llvm/lib/CodeGen/DetectDeadLanes.cpp - toolchain/llvm-project - Gitiles

 //===- DetectDeadLanes.cpp - SubRegister Lane Usage Analysis --*- 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
 /// Analysis that tracks defined/used subregister lanes across COPY instructions
 /// and instructions that get lowered to a COPY (PHI, REG_SEQUENCE,
 /// INSERT_SUBREG, EXTRACT_SUBREG).
 /// The information is used to detect dead definitions and the usage of
 /// (completely) undefined values and mark the operands as such.
 /// This pass is necessary because the dead/undef status is not obvious anymore
 /// when subregisters are involved.
 ///
 /// Example:
 ///    %0 = some definition
 ///    %1 = IMPLICIT_DEF
 ///    %2 = REG_SEQUENCE %0, sub0, %1, sub1
 ///    %3 = EXTRACT_SUBREG %2, sub1
 ///       = use %3
 /// The %0 definition is dead and %3 contains an undefined value.
 //
 //===----------------------------------------------------------------------===//

 #include <deque>
 #include <vector>

 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Pass.h"
 #include "llvm/PassRegistry.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"

 using namespace llvm;

 #define DEBUG_TYPE "detect-dead-lanes"

 namespace {

 /// Contains a bitmask of which lanes of a given virtual register are
 /// defined and which ones are actually used.
 struct VRegInfo {
   LaneBitmask UsedLanes;
   LaneBitmask DefinedLanes;
 };

 class DetectDeadLanes : public MachineFunctionPass {
 public:
   bool runOnMachineFunction(MachineFunction &MF) override;

   static char ID;
   DetectDeadLanes() : MachineFunctionPass(ID) {}

   StringRef getPassName() const override { return "Detect Dead Lanes"; }

   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
     MachineFunctionPass::getAnalysisUsage(AU);
   }

 private:
   /// Add used lane bits on the register used by operand \p MO. This translates
   /// the bitmask based on the operands subregister, and puts the register into
   /// the worklist if any new bits were added.
   void addUsedLanesOnOperand(const MachineOperand &MO, LaneBitmask UsedLanes);

   /// Given a bitmask \p UsedLanes for the used lanes on a def output of a
   /// COPY-like instruction determine the lanes used on the use operands
   /// and call addUsedLanesOnOperand() for them.
   void transferUsedLanesStep(const MachineInstr &MI, LaneBitmask UsedLanes);

   /// Given a use regiser operand \p Use and a mask of defined lanes, check
   /// if the operand belongs to a lowersToCopies() instruction, transfer the
   /// mask to the def and put the instruction into the worklist.
   void transferDefinedLanesStep(const MachineOperand &Use,
                                 LaneBitmask DefinedLanes);

   /// Given a mask \p DefinedLanes of lanes defined at operand \p OpNum
   /// of COPY-like instruction, determine which lanes are defined at the output
   /// operand \p Def.
   LaneBitmask transferDefinedLanes(const MachineOperand &Def, unsigned OpNum,
                                    LaneBitmask DefinedLanes) const;

   /// Given a mask \p UsedLanes used from the output of instruction \p MI
   /// determine which lanes are used from operand \p MO of this instruction.
   LaneBitmask transferUsedLanes(const MachineInstr &MI, LaneBitmask UsedLanes,
                                 const MachineOperand &MO) const;

   bool runOnce(MachineFunction &MF);

   LaneBitmask determineInitialDefinedLanes(unsigned Reg);
   LaneBitmask determineInitialUsedLanes(unsigned Reg);

   bool isUndefRegAtInput(const MachineOperand &MO,
                          const VRegInfo &RegInfo) const;

   bool isUndefInput(const MachineOperand &MO, bool *CrossCopy) const;

   const MachineRegisterInfo *MRI;
   const TargetRegisterInfo *TRI;

   void PutInWorklist(unsigned RegIdx) {
     if (WorklistMembers.test(RegIdx))
       return;
     WorklistMembers.set(RegIdx);
     Worklist.push_back(RegIdx);
   }

   VRegInfo *VRegInfos;
   /// Worklist containing virtreg indexes.
   std::deque<unsigned> Worklist;
   BitVector WorklistMembers;
   /// This bitvector is set for each vreg index where the vreg is defined
   /// by an instruction where lowersToCopies()==true.
   BitVector DefinedByCopy;
 };

 } // end anonymous namespace

 char DetectDeadLanes::ID = 0;
 char &llvm::DetectDeadLanesID = DetectDeadLanes::ID;

 INITIALIZE_PASS(DetectDeadLanes, DEBUG_TYPE, "Detect Dead Lanes", false, false)

 /// Returns true if \p MI will get lowered to a series of COPY instructions.
 /// We call this a COPY-like instruction.
 static bool lowersToCopies(const MachineInstr &MI) {
   // Note: We could support instructions with MCInstrDesc::isRegSequenceLike(),
   // isExtractSubRegLike(), isInsertSubregLike() in the future even though they
   // are not lowered to a COPY.
   switch (MI.getOpcode()) {
   case TargetOpcode::COPY:
   case TargetOpcode::PHI:
   case TargetOpcode::INSERT_SUBREG:
   case TargetOpcode::REG_SEQUENCE:
   case TargetOpcode::EXTRACT_SUBREG:
     return true;
   }
   return false;
 }

 static bool isCrossCopy(const MachineRegisterInfo &MRI,
                         const MachineInstr &MI,
                         const TargetRegisterClass *DstRC,
                         const MachineOperand &MO) {
   assert(lowersToCopies(MI));
   unsigned SrcReg = MO.getReg();
   const TargetRegisterClass *SrcRC = MRI.getRegClass(SrcReg);
   if (DstRC == SrcRC)
     return false;

   unsigned SrcSubIdx = MO.getSubReg();

   const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
   unsigned DstSubIdx = 0;
   switch (MI.getOpcode()) {
   case TargetOpcode::INSERT_SUBREG:
     if (MI.getOperandNo(&MO) == 2)
       DstSubIdx = MI.getOperand(3).getImm();
     break;
   case TargetOpcode::REG_SEQUENCE: {
     unsigned OpNum = MI.getOperandNo(&MO);
     DstSubIdx = MI.getOperand(OpNum+1).getImm();
     break;
   }
   case TargetOpcode::EXTRACT_SUBREG: {
     unsigned SubReg = MI.getOperand(2).getImm();
     SrcSubIdx = TRI.composeSubRegIndices(SubReg, SrcSubIdx);
   }
   }

