[PDB] Split TypeServerSource and extend type index map lifetime

[rnk]

Summary

Extending the lifetime of these type index mappings does increase memory
usage (+2% in my case), but it decouples type merging from symbol
merging. This is a pre-requisite for two changes that I have in mind:

• parallel type merging: speeds up slow type merging
• defered symbol merging: avoid heap allocating (relocating) all symbols

This eliminates CVIndexMap and moves its data into TpiSource. The maps
are also split into a SmallVector and ArrayRef component, so that the
ipiMap can alias the tpiMap for /Z7 object files, and so that both maps
can simply alias the PDB type server maps for /Zi files.

Splitting TypeServerSource establishes that all input types to be merged
can be identified with two 32-bit indices:

• The index of the TpiSource object
• The type index of the record

This is useful, because this information can be stored in a single
64-bit atomic word to enable concurrent hashtable insertion.

[aganea]

I like a lot the change, it makes the API and the ownership more clear. But I am wondering: why not simply remove the CVIndexMap type? Do we need this extra indirection now? We could simply use a TpiSource API, instead of doing the if (indexMap->isTypeServerMap) in PDB.cpp? e.g. class TpiSource { ... virtual ArrayRef<TypeIndex> indexMap()/typeMap(); }.

[rnk]

• eliminate CVIndexMap entirely

[rnk]

• remove debugging noinline

[rnk]

Thanks, looks like I can remove it. I made things more uniform by having ipiMap always point to something, so if we have an item reference, always use ipiMap.

[aganea]

Just a few more things and we should be good to go:

lld/COFF/PDB.cpp

191

Can we only have TpiSource * here and infer file?

276

Don't we always expect a source at this point?

755

source isn't checked here. Perhaps assert(source) in the DebugSHandler constructor?

921

...otherwise if you don't want the if, it is worth adding a line to explain in which situation the source doesn't have a file.

923

No need to pass down both file and source.

931

I liked it better with this if as it is self-explantory. Is there another situation where we should return here? The TypeServerIpiSource is never returned, right?

[rnk]

• Ensure all objs with symbols have TpiSources

lld/COFF/PDB.cpp

191

Almost, but there are ObjFiles which lack a corresponding TpiSource, and we must add their symbols. So TpiSource is optional (may be null) and ObjFile is required (reference). I checked, and this case is tested in tree.

276

I wish. Perhaps we should fix this by making a source for all objs, even if they lack types to avoid the null check.

755

Huh, I thought I added a check above and had it warn and return. Must have gotten lost.

921

I can make the comment more explicit: PDB sources don't have symbols and don't have an object file, so only add symbols if the source is for an object file.

923

I think we can eliminate that if we make empty TpiSources for objects with symbols but no types, I'll try that.

931

Actually, we do come here for IPI sources. We iterate over all TpiSource objects and call addDebug on them, so if we keep this if, it will need two conditions. IMO the new if handles it more neatly: only object files have symbols, so if this is a source for an object file, add symbols.

[aganea]

Looks great, thanks!

lld/COFF/InputFiles.cpp

793 ↗

(On Diff #292564)

I am fine without this, I simply forgot about this case, where we had symbols but no types. Would that affect your tpi-index in D87805? This empty TpiSource will also receive an index and will be scheduled for doing, essentially, nothing? I am fine if you want to revert to an extra if below, whichever you think is the best.

lld/COFF/PDB.cpp

931

Agreed.

[rnk]

Thanks!

lld/COFF/InputFiles.cpp

793 ↗

(On Diff #292564)

I think I like this version better: it's one less nullable pointer to check for later. There aren't many of these object files without types, they are mostly assembly files .debug$F FPO data, so the extra memory usage shouldn't be a concern.

lld/COFF/DebugTypes.h

 //===- DebugTypes.h ---------------------------------------------*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLD_COFF_DEBUGTYPES_H
 #define LLD_COFF_DEBUGTYPES_H
 
+#include "lld/Common/LLVM.h"
+#include "llvm/DebugInfo/CodeView/TypeIndex.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/MemoryBuffer.h"
 
 namespace llvm {
 namespace codeview {
 class PrecompRecord;
 class TypeServer2Record;
 } // namespace codeview
 namespace pdb {
 class NativeSession;
 }
 } // namespace llvm
 
 namespace lld {
 namespace coff {
 
+using llvm::codeview::TypeIndex;
+
 class ObjFile;
 class PDBInputFile;
-struct CVIndexMap;
 class TypeMerger;
 
 class TpiSource {
 public:
-  enum TpiKind { Regular, PCH, UsingPCH, PDB, UsingPDB };
+  enum TpiKind { Regular, PCH, UsingPCH, PDB, PDBIpi, UsingPDB };
 
   TpiSource(TpiKind k, ObjFile *f);
   virtual ~TpiSource();
 
