lib/Target/SystemZ/SystemZElimCompare.cpp

   1 //===-- SystemZElimCompare.cpp - Eliminate comparison instructions --------===//
   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 // This pass:
  10 // (1) tries to remove compares if CC already contains the required information
  11 // (2) fuses compares and branches into COMPARE AND BRANCH instructions
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "SystemZ.h"
  16 #include "SystemZInstrInfo.h"
  17 #include "SystemZTargetMachine.h"
  18 #include "llvm/ADT/SmallVector.h"
  19 #include "llvm/ADT/Statistic.h"
  20 #include "llvm/ADT/StringRef.h"
  21 #include "llvm/CodeGen/MachineBasicBlock.h"
  22 #include "llvm/CodeGen/MachineFunction.h"
  23 #include "llvm/CodeGen/MachineFunctionPass.h"
  24 #include "llvm/CodeGen/MachineInstr.h"
  25 #include "llvm/CodeGen/MachineInstrBuilder.h"
  26 #include "llvm/CodeGen/MachineOperand.h"
  27 #include "llvm/CodeGen/TargetRegisterInfo.h"
  28 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  29 #include "llvm/MC/MCInstrDesc.h"
  30 #include <cassert>
  31 #include <cstdint>
  32
  33 using namespace llvm;
  34
  35 #define DEBUG_TYPE "systemz-elim-compare"
  36
  37 STATISTIC(BranchOnCounts, "Number of branch-on-count instructions");
  38 STATISTIC(LoadAndTraps, "Number of load-and-trap instructions");
  39 STATISTIC(EliminatedComparisons, "Number of eliminated comparisons");
  40 STATISTIC(FusedComparisons, "Number of fused compare-and-branch instructions");
  41
  42 namespace {
  43
  44 // Represents the references to a particular register in one or more
  45 // instructions.
  46 struct Reference {
  47   Reference() = default;
  48
  49   Reference &operator|=(const Reference &Other) {
  50     Def |= Other.Def;
  51     Use |= Other.Use;
  52     return *this;
  53   }
  54
  55   explicit operator bool() const { return Def || Use; }
  56
  57   // True if the register is defined or used in some form, either directly or
  58   // via a sub- or super-register.
  59   bool Def = false;
  60   bool Use = false;
  61 };
  62
  63 class SystemZElimCompare : public MachineFunctionPass {
  64 public:
  65   static char ID;
  66
  67   SystemZElimCompare(const SystemZTargetMachine &tm)
  68     : MachineFunctionPass(ID) {}
  69
  70   StringRef getPassName() const override {
  71     return "SystemZ Comparison Elimination";
  72   }
  73
  74   bool processBlock(MachineBasicBlock &MBB);
  75   bool runOnMachineFunction(MachineFunction &F) override;
  76
  77   MachineFunctionProperties getRequiredProperties() const override {
  78     return MachineFunctionProperties().set(
  79         MachineFunctionProperties::Property::NoVRegs);
  80   }
  81
  82 private:
  83   Reference getRegReferences(MachineInstr &MI, unsigned Reg);
  84   bool convertToBRCT(MachineInstr &MI, MachineInstr &Compare,
  85                      SmallVectorImpl<MachineInstr *> &CCUsers);
  86   bool convertToLoadAndTrap(MachineInstr &MI, MachineInstr &Compare,
  87                             SmallVectorImpl<MachineInstr *> &CCUsers);
  88   bool convertToLoadAndTest(MachineInstr &MI, MachineInstr &Compare,
  89                             SmallVectorImpl<MachineInstr *> &CCUsers);
  90   bool adjustCCMasksForInstr(MachineInstr &MI, MachineInstr &Compare,
  91                              SmallVectorImpl<MachineInstr *> &CCUsers,
  92                              unsigned ConvOpc = 0);
  93   bool optimizeCompareZero(MachineInstr &Compare,
  94                            SmallVectorImpl<MachineInstr *> &CCUsers);
  95   bool fuseCompareOperations(MachineInstr &Compare,
  96                              SmallVectorImpl<MachineInstr *> &CCUsers);
  97
  98   const SystemZInstrInfo *TII = nullptr;
  99   const TargetRegisterInfo *TRI = nullptr;
 100 };
 101
 102 char SystemZElimCompare::ID = 0;
 103
 104 } // end anonymous namespace
 105
 106 // Return true if CC is live out of MBB.
