.. _full_cross_build:

================ Full Cross Build

.. note:: Fullbuild requires running headergen, which is a python program that depends on pyyaml. The minimum versions are listed on the :ref:header_generation page, as well as additional information.

In this document, we will present recipes to cross build the full libc. When we say cross build a full libc, we mean that we will build the full libc for a target system which is not the same as the system on which the libc is being built. For example, you could be building for a bare metal aarch64 target on a Linux x86_64 host.

There are two main recipes to cross build the full libc. Each one serves a different use case. Below is a short description of these recipes to help users pick the recipe that best suites their needs and contexts.

Standalone cross build - Using this recipe one can build the libc using a compiler of their choice. One should use this recipe if their compiler can build for the host as well as the target.
Bootstrap cross build - In this recipe, one will build the clang compiler and the libc build tools for the host first, and then use them to build the libc for the target. Unlike with the standalone build recipe, the user does not have explicitly build clang and other build tools. They get built automatically before building the libc. One should use this recipe if they intend use the built clang and the libc as part of their toolchain for the target.

The following sections present the two recipes in detail.

Standalone cross build

In the standalone crossbuild recipe, the system compiler or a custom compiler of user's choice is used to build the libc. The necessary build tools for the host are built first, and those build tools are then used to build the libc for the target. Both these steps happen automatically, as in, the user does not have to explicitly build the build tools first and then build the libc. A point to keep in mind is that the compiler used should be capable of building for the host as well as the target.

.. note:: Even though the LLVM libc provides its own complete C library implementation, compiling it for a Linux target still requires the Linux kernel API headers for that architecture. On Debian-based systems, these and other standard cross-compilation runtimes (like libgcc) can be installed via packages like gcc-riscv64-linux-gnu and linux-libc-dev-riscv64-cross (or similar for other architectures). You will need to point CMake to the kernel headers using -DLIBC_KERNEL_HEADERS (e.g., -DLIBC_KERNEL_HEADERS=/usr/riscv64-linux-gnu/include) so the libc build can find headers like asm/unistd.h.

CMake configure step

First, set up the environment variables for your compiler and target:

.. code-block:: sh

C_COMPILER=clang CXX_COMPILER=clang++ TARGET_TRIPLE=aarch64-linux-gnu

Below is the CMake command to configure the standalone crossbuild of the libc.

.. code-block:: sh

cmake
-B build
-S runtimes
-G Ninja
-DLLVM_ENABLE_RUNTIMES=libc
-DCMAKE_C_COMPILER=$C_COMPILER
-DCMAKE_CXX_COMPILER=$CXX_COMPILER
-DCMAKE_C_COMPILER_TARGET=$TARGET_TRIPLE
-DCMAKE_CXX_COMPILER_TARGET=$TARGET_TRIPLE
-DLLVM_LIBC_FULL_BUILD=ON
-DLIBC_TARGET_TRIPLE=$TARGET_TRIPLE
-DCMAKE_BUILD_TYPE=<Release|Debug>

We will go over the special options passed to the cmake command above.

Enabled Runtimes - Since we want to build LLVM-libc, we list libc as the enabled runtime.
The full build option - Since we want to build the full libc, we pass -DLLVM_LIBC_FULL_BUILD=ON.
The target triple - This is the target triple of the target for which we are building the libc. For example, for a Linux 32-bit Arm target, one can specify it as arm-linux-eabi.

Build step

After configuring the build with the above cmake command, one can build the the libc for the target with the following command:

.. code-block:: sh

ninja -C build libc libm

The above ninja command will build the libc static archives libc.a and libm.a for the target specified with -DLIBC_TARGET_TRIPLE in the CMake configure step.

Bootstrap cross build

In this recipe, the clang compiler is built automatically before building the libc for the target.