   /// Produce a mapping from the type and item indices used in the object
   /// file to those in the destination PDB.
   ///
   /// If the object file uses a type server PDB (compiled with /Zi), merge TPI
   /// and IPI from the type server PDB and return a map for it. Each unique type
   /// server PDB is merged at most once, so this may return an existing index
   /// mapping.
   ///
   /// If the object does not use a type server PDB (compiled with /Z7), we merge
   /// all the type and item records from the .debug$S stream and fill in the
   /// caller-provided ObjectIndexMap.
-  virtual llvm::Expected<const CVIndexMap *> mergeDebugT(TypeMerger *m,
-                                                         CVIndexMap *indexMap);
+  virtual Error mergeDebugT(TypeMerger *m);
+
   /// Is this a dependent file that needs to be processed first, before other
   /// OBJs?
   virtual bool isDependency() const { return false; }
 
   static void forEachSource(llvm::function_ref<void(TpiSource *)> fn);
 
   static uint32_t countTypeServerPDBs();
   static uint32_t countPrecompObjs();
 
   /// Clear global data structures for TpiSources.
   static void clear();
 
   const TpiKind kind;
   ObjFile *file;
+
+  // Storage for tpiMap or ipiMap, depending on the kind of source.
+  llvm::SmallVector<TypeIndex, 0> indexMapStorage;
+
+  // Source type index to PDB type index mapping for type and item records.
+  // These mappings will be the same for /Z7 objects, and distinct for /Zi
+  // objects.
+  llvm::ArrayRef<TypeIndex> tpiMap;
+  llvm::ArrayRef<TypeIndex> ipiMap;
 };
 
 TpiSource *makeTpiSource(ObjFile *file);
 TpiSource *makeTypeServerSource(PDBInputFile *pdbInputFile);
 TpiSource *makeUseTypeServerSource(ObjFile *file,
                                    llvm::codeview::TypeServer2Record ts);
 TpiSource *makePrecompSource(ObjFile *file);
 TpiSource *makeUsePrecompSource(ObjFile *file,
                                 llvm::codeview::PrecompRecord ts);
 
 } // namespace coff
 } // namespace lld
 
 #endif

lld/COFF/DebugTypes.cpp

 #include "llvm/Support/Path.h"
 
 using namespace llvm;
 using namespace llvm::codeview;
 using namespace lld;
 using namespace lld::coff;
 
 namespace {
+class TypeServerIpiSource;
+
 // The TypeServerSource class represents a PDB type server, a file referenced by
 // OBJ files compiled with MSVC /Zi. A single PDB can be shared by several OBJ
 // files, therefore there must be only once instance per OBJ lot. The file path
 // is discovered from the dependent OBJ's debug type stream. The
 // TypeServerSource object is then queued and loaded by the COFF Driver. The
 // debug type stream for such PDB files will be merged first in the final PDB,
 // before any dependent OBJ.
 class TypeServerSource : public TpiSource {
 public:
   explicit TypeServerSource(PDBInputFile *f)
       : TpiSource(PDB, nullptr), pdbInputFile(f) {
     if (f->loadErr && *f->loadErr)
       return;
     pdb::PDBFile &file = f->session->getPDBFile();
     auto expectedInfo = file.getPDBInfoStream();
     if (!expectedInfo)
       return;
     auto it = mappings.emplace(expectedInfo->getGuid(), this);
     assert(it.second);
     (void)it;
-    tsIndexMap.isTypeServerMap = true;
   }
 
-  Expected<const CVIndexMap *> mergeDebugT(TypeMerger *m,
-                                           CVIndexMap *indexMap) override;
+  Error mergeDebugT(TypeMerger *m) override;
   bool isDependency() const override { return true; }
 
   PDBInputFile *pdbInputFile = nullptr;
 
-  CVIndexMap tsIndexMap;
+  // TpiSource for IPI stream.
+  TypeServerIpiSource *ipiSrc = nullptr;
 
   static std::map<codeview::GUID, TypeServerSource *> mappings;
 };
 
+// Companion to TypeServerSource. Stores the index map for the IPI stream in the
+// PDB. Modeling PDBs with two sources for TPI and IPI helps establish the
+// invariant of one type index space per source.
+class TypeServerIpiSource : public TpiSource {
+public:
+  explicit TypeServerIpiSource() : TpiSource(PDBIpi, nullptr) {}
+
+  friend class TypeServerSource;
+
+  // IPI merging is handled in TypeServerSource::mergeDebugT, since it depends
+  // directly on type merging.
+  Error mergeDebugT(TypeMerger *m) override { return Error::success(); }
+
+  bool isDependency() const override { return true; }
+};
+
 // This class represents the debug type stream of an OBJ file that depends on a
 // PDB type server (see TypeServerSource).
 class UseTypeServerSource : public TpiSource {
 public:
   UseTypeServerSource(ObjFile *f, TypeServer2Record ts)
       : TpiSource(UsingPDB, f), typeServerDependency(ts) {}
 
-  Expected<const CVIndexMap *> mergeDebugT(TypeMerger *m,
-                                           CVIndexMap *indexMap) override;
+  Error mergeDebugT(TypeMerger *m) override;
 
   // Information about the PDB type server dependency, that needs to be loaded
   // in before merging this OBJ.
   TypeServer2Record typeServerDependency;
 };
 
