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      1 /* mpfr_div_2ui -- divide a floating-point number by a power of two
      2 
      3 Copyright 1999, 2001-2023 Free Software Foundation, Inc.
      4 Contributed by the AriC and Caramba projects, INRIA.
      5 
      6 This file is part of the GNU MPFR Library.
      7 
      8 The GNU MPFR Library is free software; you can redistribute it and/or modify
      9 it under the terms of the GNU Lesser General Public License as published by
     10 the Free Software Foundation; either version 3 of the License, or (at your
     11 option) any later version.
     12 
     13 The GNU MPFR Library is distributed in the hope that it will be useful, but
     14 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
     15 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
     16 License for more details.
     17 
     18 You should have received a copy of the GNU Lesser General Public License
     19 along with the GNU MPFR Library; see the file COPYING.LESSER.  If not, see
     20 https://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
     21 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
     22 
     23 #include "mpfr-impl.h"
     24 
     25 int
     26 mpfr_div_2ui (mpfr_ptr y, mpfr_srcptr x, unsigned long n, mpfr_rnd_t rnd_mode)
     27 {
     28   int inexact;
     29 
     30   MPFR_LOG_FUNC (
     31     ("x[%Pd]=%.*Rg n=%lu rnd=%d", mpfr_get_prec(x), mpfr_log_prec, x, n,
     32      rnd_mode),
     33     ("y[%Pd]=%.*Rg inexact=%d", mpfr_get_prec(y), mpfr_log_prec, y, inexact));
     34 
     35   if (MPFR_UNLIKELY (n == 0 || MPFR_IS_SINGULAR (x)))
     36     return mpfr_set (y, x, rnd_mode);
     37   else
     38     {
     39       mpfr_exp_t exp = MPFR_GET_EXP (x);
     40       mpfr_uexp_t diffexp;
     41 
     42       MPFR_SETRAW (inexact, y, x, exp, rnd_mode);
     43       /* Warning! exp may have increased by 1 due to rounding. Thus the
     44          difference below may overflow in a mpfr_exp_t; but mpfr_uexp_t
     45          is OK to hold the value, with the difference done in unsigned
     46          integer arithmetic in this type. */
     47       diffexp = (mpfr_uexp_t) exp - (mpfr_uexp_t) (__gmpfr_emin - 1);
     48       if (MPFR_UNLIKELY (n >= diffexp))  /* exp - n <= emin - 1 */
     49         {
     50           if (rnd_mode == MPFR_RNDN &&
     51               (n > diffexp ||
     52                ((MPFR_IS_NEG (y) ? inexact <= 0 : inexact >= 0) &&
     53                 mpfr_powerof2_raw (y))))
     54             rnd_mode = MPFR_RNDZ;
     55           return mpfr_underflow (y, rnd_mode, MPFR_SIGN (y));
     56         }
     57       /* Now, n < diffexp, i.e. n <= exp - emin, which a difference of
     58        * two valid exponents + 0 or 1, thus fits in a mpfr_exp_t (from
     59        * the constraints on valid exponents). Moreover, there cannot be
     60        * an overflow (if exp had been increased by 1 due to rounding)
     61        * since the case n = 0 has been filtered out.
     62        */
     63       MPFR_ASSERTD (n <= MPFR_EXP_MAX);
     64       MPFR_ASSERTD (n >= 1);
     65       MPFR_SET_EXP (y, exp - (mpfr_exp_t) n);
     66     }
     67 
     68   MPFR_RET (inexact);
     69 }
     70