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      1 //===-- SystemZElimCompare.cpp - Eliminate comparison instructions --------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This pass:
     11 // (1) tries to remove compares if CC already contains the required information
     12 // (2) fuses compares and branches into COMPARE AND BRANCH instructions
     13 //
     14 //===----------------------------------------------------------------------===//
     15 
     16 #include "SystemZTargetMachine.h"
     17 #include "llvm/ADT/Statistic.h"
     18 #include "llvm/CodeGen/MachineFunctionPass.h"
     19 #include "llvm/CodeGen/MachineInstrBuilder.h"
     20 #include "llvm/IR/Function.h"
     21 #include "llvm/Support/CommandLine.h"
     22 #include "llvm/Support/MathExtras.h"
     23 #include "llvm/Target/TargetInstrInfo.h"
     24 #include "llvm/Target/TargetMachine.h"
     25 #include "llvm/Target/TargetRegisterInfo.h"
     26 
     27 using namespace llvm;
     28 
     29 #define DEBUG_TYPE "systemz-elim-compare"
     30 
     31 STATISTIC(BranchOnCounts, "Number of branch-on-count instructions");
     32 STATISTIC(EliminatedComparisons, "Number of eliminated comparisons");
     33 STATISTIC(FusedComparisons, "Number of fused compare-and-branch instructions");
     34 
     35 namespace {
     36 // Represents the references to a particular register in one or more
     37 // instructions.
     38 struct Reference {
     39   Reference()
     40     : Def(false), Use(false), IndirectDef(false), IndirectUse(false) {}
     41 
     42   Reference &operator|=(const Reference &Other) {
     43     Def |= Other.Def;
     44     IndirectDef |= Other.IndirectDef;
     45     Use |= Other.Use;
     46     IndirectUse |= Other.IndirectUse;
     47     return *this;
     48   }
     49 
     50   operator bool() const { return Def || Use; }
     51 
     52   // True if the register is defined or used in some form, either directly or
     53   // via a sub- or super-register.
     54   bool Def;
     55   bool Use;
     56 
     57   // True if the register is defined or used indirectly, by a sub- or
     58   // super-register.
     59   bool IndirectDef;
     60   bool IndirectUse;
     61 };
     62 
     63 class SystemZElimCompare : public MachineFunctionPass {
     64 public:
     65   static char ID;
     66   SystemZElimCompare(const SystemZTargetMachine &tm)
     67     : MachineFunctionPass(ID), TII(nullptr), TRI(nullptr) {}
     68 
     69   const char *getPassName() const override {
     70     return "SystemZ Comparison Elimination";
     71   }
     72 
     73   bool processBlock(MachineBasicBlock &MBB);
     74   bool runOnMachineFunction(MachineFunction &F) override;
     75 
     76 private:
     77   Reference getRegReferences(MachineInstr *MI, unsigned Reg);
     78   bool convertToBRCT(MachineInstr *MI, MachineInstr *Compare,
     79                      SmallVectorImpl<MachineInstr *> &CCUsers);
     80   bool convertToLoadAndTest(MachineInstr *MI);
     81   bool adjustCCMasksForInstr(MachineInstr *MI, MachineInstr *Compare,
     82                              SmallVectorImpl<MachineInstr *> &CCUsers);
     83   bool optimizeCompareZero(MachineInstr *Compare,
     84                            SmallVectorImpl<MachineInstr *> &CCUsers);
     85   bool fuseCompareAndBranch(MachineInstr *Compare,
     86                             SmallVectorImpl<MachineInstr *> &CCUsers);
     87 
     88   const SystemZInstrInfo *TII;
     89   const TargetRegisterInfo *TRI;
     90 };
     91 
     92 char SystemZElimCompare::ID = 0;
     93 } // end anonymous namespace
     94 
     95 FunctionPass *llvm::createSystemZElimComparePass(SystemZTargetMachine &TM) {
     96   return new SystemZElimCompare(TM);
     97 }
     98 
     99 // Return true if CC is live out of MBB.
    100 static bool isCCLiveOut(MachineBasicBlock &MBB) {
    101   for (auto SI = MBB.succ_begin(), SE = MBB.succ_end(); SI != SE; ++SI)
    102     if ((*SI)->isLiveIn(SystemZ::CC))
    103       return true;
    104   return false;
    105 }
    106 
    107 // Return true if any CC result of MI would reflect the value of subreg
    108 // SubReg of Reg.
