Re: e_pow.c bug

[Bryce McKinlay <mckinlay at redhat dot com>]

Steve Moshier wrote:

>  Math libraries copied from Sun Microsystem's fdlibm (www.netlib.org/fdlibm)
>  have a bug in the real power function, e_pow.c.  The bug is that
>  pow(x,y) returns 0 when x is very close to -1.0 and y is very large.
>  The following test program (in C) prints
>
>  pow(1.0000000000000002e+00 4.5035996273704970e+15) = 2.7182818284590455e+00
>  pow(-1.0000000000000002e+00 4.5035996273704970e+15) =0.0000000000000000e+00
>  pow(9.9999999999999978e-01 4.5035996273704970e+15) = 3.6787944117144222e-01
>  pow(-9.9999999999999978e-01 4.5035996273704970e+15) = 0.0000000000000000e+00
>
>  which is incorrect for the negative arguments raised to an odd integer
>  power.
>  
Interesting. It looks like our StrictMath.java, which is a port of the C 
code, also suffers from this problem. As does Sun's JRE as of 1.5-beta, 
from looking at the following test case. Any idea if this issue has been 
reported to Sun?

Bryce



public class PowTest
{
public static void main(String[] args)
{
  math();
  System.out.println("--");
  strictmath();
}

static void strictmath()
{
  double x, y, z;

  x = 1.0 + StrictMath.pow (2.0, -52.0);
  y = 1.0 + StrictMath.pow (2.0, 52.0);
  z = StrictMath.pow (x, y);
  System.out.println ("pow (" + x + " " + y + ") = " + z);
  x = -x;
  z = StrictMath.pow (x, y);
  System.out.println ("pow (" + x + " " + y + ") = " + z);
  x = 1.0 - StrictMath.pow (2.0, -52.0);
  z = StrictMath.pow (x, y);
  System.out.println ("pow (" + x + " " + y + ") = " + z);
  x = -x;
  z = StrictMath.pow (x, y);
  System.out.println ("pow (" + x + " " + y + ") = " + z);
}

static void math()
{
  double x, y, z;

  x = 1.0 + Math.pow (2.0, -52.0);
  y = 1.0 + Math.pow (2.0, 52.0);
  z = Math.pow (x, y);
  System.out.println ("pow (" + x + " " + y + ") = " + z);
  x = -x;
  z = Math.pow (x, y);
  System.out.println ("pow (" + x + " " + y + ") = " + z);
  x = 1.0 - Math.pow (2.0, -52.0);
  z = Math.pow (x, y);
  System.out.println ("pow (" + x + " " + y + ") = " + z);
  x = -x;
  z = Math.pow (x, y);
  System.out.println ("pow (" + x + " " + y + ") = " + z);
}
}