lib/libc/gen/ldexp_ieee754.c

   1 /*      $NetBSD: ldexp_ieee754.c,v 1.5 2008/04/28 20:22:59 martin Exp $ */
   2
   3 /*-
   4  * Copyright (c) 1999 The NetBSD Foundation, Inc.
   5  * All rights reserved.
   6  *
   7  * This code is derived from software contributed to The NetBSD Foundation
   8  * by Charles M. Hannum.
   9  *
  10  * Redistribution and use in source and binary forms, with or without
  11  * modification, are permitted provided that the following conditions
  12  * are met:
  13  * 1. Redistributions of source code must retain the above copyright
  14  *    notice, this list of conditions and the following disclaimer.
  15  * 2. Redistributions in binary form must reproduce the above copyright
  16  *    notice, this list of conditions and the following disclaimer in the
  17  *    documentation and/or other materials provided with the distribution.
  18  *
  19  * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
  20  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
  21  * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  22  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
  23  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  24  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  25  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  26  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  27  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  28  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  29  * POSSIBILITY OF SUCH DAMAGE.
  30  */
  31
  32 #include <sys/cdefs.h>
  33 #if defined(LIBC_SCCS) && !defined(lint)
  34 __RCSID("$NetBSD: ldexp_ieee754.c,v 1.5 2008/04/28 20:22:59 martin Exp $");
  35 #endif /* LIBC_SCCS and not lint */
  36
  37 #include <sys/types.h>
  38 #include <machine/ieee.h>
  39 #include <errno.h>
  40 #include <math.h>
  41
  42 /*
  43  * Multiply the given value by 2^expon.
  44  */
  45 double
  46 ldexp(double val, int expon)
  47 {
  48         int oldexp, newexp;
  49         union ieee_double_u u, mul;
  50
  51         u.dblu_d = val;
  52         oldexp = u.dblu_dbl.dbl_exp;
  53
  54         /*
  55          * If input is zero, Inf or NaN, just return it.
  56          */
  57         if (u.dblu_d == 0.0 || oldexp == DBL_EXP_INFNAN)
  58                 return (val);
  59
  60         if (oldexp == 0) {
  61                 /*
  62                  * u.v is denormal.  We must adjust it so that the exponent
  63                  * arithmetic below will work.
  64                  */
  65                 if (expon <= DBL_EXP_BIAS) {
  66                         /*
  67                          * Optimization: if the scaling can be done in a single
  68                          * multiply, or underflows, just do it now.
  69                          */
  70                         if (expon <= -DBL_FRACBITS) {
  71                                 errno = ERANGE;
  72                                 return (val < 0.0 ? -0.0 : 0.0);
  73                         }
  74                         mul.dblu_d = 0.0;
  75                         mul.dblu_dbl.dbl_exp = expon + DBL_EXP_BIAS;
  76                         u.dblu_d *= mul.dblu_d;
  77                         if (u.dblu_d == 0.0) {
  78                                 errno = ERANGE;
  79                                 return (val < 0.0 ? -0.0 : 0.0);
  80                         }
  81                         return (u.dblu_d);
  82                 } else {
  83                         /*
  84                          * We know that expon is very large, and therefore the
  85                          * result cannot be denormal (though it may be Inf).
  86                          * Shift u.v by just enough to make it normal.
  87                          */
  88                         mul.dblu_d = 0.0;
  89                         mul.dblu_dbl.dbl_exp = DBL_FRACBITS + DBL_EXP_BIAS;
  90                         u.dblu_d *= mul.dblu_d;
  91                         expon -= DBL_FRACBITS;
  92                         oldexp = u.dblu_dbl.dbl_exp;
  93                 }
  94         }
  95
  96         /*
  97          * u.v is now normalized and oldexp has been adjusted if necessary.
  98          * Calculate the new exponent and check for underflow and overflow.
  99          */
 100         newexp = oldexp + expon;
 101
 102         if (newexp <= 0) {
 103                 /*
 104                  * The output number is either denormal or underflows (see
 105                  * comments in machine/ieee.h).
 106                  */
 107                 if (newexp <= -DBL_FRACBITS) {
 108                         errno = ERANGE;
 109                         return (val < 0.0 ? -0.0 : 0.0);
 110                 }
 111                 /*
 112                  * Denormalize the result.  We do this with a multiply.  If
 113                  * expon is very large, it won't fit in a double, so we have
 114                  * to adjust the exponent first.  This is safe because we know
 115                  * that u.v is normal at this point.
 116                  */
 117                 if (expon <= -DBL_EXP_BIAS) {
 118                         u.dblu_dbl.dbl_exp = 1;
 119                         expon += oldexp - 1;
 120                 }
 121                 mul.dblu_d = 0.0;
 122                 mul.dblu_dbl.dbl_exp = expon + DBL_EXP_BIAS;
 123                 u.dblu_d *= mul.dblu_d;
 124                 return (u.dblu_d);
 125         } else if (newexp >= DBL_EXP_INFNAN) {
 126                 /*
 127                  * The result overflowed; return +/-Inf.
 128                  */
 129                 u.dblu_dbl.dbl_exp = DBL_EXP_INFNAN;
 130                 u.dblu_dbl.dbl_frach = 0;
 131                 u.dblu_dbl.dbl_fracl = 0;
 132                 errno = ERANGE;
 133                 return (u.dblu_d);
 134         } else {
 135                 /*
 136                  * The result is normal; just replace the old exponent with the
 137                  * new one.
 138                  */
 139                 u.dblu_dbl.dbl_exp = newexp;
 140                 return (u.dblu_d);
 141         }
 142 }