83 lines
2.4 KiB
C
83 lines
2.4 KiB
C
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/* Cray PVP mpn_sub_n -- subtract two limb vectors and store their difference
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in a third limb vector.
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Copyright 1996, 2000, 2001 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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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
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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The GNU MP Library is distributed in the hope that it will be useful, but
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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
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA. */
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/* This code runs at 4 cycles/limb. It may be possible to bring it down
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to 3 cycles/limb. */
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#include "gmp.h"
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#include "gmp-impl.h"
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mp_limb_t
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mpn_sub_n (mp_ptr rp, mp_srcptr up, mp_srcptr vp, mp_size_t n)
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{
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mp_limb_t cy[n];
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mp_limb_t a, b, r, s0, c0, c1;
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mp_size_t i;
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int more_carries;
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/* Main subtract loop. Generate a raw output difference in rp[] and a
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borrow vector in cy[]. */
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#pragma _CRI ivdep
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for (i = 0; i < n; i++)
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{
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a = up[i];
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b = vp[i];
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s0 = a - b; /* a = s0 + b */
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rp[i] = s0;
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c0 = ((s0 & b) | ((s0 | b) & ~a)) >> 63;
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cy[i] = c0;
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}
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/* Borrow subtract loop. Subtract the borrow vector cy[] from the raw
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difference rp[] and store the new difference back to rp[0]. If this
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generates further borrow, set more_carries. */
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more_carries = 0;
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#pragma _CRI ivdep
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for (i = 1; i < n; i++)
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{
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r = rp[i];
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c0 = cy[i - 1];
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s0 = r - c0; /* r = s0 + c0 */
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rp[i] = s0;
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c0 = (s0 & ~r) >> 63;
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more_carries += c0;
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}
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/* If that second loop generated borrow, handle that in scalar loop. */
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if (more_carries)
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{
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mp_limb_t cyrec = 0;
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/* Look for places where rp[k] contains just ones and cy[k-1] is
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non-zero. These are where we got a recurrency borrow. */
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for (i = 1; i < n; i++)
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{
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r = rp[i];
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c0 = (~r == 0 && cy[i - 1] != 0);
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s0 = r - cyrec;
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rp[i] = s0;
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c1 = (s0 & ~r) >> 63;
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cyrec = c0 | c1;
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}
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return cyrec | cy[n - 1];
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}
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return cy[n - 1];
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}
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