<html>
<head>
<title>LLVM: Frequently Asked Questions</title>
- <style>
+ <style type="text/css">
@import url("llvm.css");
.question { font-weight: bold }
.answer { margin-left: 2em }
<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>
+ <li>After CVS update, rebuilding gives the error "No rule to make
+ target".</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>
+
+ <li>
+ How can I disable all optimizations when compiling code using the LLVM GCC front end?
+ </li>
+
</ol>
</li>
<li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
<ol>
- <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
+ <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
+ <tt>main()</tt>?</a></li>
+ <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
- #include <iostream>?</li>
+ #include <iostream>?</a></li>
+ <li><a href="#codedce">Where did all of my code go??</a></li>
+ <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
</ol>
</li>
</ol>
+<div class="doc_author">
+ <p>Written by <a href="http://llvm.org">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/1.2/LICENSE.TXT">LLVM license</a>.</p>
+href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
</div>
<div class="question">
<li>The GCC front end code is not as portable as the LLVM suite, so it may not
compile as well on unsupported platforms.</li>
- <li>The Python test classes are more UNIX-centric than they should be, so
- porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
- effort.</li>
-
<li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
require more effort.</li>
<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>After CVS update, rebuilding gives the error "No rule to make target".</p>
+</div>
+
+<div class="answer">
+<p>If the error is of the form:</p>
+
+<div class="doc_code">
+<tt>
+gmake[2]: *** No rule to make target `/path/to/somefile', needed by
+`/path/to/another/file.d'.<br>
+Stop.
+</tt>
+</div>
+
+<p>This may occur anytime files are moved within the CVS repository or removed
+entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
+which list dependencies for source files, and rebuild:</p>
+
+<div class="doc_code">
+<pre>
+% cd $LLVM_OBJ_DIR
+% rm -f `find . -name \*\.d`
+% gmake
+</pre>
+</div>
+
+<p>In other cases, it may be necessary to run <tt>make clean</tt> before
+rebuilding.</p>
+</div>
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="cfe">Using the GCC Front End</a>
<div class="answer">
<p>
-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.
+The only way this can happen is if you haven't installed the runtime library. To
+correct this, do:</p>
+<pre>
+ % 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="doc_section">
</div>
<div class="question"><p>
+<a name="__main"></a>
What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
</p></div>
<!--=========================================================================-->
+<div class="question">
+<a name="iosinit"></a>
+<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 <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"></a>
Where did all of my code go??
</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>?
+<a name="undef"></a>
+<p>What is this "<tt>undef</tt>" thing that shows up in my code?
</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>
+<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>
-<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>
+<div class="doc_code">
+ <tt>int X() { int i; return i; }</tt>
+</div>
-<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>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
+specified for it.
</p>
</div>
-
- <li>
-
-
-<!-- *********************************************************************** -->
<!-- *********************************************************************** -->
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-<div class="doc_footer">
- <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
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