Fix r194019 as requested by Eric Christopher.

Submit the basic port of the rest of ARM constant islands code to Mips. 
Two test cases are added which reflect the next level of functionality:
constants getting moved to water areas that are out of range from the
initial placement at the end of the function and basic blocks being split to
create water when none exists that can be used. There is a bunch of this
code that is not complete and has been marked with IN_PROGRESS. I will
finish cleaning this all up during the next week or two and submit the
rest of the test cases. I have elminated some code for dealing with
inline assembly because to me it unecessarily complicates things and
some of the newer features of llvm like function attributies and builtin
assembler give me better tools to solve the alignment issues created
there. Also, for Mips16 I even have the option of not doing constant
islands in the present of inline assembler if I chose. When everything
has been completed I will summarize the port and notify people that
are knowledgable regarding the ARM Constant Islands code so they can
review it in it's entirety if they wish.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194053 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 1444 insertions, 13 deletions

diff --git a/lib/Target/Mips/Mips16InstrInfo.td b/lib/Target/Mips/Mips16InstrInfo.td
index 725a3a7..9ce0139 100644
--- a/lib/Target/Mips/Mips16InstrInfo.td
+++ b/lib/Target/Mips/Mips16InstrInfo.td
@@ -60,6 +60,11 @@ class FRI16_ins<bits<5> op, string asmstr,
                 InstrItinClass itin>:
   FRI16_ins_base<op, asmstr, "\t$rx, $imm \t# 16 bit inst", itin>;
 
+class FRI16_TCP_ins<bits<5> _op, string asmstr,
+                    InstrItinClass itin>:
+  FRI16<_op, (outs CPU16Regs:$rx), (ins pcrel16:$imm, i32imm:$size),
+            !strconcat(asmstr, "\t$rx, $imm\t# 16 bit inst"), [], itin>;
+            
 class FRI16R_ins_base<bits<5> op, string asmstr, string asmstr2,
                      InstrItinClass itin>:
   FRI16<op, (outs), (ins CPU16Regs:$rx, simm16:$imm),
@@ -174,7 +179,7 @@ class FEXT_RI16_B_ins<bits<5> _op, string asmstr,
 
 class FEXT_RI16_TCP_ins<bits<5> _op, string asmstr,
                         InstrItinClass itin>:
-  FEXT_RI16<_op, (outs CPU16Regs:$rx), (ins pcrel16:$imm),
+  FEXT_RI16<_op, (outs CPU16Regs:$rx), (ins pcrel16:$imm, i32imm:$size),
             !strconcat(asmstr, "\t$rx, $imm"), [], itin>;
 
 class FEXT_2RI16_ins<bits<5> _op, string asmstr,
@@ -802,6 +807,8 @@ def LwRxSpImmX16: FEXT_RI16_SP_explicit_ins<0b10010, "lw", IILoad>, MayLoad{
   let Uses = [SP];
 }
 
+def LwRxPcTcp16: FRI16_TCP_ins<0b10110, "lw", IILoad>, MayLoad;
+
 def LwRxPcTcpX16: FEXT_RI16_TCP_ins<0b10110, "lw", IILoad>, MayLoad;
 //
 // Format: MOVE r32, rz MIPS16e
@@ -1869,3 +1876,4 @@ let neverHasSideEffects = 1, isNotDuplicable = 1 in
 def CONSTPOOL_ENTRY :
 MipsPseudo16<(outs), (ins cpinst_operand:$instid, cpinst_operand:$cpidx,
                       i32imm:$size), "foo", []>;
+
diff --git a/lib/Target/Mips/MipsConstantIslandPass.cpp b/lib/Target/Mips/MipsConstantIslandPass.cpp
index 3209c0c..1c1652f 100644
--- a/lib/Target/Mips/MipsConstantIslandPass.cpp
+++ b/lib/Target/Mips/MipsConstantIslandPass.cpp
@@ -27,6 +27,7 @@
 
 #include "Mips.h"
 #include "MCTargetDesc/MipsBaseInfo.h"
+#include "MipsMachineFunction.h"
 #include "MipsTargetMachine.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
@@ -42,30 +43,197 @@
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetRegisterInfo.h"
+#include "llvm/Support/Format.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");
 
 // FIXME: This option should be removed once it has received sufficient testing.
 static cl::opt<bool>
 AlignConstantIslands("mips-align-constant-islands", cl::Hidden, cl::init(true),
           cl::desc("Align constant islands in code"));
 
