lib/CodeGen/PHIElimination.cpp - llvm - Git at Google

 //===-- PhiElimination.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 machine instruction PHI nodes by inserting copy
 // instructions.  This destroys SSA information, but is the desired input for
 // some register allocators.
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "phielim"
 #include "llvm/CodeGen/LiveVariables.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/Compiler.h"
 #include <algorithm>
 #include <map>
 using namespace llvm;

 STATISTIC(NumAtomic, "Number of atomic phis lowered");

 namespace {
   class VISIBILITY_HIDDEN PNE : public MachineFunctionPass {
     MachineRegisterInfo  *MRI; // Machine register information

   public:
     static char ID; // Pass identification, replacement for typeid
     PNE() : MachineFunctionPass((intptr_t)&ID) {}

     virtual bool runOnMachineFunction(MachineFunction &Fn);

     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.addPreserved<LiveVariables>();
       AU.addPreservedID(MachineLoopInfoID);
       AU.addPreservedID(MachineDominatorsID);
       MachineFunctionPass::getAnalysisUsage(AU);
     }

   private:
     /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
     /// in predecessor basic blocks.
     ///
     bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
     void LowerAtomicPHINode(MachineBasicBlock &MBB,
                             MachineBasicBlock::iterator AfterPHIsIt);

     /// analyzePHINodes - Gather information about the PHI nodes in
     /// here. In particular, we want to map the number of uses of a virtual
     /// register which is used in a PHI node. We map that to the BB the
     /// vreg is coming from. This is used later to determine when the vreg
     /// is killed in the BB.
     ///
     void analyzePHINodes(const MachineFunction& Fn);

     typedef std::pair<const MachineBasicBlock*, unsigned> BBVRegPair;
     typedef std::map<BBVRegPair, unsigned> VRegPHIUse;

     VRegPHIUse VRegPHIUseCount;

     // Defs of PHI sources which are implicit_def.
     SmallPtrSet<MachineInstr*, 4> ImpDefs;
   };

   char PNE::ID = 0;
   RegisterPass<PNE> X("phi-node-elimination",
                       "Eliminate PHI nodes for register allocation");
 }

 const PassInfo *llvm::PHIEliminationID = X.getPassInfo();

 bool PNE::runOnMachineFunction(MachineFunction &Fn) {
   MRI = &Fn.getRegInfo();

   analyzePHINodes(Fn);

   bool Changed = false;

   // Eliminate PHI instructions by inserting copies into predecessor blocks.
   for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
     Changed |= EliminatePHINodes(Fn, *I);

   // Remove dead IMPLICIT_DEF instructions.
   for (SmallPtrSet<MachineInstr*,4>::iterator I = ImpDefs.begin(),
          E = ImpDefs.end(); I != E; ++I) {
     MachineInstr *DefMI = *I;
     unsigned DefReg = DefMI->getOperand(0).getReg();
     if (MRI->use_begin(DefReg) == MRI->use_end())
       DefMI->eraseFromParent();
   }

   ImpDefs.clear();
   VRegPHIUseCount.clear();
   return Changed;
 }


 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
 /// predecessor basic blocks.
 ///
 bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
   if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI)
     return false;   // Quick exit for basic blocks without PHIs.

   // Get an iterator to the first instruction after the last PHI node (this may
   // also be the end of the basic block).
   MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
   while (AfterPHIsIt != MBB.end() &&
          AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI)
     ++AfterPHIsIt;    // Skip over all of the PHI nodes...

   while (MBB.front().getOpcode() == TargetInstrInfo::PHI)
     LowerAtomicPHINode(MBB, AfterPHIsIt);

   return true;
 }

 static bool isSourceDefinedByImplicitDef(MachineInstr *MPhi,
                                          MachineRegisterInfo  *MRI) {
   for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) {
     unsigned SrcReg = MPhi->getOperand(i).getReg();
     MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
     if (!DefMI || DefMI->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
       return false;
   }
   return true;
 }

 /// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
 /// under the assuption that it needs to be lowered in a way that supports
 /// atomic execution of PHIs.  This lowering method is always correct all of the
 /// time.
 void PNE::LowerAtomicPHINode(MachineBasicBlock &MBB,
                              MachineBasicBlock::iterator AfterPHIsIt) {
   // Unlink the PHI node from the basic block, but don't delete the PHI yet.
   MachineInstr *MPhi = MBB.remove(MBB.begin());

   unsigned NumSrcs = (MPhi->getNumOperands() - 1) / 2;
   unsigned DestReg = MPhi->getOperand(0).getReg();

