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From: "simplyrahul17 at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug math/15388] New: Erroneous output from floating point tan
Date: Tue, 23 Apr 2013 02:59:00 -0000
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http://sourceware.org/bugzilla/show_bug.cgi?id=15388

             Bug #: 15388
           Summary: Erroneous output from floating point tan
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: simplyrahul17@gmail.com
    Classification: Unclassified


On executing the following code (compiled with g++ 4.6.3 on Intel(R) Core(TM)
i7-2600 CPU @ 3.40GHz running Ubuntu 12.04.1 LTS):

#include<cmath>
#include<stdio.h>
using namespace std;
int main()
{
 float f = 4386557896710122704971697171202048.0;
 printf("%.16f\n", tan(f));
 return 0;
}

I get the output as 33554432.0
The expected output is 97800744.0

In hex, this means libm says
tan(0x77584625) = 0x4C000000
However, the output should be 0x4CBA8A45.

The "correct" output was obtained using WolframAlpha
(97800745.2669871207241704332677514328806018068212018723832)
and one other compiler does produce the right output
(in hex 0x4CBA8A45, in decimal 97800744.0)
for the same program.

I know that the input is a bit strange, but looking at the hex
it seems to be a valid representable floating point number.
Using double instead of float makes the problem go away.

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