llvm/lib/CodeGen/LiveRangeCalc.cpp - toolchain/llvm-project - Gitiles

 //===---- LiveRangeCalc.cpp - Calculate live ranges -----------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // Implementation of the LiveRangeCalc class.
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "regalloc"
 #include "LiveRangeCalc.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"

 using namespace llvm;

 void LiveRangeCalc::reset(const MachineFunction *mf,
                           SlotIndexes *SI,
                           MachineDominatorTree *MDT,
                           VNInfo::Allocator *VNIA) {
   MF = mf;
   MRI = &MF->getRegInfo();
   Indexes = SI;
   DomTree = MDT;
   Alloc = VNIA;

   unsigned N = MF->getNumBlockIDs();
   Seen.clear();
   Seen.resize(N);
   LiveOut.resize(N);
   LiveIn.clear();
 }


 void LiveRangeCalc::createDeadDefs(LiveRange &LR, unsigned Reg) {
   assert(MRI && Indexes && "call reset() first");

   // Visit all def operands. If the same instruction has multiple defs of Reg,
   // LR.createDeadDef() will deduplicate.
   for (MachineRegisterInfo::def_iterator
        I = MRI->def_begin(Reg), E = MRI->def_end(); I != E; ++I) {
     const MachineInstr *MI = &*I;
     // Find the corresponding slot index.
     SlotIndex Idx;
     if (MI->isPHI())
       // PHI defs begin at the basic block start index.
       Idx = Indexes->getMBBStartIdx(MI->getParent());
     else
       // Instructions are either normal 'r', or early clobber 'e'.
       Idx = Indexes->getInstructionIndex(MI)
         .getRegSlot(I.getOperand().isEarlyClobber());

     // Create the def in LR. This may find an existing def.
     LR.createDeadDef(Idx, *Alloc);
   }
 }


 void LiveRangeCalc::extendToUses(LiveRange &LR, unsigned Reg) {
   assert(MRI && Indexes && "call reset() first");

   // Visit all operands that read Reg. This may include partial defs.
   for (MachineRegisterInfo::reg_nodbg_iterator I = MRI->reg_nodbg_begin(Reg),
        E = MRI->reg_nodbg_end(); I != E; ++I) {
     MachineOperand &MO = I.getOperand();
     // Clear all kill flags. They will be reinserted after register allocation
     // by LiveIntervalAnalysis::addKillFlags().
     if (MO.isUse())
       MO.setIsKill(false);
     if (!MO.readsReg())
       continue;
     // MI is reading Reg. We may have visited MI before if it happens to be
     // reading Reg multiple times. That is OK, extend() is idempotent.
     const MachineInstr *MI = &*I;

     // Find the SlotIndex being read.
     SlotIndex Idx;
     if (MI->isPHI()) {
       assert(!MO.isDef() && "Cannot handle PHI def of partial register.");
       // PHI operands are paired: (Reg, PredMBB).
       // Extend the live range to be live-out from PredMBB.
       Idx = Indexes->getMBBEndIdx(MI->getOperand(I.getOperandNo()+1).getMBB());
     } else {
       // This is a normal instruction.
       Idx = Indexes->getInstructionIndex(MI).getRegSlot();
       // Check for early-clobber redefs.
       unsigned DefIdx;
       if (MO.isDef()) {
         if (MO.isEarlyClobber())
           Idx = Idx.getRegSlot(true);
       } else if (MI->isRegTiedToDefOperand(I.getOperandNo(), &DefIdx)) {
         // FIXME: This would be a lot easier if tied early-clobber uses also
         // had an early-clobber flag.
         if (MI->getOperand(DefIdx).isEarlyClobber())
           Idx = Idx.getRegSlot(true);
       }
     }
     extend(LR, Idx, Reg);
   }
 }


