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Annotation of OpenXM_contrib/gmp/mpn/generic/diveby3.c, Revision 1.1.1.2

1.1       maekawa     1: /* mpn_divexact_by3 -- mpn division by 3, expecting no remainder. */
                      2:
                      3: /*
1.1.1.2 ! ohara       4: Copyright 2000, 2001, 2002 Free Software Foundation, Inc.
1.1       maekawa     5:
                      6: This file is part of the GNU MP Library.
                      7:
                      8: The GNU MP Library is free software; you can redistribute it and/or modify
                      9: it under the terms of the GNU Lesser General Public License as published by
                     10: the Free Software Foundation; either version 2.1 of the License, or (at your
                     11: option) any later version.
                     12:
                     13: The GNU MP Library is distributed in the hope that it will be useful, but
                     14: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
                     15: or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
                     16: License for more details.
                     17:
                     18: You should have received a copy of the GNU Lesser General Public License
                     19: along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
                     20: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
                     21: MA 02111-1307, USA.
                     22: */
                     23:
                     24:
                     25: #include "gmp.h"
                     26: #include "gmp-impl.h"
                     27:
                     28:
                     29: /* The "c += ..."s are adding the high limb of 3*l to c.  That high limb
                     30:    will be 0, 1 or 2.  Doing two separate "+="s seems to turn out better
                     31:    code on gcc (as of 2.95.2 at least).
                     32:
                     33:    When a subtraction of a 0,1,2 carry value causes a borrow, that leaves a
1.1.1.2 ! ohara      34:    limb value of either 0xFF...FF or 0xFF...FE and the multiply by
        !            35:    MODLIMB_INVERSE_3 gives 0x55...55 or 0xAA...AA respectively, producing a
        !            36:    further borrow of only 0 or 1 respectively.  Hence the carry out of each
        !            37:    stage and for the return value is always only 0, 1 or 2.
        !            38:
        !            39:    This implementation has each multiply successively dependent due to the
        !            40:    "l=s-c", but the multiply src[i]*MODLIMB_INVERSE_3 could be scheduled
        !            41:    back as far as desired, and the effect of the "-c" applied by subtracting
        !            42:    0, 1 or 2 copies or MODLIMB_INVERSE_3 from the product.  With some good
        !            43:    scheduling in assembler it might come down to a dependent chain of maybe
        !            44:    5 simple operations per limb.  */
1.1       maekawa    45:
                     46: mp_limb_t
                     47: mpn_divexact_by3c (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t c)
                     48: {
                     49:   mp_size_t  i;
                     50:
                     51:   ASSERT (size >= 1);
1.1.1.2 ! ohara      52:   ASSERT (c == 0 || c == 1 || c == 2);
        !            53:   ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));
1.1       maekawa    54:
                     55:   i = 0;
                     56:   do
                     57:     {
                     58:       mp_limb_t  l, s;
                     59:
                     60:       s = src[i];
                     61:       l = s - c;
                     62:       c = (l > s);
                     63:
1.1.1.2 ! ohara      64:       l = (l * MODLIMB_INVERSE_3) & GMP_NUMB_MASK;
1.1       maekawa    65:       dst[i] = l;
                     66:
1.1.1.2 ! ohara      67:       c += (l > GMP_NUMB_MAX/3);
        !            68:       c += (l > (GMP_NUMB_MAX/3)*2);
1.1       maekawa    69:     }
                     70:   while (++i < size);
                     71:
                     72:   return c;
                     73: }