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530 lines
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530 lines
17 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>
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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>I compile the code, and I get some error about <tt>/localhome</tt>.</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 CVS, and now my build is trying to use a
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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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</ol></li>
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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 crtend.o.
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</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>What is this <tt>__main()</tt> call that gets inserted into
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<tt>main()</tt>?</li>
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<li>Where did all of my code go??</li>
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<li>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>?</li>
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</ol>
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</li>
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</ol>
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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.cs.uiuc.edu/releases/1.2/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 Python test classes are more UNIX-centric than they should be, so
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porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
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effort.</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) will
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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>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
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</div>
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<div class="answer">
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<p>There are several possible causes for this. The first is that you didn't set
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a pathname properly when using <tt>configure</tt>, and it defaulted to a
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pathname that we use on our research machines.</p>
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<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
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this, please email the LLVM bug mailing list with the name of the offending
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Makefile and a description of what is wrong with it.</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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<p><tt>PATH=<the path without the bad program> ./configure ...</tt></p>
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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 CVS, and now my build is trying to use a
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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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<p><tt>./config.status <relative path to Makefile></tt><p>
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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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<p><tt>gmake ENABLE_PROFILING=1</tt>
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<p>...then you must run the tests with the following commands:</p>
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<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
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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>
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When I use the test suite, all of the C Backend tests fail. What is
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wrong?
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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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If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
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then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
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environment variable does not contain the libcrtend.a library.
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</p>
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<p>
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To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
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and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
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should be the absolute path to
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<tt>cfrontend/<<i>platform</i>>/llvm-gcc/bytecode-libs</tt>. If you've
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built your own LLVM GCC front end, then ensure that you've built and installed
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the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
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to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
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</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>
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Make sure the CC and CXX environment variables contains the full path to the
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LLVM GCC front end.
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</li>
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<li>
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Make sure that the regular C compiler is first in your PATH.
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</li>
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<li>
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Add the string "-Wl,-native" to your CFLAGS environment variable.
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</li>
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</ol>
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<p>
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This will allow the gccld linker to create a native code executable instead of
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a shell script that runs the JIT. Creating native code requires standard
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linkage, which in turn will allow the configure script to find out if code is
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not linking on your system because the feature isn't available on your system.
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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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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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In order to find libcrtend.a, you must have the directory in which it lives in
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your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
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the LLVM GCC front end, this will be the full path of the bytecode-libs
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directory inside of the LLVM GCC distribution.
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</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>
|
|
|
|
<div class="question"><p>
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What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
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</p></div>
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<div class="answer">
|
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<p>
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The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
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that static constructors and destructors are called when the program starts up
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and shuts down. In C, you can create static constructors and destructors by
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using GCC extensions, and in C++ you can do so by creating a global variable
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whose class has a ctor or dtor.
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</p>
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<p>
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The actual implementation of <tt>__main</tt> lives in the
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<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
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linked in automatically when you link the program.
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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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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>
|
|
What is this <tt>llvm.global_ctors</tt> and <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include <iostream>?
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|
</p></div>
|
|
|
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<div class="answer">
|
|
<p>
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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.
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</p>
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<p>
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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 <iostream>. This object has a static constructor and
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destructor that initializes and destroys the global iostream objects before they
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could possibly be used in the file. The code that you see in the .ll file
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|
corresponds to the constructor and destructor registration code.
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</p>
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|
<p>
|
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If you would like to make it easier to <b>understand</b> the LLVM code generated
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|
by the compiler in the demo page, consider using printf instead of iostreams to
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|
print values.
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</p>
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</div>
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<li>
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<!-- *********************************************************************** -->
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<!-- *********************************************************************** -->
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<hr>
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<div class="doc_footer">
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<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
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<br>
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Last modified: $Date$
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</div>
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</body>
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</html>
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