xref: /llvm-project-15.0.7/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp (revision c1121908)

//===- llvm/CodeGen/DwarfDebug.cpp - Dwarf Debug Framework ----------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file contains support for writing dwarf debug info into asm files.
//
//===----------------------------------------------------------------------===//

#include "DwarfDebug.h"
#include "ByteStreamer.h"
#include "DIEHash.h"
#include "DebugLocEntry.h"
#include "DebugLocStream.h"
#include "DwarfCompileUnit.h"
#include "DwarfExpression.h"
#include "DwarfFile.h"
#include "DwarfUnit.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Triple.h"
#include "llvm/ADT/Twine.h"
#include "llvm/BinaryFormat/Dwarf.h"
#include "llvm/CodeGen/AccelTable.h"
#include "llvm/CodeGen/AsmPrinter.h"
#include "llvm/CodeGen/DIE.h"
#include "llvm/CodeGen/LexicalScopes.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/DebugInfo/DWARF/DWARFExpression.h"
#include "llvm/DebugInfo/DWARF/DWARFDataExtractor.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Module.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCDwarf.h"
#include "llvm/MC/MCSection.h"
#include "llvm/MC/MCStreamer.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCTargetOptions.h"
#include "llvm/MC/MachineLocation.h"
#include "llvm/MC/SectionKind.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MD5.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/Timer.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetLoweringObjectFile.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <string>
#include <utility>
#include <vector>

using namespace llvm;

#define DEBUG_TYPE "dwarfdebug"

STATISTIC(NumCSParams, "Number of dbg call site params created");

static cl::opt<bool>
DisableDebugInfoPrinting("disable-debug-info-print", cl::Hidden,
                         cl::desc("Disable debug info printing"));

static cl::opt<bool> UseDwarfRangesBaseAddressSpecifier(
    "use-dwarf-ranges-base-address-specifier", cl::Hidden,
    cl::desc("Use base address specifiers in debug_ranges"), cl::init(false));

static cl::opt<bool> GenerateARangeSection("generate-arange-section",
                                           cl::Hidden,
                                           cl::desc("Generate dwarf aranges"),
                                           cl::init(false));

static cl::opt<bool>
    GenerateDwarfTypeUnits("generate-type-units", cl::Hidden,
                           cl::desc("Generate DWARF4 type units."),
                           cl::init(false));

static cl::opt<bool> SplitDwarfCrossCuReferences(
    "split-dwarf-cross-cu-references", cl::Hidden,
    cl::desc("Enable cross-cu references in DWO files"), cl::init(false));

enum DefaultOnOff { Default, Enable, Disable };

static cl::opt<DefaultOnOff> UnknownLocations(
    "use-unknown-locations", cl::Hidden,
    cl::desc("Make an absence of debug location information explicit."),
    cl::values(clEnumVal(Default, "At top of block or after label"),
               clEnumVal(Enable, "In all cases"), clEnumVal(Disable, "Never")),
    cl::init(Default));

static cl::opt<AccelTableKind> AccelTables(
    "accel-tables", cl::Hidden, cl::desc("Output dwarf accelerator tables."),
    cl::values(clEnumValN(AccelTableKind::Default, "Default",
                          "Default for platform"),
               clEnumValN(AccelTableKind::None, "Disable", "Disabled."),
               clEnumValN(AccelTableKind::Apple, "Apple", "Apple"),
               clEnumValN(AccelTableKind::Dwarf, "Dwarf", "DWARF")),
    cl::init(AccelTableKind::Default));

static cl::opt<DefaultOnOff>
DwarfInlinedStrings("dwarf-inlined-strings", cl::Hidden,
                 cl::desc("Use inlined strings rather than string section."),
                 cl::values(clEnumVal(Default, "Default for platform"),
                            clEnumVal(Enable, "Enabled"),
                            clEnumVal(Disable, "Disabled")),
                 cl::init(Default));

static cl::opt<bool>
    NoDwarfRangesSection("no-dwarf-ranges-section", cl::Hidden,
                         cl::desc("Disable emission .debug_ranges section."),
                         cl::init(false));

static cl::opt<DefaultOnOff> DwarfSectionsAsReferences(
    "dwarf-sections-as-references", cl::Hidden,
    cl::desc("Use sections+offset as references rather than labels."),
    cl::values(clEnumVal(Default, "Default for platform"),
               clEnumVal(Enable, "Enabled"), clEnumVal(Disable, "Disabled")),
    cl::init(Default));

enum LinkageNameOption {
  DefaultLinkageNames,
  AllLinkageNames,
  AbstractLinkageNames
};

static cl::opt<LinkageNameOption>
    DwarfLinkageNames("dwarf-linkage-names", cl::Hidden,
                      cl::desc("Which DWARF linkage-name attributes to emit."),
                      cl::values(clEnumValN(DefaultLinkageNames, "Default",
                                            "Default for platform"),
                                 clEnumValN(AllLinkageNames, "All", "All"),
                                 clEnumValN(AbstractLinkageNames, "Abstract",
                                            "Abstract subprograms")),
                      cl::init(DefaultLinkageNames));

static const char *const DWARFGroupName = "dwarf";
static const char *const DWARFGroupDescription = "DWARF Emission";
static const char *const DbgTimerName = "writer";
static const char *const DbgTimerDescription = "DWARF Debug Writer";
static constexpr unsigned ULEB128PadSize = 4;

void DebugLocDwarfExpression::emitOp(uint8_t Op, const char *Comment) {
  getActiveStreamer().EmitInt8(
      Op, Comment ? Twine(Comment) + " " + dwarf::OperationEncodingString(Op)
                  : dwarf::OperationEncodingString(Op));
}

void DebugLocDwarfExpression::emitSigned(int64_t Value) {
  getActiveStreamer().emitSLEB128(Value, Twine(Value));
}

void DebugLocDwarfExpression::emitUnsigned(uint64_t Value) {
  getActiveStreamer().emitULEB128(Value, Twine(Value));
}

void DebugLocDwarfExpression::emitData1(uint8_t Value) {
  getActiveStreamer().EmitInt8(Value, Twine(Value));
}

void DebugLocDwarfExpression::emitBaseTypeRef(uint64_t Idx) {
  assert(Idx < (1ULL << (ULEB128PadSize * 7)) && "Idx wont fit");
  getActiveStreamer().emitULEB128(Idx, Twine(Idx), ULEB128PadSize);
}

bool DebugLocDwarfExpression::isFrameRegister(const TargetRegisterInfo &TRI,
                                              unsigned MachineReg) {
  // This information is not available while emitting .debug_loc entries.
  return false;
}

void DebugLocDwarfExpression::enableTemporaryBuffer() {
  assert(!IsBuffering && "Already buffering?");
  if (!TmpBuf)
    TmpBuf = std::make_unique<TempBuffer>(OutBS.GenerateComments);
  IsBuffering = true;
}

void DebugLocDwarfExpression::disableTemporaryBuffer() { IsBuffering = false; }

unsigned DebugLocDwarfExpression::getTemporaryBufferSize() {
  return TmpBuf ? TmpBuf->Bytes.size() : 0;
}

void DebugLocDwarfExpression::commitTemporaryBuffer() {
  if (!TmpBuf)
    return;
  for (auto Byte : enumerate(TmpBuf->Bytes)) {
    const char *Comment = (Byte.index() < TmpBuf->Comments.size())
                              ? TmpBuf->Comments[Byte.index()].c_str()
                              : "";
    OutBS.EmitInt8(Byte.value(), Comment);
  }
  TmpBuf->Bytes.clear();
  TmpBuf->Comments.clear();
}

const DIType *DbgVariable::getType() const {
  return getVariable()->getType();
}

/// Get .debug_loc entry for the instruction range starting at MI.
static DbgValueLoc getDebugLocValue(const MachineInstr *MI) {
  const DIExpression *Expr = MI->getDebugExpression();
  assert(MI->getNumOperands() == 4);
  if (MI->getOperand(0).isReg()) {
    auto RegOp = MI->getOperand(0);
    auto Op1 = MI->getOperand(1);
    // If the second operand is an immediate, this is a
    // register-indirect address.
    assert((!Op1.isImm() || (Op1.getImm() == 0)) && "unexpected offset");
    MachineLocation MLoc(RegOp.getReg(), Op1.isImm());
    return DbgValueLoc(Expr, MLoc);
  }
  if (MI->getOperand(0).isTargetIndex()) {
    auto Op = MI->getOperand(0);
    return DbgValueLoc(Expr,
                       TargetIndexLocation(Op.getIndex(), Op.getOffset()));
  }
  if (MI->getOperand(0).isImm())
    return DbgValueLoc(Expr, MI->getOperand(0).getImm());
  if (MI->getOperand(0).isFPImm())
    return DbgValueLoc(Expr, MI->getOperand(0).getFPImm());
  if (MI->getOperand(0).isCImm())
    return DbgValueLoc(Expr, MI->getOperand(0).getCImm());

  llvm_unreachable("Unexpected 4-operand DBG_VALUE instruction!");
}

void DbgVariable::initializeDbgValue(const MachineInstr *DbgValue) {
  assert(FrameIndexExprs.empty() && "Already initialized?");
  assert(!ValueLoc.get() && "Already initialized?");

  assert(getVariable() == DbgValue->getDebugVariable() && "Wrong variable");
  assert(getInlinedAt() == DbgValue->getDebugLoc()->getInlinedAt() &&
         "Wrong inlined-at");

  ValueLoc = std::make_unique<DbgValueLoc>(getDebugLocValue(DbgValue));
  if (auto *E = DbgValue->getDebugExpression())
    if (E->getNumElements())
      FrameIndexExprs.push_back({0, E});
}

ArrayRef<DbgVariable::FrameIndexExpr> DbgVariable::getFrameIndexExprs() const {
  if (FrameIndexExprs.size() == 1)
    return FrameIndexExprs;

  assert(llvm::all_of(FrameIndexExprs,
                      [](const FrameIndexExpr &A) {
                        return A.Expr->isFragment();
                      }) &&
         "multiple FI expressions without DW_OP_LLVM_fragment");
  llvm::sort(FrameIndexExprs,
             [](const FrameIndexExpr &A, const FrameIndexExpr &B) -> bool {
               return A.Expr->getFragmentInfo()->OffsetInBits <
                      B.Expr->getFragmentInfo()->OffsetInBits;
             });

  return FrameIndexExprs;
}

void DbgVariable::addMMIEntry(const DbgVariable &V) {
  assert(DebugLocListIndex == ~0U && !ValueLoc.get() && "not an MMI entry");
  assert(V.DebugLocListIndex == ~0U && !V.ValueLoc.get() && "not an MMI entry");
  assert(V.getVariable() == getVariable() && "conflicting variable");
  assert(V.getInlinedAt() == getInlinedAt() && "conflicting inlined-at location");

  assert(!FrameIndexExprs.empty() && "Expected an MMI entry");
  assert(!V.FrameIndexExprs.empty() && "Expected an MMI entry");

  // FIXME: This logic should not be necessary anymore, as we now have proper
  // deduplication. However, without it, we currently run into the assertion
  // below, which means that we are likely dealing with broken input, i.e. two
  // non-fragment entries for the same variable at different frame indices.
  if (FrameIndexExprs.size()) {
    auto *Expr = FrameIndexExprs.back().Expr;
    if (!Expr || !Expr->isFragment())
      return;
  }

  for (const auto &FIE : V.FrameIndexExprs)
    // Ignore duplicate entries.
    if (llvm::none_of(FrameIndexExprs, [&](const FrameIndexExpr &Other) {
          return FIE.FI == Other.FI && FIE.Expr == Other.Expr;
        }))
      FrameIndexExprs.push_back(FIE);

  assert((FrameIndexExprs.size() == 1 ||
          llvm::all_of(FrameIndexExprs,
                       [](FrameIndexExpr &FIE) {
                         return FIE.Expr && FIE.Expr->isFragment();
                       })) &&
         "conflicting locations for variable");
}

static AccelTableKind computeAccelTableKind(unsigned DwarfVersion,
                                            bool GenerateTypeUnits,
                                            DebuggerKind Tuning,
                                            const Triple &TT) {
  // Honor an explicit request.
  if (AccelTables != AccelTableKind::Default)
    return AccelTables;

  // Accelerator tables with type units are currently not supported.
  if (GenerateTypeUnits)
    return AccelTableKind::None;

  // Accelerator tables get emitted if targetting DWARF v5 or LLDB.  DWARF v5
  // always implies debug_names. For lower standard versions we use apple
  // accelerator tables on apple platforms and debug_names elsewhere.
  if (DwarfVersion >= 5)
    return AccelTableKind::Dwarf;
  if (Tuning == DebuggerKind::LLDB)
    return TT.isOSBinFormatMachO() ? AccelTableKind::Apple
                                   : AccelTableKind::Dwarf;
  return AccelTableKind::None;
}

DwarfDebug::DwarfDebug(AsmPrinter *A, Module *M)
    : DebugHandlerBase(A), DebugLocs(A->OutStreamer->isVerboseAsm()),
      InfoHolder(A, "info_string", DIEValueAllocator),
      SkeletonHolder(A, "skel_string", DIEValueAllocator),
      IsDarwin(A->TM.getTargetTriple().isOSDarwin()) {
  const Triple &TT = Asm->TM.getTargetTriple();

  // Make sure we know our "debugger tuning".  The target option takes
  // precedence; fall back to triple-based defaults.
  if (Asm->TM.Options.DebuggerTuning != DebuggerKind::Default)
    DebuggerTuning = Asm->TM.Options.DebuggerTuning;
  else if (IsDarwin)
    DebuggerTuning = DebuggerKind::LLDB;
  else if (TT.isPS4CPU())
    DebuggerTuning = DebuggerKind::SCE;
  else
    DebuggerTuning = DebuggerKind::GDB;

  if (DwarfInlinedStrings == Default)
    UseInlineStrings = TT.isNVPTX();
  else
    UseInlineStrings = DwarfInlinedStrings == Enable;

  UseLocSection = !TT.isNVPTX();

  HasAppleExtensionAttributes = tuneForLLDB();

  // Handle split DWARF.
  HasSplitDwarf = !Asm->TM.Options.MCOptions.SplitDwarfFile.empty();

  // SCE defaults to linkage names only for abstract subprograms.
  if (DwarfLinkageNames == DefaultLinkageNames)
    UseAllLinkageNames = !tuneForSCE();
  else
    UseAllLinkageNames = DwarfLinkageNames == AllLinkageNames;

  unsigned DwarfVersionNumber = Asm->TM.Options.MCOptions.DwarfVersion;
  unsigned DwarfVersion = DwarfVersionNumber ? DwarfVersionNumber
                                    : MMI->getModule()->getDwarfVersion();
  // Use dwarf 4 by default if nothing is requested. For NVPTX, use dwarf 2.
  DwarfVersion =
      TT.isNVPTX() ? 2 : (DwarfVersion ? DwarfVersion : dwarf::DWARF_VERSION);

  UseRangesSection = !NoDwarfRangesSection && !TT.isNVPTX();

  // Use sections as references. Force for NVPTX.
  if (DwarfSectionsAsReferences == Default)
    UseSectionsAsReferences = TT.isNVPTX();
  else
    UseSectionsAsReferences = DwarfSectionsAsReferences == Enable;

  // Don't generate type units for unsupported object file formats.
  GenerateTypeUnits =
      A->TM.getTargetTriple().isOSBinFormatELF() && GenerateDwarfTypeUnits;

  TheAccelTableKind = computeAccelTableKind(
      DwarfVersion, GenerateTypeUnits, DebuggerTuning, A->TM.getTargetTriple());

  // Work around a GDB bug. GDB doesn't support the standard opcode;
  // SCE doesn't support GNU's; LLDB prefers the standard opcode, which
  // is defined as of DWARF 3.
  // See GDB bug 11616 - DW_OP_form_tls_address is unimplemented
  // https://sourceware.org/bugzilla/show_bug.cgi?id=11616
  UseGNUTLSOpcode = tuneForGDB() || DwarfVersion < 3;

  // GDB does not fully support the DWARF 4 representation for bitfields.
  UseDWARF2Bitfields = (DwarfVersion < 4) || tuneForGDB();

  // The DWARF v5 string offsets table has - possibly shared - contributions
  // from each compile and type unit each preceded by a header. The string
  // offsets table used by the pre-DWARF v5 split-DWARF implementation uses
  // a monolithic string offsets table without any header.
  UseSegmentedStringOffsetsTable = DwarfVersion >= 5;

