X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTarget%2FMips%2FMipsDelaySlotFiller.cpp;h=34286db16af664ec0d8bf680e950ad327922ce4f;hb=6ff59a16a05d43fdda587ce600b5b42a63cf3d33;hp=a2b615d8add2aa929154020778fa38a40b639590;hpb=948175785d1e5e7d3d627d11dbb0392b976d135d;p=oota-llvm.git

diff --git a/lib/Target/Mips/MipsDelaySlotFiller.cpp b/lib/Target/Mips/MipsDelaySlotFiller.cpp
index a2b615d8add..34286db16af 100644
--- a/lib/Target/Mips/MipsDelaySlotFiller.cpp
+++ b/lib/Target/Mips/MipsDelaySlotFiller.cpp
@@ -1,4 +1,4 @@
-//===-- DelaySlotFiller.cpp - Mips delay slot filler ---------------------===//
+//===-- MipsDelaySlotFiller.cpp - Mips Delay Slot Filler ------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -7,38 +7,182 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// Simple pass to fills delay slots with NOPs.
+// Simple pass to fill delay slots with useful instructions.
 //
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "delay-slot-filler"
 
 #include "Mips.h"
+#include "MipsInstrInfo.h"
 #include "MipsTargetMachine.h"
+#include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/PseudoSourceValue.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Target/TargetInstrInfo.h"
-#include "llvm/ADT/Statistic.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/Target/TargetRegisterInfo.h"
 
 using namespace llvm;
 
 STATISTIC(FilledSlots, "Number of delay slots filled");
+STATISTIC(UsefulSlots, "Number of delay slots filled with instructions that"
+                       " are not NOP.");
+
+static cl::opt<bool> DisableDelaySlotFiller(
+  "disable-mips-delay-filler",
+  cl::init(false),
+  cl::desc("Fill all delay slots with NOPs."),
+  cl::Hidden);
+
+static cl::opt<bool> DisableForwardSearch(
+  "disable-mips-df-forward-search",
+  cl::init(true),
+  cl::desc("Disallow MIPS delay filler to search forward."),
+  cl::Hidden);
+
+static cl::opt<bool> DisableSuccBBSearch(
+  "disable-mips-df-succbb-search",
+  cl::init(true),
+  cl::desc("Disallow MIPS delay filler to search successor basic blocks."),
+  cl::Hidden);
+
+static cl::opt<bool> DisableBackwardSearch(
+  "disable-mips-df-backward-search",
+  cl::init(false),
+  cl::desc("Disallow MIPS delay filler to search backward."),
+  cl::Hidden);
 
 namespace {
-  struct Filler : public MachineFunctionPass {
+  typedef MachineBasicBlock::iterator Iter;
+  typedef MachineBasicBlock::reverse_iterator ReverseIter;
+  typedef SmallDenseMap<MachineBasicBlock*, MachineInstr*, 2> BB2BrMap;
 
-    TargetMachine &TM;
-    const TargetInstrInfo *TII;
+  /// \brief A functor comparing edge weight of two blocks.
+  struct CmpWeight {
+    CmpWeight(const MachineBasicBlock &S,
+              const MachineBranchProbabilityInfo &P) : Src(S), Prob(P) {}
 
