[CMake] Eliminating spurious dependencies

[Tom Birch]

I'm using add_excecutable to generate a .s file which I then parse to generate a header file. This all works fine and all the dependencies seem to be working, but it gets built every time, even if nothing has changed.

I have a function to generate the asm file:

function (make_asm_file source dest defs flags)
set_source_files_properties(
	${source}
	PROPERTIES
	COMPILE_DEFINITIONS "${${defs}}"
	COMPILE_FLAGS "${${flags}} -S"
	)
add_executable (
	${dest}
	EXCLUDE_FROM_ALL
	${source}
	)
set_target_properties(
	${dest}
	PROPERTIES
	OUTPUT_NAME ${dest}
	RULE_LAUNCH_LINK "sh build_tools/copier.sh ${CMAKE_BINARY_DIR} <OBJECTS> --"
	)
endfunction(make_asm_file)

and I invoke that, and then add a custom command to convert the .s into a .h like this:

make_asm_file(${input_c} ${output_s} c_defs c_flags)

add_custom_command (
	OUTPUT ${header_path}
	COMMAND ./make_header ${header_path} ${dest}
	DEPENDS ${output_s}
	)

and then I use set_source_file_properties(OBJECT_DEPENDS) to make the sources depend on this generated header.

Every time I get type make (even on incremental builds) I get

[  0%] Built target genassym.c.s

but I only see the custom command on a build when the header hasn't previously been generated, i.e.

[  1%] Generating assym.h

So my question is, what is it about this invocation of add_excecutable that makes it always run? Shouldn't EXCLUDE_FROM_ALL get rid of that?

cheers,
Tom