source: asp3_tinet_ecnl_arm/trunk/musl-1.1.18/src/math/log2f.c@ 352

/* origin: FreeBSD /usr/src/lib/msun/src/e_log2f.c */
/*
 * ====================================================
 * 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.
 * ====================================================
 */
/*
 * See comments in log2.c.
 */

#include <math.h>
#include <stdint.h>

static const float
ivln2hi =  1.4428710938e+00, /* 0x3fb8b000 */
ivln2lo = -1.7605285393e-04, /* 0xb9389ad4 */
/* |(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 */

float log2f(float x)
{
        union {float f; uint32_t i;} u = {x};
        float_t hfsq,f,s,z,R,w,t1,t2,hi,lo;
        uint32_t ix;
        int k;

        ix = u.i;
        k = 0;
        if (ix < 0x00800000 || ix>>31) {  /* x < 2**-126  */
                if (ix<<1 == 0)
                        return -1/(x*x);  /* log(+-0)=-inf */
                if (ix>>31)
                        return (x-x)/0.0f; /* log(-#) = NaN */
                /* subnormal number, scale up x */
                k -= 25;
                x *= 0x1p25f;
                u.f = x;
                ix = u.i;
        } else if (ix >= 0x7f800000) {
                return x;
        } else if (ix == 0x3f800000)
                return 0;

        /* reduce x into [sqrt(2)/2, sqrt(2)] */
        ix += 0x3f800000 - 0x3f3504f3;
        k += (int)(ix>>23) - 0x7f;
        ix = (ix&0x007fffff) + 0x3f3504f3;
        u.i = ix;
        x = u.f;

        f = x - 1.0f;
        s = f/(2.0f + f);
        z = s*s;
        w = z*z;
        t1= w*(Lg2+w*Lg4);
        t2= z*(Lg1+w*Lg3);
        R = t2 + t1;
        hfsq = 0.5f*f*f;

        hi = f - hfsq;
        u.f = hi;
        u.i &= 0xfffff000;
        hi = u.f;
        lo = f - hi - hfsq + s*(hfsq+R);
        return (lo+hi)*ivln2lo + lo*ivln2hi + hi*ivln2hi + k;
}