llvm/lib/Target/X86/X86CondBrFolding.cpp - toolchain/llvm-project - Gitiles

 //===---- X86CondBrFolding.cpp - optimize conditional branches ------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 // This file defines a pass that optimizes condition branches on x86 by taking
 // advantage of the three-way conditional code generated by compare
 // instructions.
 // Currently, it tries to hoisting EQ and NE conditional branch to a dominant
 // conditional branch condition where the same EQ/NE conditional code is
 // computed. An example:
 //   bb_0:
 //     cmp %0, 19
 //     jg bb_1
 //     jmp bb_2
 //   bb_1:
 //     cmp %0, 40
 //     jg bb_3
 //     jmp bb_4
 //   bb_4:
 //     cmp %0, 20
 //     je bb_5
 //     jmp bb_6
 // Here we could combine the two compares in bb_0 and bb_4 and have the
 // following code:
 //   bb_0:
 //     cmp %0, 20
 //     jg bb_1
 //     jl bb_2
 //     jmp bb_5
 //   bb_1:
 //     cmp %0, 40
 //     jg bb_3
 //     jmp bb_6
 // For the case of %0 == 20 (bb_5), we eliminate two jumps, and the control
 // height for bb_6 is also reduced. bb_4 is gone after the optimization.
 //
 // There are plenty of this code patterns, especially from the switch case
 // lowing where we generate compare of "pivot-1" for the inner nodes in the
 // binary search tree.
 //===----------------------------------------------------------------------===//

 #include "X86.h"
 #include "X86InstrInfo.h"
 #include "X86Subtarget.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/Support/BranchProbability.h"

 using namespace llvm;

 #define DEBUG_TYPE "x86-condbr-folding"

 STATISTIC(NumFixedCondBrs, "Number of x86 condbr folded");

 namespace {
 class X86CondBrFoldingPass : public MachineFunctionPass {
 public:
   X86CondBrFoldingPass() : MachineFunctionPass(ID) {
     initializeX86CondBrFoldingPassPass(*PassRegistry::getPassRegistry());
   }
   StringRef getPassName() const override { return "X86 CondBr Folding"; }

   bool runOnMachineFunction(MachineFunction &MF) override;

   void getAnalysisUsage(AnalysisUsage &AU) const override {
     MachineFunctionPass::getAnalysisUsage(AU);
     AU.addRequired<MachineBranchProbabilityInfo>();
   }

 public:
   static char ID;
 };
 } // namespace

 char X86CondBrFoldingPass::ID = 0;
 INITIALIZE_PASS(X86CondBrFoldingPass, "X86CondBrFolding", "X86CondBrFolding", false, false)

 FunctionPass *llvm::createX86CondBrFolding() {
   return new X86CondBrFoldingPass();
 }

 namespace {
 // A class the stores the auxiliary information for each MBB.
 struct TargetMBBInfo {
   MachineBasicBlock *TBB;
   MachineBasicBlock *FBB;
   MachineInstr *BrInstr;
   MachineInstr *CmpInstr;
   X86::CondCode BranchCode;
   unsigned SrcReg;
   int CmpValue;
   bool Modified;
   bool CmpBrOnly;
 };

 // A class that optimizes the conditional branch by hoisting and merge CondCode.
 class X86CondBrFolding {
 public:
   X86CondBrFolding(const X86InstrInfo *TII,
                    const MachineBranchProbabilityInfo *MBPI,
                    MachineFunction &MF)
       : TII(TII), MBPI(MBPI), MF(MF) {}
   bool optimize();

 private:
   const X86InstrInfo *TII;
   const MachineBranchProbabilityInfo *MBPI;
   MachineFunction &MF;
   std::vector<std::unique_ptr<TargetMBBInfo>> MBBInfos;
   SmallVector<MachineBasicBlock *, 4> RemoveList;

   void optimizeCondBr(MachineBasicBlock &MBB,
                       SmallVectorImpl<MachineBasicBlock *> &BranchPath);
   void fixBranchProb(MachineBasicBlock *NextMBB, MachineBasicBlock *RootMBB,
                      SmallVectorImpl<MachineBasicBlock *> &BranchPath);
   void replaceBrDest(MachineBasicBlock *MBB, MachineBasicBlock *OrigDest,
                      MachineBasicBlock *NewDest);
   void fixupModifiedCond(MachineBasicBlock *MBB);
   std::unique_ptr<TargetMBBInfo> analyzeMBB(MachineBasicBlock &MBB);
   static bool analyzeCompare(const MachineInstr &MI, unsigned &SrcReg,
                              int &CmpValue);
   bool findPath(MachineBasicBlock *MBB,
                 SmallVectorImpl<MachineBasicBlock *> &BranchPath);
   TargetMBBInfo *getMBBInfo(MachineBasicBlock *MBB) const {
     return MBBInfos[MBB->getNumber()].get();
   }
 };
 } // namespace

 // Find a valid path that we can reuse the CondCode.
 // The resulted path (if return true) is stored in BranchPath.
 // Return value:
 //  false: is no valid path is found.
 //  true: a valid path is found and the targetBB can be reached.
 bool X86CondBrFolding::findPath(
     MachineBasicBlock *MBB, SmallVectorImpl<MachineBasicBlock *> &BranchPath) {
   TargetMBBInfo *MBBInfo = getMBBInfo(MBB);
   assert(MBBInfo && "Expecting a candidate MBB");
   int CmpValue = MBBInfo->CmpValue;

   MachineBasicBlock *PredMBB = *MBB->pred_begin();
   MachineBasicBlock *SaveMBB = MBB;
   while (PredMBB) {
     TargetMBBInfo *PredMBBInfo = getMBBInfo(PredMBB);
     if (!PredMBBInfo || PredMBBInfo->SrcReg != MBBInfo->SrcReg)
       return false;

     assert(SaveMBB == PredMBBInfo->TBB || SaveMBB == PredMBBInfo->FBB);
     bool IsFalseBranch = (SaveMBB == PredMBBInfo->FBB);