 107 static bool isCCLiveOut(MachineBasicBlock &MBB) {
 108   for (auto SI = MBB.succ_begin(), SE = MBB.succ_end(); SI != SE; ++SI)
 109     if ((*SI)->isLiveIn(SystemZ::CC))
 110       return true;
 111   return false;
 112 }
 113
 114 // Returns true if MI is an instruction whose output equals the value in Reg.
 115 static bool preservesValueOf(MachineInstr &MI, unsigned Reg) {
 116   switch (MI.getOpcode()) {
 117   case SystemZ::LR:
 118   case SystemZ::LGR:
 119   case SystemZ::LGFR:
 120   case SystemZ::LTR:
 121   case SystemZ::LTGR:
 122   case SystemZ::LTGFR:
 123   case SystemZ::LER:
 124   case SystemZ::LDR:
 125   case SystemZ::LXR:
 126   case SystemZ::LTEBR:
 127   case SystemZ::LTDBR:
 128   case SystemZ::LTXBR:
 129     if (MI.getOperand(1).getReg() == Reg)
 130       return true;
 131   }
 132
 133   return false;
 134 }
 135
 136 // Return true if any CC result of MI would (perhaps after conversion)
 137 // reflect the value of Reg.
 138 static bool resultTests(MachineInstr &MI, unsigned Reg) {
 139   if (MI.getNumOperands() > 0 && MI.getOperand(0).isReg() &&
 140       MI.getOperand(0).isDef() && MI.getOperand(0).getReg() == Reg)
 141     return true;
 142
 143   return (preservesValueOf(MI, Reg));
 144 }
 145
 146 // Describe the references to Reg or any of its aliases in MI.
 147 Reference SystemZElimCompare::getRegReferences(MachineInstr &MI, unsigned Reg) {
 148   Reference Ref;
 149   if (MI.isDebugInstr())
 150     return Ref;
 151
 152   for (unsigned I = 0, E = MI.getNumOperands(); I != E; ++I) {
 153     const MachineOperand &MO = MI.getOperand(I);
 154     if (MO.isReg()) {
 155       if (Register MOReg = MO.getReg()) {
 156         if (TRI->regsOverlap(MOReg, Reg)) {
 157           if (MO.isUse())
 158             Ref.Use = true;
 159           else if (MO.isDef())
 160             Ref.Def = true;
 161         }
 162       }
 163     }
 164   }
 165   return Ref;
 166 }
 167
 168 // Return true if this is a load and test which can be optimized the
 169 // same way as compare instruction.
 170 static bool isLoadAndTestAsCmp(MachineInstr &MI) {
 171   // If we during isel used a load-and-test as a compare with 0, the
 172   // def operand is dead.
 173   return (MI.getOpcode() == SystemZ::LTEBR ||
 174           MI.getOpcode() == SystemZ::LTDBR ||
 175           MI.getOpcode() == SystemZ::LTXBR) &&
 176          MI.getOperand(0).isDead();
 177 }
 178
 179 // Return the source register of Compare, which is the unknown value
 180 // being tested.
 181 static unsigned getCompareSourceReg(MachineInstr &Compare) {
 182   unsigned reg = 0;
 183   if (Compare.isCompare())
 184     reg = Compare.getOperand(0).getReg();
 185   else if (isLoadAndTestAsCmp(Compare))
 186     reg = Compare.getOperand(1).getReg();
 187   assert(reg);
 188
 189   return reg;
 190 }
 191
 192 // Compare compares the result of MI against zero.  If MI is an addition
 193 // of -1 and if CCUsers is a single branch on nonzero, eliminate the addition
 194 // and convert the branch to a BRCT(G) or BRCTH.  Return true on success.
 195 bool SystemZElimCompare::convertToBRCT(
 196     MachineInstr &MI, MachineInstr &Compare,
 197     SmallVectorImpl<MachineInstr *> &CCUsers) {
 198   // Check whether we have an addition of -1.