   // Create a new register for the incoming PHI arguments.
   MachineFunction &MF = *MBB.getParent();
   const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg);
   unsigned IncomingReg = MF.getRegInfo().createVirtualRegister(RC);

   // Insert a register to register copy in the top of the current block (but
   // after any remaining phi nodes) which copies the new incoming register
   // into the phi node destination.
   //
   const TargetInstrInfo *TII = MF.getTarget().getInstrInfo();
   if (isSourceDefinedByImplicitDef(MPhi, MRI))
     // If all sources of a PHI node are implicit_def, just emit an implicit_def
     // instead of a copy.
     BuildMI(MBB, AfterPHIsIt, TII->get(TargetInstrInfo::IMPLICIT_DEF), DestReg);
   else
     TII->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC, RC);

   // Update live variable information if there is any...
   LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
   if (LV) {
     MachineInstr *PHICopy = prior(AfterPHIsIt);

     // Increment use count of the newly created virtual register.
     LV->getVarInfo(IncomingReg).NumUses++;

     // Add information to LiveVariables to know that the incoming value is
     // killed.  Note that because the value is defined in several places (once
     // each for each incoming block), the "def" block and instruction fields
     // for the VarInfo is not filled in.
     //
     LV->addVirtualRegisterKilled(IncomingReg, PHICopy);

     // Since we are going to be deleting the PHI node, if it is the last use
     // of any registers, or if the value itself is dead, we need to move this
     // information over to the new copy we just inserted.
     //
     LV->removeVirtualRegistersKilled(MPhi);

     // If the result is dead, update LV.
     if (MPhi->registerDefIsDead(DestReg)) {
       LV->addVirtualRegisterDead(DestReg, PHICopy);
       LV->removeVirtualRegistersDead(MPhi);
     }

     LV->getVarInfo(IncomingReg).UsedBlocks[MBB.getNumber()] = true;
   }

   // Adjust the VRegPHIUseCount map to account for the removal of this PHI
   // node.
   for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
     --VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i + 1).getMBB(),
                                  MPhi->getOperand(i).getReg())];

   // Now loop over all of the incoming arguments, changing them to copy into
   // the IncomingReg register in the corresponding predecessor basic block.
   //
   SmallPtrSet<MachineBasicBlock*, 8> MBBsInsertedInto;
   for (int i = NumSrcs - 1; i >= 0; --i) {
     unsigned SrcReg = MPhi->getOperand(i*2+1).getReg();
     assert(TargetRegisterInfo::isVirtualRegister(SrcReg) &&
            "Machine PHI Operands must all be virtual registers!");

     // If source is defined by an implicit def, there is no need to insert
     // a copy unless it's the only source.
     MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
     if (DefMI->getOpcode() == TargetInstrInfo::IMPLICIT_DEF) {
       ImpDefs.insert(DefMI);
       continue;
     }

     // Get the MachineBasicBlock equivalent of the BasicBlock that is the
     // source path the PHI.
     MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB();

     // Check to make sure we haven't already emitted the copy for this block.
     // This can happen because PHI nodes may have multiple entries for the
     // same basic block.
     if (!MBBsInsertedInto.insert(&opBlock))
       continue;  // If the copy has already been emitted, we're done.

     // Find a safe location to insert the copy, this may be the first
     // terminator in the block (or end()).
     MachineBasicBlock::iterator InsertPos = opBlock.getFirstTerminator();

     // Insert the copy.
     TII->copyRegToReg(opBlock, InsertPos, IncomingReg, SrcReg, RC, RC);

     // Now update live variable information if we have it.  Otherwise we're done
     if (!LV) continue;

     // We want to be able to insert a kill of the register if this PHI
     // (aka, the copy we just inserted) is the last use of the source
     // value.  Live variable analysis conservatively handles this by
     // saying that the value is live until the end of the block the PHI
     // entry lives in.  If the value really is dead at the PHI copy, there
     // will be no successor blocks which have the value live-in.
     //
     // Check to see if the copy is the last use, and if so, update the
     // live variables information so that it knows the copy source
     // instruction kills the incoming value.
     //
     LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
     InRegVI.UsedBlocks[opBlock.getNumber()] = true;

