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716 lines
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716 lines
24 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>LLVM: Frequently Asked Questions</title>
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<style type="text/css">
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@import url("llvm.css");
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.question { font-weight: bold }
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.answer { margin-left: 2em }
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</style>
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</head>
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<body>
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<div class="doc_title">
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LLVM: Frequently Asked Questions
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</div>
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<ol>
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<li><a href="#license">License</a>
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<ol>
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<li>Why are the LLVM source code and the front-end distributed under different
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licenses?</li>
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<li>Does the University of Illinois Open Source License really qualify as an
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"open source" license?</li>
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<li>Can I modify LLVM source code and redistribute the modified source?</li>
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<li>Can I modify LLVM source code and redistribute binaries or other tools
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based on it, without redistributing the source?</li>
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</ol></li>
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<li><a href="#source">Source code</a>
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<ol>
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<li>In what language is LLVM written?</li>
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<li>How portable is the LLVM source code?</li>
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</ol></li>
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<li><a href="#build">Build Problems</a>
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<ol>
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<li>When I run configure, it finds the wrong C compiler.</li>
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<li>The <tt>configure</tt> script finds the right C compiler, but it uses the
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LLVM linker from a previous build. What do I do?</li>
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<li>When creating a dynamic library, I get a strange GLIBC error.</li>
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<li>I've updated my source tree from Subversion, and now my build is trying
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to use a file/directory that doesn't exist.</li>
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<li>I've modified a Makefile in my source tree, but my build tree keeps using
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the old version. What do I do?</li>
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<li>I've upgraded to a new version of LLVM, and I get strange build
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errors.</li>
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<li>I've built LLVM and am testing it, but the tests freeze.</li>
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<li>Why do test results differ when I perform different types of builds?</li>
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<li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
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<li>When I use the test suite, all of the C Backend tests fail. What is
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wrong?</li>
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<li>After Subversion update, rebuilding gives the error "No rule to make
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target".</li>
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<li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
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work.</a></li>
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</ol></li>
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<li><a href="#felangs">Source Languages</a>
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<ol>
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<li><a href="#langs">What source languages are supported?</a></li>
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<li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How
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should I interface with the LLVM middle-end optimizers and back-end code
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generators?</a></li>
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<li><a href="#langhlsupp">What support is there for higher level source
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language constructs for building a compiler?</a></li>
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<li><a href="GetElementPtr.html">I don't understand the GetElementPtr
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instruction. Help!</a></li>
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</ol>
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<li><a href="#cfe">Using the GCC Front End</a>
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<ol>
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<li>
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When I compile software that uses a configure script, the configure script
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thinks my system has all of the header files and libraries it is testing
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for. How do I get configure to work correctly?
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</li>
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<li>
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When I compile code using the LLVM GCC front end, it complains that it
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cannot find libcrtend.a.
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</li>
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<li>
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How can I disable all optimizations when compiling code using the LLVM GCC front end?
