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

["Andre Vieira (lists)" <andre.simoesdiasvieira@arm.com>]

On 24/10/2022 13:46, Richard Biener wrote:
> On Mon, 24 Oct 2022, Andre Vieira (lists) wrote:
>
>> On 24/10/2022 08:17, Richard Biener wrote:
>>> Can you check why vect_find_stmt_data_reference doesn't trip on the
>>>
>>>     if (TREE_CODE (DR_REF (dr)) == COMPONENT_REF
>>>         && DECL_BIT_FIELD (TREE_OPERAND (DR_REF (dr), 1)))
>>>       {
>>>         free_data_ref (dr);
>>>         return opt_result::failure_at (stmt,
>>>                                        "not vectorized:"
>>>                                        " statement is an unsupported"
>>>                                        " bitfield access %G", stmt);
>>>       }
>> It used to, which is why this test didn't trigger the error before my patch,
>> but we lower it to BIT_FIELD_REFs in ifcvt now so it is no longer a
>> DECL_BIT_FIELD.
>>
>> But that is a red-herring, if you change the test structure's 'type Int24 is
>> mod 2**24;' to 'type Int24 is mod 2**32;', thus making the field we access a
>> normal 32-bit integer, the field no longer is a DECL_BIT_FIELD and thus my
>> lowering does nothing. However, you will still get the failure because the
>> field before it is a packed 4-bit field, making the offset to the field we are
>> accessing less than BITS_PER_UNIT.
> Hmm, so the _intent_ of DECL_BIT_FIELD_REPRESENTATIVE is to definitely
> _not_ be a DECL_BIT_FIELD (well, that's the whole point!).   So this
> shows an issue with setting up DECL_BIT_FIELD_REPRESENTATIVE?  Of course
> for a type with an alignment less than BITS_PER_UNIT (is StructB actually
> such a type?) there cannot be a representative that isn't, so maybe
> we should then set DECL_BIT_FIELD on it with a condition like Eric
> mentions?
I could do this, but it would not resolve the latent issue as I could 
still reproduce it without using any of the bitfield lowering code, see 
below.
>
>>> ?  I think we should amend this check and I guess that
>>> checking multiple_p on DECL_FIELD_BIT_OFFSET should be enough?
>> That won't work either, unless we do the same walk-through the full access as
>> we do in get_inner_reference.
> I suppose we should not "if-convert" bit field accesses with a
> DECL_BIT_FIELD representative.  There isn't any benefit doing that
> (not for general bitfield lowering either).
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 ?