sysdeps/generic/sub_n.c

   1 /* __mpn_sub_n -- Subtract two limb vectors of equal, non-zero length.
   2
   3 Copyright (C) 1992, 1993, 1994 Free Software Foundation, Inc.
   4
   5 This file is part of the GNU MP Library.
   6
   7 The GNU MP Library is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU Library General Public License as published by
   9 the Free Software Foundation; either version 2 of the License, or (at your
  10 option) any later version.
  11
  12 The GNU MP Library is distributed in the hope that it will be useful, but
  13 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  14 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
  15 License for more details.
  16
  17 You should have received a copy of the GNU Library General Public License
  18 along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
  19 the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */
  20
  21 #include "gmp.h"
  22 #include "gmp-impl.h"
  23
  24 mp_limb
  25 #if __STDC__
  26 __mpn_sub_n (mp_ptr res_ptr, mp_srcptr s1_ptr, mp_srcptr s2_ptr, mp_size_t size)
  27 #else
  28 __mpn_sub_n (res_ptr, s1_ptr, s2_ptr, size)
  29      register mp_ptr res_ptr;
  30      register mp_srcptr s1_ptr;
  31      register mp_srcptr s2_ptr;
  32      mp_size_t size;
  33 #endif
  34 {
  35   register mp_limb x, y, cy;
  36   register mp_size_t j;
  37
  38   /* The loop counter and index J goes from -SIZE to -1.  This way
  39      the loop becomes faster.  */
  40   j = -size;
  41
  42   /* Offset the base pointers to compensate for the negative indices.  */
  43   s1_ptr -= j;
  44   s2_ptr -= j;
  45   res_ptr -= j;
  46
  47   cy = 0;
  48   do
  49     {
  50       y = s2_ptr[j];
  51       x = s1_ptr[j];
  52       y += cy;                  /* add previous carry to subtrahend */
  53       cy = (y < cy);            /* get out carry from that addition */
  54       y = x - y;                /* main subtract */
  55       cy = (y > x) + cy;        /* get out carry from the subtract, combine */
  56       res_ptr[j] = y;
  57     }
  58   while (++j != 0);
  59
  60   return cy;
  61 }