1  /* mpn_gcd_1 -- mpn and limb greatest common divisor.
       2  
       3  Copyright 1994, 1996, 2000, 2001, 2009, 2012, 2019 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 either:
       9  
      10    * the GNU Lesser General Public License as published by the Free
      11      Software Foundation; either version 3 of the License, or (at your
      12      option) any later version.
      13  
      14  or
      15  
      16    * the GNU General Public License as published by the Free Software
      17      Foundation; either version 2 of the License, or (at your option) any
      18      later version.
      19  
      20  or both in parallel, as here.
      21  
      22  The GNU MP Library is distributed in the hope that it will be useful, but
      23  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
      24  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
      25  for more details.
      26  
      27  You should have received copies of the GNU General Public License and the
      28  GNU Lesser General Public License along with the GNU MP Library.  If not,
      29  see https://www.gnu.org/licenses/.  */
      30  
      31  #include "gmp-impl.h"
      32  #include "longlong.h"
      33  
      34  /* Does not work for U == 0 or V == 0.  It would be tough to make it work for
      35     V == 0 since gcd(x,0) = x, and U does not generally fit in an mp_limb_t.
      36  
      37     The threshold for doing u%v when size==1 will vary by CPU according to
      38     the speed of a division and the code generated for the main loop.  Any
      39     tuning for this is left to a CPU specific implementation.  */
      40  
      41  mp_limb_t
      42  mpn_gcd_1 (mp_srcptr up, mp_size_t size, mp_limb_t vlimb)
      43  {
      44    mp_limb_t      ulimb;
      45    unsigned long  zero_bits, u_low_zero_bits;
      46    int c;
      47  
      48    ASSERT (size >= 1);
      49    ASSERT (vlimb != 0);
      50    ASSERT_MPN_NONZERO_P (up, size);
      51  
      52    ulimb = up[0];
      53  
      54    /* Need vlimb odd for modexact, want it odd to get common zeros. */
      55    count_trailing_zeros (zero_bits, vlimb);
      56    vlimb >>= zero_bits;
      57  
      58    if (size > 1)
      59      {
      60        /* Must get common zeros before the mod reduction.  If ulimb==0 then
      61  	 vlimb already gives the common zeros.  */
      62        if (ulimb != 0)
      63  	{
      64  	  count_trailing_zeros (u_low_zero_bits, ulimb);
      65  	  zero_bits = MIN (zero_bits, u_low_zero_bits);
      66  	}
      67  
      68        ulimb = MPN_MOD_OR_MODEXACT_1_ODD (up, size, vlimb);
      69        if (ulimb == 0)
      70  	goto done;
      71  
      72        count_trailing_zeros (c, ulimb);
      73        ulimb >>= c;
      74      }
      75    else
      76      {
      77        /* size==1, so up[0]!=0 */
      78        count_trailing_zeros (u_low_zero_bits, ulimb);
      79        ulimb >>= u_low_zero_bits;
      80        zero_bits = MIN (zero_bits, u_low_zero_bits);
      81  
      82        /* make u bigger */
      83        if (vlimb > ulimb)
      84  	MP_LIMB_T_SWAP (ulimb, vlimb);
      85  
      86        /* if u is much bigger than v, reduce using a division rather than
      87  	 chipping away at it bit-by-bit */
      88        if ((ulimb >> 16) > vlimb)
      89  	{
      90  	  ulimb %= vlimb;
      91  	  if (ulimb == 0)
      92  	    goto done;
      93  
      94  	  count_trailing_zeros (c, ulimb);
      95  	  ulimb >>= c;
      96  	}
      97      }
      98  
      99    vlimb = mpn_gcd_11 (ulimb, vlimb);
     100  
     101   done:
     102    return vlimb << zero_bits;
     103  }