sub_n.c
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- /* Cray PVP mpn_sub_n -- subtract two limb vectors and store their difference
- in a third limb vector.
- Copyright 1996, 2000, 2001 Free Software Foundation, Inc.
- This file is part of the GNU MP Library.
- The GNU MP Library is free software; you can redistribute it and/or modify
- it under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 3 of the License, or (at your
- 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
- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
- License for more details.
- You should have received a copy of the GNU Lesser General Public License
- along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
- /* This code runs at 4 cycles/limb. It may be possible to bring it down
- to 3 cycles/limb. */
- #include "gmp.h"
- #include "gmp-impl.h"
- mp_limb_t
- mpn_sub_n (mp_ptr rp, mp_srcptr up, mp_srcptr vp, mp_size_t n)
- {
- mp_limb_t cy[n];
- mp_limb_t a, b, r, s0, c0, c1;
- mp_size_t i;
- int more_carries;
- /* Main subtract loop. Generate a raw output difference in rp[] and a
- borrow vector in cy[]. */
- #pragma _CRI ivdep
- for (i = 0; i < n; i++)
- {
- a = up[i];
- b = vp[i];
- s0 = a - b; /* a = s0 + b */
- rp[i] = s0;
- c0 = ((s0 & b) | ((s0 | b) & ~a)) >> 63;
- cy[i] = c0;
- }
- /* Borrow subtract loop. Subtract the borrow vector cy[] from the raw
- difference rp[] and store the new difference back to rp[0]. If this
- generates further borrow, set more_carries. */
- more_carries = 0;
- #pragma _CRI ivdep
- for (i = 1; i < n; i++)
- {
- r = rp[i];
- c0 = cy[i - 1];
- s0 = r - c0; /* r = s0 + c0 */
- rp[i] = s0;
- c0 = (s0 & ~r) >> 63;
- more_carries += c0;
- }
- /* If that second loop generated borrow, handle that in scalar loop. */
- if (more_carries)
- {
- mp_limb_t cyrec = 0;
- /* Look for places where rp[k] contains just ones and cy[k-1] is
- non-zero. These are where we got a recurrency borrow. */
- for (i = 1; i < n; i++)
- {
- r = rp[i];
- c0 = (~r == 0 && cy[i - 1] != 0);
- s0 = r - cyrec;
- rp[i] = s0;
- c1 = (s0 & ~r) >> 63;
- cyrec = c0 | c1;
- }
- return cyrec | cy[n - 1];
- }
- return cy[n - 1];
- }