--- /dev/null
+/* mpn_jacobi_base -- limb/limb Jacobi symbol with restricted arguments.
+
+ THIS INTERFACE IS PRELIMINARY AND MIGHT DISAPPEAR OR BE SUBJECT TO
+ INCOMPATIBLE CHANGES IN A FUTURE RELEASE OF GMP.
+
+Copyright 1999, 2000, 2001, 2002 Free Software Foundation, Inc.
+
+This file is part of the GNU MP Library.
+
+The GNU MP Library is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 3 of the License, or (at your
+option) any later version.
+
+The GNU MP Library is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
+License for more details.
+
+You should have received a copy of the GNU Lesser General Public License
+along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+#include "longlong.h"
+
+
+/* Use the simple loop by default. The generic count_trailing_zeros is not
+ very fast, and the extra trickery of method 3 has proven to be less use
+ than might have been though. */
+#ifndef JACOBI_BASE_METHOD
+#define JACOBI_BASE_METHOD 2
+#endif
+
+
+/* Use count_trailing_zeros. */
+#if JACOBI_BASE_METHOD == 1
+#define PROCESS_TWOS_ANY \
+ { \
+ mp_limb_t twos; \
+ count_trailing_zeros (twos, a); \
+ result_bit1 ^= JACOBI_TWOS_U_BIT1 (twos, b); \
+ a >>= twos; \
+ }
+#define PROCESS_TWOS_EVEN PROCESS_TWOS_ANY
+#endif
+
+/* Use a simple loop. A disadvantage of this is that there's a branch on a
+ 50/50 chance of a 0 or 1 low bit. */
+#if JACOBI_BASE_METHOD == 2
+#define PROCESS_TWOS_EVEN \
+ { \
+ int two; \
+ two = JACOBI_TWO_U_BIT1 (b); \
+ do \
+ { \
+ a >>= 1; \
+ result_bit1 ^= two; \
+ ASSERT (a != 0); \
+ } \
+ while ((a & 1) == 0); \
+ }
+#define PROCESS_TWOS_ANY \
+ if ((a & 1) == 0) \
+ PROCESS_TWOS_EVEN;
+#endif
+
+/* Process one bit arithmetically, then a simple loop. This cuts the loop
+ condition down to a 25/75 chance, which should branch predict better.
+ The CPU will need a reasonable variable left shift. */
+#if JACOBI_BASE_METHOD == 3
+#define PROCESS_TWOS_EVEN \
+ { \
+ int two, mask, shift; \
+ \
+ two = JACOBI_TWO_U_BIT1 (b); \
+ mask = (~a & 2); \
+ a >>= 1; \
+ \
+ shift = (~a & 1); \
+ a >>= shift; \
+ result_bit1 ^= two ^ (two & mask); \
+ \
+ while ((a & 1) == 0) \
+ { \
+ a >>= 1; \
+ result_bit1 ^= two; \
+ ASSERT (a != 0); \
+ } \
+ }
+#define PROCESS_TWOS_ANY \
+ { \
+ int two, mask, shift; \
+ \
+ two = JACOBI_TWO_U_BIT1 (b); \
+ shift = (~a & 1); \
+ a >>= shift; \
+ \
+ mask = shift << 1; \
+ result_bit1 ^= (two & mask); \
+ \
+ while ((a & 1) == 0) \
+ { \
+ a >>= 1; \
+ result_bit1 ^= two; \
+ ASSERT (a != 0); \
+ } \
+ }
+#endif
+
+
+/* Calculate the value of the Jacobi symbol (a/b) of two mp_limb_t's, but
+ with a restricted range of inputs accepted, namely b>1, b odd, and a<=b.
+
+ The initial result_bit1 is taken as a parameter for the convenience of
+ mpz_kronecker_ui() et al. The sign changes both here and in those
+ routines accumulate nicely in bit 1, see the JACOBI macros.
+
+ The return value here is the normal +1, 0, or -1. Note that +1 and -1
+ have bit 1 in the "BIT1" sense, which could be useful if the caller is
+ accumulating it into some extended calculation.
+
+ Duplicating the loop body to avoid the MP_LIMB_T_SWAP(a,b) would be
+ possible, but a couple of tests suggest it's not a significant speedup,
+ and may even be a slowdown, so what's here is good enough for now.
+
+ Future: The code doesn't demand a<=b actually, so maybe this could be
+ relaxed. All the places this is used currently call with a<=b though. */
+
+int
+mpn_jacobi_base (mp_limb_t a, mp_limb_t b, int result_bit1)
+{
+ ASSERT (b & 1); /* b odd */
+ ASSERT (b != 1);
+ ASSERT (a <= b);
+
+ if (a == 0)
+ return 0;
+
+ PROCESS_TWOS_ANY;
+ if (a == 1)
+ goto done;
+
+ for (;;)
+ {
+ result_bit1 ^= JACOBI_RECIP_UU_BIT1 (a, b);
+ MP_LIMB_T_SWAP (a, b);
+
+ do
+ {
+ /* working on (a/b), a,b odd, a>=b */
+ ASSERT (a & 1);
+ ASSERT (b & 1);
+ ASSERT (a >= b);
+
+ if ((a -= b) == 0)
+ return 0;
+
+ PROCESS_TWOS_EVEN;
+ if (a == 1)
+ goto done;
+ }
+ while (a >= b);
+ }
+
+ done:
+ return JACOBI_BIT1_TO_PN (result_bit1);
+}
projects / msp430-gcc.git / blobdiff