 // Transfer information from the LiveIn vector to the live ranges.
 void LiveRangeCalc::updateLiveIns() {
   LiveRangeUpdater Updater;
   for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
          E = LiveIn.end(); I != E; ++I) {
     if (!I->DomNode)
       continue;
     MachineBasicBlock *MBB = I->DomNode->getBlock();
     assert(I->Value && "No live-in value found");
     SlotIndex Start, End;
     tie(Start, End) = Indexes->getMBBRange(MBB);

     if (I->Kill.isValid())
       // Value is killed inside this block.
       End = I->Kill;
     else {
       // The value is live-through, update LiveOut as well.
       // Defer the Domtree lookup until it is needed.
       assert(Seen.test(MBB->getNumber()));
       LiveOut[MBB] = LiveOutPair(I->Value, (MachineDomTreeNode *)0);
     }
     Updater.setDest(&I->LR);
     Updater.add(Start, End, I->Value);
   }
   LiveIn.clear();
 }


 void LiveRangeCalc::extend(LiveRange &LR, SlotIndex Kill, unsigned PhysReg) {
   assert(Kill.isValid() && "Invalid SlotIndex");
   assert(Indexes && "Missing SlotIndexes");
   assert(DomTree && "Missing dominator tree");

   MachineBasicBlock *KillMBB = Indexes->getMBBFromIndex(Kill.getPrevSlot());
   assert(KillMBB && "No MBB at Kill");

   // Is there a def in the same MBB we can extend?
   if (LR.extendInBlock(Indexes->getMBBStartIdx(KillMBB), Kill))
     return;

   // Find the single reaching def, or determine if Kill is jointly dominated by
   // multiple values, and we may need to create even more phi-defs to preserve
   // VNInfo SSA form.  Perform a search for all predecessor blocks where we
   // know the dominating VNInfo.
   if (findReachingDefs(LR, *KillMBB, Kill, PhysReg))
     return;

   // When there were multiple different values, we may need new PHIs.
   calculateValues();
 }


 // This function is called by a client after using the low-level API to add
 // live-out and live-in blocks.  The unique value optimization is not
 // available, SplitEditor::transferValues handles that case directly anyway.
 void LiveRangeCalc::calculateValues() {
   assert(Indexes && "Missing SlotIndexes");
   assert(DomTree && "Missing dominator tree");
   updateSSA();
   updateLiveIns();
 }


 bool LiveRangeCalc::findReachingDefs(LiveRange &LR, MachineBasicBlock &KillMBB,
                                      SlotIndex Kill, unsigned PhysReg) {
   unsigned KillMBBNum = KillMBB.getNumber();

   // Block numbers where LR should be live-in.
   SmallVector<unsigned, 16> WorkList(1, KillMBBNum);

   // Remember if we have seen more than one value.
   bool UniqueVNI = true;
   VNInfo *TheVNI = 0;

   // Using Seen as a visited set, perform a BFS for all reaching defs.
   for (unsigned i = 0; i != WorkList.size(); ++i) {
     MachineBasicBlock *MBB = MF->getBlockNumbered(WorkList[i]);

 #ifndef NDEBUG
     if (MBB->pred_empty()) {
       MBB->getParent()->verify();
       llvm_unreachable("Use not jointly dominated by defs.");
     }

     if (TargetRegisterInfo::isPhysicalRegister(PhysReg) &&
         !MBB->isLiveIn(PhysReg)) {
       MBB->getParent()->verify();
       errs() << "The register needs to be live in to BB#" << MBB->getNumber()
              << ", but is missing from the live-in list.\n";
       llvm_unreachable("Invalid global physical register");
     }
 #endif

     for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
          PE = MBB->pred_end(); PI != PE; ++PI) {
        MachineBasicBlock *Pred = *PI;

        // Is this a known live-out block?
        if (Seen.test(Pred->getNumber())) {
          if (VNInfo *VNI = LiveOut[Pred].first) {
            if (TheVNI && TheVNI != VNI)
              UniqueVNI = false;
            TheVNI = VNI;
          }
          continue;
        }

        SlotIndex Start, End;
        tie(Start, End) = Indexes->getMBBRange(Pred);

