Quelle e_logf.cpp Sprache: C

/* e_logf.c -- float version of e_log.c.
* Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
*/

/*
* ====================================================
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
*
* Developed at SunPro, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/

//#include <sys/cdefs.h>
//__FBSDID("$FreeBSD$");

#include "math_private.h"

static const float
ln2_hi =   6.9313812256e-01, /* 0x3f317180 */
ln2_lo =   9.0580006145e-06, /* 0x3717f7d1 */
two25 =    3.355443200e+07, /* 0x4c000000 */
/* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */
Lg1 =      0xaaaaaa.0p-24, /* 0.66666662693 */
Lg2 =      0xccce13.0p-25, /* 0.40000972152 */
Lg3 =      0x91e9ee.0p-25, /* 0.28498786688 */
Lg4 =      0xf89e26.0p-26; /* 0.24279078841 */

static const float zero   =  0.0;
static volatile float vzero = 0.0;

float
__ieee754_logf(float x)
{
float hfsq,f,s,z,R,w,t1,t2,dk;
int32_t k,ix,i,j;

GET_FLOAT_WORD(ix,x);

k=0;
if (ix < 0x00800000) {   /* x < 2**-126  */
     if ((ix&0x7fffffff)==0)
  return -two25/vzero;  /* log(+-0)=-inf */
     if (ix<0) return (x-x)/zero; /* log(-#) = NaN */
     k -= 25; x *= two25; /* subnormal number, scale up x */
     GET_FLOAT_WORD(ix,x);
}
if (ix >= 0x7f800000) return x+x;
k += (ix>>23)-127;
ix &= 0x007fffff;
i = (ix+(0x95f64<<3))&0x800000;
SET_FLOAT_WORD(x,ix|(i^0x3f800000)); /* normalize x or x/2 */
k += (i>>23);
f = x-(float)1.0;
if((0x007fffff&(0x8000+ix))<0xc000) { /* -2**-9 <= f < 2**-9 */
     if(f==zero) {
  if(k==0) {
      return zero;
  } else {
      dk=(float)k;
      return dk*ln2_hi+dk*ln2_lo;
  }
     }
     R = f*f*((float)0.5-(float)0.33333333333333333*f);
     if(k==0) return f-R; else {dk=(float)k;
           return dk*ln2_hi-((R-dk*ln2_lo)-f);}
}
  s = f/((float)2.0+f);
dk = (float)k;
z = s*s;
i = ix-(0x6147a<<3);
w = z*z;
j = (0x6b851<<3)-ix;
t1= w*(Lg2+w*Lg4);
t2= z*(Lg1+w*Lg3);
i |= j;
R = t2+t1;
if(i>0) {
     hfsq=(float)0.5*f*f;
     if(k==0) return f-(hfsq-s*(hfsq+R)); else
       return dk*ln2_hi-((hfsq-(s*(hfsq+R)+dk*ln2_lo))-f);
} else {
     if(k==0) return f-s*(f-R); else
       return dk*ln2_hi-((s*(f-R)-dk*ln2_lo)-f);
}
}

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