lib/Target/ARM/ARMConstantIslandPass.cpp

//===-- ARMConstantIslandPass.cpp - ARM constant islands --------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Chris Lattner and is distributed under the
// University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains a pass that splits the constant pool up into 'islands'
// which are scattered through-out the function.  This is required due to the
// limited pc-relative displacements that ARM has.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "arm-cp-islands"
#include "ARM.h"
#include "ARMMachineFunctionInfo.h"
#include "ARMInstrInfo.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

STATISTIC(NumCPEs,     "Number of constpool entries");
STATISTIC(NumSplit,    "Number of uncond branches inserted");
STATISTIC(NumCBrFixed, "Number of cond branches fixed");
STATISTIC(NumUBrFixed, "Number of uncond branches fixed");

namespace {
  /// ARMConstantIslands - Due to limited PC-relative displacements, ARM
  /// requires constant pool entries to be scattered among the instructions
  /// inside a function.  To do this, it completely ignores the normal LLVM
  /// constant pool; instead, it places constants wherever it feels like with
  /// special instructions.
  ///
  /// The terminology used in this pass includes:
  ///   Islands - Clumps of constants placed in the function.
  ///   Water   - Potential places where an island could be formed.
  ///   CPE     - A constant pool entry that has been placed somewhere, which
  ///             tracks a list of users.
  class VISIBILITY_HIDDEN ARMConstantIslands : public MachineFunctionPass {
    /// NextUID - Assign unique ID's to CPE's.
    unsigned NextUID;

    /// BBSizes - The size of each MachineBasicBlock in bytes of code, indexed
    /// by MBB Number.  The two-byte pads required for Thumb alignment are
    /// counted as part of the following block (i.e., the offset and size for
    /// a padded block will both be ==2 mod 4).
    std::vector<unsigned> BBSizes;
    
    /// BBOffsets - the offset of each MBB in bytes, starting from 0.
    /// The two-byte pads required for Thumb alignment are counted as part of
    /// the following block.
    std::vector<unsigned> BBOffsets;

    /// WaterList - A sorted list of basic blocks where islands could be placed
    /// (i.e. blocks that don't fall through to the following block, due
    /// to a return, unreachable, or unconditional branch).
    std::vector<MachineBasicBlock*> WaterList;

    /// CPUser - One user of a constant pool, keeping the machine instruction
    /// pointer, the constant pool being referenced, and the max displacement
    /// allowed from the instruction to the CP.
    struct CPUser {
      MachineInstr *MI;
      MachineInstr *CPEMI;
      unsigned MaxDisp;
      CPUser(MachineInstr *mi, MachineInstr *cpemi, unsigned maxdisp)
        : MI(mi), CPEMI(cpemi), MaxDisp(maxdisp) {}
    };
    
    /// CPUsers - Keep track of all of the machine instructions that use various
    /// constant pools and their max displacement.
    std::vector<CPUser> CPUsers;
    
    /// CPEntry - One per constant pool entry, keeping the machine instruction
    /// pointer, the constpool index, and the number of CPUser's which
    /// reference this entry.
    struct CPEntry {
      MachineInstr *CPEMI;
      unsigned CPI;
      unsigned RefCount;
      CPEntry(MachineInstr *cpemi, unsigned cpi, unsigned rc = 0)
        : CPEMI(cpemi), CPI(cpi), RefCount(rc) {}
    };

    /// CPEntries - Keep track of all of the constant pool entry machine
    /// instructions. For each original constpool index (i.e. those that
    /// existed upon entry to this pass), it keeps a vector of entries.
    /// Original elements are cloned as we go along; the clones are
    /// put in the vector of the original element, but have distinct CPIs.
    std::vector<std::vector<CPEntry> > CPEntries;
    
    /// ImmBranch - One per immediate branch, keeping the machine instruction
    /// pointer, conditional or unconditional, the max displacement,
    /// and (if isCond is true) the corresponding unconditional branch
    /// opcode.
    struct ImmBranch {
      MachineInstr *MI;
      unsigned MaxDisp : 31;
      bool isCond : 1;
      int UncondBr;
      ImmBranch(MachineInstr *mi, unsigned maxdisp, bool cond, int ubr)
        : MI(mi), MaxDisp(maxdisp), isCond(cond), UncondBr(ubr) {}
    };

    /// Branches - Keep track of all the immediate branch instructions.
    ///
    std::vector<ImmBranch> ImmBranches;

    /// PushPopMIs - Keep track of all the Thumb push / pop instructions.
    ///
    SmallVector<MachineInstr*, 4> PushPopMIs;

    /// HasFarJump - True if any far jump instruction has been emitted during
    /// the branch fix up pass.
    bool HasFarJump;

    const TargetInstrInfo *TII;
    ARMFunctionInfo *AFI;
    bool isThumb;
  public:
    static char ID;
    ARMConstantIslands() : MachineFunctionPass((intptr_t)&ID) {}

    virtual bool runOnMachineFunction(MachineFunction &Fn);

    virtual const char *getPassName() const {
      return "ARM constant island placement and branch shortening pass";
    }
    
  private:
    void DoInitialPlacement(MachineFunction &Fn,
                            std::vector<MachineInstr*> &CPEMIs);
    CPEntry *findConstPoolEntry(unsigned CPI, const MachineInstr *CPEMI);
    void InitialFunctionScan(MachineFunction &Fn,
                             const std::vector<MachineInstr*> &CPEMIs);
    MachineBasicBlock *SplitBlockBeforeInstr(MachineInstr *MI);
    void UpdateForInsertedWaterBlock(MachineBasicBlock *NewBB);
    void AdjustBBOffsetsAfter(MachineBasicBlock *BB, int delta);
    bool DecrementOldEntry(unsigned CPI, MachineInstr* CPEMI);
    int LookForExistingCPEntry(CPUser& U, unsigned UserOffset);
    bool LookForWater(CPUser&U, unsigned UserOffset, 
                      MachineBasicBlock** NewMBB);
    MachineBasicBlock* AcceptWater(MachineBasicBlock *WaterBB, 
                        std::vector<MachineBasicBlock*>::iterator IP);
    void CreateNewWater(unsigned CPUserIndex, unsigned UserOffset,
                      MachineBasicBlock** NewMBB);
    bool HandleConstantPoolUser(MachineFunction &Fn, unsigned CPUserIndex);
    void RemoveDeadCPEMI(MachineInstr *CPEMI);
    bool RemoveUnusedCPEntries();
    bool CPEIsInRange(MachineInstr *MI, unsigned UserOffset, 
                      MachineInstr *CPEMI, unsigned Disp,
                      bool DoDump);
    bool WaterIsInRange(unsigned UserOffset, MachineBasicBlock *Water,
                        unsigned Disp);
    bool OffsetIsInRange(unsigned UserOffset, unsigned TrialOffset,
                        unsigned Disp, bool NegativeOK);
    bool BBIsInRange(MachineInstr *MI, MachineBasicBlock *BB, unsigned Disp);
    bool FixUpImmediateBr(MachineFunction &Fn, ImmBranch &Br);
    bool FixUpConditionalBr(MachineFunction &Fn, ImmBranch &Br);
    bool FixUpUnconditionalBr(MachineFunction &Fn, ImmBranch &Br);
    bool UndoLRSpillRestore();

    unsigned GetOffsetOf(MachineInstr *MI) const;
    void dumpBBs();
    void verify(MachineFunction &Fn);
  };
  char ARMConstantIslands::ID = 0;
}

