llvm/lib/CodeGen/LiveRangeCalc.cpp - rust-lang/llvm-project - Git at Google

 //===- LiveRangeCalc.cpp - Calculate live ranges -------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Implementation of the LiveRangeCalc class.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/CodeGen/LiveRangeCalc.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/CodeGen/LiveInterval.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/SlotIndexes.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
 #include <iterator>
 #include <tuple>
 #include <utility>

 using namespace llvm;

 #define DEBUG_TYPE "regalloc"

 // Reserve an address that indicates a value that is known to be "undef".
 static VNInfo UndefVNI(0xbad, SlotIndex());

 void LiveRangeCalc::resetLiveOutMap() {
   unsigned NumBlocks = MF->getNumBlockIDs();
   Seen.clear();
   Seen.resize(NumBlocks);
   EntryInfos.clear();
   Map.resize(NumBlocks);
 }

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

 void LiveRangeCalc::updateFromLiveIns() {
   LiveRangeUpdater Updater;
   for (const LiveInBlock &I : LiveIn) {
     if (!I.DomNode)
       continue;
     MachineBasicBlock *MBB = I.DomNode->getBlock();
     assert(I.Value && "No live-in value found");
     SlotIndex Start, End;
     std::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()));
       Map[MBB] = LiveOutPair(I.Value, nullptr);
     }
     Updater.setDest(&I.LR);
     Updater.add(Start, End, I.Value);
   }
   LiveIn.clear();
 }

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

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

   // Is there a def in the same MBB we can extend?
   auto EP = LR.extendInBlock(Undefs, Indexes->getMBBStartIdx(UseMBB), Use);
   if (EP.first != nullptr || EP.second)
     return;

   // Find the single reaching def, or determine if Use 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, *UseMBB, Use, PhysReg, Undefs))
     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();
   updateFromLiveIns();
 }

 bool LiveRangeCalc::isDefOnEntry(LiveRange &LR, ArrayRef<SlotIndex> Undefs,
                                  MachineBasicBlock &MBB, BitVector &DefOnEntry,
                                  BitVector &UndefOnEntry) {
   unsigned BN = MBB.getNumber();
   if (DefOnEntry[BN])
     return true;
   if (UndefOnEntry[BN])
     return false;

   auto MarkDefined = [BN, &DefOnEntry](MachineBasicBlock &B) -> bool {
     for (MachineBasicBlock *S : B.successors())
       DefOnEntry[S->getNumber()] = true;
     DefOnEntry[BN] = true;
     return true;
   };

   SetVector<unsigned> WorkList;
   // Checking if the entry of MBB is reached by some def: add all predecessors
   // that are potentially defined-on-exit to the work list.
   for (MachineBasicBlock *P : MBB.predecessors())
     WorkList.insert(P->getNumber());

   for (unsigned i = 0; i != WorkList.size(); ++i) {
     // Determine if the exit from the block is reached by some def.
     unsigned N = WorkList[i];
     MachineBasicBlock &B = *MF->getBlockNumbered(N);
     if (Seen[N]) {
       const LiveOutPair &LOB = Map[&B];
       if (LOB.first != nullptr && LOB.first != &UndefVNI)
         return MarkDefined(B);
     }
     SlotIndex Begin, End;
     std::tie(Begin, End) = Indexes->getMBBRange(&B);
     // Treat End as not belonging to B.
     // If LR has a segment S that starts at the next block, i.e. [End, ...),
     // std::upper_bound will return the segment following S. Instead,
     // S should be treated as the first segment that does not overlap B.
     LiveRange::iterator UB = upper_bound(LR, End.getPrevSlot());
     if (UB != LR.begin()) {
       LiveRange::Segment &Seg = *std::prev(UB);
       if (Seg.end > Begin) {
         // There is a segment that overlaps B. If the range is not explicitly
         // undefined between the end of the segment and the end of the block,
         // treat the block as defined on exit. If it is, go to the next block
         // on the work list.
         if (LR.isUndefIn(Undefs, Seg.end, End))
           continue;
         return MarkDefined(B);
       }
     }

     // No segment overlaps with this block. If this block is not defined on
     // entry, or it undefines the range, do not process its predecessors.
     if (UndefOnEntry[N] || LR.isUndefIn(Undefs, Begin, End)) {
       UndefOnEntry[N] = true;
       continue;
     }
     if (DefOnEntry[N])
       return MarkDefined(B);

