[PATCH] D73181: [SCEV] Do not use backedge SCEV of PHI if its input is another PHI

[Sanjoy Das (Work Account) via Phabricator via llvm-commits]

sanjoy.google added inline comments.


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Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:8238
           }
           // Do we have a loop invariant value flowing around the backedge
           // for a loop which must execute the backedge?
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This comment needs to be fixed IMO.  It isn't necessarily a loop invariant value; we're really checking for a value that can be materialized outside the loop.


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Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:8247
+            if (!isa<PHINode>(BackedgeVal) &&
+                IsAvailableOnEntry(LI, DT, OnBackedge, PN->getParent()))
               return OnBackedge;
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dantrushin wrote:
> Bailing out here when any PHI is seen, because I did not find convinient API to get SCEV at previous iteration.
> Is there one?
I don't think you need to go through SCEV here.  You can just pick the `llvm::Value` for the PHI for the backedge and get the SCEV for that.


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https://reviews.llvm.org/D73181