lib/CodeGen/DetectDeadLanes.cpp

   1 //===- DetectDeadLanes.cpp - SubRegister Lane Usage Analysis --*- C++ -*---===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 /// \file
  10 /// Analysis that tracks defined/used subregister lanes across COPY instructions
  11 /// and instructions that get lowered to a COPY (PHI, REG_SEQUENCE,
  12 /// INSERT_SUBREG, EXTRACT_SUBREG).
  13 /// The information is used to detect dead definitions and the usage of
  14 /// (completely) undefined values and mark the operands as such.
  15 /// This pass is necessary because the dead/undef status is not obvious anymore
  16 /// when subregisters are involved.
  17 ///
  18 /// Example:
  19 ///    %0 = some definition
  20 ///    %1 = IMPLICIT_DEF
  21 ///    %2 = REG_SEQUENCE %0, sub0, %1, sub1
  22 ///    %3 = EXTRACT_SUBREG %2, sub1
  23 ///       = use %3
  24 /// The %0 definition is dead and %3 contains an undefined value.
  25 //
  26 //===----------------------------------------------------------------------===//
  27
  28 #include <deque>
  29 #include <vector>
  30
  31 #include "llvm/ADT/BitVector.h"
  32 #include "llvm/ADT/SetVector.h"
  33 #include "llvm/CodeGen/MachineFunctionPass.h"
  34 #include "llvm/CodeGen/MachineRegisterInfo.h"
  35 #include "llvm/CodeGen/Passes.h"
  36 #include "llvm/CodeGen/TargetRegisterInfo.h"
  37 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  38 #include "llvm/InitializePasses.h"
  39 #include "llvm/Pass.h"
  40 #include "llvm/PassRegistry.h"
  41 #include "llvm/Support/Debug.h"
  42 #include "llvm/Support/raw_ostream.h"
  43
  44 using namespace llvm;
  45
  46 #define DEBUG_TYPE "detect-dead-lanes"
  47
  48 namespace {
  49
  50 /// Contains a bitmask of which lanes of a given virtual register are
  51 /// defined and which ones are actually used.
  52 struct VRegInfo {
  53   LaneBitmask UsedLanes;
  54   LaneBitmask DefinedLanes;
  55 };
  56
  57 class DetectDeadLanes : public MachineFunctionPass {
  58 public:
  59   bool runOnMachineFunction(MachineFunction &MF) override;
  60
  61   static char ID;
  62   DetectDeadLanes() : MachineFunctionPass(ID) {}
  63
  64   StringRef getPassName() const override { return "Detect Dead Lanes"; }
  65
  66   void getAnalysisUsage(AnalysisUsage &AU) const override {
  67     AU.setPreservesCFG();
  68     MachineFunctionPass::getAnalysisUsage(AU);
  69   }
  70
  71 private:
  72   /// Add used lane bits on the register used by operand \p MO. This translates
  73   /// the bitmask based on the operands subregister, and puts the register into
  74   /// the worklist if any new bits were added.
  75   void addUsedLanesOnOperand(const MachineOperand &MO, LaneBitmask UsedLanes);
  76
  77   /// Given a bitmask \p UsedLanes for the used lanes on a def output of a
  78   /// COPY-like instruction determine the lanes used on the use operands
  79   /// and call addUsedLanesOnOperand() for them.
  80   void transferUsedLanesStep(const MachineInstr &MI, LaneBitmask UsedLanes);
  81
  82   /// Given a use regiser operand \p Use and a mask of defined lanes, check
  83   /// if the operand belongs to a lowersToCopies() instruction, transfer the
  84   /// mask to the def and put the instruction into the worklist.
  85   void transferDefinedLanesStep(const MachineOperand &Use,
  86                                 LaneBitmask DefinedLanes);
  87
  88   /// Given a mask \p DefinedLanes of lanes defined at operand \p OpNum
  89   /// of COPY-like instruction, determine which lanes are defined at the output
  90   /// operand \p Def.
