[LV] Use SmallVector instead of DenseMap in calculateRegisterUsage (NFC).

[fhahn]

Summary

We assign indices sequentially for seen instructions, so we can just use
a vector and push back the seen instructions. No need for using a
DenseMap.

[rengolin]

Good catch! LGTM, thanks!

lib/Transforms/Vectorize/LoopVectorize.cpp

LoopVectorizationCostModel::calculateRegisterUsage(ArrayRef<unsigned> VFs) {
   RegisterUsage RU;
 
   // Each 'key' in the map opens a new interval. The values
   // of the map are the index of the 'last seen' usage of the
   // instruction that is the key.
   using IntervalMap = DenseMap<Instruction *, unsigned>;
 
   // Maps instruction to its index.
-  DenseMap<unsigned, Instruction *> IdxToInstr;
+  SmallVector<Instruction *, 64> IdxToInstr;
   // Marks the end of each interval.
   IntervalMap EndPoint;
   // Saves the list of instruction indices that are used in the loop.
   SmallPtrSet<Instruction *, 8> Ends;
   // Saves the list of values that are used in the loop but are
   // defined outside the loop, such as arguments and constants.
   SmallPtrSet<Value *, 8> LoopInvariants;
 
-  unsigned Index = 0;
   for (BasicBlock *BB : make_range(DFS.beginRPO(), DFS.endRPO())) {
     for (Instruction &I : BB->instructionsWithoutDebug()) {
-      IdxToInstr[Index++] = &I;
+      IdxToInstr.push_back(&I);
 
       // Save the end location of each USE.
       for (Value *U : I.operands()) {
         auto *Instr = dyn_cast<Instruction>(U);
 
         // Ignore non-instruction values such as arguments, constants, etc.
         if (!Instr)
           continue;
 
         // If this instruction is outside the loop then record it and continue.
         if (!TheLoop->contains(Instr)) {
           LoopInvariants.insert(Instr);
           continue;
         }
 
         // Overwrite previous end points.
-        EndPoint[Instr] = Index;
+        EndPoint[Instr] = IdxToInstr.size();
         Ends.insert(Instr);
       }
     }
   }
 
   // Saves the list of intervals that end with the index in 'key'.
   using InstrList = SmallVector<Instruction *, 2>;
   DenseMap<unsigned, InstrList> TransposeEnds;
LoopVectorizationCostModel::calculateRegisterUsage(ArrayRef<unsigned> VFs) {
   // A lambda that gets the register usage for the given type and VF.
   auto GetRegUsage = [&DL, WidestRegister](Type *Ty, unsigned VF) {
     if (Ty->isTokenTy())
       return 0U;
     unsigned TypeSize = DL.getTypeSizeInBits(Ty->getScalarType());
     return std::max<unsigned>(1, VF * TypeSize / WidestRegister);
   };
 
-  for (unsigned int i = 0; i < Index; ++i) {
+  for (unsigned int i = 0, s = IdxToInstr.size(); i < s; ++i) {
     Instruction *I = IdxToInstr[i];
 
     // Remove all of the instructions that end at this location.
     InstrList &List = TransposeEnds[i];
     for (Instruction *ToRemove : List)
       OpenIntervals.erase(ToRemove);
 
     // Ignore instructions that are never used within the loop.