     X86::CondCode CC = PredMBBInfo->BranchCode;
     assert(CC == X86::COND_L || CC == X86::COND_G || CC == X86::COND_E);
     int PredCmpValue = PredMBBInfo->CmpValue;
     bool ValueCmpTrue = ((CmpValue < PredCmpValue && CC == X86::COND_L) ||
                          (CmpValue > PredCmpValue && CC == X86::COND_G) ||
                          (CmpValue == PredCmpValue && CC == X86::COND_E));
     // Check if both the result of value compare and the branch target match.
     if (!(ValueCmpTrue ^ IsFalseBranch)) {
       LLVM_DEBUG(dbgs() << "Dead BB detected!\n");
       return false;
     }

     BranchPath.push_back(PredMBB);
     // These are the conditions on which we could combine the compares.
     if ((CmpValue == PredCmpValue) ||
         (CmpValue == PredCmpValue - 1 && CC == X86::COND_L) ||
         (CmpValue == PredCmpValue + 1 && CC == X86::COND_G))
       return true;

     // If PredMBB has more than on preds, or not a pure cmp and br, we bailout.
     if (PredMBB->pred_size() != 1 || !PredMBBInfo->CmpBrOnly)
       return false;

     SaveMBB = PredMBB;
     PredMBB = *PredMBB->pred_begin();
   }
   return false;
 }

 // Fix up any PHI node in the successor of MBB.
 static void fixPHIsInSucc(MachineBasicBlock *MBB, MachineBasicBlock *OldMBB,
                           MachineBasicBlock *NewMBB) {
   if (NewMBB == OldMBB)
     return;
   for (auto MI = MBB->instr_begin(), ME = MBB->instr_end();
        MI != ME && MI->isPHI(); ++MI)
     for (unsigned i = 2, e = MI->getNumOperands() + 1; i != e; i += 2) {
       MachineOperand &MO = MI->getOperand(i);
       if (MO.getMBB() == OldMBB)
         MO.setMBB(NewMBB);
     }
 }

 // Utility function to set branch probability for edge MBB->SuccMBB.
 static inline bool setBranchProb(MachineBasicBlock *MBB,
                                  MachineBasicBlock *SuccMBB,
                                  BranchProbability Prob) {
   auto MBBI = std::find(MBB->succ_begin(), MBB->succ_end(), SuccMBB);
   if (MBBI == MBB->succ_end())
     return false;
   MBB->setSuccProbability(MBBI, Prob);
   return true;
 }

 // Utility function to find the unconditional br instruction in MBB.
 static inline MachineBasicBlock::iterator
 findUncondBrI(MachineBasicBlock *MBB) {
   return std::find_if(MBB->begin(), MBB->end(), [](MachineInstr &MI) -> bool {
     return MI.getOpcode() == X86::JMP_1;
   });
 }

 // Replace MBB's original successor, OrigDest, with NewDest.
 // Also update the MBBInfo for MBB.
 void X86CondBrFolding::replaceBrDest(MachineBasicBlock *MBB,
                                      MachineBasicBlock *OrigDest,
                                      MachineBasicBlock *NewDest) {
   TargetMBBInfo *MBBInfo = getMBBInfo(MBB);
   MachineInstr *BrMI;
   if (MBBInfo->TBB == OrigDest) {
     BrMI = MBBInfo->BrInstr;
     unsigned JNCC = GetCondBranchFromCond(MBBInfo->BranchCode);
     MachineInstrBuilder MIB =
         BuildMI(*MBB, BrMI, MBB->findDebugLoc(BrMI), TII->get(JNCC))
             .addMBB(NewDest);
     MBBInfo->TBB = NewDest;
     MBBInfo->BrInstr = MIB.getInstr();
   } else { // Should be the unconditional jump stmt.
     MachineBasicBlock::iterator UncondBrI = findUncondBrI(MBB);
     BuildMI(*MBB, UncondBrI, MBB->findDebugLoc(UncondBrI), TII->get(X86::JMP_1))
         .addMBB(NewDest);
     MBBInfo->FBB = NewDest;
     BrMI = &*UncondBrI;
   }
   fixPHIsInSucc(NewDest, OrigDest, MBB);
   BrMI->eraseFromParent();
   MBB->addSuccessor(NewDest);
   setBranchProb(MBB, NewDest, MBPI->getEdgeProbability(MBB, OrigDest));
   MBB->removeSuccessor(OrigDest);
 }

 // Change the CondCode and BrInstr according to MBBInfo.
 void X86CondBrFolding::fixupModifiedCond(MachineBasicBlock *MBB) {
   TargetMBBInfo *MBBInfo = getMBBInfo(MBB);
   if (!MBBInfo->Modified)
     return;

   MachineInstr *BrMI = MBBInfo->BrInstr;
   X86::CondCode CC = MBBInfo->BranchCode;
   MachineInstrBuilder MIB = BuildMI(*MBB, BrMI, MBB->findDebugLoc(BrMI),
                                     TII->get(GetCondBranchFromCond(CC)))
                                 .addMBB(MBBInfo->TBB);
   BrMI->eraseFromParent();
   MBBInfo->BrInstr = MIB.getInstr();

   MachineBasicBlock::iterator UncondBrI = findUncondBrI(MBB);
   BuildMI(*MBB, UncondBrI, MBB->findDebugLoc(UncondBrI), TII->get(X86::JMP_1))
       .addMBB(MBBInfo->FBB);
   MBB->erase(UncondBrI);
   MBBInfo->Modified = false;
 }

 //
 // Apply the transformation:
 //  RootMBB -1-> ... PredMBB -3-> MBB -5-> TargetMBB
 //     \-2->           \-4->       \-6-> FalseMBB
 // ==>
 //             RootMBB -1-> ... PredMBB -7-> FalseMBB
 // TargetMBB <-8-/ \-2->           \-4->
 //
 // Note that PredMBB and RootMBB could be the same.
 // And in the case of dead TargetMBB, we will not have TargetMBB and edge 8.
 //
 // There are some special handling where the RootMBB is COND_E in which case
 // we directly short-cycle the brinstr.
 //
 void X86CondBrFolding::optimizeCondBr(
     MachineBasicBlock &MBB, SmallVectorImpl<MachineBasicBlock *> &BranchPath) {

