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

fusl/src/math/acosf.c

+/* origin: FreeBSD /usr/src/lib/msun/src/e_acosf.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 "libm.h"
+
+static const float
+pio2_hi = 1.5707962513e+00, /* 0x3fc90fda */
+pio2_lo = 7.5497894159e-08, /* 0x33a22168 */
+pS0 =  1.6666586697e-01,
+pS1 = -4.2743422091e-02,
+pS2 = -8.6563630030e-03,
+qS1 = -7.0662963390e-01;
+
+static float R(float z)
+{
+        float_t p, q;
+        p = z*(pS0+z*(pS1+z*pS2));
+        q = 1.0f+z*qS1;
+        return p/q;
+}
+
+float acosf(float x)
+{
+        float z,w,s,c,df;
+        uint32_t hx,ix;
+
+        GET_FLOAT_WORD(hx, x);
+        ix = hx & 0x7fffffff;
+        /* |x| >= 1 or nan */
+        if (ix >= 0x3f800000) {
+                if (ix == 0x3f800000) {
+                        if (hx >> 31)
+                                return 2*pio2_hi + 0x1p-120f;
+                        return 0;
+                }
+                return 0/(x-x);
+        }
+        /* |x| < 0.5 */
+        if (ix < 0x3f000000) {
+                if (ix <= 0x32800000) /* |x| < 2**-26 */
+                        return pio2_hi + 0x1p-120f;
+                return pio2_hi - (x - (pio2_lo-x*R(x*x)));
+        }
+        /* x < -0.5 */
+        if (hx >> 31) {
+                z = (1+x)*0.5f;
+                s = sqrtf(z);
+                w = R(z)*s-pio2_lo;
+                return 2*(pio2_hi - (s+w));
+        }
+        /* x > 0.5 */
+        z = (1-x)*0.5f;
+        s = sqrtf(z);
+        GET_FLOAT_WORD(hx,s);
+        SET_FLOAT_WORD(df,hx&0xfffff000);
+        c = (z-df*df)/(s+df);
+        w = R(z)*s+c;
+        return 2*(df+w);
+}