lib/CodeGen/StrongPHIElimination.cpp - platform/external/llvm - Gitiles

 //===- 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 machine instruction PHI nodes by inserting copy
 // instructions, using an intelligent copy-folding technique based on
 // dominator information.  This is technique is 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.
 //    DOI= http://doi.acm.org/10.1145/512529.512534
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "strongphielim"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/CodeGen/LiveVariables.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/SSARegMap.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/Compiler.h"
 using namespace llvm;


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

     DenseMap<MachineBasicBlock*,
              std::map<unsigned, unsigned> > Waiting;

     std::map<unsigned, std::vector<unsigned> > Stacks;
     std::set<unsigned> UsedByAnother;

     bool runOnMachineFunction(MachineFunction &Fn);

     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.addPreserved<LiveVariables>();
       AU.addPreservedID(PHIEliminationID);
       AU.addRequired<MachineDominatorTree>();
       AU.addRequired<LiveVariables>();
       AU.setPreservesAll();
       MachineFunctionPass::getAnalysisUsage(AU);
     }

     virtual void releaseMemory() {
       preorder.clear();
       maxpreorder.clear();

       Waiting.clear();
     }

   private:
     struct DomForestNode {
     private:
       std::vector<DomForestNode*> children;
       unsigned reg;

       void addChild(DomForestNode* DFN) { children.push_back(DFN); }

     public:
       typedef std::vector<DomForestNode*>::iterator iterator;

       DomForestNode(unsigned r, DomForestNode* parent) : reg(r) {
         if (parent)
           parent->addChild(this);
       }

       ~DomForestNode() {
         for (iterator I = begin(), E = end(); I != E; ++I)
           delete *I;
       }

       inline unsigned getReg() { return reg; }

       inline DomForestNode::iterator begin() { return children.begin(); }
       inline DomForestNode::iterator end() { return children.end(); }
     };

     DenseMap<MachineBasicBlock*, unsigned> preorder;
     DenseMap<MachineBasicBlock*, unsigned> maxpreorder;


     void computeDFS(MachineFunction& MF);
     void processBlock(MachineBasicBlock* MBB);

     std::vector<DomForestNode*> computeDomForest(std::set<unsigned>& instrs);
     void processPHIUnion(MachineInstr* Inst,
                          std::set<unsigned>& PHIUnion,
                          std::vector<StrongPHIElimination::DomForestNode*>& DF,
                          std::vector<std::pair<unsigned, unsigned> >& locals);
     void ScheduleCopies(MachineBasicBlock* MBB, std::set<unsigned>& pushed);
     void InsertCopies(MachineBasicBlock* MBB);
   };

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

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

 /// computeDFS - Computes the DFS-in and DFS-out numbers of the dominator tree
 /// of the given MachineFunction.  These numbers are then used in other parts
 /// of the PHI elimination process.
 void StrongPHIElimination::computeDFS(MachineFunction& MF) {
   SmallPtrSet<MachineDomTreeNode*, 8> frontier;
   SmallPtrSet<MachineDomTreeNode*, 8> visited;

   unsigned time = 0;

   MachineDominatorTree& DT = getAnalysis<MachineDominatorTree>();

   MachineDomTreeNode* node = DT.getRootNode();

   std::vector<MachineDomTreeNode*> worklist;
   worklist.push_back(node);

   while (!worklist.empty()) {
     MachineDomTreeNode* currNode = worklist.back();

     if (!frontier.count(currNode)) {
       frontier.insert(currNode);
       ++time;
       preorder.insert(std::make_pair(currNode->getBlock(), time));
     }

     bool inserted = false;
     for (MachineDomTreeNode::iterator I = node->begin(), E = node->end();
          I != E; ++I)
       if (!frontier.count(*I) && !visited.count(*I)) {
         worklist.push_back(*I);
         inserted = true;
         break;
       }

     if (!inserted) {
       frontier.erase(currNode);
       visited.insert(currNode);
       maxpreorder.insert(std::make_pair(currNode->getBlock(), time));

       worklist.pop_back();
     }
   }
 }

 /// PreorderSorter - a helper class that is used to sort registers
 /// according to the preorder number of their defining blocks
 class PreorderSorter {
 private:
   DenseMap<MachineBasicBlock*, unsigned>& preorder;
   LiveVariables& LV;

 public:
   PreorderSorter(DenseMap<MachineBasicBlock*, unsigned>& p,
                 LiveVariables& L) : preorder(p), LV(L) { }

   bool operator()(unsigned A, unsigned B) {
     if (A == B)
       return false;

     MachineBasicBlock* ABlock = LV.getVarInfo(A).DefInst->getParent();
     MachineBasicBlock* BBlock = LV.getVarInfo(A).DefInst->getParent();

     if (preorder[ABlock] < preorder[BBlock])
       return true;
     else if (preorder[ABlock] > preorder[BBlock])
       return false;

     assert(0 && "Error sorting by dominance!");
     return false;
   }
 };

 /// computeDomForest - compute the subforest of the DomTree corresponding
 /// to the defining blocks of the registers in question
 std::vector<StrongPHIElimination::DomForestNode*>
 StrongPHIElimination::computeDomForest(std::set<unsigned>& regs) {
   LiveVariables& LV = getAnalysis<LiveVariables>();

