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

[Hal Finkel via llvm-dev]

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 
> <mailto: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 <mailto: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

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