[C++20] [Modules] Don't import initializer/pending implicit instantiations from other named module (#167468)

Close #166068

The cause of the problem is that we would import initializers and
pending implicit instantiations from other named module. This is very
bad and it may waste a lot of time.

And we didn't observe it as the weak symbols can live together and the
strong symbols would be removed by other mechanism. So we didn't observe
the bad behavior for a long time. But it indeeds waste compilation time.

Author: ChuanqiXu9

clang/lib/Serialization/ASTReader.cpp

@@ -4087,10 +4087,14 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
             std::errc::illegal_byte_sequence,
             "Invalid PENDING_IMPLICIT_INSTANTIATIONS block");
 
-      for (unsigned I = 0, N = Record.size(); I != N; /* in loop */) {
-        PendingInstantiations.push_back(
-            {ReadDeclID(F, Record, I),
-             ReadSourceLocation(F, Record, I).getRawEncoding()});
+      // For standard C++20 module, we will only reads the instantiations
+      // if it is the main file.
+      if (!F.StandardCXXModule || F.Kind == MK_MainFile) {
+        for (unsigned I = 0, N = Record.size(); I != N; /* in loop */) {
+          PendingInstantiations.push_back(
+              {ReadDeclID(F, Record, I),
+               ReadSourceLocation(F, Record, I).getRawEncoding()});
+        }
       }
       break;
 
@@ -6438,10 +6442,13 @@ llvm::Error ASTReader::ReadSubmoduleBlock(ModuleFile &F,
     case SUBMODULE_INITIALIZERS: {
       if (!ContextObj)
         break;
-      SmallVector<GlobalDeclID, 16> Inits;
-      for (unsigned I = 0; I < Record.size(); /*in loop*/)
-        Inits.push_back(ReadDeclID(F, Record, I));
-      ContextObj->addLazyModuleInitializers(CurrentModule, Inits);
+      // Standard C++ module has its own way to initialize variables.
+      if (!F.StandardCXXModule || F.Kind == MK_MainFile) {
+        SmallVector<GlobalDeclID, 16> Inits;
+        for (unsigned I = 0; I < Record.size(); /*in loop*/)
+          Inits.push_back(ReadDeclID(F, Record, I));
+        ContextObj->addLazyModuleInitializers(CurrentModule, Inits);
+      }
       break;
     }
 

clang/lib/Serialization/ASTWriter.cpp

@@ -3247,7 +3247,7 @@ void ASTWriter::WriteSubmodules(Module *WritingModule, ASTContext *Context) {
 
     // Emit the reachable initializers.
     // The initializer may only be unreachable in reduced BMI.
-    if (Context) {
+    if (Context && !GeneratingReducedBMI) {
       RecordData Inits;
       for (Decl *D : Context->getModuleInitializers(Mod))
         if (wasDeclEmitted(D))
@@ -5827,17 +5827,19 @@ void ASTWriter::WriteSpecialDeclRecords(Sema &SemaRef) {
     Stream.EmitRecord(UNUSED_LOCAL_TYPEDEF_NAME_CANDIDATES,
                       UnusedLocalTypedefNameCandidates);
 
-  // Write the record containing pending implicit instantiations.
-  RecordData PendingInstantiations;
-  for (const auto &I : SemaRef.PendingInstantiations) {
-    if (!wasDeclEmitted(I.first))
-      continue;
+  if (!GeneratingReducedBMI) {
+    // Write the record containing pending implicit instantiations.
+    RecordData PendingInstantiations;
+    for (const auto &I : SemaRef.PendingInstantiations) {
+      if (!wasDeclEmitted(I.first))
+        continue;
 
-    AddDeclRef(I.first, PendingInstantiations);
-    AddSourceLocation(I.second, PendingInstantiations);
+      AddDeclRef(I.first, PendingInstantiations);
+      AddSourceLocation(I.second, PendingInstantiations);
+    }
+    if (!PendingInstantiations.empty())
+      Stream.EmitRecord(PENDING_IMPLICIT_INSTANTIATIONS, PendingInstantiations);
   }
-  if (!PendingInstantiations.empty())
-    Stream.EmitRecord(PENDING_IMPLICIT_INSTANTIATIONS, PendingInstantiations);
 
   // Write the record containing declaration references of Sema.
   RecordData SemaDeclRefs;

clang/test/Modules/pr166068.cppm

@@ -0,0 +1,38 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/flyweight.cppm -emit-reduced-module-interface -o %t/flyweight.pcm
+// RUN: %clang_cc1 -std=c++20 %t/account.cppm -emit-reduced-module-interface -o %t/account.pcm -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/core.cppm -emit-reduced-module-interface -o %t/core.pcm -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/core.cppm -fprebuilt-module-path=%t -emit-llvm -disable-llvm-passes -o - | FileCheck %t/core.cppm
+
+//--- flyweight.cppm
+module;
+template <typename> struct flyweight_core {
+  static bool init() { (void)__builtin_operator_new(2); return true; }
+  static bool static_initializer;
+};
+template <typename T> bool flyweight_core<T>::static_initializer = init();
+export module flyweight;
+export template <class> void flyweight() {
+  (void)flyweight_core<int>::static_initializer;
+}
+
+//--- account.cppm
+export module account;
+import flyweight;
+export void account() {
+  (void)::flyweight<int>;
+}
+
+//--- core.cppm
+export module core;
+import account;
+
+extern "C" void core() {}
+
+// Fine enough to check it won't crash.
+// CHECK-NOT: init
+// CHECK-NOT: static_initializer
+// CHECK: define {{.*}}@core(