  Asm->OutStreamer->getContext().setDwarfVersion(DwarfVersion);
}

// Define out of line so we don't have to include DwarfUnit.h in DwarfDebug.h.
DwarfDebug::~DwarfDebug() = default;

static bool isObjCClass(StringRef Name) {
  return Name.startswith("+") || Name.startswith("-");
}

static bool hasObjCCategory(StringRef Name) {
  if (!isObjCClass(Name))
    return false;

  return Name.find(") ") != StringRef::npos;
}

static void getObjCClassCategory(StringRef In, StringRef &Class,
                                 StringRef &Category) {
  if (!hasObjCCategory(In)) {
    Class = In.slice(In.find('[') + 1, In.find(' '));
    Category = "";
    return;
  }

  Class = In.slice(In.find('[') + 1, In.find('('));
  Category = In.slice(In.find('[') + 1, In.find(' '));
}

static StringRef getObjCMethodName(StringRef In) {
  return In.slice(In.find(' ') + 1, In.find(']'));
}

// Add the various names to the Dwarf accelerator table names.
void DwarfDebug::addSubprogramNames(const DICompileUnit &CU,
                                    const DISubprogram *SP, DIE &Die) {
  if (getAccelTableKind() != AccelTableKind::Apple &&
      CU.getNameTableKind() == DICompileUnit::DebugNameTableKind::None)
    return;

  if (!SP->isDefinition())
    return;

  if (SP->getName() != "")
    addAccelName(CU, SP->getName(), Die);

  // If the linkage name is different than the name, go ahead and output that as
  // well into the name table. Only do that if we are going to actually emit
  // that name.
  if (SP->getLinkageName() != "" && SP->getName() != SP->getLinkageName() &&
      (useAllLinkageNames() || InfoHolder.getAbstractSPDies().lookup(SP)))
    addAccelName(CU, SP->getLinkageName(), Die);

  // If this is an Objective-C selector name add it to the ObjC accelerator
  // too.
  if (isObjCClass(SP->getName())) {
    StringRef Class, Category;
    getObjCClassCategory(SP->getName(), Class, Category);
    addAccelObjC(CU, Class, Die);
    if (Category != "")
      addAccelObjC(CU, Category, Die);
    // Also add the base method name to the name table.
    addAccelName(CU, getObjCMethodName(SP->getName()), Die);
  }
}

/// Check whether we should create a DIE for the given Scope, return true
/// if we don't create a DIE (the corresponding DIE is null).
bool DwarfDebug::isLexicalScopeDIENull(LexicalScope *Scope) {
  if (Scope->isAbstractScope())
    return false;

  // We don't create a DIE if there is no Range.
  const SmallVectorImpl<InsnRange> &Ranges = Scope->getRanges();
  if (Ranges.empty())
    return true;

  if (Ranges.size() > 1)
    return false;

  // We don't create a DIE if we have a single Range and the end label
  // is null.
  return !getLabelAfterInsn(Ranges.front().second);
}

template <typename Func> static void forBothCUs(DwarfCompileUnit &CU, Func F) {
  F(CU);
  if (auto *SkelCU = CU.getSkeleton())
    if (CU.getCUNode()->getSplitDebugInlining())
      F(*SkelCU);
}

bool DwarfDebug::shareAcrossDWOCUs() const {
  return SplitDwarfCrossCuReferences;
}

void DwarfDebug::constructAbstractSubprogramScopeDIE(DwarfCompileUnit &SrcCU,
                                                     LexicalScope *Scope) {
  assert(Scope && Scope->getScopeNode());
  assert(Scope->isAbstractScope());
  assert(!Scope->getInlinedAt());

  auto *SP = cast<DISubprogram>(Scope->getScopeNode());

  // Find the subprogram's DwarfCompileUnit in the SPMap in case the subprogram
  // was inlined from another compile unit.
  if (useSplitDwarf() && !shareAcrossDWOCUs() && !SP->getUnit()->getSplitDebugInlining())
    // Avoid building the original CU if it won't be used
    SrcCU.constructAbstractSubprogramScopeDIE(Scope);
  else {
    auto &CU = getOrCreateDwarfCompileUnit(SP->getUnit());
    if (auto *SkelCU = CU.getSkeleton()) {
      (shareAcrossDWOCUs() ? CU : SrcCU)
          .constructAbstractSubprogramScopeDIE(Scope);
      if (CU.getCUNode()->getSplitDebugInlining())
        SkelCU->constructAbstractSubprogramScopeDIE(Scope);
    } else
      CU.constructAbstractSubprogramScopeDIE(Scope);
  }
}

DIE &DwarfDebug::constructSubprogramDefinitionDIE(const DISubprogram *SP) {
  DICompileUnit *Unit = SP->getUnit();
  assert(SP->isDefinition() && "Subprogram not a definition");
  assert(Unit && "Subprogram definition without parent unit");
  auto &CU = getOrCreateDwarfCompileUnit(Unit);
  return *CU.getOrCreateSubprogramDIE(SP);
}

/// Try to interpret values loaded into registers that forward parameters
/// for \p CallMI. Store parameters with interpreted value into \p Params.
static void collectCallSiteParameters(const MachineInstr *CallMI,
                                      ParamSet &Params) {
  auto *MF = CallMI->getMF();
  auto CalleesMap = MF->getCallSitesInfo();
  auto CallFwdRegsInfo = CalleesMap.find(CallMI);

  // There is no information for the call instruction.
  if (CallFwdRegsInfo == CalleesMap.end())
    return;

  auto *MBB = CallMI->getParent();
  const auto &TRI = MF->getSubtarget().getRegisterInfo();
  const auto &TII = MF->getSubtarget().getInstrInfo();
  const auto &TLI = MF->getSubtarget().getTargetLowering();

  // Skip the call instruction.
  auto I = std::next(CallMI->getReverseIterator());

  // Register worklist. Each register has an associated list of parameter
  // registers whose call site values potentially can be described using that
  // register.
  using FwdRegWorklist = MapVector<unsigned, SmallVector<unsigned, 2>>;

  FwdRegWorklist ForwardedRegWorklist;

  // If an instruction defines more than one item in the worklist, we may run
  // into situations where a worklist register's value is (potentially)
  // described by the previous value of another register that is also defined
  // by that instruction.
  //
  // This can for example occur in cases like this:
  //
  //   $r1 = mov 123
  //   $r0, $r1 = mvrr $r1, 456
  //   call @foo, $r0, $r1
  //
  // When describing $r1's value for the mvrr instruction, we need to make sure
  // that we don't finalize an entry value for $r0, as that is dependent on the
  // previous value of $r1 (123 rather than 456).
  //
  // In order to not have to distinguish between those cases when finalizing
  // entry values, we simply postpone adding new parameter registers to the
  // worklist, by first keeping them in this temporary container until the
  // instruction has been handled.
  FwdRegWorklist NewWorklistItems;

  // Add all the forwarding registers into the ForwardedRegWorklist.
  for (auto ArgReg : CallFwdRegsInfo->second) {
    bool InsertedReg =
        ForwardedRegWorklist.insert({ArgReg.Reg, {ArgReg.Reg}}).second;
    assert(InsertedReg && "Single register used to forward two arguments?");
    (void)InsertedReg;
  }

  // We erase, from the ForwardedRegWorklist, those forwarding registers for
  // which we successfully describe a loaded value (by using
  // the describeLoadedValue()). For those remaining arguments in the working
  // list, for which we do not describe a loaded value by
  // the describeLoadedValue(), we try to generate an entry value expression
  // for their call site value description, if the call is within the entry MBB.
  // TODO: Handle situations when call site parameter value can be described
  // as the entry value within basic blocks other than the first one.
  bool ShouldTryEmitEntryVals = MBB->getIterator() == MF->begin();

  // If the MI is an instruction defining one or more parameters' forwarding
  // registers, add those defines.
  auto getForwardingRegsDefinedByMI = [&](const MachineInstr &MI,
                                          SmallSetVector<unsigned, 4> &Defs) {
    if (MI.isDebugInstr())
      return;

    for (const MachineOperand &MO : MI.operands()) {
      if (MO.isReg() && MO.isDef() &&
          Register::isPhysicalRegister(MO.getReg())) {
        for (auto FwdReg : ForwardedRegWorklist)
          if (TRI->regsOverlap(FwdReg.first, MO.getReg()))
            Defs.insert(FwdReg.first);
      }
    }
  };

  auto finishCallSiteParams = [&](DbgValueLoc DbgLocVal,
                                  ArrayRef<unsigned> ParamRegs) {
    for (auto FwdReg : ParamRegs) {
      DbgCallSiteParam CSParm(FwdReg, DbgLocVal);
      Params.push_back(CSParm);
      ++NumCSParams;
    }
  };

  // Add Reg to the worklist, if it's not already present, and mark that the
  // given parameter registers' values can (potentially) be described using
  // that register.
  auto addToWorklist = [](FwdRegWorklist &Worklist, unsigned Reg,
                          ArrayRef<unsigned> ParamRegs) {
    auto I = Worklist.insert({Reg, {}});
    for (auto ParamReg : ParamRegs) {
      assert(!is_contained(I.first->second, ParamReg) &&
             "Same parameter described twice by forwarding reg");
      I.first->second.push_back(ParamReg);
    }
  };

  // Search for a loading value in forwarding registers.
  for (; I != MBB->rend(); ++I) {
    // Skip bundle headers.
    if (I->isBundle())
      continue;

    // If the next instruction is a call we can not interpret parameter's
    // forwarding registers or we finished the interpretation of all parameters.
    if (I->isCall())
      return;

    if (ForwardedRegWorklist.empty())
      return;

    // Set of worklist registers that are defined by this instruction.
    SmallSetVector<unsigned, 4> FwdRegDefs;

    getForwardingRegsDefinedByMI(*I, FwdRegDefs);
    if (FwdRegDefs.empty())
      continue;

    for (auto ParamFwdReg : FwdRegDefs) {
      if (auto ParamValue = TII->describeLoadedValue(*I, ParamFwdReg)) {
        if (ParamValue->first.isImm()) {
          int64_t Val = ParamValue->first.getImm();
          DbgValueLoc DbgLocVal(ParamValue->second, Val);
          finishCallSiteParams(DbgLocVal, ForwardedRegWorklist[ParamFwdReg]);
        } else if (ParamValue->first.isReg()) {
          Register RegLoc = ParamValue->first.getReg();
          // TODO: For now, there is no use of describing the value loaded into the
          //       register that is also the source registers (e.g. $r0 = add $r0, x).
          if (ParamFwdReg == RegLoc)
            continue;

          unsigned SP = TLI->getStackPointerRegisterToSaveRestore();
          Register FP = TRI->getFrameRegister(*MF);
          bool IsSPorFP = (RegLoc == SP) || (RegLoc == FP);
          if (TRI->isCalleeSavedPhysReg(RegLoc, *MF) || IsSPorFP) {
            DbgValueLoc DbgLocVal(ParamValue->second,
                                  MachineLocation(RegLoc,
                                                  /*IsIndirect=*/IsSPorFP));
            finishCallSiteParams(DbgLocVal, ForwardedRegWorklist[ParamFwdReg]);
          // TODO: Add support for entry value plus an expression.
          } else if (ShouldTryEmitEntryVals &&
                     ParamValue->second->getNumElements() == 0) {
            assert(RegLoc != ParamFwdReg &&
                   "Can't handle a register that is described by itself");
            // ParamFwdReg was described by the non-callee saved register
            // RegLoc. Mark that the call site values for the parameters are
            // dependent on that register instead of ParamFwdReg. Since RegLoc
            // may be a register that will be handled in this iteration, we
            // postpone adding the items to the worklist, and instead keep them
            // in a temporary container.
            addToWorklist(NewWorklistItems, RegLoc,
                          ForwardedRegWorklist[ParamFwdReg]);
          }
        }
      }
    }

    // Remove all registers that this instruction defines from the worklist.
    for (auto ParamFwdReg : FwdRegDefs)
      ForwardedRegWorklist.erase(ParamFwdReg);

    // Now that we are done handling this instruction, add items from the
    // temporary worklist to the real one.
    for (auto New : NewWorklistItems)
      addToWorklist(ForwardedRegWorklist, New.first, New.second);
    NewWorklistItems.clear();
  }

  // Emit the call site parameter's value as an entry value.
  if (ShouldTryEmitEntryVals) {
    // Create an expression where the register's entry value is used.
    DIExpression *EntryExpr = DIExpression::get(
        MF->getFunction().getContext(), {dwarf::DW_OP_LLVM_entry_value, 1});
    for (auto RegEntry : ForwardedRegWorklist) {
      DbgValueLoc DbgLocVal(EntryExpr, MachineLocation(RegEntry.first));
      finishCallSiteParams(DbgLocVal, RegEntry.second);
    }
  }
}

void DwarfDebug::constructCallSiteEntryDIEs(const DISubprogram &SP,
                                            DwarfCompileUnit &CU, DIE &ScopeDIE,
                                            const MachineFunction &MF) {
  // Add a call site-related attribute (DWARF5, Sec. 3.3.1.3). Do this only if
  // the subprogram is required to have one.
  if (!SP.areAllCallsDescribed() || !SP.isDefinition())
    return;

  // Use DW_AT_call_all_calls to express that call site entries are present
  // for both tail and non-tail calls. Don't use DW_AT_call_all_source_calls
  // because one of its requirements is not met: call site entries for
  // optimized-out calls are elided.
  CU.addFlag(ScopeDIE, CU.getDwarf5OrGNUAttr(dwarf::DW_AT_call_all_calls));

  const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
  assert(TII && "TargetInstrInfo not found: cannot label tail calls");

  // Emit call site entries for each call or tail call in the function.
  for (const MachineBasicBlock &MBB : MF) {
    for (const MachineInstr &MI : MBB.instrs()) {
      // Bundles with call in them will pass the isCall() test below but do not
      // have callee operand information so skip them here. Iterator will
      // eventually reach the call MI.
      if (MI.isBundle())
        continue;

      // Skip instructions which aren't calls. Both calls and tail-calling jump
      // instructions (e.g TAILJMPd64) are classified correctly here.
      if (!MI.isCandidateForCallSiteEntry())
        continue;

      // Skip instructions marked as frame setup, as they are not interesting to
      // the user.
      if (MI.getFlag(MachineInstr::FrameSetup))
        continue;

      // TODO: Add support for targets with delay slots (see: beginInstruction).
      if (MI.hasDelaySlot())
        return;

      // If this is a direct call, find the callee's subprogram.
      // In the case of an indirect call find the register that holds
      // the callee.
      const MachineOperand &CalleeOp = MI.getOperand(0);
      if (!CalleeOp.isGlobal() && !CalleeOp.isReg())
        continue;

      unsigned CallReg = 0;
      DIE *CalleeDIE = nullptr;
      const Function *CalleeDecl = nullptr;
      if (CalleeOp.isReg()) {
        CallReg = CalleeOp.getReg();
        if (!CallReg)
          continue;
      } else {
        CalleeDecl = dyn_cast<Function>(CalleeOp.getGlobal());
        if (!CalleeDecl || !CalleeDecl->getSubprogram())
          continue;
        const DISubprogram *CalleeSP = CalleeDecl->getSubprogram();

        if (CalleeSP->isDefinition()) {
          // Ensure that a subprogram DIE for the callee is available in the
          // appropriate CU.
          CalleeDIE = &constructSubprogramDefinitionDIE(CalleeSP);
        } else {
          // Create the declaration DIE if it is missing. This is required to
          // support compilation of old bitcode with an incomplete list of
          // retained metadata.
          CalleeDIE = CU.getOrCreateSubprogramDIE(CalleeSP);
        }
        assert(CalleeDIE && "Must have a DIE for the callee");
      }

      // TODO: Omit call site entries for runtime calls (objc_msgSend, etc).

      bool IsTail = TII->isTailCall(MI);

      // If MI is in a bundle, the label was created after the bundle since
      // EmitFunctionBody iterates over top-level MIs. Get that top-level MI
      // to search for that label below.
      const MachineInstr *TopLevelCallMI =
          MI.isInsideBundle() ? &*getBundleStart(MI.getIterator()) : &MI;