 // This class represents the debug type stream of a Microsoft precompiled
 // headers OBJ (PCH OBJ). This OBJ kind needs to be merged first in the output
 // PDB, before any other OBJs that depend on this. Note that only MSVC generate
 // such files, clang does not.
 class PrecompSource : public TpiSource {
 public:
   PrecompSource(ObjFile *f) : TpiSource(PCH, f) {
     if (!f->pchSignature || !*f->pchSignature)
       fatal(toString(f) +
             " claims to be a PCH object, but does not have a valid signature");
     auto it = mappings.emplace(*f->pchSignature, this);
     if (!it.second)
       fatal("a PCH object with the same signature has already been provided (" +
             toString(it.first->second->file) + " and " + toString(file) + ")");
-    precompIndexMap.isPrecompiledTypeMap = true;
   }
 
-  Expected<const CVIndexMap *> mergeDebugT(TypeMerger *m,
-                                           CVIndexMap *indexMap) override;
   bool isDependency() const override { return true; }
 
-  CVIndexMap precompIndexMap;
-
   static std::map<uint32_t, PrecompSource *> mappings;
 };
 
 // This class represents the debug type stream of an OBJ file that depends on a
 // Microsoft precompiled headers OBJ (see PrecompSource).
 class UsePrecompSource : public TpiSource {
 public:
   UsePrecompSource(ObjFile *f, PrecompRecord precomp)
       : TpiSource(UsingPCH, f), precompDependency(precomp) {}
 
-  Expected<const CVIndexMap *> mergeDebugT(TypeMerger *m,
-                                           CVIndexMap *indexMap) override;
+  Error mergeDebugT(TypeMerger *m) override;
 
   // Information about the Precomp OBJ dependency, that needs to be loaded in
   // before merging this OBJ.
   PrecompRecord precompDependency;
 };
 } // namespace
 
 static std::vector<TpiSource *> gc;
 
 TpiSource::TpiSource(TpiKind k, ObjFile *f) : kind(k), file(f) {
   gc.push_back(this);
 }
 
 // Vtable key method.
 TpiSource::~TpiSource() = default;
 
 TpiSource *lld::coff::makeTpiSource(ObjFile *file) {
   return make<TpiSource>(TpiSource::Regular, file);
 }
 
 TpiSource *lld::coff::makeTypeServerSource(PDBInputFile *pdbInputFile) {
-  return make<TypeServerSource>(pdbInputFile);
+  // Type server sources come in pairs: the TPI stream, and the IPI stream.
+  auto *tpiSource = make<TypeServerSource>(pdbInputFile);
+  if (pdbInputFile->session->getPDBFile().hasPDBIpiStream())
+    tpiSource->ipiSrc = make<TypeServerIpiSource>();
+  return tpiSource;
 }
 
 TpiSource *lld::coff::makeUseTypeServerSource(ObjFile *file,
                                               TypeServer2Record ts) {
   return make<UseTypeServerSource>(file, ts);
 }
 
 TpiSource *lld::coff::makePrecompSource(ObjFile *file) {
getHashesFromDebugH(ArrayRef<uint8_t> debugH) {
   assert(canUseDebugH(debugH));
 
   debugH = debugH.drop_front(sizeof(object::debug_h_header));
   uint32_t count = debugH.size() / sizeof(GloballyHashedType);
   return {reinterpret_cast<const GloballyHashedType *>(debugH.data()), count};
 }
 
 // Merge .debug$T for a generic object file.
-Expected<const CVIndexMap *> TpiSource::mergeDebugT(TypeMerger *m,
-                                                    CVIndexMap *indexMap) {
+Error TpiSource::mergeDebugT(TypeMerger *m) {
   CVTypeArray types;
   BinaryStreamReader reader(file->debugTypes, support::little);
   cantFail(reader.readArray(types, reader.getLength()));
 
   if (config->debugGHashes) {
     ArrayRef<GloballyHashedType> hashes;
     std::vector<GloballyHashedType> ownedHashes;
     if (Optional<ArrayRef<uint8_t>> debugH = getDebugH(file))
       hashes = getHashesFromDebugH(*debugH);
     else {
       ownedHashes = GloballyHashedType::hashTypes(types);
       hashes = ownedHashes;
     }
 
     if (auto err = mergeTypeAndIdRecords(m->globalIDTable, m->globalTypeTable,
-                                         indexMap->tpiMap, types, hashes,
+                                         indexMapStorage, types, hashes,
                                          file->pchSignature))
       fatal("codeview::mergeTypeAndIdRecords failed: " +
             toString(std::move(err)));
   } else {
     if (auto err =
-            mergeTypeAndIdRecords(m->idTable, m->typeTable, indexMap->tpiMap,
+            mergeTypeAndIdRecords(m->idTable, m->typeTable, indexMapStorage,
                                   types, file->pchSignature))
       fatal("codeview::mergeTypeAndIdRecords failed: " +
             toString(std::move(err)));
   }
 