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</li>
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<li><a href="#translatecxx">Can I use LLVM to convert C++ code to C code?</a></li>
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</ol>
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</li>
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<li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
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<ol>
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<li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
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<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
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#include <iostream>?</a></li>
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<li><a href="#codedce">Where did all of my code go??</a></li>
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<li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
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</ol>
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</li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="license">License</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="question">
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<p>Why are the LLVM source code and the front-end distributed under different
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licenses?</p>
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</div>
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<div class="answer">
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<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
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Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
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license, in particular one that does not compel users who distribute tools based
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on modifying the source to redistribute the modified source code as well.</p>
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</div>
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<div class="question">
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<p>Does the University of Illinois Open Source License really qualify as an
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"open source" license?</p>
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</div>
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<div class="answer">
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<p>Yes, the license is <a
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href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
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Source Initiative (OSI).</p>
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</div>
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<div class="question">
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<p>Can I modify LLVM source code and redistribute the modified source?</p>
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</div>
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<div class="answer">
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<p>Yes. The modified source distribution must retain the copyright notice and
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follow the three bulletted conditions listed in the <a
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href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM license</a>.</p>
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</div>
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<div class="question">
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<p>Can I modify LLVM source code and redistribute binaries or other tools based
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on it, without redistributing the source?</p>
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</div>
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<div class="answer">
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<p>Yes, this is why we distribute LLVM under a less restrictive license than
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GPL, as explained in the first question above.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="source">Source Code</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="question">
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<p>In what language is LLVM written?</p>
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</div>
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<div class="answer">
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<p>All of the LLVM tools and libraries are written in C++ with extensive use of
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the STL.</p>
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</div>
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<div class="question">
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<p>How portable is the LLVM source code?</p>
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</div>
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<div class="answer">
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<p>The LLVM source code should be portable to most modern UNIX-like operating
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systems. Most of the code is written in standard C++ with operating system
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services abstracted to a support library. The tools required to build and test
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LLVM have been ported to a plethora of platforms.</p>
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<p>Some porting problems may exist in the following areas:</p>
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<ul>
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<li>The GCC front end code is not as portable as the LLVM suite, so it may not
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compile as well on unsupported platforms.</li>
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<li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
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Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9)
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will require more effort.</li>
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</ul>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="build">Build Problems</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="question">
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<p>When I run configure, it finds the wrong C compiler.</p>
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</div>
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<div class="answer">
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<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
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<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
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for the C and C++ compiler, respectively.</p>
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<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
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<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
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explicitly.</p>
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</div>
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<div class="question">
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<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
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LLVM linker from a previous build. What do I do?</p>
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</div>
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<div class="answer">
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<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
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if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
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it:</p>
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<ol>
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<li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
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program appears first in the <tt>PATH</tt>. This may work, but may not be
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convenient when you want them <i>first</i> in your path for other
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work.</p></li>
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<li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
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correct. In a Borne compatible shell, the syntax would be:</p>
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<pre class="doc_code">
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% PATH=[the path without the bad program] ./configure ...
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</pre>
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<p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
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to do its work without having to adjust your <tt>PATH</tt>
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permanently.</p></li>
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</ol>
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</div>
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<div class="question">
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<p>When creating a dynamic library, I get a strange GLIBC error.</p>
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</div>
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<div class="answer">
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<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
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GCC was compiled with the --disable-shared option. To work around this, install
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your own version of GCC that has shared libraries enabled by default.</p>
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</div>
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<div class="question">
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<p>I've updated my source tree from Subversion, and now my build is trying to
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use a file/directory that doesn't exist.</p>
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</div>
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<div class="answer">
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<p>You need to re-run configure in your object directory. When new Makefiles
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are added to the source tree, they have to be copied over to the object tree in
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order to be used by the build.</p>
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</div>
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<div class="question">
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<p>I've modified a Makefile in my source tree, but my build tree keeps using the
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old version. What do I do?</p>
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</div>
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<div class="answer">
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<p>If the Makefile already exists in your object tree, you
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can just run the following command in the top level directory of your object
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tree:</p>
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<pre class="doc_code">
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% ./config.status <relative path to Makefile>
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</pre>
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<p>If the Makefile is new, you will have to modify the configure script to copy
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it over.</p>
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</div>
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<div class="question">
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<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
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</div>
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<div class="answer">
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<p>Sometimes, changes to the LLVM source code alters how the build system works.