      // For tail calls, no return PC information is needed.
      // For regular calls (and tail calls in GDB tuning), the return PC
      // is needed to disambiguate paths in the call graph which could lead to
      // some target function.
      const MCSymbol *PCAddr =
          (IsTail && !tuneForGDB())
              ? nullptr
              : const_cast<MCSymbol *>(getLabelAfterInsn(TopLevelCallMI));

      assert((IsTail || PCAddr) && "Call without return PC information");

      LLVM_DEBUG(dbgs() << "CallSiteEntry: " << MF.getName() << " -> "
                        << (CalleeDecl ? CalleeDecl->getName()
                                       : StringRef(MF.getSubtarget()
                                                       .getRegisterInfo()
                                                       ->getName(CallReg)))
                        << (IsTail ? " [IsTail]" : "") << "\n");

      DIE &CallSiteDIE = CU.constructCallSiteEntryDIE(ScopeDIE, CalleeDIE,
                                                      IsTail, PCAddr, CallReg);

      // GDB and LLDB support call site parameter debug info.
      if (Asm->TM.Options.EnableDebugEntryValues &&
          (tuneForGDB() || tuneForLLDB())) {
        ParamSet Params;
        // Try to interpret values of call site parameters.
        collectCallSiteParameters(&MI, Params);
        CU.constructCallSiteParmEntryDIEs(CallSiteDIE, Params);
      }
    }
  }
}

void DwarfDebug::addGnuPubAttributes(DwarfCompileUnit &U, DIE &D) const {
  if (!U.hasDwarfPubSections())
    return;

  U.addFlag(D, dwarf::DW_AT_GNU_pubnames);
}

void DwarfDebug::finishUnitAttributes(const DICompileUnit *DIUnit,
                                      DwarfCompileUnit &NewCU) {
  DIE &Die = NewCU.getUnitDie();
  StringRef FN = DIUnit->getFilename();

  StringRef Producer = DIUnit->getProducer();
  StringRef Flags = DIUnit->getFlags();
  if (!Flags.empty() && !useAppleExtensionAttributes()) {
    std::string ProducerWithFlags = Producer.str() + " " + Flags.str();
    NewCU.addString(Die, dwarf::DW_AT_producer, ProducerWithFlags);
  } else
    NewCU.addString(Die, dwarf::DW_AT_producer, Producer);

  NewCU.addUInt(Die, dwarf::DW_AT_language, dwarf::DW_FORM_data2,
                DIUnit->getSourceLanguage());
  NewCU.addString(Die, dwarf::DW_AT_name, FN);
  StringRef SysRoot = DIUnit->getSysRoot();
  if (!SysRoot.empty())
    NewCU.addString(Die, dwarf::DW_AT_LLVM_sysroot, SysRoot);

  // Add DW_str_offsets_base to the unit DIE, except for split units.
  if (useSegmentedStringOffsetsTable() && !useSplitDwarf())
    NewCU.addStringOffsetsStart();

  if (!useSplitDwarf()) {
    NewCU.initStmtList();

    // If we're using split dwarf the compilation dir is going to be in the
    // skeleton CU and so we don't need to duplicate it here.
    if (!CompilationDir.empty())
      NewCU.addString(Die, dwarf::DW_AT_comp_dir, CompilationDir);
    addGnuPubAttributes(NewCU, Die);
  }

  if (useAppleExtensionAttributes()) {
    if (DIUnit->isOptimized())
      NewCU.addFlag(Die, dwarf::DW_AT_APPLE_optimized);

    StringRef Flags = DIUnit->getFlags();
    if (!Flags.empty())
      NewCU.addString(Die, dwarf::DW_AT_APPLE_flags, Flags);

    if (unsigned RVer = DIUnit->getRuntimeVersion())
      NewCU.addUInt(Die, dwarf::DW_AT_APPLE_major_runtime_vers,
                    dwarf::DW_FORM_data1, RVer);
  }

  if (DIUnit->getDWOId()) {
    // This CU is either a clang module DWO or a skeleton CU.
    NewCU.addUInt(Die, dwarf::DW_AT_GNU_dwo_id, dwarf::DW_FORM_data8,
                  DIUnit->getDWOId());
    if (!DIUnit->getSplitDebugFilename().empty()) {
      // This is a prefabricated skeleton CU.
      dwarf::Attribute attrDWOName = getDwarfVersion() >= 5
                                         ? dwarf::DW_AT_dwo_name
                                         : dwarf::DW_AT_GNU_dwo_name;
      NewCU.addString(Die, attrDWOName, DIUnit->getSplitDebugFilename());
    }
  }
}
// Create new DwarfCompileUnit for the given metadata node with tag
// DW_TAG_compile_unit.
DwarfCompileUnit &
DwarfDebug::getOrCreateDwarfCompileUnit(const DICompileUnit *DIUnit) {
  if (auto *CU = CUMap.lookup(DIUnit))
    return *CU;

  CompilationDir = DIUnit->getDirectory();

  auto OwnedUnit = std::make_unique<DwarfCompileUnit>(
      InfoHolder.getUnits().size(), DIUnit, Asm, this, &InfoHolder);
  DwarfCompileUnit &NewCU = *OwnedUnit;
  InfoHolder.addUnit(std::move(OwnedUnit));

  for (auto *IE : DIUnit->getImportedEntities())
    NewCU.addImportedEntity(IE);

  // LTO with assembly output shares a single line table amongst multiple CUs.
  // To avoid the compilation directory being ambiguous, let the line table
  // explicitly describe the directory of all files, never relying on the
  // compilation directory.
  if (!Asm->OutStreamer->hasRawTextSupport() || SingleCU)
    Asm->OutStreamer->emitDwarfFile0Directive(
        CompilationDir, DIUnit->getFilename(),
        NewCU.getMD5AsBytes(DIUnit->getFile()), DIUnit->getSource(),
        NewCU.getUniqueID());

  if (useSplitDwarf()) {
    NewCU.setSkeleton(constructSkeletonCU(NewCU));
    NewCU.setSection(Asm->getObjFileLowering().getDwarfInfoDWOSection());
  } else {
    finishUnitAttributes(DIUnit, NewCU);
    NewCU.setSection(Asm->getObjFileLowering().getDwarfInfoSection());
  }

  CUMap.insert({DIUnit, &NewCU});
  CUDieMap.insert({&NewCU.getUnitDie(), &NewCU});
  return NewCU;
}

void DwarfDebug::constructAndAddImportedEntityDIE(DwarfCompileUnit &TheCU,
                                                  const DIImportedEntity *N) {
  if (isa<DILocalScope>(N->getScope()))
    return;
  if (DIE *D = TheCU.getOrCreateContextDIE(N->getScope()))
    D->addChild(TheCU.constructImportedEntityDIE(N));
}

/// Sort and unique GVEs by comparing their fragment offset.
static SmallVectorImpl<DwarfCompileUnit::GlobalExpr> &
sortGlobalExprs(SmallVectorImpl<DwarfCompileUnit::GlobalExpr> &GVEs) {
  llvm::sort(
      GVEs, [](DwarfCompileUnit::GlobalExpr A, DwarfCompileUnit::GlobalExpr B) {
        // Sort order: first null exprs, then exprs without fragment
        // info, then sort by fragment offset in bits.
        // FIXME: Come up with a more comprehensive comparator so
        // the sorting isn't non-deterministic, and so the following
        // std::unique call works correctly.
        if (!A.Expr || !B.Expr)
          return !!B.Expr;
        auto FragmentA = A.Expr->getFragmentInfo();
        auto FragmentB = B.Expr->getFragmentInfo();
        if (!FragmentA || !FragmentB)
          return !!FragmentB;
        return FragmentA->OffsetInBits < FragmentB->OffsetInBits;
      });
  GVEs.erase(std::unique(GVEs.begin(), GVEs.end(),
                         [](DwarfCompileUnit::GlobalExpr A,
                            DwarfCompileUnit::GlobalExpr B) {
                           return A.Expr == B.Expr;
                         }),
             GVEs.end());
  return GVEs;
}

// Emit all Dwarf sections that should come prior to the content. Create
// global DIEs and emit initial debug info sections. This is invoked by
// the target AsmPrinter.
void DwarfDebug::beginModule() {
  NamedRegionTimer T(DbgTimerName, DbgTimerDescription, DWARFGroupName,
                     DWARFGroupDescription, TimePassesIsEnabled);
  if (DisableDebugInfoPrinting) {
    MMI->setDebugInfoAvailability(false);
    return;
  }

  const Module *M = MMI->getModule();

  unsigned NumDebugCUs = std::distance(M->debug_compile_units_begin(),
                                       M->debug_compile_units_end());
  // Tell MMI whether we have debug info.
  assert(MMI->hasDebugInfo() == (NumDebugCUs > 0) &&
         "DebugInfoAvailabilty initialized unexpectedly");
  SingleCU = NumDebugCUs == 1;
  DenseMap<DIGlobalVariable *, SmallVector<DwarfCompileUnit::GlobalExpr, 1>>
      GVMap;
  for (const GlobalVariable &Global : M->globals()) {
    SmallVector<DIGlobalVariableExpression *, 1> GVs;
    Global.getDebugInfo(GVs);
    for (auto *GVE : GVs)
      GVMap[GVE->getVariable()].push_back({&Global, GVE->getExpression()});
  }

  // Create the symbol that designates the start of the unit's contribution
  // to the string offsets table. In a split DWARF scenario, only the skeleton
  // unit has the DW_AT_str_offsets_base attribute (and hence needs the symbol).
  if (useSegmentedStringOffsetsTable())
    (useSplitDwarf() ? SkeletonHolder : InfoHolder)
        .setStringOffsetsStartSym(Asm->createTempSymbol("str_offsets_base"));


  // Create the symbols that designates the start of the DWARF v5 range list
  // and locations list tables. They are located past the table headers.
  if (getDwarfVersion() >= 5) {
    DwarfFile &Holder = useSplitDwarf() ? SkeletonHolder : InfoHolder;
    Holder.setRnglistsTableBaseSym(
        Asm->createTempSymbol("rnglists_table_base"));

    if (useSplitDwarf())
      InfoHolder.setRnglistsTableBaseSym(
          Asm->createTempSymbol("rnglists_dwo_table_base"));
  }

  // Create the symbol that points to the first entry following the debug
  // address table (.debug_addr) header.
  AddrPool.setLabel(Asm->createTempSymbol("addr_table_base"));
  DebugLocs.setSym(Asm->createTempSymbol("loclists_table_base"));

  for (DICompileUnit *CUNode : M->debug_compile_units()) {
    // FIXME: Move local imported entities into a list attached to the
    // subprogram, then this search won't be needed and a
    // getImportedEntities().empty() test should go below with the rest.
    bool HasNonLocalImportedEntities = llvm::any_of(
        CUNode->getImportedEntities(), [](const DIImportedEntity *IE) {
          return !isa<DILocalScope>(IE->getScope());
        });

    if (!HasNonLocalImportedEntities && CUNode->getEnumTypes().empty() &&
        CUNode->getRetainedTypes().empty() &&
        CUNode->getGlobalVariables().empty() && CUNode->getMacros().empty())
      continue;

    DwarfCompileUnit &CU = getOrCreateDwarfCompileUnit(CUNode);

    // Global Variables.
    for (auto *GVE : CUNode->getGlobalVariables()) {
      // Don't bother adding DIGlobalVariableExpressions listed in the CU if we
      // already know about the variable and it isn't adding a constant
      // expression.
      auto &GVMapEntry = GVMap[GVE->getVariable()];
      auto *Expr = GVE->getExpression();
      if (!GVMapEntry.size() || (Expr && Expr->isConstant()))
        GVMapEntry.push_back({nullptr, Expr});
    }
    DenseSet<DIGlobalVariable *> Processed;
    for (auto *GVE : CUNode->getGlobalVariables()) {
      DIGlobalVariable *GV = GVE->getVariable();
      if (Processed.insert(GV).second)
        CU.getOrCreateGlobalVariableDIE(GV, sortGlobalExprs(GVMap[GV]));
    }

    for (auto *Ty : CUNode->getEnumTypes()) {
      // The enum types array by design contains pointers to
      // MDNodes rather than DIRefs. Unique them here.
      CU.getOrCreateTypeDIE(cast<DIType>(Ty));
    }
    for (auto *Ty : CUNode->getRetainedTypes()) {
      // The retained types array by design contains pointers to
      // MDNodes rather than DIRefs. Unique them here.
      if (DIType *RT = dyn_cast<DIType>(Ty))
          // There is no point in force-emitting a forward declaration.
          CU.getOrCreateTypeDIE(RT);
    }
    // Emit imported_modules last so that the relevant context is already
    // available.
    for (auto *IE : CUNode->getImportedEntities())
      constructAndAddImportedEntityDIE(CU, IE);
  }
}

void DwarfDebug::finishEntityDefinitions() {
  for (const auto &Entity : ConcreteEntities) {
    DIE *Die = Entity->getDIE();
    assert(Die);
    // FIXME: Consider the time-space tradeoff of just storing the unit pointer
    // in the ConcreteEntities list, rather than looking it up again here.
    // DIE::getUnit isn't simple - it walks parent pointers, etc.
    DwarfCompileUnit *Unit = CUDieMap.lookup(Die->getUnitDie());
    assert(Unit);
    Unit->finishEntityDefinition(Entity.get());
  }
}

void DwarfDebug::finishSubprogramDefinitions() {
  for (const DISubprogram *SP : ProcessedSPNodes) {
    assert(SP->getUnit()->getEmissionKind() != DICompileUnit::NoDebug);
    forBothCUs(
        getOrCreateDwarfCompileUnit(SP->getUnit()),
        [&](DwarfCompileUnit &CU) { CU.finishSubprogramDefinition(SP); });
  }
}

void DwarfDebug::finalizeModuleInfo() {
  const TargetLoweringObjectFile &TLOF = Asm->getObjFileLowering();

  finishSubprogramDefinitions();

  finishEntityDefinitions();

  // Include the DWO file name in the hash if there's more than one CU.
  // This handles ThinLTO's situation where imported CUs may very easily be
  // duplicate with the same CU partially imported into another ThinLTO unit.
  StringRef DWOName;
  if (CUMap.size() > 1)
    DWOName = Asm->TM.Options.MCOptions.SplitDwarfFile;

  // Handle anything that needs to be done on a per-unit basis after
  // all other generation.
  for (const auto &P : CUMap) {
    auto &TheCU = *P.second;
    if (TheCU.getCUNode()->isDebugDirectivesOnly())
      continue;
    // Emit DW_AT_containing_type attribute to connect types with their
    // vtable holding type.
    TheCU.constructContainingTypeDIEs();

    // Add CU specific attributes if we need to add any.
    // If we're splitting the dwarf out now that we've got the entire
    // CU then add the dwo id to it.
    auto *SkCU = TheCU.getSkeleton();

    bool HasSplitUnit = SkCU && !TheCU.getUnitDie().children().empty();

    if (HasSplitUnit) {
      dwarf::Attribute attrDWOName = getDwarfVersion() >= 5
                                         ? dwarf::DW_AT_dwo_name
                                         : dwarf::DW_AT_GNU_dwo_name;
      finishUnitAttributes(TheCU.getCUNode(), TheCU);
      TheCU.addString(TheCU.getUnitDie(), attrDWOName,
                      Asm->TM.Options.MCOptions.SplitDwarfFile);
      SkCU->addString(SkCU->getUnitDie(), attrDWOName,
                      Asm->TM.Options.MCOptions.SplitDwarfFile);
      // Emit a unique identifier for this CU.
      uint64_t ID =
          DIEHash(Asm).computeCUSignature(DWOName, TheCU.getUnitDie());
      if (getDwarfVersion() >= 5) {
        TheCU.setDWOId(ID);
        SkCU->setDWOId(ID);
      } else {
        TheCU.addUInt(TheCU.getUnitDie(), dwarf::DW_AT_GNU_dwo_id,
                      dwarf::DW_FORM_data8, ID);
        SkCU->addUInt(SkCU->getUnitDie(), dwarf::DW_AT_GNU_dwo_id,
                      dwarf::DW_FORM_data8, ID);
      }

      if (getDwarfVersion() < 5 && !SkeletonHolder.getRangeLists().empty()) {
        const MCSymbol *Sym = TLOF.getDwarfRangesSection()->getBeginSymbol();
        SkCU->addSectionLabel(SkCU->getUnitDie(), dwarf::DW_AT_GNU_ranges_base,
                              Sym, Sym);
      }
    } else if (SkCU) {
      finishUnitAttributes(SkCU->getCUNode(), *SkCU);
    }

