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This document contains information on how to cross-compile the compiler-rt builtins library for several flavours of Arm target and how to test the libraries using qemu. Differential Revision: https://reviews.llvm.org/D39600 llvm-svn: 317554
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8.8 KiB
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202 lines
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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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* ``The qemu-arm user mode emulator``
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* ``An arm-linux-gnueabihf sysroot``
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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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``qemu-arm`` should be available as a package for your Linux distribution.
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The most complicated of the prequisites to satisfy is the arm-linux-gnueabihf
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sysroot. The :doc:`HowToCrossCompileLLVM` has information about how to use the
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Linux distributions multiarch support to fulfill the dependencies for building
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LLVM. Alternatively, as building and testing just the compiler-rt builtins
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requires fewer dependencies than LLVM, it is possible to use the Linaro
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arm-linux-gnueabihf gcc installation as our sysroot.
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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/llvm/projects/compiler-rt``
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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=/path/to/clang``
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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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* ``-DCMAKE_EXE_LINKER_FLAGS="-fuse-ld=lld"``
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* ``-DCMAKE_C_COMPILER_TARGET="arm-linux-gnueabihf"``
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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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The build-c-flags need to be sufficient to pass the C-make compiler check and
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to compile compiler-rt. When using a GCC 7 Linaro arm-linux-gnueabihf
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installation the following flags are needed:
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* ``--target=arm-linux-gnueabihf``
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* ``--march=armv7a``
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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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Depending on how your sysroot is laid out, you may not need ``--gcc-toolchain``.
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For example if you have added armhf as an architecture using your Linux
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distributions multiarch support then you should be able to use ``--sysroot=/``.
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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" can be the same as the "build-c-flags", with the addition
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of ``"-fuse-ld=lld`` if you wish to use lld to link the tests.
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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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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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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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If you wish to build, but not test compiler-rt for Armv6-M, Armv7-M or Armv7E-M
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then the easiest way is to use the BaremetalARM.cmake recipe in
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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/sysroot``
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* ``-DBAREMETAL_ARMV7M_SYSROOT=/path/to/bare/metal/sysroot``
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* ``-DBAREMETAL_ARMV7EM_SYSROOT=/path/to/bare/metal/sysroot``
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* ``-C /path/to/llvm/source/tools/clang/cmake/caches/BaremetalARM.cmake``
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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 and not llvm/projects.
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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 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.
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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 not catch instructions that are
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supported on Armv7-A but not Armv6-M, Armv7-M and Armv7E-M.
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To get the cmake compile test to pass the libraries needed to successfully link
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the test application will need to be manually added to ``CMAKE_CFLAGS``.
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Alternatively if you are using version 3.6 or above of cmake 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=${host_install_dir}/bin/clang``
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* ``-DCMAKE_C_COMPILER_TARGET="your *-none-eabi target"``
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* ``-DCMAKE_AR=/path/to/llvm-ar``
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* ``-DCMAKE_NM=/path/to/llvm-nm``
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* ``-DCMAKE_RANLIB=/path/to/llvm-ranlib``
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* ``-DCOMPILER_RT_BAREMETAL_BUILD=ON``
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* ``-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON``
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* ``-DLLVM_CONFIG_PATH=/path/to/llvm-config``
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* ``-DCMAKE_C_FLAGS="build-c-flags"``
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* ``-DCMAKE_ASM_FLAGS="${arm_cflags}"``
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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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Unfortunately at time of writing the Armv7-M and Armv7E-M builds of
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compiler-rt will always include assembler files including floating point
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instructions. This means that building for a cpu without a floating point unit
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requires something like removing the arm_Thumb1_VFPv2_SOURCES from the
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arm_Thumb1_SOURCES in builtins/CMakeLists.txt. The float-abi of the compiler-rt
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library must be matched by the float abi of the Armv7-A sysroot used by
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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 BuildAttributes so it
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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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