--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/Target/PowerPC/PPCBranchCoalescing.cpp	Fri Oct 27 17:07:41 2017 +0900
@@ -0,0 +1,784 @@
+//===-- CoalesceBranches.cpp - Coalesce blocks with the same condition ---===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Coalesce basic blocks guarded by the same branch condition into a single
+/// basic block.
+///
+//===----------------------------------------------------------------------===//
+
+#include "PPC.h"
+#include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachinePostDominators.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Target/TargetFrameLowering.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetSubtargetInfo.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "ppc-branch-coalescing"
+
+STATISTIC(NumBlocksCoalesced, "Number of blocks coalesced");
+STATISTIC(NumPHINotMoved, "Number of PHI Nodes that cannot be merged");
+STATISTIC(NumBlocksNotCoalesced, "Number of blocks not coalesced");
+
+namespace llvm {
+    void initializePPCBranchCoalescingPass(PassRegistry&);
+}
+
+//===----------------------------------------------------------------------===//
+//                               PPCBranchCoalescing
+//===----------------------------------------------------------------------===//
+///
+/// Improve scheduling by coalescing branches that depend on the same condition.
+/// This pass looks for blocks that are guarded by the same branch condition
+/// and attempts to merge the blocks together. Such opportunities arise from
+/// the expansion of select statements in the IR.
+///
+/// This pass does not handle implicit operands on branch statements. In order
+/// to run on targets that use implicit operands, changes need to be made in the
+/// canCoalesceBranch and canMerge methods.
+///
+/// Example: the following LLVM IR
+///
+///     %test = icmp eq i32 %x 0
+///     %tmp1 = select i1 %test, double %a, double 2.000000e-03
+///     %tmp2 = select i1 %test, double %b, double 5.000000e-03
+///
+/// expands to the following machine code:
+///
+/// BB#0: derived from LLVM BB %entry
+///    Live Ins: %F1 %F3 %X6
+///        <SNIP1>
+///        %vreg0<def> = COPY %F1; F8RC:%vreg0
+///        %vreg5<def> = CMPLWI %vreg4<kill>, 0; CRRC:%vreg5 GPRC:%vreg4
+///        %vreg8<def> = LXSDX %ZERO8, %vreg7<kill>, %RM<imp-use>;
+///                    mem:LD8[ConstantPool] F8RC:%vreg8 G8RC:%vreg7
+///        BCC 76, %vreg5, <BB#2>; CRRC:%vreg5
+///    Successors according to CFG: BB#1(?%) BB#2(?%)
+///
+/// BB#1: derived from LLVM BB %entry
+///    Predecessors according to CFG: BB#0
+///    Successors according to CFG: BB#2(?%)
+///
+/// BB#2: derived from LLVM BB %entry
+///    Predecessors according to CFG: BB#0 BB#1
+///        %vreg9<def> = PHI %vreg8, <BB#1>, %vreg0, <BB#0>;
+///                    F8RC:%vreg9,%vreg8,%vreg0
+///        <SNIP2>
+///        BCC 76, %vreg5, <BB#4>; CRRC:%vreg5
+///    Successors according to CFG: BB#3(?%) BB#4(?%)
+///
+/// BB#3: derived from LLVM BB %entry
+///    Predecessors according to CFG: BB#2
+///    Successors according to CFG: BB#4(?%)
+///
+/// BB#4: derived from LLVM BB %entry
+///    Predecessors according to CFG: BB#2 BB#3
+///        %vreg13<def> = PHI %vreg12, <BB#3>, %vreg2, <BB#2>;
+///                     F8RC:%vreg13,%vreg12,%vreg2
+///        <SNIP3>
+///        BLR8 %LR8<imp-use>, %RM<imp-use>, %F1<imp-use>
+///
+/// When this pattern is detected, branch coalescing will try to collapse
+/// it by moving code in BB#2 to BB#0 and/or BB#4 and removing BB#3.