 199   unsigned Opcode = MI.getOpcode();
 200   unsigned BRCT;
 201   if (Opcode == SystemZ::AHI)
 202     BRCT = SystemZ::BRCT;
 203   else if (Opcode == SystemZ::AGHI)
 204     BRCT = SystemZ::BRCTG;
 205   else if (Opcode == SystemZ::AIH)
 206     BRCT = SystemZ::BRCTH;
 207   else
 208     return false;
 209   if (MI.getOperand(2).getImm() != -1)
 210     return false;
 211
 212   // Check whether we have a single JLH.
 213   if (CCUsers.size() != 1)
 214     return false;
 215   MachineInstr *Branch = CCUsers[0];
 216   if (Branch->getOpcode() != SystemZ::BRC ||
 217       Branch->getOperand(0).getImm() != SystemZ::CCMASK_ICMP ||
 218       Branch->getOperand(1).getImm() != SystemZ::CCMASK_CMP_NE)
 219     return false;
 220
 221   // We already know that there are no references to the register between
 222   // MI and Compare.  Make sure that there are also no references between
 223   // Compare and Branch.
 224   unsigned SrcReg = getCompareSourceReg(Compare);
 225   MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
 226   for (++MBBI; MBBI != MBBE; ++MBBI)
 227     if (getRegReferences(*MBBI, SrcReg))
 228       return false;
 229
 230   // The transformation is OK.  Rebuild Branch as a BRCT(G) or BRCTH.
 231   MachineOperand Target(Branch->getOperand(2));
 232   while (Branch->getNumOperands())
 233     Branch->RemoveOperand(0);
 234   Branch->setDesc(TII->get(BRCT));
 235   MachineInstrBuilder MIB(*Branch->getParent()->getParent(), Branch);
 236   MIB.add(MI.getOperand(0)).add(MI.getOperand(1)).add(Target);
 237   // Add a CC def to BRCT(G), since we may have to split them again if the
 238   // branch displacement overflows.  BRCTH has a 32-bit displacement, so
 239   // this is not necessary there.
 240   if (BRCT != SystemZ::BRCTH)
 241     MIB.addReg(SystemZ::CC, RegState::ImplicitDefine | RegState::Dead);
 242   MI.eraseFromParent();
 243   return true;
 244 }
 245
 246 // Compare compares the result of MI against zero.  If MI is a suitable load
 247 // instruction and if CCUsers is a single conditional trap on zero, eliminate
 248 // the load and convert the branch to a load-and-trap.  Return true on success.
 249 bool SystemZElimCompare::convertToLoadAndTrap(
 250     MachineInstr &MI, MachineInstr &Compare,
 251     SmallVectorImpl<MachineInstr *> &CCUsers) {
 252   unsigned LATOpcode = TII->getLoadAndTrap(MI.getOpcode());
 253   if (!LATOpcode)
 254     return false;
 255
 256   // Check whether we have a single CondTrap that traps on zero.
 257   if (CCUsers.size() != 1)
 258     return false;
 259   MachineInstr *Branch = CCUsers[0];
 260   if (Branch->getOpcode() != SystemZ::CondTrap ||
 261       Branch->getOperand(0).getImm() != SystemZ::CCMASK_ICMP ||
 262       Branch->getOperand(1).getImm() != SystemZ::CCMASK_CMP_EQ)
 263     return false;
 264
 265   // We already know that there are no references to the register between
 266   // MI and Compare.  Make sure that there are also no references between
 267   // Compare and Branch.
 268   unsigned SrcReg = getCompareSourceReg(Compare);
 269   MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
 270   for (++MBBI; MBBI != MBBE; ++MBBI)
 271     if (getRegReferences(*MBBI, SrcReg))
 272       return false;
 273
 274   // The transformation is OK.  Rebuild Branch as a load-and-trap.
 275   while (Branch->getNumOperands())
 276     Branch->RemoveOperand(0);
 277   Branch->setDesc(TII->get(LATOpcode));
 278   MachineInstrBuilder(*Branch->getParent()->getParent(), Branch)
 279       .add(MI.getOperand(0))
 280       .add(MI.getOperand(1))
 281       .add(MI.getOperand(2))
 282       .add(MI.getOperand(3));
 283   MI.eraseFromParent();
 284   return true;
 285 }
 286
 287 // If MI is a load instruction, try to convert it into a LOAD AND TEST.
