[PATCH] D35741: Add MemorySSA alternative to AliasSetTracker in LICM.

[Alina Sbirlea via llvm-commits]

>
>
>> > FWIW I suspect MemorySSAAliasSets will be too slow for use by anything
>> for
>> > real.
>>
>> I was under the impression is that it should be no worse than
>> AliasSetTracker,
>
>
> This is true, but AST is badly N^2 already, so realize i'm in the boat of
> "AST is too slow for anything to use for real".
>
> I also very much do not want to see *new* passes using MemorySSAAliasSets
> to do anything.
>
>
>> and better in some cases (large blocks with very few
>> memory operations).
>
>
> In the case of promoting loads alone, AST is not necessary at all.
> In the case of promoting loads and stores, the way our LICM does it means
> it's still going to be N^2.
> You will save some walking time.
>
> You also end up back in a world where clients are trying to do direct
> aliasing queries, which sucks.
>
>
>> Otherwise I don't see a reason to do this.
>>
>
> I'm torn.
> It buys you something, but i also suspect at this level of complexity, we
> could have just rewritten licm and build a memoryssi or a multi-phi
> memoryssa or whatever it took to make handling stores fast.
>
> Sigh.
>

Agreed on those points; the patch description talks about the problem with
promotion.
Question is, is it ok to have this functionality checked in as is until a
rewrite of licm or an alternative for aliasing stores exists?

The hope I have is that the changes can be incremental.
Current patch only adds the core functionality, without enabling it by
default.
Action item for this to be usable:
    - teach all loop passes to preserve MemorySSA   ----> at this point we
can compare performance of AST vs MemorySSA approaches.
Action items for better results:
   - incremental: expose alias information for Uses in MemorySSA
   - long term: complete rewrite of promotion ----> at this point,
MemorySSAAliasSets
can be removed (I can add a comment to warn against this being picked up by
other passes)


>
>
>
>
>
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20170927/4fa4863f/attachment.html>