/// verify - check BBOffsets, BBSizes, alignment of islands
void ARMConstantIslands::verify(MachineFunction &Fn) {
  assert(BBOffsets.size() == BBSizes.size());
  for (unsigned i = 1, e = BBOffsets.size(); i != e; ++i)
    assert(BBOffsets[i-1]+BBSizes[i-1] == BBOffsets[i]);
  if (isThumb) {
    for (MachineFunction::iterator MBBI = Fn.begin(), E = Fn.end();
         MBBI != E; ++MBBI) {
      MachineBasicBlock *MBB = MBBI;
      if (!MBB->empty() &&
          MBB->begin()->getOpcode() == ARM::CONSTPOOL_ENTRY)
        assert((BBOffsets[MBB->getNumber()]%4 == 0 &&
                BBSizes[MBB->getNumber()]%4 == 0) ||
               (BBOffsets[MBB->getNumber()]%4 != 0 &&
                BBSizes[MBB->getNumber()]%4 != 0));
    }
  }
}

/// print block size and offset information - debugging
void ARMConstantIslands::dumpBBs() {
  for (unsigned J = 0, E = BBOffsets.size(); J !=E; ++J) {
    DOUT << "block" << J << " offset" << BBOffsets[J] << 
                            " size" << BBSizes[J] << "\n";
  }
}

/// createARMConstantIslandPass - returns an instance of the constpool
/// island pass.
FunctionPass *llvm::createARMConstantIslandPass() {
  return new ARMConstantIslands();
}

bool ARMConstantIslands::runOnMachineFunction(MachineFunction &Fn) {
  MachineConstantPool &MCP = *Fn.getConstantPool();
  
  TII = Fn.getTarget().getInstrInfo();
  AFI = Fn.getInfo<ARMFunctionInfo>();
  isThumb = AFI->isThumbFunction();

  HasFarJump = false;

  // Renumber all of the machine basic blocks in the function, guaranteeing that
  // the numbers agree with the position of the block in the function.
  Fn.RenumberBlocks();

  /// Thumb functions containing constant pools get 2-byte alignment.  This is so
  /// we can keep exact track of where the alignment padding goes.  Set default.
  AFI->setAlign(isThumb ? 1U : 2U);

  // Perform the initial placement of the constant pool entries.  To start with,
  // we put them all at the end of the function.
  std::vector<MachineInstr*> CPEMIs;
  if (!MCP.isEmpty()) {
    DoInitialPlacement(Fn, CPEMIs);
    if (isThumb)
      AFI->setAlign(2U);
  }
  
  /// The next UID to take is the first unused one.
  NextUID = CPEMIs.size();
  
  // Do the initial scan of the function, building up information about the
  // sizes of each block, the location of all the water, and finding all of the
  // constant pool users.
  InitialFunctionScan(Fn, CPEMIs);
  CPEMIs.clear();
  
  /// Remove dead constant pool entries.
  RemoveUnusedCPEntries();

  // Iteratively place constant pool entries and fix up branches until there
  // is no change.
  bool MadeChange = false;
  while (true) {
    bool Change = false;
    for (unsigned i = 0, e = CPUsers.size(); i != e; ++i)
      Change |= HandleConstantPoolUser(Fn, i);
    DEBUG(dumpBBs());
    for (unsigned i = 0, e = ImmBranches.size(); i != e; ++i)
      Change |= FixUpImmediateBr(Fn, ImmBranches[i]);
    DEBUG(dumpBBs());
    if (!Change)
      break;
    MadeChange = true;
  }

  // After a while, this might be made debug-only, but it is not expensive.
  verify(Fn);

  // If LR has been forced spilled and no far jumps (i.e. BL) has been issued.
  // Undo the spill / restore of LR if possible.
  if (!HasFarJump && AFI->isLRSpilledForFarJump() && isThumb)
    MadeChange |= UndoLRSpillRestore();

  BBSizes.clear();
  BBOffsets.clear();
  WaterList.clear();
  CPUsers.clear();
  CPEntries.clear();
  ImmBranches.clear();
  PushPopMIs.clear();

  return MadeChange;
}

/// DoInitialPlacement - Perform the initial placement of the constant pool
/// entries.  To start with, we put them all at the end of the function.
void ARMConstantIslands::DoInitialPlacement(MachineFunction &Fn,
                                        std::vector<MachineInstr*> &CPEMIs){
  // Create the basic block to hold the CPE's.
  MachineBasicBlock *BB = new MachineBasicBlock();
  Fn.getBasicBlockList().push_back(BB);
  
  // Add all of the constants from the constant pool to the end block, use an
  // identity mapping of CPI's to CPE's.
  const std::vector<MachineConstantPoolEntry> &CPs =
    Fn.getConstantPool()->getConstants();
  
  const TargetData &TD = *Fn.getTarget().getTargetData();
  for (unsigned i = 0, e = CPs.size(); i != e; ++i) {
    unsigned Size = TD.getTypeSize(CPs[i].getType());
    // Verify that all constant pool entries are a multiple of 4 bytes.  If not,
    // we would have to pad them out or something so that instructions stay
    // aligned.
    assert((Size & 3) == 0 && "CP Entry not multiple of 4 bytes!");
    MachineInstr *CPEMI =
      BuildMI(BB, TII->get(ARM::CONSTPOOL_ENTRY))
                           .addImm(i).addConstantPoolIndex(i).addImm(Size);
    CPEMIs.push_back(CPEMI);

    // Add a new CPEntry, but no corresponding CPUser yet.
    std::vector<CPEntry> CPEs;
    CPEs.push_back(CPEntry(CPEMI, i));
    CPEntries.push_back(CPEs);
    NumCPEs++;
    DOUT << "Moved CPI#" << i << " to end of function as #" << i << "\n";
  }
}

/// BBHasFallthrough - Return true if the specified basic block can fallthrough
/// into the block immediately after it.
static bool BBHasFallthrough(MachineBasicBlock *MBB) {
  // Get the next machine basic block in the function.
  MachineFunction::iterator MBBI = MBB;
  if (next(MBBI) == MBB->getParent()->end())  // Can't fall off end of function.
    return false;
  
