llvm/docs/ProgrammersManual.html - toolchain/llvm-project - Gitiles

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 <div class="doc_title">
   LLVM Programmer's Manual
 </div>

 <ol>
   <li><a href="#introduction">Introduction</a></li>
   <li><a href="#general">General Information</a>
     <ul>
       <li><a href="#stl">The C++ Standard Template Library</a></li>
 <!--
       <li>The <tt>-time-passes</tt> option</li>
       <li>How to use the LLVM Makefile system</li>
       <li>How to write a regression test</li>

 -->
     </ul>
   </li>
   <li><a href="#apis">Important and useful LLVM APIs</a>
     <ul>
       <li><a href="#isa">The <tt>isa&lt;&gt;</tt>, <tt>cast&lt;&gt;</tt>
 and <tt>dyn_cast&lt;&gt;</tt> templates</a> </li>
       <li><a href="#DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt>
 option</a>
         <ul>
           <li><a href="#DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt>
 and the <tt>-debug-only</tt> option</a> </li>
         </ul>
       </li>
       <li><a href="#Statistic">The <tt>Statistic</tt> class &amp; <tt>-stats</tt>
 option</a></li>
 <!--
       <li>The <tt>InstVisitor</tt> template
       <li>The general graph API
 -->
       <li><a href="#ViewGraph">Viewing graphs while debugging code</a></li>
     </ul>
   </li>
   <li><a href="#datastructure">Picking the Right Data Structure for a Task</a>
     <ul>
     <li><a href="#ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
     <ul>
       <li><a href="#dss_fixedarrays">Fixed Size Arrays</a></li>
       <li><a href="#dss_heaparrays">Heap Allocated Arrays</a></li>
       <li><a href="#dss_smallvector">"llvm/ADT/SmallVector.h"</a></li>
       <li><a href="#dss_vector">&lt;vector&gt;</a></li>
       <li><a href="#dss_deque">&lt;deque&gt;</a></li>
       <li><a href="#dss_list">&lt;list&gt;</a></li>
       <li><a href="#dss_ilist">llvm/ADT/ilist</a></li>
       <li><a href="#dss_other">Other Sequential Container Options</a></li>
     </ul></li>
     <li><a href="#ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
     <ul>
       <li><a href="#dss_sortedvectorset">A sorted 'vector'</a></li>
       <li><a href="#dss_smallset">"llvm/ADT/SmallSet.h"</a></li>
       <li><a href="#dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a></li>
       <li><a href="#dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a></li>
       <li><a href="#dss_set">&lt;set&gt;</a></li>
       <li><a href="#dss_setvector">"llvm/ADT/SetVector.h"</a></li>
       <li><a href="#dss_uniquevector">"llvm/ADT/UniqueVector.h"</a></li>
       <li><a href="#dss_otherset">Other Set-Like ContainerOptions</a></li>
     </ul></li>
     <li><a href="#ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
     <ul>
       <li><a href="#dss_sortedvectormap">A sorted 'vector'</a></li>
       <li><a href="#dss_cstringmap">"llvm/ADT/CStringMap.h"</a></li>
       <li><a href="#dss_indexedmap">"llvm/ADT/IndexedMap.h"</a></li>
       <li><a href="#dss_densemap">"llvm/ADT/DenseMap.h"</a></li>
       <li><a href="#dss_map">&lt;map&gt;</a></li>
       <li><a href="#dss_othermap">Other Map-Like Container Options</a></li>
     </ul></li>
   </ul>
   </li>
   <li><a href="#common">Helpful Hints for Common Operations</a>
     <ul>
       <li><a href="#inspection">Basic Inspection and Traversal Routines</a>
         <ul>
           <li><a href="#iterate_function">Iterating over the <tt>BasicBlock</tt>s
 in a <tt>Function</tt></a> </li>
           <li><a href="#iterate_basicblock">Iterating over the <tt>Instruction</tt>s
 in a <tt>BasicBlock</tt></a> </li>
           <li><a href="#iterate_institer">Iterating over the <tt>Instruction</tt>s
 in a <tt>Function</tt></a> </li>
           <li><a href="#iterate_convert">Turning an iterator into a
 class pointer</a> </li>
           <li><a href="#iterate_complex">Finding call sites: a more
 complex example</a> </li>
           <li><a href="#calls_and_invokes">Treating calls and invokes
 the same way</a> </li>
           <li><a href="#iterate_chains">Iterating over def-use &amp;
 use-def chains</a> </li>
         </ul>
       </li>
       <li><a href="#simplechanges">Making simple changes</a>
         <ul>
           <li><a href="#schanges_creating">Creating and inserting new
 		 <tt>Instruction</tt>s</a> </li>
           <li><a href="#schanges_deleting">Deleting 		 <tt>Instruction</tt>s</a> </li>
           <li><a href="#schanges_replacing">Replacing an 		 <tt>Instruction</tt>
 with another <tt>Value</tt></a> </li>
         </ul>
       </li>
 <!--
     <li>Working with the Control Flow Graph
     <ul>
       <li>Accessing predecessors and successors of a <tt>BasicBlock</tt>
       <li>
       <li>
     </ul>
 -->
     </ul>
   </li>

   <li><a href="#advanced">Advanced Topics</a>
   <ul>
   <li><a href="#TypeResolve">LLVM Type Resolution</a>
   <ul>
     <li><a href="#BuildRecType">Basic Recursive Type Construction</a></li>
     <li><a href="#refineAbstractTypeTo">The <tt>refineAbstractTypeTo</tt> method</a></li>
     <li><a href="#PATypeHolder">The PATypeHolder Class</a></li>
     <li><a href="#AbstractTypeUser">The AbstractTypeUser Class</a></li>
   </ul></li>

   <li><a href="#SymbolTable">The <tt>SymbolTable</tt> class </a></li>
   </ul></li>

   <li><a href="#coreclasses">The Core LLVM Class Hierarchy Reference</a>
     <ul>
       <li><a href="#Type">The <tt>Type</tt> class</a> </li>
       <li><a href="#Module">The <tt>Module</tt> class</a></li>
       <li><a href="#Value">The <tt>Value</tt> class</a>
       <ul>
         <li><a href="#User">The <tt>User</tt> class</a>
         <ul>
           <li><a href="#Instruction">The <tt>Instruction</tt> class</a></li>
           <li><a href="#Constant">The <tt>Constant</tt> class</a>
           <ul>
             <li><a href="#GlobalValue">The <tt>GlobalValue</tt> class</a>
             <ul>
               <li><a href="#Function">The <tt>Function</tt> class</a></li>
               <li><a href="#GlobalVariable">The <tt>GlobalVariable</tt> class</a></li>
             </ul>
             </li>
           </ul>
           </li>
         </ul>
         </li>
         <li><a href="#BasicBlock">The <tt>BasicBlock</tt> class</a></li>
         <li><a href="#Argument">The <tt>Argument</tt> class</a></li>
       </ul>
       </li>
     </ul>
   </li>
 </ol>

 <div class="doc_author">
   <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
                 <a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>,
                 <a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a>, and
                 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="introduction">Introduction </a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>This document is meant to highlight some of the important classes and
 interfaces available in the LLVM source-base.  This manual is not
 intended to explain what LLVM is, how it works, and what LLVM code looks
 like.  It assumes that you know the basics of LLVM and are interested
 in writing transformations or otherwise analyzing or manipulating the
 code.</p>

 <p>This document should get you oriented so that you can find your
 way in the continuously growing source code that makes up the LLVM
 infrastructure. Note that this manual is not intended to serve as a
 replacement for reading the source code, so if you think there should be
 a method in one of these classes to do something, but it's not listed,
 check the source.  Links to the <a href="/doxygen/">doxygen</a> sources
 are provided to make this as easy as possible.</p>

 <p>The first section of this document describes general information that is
 useful to know when working in the LLVM infrastructure, and the second describes
 the Core LLVM classes.  In the future this manual will be extended with
 information describing how to use extension libraries, such as dominator
 information, CFG traversal routines, and useful utilities like the <tt><a
 href="/doxygen/InstVisitor_8h-source.html">InstVisitor</a></tt> template.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="general">General Information</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>This section contains general information that is useful if you are working
 in the LLVM source-base, but that isn't specific to any particular API.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="stl">The C++ Standard Template Library</a>
 </div>

 <div class="doc_text">

 <p>LLVM makes heavy use of the C++ Standard Template Library (STL),
 perhaps much more than you are used to, or have seen before.  Because of
 this, you might want to do a little background reading in the
 techniques used and capabilities of the library.  There are many good
 pages that discuss the STL, and several books on the subject that you
 can get, so it will not be discussed in this document.</p>

 <p>Here are some useful links:</p>

 <ol>

 <li><a href="http://www.dinkumware.com/refxcpp.html">Dinkumware C++ Library
 reference</a> - an excellent reference for the STL and other parts of the
 standard C++ library.</li>

 <li><a href="http://www.tempest-sw.com/cpp/">C++ In a Nutshell</a> - This is an
 O'Reilly book in the making.  It has a decent
 Standard Library
 Reference that rivals Dinkumware's, and is unfortunately no longer free since the book has been
 published.</li>

 <li><a href="http://www.parashift.com/c++-faq-lite/">C++ Frequently Asked
 Questions</a></li>

 <li><a href="http://www.sgi.com/tech/stl/">SGI's STL Programmer's Guide</a> -
 Contains a useful <a
 href="http://www.sgi.com/tech/stl/stl_introduction.html">Introduction to the
 STL</a>.</li>

 <li><a href="http://www.research.att.com/%7Ebs/C++.html">Bjarne Stroustrup's C++
 Page</a></li>

 <li><a href="http://64.78.49.204/">
 Bruce Eckel's Thinking in C++, 2nd ed. Volume 2 Revision 4.0 (even better, get
 the book).</a></li>

 </ol>

 <p>You are also encouraged to take a look at the <a
 href="CodingStandards.html">LLVM Coding Standards</a> guide which focuses on how
 to write maintainable code more than where to put your curly braces.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="stl">Other useful references</a>
 </div>

 <div class="doc_text">

 <ol>
 <li><a href="http://www.psc.edu/%7Esemke/cvs_branches.html">CVS
 Branch and Tag Primer</a></li>
 <li><a href="http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html">Using
 static and shared libraries across platforms</a></li>
 </ol>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="apis">Important and useful LLVM APIs</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>Here we highlight some LLVM APIs that are generally useful and good to
 know about when writing transformations.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="isa">The <tt>isa&lt;&gt;</tt>, <tt>cast&lt;&gt;</tt> and
   <tt>dyn_cast&lt;&gt;</tt> templates</a>
 </div>

 <div class="doc_text">

 <p>The LLVM source-base makes extensive use of a custom form of RTTI.
 These templates have many similarities to the C++ <tt>dynamic_cast&lt;&gt;</tt>
 operator, but they don't have some drawbacks (primarily stemming from
 the fact that <tt>dynamic_cast&lt;&gt;</tt> only works on classes that
 have a v-table). Because they are used so often, you must know what they
 do and how they work. All of these templates are defined in the <a
  href="/doxygen/Casting_8h-source.html"><tt>llvm/Support/Casting.h</tt></a>
 file (note that you very rarely have to include this file directly).</p>

 <dl>
   <dt><tt>isa&lt;&gt;</tt>: </dt>

   <dd><p>The <tt>isa&lt;&gt;</tt> operator works exactly like the Java
   "<tt>instanceof</tt>" operator.  It returns true or false depending on whether
   a reference or pointer points to an instance of the specified class.  This can
   be very useful for constraint checking of various sorts (example below).</p>
   </dd>

   <dt><tt>cast&lt;&gt;</tt>: </dt>

   <dd><p>The <tt>cast&lt;&gt;</tt> operator is a "checked cast" operation. It
   converts a pointer or reference from a base class to a derived cast, causing
   an assertion failure if it is not really an instance of the right type.  This
   should be used in cases where you have some information that makes you believe
   that something is of the right type.  An example of the <tt>isa&lt;&gt;</tt>
   and <tt>cast&lt;&gt;</tt> template is:</p>

 <div class="doc_code">
 <pre>
 static bool isLoopInvariant(const <a href="#Value">Value</a> *V, const Loop *L) {
   if (isa&lt;<a href="#Constant">Constant</a>&gt;(V) || isa&lt;<a href="#Argument">Argument</a>&gt;(V) || isa&lt;<a href="#GlobalValue">GlobalValue</a>&gt;(V))
     return true;

   // <i>Otherwise, it must be an instruction...</i>
   return !L-&gt;contains(cast&lt;<a href="#Instruction">Instruction</a>&gt;(V)-&gt;getParent());
 }
 </pre>
 </div>

   <p>Note that you should <b>not</b> use an <tt>isa&lt;&gt;</tt> test followed
   by a <tt>cast&lt;&gt;</tt>, for that use the <tt>dyn_cast&lt;&gt;</tt>
   operator.</p>