   DomForestNode* VirtualRoot = new DomForestNode(0, 0);
   maxpreorder.insert(std::make_pair((MachineBasicBlock*)0, ~0UL));

   std::vector<unsigned> worklist;
   worklist.reserve(regs.size());
   for (std::set<unsigned>::iterator I = regs.begin(), E = regs.end();
        I != E; ++I)
     worklist.push_back(*I);

   PreorderSorter PS(preorder, LV);
   std::sort(worklist.begin(), worklist.end(), PS);

   DomForestNode* CurrentParent = VirtualRoot;
   std::vector<DomForestNode*> stack;
   stack.push_back(VirtualRoot);

   for (std::vector<unsigned>::iterator I = worklist.begin(), E = worklist.end();
        I != E; ++I) {
     unsigned pre = preorder[LV.getVarInfo(*I).DefInst->getParent()];
     MachineBasicBlock* parentBlock =
       LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();

     while (pre > maxpreorder[parentBlock]) {
       stack.pop_back();
       CurrentParent = stack.back();

       parentBlock = LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
     }

     DomForestNode* child = new DomForestNode(*I, CurrentParent);
     stack.push_back(child);
     CurrentParent = child;
   }

   std::vector<DomForestNode*> ret;
   ret.insert(ret.end(), VirtualRoot->begin(), VirtualRoot->end());
   return ret;
 }

 /// isLiveIn - helper method that determines, from a VarInfo, if a register
 /// is live into a block
 bool isLiveIn(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
   if (V.AliveBlocks.test(MBB->getNumber()))
     return true;

   if (V.DefInst->getParent() != MBB &&
       V.UsedBlocks.test(MBB->getNumber()))
     return true;

   return false;
 }

 /// isLiveOut - help method that determines, from a VarInfo, if a register is
 /// live out of a block.
 bool isLiveOut(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
   if (MBB == V.DefInst->getParent() ||
       V.UsedBlocks.test(MBB->getNumber())) {
     for (std::vector<MachineInstr*>::iterator I = V.Kills.begin(),
          E = V.Kills.end(); I != E; ++I)
       if ((*I)->getParent() == MBB)
         return false;

     return true;
   }

   return false;
 }

 /// isKillInst - helper method that determines, from a VarInfo, if an
 /// instruction kills a given register
 bool isKillInst(LiveVariables::VarInfo& V, MachineInstr* MI) {
   return std::find(V.Kills.begin(), V.Kills.end(), MI) != V.Kills.end();
 }

 /// interferes - checks for local interferences by scanning a block.  The only
 /// trick parameter is 'mode' which tells it the relationship of the two
 /// registers. 0 - defined in the same block, 1 - first properly dominates
 /// second, 2 - second properly dominates first
 bool interferes(LiveVariables::VarInfo& First, LiveVariables::VarInfo& Second,
                 MachineBasicBlock* scan, unsigned mode) {
   MachineInstr* def = 0;
   MachineInstr* kill = 0;

   bool interference = false;

   // Wallk the block, checking for interferences
   for (MachineBasicBlock::iterator MBI = scan->begin(), MBE = scan->end();
        MBI != MBE; ++MBI) {
     MachineInstr* curr = MBI;

     // Same defining block...
     if (mode == 0) {
       if (curr == First.DefInst) {
         // If we find our first DefInst, save it
         if (!def) {
           def = curr;
         // If there's already an unkilled DefInst, then
         // this is an interference
         } else if (!kill) {
           interference = true;
           break;
         // If there's a DefInst followed by a KillInst, then
         // they can't interfere
         } else {
           interference = false;
           break;
         }
       // Symmetric with the above
       } else if (curr == Second.DefInst ) {
         if (!def) {
           def = curr;
         } else if (!kill) {
           interference = true;
           break;
         } else {
           interference = false;
           break;
         }
       // Store KillInsts if they match up with the DefInst
       } else if (isKillInst(First, curr)) {
         if (def == First.DefInst) {
           kill = curr;
         } else if (isKillInst(Second, curr)) {
           if (def == Second.DefInst) {
             kill = curr;
           }
         }
       }
     // First properly dominates second...
     } else if (mode == 1) {
       if (curr == Second.DefInst) {
         // DefInst of second without kill of first is an interference
         if (!kill) {
           interference = true;
           break;
         // DefInst after a kill is a non-interference
         } else {
           interference = false;
           break;
         }
       // Save KillInsts of First
       } else if (isKillInst(First, curr)) {
         kill = curr;
       }
     // Symmetric with the above
     } else if (mode == 2) {
       if (curr == First.DefInst) {
         if (!kill) {
           interference = true;
           break;
         } else {
           interference = false;
           break;
         }
       } else if (isKillInst(Second, curr)) {
         kill = curr;
       }
     }
   }

   return interference;
 }

 /// processBlock - Eliminate PHIs in the given block
 void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
   LiveVariables& LV = getAnalysis<LiveVariables>();

   // Holds names that have been added to a set in any PHI within this block
   // before the current one.
   std::set<unsigned> ProcessedNames;

