# Advanced Build Configurations

## Introduction

CMake is a cross-platform build-generator tool. CMake does not build the project, it generates the files needed by your build tool (GNU make, Visual Studio, etc.) for building LLVM.

If you are a new contributor, please start with the Getting Started with the LLVM System or Building LLVM with CMake pages. This page is intended for users doing more complex builds.

Many of the examples below are written assuming specific CMake Generators. Unless otherwise explicitly called out these commands should work with any CMake generator.

Many of the build configurations mentioned on this documentation page can be utilized by using a CMake cache. A CMake cache is essentially a configuration file that sets the necessary flags for a specific build configuration. The caches for Clang are located in `/clang/cmake/caches` within the monorepo. They can be passed to CMake using the `-C` flag as demonstrated in the examples below along with additional configuration flags.

## Bootstrap Builds

The Clang CMake build system supports bootstrap (aka multi-stage) builds. At a high level a multi-stage build is a chain of builds that pass data from one stage into the next. The most common and simple version of this is a traditional bootstrap build.

In a simple two-stage bootstrap build, we build clang using the system compiler, then use that just-built clang to build clang again. In CMake this simplest form of a bootstrap build can be configured with a single option, CLANG_ENABLE_BOOTSTRAP.

    $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
        -DCLANG_ENABLE_BOOTSTRAP=On \
        -DLLVM_ENABLE_PROJECTS="clang" \
        <path to source>/llvm
    $ ninja stage2

This command itself isn’t terribly useful because it assumes default configurations for each stage. The next series of examples utilize CMake cache scripts to provide more complex options.

By default, only a few CMake options will be passed between stages. The list, called _BOOTSTRAP_DEFAULT_PASSTHROUGH, is defined in clang/CMakeLists.txt. To force the passing of the variables between stages, use the -DCLANG_BOOTSTRAP_PASSTHROUGH CMake option, each variable separated by a “;”. As example:

    $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
        -DCLANG_ENABLE_BOOTSTRAP=On \
        -DCLANG_BOOTSTRAP_PASSTHROUGH="CMAKE_INSTALL_PREFIX;CMAKE_VERBOSE_MAKEFILE" \
        -DLLVM_ENABLE_PROJECTS="clang" \
        <path to source>/llvm
    $ ninja stage2

CMake options starting by `BOOTSTRAP_` will be passed only to the stage2 build. This gives the opportunity to use Clang specific build flags. For example, the following CMake call will enabled ‘-fno-addrsig’ only during the stage2 build for C and C++.

    $ cmake [..]  -DBOOTSTRAP_CMAKE_CXX_FLAGS='-fno-addrsig' -DBOOTSTRAP_CMAKE_C_FLAGS='-fno-addrsig' [..]

The clang build system refers to builds as stages. A stage1 build is a standard build using the compiler installed on the host, and a stage2 build is built using the stage1 compiler. This nomenclature holds up to more stages too. In general a stage*n* build is built using the output from stage*n-1*.

## Apple Clang Builds (A More Complex Bootstrap)

Apple’s Clang builds are a slightly more complicated example of the simple bootstrapping scenario. Apple Clang is built using a 2-stage build.

The stage1 compiler is a host-only compiler with some options set. The stage1 compiler is a balance of optimization vs build time because it is a throwaway. The stage2 compiler is the fully optimized compiler intended to ship to users.

Setting up these compilers requires a lot of options. To simplify the configuration the Apple Clang build settings are contained in CMake Cache files. You can build an Apple Clang compiler using the following commands:

    $ cmake -G Ninja -C <path to source>/clang/cmake/caches/Apple-stage1.cmake <path to source>/llvm
    $ ninja stage2-distribution

This CMake invocation configures the stage1 host compiler, and sets CLANG_BOOTSTRAP_CMAKE_ARGS to pass the Apple-stage2.cmake cache script to the stage2 configuration step.

When you build the stage2-distribution target it builds the minimal stage1 compiler and required tools, then configures and builds the stage2 compiler based on the settings in Apple-stage2.cmake.

This pattern of using cache scripts to set complex settings, and specifically to make later stage builds include cache scripts is common in our more advanced build configurations.

## Multi-stage PGO

Profile-Guided Optimizations (PGO) is a really great way to optimize the code clang generates. Our multi-stage PGO builds are a workflow for generating PGO profiles that can be used to optimize clang.

At a high level, the way PGO works is that you build an instrumented compiler, then you run the instrumented compiler against sample source files. While the instrumented compiler runs it will output a bunch of files containing performance counters (.profraw files). After generating all the profraw files you use llvm-profdata to merge the files into a single profdata file that you can feed into the LLVM_PROFDATA_FILE option.

