138 lines
4.0 KiB
C
138 lines
4.0 KiB
C
/* mpn_preinv_divrem_1 -- mpn by limb division with pre-inverted divisor.
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THE FUNCTIONS IN THIS FILE ARE FOR INTERNAL USE ONLY. THEY'RE ALMOST
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CERTAIN TO BE SUBJECT TO INCOMPATIBLE CHANGES OR DISAPPEAR COMPLETELY IN
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FUTURE GNU MP RELEASES.
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Copyright 2000, 2001, 2002, 2003 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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#include "gmp.h"
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#include "gmp-impl.h"
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#include "longlong.h"
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/* Don't bloat a shared library with unused code. */
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#if USE_PREINV_DIVREM_1
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/* Same test here for skipping one divide step as in mpn_divrem_1.
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The main reason for a separate shift==0 case is that not all CPUs give
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zero for "n0 >> BITS_PER_MP_LIMB" which would arise in the general case
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code used on shift==0. shift==0 is also reasonably common in __mp_bases
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big_base, for instance base==10 on a 64-bit limb.
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Under shift!=0 it would be possible to call mpn_lshift to adjust the
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dividend all in one go (into the quotient space say), rather than
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limb-by-limb in the loop. This might help if mpn_lshift is a lot faster
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than what the compiler can generate for EXTRACT. But this is left to CPU
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specific implementations to consider, especially since EXTRACT isn't on
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the dependent chain.
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If size==0 then the result is simply xsize limbs of zeros, but nothing
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special is done for that, since it wouldn't be a usual call, and
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certainly never arises from mpn_get_str which is our main caller. */
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mp_limb_t
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mpn_preinv_divrem_1 (mp_ptr qp, mp_size_t xsize,
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mp_srcptr ap, mp_size_t size, mp_limb_t d_unnorm,
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mp_limb_t dinv, int shift)
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{
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mp_limb_t ahigh, qhigh, r;
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mp_size_t i;
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mp_limb_t n1, n0;
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mp_limb_t d;
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ASSERT (xsize >= 0);
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ASSERT (size >= 1);
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ASSERT (d_unnorm != 0);
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#if WANT_ASSERT
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{
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int want_shift;
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mp_limb_t want_dinv;
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count_leading_zeros (want_shift, d_unnorm);
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ASSERT (shift == want_shift);
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invert_limb (want_dinv, d_unnorm << shift);
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ASSERT (dinv == want_dinv);
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}
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#endif
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/* FIXME: What's the correct overlap rule when xsize!=0? */
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ASSERT (MPN_SAME_OR_SEPARATE_P (qp+xsize, ap, size));
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ahigh = ap[size-1];
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d = d_unnorm << shift;
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qp += (size + xsize - 1); /* dest high limb */
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if (shift == 0)
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{
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/* High quotient limb is 0 or 1, and skip a divide step. */
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r = ahigh;
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qhigh = (r >= d);
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r = (qhigh ? r-d : r);
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*qp-- = qhigh;
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size--;
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for (i = size-1; i >= 0; i--)
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{
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n0 = ap[i];
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udiv_qrnnd_preinv (*qp, r, r, n0, d, dinv);
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qp--;
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}
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}
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else
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{
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r = 0;
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if (ahigh < d_unnorm)
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{
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r = ahigh << shift;
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*qp-- = 0;
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size--;
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if (size == 0)
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goto done_integer;
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}
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n1 = ap[size-1];
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r |= n1 >> (BITS_PER_MP_LIMB - shift);
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for (i = size-2; i >= 0; i--)
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{
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ASSERT (r < d);
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n0 = ap[i];
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udiv_qrnnd_preinv (*qp, r, r,
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((n1 << shift) | (n0 >> (BITS_PER_MP_LIMB - shift))),
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d, dinv);
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qp--;
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n1 = n0;
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}
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udiv_qrnnd_preinv (*qp, r, r, n1 << shift, d, dinv);
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qp--;
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}
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done_integer:
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for (i = 0; i < xsize; i++)
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{
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udiv_qrnnd_preinv (*qp, r, r, CNST_LIMB(0), d, dinv);
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qp--;
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}
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return r >> shift;
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}
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#endif /* USE_PREINV_DIVREM_1 */
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