1 /* mpn_jacobi_base -- limb/limb Jacobi symbol with restricted arguments.
3 THIS INTERFACE IS PRELIMINARY AND MIGHT DISAPPEAR OR BE SUBJECT TO
4 INCOMPATIBLE CHANGES IN A FUTURE RELEASE OF GMP. */
7 Copyright (C) 1999, 2000 Free Software Foundation, Inc.
9 This file is part of the GNU MP Library.
11 The GNU MP Library is free software; you can redistribute it and/or modify
12 it under the terms of the GNU Lesser General Public License as published by
13 the Free Software Foundation; either version 2.1 of the License, or (at your
14 option) any later version.
16 The GNU MP Library is distributed in the hope that it will be useful, but
17 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
18 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
19 License for more details.
21 You should have received a copy of the GNU Lesser General Public License
22 along with the GNU MP Library; see the file COPYING.LIB. If not, write to
23 the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
24 MA 02111-1307, USA. */
31 #if COUNT_TRAILING_ZEROS_TIME <= 7
32 /* If count_trailing_zeros is fast, use it.
33 K7 at 7 cycles and P6 at 2 are good here. K6 at 12-27 and P5 at 18-42
34 are not. The default 15 in longlong.h is meant to mean not good here. */
36 #define PROCESS_TWOS_ANY \
39 count_trailing_zeros (twos, a); \
40 result_bit1 ^= JACOBI_TWOS_U_BIT1 (twos, b); \
44 #define PROCESS_TWOS_EVEN PROCESS_TWOS_ANY
47 /* Use a loop instead. With "a" uniformly distributed there will usually be
48 only a few trailing zeros.
50 Unfortunately the branch for the while loop here will be on a 50/50
51 chance of a 1 or 0, which is bad for branch prediction. */
53 #define PROCESS_TWOS_EVEN \
56 two = JACOBI_TWO_U_BIT1 (b); \
63 while ((a & 1) == 0); \
66 #define PROCESS_TWOS_ANY \
73 /* Calculate the value of the Jacobi symbol (a/b) of two mp_limb_t's, but
74 with a restricted range of inputs accepted, namely b>1, b odd, and a<=b.
76 The initial result_bit1 is taken as a parameter for the convenience of
77 mpz_kronecker_zi_ui() et al. The sign changes both here and in those
78 routines accumulate nicely in bit 1, see the JACOBI macros.
80 The return value here is the normal +1, 0, or -1. Note that +1 and -1
81 have bit 1 in the "BIT1" sense, which could be useful if the caller is
82 accumulating it into some extended calculation.
84 Duplicating the loop body to avoid the MP_LIMB_T_SWAP(a,b) would be
85 possible, but a couple of tests suggest it's not a significant speedup,
86 and may even be a slowdown, so what's here is good enough for now.
88 Future: The code doesn't demand a<=b actually, so maybe this could be
89 relaxed. All the places this is used currently call with a<=b though. */
93 mpn_jacobi_base (mp_limb_t a, mp_limb_t b, int result_bit1)
95 mpn_jacobi_base (a, b, result_bit1)
101 ASSERT (b & 1); /* b odd */
114 result_bit1 ^= JACOBI_RECIP_UU_BIT1 (a, b);
115 MP_LIMB_T_SWAP (a, b);
119 /* working on (a/b), a,b odd, a>=b */
135 return JACOBI_BIT1_TO_PN (result_bit1);