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
19mpi_limb_t
20mpihelp_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}
49