lib/Target/ARM/ARMConstantIslandPass.cpp

//===-- ARMConstantIslandPass.cpp - ARM constant islands ------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file 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 "Thumb2InstrInfo.h"
#include "MCTargetDesc/ARMAddressingModes.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/Format.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/CommandLine.h"
#include <algorithm>
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");
STATISTIC(NumTBs,        "Number of table branches generated");
STATISTIC(NumT2CPShrunk, "Number of Thumb2 constantpool instructions shrunk");
STATISTIC(NumT2BrShrunk, "Number of Thumb2 immediate branches shrunk");
STATISTIC(NumCBZ,        "Number of CBZ / CBNZ formed");
STATISTIC(NumJTMoved,    "Number of jump table destination blocks moved");
STATISTIC(NumJTInserted, "Number of jump table intermediate blocks inserted");


static cl::opt<bool>
AdjustJumpTableBlocks("arm-adjust-jump-tables", cl::Hidden, cl::init(true),
          cl::desc("Adjust basic block layout to better use TB[BH]"));

static cl::opt<bool>
AlignConstantIslands("arm-align-constant-island", cl::Hidden,
          cl::desc("Align constant islands in code"));

/// UnknownPadding - Return the worst case padding that could result from
/// unknown offset bits.  This does not include alignment padding caused by
/// known offset bits.
///
/// @param LogAlign log2(alignment)
/// @param KnownBits Number of known low offset bits.
static inline unsigned UnknownPadding(unsigned LogAlign, unsigned KnownBits) {
  if (KnownBits < LogAlign)
    return (1u << LogAlign) - (1u << KnownBits);
  return 0;
}

/// WorstCaseAlign - Assuming only the low KnownBits bits in Offset are exact,
/// add padding such that:
///
/// 1. The result is aligned to 1 << LogAlign.
///
/// 2. No other value of the unknown bits would require more padding.
///
/// This may add more padding than is required to satisfy just one of the
/// constraints.  It is necessary to compute alignment this way to guarantee
/// that we don't underestimate the padding before an aligned block.  If the
/// real padding before a block is larger than we think, constant pool entries
/// may go out of range.
static inline unsigned WorstCaseAlign(unsigned Offset, unsigned LogAlign,
                                      unsigned KnownBits) {
  // Add the worst possible padding that the unknown bits could cause.
  Offset += UnknownPadding(LogAlign, KnownBits);

  // Then align the result.
  return RoundUpToAlignment(Offset, 1u << LogAlign);
}

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 ARMConstantIslands : public MachineFunctionPass {
    /// BasicBlockInfo - Information about the offset and size of a single
    /// basic block.
    struct BasicBlockInfo {
      /// Offset - Distance from the beginning of the function to the beginning
      /// of this basic block.
      ///
      /// The offset is always aligned as required by the basic block.
      unsigned Offset;

      /// Size - Size of the basic block in bytes.  If the block contains
      /// inline assembly, this is a worst case estimate.
      ///
      /// The size does not include any alignment padding whether from the
      /// beginning of the block, or from an aligned jump table at the end.
      unsigned Size;

      /// KnownBits - The number of low bits in Offset that are known to be
      /// exact.  The remaining bits of Offset are an upper bound.
      uint8_t KnownBits;

      /// Unalign - When non-zero, the block contains instructions (inline asm)
      /// of unknown size.  The real size may be smaller than Size bytes by a
      /// multiple of 1 << Unalign.
      uint8_t Unalign;

      /// PostAlign - When non-zero, the block terminator contains a .align
      /// directive, so the end of the block is aligned to 1 << PostAlign
      /// bytes.
      uint8_t PostAlign;

      BasicBlockInfo() : Offset(0), Size(0), KnownBits(0), Unalign(0),
        PostAlign(0) {}

      /// Compute the number of known offset bits internally to this block.
      /// This number should be used to predict worst case padding when
      /// splitting the block.
      unsigned internalKnownBits() const {
        return Unalign ? Unalign : KnownBits;
      }

      /// Compute the offset immediately following this block.
      unsigned postOffset() const {
        unsigned PO = Offset + Size;
        if (!PostAlign)
          return PO;
        // Add alignment padding from the terminator.
        return WorstCaseAlign(PO, PostAlign, internalKnownBits());
      }

      /// Compute the number of known low bits of postOffset.  If this block
      /// contains inline asm, the number of known bits drops to the
      /// instruction alignment.  An aligned terminator may increase the number
      /// of know bits.
      unsigned postKnownBits() const {
        return std::max(unsigned(PostAlign), internalKnownBits());
      }
    };

    std::vector<BasicBlockInfo> BBInfo;

    /// 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;

    /// NewWaterList - The subset of WaterList that was created since the
    /// previous iteration by inserting unconditional branches.
    SmallSet<MachineBasicBlock*, 4> NewWaterList;

    typedef std::vector<MachineBasicBlock*>::iterator water_iterator;

    /// 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.  The HighWaterMark records the
    /// highest basic block where a new CPEntry can be placed.  To ensure this
    /// pass terminates, the CP entries are initially placed at the end of the
    /// function and then move monotonically to lower addresses.  The
    /// exception to this rule is when the current CP entry for a particular
    /// CPUser is out of range, but there is another CP entry for the same
    /// constant value in range.  We want to use the existing in-range CP
    /// entry, but if it later moves out of range, the search for new water
    /// should resume where it left off.  The HighWaterMark is used to record
    /// that point.
    struct CPUser {
      MachineInstr *MI;
      MachineInstr *CPEMI;
      MachineBasicBlock *HighWaterMark;
      unsigned MaxDisp;
      bool NegOk;
      bool IsSoImm;
      CPUser(MachineInstr *mi, MachineInstr *cpemi, unsigned maxdisp,
             bool neg, bool soimm)
        : MI(mi), CPEMI(cpemi), MaxDisp(maxdisp), NegOk(neg), IsSoImm(soimm) {
        HighWaterMark = CPEMI->getParent();
      }
    };

    /// 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) {}
    };

    /// ImmBranches - 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;

    /// T2JumpTables - Keep track of all the Thumb2 jumptable instructions.
    SmallVector<MachineInstr*, 4> T2JumpTables;

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

    MachineFunction *MF;
    MachineConstantPool *MCP;
    const ARMInstrInfo *TII;
    const ARMSubtarget *STI;
    ARMFunctionInfo *AFI;
    bool isThumb;
    bool isThumb1;
    bool isThumb2;
  public:
    static char ID;
    ARMConstantIslands() : MachineFunctionPass(ID) {}

    virtual bool runOnMachineFunction(MachineFunction &MF);

    virtual const char *getPassName() const {
      return "ARM constant island placement and branch shortening pass";
    }

