[PATCH] D86133: [LoopNest] False negative of `arePerfectlyNested` with LCSSA loops

[Whitney Tsang via Phabricator via llvm-commits]

Whitney added inline comments.


================
Comment at: llvm/lib/Analysis/LoopNestAnalysis.cpp:239
+      auto *PN = dyn_cast<PHINode>(&I);
+      return PN && PN->getName().endswith(".lcssa");
+    });
----------------
TaWeiTu wrote:
> Whitney wrote:
> > Don't think checking name is ideal, because it can be changed in between passes. 
> > How about PHINodes that have single incoming entry?
> The LCSSA PHINodes may have two incoming entries, one from the exit block of the inner loop and the other from the header of the outer loop.
> I'm not entirely sure about whether the condition is correct and sufficient, though.
> The LCSSA PHINodes may have two incoming entries, one from the exit block of the inner loop and the other from the header of the outer loop.

I guess you mean from the exiting block of the inner loop? I thought LCSSA blocks are the exit blocks.
https://llvm.org/docs/LoopTerminology.html#loop-closed-ssa-lcssa
`they can just iterate over all the (loop closing) PHI nodes in the exit blocks`

I see what you mean by looking at the LIT test, in function `f`, `%split` is actually the LCSSA phi, not `%res.1.lcssa`. `%res.1.lcssa` is just a generic PHINode. 


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https://reviews.llvm.org/D86133