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

fusl/src/math/tanh.c

+#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;
+}