path: root/src/sum.c

/* Sum -- efficiently sum a list of floating-point numbers

Copyright 2014-2016 Free Software Foundation, Inc.
Contributed by the AriC and Caramba projects, INRIA.

This file is part of the GNU MPFR Library.

The GNU MPFR 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 MPFR 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 MPFR Library; see the file COPYING.LESSER.  If not, see
http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */

#define MPFR_NEED_LONGLONG_H
#include "mpfr-impl.h"

/* FIXME: In the case where one or several input pointers point to the
   output variable, we need to store the significand in a new temporary
   area (as usual), because these inputs may still need to be read for
   the possible TMD resolution. Alternatively, since this is not
   necessarily a rare case (doing s += sum(x[i],0<=i<n) should not be
   regarded as uncommon), it may be better to optimize it by allocating
   a bit more for the second sum_raw invocation and delaying the copy of
   the significand when this occurs. Add a testcase to "tsum.c".
   Remove the sentences about overlapping from doc/mpfr.texi once this is
   fixed. */

/* See the doc/sum.txt file for the algorithm and a part of its proof
(this will later go into algorithms.tex).

TODO [VL, after a discussion with James Demmel]: Compared to
  James Demmel and Yozo Hida, Fast and accurate floating-point summation
  with application to computational geometry, Numerical Algorithms,
  volume 37, number 1-4, pages 101--112, 2004.
sorting is not necessary here. It is not done because in the most common
cases (where big cancellations are rare), it would take time and be
useless. However the lack of sorting increases the worst case complexity.
For instance, consider many inputs that cancel one another (two by two).
One would need n/2 iterations, where each iteration reads the exponent
of each input, therefore n*n/2 read operations. Using a worst-case sort
in O(n log n) could give a O(n log n) worst-case complexity. As we don't
want to slow down the most common cases, this could be done at the 3rd
iteration. But are there practical applications which would be used as
tests?

Note: see the following paper and its references:
http://www.eecs.berkeley.edu/~hdnguyen/public/papers/ARITH21_Fast_Sum.pdf
VL: This is very different:
          In MPFR             In the paper & references
    arbitrary precision            fixed precision
     correct rounding        just reproducible rounding
    integer operations        floating-point operations
        sequencial             parallel (& sequential)
*/

#ifdef MPFR_COV_CHECK
int __gmpfr_cov_sum_tmd[MPFR_RND_MAX][2][2][3][2][2] = { 0 };
#endif

/* Update minexp after detecting a potential integer overflow in extreme
   cases (only a 32-bit ABI may be concerned in practice).
   Instead of an assertion failure below, we could
   1. check that the ulp of each regular input has an exponent >= MPFR_EXP_MIN
      (with an assertion failure if this is not the case);
   2. set minexp to MPFR_EXP_MIN and shift the accumulator accordingly
      (the sum will then be exact).
   However, such cases, which involve huge precisions, will probably
   never occur in practice (at least with a 64-bit ABI) and are not
   easily testable due to these huge precisions. Moreover, switching
   to a 64-bit ABI would be a better solution for such computations.
   So, let's leave this unimplemented. */
#define UPDATE_MINEXP(E,SH)                     \
  do                                            \
    {                                           \
      mpfr_prec_t sh = (SH);                    \
      MPFR_ASSERTN ((E) >= MPFR_EXP_MIN + sh);  \
      minexp = (E) - sh;                        \
    }                                           \
  while (0)

/* Function sum_raw
 * ================
 *
 * Accumulate a new [minexp,maxexp[ block into (wp,ws). If e and err denote
 * the exponents of the computed result and of the error bound respectively,
 * while e - err is less than some given bound (due to cancellation), shift
 * the accumulator and reiterate.
 *
 * Inputs:
 *   wp: pointer to the accumulator (least significant limb first).
 *   ws: size of the accumulator (in limbs).
 *   wq: precision of the accumulator (ws * GMP_NUMB_BITS).
 *   x: array of the input numbers.
 *   n: size of this array (number of inputs, regular or not).
 *   minexp: exponent of the least significant bit of the first block.
 *   maxexp: exponent of the first block (exponent of its MSB + 1).
 *   tp: pointer to a temporary area (pre-allocated).
 *   ts: size of this temporary area.
 *   logn: ceil(log2(rn)), where rn is the number of regular inputs.
 *   prec: lower bound for e - err (as described above).
 *   ep: pointer to mpfr_exp_t (see below), or a null pointer.
 *   minexpp: pointer to mpfr_exp_t (see below).
 *   maxexpp: pointer to mpfr_exp_t (see below).
 *
 * Preconditions:
 *   prec >= 1
 *   wq >= logn + prec + 2
 *
 * This function returns 0 if the accumulator is 0 (which implies that
 * the exact sum for this sum_raw invocation is 0), otherwise the number
 * of cancelled bits (>= 1), defined as the number of identical bits on
 * the most significant part of the accumulator. In the latter case, it
 * also returns the following data in variables passed by reference:
 * - in ep: the exponent e of the computed result (unless ep is NULL);
 * - in minexpp: the last value of minexp;
 * - in maxexpp: the new value of maxexp (for the next iteration after
 *   the first invocation of sum_raw in the main code).
 *
 * Notes:
 * - minexp is also the exponent of the least significant bit of the
 *   accumulator;
 * - the temporary area must be large enough to hold a shifted input
 *   block, and the value of ts is used only when the full assertions
 *   are checked (i.e. with the --enable-assert configure option), to
 *   check that a buffer overflow doesn't occur;
 * - contrary to the returned value of minexp (the value in the last
 *   iteration), the returned value of maxexp is the one for the next
 *   iteration (= maxexp2 of the last iteration).
 */
static mpfr_prec_t
sum_raw (mp_limb_t *wp, mp_size_t ws, mpfr_prec_t wq, mpfr_ptr *const x,
         unsigned long n, mpfr_exp_t minexp, mpfr_exp_t maxexp,
         mp_limb_t *tp, mp_size_t ts, int logn, mpfr_prec_t prec,
         mpfr_exp_t *ep, mpfr_exp_t *minexpp, mpfr_exp_t *maxexpp)
{
  MPFR_LOG_FUNC
    (("ws=%Pd ts=%Pd prec=%Pd", (mpfr_prec_t) ws, (mpfr_prec_t) ts, prec),
     ("", 0));

