[Serialization] Don't read all declaration id eagerly when merge the tables

See the post commit message in
llvm#92083 for rationale.

Previously, when we merge the lookup tables, we will read the tables
completely to get the data. But the above page shows that it might be
problematic in specific cases where there a lot of DeclIDs to be read.

In this patch, we mitigated the problem by not reading the contents of
the merged tables. But we may need to pay for the additional memory
usages.

Author: ChuanqiXu9

clang/lib/Serialization/MultiOnDiskHashTable.h

@@ -73,6 +73,51 @@ template<typename Info> class MultiOnDiskHashTable {
   struct MergedTable {
     std::vector<file_type> Files;
     llvm::DenseMap<internal_key_type, data_type> Data;
+
+    struct UnreadDataInfo {
+      Info *InfoObj;
+      const unsigned char *start;
+      unsigned DataLen;
+    };
+
+    llvm::DenseMap<internal_key_type, llvm::SmallVector<UnreadDataInfo, 16>>
+        UnReadData;
+
+    std::vector<OnDiskTable *> MergedOnDiskTables;
+
+    void clear() {
+      for (OnDiskTable *ODT : MergedOnDiskTables)
+        delete ODT;
+
+      MergedOnDiskTables.clear();
+    }
+
+    void readAll() {
+      for (const auto &Iter : UnReadData) {
+        internal_key_type Key = Iter.first;
+        data_type_builder ValueBuilder(Data[Key]);
+
+        for (const UnreadDataInfo &I : Iter.second)
+          I.InfoObj->ReadDataInto(Key, I.start, I.DataLen, ValueBuilder);
+      }
+
+      UnReadData.clear();
+      clear();
+    }
+
+    data_type find(internal_key_type Key) {
+      auto UnreadIter = UnReadData.find(Key);
+      if (UnreadIter != UnReadData.end()) {
+        data_type_builder ValueBuilder(Data[Key]);
+        for (auto &I : UnreadIter->second)
+          I.InfoObj->ReadDataInto(Key, I.start, I.DataLen, ValueBuilder);
+        UnReadData.erase(UnreadIter);
+      }
+
+      return Data[Key];
+    }
+
+    ~MergedTable() { clear(); }
   };
 
   using Table = llvm::PointerUnion<OnDiskTable *, MergedTable *>;
@@ -159,13 +204,14 @@ template<typename Info> class MultiOnDiskHashTable {
         // FIXME: Don't rely on the OnDiskHashTable format here.
         auto L = InfoObj.ReadKeyDataLength(LocalPtr);
         const internal_key_type &Key = InfoObj.ReadKey(LocalPtr, L.first);
-        data_type_builder ValueBuilder(Merged->Data[Key]);
-        InfoObj.ReadDataInto(Key, LocalPtr + L.first, L.second,
-                             ValueBuilder);
+        // Make sure Merged->Data contains every key.
+        (void)Merged->Data[Key];
+        Merged->UnReadData[Key].push_back(
+            {&InfoObj, LocalPtr + L.first, L.second});
       }
 
       Merged->Files.push_back(ODT->File);
-      delete ODT;
+      Merged->MergedOnDiskTables.push_back(ODT);
     }
 
     Tables.clear();
@@ -239,11 +285,8 @@ template<typename Info> class MultiOnDiskHashTable {
     internal_key_type Key = Info::GetInternalKey(EKey);
     auto KeyHash = Info::ComputeHash(Key);
 
-    if (MergedTable *M = getMergedTable()) {
-      auto It = M->Data.find(Key);
-      if (It != M->Data.end())
-        Result = It->second;
-    }
+    if (MergedTable *M = getMergedTable())
+      Result = M->find(Key);
 
     data_type_builder ResultBuilder(Result);
 
@@ -268,6 +311,7 @@ template<typename Info> class MultiOnDiskHashTable {
       removeOverriddenTables();
 
     if (MergedTable *M = getMergedTable()) {
+      M->readAll();
       for (auto &KV : M->Data)
         Info::MergeDataInto(KV.second, ResultBuilder);
     }
@@ -327,7 +371,7 @@ class MultiOnDiskHashTableGenerator {
         // Add all merged entries from Base to the generator.
         for (auto &KV : Merged->Data) {
           if (!Gen.contains(KV.first, Info))
-            Gen.insert(KV.first, Info.ImportData(KV.second), Info);
+            Gen.insert(KV.first, Info.ImportData(Merged->find(KV.first)), Info);
         }
       } else {
         Writer.write<uint32_t>(0);