  private:
    void DoInitialPlacement(std::vector<MachineInstr*> &CPEMIs);
    CPEntry *findConstPoolEntry(unsigned CPI, const MachineInstr *CPEMI);
    unsigned getCPELogAlign(const MachineInstr *CPEMI);
    void JumpTableFunctionScan();
    void InitialFunctionScan(const std::vector<MachineInstr*> &CPEMIs);
    MachineBasicBlock *SplitBlockBeforeInstr(MachineInstr *MI);
    void UpdateForInsertedWaterBlock(MachineBasicBlock *NewBB);
    void AdjustBBOffsetsAfter(MachineBasicBlock *BB);
    bool DecrementOldEntry(unsigned CPI, MachineInstr* CPEMI);
    int LookForExistingCPEntry(CPUser& U, unsigned UserOffset);
    bool LookForWater(CPUser&U, unsigned UserOffset, water_iterator &WaterIter);
    void CreateNewWater(unsigned CPUserIndex, unsigned UserOffset,
                        MachineBasicBlock *&NewMBB);
    bool HandleConstantPoolUser(unsigned CPUserIndex);
    void RemoveDeadCPEMI(MachineInstr *CPEMI);
    bool RemoveUnusedCPEntries();
    bool CPEIsInRange(MachineInstr *MI, unsigned UserOffset,
                      MachineInstr *CPEMI, unsigned Disp, bool NegOk,
                      bool DoDump = false);
    bool WaterIsInRange(unsigned UserOffset, MachineBasicBlock *Water,
                        CPUser &U);
    bool BBIsInRange(MachineInstr *MI, MachineBasicBlock *BB, unsigned Disp);
    bool FixUpImmediateBr(ImmBranch &Br);
    bool FixUpConditionalBr(ImmBranch &Br);
    bool FixUpUnconditionalBr(ImmBranch &Br);
    bool UndoLRSpillRestore();
    bool OptimizeThumb2Instructions();
    bool OptimizeThumb2Branches();
    bool ReorderThumb2JumpTables();
    bool OptimizeThumb2JumpTables();
    MachineBasicBlock *AdjustJTTargetBlockForward(MachineBasicBlock *BB,
                                                  MachineBasicBlock *JTBB);

    void ComputeBlockSize(MachineBasicBlock *MBB);
    unsigned GetOffsetOf(MachineInstr *MI) const;
    void dumpBBs();
    void verify();

    bool OffsetIsInRange(unsigned UserOffset, unsigned TrialOffset,
                         unsigned Disp, bool NegativeOK, bool IsSoImm = false);
    bool OffsetIsInRange(unsigned UserOffset, unsigned TrialOffset,
                         const CPUser &U) {
      return OffsetIsInRange(UserOffset, TrialOffset,
                             U.MaxDisp, U.NegOk, U.IsSoImm);
    }
  };
  char ARMConstantIslands::ID = 0;
}

/// verify - check BBOffsets, BBSizes, alignment of islands
void ARMConstantIslands::verify() {
#ifndef NDEBUG
  for (MachineFunction::iterator MBBI = MF->begin(), E = MF->end();
       MBBI != E; ++MBBI) {
    MachineBasicBlock *MBB = MBBI;
    unsigned Align = MBB->getAlignment();
    unsigned MBBId = MBB->getNumber();
    assert(BBInfo[MBBId].Offset % (1u << Align) == 0);
    assert(!MBBId || BBInfo[MBBId - 1].postOffset() <= BBInfo[MBBId].Offset);
  }
  for (unsigned i = 0, e = CPUsers.size(); i != e; ++i) {
    CPUser &U = CPUsers[i];
    unsigned UserOffset = GetOffsetOf(U.MI) + (isThumb ? 4 : 8);
    unsigned CPEOffset  = GetOffsetOf(U.CPEMI);
    unsigned Disp = UserOffset < CPEOffset ? CPEOffset - UserOffset :
      UserOffset - CPEOffset;
    assert(Disp <= U.MaxDisp || "Constant pool entry out of range!");
  }
#endif
}

/// print block size and offset information - debugging
void ARMConstantIslands::dumpBBs() {
  DEBUG({
    for (unsigned J = 0, E = BBInfo.size(); J !=E; ++J) {
      const BasicBlockInfo &BBI = BBInfo[J];
      dbgs() << format("%08x BB#%u\t", BBI.Offset, J)
             << " kb=" << unsigned(BBI.KnownBits)
             << " ua=" << unsigned(BBI.Unalign)
             << " pa=" << unsigned(BBI.PostAlign)
             << format(" size=%#x\n", BBInfo[J].Size);
    }
  });
}

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

bool ARMConstantIslands::runOnMachineFunction(MachineFunction &mf) {
  MF = &mf;
  MCP = mf.getConstantPool();

  DEBUG(dbgs() << "***** ARMConstantIslands: "
               << MCP->getConstants().size() << " CP entries, aligned to "
               << MCP->getConstantPoolAlignment() << " bytes *****\n");

  TII = (const ARMInstrInfo*)MF->getTarget().getInstrInfo();
  AFI = MF->getInfo<ARMFunctionInfo>();
  STI = &MF->getTarget().getSubtarget<ARMSubtarget>();

  isThumb = AFI->isThumbFunction();
  isThumb1 = AFI->isThumb1OnlyFunction();
  isThumb2 = AFI->isThumb2Function();

  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.
  MF->RenumberBlocks();

  // Try to reorder and otherwise adjust the block layout to make good use
  // of the TB[BH] instructions.
  bool MadeChange = false;
  if (isThumb2 && AdjustJumpTableBlocks) {
    JumpTableFunctionScan();
    MadeChange |= ReorderThumb2JumpTables();
    // Data is out of date, so clear it. It'll be re-computed later.
    T2JumpTables.clear();
    // Blocks may have shifted around. Keep the numbering up to date.
    MF->RenumberBlocks();
  }

  // Thumb1 functions containing constant pools get 4-byte alignment.
  // This is so we can keep exact track of where the alignment padding goes.

  // ARM and Thumb2 functions need to be 4-byte aligned.
  if (!isThumb1)
    MF->EnsureAlignment(2);  // 2 = log2(4)

  // 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(CPEMIs);

  /// The next UID to take is the first unused one.
  AFI->initPICLabelUId(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(CPEMIs);
  CPEMIs.clear();
  DEBUG(dumpBBs());


  /// Remove dead constant pool entries.
  MadeChange |= RemoveUnusedCPEntries();

  // Iteratively place constant pool entries and fix up branches until there
  // is no change.
  unsigned NoCPIters = 0, NoBRIters = 0;
  while (true) {
    DEBUG(dbgs() << "Beginning CP iteration #" << NoCPIters << '\n');
    bool CPChange = false;
    for (unsigned i = 0, e = CPUsers.size(); i != e; ++i)
      CPChange |= HandleConstantPoolUser(i);
    if (CPChange && ++NoCPIters > 30)
      llvm_unreachable("Constant Island pass failed to converge!");
    DEBUG(dumpBBs());

    // Clear NewWaterList now.  If we split a block for branches, it should
    // appear as "new water" for the next iteration of constant pool placement.
    NewWaterList.clear();

    DEBUG(dbgs() << "Beginning BR iteration #" << NoBRIters << '\n');
    bool BRChange = false;
    for (unsigned i = 0, e = ImmBranches.size(); i != e; ++i)
      BRChange |= FixUpImmediateBr(ImmBranches[i]);
    if (BRChange && ++NoBRIters > 30)
      llvm_unreachable("Branch Fix Up pass failed to converge!");
    DEBUG(dumpBBs());

    if (!CPChange && !BRChange)
      break;
    MadeChange = true;
  }

  // Shrink 32-bit Thumb2 branch, load, and store instructions.
  if (isThumb2 && !STI->prefers32BitThumb())
    MadeChange |= OptimizeThumb2Instructions();

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

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

  // Save the mapping between original and cloned constpool entries.
  for (unsigned i = 0, e = CPEntries.size(); i != e; ++i) {
    for (unsigned j = 0, je = CPEntries[i].size(); j != je; ++j) {
      const CPEntry & CPE = CPEntries[i][j];
      AFI->recordCPEClone(i, CPE.CPI);
    }
  }

