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 equivalents.

  • A build of LLVM/clang for the llvm-tools and llvm-config

  • A clang executable with support for the ARM target

  • compiler-rt sources

  • The qemu-arm user mode emulator

  • An arm-linux-gnueabihf sysroot

In this example we will be using ninja.

See for more information about the dependencies on clang and LLVM.

See 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.

qemu-arm should be available as a package for your Linux distribution.

The most complicated of the prerequisites 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: * for gcc 8 and above * for gcc 4.9 to 7.3

Building compiler-rt builtins for Arm

We will be doing a standalone build of compiler-rt using the following cmake options.

  • path/to/compiler-rt

  • -G Ninja

  • -DCMAKE_AR=/path/to/llvm-ar

  • -DCMAKE_ASM_COMPILER_TARGET="arm-linux-gnueabihf"

  • -DCMAKE_ASM_FLAGS="build-c-flags"

  • -DCMAKE_C_COMPILER=/path/to/clang

  • -DCMAKE_C_COMPILER_TARGET="arm-linux-gnueabihf"

  • -DCMAKE_C_FLAGS="build-c-flags"

  • -DCMAKE_EXE_LINKER_FLAGS="-fuse-ld=lld"

  • -DCMAKE_NM=/path/to/llvm-nm

  • -DCMAKE_RANLIB=/path/to/llvm-ranlib








  • -DLLVM_CONFIG_PATH=/path/to/llvm-config

The build-c-flags 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:

  • --target=arm-linux-gnueabihf

  • -march=armv7a

  • -mthumb

When using a GCC arm-linux-gnueabihf toolchain the following flags are needed to pick up the includes and libraries:

  • --gcc-toolchain=/path/to/dir/toolchain

  • --sysroot=/path/to/toolchain/arm-linux-gnueabihf/libc

In this example we will be adding all of the command line options to both CMAKE_C_FLAGS and CMAKE_ASM_FLAGS. There are cmake flags to pass some of these options individually which can be used to simplify the build-c-flags:

  • -DCMAKE_C_COMPILER_TARGET="arm-linux-gnueabihf"

  • -DCMAKE_ASM_COMPILER_TARGET="arm-linux-gnueabihf"

  • -DCMAKE_C_COMPILER_EXTERNAL_TOOLCHAIN=/path/to/dir/toolchain

  • -DCMAKE_SYSROOT=/path/to/dir/toolchain/arm-linux-gnueabihf/libc

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


  • -DCOMPILER_RT_TEST_COMPILER="/path/to/clang"


The /path/to/armhf/sysroot should be the same as the one passed to --sysroot in the “build-c-flags”.

The “test-c-flags” need to include the target, architecture, gcc-toolchain, sysroot and arm/thumb state. The additional cmake defines such as CMAKE_C_COMPILER_EXTERNAL_TOOLCHAIN 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 "-fuse-ld=lld.

Once cmake has completed the tests can be built and run using ninja check-builtins


The cmake try compile stage fails

At an early stage cmake will attempt to compile and link a simple C program to test if the toolchain is working.

This stage can often fail at link time if the --sysroot= and --gcc-toolchain= options are not passed to the compiler. Check the CMAKE_C_FLAGS and CMAKE_C_COMPILER_TARGET flags.

It can be useful to build a simple example outside of cmake with your toolchain to make sure it is working. For example: clang --target=arm-linux-gnueabi -march=armv7a --gcc-toolchain=/path/to/gcc-toolchain --sysroot=/path/to/gcc-toolchain/arm-linux-gnueabihf/libc helloworld.c

Clang uses the host header files

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 /usr/local/include before /usr/include/arm-linux-gnueabihf leading to errors when compiling the hosts header files.

The multiarch support is not sufficient to build the builtins you will need to use a separate arm-linux-gnueabihf toolchain.

No target passed to clang

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 error: unknown directive .syntax unified.

You can check the clang invocation in the error message to see if there is no --target or if it is set incorrectly. The cause is usually CMAKE_ASM_FLAGS not containing --target or CMAKE_ASM_COMPILER_TARGET not being present.

Arm architecture not given

The --target=arm-linux-gnueabihf will default to arm architecture v4t which cannot assemble the barrier instructions used in the synch_and_fetch source files.

The cause is usually a missing -march=armv7a from the CMAKE_ASM_FLAGS.

Compiler-rt builds but the tests fail to build

The flags used to build the tests are not the same as those used to build the builtins. The c flags are provided by COMPILER_RT_TEST_COMPILE_CFLAGS and the CMAKE_C_COMPILER_TARGET, CMAKE_ASM_COMPILER_TARGET, CMAKE_C_COMPILER_EXTERNAL_TOOLCHAIN and CMAKE_SYSROOT flags are not applied.

Make sure that COMPILER_RT_TEST_COMPILE_CFLAGS contains all the necessary information.

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:

  • -DCMAKE_C_COMPILER_TARGET=arm-linux-gnueabi

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 emulation.

You will need to use an arm-linux-gnueabi GNU toolchain for soft-float.

AArch64 Target

The instructions for Arm can be used for AArch64 by substituting AArch64 equivalents for the sysroot, emulator and target.

  • -DCMAKE_C_COMPILER_TARGET=aarch64-linux-gnu

  • -DCOMPILER_RT_EMULATOR="qemu-aarch64 -L /path/to/aarch64/sysroot

The CMAKE_C_FLAGS and COMPILER_RT_TEST_COMPILER_CFLAGS may also need: "--sysroot=/path/to/aarch64/sysroot --gcc-toolchain=/path/to/gcc-toolchain"

Armv6-m, Armv7-m and Armv7E-M targets

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 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.

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 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.

To get the cmake compile test to pass you will need to pass the libraries needed to successfully link the cmake test via CMAKE_CFLAGS. It is strongly recommended that you use version 3.6 or above of cmake so you can use CMAKE_TRY_COMPILE_TARGET=STATIC_LIBRARY to skip the link step.


  • -DCOMPILER_RT_OS_DIR="baremetal"






  • -DCMAKE_C_COMPILER=${host_install_dir}/bin/clang

  • -DCMAKE_C_COMPILER_TARGET="your *-none-eabi target"

  • -DCMAKE_ASM_COMPILER_TARGET="your *-none-eabi target"

  • -DCMAKE_AR=/path/to/llvm-ar

  • -DCMAKE_NM=/path/to/llvm-nm

  • -DCMAKE_RANLIB=/path/to/llvm-ranlib



  • -DLLVM_CONFIG_PATH=/path/to/llvm-config

  • -DCMAKE_C_FLAGS="build-c-flags"

  • -DCMAKE_ASM_FLAGS="build-c-flags"

  • -DCOMPILER_RT_EMULATOR="qemu-arm -L /path/to/armv7-A/sysroot"


  • -DCOMPILER_RT_TEST_COMPILER="/path/to/clang"


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.

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 COMPILER_RT_TEST_COMPILER_CFLAGS.

Alternative using a cmake cache

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.

You will need a bare metal sysroot such as that provided by the GNU ARM Embedded toolchain.

The libraries can be built with the cmake options:

  • -DBAREMETAL_ARMV6M_SYSROOT=/path/to/bare/metal/toolchain/arm-none-eabi

  • -DBAREMETAL_ARMV7M_SYSROOT=/path/to/bare/metal/toolchain/arm-none-eabi

  • -DBAREMETAL_ARMV7EM_SYSROOT=/path/to/bare/metal/toolchain/arm-none-eabi

  • -C /path/to/llvm/source/tools/clang/cmake/caches/BaremetalARM.cmake

  • /path/to/llvm

Note 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.