1/* mpn_gcd_11 -- limb greatest common divisor. 2 3Copyright 1994, 1996, 2000, 2001, 2009, 2012, 2019 Free Software Foundation, 4Inc. 5 6This file is part of the GNU MP Library. 7 8The GNU MP Library is free software; you can redistribute it and/or modify 9it under the terms of either: 10 11 * the GNU Lesser General Public License as published by the Free 12 Software Foundation; either version 3 of the License, or (at your 13 option) any later version. 14 15or 16 17 * the GNU General Public License as published by the Free Software 18 Foundation; either version 2 of the License, or (at your option) any 19 later version. 20 21or both in parallel, as here. 22 23The GNU MP Library is distributed in the hope that it will be useful, but 24WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 25or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 26for more details. 27 28You should have received copies of the GNU General Public License and the 29GNU Lesser General Public License along with the GNU MP Library. If not, 30see https://www.gnu.org/licenses/. */ 31 32#include "gmp-impl.h" 33#include "longlong.h" 34 35mp_limb_t 36mpn_gcd_11 (mp_limb_t u, mp_limb_t v) 37{ 38 ASSERT (u & v & 1); 39 40 /* In this loop, we represent the odd numbers ulimb and vlimb 41 without the redundant least significant one bit. This reduction 42 in size by one bit ensures that the high bit of t, below, is set 43 if and only if vlimb > ulimb. */ 44 45 u >>= 1; 46 v >>= 1; 47 48 while (u != v) 49 { 50 mp_limb_t t; 51 mp_limb_t vgtu; 52 int c; 53 54 t = u - v; 55 vgtu = LIMB_HIGHBIT_TO_MASK (t); 56 57 /* v <-- min (u, v) */ 58 v += (vgtu & t); 59 60 /* u <-- |u - v| */ 61 u = (t ^ vgtu) - vgtu; 62 63 count_trailing_zeros (c, t); 64 /* We have c <= GMP_LIMB_BITS - 2 here, so that 65 66 ulimb >>= (c + 1); 67 68 would be safe. But unlike the addition c + 1, a separate 69 shift by 1 is independent of c, and can be executed in 70 parallel with count_trailing_zeros. */ 71 u = (u >> 1) >> c; 72 } 73 return (u << 1) + 1; 74} 75