[cfe-dev] [clangd-dev] Tooling RFC: Find build root from source root with a symlink?

Sam McCall via cfe-dev cfe-dev at lists.llvm.org
Thu Oct 11 08:59:29 PDT 2018


On Thu, Oct 11, 2018 at 5:40 PM Doug Schaefer <dschaefer at blackberry.com>
wrote:

> +1, been thinking about this as well as I end up setting
> –compile-commands-dir all the time as a big user of cmake. And I’m on
> Windows where symlinks are generally suspect. clangd doing this for us
> would be definitely helpful.
>
Just trying to unpack this a bit:
 - today, it's possible to get this to work by symlinking
src/compile_commands.json -> build/compile_commands.json. I'm proposing to
generalize this by supporting symlinking src/buildroot -> build/. But if
symlinks don't work for you now, I don't think this would fix that problem.
 - you could indeed put your physical build dir in src/buildroot, though
obviously this limits your flexibility.

Though, one for the bikeshed. As an example, the llvm .gitignore suggests
> using /build, as do many other projects. I’m just wondering if the cost of
> conflicts outweighs the convenience of not having to change.
>
So we could check src/build and if there's no CDB there, fall back to
checking src/.
The problem is I don't know what projects that have an existing src/build
and also use an external build dir are supposed to do. Chromium is one such
project. The cost is probably *also* adding a fallback to src/buildroot,
which increases the number of stats we need to do when walking up from your
source files to find the root.


>
>
> Doug.
>
>
>
>
>
> *From:* clangd-dev <clangd-dev-bounces at lists.llvm.org> * On Behalf Of *Sam
> McCall via clangd-dev
> *Sent:* Thursday, October 11, 2018 6:42 AM
> *To:* clang developer list <cfe-dev at lists.llvm.org>;
> clangd-dev at lists.llvm.org
> *Subject:* [clangd-dev] Tooling RFC: Find build root from source root
> with a symlink?
>
>
>
> Many projects/build systems have the concept of a "build tree" that can be
> separate from the source tree. (e.g. CMake supports this, and LLVM
> recommends this approach).
>
>
>
> Tools sometimes need to find the build tree.
>
> I'd like to suggest a convention for the Tooling library to support:
>
>     If $SRC/buildroot exists, it's the default build directory for the
> source tree $SRC.
>
>     Otherwise, the build directory is $SRC itself.
>
>     $SRC/buildroot may be a symlink.
>
>     Configuration files like compile_commands.json are searched for in the
> build directory.
>
> (Bikeshedding the "buildroot" string is welcome. "build" will conflict too
> often, sadly).
>
>
>
> Implications:
>
>  - for /foo/bar/baz.cc, we'd search for compile_commands.json in
> {/foo/bar/buildroot/,/foo/bar/, /foo/buildroot/, etc}. This is
> backwards-compatible.
>
>  - where users currently symlink compile_commands.json itself, they could
> create the buildroot symlink instead. This would work in the same way, and
> enable new cases.
>
>  - if we implement e.g. a Ninja-backed compilation DB that doesn't need
> compile_commands.json, the same symlink convention would work.
>
>  - it provides a simple model for multi-configuration-aware tools: a
> configuration is defined by a build dir, there's a default configuration,
> tools can let the user override the build dir. e.g. clangd can write its
> index files into the build dir instead of the source dir, which is
> multi-configuration-friendly (tinyurl.com/clangd-automatic-index)
>
>  - non-clang tools that consume compile_commands.json will need to be
> updated over time.
>
>
>
> What do you think?
>
> Cheers, Sam
>
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