+
+// Rather than do make check tests with huge amounts of code, we force
+// the test to use this amount.
+//
+static cl::opt<int> ConstantIslandsSmallOffset(
+  "mips-constant-islands-small-offset",
+  cl::init(0),
+  cl::desc("Make small offsets be this amount for testing purposes"),
+  cl::Hidden);
+
+/// 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;
+}
+
 namespace {
+
+
   typedef MachineBasicBlock::iterator Iter;
   typedef MachineBasicBlock::reverse_iterator ReverseIter;
 
+  /// MipsConstantIslands - Due to limited PC-relative displacements, Mips
+  /// 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 MipsConstantIslands : public MachineFunctionPass {
 
-  const TargetMachine &TM;
-  bool IsPIC;
-  unsigned ABI;
-  const MipsSubtarget *STI;
-  const MipsInstrInfo *TII;
-  MachineFunction *MF;
-  MachineConstantPool *MCP;
+    /// 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.
+      ///
+      /// Offsets are computed assuming worst case padding before an aligned
+      /// block. This means that subtracting basic block offsets always gives a
+      /// conservative estimate of the real distance which may be smaller.
+      ///
+      /// Because worst case padding is used, the computed offset of an aligned
+      /// block may not actually be aligned.
+      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 {
+        unsigned Bits = Unalign ? Unalign : KnownBits;
+        // If the block size isn't a multiple of the known bits, assume the
+        // worst case padding.
+        if (Size & ((1u << Bits) - 1))
+          Bits = countTrailingZeros(Size);
+        return Bits;
+      }
+
+      /// Compute the offset immediately following this block.  If LogAlign is
+      /// specified, return the offset the successor block will get if it has
+      /// this alignment.
+      unsigned postOffset(unsigned LogAlign = 0) const {
+        unsigned PO = Offset + Size;
+        return PO;
+      }
+
+      /// 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.
+      /// If LogAlign is given, also consider the alignment of the next block.
+      unsigned postKnownBits(unsigned LogAlign = 0) const {
+        return std::max(std::max(unsigned(PostAlign), LogAlign),
+                        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;
+    private:
+      unsigned MaxDisp;
+      unsigned LongFormMaxDisp; // mips16 has 16/32 bit instructions
+                                // with different displacements
+      unsigned LongFormOpcode;
+    public:
+      bool NegOk;
+      bool IsSoImm;
+      bool KnownAlignment;
+      CPUser(MachineInstr *mi, MachineInstr *cpemi, unsigned maxdisp,
+             bool neg, bool soimm,
+             unsigned longformmaxdisp, unsigned longformopcode)
+        : MI(mi), CPEMI(cpemi), MaxDisp(maxdisp),
+          LongFormMaxDisp(longformmaxdisp), LongFormOpcode(longformopcode),
+          NegOk(neg), IsSoImm(soimm), KnownAlignment(false)  {
+        HighWaterMark = CPEMI->getParent();
+      }
+      /// getMaxDisp - Returns the maximum displacement supported by MI.
+      /// Correct for unknown alignment.
+      /// Conservatively subtract 2 bytes to handle weird alignment effects.
+      unsigned getMaxDisp() const {
+        unsigned xMaxDisp = ConstantIslandsSmallOffset?
+                            ConstantIslandsSmallOffset: MaxDisp;
+        return (KnownAlignment ? xMaxDisp : xMaxDisp - 2) - 2;
+      }
+      unsigned getLongFormMaxDisp() const {
+        return (KnownAlignment ? LongFormMaxDisp : LongFormMaxDisp - 2) - 2;
+      }
+      unsigned getLongFormOpcode() const {
+          return LongFormOpcode;
+      }
+    };
+
+    /// 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
@@ -85,13 +253,56 @@ namespace {
   /// 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;
+
+  /// HasFarJump - True if any far jump instruction has been emitted during
+  /// the branch fix up pass.
+  bool HasFarJump;
+
+  const TargetMachine &TM;
+  bool IsPIC;
+  unsigned ABI;
+  const MipsSubtarget *STI;
+  const MipsInstrInfo *TII;
+  MipsFunctionInfo *MFI;
+  MachineFunction *MF;
+  MachineConstantPool *MCP;
+
+  unsigned PICLabelUId;
+  bool PrescannedForConstants;
+
+  void initPICLabelUId(unsigned UId) {
+    PICLabelUId = UId;
+  }
+
+
+  unsigned createPICLabelUId() {
+    return PICLabelUId++;
+  }
+
   public:
     static char ID;
     MipsConstantIslands(TargetMachine &tm)
       : MachineFunctionPass(ID), TM(tm),
         IsPIC(TM.getRelocationModel() == Reloc::PIC_),
         ABI(TM.getSubtarget<MipsSubtarget>().getTargetABI()),
-        STI(&TM.getSubtarget<MipsSubtarget>()), MF(0), MCP(0){}
+        STI(&TM.getSubtarget<MipsSubtarget>()), MF(0), MCP(0),
+        PrescannedForConstants(false){}
 
