Cross-Compiling With Bazel: AArch64 Guide

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Cross-Compiling with Bazel: AArch64 Guide

Hey guys! Running into cross-compilation headaches with Bazel, especially when targeting linux-aarch64 from a linux-x86_64 host? You're not alone! Let's dive into how to get this working using toolchains_llvm. This guide will break down the common pitfalls and provide a step-by-step approach to successful cross-compilation.

Understanding the Problem

The core issue revolves around setting up the correct toolchains and sysroots for your target architecture. The toolchains_llvm extension aims to simplify this, but misconfigurations can lead to cryptic build errors. The error messages you're seeing, like "No matching toolchains found," indicate that Bazel can't find a suitable toolchain for your target platform (linux-aarch64 in this case).

Initial Setup: Providing the Sysroot

Your initial attempt involved providing a sysroot, which is a good starting point. A sysroot essentially provides the necessary libraries and headers for your target platform. You created these sysroots using docker export after installing clang-16 in an Ubuntu 22.04 container. This approach is generally sound, but let's examine the Bazel configuration to ensure everything is correctly wired up.

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")

Potential Issues and Solutions

  1. Toolchain Naming Conflicts: The name attribute for both llvm.sysroot calls is the same (llvm_toolchain). While this might not be the direct cause, it's cleaner to differentiate them. Use llvm_sysroot_amd64 and llvm_sysroot_arm64 respectively. This helps in debugging and avoids potential confusion.

  2. Sysroot Contents: Double-check that your sysroot archives (linux-amd64-sysroot-bazel.tar.gz and linux-arm64-sysroot-bazel.tar.gz) contain the necessary files. A common mistake is missing essential libraries or headers. Inside the Docker container, ensure you've installed the *-dev packages for your target architecture. For example, you might need libc6-dev-arm64.

  3. Toolchain Registration: The register_toolchains("@llvm_toolchain//:all") line registers all toolchains defined within the @llvm_toolchain repository. Ensure that the generated toolchains actually exist. Use bazel query 'kind(cc_toolchain, @llvm_toolchain//...)' to verify the toolchains are being created as expected.

Refined Approach: Individual Toolchains

Your second attempt involved defining individual toolchains for each platform. This is a more explicit and often more reliable approach, especially for cross-compilation.

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")

Addressing the Execution Platform Issue

The error message "Incompatible execution platform @@platforms//host:host; mismatching values: aarch64" is crucial. It indicates that Bazel is trying to use the aarch64 toolchain on your host machine (x86_64), which is impossible. The exec_os and exec_arch attributes in the llvm.toolchain definition are meant to specify the platform where the toolchain runs, not the target platform.

The correct way to handle this is to remove the exec_os and exec_arch attributes. Bazel should automatically infer the execution platform based on where it's running. The target platform is determined by the --platforms flag passed to bazel build.

Revised Configuration

Here's the updated and recommended configuration:

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,
)

llvm.sysroot(
    name = "llvm_sysroot_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,
)

llvm.sysroot(
    name = "llvm_sysroot_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")

Key changes:

  • Removed exec_os and exec_arch from the llvm.toolchain definitions.
  • Renamed llvm.sysroot names for clarity.

Building with the Correct Platform

Ensure you're building with the --platforms flag to specify the target platform:

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

Replace :your_target with the actual target you want to build.

Troubleshooting Tips

  1. Verbose Logging: Use --verbose_failures to get more detailed error messages during the build.

  2. Toolchain Resolution Debugging: Your original command --toolchain_resolution_debug=@@bazel_tools//tools/cpp:toolchain_type is helpful for understanding why Bazel is rejecting certain toolchains. Pay close attention to the "mismatching values" reported.

  3. Sysroot Verification: After creating your sysroot tarballs, extract them and inspect their contents. Make sure the necessary headers and libraries for your target architecture are present.

  4. Crosstool Version: Ensure the crosstool version used to build the sysroot matches the version expected by the toolchain. Mismatched versions can lead to subtle incompatibilities.

  5. CC_FLAGS and LD_FLAGS: Sometimes, you might need to explicitly set CC_FLAGS and LD_FLAGS to point to the correct sysroot. This can be done in your BUILD file or through Bazel's command-line arguments. For example:

    cc_binary(
    name = "my_program",
    srcs = ["my_program.c"],
    copts = ["--sysroot=/path/to/your/sysroot"],
    linkopts = ["--sysroot=/path/to/your/sysroot"],
)

Example BUILD file:

load("@rules_cc//cc:defs.bzl", "cc_binary")

cc_binary(
    name = "workspace",
    srcs = ["workspace.cpp"],
)

Conclusion

Cross-compiling with Bazel and toolchains_llvm can be tricky, but by understanding the roles of toolchains, sysroots, and platforms, you can overcome the common hurdles. Remember to double-check your configurations, verify your sysroot contents, and use the debugging tools available in Bazel. Good luck, and happy building!