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<title>LLVM: Frequently Asked Questions</title>
- <style>
+ <style type="text/css">
@import url("llvm.css");
.question { font-weight: bold }
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<li>I've built LLVM and am testing it, but the tests freeze.</li>
<li>Why do test results differ when I perform different types of builds?</li>
<li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
+ <li>When I use the test suite, all of the C Backend tests fail. What is
+ wrong?</li>
</ol></li>
<li><a href="#cfe">Using the GCC Front End</a>
<li>
When I compile code using the LLVM GCC front end, it complains that it
- cannot find crtend.o.
+ cannot find libcrtend.a.
</li>
</ol>
</li>
<li>What is this <tt>__main()</tt> call that gets inserted into
<tt>main()</tt>?</li>
<li>Where did all of my code go??</li>
+ <li>What is this <tt>llvm.global_ctors</tt> and
+ <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
+ #include <iostream>?</li>
</ol>
</li>
</ol>
+<div class="doc_author">
+ <p>Written by <a href="http://llvm.cs.uiuc.edu">The LLVM Team</a></p>
+</div>
+
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="license">License</a>
<div class="answer">
<p>Yes. The modified source distribution must retain the copyright notice and
follow the three bulletted conditions listed in the <a
-href="http://llvm.cs.uiuc.edu/releases/1.0/LICENSE.TXT">LLVM license</a>.</p>
+href="http://llvm.cs.uiuc.edu/releases/1.2/LICENSE.TXT">LLVM license</a>.</p>
</div>
<div class="question">
<li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
correct. In a Borne compatible shell, the syntax would be:</p>
- <p><tt>PATH=<the path without the bad program> ./configure ...</tt></p>
+ <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
<p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
to do its work without having to adjust your <tt>PATH</tt>
affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
</div>
+<div class="question">
+<p>
+When I use the test suite, all of the C Backend tests fail. What is
+wrong?
+</p>
+</div>
+
+<div class="answer">
+<p>
+If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
+then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
+environment variable does not contain the libcrtend.a library.
+</p>
+
+<p>
+To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
+and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
+should be the absolute path to
+<tt>cfrontend/<<i>platform</i>>/llvm-gcc/bytecode-libs</tt>. If you've
+built your own LLVM GCC front end, then ensure that you've built and installed
+the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
+to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
+</p>
+</div>
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="cfe">Using the GCC Front End</a>
<div class="question">
<p>
When I compile code using the LLVM GCC front end, it complains that it cannot
-find crtend.o.
+find libcrtend.a.
</p>
</div>
<div class="answer">
<p>
-In order to find crtend.o, you must have the directory in which it lives in
+In order to find libcrtend.a, you must have the directory in which it lives in
your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
the LLVM GCC front end, this will be the full path of the bytecode-libs
directory inside of the LLVM GCC distribution.
<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 you code doesn't actually do anything
+code through the LLVM optimizers, so if your code doesn't actually do anything
useful, it might all be deleted.
</p>
</p>
</div>
-<!-- *********************************************************************** -->
+<!--=========================================================================-->
+
+<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>?
+</p></div>
+
+<div class="answer">
+<p>
+If you #include the <iostream> 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 <iostream>. 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 printf instead of iostreams to
+print values.
+</p>
+</div>
+
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