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[Bug math/13955] New: logbf() produces wrong results for subnormal 'float' 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 14:17:20 +0000
- Subject: [Bug math/13955] New: logbf() produces wrong results for subnormal 'float' numbers
- Auto-submitted: auto-generated
http://sourceware.org/bugzilla/show_bug.cgi?id=13955
Bug #: 13955
Summary: logbf() produces wrong results for subnormal 'float'
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 6325
--> http://sourceware.org/bugzilla/attachment.cgi?id=6325
test case
Seen on glibc 2.11 Linux/PowerPC,
on glibc 2.7 Linux/SPARC,
on glibc 2.7 Linux/HP-PA.
The function logbf() 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 i < FLT_MIN_EXP-1, logbf(x) is too
large.
How to reproduce:
================================== foo.c ==================================
#include <float.h>
#include <math.h>
volatile float x;
int main ()
{
int i;
for (i = 0, x = 1.0f; i >= FLT_MIN_EXP - 1; i--, x *= 0.5f)
;
/* Here i = FLT_MIN_EXP - 2. Either x = 2^i is subnormal or x = 0.0. */
if (x > 0.0f && !(logbf (x) == (float)i))
return 1;
return 0;
}
===========================================================================
$ gcc foo.c -Wall -lm
$ ./a.out; echo $?
Expected result:
0
Actual result:
1
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