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<h1>Getting Started with the LLVM System<br><font size=3>By: <a
href="mailto:gshi1@uiuc.edu">Guochun Shi</a> and <a
href="mailto:sabre@nondot.org">Chris Lattner</a></font></h1>
<ul>
<li><a href="#quickstart">Getting started with LLVM</a>
<ol>
<li><a href="#cvs">Checkout LLVM from CVS</a>
<li><a href="#shared">Access to <tt>/shared</tt></a>
<li><a href="#environment">Set up your environment</a>
<li><a href="#compile">Compiling the Source Code</a>
</ol>
<li><a href="#layout">Program layout</a>
<ol>
<li><a href="#cvsdir">CVS directories</a>
<li><a href="#dd"><tt>Depend</tt>, <tt>Debug</tt>, &
<tt>Release</tt> directories</a></li>
<li><a href="#include"><tt>llvm/include</tt></a>
<li><a href="#lib"><tt>llvm/lib</tt></a>
<li><a href="#test"><tt>llvm/test</tt></a>
<li><a href="#tools"><tt>llvm/tools</tt></a>
</ol>
<li><a href="#tutorial">An example using the LLVM tool chain</a>
<li><a href="#links">Links</a>
</ul>
<!--=====================================================================-->
<h2><a name="quickstart">Getting Started with LLVM</a></h2>
<!--=====================================================================-->
<p>This guide is meant to get you up and running with LLVM as quickly as
possible. Once you get the basic system running you can choose an area to
dive into and learn more about. If you get stuck or something is missing
from this document, please email <a
href="mailto:sabre@nondot.org">Chris</a>.</p>
<!------------------------------------------------------------------------->
<h3><a name="tools">Checkout LLVM from CVS</a></h3>
<!------------------------------------------------------------------------->
<p>First step is to get the actual source code. To do this, all you need to
do is check it out from CVS. From your home directory, just enter:</p>
<p><tt>cvs -d /home/vadve/vadve/Research/DynOpt/CVSRepository checkout
llvm</tt></p>
<p>This will create an '<tt>llvm</tt>' directory in your home directory and
fully populate it with the source code for LLVM.</p>
<!------------------------------------------------------------------------->
<h3><a name="shared">Access to <tt>/shared</tt></a></h3>
<!------------------------------------------------------------------------->
By default, LLVM is configured to send all compiled files into the
<tt>/shared/[yourloginname]/</tt> directory. The idea is that this
directory is local to the machine you're working on, so the huge libraries
and <tt>.o</tt> files you will be compiling will not have to be sent over
NFS. If you are in a situation where this setup is correct, you don't have
to do anything. If you don't have a <tt>/shared</tt> directory, you will
have to make a couple of modifications to your setup.<p>
The first modification is that you need to enable the "<tt>BUILD_ROOT =
.</tt> line in the top level <tt>Makefile.common</tt>. This will instruct
LLVM to build into the current directory tree instead of
<tt>/shared</tt>.<p>
<!------------------------------------------------------------------------->
<h3><a name="tools">Set up your environment</a></h3>
<!------------------------------------------------------------------------->
<p>Now that you have the source code available, you should set up your
environment to be able to use the LLVM tools (once compiled) with as little
hassle as possible. To do this, we recommend that you add the following
lines to your <tt>.cshrc</tt> (or the corresponding lines to your
<tt>.profile</tt> if you use a bourne shell derivative):