  91   LaneBitmask transferDefinedLanes(const MachineOperand &Def, unsigned OpNum,
  92                                    LaneBitmask DefinedLanes) const;
  93
  94   /// Given a mask \p UsedLanes used from the output of instruction \p MI
  95   /// determine which lanes are used from operand \p MO of this instruction.
  96   LaneBitmask transferUsedLanes(const MachineInstr &MI, LaneBitmask UsedLanes,
  97                                 const MachineOperand &MO) const;
  98
  99   bool runOnce(MachineFunction &MF);
 100
 101   LaneBitmask determineInitialDefinedLanes(unsigned Reg);
 102   LaneBitmask determineInitialUsedLanes(unsigned Reg);
 103
 104   bool isUndefRegAtInput(const MachineOperand &MO,
 105                          const VRegInfo &RegInfo) const;
 106
 107   bool isUndefInput(const MachineOperand &MO, bool *CrossCopy) const;
 108
 109   const MachineRegisterInfo *MRI;
 110   const TargetRegisterInfo *TRI;
 111
 112   void PutInWorklist(unsigned RegIdx) {
 113     if (WorklistMembers.test(RegIdx))
 114       return;
 115     WorklistMembers.set(RegIdx);
 116     Worklist.push_back(RegIdx);
 117   }
 118
 119   VRegInfo *VRegInfos;
 120   /// Worklist containing virtreg indexes.
 121   std::deque<unsigned> Worklist;
 122   BitVector WorklistMembers;
 123   /// This bitvector is set for each vreg index where the vreg is defined
 124   /// by an instruction where lowersToCopies()==true.
 125   BitVector DefinedByCopy;
 126 };
 127
 128 } // end anonymous namespace
 129
 130 char DetectDeadLanes::ID = 0;
 131 char &llvm::DetectDeadLanesID = DetectDeadLanes::ID;
 132
 133 INITIALIZE_PASS(DetectDeadLanes, DEBUG_TYPE, "Detect Dead Lanes", false, false)
 134
 135 /// Returns true if \p MI will get lowered to a series of COPY instructions.
 136 /// We call this a COPY-like instruction.
 137 static bool lowersToCopies(const MachineInstr &MI) {
 138   // Note: We could support instructions with MCInstrDesc::isRegSequenceLike(),
 139   // isExtractSubRegLike(), isInsertSubregLike() in the future even though they
 140   // are not lowered to a COPY.
 141   switch (MI.getOpcode()) {
 142   case TargetOpcode::COPY:
 143   case TargetOpcode::PHI:
 144   case TargetOpcode::INSERT_SUBREG:
 145   case TargetOpcode::REG_SEQUENCE:
 146   case TargetOpcode::EXTRACT_SUBREG:
 147     return true;
 148   }
 149   return false;
 150 }
 151
 152 static bool isCrossCopy(const MachineRegisterInfo &MRI,
 153                         const MachineInstr &MI,
 154                         const TargetRegisterClass *DstRC,
 155                         const MachineOperand &MO) {
 156   assert(lowersToCopies(MI));
 157   unsigned SrcReg = MO.getReg();
 158   const TargetRegisterClass *SrcRC = MRI.getRegClass(SrcReg);
 159   if (DstRC == SrcRC)
 160     return false;
 161
 162   unsigned SrcSubIdx = MO.getSubReg();
 163
 164   const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
 165   unsigned DstSubIdx = 0;
 166   switch (MI.getOpcode()) {
 167   case TargetOpcode::INSERT_SUBREG:
 168     if (MI.getOperandNo(&MO) == 2)
 169       DstSubIdx = MI.getOperand(3).getImm();
 170     break;
 171   case TargetOpcode::REG_SEQUENCE: {
 172     unsigned OpNum = MI.getOperandNo(&MO);
 173     DstSubIdx = MI.getOperand(OpNum+1).getImm();
 174     break;
 175   }
 176   case TargetOpcode::EXTRACT_SUBREG: {
 177     unsigned SubReg = MI.getOperand(2).getImm();
 178     SrcSubIdx = TRI.composeSubRegIndices(SubReg, SrcSubIdx);
 179   }
 180   }
 181
 182   unsigned PreA, PreB; // Unused.