   unsigned PreA, PreB; // Unused.
   if (SrcSubIdx && DstSubIdx)
     return !TRI.getCommonSuperRegClass(SrcRC, SrcSubIdx, DstRC, DstSubIdx, PreA,
                                        PreB);
   if (SrcSubIdx)
     return !TRI.getMatchingSuperRegClass(SrcRC, DstRC, SrcSubIdx);
   if (DstSubIdx)
     return !TRI.getMatchingSuperRegClass(DstRC, SrcRC, DstSubIdx);
   return !TRI.getCommonSubClass(SrcRC, DstRC);
 }

 void DetectDeadLanes::addUsedLanesOnOperand(const MachineOperand &MO,
                                             LaneBitmask UsedLanes) {
   if (!MO.readsReg())
     return;
   unsigned MOReg = MO.getReg();
   if (!TargetRegisterInfo::isVirtualRegister(MOReg))
     return;

   unsigned MOSubReg = MO.getSubReg();
   if (MOSubReg != 0)
     UsedLanes = TRI->composeSubRegIndexLaneMask(MOSubReg, UsedLanes);
   UsedLanes &= MRI->getMaxLaneMaskForVReg(MOReg);

   unsigned MORegIdx = TargetRegisterInfo::virtReg2Index(MOReg);
   VRegInfo &MORegInfo = VRegInfos[MORegIdx];
   LaneBitmask PrevUsedLanes = MORegInfo.UsedLanes;
   // Any change at all?
   if ((UsedLanes & ~PrevUsedLanes).none())
     return;

   // Set UsedLanes and remember instruction for further propagation.
   MORegInfo.UsedLanes = PrevUsedLanes | UsedLanes;
   if (DefinedByCopy.test(MORegIdx))
     PutInWorklist(MORegIdx);
 }

 void DetectDeadLanes::transferUsedLanesStep(const MachineInstr &MI,
                                             LaneBitmask UsedLanes) {
   for (const MachineOperand &MO : MI.uses()) {
     if (!MO.isReg() || !TargetRegisterInfo::isVirtualRegister(MO.getReg()))
       continue;
     LaneBitmask UsedOnMO = transferUsedLanes(MI, UsedLanes, MO);
     addUsedLanesOnOperand(MO, UsedOnMO);
   }
 }

 LaneBitmask DetectDeadLanes::transferUsedLanes(const MachineInstr &MI,
                                                LaneBitmask UsedLanes,
                                                const MachineOperand &MO) const {
   unsigned OpNum = MI.getOperandNo(&MO);
   assert(lowersToCopies(MI) && DefinedByCopy[
            TargetRegisterInfo::virtReg2Index(MI.getOperand(0).getReg())]);

   switch (MI.getOpcode()) {
   case TargetOpcode::COPY:
   case TargetOpcode::PHI:
     return UsedLanes;
   case TargetOpcode::REG_SEQUENCE: {
     assert(OpNum % 2 == 1);
     unsigned SubIdx = MI.getOperand(OpNum + 1).getImm();
     return TRI->reverseComposeSubRegIndexLaneMask(SubIdx, UsedLanes);
   }
   case TargetOpcode::INSERT_SUBREG: {
     unsigned SubIdx = MI.getOperand(3).getImm();
     LaneBitmask MO2UsedLanes =
         TRI->reverseComposeSubRegIndexLaneMask(SubIdx, UsedLanes);
     if (OpNum == 2)
       return MO2UsedLanes;

     const MachineOperand &Def = MI.getOperand(0);
     unsigned DefReg = Def.getReg();
     const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
     LaneBitmask MO1UsedLanes;
     if (RC->CoveredBySubRegs)
       MO1UsedLanes = UsedLanes & ~TRI->getSubRegIndexLaneMask(SubIdx);
     else
       MO1UsedLanes = RC->LaneMask;

     assert(OpNum == 1);
     return MO1UsedLanes;
   }
   case TargetOpcode::EXTRACT_SUBREG: {
     assert(OpNum == 1);
     unsigned SubIdx = MI.getOperand(2).getImm();
     return TRI->composeSubRegIndexLaneMask(SubIdx, UsedLanes);
   }
   default:
     llvm_unreachable("function must be called with COPY-like instruction");
   }
 }

 void DetectDeadLanes::transferDefinedLanesStep(const MachineOperand &Use,
                                                LaneBitmask DefinedLanes) {
   if (!Use.readsReg())
     return;
   // Check whether the operand writes a vreg and is part of a COPY-like
   // instruction.
   const MachineInstr &MI = *Use.getParent();
   if (MI.getDesc().getNumDefs() != 1)
     return;
   // FIXME: PATCHPOINT instructions announce a Def that does not always exist,
   // they really need to be modeled differently!
   if (MI.getOpcode() == TargetOpcode::PATCHPOINT)
     return;
   const MachineOperand &Def = *MI.defs().begin();
   unsigned DefReg = Def.getReg();
   if (!TargetRegisterInfo::isVirtualRegister(DefReg))
     return;
   unsigned DefRegIdx = TargetRegisterInfo::virtReg2Index(DefReg);
   if (!DefinedByCopy.test(DefRegIdx))
     return;

   unsigned OpNum = MI.getOperandNo(&Use);
   DefinedLanes =
       TRI->reverseComposeSubRegIndexLaneMask(Use.getSubReg(), DefinedLanes);
   DefinedLanes = transferDefinedLanes(Def, OpNum, DefinedLanes);

   VRegInfo &RegInfo = VRegInfos[DefRegIdx];
   LaneBitmask PrevDefinedLanes = RegInfo.DefinedLanes;
   // Any change at all?
   if ((DefinedLanes & ~PrevDefinedLanes).none())
     return;

   RegInfo.DefinedLanes = PrevDefinedLanes | DefinedLanes;
   PutInWorklist(DefRegIdx);
 }