   </dd>

   <dt><tt>dyn_cast&lt;&gt;</tt>:</dt>

   <dd><p>The <tt>dyn_cast&lt;&gt;</tt> operator is a "checking cast" operation.
   It checks to see if the operand is of the specified type, and if so, returns a
   pointer to it (this operator does not work with references). If the operand is
   not of the correct type, a null pointer is returned.  Thus, this works very
   much like the <tt>dynamic_cast&lt;&gt;</tt> operator in C++, and should be
   used in the same circumstances.  Typically, the <tt>dyn_cast&lt;&gt;</tt>
   operator is used in an <tt>if</tt> statement or some other flow control
   statement like this:</p>

 <div class="doc_code">
 <pre>
 if (<a href="#AllocationInst">AllocationInst</a> *AI = dyn_cast&lt;<a href="#AllocationInst">AllocationInst</a>&gt;(Val)) {
   // <i>...</i>
 }
 </pre>
 </div>

   <p>This form of the <tt>if</tt> statement effectively combines together a call
   to <tt>isa&lt;&gt;</tt> and a call to <tt>cast&lt;&gt;</tt> into one
   statement, which is very convenient.</p>

   <p>Note that the <tt>dyn_cast&lt;&gt;</tt> operator, like C++'s
   <tt>dynamic_cast&lt;&gt;</tt> or Java's <tt>instanceof</tt> operator, can be
   abused.  In particular, you should not use big chained <tt>if/then/else</tt>
   blocks to check for lots of different variants of classes.  If you find
   yourself wanting to do this, it is much cleaner and more efficient to use the
   <tt>InstVisitor</tt> class to dispatch over the instruction type directly.</p>

   </dd>

   <dt><tt>cast_or_null&lt;&gt;</tt>: </dt>

   <dd><p>The <tt>cast_or_null&lt;&gt;</tt> operator works just like the
   <tt>cast&lt;&gt;</tt> operator, except that it allows for a null pointer as an
   argument (which it then propagates).  This can sometimes be useful, allowing
   you to combine several null checks into one.</p></dd>

   <dt><tt>dyn_cast_or_null&lt;&gt;</tt>: </dt>

   <dd><p>The <tt>dyn_cast_or_null&lt;&gt;</tt> operator works just like the
   <tt>dyn_cast&lt;&gt;</tt> operator, except that it allows for a null pointer
   as an argument (which it then propagates).  This can sometimes be useful,
   allowing you to combine several null checks into one.</p></dd>

 </dl>

 <p>These five templates can be used with any classes, whether they have a
 v-table or not.  To add support for these templates, you simply need to add
 <tt>classof</tt> static methods to the class you are interested casting
 to. Describing this is currently outside the scope of this document, but there
 are lots of examples in the LLVM source base.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt> option</a>
 </div>

 <div class="doc_text">

 <p>Often when working on your pass you will put a bunch of debugging printouts
 and other code into your pass.  After you get it working, you want to remove
 it, but you may need it again in the future (to work out new bugs that you run
 across).</p>

 <p> Naturally, because of this, you don't want to delete the debug printouts,
 but you don't want them to always be noisy.  A standard compromise is to comment
 them out, allowing you to enable them if you need them in the future.</p>

 <p>The "<tt><a href="/doxygen/Debug_8h-source.html">llvm/Support/Debug.h</a></tt>"
 file provides a macro named <tt>DEBUG()</tt> that is a much nicer solution to
 this problem.  Basically, you can put arbitrary code into the argument of the
 <tt>DEBUG</tt> macro, and it is only executed if '<tt>opt</tt>' (or any other
 tool) is run with the '<tt>-debug</tt>' command line argument:</p>

 <div class="doc_code">
 <pre>
 DOUT &lt;&lt; "I am here!\n";
 </pre>
 </div>

 <p>Then you can run your pass like this:</p>

 <div class="doc_code">
 <pre>
 $ opt &lt; a.bc &gt; /dev/null -mypass
 <i>&lt;no output&gt;</i>
 $ opt &lt; a.bc &gt; /dev/null -mypass -debug
 I am here!
 </pre>
 </div>

 <p>Using the <tt>DEBUG()</tt> macro instead of a home-brewed solution allows you
 to not have to create "yet another" command line option for the debug output for
 your pass.  Note that <tt>DEBUG()</tt> macros are disabled for optimized builds,
 so they do not cause a performance impact at all (for the same reason, they
 should also not contain side-effects!).</p>

 <p>One additional nice thing about the <tt>DEBUG()</tt> macro is that you can
 enable or disable it directly in gdb.  Just use "<tt>set DebugFlag=0</tt>" or
 "<tt>set DebugFlag=1</tt>" from the gdb if the program is running.  If the
 program hasn't been started yet, you can always just run it with
 <tt>-debug</tt>.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt> and
   the <tt>-debug-only</tt> option</a>
 </div>

 <div class="doc_text">

 <p>Sometimes you may find yourself in a situation where enabling <tt>-debug</tt>
 just turns on <b>too much</b> information (such as when working on the code
 generator).  If you want to enable debug information with more fine-grained
 control, you define the <tt>DEBUG_TYPE</tt> macro and the <tt>-debug</tt> only
 option as follows:</p>

 <div class="doc_code">
 <pre>
 DOUT &lt;&lt; "No debug type\n";
 #undef  DEBUG_TYPE
 #define DEBUG_TYPE "foo"
 DOUT &lt;&lt; "'foo' debug type\n";
 #undef  DEBUG_TYPE
 #define DEBUG_TYPE "bar"
 DOUT &lt;&lt; "'bar' debug type\n";
 #undef  DEBUG_TYPE
 #define DEBUG_TYPE ""
 DOUT &lt;&lt; "No debug type (2)\n";
 </pre>
 </div>

 <p>Then you can run your pass like this:</p>

 <div class="doc_code">
 <pre>
 $ opt &lt; a.bc &gt; /dev/null -mypass
 <i>&lt;no output&gt;</i>
 $ opt &lt; a.bc &gt; /dev/null -mypass -debug
 No debug type
 'foo' debug type
 'bar' debug type
 No debug type (2)
 $ opt &lt; a.bc &gt; /dev/null -mypass -debug-only=foo
 'foo' debug type
 $ opt &lt; a.bc &gt; /dev/null -mypass -debug-only=bar
 'bar' debug type
 </pre>
 </div>

 <p>Of course, in practice, you should only set <tt>DEBUG_TYPE</tt> at the top of
 a file, to specify the debug type for the entire module (if you do this before
 you <tt>#include "llvm/Support/Debug.h"</tt>, you don't have to insert the ugly
 <tt>#undef</tt>'s).  Also, you should use names more meaningful than "foo" and
 "bar", because there is no system in place to ensure that names do not
 conflict. If two different modules use the same string, they will all be turned
 on when the name is specified. This allows, for example, all debug information
 for instruction scheduling to be enabled with <tt>-debug-type=InstrSched</tt>,
 even if the source lives in multiple files.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="Statistic">The <tt>Statistic</tt> class &amp; <tt>-stats</tt>
   option</a>
 </div>

 <div class="doc_text">

 <p>The "<tt><a
 href="/doxygen/Statistic_8h-source.html">llvm/ADT/Statistic.h</a></tt>" file
 provides a class named <tt>Statistic</tt> that is used as a unified way to
 keep track of what the LLVM compiler is doing and how effective various
 optimizations are.  It is useful to see what optimizations are contributing to
 making a particular program run faster.</p>

 <p>Often you may run your pass on some big program, and you're interested to see
 how many times it makes a certain transformation.  Although you can do this with
 hand inspection, or some ad-hoc method, this is a real pain and not very useful
 for big programs.  Using the <tt>Statistic</tt> class makes it very easy to
 keep track of this information, and the calculated information is presented in a
 uniform manner with the rest of the passes being executed.</p>

 <p>There are many examples of <tt>Statistic</tt> uses, but the basics of using
 it are as follows:</p>

 <ol>
     <li><p>Define your statistic like this:</p>

 <div class="doc_code">
 <pre>
 #define <a href="#DEBUG_TYPE">DEBUG_TYPE</a> "mypassname"   <i>// This goes before any #includes.</i>
 STATISTIC(NumXForms, "The # of times I did stuff");
 </pre>
 </div>

   <p>The <tt>STATISTIC</tt> macro defines a static variable, whose name is
     specified by the first argument.  The pass name is taken from the DEBUG_TYPE
     macro, and the description is taken from the second argument.  The variable
     defined ("NumXForms" in this case) acts like an unsigned integer.</p></li>

     <li><p>Whenever you make a transformation, bump the counter:</p>

 <div class="doc_code">
 <pre>
 ++NumXForms;   // <i>I did stuff!</i>
 </pre>
 </div>

     </li>
   </ol>

   <p>That's all you have to do.  To get '<tt>opt</tt>' to print out the
   statistics gathered, use the '<tt>-stats</tt>' option:</p>

 <div class="doc_code">
 <pre>
 $ opt -stats -mypassname &lt; program.bc &gt; /dev/null
 <i>... statistics output ...</i>
 </pre>
 </div>

   <p> When running <tt>gccas</tt> on a C file from the SPEC benchmark
 suite, it gives a report that looks like this:</p>

 <div class="doc_code">
 <pre>
    7646 bytecodewriter  - Number of normal instructions
     725 bytecodewriter  - Number of oversized instructions
  129996 bytecodewriter  - Number of bytecode bytes written
    2817 raise           - Number of insts DCEd or constprop'd
    3213 raise           - Number of cast-of-self removed
    5046 raise           - Number of expression trees converted
      75 raise           - Number of other getelementptr's formed
     138 raise           - Number of load/store peepholes
      42 deadtypeelim    - Number of unused typenames removed from symtab
     392 funcresolve     - Number of varargs functions resolved
      27 globaldce       - Number of global variables removed
       2 adce            - Number of basic blocks removed
     134 cee             - Number of branches revectored
      49 cee             - Number of setcc instruction eliminated
     532 gcse            - Number of loads removed
    2919 gcse            - Number of instructions removed
      86 indvars         - Number of canonical indvars added
      87 indvars         - Number of aux indvars removed
      25 instcombine     - Number of dead inst eliminate
     434 instcombine     - Number of insts combined
     248 licm            - Number of load insts hoisted
    1298 licm            - Number of insts hoisted to a loop pre-header
       3 licm            - Number of insts hoisted to multiple loop preds (bad, no loop pre-header)
      75 mem2reg         - Number of alloca's promoted
    1444 cfgsimplify     - Number of blocks simplified
 </pre>
 </div>

 <p>Obviously, with so many optimizations, having a unified framework for this
 stuff is very nice.  Making your pass fit well into the framework makes it more
 maintainable and useful.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="ViewGraph">Viewing graphs while debugging code</a>
 </div>

 <div class="doc_text">

 <p>Several of the important data structures in LLVM are graphs: for example
 CFGs made out of LLVM <a href="#BasicBlock">BasicBlock</a>s, CFGs made out of
 LLVM <a href="CodeGenerator.html#machinebasicblock">MachineBasicBlock</a>s, and
 <a href="CodeGenerator.html#selectiondag_intro">Instruction Selection
 DAGs</a>.  In many cases, while debugging various parts of the compiler, it is
 nice to instantly visualize these graphs.</p>

 <p>LLVM provides several callbacks that are available in a debug build to do
 exactly that.  If you call the <tt>Function::viewCFG()</tt> method, for example,
 the current LLVM tool will pop up a window containing the CFG for the function
 where each basic block is a node in the graph, and each node contains the
 instructions in the block.  Similarly, there also exists
 <tt>Function::viewCFGOnly()</tt> (does not include the instructions), the
 <tt>MachineFunction::viewCFG()</tt> and <tt>MachineFunction::viewCFGOnly()</tt>,
 and the <tt>SelectionDAG::viewGraph()</tt> methods.  Within GDB, for example,
 you can usually use something like <tt>call DAG.viewGraph()</tt> to pop
 up a window.  Alternatively, you can sprinkle calls to these functions in your
 code in places you want to debug.</p>

 <p>Getting this to work requires a small amount of configuration.  On Unix
 systems with X11, install the <a href="http://www.graphviz.org">graphviz</a>
 toolkit, and make sure 'dot' and 'gv' are in your path.  If you are running on
 Mac OS/X, download and install the Mac OS/X <a
 href="http://www.pixelglow.com/graphviz/">Graphviz program</a>, and add
 <tt>/Applications/Graphviz.app/Contents/MacOS/</tt> (or whereever you install
 it) to your path.  Once in your system and path are set up, rerun the LLVM
 configure script and rebuild LLVM to enable this functionality.</p>

