Add a "fork" of musl as //fusl.

fusl/src/math/log2f.c

+/* 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;
+}