LiveRangeEdit.h

//===- LiveRangeEdit.h - Basic tools for split and spill --------*- 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
//
//===----------------------------------------------------------------------===//
//
// The LiveRangeEdit class represents changes done to a virtual register when it
// is spilled or split.
//
// The parent register is never changed. Instead, a number of new virtual
// registers are created and added to the newRegs vector.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_CODEGEN_LIVERANGEEDIT_H
#define LLVM_CODEGEN_LIVERANGEEDIT_H

#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/None.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include <cassert>

namespace llvm {

class LiveIntervals;
class MachineInstr;
class MachineOperand;
class TargetInstrInfo;
class TargetRegisterInfo;
class VirtRegMap;
class VirtRegAuxInfo;

class LiveRangeEdit : private MachineRegisterInfo::Delegate {
public:
  /// Callback methods for LiveRangeEdit owners.
  class Delegate {
    virtual void anchor();

  public:
    virtual ~Delegate() = default;

    /// Called immediately before erasing a dead machine instruction.
    virtual void LRE_WillEraseInstruction(MachineInstr *MI) {}

    /// Called when a virtual register is no longer used. Return false to defer
    /// its deletion from LiveIntervals.
    virtual bool LRE_CanEraseVirtReg(Register) { return true; }

    /// Called before shrinking the live range of a virtual register.
    virtual void LRE_WillShrinkVirtReg(Register) {}

    /// Called after cloning a virtual register.
    /// This is used for new registers representing connected components of Old.
    virtual void LRE_DidCloneVirtReg(Register New, Register Old) {}
  };

private:
  const LiveInterval *const Parent;
  SmallVectorImpl<Register> &NewRegs;
  MachineRegisterInfo &MRI;
  LiveIntervals &LIS;
  VirtRegMap *VRM;
  const TargetInstrInfo &TII;
  Delegate *const TheDelegate;

  /// FirstNew - Index of the first register added to NewRegs.
  const unsigned FirstNew;

  /// ScannedRemattable - true when remattable values have been identified.
  bool ScannedRemattable = false;

  /// DeadRemats - The saved instructions which have already been dead after
  /// rematerialization but not deleted yet -- to be done in postOptimization.
  SmallPtrSet<MachineInstr *, 32> *DeadRemats;

  /// Remattable - Values defined by remattable instructions as identified by
  /// tii.isTriviallyReMaterializable().
  SmallPtrSet<const VNInfo *, 4> Remattable;

  /// Rematted - Values that were actually rematted, and so need to have their
  /// live range trimmed or entirely removed.
  SmallPtrSet<const VNInfo *, 4> Rematted;

  /// scanRemattable - Identify the Parent values that may rematerialize.
  void scanRemattable();

  /// foldAsLoad - If LI has a single use and a single def that can be folded as
  /// a load, eliminate the register by folding the def into the use.
  bool foldAsLoad(LiveInterval *LI, SmallVectorImpl<MachineInstr *> &Dead);

  using ToShrinkSet = SetVector<LiveInterval *, SmallVector<LiveInterval *, 8>,
                                SmallPtrSet<LiveInterval *, 8>>;

  /// Helper for eliminateDeadDefs.
  void eliminateDeadDef(MachineInstr *MI, ToShrinkSet &ToShrink);

  /// MachineRegisterInfo callback to notify when new virtual
  /// registers are created.
  void MRI_NoteNewVirtualRegister(Register VReg) override;

  /// Check if MachineOperand \p MO is a last use/kill either in the
  /// main live range of \p LI or in one of the matching subregister ranges.
  bool useIsKill(const LiveInterval &LI, const MachineOperand &MO) const;

  /// Create a new empty interval based on OldReg.
  LiveInterval &createEmptyIntervalFrom(Register OldReg, bool createSubRanges);

public:
  /// Create a LiveRangeEdit for breaking down parent into smaller pieces.
  /// @param parent The register being spilled or split.
  /// @param newRegs List to receive any new registers created. This needn't be
  ///                empty initially, any existing registers are ignored.
  /// @param MF The MachineFunction the live range edit is taking place in.
  /// @param lis The collection of all live intervals in this function.
  /// @param vrm Map of virtual registers to physical registers for this
  ///            function.  If NULL, no virtual register map updates will
  ///            be done.  This could be the case if called before Regalloc.
  /// @param deadRemats The collection of all the instructions defining an
  ///                   original reg and are dead after remat.
  LiveRangeEdit(const LiveInterval *parent, SmallVectorImpl<Register> &newRegs,
                MachineFunction &MF, LiveIntervals &lis, VirtRegMap *vrm,
                Delegate *delegate = nullptr,
                SmallPtrSet<MachineInstr *, 32> *deadRemats = nullptr)
      : Parent(parent), NewRegs(newRegs), MRI(MF.getRegInfo()), LIS(lis),
        VRM(vrm), TII(*MF.getSubtarget().getInstrInfo()), TheDelegate(delegate),
        FirstNew(newRegs.size()), DeadRemats(deadRemats) {
    MRI.setDelegate(this);
  }

