Re: [PATCH] [RFC] vect: Fix infinite loop while determining peeling amount

[Stefan Schulze Frielinghaus <stefansf@linux.ibm.com>]

On Tue, Jul 28, 2020 at 08:55:57AM +0200, Richard Biener wrote:
> On Mon, Jul 27, 2020 at 4:20 PM Stefan Schulze Frielinghaus
> <stefansf@linux.ibm.com> wrote:
> >
> > On Mon, Jul 27, 2020 at 12:29:11PM +0200, Richard Biener wrote:
> > > On Mon, Jul 27, 2020 at 11:45 AM Richard Sandiford
> > > <richard.sandiford@arm.com> wrote:
> > > >
> > > > Richard Biener <richard.guenther@gmail.com> writes:
> > > > > On Mon, Jul 27, 2020 at 11:09 AM Richard Sandiford
> > > > > <richard.sandiford@arm.com> wrote:
> > > > >>
> > > > >> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > > > >> > On Wed, Jul 22, 2020 at 5:18 PM Stefan Schulze Frielinghaus via
> > > > >> > Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> > > > >> >>
> > > > >> >> This is a follow up to commit 5c9669a0e6c respectively discussion
> > > > >> >> https://gcc.gnu.org/pipermail/gcc-patches/2020-June/549132.html
> > > > >> >>
> > > > >> >> In case that an alignment constraint is less than the size of a
> > > > >> >> corresponding scalar type, ensure that we advance at least by one
> > > > >> >> iteration.  For example, on s390x we have for a long double an alignment
> > > > >> >> constraint of 8 bytes whereas the size is 16 bytes.  Therefore,
> > > > >> >> TARGET_ALIGN / DR_SIZE equals zero resulting in an infinite loop which
> > > > >> >> can be reproduced by the following MWE:
> > > > >> >
> > > > >> > But we guard this case with vector_alignment_reachable_p, so we shouldn't
> > > > >> > have ended up here and the patch looks bogus.
> > > > >>
> > > > >> The above sounds like it ought to count as reachable alignment though.
> > > > >> If a type requires a lower alignment than its size, then that's even
> > > > >> more easily reachable than a type that requires the same alignment as
> > > > >> the size.  I guess at one extreme, a target alignment of 1 is always
> > > > >> reachable.
> > > > >
> > > > > Well, if the element alignment is 8 but its size is 16 then when presumably
> > > > > the desired vector alignment is a multiple of 16 we can never reach it.
> > > > > Isn't this the case here?
> > > >
> > > > If the desired vector alignment (TARGET_ALIGN) is a multiple of 16 then
> > > > TARGET_ALIGN / DR_SIZE will be nonzero and the problem the patch is
> > > > fixing wouldn't occur.  I agree that we might never be able to reach
> > > > that alignment if the pointer starts out misaligned by 8 bytes.
> > > >
> > > > But I think that's why it makes sense for the target to only ask
> > > > for 8-byte alignment for vectors too, if it can cope with it.  8-byte
> > > > alignment should always be achievable if the scalars are ABI-aligned.
> > > > And if the target does ask for only 8-byte alignment, TARGET_ALIGN /
> > > > DR_SIZE would be zero and the loop would never progress, which is the
> > > > problem that the patch is fixing.
> > > >
> > > > It would even make sense for the target to ask for 1-byte alignment,
> > > > if the target doesn't care about alignment at all.
> > >
> > > Hmm, OK.  Guess I still think we should detect this somewhere upward
> > > and avoid this peeling compute at all.  Somehow.
> >
> > I've been playing around with another solution which works for me by
> > changing vector_alignment_reachable_p to return also false if the
> > alignment requirements are already satisfied, i.e., by adding:
> >
> > if (known_alignment_for_access_p (dr_info) && aligned_access_p (dr_info))
> >   return false;
> 
> That sounds wrong, instead ...

Can you elaborate on that?  A similar test exists for predicate
vector_alignment_reachable_p where the second conjunct is the same but
negated in order to test for the case where a misalignment is known:
https://gcc.gnu.org/git?p=gcc.git;a=blob;f=gcc/tree-vect-data-refs.c;h=e35a215e042478d11d6545f1f829d816d0c3620f;hb=refs/heads/master#l1263
Therefore, I'm wondering why the non-negated case should be wrong.

> > Though, I'm not entirely sure whether this makes it better or not.
> > Strictly speaking if the alignment was reachable before peeling, then
> > reaching alignment with peeling is also possible but probably not what
> > was intended.  So I guess returning false in this case is sensible.  Any
> > comments?
> 
> ... why is the DR considered for peeling at all?  If it is already
> aligned there's
> no point to do that.

Isn't the whole point of vector_alignment_reachable_p to check DRs in
order to decide whether peeling should be done or not?  At least this is
my intuition and the reason why I was suggesting to return false in case
it is aligned.

Cheers,
Stefan

> If we want to align another DR then the loop you fix
> should run on that DRs align/size, no?
> 
> Richard.
> 
> > Thanks,
> > Stefan
> >
> > >
> > > Richard.
> > >
> > > > Thanks,
> > > > Richard