        // First time we see Pred.  Try to determine the live-out value, but set
        // it as null if Pred is live-through with an unknown value.
        VNInfo *VNI = LR.extendInBlock(Start, End);
        setLiveOutValue(Pred, VNI);
        if (VNI) {
          if (TheVNI && TheVNI != VNI)
            UniqueVNI = false;
          TheVNI = VNI;
          continue;
        }

        // No, we need a live-in value for Pred as well
        if (Pred != &KillMBB)
           WorkList.push_back(Pred->getNumber());
        else
           // Loopback to KillMBB, so value is really live through.
          Kill = SlotIndex();
     }
   }

   LiveIn.clear();

   // Both updateSSA() and LiveRangeUpdater benefit from ordered blocks, but
   // neither require it. Skip the sorting overhead for small updates.
   if (WorkList.size() > 4)
     array_pod_sort(WorkList.begin(), WorkList.end());

   // If a unique reaching def was found, blit in the live ranges immediately.
   if (UniqueVNI) {
     LiveRangeUpdater Updater(&LR);
     for (SmallVectorImpl<unsigned>::const_iterator I = WorkList.begin(),
          E = WorkList.end(); I != E; ++I) {
        SlotIndex Start, End;
        tie(Start, End) = Indexes->getMBBRange(*I);
        // Trim the live range in KillMBB.
        if (*I == KillMBBNum && Kill.isValid())
          End = Kill;
        else
          LiveOut[MF->getBlockNumbered(*I)] =
            LiveOutPair(TheVNI, (MachineDomTreeNode *)0);
        Updater.add(Start, End, TheVNI);
     }
     return true;
   }

   // Multiple values were found, so transfer the work list to the LiveIn array
   // where UpdateSSA will use it as a work list.
   LiveIn.reserve(WorkList.size());
   for (SmallVectorImpl<unsigned>::const_iterator
        I = WorkList.begin(), E = WorkList.end(); I != E; ++I) {
     MachineBasicBlock *MBB = MF->getBlockNumbered(*I);
     addLiveInBlock(LR, DomTree->getNode(MBB));
     if (MBB == &KillMBB)
       LiveIn.back().Kill = Kill;
   }

   return false;
 }


 // This is essentially the same iterative algorithm that SSAUpdater uses,
 // except we already have a dominator tree, so we don't have to recompute it.
 void LiveRangeCalc::updateSSA() {
   assert(Indexes && "Missing SlotIndexes");
   assert(DomTree && "Missing dominator tree");

   // Interate until convergence.
   unsigned Changes;
   do {
     Changes = 0;
     // Propagate live-out values down the dominator tree, inserting phi-defs
     // when necessary.
     for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
            E = LiveIn.end(); I != E; ++I) {
       MachineDomTreeNode *Node = I->DomNode;
       // Skip block if the live-in value has already been determined.
       if (!Node)
         continue;
       MachineBasicBlock *MBB = Node->getBlock();
       MachineDomTreeNode *IDom = Node->getIDom();
       LiveOutPair IDomValue;

       // We need a live-in value to a block with no immediate dominator?
       // This is probably an unreachable block that has survived somehow.
       bool needPHI = !IDom || !Seen.test(IDom->getBlock()->getNumber());

       // IDom dominates all of our predecessors, but it may not be their
       // immediate dominator. Check if any of them have live-out values that are
       // properly dominated by IDom. If so, we need a phi-def here.
       if (!needPHI) {
         IDomValue = LiveOut[IDom->getBlock()];

         // Cache the DomTree node that defined the value.
         if (IDomValue.first && !IDomValue.second)
           LiveOut[IDom->getBlock()].second = IDomValue.second =
             DomTree->getNode(Indexes->getMBBFromIndex(IDomValue.first->def));

         for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
                PE = MBB->pred_end(); PI != PE; ++PI) {
           LiveOutPair &Value = LiveOut[*PI];
           if (!Value.first || Value.first == IDomValue.first)
             continue;