  MachineBasicBlock *NextBB = next(MBBI);
  for (MachineBasicBlock::succ_iterator I = MBB->succ_begin(),
       E = MBB->succ_end(); I != E; ++I)
    if (*I == NextBB)
      return true;
  
  return false;
}

/// findConstPoolEntry - Given the constpool index and CONSTPOOL_ENTRY MI,
/// look up the corresponding CPEntry.
ARMConstantIslands::CPEntry
*ARMConstantIslands::findConstPoolEntry(unsigned CPI,
                                        const MachineInstr *CPEMI) {
  std::vector<CPEntry> &CPEs = CPEntries[CPI];
  // Number of entries per constpool index should be small, just do a
  // linear search.
  for (unsigned i = 0, e = CPEs.size(); i != e; ++i) {
    if (CPEs[i].CPEMI == CPEMI)
      return &CPEs[i];
  }
  return NULL;
}

/// InitialFunctionScan - Do the initial scan of the function, building up
/// information about the sizes of each block, the location of all the water,
/// and finding all of the constant pool users.
void ARMConstantIslands::InitialFunctionScan(MachineFunction &Fn,
                                 const std::vector<MachineInstr*> &CPEMIs) {
  unsigned Offset = 0;
  for (MachineFunction::iterator MBBI = Fn.begin(), E = Fn.end();
       MBBI != E; ++MBBI) {
    MachineBasicBlock &MBB = *MBBI;
    
    // If this block doesn't fall through into the next MBB, then this is
    // 'water' that a constant pool island could be placed.
    if (!BBHasFallthrough(&MBB))
      WaterList.push_back(&MBB);
    
    unsigned MBBSize = 0;
    for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
         I != E; ++I) {
      // Add instruction size to MBBSize.
      MBBSize += ARM::GetInstSize(I);

      int Opc = I->getOpcode();
      if (TII->isBranch(Opc)) {
        bool isCond = false;
        unsigned Bits = 0;
        unsigned Scale = 1;
        int UOpc = Opc;
        switch (Opc) {
        case ARM::tBR_JTr:
          // A Thumb table jump may involve padding; for the offsets to
          // be right, functions containing these must be 4-byte aligned.
          AFI->setAlign(2U);
          if ((Offset+MBBSize)%4 != 0)
            MBBSize += 2;           // padding
          continue;   // Does not get an entry in ImmBranches
        default:
          continue;  // Ignore other JT branches
        case ARM::Bcc:
          isCond = true;
          UOpc = ARM::B;
          // Fallthrough
        case ARM::B:
          Bits = 24;
          Scale = 4;
          break;
        case ARM::tBcc:
          isCond = true;
          UOpc = ARM::tB;
          Bits = 8;
          Scale = 2;
          break;
        case ARM::tB:
          Bits = 11;
          Scale = 2;
          break;
        }

        // Record this immediate branch.
        unsigned MaxOffs = ((1 << (Bits-1))-1) * Scale;
        ImmBranches.push_back(ImmBranch(I, MaxOffs, isCond, UOpc));
      }

      if (Opc == ARM::tPUSH || Opc == ARM::tPOP_RET)
        PushPopMIs.push_back(I);

      // Scan the instructions for constant pool operands.
      for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op)
        if (I->getOperand(op).isConstantPoolIndex()) {
          // We found one.  The addressing mode tells us the max displacement
          // from the PC that this instruction permits.
          
          // Basic size info comes from the TSFlags field.
          unsigned Bits = 0;
          unsigned Scale = 1;
          unsigned TSFlags = I->getInstrDescriptor()->TSFlags;
          switch (TSFlags & ARMII::AddrModeMask) {
          default: 
            // Constant pool entries can reach anything.
            if (I->getOpcode() == ARM::CONSTPOOL_ENTRY)
              continue;
            if (I->getOpcode() == ARM::tLEApcrel) {
              Bits = 8;  // Taking the address of a CP entry.
              break;
            }
            assert(0 && "Unknown addressing mode for CP reference!");
          case ARMII::AddrMode1: // AM1: 8 bits << 2
            Bits = 8;
            Scale = 4;  // Taking the address of a CP entry.
            break;
          case ARMII::AddrMode2:
            Bits = 12;  // +-offset_12
            break;
          case ARMII::AddrMode3:
            Bits = 8;   // +-offset_8
            break;
            // addrmode4 has no immediate offset.
          case ARMII::AddrMode5:
            Bits = 8;
            Scale = 4;  // +-(offset_8*4)
            break;
          case ARMII::AddrModeT1:
            Bits = 5;  // +offset_5
            break;
          case ARMII::AddrModeT2:
            Bits = 5;
            Scale = 2;  // +(offset_5*2)
            break;
          case ARMII::AddrModeT4:
            Bits = 5;
            Scale = 4;  // +(offset_5*4)
            break;
          case ARMII::AddrModeTs:
            Bits = 8;
            Scale = 4;  // +(offset_8*4)
            break;
          }

          // Remember that this is a user of a CP entry.
          unsigned CPI = I->getOperand(op).getConstantPoolIndex();
          MachineInstr *CPEMI = CPEMIs[CPI];
          unsigned MaxOffs = ((1 << Bits)-1) * Scale;          
          CPUsers.push_back(CPUser(I, CPEMI, MaxOffs));

          // Increment corresponding CPEntry reference count.
          CPEntry *CPE = findConstPoolEntry(CPI, CPEMI);
          assert(CPE && "Cannot find a corresponding CPEntry!");
          CPE->RefCount++;
          
          // Instructions can only use one CP entry, don't bother scanning the
          // rest of the operands.
          break;
        }
    }

    // In thumb mode, if this block is a constpool island, we may need padding
    // so it's aligned on 4 byte boundary.
    if (isThumb &&
        !MBB.empty() &&
        MBB.begin()->getOpcode() == ARM::CONSTPOOL_ENTRY &&
        (Offset%4) != 0)
      MBBSize += 2;

    BBSizes.push_back(MBBSize);
    BBOffsets.push_back(Offset);
    Offset += MBBSize;
  }
}

/// GetOffsetOf - Return the current offset of the specified machine instruction
/// from the start of the function.  This offset changes as stuff is moved
/// around inside the function.
unsigned ARMConstantIslands::GetOffsetOf(MachineInstr *MI) const {
  MachineBasicBlock *MBB = MI->getParent();
  
  // The offset is composed of two things: the sum of the sizes of all MBB's
  // before this instruction's block, and the offset from the start of the block
  // it is in.
  unsigned Offset = BBOffsets[MBB->getNumber()];

  // If we're looking for a CONSTPOOL_ENTRY in Thumb, see if this block has
  // alignment padding, and compensate if so.
  if (isThumb && 
      MI->getOpcode() == ARM::CONSTPOOL_ENTRY && 
      Offset%4 != 0)
    Offset += 2;

  // Sum instructions before MI in MBB.
  for (MachineBasicBlock::iterator I = MBB->begin(); ; ++I) {
    assert(I != MBB->end() && "Didn't find MI in its own basic block?");
    if (&*I == MI) return Offset;
    Offset += ARM::GetInstSize(I);
  }
}