 183   if (SrcSubIdx && DstSubIdx)
 184     return !TRI.getCommonSuperRegClass(SrcRC, SrcSubIdx, DstRC, DstSubIdx, PreA,
 185                                        PreB);
 186   if (SrcSubIdx)
 187     return !TRI.getMatchingSuperRegClass(SrcRC, DstRC, SrcSubIdx);
 188   if (DstSubIdx)
 189     return !TRI.getMatchingSuperRegClass(DstRC, SrcRC, DstSubIdx);
 190   return !TRI.getCommonSubClass(SrcRC, DstRC);
 191 }
 192
 193 void DetectDeadLanes::addUsedLanesOnOperand(const MachineOperand &MO,
 194                                             LaneBitmask UsedLanes) {
 195   if (!MO.readsReg())
 196     return;
 197   unsigned MOReg = MO.getReg();
 198   if (!Register::isVirtualRegister(MOReg))
 199     return;
 200
 201   unsigned MOSubReg = MO.getSubReg();
 202   if (MOSubReg != 0)
 203     UsedLanes = TRI->composeSubRegIndexLaneMask(MOSubReg, UsedLanes);
 204   UsedLanes &= MRI->getMaxLaneMaskForVReg(MOReg);
 205
 206   unsigned MORegIdx = Register::virtReg2Index(MOReg);
 207   VRegInfo &MORegInfo = VRegInfos[MORegIdx];
 208   LaneBitmask PrevUsedLanes = MORegInfo.UsedLanes;
 209   // Any change at all?
 210   if ((UsedLanes & ~PrevUsedLanes).none())
 211     return;
 212
 213   // Set UsedLanes and remember instruction for further propagation.
 214   MORegInfo.UsedLanes = PrevUsedLanes | UsedLanes;
 215   if (DefinedByCopy.test(MORegIdx))
 216     PutInWorklist(MORegIdx);
 217 }
 218
 219 void DetectDeadLanes::transferUsedLanesStep(const MachineInstr &MI,
 220                                             LaneBitmask UsedLanes) {
 221   for (const MachineOperand &MO : MI.uses()) {
 222     if (!MO.isReg() || !Register::isVirtualRegister(MO.getReg()))
 223       continue;
 224     LaneBitmask UsedOnMO = transferUsedLanes(MI, UsedLanes, MO);
 225     addUsedLanesOnOperand(MO, UsedOnMO);
 226   }
 227 }
 228
 229 LaneBitmask DetectDeadLanes::transferUsedLanes(const MachineInstr &MI,
 230                                                LaneBitmask UsedLanes,
 231                                                const MachineOperand &MO) const {
 232   unsigned OpNum = MI.getOperandNo(&MO);
 233   assert(lowersToCopies(MI) &&
 234          DefinedByCopy[Register::virtReg2Index(MI.getOperand(0).getReg())]);
 235
 236   switch (MI.getOpcode()) {
 237   case TargetOpcode::COPY:
 238   case TargetOpcode::PHI:
 239     return UsedLanes;
 240   case TargetOpcode::REG_SEQUENCE: {
 241     assert(OpNum % 2 == 1);
 242     unsigned SubIdx = MI.getOperand(OpNum + 1).getImm();
 243     return TRI->reverseComposeSubRegIndexLaneMask(SubIdx, UsedLanes);
 244   }
 245   case TargetOpcode::INSERT_SUBREG: {
 246     unsigned SubIdx = MI.getOperand(3).getImm();
 247     LaneBitmask MO2UsedLanes =
 248         TRI->reverseComposeSubRegIndexLaneMask(SubIdx, UsedLanes);
 249     if (OpNum == 2)
 250       return MO2UsedLanes;
 251
 252     const MachineOperand &Def = MI.getOperand(0);
 253     unsigned DefReg = Def.getReg();
 254     const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
 255     LaneBitmask MO1UsedLanes;
 256     if (RC->CoveredBySubRegs)
 257       MO1UsedLanes = UsedLanes & ~TRI->getSubRegIndexLaneMask(SubIdx);
 258     else
 259       MO1UsedLanes = RC->LaneMask;
 260
 261     assert(OpNum == 1);
 262     return MO1UsedLanes;
 263   }
 264   case TargetOpcode::EXTRACT_SUBREG: {
 265     assert(OpNum == 1);
 266     unsigned SubIdx = MI.getOperand(2).getImm();
 267     return TRI->composeSubRegIndexLaneMask(SubIdx, UsedLanes);
 268   }
 269   default:
 270     llvm_unreachable("function must be called with COPY-like instruction");
 271   }
 272 }
 273
 274 void DetectDeadLanes::transferDefinedLanesStep(const MachineOperand &Use,
 275                                                LaneBitmask DefinedLanes) {
 276   if (!Use.readsReg())
 277     return;
 278   // Check whether the operand writes a vreg and is part of a COPY-like
 279   // instruction.
