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      1 ===================================================================
      2 How to Cross Compile Compiler-rt Builtins For Arm
      3 ===================================================================
      4 
      5 Introduction
      6 ============
      7 
      8 This document contains information about building and testing the builtins part
      9 of compiler-rt for an Arm target, from an x86_64 Linux machine.
     10 
     11 While this document concentrates on Arm and Linux the general principles should
     12 apply to other targets supported by compiler-rt. Further contributions for other
     13 targets are welcome.
     14 
     15 The instructions in this document depend on libraries and programs external to
     16 LLVM, there are many ways to install and configure these dependencies so you
     17 may need to adapt the instructions here to fit your own local situation.
     18 
     19 Prerequisites
     20 =============
     21 
     22 In this use case we'll be using CMake on a Debian-based Linux system,
     23 cross-compiling from an x86_64 host to a hard-float Armv7-A target. We'll be
     24 using as many of the LLVM tools as we can, but it is possible to use GNU
     25 equivalents.
     26 
     27  * ``A build of LLVM/clang for the llvm-tools and llvm-config``
     28  * ``The qemu-arm user mode emulator``
     29  * ``An arm-linux-gnueabihf sysroot``
     30 
     31 See https://compiler-rt.llvm.org/ for more information about the dependencies
     32 on clang and LLVM.
     33 
     34 ``qemu-arm`` should be available as a package for your Linux distribution.
     35 
     36 The most complicated of the prequisites to satisfy is the arm-linux-gnueabihf
     37 sysroot. The :doc:`HowToCrossCompileLLVM` has information about how to use the
     38 Linux distributions multiarch support to fulfill the dependencies for building
     39 LLVM. Alternatively, as building and testing just the compiler-rt builtins
     40 requires fewer dependencies than LLVM, it is possible to use the Linaro
     41 arm-linux-gnueabihf gcc installation as our sysroot.
     42 
     43 Building compiler-rt builtins for Arm
     44 =====================================
     45 We will be doing a standalone build of compiler-rt using the following cmake
     46 options.
     47 
     48 * ``path/to/llvm/projects/compiler-rt``
     49 * ``-DCOMPILER_RT_BUILD_BUILTINS=ON``
     50 * ``-DCOMPILER_RT_BUILD_SANITIZERS=OFF``
     51 * ``-DCOMPILER_RT_BUILD_XRAY=OFF``
     52 * ``-DCOMPILER_RT_BUILD_LIBFUZZER=OFF``
     53 * ``-DCOMPILER_RT_BUILD_PROFILE=OFF``
     54 * ``-DCMAKE_C_COMPILER=/path/to/clang``
     55 * ``-DCMAKE_AR=/path/to/llvm-ar``
     56 * ``-DCMAKE_NM=/path/to/llvm-nm``
     57 * ``-DCMAKE_RANLIB=/path/to/llvm-ranlib``
     58 * ``-DCMAKE_EXE_LINKER_FLAGS="-fuse-ld=lld"``
     59 * ``-DCMAKE_C_COMPILER_TARGET="arm-linux-gnueabihf"``
     60 * ``-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON``
     61 * ``-DLLVM_CONFIG_PATH=/path/to/llvm-config``
     62 * ``-DCMAKE_C_FLAGS="build-c-flags"``
     63 
     64 The build-c-flags need to be sufficient to pass the C-make compiler check and
     65 to compile compiler-rt. When using a GCC 7 Linaro arm-linux-gnueabihf
     66 installation the following flags are needed:
     67 
     68 * ``--target=arm-linux-gnueabihf``
     69 * ``--march=armv7a``
     70 * ``--gcc-toolchain=/path/to/dir/toolchain``
     71 * ``--sysroot=/path/to/toolchain/arm-linux-gnueabihf/libc``
     72 
     73 Depending on how your sysroot is laid out, you may not need ``--gcc-toolchain``.
     74 For example if you have added armhf as an architecture using your Linux
     75 distributions multiarch support then you should be able to use ``--sysroot=/``.