/// CompareMBBNumbers - Little predicate function to sort the WaterList by MBB
/// ID.
static bool CompareMBBNumbers(const MachineBasicBlock *LHS,
                              const MachineBasicBlock *RHS) {
  return LHS->getNumber() < RHS->getNumber();
}

/// UpdateForInsertedWaterBlock - When a block is newly inserted into the
/// machine function, it upsets all of the block numbers.  Renumber the blocks
/// and update the arrays that parallel this numbering.
void ARMConstantIslands::UpdateForInsertedWaterBlock(MachineBasicBlock *NewBB) {
  // Renumber the MBB's to keep them consequtive.
  NewBB->getParent()->RenumberBlocks(NewBB);
  
  // Insert a size into BBSizes to align it properly with the (newly
  // renumbered) block numbers.
  BBSizes.insert(BBSizes.begin()+NewBB->getNumber(), 0);

  // Likewise for BBOffsets.
  BBOffsets.insert(BBOffsets.begin()+NewBB->getNumber(), 0);
  
  // Next, update WaterList.  Specifically, we need to add NewMBB as having 
  // available water after it.
  std::vector<MachineBasicBlock*>::iterator IP =
    std::lower_bound(WaterList.begin(), WaterList.end(), NewBB,
                     CompareMBBNumbers);
  WaterList.insert(IP, NewBB);
}


/// Split the basic block containing MI into two blocks, which are joined by
/// an unconditional branch.  Update datastructures and renumber blocks to
/// account for this change and returns the newly created block.
MachineBasicBlock *ARMConstantIslands::SplitBlockBeforeInstr(MachineInstr *MI) {
  MachineBasicBlock *OrigBB = MI->getParent();

  // Create a new MBB for the code after the OrigBB.
  MachineBasicBlock *NewBB = new MachineBasicBlock(OrigBB->getBasicBlock());
  MachineFunction::iterator MBBI = OrigBB; ++MBBI;
  OrigBB->getParent()->getBasicBlockList().insert(MBBI, NewBB);
  
  // Splice the instructions starting with MI over to NewBB.
  NewBB->splice(NewBB->end(), OrigBB, MI, OrigBB->end());
  
  // Add an unconditional branch from OrigBB to NewBB.
  // Note the new unconditional branch is not being recorded.
  BuildMI(OrigBB, TII->get(isThumb ? ARM::tB : ARM::B)).addMBB(NewBB);
  NumSplit++;
  
  // Update the CFG.  All succs of OrigBB are now succs of NewBB.
  while (!OrigBB->succ_empty()) {
    MachineBasicBlock *Succ = *OrigBB->succ_begin();
    OrigBB->removeSuccessor(Succ);
    NewBB->addSuccessor(Succ);
    
    // This pass should be run after register allocation, so there should be no
    // PHI nodes to update.
    assert((Succ->empty() || Succ->begin()->getOpcode() != TargetInstrInfo::PHI)
           && "PHI nodes should be eliminated by now!");
  }
  
  // OrigBB branches to NewBB.
  OrigBB->addSuccessor(NewBB);
  
  // Update internal data structures to account for the newly inserted MBB.
  // This is almost the same as UpdateForInsertedWaterBlock, except that
  // the Water goes after OrigBB, not NewBB.
  NewBB->getParent()->RenumberBlocks(NewBB);
  
  // Insert a size into BBSizes to align it properly with the (newly
  // renumbered) block numbers.
  BBSizes.insert(BBSizes.begin()+NewBB->getNumber(), 0);
  
  // Likewise for BBOffsets.
  BBOffsets.insert(BBOffsets.begin()+NewBB->getNumber(), 0);

  // Next, update WaterList.  Specifically, we need to add OrigMBB as having 
  // available water after it (but not if it's already there, which happens
  // when splitting before a conditional branch that is followed by an
  // unconditional branch - in that case we want to insert NewBB).
  std::vector<MachineBasicBlock*>::iterator IP =
    std::lower_bound(WaterList.begin(), WaterList.end(), OrigBB,
                     CompareMBBNumbers);
  MachineBasicBlock* WaterBB = *IP;
  if (WaterBB == OrigBB)
    WaterList.insert(next(IP), NewBB);
  else
    WaterList.insert(IP, OrigBB);

  // Figure out how large the first NewMBB is.  (It cannot
  // contain a constpool_entry or tablejump.)
  unsigned NewBBSize = 0;
  for (MachineBasicBlock::iterator I = NewBB->begin(), E = NewBB->end();
       I != E; ++I)
    NewBBSize += ARM::GetInstSize(I);
  
  unsigned OrigBBI = OrigBB->getNumber();
  unsigned NewBBI = NewBB->getNumber();
  // Set the size of NewBB in BBSizes.
  BBSizes[NewBBI] = NewBBSize;
  
  // We removed instructions from UserMBB, subtract that off from its size.
  // Add 2 or 4 to the block to count the unconditional branch we added to it.
  unsigned delta = isThumb ? 2 : 4;
  BBSizes[OrigBBI] -= NewBBSize - delta;

  // ...and adjust BBOffsets for NewBB accordingly.
  BBOffsets[NewBBI] = BBOffsets[OrigBBI] + BBSizes[OrigBBI];

  // All BBOffsets following these blocks must be modified.
  AdjustBBOffsetsAfter(NewBB, delta);

  return NewBB;
}

/// OffsetIsInRange - Checks whether UserOffset (the location of a constant pool
/// reference) is within MaxDisp of TrialOffset (a proposed location of a 
/// constant pool entry).
bool ARMConstantIslands::OffsetIsInRange(unsigned UserOffset, 
                      unsigned TrialOffset, unsigned MaxDisp, bool NegativeOK) {
  // On Thumb offsets==2 mod 4 are rounded down by the hardware for 
  // purposes of the displacement computation; compensate for that here.  
  // Effectively, the valid range of displacements is 2 bytes smaller for such
  // references.
  if (isThumb && UserOffset%4 !=0)
    UserOffset -= 2;
  // CPEs will be rounded up to a multiple of 4.
  if (isThumb && TrialOffset%4 != 0)
    TrialOffset += 2;

  if (UserOffset <= TrialOffset) {
    // User before the Trial.
    if (TrialOffset-UserOffset <= MaxDisp)
      return true;
  } else if (NegativeOK) {
    if (UserOffset-TrialOffset <= MaxDisp)
      return true;
  }
  return false;
}

/// WaterIsInRange - Returns true if a CPE placed after the specified
/// Water (a basic block) will be in range for the specific MI.

bool ARMConstantIslands::WaterIsInRange(unsigned UserOffset,
                         MachineBasicBlock* Water, unsigned MaxDisp)
{
  MachineFunction::iterator I = next(MachineFunction::iterator(Water));
  unsigned CPEOffset = BBOffsets[Water->getNumber()] + 
                       BBSizes[Water->getNumber()];

