Richard Sandiford <richard.sandif...@linaro.org> writes:
> "Bin.Cheng" <amker.ch...@gmail.com> writes:
>> On Wed, May 3, 2017 at 11:07 AM, Richard Biener
>> <richard.guent...@gmail.com> wrote:
>>> On Wed, May 3, 2017 at 9:54 AM, Richard Sandiford
>>> <richard.sandif...@linaro.org> wrote:
>>>> vect_find_same_alignment_drs uses the ddr dependence distance
>>>> to tell whether two references have the same alignment. Although
>>>> that's safe with the current code, there's no particular reason
>>>> why a dependence distance of 0 should mean that the accesses start
>>>> on the same byte. E.g. a reference to a full complex value could
>>>> in principle depend on a reference to the imaginary component.
>>>> A later patch adds support for this kind of dependence.
>>>>
>>>> On the other side, checking modulo vf is pessimistic when the step
>>>> divided by the element size is a factor of 2.
>>>>
>>>> This patch instead looks for cases in which the drs have the same
>>>> base, offset and step, and for which the difference in their constant
>>>> initial values is a multiple of the alignment.
>>>
>>> I'm a bit wary about trusting operand_equal_p over dependence analysis.
>>> So, did you (can you) add an assert that the new code computes
>>> same alignment in all cases where the old one did?
>
> FWIW, the operand_equal_p for the base addresses is the same as the one
> used by the dependence analysis:
>
> /* If the references do not access the same object, we do not know
> whether they alias or not. We do not care about TBAA or alignment
> info so we can use OEP_ADDRESS_OF to avoid false negatives.
> But the accesses have to use compatible types as otherwise the
> built indices would not match. */
> if (!operand_equal_p (DR_BASE_OBJECT (a), DR_BASE_OBJECT (b),
> OEP_ADDRESS_OF)
> || !types_compatible_p (TREE_TYPE (DR_BASE_OBJECT (a)),
> TREE_TYPE (DR_BASE_OBJECT (b))))
> {
> DDR_ARE_DEPENDENT (res) = chrec_dont_know;
> return res;
> }
>
>> At the moment operand_equal_p method looks more powerful than
>> dependence analysis, for example, it can handle the same memory
>> reference with pointer/array_ref forms like in PR65206. However,
>> given dependence check is not expensive here, is it possible to build
>> the patch on top of it when it fails?
>
> The old check isn't valid after my later patches, because there's
> no guarantee that the accesses start on the same byte. And like
> you say, the new check is more powerful in some ways (including
> the modulo vf thing I mentioned).
>
> So I'm not sure we can do anything useful with the dependence distance
> information. Sometimes it would give false positives and sometimes
> it would give false negatives.
Upthread you said "otherwise ok" apart from the "I'm a bit wary..." part.
Is the original patch OK given the above?
Thanks,
Richard
