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

fusl/src/math/log1pf.c

+/* origin: FreeBSD /usr/src/lib/msun/src/s_log1pf.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.
+ * ====================================================
+ */
+
+#include "libm.h"
+
+static const float
+ln2_hi = 6.9313812256e-01, /* 0x3f317180 */
+ln2_lo = 9.0580006145e-06, /* 0x3717f7d1 */
+/* |(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 log1pf(float x)
+{
+        union {float f; uint32_t i;} u = {x};
+        float_t hfsq,f,c,s,z,R,w,t1,t2,dk;
+        uint32_t ix,iu;
+        int k;
+
+        ix = u.i;
+        k = 1;
+        if (ix < 0x3ed413d0 || ix>>31) {  /* 1+x < sqrt(2)+  */
+                if (ix >= 0xbf800000) {  /* x <= -1.0 */
+                        if (x == -1)
+                                return x/0.0f; /* log1p(-1)=+inf */
+                        return (x-x)/0.0f;     /* log1p(x<-1)=NaN */
+                }
+                if (ix<<1 < 0x33800000<<1) {   /* |x| < 2**-24 */
+                        /* underflow if subnormal */
+                        if ((ix&0x7f800000) == 0)
+                                FORCE_EVAL(x*x);
+                        return x;
+                }
+                if (ix <= 0xbe95f619) { /* sqrt(2)/2- <= 1+x < sqrt(2)+ */
+                        k = 0;
+                        c = 0;
+                        f = x;
+                }
+        } else if (ix >= 0x7f800000)
+                return x;
+        if (k) {
+                u.f = 1 + x;
+                iu = u.i;
+                iu += 0x3f800000 - 0x3f3504f3;
+                k = (int)(iu>>23) - 0x7f;
+                /* correction term ~ log(1+x)-log(u), avoid underflow in c/u */
+                if (k < 25) {
+                        c = k >= 2 ? 1-(u.f-x) : x-(u.f-1);
+                        c /= u.f;
+                } else
+                        c = 0;
+                /* reduce u into [sqrt(2)/2, sqrt(2)] */
+                iu = (iu&0x007fffff) + 0x3f3504f3;
+                u.i = iu;
+                f = u.f - 1;
+        }
+        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;
+        dk = k;
+        return s*(hfsq+R) + (dk*ln2_lo+c) - hfsq + f + dk*ln2_hi;
+}