 280   const MachineInstr &MI = *Use.getParent();
 281   if (MI.getDesc().getNumDefs() != 1)
 282     return;
 283   // FIXME: PATCHPOINT instructions announce a Def that does not always exist,
 284   // they really need to be modeled differently!
 285   if (MI.getOpcode() == TargetOpcode::PATCHPOINT)
 286     return;
 287   const MachineOperand &Def = *MI.defs().begin();
 288   unsigned DefReg = Def.getReg();
 289   if (!Register::isVirtualRegister(DefReg))
 290     return;
 291   unsigned DefRegIdx = Register::virtReg2Index(DefReg);
 292   if (!DefinedByCopy.test(DefRegIdx))
 293     return;
 294
 295   unsigned OpNum = MI.getOperandNo(&Use);
 296   DefinedLanes =
 297       TRI->reverseComposeSubRegIndexLaneMask(Use.getSubReg(), DefinedLanes);
 298   DefinedLanes = transferDefinedLanes(Def, OpNum, DefinedLanes);
 299
 300   VRegInfo &RegInfo = VRegInfos[DefRegIdx];
 301   LaneBitmask PrevDefinedLanes = RegInfo.DefinedLanes;
 302   // Any change at all?
 303   if ((DefinedLanes & ~PrevDefinedLanes).none())
 304     return;
 305
 306   RegInfo.DefinedLanes = PrevDefinedLanes | DefinedLanes;
 307   PutInWorklist(DefRegIdx);
 308 }
 309
 310 LaneBitmask DetectDeadLanes::transferDefinedLanes(const MachineOperand &Def,
 311     unsigned OpNum, LaneBitmask DefinedLanes) const {
 312   const MachineInstr &MI = *Def.getParent();
 313   // Translate DefinedLanes if necessary.
 314   switch (MI.getOpcode()) {
 315   case TargetOpcode::REG_SEQUENCE: {
 316     unsigned SubIdx = MI.getOperand(OpNum + 1).getImm();
 317     DefinedLanes = TRI->composeSubRegIndexLaneMask(SubIdx, DefinedLanes);
 318     DefinedLanes &= TRI->getSubRegIndexLaneMask(SubIdx);
 319     break;
 320   }
 321   case TargetOpcode::INSERT_SUBREG: {
 322     unsigned SubIdx = MI.getOperand(3).getImm();
 323     if (OpNum == 2) {
 324       DefinedLanes = TRI->composeSubRegIndexLaneMask(SubIdx, DefinedLanes);
 325       DefinedLanes &= TRI->getSubRegIndexLaneMask(SubIdx);
 326     } else {
 327       assert(OpNum == 1 && "INSERT_SUBREG must have two operands");
 328       // Ignore lanes defined by operand 2.
