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<div class="section" id="how-to-cross-compile-compiler-rt-builtins-for-arm">
<h1>How to Cross Compile Compiler-rt Builtins For Arm<a class="headerlink" href="#how-to-cross-compile-compiler-rt-builtins-for-arm" title="Permalink to this headline">¶</a></h1>
<div class="section" id="introduction">
<h2>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>This document contains information about building and testing the builtins part
of compiler-rt for an Arm target, from an x86_64 Linux machine.</p>
<p>While this document concentrates on Arm and Linux the general principles should
apply to other targets supported by compiler-rt. Further contributions for other
targets are welcome.</p>
<p>The instructions in this document depend on libraries and programs external to
LLVM, there are many ways to install and configure these dependencies so you
may need to adapt the instructions here to fit your own local situation.</p>
</div>
<div class="section" id="prerequisites">
<h2>Prerequisites<a class="headerlink" href="#prerequisites" title="Permalink to this headline">¶</a></h2>
<p>In this use case we’ll be using cmake on a Debian-based Linux system,
cross-compiling from an x86_64 host to a hard-float Armv7-A target. We’ll be
using as many of the LLVM tools as we can, but it is possible to use GNU
equivalents.</p>
<blockquote>
<div><ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">build</span> <span class="pre">of</span> <span class="pre">LLVM/clang</span> <span class="pre">for</span> <span class="pre">the</span> <span class="pre">llvm-tools</span> <span class="pre">and</span> <span class="pre">llvm-config</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">clang</span> <span class="pre">executable</span> <span class="pre">with</span> <span class="pre">support</span> <span class="pre">for</span> <span class="pre">the</span> <span class="pre">ARM</span> <span class="pre">target</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">compiler-rt</span> <span class="pre">sources</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">The</span> <span class="pre">qemu-arm</span> <span class="pre">user</span> <span class="pre">mode</span> <span class="pre">emulator</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">An</span> <span class="pre">arm-linux-gnueabihf</span> <span class="pre">sysroot</span></code></li>
</ul>
</div></blockquote>
<p>In this example we will be using ninja.</p>
<p>See <a class="reference external" href="https://compiler-rt.llvm.org/">https://compiler-rt.llvm.org/</a> for more information about the dependencies
on clang and LLVM.</p>
<p>See <a class="reference external" href="https://llvm.org/docs/GettingStarted.html">https://llvm.org/docs/GettingStarted.html</a> for information about obtaining
the source for LLVM and compiler-rt. Note that the getting started guide
places compiler-rt in the projects subdirectory, but this is not essential and
if you are using the BaremetalARM.cmake cache for v6-M, v7-M and v7-EM then
compiler-rt must be placed in the runtimes directory.</p>
<p><code class="docutils literal notranslate"><span class="pre">qemu-arm</span></code> should be available as a package for your Linux distribution.</p>
<p>The most complicated of the prequisites to satisfy is the arm-linux-gnueabihf
sysroot. In theory it is possible to use the Linux distributions multiarch
support to fulfill the dependencies for building but unfortunately due to
/usr/local/include being added some host includes are selected. The easiest way
to supply a sysroot is to download the arm-linux-gnueabihf toolchain. This can
be found at:
* <a class="reference external" href="https://developer.arm.com/open-source/gnu-toolchain/gnu-a/downloads">https://developer.arm.com/open-source/gnu-toolchain/gnu-a/downloads</a> for gcc 8 and above
* <a class="reference external" href="https://releases.linaro.org/components/toolchain/binaries/">https://releases.linaro.org/components/toolchain/binaries/</a> for gcc 4.9 to 7.3</p>
</div>
<div class="section" id="building-compiler-rt-builtins-for-arm">
<h2>Building compiler-rt builtins for Arm<a class="headerlink" href="#building-compiler-rt-builtins-for-arm" title="Permalink to this headline">¶</a></h2>
<p>We will be doing a standalone build of compiler-rt using the following cmake
options.</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">path/to/compiler-rt</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-G</span> <span class="pre">Ninja</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_BUILTINS=ON</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_SANITIZERS=OFF</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_XRAY=OFF</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_LIBFUZZER=OFF</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_PROFILE=OFF</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_COMPILER=/path/to/clang</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_AR=/path/to/llvm-ar</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_NM=/path/to/llvm-nm</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_RANLIB=/path/to/llvm-ranlib</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_EXE_LINKER_FLAGS="-fuse-ld=lld"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_COMPILER_TARGET="arm-linux-gnueabihf"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_ASM_COMPILER_TARGET="arm-linux-gnueabihf"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DLLVM_CONFIG_PATH=/path/to/llvm-config</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_FLAGS="build-c-flags"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_ASM_FLAGS="build-c-flags"</span></code></li>
</ul>
<p>The <code class="docutils literal notranslate"><span class="pre">build-c-flags</span></code> need to be sufficient to pass the C-make compiler check,
compile compiler-rt, and if you are running the tests, compile and link the
tests. When cross-compiling with clang we will need to pass sufficient
information to generate code for the Arm architecture we are targeting. We will
need to select the Arm target, select the Armv7-A architecture and choose
between using Arm or Thumb.
