llvm/docs/AdvancedBuilds.rst - toolchain/llvm-project - Gitiles

 =============================
 Advanced Build Configurations
 =============================

 .. contents::
    :local:

 Introduction
 ============

 `CMake <http://www.cmake.org/>`_ 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 :doc:`GettingStarted` or
 :doc:`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.

 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.

 .. code-block:: console

   $ cmake -G Ninja -DCLANG_ENABLE_BOOTSTRAP=On <path to source>
   $ 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:

 .. code-block:: console

   $ cmake -G Ninja -DCLANG_ENABLE_BOOTSTRAP=On -DCLANG_BOOTSTRAP_PASSTHROUGH="CMAKE_INSTALL_PREFIX;CMAKE_VERBOSE_MAKEFILE" <path to source>
   $ ninja stage2

 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:

 .. code-block:: console

   $ cmake -G Ninja -C <path to clang>/cmake/caches/Apple-stage1.cmake <path to source>
   $ 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 script automates that whole process. You can use it by
 running:

 .. code-block:: console

   $ cmake -G Ninja -C <path_to_clang>/cmake/caches/PGO.cmake <source dir>
   $ 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:

 .. code-block:: console

   <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.

 The PGO came 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 came cache generates the following additional targets:

 **stage2-instrumented**
   Builds a stage1 x86 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 of "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 stage3 compiler (requires
   in-tree test-suite).

 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 & clang using the
 following commands:

 .. code-block:: console

   $ cmake -G Ninja -C <path_to_clang>/cmake/caches/3-stage.cmake <source dir>
   $ ninja stage3

 After the build you can compare the stage2 & stage3 compilers. We have a bot
 setup `here <http://lab.llvm.org:8011/builders/clang-3stage-ubuntu>`_ that runs
 this build and compare configuration.
	=============================
	Advanced Build Configurations
	=============================

	.. contents::
	:local:

	Introduction
	============

	`CMake <http://www.cmake.org/>`_ 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 :doc:`GettingStarted` or
	:doc:`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.

	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.

	.. code-block:: console

	$ cmake -G Ninja -DCLANG_ENABLE_BOOTSTRAP=On <path to source>
	$ 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:

	.. code-block:: console

	$ cmake -G Ninja -DCLANG_ENABLE_BOOTSTRAP=On -DCLANG_BOOTSTRAP_PASSTHROUGH="CMAKE_INSTALL_PREFIX;CMAKE_VERBOSE_MAKEFILE" <path to source>
	$ ninja stage2

	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 stagen build is built using the output from stagen-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:

	.. code-block:: console

	$ cmake -G Ninja -C <path to clang>/cmake/caches/Apple-stage1.cmake <path to source>
	$ 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 script automates that whole process. You can use it by
	running:

	.. code-block:: console

	$ cmake -G Ninja -C <path_to_clang>/cmake/caches/PGO.cmake <source dir>
	$ 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:

	.. code-block:: console

	<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.

	The PGO came 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 came cache generates the following additional targets:

	stage2-instrumented
	Builds a stage1 x86 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 of "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 stage3 compiler (requires
	in-tree test-suite).

	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 & clang using the
	following commands:

	.. code-block:: console

	$ cmake -G Ninja -C <path_to_clang>/cmake/caches/3-stage.cmake <source dir>
	$ ninja stage3

	After the build you can compare the stage2 & stage3 compilers. We have a bot
	setup `here <http://lab.llvm.org:8011/builders/clang-3stage-ubuntu>`_ that runs
	this build and compare configuration.