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ref:cde9a383711a11544ce7e107a78147fb96cc4029

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diff --git a/contrib/libs/llvm12/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp b/contrib/libs/llvm12/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp
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index 0000000000..e2daa46fe6
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+++ b/contrib/libs/llvm12/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp
@@ -0,0 +1,1994 @@
+//===- VarLocBasedImpl.cpp - Tracking Debug Value MIs with VarLoc class----===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file VarLocBasedImpl.cpp
+///
+/// LiveDebugValues is an optimistic "available expressions" dataflow
+/// algorithm. The set of expressions is the set of machine locations
+/// (registers, spill slots, constants) that a variable fragment might be
+/// located, qualified by a DIExpression and indirect-ness flag, while each
+/// variable is identified by a DebugVariable object. The availability of an
+/// expression begins when a DBG_VALUE instruction specifies the location of a
+/// DebugVariable, and continues until that location is clobbered or
+/// re-specified by a different DBG_VALUE for the same DebugVariable.
+///
+/// The output of LiveDebugValues is additional DBG_VALUE instructions,
+/// placed to extend variable locations as far they're available. This file
+/// and the VarLocBasedLDV class is an implementation that explicitly tracks
+/// locations, using the VarLoc class.
+///
+/// The canonical "available expressions" problem doesn't have expression
+/// clobbering, instead when a variable is re-assigned, any expressions using
+/// that variable get invalidated. LiveDebugValues can map onto "available
+/// expressions" by having every register represented by a variable, which is
+/// used in an expression that becomes available at a DBG_VALUE instruction.
+/// When the register is clobbered, its variable is effectively reassigned, and
+/// expressions computed from it become unavailable. A similar construct is
+/// needed when a DebugVariable has its location re-specified, to invalidate
+/// all other locations for that DebugVariable.
+///
+/// Using the dataflow analysis to compute the available expressions, we create
+/// a DBG_VALUE at the beginning of each block where the expression is
+/// live-in. This propagates variable locations into every basic block where
+/// the location can be determined, rather than only having DBG_VALUEs in blocks
+/// where locations are specified due to an assignment or some optimization.
+/// Movements of values between registers and spill slots are annotated with
+/// DBG_VALUEs too to track variable values bewteen locations. All this allows
+/// DbgEntityHistoryCalculator to focus on only the locations within individual
+/// blocks, facilitating testing and improving modularity.
+///
+/// We follow an optimisic dataflow approach, with this lattice:
+///
+/// \verbatim
+///                    ┬ "Unknown"
+///                          |
+///                          v
+///                         True
+///                          |
+///                          v
+///                      ⊥ False
+/// \endverbatim With "True" signifying that the expression is available (and
+/// thus a DebugVariable's location is the corresponding register), while
+/// "False" signifies that the expression is unavailable. "Unknown"s never
+/// survive to the end of the analysis (see below).
+///
+/// Formally, all DebugVariable locations that are live-out of a block are
+/// initialized to \top.  A blocks live-in values take the meet of the lattice
+/// value for every predecessors live-outs, except for the entry block, where
+/// all live-ins are \bot. The usual dataflow propagation occurs: the transfer
+/// function for a block assigns an expression for a DebugVariable to be "True"
+/// if a DBG_VALUE in the block specifies it; "False" if the location is
+/// clobbered; or the live-in value if it is unaffected by the block. We
+/// visit each block in reverse post order until a fixedpoint is reached. The
+/// solution produced is maximal.
+///
+/// Intuitively, we start by assuming that every expression / variable location
+/// is at least "True", and then propagate "False" from the entry block and any
+/// clobbers until there are no more changes to make. This gives us an accurate
+/// solution because all incorrect locations will have a "False" propagated into
+/// them. It also gives us a solution that copes well with loops by assuming
+/// that variable locations are live-through every loop, and then removing those
+/// that are not through dataflow.
+///
+/// Within LiveDebugValues: each variable location is represented by a
+/// VarLoc object that identifies the source variable, its current
+/// machine-location, and the DBG_VALUE inst that specifies the location. Each
+/// VarLoc is indexed in the (function-scope) \p VarLocMap, giving each VarLoc a
+/// unique index. Rather than operate directly on machine locations, the
+/// dataflow analysis in this pass identifies locations by their index in the
+/// VarLocMap, meaning all the variable locations in a block can be described
+/// by a sparse vector of VarLocMap indicies.
+///
+/// All the storage for the dataflow analysis is local to the ExtendRanges
+/// method and passed down to helper methods. "OutLocs" and "InLocs" record the
+/// in and out lattice values for each block. "OpenRanges" maintains a list of
+/// variable locations and, with the "process" method, evaluates the transfer
+/// function of each block. "flushPendingLocs" installs DBG_VALUEs for each
+/// live-in location at the start of blocks, while "Transfers" records
+/// transfers of values between machine-locations.
+///
+/// We avoid explicitly representing the "Unknown" (\top) lattice value in the
+/// implementation. Instead, unvisited blocks implicitly have all lattice
+/// values set as "Unknown". After being visited, there will be path back to
+/// the entry block where the lattice value is "False", and as the transfer
+/// function cannot make new "Unknown" locations, there are no scenarios where
+/// a block can have an "Unknown" location after being visited. Similarly, we
+/// don't enumerate all possible variable locations before exploring the
+/// function: when a new location is discovered, all blocks previously explored
+/// were implicitly "False" but unrecorded, and become explicitly "False" when
+/// a new VarLoc is created with its bit not set in predecessor InLocs or
+/// OutLocs.
+///
+//===----------------------------------------------------------------------===//
+
+#include "LiveDebugValues.h"
+
+#include "llvm/ADT/CoalescingBitVector.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/UniqueVector.h"
+#include "llvm/CodeGen/LexicalScopes.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFrameInfo.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineMemOperand.h"
+#include "llvm/CodeGen/MachineOperand.h"
+#include "llvm/CodeGen/PseudoSourceValue.h"
+#include "llvm/CodeGen/RegisterScavenging.h"
+#include "llvm/CodeGen/TargetFrameLowering.h"
+#include "llvm/CodeGen/TargetInstrInfo.h"
+#include "llvm/CodeGen/TargetLowering.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
+#include "llvm/CodeGen/TargetRegisterInfo.h"
+#include "llvm/CodeGen/TargetSubtargetInfo.h"
+#include "llvm/Config/llvm-config.h"
+#include "llvm/IR/DIBuilder.h"
+#include "llvm/IR/DebugInfoMetadata.h"
+#include "llvm/IR/DebugLoc.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Module.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/MC/MCRegisterInfo.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/TypeSize.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetMachine.h"
+#include <algorithm>
+#include <cassert>
+#include <cstdint>
+#include <functional>
+#include <queue>
+#include <tuple>
+#include <utility>
+#include <vector>
+
+using namespace llvm;
+
+#define DEBUG_TYPE "livedebugvalues"
+
+STATISTIC(NumInserted, "Number of DBG_VALUE instructions inserted");
+
+// Options to prevent pathological compile-time behavior. If InputBBLimit and
+// InputDbgValueLimit are both exceeded, range extension is disabled.
+static cl::opt<unsigned> InputBBLimit(
+    "livedebugvalues-input-bb-limit",
+    cl::desc("Maximum input basic blocks before DBG_VALUE limit applies"),
+    cl::init(10000), cl::Hidden);
+static cl::opt<unsigned> InputDbgValueLimit(
+    "livedebugvalues-input-dbg-value-limit",
+    cl::desc(
+        "Maximum input DBG_VALUE insts supported by debug range extension"),
+    cl::init(50000), cl::Hidden);
+
+// If @MI is a DBG_VALUE with debug value described by a defined
+// register, returns the number of this register. In the other case, returns 0.
+static Register isDbgValueDescribedByReg(const MachineInstr &MI) {
+  assert(MI.isDebugValue() && "expected a DBG_VALUE");
+  assert(MI.getNumOperands() == 4 && "malformed DBG_VALUE");
+  // If location of variable is described using a register (directly
+  // or indirectly), this register is always a first operand.
+  return MI.getDebugOperand(0).isReg() ? MI.getDebugOperand(0).getReg()
+                                       : Register();
+}
+
+/// If \p Op is a stack or frame register return true, otherwise return false.
+/// This is used to avoid basing the debug entry values on the registers, since
+/// we do not support it at the moment.
+static bool isRegOtherThanSPAndFP(const MachineOperand &Op,
+                                  const MachineInstr &MI,
+                                  const TargetRegisterInfo *TRI) {
+  if (!Op.isReg())
+    return false;
+
+  const MachineFunction *MF = MI.getParent()->getParent();
+  const TargetLowering *TLI = MF->getSubtarget().getTargetLowering();
+  Register SP = TLI->getStackPointerRegisterToSaveRestore();
+  Register FP = TRI->getFrameRegister(*MF);
+  Register Reg = Op.getReg();
+
+  return Reg && Reg != SP && Reg != FP;
+}
+
+namespace {
+
+// Max out the number of statically allocated elements in DefinedRegsSet, as
+// this prevents fallback to std::set::count() operations.
+using DefinedRegsSet = SmallSet<Register, 32>;
+
+using VarLocSet = CoalescingBitVector<uint64_t>;
+
+/// A type-checked pair of {Register Location (or 0), Index}, used to index
+/// into a \ref VarLocMap. This can be efficiently converted to a 64-bit int
+/// for insertion into a \ref VarLocSet, and efficiently converted back. The
+/// type-checker helps ensure that the conversions aren't lossy.
+///
+/// Why encode a location /into/ the VarLocMap index? This makes it possible
+/// to find the open VarLocs killed by a register def very quickly. This is a
+/// performance-critical operation for LiveDebugValues.
+struct LocIndex {
+  using u32_location_t = uint32_t;
+  using u32_index_t = uint32_t;
+
+  u32_location_t Location; // Physical registers live in the range [1;2^30) (see
+                           // \ref MCRegister), so we have plenty of range left
+                           // here to encode non-register locations.
+  u32_index_t Index;
+
+  /// The first location greater than 0 that is not reserved for VarLocs of
+  /// kind RegisterKind.
+  static constexpr u32_location_t kFirstInvalidRegLocation = 1 << 30;
+
+  /// A special location reserved for VarLocs of kind SpillLocKind.
+  static constexpr u32_location_t kSpillLocation = kFirstInvalidRegLocation;
+
+  /// A special location reserved for VarLocs of kind EntryValueBackupKind and
+  /// EntryValueCopyBackupKind.
+  static constexpr u32_location_t kEntryValueBackupLocation =
+      kFirstInvalidRegLocation + 1;
+
+  LocIndex(u32_location_t Location, u32_index_t Index)
+      : Location(Location), Index(Index) {}
+
+  uint64_t getAsRawInteger() const {
+    return (static_cast<uint64_t>(Location) << 32) | Index;
+  }
+
+  template<typename IntT> static LocIndex fromRawInteger(IntT ID) {
+    static_assert(std::is_unsigned<IntT>::value &&
+                      sizeof(ID) == sizeof(uint64_t),
+                  "Cannot convert raw integer to LocIndex");
+    return {static_cast<u32_location_t>(ID >> 32),
+            static_cast<u32_index_t>(ID)};
+  }
+
+  /// Get the start of the interval reserved for VarLocs of kind RegisterKind
+  /// which reside in \p Reg. The end is at rawIndexForReg(Reg+1)-1.
