[clang][modules] Shrink the size of Module::Headers

This patch shrinks the size of the `Module` class from 2112B to 1624B. I wasn't able to get a good data on the actual impact on memory usage, but given my `clang-scan-deps` workload at hand (with tens of thousands of instances), I think there should be some win here. This also speeds up my benchmark by under 0.1%.

Author: jansvoboda11

clang/include/clang/Basic/Module.h

@@ -271,8 +271,22 @@ class alignas(8) Module {
     DirectoryEntryRef Entry;
   };
 
-  /// The headers that are part of this module.
-  SmallVector<Header, 2> Headers[5];
+private:
+  unsigned HeaderKindBeginIndex[6] = {};
+  SmallVector<Header, 2> HeadersStorage;
+
+public:
+  ArrayRef<Header> getHeaders(HeaderKind HK) const {
+    auto BeginIt = HeadersStorage.begin() + HeaderKindBeginIndex[HK];
+    auto EndIt = HeadersStorage.begin() + HeaderKindBeginIndex[HK + 1];
+    return {BeginIt, EndIt};
+  }
+  void addHeader(HeaderKind HK, Header H) {
+    auto EndIt = HeadersStorage.begin() + HeaderKindBeginIndex[HK + 1];
+    HeadersStorage.insert(EndIt, std::move(H));
+    for (unsigned HKI = HK + 1; HKI != 6; ++HKI)
+      ++HeaderKindBeginIndex[HKI];
+  }
 
   /// Stored information about a header directive that was found in the
   /// module map file but has not been resolved to a file.

clang/lib/Basic/Module.cpp

@@ -528,7 +528,7 @@ void Module::print(raw_ostream &OS, unsigned Indent, bool Dump) const {
 
   for (auto &K : Kinds) {
     assert(&K == &Kinds[K.Kind] && "kinds in wrong order");
-    for (auto &H : Headers[K.Kind]) {
+    for (auto &H : getHeaders(K.Kind)) {
       OS.indent(Indent + 2);
       OS << K.Prefix << "header \"";
       OS.write_escaped(H.NameAsWritten);

clang/lib/Frontend/FrontendAction.cpp

@@ -358,7 +358,7 @@ static std::error_code collectModuleHeaderIncludes(
 
   // Add includes for each of these headers.
   for (auto HK : {Module::HK_Normal, Module::HK_Private}) {
-    for (Module::Header &H : Module->Headers[HK]) {
+    for (const Module::Header &H : Module->getHeaders(HK)) {
       Module->addTopHeader(H.Entry);
       // Use the path as specified in the module map file. We'll look for this
       // file relative to the module build directory (the directory containing

clang/lib/Lex/ModuleMap.cpp

@@ -472,12 +472,12 @@ static bool violatesPrivateInclude(Module *RequestingModule,
     // as obtained from the lookup and as obtained from the module.
     // This check is not cheap, so enable it only for debugging.
     bool IsPrivate = false;
-    SmallVectorImpl<Module::Header> *HeaderList[] = {
-        &Header.getModule()->Headers[Module::HK_Private],
-        &Header.getModule()->Headers[Module::HK_PrivateTextual]};
-    for (auto *Hs : HeaderList)
+    ArrayRef<Module::Header> HeaderList[] = {
+        Header.getModule()->getHeaders(Module::HK_Private),
+        Header.getModule()->getHeaders(Module::HK_PrivateTextual)};
+    for (auto Hs : HeaderList)
       IsPrivate |= llvm::any_of(
-          *Hs, [&](const Module::Header &H) { return H.Entry == IncFileEnt; });
+          Hs, [&](const Module::Header &H) { return H.Entry == IncFileEnt; });
     assert(IsPrivate && "inconsistent headers and roles");
   }
 #endif
@@ -1296,27 +1296,29 @@ void ModuleMap::addHeader(Module *Mod, Module::Header Header,
                           ModuleHeaderRole Role, bool Imported) {
   KnownHeader KH(Mod, Role);
 
+  FileEntryRef HeaderEntry = Header.Entry;
+
   // Only add each header to the headers list once.
   // FIXME: Should we diagnose if a header is listed twice in the
   // same module definition?
-  auto &HeaderList = Headers[Header.Entry];
+  auto &HeaderList = Headers[HeaderEntry];
   if (llvm::is_contained(HeaderList, KH))
     return;
 
   HeaderList.push_back(KH);
-  Mod->Headers[headerRoleToKind(Role)].push_back(Header);
+  Mod->addHeader(headerRoleToKind(Role), std::move(Header));
 
   bool isCompilingModuleHeader = Mod->isForBuilding(LangOpts);
   if (!Imported || isCompilingModuleHeader) {
     // When we import HeaderFileInfo, the external source is expected to
     // set the isModuleHeader flag itself.
-    HeaderInfo.MarkFileModuleHeader(Header.Entry, Role,
+    HeaderInfo.MarkFileModuleHeader(HeaderEntry, Role,
                                     isCompilingModuleHeader);
   }
 
   // Notify callbacks that we just added a new header.
   for (const auto &Cb : Callbacks)
-    Cb->moduleMapAddHeader(Header.Entry.getName());
+    Cb->moduleMapAddHeader(HeaderEntry.getName());
 }
 
 FileID ModuleMap::getContainingModuleMapFileID(const Module *Module) const {

clang/lib/Serialization/ASTWriter.cpp

@@ -3070,9 +3070,9 @@ void ASTWriter::WriteSubmodules(Module *WritingModule) {
         Module::HK_PrivateTextual},
       {SUBMODULE_EXCLUDED_HEADER, ExcludedHeaderAbbrev, Module::HK_Excluded}
     };
-    for (auto &HL : HeaderLists) {
+    for (const auto &HL : HeaderLists) {
       RecordData::value_type Record[] = {HL.RecordKind};
-      for (auto &H : Mod->Headers[HL.HeaderKind])
+      for (const auto &H : Mod->getHeaders(HL.HeaderKind))
         Stream.EmitRecordWithBlob(HL.Abbrev, Record, H.NameAsWritten);
     }