CMake configure step

First, set up the environment variables for your compiler and target:

.. code-block:: sh

C_COMPILER=clang CXX_COMPILER=clang++ TARGET_TRIPLE=aarch64-linux-gnu

Then, configure the CMake build for the bootstrap build:

.. code-block:: sh

cmake
-B build
-S llvm
-G Ninja
-DCMAKE_C_COMPILER=$C_COMPILER
-DCMAKE_CXX_COMPILER=$CXX_COMPILER
-DLLVM_ENABLE_PROJECTS=clang
-DLLVM_ENABLE_RUNTIMES=libc
-DLLVM_LIBC_FULL_BUILD=ON
-DLLVM_RUNTIME_TARGETS=$TARGET_TRIPLE
-DCMAKE_BUILD_TYPE=Debug

Note how the above cmake command differs from the one used in the other recipe:

clang is listed in -DLLVM_ENABLE_PROJECTS and libc is listed in -DLLVM_ENABLE_RUNTIMES.
The CMake root source directory is llvm-project/llvm.
The target triple is specified with -DLLVM_RUNTIME_TARGETS.

Build step

The build step is similar to the other recipe:

.. code-block:: sh

ninja -C build libc

The above ninja command should build the libc static archives for the target specified with -DLLVM_RUNTIME_TARGETS.

Building for bare metal

To build for bare metal, all one has to do is to specify the system <https://clang.llvm.org/docs/CrossCompilation.html#target-triple>_ component of the target triple as none. For example, to build for a 32-bit arm target on bare metal, one can use a target triple like arm-none-eabi. Other than that, the libc for a bare metal target can be built using any of the three recipes described above.

Building for the GPU

To build for a GPU architecture, it should only be necessary to specify the target triple as one of the supported GPU targets. Currently, this is either nvptx64-nvidia-cuda for NVIDIA GPUs or amdgcn-amd-amdhsa for AMD GPUs. More detailed information is provided in the :ref:GPU documentation<libc_gpu_building>.

Building and Testing with an Emulator

If you are cross-compiling the libc for a different architecture, you can use an emulator such as QEMU to run the tests. For instance, to cross-compile for riscv64 and run tests using qemu-riscv64, you can use the standalone cross build recipe with a few additional CMake flags.

CMake configure step

Assuming your system compiler (e.g., clang++) supports the RISC-V target, you can configure the build as follows:

.. code-block:: sh

cmake
-B build
-S runtimes
-G Ninja
-DLLVM_ENABLE_RUNTIMES=libc
-DCMAKE_C_COMPILER=clang
-DCMAKE_CXX_COMPILER=clang++
-DCMAKE_C_COMPILER_TARGET=riscv64-linux-gnu
-DCMAKE_CXX_COMPILER_TARGET=riscv64-linux-gnu
-DLLVM_LIBC_FULL_BUILD=ON
-DLIBC_TARGET_TRIPLE=riscv64-linux-gnu
-DLIBC_KERNEL_HEADERS=/usr/riscv64-linux-gnu/include
-DCMAKE_CROSSCOMPILING_EMULATOR=qemu-riscv64
-DLLVM_ENABLE_LLD=ON
-DCMAKE_BUILD_TYPE=Debug

The notable additions are:

The compiler target - We set -DCMAKE_C_COMPILER_TARGET=riscv64-linux-gnu and -DCMAKE_CXX_COMPILER_TARGET=riscv64-linux-gnu to tell CMake to pass the correct --target flags to clang so that it cross-compiles rather than building for the host.
The target triple - We set -DLIBC_TARGET_TRIPLE=riscv64-linux-gnu
The kernel headers - We set -DLIBC_KERNEL_HEADERS=/usr/riscv64-linux-gnu/include to point to the target's Linux API headers (e.g. for asm/unistd.h).
The cross-compiling emulator - We set -DCMAKE_CROSSCOMPILING_EMULATOR=qemu-riscv64 to tell CMake how to execute the compiled unittests. Note that this requires that qemu-riscv64 is installed and available in your $PATH.
LLD Linker - We set -DLLVM_ENABLE_LLD=ON to ensure the test suite is linked using lld, which is necessary for cross-compilation.

Build and Test step

You can then build the libc using:

.. code-block:: sh

ninja -C build libc

To run the tests for the cross-compiled libc, you must use the hermetic test suite, which is entirely self-hosted.

.. code-block:: sh

ninja -C build libc-hermetic-tests

.. note:: The standard check-libc target relies on the target's system C++ and C library headers. Because these tests aren't hermetic, they are not expected to work for a standalone cross-compilation build.