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

fusl/src/math/fmaf.c

+/* origin: FreeBSD /usr/src/lib/msun/src/s_fmaf.c */
+/*-
+ * Copyright (c) 2005-2011 David Schultz <das@FreeBSD.ORG>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <fenv.h>
+#include <math.h>
+#include <stdint.h>
+
+/*
+ * Fused multiply-add: Compute x * y + z with a single rounding error.
+ *
+ * A double has more than twice as much precision than a float, so
+ * direct double-precision arithmetic suffices, except where double
+ * rounding occurs.
+ */
+float fmaf(float x, float y, float z)
+{
+        #pragma STDC FENV_ACCESS ON
+        double xy, result;
+        union {double f; uint64_t i;} u;
+        int e;
+
+        xy = (double)x * y;
+        result = xy + z;
+        u.f = result;
+        e = u.i>>52 & 0x7ff;
+        /* Common case: The double precision result is fine. */
+        if ((u.i & 0x1fffffff) != 0x10000000 || /* not a halfway case */
+                e == 0x7ff ||                   /* NaN */
+                result - xy == z ||                 /* exact */
+                fegetround() != FE_TONEAREST)       /* not round-to-nearest */
+        {
+                /*
+                underflow may not be raised correctly, example:
+                fmaf(0x1p-120f, 0x1p-120f, 0x1p-149f)
+                */
+#if defined(FE_INEXACT) && defined(FE_UNDERFLOW)
+                if (e < 0x3ff-126 && e >= 0x3ff-149 && fetestexcept(FE_INEXACT)) {
+                        feclearexcept(FE_INEXACT);
+                        /* TODO: gcc and clang bug workaround */
+                        volatile float vz = z;
+                        result = xy + vz;
+                        if (fetestexcept(FE_INEXACT))
+                                feraiseexcept(FE_UNDERFLOW);
+                        else
+                                feraiseexcept(FE_INEXACT);
+                }
+#endif
+                z = result;
+                return z;
+        }
+
+        /*
+         * If result is inexact, and exactly halfway between two float values,
+         * we need to adjust the low-order bit in the direction of the error.
+         */
+#ifdef FE_TOWARDZERO
+        fesetround(FE_TOWARDZERO);
+#endif
+        volatile double vxy = xy;  /* XXX work around gcc CSE bug */
+        double adjusted_result = vxy + z;
+        fesetround(FE_TONEAREST);
+        if (result == adjusted_result) {
+                u.f = adjusted_result;
+                u.i++;
+                adjusted_result = u.f;
+        }
+        z = adjusted_result;
+        return z;
+}