   X86::CondCode CC;
   TargetMBBInfo *MBBInfo = getMBBInfo(&MBB);
   assert(MBBInfo && "Expecting a candidate MBB");
   MachineBasicBlock *TargetMBB = MBBInfo->TBB;
   BranchProbability TargetProb = MBPI->getEdgeProbability(&MBB, MBBInfo->TBB);

   // Forward the jump from MBB's predecessor to MBB's false target.
   MachineBasicBlock *PredMBB = BranchPath.front();
   TargetMBBInfo *PredMBBInfo = getMBBInfo(PredMBB);
   assert(PredMBBInfo && "Expecting a candidate MBB");
   if (PredMBBInfo->Modified)
     fixupModifiedCond(PredMBB);
   CC = PredMBBInfo->BranchCode;
   // Don't do this if depth of BranchPath is 1 and PredMBB is of COND_E.
   // We will short-cycle directly for this case.
   if (!(CC == X86::COND_E && BranchPath.size() == 1))
     replaceBrDest(PredMBB, &MBB, MBBInfo->FBB);

   MachineBasicBlock *RootMBB = BranchPath.back();
   TargetMBBInfo *RootMBBInfo = getMBBInfo(RootMBB);
   assert(RootMBBInfo && "Expecting a candidate MBB");
   if (RootMBBInfo->Modified)
     fixupModifiedCond(RootMBB);
   CC = RootMBBInfo->BranchCode;

   if (CC != X86::COND_E) {
     MachineBasicBlock::iterator UncondBrI = findUncondBrI(RootMBB);
     // RootMBB: Cond jump to the original not-taken MBB.
     X86::CondCode NewCC;
     switch (CC) {
     case X86::COND_L:
       NewCC = X86::COND_G;
       break;
     case X86::COND_G:
       NewCC = X86::COND_L;
       break;
     default:
       llvm_unreachable("unexpected condtional code.");
     }
     BuildMI(*RootMBB, UncondBrI, RootMBB->findDebugLoc(UncondBrI),
             TII->get(GetCondBranchFromCond(NewCC)))
         .addMBB(RootMBBInfo->FBB);

     // RootMBB: Jump to TargetMBB
     BuildMI(*RootMBB, UncondBrI, RootMBB->findDebugLoc(UncondBrI),
             TII->get(X86::JMP_1))
         .addMBB(TargetMBB);
     RootMBB->addSuccessor(TargetMBB);
     fixPHIsInSucc(TargetMBB, &MBB, RootMBB);
     RootMBB->erase(UncondBrI);
   } else {
     replaceBrDest(RootMBB, RootMBBInfo->TBB, TargetMBB);
   }

   // Fix RootMBB's CmpValue to MBB's CmpValue to TargetMBB. Don't set Imm
   // directly. Move MBB's stmt to here as the opcode might be different.
   if (RootMBBInfo->CmpValue != MBBInfo->CmpValue) {
     MachineInstr *NewCmp = MBBInfo->CmpInstr;
     NewCmp->removeFromParent();
     RootMBB->insert(RootMBBInfo->CmpInstr, NewCmp);
     RootMBBInfo->CmpInstr->eraseFromParent();
   }

   // Fix branch Probabilities.
   auto fixBranchProb = [&](MachineBasicBlock *NextMBB) {
     BranchProbability Prob;
     for (auto &I : BranchPath) {
       MachineBasicBlock *ThisMBB = I;
       if (!ThisMBB->hasSuccessorProbabilities() ||
           !ThisMBB->isSuccessor(NextMBB))
         break;
       Prob = MBPI->getEdgeProbability(ThisMBB, NextMBB);
       if (Prob.isUnknown())
         break;
       TargetProb = Prob * TargetProb;
       Prob = Prob - TargetProb;
       setBranchProb(ThisMBB, NextMBB, Prob);
       if (ThisMBB == RootMBB) {
         setBranchProb(ThisMBB, TargetMBB, TargetProb);
       }
       ThisMBB->normalizeSuccProbs();
       if (ThisMBB == RootMBB)
         break;
       NextMBB = ThisMBB;
     }
     return true;
   };
   if (CC != X86::COND_E && !TargetProb.isUnknown())
     fixBranchProb(MBBInfo->FBB);

   if (CC != X86::COND_E)
     RemoveList.push_back(&MBB);

   // Invalidate MBBInfo just in case.
   MBBInfos[MBB.getNumber()] = nullptr;
   MBBInfos[RootMBB->getNumber()] = nullptr;

   LLVM_DEBUG(dbgs() << "After optimization:\nRootMBB is: " << *RootMBB << "\n");
   if (BranchPath.size() > 1)
     LLVM_DEBUG(dbgs() << "PredMBB is: " << *(BranchPath[0]) << "\n");
 }

 // Driver function for optimization: find the valid candidate and apply
 // the transformation.
 bool X86CondBrFolding::optimize() {
   bool Changed = false;
   LLVM_DEBUG(dbgs() << "***** X86CondBr Folding on Function: " << MF.getName()
                     << " *****\n");
   // Setup data structures.
   MBBInfos.resize(MF.getNumBlockIDs());
   for (auto &MBB : MF)
     MBBInfos[MBB.getNumber()] = analyzeMBB(MBB);

   for (auto &MBB : MF) {
     TargetMBBInfo *MBBInfo = getMBBInfo(&MBB);
     if (!MBBInfo || !MBBInfo->CmpBrOnly)
       continue;
     if (MBB.pred_size() != 1)
       continue;
     LLVM_DEBUG(dbgs() << "Work on MBB." << MBB.getNumber()
                       << " CmpValue: " << MBBInfo->CmpValue << "\n");
     SmallVector<MachineBasicBlock *, 4> BranchPath;
     if (!findPath(&MBB, BranchPath))
       continue;

