lib/mpi/generic_mpih-add1.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /* mpihelp-add_1.c  -  MPI helper functions
   3  * Copyright (C) 1994, 1996, 1997, 1998,
   4  *               2000 Free Software Foundation, Inc.
   5  *
   6  * This file is part of GnuPG.
   7  *
   8  * Note: This code is heavily based on the GNU MP Library.
   9  *       Actually it's the same code with only minor changes in the
  10  *       way the data is stored; this is to support the abstraction
  11  *       of an optional secure memory allocation which may be used
  12  *       to avoid revealing of sensitive data due to paging etc.
  13  *       The GNU MP Library itself is published under the LGPL;
  14  *       however I decided to publish this code under the plain GPL.
  15  */
  16
  17 #include "mpi-internal.h"
  18 #include "longlong.h"
  19
  20 mpi_limb_t
  21 mpihelp_add_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
  22               mpi_ptr_t s2_ptr, mpi_size_t size)
  23 {
  24         mpi_limb_t x, y, cy;
  25         mpi_size_t j;
  26
  27         /* The loop counter and index J goes from -SIZE to -1.  This way
  28            the loop becomes faster.  */
  29         j = -size;
  30
  31         /* Offset the base pointers to compensate for the negative indices. */
  32         s1_ptr -= j;
  33         s2_ptr -= j;
  34         res_ptr -= j;
  35
  36         cy = 0;
  37         do {
  38                 y = s2_ptr[j];
  39                 x = s1_ptr[j];
  40                 y += cy;        /* add previous carry to one addend */
  41                 cy = y < cy;    /* get out carry from that addition */
  42                 y += x;         /* add other addend */
  43                 cy += y < x;    /* get out carry from that add, combine */
  44                 res_ptr[j] = y;
  45         } while (++j);
  46
  47         return cy;
  48 }