+///
+/// If all conditions are meet, IR should collapse to:
+///
+/// BB#0: derived from LLVM BB %entry
+///    Live Ins: %F1 %F3 %X6
+///        <SNIP1>
+///        %vreg0<def> = COPY %F1; F8RC:%vreg0
+///        %vreg5<def> = CMPLWI %vreg4<kill>, 0; CRRC:%vreg5 GPRC:%vreg4
+///        %vreg8<def> = LXSDX %ZERO8, %vreg7<kill>, %RM<imp-use>;
+///                     mem:LD8[ConstantPool] F8RC:%vreg8 G8RC:%vreg7
+///        <SNIP2>
+///        BCC 76, %vreg5, <BB#4>; CRRC:%vreg5
+///    Successors according to CFG: BB#1(0x2aaaaaaa / 0x80000000 = 33.33%)
+///      BB#4(0x55555554 / 0x80000000 = 66.67%)
+///
+/// BB#1: derived from LLVM BB %entry
+///    Predecessors according to CFG: BB#0
+///    Successors according to CFG: BB#4(0x40000000 / 0x80000000 = 50.00%)
+///
+/// BB#4: derived from LLVM BB %entry
+///    Predecessors according to CFG: BB#0 BB#1
+///        %vreg9<def> = PHI %vreg8, <BB#1>, %vreg0, <BB#0>;
+///                    F8RC:%vreg9,%vreg8,%vreg0
+///        %vreg13<def> = PHI %vreg12, <BB#1>, %vreg2, <BB#0>;
+///                     F8RC:%vreg13,%vreg12,%vreg2
+///        <SNIP3>
+///        BLR8 %LR8<imp-use>, %RM<imp-use>, %F1<imp-use>
+///
+/// Branch Coalescing does not split blocks, it moves everything in the same
+/// direction ensuring it does not break use/definition semantics.
+///
+/// PHI nodes and its corresponding use instructions are moved to its successor
+/// block if there are no uses within the successor block PHI nodes.  PHI
+/// node ordering cannot be assumed.
+///
+/// Non-PHI can be moved up to the predecessor basic block or down to the
+/// successor basic block following any PHI instructions. Whether it moves
+/// up or down depends on whether the register(s) defined in the instructions
+/// are used in current block or in any PHI instructions at the beginning of
+/// the successor block.
+
+namespace {
+
+class PPCBranchCoalescing : public MachineFunctionPass {
+  struct CoalescingCandidateInfo {
+    MachineBasicBlock *BranchBlock;       // Block containing the branch
+    MachineBasicBlock *BranchTargetBlock; // Block branched to
+    MachineBasicBlock *FallThroughBlock;  // Fall-through if branch not taken
+    SmallVector<MachineOperand, 4> Cond;
+    bool MustMoveDown;
+    bool MustMoveUp;
+
+    CoalescingCandidateInfo();
+    void clear();
+  };
+
+  MachineDominatorTree *MDT;
+  MachinePostDominatorTree *MPDT;
+  const TargetInstrInfo *TII;
+  MachineRegisterInfo *MRI;
+
+  void initialize(MachineFunction &F);
+  bool canCoalesceBranch(CoalescingCandidateInfo &Cand);
+  bool identicalOperands(ArrayRef<MachineOperand> OperandList1,
+                         ArrayRef<MachineOperand> OperandList2) const;
+  bool validateCandidates(CoalescingCandidateInfo &SourceRegion,
+                          CoalescingCandidateInfo &TargetRegion) const;
+
+public:
+  static char ID;
+
+  PPCBranchCoalescing() : MachineFunctionPass(ID) {
+    initializePPCBranchCoalescingPass(*PassRegistry::getPassRegistry());
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<MachineDominatorTree>();
+    AU.addRequired<MachinePostDominatorTree>();
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  StringRef getPassName() const override { return "Branch Coalescing"; }
+
+  bool mergeCandidates(CoalescingCandidateInfo &SourceRegion,
+                       CoalescingCandidateInfo &TargetRegion);
+  bool canMoveToBeginning(const MachineInstr &MI,
+                          const MachineBasicBlock &MBB) const;
+  bool canMoveToEnd(const MachineInstr &MI,
+                    const MachineBasicBlock &MBB) const;
+  bool canMerge(CoalescingCandidateInfo &SourceRegion,
+                CoalescingCandidateInfo &TargetRegion) const;
+  void moveAndUpdatePHIs(MachineBasicBlock *SourceRegionMBB,
+                         MachineBasicBlock *TargetRegionMBB);
+  bool runOnMachineFunction(MachineFunction &MF) override;
+};
+} // End anonymous namespace.