     76 
     77 Once cmake has completed the builtins can be built with ``ninja builtins``
     78 
     79 Testing compiler-rt builtins using qemu-arm
     80 ===========================================
     81 To test the builtins library we need to add a few more cmake flags to enable
     82 testing and set up the compiler and flags for test case. We must also tell
     83 cmake that we wish to run the tests on ``qemu-arm``.
     84 
     85 * ``-DCOMPILER_RT_EMULATOR="qemu-arm -L /path/to/armhf/sysroot``
     86 * ``-DCOMPILER_RT_INCLUDE_TESTS=ON``
     87 * ``-DCOMPILER_RT_TEST_COMPILER="/path/to/clang"``
     88 * ``-DCOMPILER_RT_TEST_COMPILER_CFLAGS="test-c-flags"``
     89 
     90 The ``/path/to/armhf/sysroot`` should be the same as the one passed to
     91 ``--sysroot`` in the "build-c-flags".
     92 
     93 The "test-c-flags" can be the same as the "build-c-flags", with the addition
     94 of ``"-fuse-ld=lld`` if you wish to use lld to link the tests.
     95 
     96 Once cmake has completed the tests can be built and run using
     97 ``ninja check-builtins``
     98 
     99 Modifications for other Targets
    100 ===============================
    101 
    102 Arm Soft-Float Target
    103 ---------------------
    104 The instructions for the Arm hard-float target can be used for the soft-float
    105 target by substituting soft-float equivalents for the sysroot and target. The
    106 target to use is:
    107 
    108 * ``-DCMAKE_C_COMPILER_TARGET=arm-linux-gnueabi``
    109 
    110 Depending on whether you want to use floating point instructions or not you
    111 may need extra c-flags such as ``-mfloat-abi=softfp`` for use of floating-point
    112 instructions, and ``-mfloat-abi=soft -mfpu=none`` for software floating-point
    113 emulation.
    114 
    115 AArch64 Target
    116 --------------
    117 The instructions for Arm can be used for AArch64 by substituting AArch64
    118 equivalents for the sysroot, emulator and target.
    119 
    120 * ``-DCMAKE_C_COMPILER_TARGET=aarch64-linux-gnu``
    121 * ``-DCOMPILER_RT_EMULATOR="qemu-aarch64 -L /path/to/aarch64/sysroot``
    122 
    123 The CMAKE_C_FLAGS and COMPILER_RT_TEST_COMPILER_CFLAGS may also need:
    124 ``"--sysroot=/path/to/aarch64/sysroot --gcc-toolchain=/path/to/gcc-toolchain"``
    125 
    126 Armv6-m, Armv7-m and Armv7E-M targets
    127 -------------------------------------
    128 If you wish to build, but not test compiler-rt for Armv6-M, Armv7-M or Armv7E-M
    129 then the easiest way is to use the BaremetalARM.cmake recipe in
    130 clang/cmake/caches.
    131 
    132 You will need a bare metal sysroot such as that provided by the GNU ARM
    133 Embedded toolchain.
    134 
    135 The libraries can be built with the cmake options:
    136 
    137 * ``-DBAREMETAL_ARMV6M_SYSROOT=/path/to/bare/metal/sysroot``
    138 * ``-DBAREMETAL_ARMV7M_SYSROOT=/path/to/bare/metal/sysroot``
    139 * ``-DBAREMETAL_ARMV7EM_SYSROOT=/path/to/bare/metal/sysroot``
    140 * ``-C /path/to/llvm/source/tools/clang/cmake/caches/BaremetalARM.cmake``
    141 
    142 **Note** that for the recipe to work the compiler-rt source must be checked out
    143 into the directory llvm/runtimes and not llvm/projects.
    144 
    145 To build and test the libraries using a similar method to Armv7-A is possible
    146 but more difficult. The main problems are:
    147 
    148 * 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.