 <p><tt>SelectionDAG</tt> has been extended to make it easier to locate
 <i>interesting</i> nodes in large complex graphs.  From gdb, if you
 <tt>call DAG.setGraphColor(<i>node</i>, "<i>color</i>")</tt>, then the
 next <tt>call DAG.viewGraph()</tt> would hilight the node in the
 specified color (choices of colors can be found at <a
 href="http://www.graphviz.org/doc/info/colors.html">colors</a>.) More
 complex node attributes can be provided with <tt>call
 DAG.setGraphAttrs(<i>node</i>, "<i>attributes</i>")</tt> (choices can be
 found at <a href="http://www.graphviz.org/doc/info/attrs.html">Graph
 Attributes</a>.)  If you want to restart and clear all the current graph
 attributes, then you can <tt>call DAG.clearGraphAttrs()</tt>. </p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="datastructure">Picking the Right Data Structure for a Task</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>LLVM has a plethora of datastructures in the <tt>llvm/ADT/</tt> directory,
  and we commonly use STL datastructures.  This section describes the tradeoffs
  you should consider when you pick one.</p>

 <p>
 The first step is a choose your own adventure: do you want a sequential
 container, a set-like container, or a map-like container?  The most important
 thing when choosing a container is the algorithmic properties of how you plan to
 access the container.  Based on that, you should use:</p>

 <ul>
 <li>a <a href="#ds_map">map-like</a> container if you need efficient lookup
     of an value based on another value.  Map-like containers also support
     efficient queries for containment (whether a key is in the map).  Map-like
     containers generally do not support efficient reverse mapping (values to
     keys).  If you need that, use two maps.  Some map-like containers also
     support efficient iteration through the keys in sorted order.  Map-like
     containers are the most expensive sort, only use them if you need one of
     these capabilities.</li>

 <li>a <a href="#ds_set">set-like</a> container if you need to put a bunch of
     stuff into a container that automatically eliminates duplicates.  Some
     set-like containers support efficient iteration through the elements in
     sorted order.  Set-like containers are more expensive than sequential
     containers.
 </li>

 <li>a <a href="#ds_sequential">sequential</a> container provides
     the most efficient way to add elements and keeps track of the order they are
     added to the collection.  They permit duplicates and support efficient
     iteration, but do not support efficient lookup based on a key.
 </li>

 </ul>

 <p>
 Once the proper catagory of container is determined, you can fine tune the
 memory use, constant factors, and cache behaviors of access by intelligently
 picking a member of the catagory.  Note that constant factors and cache behavior
 can be a big deal.  If you have a vector that usually only contains a few
 elements (but could contain many), for example, it's much better to use
 <a href="#dss_smallvector">SmallVector</a> than <a href="#dss_vector">vector</a>
 .  Doing so avoids (relatively) expensive malloc/free calls, which dwarf the
 cost of adding the elements to the container. </p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
 </div>

 <div class="doc_text">
 There are a variety of sequential containers available for you, based on your
 needs.  Pick the first in this section that will do what you want.
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_fixedarrays">Fixed Size Arrays</a>
 </div>

 <div class="doc_text">
 <p>Fixed size arrays are very simple and very fast.  They are good if you know
 exactly how many elements you have, or you have a (low) upper bound on how many
 you have.</p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_heaparrays">Heap Allocated Arrays</a>
 </div>

 <div class="doc_text">
 <p>Heap allocated arrays (new[] + delete[]) are also simple.  They are good if
 the number of elements is variable, if you know how many elements you will need
 before the array is allocated, and if the array is usually large (if not,
 consider a <a href="#dss_smallvector">SmallVector</a>).  The cost of a heap
 allocated array is the cost of the new/delete (aka malloc/free).  Also note that
 if you are allocating an array of a type with a constructor, the constructor and
 destructors will be run for every element in the array (resizable vectors only
 construct those elements actually used).</p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_smallvector">"llvm/ADT/SmallVector.h"</a>
 </div>

 <div class="doc_text">
 <p><tt>SmallVector&lt;Type, N&gt;</tt> is a simple class that looks and smells
 just like <tt>vector&lt;Type&gt;</tt>:
 it supports efficient iteration, lays out elements in memory order (so you can
 do pointer arithmetic between elements), supports efficient push_back/pop_back
 operations, supports efficient random access to its elements, etc.</p>

 <p>The advantage of SmallVector is that it allocates space for
 some number of elements (N) <b>in the object itself</b>.  Because of this, if
 the SmallVector is dynamically smaller than N, no malloc is performed.  This can
 be a big win in cases where the malloc/free call is far more expensive than the
 code that fiddles around with the elements.</p>

 <p>This is good for vectors that are "usually small" (e.g. the number of
 predecessors/successors of a block is usually less than 8).  On the other hand,
 this makes the size of the SmallVector itself large, so you don't want to
 allocate lots of them (doing so will waste a lot of space).  As such,
 SmallVectors are most useful when on the stack.</p>

 <p>SmallVector also provides a nice portable and efficient replacement for
 <tt>alloca</tt>.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_vector">&lt;vector&gt;</a>
 </div>

 <div class="doc_text">
 <p>
 std::vector is well loved and respected.  It is useful when SmallVector isn't:
 when the size of the vector is often large (thus the small optimization will
 rarely be a benefit) or if you will be allocating many instances of the vector
 itself (which would waste space for elements that aren't in the container).
 vector is also useful when interfacing with code that expects vectors :).
 </p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_deque">&lt;deque&gt;</a>
 </div>

 <div class="doc_text">
 <p>std::deque is, in some senses, a generalized version of std::vector.  Like
 std::vector, it provides constant time random access and other similar
 properties, but it also provides efficient access to the front of the list.  It
 does not guarantee continuity of elements within memory.</p>

 <p>In exchange for this extra flexibility, std::deque has significantly higher
 constant factor costs than std::vector.  If possible, use std::vector or
 something cheaper.</p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_list">&lt;list&gt;</a>
 </div>

 <div class="doc_text">
 <p>std::list is an extremely inefficient class that is rarely useful.
 It performs a heap allocation for every element inserted into it, thus having an
 extremely high constant factor, particularly for small data types.  std::list
 also only supports bidirectional iteration, not random access iteration.</p>

 <p>In exchange for this high cost, std::list supports efficient access to both
 ends of the list (like std::deque, but unlike std::vector or SmallVector).  In
 addition, the iterator invalidation characteristics of std::list are stronger
 than that of a vector class: inserting or removing an element into the list does
 not invalidate iterator or pointers to other elements in the list.</p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_ilist">llvm/ADT/ilist</a>
 </div>

 <div class="doc_text">
 <p><tt>ilist&lt;T&gt;</tt> implements an 'intrusive' doubly-linked list.  It is
 intrusive, because it requires the element to store and provide access to the
 prev/next pointers for the list.</p>

 <p>ilist has the same drawbacks as std::list, and additionally requires an
 ilist_traits implementation for the element type, but it provides some novel
 characteristics.  In particular, it can efficiently store polymorphic objects,
 the traits class is informed when an element is inserted or removed from the
 list, and ilists are guaranteed to support a constant-time splice operation.
 </p>

 <p>These properties are exactly what we want for things like Instructions and
 basic blocks, which is why these are implemented with ilists.</p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_other">Other Sequential Container options</a>
 </div>

 <div class="doc_text">
 <p>Other STL containers are available, such as std::string.</p>

 <p>There are also various STL adapter classes such as std::queue,
 std::priority_queue, std::stack, etc.  These provide simplified access to an
 underlying container but don't affect the cost of the container itself.</p>

 </div>


 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
 </div>

 <div class="doc_text">

 <p>Set-like containers are useful when you need to canonicalize multiple values
 into a single representation.  There are several different choices for how to do
 this, providing various trade-offs.</p>

 </div>


 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_sortedvectorset">A sorted 'vector'</a>
 </div>

 <div class="doc_text">

 <p>If you intend to insert a lot of elements, then do a lot of queries, a
 great approach is to use a vector (or other sequential container) with
 std::sort+std::unique to remove duplicates.  This approach works really well if
 your usage pattern has these two distinct phases (insert then query), and can be
 coupled with a good choice of <a href="#ds_sequential">sequential container</a>.
 </p>

 <p>
 This combination provides the several nice properties: the result data is
 contiguous in memory (good for cache locality), has few allocations, is easy to
 address (iterators in the final vector are just indices or pointers), and can be
 efficiently queried with a standard binary or radix search.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_smallset">"llvm/ADT/SmallSet.h"</a>
 </div>

 <div class="doc_text">

 <p>If you have a set-like datastructure that is usually small and whose elements
 are reasonably small, a <tt>SmallSet&lt;Type, N&gt;</tt> is a good choice.  This set
 has space for N elements in place (thus, if the set is dynamically smaller than
 N, no malloc traffic is required) and accesses them with a simple linear search.
 When the set grows beyond 'N' elements, it allocates a more expensive representation that
 guarantees efficient access (for most types, it falls back to std::set, but for
 pointers it uses something far better, <a
 href="#dss_smallptrset">SmallPtrSet</a>).</p>

 <p>The magic of this class is that it handles small sets extremely efficiently,
 but gracefully handles extremely large sets without loss of efficiency.  The
 drawback is that the interface is quite small: it supports insertion, queries
 and erasing, but does not support iteration.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a>
 </div>

 <div class="doc_text">

 <p>SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is
 transparently implemented with a SmallPtrSet), but also suports iterators.  If
 more than 'N' insertions are performed, a single quadratically
 probed hash table is allocated and grows as needed, providing extremely
 efficient access (constant time insertion/deleting/queries with low constant
 factors) and is very stingy with malloc traffic.</p>

 <p>Note that, unlike std::set, the iterators of SmallPtrSet are invalidated
 whenever an insertion occurs.  Also, the values visited by the iterators are not
 visited in sorted order.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a>
 </div>

 <div class="doc_text">

 <p>
 FoldingSet is an aggregate class that is really good at uniquing
 expensive-to-create or polymorphic objects.  It is a combination of a chained
 hash table with intrusive links (uniqued objects are required to inherit from
 FoldingSetNode) that uses <a href="#dss_smallvector">SmallVector</a> as part of
 its ID process.</p>

 <p>Consider a case where you want to implement a "getOrCreateFoo" method for
 a complex object (for example, a node in the code generator).  The client has a
 description of *what* it wants to generate (it knows the opcode and all the
 operands), but we don't want to 'new' a node, then try inserting it into a set
 only to find out it already exists, at which point we would have to delete it
 and return the node that already exists.
 </p>

 <p>To support this style of client, FoldingSet perform a query with a
 FoldingSetNodeID (which wraps SmallVector) that can be used to describe the
 element that we want to query for.  The query either returns the element
 matching the ID or it returns an opaque ID that indicates where insertion should
 take place.  Construction of the ID usually does not require heap traffic.</p>

 <p>Because FoldingSet uses intrusive links, it can support polymorphic objects
 in the set (for example, you can have SDNode instances mixed with LoadSDNodes).
 Because the elements are individually allocated, pointers to the elements are
 stable: inserting or removing elements does not invalidate any pointers to other
 elements.
 </p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_set">&lt;set&gt;</a>
 </div>

 <div class="doc_text">

 <p><tt>std::set</tt> is a reasonable all-around set class, which is decent at
 many things but great at nothing.  std::set allocates memory for each element
 inserted (thus it is very malloc intensive) and typically stores three pointers
 per element in the set (thus adding a large amount of per-element space
 overhead).  It offers guaranteed log(n) performance, which is not particularly
 fast from a complexity standpoint (particularly if the elements of the set are
 expensive to compare, like strings), and has extremely high constant factors for
 lookup, insertion and removal.</p>

 <p>The advantages of std::set are that its iterators are stable (deleting or
 inserting an element from the set does not affect iterators or pointers to other
 elements) and that iteration over the set is guaranteed to be in sorted order.
 If the elements in the set are large, then the relative overhead of the pointers
 and malloc traffic is not a big deal, but if the elements of the set are small,
 std::set is almost never a good choice.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_setvector">"llvm/ADT/SetVector.h"</a>
 </div>

 <div class="doc_text">
 <p>LLVM's SetVector&lt;Type&gt; is actually a combination of a set along with
 a <a href="#ds_sequential">Sequential Container</a>.  The important property
 that this provides is efficient insertion with uniquing (duplicate elements are
 ignored) with iteration support.  It implements this by inserting elements into
 both a set-like container and the sequential container, using the set-like
 container for uniquing and the sequential container for iteration.
 </p>

 <p>The difference between SetVector and other sets is that the order of
 iteration is guaranteed to match the order of insertion into the SetVector.
 This property is really important for things like sets of pointers.  Because
 pointer values are non-deterministic (e.g. vary across runs of the program on
 different machines), iterating over the pointers in a std::set or other set will
 not be in a well-defined order.</p>

 <p>
 The drawback of SetVector is that it requires twice as much space as a normal
 set and has the sum of constant factors from the set-like container and the
 sequential container that it uses.  Use it *only* if you need to iterate over
 the elements in a deterministic order.  SetVector is also expensive to delete
 elements out of (linear time).
 </p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_uniquevector">"llvm/ADT/UniqueVector.h"</a>
 </div>

 <div class="doc_text">

 <p>
 UniqueVector is similar to <a href="#dss_setvector">SetVector</a>, but it
 retains a unique ID for each element inserted into the set.  It internally
 contains a map and a vector, and it assigns a unique ID for each value inserted
 into the set.</p>

