MachineSSAUpdater: Allow initialization with just a register class

[nhaehnle]

Summary

The register class is required for inserting PHIs, but the "current
virtual register" isn't actually used for anything, so let's remove it
while we're at it.

[arsenm]

This should probably support using an LLT/generic virtual register without a class, but that’s a separate change

llvm/include/llvm/CodeGen/MachineSSAUpdater.h

class MachineSSAUpdater {
   friend class SSAUpdaterTraits<MachineSSAUpdater>;
 
 private:
   /// AvailableVals - This keeps track of which value to use on a per-block
   /// basis.  When we insert PHI nodes, we keep track of them here.
   //typedef DenseMap<MachineBasicBlock*, Register> AvailableValsTy;
   void *AV = nullptr;
 
-  /// VR - Current virtual register whose uses are being updated.
-  Register VR;
-
   /// VRC - Register class of the current virtual register.
   const TargetRegisterClass *VRC;
 
   /// InsertedPHIs - If this is non-null, the MachineSSAUpdater adds all PHI
   /// nodes that it creates to the vector.
   SmallVectorImpl<MachineInstr*> *InsertedPHIs;
 
   const TargetInstrInfo *TII;
   MachineRegisterInfo *MRI;
 
 public:
   /// MachineSSAUpdater constructor.  If InsertedPHIs is specified, it will be
   /// filled in with all PHI Nodes created by rewriting.
   explicit MachineSSAUpdater(MachineFunction &MF,
                         SmallVectorImpl<MachineInstr*> *NewPHI = nullptr);
   MachineSSAUpdater(const MachineSSAUpdater &) = delete;
   MachineSSAUpdater &operator=(const MachineSSAUpdater &) = delete;
   ~MachineSSAUpdater();
 
   /// Initialize - Reset this object to get ready for a new set of SSA
   /// updates.
   void Initialize(Register V);
+  void Initialize(const TargetRegisterClass *RC);
 
   /// AddAvailableValue - Indicate that a rewritten value is available at the
   /// end of the specified block with the specified value.
   void AddAvailableValue(MachineBasicBlock *BB, Register V);
 
   /// HasValueForBlock - Return true if the MachineSSAUpdater already has a
   /// value for the specified block.
   bool HasValueForBlock(MachineBasicBlock *BB) const;

llvm/lib/CodeGen/MachineSSAUpdater.cpp

MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
   : InsertedPHIs(NewPHI), TII(MF.getSubtarget().getInstrInfo()),
     MRI(&MF.getRegInfo()) {}
 
 MachineSSAUpdater::~MachineSSAUpdater() {
   delete static_cast<AvailableValsTy*>(AV);
 }
 
 /// Initialize - Reset this object to get ready for a new set of SSA
-/// updates.  ProtoValue is the value used to name PHI nodes.
-void MachineSSAUpdater::Initialize(Register V) {
+/// updates.
+void MachineSSAUpdater::Initialize(const TargetRegisterClass *RC) {
   if (!AV)
     AV = new AvailableValsTy();
   else
     getAvailableVals(AV).clear();
 
-  VR = V;
-  VRC = MRI->getRegClass(VR);
+  VRC = RC;
+}
+
+void MachineSSAUpdater::Initialize(Register V) {
+  Initialize(MRI->getRegClass(V));
 }
 
 /// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for
 /// the specified block.
 bool MachineSSAUpdater::HasValueForBlock(MachineBasicBlock *BB) const {
   return getAvailableVals(AV).count(BB);
 }