[cmake-developers] Using a custom DSL compiler in CMake

[Brad King]

On 06/21/2016 02:18 PM, Chris Bieneman wrote:
> Our current solution to work with this tool in CMake

For reference, a quick look at LLVM's `cmake/modules/TableGen.cmake`
indicates that the current solution is based on add_custom_command.

> The harder part is that our DSL supports C-like includes.

The add_custom_command signature has an IMPLICIT_DEPENDS option
for this, but it only works for Makefile generators.

> Our DSL compiler can generate .d files, but hooking that up to
> CMake is a harder problem.

With some work it may be possible to teach our Ninja generator how
to consume .d files in custom commands.  Perhaps such support could
even be extended to the Makefile generators instead of using our
approximate scanning implementation.

In general implicit dependencies cannot be cleanly supported by
the Visual Studio or Xcode generators.  Those environments provide
no hooks for custom build-time dependency scanning.  At best one
can have the custom command produce a file as a side effect that
can be consumed by CMake on the next run.  This would work for
all generators but causes lots of CMake re-runs.

> My thought was to try and treat TableGen as a language.
> There are some complications with that because we don’t
> actually have a compiler for it at configuration time.

I don't think that is a good approach both because we won't
be able to enable the language without the compiler already
existing and because custom languages do not work well in
the IDE generators (and will have the same dependency scanning
problems as above).  See above approaches instead.

-Brad