-    static char ID;
-    Filler(TargetMachine &tm) 
-      : MachineFunctionPass(&ID), TM(tm), TII(tm.getInstrInfo()) { }
+    bool operator()(const MachineBasicBlock *Dst0,
+                    const MachineBasicBlock *Dst1) const {
+      return Prob.getEdgeWeight(&Src, Dst0) < Prob.getEdgeWeight(&Src, Dst1);
+    }
+
+    const MachineBasicBlock &Src;
+    const MachineBranchProbabilityInfo &Prob;
+  };
+
+  class RegDefsUses {
+  public:
+    RegDefsUses(TargetMachine &TM);
+    void init(const MachineInstr &MI);
+
+    /// This function sets all caller-saved registers in Defs.
+    void setCallerSaved(const MachineInstr &MI);
+
+    /// This function sets all unallocatable registers in Defs.
+    void setUnallocatableRegs(const MachineFunction &MF);
+
+    /// Set bits in Uses corresponding to MBB's live-out registers except for
+    /// the registers that are live-in to SuccBB.
+    void addLiveOut(const MachineBasicBlock &MBB,
+                    const MachineBasicBlock &SuccBB);
+
+    bool update(const MachineInstr &MI, unsigned Begin, unsigned End);
+
+  private:
+    bool checkRegDefsUses(BitVector &NewDefs, BitVector &NewUses, unsigned Reg,
+                          bool IsDef) const;
+
+    /// Returns true if Reg or its alias is in RegSet.
+    bool isRegInSet(const BitVector &RegSet, unsigned Reg) const;
+
+    const TargetRegisterInfo &TRI;
+    BitVector Defs, Uses;
+  };
+
+  /// Base class for inspecting loads and stores.
+  class InspectMemInstr {
+  public:
+    InspectMemInstr(bool ForbidMemInstr_)
+      : OrigSeenLoad(false), OrigSeenStore(false), SeenLoad(false),
+        SeenStore(false), ForbidMemInstr(ForbidMemInstr_) {}
+
+    /// Return true if MI cannot be moved to delay slot.
+    bool hasHazard(const MachineInstr &MI);
+
+    virtual ~InspectMemInstr() {}
+
+  protected:
+    /// Flags indicating whether loads or stores have been seen.
+    bool OrigSeenLoad, OrigSeenStore, SeenLoad, SeenStore;
+
+    /// Memory instructions are not allowed to move to delay slot if this flag
+    /// is true.
+    bool ForbidMemInstr;
+
+  private:
+    virtual bool hasHazard_(const MachineInstr &MI) = 0;
+  };
+
+  /// This subclass rejects any memory instructions.
+  class NoMemInstr : public InspectMemInstr {
+  public:
+    NoMemInstr() : InspectMemInstr(true) {}
+  private:
+    virtual bool hasHazard_(const MachineInstr &MI) { return true; }
+  };
+
+  /// This subclass accepts loads from stacks and constant loads.
+  class LoadFromStackOrConst : public InspectMemInstr {
+  public:
+    LoadFromStackOrConst() : InspectMemInstr(false) {}
+  private:
+    virtual bool hasHazard_(const MachineInstr &MI);
+  };
+
+  /// This subclass uses memory dependence information to determine whether a
+  /// memory instruction can be moved to a delay slot.
+  class MemDefsUses : public InspectMemInstr {
+  public:
+    MemDefsUses(const MachineFrameInfo *MFI);
+
+  private:
+    virtual bool hasHazard_(const MachineInstr &MI);
+
+    /// Update Defs and Uses. Return true if there exist dependences that
+    /// disqualify the delay slot candidate between V and values in Uses and
+    /// Defs.
+    bool updateDefsUses(const Value *V, bool MayStore);
+
+    /// Get the list of underlying objects of MI's memory operand.
+    bool getUnderlyingObjects(const MachineInstr &MI,
+                              SmallVectorImpl<const Value *> &Objects) const;
+
+    const MachineFrameInfo *MFI;
+    SmallPtrSet<const Value*, 4> Uses, Defs;
+
+    /// Flags indicating whether loads or stores with no underlying objects have
+    /// been seen.
+    bool SeenNoObjLoad, SeenNoObjStore;
+  };
+
+  class Filler : public MachineFunctionPass {
+  public:
+    Filler(TargetMachine &tm)
+      : MachineFunctionPass(ID), TM(tm) { }
 
     virtual const char *getPassName() const {
       return "Mips Delay Slot Filler";
     }
 
-    bool runOnMachineBasicBlock(MachineBasicBlock &MBB);
     bool runOnMachineFunction(MachineFunction &F) {
       bool Changed = false;
       for (MachineFunction::iterator FI = F.begin(), FE = F.end();
@@ -47,25 +191,333 @@ namespace {
       return Changed;
     }
 
