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<head>

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<title>Getting Started with LLVM System</title>

</head>

<h1>Getting Started with the LLVM System<br>

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<font size="3">By: <a href="mailto:gshi1@uiuc.edu">Guochun Shi</a>, <a

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href="mailto:sabre@nondot.org">Chris Lattner</a> and <a

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href="http://www.cs.uiuc.edu/%7Evadve">Vikram Adve</a> </font></h1>

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</center>

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<h2><a name="Contents">Contents</a></h2>

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<ul>

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<li><a href="#overview">Overview</a> </li>

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<li><a href="#starting">Getting started with LLVM</a>

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<ol>

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<li><a href="#quickstart">Getting started quickly (a summary)</a>

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</li>

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<li><a href="#checkout">Checkout LLVM from CVS</a> </li>

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<li><a href="#terminology">Terminology and Notation</a> </li>

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<li><a href="#objfiles">The location for object files</a> </li>

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<li><a href="#config">Local Configuration Options</a> </li>

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<li><a href="#environment">Setting up your environment</a>

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</li>

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<li><a href="#compile">Compiling the source code</a> </li>

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</ol>

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</li>

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<li><a href="#layout">Program layout</a>

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<ol>

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<li><a href="#cvsdir">CVS directories</a> </li>

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<li><a href="#dd"><tt>Depend</tt>, <tt>Debug</tt>, &

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<tt>Release</tt> directories</a></li>

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<li><a href="#include"><tt>llvm/include</tt></a> </li>

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<li><a href="#tools"><tt>llvm/tools</tt></a> </li>

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</ol>

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</li>

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<li><a href="#tutorial">An example using the LLVM tool chain</a>

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</li>

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<li><a href="#links">Links</a> </li>

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</ul>

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<h2><a name="overview"><b>Overview</b></a></h2>

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</center>

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<p>The <a href="" starting="">next section</a> of this guide is meant to

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get you up and running with LLVM, and to give you some basic information

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about the LLVM environment. The <a href="" #quickstart="">first subsection</a>

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gives a short summary for those who are already familiar with the system

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and want to get started as quickly as possible. </p>

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<p>The later sections of this guide describe the <a href="" #layout="">general

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layout</a> of the LLVM source-tree, a <a href="#tutorial">simple example</a>

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using the LLVM tool chain, and <a href="#links">links</a> to find more information

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about LLVM or to get help via e-mail.

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</p>

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<h2><a name="starting"><b>Getting Started</b></a></h2>

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</center>

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<h3><a name="quickstart"><b>Getting Started Quickly (A Summary)</b></a></h3>

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Here's the short story for getting up and running quickly with LLVM:

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<ol>

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<li>Find the path to the CVS repository containing LLVM (we'll call

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this <i>CVSROOTDIR</i>). </li>

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<li><tt>cd <i>where-you-want-llvm-to-live</i></tt> </li>

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<li><tt>cvs -d <i>CVSROOTDIR</i> checkout llvm</tt> </li>

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<li>Edit <tt>Makefile.config</tt> to set local paths. This includes

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setting the install location of the C frontend and the various paths

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to the C and C++ compilers used to build LLVM itself. </li>

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<li>Set your LLVM_LIB_SEARCH_PATH environment variable. </li>

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<li><tt>gmake -k |& tee gnumake.out    # this is

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csh or tcsh syntax</tt> </li>

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</ol>

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<p>See <a href="#environment">Setting up your environment</a> on tips to

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simplify working with the LLVM front-end and compiled tools. See the

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other sub-sections below for other useful details in working with LLVM,

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or go straight to <a href="#layout">Program Layout</a> to learn about

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the layout of the source code tree.

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</p>

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<h3><a name="terminology">Terminology and Notation</a></h3>

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<p>Through this manual, the following names are used to denote paths

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specific to the local system and working environment. <i>These are not

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environment variables you need to set, but just strings used in the rest

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of this document below</i>. In any of the examples below, simply replace

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each of these names with the appropriate pathname on your local system.

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All these paths are absolute:</p>

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<ul>

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<h3><a name="checkout">Checkout LLVM from CVS</a></h3>

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<p>Before checking out the source code, you will need to know the path to

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the CVS repository containing LLVM source code (we'll call this <i>CVSROOTDIR</i>

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below). Ask the person responsible for your local LLVM installation

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to give you this path. </p>

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<p>To get a fresh copy of the entire source code, all you need to do

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is check it out from CVS as follows: </p>

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<ul>

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<li><tt>cd <i>where-you-want-llvm-to-live</i></tt> </li>

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<li><tt>cvs -d <i>CVSROOTDIR</i> checkout llvm</tt>

<p></p>

</li>

</ul>

<p>This will create an '<tt>llvm</tt>' directory in the current directory

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and fully populate it with the LLVM source code, Makefiles, test directories,

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and local copies of documentation files.</p>

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<h3><a name="config">Local Configuration Options</a></h3>

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<p>The file <tt>llvm/Makefile.config</tt> defines the following path

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variables which are specific to a particular installation of LLVM.

