[Scons-users] CompilationDatabase support for SCons

Gary Oberbrunner garyo at oberbrunner.com
Sun Jan 11 12:33:53 EST 2015

On Sun, Jan 11, 2015 at 11:27 AM, Andrew C. Morrow <
andrew.c.morrow at gmail.com> wrote:

> Hi Gary -
> Thanks for getting back to me.
> The utility of a compilation database
> <http://clang.llvm.org/docs/JSONCompilationDatabase.html> is that it lets
> you capture with fidelity the compile actions that the build system would
> take (given the current options, configure checks, etc), and then load that
> information into other tools. A good overview of why this is needed can be
> found here:
> http://eli.thegreenplace.net/2014/05/21/compilation-databases-for-clang-based-tools
> OK, from that I gather that it's a clang-specific database of which flags
were used to compile which source files -- presumably from the last build.

> For some additional practical examples, here are some examples of things
> that can be done once you have a compilation database (usually named
> compile_commands.json):
> - Run clang-modernize <http://clang.llvm.org/extra/clang-modernize.html> (convert
> C++03 to C++11) and clang-tidy
> <http://clang.llvm.org/extra/clang-tidy.html> (AST aware linter)
> - Configure emacs or VI to have a "compile current buffer" facility, by
> extracting the last compile command for the current buffer (
> https://github.com/randomphrase/ede-compdb)

This in particular seems like a bad idea.  If you tell SCons to recompile
the current buffer (its .o file) it will check for any modified headers,
different compile flags, generated sources that need generating, and so on,
before issuing the compile command -- and that only if needed.  Re-running
the same last command seems at best error-prone, and at worst it could
confuse SCons because it doesn't know you updated the .o outside of it (so
it doesn't get into the repository if you're using one, and so on).

- Drive your own AST level source introspection or transformation tools
> implemented with clang's LibTooling
> <http://clang.llvm.org/docs/LibTooling.html>
> - Simplify configuration of emacs flycheck or VI you-complete-me tools for
> real-time in-editor syntax checking
> I'm sure there are many more I'm not aware of.
> I acknowledge that this is all fairly clang specific, which is why my hope
> had been that it would be possible to generate a compile_commands.json file
> in the build system (by writing a SCons Tool), rather than by the build
> system (needing to implement it as an intrinsic SCons feature, as I believe
> CMake and Ninja do).
> Also, I think it is very important that it be possible to produce the
> compilation database without needing to actually compile the entire source
> tree. For some projects a complete rebuild may be prohibitively expensive,
> but this should not preclude using the above tools. My second attempt above
> does work, but has the unwanted side effect that all objects are built, so
> does not meet this goal.

I'm pretty sure this can't work in the general case.  It's easy with SCons
to create generated headers and sources, for instance, and even introspect
sources (which may be generated) to see what needs to be compiled, and how.
 (My own day-job build does exactly this, by building a precompiler
(written in C++), precompiling a proprietary high-level language into C++
code which is then compiled.) You can't just do an old-school "make -n" and
have it be likely to resemble the real list of commands, except in very
simple, straightforward cases.

In a simple C++-only single-pass build, I can see this working to some
extent.  I suspect the simplest method is to wrap your compiler with a
shell script that emits $* to the database before (or after?) running the
comand, and use that shell script as $CC/$CXX etc.  Modifying the existing
tools as you've started to do could also work, as could overriding the
CC*OMSTR variables perhaps.  However, these will only produce the partial
results of what actually got compiled -- if you change flags or compiler
versions or paths, you could end up with an out-of-sync database.

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