           // Cache the DomTree node that defined the value.
           if (!Value.second)
             Value.second =
               DomTree->getNode(Indexes->getMBBFromIndex(Value.first->def));

           // This predecessor is carrying something other than IDomValue.
           // It could be because IDomValue hasn't propagated yet, or it could be
           // because MBB is in the dominance frontier of that value.
           if (DomTree->dominates(IDom, Value.second)) {
             needPHI = true;
             break;
           }
         }
       }

       // The value may be live-through even if Kill is set, as can happen when
       // we are called from extendRange. In that case LiveOutSeen is true, and
       // LiveOut indicates a foreign or missing value.
       LiveOutPair &LOP = LiveOut[MBB];

       // Create a phi-def if required.
       if (needPHI) {
         ++Changes;
         assert(Alloc && "Need VNInfo allocator to create PHI-defs");
         SlotIndex Start, End;
         tie(Start, End) = Indexes->getMBBRange(MBB);
         LiveRange &LR = I->LR;
         VNInfo *VNI = LR.getNextValue(Start, *Alloc);
         I->Value = VNI;
         // This block is done, we know the final value.
         I->DomNode = 0;

         // Add liveness since updateLiveIns now skips this node.
         if (I->Kill.isValid())
           LR.addSegment(LiveInterval::Segment(Start, I->Kill, VNI));
         else {
           LR.addSegment(LiveInterval::Segment(Start, End, VNI));
           LOP = LiveOutPair(VNI, Node);
         }
       } else if (IDomValue.first) {
         // No phi-def here. Remember incoming value.
         I->Value = IDomValue.first;

         // If the IDomValue is killed in the block, don't propagate through.
         if (I->Kill.isValid())
           continue;

         // Propagate IDomValue if it isn't killed:
         // MBB is live-out and doesn't define its own value.
         if (LOP.first == IDomValue.first)
           continue;
         ++Changes;
         LOP = IDomValue;
       }
     }
   } while (Changes);
 }
	//===---- LiveRangeCalc.cpp - Calculate live ranges -----------------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// Implementation of the LiveRangeCalc class.
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "regalloc"
	#include "LiveRangeCalc.h"
	#include "llvm/CodeGen/MachineDominators.h"
	#include "llvm/CodeGen/MachineRegisterInfo.h"

	using namespace llvm;

	void LiveRangeCalc::reset(const MachineFunction *mf,
	SlotIndexes *SI,
	MachineDominatorTree *MDT,
	VNInfo::Allocator *VNIA) {
	MF = mf;
	MRI = &MF->getRegInfo();
	Indexes = SI;
	DomTree = MDT;
	Alloc = VNIA;

	unsigned N = MF->getNumBlockIDs();
	Seen.clear();
	Seen.resize(N);
	LiveOut.resize(N);
	LiveIn.clear();
	}


	void LiveRangeCalc::createDeadDefs(LiveRange &LR, unsigned Reg) {
	assert(MRI && Indexes && "call reset() first");

	// Visit all def operands. If the same instruction has multiple defs of Reg,
	// LR.createDeadDef() will deduplicate.
	for (MachineRegisterInfo::def_iterator
	I = MRI->def_begin(Reg), E = MRI->def_end(); I != E; ++I) {
	const MachineInstr MI = &I;
	// Find the corresponding slot index.
	SlotIndex Idx;
	if (MI->isPHI())
	// PHI defs begin at the basic block start index.
	Idx = Indexes->getMBBStartIdx(MI->getParent());
	else
	// Instructions are either normal 'r', or early clobber 'e'.
	Idx = Indexes->getInstructionIndex(MI)
	.getRegSlot(I.getOperand().isEarlyClobber());

	// Create the def in LR. This may find an existing def.
	LR.createDeadDef(Idx, *Alloc);
	}
	}


	void LiveRangeCalc::extendToUses(LiveRange &LR, unsigned Reg) {
	assert(MRI && Indexes && "call reset() first");

