Remove the now unused strong phi elimination pass.

llvm-svn: 192604

llvm/include/llvm/CodeGen/Passes.h

@@ -381,14 +381,6 @@ namespace llvm {
   /// these register allocator like this: AU.addRequiredID(PHIEliminationID);
   extern char &PHIEliminationID;
 
-  /// StrongPHIElimination - This pass eliminates machine instruction PHI
-  /// nodes by inserting copy instructions.  This destroys SSA information, but
-  /// is the desired input for some register allocators.  This pass is
-  /// "required" by these register allocator like this:
-  ///    AU.addRequiredID(PHIEliminationID);
-  ///  This pass is still in development
-  extern char &StrongPHIEliminationID;
-
   /// LiveIntervals - This analysis keeps track of the live ranges of virtual
   /// and physical registers.
   extern char &LiveIntervalsID;

llvm/include/llvm/InitializePasses.h

@@ -242,7 +242,6 @@ void initializeStripDeadPrototypesPassPass(PassRegistry&);
 void initializeStripDebugDeclarePass(PassRegistry&);
 void initializeStripNonDebugSymbolsPass(PassRegistry&);
 void initializeStripSymbolsPass(PassRegistry&);
-void initializeStrongPHIEliminationPass(PassRegistry&);
 void initializeTailCallElimPass(PassRegistry&);
 void initializeTailDuplicatePassPass(PassRegistry&);
 void initializeTargetPassConfigPass(PassRegistry&);

llvm/lib/CodeGen/CMakeLists.txt

@@ -97,7 +97,6 @@ add_llvm_library(LLVMCodeGen
   StackColoring.cpp
   StackProtector.cpp
   StackSlotColoring.cpp
-  StrongPHIElimination.cpp
   TailDuplication.cpp
   TargetFrameLoweringImpl.cpp
   TargetInstrInfo.cpp

llvm/lib/CodeGen/CodeGen.cpp

@@ -60,7 +60,6 @@ void llvm::initializeCodeGen(PassRegistry &Registry) {
   initializeStackProtectorPass(Registry);
   initializeStackColoringPass(Registry);
   initializeStackSlotColoringPass(Registry);
-  initializeStrongPHIEliminationPass(Registry);
   initializeTailDuplicatePassPass(Registry);
   initializeTargetPassConfigPass(Registry);
   initializeTwoAddressInstructionPassPass(Registry);

llvm/lib/CodeGen/Passes.cpp

@@ -58,8 +58,6 @@ OptimizeRegAlloc("optimize-regalloc", cl::Hidden,
 static cl::opt<cl::boolOrDefault>
 EnableMachineSched("enable-misched", cl::Hidden,
     cl::desc("Enable the machine instruction scheduling pass."));
-static cl::opt<bool> EnableStrongPHIElim("strong-phi-elim", cl::Hidden,
-    cl::desc("Use strong PHI elimination."));
 static cl::opt<bool> DisablePostRAMachineLICM("disable-postra-machine-licm",
     cl::Hidden,
     cl::desc("Disable Machine LICM"));
@@ -675,24 +673,15 @@ void TargetPassConfig::addOptimizedRegAlloc(FunctionPass *RegAllocPass) {
   // preferably fix the scavenger to not depend on them).
   addPass(&LiveVariablesID);
 
-  // Add passes that move from transformed SSA into conventional SSA. This is a
+  // Edge splitting is smarter with machine loop info.
-  // "copy coalescing" problem.
+  addPass(&MachineLoopInfoID);
-  //
+  addPass(&PHIEliminationID);
-  if (!EnableStrongPHIElim) {
-    // Edge splitting is smarter with machine loop info.
-    addPass(&MachineLoopInfoID);
-    addPass(&PHIEliminationID);
-  }
 
   // Eventually, we want to run LiveIntervals before PHI elimination.
   if (EarlyLiveIntervals)
     addPass(&LiveIntervalsID);
 
   addPass(&TwoAddressInstructionPassID);
-
-  if (EnableStrongPHIElim)
-    addPass(&StrongPHIEliminationID);
-
   addPass(&RegisterCoalescerID);
 
   // PreRA instruction scheduling.