+
+char PPCBranchCoalescing::ID = 0;
+/// createPPCBranchCoalescingPass - returns an instance of the Branch Coalescing
+/// Pass
+FunctionPass *llvm::createPPCBranchCoalescingPass() {
+  return new PPCBranchCoalescing();
+}
+
+INITIALIZE_PASS_BEGIN(PPCBranchCoalescing, DEBUG_TYPE,
+                      "Branch Coalescing", false, false)
+INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
+INITIALIZE_PASS_DEPENDENCY(MachinePostDominatorTree)
+INITIALIZE_PASS_END(PPCBranchCoalescing, DEBUG_TYPE, "Branch Coalescing",
+                    false, false)
+
+PPCBranchCoalescing::CoalescingCandidateInfo::CoalescingCandidateInfo()
+    : BranchBlock(nullptr), BranchTargetBlock(nullptr),
+      FallThroughBlock(nullptr), MustMoveDown(false), MustMoveUp(false) {}
+
+void PPCBranchCoalescing::CoalescingCandidateInfo::clear() {
+  BranchBlock = nullptr;
+  BranchTargetBlock = nullptr;
+  FallThroughBlock = nullptr;
+  Cond.clear();
+  MustMoveDown = false;
+  MustMoveUp = false;
+}
+
+void PPCBranchCoalescing::initialize(MachineFunction &MF) {
+  MDT = &getAnalysis<MachineDominatorTree>();
+  MPDT = &getAnalysis<MachinePostDominatorTree>();
+  TII = MF.getSubtarget().getInstrInfo();
+  MRI = &MF.getRegInfo();
+}
+
+///
+/// Analyze the branch statement to determine if it can be coalesced. This
+/// method analyses the branch statement for the given candidate to determine
+/// if it can be coalesced. If the branch can be coalesced, then the
+/// BranchTargetBlock and the FallThroughBlock are recorded in the specified
+/// Candidate.
+///
+///\param[in,out] Cand The coalescing candidate to analyze
+///\return true if and only if the branch can be coalesced, false otherwise
+///
+bool PPCBranchCoalescing::canCoalesceBranch(CoalescingCandidateInfo &Cand) {
+  DEBUG(dbgs() << "Determine if branch block " << Cand.BranchBlock->getNumber()
+               << " can be coalesced:");
+  MachineBasicBlock *FalseMBB = nullptr;
+
+  if (TII->analyzeBranch(*Cand.BranchBlock, Cand.BranchTargetBlock, FalseMBB,
+                         Cand.Cond)) {
+    DEBUG(dbgs() << "TII unable to Analyze Branch - skip\n");
+    return false;
+  }
+
+  for (auto &I : Cand.BranchBlock->terminators()) {
+    DEBUG(dbgs() << "Looking at terminator : " << I << "\n");
+    if (!I.isBranch())
+      continue;
+
+    // The analyzeBranch method does not include any implicit operands.
+    // This is not an issue on PPC but must be handled on other targets.
+    // For this pass to be made target-independent, the analyzeBranch API
+    // need to be updated to support implicit operands and there would
+    // need to be a way to verify that any implicit operands would not be
+    // clobbered by merging blocks.  This would include identifying the
+    // implicit operands as well as the basic block they are defined in.
+    // This could be done by changing the analyzeBranch API to have it also
+    // record and return the implicit operands and the blocks where they are
+    // defined. Alternatively, the BranchCoalescing code would need to be
+    // extended to identify the implicit operands.  The analysis in canMerge
+    // must then be extended to prove that none of the implicit operands are
+    // changed in the blocks that are combined during coalescing.
+    if (I.getNumOperands() != I.getNumExplicitOperands()) {
+      DEBUG(dbgs() << "Terminator contains implicit operands - skip : " << I
+                   << "\n");
+      return false;
+    }
+  }
+
+  if (Cand.BranchBlock->isEHPad() || Cand.BranchBlock->hasEHPadSuccessor()) {
+    DEBUG(dbgs() << "EH Pad - skip\n");
+    return false;
+  }
+
+  // For now only consider triangles (i.e, BranchTargetBlock is set,
+  // FalseMBB is null, and BranchTargetBlock is a successor to BranchBlock)
+  if (!Cand.BranchTargetBlock || FalseMBB ||
+      !Cand.BranchBlock->isSuccessor(Cand.BranchTargetBlock)) {
+    DEBUG(dbgs() << "Does not form a triangle - skip\n");
+    return false;
+  }
+
+  // Ensure there are only two successors
+  if (Cand.BranchBlock->succ_size() != 2) {
+    DEBUG(dbgs() << "Does not have 2 successors - skip\n");
+    return false;
+  }
+
+  // Sanity check - the block must be able to fall through
+  assert(Cand.BranchBlock->canFallThrough() &&
+         "Expecting the block to fall through!");
+
+  // We have already ensured there are exactly two successors to
+  // BranchBlock and that BranchTargetBlock is a successor to BranchBlock.