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Changes in libtool, autoconf, or header file dependencies are especially prone
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to this sort of problem.</p>
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<p>The best thing to try is to remove the old files and re-build. In most
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cases, this takes care of the problem. To do this, just type <tt>make
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clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
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</div>
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<div class="question">
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<p>I've built LLVM and am testing it, but the tests freeze.</p>
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</div>
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<div class="answer">
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<p>This is most likely occurring because you built a profile or release
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(optimized) build of LLVM and have not specified the same information on the
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<tt>gmake</tt> command line.</p>
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<p>For example, if you built LLVM with the command:</p>
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<pre class="doc_code">
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% gmake ENABLE_PROFILING=1
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</pre>
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<p>...then you must run the tests with the following commands:</p>
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<pre class="doc_code">
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% cd llvm/test
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% gmake ENABLE_PROFILING=1
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</pre>
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</div>
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<div class="question">
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<p>Why do test results differ when I perform different types of builds?</p>
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</div>
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<div class="answer">
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<p>The LLVM test suite is dependent upon several features of the LLVM tools and
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libraries.</p>
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<p>First, the debugging assertions in code are not enabled in optimized or
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profiling builds. Hence, tests that used to fail may pass.</p>
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<p>Second, some tests may rely upon debugging options or behavior that is only
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available in the debug build. These tests will fail in an optimized or profile
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build.</p>
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</div>
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<div class="question">
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<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
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</div>
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<div class="answer">
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<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
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affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
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</div>
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<div class="question">
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<p>After Subversion update, rebuilding gives the error "No rule to make
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target".</p>
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</div>
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<div class="answer">
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<p>If the error is of the form:</p>
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<pre class="doc_code">
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gmake[2]: *** No rule to make target `/path/to/somefile', needed by
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`/path/to/another/file.d'.<br>
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Stop.
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</pre>
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<p>This may occur anytime files are moved within the Subversion repository or
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removed entirely. In this case, the best solution is to erase all
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<tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
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<pre class="doc_code">
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% cd $LLVM_OBJ_DIR
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% rm -f `find . -name \*\.d`
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% gmake
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</pre>
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<p>In other cases, it may be necessary to run <tt>make clean</tt> before
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rebuilding.</p>
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</div>
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<div class="question"><p><a name="llvmc">
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The <tt>llvmc</tt> program gives me errors/doesn't work.</a></p>
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</div>
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<div class="answer">
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<p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
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using <tt>llvm-gcc</tt> instead.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="felangs">Source Languages</a></div>
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<div class="question"><p>
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<a name="langs">What source languages are supported?</a></p>
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</div>
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<div class="answer">
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<p>LLVM currently has full support for C and C++ source languages. These are
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available through a special version of GCC that LLVM calls the
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<a href="#cfe">C Front End</a></p>
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<p>There is an incomplete version of a Java front end available in the
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<tt>java</tt> module. There is no documentation on this yet so
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you'll need to download the code, compile it, and try it.</p>
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<p>The PyPy developers are working on integrating LLVM into the PyPy backend
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so that PyPy language can translate to LLVM.</p>
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</div>
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<div class="question"><p><a name="langirgen">
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I'd like to write a self-hosting LLVM compiler. How should I interface with
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the LLVM middle-end optimizers and back-end code generators?
