github.com/aergoio/aergo@v1.3.1/libtool/src/gmp-6.1.2/mpn/generic/dive_1.c (about)

     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 2000-2003, 2005, 2013 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  
    41  /* Divide a={src,size} by d=divisor and store the quotient in q={dst,size}.
    42     q will only be correct if d divides a exactly.
    43  
    44     A separate loop is used for shift==0 because n<<GMP_LIMB_BITS doesn't
    45     give zero on all CPUs (for instance it doesn't on the x86s).  This
    46     separate loop might run faster too, helping odd divisors.
    47  
    48     Possibilities:
    49  
    50     mpn_divexact_1c could be created, accepting and returning c.  This would
    51     let a long calculation be done piece by piece.  Currently there's no
    52     particular need for that, and not returning c means that a final umul can
    53     be skipped.
    54  
    55     Another use for returning c would be letting the caller know whether the
    56     division was in fact exact.  It would work just to return the carry bit
    57     "c=(l>s)" and let the caller do a final umul if interested.
    58  
    59     When the divisor is even, the factors of two could be handled with a
    60     separate mpn_rshift, instead of shifting on the fly.  That might be
    61     faster on some CPUs and would mean just the shift==0 style loop would be
    62     needed.
    63  
    64     If n<<GMP_LIMB_BITS gives zero on a particular CPU then the separate
    65     shift==0 loop is unnecessary, and could be eliminated if there's no great
    66     speed difference.
    67  
    68     It's not clear whether "/" is the best way to handle size==1.  Alpha gcc
    69     2.95 for instance has a poor "/" and might prefer the modular method.
    70     Perhaps a tuned parameter should control this.
    71  
    72     If src[size-1] < divisor then dst[size-1] will be zero, and one divide
    73     step could be skipped.  A test at last step for s<divisor (or ls in the
    74     even case) might be a good way to do that.  But if this code is often
    75     used with small divisors then it might not be worth bothering  */
    76  
    77  void
    78  mpn_divexact_1 (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t divisor)
    79  {
    80    mp_size_t  i;
    81    mp_limb_t  c, h, l, ls, s, s_next, inverse, dummy;
    82    unsigned   shift;
    83  
    84    ASSERT (size >= 1);
    85    ASSERT (divisor != 0);
    86    ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));
    87    ASSERT_MPN (src, size);
    88    ASSERT_LIMB (divisor);
    89  
    90    if ((divisor & 1) == 0)
    91      {
    92        count_trailing_zeros (shift, divisor);
    93        divisor >>= shift;
    94      }
    95    else
    96      shift = 0;
    97  
    98    binvert_limb (inverse, divisor);
    99    divisor <<= GMP_NAIL_BITS;
   100  
   101    if (shift != 0)
   102      {
   103        c = 0;
   104  
   105        s = src[0];
   106  
   107        for (i = 1; i < size; i++)
   108  	{
   109  	  s_next = src[i];
   110  	  ls = ((s >> shift) | (s_next << (GMP_NUMB_BITS-shift))) & GMP_NUMB_MASK;
   111  	  s = s_next;
   112  
   113  	  SUBC_LIMB (c, l, ls, c);
   114  
   115  	  l = (l * inverse) & GMP_NUMB_MASK;
   116  	  dst[i - 1] = l;
   117  
   118  	  umul_ppmm (h, dummy, l, divisor);
   119  	  c += h;
   120  	}
   121  
   122        ls = s >> shift;
   123        l = ls - c;
   124        l = (l * inverse) & GMP_NUMB_MASK;
   125        dst[size - 1] = l;
   126      }
   127    else
   128      {
   129        s = src[0];
   130  
   131        l = (s * inverse) & GMP_NUMB_MASK;
   132        dst[0] = l;
   133        c = 0;
   134  
   135        for (i = 1; i < size; i++)
   136  	{
   137  	  umul_ppmm (h, dummy, l, divisor);
   138  	  c += h;
   139  
   140  	  s = src[i];
   141  	  SUBC_LIMB (c, l, s, c);
   142  
   143  	  l = (l * inverse) & GMP_NUMB_MASK;
   144  	  dst[i] = l;
   145  	}
   146      }
   147  }