Richard Biener <rguent...@suse.de> writes:
> On Thu, 7 Nov 2019, Andre Vieira (lists) wrote:
>> On 07/11/2019 14:00, Richard Biener wrote:
>> > On Thu, 7 Nov 2019, Andre Vieira (lists) wrote:
>> >
>> >> Hi,
>> >>
>> >> PR92351 reports a bug in which a wrongly aligned load is generated for an
>> >> epilogue of a main loop for which we peeled for alignment. There is no
>> >> way
>> >> to
>> >> guarantee that epilogue data accesses are aligned when the main loop is
>> >> peeling for alignment.
>> >>
>> >> I also had to split vect-peel-2.c as there were scans there for the number
>> >> of
>> >> unaligned accesses that were vectorized, thanks to this change that now
>> >> depends on whether we are vectorizing the epilogue, which will also
>> >> contain
>> >> unaligned accesses. Since not all targets need to be able to vectorize
>> >> the
>> >> epilogue I decided to disable epilogue vectorization for the version in
>> >> which
>> >> we scan the dumps and add a version that attempts epilogue vectorization
>> >> but
>> >> does not scan the dumps.
>> >>
>> >> Bootstrapped and regression tested on x86_64 and aarch64.
>> >>
>> >> Is this OK for trunk?
>> >
>> > @@ -938,6 +938,18 @@ vect_compute_data_ref_alignment (dr_vec_info
>> > *dr_info)
>> > = exact_div (vect_calculate_target_alignment (dr_info),
>> > BITS_PER_UNIT);
>> > DR_TARGET_ALIGNMENT (dr_info) = vector_alignment;
>> > + /* If the main loop has peeled for alignment we have no way of knowing
>> > + whether the data accesses in the epilogues are aligned. We can't at
>> > + compile time answer the question whether we have entered the main
>> > loop
>> > or
>> > + not. Fixes PR 92351. */
>> > + if (loop_vinfo)
>> > + {
>> > + loop_vec_info orig_loop_vinfo = LOOP_VINFO_ORIG_LOOP_INFO
>> > (loop_vinfo);
>> > + if (orig_loop_vinfo
>> > + && LOOP_VINFO_PEELING_FOR_ALIGNMENT (orig_loop_vinfo) != 0)
>> > + return;
>> > + }
>> >
>> > so I'm not sure this is the correct place to do the fixup. Isn't the
>> > above done when analyzing the loops with different vector size/mode?
>> > So we don't yet know whether we analyze the loop as epilogue or
>> > not epilogue? Looks like we at the moment always choose the
>> > very first loop we analyze successfully as "main" loop?
>> >
>> > So, can we do this instead in update_epilogue_loop_vinfo? There
>> > we should also know whether we created the jump-around the
>> > main vect loop.
>> >
>>
>> So we do know we are analyzing it as an epilogue, that is the only case
>> orig_loop_vinfo is set.
>>
>> The reason why we shouldn't do it in update_epilogue_loop_vinfo is that the
>> target might not know how to vectorize memory accesses for unaligned memory
>> for the given VF. Or maybe it does but is too expensive don't know if we
>> currently check that though. I do not have an example but this is why I
>> believe it would be better to do it during analysis. I thought it had been
>> you
>> who alerted me to this, but maybe it was Sandiford, or maybe I dreamt it up
>> ;)
>
> It was probably me, yes. But don't we have a catch-22 now? If we
> have multiple vector sizes and as Richard, want to first compute
> the "cheapest" to use as the main vectorized body don't we then have
> to re-analyze the smaller vector sizes for epilogue use?
It was a nice hack that we could vectorise as an epilogue even when
choosing main loops, and optionally "promote" them later, but it's
probably going to have to yield at some point anyway. E.g. from what
Andre said on IRC yesterday, he might have to take peeling for gaps
into account too.
> So how do we handle this situation at the moment?
>
> I think during alignment peeling analysis we look whether a DR
> absolutely needs to be aligned, that is, we use
> vect_supportable_dr_alignment (*, true). If that returns
> dr_unaligned_unsupported we should probably simply disable
> epilogue vectorization if we didn't version for alignment
> (or we know the vectorized loop was entered).
I guess doing this based on the main loop would hard-code an assumption
that the shorter vectors have the same sensitivity to alignment as
longer vectors. Which is probably fine in practice, but it would
be good to avoid if possible.
> So, during analysis reject epilogues that have DRs with
> dr_unaligned_unsupported but allow them as "main" loops still
> (so disable epilogue vectorization for a main loop with such DRs).
>
> Then at update_epilogue_loop_vinfo time simply make alignment
> unknown.
>
> Would that work?
Agree it sounds like it would work. But at the moment we don't yet have
a dr_unaligned_unsupported target that wants the "best loop" behaviour.
Given that we might have to do what the vect_analyze_loop comment in
https://gcc.gnu.org/ml/gcc-patches/2019-11/msg00296.html
explains away anyway, it might not be worth the effort to support
that case.
Thanks,
Richard