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</a></p></div>
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<div class="answer">
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<p>Your compiler front-end will communicate with LLVM by creating a module in
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the LLVM intermediate representation (IR) format. Assuming you want to
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write your language's compiler in the language itself (rather than C++),
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there are 3 major ways to tackle generating LLVM IR from a front-end:</p>
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<ul>
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<li>
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<strong>Call into the LLVM libraries code using your language's FFI
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(foreign function interface).</strong>
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<ul>
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<li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax,
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and .bc format</li>
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<li><em>for:</em> enables running LLVM optimization passes without a
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emit/parse overhead</li>
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<li><em>for:</em> adapts well to a JIT context</li>
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<li><em>against:</em> lots of ugly glue code to write</li>
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</ul>
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</li>
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<li>
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<strong>Emit LLVM assembly from your compiler's native language.</strong>
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<ul>
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<li><em>for:</em> very straightforward to get started</li>
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<li><em>against:</em> the .ll parser is slower than the bitcode reader
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when interfacing to the middle end</li>
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<li><em>against:</em> you'll have to re-engineer the LLVM IR object
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model and asm writer in your language</li>
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<li><em>against:</em> it may be harder to track changes to the IR</li>
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</ul>
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</li>
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<li>
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<strong>Emit LLVM bitcode from your compiler's native language.</strong>
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<ul>
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<li><em>for:</em> can use the more-efficient bitcode reader when
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interfacing to the middle end</li>
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<li><em>against:</em> you'll have to re-engineer the LLVM IR object
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model and bitcode writer in your language</li>
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<li><em>against:</em> it may be harder to track changes to the IR</li>
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</ul>
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</li>
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</ul>
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<p>If you go with the first option, the C bindings in include/llvm-c should
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help a lot, since most languages have strong support for interfacing with
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C. The most common hurdle with calling C from managed code is interfacing
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with the garbage collector. The C interface was designed to require very
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little memory management, and so is straightforward in this regard.</p>
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</div>
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<div class="question"><p><a name="langhlsupp">
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What support is there for a higher level source language constructs for
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|
building a compiler?</a></p>
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|
</div>
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<div class="answer">
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|
<p>Currently, there isn't much. LLVM supports an intermediate representation
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which is useful for code representation but will not support the high level
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(abstract syntax tree) representation needed by most compilers. There are no
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facilities for lexical nor semantic analysis. There is, however, a <i>mostly
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implemented</i> configuration-driven
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<a href="CompilerDriver.html">compiler driver</a> which simplifies the task
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of running optimizations, linking, and executable generation.</p>
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</div>
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|
|
<div class="question"><p><a name="getelementptr">
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|
I don't understand the GetElementPtr instruction. Help!</a></p>
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|
</div>
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|
<div class="answer">
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|
<p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
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Instruction</a>.</p>
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</div>
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|
<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="cfe">Using the GCC Front End</a>
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</div>
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<div class="question">
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<p>
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When I compile software that uses a configure script, the configure script
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thinks my system has all of the header files and libraries it is testing for.
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How do I get configure to work correctly?
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</p>
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</div>
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<div class="answer">
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<p>
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The configure script is getting things wrong because the LLVM linker allows
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symbols to be undefined at link time (so that they can be resolved during JIT
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or translation to the C back end). That is why configure thinks your system
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"has everything."
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</p>
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<p>
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To work around this, perform the following steps:
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</p>
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<ol>
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<li>Make sure the CC and CXX environment variables contains the full path to
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the LLVM GCC front end.</li>
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<li>Make sure that the regular C compiler is first in your PATH. </li>
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<li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
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</ol>
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<p>
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This will allow the <tt>llvm-ld</tt> linker to create a native code executable
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instead of shell script that runs the JIT. Creating native code requires
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standard linkage, which in turn will allow the configure script to find out if
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code is not linking on your system because the feature isn't available on your
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system.</p>
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</div>
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<div class="question">
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<p>
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When I compile code using the LLVM GCC front end, it complains that it cannot
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find libcrtend.a.
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</p>
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</div>
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<div class="answer">
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<p>
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The only way this can happen is if you haven't installed the runtime library. To
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correct this, do:</p>
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<pre class="doc_code">
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% cd llvm/runtime
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% make clean ; make install-bytecode
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</pre>
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</div>
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<div class="question">
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<p>
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How can I disable all optimizations when compiling code using the LLVM GCC front end?
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</p>
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</div>
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<div class="answer">
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<p>
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Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
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optimizations done at the llvm level, leaving you with the truly horrible
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code that you desire.
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</p>
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</div>
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<div class="question">
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<p>
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<a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a>
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</p>
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</div>
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<div class="answer">
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<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
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Note that the generated C code will be very low level (all loops are lowered
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to gotos, etc) and not very pretty (comments are stripped, original source
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formatting is totally lost, variables are renamed, expressions are regrouped),
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so this may not be what you're looking for. However, this is a good way to add
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C++ support for a processor that does not otherwise have a C++ compiler.