+  // Ensure the single fall though block is empty.
+  MachineBasicBlock *Succ =
+    (*Cand.BranchBlock->succ_begin() == Cand.BranchTargetBlock)
+    ? *Cand.BranchBlock->succ_rbegin()
+    : *Cand.BranchBlock->succ_begin();
+
+  assert(Succ && "Expecting a valid fall-through block\n");
+
+  if (!Succ->empty()) {
+      DEBUG(dbgs() << "Fall-through block contains code -- skip\n");
+      return false;
+  }
+
+  if (!Succ->isSuccessor(Cand.BranchTargetBlock)) {
+      DEBUG(dbgs()
+            << "Successor of fall through block is not branch taken block\n");
+      return false;
+  }
+
+  Cand.FallThroughBlock = Succ;
+  DEBUG(dbgs() << "Valid Candidate\n");
+  return true;
+}
+
+///
+/// Determine if the two operand lists are identical
+///
+/// \param[in] OpList1 operand list
+/// \param[in] OpList2 operand list
+/// \return true if and only if the operands lists are identical
+///
+bool PPCBranchCoalescing::identicalOperands(
+    ArrayRef<MachineOperand> OpList1, ArrayRef<MachineOperand> OpList2) const {
+
+  if (OpList1.size() != OpList2.size()) {
+    DEBUG(dbgs() << "Operand list is different size\n");
+    return false;
+  }
+
+  for (unsigned i = 0; i < OpList1.size(); ++i) {
+    const MachineOperand &Op1 = OpList1[i];
+    const MachineOperand &Op2 = OpList2[i];
+
+    DEBUG(dbgs() << "Op1: " << Op1 << "\n"
+                 << "Op2: " << Op2 << "\n");
+
+    if (Op1.isIdenticalTo(Op2)) {
+      // filter out instructions with physical-register uses
+      if (Op1.isReg() && TargetRegisterInfo::isPhysicalRegister(Op1.getReg())
+        // If the physical register is constant then we can assume the value
+        // has not changed between uses.
+          && !(Op1.isUse() && MRI->isConstantPhysReg(Op1.getReg()))) {
+        DEBUG(dbgs() << "The operands are not provably identical.\n");
+        return false;
+      }
+      DEBUG(dbgs() << "Op1 and Op2 are identical!\n");
+      continue;
+    }
+
+    // If the operands are not identical, but are registers, check to see if the
+    // definition of the register produces the same value. If they produce the
+    // same value, consider them to be identical.
+    if (Op1.isReg() && Op2.isReg() &&
+        TargetRegisterInfo::isVirtualRegister(Op1.getReg()) &&
+        TargetRegisterInfo::isVirtualRegister(Op2.getReg())) {
+      MachineInstr *Op1Def = MRI->getVRegDef(Op1.getReg());
+      MachineInstr *Op2Def = MRI->getVRegDef(Op2.getReg());
+      if (TII->produceSameValue(*Op1Def, *Op2Def, MRI)) {
+        DEBUG(dbgs() << "Op1Def: " << *Op1Def << " and " << *Op2Def
+                     << " produce the same value!\n");
+      } else {
+        DEBUG(dbgs() << "Operands produce different values\n");
+        return false;
+      }
+    } else {
+      DEBUG(dbgs() << "The operands are not provably identical.\n");
+      return false;
+    }
+  }
+
+  return true;
+}
+
+///
+/// Moves ALL PHI instructions in SourceMBB to beginning of TargetMBB
+/// and update them to refer to the new block.  PHI node ordering
+/// cannot be assumed so it does not matter where the PHI instructions
+/// are moved to in TargetMBB.