 329       DefinedLanes &= ~TRI->getSubRegIndexLaneMask(SubIdx);
 330     }
 331     break;
 332   }
 333   case TargetOpcode::EXTRACT_SUBREG: {
 334     unsigned SubIdx = MI.getOperand(2).getImm();
 335     assert(OpNum == 1 && "EXTRACT_SUBREG must have one register operand only");
 336     DefinedLanes = TRI->reverseComposeSubRegIndexLaneMask(SubIdx, DefinedLanes);
 337     break;
 338   }
 339   case TargetOpcode::COPY:
 340   case TargetOpcode::PHI:
 341     break;
 342   default:
 343     llvm_unreachable("function must be called with COPY-like instruction");
 344   }
 345
 346   assert(Def.getSubReg() == 0 &&
 347          "Should not have subregister defs in machine SSA phase");
 348   DefinedLanes &= MRI->getMaxLaneMaskForVReg(Def.getReg());
 349   return DefinedLanes;
 350 }
 351
 352 LaneBitmask DetectDeadLanes::determineInitialDefinedLanes(unsigned Reg) {
 353   // Live-In or unused registers have no definition but are considered fully
 354   // defined.
 355   if (!MRI->hasOneDef(Reg))
 356     return LaneBitmask::getAll();
 357
 358   const MachineOperand &Def = *MRI->def_begin(Reg);
 359   const MachineInstr &DefMI = *Def.getParent();
 360   if (lowersToCopies(DefMI)) {
 361     // Start optimisatically with no used or defined lanes for copy
 362     // instructions. The following dataflow analysis will add more bits.
 363     unsigned RegIdx = Register::virtReg2Index(Reg);
 364     DefinedByCopy.set(RegIdx);
 365     PutInWorklist(RegIdx);
 366
 367     if (Def.isDead())
 368       return LaneBitmask::getNone();
 369
 370     // COPY/PHI can copy across unrelated register classes (example: float/int)
 371     // with incompatible subregister structure. Do not include these in the
 372     // dataflow analysis since we cannot transfer lanemasks in a meaningful way.
 373     const TargetRegisterClass *DefRC = MRI->getRegClass(Reg);
 374
 375     // Determine initially DefinedLanes.
 376     LaneBitmask DefinedLanes;
 377     for (const MachineOperand &MO : DefMI.uses()) {
 378       if (!MO.isReg() || !MO.readsReg())
 379         continue;
 380       unsigned MOReg = MO.getReg();
 381       if (!MOReg)
 382         continue;
 383
 384       LaneBitmask MODefinedLanes;
 385       if (Register::isPhysicalRegister(MOReg)) {
 386         MODefinedLanes = LaneBitmask::getAll();
 387       } else if (isCrossCopy(*MRI, DefMI, DefRC, MO)) {
 388         MODefinedLanes = LaneBitmask::getAll();
 389       } else {
 390         assert(Register::isVirtualRegister(MOReg));
 391         if (MRI->hasOneDef(MOReg)) {
 392           const MachineOperand &MODef = *MRI->def_begin(MOReg);
 393           const MachineInstr &MODefMI = *MODef.getParent();
 394           // Bits from copy-like operations will be added later.
 395           if (lowersToCopies(MODefMI) || MODefMI.isImplicitDef())
 396             continue;
 397         }
 398         unsigned MOSubReg = MO.getSubReg();
 399         MODefinedLanes = MRI->getMaxLaneMaskForVReg(MOReg);
 400         MODefinedLanes = TRI->reverseComposeSubRegIndexLaneMask(
 401             MOSubReg, MODefinedLanes);
 402       }
 403
 404       unsigned OpNum = DefMI.getOperandNo(&MO);
 405       DefinedLanes |= transferDefinedLanes(Def, OpNum, MODefinedLanes);
 406     }
 407     return DefinedLanes;
 408   }
 409   if (DefMI.isImplicitDef() || Def.isDead())
 410     return LaneBitmask::getNone();
 411
 412   assert(Def.getSubReg() == 0 &&
 413          "Should not have subregister defs in machine SSA phase");
 414   return MRI->getMaxLaneMaskForVReg(Reg);
 415 }
 416
 417 LaneBitmask DetectDeadLanes::determineInitialUsedLanes(unsigned Reg) {
 418   LaneBitmask UsedLanes = LaneBitmask::getNone();
 419   for (const MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
 420     if (!MO.readsReg())
 421       continue;
 422
 423     const MachineInstr &UseMI = *MO.getParent();
 424     if (UseMI.isKill())
 425       continue;
 426
 427     unsigned SubReg = MO.getSubReg();
 428     if (lowersToCopies(UseMI)) {
 429       assert(UseMI.getDesc().getNumDefs() == 1);
 430       const MachineOperand &Def = *UseMI.defs().begin();
 431       unsigned DefReg = Def.getReg();
 432       // The used lanes of COPY-like instruction operands are determined by the
 433       // following dataflow analysis.