     // Loop over all of the successors of the basic block, checking to see
     // if the value is either live in the block, or if it is killed in the
     // block.  Also check to see if this register is in use by another PHI
     // node which has not yet been eliminated.  If so, it will be killed
     // at an appropriate point later.
     //

     // Is it used by any PHI instructions in this block?
     bool ValueIsLive = VRegPHIUseCount[BBVRegPair(&opBlock, SrcReg)] != 0;

     std::vector<MachineBasicBlock*> OpSuccBlocks;

     // Otherwise, scan successors, including the BB the PHI node lives in.
     for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(),
            E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) {
       MachineBasicBlock *SuccMBB = *SI;

       // Is it alive in this successor?
       unsigned SuccIdx = SuccMBB->getNumber();
       if (SuccIdx < InRegVI.AliveBlocks.size() &&
           InRegVI.AliveBlocks[SuccIdx]) {
         ValueIsLive = true;
         break;
       }

       OpSuccBlocks.push_back(SuccMBB);
     }

     // Check to see if this value is live because there is a use in a successor
     // that kills it.
     if (!ValueIsLive) {
       switch (OpSuccBlocks.size()) {
       case 1: {
         MachineBasicBlock *MBB = OpSuccBlocks[0];
         for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
           if (InRegVI.Kills[i]->getParent() == MBB) {
             ValueIsLive = true;
             break;
           }
         break;
       }
       case 2: {
         MachineBasicBlock *MBB1 = OpSuccBlocks[0], *MBB2 = OpSuccBlocks[1];
         for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
           if (InRegVI.Kills[i]->getParent() == MBB1 ||
               InRegVI.Kills[i]->getParent() == MBB2) {
             ValueIsLive = true;
             break;
           }
         break;
       }
       default:
         std::sort(OpSuccBlocks.begin(), OpSuccBlocks.end());
         for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
           if (std::binary_search(OpSuccBlocks.begin(), OpSuccBlocks.end(),
                                  InRegVI.Kills[i]->getParent())) {
             ValueIsLive = true;
             break;
           }
       }
     }

     // Okay, if we now know that the value is not live out of the block,
     // we can add a kill marker in this block saying that it kills the incoming
     // value!
     if (!ValueIsLive) {
       // In our final twist, we have to decide which instruction kills the
       // register.  In most cases this is the copy, however, the first
       // terminator instruction at the end of the block may also use the value.
       // In this case, we should mark *it* as being the killing block, not the
       // copy.
       MachineBasicBlock::iterator KillInst = prior(InsertPos);
       MachineBasicBlock::iterator Term = opBlock.getFirstTerminator();
       if (Term != opBlock.end()) {
         if (Term->readsRegister(SrcReg))
           KillInst = Term;

         // Check that no other terminators use values.
 #ifndef NDEBUG
         for (MachineBasicBlock::iterator TI = next(Term); TI != opBlock.end();
              ++TI) {
           assert(!TI->readsRegister(SrcReg) &&
                  "Terminator instructions cannot use virtual registers unless"
                  "they are the first terminator in a block!");
         }
 #endif
       }

       // Finally, mark it killed.
       LV->addVirtualRegisterKilled(SrcReg, KillInst);

       // This vreg no longer lives all of the way through opBlock.
       unsigned opBlockNum = opBlock.getNumber();
       if (opBlockNum < InRegVI.AliveBlocks.size())
         InRegVI.AliveBlocks[opBlockNum] = false;
     }
   }

   // Really delete the PHI instruction now!
   delete MPhi;
   ++NumAtomic;
 }

 /// analyzePHINodes - Gather information about the PHI nodes in here. In
 /// particular, we want to map the number of uses of a virtual register which is
 /// used in a PHI node. We map that to the BB the vreg is coming from. This is
 /// used later to determine when the vreg is killed in the BB.
 ///
 void PNE::analyzePHINodes(const MachineFunction& Fn) {
   for (MachineFunction::const_iterator I = Fn.begin(), E = Fn.end();
        I != E; ++I)
     for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end();
          BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
       for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
         ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i + 1).getMBB(),
                                      BBI->getOperand(i).getReg())];
 }
	//===-- PhiElimination.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 machine instruction PHI nodes by inserting copy
	// instructions. This destroys SSA information, but is the desired input for
	// some register allocators.
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "phielim"
	#include "llvm/CodeGen/LiveVariables.h"
	#include "llvm/CodeGen/Passes.h"
	#include "llvm/CodeGen/MachineFunctionPass.h"
	#include "llvm/CodeGen/MachineInstr.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/CodeGen/MachineRegisterInfo.h"
	#include "llvm/Target/TargetInstrInfo.h"
	#include "llvm/Target/TargetMachine.h"
	#include "llvm/ADT/SmallPtrSet.h"
	#include "llvm/ADT/STLExtras.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Support/Compiler.h"
	#include <algorithm>
	#include <map>
	using namespace llvm;

