DependencyTracker.cpp

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//=== DependencyTracker.cpp -----------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
 
#include "DependencyTracker.h"
#include "llvm/Support/FormatVariadic.h"
 
using namespace llvm;
using namespace dwarf_linker;
using namespace dwarf_linker::parallel;
 
/// A broken link in the keep chain. By recording both the parent and the child
/// we can show only broken links for DIEs with multiple children.
struct BrokenLink {
  BrokenLink(DWARFDie Parent, DWARFDie Child, const char *Message)
      : Parent(Parent), Child(Child), Message(Message) {}
  DWARFDie Parent;
  DWARFDie Child;
  std::string Message;
};
 
/// Verify the keep chain by looking for DIEs that are kept but who's parent
/// isn't.
void DependencyTracker::verifyKeepChain() {
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
  SmallVector<DWARFDie> Worklist;
  Worklist.push_back(CU.getOrigUnit().getUnitDIE());
 
  // List of broken links.
  SmallVector<BrokenLink> BrokenLinks;
 
  while (!Worklist.empty()) {
    const DWARFDie Current = Worklist.back();
    Worklist.pop_back();
 
    if (!Current.isValid())
      continue;
 
    CompileUnit::DIEInfo &CurrentInfo =
        CU.getDIEInfo(Current.getDebugInfoEntry());
    const bool ParentPlainDieIsKept = CurrentInfo.needToKeepInPlainDwarf();
    const bool ParentTypeDieIsKept = CurrentInfo.needToPlaceInTypeTable();
 
    for (DWARFDie Child : reverse(Current.children())) {
      Worklist.push_back(Child);
 
      CompileUnit::DIEInfo &ChildInfo =
          CU.getDIEInfo(Child.getDebugInfoEntry());
      const bool ChildPlainDieIsKept = ChildInfo.needToKeepInPlainDwarf();
      const bool ChildTypeDieIsKept = ChildInfo.needToPlaceInTypeTable();
 
      if (!ParentPlainDieIsKept && ChildPlainDieIsKept)
        BrokenLinks.emplace_back(Current, Child,
                                 "Found invalid link in keep chain");
 
      if (Child.getTag() == dwarf::DW_TAG_subprogram) {
        if (!ChildInfo.getKeep() && isLiveSubprogramEntry(UnitEntryPairTy(
                                        &CU, Child.getDebugInfoEntry()))) {
          BrokenLinks.emplace_back(Current, Child,
                                   "Live subprogram is not marked as kept");
        }
      }
 
      if (!ChildInfo.getODRAvailable()) {
        assert(!ChildTypeDieIsKept);
        continue;
      }
 
      if (!ParentTypeDieIsKept && ChildTypeDieIsKept)
        BrokenLinks.emplace_back(Current, Child,
                                 "Found invalid link in keep chain");
 
      if (CurrentInfo.getIsInAnonNamespaceScope() &&
          ChildInfo.needToPlaceInTypeTable()) {
        BrokenLinks.emplace_back(Current, Child,
                                 "Found invalid placement marking for member "
                                 "of anonymous namespace");
      }
    }
  }
 
  if (!BrokenLinks.empty()) {
    for (BrokenLink Link : BrokenLinks) {
      errs() << "\n=================================\n";
      WithColor::error() << formatv("{0} between {1:x} and {2:x}", Link.Message,
                                    Link.Parent.getOffset(),
                                    Link.Child.getOffset());
 
      errs() << "\nParent:";
      Link.Parent.dump(errs(), 0, {});
      errs() << "\n";
      CU.getDIEInfo(Link.Parent).dump();
 
      errs() << "\nChild:";
      Link.Child.dump(errs(), 2, {});
      errs() << "\n";
      CU.getDIEInfo(Link.Child).dump();
    }
    report_fatal_error("invalid keep chain");
  }
#endif
}
 
static bool isNamespaceLikeEntry(const DWARFDebugInfoEntry *Entry) {
  switch (Entry->getTag()) {
  case dwarf::DW_TAG_compile_unit:
  case dwarf::DW_TAG_module:
  case dwarf::DW_TAG_namespace:
    return true;
 
  default:
    return false;
  }
}
 
bool DependencyTracker::resolveDependenciesAndMarkLiveness(
    bool InterCUProcessingStarted, std::atomic<bool> &HasNewInterconnectedCUs) {
  RootEntriesWorkList.clear();
 
  // Search for live root DIEs.
  CompileUnit::DIEInfo &CUInfo = CU.getDIEInfo(CU.getDebugInfoEntry(0));
  CUInfo.setPlacement(CompileUnit::PlainDwarf);
  collectRootsToKeep(UnitEntryPairTy{&CU, CU.getDebugInfoEntry(0)},
                     std::nullopt, false);
 
  // Mark live DIEs as kept.
  return markCollectedLiveRootsAsKept(InterCUProcessingStarted,
                                      HasNewInterconnectedCUs);
}
 
void DependencyTracker::addActionToRootEntriesWorkList(
    LiveRootWorklistActionTy Action, const UnitEntryPairTy &Entry,
    std::optional<UnitEntryPairTy> ReferencedBy,
    const DWARFDebugInfoEntry *ReferencedTypeDieEntry) {
  if (ReferencedBy) {
    RootEntriesWorkList.emplace_back(Action, Entry, *ReferencedBy,
                                     ReferencedTypeDieEntry);
    return;
  }
 