    // If we have code split among multiple sections or non-contiguous
    // ranges of code then emit a DW_AT_ranges attribute on the unit that will
    // remain in the .o file, otherwise add a DW_AT_low_pc.
    // FIXME: We should use ranges allow reordering of code ala
    // .subsections_via_symbols in mach-o. This would mean turning on
    // ranges for all subprogram DIEs for mach-o.
    DwarfCompileUnit &U = SkCU ? *SkCU : TheCU;

    if (unsigned NumRanges = TheCU.getRanges().size()) {
      if (NumRanges > 1 && useRangesSection())
        // A DW_AT_low_pc attribute may also be specified in combination with
        // DW_AT_ranges to specify the default base address for use in
        // location lists (see Section 2.6.2) and range lists (see Section
        // 2.17.3).
        U.addUInt(U.getUnitDie(), dwarf::DW_AT_low_pc, dwarf::DW_FORM_addr, 0);
      else
        U.setBaseAddress(TheCU.getRanges().front().Begin);
      U.attachRangesOrLowHighPC(U.getUnitDie(), TheCU.takeRanges());
    }

    // We don't keep track of which addresses are used in which CU so this
    // is a bit pessimistic under LTO.
    if ((!AddrPool.isEmpty() || TheCU.hasRangeLists()) &&
        (getDwarfVersion() >= 5 || HasSplitUnit))
      U.addAddrTableBase();

    if (getDwarfVersion() >= 5) {
      if (U.hasRangeLists())
        U.addRnglistsBase();

      if (!DebugLocs.getLists().empty()) {
        if (!useSplitDwarf())
          U.addSectionLabel(U.getUnitDie(), dwarf::DW_AT_loclists_base,
                            DebugLocs.getSym(),
                            TLOF.getDwarfLoclistsSection()->getBeginSymbol());
      }
    }

    auto *CUNode = cast<DICompileUnit>(P.first);
    // If compile Unit has macros, emit "DW_AT_macro_info" attribute.
    if (CUNode->getMacros()) {
      if (useSplitDwarf())
        TheCU.addSectionDelta(TheCU.getUnitDie(), dwarf::DW_AT_macro_info,
                            U.getMacroLabelBegin(),
                            TLOF.getDwarfMacinfoDWOSection()->getBeginSymbol());
      else
        U.addSectionLabel(U.getUnitDie(), dwarf::DW_AT_macro_info,
                          U.getMacroLabelBegin(),
                          TLOF.getDwarfMacinfoSection()->getBeginSymbol());
    }
  }

  // Emit all frontend-produced Skeleton CUs, i.e., Clang modules.
  for (auto *CUNode : MMI->getModule()->debug_compile_units())
    if (CUNode->getDWOId())
      getOrCreateDwarfCompileUnit(CUNode);

  // Compute DIE offsets and sizes.
  InfoHolder.computeSizeAndOffsets();
  if (useSplitDwarf())
    SkeletonHolder.computeSizeAndOffsets();
}

// Emit all Dwarf sections that should come after the content.
void DwarfDebug::endModule() {
  assert(CurFn == nullptr);
  assert(CurMI == nullptr);

  for (const auto &P : CUMap) {
    auto &CU = *P.second;
    CU.createBaseTypeDIEs();
  }

  // If we aren't actually generating debug info (check beginModule -
  // conditionalized on !DisableDebugInfoPrinting and the presence of the
  // llvm.dbg.cu metadata node)
  if (!MMI->hasDebugInfo())
    return;

  // Finalize the debug info for the module.
  finalizeModuleInfo();

  if (useSplitDwarf())
    // Emit debug_loc.dwo/debug_loclists.dwo section.
    emitDebugLocDWO();
  else
    // Emit debug_loc/debug_loclists section.
    emitDebugLoc();

  // Corresponding abbreviations into a abbrev section.
  emitAbbreviations();

  // Emit all the DIEs into a debug info section.
  emitDebugInfo();

  // Emit info into a debug aranges section.
  if (GenerateARangeSection)
    emitDebugARanges();

  // Emit info into a debug ranges section.
  emitDebugRanges();

  if (useSplitDwarf())
  // Emit info into a debug macinfo.dwo section.
    emitDebugMacinfoDWO();
  else
  // Emit info into a debug macinfo section.
    emitDebugMacinfo();

  emitDebugStr();

  if (useSplitDwarf()) {
    emitDebugStrDWO();
    emitDebugInfoDWO();
    emitDebugAbbrevDWO();
    emitDebugLineDWO();
    emitDebugRangesDWO();
  }

  emitDebugAddr();

  // Emit info into the dwarf accelerator table sections.
  switch (getAccelTableKind()) {
  case AccelTableKind::Apple:
    emitAccelNames();
    emitAccelObjC();
    emitAccelNamespaces();
    emitAccelTypes();
    break;
  case AccelTableKind::Dwarf:
    emitAccelDebugNames();
    break;
  case AccelTableKind::None:
    break;
  case AccelTableKind::Default:
    llvm_unreachable("Default should have already been resolved.");
  }

  // Emit the pubnames and pubtypes sections if requested.
  emitDebugPubSections();

  // clean up.
  // FIXME: AbstractVariables.clear();
}

void DwarfDebug::ensureAbstractEntityIsCreated(DwarfCompileUnit &CU,
                                               const DINode *Node,
                                               const MDNode *ScopeNode) {
  if (CU.getExistingAbstractEntity(Node))
    return;

  CU.createAbstractEntity(Node, LScopes.getOrCreateAbstractScope(
                                       cast<DILocalScope>(ScopeNode)));
}

void DwarfDebug::ensureAbstractEntityIsCreatedIfScoped(DwarfCompileUnit &CU,
    const DINode *Node, const MDNode *ScopeNode) {
  if (CU.getExistingAbstractEntity(Node))
    return;

  if (LexicalScope *Scope =
          LScopes.findAbstractScope(cast_or_null<DILocalScope>(ScopeNode)))
    CU.createAbstractEntity(Node, Scope);
}

// Collect variable information from side table maintained by MF.
void DwarfDebug::collectVariableInfoFromMFTable(
    DwarfCompileUnit &TheCU, DenseSet<InlinedEntity> &Processed) {
  SmallDenseMap<InlinedEntity, DbgVariable *> MFVars;
  LLVM_DEBUG(dbgs() << "DwarfDebug: collecting variables from MF side table\n");
  for (const auto &VI : Asm->MF->getVariableDbgInfo()) {
    if (!VI.Var)
      continue;
    assert(VI.Var->isValidLocationForIntrinsic(VI.Loc) &&
           "Expected inlined-at fields to agree");

    InlinedEntity Var(VI.Var, VI.Loc->getInlinedAt());
    Processed.insert(Var);
    LexicalScope *Scope = LScopes.findLexicalScope(VI.Loc);

    // If variable scope is not found then skip this variable.
    if (!Scope) {
      LLVM_DEBUG(dbgs() << "Dropping debug info for " << VI.Var->getName()
                        << ", no variable scope found\n");
      continue;
    }

    ensureAbstractEntityIsCreatedIfScoped(TheCU, Var.first, Scope->getScopeNode());
    auto RegVar = std::make_unique<DbgVariable>(
                    cast<DILocalVariable>(Var.first), Var.second);
    RegVar->initializeMMI(VI.Expr, VI.Slot);
    LLVM_DEBUG(dbgs() << "Created DbgVariable for " << VI.Var->getName()
                      << "\n");
    if (DbgVariable *DbgVar = MFVars.lookup(Var))
      DbgVar->addMMIEntry(*RegVar);
    else if (InfoHolder.addScopeVariable(Scope, RegVar.get())) {
      MFVars.insert({Var, RegVar.get()});
      ConcreteEntities.push_back(std::move(RegVar));
    }
  }
}

/// Determine whether a *singular* DBG_VALUE is valid for the entirety of its
/// enclosing lexical scope. The check ensures there are no other instructions
/// in the same lexical scope preceding the DBG_VALUE and that its range is
/// either open or otherwise rolls off the end of the scope.
static bool validThroughout(LexicalScopes &LScopes,
                            const MachineInstr *DbgValue,
                            const MachineInstr *RangeEnd) {
  assert(DbgValue->getDebugLoc() && "DBG_VALUE without a debug location");
  auto MBB = DbgValue->getParent();
  auto DL = DbgValue->getDebugLoc();
  auto *LScope = LScopes.findLexicalScope(DL);
  // Scope doesn't exist; this is a dead DBG_VALUE.
  if (!LScope)
    return false;
  auto &LSRange = LScope->getRanges();
  if (LSRange.size() == 0)
    return false;

  // Determine if the DBG_VALUE is valid at the beginning of its lexical block.
  const MachineInstr *LScopeBegin = LSRange.front().first;
  // Early exit if the lexical scope begins outside of the current block.
  if (LScopeBegin->getParent() != MBB)
    return false;
  MachineBasicBlock::const_reverse_iterator Pred(DbgValue);
  for (++Pred; Pred != MBB->rend(); ++Pred) {
    if (Pred->getFlag(MachineInstr::FrameSetup))
      break;
    auto PredDL = Pred->getDebugLoc();
    if (!PredDL || Pred->isMetaInstruction())
      continue;
    // Check whether the instruction preceding the DBG_VALUE is in the same
    // (sub)scope as the DBG_VALUE.
    if (DL->getScope() == PredDL->getScope())
      return false;
    auto *PredScope = LScopes.findLexicalScope(PredDL);
    if (!PredScope || LScope->dominates(PredScope))
      return false;
  }

  // If the range of the DBG_VALUE is open-ended, report success.
  if (!RangeEnd)
    return true;

  // Fail if there are instructions belonging to our scope in another block.
  const MachineInstr *LScopeEnd = LSRange.back().second;
  if (LScopeEnd->getParent() != MBB)
    return false;

  // Single, constant DBG_VALUEs in the prologue are promoted to be live
  // throughout the function. This is a hack, presumably for DWARF v2 and not
  // necessarily correct. It would be much better to use a dbg.declare instead
  // if we know the constant is live throughout the scope.
  if (DbgValue->getOperand(0).isImm() && MBB->pred_empty())
    return true;

  return false;
}

/// Build the location list for all DBG_VALUEs in the function that
/// describe the same variable. The resulting DebugLocEntries will have
/// strict monotonically increasing begin addresses and will never
/// overlap. If the resulting list has only one entry that is valid
/// throughout variable's scope return true.
//
// See the definition of DbgValueHistoryMap::Entry for an explanation of the
// different kinds of history map entries. One thing to be aware of is that if
// a debug value is ended by another entry (rather than being valid until the
// end of the function), that entry's instruction may or may not be included in
// the range, depending on if the entry is a clobbering entry (it has an
// instruction that clobbers one or more preceding locations), or if it is an
// (overlapping) debug value entry. This distinction can be seen in the example
// below. The first debug value is ended by the clobbering entry 2, and the
// second and third debug values are ended by the overlapping debug value entry
// 4.
//
// Input:
//
//   History map entries [type, end index, mi]
//
// 0 |      [DbgValue, 2, DBG_VALUE $reg0, [...] (fragment 0, 32)]
// 1 | |    [DbgValue, 4, DBG_VALUE $reg1, [...] (fragment 32, 32)]
// 2 | |    [Clobber, $reg0 = [...], -, -]
// 3   | |  [DbgValue, 4, DBG_VALUE 123, [...] (fragment 64, 32)]
// 4        [DbgValue, ~0, DBG_VALUE @g, [...] (fragment 0, 96)]
//
// Output [start, end) [Value...]:
//
// [0-1)    [(reg0, fragment 0, 32)]
// [1-3)    [(reg0, fragment 0, 32), (reg1, fragment 32, 32)]
// [3-4)    [(reg1, fragment 32, 32), (123, fragment 64, 32)]
// [4-)     [(@g, fragment 0, 96)]
bool DwarfDebug::buildLocationList(SmallVectorImpl<DebugLocEntry> &DebugLoc,
                                   const DbgValueHistoryMap::Entries &Entries) {
  using OpenRange =
      std::pair<DbgValueHistoryMap::EntryIndex, DbgValueLoc>;
  SmallVector<OpenRange, 4> OpenRanges;
  bool isSafeForSingleLocation = true;
  const MachineInstr *StartDebugMI = nullptr;
  const MachineInstr *EndMI = nullptr;

  for (auto EB = Entries.begin(), EI = EB, EE = Entries.end(); EI != EE; ++EI) {
    const MachineInstr *Instr = EI->getInstr();

    // Remove all values that are no longer live.
    size_t Index = std::distance(EB, EI);
    auto Last =
        remove_if(OpenRanges, [&](OpenRange &R) { return R.first <= Index; });
    OpenRanges.erase(Last, OpenRanges.end());

    // If we are dealing with a clobbering entry, this iteration will result in
    // a location list entry starting after the clobbering instruction.
    const MCSymbol *StartLabel =
        EI->isClobber() ? getLabelAfterInsn(Instr) : getLabelBeforeInsn(Instr);
    assert(StartLabel &&
           "Forgot label before/after instruction starting a range!");

    const MCSymbol *EndLabel;
    if (std::next(EI) == Entries.end()) {
      EndLabel = Asm->getFunctionEnd();
      if (EI->isClobber())
        EndMI = EI->getInstr();
    }
    else if (std::next(EI)->isClobber())
      EndLabel = getLabelAfterInsn(std::next(EI)->getInstr());
    else
      EndLabel = getLabelBeforeInsn(std::next(EI)->getInstr());
    assert(EndLabel && "Forgot label after instruction ending a range!");

    if (EI->isDbgValue())
      LLVM_DEBUG(dbgs() << "DotDebugLoc: " << *Instr << "\n");

    // If this history map entry has a debug value, add that to the list of
    // open ranges and check if its location is valid for a single value
    // location.
    if (EI->isDbgValue()) {
      // Do not add undef debug values, as they are redundant information in
      // the location list entries. An undef debug results in an empty location
      // description. If there are any non-undef fragments then padding pieces
      // with empty location descriptions will automatically be inserted, and if
      // all fragments are undef then the whole location list entry is
      // redundant.
      if (!Instr->isUndefDebugValue()) {
        auto Value = getDebugLocValue(Instr);
        OpenRanges.emplace_back(EI->getEndIndex(), Value);

        // TODO: Add support for single value fragment locations.
        if (Instr->getDebugExpression()->isFragment())
          isSafeForSingleLocation = false;

        if (!StartDebugMI)
          StartDebugMI = Instr;
      } else {
        isSafeForSingleLocation = false;
      }
    }

    // Location list entries with empty location descriptions are redundant
    // information in DWARF, so do not emit those.
    if (OpenRanges.empty())
      continue;

    // Omit entries with empty ranges as they do not have any effect in DWARF.
    if (StartLabel == EndLabel) {
      LLVM_DEBUG(dbgs() << "Omitting location list entry with empty range.\n");
      continue;
    }

    SmallVector<DbgValueLoc, 4> Values;
    for (auto &R : OpenRanges)
      Values.push_back(R.second);
    DebugLoc.emplace_back(StartLabel, EndLabel, Values);

    // Attempt to coalesce the ranges of two otherwise identical
    // DebugLocEntries.
    auto CurEntry = DebugLoc.rbegin();
    LLVM_DEBUG({
      dbgs() << CurEntry->getValues().size() << " Values:\n";
      for (auto &Value : CurEntry->getValues())
        Value.dump();
      dbgs() << "-----\n";
    });

    auto PrevEntry = std::next(CurEntry);
    if (PrevEntry != DebugLoc.rend() && PrevEntry->MergeRanges(*CurEntry))
      DebugLoc.pop_back();
  }

  return DebugLoc.size() == 1 && isSafeForSingleLocation &&
         validThroughout(LScopes, StartDebugMI, EndMI);
}

DbgEntity *DwarfDebug::createConcreteEntity(DwarfCompileUnit &TheCU,
                                            LexicalScope &Scope,
                                            const DINode *Node,
                                            const DILocation *Location,
                                            const MCSymbol *Sym) {
  ensureAbstractEntityIsCreatedIfScoped(TheCU, Node, Scope.getScopeNode());
  if (isa<const DILocalVariable>(Node)) {
    ConcreteEntities.push_back(
        std::make_unique<DbgVariable>(cast<const DILocalVariable>(Node),
                                       Location));
    InfoHolder.addScopeVariable(&Scope,
        cast<DbgVariable>(ConcreteEntities.back().get()));
  } else if (isa<const DILabel>(Node)) {
    ConcreteEntities.push_back(
        std::make_unique<DbgLabel>(cast<const DILabel>(Node),
                                    Location, Sym));
    InfoHolder.addScopeLabel(&Scope,
        cast<DbgLabel>(ConcreteEntities.back().get()));
  }
  return ConcreteEntities.back().get();
}

