generic_mpih-mul3.c

// SPDX-License-Identifier: GPL-2.0-or-later
/* mpihelp-mul_3.c  -  MPI helper functions
 * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.
 *
 * This file is part of GnuPG.
 *
 * Note: This code is heavily based on the GNU MP Library.
 *	 Actually it's the same code with only minor changes in the
 *	 way the data is stored; this is to support the abstraction
 *	 of an optional secure memory allocation which may be used
 *	 to avoid revealing of sensitive data due to paging etc.
 *	 The GNU MP Library itself is published under the LGPL;
 *	 however I decided to publish this code under the plain GPL.
 */

#include "mpi-internal.h"
#include "longlong.h"

mpi_limb_t
mpihelp_submul_1(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
		 mpi_size_t s1_size, mpi_limb_t s2_limb)
{
	mpi_limb_t cy_limb;
	mpi_size_t j;
	mpi_limb_t prod_high, prod_low;
	mpi_limb_t x;

	/* The loop counter and index J goes from -SIZE to -1.  This way
	 * the loop becomes faster.  */
	j = -s1_size;
	res_ptr -= j;
	s1_ptr -= j;

	cy_limb = 0;
	do {
		umul_ppmm(prod_high, prod_low, s1_ptr[j], s2_limb);

		prod_low += cy_limb;
		cy_limb = (prod_low < cy_limb ? 1 : 0) + prod_high;

		x = res_ptr[j];
		prod_low = x - prod_low;
		cy_limb += prod_low > x ? 1 : 0;
		res_ptr[j] = prod_low;
	} while (++j);

	return cy_limb;
}