1*2a598d0bSHerbert Xu // SPDX-License-Identifier: GPL-2.0-or-later
2*2a598d0bSHerbert Xu /* mpihelp-add_2.c - MPI helper functions
3*2a598d0bSHerbert Xu * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.
4*2a598d0bSHerbert Xu *
5*2a598d0bSHerbert Xu * This file is part of GnuPG.
6*2a598d0bSHerbert Xu *
7*2a598d0bSHerbert Xu * Note: This code is heavily based on the GNU MP Library.
8*2a598d0bSHerbert Xu * Actually it's the same code with only minor changes in the
9*2a598d0bSHerbert Xu * way the data is stored; this is to support the abstraction
10*2a598d0bSHerbert Xu * of an optional secure memory allocation which may be used
11*2a598d0bSHerbert Xu * to avoid revealing of sensitive data due to paging etc.
12*2a598d0bSHerbert Xu * The GNU MP Library itself is published under the LGPL;
13*2a598d0bSHerbert Xu * however I decided to publish this code under the plain GPL.
14*2a598d0bSHerbert Xu */
15*2a598d0bSHerbert Xu
16*2a598d0bSHerbert Xu #include "mpi-internal.h"
17*2a598d0bSHerbert Xu #include "longlong.h"
18*2a598d0bSHerbert Xu
19*2a598d0bSHerbert Xu 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*2a598d0bSHerbert Xu mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
21*2a598d0bSHerbert Xu mpi_ptr_t s2_ptr, mpi_size_t size)
22*2a598d0bSHerbert Xu {
23*2a598d0bSHerbert Xu mpi_limb_t x, y, cy;
24*2a598d0bSHerbert Xu mpi_size_t j;
25*2a598d0bSHerbert Xu
26*2a598d0bSHerbert Xu /* The loop counter and index J goes from -SIZE to -1. This way
27*2a598d0bSHerbert Xu the loop becomes faster. */
28*2a598d0bSHerbert Xu j = -size;
29*2a598d0bSHerbert Xu
30*2a598d0bSHerbert Xu /* Offset the base pointers to compensate for the negative indices. */
31*2a598d0bSHerbert Xu s1_ptr -= j;
32*2a598d0bSHerbert Xu s2_ptr -= j;
33*2a598d0bSHerbert Xu res_ptr -= j;
34*2a598d0bSHerbert Xu
35*2a598d0bSHerbert Xu cy = 0;
36*2a598d0bSHerbert Xu do {
37*2a598d0bSHerbert Xu y = s2_ptr[j];
38*2a598d0bSHerbert Xu x = s1_ptr[j];
39*2a598d0bSHerbert Xu y += cy; /* add previous carry to subtrahend */
40*2a598d0bSHerbert Xu cy = y < cy; /* get out carry from that addition */
41*2a598d0bSHerbert Xu y = x - y; /* main subtract */
42*2a598d0bSHerbert Xu cy += y > x; /* get out carry from the subtract, combine */
43*2a598d0bSHerbert Xu res_ptr[j] = y;
44*2a598d0bSHerbert Xu } while (++j);
45*2a598d0bSHerbert Xu
46*2a598d0bSHerbert Xu return cy;
47*2a598d0bSHerbert Xu }
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