Coverage Report

Created: 2025-03-06 06:58

/src/gmp/mpn/perfsqr.c
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/* mpn_perfect_square_p(u,usize) -- Return non-zero if U is a perfect square,
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   zero otherwise.
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Copyright 1991, 1993, 1994, 1996, 1997, 2000-2002, 2005, 2012 Free Software
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Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of either:
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  * the GNU Lesser General Public License as published by the Free
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    Software Foundation; either version 3 of the License, or (at your
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    option) any later version.
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or
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  * the GNU General Public License as published by the Free Software
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    Foundation; either version 2 of the License, or (at your option) any
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    later version.
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or both in parallel, as here.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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for more details.
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You should have received copies of the GNU General Public License and the
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GNU Lesser General Public License along with the GNU MP Library.  If not,
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see https://www.gnu.org/licenses/.  */
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#include <stdio.h> /* for NULL */
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#include "gmp-impl.h"
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#include "longlong.h"
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#include "perfsqr.h"
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/* change this to "#define TRACE(x) x" for diagnostics */
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#define TRACE(x)
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/* PERFSQR_MOD_* detects non-squares using residue tests.
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   A macro PERFSQR_MOD_TEST is setup by gen-psqr.c in perfsqr.h.  It takes
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   {up,usize} modulo a selected modulus to get a remainder r.  For 32-bit or
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   64-bit limbs this modulus will be 2^24-1 or 2^48-1 using PERFSQR_MOD_34,
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   or for other limb or nail sizes a PERFSQR_PP is chosen and PERFSQR_MOD_PP
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   used.  PERFSQR_PP_NORM and PERFSQR_PP_INVERTED are pre-calculated in this
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   case too.
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   PERFSQR_MOD_TEST then makes various calls to PERFSQR_MOD_1 or
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   PERFSQR_MOD_2 with divisors d which are factors of the modulus, and table
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   data indicating residues and non-residues modulo those divisors.  The
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   table data is in 1 or 2 limbs worth of bits respectively, per the size of
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   each d.
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   A "modexact" style remainder is taken to reduce r modulo d.
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   PERFSQR_MOD_IDX implements this, producing an index "idx" for use with
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   the table data.  Notice there's just one multiplication by a constant
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   "inv", for each d.
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   The modexact doesn't produce a true r%d remainder, instead idx satisfies
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   "-(idx<<PERFSQR_MOD_BITS) == r mod d".  Because d is odd, this factor
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   -2^PERFSQR_MOD_BITS is a one-to-one mapping between r and idx, and is
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   accounted for by having the table data suitably permuted.
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   The remainder r fits within PERFSQR_MOD_BITS which is less than a limb.
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   In fact the GMP_LIMB_BITS - PERFSQR_MOD_BITS spare bits are enough to fit
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   each divisor d meaning the modexact multiply can take place entirely
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   within one limb, giving the compiler the chance to optimize it, in a way
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   that say umul_ppmm would not give.
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   There's no need for the divisors d to be prime, in fact gen-psqr.c makes
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   a deliberate effort to combine factors so as to reduce the number of
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   separate tests done on r.  But such combining is limited to d <=
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   2*GMP_LIMB_BITS so that the table data fits in at most 2 limbs.
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   Alternatives:
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   It'd be possible to use bigger divisors d, and more than 2 limbs of table
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   data, but this doesn't look like it would be of much help to the prime
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   factors in the usual moduli 2^24-1 or 2^48-1.
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   The moduli 2^24-1 or 2^48-1 are nothing particularly special, they're
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   just easy to calculate (see mpn_mod_34lsub1) and have a nice set of prime
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   factors.  2^32-1 and 2^64-1 would be equally easy to calculate, but have
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   fewer prime factors.
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   The nails case usually ends up using mpn_mod_1, which is a lot slower
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   than mpn_mod_34lsub1.  Perhaps other such special moduli could be found
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   for the nails case.  Two-term things like 2^30-2^15-1 might be
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   candidates.  Or at worst some on-the-fly de-nailing would allow the plain
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   2^24-1 to be used.  Currently nails are too preliminary to be worried
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   about.
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*/
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#define PERFSQR_MOD_MASK       ((CNST_LIMB(1) << PERFSQR_MOD_BITS) - 1)
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#define MOD34_BITS  (GMP_NUMB_BITS / 4 * 3)
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#define MOD34_MASK  ((CNST_LIMB(1) << MOD34_BITS) - 1)
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#define PERFSQR_MOD_34(r, up, usize)        \
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  do {               \
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    (r) = mpn_mod_34lsub1 (up, usize);       \
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    (r) = ((r) & MOD34_MASK) + ((r) >> MOD34_BITS);    \
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  } while (0)
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/* FIXME: The %= here isn't good, and might destroy any savings from keeping
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   the PERFSQR_MOD_IDX stuff within a limb (rather than needing umul_ppmm).