+  // In an object, there is only one mapping for both types and items.
+  tpiMap = indexMapStorage;
+  ipiMap = indexMapStorage;
+
   if (config->showSummary) {
     // Count how many times we saw each type record in our input. This
     // calculation requires a second pass over the type records to classify each
     // record as a type or index. This is slow, but this code executes when
     // collecting statistics.
     m->tpiCounts.resize(m->getTypeTable().size());
     m->ipiCounts.resize(m->getIDTable().size());
     uint32_t srcIdx = 0;
     for (CVType &ty : types) {
-      TypeIndex dstIdx = indexMap->tpiMap[srcIdx++];
+      TypeIndex dstIdx = tpiMap[srcIdx++];
       // Type merging may fail, so a complex source type may become the simple
       // NotTranslated type, which cannot be used as an array index.
       if (dstIdx.isSimple())
         continue;
       SmallVectorImpl<uint32_t> &counts =
           isIdRecord(ty.kind()) ? m->ipiCounts : m->tpiCounts;
       ++counts[dstIdx.toArrayIndex()];
     }
   }
 
-  return indexMap;
+  return Error::success();
 }
 
 // Merge types from a type server PDB.
-Expected<const CVIndexMap *> TypeServerSource::mergeDebugT(TypeMerger *m,
-                                                           CVIndexMap *) {
+Error TypeServerSource::mergeDebugT(TypeMerger *m) {
   pdb::PDBFile &pdbFile = pdbInputFile->session->getPDBFile();
   Expected<pdb::TpiStream &> expectedTpi = pdbFile.getPDBTpiStream();
   if (auto e = expectedTpi.takeError())
     fatal("Type server does not have TPI stream: " + toString(std::move(e)));
   pdb::TpiStream *maybeIpi = nullptr;
   if (pdbFile.hasPDBIpiStream()) {
     Expected<pdb::TpiStream &> expectedIpi = pdbFile.getPDBIpiStream();
     if (auto e = expectedIpi.takeError())
       fatal("Error getting type server IPI stream: " + toString(std::move(e)));
     maybeIpi = &*expectedIpi;
   }
 
   if (config->debugGHashes) {
     // PDBs do not actually store global hashes, so when merging a type server
     // PDB we have to synthesize global hashes.  To do this, we first synthesize
     // global hashes for the TPI stream, since it is independent, then we
     // synthesize hashes for the IPI stream, using the hashes for the TPI stream
     // as inputs.
     auto tpiHashes = GloballyHashedType::hashTypes(expectedTpi->typeArray());
     Optional<uint32_t> endPrecomp;
     // Merge TPI first, because the IPI stream will reference type indices.
     if (auto err =
-            mergeTypeRecords(m->globalTypeTable, tsIndexMap.tpiMap,
+            mergeTypeRecords(m->globalTypeTable, indexMapStorage,
                              expectedTpi->typeArray(), tpiHashes, endPrecomp))
       fatal("codeview::mergeTypeRecords failed: " + toString(std::move(err)));
+    tpiMap = indexMapStorage;
 
     // Merge IPI.
     if (maybeIpi) {
       auto ipiHashes =
           GloballyHashedType::hashIds(maybeIpi->typeArray(), tpiHashes);
-      if (auto err = mergeIdRecords(m->globalIDTable, tsIndexMap.tpiMap,
-                                    tsIndexMap.ipiMap, maybeIpi->typeArray(),
-                                    ipiHashes))
+      if (auto err =
+              mergeIdRecords(m->globalIDTable, tpiMap, ipiSrc->indexMapStorage,
+                             maybeIpi->typeArray(), ipiHashes))
         fatal("codeview::mergeIdRecords failed: " + toString(std::move(err)));
+      ipiMap = ipiSrc->indexMapStorage;
     }
   } else {
     // Merge TPI first, because the IPI stream will reference type indices.
-    if (auto err = mergeTypeRecords(m->typeTable, tsIndexMap.tpiMap,
+    if (auto err = mergeTypeRecords(m->typeTable, indexMapStorage,
                                     expectedTpi->typeArray()))
       fatal("codeview::mergeTypeRecords failed: " + toString(std::move(err)));
+    tpiMap = indexMapStorage;
 
     // Merge IPI.
     if (maybeIpi) {
-      if (auto err = mergeIdRecords(m->idTable, tsIndexMap.tpiMap,
-                                    tsIndexMap.ipiMap, maybeIpi->typeArray()))
+      if (auto err = mergeIdRecords(m->idTable, tpiMap, ipiSrc->indexMapStorage,
+                                    maybeIpi->typeArray()))
         fatal("codeview::mergeIdRecords failed: " + toString(std::move(err)));
+      ipiMap = ipiSrc->indexMapStorage;
     }
   }
 
   if (config->showSummary) {
     // Count how many times we saw each type record in our input. If a
     // destination type index is present in the source to destination type index
     // map, that means we saw it once in the input. Add it to our histogram.
     m->tpiCounts.resize(m->getTypeTable().size());
     m->ipiCounts.resize(m->getIDTable().size());
-    for (TypeIndex ti : tsIndexMap.tpiMap)
+    for (TypeIndex ti : tpiMap)
       if (!ti.isSimple())
         ++m->tpiCounts[ti.toArrayIndex()];
-    for (TypeIndex ti : tsIndexMap.ipiMap)
+    for (TypeIndex ti : ipiMap)
       if (!ti.isSimple())
         ++m->ipiCounts[ti.toArrayIndex()];
   }
 