     // Still don't know: add all predecessors to the work list.
     for (MachineBasicBlock *P : B.predecessors())
       WorkList.insert(P->getNumber());
   }

   UndefOnEntry[BN] = true;
   return false;
 }

 bool LiveRangeCalc::findReachingDefs(LiveRange &LR, MachineBasicBlock &UseMBB,
                                      SlotIndex Use, unsigned PhysReg,
                                      ArrayRef<SlotIndex> Undefs) {
   unsigned UseMBBNum = UseMBB.getNumber();

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

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

   bool FoundUndef = false;

   // 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();
       errs() << "Use of " << printReg(PhysReg, MRI->getTargetRegisterInfo())
              << " does not have a corresponding definition on every path:\n";
       const MachineInstr *MI = Indexes->getInstructionFromIndex(Use);
       if (MI != nullptr)
         errs() << Use << " " << *MI;
       report_fatal_error("Use not jointly dominated by defs.");
     }

     if (Register::isPhysicalRegister(PhysReg) && !MBB->isLiveIn(PhysReg)) {
       MBB->getParent()->verify();
       const TargetRegisterInfo *TRI = MRI->getTargetRegisterInfo();
       errs() << "The register " << printReg(PhysReg, TRI)
              << " needs to be live in to " << printMBBReference(*MBB)
              << ", but is missing from the live-in list.\n";
       report_fatal_error("Invalid global physical register");
     }
 #endif
     FoundUndef |= MBB->pred_empty();

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

        SlotIndex Start, End;
        std::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.
        auto EP = LR.extendInBlock(Undefs, Start, End);
        VNInfo *VNI = EP.first;
        FoundUndef |= EP.second;
        setLiveOutValue(Pred, EP.second ? &UndefVNI : VNI);
        if (VNI) {
          if (TheVNI && TheVNI != VNI)
            UniqueVNI = false;
          TheVNI = VNI;
        }
        if (VNI || EP.second)
          continue;

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

   LiveIn.clear();
   FoundUndef |= (TheVNI == nullptr || TheVNI == &UndefVNI);
   if (!Undefs.empty() && FoundUndef)
     UniqueVNI = false;

   // 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) {
     assert(TheVNI != nullptr && TheVNI != &UndefVNI);
     LiveRangeUpdater Updater(&LR);
     for (unsigned BN : WorkList) {
       SlotIndex Start, End;
       std::tie(Start, End) = Indexes->getMBBRange(BN);
       // Trim the live range in UseMBB.
       if (BN == UseMBBNum && Use.isValid())
         End = Use;
       else
         Map[MF->getBlockNumbered(BN)] = LiveOutPair(TheVNI, nullptr);
       Updater.add(Start, End, TheVNI);
     }
     return true;
   }

   // Prepare the defined/undefined bit vectors.
   EntryInfoMap::iterator Entry;
   bool DidInsert;
   std::tie(Entry, DidInsert) = EntryInfos.insert(
       std::make_pair(&LR, std::make_pair(BitVector(), BitVector())));
   if (DidInsert) {
     // Initialize newly inserted entries.
     unsigned N = MF->getNumBlockIDs();
     Entry->second.first.resize(N);
     Entry->second.second.resize(N);
   }
   BitVector &DefOnEntry = Entry->second.first;
   BitVector &UndefOnEntry = Entry->second.second;

   // 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 (unsigned BN : WorkList) {
     MachineBasicBlock *MBB = MF->getBlockNumbered(BN);
     if (!Undefs.empty() &&
         !isDefOnEntry(LR, Undefs, *MBB, DefOnEntry, UndefOnEntry))
       continue;
     addLiveInBlock(LR, DomTree->getNode(MBB));
     if (MBB == &UseMBB)
       LiveIn.back().Kill = Use;
   }

   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.
   bool Changed;
   do {
     Changed = false;
     // Propagate live-out values down the dominator tree, inserting phi-defs
     // when necessary.
     for (LiveInBlock &I : LiveIn) {
       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 = Map[IDom->getBlock()];

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

         for (MachineBasicBlock *Pred : MBB->predecessors()) {
           LiveOutPair &Value = Map[Pred];
           if (!Value.first || Value.first == IDomValue.first)
             continue;
           if (Value.first == &UndefVNI) {
             needPHI = true;
             break;
           }

