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

fusl/src/math/acoshf.c

Index: fusl/src/math/acoshf.c
diff --git a/fusl/src/math/acoshf.c b/fusl/src/math/acoshf.c
new file mode 100644
index 0000000000000000000000000000000000000000..8a4ec4d57ead3cbcd473d2ddfdd43d910956f1b9
--- /dev/null
+++ b/fusl/src/math/acoshf.c
@@ -0,0 +1,26 @@
+#include "libm.h"
+
+#if FLT_EVAL_METHOD==2
+#undef sqrtf
+#define sqrtf sqrtl
+#elif FLT_EVAL_METHOD==1
+#undef sqrtf
+#define sqrtf sqrt
+#endif
+
+/* acosh(x) = log(x + sqrt(x*x-1)) */
+float acoshf(float x)
+{
+	union {float f; uint32_t i;} u = {x};
+	uint32_t a = u.i & 0x7fffffff;
+
+	if (a < 0x3f800000+(1<<23))
+		/* |x| < 2, invalid if x < 1 or nan */
+		/* up to 2ulp error in [1,1.125] */
+		return log1pf(x-1 + sqrtf((x-1)*(x-1)+2*(x-1)));
+	if (a < 0x3f800000+(12<<23))
+		/* |x| < 0x1p12 */
+		return logf(2*x - 1/(x+sqrtf(x*x-1)));
+	/* x >= 0x1p12 */
+	return logf(x) + 0.693147180559945309417232121458176568f;
+}