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939 lines
31 KiB
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939 lines
31 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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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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
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different 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
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the 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
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using 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
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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>Compiling LLVM with GCC succeeds, but the resulting tools do not work,
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what can be wrong?</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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<li><a href="#srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir,
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it fails. Why?</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>When I compile software that uses a configure script, the configure
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script thinks my system has all of the header files and libraries it is
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testing for. How do I get configure to work correctly?</li>
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<li>When I compile code using the LLVM GCC front end, it complains that it
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cannot find libcrtend.a?</li>
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<li>How can I disable all optimizations when compiling code using the LLVM
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GCC front end?</li>
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<li><a href="#translatecxx">Can I use LLVM to convert C++ code to C
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code?</a></li>
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<li><a href="#platformindependent">Can I compile C or C++ code to
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platform-independent LLVM bitcode?</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
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my code?</a></li>
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<li><a href="#callconvwrong">Why does instcombine + simplifycfg turn
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a call to a function with a mismatched calling convention into "unreachable"?
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Why not make the verifier reject it?</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
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restrictive</em> license, in particular one that does not compel users who
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distribute tools based on modifying the source to redistribute the modified
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source code as well.</p>
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</div>
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|
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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
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is <a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by
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the Open 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
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the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM
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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>
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and <tt>CXX</tt> 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">
|
|
<p>When creating a dynamic library, I get a strange GLIBC error.</p>
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</div>
|
|
|
|
<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,
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install your own version of GCC that has shared libraries enabled by
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default.</p>
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</div>
|
|
|
|
<div class="question">
|
|
<p>I've updated my source tree from Subversion, and now my build is trying to
|
|
use a file/directory that doesn't exist.</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<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
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in order to be used by the build.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>I've modified a Makefile in my source tree, but my build tree keeps using the
|
|
old version. What do I do?</p>
|
|
</div>
|
|
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|
<div class="answer">
|
|
<p>If the Makefile already exists in your object tree, you can just run the
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following command in the top level directory of your object 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>
|
|
|
|
<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">
|
|
<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
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prone 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>
|
|
|
|
<div class="question">
|
|
<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>
|
|
|
|
<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
|
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profile build.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
|
|
</div>
|
|
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|
<div class="answer">
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<p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392">a bug in
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GCC</a>, and affects projects other than LLVM. Try upgrading or downgrading
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your GCC.</p>
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</div>
|
|
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<div class="question">
|
|
<p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what
|
|
can be wrong?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>Several versions of GCC have shown a weakness in miscompiling the LLVM
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codebase. Please consult your compiler version (<tt>gcc --version</tt>) to
