[LoopVectorize] NFC: Change computeFeasibleMaxVF to operate on ElementCount.

[sdesmalen]

Summary

This patch is NFC and changes occurrences of unsigned MaxVectorSize
to work on type ElementCount.

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.

[sdesmalen]

Just FYI, this patch is part of some refactoring that prepares the LV for creating VPlans with a scalable VF.
I've posted a few more patches (without adding reviewers yet) to provide some context of why these NFC changes are required and what they're working towards.

Hope that's helpful!

[kmclaughlin]

LGTM!

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

if (UserVF.isNonZero() && !IgnoreScalableUserVF) {
     });
     return MaxSafeVF;
   }
 
   WidestRegister = std::min(WidestRegister, MaxSafeVectorWidthInBits);
 
   // Ensure MaxVF is a power of 2; the dependence distance bound may not be.
   // Note that both WidestRegister and WidestType may not be a powers of 2.
-  unsigned MaxVectorSize = PowerOf2Floor(WidestRegister / WidestType);
+  auto MaxVectorSize =
+      ElementCount::getFixed(PowerOf2Floor(WidestRegister / WidestType));
 
   LLVM_DEBUG(dbgs() << "LV: The Smallest and Widest types: " << SmallestType
                     << " / " << WidestType << " bits.\n");
   LLVM_DEBUG(dbgs() << "LV: The Widest register safe to use is: "
                     << WidestRegister << " bits.\n");
 
-  assert(MaxVectorSize <= WidestRegister &&
+  assert(MaxVectorSize.getFixedValue() <= WidestRegister &&
          "Did not expect to pack so many elements"
          " into one vector!");
-  if (MaxVectorSize == 0) {
+  if (MaxVectorSize.getFixedValue() == 0) {
     LLVM_DEBUG(dbgs() << "LV: The target has no vector registers.\n");
-    MaxVectorSize = 1;
-    return ElementCount::getFixed(MaxVectorSize);
-  } else if (ConstTripCount && ConstTripCount < MaxVectorSize &&
+    return ElementCount::getFixed(1);
+  } else if (ConstTripCount && ConstTripCount < MaxVectorSize.getFixedValue() &&
              isPowerOf2_32(ConstTripCount)) {
     // We need to clamp the VF to be the ConstTripCount. There is no point in
     // choosing a higher viable VF as done in the loop below.
     LLVM_DEBUG(dbgs() << "LV: Clamping the MaxVF to the constant trip count: "
                       << ConstTripCount << "\n");
-    MaxVectorSize = ConstTripCount;
-    return ElementCount::getFixed(MaxVectorSize);
+    return ElementCount::getFixed(ConstTripCount);
   }
 
-  unsigned MaxVF = MaxVectorSize;
+  ElementCount MaxVF = MaxVectorSize;
   if (TTI.shouldMaximizeVectorBandwidth(!isScalarEpilogueAllowed()) ||
       (MaximizeBandwidth && isScalarEpilogueAllowed())) {
     // Collect all viable vectorization factors larger than the default MaxVF
     // (i.e. MaxVectorSize).
     SmallVector<ElementCount, 8> VFs;
-    unsigned NewMaxVectorSize = WidestRegister / SmallestType;
-    for (unsigned VS = MaxVectorSize * 2; VS <= NewMaxVectorSize; VS *= 2)
-      VFs.push_back(ElementCount::getFixed(VS));
+    auto MaxVectorSizeMaxBW =
+        ElementCount::getFixed(WidestRegister / SmallestType);
+    for (ElementCount VS = MaxVectorSize * 2;
+         ElementCount::isKnownLE(VS, MaxVectorSizeMaxBW); VS *= 2)
+      VFs.push_back(VS);
 
     // For each VF calculate its register usage.
     auto RUs = calculateRegisterUsage(VFs);
 
     // Select the largest VF which doesn't require more registers than existing
     // ones.
     for (int i = RUs.size() - 1; i >= 0; --i) {
       bool Selected = true;
-      for (auto& pair : RUs[i].MaxLocalUsers) {
+      for (auto &pair : RUs[i].MaxLocalUsers) {
         unsigned TargetNumRegisters = TTI.getNumberOfRegisters(pair.first);
         if (pair.second > TargetNumRegisters)
           Selected = false;
       }
       if (Selected) {
-        MaxVF = VFs[i].getKnownMinValue();
+        MaxVF = VFs[i];
         break;
       }
     }
-    if (unsigned MinVF = TTI.getMinimumVF(SmallestType)) {
-      if (MaxVF < MinVF) {
+    if (auto MinVF = ElementCount::getFixed(TTI.getMinimumVF(SmallestType))) {
+      if (ElementCount::isKnownLT(MaxVF, MinVF)) {
         LLVM_DEBUG(dbgs() << "LV: Overriding calculated MaxVF(" << MaxVF
                           << ") with target's minimum: " << MinVF << '\n');
         MaxVF = MinVF;
       }
     }
   }
-  return ElementCount::getFixed(MaxVF);
+  return MaxVF;
 }
 
 VectorizationFactor
 LoopVectorizationCostModel::selectVectorizationFactor(ElementCount MaxVF) {
   // FIXME: This can be fixed for scalable vectors later, because at this stage
   // 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");