[CMake] [cmake-developers] Obtaining header file dependencies of a source file manually

[Petr Kmoch]

Hi Dan,

you could look into the IMPLICIT_DEPENDS argument of add_custom_command:
https://cmake.org/cmake/help/latest/command/add_custom_command.html

I don't have direct experience with it, but it looks like it could do what
you're looking for.

Petr

On Sun, Nov 29, 2015 at 10:47 AM, Dan Liew <dan at 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.
> --
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