-  return &tsIndexMap;
+  return Error::success();
 }
 
-Expected<const CVIndexMap *>
-UseTypeServerSource::mergeDebugT(TypeMerger *m, CVIndexMap *indexMap) {
+Error UseTypeServerSource::mergeDebugT(TypeMerger *m) {
   const codeview::GUID &tsId = typeServerDependency.getGuid();
   StringRef tsPath = typeServerDependency.getName();
 
   TypeServerSource *tsSrc;
   auto it = TypeServerSource::mappings.find(tsId);
   if (it != TypeServerSource::mappings.end()) {
     tsSrc = it->second;
   } else {
     // The file failed to load, lookup by name
     PDBInputFile *pdb = PDBInputFile::findFromRecordPath(tsPath, file);
     if (!pdb)
       return createFileError(tsPath, errorCodeToError(std::error_code(
                                          ENOENT, std::generic_category())));
     // If an error occurred during loading, throw it now
     if (pdb->loadErr && *pdb->loadErr)
       return createFileError(tsPath, std::move(*pdb->loadErr));
 
     tsSrc = (TypeServerSource *)pdb->debugTypesObj;
   }
 
   pdb::PDBFile &pdbSession = tsSrc->pdbInputFile->session->getPDBFile();
   auto expectedInfo = pdbSession.getPDBInfoStream();
   if (!expectedInfo)
-    return &tsSrc->tsIndexMap;
+    return expectedInfo.takeError();
 
   // Just because a file with a matching name was found and it was an actual
   // PDB file doesn't mean it matches.  For it to match the InfoStream's GUID
   // must match the GUID specified in the TypeServer2 record.
   if (expectedInfo->getGuid() != typeServerDependency.getGuid())
     return createFileError(
         tsPath,
         make_error<pdb::PDBError>(pdb::pdb_error_code::signature_out_of_date));
 
-  return &tsSrc->tsIndexMap;
+  // Reuse the type index map of the type server.
+  tpiMap = tsSrc->tpiMap;
+  ipiMap = tsSrc->ipiMap;
+  return Error::success();
 }
 
 static bool equalsPath(StringRef path1, StringRef path2) {
 #if defined(_WIN32)
   return path1.equals_lower(path2);
 #else
   return path1.equals(path2);
 #endif
 }
 
 // Find by name an OBJ provided on the command line
 static PrecompSource *findObjByName(StringRef fileNameOnly) {
   SmallString<128> currentPath;
   for (auto kv : PrecompSource::mappings) {
     StringRef currentFileName = sys::path::filename(kv.second->file->getName(),
                                                     sys::path::Style::windows);
 
     // Compare based solely on the file name (link.exe behavior)
     if (equalsPath(currentFileName, fileNameOnly))
       return kv.second;
   }
   return nullptr;
 }
 
-static Expected<const CVIndexMap *> findPrecompMap(ObjFile *file,
-                                                   PrecompRecord &pr) {
+static Expected<PrecompSource *> findPrecompMap(ObjFile *file,
   // Cross-compile warning: given that Clang doesn't generate LF_PRECOMP
   // records, we assume the OBJ comes from a Windows build of cl.exe. Thusly,
   // the paths embedded in the OBJs are in the Windows format.
   SmallString<128> prFileName =
       sys::path::filename(pr.getPrecompFilePath(), sys::path::Style::windows);
 
   PrecompSource *precomp;
   auto it = PrecompSource::mappings.find(pr.getSignature());
return createFileError(
         toString(file),
         make_error<pdb::PDBError>(pdb::pdb_error_code::no_matching_pch));
 
   if (pr.getSignature() != *precomp->file->pchSignature)
     return createFileError(
         toString(precomp->file),
         make_error<pdb::PDBError>(pdb::pdb_error_code::no_matching_pch));
 
-  return &precomp->precompIndexMap;
+  return precomp;
 }
 
 /// Merges a precompiled headers TPI map into the current TPI map. The
 /// precompiled headers object will also be loaded and remapped in the
 /// process.
-static Expected<const CVIndexMap *>
-mergeInPrecompHeaderObj(ObjFile *file, CVIndexMap *indexMap,
+static Error
+mergeInPrecompHeaderObj(ObjFile *file,
+                        SmallVectorImpl<TypeIndex> &indexMapStorage,
                         PrecompRecord &precomp) {
   auto e = findPrecompMap(file, precomp);
   if (!e)
     return e.takeError();
 
-  const CVIndexMap *precompIndexMap = *e;
-  assert(precompIndexMap->isPrecompiledTypeMap);
-
-  if (precompIndexMap->tpiMap.empty())
-    return precompIndexMap;
+  PrecompSource *precompSrc = *e;
+  if (precompSrc->tpiMap.empty())
+    return Error::success();
 
   assert(precomp.getStartTypeIndex() == TypeIndex::FirstNonSimpleIndex);
-  assert(precomp.getTypesCount() <= precompIndexMap->tpiMap.size());
+  assert(precomp.getTypesCount() <= precompSrc->tpiMap.size());
   // Use the previously remapped index map from the precompiled headers.
-  indexMap->tpiMap.append(precompIndexMap->tpiMap.begin(),
-                          precompIndexMap->tpiMap.begin() +
-                              precomp.getTypesCount());
-  return indexMap;
+  indexMapStorage.append(precompSrc->tpiMap.begin(),
+                         precompSrc->tpiMap.begin() + precomp.getTypesCount());
+  return Error::success();
 }
 
