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|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>LLVM: Frequently Asked Questions</title>
<style type="text/css">
@import url("_static/llvm.css");
.question { font-weight: bold }
.answer { margin-left: 2em }
</style>
</head>
<body>
<h1>
LLVM: Frequently Asked Questions
</h1>
<ol>
<li><a href="#license">License</a>
<ol>
<li>Does the University of Illinois Open Source License really qualify as an
"open source" license?</li>
<li>Can I modify LLVM source code and redistribute the modified source?</li>
<li>Can I modify LLVM source code and redistribute binaries or other tools
based on it, without redistributing the source?</li>
</ol></li>
<li><a href="#source">Source code</a>
<ol>
<li>In what language is LLVM written?</li>
<li>How portable is the LLVM source code?</li>
</ol></li>
<li><a href="#build">Build Problems</a>
<ol>
<li>When I run configure, it finds the wrong C compiler.</li>
<li>The <tt>configure</tt> script finds the right C compiler, but it uses
the LLVM linker from a previous build. What do I do?</li>
<li>When creating a dynamic library, I get a strange GLIBC error.</li>
<li>I've updated my source tree from Subversion, and now my build is trying
to use a file/directory that doesn't exist.</li>
<li>I've modified a Makefile in my source tree, but my build tree keeps
using the old version. What do I do?</li>
<li>I've upgraded to a new version of LLVM, and I get strange build
errors.</li>
<li>I've built LLVM and am testing it, but the tests freeze.</li>
<li>Why do test results differ when I perform different types of
builds?</li>
<li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
<li>Compiling LLVM with GCC succeeds, but the resulting tools do not work,
what can be wrong?</li>
<li>When I use the test suite, all of the C Backend tests fail. What is
wrong?</li>
<li>After Subversion update, rebuilding gives the error "No rule to make
target".</li>
</ol></li>
<li><a href="#felangs">Source Languages</a>
<ol>
<li><a href="#langs">What source languages are supported?</a></li>
<li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How
should I interface with the LLVM middle-end optimizers and back-end code
generators?</a></li>
<li><a href="#langhlsupp">What support is there for higher level source
language constructs for building a compiler?</a></li>
<li><a href="GetElementPtr.html">I don't understand the GetElementPtr
instruction. Help!</a></li>
</ol>
<li><a href="#cfe">Using the C and C++ Front Ends</a>
<ol>
<li><a href="#platformindependent">Can I compile C or C++ code to
platform-independent LLVM bitcode?</a></li>
</ol>
</li>
<li><a href="#cfe_code">Questions about code generated by the demo page</a>
<ol>
<li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
<tt>_GLOBAL__I_a...</tt> stuff that happens when I
#include <iostream>?</a></li>
<li><a href="#codedce">Where did all of my code go??</a></li>
<li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in
my code?</a></li>
<li><a href="#callconvwrong">Why does instcombine + simplifycfg turn
a call to a function with a mismatched calling convention into "unreachable"?
Why not make the verifier reject it?</a></li>
</ol>
</li>
</ol>
<div class="doc_author">
<p>Written by <a href="http://llvm.org/">The LLVM Team</a></p>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="license">License</a>
</h2>
<!-- *********************************************************************** -->
<div>
<div class="question">
<p>Does the University of Illinois Open Source License really qualify as an
"open source" license?</p>
</div>
<div class="answer">
<p>Yes, the license
is <a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by
the Open Source Initiative (OSI).</p>
</div>
<div class="question">
<p>Can I modify LLVM source code and redistribute the modified source?</p>
</div>
<div class="answer">
<p>Yes. The modified source distribution must retain the copyright notice and
follow the three bulletted conditions listed in
the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM
license</a>.</p>
</div>
<div class="question">
<p>Can I modify LLVM source code and redistribute binaries or other tools based
on it, without redistributing the source?</p>
</div>
<div class="answer">
<p>Yes. This is why we distribute LLVM under a less restrictive license than
GPL, as explained in the first question above.</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="source">Source Code</a>
</h2>
<!-- *********************************************************************** -->
<div>
<div class="question">
<p>In what language is LLVM written?</p>
</div>
<div class="answer">
<p>All of the LLVM tools and libraries are written in C++ with extensive use of
the STL.</p>
</div>
<div class="question">
<p>How portable is the LLVM source code?</p>
</div>
<div class="answer">
<p>The LLVM source code should be portable to most modern UNIX-like operating
systems. Most of the code is written in standard C++ with operating system
services abstracted to a support library. The tools required to build and test
LLVM have been ported to a plethora of platforms.</p>
<p>Some porting problems may exist in the following areas:</p>
<ul>
<li>The autoconf/makefile build system relies heavily on UNIX shell tools,
like the Bourne Shell and sed. Porting to systems without these tools
(MacOS 9, Plan 9) Will require more effort.</li>
</ul>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="build">Build Problems</a>
</h2>
<!-- *********************************************************************** -->
<div>
<div class="question">
<p>When I run configure, it finds the wrong C compiler.</p>
</div>
<div class="answer">
<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt>
and <tt>CXX</tt> for the C and C++ compiler, respectively.</p>
<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
explicitly.</p>
</div>
<div class="question">
<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
LLVM tools from a previous build. What do I do?</p>
</div>
<div class="answer">
<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
it:</p>
<ol>
<li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
program appears first in the <tt>PATH</tt>. This may work, but may not be
convenient when you want them <i>first</i> in your path for other
work.</p></li>
|