 <p>UniqueVector is very expensive: its cost is the sum of the cost of
 maintaining both the map and vector, it has high complexity, high constant
 factors, and produces a lot of malloc traffic.  It should be avoided.</p>

 </div>


 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_otherset">Other Set-Like Container Options</a>
 </div>

 <div class="doc_text">

 <p>
 The STL provides several other options, such as std::multiset and the various
 "hash_set" like containers (whether from C++ TR1 or from the SGI library).</p>

 <p>std::multiset is useful if you're not interested in elimination of
 duplicates, but has all the drawbacks of std::set.  A sorted vector (where you
 don't delete duplicate entries) or some other approach is almost always
 better.</p>

 <p>The various hash_set implementations (exposed portably by
 "llvm/ADT/hash_set") is a simple chained hashtable.  This algorithm is as malloc
 intensive as std::set (performing an allocation for each element inserted,
 thus having really high constant factors) but (usually) provides O(1)
 insertion/deletion of elements.  This can be useful if your elements are large
 (thus making the constant-factor cost relatively low) or if comparisons are
 expensive.  Element iteration does not visit elements in a useful order.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
 </div>

 <div class="doc_text">
 Map-like containers are useful when you want to associate data to a key.  As
 usual, there are a lot of different ways to do this. :)
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_sortedvectormap">A sorted 'vector'</a>
 </div>

 <div class="doc_text">

 <p>
 If your usage pattern follows a strict insert-then-query approach, you can
 trivially use the same approach as <a href="#dss_sortedvectorset">sorted vectors
 for set-like containers</a>.  The only difference is that your query function
 (which uses std::lower_bound to get efficient log(n) lookup) should only compare
 the key, not both the key and value.  This yields the same advantages as sorted
 vectors for sets.
 </p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_cstringmap">"llvm/ADT/CStringMap.h"</a>
 </div>

 <div class="doc_text">

 <p>
 Strings are commonly used as keys in maps, and they are difficult to support
 efficiently: they are variable length, inefficient to hash and compare when
 long, expensive to copy, etc.  CStringMap is a specialized container designed to
 cope with these issues.  It supports mapping an arbitrary range of bytes that
 does not have an embedded nul character in it ("C strings") to an arbitrary
 other object.</p>

 <p>The CStringMap implementation uses a quadratically-probed hash table, where
 the buckets store a pointer to the heap allocated entries (and some other
 stuff).  The entries in the map must be heap allocated because the strings are
 variable length.  The string data (key) and the element object (value) are
 stored in the same allocation with the string data immediately after the element
 object.  This container guarantees the "<tt>(char*)(&amp;Value+1)</tt>" points
 to the key string for a value.</p>

 <p>The CStringMap is very fast for several reasons: quadratic probing is very
 cache efficient for lookups, the hash value of strings in buckets is not
 recomputed when lookup up an element, CStringMap rarely has to touch the
 memory for unrelated objects when looking up a value (even when hash collisions
 happen), hash table growth does not recompute the hash values for strings
 already in the table, and each pair in the map is store in a single allocation
 (the string data is stored in the same allocation as the Value of a pair).</p>

 <p>CStringMap also provides query methods that take byte ranges, so it only ever
 copies a string if a value is inserted into the table.</p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_indexedmap">"llvm/ADT/IndexedMap.h"</a>
 </div>

 <div class="doc_text">
 <p>
 IndexedMap is a specialized container for mapping small dense integers (or
 values that can be mapped to small dense integers) to some other type.  It is
 internally implemented as a vector with a mapping function that maps the keys to
 the dense integer range.
 </p>

 <p>
 This is useful for cases like virtual registers in the LLVM code generator: they
 have a dense mapping that is offset by a compile-time constant (the first
 virtual register ID).</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_densemap">"llvm/ADT/DenseMap.h"</a>
 </div>

 <div class="doc_text">

 <p>
 DenseMap is a simple quadratically probed hash table.  It excels at supporting
 small keys and values: it uses a single allocation to hold all of the pairs that
 are currently inserted in the map.  DenseMap is a great way to map pointers to
 pointers, or map other small types to each other.
 </p>

 <p>
 There are several aspects of DenseMap that you should be aware of, however.  The
 iterators in a densemap are invalidated whenever an insertion occurs, unlike
 map.  Also, because DenseMap allocates space for a large number of key/value
 pairs (it starts with 64 by default), it will waste a lot of space if your keys
 or values are large.  Finally, you must implement a partial specialization of
 DenseMapKeyInfo for the key that you want, if it isn't already supported.  This
 is required to tell DenseMap about two special marker values (which can never be
 inserted into the map) that it needs internally.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_map">&lt;map&gt;</a>
 </div>

 <div class="doc_text">

 <p>
 std::map has similar characteristics to <a href="#dss_set">std::set</a>: it uses
 a single allocation per pair inserted into the map, it offers log(n) lookup with
 an extremely large constant factor, imposes a space penalty of 3 pointers per
 pair in the map, etc.</p>

 <p>std::map is most useful when your keys or values are very large, if you need
 to iterate over the collection in sorted order, or if you need stable iterators
 into the map (i.e. they don't get invalidated if an insertion or deletion of
 another element takes place).</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="dss_othermap">Other Map-Like Container Options</a>
 </div>

 <div class="doc_text">

 <p>
 The STL provides several other options, such as std::multimap and the various
 "hash_map" like containers (whether from C++ TR1 or from the SGI library).</p>

 <p>std::multimap is useful if you want to map a key to multiple values, but has
 all the drawbacks of std::map.  A sorted vector or some other approach is almost
 always better.</p>

 <p>The various hash_map implementations (exposed portably by
 "llvm/ADT/hash_map") are simple chained hash tables.  This algorithm is as
 malloc intensive as std::map (performing an allocation for each element
 inserted, thus having really high constant factors) but (usually) provides O(1)
 insertion/deletion of elements.  This can be useful if your elements are large
 (thus making the constant-factor cost relatively low) or if comparisons are
 expensive.  Element iteration does not visit elements in a useful order.</p>

 </div>


 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="common">Helpful Hints for Common Operations</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>This section describes how to perform some very simple transformations of
 LLVM code.  This is meant to give examples of common idioms used, showing the
 practical side of LLVM transformations.  <p> Because this is a "how-to" section,
 you should also read about the main classes that you will be working with.  The
 <a href="#coreclasses">Core LLVM Class Hierarchy Reference</a> contains details
 and descriptions of the main classes that you should know about.</p>

 </div>

 <!-- NOTE: this section should be heavy on example code -->
 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="inspection">Basic Inspection and Traversal Routines</a>
 </div>

 <div class="doc_text">

 <p>The LLVM compiler infrastructure have many different data structures that may
 be traversed.  Following the example of the C++ standard template library, the
 techniques used to traverse these various data structures are all basically the
 same.  For a enumerable sequence of values, the <tt>XXXbegin()</tt> function (or
 method) returns an iterator to the start of the sequence, the <tt>XXXend()</tt>
 function returns an iterator pointing to one past the last valid element of the
 sequence, and there is some <tt>XXXiterator</tt> data type that is common
 between the two operations.</p>

 <p>Because the pattern for iteration is common across many different aspects of
 the program representation, the standard template library algorithms may be used
 on them, and it is easier to remember how to iterate. First we show a few common
 examples of the data structures that need to be traversed.  Other data
 structures are traversed in very similar ways.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="iterate_function">Iterating over the </a><a
   href="#BasicBlock"><tt>BasicBlock</tt></a>s in a <a
   href="#Function"><tt>Function</tt></a>
 </div>

 <div class="doc_text">

 <p>It's quite common to have a <tt>Function</tt> instance that you'd like to
 transform in some way; in particular, you'd like to manipulate its
 <tt>BasicBlock</tt>s.  To facilitate this, you'll need to iterate over all of
 the <tt>BasicBlock</tt>s that constitute the <tt>Function</tt>. The following is
 an example that prints the name of a <tt>BasicBlock</tt> and the number of
 <tt>Instruction</tt>s it contains:</p>

 <div class="doc_code">
 <pre>
 // <i>func is a pointer to a Function instance</i>
 for (Function::iterator i = func-&gt;begin(), e = func-&gt;end(); i != e; ++i)
   // <i>Print out the name of the basic block if it has one, and then the</i>
   // <i>number of instructions that it contains</i>
   llvm::cerr &lt;&lt; "Basic block (name=" &lt;&lt; i-&gt;getName() &lt;&lt; ") has "
              &lt;&lt; i-&gt;size() &lt;&lt; " instructions.\n";
 </pre>
 </div>

 <p>Note that i can be used as if it were a pointer for the purposes of
 invoking member functions of the <tt>Instruction</tt> class.  This is
 because the indirection operator is overloaded for the iterator
 classes.  In the above code, the expression <tt>i-&gt;size()</tt> is
 exactly equivalent to <tt>(*i).size()</tt> just like you'd expect.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="iterate_basicblock">Iterating over the </a><a
   href="#Instruction"><tt>Instruction</tt></a>s in a <a
   href="#BasicBlock"><tt>BasicBlock</tt></a>
 </div>

 <div class="doc_text">

 <p>Just like when dealing with <tt>BasicBlock</tt>s in <tt>Function</tt>s, it's
 easy to iterate over the individual instructions that make up
 <tt>BasicBlock</tt>s. Here's a code snippet that prints out each instruction in
 a <tt>BasicBlock</tt>:</p>

 <div class="doc_code">
 <pre>
 // <i>blk is a pointer to a BasicBlock instance</i>
 for (BasicBlock::iterator i = blk-&gt;begin(), e = blk-&gt;end(); i != e; ++i)
    // <i>The next statement works since operator&lt;&lt;(ostream&amp;,...)</i>
    // <i>is overloaded for Instruction&amp;</i>
    llvm::cerr &lt;&lt; *i &lt;&lt; "\n";
 </pre>
 </div>

 <p>However, this isn't really the best way to print out the contents of a
 <tt>BasicBlock</tt>!  Since the ostream operators are overloaded for virtually
 anything you'll care about, you could have just invoked the print routine on the
 basic block itself: <tt>llvm::cerr &lt;&lt; *blk &lt;&lt; "\n";</tt>.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="iterate_institer">Iterating over the </a><a
   href="#Instruction"><tt>Instruction</tt></a>s in a <a
   href="#Function"><tt>Function</tt></a>
 </div>

 <div class="doc_text">

 <p>If you're finding that you commonly iterate over a <tt>Function</tt>'s
 <tt>BasicBlock</tt>s and then that <tt>BasicBlock</tt>'s <tt>Instruction</tt>s,
 <tt>InstIterator</tt> should be used instead. You'll need to include <a
 href="/doxygen/InstIterator_8h-source.html"><tt>llvm/Support/InstIterator.h</tt></a>,
 and then instantiate <tt>InstIterator</tt>s explicitly in your code.  Here's a
 small example that shows how to dump all instructions in a function to the standard error stream:<p>

 <div class="doc_code">
 <pre>
 #include "<a href="/doxygen/InstIterator_8h-source.html">llvm/Support/InstIterator.h</a>"

 // <i>F is a ptr to a Function instance</i>
 for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
   llvm::cerr &lt;&lt; *i &lt;&lt; "\n";
 </pre>
 </div>

 <p>Easy, isn't it?  You can also use <tt>InstIterator</tt>s to fill a
 worklist with its initial contents.  For example, if you wanted to
 initialize a worklist to contain all instructions in a <tt>Function</tt>
 F, all you would need to do is something like:</p>

 <div class="doc_code">
 <pre>
 std::set&lt;Instruction*&gt; worklist;
 worklist.insert(inst_begin(F), inst_end(F));
 </pre>
 </div>

 <p>The STL set <tt>worklist</tt> would now contain all instructions in the
 <tt>Function</tt> pointed to by F.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="iterate_convert">Turning an iterator into a class pointer (and
   vice-versa)</a>
 </div>

 <div class="doc_text">

 <p>Sometimes, it'll be useful to grab a reference (or pointer) to a class
 instance when all you've got at hand is an iterator.  Well, extracting
 a reference or a pointer from an iterator is very straight-forward.
 Assuming that <tt>i</tt> is a <tt>BasicBlock::iterator</tt> and <tt>j</tt>
 is a <tt>BasicBlock::const_iterator</tt>:</p>

 <div class="doc_code">
 <pre>
 Instruction&amp; inst = *i;   // <i>Grab reference to instruction reference</i>
 Instruction* pinst = &amp;*i; // <i>Grab pointer to instruction reference</i>
 const Instruction&amp; inst = *j;
 </pre>
 </div>

 <p>However, the iterators you'll be working with in the LLVM framework are
 special: they will automatically convert to a ptr-to-instance type whenever they
 need to.  Instead of dereferencing the iterator and then taking the address of
 the result, you can simply assign the iterator to the proper pointer type and
 you get the dereference and address-of operation as a result of the assignment
 (behind the scenes, this is a result of overloading casting mechanisms).  Thus
 the last line of the last example,</p>

