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 }
49