lib/libc/compat/gen/compat_ldexp_ieee754.c

   1 /* $NetBSD: compat_ldexp_ieee754.c,v 1.5 2010/04/23 19:04:54 drochner 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: compat_ldexp_ieee754.c,v 1.5 2010/04/23 19:04:54 drochner Exp $");
  35 #endif /* LIBC_SCCS and not lint */
  36
  37 #include <sys/types.h>
  38 #include <machine/ieee.h>
  39 #include <errno.h>
  40
  41 double ldexp(double, int);
  42
  43 /*
  44  * Multiply the given value by 2^expon.
  45  */
  46 double
  47 ldexp(double val, int expon)
  48 {
  49         int oldexp, newexp;
  50         union ieee_double_u u, mul;
  51
  52         u.dblu_d = val;
  53         oldexp = u.dblu_dbl.dbl_exp;
  54
  55         /*
  56          * If input is zero, Inf or NaN, just return it.
  57          */
  58         if (u.dblu_d == 0.0 || oldexp == DBL_EXP_INFNAN)
  59                 return (val);
  60
  61         if (oldexp == 0) {
  62                 /*
  63                  * u.v is denormal.  We must adjust it so that the exponent
  64                  * arithmetic below will work.
  65                  */
  66                 if (expon <= DBL_EXP_BIAS) {
  67                         /*
  68                          * Optimization: if the scaling can be done in a single
  69                          * multiply, or underflows, just do it now.
  70                          */
  71                         if (expon <= -DBL_FRACBITS) {
  72                                 errno = ERANGE;
  73                                 return (val < 0.0 ? -0.0 : 0.0);
  74                         }
  75                         mul.dblu_d = 0.0;
  76                         mul.dblu_dbl.dbl_exp = expon + DBL_EXP_BIAS;
  77                         u.dblu_d *= mul.dblu_d;
  78                         if (u.dblu_d == 0.0) {
  79                                 errno = ERANGE;
  80                                 return (val < 0.0 ? -0.0 : 0.0);
  81                         }
  82                         return (u.dblu_d);
  83                 } else {
  84                         /*
  85                          * We know that expon is very large, and therefore the
  86                          * result cannot be denormal (though it may be Inf).
  87                          * Shift u.v by just enough to make it normal.
  88                          */
  89                         mul.dblu_d = 0.0;
  90                         mul.dblu_dbl.dbl_exp = DBL_FRACBITS + DBL_EXP_BIAS;
  91                         u.dblu_d *= mul.dblu_d;
  92                         expon -= DBL_FRACBITS;
  93                         oldexp = u.dblu_dbl.dbl_exp;
  94                 }
  95         }
  96
  97         /*
  98          * u.v is now normalized and oldexp has been adjusted if necessary.
  99          * Calculate the new exponent and check for underflow and overflow.
 100          */
 101         newexp = oldexp + expon;
 102
 103         if (newexp <= 0) {
 104                 /*
 105                  * The output number is either denormal or underflows (see
 106                  * comments in machine/ieee.h).
 107                  */
 108                 if (newexp <= -DBL_FRACBITS) {
 109                         errno = ERANGE;
 110                         return (val < 0.0 ? -0.0 : 0.0);
 111                 }
 112                 /*
 113                  * Denormalize the result.  We do this with a multiply.  If
 114                  * expon is very large, it won't fit in a double, so we have
 115                  * to adjust the exponent first.  This is safe because we know
 116                  * that u.v is normal at this point.
 117                  */
 118                 if (expon <= -DBL_EXP_BIAS) {
 119                         u.dblu_dbl.dbl_exp = 1;
 120                         expon += oldexp - 1;
 121                 }
 122                 mul.dblu_d = 0.0;
 123                 mul.dblu_dbl.dbl_exp = expon + DBL_EXP_BIAS;
 124                 u.dblu_d *= mul.dblu_d;
 125                 return (u.dblu_d);
 126         } else if (newexp >= DBL_EXP_INFNAN) {
 127                 /*
 128                  * The result overflowed; return +/-Inf.
 129                  */
 130                 u.dblu_dbl.dbl_exp = DBL_EXP_INFNAN;
 131                 u.dblu_dbl.dbl_frach = 0;
 132                 u.dblu_dbl.dbl_fracl = 0;
 133                 errno = ERANGE;
 134                 return (u.dblu_d);
 135         } else {
 136                 /*
 137                  * The result is normal; just replace the old exponent with the
 138                  * new one.
 139                  */
 140                 u.dblu_dbl.dbl_exp = newexp;
 141                 return (u.dblu_d);
 142         }
 143 }