 <div class="doc_code">
 <pre>
 Instruction* pinst = &amp;*i;
 </pre>
 </div>

 <p>is semantically equivalent to</p>

 <div class="doc_code">
 <pre>
 Instruction* pinst = i;
 </pre>
 </div>

 <p>It's also possible to turn a class pointer into the corresponding iterator,
 and this is a constant time operation (very efficient).  The following code
 snippet illustrates use of the conversion constructors provided by LLVM
 iterators.  By using these, you can explicitly grab the iterator of something
 without actually obtaining it via iteration over some structure:</p>

 <div class="doc_code">
 <pre>
 void printNextInstruction(Instruction* inst) {
   BasicBlock::iterator it(inst);
   ++it; // <i>After this line, it refers to the instruction after *inst</i>
   if (it != inst-&gt;getParent()-&gt;end()) llvm::cerr &lt;&lt; *it &lt;&lt; "\n";
 }
 </pre>
 </div>

 </div>

 <!--_______________________________________________________________________-->
 <div class="doc_subsubsection">
   <a name="iterate_complex">Finding call sites: a slightly more complex
   example</a>
 </div>

 <div class="doc_text">

 <p>Say that you're writing a FunctionPass and would like to count all the
 locations in the entire module (that is, across every <tt>Function</tt>) where a
 certain function (i.e., some <tt>Function</tt>*) is already in scope.  As you'll
 learn later, you may want to use an <tt>InstVisitor</tt> to accomplish this in a
 much more straight-forward manner, but this example will allow us to explore how
 you'd do it if you didn't have <tt>InstVisitor</tt> around. In pseudocode, this
 is what we want to do:</p>

 <div class="doc_code">
 <pre>
 initialize callCounter to zero
 for each Function f in the Module
   for each BasicBlock b in f
     for each Instruction i in b
       if (i is a CallInst and calls the given function)
         increment callCounter
 </pre>
 </div>

 <p>And the actual code is (remember, because we're writing a
 <tt>FunctionPass</tt>, our <tt>FunctionPass</tt>-derived class simply has to
 override the <tt>runOnFunction</tt> method):</p>

 <div class="doc_code">
 <pre>
 Function* targetFunc = ...;

 class OurFunctionPass : public FunctionPass {
   public:
     OurFunctionPass(): callCounter(0) { }

     virtual runOnFunction(Function&amp; F) {
       for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) {
         for (BasicBlock::iterator i = b-&gt;begin(); ie = b-&gt;end(); i != ie; ++i) {
           if (<a href="#CallInst">CallInst</a>* callInst = <a href="#isa">dyn_cast</a>&lt;<a
  href="#CallInst">CallInst</a>&gt;(&amp;*i)) {
             // <i>We know we've encountered a call instruction, so we</i>
             // <i>need to determine if it's a call to the</i>
             // <i>function pointed to by m_func or not</i>

             if (callInst-&gt;getCalledFunction() == targetFunc)
               ++callCounter;
           }
         }
       }
     }

   private:
     unsigned  callCounter;
 };
 </pre>
 </div>

 </div>

 <!--_______________________________________________________________________-->
 <div class="doc_subsubsection">
   <a name="calls_and_invokes">Treating calls and invokes the same way</a>
 </div>

 <div class="doc_text">

 <p>You may have noticed that the previous example was a bit oversimplified in
 that it did not deal with call sites generated by 'invoke' instructions. In
 this, and in other situations, you may find that you want to treat
 <tt>CallInst</tt>s and <tt>InvokeInst</tt>s the same way, even though their
 most-specific common base class is <tt>Instruction</tt>, which includes lots of
 less closely-related things. For these cases, LLVM provides a handy wrapper
 class called <a
 href="http://llvm.org/doxygen/classllvm_1_1CallSite.html"><tt>CallSite</tt></a>.
 It is essentially a wrapper around an <tt>Instruction</tt> pointer, with some
 methods that provide functionality common to <tt>CallInst</tt>s and
 <tt>InvokeInst</tt>s.</p>

 <p>This class has "value semantics": it should be passed by value, not by
 reference and it should not be dynamically allocated or deallocated using
 <tt>operator new</tt> or <tt>operator delete</tt>. It is efficiently copyable,
 assignable and constructable, with costs equivalents to that of a bare pointer.
 If you look at its definition, it has only a single pointer member.</p>

 </div>

 <!--_______________________________________________________________________-->
 <div class="doc_subsubsection">
   <a name="iterate_chains">Iterating over def-use &amp; use-def chains</a>
 </div>

 <div class="doc_text">

 <p>Frequently, we might have an instance of the <a
 href="/doxygen/classllvm_1_1Value.html">Value Class</a> and we want to
 determine which <tt>User</tt>s use the <tt>Value</tt>.  The list of all
 <tt>User</tt>s of a particular <tt>Value</tt> is called a <i>def-use</i> chain.
 For example, let's say we have a <tt>Function*</tt> named <tt>F</tt> to a
 particular function <tt>foo</tt>. Finding all of the instructions that
 <i>use</i> <tt>foo</tt> is as simple as iterating over the <i>def-use</i> chain
 of <tt>F</tt>:</p>

 <div class="doc_code">
 <pre>
 Function* F = ...;

 for (Value::use_iterator i = F-&gt;use_begin(), e = F-&gt;use_end(); i != e; ++i)
   if (Instruction *Inst = dyn_cast&lt;Instruction&gt;(*i)) {
     llvm::cerr &lt;&lt; "F is used in instruction:\n";
     llvm::cerr &lt;&lt; *Inst &lt;&lt; "\n";
   }
 </pre>
 </div>

 <p>Alternately, it's common to have an instance of the <a
 href="/doxygen/classllvm_1_1User.html">User Class</a> and need to know what
 <tt>Value</tt>s are used by it.  The list of all <tt>Value</tt>s used by a
 <tt>User</tt> is known as a <i>use-def</i> chain.  Instances of class
 <tt>Instruction</tt> are common <tt>User</tt>s, so we might want to iterate over
 all of the values that a particular instruction uses (that is, the operands of
 the particular <tt>Instruction</tt>):</p>

 <div class="doc_code">
 <pre>
 Instruction* pi = ...;

 for (User::op_iterator i = pi-&gt;op_begin(), e = pi-&gt;op_end(); i != e; ++i) {
   Value* v = *i;
   // <i>...</i>
 }
 </pre>
 </div>

 <!--
   def-use chains ("finding all users of"): Value::use_begin/use_end
   use-def chains ("finding all values used"): User::op_begin/op_end [op=operand]
 -->

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="simplechanges">Making simple changes</a>
 </div>

 <div class="doc_text">

 <p>There are some primitive transformation operations present in the LLVM
 infrastructure that are worth knowing about.  When performing
 transformations, it's fairly common to manipulate the contents of basic
 blocks. This section describes some of the common methods for doing so
 and gives example code.</p>

 </div>

 <!--_______________________________________________________________________-->
 <div class="doc_subsubsection">
   <a name="schanges_creating">Creating and inserting new
   <tt>Instruction</tt>s</a>
 </div>

 <div class="doc_text">

 <p><i>Instantiating Instructions</i></p>

 <p>Creation of <tt>Instruction</tt>s is straight-forward: simply call the
 constructor for the kind of instruction to instantiate and provide the necessary
 parameters. For example, an <tt>AllocaInst</tt> only <i>requires</i> a
 (const-ptr-to) <tt>Type</tt>. Thus:</p>

 <div class="doc_code">
 <pre>
 AllocaInst* ai = new AllocaInst(Type::IntTy);
 </pre>
 </div>

 <p>will create an <tt>AllocaInst</tt> instance that represents the allocation of
 one integer in the current stack frame, at runtime. Each <tt>Instruction</tt>
 subclass is likely to have varying default parameters which change the semantics
 of the instruction, so refer to the <a
 href="/doxygen/classllvm_1_1Instruction.html">doxygen documentation for the subclass of
 Instruction</a> that you're interested in instantiating.</p>

 <p><i>Naming values</i></p>

 <p>It is very useful to name the values of instructions when you're able to, as
 this facilitates the debugging of your transformations.  If you end up looking
 at generated LLVM machine code, you definitely want to have logical names
 associated with the results of instructions!  By supplying a value for the
 <tt>Name</tt> (default) parameter of the <tt>Instruction</tt> constructor, you
 associate a logical name with the result of the instruction's execution at
 runtime.  For example, say that I'm writing a transformation that dynamically
 allocates space for an integer on the stack, and that integer is going to be
 used as some kind of index by some other code.  To accomplish this, I place an
 <tt>AllocaInst</tt> at the first point in the first <tt>BasicBlock</tt> of some
 <tt>Function</tt>, and I'm intending to use it within the same
 <tt>Function</tt>. I might do:</p>

 <div class="doc_code">
 <pre>
 AllocaInst* pa = new AllocaInst(Type::IntTy, 0, "indexLoc");
 </pre>
 </div>

 <p>where <tt>indexLoc</tt> is now the logical name of the instruction's
 execution value, which is a pointer to an integer on the runtime stack.</p>

 <p><i>Inserting instructions</i></p>

 <p>There are essentially two ways to insert an <tt>Instruction</tt>
 into an existing sequence of instructions that form a <tt>BasicBlock</tt>:</p>

 <ul>
   <li>Insertion into an explicit instruction list

     <p>Given a <tt>BasicBlock* pb</tt>, an <tt>Instruction* pi</tt> within that
     <tt>BasicBlock</tt>, and a newly-created instruction we wish to insert
     before <tt>*pi</tt>, we do the following: </p>

 <div class="doc_code">
 <pre>
 BasicBlock *pb = ...;
 Instruction *pi = ...;
 Instruction *newInst = new Instruction(...);

 pb-&gt;getInstList().insert(pi, newInst); // <i>Inserts newInst before pi in pb</i>
 </pre>
 </div>

     <p>Appending to the end of a <tt>BasicBlock</tt> is so common that
     the <tt>Instruction</tt> class and <tt>Instruction</tt>-derived
     classes provide constructors which take a pointer to a
     <tt>BasicBlock</tt> to be appended to. For example code that
     looked like: </p>

 <div class="doc_code">
 <pre>
 BasicBlock *pb = ...;
 Instruction *newInst = new Instruction(...);

 pb-&gt;getInstList().push_back(newInst); // <i>Appends newInst to pb</i>
 </pre>
 </div>

     <p>becomes: </p>

 <div class="doc_code">
 <pre>
 BasicBlock *pb = ...;
 Instruction *newInst = new Instruction(..., pb);
 </pre>
 </div>

     <p>which is much cleaner, especially if you are creating
     long instruction streams.</p></li>

   <li>Insertion into an implicit instruction list

     <p><tt>Instruction</tt> instances that are already in <tt>BasicBlock</tt>s
     are implicitly associated with an existing instruction list: the instruction
     list of the enclosing basic block. Thus, we could have accomplished the same
     thing as the above code without being given a <tt>BasicBlock</tt> by doing:
     </p>

 <div class="doc_code">
 <pre>
 Instruction *pi = ...;
 Instruction *newInst = new Instruction(...);

 pi-&gt;getParent()-&gt;getInstList().insert(pi, newInst);
 </pre>
 </div>

     <p>In fact, this sequence of steps occurs so frequently that the
     <tt>Instruction</tt> class and <tt>Instruction</tt>-derived classes provide
     constructors which take (as a default parameter) a pointer to an
     <tt>Instruction</tt> which the newly-created <tt>Instruction</tt> should
     precede.  That is, <tt>Instruction</tt> constructors are capable of
     inserting the newly-created instance into the <tt>BasicBlock</tt> of a
     provided instruction, immediately before that instruction.  Using an
     <tt>Instruction</tt> constructor with a <tt>insertBefore</tt> (default)
     parameter, the above code becomes:</p>

 <div class="doc_code">
 <pre>
 Instruction* pi = ...;
 Instruction* newInst = new Instruction(..., pi);
 </pre>
 </div>

     <p>which is much cleaner, especially if you're creating a lot of
     instructions and adding them to <tt>BasicBlock</tt>s.</p></li>
 </ul>

 </div>

 <!--_______________________________________________________________________-->
 <div class="doc_subsubsection">
   <a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a>
 </div>

 <div class="doc_text">

 <p>Deleting an instruction from an existing sequence of instructions that form a
 <a href="#BasicBlock"><tt>BasicBlock</tt></a> is very straight-forward. First,
 you must have a pointer to the instruction that you wish to delete.  Second, you
 need to obtain the pointer to that instruction's basic block. You use the
 pointer to the basic block to get its list of instructions and then use the
 erase function to remove your instruction. For example:</p>

 <div class="doc_code">
 <pre>
 <a href="#Instruction">Instruction</a> *I = .. ;
 <a href="#BasicBlock">BasicBlock</a> *BB = I-&gt;getParent();