<pre>
# Make the C frontend easy to use...
alias llvmgcc /home/vadve/lattner/cvs/gcc_install/bin/gcc
# Make the LLVM tools easy to use...
setenv PATH /shared/[yourloginname]/llvm/tools/Debug:${PATH}
</pre>
<p>The C compiler is not included in the CVS tree you just checked out, so
we just point to the cannonical location, and access it with the
<tt>llvmgcc</tt> command. The rest of the <a href="#tools">LLVM tools</a>
will be built into the <tt>llvm/tools/Debug</tt> directory. If you do not
have access to <tt>/shared</tt>, use this line instead:</p>
<pre>
setenv PATH ~/llvm/tools/Debug:${PATH}
</pre>
Adding these two lines to your path will make it much easier to use the LLVM
tools.</p>
<!------------------------------------------------------------------------->
<h3><a name="compile">Compiling the Source Code</a></h3>
<!------------------------------------------------------------------------->
<p>Every directory in the LLVM source tree includes a Makefile to build it,
and any subdirectories that it contains. These makefiles require that you
use <tt>gmake</tt>, instead of <tt>make</tt> to build them, but can
otherwise be used freely. To build the entire LLVM system, just enter the
top level <tt>llvm</tt> directory and type <tt>gmake</tt>. A few minutes
later you will hopefully have a freshly compiled toolchain waiting for you
in <tt>llvm/tools/Debug</tt>. If you want to look at the libraries that
were compiled, look in <tt>llvm/lib/Debug</tt>.</p>
<p>By default, the LLVM build process sends all temporary (<tt>.o</tt>,
<tt>.so</tt>, <tt>.a</tt>) files into a <tt>/shared/[your login
name]/...</tt> directory, which is supposed to be on a disk local to the
current machine. If you get an error talking about a <tt>/shared</tt>
directory, follow the instructions in the <a href="#shared">section about
<tt>/shared</tt></a>.<p>
<!--=====================================================================-->
<h2><a name="layout">Program Layout</a></h2>
<!--=====================================================================-->
<p>One useful source of infomation about the LLVM sourcebase is the LLVM <a
href="http://www.doxygen.org">doxygen</a> documentation, available at <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/">http://llvm.cs.uiuc.edu/doxygen/</a></tt>. The
following is a brief introduction to code layout:</p>
<!------------------------------------------------------------------------->
<h3><a name="cvsdir"><tt>CVS</tt> directories</a></h3>
<!------------------------------------------------------------------------->
Every directory checked out of CVS will contain a <tt>CVS</tt> directory,
for the most part these can just be ignored.
<!------------------------------------------------------------------------->
<h3><a name="ddr"><tt>Depend</tt>, <tt>Debug</tt>, & <tt>Release</tt>
directories</a></h3>
<!------------------------------------------------------------------------->
If you are building with the "<tt>BUILD_ROOT=.</tt>" option enabled in the
<tt>Makefile.common</tt> file, most source directories will contain two
directories, <tt>Depend</tt> and <tt>Debug</tt>. The <tt>Depend</tt>
directory contains automatically generated dependance files which are used
during compilation to make sure that source files get rebuilt if a header
file they use is modified. The <tt>Debug</tt> directory holds the object
files, library files and executables that are used for building a debug
enabled build. The <tt>Release</tt> directory is created to hold the same
files when the <tt>ENABLE_OPTIMIZED=1</tt> flag is passed to <tt>gmake</tt>,
causing an optimized built to be performed.<p>
<!------------------------------------------------------------------------->
<h3><a name="include"><tt>llvm/include</tt></a></h3>
<!------------------------------------------------------------------------->
This directory contains public header files exported from the LLVM
library. The two main subdirectories of this directory are:<p>
<ol>
<li><tt>llvm/include/llvm</tt> - This directory contains all of the LLVM
specific header files. This directory also has subdirectories for
different portions of llvm: <tt>Analysis</tt>, <tt>CodeGen</tt>,
<tt>Reoptimizer</tt>, <tt>Target</tt>, <tt>Transforms</tt>, etc...
<li><tt>llvm/include/Support</tt> - This directory contains generic
support libraries that are independant of LLVM, but are used by LLVM.
For example, some C++ STL utilities and a Command Line option processing
library.
</ol>
<!------------------------------------------------------------------------->
<h3><a name="lib"><tt>llvm/lib</tt></a></h3>
<!------------------------------------------------------------------------->
This directory contains most source files of LLVM system. In LLVM almost all
code exists in libraries, making it very easy to share code among the
different <a href="#tools">tools</a>.<p>
<dl compact>
<dt><tt>llvm/lib/VMCore/</tt><dd> This directory holds the core LLVM
source files that implement core classes like Instruction and BasicBlock.