  RootEntriesWorkList.emplace_back(Action, Entry);
}
 
void DependencyTracker::collectRootsToKeep(
    const UnitEntryPairTy &Entry, std::optional<UnitEntryPairTy> ReferencedBy,
    bool IsLiveParent) {
  for (const DWARFDebugInfoEntry *CurChild =
           Entry.CU->getFirstChildEntry(Entry.DieEntry);
       CurChild && CurChild->getAbbreviationDeclarationPtr();
       CurChild = Entry.CU->getSiblingEntry(CurChild)) {
    UnitEntryPairTy ChildEntry(Entry.CU, CurChild);
    CompileUnit::DIEInfo &ChildInfo = Entry.CU->getDIEInfo(CurChild);
 
    bool IsLiveChild = false;
 
    switch (CurChild->getTag()) {
    case dwarf::DW_TAG_label: {
      IsLiveChild = isLiveSubprogramEntry(ChildEntry);
 
      // Keep label referencing live address.
      // Keep label which is child of live parent entry.
      if (IsLiveChild || (IsLiveParent && ChildInfo.getHasAnAddress())) {
        addActionToRootEntriesWorkList(
            LiveRootWorklistActionTy::MarkLiveEntryRec, ChildEntry,
            ReferencedBy);
      }
    } break;
    case dwarf::DW_TAG_subprogram: {
      IsLiveChild = isLiveSubprogramEntry(ChildEntry);
 
      // Keep subprogram referencing live address.
      if (IsLiveChild) {
        // If subprogram is in module scope and this module allows ODR
        // deduplication set "TypeTable" placement, otherwise set "" placement
        LiveRootWorklistActionTy Action =
            (ChildInfo.getIsInMouduleScope() && ChildInfo.getODRAvailable())
                ? LiveRootWorklistActionTy::MarkTypeEntryRec
                : LiveRootWorklistActionTy::MarkLiveEntryRec;
 
        addActionToRootEntriesWorkList(Action, ChildEntry, ReferencedBy);
      }
    } break;
    case dwarf::DW_TAG_constant:
    case dwarf::DW_TAG_variable: {
      IsLiveChild = isLiveVariableEntry(ChildEntry, IsLiveParent);
 
      // Keep variable referencing live address.
      if (IsLiveChild) {
        // If variable is in module scope and this module allows ODR
        // deduplication set "TypeTable" placement, otherwise set "" placement
 
        LiveRootWorklistActionTy Action =
            (ChildInfo.getIsInMouduleScope() && ChildInfo.getODRAvailable())
                ? LiveRootWorklistActionTy::MarkTypeEntryRec
                : LiveRootWorklistActionTy::MarkLiveEntryRec;
 
        addActionToRootEntriesWorkList(Action, ChildEntry, ReferencedBy);
      }
    } break;
    case dwarf::DW_TAG_base_type: {
      // Always keep base types.
      addActionToRootEntriesWorkList(
          LiveRootWorklistActionTy::MarkSingleLiveEntry, ChildEntry,
          ReferencedBy);
    } break;
    case dwarf::DW_TAG_imported_module:
    case dwarf::DW_TAG_imported_declaration:
    case dwarf::DW_TAG_imported_unit: {
      // Always keep DIEs having DW_AT_import attribute.
      if (Entry.DieEntry->getTag() == dwarf::DW_TAG_compile_unit) {
        addActionToRootEntriesWorkList(
            LiveRootWorklistActionTy::MarkSingleLiveEntry, ChildEntry,
            ReferencedBy);
        break;
      }
 
      addActionToRootEntriesWorkList(
          LiveRootWorklistActionTy::MarkSingleTypeEntry, ChildEntry,
          ReferencedBy);
    } break;
    case dwarf::DW_TAG_type_unit:
    case dwarf::DW_TAG_partial_unit:
    case dwarf::DW_TAG_compile_unit: {
      llvm_unreachable("Called for incorrect DIE");
    } break;
    default:
      // A module compile unit has no relocations, so liveness analysis never
      // reaches a type definition that nothing else in the unit references. The
      // module owns the only copy of those definitions, so keep them.
      if (Entry.CU->isClangModule() && isNamespaceLikeEntry(Entry.DieEntry) &&
          dwarf::isType(CurChild->getTag())) {
        addActionToRootEntriesWorkList(
            LiveRootWorklistActionTy::MarkTypeEntryRec, ChildEntry,
            ReferencedBy);
        break;
      }
 
      // An importing unit emits a skeleton of the module it imports, so a
      // forward-declared type nested in a DW_TAG_module there is the module's
      // record that the name exists, even when no full definition has been
      // emitted. Route it through the type pool: when another CU emits a
      // real definition for the same synthetic name, the existing
      // decl-vs-def race resolution in allocateTypeDie + getFinalDie keeps
      // the definition and drops this declaration at emission time. For
      // non-ODR languages getFinalPlacementForEntry forces PlainDwarf,
      // so the forward decl is kept in place under its module.
      if (!Entry.CU->isClangModule() &&
          Entry.DieEntry->getTag() == dwarf::DW_TAG_module &&
          dwarf::isType(CurChild->getTag()) &&
          dwarf::toUnsigned(Entry.CU->find(CurChild, dwarf::DW_AT_declaration),
                            0)) {
        addActionToRootEntriesWorkList(
            LiveRootWorklistActionTy::MarkTypeEntryRec, ChildEntry,
            ReferencedBy);
      }
      break;
    }
 
    collectRootsToKeep(ChildEntry, ReferencedBy, IsLiveChild || IsLiveParent);
  }
}
 
bool DependencyTracker::markCollectedLiveRootsAsKept(
    bool InterCUProcessingStarted, std::atomic<bool> &HasNewInterconnectedCUs) {
  bool Res = true;
 
