Enable Unity(Jumbo) builds (#16454)

**Description**
Added a cmake option to enable Unity builds for libprotobuf-lite.
Added missing symmetric include for port_def.inc
Added preproc guards for PROTOBUF_PRAGMA_INIT_SEG so it does not get included multiple times in unity build.

**Motivation and Context**
For large code base, especially in the game dev industry, Unity builds (https://en.wikipedia.org/wiki/Unity_build) are used to speed up the compilations.
Special care must be taken to enable Unity builds for a project, since many translation units will be merged togheter.

Closes #16454

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/16454 from jslap-ubi:enable-unity-build a387ffd862
PiperOrigin-RevId: 646581094

CMakeLists.txt

@@ -35,6 +35,9 @@ option(protobuf_BUILD_LIBUPB "Build libupb" ON)
 option(protobuf_DISABLE_RTTI "Remove runtime type information in the binaries" OFF)
 option(protobuf_TEST_XML_OUTDIR "Output directory for XML logs from tests." "")
 option(protobuf_ALLOW_CCACHE "Adjust build flags to allow for ccache support." OFF)
+
+# We support Unity (Jumbo) builds best-effort.
+option(protobuf_USE_UNITY_BUILD "Enable Unity (Jumbo) build for" OFF)
 if (BUILD_SHARED_LIBS)
   set(protobuf_BUILD_SHARED_LIBS_DEFAULT ON)
 else (BUILD_SHARED_LIBS)

cmake/libprotobuf-lite.cmake

@@ -29,6 +29,7 @@ target_include_directories(libprotobuf-lite PUBLIC
 )
 target_link_libraries(libprotobuf-lite PUBLIC ${protobuf_ABSL_USED_TARGETS})
 protobuf_configure_target(libprotobuf-lite)
+protobuf_configure_unity_target(libprotobuf-lite)
 if(protobuf_BUILD_SHARED_LIBS)
   target_compile_definitions(libprotobuf-lite
     PUBLIC  PROTOBUF_USE_DLLS

cmake/protobuf-configure-target.cmake

@@ -52,4 +52,21 @@ function(protobuf_configure_target target)
     if (HAVE_ZLIB)
         target_compile_definitions("${target}" PRIVATE -DHAVE_ZLIB)
     endif ()
+
+
+endfunction ()
+
+function(protobuf_configure_unity_target target)
+    if(protobuf_USE_UNITY_BUILD)
+        # UNITY_BUILD available in CMake >=3.18.
+        if(${CMAKE_VERSION} VERSION_GREATER 3.18 OR ${CMAKE_VERSION} VERSION_EQUAL 3.18)
+            # If protobuf_USE_UNITY_BUILD is set to ON, set target to use Unity builds.
+            set_target_properties("${target}"
+                PROPERTIES 
+                UNITY_BUILD ON
+                UNITY_BUILD_MODE BATCH
+                UNITY_BUILD_BATCH_SIZE 50
+            )
+        endif()
+    endif()
 endfunction ()

cmake/tests.cmake

@@ -103,6 +103,7 @@ add_executable(tests ${tests_files} ${common_test_files})
 if (MSVC)
   target_compile_options(tests PRIVATE
     /wd4146 # unary minus operator applied to unsigned type, result still unsigned
+    /bigobj
   )
 endif()
 target_link_libraries(tests

src/google/protobuf/arenaz_sampler.cc

@@ -221,3 +221,5 @@ void SetThreadSafeArenazGlobalNextSample(int64_t next_sample) {}
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google
+
+#include "google/protobuf/port_undef.inc"

src/google/protobuf/generated_enum_util.cc

@@ -219,3 +219,5 @@ std::vector<uint32_t> GenerateEnumData(absl::Span<const int32_t> values) {
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google
+
+#include "google/protobuf/port_undef.inc"

src/google/protobuf/generated_message_tctable_lite.cc

@@ -2938,3 +2938,5 @@ const char* TcParser::DiscardEverythingFallback(PROTOBUF_TC_PARAM_DECL) {
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google
+
+#include "google/protobuf/port_undef.inc"

src/google/protobuf/generated_message_util.cc

@@ -28,7 +28,10 @@
 // Must be included last
 #include "google/protobuf/port_def.inc"
 
+#ifndef PROTOBUF_PRAGMA_INIT_SEG_DONE
 PROTOBUF_PRAGMA_INIT_SEG
+#define PROTOBUF_PRAGMA_INIT_SEG_DONE
+#endif
 
 
 namespace google {

src/google/protobuf/implicit_weak_message.cc

@@ -14,7 +14,12 @@
 // Must be included last.
 #include "google/protobuf/port_def.inc"
 
+// Since we could be merging Translation units, we must check if this was done
+// before.
+#ifndef PROTOBUF_PRAGMA_INIT_SEG_DONE
 PROTOBUF_PRAGMA_INIT_SEG
+#define PROTOBUF_PRAGMA_INIT_SEG_DONE
+#endif
 
 namespace google {
 namespace protobuf {

src/google/protobuf/inlined_string_field.cc

@@ -116,3 +116,5 @@ void InlinedStringField::ClearToDefault(const LazyString& default_value,
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google
+
+#include "google/protobuf/port_undef.inc"

src/google/protobuf/io/zero_copy_stream.cc

@@ -119,3 +119,5 @@ bool ZeroCopyOutputStream::WriteAliasedRaw(const void* /* data */,
 }  // namespace io
 }  // namespace protobuf
 }  // namespace google
+
+#include "google/protobuf/port_undef.inc"

src/google/protobuf/io/zero_copy_stream_impl_lite.cc

@@ -683,3 +683,5 @@ absl::Cord CordOutputStream::Consume() {
 }  // namespace io
 }  // namespace protobuf
 }  // namespace google
+
+#include "google/protobuf/port_undef.inc"

src/google/protobuf/port.cc

@@ -27,3 +27,5 @@ void protobuf_assumption_failed(const char* pred, const char* file, int line) {
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google
+
+#include "google/protobuf/port_undef.inc"

src/google/protobuf/raw_ptr.cc

@@ -23,3 +23,5 @@ ABSL_CONST_INIT PROTOBUF_EXPORT
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google
+
+#include "google/protobuf/port_undef.inc"