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docs: Add initial docs on LLVMBuild organization.
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Daniel Dunbar
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docs/LLVMBuild.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>LLVMBuild Documentation</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<h1>LLVMBuild Guide</h1>
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<ol>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#projectorg">Project Organization</a></li>
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<li><a href="#buildintegration">Build Integration</a></li>
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<li><a href="#componentoverview">Component Overview</a></li>
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<li><a href="#formatreference">Format Reference</a></li>
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</ol>
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<!-- *********************************************************************** -->
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<h2><a name="introduction">Introduction</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>This document describes the <tt>LLVMBuild</tt> organization and files which
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we use to describe parts of the LLVM ecosystem. For description of specific
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LLVMBuild related tools, please see the command guide.</p>
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<p>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, JITs,
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custom code generators, optimization passes, interpreters, and so on. Related
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projects in the LLVM system like Clang and LLDB also tend to follow this
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philosophy.</p>
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<p>In order to support this usage style, LLVM has a fairly strict structure as
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to how the source code and various components are organized. The
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<tt>LLVMBuild.txt</tt> files are the explicit specification of that structure,
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and are used by the build systems and other tools in order to develop the LLVM
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project.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="projectorg">Project Organization</a></h2>
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<!-- *********************************************************************** -->
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<!-- FIXME: We should probably have an explicit top level project object. Good
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place to hang project level data, name, etc. Also useful for serving as the
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$ROOT of project trees for things which can be checked out separately. -->
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<div>
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<p>The source code for LLVM projects using the LLVMBuild system (LLVM, Clang,
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and LLDB) is organized into <em>components</em>, which define the separate
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pieces of functionality that make up the project. These projects may consist
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of many libraries, associated tools, build tools, or other utility tools (for
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example, testing tools).</p>
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<p>For the most part, the project contents are organized around defining one
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main component per each subdirectory. Each such directory contains
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an <tt>LLVMBuild.txt</tt> which contains the component definitions.</p>
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<p>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 source
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directory structure to find all of the component description files. NOTE: For
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performance/sanity reasons, we only traverse into subdirectories when the
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parent itself contains an <tt>LLVMBuild.txt</tt> description file.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="buildintegration">Build Integration</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>The LLVMBuild files themselves are just a declarative way to describe the
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project structure. The actual building of the LLVM project is handled by
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another build system (currently we support
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both <a href="MakefileGuide.html">Makefiles</a>
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and <a href="CMake.html">CMake</a>.</p>
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<p>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. Typically, the
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build system will only need to load this information at "configure" time, and
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use it to generative native information. Build systems will also handle
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automatically reconfiguring their information when the contents of
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the <i>LLVMBuild.txt</i> files change.</p>
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<p>Developers generally are not expected to need to be aware of the details of
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how the LLVMBuild system is integrated into their build. Ideally, LLVM
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developers who are not working on the build system would only ever need to
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modify the contents of the <i>LLVMBuild.txt</i> description files (although we
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have not reached this goal yet).</p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="componentoverview">Component Overview</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>As mentioned earlier, LLVM projects are organized into
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logical <em>components</em>. Every component is typically grouped into it's
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own subdirectory. Generally, a component is organized around a coherent group
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of sources which have some kind of clear API separation from other parts of
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the code.</p>
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<p>LLVM primarily uses the following types of components:</p>
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<ul>
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<li><em>Libraries</em> - Library components define a distinct API which can
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be 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.</li>
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<li><em>Build Tools</em> - Build tools are applications which are designed
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to be run as part of the build process (typically to generate other source
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files). Currently, LLVM uses one main build tool
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called <a href="TableGenFundamentals.html">TableGen</a> to generate a
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variety of source files.</li>
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<li><em>Tools</em> - Command line applications which are built using the
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LLVM component libraries. Most LLVM tools are small and are primarily
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frontends to the library interfaces.</li>
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<!-- FIXME: We also need shared libraries as a first class component, but this
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is not yet implemented. -->
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</ul>
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<p>Components are described using <em>LLVMBuild.txt</em> files in the
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directories that define the component. See
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the <a href="#formatreference">Format Reference</a> section for information on
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the exact format of these files.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="formatref">LLVMBuild Format Reference</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>LLVMBuild files are written in a simple variant of the INI or configuration
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file format (<a href="http://en.wikipedia.org/wiki/INI_file">Wikipedia
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entry</a>). The format defines a list of sections each of which may contain
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some number of properties. A simple example of the file format is below:</p>
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<div class="doc_code">
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<pre>
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<i>; Comments start with a semi-colon.</i>
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<i>; Sections are declared using square brackets.</i>
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[component 0]
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<i>; Properties are declared using '=' and are contained in the previous section.
