1*4882a593Smuzhiyun // SPDX-License-Identifier: GPL-2.0-or-later
2*4882a593Smuzhiyun /* mpihelp-add_2.c - MPI helper functions
3*4882a593Smuzhiyun * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.
4*4882a593Smuzhiyun *
5*4882a593Smuzhiyun * This file is part of GnuPG.
6*4882a593Smuzhiyun *
7*4882a593Smuzhiyun * Note: This code is heavily based on the GNU MP Library.
8*4882a593Smuzhiyun * Actually it's the same code with only minor changes in the
9*4882a593Smuzhiyun * way the data is stored; this is to support the abstraction
10*4882a593Smuzhiyun * of an optional secure memory allocation which may be used
11*4882a593Smuzhiyun * to avoid revealing of sensitive data due to paging etc.
12*4882a593Smuzhiyun * The GNU MP Library itself is published under the LGPL;
13*4882a593Smuzhiyun * however I decided to publish this code under the plain GPL.
14*4882a593Smuzhiyun */
15*4882a593Smuzhiyun
16*4882a593Smuzhiyun #include "mpi-internal.h"
17*4882a593Smuzhiyun #include "longlong.h"
18*4882a593Smuzhiyun
19*4882a593Smuzhiyun mpi_limb_t
mpihelp_sub_n(mpi_ptr_t res_ptr,mpi_ptr_t s1_ptr,mpi_ptr_t s2_ptr,mpi_size_t size)20*4882a593Smuzhiyun mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
21*4882a593Smuzhiyun mpi_ptr_t s2_ptr, mpi_size_t size)
22*4882a593Smuzhiyun {
23*4882a593Smuzhiyun mpi_limb_t x, y, cy;
24*4882a593Smuzhiyun mpi_size_t j;
25*4882a593Smuzhiyun
26*4882a593Smuzhiyun /* The loop counter and index J goes from -SIZE to -1. This way
27*4882a593Smuzhiyun the loop becomes faster. */
28*4882a593Smuzhiyun j = -size;
29*4882a593Smuzhiyun
30*4882a593Smuzhiyun /* Offset the base pointers to compensate for the negative indices. */
31*4882a593Smuzhiyun s1_ptr -= j;
32*4882a593Smuzhiyun s2_ptr -= j;
33*4882a593Smuzhiyun res_ptr -= j;
34*4882a593Smuzhiyun
35*4882a593Smuzhiyun cy = 0;
36*4882a593Smuzhiyun do {
37*4882a593Smuzhiyun y = s2_ptr[j];
38*4882a593Smuzhiyun x = s1_ptr[j];
39*4882a593Smuzhiyun y += cy; /* add previous carry to subtrahend */
40*4882a593Smuzhiyun cy = y < cy; /* get out carry from that addition */
41*4882a593Smuzhiyun y = x - y; /* main subtract */
42*4882a593Smuzhiyun cy += y > x; /* get out carry from the subtract, combine */
43*4882a593Smuzhiyun res_ptr[j] = y;
44*4882a593Smuzhiyun } while (++j);
45*4882a593Smuzhiyun
46*4882a593Smuzhiyun return cy;
47*4882a593Smuzhiyun }
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