  /* The C code below requires prec >= 0 due to the use of unsigned
     integer arithmetic on it. Actually the computation makes sense
     only with prec >= 1 (otherwise one can't even know the sign of
     the result), hence the following assertion. */
  MPFR_ASSERTD (prec >= 1);

  /* Consistency check. */
  MPFR_ASSERTD (wq == (mpfr_prec_t) ws * GMP_NUMB_BITS);

  /* The following precondition together with prec >= 1 will imply:
     minexp - shiftq < maxexp2, as required by the algorithm. */
  MPFR_ASSERTD (wq >= logn + prec + 2);

  while (1)
    {
      mpfr_exp_t maxexp2 = MPFR_EXP_MIN;
      unsigned long i;

      MPFR_LOG_MSG (("sum_raw loop: "
                     "maxexp=%" MPFR_EXP_FSPEC "d "
                     "minexp=%" MPFR_EXP_FSPEC "d\n",
                     (mpfr_eexp_t) maxexp, (mpfr_eexp_t) minexp));

      MPFR_ASSERTD (maxexp > minexp);

      for (i = 0; i < n; i++)
        if (! MPFR_IS_SINGULAR (x[i]))  /* Step 1 (see sum_raw in sum.txt) */
          {
            mp_limb_t *dp, *vp;
            mp_size_t ds, vs, vds;
            mpfr_exp_t xe, vd;
            mpfr_prec_t xq;
            int tr;

            xe = MPFR_GET_EXP (x[i]);
            xq = MPFR_GET_PREC (x[i]);

            vp = MPFR_MANT (x[i]);
            vs = MPFR_PREC2LIMBS (xq);
            vd = xe - vs * GMP_NUMB_BITS - minexp;
            /* vd is the exponent of the least significant represented bit of
               x[i] (including the trailing bits, whose value is 0) minus the
               exponent of the least significant bit of the accumulator. To
               make the code simpler, we won't try to filter out the trailing
               bits of x[i]. */

            /* Steps 2, 3, 4 (see sum_raw in sum.txt) */

            if (vd < 0)
              {
                /* This covers the following cases:
                 *     [-+- accumulator ---]
                 *   [---|----- x[i] ------|--]
                 *       |   [----- x[i] --|--]
                 *       |                 |[----- x[i] -----]
                 *       |                 |    [----- x[i] -----]
                 *     maxexp           minexp
                 */

                /* Step 2 for subcase vd < 0 */

                if (xe <= minexp)
                  {
                    /* x[i] is entirely after the LSB of the accumulator,
                       so that it will be ignored at this iteration. */
                    if (xe > maxexp2)
                      {
                        maxexp2 = xe;
                        /* And since the exponent of x[i] is valid... */
                        MPFR_ASSERTD (maxexp2 >= MPFR_EMIN_MIN);
                      }
                    continue;
                  }

                /* Step 3 for subcase vd < 0 */

                /* If some significant bits of x[i] are after the LSB of the
                   accumulator, then maxexp2 will necessarily be minexp. */
                if (MPFR_LIKELY (xe - xq < minexp))
                  maxexp2 = minexp;

                /* Step 4 for subcase vd < 0 */

                /* We need to ignore the least |vd| significant bits of x[i].
                   First, let's ignore the least vds = |vd| / GMP_NUMB_BITS
                   limbs. */
                vd = - vd;
                vds = vd / GMP_NUMB_BITS;
                vs -= vds;
                MPFR_ASSERTD (vs > 0);  /* see xe <= minexp test above */
                vp += vds;
                vd -= vds * GMP_NUMB_BITS;
                MPFR_ASSERTD (vd >= 0 && vd < GMP_NUMB_BITS);

                if (xe > maxexp)
                  {
                    vs -= (xe - maxexp) / GMP_NUMB_BITS;
                    MPFR_ASSERTD (vs > 0);
                    tr = (xe - maxexp) % GMP_NUMB_BITS;
                  }
                else
                  tr = 0;

                if (vd != 0)
                  {
                    MPFR_ASSERTD (vs <= ts);
                    mpn_rshift (tp, vp, vs, vd);
                    vp = tp;
                    tr += vd;
                    if (tr >= GMP_NUMB_BITS)
                      {
                        vs--;
                        tr -= GMP_NUMB_BITS;
                      }
                    MPFR_ASSERTD (vs >= 1);
                    MPFR_ASSERTD (tr >= 0 && tr < GMP_NUMB_BITS);
                    if (tr != 0)
                      {
                        tp[vs-1] &= MPFR_LIMB_MASK (GMP_NUMB_BITS - tr);
                        tr = 0;
                      }
                    /* Truncation has now been taken into account. */
                    MPFR_ASSERTD (tr == 0);
                  }

                dp = wp;
                ds = ws;
              }
            else  /* vd >= 0 */
              {
                /* This covers the following cases:
                 *               [-+- accumulator ---]
                 *   [- x[i] -]    |                 |
                 *             [---|-- x[i] ------]  |
                 *          [------|-- x[i] ---------]
                 *                 |   [- x[i] -]    |
                 *               maxexp           minexp
                 */

                /* Steps 2 and 3 for subcase vd >= 0 */

                MPFR_ASSERTD (xe - xq >= minexp);  /* see definition of vd */

                /* Step 4 for subcase vd >= 0 */

                /* We need to ignore the least vd significant bits
                   of the accumulator. First, let's ignore the least
                   vds = vd / GMP_NUMB_BITS limbs. -> (dp,ds) */
                vds = vd / GMP_NUMB_BITS;
                ds = ws - vds;
                if (ds <= 0)
                  continue;
                dp = wp + vds;
                vd -= vds * GMP_NUMB_BITS;
                MPFR_ASSERTD (vd >= 0 && vd < GMP_NUMB_BITS);

                /* The low part of x[i] (to be determined) will have to be
                   shifted vd bits to the left if vd != 0. */

                if (xe > maxexp)
                  {
                    vs -= (xe - maxexp) / GMP_NUMB_BITS;
                    if (vs <= 0)
                      continue;
                    tr = (xe - maxexp) % GMP_NUMB_BITS;
                  }
                else
                  tr = 0;

                MPFR_ASSERTD (tr >= 0 && tr < GMP_NUMB_BITS && vs > 0);

                /* We need to consider the least significant vs limbs of x[i]
                   except the most significant tr bits. */

                if (vd != 0)
                  {
                    mp_limb_t carry;

