/* $OpenBSD: ldexp.c,v 1.4 2005/08/07 16:40:13 espie Exp $ */ /* $NetBSD: ldexp.c,v 1.1 1995/02/10 17:50:24 cgd Exp $ */ /* * Copyright (c) 1994, 1995 Carnegie-Mellon University. * All rights reserved. * * Author: Chris G. Demetriou * * Permission to use, copy, modify and distribute this software and * its documentation is hereby granted, provided that both the copyright * notice and this permission notice appear in all copies of the * software, derivative works or modified versions, and any portions * thereof, and that both notices appear in supporting documentation. * * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS" * CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE. * * Carnegie Mellon requests users of this software to return to * * Software Distribution Coordinator or Software.Distribution@CS.CMU.EDU * School of Computer Science * Carnegie Mellon University * Pittsburgh PA 15213-3890 * * any improvements or extensions that they make and grant Carnegie the * rights to redistribute these changes. */ #include #include #include #include /* * double ldexp(double val, int exp) * returns: val * (2**exp) */ double ldexp(val, exp) double val; int exp; { register int oldexp, newexp, mulexp; union doub { double v; struct ieee_double s; } u, mul; /* * If input is zero, or no change, just return input. * Likewise, if input is Inf or NaN, just return it. */ u.v = val; oldexp = u.s.dbl_exp; if (val == 0 || exp == 0 || oldexp == DBL_EXP_INFNAN) return (val); /* * Compute new exponent and check for over/under flow. * Underflow, unfortunately, could mean switching to denormal. * If result out of range, set ERANGE and return 0 if too small * or Inf if too big, with the same sign as the input value. */ newexp = oldexp + exp; if (newexp >= DBL_EXP_INFNAN) { /* u.s.dbl_sign = val < 0; -- already set */ u.s.dbl_exp = DBL_EXP_INFNAN; u.s.dbl_frach = u.s.dbl_fracl = 0; errno = ERANGE; return (u.v); /* Inf */ } if (newexp <= 0) { /* * The output number is either a denormal or underflows * (see comments in machine/ieee.h). */ if (newexp <= -DBL_FRACBITS) { /* u.s.dbl_sign = val < 0; -- already set */ u.s.dbl_exp = 0; u.s.dbl_frach = u.s.dbl_fracl = 0; errno = ERANGE; return (u.v); /* zero */ } /* * We are going to produce a denorm. Our `exp' argument * might be as small as -2097, and we cannot compute * 2^-2097, so we may have to do this as many as three * steps (not just two, as for positive `exp's below). */ mul.v = 0; while (exp <= -DBL_EXP_BIAS) { mul.s.dbl_exp = 1; val *= mul.v; exp += DBL_EXP_BIAS - 1; } mul.s.dbl_exp = exp + DBL_EXP_BIAS; val *= mul.v; return (val); } /* * Newexp is positive. * * If oldexp is zero, we are starting with a denorm, and simply * adjusting the exponent will produce bogus answers. We need * to fix that first. */ if (oldexp == 0) { /* * Multiply by 2^mulexp to make the number normalizable. * We cannot multiply by more than 2^1023, but `exp' * argument might be as large as 2046. A single * adjustment, however, will normalize the number even * for huge `exp's, and then we can use exponent * arithmetic just as for normal `double's. */ mulexp = exp <= DBL_EXP_BIAS ? exp : DBL_EXP_BIAS; mul.v = 0; mul.s.dbl_exp = mulexp + DBL_EXP_BIAS; val *= mul.v; if (mulexp == exp) return (val); u.v = val; newexp -= mulexp; } /* * Both oldexp and newexp are positive; just replace the * old exponent with the new one. */ u.s.dbl_exp = newexp; return (u.v); }