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These should need to be modified only once after checking out a copy

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of LLVM (if the default values do not already match your system):

</p>

<ul>

<p></p>

<li><i>CXX</i> = Path to C++ compiler to use.

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<p></p>

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</li>

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<li><i>OBJ_ROOT</i> = Path to the llvm directory where object files

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should be placed. (See the Section on <a href="#objfiles"> The

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location for LLVM object files</a> for more information.)

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<p></p>

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</li>

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<li><i>LLVMGCCDIR</i> = Path to the location of the LLVM front-end

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binaries and associated libraries.

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<p></p>

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</li>

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<li><i>PURIFY</i> = Path to the purify program. </li>

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</ul>

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In addition to settings in this file, you must set a <tt>LLVM_LIB_SEARCH_PATH</tt>

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environment variable in your startup scripts. This environment variable

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is used to locate "system" libraries like "<tt>-lc</tt>" and "<tt>-lm</tt>"

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when linking. This variable should be set to the absolute path for the

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bytecode-libs subdirectory of the C front-end install. For example,

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<tt>/home/vadve/lattner/local/x86/llvm-gcc/bytecode-libs</tt> is used

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for the X86 version of the C front-end on our research machines.

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</p>

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<h3><a name="objfiles">The location for LLVM object files</a></h3>

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<p>The LLVM make system sends most output files generated during the build

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into the directory defined by the variable OBJ_ROOT in <tt>llvm/Makefile.config</tt>.

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This can be either just your normal LLVM source tree or some other directory

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writable by you. You may wish to put object files on a different filesystem

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either to keep them from being backed up or to speed up local builds.

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</p>

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<p>If you do not wish to use a different location for object files (i.e.

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building into the source tree directly), just set this variable to ".".</p>

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</p>

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<h3><a name="environment">Setting up your environment</a></h3>

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<i>NOTE: This step is optional but will set up your environment so you

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can use the compiled LLVM tools with as little hassle as possible.</i>)

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<p>Add the following lines to your <tt>.cshrc</tt> (or the corresponding

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lines to your <tt>.profile</tt> if you use a bourne shell derivative).

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</p>

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<pre> # Make the C front end easy to use...<br> alias llvmgcc <i>LLVMGCCDIR</i><tt>/bin/llvm-gcc</tt>

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# Make the LLVM tools easy to use...

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setenv PATH <i>OBJ_ROOT</i>/llvm/tools/Debug:${PATH}<br> </pre>

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The <tt>llvmgcc</tt> alias is useful because the C compiler is not

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included in the CVS tree you just checked out.

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<p>The other <a href="#tools">LLVM tools</a> are part of the LLVM source

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base and are built when compiling LLVM. They will be built into the

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<tt><i>OBJ_ROOT</i>/tools/Debug</tt> directory.</p>

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<h3><a name="compile">Compiling the source code</a></h3>

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<p>Every directory in the LLVM source tree includes a <tt>Makefile</tt> to

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build it and any subdirectories that it contains. These makefiles require

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GNU Make (<tt>gmake)</tt> instead of <tt>make</tt> to build them, but

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can otherwise be used freely. To build the entire LLVM system, just

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enter the top level <tt>llvm</tt> directory and type <tt>gmake</tt>.

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A few minutes later you will hopefully have a freshly compiled toolchain

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waiting for you in <tt><i>OBJ_ROOT</i></tt><tt>/llvm/tools/Debug</tt>.

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If you want to look at the libraries that were compiled, look in <tt><i>OBJ_ROOT</i></tt><tt>/llvm/lib/Debug</tt>.</p>

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If you get an error about the <tt>/localhome</tt> directory, chances

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are good that something has been misconfigured.  Follow the instructions

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in the section about <a href="#environment">Setting Up Your Environment.</a>

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<h2><a name="layout"><b>Program Layout</b></a></h2>

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</center>

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<p>One useful source of infomation about the LLVM sourcebase is the LLVM

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<a href="http://www.doxygen.org">doxygen</a> documentation, available at

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<tt><a href="http://llvm.cs.uiuc.edu/doxygen/">http://llvm.cs.uiuc.edu/doxygen/</a></tt>.

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The following is a brief introduction to code layout:</p>

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<h3><a name="cvsdir"><tt>CVS</tt> directories</a></h3>

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Every directory checked out of CVS will contain a <tt>CVS</tt> directory;

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for the most part, these can just be ignored.

