[LoopVectorize] In LoopVectorize start using getSymbolicMaxBackedgeTakenCount

LoopVectorizationLegality currently only treats a loop as legal
to vectorise if PredicatedScalarEvolution::getBackedgeTakenCount
returns a valid SCEV, or more precisely that the loop must have
an exact backedge taken count. Therefore, in LoopVectorize.cpp
we can safely replace all calls to getBackedgeTakenCount with
calls to getSymbolicMaxBackedgeTakenCount, since the result is
the same.

This also helps prepare the loop vectoriser for PR #88385.

Author: david-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -4075,7 +4075,12 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
     unsigned MaxVFtimesIC =
         UserIC ? *MaxPowerOf2RuntimeVF * UserIC : *MaxPowerOf2RuntimeVF;
     ScalarEvolution *SE = PSE.getSE();
-    const SCEV *BackedgeTakenCount = PSE.getBackedgeTakenCount();
+    // Currently only loops with countable exits are vectorized so it's safe to
+    // use getSymbolicMaxBackedgeTakenCount as it should give the same result
+    // as getBackedgeTakenCount.
+    const SCEV *BackedgeTakenCount = PSE.getSymbolicMaxBackedgeTakenCount();
+    assert(!isa<SCEVCouldNotCompute>(BackedgeTakenCount) &&
+           "Invalid loop count");
     const SCEV *ExitCount = SE->getAddExpr(
         BackedgeTakenCount, SE->getOne(BackedgeTakenCount->getType()));
     const SCEV *Rem = SE->getURemExpr(

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -880,11 +880,16 @@ VPlanPtr VPlan::createInitialVPlan(Type *InductionTy,
   auto Plan = std::make_unique<VPlan>(Entry, VecPreheader);
 
   // Create SCEV and VPValue for the trip count.
-  const SCEV *BackedgeTakenCount = PSE.getBackedgeTakenCount();
-  assert(!isa<SCEVCouldNotCompute>(BackedgeTakenCount) && "Invalid loop count");
+
+  // Using getSymbolicMaxBackedgeTakenCount instead of getBackedgeTakenCount,
+  // since they should be identical as we currently only vectorize loops when
+  // all exits are countable.
+  const SCEV *BackedgeTakenCountSCEV = PSE.getSymbolicMaxBackedgeTakenCount();
+  assert(!isa<SCEVCouldNotCompute>(BackedgeTakenCountSCEV) &&
+         "Invalid loop count");
   ScalarEvolution &SE = *PSE.getSE();
-  const SCEV *TripCount =
-      SE.getTripCountFromExitCount(BackedgeTakenCount, InductionTy, TheLoop);
+  const SCEV *TripCount = SE.getTripCountFromExitCount(BackedgeTakenCountSCEV,
+                                                       InductionTy, TheLoop);
   Plan->TripCount =
       vputils::getOrCreateVPValueForSCEVExpr(*Plan, TripCount, SE);