 LaneBitmask DetectDeadLanes::transferDefinedLanes(const MachineOperand &Def,
     unsigned OpNum, LaneBitmask DefinedLanes) const {
   const MachineInstr &MI = *Def.getParent();
   // Translate DefinedLanes if necessary.
   switch (MI.getOpcode()) {
   case TargetOpcode::REG_SEQUENCE: {
     unsigned SubIdx = MI.getOperand(OpNum + 1).getImm();
     DefinedLanes = TRI->composeSubRegIndexLaneMask(SubIdx, DefinedLanes);
     DefinedLanes &= TRI->getSubRegIndexLaneMask(SubIdx);
     break;
   }
   case TargetOpcode::INSERT_SUBREG: {
     unsigned SubIdx = MI.getOperand(3).getImm();
     if (OpNum == 2) {
       DefinedLanes = TRI->composeSubRegIndexLaneMask(SubIdx, DefinedLanes);
       DefinedLanes &= TRI->getSubRegIndexLaneMask(SubIdx);
     } else {
       assert(OpNum == 1 && "INSERT_SUBREG must have two operands");
       // Ignore lanes defined by operand 2.
       DefinedLanes &= ~TRI->getSubRegIndexLaneMask(SubIdx);
     }
     break;
   }
   case TargetOpcode::EXTRACT_SUBREG: {
     unsigned SubIdx = MI.getOperand(2).getImm();
     assert(OpNum == 1 && "EXTRACT_SUBREG must have one register operand only");
     DefinedLanes = TRI->reverseComposeSubRegIndexLaneMask(SubIdx, DefinedLanes);
     break;
   }
   case TargetOpcode::COPY:
   case TargetOpcode::PHI:
     break;
   default:
     llvm_unreachable("function must be called with COPY-like instruction");
   }

   assert(Def.getSubReg() == 0 &&
          "Should not have subregister defs in machine SSA phase");
   DefinedLanes &= MRI->getMaxLaneMaskForVReg(Def.getReg());
   return DefinedLanes;
 }

 LaneBitmask DetectDeadLanes::determineInitialDefinedLanes(unsigned Reg) {
   // Live-In or unused registers have no definition but are considered fully
   // defined.
   if (!MRI->hasOneDef(Reg))
     return LaneBitmask::getAll();

   const MachineOperand &Def = *MRI->def_begin(Reg);
   const MachineInstr &DefMI = *Def.getParent();
   if (lowersToCopies(DefMI)) {
     // Start optimisatically with no used or defined lanes for copy
     // instructions. The following dataflow analysis will add more bits.
     unsigned RegIdx = TargetRegisterInfo::virtReg2Index(Reg);
     DefinedByCopy.set(RegIdx);
     PutInWorklist(RegIdx);

     if (Def.isDead())
       return LaneBitmask::getNone();

     // COPY/PHI can copy across unrelated register classes (example: float/int)
     // with incompatible subregister structure. Do not include these in the
     // dataflow analysis since we cannot transfer lanemasks in a meaningful way.
     const TargetRegisterClass *DefRC = MRI->getRegClass(Reg);

     // Determine initially DefinedLanes.
     LaneBitmask DefinedLanes;
     for (const MachineOperand &MO : DefMI.uses()) {
       if (!MO.isReg() || !MO.readsReg())
         continue;
       unsigned MOReg = MO.getReg();
       if (!MOReg)
         continue;

       LaneBitmask MODefinedLanes;
       if (TargetRegisterInfo::isPhysicalRegister(MOReg)) {
         MODefinedLanes = LaneBitmask::getAll();
       } else if (isCrossCopy(*MRI, DefMI, DefRC, MO)) {
         MODefinedLanes = LaneBitmask::getAll();
       } else {
         assert(TargetRegisterInfo::isVirtualRegister(MOReg));
         if (MRI->hasOneDef(MOReg)) {
           const MachineOperand &MODef = *MRI->def_begin(MOReg);
           const MachineInstr &MODefMI = *MODef.getParent();
           // Bits from copy-like operations will be added later.
           if (lowersToCopies(MODefMI) || MODefMI.isImplicitDef())
             continue;
         }
         unsigned MOSubReg = MO.getSubReg();
         MODefinedLanes = MRI->getMaxLaneMaskForVReg(MOReg);
         MODefinedLanes = TRI->reverseComposeSubRegIndexLaneMask(
             MOSubReg, MODefinedLanes);
       }

       unsigned OpNum = DefMI.getOperandNo(&MO);
       DefinedLanes |= transferDefinedLanes(Def, OpNum, MODefinedLanes);
     }
     return DefinedLanes;
   }
   if (DefMI.isImplicitDef() || Def.isDead())
     return LaneBitmask::getNone();

   assert(Def.getSubReg() == 0 &&
          "Should not have subregister defs in machine SSA phase");
   return MRI->getMaxLaneMaskForVReg(Reg);
 }

 LaneBitmask DetectDeadLanes::determineInitialUsedLanes(unsigned Reg) {
   LaneBitmask UsedLanes = LaneBitmask::getNone();
   for (const MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
     if (!MO.readsReg())
       continue;

     const MachineInstr &UseMI = *MO.getParent();
     if (UseMI.isKill())
       continue;

     unsigned SubReg = MO.getSubReg();
     if (lowersToCopies(UseMI)) {
       assert(UseMI.getDesc().getNumDefs() == 1);
       const MachineOperand &Def = *UseMI.defs().begin();
       unsigned DefReg = Def.getReg();
       // The used lanes of COPY-like instruction operands are determined by the
       // following dataflow analysis.
       if (TargetRegisterInfo::isVirtualRegister(DefReg)) {
         // But ignore copies across incompatible register classes.
         bool CrossCopy = false;
         if (lowersToCopies(UseMI)) {
           const TargetRegisterClass *DstRC = MRI->getRegClass(DefReg);
           CrossCopy = isCrossCopy(*MRI, UseMI, DstRC, MO);
           if (CrossCopy)
             LLVM_DEBUG(dbgs() << "Copy across incompatible classes: " << UseMI);
         }

         if (!CrossCopy)
           continue;
       }
     }

     // Shortcut: All lanes are used.
     if (SubReg == 0)
       return MRI->getMaxLaneMaskForVReg(Reg);

