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.

Note: you may need to add build --incompatible_enable_cc_toolchain_resolution to your .bazelrc to enable toolchain resolution for cc toolchains (see this issue). If you do this, the llvm_register_toolchains call in WORKSPACE shown in the example above should be sufficient to get Bazel to use the toolchain.

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.

Setting Up Toolchains for Other Targets

Using extra_targets

load("@com_grail_bazel_toolchain//toolchain:rules.bzl", "llvm_toolchain")
llvm_toolchain(
    name = "llvm_toolchain",
    llvm_version = "8.0.0",
    extra_targets = [
        "wasm32-unknown-wasi",
    ],

    # Extra targets can have their sysroots overriden too:
    sysroots: {
        "linux": "@some_example_sysroot_repo//:linux_sysroot",
        "darwin": "@some_example_sysroot_repo//:macos_sysroot",

        "linux_wasm32-unknown-wasi": "@some_example_sysroot_repo//:wasi_sysroot",
        "darwin_wasm32-unknown-wasi": "@some_example_sysroot_repo//:wasi_sysroot",
    }
)

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

http_archive(
    name = "some_example_sysroot_repo",
    ...
)

The toolchain that is created will have the appropriate constraints so that Bazel will pick it when resolving a toolchain for a particular platform. For example:

platform(
    name = "wasi",
    constraints = [
        "@platforms//os:wasi",
        "@platforms//cpu:wasm32",
    ]
)

cc_library(
    name = "test",
    srcs = [...],
)

Running bazel build //:test --platforms //:wasm should use the configured wasm32-unknown-wasi toolchain and produce an object file with wasm32 assembly in it.

Note that this should work also work with rules that apply a transition to require that a target be built for a particular platform.

Also note that the order of the triples in extra_targets influences how toolchains will be considered during toolchain resolution as does using a target triple ending with -unknown instead of -none (as a rule of thumb, prefer -none over -unknown unless you have a good reason not to). See this comment for some context.

Currently only the following extra targets are implemented/tested at all:

• wasm32-unknown-wasi
• wasm32-unknown-none

Other targets can be specified but are unlikely to work as the glue needed to fetch their sysroots/compiler-rt (i.e. libclang_rt.builtins-...) is not yet implemented.

tests/extra_targets contains some examples that use the extra toolchains configured in this repo's workspace.

Manually

For other targets (or if you just want to make modifications to the toolchains that the machinery in this repo produces) you can set up a toolchain manually. The process for doing so looks something like this:

# WORKSPACE
# (parts to set up `@llvm_toolchain` have been elided; see above)

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

    # NOTE: This is required to set up toolchains outside of `@llvm_toolchain`, unfortunately
    absolute_paths = True,
 )

# This registers the default toolchains.
load("@llvm_toolchain//:toolchains.bzl", "llvm_register_toolchains", "register_toolchain")

llvm_register_toolchains()

# Now let's make our own:
http_archive(
    name = "thumbv7-sysroot",
    urls = ["example.com"],
)
register_toolchain("//tests:custom_toolchain_example")

# BUILD file:
# Example Custom Toolchain:
load("@llvm_toolchain//:cc_toolchain_config.bzl", "cc_toolchain_config")

# Docs for this function and `overrides` are in `cc_toolchain_config.bzl.tpl`.
cc_toolchain_config(
    name = "custom_toolchain_example_config",
    host_platform = "linux",
    custom_target_triple = "thumbv7em-unknown-none-gnueabihf",
    overrides = {
        "target_system_name": "thumbv7em-unknown-none-gnueabihf",
        "target_cpu": "thumbv7em",
        "target_libc": "unknown",
        "abi_libc_version": "unknown",

        # If you omit this, be sure to depend on
        # `@llvm_toolchain:host_sysroot_components`.
        # "sysroot_path": "external/thumbv7-sysroot/sysroot",

        "extra_compile_flags": [
            "-mthumb",
            "-mcpu=cortex-m4",
            "-mfpu=fpv4-sp-d16",
            "-mfloat-abi=hard",
        ],
        "omit_hosted_linker_flags": True,
        "omit_cxx_stdlib_flag": False,
        "use_llvm_ar_instead_of_libtool_on_macos": True,
    }
)

load("@com_grail_bazel_toolchain//toolchain:rules.bzl", "conditional_cc_toolchain")
conditional_cc_toolchain(
    name = "custom_toolchain",
    toolchain_config = ":custom_toolchain_example_config",
    host_is_darwin = False,

    sysroot_label = "@llvm_toolchain//:host_sysroot_components", # use this if not overriding
    # sysroot_label = "@thumbv7-sysroot//:sysroot", # override

    absolute_paths = True, # this is required for toolchains set up outside of `@llvm_toolchain`, unfortunately
    llvm_repo_label_prefix = "@llvm_toolchain//",
)

# Constraints come from here: https://github.com/bazelbuild/platforms
toolchain(
    name = "custom_toolchain_example",
    exec_compatible_with = [
        "@platforms//cpu:x86_64",
        "@platforms//os:linux",
    ],
    target_compatible_with = [
        "@platforms//cpu:armv7", # `v7e-mf` has not yet made it to stable Bazel?
        # "@platforms//os:none",
    ],
    toolchain = ":custom_toolchain",
    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
)

As with the wasm32-unknown-wasi example above, you can "use" the toolchain by creating a platform matching the constraints which the toolchain satisfies and then either specifying that platform globally (on the command line) or for a particular target via a transition.

Here's an example of using target_compatible_with on a target to get it to only build when an appropriate target platform is specified:

platform(
    name = "arm",
    constraint_values = [
        "@platforms//cpu:armv7",
        # "@platforms//os:none",
    ]
)

cc_library(
    name = "custom_target_test",
    srcs = ["test.cc"],
    target_compatible_with = [
        "@platforms//cpu:armv7",
    ]
)

Ultimately the goal is to add support for extra targets directly in this repo; PRs are very welcome :-).

Misc

Other examples of toolchain configuration:

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

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