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+//===- RegisterScavenging.h - Machine register scavenging -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// This file declares the machine register scavenger class. It can provide
+/// information such as unused register at any point in a machine basic block.
+/// It also provides a mechanism to make registers available by evicting them
+/// to spill slots.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_REGISTERSCAVENGING_H
+#define LLVM_CODEGEN_REGISTERSCAVENGING_H
+
+#include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/CodeGen/LiveRegUnits.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/MC/LaneBitmask.h"
+
+namespace llvm {
+
+class MachineInstr;
+class TargetInstrInfo;
+class TargetRegisterClass;
+class TargetRegisterInfo;
+
+class RegScavenger {
+  const TargetRegisterInfo *TRI;
+  const TargetInstrInfo *TII;
+  MachineRegisterInfo* MRI;
+  MachineBasicBlock *MBB = nullptr;
+  MachineBasicBlock::iterator MBBI;
+  unsigned NumRegUnits = 0;
+
+  /// True if RegScavenger is currently tracking the liveness of registers.
+  bool Tracking = false;
+
+  /// Information on scavenged registers (held in a spill slot).
+  struct ScavengedInfo {
+    ScavengedInfo(int FI = -1) : FrameIndex(FI) {}
+
+    /// A spill slot used for scavenging a register post register allocation.
+    int FrameIndex;
+
+    /// If non-zero, the specific register is currently being
+    /// scavenged. That is, it is spilled to this scavenging stack slot.
+    unsigned Reg = 0;
+
+    /// The instruction that restores the scavenged register from stack.
+    const MachineInstr *Restore = nullptr;
+  };
+
+  /// A vector of information on scavenged registers.
+  SmallVector<ScavengedInfo, 2> Scavenged;
+
+  LiveRegUnits LiveUnits;
+
+  // These BitVectors are only used internally to forward(). They are members
+  // to avoid frequent reallocations.
+  BitVector KillRegUnits, DefRegUnits;
+  BitVector TmpRegUnits;
+
+public:
+  RegScavenger() = default;
+
+  /// Start tracking liveness from the begin of basic block \p MBB.
+  void enterBasicBlock(MachineBasicBlock &MBB);
+
+  /// Start tracking liveness from the end of basic block \p MBB.
+  /// Use backward() to move towards the beginning of the block. This is
+  /// preferred to enterBasicBlock() and forward() because it does not depend
+  /// on the presence of kill flags.
+  void enterBasicBlockEnd(MachineBasicBlock &MBB);
+
+  /// Move the internal MBB iterator and update register states.
+  void forward();
+
+  /// Move the internal MBB iterator and update register states until
+  /// it has processed the specific iterator.
+  void forward(MachineBasicBlock::iterator I) {
+    if (!Tracking && MBB->begin() != I) forward();
+    while (MBBI != I) forward();
+  }
+
+  /// Invert the behavior of forward() on the current instruction (undo the
+  /// changes to the available registers made by forward()).
+  void unprocess();
+
+  /// Unprocess instructions until you reach the provided iterator.
+  void unprocess(MachineBasicBlock::iterator I) {
+    while (MBBI != I) unprocess();
+  }
+
+  /// Update internal register state and move MBB iterator backwards.
+  /// Contrary to unprocess() this method gives precise results even in the
+  /// absence of kill flags.
+  void backward();
+
+  /// Call backward() as long as the internal iterator does not point to \p I.
+  void backward(MachineBasicBlock::iterator I) {
+    while (MBBI != I)
+      backward();
+  }
+
+  /// Move the internal MBB iterator but do not update register states.
+  void skipTo(MachineBasicBlock::iterator I) {
+    if (I == MachineBasicBlock::iterator(nullptr))
+      Tracking = false;
+    MBBI = I;
+  }
+
+  MachineBasicBlock::iterator getCurrentPosition() const { return MBBI; }
+
+  /// Return if a specific register is currently used.
+  bool isRegUsed(unsigned Reg, bool includeReserved = true) const;
+
+  /// Return all available registers in the register class in Mask.
+  BitVector getRegsAvailable(const TargetRegisterClass *RC);
+
+  /// Find an unused register of the specified register class.
+  /// Return 0 if none is found.
+  unsigned FindUnusedReg(const TargetRegisterClass *RC) const;
+
+  /// Add a scavenging frame index.
