On Wed, 12 Jan 2022, Richard Sandiford wrote:
> Richard Biener <[email protected]> writes:
> > On Wed, 12 Jan 2022, Andre Vieira (lists) wrote:
> >
> >> Hi,
> >>
> >> This a fix for the regression caused by '[vect] Re-analyze all modes for
> >> epilogues'. The earlier patch assumed there was always at least one other
> >> mode
> >> than VOIDmode, but that does not need to be the case.
> >> If we are dealing with a target that does not define more modes for
> >> 'autovectorize_vector_modes', the behaviour before the patch would be to
> >> try
> >> to create an epilogue for the same autodetected_vector_mode, which unless
> >> the
> >> target supported partial vectors would always fail. So as a fix I suggest
> >> trying to vectorize the epilogue with the preferred_simd_mode for QI,
> >> mimicking autovectorize_vector_mode, which will be skipped if it is not a
> >> vector_mode (since that already should indicate partial vectors aren't
> >> possible) or if no partial vectors are supported and its pessimistic
> >> NUNITS is
> >> larger than the main loop's VF.
> >>
> >> Currently bootstrapping and regression testing, otherwise OK for trunk? Can
> >> someone verify this fixes the issue for PR103971 on powerpc?
> >
> > Why not simply start at mode_i = 0 which means autodetecting the mode
> > to use for the epilogue? That appears to be a much simpler solution to
> > me, including for targets where there are more than one element in the
> > vector.
>
> VOIDmode doesn't tell us anything about what the autodetected mode
> will be, so current short-circuit:
>
> /* If the target does not support partial vectors we can shorten the
> number of modes to analyze for the epilogue as we know we can't pick a
> mode that has at least as many NUNITS as the main loop's vectorization
> factor, since that would imply the epilogue's vectorization factor
> would be at least as high as the main loop's and we would be
> vectorizing for more scalar iterations than there would be left. */
> if (!supports_partial_vectors
> && maybe_ge (GET_MODE_NUNITS (vector_modes[mode_i]), first_vinfo_vf))
> {
> mode_i++;
> if (mode_i == vector_modes.length ())
> break;
> continue;
> }
>
> wouldn't be effective.
Well, before this change we simply did
- /* Handle the case that the original loop can use partial
- vectorization, but want to only adopt it for the epilogue.
- The retry should be in the same mode as original. */
- if (LOOP_VINFO_EPIL_USING_PARTIAL_VECTORS_P (first_loop_vinfo))
...
- else
- {
- mode_i = first_loop_next_i;
- if (mode_i == vector_modes.length ())
- return first_loop_vinfo;
- }
and thus didn't bother with epilogue vectorization. I think we should
then just restore this behavior, not doing epilogue vectorization
if vector_modes.length () == 1?
Richard.