llvm/lib/CodeGen/StrongPHIElimination.cpp

@@ -1,825 +0,0 @@
-//===- StrongPHIElimination.cpp - Eliminate PHI nodes by inserting copies -===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===----------------------------------------------------------------------===//
-//
-// This pass eliminates PHI instructions by aggressively coalescing the copies
-// that would be inserted by a naive algorithm and only inserting the copies
-// that are necessary. The coalescing technique initially assumes that all
-// registers appearing in a PHI instruction do not interfere. It then eliminates
-// proven interferences, using dominators to only perform a linear number of
-// interference tests instead of the quadratic number of interference tests
-// that this would naively require. This is a technique derived from:
-// 
-//    Budimlic, et al. Fast copy coalescing and live-range identification.
-//    In Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language
-//    Design and Implementation (Berlin, Germany, June 17 - 19, 2002).
-//    PLDI '02. ACM, New York, NY, 25-32.
-//
-// The original implementation constructs a data structure they call a dominance
-// forest for this purpose. The dominance forest was shown to be unnecessary,
-// as it is possible to emulate the creation and traversal of a dominance forest
-// by directly using the dominator tree, rather than actually constructing the
-// dominance forest.  This technique is explained in:
-//
-//   Boissinot, et al. Revisiting Out-of-SSA Translation for Correctness, Code
-//     Quality and Efficiency,
-//   In Proceedings of the 7th annual IEEE/ACM International Symposium on Code
-//   Generation and Optimization (Seattle, Washington, March 22 - 25, 2009).
-//   CGO '09. IEEE, Washington, DC, 114-125.
-//
-// Careful implementation allows for all of the dominator forest interference
-// checks to be performed at once in a single depth-first traversal of the
-// dominator tree, which is what is implemented here.
-//
-//===----------------------------------------------------------------------===//
-
-#define DEBUG_TYPE "strongphielim"
-#include "llvm/CodeGen/Passes.h"
-#include "PHIEliminationUtils.h"
-#include "llvm/ADT/DenseSet.h"
-#include "llvm/ADT/Statistic.h"
-#include "llvm/CodeGen/LiveIntervalAnalysis.h"
-#include "llvm/CodeGen/MachineDominators.h"
-#include "llvm/CodeGen/MachineFunctionPass.h"
-#include "llvm/CodeGen/MachineInstrBuilder.h"
-#include "llvm/CodeGen/MachineRegisterInfo.h"
-#include "llvm/Support/Debug.h"
-#include "llvm/Target/TargetInstrInfo.h"
-using namespace llvm;
-
-namespace {
-  class StrongPHIElimination : public MachineFunctionPass {
-  public:
-    static char ID; // Pass identification, replacement for typeid
-    StrongPHIElimination() : MachineFunctionPass(ID) {
-      initializeStrongPHIEliminationPass(*PassRegistry::getPassRegistry());
-    }
-
-    virtual void getAnalysisUsage(AnalysisUsage&) const;
-    bool runOnMachineFunction(MachineFunction&);
-
-  private:
-    /// This struct represents a single node in the union-find data structure
-    /// representing the variable congruence classes. There is one difference
-    /// from a normal union-find data structure. We steal two bits from the parent
-    /// pointer . One of these bits is used to represent whether the register
-    /// itself has been isolated, and the other is used to represent whether the
-    /// PHI with that register as its destination has been isolated.
-    ///
-    /// Note that this leads to the strange situation where the leader of a
-    /// congruence class may no longer logically be a member, due to being
-    /// isolated.
-    struct Node {
-      enum Flags {
-        kRegisterIsolatedFlag = 1,
-        kPHIIsolatedFlag = 2
-      };
-      Node(unsigned v) : value(v), rank(0) { parent.setPointer(this); }
-
-      Node *getLeader();
-
-      PointerIntPair<Node*, 2> parent;
-      unsigned value;
-      unsigned rank;
-    };
-
-    /// Add a register in a new congruence class containing only itself.
-    void addReg(unsigned);
-
-    /// Join the congruence classes of two registers. This function is biased
-    /// towards the left argument, i.e. after
-    ///
-    /// addReg(r2);
-    /// unionRegs(r1, r2);
-    ///
-    /// the leader of the unioned congruence class is the same as the leader of
-    /// r1's congruence class prior to the union. This is actually relied upon
-    /// in the copy insertion code.
-    void unionRegs(unsigned, unsigned);
-
-    /// Get the color of a register. The color is 0 if the register has been
-    /// isolated.
-    unsigned getRegColor(unsigned);
-
-    // Isolate a register.
-    void isolateReg(unsigned);
-
-    /// Get the color of a PHI. The color of a PHI is 0 if the PHI has been
