</head>
<body>
-<div class="doc_title">
+<h1>
LLVM: Frequently Asked Questions
-</div>
+</h1>
<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>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>
-
- <li><a href="#srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir,
- it fails. Why?</a></li>
</ol></li>
<li><a href="#felangs">Source Languages</a>
instruction. Help!</a></li>
</ol>
- <li><a href="#cfe">Using the GCC Front End</a>
+ <li><a href="#cfe">Using the C and C++ Front Ends</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>
+ <li><a href="#cfe_code">Questions about code generated by the demo page</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
+ <tt>_GLOBAL__I_a...</tt> stuff that happens when I
#include <iostream>?</a></li>
<li><a href="#codedce">Where did all of my code go??</a></li>
</ol>
<div class="doc_author">
- <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
+ <p>Written by <a href="http://llvm.org/">The LLVM Team</a></p>
</div>
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="license">License</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<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>
<div class="question">
<p>Does the University of Illinois Open Source License really qualify as an
GPL, as explained in the first question above.</p>
</div>
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="source">Source Code</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
+<div>
+
<div class="question">
<p>In what language is LLVM written?</p>
</div>
<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>
+ <li>The autoconf/makefile 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>
+
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="build">Build Problems</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
+<div>
+
<div class="question">
<p>When I run configure, it finds the wrong C compiler.</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>
+ LLVM tools from a previous build. What do I do?</p>
</div>
<div class="answer">
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="question">
-<p><a name="srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir, it
- fails. Why?</a></p>
-</div>
-
-<div class="answer">
-<p>The <tt>GNUmakefile</tt> in the top-level directory of LLVM-GCC is a special
- <tt>Makefile</tt> used by Apple to invoke the <tt>build_gcc</tt> script after
- setting up a special environment. This has the unfortunate side-effect that
- trying to build LLVM-GCC with srcdir == objdir in a "non-Apple way" invokes
- the <tt>GNUmakefile</tt> instead of <tt>Makefile</tt>. Because the
- environment isn't set up correctly to do this, the build fails.</p>
-
-<p>People not building LLVM-GCC the "Apple way" need to build LLVM-GCC with
- srcdir != objdir, or simply remove the GNUmakefile entirely.</p>
-
-<p>We regret the inconvenience.</p>
</div>
<!-- *********************************************************************** -->
-<div class="doc_section"><a name="felangs">Source Languages</a></div>
+<h2>
+ <a name="felangs">Source Languages</a>
+</h2>
+
+<div>
<div class="question">
<p><a name="langs">What source languages are supported?</a></p>
<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>
+ available through both <a href="http://clang.llvm.org/">Clang</a> and
+ <a href="http://dragonegg.llvm.org/">DragonEgg</a>.</p>
<p>The PyPy developers are working on integrating LLVM into the PyPy backend so
that PyPy language can translate to LLVM.</p>
<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>
+ facilities for lexical nor semantic analysis.</p>
</div>
<div class="question">
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>
+<!-- *********************************************************************** -->
+<h2>
+ <a name="cfe">Using the C and C++ Front Ends</a>
+</h2>
-<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 incompatibilities with
- standard C++ libraries.</p>
-</div>
+<div>
<div class="question">
<p><a name="platformindependent">Can I compile C or C++ code to
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>
+<!-- *********************************************************************** -->
+<h2>
+ <a name="cfe_code">Questions about code generated by the demo page</a>
+</h2>
+
+<div>
+
<div class="question">
<p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and
- <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I <tt>#include
+ <tt>_GLOBAL__I_a...</tt> stuff that happens when I <tt>#include
<iostream></tt>?</a></p>
</div>
ret void
}
define void @bar() {
- call void @foo( )
+ call void @foo()
ret void
}
</pre>
</div>
+</div>
+
<!-- *********************************************************************** -->
<hr>
<a href="http://validator.w3.org/check/referer"><img
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
- <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
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