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exp() very wrong

Jumat, 27 Mei 2011

Reference Number : 
DPD200161380

Version : 

Intel c++ compiler 11.1

Operating System : 

Linux

Problem Description : 
Using Intel C++ compiler 11.1 and using both -lm and -limf, exp() computes the wrong values as follows:

Test vector 2723: FUT not close enough: Ulp error=402e788fe0000000=+1.52354726791381836e+01
With rounding(Downward)
Input arg =3fe5240000000000=+6.60644531250000000e-01
Computed =3ffefa04d69fe921=+1.93603977048639586e+00
Expect hi=3ffefa04d69fe930=+1.93603977048639919e+00

The following test case demonstrates the issue.
// tstcase.cpp  /* exp example */  #include <stdio.h>  #include <fenv.h>  #include <math.h>    int main ()  {    double param, result;      fesetround(FE_DOWNWARD);      param = 6.60644531250000000e-01;    result = exp (param);    printf ("The exponential value of %.20g is %.20g.\n", param, result );    return 0;  }    
$ icc -std=c99 -O0 tstcase.cpp
$ ./a.out
The exponential value of 0.66064453125 is 1.9360397704863958612.


Resolution Status : 
The issue is resolved in Intel C++ compiler 12.0 update 4.


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26 May, 2011


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Source: http://software.intel.com/en-us/articles/exp-very-wrong/
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