     UsedLanes |= TRI->getSubRegIndexLaneMask(SubReg);
   }
   return UsedLanes;
 }

 bool DetectDeadLanes::isUndefRegAtInput(const MachineOperand &MO,
                                         const VRegInfo &RegInfo) const {
   unsigned SubReg = MO.getSubReg();
   LaneBitmask Mask = TRI->getSubRegIndexLaneMask(SubReg);
   return (RegInfo.DefinedLanes & RegInfo.UsedLanes & Mask).none();
 }

 bool DetectDeadLanes::isUndefInput(const MachineOperand &MO,
                                    bool *CrossCopy) const {
   if (!MO.isUse())
     return false;
   const MachineInstr &MI = *MO.getParent();
   if (!lowersToCopies(MI))
     return false;
   const MachineOperand &Def = MI.getOperand(0);
   unsigned DefReg = Def.getReg();
   if (!TargetRegisterInfo::isVirtualRegister(DefReg))
     return false;
   unsigned DefRegIdx = TargetRegisterInfo::virtReg2Index(DefReg);
   if (!DefinedByCopy.test(DefRegIdx))
     return false;

   const VRegInfo &DefRegInfo = VRegInfos[DefRegIdx];
   LaneBitmask UsedLanes = transferUsedLanes(MI, DefRegInfo.UsedLanes, MO);
   if (UsedLanes.any())
     return false;

   unsigned MOReg = MO.getReg();
   if (TargetRegisterInfo::isVirtualRegister(MOReg)) {
     const TargetRegisterClass *DstRC = MRI->getRegClass(DefReg);
     *CrossCopy = isCrossCopy(*MRI, MI, DstRC, MO);
   }
   return true;
 }

 bool DetectDeadLanes::runOnce(MachineFunction &MF) {
   // First pass: Populate defs/uses of vregs with initial values
   unsigned NumVirtRegs = MRI->getNumVirtRegs();
   for (unsigned RegIdx = 0; RegIdx < NumVirtRegs; ++RegIdx) {
     unsigned Reg = TargetRegisterInfo::index2VirtReg(RegIdx);

     // Determine used/defined lanes and add copy instructions to worklist.
     VRegInfo &Info = VRegInfos[RegIdx];
     Info.DefinedLanes = determineInitialDefinedLanes(Reg);
     Info.UsedLanes = determineInitialUsedLanes(Reg);
   }

   // Iterate as long as defined lanes/used lanes keep changing.
   while (!Worklist.empty()) {
     unsigned RegIdx = Worklist.front();
     Worklist.pop_front();
     WorklistMembers.reset(RegIdx);
     VRegInfo &Info = VRegInfos[RegIdx];
     unsigned Reg = TargetRegisterInfo::index2VirtReg(RegIdx);

     // Transfer UsedLanes to operands of DefMI (backwards dataflow).
     MachineOperand &Def = *MRI->def_begin(Reg);
     const MachineInstr &MI = *Def.getParent();
     transferUsedLanesStep(MI, Info.UsedLanes);
     // Transfer DefinedLanes to users of Reg (forward dataflow).
     for (const MachineOperand &MO : MRI->use_nodbg_operands(Reg))
       transferDefinedLanesStep(MO, Info.DefinedLanes);
   }

   LLVM_DEBUG(dbgs() << "Defined/Used lanes:\n"; for (unsigned RegIdx = 0;
                                                      RegIdx < NumVirtRegs;
                                                      ++RegIdx) {
     unsigned Reg = TargetRegisterInfo::index2VirtReg(RegIdx);
     const VRegInfo &Info = VRegInfos[RegIdx];
     dbgs() << printReg(Reg, nullptr)
            << " Used: " << PrintLaneMask(Info.UsedLanes)
            << " Def: " << PrintLaneMask(Info.DefinedLanes) << '\n';
   } dbgs() << "\n";);

   bool Again = false;
   // Mark operands as dead/unused.
   for (MachineBasicBlock &MBB : MF) {
     for (MachineInstr &MI : MBB) {
       for (MachineOperand &MO : MI.operands()) {
         if (!MO.isReg())
           continue;
         unsigned Reg = MO.getReg();
         if (!TargetRegisterInfo::isVirtualRegister(Reg))
           continue;
         unsigned RegIdx = TargetRegisterInfo::virtReg2Index(Reg);
         const VRegInfo &RegInfo = VRegInfos[RegIdx];
         if (MO.isDef() && !MO.isDead() && RegInfo.UsedLanes.none()) {
           LLVM_DEBUG(dbgs()
                      << "Marking operand '" << MO << "' as dead in " << MI);
           MO.setIsDead();
         }
         if (MO.readsReg()) {
           bool CrossCopy = false;
           if (isUndefRegAtInput(MO, RegInfo)) {
             LLVM_DEBUG(dbgs()
                        << "Marking operand '" << MO << "' as undef in " << MI);
             MO.setIsUndef();
           } else if (isUndefInput(MO, &CrossCopy)) {
             LLVM_DEBUG(dbgs()
                        << "Marking operand '" << MO << "' as undef in " << MI);
             MO.setIsUndef();
             if (CrossCopy)
               Again = true;
           }
         }
       }
     }
   }

   return Again;
 }

 bool DetectDeadLanes::runOnMachineFunction(MachineFunction &MF) {
   // Don't bother if we won't track subregister liveness later.  This pass is
   // required for correctness if subregister liveness is enabled because the
   // register coalescer cannot deal with hidden dead defs. However without
   // subregister liveness enabled, the expected benefits of this pass are small
   // so we safe the compile time.
   MRI = &MF.getRegInfo();
   if (!MRI->subRegLivenessEnabled()) {
     LLVM_DEBUG(dbgs() << "Skipping Detect dead lanes pass\n");
     return false;
   }

   TRI = MRI->getTargetRegisterInfo();

   unsigned NumVirtRegs = MRI->getNumVirtRegs();
   VRegInfos = new VRegInfo[NumVirtRegs];
   WorklistMembers.resize(NumVirtRegs);
   DefinedByCopy.resize(NumVirtRegs);

   bool Again;
   do {
     Again = runOnce(MF);
   } while(Again);

