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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``.