+    void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.addRequired<MachineBranchProbabilityInfo>();
+      MachineFunctionPass::getAnalysisUsage(AU);
+    }
+
+  private:
+    bool runOnMachineBasicBlock(MachineBasicBlock &MBB);
+
+    /// This function checks if it is valid to move Candidate to the delay slot
+    /// and returns true if it isn't. It also updates memory and register
+    /// dependence information.
+    bool delayHasHazard(const MachineInstr &Candidate, RegDefsUses &RegDU,
+                        InspectMemInstr &IM) const;
+
+    /// This function searches range [Begin, End) for an instruction that can be
+    /// moved to the delay slot. Returns true on success.
+    template<typename IterTy>
+    bool searchRange(MachineBasicBlock &MBB, IterTy Begin, IterTy End,
+                     RegDefsUses &RegDU, InspectMemInstr &IM,
+                     IterTy &Filler) const;
+
+    /// This function searches in the backward direction for an instruction that
+    /// can be moved to the delay slot. Returns true on success.
+    bool searchBackward(MachineBasicBlock &MBB, Iter Slot) const;
+
+    /// This function searches MBB in the forward direction for an instruction
+    /// that can be moved to the delay slot. Returns true on success.
+    bool searchForward(MachineBasicBlock &MBB, Iter Slot) const;
+
+    /// This function searches one of MBB's successor blocks for an instruction
+    /// that can be moved to the delay slot and inserts clones of the
+    /// instruction into the successor's predecessor blocks.
+    bool searchSuccBBs(MachineBasicBlock &MBB, Iter Slot) const;
+
+    /// Pick a successor block of MBB. Return NULL if MBB doesn't have a
+    /// successor block that is not a landing pad.
+    MachineBasicBlock *selectSuccBB(MachineBasicBlock &B) const;
+
+    /// This function analyzes MBB and returns an instruction with an unoccupied
+    /// slot that branches to Dst.
+    std::pair<MipsInstrInfo::BranchType, MachineInstr *>
+    getBranch(MachineBasicBlock &MBB, const MachineBasicBlock &Dst) const;
+
+    /// Examine Pred and see if it is possible to insert an instruction into
+    /// one of its branches delay slot or its end.
+    bool examinePred(MachineBasicBlock &Pred, const MachineBasicBlock &Succ,
+                     RegDefsUses &RegDU, bool &HasMultipleSuccs,
+                     BB2BrMap &BrMap) const;
+
+    bool terminateSearch(const MachineInstr &Candidate) const;
+
+    TargetMachine &TM;
+
+    static char ID;
   };
   char Filler::ID = 0;
 } // end of anonymous namespace
 
