c10736aa49
Someone asked about this on Discord and it was a bit hard to follow. I found them a config that worked, but the doc was not as much help as it should have been. It probably needs some updates for the runtime build era, but for now, I'm just making it easier to read. I know the basic build can work at least. Some aspects of it may be literally wrong now, but I'll check that later. * Remove contractions. * Remove references to the old separate llvm repo layout. * Remove mentions of cmake versions older than what llvm requires now. * Make a bunch of things plain text. * Make a bunch of things code blocks so they are easier to copy and paste from.
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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 situation.
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Prerequisites
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=============
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In this use case we will 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 will 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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You will need:
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* A build of LLVM 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`` as the build tool.
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See https://compiler-rt.llvm.org/ for 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.
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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.
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The easiest way to supply a sysroot is to download an ``arm-linux-gnueabihf``
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toolchain from https://developer.arm.com/open-source/gnu-toolchain/gnu-a/downloads.
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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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cmake 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.
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We will need to select:
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* The Arm target and Armv7-A architecture with ``--target=arm-linux-gnueabihf -march=armv7a``.
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* Whether to generate Arm (the default) or Thumb instructions (``-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::
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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``
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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 is not a ``qemu-arm`` user-mode emulator for bare-metal systems.
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``qemu-system-arm`` can be used but this is significantly more difficult
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to setup.
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* The targets to compile compiler-rt have the suffix ``-none-eabi``. This uses
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the BareMetal driver in clang and by default will not find the libraries
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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``::
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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 Embedded
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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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