lib/mpi/generic_mpih-mul3.c

   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 }