+///
+/// \param[in] SourceMBB block to move PHI instructions from
+/// \param[in] TargetMBB block to move PHI instructions to
+///
+void PPCBranchCoalescing::moveAndUpdatePHIs(MachineBasicBlock *SourceMBB,
+                                         MachineBasicBlock *TargetMBB) {
+
+  MachineBasicBlock::iterator MI = SourceMBB->begin();
+  MachineBasicBlock::iterator ME = SourceMBB->getFirstNonPHI();
+
+  if (MI == ME) {
+    DEBUG(dbgs() << "SourceMBB contains no PHI instructions.\n");
+    return;
+  }
+
+  // Update all PHI instructions in SourceMBB and move to top of TargetMBB
+  for (MachineBasicBlock::iterator Iter = MI; Iter != ME; Iter++) {
+    MachineInstr &PHIInst = *Iter;
+    for (unsigned i = 2, e = PHIInst.getNumOperands() + 1; i != e; i += 2) {
+      MachineOperand &MO = PHIInst.getOperand(i);
+      if (MO.getMBB() == SourceMBB)
+        MO.setMBB(TargetMBB);
+    }
+  }
+  TargetMBB->splice(TargetMBB->begin(), SourceMBB, MI, ME);
+}
+
+///
+/// This function checks if MI can be moved to the beginning of the TargetMBB
+/// following PHI instructions. A MI instruction can be moved to beginning of
+/// the TargetMBB if there are no uses of it within the TargetMBB PHI nodes.
+///
+/// \param[in] MI the machine instruction to move.
+/// \param[in] TargetMBB the machine basic block to move to
+/// \return true if it is safe to move MI to beginning of TargetMBB,
+///         false otherwise.
+///
+bool PPCBranchCoalescing::canMoveToBeginning(const MachineInstr &MI,
+                                          const MachineBasicBlock &TargetMBB
+                                          ) const {
+
+  DEBUG(dbgs() << "Checking if " << MI << " can move to beginning of "
+        << TargetMBB.getNumber() << "\n");
+
+  for (auto &Def : MI.defs()) { // Looking at Def
+    for (auto &Use : MRI->use_instructions(Def.getReg())) {
+      if (Use.isPHI() && Use.getParent() == &TargetMBB) {
+        DEBUG(dbgs() << "    *** used in a PHI -- cannot move ***\n");
+       return false;
+      }
+    }
+  }
+
+  DEBUG(dbgs() << "  Safe to move to the beginning.\n");
+  return true;
+}
+
+///
+/// This function checks if MI can be moved to the end of the TargetMBB,
+/// immediately before the first terminator.  A MI instruction can be moved
+/// to then end of the TargetMBB if no PHI node defines what MI uses within
+/// it's own MBB.
+///
+/// \param[in] MI the machine instruction to move.
+/// \param[in] TargetMBB the machine basic block to move to
+/// \return true if it is safe to move MI to end of TargetMBB,
+///         false otherwise.
+///
+bool PPCBranchCoalescing::canMoveToEnd(const MachineInstr &MI,
+                                    const MachineBasicBlock &TargetMBB
+                                    ) const {
+
+  DEBUG(dbgs() << "Checking if " << MI << " can move to end of "
+        << TargetMBB.getNumber() << "\n");
+
+  for (auto &Use : MI.uses()) {
+    if (Use.isReg() && TargetRegisterInfo::isVirtualRegister(Use.getReg())) {
+      MachineInstr *DefInst = MRI->getVRegDef(Use.getReg());
+      if (DefInst->isPHI() && DefInst->getParent() == MI.getParent()) {
+        DEBUG(dbgs() << "    *** Cannot move this instruction ***\n");
+        return false;
+      } else {
+        DEBUG(dbgs() << "    *** def is in another block -- safe to move!\n");
+      }
+    }
+  }
+
+  DEBUG(dbgs() << "  Safe to move to the end.\n");
+  return true;
+}
+
+///
+/// This method checks to ensure the two coalescing candidates follows the
+/// expected pattern required for coalescing.
+///
+/// \param[in] SourceRegion The candidate to move statements from
+/// \param[in] TargetRegion The candidate to move statements to
+/// \return true if all instructions in SourceRegion.BranchBlock can be merged
+/// into a block in TargetRegion; false otherwise.