 434       if (Register::isVirtualRegister(DefReg)) {
 435         // But ignore copies across incompatible register classes.
 436         bool CrossCopy = false;
 437         if (lowersToCopies(UseMI)) {
 438           const TargetRegisterClass *DstRC = MRI->getRegClass(DefReg);
 439           CrossCopy = isCrossCopy(*MRI, UseMI, DstRC, MO);
 440           if (CrossCopy)
 441             LLVM_DEBUG(dbgs() << "Copy across incompatible classes: " << UseMI);
 442         }
 443
 444         if (!CrossCopy)
 445           continue;
 446       }
 447     }
 448
 449     // Shortcut: All lanes are used.
 450     if (SubReg == 0)
 451       return MRI->getMaxLaneMaskForVReg(Reg);
 452
 453     UsedLanes |= TRI->getSubRegIndexLaneMask(SubReg);
 454   }
 455   return UsedLanes;
 456 }
 457
 458 bool DetectDeadLanes::isUndefRegAtInput(const MachineOperand &MO,
 459                                         const VRegInfo &RegInfo) const {
 460   unsigned SubReg = MO.getSubReg();
 461   LaneBitmask Mask = TRI->getSubRegIndexLaneMask(SubReg);
 462   return (RegInfo.DefinedLanes & RegInfo.UsedLanes & Mask).none();
 463 }
 464
 465 bool DetectDeadLanes::isUndefInput(const MachineOperand &MO,
 466                                    bool *CrossCopy) const {
 467   if (!MO.isUse())
 468     return false;
 469   const MachineInstr &MI = *MO.getParent();
 470   if (!lowersToCopies(MI))
 471     return false;
 472   const MachineOperand &Def = MI.getOperand(0);
 473   unsigned DefReg = Def.getReg();
 474   if (!Register::isVirtualRegister(DefReg))
 475     return false;
 476   unsigned DefRegIdx = Register::virtReg2Index(DefReg);
 477   if (!DefinedByCopy.test(DefRegIdx))
 478     return false;
 479
 480   const VRegInfo &DefRegInfo = VRegInfos[DefRegIdx];
 481   LaneBitmask UsedLanes = transferUsedLanes(MI, DefRegInfo.UsedLanes, MO);
 482   if (UsedLanes.any())
 483     return false;
 484
 485   unsigned MOReg = MO.getReg();
 486   if (Register::isVirtualRegister(MOReg)) {
 487     const TargetRegisterClass *DstRC = MRI->getRegClass(DefReg);
 488     *CrossCopy = isCrossCopy(*MRI, MI, DstRC, MO);
 489   }
 490   return true;
 491 }
 492
 493 bool DetectDeadLanes::runOnce(MachineFunction &MF) {
 494   // First pass: Populate defs/uses of vregs with initial values
 495   unsigned NumVirtRegs = MRI->getNumVirtRegs();
 496   for (unsigned RegIdx = 0; RegIdx < NumVirtRegs; ++RegIdx) {
 497     unsigned Reg = Register::index2VirtReg(RegIdx);
 498
 499     // Determine used/defined lanes and add copy instructions to worklist.
 500     VRegInfo &Info = VRegInfos[RegIdx];
 501     Info.DefinedLanes = determineInitialDefinedLanes(Reg);
 502     Info.UsedLanes = determineInitialUsedLanes(Reg);
 503   }
 504
 505   // Iterate as long as defined lanes/used lanes keep changing.
 506   while (!Worklist.empty()) {
 507     unsigned RegIdx = Worklist.front();
 508     Worklist.pop_front();
 509     WorklistMembers.reset(RegIdx);
 510     VRegInfo &Info = VRegInfos[RegIdx];
 511     unsigned Reg = Register::index2VirtReg(RegIdx);
 512
 513     // Transfer UsedLanes to operands of DefMI (backwards dataflow).