-    /// isolated. Otherwise, it is the original color of its destination and
-    /// all of its operands (before they were isolated, if they were).
-    unsigned getPHIColor(MachineInstr*);
-
-    /// Isolate a PHI.
-    void isolatePHI(MachineInstr*);
-
-    /// Traverses a basic block, splitting any interferences found between
-    /// registers in the same congruence class. It takes two DenseMaps as
-    /// arguments that it also updates: CurrentDominatingParent, which maps
-    /// a color to the register in that congruence class whose definition was
-    /// most recently seen, and ImmediateDominatingParent, which maps a register
-    /// to the register in the same congruence class that most immediately
-    /// dominates it.
-    ///
-    /// This function assumes that it is being called in a depth-first traversal
-    /// of the dominator tree.
-    void SplitInterferencesForBasicBlock(
-      MachineBasicBlock&,
-      DenseMap<unsigned, unsigned> &CurrentDominatingParent,
-      DenseMap<unsigned, unsigned> &ImmediateDominatingParent);
-
-    // Lowers a PHI instruction, inserting copies of the source and destination
-    // registers as necessary.
-    void InsertCopiesForPHI(MachineInstr*, MachineBasicBlock*);
-
-    // Merges the live interval of Reg into NewReg and renames Reg to NewReg
-    // everywhere that Reg appears. Requires Reg and NewReg to have non-
-    // overlapping lifetimes.
-    void MergeLIsAndRename(unsigned Reg, unsigned NewReg);
-
-    MachineRegisterInfo *MRI;
-    const TargetInstrInfo *TII;
-    MachineDominatorTree *DT;
-    LiveIntervals *LI;
-
-    BumpPtrAllocator Allocator;
-
-    DenseMap<unsigned, Node*> RegNodeMap;
-
-    // Maps a basic block to a list of its defs of registers that appear as PHI
-    // sources.
-    DenseMap<MachineBasicBlock*, std::vector<MachineInstr*> > PHISrcDefs;
-
-    // Maps a color to a pair of a MachineInstr* and a virtual register, which
-    // is the operand of that PHI corresponding to the current basic block.
-    DenseMap<unsigned, std::pair<MachineInstr*, unsigned> > CurrentPHIForColor;
-
-    // FIXME: Can these two data structures be combined? Would a std::multimap
-    // be any better?
-
-    // Stores pairs of predecessor basic blocks and the source registers of
-    // inserted copy instructions.
-    typedef DenseSet<std::pair<MachineBasicBlock*, unsigned> > SrcCopySet;
-    SrcCopySet InsertedSrcCopySet;
-
-    // Maps pairs of predecessor basic blocks and colors to their defining copy
-    // instructions.
-    typedef DenseMap<std::pair<MachineBasicBlock*, unsigned>, MachineInstr*>
-      SrcCopyMap;
-    SrcCopyMap InsertedSrcCopyMap;
-
-    // Maps inserted destination copy registers to their defining copy
-    // instructions.
-    typedef DenseMap<unsigned, MachineInstr*> DestCopyMap;
-    DestCopyMap InsertedDestCopies;
-  };
-
-  struct MIIndexCompare {
-    MIIndexCompare(LiveIntervals *LiveIntervals) : LI(LiveIntervals) { }
-
-    bool operator()(const MachineInstr *LHS, const MachineInstr *RHS) const {
-      return LI->getInstructionIndex(LHS) < LI->getInstructionIndex(RHS);
-    }
-
-    LiveIntervals *LI;
-  };
-} // namespace
-
-STATISTIC(NumPHIsLowered, "Number of PHIs lowered");
-STATISTIC(NumDestCopiesInserted, "Number of destination copies inserted");
-STATISTIC(NumSrcCopiesInserted, "Number of source copies inserted");
-
-char StrongPHIElimination::ID = 0;
-INITIALIZE_PASS_BEGIN(StrongPHIElimination, "strong-phi-node-elimination",
-  "Eliminate PHI nodes for register allocation, intelligently", false, false)
-INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
-INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
-INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
-INITIALIZE_PASS_END(StrongPHIElimination, "strong-phi-node-elimination",
-  "Eliminate PHI nodes for register allocation, intelligently", false, false)
-
-char &llvm::StrongPHIEliminationID = StrongPHIElimination::ID;
-
-void StrongPHIElimination::getAnalysisUsage(AnalysisUsage &AU) const {
-  AU.setPreservesCFG();
-  AU.addRequired<MachineDominatorTree>();
-  AU.addRequired<SlotIndexes>();
-  AU.addPreserved<SlotIndexes>();
-  AU.addRequired<LiveIntervals>();
-  AU.addPreserved<LiveIntervals>();
-  MachineFunctionPass::getAnalysisUsage(AU);
-}
-
-static MachineOperand *findLastUse(MachineBasicBlock *MBB, unsigned Reg) {
-  // FIXME: This only needs to check from the first terminator, as only the
-  // first terminator can use a virtual register.
-  for (MachineBasicBlock::reverse_iterator RI = MBB->rbegin(); ; ++RI) {
-    assert (RI != MBB->rend());
-    MachineInstr *MI = &*RI;
-
-    for (MachineInstr::mop_iterator OI = MI->operands_begin(),
-         OE = MI->operands_end(); OI != OE; ++OI) {
-      MachineOperand &MO = *OI;