   MachineBasicBlock::iterator P = MBB->begin();
   while (P->getOpcode() == TargetInstrInfo::PHI) {
     LiveVariables::VarInfo& PHIInfo = LV.getVarInfo(P->getOperand(0).getReg());

     unsigned DestReg = P->getOperand(0).getReg();

     // Hold the names that are currently in the candidate set.
     std::set<unsigned> PHIUnion;
     std::set<MachineBasicBlock*> UnionedBlocks;

     for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
       unsigned SrcReg = P->getOperand(i-1).getReg();
       LiveVariables::VarInfo& SrcInfo = LV.getVarInfo(SrcReg);

       // Check for trivial interferences
       if (isLiveIn(SrcInfo, P->getParent()) ||
           isLiveOut(PHIInfo, SrcInfo.DefInst->getParent()) ||
           ( PHIInfo.DefInst->getOpcode() == TargetInstrInfo::PHI &&
             isLiveIn(PHIInfo, SrcInfo.DefInst->getParent()) ) ||
           ProcessedNames.count(SrcReg) ||
           UnionedBlocks.count(SrcInfo.DefInst->getParent())) {

         // add a copy from a_i to p in Waiting[From[a_i]]
         MachineBasicBlock* From = P->getOperand(i).getMachineBasicBlock();
         Waiting[From].insert(std::make_pair(SrcReg, DestReg));
         UsedByAnother.insert(SrcReg);
       } else {
         PHIUnion.insert(SrcReg);
         UnionedBlocks.insert(SrcInfo.DefInst->getParent());
       }
     }

     std::vector<StrongPHIElimination::DomForestNode*> DF =
                                                      computeDomForest(PHIUnion);

     // Walk DomForest to resolve interferences
     std::vector<std::pair<unsigned, unsigned> > localInterferences;
     processPHIUnion(P, PHIUnion, DF, localInterferences);

     // Check for local interferences
     for (std::vector<std::pair<unsigned, unsigned> >::iterator I =
         localInterferences.begin(), E = localInterferences.end(); I != E; ++I) {
       std::pair<unsigned, unsigned> p = *I;

       LiveVariables::VarInfo& FirstInfo = LV.getVarInfo(p.first);
       LiveVariables::VarInfo& SecondInfo = LV.getVarInfo(p.second);

       MachineDominatorTree& MDT = getAnalysis<MachineDominatorTree>();

       // Determine the block we need to scan and the relationship between
       // the two registers
       MachineBasicBlock* scan = 0;
       unsigned mode = 0;
       if (FirstInfo.DefInst->getParent() == SecondInfo.DefInst->getParent()) {
         scan = FirstInfo.DefInst->getParent();
         mode = 0; // Same block
       } else if (MDT.dominates(FirstInfo.DefInst->getParent(),
                              SecondInfo.DefInst->getParent())) {
         scan = SecondInfo.DefInst->getParent();
         mode = 1; // First dominates second
       } else {
         scan = FirstInfo.DefInst->getParent();
         mode = 2; // Second dominates first
       }

       // If there's an interference, we need to insert  copies
       if (interferes(FirstInfo, SecondInfo, scan, mode)) {
         // Insert copies for First
         for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
           if (P->getOperand(i-1).getReg() == p.first) {
             unsigned SrcReg = p.first;
             MachineBasicBlock* From = P->getOperand(i).getMBB();

             Waiting[From].insert(std::make_pair(SrcReg,
                                                 P->getOperand(0).getReg()));
             UsedByAnother.insert(SrcReg);

             PHIUnion.erase(SrcReg);
           }
         }
       }
     }

     // FIXME: Cache renaming information

     ProcessedNames.insert(PHIUnion.begin(), PHIUnion.end());
     ++P;
   }
 }

 /// processPHIUnion - Take a set of candidate registers to be coallesced when
 /// decomposing the PHI instruction.  Use the DominanceForest to remove the ones
 /// that are known to interfere, and flag others that need to be checked for
 /// local interferences.
 void StrongPHIElimination::processPHIUnion(MachineInstr* Inst,
                                            std::set<unsigned>& PHIUnion,
                         std::vector<StrongPHIElimination::DomForestNode*>& DF,
                         std::vector<std::pair<unsigned, unsigned> >& locals) {

   std::vector<DomForestNode*> worklist(DF.begin(), DF.end());
   SmallPtrSet<DomForestNode*, 4> visited;

   LiveVariables& LV = getAnalysis<LiveVariables>();
   unsigned DestReg = Inst->getOperand(0).getReg();

   // DF walk on the DomForest
   while (!worklist.empty()) {
     DomForestNode* DFNode = worklist.back();

     LiveVariables::VarInfo& Info = LV.getVarInfo(DFNode->getReg());
     visited.insert(DFNode);

     bool inserted = false;
     for (DomForestNode::iterator CI = DFNode->begin(), CE = DFNode->end();
          CI != CE; ++CI) {
       DomForestNode* child = *CI;
       LiveVariables::VarInfo& CInfo = LV.getVarInfo(child->getReg());

       if (isLiveOut(Info, CInfo.DefInst->getParent())) {
         // Insert copies for parent
         for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
           if (Inst->getOperand(i-1).getReg() == DFNode->getReg()) {
             unsigned SrcReg = DFNode->getReg();
             MachineBasicBlock* From = Inst->getOperand(i).getMBB();

