Support GCC C++20+ modules.

example/CMakeLists.txt

@@ -14,8 +14,10 @@ message(STATUS "")
 message(STATUS "C++ latest standard  = ${std_latest_ver}")
 message(STATUS "    modules support  = ${cxx_modules_latest}")
 
+set(MAPPER_FILE ${CMAKE_BINARY_DIR}/mapper.txt)
+
 # Module library
-add_module_library(hello hello.cc FALLBACK hello_fallback.cc)
+add_module_library(hello hello.cc FALLBACK hello_fallback.cc MAPPER_FILE ${MAPPER_FILE})
 target_include_directories(hello PUBLIC include)
 
 # Demo application

mapper.cmake

@@ -0,0 +1,15 @@
+if(NOT(EXISTS ${MAPPER_FILE}))
+  # create file if doesn't exist
+  write_file(${MAPPER_FILE} "") 
+endif()
+FILE(GLOB children RELATIVE ${CMAKE_CURRENT_BINARY_DIR}/gcm.cache/ ${CMAKE_CURRENT_BINARY_DIR}/gcm.cache/*)
+FOREACH(child ${children})
+  IF(NOT (IS_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/gcm.cache/${child}))
+    # remove module from mapper file if exists, then append new module to mapper file
+    get_filename_component(inc ${child} NAME_WE)
+    file(READ ${MAPPER_FILE} file_data)
+    string(REGEX REPLACE "[ \t]*${inc}[ \t]+[^\n]+\n" "" file_data ${file_data})
+    set(file_data "${file_data}\n${inc} ${CMAKE_CURRENT_BINARY_DIR}/gcm.cache/${child}\n")
+    file(WRITE ${MAPPER_FILE} "${file_data}")
+  ENDIF()
+ENDFOREACH()

modules.cmake

@@ -129,8 +129,9 @@ endfunction()
 # Usage:
 #   add_module_library(<name> [sources...] FALLBACK [sources...] [IF enabled])
 function(add_module_library name)
-  cmake_parse_arguments(AML "" "IF" "FALLBACK" ${ARGN})
+  cmake_parse_arguments(AML "" "IF;MAPPER_FILE" "FALLBACK" ${ARGN})
   set(sources ${AML_UNPARSED_ARGUMENTS})
+  set(mapper_file ${AML_MAPPER_FILE})
 
   add_library(${name})
   set_target_properties(${name} PROPERTIES LINKER_LANGUAGE CXX)
@@ -203,6 +204,47 @@ function(add_module_library name)
     endforeach ()
   endif ()
 
+
+  if (CMAKE_COMPILER_IS_GNUCXX)
+    get_target_property(std ${name} CXX_STANDARD)
+
+    # Propagate -fmodule-mapper=mapper_file to targets that link with this library.
+    target_compile_options(
+      ${name} INTERFACE -fmodule-mapper=${MAPPER_FILE})
+
+
+    # Add .gcm/.o files as sources to make sure they are built before the library.
+    set(sources_in ${sources})
+    set(sources)
+    set(gcms)
+    foreach (src ${sources_in})
+      get_filename_component(src_we ${src} NAME_WE)
+      set(obj ${src_we}.o)
+      set(sources ${sources} ${CMAKE_CURRENT_BINARY_DIR}/${obj})
+      set(gcm ${CMAKE_CURRENT_BINARY_DIR}/gcm.cache/${src_we}.gcm)
+      set(gcms ${gcms} ${gcm})
+      add_custom_command(
+        OUTPUT ${obj} ${gcm}
+        COMMAND ${CMAKE_CXX_COMPILER} $<TARGET_PROPERTY:${name},COMPILE_OPTIONS>
+        -std=c++${std} -fmodules-ts
+        -xc++ -c -o ${obj} ${CMAKE_CURRENT_SOURCE_DIR}/${src}
+        # Required by the generator expression above.
+        COMMAND_EXPAND_LISTS
+        DEPENDS ${src})
+    endforeach ()
+
+    # Generate moduler mapper `-fmodule-mapper`
+    add_custom_command(
+      OUTPUT ${mapper_file}
+      COMMAND ${CMAKE_COMMAND} -DCMAKE_CURRENT_BINARY_DIR=${CMAKE_CURRENT_BINARY_DIR} -DMAPPER_FILE=${mapper_file} -P ${CMAKE_CURRENT_LIST_DIR}/../mapper.cmake
+      # Required by the generator expression above.
+      COMMAND_EXPAND_LISTS
+      DEPENDS ${gcms})
+
+    set_source_files_properties(
+      ${sources} PROPERTIES OBJECT_DEPENDS ${mapper_file})
+  endif ()
+
   target_sources(${name} PRIVATE ${sources})
 
   if (MSVC)