e_log2f.c

/*
 *                               POK header
 * 
 * The following file is a part of the POK project. Any modification should
 * made according to the POK licence. You CANNOT use this file or a part of
 * this file is this part of a file for your own project
 *
 * For more information on the POK licence, please see our LICENCE FILE
 *
 * Please follow the coding guidelines described in doc/CODING_GUIDELINES
 *
 *                                      Copyright (c) 2007-2009 POK team 
 *
 * Created by julien on Fri Jan 30 14:41:34 2009 
 */

/* 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.
 * ====================================================
 */

#ifdef POK_NEEDS_LIBMATH

#include "math_private.h"

static const float
ln2 = 0.6931471805599452862268, 
two25 =    3.355443200e+07,     /* 0x4c000000 */
Lg1 = 6.6666668653e-01, /* 3F2AAAAB */
Lg2 = 4.0000000596e-01, /* 3ECCCCCD */
Lg3 = 2.8571429849e-01, /* 3E924925 */
Lg4 = 2.2222198546e-01, /* 3E638E29 */
Lg5 = 1.8183572590e-01, /* 3E3A3325 */
Lg6 = 1.5313838422e-01, /* 3E1CD04F */
Lg7 = 1.4798198640e-01; /* 3E178897 */

static const float zero   =  0.0;

float
__ieee754_log2f(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/zero;             /* 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);
        dk = (float)k;
        f = x-(float)1.0;
        if((0x007fffff&(15+ix))<16) {   /* |f| < 2**-20 */
            if (f==zero) 
                    return (dk);
            R = f*f*((float)0.5-(float)0.33333333333333333*f);
            return (dk-(R-f)/ln2);
        }
        s = f/((float)2.0+f);
        z = s*s;
        i = ix-(0x6147a<<3);
        w = z*z;
        j = (0x6b851<<3)-ix;
        t1= w*(Lg2+w*(Lg4+w*Lg6));
        t2= z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7)));
        i |= j;
        R = t2+t1;
        if(i>0) {
            hfsq=(float)0.5*f*f;
            return (dk-(hfsq-s*(hfsq+R)-f)/ln2);
        } else
                return (dk-((s*(f-R))-f)/ln2);
}
#endif