<dt><tt>llvm/lib/AsmParser/</tt><dd> This directory holds the source code
for the LLVM assembly language parser library.
<dt><tt>llvm/lib/ByteCode/</tt><dd> This directory holds code for reading
and write LLVM bytecode.
<dt><tt>llvm/lib/CWrite/</tt><dd> This directory implements the LLVM to C
converter.
<dt><tt>llvm/lib/Analysis/</tt><dd> This directory contains a variety of
different program analyses, such as Dominator Information, Call Graphs,
Induction Variables, Interval Identification, Natural Loop Identification,
etc...
<dt><tt>llvm/lib/Transforms/</tt><dd> This directory contains the source
code for the LLVM to LLVM program transformations, such as Aggressive Dead
Code Elimination, Sparse Conditional Constant Propogation, Inlining, Loop
Invarient Code Motion, Dead Global Elimination, Pool Allocation, and many
others...
<dt><tt>llvm/lib/Target/</tt><dd> This directory contains files that
describe various target architectures for code generation. For example,
the llvm/lib/Target/Sparc directory holds the Sparc machine
description.<br>
<dt><tt>llvm/lib/CodeGen/</tt><dd> This directory contains the major parts
of the code generator: Instruction Selector, Instruction Scheduling, and
Register Allocation.
<dt><tt>llvm/lib/Reoptimizer/</tt><dd> This directory holds code related
to the runtime reoptimizer framework that is currently under development.
<dt><tt>llvm/lib/Support/</tt><dd> This directory contains the source code
that corresponds to the header files located in
<tt>llvm/include/Support/</tt>.
</dl>
<!------------------------------------------------------------------------->
<h3><a name="test"><tt>llvm/test</tt></a></h3>
<!------------------------------------------------------------------------->
<p>This directory contains regression tests and source code that is used to
test the LLVM infrastructure...</p>
<!------------------------------------------------------------------------->
<h3><a name="tools"><tt>llvm/tools</tt></a></h3>
<!------------------------------------------------------------------------->
<p>The <b>tools</b> directory contains the executables built out of the
libraries above, which form the main part of the user interface. You can
always get help for a tool by typing <tt>tool_name --help</tt>. The
following is a brief introduction to the most important tools.</p>
<dl compact>
<dt><tt><b>as</b></tt><dd>The assembler transforms the human readable
llvm assembly to llvm bytecode.<p>
<dt><tt><b>dis</b></tt><dd>The disassembler transforms the llvm bytecode
to human readable llvm assembly. Additionally it can convert LLVM
bytecode to C, which is enabled with the <tt>-c</tt> option.<p>
<dt><tt><b>lli</b></tt><dd> <tt>lli</tt> is the LLVM interpreter, which
can directly execute LLVM bytecode (although very slowly...). In addition
to a simple intepreter, <tt>lli</tt> is also has debugger and tracing
modes (entered by specifying <tt>-debug</tt> or <tt>-trace</tt> on the
command line, respectively).<p>
<dt><tt><b>llc</b></tt><dd> <tt>llc</tt> is the LLVM backend compiler,
which translates LLVM bytecode to a SPARC assembly file.<p>
<dt><tt><b>llvmgcc</b></tt><dd> <tt>llvmgcc</tt> is a GCC based C frontend
that has been retargeted to emit LLVM code as the machine code output. It
works just like any other GCC compiler, taking the typical <tt>-c, -S, -E,
-o</tt> options that are typically used. The source code for the
<tt>llvmgcc</tt> tool is currently not included in the LLVM cvs tree
because it is quite large and not very interesting.<p>
<ol>
<dt><tt><b>gccas</b></tt><dd> This took is invoked by the
<tt>llvmgcc</tt> frontend as the "assembler" part of the compiler. This
tool actually assembles its input, performs a variety of optimizations,
and outputs LLVM bytecode. Thus when you invoke <tt>llvmgcc -c x.c -o
