X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=docs%2FFAQ.html;h=b67fae3abe547d3d281e693f30b42f533a5d7667;hb=550211711acc12a470a66cf55147796f61e923cd;hp=d0c5148763acae2fc191979b4c28b2198a53b155;hpb=76c1e380395abd608fd3a1e8f9835b21d1b5e081;p=oota-llvm.git

diff --git a/docs/FAQ.html b/docs/FAQ.html
index d0c5148763a..b67fae3abe5 100644
--- a/docs/FAQ.html
+++ b/docs/FAQ.html
@@ -3,7 +3,7 @@
 <html>
 <head>
   <title>LLVM: Frequently Asked Questions</title>
-  <style>
+  <style type="text/css">
     @import url("llvm.css");
     .question { font-weight: bold }
     .answer   { margin-left: 2em  }
@@ -36,7 +36,6 @@
   <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>
@@ -48,9 +47,23 @@
   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>
   </ol></li>
 
-  <li><a href="#cfe">GCC Front End</a>
+  <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
@@ -60,12 +73,36 @@
 
     <li>
     When I compile code using the LLVM GCC front end, it complains that it
-    cannot find crtend.o.
+    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>
@@ -102,7 +139,7 @@ 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.cs.uiuc.edu/releases/1.0/LICENSE.TXT">LLVM license</a>.</p>
+href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
 </div>
 
 <div class="question">
@@ -147,10 +184,6 @@ LLVM have been ported to a plethora of platforms.</p>
   <li>The GCC front end code is not as portable as the LLVM suite, so it may not
   compile as well on unsupported platforms.</li>
 
-  <li>The Python test classes are more UNIX-centric than they should be, so
-  porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
-  effort.</li>
-
   <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
   Shell and sed.  Porting to systems without these tools (MacOS 9, Plan 9) will
   require more effort.</li>
@@ -181,22 +214,6 @@ 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>
-
-</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>
@@ -217,7 +234,7 @@ it:</p>
   <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><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>
@@ -320,9 +337,92 @@ 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 CVS 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>
+gmake[2]: *** No rule to make target `/path/to/somefile', needed by
+`/path/to/another/file.d'.<br>
+Stop.
+</tt>
+</div>
+
+<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>
+
+<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="doc_section"><a name="felangs">Source Languages</a></div>
+
+<div class="question"><p>
+  <a name="langs">What source languages are supported?</a></p>
+</div>
+<div class="answer">
+  <p>LLVM currently has full support for C and C++ source languages. These are
+  available through a special version of GCC that LLVM calls the 
+  <a href="#cfe">C Front End</a></p>
+  <p>There is an incomplete version of a Java front end available in the
+  <tt>llvm-java</tt> CVS repository. There is no documentation on this yet so
+  you'll need to download the code, compile it, and try it.</p>
+  <p>In the <tt>examples/BFtoLLVM</tt> directory is a translator for the 
+  BrainF*** language (2002 Language Specification).</p>
+  <p>In the <tt>projects/Stacker</tt> directory is a compiler and runtime
+  library for the Stacker language, a "toy" language loosely based on Forth.</p>
+  <p>The PyPy developers are working on integrating LLVM into the PyPy backend
+  so that PyPy language can translate to LLVM.</p>
+</div>
+<div class="question"><a name="langhlsupp">
+  <p>What support is there for a higher level source language constructs for 
+  building a compiler?</a></p>
+</div>
+<div class="answer">
+  <p>Currently, there isn't much. LLVM supports an intermediate representation
+  which is useful for code representation but will not support the high level
+  (abstract syntax tree) representation needed by most compilers. There are no
+  facilities for lexical nor semantic analysis. There is, however, a <i>mostly
+    implemented</i> configuration-driven 
+  <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
+  of running optimizations, linking, and executable generation.</p>
+</div>
+
+<div class="question"><a name="langhlsupp">
+  <p>I don't understand the GetElementPtr
+      instruction. Help!</a></p>
+</div>
+<div class="answer">
+  <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
+   Instruction</a>.</li>
+</div>
+
 <!-- *********************************************************************** -->
 <div class="doc_section">
-  <a name="cfe">GCC Front End</a>
+  <a name="cfe">Using the GCC Front End</a>
 </div>
 