             Waiting[From].insert(std::make_pair(SrcReg, DestReg));
             UsedByAnother.insert(SrcReg);

             PHIUnion.erase(SrcReg);
           }
         }
       } else if (isLiveIn(Info, CInfo.DefInst->getParent()) ||
                  Info.DefInst->getParent() == CInfo.DefInst->getParent()) {
         // Add (p, c) to possible local interferences
         locals.push_back(std::make_pair(DFNode->getReg(), child->getReg()));
       }

       if (!visited.count(child)) {
         worklist.push_back(child);
         inserted = true;
       }
     }

     if (!inserted) worklist.pop_back();
   }
 }

 /// ScheduleCopies - Insert copies into predecessor blocks, scheduling
 /// them properly so as to avoid the 'lost copy' and the 'virtual swap'
 /// problems.
 ///
 /// Based on "Practical Improvements to the Construction and Destruction
 /// of Static Single Assignment Form" by Briggs, et al.
 void StrongPHIElimination::ScheduleCopies(MachineBasicBlock* MBB,
                                           std::set<unsigned>& pushed) {
   std::map<unsigned, unsigned>& copy_set= Waiting[MBB];

   std::map<unsigned, unsigned> worklist;
   std::map<unsigned, unsigned> map;

   // Setup worklist of initial copies
   for (std::map<unsigned, unsigned>::iterator I = copy_set.begin(),
        E = copy_set.end(); I != E; ) {
     map.insert(std::make_pair(I->first, I->first));
     map.insert(std::make_pair(I->second, I->second));

     if (!UsedByAnother.count(I->first)) {
       worklist.insert(*I);

       // Avoid iterator invalidation
       unsigned first = I->first;
       ++I;
       copy_set.erase(first);
     } else {
       ++I;
     }
   }

   LiveVariables& LV = getAnalysis<LiveVariables>();

   // Iterate over the worklist, inserting copies
   while (!worklist.empty() || !copy_set.empty()) {
     while (!worklist.empty()) {
       std::pair<unsigned, unsigned> curr = *worklist.begin();
       worklist.erase(curr.first);

       if (isLiveOut(LV.getVarInfo(curr.second), MBB)) {
         // Insert copy from curr.second to a temporary
         // Push temporary on Stacks
         // Insert temporary in pushed
       }

       // Insert copy from map[curr.first] to curr.second
       map[curr.first] = curr.second;

       // If curr.first is a destination in copy_set...
       for (std::map<unsigned, unsigned>::iterator I = copy_set.begin(),
            E = copy_set.end(); I != E; )
         if (curr.first == I->second) {
           std::pair<unsigned, unsigned> temp = *I;

           // Avoid iterator invalidation
           ++I;
           copy_set.erase(temp.first);
           worklist.insert(temp);

           break;
         } else {
           ++I;
         }
     }

     if (!copy_set.empty()) {
       std::pair<unsigned, unsigned> curr = *copy_set.begin();
       copy_set.erase(curr.first);

       // Insert a copy from dest to a new temporary t at the end of b
       // map[curr.second] = t;

       worklist.insert(curr);
     }
   }
 }

 /// InsertCopies - insert copies into MBB and all of its successors
 void StrongPHIElimination::InsertCopies(MachineBasicBlock* MBB) {
   std::set<unsigned> pushed;

   // Rewrite register uses from Stacks
   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
       I != E; ++I)
     for (unsigned i = 0; i < I->getNumOperands(); ++i)
       if (I->getOperand(i).isRegister() &&
           Stacks[I->getOperand(i).getReg()].size()) {
         I->getOperand(i).setReg(Stacks[I->getOperand(i).getReg()].back());
       }

   // Schedule the copies for this block
   ScheduleCopies(MBB, pushed);

   // Recur to our successors
   for (GraphTraits<MachineBasicBlock*>::ChildIteratorType I =
        GraphTraits<MachineBasicBlock*>::child_begin(MBB), E =
        GraphTraits<MachineBasicBlock*>::child_end(MBB); I != E; ++I)
     InsertCopies(*I);

   // As we exit this block, pop the names we pushed while processing it
   for (std::set<unsigned>::iterator I = pushed.begin(),
        E = pushed.end(); I != E; ++I)
     Stacks[*I].pop_back();
 }

 bool StrongPHIElimination::runOnMachineFunction(MachineFunction &Fn) {
   // Compute DFS numbers of each block
   computeDFS(Fn);

   // Determine which phi node operands need copies
   for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
     if (!I->empty() &&
         I->begin()->getOpcode() == TargetInstrInfo::PHI)
       processBlock(I);

   // Insert copies
   // FIXME: This process should probably preserve LiveVariables
   InsertCopies(Fn.begin());