	STATISTIC(NumAtomic, "Number of atomic phis lowered");

	namespace {
	class VISIBILITY_HIDDEN PNE : public MachineFunctionPass {
	MachineRegisterInfo *MRI; // Machine register information

	public:
	static char ID; // Pass identification, replacement for typeid
	PNE() : MachineFunctionPass((intptr_t)&ID) {}

	virtual bool runOnMachineFunction(MachineFunction &Fn);

	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	AU.addPreserved<LiveVariables>();
	AU.addPreservedID(MachineLoopInfoID);
	AU.addPreservedID(MachineDominatorsID);
	MachineFunctionPass::getAnalysisUsage(AU);
	}

	private:
	/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
	/// in predecessor basic blocks.
	///
	bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
	void LowerAtomicPHINode(MachineBasicBlock &MBB,
	MachineBasicBlock::iterator AfterPHIsIt);

	/// analyzePHINodes - Gather information about the PHI nodes in
	/// here. In particular, we want to map the number of uses of a virtual
	/// register which is used in a PHI node. We map that to the BB the
	/// vreg is coming from. This is used later to determine when the vreg
	/// is killed in the BB.
	///
	void analyzePHINodes(const MachineFunction& Fn);

	typedef std::pair<const MachineBasicBlock*, unsigned> BBVRegPair;
	typedef std::map<BBVRegPair, unsigned> VRegPHIUse;

	VRegPHIUse VRegPHIUseCount;

	// Defs of PHI sources which are implicit_def.
	SmallPtrSet<MachineInstr*, 4> ImpDefs;
	};

	char PNE::ID = 0;
	RegisterPass<PNE> X("phi-node-elimination",
	"Eliminate PHI nodes for register allocation");
	}

	const PassInfo *llvm::PHIEliminationID = X.getPassInfo();

	bool PNE::runOnMachineFunction(MachineFunction &Fn) {
	MRI = &Fn.getRegInfo();

	analyzePHINodes(Fn);

	bool Changed = false;

	// Eliminate PHI instructions by inserting copies into predecessor blocks.
	for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
	Changed \|= EliminatePHINodes(Fn, *I);

	// Remove dead IMPLICIT_DEF instructions.
	for (SmallPtrSet<MachineInstr*,4>::iterator I = ImpDefs.begin(),
	E = ImpDefs.end(); I != E; ++I) {
	MachineInstr DefMI = I;
	unsigned DefReg = DefMI->getOperand(0).getReg();
	if (MRI->use_begin(DefReg) == MRI->use_end())
	DefMI->eraseFromParent();
	}

	ImpDefs.clear();
	VRegPHIUseCount.clear();
	return Changed;
	}


	/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
	/// predecessor basic blocks.
	///
	bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
	if (MBB.empty() \|\| MBB.front().getOpcode() != TargetInstrInfo::PHI)
	return false; // Quick exit for basic blocks without PHIs.

	// Get an iterator to the first instruction after the last PHI node (this may
	// also be the end of the basic block).
	MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
	while (AfterPHIsIt != MBB.end() &&
	AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI)
	++AfterPHIsIt; // Skip over all of the PHI nodes...

	while (MBB.front().getOpcode() == TargetInstrInfo::PHI)
	LowerAtomicPHINode(MBB, AfterPHIsIt);

	return true;
	}

	static bool isSourceDefinedByImplicitDef(MachineInstr *MPhi,
	MachineRegisterInfo *MRI) {
	for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) {
	unsigned SrcReg = MPhi->getOperand(i).getReg();
	MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
	if (!DefMI \|\| DefMI->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
	return false;
	}
	return true;
	}