+///
+bool PPCBranchCoalescing::validateCandidates(
+    CoalescingCandidateInfo &SourceRegion,
+    CoalescingCandidateInfo &TargetRegion) const {
+
+  if (TargetRegion.BranchTargetBlock != SourceRegion.BranchBlock)
+    llvm_unreachable("Expecting SourceRegion to immediately follow TargetRegion");
+  else if (!MDT->dominates(TargetRegion.BranchBlock, SourceRegion.BranchBlock))
+    llvm_unreachable("Expecting TargetRegion to dominate SourceRegion");
+  else if (!MPDT->dominates(SourceRegion.BranchBlock, TargetRegion.BranchBlock))
+    llvm_unreachable("Expecting SourceRegion to post-dominate TargetRegion");
+  else if (!TargetRegion.FallThroughBlock->empty() ||
+           !SourceRegion.FallThroughBlock->empty())
+    llvm_unreachable("Expecting fall-through blocks to be empty");
+
+  return true;
+}
+
+///
+/// This method determines whether the two coalescing candidates can be merged.
+/// In order to be merged, all instructions must be able to
+///   1. Move to the beginning of the SourceRegion.BranchTargetBlock;
+///   2. Move to the end of the TargetRegion.BranchBlock.
+/// Merging involves moving the instructions in the
+/// TargetRegion.BranchTargetBlock (also SourceRegion.BranchBlock).
+///
+/// This function first try to move instructions from the
+/// TargetRegion.BranchTargetBlock down, to the beginning of the
+/// SourceRegion.BranchTargetBlock. This is not possible if any register defined
+/// in TargetRegion.BranchTargetBlock is used in a PHI node in the
+/// SourceRegion.BranchTargetBlock. In this case, check whether the statement
+/// can be moved up, to the end of the TargetRegion.BranchBlock (immediately
+/// before the branch statement). If it cannot move, then these blocks cannot
+/// be merged.
+///
+/// Note that there is no analysis for moving instructions past the fall-through
+/// blocks because they are confirmed to be empty. An assert is thrown if they
+/// are not.
+///
+/// \param[in] SourceRegion The candidate to move statements from
+/// \param[in] TargetRegion The candidate to move statements to
+/// \return true if all instructions in SourceRegion.BranchBlock can be merged
+///         into a block in TargetRegion, false otherwise.
+///
+bool PPCBranchCoalescing::canMerge(CoalescingCandidateInfo &SourceRegion,
+                                CoalescingCandidateInfo &TargetRegion) const {
+  if (!validateCandidates(SourceRegion, TargetRegion))
+    return false;
+
+  // Walk through PHI nodes first and see if they force the merge into the
+  // SourceRegion.BranchTargetBlock.
+  for (MachineBasicBlock::iterator
+           I = SourceRegion.BranchBlock->instr_begin(),
+           E = SourceRegion.BranchBlock->getFirstNonPHI();
+       I != E; ++I) {
+    for (auto &Def : I->defs())
+      for (auto &Use : MRI->use_instructions(Def.getReg())) {
+        if (Use.isPHI() && Use.getParent() == SourceRegion.BranchTargetBlock) {
+          DEBUG(dbgs() << "PHI " << *I << " defines register used in another "
+                          "PHI within branch target block -- can't merge\n");
+          NumPHINotMoved++;
+          return false;
+        }
+        if (Use.getParent() == SourceRegion.BranchBlock) {
+          DEBUG(dbgs() << "PHI " << *I
+                       << " defines register used in this "
+                          "block -- all must move down\n");
+          SourceRegion.MustMoveDown = true;
+        }
+      }
+  }
+
+  // Walk through the MI to see if they should be merged into
+  // TargetRegion.BranchBlock (up) or SourceRegion.BranchTargetBlock (down)
+  for (MachineBasicBlock::iterator
+           I = SourceRegion.BranchBlock->getFirstNonPHI(),
+           E = SourceRegion.BranchBlock->end();
+       I != E; ++I) {
+    if (!canMoveToBeginning(*I, *SourceRegion.BranchTargetBlock)) {
+      DEBUG(dbgs() << "Instruction " << *I
+                   << " cannot move down - must move up!\n");
+      SourceRegion.MustMoveUp = true;
+    }
+    if (!canMoveToEnd(*I, *TargetRegion.BranchBlock)) {
+      DEBUG(dbgs() << "Instruction " << *I
+                   << " cannot move up - must move down!\n");
+      SourceRegion.MustMoveDown = true;
+    }
+  }
+
+  return (SourceRegion.MustMoveUp && SourceRegion.MustMoveDown) ? false : true;
+}
+
+/// Merge the instructions from SourceRegion.BranchBlock,
+/// SourceRegion.BranchTargetBlock, and SourceRegion.FallThroughBlock into
+/// TargetRegion.BranchBlock, TargetRegion.BranchTargetBlock and
+/// TargetRegion.FallThroughBlock respectively.