   // FIXME: Perform renaming

   return false;
 }
	//===- 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 machine instruction PHI nodes by inserting copy
	// instructions, using an intelligent copy-folding technique based on
	// dominator information. This is technique is 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.
	// DOI= http://doi.acm.org/10.1145/512529.512534
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "strongphielim"
	#include "llvm/CodeGen/Passes.h"
	#include "llvm/CodeGen/LiveVariables.h"
	#include "llvm/CodeGen/MachineDominators.h"
	#include "llvm/CodeGen/MachineFunctionPass.h"
	#include "llvm/CodeGen/MachineInstr.h"
	#include "llvm/CodeGen/SSARegMap.h"
	#include "llvm/Target/TargetInstrInfo.h"
	#include "llvm/Target/TargetMachine.h"
	#include "llvm/ADT/DepthFirstIterator.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Support/Compiler.h"
	using namespace llvm;


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

	DenseMap<MachineBasicBlock*,
	std::map<unsigned, unsigned> > Waiting;

	std::map<unsigned, std::vector<unsigned> > Stacks;
	std::set<unsigned> UsedByAnother;

	bool runOnMachineFunction(MachineFunction &Fn);

	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	AU.addPreserved<LiveVariables>();
	AU.addPreservedID(PHIEliminationID);
	AU.addRequired<MachineDominatorTree>();
	AU.addRequired<LiveVariables>();
	AU.setPreservesAll();
	MachineFunctionPass::getAnalysisUsage(AU);
	}

	virtual void releaseMemory() {
	preorder.clear();
	maxpreorder.clear();

	Waiting.clear();
	}

	private:
	struct DomForestNode {
	private:
	std::vector<DomForestNode*> children;
	unsigned reg;

	void addChild(DomForestNode* DFN) { children.push_back(DFN); }

	public:
	typedef std::vector<DomForestNode*>::iterator iterator;

	DomForestNode(unsigned r, DomForestNode* parent) : reg(r) {
	if (parent)
	parent->addChild(this);
	}

	~DomForestNode() {
	for (iterator I = begin(), E = end(); I != E; ++I)
	delete *I;
	}

	inline unsigned getReg() { return reg; }

	inline DomForestNode::iterator begin() { return children.begin(); }
	inline DomForestNode::iterator end() { return children.end(); }
	};

	DenseMap<MachineBasicBlock*, unsigned> preorder;
	DenseMap<MachineBasicBlock*, unsigned> maxpreorder;


	void computeDFS(MachineFunction& MF);
	void processBlock(MachineBasicBlock* MBB);

	std::vector<DomForestNode*> computeDomForest(std::set<unsigned>& instrs);
	void processPHIUnion(MachineInstr* Inst,
	std::set<unsigned>& PHIUnion,
	std::vector<StrongPHIElimination::DomForestNode*>& DF,
	std::vector<std::pair<unsigned, unsigned> >& locals);
	void ScheduleCopies(MachineBasicBlock* MBB, std::set<unsigned>& pushed);
	void InsertCopies(MachineBasicBlock* MBB);
	};

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

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

	/// computeDFS - Computes the DFS-in and DFS-out numbers of the dominator tree
	/// of the given MachineFunction. These numbers are then used in other parts
	/// of the PHI elimination process.
	void StrongPHIElimination::computeDFS(MachineFunction& MF) {
	SmallPtrSet<MachineDomTreeNode*, 8> frontier;
	SmallPtrSet<MachineDomTreeNode*, 8> visited;

	unsigned time = 0;

	MachineDominatorTree& DT = getAnalysis<MachineDominatorTree>();

	MachineDomTreeNode* node = DT.getRootNode();

	std::vector<MachineDomTreeNode*> worklist;
	worklist.push_back(node);

	while (!worklist.empty()) {
	MachineDomTreeNode* currNode = worklist.back();

	if (!frontier.count(currNode)) {
	frontier.insert(currNode);
	++time;
	preorder.insert(std::make_pair(currNode->getBlock(), time));
	}

	bool inserted = false;
	for (MachineDomTreeNode::iterator I = node->begin(), E = node->end();
	I != E; ++I)
	if (!frontier.count(I) && !visited.count(I)) {
	worklist.push_back(*I);
	inserted = true;
	break;
	}

	if (!inserted) {
	frontier.erase(currNode);
	visited.insert(currNode);
	maxpreorder.insert(std::make_pair(currNode->getBlock(), time));

	worklist.pop_back();
	}
	}
	}

	/// PreorderSorter - a helper class that is used to sort registers
	/// according to the preorder number of their defining blocks
	class PreorderSorter {
	private:
	DenseMap<MachineBasicBlock*, unsigned>& preorder;
	LiveVariables& LV;

	public:
	PreorderSorter(DenseMap<MachineBasicBlock*, unsigned>& p,
	LiveVariables& L) : preorder(p), LV(L) { }

	bool operator()(unsigned A, unsigned B) {
	if (A == B)
	return false;

	MachineBasicBlock* ABlock = LV.getVarInfo(A).DefInst->getParent();
	MachineBasicBlock* BBlock = LV.getVarInfo(A).DefInst->getParent();

	if (preorder[ABlock] < preorder[BBlock])
	return true;
	else if (preorder[ABlock] > preorder[BBlock])
	return false;

	assert(0 && "Error sorting by dominance!");
	return false;
	}
	};