	/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
	/// under the assuption that it needs to be lowered in a way that supports
	/// atomic execution of PHIs. This lowering method is always correct all of the
	/// time.
	void PNE::LowerAtomicPHINode(MachineBasicBlock &MBB,
	MachineBasicBlock::iterator AfterPHIsIt) {
	// Unlink the PHI node from the basic block, but don't delete the PHI yet.
	MachineInstr *MPhi = MBB.remove(MBB.begin());

	unsigned NumSrcs = (MPhi->getNumOperands() - 1) / 2;
	unsigned DestReg = MPhi->getOperand(0).getReg();

	// Create a new register for the incoming PHI arguments.
	MachineFunction &MF = *MBB.getParent();
	const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg);
	unsigned IncomingReg = MF.getRegInfo().createVirtualRegister(RC);

	// Insert a register to register copy in the top of the current block (but
	// after any remaining phi nodes) which copies the new incoming register
	// into the phi node destination.
	//
	const TargetInstrInfo *TII = MF.getTarget().getInstrInfo();
	if (isSourceDefinedByImplicitDef(MPhi, MRI))
	// If all sources of a PHI node are implicit_def, just emit an implicit_def
	// instead of a copy.
	BuildMI(MBB, AfterPHIsIt, TII->get(TargetInstrInfo::IMPLICIT_DEF), DestReg);
	else
	TII->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC, RC);

	// Update live variable information if there is any...
	LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
	if (LV) {
	MachineInstr *PHICopy = prior(AfterPHIsIt);

	// Increment use count of the newly created virtual register.
	LV->getVarInfo(IncomingReg).NumUses++;

	// Add information to LiveVariables to know that the incoming value is
	// killed. Note that because the value is defined in several places (once
	// each for each incoming block), the "def" block and instruction fields
	// for the VarInfo is not filled in.
	//
	LV->addVirtualRegisterKilled(IncomingReg, PHICopy);

	// Since we are going to be deleting the PHI node, if it is the last use
	// of any registers, or if the value itself is dead, we need to move this
	// information over to the new copy we just inserted.
	//
	LV->removeVirtualRegistersKilled(MPhi);

	// If the result is dead, update LV.
	if (MPhi->registerDefIsDead(DestReg)) {
	LV->addVirtualRegisterDead(DestReg, PHICopy);
	LV->removeVirtualRegistersDead(MPhi);
	}

	LV->getVarInfo(IncomingReg).UsedBlocks[MBB.getNumber()] = true;
	}

	// Adjust the VRegPHIUseCount map to account for the removal of this PHI
	// node.
	for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
	--VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i + 1).getMBB(),
	MPhi->getOperand(i).getReg())];

	// Now loop over all of the incoming arguments, changing them to copy into
	// the IncomingReg register in the corresponding predecessor basic block.
	//
	SmallPtrSet<MachineBasicBlock*, 8> MBBsInsertedInto;
	for (int i = NumSrcs - 1; i >= 0; --i) {
	unsigned SrcReg = MPhi->getOperand(i*2+1).getReg();
	assert(TargetRegisterInfo::isVirtualRegister(SrcReg) &&
	"Machine PHI Operands must all be virtual registers!");

	// If source is defined by an implicit def, there is no need to insert
	// a copy unless it's the only source.
	MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
	if (DefMI->getOpcode() == TargetInstrInfo::IMPLICIT_DEF) {
	ImpDefs.insert(DefMI);
	continue;
	}

	// Get the MachineBasicBlock equivalent of the BasicBlock that is the
	// source path the PHI.
	MachineBasicBlock &opBlock = MPhi->getOperand(i2+2).getMBB();

	// Check to make sure we haven't already emitted the copy for this block.
	// This can happen because PHI nodes may have multiple entries for the
	// same basic block.
	if (!MBBsInsertedInto.insert(&opBlock))
	continue; // If the copy has already been emitted, we're done.

	// Find a safe location to insert the copy, this may be the first
	// terminator in the block (or end()).
	MachineBasicBlock::iterator InsertPos = opBlock.getFirstTerminator();

	// Insert the copy.
	TII->copyRegToReg(opBlock, InsertPos, IncomingReg, SrcReg, RC, RC);

	// Now update live variable information if we have it. Otherwise we're done
	if (!LV) continue;

	// We want to be able to insert a kill of the register if this PHI
	// (aka, the copy we just inserted) is the last use of the source
	// value. Live variable analysis conservatively handles this by
	// saying that the value is live until the end of the block the PHI
	// entry lives in. If the value really is dead at the PHI copy, there
	// will be no successor blocks which have the value live-in.
	//
	// Check to see if the copy is the last use, and if so, update the
	// live variables information so that it knows the copy source
	// instruction kills the incoming value.
	//
	LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
	InRegVI.UsedBlocks[opBlock.getNumber()] = true;