  // Mark roots as kept.
  while (!RootEntriesWorkList.empty()) {
    LiveRootWorklistItemTy Root = RootEntriesWorkList.pop_back_val();
 
    if (markDIEEntryAsKeptRec(Root.getAction(), Root.getRootEntry(),
                              Root.getRootEntry(), InterCUProcessingStarted,
                              HasNewInterconnectedCUs)) {
      if (Root.hasReferencedByOtherEntry())
        Dependencies.push_back(Root);
    } else
      Res = false;
  }
 
  return Res;
}
 
bool DependencyTracker::updateDependenciesCompleteness() {
  bool HasNewDependency = false;
  for (LiveRootWorklistItemTy &Root : Dependencies) {
    assert(Root.hasReferencedByOtherEntry() &&
           "Root entry without dependency inside the dependencies list");
 
    UnitEntryPairTy RootEntry = Root.getRootEntry();
 
    // Completeness must be checked against the actual referenced DIE, not its
    // enclosing root. A nested type can be demoted to plain DWARF while its
    // root stays in the type table, and a type-table DIE may only reference
    // DIEs that are themselves in the type table. Checking the root instead
    // leaves such a DIE in the type table, later tripping the type-unit
    // reference assertion in DIEAttributeCloner::cloneDieRefAttr.
    const DWARFDebugInfoEntry *ReferencedDieEntry =
        Root.getReferencedTypeDieEntry() ? Root.getReferencedTypeDieEntry()
                                         : RootEntry.DieEntry;
    CompileUnit::DIEInfo &RootInfo =
        RootEntry.CU->getDIEInfo(ReferencedDieEntry);
 
    UnitEntryPairTy ReferencedByEntry = Root.getReferencedByEntry();
    CompileUnit::DIEInfo &ReferencedByInfo =
        ReferencedByEntry.CU->getDIEInfo(ReferencedByEntry.DieEntry);
 
    if (!RootInfo.needToPlaceInTypeTable() &&
        ReferencedByInfo.needToPlaceInTypeTable()) {
      HasNewDependency = true;
      setPlainDwarfPlacementRec(ReferencedByEntry);
 
      // FIXME: we probably need to update getKeepTypeChildren status for
      // parents of *Root.ReferencedBy.
    }
  }
 
  return HasNewDependency;
}
 
void DependencyTracker::setPlainDwarfPlacementRec(
    const UnitEntryPairTy &Entry) {
  CompileUnit::DIEInfo &Info = Entry.CU->getDIEInfo(Entry.DieEntry);
  if (Info.getPlacement() == CompileUnit::PlainDwarf &&
      !Info.getKeepTypeChildren())
    return;
 
  Info.setPlacement(CompileUnit::PlainDwarf);
  Info.unsetKeepTypeChildren();
  markParentsAsKeepingChildren(Entry);
 
  for (const DWARFDebugInfoEntry *CurChild =
           Entry.CU->getFirstChildEntry(Entry.DieEntry);
       CurChild && CurChild->getAbbreviationDeclarationPtr();
       CurChild = Entry.CU->getSiblingEntry(CurChild))
    setPlainDwarfPlacementRec(UnitEntryPairTy{Entry.CU, CurChild});
}
 
bool isAlreadyMarked(const CompileUnit::DIEInfo &Info,
                     CompileUnit::DieOutputPlacement NewPlacement) {
  if (!Info.getKeep())
    return false;
 
  switch (NewPlacement) {
  case CompileUnit::TypeTable:
    return Info.needToPlaceInTypeTable();
 
  case CompileUnit::PlainDwarf:
    return Info.needToKeepInPlainDwarf();
 
  case CompileUnit::Both:
    return Info.needToPlaceInTypeTable() && Info.needToKeepInPlainDwarf();
 
  case CompileUnit::NotSet:
    llvm_unreachable("Unset placement type is specified.");
  };
 
  llvm_unreachable("Unknown CompileUnit::DieOutputPlacement enum");
}
 
bool isAlreadyMarked(const UnitEntryPairTy &Entry,
                     CompileUnit::DieOutputPlacement NewPlacement) {
  return isAlreadyMarked(Entry.CU->getDIEInfo(Entry.DieEntry), NewPlacement);
}
 
void DependencyTracker::markParentsAsKeepingChildren(
    const UnitEntryPairTy &Entry) {
  if (Entry.DieEntry->getAbbreviationDeclarationPtr() == nullptr)
    return;
 
  CompileUnit::DIEInfo &Info = Entry.CU->getDIEInfo(Entry.DieEntry);
  bool NeedKeepTypeChildren = Info.needToPlaceInTypeTable();
  bool NeedKeepPlainChildren = Info.needToKeepInPlainDwarf();
 
  bool AreTypeParentsDone = !NeedKeepTypeChildren;
  bool ArePlainParentsDone = !NeedKeepPlainChildren;
 
