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https://github.com/bazel-contrib/toolchains_llvm

LLVM toolchain for bazel
https://github.com/bazel-contrib/toolchains_llvm

bazel clang llvm-toolchain toolchain

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LLVM toolchain for bazel

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# LLVM toolchain for Bazel [![Tests](https://github.com/bazel-contrib/toolchains_llvm/actions/workflows/tests.yml/badge.svg)](https://github.com/bazel-contrib/toolchains_llvm/actions/workflows/tests.yml)



## Quickstart



See notes on the [release](https://github.com/bazel-contrib/toolchains_llvm/releases)

for how to get started.



NOTE: For releases prior to 0.10.1, please also see [these notes](REPO_RENAME.md).



## Basic Usage



The toolchain can automatically detect your OS and arch type, and use the right

pre-built binary LLVM distribution. See the section on "Bring Your Own LLVM"

below for more options.



See in-code documentation in [rules.bzl](toolchain/rules.bzl) for available

attributes to `llvm_toolchain`.



## Advanced Usage



### Per host architecture LLVM version



LLVM does not come with distributions for all host architectures in each

version. In particular patch versions often come with few prebuilt packages.

This means that a single version probably is not enough to address all hosts

one wants to support.



This can be solved by providing a target/version map with a default version.

The example below selects `15.0.6` as the default version for all targets not

specified explicitly. This is like providing `llvm_version = "15.0.6"`, just

like in the example on the top. However, here we provide two more entries that

map their respective target to a distinct version:



```starlark

llvm_toolchain(

    name = "llvm_toolchain",

    llvm_versions = {

        "": "15.0.6",

        "darwin-aarch64": "15.0.7",

        "darwin-x86_64": "15.0.7",

    },

)

```



### Customizations



We currently offer limited customizability through attributes of the

[llvm_toolchain\_\* rules](toolchain/rules.bzl). You can send us a PR to add

more configuration attributes.



A majority of the complexity of this project is to make it generic for multiple

use cases. For one-off experiments with new architectures, cross-compilations,

new compiler features, etc., my advice would be to look at the toolchain

configurations generated by this repo, and copy-paste/edit to make your own in

any package in your own workspace.



```sh

bazel query --output=build @llvm_toolchain//:all | grep -v -e '^#' -e '^  generator'

```



Besides defining your toolchain in your package BUILD file, and until this

[issue](https://github.com/bazelbuild/bazel/issues/7746) is resolved, you would

also need a way for bazel to access the tools in LLVM distribution as relative

paths from your package without using `..` up-references. For this, you can

create a symlink that uses up-references to point to the LLVM distribution

directory, and also create a wrapper script for clang such that the actual

clang invocation is not through the symlinked path. See the files in the

`@llvm_toolchain//:` package as a reference.



```sh

# See generated files for reference.

ls -lR "$(bazel info output_base)/external/llvm_toolchain"



# Create symlink to LLVM distribution.

cd _your_package_directory_

ln -s ../....../external/llvm_toolchain_llvm llvm



# Create CC wrapper script.

mkdir bin

cp "$(bazel info output_base)/external/llvm_toolchain/bin/cc_wrapper.sh" bin/cc_wrapper.sh

vim bin/cc_wrapper.sh # Review to ensure relative paths, etc. are good.

```



See [bazel

tutorial](https://docs.bazel.build/versions/main/tutorial/cc-toolchain-config.html)

for how CC toolchains work in general.



### Selecting Toolchains



If toolchains are registered (see Quickstart section above), you do not need to

do anything special for bazel to find the toolchain. You may want to check once

with the `--toolchain_resolution_debug` flag to see which toolchains were

selected by bazel for your target platform.



For specifying unregistered toolchains on the command line, please use the

`--extra_toolchains` flag. For example,

`--extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux`.



### Bring Your Own LLVM



The following mechanisms are available for using an LLVM toolchain:



1. Host OS information is used to find the right pre-built binary distribution

   from llvm.org, given the `llvm_version` or `llvm_versions` attribute. The

   LLVM toolchain archive is downloaded and extracted as a separate repository

   with the suffix `_llvm`. The detection logic for `llvm_version` is not

   perfect, so you may have to use `llvm_versions` for some host OS type and

   versions. We expect the detection logic to grow through community

   contributions. We welcome PRs.

2. You can use the `urls` attribute to specify your own URLs for each OS type,

   version and architecture. For example, you can specify a different URL for

   Arch Linux and a different one for Ubuntu. Just as with the option above,

   the archive is downloaded and extracted as a separate repository with the

   suffix `_llvm`.

3. You can also specify your own bazel package paths or local absolute paths

   for each host os-arch pair through the `toolchain_roots` attribute (without

   bzlmod) or the `toolchain_root` module extension tags (with bzlmod). Note

   that the keys here are different and less granular than the keys in the `urls`

   attribute. When using a bazel package path, each of the values is typically

   a package in the user's workspace or configured through `local_repository` or

   `http_archive`; the BUILD file of the package should be similar to

   `@toolchains_llvm//toolchain:BUILD.llvm_repo`. If using only

   `http_archive`, maybe consider using the `urls` attribute instead to get more

   flexibility if you need.

