Appending compiler flags to a file with CMake

Try this:

set_property(SOURCE MyFile.cpp APPEND PROPERTY CMAKE_CXX_FLAGS "-msse4.1")

By the way, a few properties are always appended, for example, COMPILE_FLAGS. For those you don't need to do anything special, just set them and they get appended :)

You're almost there, this should work:

set_property(SOURCE MyFile.cpp APPEND PROPERTY CMAKE_CXX_FLAGS -msse4.1)

The kind-specific helpers (like set_source_files_properties()) can be handy at times, but they have a very simiplified interface. For non-trivial things, you have to use set_property(). I've found that I actually rarely use the helpers at all.

For CMake 3.0 or later, use the COMPILE_OPTIONS property to add a flag to a single translation unit, i.e.:

set_property(SOURCE MyFile.cpp APPEND PROPERTY COMPILE_OPTIONS "-msse4.1")

For earlier versions of CMake, use the COMPILE_FLAGS property. COMPILE_FLAGS is a string property. Therefore the correct way to append additional options to it is to use the APPEND_STRING variant of the set_property command:

set_property(SOURCE MyFile.cpp APPEND_STRING PROPERTY COMPILE_FLAGS " -msse4.1 ")