instructions. For example:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">--target=arm-linux-gnueabihf</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-march=armv7a</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-mthumb</span></code></li>
</ul>
<p>When using a GCC arm-linux-gnueabihf toolchain the following flags are
needed to pick up the includes and libraries:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">--gcc-toolchain=/path/to/dir/toolchain</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">--sysroot=/path/to/toolchain/arm-linux-gnueabihf/libc</span></code></li>
</ul>
<p>In this example we will be adding all of the command line options to both
<code class="docutils literal notranslate"><span class="pre">CMAKE_C_FLAGS</span></code> and <code class="docutils literal notranslate"><span class="pre">CMAKE_ASM_FLAGS</span></code>. There are cmake flags to pass some of
these options individually which can be used to simplify the <code class="docutils literal notranslate"><span class="pre">build-c-flags</span></code>:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_COMPILER_TARGET="arm-linux-gnueabihf"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_ASM_COMPILER_TARGET="arm-linux-gnueabihf"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_COMPILER_EXTERNAL_TOOLCHAIN=/path/to/dir/toolchain</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_SYSROOT=/path/to/dir/toolchain/arm-linux-gnueabihf/libc</span></code></li>
</ul>
<p>Once cmake has completed the builtins can be built with <code class="docutils literal notranslate"><span class="pre">ninja</span> <span class="pre">builtins</span></code></p>
</div>
<div class="section" id="testing-compiler-rt-builtins-using-qemu-arm">
<h2>Testing compiler-rt builtins using qemu-arm<a class="headerlink" href="#testing-compiler-rt-builtins-using-qemu-arm" title="Permalink to this headline">¶</a></h2>
<p>To test the builtins library we need to add a few more cmake flags to enable
testing and set up the compiler and flags for test case. We must also tell
cmake that we wish to run the tests on <code class="docutils literal notranslate"><span class="pre">qemu-arm</span></code>.</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_EMULATOR="qemu-arm</span> <span class="pre">-L</span> <span class="pre">/path/to/armhf/sysroot</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_INCLUDE_TESTS=ON</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_TEST_COMPILER="/path/to/clang"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_TEST_COMPILER_CFLAGS="test-c-flags"</span></code></li>
</ul>
<p>The <code class="docutils literal notranslate"><span class="pre">/path/to/armhf/sysroot</span></code> should be the same as the one passed to
<code class="docutils literal notranslate"><span class="pre">--sysroot</span></code> in the “build-c-flags”.</p>
<p>The “test-c-flags” need to include the target, architecture, gcc-toolchain,
sysroot and arm/thumb state. The additional cmake defines such as
<code class="docutils literal notranslate"><span class="pre">CMAKE_C_COMPILER_EXTERNAL_TOOLCHAIN</span></code> do not apply when building the tests. If
you have put all of these in “build-c-flags” then these can be repeated. If you
wish to use lld to link the tests then add <code class="docutils literal notranslate"><span class="pre">"-fuse-ld=lld</span></code>.</p>
<p>Once cmake has completed the tests can be built and run using
<code class="docutils literal notranslate"><span class="pre">ninja</span> <span class="pre">check-builtins</span></code></p>
</div>
<div class="section" id="troubleshooting">
<h2>Troubleshooting<a class="headerlink" href="#troubleshooting" title="Permalink to this headline">¶</a></h2>
<div class="section" id="the-cmake-try-compile-stage-fails">
<h3>The cmake try compile stage fails<a class="headerlink" href="#the-cmake-try-compile-stage-fails" title="Permalink to this headline">¶</a></h3>
<p>At an early stage cmake will attempt to compile and link a simple C program to
test if the toolchain is working.</p>
<p>This stage can often fail at link time if the <code class="docutils literal notranslate"><span class="pre">--sysroot</span></code> and