+  static uint64_t rawIndexForReg(uint32_t Reg) {
+    return LocIndex(Reg, 0).getAsRawInteger();
+  }
+
+  /// Return a range covering all set indices in the interval reserved for
+  /// \p Location in \p Set.
+  static auto indexRangeForLocation(const VarLocSet &Set,
+                                    u32_location_t Location) {
+    uint64_t Start = LocIndex(Location, 0).getAsRawInteger();
+    uint64_t End = LocIndex(Location + 1, 0).getAsRawInteger();
+    return Set.half_open_range(Start, End);
+  }
+};
+
+class VarLocBasedLDV : public LDVImpl {
+private:
+  const TargetRegisterInfo *TRI;
+  const TargetInstrInfo *TII;
+  const TargetFrameLowering *TFI;
+  TargetPassConfig *TPC;
+  BitVector CalleeSavedRegs;
+  LexicalScopes LS;
+  VarLocSet::Allocator Alloc;
+
+  enum struct TransferKind { TransferCopy, TransferSpill, TransferRestore };
+
+  using FragmentInfo = DIExpression::FragmentInfo;
+  using OptFragmentInfo = Optional<DIExpression::FragmentInfo>;
+
+  /// A pair of debug variable and value location.
+  struct VarLoc {
+    // The location at which a spilled variable resides. It consists of a
+    // register and an offset.
+    struct SpillLoc {
+      unsigned SpillBase;
+      StackOffset SpillOffset;
+      bool operator==(const SpillLoc &Other) const {
+        return SpillBase == Other.SpillBase && SpillOffset == Other.SpillOffset;
+      }
+      bool operator!=(const SpillLoc &Other) const {
+        return !(*this == Other);
+      }
+    };
+
+    /// Identity of the variable at this location.
+    const DebugVariable Var;
+
+    /// The expression applied to this location.
+    const DIExpression *Expr;
+
+    /// DBG_VALUE to clone var/expr information from if this location
+    /// is moved.
+    const MachineInstr &MI;
+
+    enum VarLocKind {
+      InvalidKind = 0,
+      RegisterKind,
+      SpillLocKind,
+      ImmediateKind,
+      EntryValueKind,
+      EntryValueBackupKind,
+      EntryValueCopyBackupKind
+    } Kind = InvalidKind;
+
+    /// The value location. Stored separately to avoid repeatedly
+    /// extracting it from MI.
+    union LocUnion {
+      uint64_t RegNo;
+      SpillLoc SpillLocation;
+      uint64_t Hash;
+      int64_t Immediate;
+      const ConstantFP *FPImm;
+      const ConstantInt *CImm;
+      LocUnion() : Hash(0) {}
+    } Loc;
+
+    VarLoc(const MachineInstr &MI, LexicalScopes &LS)
+        : Var(MI.getDebugVariable(), MI.getDebugExpression(),
+              MI.getDebugLoc()->getInlinedAt()),
+          Expr(MI.getDebugExpression()), MI(MI) {
+      assert(MI.isDebugValue() && "not a DBG_VALUE");
+      assert(MI.getNumOperands() == 4 && "malformed DBG_VALUE");
+      if (int RegNo = isDbgValueDescribedByReg(MI)) {
+        Kind = RegisterKind;
+        Loc.RegNo = RegNo;
+      } else if (MI.getDebugOperand(0).isImm()) {
+        Kind = ImmediateKind;
+        Loc.Immediate = MI.getDebugOperand(0).getImm();
+      } else if (MI.getDebugOperand(0).isFPImm()) {
+        Kind = ImmediateKind;
+        Loc.FPImm = MI.getDebugOperand(0).getFPImm();
+      } else if (MI.getDebugOperand(0).isCImm()) {
+        Kind = ImmediateKind;
+        Loc.CImm = MI.getDebugOperand(0).getCImm();
+      }
+
+      // We create the debug entry values from the factory functions rather than
+      // from this ctor.
+      assert(Kind != EntryValueKind && !isEntryBackupLoc());
+    }
+
+    /// Take the variable and machine-location in DBG_VALUE MI, and build an
+    /// entry location using the given expression.
+    static VarLoc CreateEntryLoc(const MachineInstr &MI, LexicalScopes &LS,
+                                 const DIExpression *EntryExpr, Register Reg) {
+      VarLoc VL(MI, LS);
+      assert(VL.Kind == RegisterKind);
+      VL.Kind = EntryValueKind;
+      VL.Expr = EntryExpr;
+      VL.Loc.RegNo = Reg;
+      return VL;
+    }
+
+    /// Take the variable and machine-location from the DBG_VALUE (from the
+    /// function entry), and build an entry value backup location. The backup
+    /// location will turn into the normal location if the backup is valid at
+    /// the time of the primary location clobbering.
+    static VarLoc CreateEntryBackupLoc(const MachineInstr &MI,
+                                       LexicalScopes &LS,
+                                       const DIExpression *EntryExpr) {
+      VarLoc VL(MI, LS);
+      assert(VL.Kind == RegisterKind);
+      VL.Kind = EntryValueBackupKind;
+      VL.Expr = EntryExpr;
+      return VL;
+    }
+
+    /// Take the variable and machine-location from the DBG_VALUE (from the
+    /// function entry), and build a copy of an entry value backup location by
+    /// setting the register location to NewReg.
+    static VarLoc CreateEntryCopyBackupLoc(const MachineInstr &MI,
+                                           LexicalScopes &LS,
+                                           const DIExpression *EntryExpr,
+                                           Register NewReg) {
+      VarLoc VL(MI, LS);
+      assert(VL.Kind == RegisterKind);
+      VL.Kind = EntryValueCopyBackupKind;
+      VL.Expr = EntryExpr;
+      VL.Loc.RegNo = NewReg;
+      return VL;
+    }
+
+    /// Copy the register location in DBG_VALUE MI, updating the register to
+    /// be NewReg.
+    static VarLoc CreateCopyLoc(const MachineInstr &MI, LexicalScopes &LS,
+                                Register NewReg) {
+      VarLoc VL(MI, LS);
+      assert(VL.Kind == RegisterKind);
+      VL.Loc.RegNo = NewReg;
+      return VL;
+    }
+
+    /// Take the variable described by DBG_VALUE MI, and create a VarLoc
+    /// locating it in the specified spill location.
+    static VarLoc CreateSpillLoc(const MachineInstr &MI, unsigned SpillBase,
+                                 StackOffset SpillOffset, LexicalScopes &LS) {
+      VarLoc VL(MI, LS);
+      assert(VL.Kind == RegisterKind);
+      VL.Kind = SpillLocKind;
+      VL.Loc.SpillLocation = {SpillBase, SpillOffset};
+      return VL;
+    }
+
+    /// Create a DBG_VALUE representing this VarLoc in the given function.
+    /// Copies variable-specific information such as DILocalVariable and
+    /// inlining information from the original DBG_VALUE instruction, which may
+    /// have been several transfers ago.
+    MachineInstr *BuildDbgValue(MachineFunction &MF) const {
+      const DebugLoc &DbgLoc = MI.getDebugLoc();
+      bool Indirect = MI.isIndirectDebugValue();
+      const auto &IID = MI.getDesc();
+      const DILocalVariable *Var = MI.getDebugVariable();
+      const DIExpression *DIExpr = MI.getDebugExpression();
+      NumInserted++;
+
+      switch (Kind) {
+      case EntryValueKind:
+        // An entry value is a register location -- but with an updated
+        // expression. The register location of such DBG_VALUE is always the one
+        // from the entry DBG_VALUE, it does not matter if the entry value was
+        // copied in to another register due to some optimizations.
+        return BuildMI(MF, DbgLoc, IID, Indirect,
+                       MI.getDebugOperand(0).getReg(), Var, Expr);
+      case RegisterKind:
+        // Register locations are like the source DBG_VALUE, but with the
+        // register number from this VarLoc.
+        return BuildMI(MF, DbgLoc, IID, Indirect, Loc.RegNo, Var, DIExpr);
+      case SpillLocKind: {
+        // Spills are indirect DBG_VALUEs, with a base register and offset.
+        // Use the original DBG_VALUEs expression to build the spilt location
+        // on top of. FIXME: spill locations created before this pass runs
+        // are not recognized, and not handled here.
+        auto *TRI = MF.getSubtarget().getRegisterInfo();
+        auto *SpillExpr = TRI->prependOffsetExpression(
+            DIExpr, DIExpression::ApplyOffset, Loc.SpillLocation.SpillOffset);
+        unsigned Base = Loc.SpillLocation.SpillBase;
+        return BuildMI(MF, DbgLoc, IID, true, Base, Var, SpillExpr);
+      }
+      case ImmediateKind: {
+        MachineOperand MO = MI.getDebugOperand(0);
+        return BuildMI(MF, DbgLoc, IID, Indirect, MO, Var, DIExpr);
+      }
+      case EntryValueBackupKind:
+      case EntryValueCopyBackupKind:
+      case InvalidKind:
+        llvm_unreachable(
+            "Tried to produce DBG_VALUE for invalid or backup VarLoc");
+      }
+      llvm_unreachable("Unrecognized VarLocBasedLDV.VarLoc.Kind enum");
+    }
+
+    /// Is the Loc field a constant or constant object?
+    bool isConstant() const { return Kind == ImmediateKind; }
+
+    /// Check if the Loc field is an entry backup location.
+    bool isEntryBackupLoc() const {
+      return Kind == EntryValueBackupKind || Kind == EntryValueCopyBackupKind;
+    }
+
+    /// If this variable is described by a register holding the entry value,
+    /// return it, otherwise return 0.
+    unsigned getEntryValueBackupReg() const {
+      if (Kind == EntryValueBackupKind)
+        return Loc.RegNo;
+      return 0;
+    }
+
+    /// If this variable is described by a register holding the copy of the
+    /// entry value, return it, otherwise return 0.
+    unsigned getEntryValueCopyBackupReg() const {
+      if (Kind == EntryValueCopyBackupKind)
+        return Loc.RegNo;
+      return 0;
+    }
+
+    /// If this variable is described by a register, return it,
+    /// otherwise return 0.
+    unsigned isDescribedByReg() const {
+      if (Kind == RegisterKind)
+        return Loc.RegNo;
+      return 0;
+    }
+
+    /// Determine whether the lexical scope of this value's debug location
+    /// dominates MBB.
+    bool dominates(LexicalScopes &LS, MachineBasicBlock &MBB) const {
+      return LS.dominates(MI.getDebugLoc().get(), &MBB);
+    }
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+    // TRI can be null.
