[PATCH] D71492: [SCEV] Generate AddRec for trivial and LCSSA phis outside of loop header.

[Florian Hahn via Phabricator via llvm-commits]

fhahn added inline comments.


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Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:5332-5335
   // If the PHI has a single incoming value, follow that value, unless the
   // PHI's incoming blocks are in a different loop, in which case doing so
   // risks breaking LCSSA form. Instcombine would normally zap these, but
   // it doesn't have DominatorTree information, so it may miss cases.
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Meinersbur wrote:
> [serious] Shouldn't this code handle the case? It explicitly mentions the risk of breaking LCSSA, why isn't this the case for your code? 
I think the problem here is that SCEVExpander does not preserve LCSSA at the moment while expanding expressions and looking through LCSSA would break LCSSA form while expanding SCEVs that would refer to a LCSSA instruction without looking through them.

Getting rid of that restriction was discussed in D68194 as well and I have been working on set of follow up patches that make SCEVExpander preserve LCSSA while expanding. I've put up the WIP patches at D71538 and D71539. There are still a few problems/refactors pending before they are ready though.


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https://reviews.llvm.org/D71492