 #ifndef NDEBUG
     LLVM_DEBUG(dbgs() << "Found one path (len=" << BranchPath.size() << "):\n");
     int Index = 1;
     LLVM_DEBUG(dbgs() << "Target MBB is: " << MBB << "\n");
     for (auto I = BranchPath.rbegin(); I != BranchPath.rend(); ++I, ++Index) {
       MachineBasicBlock *PMBB = *I;
       TargetMBBInfo *PMBBInfo = getMBBInfo(PMBB);
       LLVM_DEBUG(dbgs() << "Path MBB (" << Index << " of " << BranchPath.size()
                         << ") is " << *PMBB);
       LLVM_DEBUG(dbgs() << "CC=" << PMBBInfo->BranchCode
                         << "  Val=" << PMBBInfo->CmpValue
                         << "  CmpBrOnly=" << PMBBInfo->CmpBrOnly << "\n\n");
     }
 #endif
     optimizeCondBr(MBB, BranchPath);
     Changed = true;
   }
   NumFixedCondBrs += RemoveList.size();
   for (auto MBBI : RemoveList) {
     while (!MBBI->succ_empty())
       MBBI->removeSuccessor(MBBI->succ_end() - 1);

     MBBI->eraseFromParent();
   }

   return Changed;
 }

 // Analyze instructions that generate CondCode and extract information.
 bool X86CondBrFolding::analyzeCompare(const MachineInstr &MI, unsigned &SrcReg,
                                       int &CmpValue) {
   unsigned SrcRegIndex = 0;
   unsigned ValueIndex = 0;
   switch (MI.getOpcode()) {
   // TODO: handle test instructions.
   default:
     return false;
   case X86::CMP64ri32:
   case X86::CMP64ri8:
   case X86::CMP32ri:
   case X86::CMP32ri8:
   case X86::CMP16ri:
   case X86::CMP16ri8:
   case X86::CMP8ri:
     SrcRegIndex = 0;
     ValueIndex = 1;
     break;
   case X86::SUB64ri32:
   case X86::SUB64ri8:
   case X86::SUB32ri:
   case X86::SUB32ri8:
   case X86::SUB16ri:
   case X86::SUB16ri8:
   case X86::SUB8ri:
     SrcRegIndex = 1;
     ValueIndex = 2;
     break;
   }
   SrcReg = MI.getOperand(SrcRegIndex).getReg();
   if (!MI.getOperand(ValueIndex).isImm())
     return false;
   CmpValue = MI.getOperand(ValueIndex).getImm();
   return true;
 }

 // Analyze a candidate MBB and set the extract all the information needed.
 // The valid candidate will have two successors.
 // It also should have a sequence of
 //  Branch_instr,
 //  CondBr,
 //  UnCondBr.
 // Return TargetMBBInfo if MBB is a valid candidate and nullptr otherwise.
 std::unique_ptr<TargetMBBInfo>
 X86CondBrFolding::analyzeMBB(MachineBasicBlock &MBB) {
   MachineBasicBlock *TBB;
   MachineBasicBlock *FBB;
   MachineInstr *BrInstr;
   MachineInstr *CmpInstr;
   X86::CondCode CC;
   unsigned SrcReg;
   int CmpValue;
   bool Modified;
   bool CmpBrOnly;

   if (MBB.succ_size() != 2)
     return nullptr;

   CmpBrOnly = true;
   FBB = TBB = nullptr;
   CmpInstr = nullptr;
   MachineBasicBlock::iterator I = MBB.end();
   while (I != MBB.begin()) {
     --I;
     if (I->isDebugValue())
       continue;
     if (I->getOpcode() == X86::JMP_1) {
       if (FBB)
         return nullptr;
       FBB = I->getOperand(0).getMBB();
       continue;
     }
     if (I->isBranch()) {
       if (TBB)
         return nullptr;
       CC = X86::getCondFromBranchOpc(I->getOpcode());
       switch (CC) {
       default:
         return nullptr;
       case X86::COND_E:
       case X86::COND_L:
       case X86::COND_G:
       case X86::COND_NE:
       case X86::COND_LE:
       case X86::COND_GE:
         break;
       }
       TBB = I->getOperand(0).getMBB();
       BrInstr = &*I;
       continue;
     }
     if (analyzeCompare(*I, SrcReg, CmpValue)) {
       if (CmpInstr)
         return nullptr;
       CmpInstr = &*I;
       continue;
     }
     CmpBrOnly = false;
     break;
   }

   if (!TBB || !FBB || !CmpInstr)
     return nullptr;

   // Simplify CondCode. Note this is only to simplify the findPath logic
   // and will not change the instruction here.
   switch (CC) {
   case X86::COND_NE:
     CC = X86::COND_E;
     std::swap(TBB, FBB);
     Modified = true;
     break;
   case X86::COND_LE:
     if (CmpValue == INT_MAX)
       return nullptr;
     CC = X86::COND_L;
     CmpValue += 1;
     Modified = true;
     break;
   case X86::COND_GE:
     if (CmpValue == INT_MIN)
       return nullptr;
     CC = X86::COND_G;
     CmpValue -= 1;
     Modified = true;
     break;
   default:
     Modified = false;
     break;
   }
   return llvm::make_unique<TargetMBBInfo>(TargetMBBInfo{
       TBB, FBB, BrInstr, CmpInstr, CC, SrcReg, CmpValue, Modified, CmpBrOnly});
 }

 bool X86CondBrFoldingPass::runOnMachineFunction(MachineFunction &MF) {
   const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
   if (!ST.threewayBranchProfitable())
     return false;
   const X86InstrInfo *TII = ST.getInstrInfo();
   const MachineBranchProbabilityInfo *MBPI =
       &getAnalysis<MachineBranchProbabilityInfo>();