  // Mark parents as 'Keep*Children'.
  std::optional<uint32_t> ParentIdx = Entry.DieEntry->getParentIdx();
  while (ParentIdx) {
    const DWARFDebugInfoEntry *ParentEntry =
        Entry.CU->getDebugInfoEntry(*ParentIdx);
    CompileUnit::DIEInfo &ParentInfo = Entry.CU->getDIEInfo(*ParentIdx);
 
    if (!AreTypeParentsDone && NeedKeepTypeChildren) {
      if (ParentInfo.getKeepTypeChildren())
        AreTypeParentsDone = true;
      else {
        bool AddToWorklist = !isAlreadyMarked(
            ParentInfo, CompileUnit::DieOutputPlacement::TypeTable);
        ParentInfo.setKeepTypeChildren();
        if (AddToWorklist && !isNamespaceLikeEntry(ParentEntry)) {
          addActionToRootEntriesWorkList(
              LiveRootWorklistActionTy::MarkTypeChildrenRec,
              UnitEntryPairTy{Entry.CU, ParentEntry}, std::nullopt);
        }
      }
    }
 
    if (!ArePlainParentsDone && NeedKeepPlainChildren) {
      if (ParentInfo.getKeepPlainChildren())
        ArePlainParentsDone = true;
      else {
        bool AddToWorklist = !isAlreadyMarked(
            ParentInfo, CompileUnit::DieOutputPlacement::PlainDwarf);
        ParentInfo.setKeepPlainChildren();
        if (AddToWorklist && !isNamespaceLikeEntry(ParentEntry)) {
          addActionToRootEntriesWorkList(
              LiveRootWorklistActionTy::MarkLiveChildrenRec,
              UnitEntryPairTy{Entry.CU, ParentEntry}, std::nullopt);
        }
      }
    }
 
    if (AreTypeParentsDone && ArePlainParentsDone)
      break;
 
    ParentIdx = ParentEntry->getParentIdx();
  }
}
 
namespace {
struct FinalPlacement {
  CompileUnit::DieOutputPlacement Placement;
 
  /// How Placement combines with the DIE's current placement when applied.
  enum ApplyMode {
    /// Overwrite the current placement. Used for entries whose placement is
    /// fully determined regardless of how they were reached, so every mark
    /// agrees on the value (ODR-unavailable entries and static data member
    /// declarations).
    Overwrite,
    /// OR-join into the current placement (the common monotone-lattice case):
    /// a DIE reached by both a live and a type mark ends up in Both.
    Join,
    /// Join for a DW_TAG_variable, which cannot occupy the type table and plain
    /// DWARF at once: PlainDwarf is absorbing so the variable never lands in
    /// Both.
    JoinVariable,
  } Mode;
};
} // namespace
 
// Computes the placement to apply to \p Entry for a mark requesting \p
// Placement (PlainDwarf for a live action, TypeTable for a type action), along
// with how it combines with the DIE's current placement. Most entries join, so
// a DIE reached by both actions ends up in Both. Entries whose placement is
// fully determined regardless of how they were reached instead overwrite with
// an exact placement: ODR-unavailable entries cannot be deduplicated into the
// type table, and a DW_TAG_variable cannot occupy the type table and plain
// DWARF at once.
static FinalPlacement
getFinalPlacementForEntry(const UnitEntryPairTy &Entry,
                          CompileUnit::DieOutputPlacement Placement) {
  assert((Placement != CompileUnit::NotSet) && "Placement is not set");
  CompileUnit::DIEInfo &EntryInfo = Entry.CU->getDIEInfo(Entry.DieEntry);
 
  if (!EntryInfo.getODRAvailable())
    return {CompileUnit::PlainDwarf, FinalPlacement::Overwrite};
 
  if (Entry.DieEntry->getTag() == dwarf::DW_TAG_variable) {
    // In-class static member declarations (e.g. "static constexpr int x = 1;")
    // are DW_TAG_variable children of a DW_TAG_class_type /
    // DW_TAG_structure_type / DW_TAG_union_type with DW_AT_declaration set.
    // They are part of the class type and belong in the TypeTable together with
    // the class. Forcing them into PlainDwarf would also drag the parent class
    // into PlainDwarf (via markParentsAsKeepingChildren), producing a duplicate
    // empty class declaration DIE alongside the full class definition emitted
    // in another CU.
    bool IsDeclaration = dwarf::toUnsigned(
        Entry.CU->find(Entry.DieEntry, dwarf::DW_AT_declaration), 0);
    bool ParentIsType = false;
    if (IsDeclaration) {
      if (std::optional<uint32_t> ParentIdx = Entry.DieEntry->getParentIdx()) {
        dwarf::Tag ParentTag =
            Entry.CU->getDebugInfoEntry(*ParentIdx)->getTag();
        ParentIsType = ParentTag == dwarf::DW_TAG_class_type ||
                       ParentTag == dwarf::DW_TAG_structure_type ||
                       ParentTag == dwarf::DW_TAG_union_type;
      }
    }
    if (IsDeclaration && ParentIsType) {
      // Pure declarations have no runtime address; they belong with the class
      // type. Always place in TypeTable regardless of how they were reached.
      return {CompileUnit::TypeTable, FinalPlacement::Overwrite};
    }
 
