On Thu, 13 Jan 2022, Andre Vieira (lists) wrote:
>
> On 13/01/2022 12:36, Richard Biener wrote:
> > 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.
> True, I'll start with QImode instead of the inner of vector_modes[mode_i] too
> since we can't guarantee the mode is a VECTOR_MODE_P and it is actually better
> too since we can't possible guarantee the element size of the
> USED_VECTOR_MODES is smaller than that of the first vector mode...
>
> > 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
> I suspect that should be fine though, since if we use the largest element size
> of all used_vector_modes then that should gives us the least possible number
> of NUNITS and thus only conservatively skip. That said, that does assume that
> no vector mode used may be larger than the size of the loop's vector_mode. Can
> I assume that?
No idea, but I would lean towards a no ;) I think the loops vector_mode
doesn't have to match vector_modes[mode_i] either, does it? At least
autodetected_vector_mode will be not QImode based.
> >
> > (and you probably also want to exclude
> > VECTOR_BOOLEAN_TYPE_P from the search?)
> Yeah I think so too, thanks!
>
> I keep going back to thinking (as I brought up in the bugzilla ticket), maybe
> we ought to only skip if the NUNITS of the vector mode with the same vector
> size as vector_modes[mode_i] is larger than first_info_vf, or just don't skip
> at all...
The question is how much work we do before realizing the chosen mode
cannot be used because there's not enough iterations? Maybe we can
improve there easily?
Also for targets that for the main loop do not perform cost
comparison (like x86) but have lots of vector modes the previous
mode of operation really made sense (start at next_mode_i or
mode_i when unrolling).
