Re: vect: Make vect_check_gather_scatter reject offsets that aren't multiples of BITS_PER_UNIT [PR107346]

[Richard Biener <rguenther@suse.de>]

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On Fri, 28 Oct 2022, Andre Vieira (lists) wrote:

> 
> On 24/10/2022 14:29, Richard Biener wrote:
> > On Mon, 24 Oct 2022, Andre Vieira (lists) wrote:
> >
> >> Changing if-convert would merely change this testcase but we could still
> >> trigger using a different structure type, changing the size of Int24 to 32
> >> bits rather than 24:
> >> package Loop_Optimization23_Pkg is
> >>    type Nibble is mod 2**4;
> >>    type Int24  is mod 2**32;  -- Changed this from 24->32
> >>    type StructA is record
> >>      a : Nibble;
> >>      b : Int24;
> >>    end record;
> >>    pragma Pack(StructA);
> >>    type StructB is record
> >>      a : Nibble;
> >>      b : StructA;
> >>    end record;
> >>    pragma Pack(StructB);
> >>    type ArrayOfStructB is array(0..100) of StructB;
> >>    procedure Foo (X : in out ArrayOfStructB);
> >> end Loop_Optimization23_Pkg;
> >>
> >> This would yield a DR_REF (dr): (*x_7(D))[_1].b.b  where the last 'b' isn't
> >> a
> >> DECL_BIT_FIELD anymore, but the first one still is and still has the
> >> non-multiple of BITS_PER_UNIT offset. Thus passing the
> >> vect_find_stmt_data_reference check and triggering the
> >> vect_check_gather_scatter failure. So unless we go and make sure we always
> >> set
> >> the DECL_BIT_FIELD on all subsequent accesses of a DECL_BIT_FIELD 'struct'
> >> (which is odd enough on its own) then we are better off catching the issue
> >> in
> >> vect_check_gather_scatter ?
> > But it's not only an issue with scatter-gather, other load/store handling
> > assumes it can create a pointer to the start of the access and thus
> > requires BITS_PER_UNIT alignment for each of them.  So we need to fail
> > at data-ref analysis somehow.
> >
> > Richard.
> 
> Sorry for the delay on this, had some other things come in between. After our
> IRC discussion I believe we agreed that it would be neater to check this in
> vect_check_gather_scatter as I did in the original patch in
> https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604139.html
> The main reasons being that to check earlier we'd need to walk the DR_REF to
> look for any FIELD_DECL that has DECL_BIT_FIELD set and we decided against
> that.
> 
> Can you confirm the original patch is OK for trunk?

Yes.

Thanks,
Richard.

> Kind regards,
> Andre