-Expected<const CVIndexMap *>
-UsePrecompSource::mergeDebugT(TypeMerger *m, CVIndexMap *indexMap) {
+Error UsePrecompSource::mergeDebugT(TypeMerger *m) {
   // This object was compiled with /Yu, so process the corresponding
   // precompiled headers object (/Yc) first. Some type indices in the current
   // object are referencing data in the precompiled headers object, so we need
   // both to be loaded.
-  auto e = mergeInPrecompHeaderObj(file, indexMap, precompDependency);
-  if (!e)
-    return e.takeError();
-
-  return TpiSource::mergeDebugT(m, indexMap);
-}
+  if (Error e =
+          mergeInPrecompHeaderObj(file, indexMapStorage, precompDependency))
+    return e;
 
-Expected<const CVIndexMap *> PrecompSource::mergeDebugT(TypeMerger *m,
-                                                        CVIndexMap *) {
-  // Note that we're not using the provided CVIndexMap. Instead, we use our
-  // local one. Precompiled headers objects need to save the index map for
-  // further reference by other objects which use the precompiled headers.
-  return TpiSource::mergeDebugT(m, &precompIndexMap);
+  return TpiSource::mergeDebugT(m);
 }
 
 uint32_t TpiSource::countTypeServerPDBs() {
   return TypeServerSource::mappings.size();
 }
 
 uint32_t TpiSource::countPrecompObjs() {
   return PrecompSource::mappings.size();
 }
 
 void TpiSource::clear() {
   gc.clear();
   TypeServerSource::mappings.clear();
   PrecompSource::mappings.clear();
 }

lld/COFF/PDB.cpp

public:
   /// Link info for each import file in the symbol table into the PDB.
   void addImportFilesToPDB(ArrayRef<OutputSection *> outputSections);
 
   /// Link CodeView from a single object file into the target (output) PDB.
   /// When a precompiled headers object is linked, its TPI map might be provided
   /// externally.
   void addDebug(TpiSource *source);
 
-  const CVIndexMap *mergeTypeRecords(TpiSource *source, CVIndexMap *localMap);
+  bool mergeTypeRecords(TpiSource *source);
 
-  void addDebugSymbols(ObjFile *file, const CVIndexMap *indexMap);
+  void addDebugSymbols(ObjFile *file, TpiSource *source);
 
-  void mergeSymbolRecords(ObjFile *file, const CVIndexMap &indexMap,
+  void mergeSymbolRecords(ObjFile *file, TpiSource *source,
                           std::vector<ulittle32_t *> &stringTableRefs,
                           BinaryStreamRef symData);
 
   /// Add the section map and section contributions to the PDB.
   void addSections(ArrayRef<OutputSection *> outputSections,
                    ArrayRef<uint8_t> sectionTable);
 
   /// Write the PDB to disk and store the Guid generated for it in *Guid.

 
 class DebugSHandler {
   PDBLinker &linker;
 
   /// The object file whose .debug$S sections we're processing.
   ObjFile &file;
 
   /// The result of merging type indices.
-  const CVIndexMap *indexMap;
+  TpiSource *source;
 
   /// The DEBUG_S_STRINGTABLE subsection.  These strings are referred to by
   /// index from other records in the .debug$S section.  All of these strings
   /// need to be added to the global PDB string table, and all references to
   /// these strings need to have their indices re-written to refer to the
   /// global PDB string table.
   DebugStringTableSubsectionRef cvStrTab;
 
class DebugSHandler {
   /// up this list as we go.  At the end, we use the string table (which must
   /// have been discovered by now else it is an error) to re-write these
   /// references.
   std::vector<ulittle32_t *> stringTableReferences;
 
   void mergeInlineeLines(const DebugSubsectionRecord &inlineeLines);
 
 public:
-  DebugSHandler(PDBLinker &linker, ObjFile &file, const CVIndexMap *indexMap)
-      : linker(linker), file(file), indexMap(indexMap) {}
+  DebugSHandler(PDBLinker &linker, ObjFile &file, TpiSource *source)
+      : linker(linker), file(file), source(source) {}
 
   void handleDebugS(ArrayRef<uint8_t> relocatedDebugContents);
 
   void finish();
 };
 }
 
 // Visual Studio's debugger requires absolute paths in various places in the
static bool remapTypeIndex(TypeIndex &ti, ArrayRef<TypeIndex> typeIndexMap) {
   if (ti.toArrayIndex() >= typeIndexMap.size())
     return false;
   ti = typeIndexMap[ti.toArrayIndex()];
   return true;
 }
 
 static void remapTypesInSymbolRecord(ObjFile *file, SymbolKind symKind,
                                      MutableArrayRef<uint8_t> recordBytes,
-                                     const CVIndexMap &indexMap,
+                                     TpiSource *source,
                                      ArrayRef<TiReference> typeRefs) {
   MutableArrayRef<uint8_t> contents =
       recordBytes.drop_front(sizeof(RecordPrefix));
   for (const TiReference &ref : typeRefs) {
     unsigned byteSize = ref.Count * sizeof(TypeIndex);
     if (contents.size() < ref.Offset + byteSize)
       fatal("symbol record too short");
 
