[clang][modules] Remove preloaded SLocEntries from PCM files

This commit removes the list of SLocEntry offsets to preload eagerly from PCM files. Commit introducing this functionality (258ae54) doesn't clarify why this would be more performant than the lazy approach used regularly.

Currently, the only SLocEntry the reader is supposed to preload is the predefines buffer, but in my experience, it's not actually referenced in most modules, so the time spent deserializing its SLocEntry is wasted. This is especially noticeable in the dependency scanner, where this change brings 4.56% speedup on my benchmark.

Author: jansvoboda11

clang/include/clang/Serialization/ASTBitCodes.h

@@ -51,7 +51,7 @@ const unsigned VERSION_MAJOR = 29;
 /// for the previous version could still support reading the new
 /// version by ignoring new kinds of subblocks), this number
 /// should be increased.
-const unsigned VERSION_MINOR = 0;
+const unsigned VERSION_MINOR = 1;
 
 /// An ID number that refers to an identifier in an AST file.
 ///
@@ -524,13 +524,7 @@ enum ASTRecordTypes {
   /// of source-location information.
   SOURCE_LOCATION_OFFSETS = 14,
 
-  /// Record code for the set of source location entries
-  /// that need to be preloaded by the AST reader.
-  ///
-  /// This set contains the source location entry for the
-  /// predefines buffer and for any file entries that need to be
-  /// preloaded.
-  SOURCE_LOCATION_PRELOADS = 15,
+  // ID 15 used to be for source location entry preloads.
 
   /// Record code for the set of ext_vector type names.
   EXT_VECTOR_DECLS = 16,

clang/include/clang/Serialization/ModuleFile.h

@@ -292,9 +292,6 @@ class ModuleFile {
   /// AST file.
   const uint32_t *SLocEntryOffsets = nullptr;
 
-  /// SLocEntries that we're going to preload.
-  SmallVector<uint64_t, 4> PreloadSLocEntries;
-
   /// Remapping table for source locations in this module.
   ContinuousRangeMap<SourceLocation::UIntTy, SourceLocation::IntTy, 2>
       SLocRemap;

clang/lib/Serialization/ASTReader.cpp

@@ -3226,7 +3226,6 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
       case SOURCE_LOCATION_OFFSETS:
       case MODULE_OFFSET_MAP:
       case SOURCE_MANAGER_LINE_TABLE:
-      case SOURCE_LOCATION_PRELOADS:
       case PPD_ENTITIES_OFFSETS:
       case HEADER_SEARCH_TABLE:
       case IMPORTED_MODULES:
@@ -3576,18 +3575,6 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
       ParseLineTable(F, Record);
       break;
 
-    case SOURCE_LOCATION_PRELOADS: {
-      // Need to transform from the local view (1-based IDs) to the global view,
-      // which is based off F.SLocEntryBaseID.
-      if (!F.PreloadSLocEntries.empty())
-        return llvm::createStringError(
-            std::errc::illegal_byte_sequence,
-            "Multiple SOURCE_LOCATION_PRELOADS records in AST file");
-
-      F.PreloadSLocEntries.swap(Record);
-      break;
-    }
-
     case EXT_VECTOR_DECLS:
       for (unsigned I = 0, N = Record.size(); I != N; ++I)
         ExtVectorDecls.push_back(getGlobalDeclID(F, Record[I]));
@@ -4417,16 +4404,6 @@ ASTReader::ASTReadResult ASTReader::ReadAST(StringRef FileName, ModuleKind Type,
   for (ImportedModule &M : Loaded) {
     ModuleFile &F = *M.Mod;
 
-    // Preload SLocEntries.
-    for (unsigned I = 0, N = F.PreloadSLocEntries.size(); I != N; ++I) {
-      int Index = int(F.PreloadSLocEntries[I] - 1) + F.SLocEntryBaseID;
-      // Load it through the SourceManager and don't call ReadSLocEntry()
-      // directly because the entry may have already been loaded in which case
-      // calling ReadSLocEntry() directly would trigger an assertion in
-      // SourceManager.
-      SourceMgr.getLoadedSLocEntryByID(Index);
-    }
-
     // Map the original source file ID into the ID space of the current
     // compilation.
     if (F.OriginalSourceFileID.isValid())

clang/lib/Serialization/ASTWriter.cpp

@@ -838,7 +838,6 @@ void ASTWriter::WriteBlockInfoBlock() {
   RECORD(METHOD_POOL);
   RECORD(PP_COUNTER_VALUE);
   RECORD(SOURCE_LOCATION_OFFSETS);
-  RECORD(SOURCE_LOCATION_PRELOADS);
   RECORD(EXT_VECTOR_DECLS);
   RECORD(UNUSED_FILESCOPED_DECLS);
   RECORD(PPD_ENTITIES_OFFSETS);
@@ -2137,7 +2136,6 @@ void ASTWriter::WriteSourceManagerBlock(SourceManager &SourceMgr,
   // entry, which is always the same dummy entry.
   std::vector<uint32_t> SLocEntryOffsets;
   uint64_t SLocEntryOffsetsBase = Stream.GetCurrentBitNo();
-  RecordData PreloadSLocs;
   SLocEntryOffsets.reserve(SourceMgr.local_sloc_entry_size() - 1);
   for (unsigned I = 1, N = SourceMgr.local_sloc_entry_size();
        I != N; ++I) {
@@ -2213,9 +2211,6 @@ void ASTWriter::WriteSourceManagerBlock(SourceManager &SourceMgr,
         Stream.EmitRecordWithBlob(SLocBufferAbbrv, Record,
                                   StringRef(Name.data(), Name.size() + 1));
         EmitBlob = true;
-
-        if (Name == "<built-in>")
-          PreloadSLocs.push_back(SLocEntryOffsets.size());
       }
 
       if (EmitBlob) {
@@ -2277,9 +2272,6 @@ void ASTWriter::WriteSourceManagerBlock(SourceManager &SourceMgr,
     Stream.EmitRecordWithBlob(SLocOffsetsAbbrev, Record,
                               bytes(SLocEntryOffsets));
   }
-  // Write the source location entry preloads array, telling the AST
-  // reader which source locations entries it should load eagerly.
-  Stream.EmitRecord(SOURCE_LOCATION_PRELOADS, PreloadSLocs);
 
   // Write the line table. It depends on remapping working, so it must come
   // after the source location offsets.