[C++20] [Modules] Introduce reduced BMI (#75894)

Close https://github.com/llvm/llvm-project/issues/71034

See

https://discourse.llvm.org/t/rfc-c-20-modules-introduce-thin-bmi-and-decls-hash/74755

This patch introduces reduced BMI, which doesn't contain the definitions
of functions and variables if its definitions won't contribute to the
ABI.

Testing is a big part of the patch. We want to make sure the reduced BMI
contains the same behavior with the existing and relatively stable
fatBMI. This is pretty helpful for further reduction.

The user interfaces part it left to following patches to ease the
reviewing.

1 files changed, 31 insertions, 6 deletions

diff --git a/clang/lib/Frontend/FrontendActions.cpp b/clang/lib/Frontend/FrontendActions.cpp
index b9ed5de..cd9b992 100644
--- a/clang/lib/Frontend/FrontendActions.cpp
+++ b/clang/lib/Frontend/FrontendActions.cpp
@@ -184,12 +184,12 @@ bool GeneratePCHAction::BeginSourceFileAction(CompilerInstance &CI) {
   return true;
 }
 
-std::unique_ptr<ASTConsumer>
-GenerateModuleAction::CreateASTConsumer(CompilerInstance &CI,
-                                        StringRef InFile) {
+std::vector<std::unique_ptr<ASTConsumer>>
+GenerateModuleAction::CreateMultiplexConsumer(CompilerInstance &CI,
+                                              StringRef InFile) {
   std::unique_ptr<raw_pwrite_stream> OS = CreateOutputFile(CI, InFile);
   if (!OS)
-    return nullptr;
+    return {};
 
   std::string OutputFile = CI.getFrontendOpts().OutputFile;
   std::string Sysroot;
@@ -210,6 +210,17 @@ GenerateModuleAction::CreateASTConsumer(CompilerInstance &CI,
       +CI.getFrontendOpts().BuildingImplicitModule));
   Consumers.push_back(CI.getPCHContainerWriter().CreatePCHContainerGenerator(
       CI, std::string(InFile), OutputFile, std::move(OS), Buffer));
+  return std::move(Consumers);
+}
+
+std::unique_ptr<ASTConsumer>
+GenerateModuleAction::CreateASTConsumer(CompilerInstance &CI,
+                                        StringRef InFile) {
+  std::vector<std::unique_ptr<ASTConsumer>> Consumers =
+      CreateMultiplexConsumer(CI, InFile);
+  if (Consumers.empty())
+    return nullptr;
+
   return std::make_unique<MultiplexConsumer>(std::move(Consumers));
 }
 
@@ -265,7 +276,12 @@ GenerateModuleInterfaceAction::CreateASTConsumer(CompilerInstance &CI,
   CI.getHeaderSearchOpts().ModulesSkipHeaderSearchPaths = true;
   CI.getHeaderSearchOpts().ModulesSkipPragmaDiagnosticMappings = true;
 
-  return GenerateModuleAction::CreateASTConsumer(CI, InFile);
+  std::vector<std::unique_ptr<ASTConsumer>> Consumers =
+      CreateMultiplexConsumer(CI, InFile);
+  if (Consumers.empty())
+    return nullptr;
+
+  return std::make_unique<MultiplexConsumer>(std::move(Consumers));
 }
 
 std::unique_ptr<raw_pwrite_stream>
@@ -274,6 +290,16 @@ GenerateModuleInterfaceAction::CreateOutputFile(CompilerInstance &CI,
   return CI.createDefaultOutputFile(/*Binary=*/true, InFile, "pcm");
 }
 
+std::unique_ptr<ASTConsumer>
+GenerateReducedModuleInterfaceAction::CreateASTConsumer(CompilerInstance &CI,
+                                                        StringRef InFile) {
+  auto Buffer = std::make_shared<PCHBuffer>();
+  return std::make_unique<ReducedBMIGenerator>(
+      CI.getPreprocessor(), CI.getModuleCache(),
+      CI.getFrontendOpts().OutputFile, Buffer,
+      /*IncludeTimestamps=*/+CI.getFrontendOpts().IncludeTimestamps);
+}
+
 bool GenerateHeaderUnitAction::BeginSourceFileAction(CompilerInstance &CI) {
   if (!CI.getLangOpts().CPlusPlusModules) {
     CI.getDiagnostics().Report(diag::err_module_interface_requires_cpp_modules);
@@ -839,7 +865,6 @@ void DumpModuleInfoAction::ExecuteAction() {
 
   const LangOptions &LO = getCurrentASTUnit().getLangOpts();
   if (LO.CPlusPlusModules && !LO.CurrentModule.empty()) {
-
     ASTReader *R = getCurrentASTUnit().getASTReader().get();
     unsigned SubModuleCount = R->getTotalNumSubmodules();
     serialization::ModuleFile &MF = R->getModuleManager().getPrimaryModule();