 288 // Return true on success.
 289 bool SystemZElimCompare::convertToLoadAndTest(
 290     MachineInstr &MI, MachineInstr &Compare,
 291     SmallVectorImpl<MachineInstr *> &CCUsers) {
 292
 293   // Try to adjust CC masks for the LOAD AND TEST opcode that could replace MI.
 294   unsigned Opcode = TII->getLoadAndTest(MI.getOpcode());
 295   if (!Opcode || !adjustCCMasksForInstr(MI, Compare, CCUsers, Opcode))
 296     return false;
 297
 298   // Rebuild to get the CC operand in the right place.
 299   auto MIB = BuildMI(*MI.getParent(), MI, MI.getDebugLoc(), TII->get(Opcode));
 300   for (const auto &MO : MI.operands())
 301     MIB.add(MO);
 302   MIB.setMemRefs(MI.memoperands());
 303   MI.eraseFromParent();
 304
 305   return true;
 306 }
 307
 308 // The CC users in CCUsers are testing the result of a comparison of some
 309 // value X against zero and we know that any CC value produced by MI would
 310 // also reflect the value of X.  ConvOpc may be used to pass the transfomed
 311 // opcode MI will have if this succeeds.  Try to adjust CCUsers so that they
 312 // test the result of MI directly, returning true on success.  Leave
 313 // everything unchanged on failure.
 314 bool SystemZElimCompare::adjustCCMasksForInstr(
 315     MachineInstr &MI, MachineInstr &Compare,
 316     SmallVectorImpl<MachineInstr *> &CCUsers,
 317     unsigned ConvOpc) {
 318   int Opcode = (ConvOpc ? ConvOpc : MI.getOpcode());
 319   const MCInstrDesc &Desc = TII->get(Opcode);
 320   unsigned MIFlags = Desc.TSFlags;
 321
 322   // See which compare-style condition codes are available.
 323   unsigned ReusableCCMask = SystemZII::getCompareZeroCCMask(MIFlags);
 324
 325   // For unsigned comparisons with zero, only equality makes sense.
 326   unsigned CompareFlags = Compare.getDesc().TSFlags;
 327   if (CompareFlags & SystemZII::IsLogical)
 328     ReusableCCMask &= SystemZ::CCMASK_CMP_EQ;
 329
 330   if (ReusableCCMask == 0)
 331     return false;
 332
 333   unsigned CCValues = SystemZII::getCCValues(MIFlags);
 334   assert((ReusableCCMask & ~CCValues) == 0 && "Invalid CCValues");
 335
 336   bool MIEquivalentToCmp =
 337     (ReusableCCMask == CCValues &&
 338      CCValues == SystemZII::getCCValues(CompareFlags));
 339
 340   if (!MIEquivalentToCmp) {
 341     // Now check whether these flags are enough for all users.
 342     SmallVector<MachineOperand *, 4> AlterMasks;
 343     for (unsigned int I = 0, E = CCUsers.size(); I != E; ++I) {
 344       MachineInstr *MI = CCUsers[I];
 345
 346       // Fail if this isn't a use of CC that we understand.
 347       unsigned Flags = MI->getDesc().TSFlags;
 348       unsigned FirstOpNum;
 349       if (Flags & SystemZII::CCMaskFirst)
 350         FirstOpNum = 0;
 351       else if (Flags & SystemZII::CCMaskLast)
 352         FirstOpNum = MI->getNumExplicitOperands() - 2;
 353       else
 354         return false;
 355
 356       // Check whether the instruction predicate treats all CC values
 357       // outside of ReusableCCMask in the same way.  In that case it
 358       // doesn't matter what those CC values mean.
 359       unsigned CCValid = MI->getOperand(FirstOpNum).getImm();
 360       unsigned CCMask = MI->getOperand(FirstOpNum + 1).getImm();
 361       unsigned OutValid = ~ReusableCCMask & CCValid;
 362       unsigned OutMask = ~ReusableCCMask & CCMask;
 363       if (OutMask != 0 && OutMask != OutValid)
 364         return false;
 365
 366       AlterMasks.push_back(&MI->getOperand(FirstOpNum));
 367       AlterMasks.push_back(&MI->getOperand(FirstOpNum + 1));
 368     }
 369
 370     // All users are OK.  Adjust the masks for MI.
