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

fusl/src/math/cosh.c

Index: fusl/src/math/cosh.c
diff --git a/fusl/src/math/cosh.c b/fusl/src/math/cosh.c
new file mode 100644
index 0000000000000000000000000000000000000000..100f8231d8829604b9e293491a9abb1774b4b29a
--- /dev/null
+++ b/fusl/src/math/cosh.c
@@ -0,0 +1,40 @@
+#include "libm.h"
+
+/* cosh(x) = (exp(x) + 1/exp(x))/2
+ *         = 1 + 0.5*(exp(x)-1)*(exp(x)-1)/exp(x)
+ *         = 1 + x*x/2 + o(x^4)
+ */
+double cosh(double x)
+{
+	union {double f; uint64_t i;} u = {.f = x};
+	uint32_t w;
+	double t;
+
+	/* |x| */
+	u.i &= (uint64_t)-1/2;
+	x = u.f;
+	w = u.i >> 32;
+
+	/* |x| < log(2) */
+	if (w < 0x3fe62e42) {
+		if (w < 0x3ff00000 - (26<<20)) {
+			/* raise inexact if x!=0 */
+			FORCE_EVAL(x + 0x1p120f);
+			return 1;
+		}
+		t = expm1(x);
+		return 1 + t*t/(2*(1+t));
+	}
+
+	/* |x| < log(DBL_MAX) */
+	if (w < 0x40862e42) {
+		t = exp(x);
+		/* note: if x>log(0x1p26) then the 1/t is not needed */
+		return 0.5*(t + 1/t);
+	}
+
+	/* |x| > log(DBL_MAX) or nan */
+	/* note: the result is stored to handle overflow */
+	t = __expo2(x);
+	return t;
+}