           // 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 = Map[MBB];

       // Create a phi-def if required.
       if (needPHI) {
         Changed = true;
         assert(Alloc && "Need VNInfo allocator to create PHI-defs");
         SlotIndex Start, End;
         std::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 = nullptr;

         // Add liveness since updateFromLiveIns now skips this node.
         if (I.Kill.isValid()) {
           if (VNI)
             LR.addSegment(LiveInterval::Segment(Start, I.Kill, VNI));
         } else {
           if (VNI)
             LR.addSegment(LiveInterval::Segment(Start, End, VNI));
           LOP = LiveOutPair(VNI, Node);
         }
       } else if (IDomValue.first && IDomValue.first != &UndefVNI) {
         // 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;
         Changed = true;
         LOP = IDomValue;
       }
     }
   } while (Changed);
 }

 bool LiveRangeCalc::isJointlyDominated(const MachineBasicBlock *MBB,
                                        ArrayRef<SlotIndex> Defs,
                                        const SlotIndexes &Indexes) {
   const MachineFunction &MF = *MBB->getParent();
   BitVector DefBlocks(MF.getNumBlockIDs());
   for (SlotIndex I : Defs)
     DefBlocks.set(Indexes.getMBBFromIndex(I)->getNumber());

   SetVector<unsigned> PredQueue;
   PredQueue.insert(MBB->getNumber());
   for (unsigned i = 0; i != PredQueue.size(); ++i) {
     unsigned BN = PredQueue[i];
     if (DefBlocks[BN])
       return true;
     const MachineBasicBlock *B = MF.getBlockNumbered(BN);
     for (const MachineBasicBlock *P : B->predecessors())
       PredQueue.insert(P->getNumber());
   }
   return false;
 }
	//===- LiveRangeCalc.cpp - Calculate live ranges -------------------------===//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//
	//
	// Implementation of the LiveRangeCalc class.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/CodeGen/LiveRangeCalc.h"
	#include "llvm/ADT/BitVector.h"
	#include "llvm/ADT/STLExtras.h"
	#include "llvm/ADT/SetVector.h"
	#include "llvm/ADT/SmallVector.h"
	#include "llvm/CodeGen/LiveInterval.h"
	#include "llvm/CodeGen/MachineBasicBlock.h"
	#include "llvm/CodeGen/MachineDominators.h"
	#include "llvm/CodeGen/MachineFunction.h"
	#include "llvm/CodeGen/MachineInstr.h"
	#include "llvm/CodeGen/MachineRegisterInfo.h"
	#include "llvm/CodeGen/SlotIndexes.h"
	#include "llvm/CodeGen/TargetRegisterInfo.h"
	#include "llvm/Support/ErrorHandling.h"
	#include "llvm/Support/raw_ostream.h"
	#include <algorithm>
	#include <cassert>
	#include <iterator>
	#include <tuple>
	#include <utility>

	using namespace llvm;

	#define DEBUG_TYPE "regalloc"

	// Reserve an address that indicates a value that is known to be "undef".
	static VNInfo UndefVNI(0xbad, SlotIndex());

	void LiveRangeCalc::resetLiveOutMap() {
	unsigned NumBlocks = MF->getNumBlockIDs();
	Seen.clear();
	Seen.resize(NumBlocks);
	EntryInfos.clear();
	Map.resize(NumBlocks);
	}

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

	void LiveRangeCalc::updateFromLiveIns() {
	LiveRangeUpdater Updater;
	for (const LiveInBlock &I : LiveIn) {
	if (!I.DomNode)
	continue;
	MachineBasicBlock *MBB = I.DomNode->getBlock();
	assert(I.Value && "No live-in value found");
	SlotIndex Start, End;
	std::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()));
	Map[MBB] = LiveOutPair(I.Value, nullptr);
	}
	Updater.setDest(&I.LR);
	Updater.add(Start, End, I.Value);
	}
	LiveIn.clear();
	}

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

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

	// Is there a def in the same MBB we can extend?
	auto EP = LR.extendInBlock(Undefs, Indexes->getMBBStartIdx(UseMBB), Use);
	if (EP.first != nullptr \|\| EP.second)
	return;