+    void dump(const TargetRegisterInfo *TRI, raw_ostream &Out = dbgs()) const {
+      Out << "VarLoc(";
+      switch (Kind) {
+      case RegisterKind:
+      case EntryValueKind:
+      case EntryValueBackupKind:
+      case EntryValueCopyBackupKind:
+        Out << printReg(Loc.RegNo, TRI);
+        break;
+      case SpillLocKind:
+        Out << printReg(Loc.SpillLocation.SpillBase, TRI);
+        Out << "[" << Loc.SpillLocation.SpillOffset.getFixed() << " + "
+            << Loc.SpillLocation.SpillOffset.getScalable() << "x vscale"
+            << "]";
+        break;
+      case ImmediateKind:
+        Out << Loc.Immediate;
+        break;
+      case InvalidKind:
+        llvm_unreachable("Invalid VarLoc in dump method");
+      }
+
+      Out << ", \"" << Var.getVariable()->getName() << "\", " << *Expr << ", ";
+      if (Var.getInlinedAt())
+        Out << "!" << Var.getInlinedAt()->getMetadataID() << ")\n";
+      else
+        Out << "(null))";
+
+      if (isEntryBackupLoc())
+        Out << " (backup loc)\n";
+      else
+        Out << "\n";
+    }
+#endif
+
+    bool operator==(const VarLoc &Other) const {
+      if (Kind != Other.Kind || !(Var == Other.Var) || Expr != Other.Expr)
+        return false;
+
+      switch (Kind) {
+      case SpillLocKind:
+        return Loc.SpillLocation == Other.Loc.SpillLocation;
+      case RegisterKind:
+      case ImmediateKind:
+      case EntryValueKind:
+      case EntryValueBackupKind:
+      case EntryValueCopyBackupKind:
+        return Loc.Hash == Other.Loc.Hash;
+      default:
+        llvm_unreachable("Invalid kind");
+      }
+    }
+
+    /// This operator guarantees that VarLocs are sorted by Variable first.
+    bool operator<(const VarLoc &Other) const {
+      switch (Kind) {
+      case SpillLocKind:
+        return std::make_tuple(Var, Kind, Loc.SpillLocation.SpillBase,
+                               Loc.SpillLocation.SpillOffset.getFixed(),
+                               Loc.SpillLocation.SpillOffset.getScalable(),
+                               Expr) <
+               std::make_tuple(
+                   Other.Var, Other.Kind, Other.Loc.SpillLocation.SpillBase,
+                   Other.Loc.SpillLocation.SpillOffset.getFixed(),
+                   Other.Loc.SpillLocation.SpillOffset.getScalable(),
+                   Other.Expr);
+      case RegisterKind:
+      case ImmediateKind:
+      case EntryValueKind:
+      case EntryValueBackupKind:
+      case EntryValueCopyBackupKind:
+        return std::tie(Var, Kind, Loc.Hash, Expr) <
+               std::tie(Other.Var, Other.Kind, Other.Loc.Hash, Other.Expr);
+      default:
+        llvm_unreachable("Invalid kind");
+      }
+    }
+  };
+
+  /// VarLocMap is used for two things:
+  /// 1) Assigning a unique LocIndex to a VarLoc. This LocIndex can be used to
+  ///    virtually insert a VarLoc into a VarLocSet.
+  /// 2) Given a LocIndex, look up the unique associated VarLoc.
+  class VarLocMap {
+    /// Map a VarLoc to an index within the vector reserved for its location
+    /// within Loc2Vars.
+    std::map<VarLoc, LocIndex::u32_index_t> Var2Index;
+
+    /// Map a location to a vector which holds VarLocs which live in that
+    /// location.
+    SmallDenseMap<LocIndex::u32_location_t, std::vector<VarLoc>> Loc2Vars;
+
+    /// Determine the 32-bit location reserved for \p VL, based on its kind.
+    static LocIndex::u32_location_t getLocationForVar(const VarLoc &VL) {
+      switch (VL.Kind) {
+      case VarLoc::RegisterKind:
+        assert((VL.Loc.RegNo < LocIndex::kFirstInvalidRegLocation) &&
+               "Physreg out of range?");
+        return VL.Loc.RegNo;
+      case VarLoc::SpillLocKind:
+        return LocIndex::kSpillLocation;
+      case VarLoc::EntryValueBackupKind:
+      case VarLoc::EntryValueCopyBackupKind:
+        return LocIndex::kEntryValueBackupLocation;
+      default:
+        return 0;
+      }
+    }
+
+  public:
+    /// Retrieve a unique LocIndex for \p VL.
+    LocIndex insert(const VarLoc &VL) {
+      LocIndex::u32_location_t Location = getLocationForVar(VL);
+      LocIndex::u32_index_t &Index = Var2Index[VL];
+      if (!Index) {
+        auto &Vars = Loc2Vars[Location];
+        Vars.push_back(VL);
+        Index = Vars.size();
+      }
+      return {Location, Index - 1};
+    }
+
+    /// Retrieve the unique VarLoc associated with \p ID.
+    const VarLoc &operator[](LocIndex ID) const {
+      auto LocIt = Loc2Vars.find(ID.Location);
+      assert(LocIt != Loc2Vars.end() && "Location not tracked");
+      return LocIt->second[ID.Index];
+    }
+  };
+
+  using VarLocInMBB =
+      SmallDenseMap<const MachineBasicBlock *, std::unique_ptr<VarLocSet>>;
+  struct TransferDebugPair {
+    MachineInstr *TransferInst; ///< Instruction where this transfer occurs.
+    LocIndex LocationID;        ///< Location number for the transfer dest.
+  };
+  using TransferMap = SmallVector<TransferDebugPair, 4>;
+
+  // Types for recording sets of variable fragments that overlap. For a given
+  // local variable, we record all other fragments of that variable that could
+  // overlap it, to reduce search time.
+  using FragmentOfVar =
+      std::pair<const DILocalVariable *, DIExpression::FragmentInfo>;
+  using OverlapMap =
+      DenseMap<FragmentOfVar, SmallVector<DIExpression::FragmentInfo, 1>>;
+
+  // Helper while building OverlapMap, a map of all fragments seen for a given
+  // DILocalVariable.
+  using VarToFragments =
+      DenseMap<const DILocalVariable *, SmallSet<FragmentInfo, 4>>;
+
+  /// This holds the working set of currently open ranges. For fast
+  /// access, this is done both as a set of VarLocIDs, and a map of
+  /// DebugVariable to recent VarLocID. Note that a DBG_VALUE ends all
+  /// previous open ranges for the same variable. In addition, we keep
+  /// two different maps (Vars/EntryValuesBackupVars), so erase/insert
+  /// methods act differently depending on whether a VarLoc is primary
+  /// location or backup one. In the case the VarLoc is backup location
+  /// we will erase/insert from the EntryValuesBackupVars map, otherwise
+  /// we perform the operation on the Vars.
+  class OpenRangesSet {
+    VarLocSet VarLocs;
+    // Map the DebugVariable to recent primary location ID.
+    SmallDenseMap<DebugVariable, LocIndex, 8> Vars;
+    // Map the DebugVariable to recent backup location ID.
+    SmallDenseMap<DebugVariable, LocIndex, 8> EntryValuesBackupVars;
+    OverlapMap &OverlappingFragments;
+
+  public:
+    OpenRangesSet(VarLocSet::Allocator &Alloc, OverlapMap &_OLapMap)
+        : VarLocs(Alloc), OverlappingFragments(_OLapMap) {}
+
+    const VarLocSet &getVarLocs() const { return VarLocs; }
+
+    /// Terminate all open ranges for VL.Var by removing it from the set.
+    void erase(const VarLoc &VL);
+
+    /// Terminate all open ranges listed in \c KillSet by removing
+    /// them from the set.
+    void erase(const VarLocSet &KillSet, const VarLocMap &VarLocIDs);
+
+    /// Insert a new range into the set.
+    void insert(LocIndex VarLocID, const VarLoc &VL);
+
+    /// Insert a set of ranges.
+    void insertFromLocSet(const VarLocSet &ToLoad, const VarLocMap &Map) {
+      for (uint64_t ID : ToLoad) {
+        LocIndex Idx = LocIndex::fromRawInteger(ID);
+        const VarLoc &VarL = Map[Idx];
+        insert(Idx, VarL);
+      }
+    }
+
+    llvm::Optional<LocIndex> getEntryValueBackup(DebugVariable Var);
+
+    /// Empty the set.
+    void clear() {
+      VarLocs.clear();
+      Vars.clear();
+      EntryValuesBackupVars.clear();
+    }
+
+    /// Return whether the set is empty or not.
+    bool empty() const {
+      assert(Vars.empty() == EntryValuesBackupVars.empty() &&
+             Vars.empty() == VarLocs.empty() &&
+             "open ranges are inconsistent");
+      return VarLocs.empty();
+    }
+
+    /// Get an empty range of VarLoc IDs.
+    auto getEmptyVarLocRange() const {
+      return iterator_range<VarLocSet::const_iterator>(getVarLocs().end(),
+                                                       getVarLocs().end());
+    }
+
+    /// Get all set IDs for VarLocs of kind RegisterKind in \p Reg.
+    auto getRegisterVarLocs(Register Reg) const {
+      return LocIndex::indexRangeForLocation(getVarLocs(), Reg);
+    }
+
+    /// Get all set IDs for VarLocs of kind SpillLocKind.
+    auto getSpillVarLocs() const {
+      return LocIndex::indexRangeForLocation(getVarLocs(),
+                                             LocIndex::kSpillLocation);
+    }
+
+    /// Get all set IDs for VarLocs of kind EntryValueBackupKind or
+    /// EntryValueCopyBackupKind.
+    auto getEntryValueBackupVarLocs() const {
+      return LocIndex::indexRangeForLocation(
+          getVarLocs(), LocIndex::kEntryValueBackupLocation);
+    }
+  };
+
+  /// Collect all VarLoc IDs from \p CollectFrom for VarLocs of kind
+  /// RegisterKind which are located in any reg in \p Regs. Insert collected IDs
+  /// into \p Collected.
+  void collectIDsForRegs(VarLocSet &Collected, const DefinedRegsSet &Regs,
+                         const VarLocSet &CollectFrom) const;
+
+  /// Get the registers which are used by VarLocs of kind RegisterKind tracked
+  /// by \p CollectFrom.
+  void getUsedRegs(const VarLocSet &CollectFrom,
+                   SmallVectorImpl<uint32_t> &UsedRegs) const;
+
+  VarLocSet &getVarLocsInMBB(const MachineBasicBlock *MBB, VarLocInMBB &Locs) {
+    std::unique_ptr<VarLocSet> &VLS = Locs[MBB];
+    if (!VLS)
+      VLS = std::make_unique<VarLocSet>(Alloc);
+    return *VLS.get();
+  }
+
+  const VarLocSet &getVarLocsInMBB(const MachineBasicBlock *MBB,
+                                   const VarLocInMBB &Locs) const {
+    auto It = Locs.find(MBB);
+    assert(It != Locs.end() && "MBB not in map");
+    return *It->second.get();
+  }
+
+  /// Tests whether this instruction is a spill to a stack location.
+  bool isSpillInstruction(const MachineInstr &MI, MachineFunction *MF);
+
+  /// Decide if @MI is a spill instruction and return true if it is. We use 2
+  /// criteria to make this decision:
+  /// - Is this instruction a store to a spill slot?
+  /// - Is there a register operand that is both used and killed?
+  /// TODO: Store optimization can fold spills into other stores (including
+  /// other spills). We do not handle this yet (more than one memory operand).
+  bool isLocationSpill(const MachineInstr &MI, MachineFunction *MF,
+                       Register &Reg);
+
+  /// Returns true if the given machine instruction is a debug value which we
+  /// can emit entry values for.