 371     for (unsigned I = 0, E = AlterMasks.size(); I != E; I += 2) {
 372       AlterMasks[I]->setImm(CCValues);
 373       unsigned CCMask = AlterMasks[I + 1]->getImm();
 374       if (CCMask & ~ReusableCCMask)
 375         AlterMasks[I + 1]->setImm((CCMask & ReusableCCMask) |
 376                                   (CCValues & ~ReusableCCMask));
 377     }
 378   }
 379
 380   // CC is now live after MI.
 381   if (!ConvOpc)
 382     MI.clearRegisterDeads(SystemZ::CC);
 383
 384   // Check if MI lies before Compare.
 385   bool BeforeCmp = false;
 386   MachineBasicBlock::iterator MBBI = MI, MBBE = MI.getParent()->end();
 387   for (++MBBI; MBBI != MBBE; ++MBBI)
 388     if (MBBI == Compare) {
 389       BeforeCmp = true;
 390       break;
 391     }
 392
 393   // Clear any intervening kills of CC.
 394   if (BeforeCmp) {
 395     MachineBasicBlock::iterator MBBI = MI, MBBE = Compare;
 396     for (++MBBI; MBBI != MBBE; ++MBBI)
 397       MBBI->clearRegisterKills(SystemZ::CC, TRI);
 398   }
 399
 400   return true;
 401 }
 402
 403 // Return true if Compare is a comparison against zero.
 404 static bool isCompareZero(MachineInstr &Compare) {
 405   switch (Compare.getOpcode()) {
 406   case SystemZ::LTEBRCompare:
 407   case SystemZ::LTDBRCompare:
 408   case SystemZ::LTXBRCompare:
 409     return true;
 410
 411   default:
 412     if (isLoadAndTestAsCmp(Compare))
 413       return true;
 414     return Compare.getNumExplicitOperands() == 2 &&
 415            Compare.getOperand(1).isImm() && Compare.getOperand(1).getImm() == 0;
 416   }
 417 }
 418
 419 // Try to optimize cases where comparison instruction Compare is testing
 420 // a value against zero.  Return true on success and if Compare should be
 421 // deleted as dead.  CCUsers is the list of instructions that use the CC
 422 // value produced by Compare.
 423 bool SystemZElimCompare::optimizeCompareZero(
 424     MachineInstr &Compare, SmallVectorImpl<MachineInstr *> &CCUsers) {
 425   if (!isCompareZero(Compare))
 426     return false;
 427
 428   // Search back for CC results that are based on the first operand.
 429   unsigned SrcReg = getCompareSourceReg(Compare);
 430   MachineBasicBlock &MBB = *Compare.getParent();
 431   Reference CCRefs;
 432   Reference SrcRefs;
 433   for (MachineBasicBlock::reverse_iterator MBBI =
 434          std::next(MachineBasicBlock::reverse_iterator(&Compare)),
 435          MBBE = MBB.rend(); MBBI != MBBE;) {
 436     MachineInstr &MI = *MBBI++;
 437     if (resultTests(MI, SrcReg)) {
 438       // Try to remove both MI and Compare by converting a branch to BRCT(G).
 439       // or a load-and-trap instruction.  We don't care in this case whether
 440       // CC is modified between MI and Compare.
 441       if (!CCRefs.Use && !SrcRefs) {
 442         if (convertToBRCT(MI, Compare, CCUsers)) {
 443           BranchOnCounts += 1;
 444           return true;
 445         }
 446         if (convertToLoadAndTrap(MI, Compare, CCUsers)) {
 447           LoadAndTraps += 1;
 448           return true;
 449         }
 450       }
 451       // Try to eliminate Compare by reusing a CC result from MI.