     virtual const char *getPassName() const {
       return "Mips Constant Islands";
@@ -100,6 +311,45 @@ namespace {
     bool runOnMachineFunction(MachineFunction &F);
 
     void doInitialPlacement(std::vector<MachineInstr*> &CPEMIs);
+    CPEntry *findConstPoolEntry(unsigned CPI, const MachineInstr *CPEMI);
+    unsigned getCPELogAlign(const MachineInstr *CPEMI);
+    void initializeFunctionInfo(const std::vector<MachineInstr*> &CPEMIs);
+    unsigned getOffsetOf(MachineInstr *MI) const;
+    unsigned getUserOffset(CPUser&) const;
+    void dumpBBs();
+    void verify();
+
+    bool isOffsetInRange(unsigned UserOffset, unsigned TrialOffset,
+                         unsigned Disp, bool NegativeOK, bool IsSoImm = false);
+    bool isOffsetInRange(unsigned UserOffset, unsigned TrialOffset,
+                         const CPUser &U);
+
+    bool isLongFormOffsetInRange(unsigned UserOffset, unsigned TrialOffset,
+                                const CPUser &U);
+
+    void computeBlockSize(MachineBasicBlock *MBB);
+    MachineBasicBlock *splitBlockBeforeInstr(MachineInstr *MI);
+    void updateForInsertedWaterBlock(MachineBasicBlock *NewBB);
+    void adjustBBOffsetsAfter(MachineBasicBlock *BB);
+    bool decrementCPEReferenceCount(unsigned CPI, MachineInstr* CPEMI);
+    int findInRangeCPEntry(CPUser& U, unsigned UserOffset);
+    int findLongFormInRangeCPEntry(CPUser& U, unsigned UserOffset);
+    bool findAvailableWater(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 isCPEntryInRange(MachineInstr *MI, unsigned UserOffset,
+                          MachineInstr *CPEMI, unsigned Disp, bool NegOk,
+                          bool DoDump = false);
+    bool isWaterInRange(unsigned UserOffset, MachineBasicBlock *Water,
+                        CPUser &U, unsigned &Growth);
+    bool isBBInRange(MachineInstr *MI, MachineBasicBlock *BB, unsigned Disp);
+    bool fixupImmediateBr(ImmBranch &Br);
+    bool fixupConditionalBr(ImmBranch &Br);
+    bool fixupUnconditionalBr(ImmBranch &Br);
 
     void prescanForConstants();
 
@@ -110,6 +360,33 @@ namespace {
   char MipsConstantIslands::ID = 0;
 } // end of anonymous namespace
 
+
+bool MipsConstantIslands::isLongFormOffsetInRange
+  (unsigned UserOffset, unsigned TrialOffset,
+   const CPUser &U) {
+  return isOffsetInRange(UserOffset, TrialOffset,
+                         U.getLongFormMaxDisp(), U.NegOk, U.IsSoImm);
+}
+
+bool MipsConstantIslands::isOffsetInRange
+  (unsigned UserOffset, unsigned TrialOffset,
+   const CPUser &U) {
+  return isOffsetInRange(UserOffset, TrialOffset,
+                         U.getMaxDisp(), U.NegOk, U.IsSoImm);
+}
+/// print block size and offset information - debugging
+void MipsConstantIslands::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);
+    }
+  });
+}
 /// createMipsLongBranchPass - Returns a pass that converts branches to long
 /// branches.
 FunctionPass *llvm::createMipsConstantIslandPass(MipsTargetMachine &tm) {
@@ -127,13 +404,15 @@ bool MipsConstantIslands::runOnMachineFunction(MachineFunction &mf) {
     return false;
   }
   TII = (const MipsInstrInfo*)MF->getTarget().getInstrInfo();
+  MFI = MF->getInfo<MipsFunctionInfo>();
   DEBUG(dbgs() << "constant island processing " << "\n");
   //
   // will need to make predermination if there is any constants we need to
   // put in constant islands. TBD.
   //
-  prescanForConstants();
+  if (!PrescannedForConstants) prescanForConstants();
 