<code class="docutils literal notranslate"><span class="pre">--gcc-toolchain</span></code> options are not passed to the compiler. Check the
<code class="docutils literal notranslate"><span class="pre">CMAKE_C_FLAGS</span></code> and <code class="docutils literal notranslate"><span class="pre">CMAKE_C_COMPILER_TARGET</span></code> flags.</p>
<p>It can be useful to build a simple example outside of cmake with your toolchain
to make sure it is working. For example: <code class="docutils literal notranslate"><span class="pre">clang</span> <span class="pre">--target=arm-linux-gnueabi</span> <span class="pre">-march=armv7a</span> <span class="pre">--gcc-toolchain=/path/to/gcc-toolchain</span> <span class="pre">--sysroot=/path/to/gcc-toolchain/arm-linux-gnueabihf/libc</span> <span class="pre">helloworld.c</span></code></p>
</div>
<div class="section" id="clang-uses-the-host-header-files">
<h3>Clang uses the host header files<a class="headerlink" href="#clang-uses-the-host-header-files" title="Permalink to this headline">¶</a></h3>
<p>On debian based systems it is possible to install multiarch support for
arm-linux-gnueabi and arm-linux-gnueabihf. In many cases clang can successfully
use this multiarch support when -gcc-toolchain and –sysroot are not supplied.
Unfortunately clang adds <code class="docutils literal notranslate"><span class="pre">/usr/local/include</span></code> before
<code class="docutils literal notranslate"><span class="pre">/usr/include/arm-linux-gnueabihf</span></code> leading to errors when compiling the hosts
header files.</p>
<p>The multiarch support is not sufficient to build the builtins you will need to
use a separate arm-linux-gnueabihf toolchain.</p>
</div>
<div class="section" id="no-target-passed-to-clang">
<h3>No target passed to clang<a class="headerlink" href="#no-target-passed-to-clang" title="Permalink to this headline">¶</a></h3>
<p>If clang is not given a target it will typically use the host target, this will
not understand the Arm assembly language files resulting in error messages such
as <code class="docutils literal notranslate"><span class="pre">error:</span> <span class="pre">unknown</span> <span class="pre">directive</span> <span class="pre">.syntax</span> <span class="pre">unified</span></code>.</p>
<p>You can check the clang invocation in the error message to see if there is no
<code class="docutils literal notranslate"><span class="pre">--target</span></code> or if it is set incorrectly. The cause is usually
<code class="docutils literal notranslate"><span class="pre">CMAKE_ASM_FLAGS</span></code> not containing <code class="docutils literal notranslate"><span class="pre">--target</span></code> or <code class="docutils literal notranslate"><span class="pre">CMAKE_ASM_COMPILER_TARGET</span></code> not being present.</p>
</div>
<div class="section" id="arm-architecture-not-given">
<h3>Arm architecture not given<a class="headerlink" href="#arm-architecture-not-given" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">--target=arm-linux-gnueabihf</span></code> will default to arm architecture v4t which
cannot assemble the barrier instructions used in the synch_and_fetch source
files.</p>
<p>The cause is usually a missing <code class="docutils literal notranslate"><span class="pre">-march=armv7a</span></code> from the <code class="docutils literal notranslate"><span class="pre">CMAKE_ASM_FLAGS</span></code>.</p>
</div>
<div class="section" id="compiler-rt-builds-but-the-tests-fail-to-build">
<h3>Compiler-rt builds but the tests fail to build<a class="headerlink" href="#compiler-rt-builds-but-the-tests-fail-to-build" title="Permalink to this headline">¶</a></h3>
<p>The flags used to build the tests are not the same as those used to build the
builtins. The c flags are provided by <code class="docutils literal notranslate"><span class="pre">COMPILER_RT_TEST_COMPILE_CFLAGS</span></code> and
the <code class="docutils literal notranslate"><span class="pre">CMAKE_C_COMPILER_TARGET</span></code>, <code class="docutils literal notranslate"><span class="pre">CMAKE_ASM_COMPILER_TARGET</span></code>,