-      if (MO.isReg() && MO.isUse() && MO.getReg() == Reg)
-        return &MO;
-    }
-  }
-}
-
-bool StrongPHIElimination::runOnMachineFunction(MachineFunction &MF) {
-  MRI = &MF.getRegInfo();
-  TII = MF.getTarget().getInstrInfo();
-  DT = &getAnalysis<MachineDominatorTree>();
-  LI = &getAnalysis<LiveIntervals>();
-
-  for (MachineFunction::iterator I = MF.begin(), E = MF.end();
-       I != E; ++I) {
-    for (MachineBasicBlock::iterator BBI = I->begin(), BBE = I->end();
-         BBI != BBE && BBI->isPHI(); ++BBI) {
-      unsigned DestReg = BBI->getOperand(0).getReg();
-      addReg(DestReg);
-      PHISrcDefs[I].push_back(BBI);
-
-      for (unsigned i = 1; i < BBI->getNumOperands(); i += 2) {
-        MachineOperand &SrcMO = BBI->getOperand(i);
-        unsigned SrcReg = SrcMO.getReg();
-        addReg(SrcReg);
-        unionRegs(DestReg, SrcReg);
-
-        MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
-        if (DefMI)
-          PHISrcDefs[DefMI->getParent()].push_back(DefMI);
-      }
-    }
-  }
-
-  // Perform a depth-first traversal of the dominator tree, splitting
-  // interferences amongst PHI-congruence classes.
-  DenseMap<unsigned, unsigned> CurrentDominatingParent;
-  DenseMap<unsigned, unsigned> ImmediateDominatingParent;
-  for (df_iterator<MachineDomTreeNode*> DI = df_begin(DT->getRootNode()),
-       DE = df_end(DT->getRootNode()); DI != DE; ++DI) {
-    SplitInterferencesForBasicBlock(*DI->getBlock(),
-                                    CurrentDominatingParent,
-                                    ImmediateDominatingParent);
-  }
-
-  // Insert copies for all PHI source and destination registers.
-  for (MachineFunction::iterator I = MF.begin(), E = MF.end();
-       I != E; ++I) {
-    for (MachineBasicBlock::iterator BBI = I->begin(), BBE = I->end();
-         BBI != BBE && BBI->isPHI(); ++BBI) {
-      InsertCopiesForPHI(BBI, I);
-    }
-  }
-
-  // FIXME: Preserve the equivalence classes during copy insertion and use
-  // the preversed equivalence classes instead of recomputing them.
-  RegNodeMap.clear();
-  for (MachineFunction::iterator I = MF.begin(), E = MF.end();
-       I != E; ++I) {
-    for (MachineBasicBlock::iterator BBI = I->begin(), BBE = I->end();
-         BBI != BBE && BBI->isPHI(); ++BBI) {
-      unsigned DestReg = BBI->getOperand(0).getReg();
-      addReg(DestReg);
-
-      for (unsigned i = 1; i < BBI->getNumOperands(); i += 2) {
-        unsigned SrcReg = BBI->getOperand(i).getReg();
-        addReg(SrcReg);
-        unionRegs(DestReg, SrcReg);
-      }
-    }
-  }
-
-  DenseMap<unsigned, unsigned> RegRenamingMap;
-  bool Changed = false;
-  for (MachineFunction::iterator I = MF.begin(), E = MF.end();
-       I != E; ++I) {
-    MachineBasicBlock::iterator BBI = I->begin(), BBE = I->end();
-    while (BBI != BBE && BBI->isPHI()) {
-      MachineInstr *PHI = BBI;
-
-      assert(PHI->getNumOperands() > 0);
-
-      unsigned SrcReg = PHI->getOperand(1).getReg();
-      unsigned SrcColor = getRegColor(SrcReg);
-      unsigned NewReg = RegRenamingMap[SrcColor];
-      if (!NewReg) {
-        NewReg = SrcReg;
-        RegRenamingMap[SrcColor] = SrcReg;
-      }
-      MergeLIsAndRename(SrcReg, NewReg);
-
-      unsigned DestReg = PHI->getOperand(0).getReg();
-      if (!InsertedDestCopies.count(DestReg))
-        MergeLIsAndRename(DestReg, NewReg);
-
-      for (unsigned i = 3; i < PHI->getNumOperands(); i += 2) {
-        unsigned SrcReg = PHI->getOperand(i).getReg();
-        MergeLIsAndRename(SrcReg, NewReg);
-      }
-
-      ++BBI;
-      LI->RemoveMachineInstrFromMaps(PHI);
-      PHI->eraseFromParent();
-      Changed = true;
-    }
-  }
-
-  // Due to the insertion of copies to split live ranges, the live intervals are
-  // guaranteed to not overlap, except in one case: an original PHI source and a
-  // PHI destination copy. In this case, they have the same value and thus don't
-  // truly intersect, so we merge them into the value live at that point.
-  // FIXME: Is there some better way we can handle this?
-  for (DestCopyMap::iterator I = InsertedDestCopies.begin(),
-       E = InsertedDestCopies.end(); I != E; ++I) {
-    unsigned DestReg = I->first;
-    unsigned DestColor = getRegColor(DestReg);
-    unsigned NewReg = RegRenamingMap[DestColor];
-
-    LiveInterval &DestLI = LI->getInterval(DestReg);
-    LiveInterval &NewLI = LI->getInterval(NewReg);
-
-    assert(DestLI.size() == 1
-           && "PHI destination copy's live interval should be a single live "
-               "range from the beginning of the BB to the copy instruction.");
-    LiveInterval::Segment *DestS = DestLI.begin();
-    VNInfo *NewVNI = NewLI.getVNInfoAt(DestS->start);
-    if (!NewVNI) {
-      NewVNI = NewLI.createValueCopy(DestS->valno, LI->getVNInfoAllocator());
