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 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.

> ?  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.

Let me elaborate, the 'offending' stmt here is:
_ifc__23 = (*x_7(D))[_1].b.D.3707;

And the struct in question is:
package Loop_Optimization23_Pkg is
   type Nibble is mod 2**4;
   type Int24  is mod 2**24;
   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;

That D.3707 is the 'container'm i.e. the DECL_BIT_FIELD_REPRESENTATIVE 
of the original bitfield of type Int24.
So in vect_find_stmt_data_reference , the dr is: (*x_7(D))[_1].b.D.3707 and
TREE_OPERAND (DR_REF (dr), 1): D.3707,
which has DECL_FIELD_BIT_OFFSET: 0

So that check would also pass. However, get_inner_reference, walks the 
full access and comes across '.b', the member access for StructA inside 
StructB, that has DECL_FIELD_BIT_OFFSET: 4
Which is where we get into trouble. So to catch that here, we would need 
to do the same type of walking through all the member accesses, like 
get_inner_reference does.