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 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?

Kind regards,
Andre