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===============
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LLVMBuild Guide
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===============
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.. contents::
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:local:
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Introduction
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============
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This document describes the ``LLVMBuild`` organization and files which
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we use to describe parts of the LLVM ecosystem. For description of
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specific LLVMBuild related tools, please see the command guide.
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LLVM is designed to be a modular set of libraries which can be flexibly
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mixed together in order to build a variety of tools, like compilers,
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JITs, custom code generators, optimization passes, interpreters, and so
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on. Related projects in the LLVM system like Clang and LLDB also tend to
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follow this philosophy.
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In order to support this usage style, LLVM has a fairly strict structure
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as to how the source code and various components are organized. The
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``LLVMBuild.txt`` files are the explicit specification of that
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structure, and are used by the build systems and other tools in order to
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develop the LLVM project.
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Project Organization
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====================
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The source code for LLVM projects using the LLVMBuild system (LLVM,
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Clang, and LLDB) is organized into *components*, which define the
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separate pieces of functionality that make up the project. These
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projects may consist of many libraries, associated tools, build tools,
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or other utility tools (for example, testing tools).
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For the most part, the project contents are organized around defining
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one main component per each subdirectory. Each such directory contains
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an ``LLVMBuild.txt`` which contains the component definitions.
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The component descriptions for the project as a whole are automatically
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gathered by the LLVMBuild tools. The tools automatically traverse the
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source directory structure to find all of the component description
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files. NOTE: For performance/sanity reasons, we only traverse into
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subdirectories when the parent itself contains an ``LLVMBuild.txt``
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description file.
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Build Integration
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=================
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The LLVMBuild files themselves are just a declarative way to describe
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the project structure. The actual building of the LLVM project is
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handled by another build system (currently we support both
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:doc:`Makefiles <MakefileGuide>` and :doc:`CMake <CMake>`).
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The build system implementation will load the relevant contents of the
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LLVMBuild files and use that to drive the actual project build.
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Typically, the build system will only need to load this information at
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"configure" time, and use it to generative native information. Build
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systems will also handle automatically reconfiguring their information
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when the contents of the ``LLVMBuild.txt`` files change.
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Developers generally are not expected to need to be aware of the details
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of how the LLVMBuild system is integrated into their build. Ideally,
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LLVM developers who are not working on the build system would only ever
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need to modify the contents of the ``LLVMBuild.txt`` description files
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(although we have not reached this goal yet).
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For more information on the utility tool we provide to help interfacing
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with the build system, please see the :doc:`llvm-build
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<CommandGuide/llvm-build>` documentation.
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Component Overview
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==================
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As mentioned earlier, LLVM projects are organized into logical
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*components*. Every component is typically grouped into its own
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subdirectory. Generally, a component is organized around a coherent
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group of sources which have some kind of clear API separation from other
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parts of the code.
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LLVM primarily uses the following types of components:
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- *Libraries* - Library components define a distinct API which can be
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independently linked into LLVM client applications. Libraries typically
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have private and public header files, and may specify a link of required
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libraries that they build on top of.
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- *Build Tools* - Build tools are applications which are designed to be run
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as part of the build process (typically to generate other source files).
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Currently, LLVM uses one main build tool called :doc:`TableGen/index`
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to generate a variety of source files.
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- *Tools* - Command line applications which are built using the LLVM
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component libraries. Most LLVM tools are small and are primarily
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frontends to the library interfaces.
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Components are described using ``LLVMBuild.txt`` files in the directories
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that define the component. See the `LLVMBuild Format Reference`_ section
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for information on the exact format of these files.
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LLVMBuild Format Reference
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==========================
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LLVMBuild files are written in a simple variant of the INI or configuration
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file format (`Wikipedia entry`_). The format defines a list of sections
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each of which may contain some number of properties. A simple example of
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the file format is below:
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.. _Wikipedia entry: http://en.wikipedia.org/wiki/INI_file
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.. code-block:: ini
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; Comments start with a semi-colon.
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; Sections are declared using square brackets.
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[component_0]
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; Properties are declared using '=' and are contained in the previous section.
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;
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; We support simple string and boolean scalar values and list values, where
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; items are separated by spaces. There is no support for quoting, and so
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; property values may not contain spaces.
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property_name = property_value
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list_property_name = value_1 value_2 ... value_n
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boolean_property_name = 1 (or 0)
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LLVMBuild files are expected to define a strict set of sections and
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properties. A typical component description file for a library
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component would look like the following example:
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.. code-block:: ini
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[component_0]
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type = Library
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name = Linker
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parent = Libraries
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required_libraries = Archive BitReader Core Support TransformUtils
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A full description of the exact sections and properties which are
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allowed follows.
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Each file may define exactly one common component, named ``common``. The
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common component may define the following properties:
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- ``subdirectories`` **[optional]**
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If given, a list of the names of the subdirectories from the current
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subpath to search for additional LLVMBuild files.
