lib/mpi/generic_mpih-sub1.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /* mpihelp-add_2.c  -  MPI helper functions
   3  * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.
   4  *
   5  * This file is part of GnuPG.
   6  *
   7  * Note: This code is heavily based on the GNU MP Library.
   8  *       Actually it's the same code with only minor changes in the
   9  *       way the data is stored; this is to support the abstraction
  10  *       of an optional secure memory allocation which may be used
  11  *       to avoid revealing of sensitive data due to paging etc.
  12  *       The GNU MP Library itself is published under the LGPL;
  13  *       however I decided to publish this code under the plain GPL.
  14  */
  15
  16 #include "mpi-internal.h"
  17 #include "longlong.h"
  18
  19 mpi_limb_t
  20 mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
  21               mpi_ptr_t s2_ptr, mpi_size_t size)
  22 {
  23         mpi_limb_t x, y, cy;
  24         mpi_size_t j;
  25
  26         /* The loop counter and index J goes from -SIZE to -1.  This way
  27            the loop becomes faster.  */
  28         j = -size;
  29
  30         /* Offset the base pointers to compensate for the negative indices.  */
  31         s1_ptr -= j;
  32         s2_ptr -= j;
  33         res_ptr -= j;
  34
  35         cy = 0;
  36         do {
  37                 y = s2_ptr[j];
  38                 x = s1_ptr[j];
  39                 y += cy;        /* add previous carry to subtrahend */
  40                 cy = y < cy;    /* get out carry from that addition */
  41                 y = x - y;      /* main subtract */
  42                 cy += y > x;    /* get out carry from the subtract, combine */
  43                 res_ptr[j] = y;
  44         } while (++j);
  45
  46         return cy;
  47 }