   DefinedByCopy.clear();
   WorklistMembers.clear();
   delete[] VRegInfos;
   return true;
 }
	//===- DetectDeadLanes.cpp - SubRegister Lane Usage Analysis --- 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
	/// Analysis that tracks defined/used subregister lanes across COPY instructions
	/// and instructions that get lowered to a COPY (PHI, REG_SEQUENCE,
	/// INSERT_SUBREG, EXTRACT_SUBREG).
	/// The information is used to detect dead definitions and the usage of
	/// (completely) undefined values and mark the operands as such.
	/// This pass is necessary because the dead/undef status is not obvious anymore
	/// when subregisters are involved.
	///
	/// Example:
	/// %0 = some definition
	/// %1 = IMPLICIT_DEF
	/// %2 = REG_SEQUENCE %0, sub0, %1, sub1
	/// %3 = EXTRACT_SUBREG %2, sub1
	/// = use %3
	/// The %0 definition is dead and %3 contains an undefined value.
	//
	//===----------------------------------------------------------------------===//

	#include <deque>
	#include <vector>

	#include "llvm/ADT/BitVector.h"
	#include "llvm/ADT/SetVector.h"
	#include "llvm/CodeGen/MachineFunctionPass.h"
	#include "llvm/CodeGen/MachineRegisterInfo.h"
	#include "llvm/CodeGen/Passes.h"
	#include "llvm/CodeGen/TargetRegisterInfo.h"
	#include "llvm/CodeGen/TargetSubtargetInfo.h"
	#include "llvm/InitializePasses.h"
	#include "llvm/Pass.h"
	#include "llvm/PassRegistry.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/raw_ostream.h"

	using namespace llvm;

	#define DEBUG_TYPE "detect-dead-lanes"

	namespace {

	/// Contains a bitmask of which lanes of a given virtual register are
	/// defined and which ones are actually used.
	struct VRegInfo {
	LaneBitmask UsedLanes;
	LaneBitmask DefinedLanes;
	};

	class DetectDeadLanes : public MachineFunctionPass {
	public:
	bool runOnMachineFunction(MachineFunction &MF) override;

	static char ID;
	DetectDeadLanes() : MachineFunctionPass(ID) {}

	StringRef getPassName() const override { return "Detect Dead Lanes"; }

	void getAnalysisUsage(AnalysisUsage &AU) const override {
	AU.setPreservesCFG();
	MachineFunctionPass::getAnalysisUsage(AU);
	}

	private:
	/// Add used lane bits on the register used by operand \p MO. This translates
	/// the bitmask based on the operands subregister, and puts the register into
	/// the worklist if any new bits were added.
	void addUsedLanesOnOperand(const MachineOperand &MO, LaneBitmask UsedLanes);

	/// Given a bitmask \p UsedLanes for the used lanes on a def output of a
	/// COPY-like instruction determine the lanes used on the use operands
	/// and call addUsedLanesOnOperand() for them.
	void transferUsedLanesStep(const MachineInstr &MI, LaneBitmask UsedLanes);

	/// Given a use regiser operand \p Use and a mask of defined lanes, check
	/// if the operand belongs to a lowersToCopies() instruction, transfer the
	/// mask to the def and put the instruction into the worklist.
	void transferDefinedLanesStep(const MachineOperand &Use,
	LaneBitmask DefinedLanes);

	/// Given a mask \p DefinedLanes of lanes defined at operand \p OpNum
	/// of COPY-like instruction, determine which lanes are defined at the output
	/// operand \p Def.
	LaneBitmask transferDefinedLanes(const MachineOperand &Def, unsigned OpNum,
	LaneBitmask DefinedLanes) const;

	/// Given a mask \p UsedLanes used from the output of instruction \p MI
	/// determine which lanes are used from operand \p MO of this instruction.
	LaneBitmask transferUsedLanes(const MachineInstr &MI, LaneBitmask UsedLanes,
	const MachineOperand &MO) const;

	bool runOnce(MachineFunction &MF);

	LaneBitmask determineInitialDefinedLanes(unsigned Reg);
	LaneBitmask determineInitialUsedLanes(unsigned Reg);

	bool isUndefRegAtInput(const MachineOperand &MO,
	const VRegInfo &RegInfo) const;

	bool isUndefInput(const MachineOperand &MO, bool *CrossCopy) const;

	const MachineRegisterInfo *MRI;
	const TargetRegisterInfo *TRI;

	void PutInWorklist(unsigned RegIdx) {
	if (WorklistMembers.test(RegIdx))
	return;
	WorklistMembers.set(RegIdx);
	Worklist.push_back(RegIdx);
	}

	VRegInfo *VRegInfos;
	/// Worklist containing virtreg indexes.
	std::deque<unsigned> Worklist;
	BitVector WorklistMembers;
	/// This bitvector is set for each vreg index where the vreg is defined
	/// by an instruction where lowersToCopies()==true.
	BitVector DefinedByCopy;
	};

	} // end anonymous namespace

	char DetectDeadLanes::ID = 0;
	char &llvm::DetectDeadLanesID = DetectDeadLanes::ID;

	INITIALIZE_PASS(DetectDeadLanes, DEBUG_TYPE, "Detect Dead Lanes", false, false)

	/// Returns true if \p MI will get lowered to a series of COPY instructions.
	/// We call this a COPY-like instruction.
	static bool lowersToCopies(const MachineInstr &MI) {
	// Note: We could support instructions with MCInstrDesc::isRegSequenceLike(),
	// isExtractSubRegLike(), isInsertSubregLike() in the future even though they
	// are not lowered to a COPY.
	switch (MI.getOpcode()) {
	case TargetOpcode::COPY:
	case TargetOpcode::PHI:
	case TargetOpcode::INSERT_SUBREG:
	case TargetOpcode::REG_SEQUENCE:
	case TargetOpcode::EXTRACT_SUBREG:
	return true;
	}
	return false;
	}

	static bool isCrossCopy(const MachineRegisterInfo &MRI,
	const MachineInstr &MI,
	const TargetRegisterClass *DstRC,
	const MachineOperand &MO) {
	assert(lowersToCopies(MI));
	unsigned SrcReg = MO.getReg();
	const TargetRegisterClass *SrcRC = MRI.getRegClass(SrcReg);
	if (DstRC == SrcRC)
	return false;

	unsigned SrcSubIdx = MO.getSubReg();