<code class="docutils literal notranslate"><span class="pre">CMAKE_C_COMPILER_EXTERNAL_TOOLCHAIN</span></code> and <code class="docutils literal notranslate"><span class="pre">CMAKE_SYSROOT</span></code> flags are not
applied.</p>
<p>Make sure that <code class="docutils literal notranslate"><span class="pre">COMPILER_RT_TEST_COMPILE_CFLAGS</span></code> contains all the necessary
information.</p>
</div>
</div>
<div class="section" id="modifications-for-other-targets">
<h2>Modifications for other Targets<a class="headerlink" href="#modifications-for-other-targets" title="Permalink to this headline">¶</a></h2>
<div class="section" id="arm-soft-float-target">
<h3>Arm Soft-Float Target<a class="headerlink" href="#arm-soft-float-target" title="Permalink to this headline">¶</a></h3>
<p>The instructions for the Arm hard-float target can be used for the soft-float
target by substituting soft-float equivalents for the sysroot and target. The
target to use is:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_COMPILER_TARGET=arm-linux-gnueabi</span></code></li>
</ul>
<p>Depending on whether you want to use floating point instructions or not you
may need extra c-flags such as <code class="docutils literal notranslate"><span class="pre">-mfloat-abi=softfp</span></code> for use of floating-point
instructions, and <code class="docutils literal notranslate"><span class="pre">-mfloat-abi=soft</span> <span class="pre">-mfpu=none</span></code> for software floating-point
emulation.</p>
<p>You will need to use an arm-linux-gnueabi GNU toolchain for soft-float.</p>
</div>
<div class="section" id="aarch64-target">
<h3>AArch64 Target<a class="headerlink" href="#aarch64-target" title="Permalink to this headline">¶</a></h3>
<p>The instructions for Arm can be used for AArch64 by substituting AArch64
equivalents for the sysroot, emulator and target.</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_COMPILER_TARGET=aarch64-linux-gnu</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_EMULATOR="qemu-aarch64</span> <span class="pre">-L</span> <span class="pre">/path/to/aarch64/sysroot</span></code></li>
</ul>
<p>The CMAKE_C_FLAGS and COMPILER_RT_TEST_COMPILER_CFLAGS may also need:
<code class="docutils literal notranslate"><span class="pre">"--sysroot=/path/to/aarch64/sysroot</span> <span class="pre">--gcc-toolchain=/path/to/gcc-toolchain"</span></code></p>
</div>
<div class="section" id="armv6-m-armv7-m-and-armv7e-m-targets">
<h3>Armv6-m, Armv7-m and Armv7E-M targets<a class="headerlink" href="#armv6-m-armv7-m-and-armv7e-m-targets" title="Permalink to this headline">¶</a></h3>
<p>To build and test the libraries using a similar method to Armv7-A is possible
but more difficult. The main problems are:</p>
<ul class="simple">
<li>There isn’t a <code class="docutils literal notranslate"><span class="pre">qemu-arm</span></code> user-mode emulator for bare-metal systems. The <code class="docutils literal notranslate"><span class="pre">qemu-system-arm</span></code> can be used but this is significantly more difficult to setup.</li>
<li>The targets to compile compiler-rt have the suffix -none-eabi. This uses the BareMetal driver in clang and by default won’t find the libraries needed to pass the cmake compiler check.</li>
</ul>
<p>As the Armv6-M, Armv7-M and Armv7E-M builds of compiler-rt only use instructions
that are supported on Armv7-A we can still get most of the value of running the
tests using the same <code class="docutils literal notranslate"><span class="pre">qemu-arm</span></code> that we used for Armv7-A by building and
running the test cases for Armv7-A but using the builtins compiled for
Armv6-M, Armv7-M or Armv7E-M. This will test that the builtins can be linked
into a binary and execute the tests correctly but it will not catch if the
builtins use instructions that are supported on Armv7-A but not Armv6-M,
Armv7-M and Armv7E-M.</p>
<p>To get the cmake compile test to pass you will need to pass the libraries
needed to successfully link the cmake test via <code class="docutils literal notranslate"><span class="pre">CMAKE_CFLAGS</span></code>. It is
strongly recommended that you use version 3.6 or above of cmake so you can use