  DEBUG(dbgs() << '\n'; dumpBBs());

  BBInfo.clear();
  WaterList.clear();
  CPUsers.clear();
  CPEntries.clear();
  ImmBranches.clear();
  PushPopMIs.clear();
  T2JumpTables.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(std::vector<MachineInstr*> &CPEMIs) {
  // Create the basic block to hold the CPE's.
  MachineBasicBlock *BB = MF->CreateMachineBasicBlock();
  MF->push_back(BB);

  // MachineConstantPool measures alignment in bytes. We measure in log2(bytes).
  unsigned MaxAlign = Log2_32(MF->getConstantPool()->getConstantPoolAlignment());

  // Mark the basic block as required by the const-pool.
  // If AlignConstantIslands isn't set, use 4-byte alignment for everything.
  BB->setAlignment(AlignConstantIslands ? MaxAlign : 2);

  // The function needs to be as aligned as the basic blocks. The linker may
  // move functions around based on their alignment.
  MF->EnsureAlignment(BB->getAlignment());

  // Order the entries in BB by descending alignment.  That ensures correct
  // alignment of all entries as long as BB is sufficiently aligned.  Keep
  // track of the insertion point for each alignment.  We are going to bucket
  // sort the entries as they are created.
  SmallVector<MachineBasicBlock::iterator, 8> InsPoint(MaxAlign + 1, BB->end());

  // 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 =
    MF->getConstantPool()->getConstants();

  const TargetData &TD = *MF->getTarget().getTargetData();
  for (unsigned i = 0, e = CPs.size(); i != e; ++i) {
    unsigned Size = TD.getTypeAllocSize(CPs[i].getType());
    assert(Size >= 4 && "Too small constant pool entry");
    unsigned Align = CPs[i].getAlignment();
    assert(isPowerOf2_32(Align) && "Invalid alignment");
    // Verify that all constant pool entries are a multiple of their alignment.
    // If not, we would have to pad them out so that instructions stay aligned.
    assert((Size % Align) == 0 && "CP Entry not multiple of 4 bytes!");

    // Insert CONSTPOOL_ENTRY before entries with a smaller alignment.
    unsigned LogAlign = Log2_32(Align);
    MachineBasicBlock::iterator InsAt = InsPoint[LogAlign];
    MachineInstr *CPEMI =
      BuildMI(*BB, InsAt, DebugLoc(), TII->get(ARM::CONSTPOOL_ENTRY))
        .addImm(i).addConstantPoolIndex(i).addImm(Size);
    CPEMIs.push_back(CPEMI);

    // Ensure that future entries with higher alignment get inserted before
    // CPEMI. This is bucket sort with iterators.
    for (unsigned a = LogAlign + 1; a < MaxAlign; ++a)
      if (InsPoint[a] == InsAt)
        InsPoint[a] = 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;
    DEBUG(dbgs() << "Moved CPI#" << i << " to end of function\n");
  }
  DEBUG(BB->dump());
}

/// 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;
  // Can't fall off end of function.
  if (llvm::next(MBBI) == MBB->getParent()->end())
    return false;

  MachineBasicBlock *NextBB = llvm::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;
}

/// getCPELogAlign - Returns the required alignment of the constant pool entry
/// represented by CPEMI.  Alignment is measured in log2(bytes) units.
unsigned ARMConstantIslands::getCPELogAlign(const MachineInstr *CPEMI) {
  assert(CPEMI && CPEMI->getOpcode() == ARM::CONSTPOOL_ENTRY);

  // Everything is 4-byte aligned unless AlignConstantIslands is set.
  if (!AlignConstantIslands)
    return 2;

  unsigned CPI = CPEMI->getOperand(1).getIndex();
  assert(CPI < MCP->getConstants().size() && "Invalid constant pool index.");
  unsigned Align = MCP->getConstants()[CPI].getAlignment();
  assert(isPowerOf2_32(Align) && "Invalid CPE alignment");
  return Log2_32(Align);
}

/// JumpTableFunctionScan - Do a scan of the function, building up
/// information about the sizes of each block and the locations of all
/// the jump tables.
void ARMConstantIslands::JumpTableFunctionScan() {
  for (MachineFunction::iterator MBBI = MF->begin(), E = MF->end();
       MBBI != E; ++MBBI) {
    MachineBasicBlock &MBB = *MBBI;

    for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
         I != E; ++I)
      if (I->isBranch() && I->getOpcode() == ARM::t2BR_JT)
        T2JumpTables.push_back(I);
  }
}

/// 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(const std::vector<MachineInstr*> &CPEMIs) {
  BBInfo.clear();
  BBInfo.resize(MF->getNumBlockIDs());

  // First thing, compute the size of all basic blocks, and see if the function
  // has any inline assembly in it. If so, we have to be conservative about
  // alignment assumptions, as we don't know for sure the size of any
  // instructions in the inline assembly.
  for (MachineFunction::iterator I = MF->begin(), E = MF->end(); I != E; ++I)
    ComputeBlockSize(I);

  // The known bits of the entry block offset are determined by the function
  // alignment.
  BBInfo.front().KnownBits = MF->getAlignment();

  // Compute block offsets and known bits.
  AdjustBBOffsetsAfter(MF->begin());

  // Now go back through the instructions and build up our data structures.
  for (MachineFunction::iterator MBBI = MF->begin(), E = MF->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);

    for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
         I != E; ++I) {
      if (I->isDebugValue())
        continue;

      int Opc = I->getOpcode();
      if (I->isBranch()) {
        bool isCond = false;
        unsigned Bits = 0;
        unsigned Scale = 1;
        int UOpc = Opc;
        switch (Opc) {
        default:
          continue;  // Ignore other JT branches
        case ARM::t2BR_JT:
          T2JumpTables.push_back(I);
          continue;   // Does not get an entry in ImmBranches
        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;
        case ARM::t2Bcc:
          isCond = true;
          UOpc = ARM::t2B;
          Bits = 20;
          Scale = 2;
          break;
        case ARM::t2B:
          Bits = 24;
          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);

      if (Opc == ARM::CONSTPOOL_ENTRY)
        continue;

      // Scan the instructions for constant pool operands.
      for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op)
        if (I->getOperand(op).isCPI()) {
          // 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;
          bool NegOk = false;
          bool IsSoImm = false;

          switch (Opc) {
          default:
            llvm_unreachable("Unknown addressing mode for CP reference!");
            break;

          // Taking the address of a CP entry.
          case ARM::LEApcrel:
            // This takes a SoImm, which is 8 bit immediate rotated. We'll
            // pretend the maximum offset is 255 * 4. Since each instruction
            // 4 byte wide, this is always correct. We'll check for other
            // displacements that fits in a SoImm as well.
            Bits = 8;
            Scale = 4;
            NegOk = true;
            IsSoImm = true;
            break;
          case ARM::t2LEApcrel:
            Bits = 12;
            NegOk = true;
            break;
          case ARM::tLEApcrel:
            Bits = 8;
            Scale = 4;
            break;

          case ARM::LDRi12:
          case ARM::LDRcp:
          case ARM::t2LDRpci:
            Bits = 12;  // +-offset_12
            NegOk = true;
            break;

          case ARM::tLDRpci:
            Bits = 8;
            Scale = 4;  // +(offset_8*4)
            break;

          case ARM::VLDRD:
          case ARM::VLDRS:
            Bits = 8;
            Scale = 4;  // +-(offset_8*4)
            NegOk = true;
            break;
          }

