[PATCH] Fix PR90332 by extending half size vector mode

[Richard Biener]

On Wed, Mar 18, 2020 at 11:06 AM Kewen.Lin <linkw@linux.ibm.com> wrote:
>
> Hi,
>
> As PR90332 shows, the current scalar epilogue peeling for gaps
> elimination requires expected vec_init optab with two half size
> vector mode.  On Power, we don't support vector mode like V8QI,
> so can't support optab like vec_initv16qiv8qi.  But we want to
> leverage existing scalar mode like DI to init the desirable
> vector mode.  This patch is to extend the existing support for
> Power, as evaluated on Power9 we can see expected 1.9% speed up
> on SPEC2017 525.x264_r.
>
> Bootstrapped/regtested on powerpc64le-linux-gnu (LE) P8 and P9.
>
> Is it ok for trunk?

There's already code exercising such a case in vectorizable_load
(VMAT_STRIDED_SLP) which you could have factored out.

 vectype, bool slp,
             than the alignment boundary B.  Every vector access will
             be a multiple of B and so we are guaranteed to access a
             non-gap element in the same B-sized block.  */
+         machine_mode half_mode;
          if (overrun_p
              && gap < (vect_known_alignment_in_bytes (first_dr_info)
                        / vect_get_scalar_dr_size (first_dr_info)))
-           overrun_p = false;
-
+           {
+             overrun_p = false;
+             if (known_eq (nunits, (group_size - gap) * 2)
+                 && known_eq (nunits, group_size)
+                 && get_half_mode_for_vector (vectype, &half_mode))
+               DR_GROUP_HALF_MODE (first_stmt_info) = half_mode;
+           }

why do you need to amend this case?

I don't like storing DR_GROUP_HALF_MODE very much, later
you need a vector type and it looks cheap enough to recompute
it where you need it?  Iff then it doesn't belong to DR_GROUP
but to the stmt-info.

I realize the original optimization was kind of a hack (and I was too
lazy to implement the integer mode construction path ...).

So, can you factor out the existing code into a function returning
the vector type for construction for a vector type and a
pieces size?  So for V16QI and a pieces-size of 4 we'd
get either V16QI back (then construction from V4QI pieces
should work) or V4SI (then construction from SImode pieces
should work)?  Eventually as secondary output provide that
piece type (SI / V4QI).

Thanks,
Richard.

> BR,
> Kewen
> -----------
>
> gcc/ChangeLog
>
> 2020-MM-DD  Kewen Lin  <linkw@gcc.gnu.org>
>
>         PR tree-optimization/90332
>         * gcc/tree-vectorizer.h (struct _stmt_vec_info): Add half_mode field.
>         (DR_GROUP_HALF_MODE): New macro.
>         * gcc/tree-vect-stmts.c (get_half_mode_for_vector): New function.
>         (get_group_load_store_type): Call get_half_mode_for_vector to query target
>         whether support half size mode and update DR_GROUP_HALF_MODE if yes.
>         (vectorizable_load): Build appropriate vector type based on
>         DR_GROUP_HALF_MODE.