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 two25 =    3.355443200e+07,     /* 0x4c000000 */
  27 Lg1 = 6.6666668653e-01, /* 3F2AAAAB */
  28 Lg2 = 4.0000000596e-01, /* 3ECCCCCD */
  29 Lg3 = 2.8571429849e-01, /* 3E924925 */
  30 Lg4 = 2.2222198546e-01, /* 3E638E29 */
  31 Lg5 = 1.8183572590e-01, /* 3E3A3325 */
  32 Lg6 = 1.5313838422e-01, /* 3E1CD04F */
  33 Lg7 = 1.4798198640e-01; /* 3E178897 */
  34
  35 #ifdef __STDC__
  36 static const float zero   =  0.0;
  37 #else
  38 static float zero   =  0.0;
  39 #endif
  40
  41 #ifdef __STDC__
  42         float __log2f(float x)
  43 #else
  44         float __log2f(x)
  45         float x;
  46 #endif
  47 {
  48         float hfsq,f,s,z,R,w,t1,t2,dk;
  49         int32_t k,ix,i,j;
  50
  51         GET_FLOAT_WORD(ix,x);
  52
  53         k=0;
  54         if (ix < 0x00800000) {                  /* x < 2**-126  */
  55             if ((ix&0x7fffffff)==0)
  56                 return -two25/zero;             /* log(+-0)=-inf */
  57             if (ix<0) return (x-x)/zero;        /* log(-#) = NaN */
  58             k -= 25; x *= two25; /* subnormal number, scale up x */
  59             GET_FLOAT_WORD(ix,x);
  60         }
  61         if (ix >= 0x7f800000) return x+x;
  62         k += (ix>>23)-127;
  63         ix &= 0x007fffff;
  64         i = (ix+(0x95f64<<3))&0x800000;
  65         SET_FLOAT_WORD(x,ix|(i^0x3f800000));    /* normalize x or x/2 */
  66         k += (i>>23);
  67         dk = (float)k;
  68         f = x-(float)1.0;
  69         if((0x007fffff&(15+ix))<16) {   /* |f| < 2**-20 */
  70             if(f==zero) return dk;
  71             R = f*f*((float)0.5-(float)0.33333333333333333*f);
  72             return dk-(R-f);
  73         }
  74         s = f/((float)2.0+f);
  75         z = s*s;
  76         i = ix-(0x6147a<<3);
  77         w = z*z;
  78         j = (0x6b851<<3)-ix;
  79         t1= w*(Lg2+w*(Lg4+w*Lg6));
  80         t2= z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7)));
  81         i |= j;
  82         R = t2+t1;
  83         if(i>0) {
  84             hfsq=(float)0.5*f*f;
  85             return dk-((hfsq-(s*(hfsq+R)))-f);
  86         } else {
  87             return dk-((s*(f-R))-f);
  88         }
  89 }
  90 weak_alias (__log2f, log2f)