                    MPFR_ASSERTD (vs <= ts);
                    carry = mpn_lshift (tp, vp, vs, vd);
                    tr -= vd;
                    if (tr < 0)
                      {
                        tr += GMP_NUMB_BITS;
                        MPFR_ASSERTD (vs + 1 <= ts);
                        tp[vs++] = carry;
                      }
                    MPFR_ASSERTD (tr >= 0 && tr < GMP_NUMB_BITS);
                    vp = tp;
                  }
              }  /* vd >= 0 */

            MPFR_ASSERTD (vs > 0 && vs <= ds);

            /* We can't truncate the most significant limb of the input
               (in case it hasn't been shifted to the temporary area).
               So, let's ignore it now. It will be taken into account
               via carry propagation after the addition. */
            if (tr != 0)
              vs--;

            /* Step 5 (see sum_raw in sum.txt) */

            if (MPFR_IS_POS (x[i]))
              {
                mp_limb_t carry;

                carry = vs > 0 ? mpn_add_n (dp, dp, vp, vs) : 0;
                MPFR_ASSERTD (carry <= 1);
                if (tr != 0)
                  carry += vp[vs] & MPFR_LIMB_MASK (GMP_NUMB_BITS - tr);
                if (ds > vs)
                  mpn_add_1 (dp + vs, dp + vs, ds - vs, carry);
              }
            else
              {
                mp_limb_t borrow;

                borrow = vs > 0 ? mpn_sub_n (dp, dp, vp, vs) : 0;
                MPFR_ASSERTD (borrow <= 1);
                if (tr != 0)
                  borrow += vp[vs] & MPFR_LIMB_MASK (GMP_NUMB_BITS - tr);
                if (ds > vs)
                  mpn_sub_1 (dp + vs, dp + vs, ds - vs, borrow);
              }
          }

      {
        mpfr_prec_t cancel;  /* number of cancelled bits */
        mp_size_t wi;        /* index in the accumulator */
        mp_limb_t a, b;
        int cnt;

        cancel = 0;
        wi = ws - 1;
        MPFR_ASSERTD (wi >= 0);
        a = wp[wi] >> (GMP_NUMB_BITS - 1) ? MPFR_LIMB_MAX : MPFR_LIMB_ZERO;

        while (wi >= 0)
          if ((b = wp[wi]) == a)
            {
              cancel += GMP_NUMB_BITS;
              wi--;
            }
          else
            {
              b ^= a;
              MPFR_ASSERTD (b != 0);
              count_leading_zeros (cnt, b);
              cancel += cnt;
              break;
            }

        if (wi >= 0 || a != MPFR_LIMB_ZERO)  /* accumulator != 0 */
          {
            mpfr_exp_t e;        /* exponent of the computed result */
            mpfr_exp_t err;      /* exponent of the error bound */

            MPFR_LOG_MSG (("accumulator %s 0, cancel=%Pd\n",
                           a != MPFR_LIMB_ZERO ? "<" : ">", cancel));

            MPFR_ASSERTD (cancel > 0);
            e = minexp + wq - cancel;
            MPFR_ASSERTD (e >= minexp);
            err = maxexp2 + logn;  /* OK even if maxexp2 == MPFR_EXP_MIN */

            /* The absolute value of the truncated sum is in the binade
               [2^(e-1),2^e] (closed on both ends due to two's complement).
               The error is strictly less than 2^err (and is 0 if
               maxexp2 == MPFR_EXP_MIN). */

            /* This basically tests whether err <= e - prec without
               potential integer overflow (since prec >= 0)...
               Note that the maxexp2 == MPFR_EXP_MIN test is there just for
               the potential corner case e - prec < MPFR_EXP_MIN + logn.
               Such corner cases, involving specific huge-precision numbers,
               are probably not supported in many places in MPFR, but this
               test doesn't hurt... */
            if (maxexp2 == MPFR_EXP_MIN ||
                (err <= e && SAFE_DIFF (mpfr_uexp_t, e, err) >= prec))
              {
                MPFR_LOG_MSG (("(err=%" MPFR_EXP_FSPEC "d) <= (e=%"
                               MPFR_EXP_FSPEC "d) - (prec=%Pd)\n",
                               (mpfr_eexp_t) err, (mpfr_eexp_t) e, prec));
                if (ep != NULL)
                  *ep = e;
                *minexpp = minexp;
                *maxexpp = maxexp2;
                MPFR_LOG_MSG (("return with minexp=%" MPFR_EXP_FSPEC
                               "d maxexp2=%" MPFR_EXP_FSPEC "d%s\n",
                               (mpfr_eexp_t) minexp, (mpfr_eexp_t) maxexp2,
                               maxexp2 == MPFR_EXP_MIN ?
                               " (MPFR_EXP_MIN)" : ""));
                return cancel;
              }
            else
              {
                mpfr_exp_t diffexp;
                mpfr_prec_t shiftq;
                mpfr_size_t shifts;
                int shiftc;

                MPFR_LOG_MSG (("e=%" MPFR_EXP_FSPEC "d err=%" MPFR_EXP_FSPEC
                               "d maxexp2=%" MPFR_EXP_FSPEC "d%s\n",
                               (mpfr_eexp_t) e, (mpfr_eexp_t) err,
                               (mpfr_eexp_t) maxexp2,
                               maxexp2 == MPFR_EXP_MIN ?
                               " (MPFR_EXP_MIN)" : ""));

                diffexp = err - e;
                if (diffexp < 0)
                  diffexp = 0;
                /* diffexp = max(0, err - e) */

                MPFR_LOG_MSG (("diffexp=%" MPFR_EXP_FSPEC "d\n",
                                (mpfr_eexp_t) diffexp));