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   Maybe a new sort of mpn_preinv_mod_1 could accept an unnormalized divisor
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   and a shift count, like mpn_preinv_divrem_1.  But mod_34lsub1 is our
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   normal case, so lets not worry too much about mod_1.  */
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#define PERFSQR_MOD_PP(r, up, usize)          \
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  do {                  \
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    if (BELOW_THRESHOLD (usize, PREINV_MOD_1_TO_MOD_1_THRESHOLD)) \
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      {                 \
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  (r) = mpn_preinv_mod_1 (up, usize, PERFSQR_PP_NORM,   \
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        PERFSQR_PP_INVERTED);     \
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  (r) %= PERFSQR_PP;            \
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      }                 \
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    else                \
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      {                 \
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  (r) = mpn_mod_1 (up, usize, PERFSQR_PP);      \
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      }                 \
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  } while (0)
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#define PERFSQR_MOD_IDX(idx, r, d, inv)       \
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  do {               \
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    mp_limb_t  q;           \
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    ASSERT ((r) <= PERFSQR_MOD_MASK);       \
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    ASSERT ((((inv) * (d)) & PERFSQR_MOD_MASK) == 1);   \
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    ASSERT (MP_LIMB_T_MAX / (d) >= PERFSQR_MOD_MASK);   \
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                \
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    q = ((r) * (inv)) & PERFSQR_MOD_MASK;     \
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    ASSERT (r == ((q * (d)) & PERFSQR_MOD_MASK));   \
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    (idx) = (q * (d)) >> PERFSQR_MOD_BITS;     \
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  } while (0)
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#define PERFSQR_MOD_1(r, d, inv, mask)        \
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  do {               \
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    unsigned   idx;           \
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    ASSERT ((d) <= GMP_LIMB_BITS);        \
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    PERFSQR_MOD_IDX(idx, r, d, inv);       \
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    TRACE (printf ("  PERFSQR_MOD_1 d=%u r=%lu idx=%u\n", \
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       d, r%d, idx));       \
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    if ((((mask) >> idx) & 1) == 0)       \
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      {               \
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  TRACE (printf ("  non-square\n"));      \
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  return 0;           \
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      }                \
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  } while (0)
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/* The expression "(int) idx - GMP_LIMB_BITS < 0" lets the compiler use the
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   sign bit from "idx-GMP_LIMB_BITS", which might help avoid a branch.  */
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#define PERFSQR_MOD_2(r, d, inv, mhi, mlo)      \
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  do {               \
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    mp_limb_t  m;           \
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    unsigned   idx;           \
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    ASSERT ((d) <= 2*GMP_LIMB_BITS);        \
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                \
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    PERFSQR_MOD_IDX (idx, r, d, inv);        \
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    TRACE (printf ("  PERFSQR_MOD_2 d=%u r=%lu idx=%u\n", \
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       d, r%d, idx));       \
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    m = ((int) idx - GMP_LIMB_BITS < 0 ? (mlo) : (mhi));  \
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    idx %= GMP_LIMB_BITS;         \
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    if (((m >> idx) & 1) == 0)         \
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      {               \
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  TRACE (printf ("  non-square\n"));      \
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  return 0;           \
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      }                \
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  } while (0)
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int
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mpn_perfect_square_p (mp_srcptr up, mp_size_t usize)
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{
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  ASSERT (usize >= 1);
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  TRACE (gmp_printf ("mpn_perfect_square_p %Nd\n", up, usize));
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  /* The first test excludes 212/256 (82.8%) of the perfect square candidates
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     in O(1) time.  */
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  {
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    unsigned  idx = up[0] % 0x100;
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    if (((sq_res_0x100[idx / GMP_LIMB_BITS]
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    >> (idx % GMP_LIMB_BITS)) & 1) == 0)
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      return 0;
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  }
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#if 0
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  /* Check that we have even multiplicity of 2, and then check that the rest is
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     a possible perfect square.  Leave disabled until we can determine this
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     really is an improvement.  If it is, it could completely replace the
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     simple probe above, since this should throw out more non-squares, but at
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     the expense of somewhat more cycles.  */
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  {
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    mp_limb_t lo;
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    int cnt;
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    lo = up[0];
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    while (lo == 0)
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      up++, lo = up[0], usize--;
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    count_trailing_zeros (cnt, lo);
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    if ((cnt & 1) != 0)
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      return 0;     /* return of not even multiplicity of 2 */
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    lo >>= cnt;     /* shift down to align lowest non-zero bit */
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    if ((lo & 6) != 0)
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      return 0;
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  }
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#endif
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  /* The second test uses mpn_mod_34lsub1 or mpn_mod_1 to detect non-squares
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     according to their residues modulo small primes (or powers of
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     primes).  See perfsqr.h.  */
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  PERFSQR_MOD_TEST (up, usize);
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  /* For the third and last test, we finally compute the square root,
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     to make sure we've really got a perfect square.  */
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  {
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    mp_ptr root_ptr;
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    int res;
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    TMP_DECL;
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    TMP_MARK;
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    root_ptr = TMP_ALLOC_LIMBS ((usize + 1) / 2);
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    /* Iff mpn_sqrtrem returns zero, the square is perfect.  */
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    res = ! mpn_sqrtrem (root_ptr, NULL, up, usize);
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    TMP_FREE;
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    return res;
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  }
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}