Fix overwriting of unity files in reconfiguration.

One flaw in the unity build system is that it always generates the unity
inclusion files when the build system is (re)configured. This results in a lot
of extra compilation.

This patch fixes the issue by first writing to a temporary file and then
moving it only if it is not equal to the current unity file.

diff --git a/cmake/macros/macro_setup_unity_target.cmake b/cmake/macros/macro_setup_unity_target.cmake
index 9293065d1fb9dd5a3d6af92755c0b8fa403e2a27..230bb0aab6f4a3e3bc02268b5ca34cdd136f0609 100644 (file)
--- a/cmake/macros/macro_setup_unity_target.cmake
+++ b/cmake/macros/macro_setup_unity_target.cmake
@@ -46,6 +46,8 @@ MACRO(SETUP_UNITY_TARGET _unity_include_src _n_includes_per_unity_file _output_s
 
   FOREACH(_unity_prefix ${_unity_prefixes})
     SET(_unity_file_name "${CMAKE_CURRENT_BINARY_DIR}/${_unity_prefix}.cc")
+    SET(_tmp_unity_file_name "${CMAKE_CURRENT_BINARY_DIR}/${_unity_prefix}_tmp.cc")
+
     #
     # Note the CMake weirdness: _output_src is actually a string containing the
     # variable name since we are in a macro...
@@ -54,13 +56,21 @@ MACRO(SETUP_UNITY_TARGET _unity_include_src _n_includes_per_unity_file _output_s
       ${${_output_src}}
       ${_unity_file_name}
       )
-    FILE(WRITE "${_unity_file_name}"
+    FILE(WRITE "${_tmp_unity_file_name}"
 "// This file was automatically generated by the deal.II CMake configuration as
 // part of the unity build subsystem. Please do not edit this file directly;
 // instead, make appropriate changes to the relevant CMakeLists.txt file.\n\n"
       )
     FOREACH(_unity_file ${${_unity_prefix}})
-      FILE(APPEND "${_unity_file_name}" "#include \"${_unity_file}\"\n")
+      FILE(APPEND "${_tmp_unity_file_name}" "#include \"${_unity_file}\"\n")
     ENDFOREACH()
+
+    #
+    # CMake regenerates all unity files as part of the configuration process,
+    # even though they usually have not changed. Avoid some recompilation by
+    # using the CONFIGURE_FILE command, which won't update the time stamp unless
+    # the content has changed.
+    #
+    CONFIGURE_FILE(${_tmp_unity_file_name} ${_unity_file_name} COPYONLY)
   ENDFOREACH()
 ENDMACRO()