	const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
	unsigned DstSubIdx = 0;
	switch (MI.getOpcode()) {
	case TargetOpcode::INSERT_SUBREG:
	if (MI.getOperandNo(&MO) == 2)
	DstSubIdx = MI.getOperand(3).getImm();
	break;
	case TargetOpcode::REG_SEQUENCE: {
	unsigned OpNum = MI.getOperandNo(&MO);
	DstSubIdx = MI.getOperand(OpNum+1).getImm();
	break;
	}
	case TargetOpcode::EXTRACT_SUBREG: {
	unsigned SubReg = MI.getOperand(2).getImm();
	SrcSubIdx = TRI.composeSubRegIndices(SubReg, SrcSubIdx);
	}
	}

	unsigned PreA, PreB; // Unused.
	if (SrcSubIdx && DstSubIdx)
	return !TRI.getCommonSuperRegClass(SrcRC, SrcSubIdx, DstRC, DstSubIdx, PreA,
	PreB);
	if (SrcSubIdx)
	return !TRI.getMatchingSuperRegClass(SrcRC, DstRC, SrcSubIdx);
	if (DstSubIdx)
	return !TRI.getMatchingSuperRegClass(DstRC, SrcRC, DstSubIdx);
	return !TRI.getCommonSubClass(SrcRC, DstRC);
	}

	void DetectDeadLanes::addUsedLanesOnOperand(const MachineOperand &MO,
	LaneBitmask UsedLanes) {
	if (!MO.readsReg())
	return;
	unsigned MOReg = MO.getReg();
	if (!TargetRegisterInfo::isVirtualRegister(MOReg))
	return;

	unsigned MOSubReg = MO.getSubReg();
	if (MOSubReg != 0)
	UsedLanes = TRI->composeSubRegIndexLaneMask(MOSubReg, UsedLanes);
	UsedLanes &= MRI->getMaxLaneMaskForVReg(MOReg);

	unsigned MORegIdx = TargetRegisterInfo::virtReg2Index(MOReg);
	VRegInfo &MORegInfo = VRegInfos[MORegIdx];
	LaneBitmask PrevUsedLanes = MORegInfo.UsedLanes;
	// Any change at all?
	if ((UsedLanes & ~PrevUsedLanes).none())
	return;

	// Set UsedLanes and remember instruction for further propagation.
	MORegInfo.UsedLanes = PrevUsedLanes \| UsedLanes;
	if (DefinedByCopy.test(MORegIdx))
	PutInWorklist(MORegIdx);
	}

	void DetectDeadLanes::transferUsedLanesStep(const MachineInstr &MI,
	LaneBitmask UsedLanes) {
	for (const MachineOperand &MO : MI.uses()) {
	if (!MO.isReg() \|\| !TargetRegisterInfo::isVirtualRegister(MO.getReg()))
	continue;
	LaneBitmask UsedOnMO = transferUsedLanes(MI, UsedLanes, MO);
	addUsedLanesOnOperand(MO, UsedOnMO);
	}
	}

	LaneBitmask DetectDeadLanes::transferUsedLanes(const MachineInstr &MI,
	LaneBitmask UsedLanes,
	const MachineOperand &MO) const {
	unsigned OpNum = MI.getOperandNo(&MO);
	assert(lowersToCopies(MI) && DefinedByCopy[
	TargetRegisterInfo::virtReg2Index(MI.getOperand(0).getReg())]);

	switch (MI.getOpcode()) {
	case TargetOpcode::COPY:
	case TargetOpcode::PHI:
	return UsedLanes;
	case TargetOpcode::REG_SEQUENCE: {
	assert(OpNum % 2 == 1);
	unsigned SubIdx = MI.getOperand(OpNum + 1).getImm();
	return TRI->reverseComposeSubRegIndexLaneMask(SubIdx, UsedLanes);
	}
	case TargetOpcode::INSERT_SUBREG: {
	unsigned SubIdx = MI.getOperand(3).getImm();
	LaneBitmask MO2UsedLanes =
	TRI->reverseComposeSubRegIndexLaneMask(SubIdx, UsedLanes);
	if (OpNum == 2)
	return MO2UsedLanes;

	const MachineOperand &Def = MI.getOperand(0);
	unsigned DefReg = Def.getReg();
	const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
	LaneBitmask MO1UsedLanes;
	if (RC->CoveredBySubRegs)
	MO1UsedLanes = UsedLanes & ~TRI->getSubRegIndexLaneMask(SubIdx);
	else
	MO1UsedLanes = RC->LaneMask;

	assert(OpNum == 1);
	return MO1UsedLanes;
	}
	case TargetOpcode::EXTRACT_SUBREG: {
	assert(OpNum == 1);
	unsigned SubIdx = MI.getOperand(2).getImm();
	return TRI->composeSubRegIndexLaneMask(SubIdx, UsedLanes);
	}
	default:
	llvm_unreachable("function must be called with COPY-like instruction");
	}
	}

	void DetectDeadLanes::transferDefinedLanesStep(const MachineOperand &Use,
	LaneBitmask DefinedLanes) {
	if (!Use.readsReg())
	return;
	// Check whether the operand writes a vreg and is part of a COPY-like
	// instruction.
	const MachineInstr &MI = *Use.getParent();
	if (MI.getDesc().getNumDefs() != 1)
	return;
	// FIXME: PATCHPOINT instructions announce a Def that does not always exist,
	// they really need to be modeled differently!
	if (MI.getOpcode() == TargetOpcode::PATCHPOINT)
	return;
	const MachineOperand &Def = *MI.defs().begin();
	unsigned DefReg = Def.getReg();
	if (!TargetRegisterInfo::isVirtualRegister(DefReg))
	return;
	unsigned DefRegIdx = TargetRegisterInfo::virtReg2Index(DefReg);
	if (!DefinedByCopy.test(DefRegIdx))
	return;

	unsigned OpNum = MI.getOperandNo(&Use);
	DefinedLanes =
	TRI->reverseComposeSubRegIndexLaneMask(Use.getSubReg(), DefinedLanes);
	DefinedLanes = transferDefinedLanes(Def, OpNum, DefinedLanes);

	VRegInfo &RegInfo = VRegInfos[DefRegIdx];
	LaneBitmask PrevDefinedLanes = RegInfo.DefinedLanes;
	// Any change at all?
	if ((DefinedLanes & ~PrevDefinedLanes).none())
	return;

	RegInfo.DefinedLanes = PrevDefinedLanes \| DefinedLanes;
	PutInWorklist(DefRegIdx);
	}

