Re: [3/8] Add flags to dr_with_seg_len_pair_t

[Richard Sandiford <richard dot sandiford at arm dot com>]

Richard Biener <richard.guenther@gmail.com> writes:
> On Mon, Nov 11, 2019 at 7:47 PM Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>>
>> This patch adds a bunch of flags to dr_with_seg_len_pair_t,
>> for use by later patches.  The update to tree-loop-distribution.c
>> is conservatively correct, but might be tweakable later.
>
> Does this all work with interleaved SLP loads/stores like
>
>   a[i] = b[i];
>   tem1 = b[i+1];
>   tem2 = b[i+2];
>   a[i+2] = tem2;
>   a[i+1] = tem1;
>
> where we don't preserve the scalar order but emit code at the
> latest stmt of the grouped access?

Yeah.  vect-alias-check-9.c is a less sophisticated example of that.
In both cases vect_preserves_scalar_order_p is false.

> That is, what does "preserve scalar oder" actually mean?

It's supposed to mean that if a memory access A from the first
group and a memory access B from the second group occur within
the same scalar iteration, the order of the accesses in the
vector loop body will be the same as it was in the scalar loop body.
In other words, the order of the vector stmts honours any dependencies
between the accesses that occur within one iteration of the scalar loop.

Thanks,
Richard