--- /dev/null
+/* mpn_mod_34lsub1 -- remainder modulo 2^(GMP_NUMB_BITS*3/4)-1.
+
+ THE FUNCTIONS IN THIS FILE ARE FOR INTERNAL USE ONLY. THEY'RE ALMOST
+ CERTAIN TO BE SUBJECT TO INCOMPATIBLE CHANGES OR DISAPPEAR COMPLETELY IN
+ FUTURE GNU MP RELEASES.
+
+Copyright 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"
+
+
+/* Calculate a remainder from {p,n} divided by 2^(GMP_NUMB_BITS*3/4)-1.
+ The remainder is not fully reduced, it's any limb value congruent to
+ {p,n} modulo that divisor.
+
+ This implementation is only correct when GMP_NUMB_BITS is a multiple of
+ 4.
+
+ FIXME: If GMP_NAIL_BITS is some silly big value during development then
+ it's possible the carry accumulators c0,c1,c2 could overflow.
+
+ General notes:
+
+ The basic idea is to use a set of N accumulators (N=3 in this case) to
+ effectively get a remainder mod 2^(GMP_NUMB_BITS*N)-1 followed at the end
+ by a reduction to GMP_NUMB_BITS*N/M bits (M=4 in this case) for a
+ remainder mod 2^(GMP_NUMB_BITS*N/M)-1. N and M are chosen to give a good
+ set of small prime factors in 2^(GMP_NUMB_BITS*N/M)-1.
+
+ N=3 M=4 suits GMP_NUMB_BITS==32 and GMP_NUMB_BITS==64 quite well, giving
+ a few more primes than a single accumulator N=1 does, and for no extra
+ cost (assuming the processor has a decent number of registers).
+
+ For strange nailified values of GMP_NUMB_BITS the idea would be to look
+ for what N and M give good primes. With GMP_NUMB_BITS not a power of 2
+ the choices for M may be opened up a bit. But such things are probably
+ best done in separate code, not grafted on here. */
+
+#if GMP_NUMB_BITS % 4 == 0
+
+#define B1 (GMP_NUMB_BITS / 4)
+#define B2 (B1 * 2)
+#define B3 (B1 * 3)
+
+#define M1 ((CNST_LIMB(1) << B1) - 1)
+#define M2 ((CNST_LIMB(1) << B2) - 1)
+#define M3 ((CNST_LIMB(1) << B3) - 1)
+
+#define LOW0(n) ((n) & M3)
+#define HIGH0(n) ((n) >> B3)
+
+#define LOW1(n) (((n) & M2) << B1)
+#define HIGH1(n) ((n) >> B2)
+
+#define LOW2(n) (((n) & M1) << B2)
+#define HIGH2(n) ((n) >> B1)
+
+#define PARTS0(n) (LOW0(n) + HIGH0(n))
+#define PARTS1(n) (LOW1(n) + HIGH1(n))
+#define PARTS2(n) (LOW2(n) + HIGH2(n))
+
+#define ADD(c,a,val) \
+ do { \
+ mp_limb_t new_c; \
+ ADDC_LIMB (new_c, a, a, val); \
+ (c) += new_c; \
+ } while (0)
+
+mp_limb_t
+mpn_mod_34lsub1 (mp_srcptr p, mp_size_t n)
+{
+ mp_limb_t c0 = 0;
+ mp_limb_t c1 = 0;
+ mp_limb_t c2 = 0;
+ mp_limb_t a0, a1, a2;
+
+ ASSERT (n >= 1);
+ ASSERT (n/3 < GMP_NUMB_MAX);
+
+ a0 = a1 = a2 = 0;
+ c0 = c1 = c2 = 0;
+
+ while ((n -= 3) >= 0)
+ {
+ ADD (c0, a0, p[0]);
+ ADD (c1, a1, p[1]);
+ ADD (c2, a2, p[2]);
+ p += 3;
+ }
+
+ if (n != -3)
+ {
+ ADD (c0, a0, p[0]);
+ if (n != -2)
+ ADD (c1, a1, p[1]);
+ }
+
+ return
+ PARTS0 (a0) + PARTS1 (a1) + PARTS2 (a2)
+ + PARTS1 (c0) + PARTS2 (c1) + PARTS0 (c2);
+}
+
+#endif
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