add_n.c

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

/* Cray PVP mpn_add_n -- add two limb vectors and store their sum 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_add_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 add loop.  Generate a raw output sum in rp[] and a carry vector
     in cy[].  */
#pragma _CRI ivdep
  for (i = 0; i < n; i++)
    {
      a = up[i];
      b = vp[i];
      s0 = a + b;
      rp[i] = s0;
      c0 = ((a & b) | ((a | b) & ~s0)) >> 63;
      cy[i] = c0;
    }
  /* Carry add loop.  Add the carry vector cy[] to the raw sum rp[] and
     store the new sum back to rp[0].  If this generates further carry, 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;
      rp[i] = s0;
      c0 = (r & ~s0) >> 63;
      more_carries += c0;
    }
  /* If that second loop generated carry, handle that in scalar loop.  */
  if (more_carries)
    {
      mp_limb_t cyrec = 0;
      /* Look for places where rp[k] is zero and cy[k-1] is non-zero.
	 These are where we got a recurrency carry.  */
      for (i = 1; i < n; i++)
	{
	  r = rp[i];
	  c0 = (r == 0 && cy[i - 1] != 0);
	  s0 = r + cyrec;
	  rp[i] = s0;
	  c1 = (r & ~s0) >> 63;
	  cyrec = c0 | c1;
	}
      return cyrec | cy[n - 1];
    }

  return cy[n - 1];
}