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</p>
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<p>Use commands like this:</p>
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<ol>
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<li><p>Compile your program as normal with llvm-g++:</p>
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<pre class="doc_code">
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% llvm-g++ x.cpp -o program
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</pre>
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<p>or:</p>
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<pre class="doc_code">
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% llvm-g++ a.cpp -c
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% llvm-g++ b.cpp -c
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% llvm-g++ a.o b.o -o program
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</pre>
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<p>With llvm-gcc3, this will generate program and program.bc. The .bc
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file is the LLVM version of the program all linked together.</p></li>
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<li><p>Convert the LLVM code to C code, using the LLC tool with the C
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backend:</p>
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<pre class="doc_code">
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% llc -march=c program.bc -o program.c
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</pre></li>
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<li><p>Finally, compile the C file:</p>
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<pre class="doc_code">
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% cc x.c
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</pre></li>
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</ol>
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<p>Note that, by default, the C backend does not support exception handling. If
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you want/need it for a certain program, you can enable it by passing
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"-enable-correct-eh-support" to the llc program. The resultant code will use
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setjmp/longjmp to implement exception support that is correct but relatively
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slow.</p>
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<p>Also note: this specific sequence of commands won't work if you use a
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function defined in the C++ runtime library (or any other C++ library). To
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access an external C++ library, you must manually compile libstdc++ to LLVM
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bitcode, statically link it into your program, then use the commands above to
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convert the whole result into C code. Alternatively, you can compile the
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libraries and your application into two different chunks of C code and link
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them.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="cfe_code">Questions about code generated by the GCC front-end</a>
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</div>
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<div class="question">
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<a name="iosinit"></a>
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<p> What is this <tt>llvm.global_ctors</tt> and
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<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
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<iostream>?</p>
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</div>
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<div class="answer">
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<p>If you #include the <iostream> header into a C++ translation unit, the
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file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
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objects. However, C++ does not guarantee an order of initialization between
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static objects in different translation units, so if a static ctor/dtor in your
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.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
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be automatically initialized before your use.</p>
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<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
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STL that we use declares a static object that gets created in every translation
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unit that includes <tt><iostream></tt>. This object has a static
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constructor and destructor that initializes and destroys the global iostream
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objects before they could possibly be used in the file. The code that you see
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in the .ll file corresponds to the constructor and destructor registration code.
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</p>
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<p>If you would like to make it easier to <b>understand</b> the LLVM code
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generated by the compiler in the demo page, consider using <tt>printf()</tt>
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instead of <tt>iostream</tt>s to print values.</p>
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</div>
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<!--=========================================================================-->
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<div class="question"><p>
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<a name="codedce"></a>
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Where did all of my code go??
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</p></div>
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<div class="answer">
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<p>
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If you are using the LLVM demo page, you may often wonder what happened to all
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of the code that you typed in. Remember that the demo script is running the
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code through the LLVM optimizers, so if your code doesn't actually do anything
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useful, it might all be deleted.
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</p>
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<p>
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To prevent this, make sure that the code is actually needed. For example, if
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you are computing some expression, return the value from the function instead of
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leaving it in a local variable. If you really want to constrain the optimizer,
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you can read from and assign to <tt>volatile</tt> global variables.
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</p>
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</div>
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<!--=========================================================================-->
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|
|
<div class="question"><p>
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<a name="undef"></a>
|
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<p>What is this "<tt>undef</tt>" thing that shows up in my code?
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</p></div>
|
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|
|
<div class="answer">
|
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<p>
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<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
|
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a value that is not defined. You can get these if you do not initialize a
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variable before you use it. For example, the C function:</p>
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|
|
<pre class="doc_code">
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|
int X() { int i; return i; }
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</pre>
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|
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<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
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a value specified for it.</p>
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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">LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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</address>
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