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

          // 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;
        }
    }
  }
}

/// ComputeBlockSize - Compute the size and some alignment information for MBB.
/// This function updates BBInfo directly.
void ARMConstantIslands::ComputeBlockSize(MachineBasicBlock *MBB) {
  BasicBlockInfo &BBI = BBInfo[MBB->getNumber()];
  BBI.Size = 0;
  BBI.Unalign = 0;
  BBI.PostAlign = 0;

  for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;
       ++I) {
    BBI.Size += TII->GetInstSizeInBytes(I);
    // For inline asm, GetInstSizeInBytes returns a conservative estimate.
    // The actual size may be smaller, but still a multiple of the instr size.
    if (I->isInlineAsm())
      BBI.Unalign = isThumb ? 1 : 2;
  }

  // tBR_JTr contains a .align 2 directive.
  if (!MBB->empty() && MBB->back().getOpcode() == ARM::tBR_JTr) {
    BBI.PostAlign = 2;
    MBB->getParent()->EnsureAlignment(2);
  }
}

/// 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 = BBInfo[MBB->getNumber()].Offset;

  // 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 += TII->GetInstSizeInBytes(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 consecutive.
  NewBB->getParent()->RenumberBlocks(NewBB);

  // Insert an entry into BBInfo to align it properly with the (newly
  // renumbered) block numbers.
  BBInfo.insert(BBInfo.begin() + NewBB->getNumber(), BasicBlockInfo());

  // Next, update WaterList.  Specifically, we need to add NewMBB as having
  // available water after it.
  water_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 data structures 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 =
    MF->CreateMachineBasicBlock(OrigBB->getBasicBlock());
  MachineFunction::iterator MBBI = OrigBB; ++MBBI;
  MF->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.
  // There doesn't seem to be meaningful DebugInfo available; this doesn't
  // correspond to anything in the source.
  unsigned Opc = isThumb ? (isThumb2 ? ARM::t2B : ARM::tB) : ARM::B;
  if (!isThumb)
    BuildMI(OrigBB, DebugLoc(), TII->get(Opc)).addMBB(NewBB);
  else
    BuildMI(OrigBB, DebugLoc(), TII->get(Opc)).addMBB(NewBB)
            .addImm(ARMCC::AL).addReg(0);
  ++NumSplit;

  // Update the CFG.  All succs of OrigBB are now succs of NewBB.
  NewBB->transferSuccessors(OrigBB);

  // 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.
  MF->RenumberBlocks(NewBB);

  // Insert an entry into BBInfo to align it properly with the (newly
  // renumbered) block numbers.
  BBInfo.insert(BBInfo.begin() + NewBB->getNumber(), BasicBlockInfo());

  // 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).
  water_iterator IP =
    std::lower_bound(WaterList.begin(), WaterList.end(), OrigBB,
                     CompareMBBNumbers);
  MachineBasicBlock* WaterBB = *IP;
  if (WaterBB == OrigBB)
    WaterList.insert(llvm::next(IP), NewBB);
  else
    WaterList.insert(IP, OrigBB);
  NewWaterList.insert(OrigBB);

  // Figure out how large the OrigBB is.  As the first half of the original
  // block, it cannot contain a tablejump.  The size includes
  // the new jump we added.  (It should be possible to do this without
  // recounting everything, but it's very confusing, and this is rarely
  // executed.)
  ComputeBlockSize(OrigBB);

  // Figure out how large the NewMBB is.  As the second half of the original
  // block, it may contain a tablejump.
  ComputeBlockSize(NewBB);

  // All BBOffsets following these blocks must be modified.
  AdjustBBOffsetsAfter(OrigBB);

  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, bool IsSoImm) {
  // 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.
  unsigned TotalAdj = 0;
  if (isThumb && UserOffset%4 !=0) {
    UserOffset -= 2;
    TotalAdj = 2;
  }
  // CPEs will be rounded up to a multiple of 4.
  if (isThumb && TrialOffset%4 != 0) {
    TrialOffset += 2;
    TotalAdj += 2;
  }

  // In Thumb2 mode, later branch adjustments can shift instructions up and
  // cause alignment change. In the worst case scenario this can cause the
  // user's effective address to be subtracted by 2 and the CPE's address to
  // be plus 2.
  if (isThumb2 && TotalAdj != 4)
    MaxDisp -= (4 - TotalAdj);

  if (UserOffset <= TrialOffset) {
    // User before the Trial.
    if (TrialOffset - UserOffset <= MaxDisp)
      return true;
    // FIXME: Make use full range of soimm values.
  } else if (NegativeOK) {
    if (UserOffset - TrialOffset <= MaxDisp)
      return true;
    // FIXME: Make use full range of soimm values.
  }
  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, CPUser &U) {
  unsigned CPEOffset = BBInfo[Water->getNumber()].postOffset();

  // If the CPE is to be inserted before the instruction, that will raise
  // the offset of the instruction.
  if (CPEOffset < UserOffset)
    UserOffset += U.CPEMI->getOperand(2).getImm();

  return OffsetIsInRange(UserOffset, CPEOffset, U);
}

/// 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 NegOk, bool DoDump) {
  unsigned CPEOffset  = GetOffsetOf(CPEMI);
  assert(CPEOffset % 4 == 0 && "Misaligned CPE");

  if (DoDump) {
    DEBUG({
      unsigned Block = MI->getParent()->getNumber();
      const BasicBlockInfo &BBI = BBInfo[Block];
      dbgs() << "User of CPE#" << CPEMI->getOperand(0).getImm()
             << " max delta=" << MaxDisp
             << format(" insn address=%#x", UserOffset)
             << " in BB#" << Block << ": "
             << format("%#x-%x\t", BBI.Offset, BBI.postOffset()) << *MI
             << format("CPE address=%#x offset=%+d: ", CPEOffset,
                       int(CPEOffset-UserOffset));
    });
  }

  return OffsetIsInRange(UserOffset, CPEOffset, MaxDisp, NegOk);
}

#ifndef NDEBUG
/// 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
      || PredMI->getOpcode() == ARM::t2B)
    return PredMI->getOperand(0).getMBB() == Succ;
  return false;
}
#endif // NDEBUG

void ARMConstantIslands::AdjustBBOffsetsAfter(MachineBasicBlock *BB) {
  for(unsigned i = BB->getNumber() + 1, e = MF->getNumBlockIDs(); i < e; ++i) {
    // Get the offset and known bits at the end of the layout predecessor.
    unsigned Offset = BBInfo[i - 1].postOffset();
    unsigned KnownBits = BBInfo[i - 1].postKnownBits();

    // Add padding before an aligned block. This may teach us more bits.
    if (unsigned Align = MF->getBlockNumbered(i)->getAlignment()) {
      Offset = WorstCaseAlign(Offset, Align, KnownBits);
      KnownBits = std::max(KnownBits, Align);
    }

    // This is where block i begins.
    BBInfo[i].Offset = Offset;
    BBInfo[i].KnownBits = KnownBits;
  }
}

/// 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, U.NegOk, true)) {
    DEBUG(dbgs() << "In range\n");
    return 1;
  }