// Find variables for each lexical scope.
void DwarfDebug::collectEntityInfo(DwarfCompileUnit &TheCU,
                                   const DISubprogram *SP,
                                   DenseSet<InlinedEntity> &Processed) {
  // Grab the variable info that was squirreled away in the MMI side-table.
  collectVariableInfoFromMFTable(TheCU, Processed);

  for (const auto &I : DbgValues) {
    InlinedEntity IV = I.first;
    if (Processed.count(IV))
      continue;

    // Instruction ranges, specifying where IV is accessible.
    const auto &HistoryMapEntries = I.second;
    if (HistoryMapEntries.empty())
      continue;

    LexicalScope *Scope = nullptr;
    const DILocalVariable *LocalVar = cast<DILocalVariable>(IV.first);
    if (const DILocation *IA = IV.second)
      Scope = LScopes.findInlinedScope(LocalVar->getScope(), IA);
    else
      Scope = LScopes.findLexicalScope(LocalVar->getScope());
    // If variable scope is not found then skip this variable.
    if (!Scope)
      continue;

    Processed.insert(IV);
    DbgVariable *RegVar = cast<DbgVariable>(createConcreteEntity(TheCU,
                                            *Scope, LocalVar, IV.second));

    const MachineInstr *MInsn = HistoryMapEntries.front().getInstr();
    assert(MInsn->isDebugValue() && "History must begin with debug value");

    // Check if there is a single DBG_VALUE, valid throughout the var's scope.
    // If the history map contains a single debug value, there may be an
    // additional entry which clobbers the debug value.
    size_t HistSize = HistoryMapEntries.size();
    bool SingleValueWithClobber =
        HistSize == 2 && HistoryMapEntries[1].isClobber();
    if (HistSize == 1 || SingleValueWithClobber) {
      const auto *End =
          SingleValueWithClobber ? HistoryMapEntries[1].getInstr() : nullptr;
      if (validThroughout(LScopes, MInsn, End)) {
        RegVar->initializeDbgValue(MInsn);
        continue;
      }
    }

    // Do not emit location lists if .debug_loc secton is disabled.
    if (!useLocSection())
      continue;

    // Handle multiple DBG_VALUE instructions describing one variable.
    DebugLocStream::ListBuilder List(DebugLocs, TheCU, *Asm, *RegVar, *MInsn);

    // Build the location list for this variable.
    SmallVector<DebugLocEntry, 8> Entries;
    bool isValidSingleLocation = buildLocationList(Entries, HistoryMapEntries);

    // Check whether buildLocationList managed to merge all locations to one
    // that is valid throughout the variable's scope. If so, produce single
    // value location.
    if (isValidSingleLocation) {
      RegVar->initializeDbgValue(Entries[0].getValues()[0]);
      continue;
    }

    // If the variable has a DIBasicType, extract it.  Basic types cannot have
    // unique identifiers, so don't bother resolving the type with the
    // identifier map.
    const DIBasicType *BT = dyn_cast<DIBasicType>(
        static_cast<const Metadata *>(LocalVar->getType()));

    // Finalize the entry by lowering it into a DWARF bytestream.
    for (auto &Entry : Entries)
      Entry.finalize(*Asm, List, BT, TheCU);
  }

  // For each InlinedEntity collected from DBG_LABEL instructions, convert to
  // DWARF-related DbgLabel.
  for (const auto &I : DbgLabels) {
    InlinedEntity IL = I.first;
    const MachineInstr *MI = I.second;
    if (MI == nullptr)
      continue;

    LexicalScope *Scope = nullptr;
    const DILabel *Label = cast<DILabel>(IL.first);
    // The scope could have an extra lexical block file.
    const DILocalScope *LocalScope =
        Label->getScope()->getNonLexicalBlockFileScope();
    // Get inlined DILocation if it is inlined label.
    if (const DILocation *IA = IL.second)
      Scope = LScopes.findInlinedScope(LocalScope, IA);
    else
      Scope = LScopes.findLexicalScope(LocalScope);
    // If label scope is not found then skip this label.
    if (!Scope)
      continue;

    Processed.insert(IL);
    /// At this point, the temporary label is created.
    /// Save the temporary label to DbgLabel entity to get the
    /// actually address when generating Dwarf DIE.
    MCSymbol *Sym = getLabelBeforeInsn(MI);
    createConcreteEntity(TheCU, *Scope, Label, IL.second, Sym);
  }

  // Collect info for variables/labels that were optimized out.
  for (const DINode *DN : SP->getRetainedNodes()) {
    if (!Processed.insert(InlinedEntity(DN, nullptr)).second)
      continue;
    LexicalScope *Scope = nullptr;
    if (auto *DV = dyn_cast<DILocalVariable>(DN)) {
      Scope = LScopes.findLexicalScope(DV->getScope());
    } else if (auto *DL = dyn_cast<DILabel>(DN)) {
      Scope = LScopes.findLexicalScope(DL->getScope());
    }

    if (Scope)
      createConcreteEntity(TheCU, *Scope, DN, nullptr);
  }
}

// Process beginning of an instruction.
void DwarfDebug::beginInstruction(const MachineInstr *MI) {
  DebugHandlerBase::beginInstruction(MI);
  assert(CurMI);

  const auto *SP = MI->getMF()->getFunction().getSubprogram();
  if (!SP || SP->getUnit()->getEmissionKind() == DICompileUnit::NoDebug)
    return;

  // Check if source location changes, but ignore DBG_VALUE and CFI locations.
  // If the instruction is part of the function frame setup code, do not emit
  // any line record, as there is no correspondence with any user code.
  if (MI->isMetaInstruction() || MI->getFlag(MachineInstr::FrameSetup))
    return;
  const DebugLoc &DL = MI->getDebugLoc();
  // When we emit a line-0 record, we don't update PrevInstLoc; so look at
  // the last line number actually emitted, to see if it was line 0.
  unsigned LastAsmLine =
      Asm->OutStreamer->getContext().getCurrentDwarfLoc().getLine();

  // Request a label after the call in order to emit AT_return_pc information
  // in call site entries. TODO: Add support for targets with delay slots.
  if (SP->areAllCallsDescribed() && MI->isCall() && !MI->hasDelaySlot())
    requestLabelAfterInsn(MI);

  if (DL == PrevInstLoc) {
    // If we have an ongoing unspecified location, nothing to do here.
    if (!DL)
      return;
    // We have an explicit location, same as the previous location.
    // But we might be coming back to it after a line 0 record.
    if (LastAsmLine == 0 && DL.getLine() != 0) {
      // Reinstate the source location but not marked as a statement.
      const MDNode *Scope = DL.getScope();
      recordSourceLine(DL.getLine(), DL.getCol(), Scope, /*Flags=*/0);
    }
    return;
  }

  if (!DL) {
    // We have an unspecified location, which might want to be line 0.
    // If we have already emitted a line-0 record, don't repeat it.
    if (LastAsmLine == 0)
      return;
    // If user said Don't Do That, don't do that.
    if (UnknownLocations == Disable)
      return;
    // See if we have a reason to emit a line-0 record now.
    // Reasons to emit a line-0 record include:
    // - User asked for it (UnknownLocations).
    // - Instruction has a label, so it's referenced from somewhere else,
    //   possibly debug information; we want it to have a source location.
    // - Instruction is at the top of a block; we don't want to inherit the
    //   location from the physically previous (maybe unrelated) block.
    if (UnknownLocations == Enable || PrevLabel ||
        (PrevInstBB && PrevInstBB != MI->getParent())) {
      // Preserve the file and column numbers, if we can, to save space in
      // the encoded line table.
      // Do not update PrevInstLoc, it remembers the last non-0 line.
      const MDNode *Scope = nullptr;
      unsigned Column = 0;
      if (PrevInstLoc) {
        Scope = PrevInstLoc.getScope();
        Column = PrevInstLoc.getCol();
      }
      recordSourceLine(/*Line=*/0, Column, Scope, /*Flags=*/0);
    }
    return;
  }

  // We have an explicit location, different from the previous location.
  // Don't repeat a line-0 record, but otherwise emit the new location.
  // (The new location might be an explicit line 0, which we do emit.)
  if (DL.getLine() == 0 && LastAsmLine == 0)
    return;
  unsigned Flags = 0;
  if (DL == PrologEndLoc) {
    Flags |= DWARF2_FLAG_PROLOGUE_END | DWARF2_FLAG_IS_STMT;
    PrologEndLoc = DebugLoc();
  }
  // If the line changed, we call that a new statement; unless we went to
  // line 0 and came back, in which case it is not a new statement.
  unsigned OldLine = PrevInstLoc ? PrevInstLoc.getLine() : LastAsmLine;
  if (DL.getLine() && DL.getLine() != OldLine)
    Flags |= DWARF2_FLAG_IS_STMT;

  const MDNode *Scope = DL.getScope();
  recordSourceLine(DL.getLine(), DL.getCol(), Scope, Flags);

  // If we're not at line 0, remember this location.
  if (DL.getLine())
    PrevInstLoc = DL;
}

static DebugLoc findPrologueEndLoc(const MachineFunction *MF) {
  // First known non-DBG_VALUE and non-frame setup location marks
  // the beginning of the function body.
  for (const auto &MBB : *MF)
    for (const auto &MI : MBB)
      if (!MI.isMetaInstruction() && !MI.getFlag(MachineInstr::FrameSetup) &&
          MI.getDebugLoc())
        return MI.getDebugLoc();
  return DebugLoc();
}

/// Register a source line with debug info. Returns the  unique label that was
/// emitted and which provides correspondence to the source line list.
static void recordSourceLine(AsmPrinter &Asm, unsigned Line, unsigned Col,
                             const MDNode *S, unsigned Flags, unsigned CUID,
                             uint16_t DwarfVersion,
                             ArrayRef<std::unique_ptr<DwarfCompileUnit>> DCUs) {
  StringRef Fn;
  unsigned FileNo = 1;
  unsigned Discriminator = 0;
  if (auto *Scope = cast_or_null<DIScope>(S)) {
    Fn = Scope->getFilename();
    if (Line != 0 && DwarfVersion >= 4)
      if (auto *LBF = dyn_cast<DILexicalBlockFile>(Scope))
        Discriminator = LBF->getDiscriminator();

    FileNo = static_cast<DwarfCompileUnit &>(*DCUs[CUID])
                 .getOrCreateSourceID(Scope->getFile());
  }
  Asm.OutStreamer->emitDwarfLocDirective(FileNo, Line, Col, Flags, 0,
                                         Discriminator, Fn);
}

DebugLoc DwarfDebug::emitInitialLocDirective(const MachineFunction &MF,
                                             unsigned CUID) {
  // Get beginning of function.
  if (DebugLoc PrologEndLoc = findPrologueEndLoc(&MF)) {
    // Ensure the compile unit is created if the function is called before
    // beginFunction().
    (void)getOrCreateDwarfCompileUnit(
        MF.getFunction().getSubprogram()->getUnit());
    // We'd like to list the prologue as "not statements" but GDB behaves
    // poorly if we do that. Revisit this with caution/GDB (7.5+) testing.
    const DISubprogram *SP = PrologEndLoc->getInlinedAtScope()->getSubprogram();
    ::recordSourceLine(*Asm, SP->getScopeLine(), 0, SP, DWARF2_FLAG_IS_STMT,
                       CUID, getDwarfVersion(), getUnits());
    return PrologEndLoc;
  }
  return DebugLoc();
}

// Gather pre-function debug information.  Assumes being called immediately
// after the function entry point has been emitted.
void DwarfDebug::beginFunctionImpl(const MachineFunction *MF) {
  CurFn = MF;

  auto *SP = MF->getFunction().getSubprogram();
  assert(LScopes.empty() || SP == LScopes.getCurrentFunctionScope()->getScopeNode());
  if (SP->getUnit()->getEmissionKind() == DICompileUnit::NoDebug)
    return;

  DwarfCompileUnit &CU = getOrCreateDwarfCompileUnit(SP->getUnit());

  // Set DwarfDwarfCompileUnitID in MCContext to the Compile Unit this function
  // belongs to so that we add to the correct per-cu line table in the
  // non-asm case.
  if (Asm->OutStreamer->hasRawTextSupport())
    // Use a single line table if we are generating assembly.
    Asm->OutStreamer->getContext().setDwarfCompileUnitID(0);
  else
    Asm->OutStreamer->getContext().setDwarfCompileUnitID(CU.getUniqueID());

  // Record beginning of function.
  PrologEndLoc = emitInitialLocDirective(
      *MF, Asm->OutStreamer->getContext().getDwarfCompileUnitID());
}

void DwarfDebug::skippedNonDebugFunction() {
  // If we don't have a subprogram for this function then there will be a hole
  // in the range information. Keep note of this by setting the previously used
  // section to nullptr.
  PrevCU = nullptr;
  CurFn = nullptr;
}

// Gather and emit post-function debug information.
void DwarfDebug::endFunctionImpl(const MachineFunction *MF) {
  const DISubprogram *SP = MF->getFunction().getSubprogram();

  assert(CurFn == MF &&
      "endFunction should be called with the same function as beginFunction");

  // Set DwarfDwarfCompileUnitID in MCContext to default value.
  Asm->OutStreamer->getContext().setDwarfCompileUnitID(0);

  LexicalScope *FnScope = LScopes.getCurrentFunctionScope();
  assert(!FnScope || SP == FnScope->getScopeNode());
  DwarfCompileUnit &TheCU = *CUMap.lookup(SP->getUnit());
  if (TheCU.getCUNode()->isDebugDirectivesOnly()) {
    PrevLabel = nullptr;
    CurFn = nullptr;
    return;
  }

  DenseSet<InlinedEntity> Processed;
  collectEntityInfo(TheCU, SP, Processed);

  // Add the range of this function to the list of ranges for the CU.
  TheCU.addRange({Asm->getFunctionBegin(), Asm->getFunctionEnd()});

  // Under -gmlt, skip building the subprogram if there are no inlined
  // subroutines inside it. But with -fdebug-info-for-profiling, the subprogram
  // is still needed as we need its source location.
  if (!TheCU.getCUNode()->getDebugInfoForProfiling() &&
      TheCU.getCUNode()->getEmissionKind() == DICompileUnit::LineTablesOnly &&
      LScopes.getAbstractScopesList().empty() && !IsDarwin) {
    assert(InfoHolder.getScopeVariables().empty());
    PrevLabel = nullptr;
    CurFn = nullptr;
    return;
  }

#ifndef NDEBUG
  size_t NumAbstractScopes = LScopes.getAbstractScopesList().size();
#endif
  // Construct abstract scopes.
  for (LexicalScope *AScope : LScopes.getAbstractScopesList()) {
    auto *SP = cast<DISubprogram>(AScope->getScopeNode());
    for (const DINode *DN : SP->getRetainedNodes()) {
      if (!Processed.insert(InlinedEntity(DN, nullptr)).second)
        continue;

      const MDNode *Scope = nullptr;
      if (auto *DV = dyn_cast<DILocalVariable>(DN))
        Scope = DV->getScope();
      else if (auto *DL = dyn_cast<DILabel>(DN))
        Scope = DL->getScope();
      else
        llvm_unreachable("Unexpected DI type!");

      // Collect info for variables/labels that were optimized out.
      ensureAbstractEntityIsCreated(TheCU, DN, Scope);
      assert(LScopes.getAbstractScopesList().size() == NumAbstractScopes
             && "ensureAbstractEntityIsCreated inserted abstract scopes");
    }
    constructAbstractSubprogramScopeDIE(TheCU, AScope);
  }

  ProcessedSPNodes.insert(SP);
  DIE &ScopeDIE = TheCU.constructSubprogramScopeDIE(SP, FnScope);
  if (auto *SkelCU = TheCU.getSkeleton())
    if (!LScopes.getAbstractScopesList().empty() &&
        TheCU.getCUNode()->getSplitDebugInlining())
      SkelCU->constructSubprogramScopeDIE(SP, FnScope);