+  ///
+  /// Currently, we generate debug entry values only for parameters that are
+  /// unmodified throughout the function and located in a register.
+  bool isEntryValueCandidate(const MachineInstr &MI,
+                             const DefinedRegsSet &Regs) const;
+
+  /// If a given instruction is identified as a spill, return the spill location
+  /// and set \p Reg to the spilled register.
+  Optional<VarLoc::SpillLoc> isRestoreInstruction(const MachineInstr &MI,
+                                                  MachineFunction *MF,
+                                                  Register &Reg);
+  /// Given a spill instruction, extract the register and offset used to
+  /// address the spill location in a target independent way.
+  VarLoc::SpillLoc extractSpillBaseRegAndOffset(const MachineInstr &MI);
+  void insertTransferDebugPair(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                               TransferMap &Transfers, VarLocMap &VarLocIDs,
+                               LocIndex OldVarID, TransferKind Kind,
+                               Register NewReg = Register());
+
+  void transferDebugValue(const MachineInstr &MI, OpenRangesSet &OpenRanges,
+                          VarLocMap &VarLocIDs);
+  void transferSpillOrRestoreInst(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                                  VarLocMap &VarLocIDs, TransferMap &Transfers);
+  bool removeEntryValue(const MachineInstr &MI, OpenRangesSet &OpenRanges,
+                        VarLocMap &VarLocIDs, const VarLoc &EntryVL);
+  void emitEntryValues(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                       VarLocMap &VarLocIDs, TransferMap &Transfers,
+                       VarLocSet &KillSet);
+  void recordEntryValue(const MachineInstr &MI,
+                        const DefinedRegsSet &DefinedRegs,
+                        OpenRangesSet &OpenRanges, VarLocMap &VarLocIDs);
+  void transferRegisterCopy(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                            VarLocMap &VarLocIDs, TransferMap &Transfers);
+  void transferRegisterDef(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                           VarLocMap &VarLocIDs, TransferMap &Transfers);
+  bool transferTerminator(MachineBasicBlock *MBB, OpenRangesSet &OpenRanges,
+                          VarLocInMBB &OutLocs, const VarLocMap &VarLocIDs);
+
+  void process(MachineInstr &MI, OpenRangesSet &OpenRanges,
+               VarLocMap &VarLocIDs, TransferMap &Transfers);
+
+  void accumulateFragmentMap(MachineInstr &MI, VarToFragments &SeenFragments,
+                             OverlapMap &OLapMap);
+
+  bool join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs, VarLocInMBB &InLocs,
+            const VarLocMap &VarLocIDs,
+            SmallPtrSet<const MachineBasicBlock *, 16> &Visited,
+            SmallPtrSetImpl<const MachineBasicBlock *> &ArtificialBlocks);
+
+  /// Create DBG_VALUE insts for inlocs that have been propagated but
+  /// had their instruction creation deferred.
+  void flushPendingLocs(VarLocInMBB &PendingInLocs, VarLocMap &VarLocIDs);
+
+  bool ExtendRanges(MachineFunction &MF, TargetPassConfig *TPC) override;
+
+public:
+  /// Default construct and initialize the pass.
+  VarLocBasedLDV();
+
+  ~VarLocBasedLDV();
+
+  /// Print to ostream with a message.
+  void printVarLocInMBB(const MachineFunction &MF, const VarLocInMBB &V,
+                        const VarLocMap &VarLocIDs, const char *msg,
+                        raw_ostream &Out) const;
+};
+
+} // end anonymous namespace
+
+//===----------------------------------------------------------------------===//
+//            Implementation
+//===----------------------------------------------------------------------===//
+
+VarLocBasedLDV::VarLocBasedLDV() { }
+
+VarLocBasedLDV::~VarLocBasedLDV() { }
+
+/// Erase a variable from the set of open ranges, and additionally erase any
+/// fragments that may overlap it. If the VarLoc is a backup location, erase
+/// the variable from the EntryValuesBackupVars set, indicating we should stop
+/// tracking its backup entry location. Otherwise, if the VarLoc is primary
+/// location, erase the variable from the Vars set.
+void VarLocBasedLDV::OpenRangesSet::erase(const VarLoc &VL) {
+  // Erasure helper.
+  auto DoErase = [VL, this](DebugVariable VarToErase) {
+    auto *EraseFrom = VL.isEntryBackupLoc() ? &EntryValuesBackupVars : &Vars;
+    auto It = EraseFrom->find(VarToErase);
+    if (It != EraseFrom->end()) {
+      LocIndex ID = It->second;
+      VarLocs.reset(ID.getAsRawInteger());
+      EraseFrom->erase(It);
+    }
+  };
+
+  DebugVariable Var = VL.Var;
+
+  // Erase the variable/fragment that ends here.
+  DoErase(Var);
+
+  // Extract the fragment. Interpret an empty fragment as one that covers all
+  // possible bits.
+  FragmentInfo ThisFragment = Var.getFragmentOrDefault();
+
+  // There may be fragments that overlap the designated fragment. Look them up
+  // in the pre-computed overlap map, and erase them too.
+  auto MapIt = OverlappingFragments.find({Var.getVariable(), ThisFragment});
+  if (MapIt != OverlappingFragments.end()) {
+    for (auto Fragment : MapIt->second) {
+      VarLocBasedLDV::OptFragmentInfo FragmentHolder;
+      if (!DebugVariable::isDefaultFragment(Fragment))
+        FragmentHolder = VarLocBasedLDV::OptFragmentInfo(Fragment);
+      DoErase({Var.getVariable(), FragmentHolder, Var.getInlinedAt()});
+    }
+  }
+}
+
+void VarLocBasedLDV::OpenRangesSet::erase(const VarLocSet &KillSet,
+                                           const VarLocMap &VarLocIDs) {
+  VarLocs.intersectWithComplement(KillSet);
+  for (uint64_t ID : KillSet) {
+    const VarLoc *VL = &VarLocIDs[LocIndex::fromRawInteger(ID)];
+    auto *EraseFrom = VL->isEntryBackupLoc() ? &EntryValuesBackupVars : &Vars;
+    EraseFrom->erase(VL->Var);
+  }
+}
+
+void VarLocBasedLDV::OpenRangesSet::insert(LocIndex VarLocID,
+                                            const VarLoc &VL) {
+  auto *InsertInto = VL.isEntryBackupLoc() ? &EntryValuesBackupVars : &Vars;
+  VarLocs.set(VarLocID.getAsRawInteger());
+  InsertInto->insert({VL.Var, VarLocID});
+}
+
+/// Return the Loc ID of an entry value backup location, if it exists for the
+/// variable.
+llvm::Optional<LocIndex>
+VarLocBasedLDV::OpenRangesSet::getEntryValueBackup(DebugVariable Var) {
+  auto It = EntryValuesBackupVars.find(Var);
+  if (It != EntryValuesBackupVars.end())
+    return It->second;
+
+  return llvm::None;
+}
+
+void VarLocBasedLDV::collectIDsForRegs(VarLocSet &Collected,
+                                        const DefinedRegsSet &Regs,
+                                        const VarLocSet &CollectFrom) const {
+  assert(!Regs.empty() && "Nothing to collect");
+  SmallVector<uint32_t, 32> SortedRegs;
+  for (Register Reg : Regs)
+    SortedRegs.push_back(Reg);
+  array_pod_sort(SortedRegs.begin(), SortedRegs.end());
+  auto It = CollectFrom.find(LocIndex::rawIndexForReg(SortedRegs.front()));
+  auto End = CollectFrom.end();
+  for (uint32_t Reg : SortedRegs) {
+    // The half-open interval [FirstIndexForReg, FirstInvalidIndex) contains all
+    // possible VarLoc IDs for VarLocs of kind RegisterKind which live in Reg.
+    uint64_t FirstIndexForReg = LocIndex::rawIndexForReg(Reg);
+    uint64_t FirstInvalidIndex = LocIndex::rawIndexForReg(Reg + 1);
+    It.advanceToLowerBound(FirstIndexForReg);
+
+    // Iterate through that half-open interval and collect all the set IDs.
+    for (; It != End && *It < FirstInvalidIndex; ++It)
+      Collected.set(*It);
+
+    if (It == End)
+      return;
+  }
+}
+
+void VarLocBasedLDV::getUsedRegs(const VarLocSet &CollectFrom,
+                                  SmallVectorImpl<uint32_t> &UsedRegs) const {
+  // All register-based VarLocs are assigned indices greater than or equal to
+  // FirstRegIndex.
+  uint64_t FirstRegIndex = LocIndex::rawIndexForReg(1);
+  uint64_t FirstInvalidIndex =
+      LocIndex::rawIndexForReg(LocIndex::kFirstInvalidRegLocation);
+  for (auto It = CollectFrom.find(FirstRegIndex),
+            End = CollectFrom.find(FirstInvalidIndex);
+       It != End;) {
+    // We found a VarLoc ID for a VarLoc that lives in a register. Figure out
+    // which register and add it to UsedRegs.
+    uint32_t FoundReg = LocIndex::fromRawInteger(*It).Location;
+    assert((UsedRegs.empty() || FoundReg != UsedRegs.back()) &&
+           "Duplicate used reg");
+    UsedRegs.push_back(FoundReg);
+
+    // Skip to the next /set/ register. Note that this finds a lower bound, so
+    // even if there aren't any VarLocs living in `FoundReg+1`, we're still
+    // guaranteed to move on to the next register (or to end()).
+    uint64_t NextRegIndex = LocIndex::rawIndexForReg(FoundReg + 1);
+    It.advanceToLowerBound(NextRegIndex);
+  }
+}
+
+//===----------------------------------------------------------------------===//
+//            Debug Range Extension Implementation
+//===----------------------------------------------------------------------===//
+
+#ifndef NDEBUG
+void VarLocBasedLDV::printVarLocInMBB(const MachineFunction &MF,
+                                       const VarLocInMBB &V,
+                                       const VarLocMap &VarLocIDs,
+                                       const char *msg,
+                                       raw_ostream &Out) const {
+  Out << '\n' << msg << '\n';
+  for (const MachineBasicBlock &BB : MF) {
+    if (!V.count(&BB))
+      continue;
+    const VarLocSet &L = getVarLocsInMBB(&BB, V);
+    if (L.empty())
+      continue;
+    Out << "MBB: " << BB.getNumber() << ":\n";
+    for (uint64_t VLL : L) {
+      const VarLoc &VL = VarLocIDs[LocIndex::fromRawInteger(VLL)];
+      Out << " Var: " << VL.Var.getVariable()->getName();
+      Out << " MI: ";
+      VL.dump(TRI, Out);
+    }
+  }
+  Out << "\n";
+}
+#endif
+
+VarLocBasedLDV::VarLoc::SpillLoc
+VarLocBasedLDV::extractSpillBaseRegAndOffset(const MachineInstr &MI) {
+  assert(MI.hasOneMemOperand() &&
+         "Spill instruction does not have exactly one memory operand?");
+  auto MMOI = MI.memoperands_begin();
+  const PseudoSourceValue *PVal = (*MMOI)->getPseudoValue();
+  assert(PVal->kind() == PseudoSourceValue::FixedStack &&
+         "Inconsistent memory operand in spill instruction");
+  int FI = cast<FixedStackPseudoSourceValue>(PVal)->getFrameIndex();
+  const MachineBasicBlock *MBB = MI.getParent();
+  Register Reg;
+  StackOffset Offset = TFI->getFrameIndexReference(*MBB->getParent(), FI, Reg);
+  return {Reg, Offset};
+}
+
+/// Try to salvage the debug entry value if we encounter a new debug value
+/// describing the same parameter, otherwise stop tracking the value. Return
+/// true if we should stop tracking the entry value, otherwise return false.