    109 static bool resultTests(MachineInstr *MI, unsigned Reg, unsigned SubReg) {
    110   if (MI->getNumOperands() > 0 &&
    111       MI->getOperand(0).isReg() &&
    112       MI->getOperand(0).isDef() &&
    113       MI->getOperand(0).getReg() == Reg &&
    114       MI->getOperand(0).getSubReg() == SubReg)
    115     return true;
    116 
    117   switch (MI->getOpcode()) {
    118   case SystemZ::LR:
    119   case SystemZ::LGR:
    120   case SystemZ::LGFR:
    121   case SystemZ::LTR:
    122   case SystemZ::LTGR:
    123   case SystemZ::LTGFR:
    124   case SystemZ::LER:
    125   case SystemZ::LDR:
    126   case SystemZ::LXR:
    127   case SystemZ::LTEBR:
    128   case SystemZ::LTDBR:
    129   case SystemZ::LTXBR:
    130     if (MI->getOperand(1).getReg() == Reg &&
    131         MI->getOperand(1).getSubReg() == SubReg)
    132       return true;
    133   }
    134 
    135   return false;
    136 }
    137 
    138 // Describe the references to Reg in MI, including sub- and super-registers.
    139 Reference SystemZElimCompare::getRegReferences(MachineInstr *MI, unsigned Reg) {
    140   Reference Ref;
    141   for (unsigned I = 0, E = MI->getNumOperands(); I != E; ++I) {
    142     const MachineOperand &MO = MI->getOperand(I);
    143     if (MO.isReg()) {
    144       if (unsigned MOReg = MO.getReg()) {
    145         if (MOReg == Reg || TRI->regsOverlap(MOReg, Reg)) {
    146           if (MO.isUse()) {
    147             Ref.Use = true;
    148             Ref.IndirectUse |= (MOReg != Reg);
    149           }
    150           if (MO.isDef()) {
    151             Ref.Def = true;
    152             Ref.IndirectDef |= (MOReg != Reg);
    153           }
    154         }
    155       }
    156     }
    157   }
    158   return Ref;
    159 }
    160 
    161 // Compare compares the result of MI against zero.  If MI is an addition
    162 // of -1 and if CCUsers is a single branch on nonzero, eliminate the addition
    163 // and convert the branch to a BRCT(G).  Return true on success.
    164 bool
    165 SystemZElimCompare::convertToBRCT(MachineInstr *MI, MachineInstr *Compare,
    166                                   SmallVectorImpl<MachineInstr *> &CCUsers) {
    167   // Check whether we have an addition of -1.
    168   unsigned Opcode = MI->getOpcode();
    169   unsigned BRCT;
    170   if (Opcode == SystemZ::AHI)
    171     BRCT = SystemZ::BRCT;
    172   else if (Opcode == SystemZ::AGHI)
    173     BRCT = SystemZ::BRCTG;
    174   else
    175     return false;
    176   if (MI->getOperand(2).getImm() != -1)
    177     return false;
    178 
    179   // Check whether we have a single JLH.
    180   if (CCUsers.size() != 1)
    181     return false;
    182   MachineInstr *Branch = CCUsers[0];
    183   if (Branch->getOpcode() != SystemZ::BRC ||
    184       Branch->getOperand(0).getImm() != SystemZ::CCMASK_ICMP ||
    185       Branch->getOperand(1).getImm() != SystemZ::CCMASK_CMP_NE)
    186     return false;
    187 
    188   // We already know that there are no references to the register between
    189   // MI and Compare.  Make sure that there are also no references between
    190   // Compare and Branch.
    191   unsigned SrcReg = Compare->getOperand(0).getReg();
    192   MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
    193   for (++MBBI; MBBI != MBBE; ++MBBI)
    194     if (getRegReferences(MBBI, SrcReg))
    195       return false;
    196 
    197   // The transformation is OK.  Rebuild Branch as a BRCT(G).
