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<title>LLVM: Frequently Asked Questions</title>
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<div class="doc_title">
LLVM: Frequently Asked Questions
</div>
<ol>
<li><a href="#license">License</a>
<ol>
<li>Why are the LLVM source code and the front-end distributed under different
licenses?</li>
<li>Does the University of Illinois Open Source License really qualify as an
"open source" license?</li>
<li>Can I modify LLVM source code and redistribute the modified source?</li>
<li>Can I modify LLVM source code and redistribute binaries or other tools
based on it, without redistributing the source?</li>
</ol></li>
<li><a href="#source">Source code</a>
<ol>
<li>In what language is LLVM written?</li>
<li>How portable is the LLVM source code?</li>
</ol></li>
<li><a href="#build">Build Problems</a>
<ol>
<li>When I run configure, it finds the wrong C compiler.</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 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 3.3.2 fails, what should I do?</li>
<li>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what can be wrong?</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="#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></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>
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?
</li>
<li>
When I compile code using the LLVM GCC front end, it complains that it
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="#translatecxx">Can I use LLVM to convert C++ code to C code?</a></li>
<li><a href="#platformindependent">Can I compile C or C++ code to platform-independent LLVM bitcode?</a></li>
</ol>
</li>
<li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
<ol>
<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 &lt;iostream&gt;?</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>
<!-- *********************************************************************** -->
<div class="question">
<p>Why are the LLVM source code and the front-end distributed under different
licenses?</p>
</div>
<div class="answer">
<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
license, in particular one that does not compel users who distribute tools based
on modifying the source to redistribute the modified source code as well.</p>
</div>
<div class="question">
<p>Does the University of Illinois Open Source License really qualify as an
"open source" license?</p>
</div>
<div class="answer">
<p>Yes, the license is <a
href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
Source Initiative (OSI).</p>
</div>
<div class="question">
<p>Can I modify LLVM source code and redistribute the modified source?</p>
</div>
<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.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM license</a>.</p>
</div>
<div class="question">
<p>Can I modify LLVM source code and redistribute binaries or other tools based
on it, without redistributing the source?</p>
</div>
<div class="answer">
<p>Yes, this is why we distribute LLVM under a less restrictive license than
GPL, as explained in the first question above.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="source">Source Code</a>
</div>
<!-- *********************************************************************** -->
<div class="question">
<p>In what language is LLVM written?</p>
</div>
<div class="answer">
<p>All of the LLVM tools and libraries are written in C++ with extensive use of
the STL.</p>
</div>
<div class="question">
<p>How portable is the LLVM source code?</p>
</div>
<div class="answer">
<p>The LLVM source code should be portable to most modern UNIX-like operating
systems. Most of the code is written in standard C++ with operating system
services abstracted to a support library. The tools required to build and test
LLVM have been ported to a plethora of platforms.</p>
<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 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>
</ul>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="build">Build Problems</a>
</div>
<!-- *********************************************************************** -->
<div class="question">
<p>When I run configure, it finds the wrong C compiler.</p>
</div>
<div class="answer">
<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
for the C and C++ compiler, respectively.</p>
<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
explicitly.</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>
</div>
<div class="answer">
<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
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>
<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>
<pre class="doc_code">
% PATH=[the path without the bad program] ./configure ...
</pre>
<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>
</ol>
</div>
<div class="question">
<p>When creating a dynamic library, I get a strange GLIBC error.</p>
</div>
<div class="answer">
<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
GCC was compiled with the --disable-shared option. To work around this, install
your own version of GCC that has shared libraries enabled by default.</p>
</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
are added to the source tree, they have to be copied over to the object tree 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>
<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>
<pre class="doc_code">
% ./config.status &lt;relative path to Makefile&gt;
</pre>
<p>If the Makefile is new, you will have to modify the configure script to copy
it over.</p>
</div>
<div class="question">
<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
</div>
<div class="answer">
<p>Sometimes, changes to the LLVM source code alters how the build system works.
Changes in libtool, autoconf, or header file dependencies are especially prone
to this sort of problem.</p>
<p>The best thing to try is to remove the old files and re-build. In most
cases, this takes care of the problem. To do this, just type <tt>make
clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
</div>
<div class="question">
<p>I've built LLVM and am testing it, but the tests freeze.</p>
</div>
<div class="answer">
<p>This is most likely occurring because you built a profile or release
(optimized) build of LLVM and have not specified the same information on the
<tt>gmake</tt> command line.</p>
<p>For example, if you built LLVM with the command:</p>
<pre class="doc_code">
% gmake ENABLE_PROFILING=1
</pre>
<p>...then you must run the tests with the following commands:</p>
<pre class="doc_code">
% cd llvm/test
% gmake ENABLE_PROFILING=1
</pre>
</div>
<div class="question">
<p>Why do test results differ when I perform different types of builds?</p>
</div>
<div class="answer">
<p>The LLVM test suite is dependent upon several features of the LLVM tools and
libraries.</p>
<p>First, the debugging assertions in code are not enabled in optimized or
profiling builds. Hence, tests that used to fail may pass.</p>
<p>Second, some tests may rely upon debugging options or behavior that is only
available in the debug build. These tests will fail in an optimized or profile
build.</p>
</div>
<div class="question">
<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/bugzilla/show_bug.cgi?id=13392">a bug in GCC</a>, and
affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
</div>
<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 codebase. Please
consult your compiler version (<tt>gcc --version</tt>) to find out whether it is
<a href="GettingStarted.html#brokengcc">broken</a>. If so, your only option is to upgrade
GCC to a known good version.</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>
<pre class="doc_code">
gmake[2]: *** No rule to make target `/path/to/somefile', needed by
`/path/to/another/file.d'.<br>
Stop.
</pre>
<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>
<pre class="doc_code">
% cd $LLVM_OBJ_DIR
% rm -f `find . -name \*\.d`
% gmake
</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="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>
<li>
<strong>Call into the LLVM libraries code using your language's FFI
(foreign function interface).</strong>
<ul>
<li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax,
and .bc format</li>
<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 incompatiblities
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 targetting.</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 hardwaring 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">
<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
&lt;iostream&gt;?</p>
</div>
<div class="answer">
<p>If you #include the &lt;iostream&gt; 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>&lt;iostream&gt;</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="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"></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>
<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>
<!-- *********************************************************************** -->
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