  // Construct call site entries.
  constructCallSiteEntryDIEs(*SP, TheCU, ScopeDIE, *MF);

  // Clear debug info
  // Ownership of DbgVariables is a bit subtle - ScopeVariables owns all the
  // DbgVariables except those that are also in AbstractVariables (since they
  // can be used cross-function)
  InfoHolder.getScopeVariables().clear();
  InfoHolder.getScopeLabels().clear();
  PrevLabel = nullptr;
  CurFn = nullptr;
}

// Register a source line with debug info. Returns the  unique label that was
// emitted and which provides correspondence to the source line list.
void DwarfDebug::recordSourceLine(unsigned Line, unsigned Col, const MDNode *S,
                                  unsigned Flags) {
  ::recordSourceLine(*Asm, Line, Col, S, Flags,
                     Asm->OutStreamer->getContext().getDwarfCompileUnitID(),
                     getDwarfVersion(), getUnits());
}

//===----------------------------------------------------------------------===//
// Emit Methods
//===----------------------------------------------------------------------===//

// Emit the debug info section.
void DwarfDebug::emitDebugInfo() {
  DwarfFile &Holder = useSplitDwarf() ? SkeletonHolder : InfoHolder;
  Holder.emitUnits(/* UseOffsets */ false);
}

// Emit the abbreviation section.
void DwarfDebug::emitAbbreviations() {
  DwarfFile &Holder = useSplitDwarf() ? SkeletonHolder : InfoHolder;

  Holder.emitAbbrevs(Asm->getObjFileLowering().getDwarfAbbrevSection());
}

void DwarfDebug::emitStringOffsetsTableHeader() {
  DwarfFile &Holder = useSplitDwarf() ? SkeletonHolder : InfoHolder;
  Holder.getStringPool().emitStringOffsetsTableHeader(
      *Asm, Asm->getObjFileLowering().getDwarfStrOffSection(),
      Holder.getStringOffsetsStartSym());
}

template <typename AccelTableT>
void DwarfDebug::emitAccel(AccelTableT &Accel, MCSection *Section,
                           StringRef TableName) {
  Asm->OutStreamer->SwitchSection(Section);

  // Emit the full data.
  emitAppleAccelTable(Asm, Accel, TableName, Section->getBeginSymbol());
}

void DwarfDebug::emitAccelDebugNames() {
  // Don't emit anything if we have no compilation units to index.
  if (getUnits().empty())
    return;

  emitDWARF5AccelTable(Asm, AccelDebugNames, *this, getUnits());
}

// Emit visible names into a hashed accelerator table section.
void DwarfDebug::emitAccelNames() {
  emitAccel(AccelNames, Asm->getObjFileLowering().getDwarfAccelNamesSection(),
            "Names");
}

// Emit objective C classes and categories into a hashed accelerator table
// section.
void DwarfDebug::emitAccelObjC() {
  emitAccel(AccelObjC, Asm->getObjFileLowering().getDwarfAccelObjCSection(),
            "ObjC");
}

// Emit namespace dies into a hashed accelerator table.
void DwarfDebug::emitAccelNamespaces() {
  emitAccel(AccelNamespace,
            Asm->getObjFileLowering().getDwarfAccelNamespaceSection(),
            "namespac");
}

// Emit type dies into a hashed accelerator table.
void DwarfDebug::emitAccelTypes() {
  emitAccel(AccelTypes, Asm->getObjFileLowering().getDwarfAccelTypesSection(),
            "types");
}

// Public name handling.
// The format for the various pubnames:
//
// dwarf pubnames - offset/name pairs where the offset is the offset into the CU
// for the DIE that is named.
//
// gnu pubnames - offset/index value/name tuples where the offset is the offset
// into the CU and the index value is computed according to the type of value
// for the DIE that is named.
//
// For type units the offset is the offset of the skeleton DIE. For split dwarf
// it's the offset within the debug_info/debug_types dwo section, however, the
// reference in the pubname header doesn't change.

/// computeIndexValue - Compute the gdb index value for the DIE and CU.
static dwarf::PubIndexEntryDescriptor computeIndexValue(DwarfUnit *CU,
                                                        const DIE *Die) {
  // Entities that ended up only in a Type Unit reference the CU instead (since
  // the pub entry has offsets within the CU there's no real offset that can be
  // provided anyway). As it happens all such entities (namespaces and types,
  // types only in C++ at that) are rendered as TYPE+EXTERNAL. If this turns out
  // not to be true it would be necessary to persist this information from the
  // point at which the entry is added to the index data structure - since by
  // the time the index is built from that, the original type/namespace DIE in a
  // type unit has already been destroyed so it can't be queried for properties
  // like tag, etc.
  if (Die->getTag() == dwarf::DW_TAG_compile_unit)
    return dwarf::PubIndexEntryDescriptor(dwarf::GIEK_TYPE,
                                          dwarf::GIEL_EXTERNAL);
  dwarf::GDBIndexEntryLinkage Linkage = dwarf::GIEL_STATIC;

  // We could have a specification DIE that has our most of our knowledge,
  // look for that now.
  if (DIEValue SpecVal = Die->findAttribute(dwarf::DW_AT_specification)) {
    DIE &SpecDIE = SpecVal.getDIEEntry().getEntry();
    if (SpecDIE.findAttribute(dwarf::DW_AT_external))
      Linkage = dwarf::GIEL_EXTERNAL;
  } else if (Die->findAttribute(dwarf::DW_AT_external))
    Linkage = dwarf::GIEL_EXTERNAL;

  switch (Die->getTag()) {
  case dwarf::DW_TAG_class_type:
  case dwarf::DW_TAG_structure_type:
  case dwarf::DW_TAG_union_type:
  case dwarf::DW_TAG_enumeration_type:
    return dwarf::PubIndexEntryDescriptor(
        dwarf::GIEK_TYPE,
        dwarf::isCPlusPlus((dwarf::SourceLanguage)CU->getLanguage())
            ? dwarf::GIEL_EXTERNAL
            : dwarf::GIEL_STATIC);
  case dwarf::DW_TAG_typedef:
  case dwarf::DW_TAG_base_type:
  case dwarf::DW_TAG_subrange_type:
    return dwarf::PubIndexEntryDescriptor(dwarf::GIEK_TYPE, dwarf::GIEL_STATIC);
  case dwarf::DW_TAG_namespace:
    return dwarf::GIEK_TYPE;
  case dwarf::DW_TAG_subprogram:
    return dwarf::PubIndexEntryDescriptor(dwarf::GIEK_FUNCTION, Linkage);
  case dwarf::DW_TAG_variable:
    return dwarf::PubIndexEntryDescriptor(dwarf::GIEK_VARIABLE, Linkage);
  case dwarf::DW_TAG_enumerator:
    return dwarf::PubIndexEntryDescriptor(dwarf::GIEK_VARIABLE,
                                          dwarf::GIEL_STATIC);
  default:
    return dwarf::GIEK_NONE;
  }
}

/// emitDebugPubSections - Emit visible names and types into debug pubnames and
/// pubtypes sections.
void DwarfDebug::emitDebugPubSections() {
  for (const auto &NU : CUMap) {
    DwarfCompileUnit *TheU = NU.second;
    if (!TheU->hasDwarfPubSections())
      continue;

    bool GnuStyle = TheU->getCUNode()->getNameTableKind() ==
                    DICompileUnit::DebugNameTableKind::GNU;

    Asm->OutStreamer->SwitchSection(
        GnuStyle ? Asm->getObjFileLowering().getDwarfGnuPubNamesSection()
                 : Asm->getObjFileLowering().getDwarfPubNamesSection());
    emitDebugPubSection(GnuStyle, "Names", TheU, TheU->getGlobalNames());

    Asm->OutStreamer->SwitchSection(
        GnuStyle ? Asm->getObjFileLowering().getDwarfGnuPubTypesSection()
                 : Asm->getObjFileLowering().getDwarfPubTypesSection());
    emitDebugPubSection(GnuStyle, "Types", TheU, TheU->getGlobalTypes());
  }
}

void DwarfDebug::emitSectionReference(const DwarfCompileUnit &CU) {
  if (useSectionsAsReferences())
    Asm->emitDwarfOffset(CU.getSection()->getBeginSymbol(),
                         CU.getDebugSectionOffset());
  else
    Asm->emitDwarfSymbolReference(CU.getLabelBegin());
}

void DwarfDebug::emitDebugPubSection(bool GnuStyle, StringRef Name,
                                     DwarfCompileUnit *TheU,
                                     const StringMap<const DIE *> &Globals) {
  if (auto *Skeleton = TheU->getSkeleton())
    TheU = Skeleton;

  // Emit the header.
  Asm->OutStreamer->AddComment("Length of Public " + Name + " Info");
  MCSymbol *BeginLabel = Asm->createTempSymbol("pub" + Name + "_begin");
  MCSymbol *EndLabel = Asm->createTempSymbol("pub" + Name + "_end");
  Asm->emitLabelDifference(EndLabel, BeginLabel, 4);

  Asm->OutStreamer->emitLabel(BeginLabel);

  Asm->OutStreamer->AddComment("DWARF Version");
  Asm->emitInt16(dwarf::DW_PUBNAMES_VERSION);

  Asm->OutStreamer->AddComment("Offset of Compilation Unit Info");
  emitSectionReference(*TheU);

  Asm->OutStreamer->AddComment("Compilation Unit Length");
  Asm->emitInt32(TheU->getLength());

  // Emit the pubnames for this compilation unit.
  for (const auto &GI : Globals) {
    const char *Name = GI.getKeyData();
    const DIE *Entity = GI.second;

    Asm->OutStreamer->AddComment("DIE offset");
    Asm->emitInt32(Entity->getOffset());

    if (GnuStyle) {
      dwarf::PubIndexEntryDescriptor Desc = computeIndexValue(TheU, Entity);
      Asm->OutStreamer->AddComment(
          Twine("Attributes: ") + dwarf::GDBIndexEntryKindString(Desc.Kind) +
          ", " + dwarf::GDBIndexEntryLinkageString(Desc.Linkage));
      Asm->emitInt8(Desc.toBits());
    }

    Asm->OutStreamer->AddComment("External Name");
    Asm->OutStreamer->emitBytes(StringRef(Name, GI.getKeyLength() + 1));
  }

  Asm->OutStreamer->AddComment("End Mark");
  Asm->emitInt32(0);
  Asm->OutStreamer->emitLabel(EndLabel);
}

/// Emit null-terminated strings into a debug str section.
void DwarfDebug::emitDebugStr() {
  MCSection *StringOffsetsSection = nullptr;
  if (useSegmentedStringOffsetsTable()) {
    emitStringOffsetsTableHeader();
    StringOffsetsSection = Asm->getObjFileLowering().getDwarfStrOffSection();
  }
  DwarfFile &Holder = useSplitDwarf() ? SkeletonHolder : InfoHolder;
  Holder.emitStrings(Asm->getObjFileLowering().getDwarfStrSection(),
                     StringOffsetsSection, /* UseRelativeOffsets = */ true);
}

void DwarfDebug::emitDebugLocEntry(ByteStreamer &Streamer,
                                   const DebugLocStream::Entry &Entry,
                                   const DwarfCompileUnit *CU) {
  auto &&Comments = DebugLocs.getComments(Entry);
  auto Comment = Comments.begin();
  auto End = Comments.end();

  // The expressions are inserted into a byte stream rather early (see
  // DwarfExpression::addExpression) so for those ops (e.g. DW_OP_convert) that
  // need to reference a base_type DIE the offset of that DIE is not yet known.
  // To deal with this we instead insert a placeholder early and then extract
  // it here and replace it with the real reference.
  unsigned PtrSize = Asm->MAI->getCodePointerSize();
  DWARFDataExtractor Data(StringRef(DebugLocs.getBytes(Entry).data(),
                                    DebugLocs.getBytes(Entry).size()),
                          Asm->getDataLayout().isLittleEndian(), PtrSize);
  DWARFExpression Expr(Data, PtrSize);

  using Encoding = DWARFExpression::Operation::Encoding;
  uint64_t Offset = 0;
  for (auto &Op : Expr) {
    assert(Op.getCode() != dwarf::DW_OP_const_type &&
           "3 operand ops not yet supported");
    Streamer.EmitInt8(Op.getCode(), Comment != End ? *(Comment++) : "");
    Offset++;
    for (unsigned I = 0; I < 2; ++I) {
      if (Op.getDescription().Op[I] == Encoding::SizeNA)
        continue;
      if (Op.getDescription().Op[I] == Encoding::BaseTypeRef) {
        uint64_t Offset =
            CU->ExprRefedBaseTypes[Op.getRawOperand(I)].Die->getOffset();
        assert(Offset < (1ULL << (ULEB128PadSize * 7)) && "Offset wont fit");
        Streamer.emitULEB128(Offset, "", ULEB128PadSize);
        // Make sure comments stay aligned.
        for (unsigned J = 0; J < ULEB128PadSize; ++J)
          if (Comment != End)
            Comment++;
      } else {
        for (uint64_t J = Offset; J < Op.getOperandEndOffset(I); ++J)
          Streamer.EmitInt8(Data.getData()[J], Comment != End ? *(Comment++) : "");
      }
      Offset = Op.getOperandEndOffset(I);
    }
    assert(Offset == Op.getEndOffset());
  }
}

void DwarfDebug::emitDebugLocValue(const AsmPrinter &AP, const DIBasicType *BT,
                                   const DbgValueLoc &Value,
                                   DwarfExpression &DwarfExpr) {
  auto *DIExpr = Value.getExpression();
  DIExpressionCursor ExprCursor(DIExpr);
  DwarfExpr.addFragmentOffset(DIExpr);
  // Regular entry.
  if (Value.isInt()) {
    if (BT && (BT->getEncoding() == dwarf::DW_ATE_signed ||
               BT->getEncoding() == dwarf::DW_ATE_signed_char))
      DwarfExpr.addSignedConstant(Value.getInt());
    else
      DwarfExpr.addUnsignedConstant(Value.getInt());
  } else if (Value.isLocation()) {
    MachineLocation Location = Value.getLoc();
    if (Location.isIndirect())
      DwarfExpr.setMemoryLocationKind();
    DIExpressionCursor Cursor(DIExpr);

    if (DIExpr->isEntryValue()) {
      DwarfExpr.setEntryValueFlag();
      DwarfExpr.beginEntryValueExpression(Cursor);
    }

    const TargetRegisterInfo &TRI = *AP.MF->getSubtarget().getRegisterInfo();
    if (!DwarfExpr.addMachineRegExpression(TRI, Cursor, Location.getReg()))
      return;
    return DwarfExpr.addExpression(std::move(Cursor));
  } else if (Value.isTargetIndexLocation()) {
    TargetIndexLocation Loc = Value.getTargetIndexLocation();
    // TODO TargetIndexLocation is a target-independent. Currently only the WebAssembly-specific
    // encoding is supported.
    DwarfExpr.addWasmLocation(Loc.Index, Loc.Offset);
  } else if (Value.isConstantFP()) {
    APInt RawBytes = Value.getConstantFP()->getValueAPF().bitcastToAPInt();
    DwarfExpr.addUnsignedConstant(RawBytes);
  }
  DwarfExpr.addExpression(std::move(ExprCursor));
}

void DebugLocEntry::finalize(const AsmPrinter &AP,
                             DebugLocStream::ListBuilder &List,
                             const DIBasicType *BT,
                             DwarfCompileUnit &TheCU) {
  assert(!Values.empty() &&
         "location list entries without values are redundant");
  assert(Begin != End && "unexpected location list entry with empty range");
  DebugLocStream::EntryBuilder Entry(List, Begin, End);
  BufferByteStreamer Streamer = Entry.getStreamer();
  DebugLocDwarfExpression DwarfExpr(AP.getDwarfVersion(), Streamer, TheCU);
  const DbgValueLoc &Value = Values[0];
  if (Value.isFragment()) {
    // Emit all fragments that belong to the same variable and range.
    assert(llvm::all_of(Values, [](DbgValueLoc P) {
          return P.isFragment();
        }) && "all values are expected to be fragments");
    assert(std::is_sorted(Values.begin(), Values.end()) &&
           "fragments are expected to be sorted");

    for (auto Fragment : Values)
      DwarfDebug::emitDebugLocValue(AP, BT, Fragment, DwarfExpr);