 BB-&gt;getInstList().erase(I);
 </pre>
 </div>

 </div>

 <!--_______________________________________________________________________-->
 <div class="doc_subsubsection">
   <a name="schanges_replacing">Replacing an <tt>Instruction</tt> with another
   <tt>Value</tt></a>
 </div>

 <div class="doc_text">

 <p><i>Replacing individual instructions</i></p>

 <p>Including "<a href="/doxygen/BasicBlockUtils_8h-source.html">llvm/Transforms/Utils/BasicBlockUtils.h</a>"
 permits use of two very useful replace functions: <tt>ReplaceInstWithValue</tt>
 and <tt>ReplaceInstWithInst</tt>.</p>

 <h4><a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a></h4>

 <ul>
   <li><tt>ReplaceInstWithValue</tt>

     <p>This function replaces all uses (within a basic block) of a given
     instruction with a value, and then removes the original instruction. The
     following example illustrates the replacement of the result of a particular
     <tt>AllocaInst</tt> that allocates memory for a single integer with a null
     pointer to an integer.</p>

 <div class="doc_code">
 <pre>
 AllocaInst* instToReplace = ...;
 BasicBlock::iterator ii(instToReplace);

 ReplaceInstWithValue(instToReplace-&gt;getParent()-&gt;getInstList(), ii,
                      Constant::getNullValue(PointerType::get(Type::IntTy)));
 </pre></div></li>

   <li><tt>ReplaceInstWithInst</tt>

     <p>This function replaces a particular instruction with another
     instruction. The following example illustrates the replacement of one
     <tt>AllocaInst</tt> with another.</p>

 <div class="doc_code">
 <pre>
 AllocaInst* instToReplace = ...;
 BasicBlock::iterator ii(instToReplace);

 ReplaceInstWithInst(instToReplace-&gt;getParent()-&gt;getInstList(), ii,
                     new AllocaInst(Type::IntTy, 0, "ptrToReplacedInt"));
 </pre></div></li>
 </ul>

 <p><i>Replacing multiple uses of <tt>User</tt>s and <tt>Value</tt>s</i></p>

 <p>You can use <tt>Value::replaceAllUsesWith</tt> and
 <tt>User::replaceUsesOfWith</tt> to change more than one use at a time.  See the
 doxygen documentation for the <a href="/doxygen/classllvm_1_1Value.html">Value Class</a>
 and <a href="/doxygen/classllvm_1_1User.html">User Class</a>, respectively, for more
 information.</p>

 <!-- Value::replaceAllUsesWith User::replaceUsesOfWith Point out:
 include/llvm/Transforms/Utils/ especially BasicBlockUtils.h with:
 ReplaceInstWithValue, ReplaceInstWithInst -->

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="advanced">Advanced Topics</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">
 <p>
 This section describes some of the advanced or obscure API's that most clients
 do not need to be aware of.  These API's tend manage the inner workings of the
 LLVM system, and only need to be accessed in unusual circumstances.
 </p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="TypeResolve">LLVM Type Resolution</a>
 </div>

 <div class="doc_text">

 <p>
 The LLVM type system has a very simple goal: allow clients to compare types for
 structural equality with a simple pointer comparison (aka a shallow compare).
 This goal makes clients much simpler and faster, and is used throughout the LLVM
 system.
 </p>

 <p>
 Unfortunately achieving this goal is not a simple matter.  In particular,
 recursive types and late resolution of opaque types makes the situation very
 difficult to handle.  Fortunately, for the most part, our implementation makes
 most clients able to be completely unaware of the nasty internal details.  The
 primary case where clients are exposed to the inner workings of it are when
 building a recursive type.  In addition to this case, the LLVM bytecode reader,
 assembly parser, and linker also have to be aware of the inner workings of this
 system.
 </p>

 <p>
 For our purposes below, we need three concepts.  First, an "Opaque Type" is
 exactly as defined in the <a href="LangRef.html#t_opaque">language
 reference</a>.  Second an "Abstract Type" is any type which includes an
 opaque type as part of its type graph (for example "<tt>{ opaque, i32 }</tt>").
 Third, a concrete type is a type that is not an abstract type (e.g. "<tt>{ i32,
 float }</tt>").
 </p>

 </div>

 <!-- ______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="BuildRecType">Basic Recursive Type Construction</a>
 </div>

 <div class="doc_text">

 <p>
 Because the most common question is "how do I build a recursive type with LLVM",
 we answer it now and explain it as we go.  Here we include enough to cause this
 to be emitted to an output .ll file:
 </p>

 <div class="doc_code">
 <pre>
 %mylist = type { %mylist*, i32 }
 </pre>
 </div>

 <p>
 To build this, use the following LLVM APIs:
 </p>

 <div class="doc_code">
 <pre>
 // <i>Create the initial outer struct</i>
 <a href="#PATypeHolder">PATypeHolder</a> StructTy = OpaqueType::get();
 std::vector&lt;const Type*&gt; Elts;
 Elts.push_back(PointerType::get(StructTy));
 Elts.push_back(Type::IntTy);
 StructType *NewSTy = StructType::get(Elts);

 // <i>At this point, NewSTy = "{ opaque*, i32 }". Tell VMCore that</i>
 // <i>the struct and the opaque type are actually the same.</i>
 cast&lt;OpaqueType&gt;(StructTy.get())-&gt;<a href="#refineAbstractTypeTo">refineAbstractTypeTo</a>(NewSTy);

 // <i>NewSTy is potentially invalidated, but StructTy (a <a href="#PATypeHolder">PATypeHolder</a>) is</i>
 // <i>kept up-to-date</i>
 NewSTy = cast&lt;StructType&gt;(StructTy.get());

 // <i>Add a name for the type to the module symbol table (optional)</i>
 MyModule-&gt;addTypeName("mylist", NewSTy);
 </pre>
 </div>

 <p>
 This code shows the basic approach used to build recursive types: build a
 non-recursive type using 'opaque', then use type unification to close the cycle.
 The type unification step is performed by the <tt><a
 href="#refineAbstractTypeTo">refineAbstractTypeTo</a></tt> method, which is
 described next.  After that, we describe the <a
 href="#PATypeHolder">PATypeHolder class</a>.
 </p>

 </div>

 <!-- ______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="refineAbstractTypeTo">The <tt>refineAbstractTypeTo</tt> method</a>
 </div>

 <div class="doc_text">
 <p>
 The <tt>refineAbstractTypeTo</tt> method starts the type unification process.
 While this method is actually a member of the DerivedType class, it is most
 often used on OpaqueType instances.  Type unification is actually a recursive
 process.  After unification, types can become structurally isomorphic to
 existing types, and all duplicates are deleted (to preserve pointer equality).
 </p>

 <p>
 In the example above, the OpaqueType object is definitely deleted.
 Additionally, if there is an "{ \2*, i32}" type already created in the system,
 the pointer and struct type created are <b>also</b> deleted.  Obviously whenever
 a type is deleted, any "Type*" pointers in the program are invalidated.  As
 such, it is safest to avoid having <i>any</i> "Type*" pointers to abstract types
 live across a call to <tt>refineAbstractTypeTo</tt> (note that non-abstract
 types can never move or be deleted).  To deal with this, the <a
 href="#PATypeHolder">PATypeHolder</a> class is used to maintain a stable
 reference to a possibly refined type, and the <a
 href="#AbstractTypeUser">AbstractTypeUser</a> class is used to update more
 complex datastructures.
 </p>

 </div>

 <!-- ______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="PATypeHolder">The PATypeHolder Class</a>
 </div>

 <div class="doc_text">
 <p>
 PATypeHolder is a form of a "smart pointer" for Type objects.  When VMCore
 happily goes about nuking types that become isomorphic to existing types, it
 automatically updates all PATypeHolder objects to point to the new type.  In the
 example above, this allows the code to maintain a pointer to the resultant
 resolved recursive type, even though the Type*'s are potentially invalidated.
 </p>

 <p>
 PATypeHolder is an extremely light-weight object that uses a lazy union-find
 implementation to update pointers.  For example the pointer from a Value to its
 Type is maintained by PATypeHolder objects.
 </p>

 </div>

 <!-- ______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="AbstractTypeUser">The AbstractTypeUser Class</a>
 </div>

 <div class="doc_text">

 <p>
 Some data structures need more to perform more complex updates when types get
 resolved.  The <a href="#SymbolTable">SymbolTable</a> class, for example, needs
 move and potentially merge type planes in its representation when a pointer
 changes.</p>

 <p>
 To support this, a class can derive from the AbstractTypeUser class.  This class
 allows it to get callbacks when certain types are resolved.  To register to get
 callbacks for a particular type, the DerivedType::{add/remove}AbstractTypeUser
 methods can be called on a type.  Note that these methods only work for <i>
   abstract</i> types.  Concrete types (those that do not include any opaque
 objects) can never be refined.
 </p>
 </div>


 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="SymbolTable">The <tt>SymbolTable</tt> class</a>
 </div>

 <div class="doc_text">
 <p>This class provides a symbol table that the <a
 href="#Function"><tt>Function</tt></a> and <a href="#Module">
 <tt>Module</tt></a> classes use for naming definitions. The symbol table can
 provide a name for any <a href="#Value"><tt>Value</tt></a>.
 <tt>SymbolTable</tt> is an abstract data type. It hides the data it contains
 and provides access to it through a controlled interface.</p>

 <p>Note that the <tt>SymbolTable</tt> class should not be directly accessed
 by most clients.  It should only be used when iteration over the symbol table
 names themselves are required, which is very special purpose.  Note that not
 all LLVM
 <a href="#Value">Value</a>s have names, and those without names (i.e. they have
 an empty name) do not exist in the symbol table.
 </p>

 <p>To use the <tt>SymbolTable</tt> well, you need to understand the
 structure of the information it holds. The class contains two
 <tt>std::map</tt> objects. The first, <tt>pmap</tt>, is a map of
 <tt>Type*</tt> to maps of name (<tt>std::string</tt>) to <tt>Value*</tt>.
 Thus, Values are stored in two-dimensions and accessed by <tt>Type</tt> and
 name.</p>

 <p>The interface of this class provides three basic types of operations:
 <ol>
   <li><em>Accessors</em>. Accessors provide read-only access to information
   such as finding a value for a name with the
   <a href="#SymbolTable_lookup">lookup</a> method.</li>
   <li><em>Mutators</em>. Mutators allow the user to add information to the
   <tt>SymbolTable</tt> with methods like
   <a href="#SymbolTable_insert"><tt>insert</tt></a>.</li>
   <li><em>Iterators</em>. Iterators allow the user to traverse the content
   of the symbol table in well defined ways, such as the method
   <a href="#SymbolTable_plane_begin"><tt>plane_begin</tt></a>.</li>
 </ol>

 <h3>Accessors</h3>
 <dl>
   <dt><tt>Value* lookup(const Type* Ty, const std::string&amp; name) const</tt>:
   </dt>
   <dd>The <tt>lookup</tt> method searches the type plane given by the
   <tt>Ty</tt> parameter for a <tt>Value</tt> with the provided <tt>name</tt>.
   If a suitable <tt>Value</tt> is not found, null is returned.</dd>

   <dt><tt>bool isEmpty() const</tt>:</dt>
   <dd>This function returns true if both the value and types maps are
   empty</dd>
 </dl>

 <h3>Mutators</h3>
 <dl>
   <dt><tt>void insert(Value *Val)</tt>:</dt>
   <dd>This method adds the provided value to the symbol table.  The Value must
   have both a name and a type which are extracted and used to place the value
   in the correct type plane under the value's name.</dd>

   <dt><tt>void insert(const std::string&amp; Name, Value *Val)</tt>:</dt>
   <dd> Inserts a constant or type into the symbol table with the specified
   name. There can be a many to one mapping between names and constants
   or types.</dd>

   <dt><tt>void remove(Value* Val)</tt>:</dt>
  <dd> This method removes a named value from the symbol table. The
   type and name of the Value are extracted from \p N and used to
   lookup the Value in the correct type plane. If the Value is
   not in the symbol table, this method silently ignores the
   request.</dd>

   <dt><tt>Value* remove(const std::string&amp; Name, Value *Val)</tt>:</dt>
   <dd> Remove a constant or type with the specified name from the
   symbol table.</dd>

   <dt><tt>Value *remove(const value_iterator&amp; It)</tt>:</dt>
   <dd> Removes a specific value from the symbol table.
   Returns the removed value.</dd>

   <dt><tt>bool strip()</tt>:</dt>
   <dd> This method will strip the symbol table of its names leaving
   the type and values. </dd>

   <dt><tt>void clear()</tt>:</dt>
   <dd>Empty the symbol table completely.</dd>
 </dl>

 <h3>Iteration</h3>
 <p>The following functions describe three types of iterators you can obtain
 the beginning or end of the sequence for both const and non-const. It is
 important to keep track of the different kinds of iterators. There are
 three idioms worth pointing out:</p>

