[PEI][Mips] Switch to backwards frame index elimination

[foad]

Summary

This adds support for running PEI::replaceFrameIndicesBackward with no
RegisterScavenger, and basic support for eliminating call frame pseudo
instructions.

llvm/lib/CodeGen/PrologEpilogInserter.cpp

bool PEI::replaceFrameIndexDebugInstr(MachineFunction &MF, MachineInstr &MI,
   return false;
 }
 
 void PEI::replaceFrameIndicesBackward(MachineBasicBlock *BB,
                                       MachineFunction &MF, int &SPAdj) {
   assert(MF.getSubtarget().getRegisterInfo() &&
          "getRegisterInfo() must be implemented!");
 
+  const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
   const TargetRegisterInfo &TRI = *MF.getSubtarget().getRegisterInfo();
+  const TargetFrameLowering &TFI = *MF.getSubtarget().getFrameLowering();
 
-  RS->enterBasicBlockEnd(*BB);
+  RegScavenger *LocalRS = FrameIndexEliminationScavenging ? RS : nullptr;
+  if (LocalRS)
+    LocalRS->enterBasicBlockEnd(*BB);
 
   for (MachineInstr &MI : make_early_inc_range(reverse(*BB))) {
+    if (TII.isFrameInstr(MI)) {
+      TFI.eliminateCallFramePseudoInstr(MF, *BB, &MI);
+      continue;
+    }
+
     // Step backwards to get the liveness state at (immedately after) MI.
-    RS->backward(MI);
+    if (LocalRS)
+      LocalRS->backward(MI);
 
     for (unsigned i = 0; i != MI.getNumOperands(); ++i) {
       if (!MI.getOperand(i).isFI())
         continue;
 
       if (replaceFrameIndexDebugInstr(MF, MI, i, SPAdj))
         continue;
 
       // Eliminate this FrameIndex operand.
       //
       // Save and restore the scavenger's position around the call to
       // eliminateFrameIndex in case it erases MI and invalidates the iterator.
-      MachineBasicBlock::iterator Save = std::next(RS->getCurrentPosition());
+      MachineBasicBlock::iterator Save;
+      if (LocalRS)
+  Save = std::next(LocalRS->getCurrentPosition());
       bool Removed = TRI.eliminateFrameIndex(MI, SPAdj, i, RS);
-      RS->skipTo(std::prev(Save));
+      if (LocalRS)
+  LocalRS->skipTo(std::prev(Save));
 
       if (Removed)
         break;
     }
   }
 }
 
 void PEI::replaceFrameIndices(MachineBasicBlock *BB, MachineFunction &MF,
                               int &SPAdj) {
   assert(MF.getSubtarget().getRegisterInfo() &&
          "getRegisterInfo() must be implemented!");
   const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
   const TargetRegisterInfo &TRI = *MF.getSubtarget().getRegisterInfo();
   const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
 
-  if (RS && TRI.supportsBackwardScavenger())
+  if (TRI.supportsBackwardScavenger())
     return replaceFrameIndicesBackward(BB, MF, SPAdj);
 
   if (RS && FrameIndexEliminationScavenging)
     RS->enterBasicBlock(*BB);
 
   bool InsideCallSequence = false;
 
   for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ) {

llvm/lib/Target/Mips/MipsRegisterInfo.h

public:
   bool canRealignStack(const MachineFunction &MF) const override;
 
   /// Debug information queries.
   Register getFrameRegister(const MachineFunction &MF) const override;
 
   /// Return GPR register class.
   virtual const TargetRegisterClass *intRegClass(unsigned Size) const = 0;
 
+  bool supportsBackwardScavenger() const override { return true; }
+
 private:
   virtual void eliminateFI(MachineBasicBlock::iterator II, unsigned OpNo,
                            int FrameIndex, uint64_t StackSize,
                            int64_t SPOffset) const = 0;
 };
 
 } // end namespace llvm
 
 #endif // LLVM_LIB_TARGET_MIPS_MIPSREGISTERINFO_H