+  HasFarJump = false;
   // This pass invalidates liveness information when it splits basic blocks.
   MF->getRegInfo().invalidateLiveness();
 
@@ -141,13 +420,66 @@ bool MipsConstantIslands::runOnMachineFunction(MachineFunction &mf) {
   // the numbers agree with the position of the block in the function.
   MF->RenumberBlocks();
 
+  bool MadeChange = false;
+
   // 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);
 
-  return true;
+  /// The next UID to take is the first unused one.
+  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.
+  initializeFunctionInfo(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;
+  (void)NoBRIters;
+  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)
+      report_fatal_error("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;
+#ifdef IN_PROGRESS
+    for (unsigned i = 0, e = ImmBranches.size(); i != e; ++i)
+      BRChange |= fixupImmediateBr(ImmBranches[i]);
+    if (BRChange && ++NoBRIters > 30)
+      report_fatal_error("Branch Fix Up pass failed to converge!");
+    DEBUG(dumpBBs());
+#endif
+    if (!CPChange && !BRChange)
+      break;
+    MadeChange = true;
+  }
+
+  DEBUG(dbgs() << '\n'; dumpBBs());
+
+  BBInfo.clear();
+  WaterList.clear();
+  CPUsers.clear();
+  CPEntries.clear();
+  ImmBranches.clear();
+  return MadeChange;
 }
 
