1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /* mpihelp-add_1.c  -  MPI helper functions
3  * Copyright (C) 1994, 1996, 1997, 1998,
4  *               2000 Free Software Foundation, Inc.
5  *
6  * This file is part of GnuPG.
7  *
8  * Note: This code is heavily based on the GNU MP Library.
9  *	 Actually it's the same code with only minor changes in the
10  *	 way the data is stored; this is to support the abstraction
11  *	 of an optional secure memory allocation which may be used
12  *	 to avoid revealing of sensitive data due to paging etc.
13  *	 The GNU MP Library itself is published under the LGPL;
14  *	 however I decided to publish this code under the plain GPL.
15  */
16 
17 #include "mpi-internal.h"
18 #include "longlong.h"
19 
20 mpi_limb_t
mpihelp_add_n(mpi_ptr_t res_ptr,mpi_ptr_t s1_ptr,mpi_ptr_t s2_ptr,mpi_size_t size)21 mpihelp_add_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
22 	      mpi_ptr_t s2_ptr, mpi_size_t size)
23 {
24 	mpi_limb_t x, y, cy;
25 	mpi_size_t j;
26 
27 	/* The loop counter and index J goes from -SIZE to -1.  This way
28 	   the loop becomes faster.  */
29 	j = -size;
30 
31 	/* Offset the base pointers to compensate for the negative indices. */
32 	s1_ptr -= j;
33 	s2_ptr -= j;
34 	res_ptr -= j;
35 
36 	cy = 0;
37 	do {
38 		y = s2_ptr[j];
39 		x = s1_ptr[j];
40 		y += cy;	/* add previous carry to one addend */
41 		cy = y < cy;	/* get out carry from that addition */
42 		y += x;		/* add other addend */
43 		cy += y < x;	/* get out carry from that add, combine */
44 		res_ptr[j] = y;
45 	} while (++j);
46 
47 	return cy;
48 }
49