X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=docs%2FFAQ.html;h=bf050c492e2f1633644fe37ba4b03d8db2609367;hb=aa02ff1a2beaef81d24f9a59324c8a7b02ad9c90;hp=cc1fef79427e43db57e232dd5808e2b85a5cb942;hpb=05fe4b02561f4431626a73ac2900be523e233cd6;p=oota-llvm.git

diff --git a/docs/FAQ.html b/docs/FAQ.html
index cc1fef79427..bf050c492e2 100644
--- a/docs/FAQ.html
+++ b/docs/FAQ.html
@@ -2,6 +2,7 @@
                       "http://www.w3.org/TR/html4/strict.dtd">
 <html>
 <head>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
   <title>LLVM: Frequently Asked Questions</title>
   <style type="text/css">
     @import url("llvm.css");
@@ -18,72 +19,115 @@
 <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>
+    <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>
+    <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>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 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 CVS update, rebuilding gives the error "No rule to make
-  target".</li>
+    <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>Compiling LLVM with GCC succeeds, but the resulting tools do not work,
+        what can be wrong?</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.</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>
+  <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
+      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 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>
-    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>
-    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>
   <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>
+
+     <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in
+         my code?</a></li>
+         
+      <li><a href="#callconvwrong">Why does instcombine + simplifycfg turn
+   a call to a function with a mismatched calling convention into "unreachable"?
+   Why not make the verifier reject it?</a></li>
   </ol>
   </li>
 </ol>
@@ -101,25 +145,26 @@
 
 <div class="question">
 <p>Why are the LLVM source code and the front-end distributed under different
-licenses?</p>
+   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>
+   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>
+   "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>
+<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">
@@ -128,18 +173,19 @@ Source Initiative (OSI).</p>
 
 <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>
+   follow the three bulletted conditions listed in
+   the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/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>
+   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>
+<p>Yes. This is why we distribute LLVM under a less restrictive license than
+   GPL, as explained in the first question above.</p>
 </div>
 
 <!-- *********************************************************************** -->
@@ -154,7 +200,7 @@ GPL, as explained in the first question above.</p>
 
 <div class="answer">
 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
-the STL.</p>
+   the STL.</p>
 </div>
 
 <div class="question">
@@ -170,14 +216,12 @@ 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>
+      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>
@@ -193,61 +237,43 @@ LLVM have been ported to a plethora of platforms.</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>
+   <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>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>
+   <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>
+   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>
+   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>
+      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 class="question">
@@ -256,37 +282,37 @@ it:</p>
 
 <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>
+   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 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">
 <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>
+   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>
+   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>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>
+<pre class="doc_code">
+% ./config.status &lt;relative path to Makefile&gt;
+</pre>
 
 <p>If the Makefile is new, you will have to modify the configure script to copy
-it over.</p>
-
+   it over.</p>
 </div>
 
 <div class="question">
@@ -296,13 +322,12 @@ it over.</p>
 <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>
+   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>
-
+   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">
@@ -310,19 +335,22 @@ clean</tt> and then <tt>make</tt> in the directory that fails to build.</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>
+   (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>
 
-<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="question">
@@ -330,17 +358,15 @@ clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
 </div>
 
 <div class="answer">
-
 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
-libraries.</p>
+   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>
+   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>
-
+   available in the debug build.  These tests will fail in an optimized or
+   profile build.</p>
 </div>
 
 <div class="question">
@@ -348,39 +374,184 @@ build.</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>
+<p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=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 CVS update, rebuilding gives the error "No rule to make target".</p>
+<p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what
+   can be wrong?</p>
+</div>
+
+<div class="answer">
+<p>Several versions of GCC have shown a weakness in miscompiling the LLVM
+   codebase. Please consult your compiler version (<tt>gcc --version</tt>) to
+   find out whether it is <a href="GettingStarted.html#brokengcc">broken</a>.
+   If so, your only option is to upgrade GCC to a known good version.</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">
-<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>
+   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>
+
+<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>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">
+<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">
+<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">
+<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>.</p>
 </div>
 