 <div class="question">
@@ -343,54 +443,233 @@ or translation to the C back end).  That is why configure thinks your system
 <p>
 To work around this, perform the following steps:
 </p>
-
 <ol>
-  <li>
-  Make sure the CC and CXX environment variables contains the full path to the
-  LLVM GCC front end.
-  </li>
+  <li>Make sure the CC and CXX environment variables contains the full path to 
+  the LLVM GCC front end.</li>
 
-  <li>
-  Make sure that the regular C compiler is first in your PATH.
-  </li>
+  <li>Make sure that the regular C compiler is first in your PATH. </li>
 
-  <li>
-  Add the string "-Wl,-native" to your CFLAGS environment variable.
-  </li>
+  <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
 </ol>
 
 <p>
-This will allow the gccld linker to create a native code executable instead of
-a shell script that runs the JIT.  Creating native code requires standard
-linkage, which in turn will allow the configure script to find out if code is
-not linking on your system because the feature isn't available on your system.
-</p>
+This will allow the <tt>llvm-ld</tt> linker to create a native code executable 
+instead of shell script that runs the JIT.  Creating native code requires 
+standard linkage, which in turn will allow the configure script to find out if 
+code is not linking on your system because the feature isn't available on your 
+system.</p>
 </div>
 
-<div class="question"
+<div class="question">
 <p>
 When I compile code using the LLVM GCC front end, it complains that it cannot
-find crtend.o.
+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
+</pre>
+</div>
+
+<div class="question">
+<p>
+How can I disable all optimizations when compiling code using the LLVM GCC front end?
 </p>
 </div>
 
-<div class="answer"
+<div class="answer">
 <p>
-In order to find crtend.o, you must have the directory in which it lives in
-your LLVM_LIB_SEARCH_PATH environment variable.  For the binary distribution of
-the LLVM GCC front end, this will be the full path of the bytecode-libs
-directory inside of the LLVM GCC distribution.
+Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
+optimizations done at the llvm level, leaving you with the truly horrible
+code that you desire.
 </p>
 </div>
+
+
+<div class="question">
+<p>
+<a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
+</p>
+</div>
+
+<div class="answer">
+<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
+Note that the generated C code will be very low level (all loops are lowered
+to gotos, etc) and not very pretty (comments are stripped, original source
+formatting is totally lost, variables are renamed, expressions are regrouped), 
+so this may not be what you're looking for.  However, this is a good way to add
+C++ support for a processor that does not otherwise have a C++ compiler.
+</p>
+
+<p>Use commands like this:</p>
+
+<ol>
+<li><p>Compile your program as normal with llvm-g++:</p></li>
+
+<div class="doc_code">$ llvm-g++ x.cpp -o program</div>
+
+<p>or:</p>
+
+<div class="doc_code">
+ llvm-g++ a.cpp -c<br>
+ llvm-g++ b.cpp -c<br>
+ llvm-g++ a.o b.o -o program
+</div>
+
+<p>With llvm-gcc3, this will generate program and program.bc.  The .bc file is 
+the LLVM version of the program all linked together.</p>
+
+<li><p>Convert the LLVM code to C code, using the LLC tool with the C
+backend:</p></li>
+
+<div class="doc_code">$ llc -march=c program.bc -o program.c</div>
+
+<li><p>Finally, compile the c file:</p></li>
+
+<div class="doc_code">$ cc x.c</div>
+
+</ol>
+
+<p>Note that, by default, the C backend does not support exception handling.
+If you want/need it for a certain program, you can enable it by passing
+"-enable-correct-eh-support" to the llc program.  The resultant code will
+use setjmp/longjmp to implement exception support that is correct but
+relatively slow.
+</p>
+
+<p>Also note: this specific sequence of commands won't work if you use a 
+function defined in the C++ runtime library (or any other C++ library).  To 
+access an external C++ library, you must manually 
+compile libstdc++ to LLVM bytecode, statically link it into your program, then
+use the commands above to convert the whole result into C code.  Alternatively,
+you can compile the libraries and your application into two different chunks
+of C code and link them.</p>
+
+</div>
+
+
+
 <!-- *********************************************************************** -->
+<div class="doc_section">
+  <a name="cfe_code">Questions about code generated by the GCC front-end</a>
+</div>
+
+<div class="question"><p>
+<a name="__main"></a>
+What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
+</p></div>
+
+<div class="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">
+  <tt>int X() { int i; return i; }</tt>
+</div>
+
+<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value 
+specified for it.
+</p>
+</div>
+
 <!-- *********************************************************************** -->
 
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