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 "PHIElimination.h"
 #include "llvm/BasicBlock.h"
 #include "llvm/Instructions.h"
 #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/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");

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

 const PassInfo *const llvm::PHIEliminationID = &X;

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

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

   PHIDefs.clear();
   PHIKills.clear();
   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_empty(DefReg))
       DefMI->eraseFromParent();
   }

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


 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
 /// predecessor basic blocks.
 ///
 bool llvm::PHIElimination::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 = SkipPHIsAndLabels(MBB, MBB.begin());

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

   return true;
 }

 /// isSourceDefinedByImplicitDef - Return true if all sources of the phi node
 /// are implicit_def's.
 static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi,
                                          const MachineRegisterInfo *MRI) {
   for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) {
     unsigned SrcReg = MPhi->getOperand(i).getReg();
     const MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
     if (!DefMI || DefMI->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
       return false;
   }
   return true;
 }

 // FindCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg.
 // This needs to be after any def or uses of SrcReg, but before any subsequent
 // point where control flow might jump out of the basic block.
 MachineBasicBlock::iterator
 llvm::PHIElimination::FindCopyInsertPoint(MachineBasicBlock &MBB,
                                           unsigned SrcReg) {
   // Handle the trivial case trivially.
   if (MBB.empty())
     return MBB.begin();

   // If this basic block does not contain an invoke, then control flow always
   // reaches the end of it, so place the copy there.  The logic below works in
   // this case too, but is more expensive.
   if (!isa<InvokeInst>(MBB.getBasicBlock()->getTerminator()))
     return MBB.getFirstTerminator();

   // Discover any definition/uses in this basic block.
   SmallPtrSet<MachineInstr*, 8> DefUsesInMBB;
   for (MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(SrcReg),
        RE = MRI->reg_end(); RI != RE; ++RI) {
     MachineInstr *DefUseMI = &*RI;
     if (DefUseMI->getParent() == &MBB)
       DefUsesInMBB.insert(DefUseMI);
   }

   MachineBasicBlock::iterator InsertPoint;
   if (DefUsesInMBB.empty()) {
     // No def/uses.  Insert the copy at the start of the basic block.
     InsertPoint = MBB.begin();
   } else if (DefUsesInMBB.size() == 1) {
     // Insert the copy immediately after the definition/use.
     InsertPoint = *DefUsesInMBB.begin();
     ++InsertPoint;
   } else {
     // Insert the copy immediately after the last definition/use.
     InsertPoint = MBB.end();
     while (!DefUsesInMBB.count(&*--InsertPoint)) {}
     ++InsertPoint;
   }

   // Make sure the copy goes after any phi nodes however.
   return SkipPHIsAndLabels(MBB, InsertPoint);
 }

 /// 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 llvm::PHIElimination::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();
   bool isDead = MPhi->getOperand(0).isDead();

   // Create a new register for the incoming PHI arguments.
   MachineFunction &MF = *MBB.getParent();
   const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg);
   unsigned IncomingReg = 0;

   // Insert a register to register copy at 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, MPhi->getDebugLoc(),
             TII->get(TargetInstrInfo::IMPLICIT_DEF), DestReg);
   else {
     IncomingReg = MF.getRegInfo().createVirtualRegister(RC);
     TII->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC, RC);
   }

   // Record PHI def.
   assert(!hasPHIDef(DestReg) && "Vreg has multiple phi-defs?");
   PHIDefs[DestReg] = &MBB;

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

     if (IncomingReg) {
       // 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 (isDead) {
       LV->addVirtualRegisterDead(DestReg, PHICopy);
       LV->removeVirtualRegisterDead(DestReg, MPhi);
     }
   }

   // 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!");

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

     // Record the kill.
     PHIKills[SrcReg].insert(&opBlock);

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

     // 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 = FindCopyInsertPoint(opBlock, SrcReg);

     // 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);

     // 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 (InRegVI.AliveBlocks.test(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();
       InRegVI.AliveBlocks.reset(opBlockNum);
     }
   }