    198   MachineOperand Target(Branch->getOperand(2));
    199   Branch->RemoveOperand(2);
    200   Branch->RemoveOperand(1);
    201   Branch->RemoveOperand(0);
    202   Branch->setDesc(TII->get(BRCT));
    203   MachineInstrBuilder(*Branch->getParent()->getParent(), Branch)
    204     .addOperand(MI->getOperand(0))
    205     .addOperand(MI->getOperand(1))
    206     .addOperand(Target)
    207     .addReg(SystemZ::CC, RegState::ImplicitDefine);
    208   MI->removeFromParent();
    209   return true;
    210 }
    211 
    212 // If MI is a load instruction, try to convert it into a LOAD AND TEST.
    213 // Return true on success.
    214 bool SystemZElimCompare::convertToLoadAndTest(MachineInstr *MI) {
    215   unsigned Opcode = TII->getLoadAndTest(MI->getOpcode());
    216   if (!Opcode)
    217     return false;
    218 
    219   MI->setDesc(TII->get(Opcode));
    220   MachineInstrBuilder(*MI->getParent()->getParent(), MI)
    221     .addReg(SystemZ::CC, RegState::ImplicitDefine);
    222   return true;
    223 }
    224 
    225 // The CC users in CCUsers are testing the result of a comparison of some
    226 // value X against zero and we know that any CC value produced by MI
    227 // would also reflect the value of X.  Try to adjust CCUsers so that
    228 // they test the result of MI directly, returning true on success.
    229 // Leave everything unchanged on failure.
    230 bool SystemZElimCompare::
    231 adjustCCMasksForInstr(MachineInstr *MI, MachineInstr *Compare,
    232                       SmallVectorImpl<MachineInstr *> &CCUsers) {
    233   int Opcode = MI->getOpcode();
    234   const MCInstrDesc &Desc = TII->get(Opcode);
    235   unsigned MIFlags = Desc.TSFlags;
    236 
    237   // See which compare-style condition codes are available.
    238   unsigned ReusableCCMask = SystemZII::getCompareZeroCCMask(MIFlags);
    239 
    240   // For unsigned comparisons with zero, only equality makes sense.
    241   unsigned CompareFlags = Compare->getDesc().TSFlags;
    242   if (CompareFlags & SystemZII::IsLogical)
    243     ReusableCCMask &= SystemZ::CCMASK_CMP_EQ;
    244 
    245   if (ReusableCCMask == 0)
    246     return false;
    247 
    248   unsigned CCValues = SystemZII::getCCValues(MIFlags);
    249   assert((ReusableCCMask & ~CCValues) == 0 && "Invalid CCValues");
    250 
    251   // Now check whether these flags are enough for all users.
    252   SmallVector<MachineOperand *, 4> AlterMasks;
    253   for (unsigned int I = 0, E = CCUsers.size(); I != E; ++I) {
    254     MachineInstr *MI = CCUsers[I];
    255 
    256     // Fail if this isn't a use of CC that we understand.
    257     unsigned Flags = MI->getDesc().TSFlags;
    258     unsigned FirstOpNum;
    259     if (Flags & SystemZII::CCMaskFirst)
    260       FirstOpNum = 0;
    261     else if (Flags & SystemZII::CCMaskLast)
    262       FirstOpNum = MI->getNumExplicitOperands() - 2;
    263     else
    264       return false;
    265 
    266     // Check whether the instruction predicate treats all CC values
    267     // outside of ReusableCCMask in the same way.  In that case it
    268     // doesn't matter what those CC values mean.
    269     unsigned CCValid = MI->getOperand(FirstOpNum).getImm();
    270     unsigned CCMask = MI->getOperand(FirstOpNum + 1).getImm();
    271     unsigned OutValid = ~ReusableCCMask & CCValid;
    272     unsigned OutMask = ~ReusableCCMask & CCMask;
    273     if (OutMask != 0 && OutMask != OutValid)
    274       return false;
    275 
    276     AlterMasks.push_back(&MI->getOperand(FirstOpNum));
    277     AlterMasks.push_back(&MI->getOperand(FirstOpNum + 1));
    278   }
    279 
    280   // All users are OK.  Adjust the masks for MI.
    281   for (unsigned I = 0, E = AlterMasks.size(); I != E; I += 2) {
    282     AlterMasks[I]->setImm(CCValues);
    283     unsigned CCMask = AlterMasks[I + 1]->getImm();
    284     if (CCMask & ~ReusableCCMask)
    285       AlterMasks[I + 1]->setImm((CCMask & ReusableCCMask) |
    286                                 (CCValues & ~ReusableCCMask));
    287   }
    288 
    289   // CC is now live after MI.
