Cross-Compiling With Bazel And Toolchains_llvm: A Guide

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Cross-Compiling with Bazel and toolchains_llvm: A Guide

Hey guys, ever found yourself scratching your head trying to cross-compile a Bazel project? Specifically, trying to go from your trusty linux-x86_64 host to a linux-aarch64 target? It can be a bit of a maze, but let's break it down and get you building successfully. This guide walks you through the process, highlighting common pitfalls and offering solutions.

Understanding the Basics of Cross-Compilation

Cross-compilation, in simple terms, means building code on one platform (your host) that will run on another (your target). This is super common in embedded systems development, or when you're targeting architectures different from your development machine. Bazel, with its powerful build system, makes this possible, but requires careful configuration, especially when using toolchains_llvm. Understanding the toolchain concept is essential. A toolchain is a set of tools (compiler, linker, assembler, etc.) configured to build code for a specific target. When cross-compiling, you need a toolchain that can generate code compatible with your target architecture, even though you're building on a different architecture.

When diving into cross-compilation, the initial setup can feel like navigating a labyrinth. The key is to ensure that your build environment is correctly configured to produce executables tailored for the target architecture, rather than your host system. This involves not only selecting the right compiler and linker but also providing the necessary libraries and system headers that match the target environment.

For instance, if you are developing on an x86_64 machine but targeting an ARM architecture, you need a cross-compilation toolchain that includes an ARM compiler and linker, along with the ARM-compatible versions of standard libraries. This setup ensures that the compiled code is not only executable on the target ARM device but also correctly linked against the necessary dependencies.

Configuring the build system to use this toolchain typically involves specifying the target architecture, the location of the cross-compiler, and any target-specific flags or settings. This process is crucial for resolving dependencies correctly and avoiding errors during the build process. Without a properly configured cross-compilation environment, the build system might default to using the host architecture’s tools and libraries, resulting in executables that are incompatible with the target device. Therefore, meticulous attention to detail during the setup phase is essential for a successful cross-compilation endeavor.

Initial Attempt: Providing Sysroots

So, the initial approach involves defining sysroots for both the host and target architectures. A sysroot is essentially a directory containing the root file system for your target. It includes the libraries, headers, and other essential files needed to build code for that system. The idea is that toolchains_llvm should pick up the correct sysroot based on the target platform. Let's examine the initial MODULE.bazel setup:

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")

LLVM_VERSION = "16.0.4"

HOSTS = ["linux-x86_64"]
TARGETS = ["linux-x86_64", "linux-aarch64"]

llvm.toolchain(
    name = "llvm_toolchain",
    llvm_version = LLVM_VERSION,
)

llvm.sysroot(
    name = "llvm_toolchain",
    label = "//:linux-amd64-sysroot-bazel.tar.gz",
    targets = ["linux-x86_64"],
)

llvm.sysroot(
    name = "llvm_toolchain",
    label = "//:linux-arm64-sysroot-bazel.tar.gz",
    targets = ["linux-aarch64"],
)

use_repo(llvm, "llvm_toolchain")
register_toolchains("@llvm_toolchain//:all")

The problem here, as the error message indicates, is that Bazel can't find a suitable toolchain for the linux-aarch64 target. It rejects the available toolchains because of mismatching values. Specifically, it's looking for a toolchain that matches the target platform @@toolchains_llvm+//platforms:linux-aarch64, but none of the registered toolchains are a direct fit. The error messages like ToolchainResolution: No @@bazel_tools//tools/cpp:toolchain_type toolchain found confirm this.

This is often because the toolchains haven't been configured to explicitly target the aarch64 architecture. Bazel needs to be told, in no uncertain terms, that you want to build for ARM64 when targeting that platform. The sysroot is necessary, but not sufficient; you also need the right toolchain configuration.

Second Attempt: Individual Toolchains

The second attempt involves creating separate toolchains for each platform, explicitly defining the execution architecture and OS. This is a step in the right direction, but still falls short:

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")

LLVM_VERSION = "16.0.4"

HOSTS = ["linux-x86_64"]
TARGETS = ["linux-x86_64", "linux-aarch64"]

llvm.toolchain(
    name = "llvm_toolchain-linux-amd64",
    llvm_version = LLVM_VERSION,
    exec_os = "linux",
    exec_arch = "x86_64",
)

llvm.sysroot(
    name = "llvm_toolchain-linux-amd64",
    label = "//:linux-amd64-sysroot-bazel.tar.gz",
    targets = ["linux-x86_64"],
)

use_repo(llvm, "llvm_toolchain-linux-amd64")
register_toolchains("@llvm_toolchain-linux-amd64//:cc-toolchain-x86_64-linux")

llvm.toolchain(
    name = "llvm_toolchain-linux-arm64",
    llvm_version = LLVM_VERSION,
    exec_os = "linux",
    exec_arch = "aarch64",
)

llvm.sysroot(
    name = "llvm_toolchain-linux-arm64",
    label = "//:linux-arm64-sysroot-bazel.tar.gz",
    targets = ["linux-aarch64"],
)

use_repo(llvm, "llvm_toolchain-linux-arm64")
register_toolchains("@llvm_toolchain-linux-arm64//:cc-toolchain-aarch64-linux")

The key issue here is the exec_arch. The exec_arch specifies the architecture on which the toolchain executes, not the architecture it targets. In this case, you're telling Bazel that the llvm_toolchain-linux-arm64 toolchain runs on aarch64, which is incorrect since you're building on an x86_64 machine. This is why Bazel complains about Incompatible execution platform @@platforms//host:host; mismatching values: aarch64.