     // This can be an item index or a type index. Choose the appropriate map.
-    ArrayRef<TypeIndex> typeOrItemMap = indexMap.tpiMap;
     bool isItemIndex = ref.Kind == TiRefKind::IndexRef;
-    if (isItemIndex && indexMap.isTypeServerMap)
-      typeOrItemMap = indexMap.ipiMap;
+    ArrayRef<TypeIndex> typeOrItemMap;
+    if (source)
+      typeOrItemMap = isItemIndex ? source->ipiMap : source->tpiMap;
 
     MutableArrayRef<TypeIndex> tIs(
         reinterpret_cast<TypeIndex *>(contents.data() + ref.Offset), ref.Count);
     for (TypeIndex &ti : tIs) {
       if (!remapTypeIndex(ti, typeOrItemMap)) {
         log("ignoring symbol record of kind 0x" + utohexstr(symKind) + " in " +
             file->getName() + " with bad " + (isItemIndex ? "item" : "type") +
             " index 0x" + utohexstr(ti.getIndex()));
case SymbolKind::S_LPROC32: {
     builder.addGlobalSymbol(ps);
     break;
   }
   default:
     llvm_unreachable("Invalid symbol kind!");
   }
 }
 
-void PDBLinker::mergeSymbolRecords(ObjFile *file, const CVIndexMap &indexMap,
+void PDBLinker::mergeSymbolRecords(ObjFile *file, TpiSource *source,
                                    std::vector<ulittle32_t *> &stringTableRefs,
                                    BinaryStreamRef symData) {
   ArrayRef<uint8_t> symsBuffer;
   cantFail(symData.readBytes(0, symData.getLength(), symsBuffer));
   SmallVector<SymbolScope, 4> scopes;
 
   // Iterate every symbol to check if any need to be realigned, and if so, how
   // much space we need to allocate for them.
cantFail(forEachCodeViewRecord<CVSymbol>(
         SmallVector<TiReference, 32> typeRefs;
         if (!discoverTypeIndicesInSymbol(sym, typeRefs)) {
           log("ignoring unknown symbol record with kind 0x" +
               utohexstr(sym.kind()));
           return Error::success();
         }
 
         // Re-map all the type index references.
-        remapTypesInSymbolRecord(file, sym.kind(), recordBytes, indexMap,
+        remapTypesInSymbolRecord(file, sym.kind(), recordBytes, source,
                                  typeRefs);
 
         // An object file may have S_xxx_ID symbols, but these get converted to
         // "real" symbols in a PDB.
         translateIdSymbols(recordBytes, tMerger.getIDTable());
         sym = CVSymbol(recordBytes);
 
         // If this record refers to an offset in the object file's string table,

 void DebugSHandler::handleDebugS(ArrayRef<uint8_t> relocatedDebugContents) {
   relocatedDebugContents =
       SectionChunk::consumeDebugMagic(relocatedDebugContents, ".debug$S");
 
   DebugSubsectionArray subsections;
   BinaryStreamReader reader(relocatedDebugContents, support::little);
   exitOnErr(reader.readArray(subsections, relocatedDebugContents.size()));
 
-  // If there is no index map, use an empty one.
-  CVIndexMap tempIndexMap;
-  if (!indexMap)
-    indexMap = &tempIndexMap;
-
   for (const DebugSubsectionRecord &ss : subsections) {
     // Ignore subsections with the 'ignore' bit. Some versions of the Visual C++
     // runtime have subsections with this bit set.
     if (uint32_t(ss.kind()) & codeview::SubsectionIgnoreFlag)
       continue;
 
     switch (ss.kind()) {
     case DebugSubsectionKind::StringTable: {
case DebugSubsectionKind::FrameData: {
       // frame data subsections until we've processed the entire list of
       // subsections so that we can be sure we have the string table.
       DebugFrameDataSubsectionRef fds;
       exitOnErr(fds.initialize(ss.getRecordData()));
       newFpoFrames.push_back(std::move(fds));
       break;
     }
     case DebugSubsectionKind::Symbols: {
-      linker.mergeSymbolRecords(&file, *indexMap, stringTableReferences,
+      linker.mergeSymbolRecords(&file, source, stringTableReferences,
                                 ss.getRecordData());
       break;
     }
 
     case DebugSubsectionKind::CrossScopeImports:
     case DebugSubsectionKind::CrossScopeExports:
       // These appear to relate to cross-module optimization, so we might use
       // these for ThinLTO.

 void DebugSHandler::mergeInlineeLines(
     const DebugSubsectionRecord &inlineeSubsection) {
   DebugInlineeLinesSubsectionRef inlineeLines;
   exitOnErr(inlineeLines.initialize(inlineeSubsection.getRecordData()));
 