-      MachineInstr *CopyInstr = I->second;
-      CopyInstr->getOperand(1).setIsKill(true);
-    }
-
-    LiveInterval::Segment NewS(DestS->start, DestS->end, NewVNI);
-    NewLI.addSegment(NewS);
-
-    LI->removeInterval(DestReg);
-    MRI->replaceRegWith(DestReg, NewReg);
-  }
-
-  // Adjust the live intervals of all PHI source registers to handle the case
-  // where the PHIs in successor blocks were the only later uses of the source
-  // register.
-  for (SrcCopySet::iterator I = InsertedSrcCopySet.begin(),
-       E = InsertedSrcCopySet.end(); I != E; ++I) {
-    MachineBasicBlock *MBB = I->first;
-    unsigned SrcReg = I->second;
-    if (unsigned RenamedRegister = RegRenamingMap[getRegColor(SrcReg)])
-      SrcReg = RenamedRegister;
-
-    LiveInterval &SrcLI = LI->getInterval(SrcReg);
-
-    bool isLiveOut = false;
-    for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
-         SE = MBB->succ_end(); SI != SE; ++SI) {
-      if (SrcLI.liveAt(LI->getMBBStartIdx(*SI))) {
-        isLiveOut = true;
-        break;
-      }
-    }
-
-    if (isLiveOut)
-      continue;
-
-    MachineOperand *LastUse = findLastUse(MBB, SrcReg);
-    assert(LastUse);
-    SlotIndex LastUseIndex = LI->getInstructionIndex(LastUse->getParent());
-    SrcLI.removeSegment(LastUseIndex.getRegSlot(), LI->getMBBEndIdx(MBB));
-    LastUse->setIsKill(true);
-  }
-
-  Allocator.Reset();
-  RegNodeMap.clear();
-  PHISrcDefs.clear();
-  InsertedSrcCopySet.clear();
-  InsertedSrcCopyMap.clear();
-  InsertedDestCopies.clear();
-
-  return Changed;
-}
-
-void StrongPHIElimination::addReg(unsigned Reg) {
-  Node *&N = RegNodeMap[Reg];
-  if (!N)
-    N = new (Allocator) Node(Reg);
-}
-
-StrongPHIElimination::Node*
-StrongPHIElimination::Node::getLeader() {
-  Node *N = this;
-  Node *Parent = parent.getPointer();
-  Node *Grandparent = Parent->parent.getPointer();
-
-  while (Parent != Grandparent) {
-    N->parent.setPointer(Grandparent);
-    N = Grandparent;
-    Parent = Parent->parent.getPointer();
-    Grandparent = Parent->parent.getPointer();
-  }
-
-  return Parent;
-}
-
-unsigned StrongPHIElimination::getRegColor(unsigned Reg) {
-  DenseMap<unsigned, Node*>::iterator RI = RegNodeMap.find(Reg);
-  if (RI == RegNodeMap.end())
-    return 0;
-  Node *Node = RI->second;
-  if (Node->parent.getInt() & Node::kRegisterIsolatedFlag)
-    return 0;
-  return Node->getLeader()->value;
-}
-
-void StrongPHIElimination::unionRegs(unsigned Reg1, unsigned Reg2) {
-  Node *Node1 = RegNodeMap[Reg1]->getLeader();
-  Node *Node2 = RegNodeMap[Reg2]->getLeader();
-
-  if (Node1->rank > Node2->rank) {
-    Node2->parent.setPointer(Node1->getLeader());
-  } else if (Node1->rank < Node2->rank) {
-    Node1->parent.setPointer(Node2->getLeader());
-  } else if (Node1 != Node2) {
-    Node2->parent.setPointer(Node1->getLeader());
-    Node1->rank++;
-  }
-}
-
-void StrongPHIElimination::isolateReg(unsigned Reg) {
-  Node *Node = RegNodeMap[Reg];
-  Node->parent.setInt(Node->parent.getInt() | Node::kRegisterIsolatedFlag);
-}
-
-unsigned StrongPHIElimination::getPHIColor(MachineInstr *PHI) {
-  assert(PHI->isPHI());
-
-  unsigned DestReg = PHI->getOperand(0).getReg();
-  Node *DestNode = RegNodeMap[DestReg];
-  if (DestNode->parent.getInt() & Node::kPHIIsolatedFlag)
-    return 0;
-
-  for (unsigned i = 1; i < PHI->getNumOperands(); i += 2) {
-    unsigned SrcColor = getRegColor(PHI->getOperand(i).getReg());
-    if (SrcColor)
-      return SrcColor;
-  }
-  return 0;
-}
-
-void StrongPHIElimination::isolatePHI(MachineInstr *PHI) {
-  assert(PHI->isPHI());
-  Node *Node = RegNodeMap[PHI->getOperand(0).getReg()];
-  Node->parent.setInt(Node->parent.getInt() | Node::kPHIIsolatedFlag);
-}
-
-/// SplitInterferencesForBasicBlock - traverses a basic block, splitting any
-/// interferences found between registers in the same congruence class. It
-/// takes two DenseMaps as arguments that it also updates:
-///
-/// 1) CurrentDominatingParent, which maps a color to the register in that
-///    congruence class whose definition was most recently seen.
-///
-/// 2) ImmediateDominatingParent, which maps a register to the register in the
-///    same congruence class that most immediately dominates it.
-///
-/// This function assumes that it is being called in a depth-first traversal
-/// of the dominator tree.
-///
-/// The algorithm used here is a generalization of the dominance-based SSA test
-/// for two variables. If there are variables a_1, ..., a_n such that
-///
-///   def(a_1) dom ... dom def(a_n),
-///
-/// then we can test for an interference between any two a_i by only using O(n)
-/// interference tests between pairs of variables. If i < j and a_i and a_j
-/// interfere, then a_i is alive at def(a_j), so it is also alive at def(a_i+1).