+  void addScavengingFrameIndex(int FI) {
+    Scavenged.push_back(ScavengedInfo(FI));
+  }
+
+  /// Query whether a frame index is a scavenging frame index.
+  bool isScavengingFrameIndex(int FI) const {
+    for (SmallVectorImpl<ScavengedInfo>::const_iterator I = Scavenged.begin(),
+         IE = Scavenged.end(); I != IE; ++I)
+      if (I->FrameIndex == FI)
+        return true;
+
+    return false;
+  }
+
+  /// Get an array of scavenging frame indices.
+  void getScavengingFrameIndices(SmallVectorImpl<int> &A) const {
+    for (SmallVectorImpl<ScavengedInfo>::const_iterator I = Scavenged.begin(),
+         IE = Scavenged.end(); I != IE; ++I)
+      if (I->FrameIndex >= 0)
+        A.push_back(I->FrameIndex);
+  }
+
+  /// Make a register of the specific register class
+  /// available and do the appropriate bookkeeping. SPAdj is the stack
+  /// adjustment due to call frame, it's passed along to eliminateFrameIndex().
+  /// Returns the scavenged register.
+  /// This is deprecated as it depends on the quality of the kill flags being
+  /// present; Use scavengeRegisterBackwards() instead!
+  unsigned scavengeRegister(const TargetRegisterClass *RC,
+                            MachineBasicBlock::iterator I, int SPAdj);
+  unsigned scavengeRegister(const TargetRegisterClass *RegClass, int SPAdj) {
+    return scavengeRegister(RegClass, MBBI, SPAdj);
+  }
+
+  /// Make a register of the specific register class available from the current
+  /// position backwards to the place before \p To. If \p RestoreAfter is true
+  /// this includes the instruction following the current position.
+  /// SPAdj is the stack adjustment due to call frame, it's passed along to
+  /// eliminateFrameIndex().
+  /// Returns the scavenged register.
+  unsigned scavengeRegisterBackwards(const TargetRegisterClass &RC,
+                                     MachineBasicBlock::iterator To,
+                                     bool RestoreAfter, int SPAdj);
+
+  /// Tell the scavenger a register is used.
+  void setRegUsed(unsigned Reg, LaneBitmask LaneMask = LaneBitmask::getAll());
+
+private:
+  /// Returns true if a register is reserved. It is never "unused".
+  bool isReserved(unsigned Reg) const { return MRI->isReserved(Reg); }
+
+  /// setUsed / setUnused - Mark the state of one or a number of register units.
+  ///
+  void setUsed(const BitVector &RegUnits) {
+    LiveUnits.addUnits(RegUnits);
+  }
+  void setUnused(const BitVector &RegUnits) {
+    LiveUnits.removeUnits(RegUnits);
+  }
+
+  /// Processes the current instruction and fill the KillRegUnits and
+  /// DefRegUnits bit vectors.
+  void determineKillsAndDefs();
+
+  /// Add all Reg Units that Reg contains to BV.
+  void addRegUnits(BitVector &BV, unsigned Reg);
+
+  /// Remove all Reg Units that \p Reg contains from \p BV.
+  void removeRegUnits(BitVector &BV, unsigned Reg);
+
+  /// Return the candidate register that is unused for the longest after
+  /// StartMI. UseMI is set to the instruction where the search stopped.
+  ///
+  /// No more than InstrLimit instructions are inspected.
+  unsigned findSurvivorReg(MachineBasicBlock::iterator StartMI,
+                           BitVector &Candidates,
+                           unsigned InstrLimit,
+                           MachineBasicBlock::iterator &UseMI);
+
+  /// Initialize RegisterScavenger.
+  void init(MachineBasicBlock &MBB);
+
+  /// Mark live-in registers of basic block as used.
+  void setLiveInsUsed(const MachineBasicBlock &MBB);
+
+  /// Spill a register after position \p After and reload it before position
+  /// \p UseMI.
+  ScavengedInfo &spill(unsigned Reg, const TargetRegisterClass &RC, int SPAdj,
+                       MachineBasicBlock::iterator Before,
+                       MachineBasicBlock::iterator &UseMI);
+};
+
+/// Replaces all frame index virtual registers with physical registers. Uses the
+/// register scavenger to find an appropriate register to use.
+void scavengeFrameVirtualRegs(MachineFunction &MF, RegScavenger &RS);
+
+} // end namespace llvm
+
+#endif // LLVM_CODEGEN_REGISTERSCAVENGING_H