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find out whether it is <a href="GettingStarted.html#brokengcc">broken</a>.
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If so, your only option is to upgrade GCC to a known good version.</p>
|
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</div>
|
|
|
|
<div class="question">
|
|
<p>After Subversion update, rebuilding gives the error "No rule to make
|
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target".</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>If the error is of the form:</p>
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|
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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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|
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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>
|
|
|
|
<p>In other cases, it may be necessary to run <tt>make clean</tt> before
|
|
rebuilding.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p><a name="llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
|
|
work.</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
|
|
using <tt>llvm-gcc</tt> instead.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p><a name="srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir, it
|
|
fails. Why?</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>The <tt>GNUmakefile</tt> in the top-level directory of LLVM-GCC is a special
|
|
<tt>Makefile</tt> used by Apple to invoke the <tt>build_gcc</tt> script after
|
|
setting up a special environment. This has the unforunate side-effect that
|
|
trying to build LLVM-GCC with srcdir == objdir in a "non-Apple way" invokes
|
|
the <tt>GNUmakefile</tt> instead of <tt>Makefile</tt>. Because the
|
|
environment isn't set up correctly to do this, the build fails.</p>
|
|
|
|
<p>People not building LLVM-GCC the "Apple way" need to build LLVM-GCC with
|
|
srcdir != objdir, or simply remove the GNUmakefile entirely.</p>
|
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<p>We regret the inconvenience.</p>
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|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="felangs">Source Languages</a></div>
|
|
|
|
<div class="question">
|
|
<p><a name="langs">What source languages are supported?</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>LLVM currently has full support for C and C++ source languages. These are
|
|
available through a special version of GCC that LLVM calls the
|
|
<a href="#cfe">C Front End</a></p>
|
|
|
|
<p>There is an incomplete version of a Java front end available in the
|
|
<tt>java</tt> module. There is no documentation on this yet so you'll need to
|
|
download the code, compile it, and try it.</p>
|
|
|
|
<p>The PyPy developers are working on integrating LLVM into the PyPy backend so
|
|
that PyPy language can translate to LLVM.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p><a name="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></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>Your compiler front-end will communicate with LLVM by creating a module in
|
|
the LLVM intermediate representation (IR) format. Assuming you want to write
|
|
your language's compiler in the language itself (rather than C++), there are
|
|
3 major ways to tackle generating LLVM IR from a front-end:</p>
|
|
|
|
<ul>
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|
<li><strong>Call into the LLVM libraries code using your language's FFI
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|
(foreign function interface).</strong>
|
|
|
|
<ul>
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|
<li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax, and .bc
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|
format</li>
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|
|
|
<li><em>for:</em> enables running LLVM optimization passes without a
|
|
emit/parse overhead</li>
|
|
|
|
<li><em>for:</em> adapts well to a JIT context</li>
|
|
|
|
<li><em>against:</em> lots of ugly glue code to write</li>
|
|
</ul></li>
|
|
|
|
<li> <strong>Emit LLVM assembly from your compiler's native language.</strong>
|
|
<ul>
|
|
<li><em>for:</em> very straightforward to get started</li>
|
|
|
|
<li><em>against:</em> the .ll parser is slower than the bitcode reader
|
|
when interfacing to the middle end</li>
|
|
|
|
<li><em>against:</em> you'll have to re-engineer the LLVM IR object model
|
|
and asm writer in your language</li>
|
|
|
|
<li><em>against:</em> it may be harder to track changes to the IR</li>
|
|
</ul></li>
|
|
|
|
<li><strong>Emit LLVM bitcode from your compiler's native language.</strong>
|
|
|
|
<ul>
|
|
<li><em>for:</em> can use the more-efficient bitcode reader when
|
|
interfacing to the middle end</li>
|
|
|
|
<li><em>against:</em> you'll have to re-engineer the LLVM IR object
|
|
model and bitcode writer in your language</li>
|
|
|
|
<li><em>against:</em> it may be harder to track changes to the IR</li>
|
|
</ul></li>
|
|
</ul>
|
|
|
|
<p>If you go with the first option, the C bindings in include/llvm-c should help
|
|
a lot, since most languages have strong support for interfacing with C. The
|
|
most common hurdle with calling C from managed code is interfacing with the
|
|
garbage collector. The C interface was designed to require very little memory
|
|
management, and so is straightforward in this regard.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p><a name="langhlsupp">What support is there for a higher level source language
|
|
constructs for building a compiler?</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>Currently, there isn't much. LLVM supports an intermediate representation
|
|
which is useful for code representation but will not support the high level
|
|
(abstract syntax tree) representation needed by most compilers. There are no
|
|
facilities for lexical nor semantic analysis. There is, however, a <i>mostly
|
|
implemented</i> configuration-driven
|
|
<a href="CompilerDriver.html">compiler driver</a> which simplifies the task
|
|
of running optimizations, linking, and executable generation.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p><a name="getelementptr">I don't understand the GetElementPtr
|
|
instruction. Help!</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
|
|
Instruction</a>.</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="cfe">Using the GCC Front End</a>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>When I compile software that uses a configure script, the configure script
|
|
thinks my system has all of the header files and libraries it is testing for.
