3.1-16).
I've just tested it again on the same machine with libc6 2.3.5-8 and
I no longer get this problem. As reported on
http://www.vinc17.org/research/testlibm/
the log now seems to be correctly rounded, as with libc6 2.3.1-16.
So, I'm closing the bug.
--
Vincent Lefèvre <vincent@vinc17.org> - Web: <http://www.vinc17.org/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.org/blog/>
Work: CR INRIA - computer arithmetic / SPACES project at LORIA
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