contrib/gmp/mpn/generic/dive_1.c

   1 /* mpn_divexact_1 -- mpn by limb exact division.
   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, 2001, 2002, 2003, 2005 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 the GNU Lesser General Public License as published by
  13 the Free Software Foundation; either version 3 of the License, or (at your
  14 option) any later version.
  15
  16 The GNU MP Library is distributed in the hope that it will be useful, but
  17 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  18 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  19 License for more details.
  20
  21 You should have received a copy of the GNU Lesser General Public License
  22 along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
  23
  24 #include "gmp.h"
  25 #include "gmp-impl.h"
  26 #include "longlong.h"
  27
  28
  29
  30 /* Divide a={src,size} by d=divisor and store the quotient in q={dst,size}.
  31    q will only be correct if d divides a exactly.
  32
  33    A separate loop is used for shift==0 because n<<BITS_PER_MP_LIMB doesn't
  34    give zero on all CPUs (for instance it doesn't on the x86s).  This
  35    separate loop might run faster too, helping odd divisors.
  36
  37    Possibilities:
  38
  39    mpn_divexact_1c could be created, accepting and returning c.  This would
  40    let a long calculation be done piece by piece.  Currently there's no
  41    particular need for that, and not returning c means that a final umul can
  42    be skipped.
  43
  44    Another use for returning c would be letting the caller know whether the
  45    division was in fact exact.  It would work just to return the carry bit
  46    "c=(l>s)" and let the caller do a final umul if interested.
  47
  48    When the divisor is even, the factors of two could be handled with a
  49    separate mpn_rshift, instead of shifting on the fly.  That might be
  50    faster on some CPUs and would mean just the shift==0 style loop would be
  51    needed.
  52
  53    If n<<BITS_PER_MP_LIMB gives zero on a particular CPU then the separate
  54    shift==0 loop is unnecessary, and could be eliminated if there's no great
  55    speed difference.
  56
  57    It's not clear whether "/" is the best way to handle size==1.  Alpha gcc
  58    2.95 for instance has a poor "/" and might prefer the modular method.
  59    Perhaps a tuned parameter should control this.
  60
  61    If src[size-1] < divisor then dst[size-1] will be zero, and one divide
  62    step could be skipped.  A test at last step for s<divisor (or ls in the
  63    even case) might be a good way to do that.  But if this code is often
  64    used with small divisors then it might not be worth bothering  */
  65
  66 void
  67 mpn_divexact_1 (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t divisor)
  68 {
  69   mp_size_t  i;
  70   mp_limb_t  c, h, l, ls, s, s_next, inverse, dummy;
  71   unsigned   shift;
  72
  73   ASSERT (size >= 1);
  74   ASSERT (divisor != 0);
  75   ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));
  76   ASSERT_MPN (src, size);
  77   ASSERT_LIMB (divisor);
  78
  79   s = src[0];
  80
  81   if (size == 1)
  82     {
  83       dst[0] = s / divisor;
  84       return;
  85     }
  86
  87   if ((divisor & 1) == 0)
  88     {
  89       count_trailing_zeros (shift, divisor);
  90       divisor >>= shift;
  91     }
  92   else
  93     shift = 0;
  94
  95   binvert_limb (inverse, divisor);
  96   divisor <<= GMP_NAIL_BITS;
  97
  98   if (shift != 0)
  99     {
 100       c = 0;
 101       i = 0;
 102       size--;
 103
 104       do
 105         {
 106           s_next = src[i+1];
 107           ls = ((s >> shift) | (s_next << (GMP_NUMB_BITS-shift))) & GMP_NUMB_MASK;
 108           s = s_next;
 109
 110           SUBC_LIMB (c, l, ls, c);
 111
 112           l = (l * inverse) & GMP_NUMB_MASK;
 113           dst[i] = l;
 114
 115           umul_ppmm (h, dummy, l, divisor);
 116           c += h;
 117
 118           i++;
 119         }
 120       while (i < size);
 121
 122       ls = s >> shift;
 123       l = ls - c;
 124       l = (l * inverse) & GMP_NUMB_MASK;
 125       dst[i] = l;
 126     }
 127   else
 128     {
 129       l = (s * inverse) & GMP_NUMB_MASK;
 130       dst[0] = l;
 131       i = 1;
 132       c = 0;
 133
 134       do
 135         {
 136           umul_ppmm (h, dummy, l, divisor);
 137           c += h;
 138
 139           s = src[i];
 140           SUBC_LIMB (c, l, s, c);
 141
 142           l = (l * inverse) & GMP_NUMB_MASK;
 143           dst[i] = l;
 144           i++;
 145         }
 146       while (i < size);
 147     }
 148 }