source/libs/gmp/gmp-src/mpn/generic/pre_divrem_1.c

   1 /* mpn_preinv_divrem_1 -- mpn by limb division with pre-inverted divisor.
   2
   3    THE FUNCTIONS IN THIS FILE ARE FOR INTERNAL USE ONLY.  THEY'RE ALMOST
   4    CERTAIN TO BE SUBJECT TO INCOMPATIBLE CHANGES OR DISAPPEAR COMPLETELY IN
   5    FUTURE GNU MP RELEASES.
   6
   7 Copyright 2000-2003 Free Software Foundation, Inc.
   8
   9 This file is part of the GNU MP Library.
  10
  11 The GNU MP Library is free software; you can redistribute it and/or modify
  12 it under the terms of either:
  13
  14   * the GNU Lesser General Public License as published by the Free
  15     Software Foundation; either version 3 of the License, or (at your
  16     option) any later version.
  17
  18 or
  19
  20   * the GNU General Public License as published by the Free Software
  21     Foundation; either version 2 of the License, or (at your option) any
  22     later version.
  23
  24 or both in parallel, as here.
  25
  26 The GNU MP Library is distributed in the hope that it will be useful, but
  27 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  28 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  29 for more details.
  30
  31 You should have received copies of the GNU General Public License and the
  32 GNU Lesser General Public License along with the GNU MP Library.  If not,
  33 see https://www.gnu.org/licenses/.  */
  34
  35 #include "gmp.h"
  36 #include "gmp-impl.h"
  37 #include "longlong.h"
  38
  39
  40 /* Don't bloat a shared library with unused code. */
  41 #if USE_PREINV_DIVREM_1
  42
  43 /* Same test here for skipping one divide step as in mpn_divrem_1.
  44
  45    The main reason for a separate shift==0 case is that not all CPUs give
  46    zero for "n0 >> GMP_LIMB_BITS" which would arise in the general case
  47    code used on shift==0.  shift==0 is also reasonably common in mp_bases
  48    big_base, for instance base==10 on a 64-bit limb.
  49
  50    Under shift!=0 it would be possible to call mpn_lshift to adjust the
  51    dividend all in one go (into the quotient space say), rather than
  52    limb-by-limb in the loop.  This might help if mpn_lshift is a lot faster
  53    than what the compiler can generate for EXTRACT.  But this is left to CPU
  54    specific implementations to consider, especially since EXTRACT isn't on
  55    the dependent chain.
  56
  57    If size==0 then the result is simply xsize limbs of zeros, but nothing
  58    special is done for that, since it wouldn't be a usual call, and
  59    certainly never arises from mpn_get_str which is our main caller.  */
  60
  61 mp_limb_t
  62 mpn_preinv_divrem_1 (mp_ptr qp, mp_size_t xsize,
  63                      mp_srcptr ap, mp_size_t size, mp_limb_t d_unnorm,
  64                      mp_limb_t dinv, int shift)
  65 {
  66   mp_limb_t  ahigh, qhigh, r;
  67   mp_size_t  i;
  68   mp_limb_t  n1, n0;
  69   mp_limb_t  d;
  70
  71   ASSERT (xsize >= 0);
  72   ASSERT (size >= 1);
  73   ASSERT (d_unnorm != 0);
  74 #if WANT_ASSERT
  75   {
  76     int        want_shift;
  77     mp_limb_t  want_dinv;
  78     count_leading_zeros (want_shift, d_unnorm);
  79     ASSERT (shift == want_shift);
  80     invert_limb (want_dinv, d_unnorm << shift);
  81     ASSERT (dinv == want_dinv);
  82   }
  83 #endif
  84   /* FIXME: What's the correct overlap rule when xsize!=0? */
  85   ASSERT (MPN_SAME_OR_SEPARATE_P (qp+xsize, ap, size));
  86
  87   ahigh = ap[size-1];
  88   d = d_unnorm << shift;
  89   qp += (size + xsize - 1);   /* dest high limb */
  90
  91   if (shift == 0)
  92     {
  93       /* High quotient limb is 0 or 1, and skip a divide step. */
  94       r = ahigh;
  95       qhigh = (r >= d);
  96       r = (qhigh ? r-d : r);
  97       *qp-- = qhigh;
  98       size--;
  99
 100       for (i = size-1; i >= 0; i--)
 101         {
 102           n0 = ap[i];
 103           udiv_qrnnd_preinv (*qp, r, r, n0, d, dinv);
 104           qp--;
 105         }
 106     }
 107   else
 108     {
 109       r = 0;
 110       if (ahigh < d_unnorm)
 111         {
 112           r = ahigh << shift;
 113           *qp-- = 0;
 114           size--;
 115           if (size == 0)
 116             goto done_integer;
 117         }
 118
 119       n1 = ap[size-1];
 120       r |= n1 >> (GMP_LIMB_BITS - shift);
 121
 122       for (i = size-2; i >= 0; i--)
 123         {
 124           ASSERT (r < d);
 125           n0 = ap[i];
 126           udiv_qrnnd_preinv (*qp, r, r,
 127                              ((n1 << shift) | (n0 >> (GMP_LIMB_BITS - shift))),
 128                              d, dinv);
 129           qp--;
 130           n1 = n0;
 131         }
 132       udiv_qrnnd_preinv (*qp, r, r, n1 << shift, d, dinv);
 133       qp--;
 134     }
 135
 136  done_integer:
 137   for (i = 0; i < xsize; i++)
 138     {
 139       udiv_qrnnd_preinv (*qp, r, r, CNST_LIMB(0), d, dinv);
 140       qp--;
 141     }
 142
 143   return r >> shift;
 144 }
 145
 146 #endif /* USE_PREINV_DIVREM_1 */