<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
<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>
+ <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
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>
+ <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="#__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>?</a></li>
<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/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
+href="http://llvm.org/svn/llvm-project/llvm/trunk/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>
+ 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>
+
+<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>
-
+ 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>
+<pre class="doc_code">
+% ./config.status <relative path to Makefile>
+</pre>
<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>
+<pre class="doc_code">
+% gmake ENABLE_PROFILING=1
+</pre>
<p>...then you must run the tests with the following commands:</p>
-<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
+<pre class="doc_code">
+% cd llvm/test
+% gmake ENABLE_PROFILING=1
+</pre>
</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>
+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>
+<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">
-<tt>
+<pre class="doc_code">
gmake[2]: *** No rule to make target `/path/to/somefile', needed by
`/path/to/another/file.d'.<br>
Stop.
-</tt>
-</div>
+</pre>
-<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>
+<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>
+<pre class="doc_code">
% 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>
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
+ <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>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
+
+<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">
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 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>.</li>
+ 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>
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 class="doc_code">
+% cd llvm/runtime
+% make clean ; make install-bytecode
</pre>
</div>
<div class="question">
<p>
-<a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
+<a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a>
</p>
</div>
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>
+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></li>
+ <li><p>Compile your program as normal with llvm-g++:</p>
-<div class="doc_code">$ llvm-g++ x.cpp -o program</div>
+<pre class="doc_code">
+% llvm-g++ x.cpp -o program
+</pre>
-<p>or:</p>
-<div class="doc_code">
- llvm-g++ a.cpp -c
- llvm-g++ b.cpp -c
- llvm-g++ a.o b.o -o program
-</div>
+<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>
+ <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></li>
+
<li><p>Convert the LLVM code to C code, using the LLC tool with the C
+ backend:</p>
-<div class="doc_code">$ llc -march=c program.bc -o program.c</div>
+<pre class="doc_code">
+% llc -march=c program.bc -o program.c
+</pre></li>
-<li><p>Finally, compile the c file:</p></li>
+<li><p>Finally, compile the C file:</p>
-<div class="doc_code">$ cc x.c</div>
+<pre class="doc_code">
+% cc x.c
+</pre></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>
-</div>
+<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>
-<!-- *********************************************************************** -->
-<div class="doc_section">
- <a name="cfe_code">Questions about code generated by the GCC front-end</a>
-</div>
+<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 class="question"><p>
-<a name="__main"></a>
-What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
-</p></div>
+</div>
-<div class="answer">
+<div class="question">
<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.
+<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>
-<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>
</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>
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">
- <tt>int X() { int i; return i; }</tt>
-</div>
+<pre class="doc_code">
+int X() { int i; return i; }
+</pre>
-<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
-specified for it.
-</p>
+<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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