LLVM toolchain for Bazel

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:configure.bzl", "llvm_toolchain")

llvm_toolchain(
    name = "llvm_toolchain",
    llvm_version = "6.0.0",
)

The in-built dictionary of URLs is good for Ubuntu 16.04 and macOS 10.13. For other environments, provide your own sources through the urls attribute.

For making changes to default settings for these toolchains, edit the CROSSTOOL.tpl file. The file is in ASCII protobuf format.

https://github.com/bazelbuild/bazel/wiki/About-the-CROSSTOOL

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.
Sandboxing the toolchain introduces a significant overhead (100ms per action). 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. That is why we have templated the 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.
The toolchain is known to also work with rules_go, both with absolute and relative paths.
There is no R toolchain yet, so the Makevars file has been repurposed to provide the right configuration, but only when using cc_deps attribute of r_pkg. All packages not using cc_deps are free to configure themselves. The Makevars file provided will always use absolute paths. This implies that the build actions are not reproducible across multiple workspaces, although the build outputs can be.
The toolchain is almost hermetic but borrows system headers and libraries from the user's system. If needed, we can package a sysroot for our build environment and set builtin_sysroot in CROSSTOOL.

Other examples of toolchain configuration:

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

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

promiseofcake/bazel-toolchain

LLVM toolchain for Bazel