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 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).

Richard.

> 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.
> 
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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