	/// computeDomForest - compute the subforest of the DomTree corresponding
	/// to the defining blocks of the registers in question
	std::vector<StrongPHIElimination::DomForestNode*>
	StrongPHIElimination::computeDomForest(std::set<unsigned>& regs) {
	LiveVariables& LV = getAnalysis<LiveVariables>();

	DomForestNode* VirtualRoot = new DomForestNode(0, 0);
	maxpreorder.insert(std::make_pair((MachineBasicBlock*)0, ~0UL));

	std::vector<unsigned> worklist;
	worklist.reserve(regs.size());
	for (std::set<unsigned>::iterator I = regs.begin(), E = regs.end();
	I != E; ++I)
	worklist.push_back(*I);

	PreorderSorter PS(preorder, LV);
	std::sort(worklist.begin(), worklist.end(), PS);

	DomForestNode* CurrentParent = VirtualRoot;
	std::vector<DomForestNode*> stack;
	stack.push_back(VirtualRoot);

	for (std::vector<unsigned>::iterator I = worklist.begin(), E = worklist.end();
	I != E; ++I) {
	unsigned pre = preorder[LV.getVarInfo(*I).DefInst->getParent()];
	MachineBasicBlock* parentBlock =
	LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();

	while (pre > maxpreorder[parentBlock]) {
	stack.pop_back();
	CurrentParent = stack.back();

	parentBlock = LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
	}

	DomForestNode* child = new DomForestNode(*I, CurrentParent);
	stack.push_back(child);
	CurrentParent = child;
	}

	std::vector<DomForestNode*> ret;
	ret.insert(ret.end(), VirtualRoot->begin(), VirtualRoot->end());
	return ret;
	}

	/// isLiveIn - helper method that determines, from a VarInfo, if a register
	/// is live into a block
	bool isLiveIn(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
	if (V.AliveBlocks.test(MBB->getNumber()))
	return true;

	if (V.DefInst->getParent() != MBB &&
	V.UsedBlocks.test(MBB->getNumber()))
	return true;

	return false;
	}

	/// isLiveOut - help method that determines, from a VarInfo, if a register is
	/// live out of a block.
	bool isLiveOut(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
	if (MBB == V.DefInst->getParent() \|\|
	V.UsedBlocks.test(MBB->getNumber())) {
	for (std::vector<MachineInstr*>::iterator I = V.Kills.begin(),
	E = V.Kills.end(); I != E; ++I)
	if ((*I)->getParent() == MBB)
	return false;

	return true;
	}

	return false;
	}

	/// isKillInst - helper method that determines, from a VarInfo, if an
	/// instruction kills a given register
	bool isKillInst(LiveVariables::VarInfo& V, MachineInstr* MI) {
	return std::find(V.Kills.begin(), V.Kills.end(), MI) != V.Kills.end();
	}

	/// interferes - checks for local interferences by scanning a block. The only
	/// trick parameter is 'mode' which tells it the relationship of the two
	/// registers. 0 - defined in the same block, 1 - first properly dominates
	/// second, 2 - second properly dominates first
	bool interferes(LiveVariables::VarInfo& First, LiveVariables::VarInfo& Second,
	MachineBasicBlock* scan, unsigned mode) {
	MachineInstr* def = 0;
	MachineInstr* kill = 0;

	bool interference = false;

	// Wallk the block, checking for interferences
	for (MachineBasicBlock::iterator MBI = scan->begin(), MBE = scan->end();
	MBI != MBE; ++MBI) {
	MachineInstr* curr = MBI;

	// Same defining block...
	if (mode == 0) {
	if (curr == First.DefInst) {
	// If we find our first DefInst, save it
	if (!def) {
	def = curr;
	// If there's already an unkilled DefInst, then
	// this is an interference
	} else if (!kill) {
	interference = true;
	break;
	// If there's a DefInst followed by a KillInst, then
	// they can't interfere
	} else {
	interference = false;
	break;
	}
	// Symmetric with the above
	} else if (curr == Second.DefInst ) {
	if (!def) {
	def = curr;
	} else if (!kill) {
	interference = true;
	break;
	} else {
	interference = false;
	break;
	}
	// Store KillInsts if they match up with the DefInst
	} else if (isKillInst(First, curr)) {
	if (def == First.DefInst) {
	kill = curr;
	} else if (isKillInst(Second, curr)) {
	if (def == Second.DefInst) {
	kill = curr;
	}
	}
	}
	// First properly dominates second...
	} else if (mode == 1) {
	if (curr == Second.DefInst) {
	// DefInst of second without kill of first is an interference
	if (!kill) {
	interference = true;
	break;
	// DefInst after a kill is a non-interference
	} else {
	interference = false;
	break;
	}
	// Save KillInsts of First
	} else if (isKillInst(First, curr)) {
	kill = curr;
	}
	// Symmetric with the above
	} else if (mode == 2) {
	if (curr == First.DefInst) {
	if (!kill) {
	interference = true;
	break;
	} else {
	interference = false;
	break;
	}
	} else if (isKillInst(Second, curr)) {
	kill = curr;
	}
	}
	}

	return interference;
	}

	/// processBlock - Eliminate PHIs in the given block
	void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
	LiveVariables& LV = getAnalysis<LiveVariables>();