<code class="docutils literal notranslate"><span class="pre">CMAKE_TRY_COMPILE_TARGET=STATIC_LIBRARY</span></code> to skip the link step.</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_OS_DIR="baremetal"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_BUILTINS=ON</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_SANITIZERS=OFF</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_XRAY=OFF</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_LIBFUZZER=OFF</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BUILD_PROFILE=OFF</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_COMPILER=${host_install_dir}/bin/clang</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_COMPILER_TARGET="your</span> <span class="pre">*-none-eabi</span> <span class="pre">target"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_ASM_COMPILER_TARGET="your</span> <span class="pre">*-none-eabi</span> <span class="pre">target"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_AR=/path/to/llvm-ar</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_NM=/path/to/llvm-nm</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_RANLIB=/path/to/llvm-ranlib</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_BAREMETAL_BUILD=ON</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DLLVM_CONFIG_PATH=/path/to/llvm-config</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_C_FLAGS="build-c-flags"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCMAKE_ASM_FLAGS="build-c-flags"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_EMULATOR="qemu-arm</span> <span class="pre">-L</span> <span class="pre">/path/to/armv7-A/sysroot"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_INCLUDE_TESTS=ON</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_TEST_COMPILER="/path/to/clang"</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DCOMPILER_RT_TEST_COMPILER_CFLAGS="test-c-flags"</span></code></li>
</ul>
<p>The Armv6-M builtins will use the soft-float ABI. When compiling the tests for
Armv7-A we must include <code class="docutils literal notranslate"><span class="pre">"-mthumb</span> <span class="pre">-mfloat-abi=soft</span> <span class="pre">-mfpu=none"</span></code> in the
test-c-flags. We must use an Armv7-A soft-float abi sysroot for <code class="docutils literal notranslate"><span class="pre">qemu-arm</span></code>.</p>
<p>Depending on the linker used for the test cases you may encounter BuildAttribute
mismatches between the M-profile objects from compiler-rt and the A-profile
objects from the test. The lld linker does not check the profile
BuildAttribute so it can be used to link the tests by adding -fuse-ld=lld to the
<code class="docutils literal notranslate"><span class="pre">COMPILER_RT_TEST_COMPILER_CFLAGS</span></code>.</p>
</div>
<div class="section" id="alternative-using-a-cmake-cache">
<h3>Alternative using a cmake cache<a class="headerlink" href="#alternative-using-a-cmake-cache" title="Permalink to this headline">¶</a></h3>
<p>If you wish to build, but not test compiler-rt for Armv6-M, Armv7-M or Armv7E-M
the easiest way is to use the BaremetalARM.cmake recipe in clang/cmake/caches.</p>
<p>You will need a bare metal sysroot such as that provided by the GNU ARM
Embedded toolchain.</p>
<p>The libraries can be built with the cmake options:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">-DBAREMETAL_ARMV6M_SYSROOT=/path/to/bare/metal/toolchain/arm-none-eabi</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DBAREMETAL_ARMV7M_SYSROOT=/path/to/bare/metal/toolchain/arm-none-eabi</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-DBAREMETAL_ARMV7EM_SYSROOT=/path/to/bare/metal/toolchain/arm-none-eabi</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">-C</span> <span class="pre">/path/to/llvm/source/tools/clang/cmake/caches/BaremetalARM.cmake</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">/path/to/llvm</span></code></li>
</ul>
<p><strong>Note</strong> that for the recipe to work the compiler-rt source must be checked out
into the directory llvm/runtimes. You will also need clang and lld checked out.</p>
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