                MPFR_ASSERTD (diffexp < cancel - 2);
                shiftq = cancel - 2 - (mpfr_prec_t) diffexp;
                MPFR_ASSERTD (shiftq > 0);
                shifts = shiftq / GMP_NUMB_BITS;
                shiftc = shiftq % GMP_NUMB_BITS;
                MPFR_LOG_MSG (("shiftq = %Pd = %Pd * GMP_NUMB_BITS + %d\n",
                               shiftq, (mpfr_prec_t) shifts, shiftc));
                if (MPFR_LIKELY (shiftc != 0))
                  mpn_lshift (wp + shifts, wp, ws - shifts, shiftc);
                else
                  MPN_COPY_DECR (wp + shifts, wp, ws - shifts);
                MPN_ZERO (wp, shifts);
                /* Compute minexp = minexp - shiftq safely. */
                UPDATE_MINEXP (minexp, shiftq);
                MPFR_ASSERTD (minexp < maxexp2);
              }
          }
        else if (maxexp2 == MPFR_EXP_MIN)
          {
            MPFR_LOG_MSG (("accumulator = 0, maxexp2 = MPFR_EXP_MIN\n", 0));
            return 0;
          }
        else
          {
            MPFR_LOG_MSG (("accumulator = 0, reiterate\n", 0));
            /* Compute minexp = maxexp2 - (wq - (logn + 1)) safely. */
            UPDATE_MINEXP (maxexp2, wq - (logn + 1));
            /* Note: the logn + 1 corresponds to cq in the main code. */
          }
      }

      maxexp = maxexp2;
    }
}

/**********************************************************************/

/* Generic case: all the inputs are finite numbers,
   with at least 3 regular numbers. */
static int
sum_aux (mpfr_ptr sum, mpfr_ptr *const x, unsigned long n, mpfr_rnd_t rnd,
         mpfr_exp_t maxexp, unsigned long rn)
{
  mp_limb_t *sump;
  mp_limb_t *tp;  /* pointer to a temporary area */
  mp_limb_t *wp;  /* pointer to the accumulator */
  mp_size_t ts;   /* size of the temporary area, in limbs */
  mp_size_t ws;   /* size of the accumulator, in limbs */
  mpfr_prec_t wq; /* size of the accumulator, in bits */
  int logn;       /* ceil(log2(rn)) */
  int cq;
  mpfr_prec_t sq;
  int inex;
  MPFR_TMP_DECL (marker);

  MPFR_LOG_FUNC
    (("n=%lu rnd=%d maxexp=%" MPFR_EXP_FSPEC "d rn=%lu",
      n, rnd, (mpfr_eexp_t) maxexp, rn),
     ("sum[%Pu]=%.*Rg", mpfr_get_prec (sum), mpfr_log_prec, sum));

  MPFR_ASSERTD (rn >= 3 && rn <= n);

  /* In practice, no integer overflow on the exponent. */
  MPFR_STAT_STATIC_ASSERT (MPFR_EXP_MAX - MPFR_EMAX_MAX >=
                           sizeof (unsigned long) * CHAR_BIT);

  /* Set up some variables and the accumulator. */

  sump = MPFR_MANT (sum);

  /* rn is the number of regular inputs (the singular ones will be
     ignored). Compute logn = ceil(log2(rn)). */
  logn = MPFR_INT_CEIL_LOG2 (rn);
  MPFR_ASSERTD (logn >= 2);

  MPFR_LOG_MSG (("logn=%d maxexp=%" MPFR_EXP_FSPEC "d\n",
                 logn, (mpfr_eexp_t) maxexp));

  sq = MPFR_GET_PREC (sum);
  cq = logn + 1;

  /* First determine the size of the accumulator.
   * cq + sq + logn + 2 >= logn + sq + 5, which will be used later.
   * The assertion wq - cq - sq >= 4 is another way to check that.
   */
  ws = MPFR_PREC2LIMBS (cq + sq + logn + 2);
  wq = (mpfr_prec_t) ws * GMP_NUMB_BITS;
  MPFR_ASSERTD (wq - cq - sq >= 4);

  MPFR_LOG_MSG (("cq=%d sq=%Pd logn=%d wq=%Pd\n", cq, sq, logn, wq));

  /* An input block will have up to wq - cq bits, and its shifted value
     (to be correctly aligned) may take GMP_NUMB_BITS - 1 additional bits. */
  ts = MPFR_PREC2LIMBS (wq - cq + GMP_NUMB_BITS - 1);

  MPFR_TMP_MARK (marker);

  tp = MPFR_TMP_LIMBS_ALLOC (ts + ws);
  wp = tp + ts;

  MPN_ZERO (wp, ws);  /* zero the accumulator */

  {
    mpfr_exp_t minexp;   /* exponent of the LSB of the block */
    mpfr_prec_t cancel;  /* number of cancelled bits */
    mpfr_exp_t e;        /* temporary exponent of the result */
    mpfr_exp_t u;        /* temporary exponent of the ulp (quantum) */
    mp_limb_t rbit;      /* rounding bit (corrected in halfway case) */
    int corr;            /* correction term (from -1 to 2) */
    int sd, sh;          /* shift counts */
    mp_size_t sn;        /* size of the output number */
    int tmd;             /* 0: the TMD does not occur
                            1: the TMD occurs on a machine number
                            2: the TMD occurs on a midpoint */
    int pos;             /* 0 if negative sum, 1 if positive */

    MPFR_LOG_MSG (("Compute an approximation with sum_raw...\n", 0));

    /* Compute minexp = maxexp - (wq - cq) safely. */
    UPDATE_MINEXP (maxexp, wq - cq);
    MPFR_ASSERTD (wq >= logn + sq + 5);
    cancel = sum_raw (wp, ws, wq, x, n, minexp, maxexp, tp, ts,
                      logn, sq + 3, &e, &minexp, &maxexp);

    if (MPFR_UNLIKELY (cancel == 0))
      {
        /* The exact sum is zero. Since not all inputs are 0, the sum
         * is +0 except in MPFR_RNDD, as specified according to the
         * IEEE 754 rules for the addition of two numbers.
         */
        MPFR_SET_SIGN (sum, (rnd != MPFR_RNDD ?
                             MPFR_SIGN_POS : MPFR_SIGN_NEG));
        MPFR_SET_ZERO (sum);
        MPFR_TMP_FREE (marker);
        MPFR_RET (0);
      }

    /* The absolute value of the truncated sum is in the binade
       [2^(e-1),2^e] (closed on both ends due to two's complement).
       The error is strictly less than 2^(maxexp + logn) (and is 0
       if maxexp == MPFR_EXP_MIN). */

    u = e - sq;  /* e being the exponent, u is the ulp of the target */

    MPFR_LOG_MSG (("cancel=%Pd"
                   " e=%" MPFR_EXP_FSPEC "d"
                   " u=%" MPFR_EXP_FSPEC "d"
                   " maxexp=%" MPFR_EXP_FSPEC "d%s\n",
                   cancel, (mpfr_eexp_t) e, (mpfr_eexp_t) u,
                   (mpfr_eexp_t) maxexp,
                   maxexp == MPFR_EXP_MIN ? " (MPFR_EXP_MIN)" : ""));

