[LoopVectorize] NFC: Change selectVectorizationFactor to work on ElementCount.

[sdesmalen]

Summary

This patch is NFC and changes occurrences of unsigned Width
and unsigned i to work on type ElementCount instead.

This patch is a preparatory patch with the ultimate goal of making
computeMaxVF() return both a max fixed VF and a max scalable VF,
so that selectVectorizationFactor() can pick the most cost-effective
vectorization factor.

[david-arm]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

5856

Perhaps this should be getFixedValue() since it can never be scalable?

[sdesmalen]

s/getKnownMinValue/getFixedValue/

[david-arm]

LGTM!

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

LoopVectorizationCostModel::selectVectorizationFactor(ElementCount MaxVF) {
   // the LoopVectorizer will only consider vectorizing a loop with scalable
   // vectors when the loop has a hint to enable vectorization for a given VF.
   assert(!MaxVF.isScalable() && "scalable vectors not yet supported");
 
   InstructionCost ExpectedCost = expectedCost(ElementCount::getFixed(1)).first;
   LLVM_DEBUG(dbgs() << "LV: Scalar loop costs: " << ExpectedCost << ".\n");
   assert(ExpectedCost.isValid() && "Unexpected invalid cost for scalar loop");
 
-  unsigned Width = 1;
+  auto Width = ElementCount::getFixed(1);
   const float ScalarCost = *ExpectedCost.getValue();
   float Cost = ScalarCost;
 
   bool ForceVectorization = Hints->getForce() == LoopVectorizeHints::FK_Enabled;
   if (ForceVectorization && MaxVF.isVector()) {
     // Ignore scalar width, because the user explicitly wants vectorization.
     // Initialize cost to max so that VF = 2 is, at least, chosen during cost
     // evaluation.
     Cost = std::numeric_limits<float>::max();
   }
 
-  for (unsigned i = 2; i <= MaxVF.getFixedValue(); i *= 2) {
+  for (auto i = ElementCount::getFixed(2); ElementCount::isKnownLE(i, MaxVF);
+       i *= 2) {
     // Notice that the vector loop needs to be executed less times, so
     // we need to divide the cost of the vector loops by the width of
     // the vector elements.
-    VectorizationCostTy C = expectedCost(ElementCount::getFixed(i));
+    VectorizationCostTy C = expectedCost(i);
     assert(C.first.isValid() && "Unexpected invalid cost for vector loop");
-    float VectorCost = *C.first.getValue() / (float)i;
+    float VectorCost = *C.first.getValue() / (float)i.getFixedValue();
     LLVM_DEBUG(dbgs() << "LV: Vector loop of width " << i
                       << " costs: " << (int)VectorCost << ".\n");
     if (!C.second && !ForceVectorization) {
       LLVM_DEBUG(
           dbgs() << "LV: Not considering vector loop of width " << i
                  << " because it will not generate any vector instructions.\n");
       continue;
     }
 
     // If profitable add it to ProfitableVF list.
     if (VectorCost < ScalarCost) {
       ProfitableVFs.push_back(VectorizationFactor(
-          {ElementCount::getFixed(i), (unsigned)VectorCost}));
+          {i, (unsigned)VectorCost}));
     }
 
     if (VectorCost < Cost) {
       Cost = VectorCost;
       Width = i;
     }
   }
 
   if (!EnableCondStoresVectorization && NumPredStores) {
     reportVectorizationFailure("There are conditional stores.",
         "store that is conditionally executed prevents vectorization",
         "ConditionalStore", ORE, TheLoop);
-    Width = 1;
+    Width = ElementCount::getFixed(1);
     Cost = ScalarCost;
   }
 
-  LLVM_DEBUG(if (ForceVectorization && Width > 1 && Cost >= ScalarCost) dbgs()
+  LLVM_DEBUG(if (ForceVectorization && !Width.isScalar() && Cost >= ScalarCost) dbgs()
              << "LV: Vectorization seems to be not beneficial, "
              << "but was forced by a user.\n");
   LLVM_DEBUG(dbgs() << "LV: Selecting VF: " << Width << ".\n");
-  VectorizationFactor Factor = {ElementCount::getFixed(Width),
-                                (unsigned)(Width * Cost)};
+  VectorizationFactor Factor = {Width,
+                                (unsigned)(Width.getKnownMinValue() * Cost)};
   return Factor;
 }
 
 bool LoopVectorizationCostModel::isCandidateForEpilogueVectorization(
     const Loop &L, ElementCount VF) const {
   // Cross iteration phis such as reductions need special handling and are
   // currently unsupported.
   if (any_of(L.getHeader()->phis(), [&](PHINode &Phi) {