    // A live (PlainDwarf) mark pins the variable to plain DWARF.
    if (Placement == CompileUnit::PlainDwarf || Placement == CompileUnit::Both)
      return {CompileUnit::PlainDwarf, FinalPlacement::Overwrite};
 
    // Only a type-table mark reaches here. The variable join keeps a PlainDwarf
    // mark racing this one from turning the variable into Both.
    return {Placement, FinalPlacement::JoinVariable};
  }
 
  return {Placement, FinalPlacement::Join};
}
 
bool DependencyTracker::markDIEEntryAsKeptRec(
    LiveRootWorklistActionTy Action, const UnitEntryPairTy &RootEntry,
    const UnitEntryPairTy &Entry, bool InterCUProcessingStarted,
    std::atomic<bool> &HasNewInterconnectedCUs, bool RecordDepsOnly) {
  if (Entry.DieEntry->getAbbreviationDeclarationPtr() == nullptr)
    return true;
 
  CompileUnit::DIEInfo &Info = Entry.CU->getDIEInfo(Entry.DieEntry);
 
  // Calculate final placement.
  FinalPlacement Final = getFinalPlacementForEntry(
      Entry,
      isLiveAction(Action) ? CompileUnit::PlainDwarf : CompileUnit::TypeTable);
  CompileUnit::DieOutputPlacement Placement = Final.Placement;
  assert((Info.getODRAvailable() || isLiveAction(Action) ||
          Placement == CompileUnit::PlainDwarf) &&
         "Wrong kind of placement for ODR unavailable entry");
 
  if (!RecordDepsOnly && !isChildrenAction(Action) &&
      isAlreadyMarked(Entry, Placement)) {
    // Entry (and its subtree) were already marked, possibly by a racing CU or
    // another referencing root, and which one wins is non-deterministic. Skip
    // the redundant marking, but re-walk the subtree in record-deps-only mode
    // so this referencing root still contributes its outgoing completeness
    // dependencies. Otherwise the recorded dependency set depends on thread
    // interleaving, the demotion fixpoint misses demotions, and whole type
    // subtrees are left in the artificial type unit non-deterministically.
    // Recording extra dependencies is harmless: a dependency only triggers a
    // demotion when the referenced type is actually placed in plain DWARF.
    return markDIEEntryAsKeptRec(Action, RootEntry, Entry,
                                 InterCUProcessingStarted,
                                 HasNewInterconnectedCUs,
                                 /*RecordDepsOnly=*/true);
  }
 
  if (!RecordDepsOnly) {
    // Mark current DIE as kept.
    Info.setKeep();
    // Marks compose monotonically so no interleaving loses an update: a general
    // mark only raises the placement in the lattice, and a forced placement is
    // a value every mark agrees on.
    switch (Final.Mode) {
    case FinalPlacement::Overwrite:
      Info.setPlacement(Placement);
      break;
    case FinalPlacement::Join:
      Info.joinPlacement(Placement);
      break;
    case FinalPlacement::JoinVariable:
      Info.joinVariablePlacement(Placement);
      break;
    }
 
    // Set keep children property for parents.
    markParentsAsKeepingChildren(Entry);
  }
 
  UnitEntryPairTy FinalRootEntry =
      Entry.DieEntry->getTag() == dwarf::DW_TAG_subprogram ? Entry : RootEntry;
 
  // Analyse referenced DIEs.
  bool Res = true;
  if (!maybeAddReferencedRoots(Action, FinalRootEntry, Entry,
                               InterCUProcessingStarted,
                               HasNewInterconnectedCUs, RecordDepsOnly))
    Res = false;
 
  // Return if we do not need to process children.
  if (isSingleAction(Action))
    return Res;
 
  // Process children.
  // Check for subprograms special case.
  if (Entry.DieEntry->getTag() == dwarf::DW_TAG_subprogram &&
      Info.getODRAvailable()) {
    // Subprograms is a special case. As it can be root for type DIEs
    // and itself may be subject to move into the artificial type unit.
    //  a) Non removable children(like DW_TAG_formal_parameter) should always
    //     be cloned. They are placed into the "PlainDwarf" and into the
    //     "TypeTable".
    //  b) ODR deduplication candidates(type DIEs) children should not be put
    //  into the "PlainDwarf".
    //  c) Children keeping addresses and locations(like DW_TAG_call_site)
    //  should not be put into the "TypeTable".
    for (const DWARFDebugInfoEntry *CurChild =
             Entry.CU->getFirstChildEntry(Entry.DieEntry);
         CurChild && CurChild->getAbbreviationDeclarationPtr();
         CurChild = Entry.CU->getSiblingEntry(CurChild)) {
      CompileUnit::DIEInfo ChildInfo = Entry.CU->getDIEInfo(CurChild);
 
      switch (CurChild->getTag()) {
      case dwarf::DW_TAG_variable:
      case dwarf::DW_TAG_constant:
      case dwarf::DW_TAG_subprogram:
      case dwarf::DW_TAG_label: {
        if (ChildInfo.getHasAnAddress())
          continue;
      } break;
 