 <table>
   <tr><th>Units</th><th>Iterator</th><th>Idiom</th></tr>
   <tr>
     <td align="left">Planes Of name/Value maps</td><td>PI</td>
     <td align="left"><pre><tt>
 for (SymbolTable::plane_const_iterator PI = ST.plane_begin(),
      PE = ST.plane_end(); PI != PE; ++PI ) {
   PI-&gt;first  // <i>This is the Type* of the plane</i>
   PI-&gt;second // <i>This is the SymbolTable::ValueMap of name/Value pairs</i>
 }
     </tt></pre></td>
   </tr>
   <tr>
     <td align="left">name/Value pairs in a plane</td><td>VI</td>
     <td align="left"><pre><tt>
 for (SymbolTable::value_const_iterator VI = ST.value_begin(SomeType),
      VE = ST.value_end(SomeType); VI != VE; ++VI ) {
   VI-&gt;first  // <i>This is the name of the Value</i>
   VI-&gt;second // <i>This is the Value* value associated with the name</i>
 }
     </tt></pre></td>
   </tr>
 </table>

 <p>Using the recommended iterator names and idioms will help you avoid
 making mistakes. Of particular note, make sure that whenever you use
 value_begin(SomeType) that you always compare the resulting iterator
 with value_end(SomeType) not value_end(SomeOtherType) or else you
 will loop infinitely.</p>

 <dl>

   <dt><tt>plane_iterator plane_begin()</tt>:</dt>
   <dd>Get an iterator that starts at the beginning of the type planes.
   The iterator will iterate over the Type/ValueMap pairs in the
   type planes. </dd>

   <dt><tt>plane_const_iterator plane_begin() const</tt>:</dt>
   <dd>Get a const_iterator that starts at the beginning of the type
   planes.  The iterator will iterate over the Type/ValueMap pairs
   in the type planes. </dd>

   <dt><tt>plane_iterator plane_end()</tt>:</dt>
   <dd>Get an iterator at the end of the type planes. This serves as
   the marker for end of iteration over the type planes.</dd>

   <dt><tt>plane_const_iterator plane_end() const</tt>:</dt>
   <dd>Get a const_iterator at the end of the type planes. This serves as
   the marker for end of iteration over the type planes.</dd>

   <dt><tt>value_iterator value_begin(const Type *Typ)</tt>:</dt>
   <dd>Get an iterator that starts at the beginning of a type plane.
   The iterator will iterate over the name/value pairs in the type plane.
   Note: The type plane must already exist before using this.</dd>

   <dt><tt>value_const_iterator value_begin(const Type *Typ) const</tt>:</dt>
   <dd>Get a const_iterator that starts at the beginning of a type plane.
   The iterator will iterate over the name/value pairs in the type plane.
   Note: The type plane must already exist before using this.</dd>

   <dt><tt>value_iterator value_end(const Type *Typ)</tt>:</dt>
   <dd>Get an iterator to the end of a type plane. This serves as the marker
   for end of iteration of the type plane.
   Note: The type plane must already exist before using this.</dd>

   <dt><tt>value_const_iterator value_end(const Type *Typ) const</tt>:</dt>
   <dd>Get a const_iterator to the end of a type plane. This serves as the
   marker for end of iteration of the type plane.
   Note: the type plane must already exist before using this.</dd>

   <dt><tt>plane_const_iterator find(const Type* Typ ) const</tt>:</dt>
   <dd>This method returns a plane_const_iterator for iteration over
   the type planes starting at a specific plane, given by \p Ty.</dd>

   <dt><tt>plane_iterator find( const Type* Typ </tt>:</dt>
   <dd>This method returns a plane_iterator for iteration over the
   type planes starting at a specific plane, given by \p Ty.</dd>

 </dl>
 </div>


 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="coreclasses">The Core LLVM Class Hierarchy Reference </a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">
 <p><tt>#include "<a href="/doxygen/Type_8h-source.html">llvm/Type.h</a>"</tt>
 <br>doxygen info: <a href="/doxygen/classllvm_1_1Type.html">Type Class</a></p>

 <p>The Core LLVM classes are the primary means of representing the program
 being inspected or transformed.  The core LLVM classes are defined in
 header files in the <tt>include/llvm/</tt> directory, and implemented in
 the <tt>lib/VMCore</tt> directory.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="Type">The <tt>Type</tt> class and Derived Types</a>
 </div>

 <div class="doc_text">

   <p><tt>Type</tt> is a superclass of all type classes. Every <tt>Value</tt> has
   a <tt>Type</tt>. <tt>Type</tt> cannot be instantiated directly but only
   through its subclasses. Certain primitive types (<tt>VoidType</tt>,
   <tt>LabelType</tt>, <tt>FloatType</tt> and <tt>DoubleType</tt>) have hidden
   subclasses. They are hidden because they offer no useful functionality beyond
   what the <tt>Type</tt> class offers except to distinguish themselves from
   other subclasses of <tt>Type</tt>.</p>
   <p>All other types are subclasses of <tt>DerivedType</tt>.  Types can be
   named, but this is not a requirement. There exists exactly
   one instance of a given shape at any one time.  This allows type equality to
   be performed with address equality of the Type Instance. That is, given two
   <tt>Type*</tt> values, the types are identical if the pointers are identical.
   </p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_Value">Important Public Methods</a>
 </div>

 <div class="doc_text">

 <ul>
   <li><tt>bool isInteger() const</tt>: Returns true for any integer type.</li>

   <li><tt>bool isFloatingPoint()</tt>: Return true if this is one of the two
   floating point types.</li>

   <li><tt>bool isAbstract()</tt>: Return true if the type is abstract (contains
   an OpaqueType anywhere in its definition).</li>

   <li><tt>bool isSized()</tt>: Return true if the type has known size. Things
   that don't have a size are abstract types, labels and void.</li>

 </ul>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_Value">Important Derived Types</a>
 </div>
 <div class="doc_text">
 <dl>
   <dt><tt>IntegerType</tt></dt>
   <dd>Subclass of DerivedType that represents integer types of any bit width.
   Any bit width between <tt>IntegerType::MIN_INT_BITS</tt> (1) and
   <tt>IntegerType::MAX_INT_BITS</tt> (~8 million) can be represented.
   <ul>
     <li><tt>static const IntegerType* get(unsigned NumBits)</tt>: get an integer
     type of a specific bit width.</li>
     <li><tt>unsigned getBitWidth() const</tt>: Get the bit width of an integer
     type.</li>
   </ul>
   </dd>
   <dt><tt>SequentialType</tt></dt>
   <dd>This is subclassed by ArrayType and PointerType
     <ul>
       <li><tt>const Type * getElementType() const</tt>: Returns the type of each
       of the elements in the sequential type. </li>
     </ul>
   </dd>
   <dt><tt>ArrayType</tt></dt>
   <dd>This is a subclass of SequentialType and defines the interface for array
   types.
     <ul>
       <li><tt>unsigned getNumElements() const</tt>: Returns the number of
       elements in the array. </li>
     </ul>
   </dd>
   <dt><tt>PointerType</tt></dt>
   <dd>Subclass of SequentialType for pointer types.</dd>
   <dt><tt>PackedType</tt></dt>
   <dd>Subclass of SequentialType for packed (vector) types. A
   packed type is similar to an ArrayType but is distinguished because it is
   a first class type wherease ArrayType is not. Packed types are used for
   vector operations and are usually small vectors of of an integer or floating
   point type.</dd>
   <dt><tt>StructType</tt></dt>
   <dd>Subclass of DerivedTypes for struct types.</dd>
   <dt><tt>FunctionType</tt></dt>
   <dd>Subclass of DerivedTypes for function types.
     <ul>
       <li><tt>bool isVarArg() const</tt>: Returns true if its a vararg
       function</li>
       <li><tt> const Type * getReturnType() const</tt>: Returns the
       return type of the function.</li>
       <li><tt>const Type * getParamType (unsigned i)</tt>: Returns
       the type of the ith parameter.</li>
       <li><tt> const unsigned getNumParams() const</tt>: Returns the
       number of formal parameters.</li>
     </ul>
   </dd>
   <dt><tt>OpaqueType</tt></dt>
   <dd>Sublcass of DerivedType for abstract types. This class
   defines no content and is used as a placeholder for some other type. Note
   that OpaqueType is used (temporarily) during type resolution for forward
   references of types. Once the referenced type is resolved, the OpaqueType
   is replaced with the actual type. OpaqueType can also be used for data
   abstraction. At link time opaque types can be resolved to actual types
   of the same name.</dd>
 </dl>
 </div>


 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="Module">The <tt>Module</tt> class</a>
 </div>

 <div class="doc_text">

 <p><tt>#include "<a
 href="/doxygen/Module_8h-source.html">llvm/Module.h</a>"</tt><br> doxygen info:
 <a href="/doxygen/classllvm_1_1Module.html">Module Class</a></p>

 <p>The <tt>Module</tt> class represents the top level structure present in LLVM
 programs.  An LLVM module is effectively either a translation unit of the
 original program or a combination of several translation units merged by the
 linker.  The <tt>Module</tt> class keeps track of a list of <a
 href="#Function"><tt>Function</tt></a>s, a list of <a
 href="#GlobalVariable"><tt>GlobalVariable</tt></a>s, and a <a
 href="#SymbolTable"><tt>SymbolTable</tt></a>.  Additionally, it contains a few
 helpful member functions that try to make common operations easy.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_Module">Important Public Members of the <tt>Module</tt> class</a>
 </div>

 <div class="doc_text">

 <ul>
   <li><tt>Module::Module(std::string name = "")</tt></li>
 </ul>

 <p>Constructing a <a href="#Module">Module</a> is easy. You can optionally
 provide a name for it (probably based on the name of the translation unit).</p>

 <ul>
   <li><tt>Module::iterator</tt> - Typedef for function list iterator<br>
     <tt>Module::const_iterator</tt> - Typedef for const_iterator.<br>

     <tt>begin()</tt>, <tt>end()</tt>
     <tt>size()</tt>, <tt>empty()</tt>

     <p>These are forwarding methods that make it easy to access the contents of
     a <tt>Module</tt> object's <a href="#Function"><tt>Function</tt></a>
     list.</p></li>

   <li><tt>Module::FunctionListType &amp;getFunctionList()</tt>

     <p> Returns the list of <a href="#Function"><tt>Function</tt></a>s.  This is
     necessary to use when you need to update the list or perform a complex
     action that doesn't have a forwarding method.</p>

     <p><!--  Global Variable --></p></li>
 </ul>

 <hr>

 <ul>
   <li><tt>Module::global_iterator</tt> - Typedef for global variable list iterator<br>

     <tt>Module::const_global_iterator</tt> - Typedef for const_iterator.<br>

     <tt>global_begin()</tt>, <tt>global_end()</tt>
     <tt>global_size()</tt>, <tt>global_empty()</tt>

     <p> These are forwarding methods that make it easy to access the contents of
     a <tt>Module</tt> object's <a
     href="#GlobalVariable"><tt>GlobalVariable</tt></a> list.</p></li>

   <li><tt>Module::GlobalListType &amp;getGlobalList()</tt>

     <p>Returns the list of <a
     href="#GlobalVariable"><tt>GlobalVariable</tt></a>s.  This is necessary to
     use when you need to update the list or perform a complex action that
     doesn't have a forwarding method.</p>

     <p><!--  Symbol table stuff --> </p></li>
 </ul>

 <hr>

 <ul>
   <li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>

     <p>Return a reference to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
     for this <tt>Module</tt>.</p>

     <p><!--  Convenience methods --></p></li>
 </ul>

 <hr>

 <ul>
   <li><tt><a href="#Function">Function</a> *getFunction(const std::string
   &amp;Name, const <a href="#FunctionType">FunctionType</a> *Ty)</tt>

     <p>Look up the specified function in the <tt>Module</tt> <a
     href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, return
     <tt>null</tt>.</p></li>

   <li><tt><a href="#Function">Function</a> *getOrInsertFunction(const
   std::string &amp;Name, const <a href="#FunctionType">FunctionType</a> *T)</tt>

     <p>Look up the specified function in the <tt>Module</tt> <a
     href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, add an
     external declaration for the function and return it.</p></li>

   <li><tt>std::string getTypeName(const <a href="#Type">Type</a> *Ty)</tt>

     <p>If there is at least one entry in the <a
     href="#SymbolTable"><tt>SymbolTable</tt></a> for the specified <a
     href="#Type"><tt>Type</tt></a>, return it.  Otherwise return the empty
     string.</p></li>

   <li><tt>bool addTypeName(const std::string &amp;Name, const <a
   href="#Type">Type</a> *Ty)</tt>

     <p>Insert an entry in the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
     mapping <tt>Name</tt> to <tt>Ty</tt>. If there is already an entry for this
     name, true is returned and the <a
     href="#SymbolTable"><tt>SymbolTable</tt></a> is not modified.</p></li>
 </ul>

 </div>


 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="Value">The <tt>Value</tt> class</a>
 </div>

 <div class="doc_text">

 <p><tt>#include "<a href="/doxygen/Value_8h-source.html">llvm/Value.h</a>"</tt>
 <br>
 doxygen info: <a href="/doxygen/classllvm_1_1Value.html">Value Class</a></p>

