[tablegen] Improve performance on *GenRegisterInfo.inc by replacing SparseVector with BitVector. NFC

[dsanders]

Summary

Generating X86GenRegisterInfo.inc and AArch64GenRegisterInfo.inc is 8-9% faster on my build.

[bogner]

This makes a lot of sense, especially given that the change to use SparseBitVector here instead of std::set specifically called out that this was to improve cases where there were dense register files. LGTM.

utils/TableGen/CodeGenRegisters.cpp

 #include "CodeGenTarget.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/IntEqClasses.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/SparseBitVector.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TableGen/Error.h"

 // Side effects:
 // - creates new adopted register units
 // - causes superregisters to inherit adopted units
 // - increases the weight of "singular" units
 // - induces recomputation of UberWeights.
 static bool normalizeWeight(CodeGenRegister *Reg,
                             std::vector<UberRegSet> &UberSets,
                             std::vector<UberRegSet*> &RegSets,
-                            SparseBitVector<> &NormalRegs,
+                            BitVector &NormalRegs,
                             CodeGenRegister::RegUnitList &NormalUnits,
                             CodeGenRegBank &RegBank) {
+  NormalRegs.resize(std::max(Reg->EnumValue + 1, NormalRegs.size()));
   if (NormalRegs.test(Reg->EnumValue))
     return false;
   NormalRegs.set(Reg->EnumValue);
 
   bool Changed = false;
   const CodeGenRegister::SubRegMap &SRM = Reg->getSubRegs();
   for (CodeGenRegister::SubRegMap::const_iterator SRI = SRM.begin(),
          SRE = SRM.end(); SRI != SRE; ++SRI) {
void CodeGenRegBank::computeRegUnitWeights() {
   // Iterate over each Register, normalizing the unit weights until reaching
   // a fix point.
   unsigned NumIters = 0;
   for (bool Changed = true; Changed; ++NumIters) {
     assert(NumIters <= NumNativeRegUnits && "Runaway register unit weights");
     Changed = false;
     for (auto &Reg : Registers) {
       CodeGenRegister::RegUnitList NormalUnits;
-      SparseBitVector<> NormalRegs;
+      BitVector NormalRegs;
       Changed |= normalizeWeight(&Reg, UberSets, RegSets, NormalRegs,
                                  NormalUnits, *this);
     }
   }
 }
 
 // Find a set in UniqueSets with the same elements as Set.
 // Return an iterator into UniqueSets.