	// Holds names that have been added to a set in any PHI within this block
	// before the current one.
	std::set<unsigned> ProcessedNames;

	MachineBasicBlock::iterator P = MBB->begin();
	while (P->getOpcode() == TargetInstrInfo::PHI) {
	LiveVariables::VarInfo& PHIInfo = LV.getVarInfo(P->getOperand(0).getReg());

	unsigned DestReg = P->getOperand(0).getReg();

	// Hold the names that are currently in the candidate set.
	std::set<unsigned> PHIUnion;
	std::set<MachineBasicBlock*> UnionedBlocks;

	for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
	unsigned SrcReg = P->getOperand(i-1).getReg();
	LiveVariables::VarInfo& SrcInfo = LV.getVarInfo(SrcReg);

	// Check for trivial interferences
	if (isLiveIn(SrcInfo, P->getParent()) \|\|
	isLiveOut(PHIInfo, SrcInfo.DefInst->getParent()) \|\|
	( PHIInfo.DefInst->getOpcode() == TargetInstrInfo::PHI &&
	isLiveIn(PHIInfo, SrcInfo.DefInst->getParent()) ) \|\|
	ProcessedNames.count(SrcReg) \|\|
	UnionedBlocks.count(SrcInfo.DefInst->getParent())) {

	// add a copy from a_i to p in Waiting[From[a_i]]
	MachineBasicBlock* From = P->getOperand(i).getMachineBasicBlock();
	Waiting[From].insert(std::make_pair(SrcReg, DestReg));
	UsedByAnother.insert(SrcReg);
	} else {
	PHIUnion.insert(SrcReg);
	UnionedBlocks.insert(SrcInfo.DefInst->getParent());
	}
	}

	std::vector<StrongPHIElimination::DomForestNode*> DF =
	computeDomForest(PHIUnion);

	// Walk DomForest to resolve interferences
	std::vector<std::pair<unsigned, unsigned> > localInterferences;
	processPHIUnion(P, PHIUnion, DF, localInterferences);

	// Check for local interferences
	for (std::vector<std::pair<unsigned, unsigned> >::iterator I =
	localInterferences.begin(), E = localInterferences.end(); I != E; ++I) {
	std::pair<unsigned, unsigned> p = *I;

	LiveVariables::VarInfo& FirstInfo = LV.getVarInfo(p.first);
	LiveVariables::VarInfo& SecondInfo = LV.getVarInfo(p.second);

	MachineDominatorTree& MDT = getAnalysis<MachineDominatorTree>();

	// Determine the block we need to scan and the relationship between
	// the two registers
	MachineBasicBlock* scan = 0;
	unsigned mode = 0;
	if (FirstInfo.DefInst->getParent() == SecondInfo.DefInst->getParent()) {
	scan = FirstInfo.DefInst->getParent();
	mode = 0; // Same block
	} else if (MDT.dominates(FirstInfo.DefInst->getParent(),
	SecondInfo.DefInst->getParent())) {
	scan = SecondInfo.DefInst->getParent();
	mode = 1; // First dominates second
	} else {
	scan = FirstInfo.DefInst->getParent();
	mode = 2; // Second dominates first
	}

	// If there's an interference, we need to insert copies
	if (interferes(FirstInfo, SecondInfo, scan, mode)) {
	// Insert copies for First
	for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
	if (P->getOperand(i-1).getReg() == p.first) {
	unsigned SrcReg = p.first;
	MachineBasicBlock* From = P->getOperand(i).getMBB();

	Waiting[From].insert(std::make_pair(SrcReg,
	P->getOperand(0).getReg()));
	UsedByAnother.insert(SrcReg);

	PHIUnion.erase(SrcReg);
	}
	}
	}
	}

	// FIXME: Cache renaming information

	ProcessedNames.insert(PHIUnion.begin(), PHIUnion.end());
	++P;
	}
	}

	/// processPHIUnion - Take a set of candidate registers to be coallesced when
	/// decomposing the PHI instruction. Use the DominanceForest to remove the ones
	/// that are known to interfere, and flag others that need to be checked for
	/// local interferences.
	void StrongPHIElimination::processPHIUnion(MachineInstr* Inst,
	std::set<unsigned>& PHIUnion,
	std::vector<StrongPHIElimination::DomForestNode*>& DF,
	std::vector<std::pair<unsigned, unsigned> >& locals) {

	std::vector<DomForestNode*> worklist(DF.begin(), DF.end());
	SmallPtrSet<DomForestNode*, 4> visited;

	LiveVariables& LV = getAnalysis<LiveVariables>();
	unsigned DestReg = Inst->getOperand(0).getReg();

	// DF walk on the DomForest
	while (!worklist.empty()) {
	DomForestNode* DFNode = worklist.back();