  // No.  Look for previously created clones of the CPE that are in range.
  unsigned CPI = CPEMI->getOperand(1).getIndex();
  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, U.NegOk)) {
      DEBUG(dbgs() << "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).isCPI()) {
          UserMI->getOperand(j).setIndex(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) {
  switch (Opc) {
  case ARM::tB:
    return ((1<<10)-1)*2;
  case ARM::t2B:
    return ((1<<23)-1)*2;
  default:
    break;
  }

  return ((1<<23)-1)*4;
}

/// 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, WaterIter
/// is set to the WaterList entry.  For Thumb, prefer water that will not
/// introduce padding to water that will.  To ensure that this pass
/// terminates, the CPE location for a particular CPUser is only allowed to
/// move to a lower address, so search backward from the end of the list and
/// prefer the first water that is in range.
bool ARMConstantIslands::LookForWater(CPUser &U, unsigned UserOffset,
                                      water_iterator &WaterIter) {
  if (WaterList.empty())
    return false;

  bool FoundWaterThatWouldPad = false;
  water_iterator IPThatWouldPad;
  for (water_iterator IP = prior(WaterList.end()),
         B = WaterList.begin();; --IP) {
    MachineBasicBlock* WaterBB = *IP;
    // Check if water is in range and is either at a lower address than the
    // current "high water mark" or a new water block that was created since
    // the previous iteration by inserting an unconditional branch.  In the
    // latter case, we want to allow resetting the high water mark back to
    // this new water since we haven't seen it before.  Inserting branches
    // should be relatively uncommon and when it does happen, we want to be
    // sure to take advantage of it for all the CPEs near that block, so that
    // we don't insert more branches than necessary.
    if (WaterIsInRange(UserOffset, WaterBB, U) &&
        (WaterBB->getNumber() < U.HighWaterMark->getNumber() ||
         NewWaterList.count(WaterBB))) {
      unsigned WBBId = WaterBB->getNumber();
      if (isThumb && BBInfo[WBBId].postOffset()%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 (!FoundWaterThatWouldPad) {
          FoundWaterThatWouldPad = true;
          IPThatWouldPad = IP;
        }
      } else {
        WaterIter = IP;
        return true;
      }
    }
    if (IP == B)
      break;
  }
  if (FoundWaterThatWouldPad) {
    WaterIter = 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();
  const BasicBlockInfo &UserBBI = BBInfo[UserMBB->getNumber()];
  unsigned OffsetOfNextBlock = UserBBI.postOffset();

  // If the block does not end in an unconditional branch already, and if 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 +
  // Thumb2, 2 on Thumb1.  Possible Thumb1 alignment padding is allowed for
  // inside OffsetIsInRange.
  if (BBHasFallthrough(UserMBB) &&
      OffsetIsInRange(UserOffset, OffsetOfNextBlock + (isThumb1 ? 2: 4), U)) {
    DEBUG(dbgs() << "Split at end of block\n");
    if (&UserMBB->back() == UserMI)
      assert(BBHasFallthrough(UserMBB) && "Expected a fallthrough BB!");
    NewMBB = llvm::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 ? ((isThumb2) ? ARM::t2B : ARM::tB) : ARM::B;
    if (!isThumb)
      BuildMI(UserMBB, DebugLoc(), TII->get(UncondBr)).addMBB(NewMBB);
    else
      BuildMI(UserMBB, DebugLoc(), TII->get(UncondBr)).addMBB(NewMBB)
              .addImm(ARMCC::AL).addReg(0);
    unsigned MaxDisp = getUnconditionalBrDisp(UncondBr);
    ImmBranches.push_back(ImmBranch(&UserMBB->back(),
                          MaxDisp, false, UncondBr));
    int delta = isThumb1 ? 2 : 4;
    BBInfo[UserMBB->getNumber()].Size += delta;
    AdjustBBOffsetsAfter(UserMBB);
  } else {
    // What a big block.  Find a place within the block to split it.
    // This is a little tricky on Thumb1 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.

    // Try to split the block so it's fully aligned.  Compute the latest split
    // point where we can add a 4-byte branch instruction, and then
    // WorstCaseAlign to LogAlign.
    unsigned LogAlign = UserMBB->getParent()->getAlignment();
    unsigned KnownBits = UserBBI.internalKnownBits();
    unsigned UPad = UnknownPadding(LogAlign, KnownBits);
    unsigned BaseInsertOffset = UserOffset + U.MaxDisp;
    DEBUG(dbgs() << format("Split in middle of big block before %#x",
                           BaseInsertOffset));

    // Account for alignment and unknown padding.
    BaseInsertOffset &= ~((1u << LogAlign) - 1);
    BaseInsertOffset -= UPad;

    // The 4 in the following is for the unconditional branch we'll be
    // inserting (allows for long branch on Thumb1).  Alignment of the
    // island is handled inside OffsetIsInRange.
    BaseInsertOffset -= 4;

    DEBUG(dbgs() << format(", adjusted to %#x", BaseInsertOffset)
                 << " la=" << LogAlign
                 << " kb=" << KnownBits
                 << " up=" << UPad << '\n');

    // 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 >= BBInfo[UserMBB->getNumber()+1].Offset)
      BaseInsertOffset = BBInfo[UserMBB->getNumber()+1].Offset -
                              (isThumb1 ? 6 : 8);
    unsigned EndInsertOffset =
      WorstCaseAlign(BaseInsertOffset + 4, LogAlign, KnownBits) +
      CPEMI->getOperand(2).getImm();
    MachineBasicBlock::iterator MI = UserMI;
    ++MI;
    unsigned CPUIndex = CPUserIndex+1;
    unsigned NumCPUsers = CPUsers.size();
    MachineInstr *LastIT = 0;
    for (unsigned Offset = UserOffset+TII->GetInstSizeInBytes(UserMI);
         Offset < BaseInsertOffset;
         Offset += TII->GetInstSizeInBytes(MI),
           MI = llvm::next(MI)) {
      if (CPUIndex < NumCPUsers && CPUsers[CPUIndex].MI == MI) {
        CPUser &U = CPUsers[CPUIndex];
        if (!OffsetIsInRange(Offset, EndInsertOffset, U)) {
          BaseInsertOffset -= 1u << LogAlign;
          EndInsertOffset  -= 1u << LogAlign;
        }
        // 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++;
      }

      // Remember the last IT instruction.
      if (MI->getOpcode() == ARM::t2IT)
        LastIT = MI;
    }

    --MI;

    // Avoid splitting an IT block.
    if (LastIT) {
      unsigned PredReg = 0;
      ARMCC::CondCodes CC = llvm::getITInstrPredicate(MI, PredReg);
      if (CC != ARMCC::AL)
        MI = LastIT;
    }
    NewMBB = SplitBlockBeforeInstr(MI);
  }
}

/// HandleConstantPoolUser - Analyze the specified user, checking to see if it
/// is out-of-range.  If so, pick 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(unsigned CPUserIndex) {
  CPUser &U = CPUsers[CPUserIndex];
  MachineInstr *UserMI = U.MI;
  MachineInstr *CPEMI  = U.CPEMI;
  unsigned CPI = CPEMI->getOperand(1).getIndex();
  unsigned Size = CPEMI->getOperand(2).getImm();
  // Compute this only once, it's expensive.  The 4 or 8 is the value the
  // hardware keeps in the PC.
  unsigned UserOffset = GetOffsetOf(UserMI) + (isThumb ? 4 : 8);

  // 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 = AFI->createPICLabelUId();

