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Fequently Asked Questions about LLVM. Mainly focuses on build problems, but
anything FAQ'ish should go here if it's not heavily documented elsewhere (like the README.txt file or the Getting Started Guide). llvm-svn: 9086
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John Criswell
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<h1>
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<center>
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LLVM: Frequently Asked Questions
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</center>
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</h1>
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<hr>
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<!--=====================================================================-->
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<h2>
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<a name="source">Source Code</a>
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</h2>
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<!--=====================================================================-->
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<dl compact>
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<dt> <b>In what language is LLVM written?</b>
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<dd>
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All of the LLVM tools and libraries are written in C++ with extensive use
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of the STL.
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<p>
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<dt><b>How portable is the LLVM source code?</b>
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<dd>
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The LLVM source code should be portable to most modern UNIX-like operating
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systems. Most of the code is written in standard C++ with operating
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system services abstracted to a support library. The tools required to
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build and test LLVM have been ported to a plethora of platforms.
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<p>
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Some porting problems may exist in the following areas:
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<ul>
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<li>The GCC front end code is not as portable as the LLVM suite, so it
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may not compile as well on unsupported platforms.
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<p>
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<li>The Python test classes are more UNIX-centric than they should be,
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so porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will
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require some effort.
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<p>
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<li>The LLVM build system relies heavily on UNIX shell tools, like the
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Bourne Shell and sed. Porting to systems without these tools (MacOS 9,
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Plan 9) will require more effort.
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</ul>
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</dl>
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<hr>
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<!--=====================================================================-->
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<h2>
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<a name="build">Build Problems</a>
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</h2>
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<!--=====================================================================-->
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<dl compact>
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<dt><b>When I run configure, it finds the wrong C compiler.</b>
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<dd>
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The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and
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then <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and
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<tt>CXX</tt> for the C and C++ compiler, respectively.
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If <tt>configure</tt> finds the wrong compiler, either adjust your
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<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
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explicitly.
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<p>
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<dt><b>I compile the code, and I get some error about /localhome</b>.
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<dd>
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There are several possible causes for this. The first is that you
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didn't set a pathname properly when using <tt>configure</tt>, and it
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defaulted to a pathname that we use on our research machines.
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<p>
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Another possibility is that we hardcoded a path in our Makefiles. If
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you see this, please email the LLVM bug mailing list with the name of
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the offending Makefile and a description of what is wrong with it.
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<dt><b>The <tt>configure</tt> script finds the right C compiler, but it
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uses the LLVM linker from a previous build. What do I do?</b>
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<dd>
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The <tt>configure</tt> script uses the <tt>PATH</tt> to find
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executables, so if it's grabbing the wrong linker/assembler/etc, there
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are two ways to fix it:
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<ol>
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<li>Adjust your <tt>PATH</tt> environment variable so that the
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correct program appears first in the <tt>PATH</tt>. This may work,
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but may not be convenient when you want them <i>first</i> in your
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path for other work.
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<p>
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<li>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that
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is correct. In a Borne compatible shell, the syntax would be:
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<p>
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<tt>PATH=<the path without the bad program> ./configure ...</tt>
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<p>
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This is still somewhat inconvenient, but it allows
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<tt>configure</tt> to do its work without having to adjust your
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<tt>PATH</tt> permanently.
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</ol>
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<dt><b>When creating a dynamic library, I get a strange GLIBC error.</b>
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<dd>
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Under some operating systems (i.e. Linux), libtool does not work correctly
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if GCC was compiled with the --disable-shared option. To work around this,
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install your own version of GCC that has shared libraries enabled by
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default.
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<p>
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<dt><b>I've updated my source tree from CVS, and now my build is trying to
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use a file/directory that doesn't exist.</b>
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<dd>
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You need to re-run configure in your object directory. When new Makefiles
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are added to the source tree, they have to be copied over to the object
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tree in order to be used by the build.
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<p>
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<dt><b>I've modified a Makefile in my source tree, but my build tree keeps
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using the old version. What do I do?</b>
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<dd>
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If the Makefile already exists in your object tree, you can just run the
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following command in the top level directory of your object tree:
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<p>
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<tt>./config.status <relative path to Makefile></tt>
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<p>
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If the Makefile is new, you will have to modify the configure script to copy
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it over.
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<p>
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<dt><b>I've upgraded to a new version of LLVM, and I get strange build
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errors.</b>
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<dd>
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Sometimes changes to the LLVM source code alters how the build system
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works. Changes in libtool, autoconf, or header file dependencies are
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especially prone to this sort of problem.
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<p>
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The best thing to try is to remove the old files and re-build. In most
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cases, this takes care of the problem. To do this, just type <tt>make
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clean</tt> and then <tt>make</tt> in the directory that fails to build.
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<p>
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</dl>
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<hr>
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</body>
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</html>
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