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<h3><a name="ddr"><tt>Depend</tt>, <tt>Debug</tt>, & <tt>Release</tt>

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directories</a></h3>

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If you are building with the "<tt>OBJ_ROOT=.</tt>" option enabled in

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the <tt>Makefile.config</tt> file, most source directories will contain

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two directories, <tt>Depend</tt> and <tt>Debug</tt>. The <tt>Depend</tt>

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directory contains automatically generated dependance files which are

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used during compilation to make sure that source files get rebuilt if

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a header file they use is modified. The <tt>Debug</tt> directory holds

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the object files, library files, and executables that are used for building

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a debug enabled build. The <tt>Release</tt> directory is created to

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hold the same files when the <tt>ENABLE_OPTIMIZED=1</tt> flag is passed

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to <tt>gmake</tt>, causing an optimized built to be performed.

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</p>

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<h3><a name="include"><tt>llvm/include</tt></a></h3>

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This directory contains public header files exported from the LLVM

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library. The two main subdirectories of this directory are:

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<ol>

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<li><tt>llvm/include/llvm</tt> - This directory contains all of the

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LLVM specific header files. This directory also has subdirectories

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for different portions of LLVM: <tt>Analysis</tt>, <tt>CodeGen</tt>,

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<tt>Reoptimizer</tt>, <tt>Target</tt>, <tt>Transforms</tt>, etc...

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</li>

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<li><tt>llvm/include/Support</tt> - This directory contains generic

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support libraries that are independant of LLVM, but are used by LLVM.

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For example, some C++ STL utilities and a Command Line option processing

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library. </li>

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</ol>

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This directory contains most source files of LLVM system. In LLVM almost

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all code exists in libraries, making it very easy to share code among

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the different <a href="#tools">tools</a>.

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<dt><tt>llvm/lib/VMCore/</tt></dt>

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<dd> This directory holds the core LLVM source files that implement

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core classes like Instruction and BasicBlock. </dd>

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<dt><tt>llvm/lib/AsmParser/</tt></dt>

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<dd> This directory holds the source code for the LLVM assembly language

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parser library. </dd>

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<dt><tt>llvm/lib/ByteCode/</tt></dt>

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<dd> This directory holds code for reading and write LLVM bytecode.

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</dd>

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<dt><tt>llvm/lib/CWriter/</tt></dt>

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<dd> This directory implements the LLVM to C converter. </dd>

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<dt><tt>llvm/lib/Analysis/</tt></dt>

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<dd> This directory contains a variety of different program analyses,

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such as Dominator Information, Call Graphs, Induction Variables, Interval

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Identification, Natural Loop Identification, etc... </dd>

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<dt><tt>llvm/lib/Transforms/</tt></dt>

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<dd> This directory contains the source code for the LLVM to LLVM

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program transformations, such as Aggressive Dead Code Elimination,

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Sparse Conditional Constant Propagation, Inlining, Loop Invarient Code

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Motion, Dead Global Elimination, Pool Allocation, and many others...

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</dd>

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<dt><tt>llvm/lib/Target/</tt></dt>

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<dd> This directory contains files that describe various target architectures

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for code generation. For example, the llvm/lib/Target/Sparc directory

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holds the Sparc machine description.<br>

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</dd>

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<dt><tt>llvm/lib/CodeGen/</tt></dt>

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<dd> This directory contains the major parts of the code generator:

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Instruction Selector, Instruction Scheduling, and Register Allocation.

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</dd>

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<dt><tt>llvm/lib/Reoptimizer/</tt></dt>

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<dd> This directory holds code related to the runtime reoptimizer

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framework that is currently under development. </dd>

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<dt><tt>llvm/lib/Support/</tt></dt>

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<dd> This directory contains the source code that corresponds to

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the header files located in <tt>llvm/include/Support/</tt>. </dd>

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</dl>

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<p>This directory contains regression tests and source code that is used

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to test the LLVM infrastructure...</p>

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<h3><a name="tools"><tt>llvm/tools</tt></a></h3>

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<p>The <b>tools</b> directory contains the executables built out of the

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libraries above, which form the main part of the user interface. You

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can always get help for a tool by typing <tt>tool_name --help</tt>.

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The following is a brief introduction to the most important tools.</p>

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<dd>The assembler transforms the human readable LLVM assembly to

LLVM bytecode.

</dd>

<dd>The disassembler transforms the LLVM bytecode to human readable

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LLVM assembly. Additionally it can convert LLVM bytecode to C, which

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is enabled with the <tt>-c</tt> option.

</dd>

<dd> <tt>lli</tt> is the LLVM interpreter, which can directly execute

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LLVM bytecode (although very slowly...). In addition to a simple intepreter,

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<tt>lli</tt> is also has debugger and tracing modes (entered by

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specifying <tt>-debug</tt> or <tt>-trace</tt> on the command line,

respectively).

</dd>

<dd> <tt>llc</tt> is the LLVM backend compiler, which translates

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LLVM bytecode to a SPARC assembly file.