Our PGO.cmake cache automates that whole process. You can use it for configuration with CMake with the following command:

    $ cmake -G Ninja -C <path to source>/clang/cmake/caches/PGO.cmake \
        <path to source>/llvm

There are several additional options that the cache file also accepts to modify the build, particularly the PGO_INSTRUMENT_LTO option. Setting this option to Thin or Full will enable ThinLTO or full LTO respectively, further enhancing the performance gains from a PGO build by enabling interprocedural optimizations. For example, to run a CMake configuration for a PGO build that also enables ThinTLO, use the following command:

    $ cmake -G Ninja -C <path to source>/clang/cmake/caches/PGO.cmake \
        -DPGO_INSTRUMENT_LTO=Thin \
        <path to source>/llvm

After configuration, building the stage2-instrumented-generate-profdata target will automatically build the stage1 compiler, build the instrumented compiler with the stage1 compiler, and then run the instrumented compiler against the perf training data:

    $ ninja stage2-instrumented-generate-profdata

If you let that run for a few hours or so, it will place a profdata file in your build directory. This takes a really long time because it builds clang twice, and you must have compiler-rt in your build tree.

This process uses any source files under the perf-training directory as training data as long as the source files are marked up with LIT-style RUN lines.

After it finishes you can use `find . -name clang.profdata` to find it, but it should be at a path something like:

    <build dir>/tools/clang/stage2-instrumented-bins/utils/perf-training/clang.profdata

You can feed that file into the LLVM_PROFDATA_FILE option when you build your optimized compiler.

It may be necessary to build additional targets before running perf training, such as builtins and runtime libraries. You can use the `CLANG_PERF_TRAINING_DEPS` CMake variable for that purpose:

    set(CLANG_PERF_TRAINING_DEPS builtins runtimes CACHE STRING "")

The PGO cache has a slightly different stage naming scheme than other multi-stage builds. It generates three stages: stage1, stage2-instrumented, and stage2. Both of the stage2 builds are built using the stage1 compiler.

The PGO cache generates the following additional targets:

stage2-instrumented

  Builds a stage1 compiler, runtime, and required tools (llvm-config, llvm-profdata) then uses that compiler to build an instrumented stage2 compiler.

stage2-instrumented-generate-profdata

  Depends on stage2-instrumented and will use the instrumented compiler to generate profdata based on the training files in clang/utils/perf-training

stage2

  Depends on stage2-instrumented-generate-profdata and will use the stage1 compiler with the stage2 profdata to build a PGO-optimized compiler.

stage2-check-llvm

  Depends on stage2 and runs check-llvm using the stage2 compiler.

stage2-check-clang

  Depends on stage2 and runs check-clang using the stage2 compiler.

stage2-check-all

  Depends on stage2 and runs check-all using the stage2 compiler.

stage2-test-suite

  Depends on stage2 and runs the test-suite using the stage2 compiler (requires in-tree test-suite).

## BOLT

BOLT (Binary Optimization and Layout Tool) is a tool that optimizes binaries post-link by profiling them at runtime and then using that information to optimize the layout of the final binary among other optimizations performed at the binary level. There are also CMake caches available to build LLVM/Clang with BOLT.

To configure a single-stage build that builds LLVM/Clang and then optimizes it with BOLT, use the following CMake configuration:

    $ cmake <path to source>/llvm -C <path to source>/clang/cmake/caches/BOLT.cmake

Then, build the BOLT-optimized binary by running the following ninja command:

    $ ninja clang-bolt

If you’re seeing errors in the build process, try building with a recent version of Clang/LLVM by setting the CMAKE_C_COMPILER and CMAKE_CXX_COMPILER flags to the appropriate values.

It is also possible to use BOLT on top of PGO and (Thin)LTO for an even more significant runtime speedup. To configure a three stage PGO build with ThinLTO that optimizes the resulting binary with BOLT, use the following CMake configuration command:

    $ cmake -G Ninja <path to source>/llvm \
        -C <path to source>/clang/cmake/caches/BOLT-PGO.cmake \
        -DBOOTSTRAP_LLVM_ENABLE_LLD=ON \
        -DBOOTSTRAP_BOOTSTRAP_LLVM_ENABLE_LLD=ON \
        -DPGO_INSTRUMENT_LTO=Thin

Then, to build the final optimized binary, build the stage2-clang-bolt target:

    $ ninja stage2-clang-bolt

## 3-Stage Non-Determinism

In the ancient lore of compilers non-determinism is like the multi-headed hydra. Whenever its head pops up, terror and chaos ensue.

Historically one of the tests to verify that a compiler was deterministic would be a three stage build. The idea of a three stage build is you take your sources and build a compiler (stage1), then use that compiler to rebuild the sources (stage2), then you use that compiler to rebuild the sources a third time (stage3) with an identical configuration to the stage2 build. At the end of this, you have a stage2 and stage3 compiler that should be bit-for-bit identical.

You can perform one of these 3-stage builds with LLVM and clang using the following commands:

    $ cmake -G Ninja -C <path to source>/clang/cmake/caches/3-stage.cmake <path to source>/llvm
    $ ninja stage3

After the build you can compare the stage2 and stage3 compilers.