+static bool hasUnoccupiedSlot(const MachineInstr *MI) {
+  return MI->hasDelaySlot() && !MI->isBundledWithSucc();
+}
+
+/// This function inserts clones of Filler into predecessor blocks.
+static void insertDelayFiller(Iter Filler, const BB2BrMap &BrMap) {
+  MachineFunction *MF = Filler->getParent()->getParent();
+
+  for (BB2BrMap::const_iterator I = BrMap.begin(); I != BrMap.end(); ++I) {
+    if (I->second) {
+      MIBundleBuilder(I->second).append(MF->CloneMachineInstr(&*Filler));
+      ++UsefulSlots;
+    } else {
+      I->first->insert(I->first->end(), MF->CloneMachineInstr(&*Filler));
+    }
+  }
+}
+
+/// This function adds registers Filler defines to MBB's live-in register list.
+static void addLiveInRegs(Iter Filler, MachineBasicBlock &MBB) {
+  for (unsigned I = 0, E = Filler->getNumOperands(); I != E; ++I) {
+    const MachineOperand &MO = Filler->getOperand(I);
+    unsigned R;
+
+    if (!MO.isReg() || !MO.isDef() || !(R = MO.getReg()))
+      continue;
+
+#ifndef NDEBUG
+    const MachineFunction &MF = *MBB.getParent();
+    assert(MF.getTarget().getRegisterInfo()->getAllocatableSet(MF).test(R) &&
+           "Shouldn't move an instruction with unallocatable registers across "
+           "basic block boundaries.");
+#endif
+
+    if (!MBB.isLiveIn(R))
+      MBB.addLiveIn(R);
+  }
+}
+
+RegDefsUses::RegDefsUses(TargetMachine &TM)
+  : TRI(*TM.getRegisterInfo()), Defs(TRI.getNumRegs(), false),
+    Uses(TRI.getNumRegs(), false) {}
+
+void RegDefsUses::init(const MachineInstr &MI) {
+  // Add all register operands which are explicit and non-variadic.
+  update(MI, 0, MI.getDesc().getNumOperands());
+
+  // If MI is a call, add RA to Defs to prevent users of RA from going into
+  // delay slot.
+  if (MI.isCall())
+    Defs.set(Mips::RA);
+
+  // Add all implicit register operands of branch instructions except
+  // register AT.
+  if (MI.isBranch()) {
+    update(MI, MI.getDesc().getNumOperands(), MI.getNumOperands());
+    Defs.reset(Mips::AT);
+  }
+}
+
+void RegDefsUses::setCallerSaved(const MachineInstr &MI) {
+  assert(MI.isCall());
+
+  // If MI is a call, add all caller-saved registers to Defs.
+  BitVector CallerSavedRegs(TRI.getNumRegs(), true);
+
+  CallerSavedRegs.reset(Mips::ZERO);
+  CallerSavedRegs.reset(Mips::ZERO_64);
+
+  for (const MCPhysReg *R = TRI.getCalleeSavedRegs(); *R; ++R)
+    for (MCRegAliasIterator AI(*R, &TRI, true); AI.isValid(); ++AI)
+      CallerSavedRegs.reset(*AI);
+
+  Defs |= CallerSavedRegs;
+}
+
+void RegDefsUses::setUnallocatableRegs(const MachineFunction &MF) {
+  BitVector AllocSet = TRI.getAllocatableSet(MF);
+
+  for (int R = AllocSet.find_first(); R != -1; R = AllocSet.find_next(R))
+    for (MCRegAliasIterator AI(R, &TRI, false); AI.isValid(); ++AI)
+      AllocSet.set(*AI);
+
+  AllocSet.set(Mips::ZERO);
+  AllocSet.set(Mips::ZERO_64);
+
+  Defs |= AllocSet.flip();
+}
+
+void RegDefsUses::addLiveOut(const MachineBasicBlock &MBB,
+                             const MachineBasicBlock &SuccBB) {
+  for (MachineBasicBlock::const_succ_iterator SI = MBB.succ_begin(),
+       SE = MBB.succ_end(); SI != SE; ++SI)
+    if (*SI != &SuccBB)
+      for (MachineBasicBlock::livein_iterator LI = (*SI)->livein_begin(),
+           LE = (*SI)->livein_end(); LI != LE; ++LI)
+        Uses.set(*LI);
+}
+
+bool RegDefsUses::update(const MachineInstr &MI, unsigned Begin, unsigned End) {
+  BitVector NewDefs(TRI.getNumRegs()), NewUses(TRI.getNumRegs());
+  bool HasHazard = false;
+
+  for (unsigned I = Begin; I != End; ++I) {
+    const MachineOperand &MO = MI.getOperand(I);
+
+    if (MO.isReg() && MO.getReg())
+      HasHazard |= checkRegDefsUses(NewDefs, NewUses, MO.getReg(), MO.isDef());
+  }
+
+  Defs |= NewDefs;
+  Uses |= NewUses;
+
+  return HasHazard;
+}
+
+bool RegDefsUses::checkRegDefsUses(BitVector &NewDefs, BitVector &NewUses,
+                                   unsigned Reg, bool IsDef) const {
+  if (IsDef) {
+    NewDefs.set(Reg);
+    // check whether Reg has already been defined or used.
+    return (isRegInSet(Defs, Reg) || isRegInSet(Uses, Reg));
+  }
+
+  NewUses.set(Reg);
+  // check whether Reg has already been defined.
+  return isRegInSet(Defs, Reg);
+}
+
+bool RegDefsUses::isRegInSet(const BitVector &RegSet, unsigned Reg) const {
+  // Check Reg and all aliased Registers.
+  for (MCRegAliasIterator AI(Reg, &TRI, true); AI.isValid(); ++AI)