      // Entries having following tags could not be removed from the subprogram.
      case dwarf::DW_TAG_lexical_block:
      case dwarf::DW_TAG_friend:
      case dwarf::DW_TAG_inheritance:
      case dwarf::DW_TAG_formal_parameter:
      case dwarf::DW_TAG_unspecified_parameters:
      case dwarf::DW_TAG_template_type_parameter:
      case dwarf::DW_TAG_template_value_parameter:
      case dwarf::DW_TAG_GNU_template_parameter_pack:
      case dwarf::DW_TAG_GNU_formal_parameter_pack:
      case dwarf::DW_TAG_GNU_template_template_param:
      case dwarf::DW_TAG_thrown_type: {
        // Go to the default child handling.
      } break;
 
      default: {
        bool ChildIsTypeTableCandidate = isTypeTableCandidate(CurChild);
 
        // Skip child marked to be copied into the artificial type unit.
        if (isLiveAction(Action) && ChildIsTypeTableCandidate)
          continue;
 
        // Skip child marked to be copied into the plain unit.
        if (isTypeAction(Action) && !ChildIsTypeTableCandidate)
          continue;
 
        // Go to the default child handling.
      } break;
      }
 
      if (!markDIEEntryAsKeptRec(Action, FinalRootEntry,
                                 UnitEntryPairTy{Entry.CU, CurChild},
                                 InterCUProcessingStarted,
                                 HasNewInterconnectedCUs, RecordDepsOnly))
        Res = false;
    }
 
    return Res;
  }
 
  // Recursively process children.
  for (const DWARFDebugInfoEntry *CurChild =
           Entry.CU->getFirstChildEntry(Entry.DieEntry);
       CurChild && CurChild->getAbbreviationDeclarationPtr();
       CurChild = Entry.CU->getSiblingEntry(CurChild)) {
    CompileUnit::DIEInfo ChildInfo = Entry.CU->getDIEInfo(CurChild);
    switch (CurChild->getTag()) {
    case dwarf::DW_TAG_variable:
    case dwarf::DW_TAG_constant:
    case dwarf::DW_TAG_subprogram:
    case dwarf::DW_TAG_label: {
      if (ChildInfo.getHasAnAddress())
        continue;
    } break;
    default:
      break; // Nothing to do.
    };
 
    if (!markDIEEntryAsKeptRec(
            Action, FinalRootEntry, UnitEntryPairTy{Entry.CU, CurChild},
            InterCUProcessingStarted, HasNewInterconnectedCUs, RecordDepsOnly))
      Res = false;
  }
 
  return Res;
}
 
bool DependencyTracker::isTypeTableCandidate(
    const DWARFDebugInfoEntry *DIEEntry) {
  switch (DIEEntry->getTag()) {
  default:
    return false;
 
  case dwarf::DW_TAG_imported_module:
  case dwarf::DW_TAG_imported_declaration:
  case dwarf::DW_TAG_imported_unit:
  case dwarf::DW_TAG_array_type:
  case dwarf::DW_TAG_class_type:
  case dwarf::DW_TAG_enumeration_type:
  case dwarf::DW_TAG_pointer_type:
  case dwarf::DW_TAG_reference_type:
  case dwarf::DW_TAG_string_type:
  case dwarf::DW_TAG_structure_type:
  case dwarf::DW_TAG_subroutine_type:
  case dwarf::DW_TAG_typedef:
  case dwarf::DW_TAG_union_type:
  case dwarf::DW_TAG_variant:
  case dwarf::DW_TAG_module:
  case dwarf::DW_TAG_ptr_to_member_type:
  case dwarf::DW_TAG_set_type:
  case dwarf::DW_TAG_subrange_type:
  case dwarf::DW_TAG_base_type:
  case dwarf::DW_TAG_const_type:
  case dwarf::DW_TAG_enumerator:
  case dwarf::DW_TAG_file_type:
  case dwarf::DW_TAG_packed_type:
  case dwarf::DW_TAG_thrown_type:
  case dwarf::DW_TAG_volatile_type:
  case dwarf::DW_TAG_dwarf_procedure:
  case dwarf::DW_TAG_restrict_type:
  case dwarf::DW_TAG_interface_type:
  case dwarf::DW_TAG_namespace:
  case dwarf::DW_TAG_unspecified_type:
  case dwarf::DW_TAG_shared_type:
  case dwarf::DW_TAG_rvalue_reference_type:
  case dwarf::DW_TAG_coarray_type:
  case dwarf::DW_TAG_dynamic_type:
  case dwarf::DW_TAG_atomic_type:
  case dwarf::DW_TAG_immutable_type:
  case dwarf::DW_TAG_function_template:
  case dwarf::DW_TAG_class_template:
    return true;
  }
}
 
bool DependencyTracker::maybeAddReferencedRoots(
    LiveRootWorklistActionTy Action, const UnitEntryPairTy &RootEntry,
    const UnitEntryPairTy &Entry, bool InterCUProcessingStarted,
    std::atomic<bool> &HasNewInterconnectedCUs, bool RecordDepsOnly) {
  const auto *Abbrev = Entry.DieEntry->getAbbreviationDeclarationPtr();
  if (Abbrev == nullptr)
    return true;
 
  // In record-deps-only mode the referenced root is not scheduled for marking.
  // The completeness dependency is appended directly so it participates in the
  // demotion fixpoint without triggering any reference-following recursion.
  auto AddRoot = [&](LiveRootWorklistActionTy RootAction,
                     const UnitEntryPairTy &Root,
                     const DWARFDebugInfoEntry *ReferencedTypeDieEntry) {
    if (RecordDepsOnly) {
      Dependencies.emplace_back(RootAction, Root, RootEntry,
                                ReferencedTypeDieEntry);
      return;
    }
    addActionToRootEntriesWorkList(RootAction, Root, RootEntry,
                                   ReferencedTypeDieEntry);
  };
 