    /* Let's copy/shift the bits [max(u,minexp),e) to the
       most significant part of the destination, and zero
       the least significant part (there can be one only if
       u < minexp). The trailing bits of the destination may
       contain garbage at this point. Then, at the same time,
       take the absolute value and do an initial rounding,
       zeroing the trailing bits at this point.
       TODO: This may be improved by merging some operations
       in particular cases. The average speed-up may not be
       significant, though. To be tested... */

    sn = MPFR_PREC2LIMBS (sq);
    sd = (mpfr_prec_t) sn * GMP_NUMB_BITS - sq;
    sh = cancel % GMP_NUMB_BITS;

    MPFR_ASSERTD (sd >= 0 && sd < GMP_NUMB_BITS);

    if (MPFR_LIKELY (u > minexp))
      {
        mpfr_prec_t tq;
        mp_size_t ei, fi, wi;
        int td;

        tq = u - minexp;
        MPFR_ASSERTD (tq > 0); /* number of trailing bits */
        MPFR_LOG_MSG (("tq=%Pd\n", tq));

        wi = tq / GMP_NUMB_BITS;

        if (MPFR_LIKELY (sh != 0))
          {
            ei = (e - minexp) / GMP_NUMB_BITS;
            fi = ei - (sn - 1);
            MPFR_ASSERTD (fi == wi || fi == wi + 1);
            mpn_lshift (sump, wp + fi, sn, sh);
            if (fi != wi)
              sump[0] |= wp[wi] >> (GMP_NUMB_BITS - sh);
          }
        else
          {
            MPFR_ASSERTD ((mpfr_prec_t) (ws - (wi + sn)) * GMP_NUMB_BITS
                          == cancel);
            MPN_COPY (sump, wp + wi, sn);
          }

        /* Determine the rounding bit, which is represented. */
        td = tq % GMP_NUMB_BITS;
        rbit = td >= 1 ? ((wp[wi] >> (td - 1)) & MPFR_LIMB_ONE) :
          (MPFR_ASSERTD (wi >= 1), wp[wi-1] >> (GMP_NUMB_BITS - 1));
        MPFR_ASSERTD (rbit == 0 || rbit == 1);
        MPFR_LOG_MSG (("rbit=%d\n", (int) rbit));

        if (maxexp == MPFR_EXP_MIN)
          {
            /* The sum in the accumulator is exact. Determine inex:
               inex = 0 if the final sum is exact, else 1, i.e.
               inex = rounding bit || sticky bit. In round to nearest,
               also determine the rounding direction: obtained from
               the rounding bit possibly except in halfway cases. */
            if (MPFR_LIKELY (rbit == 0 ||
                             (rnd == MPFR_RNDN && ((wp[wi] >> td) & 1) == 0)))
              {
                /* We need to determine the sticky bit, either to set inex
                   (if the rounding bit is 0) or to possibly "correct" rbit
                   (round to nearest, halfway case rounded downward) from
                   which the rounding direction will be determined. */
                MPFR_LOG_MSG (("Determine the sticky bit...\n", 0));

                inex = td >= 2 ? (wp[wi] & MPFR_LIMB_MASK (td - 1)) != 0
                  : td == 0 ?
                  (MPFR_ASSERTD (wi >= 1),
                   (wp[--wi] & MPFR_LIMB_MASK (GMP_NUMB_BITS - 1)) != 0)
                  : 0;

                if (!inex)
                  {
                    mp_size_t wj = wi;

                    while (!inex && wj > 0)
                      inex = wp[--wj] != 0;
                    if (!inex && rbit != 0)
                      {
                        /* sticky bit = 0, rounding bit = 1,
                           i.e. halfway case, which will be
                           rounded downward (see earlier if). */
                        MPFR_ASSERTD (rnd == MPFR_RNDN);
                        inex = 1;
                        rbit = 0;  /* even rounding downward */
                        MPFR_LOG_MSG (("Halfway case rounded downward;"
                                       " set inex=1 rbit=0\n", 0));
                      }
                  }
              }
            else
              inex = 1;
            tmd = 0;  /* We can round correctly -> no TMD. */
          }
        else  /* maxexp > MPFR_EXP_MIN */
          {
            mpfr_exp_t d;
            mp_limb_t limb, mask;
            int nbits;

            /* Since maxexp was set to either the exponent of a x[i] or
               to minexp... */
            MPFR_ASSERTD (maxexp >= MPFR_EMIN_MIN || maxexp == minexp);

            inex = 1;  /* We do not know whether the sum is exact. */

            MPFR_ASSERTD (u <= MPFR_EMAX_MAX && u <= minexp + wq);
            d = u - (maxexp + logn);  /* representable */
            MPFR_ASSERTD (d >= 3);  /* due to prec = sq + 3 in sum_raw */

            /* Let's see whether the TMD occurs by looking at the d bits
               following the ulp bit, or the d-1 bits after the rounding
               bit. */

            /* First chunk after the rounding bit... It starts at:
               (wi,td-2) if td >= 2,
               (wi-1,td-2+GMP_NUMB_BITS) if td < 2. */
            if (td == 0)
              {
                MPFR_ASSERTD (wi >= 1);
                limb = wp[--wi];
                mask = MPFR_LIMB_MASK (GMP_NUMB_BITS - 1);
                nbits = GMP_NUMB_BITS;
              }
            else if (td == 1)
              {
                limb = wi >= 1 ? wp[--wi] : MPFR_LIMB_ZERO;
                mask = MPFR_LIMB_MAX;
                nbits = GMP_NUMB_BITS + 1;
              }
            else  /* td >= 2 */
              {
                MPFR_ASSERTD (td >= 2);
                limb = wp[wi];
                mask = MPFR_LIMB_MASK (td - 1);
                nbits = td;
              }

            /* nbits: number of bits of the first chunk + 1
               (the +1 is for the rounding bit). */

            if (nbits > d)
              {
                /* Some low significant bits must be ignored. */
                limb >>= nbits - d;
                mask >>= nbits - d;
                d = 0;
              }
            else
              {
                d -= nbits;
                MPFR_ASSERTD (d >= 0);
              }

            limb &= mask;
            tmd =
              limb == MPFR_LIMB_ZERO ?
                (rbit == 0 ? 1 : rnd == MPFR_RNDN ? 2 : 0) :
              limb == mask ?
                (limb = MPFR_LIMB_MAX,
                 rbit != 0 ? 1 : rnd == MPFR_RNDN ? 2 : 0) : 0;

            while (tmd != 0 && d != 0)
              {
                mp_limb_t limb2;