	LiveVariables::VarInfo& Info = LV.getVarInfo(DFNode->getReg());
	visited.insert(DFNode);

	bool inserted = false;
	for (DomForestNode::iterator CI = DFNode->begin(), CE = DFNode->end();
	CI != CE; ++CI) {
	DomForestNode* child = *CI;
	LiveVariables::VarInfo& CInfo = LV.getVarInfo(child->getReg());

	if (isLiveOut(Info, CInfo.DefInst->getParent())) {
	// Insert copies for parent
	for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
	if (Inst->getOperand(i-1).getReg() == DFNode->getReg()) {
	unsigned SrcReg = DFNode->getReg();
	MachineBasicBlock* From = Inst->getOperand(i).getMBB();

	Waiting[From].insert(std::make_pair(SrcReg, DestReg));
	UsedByAnother.insert(SrcReg);

	PHIUnion.erase(SrcReg);
	}
	}
	} else if (isLiveIn(Info, CInfo.DefInst->getParent()) \|\|
	Info.DefInst->getParent() == CInfo.DefInst->getParent()) {
	// Add (p, c) to possible local interferences
	locals.push_back(std::make_pair(DFNode->getReg(), child->getReg()));
	}

	if (!visited.count(child)) {
	worklist.push_back(child);
	inserted = true;
	}
	}

	if (!inserted) worklist.pop_back();
	}
	}

	/// ScheduleCopies - Insert copies into predecessor blocks, scheduling
	/// them properly so as to avoid the 'lost copy' and the 'virtual swap'
	/// problems.
	///
	/// Based on "Practical Improvements to the Construction and Destruction
	/// of Static Single Assignment Form" by Briggs, et al.
	void StrongPHIElimination::ScheduleCopies(MachineBasicBlock* MBB,
	std::set<unsigned>& pushed) {
	std::map<unsigned, unsigned>& copy_set= Waiting[MBB];

	std::map<unsigned, unsigned> worklist;
	std::map<unsigned, unsigned> map;

	// Setup worklist of initial copies
	for (std::map<unsigned, unsigned>::iterator I = copy_set.begin(),
	E = copy_set.end(); I != E; ) {
	map.insert(std::make_pair(I->first, I->first));
	map.insert(std::make_pair(I->second, I->second));

	if (!UsedByAnother.count(I->first)) {
	worklist.insert(*I);

	// Avoid iterator invalidation
	unsigned first = I->first;
	++I;
	copy_set.erase(first);
	} else {
	++I;
	}
	}

	LiveVariables& LV = getAnalysis<LiveVariables>();

	// Iterate over the worklist, inserting copies
	while (!worklist.empty() \|\| !copy_set.empty()) {
	while (!worklist.empty()) {
	std::pair<unsigned, unsigned> curr = *worklist.begin();
	worklist.erase(curr.first);

	if (isLiveOut(LV.getVarInfo(curr.second), MBB)) {
	// Insert copy from curr.second to a temporary
	// Push temporary on Stacks
	// Insert temporary in pushed
	}

	// Insert copy from map[curr.first] to curr.second
	map[curr.first] = curr.second;

	// If curr.first is a destination in copy_set...
	for (std::map<unsigned, unsigned>::iterator I = copy_set.begin(),
	E = copy_set.end(); I != E; )
	if (curr.first == I->second) {
	std::pair<unsigned, unsigned> temp = *I;

	// Avoid iterator invalidation
	++I;
	copy_set.erase(temp.first);
	worklist.insert(temp);

	break;
	} else {
	++I;
	}
	}

	if (!copy_set.empty()) {
	std::pair<unsigned, unsigned> curr = *copy_set.begin();
	copy_set.erase(curr.first);

	// Insert a copy from dest to a new temporary t at the end of b
	// map[curr.second] = t;

	worklist.insert(curr);
	}
	}
	}

	/// InsertCopies - insert copies into MBB and all of its successors
	void StrongPHIElimination::InsertCopies(MachineBasicBlock* MBB) {
	std::set<unsigned> pushed;

	// Rewrite register uses from Stacks
	for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
	I != E; ++I)
	for (unsigned i = 0; i < I->getNumOperands(); ++i)
	if (I->getOperand(i).isRegister() &&
	Stacks[I->getOperand(i).getReg()].size()) {
	I->getOperand(i).setReg(Stacks[I->getOperand(i).getReg()].back());
	}

	// Schedule the copies for this block
	ScheduleCopies(MBB, pushed);

	// Recur to our successors
	for (GraphTraits<MachineBasicBlock*>::ChildIteratorType I =
	GraphTraits<MachineBasicBlock*>::child_begin(MBB), E =
	GraphTraits<MachineBasicBlock*>::child_end(MBB); I != E; ++I)
	InsertCopies(*I);

	// As we exit this block, pop the names we pushed while processing it
	for (std::set<unsigned>::iterator I = pushed.begin(),
	E = pushed.end(); I != E; ++I)
	Stacks[*I].pop_back();
	}

	bool StrongPHIElimination::runOnMachineFunction(MachineFunction &Fn) {
	// Compute DFS numbers of each block
	computeDFS(Fn);

	// Determine which phi node operands need copies
	for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
	if (!I->empty() &&
	I->begin()->getOpcode() == TargetInstrInfo::PHI)
	processBlock(I);

	// Insert copies
	// FIXME: This process should probably preserve LiveVariables
	InsertCopies(Fn.begin());

	// FIXME: Perform renaming

	return false;
	}