-/// Thus, in order to test for an interference involving a_i, we need only check
-/// for a potential interference with a_i+1.
-///
-/// This method can be generalized to arbitrary sets of variables by performing
-/// a depth-first traversal of the dominator tree. As we traverse down a branch
-/// of the dominator tree, we keep track of the current dominating variable and
-/// only perform an interference test with that variable. However, when we go to
-/// another branch of the dominator tree, the definition of the current dominating
-/// variable may no longer dominate the current block. In order to correct this,
-/// we need to use a stack of past choices of the current dominating variable
-/// and pop from this stack until we find a variable whose definition actually
-/// dominates the current block.
-/// 
-/// There will be one push on this stack for each variable that has become the
-/// current dominating variable, so instead of using an explicit stack we can
-/// simply associate the previous choice for a current dominating variable with
-/// the new choice. This works better in our implementation, where we test for
-/// interference in multiple distinct sets at once.
-void
-StrongPHIElimination::SplitInterferencesForBasicBlock(
-    MachineBasicBlock &MBB,
-    DenseMap<unsigned, unsigned> &CurrentDominatingParent,
-    DenseMap<unsigned, unsigned> &ImmediateDominatingParent) {
-  // Sort defs by their order in the original basic block, as the code below
-  // assumes that it is processing definitions in dominance order.
-  std::vector<MachineInstr*> &DefInstrs = PHISrcDefs[&MBB];
-  std::sort(DefInstrs.begin(), DefInstrs.end(), MIIndexCompare(LI));
-
-  for (std::vector<MachineInstr*>::const_iterator BBI = DefInstrs.begin(),
-       BBE = DefInstrs.end(); BBI != BBE; ++BBI) {
-    for (MachineInstr::const_mop_iterator I = (*BBI)->operands_begin(),
-         E = (*BBI)->operands_end(); I != E; ++I) {
-      const MachineOperand &MO = *I;
-
-      // FIXME: This would be faster if it were possible to bail out of checking
-      // an instruction's operands after the explicit defs, but this is incorrect
-      // for variadic instructions, which may appear before register allocation
-      // in the future.
-      if (!MO.isReg() || !MO.isDef())
-        continue;
-
-      unsigned DestReg = MO.getReg();
-      if (!DestReg || !TargetRegisterInfo::isVirtualRegister(DestReg))
-        continue;
-
-      // If the virtual register being defined is not used in any PHI or has
-      // already been isolated, then there are no more interferences to check.
-      unsigned DestColor = getRegColor(DestReg);
-      if (!DestColor)
-        continue;
-
-      // The input to this pass sometimes is not in SSA form in every basic
-      // block, as some virtual registers have redefinitions. We could eliminate
-      // this by fixing the passes that generate the non-SSA code, or we could
-      // handle it here by tracking defining machine instructions rather than
-      // virtual registers. For now, we just handle the situation conservatively
-      // in a way that will possibly lead to false interferences.
-      unsigned &CurrentParent = CurrentDominatingParent[DestColor];
-      unsigned NewParent = CurrentParent;
-      if (NewParent == DestReg)
-        continue;
-
-      // Pop registers from the stack represented by ImmediateDominatingParent
-      // until we find a parent that dominates the current instruction.
-      while (NewParent && (!DT->dominates(MRI->getVRegDef(NewParent), *BBI)
-                           || !getRegColor(NewParent)))
-        NewParent = ImmediateDominatingParent[NewParent];
-
-      // If NewParent is nonzero, then its definition dominates the current
-      // instruction, so it is only necessary to check for the liveness of
-      // NewParent in order to check for an interference.
-      if (NewParent
-          && LI->getInterval(NewParent).liveAt(LI->getInstructionIndex(*BBI))) {
-        // If there is an interference, always isolate the new register. This
-        // could be improved by using a heuristic that decides which of the two
-        // registers to isolate.
-        isolateReg(DestReg);
-        CurrentParent = NewParent;
-      } else {
-        // If there is no interference, update ImmediateDominatingParent and set
-        // the CurrentDominatingParent for this color to the current register.
-        ImmediateDominatingParent[DestReg] = NewParent;
-        CurrentParent = DestReg;
-      }
-    }
-  }
-
-  // We now walk the PHIs in successor blocks and check for interferences. This
-  // is necessary because the use of a PHI's operands are logically contained in
-  // the predecessor block. The def of a PHI's destination register is processed