  // If the CPE is to be inserted before the instruction, that will raise
  // the offset of the instruction.  (Currently applies only to ARM, so
  // no alignment compensation attempted here.)
  if (CPEOffset < UserOffset)
    UserOffset += 4;

  return OffsetIsInRange (UserOffset, CPEOffset, MaxDisp, !isThumb);
}

/// CPEIsInRange - Returns true if the distance between specific MI and
/// specific ConstPool entry instruction can fit in MI's displacement field.
bool ARMConstantIslands::CPEIsInRange(MachineInstr *MI, unsigned UserOffset,
                                      MachineInstr *CPEMI,
                                      unsigned MaxDisp, bool DoDump) {
  unsigned CPEOffset  = GetOffsetOf(CPEMI);
  assert(CPEOffset%4 == 0 && "Misaligned CPE");

  if (DoDump) {
    DOUT << "User of CPE#" << CPEMI->getOperand(0).getImm()
         << " max delta=" << MaxDisp
         << " insn address=" << UserOffset
         << " CPE address=" << CPEOffset
         << " offset=" << int(CPEOffset-UserOffset) << "\t" << *MI;
  }

  return OffsetIsInRange(UserOffset, CPEOffset, MaxDisp, !isThumb);
}

/// BBIsJumpedOver - Return true of the specified basic block's only predecessor
/// unconditionally branches to its only successor.
static bool BBIsJumpedOver(MachineBasicBlock *MBB) {
  if (MBB->pred_size() != 1 || MBB->succ_size() != 1)
    return false;

  MachineBasicBlock *Succ = *MBB->succ_begin();
  MachineBasicBlock *Pred = *MBB->pred_begin();
  MachineInstr *PredMI = &Pred->back();
  if (PredMI->getOpcode() == ARM::B || PredMI->getOpcode() == ARM::tB)
    return PredMI->getOperand(0).getMBB() == Succ;
  return false;
}

void ARMConstantIslands::AdjustBBOffsetsAfter(MachineBasicBlock *BB, 
                                              int delta) {
  MachineFunction::iterator MBBI = BB; MBBI = next(MBBI);
  for(unsigned i=BB->getNumber()+1; i<BB->getParent()->getNumBlockIDs(); i++) {
    BBOffsets[i] += delta;
    // If some existing blocks have padding, adjust the padding as needed, a
    // bit tricky.  delta can be negative so don't use % on that.
    if (isThumb) {
      MachineBasicBlock *MBB = MBBI;
      if (!MBB->empty()) {
        // Constant pool entries require padding.
        if (MBB->begin()->getOpcode() == ARM::CONSTPOOL_ENTRY) {
          unsigned oldOffset = BBOffsets[i] - delta;
          if (oldOffset%4==0 && BBOffsets[i]%4!=0) {
            // add new padding
            BBSizes[i] += 2;
            delta += 2;
          } else if (oldOffset%4!=0 && BBOffsets[i]%4==0) {
            // remove existing padding
            BBSizes[i] -=2;
            delta -= 2;
          }
        }
        // Thumb jump tables require padding.  They can be at the end, or
        // followed by an unconditional branch.
        MachineInstr *ThumbJTMI = NULL;
        if (prior(MBB->end())->getOpcode() == ARM::tBR_JTr)
          ThumbJTMI = prior(MBB->end());
        else if (prior(MBB->end()) != MBB->begin() &&
                prior(prior(MBB->end()))->getOpcode() == ARM::tBR_JTr)
          ThumbJTMI = prior(prior(MBB->end()));
        if (ThumbJTMI) {
          unsigned newMIOffset = GetOffsetOf(ThumbJTMI);
          unsigned oldMIOffset = newMIOffset - delta;
          if (oldMIOffset%4 == 0 && newMIOffset%4 != 0) {
            // remove existing padding
            BBSizes[i] -= 2;
            delta -= 2;
          } else if (oldMIOffset%4 != 0 && newMIOffset%4 == 0) {
            // add new padding
            BBSizes[i] += 2;
            delta += 2;
          }
        }
        if (delta==0)
          return;
      }
      MBBI = next(MBBI);
    }
  }
}

/// DecrementOldEntry - find the constant pool entry with index CPI
/// and instruction CPEMI, and decrement its refcount.  If the refcount
/// becomes 0 remove the entry and instruction.  Returns true if we removed 
/// the entry, false if we didn't.

bool ARMConstantIslands::DecrementOldEntry(unsigned CPI, MachineInstr *CPEMI) {
  // Find the old entry. Eliminate it if it is no longer used.
  CPEntry *CPE = findConstPoolEntry(CPI, CPEMI);
  assert(CPE && "Unexpected!");
  if (--CPE->RefCount == 0) {
    RemoveDeadCPEMI(CPEMI);
    CPE->CPEMI = NULL;
    NumCPEs--;
    return true;
  }
  return false;
}

/// LookForCPEntryInRange - see if the currently referenced CPE is in range;
/// if not, see if an in-range clone of the CPE is in range, and if so,
/// change the data structures so the user references the clone.  Returns:
/// 0 = no existing entry found
/// 1 = entry found, and there were no code insertions or deletions
/// 2 = entry found, and there were code insertions or deletions
int ARMConstantIslands::LookForExistingCPEntry(CPUser& U, unsigned UserOffset)
{
  MachineInstr *UserMI = U.MI;
  MachineInstr *CPEMI  = U.CPEMI;

  // Check to see if the CPE is already in-range.
  if (CPEIsInRange(UserMI, UserOffset, CPEMI, U.MaxDisp, true)) {
    DOUT << "In range\n";
    return 1;
  }

  // No.  Look for previously created clones of the CPE that are in range.
  unsigned CPI = CPEMI->getOperand(1).getConstantPoolIndex();
  std::vector<CPEntry> &CPEs = CPEntries[CPI];
  for (unsigned i = 0, e = CPEs.size(); i != e; ++i) {
    // We already tried this one
    if (CPEs[i].CPEMI == CPEMI)
      continue;
    // Removing CPEs can leave empty entries, skip
    if (CPEs[i].CPEMI == NULL)
      continue;
    if (CPEIsInRange(UserMI, UserOffset, CPEs[i].CPEMI, U.MaxDisp, false)) {
      DOUT << "Replacing CPE#" << CPI << " with CPE#" << CPEs[i].CPI << "\n";
      // Point the CPUser node to the replacement
      U.CPEMI = CPEs[i].CPEMI;
      // Change the CPI in the instruction operand to refer to the clone.
      for (unsigned j = 0, e = UserMI->getNumOperands(); j != e; ++j)
        if (UserMI->getOperand(j).isConstantPoolIndex()) {
          UserMI->getOperand(j).setConstantPoolIndex(CPEs[i].CPI);
          break;
        }
      // Adjust the refcount of the clone...
      CPEs[i].RefCount++;
      // ...and the original.  If we didn't remove the old entry, none of the
      // addresses changed, so we don't need another pass.
      return DecrementOldEntry(CPI, CPEMI) ? 2 : 1;
    }
  }
  return 0;
}

/// getUnconditionalBrDisp - Returns the maximum displacement that can fit in
/// the specific unconditional branch instruction.
static inline unsigned getUnconditionalBrDisp(int Opc) {
  return (Opc == ARM::tB) ? ((1<<10)-1)*2 : ((1<<23)-1)*4;
}

/// AcceptWater - Small amount of common code factored out of the following.