+    if (RegSet.test(*AI))
+      return true;
+  return false;
+}
+
+bool InspectMemInstr::hasHazard(const MachineInstr &MI) {
+  if (!MI.mayStore() && !MI.mayLoad())
+    return false;
+
+  if (ForbidMemInstr)
+    return true;
+
+  OrigSeenLoad = SeenLoad;
+  OrigSeenStore = SeenStore;
+  SeenLoad |= MI.mayLoad();
+  SeenStore |= MI.mayStore();
+
+  // If MI is an ordered or volatile memory reference, disallow moving
+  // subsequent loads and stores to delay slot.
+  if (MI.hasOrderedMemoryRef() && (OrigSeenLoad || OrigSeenStore)) {
+    ForbidMemInstr = true;
+    return true;
+  }
+
+  return hasHazard_(MI);
+}
+
+bool LoadFromStackOrConst::hasHazard_(const MachineInstr &MI) {
+  if (MI.mayStore())
+    return true;
+
+  if (!MI.hasOneMemOperand() || !(*MI.memoperands_begin())->getValue())
+    return true;
+
+  const Value *V = (*MI.memoperands_begin())->getValue();
+
+  if (isa<FixedStackPseudoSourceValue>(V))
+    return false;
+
+  if (const PseudoSourceValue *PSV = dyn_cast<const PseudoSourceValue>(V))
+    return !PSV->isConstant(0) && V != PseudoSourceValue::getStack();
+
+  return true;
+}
+
+MemDefsUses::MemDefsUses(const MachineFrameInfo *MFI_)
+  : InspectMemInstr(false), MFI(MFI_), SeenNoObjLoad(false),
+    SeenNoObjStore(false) {}
+
+bool MemDefsUses::hasHazard_(const MachineInstr &MI) {
+  bool HasHazard = false;
+  SmallVector<const Value *, 4> Objs;
+
+  // Check underlying object list.
+  if (getUnderlyingObjects(MI, Objs)) {
+    for (SmallVectorImpl<const Value *>::const_iterator I = Objs.begin();
+         I != Objs.end(); ++I)
+      HasHazard |= updateDefsUses(*I, MI.mayStore());
+
+    return HasHazard;
+  }
+
+  // No underlying objects found.
+  HasHazard = MI.mayStore() && (OrigSeenLoad || OrigSeenStore);
+  HasHazard |= MI.mayLoad() || OrigSeenStore;
+
+  SeenNoObjLoad |= MI.mayLoad();
+  SeenNoObjStore |= MI.mayStore();
+
+  return HasHazard;
+}
+
+bool MemDefsUses::updateDefsUses(const Value *V, bool MayStore) {
+  if (MayStore)
+    return !Defs.insert(V) || Uses.count(V) || SeenNoObjStore || SeenNoObjLoad;
+
+  Uses.insert(V);
+  return Defs.count(V) || SeenNoObjStore;
+}
+
+bool MemDefsUses::
+getUnderlyingObjects(const MachineInstr &MI,
+                     SmallVectorImpl<const Value *> &Objects) const {
+  if (!MI.hasOneMemOperand() || !(*MI.memoperands_begin())->getValue())
+    return false;
+
+  const Value *V = (*MI.memoperands_begin())->getValue();
+
+  SmallVector<Value *, 4> Objs;
+  GetUnderlyingObjects(const_cast<Value *>(V), Objs);
+
+  for (SmallVectorImpl<Value *>::iterator I = Objs.begin(), E = Objs.end();
+       I != E; ++I) {
+    if (const PseudoSourceValue *PSV = dyn_cast<PseudoSourceValue>(*I)) {
+      if (PSV->isAliased(MFI))
+        return false;
+    } else if (!isIdentifiedObject(V))
+      return false;
+
+    Objects.push_back(*I);
+  }
+
+  return true;
+}
+
 /// runOnMachineBasicBlock - Fill in delay slots for the given basic block.
-/// Currently, we fill delay slots with NOPs. We assume there is only one
-/// delay slot per delayed instruction.
-bool Filler::
-runOnMachineBasicBlock(MachineBasicBlock &MBB) 
-{
+/// We assume there is only one delay slot per delayed instruction.
+bool Filler::runOnMachineBasicBlock(MachineBasicBlock &MBB) {
   bool Changed = false;
-  for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ++I)
-    if (I->getDesc().hasDelaySlot()) {
-      MachineBasicBlock::iterator J = I;
-      ++J;
-      BuildMI(MBB, J, I->getDebugLoc(), TII->get(Mips::NOP));
-      ++FilledSlots;
-      Changed = true;
+
+  for (Iter I = MBB.begin(); I != MBB.end(); ++I) {
+    if (!hasUnoccupiedSlot(&*I))
+      continue;
+
+    ++FilledSlots;
+    Changed = true;
+
+    // Delay slot filling is disabled at -O0.
+    if (!DisableDelaySlotFiller && (TM.getOptLevel() != CodeGenOpt::None)) {
+      if (searchBackward(MBB, I))
+        continue;
+
+      if (I->isTerminator()) {
+        if (searchSuccBBs(MBB, I))
+          continue;
+      } else if (searchForward(MBB, I)) {
+        continue;
+      }
     }
+
+    // Bundle the NOP to the instruction with the delay slot.
+    const MipsInstrInfo *TII =
+      static_cast<const MipsInstrInfo*>(TM.getInstrInfo());
+    BuildMI(MBB, llvm::next(I), I->getDebugLoc(), TII->get(Mips::NOP));
+    MIBundleBuilder(MBB, I, llvm::next(llvm::next(I)));
+  }
+
   return Changed;
 }
 