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Each file may define multiple components. Each component is described by a
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section who name starts with ``component``. The remainder of the section
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name is ignored, but each section name must be unique. Typically components
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are just number in order for files with multiple components
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(``component_0``, ``component_1``, and so on).
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.. warning::
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Section names not matching this format (or the ``common`` section) are
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currently unused and are disallowed.
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Every component is defined by the properties in the section. The exact
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list of properties that are allowed depends on the component type.
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Components **may not** define any properties other than those expected
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by the component type.
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Every component must define the following properties:
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- ``type`` **[required]**
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The type of the component. Supported component types are detailed
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below. Most components will define additional properties which may be
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required or optional.
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- ``name`` **[required]**
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The name of the component. Names are required to be unique across the
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entire project.
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- ``parent`` **[required]**
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The name of the logical parent of the component. Components are
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organized into a logical tree to make it easier to navigate and
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organize groups of components. The parents have no semantics as far
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as the project build is concerned, however. Typically, the parent
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will be the main component of the parent directory.
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Components may reference the root pseudo component using ``$ROOT`` to
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indicate they should logically be grouped at the top-level.
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Components may define the following properties:
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- ``dependencies`` **[optional]**
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If specified, a list of names of components which *must* be built
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prior to this one. This should only be exactly those components which
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produce some tool or source code required for building the component.
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.. note::
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``Group`` and ``LibraryGroup`` components have no semantics for the
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actual build, and are not allowed to specify dependencies.
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The following section lists the available component types, as well as
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the properties which are associated with that component.
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- ``type = Group``
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Group components exist purely to allow additional arbitrary structuring
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of the logical components tree. For example, one might define a
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``Libraries`` group to hold all of the root library components.
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``Group`` components have no additionally properties.
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- ``type = Library``
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Library components define an individual library which should be built
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from the source code in the component directory.
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Components with this type use the following properties:
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- ``library_name`` **[optional]**
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If given, the name to use for the actual library file on disk. If
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not given, the name is derived from the component name itself.
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- ``required_libraries`` **[optional]**
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If given, a list of the names of ``Library`` or ``LibraryGroup``
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components which must also be linked in whenever this library is
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used. That is, the link time dependencies for this component. When
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tools are built, the build system will include the transitive closure
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of all ``required_libraries`` for the components the tool needs.
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- ``add_to_library_groups`` **[optional]**
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If given, a list of the names of ``LibraryGroup`` components which
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this component is also part of. This allows nesting groups of
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components. For example, the ``X86`` target might define a library
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group for all of the ``X86`` components. That library group might
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then be included in the ``all-targets`` library group.
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- ``installed`` **[optional]** **[boolean]**
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Whether this library is installed. Libraries that are not installed
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are only reported by ``llvm-config`` when it is run as part of a
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development directory.
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- ``type = LibraryGroup``
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``LibraryGroup`` components are a mechanism to allow easy definition of
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useful sets of related components. In particular, we use them to easily
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specify things like "all targets", or "all assembly printers".
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Components with this type use the following properties:
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- ``required_libraries`` **[optional]**
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See the ``Library`` type for a description of this property.
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- ``add_to_library_groups`` **[optional]**
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See the ``Library`` type for a description of this property.
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- ``type = TargetGroup``
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``TargetGroup`` components are an extension of ``LibraryGroup``\s,
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specifically for defining LLVM targets (which are handled specially in a
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few places).
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The name of the component should always be the name of the target.
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Components with this type use the ``LibraryGroup`` properties in
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addition to:
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- ``has_asmparser`` **[optional]** **[boolean]**
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Whether this target defines an assembly parser.
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- ``has_asmprinter`` **[optional]** **[boolean]**
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Whether this target defines an assembly printer.
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- ``has_disassembler`` **[optional]** **[boolean]**
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Whether this target defines a disassembler.
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- ``has_jit`` **[optional]** **[boolean]**
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Whether this target supports JIT compilation.
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- ``type = Tool``
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``Tool`` components define standalone command line tools which should be
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built from the source code in the component directory and linked.
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Components with this type use the following properties:
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- ``required_libraries`` **[optional]**
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If given, a list of the names of ``Library`` or ``LibraryGroup``
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components which this tool is required to be linked with.
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.. note::
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The values should be the component names, which may not always
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match up with the actual library names on disk.
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Build systems are expected to properly include all of the libraries
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required by the linked components (i.e., the transitive closure of
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``required_libraries``).
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Build systems are also expected to understand that those library
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components must be built prior to linking -- they do not also need
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to be listed under ``dependencies``.
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- ``type = BuildTool``
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``BuildTool`` components are like ``Tool`` components, except that the
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tool is supposed to be built for the platform where the build is running
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(instead of that platform being targeted). Build systems are expected
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to handle the fact that required libraries may need to be built for
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multiple platforms in order to be able to link this tool.
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``BuildTool`` components currently use the exact same properties as
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``Tool`` components, the type distinction is only used to differentiate
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what the tool is built for.
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