lib/libm/src/e_log10f.c

   1 /* e_log10f.c -- float version of e_log10.c.
   2  * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
   3  */
   4
   5 /*
   6  * ====================================================
   7  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   8  *
   9  * Developed at SunPro, a Sun Microsystems, Inc. business.
  10  * Permission to use, copy, modify, and distribute this
  11  * software is freely granted, provided that this notice
  12  * is preserved.
  13  * ====================================================
  14  */
  15
  16 #include <sys/cdefs.h>
  17 #if defined(LIBM_SCCS) && !defined(lint)
  18 __RCSID("$NetBSD: e_log10f.c,v 1.8 2002/05/26 22:01:51 wiz Exp $");
  19 #endif
  20
  21 #include "math.h"
  22 #include "math_private.h"
  23
  24 static const float
  25 two25      =  3.3554432000e+07, /* 0x4c000000 */
  26 ivln10     =  4.3429449201e-01, /* 0x3ede5bd9 */
  27 log10_2hi  =  3.0102920532e-01, /* 0x3e9a2080 */
  28 log10_2lo  =  7.9034151668e-07; /* 0x355427db */
  29
  30 static const float zero   =  0.0;
  31
  32 float
  33 __ieee754_log10f(float x)
  34 {
  35         float y,z;
  36         int32_t i,k,hx;
  37
  38         GET_FLOAT_WORD(hx,x);
  39
  40         k=0;
  41         if (hx < 0x00800000) {                  /* x < 2**-126  */
  42             if ((hx&0x7fffffff)==0)
  43                 return -two25/zero;             /* log(+-0)=-inf */
  44             if (hx<0) return (x-x)/zero;        /* log(-#) = NaN */
  45             k -= 25; x *= two25; /* subnormal number, scale up x */
  46             GET_FLOAT_WORD(hx,x);
  47         }
  48         if (hx >= 0x7f800000) return x+x;
  49         k += (hx>>23)-127;
  50         i  = ((u_int32_t)k&0x80000000)>>31;
  51         hx = (hx&0x007fffff)|((0x7f-i)<<23);
  52         y  = (float)(k+i);
  53         SET_FLOAT_WORD(x,hx);
  54         z  = y*log10_2lo + ivln10*__ieee754_logf(x);
  55         return  z+y*log10_2hi;
  56 }