[PATCH] D104140: [SCEV] Allow negative steps for LT exit count computation

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


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Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:11559
   APInt One(BitWidth, 1, IsSigned);
   StrideForMaxBECount = APIntOps::smax(One, StrideForMaxBECount);
 
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Is the unconditional use of "smax" here going to cause issues?


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Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:11570
   APInt MaxEnd = IsSigned ? APIntOps::smin(getSignedRangeMax(End), Limit)
                           : APIntOps::umin(getUnsignedRangeMax(End), Limit);
 
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Instead of computing `min(End, Limit) - Start`, should we be using `max(End, Start) - Start` like we do elsewhere?


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Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:11634
+    // Precondition b) implies that if the stride is zero, this is a single
+    // trip loop. The backedge taken count formula reduces to zero in this case.
     //
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This is inductive logic, right?  If the first iteration doesn't the loop, the following iterations also can't exit the loop.

I think this logic requires that RHS is invariant?  Not that we would compute BECount anyway in that case, but I think we might underestimate MaxBECount.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D104140/new/

https://reviews.llvm.org/D104140