x.o</tt>, you are causing <tt>gccas</tt> to be run, which writes the
<tt>x.o</tt> file (which is an LLVM bytecode file that can be
disassembled or manipulated just like any other bytecode file). The
command line interface to <tt>gccas</tt> is designed to be as close as
possible to the <b>system</b> <tt>as</tt> utility so that the gcc
frontend itself did not have to be modified to interface to a "wierd"
assembler.<p>
<dt><tt><b>gccld</b></tt><dd> <tt>gccld</tt> links together several llvm
bytecode files into one bytecode file and does some optimization. It is
the linker invoked by the gcc frontend when multiple .o files need to be
linked together. Like <tt>gccas</tt> the command line interface of
<tt>gccld</tt> is designed to match the system linker, to aid
interfacing with the GCC frontend.<p>
</ol>
<dt><tt><b>opt</b></tt><dd> <tt>opt</tt> reads llvm bytecode, applies a
series of LLVM to LLVM transformations (which are specified on the command
line), and then outputs the resultant bytecode. The '<tt>opt --help</tt>'
command is a good way to get a list of the program transformations
available in LLVM.<p>
<dt><tt><b>analyze</b></tt><dd> <tt>analyze</tt> is used to run a specific
analysis on an input LLVM bytecode file and print out the results. It is
primarily useful for debugging analyses, or familiarizing yourself with
what an analysis does.<p>
</dl>
<!--=====================================================================-->
<h2><a name="tutorial">An example using the LLVM tool chain</h2>
<!--=====================================================================-->
<ol>
<li>First, create a simple C file, name it 'hello.c':
<pre>
#include <stdio.h>
int main() {
printf("hello world\n");
return 0;
}
</pre>
<li>Next, compile the C file into a LLVM bytecode file:<p>
<tt>% llvmgcc hello.c -o hello</tt><p>
This will create two result files: <tt>hello</tt> and
<tt>hello.bc</tt>. The <tt>hello.bc</tt> is the LLVM bytecode that
corresponds the the compiled program and the library facilities that it
required. <tt>hello</tt> is a simple shell script that runs the bytecode
file with <tt>lli</tt>, making the result directly executable.<p>
<li>Run the program. To make sure the program ran, execute one of the
following commands:<p>
<tt>% ./hello</tt><p>
or<p>
<tt>% lli hello.bc</tt><p>
<li>Use the <tt>dis</tt> utility to take a look at the LLVM assembly
code:<p>
<tt>% dis < hello.bc | less</tt><p>
<li>Compile the program to native Sparc assembly using the code
generator:<p>
<tt>% llc hello.bc -o hello.s</tt><p>
<li>Assemble the native sparc assemble file into a program:<p>
<tt>% /opt/SUNWspro/bin/cc -xarch=v9 hello.s -o hello.sparc</tt><p>
<li>Execute the native sparc program:<p>
<tt>% ./hello.sparc</tt><p>
</ol>
<!--=====================================================================-->
<h2><a name="links">Links</a></h2>
<!--=====================================================================-->
<p>This document is just an <b>introduction</b> to how to use LLVM to do
some simple things... there are many more interesting and complicated things
that you can do that aren't documented here (but we'll gladly accept a patch
if you want to write something up!). For more information about LLVM, check
out:</p>
<ul>
<li><a href="http://llvm.cs.uiuc.edu/">LLVM homepage</a></li>
<li><a href="http://tank.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
additions...), please send an email to <a
href="mailto:sabre@nondot.org">Chris Lattner</a> or <a
href="mailto:gshi1@uiuc.edu">Guochun Shi</a>. <p>
<!-- Created: Mon Jul 1 02:29:02 CDT 2002 -->
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Last modified: Tue Aug 13 16:09:25 CDT 2002
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