Index: docs/HowToCrossCompileBuiltinsOnArm.rst =================================================================== --- /dev/null +++ docs/HowToCrossCompileBuiltinsOnArm.rst @@ -0,0 +1,201 @@ +=================================================================== +How to Cross Compile Compiler-rt Builtins For Arm +=================================================================== + +Introduction +============ + +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. + +Prerequisites +============= + +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`` + * ``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 +options. + +* ``path/to/llvm/projects/compiler-rt`` +* ``-DCOMPILER_RT_BUILD_BUILTINS=ON`` +* ``-DCOMPILER_RT_BUILD_SANITIZERS=OFF`` +* ``-DCOMPILER_RT_BUILD_XRAY=OFF`` +* ``-DCOMPILER_RT_BUILD_LIBFUZZER=OFF`` +* ``-DCOMPILER_RT_BUILD_PROFILE=OFF`` +* ``-DCMAKE_C_COMPILER=/path/to/clang`` +* ``-DCMAKE_AR=/path/to/llvm-ar`` +* ``-DCMAKE_NM=/path/to/llvm-nm`` +* ``-DCMAKE_RANLIB=/path/to/llvm-ranlib`` +* ``-DCMAKE_EXE_LINKER_FLAGS="-fuse-ld=lld"`` +* ``-DCMAKE_C_COMPILER_TARGET="arm-linux-gnueabihf"`` +* ``-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON`` +* ``-DLLVM_CONFIG_PATH=/path/to/llvm-config`` +* ``-DCMAKE_C_FLAGS="build-c-flags"`` + +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: + +* ``--target=arm-linux-gnueabihf`` +* ``--march=armv7a`` +* ``--gcc-toolchain=/path/to/dir/toolchain`` +* ``--sysroot=/path/to/toolchain/arm-linux-gnueabihf/libc`` + +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`` +* ``-DCOMPILER_RT_INCLUDE_TESTS=ON`` +* ``-DCOMPILER_RT_TEST_COMPILER="/path/to/clang"`` +* ``-DCOMPILER_RT_TEST_COMPILER_CFLAGS="test-c-flags"`` + +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 +``ninja check-builtins`` + +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. + +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 +------------------------------------- +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 +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/sysroot`` +* ``-DBAREMETAL_ARMV7M_SYSROOT=/path/to/bare/metal/sysroot`` +* ``-DBAREMETAL_ARMV7EM_SYSROOT=/path/to/bare/metal/sysroot`` +* ``-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_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY`` +* ``-DCOMPILER_RT_OS_DIR="baremetal"`` +* ``-DCOMPILER_RT_BUILD_BUILTINS=ON`` +* ``-DCOMPILER_RT_BUILD_SANITIZERS=OFF`` +* ``-DCOMPILER_RT_BUILD_XRAY=OFF`` +* ``-DCOMPILER_RT_BUILD_LIBFUZZER=OFF`` +* ``-DCOMPILER_RT_BUILD_PROFILE=OFF`` +* ``-DCMAKE_C_COMPILER=${host_install_dir}/bin/clang`` +* ``-DCMAKE_C_COMPILER_TARGET="your *-none-eabi target"`` +* ``-DCMAKE_AR=/path/to/llvm-ar`` +* ``-DCMAKE_NM=/path/to/llvm-nm`` +* ``-DCMAKE_RANLIB=/path/to/llvm-ranlib`` +* ``-DCOMPILER_RT_BAREMETAL_BUILD=ON`` +* ``-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON`` +* ``-DLLVM_CONFIG_PATH=/path/to/llvm-config`` +* ``-DCMAKE_C_FLAGS="build-c-flags"`` +* ``-DCMAKE_ASM_FLAGS="${arm_cflags}"`` +* ``-DCOMPILER_RT_EMULATOR="qemu-arm -L /path/to/armv7-A/sysroot"`` +* ``-DCOMPILER_RT_INCLUDE_TESTS=ON`` +* ``-DCOMPILER_RT_TEST_COMPILER="/path/to/clang"`` +* ``-DCOMPILER_RT_TEST_COMPILER_CFLAGS="test-c-flags"`` + +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 +qemu-ar.m + +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 +``COMPILER_RT_TEST_COMPILER_CFLAGS``. Index: docs/index.rst =================================================================== --- docs/index.rst +++ docs/index.rst @@ -68,6 +68,7 @@ CMakePrimer AdvancedBuilds HowToBuildOnARM + HowToCrossCompileBuiltinsOnArm HowToCrossCompileLLVM CommandGuide/index GettingStarted @@ -105,6 +106,9 @@ :doc:`HowToBuildOnARM` Notes on building and testing LLVM/Clang on ARM. +:doc:`HowToCrossCompileBuiltinsOnArm` + Notes on cross-building and testing the compiler-rt builtins for Arm. + :doc:`HowToCrossCompileLLVM` Notes on cross-building and testing LLVM/Clang.