4. All the above options rely on host OS information, and are not suited for

   docker based sandboxed builds or remote execution builds. Such builds will

   need a single distribution version specified through the `distribution`

   attribute, or URLs specified through the `urls` attribute with an empty key, or

   a toolchain root specified through the `toolchain_roots` attribute with an

   empty key.



### Sysroots



A sysroot can be specified through the `sysroot` attribute (without bzlmod) or

the `sysroot` module extension tag (with bzlmod). 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](https://chromium.googlesource.com/chromium/src/+/HEAD/docs/linux/sysroot.md).



### Cross-compilation



The toolchain supports cross-compilation if you bring your own sysroot. When

cross-compiling, we link against the libstdc++ from the sysroot

(single-platform build behavior is to link against libc++ bundled with LLVM).

The following pairs have been tested to work for some hello-world binaries:



- {linux, x86_64} -> {linux, aarch64}

- {linux, aarch64} -> {linux, x86_64}

- {darwin, x86_64} -> {linux, x86_64}

- {darwin, x86_64} -> {linux, aarch64}



A recommended approach would be to define two toolchains, one without sysroot

for single-platform builds, and one with sysroot for cross-compilation builds.

Then, when cross-compiling, explicitly specify the toolchain with the sysroot

and the target platform. For example, see the [MODULE.bazel](tests/MODULE.bazel)

file for `llvm_toolchain_with_sysroot` and the [test

script](tests/scripts/run_xcompile_tests.sh) for cross-compilation.



```sh

bazel build \

  --platforms=@toolchains_llvm//platforms:linux-x86_64 \

  --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux \

  //...

```



### Multi-platform builds



The toolchain supports multi-platform builds through the combination of the

`exec_os`, `exec_arch` attribute pair, and either the `distribution` attribute,

or the `urls` attribute. This allows one to run their builds on one platform

(e.g. macOS) and their build actions to run on another (e.g. Linux), enabling

remote build execution (RBE). For example, see the [MODULE.bazel](tests/MODULE.bazel)

file for `llvm_toolchain_linux_exec` and the [test

script](tests/scripts/run_docker_exec_test.sh) for running the build actions on

Linux even if the build is being run from macOS.



```sh

bazel build \

  --platforms=@toolchains_llvm//platforms:linux-x86_64 \

  --extra_execution_platforms=@toolchains_llvm//platforms:linux-x86_64 \

  --extra_toolchains=@llvm_toolchain_linux_exec//:cc-toolchain-x86_64-linux \

  //...

```



### Supporting New Target Platforms



The following is a rough (untested) list of steps:



1. To help us detect if you are cross-compiling or not, note the arch string as

   given by `python3 -c 'import platform; print(platform.machine())`.

2. Edit `SUPPORTED_TARGETS` in

   [toolchain/internal/common.bzl](toolchain/internal/common.bzl) with the os

   and the arch string from above.

3. Add `target_system_name`, etc. in

   [toolchain/cc_toolchain_config.bzl](toolchain/cc_toolchain_config.bzl).

4. For cross-compiling, add a `platform` bazel type for your target platform in

   [platforms/BUILD.bazel](platforms/BUILD.bazel), and add an appropriate

   sysroot entry to your `llvm_toolchain` repository definition.

5. If not cross-compiling, bring your own LLVM (see section above) through the

   `toolchain_roots` or `urls` attribute.

6. Test your build.



### Sandbox



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.



### Compatibility



The toolchain is tested to work with `rules_go`, `rules_rust`, and

`rules_foreign_cc`.



### Accessing tools



The LLVM distribution also provides several tools like `clang-format`. You can

depend on these tools directly in the bin directory of the distribution. When

not using the `toolchain_roots` attribute, the distribution is available in the

repo with the suffix `_llvm` appended to the name you used for the

`llvm_toolchain` rule. For example, `@llvm_toolchain_llvm//:bin/clang-format`

is a valid and visible target in the quickstart example above.



When using the `toolchain_roots` attribute, there is currently no single target

that you can reference, and you may have to alias the tools you want with a

`select` clause in your workspace.



As a convenience, some targets are aliased appropriately in the configuration

repo (as opposed to the LLVM distribution repo) for you to use and will work

even when using `toolchain_roots`. The complete list is in the file

[aliases.bzl](toolchain/aliases.bzl). If your repo is named `llvm_toolchain`,

then they can be referenced as:



- `@llvm_toolchain//:omp`

- `@llvm_toolchain//:clang-format`

- `@llvm_toolchain//:llvm-cov`



### Strict header deps (Linux only)



The toolchain supports Bazel's `layering_check` feature, which relies on

[Clang modules](https://clang.llvm.org/docs/Modules.html) to implement strict

deps (also known as "depend on what you use") for `cc_*` rules. This feature

can be enabled by enabling the `layering_check` feature on a per-target,

per-package or global basis.



## Prior Art



Other examples of toolchain configuration:



https://bazel.build/tutorials/ccp-toolchain-config



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