+bool VarLocBasedLDV::removeEntryValue(const MachineInstr &MI,
+                                       OpenRangesSet &OpenRanges,
+                                       VarLocMap &VarLocIDs,
+                                       const VarLoc &EntryVL) {
+  // Skip the DBG_VALUE which is the debug entry value itself.
+  if (MI.isIdenticalTo(EntryVL.MI))
+    return false;
+
+  // If the parameter's location is not register location, we can not track
+  // the entry value any more. In addition, if the debug expression from the
+  // DBG_VALUE is not empty, we can assume the parameter's value has changed
+  // indicating that we should stop tracking its entry value as well.
+  if (!MI.getDebugOperand(0).isReg() ||
+      MI.getDebugExpression()->getNumElements() != 0)
+    return true;
+
+  // If the DBG_VALUE comes from a copy instruction that copies the entry value,
+  // it means the parameter's value has not changed and we should be able to use
+  // its entry value.
+  bool TrySalvageEntryValue = false;
+  Register Reg = MI.getDebugOperand(0).getReg();
+  auto I = std::next(MI.getReverseIterator());
+  const MachineOperand *SrcRegOp, *DestRegOp;
+  if (I != MI.getParent()->rend()) {
+    // TODO: Try to keep tracking of an entry value if we encounter a propagated
+    // DBG_VALUE describing the copy of the entry value. (Propagated entry value
+    // does not indicate the parameter modification.)
+    auto DestSrc = TII->isCopyInstr(*I);
+    if (!DestSrc)
+      return true;
+
+    SrcRegOp = DestSrc->Source;
+    DestRegOp = DestSrc->Destination;
+    if (Reg != DestRegOp->getReg())
+      return true;
+    TrySalvageEntryValue = true;
+  }
+
+  if (TrySalvageEntryValue) {
+    for (uint64_t ID : OpenRanges.getEntryValueBackupVarLocs()) {
+      const VarLoc &VL = VarLocIDs[LocIndex::fromRawInteger(ID)];
+      if (VL.getEntryValueCopyBackupReg() == Reg &&
+          VL.MI.getDebugOperand(0).getReg() == SrcRegOp->getReg())
+        return false;
+    }
+  }
+
+  return true;
+}
+
+/// End all previous ranges related to @MI and start a new range from @MI
+/// if it is a DBG_VALUE instr.
+void VarLocBasedLDV::transferDebugValue(const MachineInstr &MI,
+                                         OpenRangesSet &OpenRanges,
+                                         VarLocMap &VarLocIDs) {
+  if (!MI.isDebugValue())
+    return;
+  const DILocalVariable *Var = MI.getDebugVariable();
+  const DIExpression *Expr = MI.getDebugExpression();
+  const DILocation *DebugLoc = MI.getDebugLoc();
+  const DILocation *InlinedAt = DebugLoc->getInlinedAt();
+  assert(Var->isValidLocationForIntrinsic(DebugLoc) &&
+         "Expected inlined-at fields to agree");
+
+  DebugVariable V(Var, Expr, InlinedAt);
+
+  // Check if this DBG_VALUE indicates a parameter's value changing.
+  // If that is the case, we should stop tracking its entry value.
+  auto EntryValBackupID = OpenRanges.getEntryValueBackup(V);
+  if (Var->isParameter() && EntryValBackupID) {
+    const VarLoc &EntryVL = VarLocIDs[*EntryValBackupID];
+    if (removeEntryValue(MI, OpenRanges, VarLocIDs, EntryVL)) {
+      LLVM_DEBUG(dbgs() << "Deleting a DBG entry value because of: ";
+                 MI.print(dbgs(), /*IsStandalone*/ false,
+                          /*SkipOpers*/ false, /*SkipDebugLoc*/ false,
+                          /*AddNewLine*/ true, TII));
+      OpenRanges.erase(EntryVL);
+    }
+  }
+
+  if (isDbgValueDescribedByReg(MI) || MI.getDebugOperand(0).isImm() ||
+      MI.getDebugOperand(0).isFPImm() || MI.getDebugOperand(0).isCImm()) {
+    // Use normal VarLoc constructor for registers and immediates.
+    VarLoc VL(MI, LS);
+    // End all previous ranges of VL.Var.
+    OpenRanges.erase(VL);
+
+    LocIndex ID = VarLocIDs.insert(VL);
+    // Add the VarLoc to OpenRanges from this DBG_VALUE.
+    OpenRanges.insert(ID, VL);
+  } else if (MI.hasOneMemOperand()) {
+    llvm_unreachable("DBG_VALUE with mem operand encountered after regalloc?");
+  } else {
+    // This must be an undefined location. If it has an open range, erase it.
+    assert(MI.getDebugOperand(0).isReg() &&
+           MI.getDebugOperand(0).getReg() == 0 &&
+           "Unexpected non-undef DBG_VALUE encountered");
+    VarLoc VL(MI, LS);
+    OpenRanges.erase(VL);
+  }
+}
+
+/// Turn the entry value backup locations into primary locations.
+void VarLocBasedLDV::emitEntryValues(MachineInstr &MI,
+                                      OpenRangesSet &OpenRanges,
+                                      VarLocMap &VarLocIDs,
+                                      TransferMap &Transfers,
+                                      VarLocSet &KillSet) {
+  // Do not insert entry value locations after a terminator.
+  if (MI.isTerminator())
+    return;
+
+  for (uint64_t ID : KillSet) {
+    LocIndex Idx = LocIndex::fromRawInteger(ID);
+    const VarLoc &VL = VarLocIDs[Idx];
+    if (!VL.Var.getVariable()->isParameter())
+      continue;
+
+    auto DebugVar = VL.Var;
+    Optional<LocIndex> EntryValBackupID =
+        OpenRanges.getEntryValueBackup(DebugVar);
+
+    // If the parameter has the entry value backup, it means we should
+    // be able to use its entry value.
+    if (!EntryValBackupID)
+      continue;
+
+    const VarLoc &EntryVL = VarLocIDs[*EntryValBackupID];
+    VarLoc EntryLoc =
+        VarLoc::CreateEntryLoc(EntryVL.MI, LS, EntryVL.Expr, EntryVL.Loc.RegNo);
+    LocIndex EntryValueID = VarLocIDs.insert(EntryLoc);
+    Transfers.push_back({&MI, EntryValueID});
+    OpenRanges.insert(EntryValueID, EntryLoc);
+  }
+}
+
+/// Create new TransferDebugPair and insert it in \p Transfers. The VarLoc
+/// with \p OldVarID should be deleted form \p OpenRanges and replaced with
+/// new VarLoc. If \p NewReg is different than default zero value then the
+/// new location will be register location created by the copy like instruction,
+/// otherwise it is variable's location on the stack.
+void VarLocBasedLDV::insertTransferDebugPair(
+    MachineInstr &MI, OpenRangesSet &OpenRanges, TransferMap &Transfers,
+    VarLocMap &VarLocIDs, LocIndex OldVarID, TransferKind Kind,
+    Register NewReg) {
+  const MachineInstr *DebugInstr = &VarLocIDs[OldVarID].MI;
+
+  auto ProcessVarLoc = [&MI, &OpenRanges, &Transfers, &VarLocIDs](VarLoc &VL) {
+    LocIndex LocId = VarLocIDs.insert(VL);
+
+    // Close this variable's previous location range.
+    OpenRanges.erase(VL);
+
+    // Record the new location as an open range, and a postponed transfer
+    // inserting a DBG_VALUE for this location.
+    OpenRanges.insert(LocId, VL);
+    assert(!MI.isTerminator() && "Cannot insert DBG_VALUE after terminator");
+    TransferDebugPair MIP = {&MI, LocId};
+    Transfers.push_back(MIP);
+  };
+
+  // End all previous ranges of VL.Var.
+  OpenRanges.erase(VarLocIDs[OldVarID]);
+  switch (Kind) {
+  case TransferKind::TransferCopy: {
+    assert(NewReg &&
+           "No register supplied when handling a copy of a debug value");
+    // Create a DBG_VALUE instruction to describe the Var in its new
+    // register location.
+    VarLoc VL = VarLoc::CreateCopyLoc(*DebugInstr, LS, NewReg);
+    ProcessVarLoc(VL);
+    LLVM_DEBUG({
+      dbgs() << "Creating VarLoc for register copy:";
+      VL.dump(TRI);
+    });
+    return;
+  }
+  case TransferKind::TransferSpill: {
+    // Create a DBG_VALUE instruction to describe the Var in its spilled
+    // location.
+    VarLoc::SpillLoc SpillLocation = extractSpillBaseRegAndOffset(MI);
+    VarLoc VL = VarLoc::CreateSpillLoc(*DebugInstr, SpillLocation.SpillBase,
+                                       SpillLocation.SpillOffset, LS);
+    ProcessVarLoc(VL);
+    LLVM_DEBUG({
+      dbgs() << "Creating VarLoc for spill:";
+      VL.dump(TRI);
+    });
+    return;
+  }
+  case TransferKind::TransferRestore: {
+    assert(NewReg &&
+           "No register supplied when handling a restore of a debug value");
+    // DebugInstr refers to the pre-spill location, therefore we can reuse
+    // its expression.
+    VarLoc VL = VarLoc::CreateCopyLoc(*DebugInstr, LS, NewReg);
+    ProcessVarLoc(VL);
+    LLVM_DEBUG({
+      dbgs() << "Creating VarLoc for restore:";
+      VL.dump(TRI);
+    });
+    return;
+  }
+  }
+  llvm_unreachable("Invalid transfer kind");
+}
+
+/// A definition of a register may mark the end of a range.
+void VarLocBasedLDV::transferRegisterDef(
+    MachineInstr &MI, OpenRangesSet &OpenRanges, VarLocMap &VarLocIDs,
+    TransferMap &Transfers) {
+
+  // Meta Instructions do not affect the debug liveness of any register they
+  // define.
+  if (MI.isMetaInstruction())
+    return;
+
+  MachineFunction *MF = MI.getMF();
+  const TargetLowering *TLI = MF->getSubtarget().getTargetLowering();
+  Register SP = TLI->getStackPointerRegisterToSaveRestore();
+
+  // Find the regs killed by MI, and find regmasks of preserved regs.
+  DefinedRegsSet DeadRegs;
+  SmallVector<const uint32_t *, 4> RegMasks;
+  for (const MachineOperand &MO : MI.operands()) {
+    // Determine whether the operand is a register def.
+    if (MO.isReg() && MO.isDef() && MO.getReg() &&
+        Register::isPhysicalRegister(MO.getReg()) &&
+        !(MI.isCall() && MO.getReg() == SP)) {
+      // Remove ranges of all aliased registers.
