[NFCI][WPD]Use unique string saver to store type id (#106932)

Currently, both
[TypeIdMap](https://github.com/llvm/llvm-project/blob/67a1fdb014790a38a205d28e1748634de34471dd/llvm/include/llvm/IR/ModuleSummaryIndex.h#L1356)
and
[TypeIdCompatibleVtableMap](https://github.com/llvm/llvm-project/blob/67a1fdb014790a38a205d28e1748634de34471dd/llvm/include/llvm/IR/ModuleSummaryIndex.h#L1363)
keep type-id as `std::string` in the combined index for LTO indexing
analysis.

With this change, index uses a unique-string-saver to own the string
copies and two maps above can use string references to save some memory.

This shows a 3% memory reduction (from 8.2GiB to 7.9GiB) in an internal
binary with high indexing memory usage.

Author: mingmingl-llvm

llvm/include/llvm/IR/ModuleSummaryIndex.h

@@ -1315,7 +1315,7 @@ using GVSummaryPtrSet = std::unordered_set<GlobalValueSummary *>;
 /// Map of a type GUID to type id string and summary (multimap used
 /// in case of GUID conflicts).
 using TypeIdSummaryMapTy =
-    std::multimap<GlobalValue::GUID, std::pair<std::string, TypeIdSummary>>;
+    std::multimap<GlobalValue::GUID, std::pair<StringRef, TypeIdSummary>>;
 
 /// The following data structures summarize type metadata information.
 /// For type metadata overview see https://llvm.org/docs/TypeMetadata.html.
@@ -1351,6 +1351,9 @@ class ModuleSummaryIndex {
   /// Holds strings for combined index, mapping to the corresponding module ID.
   ModulePathStringTableTy ModulePathStringTable;
 
+  BumpPtrAllocator TypeIdSaverAlloc;
+  UniqueStringSaver TypeIdSaver;
+
   /// Mapping from type identifier GUIDs to type identifier and its summary
   /// information. Produced by thin link.
   TypeIdSummaryMapTy TypeIdMap;
@@ -1359,7 +1362,7 @@ class ModuleSummaryIndex {
   /// with that type identifier's metadata. Produced by per module summary
   /// analysis and consumed by thin link. For more information, see description
   /// above where TypeIdCompatibleVtableInfo is defined.
-  std::map<std::string, TypeIdCompatibleVtableInfo, std::less<>>
+  std::map<StringRef, TypeIdCompatibleVtableInfo, std::less<>>
       TypeIdCompatibleVtableMap;
 
   /// Mapping from original ID to GUID. If original ID can map to multiple
@@ -1455,8 +1458,9 @@ class ModuleSummaryIndex {
   // See HaveGVs variable comment.
   ModuleSummaryIndex(bool HaveGVs, bool EnableSplitLTOUnit = false,
                      bool UnifiedLTO = false)
-      : HaveGVs(HaveGVs), EnableSplitLTOUnit(EnableSplitLTOUnit),
-        UnifiedLTO(UnifiedLTO), Saver(Alloc) {}
+      : TypeIdSaver(TypeIdSaverAlloc), HaveGVs(HaveGVs),
+        EnableSplitLTOUnit(EnableSplitLTOUnit), UnifiedLTO(UnifiedLTO),
+        Saver(Alloc) {}
 
   // Current version for the module summary in bitcode files.
   // The BitcodeSummaryVersion should be bumped whenever we introduce changes
@@ -1829,8 +1833,8 @@ class ModuleSummaryIndex {
     for (auto &[GUID, TypeIdPair] : make_range(TidIter))
       if (TypeIdPair.first == TypeId)
         return TypeIdPair.second;
-    auto It = TypeIdMap.insert(
-        {GlobalValue::getGUID(TypeId), {std::string(TypeId), TypeIdSummary()}});
+    auto It = TypeIdMap.insert({GlobalValue::getGUID(TypeId),
+                                {TypeIdSaver.save(TypeId), TypeIdSummary()}});
     return It->second.second;
   }
 
@@ -1859,7 +1863,7 @@ class ModuleSummaryIndex {
   /// the ThinLTO backends.
   TypeIdCompatibleVtableInfo &
   getOrInsertTypeIdCompatibleVtableSummary(StringRef TypeId) {
-    return TypeIdCompatibleVtableMap[std::string(TypeId)];
+    return TypeIdCompatibleVtableMap[TypeIdSaver.save(TypeId)];
   }
 
   /// For the given \p TypeId, this returns the TypeIdCompatibleVtableMap

llvm/include/llvm/IR/ModuleSummaryIndexYAML.h

@@ -313,11 +313,11 @@ template <> struct CustomMappingTraits<TypeIdSummaryMapTy> {
   static void inputOne(IO &io, StringRef Key, TypeIdSummaryMapTy &V) {
     TypeIdSummary TId;
     io.mapRequired(Key.str().c_str(), TId);
-    V.insert({GlobalValue::getGUID(Key), {std::string(Key), TId}});
+    V.insert({GlobalValue::getGUID(Key), {Key, TId}});
   }
   static void output(IO &io, TypeIdSummaryMapTy &V) {
     for (auto &TidIter : V)
-      io.mapRequired(TidIter.second.first.c_str(), TidIter.second.second);
+      io.mapRequired(TidIter.second.first.str().c_str(), TidIter.second.second);
   }
 };
 
@@ -327,7 +327,21 @@ template <> struct MappingTraits<ModuleSummaryIndex> {
     if (!io.outputting())
       CustomMappingTraits<GlobalValueSummaryMapTy>::fixAliaseeLinks(
           index.GlobalValueMap);
-    io.mapOptional("TypeIdMap", index.TypeIdMap);
+
+    if (io.outputting()) {
+      io.mapOptional("TypeIdMap", index.TypeIdMap);
+    } else {
+      TypeIdSummaryMapTy TypeIdMap;
+      io.mapOptional("TypeIdMap", TypeIdMap);
+      for (auto &[TypeGUID, TypeIdSummaryMap] : TypeIdMap) {
+        // Save type id references in index and point TypeIdMap to use the
+        // references owned by index.
+        StringRef KeyRef = index.TypeIdSaver.save(TypeIdSummaryMap.first);
+        index.TypeIdMap.insert(
+            {TypeGUID, {KeyRef, std::move(TypeIdSummaryMap.second)}});
+      }
+    }
+
     io.mapOptional("WithGlobalValueDeadStripping",
                    index.WithGlobalValueDeadStripping);
 

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -4165,7 +4165,7 @@ static void writeWholeProgramDevirtResolution(
 
 static void writeTypeIdSummaryRecord(SmallVector<uint64_t, 64> &NameVals,
                                      StringTableBuilder &StrtabBuilder,
-                                     const std::string &Id,
+                                     StringRef Id,
                                      const TypeIdSummary &Summary) {
   NameVals.push_back(StrtabBuilder.add(Id));
   NameVals.push_back(Id.size());
@@ -4184,7 +4184,7 @@ static void writeTypeIdSummaryRecord(SmallVector<uint64_t, 64> &NameVals,
 
 static void writeTypeIdCompatibleVtableSummaryRecord(
     SmallVector<uint64_t, 64> &NameVals, StringTableBuilder &StrtabBuilder,
-    const std::string &Id, const TypeIdCompatibleVtableInfo &Summary,
+    StringRef Id, const TypeIdCompatibleVtableInfo &Summary,
     ValueEnumerator &VE) {
   NameVals.push_back(StrtabBuilder.add(Id));
   NameVals.push_back(Id.size());