sysdeps/libm-ieee754/s_log2f.c

   1 /* e_logf.c -- float version of e_log.c.
   2  * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
   3  * adapted for log2 by Ulrich Drepper <drepper@cygnus.com>
   4  */
   5
   6 /*
   7  * ====================================================
   8  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   9  *
  10  * Developed at SunPro, a Sun Microsystems, Inc. business.
  11  * Permission to use, copy, modify, and distribute this
  12  * software is freely granted, provided that this notice
  13  * is preserved.
  14  * ====================================================
  15  */
  16
  17
  18 #include "math.h"
  19 #include "math_private.h"
  20
  21 #ifdef __STDC__
  22 static const float
  23 #else
  24 static float
  25 #endif
  26 ln2 = 0.69314718055994530942,
  27 two25 =    3.355443200e+07,     /* 0x4c000000 */
  28 Lg1 = 6.6666668653e-01, /* 3F2AAAAB */
  29 Lg2 = 4.0000000596e-01, /* 3ECCCCCD */
  30 Lg3 = 2.8571429849e-01, /* 3E924925 */
  31 Lg4 = 2.2222198546e-01, /* 3E638E29 */
  32 Lg5 = 1.8183572590e-01, /* 3E3A3325 */
  33 Lg6 = 1.5313838422e-01, /* 3E1CD04F */
  34 Lg7 = 1.4798198640e-01; /* 3E178897 */
  35
  36 #ifdef __STDC__
  37 static const float zero   =  0.0;
  38 #else
  39 static float zero   =  0.0;
  40 #endif
  41
  42 #ifdef __STDC__
  43         float __log2f(float x)
  44 #else
  45         float __log2f(x)
  46         float x;
  47 #endif
  48 {
  49         float hfsq,f,s,z,R,w,t1,t2,dk;
  50         int32_t k,ix,i,j;
  51
  52         GET_FLOAT_WORD(ix,x);
  53
  54         k=0;
  55         if (ix < 0x00800000) {                  /* x < 2**-126  */
  56             if ((ix&0x7fffffff)==0)
  57                 return -two25/(x-x);            /* log(+-0)=-inf */
  58             if (ix<0) return (x-x)/(x-x);       /* log(-#) = NaN */
  59             k -= 25; x *= two25; /* subnormal number, scale up x */
  60             GET_FLOAT_WORD(ix,x);
  61         }
  62         if (ix >= 0x7f800000) return x+x;
  63         k += (ix>>23)-127;
  64         ix &= 0x007fffff;
  65         i = (ix+(0x95f64<<3))&0x800000;
  66         SET_FLOAT_WORD(x,ix|(i^0x3f800000));    /* normalize x or x/2 */
  67         k += (i>>23);
  68         dk = (float)k;
  69         f = x-(float)1.0;
  70         if((0x007fffff&(15+ix))<16) {   /* |f| < 2**-20 */
  71             if(f==zero) return dk;
  72             R = f*f*((float)0.5-(float)0.33333333333333333*f);
  73             return dk-(R-f)/ln2;
  74         }
  75         s = f/((float)2.0+f);
  76         z = s*s;
  77         i = ix-(0x6147a<<3);
  78         w = z*z;
  79         j = (0x6b851<<3)-ix;
  80         t1= w*(Lg2+w*(Lg4+w*Lg6));
  81         t2= z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7)));
  82         i |= j;
  83         R = t2+t1;
  84         if(i>0) {
  85             hfsq=(float)0.5*f*f;
  86             return dk-((hfsq-(s*(hfsq+R)))-f)/ln2;
  87         } else {
  88             return dk-((s*(f-R))-f)/ln2;
  89         }
  90 }
  91 weak_alias (__log2f, log2f)