	// Find the single reaching def, or determine if Use 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, *UseMBB, Use, PhysReg, Undefs))
	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();
	updateFromLiveIns();
	}

	bool LiveRangeCalc::isDefOnEntry(LiveRange &LR, ArrayRef<SlotIndex> Undefs,
	MachineBasicBlock &MBB, BitVector &DefOnEntry,
	BitVector &UndefOnEntry) {
	unsigned BN = MBB.getNumber();
	if (DefOnEntry[BN])
	return true;
	if (UndefOnEntry[BN])
	return false;

	auto MarkDefined = [BN, &DefOnEntry](MachineBasicBlock &B) -> bool {
	for (MachineBasicBlock *S : B.successors())
	DefOnEntry[S->getNumber()] = true;
	DefOnEntry[BN] = true;
	return true;
	};

	SetVector<unsigned> WorkList;
	// Checking if the entry of MBB is reached by some def: add all predecessors
	// that are potentially defined-on-exit to the work list.
	for (MachineBasicBlock *P : MBB.predecessors())
	WorkList.insert(P->getNumber());

	for (unsigned i = 0; i != WorkList.size(); ++i) {
	// Determine if the exit from the block is reached by some def.
	unsigned N = WorkList[i];
	MachineBasicBlock &B = *MF->getBlockNumbered(N);
	if (Seen[N]) {
	const LiveOutPair &LOB = Map[&B];
	if (LOB.first != nullptr && LOB.first != &UndefVNI)
	return MarkDefined(B);
	}
	SlotIndex Begin, End;
	std::tie(Begin, End) = Indexes->getMBBRange(&B);
	// Treat End as not belonging to B.
	// If LR has a segment S that starts at the next block, i.e. [End, ...),
	// std::upper_bound will return the segment following S. Instead,
	// S should be treated as the first segment that does not overlap B.
	LiveRange::iterator UB = upper_bound(LR, End.getPrevSlot());
	if (UB != LR.begin()) {
	LiveRange::Segment &Seg = *std::prev(UB);
	if (Seg.end > Begin) {
	// There is a segment that overlaps B. If the range is not explicitly
	// undefined between the end of the segment and the end of the block,
	// treat the block as defined on exit. If it is, go to the next block
	// on the work list.
	if (LR.isUndefIn(Undefs, Seg.end, End))
	continue;
	return MarkDefined(B);
	}
	}

	// No segment overlaps with this block. If this block is not defined on
	// entry, or it undefines the range, do not process its predecessors.
	if (UndefOnEntry[N] \|\| LR.isUndefIn(Undefs, Begin, End)) {
	UndefOnEntry[N] = true;
	continue;
	}
	if (DefOnEntry[N])
	return MarkDefined(B);

	// Still don't know: add all predecessors to the work list.
	for (MachineBasicBlock *P : B.predecessors())
	WorkList.insert(P->getNumber());
	}

	UndefOnEntry[BN] = true;
	return false;
	}

	bool LiveRangeCalc::findReachingDefs(LiveRange &LR, MachineBasicBlock &UseMBB,
	SlotIndex Use, unsigned PhysReg,
	ArrayRef<SlotIndex> Undefs) {
	unsigned UseMBBNum = UseMBB.getNumber();

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

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

	bool FoundUndef = false;

	// 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();
	errs() << "Use of " << printReg(PhysReg, MRI->getTargetRegisterInfo())
	<< " does not have a corresponding definition on every path:\n";
	const MachineInstr *MI = Indexes->getInstructionFromIndex(Use);
	if (MI != nullptr)
	errs() << Use << " " << *MI;
	report_fatal_error("Use not jointly dominated by defs.");
	}

	if (Register::isPhysicalRegister(PhysReg) && !MBB->isLiveIn(PhysReg)) {
	MBB->getParent()->verify();
	const TargetRegisterInfo *TRI = MRI->getTargetRegisterInfo();
	errs() << "The register " << printReg(PhysReg, TRI)
	<< " needs to be live in to " << printMBBReference(*MBB)
	<< ", but is missing from the live-in list.\n";
	report_fatal_error("Invalid global physical register");
	}
	#endif
	FoundUndef \|= MBB->pred_empty();

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

	SlotIndex Start, End;
	std::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.
	auto EP = LR.extendInBlock(Undefs, Start, End);
	VNInfo *VNI = EP.first;
	FoundUndef \|= EP.second;
	setLiveOutValue(Pred, EP.second ? &UndefVNI : VNI);
	if (VNI) {
	if (TheVNI && TheVNI != VNI)
	UniqueVNI = false;
	TheVNI = VNI;
	}
	if (VNI \|\| EP.second)
	continue;

