How to Cross Compile Compiler-rt Builtins For Arm
This document contains information about building and testing the builtins part
of compiler-rt for an Arm target, from an x86_64 Linux machine.
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.
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.
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
* ``A build of LLVM/clang for the llvm-tools and llvm-config``
* ``The qemu-arm user mode emulator``
* ``An arm-linux-gnueabihf sysroot``
See https://compiler-rt.llvm.org/ for more information about the dependencies
on clang and LLVM.
``qemu-arm`` should be available as a package for your Linux distribution.
The most complicated of the prequisites to satisfy is the arm-linux-gnueabihf
sysroot. The :doc:`HowToCrossCompileLLVM` has information about how to use the
Linux distributions multiarch support to fulfill the dependencies for building
LLVM. Alternatively, as building and testing just the compiler-rt builtins
requires fewer dependencies than LLVM, it is possible to use the Linaro
arm-linux-gnueabihf gcc installation as our sysroot.
Building compiler-rt builtins for Arm
We will be doing a standalone build of compiler-rt using the following cmake
The build-c-flags need to be sufficient to pass the C-make compiler check and
to compile compiler-rt. When using a GCC 7 Linaro arm-linux-gnueabihf
installation the following flags are needed:
Depending on how your sysroot is laid out, you may not need ``--gcc-toolchain``.
For example if you have added armhf as an architecture using your Linux
distributions multiarch support then you should be able to use ``--sysroot=/``.
Once cmake has completed the builtins can be built with ``ninja builtins``
Testing compiler-rt builtins using qemu-arm
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 ``qemu-arm``.
* ``-DCOMPILER_RT_EMULATOR="qemu-arm -L /path/to/armhf/sysroot``
The ``/path/to/armhf/sysroot`` should be the same as the one passed to
``--sysroot`` in the "build-c-flags".
The "test-c-flags" can be the same as the "build-c-flags", with the addition
of ``"-fuse-ld=lld`` if you wish to use lld to link the tests.
Once cmake has completed the tests can be built and run using
Modifications for other Targets
Arm Soft-Float Target
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:
Depending on whether you want to use floating point instructions or not you
may need extra c-flags such as ``-mfloat-abi=softfp`` for use of floating-point
instructions, and ``-mfloat-abi=soft -mfpu=none`` for software floating-point
The instructions for Arm can be used for AArch64 by substituting AArch64
equivalents for the sysroot, emulator and target.
* ``-DCOMPILER_RT_EMULATOR="qemu-aarch64 -L /path/to/aarch64/sysroot``
The CMAKE_C_FLAGS and COMPILER_RT_TEST_COMPILER_CFLAGS may also need:
Armv6-m, Armv7-m and Armv7E-M targets
If you wish to build, but not test compiler-rt for Armv6-M, Armv7-M or Armv7E-M
then the easiest way is to use the BaremetalARM.cmake recipe in
You will need a bare metal sysroot such as that provided by the GNU ARM
The libraries can be built with the cmake options:
* ``-C /path/to/llvm/source/tools/clang/cmake/caches/BaremetalARM.cmake``
**Note** that for the recipe to work the compiler-rt source must be checked out
into the directory llvm/runtimes and not llvm/projects.
To build and test the libraries using a similar method to Armv7-A is possible
but more difficult. The main problems are:
* There isn't a ``qemu-arm`` user-mode emulator for bare-metal systems. The ``qemu-system-arm`` can be used but this is significantly more difficult to setup.
* The target 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.
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 ``qemu-arm`` 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 not catch instructions that are
supported on Armv7-A but not Armv6-M, Armv7-M and Armv7E-M.
To get the cmake compile test to pass the libraries needed to successfully link
the test application will need to be manually added to ``CMAKE_CFLAGS``.
Alternatively if you are using version 3.6 or above of cmake you can use
``CMAKE_TRY_COMPILE_TARGET=STATIC_LIBRARY`` to skip the link step.
* ``-DCMAKE_C_COMPILER_TARGET="your *-none-eabi target"``
* ``-DCOMPILER_RT_EMULATOR="qemu-arm -L /path/to/armv7-A/sysroot"``
The Armv6-M builtins will use the soft-float ABI. When compiling the tests for
Armv7-A we must include ``"-mthumb -mfloat-abi=soft -mfpu=none"`` in the
test-c-flags. We must use an Armv7-A soft-float abi sysroot for ``qemu-arm``.
Unfortunately at time of writing the Armv7-M and Armv7E-M builds of
compiler-rt will always include assembler files including floating point
instructions. This means that building for a cpu without a floating point unit
requires something like removing the arm_Thumb1_VFPv2_SOURCES from the
arm_Thumb1_SOURCES in builtins/CMakeLists.txt. The float-abi of the compiler-rt
library must be matched by the float abi of the Armv7-A sysroot used by
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 BuildAttributes so it
can be used to link the tests by adding -fuse-ld=lld to the