MachineBasicBlock* ARMConstantIslands::AcceptWater(MachineBasicBlock *WaterBB, 
                          std::vector<MachineBasicBlock*>::iterator IP) {
  DOUT << "found water in range\n";
  // Remove the original WaterList entry; we want subsequent
  // insertions in this vicinity to go after the one we're
  // about to insert.  This considerably reduces the number
  // of times we have to move the same CPE more than once.
  WaterList.erase(IP);
  // CPE goes before following block (NewMBB).
  return next(MachineFunction::iterator(WaterBB));
}

/// LookForWater - look for an existing entry in the WaterList in which
/// we can place the CPE referenced from U so it's within range of U's MI.
/// Returns true if found, false if not.  If it returns true, *NewMBB
/// is set to the WaterList entry.
/// For ARM, we prefer the water that's farthest away.  For Thumb, prefer
/// water that will not introduce padding to water that will; within each
/// group, prefer the water that's farthest away.

bool ARMConstantIslands::LookForWater(CPUser &U, unsigned UserOffset,
                                      MachineBasicBlock** NewMBB) {
  std::vector<MachineBasicBlock*>::iterator IPThatWouldPad;
  MachineBasicBlock* WaterBBThatWouldPad = NULL;
  if (!WaterList.empty()) {
    for (std::vector<MachineBasicBlock*>::iterator IP = prior(WaterList.end()),
        B = WaterList.begin();; --IP) {
      MachineBasicBlock* WaterBB = *IP;
      if (WaterIsInRange(UserOffset, WaterBB, U.MaxDisp)) {
        if (isThumb &&
            (BBOffsets[WaterBB->getNumber()] + 
             BBSizes[WaterBB->getNumber()])%4 != 0) {
          // This is valid Water, but would introduce padding.  Remember
          // it in case we don't find any Water that doesn't do this.
          if (!WaterBBThatWouldPad) {
            WaterBBThatWouldPad = WaterBB;
            IPThatWouldPad = IP;
          }
        } else {
          *NewMBB = AcceptWater(WaterBB, IP);
          return true;
        }
    }
      if (IP == B)
        break;
    }
  }
  if (isThumb && WaterBBThatWouldPad) {
    *NewMBB = AcceptWater(WaterBBThatWouldPad, IPThatWouldPad);
    return true;
  }
  return false;
}

/// CreateNewWater - No existing WaterList entry will work for 
/// CPUsers[CPUserIndex], so create a place to put the CPE.  The end of the
/// block is used if in range, and the conditional branch munged so control
/// flow is correct.  Otherwise the block is split to create a hole with an
/// unconditional branch around it.  In either case *NewMBB is set to a
/// block following which the new island can be inserted (the WaterList
/// is not adjusted).

void ARMConstantIslands::CreateNewWater(unsigned CPUserIndex, 
                        unsigned UserOffset, MachineBasicBlock** NewMBB) {
  CPUser &U = CPUsers[CPUserIndex];
  MachineInstr *UserMI = U.MI;
  MachineInstr *CPEMI  = U.CPEMI;
  MachineBasicBlock *UserMBB = UserMI->getParent();
  unsigned OffsetOfNextBlock = BBOffsets[UserMBB->getNumber()] + 
                               BBSizes[UserMBB->getNumber()];
  assert(OffsetOfNextBlock== BBOffsets[UserMBB->getNumber()+1]);

  // If the use is at the end of the block, or the end of the block
  // is within range, make new water there.  (The addition below is
  // for the unconditional branch we will be adding:  4 bytes on ARM,
  // 2 on Thumb.  Possible Thumb alignment padding is allowed for
  // inside OffsetIsInRange.
  // If the block ends in an unconditional branch already, it is water, 
  // and is known to be out of range, so we'll always be adding a branch.)
  if (&UserMBB->back() == UserMI ||
      OffsetIsInRange(UserOffset, OffsetOfNextBlock + (isThumb ? 2: 4),
           U.MaxDisp, !isThumb)) {
    DOUT << "Split at end of block\n";
    if (&UserMBB->back() == UserMI)
      assert(BBHasFallthrough(UserMBB) && "Expected a fallthrough BB!");
    *NewMBB = next(MachineFunction::iterator(UserMBB));
    // Add an unconditional branch from UserMBB to fallthrough block.
    // Record it for branch lengthening; this new branch will not get out of
    // range, but if the preceding conditional branch is out of range, the
    // targets will be exchanged, and the altered branch may be out of
    // range, so the machinery has to know about it.
    int UncondBr = isThumb ? ARM::tB : ARM::B;
    BuildMI(UserMBB, TII->get(UncondBr)).addMBB(*NewMBB);
    unsigned MaxDisp = getUnconditionalBrDisp(UncondBr);
    ImmBranches.push_back(ImmBranch(&UserMBB->back(), 
                          MaxDisp, false, UncondBr));
    int delta = isThumb ? 2 : 4;
    BBSizes[UserMBB->getNumber()] += delta;
    AdjustBBOffsetsAfter(UserMBB, delta);
  } else {
    // What a big block.  Find a place within the block to split it.
    // This is a little tricky on Thumb since instructions are 2 bytes
    // and constant pool entries are 4 bytes: if instruction I references
    // island CPE, and instruction I+1 references CPE', it will
    // not work well to put CPE as far forward as possible, since then
    // CPE' cannot immediately follow it (that location is 2 bytes
    // farther away from I+1 than CPE was from I) and we'd need to create
    // a new island.  So, we make a first guess, then walk through the
    // instructions between the one currently being looked at and the
    // possible insertion point, and make sure any other instructions
    // that reference CPEs will be able to use the same island area;
    // if not, we back up the insertion point.