+///
+/// The successors for blocks in TargetRegion will be updated to use the
+/// successors from blocks in SourceRegion. Finally, the blocks in SourceRegion
+/// will be removed from the function.
+///
+/// A region consists of a BranchBlock, a FallThroughBlock, and a
+/// BranchTargetBlock. Branch coalesce works on patterns where the
+/// TargetRegion's BranchTargetBlock must also be the SourceRegions's
+/// BranchBlock.
+///
+///  Before mergeCandidates:
+///
+///  +---------------------------+
+///  |  TargetRegion.BranchBlock |
+///  +---------------------------+
+///     /        |
+///    /   +--------------------------------+
+///   |    |  TargetRegion.FallThroughBlock |
+///    \   +--------------------------------+
+///     \        |
+///  +----------------------------------+
+///  |  TargetRegion.BranchTargetBlock  |
+///  |  SourceRegion.BranchBlock        |
+///  +----------------------------------+
+///     /        |
+///    /   +--------------------------------+
+///   |    |  SourceRegion.FallThroughBlock |
+///    \   +--------------------------------+
+///     \        |
+///  +----------------------------------+
+///  |  SourceRegion.BranchTargetBlock  |
+///  +----------------------------------+
+///
+///  After mergeCandidates:
+///
+///  +-----------------------------+
+///  |  TargetRegion.BranchBlock   |
+///  |  SourceRegion.BranchBlock   |
+///  +-----------------------------+
+///     /        |
+///    /   +---------------------------------+
+///   |    |  TargetRegion.FallThroughBlock  |
+///   |    |  SourceRegion.FallThroughBlock  |
+///    \   +---------------------------------+
+///     \        |
+///  +----------------------------------+
+///  |  SourceRegion.BranchTargetBlock  |
+///  +----------------------------------+
+///
+/// \param[in] SourceRegion The candidate to move blocks from
+/// \param[in] TargetRegion The candidate to move blocks to
+///
+bool PPCBranchCoalescing::mergeCandidates(CoalescingCandidateInfo &SourceRegion,
+                                       CoalescingCandidateInfo &TargetRegion) {
+
+  if (SourceRegion.MustMoveUp && SourceRegion.MustMoveDown) {
+    llvm_unreachable("Cannot have both MustMoveDown and MustMoveUp set!");
+    return false;
+  }
+
+  if (!validateCandidates(SourceRegion, TargetRegion))
+    return false;
+
+  // Start the merging process by first handling the BranchBlock.
+  // Move any PHIs in SourceRegion.BranchBlock down to the branch-taken block
+  moveAndUpdatePHIs(SourceRegion.BranchBlock, SourceRegion.BranchTargetBlock);
+
+  // Move remaining instructions in SourceRegion.BranchBlock into
+  // TargetRegion.BranchBlock
+  MachineBasicBlock::iterator firstInstr =
+      SourceRegion.BranchBlock->getFirstNonPHI();
+  MachineBasicBlock::iterator lastInstr =
+      SourceRegion.BranchBlock->getFirstTerminator();
+
+  MachineBasicBlock *Source = SourceRegion.MustMoveDown
+                                  ? SourceRegion.BranchTargetBlock
+                                  : TargetRegion.BranchBlock;
+
+  MachineBasicBlock::iterator Target =
+      SourceRegion.MustMoveDown
+          ? SourceRegion.BranchTargetBlock->getFirstNonPHI()
+          : TargetRegion.BranchBlock->getFirstTerminator();
+
+  Source->splice(Target, SourceRegion.BranchBlock, firstInstr, lastInstr);
+
+  // Once PHI and instructions have been moved we need to clean up the
+  // control flow.
+
+  // Remove SourceRegion.FallThroughBlock before transferring successors of
+  // SourceRegion.BranchBlock to TargetRegion.BranchBlock.
+  SourceRegion.BranchBlock->removeSuccessor(SourceRegion.FallThroughBlock);
+  TargetRegion.BranchBlock->transferSuccessorsAndUpdatePHIs(
+      SourceRegion.BranchBlock);
+  // Update branch in TargetRegion.BranchBlock to jump to
+  // SourceRegion.BranchTargetBlock
+  // In this case, TargetRegion.BranchTargetBlock == SourceRegion.BranchBlock.
