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

fusl/src/math/hypot.c

+#include <math.h>
+#include <stdint.h>
+#include <float.h>
+
+#if FLT_EVAL_METHOD > 1U && LDBL_MANT_DIG == 64
+#define SPLIT (0x1p32 + 1)
+#else
+#define SPLIT (0x1p27 + 1)
+#endif
+
+static void sq(double_t *hi, double_t *lo, double x)
+{
+        double_t xh, xl, xc;
+
+        xc = (double_t)x*SPLIT;
+        xh = x - xc + xc;
+        xl = x - xh;
+        *hi = (double_t)x*x;
+        *lo = xh*xh - *hi + 2*xh*xl + xl*xl;
+}
+
+double hypot(double x, double y)
+{
+        union {double f; uint64_t i;} ux = {x}, uy = {y}, ut;
+        int ex, ey;
+        double_t hx, lx, hy, ly, z;
+
+        /* arrange |x| >= |y| */
+        ux.i &= -1ULL>>1;
+        uy.i &= -1ULL>>1;
+        if (ux.i < uy.i) {
+                ut = ux;
+                ux = uy;
+                uy = ut;
+        }
+
+        /* special cases */
+        ex = ux.i>>52;
+        ey = uy.i>>52;
+        x = ux.f;
+        y = uy.f;
+        /* note: hypot(inf,nan) == inf */
+        if (ey == 0x7ff)
+                return y;
+        if (ex == 0x7ff || uy.i == 0)
+                return x;
+        /* note: hypot(x,y) ~= x + y*y/x/2 with inexact for small y/x */
+        /* 64 difference is enough for ld80 double_t */
+        if (ex - ey > 64)
+                return x + y;
+
+        /* precise sqrt argument in nearest rounding mode without overflow */
+        /* xh*xh must not overflow and xl*xl must not underflow in sq */
+        z = 1;
+        if (ex > 0x3ff+510) {
+                z = 0x1p700;
+                x *= 0x1p-700;
+                y *= 0x1p-700;
+        } else if (ey < 0x3ff-450) {
+                z = 0x1p-700;
+                x *= 0x1p700;
+                y *= 0x1p700;
+        }
+        sq(&hx, &lx, x);
+        sq(&hy, &ly, y);
+        return z*sqrt(ly+lx+hy+hx);
+}