On Wed, May 31, 2017 at 8:55 AM, Richard Sandiford
<richard.sandif...@linaro.org> wrote:
> 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?
Yes.
Thanks,
Richard.
> Thanks,
> Richard
