1============================= 2Advanced Build Configurations 3============================= 4 5.. contents:: 6 :local: 7 8Introduction 9============ 10 11`CMake <http://www.cmake.org/>`_ is a cross-platform build-generator tool. CMake 12does not build the project, it generates the files needed by your build tool 13(GNU make, Visual Studio, etc.) for building LLVM. 14 15If **you are a new contributor**, please start with the :doc:`GettingStarted` or 16:doc:`CMake` pages. This page is intended for users doing more complex builds. 17 18Many of the examples below are written assuming specific CMake Generators. 19Unless otherwise explicitly called out these commands should work with any CMake 20generator. 21 22Bootstrap Builds 23================ 24 25The Clang CMake build system supports bootstrap (aka multi-stage) builds. At a 26high level a multi-stage build is a chain of builds that pass data from one 27stage into the next. The most common and simple version of this is a traditional 28bootstrap build. 29 30In a simple two-stage bootstrap build, we build clang using the system compiler, 31then use that just-built clang to build clang again. In CMake this simplest form 32of a bootstrap build can be configured with a single option, 33CLANG_ENABLE_BOOTSTRAP. 34 35.. code-block:: console 36 37 $ cmake -G Ninja -DCLANG_ENABLE_BOOTSTRAP=On <path to source> 38 $ ninja stage2 39 40This command itself isn't terribly useful because it assumes default 41configurations for each stage. The next series of examples utilize CMake cache 42scripts to provide more complex options. 43 44The clang build system refers to builds as stages. A stage1 build is a standard 45build using the compiler installed on the host, and a stage2 build is built 46using the stage1 compiler. This nomenclature holds up to more stages too. In 47general a stage*n* build is built using the output from stage*n-1*. 48 49Apple Clang Builds (A More Complex Bootstrap) 50============================================= 51 52Apple's Clang builds are a slightly more complicated example of the simple 53bootstrapping scenario. Apple Clang is built using a 2-stage build. 54 55The stage1 compiler is a host-only compiler with some options set. The stage1 56compiler is a balance of optimization vs build time because it is a throwaway. 57The stage2 compiler is the fully optimized compiler intended to ship to users. 58 59Setting up these compilers requires a lot of options. To simplify the 60configuration the Apple Clang build settings are contained in CMake Cache files. 61You can build an Apple Clang compiler using the following commands: 62 63.. code-block:: console 64 65 $ cmake -G Ninja -C <path to clang>/cmake/caches/Apple-stage1.cmake <path to source> 66 $ ninja stage2-distribution 67 68This CMake invocation configures the stage1 host compiler, and sets 69CLANG_BOOTSTRAP_CMAKE_ARGS to pass the Apple-stage2.cmake cache script to the 70stage2 configuration step. 71 72When you build the stage2-distribution target it builds the minimal stage1 73compiler and required tools, then configures and builds the stage2 compiler 74based on the settings in Apple-stage2.cmake. 75 76This pattern of using cache scripts to set complex settings, and specifically to 77make later stage builds include cache scripts is common in our more advanced 78build configurations. 79 80Multi-stage PGO 81=============== 82 83Profile-Guided Optimizations (PGO) is a really great way to optimize the code 84clang generates. Our multi-stage PGO builds are a workflow for generating PGO 85profiles that can be used to optimize clang. 86 87At a high level, the way PGO works is that you build an instrumented compiler, 88then you run the instrumented compiler against sample source files. While the 89instrumented compiler runs it will output a bunch of files containing 90performance counters (.profraw files). After generating all the profraw files 91you use llvm-profdata to merge the files into a single profdata file that you 92can feed into the LLVM_PROFDATA_FILE option. 93 94Our PGO.cmake cache script automates that whole process. You can use it by 95running: 96 97.. code-block:: console 98 99 $ cmake -G Ninja -C <path_to_clang>/cmake/caches/PGO.cmake <source dir> 100 $ ninja stage2-instrumented-generate-profdata 101 102If you let that run for a few hours or so, it will place a profdata file in your 103build directory. This takes a really long time because it builds clang twice, 104and you *must* have compiler-rt in your build tree. 105 106This process uses any source files under the perf-training directory as training 107data as long as the source files are marked up with LIT-style RUN lines. 108 109After it finishes you can use “find . -name clang.profdata” to find it, but it 110should be at a path something like: 111 112.. code-block:: console 113 114 <build dir>/tools/clang/stage2-instrumented-bins/utils/perf-training/clang.profdata 115 116You can feed that file into the LLVM_PROFDATA_FILE option when you build your 117optimized compiler. 118 119The PGO came cache has a slightly different stage naming scheme than other 120multi-stage builds. It generates three stages; stage1, stage2-instrumented, and 121stage2. Both of the stage2 builds are built using the stage1 compiler. 122 123The PGO came cache generates the following additional targets: 124 125**stage2-instrumented** 126 Builds a stage1 x86 compiler, runtime, and required tools (llvm-config, 127 llvm-profdata) then uses that compiler to build an instrumented stage2 compiler. 128 129**stage2-instrumented-generate-profdata** 130 Depends on "stage2-instrumented" and will use the instrumented compiler to 131 generate profdata based on the training files in <clang>/utils/perf-training 132 133**stage2** 134 Depends of "stage2-instrumented-generate-profdata" and will use the stage1 135 compiler with the stage2 profdata to build a PGO-optimized compiler. 136 137**stage2-check-llvm** 138 Depends on stage2 and runs check-llvm using the stage2 compiler. 139 140**stage2-check-clang** 141 Depends on stage2 and runs check-clang using the stage2 compiler. 142 143**stage2-check-all** 144 Depends on stage2 and runs check-all using the stage2 compiler. 145 146**stage2-test-suite** 147 Depends on stage2 and runs the test-suite using the stage3 compiler (requires 148 in-tree test-suite). 149 1503-Stage Non-Determinism 151======================= 152 153In the ancient lore of compilers non-determinism is like the multi-headed hydra. 154Whenever it's head pops up, terror and chaos ensue. 155 156Historically one of the tests to verify that a compiler was deterministic would 157be a three stage build. The idea of a three stage build is you take your sources 158and build a compiler (stage1), then use that compiler to rebuild the sources 159(stage2), then you use that compiler to rebuild the sources a third time 160(stage3) with an identical configuration to the stage2 build. At the end of 161this, you have a stage2 and stage3 compiler that should be bit-for-bit 162identical. 163 164You can perform one of these 3-stage builds with LLVM & clang using the 165following commands: 166 167.. code-block:: console 168 169 $ cmake -G Ninja -C <path_to_clang>/cmake/caches/3-stage.cmake <source dir> 170 $ ninja stage3 171 172After the build you can compare the stage2 & stage3 compilers. We have a bot 173setup `here <http://lab.llvm.org:8011/builders/clang-3stage-ubuntu>`_ that runs 174this build and compare configuration. 175