Re: [PATCH V15] VECT: Add decrement IV iteration loop control by variable amount support

[Richard Sandiford via Gcc-patches]

"juzhe.zh...@rivai.ai" <juzhe.zh...@rivai.ai> writes:
> Hi， Richard. Thanks for the comments.
>
>>> if (!LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo)
>>>     || !iv_rgc
>>>     || (iv_rgc->max_nscalars_per_iter * iv_rgc->factor
>>> != rgc->max_nscalars_per_iter * rgc->factor))
>>>           {
>   >>           /* See whether zero-based IV would ever generate all-false 
> masks
>    >>             or zero length before wrapping around.  */
>    >>          bool might_wrap_p = vect_rgroup_iv_might_wrap_p (loop_vinfo, 
> rgc);
>  
>    >>          /* Set up all controls for this group.  */
>      >>        test_ctrl = vect_set_loop_controls_directly (loop, loop_vinfo,
>     >>                                                      &preheader_seq,
>         >>                                                  &header_seq,
>     >>                                                      loop_cond_gsi, 
> rgc,
>     >>                                                      niters, 
> niters_skip,
>     >>                                                      might_wrap_p);
>  
>    >>  iv_rgc = rgc;
>   >> }
>
>
> Could you tell me why you add:
> (iv_rgc->max_nscalars_per_iter * iv_rgc->factor
>>> != rgc->max_nscalars_per_iter * rgc->factor) ?

The patch creates IVs with the following step:

      gimple_seq_add_stmt (header_seq, gimple_build_assign (step, MIN_EXPR,
                                                            index_before_incr,
                                                            nitems_step));

If nitems_step is the same for two IVs, those IVs will always be equal.

So having multiple IVs with the same nitems_step is redundant.

nitems_step is calculated as follows:

  unsigned int nitems_per_iter = rgc->max_nscalars_per_iter * rgc->factor;
  ...
  poly_uint64 vf = LOOP_VINFO_VECT_FACTOR (loop_vinfo);
  ...

  if (nitems_per_iter != 1)
    {
      ...
      tree iv_factor = build_int_cst (iv_type, nitems_per_iter);
      ...
      nitems_step = gimple_build (preheader_seq, MULT_EXPR, iv_type,
                                  nitems_step, iv_factor);
      ...
    }

so nitems_per_step is equal to:

  rgc->max_nscalars_per_iter * rgc->factor * VF

VF is fixed for a loop, so nitems_step is equal for two different
rgroup_controls if:

  rgc->max_nscalars_per_iter * rgc->factor

is the same for those rgroup_controls.

Please try the example I posted earlier today. I think you'll see that,
without the:

  (iv_rgc->max_nscalars_per_iter * iv_rgc->factor
   != rgc->max_nscalars_per_iter * rgc->factor)

you'll have two IVs with the same step (because their MIN_EXPRs have
the same bound).

Thanks,
Richard