-  // along with the other defs in a basic block.
-
-  CurrentPHIForColor.clear();
-
-  for (MachineBasicBlock::succ_iterator SI = MBB.succ_begin(),
-       SE = MBB.succ_end(); SI != SE; ++SI) {
-    for (MachineBasicBlock::iterator BBI = (*SI)->begin(), BBE = (*SI)->end();
-         BBI != BBE && BBI->isPHI(); ++BBI) {
-      MachineInstr *PHI = BBI;
-
-      // If a PHI is already isolated, either by being isolated directly or
-      // having all of its operands isolated, ignore it.
-      unsigned Color = getPHIColor(PHI);
-      if (!Color)
-        continue;
-
-      // Find the index of the PHI operand that corresponds to this basic block.
-      unsigned PredIndex;
-      for (PredIndex = 1; PredIndex < PHI->getNumOperands(); PredIndex += 2) {
-        if (PHI->getOperand(PredIndex + 1).getMBB() == &MBB)
-          break;
-      }
-      assert(PredIndex < PHI->getNumOperands());
-      unsigned PredOperandReg = PHI->getOperand(PredIndex).getReg();
-
-      // Pop registers from the stack represented by ImmediateDominatingParent
-      // until we find a parent that dominates the current instruction.
-      unsigned &CurrentParent = CurrentDominatingParent[Color];
-      unsigned NewParent = CurrentParent;
-      while (NewParent
-             && (!DT->dominates(MRI->getVRegDef(NewParent)->getParent(), &MBB)
-                 || !getRegColor(NewParent)))
-        NewParent = ImmediateDominatingParent[NewParent];
-      CurrentParent = NewParent;
-
-      // If there is an interference with a register, always isolate the
-      // register rather than the PHI. It is also possible to isolate the
-      // PHI, but that introduces copies for all of the registers involved
-      // in that PHI.
-      if (NewParent && LI->isLiveOutOfMBB(LI->getInterval(NewParent), &MBB)
-                    && NewParent != PredOperandReg)
-        isolateReg(NewParent);
-
-      std::pair<MachineInstr*, unsigned>
-        &CurrentPHI = CurrentPHIForColor[Color];
-
-      // If two PHIs have the same operand from every shared predecessor, then
-      // they don't actually interfere. Otherwise, isolate the current PHI. This
-      // could possibly be improved, e.g. we could isolate the PHI with the
-      // fewest operands.
-      if (CurrentPHI.first && CurrentPHI.second != PredOperandReg)
-        isolatePHI(PHI);
-      else
-        CurrentPHI = std::make_pair(PHI, PredOperandReg);
-    }
-  }
-}
-
-void StrongPHIElimination::InsertCopiesForPHI(MachineInstr *PHI,
-                                              MachineBasicBlock *MBB) {
-  assert(PHI->isPHI());
-  ++NumPHIsLowered;
-  unsigned PHIColor = getPHIColor(PHI);
-
-  for (unsigned i = 1; i < PHI->getNumOperands(); i += 2) {
-    MachineOperand &SrcMO = PHI->getOperand(i);
-
-    // If a source is defined by an implicit def, there is no need to insert a
-    // copy in the predecessor.
-    if (SrcMO.isUndef())
-      continue;
-
-    unsigned SrcReg = SrcMO.getReg();
-    assert(TargetRegisterInfo::isVirtualRegister(SrcReg) &&
-           "Machine PHI Operands must all be virtual registers!");
-
-    MachineBasicBlock *PredBB = PHI->getOperand(i + 1).getMBB();
-    unsigned SrcColor = getRegColor(SrcReg);
-
-    // If neither the PHI nor the operand were isolated, then we only need to
-    // set the phi-kill flag on the VNInfo at this PHI.
-    if (PHIColor && SrcColor == PHIColor) {
-      LiveInterval &SrcInterval = LI->getInterval(SrcReg);
-      SlotIndex PredIndex = LI->getMBBEndIdx(PredBB);
-      VNInfo *SrcVNI = SrcInterval.getVNInfoBefore(PredIndex);
-      (void)SrcVNI;
-      assert(SrcVNI);
-      continue;
-    }
-
-    unsigned CopyReg = 0;
-    if (PHIColor) {
-      SrcCopyMap::const_iterator I
-        = InsertedSrcCopyMap.find(std::make_pair(PredBB, PHIColor));
-      CopyReg
-        = I != InsertedSrcCopyMap.end() ? I->second->getOperand(0).getReg() : 0;
-    }
-
-    if (!CopyReg) {
-      const TargetRegisterClass *RC = MRI->getRegClass(SrcReg);
-      CopyReg = MRI->createVirtualRegister(RC);
-
-      MachineBasicBlock::iterator
-        CopyInsertPoint = findPHICopyInsertPoint(PredBB, MBB, SrcReg);
-      unsigned SrcSubReg = SrcMO.getSubReg();
-      MachineInstr *CopyInstr = BuildMI(*PredBB,
-                                        CopyInsertPoint,
-                                        PHI->getDebugLoc(),
-                                        TII->get(TargetOpcode::COPY),
-                                        CopyReg).addReg(SrcReg, 0, SrcSubReg);
-      LI->InsertMachineInstrInMaps(CopyInstr);
-      ++NumSrcCopiesInserted;
-
-      // addLiveRangeToEndOfBlock() also adds the phikill flag to the VNInfo for
-      // the newly added range.
-      LI->addSegmentToEndOfBlock(CopyReg, CopyInstr);
-      InsertedSrcCopySet.insert(std::make_pair(PredBB, SrcReg));