|
|
How do I get configure to work correctly?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>The configure script is getting things wrong because the LLVM linker allows
|
|
symbols to be undefined at link time (so that they can be resolved during JIT
|
|
or translation to the C back end). That is why configure thinks your system
|
|
"has everything."</p>
|
|
|
|
<p>To work around this, perform the following steps:</p>
|
|
|
|
<ol>
|
|
<li>Make sure the CC and CXX environment variables contains the full path to
|
|
the LLVM GCC front end.</li>
|
|
|
|
<li>Make sure that the regular C compiler is first in your PATH. </li>
|
|
|
|
<li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
|
|
</ol>
|
|
|
|
<p>This will allow the <tt>llvm-ld</tt> linker to create a native code
|
|
executable instead of shell script that runs the JIT. Creating native code
|
|
requires standard linkage, which in turn will allow the configure script to
|
|
find out if code is not linking on your system because the feature isn't
|
|
available on your system.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>When I compile code using the LLVM GCC front end, it complains that it cannot
|
|
find libcrtend.a.
|
|
</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>The only way this can happen is if you haven't installed the runtime
|
|
library. To correct this, do:</p>
|
|
|
|
<pre class="doc_code">
|
|
% cd llvm/runtime
|
|
% make clean ; make install-bytecode
|
|
</pre>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>How can I disable all optimizations when compiling code using the LLVM GCC
|
|
front end?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
|
|
optimizations done at the llvm level, leaving you with the truly horrible
|
|
code that you desire.</p>
|
|
</div>
|
|
|
|
|
|
<div class="question">
|
|
<p><a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
|
|
Note that the generated C code will be very low level (all loops are lowered
|
|
to gotos, etc) and not very pretty (comments are stripped, original source
|
|
formatting is totally lost, variables are renamed, expressions are
|
|
regrouped), so this may not be what you're looking for. Also, there are
|
|
several limitations noted below.<p>
|
|
|
|
<p>Use commands like this:</p>
|
|
|
|
<ol>
|
|
<li><p>Compile your program as normal with llvm-g++:</p>
|
|
|
|
<pre class="doc_code">
|
|
% llvm-g++ x.cpp -o program
|
|
</pre>
|
|
|
|
<p>or:</p>
|
|
|
|
<pre class="doc_code">
|
|
% llvm-g++ a.cpp -c
|
|
% llvm-g++ b.cpp -c
|
|
% llvm-g++ a.o b.o -o program
|
|
</pre>
|
|
|
|
<p>With llvm-gcc3, this will generate program and program.bc. The .bc
|
|
file is the LLVM version of the program all linked together.</p></li>
|
|
|
|
<li><p>Convert the LLVM code to C code, using the LLC tool with the C
|
|
backend:</p>
|
|
|
|
<pre class="doc_code">
|
|
% llc -march=c program.bc -o program.c
|
|
</pre></li>
|
|
|
|
<li><p>Finally, compile the C file:</p>
|
|
|
|
<pre class="doc_code">
|
|
% cc x.c
|
|
</pre></li>
|
|
|
|
</ol>
|
|
|
|
<p>Using LLVM does not eliminate the need for C++ library support. If you use
|
|
the llvm-g++ front-end, the generated code will depend on g++'s C++ support
|
|
libraries in the same way that code generated from g++ would. If you use
|
|
another C++ front-end, the generated code will depend on whatever library
|
|
that front-end would normally require.</p>
|
|
|
|
<p>If you are working on a platform that does not provide any C++ libraries, you
|
|
may be able to manually compile libstdc++ to LLVM bitcode, statically link it
|
|
into your program, then use the commands above to convert the whole result
|
|
into C code. Alternatively, you might compile the libraries and your
|
|
application into two different chunks of C code and link them.</p>
|
|
|
|
<p>Note that, by default, the C back end does not support exception handling.
