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 ?
