first commit of the new libm!

[musl] / src / math / fmaf.c

diff --git a/src/math/fmaf.c b/src/math/fmaf.c
new file mode 100644 (file)
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--- /dev/null
+++ b/src/math/fmaf.c
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+/* 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 "libm.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)
+{
+       double xy, result;
+       uint32_t hr, lr;
+
+       xy = (double)x * y;
+       result = xy + z;
+       EXTRACT_WORDS(hr, lr, result);
+       /* Common case: The double precision result is fine. */
+       if ((lr & 0x1fffffff) != 0x10000000 ||  /* not a halfway case */
+               (hr & 0x7ff00000) == 0x7ff00000 ||  /* NaN */
+               result - xy == z ||                 /* exact */
+               fegetround() != FE_TONEAREST)       /* not round-to-nearest */
+               return (result);
+
+       /*
+        * 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.
+        */
+       fesetround(FE_TOWARDZERO);
+       volatile double vxy = xy;  /* XXX work around gcc CSE bug */
+       double adjusted_result = vxy + z;
+       fesetround(FE_TONEAREST);
+       if (result == adjusted_result)
+               SET_LOW_WORD(adjusted_result, lr + 1);
+       return (adjusted_result);
+}