+      for (MCRegAliasIterator RAI(MO.getReg(), TRI, true); RAI.isValid(); ++RAI)
+        // FIXME: Can we break out of this loop early if no insertion occurs?
+        DeadRegs.insert(*RAI);
+    } else if (MO.isRegMask()) {
+      RegMasks.push_back(MO.getRegMask());
+    }
+  }
+
+  // Erase VarLocs which reside in one of the dead registers. For performance
+  // reasons, it's critical to not iterate over the full set of open VarLocs.
+  // Iterate over the set of dying/used regs instead.
+  if (!RegMasks.empty()) {
+    SmallVector<uint32_t, 32> UsedRegs;
+    getUsedRegs(OpenRanges.getVarLocs(), UsedRegs);
+    for (uint32_t Reg : UsedRegs) {
+      // Remove ranges of all clobbered registers. Register masks don't usually
+      // list SP as preserved. Assume that call instructions never clobber SP,
+      // because some backends (e.g., AArch64) never list SP in the regmask.
+      // While the debug info may be off for an instruction or two around
+      // callee-cleanup calls, transferring the DEBUG_VALUE across the call is
+      // still a better user experience.
+      if (Reg == SP)
+        continue;
+      bool AnyRegMaskKillsReg =
+          any_of(RegMasks, [Reg](const uint32_t *RegMask) {
+            return MachineOperand::clobbersPhysReg(RegMask, Reg);
+          });
+      if (AnyRegMaskKillsReg)
+        DeadRegs.insert(Reg);
+    }
+  }
+
+  if (DeadRegs.empty())
+    return;
+
+  VarLocSet KillSet(Alloc);
+  collectIDsForRegs(KillSet, DeadRegs, OpenRanges.getVarLocs());
+  OpenRanges.erase(KillSet, VarLocIDs);
+
+  if (TPC) {
+    auto &TM = TPC->getTM<TargetMachine>();
+    if (TM.Options.ShouldEmitDebugEntryValues())
+      emitEntryValues(MI, OpenRanges, VarLocIDs, Transfers, KillSet);
+  }
+}
+
+bool VarLocBasedLDV::isSpillInstruction(const MachineInstr &MI,
+                                         MachineFunction *MF) {
+  // TODO: Handle multiple stores folded into one.
+  if (!MI.hasOneMemOperand())
+    return false;
+
+  if (!MI.getSpillSize(TII) && !MI.getFoldedSpillSize(TII))
+    return false; // This is not a spill instruction, since no valid size was
+                  // returned from either function.
+
+  return true;
+}
+
+bool VarLocBasedLDV::isLocationSpill(const MachineInstr &MI,
+                                      MachineFunction *MF, Register &Reg) {
+  if (!isSpillInstruction(MI, MF))
+    return false;
+
+  auto isKilledReg = [&](const MachineOperand MO, Register &Reg) {
+    if (!MO.isReg() || !MO.isUse()) {
+      Reg = 0;
+      return false;
+    }
+    Reg = MO.getReg();
+    return MO.isKill();
+  };
+
+  for (const MachineOperand &MO : MI.operands()) {
+    // In a spill instruction generated by the InlineSpiller the spilled
+    // register has its kill flag set.
+    if (isKilledReg(MO, Reg))
+      return true;
+    if (Reg != 0) {
+      // Check whether next instruction kills the spilled register.
+      // FIXME: Current solution does not cover search for killed register in
+      // bundles and instructions further down the chain.
+      auto NextI = std::next(MI.getIterator());
+      // Skip next instruction that points to basic block end iterator.
+      if (MI.getParent()->end() == NextI)
+        continue;
+      Register RegNext;
+      for (const MachineOperand &MONext : NextI->operands()) {
+        // Return true if we came across the register from the
+        // previous spill instruction that is killed in NextI.
+        if (isKilledReg(MONext, RegNext) && RegNext == Reg)
+          return true;
+      }
+    }
+  }
+  // Return false if we didn't find spilled register.
+  return false;
+}
+
+Optional<VarLocBasedLDV::VarLoc::SpillLoc>
+VarLocBasedLDV::isRestoreInstruction(const MachineInstr &MI,
+                                      MachineFunction *MF, Register &Reg) {
+  if (!MI.hasOneMemOperand())
+    return None;
+
+  // FIXME: Handle folded restore instructions with more than one memory
+  // operand.
+  if (MI.getRestoreSize(TII)) {
+    Reg = MI.getOperand(0).getReg();
+    return extractSpillBaseRegAndOffset(MI);
+  }
+  return None;
+}
+
+/// A spilled register may indicate that we have to end the current range of
+/// a variable and create a new one for the spill location.
+/// A restored register may indicate the reverse situation.
+/// We don't want to insert any instructions in process(), so we just create
+/// the DBG_VALUE without inserting it and keep track of it in \p Transfers.
+/// It will be inserted into the BB when we're done iterating over the
+/// instructions.
+void VarLocBasedLDV::transferSpillOrRestoreInst(MachineInstr &MI,
+                                                 OpenRangesSet &OpenRanges,
+                                                 VarLocMap &VarLocIDs,
+                                                 TransferMap &Transfers) {
+  MachineFunction *MF = MI.getMF();
+  TransferKind TKind;
+  Register Reg;
+  Optional<VarLoc::SpillLoc> Loc;
+
+  LLVM_DEBUG(dbgs() << "Examining instruction: "; MI.dump(););
+
+  // First, if there are any DBG_VALUEs pointing at a spill slot that is
+  // written to, then close the variable location. The value in memory
+  // will have changed.
+  VarLocSet KillSet(Alloc);
+  if (isSpillInstruction(MI, MF)) {
+    Loc = extractSpillBaseRegAndOffset(MI);
+    for (uint64_t ID : OpenRanges.getSpillVarLocs()) {
+      LocIndex Idx = LocIndex::fromRawInteger(ID);
+      const VarLoc &VL = VarLocIDs[Idx];
+      assert(VL.Kind == VarLoc::SpillLocKind && "Broken VarLocSet?");
+      if (VL.Loc.SpillLocation == *Loc) {
+        // This location is overwritten by the current instruction -- terminate
+        // the open range, and insert an explicit DBG_VALUE $noreg.
+        //
+        // Doing this at a later stage would require re-interpreting all
+        // DBG_VALUes and DIExpressions to identify whether they point at
+        // memory, and then analysing all memory writes to see if they
+        // overwrite that memory, which is expensive.
+        //
+        // At this stage, we already know which DBG_VALUEs are for spills and
+        // where they are located; it's best to fix handle overwrites now.
+        KillSet.set(ID);
+        VarLoc UndefVL = VarLoc::CreateCopyLoc(VL.MI, LS, 0);
+        LocIndex UndefLocID = VarLocIDs.insert(UndefVL);
+        Transfers.push_back({&MI, UndefLocID});
+      }
+    }
+    OpenRanges.erase(KillSet, VarLocIDs);
+  }
+
+  // Try to recognise spill and restore instructions that may create a new
+  // variable location.
+  if (isLocationSpill(MI, MF, Reg)) {
+    TKind = TransferKind::TransferSpill;
+    LLVM_DEBUG(dbgs() << "Recognized as spill: "; MI.dump(););
+    LLVM_DEBUG(dbgs() << "Register: " << Reg << " " << printReg(Reg, TRI)
+                      << "\n");
+  } else {
+    if (!(Loc = isRestoreInstruction(MI, MF, Reg)))
+      return;
+    TKind = TransferKind::TransferRestore;
+    LLVM_DEBUG(dbgs() << "Recognized as restore: "; MI.dump(););
+    LLVM_DEBUG(dbgs() << "Register: " << Reg << " " << printReg(Reg, TRI)
+                      << "\n");
+  }
+  // Check if the register or spill location is the location of a debug value.
+  auto TransferCandidates = OpenRanges.getEmptyVarLocRange();
+  if (TKind == TransferKind::TransferSpill)
+    TransferCandidates = OpenRanges.getRegisterVarLocs(Reg);
+  else if (TKind == TransferKind::TransferRestore)
+    TransferCandidates = OpenRanges.getSpillVarLocs();
+  for (uint64_t ID : TransferCandidates) {
+    LocIndex Idx = LocIndex::fromRawInteger(ID);
+    const VarLoc &VL = VarLocIDs[Idx];
+    if (TKind == TransferKind::TransferSpill) {
+      assert(VL.isDescribedByReg() == Reg && "Broken VarLocSet?");
+      LLVM_DEBUG(dbgs() << "Spilling Register " << printReg(Reg, TRI) << '('
+                        << VL.Var.getVariable()->getName() << ")\n");
+    } else {
+      assert(TKind == TransferKind::TransferRestore &&
+             VL.Kind == VarLoc::SpillLocKind && "Broken VarLocSet?");
+      if (VL.Loc.SpillLocation != *Loc)
+        // The spill location is not the location of a debug value.
+        continue;
+      LLVM_DEBUG(dbgs() << "Restoring Register " << printReg(Reg, TRI) << '('
+                        << VL.Var.getVariable()->getName() << ")\n");
+    }
+    insertTransferDebugPair(MI, OpenRanges, Transfers, VarLocIDs, Idx, TKind,
+                            Reg);
+    // FIXME: A comment should explain why it's correct to return early here,
+    // if that is in fact correct.
+    return;
+  }
+}
+
+/// If \p MI is a register copy instruction, that copies a previously tracked
+/// value from one register to another register that is callee saved, we
+/// create new DBG_VALUE instruction  described with copy destination register.
+void VarLocBasedLDV::transferRegisterCopy(MachineInstr &MI,
+                                           OpenRangesSet &OpenRanges,
+                                           VarLocMap &VarLocIDs,
+                                           TransferMap &Transfers) {
+  auto DestSrc = TII->isCopyInstr(MI);
+  if (!DestSrc)
+    return;
+
+  const MachineOperand *DestRegOp = DestSrc->Destination;
+  const MachineOperand *SrcRegOp = DestSrc->Source;
+
+  if (!DestRegOp->isDef())
+    return;
+
+  auto isCalleeSavedReg = [&](Register Reg) {
+    for (MCRegAliasIterator RAI(Reg, TRI, true); RAI.isValid(); ++RAI)
+      if (CalleeSavedRegs.test(*RAI))
+        return true;
+    return false;
+  };
+
+  Register SrcReg = SrcRegOp->getReg();
+  Register DestReg = DestRegOp->getReg();
+
+  // We want to recognize instructions where destination register is callee
+  // saved register. If register that could be clobbered by the call is
+  // included, there would be a great chance that it is going to be clobbered
+  // soon. It is more likely that previous register location, which is callee
+  // saved, is going to stay unclobbered longer, even if it is killed.
+  if (!isCalleeSavedReg(DestReg))
+    return;
+
+  // Remember an entry value movement. If we encounter a new debug value of
+  // a parameter describing only a moving of the value around, rather then
+  // modifying it, we are still able to use the entry value if needed.
