[llvm-dev] RFC: Mark BasicAA as a CFG-only pass.

[Alina Sbirlea via llvm-dev]

Got it, I updated the patch. Please let me know fi I should detail it
further.

Thank you,
Alina

On Mon, Feb 10, 2020 at 3:10 PM Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 2/10/20 4:09 PM, Alina Sbirlea wrote:
>
> On Mon, Feb 10, 2020 at 1:48 PM Hal Finkel <hfinkel at anl.gov> wrote:
>
>>
>> On 2/10/20 2:35 PM, Alina Sbirlea wrote:
>>
>> Hi,
>>
>> Here's a tentative patch of the changes for this: D74353
>> <https://reviews.llvm.org/D74353>.
>>
>>
>> I suppose that, as expected, it's invalidated less often this way. Given
>> that it's generally stateless, does this really represent a cost savings?
>>
>>  -Hal
>>
>
> I don't think there is a cost savings now, the current patch shouldn't
> have much or any impact.
> But I think this can affect future infrastructure changes.
> I got here by looking at MemCpyOpt, as part of a longer term goal of
> having the pass sequence [GVN, MemCpyOpt, DSE] use MemorySSA instead of
> MemDepAnalysis. Currently we're explicitly checking that, after MemCpyOpt,
> BasicAA is freed. This wouldn't make sense when using MemorySSA in all 3
> passes, as we'd end up rebuilding MemorySSA due to invalidating BasicAA.
> I kept the changes needed to MemCpyOpt to preserve passes separate from
> this discussion.
> Does this clarify some of the motivation?
>
>
> Yes, please add this information to the patch description.
>
> I think that it makes sense to mark BasicAA as not being invalidated on
> non-CFG-altering IR changes if it's not.
>
>  -Hal
>
>
>
> Alina
>
>>
>>
>> Thank you,
>> Alina
>>
>>
>> On Mon, Feb 10, 2020 at 11:34 AM Alina Sbirlea <alina.sbirlea at gmail.com>
>> wrote:
>>
>>> Hi,
>>>
>>> I'd like to understand if it makes sense to keep BasicAA as a not
>>> CFG-only pass, or if it can be updated to CFG-only. The change was made in
>>> D44564 <https://reviews.llvm.org/D44564>.
>>>
>>> From what I gathered the motivation was PhiValuesAnalysis not being
>>> properly updated, and BasicAA having an instance of it.
>>> PhiValuesAnalysis now uses callback values to invalidate deleted values (
>>> r340613 <https://reviews.llvm.org/rL340613>), PhiValuesAnalysis is also
>>> being updated in MemDepAnalysis (D48489
>>> <https://reviews.llvm.org/D48489>) and BasicAA is invalidated if PhiValuesAnalysis
>>> gets invalidated.
>>>
>>> I may not have the full context here, so I'd like some feedback: does it
>>> make sense to make BasicAA a CFG-only pass again?
>>>
>>> Thank you,
>>> Alina
>>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200210/56123356/attachment-0001.html>