<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>
target".</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>
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="#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>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.cs.uiuc.edu">The LLVM Team</a></p>
+ <p>Written by <a href="http://llvm.org">The LLVM Team</a></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.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>
</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>
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="question">
<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
</div>
rebuilding.</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>llvm-java</tt> CVS repository. 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>examples/BFtoLLVM</tt> directory is a translator for the
+ BrainF*** language (2002 Language Specification).</p>
+ <p>In the <tt>projects/Stacker</tt> directory 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"><a name="langhlsupp">
+ <p>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"><a name="langhlsupp">
+ <p>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>.</li>
+</div>
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="cfe">Using the GCC Front End</a>
<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">
<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="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></li>
+
+<div class="doc_code">$ llvm-g++ x.cpp -o program</div>
+
+<p>or:</p>
+
+<div class="doc_code">
+ llvm-g++ a.cpp -c<br>
+ llvm-g++ b.cpp -c<br>
+ llvm-g++ a.o b.o -o program
+</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><p>Convert the LLVM code to C code, using the LLC tool with the C
+backend:</p></li>
+
+<div class="doc_code">$ llc -march=c program.bc -o program.c</div>
+
+<li><p>Finally, compile the c file:</p></li>
+
+<div class="doc_code">$ cc x.c</div>
+
+</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 bytecode, 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>
+<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>
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src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
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+ <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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