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

	LiveIn.clear();
	FoundUndef \|= (TheVNI == nullptr \|\| TheVNI == &UndefVNI);
	if (!Undefs.empty() && FoundUndef)
	UniqueVNI = false;

	// 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) {
	assert(TheVNI != nullptr && TheVNI != &UndefVNI);
	LiveRangeUpdater Updater(&LR);
	for (unsigned BN : WorkList) {
	SlotIndex Start, End;
	std::tie(Start, End) = Indexes->getMBBRange(BN);
	// Trim the live range in UseMBB.
	if (BN == UseMBBNum && Use.isValid())
	End = Use;
	else
	Map[MF->getBlockNumbered(BN)] = LiveOutPair(TheVNI, nullptr);
	Updater.add(Start, End, TheVNI);
	}
	return true;
	}

	// Prepare the defined/undefined bit vectors.
	EntryInfoMap::iterator Entry;
	bool DidInsert;
	std::tie(Entry, DidInsert) = EntryInfos.insert(
	std::make_pair(&LR, std::make_pair(BitVector(), BitVector())));
	if (DidInsert) {
	// Initialize newly inserted entries.
	unsigned N = MF->getNumBlockIDs();
	Entry->second.first.resize(N);
	Entry->second.second.resize(N);
	}
	BitVector &DefOnEntry = Entry->second.first;
	BitVector &UndefOnEntry = Entry->second.second;

	// 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 (unsigned BN : WorkList) {
	MachineBasicBlock *MBB = MF->getBlockNumbered(BN);
	if (!Undefs.empty() &&
	!isDefOnEntry(LR, Undefs, *MBB, DefOnEntry, UndefOnEntry))
	continue;
	addLiveInBlock(LR, DomTree->getNode(MBB));
	if (MBB == &UseMBB)
	LiveIn.back().Kill = Use;
	}

	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.
	bool Changed;
	do {
	Changed = false;
	// Propagate live-out values down the dominator tree, inserting phi-defs
	// when necessary.
	for (LiveInBlock &I : LiveIn) {
	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 = Map[IDom->getBlock()];

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

	for (MachineBasicBlock *Pred : MBB->predecessors()) {
	LiveOutPair &Value = Map[Pred];
	if (!Value.first \|\| Value.first == IDomValue.first)
	continue;
	if (Value.first == &UndefVNI) {
	needPHI = true;
	break;
	}

	// 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 = Map[MBB];

	// Create a phi-def if required.
	if (needPHI) {
	Changed = true;
	assert(Alloc && "Need VNInfo allocator to create PHI-defs");
	SlotIndex Start, End;
	std::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 = nullptr;

	// Add liveness since updateFromLiveIns now skips this node.
	if (I.Kill.isValid()) {
	if (VNI)
	LR.addSegment(LiveInterval::Segment(Start, I.Kill, VNI));
	} else {
	if (VNI)
	LR.addSegment(LiveInterval::Segment(Start, End, VNI));
	LOP = LiveOutPair(VNI, Node);
	}
	} else if (IDomValue.first && IDomValue.first != &UndefVNI) {
	// 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;
	Changed = true;
	LOP = IDomValue;
	}
	}
	} while (Changed);
	}

	bool LiveRangeCalc::isJointlyDominated(const MachineBasicBlock *MBB,
	ArrayRef<SlotIndex> Defs,
	const SlotIndexes &Indexes) {
	const MachineFunction &MF = *MBB->getParent();
	BitVector DefBlocks(MF.getNumBlockIDs());
	for (SlotIndex I : Defs)
	DefBlocks.set(Indexes.getMBBFromIndex(I)->getNumber());

	SetVector<unsigned> PredQueue;
	PredQueue.insert(MBB->getNumber());
	for (unsigned i = 0; i != PredQueue.size(); ++i) {
	unsigned BN = PredQueue[i];
	if (DefBlocks[BN])
	return true;
	const MachineBasicBlock *B = MF.getBlockNumbered(BN);
	for (const MachineBasicBlock *P : B->predecessors())
	PredQueue.insert(P->getNumber());
	}
	return false;
	}