    // The 4 in the following is for the unconditional branch we'll be
    // inserting (allows for long branch on Thumb).  Alignment of the
    // island is handled inside OffsetIsInRange.
    unsigned BaseInsertOffset = UserOffset + U.MaxDisp -4;
    // This could point off the end of the block if we've already got
    // constant pool entries following this block; only the last one is
    // in the water list.  Back past any possible branches (allow for a
    // conditional and a maximally long unconditional).
    if (BaseInsertOffset >= BBOffsets[UserMBB->getNumber()+1])
      BaseInsertOffset = BBOffsets[UserMBB->getNumber()+1] - 
                              (isThumb ? 6 : 8);
    unsigned EndInsertOffset = BaseInsertOffset +
           CPEMI->getOperand(2).getImm();
    MachineBasicBlock::iterator MI = UserMI;
    ++MI;
    unsigned CPUIndex = CPUserIndex+1;
    for (unsigned Offset = UserOffset+ARM::GetInstSize(UserMI);
         Offset < BaseInsertOffset;
         Offset += ARM::GetInstSize(MI),
            MI = next(MI)) {
      if (CPUIndex < CPUsers.size() && CPUsers[CPUIndex].MI == MI) {
        if (!OffsetIsInRange(Offset, EndInsertOffset, 
              CPUsers[CPUIndex].MaxDisp, !isThumb)) {
          BaseInsertOffset -= (isThumb ? 2 : 4);
          EndInsertOffset -= (isThumb ? 2 : 4);
        }
        // This is overly conservative, as we don't account for CPEMIs
        // being reused within the block, but it doesn't matter much.
        EndInsertOffset += CPUsers[CPUIndex].CPEMI->getOperand(2).getImm();
        CPUIndex++;
      }
    }
    DOUT << "Split in middle of big block\n";
    *NewMBB = SplitBlockBeforeInstr(prior(MI));
  }
}

/// HandleConstantPoolUser - Analyze the specified user, checking to see if it
/// is out-of-range.  If so, pick it up the constant pool value and move it some
/// place in-range.  Return true if we changed any addresses (thus must run
/// another pass of branch lengthening), false otherwise.
bool ARMConstantIslands::HandleConstantPoolUser(MachineFunction &Fn, 
                                                unsigned CPUserIndex){
  CPUser &U = CPUsers[CPUserIndex];
  MachineInstr *UserMI = U.MI;
  MachineInstr *CPEMI  = U.CPEMI;
  unsigned CPI = CPEMI->getOperand(1).getConstantPoolIndex();
  unsigned Size = CPEMI->getOperand(2).getImm();
  MachineBasicBlock *NewMBB;
  // Compute this only once, it's expensive.  The 4 or 8 is the value the
  //  hardware keeps in the PC (2 insns ahead of the reference).
  unsigned UserOffset = GetOffsetOf(UserMI) + (isThumb ? 4 : 8);

  // Special case: tLEApcrel are two instructions MI's. The actual user is the
  // second instruction.
  if (UserMI->getOpcode() == ARM::tLEApcrel)
    UserOffset += 2;
 
  // See if the current entry is within range, or there is a clone of it
  // in range.
  int result = LookForExistingCPEntry(U, UserOffset);
  if (result==1) return false;
  else if (result==2) return true;

  // No existing clone of this CPE is within range.
  // We will be generating a new clone.  Get a UID for it.
  unsigned ID  = NextUID++;

  // Look for water where we can place this CPE.  We look for the farthest one
  // away that will work.  Forward references only for now (although later
  // we might find some that are backwards).

  if (!LookForWater(U, UserOffset, &NewMBB)) {
    // No water found.
    DOUT << "No water found\n";
    CreateNewWater(CPUserIndex, UserOffset, &NewMBB);
  }

  // Okay, we know we can put an island before NewMBB now, do it!
  MachineBasicBlock *NewIsland = new MachineBasicBlock();
  Fn.getBasicBlockList().insert(NewMBB, NewIsland);

  // Update internal data structures to account for the newly inserted MBB.
  UpdateForInsertedWaterBlock(NewIsland);

  // Decrement the old entry, and remove it if refcount becomes 0.
  DecrementOldEntry(CPI, CPEMI);

  // Now that we have an island to add the CPE to, clone the original CPE and
  // add it to the island.
  U.CPEMI = BuildMI(NewIsland, TII->get(ARM::CONSTPOOL_ENTRY))
                .addImm(ID).addConstantPoolIndex(CPI).addImm(Size);
  CPEntries[CPI].push_back(CPEntry(U.CPEMI, ID, 1));
  NumCPEs++;

  BBOffsets[NewIsland->getNumber()] = BBOffsets[NewMBB->getNumber()];
  // Compensate for .align 2 in thumb mode.
  if (isThumb && BBOffsets[NewIsland->getNumber()]%4 != 0) 
    Size += 2;
  // Increase the size of the island block to account for the new entry.
  BBSizes[NewIsland->getNumber()] += Size;
  AdjustBBOffsetsAfter(NewIsland, Size);
  
  // Finally, change the CPI in the instruction operand to be ID.
  for (unsigned i = 0, e = UserMI->getNumOperands(); i != e; ++i)
    if (UserMI->getOperand(i).isConstantPoolIndex()) {
      UserMI->getOperand(i).setConstantPoolIndex(ID);
      break;
    }
      
  DOUT << "  Moved CPE to #" << ID << " CPI=" << CPI << "\t" << *UserMI;
      
  return true;
}

/// RemoveDeadCPEMI - Remove a dead constant pool entry instruction. Update
/// sizes and offsets of impacted basic blocks.
void ARMConstantIslands::RemoveDeadCPEMI(MachineInstr *CPEMI) {
  MachineBasicBlock *CPEBB = CPEMI->getParent();
  unsigned Size = CPEMI->getOperand(2).getImm();
  CPEMI->eraseFromParent();
  BBSizes[CPEBB->getNumber()] -= Size;
  // All succeeding offsets have the current size value added in, fix this.
  if (CPEBB->empty()) {
    // In thumb mode, the size of island may be  padded by two to compensate for
    // the alignment requirement.  Then it will now be 2 when the block is
    // empty, so fix this.
    // All succeeding offsets have the current size value added in, fix this.
    if (BBSizes[CPEBB->getNumber()] != 0) {
      Size += BBSizes[CPEBB->getNumber()];
      BBSizes[CPEBB->getNumber()] = 0;
    }
  }
  AdjustBBOffsetsAfter(CPEBB, -Size);
  // An island has only one predecessor BB and one successor BB. Check if
  // this BB's predecessor jumps directly to this BB's successor. This
  // shouldn't happen currently.
  assert(!BBIsJumpedOver(CPEBB) && "How did this happen?");
  // FIXME: remove the empty blocks after all the work is done?
}

/// RemoveUnusedCPEntries - Remove constant pool entries whose refcounts
/// are zero.
bool ARMConstantIslands::RemoveUnusedCPEntries() {
  unsigned MadeChange = false;
  for (unsigned i = 0, e = CPEntries.size(); i != e; ++i) {
      std::vector<CPEntry> &CPEs = CPEntries[i];
      for (unsigned j = 0, ee = CPEs.size(); j != ee; ++j) {
        if (CPEs[j].RefCount == 0 && CPEs[j].CPEMI) {
          RemoveDeadCPEMI(CPEs[j].CPEMI);
          CPEs[j].CPEMI = NULL;
          MadeChange = true;
        }
      }
  }  
  return MadeChange;
}

/// BBIsInRange - Returns true if the distance between specific MI and
/// specific BB can fit in MI's displacement field.
bool ARMConstantIslands::BBIsInRange(MachineInstr *MI,MachineBasicBlock *DestBB,
                                     unsigned MaxDisp) {
  unsigned PCAdj      = isThumb ? 4 : 8;
  unsigned BrOffset   = GetOffsetOf(MI) + PCAdj;
  unsigned DestOffset = BBOffsets[DestBB->getNumber()];

