[vect] PR103997: Fix epilogue mode skipping

[Richard Biener]

On Thu, 13 Jan 2022, Andre Vieira (lists) wrote:

> This time to the list too (sorry for double email)
> 
> Hi,
> 
> The original patch '[vect] Re-analyze all modes for epilogues', skipped modes
> that should not be skipped since it used the vector mode provided by
> autovectorize_vector_modes to derive the minimum VF required for it. However,
> those modes should only really be used to dictate vector size, so instead this
> patch looks for the mode in 'used_vector_modes' with the largest element size,
> and constructs a vector mode with the smae size as the current
> vector_modes[mode_i]. Since we are using the largest element size the NUNITs
> for this mode is the smallest possible VF required for an epilogue with this
> mode and should thus skip only the modes we are certain can not be used.
> 
> Passes bootstrap and regression on x86_64 and aarch64.

Clearly

+         /* To make sure we are conservative as to what modes we skip, we
+            should use check the smallest possible NUNITS which would be
+            derived from the mode in USED_VECTOR_MODES with the largest
+            element size.  */
+         scalar_mode max_elsize_mode = GET_MODE_INNER
(vector_modes[mode_i]);
+         for (vec_info::mode_set::iterator i =
+               first_loop_vinfo->used_vector_modes.begin ();
+             i != first_loop_vinfo->used_vector_modes.end (); ++i)
+           {
+             if (VECTOR_MODE_P (*i)
+                 && GET_MODE_SIZE (GET_MODE_INNER (*i))
+                 > GET_MODE_SIZE (max_elsize_mode))
+               max_elsize_mode = GET_MODE_INNER (*i);
+           }

can be done once before iterating over the modes for the epilogue.

Richard maybe knows whether we should take care to look at the
size of the vector mode as well since related_vector_mode when
passed 0 as nunits produces a vector mode with the same size
as vector_modes[mode_i] but not all used_vector_modes may be
of the same size (and you probably also want to exclude
VECTOR_BOOLEAN_TYPE_P from the search?)

Thanks,
Richard.

> gcc/ChangeLog:
> 
>         PR 103997
>         * tree-vect-loop.c (vect_analyze_loop): Fix mode skipping for 
> epilogue
>         vectorization.
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Ivo Totev; HRB 36809 (AG Nuernberg)