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;
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; We support simple scalar values and list values, where items are separated by
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; spaces. There is no support for quoting, and so property values may not contain
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; spaces.</i>
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property_name = property_value
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list_property_name = value_1 value_2 <em>...</em> value_n
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</pre>
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</div>
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<p>LLVMBuild files are expected to define a strict set of section and
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properties. An typical component description file for a library
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component would look typically look like the following example:</p>
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<div class="doc_code">
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<pre>
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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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</pre>
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</div class="doc_code">
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<p>A full description of the exact sections and properties which are allowed
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follows.</p>
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<p>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 name is
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ignored, but each section name must be unique. Typically components are just
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number in order for files with multiple components ("component_0",
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"component_1", and so on).<p>
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<p><b>Section names not matches this format are currently
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unused and are disallowed.</b></p>
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<p>Every component is defined by the properties in the section. The exact list
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of properties that are allowed depends on the component
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type. Components <b>may not</b> define any properties other than those
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expected by the component type.</p>
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<p>Every component must define the following properties:</p>
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<ul>
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<li><i>type</i> <b>[required]</b>
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<p>The type of the component. Supported component types are
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detailed below. Most components will define additional properties which
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may be required or optional.</p></li>
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<li><i>name</i> <b>[required]</b>
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<p>The name of the component. Names are required to be unique
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across the entire project.</p></li>
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<li><i>parent</i> <b>[required]</b>
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<p>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 organize
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groups of components. The parent's have no semantics as far as the project
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build is concerned, however. Typically, the parent will be the main
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component of the parent directory.</p>
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<!-- FIXME: Should we make the parent optional, and default to parent
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directories component? -->
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<p>Components may reference the root pseudo component using '$ROOT' to
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indicate they should logically be grouped at the top-level.</p>
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</li>
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</ul>
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<p>Components may define the following properties:</p>
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<ul>
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<li><i>dependencies</i> <b>[optional]</b>
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<p>If specified, a list of names of components which <i>must</i> 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
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component.</p>
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<p><em>NOTE:</em> Group and LibraryGroup components have no semantics for
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the actual build, and are not allowed to specify dependencies.</p></li>
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</ul>
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<p>The following section lists the available component types, as well as the
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properties which are associated with that component.</p>
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<ul>
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<li><i>type = Group</i>
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<p>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.</p>
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<p>Group components have no additionally properties.</p>
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</li>
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<li><i>type = Library</i>
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<p>Library components define an individual library which should be built
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from the source code in the component directory.</p>
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<p>Components with this type use the following properties:</p>
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<ul>
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<li><i>library_name</i> <b>[optional]</b>
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<p>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
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itself.</p></li>
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<li><i>required_libraries</i> <b>[optional]</b>
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<p>If given, a list of the names of Library or LibraryGroup components
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which must also be linked in whenever this library is used. That is,
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the link time dependencies for this component. When tools are built,
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the build system will include the transitive closer of
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all <i>required_libraries</i> for the components the tool needs.</p></li>
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<li><i>add_to_library_groups</i> <b>[optional]</b>
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<p>If given, a list of the names of LibraryGroup components which this
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component is also part of. This allows nesting groups of
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components. For example, the <i>X86</i> target might define a library
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group for all of the <i>X86</i> components. That library group might
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then be included in the <i>all-targets</i> library group.</p></li>
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</ul>
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</li>
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<li><i>type = LibraryGroup</i>
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<p>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".</p>
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<p>Components with this type use the following properties:</p>
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<ul>
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<li><i>required_libraries</i> <b>[optional]</b>
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<p>See the Library type for a description of this property.</p></li>
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<li><i>add_to_library_groups</i> <b>[optional]</b>
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<p>See the Library type for a description of this property.</p></li>
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</ul>
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</li>
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<li><i>type = Tool</i>
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<p>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.</p>
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<p>Components with this type use the following properties:</p>
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<ul>
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<li><i>required_libraries</i> <b>[optional]</b>
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<p>If given, a list of the names of Library or LibraryGroup components
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which this tool is required to be linked with. <b>NOTE:</b> The values
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should be the component names, which may not always match up with the
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actual library names on disk.</p>
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<p>Build systems are expected to properly include all of the libraries
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required by the linked components (i.e., the transitive closer
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of <em>required_libraries</em>).</p>
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<p>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 to
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be listed under <i>dependencies</i>.</p></li>
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</ul>
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</li>
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<li><i>type = BuildTool</i>
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<p>BuildTool components are like Tool components, except that the tool is
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supposed to be built for the platform where the build is running (instead
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of that platform being targetted). Build systems are expected to handle
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the fact that required libraries may need to be built for multiple
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platforms in order to be able to link this tool.</p>
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<p>BuildTool components currently use the exact same properties as Tool
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components, the type distinction is only used to differentiate what the
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tool is built for.</p>
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</li>
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</ul>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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</address>
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</body>
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</html>
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@ -226,6 +226,10 @@ templates (directory organization, Makefiles, and test tree) allow the project
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code to be located outside (or inside) the <tt>llvm/</tt> tree, while using LLVM
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header files and libraries.</li>
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<li><a href="LLVMBuild.html">LLVMBuild Documentation</a> - Describes the
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LLVMBuild organization and files used by LLVM to specify component
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descriptions.</li>
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<li><a href="MakefileGuide.html">LLVM Makefile Guide</a> - Describes how the
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LLVM makefiles work and how to use them.</li>
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