	// Visit all operands that read Reg. This may include partial defs.
	for (MachineRegisterInfo::reg_nodbg_iterator I = MRI->reg_nodbg_begin(Reg),
	E = MRI->reg_nodbg_end(); I != E; ++I) {
	MachineOperand &MO = I.getOperand();
	// Clear all kill flags. They will be reinserted after register allocation
	// by LiveIntervalAnalysis::addKillFlags().
	if (MO.isUse())
	MO.setIsKill(false);
	if (!MO.readsReg())
	continue;
	// MI is reading Reg. We may have visited MI before if it happens to be
	// reading Reg multiple times. That is OK, extend() is idempotent.
	const MachineInstr MI = &I;

	// Find the SlotIndex being read.
	SlotIndex Idx;
	if (MI->isPHI()) {
	assert(!MO.isDef() && "Cannot handle PHI def of partial register.");
	// PHI operands are paired: (Reg, PredMBB).
	// Extend the live range to be live-out from PredMBB.
	Idx = Indexes->getMBBEndIdx(MI->getOperand(I.getOperandNo()+1).getMBB());
	} else {
	// This is a normal instruction.
	Idx = Indexes->getInstructionIndex(MI).getRegSlot();
	// Check for early-clobber redefs.
	unsigned DefIdx;
	if (MO.isDef()) {
	if (MO.isEarlyClobber())
	Idx = Idx.getRegSlot(true);
	} else if (MI->isRegTiedToDefOperand(I.getOperandNo(), &DefIdx)) {
	// FIXME: This would be a lot easier if tied early-clobber uses also
	// had an early-clobber flag.
	if (MI->getOperand(DefIdx).isEarlyClobber())
	Idx = Idx.getRegSlot(true);
	}
	}
	extend(LR, Idx, Reg);
	}
	}


	// Transfer information from the LiveIn vector to the live ranges.
	void LiveRangeCalc::updateLiveIns() {
	LiveRangeUpdater Updater;
	for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
	E = LiveIn.end(); I != E; ++I) {
	if (!I->DomNode)
	continue;
	MachineBasicBlock *MBB = I->DomNode->getBlock();
	assert(I->Value && "No live-in value found");
	SlotIndex Start, End;
	tie(Start, End) = Indexes->getMBBRange(MBB);

	if (I->Kill.isValid())
	// Value is killed inside this block.
	End = I->Kill;
	else {
	// The value is live-through, update LiveOut as well.
	// Defer the Domtree lookup until it is needed.
	assert(Seen.test(MBB->getNumber()));
	LiveOut[MBB] = LiveOutPair(I->Value, (MachineDomTreeNode *)0);
	}
	Updater.setDest(&I->LR);
	Updater.add(Start, End, I->Value);
	}
	LiveIn.clear();
	}


	void LiveRangeCalc::extend(LiveRange &LR, SlotIndex Kill, unsigned PhysReg) {
	assert(Kill.isValid() && "Invalid SlotIndex");
	assert(Indexes && "Missing SlotIndexes");
	assert(DomTree && "Missing dominator tree");

	MachineBasicBlock *KillMBB = Indexes->getMBBFromIndex(Kill.getPrevSlot());
	assert(KillMBB && "No MBB at Kill");

	// Is there a def in the same MBB we can extend?
	if (LR.extendInBlock(Indexes->getMBBStartIdx(KillMBB), Kill))
	return;

	// Find the single reaching def, or determine if Kill is jointly dominated by
	// multiple values, and we may need to create even more phi-defs to preserve
	// VNInfo SSA form. Perform a search for all predecessor blocks where we
	// know the dominating VNInfo.
	if (findReachingDefs(LR, *KillMBB, Kill, PhysReg))
	return;

	// When there were multiple different values, we may need new PHIs.
	calculateValues();
	}