 /// doInitialPlacement - Perform the initial placement of the constant pool
@@ -216,9 +548,1034 @@ MipsConstantIslands::doInitialPlacement(std::vector<MachineInstr*> &CPEMIs) {
   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.
+MipsConstantIslands::CPEntry
+*MipsConstantIslands::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 MipsConstantIslands::getCPELogAlign(const MachineInstr *CPEMI) {
+  assert(CPEMI && CPEMI->getOpcode() == Mips::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);
+}
+
+/// initializeFunctionInfo - 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 MipsConstantIslands::
+initializeFunctionInfo(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.
+  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
+        }
+        // Record this immediate branch.
+        unsigned MaxOffs = ((1 << (Bits-1))-1) * Scale;
+        ImmBranches.push_back(ImmBranch(I, MaxOffs, isCond, UOpc));
+
+      }
+
+
+      if (Opc == Mips::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;
+          unsigned LongFormBits = 0;
+          unsigned LongFormScale = 0;
+          unsigned LongFormOpcode = 0;
+          switch (Opc) {
+          default:
+            llvm_unreachable("Unknown addressing mode for CP reference!");
+          case Mips::LwRxPcTcp16:
+            Bits = 8;
+            Scale = 2;
+            LongFormOpcode = Mips::LwRxPcTcpX16;
+            break;
+          case Mips::LwRxPcTcpX16:
+            Bits = 16;
+            Scale = 2;
+            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;
+          unsigned LongFormMaxOffs = ((1 << LongFormBits)-1) * LongFormScale;
+          CPUsers.push_back(CPUser(I, CPEMI, MaxOffs, NegOk,
+                                   IsSoImm, LongFormMaxOffs,
+                                   LongFormOpcode));
+
+          // 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 MipsConstantIslands::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);
+
+}
+
+/// 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 MipsConstantIslands::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 != MI; ++I) {
+    assert(I != MBB->end() && "Didn't find MI in its own basic block?");
+    Offset += TII->GetInstSizeInBytes(I);
+  }
+  return Offset;
+}
+
+/// 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 MipsConstantIslands::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);
+}
+
+/// getUserOffset - Compute the offset of U.MI as seen by the hardware
+/// displacement computation.  Update U.KnownAlignment to match its current
+/// basic block location.
+unsigned MipsConstantIslands::getUserOffset(CPUser &U) const {
+  unsigned UserOffset = getOffsetOf(U.MI);
+  const BasicBlockInfo &BBI = BBInfo[U.MI->getParent()->getNumber()];
+  unsigned KnownBits = BBI.internalKnownBits();
+
+  // The value read from PC is offset from the actual instruction address.
+
+
+  // Because of inline assembly, we may not know the alignment (mod 4) of U.MI.
+  // Make sure U.getMaxDisp() returns a constrained range.
+  U.KnownAlignment = (KnownBits >= 2);
+
+  // On Thumb, offsets==2 mod 4 are rounded down by the hardware for
+  // purposes of the displacement computation; compensate for that here.
+  // For unknown alignments, getMaxDisp() constrains the range instead.
+
+
+  return UserOffset;
+}
+
+/// 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 *MipsConstantIslands::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.
+  BuildMI(OrigBB, DebugLoc(), TII->get(Mips::BimmX16)).addMBB(NewBB);
+  ++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;
+}
+
+
+
+/// isOffsetInRange - Checks whether UserOffset (the location of a constant pool
+/// reference) is within MaxDisp of TrialOffset (a proposed location of a
+/// constant pool entry).
+/// UserOffset is computed by getUserOffset above to include PC adjustments. If
+/// the mod 4 alignment of UserOffset is not known, the uncertainty must be
+/// subtracted from MaxDisp instead. CPUser::getMaxDisp() does that.
+bool MipsConstantIslands::isOffsetInRange(unsigned UserOffset,
+                                         unsigned TrialOffset, unsigned MaxDisp,
+                                         bool NegativeOK, bool IsSoImm) {
+  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;
+}
+
+/// isWaterInRange - Returns true if a CPE placed after the specified
+/// Water (a basic block) will be in range for the specific MI.
+///
+/// Compute how much the function will grow by inserting a CPE after Water.
+bool MipsConstantIslands::isWaterInRange(unsigned UserOffset,
+                                        MachineBasicBlock* Water, CPUser &U,
+                                        unsigned &Growth) {
+  unsigned CPELogAlign = getCPELogAlign(U.CPEMI);
+  unsigned CPEOffset = BBInfo[Water->getNumber()].postOffset(CPELogAlign);
+  unsigned NextBlockOffset, NextBlockAlignment;
+  MachineFunction::const_iterator NextBlock = Water;
+  if (++NextBlock == MF->end()) {
+    NextBlockOffset = BBInfo[Water->getNumber()].postOffset();
+    NextBlockAlignment = 0;
+  } else {
+    NextBlockOffset = BBInfo[NextBlock->getNumber()].Offset;
+    NextBlockAlignment = NextBlock->getAlignment();
+  }
+  unsigned Size = U.CPEMI->getOperand(2).getImm();
+  unsigned CPEEnd = CPEOffset + Size;
+
+  // The CPE may be able to hide in the alignment padding before the next