	LaneBitmask DetectDeadLanes::transferDefinedLanes(const MachineOperand &Def,
	unsigned OpNum, LaneBitmask DefinedLanes) const {
	const MachineInstr &MI = *Def.getParent();
	// Translate DefinedLanes if necessary.
	switch (MI.getOpcode()) {
	case TargetOpcode::REG_SEQUENCE: {
	unsigned SubIdx = MI.getOperand(OpNum + 1).getImm();
	DefinedLanes = TRI->composeSubRegIndexLaneMask(SubIdx, DefinedLanes);
	DefinedLanes &= TRI->getSubRegIndexLaneMask(SubIdx);
	break;
	}
	case TargetOpcode::INSERT_SUBREG: {
	unsigned SubIdx = MI.getOperand(3).getImm();
	if (OpNum == 2) {
	DefinedLanes = TRI->composeSubRegIndexLaneMask(SubIdx, DefinedLanes);
	DefinedLanes &= TRI->getSubRegIndexLaneMask(SubIdx);
	} else {
	assert(OpNum == 1 && "INSERT_SUBREG must have two operands");
	// Ignore lanes defined by operand 2.
	DefinedLanes &= ~TRI->getSubRegIndexLaneMask(SubIdx);
	}
	break;
	}
	case TargetOpcode::EXTRACT_SUBREG: {
	unsigned SubIdx = MI.getOperand(2).getImm();
	assert(OpNum == 1 && "EXTRACT_SUBREG must have one register operand only");
	DefinedLanes = TRI->reverseComposeSubRegIndexLaneMask(SubIdx, DefinedLanes);
	break;
	}
	case TargetOpcode::COPY:
	case TargetOpcode::PHI:
	break;
	default:
	llvm_unreachable("function must be called with COPY-like instruction");
	}

	assert(Def.getSubReg() == 0 &&
	"Should not have subregister defs in machine SSA phase");
	DefinedLanes &= MRI->getMaxLaneMaskForVReg(Def.getReg());
	return DefinedLanes;
	}

	LaneBitmask DetectDeadLanes::determineInitialDefinedLanes(unsigned Reg) {
	// Live-In or unused registers have no definition but are considered fully
	// defined.
	if (!MRI->hasOneDef(Reg))
	return LaneBitmask::getAll();

	const MachineOperand &Def = *MRI->def_begin(Reg);
	const MachineInstr &DefMI = *Def.getParent();
	if (lowersToCopies(DefMI)) {
	// Start optimisatically with no used or defined lanes for copy
	// instructions. The following dataflow analysis will add more bits.
	unsigned RegIdx = TargetRegisterInfo::virtReg2Index(Reg);
	DefinedByCopy.set(RegIdx);
	PutInWorklist(RegIdx);

	if (Def.isDead())
	return LaneBitmask::getNone();

	// COPY/PHI can copy across unrelated register classes (example: float/int)
	// with incompatible subregister structure. Do not include these in the
	// dataflow analysis since we cannot transfer lanemasks in a meaningful way.
	const TargetRegisterClass *DefRC = MRI->getRegClass(Reg);

	// Determine initially DefinedLanes.
	LaneBitmask DefinedLanes;
	for (const MachineOperand &MO : DefMI.uses()) {
	if (!MO.isReg() \|\| !MO.readsReg())
	continue;
	unsigned MOReg = MO.getReg();
	if (!MOReg)
	continue;

	LaneBitmask MODefinedLanes;
	if (TargetRegisterInfo::isPhysicalRegister(MOReg)) {
	MODefinedLanes = LaneBitmask::getAll();
	} else if (isCrossCopy(*MRI, DefMI, DefRC, MO)) {
	MODefinedLanes = LaneBitmask::getAll();
	} else {
	assert(TargetRegisterInfo::isVirtualRegister(MOReg));
	if (MRI->hasOneDef(MOReg)) {
	const MachineOperand &MODef = *MRI->def_begin(MOReg);
	const MachineInstr &MODefMI = *MODef.getParent();
	// Bits from copy-like operations will be added later.
	if (lowersToCopies(MODefMI) \|\| MODefMI.isImplicitDef())
	continue;
	}
	unsigned MOSubReg = MO.getSubReg();
	MODefinedLanes = MRI->getMaxLaneMaskForVReg(MOReg);
	MODefinedLanes = TRI->reverseComposeSubRegIndexLaneMask(
	MOSubReg, MODefinedLanes);
	}

	unsigned OpNum = DefMI.getOperandNo(&MO);
	DefinedLanes \|= transferDefinedLanes(Def, OpNum, MODefinedLanes);
	}
	return DefinedLanes;
	}
	if (DefMI.isImplicitDef() \|\| Def.isDead())
	return LaneBitmask::getNone();

	assert(Def.getSubReg() == 0 &&
	"Should not have subregister defs in machine SSA phase");
	return MRI->getMaxLaneMaskForVReg(Reg);
	}

	LaneBitmask DetectDeadLanes::determineInitialUsedLanes(unsigned Reg) {
	LaneBitmask UsedLanes = LaneBitmask::getNone();
	for (const MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
	if (!MO.readsReg())
	continue;

	const MachineInstr &UseMI = *MO.getParent();
	if (UseMI.isKill())
	continue;

	unsigned SubReg = MO.getSubReg();
	if (lowersToCopies(UseMI)) {
	assert(UseMI.getDesc().getNumDefs() == 1);
	const MachineOperand &Def = *UseMI.defs().begin();
	unsigned DefReg = Def.getReg();
	// The used lanes of COPY-like instruction operands are determined by the
	// following dataflow analysis.
	if (TargetRegisterInfo::isVirtualRegister(DefReg)) {
	// But ignore copies across incompatible register classes.
	bool CrossCopy = false;
	if (lowersToCopies(UseMI)) {
	const TargetRegisterClass *DstRC = MRI->getRegClass(DefReg);
	CrossCopy = isCrossCopy(*MRI, UseMI, DstRC, MO);
	if (CrossCopy)
	LLVM_DEBUG(dbgs() << "Copy across incompatible classes: " << UseMI);
	}

	if (!CrossCopy)
	continue;
	}
	}

	// Shortcut: All lanes are used.
	if (SubReg == 0)
	return MRI->getMaxLaneMaskForVReg(Reg);