  ~LiveRangeEdit() override { MRI.resetDelegate(this); }

  const LiveInterval &getParent() const {
    assert(Parent && "No parent LiveInterval");
    return *Parent;
  }

  Register getReg() const { return getParent().reg(); }

  /// Iterator for accessing the new registers added by this edit.
  using iterator = SmallVectorImpl<Register>::const_iterator;
  iterator begin() const { return NewRegs.begin() + FirstNew; }
  iterator end() const { return NewRegs.end(); }
  unsigned size() const { return NewRegs.size() - FirstNew; }
  bool empty() const { return size() == 0; }
  Register get(unsigned idx) const { return NewRegs[idx + FirstNew]; }

  /// pop_back - It allows LiveRangeEdit users to drop new registers.
  /// The context is when an original def instruction of a register is
  /// dead after rematerialization, we still want to keep it for following
  /// rematerializations. We save the def instruction in DeadRemats,
  /// and replace the original dst register with a new dummy register so
  /// the live range of original dst register can be shrinked normally.
  /// We don't want to allocate phys register for the dummy register, so
  /// we want to drop it from the NewRegs set.
  void pop_back() { NewRegs.pop_back(); }

  ArrayRef<Register> regs() const {
    return makeArrayRef(NewRegs).slice(FirstNew);
  }

  /// createFrom - Create a new virtual register based on OldReg.
  Register createFrom(Register OldReg);

  /// create - Create a new register with the same class and original slot as
  /// parent.
  LiveInterval &createEmptyInterval() {
    return createEmptyIntervalFrom(getReg(), true);
  }

  Register create() { return createFrom(getReg()); }

  /// anyRematerializable - Return true if any parent values may be
  /// rematerializable.
  /// This function must be called before any rematerialization is attempted.
  bool anyRematerializable();

  /// checkRematerializable - Manually add VNI to the list of rematerializable
  /// values if DefMI may be rematerializable.
  bool checkRematerializable(VNInfo *VNI, const MachineInstr *DefMI);

  /// Remat - Information needed to rematerialize at a specific location.
  struct Remat {
    const VNInfo *const ParentVNI;  // parent_'s value at the remat location.
    MachineInstr *OrigMI = nullptr; // Instruction defining OrigVNI. It contains
                                    // the real expr for remat.

    explicit Remat(const VNInfo *ParentVNI) : ParentVNI(ParentVNI) {}
  };

  /// allUsesAvailableAt - Return true if all registers used by OrigMI at
  /// OrigIdx are also available with the same value at UseIdx.
  bool allUsesAvailableAt(const MachineInstr *OrigMI, SlotIndex OrigIdx,
                          SlotIndex UseIdx) const;

  /// canRematerializeAt - Determine if ParentVNI can be rematerialized at
  /// UseIdx. It is assumed that parent_.getVNINfoAt(UseIdx) == ParentVNI.
  /// When cheapAsAMove is set, only cheap remats are allowed.
  bool canRematerializeAt(Remat &RM, VNInfo *OrigVNI, SlotIndex UseIdx,
                          bool cheapAsAMove);

  /// rematerializeAt - Rematerialize RM.ParentVNI into DestReg by inserting an
  /// instruction into MBB before MI. The new instruction is mapped, but
  /// liveness is not updated.
  /// Return the SlotIndex of the new instruction.
  SlotIndex rematerializeAt(MachineBasicBlock &MBB,
                            MachineBasicBlock::iterator MI, unsigned DestReg,
                            const Remat &RM, const TargetRegisterInfo &,
                            bool Late = false);

  /// markRematerialized - explicitly mark a value as rematerialized after doing
  /// it manually.
  void markRematerialized(const VNInfo *ParentVNI) {
    Rematted.insert(ParentVNI);
  }

  /// didRematerialize - Return true if ParentVNI was rematerialized anywhere.
  bool didRematerialize(const VNInfo *ParentVNI) const {
    return Rematted.count(ParentVNI);
  }

  /// eraseVirtReg - Notify the delegate that Reg is no longer in use, and try
  /// to erase it from LIS.
  void eraseVirtReg(Register Reg);

  /// eliminateDeadDefs - Try to delete machine instructions that are now dead
  /// (allDefsAreDead returns true). This may cause live intervals to be trimmed
  /// and further dead efs to be eliminated.
  /// RegsBeingSpilled lists registers currently being spilled by the register
  /// allocator.  These registers should not be split into new intervals
  /// as currently those new intervals are not guaranteed to spill.
  void eliminateDeadDefs(SmallVectorImpl<MachineInstr *> &Dead,
                         ArrayRef<Register> RegsBeingSpilled = None);

  /// calculateRegClassAndHint - Recompute register class and hint for each new
  /// register.
  void calculateRegClassAndHint(MachineFunction &, VirtRegAuxInfo &);
};

} // end namespace llvm

#endif // LLVM_CODEGEN_LIVERANGEEDIT_H