 <!-- *********************************************************************** -->
@@ -389,179 +560,365 @@ rebuilding.</p>
 </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>
+<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>
+<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 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>
+<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>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>
-  % cd llvm/runtime
-  % make clean ; make install-bytecode
+<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>
+<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>
+<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="doc_section">
-  <a name="cfe_code">Questions about code generated by the GCC front-end</a>
+
+<div class="question">
+<p><a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a></p>
 </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>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 with llvm-g++:</p>
+
+<pre class="doc_code">
+% llvm-g++ -emit-llvm x.cpp -o program.bc -c
+</pre>
+
+      <p>or:</p>
+
+<pre class="doc_code">
+% llvm-g++ a.cpp -c -emit-llvm
+% llvm-g++ b.cpp -c -emit-llvm
+% llvm-ld a.o b.o -o program
+</pre>
+
+   <p>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 -lstdc++
+</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>
+
+<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 class="question">
+<p><a name="platformindependent">Can I compile C or C++ code to
+   platform-independent LLVM bitcode?</a></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>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
+   targeting.</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>
+<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 hard-wiring 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>
 </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>
-<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>
+<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
+   &lt;iostream&gt;</tt>?</a></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>If you <tt>#include</tt> the <tt>&lt;iostream&gt;</tt> 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.
+   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>
-
+   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><a name="codedce">Where did all of my code go??</a></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>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>
+<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="question">
+<p><a name="undef">What is this "<tt>undef</tt>" thing that shows up in my
+   code?</a></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>
+<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">
-  <tt>int X() { int i; return i; }</tt>
+<pre class="doc_code">
+int X() { int i; return i; }
+</pre>
+
+<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a
+   value specified for it.</p>
+</div>
+
+<!--=========================================================================-->
+
+<div class="question">
+<p><a name="callconvwrong">Why does instcombine + simplifycfg turn
+   a call to a function with a mismatched calling convention into "unreachable"?
+   Why not make the verifier reject it?</a></p>
 </div>
 
-<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value 
-specified for it.
+<div class="answer">
+<p>This is a common problem run into by authors of front-ends that are using
+custom calling conventions: you need to make sure to set the right calling
+convention on both the function and on each call to the function.  For example,
+this code:</p>
+
+<pre class="doc_code">
+define fastcc void @foo() {
+        ret void
+}
+define void @bar() {
+        call void @foo( )
+        ret void
+}
+</pre>
+
+<p>Is optimized to:</p>
+
+<pre class="doc_code">
+define fastcc void @foo() {
+	ret void
+}
+define void @bar() {
+	unreachable
+}
+</pre>
+
+<p>... with "opt -instcombine -simplifycfg".  This often bites people because
+"all their code disappears".  Setting the calling convention on the caller and
+callee is required for indirect calls to work, so people often ask why not make
+the verifier reject this sort of thing.</p>
+
+<p>The answer is that this code has undefined behavior, but it is not illegal.
+If we made it illegal, then every transformation that could potentially create
+this would have to ensure that it doesn't, and there is valid code that can
+create this sort of construct (in dead code).  The sorts of things that can
+cause this to happen are fairly contrived, but we still need to accept them.
+Here's an example:</p>
+
+<pre class="doc_code">
+define fastcc void @foo() {
+        ret void
+}
+define internal void @bar(void()* %FP, i1 %cond) {
+        br i1 %cond, label %T, label %F
+T:  
+        call void %FP()
+        ret void
+F:
+        call fastcc void %FP()
+        ret void
+}
+define void @test() {
+        %X = or i1 false, false
+        call void @bar(void()* @foo, i1 %X)
+        ret void
+} 
+</pre>
+
+<p>In this example, "test" always passes @foo/false into bar, which ensures that
+   it is dynamically called with the right calling conv (thus, the code is
+   perfectly well defined).  If you run this through the inliner, you get this
+   (the explicit "or" is there so that the inliner doesn't dead code eliminate
+   a bunch of stuff):
+</p>
+
+<pre class="doc_code">
+define fastcc void @foo() {
+	ret void
+}
+define void @test() {
+	%X = or i1 false, false
+	br i1 %X, label %T.i, label %F.i
+T.i:
+	call void @foo()
+	br label %bar.exit
+F.i:
+	call fastcc void @foo()
+	br label %bar.exit
+bar.exit:
+	ret void
+}
+</pre>
+
+<p>Here you can see that the inlining pass made an undefined call to @foo with
+  the wrong calling convention.  We really don't want to make the inliner have
+  to know about this sort of thing, so it needs to be valid code.  In this case,
+  dead code elimination can trivially remove the undefined code.  However, if %X
+  was an input argument to @test, the inliner would produce this:
 </p>
+
+<pre class="doc_code">
+define fastcc void @foo() {
+	ret void
+}
+
+define void @test(i1 %X) {
+	br i1 %X, label %T.i, label %F.i
+T.i:
+	call void @foo()
+	br label %bar.exit
+F.i:
+	call fastcc void @foo()
+	br label %bar.exit
+bar.exit:
+	ret void
+}
+</pre>
+
+<p>The interesting thing about this is that %X <em>must</em> be false for the
+code to be well-defined, but no amount of dead code elimination will be able to
+delete the broken call as unreachable.  However, since instcombine/simplifycfg
+turns the undefined call into unreachable, we end up with a branch on a
+condition that goes to unreachable: a branch to unreachable can never happen, so
+"-inline -instcombine -simplifycfg" is able to produce:</p>
+
+<pre class="doc_code">
+define fastcc void @foo() {
+	ret void
+}
+define void @test(i1 %X) {
+F.i:
+	call fastcc void @foo()
+	ret void
+}
+</pre>
+
 </div>
 
 <!-- *********************************************************************** -->
@@ -569,9 +926,9 @@ specified for it.
 <hr>
 <address>
   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
-  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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   <a href="http://validator.w3.org/check/referer"><img
-  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
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