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

fusl/src/math/atan2f.c

+/* origin: FreeBSD /usr/src/lib/msun/src/e_atan2f.c */
+/*
+ * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
+ */
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunPro, a Sun Microsystems, Inc. business.
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice
+ * is preserved.
+ * ====================================================
+ */
+
+#include "libm.h"
+
+static const float
+pi     = 3.1415927410e+00, /* 0x40490fdb */
+pi_lo  = -8.7422776573e-08; /* 0xb3bbbd2e */
+
+float atan2f(float y, float x)
+{
+        float z;
+        uint32_t m,ix,iy;
+
+        if (isnan(x) || isnan(y))
+                return x+y;
+        GET_FLOAT_WORD(ix, x);
+        GET_FLOAT_WORD(iy, y);
+        if (ix == 0x3f800000)  /* x=1.0 */
+                return atanf(y);
+        m = ((iy>>31)&1) | ((ix>>30)&2);  /* 2*sign(x)+sign(y) */
+        ix &= 0x7fffffff;
+        iy &= 0x7fffffff;
+
+        /* when y = 0 */
+        if (iy == 0) {
+                switch (m) {
+                case 0:
+                case 1: return y;   /* atan(+-0,+anything)=+-0 */
+                case 2: return  pi; /* atan(+0,-anything) = pi */
+                case 3: return -pi; /* atan(-0,-anything) =-pi */
+                }
+        }
+        /* when x = 0 */
+        if (ix == 0)
+                return m&1 ? -pi/2 : pi/2;
+        /* when x is INF */
+        if (ix == 0x7f800000) {
+                if (iy == 0x7f800000) {
+                        switch (m) {
+                        case 0: return  pi/4; /* atan(+INF,+INF) */
+                        case 1: return -pi/4; /* atan(-INF,+INF) */
+                        case 2: return 3*pi/4;  /*atan(+INF,-INF)*/
+                        case 3: return -3*pi/4; /*atan(-INF,-INF)*/
+                        }
+                } else {
+                        switch (m) {
+                        case 0: return  0.0f;    /* atan(+...,+INF) */
+                        case 1: return -0.0f;    /* atan(-...,+INF) */
+                        case 2: return  pi; /* atan(+...,-INF) */
+                        case 3: return -pi; /* atan(-...,-INF) */
+                        }
+                }
+        }
+        /* |y/x| > 0x1p26 */
+        if (ix+(26<<23) < iy || iy == 0x7f800000)
+                return m&1 ? -pi/2 : pi/2;
+
+        /* z = atan(|y/x|) with correct underflow */
+        if ((m&2) && iy+(26<<23) < ix)  /*|y/x| < 0x1p-26, x < 0 */
+                z = 0.0;
+        else
+                z = atanf(fabsf(y/x));
+        switch (m) {
+        case 0: return z;              /* atan(+,+) */
+        case 1: return -z;             /* atan(-,+) */
+        case 2: return pi - (z-pi_lo); /* atan(+,-) */
+        default: /* case 3 */
+                return (z-pi_lo) - pi; /* atan(-,-) */
+        }
+}