    290   int CCDef = MI->findRegisterDefOperandIdx(SystemZ::CC, false, true, TRI);
    291   assert(CCDef >= 0 && "Couldn't find CC set");
    292   MI->getOperand(CCDef).setIsDead(false);
    293 
    294   // Clear any intervening kills of CC.
    295   MachineBasicBlock::iterator MBBI = MI, MBBE = Compare;
    296   for (++MBBI; MBBI != MBBE; ++MBBI)
    297     MBBI->clearRegisterKills(SystemZ::CC, TRI);
    298 
    299   return true;
    300 }
    301 
    302 // Return true if Compare is a comparison against zero.
    303 static bool isCompareZero(MachineInstr *Compare) {
    304   switch (Compare->getOpcode()) {
    305   case SystemZ::LTEBRCompare:
    306   case SystemZ::LTDBRCompare:
    307   case SystemZ::LTXBRCompare:
    308     return true;
    309 
    310   default:
    311     return (Compare->getNumExplicitOperands() == 2 &&
    312             Compare->getOperand(1).isImm() &&
    313             Compare->getOperand(1).getImm() == 0);
    314   }
    315 }
    316 
    317 // Try to optimize cases where comparison instruction Compare is testing
    318 // a value against zero.  Return true on success and if Compare should be
    319 // deleted as dead.  CCUsers is the list of instructions that use the CC
    320 // value produced by Compare.
    321 bool SystemZElimCompare::
    322 optimizeCompareZero(MachineInstr *Compare,
    323                     SmallVectorImpl<MachineInstr *> &CCUsers) {
    324   if (!isCompareZero(Compare))
    325     return false;
    326 
    327   // Search back for CC results that are based on the first operand.
    328   unsigned SrcReg = Compare->getOperand(0).getReg();
    329   unsigned SrcSubReg = Compare->getOperand(0).getSubReg();
    330   MachineBasicBlock &MBB = *Compare->getParent();
    331   MachineBasicBlock::iterator MBBI = Compare, MBBE = MBB.begin();
    332   Reference CCRefs;
    333   Reference SrcRefs;
    334   while (MBBI != MBBE) {
    335     --MBBI;
    336     MachineInstr *MI = MBBI;
    337     if (resultTests(MI, SrcReg, SrcSubReg)) {
    338       // Try to remove both MI and Compare by converting a branch to BRCT(G).
    339       // We don't care in this case whether CC is modified between MI and
    340       // Compare.
    341       if (!CCRefs.Use && !SrcRefs && convertToBRCT(MI, Compare, CCUsers)) {
    342         BranchOnCounts += 1;
    343         return true;
    344       }
    345       // Try to eliminate Compare by reusing a CC result from MI.
    346       if ((!CCRefs && convertToLoadAndTest(MI)) ||
    347           (!CCRefs.Def && adjustCCMasksForInstr(MI, Compare, CCUsers))) {
    348         EliminatedComparisons += 1;
    349         return true;
    350       }
    351     }
    352     SrcRefs |= getRegReferences(MI, SrcReg);
    353     if (SrcRefs.Def)
    354       return false;
    355     CCRefs |= getRegReferences(MI, SystemZ::CC);
    356     if (CCRefs.Use && CCRefs.Def)
    357       return false;
    358   }
    359   return false;
    360 }
    361 
    362 // Try to fuse comparison instruction Compare into a later branch.
    363 // Return true on success and if Compare is therefore redundant.
    364 bool SystemZElimCompare::
    365 fuseCompareAndBranch(MachineInstr *Compare,
    366                      SmallVectorImpl<MachineInstr *> &CCUsers) {
    367   // See whether we have a comparison that can be fused.
    368   unsigned FusedOpcode = TII->getCompareAndBranch(Compare->getOpcode(),
    369                                                   Compare);
    370   if (!FusedOpcode)
    371     return false;
    372 
    373   // See whether we have a single branch with which to fuse.