    149 * 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.
    150 
    151 As the Armv6-M, Armv7-M and Armv7E-M builds of compiler-rt only use instructions
    152 that are supported on Armv7-A we can still get most of the value of running the
    153 tests using the same ``qemu-arm`` that we used for Armv7-A by building and
    154 running the test cases for Armv7-A but using the builtins compiled for
    155 Armv6-M, Armv7-M or Armv7E-M. This will not catch instructions that are
    156 supported on Armv7-A but not Armv6-M, Armv7-M and Armv7E-M.
    157 
    158 To get the cmake compile test to pass the libraries needed to successfully link
    159 the test application will need to be manually added to ``CMAKE_CFLAGS``.
    160 Alternatively if you are using version 3.6 or above of cmake you can use
    161 ``CMAKE_TRY_COMPILE_TARGET=STATIC_LIBRARY`` to skip the link step.
    162 
    163 * ``-DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY``
    164 * ``-DCOMPILER_RT_OS_DIR="baremetal"``
    165 * ``-DCOMPILER_RT_BUILD_BUILTINS=ON``
    166 * ``-DCOMPILER_RT_BUILD_SANITIZERS=OFF``
    167 * ``-DCOMPILER_RT_BUILD_XRAY=OFF``
    168 * ``-DCOMPILER_RT_BUILD_LIBFUZZER=OFF``
    169 * ``-DCOMPILER_RT_BUILD_PROFILE=OFF``
    170 * ``-DCMAKE_C_COMPILER=${host_install_dir}/bin/clang``
    171 * ``-DCMAKE_C_COMPILER_TARGET="your *-none-eabi target"``
    172 * ``-DCMAKE_AR=/path/to/llvm-ar``
    173 * ``-DCMAKE_NM=/path/to/llvm-nm``
    174 * ``-DCMAKE_RANLIB=/path/to/llvm-ranlib``
    175 * ``-DCOMPILER_RT_BAREMETAL_BUILD=ON``
    176 * ``-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON``
    177 * ``-DLLVM_CONFIG_PATH=/path/to/llvm-config``
    178 * ``-DCMAKE_C_FLAGS="build-c-flags"``
    179 * ``-DCMAKE_ASM_FLAGS="${arm_cflags}"``
    180 * ``-DCOMPILER_RT_EMULATOR="qemu-arm -L /path/to/armv7-A/sysroot"``
    181 * ``-DCOMPILER_RT_INCLUDE_TESTS=ON``
    182 * ``-DCOMPILER_RT_TEST_COMPILER="/path/to/clang"``
    183 * ``-DCOMPILER_RT_TEST_COMPILER_CFLAGS="test-c-flags"``
    184 
    185 The Armv6-M builtins will use the soft-float ABI. When compiling the tests for
    186 Armv7-A we must include ``"-mthumb -mfloat-abi=soft -mfpu=none"`` in the
    187 test-c-flags. We must use an Armv7-A soft-float abi sysroot for ``qemu-arm``.
    188 
    189 Unfortunately at time of writing the Armv7-M and Armv7E-M builds of
    190 compiler-rt will always include assembler files including floating point
    191 instructions. This means that building for a cpu without a floating point unit
    192 requires something like removing the arm_Thumb1_VFPv2_SOURCES from the
    193 arm_Thumb1_SOURCES in builtins/CMakeLists.txt. The float-abi of the compiler-rt
    194 library must be matched by the float abi of the Armv7-A sysroot used by
    195 qemu-arm.
    196 
    197 Depending on the linker used for the test cases you may encounter BuildAttribute
    198 mismatches between the M-profile objects from compiler-rt and the A-profile
    199 objects from the test. The lld linker does not check the BuildAttributes so it
    200 can be used to link the tests by adding -fuse-ld=lld to the
    201 ``COMPILER_RT_TEST_COMPILER_CFLAGS``.
    202