[DRAFT] [C++20] [Modules] Introduce -fgen-reduced-bmi

This is draft for the user interfaces for
llvm#75894 and required by @iains
to get a feeling about the proposed future direction.

Note that this is still in an early stage and nothing is decided.

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -232,6 +232,8 @@ def err_drv_invalid_argument_to_option : Error<
   "invalid argument '%0' to -%1">;
 def err_drv_missing_sanitizer_ignorelist : Error<
   "missing sanitizer ignorelist: '%0'">;
+def err_drv_incorrect_input_to_gen_reduced_bmi : Error <
+  "-fgen-reduced-bmi can only be used with module interface unit">;
 def err_drv_malformed_sanitizer_ignorelist : Error<
   "malformed sanitizer ignorelist: '%0'">;
 def err_drv_malformed_sanitizer_coverage_allowlist : Error<

clang/include/clang/Basic/LangOptions.def

@@ -175,6 +175,7 @@ BENIGN_LANGOPT(HeinousExtensions , 1, 0, "extensions that we really don't like a
 LANGOPT(Modules           , 1, 0, "modules semantics")
 COMPATIBLE_LANGOPT(CPlusPlusModules, 1, 0, "C++ modules syntax")
 LANGOPT(BuiltinHeadersInSystemModules, 1, 0, "builtin headers belong to system modules, and _Builtin_ modules are ignored for cstdlib headers")
+COMPATIBLE_LANGOPT(GenReducedBMI, 1, 0, "Generate Lang Options")
 BENIGN_ENUM_LANGOPT(CompilingModule, CompilingModuleKind, 3, CMK_None,
                     "compiling a module interface")
 BENIGN_LANGOPT(CompilingPCH, 1, 0, "building a pch")

clang/include/clang/CodeGen/CodeGenAction.h

@@ -57,6 +57,8 @@ class CodeGenAction : public ASTFrontendAction {
   bool loadLinkModules(CompilerInstance &CI);
 
 protected:
+  bool BeginSourceFileAction(CompilerInstance &CI) override;
+
   /// Create a new code generation action.  If the optional \p _VMContext
   /// parameter is supplied, the action uses it without taking ownership,
   /// otherwise it creates a fresh LLVM context and takes ownership.

clang/include/clang/Driver/Options.td

@@ -2977,6 +2977,11 @@ def fmodule_output : Flag<["-"], "fmodule-output">, Flags<[NoXarchOption]>,
   Visibility<[ClangOption, CC1Option]>,
   HelpText<"Save intermediate module file results when compiling a standard C++ module unit.">;
 
+defm gen_reduced_bmi : BoolFOption<"gen-reduced-bmi",
+  LangOpts<"GenReducedBMI">, DefaultFalse,
+  PosFlag<SetTrue, [], [CC1Option, ClangOption], "Generate reduced BMI.">,
+  NegFlag<SetFalse>, BothFlags<[], [ClangOption, CC1Option]>>;
+
 def fmodules_prune_interval : Joined<["-"], "fmodules-prune-interval=">, Group<i_Group>,
   Visibility<[ClangOption, CC1Option]>, MetaVarName<"<seconds>">,
   HelpText<"Specify the interval (in seconds) between attempts to prune the module cache">,

clang/include/clang/Frontend/FrontendOptions.h

@@ -553,6 +553,9 @@ class FrontendOptions {
   /// Path which stores the output files for -ftime-trace
   std::string TimeTracePath;
 
+  /// Output Path for module output file.
+  std::string ModuleOutputPath;
+
 public:
   FrontendOptions()
       : DisableFree(false), RelocatablePCH(false), ShowHelp(false),

clang/include/clang/Serialization/ASTWriter.h

@@ -825,6 +825,10 @@ class PCHGenerator : public SemaConsumer {
     return SemaPtr->getDiagnostics();
   }
 
+  const Preprocessor &getPreprocessor() const {
+    return PP;
+  }
+
 public:
   PCHGenerator(const Preprocessor &PP, InMemoryModuleCache &ModuleCache,
                StringRef OutputFile, StringRef isysroot,

clang/lib/CodeGen/CodeGenAction.cpp

@@ -26,7 +26,10 @@
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/FrontendDiagnostic.h"
+#include "clang/Frontend/FrontendActions.h"
+#include "clang/Frontend/MultiplexConsumer.h"
 #include "clang/Lex/Preprocessor.h"
+#include "clang/Serialization/ASTWriter.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
@@ -996,6 +999,12 @@ CodeGenerator *CodeGenAction::getCodeGenerator() const {
   return BEConsumer->getCodeGenerator();
 }
 
+bool CodeGenAction::BeginSourceFileAction(CompilerInstance &CI) {
+  if (CI.getLangOpts().GenReducedBMI)
+    CI.getLangOpts().setCompilingModule(LangOptions::CMK_ModuleInterface);
+  return true;
+}
+
 static std::unique_ptr<raw_pwrite_stream>
 GetOutputStream(CompilerInstance &CI, StringRef InFile, BackendAction Action) {
   switch (Action) {
@@ -1053,6 +1062,16 @@ CodeGenAction::CreateASTConsumer(CompilerInstance &CI, StringRef InFile) {
     CI.getPreprocessor().addPPCallbacks(std::move(Callbacks));
   }
 
+  if (CI.getLangOpts().GenReducedBMI && !CI.getFrontendOpts().ModuleOutputPath.empty()) {
+    std::vector<std::unique_ptr<ASTConsumer>> Consumers{2};
+    Consumers[0] = std::move(Result);
+    Consumers[1] = std::make_unique<ReducedBMIGenerator>(
+      CI.getPreprocessor(), CI.getModuleCache(),
+      CI.getFrontendOpts().OutputFile, std::make_shared<PCHBuffer>(),
+      /*IncludeTimestamps=*/+CI.getFrontendOpts().IncludeTimestamps);
+    return std::make_unique<MultiplexConsumer>(std::move(Consumers));
+  }
+
   return std::move(Result);
 }
 

clang/lib/Driver/Driver.cpp

@@ -4688,6 +4688,10 @@ Action *Driver::ConstructPhaseAction(
     if (Args.hasArg(options::OPT_extract_api))
       return C.MakeAction<ExtractAPIJobAction>(Input, types::TY_API_INFO);
 
+    if (Args.hasFlag(options::OPT_fgen_reduced_bmi,
+                     options::OPT_fno_gen_reduced_bmi, false))
+      return Input;
+
     types::ID OutputTy = getPrecompiledType(Input->getType());
     assert(OutputTy != types::TY_INVALID &&
            "Cannot precompile this input type!");

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3889,7 +3889,16 @@ static bool RenderModulesOptions(Compilation &C, const Driver &D,
 
   // Claim `-fmodule-output` and `-fmodule-output=` to avoid unused warnings.
   Args.ClaimAllArgs(options::OPT_fmodule_output);
-  Args.ClaimAllArgs(options::OPT_fmodule_output_EQ);
+  Args.AddLastArg(CmdArgs, options::OPT_fmodule_output_EQ);
+
+  if (Args.hasFlag(options::OPT_fgen_reduced_bmi,
+                   options::OPT_fno_gen_reduced_bmi, false)) {
+    if (Input.getType() != driver::types::TY_CXXModule &&
+        Input.getType() != driver::types::TY_PP_CXXModule)
+      D.Diag(diag::err_drv_incorrect_input_to_gen_reduced_bmi);
+
+    CmdArgs.push_back("-fgen-reduced-bmi");
+  }
 
   return HaveModules;
 }