On Sun, Nov 29, 2015, 10:47 Dan Liew <d...@su-root.co.uk> wrote: Hi,
# TL;DR I need a way of determining the header file dependencies of a source file and inform CMake about them. CMake doesn't do this automatically because I'm using custom commands for the compilation step so CMake doesn't do it's usual magic of automatically inferring source file header dependencies. This is because this part of the compilation step requires a separate C++ compiler (completely independent from the one that CMake detects at configure time). Is there a way of doing this that will also regenerate the dependencies if the source file changes? # Long version A C++ project that I work on [1] is (amongst other things) a compiler. In order to compile applications it needs to link in a supporting runtime that is compiled to LLVM bitcode which is linked into the applications it compiles. The supporting runtime is written in C++ and compiled with Clang. The compilation of the runtime is currently achieved using ``add_custom_command()`` and so I am not using CMake's in built support for detecting header file dependencies. The reason for doing it this way is because the LLVM bitcode compiler (i.e. Clang) **is not** the same compiler as the host C++ compiler for the project. For example the host code might be built with MSVC but the supporting runtime is **always** built with Clang. To see this concretely take a look at [2]. The build works correctly if you build from a clean build directory. It does not work correctly if you perform a rebuild and change one of the header files that the supporting runtime depends on. This is because CMake doesn't know that the runtime source files depend on the header files and so doesn't rebuild the relevant source files. I can obviously tell CMake about these manually (adding more entries under ``DEPENDS`` in the ``add_custom_command()`` invocation) but this is very cumbersome. What I really need is a way to * Automatically infer the dependencies of a source file and tell CMake about them * Have the dependencies automatically regenerated if the source file changes (someone could add or remove a header file include). In a simple Makefile build system this typically achieved by doing something like this: ``` all:: $(SRCS:.cpp:.o) SRCS := foo.cpp bar.cpp # Include SRC file dependencies generated by the compiler if they exist -include $(SRCs:.cpp=.d) %.o : %.cpp $(CXX) -c $< -o $@ -MP -MMD -MF $*.d ``` Note the ``-M*`` flags get the compiler when it runs to generate additional makefile rules that will get included next time a build happens. I don't really know how to do the same thing with CMake. One idea is at configure time invoke Clang with the ``-MP -MMD -MF`` flags on each runtime source file, extract the dependencies then pass them to ``DEPENDS`` in ``add_custom_command()``. If I wanted the dependencies regenerated if the runtime source file changes then I would need to somehow get CMake to reconfigure every time this happens. I don't like this idea very much due to * Having to invoke Clang manually to determine the dependencies. CMake already knows how to determine source file dependencies, but this functionality (AFAIK) isn't exposed to me. * Reconfiguring every time one of the runtime source file changes is annoying (configuring can be slow sometimes). Does anyone have any other ideas? CMake obviously knows how to do all this stuff already for source files being compiled for the detected host C++ compiler, I just don't know how to get at this logic for source files that need to be built with a second independent C++ compiler. [1] https://github.com/halide/Halide [2] https://github.com/halide/Halide/blob/master/src/CMakeLists.txt#L140 Thanks, Dan. Hi Dan, Not going into detail as I'm typing on the phone, but this really sounds like a case where a "SuperBuild" ( http://www.kitware.com/media/html/BuildingExternalProjectsWithCMake2.8.html) can help you to simplify things a lot. Michael
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