                MPFR_ASSERTD (d > 0);
                if (wi == 0)
                  {
                    /* The non-represented bits are 0's. */
                    if (limb != MPFR_LIMB_ZERO)
                      tmd = 0;
                    break;
                  }
                MPFR_ASSERTD (wi > 0);
                limb2 = wp[--wi];
                if (d < GMP_NUMB_BITS)
                  {
                    int c = GMP_NUMB_BITS - d;
                    MPFR_ASSERTD (c > 0 && c < GMP_NUMB_BITS);
                    if ((limb2 >> c) != (limb >> c))
                      tmd = 0;
                    break;
                  }
                if (limb2 != limb)
                  tmd = 0;
                d -= GMP_NUMB_BITS;
              }
          }
      }
    else  /* u <= minexp */
      {
        mp_size_t en;

        en = (e - minexp + (GMP_NUMB_BITS - 1)) / GMP_NUMB_BITS;
        if (MPFR_LIKELY (sh != 0))
          mpn_lshift (sump + sn - en, wp, en, sh);
        else if (MPFR_UNLIKELY (en > 0))
          MPN_COPY (sump + sn - en, wp, en);
        if (sn > en)
          MPN_ZERO (sump, sn - en);

        /* The exact value of the accumulator has been copied.
         * The TMD occurs if and only if there are bits still
         * not taken into account, and if it occurs, this is
         * necessarily on a machine number (-> tmd = 1).
         */
        rbit = 0;
        inex = tmd = maxexp != MPFR_EXP_MIN;
      }

    /* Leading bit: 1 if positive, 0 if negative. */
    pos = sump[sn-1] >> (GMP_NUMB_BITS - 1);

    MPFR_ASSERTD (rbit == 0 || rbit == 1);

    MPFR_LOG_MSG (("tmd=%d rbit=%d inex=%d pos=%d\n",
                   tmd, (int) rbit, inex, pos));

    /* Here, if the final sum is known to be exact, inex = 0, otherwise
     * inex = 1. We have a truncated significand, a trailing term t such
     * that 0 <= t < 1 ulp, and an error on the trailing term bounded by
     * t' in absolute value. Thus the error e on the truncated significand
     * satisfies -t' <= e < 1 ulp + t'. Thus one has 4 correction cases
     * denoted by a corr value between -1 and 2 depending on e, pos, rbit,
     * and the rounding mode:
     *   -1: equivalent to nextbelow;
     *    0: the truncated significand is not corrected;
     *    1: add 1 ulp;
     *    2: add 1 ulp, then nextabove.
     * The nextbelow and nextabove are used here since there may be a
     * change of the binade.
     */

    if (tmd == 0)  /* no TMD */
      {
        switch (rnd)
          {
          case MPFR_RNDD:
            corr = 0;
            break;
          case MPFR_RNDU:
            corr = inex;
            break;
          case MPFR_RNDZ:
            corr = inex && !pos;
            break;
          case MPFR_RNDA:
            corr = inex && pos;
            break;
          default:
            MPFR_ASSERTN (rnd == MPFR_RNDN);
            /* Note: for halfway cases (maxexp == MPFR_EXP_MIN) that are
               rounded downward, rbit has been changed to 0 so that corr
               is set correctly. */
            corr = rbit;
          }
        MPFR_ASSERTD (corr == 0 || corr == 1);
        if (inex && corr == 0)  /* two's complement significand decreased */
          inex = -1;
      }
    else
      {
        mpfr_exp_t err;  /* exponent of the error bound */
        mp_size_t zs;
        int sst;  /* sign of the secondary term */

        MPFR_ASSERTD (maxexp > MPFR_EXP_MIN);
        MPFR_ASSERTD (tmd == 1 || tmd == 2);

        /* New accumulator size */
        ws = MPFR_PREC2LIMBS (wq - sq);
        wq = (mpfr_prec_t) ws * GMP_NUMB_BITS;

        err = maxexp + logn;

        MPFR_LOG_MSG (("TMD with"
                       " maxexp=%" MPFR_EXP_FSPEC "d"
                       " err=%" MPFR_EXP_FSPEC "d"
                       " ws=%Pd"
                       " wq=%Pd\n",
                       (mpfr_eexp_t) maxexp, (mpfr_eexp_t) err,
                       (mpfr_prec_t) ws, wq));

        /* The d-1 bits from u-2 to u-d (= err) are identical. */

        if (err >= minexp)
          {
            mpfr_prec_t tq;
            mp_size_t wi;
            int td;

            /* Let's keep the last 2 over the d-1 identical bits and the
               following bits, i.e. the bits from err+1 to minexp. */
            tq = err - minexp + 2;  /* tq = number of such bits */
            MPFR_LOG_MSG (("[TMD] tq=%Pd\n", tq));
            MPFR_ASSERTD (tq >= 2);

            wi = tq / GMP_NUMB_BITS;
            td = tq % GMP_NUMB_BITS;

            if (td != 0)
              {
                wi++;  /* number of words with represented bits */
                td = GMP_NUMB_BITS - td;
                zs = ws - wi;
                MPFR_ASSERTD (zs >= 0 && zs < ws);
                mpn_lshift (wp + zs, wp, wi, td);
              }
            else
              {
                MPFR_ASSERTD (wi > 0);
                zs = ws - wi;
                MPFR_ASSERTD (zs >= 0 && zs < ws);
                if (zs > 0)
                  MPN_COPY_DECR (wp + zs, wp, wi);
              }

            /* Compute minexp = minexp - (zs * GMP_NUMB_BITS + td) safely. */
            UPDATE_MINEXP (minexp, zs * GMP_NUMB_BITS + td);
            MPFR_ASSERTD (minexp == err + 2 - wq);
          }
        else  /* err < minexp */
          {
            /* At least one of the identical bits is not represented,
               meaning that it is 0 and all these bits are 0's. Thus
               the accumulator will be 0. The new minexp is determined
               from maxexp, with cq bits reserved to avoid an overflow
               (as in the early steps). */
            MPFR_LOG_MSG (("[TMD] err < minexp\n", 0));
            zs = ws;

