docs/FAQ.html - platform/external/llvm - Gitiles

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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>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.</li></a>
   </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 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="#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><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 &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/releases/1.3/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>

       <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>

       <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>

 <p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>

 <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>

 <div class="doc_code">
 <pre>
 % gmake ENABLE_PROFILING=1
 </pre>
 </div>

 <p>...then you must run the tests with the following commands:</p>

 <div class="doc_code">
 <pre>
 % cd llvm/test
 % gmake ENABLE_PROFILING=1
 </pre>
 </div>

 </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/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">
   <a name="llvmc"<p>The <tt>llvmc</tt> program gives me errors/doesn't
   work.</p></a>
 </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"><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>
 </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>

 <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>
 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">
 <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><p>Convert the LLVM code to C code, using the LLC tool with the C
 backend:</p></li>

 <div class="doc_code">
 <pre>
 % llc -march=c program.bc -o program.c
 </pre>
 </div>

 <li><p>Finally, compile the c file:</p></li>

 <div class="doc_code">
 <pre>
 % cc x.c
 </pre>
 </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 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>
 <a name="__main"></a>
 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
 </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>
 </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>

 <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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