sub_n.c

a very popular packet of cryptography tools,it encloses the most common used algorithm and protocols

/* Cray PVP mpn_sub_n -- subtract two limb vectors and store their difference
   in a third limb vector.

Copyright 1996, 2000, 2001 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA.  */

/* This code runs at 4 cycles/limb.  It may be possible to bring it down
   to 3 cycles/limb.  */

#include "gmp.h"
#include "gmp-impl.h"

mp_limb_t
mpn_sub_n (mp_ptr rp, mp_srcptr up, mp_srcptr vp, mp_size_t n)
{
  mp_limb_t cy[n];
  mp_limb_t a, b, r, s0, c0, c1;
  mp_size_t i;
  int more_carries;

  /* Main subtract loop.  Generate a raw output difference in rp[] and a
     borrow vector in cy[].  */
#pragma _CRI ivdep
  for (i = 0; i < n; i++)
    {
      a = up[i];
      b = vp[i];
      s0 = a - b;		/* a = s0 + b */
      rp[i] = s0;
      c0 = ((s0 & b) | ((s0 | b) & ~a)) >> 63;
      cy[i] = c0;
    }
  /* Borrow subtract loop.  Subtract the borrow vector cy[] from the raw
     difference rp[] and store the new difference back to rp[0].  If this
     generates further borrow, set more_carries.  */
  more_carries = 0;
#pragma _CRI ivdep
  for (i = 1; i < n; i++)
    {
      r = rp[i];
      c0 = cy[i - 1];
      s0 = r - c0;		/* r = s0 + c0 */
      rp[i] = s0;
      c0 = (s0 & ~r) >> 63;
      more_carries += c0;
    }
  /* If that second loop generated borrow, handle that in scalar loop.  */
  if (more_carries)
    {
      mp_limb_t cyrec = 0;
      /* Look for places where rp[k] contains just ones and cy[k-1] is
	 non-zero.  These are where we got a recurrency borrow.  */
      for (i = 1; i < n; i++)
	{
	  r = rp[i];
	  c0 = (~r == 0 && cy[i - 1] != 0);
	  s0 = r - cyrec;
	  rp[i] = s0;
	  c1 = (s0 & ~r) >> 63;
	  cyrec = c0 | c1;
	}
      return cyrec | cy[n - 1];
    }

  return cy[n - 1];
}