  // Look for water where we can place this CPE.
  MachineBasicBlock *NewIsland = MF->CreateMachineBasicBlock();
  MachineBasicBlock *NewMBB;
  water_iterator IP;
  if (LookForWater(U, UserOffset, IP)) {
    DEBUG(dbgs() << "Found water in range\n");
    MachineBasicBlock *WaterBB = *IP;

    // If the original WaterList entry was "new water" on this iteration,
    // propagate that to the new island.  This is just keeping NewWaterList
    // updated to match the WaterList, which will be updated below.
    if (NewWaterList.count(WaterBB)) {
      NewWaterList.erase(WaterBB);
      NewWaterList.insert(NewIsland);
    }
    // The new CPE goes before the following block (NewMBB).
    NewMBB = llvm::next(MachineFunction::iterator(WaterBB));

  } else {
    // No water found.
    DEBUG(dbgs() << "No water found\n");
    CreateNewWater(CPUserIndex, UserOffset, NewMBB);

    // SplitBlockBeforeInstr adds to WaterList, which is important when it is
    // called while handling branches so that the water will be seen on the
    // next iteration for constant pools, but in this context, we don't want
    // it.  Check for this so it will be removed from the WaterList.
    // Also remove any entry from NewWaterList.
    MachineBasicBlock *WaterBB = prior(MachineFunction::iterator(NewMBB));
    IP = std::find(WaterList.begin(), WaterList.end(), WaterBB);
    if (IP != WaterList.end())
      NewWaterList.erase(WaterBB);

    // We are adding new water.  Update NewWaterList.
    NewWaterList.insert(NewIsland);
  }

  // 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 and is also important to ensure the algorithm terminates.
  if (IP != WaterList.end())
    WaterList.erase(IP);

  // Okay, we know we can put an island before NewMBB now, do it!
  MF->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.HighWaterMark = NewIsland;
  U.CPEMI = BuildMI(NewIsland, DebugLoc(), TII->get(ARM::CONSTPOOL_ENTRY))
                .addImm(ID).addConstantPoolIndex(CPI).addImm(Size);
  CPEntries[CPI].push_back(CPEntry(U.CPEMI, ID, 1));
  ++NumCPEs;

  // Mark the basic block as aligned as required by the const-pool entry.
  NewIsland->setAlignment(getCPELogAlign(U.CPEMI));

  // Increase the size of the island block to account for the new entry.
  BBInfo[NewIsland->getNumber()].Size += Size;
  AdjustBBOffsetsAfter(llvm::prior(MachineFunction::iterator(NewIsland)));

  // 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).isCPI()) {
      UserMI->getOperand(i).setIndex(ID);
      break;
    }

  DEBUG(dbgs() << "  Moved CPE to #" << ID << " CPI=" << CPI
        << format(" offset=%#x\n", BBInfo[NewIsland->getNumber()].Offset));

  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();
  BBInfo[CPEBB->getNumber()].Size -= Size;
  // All succeeding offsets have the current size value added in, fix this.
  if (CPEBB->empty()) {
    BBInfo[CPEBB->getNumber()].Size = 0;

    // This block no longer needs to be aligned. <rdar://problem/10534709>.
    CPEBB->setAlignment(0);
  } else
    // Entries are sorted by descending alignment, so realign from the front.
    CPEBB->setAlignment(getCPELogAlign(CPEBB->begin()));

  AdjustBBOffsetsAfter(CPEBB);
  // 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 = BBInfo[DestBB->getNumber()].Offset;

  DEBUG(dbgs() << "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(ImmBranch &Br) {
  MachineInstr *MI = Br.MI;
  MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();

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

  if (!Br.isCond)
    return FixUpUnconditionalBr(Br);
  return FixUpConditionalBr(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 a branch.
bool
ARMConstantIslands::FixUpUnconditionalBr(ImmBranch &Br) {
  MachineInstr *MI = Br.MI;
  MachineBasicBlock *MBB = MI->getParent();
  if (!isThumb1)
    llvm_unreachable("FixUpUnconditionalBr is Thumb1 only!");

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

  DEBUG(dbgs() << "  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(ImmBranch &Br) {
  MachineInstr *MI = Br.MI;
  MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();

  // Add an 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).getImm();
  CC = ARMCC::getOppositeCondition(CC);
  unsigned CCReg = MI->getOperand(2).getReg();

  // 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 (llvm::next(MachineBasicBlock::iterator(MI)) == prior(MBB->end()) &&
        BMI->getOpcode() == Br.UncondBr) {
      // Last MI in the BB is an unconditional branch. Can we simply invert the
      // condition and swap destinations:
      // beq L1
      // b   L2
      // =>
      // bne L2
      // b   L1
      MachineBasicBlock *NewDest = BMI->getOperand(0).getMBB();
      if (BBIsInRange(MI, NewDest, Br.MaxDisp)) {
        DEBUG(dbgs() << "  Invert Bcc condition and swap its destination with "
                     << *BMI);
        BMI->getOperand(0).setMBB(DestBB);
        MI->getOperand(0).setMBB(NewDest);
        MI->getOperand(1).setImm(CC);
        return true;
      }
    }
  }

  if (NeedSplit) {
    SplitBlockBeforeInstr(MI);
    // No need for the branch to the next block. We're adding an unconditional
    // branch to the destination.
    int delta = TII->GetInstSizeInBytes(&MBB->back());
    BBInfo[MBB->getNumber()].Size -= delta;
    MBB->back().eraseFromParent();
    // BBInfo[SplitBB].Offset is wrong temporarily, fixed below
  }
  MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));

  DEBUG(dbgs() << "  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, DebugLoc(), TII->get(MI->getOpcode()))
    .addMBB(NextBB).addImm(CC).addReg(CCReg);
  Br.MI = &MBB->back();
  BBInfo[MBB->getNumber()].Size += TII->GetInstSizeInBytes(&MBB->back());
  if (isThumb)
    BuildMI(MBB, DebugLoc(), TII->get(Br.UncondBr)).addMBB(DestBB)
            .addImm(ARMCC::AL).addReg(0);
  else
    BuildMI(MBB, DebugLoc(), TII->get(Br.UncondBr)).addMBB(DestBB);
  BBInfo[MBB->getNumber()].Size += TII->GetInstSizeInBytes(&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.
  BBInfo[MI->getParent()->getNumber()].Size -= TII->GetInstSizeInBytes(MI);
  MI->eraseFromParent();
  AdjustBBOffsetsAfter(MBB);
  return true;
}

/// UndoLRSpillRestore - Remove Thumb push / pop instructions that only spills
/// LR / restores LR to pc. FIXME: This is done here because it's only possible
/// to do this if tBfar is not used.
bool ARMConstantIslands::UndoLRSpillRestore() {
  bool MadeChange = false;
  for (unsigned i = 0, e = PushPopMIs.size(); i != e; ++i) {
    MachineInstr *MI = PushPopMIs[i];
    // First two operands are predicates.
    if (MI->getOpcode() == ARM::tPOP_RET &&
        MI->getOperand(2).getReg() == ARM::PC &&
        MI->getNumExplicitOperands() == 3) {
      // Create the new insn and copy the predicate from the old.
      BuildMI(MI->getParent(), MI->getDebugLoc(), TII->get(ARM::tBX_RET))
        .addOperand(MI->getOperand(0))
        .addOperand(MI->getOperand(1));
      MI->eraseFromParent();
      MadeChange = true;
    }
  }
  return MadeChange;
}

bool ARMConstantIslands::OptimizeThumb2Instructions() {
  bool MadeChange = false;

