<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><a href="#build">Build Problems</a>
<ol>
<li>When I run configure, it finds the wrong C compiler.</li>
- <li>I compile the code, and I get some error about <tt>/localhome</tt>.</li>
<li>The <tt>configure</tt> script finds the right C compiler, but it uses the
LLVM linker from a previous build. What do I do?</li>
<li>When creating a dynamic library, I get a strange GLIBC error.</li>
- <li>I've updated my source tree from CVS, and now my build is trying to use a
- file/directory that doesn't exist.</li>
+ <li>I've updated my source tree from Subversion, and now my build is trying
+ to use a file/directory that doesn't exist.</li>
<li>I've modified a Makefile in my source tree, but my build tree keeps using
the old version. What do I do?</li>
<li>I've upgraded to a new version of LLVM, and I get strange build
errors.</li>
<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 on SuSE 9 fails, what should I do?</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 Subversion update, rebuilding gives the error "No rule to make
+ target".</li>
+ <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
+ work.</a></li>
</ol></li>
+ <li><a href="#felangs">Source Languages</a>
+ <ol>
+ <li><a href="#langs">What source languages are supported?</a></li>
+ <li><a href="#langhlsupp">What support is there for higher level source
+ language constructs for building a compiler?</a></li>
+ <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
+ instruction. Help!</a></li>
+ </ol>
+
<li><a href="#cfe">Using the GCC Front End</a>
<ol>
<li>
<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>
+
+ <li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></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><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>?</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/1.0/LICENSE.TXT">LLVM license</a>.</p>
+href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
</div>
<div class="question">
<p>Some porting problems may exist in the following areas:</p>
<ul>
-
<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>
+ compile as well on unsupported platforms.</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>
-
+ Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9)
+ will require more effort.</li>
</ul>
</div>
</div>
-<div class="question">
-<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
-</div>
-
-<div class="answer">
-
-<p>There are several possible causes for this. The first is that you didn't set
-a pathname properly when using <tt>configure</tt>, and it defaulted to a
-pathname that we use on our research machines.</p>
-
-<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
-this, please email the LLVM bug mailing list with the name of the offending
-Makefile and a description of what is wrong with it.</p>
-
-</div>
-
<div class="question">
<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
LLVM linker from a previous build. What do I do?</p>
it:</p>
<ol>
-
<li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
- program appears first in the <tt>PATH</tt>. This may work, but may not be
- convenient when you want them <i>first</i> in your path for other
- work.</p></li>
+ program appears first in the <tt>PATH</tt>. This may work, but may not be
+ convenient when you want them <i>first</i> in your path for other
+ work.</p></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>
+ correct. In a Borne compatible shell, the syntax would be:</p>
+
+<div class="doc_code">
+<pre>
+% PATH=[the path without the bad program] ./configure ...
+</pre>
+</div>
<p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
- to do its work without having to adjust your <tt>PATH</tt>
- permanently.</p></li>
-
+ to do its work without having to adjust your <tt>PATH</tt>
+ permanently.</p></li>
</ol>
</div>
</div>
<div class="question">
-<p>I've updated my source tree from CVS, and now my build is trying to use a
-file/directory that doesn't exist.</p>
+<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">
</div>
<div class="answer">
-
<p>If the Makefile already exists in your object tree, you
can just run the following command in the top level directory of your object
tree:</p>
-<p><tt>./config.status <relative path to Makefile></tt><p>
+<div class="doc_code">
+<pre>% ./config.status <relative path to Makefile></pre>
+</div>
<p>If the Makefile is new, you will have to modify the configure script to copy
it over.</p>
<p>For example, if you built LLVM with the command:</p>
-<p><tt>gmake ENABLE_PROFILING=1</tt>
+<div class="doc_code">
+<pre>% gmake ENABLE_PROFILING=1</pre>
+</div>
<p>...then you must run the tests with the following commands:</p>
-<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
+<div class="doc_code">
+<pre>
+% cd llvm/test
+% gmake ENABLE_PROFILING=1
+</pre>
+</div>
</div>
</div>
<div class="question">
-<p>Compiling LLVM with GCC 3.3 on SuSE 9 fails, what should I do?</p>
+<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
+</div>
+
+<div class="answer">
+<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
+affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
+</div>
+
+<div class="question">
+<p>After Subversion 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">
+<pre>
+gmake[2]: *** No rule to make target `/path/to/somefile', needed by
+`/path/to/another/file.d'.<br>
+Stop.
+</pre>
+</div>
+
+<p>This may occur anytime files are moved within the Subversion 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="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="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>In the <tt>stacker</tt> module is a compiler and runtime
+ library for the Stacker language, a "toy" language loosely based on Forth.</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="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="langhlsupp">
+ I don't understand the GetElementPtr instruction. Help!</a></p>
+</div>
<div class="answer">
-<p>This is a bug in the customized version of GCC shipped with SuSE, and affects
- projects other than LLVM. Complain loudly to SuSE. :)</p>
+ <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
+ Instruction</a>.</p>
</div>
<!-- *********************************************************************** -->
<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 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>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>
+ <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
</ol>
<p>
-This will allow the gccld linker to create a native code executable instead of
-a 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>
+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 crtend.o.
+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>
+
+<div class="doc_code">
+<pre>
+% cd llvm/runtime
+% make clean ; make install-bytecode
+</pre>
+</div>
+</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>
-In order to find crtend.o, 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.
+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="translatec++">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. However, this is a good way to add
+C++ support for a processor that does not otherwise have a C++ compiler.
</p>
+
+<p>Use commands like this:</p>
+
+<ol>
+ <li><p>Compile your program as normal with llvm-g++:</p>
+
+<div class="doc_code">
+<pre>
+% llvm-g++ x.cpp -o program
+</pre>
+</div>
+
+ <p>or:</p>
+
+<div class="doc_code">
+<pre>
+% llvm-g++ a.cpp -c
+% llvm-g++ b.cpp -c
+% llvm-g++ a.o b.o -o program
+</pre>
</div>
+ <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>
+
+<div class="doc_code">
+<pre>
+% llc -march=c program.bc -o program.c
+</pre>
+</div></li>
+
+<li><p>Finally, compile the C file:</p>
+
+<div class="doc_code">
+<pre>
+% cc x.c
+</pre>
+</div></li>
+
+</ol>
+
+<p>Note that, by default, the C backend 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 correct but relatively
+slow.</p>
+
+<p>Also note: this specific sequence of commands won't work if you use a
+function defined in the C++ runtime library (or any other C++ library). To
+access an external C++ library, you must 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 can compile the
+libraries and your application into two different chunks of C code and link
+them.</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>
-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>
-The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
-that static constructors and destructors are called when the program starts up
-and shuts down. In C, you can create static constructors and destructors by
-using GCC extensions, and in C++ you can do so by creating a global variable
-whose class has a ctor or dtor.
-</p>
-<p>
-The actual implementation of <tt>__main</tt> lives in the
-<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
-linked in automatically when you link the program.
+<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>
<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>
+<a name="undef"></a>
+<p>What is this "<tt>undef</tt>" thing that shows up in my code?
+</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>
+
+<div class="doc_code">
+<pre>
+int X() { int i; return i; }
+</pre>
+</div>
+
+<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
+a value specified for it.</p>
+</div>
+
<!-- *********************************************************************** -->
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