|
|
If you want/need it for a certain program, you can enable it by passing
|
|
"-enable-correct-eh-support" to the llc program. The resultant code will use
|
|
setjmp/longjmp to implement exception support that is relatively slow, and
|
|
not C++-ABI-conforming on most platforms, but otherwise correct.</p>
|
|
|
|
<p>Also, there are a number of other limitations of the C backend that cause it
|
|
to produce code that does not fully conform to the C++ ABI on most
|
|
platforms. Some of the C++ programs in LLVM's test suite are known to fail
|
|
when compiled with the C back end because of ABI incompatibilities with
|
|
standard C++ libraries.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p><a name="platformindependent">Can I compile C or C++ code to
|
|
platform-independent LLVM bitcode?</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>No. C and C++ are inherently platform-dependent languages. The most obvious
|
|
example of this is the preprocessor. A very common way that C code is made
|
|
portable is by using the preprocessor to include platform-specific code. In
|
|
practice, information about other platforms is lost after preprocessing, so
|
|
the result is inherently dependent on the platform that the preprocessing was
|
|
targeting.</p>
|
|
|
|
<p>Another example is <tt>sizeof</tt>. It's common for <tt>sizeof(long)</tt> to
|
|
vary between platforms. In most C front-ends, <tt>sizeof</tt> is expanded to
|
|
a constant immediately, thus hard-wiring a platform-specific detail.</p>
|
|
|
|
<p>Also, since many platforms define their ABIs in terms of C, and since LLVM is
|
|
lower-level than C, front-ends currently must emit platform-specific IR in
|
|
order to have the result conform to the platform ABI.</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="cfe_code">Questions about code generated by the GCC front-end</a>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and
|
|
<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I <tt>#include
|
|
<iostream></tt>?</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>If you <tt>#include</tt> the <tt><iostream></tt> header into a C++
|
|
translation unit, the file will probably use
|
|
the <tt>std::cin</tt>/<tt>std::cout</tt>/... global objects. However, C++
|
|
does not guarantee an order of initialization between static objects in
|
|
different translation units, so if a static ctor/dtor in your .cpp file
|
|
used <tt>std::cout</tt>, for example, the object would not necessarily be
|
|
automatically initialized before your use.</p>
|
|
|
|
<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
|
|
STL that we use declares a static object that gets created in every
|
|
translation unit that includes <tt><iostream></tt>. This object has a
|
|
static constructor and destructor that initializes and destroys the global
|
|
iostream objects before they could possibly be used in the file. The code
|
|
that you see in the .ll file corresponds to the constructor and destructor
|
|
registration code.
|
|
</p>
|
|
|
|
<p>If you would like to make it easier to <b>understand</b> the LLVM code
|
|
generated by the compiler in the demo page, consider using <tt>printf()</tt>
|
|
instead of <tt>iostream</tt>s to print values.</p>
|
|
</div>
|
|
|
|
<!--=========================================================================-->
|
|
|
|
<div class="question">
|
|
<p><a name="codedce">Where did all of my code go??</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>If you are using the LLVM demo page, you may often wonder what happened to
|
|
all of the code that you typed in. Remember that the demo script is running
|
|
the code through the LLVM optimizers, so if your code doesn't actually do
|
|
anything useful, it might all be deleted.</p>
|
|
|
|
<p>To prevent this, make sure that the code is actually needed. For example, if
|
|
you are computing some expression, return the value from the function instead
|
|
of leaving it in a local variable. If you really want to constrain the
|
|
optimizer, you can read from and assign to <tt>volatile</tt> global
|
|
variables.</p>
|
|
</div>
|
|
|
|
<!--=========================================================================-->
|
|
|
|
<div class="question">
|
|
<p><a name="undef">What is this "<tt>undef</tt>" thing that shows up in my
|
|
code?</a></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p><a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of
|
|
representing a value that is not defined. You can get these if you do not
|
|
initialize a variable before you use it. For example, the C function:</p>
|
|
|
|
<pre class="doc_code">
|
|
int X() { int i; return i; }
|
|
</pre>
|
|
|
|
<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a
|
|
value specified for it.</p>
|
|
</div>
|
|
|
|
<!--=========================================================================-->
|
|
|
|
<div class="question">
|
|
<p><a name="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></p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>This is a common problem run into by authors of front-ends that are using
|
|
custom calling conventions: you need to make sure to set the right calling
|
|
convention on both the function and on each call to the function. For example,
|
|
this code:</p>
|
|
|
|
<pre class="doc_code">
|
|
define fastcc void @foo() {
|
|
ret void
|
|
}
|
|
define void @bar() {
|
|
call void @foo( )
|
|
ret void
|
|
}
|
|
</pre>
|
|
|
|
<p>Is optimized to:</p>
|
|
|
|
<pre class="doc_code">
|
|
define fastcc void @foo() {
|
|
ret void
|
|
}
|
|
define void @bar() {
|
|
unreachable
|
|
}
|
|
</pre>
|
|
|
|
<p>... with "opt -instcombine -simplifycfg". This often bites people because
|
|
"all their code disappears". Setting the calling convention on the caller and
|
|
callee is required for indirect calls to work, so people often ask why not make
|
|
the verifier reject this sort of thing.</p>
|
|
|
|
<p>The answer is that this code has undefined behavior, but it is not illegal.
