[Octave-bug-tracker] [bug #60928] Performance of sort unexpectedly slow for DIM=2

[Rik]

Follow-up Comment #12, bug #60928 (project octave):

Okay, the slowdown is entirely due to this piece of code


      for (octave_idx_type j = 0; j < iter; j++)
        {
          octave_idx_type offset = j;
          octave_idx_type offset2 = 0;

          while (offset >= stride)
            {
              offset -= stride;
              offset2++;
            }

          offset += offset2 * stride * ns;


For dimension 1, stride is 1 and offset = j will take the values 1 .. 125,000
for my benchmark data.  So for the last calculation, the code reduces to a
while loop that executes 125,000 times simply to increment offset2.

I assume that the original coder was trying to calculate


offset = floor (j / stride) * stride * ns;


in such a way as to avoid conversion to double and to avoid actual division
operation.  However, this is causing real problems.

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