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.. _libc_uefi_building:
======================
Building libc for UEFI
======================
.. contents:: Table of Contents
:depth: 4
:local:
Building LLVM libc for UEFI
===========================
This document will present recipes to build the LLVM C library for UEFI.
UEFI builds use the same :ref:`cross build<full_cross_build>` support as
the other targets. However, the UEFI target has the restriction that it *must*
be built with an up-to-date ``clang`` compiler. This is because UEFI support
in ``clang`` is still an experimental feature.
Currently, it is only possible to build LLVM libc for UEFI for ``x86_64``
CPUs. This is due to the target not being enabled for ``aarch64`` and
``riscv64``.
Once you have finished building, refer to :ref:`libc_uefi_usage` to get started
with the newly built C library.
Standard runtimes build
-----------------------
The simplest way to build for UEFI is to use the existing LLVM runtimes
support. This will automatically handle bootstrapping an up-to-date ``clang``
compiler and use it to build the C library. The following CMake invocation
will instruct it to build the ``libc`` runtime targeting ``x86_64`` CPUs.
.. code-block:: sh
$> cd llvm-project # The llvm-project checkout
$> mkdir build
$> cd build
$> cmake ../llvm -G Ninja \
-DLLVM_ENABLE_PROJECTS="clang;lld" \
-DCMAKE_BUILD_TYPE=<Debug|Release> \ # Select build type
-DCMAKE_INSTALL_PREFIX=<PATH> \ # Where the libraries will live
-DLLVM_DEFAULT_TARGET_TRIPLE=x86_64-unknown-uefi-llvm \
-DLLVM_RUNTIME_TARGETS=x86_64-unknown-uefi-llvm \
-DRUNTIMES_x86_64-unknown-uefi-llvm_LLVM_ENABLE_RUNTIMES=libc \
-DRUNTIMES_x86_64-unknown-uefi-llvm_LLVM_LIBC_FULL_BUILD=true \
$> ninja install
We need ``clang`` to build the UEFI C library and ``lld`` to link UEFI PE
executables, so we enable them in ``LLVM_ENABLE_PROJECTS``. We then set
``RUNTIMES_<triple>_LLVM_ENABLE_RUNTIMES`` to enable ``libc`` for the UEFI
targets.
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