This is an example project showing how it is possible to build an LLVM-based language with Bazel.
The LLVM pre-built binaries are downloaded and exposed at the @llvm Bazel target. This dependency can be used for any target which needs to link against LLVM.
compiler.cpp is a simple compiler built using LLVM which outputs a program which prints the original source code with "foo: " prepended to each line.
The llvm.bzl file contains all the build definitions for downloading and linking against LLVM. It also exposes an llc macro, which is used to transpile the LLVM IR into assembly. That assembly is then put into a cc_binary() which allows it to be assembled into an executable binary. The entire process is wrapped in the foo_binary() macro, which compiles a source file written in foo all the way to an executable binary. The example can be run with:
bazel run //:helloThis example exposes the ficticious foo language as a build step in Bazel and can be expanded upon. For example, tests could be written asserting the output of various input foo code. Another example would linking foo code against against other cc_library() rules as dependencies.
I would love to make this a proper module, something you can just add to your WORKSPACE and get a dependcy on @llvm, unfortunately I don't see a good way of doing that. Instead, I think the best way of using this is to just hard copy llvm.bzl and use this project as an example of how to use it.
Currently, I've only tested this on LLVM 3.9.1, but this can be overridden with any version from releases.llvm.org using the version = "X.X.X" attribute on the llvm repository rule in the WORKSPACE file. You're mileage with any given version may vary. If you manage to get one working, don't hesitate to submit a pull request!
Currently, this downloads the pre-built 64-bit Ubuntu 16.04 version of the binaries, which limits the platform. LLVM is pretty portable, so this could probably be expanded to pick the right architecture based on context.