Split generated unity build translation units into volumes of max 500 real translation units each

Author: SergeyRyabinin

cmake/utilities.cmake

@@ -11,24 +11,54 @@ endmacro()
 
 # this function is based on the unity build function described at: https://cheind.wordpress.com/2009/12/10/reducing-compilation-time-unity-builds/
 function(enable_unity_build UNITY_SUFFIX SOURCE_FILES)
+    set(MAX_TRANSLATION_UNITS_PER_UNITY_UNIT 500)
+
     set(files ${${SOURCE_FILES}})
+    list(LENGTH files length)
+
+    set(UNITY_FILE_COUNT "")
+    if (length GREATER ${MAX_TRANSLATION_UNITS_PER_UNITY_UNIT})
+        set(UNITY_FILE_COUNT 1)
+    endif()
+
+    set(SOURCE_FILE_COUNT 0)
 
     # Generate a unique filename for the unity build translation unit
-    set(unity_build_file ${CMAKE_CURRENT_BINARY_DIR}/ub_${UNITY_SUFFIX}.cpp)
+    set(unity_build_file ${CMAKE_CURRENT_BINARY_DIR}/ub${UNITY_FILE_COUNT}_${UNITY_SUFFIX}.cpp)
 
     # Exclude all translation units from compilation
     set_source_files_properties(${files} PROPERTIES HEADER_FILE_ONLY true)
 
     # Open the ub file
-    FILE(WRITE ${unity_build_file} "// Unity Build generated by CMake\n")
+    if (length GREATER ${MAX_TRANSLATION_UNITS_PER_UNITY_UNIT})
+        FILE(WRITE ${unity_build_file} "// Unity Build Volume ${UNITY_FILE_COUNT} generated by CMake\n")
+    else()
+        FILE(WRITE ${unity_build_file} "// Unity Build generated by CMake\n")
+    endif()
+
+    # Complement list of translation units with the name of ub
+    list(APPEND ${SOURCE_FILES} "${unity_build_file}")
+    set(${SOURCE_FILES} ${${SOURCE_FILES}} PARENT_SCOPE)
 
-    # Add include statement for each translation unit
     foreach(source_file ${files} )
+        if(SOURCE_FILE_COUNT EQUAL ${MAX_TRANSLATION_UNITS_PER_UNITY_UNIT})
+            MATH(EXPR UNITY_FILE_COUNT "${UNITY_FILE_COUNT}+1")
+            set(SOURCE_FILE_COUNT 0)
+
+            set(unity_build_file ${CMAKE_CURRENT_BINARY_DIR}/ub${UNITY_FILE_COUNT}_${UNITY_SUFFIX}.cpp)
+            # Open the ub file
+            FILE(WRITE ${unity_build_file} "// Unity Build Volume ${UNITY_FILE_COUNT} generated by CMake\n")
+
+            # Complement list of translation units with the name of ub
+            list(APPEND ${SOURCE_FILES} "${unity_build_file}")
+            set(${SOURCE_FILES} ${${SOURCE_FILES}} PARENT_SCOPE)
+        endif()
+
+        # Add include statement for each translation unit
         FILE( APPEND ${unity_build_file} "#include <${source_file}>\n")
-    endforeach(source_file)
 
-    # Complement list of translation units with the name of ub
-    set(${SOURCE_FILES} ${${SOURCE_FILES}} ${unity_build_file} PARENT_SCOPE)
+        MATH(EXPR SOURCE_FILE_COUNT "${SOURCE_FILE_COUNT}+1")
+    endforeach(source_file)
 endfunction(enable_unity_build)
 
 macro(setup_install)