+  TargetRegion.BranchBlock->ReplaceUsesOfBlockWith(
+      SourceRegion.BranchBlock, SourceRegion.BranchTargetBlock);
+  // Remove the branch statement(s) in SourceRegion.BranchBlock
+  MachineBasicBlock::iterator I =
+      SourceRegion.BranchBlock->terminators().begin();
+  while (I != SourceRegion.BranchBlock->terminators().end()) {
+    MachineInstr &CurrInst = *I;
+    ++I;
+    if (CurrInst.isBranch())
+      CurrInst.eraseFromParent();
+  }
+
+  // Fall-through block should be empty since this is part of the condition
+  // to coalesce the branches.
+  assert(TargetRegion.FallThroughBlock->empty() &&
+         "FallThroughBlocks should be empty!");
+
+  // Transfer successor information and move PHIs down to the
+  // branch-taken block.
+  TargetRegion.FallThroughBlock->transferSuccessorsAndUpdatePHIs(
+      SourceRegion.FallThroughBlock);
+  TargetRegion.FallThroughBlock->removeSuccessor(SourceRegion.BranchBlock);
+
+  // Remove the blocks from the function.
+  assert(SourceRegion.BranchBlock->empty() &&
+         "Expecting branch block to be empty!");
+  SourceRegion.BranchBlock->eraseFromParent();
+
+  assert(SourceRegion.FallThroughBlock->empty() &&
+         "Expecting fall-through block to be empty!\n");
+  SourceRegion.FallThroughBlock->eraseFromParent();
+
+  NumBlocksCoalesced++;
+  return true;
+}
+
+bool PPCBranchCoalescing::runOnMachineFunction(MachineFunction &MF) {
+
+  if (skipFunction(*MF.getFunction()) || MF.empty())
+    return false;
+
+  bool didSomething = false;
+
+  DEBUG(dbgs() << "******** Branch Coalescing ********\n");
+  initialize(MF);
+
+  DEBUG(dbgs() << "Function: "; MF.dump(); dbgs() << "\n");
+
+  CoalescingCandidateInfo Cand1, Cand2;
+  // Walk over blocks and find candidates to merge
+  // Continue trying to merge with the first candidate found, as long as merging
+  // is successfull.
+  for (MachineBasicBlock &MBB : MF) {
+    bool MergedCandidates = false;
+    do {
+      MergedCandidates = false;
+      Cand1.clear();
+      Cand2.clear();
+
+      Cand1.BranchBlock = &MBB;
+
+      // If unable to coalesce the branch, then continue to next block
+      if (!canCoalesceBranch(Cand1))
+        break;
+
+      Cand2.BranchBlock = Cand1.BranchTargetBlock;
+      if (!canCoalesceBranch(Cand2))
+        break;
+
+      // Sanity check
+      // The branch-taken block of the second candidate should post-dominate the
+      // first candidate
+      assert(MPDT->dominates(Cand2.BranchTargetBlock, Cand1.BranchBlock) &&
+             "Branch-taken block should post-dominate first candidate");
+
+      if (!identicalOperands(Cand1.Cond, Cand2.Cond)) {
+        DEBUG(dbgs() << "Blocks " << Cand1.BranchBlock->getNumber() << " and "
+                     << Cand2.BranchBlock->getNumber()
+                     << " have different branches\n");
+        break;
+      }
+      if (!canMerge(Cand2, Cand1)) {
+        DEBUG(dbgs() << "Cannot merge blocks " << Cand1.BranchBlock->getNumber()
+                     << " and " << Cand2.BranchBlock->getNumber() << "\n");
+        NumBlocksNotCoalesced++;
+        continue;
+      }
+      DEBUG(dbgs() << "Merging blocks " << Cand1.BranchBlock->getNumber()
+                   << " and " << Cand1.BranchTargetBlock->getNumber() << "\n");
+      MergedCandidates = mergeCandidates(Cand2, Cand1);
+      if (MergedCandidates)
+        didSomething = true;
+
+      DEBUG(dbgs() << "Function after merging: "; MF.dump(); dbgs() << "\n");
+    } while (MergedCandidates);
+  }
+
+#ifndef NDEBUG
+  // Verify MF is still valid after branch coalescing
+  if (didSomething)
+    MF.verify(nullptr, "Error in code produced by branch coalescing");
+#endif // NDEBUG
+
+  DEBUG(dbgs() << "Finished Branch Coalescing\n");
+  return didSomething;
+}