compiler/stdc/math/s_logb.c

   1 /* @(#)s_logb.c 5.1 93/09/24 */
   2 /*
   3  * ====================================================
   4  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   5  *
   6  * Developed at SunPro, a Sun Microsystems, Inc. business.
   7  * Permission to use, copy, modify, and distribute this
   8  * software is freely granted, provided that this notice
   9  * is preserved.
  10  * ====================================================
  11  */
  12
  13 #ifndef lint
  14 static char rcsid[] = "$FreeBSD: src/lib/msun/src/s_logb.c,v 1.10 2005/12/03 11:57:19 bde Exp $";
  15 #endif
  16
  17 /*
  18  * double logb(x)
  19  * IEEE 754 logb. Included to pass IEEE test suite. Not recommend.
  20  * Use ilogb instead.
  21  */
  22
  23 #include "math.h"
  24 #include "math_private.h"
  25
  26 static const double
  27 two54 = 1.80143985094819840000e+16;     /* 43500000 00000000 */
  28
  29 double
  30 logb(double x)
  31 {
  32         int32_t lx,ix;
  33         EXTRACT_WORDS(ix,lx,x);
  34         ix &= 0x7fffffff;                       /* high |x| */
  35         if((ix|lx)==0) return -1.0/fabs(x);
  36         if(ix>=0x7ff00000) return x*x;
  37         if(ix<0x00100000) {
  38                 x *= two54;              /* convert subnormal x to normal */
  39                 GET_FLOAT_WORD(ix,x);
  40                 ix &= 0x7fffffff;
  41                 return (float) ((ix>>20)-1023-54);
  42         } else
  43                 return (double) ((ix>>20)-1023);
  44 }