   X86CondBrFolding CondBr(TII, MBPI, MF);
   return CondBr.optimize();
 }
	//===---- X86CondBrFolding.cpp - optimize conditional branches ------------===//
	//
	// 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
	//
	//===----------------------------------------------------------------------===//
	// This file defines a pass that optimizes condition branches on x86 by taking
	// advantage of the three-way conditional code generated by compare
	// instructions.
	// Currently, it tries to hoisting EQ and NE conditional branch to a dominant
	// conditional branch condition where the same EQ/NE conditional code is
	// computed. An example:
	// bb_0:
	// cmp %0, 19
	// jg bb_1
	// jmp bb_2
	// bb_1:
	// cmp %0, 40
	// jg bb_3
	// jmp bb_4
	// bb_4:
	// cmp %0, 20
	// je bb_5
	// jmp bb_6
	// Here we could combine the two compares in bb_0 and bb_4 and have the
	// following code:
	// bb_0:
	// cmp %0, 20
	// jg bb_1
	// jl bb_2
	// jmp bb_5
	// bb_1:
	// cmp %0, 40
	// jg bb_3
	// jmp bb_6
	// For the case of %0 == 20 (bb_5), we eliminate two jumps, and the control
	// height for bb_6 is also reduced. bb_4 is gone after the optimization.
	//
	// There are plenty of this code patterns, especially from the switch case
	// lowing where we generate compare of "pivot-1" for the inner nodes in the
	// binary search tree.
	//===----------------------------------------------------------------------===//

	#include "X86.h"
	#include "X86InstrInfo.h"
	#include "X86Subtarget.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
	#include "llvm/CodeGen/MachineFunctionPass.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/CodeGen/MachineRegisterInfo.h"
	#include "llvm/Support/BranchProbability.h"

	using namespace llvm;

	#define DEBUG_TYPE "x86-condbr-folding"

	STATISTIC(NumFixedCondBrs, "Number of x86 condbr folded");

	namespace {
	class X86CondBrFoldingPass : public MachineFunctionPass {
	public:
	X86CondBrFoldingPass() : MachineFunctionPass(ID) {
	initializeX86CondBrFoldingPassPass(*PassRegistry::getPassRegistry());
	}
	StringRef getPassName() const override { return "X86 CondBr Folding"; }

	bool runOnMachineFunction(MachineFunction &MF) override;

	void getAnalysisUsage(AnalysisUsage &AU) const override {
	MachineFunctionPass::getAnalysisUsage(AU);
	AU.addRequired<MachineBranchProbabilityInfo>();
	}

	public:
	static char ID;
	};
	} // namespace

	char X86CondBrFoldingPass::ID = 0;
	INITIALIZE_PASS(X86CondBrFoldingPass, "X86CondBrFolding", "X86CondBrFolding", false, false)

	FunctionPass *llvm::createX86CondBrFolding() {
	return new X86CondBrFoldingPass();
	}

	namespace {
	// A class the stores the auxiliary information for each MBB.
	struct TargetMBBInfo {
	MachineBasicBlock *TBB;
	MachineBasicBlock *FBB;
	MachineInstr *BrInstr;
	MachineInstr *CmpInstr;
	X86::CondCode BranchCode;
	unsigned SrcReg;
	int CmpValue;
	bool Modified;
	bool CmpBrOnly;
	};

	// A class that optimizes the conditional branch by hoisting and merge CondCode.
	class X86CondBrFolding {
	public:
	X86CondBrFolding(const X86InstrInfo *TII,
	const MachineBranchProbabilityInfo *MBPI,
	MachineFunction &MF)
	: TII(TII), MBPI(MBPI), MF(MF) {}
	bool optimize();

	private:
	const X86InstrInfo *TII;
	const MachineBranchProbabilityInfo *MBPI;
	MachineFunction &MF;
	std::vector<std::unique_ptr<TargetMBBInfo>> MBBInfos;
	SmallVector<MachineBasicBlock *, 4> RemoveList;

	void optimizeCondBr(MachineBasicBlock &MBB,
	SmallVectorImpl<MachineBasicBlock *> &BranchPath);
	void fixBranchProb(MachineBasicBlock NextMBB, MachineBasicBlock RootMBB,
	SmallVectorImpl<MachineBasicBlock *> &BranchPath);
	void replaceBrDest(MachineBasicBlock MBB, MachineBasicBlock OrigDest,
	MachineBasicBlock *NewDest);
	void fixupModifiedCond(MachineBasicBlock *MBB);
	std::unique_ptr<TargetMBBInfo> analyzeMBB(MachineBasicBlock &MBB);
	static bool analyzeCompare(const MachineInstr &MI, unsigned &SrcReg,
	int &CmpValue);
	bool findPath(MachineBasicBlock *MBB,
	SmallVectorImpl<MachineBasicBlock *> &BranchPath);
	TargetMBBInfo getMBBInfo(MachineBasicBlock MBB) const {
	return MBBInfos[MBB->getNumber()].get();
	}
	};
	} // namespace

	// Find a valid path that we can reuse the CondCode.
	// The resulted path (if return true) is stored in BranchPath.
	// Return value:
	// false: is no valid path is found.
	// true: a valid path is found and the targetBB can be reached.
	bool X86CondBrFolding::findPath(
	MachineBasicBlock MBB, SmallVectorImpl<MachineBasicBlock > &BranchPath) {
	TargetMBBInfo *MBBInfo = getMBBInfo(MBB);
	assert(MBBInfo && "Expecting a candidate MBB");
	int CmpValue = MBBInfo->CmpValue;

	MachineBasicBlock PredMBB = MBB->pred_begin();
	MachineBasicBlock *SaveMBB = MBB;
	while (PredMBB) {
	TargetMBBInfo *PredMBBInfo = getMBBInfo(PredMBB);
	if (!PredMBBInfo \|\| PredMBBInfo->SrcReg != MBBInfo->SrcReg)
	return false;

	assert(SaveMBB == PredMBBInfo->TBB \|\| SaveMBB == PredMBBInfo->FBB);
	bool IsFalseBranch = (SaveMBB == PredMBBInfo->FBB);

	X86::CondCode CC = PredMBBInfo->BranchCode;
	assert(CC == X86::COND_L \|\| CC == X86::COND_G \|\| CC == X86::COND_E);
	int PredCmpValue = PredMBBInfo->CmpValue;
	bool ValueCmpTrue = ((CmpValue < PredCmpValue && CC == X86::COND_L) \|\|
	(CmpValue > PredCmpValue && CC == X86::COND_G) \|\|
	(CmpValue == PredCmpValue && CC == X86::COND_E));
	// Check if both the result of value compare and the branch target match.
	if (!(ValueCmpTrue ^ IsFalseBranch)) {
	LLVM_DEBUG(dbgs() << "Dead BB detected!\n");
	return false;
	}

	BranchPath.push_back(PredMBB);
	// These are the conditions on which we could combine the compares.
	if ((CmpValue == PredCmpValue) \|\|
	(CmpValue == PredCmpValue - 1 && CC == X86::COND_L) \|\|
	(CmpValue == PredCmpValue + 1 && CC == X86::COND_G))
	return true;

	// If PredMBB has more than on preds, or not a pure cmp and br, we bailout.
	if (PredMBB->pred_size() != 1 \|\| !PredMBBInfo->CmpBrOnly)
	return false;

	SaveMBB = PredMBB;
	PredMBB = *PredMBB->pred_begin();
	}
	return false;
	}

	// Fix up any PHI node in the successor of MBB.
	static void fixPHIsInSucc(MachineBasicBlock MBB, MachineBasicBlock OldMBB,
	MachineBasicBlock *NewMBB) {
	if (NewMBB == OldMBB)
	return;
	for (auto MI = MBB->instr_begin(), ME = MBB->instr_end();
	MI != ME && MI->isPHI(); ++MI)
	for (unsigned i = 2, e = MI->getNumOperands() + 1; i != e; i += 2) {
	MachineOperand &MO = MI->getOperand(i);
	if (MO.getMBB() == OldMBB)
	MO.setMBB(NewMBB);
	}
	}

	// Utility function to set branch probability for edge MBB->SuccMBB.
	static inline bool setBranchProb(MachineBasicBlock *MBB,
	MachineBasicBlock *SuccMBB,
	BranchProbability Prob) {
	auto MBBI = std::find(MBB->succ_begin(), MBB->succ_end(), SuccMBB);
	if (MBBI == MBB->succ_end())
	return false;
	MBB->setSuccProbability(MBBI, Prob);
	return true;
	}

	// Utility function to find the unconditional br instruction in MBB.
	static inline MachineBasicBlock::iterator
	findUncondBrI(MachineBasicBlock *MBB) {
	return std::find_if(MBB->begin(), MBB->end(), [](MachineInstr &MI) -> bool {
	return MI.getOpcode() == X86::JMP_1;
	});
	}

	// Replace MBB's original successor, OrigDest, with NewDest.
	// Also update the MBBInfo for MBB.
	void X86CondBrFolding::replaceBrDest(MachineBasicBlock *MBB,
	MachineBasicBlock *OrigDest,
	MachineBasicBlock *NewDest) {
	TargetMBBInfo *MBBInfo = getMBBInfo(MBB);
	MachineInstr *BrMI;
	if (MBBInfo->TBB == OrigDest) {
	BrMI = MBBInfo->BrInstr;
	unsigned JNCC = GetCondBranchFromCond(MBBInfo->BranchCode);
	MachineInstrBuilder MIB =
	BuildMI(*MBB, BrMI, MBB->findDebugLoc(BrMI), TII->get(JNCC))
	.addMBB(NewDest);
	MBBInfo->TBB = NewDest;
	MBBInfo->BrInstr = MIB.getInstr();
	} else { // Should be the unconditional jump stmt.
	MachineBasicBlock::iterator UncondBrI = findUncondBrI(MBB);
	BuildMI(*MBB, UncondBrI, MBB->findDebugLoc(UncondBrI), TII->get(X86::JMP_1))
	.addMBB(NewDest);
	MBBInfo->FBB = NewDest;
	BrMI = &*UncondBrI;
	}
	fixPHIsInSucc(NewDest, OrigDest, MBB);
	BrMI->eraseFromParent();
	MBB->addSuccessor(NewDest);
	setBranchProb(MBB, NewDest, MBPI->getEdgeProbability(MBB, OrigDest));
	MBB->removeSuccessor(OrigDest);
	}

	// Change the CondCode and BrInstr according to MBBInfo.
	void X86CondBrFolding::fixupModifiedCond(MachineBasicBlock *MBB) {
	TargetMBBInfo *MBBInfo = getMBBInfo(MBB);
	if (!MBBInfo->Modified)
	return;

	MachineInstr *BrMI = MBBInfo->BrInstr;
	X86::CondCode CC = MBBInfo->BranchCode;
	MachineInstrBuilder MIB = BuildMI(*MBB, BrMI, MBB->findDebugLoc(BrMI),
	TII->get(GetCondBranchFromCond(CC)))
	.addMBB(MBBInfo->TBB);
	BrMI->eraseFromParent();
	MBBInfo->BrInstr = MIB.getInstr();

	MachineBasicBlock::iterator UncondBrI = findUncondBrI(MBB);
	BuildMI(*MBB, UncondBrI, MBB->findDebugLoc(UncondBrI), TII->get(X86::JMP_1))
	.addMBB(MBBInfo->FBB);
	MBB->erase(UncondBrI);
	MBBInfo->Modified = false;
	}

	//
	// Apply the transformation:
	// RootMBB -1-> ... PredMBB -3-> MBB -5-> TargetMBB
	// \-2-> \-4-> \-6-> FalseMBB
	// ==>
	// RootMBB -1-> ... PredMBB -7-> FalseMBB
	// TargetMBB <-8-/ \-2-> \-4->
	//
	// Note that PredMBB and RootMBB could be the same.
	// And in the case of dead TargetMBB, we will not have TargetMBB and edge 8.
	//
	// There are some special handling where the RootMBB is COND_E in which case
	// we directly short-cycle the brinstr.
	//
	void X86CondBrFolding::optimizeCondBr(
	MachineBasicBlock &MBB, SmallVectorImpl<MachineBasicBlock *> &BranchPath) {

	X86::CondCode CC;
	TargetMBBInfo *MBBInfo = getMBBInfo(&MBB);
	assert(MBBInfo && "Expecting a candidate MBB");
	MachineBasicBlock *TargetMBB = MBBInfo->TBB;
	BranchProbability TargetProb = MBPI->getEdgeProbability(&MBB, MBBInfo->TBB);

	// Forward the jump from MBB's predecessor to MBB's false target.
	MachineBasicBlock *PredMBB = BranchPath.front();
	TargetMBBInfo *PredMBBInfo = getMBBInfo(PredMBB);
	assert(PredMBBInfo && "Expecting a candidate MBB");
	if (PredMBBInfo->Modified)
	fixupModifiedCond(PredMBB);
	CC = PredMBBInfo->BranchCode;
	// Don't do this if depth of BranchPath is 1 and PredMBB is of COND_E.
	// We will short-cycle directly for this case.
	if (!(CC == X86::COND_E && BranchPath.size() == 1))
	replaceBrDest(PredMBB, &MBB, MBBInfo->FBB);

	MachineBasicBlock *RootMBB = BranchPath.back();
	TargetMBBInfo *RootMBBInfo = getMBBInfo(RootMBB);
	assert(RootMBBInfo && "Expecting a candidate MBB");
	if (RootMBBInfo->Modified)
	fixupModifiedCond(RootMBB);
	CC = RootMBBInfo->BranchCode;

	if (CC != X86::COND_E) {
	MachineBasicBlock::iterator UncondBrI = findUncondBrI(RootMBB);
	// RootMBB: Cond jump to the original not-taken MBB.
	X86::CondCode NewCC;
	switch (CC) {
	case X86::COND_L:
	NewCC = X86::COND_G;
	break;
	case X86::COND_G:
	NewCC = X86::COND_L;
	break;
	default:
	llvm_unreachable("unexpected condtional code.");
	}
	BuildMI(*RootMBB, UncondBrI, RootMBB->findDebugLoc(UncondBrI),
	TII->get(GetCondBranchFromCond(NewCC)))
	.addMBB(RootMBBInfo->FBB);

	// RootMBB: Jump to TargetMBB
	BuildMI(*RootMBB, UncondBrI, RootMBB->findDebugLoc(UncondBrI),
	TII->get(X86::JMP_1))
	.addMBB(TargetMBB);
	RootMBB->addSuccessor(TargetMBB);
	fixPHIsInSucc(TargetMBB, &MBB, RootMBB);
	RootMBB->erase(UncondBrI);
	} else {
	replaceBrDest(RootMBB, RootMBBInfo->TBB, TargetMBB);
	}

	// Fix RootMBB's CmpValue to MBB's CmpValue to TargetMBB. Don't set Imm
	// directly. Move MBB's stmt to here as the opcode might be different.
	if (RootMBBInfo->CmpValue != MBBInfo->CmpValue) {
	MachineInstr *NewCmp = MBBInfo->CmpInstr;
	NewCmp->removeFromParent();
	RootMBB->insert(RootMBBInfo->CmpInstr, NewCmp);
	RootMBBInfo->CmpInstr->eraseFromParent();
	}

	// Fix branch Probabilities.
	auto fixBranchProb = [&](MachineBasicBlock *NextMBB) {
	BranchProbability Prob;
	for (auto &I : BranchPath) {
	MachineBasicBlock *ThisMBB = I;
	if (!ThisMBB->hasSuccessorProbabilities() \|\|
	!ThisMBB->isSuccessor(NextMBB))
	break;
	Prob = MBPI->getEdgeProbability(ThisMBB, NextMBB);
	if (Prob.isUnknown())
	break;
	TargetProb = Prob * TargetProb;
	Prob = Prob - TargetProb;
	setBranchProb(ThisMBB, NextMBB, Prob);
	if (ThisMBB == RootMBB) {
	setBranchProb(ThisMBB, TargetMBB, TargetProb);
	}
	ThisMBB->normalizeSuccProbs();
	if (ThisMBB == RootMBB)
	break;
	NextMBB = ThisMBB;
	}
	return true;
	};
	if (CC != X86::COND_E && !TargetProb.isUnknown())
	fixBranchProb(MBBInfo->FBB);

	if (CC != X86::COND_E)
	RemoveList.push_back(&MBB);

	// Invalidate MBBInfo just in case.
	MBBInfos[MBB.getNumber()] = nullptr;
	MBBInfos[RootMBB->getNumber()] = nullptr;

	LLVM_DEBUG(dbgs() << "After optimization:\nRootMBB is: " << *RootMBB << "\n");
	if (BranchPath.size() > 1)
	LLVM_DEBUG(dbgs() << "PredMBB is: " << *(BranchPath[0]) << "\n");
	}

	// Driver function for optimization: find the valid candidate and apply
	// the transformation.
	bool X86CondBrFolding::optimize() {
	bool Changed = false;
	LLVM_DEBUG(dbgs() << "***** X86CondBr Folding on Function: " << MF.getName()
	<< " *****\n");
	// Setup data structures.
	MBBInfos.resize(MF.getNumBlockIDs());
	for (auto &MBB : MF)
	MBBInfos[MBB.getNumber()] = analyzeMBB(MBB);

	for (auto &MBB : MF) {
	TargetMBBInfo *MBBInfo = getMBBInfo(&MBB);
	if (!MBBInfo \|\| !MBBInfo->CmpBrOnly)
	continue;
	if (MBB.pred_size() != 1)
	continue;
	LLVM_DEBUG(dbgs() << "Work on MBB." << MBB.getNumber()
	<< " CmpValue: " << MBBInfo->CmpValue << "\n");
	SmallVector<MachineBasicBlock *, 4> BranchPath;
	if (!findPath(&MBB, BranchPath))
	continue;

