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Date: Thu, 13 Jun 2002 12:30:49 -0700 (PDT)
From: Stephen L Moshier <steve@moshier.net>
To: freebsd-gnats-submit@FreeBSD.org
Subject: error in e_pow.c
X-Send-Pr-Version: www-1.0

>Number:         39236
>Category:       misc
>Synopsis:       error in e_pow.c
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          closed
>Quarter:        
>Keywords:       
>Date-Required:  
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Thu Jun 13 12:40:01 PDT 2002
>Closed-Date:    Mon Jun 24 03:16:31 PDT 2002
>Last-Modified:  Mon Jun 24 03:16:31 PDT 2002
>Originator:     Stephen L Moshier
>Release:        any
>Organization:
>Environment:
>Description:
 pow(x,y) returns 0 when x is very close to -1.0 and y is very large.

>How-To-Repeat:
   The following test program 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.
   double pow (double, double);

int
main ()
{
  double x, y, z;

  x = 1.0 + pow (2.0, -52.0);
  y = 1.0 + pow (2.0, 52.0);
  z = pow (x, y);
  printf ("pow(%.16e %.16e) = %.16e\n", x, y, z);
  x = -x;
  z = pow (x, y);
  printf ("pow(%.16e %.16e) = %.16e\n", x, y, z);
  x = 1.0 - pow (2.0, -52.0);
  z = pow (x, y);
  printf ("pow(%.16e %.16e) = %.16e\n", x, y, z);
  x = -x;
  z = pow (x, y);
  printf ("pow(%.16e %.16e) = %.16e\n", x, y, z);
}

>Fix:
      freebsd: src/lib/msun/src/e_pow.c
*** e_pow.c     1999/07/13 23:49:20     1.1
--- e_pow.c     2002/06/13 17:06:57
***************
*** 224,230 ****
            if(ix>0x3ff00000) return (hy>0)? C[4]*C[4]:C[5]*C[5];
        /* now |1-x| is tiny <= 2**-20, suffice to compute
           log(x) by x-x^2/2+x^3/3-x^4/4 */
!           t = x-1;            /* t has 20 trailing zeros */
            w = (t*t)*(0.5-t*(0.3333333333333333333333-t*0.25));
            u = C[25]*t;        /* ivln2_h has 21 sig. bits */
            v = t*C[26]-w*C[24];
--- 224,230 ----
            if(ix>0x3ff00000) return (hy>0)? C[4]*C[4]:C[5]*C[5];
        /* now |1-x| is tiny <= 2**-20, suffice to compute
           log(x) by x-x^2/2+x^3/3-x^4/4 */
!           t = ax-1;           /* t has 20 trailing zeros */
            w = (t*t)*(0.5-t*(0.3333333333333333333333-t*0.25));
            u = C[25]*t;        /* ivln2_h has 21 sig. bits */
            v = t*C[26]-w*C[24];

>Release-Note:
>Audit-Trail:
State-Changed-From-To: open->closed 
State-Changed-By: bde 
State-Changed-When: Mon Jun 24 03:14:33 PDT 2002 
State-Changed-Why:  
Patch applied in rev.1.9 (-current) and rev.1.6.2.1 (RELENG_4) of e_pow.c. 
Applied same patch to e_powf.c although it is just cosmetic there. 

Thanks. 

http://www.freebsd.org/cgi/query-pr.cgi?pr=39236 
>Unformatted:
