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Annotation of OpenXM_contrib/gmp/mpn/generic/pre_divrem_1.c, Revision 1.1

1.1     ! ohara       1: /* mpn_preinv_divrem_1 -- mpn by limb division with pre-inverted divisor.
        !             2:
        !             3: Copyright 2000, 2001, 2002 Free Software Foundation, Inc.
        !             4:
        !             5: This file is part of the GNU MP Library.
        !             6:
        !             7: The GNU MP Library is free software; you can redistribute it and/or modify
        !             8: it under the terms of the GNU Lesser General Public License as published by
        !             9: the Free Software Foundation; either version 2.1 of the License, or (at your
        !            10: option) any later version.
        !            11:
        !            12: The GNU MP Library is distributed in the hope that it will be useful, but
        !            13: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
        !            14: or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
        !            15: License for more details.
        !            16:
        !            17: You should have received a copy of the GNU Lesser General Public License
        !            18: along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
        !            19: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
        !            20: MA 02111-1307, USA. */
        !            21:
        !            22: #include "gmp.h"
        !            23: #include "gmp-impl.h"
        !            24: #include "longlong.h"
        !            25:
        !            26:
        !            27: /* Don't bloat a shared library with unused code. */
        !            28: #if USE_PREINV_DIVREM_1
        !            29:
        !            30: /* Same test here for skipping one divide step as in mpn_divrem_1.
        !            31:
        !            32:    The main reason for a separate shift==0 case is that not all CPUs give
        !            33:    zero for "n0 >> BITS_PER_MP_LIMB" which would arise in the general case
        !            34:    code used on shift==0.  shift==0 is also reasonably common in __mp_bases
        !            35:    big_base, for instance base==10 on a 64-bit limb.
        !            36:
        !            37:    Under shift!=0 it would be possible to call mpn_lshift to adjust the
        !            38:    dividend all in one go (into the quotient space say), rather than
        !            39:    limb-by-limb in the loop.  This might help if mpn_lshift is a lot faster
        !            40:    than what the compiler can generate for EXTRACT.  But this is left to CPU
        !            41:    specific implementations to consider, especially since EXTRACT isn't on
        !            42:    the dependent chain.
        !            43:
        !            44:    If size==0 then the result is simply xsize limbs of zeros, but nothing
        !            45:    special is done for that, since it wouldn't be a usual call, and
        !            46:    certainly never arises from mpn_get_str which is our main caller.  */
        !            47:
        !            48: mp_limb_t
        !            49: mpn_preinv_divrem_1 (mp_ptr qp, mp_size_t xsize,
        !            50:                     mp_srcptr ap, mp_size_t size, mp_limb_t d_unnorm,
        !            51:                     mp_limb_t dinv, int shift)
        !            52: {
        !            53:   mp_limb_t  ahigh, qhigh, r;
        !            54:   mp_size_t  i;
        !            55:   mp_limb_t  n1, n0;
        !            56:   mp_limb_t  d;
        !            57:
        !            58:   ASSERT (xsize >= 0);
        !            59:   ASSERT (size >= 1);
        !            60:   ASSERT (d_unnorm != 0);
        !            61: #if WANT_ASSERT
        !            62:   {
        !            63:     int        want_shift;
        !            64:     mp_limb_t  want_dinv;
        !            65:     count_leading_zeros (want_shift, d_unnorm);
        !            66:     ASSERT (shift == want_shift);
        !            67:     invert_limb (want_dinv, d_unnorm << shift);
        !            68:     ASSERT (dinv == want_dinv);
        !            69:   }
        !            70: #endif
        !            71:   /* FIXME: What's the correct overlap rule when xsize!=0? */
        !            72:   ASSERT (MPN_SAME_OR_SEPARATE_P (qp+xsize, ap, size));
        !            73:
        !            74:   ahigh = ap[size-1];
        !            75:   d = d_unnorm << shift;
        !            76:   qp += (size + xsize - 1);   /* dest high limb */
        !            77:
        !            78:   if (shift == 0)
        !            79:     {
        !            80:       /* High quotient limb is 0 or 1, and skip a divide step. */
        !            81:       r = ahigh;
        !            82:       qhigh = (r >= d);
        !            83:       r = (qhigh ? r-d : r);
        !            84:       *qp-- = qhigh;
        !            85:       size--;
        !            86:
        !            87:       for (i = size-1; i >= 0; i--)
        !            88:        {
        !            89:          n0 = ap[i];
        !            90:          udiv_qrnnd_preinv (*qp, r, r, n0, d, dinv);
        !            91:          qp--;
        !            92:        }
        !            93:     }
        !            94:   else
        !            95:     {
        !            96:       r = 0;
        !            97:       if (ahigh < d_unnorm)
        !            98:        {
        !            99:          r = ahigh << shift;
        !           100:          *qp-- = 0;
        !           101:          size--;
        !           102:          if (size == 0)
        !           103:            goto done_integer;
        !           104:        }
        !           105:
        !           106:       n1 = ap[size-1];
        !           107:       r |= n1 >> (BITS_PER_MP_LIMB - shift);
        !           108:
        !           109:       for (i = size-2; i >= 0; i--)
        !           110:        {
        !           111:          ASSERT (r < d);
        !           112:          n0 = ap[i];
        !           113:          udiv_qrnnd_preinv (*qp, r, r,
        !           114:                             ((n1 << shift) | (n0 >> (BITS_PER_MP_LIMB - shift))),
        !           115:                             d, dinv);
        !           116:          qp--;
        !           117:          n1 = n0;
        !           118:        }
        !           119:       udiv_qrnnd_preinv (*qp, r, r, n1 << shift, d, dinv);
        !           120:       qp--;
        !           121:     }
        !           122:
        !           123:  done_integer:
        !           124:   for (i = 0; i < xsize; i++)
        !           125:     {
        !           126:       udiv_qrnnd_preinv (*qp, r, r, 0, d, dinv);
        !           127:       qp--;
        !           128:     }
        !           129:
        !           130:   return r >> shift;
        !           131: }
        !           132:
        !           133: #endif /* USE_PREINV_DIVREM_1 */