    374   if (CCUsers.size() != 1)
    375     return false;
    376   MachineInstr *Branch = CCUsers[0];
    377   if (Branch->getOpcode() != SystemZ::BRC)
    378     return false;
    379 
    380   // Make sure that the operands are available at the branch.
    381   unsigned SrcReg = Compare->getOperand(0).getReg();
    382   unsigned SrcReg2 = (Compare->getOperand(1).isReg() ?
    383                       Compare->getOperand(1).getReg() : 0);
    384   MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
    385   for (++MBBI; MBBI != MBBE; ++MBBI)
    386     if (MBBI->modifiesRegister(SrcReg, TRI) ||
    387         (SrcReg2 && MBBI->modifiesRegister(SrcReg2, TRI)))
    388       return false;
    389 
    390   // Read the branch mask and target.
    391   MachineOperand CCMask(MBBI->getOperand(1));
    392   MachineOperand Target(MBBI->getOperand(2));
    393   assert((CCMask.getImm() & ~SystemZ::CCMASK_ICMP) == 0 &&
    394          "Invalid condition-code mask for integer comparison");
    395 
    396   // Clear out all current operands.
    397   int CCUse = MBBI->findRegisterUseOperandIdx(SystemZ::CC, false, TRI);
    398   assert(CCUse >= 0 && "BRC must use CC");
    399   Branch->RemoveOperand(CCUse);
    400   Branch->RemoveOperand(2);
    401   Branch->RemoveOperand(1);
    402   Branch->RemoveOperand(0);
    403 
    404   // Rebuild Branch as a fused compare and branch.
    405   Branch->setDesc(TII->get(FusedOpcode));
    406   MachineInstrBuilder(*Branch->getParent()->getParent(), Branch)
    407     .addOperand(Compare->getOperand(0))
    408     .addOperand(Compare->getOperand(1))
    409     .addOperand(CCMask)
    410     .addOperand(Target)
    411     .addReg(SystemZ::CC, RegState::ImplicitDefine);
    412 
    413   // Clear any intervening kills of SrcReg and SrcReg2.
    414   MBBI = Compare;
    415   for (++MBBI; MBBI != MBBE; ++MBBI) {
    416     MBBI->clearRegisterKills(SrcReg, TRI);
    417     if (SrcReg2)
    418       MBBI->clearRegisterKills(SrcReg2, TRI);
    419   }
    420   FusedComparisons += 1;
    421   return true;
    422 }
    423 
    424 // Process all comparison instructions in MBB.  Return true if something
    425 // changed.
    426 bool SystemZElimCompare::processBlock(MachineBasicBlock &MBB) {
    427   bool Changed = false;
    428 
    429   // Walk backwards through the block looking for comparisons, recording
    430   // all CC users as we go.  The subroutines can delete Compare and
    431   // instructions before it.
    432   bool CompleteCCUsers = !isCCLiveOut(MBB);
    433   SmallVector<MachineInstr *, 4> CCUsers;
    434   MachineBasicBlock::iterator MBBI = MBB.end();
    435   while (MBBI != MBB.begin()) {
    436     MachineInstr *MI = --MBBI;
    437     if (CompleteCCUsers &&
    438         MI->isCompare() &&
    439         (optimizeCompareZero(MI, CCUsers) ||
    440          fuseCompareAndBranch(MI, CCUsers))) {
    441       ++MBBI;
    442       MI->removeFromParent();
    443       Changed = true;
    444       CCUsers.clear();
    445       CompleteCCUsers = true;
    446       continue;
    447     }
    448 
    449     Reference CCRefs(getRegReferences(MI, SystemZ::CC));
    450     if (CCRefs.Def) {
    451       CCUsers.clear();
    452       CompleteCCUsers = !CCRefs.IndirectDef;
    453     }
    454     if (CompleteCCUsers && CCRefs.Use)
    455       CCUsers.push_back(MI);
    456   }
    457   return Changed;
    458 }
    459 
    460 bool SystemZElimCompare::runOnMachineFunction(MachineFunction &F) {
    461   TII = static_cast<const SystemZInstrInfo *>(F.getTarget().getInstrInfo());
    462   TRI = &TII->getRegisterInfo();
    463 
    464   bool Changed = false;
    465   for (auto &MBB : F)
    466     Changed |= processBlock(MBB);
    467 
    468   return Changed;
    469 }
    470