compiler/stdc/math/s_fmaf.c

   1 /*-
   2  * Copyright (c) 2005-2011 David Schultz <das@FreeBSD.ORG>
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions
   7  * are met:
   8  * 1. Redistributions of source code must retain the above copyright
   9  *    notice, this list of conditions and the following disclaimer.
  10  * 2. Redistributions in binary form must reproduce the above copyright
  11  *    notice, this list of conditions and the following disclaimer in the
  12  *    documentation and/or other materials provided with the distribution.
  13  *
  14  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  15  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  16  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  17  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  18  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  19  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  20  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  21  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  22  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  23  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  24  * SUCH DAMAGE.
  25  */
  26
  27 __FBSDID("$FreeBSD: src/lib/msun/src/s_fmaf.c,v 1.3 2011/10/15 04:16:58 das Exp $");
  28
  29 #include <aros/system.h>
  30
  31 #include <fenv.h>
  32 #include <float.h>
  33
  34 #include "math.h"
  35 #include "math_private.h"
  36
  37 /*
  38  * Fused multiply-add: Compute x * y + z with a single rounding error.
  39  *
  40  * A double has more than twice as much precision than a float, so
  41  * direct double-precision arithmetic suffices, except where double
  42  * rounding occurs.
  43  */
  44 float
  45 fmaf(float x, float y, float z)
  46 {
  47         double xy, result;
  48         uint32_t hr, lr;
  49
  50         xy = (double)x * y;
  51         result = xy + z;
  52         EXTRACT_WORDS(hr, lr, result);
  53         /* Common case: The double precision result is fine. */
  54         if ((lr & 0x1fffffff) != 0x10000000 ||  /* not a halfway case */
  55             (hr & 0x7ff00000) == 0x7ff00000 ||  /* NaN */
  56             result - xy == z ||                 /* exact */
  57             fegetround() != FE_TONEAREST)       /* not round-to-nearest */
  58                 return (result);
  59
  60         /*
  61          * If result is inexact, and exactly halfway between two float values,
  62          * we need to adjust the low-order bit in the direction of the error.
  63          */
  64         fesetround(FE_TOWARDZERO);
  65         volatile double vxy = xy;  /* XXX work around gcc CSE bug */
  66         double adjusted_result = vxy + z;
  67         fesetround(FE_TONEAREST);
  68         if (result == adjusted_result)
  69                 SET_LOW_WORD(adjusted_result, lr + 1);
  70         return (adjusted_result);
  71 }