  DOUT << "Branch of destination BB#" << DestBB->getNumber()
       << " from BB#" << MI->getParent()->getNumber()
       << " max delta=" << MaxDisp
       << " from " << GetOffsetOf(MI) << " to " << DestOffset
       << " offset " << int(DestOffset-BrOffset) << "\t" << *MI;

  if (BrOffset <= DestOffset) {
    // Branch before the Dest.
    if (DestOffset-BrOffset <= MaxDisp)
      return true;
  } else {
    if (BrOffset-DestOffset <= MaxDisp)
      return true;
  }
  return false;
}

/// FixUpImmediateBr - Fix up an immediate branch whose destination is too far
/// away to fit in its displacement field.
bool ARMConstantIslands::FixUpImmediateBr(MachineFunction &Fn, ImmBranch &Br) {
  MachineInstr *MI = Br.MI;
  MachineBasicBlock *DestBB = MI->getOperand(0).getMachineBasicBlock();

  // Check to see if the DestBB is already in-range.
  if (BBIsInRange(MI, DestBB, Br.MaxDisp))
    return false;

  if (!Br.isCond)
    return FixUpUnconditionalBr(Fn, Br);
  return FixUpConditionalBr(Fn, Br);
}

/// FixUpUnconditionalBr - Fix up an unconditional branch whose destination is
/// too far away to fit in its displacement field. If the LR register has been
/// spilled in the epilogue, then we can use BL to implement a far jump.
/// Otherwise, add an intermediate branch instruction to to a branch.
bool
ARMConstantIslands::FixUpUnconditionalBr(MachineFunction &Fn, ImmBranch &Br) {
  MachineInstr *MI = Br.MI;
  MachineBasicBlock *MBB = MI->getParent();
  assert(isThumb && "Expected a Thumb function!");

  // Use BL to implement far jump.
  Br.MaxDisp = (1 << 21) * 2;
  MI->setInstrDescriptor(TII->get(ARM::tBfar));
  BBSizes[MBB->getNumber()] += 2;
  AdjustBBOffsetsAfter(MBB, 2);
  HasFarJump = true;
  NumUBrFixed++;

  DOUT << "  Changed B to long jump " << *MI;

  return true;
}

/// FixUpConditionalBr - Fix up a conditional branch whose destination is too
/// far away to fit in its displacement field. It is converted to an inverse
/// conditional branch + an unconditional branch to the destination.
bool
ARMConstantIslands::FixUpConditionalBr(MachineFunction &Fn, ImmBranch &Br) {
  MachineInstr *MI = Br.MI;
  MachineBasicBlock *DestBB = MI->getOperand(0).getMachineBasicBlock();

  // Add a unconditional branch to the destination and invert the branch
  // condition to jump over it:
  // blt L1
  // =>
  // bge L2
  // b   L1
  // L2:
  ARMCC::CondCodes CC = (ARMCC::CondCodes)MI->getOperand(1).getImmedValue();
  CC = ARMCC::getOppositeCondition(CC);

  // If the branch is at the end of its MBB and that has a fall-through block,
  // direct the updated conditional branch to the fall-through block. Otherwise,
  // split the MBB before the next instruction.
  MachineBasicBlock *MBB = MI->getParent();
  MachineInstr *BMI = &MBB->back();
  bool NeedSplit = (BMI != MI) || !BBHasFallthrough(MBB);

  NumCBrFixed++;
  if (BMI != MI) {
    if (next(MachineBasicBlock::iterator(MI)) == MBB->back() &&
        BMI->getOpcode() == Br.UncondBr) {
      // Last MI in the BB is a unconditional branch. Can we simply invert the
      // condition and swap destinations:
      // beq L1
      // b   L2
      // =>
      // bne L2
      // b   L1
      MachineBasicBlock *NewDest = BMI->getOperand(0).getMachineBasicBlock();
      if (BBIsInRange(MI, NewDest, Br.MaxDisp)) {
        DOUT << "  Invert Bcc condition and swap its destination with " << *BMI;
        BMI->getOperand(0).setMachineBasicBlock(DestBB);
        MI->getOperand(0).setMachineBasicBlock(NewDest);
        MI->getOperand(1).setImm(CC);
        return true;
      }
    }
  }

  if (NeedSplit) {
    SplitBlockBeforeInstr(MI);
    // No need for the branch to the next block. We're adding a unconditional
    // branch to the destination.
    int delta = ARM::GetInstSize(&MBB->back());
    BBSizes[MBB->getNumber()] -= delta;
    MachineBasicBlock* SplitBB = next(MachineFunction::iterator(MBB));
    AdjustBBOffsetsAfter(SplitBB, -delta);
    MBB->back().eraseFromParent();
    // BBOffsets[SplitBB] is wrong temporarily, fixed below
  }
  MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
 
  DOUT << "  Insert B to BB#" << DestBB->getNumber()
       << " also invert condition and change dest. to BB#"
       << NextBB->getNumber() << "\n";

  // Insert a new conditional branch and a new unconditional branch.
  // Also update the ImmBranch as well as adding a new entry for the new branch.
  BuildMI(MBB, TII->get(MI->getOpcode())).addMBB(NextBB).addImm(CC);
  Br.MI = &MBB->back();
  BBSizes[MBB->getNumber()] += ARM::GetInstSize(&MBB->back());
  BuildMI(MBB, TII->get(Br.UncondBr)).addMBB(DestBB);
  BBSizes[MBB->getNumber()] += ARM::GetInstSize(&MBB->back());
  unsigned MaxDisp = getUnconditionalBrDisp(Br.UncondBr);
  ImmBranches.push_back(ImmBranch(&MBB->back(), MaxDisp, false, Br.UncondBr));

  // Remove the old conditional branch.  It may or may not still be in MBB.
  BBSizes[MI->getParent()->getNumber()] -= ARM::GetInstSize(MI);
  MI->eraseFromParent();

  // The net size change is an addition of one unconditional branch.
  int delta = ARM::GetInstSize(&MBB->back());
  AdjustBBOffsetsAfter(MBB, delta);
  return true;
}

/// UndoLRSpillRestore - Remove Thumb push / pop instructions that only spills
/// LR / restores LR to pc.
bool ARMConstantIslands::UndoLRSpillRestore() {
  bool MadeChange = false;
  for (unsigned i = 0, e = PushPopMIs.size(); i != e; ++i) {
    MachineInstr *MI = PushPopMIs[i];
    if (MI->getNumOperands() == 1) {
        if (MI->getOpcode() == ARM::tPOP_RET &&
            MI->getOperand(0).getReg() == ARM::PC)
          BuildMI(MI->getParent(), TII->get(ARM::tBX_RET));
        MI->eraseFromParent();
        MadeChange = true;
    }
  }
  return MadeChange;
}