@@ -75,3 +527,195 @@ FunctionPass *llvm::createMipsDelaySlotFillerPass(MipsTargetMachine &tm) {
   return new Filler(tm);
 }
 
+template<typename IterTy>
+bool Filler::searchRange(MachineBasicBlock &MBB, IterTy Begin, IterTy End,
+                         RegDefsUses &RegDU, InspectMemInstr& IM,
+                         IterTy &Filler) const {
+  for (IterTy I = Begin; I != End; ++I) {
+    // skip debug value
+    if (I->isDebugValue())
+      continue;
+
+    if (terminateSearch(*I))
+      break;
+
+    assert((!I->isCall() && !I->isReturn() && !I->isBranch()) &&
+           "Cannot put calls, returns or branches in delay slot.");
+
+    if (delayHasHazard(*I, RegDU, IM))
+      continue;
+
+    Filler = I;
+    return true;
+  }
+
+  return false;
+}
+
+bool Filler::searchBackward(MachineBasicBlock &MBB, Iter Slot) const {
+  if (DisableBackwardSearch)
+    return false;
+
+  RegDefsUses RegDU(TM);
+  MemDefsUses MemDU(MBB.getParent()->getFrameInfo());
+  ReverseIter Filler;
+
+  RegDU.init(*Slot);
+
+  if (searchRange(MBB, ReverseIter(Slot), MBB.rend(), RegDU, MemDU, Filler)) {
+    MBB.splice(llvm::next(Slot), &MBB, llvm::next(Filler).base());
+    MIBundleBuilder(MBB, Slot, llvm::next(llvm::next(Slot)));
+    ++UsefulSlots;
+    return true;
+  }
+
+  return false;
+}
+
+bool Filler::searchForward(MachineBasicBlock &MBB, Iter Slot) const {
+  // Can handle only calls.
+  if (DisableForwardSearch || !Slot->isCall())
+    return false;
+
+  RegDefsUses RegDU(TM);
+  NoMemInstr NM;
+  Iter Filler;
+
+  RegDU.setCallerSaved(*Slot);
+
+  if (searchRange(MBB, llvm::next(Slot), MBB.end(), RegDU, NM, Filler)) {
+    MBB.splice(llvm::next(Slot), &MBB, Filler);
+    MIBundleBuilder(MBB, Slot, llvm::next(llvm::next(Slot)));
+    ++UsefulSlots;
+    return true;
+  }
+
+  return false;
+}
+
+bool Filler::searchSuccBBs(MachineBasicBlock &MBB, Iter Slot) const {
+  if (DisableSuccBBSearch)
+    return false;
+
+  MachineBasicBlock *SuccBB = selectSuccBB(MBB);
+
+  if (!SuccBB)
+    return false;
+
+  RegDefsUses RegDU(TM);
+  bool HasMultipleSuccs = false;
+  BB2BrMap BrMap;
+  OwningPtr<InspectMemInstr> IM;
+  Iter Filler;
+
+  // Iterate over SuccBB's predecessor list.
+  for (MachineBasicBlock::pred_iterator PI = SuccBB->pred_begin(),
+       PE = SuccBB->pred_end(); PI != PE; ++PI)
+    if (!examinePred(**PI, *SuccBB, RegDU, HasMultipleSuccs, BrMap))
+      return false;
+
+  // Do not allow moving instructions which have unallocatable register operands
+  // across basic block boundaries.
+  RegDU.setUnallocatableRegs(*MBB.getParent());
+
+  // Only allow moving loads from stack or constants if any of the SuccBB's
+  // predecessors have multiple successors.
+  if (HasMultipleSuccs) {
+    IM.reset(new LoadFromStackOrConst());
+  } else {
+    const MachineFrameInfo *MFI = MBB.getParent()->getFrameInfo();
+    IM.reset(new MemDefsUses(MFI));
+  }
+
+  if (!searchRange(MBB, SuccBB->begin(), SuccBB->end(), RegDU, *IM, Filler))
+    return false;
+
+  insertDelayFiller(Filler, BrMap);
+  addLiveInRegs(Filler, *SuccBB);