|
|
If we made it illegal, then every transformation that could potentially create
|
|
this would have to ensure that it doesn't, and there is valid code that can
|
|
create this sort of construct (in dead code). The sorts of things that can
|
|
cause this to happen are fairly contrived, but we still need to accept them.
|
|
Here's an example:</p>
|
|
|
|
<pre class="doc_code">
|
|
define fastcc void @foo() {
|
|
ret void
|
|
}
|
|
define internal void @bar(void()* %FP, i1 %cond) {
|
|
br i1 %cond, label %T, label %F
|
|
T:
|
|
call void %FP()
|
|
ret void
|
|
F:
|
|
call fastcc void %FP()
|
|
ret void
|
|
}
|
|
define void @test() {
|
|
%X = or i1 false, false
|
|
call void @bar(void()* @foo, i1 %X)
|
|
ret void
|
|
}
|
|
</pre>
|
|
|
|
<p>In this example, "test" always passes @foo/false into bar, which ensures that
|
|
it is dynamically called with the right calling conv (thus, the code is
|
|
perfectly well defined). If you run this through the inliner, you get this
|
|
(the explicit "or" is there so that the inliner doesn't dead code eliminate
|
|
a bunch of stuff):
|
|
</p>
|
|
|
|
<pre class="doc_code">
|
|
define fastcc void @foo() {
|
|
ret void
|
|
}
|
|
define void @test() {
|
|
%X = or i1 false, false
|
|
br i1 %X, label %T.i, label %F.i
|
|
T.i:
|
|
call void @foo()
|
|
br label %bar.exit
|
|
F.i:
|
|
call fastcc void @foo()
|
|
br label %bar.exit
|
|
bar.exit:
|
|
ret void
|
|
}
|
|
</pre>
|
|
|
|
<p>Here you can see that the inlining pass made an undefined call to @foo with
|
|
the wrong calling convention. We really don't want to make the inliner have
|
|
to know about this sort of thing, so it needs to be valid code. In this case,
|
|
dead code elimination can trivially remove the undefined code. However, if %X
|
|
was an input argument to @test, the inliner would produce this:
|
|
</p>
|
|
|
|
<pre class="doc_code">
|
|
define fastcc void @foo() {
|
|
ret void
|
|
}
|
|
|
|
define void @test(i1 %X) {
|
|
br i1 %X, label %T.i, label %F.i
|
|
T.i:
|
|
call void @foo()
|
|
br label %bar.exit
|
|
F.i:
|
|
call fastcc void @foo()
|
|
br label %bar.exit
|
|
bar.exit:
|
|
ret void
|
|
}
|
|
</pre>
|
|
|
|
<p>The interesting thing about this is that %X <em>must</em> be false for the
|
|
code to be well-defined, but no amount of dead code elimination will be able to
|
|
delete the broken call as unreachable. However, since instcombine/simplifycfg
|
|
turns the undefined call into unreachable, we end up with a branch on a
|
|
condition that goes to unreachable: a branch to unreachable can never happen, so
|
|
"-inline -instcombine -simplifycfg" is able to produce:</p>
|
|
|
|
<pre class="doc_code">
|
|
define fastcc void @foo() {
|
|
ret void
|
|
}
|
|
define void @test(i1 %X) {
|
|
F.i:
|
|
call fastcc void @foo()
|
|
ret void
|
|
}
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
|
|
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
|
|
</body>
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</html>
|