  DWARFUnit &Unit = Entry.CU->getOrigUnit();
  DWARFDataExtractor Data = Unit.getDebugInfoExtractor();
  uint64_t Offset =
      Entry.DieEntry->getOffset() + getULEB128Size(Abbrev->getCode());
 
  // For each DIE attribute...
  for (const auto &AttrSpec : Abbrev->attributes()) {
    DWARFFormValue Val(AttrSpec.Form);
    if (!Val.isFormClass(DWARFFormValue::FC_Reference) ||
        AttrSpec.Attr == dwarf::DW_AT_sibling) {
      DWARFFormValue::skipValue(AttrSpec.Form, Data, &Offset,
                                Unit.getFormParams());
      continue;
    }
    Val.extractValue(Data, &Offset, Unit.getFormParams(), &Unit);
 
    // Resolve reference.
    std::optional<UnitEntryPairTy> RefDie = Entry.CU->resolveDIEReference(
        Val, InterCUProcessingStarted
                 ? ResolveInterCUReferencesMode::Resolve
                 : ResolveInterCUReferencesMode::AvoidResolving);
    if (!RefDie) {
      Entry.CU->warn("could not find referenced DIE", Entry.DieEntry);
      continue;
    }
 
    if (!RefDie->DieEntry) {
      // The reference could not be resolved yet. Recording dependencies
      // happens only after marking has fully resolved interconnections, so skip
      // it here. The scheduling path below handles the delayed-resolution case.
      if (RecordDepsOnly)
        continue;
 
      // Delay resolving reference.
      RefDie->CU->setInterconnectedCU();
      Entry.CU->setInterconnectedCU();
      HasNewInterconnectedCUs = true;
      return false;
    }
 
    assert((Entry.CU->getUniqueID() == RefDie->CU->getUniqueID() ||
            InterCUProcessingStarted) &&
           "Inter-CU reference while inter-CU processing is not started");
 
    CompileUnit::DIEInfo &RefInfo = RefDie->CU->getDIEInfo(RefDie->DieEntry);
    if (!RefInfo.getODRAvailable())
      Action = LiveRootWorklistActionTy::MarkLiveEntryRec;
    else if (RefInfo.getODRAvailable() &&
             llvm::is_contained(getODRAttributes(), AttrSpec.Attr))
      // Note: getODRAttributes does not include DW_AT_containing_type.
      // It should be OK as we do getRootForSpecifiedEntry(). So any containing
      // type would be found as the root for the entry.
      Action = LiveRootWorklistActionTy::MarkTypeEntryRec;
    else if (isLiveAction(Action))
      Action = LiveRootWorklistActionTy::MarkLiveEntryRec;
    else
      Action = LiveRootWorklistActionTy::MarkTypeEntryRec;
 
    if (AttrSpec.Attr == dwarf::DW_AT_import) {
      if (isNamespaceLikeEntry(RefDie->DieEntry)) {
        AddRoot(isTypeAction(Action)
                    ? LiveRootWorklistActionTy::MarkSingleTypeEntry
                    : LiveRootWorklistActionTy::MarkSingleLiveEntry,
                *RefDie, nullptr);
        continue;
      }
 
      AddRoot(Action, *RefDie, nullptr);
      continue;
    }
 
    // Mark the enclosing root type as kept, but also record the actual
    // referenced DIE: a nested type can be demoted to plain DWARF independently
    // of its root, in which case ReferencedBy must be demoted too (see
    // updateDependenciesCompleteness).
    UnitEntryPairTy RootForReferencedDie = getRootForSpecifiedEntry(*RefDie);
    AddRoot(Action, RootForReferencedDie, RefDie->DieEntry);
  }
 
  return true;
}
 
UnitEntryPairTy
DependencyTracker::getRootForSpecifiedEntry(UnitEntryPairTy Entry) {
  UnitEntryPairTy Result = Entry;
 
  do {
    switch (Entry.DieEntry->getTag()) {
    case dwarf::DW_TAG_subprogram:
    case dwarf::DW_TAG_label:
    case dwarf::DW_TAG_variable:
    case dwarf::DW_TAG_constant: {
      return Result;
    } break;
 
    default: {
      // Nothing to do.
    }
    }
 
    std::optional<uint32_t> ParentIdx = Result.DieEntry->getParentIdx();
    if (!ParentIdx)
      return Result;
 
    const DWARFDebugInfoEntry *ParentEntry =
        Result.CU->getDebugInfoEntry(*ParentIdx);
    if (isNamespaceLikeEntry(ParentEntry))
      break;
    Result.DieEntry = ParentEntry;
  } while (true);
 
  return Result;
}
 
static void dumpKeptDIE(const DWARFDie &DIE, StringRef Kind, bool Verbose) {
  if (!Verbose)
    return;
  outs() << "Keeping " << Kind << " DIE:";
  DIDumpOptions DumpOpts;
  DumpOpts.ChildRecurseDepth = 0;
  DumpOpts.Verbose = Verbose;
  DIE.dump(outs(), /*Indent=*/8, DumpOpts);
}
 
