1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /* mpihelp-mul_3.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_submul_1(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
21 mpi_size_t s1_size, mpi_limb_t s2_limb)
22 {
23 mpi_limb_t cy_limb;
24 mpi_size_t j;
25 mpi_limb_t prod_high, prod_low;
26 mpi_limb_t x;
27
28 /* The loop counter and index J goes from -SIZE to -1. This way
29 * the loop becomes faster. */
30 j = -s1_size;
31 res_ptr -= j;
32 s1_ptr -= j;
33
34 cy_limb = 0;
35 do {
36 umul_ppmm(prod_high, prod_low, s1_ptr[j], s2_limb);
37
38 prod_low += cy_limb;
39 cy_limb = (prod_low < cy_limb ? 1 : 0) + prod_high;
40
41 x = res_ptr[j];
42 prod_low = x - prod_low;
43 cy_limb += prod_low > x ? 1 : 0;
44 res_ptr[j] = prod_low;
45 } while (++j);
46
47 return cy_limb;
48 }