+  Filler->eraseFromParent();
+
+  return true;
+}
+
+MachineBasicBlock *Filler::selectSuccBB(MachineBasicBlock &B) const {
+  if (B.succ_empty())
+    return NULL;
+
+  // Select the successor with the larget edge weight.
+  CmpWeight Cmp(B, getAnalysis<MachineBranchProbabilityInfo>());
+  MachineBasicBlock *S = *std::max_element(B.succ_begin(), B.succ_end(), Cmp);
+  return S->isLandingPad() ? NULL : S;
+}
+
+std::pair<MipsInstrInfo::BranchType, MachineInstr *>
+Filler::getBranch(MachineBasicBlock &MBB, const MachineBasicBlock &Dst) const {
+  const MipsInstrInfo *TII =
+    static_cast<const MipsInstrInfo*>(TM.getInstrInfo());
+  MachineBasicBlock *TrueBB = 0, *FalseBB = 0;
+  SmallVector<MachineInstr*, 2> BranchInstrs;
+  SmallVector<MachineOperand, 2> Cond;
+
+  MipsInstrInfo::BranchType R =
+    TII->AnalyzeBranch(MBB, TrueBB, FalseBB, Cond, false, BranchInstrs);
+
+  if ((R == MipsInstrInfo::BT_None) || (R == MipsInstrInfo::BT_NoBranch))
+    return std::make_pair(R, (MachineInstr*)NULL);
+
+  if (R != MipsInstrInfo::BT_CondUncond) {
+    if (!hasUnoccupiedSlot(BranchInstrs[0]))
+      return std::make_pair(MipsInstrInfo::BT_None, (MachineInstr*)NULL);
+
+    assert(((R != MipsInstrInfo::BT_Uncond) || (TrueBB == &Dst)));
+
+    return std::make_pair(R, BranchInstrs[0]);
+  }
+
+  assert((TrueBB == &Dst) || (FalseBB == &Dst));
+
+  // Examine the conditional branch. See if its slot is occupied.
+  if (hasUnoccupiedSlot(BranchInstrs[0]))
+    return std::make_pair(MipsInstrInfo::BT_Cond, BranchInstrs[0]);
+
+  // If that fails, try the unconditional branch.
+  if (hasUnoccupiedSlot(BranchInstrs[1]) && (FalseBB == &Dst))
+    return std::make_pair(MipsInstrInfo::BT_Uncond, BranchInstrs[1]);
+
+  return std::make_pair(MipsInstrInfo::BT_None, (MachineInstr*)NULL);
+}
+
+bool Filler::examinePred(MachineBasicBlock &Pred, const MachineBasicBlock &Succ,
+                         RegDefsUses &RegDU, bool &HasMultipleSuccs,
+                         BB2BrMap &BrMap) const {
+  std::pair<MipsInstrInfo::BranchType, MachineInstr *> P =
+    getBranch(Pred, Succ);
+
+  // Return if either getBranch wasn't able to analyze the branches or there
+  // were no branches with unoccupied slots.
+  if (P.first == MipsInstrInfo::BT_None)
+    return false;
+
+  if ((P.first != MipsInstrInfo::BT_Uncond) &&
+      (P.first != MipsInstrInfo::BT_NoBranch)) {
+    HasMultipleSuccs = true;
+    RegDU.addLiveOut(Pred, Succ);
+  }
+
+  BrMap[&Pred] = P.second;
+  return true;
+}
+
+bool Filler::delayHasHazard(const MachineInstr &Candidate, RegDefsUses &RegDU,
+                            InspectMemInstr &IM) const {
+  bool HasHazard = (Candidate.isImplicitDef() || Candidate.isKill());
+
+  HasHazard |= IM.hasHazard(Candidate);
+  HasHazard |= RegDU.update(Candidate, 0, Candidate.getNumOperands());
+
+  return HasHazard;
+}
+
+bool Filler::terminateSearch(const MachineInstr &Candidate) const {
+  return (Candidate.isTerminator() || Candidate.isCall() ||
+          Candidate.isLabel() || Candidate.isInlineAsm() ||
+          Candidate.hasUnmodeledSideEffects());
+}