To effectively resolve the cross-compilation challenge, it's essential to clarify the roles of execution and target platforms within your build configuration. The execution platform defines the environment where the build tools (like compilers and linkers) are executed, while the target platform specifies the environment for which the final executable is being built.

When setting up your cross-compilation environment, ensure that the execution platform is correctly set to match your build machine's architecture. This involves configuring Bazel to recognize that the build tools are running on, say, an x86_64 Linux machine, even though the target platform is ARM64.

The separation of these two platform definitions is crucial for a successful build. It allows Bazel to select the appropriate toolchain that can run on the execution platform while producing code compatible with the target platform. Failing to properly differentiate between these can lead to the build system selecting the wrong tools, resulting in compilation errors or executables that are not compatible with the target environment.

By clearly defining both the execution and target platforms, you provide Bazel with the necessary information to optimize the build process, ensuring that the generated code meets the requirements of the target architecture without compromising the efficiency of the build environment.

How It's Supposed to Work: The Correct Approach

Here’s a breakdown of how cross-compilation with toolchains_llvm should ideally work, along with a corrected MODULE.bazel example:

  1. Define Target Platforms: You need to define your target platforms properly. The toolchains_llvm extension provides a convenient way to specify these.
  2. Configure Toolchains: You need to configure your toolchains to target the desired architecture, while running on your host architecture. This involves specifying the correct target triple.
  3. Register Toolchains: Register the configured toolchains with Bazel.
  4. Build with the Target Platform: Use the --platforms flag to tell Bazel to build for the specified target platform.

Here's a refined MODULE.bazel that should work:

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")

LLVM_VERSION = "16.0.4"

HOSTS = ["linux-x86_64"]
TARGETS = ["linux-x86_64", "linux-aarch64"]

llvm.toolchain(
    name = "llvm_toolchain-linux-aarch64",
    llvm_version = LLVM_VERSION,
    target_os = "linux",
    target_arch = "aarch64",
    # Important:  Omit exec_os and exec_arch.  Bazel will infer the execution platform.
    # Or, explicitly set exec constraints if needed, but usually not.
)

llvm.sysroot(
    name = "llvm_toolchain-linux-aarch64",
    label = "//:linux-arm64-sysroot-bazel.tar.gz",
    targets = ["linux-aarch64"],
)

use_repo(llvm, "llvm_toolchain-linux-aarch64")
register_toolchains("@llvm_toolchain-linux-aarch64//:cc-toolchain-aarch64-linux")

llvm.toolchain(
    name = "llvm_toolchain-linux-x86_64",
    llvm_version = LLVM_VERSION,
    target_os = "linux",
    target_arch = "x86_64",
)

llvm.sysroot(
    name = "llvm_toolchain-linux-x86_64",
    label = "//:linux-amd64-sysroot-bazel.tar.gz",
    targets = ["linux-x86_64"],
)

use_repo(llvm, "llvm_toolchain-linux-x86_64")
register_toolchains("@llvm_toolchain-linux-x86_64//:cc-toolchain-x86_64-linux")

Key changes and explanations:

  • target_os and target_arch: These are the critical parameters. They tell the toolchain what architecture and OS to target. This is what you're building for.
  • Omit exec_os and exec_arch: By not specifying exec_os and exec_arch, you let Bazel automatically detect the execution platform (your host machine). This is almost always what you want for cross-compilation.
  • Separate Toolchains: Defining separate toolchains for x86_64 and aarch64 can simplify things.

Now, build with:

bazel build --platforms=@toolchains_llvm//platforms:linux-aarch64 :workspace

This command tells Bazel to build the :workspace target for the linux-aarch64 platform, using the toolchain you've configured. It should now correctly pick the llvm_toolchain-linux-aarch64 toolchain and build your code for ARM64.

Additional Tips for Success

  • Sysroot Contents: Make sure your sysroots are complete and correct. They should contain all the necessary libraries and headers for your target platform. Using docker export after installing clang is a good start, but double-check that everything's there.
  • Dependencies: Ensure that any external dependencies you're using also have ARM64 versions available, or that you're building them from source as part of your Bazel build.
  • Toolchain Selection Debugging: The --toolchain_resolution_debug flag is your friend! Use it to see exactly which toolchains Bazel is considering and why it's rejecting others.
  • Constraints: For more complex scenarios, you might need to use Bazel's constraint system to define specific requirements for your toolchains and targets.
  • Check the basics: Ensure the sysroot file paths are correct in your MODULE.bazel file and that the files exist where you expect them to be. Typos are easy to make and can cause frustrating issues.

Troubleshooting Common Issues