+  if (isRegOtherThanSPAndFP(*DestRegOp, MI, TRI)) {
+    for (uint64_t ID : OpenRanges.getEntryValueBackupVarLocs()) {
+      LocIndex Idx = LocIndex::fromRawInteger(ID);
+      const VarLoc &VL = VarLocIDs[Idx];
+      if (VL.getEntryValueBackupReg() == SrcReg) {
+        LLVM_DEBUG(dbgs() << "Copy of the entry value: "; MI.dump(););
+        VarLoc EntryValLocCopyBackup =
+            VarLoc::CreateEntryCopyBackupLoc(VL.MI, LS, VL.Expr, DestReg);
+
+        // Stop tracking the original entry value.
+        OpenRanges.erase(VL);
+
+        // Start tracking the entry value copy.
+        LocIndex EntryValCopyLocID = VarLocIDs.insert(EntryValLocCopyBackup);
+        OpenRanges.insert(EntryValCopyLocID, EntryValLocCopyBackup);
+        break;
+      }
+    }
+  }
+
+  if (!SrcRegOp->isKill())
+    return;
+
+  for (uint64_t ID : OpenRanges.getRegisterVarLocs(SrcReg)) {
+    LocIndex Idx = LocIndex::fromRawInteger(ID);
+    assert(VarLocIDs[Idx].isDescribedByReg() == SrcReg && "Broken VarLocSet?");
+    insertTransferDebugPair(MI, OpenRanges, Transfers, VarLocIDs, Idx,
+                            TransferKind::TransferCopy, DestReg);
+    // FIXME: A comment should explain why it's correct to return early here,
+    // if that is in fact correct.
+    return;
+  }
+}
+
+/// Terminate all open ranges at the end of the current basic block.
+bool VarLocBasedLDV::transferTerminator(MachineBasicBlock *CurMBB,
+                                         OpenRangesSet &OpenRanges,
+                                         VarLocInMBB &OutLocs,
+                                         const VarLocMap &VarLocIDs) {
+  bool Changed = false;
+
+  LLVM_DEBUG(for (uint64_t ID
+                  : OpenRanges.getVarLocs()) {
+    // Copy OpenRanges to OutLocs, if not already present.
+    dbgs() << "Add to OutLocs in MBB #" << CurMBB->getNumber() << ":  ";
+    VarLocIDs[LocIndex::fromRawInteger(ID)].dump(TRI);
+  });
+  VarLocSet &VLS = getVarLocsInMBB(CurMBB, OutLocs);
+  Changed = VLS != OpenRanges.getVarLocs();
+  // New OutLocs set may be different due to spill, restore or register
+  // copy instruction processing.
+  if (Changed)
+    VLS = OpenRanges.getVarLocs();
+  OpenRanges.clear();
+  return Changed;
+}
+
+/// Accumulate a mapping between each DILocalVariable fragment and other
+/// fragments of that DILocalVariable which overlap. This reduces work during
+/// the data-flow stage from "Find any overlapping fragments" to "Check if the
+/// known-to-overlap fragments are present".
+/// \param MI A previously unprocessed DEBUG_VALUE instruction to analyze for
+///           fragment usage.
+/// \param SeenFragments Map from DILocalVariable to all fragments of that
+///           Variable which are known to exist.
+/// \param OverlappingFragments The overlap map being constructed, from one
+///           Var/Fragment pair to a vector of fragments known to overlap.
+void VarLocBasedLDV::accumulateFragmentMap(MachineInstr &MI,
+                                            VarToFragments &SeenFragments,
+                                            OverlapMap &OverlappingFragments) {
+  DebugVariable MIVar(MI.getDebugVariable(), MI.getDebugExpression(),
+                      MI.getDebugLoc()->getInlinedAt());
+  FragmentInfo ThisFragment = MIVar.getFragmentOrDefault();
+
+  // If this is the first sighting of this variable, then we are guaranteed
+  // there are currently no overlapping fragments either. Initialize the set
+  // of seen fragments, record no overlaps for the current one, and return.
+  auto SeenIt = SeenFragments.find(MIVar.getVariable());
+  if (SeenIt == SeenFragments.end()) {
+    SmallSet<FragmentInfo, 4> OneFragment;
+    OneFragment.insert(ThisFragment);
+    SeenFragments.insert({MIVar.getVariable(), OneFragment});
+
+    OverlappingFragments.insert({{MIVar.getVariable(), ThisFragment}, {}});
+    return;
+  }
+
+  // If this particular Variable/Fragment pair already exists in the overlap
+  // map, it has already been accounted for.
+  auto IsInOLapMap =
+      OverlappingFragments.insert({{MIVar.getVariable(), ThisFragment}, {}});
+  if (!IsInOLapMap.second)
+    return;
+
+  auto &ThisFragmentsOverlaps = IsInOLapMap.first->second;
+  auto &AllSeenFragments = SeenIt->second;
+
+  // Otherwise, examine all other seen fragments for this variable, with "this"
+  // fragment being a previously unseen fragment. Record any pair of
+  // overlapping fragments.
+  for (auto &ASeenFragment : AllSeenFragments) {
+    // Does this previously seen fragment overlap?
+    if (DIExpression::fragmentsOverlap(ThisFragment, ASeenFragment)) {
+      // Yes: Mark the current fragment as being overlapped.
+      ThisFragmentsOverlaps.push_back(ASeenFragment);
+      // Mark the previously seen fragment as being overlapped by the current
+      // one.
+      auto ASeenFragmentsOverlaps =
+          OverlappingFragments.find({MIVar.getVariable(), ASeenFragment});
+      assert(ASeenFragmentsOverlaps != OverlappingFragments.end() &&
+             "Previously seen var fragment has no vector of overlaps");
+      ASeenFragmentsOverlaps->second.push_back(ThisFragment);
+    }
+  }
+
+  AllSeenFragments.insert(ThisFragment);
+}
+
+/// This routine creates OpenRanges.
+void VarLocBasedLDV::process(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                              VarLocMap &VarLocIDs, TransferMap &Transfers) {
+  transferDebugValue(MI, OpenRanges, VarLocIDs);
+  transferRegisterDef(MI, OpenRanges, VarLocIDs, Transfers);
+  transferRegisterCopy(MI, OpenRanges, VarLocIDs, Transfers);
+  transferSpillOrRestoreInst(MI, OpenRanges, VarLocIDs, Transfers);
+}
+
+/// This routine joins the analysis results of all incoming edges in @MBB by
+/// inserting a new DBG_VALUE instruction at the start of the @MBB - if the same
+/// source variable in all the predecessors of @MBB reside in the same location.
+bool VarLocBasedLDV::join(
+    MachineBasicBlock &MBB, VarLocInMBB &OutLocs, VarLocInMBB &InLocs,
+    const VarLocMap &VarLocIDs,
+    SmallPtrSet<const MachineBasicBlock *, 16> &Visited,
+    SmallPtrSetImpl<const MachineBasicBlock *> &ArtificialBlocks) {
+  LLVM_DEBUG(dbgs() << "join MBB: " << MBB.getNumber() << "\n");
+
+  VarLocSet InLocsT(Alloc); // Temporary incoming locations.
+
+  // For all predecessors of this MBB, find the set of VarLocs that
+  // can be joined.
+  int NumVisited = 0;
+  for (auto p : MBB.predecessors()) {
+    // Ignore backedges if we have not visited the predecessor yet. As the
+    // predecessor hasn't yet had locations propagated into it, most locations
+    // will not yet be valid, so treat them as all being uninitialized and
+    // potentially valid. If a location guessed to be correct here is
+    // invalidated later, we will remove it when we revisit this block.
+    if (!Visited.count(p)) {
+      LLVM_DEBUG(dbgs() << "  ignoring unvisited pred MBB: " << p->getNumber()
+                        << "\n");
+      continue;
+    }
+    auto OL = OutLocs.find(p);
+    // Join is null in case of empty OutLocs from any of the pred.
+    if (OL == OutLocs.end())
+      return false;
+
+    // Just copy over the Out locs to incoming locs for the first visited
+    // predecessor, and for all other predecessors join the Out locs.
+    VarLocSet &OutLocVLS = *OL->second.get();
+    if (!NumVisited)
+      InLocsT = OutLocVLS;
+    else
+      InLocsT &= OutLocVLS;
+
+    LLVM_DEBUG({
+      if (!InLocsT.empty()) {
+        for (uint64_t ID : InLocsT)
+          dbgs() << "  gathered candidate incoming var: "
+                 << VarLocIDs[LocIndex::fromRawInteger(ID)]
+                        .Var.getVariable()
+                        ->getName()
+                 << "\n";
+      }
+    });
+
+    NumVisited++;
+  }
+
+  // Filter out DBG_VALUES that are out of scope.
+  VarLocSet KillSet(Alloc);
+  bool IsArtificial = ArtificialBlocks.count(&MBB);
+  if (!IsArtificial) {
+    for (uint64_t ID : InLocsT) {
+      LocIndex Idx = LocIndex::fromRawInteger(ID);
+      if (!VarLocIDs[Idx].dominates(LS, MBB)) {
+        KillSet.set(ID);
+        LLVM_DEBUG({
+          auto Name = VarLocIDs[Idx].Var.getVariable()->getName();
+          dbgs() << "  killing " << Name << ", it doesn't dominate MBB\n";
+        });
+      }
+    }
+  }
+  InLocsT.intersectWithComplement(KillSet);
+
+  // As we are processing blocks in reverse post-order we
+  // should have processed at least one predecessor, unless it
+  // is the entry block which has no predecessor.
+  assert((NumVisited || MBB.pred_empty()) &&
+         "Should have processed at least one predecessor");
+
+  VarLocSet &ILS = getVarLocsInMBB(&MBB, InLocs);
+  bool Changed = false;
+  if (ILS != InLocsT) {
+    ILS = InLocsT;
+    Changed = true;
+  }
+
+  return Changed;
+}
+
+void VarLocBasedLDV::flushPendingLocs(VarLocInMBB &PendingInLocs,
+                                       VarLocMap &VarLocIDs) {
+  // PendingInLocs records all locations propagated into blocks, which have
+  // not had DBG_VALUE insts created. Go through and create those insts now.
+  for (auto &Iter : PendingInLocs) {
+    // Map is keyed on a constant pointer, unwrap it so we can insert insts.
+    auto &MBB = const_cast<MachineBasicBlock &>(*Iter.first);
+    VarLocSet &Pending = *Iter.second.get();
+
+    for (uint64_t ID : Pending) {
+      // The ID location is live-in to MBB -- work out what kind of machine
+      // location it is and create a DBG_VALUE.
+      const VarLoc &DiffIt = VarLocIDs[LocIndex::fromRawInteger(ID)];
+      if (DiffIt.isEntryBackupLoc())
+        continue;
+      MachineInstr *MI = DiffIt.BuildDbgValue(*MBB.getParent());
+      MBB.insert(MBB.instr_begin(), MI);
+
+      (void)MI;
+      LLVM_DEBUG(dbgs() << "Inserted: "; MI->dump(););
+    }
+  }
+}
+
+bool VarLocBasedLDV::isEntryValueCandidate(
+    const MachineInstr &MI, const DefinedRegsSet &DefinedRegs) const {
+  assert(MI.isDebugValue() && "This must be DBG_VALUE.");
+
+  // TODO: Add support for local variables that are expressed in terms of
+  // parameters entry values.
+  // TODO: Add support for modified arguments that can be expressed
+  // by using its entry value.