	// This function is called by a client after using the low-level API to add
	// live-out and live-in blocks. The unique value optimization is not
	// available, SplitEditor::transferValues handles that case directly anyway.
	void LiveRangeCalc::calculateValues() {
	assert(Indexes && "Missing SlotIndexes");
	assert(DomTree && "Missing dominator tree");
	updateSSA();
	updateLiveIns();
	}


	bool LiveRangeCalc::findReachingDefs(LiveRange &LR, MachineBasicBlock &KillMBB,
	SlotIndex Kill, unsigned PhysReg) {
	unsigned KillMBBNum = KillMBB.getNumber();

	// Block numbers where LR should be live-in.
	SmallVector<unsigned, 16> WorkList(1, KillMBBNum);

	// Remember if we have seen more than one value.
	bool UniqueVNI = true;
	VNInfo *TheVNI = 0;

	// Using Seen as a visited set, perform a BFS for all reaching defs.
	for (unsigned i = 0; i != WorkList.size(); ++i) {
	MachineBasicBlock *MBB = MF->getBlockNumbered(WorkList[i]);

	#ifndef NDEBUG
	if (MBB->pred_empty()) {
	MBB->getParent()->verify();
	llvm_unreachable("Use not jointly dominated by defs.");
	}

	if (TargetRegisterInfo::isPhysicalRegister(PhysReg) &&
	!MBB->isLiveIn(PhysReg)) {
	MBB->getParent()->verify();
	errs() << "The register needs to be live in to BB#" << MBB->getNumber()
	<< ", but is missing from the live-in list.\n";
	llvm_unreachable("Invalid global physical register");
	}
	#endif

	for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
	PE = MBB->pred_end(); PI != PE; ++PI) {
	MachineBasicBlock Pred = PI;

	// Is this a known live-out block?
	if (Seen.test(Pred->getNumber())) {
	if (VNInfo *VNI = LiveOut[Pred].first) {
	if (TheVNI && TheVNI != VNI)
	UniqueVNI = false;
	TheVNI = VNI;
	}
	continue;
	}

	SlotIndex Start, End;
	tie(Start, End) = Indexes->getMBBRange(Pred);

	// First time we see Pred. Try to determine the live-out value, but set
	// it as null if Pred is live-through with an unknown value.
	VNInfo *VNI = LR.extendInBlock(Start, End);
	setLiveOutValue(Pred, VNI);
	if (VNI) {
	if (TheVNI && TheVNI != VNI)
	UniqueVNI = false;
	TheVNI = VNI;
	continue;
	}

	// No, we need a live-in value for Pred as well
	if (Pred != &KillMBB)
	WorkList.push_back(Pred->getNumber());
	else
	// Loopback to KillMBB, so value is really live through.
	Kill = SlotIndex();
	}
	}

	LiveIn.clear();

	// Both updateSSA() and LiveRangeUpdater benefit from ordered blocks, but
	// neither require it. Skip the sorting overhead for small updates.
	if (WorkList.size() > 4)
	array_pod_sort(WorkList.begin(), WorkList.end());

	// If a unique reaching def was found, blit in the live ranges immediately.
	if (UniqueVNI) {
	LiveRangeUpdater Updater(&LR);
	for (SmallVectorImpl<unsigned>::const_iterator I = WorkList.begin(),
	E = WorkList.end(); I != E; ++I) {
	SlotIndex Start, End;
	tie(Start, End) = Indexes->getMBBRange(*I);
	// Trim the live range in KillMBB.
	if (*I == KillMBBNum && Kill.isValid())
	End = Kill;
	else
	LiveOut[MF->getBlockNumbered(*I)] =
	LiveOutPair(TheVNI, (MachineDomTreeNode *)0);
	Updater.add(Start, End, TheVNI);
	}
	return true;
	}