	// Loop over all of the successors of the basic block, checking to see
	// if the value is either live in the block, or if it is killed in the
	// block. Also check to see if this register is in use by another PHI
	// node which has not yet been eliminated. If so, it will be killed
	// at an appropriate point later.
	//

	// Is it used by any PHI instructions in this block?
	bool ValueIsLive = VRegPHIUseCount[BBVRegPair(&opBlock, SrcReg)] != 0;

	std::vector<MachineBasicBlock*> OpSuccBlocks;

	// Otherwise, scan successors, including the BB the PHI node lives in.
	for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(),
	E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) {
	MachineBasicBlock SuccMBB = SI;

	// Is it alive in this successor?
	unsigned SuccIdx = SuccMBB->getNumber();
	if (SuccIdx < InRegVI.AliveBlocks.size() &&
	InRegVI.AliveBlocks[SuccIdx]) {
	ValueIsLive = true;
	break;
	}

	OpSuccBlocks.push_back(SuccMBB);
	}

	// Check to see if this value is live because there is a use in a successor
	// that kills it.
	if (!ValueIsLive) {
	switch (OpSuccBlocks.size()) {
	case 1: {
	MachineBasicBlock *MBB = OpSuccBlocks[0];
	for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
	if (InRegVI.Kills[i]->getParent() == MBB) {
	ValueIsLive = true;
	break;
	}
	break;
	}
	case 2: {
	MachineBasicBlock MBB1 = OpSuccBlocks[0], MBB2 = OpSuccBlocks[1];
	for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
	if (InRegVI.Kills[i]->getParent() == MBB1 \|\|
	InRegVI.Kills[i]->getParent() == MBB2) {
	ValueIsLive = true;
	break;
	}
	break;
	}
	default:
	std::sort(OpSuccBlocks.begin(), OpSuccBlocks.end());
	for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
	if (std::binary_search(OpSuccBlocks.begin(), OpSuccBlocks.end(),
	InRegVI.Kills[i]->getParent())) {
	ValueIsLive = true;
	break;
	}
	}
	}

	// Okay, if we now know that the value is not live out of the block,
	// we can add a kill marker in this block saying that it kills the incoming
	// value!
	if (!ValueIsLive) {
	// In our final twist, we have to decide which instruction kills the
	// register. In most cases this is the copy, however, the first
	// terminator instruction at the end of the block may also use the value.
	// In this case, we should mark it as being the killing block, not the
	// copy.
	MachineBasicBlock::iterator KillInst = prior(InsertPos);
	MachineBasicBlock::iterator Term = opBlock.getFirstTerminator();
	if (Term != opBlock.end()) {
	if (Term->readsRegister(SrcReg))
	KillInst = Term;

	// Check that no other terminators use values.
	#ifndef NDEBUG
	for (MachineBasicBlock::iterator TI = next(Term); TI != opBlock.end();
	++TI) {
	assert(!TI->readsRegister(SrcReg) &&
	"Terminator instructions cannot use virtual registers unless"
	"they are the first terminator in a block!");
	}
	#endif
	}

	// Finally, mark it killed.
	LV->addVirtualRegisterKilled(SrcReg, KillInst);

	// This vreg no longer lives all of the way through opBlock.
	unsigned opBlockNum = opBlock.getNumber();
	if (opBlockNum < InRegVI.AliveBlocks.size())
	InRegVI.AliveBlocks[opBlockNum] = false;
	}
	}

	// Really delete the PHI instruction now!
	delete MPhi;
	++NumAtomic;
	}

	/// analyzePHINodes - Gather information about the PHI nodes in here. In
	/// particular, we want to map the number of uses of a virtual register which is
	/// used in a PHI node. We map that to the BB the vreg is coming from. This is
	/// used later to determine when the vreg is killed in the BB.
	///
	void PNE::analyzePHINodes(const MachineFunction& Fn) {
	for (MachineFunction::const_iterator I = Fn.begin(), E = Fn.end();
	I != E; ++I)
	for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end();
	BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
	for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
	++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i + 1).getMBB(),
	BBI->getOperand(i).getReg())];
	}