            /* Compute minexp = maxexp - (wq - cq) safely. */
            UPDATE_MINEXP (maxexp, wq - cq);
            MPFR_ASSERTD (minexp == err + 1 - wq);
          }

        MPN_ZERO (wp, zs);

        /* We need to determine the sign sst of the secondary term.
           In sum_raw, since the truncated sum corresponding to this
           secondary term will be in [2^(e-1),2^e] and the error
           strictly less than 2^err, we can stop the iterations when
           e - err >= 1 (this bound is the 11th argument of sum_raw). */
        cancel = sum_raw (wp, ws, wq, x, n, minexp, maxexp, tp, ts,
                          logn, 1, NULL, &minexp, &maxexp);

        if (cancel != 0)
          sst = MPFR_LIMB_MSB (wp[ws-1]) == 0 ? 1 : -1;
        else if (tmd == 1)
          sst = 0;
        else
          {
            /* For halfway cases, let's virtually eliminate them
               by setting a sst equivalent to a non-halfway case,
               which depends on the last bit of the pre-rounded
               result. */
            MPFR_ASSERTD (rnd == MPFR_RNDN && tmd == 2);
            sst = (sump[0] & (MPFR_LIMB_ONE << sd)) ? 1 : -1;
          }

        MPFR_LOG_MSG (("[TMD] tmd=%d rbit=%d sst=%d\n",
                       tmd, (int) rbit, sst));

        /* Do not consider the corrected sst for MPFR_COV_SET */
        MPFR_COV_SET (sum_tmd[(int) rnd][tmd-1][rbit]
                      [cancel == 0 ? 1 : sst+1][pos][sq > MPFR_PREC_MIN]);

        inex =
          MPFR_IS_LIKE_RNDD (rnd, pos ? 1 : -1) ? (sst ? -1 : 0) :
          MPFR_IS_LIKE_RNDU (rnd, pos ? 1 : -1) ? (sst ?  1 : 0) :
          (MPFR_ASSERTD (rnd == MPFR_RNDN),
           tmd == 1 ? - sst : sst);

        if (tmd == 2 && sst == (rbit ? -1 : 1))
          corr = 1 - (int) rbit;
        else if (MPFR_IS_LIKE_RNDD (rnd, pos ? 1 : -1) && sst == -1)
          corr = (int) rbit - 1;
        else if (MPFR_IS_LIKE_RNDU (rnd, pos ? 1 : -1) && sst == +1)
          corr = (int) rbit + 1;
        else
          corr = (int) rbit;
      }

    MPFR_LOG_MSG (("pos=%d corr=%d inex=%d\n", pos, corr, inex));

    /* Sign handling (-> absolute value and sign), together with
       rounding. The most common cases are corr = 0 and corr = 1
       as this is necessarily the case when the TMD did not occur. */

    MPFR_ASSERTD (corr >= -1 && corr <= 2);

    MPFR_SIGN (sum) = pos ? MPFR_SIGN_POS : MPFR_SIGN_NEG;

    if (MPFR_UNLIKELY (sq == 1))  /* precision 1 */
      {
        sump[0] = MPFR_LIMB_HIGHBIT;
        e += pos ? corr : 1 - corr;
      }
    else if (pos)  /* positive result with sq > 1 */
      {
        MPFR_ASSERTD (MPFR_LIMB_MSB (sump[sn-1]) != 0);
        sump[0] &= ~ MPFR_LIMB_MASK (sd);

        if (corr > 0)
          {
            mp_limb_t corr2, carry_out;

            corr2 = (mp_limb_t) corr << sd;
            /* If corr == 2 && sd == GMP_NUMB_BITS - 1, this overflows
               to corr2 = 0. This case is taken into account below. */

            carry_out = corr2 != 0 ? mpn_add_1 (sump, sump, sn, corr2) :
              (MPFR_ASSERTD (sn > 1),
               mpn_add_1 (sump + 1, sump + 1, sn - 1, MPFR_LIMB_ONE));

            MPFR_ASSERTD (sump[sn-1] >> (GMP_NUMB_BITS - 1) == !carry_out);

            if (MPFR_UNLIKELY (carry_out))
              {
                /* Note: The | is important when sump[sn-1] is not 0
                   (this can occur with sn = 1 and corr = 2). TODO:
                   Add something to make sure that this is tested. */
                sump[sn-1] |= MPFR_LIMB_HIGHBIT;
                e++;
              }
          }

        if (corr < 0)
          {
            mpn_sub_1 (sump, sump, sn, MPFR_LIMB_ONE << sd);

            if (MPFR_UNLIKELY (MPFR_LIMB_MSB (sump[sn-1]) == 0))
              {
                sump[sn-1] |= MPFR_LIMB_HIGHBIT;
                e--;
              }
          }
      }
    else  /* negative result with sq > 1 */
      {
        MPFR_ASSERTD (MPFR_LIMB_MSB (sump[sn-1]) == 0);

        /* abs(x + corr) = - (x + corr) = com(x) + (1 - corr) */

        /* We want to avoid separate mpn_com (or mpn_neg) and mpn_add_1
           (or mpn_sub_1) operations, as they could yield two loops in
           some particular cases involving a long sequence of 0's in
           the low significant bits (except the least significant bit,
           which doesn't matter). */

        if (corr <= 1)
          {
            mp_limb_t corr2;

            /* Here we can just do the correction operation on the
               least significant limb, then do either a mpn_com or
               a mpn_neg on the remaining limbs, depending on the
               carry (BTW, mpn_neg is just a mpn_com with an initial
               carry propagation: after some point, mpn_neg does a
               complement). */

            corr2 = (mp_limb_t) (1 - corr) << sd;
            /* Note: If corr = -1, this can overflow to corr2 = 0.
               This case is taken into account below. */

            sump[0] = (~ (sump[0] | MPFR_LIMB_MASK (sd))) + corr2;

            if (sump[0] < corr2 || (corr2 == 0 && corr < 0))
              {
                if (sn == 1 || ! mpn_neg (sump + 1, sump + 1, sn - 1))
                  {
                    /* Note: The | is important when sump[sn-1] is not 0
                       (this can occur with sn = 1 and corr = -1). TODO:
                       Add something to make sure that this is tested. */
                    sump[sn-1] |= MPFR_LIMB_HIGHBIT;
                    e++;
                  }
              }
            else if (sn > 1)
              mpn_com (sump + 1, sump + 1, sn - 1);
          }
        else  /* corr == 2 */
          {
            mp_limb_t corr2, c;
            mp_size_t i = 1;