+  // block. It may also cause more padding to be required if it is more aligned
+  // that the next block.
+  if (CPEEnd > NextBlockOffset) {
+    Growth = CPEEnd - NextBlockOffset;
+    // Compute the padding that would go at the end of the CPE to align the next
+    // block.
+    Growth += OffsetToAlignment(CPEEnd, 1u << NextBlockAlignment);
+
+    // If the CPE is to be inserted before the instruction, that will raise
+    // the offset of the instruction. Also account for unknown alignment padding
+    // in blocks between CPE and the user.
+    if (CPEOffset < UserOffset)
+      UserOffset += Growth + UnknownPadding(MF->getAlignment(), CPELogAlign);
+  } else
+    // CPE fits in existing padding.
+    Growth = 0;
+
+  return isOffsetInRange(UserOffset, CPEOffset, U);
+}
+
+/// isCPEntryInRange - Returns true if the distance between specific MI and
+/// specific ConstPool entry instruction can fit in MI's displacement field.
+bool MipsConstantIslands::isCPEntryInRange
+  (MachineInstr *MI, unsigned UserOffset,
+   MachineInstr *CPEMI, unsigned MaxDisp,
+   bool NegOk, bool DoDump) {
+  unsigned CPEOffset  = getOffsetOf(CPEMI);
+
+  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 isOffsetInRange(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() == Mips::BimmX16)
+    return PredMI->getOperand(0).getMBB() == Succ;
+  return false;
+}
+#endif
+
+void MipsConstantIslands::adjustBBOffsetsAfter(MachineBasicBlock *BB) {
+  unsigned BBNum = BB->getNumber();
+  for(unsigned i = BBNum + 1, e = MF->getNumBlockIDs(); i < e; ++i) {
+    // Get the offset and known bits at the end of the layout predecessor.
+    // Include the alignment of the current block.
+    unsigned Offset = BBInfo[i - 1].postOffset();
+    BBInfo[i].Offset = Offset;
+  }
+}
+
+/// decrementCPEReferenceCount - 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 MipsConstantIslands::decrementCPEReferenceCount(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 MipsConstantIslands::findInRangeCPEntry(CPUser& U, unsigned UserOffset)
+{
+  MachineInstr *UserMI = U.MI;
+  MachineInstr *CPEMI  = U.CPEMI;
+
+  // Check to see if the CPE is already in-range.
+  if (isCPEntryInRange(UserMI, UserOffset, CPEMI, U.getMaxDisp(), 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 (isCPEntryInRange(UserMI, UserOffset, CPEs[i].CPEMI, U.getMaxDisp(),
+                     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 decrementCPEReferenceCount(CPI, CPEMI) ? 2 : 1;
+    }
+  }
+  return 0;
+}
+
+/// LookForCPEntryInRange - see if the currently referenced CPE is in range;
+/// This version checks if the longer form of the instruction can be used to
+/// to satisfy things.
+/// 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 MipsConstantIslands::findLongFormInRangeCPEntry
+  (CPUser& U, unsigned UserOffset)
+{
+  MachineInstr *UserMI = U.MI;
+  MachineInstr *CPEMI  = U.CPEMI;
+
+  // Check to see if the CPE is already in-range.
+  if (isCPEntryInRange(UserMI, UserOffset, CPEMI,
+                       U.getLongFormMaxDisp(), U.NegOk,
+                       true)) {
+    DEBUG(dbgs() << "In range\n");
+    UserMI->setDesc(TII->get(U.getLongFormOpcode()));
+    return 2;  // instruction is longer length now
+  }
+
+  // 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 (isCPEntryInRange(UserMI, UserOffset, CPEs[i].CPEMI,
+                         U.getLongFormMaxDisp(), 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 decrementCPEReferenceCount(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 Mips::BimmX16:
+    return ((1<<16)-1)*2;
+  default:
+    break;
+  }
+  return ((1<<16)-1)*2;
+}
+
+/// findAvailableWater - 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 MipsConstantIslands::findAvailableWater(CPUser &U, unsigned UserOffset,
+                                      water_iterator &WaterIter) {
+  if (WaterList.empty())
+    return false;
+
+  unsigned BestGrowth = ~0u;
+  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.
+    unsigned Growth;
+    if (isWaterInRange(UserOffset, WaterBB, U, Growth) &&
+        (WaterBB->getNumber() < U.HighWaterMark->getNumber() ||
+         NewWaterList.count(WaterBB)) && Growth < BestGrowth) {
+      // This is the least amount of required padding seen so far.
+      BestGrowth = Growth;
+      WaterIter = IP;
+      DEBUG(dbgs() << "Found water after BB#" << WaterBB->getNumber()
+                   << " Growth=" << Growth << '\n');
+
+      // Keep looking unless it is perfect.
+      if (BestGrowth == 0)
+        return true;
+    }
+    if (IP == B)
+      break;
+  }
+  return BestGrowth != ~0u;
+}
+
+/// 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 MipsConstantIslands::createNewWater(unsigned CPUserIndex,
+                                        unsigned UserOffset,
+                                        MachineBasicBlock *&NewMBB) {
+  CPUser &U = CPUsers[CPUserIndex];
+  MachineInstr *UserMI = U.MI;
+  MachineInstr *CPEMI  = U.CPEMI;
+  unsigned CPELogAlign = getCPELogAlign(CPEMI);
+  MachineBasicBlock *UserMBB = UserMI->getParent();
+  const BasicBlockInfo &UserBBI = BBInfo[UserMBB->getNumber()];
+
+  // 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.
+  if (BBHasFallthrough(UserMBB)) {
+    // Size of branch to insert.
+    unsigned Delta = 2;
