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

fusl/src/math/tanh.c

Index: fusl/src/math/tanh.c
diff --git a/fusl/src/math/tanh.c b/fusl/src/math/tanh.c
new file mode 100644
index 0000000000000000000000000000000000000000..20d6dbcf4175c7aad93fe70a53056f0d3e1c4f13
--- /dev/null
+++ b/fusl/src/math/tanh.c
@@ -0,0 +1,45 @@
+#include "libm.h"
+
+/* tanh(x) = (exp(x) - exp(-x))/(exp(x) + exp(-x))
+ *         = (exp(2*x) - 1)/(exp(2*x) - 1 + 2)
+ *         = (1 - exp(-2*x))/(exp(-2*x) - 1 + 2)
+ */
+double tanh(double x)
+{
+	union {double f; uint64_t i;} u = {.f = x};
+	uint32_t w;
+	int sign;
+	double_t t;
+
+	/* x = |x| */
+	sign = u.i >> 63;
+	u.i &= (uint64_t)-1/2;
+	x = u.f;
+	w = u.i >> 32;
+
+	if (w > 0x3fe193ea) {
+		/* |x| > log(3)/2 ~= 0.5493 or nan */
+		if (w > 0x40340000) {
+			/* |x| > 20 or nan */
+			/* note: this branch avoids raising overflow */
+			t = 1 - 0/x;
+		} else {
+			t = expm1(2*x);
+			t = 1 - 2/(t+2);
+		}
+	} else if (w > 0x3fd058ae) {
+		/* |x| > log(5/3)/2 ~= 0.2554 */
+		t = expm1(2*x);
+		t = t/(t+2);
+	} else if (w >= 0x00100000) {
+		/* |x| >= 0x1p-1022, up to 2ulp error in [0.1,0.2554] */
+		t = expm1(-2*x);
+		t = -t/(t+2);
+	} else {
+		/* |x| is subnormal */
+		/* note: the branch above would not raise underflow in [0x1p-1023,0x1p-1022) */
+		FORCE_EVAL((float)x);
+		t = x;
+	}
+	return sign ? -t : t;
+}