	// Multiple values were found, so transfer the work list to the LiveIn array
	// where UpdateSSA will use it as a work list.
	LiveIn.reserve(WorkList.size());
	for (SmallVectorImpl<unsigned>::const_iterator
	I = WorkList.begin(), E = WorkList.end(); I != E; ++I) {
	MachineBasicBlock MBB = MF->getBlockNumbered(I);
	addLiveInBlock(LR, DomTree->getNode(MBB));
	if (MBB == &KillMBB)
	LiveIn.back().Kill = Kill;
	}

	return false;
	}


	// This is essentially the same iterative algorithm that SSAUpdater uses,
	// except we already have a dominator tree, so we don't have to recompute it.
	void LiveRangeCalc::updateSSA() {
	assert(Indexes && "Missing SlotIndexes");
	assert(DomTree && "Missing dominator tree");

	// Interate until convergence.
	unsigned Changes;
	do {
	Changes = 0;
	// Propagate live-out values down the dominator tree, inserting phi-defs
	// when necessary.
	for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
	E = LiveIn.end(); I != E; ++I) {
	MachineDomTreeNode *Node = I->DomNode;
	// Skip block if the live-in value has already been determined.
	if (!Node)
	continue;
	MachineBasicBlock *MBB = Node->getBlock();
	MachineDomTreeNode *IDom = Node->getIDom();
	LiveOutPair IDomValue;

	// We need a live-in value to a block with no immediate dominator?
	// This is probably an unreachable block that has survived somehow.
	bool needPHI = !IDom \|\| !Seen.test(IDom->getBlock()->getNumber());

	// IDom dominates all of our predecessors, but it may not be their
	// immediate dominator. Check if any of them have live-out values that are
	// properly dominated by IDom. If so, we need a phi-def here.
	if (!needPHI) {
	IDomValue = LiveOut[IDom->getBlock()];

	// Cache the DomTree node that defined the value.
	if (IDomValue.first && !IDomValue.second)
	LiveOut[IDom->getBlock()].second = IDomValue.second =
	DomTree->getNode(Indexes->getMBBFromIndex(IDomValue.first->def));

	for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
	PE = MBB->pred_end(); PI != PE; ++PI) {
	LiveOutPair &Value = LiveOut[*PI];
	if (!Value.first \|\| Value.first == IDomValue.first)
	continue;

	// Cache the DomTree node that defined the value.
	if (!Value.second)
	Value.second =
	DomTree->getNode(Indexes->getMBBFromIndex(Value.first->def));

	// This predecessor is carrying something other than IDomValue.
	// It could be because IDomValue hasn't propagated yet, or it could be
	// because MBB is in the dominance frontier of that value.
	if (DomTree->dominates(IDom, Value.second)) {
	needPHI = true;
	break;
	}
	}
	}

	// The value may be live-through even if Kill is set, as can happen when
	// we are called from extendRange. In that case LiveOutSeen is true, and
	// LiveOut indicates a foreign or missing value.
	LiveOutPair &LOP = LiveOut[MBB];

	// Create a phi-def if required.
	if (needPHI) {
	++Changes;
	assert(Alloc && "Need VNInfo allocator to create PHI-defs");
	SlotIndex Start, End;
	tie(Start, End) = Indexes->getMBBRange(MBB);
	LiveRange &LR = I->LR;
	VNInfo VNI = LR.getNextValue(Start, Alloc);
	I->Value = VNI;
	// This block is done, we know the final value.
	I->DomNode = 0;

	// Add liveness since updateLiveIns now skips this node.
	if (I->Kill.isValid())
	LR.addSegment(LiveInterval::Segment(Start, I->Kill, VNI));
	else {
	LR.addSegment(LiveInterval::Segment(Start, End, VNI));
	LOP = LiveOutPair(VNI, Node);
	}
	} else if (IDomValue.first) {
	// No phi-def here. Remember incoming value.
	I->Value = IDomValue.first;

	// If the IDomValue is killed in the block, don't propagate through.
	if (I->Kill.isValid())
	continue;

	// Propagate IDomValue if it isn't killed:
	// MBB is live-out and doesn't define its own value.
	if (LOP.first == IDomValue.first)
	continue;
	++Changes;
	LOP = IDomValue;
	}
	}
	} while (Changes);
	}