   // Remap type indices in inlinee line records in place.
   for (const InlineeSourceLine &line : inlineeLines) {
     TypeIndex &inlinee = *const_cast<TypeIndex *>(&line.Header->Inlinee);
-    ArrayRef<TypeIndex> typeOrItemMap =
-        indexMap->isTypeServerMap ? indexMap->ipiMap : indexMap->tpiMap;
-    if (!remapTypeIndex(inlinee, typeOrItemMap)) {
+    if (!remapTypeIndex(inlinee, source->ipiMap)) {
       log("bad inlinee line record in " + file.getName() +
           " with bad inlinee index 0x" + utohexstr(inlinee.getIndex()));
     }
   }
 
   // Add the modified inlinee line subsection directly.
   file.moduleDBI->addDebugSubsection(inlineeSubsection);
 }
static void warnUnusable(InputFile *f, Error e) {
   }
   auto msg = "Cannot use debug info for '" + toString(f) + "' [LNK4099]";
   if (e)
     warn(msg + "\n>>> failed to load reference " + toString(std::move(e)));
   else
     warn(msg);
 }
 
-const CVIndexMap *PDBLinker::mergeTypeRecords(TpiSource *source,
-                                              CVIndexMap *localMap) {
+bool PDBLinker::mergeTypeRecords(TpiSource *source) {
   ScopedTimer t(typeMergingTimer);
   // Before we can process symbol substreams from .debug$S, we need to process
   // type information, file checksums, and the string table.  Add type info to
   // the PDB first, so that we can get the map from object file type and item
   // indices to PDB type and item indices.
-  Expected<const CVIndexMap *> r = source->mergeDebugT(&tMerger, localMap);
-
+  if (Error e = source->mergeDebugT(&tMerger)) {
     // If the .debug$T sections fail to merge, assume there is no debug info.
-  if (!r) {
-    warnUnusable(source->file, r.takeError());
-    return nullptr;
+    warnUnusable(source->file, std::move(e));
+    return false;
   }
-  return *r;
+  return true;
 }
 
 // Allocate memory for a .debug$S / .debug$F section and relocate it.
 static ArrayRef<uint8_t> relocateDebugChunk(SectionChunk &debugChunk) {
   uint8_t *buffer = bAlloc.Allocate<uint8_t>(debugChunk.getSize());
   assert(debugChunk.getOutputSectionIdx() == 0 &&
          "debug sections should not be in output sections");
   debugChunk.writeTo(buffer);
   return makeArrayRef(buffer, debugChunk.getSize());
 }
 
-void PDBLinker::addDebugSymbols(ObjFile *file, const CVIndexMap *indexMap) {
+void PDBLinker::addDebugSymbols(ObjFile *file, TpiSource *source) {
   ScopedTimer t(symbolMergingTimer);
   pdb::DbiStreamBuilder &dbiBuilder = builder.getDbiBuilder();
-  DebugSHandler dsh(*this, *file, indexMap);
+  DebugSHandler dsh(*this, *file, source);
   // Now do all live .debug$S and .debug$F sections.
   for (SectionChunk *debugChunk : file->getDebugChunks()) {
     if (!debugChunk->live || debugChunk->getSize() == 0)
       continue;
 
     bool isDebugS = debugChunk->getSectionName() == ".debug$S";
     bool isDebugF = debugChunk->getSectionName() == ".debug$F";
     if (!isDebugS && !isDebugF)
if (!secChunk || !secChunk->live)
       continue;
     pdb::SectionContrib sc = createSectionContrib(secChunk, modi);
     file->moduleDBI->setFirstSectionContrib(sc);
     break;
   }
 }
 
 void PDBLinker::addDebug(TpiSource *source) {
-  CVIndexMap localMap;
-  const CVIndexMap *indexMap = mergeTypeRecords(source, &localMap);
-
-  if (source->kind == TpiSource::PDB)
-    return; // No symbols in TypeServer PDBs
+  if (!mergeTypeRecords(source))
+    return;
 
-  addDebugSymbols(source->file, indexMap);
+  // If the source has an object file, add its symbol records.
+  if (source->file)
+    addDebugSymbols(source->file, source);
 }
 
 static pdb::BulkPublic createPublic(Defined *def) {
   pdb::BulkPublic pub;
   pub.Name = def->getName().data();
   pub.NameLen = def->getName().size();
 
   PublicSymFlags flags = PublicSymFlags::None;

lld/COFF/TypeMerger.h

public:
   llvm::codeview::GlobalTypeTableBuilder globalIDTable;
 
   // When showSummary is enabled, these are histograms of TPI and IPI records
   // keyed by type index.
   SmallVector<uint32_t, 0> tpiCounts;
   SmallVector<uint32_t, 0> ipiCounts;
 };
 
-/// Map from type index and item index in a type server PDB to the
-/// corresponding index in the destination PDB.
-struct CVIndexMap {
-  llvm::SmallVector<llvm::codeview::TypeIndex, 0> tpiMap;
-  llvm::SmallVector<llvm::codeview::TypeIndex, 0> ipiMap;
-  bool isTypeServerMap = false;
-  bool isPrecompiledTypeMap = false;
-};
-
 } // namespace coff
 } // namespace lld
 
 #endif