+  auto *DIVar = MI.getDebugVariable();
+  if (!DIVar->isParameter())
+    return false;
+
+  // Do not consider parameters that belong to an inlined function.
+  if (MI.getDebugLoc()->getInlinedAt())
+    return false;
+
+  // Only consider parameters that are described using registers. Parameters
+  // that are passed on the stack are not yet supported, so ignore debug
+  // values that are described by the frame or stack pointer.
+  if (!isRegOtherThanSPAndFP(MI.getDebugOperand(0), MI, TRI))
+    return false;
+
+  // If a parameter's value has been propagated from the caller, then the
+  // parameter's DBG_VALUE may be described using a register defined by some
+  // instruction in the entry block, in which case we shouldn't create an
+  // entry value.
+  if (DefinedRegs.count(MI.getDebugOperand(0).getReg()))
+    return false;
+
+  // TODO: Add support for parameters that have a pre-existing debug expressions
+  // (e.g. fragments).
+  if (MI.getDebugExpression()->getNumElements() > 0)
+    return false;
+
+  return true;
+}
+
+/// Collect all register defines (including aliases) for the given instruction.
+static void collectRegDefs(const MachineInstr &MI, DefinedRegsSet &Regs,
+                           const TargetRegisterInfo *TRI) {
+  for (const MachineOperand &MO : MI.operands())
+    if (MO.isReg() && MO.isDef() && MO.getReg())
+      for (MCRegAliasIterator AI(MO.getReg(), TRI, true); AI.isValid(); ++AI)
+        Regs.insert(*AI);
+}
+
+/// This routine records the entry values of function parameters. The values
+/// could be used as backup values. If we loose the track of some unmodified
+/// parameters, the backup values will be used as a primary locations.
+void VarLocBasedLDV::recordEntryValue(const MachineInstr &MI,
+                                       const DefinedRegsSet &DefinedRegs,
+                                       OpenRangesSet &OpenRanges,
+                                       VarLocMap &VarLocIDs) {
+  if (TPC) {
+    auto &TM = TPC->getTM<TargetMachine>();
+    if (!TM.Options.ShouldEmitDebugEntryValues())
+      return;
+  }
+
+  DebugVariable V(MI.getDebugVariable(), MI.getDebugExpression(),
+                  MI.getDebugLoc()->getInlinedAt());
+
+  if (!isEntryValueCandidate(MI, DefinedRegs) ||
+      OpenRanges.getEntryValueBackup(V))
+    return;
+
+  LLVM_DEBUG(dbgs() << "Creating the backup entry location: "; MI.dump(););
+
+  // Create the entry value and use it as a backup location until it is
+  // valid. It is valid until a parameter is not changed.
+  DIExpression *NewExpr =
+      DIExpression::prepend(MI.getDebugExpression(), DIExpression::EntryValue);
+  VarLoc EntryValLocAsBackup = VarLoc::CreateEntryBackupLoc(MI, LS, NewExpr);
+  LocIndex EntryValLocID = VarLocIDs.insert(EntryValLocAsBackup);
+  OpenRanges.insert(EntryValLocID, EntryValLocAsBackup);
+}
+
+/// Calculate the liveness information for the given machine function and
+/// extend ranges across basic blocks.
+bool VarLocBasedLDV::ExtendRanges(MachineFunction &MF, TargetPassConfig *TPC) {
+  LLVM_DEBUG(dbgs() << "\nDebug Range Extension\n");
+
+  if (!MF.getFunction().getSubprogram())
+    // VarLocBaseLDV will already have removed all DBG_VALUEs.
+    return false;
+
+  // Skip functions from NoDebug compilation units.
+  if (MF.getFunction().getSubprogram()->getUnit()->getEmissionKind() ==
+      DICompileUnit::NoDebug)
+    return false;
+
+  TRI = MF.getSubtarget().getRegisterInfo();
+  TII = MF.getSubtarget().getInstrInfo();
+  TFI = MF.getSubtarget().getFrameLowering();
+  TFI->getCalleeSaves(MF, CalleeSavedRegs);
+  this->TPC = TPC;
+  LS.initialize(MF);
+
+  bool Changed = false;
+  bool OLChanged = false;
+  bool MBBJoined = false;
+
+  VarLocMap VarLocIDs;         // Map VarLoc<>unique ID for use in bitvectors.
+  OverlapMap OverlapFragments; // Map of overlapping variable fragments.
+  OpenRangesSet OpenRanges(Alloc, OverlapFragments);
+                              // Ranges that are open until end of bb.
+  VarLocInMBB OutLocs;        // Ranges that exist beyond bb.
+  VarLocInMBB InLocs;         // Ranges that are incoming after joining.
+  TransferMap Transfers;      // DBG_VALUEs associated with transfers (such as
+                              // spills, copies and restores).
+
+  VarToFragments SeenFragments;
+
+  // Blocks which are artificial, i.e. blocks which exclusively contain
+  // instructions without locations, or with line 0 locations.
+  SmallPtrSet<const MachineBasicBlock *, 16> ArtificialBlocks;
+
+  DenseMap<unsigned int, MachineBasicBlock *> OrderToBB;
+  DenseMap<MachineBasicBlock *, unsigned int> BBToOrder;
+  std::priority_queue<unsigned int, std::vector<unsigned int>,
+                      std::greater<unsigned int>>
+      Worklist;
+  std::priority_queue<unsigned int, std::vector<unsigned int>,
+                      std::greater<unsigned int>>
+      Pending;
+
+  // Set of register defines that are seen when traversing the entry block
+  // looking for debug entry value candidates.
+  DefinedRegsSet DefinedRegs;
+
+  // Only in the case of entry MBB collect DBG_VALUEs representing
+  // function parameters in order to generate debug entry values for them.
+  MachineBasicBlock &First_MBB = *(MF.begin());
+  for (auto &MI : First_MBB) {
+    collectRegDefs(MI, DefinedRegs, TRI);
+    if (MI.isDebugValue())
+      recordEntryValue(MI, DefinedRegs, OpenRanges, VarLocIDs);
+  }
+
+  // Initialize per-block structures and scan for fragment overlaps.
+  for (auto &MBB : MF)
+    for (auto &MI : MBB)
+      if (MI.isDebugValue())
+        accumulateFragmentMap(MI, SeenFragments, OverlapFragments);
+
+  auto hasNonArtificialLocation = [](const MachineInstr &MI) -> bool {
+    if (const DebugLoc &DL = MI.getDebugLoc())
+      return DL.getLine() != 0;
+    return false;
+  };
+  for (auto &MBB : MF)
+    if (none_of(MBB.instrs(), hasNonArtificialLocation))
+      ArtificialBlocks.insert(&MBB);
+
+  LLVM_DEBUG(printVarLocInMBB(MF, OutLocs, VarLocIDs,
+                              "OutLocs after initialization", dbgs()));
+
+  ReversePostOrderTraversal<MachineFunction *> RPOT(&MF);
+  unsigned int RPONumber = 0;
+  for (auto RI = RPOT.begin(), RE = RPOT.end(); RI != RE; ++RI) {
+    OrderToBB[RPONumber] = *RI;
+    BBToOrder[*RI] = RPONumber;
+    Worklist.push(RPONumber);
+    ++RPONumber;
+  }
+
+  if (RPONumber > InputBBLimit) {
+    unsigned NumInputDbgValues = 0;
+    for (auto &MBB : MF)
+      for (auto &MI : MBB)
+        if (MI.isDebugValue())
+          ++NumInputDbgValues;
+    if (NumInputDbgValues > InputDbgValueLimit) {
+      LLVM_DEBUG(dbgs() << "Disabling VarLocBasedLDV: " << MF.getName()
+                        << " has " << RPONumber << " basic blocks and "
+                        << NumInputDbgValues
+                        << " input DBG_VALUEs, exceeding limits.\n");
+      return false;
+    }
+  }
+
+  // This is a standard "union of predecessor outs" dataflow problem.
+  // To solve it, we perform join() and process() using the two worklist method
+  // until the ranges converge.
+  // Ranges have converged when both worklists are empty.
+  SmallPtrSet<const MachineBasicBlock *, 16> Visited;
+  while (!Worklist.empty() || !Pending.empty()) {
+    // We track what is on the pending worklist to avoid inserting the same
+    // thing twice.  We could avoid this with a custom priority queue, but this
+    // is probably not worth it.
+    SmallPtrSet<MachineBasicBlock *, 16> OnPending;
+    LLVM_DEBUG(dbgs() << "Processing Worklist\n");
+    while (!Worklist.empty()) {
+      MachineBasicBlock *MBB = OrderToBB[Worklist.top()];
+      Worklist.pop();
+      MBBJoined = join(*MBB, OutLocs, InLocs, VarLocIDs, Visited,
+                       ArtificialBlocks);
+      MBBJoined |= Visited.insert(MBB).second;
+      if (MBBJoined) {
+        MBBJoined = false;
+        Changed = true;
+        // Now that we have started to extend ranges across BBs we need to
+        // examine spill, copy and restore instructions to see whether they
+        // operate with registers that correspond to user variables.
+        // First load any pending inlocs.
+        OpenRanges.insertFromLocSet(getVarLocsInMBB(MBB, InLocs), VarLocIDs);
+        for (auto &MI : *MBB)
+          process(MI, OpenRanges, VarLocIDs, Transfers);
+        OLChanged |= transferTerminator(MBB, OpenRanges, OutLocs, VarLocIDs);
+
+        LLVM_DEBUG(printVarLocInMBB(MF, OutLocs, VarLocIDs,
+                                    "OutLocs after propagating", dbgs()));
+        LLVM_DEBUG(printVarLocInMBB(MF, InLocs, VarLocIDs,
+                                    "InLocs after propagating", dbgs()));
+
+        if (OLChanged) {
+          OLChanged = false;
+          for (auto s : MBB->successors())
+            if (OnPending.insert(s).second) {
+              Pending.push(BBToOrder[s]);
+            }
+        }
+      }
+    }
+    Worklist.swap(Pending);
+    // At this point, pending must be empty, since it was just the empty
+    // worklist
+    assert(Pending.empty() && "Pending should be empty");
+  }
+
+  // Add any DBG_VALUE instructions created by location transfers.
+  for (auto &TR : Transfers) {
+    assert(!TR.TransferInst->isTerminator() &&
+           "Cannot insert DBG_VALUE after terminator");
+    MachineBasicBlock *MBB = TR.TransferInst->getParent();
+    const VarLoc &VL = VarLocIDs[TR.LocationID];
+    MachineInstr *MI = VL.BuildDbgValue(MF);
+    MBB->insertAfterBundle(TR.TransferInst->getIterator(), MI);
+  }
+  Transfers.clear();
+
+  // Deferred inlocs will not have had any DBG_VALUE insts created; do
+  // that now.
+  flushPendingLocs(InLocs, VarLocIDs);
+
+  LLVM_DEBUG(printVarLocInMBB(MF, OutLocs, VarLocIDs, "Final OutLocs", dbgs()));
+  LLVM_DEBUG(printVarLocInMBB(MF, InLocs, VarLocIDs, "Final InLocs", dbgs()));
+  return Changed;
+}
+
+LDVImpl *
+llvm::makeVarLocBasedLiveDebugValues()
+{
+  return new VarLocBasedLDV();
+}