| 1 | // SPDX-License-Identifier: GPL-2.0-or-later |
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| 2 | /* mpihelp-add_2.c - MPI helper functions |
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| 3 | * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc. |
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| 4 | * |
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| 5 | * This file is part of GnuPG. |
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| 6 | * |
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| 7 | * Note: This code is heavily based on the GNU MP Library. |
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| 8 | * Actually it's the same code with only minor changes in the |
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| 9 | * way the data is stored; this is to support the abstraction |
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| 10 | * of an optional secure memory allocation which may be used |
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| 11 | * to avoid revealing of sensitive data due to paging etc. |
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| 12 | * The GNU MP Library itself is published under the LGPL; |
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| 13 | * however I decided to publish this code under the plain GPL. |
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| 14 | */ |
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| 15 | |
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| 16 | #include "mpi-internal.h" |
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| 17 | #include "longlong.h" |
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| 18 | |
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| 19 | mpi_limb_t |
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| 20 | mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr, |
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| 21 | mpi_ptr_t s2_ptr, mpi_size_t size) |
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| 22 | { |
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| 23 | mpi_limb_t x, y, cy; |
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| 24 | mpi_size_t j; |
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| 25 | |
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| 26 | /* The loop counter and index J goes from -SIZE to -1. This way |
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| 27 | the loop becomes faster. */ |
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| 28 | j = -size; |
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| 29 | |
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| 30 | /* Offset the base pointers to compensate for the negative indices. */ |
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| 31 | s1_ptr -= j; |
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| 32 | s2_ptr -= j; |
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| 33 | res_ptr -= j; |
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| 34 | |
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| 35 | cy = 0; |
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| 36 | do { |
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| 37 | y = s2_ptr[j]; |
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| 38 | x = s1_ptr[j]; |
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| 39 | y += cy; /* add previous carry to subtrahend */ |
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| 40 | cy = y < cy; /* get out carry from that addition */ |
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| 41 | y = x - y; /* main subtract */ |
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| 42 | cy += y > x; /* get out carry from the subtract, combine */ |
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| 43 | res_ptr[j] = y; |
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| 44 | } while (++j); |
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| 45 | |
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| 46 | return cy; |
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| 47 | } |
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| 48 | |
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