 452       if ((!CCRefs && convertToLoadAndTest(MI, Compare, CCUsers)) ||
 453           (!CCRefs.Def && adjustCCMasksForInstr(MI, Compare, CCUsers))) {
 454         EliminatedComparisons += 1;
 455         return true;
 456       }
 457     }
 458     SrcRefs |= getRegReferences(MI, SrcReg);
 459     if (SrcRefs.Def)
 460       break;
 461     CCRefs |= getRegReferences(MI, SystemZ::CC);
 462     if (CCRefs.Use && CCRefs.Def)
 463       break;
 464   }
 465
 466   // Also do a forward search to handle cases where an instruction after the
 467   // compare can be converted, like
 468   // LTEBRCompare %f0s, %f0s; %f2s = LER %f0s  =>  LTEBRCompare %f2s, %f0s
 469   for (MachineBasicBlock::iterator MBBI =
 470          std::next(MachineBasicBlock::iterator(&Compare)), MBBE = MBB.end();
 471        MBBI != MBBE;) {
 472     MachineInstr &MI = *MBBI++;
 473     if (preservesValueOf(MI, SrcReg)) {
 474       // Try to eliminate Compare by reusing a CC result from MI.
 475       if (convertToLoadAndTest(MI, Compare, CCUsers)) {
 476         EliminatedComparisons += 1;
 477         return true;
 478       }
 479     }
 480     if (getRegReferences(MI, SrcReg).Def)
 481       return false;
 482     if (getRegReferences(MI, SystemZ::CC))
 483       return false;
 484   }
 485
 486   return false;
 487 }
 488
 489 // Try to fuse comparison instruction Compare into a later branch.
 490 // Return true on success and if Compare is therefore redundant.
 491 bool SystemZElimCompare::fuseCompareOperations(
 492     MachineInstr &Compare, SmallVectorImpl<MachineInstr *> &CCUsers) {
 493   // See whether we have a single branch with which to fuse.
 494   if (CCUsers.size() != 1)
 495     return false;
 496   MachineInstr *Branch = CCUsers[0];
 497   SystemZII::FusedCompareType Type;
 498   switch (Branch->getOpcode()) {
 499   case SystemZ::BRC:
 500     Type = SystemZII::CompareAndBranch;
 501     break;
 502   case SystemZ::CondReturn:
 503     Type = SystemZII::CompareAndReturn;
 504     break;
 505   case SystemZ::CallBCR:
 506     Type = SystemZII::CompareAndSibcall;
 507     break;
 508   case SystemZ::CondTrap:
 509     Type = SystemZII::CompareAndTrap;
 510     break;
 511   default:
 512     return false;
 513   }
 514
 515   // See whether we have a comparison that can be fused.
 516   unsigned FusedOpcode =
 517       TII->getFusedCompare(Compare.getOpcode(), Type, &Compare);
 518   if (!FusedOpcode)
 519     return false;
 520
 521   // Make sure that the operands are available at the branch.
 522   // SrcReg2 is the register if the source operand is a register,
 523   // 0 if the source operand is immediate, and the base register
 524   // if the source operand is memory (index is not supported).
 525   Register SrcReg = Compare.getOperand(0).getReg();
 526   Register SrcReg2 =
 527     Compare.getOperand(1).isReg() ? Compare.getOperand(1).getReg() : Register();
 528   MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
 529   for (++MBBI; MBBI != MBBE; ++MBBI)
 530     if (MBBI->modifiesRegister(SrcReg, TRI) ||
 531         (SrcReg2 && MBBI->modifiesRegister(SrcReg2, TRI)))
 532       return false;
 533
 534   // Read the branch mask, target (if applicable), regmask (if applicable).
 535   MachineOperand CCMask(MBBI->getOperand(1));
 536   assert((CCMask.getImm() & ~SystemZ::CCMASK_ICMP) == 0 &&
 537          "Invalid condition-code mask for integer comparison");
 538   // This is only valid for CompareAndBranch.
 539   MachineOperand Target(MBBI->getOperand(
 540     Type == SystemZII::CompareAndBranch ? 2 : 0));
 541   const uint32_t *RegMask;
 542   if (Type == SystemZII::CompareAndSibcall)
 543     RegMask = MBBI->getOperand(2).getRegMask();
 544
 545   // Clear out all current operands.
