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1.. _full_cross_build:2 3================4Full Cross Build5================6 7.. contents:: Table of Contents8 :depth: 19 :local:10 11.. note::12 Fullbuild requires running headergen, which is a python program that depends on13 pyyaml. The minimum versions are listed on the :ref:`header_generation`14 page, as well as additional information.15 16In this document, we will present recipes to cross build the full libc. When we17say *cross build* a full libc, we mean that we will build the full libc for a18target system which is not the same as the system on which the libc is being19built. For example, you could be building for a bare metal aarch64 *target* on a20Linux x86_64 *host*.21 22There are two main recipes to cross build the full libc. Each one serves a23different use case. Below is a short description of these recipes to help users24pick the recipe that best suites their needs and contexts.25 26* **Standalone cross build** - Using this recipe one can build the libc using a27 compiler of their choice. One should use this recipe if their compiler can28 build for the host as well as the target.29* **Bootstrap cross build** - In this recipe, one will build the ``clang``30 compiler and the libc build tools for the host first, and then use them to31 build the libc for the target. Unlike with the standalone build recipe, the32 user does not have explicitly build ``clang`` and other build tools.33 They get built automatically before building the libc. One should use this34 recipe if they intend use the built ``clang`` and the libc as part of their35 toolchain for the target.36 37The following sections present the two recipes in detail.38 39Standalone cross build40======================41 42In the *standalone crossbuild* recipe, the system compiler or a custom compiler43of user's choice is used to build the libc. The necessary build tools for the44host are built first, and those build tools are then used to build the libc for45the target. Both these steps happen automatically, as in, the user does not have46to explicitly build the build tools first and then build the libc. A point to47keep in mind is that the compiler used should be capable of building for the48host as well as the target.49 50CMake configure step51--------------------52 53Below is the CMake command to configure the standalone crossbuild of the libc.54 55.. code-block:: sh56 57 $> cd llvm-project # The llvm-project checkout58 $> mkdir build59 $> cd build60 $> C_COMPILER=<C compiler> # For example "clang"61 $> CXX_COMPILER=<C++ compiler> # For example "clang++"62 $> cmake ../runtimes \63 -G Ninja \64 -DLLVM_ENABLE_RUNTIMES=libc \65 -DCMAKE_C_COMPILER=$C_COMPILER \66 -DCMAKE_CXX_COMPILER=$CXX_COMPILER \67 -DLLVM_LIBC_FULL_BUILD=ON \68 -DLIBC_TARGET_TRIPLE=<Your target triple> \69 -DCMAKE_BUILD_TYPE=<Release|Debug>70 71We will go over the special options passed to the ``cmake`` command above.72 73* **Enabled Runtimes** - Since we want to build LLVM-libc, we list74 ``libc`` as the enabled runtime.75* **The full build option** - Since we want to build the full libc, we pass76 ``-DLLVM_LIBC_FULL_BUILD=ON``.77* **The target triple** - This is the target triple of the target for which78 we are building the libc. For example, for a Linux 32-bit Arm target,79 one can specify it as ``arm-linux-eabi``.80 81Build step82----------83 84After configuring the build with the above ``cmake`` command, one can build the85the libc for the target with the following command:86 87.. code-block:: sh88 89 $> ninja libc libm90 91The above ``ninja`` command will build the libc static archives ``libc.a`` and92``libm.a`` for the target specified with ``-DLIBC_TARGET_TRIPLE`` in the CMake93configure step.94 95Bootstrap cross build96=====================97 98In this recipe, the clang compiler is built automatically before building99the libc for the target.100 101CMake configure step102--------------------103 104.. code-block:: sh105 106 $> cd llvm-project # The llvm-project checkout107 $> mkdir build108 $> cd build109 $> C_COMPILER=<C compiler> # For example "clang"110 $> CXX_COMPILER=<C++ compiler> # For example "clang++"111 $> TARGET_TRIPLE=<Your target triple>112 $> cmake ../llvm \113 -G Ninja \114 -DCMAKE_C_COMPILER=$C_COMPILER \115 -DCMAKE_CXX_COMPILER=$CXX_COMPILER \116 -DLLVM_ENABLE_PROJECTS=clang \117 -DLLVM_ENABLE_RUNTIMES=libc \118 -DLLVM_LIBC_FULL_BUILD=ON \119 -DLLVM_RUNTIME_TARGETS=$TARGET_TRIPLE \120 -DCMAKE_BUILD_TYPE=Debug121 122Note how the above cmake command differs from the one used in the other recipe:123 124* ``clang`` is listed in ``-DLLVM_ENABLE_PROJECTS`` and ``libc`` is125 listed in ``-DLLVM_ENABLE_RUNTIMES``.126* The CMake root source directory is ``llvm-project/llvm``.127* The target triple is specified with ``-DLLVM_RUNTIME_TARGETS``.128 129Build step130----------131 132The build step is similar to the other recipe:133 134.. code-block:: sh135 136 $> ninja libc137 138The above ninja command should build the libc static archives for the target139specified with ``-DLLVM_RUNTIME_TARGETS``.140 141Building for bare metal142=======================143 144To build for bare metal, all one has to do is to specify the145`system <https://clang.llvm.org/docs/CrossCompilation.html#target-triple>`_146component of the target triple as ``none``. For example, to build for a14732-bit arm target on bare metal, one can use a target triple like148``arm-none-eabi``. Other than that, the libc for a bare metal target can be149built using any of the three recipes described above.150 151Building for the GPU152====================153 154To build for a GPU architecture, it should only be necessary to specify the155target triple as one of the supported GPU targets. Currently, this is either156``nvptx64-nvidia-cuda`` for NVIDIA GPUs or ``amdgcn-amd-amdhsa`` for AMD GPUs.157More detailed information is provided in the :ref:`GPU158documentation<libc_gpu_building>`.159