[NFC] [C++20] [Modules] [Reduced BMI] Make sure the size of reduced BMI is not large than full BMI

Before this patch, the size of the reduced BMI may be large than the
full BMI when the source codes is pretty small. This violates the design
principles. The root cause is an oversight that we skipped something
in full BMI but forgot to make it in reduced BMI.

Author: ChuanqiXu9

clang/include/clang/Serialization/ASTWriter.h

@@ -846,7 +846,7 @@ class ASTWriter : public ASTDeserializationListener,
 /// AST and semantic-analysis consumer that generates a
 /// precompiled header from the parsed source code.
 class PCHGenerator : public SemaConsumer {
-  const Preprocessor &PP;
+  Preprocessor &PP;
   std::string OutputFile;
   std::string isysroot;
   Sema *SemaPtr;
@@ -867,11 +867,12 @@ class PCHGenerator : public SemaConsumer {
   DiagnosticsEngine &getDiagnostics() const {
     return SemaPtr->getDiagnostics();
   }
+  Preprocessor &getPreprocessor() { return PP; }
 
   virtual Module *getEmittingModule(ASTContext &Ctx);
 
 public:
-  PCHGenerator(const Preprocessor &PP, InMemoryModuleCache &ModuleCache,
+  PCHGenerator(Preprocessor &PP, InMemoryModuleCache &ModuleCache,
                StringRef OutputFile, StringRef isysroot,
                std::shared_ptr<PCHBuffer> Buffer,
                ArrayRef<std::shared_ptr<ModuleFileExtension>> Extensions,
@@ -893,7 +894,7 @@ class ReducedBMIGenerator : public PCHGenerator {
   virtual Module *getEmittingModule(ASTContext &Ctx) override;
 
 public:
-  ReducedBMIGenerator(const Preprocessor &PP, InMemoryModuleCache &ModuleCache,
+  ReducedBMIGenerator(Preprocessor &PP, InMemoryModuleCache &ModuleCache,
                       StringRef OutputFile);
 
   void HandleTranslationUnit(ASTContext &Ctx) override;

clang/lib/Frontend/PrecompiledPreamble.cpp

@@ -290,8 +290,7 @@ class PrecompilePreambleAction : public ASTFrontendAction {
 
 class PrecompilePreambleConsumer : public PCHGenerator {
 public:
-  PrecompilePreambleConsumer(PrecompilePreambleAction &Action,
-                             const Preprocessor &PP,
+  PrecompilePreambleConsumer(PrecompilePreambleAction &Action, Preprocessor &PP,
                              InMemoryModuleCache &ModuleCache,
                              StringRef isysroot,
                              std::shared_ptr<PCHBuffer> Buffer)

clang/lib/Serialization/GeneratePCH.cpp

@@ -14,6 +14,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/Frontend/FrontendDiagnostic.h"
 #include "clang/Lex/HeaderSearch.h"
+#include "clang/Lex/HeaderSearchOptions.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/SemaConsumer.h"
 #include "clang/Serialization/ASTReader.h"
@@ -23,8 +24,8 @@
 using namespace clang;
 
 PCHGenerator::PCHGenerator(
-    const Preprocessor &PP, InMemoryModuleCache &ModuleCache,
-    StringRef OutputFile, StringRef isysroot, std::shared_ptr<PCHBuffer> Buffer,
+    Preprocessor &PP, InMemoryModuleCache &ModuleCache, StringRef OutputFile,
+    StringRef isysroot, std::shared_ptr<PCHBuffer> Buffer,
     ArrayRef<std::shared_ptr<ModuleFileExtension>> Extensions,
     bool AllowASTWithErrors, bool IncludeTimestamps,
     bool BuildingImplicitModule, bool ShouldCacheASTInMemory,
@@ -88,7 +89,7 @@ ASTDeserializationListener *PCHGenerator::GetASTDeserializationListener() {
   return &Writer;
 }
 
-ReducedBMIGenerator::ReducedBMIGenerator(const Preprocessor &PP,
+ReducedBMIGenerator::ReducedBMIGenerator(Preprocessor &PP,
                                          InMemoryModuleCache &ModuleCache,
                                          StringRef OutputFile)
     : PCHGenerator(
@@ -107,6 +108,18 @@ Module *ReducedBMIGenerator::getEmittingModule(ASTContext &Ctx) {
 }
 
 void ReducedBMIGenerator::HandleTranslationUnit(ASTContext &Ctx) {
+  // We need to do this to make sure the size of reduced BMI not to be larger
+  // than full BMI.
+  //
+  // FIMXE: We'd better to wrap such options to a new class ASTWriterOptions
+  // since this is not about searching header really.
+  // FIXME2: We'd better to move the class writing full BMI with reduced BMI.
+  HeaderSearchOptions &HSOpts =
+      getPreprocessor().getHeaderSearchInfo().getHeaderSearchOpts();
+  HSOpts.ModulesSkipDiagnosticOptions = true;
+  HSOpts.ModulesSkipHeaderSearchPaths = true;
+  HSOpts.ModulesSkipPragmaDiagnosticMappings = true;
+
   PCHGenerator::HandleTranslationUnit(Ctx);
 
   if (!isComplete())

clang/test/Modules/reduced-bmi-size.cppm

@@ -0,0 +1,16 @@
+// Ensure that the size of the reduced BMI is not larger than the full BMI
+// in the most simple case. 
+
+// This test requires linux commands.
+// REQUIRES: system-linux
+
+// RUN: rm -fr %t
+// RUN: mkdir %t
+//
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %s -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-reduced-module-interface %s -o %t/a.reduced.pcm
+//
+// %s implies the current source file. So we can't use it directly.
+// RUN: [ $(stat -c%\s "%t/a.pcm") -le $(stat -c%\s "%t/a.reduced.pcm") ]
+
+export module a;