Adjusting CMake compiler commands to account for weirdness

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I'm working with a third-party-modified version of clang 7.0.0 that has hijacked the -c option, replacing it with a flag -ccdsp for... reasons...

This has created two problems for CMake:

  1. The -ccdsp option seems to be required to be the very first option to clang, before anything else, including -D... and -I... parameters. There is a BEFORE modifier for target_compile_options that I can use to make sure it appears at the start of the option list, but I don't seem to have any control over the ordering of non-options like definitions and include-paths, which results in -ccdsp occurring later in the overall set of parameters. I haven't been able to find a way to ensure that this is the very first option using modern CMake methods. Instead I have had to resort to setting CMAKE_CXX_FLAGS explicitly.

  2. Now that the -c option to specify a source file is no longer available, I need to find a way to get CMake to omit it. It turns out that, as far as I can tell, this flag isn't really required, as clang -c foo.cpp -o foo.o and clang foo.cpp -o foo.o appear to do much the same thing. So I need a way to get CMake to drop the -c option.

I realise that this is an unusual question and it's a situation entirely created by the organisation that provided the modified version of clang. Unfortunately my support requests to them remain (to date) unanswered.

Does anyone know of a way I can work around this issue? Can I create a custom compiler handler in CMake, based on clang, where I add -ccdsp and remove -c somehow?

cmake c++
1 Answers

In CMake a compiler's command line is specified in CMAKE_<LANG>_COMPILE_OBJECT variable.

You may set this variable in a separate file:

my_clang_override.cmake:

set(CMAKE_CXX_COMPILE_OBJECT "<CMAKE_CXX_COMPILER> -ccdsp <FLAGS> -o <OBJECT>")

and include this file into the project's CMakeLists.txt by specifying CMAKE_USER_MAKE_RULES_OVERRIDE variable with either of two ways:

  1. Specify the variable in the CMakeLists.txt itself, before the project() call:

    CMakeLists.txt:

     # Assume file 'my_clang_override.cmake' to be in the project's source directory.
 set(CMAKE_USER_MAKE_RULES_OVERRIDE "my_clang_override.cmake")

  2. Pass the variable's setting to cmake when configure the project:

    cmake -DCMAKE_USER_MAKE_RULES_OVERRIDE=<path/to/my_clang_override.cmake> <other-parameters>
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