In OSS mode omit some extern template specializations. We have seen r…

…eports of

compilers falling over due to the size of translation units.

PiperOrigin-RevId: 549653990

src/google/protobuf/compiler/cpp/file.cc

@@ -1242,11 +1242,18 @@ class FileGenerator::ForwardDeclarations {
   }
 
   void PrintTopLevelDecl(io::Printer* p, const Options& options) const {
-    for (const auto& c : classes_) {
-      p->Emit({{"class", QualifiedClassName(c.second, options)}}, R"cc(
-        template <>
-        $dllexport_decl $$class$* Arena::CreateMaybeMessage<$class$>(Arena*);
-      )cc");
+    if (ShouldGenerateExternSpecializations(options)) {
+      for (const auto& c : classes_) {
+        // To reduce total linker input size in large binaries we make these
+        // functions extern and define then in the pb.cc file. This avoids bloat
+        // in callers by having duplicate definitions of the template.
+        // However, it increases the size of the pb.cc translation units so it
+        // is a tradeoff.
+        p->Emit({{"class", QualifiedClassName(c.second, options)}}, R"cc(
+          template <>
+          $dllexport_decl $$class$* Arena::CreateMaybeMessage<$class$>(Arena*);
+        )cc");
+      }
     }
   }
 

src/google/protobuf/compiler/cpp/helpers.h

@@ -1008,6 +1008,14 @@ void GenerateUtf8CheckCodeForCord(io::Printer* p, const FieldDescriptor* field,
                                   const Options& options, bool for_parse,
                                   absl::string_view parameters);
 
+inline bool ShouldGenerateExternSpecializations(const Options& options) {
+  // For OSS we omit the specializations to reduce codegen size.
+  // Some compilers can't handle that much input in a single translation unit.
+  // These specializations are just a link size optimization and do not affect
+  // correctness or performance, so it is ok to omit them.
+  return !options.opensource_runtime;
+}
+
 struct OneOfRangeImpl {
   struct Iterator {
     using iterator_category = std::forward_iterator_tag;

src/google/protobuf/compiler/cpp/message.cc

@@ -2953,12 +2953,14 @@ void MessageGenerator::GenerateSourceInProto2Namespace(io::Printer* p) {
   auto v = p->WithVars(ClassVars(descriptor_, options_));
   auto t = p->WithVars(MakeTrackerCalls(descriptor_, options_));
   Formatter format(p);
-  format(
-      "template<> "
-      "PROTOBUF_NOINLINE $classtype$*\n"
-      "Arena::CreateMaybeMessage< $classtype$ >(Arena* arena) {\n"
-      "  return Arena::CreateMessageInternal< $classtype$ >(arena);\n"
-      "}\n");
+  if (ShouldGenerateExternSpecializations(options_)) {
+    format(
+        "template<> "
+        "PROTOBUF_NOINLINE $classtype$*\n"
+        "Arena::CreateMaybeMessage< $classtype$ >(Arena* arena) {\n"
+        "  return Arena::CreateMessageInternal< $classtype$ >(arena);\n"
+        "}\n");
+  }
 }
 
 void MessageGenerator::GenerateClear(io::Printer* p) {