-
-      addReg(CopyReg);
-      if (PHIColor) {
-        unionRegs(PHIColor, CopyReg);
-        assert(getRegColor(CopyReg) != CopyReg);
-      } else {
-        PHIColor = CopyReg;
-        assert(getRegColor(CopyReg) == CopyReg);
-      }
-
-      // Insert into map if not already there.
-      InsertedSrcCopyMap.insert(std::make_pair(std::make_pair(PredBB, PHIColor),
-                                               CopyInstr));
-    }
-
-    SrcMO.setReg(CopyReg);
-
-    // If SrcReg is not live beyond the PHI, trim its interval so that it is no
-    // longer live-in to MBB. Note that SrcReg may appear in other PHIs that are
-    // processed later, but this is still correct to do at this point because we
-    // never rely on LiveIntervals being correct while inserting copies.
-    // FIXME: Should this just count uses at PHIs like the normal PHIElimination
-    // pass does?
-    LiveInterval &SrcLI = LI->getInterval(SrcReg);
-    SlotIndex MBBStartIndex = LI->getMBBStartIdx(MBB);
-    SlotIndex PHIIndex = LI->getInstructionIndex(PHI);
-    SlotIndex NextInstrIndex = PHIIndex.getNextIndex();
-    if (SrcLI.liveAt(MBBStartIndex) && SrcLI.expiredAt(NextInstrIndex))
-      SrcLI.removeSegment(MBBStartIndex, PHIIndex, true);
-  }
-
-  unsigned DestReg = PHI->getOperand(0).getReg();
-  unsigned DestColor = getRegColor(DestReg);
-
-  if (PHIColor && DestColor == PHIColor) {
-    LiveInterval &DestLI = LI->getInterval(DestReg);
-
-    // Set the phi-def flag for the VN at this PHI.
-    SlotIndex PHIIndex = LI->getInstructionIndex(PHI);
-    VNInfo *DestVNI = DestLI.getVNInfoAt(PHIIndex.getRegSlot());
-    assert(DestVNI);
-  
-    // Prior to PHI elimination, the live ranges of PHIs begin at their defining
-    // instruction. After PHI elimination, PHI instructions are replaced by VNs
-    // with the phi-def flag set, and the live ranges of these VNs start at the
-    // beginning of the basic block.
-    SlotIndex MBBStartIndex = LI->getMBBStartIdx(MBB);
-    DestVNI->def = MBBStartIndex;
-    DestLI.addSegment(LiveInterval::Segment(MBBStartIndex,
-                                            PHIIndex.getRegSlot(),
-                                            DestVNI));
-    return;
-  }
-
-  const TargetRegisterClass *RC = MRI->getRegClass(DestReg);
-  unsigned CopyReg = MRI->createVirtualRegister(RC);
-
-  MachineInstr *CopyInstr = BuildMI(*MBB,
-                                    MBB->SkipPHIsAndLabels(MBB->begin()),
-                                    PHI->getDebugLoc(),
-                                    TII->get(TargetOpcode::COPY),
-                                    DestReg).addReg(CopyReg);
-  LI->InsertMachineInstrInMaps(CopyInstr);
-  PHI->getOperand(0).setReg(CopyReg);
-  ++NumDestCopiesInserted;
-
-  // Add the region from the beginning of MBB to the copy instruction to
-  // CopyReg's live interval, and give the VNInfo the phidef flag.
-  LiveInterval &CopyLI = LI->createEmptyInterval(CopyReg);
-  SlotIndex MBBStartIndex = LI->getMBBStartIdx(MBB);
-  SlotIndex DestCopyIndex = LI->getInstructionIndex(CopyInstr);
-  VNInfo *CopyVNI = CopyLI.getNextValue(MBBStartIndex,
-                                        LI->getVNInfoAllocator());
-  CopyLI.addSegment(LiveInterval::Segment(MBBStartIndex,
-                                          DestCopyIndex.getRegSlot(),
-                                          CopyVNI));
-
-  // Adjust DestReg's live interval to adjust for its new definition at
-  // CopyInstr.
-  LiveInterval &DestLI = LI->createEmptyInterval(DestReg);
-  SlotIndex PHIIndex = LI->getInstructionIndex(PHI);
-  DestLI.removeSegment(PHIIndex.getRegSlot(), DestCopyIndex.getRegSlot());
-
-  VNInfo *DestVNI = DestLI.getVNInfoAt(DestCopyIndex.getRegSlot());
-  assert(DestVNI);
-  DestVNI->def = DestCopyIndex.getRegSlot();
-
-  InsertedDestCopies[CopyReg] = CopyInstr;
-}
-
-void StrongPHIElimination::MergeLIsAndRename(unsigned Reg, unsigned NewReg) {
-  if (Reg == NewReg)
-    return;
-
-  LiveInterval &OldLI = LI->getInterval(Reg);
-  LiveInterval &NewLI = LI->getInterval(NewReg);
-
-  // Merge the live ranges of the two registers.
-  DenseMap<VNInfo*, VNInfo*> VNMap;
-  for (LiveInterval::iterator SI = OldLI.begin(), SE = OldLI.end();
-       SI != SE; ++SI) {
-    LiveInterval::Segment OldS = *SI;
-    VNInfo *OldVN = OldS.valno;
-
-    VNInfo *&NewVN = VNMap[OldVN];
-    if (!NewVN) {
-      NewVN = NewLI.createValueCopy(OldVN, LI->getVNInfoAllocator());
-      VNMap[OldVN] = NewVN;
-    }
-
-    LiveInterval::Segment S(OldS.start, OldS.end, NewVN);
-    NewLI.addSegment(S);
-  }
-
-  // Remove the LiveInterval for the register being renamed and replace all
-  // of its defs and uses with the new register.
-  LI->removeInterval(Reg);
-  MRI->replaceRegWith(Reg, NewReg);
-}