Home | History | Annotate | Line # | Download | only in src 
      1 /*	$NetBSD: s_fmaf.c,v 1.3 2017/05/06 18:02:52 christos Exp $	*/
      2 
      3 /*-
      4  * Copyright (c) 2005-2011 David Schultz <das (at) FreeBSD.ORG>
      5  * All rights reserved.
      6  *
      7  * Redistribution and use in source and binary forms, with or without
      8  * modification, are permitted provided that the following conditions
      9  * are met:
     10  * 1. Redistributions of source code must retain the above copyright
     11  *    notice, this list of conditions and the following disclaimer.
     12  * 2. Redistributions in binary form must reproduce the above copyright
     13  *    notice, this list of conditions and the following disclaimer in the
     14  *    documentation and/or other materials provided with the distribution.
     15  *
     16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
     17  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     19  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
     20  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     21  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     22  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     23  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     24  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     25  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     26  * SUCH DAMAGE.
     27  */
     28 
     29 #include <sys/cdefs.h>
     30 #if 0
     31 __FBSDID("$FreeBSD: src/lib/msun/src/s_fmaf.c,v 1.3 2011/10/15 04:16:58 das Exp $");
     32 #else
     33 __RCSID("$NetBSD: s_fmaf.c,v 1.3 2017/05/06 18:02:52 christos Exp $");
     34 #endif
     35 
     36 #include "namespace.h"
     37 
     38 #include <fenv.h>
     39 
     40 #include "math.h"
     41 #include "math_private.h"
     42 
     43 /*
     44  * Fused multiply-add: Compute x * y + z with a single rounding error.
     45  *
     46  * A double has more than twice as much precision than a float, so
     47  * direct double-precision arithmetic suffices, except where double
     48  * rounding occurs.
     49  */
     50 float
     51 fmaf(float x, float y, float z)
     52 {
     53 	double xy, result;
     54 	uint32_t hr, lr;
     55 
     56 	xy = (double)x * y;
     57 	result = xy + z;
     58 	EXTRACT_WORDS(hr, lr, result);
     59 	/* Common case: The double precision result is fine. */
     60 	if ((lr & 0x1fffffff) != 0x10000000 ||	/* not a halfway case */
     61 	    (hr & 0x7ff00000) == 0x7ff00000 ||	/* NaN */
     62 	    result - xy == z ||			/* exact */
     63 	    fegetround() != FE_TONEAREST)	/* not round-to-nearest */
     64 		return (result);
     65 
     66 	/*
     67 	 * If result is inexact, and exactly halfway between two float values,
     68 	 * we need to adjust the low-order bit in the direction of the error.
     69 	 */
     70 	fesetround(FE_TOWARDZERO);
     71 	volatile double vxy = xy;  /* XXX work around gcc CSE bug */
     72 	double adjusted_result = vxy + z;
     73 	fesetround(FE_TONEAREST);
     74 	if (result == adjusted_result)
     75 		SET_LOW_WORD(adjusted_result, lr + 1);
     76 	return (adjusted_result);
     77 }
     78