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[Bug math/13956] New: logbl() produces wrong results for subnormal 'long double' numbers
- From: "bruno at clisp dot org" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sources dot redhat dot com
- Date: Fri, 06 Apr 2012 14:22:48 +0000
- Subject: [Bug math/13956] New: logbl() produces wrong results for subnormal 'long double' numbers
- Auto-submitted: auto-generated
http://sourceware.org/bugzilla/show_bug.cgi?id=13956
Bug #: 13956
Summary: logbl() produces wrong results for subnormal 'long
double' numbers
Product: glibc
Version: 2.11
Status: NEW
Severity: normal
Priority: P2
Component: math
AssignedTo: unassigned@sourceware.org
ReportedBy: bruno@clisp.org
Classification: Unclassified
Created attachment 6326
--> http://sourceware.org/bugzilla/attachment.cgi?id=6326
test case
Seen on glibc 2.11 Linux/PowerPC,
on glibc 2.7 Linux/SPARC,
on glibc 2.7 Linux/HP-PA.
The function logbl() produces a wrong result for subnormal (denormalized)
arguments.
POSIX:2008 specifies the behaviour of this function for subnormal numbers:
<http://pubs.opengroup.org/onlinepubs/9699919799/>.
In particular, if x = 2^i is > 0, logb(x) must be = i.
But in glibc on the specified platforms, for some values of i < LDBL_MIN_EXP-1,
logb(x) is too large.
How to reproduce:
================================== foo.c ==================================
#include <float.h>
#include <math.h>
volatile long double x;
int main ()
{
int i;
for (i = 0, x = 1.0L; i >= LDBL_MIN_EXP - 55; i--, x *= 0.5L)
/* Either x = 2^i or x = 0.0. */
if ((i == LDBL_MIN_EXP - 2 || i == LDBL_MIN_EXP - 55)
&& (x > 0.0L && !(logbl (x) == (long double)i)))
return 1;
return 0;
}
===========================================================================
$ gcc foo.c -Wall -lm
$ ./a.out; echo $?
Expected result:
0
Actual result:
1
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