Question about vectorization limit

[Dehao Chen]

Hi,

In tree-vect-loop.c, it limits the vectorization only to loops that have 2 BBs:

      /* Inner-most loop.  We currently require that the number of BBs is
         exactly 2 (the header and latch).  Vectorizable inner-most loops
         look like this:

                        (pre-header)
                           |
                          header <--------+
                           | |            |
                           | +--> latch --+
                           |
                        (exit-bb)  */

      if (loop->num_nodes != 2)
        {
          if (dump_enabled_p ())
            dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
                             "not vectorized: control flow in loop.");
          return NULL;
        }

Any insights why the limit is set to 2? We found that removing this
limit actually improve performance for many applications.

Thanks,
Dehao