  } else {
    assert(Values.size() == 1 && "only fragments may have >1 value");
    DwarfDebug::emitDebugLocValue(AP, BT, Value, DwarfExpr);
  }
  DwarfExpr.finalize();
  if (DwarfExpr.TagOffset)
    List.setTagOffset(*DwarfExpr.TagOffset);
}

void DwarfDebug::emitDebugLocEntryLocation(const DebugLocStream::Entry &Entry,
                                           const DwarfCompileUnit *CU) {
  // Emit the size.
  Asm->OutStreamer->AddComment("Loc expr size");
  if (getDwarfVersion() >= 5)
    Asm->emitULEB128(DebugLocs.getBytes(Entry).size());
  else if (DebugLocs.getBytes(Entry).size() <= std::numeric_limits<uint16_t>::max())
    Asm->emitInt16(DebugLocs.getBytes(Entry).size());
  else {
    // The entry is too big to fit into 16 bit, drop it as there is nothing we
    // can do.
    Asm->emitInt16(0);
    return;
  }
  // Emit the entry.
  APByteStreamer Streamer(*Asm);
  emitDebugLocEntry(Streamer, Entry, CU);
}

// Emit the common part of the DWARF 5 range/locations list tables header.
static void emitListsTableHeaderStart(AsmPrinter *Asm,
                                      MCSymbol *TableStart,
                                      MCSymbol *TableEnd) {
  // Build the table header, which starts with the length field.
  Asm->OutStreamer->AddComment("Length");
  Asm->emitLabelDifference(TableEnd, TableStart, 4);
  Asm->OutStreamer->emitLabel(TableStart);
  // Version number (DWARF v5 and later).
  Asm->OutStreamer->AddComment("Version");
  Asm->emitInt16(Asm->OutStreamer->getContext().getDwarfVersion());
  // Address size.
  Asm->OutStreamer->AddComment("Address size");
  Asm->emitInt8(Asm->MAI->getCodePointerSize());
  // Segment selector size.
  Asm->OutStreamer->AddComment("Segment selector size");
  Asm->emitInt8(0);
}

// Emit the header of a DWARF 5 range list table list table. Returns the symbol
// that designates the end of the table for the caller to emit when the table is
// complete.
static MCSymbol *emitRnglistsTableHeader(AsmPrinter *Asm,
                                         const DwarfFile &Holder) {
  MCSymbol *TableStart = Asm->createTempSymbol("debug_rnglist_table_start");
  MCSymbol *TableEnd = Asm->createTempSymbol("debug_rnglist_table_end");
  emitListsTableHeaderStart(Asm, TableStart, TableEnd);

  Asm->OutStreamer->AddComment("Offset entry count");
  Asm->emitInt32(Holder.getRangeLists().size());
  Asm->OutStreamer->emitLabel(Holder.getRnglistsTableBaseSym());

  for (const RangeSpanList &List : Holder.getRangeLists())
    Asm->emitLabelDifference(List.Label, Holder.getRnglistsTableBaseSym(), 4);

  return TableEnd;
}

// Emit the header of a DWARF 5 locations list table. Returns the symbol that
// designates the end of the table for the caller to emit when the table is
// complete.
static MCSymbol *emitLoclistsTableHeader(AsmPrinter *Asm,
                                         const DwarfDebug &DD) {
  MCSymbol *TableStart = Asm->createTempSymbol("debug_loclist_table_start");
  MCSymbol *TableEnd = Asm->createTempSymbol("debug_loclist_table_end");
  emitListsTableHeaderStart(Asm, TableStart, TableEnd);

  const auto &DebugLocs = DD.getDebugLocs();

  Asm->OutStreamer->AddComment("Offset entry count");
  Asm->emitInt32(DebugLocs.getLists().size());
  Asm->OutStreamer->emitLabel(DebugLocs.getSym());

  for (const auto &List : DebugLocs.getLists())
    Asm->emitLabelDifference(List.Label, DebugLocs.getSym(), 4);

  return TableEnd;
}

template <typename Ranges, typename PayloadEmitter>
static void emitRangeList(
    DwarfDebug &DD, AsmPrinter *Asm, MCSymbol *Sym, const Ranges &R,
    const DwarfCompileUnit &CU, unsigned BaseAddressx, unsigned OffsetPair,
    unsigned StartxLength, unsigned EndOfList,
    StringRef (*StringifyEnum)(unsigned),
    bool ShouldUseBaseAddress,
    PayloadEmitter EmitPayload) {

  auto Size = Asm->MAI->getCodePointerSize();
  bool UseDwarf5 = DD.getDwarfVersion() >= 5;

  // Emit our symbol so we can find the beginning of the range.
  Asm->OutStreamer->emitLabel(Sym);

  // Gather all the ranges that apply to the same section so they can share
  // a base address entry.
  MapVector<const MCSection *, std::vector<decltype(&*R.begin())>> SectionRanges;

  for (const auto &Range : R)
    SectionRanges[&Range.Begin->getSection()].push_back(&Range);

  const MCSymbol *CUBase = CU.getBaseAddress();
  bool BaseIsSet = false;
  for (const auto &P : SectionRanges) {
    auto *Base = CUBase;
    if (!Base && ShouldUseBaseAddress) {
      const MCSymbol *Begin = P.second.front()->Begin;
      const MCSymbol *NewBase = DD.getSectionLabel(&Begin->getSection());
      if (!UseDwarf5) {
        Base = NewBase;
        BaseIsSet = true;
        Asm->OutStreamer->emitIntValue(-1, Size);
        Asm->OutStreamer->AddComment("  base address");
        Asm->OutStreamer->emitSymbolValue(Base, Size);
      } else if (NewBase != Begin || P.second.size() > 1) {
        // Only use a base address if
        //  * the existing pool address doesn't match (NewBase != Begin)
        //  * or, there's more than one entry to share the base address
        Base = NewBase;
        BaseIsSet = true;
        Asm->OutStreamer->AddComment(StringifyEnum(BaseAddressx));
        Asm->emitInt8(BaseAddressx);
        Asm->OutStreamer->AddComment("  base address index");
        Asm->emitULEB128(DD.getAddressPool().getIndex(Base));
      }
    } else if (BaseIsSet && !UseDwarf5) {
      BaseIsSet = false;
      assert(!Base);
      Asm->OutStreamer->emitIntValue(-1, Size);
      Asm->OutStreamer->emitIntValue(0, Size);
    }

    for (const auto *RS : P.second) {
      const MCSymbol *Begin = RS->Begin;
      const MCSymbol *End = RS->End;
      assert(Begin && "Range without a begin symbol?");
      assert(End && "Range without an end symbol?");
      if (Base) {
        if (UseDwarf5) {
          // Emit offset_pair when we have a base.
          Asm->OutStreamer->AddComment(StringifyEnum(OffsetPair));
          Asm->emitInt8(OffsetPair);
          Asm->OutStreamer->AddComment("  starting offset");
          Asm->emitLabelDifferenceAsULEB128(Begin, Base);
          Asm->OutStreamer->AddComment("  ending offset");
          Asm->emitLabelDifferenceAsULEB128(End, Base);
        } else {
          Asm->emitLabelDifference(Begin, Base, Size);
          Asm->emitLabelDifference(End, Base, Size);
        }
      } else if (UseDwarf5) {
        Asm->OutStreamer->AddComment(StringifyEnum(StartxLength));
        Asm->emitInt8(StartxLength);
        Asm->OutStreamer->AddComment("  start index");
        Asm->emitULEB128(DD.getAddressPool().getIndex(Begin));
        Asm->OutStreamer->AddComment("  length");
        Asm->emitLabelDifferenceAsULEB128(End, Begin);
      } else {
        Asm->OutStreamer->emitSymbolValue(Begin, Size);
        Asm->OutStreamer->emitSymbolValue(End, Size);
      }
      EmitPayload(*RS);
    }
  }

  if (UseDwarf5) {
    Asm->OutStreamer->AddComment(StringifyEnum(EndOfList));
    Asm->emitInt8(EndOfList);
  } else {
    // Terminate the list with two 0 values.
    Asm->OutStreamer->emitIntValue(0, Size);
    Asm->OutStreamer->emitIntValue(0, Size);
  }
}

// Handles emission of both debug_loclist / debug_loclist.dwo
static void emitLocList(DwarfDebug &DD, AsmPrinter *Asm, const DebugLocStream::List &List) {
  emitRangeList(DD, Asm, List.Label, DD.getDebugLocs().getEntries(List),
                *List.CU, dwarf::DW_LLE_base_addressx,
                dwarf::DW_LLE_offset_pair, dwarf::DW_LLE_startx_length,
                dwarf::DW_LLE_end_of_list, llvm::dwarf::LocListEncodingString,
                /* ShouldUseBaseAddress */ true,
                [&](const DebugLocStream::Entry &E) {
                  DD.emitDebugLocEntryLocation(E, List.CU);
                });
}

void DwarfDebug::emitDebugLocImpl(MCSection *Sec) {
  if (DebugLocs.getLists().empty())
    return;

  Asm->OutStreamer->SwitchSection(Sec);

  MCSymbol *TableEnd = nullptr;
  if (getDwarfVersion() >= 5)
    TableEnd = emitLoclistsTableHeader(Asm, *this);

  for (const auto &List : DebugLocs.getLists())
    emitLocList(*this, Asm, List);

  if (TableEnd)
    Asm->OutStreamer->emitLabel(TableEnd);
}

// Emit locations into the .debug_loc/.debug_loclists section.
void DwarfDebug::emitDebugLoc() {
  emitDebugLocImpl(
      getDwarfVersion() >= 5
          ? Asm->getObjFileLowering().getDwarfLoclistsSection()
          : Asm->getObjFileLowering().getDwarfLocSection());
}

// Emit locations into the .debug_loc.dwo/.debug_loclists.dwo section.
void DwarfDebug::emitDebugLocDWO() {
  if (getDwarfVersion() >= 5) {
    emitDebugLocImpl(
        Asm->getObjFileLowering().getDwarfLoclistsDWOSection());

    return;
  }

  for (const auto &List : DebugLocs.getLists()) {
    Asm->OutStreamer->SwitchSection(
        Asm->getObjFileLowering().getDwarfLocDWOSection());
    Asm->OutStreamer->emitLabel(List.Label);

    for (const auto &Entry : DebugLocs.getEntries(List)) {
      // GDB only supports startx_length in pre-standard split-DWARF.
      // (in v5 standard loclists, it currently* /only/ supports base_address +
      // offset_pair, so the implementations can't really share much since they
      // need to use different representations)
      // * as of October 2018, at least
      //
      // In v5 (see emitLocList), this uses SectionLabels to reuse existing
      // addresses in the address pool to minimize object size/relocations.
      Asm->emitInt8(dwarf::DW_LLE_startx_length);
      unsigned idx = AddrPool.getIndex(Entry.Begin);
      Asm->emitULEB128(idx);
      // Also the pre-standard encoding is slightly different, emitting this as
      // an address-length entry here, but its a ULEB128 in DWARFv5 loclists.
      Asm->emitLabelDifference(Entry.End, Entry.Begin, 4);
      emitDebugLocEntryLocation(Entry, List.CU);
    }
    Asm->emitInt8(dwarf::DW_LLE_end_of_list);
  }
}

struct ArangeSpan {
  const MCSymbol *Start, *End;
};

// Emit a debug aranges section, containing a CU lookup for any
// address we can tie back to a CU.
void DwarfDebug::emitDebugARanges() {
  // Provides a unique id per text section.
  MapVector<MCSection *, SmallVector<SymbolCU, 8>> SectionMap;

  // Filter labels by section.
  for (const SymbolCU &SCU : ArangeLabels) {
    if (SCU.Sym->isInSection()) {
      // Make a note of this symbol and it's section.
      MCSection *Section = &SCU.Sym->getSection();
      if (!Section->getKind().isMetadata())
        SectionMap[Section].push_back(SCU);
    } else {
      // Some symbols (e.g. common/bss on mach-o) can have no section but still
      // appear in the output. This sucks as we rely on sections to build
      // arange spans. We can do it without, but it's icky.
      SectionMap[nullptr].push_back(SCU);
    }
  }

  DenseMap<DwarfCompileUnit *, std::vector<ArangeSpan>> Spans;

  for (auto &I : SectionMap) {
    MCSection *Section = I.first;
    SmallVector<SymbolCU, 8> &List = I.second;
    if (List.size() < 1)
      continue;

    // If we have no section (e.g. common), just write out
    // individual spans for each symbol.
    if (!Section) {
      for (const SymbolCU &Cur : List) {
        ArangeSpan Span;
        Span.Start = Cur.Sym;
        Span.End = nullptr;
        assert(Cur.CU);
        Spans[Cur.CU].push_back(Span);
      }
      continue;
    }

    // Sort the symbols by offset within the section.
    llvm::stable_sort(List, [&](const SymbolCU &A, const SymbolCU &B) {
      unsigned IA = A.Sym ? Asm->OutStreamer->GetSymbolOrder(A.Sym) : 0;
      unsigned IB = B.Sym ? Asm->OutStreamer->GetSymbolOrder(B.Sym) : 0;

      // Symbols with no order assigned should be placed at the end.
      // (e.g. section end labels)
      if (IA == 0)
        return false;
      if (IB == 0)
        return true;
      return IA < IB;
    });

    // Insert a final terminator.
    List.push_back(SymbolCU(nullptr, Asm->OutStreamer->endSection(Section)));

    // Build spans between each label.
    const MCSymbol *StartSym = List[0].Sym;
    for (size_t n = 1, e = List.size(); n < e; n++) {
      const SymbolCU &Prev = List[n - 1];
      const SymbolCU &Cur = List[n];

      // Try and build the longest span we can within the same CU.
      if (Cur.CU != Prev.CU) {
        ArangeSpan Span;
        Span.Start = StartSym;
        Span.End = Cur.Sym;
        assert(Prev.CU);
        Spans[Prev.CU].push_back(Span);
        StartSym = Cur.Sym;
      }
    }
  }

  // Start the dwarf aranges section.
  Asm->OutStreamer->SwitchSection(
      Asm->getObjFileLowering().getDwarfARangesSection());

  unsigned PtrSize = Asm->MAI->getCodePointerSize();

  // Build a list of CUs used.
  std::vector<DwarfCompileUnit *> CUs;
  for (const auto &it : Spans) {
    DwarfCompileUnit *CU = it.first;
    CUs.push_back(CU);
  }

  // Sort the CU list (again, to ensure consistent output order).
  llvm::sort(CUs, [](const DwarfCompileUnit *A, const DwarfCompileUnit *B) {
    return A->getUniqueID() < B->getUniqueID();
  });

  // Emit an arange table for each CU we used.
  for (DwarfCompileUnit *CU : CUs) {
    std::vector<ArangeSpan> &List = Spans[CU];

    // Describe the skeleton CU's offset and length, not the dwo file's.
    if (auto *Skel = CU->getSkeleton())
      CU = Skel;

    // Emit size of content not including length itself.
    unsigned ContentSize =
        sizeof(int16_t) + // DWARF ARange version number
        sizeof(int32_t) + // Offset of CU in the .debug_info section
        sizeof(int8_t) +  // Pointer Size (in bytes)
        sizeof(int8_t);   // Segment Size (in bytes)

    unsigned TupleSize = PtrSize * 2;

    // 7.20 in the Dwarf specs requires the table to be aligned to a tuple.
    unsigned Padding =
        offsetToAlignment(sizeof(int32_t) + ContentSize, Align(TupleSize));

    ContentSize += Padding;
    ContentSize += (List.size() + 1) * TupleSize;

    // For each compile unit, write the list of spans it covers.
    Asm->OutStreamer->AddComment("Length of ARange Set");
    Asm->emitInt32(ContentSize);
    Asm->OutStreamer->AddComment("DWARF Arange version number");
    Asm->emitInt16(dwarf::DW_ARANGES_VERSION);
    Asm->OutStreamer->AddComment("Offset Into Debug Info Section");
    emitSectionReference(*CU);
    Asm->OutStreamer->AddComment("Address Size (in bytes)");
    Asm->emitInt8(PtrSize);
    Asm->OutStreamer->AddComment("Segment Size (in bytes)");
    Asm->emitInt8(0);

    Asm->OutStreamer->emitFill(Padding, 0xff);

    for (const ArangeSpan &Span : List) {
      Asm->emitLabelReference(Span.Start, PtrSize);