 514     MachineOperand &Def = *MRI->def_begin(Reg);
 515     const MachineInstr &MI = *Def.getParent();
 516     transferUsedLanesStep(MI, Info.UsedLanes);
 517     // Transfer DefinedLanes to users of Reg (forward dataflow).
 518     for (const MachineOperand &MO : MRI->use_nodbg_operands(Reg))
 519       transferDefinedLanesStep(MO, Info.DefinedLanes);
 520   }
 521
 522   LLVM_DEBUG(dbgs() << "Defined/Used lanes:\n"; for (unsigned RegIdx = 0;
 523                                                      RegIdx < NumVirtRegs;
 524                                                      ++RegIdx) {
 525     unsigned Reg = Register::index2VirtReg(RegIdx);
 526     const VRegInfo &Info = VRegInfos[RegIdx];
 527     dbgs() << printReg(Reg, nullptr)
 528            << " Used: " << PrintLaneMask(Info.UsedLanes)
 529            << " Def: " << PrintLaneMask(Info.DefinedLanes) << '\n';
 530   } dbgs() << "\n";);
 531
 532   bool Again = false;
 533   // Mark operands as dead/unused.
 534   for (MachineBasicBlock &MBB : MF) {
 535     for (MachineInstr &MI : MBB) {
 536       for (MachineOperand &MO : MI.operands()) {
 537         if (!MO.isReg())
 538           continue;
 539         unsigned Reg = MO.getReg();
 540         if (!Register::isVirtualRegister(Reg))
 541           continue;
 542         unsigned RegIdx = Register::virtReg2Index(Reg);
 543         const VRegInfo &RegInfo = VRegInfos[RegIdx];
 544         if (MO.isDef() && !MO.isDead() && RegInfo.UsedLanes.none()) {
 545           LLVM_DEBUG(dbgs()
 546                      << "Marking operand '" << MO << "' as dead in " << MI);
 547           MO.setIsDead();
 548         }
 549         if (MO.readsReg()) {
 550           bool CrossCopy = false;
 551           if (isUndefRegAtInput(MO, RegInfo)) {
 552             LLVM_DEBUG(dbgs()
 553                        << "Marking operand '" << MO << "' as undef in " << MI);
 554             MO.setIsUndef();
 555           } else if (isUndefInput(MO, &CrossCopy)) {
 556             LLVM_DEBUG(dbgs()
 557                        << "Marking operand '" << MO << "' as undef in " << MI);
 558             MO.setIsUndef();
 559             if (CrossCopy)
 560               Again = true;
 561           }
 562         }
 563       }
 564     }
 565   }
 566
 567   return Again;
 568 }
 569
 570 bool DetectDeadLanes::runOnMachineFunction(MachineFunction &MF) {
 571   // Don't bother if we won't track subregister liveness later.  This pass is
 572   // required for correctness if subregister liveness is enabled because the
 573   // register coalescer cannot deal with hidden dead defs. However without
 574   // subregister liveness enabled, the expected benefits of this pass are small
 575   // so we safe the compile time.
 576   MRI = &MF.getRegInfo();
 577   if (!MRI->subRegLivenessEnabled()) {
 578     LLVM_DEBUG(dbgs() << "Skipping Detect dead lanes pass\n");
 579     return false;
 580   }
 581
 582   TRI = MRI->getTargetRegisterInfo();
 583
 584   unsigned NumVirtRegs = MRI->getNumVirtRegs();
 585   VRegInfos = new VRegInfo[NumVirtRegs];
 586   WorklistMembers.resize(NumVirtRegs);
 587   DefinedByCopy.resize(NumVirtRegs);
 588
 589   bool Again;
 590   do {
 591     Again = runOnce(MF);
 592   } while(Again);
 593
 594   DefinedByCopy.clear();
 595   WorklistMembers.clear();
 596   delete[] VRegInfos;
 597   return true;
 598 }