  // Shrink ADR and LDR from constantpool.
  for (unsigned i = 0, e = CPUsers.size(); i != e; ++i) {
    CPUser &U = CPUsers[i];
    unsigned Opcode = U.MI->getOpcode();
    unsigned NewOpc = 0;
    unsigned Scale = 1;
    unsigned Bits = 0;
    switch (Opcode) {
    default: break;
    case ARM::t2LEApcrel:
      if (isARMLowRegister(U.MI->getOperand(0).getReg())) {
        NewOpc = ARM::tLEApcrel;
        Bits = 8;
        Scale = 4;
      }
      break;
    case ARM::t2LDRpci:
      if (isARMLowRegister(U.MI->getOperand(0).getReg())) {
        NewOpc = ARM::tLDRpci;
        Bits = 8;
        Scale = 4;
      }
      break;
    }

    if (!NewOpc)
      continue;

    unsigned UserOffset = GetOffsetOf(U.MI) + 4;
    unsigned MaxOffs = ((1 << Bits) - 1) * Scale;
    // FIXME: Check if offset is multiple of scale if scale is not 4.
    if (CPEIsInRange(U.MI, UserOffset, U.CPEMI, MaxOffs, false, true)) {
      U.MI->setDesc(TII->get(NewOpc));
      MachineBasicBlock *MBB = U.MI->getParent();
      BBInfo[MBB->getNumber()].Size -= 2;
      AdjustBBOffsetsAfter(MBB);
      ++NumT2CPShrunk;
      MadeChange = true;
    }
  }

  MadeChange |= OptimizeThumb2Branches();
  MadeChange |= OptimizeThumb2JumpTables();
  return MadeChange;
}

bool ARMConstantIslands::OptimizeThumb2Branches() {
  bool MadeChange = false;

  for (unsigned i = 0, e = ImmBranches.size(); i != e; ++i) {
    ImmBranch &Br = ImmBranches[i];
    unsigned Opcode = Br.MI->getOpcode();
    unsigned NewOpc = 0;
    unsigned Scale = 1;
    unsigned Bits = 0;
    switch (Opcode) {
    default: break;
    case ARM::t2B:
      NewOpc = ARM::tB;
      Bits = 11;
      Scale = 2;
      break;
    case ARM::t2Bcc: {
      NewOpc = ARM::tBcc;
      Bits = 8;
      Scale = 2;
      break;
    }
    }
    if (NewOpc) {
      unsigned MaxOffs = ((1 << (Bits-1))-1) * Scale;
      MachineBasicBlock *DestBB = Br.MI->getOperand(0).getMBB();
      if (BBIsInRange(Br.MI, DestBB, MaxOffs)) {
        Br.MI->setDesc(TII->get(NewOpc));
        MachineBasicBlock *MBB = Br.MI->getParent();
        BBInfo[MBB->getNumber()].Size -= 2;
        AdjustBBOffsetsAfter(MBB);
        ++NumT2BrShrunk;
        MadeChange = true;
      }
    }

    Opcode = Br.MI->getOpcode();
    if (Opcode != ARM::tBcc)
      continue;

    NewOpc = 0;
    unsigned PredReg = 0;
    ARMCC::CondCodes Pred = llvm::getInstrPredicate(Br.MI, PredReg);
    if (Pred == ARMCC::EQ)
      NewOpc = ARM::tCBZ;
    else if (Pred == ARMCC::NE)
      NewOpc = ARM::tCBNZ;
    if (!NewOpc)
      continue;
    MachineBasicBlock *DestBB = Br.MI->getOperand(0).getMBB();
    // Check if the distance is within 126. Subtract starting offset by 2
    // because the cmp will be eliminated.
    unsigned BrOffset = GetOffsetOf(Br.MI) + 4 - 2;
    unsigned DestOffset = BBInfo[DestBB->getNumber()].Offset;
    if (BrOffset < DestOffset && (DestOffset - BrOffset) <= 126) {
      MachineBasicBlock::iterator CmpMI = Br.MI;
      if (CmpMI != Br.MI->getParent()->begin()) {
        --CmpMI;
        if (CmpMI->getOpcode() == ARM::tCMPi8) {
          unsigned Reg = CmpMI->getOperand(0).getReg();
          Pred = llvm::getInstrPredicate(CmpMI, PredReg);
          if (Pred == ARMCC::AL &&
              CmpMI->getOperand(1).getImm() == 0 &&
              isARMLowRegister(Reg)) {
            MachineBasicBlock *MBB = Br.MI->getParent();
            MachineInstr *NewBR =
              BuildMI(*MBB, CmpMI, Br.MI->getDebugLoc(), TII->get(NewOpc))
              .addReg(Reg).addMBB(DestBB,Br.MI->getOperand(0).getTargetFlags());
            CmpMI->eraseFromParent();
            Br.MI->eraseFromParent();
            Br.MI = NewBR;
            BBInfo[MBB->getNumber()].Size -= 2;
            AdjustBBOffsetsAfter(MBB);
            ++NumCBZ;
            MadeChange = true;
          }
        }
      }
    }
  }

  return MadeChange;
}

/// OptimizeThumb2JumpTables - Use tbb / tbh instructions to generate smaller
/// jumptables when it's possible.
bool ARMConstantIslands::OptimizeThumb2JumpTables() {
  bool MadeChange = false;

  // FIXME: After the tables are shrunk, can we get rid some of the
  // constantpool tables?
  MachineJumpTableInfo *MJTI = MF->getJumpTableInfo();
  if (MJTI == 0) return false;

  const std::vector<MachineJumpTableEntry> &JT = MJTI->getJumpTables();
  for (unsigned i = 0, e = T2JumpTables.size(); i != e; ++i) {
    MachineInstr *MI = T2JumpTables[i];
    const MCInstrDesc &MCID = MI->getDesc();
    unsigned NumOps = MCID.getNumOperands();
    unsigned JTOpIdx = NumOps - (MI->isPredicable() ? 3 : 2);
    MachineOperand JTOP = MI->getOperand(JTOpIdx);
    unsigned JTI = JTOP.getIndex();
    assert(JTI < JT.size());

    bool ByteOk = true;
    bool HalfWordOk = true;
    unsigned JTOffset = GetOffsetOf(MI) + 4;
    const std::vector<MachineBasicBlock*> &JTBBs = JT[JTI].MBBs;
    for (unsigned j = 0, ee = JTBBs.size(); j != ee; ++j) {
      MachineBasicBlock *MBB = JTBBs[j];
      unsigned DstOffset = BBInfo[MBB->getNumber()].Offset;
      // Negative offset is not ok. FIXME: We should change BB layout to make
      // sure all the branches are forward.
      if (ByteOk && (DstOffset - JTOffset) > ((1<<8)-1)*2)
        ByteOk = false;
      unsigned TBHLimit = ((1<<16)-1)*2;
      if (HalfWordOk && (DstOffset - JTOffset) > TBHLimit)
        HalfWordOk = false;
      if (!ByteOk && !HalfWordOk)
        break;
    }

    if (ByteOk || HalfWordOk) {
      MachineBasicBlock *MBB = MI->getParent();
      unsigned BaseReg = MI->getOperand(0).getReg();
      bool BaseRegKill = MI->getOperand(0).isKill();
      if (!BaseRegKill)
        continue;
      unsigned IdxReg = MI->getOperand(1).getReg();
      bool IdxRegKill = MI->getOperand(1).isKill();