	UsedLanes \|= TRI->getSubRegIndexLaneMask(SubReg);
	}
	return UsedLanes;
	}

	bool DetectDeadLanes::isUndefRegAtInput(const MachineOperand &MO,
	const VRegInfo &RegInfo) const {
	unsigned SubReg = MO.getSubReg();
	LaneBitmask Mask = TRI->getSubRegIndexLaneMask(SubReg);
	return (RegInfo.DefinedLanes & RegInfo.UsedLanes & Mask).none();
	}

	bool DetectDeadLanes::isUndefInput(const MachineOperand &MO,
	bool *CrossCopy) const {
	if (!MO.isUse())
	return false;
	const MachineInstr &MI = *MO.getParent();
	if (!lowersToCopies(MI))
	return false;
	const MachineOperand &Def = MI.getOperand(0);
	unsigned DefReg = Def.getReg();
	if (!TargetRegisterInfo::isVirtualRegister(DefReg))
	return false;
	unsigned DefRegIdx = TargetRegisterInfo::virtReg2Index(DefReg);
	if (!DefinedByCopy.test(DefRegIdx))
	return false;

	const VRegInfo &DefRegInfo = VRegInfos[DefRegIdx];
	LaneBitmask UsedLanes = transferUsedLanes(MI, DefRegInfo.UsedLanes, MO);
	if (UsedLanes.any())
	return false;

	unsigned MOReg = MO.getReg();
	if (TargetRegisterInfo::isVirtualRegister(MOReg)) {
	const TargetRegisterClass *DstRC = MRI->getRegClass(DefReg);
	CrossCopy = isCrossCopy(MRI, MI, DstRC, MO);
	}
	return true;
	}

	bool DetectDeadLanes::runOnce(MachineFunction &MF) {
	// First pass: Populate defs/uses of vregs with initial values
	unsigned NumVirtRegs = MRI->getNumVirtRegs();
	for (unsigned RegIdx = 0; RegIdx < NumVirtRegs; ++RegIdx) {
	unsigned Reg = TargetRegisterInfo::index2VirtReg(RegIdx);

	// Determine used/defined lanes and add copy instructions to worklist.
	VRegInfo &Info = VRegInfos[RegIdx];
	Info.DefinedLanes = determineInitialDefinedLanes(Reg);
	Info.UsedLanes = determineInitialUsedLanes(Reg);
	}

	// Iterate as long as defined lanes/used lanes keep changing.
	while (!Worklist.empty()) {
	unsigned RegIdx = Worklist.front();
	Worklist.pop_front();
	WorklistMembers.reset(RegIdx);
	VRegInfo &Info = VRegInfos[RegIdx];
	unsigned Reg = TargetRegisterInfo::index2VirtReg(RegIdx);

	// Transfer UsedLanes to operands of DefMI (backwards dataflow).
	MachineOperand &Def = *MRI->def_begin(Reg);
	const MachineInstr &MI = *Def.getParent();
	transferUsedLanesStep(MI, Info.UsedLanes);
	// Transfer DefinedLanes to users of Reg (forward dataflow).
	for (const MachineOperand &MO : MRI->use_nodbg_operands(Reg))
	transferDefinedLanesStep(MO, Info.DefinedLanes);
	}

	LLVM_DEBUG(dbgs() << "Defined/Used lanes:\n"; for (unsigned RegIdx = 0;
	RegIdx < NumVirtRegs;
	++RegIdx) {
	unsigned Reg = TargetRegisterInfo::index2VirtReg(RegIdx);
	const VRegInfo &Info = VRegInfos[RegIdx];
	dbgs() << printReg(Reg, nullptr)
	<< " Used: " << PrintLaneMask(Info.UsedLanes)
	<< " Def: " << PrintLaneMask(Info.DefinedLanes) << '\n';
	} dbgs() << "\n";);

	bool Again = false;
	// Mark operands as dead/unused.
	for (MachineBasicBlock &MBB : MF) {
	for (MachineInstr &MI : MBB) {
	for (MachineOperand &MO : MI.operands()) {
	if (!MO.isReg())
	continue;
	unsigned Reg = MO.getReg();
	if (!TargetRegisterInfo::isVirtualRegister(Reg))
	continue;
	unsigned RegIdx = TargetRegisterInfo::virtReg2Index(Reg);
	const VRegInfo &RegInfo = VRegInfos[RegIdx];
	if (MO.isDef() && !MO.isDead() && RegInfo.UsedLanes.none()) {
	LLVM_DEBUG(dbgs()
	<< "Marking operand '" << MO << "' as dead in " << MI);
	MO.setIsDead();
	}
	if (MO.readsReg()) {
	bool CrossCopy = false;
	if (isUndefRegAtInput(MO, RegInfo)) {
	LLVM_DEBUG(dbgs()
	<< "Marking operand '" << MO << "' as undef in " << MI);
	MO.setIsUndef();
	} else if (isUndefInput(MO, &CrossCopy)) {
	LLVM_DEBUG(dbgs()
	<< "Marking operand '" << MO << "' as undef in " << MI);
	MO.setIsUndef();
	if (CrossCopy)
	Again = true;
	}
	}
	}
	}
	}

	return Again;
	}

	bool DetectDeadLanes::runOnMachineFunction(MachineFunction &MF) {
	// Don't bother if we won't track subregister liveness later. This pass is
	// required for correctness if subregister liveness is enabled because the
	// register coalescer cannot deal with hidden dead defs. However without
	// subregister liveness enabled, the expected benefits of this pass are small
	// so we safe the compile time.
	MRI = &MF.getRegInfo();
	if (!MRI->subRegLivenessEnabled()) {
	LLVM_DEBUG(dbgs() << "Skipping Detect dead lanes pass\n");
	return false;
	}

	TRI = MRI->getTargetRegisterInfo();

	unsigned NumVirtRegs = MRI->getNumVirtRegs();
	VRegInfos = new VRegInfo[NumVirtRegs];
	WorklistMembers.resize(NumVirtRegs);
	DefinedByCopy.resize(NumVirtRegs);

	bool Again;
	do {
	Again = runOnce(MF);
	} while(Again);

	DefinedByCopy.clear();
	WorklistMembers.clear();
	delete[] VRegInfos;
	return true;
	}