+    // Compute the offset where the CPE will begin.
+    unsigned CPEOffset = UserBBI.postOffset(CPELogAlign) + Delta;
+
+    if (isOffsetInRange(UserOffset, CPEOffset, U)) {
+      DEBUG(dbgs() << "Split at end of BB#" << UserMBB->getNumber()
+            << format(", expected CPE offset %#x\n", CPEOffset));
+      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 = Mips::BimmX16;
+      BuildMI(UserMBB, DebugLoc(), TII->get(UncondBr)).addMBB(NewMBB);
+      unsigned MaxDisp = getUnconditionalBrDisp(UncondBr);
+      ImmBranches.push_back(ImmBranch(&UserMBB->back(),
+                                      MaxDisp, false, UncondBr));
+      BBInfo[UserMBB->getNumber()].Size += Delta;
+      adjustBBOffsetsAfter(UserMBB);
+      return;
+    }
+  }
+
+  // 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 align to
+  // LogAlign which is the largest possible alignment in the function.
+  unsigned LogAlign = MF->getAlignment();
+  assert(LogAlign >= CPELogAlign && "Over-aligned constant pool entry");
+  unsigned KnownBits = UserBBI.internalKnownBits();
+  unsigned UPad = UnknownPadding(LogAlign, KnownBits);
+  unsigned BaseInsertOffset = UserOffset + U.getMaxDisp() - UPad;
+  DEBUG(dbgs() << format("Split in middle of big block before %#x",
+                         BaseInsertOffset));
+
+  // 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 isOffsetInRange.
+  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 + 8 >= UserBBI.postOffset()) {
+    BaseInsertOffset = UserBBI.postOffset() - UPad - 8;
+    DEBUG(dbgs() << format("Move inside block: %#x\n", BaseInsertOffset));
+  }
+  unsigned EndInsertOffset = BaseInsertOffset + 4 + UPad +
+    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)) {
+    assert(MI != UserMBB->end() && "Fell off end of block");
+    if (CPUIndex < NumCPUsers && CPUsers[CPUIndex].MI == MI) {
+      CPUser &U = CPUsers[CPUIndex];
+      if (!isOffsetInRange(Offset, EndInsertOffset, U)) {
+        // Shift intertion point by one unit of alignment so it is within reach.
+        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.  Also assume CPEs
+      // are added in order with alignment padding.  We may eventually be able
+      // to pack the aligned CPEs better.
+      EndInsertOffset += U.CPEMI->getOperand(2).getImm();
+      CPUIndex++;
+    }
+  }
+
+  --MI;
+  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 MipsConstantIslands::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.
+  unsigned UserOffset = getUserOffset(U);
+
+  // See if the current entry is within range, or there is a clone of it
+  // in range.
+  int result = findInRangeCPEntry(U, UserOffset);
+  if (result==1) return false;
+  else if (result==2) return true;
+
+
+  // Look for water where we can place this CPE.
+  MachineBasicBlock *NewIsland = MF->CreateMachineBasicBlock();
+  MachineBasicBlock *NewMBB;
+  water_iterator IP;
+  if (findAvailableWater(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.erase(WaterBB))
+      NewWaterList.insert(NewIsland);
+
+    // The new CPE goes before the following block (NewMBB).
+    NewMBB = llvm::next(MachineFunction::iterator(WaterBB));
+
+  } else {
+    // No water found.
+    // we first see if a longer form of the instrucion could have reached
+    // the constant. in that case we won't bother to split
+#ifdef IN_PROGRESS
+    result = findLongFormInRangeCPEntry(U, UserOffset);
+#endif
+    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.
+  decrementCPEReferenceCount(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(Mips::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)));
+
+  // No existing clone of this CPE is within range.
+  // We will be generating a new clone.  Get a UID for it.
+  unsigned ID = createPICLabelUId();
+
+  // 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 MipsConstantIslands::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.
+    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 MipsConstantIslands::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;
+}
+
+/// isBBInRange - Returns true if the distance between specific MI and
+/// specific BB can fit in MI's displacement field.
+bool MipsConstantIslands::isBBInRange
+  (MachineInstr *MI,MachineBasicBlock *DestBB, unsigned MaxDisp) {
+
+unsigned PCAdj = 4;
+
+  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 MipsConstantIslands::fixupImmediateBr(ImmBranch &Br) {
+  MachineInstr *MI = Br.MI;
+  MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
+
+  // Check to see if the DestBB is already in-range.
+  if (isBBInRange(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
+MipsConstantIslands::fixupUnconditionalBr(ImmBranch &Br) {
+  MachineInstr *MI = Br.MI;
+  MachineBasicBlock *MBB = MI->getParent();
+  // Use BL to implement far jump.
+  Br.MaxDisp = ((1 << 16)-1) * 2;
+  MI->setDesc(TII->get(Mips::BimmX16));
+  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
+MipsConstantIslands::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:
+  unsigned CCReg = 0;  // FIXME
+  unsigned CC=0; //FIXME
+
+  // 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 (isBBInRange(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);
+        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());
+  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;
+}
+
 