 546   int CCUse = MBBI->findRegisterUseOperandIdx(SystemZ::CC, false, TRI);
 547   assert(CCUse >= 0 && "BRC/BCR must use CC");
 548   Branch->RemoveOperand(CCUse);
 549   // Remove target (branch) or regmask (sibcall).
 550   if (Type == SystemZII::CompareAndBranch ||
 551       Type == SystemZII::CompareAndSibcall)
 552     Branch->RemoveOperand(2);
 553   Branch->RemoveOperand(1);
 554   Branch->RemoveOperand(0);
 555
 556   // Rebuild Branch as a fused compare and branch.
 557   // SrcNOps is the number of MI operands of the compare instruction
 558   // that we need to copy over.
 559   unsigned SrcNOps = 2;
 560   if (FusedOpcode == SystemZ::CLT || FusedOpcode == SystemZ::CLGT)
 561     SrcNOps = 3;
 562   Branch->setDesc(TII->get(FusedOpcode));
 563   MachineInstrBuilder MIB(*Branch->getParent()->getParent(), Branch);
 564   for (unsigned I = 0; I < SrcNOps; I++)
 565     MIB.add(Compare.getOperand(I));
 566   MIB.add(CCMask);
 567
 568   if (Type == SystemZII::CompareAndBranch) {
 569     // Only conditional branches define CC, as they may be converted back
 570     // to a non-fused branch because of a long displacement.  Conditional
 571     // returns don't have that problem.
 572     MIB.add(Target).addReg(SystemZ::CC,
 573                            RegState::ImplicitDefine | RegState::Dead);
 574   }
 575
 576   if (Type == SystemZII::CompareAndSibcall)
 577     MIB.addRegMask(RegMask);
 578
 579   // Clear any intervening kills of SrcReg and SrcReg2.
 580   MBBI = Compare;
 581   for (++MBBI; MBBI != MBBE; ++MBBI) {
 582     MBBI->clearRegisterKills(SrcReg, TRI);
 583     if (SrcReg2)
 584       MBBI->clearRegisterKills(SrcReg2, TRI);
 585   }
 586   FusedComparisons += 1;
 587   return true;
 588 }
 589
 590 // Process all comparison instructions in MBB.  Return true if something
 591 // changed.
 592 bool SystemZElimCompare::processBlock(MachineBasicBlock &MBB) {
 593   bool Changed = false;
 594
 595   // Walk backwards through the block looking for comparisons, recording
 596   // all CC users as we go.  The subroutines can delete Compare and
 597   // instructions before it.
 598   bool CompleteCCUsers = !isCCLiveOut(MBB);
 599   SmallVector<MachineInstr *, 4> CCUsers;
 600   MachineBasicBlock::iterator MBBI = MBB.end();
 601   while (MBBI != MBB.begin()) {
 602     MachineInstr &MI = *--MBBI;
 603     if (CompleteCCUsers && (MI.isCompare() || isLoadAndTestAsCmp(MI)) &&
 604         (optimizeCompareZero(MI, CCUsers) ||
 605          fuseCompareOperations(MI, CCUsers))) {
 606       ++MBBI;
 607       MI.eraseFromParent();
 608       Changed = true;
 609       CCUsers.clear();
 610       continue;
 611     }
 612
 613     if (MI.definesRegister(SystemZ::CC)) {
 614       CCUsers.clear();
 615       CompleteCCUsers = true;
 616     }
 617     if (MI.readsRegister(SystemZ::CC) && CompleteCCUsers)
 618       CCUsers.push_back(&MI);
 619   }
 620   return Changed;
 621 }
 622
 623 bool SystemZElimCompare::runOnMachineFunction(MachineFunction &F) {
 624   if (skipFunction(F.getFunction()))
 625     return false;
 626
 627   TII = static_cast<const SystemZInstrInfo *>(F.getSubtarget().getInstrInfo());
 628   TRI = &TII->getRegisterInfo();
 629
 630   bool Changed = false;
 631   for (auto &MBB : F)
 632     Changed |= processBlock(MBB);
 633
 634   return Changed;
 635 }
 636
 637 FunctionPass *llvm::createSystemZElimComparePass(SystemZTargetMachine &TM) {
 638   return new SystemZElimCompare(TM);
 639 }