glibc/stdlib/submul_1.c

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/* mpn_submul_1 -- multiply the S1_SIZE long limb vector pointed to by S1_PTR
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by S2_LIMB, subtract the S1_SIZE least significant limbs of the product
from the limb vector pointed to by RES_PTR. Return the most significant
limb of the product, adjusted for carry-out from the subtraction.
Copyright (C) 1992-2015 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library; see the file COPYING.LIB. If not, see
<http://www.gnu.org/licenses/>. */
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#include <gmp.h>
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#include "gmp-impl.h"
#include "longlong.h"
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mp_limb_t
mpn_submul_1 (res_ptr, s1_ptr, s1_size, s2_limb)
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register mp_ptr res_ptr;
register mp_srcptr s1_ptr;
mp_size_t s1_size;
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register mp_limb_t s2_limb;
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{
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register mp_limb_t cy_limb;
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register mp_size_t j;
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register mp_limb_t prod_high, prod_low;
register mp_limb_t x;
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/* The loop counter and index J goes from -SIZE to -1. This way
the loop becomes faster. */
j = -s1_size;
/* Offset the base pointers to compensate for the negative indices. */
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) + prod_high;
x = res_ptr[j];
prod_low = x - prod_low;
cy_limb += (prod_low > x);
res_ptr[j] = prod_low;
}
while (++j != 0);
return cy_limb;
}