[NFC] [Serialization] Reordering lexcical and visible TU block after type decl offsets

This patch reorder the lexical block for the translation unit, visible update block for the TU and
the viisble upaete block for the extern C context after the type decl
offsets block.

This should be a NFC patch.

This is helpful for later optimizations for eliding unreachable
declarations in the global module fragment. See the comments in
#76930.

Simply, if we want to get the reachable sets of declaratins during the
writing process, we need to write the file-level context later than the
process of writing declarations (which is the main process to determine
the reachable set).

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriter.cpp

@@ -4959,38 +4959,12 @@ ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
     Stream.EmitRecord(METADATA_OLD_FORMAT, Record);
   }
 
-  // Create a lexical update block containing all of the declarations in the
-  // translation unit that do not come from other AST files.
   const TranslationUnitDecl *TU = Context.getTranslationUnitDecl();
-  SmallVector<uint32_t, 128> NewGlobalKindDeclPairs;
-  for (const auto *D : TU->noload_decls()) {
-    if (!D->isFromASTFile()) {
-      NewGlobalKindDeclPairs.push_back(D->getKind());
-      NewGlobalKindDeclPairs.push_back(GetDeclRef(D));
-    }
-  }
-
-  auto Abv = std::make_shared<BitCodeAbbrev>();
-  Abv->Add(llvm::BitCodeAbbrevOp(TU_UPDATE_LEXICAL));
-  Abv->Add(llvm::BitCodeAbbrevOp(llvm::BitCodeAbbrevOp::Blob));
-  unsigned TuUpdateLexicalAbbrev = Stream.EmitAbbrev(std::move(Abv));
-  {
-    RecordData::value_type Record[] = {TU_UPDATE_LEXICAL};
-    Stream.EmitRecordWithBlob(TuUpdateLexicalAbbrev, Record,
-                              bytes(NewGlobalKindDeclPairs));
-  }
 
-  // And a visible updates block for the translation unit.
-  Abv = std::make_shared<BitCodeAbbrev>();
-  Abv->Add(llvm::BitCodeAbbrevOp(UPDATE_VISIBLE));
-  Abv->Add(llvm::BitCodeAbbrevOp(llvm::BitCodeAbbrevOp::VBR, 6));
-  Abv->Add(llvm::BitCodeAbbrevOp(llvm::BitCodeAbbrevOp::Blob));
-  UpdateVisibleAbbrev = Stream.EmitAbbrev(std::move(Abv));
-  WriteDeclContextVisibleUpdate(TU);
-
-  // If we have any extern "C" names, write out a visible update for them.
-  if (Context.ExternCContext)
-    WriteDeclContextVisibleUpdate(Context.ExternCContext);
+  // Force all top level declarations to be emitted.
+  for (const auto *D : TU->noload_decls())
+    if (!D->isFromASTFile())
+      GetDeclRef(D);
 
   // If the translation unit has an anonymous namespace, and we don't already
   // have an update block for it, write it as an update block.
@@ -5131,6 +5105,14 @@ ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   for (auto *D : SemaRef.DeclsToCheckForDeferredDiags)
     DeclsToCheckForDeferredDiags.push_back(GetDeclRef(D));
 
+  {
+    auto Abv = std::make_shared<BitCodeAbbrev>();
+    Abv->Add(llvm::BitCodeAbbrevOp(UPDATE_VISIBLE));
+    Abv->Add(llvm::BitCodeAbbrevOp(llvm::BitCodeAbbrevOp::VBR, 6));
+    Abv->Add(llvm::BitCodeAbbrevOp(llvm::BitCodeAbbrevOp::Blob));
+    UpdateVisibleAbbrev = Stream.EmitAbbrev(std::move(Abv));
+  }
+
   RecordData DeclUpdatesOffsetsRecord;
 
   // Keep writing types, declarations, and declaration update records
@@ -5158,6 +5140,35 @@ ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   WriteTypeDeclOffsets();
   if (!DeclUpdatesOffsetsRecord.empty())
     Stream.EmitRecord(DECL_UPDATE_OFFSETS, DeclUpdatesOffsetsRecord);
+
+  // Create a lexical update block containing all of the declarations in the
+  // translation unit that do not come from other AST files.
+  {
+    SmallVector<uint32_t, 128> NewGlobalKindDeclPairs;
+    for (const auto *D : TU->noload_decls()) {
+      if (!D->isFromASTFile()) {
+        NewGlobalKindDeclPairs.push_back(D->getKind());
+        NewGlobalKindDeclPairs.push_back(GetDeclRef(D));
+      }
+    }
+
+    auto Abv = std::make_shared<BitCodeAbbrev>();
+    Abv->Add(llvm::BitCodeAbbrevOp(TU_UPDATE_LEXICAL));
+    Abv->Add(llvm::BitCodeAbbrevOp(llvm::BitCodeAbbrevOp::Blob));
+    unsigned TuUpdateLexicalAbbrev = Stream.EmitAbbrev(std::move(Abv));
+
+    RecordData::value_type Record[] = {TU_UPDATE_LEXICAL};
+    Stream.EmitRecordWithBlob(TuUpdateLexicalAbbrev, Record,
+                              bytes(NewGlobalKindDeclPairs));
+  }
+
+  // And a visible updates block for the translation unit.
+  WriteDeclContextVisibleUpdate(TU);
+
+  // If we have any extern "C" names, write out a visible update for them.
+  if (Context.ExternCContext)
+    WriteDeclContextVisibleUpdate(Context.ExternCContext);
+
   WriteFileDeclIDsMap();
   WriteSourceManagerBlock(Context.getSourceManager(), PP);
   WriteComments();

clang/test/Modules/language-linkage.cppm

@@ -14,5 +14,5 @@ void foo() {}
 
 extern "C" void bar() {}
 
-// CHECK: define {{.*}}@bar(
 // CHECK: define {{.*}}@_Z3foov(
+// CHECK: define {{.*}}@bar(