      // Scan backwards to find the instruction that defines the base
      // register. Due to post-RA scheduling, we can't count on it
      // immediately preceding the branch instruction.
      MachineBasicBlock::iterator PrevI = MI;
      MachineBasicBlock::iterator B = MBB->begin();
      while (PrevI != B && !PrevI->definesRegister(BaseReg))
        --PrevI;

      // If for some reason we didn't find it, we can't do anything, so
      // just skip this one.
      if (!PrevI->definesRegister(BaseReg))
        continue;

      MachineInstr *AddrMI = PrevI;
      bool OptOk = true;
      // Examine the instruction that calculates the jumptable entry address.
      // Make sure it only defines the base register and kills any uses
      // other than the index register.
      for (unsigned k = 0, eee = AddrMI->getNumOperands(); k != eee; ++k) {
        const MachineOperand &MO = AddrMI->getOperand(k);
        if (!MO.isReg() || !MO.getReg())
          continue;
        if (MO.isDef() && MO.getReg() != BaseReg) {
          OptOk = false;
          break;
        }
        if (MO.isUse() && !MO.isKill() && MO.getReg() != IdxReg) {
          OptOk = false;
          break;
        }
      }
      if (!OptOk)
        continue;

      // Now scan back again to find the tLEApcrel or t2LEApcrelJT instruction
      // that gave us the initial base register definition.
      for (--PrevI; PrevI != B && !PrevI->definesRegister(BaseReg); --PrevI)
        ;

      // The instruction should be a tLEApcrel or t2LEApcrelJT; we want
      // to delete it as well.
      MachineInstr *LeaMI = PrevI;
      if ((LeaMI->getOpcode() != ARM::tLEApcrelJT &&
           LeaMI->getOpcode() != ARM::t2LEApcrelJT) ||
          LeaMI->getOperand(0).getReg() != BaseReg)
        OptOk = false;

      if (!OptOk)
        continue;

      unsigned Opc = ByteOk ? ARM::t2TBB_JT : ARM::t2TBH_JT;
      MachineInstr *NewJTMI = BuildMI(MBB, MI->getDebugLoc(), TII->get(Opc))
        .addReg(IdxReg, getKillRegState(IdxRegKill))
        .addJumpTableIndex(JTI, JTOP.getTargetFlags())
        .addImm(MI->getOperand(JTOpIdx+1).getImm());
      // FIXME: Insert an "ALIGN" instruction to ensure the next instruction
      // is 2-byte aligned. For now, asm printer will fix it up.
      unsigned NewSize = TII->GetInstSizeInBytes(NewJTMI);
      unsigned OrigSize = TII->GetInstSizeInBytes(AddrMI);
      OrigSize += TII->GetInstSizeInBytes(LeaMI);
      OrigSize += TII->GetInstSizeInBytes(MI);

      AddrMI->eraseFromParent();
      LeaMI->eraseFromParent();
      MI->eraseFromParent();

      int delta = OrigSize - NewSize;
      BBInfo[MBB->getNumber()].Size -= delta;
      AdjustBBOffsetsAfter(MBB);

      ++NumTBs;
      MadeChange = true;
    }
  }

  return MadeChange;
}

/// ReorderThumb2JumpTables - Adjust the function's block layout to ensure that
/// jump tables always branch forwards, since that's what tbb and tbh need.
bool ARMConstantIslands::ReorderThumb2JumpTables() {
  bool MadeChange = false;

  MachineJumpTableInfo *MJTI = MF->getJumpTableInfo();
  if (MJTI == 0) return false;

  const std::vector<MachineJumpTableEntry> &JT = MJTI->getJumpTables();
  for (unsigned i = 0, e = T2JumpTables.size(); i != e; ++i) {
    MachineInstr *MI = T2JumpTables[i];
    const MCInstrDesc &MCID = MI->getDesc();
    unsigned NumOps = MCID.getNumOperands();
    unsigned JTOpIdx = NumOps - (MI->isPredicable() ? 3 : 2);
    MachineOperand JTOP = MI->getOperand(JTOpIdx);
    unsigned JTI = JTOP.getIndex();
    assert(JTI < JT.size());

    // We prefer if target blocks for the jump table come after the jump
    // instruction so we can use TB[BH]. Loop through the target blocks
    // and try to adjust them such that that's true.
    int JTNumber = MI->getParent()->getNumber();
    const std::vector<MachineBasicBlock*> &JTBBs = JT[JTI].MBBs;
    for (unsigned j = 0, ee = JTBBs.size(); j != ee; ++j) {
      MachineBasicBlock *MBB = JTBBs[j];
      int DTNumber = MBB->getNumber();

      if (DTNumber < JTNumber) {
        // The destination precedes the switch. Try to move the block forward
        // so we have a positive offset.
        MachineBasicBlock *NewBB =
          AdjustJTTargetBlockForward(MBB, MI->getParent());
        if (NewBB)
          MJTI->ReplaceMBBInJumpTable(JTI, JTBBs[j], NewBB);
        MadeChange = true;
      }
    }
  }

  return MadeChange;
}

MachineBasicBlock *ARMConstantIslands::
AdjustJTTargetBlockForward(MachineBasicBlock *BB, MachineBasicBlock *JTBB)
{
  // If the destination block is terminated by an unconditional branch,
  // try to move it; otherwise, create a new block following the jump
  // table that branches back to the actual target. This is a very simple
  // heuristic. FIXME: We can definitely improve it.
  MachineBasicBlock *TBB = 0, *FBB = 0;
  SmallVector<MachineOperand, 4> Cond;
  SmallVector<MachineOperand, 4> CondPrior;
  MachineFunction::iterator BBi = BB;
  MachineFunction::iterator OldPrior = prior(BBi);

  // If the block terminator isn't analyzable, don't try to move the block
  bool B = TII->AnalyzeBranch(*BB, TBB, FBB, Cond);

  // If the block ends in an unconditional branch, move it. The prior block
  // has to have an analyzable terminator for us to move this one. Be paranoid
  // and make sure we're not trying to move the entry block of the function.
  if (!B && Cond.empty() && BB != MF->begin() &&
      !TII->AnalyzeBranch(*OldPrior, TBB, FBB, CondPrior)) {
    BB->moveAfter(JTBB);
    OldPrior->updateTerminator();
    BB->updateTerminator();
    // Update numbering to account for the block being moved.
    MF->RenumberBlocks();
    ++NumJTMoved;
    return NULL;
  }

  // Create a new MBB for the code after the jump BB.
  MachineBasicBlock *NewBB =
    MF->CreateMachineBasicBlock(JTBB->getBasicBlock());
  MachineFunction::iterator MBBI = JTBB; ++MBBI;
  MF->insert(MBBI, NewBB);

  // Add an unconditional branch from NewBB to BB.
  // There doesn't seem to be meaningful DebugInfo available; this doesn't
  // correspond directly to anything in the source.
  assert (isThumb2 && "Adjusting for TB[BH] but not in Thumb2?");
  BuildMI(NewBB, DebugLoc(), TII->get(ARM::t2B)).addMBB(BB)
          .addImm(ARMCC::AL).addReg(0);

  // Update internal data structures to account for the newly inserted MBB.
  MF->RenumberBlocks(NewBB);

  // Update the CFG.
  NewBB->addSuccessor(BB);
  JTBB->removeSuccessor(BB);
  JTBB->addSuccessor(NewBB);

  ++NumJTInserted;
  return NewBB;
}