      // Calculate the size as being from the span start to it's end.
      if (Span.End) {
        Asm->emitLabelDifference(Span.End, Span.Start, PtrSize);
      } else {
        // For symbols without an end marker (e.g. common), we
        // write a single arange entry containing just that one symbol.
        uint64_t Size = SymSize[Span.Start];
        if (Size == 0)
          Size = 1;

        Asm->OutStreamer->emitIntValue(Size, PtrSize);
      }
    }

    Asm->OutStreamer->AddComment("ARange terminator");
    Asm->OutStreamer->emitIntValue(0, PtrSize);
    Asm->OutStreamer->emitIntValue(0, PtrSize);
  }
}

/// Emit a single range list. We handle both DWARF v5 and earlier.
static void emitRangeList(DwarfDebug &DD, AsmPrinter *Asm,
                          const RangeSpanList &List) {
  emitRangeList(DD, Asm, List.Label, List.Ranges, *List.CU,
                dwarf::DW_RLE_base_addressx, dwarf::DW_RLE_offset_pair,
                dwarf::DW_RLE_startx_length, dwarf::DW_RLE_end_of_list,
                llvm::dwarf::RangeListEncodingString,
                List.CU->getCUNode()->getRangesBaseAddress() ||
                    DD.getDwarfVersion() >= 5,
                [](auto) {});
}

void DwarfDebug::emitDebugRangesImpl(const DwarfFile &Holder, MCSection *Section) {
  if (Holder.getRangeLists().empty())
    return;

  assert(useRangesSection());
  assert(!CUMap.empty());
  assert(llvm::any_of(CUMap, [](const decltype(CUMap)::value_type &Pair) {
    return !Pair.second->getCUNode()->isDebugDirectivesOnly();
  }));

  Asm->OutStreamer->SwitchSection(Section);

  MCSymbol *TableEnd = nullptr;
  if (getDwarfVersion() >= 5)
    TableEnd = emitRnglistsTableHeader(Asm, Holder);

  for (const RangeSpanList &List : Holder.getRangeLists())
    emitRangeList(*this, Asm, List);

  if (TableEnd)
    Asm->OutStreamer->emitLabel(TableEnd);
}

/// Emit address ranges into the .debug_ranges section or into the DWARF v5
/// .debug_rnglists section.
void DwarfDebug::emitDebugRanges() {
  const auto &Holder = useSplitDwarf() ? SkeletonHolder : InfoHolder;

  emitDebugRangesImpl(Holder,
                      getDwarfVersion() >= 5
                          ? Asm->getObjFileLowering().getDwarfRnglistsSection()
                          : Asm->getObjFileLowering().getDwarfRangesSection());
}

void DwarfDebug::emitDebugRangesDWO() {
  emitDebugRangesImpl(InfoHolder,
                      Asm->getObjFileLowering().getDwarfRnglistsDWOSection());
}

void DwarfDebug::handleMacroNodes(DIMacroNodeArray Nodes, DwarfCompileUnit &U) {
  for (auto *MN : Nodes) {
    if (auto *M = dyn_cast<DIMacro>(MN))
      emitMacro(*M);
    else if (auto *F = dyn_cast<DIMacroFile>(MN))
      emitMacroFile(*F, U);
    else
      llvm_unreachable("Unexpected DI type!");
  }
}

void DwarfDebug::emitMacro(DIMacro &M) {
  Asm->emitULEB128(M.getMacinfoType());
  Asm->emitULEB128(M.getLine());
  StringRef Name = M.getName();
  StringRef Value = M.getValue();
  Asm->OutStreamer->emitBytes(Name);
  if (!Value.empty()) {
    // There should be one space between macro name and macro value.
    Asm->emitInt8(' ');
    Asm->OutStreamer->emitBytes(Value);
  }
  Asm->emitInt8('\0');
}

void DwarfDebug::emitMacroFile(DIMacroFile &F, DwarfCompileUnit &U) {
  assert(F.getMacinfoType() == dwarf::DW_MACINFO_start_file);
  Asm->emitULEB128(dwarf::DW_MACINFO_start_file);
  Asm->emitULEB128(F.getLine());
  Asm->emitULEB128(U.getOrCreateSourceID(F.getFile()));
  handleMacroNodes(F.getElements(), U);
  Asm->emitULEB128(dwarf::DW_MACINFO_end_file);
}

void DwarfDebug::emitDebugMacinfoImpl(MCSection *Section) {
  for (const auto &P : CUMap) {
    auto &TheCU = *P.second;
    auto *SkCU = TheCU.getSkeleton();
    DwarfCompileUnit &U = SkCU ? *SkCU : TheCU;
    auto *CUNode = cast<DICompileUnit>(P.first);
    DIMacroNodeArray Macros = CUNode->getMacros();
    if (Macros.empty())
      continue;
    Asm->OutStreamer->SwitchSection(Section);
    Asm->OutStreamer->emitLabel(U.getMacroLabelBegin());
    handleMacroNodes(Macros, U);
    Asm->OutStreamer->AddComment("End Of Macro List Mark");
    Asm->emitInt8(0);
  }
}

/// Emit macros into a debug macinfo section.
void DwarfDebug::emitDebugMacinfo() {
  emitDebugMacinfoImpl(Asm->getObjFileLowering().getDwarfMacinfoSection());
}

void DwarfDebug::emitDebugMacinfoDWO() {
  emitDebugMacinfoImpl(Asm->getObjFileLowering().getDwarfMacinfoDWOSection());
}

// DWARF5 Experimental Separate Dwarf emitters.

void DwarfDebug::initSkeletonUnit(const DwarfUnit &U, DIE &Die,
                                  std::unique_ptr<DwarfCompileUnit> NewU) {

  if (!CompilationDir.empty())
    NewU->addString(Die, dwarf::DW_AT_comp_dir, CompilationDir);
  addGnuPubAttributes(*NewU, Die);

  SkeletonHolder.addUnit(std::move(NewU));
}

DwarfCompileUnit &DwarfDebug::constructSkeletonCU(const DwarfCompileUnit &CU) {

  auto OwnedUnit = std::make_unique<DwarfCompileUnit>(
      CU.getUniqueID(), CU.getCUNode(), Asm, this, &SkeletonHolder,
      UnitKind::Skeleton);
  DwarfCompileUnit &NewCU = *OwnedUnit;
  NewCU.setSection(Asm->getObjFileLowering().getDwarfInfoSection());

  NewCU.initStmtList();

  if (useSegmentedStringOffsetsTable())
    NewCU.addStringOffsetsStart();

  initSkeletonUnit(CU, NewCU.getUnitDie(), std::move(OwnedUnit));

  return NewCU;
}

// Emit the .debug_info.dwo section for separated dwarf. This contains the
// compile units that would normally be in debug_info.
void DwarfDebug::emitDebugInfoDWO() {
  assert(useSplitDwarf() && "No split dwarf debug info?");
  // Don't emit relocations into the dwo file.
  InfoHolder.emitUnits(/* UseOffsets */ true);
}

// Emit the .debug_abbrev.dwo section for separated dwarf. This contains the
// abbreviations for the .debug_info.dwo section.
void DwarfDebug::emitDebugAbbrevDWO() {
  assert(useSplitDwarf() && "No split dwarf?");
  InfoHolder.emitAbbrevs(Asm->getObjFileLowering().getDwarfAbbrevDWOSection());
}

void DwarfDebug::emitDebugLineDWO() {
  assert(useSplitDwarf() && "No split dwarf?");
  SplitTypeUnitFileTable.Emit(
      *Asm->OutStreamer, MCDwarfLineTableParams(),
      Asm->getObjFileLowering().getDwarfLineDWOSection());
}

void DwarfDebug::emitStringOffsetsTableHeaderDWO() {
  assert(useSplitDwarf() && "No split dwarf?");
  InfoHolder.getStringPool().emitStringOffsetsTableHeader(
      *Asm, Asm->getObjFileLowering().getDwarfStrOffDWOSection(),
      InfoHolder.getStringOffsetsStartSym());
}

// Emit the .debug_str.dwo section for separated dwarf. This contains the
// string section and is identical in format to traditional .debug_str
// sections.
void DwarfDebug::emitDebugStrDWO() {
  if (useSegmentedStringOffsetsTable())
    emitStringOffsetsTableHeaderDWO();
  assert(useSplitDwarf() && "No split dwarf?");
  MCSection *OffSec = Asm->getObjFileLowering().getDwarfStrOffDWOSection();
  InfoHolder.emitStrings(Asm->getObjFileLowering().getDwarfStrDWOSection(),
                         OffSec, /* UseRelativeOffsets = */ false);
}

// Emit address pool.
void DwarfDebug::emitDebugAddr() {
  AddrPool.emit(*Asm, Asm->getObjFileLowering().getDwarfAddrSection());
}

MCDwarfDwoLineTable *DwarfDebug::getDwoLineTable(const DwarfCompileUnit &CU) {
  if (!useSplitDwarf())
    return nullptr;
  const DICompileUnit *DIUnit = CU.getCUNode();
  SplitTypeUnitFileTable.maybeSetRootFile(
      DIUnit->getDirectory(), DIUnit->getFilename(),
      CU.getMD5AsBytes(DIUnit->getFile()), DIUnit->getSource());
  return &SplitTypeUnitFileTable;
}

uint64_t DwarfDebug::makeTypeSignature(StringRef Identifier) {
  MD5 Hash;
  Hash.update(Identifier);
  // ... take the least significant 8 bytes and return those. Our MD5
  // implementation always returns its results in little endian, so we actually
  // need the "high" word.
  MD5::MD5Result Result;
  Hash.final(Result);
  return Result.high();
}

void DwarfDebug::addDwarfTypeUnitType(DwarfCompileUnit &CU,
                                      StringRef Identifier, DIE &RefDie,
                                      const DICompositeType *CTy) {
  // Fast path if we're building some type units and one has already used the
  // address pool we know we're going to throw away all this work anyway, so
  // don't bother building dependent types.
  if (!TypeUnitsUnderConstruction.empty() && AddrPool.hasBeenUsed())
    return;

  auto Ins = TypeSignatures.insert(std::make_pair(CTy, 0));
  if (!Ins.second) {
    CU.addDIETypeSignature(RefDie, Ins.first->second);
    return;
  }

  bool TopLevelType = TypeUnitsUnderConstruction.empty();
  AddrPool.resetUsedFlag();

  auto OwnedUnit = std::make_unique<DwarfTypeUnit>(CU, Asm, this, &InfoHolder,
                                                    getDwoLineTable(CU));
  DwarfTypeUnit &NewTU = *OwnedUnit;
  DIE &UnitDie = NewTU.getUnitDie();
  TypeUnitsUnderConstruction.emplace_back(std::move(OwnedUnit), CTy);

  NewTU.addUInt(UnitDie, dwarf::DW_AT_language, dwarf::DW_FORM_data2,
                CU.getLanguage());

  uint64_t Signature = makeTypeSignature(Identifier);
  NewTU.setTypeSignature(Signature);
  Ins.first->second = Signature;

  if (useSplitDwarf()) {
    MCSection *Section =
        getDwarfVersion() <= 4
            ? Asm->getObjFileLowering().getDwarfTypesDWOSection()
            : Asm->getObjFileLowering().getDwarfInfoDWOSection();
    NewTU.setSection(Section);
  } else {
    MCSection *Section =
        getDwarfVersion() <= 4
            ? Asm->getObjFileLowering().getDwarfTypesSection(Signature)
            : Asm->getObjFileLowering().getDwarfInfoSection(Signature);
    NewTU.setSection(Section);
    // Non-split type units reuse the compile unit's line table.
    CU.applyStmtList(UnitDie);
  }

  // Add DW_AT_str_offsets_base to the type unit DIE, but not for split type
  // units.
  if (useSegmentedStringOffsetsTable() && !useSplitDwarf())
    NewTU.addStringOffsetsStart();

  NewTU.setType(NewTU.createTypeDIE(CTy));

  if (TopLevelType) {
    auto TypeUnitsToAdd = std::move(TypeUnitsUnderConstruction);
    TypeUnitsUnderConstruction.clear();

    // Types referencing entries in the address table cannot be placed in type
    // units.
    if (AddrPool.hasBeenUsed()) {

      // Remove all the types built while building this type.
      // This is pessimistic as some of these types might not be dependent on
      // the type that used an address.
      for (const auto &TU : TypeUnitsToAdd)
        TypeSignatures.erase(TU.second);

      // Construct this type in the CU directly.
      // This is inefficient because all the dependent types will be rebuilt
      // from scratch, including building them in type units, discovering that
      // they depend on addresses, throwing them out and rebuilding them.
      CU.constructTypeDIE(RefDie, cast<DICompositeType>(CTy));
      return;
    }

    // If the type wasn't dependent on fission addresses, finish adding the type
    // and all its dependent types.
    for (auto &TU : TypeUnitsToAdd) {
      InfoHolder.computeSizeAndOffsetsForUnit(TU.first.get());
      InfoHolder.emitUnit(TU.first.get(), useSplitDwarf());
    }
  }
  CU.addDIETypeSignature(RefDie, Signature);
}

DwarfDebug::NonTypeUnitContext::NonTypeUnitContext(DwarfDebug *DD)
    : DD(DD),
      TypeUnitsUnderConstruction(std::move(DD->TypeUnitsUnderConstruction)) {
  DD->TypeUnitsUnderConstruction.clear();
  assert(TypeUnitsUnderConstruction.empty() || !DD->AddrPool.hasBeenUsed());
}

DwarfDebug::NonTypeUnitContext::~NonTypeUnitContext() {
  DD->TypeUnitsUnderConstruction = std::move(TypeUnitsUnderConstruction);
  DD->AddrPool.resetUsedFlag();
}

DwarfDebug::NonTypeUnitContext DwarfDebug::enterNonTypeUnitContext() {
  return NonTypeUnitContext(this);
}

// Add the Name along with its companion DIE to the appropriate accelerator
// table (for AccelTableKind::Dwarf it's always AccelDebugNames, for
// AccelTableKind::Apple, we use the table we got as an argument). If
// accelerator tables are disabled, this function does nothing.
template <typename DataT>
void DwarfDebug::addAccelNameImpl(const DICompileUnit &CU,
                                  AccelTable<DataT> &AppleAccel, StringRef Name,
                                  const DIE &Die) {
  if (getAccelTableKind() == AccelTableKind::None)
    return;

  if (getAccelTableKind() != AccelTableKind::Apple &&
      CU.getNameTableKind() != DICompileUnit::DebugNameTableKind::Default)
    return;

  DwarfFile &Holder = useSplitDwarf() ? SkeletonHolder : InfoHolder;
  DwarfStringPoolEntryRef Ref = Holder.getStringPool().getEntry(*Asm, Name);

  switch (getAccelTableKind()) {
  case AccelTableKind::Apple:
    AppleAccel.addName(Ref, Die);
    break;
  case AccelTableKind::Dwarf:
    AccelDebugNames.addName(Ref, Die);
    break;
  case AccelTableKind::Default:
    llvm_unreachable("Default should have already been resolved.");
  case AccelTableKind::None:
    llvm_unreachable("None handled above");
  }
}

void DwarfDebug::addAccelName(const DICompileUnit &CU, StringRef Name,
                              const DIE &Die) {
  addAccelNameImpl(CU, AccelNames, Name, Die);
}

void DwarfDebug::addAccelObjC(const DICompileUnit &CU, StringRef Name,
                              const DIE &Die) {
  // ObjC names go only into the Apple accelerator tables.
  if (getAccelTableKind() == AccelTableKind::Apple)
    addAccelNameImpl(CU, AccelObjC, Name, Die);
}

void DwarfDebug::addAccelNamespace(const DICompileUnit &CU, StringRef Name,
                                   const DIE &Die) {
  addAccelNameImpl(CU, AccelNamespace, Name, Die);
}

void DwarfDebug::addAccelType(const DICompileUnit &CU, StringRef Name,
                              const DIE &Die, char Flags) {
  addAccelNameImpl(CU, AccelTypes, Name, Die);
}

uint16_t DwarfDebug::getDwarfVersion() const {
  return Asm->OutStreamer->getContext().getDwarfVersion();
}

const MCSymbol *DwarfDebug::getSectionLabel(const MCSection *S) {
  return SectionLabels.find(S)->second;
}
void DwarfDebug::insertSectionLabel(const MCSymbol *S) {
  SectionLabels.insert(std::make_pair(&S->getSection(), S));
}