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

fusl/src/math/nextafterl.c

+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double nextafterl(long double x, long double y)
+{
+        return nextafter(x, y);
+}
+#elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384
+long double nextafterl(long double x, long double y)
+{
+        union ldshape ux, uy;
+
+        if (isnan(x) || isnan(y))
+                return x + y;
+        if (x == y)
+                return y;
+        ux.f = x;
+        if (x == 0) {
+                uy.f = y;
+                ux.i.m = 1;
+                ux.i.se = uy.i.se & 0x8000;
+        } else if ((x < y) == !(ux.i.se & 0x8000)) {
+                ux.i.m++;
+                if (ux.i.m << 1 == 0) {
+                        ux.i.m = 1ULL << 63;
+                        ux.i.se++;
+                }
+        } else {
+                if (ux.i.m << 1 == 0) {
+                        ux.i.se--;
+                        if (ux.i.se)
+                                ux.i.m = 0;
+                }
+                ux.i.m--;
+        }
+        /* raise overflow if ux is infinite and x is finite */
+        if ((ux.i.se & 0x7fff) == 0x7fff)
+                return x + x;
+        /* raise underflow if ux is subnormal or zero */
+        if ((ux.i.se & 0x7fff) == 0)
+                FORCE_EVAL(x*x + ux.f*ux.f);
+        return ux.f;
+}
+#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384
+long double nextafterl(long double x, long double y)
+{
+        union ldshape ux, uy;
+
+        if (isnan(x) || isnan(y))
+                return x + y;
+        if (x == y)
+                return y;
+        ux.f = x;
+        if (x == 0) {
+                uy.f = y;
+                ux.i.lo = 1;
+                ux.i.se = uy.i.se & 0x8000;
+        } else if ((x < y) == !(ux.i.se & 0x8000)) {
+                ux.i2.lo++;
+                if (ux.i2.lo == 0)
+                        ux.i2.hi++;
+        } else {
+                if (ux.i2.lo == 0)
+                        ux.i2.hi--;
+                ux.i2.lo--;
+        }
+        /* raise overflow if ux is infinite and x is finite */
+        if ((ux.i.se & 0x7fff) == 0x7fff)
+                return x + x;
+        /* raise underflow if ux is subnormal or zero */
+        if ((ux.i.se & 0x7fff) == 0)
+                FORCE_EVAL(x*x + ux.f*ux.f);
+        return ux.f;
+}
+#endif