 void MipsConstantIslands::prescanForConstants() {
-  unsigned int J;
+  unsigned J = 0;
+  (void)J;
+  PrescannedForConstants = true;
   for (MachineFunction::iterator B =
          MF->begin(), E = MF->end(); B != E; ++B) {
     for (MachineBasicBlock::instr_iterator I =
@@ -238,10 +1595,11 @@ void MipsConstantIslands::prescanForConstants() {
             unsigned index = MCP->getConstantPoolIndex(C, 4);
             I->getOperand(2).ChangeToImmediate(index);
             DEBUG(dbgs() << "constant island constant " << *I << "\n");
-            I->setDesc(TII->get(Mips::LwRxPcTcpX16));
+            I->setDesc(TII->get(Mips::LwRxPcTcp16));
             I->RemoveOperand(1);
             I->RemoveOperand(1);
             I->addOperand(MachineOperand::CreateCPI(index, 0));
+            I->addOperand(MachineOperand::CreateImm(4));
           }
           break;
         }
@@ -251,3 +1609,4 @@ void MipsConstantIslands::prescanForConstants() {
     }
   }
 }
+
diff --git a/test/CodeGen/Mips/const4.ll b/test/CodeGen/Mips/const4.ll
new file mode 100644
index 0000000..04da731
--- /dev/null
+++ b/test/CodeGen/Mips/const4.ll
@@ -0,0 +1,64 @@
+; RUN: llc -mtriple=mipsel-linux-gnu -march=mipsel -mcpu=mips16 -soft-float -mips16-hard-float -relocation-model=static -mips16-constant-islands -mips-constant-islands-small-offset=20  < %s | FileCheck %s -check-prefix=offset20
+
+; RUN: llc -mtriple=mipsel-linux-gnu -march=mipsel -mcpu=mips16 -soft-float -mips16-hard-float -relocation-model=static -mips16-constant-islands -mips-constant-islands-small-offset=40  < %s | FileCheck %s -check-prefix=offset40
+
+
+@i = common global i32 0, align 4
+@b = common global i32 0, align 4
+
+; Function Attrs: nounwind
+define void @t() #0 {
+entry:
+  store i32 -559023410, i32* @i, align 4
+  %0 = load i32* @b, align 4
+  %tobool = icmp ne i32 %0, 0
+  br i1 %tobool, label %if.then, label %if.else
+; offset20: lw	${{[0-9]+}}, $CPI0_1	# 16 bit inst
+; offset20:	b	$BB0_2
+; offset20:	.align	2
+; offset20: $CPI0_0:
+; offset20:	.4byte	3735943886
+; offset20: $BB0_2: 
+
+; offset40:	beqz	${{[0-9]+}}, $BB0_3
+; offset40:	jal	foo
+; offset40:	nop
+; offset40:	b	$BB0_4
+; offset40:	.align	2
+; offset40: $CPI0_0:
+; offset40:	.4byte	3735943886
+; offset40: $BB0_3:
+; offset40:	jal	goo
+
+if.then:                                          ; preds = %entry
+  call void bitcast (void (...)* @foo to void ()*)()
+  br label %if.end
+
+if.else:                                          ; preds = %entry
+  call void bitcast (void (...)* @goo to void ()*)()
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  call void bitcast (void (...)* @hoo to void ()*)()
+  call void bitcast (void (...)* @hoo to void ()*)()
+  call void bitcast (void (...)* @hoo to void ()*)()
+  call void bitcast (void (...)* @hoo to void ()*)()
+  call void bitcast (void (...)* @hoo to void ()*)()
+  call void bitcast (void (...)* @hoo to void ()*)()
+  call void bitcast (void (...)* @hoo to void ()*)()
+  call void bitcast (void (...)* @hoo to void ()*)()
+  ret void
+}
+
+declare void @foo(...) #1
+
+declare void @goo(...) #1
+
+declare void @hoo(...) #1
+
+attributes #0 = { nounwind "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="true" }
+attributes #1 = { "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="true" }
+
+!llvm.ident = !{!0}
+
+!0 = metadata !{metadata !"clang version 3.4 (gitosis@dmz-portal.mips.com:clang.git 3a50d847e098f36e3bf8bc14eea07a6cc35f7803) (gitosis@dmz-portal.mips.com:llvm.git f52db0b69f0c888bdc98bb2f13aaecc1e83288a9)"}