Bug found in nightbuilds

Torbjorn Granlund tg at gmplib.org
Sun Feb 16 16:45:04 UTC 2014

bodrato at mail.dm.unipi.it writes:

  Code says:
    if (d & GMP_NUMB_HIGHBIT)
      { /* Normalized case */
        uh = up[--n]; /* Here n goes to 0 */
            while (n > 0)
              udiv_qrnnd (...);
            return uh;
        return mpn_div_qr_1n_pi1 (qp, up, n, uh, d, dinv);
  The error arises when DIV_QR_1_NORM_THRESHOLD is zero, and the
  BELOW_THRESHOLD is unconditionally false, so that the code doesn't simply
  return uh, but calls mpn_div_qr_1n_pi1 with n = 0...
  I disallowed zero DIV_QR_1_NORM_THRESHOLD with this patch:
  and the  GMP_CHECK_RANDOMIZE=3526906869 test passes.
  I believe that we should, for the same reasons, disallow zero 
  DIV_QR_1_UNNORM_THRESHOLD in the /* Unnormalized case */ branch.
Since we control these thresholds at the source level, if it is
desirable to keep DIV_QR_1_UNNORM_THRESHOLD >= 1, that should be done in

Your patch adds some overhead when tuning, since then
DIV_QR_1_UNNORM_THRESHOLD is not really constant.

Also, keeping DIV_QR_1_UNNORM_THRESHOLD >= 1 surely does the job, but it
puts another test it the critical path.  Perhaps that's unavoidable, I
haven't checked.


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