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

fusl/src/math/acosl.c

+/* origin: FreeBSD /usr/src/lib/msun/src/e_acosl.c */
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunSoft, 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 acos.c.
+ * Converted to long double by David Schultz <das@FreeBSD.ORG>.
+ */
+
+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double acosl(long double x)
+{
+        return acos(x);
+}
+#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
+#include "__invtrigl.h"
+#if LDBL_MANT_DIG == 64
+#define CLEARBOTTOM(u) (u.i.m &= -1ULL << 32)
+#elif LDBL_MANT_DIG == 113
+#define CLEARBOTTOM(u) (u.i.lo = 0)
+#endif
+
+long double acosl(long double x)
+{
+        union ldshape u = {x};
+        long double z, s, c, f;
+        uint16_t e = u.i.se & 0x7fff;
+
+        /* |x| >= 1 or nan */
+        if (e >= 0x3fff) {
+                if (x == 1)
+                        return 0;
+                if (x == -1)
+                        return 2*pio2_hi + 0x1p-120f;
+                return 0/(x-x);
+        }
+        /* |x| < 0.5 */
+        if (e < 0x3fff - 1) {
+                if (e < 0x3fff - LDBL_MANT_DIG - 1)
+                        return pio2_hi + 0x1p-120f;
+                return pio2_hi - (__invtrigl_R(x*x)*x - pio2_lo + x);
+        }
+        /* x < -0.5 */
+        if (u.i.se >> 15) {
+                z = (1 + x)*0.5;
+                s = sqrtl(z);
+                return 2*(pio2_hi - (__invtrigl_R(z)*s - pio2_lo + s));
+        }
+        /* x > 0.5 */
+        z = (1 - x)*0.5;
+        s = sqrtl(z);
+        u.f = s;
+        CLEARBOTTOM(u);
+        f = u.f;
+        c = (z - f*f)/(s + f);
+        return 2*(__invtrigl_R(z)*s + c + f);
+}
+#endif