X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=docs%2FFAQ.html;h=2dffd203ffdc406b61f45c1892979b74c338c388;hb=14fbcbfa2b7146569c588c451825206906c3b55a;hp=9c3121ec4811418b519a4d0d6f7c638a072d3f76;hpb=7911ce2578fd143f37ef4de7aa3e5795c18df4a6;p=oota-llvm.git

diff --git a/docs/FAQ.html b/docs/FAQ.html
index 9c3121ec481..2dffd203ffd 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  }
@@ -51,8 +51,19 @@
   <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="#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>
@@ -65,23 +76,31 @@
     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>What is this <tt>__main()</tt> call that gets inserted into
-         <tt>main()</tt>?</li>
-     <li>Where did all of my code go??</li>
-     <li>What is this <tt>llvm.global_ctors</tt> and
+     <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;?</li>
+          #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.cs.uiuc.edu">The LLVM Team</a></p>
+  <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
 </div>
 
 
@@ -121,7 +140,7 @@ 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/1.2/LICENSE.TXT">LLVM license</a>.</p>
+href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
 </div>
 
 <div class="question">
@@ -166,10 +185,6 @@ 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>
@@ -236,7 +251,7 @@ 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>
@@ -349,28 +364,77 @@ build.</p>
 </div>
 
 <div class="question">
-<p>
-When I use the test suite, all of the C Backend tests fail.  What is
-wrong?
-</p>
+<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
 </div>
 
 <div class="answer">
-<p>
-If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
-then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
-environment variable does not contain the libcrtend.a library.
-</p>
+<p>If the error is of the form:</p>
 
-<p>
-To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
-and that libcrtend.a is inside.  For pre-built LLVM GCC front ends, this
-should be the absolute path to
-<tt>cfrontend/&lt;<i>platform</i>&gt;/llvm-gcc/bytecode-libs</tt>.  If you've
-built your own LLVM GCC front end, then ensure that you've built and installed
-the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
-to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
-</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>
 
 <!-- *********************************************************************** -->
@@ -429,20 +493,99 @@ find libcrtend.a.
 
 <div class="answer">
 <p>
-In order to find libcrtend.a, 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.
+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">
+<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">$ 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>
 
@@ -464,7 +607,40 @@ linked in automatically when you link the program.
 
 <!--=========================================================================-->
 
+<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>
 
@@ -487,44 +663,37 @@ you can read from and assign to <tt>volatile</tt> global variables.
 <!--=========================================================================-->
 
 <div class="question"><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;?
+<a name="undef"></a>
+<p>What is this "<tt>undef</tt>" thing that shows up in my code?
 </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>
+<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>
-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 &lt;iostream&gt;.  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>
+<div class="doc_code">
+  <tt>int X() { int i; return i; }</tt>
+</div>
 
-<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 printf instead of iostreams to
-print values.
+<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value 
+specified for it.
 </p>
 </div>
 
-<!-- *********************************************************************** -->
 <!-- *********************************************************************** -->
 
 <hr>
-<div class="doc_footer">
-  <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
-  <br>
+<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>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
   Last modified: $Date$
-</div>
+</address>
 
 </body>
 </html>
