Address review comment

* Updated comments around calls to getSymbolicMaxBackedgeTakenCount

Author: david-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -4054,9 +4054,10 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
     unsigned MaxVFtimesIC =
         UserIC ? *MaxPowerOf2RuntimeVF * UserIC : *MaxPowerOf2RuntimeVF;
     ScalarEvolution *SE = PSE.getSE();
-    // Currently only loops with countable exits are vectorized so it's safe to
-    // use getSymbolicMaxBackedgeTakenCount as it should give the same result
-    // as getBackedgeTakenCount.
+    // Currently only loops with countable exits are vectorized, but calling
+    // getSymbolicMaxBackedgeTakenCount allows enablement work for loops with
+    // uncountable exits whilst also ensuring the symbolic maximum and known
+    // back-edge taken count remain identical for loops with countable exits.
     const SCEV *BackedgeTakenCount = PSE.getSymbolicMaxBackedgeTakenCount();
     assert(!isa<SCEVCouldNotCompute>(BackedgeTakenCount) &&
            "Invalid loop count");

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -881,9 +881,10 @@ VPlanPtr VPlan::createInitialVPlan(Type *InductionTy,
 
   // Create SCEV and VPValue for the trip count.
 
-  // Using getSymbolicMaxBackedgeTakenCount instead of getBackedgeTakenCount,
-  // since they should be identical as we currently only vectorize loops when
-  // all exits are countable.
+  // Currently only loops with countable exits are vectorized, but calling
+  // getSymbolicMaxBackedgeTakenCount allows enablement work for loops with
+  // uncountable exits whilst also ensuring the symbolic maximum and known
+  // back-edge taken count remain identical for loops with countable exits.
   const SCEV *BackedgeTakenCountSCEV = PSE.getSymbolicMaxBackedgeTakenCount();
   assert(!isa<SCEVCouldNotCompute>(BackedgeTakenCountSCEV) &&
          "Invalid loop count");