	#ifndef NDEBUG
	LLVM_DEBUG(dbgs() << "Found one path (len=" << BranchPath.size() << "):\n");
	int Index = 1;
	LLVM_DEBUG(dbgs() << "Target MBB is: " << MBB << "\n");
	for (auto I = BranchPath.rbegin(); I != BranchPath.rend(); ++I, ++Index) {
	MachineBasicBlock PMBB = I;
	TargetMBBInfo *PMBBInfo = getMBBInfo(PMBB);
	LLVM_DEBUG(dbgs() << "Path MBB (" << Index << " of " << BranchPath.size()
	<< ") is " << *PMBB);
	LLVM_DEBUG(dbgs() << "CC=" << PMBBInfo->BranchCode
	<< " Val=" << PMBBInfo->CmpValue
	<< " CmpBrOnly=" << PMBBInfo->CmpBrOnly << "\n\n");
	}
	#endif
	optimizeCondBr(MBB, BranchPath);
	Changed = true;
	}
	NumFixedCondBrs += RemoveList.size();
	for (auto MBBI : RemoveList) {
	while (!MBBI->succ_empty())
	MBBI->removeSuccessor(MBBI->succ_end() - 1);

	MBBI->eraseFromParent();
	}

	return Changed;
	}

	// Analyze instructions that generate CondCode and extract information.
	bool X86CondBrFolding::analyzeCompare(const MachineInstr &MI, unsigned &SrcReg,
	int &CmpValue) {
	unsigned SrcRegIndex = 0;
	unsigned ValueIndex = 0;
	switch (MI.getOpcode()) {
	// TODO: handle test instructions.
	default:
	return false;
	case X86::CMP64ri32:
	case X86::CMP64ri8:
	case X86::CMP32ri:
	case X86::CMP32ri8:
	case X86::CMP16ri:
	case X86::CMP16ri8:
	case X86::CMP8ri:
	SrcRegIndex = 0;
	ValueIndex = 1;
	break;
	case X86::SUB64ri32:
	case X86::SUB64ri8:
	case X86::SUB32ri:
	case X86::SUB32ri8:
	case X86::SUB16ri:
	case X86::SUB16ri8:
	case X86::SUB8ri:
	SrcRegIndex = 1;
	ValueIndex = 2;
	break;
	}
	SrcReg = MI.getOperand(SrcRegIndex).getReg();
	if (!MI.getOperand(ValueIndex).isImm())
	return false;
	CmpValue = MI.getOperand(ValueIndex).getImm();
	return true;
	}

	// Analyze a candidate MBB and set the extract all the information needed.
	// The valid candidate will have two successors.
	// It also should have a sequence of
	// Branch_instr,
	// CondBr,
	// UnCondBr.
	// Return TargetMBBInfo if MBB is a valid candidate and nullptr otherwise.
	std::unique_ptr<TargetMBBInfo>
	X86CondBrFolding::analyzeMBB(MachineBasicBlock &MBB) {
	MachineBasicBlock *TBB;
	MachineBasicBlock *FBB;
	MachineInstr *BrInstr;
	MachineInstr *CmpInstr;
	X86::CondCode CC;
	unsigned SrcReg;
	int CmpValue;
	bool Modified;
	bool CmpBrOnly;

	if (MBB.succ_size() != 2)
	return nullptr;

	CmpBrOnly = true;
	FBB = TBB = nullptr;
	CmpInstr = nullptr;
	MachineBasicBlock::iterator I = MBB.end();
	while (I != MBB.begin()) {
	--I;
	if (I->isDebugValue())
	continue;
	if (I->getOpcode() == X86::JMP_1) {
	if (FBB)
	return nullptr;
	FBB = I->getOperand(0).getMBB();
	continue;
	}
	if (I->isBranch()) {
	if (TBB)
	return nullptr;
	CC = X86::getCondFromBranchOpc(I->getOpcode());
	switch (CC) {
	default:
	return nullptr;
	case X86::COND_E:
	case X86::COND_L:
	case X86::COND_G:
	case X86::COND_NE:
	case X86::COND_LE:
	case X86::COND_GE:
	break;
	}
	TBB = I->getOperand(0).getMBB();
	BrInstr = &*I;
	continue;
	}
	if (analyzeCompare(*I, SrcReg, CmpValue)) {
	if (CmpInstr)
	return nullptr;
	CmpInstr = &*I;
	continue;
	}
	CmpBrOnly = false;
	break;
	}

	if (!TBB \|\| !FBB \|\| !CmpInstr)
	return nullptr;

	// Simplify CondCode. Note this is only to simplify the findPath logic
	// and will not change the instruction here.
	switch (CC) {
	case X86::COND_NE:
	CC = X86::COND_E;
	std::swap(TBB, FBB);
	Modified = true;
	break;
	case X86::COND_LE:
	if (CmpValue == INT_MAX)
	return nullptr;
	CC = X86::COND_L;
	CmpValue += 1;
	Modified = true;
	break;
	case X86::COND_GE:
	if (CmpValue == INT_MIN)
	return nullptr;
	CC = X86::COND_G;
	CmpValue -= 1;
	Modified = true;
	break;
	default:
	Modified = false;
	break;
	}
	return llvm::make_unique<TargetMBBInfo>(TargetMBBInfo{
	TBB, FBB, BrInstr, CmpInstr, CC, SrcReg, CmpValue, Modified, CmpBrOnly});
	}

	bool X86CondBrFoldingPass::runOnMachineFunction(MachineFunction &MF) {
	const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
	if (!ST.threewayBranchProfitable())
	return false;
	const X86InstrInfo *TII = ST.getInstrInfo();
	const MachineBranchProbabilityInfo *MBPI =
	&getAnalysis<MachineBranchProbabilityInfo>();

	X86CondBrFolding CondBr(TII, MBPI, MF);
	return CondBr.optimize();
	}