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</dd>

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<dt><tt><b>llvmgcc</b></tt></dt>

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<dd> <tt>llvmgcc</tt> is a GCC based C frontend that has been retargeted

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to emit LLVM code as the machine code output. It works just like any

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other GCC compiler, taking the typical <tt>-c, -S, -E, -o</tt> options

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that are typically used. The source code for the <tt>llvmgcc</tt>

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tool is currently not included in the LLVM cvs tree because it is quite

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large and not very interesting.

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<ol>

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<dt><tt><b>gccas</b></tt></dt>

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<dd> This tool is invoked by the <tt>llvmgcc</tt> frontend

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as the "assembler" part of the compiler. This tool actually assembles

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LLVM assembly to LLVM bytecode, performs a variety of optimizations,

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and outputs LLVM bytecode. Thus when you invoke <tt>llvmgcc -c x.c

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-o x.o</tt>, you are causing <tt>gccas</tt> to be run, which writes

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the <tt>x.o</tt> file (which is an LLVM bytecode file that can be

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disassembled or manipulated just like any other bytecode file).

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The command line interface to <tt>gccas</tt> is designed to be as

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close as possible to the <b>system</b> '<tt>as</tt>' utility so that

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the gcc frontend itself did not have to be modified to interface

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to a "wierd" assembler.

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</dd>

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<dt><tt><b>gccld</b></tt></dt>

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<dd> <tt>gccld</tt> links together several LLVM bytecode files

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into one bytecode file and does some optimization. It is the linker

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invoked by the gcc frontend when multiple .o files need to be linked

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together. Like <tt>gccas</tt> the command line interface of <tt>gccld</tt>

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is designed to match the system linker, to aid interfacing with the

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GCC frontend.

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</dd>

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</ol>

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</dd>

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<dd> <tt>opt</tt> reads LLVM bytecode, applies a series of LLVM to

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LLVM transformations (which are specified on the command line), and

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then outputs the resultant bytecode. The '<tt>opt --help</tt>' command

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is a good way to get a list of the program transformations available

in LLVM.

</dd>

<dt><tt><b>analyze</b></tt></dt>

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<dd> <tt>analyze</tt> is used to run a specific analysis on an input

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LLVM bytecode file and print out the results. It is primarily useful

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for debugging analyses, or familiarizing yourself with what an analysis

does.

</dd>

</dl>

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<h2><a name="tutorial">An example using the LLVM tool chain</a></h2>

</a>

<ol>

<a name="tutorial"> <li>First, create a simple C file, name it 'hello.c':

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<pre> #include <stdio.h><br> int main() {<br> printf("hello world\n");<br> return 0;<br> }<br> </pre>

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</li>

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<li>Next, compile the C file into a LLVM bytecode file:

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<p> <tt>% llvmgcc hello.c -o hello</tt></p>

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<p> This will create two result files: <tt>hello</tt> and

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<tt>hello.bc</tt>. The <tt>hello.bc</tt> is the LLVM bytecode that

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corresponds the the compiled program and the library facilities that it

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required. <tt>hello</tt> is a simple shell script that runs the bytecode

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file with <tt>lli</tt>, making the result directly executable.</p>

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</li>

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<li>Run the program. To make sure the program ran, execute one of the

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following commands:

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<p> <tt>% ./hello</tt></p>

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<p> <tt>% lli hello.bc</tt></p>

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</li>

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<li>Use the <tt>dis</tt> utility to take a look at the LLVM assembly

code:

</li>

<li>Compile the program to native Sparc assembly using the code generator:

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<p> <tt>% llc hello.bc -o hello.s</tt></p>

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</li>

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<li>Assemble the native sparc assemble file into a program:

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<p> <tt>% /opt/SUNWspro/bin/cc -xarch=v9 hello.s -o hello.sparc</tt></p>

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</li>

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<li>Execute the native sparc program:

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<p> <tt>% ./hello.sparc</tt></p>

</li>

</a>

</ol>

</a>

<h2><a name="links">Links</a></h2>

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<p>This document is just an <b>introduction</b> to how to use LLVM to do

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some simple things... there are many more interesting and complicated

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things that you can do that aren't documented here (but we'll gladly

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accept a patch if you want to write something up!). For more information

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about LLVM, check out:</p>

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<ul>

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<li><a href="http://llvm.cs.uiuc.edu/">LLVM homepage</a></li>

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<li><a href="http://llvm.cs.uiuc.edu/doxygen/">LLVM doxygen tree</a></li>

</ul>

<hr> If you have any questions or run into any snags (or you have any

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additions...), please send an email to <a

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href="mailto:sabre@nondot.org">Chris Lattner</a>.

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<p></p>

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Last modified: Tue Jun 3 22:06:43 CDT 2003  <br>

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</body>

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