1 /* mpn_gcd_1 -- mpn and limb greatest common divisor. 2 3 Copyright 1994, 1996, 2000, 2001 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 3 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. If not, see http://www.gnu.org/licenses/. */ 19 20 #include "gmp.h" 21 #include "gmp-impl.h" 22 #include "longlong.h" 23 24 25 /* Does not work for U == 0 or V == 0. It would be tough to make it work for 26 V == 0 since gcd(x,0) = x, and U does not generally fit in an mp_limb_t. 27 28 The threshold for doing u%v when size==1 will vary by CPU according to 29 the speed of a division and the code generated for the main loop. Any 30 tuning for this is left to a CPU specific implementation. */ 31 32 mp_limb_t 33 mpn_gcd_1 (mp_srcptr up, mp_size_t size, mp_limb_t vlimb) 34 { 35 mp_limb_t ulimb; 36 unsigned long zero_bits, u_low_zero_bits; 37 38 ASSERT (size >= 1); 39 ASSERT (vlimb != 0); 40 ASSERT_MPN_NONZERO_P (up, size); 41 42 ulimb = up[0]; 43 44 /* Need vlimb odd for modexact, want it odd to get common zeros. */ 45 count_trailing_zeros (zero_bits, vlimb); 46 vlimb >>= zero_bits; 47 48 if (size > 1) 49 { 50 /* Must get common zeros before the mod reduction. If ulimb==0 then 51 vlimb already gives the common zeros. */ 52 if (ulimb != 0) 53 { 54 count_trailing_zeros (u_low_zero_bits, ulimb); 55 zero_bits = MIN (zero_bits, u_low_zero_bits); 56 } 57 58 ulimb = MPN_MOD_OR_MODEXACT_1_ODD (up, size, vlimb); 59 if (ulimb == 0) 60 goto done; 61 62 goto strip_u_maybe; 63 } 64 65 /* size==1, so up[0]!=0 */ 66 count_trailing_zeros (u_low_zero_bits, ulimb); 67 ulimb >>= u_low_zero_bits; 68 zero_bits = MIN (zero_bits, u_low_zero_bits); 69 70 /* make u bigger */ 71 if (vlimb > ulimb) 72 MP_LIMB_T_SWAP (ulimb, vlimb); 73 74 /* if u is much bigger than v, reduce using a division rather than 75 chipping away at it bit-by-bit */ 76 if ((ulimb >> 16) > vlimb) 77 { 78 ulimb %= vlimb; 79 if (ulimb == 0) 80 goto done; 81 goto strip_u_maybe; 82 } 83 84 while (ulimb != vlimb) 85 { 86 ASSERT (ulimb & 1); 87 ASSERT (vlimb & 1); 88 89 if (ulimb > vlimb) 90 { 91 ulimb -= vlimb; 92 do 93 { 94 ulimb >>= 1; 95 ASSERT (ulimb != 0); 96 strip_u_maybe: 97 ; 98 } 99 while ((ulimb & 1) == 0); 100 } 101 else /* vlimb > ulimb. */ 102 { 103 vlimb -= ulimb; 104 do 105 { 106 vlimb >>= 1; 107 ASSERT (vlimb != 0); 108 } 109 while ((vlimb & 1) == 0); 110 } 111 } 112 113 done: 114 return vlimb << zero_bits; 115 } 116