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