            /* We want to compute com(x) - 1, but GMP doesn't have an
               operation for that. The fact is that a sequence of low
               significant bits 1 is invariant. Starting at the first
               low significant bit 0, we can do the complement with
               mpn_com. */

            corr2 = MPFR_LIMB_ONE << sd;
            c = ~ (sump[0] | MPFR_LIMB_MASK (sd));
            sump[0] = c - corr2;

            if (c == 0)
              {
                while (MPFR_ASSERTD (i < sn), sump[i] == MPFR_LIMB_MAX)
                  i++;
                sump[i] = (~ sump[i]) - 1;
                i++;
              }

            if (i < sn)
              mpn_com (sump + i, sump + i, sn - i);
            else if (MPFR_UNLIKELY (MPFR_LIMB_MSB (sump[sn-1]) == 0))
              {
                /* Happens on 01111...111, whose complement is
                   10000...000, and com(x) - 1 is 01111...111. */
                sump[sn-1] |= MPFR_LIMB_HIGHBIT;
                e--;
              }
          }
      }

    MPFR_ASSERTD (MPFR_LIMB_MSB (sump[sn-1]) != 0);
    MPFR_LOG_MSG (("Set exponent e=%" MPFR_EXP_FSPEC "d\n", (mpfr_eexp_t) e));
    /* e may be outside the current exponent range, but this will be checked
       with mpfr_check_range below. */
    MPFR_EXP (sum) = e;
  }  /* main block */

  MPFR_TMP_FREE (marker);
  return mpfr_check_range (sum, inex, rnd);
}

/**********************************************************************/

int
mpfr_sum (mpfr_ptr sum, mpfr_ptr *const x, unsigned long n, mpfr_rnd_t rnd)
{
  MPFR_LOG_FUNC
    (("n=%lu rnd=%d", n, rnd),
     ("sum[%Pu]=%.*Rg", mpfr_get_prec (sum), mpfr_log_prec, sum));

  if (MPFR_UNLIKELY (n <= 2))
    {
      if (n == 0)
        {
          MPFR_SET_ZERO (sum);
          MPFR_SET_POS (sum);
          MPFR_RET (0);
        }
      else if (n == 1)
        return mpfr_set (sum, x[0], rnd);
      else
        return mpfr_add (sum, x[0], x[1], rnd);
    }
  else
    {
      mpfr_exp_t maxexp = MPFR_EXP_MIN;  /* max(Empty) */
      unsigned long i;
      unsigned long rn = 0;  /* will be the number of regular inputs */
      /* sign of infinities and zeros (0: currently unknown) */
      int sign_inf = 0, sign_zero = 0;

      MPFR_LOG_MSG (("Check for special inputs (n = %lu >= 3)\n", n));

      for (i = 0; i < n; i++)
        {
          if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (x[i])))
            {
              if (MPFR_IS_NAN (x[i]))
                {
                  /* The current value x[i] is NaN. Then the sum is NaN. */
                nan:
                  MPFR_SET_NAN (sum);
                  MPFR_RET_NAN;
                }
              else if (MPFR_IS_INF (x[i]))
                {
                  /* The current value x[i] is an infinity.
                     There are two cases:
                     1. This is the first infinity value (sign_inf == 0).
                        Then set sign_inf to its sign, and go on.
                     2. All the infinities found until now have the same
                        sign sign_inf. If this new infinity has a different
                        sign, then return NaN immediately, else go on. */
                  if (sign_inf == 0)
                    sign_inf = MPFR_SIGN (x[i]);
                  else if (MPFR_SIGN (x[i]) != sign_inf)
                    goto nan;
                }
              else if (MPFR_UNLIKELY (rn == 0))
                {
                  /* The current value x[i] is a zero. The code below matters
                     only when all values found until now are zeros, otherwise
                     it is harmless (the test rn == 0 above is just a minor
                     optimization).
                     Here we track the sign of the zero result when all inputs
                     are zeros: if all zeros have the same sign, the result
                     will have this sign, otherwise (i.e. if there is at least
                     a zero of each sign), the sign of the zero result depends
                     only on the rounding mode (note that this choice is
                     sticky when new zeros are considered). */
                  MPFR_ASSERTD (MPFR_IS_ZERO (x[i]));
                  if (sign_zero == 0)
                    sign_zero = MPFR_SIGN (x[i]);
                  else if (MPFR_SIGN (x[i]) != sign_zero)
                    sign_zero = rnd == MPFR_RNDD ? -1 : 1;
                }
            }
          else
            {
              /* The current value x[i] is a regular number. */
              mpfr_exp_t e = MPFR_GET_EXP (x[i]);
              if (e > maxexp)
                maxexp = e;  /* maximum exponent found until now */
              rn++;  /* current number of regular inputs */
            }
        }

      MPFR_LOG_MSG (("rn=%lu sign_inf=%d sign_zero=%d\n",
                     rn, sign_inf, sign_zero));

      /* At this point the result cannot be NaN (this case has already
         been filtered out). */

      if (MPFR_UNLIKELY (sign_inf != 0))
        {
          /* At least one infinity, and all of them have the same sign
             sign_inf. The sum is the infinity of this sign. */
          MPFR_SET_INF (sum);
          MPFR_SET_SIGN (sum, sign_inf);
          MPFR_RET (0);
        }

      /* At this point, all the inputs are finite numbers. */

      if (MPFR_UNLIKELY (rn == 0))
        {
          /* All the numbers were zeros (and there is at least one).
             The sum is zero with sign sign_zero. */
          MPFR_ASSERTD (sign_zero != 0);
          MPFR_SET_ZERO (sum);
          MPFR_SET_SIGN (sum, sign_zero);
          MPFR_RET (0);
        }

      /* Optimize the case where there are only two regular numbers. */
      if (MPFR_UNLIKELY (rn <= 2))
        {
          unsigned long h = ULONG_MAX;

          for (i = 0; i < n; i++)
            if (! MPFR_IS_SINGULAR (x[i]))
              {
                if (rn == 1)
                  return mpfr_set (sum, x[i], rnd);
                if (h != ULONG_MAX)
                  return mpfr_add (sum, x[h], x[i], rnd);
                h = i;
              }
          MPFR_RET_NEVER_GO_HERE();
        }

      return sum_aux (sum, x, n, rnd, maxexp, rn);
    }
}