bool DependencyTracker::isLiveVariableEntry(const UnitEntryPairTy &Entry,
                                            bool IsLiveParent) {
  DWARFDie DIE = Entry.CU->getDIE(Entry.DieEntry);
  CompileUnit::DIEInfo &Info = Entry.CU->getDIEInfo(DIE);
 
  if (Info.getTrackLiveness()) {
    const auto *Abbrev = DIE.getAbbreviationDeclarationPtr();
 
    if (!Info.getIsInFunctionScope() &&
        Abbrev->findAttributeIndex(dwarf::DW_AT_const_value)) {
      // Global variables with constant value can always be kept.
    } else {
      // See if there is a relocation to a valid debug map entry inside this
      // variable's location. The order is important here. We want to always
      // check if the variable has a location expression address. However, we
      // don't want a static variable in a function to force us to keep the
      // enclosing function, unless requested explicitly.
      std::pair<bool, std::optional<int64_t>> LocExprAddrAndRelocAdjustment =
          Entry.CU->getContaingFile().Addresses->getVariableRelocAdjustment(
              DIE, Entry.CU->getGlobalData().getOptions().Verbose);
 
      if (LocExprAddrAndRelocAdjustment.first)
        Info.setHasAnAddress();
 
      if (!LocExprAddrAndRelocAdjustment.second)
        return false;
 
      if (!IsLiveParent && Info.getIsInFunctionScope() &&
          !Entry.CU->getGlobalData().getOptions().KeepFunctionForStatic)
        return false;
    }
  }
  Info.setHasAnAddress();
 
  dumpKeptDIE(DIE, "variable", Entry.CU->getGlobalData().getOptions().Verbose);
 
  return true;
}
 
bool DependencyTracker::isLiveSubprogramEntry(const UnitEntryPairTy &Entry) {
  DWARFDie DIE = Entry.CU->getDIE(Entry.DieEntry);
  CompileUnit::DIEInfo &Info = Entry.CU->getDIEInfo(Entry.DieEntry);
  std::optional<DWARFFormValue> LowPCVal = DIE.find(dwarf::DW_AT_low_pc);
 
  const bool Verbose = Entry.CU->getGlobalData().getOptions().Verbose;
  std::optional<uint64_t> LowPc;
  std::optional<uint64_t> HighPc;
  std::optional<int64_t> RelocAdjustment;
  if (Info.getTrackLiveness()) {
    LowPc = dwarf::toAddress(LowPCVal);
    if (!LowPc)
      return false;
 
    Info.setHasAnAddress();
 
    RelocAdjustment =
        Entry.CU->getContaingFile().Addresses->getSubprogramRelocAdjustment(
            DIE, Verbose);
    if (!RelocAdjustment)
      return false;
 
    if (DIE.getTag() == dwarf::DW_TAG_subprogram) {
      // Validate subprogram address range.
 
      HighPc = DIE.getHighPC(*LowPc);
      if (!HighPc) {
        Entry.CU->warn("function without high_pc. Range will be discarded.",
                       &DIE);
        return false;
      }
 
      if (*LowPc > *HighPc) {
        Entry.CU->warn("low_pc greater than high_pc. Range will be discarded.",
                       &DIE);
        return false;
      }
    } else if (DIE.getTag() == dwarf::DW_TAG_label) {
      if (Entry.CU->hasLabelAt(*LowPc))
        return false;
 
      // FIXME: dsymutil-classic compat. dsymutil-classic doesn't consider
      // labels that don't fall into the CU's aranges. This is wrong IMO. Debug
      // info generation bugs aside, this is really wrong in the case of labels,
      // where a label marking the end of a function will have a PC == CU's
      // high_pc.
      if (dwarf::toAddress(Entry.CU->find(Entry.DieEntry, dwarf::DW_AT_high_pc))
              .value_or(UINT64_MAX) <= LowPc)
        return false;
 
      // For assembly-language CUs there are typically no DW_TAG_subprogram
      // DIEs, so labels are the only addresses we see. Fall back to the
      // symbol-range lookup to recover a function range for the line-table
      // filter; otherwise the output line table would be empty.
      uint16_t Language = dwarf::toUnsigned(
          Entry.CU->getOrigUnit().getUnitDIE().find(dwarf::DW_AT_language), 0);
      if (Language == dwarf::DW_LANG_Mips_Assembler ||
          Language == dwarf::DW_LANG_Assembly) {
        if (auto Range =
                Entry.CU->getContaingFile().Addresses->getSymbolRangeForAddress(
                    *LowPc))
          Entry.CU->addFunctionRange(Range->LowPC, Range->HighPC,
                                     *RelocAdjustment);
      }
 
      Entry.CU->addLabelLowPc(*LowPc, *RelocAdjustment);
    }
  } else
    Info.setHasAnAddress();
 
  dumpKeptDIE(DIE, "subprogram", Verbose);
 
  if (!Info.getTrackLiveness() || DIE.getTag() == dwarf::DW_TAG_label)
    return true;
 
  Entry.CU->addFunctionRange(
      *LowPc,
      Entry.CU->getContaingFile().Addresses->constrainCodeRangeHighPC(
          *LowPc, *HighPc, *RelocAdjustment),
      *RelocAdjustment);
  return true;
}