   // Really delete the PHI instruction now!
   MF.DeleteMachineInstr(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 llvm::PHIElimination::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 "PHIElimination.h"
	#include "llvm/BasicBlock.h"
	#include "llvm/Instructions.h"
	#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/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");

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

	const PassInfo *const llvm::PHIEliminationID = &X;

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

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

	PHIDefs.clear();
	PHIKills.clear();
	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_empty(DefReg))
	DefMI->eraseFromParent();
	}

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


	/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
	/// predecessor basic blocks.
	///
	bool llvm::PHIElimination::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 = SkipPHIsAndLabels(MBB, MBB.begin());

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

	return true;
	}

	/// isSourceDefinedByImplicitDef - Return true if all sources of the phi node
	/// are implicit_def's.
	static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi,
	const MachineRegisterInfo *MRI) {
	for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) {
	unsigned SrcReg = MPhi->getOperand(i).getReg();
	const MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
	if (!DefMI \|\| DefMI->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
	return false;
	}
	return true;
	}

	// FindCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg.
	// This needs to be after any def or uses of SrcReg, but before any subsequent
	// point where control flow might jump out of the basic block.
	MachineBasicBlock::iterator
	llvm::PHIElimination::FindCopyInsertPoint(MachineBasicBlock &MBB,
	unsigned SrcReg) {
	// Handle the trivial case trivially.
	if (MBB.empty())
	return MBB.begin();

	// If this basic block does not contain an invoke, then control flow always
	// reaches the end of it, so place the copy there. The logic below works in
	// this case too, but is more expensive.
	if (!isa<InvokeInst>(MBB.getBasicBlock()->getTerminator()))
	return MBB.getFirstTerminator();

	// Discover any definition/uses in this basic block.
	SmallPtrSet<MachineInstr*, 8> DefUsesInMBB;
	for (MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(SrcReg),
	RE = MRI->reg_end(); RI != RE; ++RI) {
	MachineInstr DefUseMI = &RI;
	if (DefUseMI->getParent() == &MBB)
	DefUsesInMBB.insert(DefUseMI);
	}

	MachineBasicBlock::iterator InsertPoint;
	if (DefUsesInMBB.empty()) {
	// No def/uses. Insert the copy at the start of the basic block.
	InsertPoint = MBB.begin();
	} else if (DefUsesInMBB.size() == 1) {
	// Insert the copy immediately after the definition/use.
	InsertPoint = *DefUsesInMBB.begin();
	++InsertPoint;
	} else {
	// Insert the copy immediately after the last definition/use.
	InsertPoint = MBB.end();
	while (!DefUsesInMBB.count(&*--InsertPoint)) {}
	++InsertPoint;
	}

	// Make sure the copy goes after any phi nodes however.
	return SkipPHIsAndLabels(MBB, InsertPoint);
	}

	/// 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 llvm::PHIElimination::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();
	bool isDead = MPhi->getOperand(0).isDead();

	// Create a new register for the incoming PHI arguments.
	MachineFunction &MF = *MBB.getParent();
	const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg);
	unsigned IncomingReg = 0;

	// Insert a register to register copy at 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, MPhi->getDebugLoc(),
	TII->get(TargetInstrInfo::IMPLICIT_DEF), DestReg);
	else {
	IncomingReg = MF.getRegInfo().createVirtualRegister(RC);
	TII->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC, RC);
	}

	// Record PHI def.
	assert(!hasPHIDef(DestReg) && "Vreg has multiple phi-defs?");
	PHIDefs[DestReg] = &MBB;

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

	if (IncomingReg) {
	// 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 (isDead) {
	LV->addVirtualRegisterDead(DestReg, PHICopy);
	LV->removeVirtualRegisterDead(DestReg, MPhi);
	}
	}

	// 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!");

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

	// Record the kill.
	PHIKills[SrcReg].insert(&opBlock);

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

	// 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 = FindCopyInsertPoint(opBlock, SrcReg);

	// 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);

	// 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 (InRegVI.AliveBlocks.test(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();
	InRegVI.AliveBlocks.reset(opBlockNum);
	}
	}

	// Really delete the PHI instruction now!
	MF.DeleteMachineInstr(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 llvm::PHIElimination::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())];
	}