LLVM toolchain for Bazel

NOTE: As of 2200d53, this project requires bazel 1.0 or up.

To use this toolchain, include this section in your WORKSPACE:

# Change master to the git tag you want.
http_archive(
    name = "com_grail_bazel_toolchain",
    strip_prefix = "bazel-toolchain-master",
    urls = ["https://github.com/grailbio/bazel-toolchain/archive/master.tar.gz"],
)

load("@com_grail_bazel_toolchain//toolchain:deps.bzl", "bazel_toolchain_dependencies")

bazel_toolchain_dependencies()

load("@com_grail_bazel_toolchain//toolchain:rules.bzl", "llvm_toolchain")

llvm_toolchain(
    name = "llvm_toolchain",
    llvm_version = "8.0.0",
)

load("@llvm_toolchain//:toolchains.bzl", "llvm_register_toolchains")

llvm_register_toolchains()

The toolchain can automatically detect your OS type, and use the right pre-built binary distribution from llvm.org. The detection is currently based on host OS and is not perfect, so some distributions, docker based sandboxed builds, and remote execution builds will need toolchains configured manually through the distribution attribute. We expect the detection logic to grow through community contributions. We welcome PRs! 😄

See in-code documentation in rules.bzl for available attributes to llvm_toolchain.

For making changes to default settings for these toolchains, edit the cc_toolchain_config template. See tutorial.

For overriding toolchains on the command line, please use the --extra_toolchains flag in lieu of the deprecated --crosstool_top flag. For example, --extra_toolchains=@llvm_toolchain//:cc-toolchain-linux.

If you would like to use the older method of selecting toolchains, you can continue to do so with --crosstool_top=@llvm_toolchain//:toolchain.

Notes:

The LLVM toolchain archive is downloaded and extracted in the named repository. People elsewhere have used wrapper scripts to avoid symlinking and get better control of the environment in which the toolchain binaries are run.
A sysroot can be specified through the sysroot attribute. This can be either a path on the user's system, or a bazel filegroup like label. One way to create a sysroot is to use docker export to get a single archive of the entire filesystem for the image you want. Another way is to use the build scripts provided by the Chromium project.
Sandboxing the toolchain introduces a significant overhead (100ms per action, as of mid 2018). To overcome this, one can use --experimental_sandbox_base=/dev/shm. However, not all environments might have enough shared memory available to load all the files in memory. If this is a concern, you may set the attribute for using absolute paths, which will substitute templated paths to the toolchain as absolute paths. When running bazel actions, these paths will be available from inside the sandbox as part of the / read-only mount. Note that this will make your builds non-hermetic.
The toolchain is known to also work with rules_go.
The LLVM toolchain also provides several tools like clang-format. You can depend on these tools directly in the bin directory of the toolchain. For example, @llvm_toolchain//:bin/clang-format is a valid and visible target.

Other examples of toolchain configuration:

https://github.com/bazelbuild/bazel/wiki/Building-with-a-custom-toolchain

https://github.com/vsco/bazel-toolchains

FKint/bazel-toolchain

LLVM toolchain for Bazel