On 02/06/2018 03:19 AM, Claus Klein wrote: > IMHO: This make ninja slower without much user benefit!
For reference, the change was made for CMake 3.6 here: https://gitlab.kitware.com/cmake/cmake/commit/6d74e7870b8804a5af0bc395a9fbb45c1b3d26a4 The benefit is that sources recompile when system headers change after an update to system packages. During discussion of the change from -MMD to -MD, it was claimed that the performance impact was minimal: https://cmake.org/Bug/view.php?id=14914#c40684 Maybe that's true in some environments but not others. > On Thu, Nov 30, 2017 at 12:09:55 -0800, Evan Martin wrote: >> That CMake change is interesting. While it's true that it is more correct >> to check the state of the system headers, it's also true that to be fully >> correct here you need a completely hermetic build. If CMake doesn't also >> check the compiler binary as well as any shared libraries that the compiler >> binary uses, then I think using this extra flag only serves to make ninja >> slower without much user benefit. Robustness to some system updates is still valuable even without being robust to all system updates. Some people want system headers checked for $reasons, and others don't want them checked for $other_reasons. The only way to satisfy both groups is to make it an option. The internal `CMAKE_DEPFILE_FLAGS_${lang}` table of flags used for this would need to be extended with `CMAKE_DEPFILE_NOSYSTEM_FLAGS_${lang}` alternatives. Some option would need to be introduced to choose. -Brad -- Powered by www.kitware.com Please keep messages on-topic and check the CMake FAQ at: http://www.cmake.org/Wiki/CMake_FAQ Kitware offers various services to support the CMake community. For more information on each offering, please visit: CMake Support: http://cmake.org/cmake/help/support.html CMake Consulting: http://cmake.org/cmake/help/consulting.html CMake Training Courses: http://cmake.org/cmake/help/training.html Visit other Kitware open-source projects at http://www.kitware.com/opensource/opensource.html Follow this link to subscribe/unsubscribe: https://cmake.org/mailman/listinfo/cmake-developers