 <p>The <tt>Value</tt> class is the most important class in the LLVM Source
 base.  It represents a typed value that may be used (among other things) as an
 operand to an instruction.  There are many different types of <tt>Value</tt>s,
 such as <a href="#Constant"><tt>Constant</tt></a>s,<a
 href="#Argument"><tt>Argument</tt></a>s. Even <a
 href="#Instruction"><tt>Instruction</tt></a>s and <a
 href="#Function"><tt>Function</tt></a>s are <tt>Value</tt>s.</p>

 <p>A particular <tt>Value</tt> may be used many times in the LLVM representation
 for a program.  For example, an incoming argument to a function (represented
 with an instance of the <a href="#Argument">Argument</a> class) is "used" by
 every instruction in the function that references the argument.  To keep track
 of this relationship, the <tt>Value</tt> class keeps a list of all of the <a
 href="#User"><tt>User</tt></a>s that is using it (the <a
 href="#User"><tt>User</tt></a> class is a base class for all nodes in the LLVM
 graph that can refer to <tt>Value</tt>s).  This use list is how LLVM represents
 def-use information in the program, and is accessible through the <tt>use_</tt>*
 methods, shown below.</p>

 <p>Because LLVM is a typed representation, every LLVM <tt>Value</tt> is typed,
 and this <a href="#Type">Type</a> is available through the <tt>getType()</tt>
 method. In addition, all LLVM values can be named.  The "name" of the
 <tt>Value</tt> is a symbolic string printed in the LLVM code:</p>

 <div class="doc_code">
 <pre>
 %<b>foo</b> = add i32 1, 2
 </pre>
 </div>

 <p><a name="#nameWarning">The name of this instruction is "foo".</a> <b>NOTE</b>
 that the name of any value may be missing (an empty string), so names should
 <b>ONLY</b> be used for debugging (making the source code easier to read,
 debugging printouts), they should not be used to keep track of values or map
 between them.  For this purpose, use a <tt>std::map</tt> of pointers to the
 <tt>Value</tt> itself instead.</p>

 <p>One important aspect of LLVM is that there is no distinction between an SSA
 variable and the operation that produces it.  Because of this, any reference to
 the value produced by an instruction (or the value available as an incoming
 argument, for example) is represented as a direct pointer to the instance of
 the class that
 represents this value.  Although this may take some getting used to, it
 simplifies the representation and makes it easier to manipulate.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_Value">Important Public Members of the <tt>Value</tt> class</a>
 </div>

 <div class="doc_text">

 <ul>
   <li><tt>Value::use_iterator</tt> - Typedef for iterator over the
 use-list<br>
     <tt>Value::use_const_iterator</tt> - Typedef for const_iterator over
 the use-list<br>
     <tt>unsigned use_size()</tt> - Returns the number of users of the
 value.<br>
     <tt>bool use_empty()</tt> - Returns true if there are no users.<br>
     <tt>use_iterator use_begin()</tt> - Get an iterator to the start of
 the use-list.<br>
     <tt>use_iterator use_end()</tt> - Get an iterator to the end of the
 use-list.<br>
     <tt><a href="#User">User</a> *use_back()</tt> - Returns the last
 element in the list.
     <p> These methods are the interface to access the def-use
 information in LLVM.  As with all other iterators in LLVM, the naming
 conventions follow the conventions defined by the <a href="#stl">STL</a>.</p>
   </li>
   <li><tt><a href="#Type">Type</a> *getType() const</tt>
     <p>This method returns the Type of the Value.</p>
   </li>
   <li><tt>bool hasName() const</tt><br>
     <tt>std::string getName() const</tt><br>
     <tt>void setName(const std::string &amp;Name)</tt>
     <p> This family of methods is used to access and assign a name to a <tt>Value</tt>,
 be aware of the <a href="#nameWarning">precaution above</a>.</p>
   </li>
   <li><tt>void replaceAllUsesWith(Value *V)</tt>

     <p>This method traverses the use list of a <tt>Value</tt> changing all <a
     href="#User"><tt>User</tt>s</a> of the current value to refer to
     "<tt>V</tt>" instead.  For example, if you detect that an instruction always
     produces a constant value (for example through constant folding), you can
     replace all uses of the instruction with the constant like this:</p>

 <div class="doc_code">
 <pre>
 Inst-&gt;replaceAllUsesWith(ConstVal);
 </pre>
 </div>

 </ul>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="User">The <tt>User</tt> class</a>
 </div>

 <div class="doc_text">

 <p>
 <tt>#include "<a href="/doxygen/User_8h-source.html">llvm/User.h</a>"</tt><br>
 doxygen info: <a href="/doxygen/classllvm_1_1User.html">User Class</a><br>
 Superclass: <a href="#Value"><tt>Value</tt></a></p>

 <p>The <tt>User</tt> class is the common base class of all LLVM nodes that may
 refer to <a href="#Value"><tt>Value</tt></a>s.  It exposes a list of "Operands"
 that are all of the <a href="#Value"><tt>Value</tt></a>s that the User is
 referring to.  The <tt>User</tt> class itself is a subclass of
 <tt>Value</tt>.</p>

 <p>The operands of a <tt>User</tt> point directly to the LLVM <a
 href="#Value"><tt>Value</tt></a> that it refers to.  Because LLVM uses Static
 Single Assignment (SSA) form, there can only be one definition referred to,
 allowing this direct connection.  This connection provides the use-def
 information in LLVM.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_User">Important Public Members of the <tt>User</tt> class</a>
 </div>

 <div class="doc_text">

 <p>The <tt>User</tt> class exposes the operand list in two ways: through
 an index access interface and through an iterator based interface.</p>

 <ul>
   <li><tt>Value *getOperand(unsigned i)</tt><br>
     <tt>unsigned getNumOperands()</tt>
     <p> These two methods expose the operands of the <tt>User</tt> in a
 convenient form for direct access.</p></li>

   <li><tt>User::op_iterator</tt> - Typedef for iterator over the operand
 list<br>
     <tt>op_iterator op_begin()</tt> - Get an iterator to the start of
 the operand list.<br>
     <tt>op_iterator op_end()</tt> - Get an iterator to the end of the
 operand list.
     <p> Together, these methods make up the iterator based interface to
 the operands of a <tt>User</tt>.</p></li>
 </ul>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="Instruction">The <tt>Instruction</tt> class</a>
 </div>

 <div class="doc_text">

 <p><tt>#include "</tt><tt><a
 href="/doxygen/Instruction_8h-source.html">llvm/Instruction.h</a>"</tt><br>
 doxygen info: <a href="/doxygen/classllvm_1_1Instruction.html">Instruction Class</a><br>
 Superclasses: <a href="#User"><tt>User</tt></a>, <a
 href="#Value"><tt>Value</tt></a></p>

 <p>The <tt>Instruction</tt> class is the common base class for all LLVM
 instructions.  It provides only a few methods, but is a very commonly used
 class.  The primary data tracked by the <tt>Instruction</tt> class itself is the
 opcode (instruction type) and the parent <a
 href="#BasicBlock"><tt>BasicBlock</tt></a> the <tt>Instruction</tt> is embedded
 into.  To represent a specific type of instruction, one of many subclasses of
 <tt>Instruction</tt> are used.</p>

 <p> Because the <tt>Instruction</tt> class subclasses the <a
 href="#User"><tt>User</tt></a> class, its operands can be accessed in the same
 way as for other <a href="#User"><tt>User</tt></a>s (with the
 <tt>getOperand()</tt>/<tt>getNumOperands()</tt> and
 <tt>op_begin()</tt>/<tt>op_end()</tt> methods).</p> <p> An important file for
 the <tt>Instruction</tt> class is the <tt>llvm/Instruction.def</tt> file. This
 file contains some meta-data about the various different types of instructions
 in LLVM.  It describes the enum values that are used as opcodes (for example
 <tt>Instruction::Add</tt> and <tt>Instruction::ICmp</tt>), as well as the
 concrete sub-classes of <tt>Instruction</tt> that implement the instruction (for
 example <tt><a href="#BinaryOperator">BinaryOperator</a></tt> and <tt><a
 href="#CmpInst">CmpInst</a></tt>).  Unfortunately, the use of macros in
 this file confuses doxygen, so these enum values don't show up correctly in the
 <a href="/doxygen/classllvm_1_1Instruction.html">doxygen output</a>.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="s_Instruction">Important Subclasses of the <tt>Instruction</tt>
   class</a>
 </div>
 <div class="doc_text">
   <ul>
     <li><tt><a name="BinaryOperator">BinaryOperator</a></tt>
     <p>This subclasses represents all two operand instructions whose operands
     must be the same type, except for the comparison instructions.</p></li>
     <li><tt><a name="CastInst">CastInst</a></tt>
     <p>This subclass is the parent of the 12 casting instructions. It provides
     common operations on cast instructions.</p>
     <li><tt><a name="CmpInst">CmpInst</a></tt>
     <p>This subclass respresents the two comparison instructions,
     <a href="LangRef.html#i_icmp">ICmpInst</a> (integer opreands), and
     <a href="LangRef.html#i_fcmp">FCmpInst</a> (floating point operands).</p>
     <li><tt><a name="TerminatorInst">TerminatorInst</a></tt>
     <p>This subclass is the parent of all terminator instructions (those which
     can terminate a block).</p>
   </ul>
   </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_Instruction">Important Public Members of the <tt>Instruction</tt>
   class</a>
 </div>

 <div class="doc_text">

 <ul>
   <li><tt><a href="#BasicBlock">BasicBlock</a> *getParent()</tt>
     <p>Returns the <a href="#BasicBlock"><tt>BasicBlock</tt></a> that
 this  <tt>Instruction</tt> is embedded into.</p></li>
   <li><tt>bool mayWriteToMemory()</tt>
     <p>Returns true if the instruction writes to memory, i.e. it is a
       <tt>call</tt>,<tt>free</tt>,<tt>invoke</tt>, or <tt>store</tt>.</p></li>
   <li><tt>unsigned getOpcode()</tt>
     <p>Returns the opcode for the <tt>Instruction</tt>.</p></li>
   <li><tt><a href="#Instruction">Instruction</a> *clone() const</tt>
     <p>Returns another instance of the specified instruction, identical
 in all ways to the original except that the instruction has no parent
 (ie it's not embedded into a <a href="#BasicBlock"><tt>BasicBlock</tt></a>),
 and it has no name</p></li>
 </ul>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="Constant">The <tt>Constant</tt> class and subclasses</a>
 </div>

 <div class="doc_text">

 <p>Constant represents a base class for different types of constants. It
 is subclassed by ConstantInt, ConstantArray, etc. for representing
 the various types of Constants.  <a href="#GlobalValue">GlobalValue</a> is also
 a subclass, which represents the address of a global variable or function.
 </p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">Important Subclasses of Constant </div>
 <div class="doc_text">
 <ul>
   <li>ConstantInt : This subclass of Constant represents an integer constant of
   any width.
     <ul>
       <li><tt>int64_t getSExtValue() const</tt>: Returns the underlying value of
       this constant as a sign extended signed integer value.</li>
       <li><tt>uint64_t getZExtValue() const</tt>: Returns the underlying value
       of this constant as a zero extended unsigned integer value.</li>
       <li><tt>static ConstantInt* get(const Type *Ty, uint64_t Val)</tt>:
       Returns the ConstantInt object that represents the value provided by
       <tt>Val</tt> for integer type <tt>Ty</tt>.</li>
     </ul>
   </li>
   <li>ConstantFP : This class represents a floating point constant.
     <ul>
       <li><tt>double getValue() const</tt>: Returns the underlying value of
       this constant. </li>
     </ul>
   </li>
   <li>ConstantArray : This represents a constant array.
     <ul>
       <li><tt>const std::vector&lt;Use&gt; &amp;getValues() const</tt>: Returns
       a vector of component constants that makeup this array. </li>
     </ul>
   </li>
   <li>ConstantStruct : This represents a constant struct.
     <ul>
       <li><tt>const std::vector&lt;Use&gt; &amp;getValues() const</tt>: Returns
       a vector of component constants that makeup this array. </li>
     </ul>
   </li>
   <li>GlobalValue : This represents either a global variable or a function. In
   either case, the value is a constant fixed address (after linking).
   </li>
 </ul>
 </div>


 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="GlobalValue">The <tt>GlobalValue</tt> class</a>
 </div>

 <div class="doc_text">

 <p><tt>#include "<a
 href="/doxygen/GlobalValue_8h-source.html">llvm/GlobalValue.h</a>"</tt><br>
 doxygen info: <a href="/doxygen/classllvm_1_1GlobalValue.html">GlobalValue
 Class</a><br>
 Superclasses: <a href="#Constant"><tt>Constant</tt></a>,
 <a href="#User"><tt>User</tt></a>, <a href="#Value"><tt>Value</tt></a></p>

 <p>Global values (<a href="#GlobalVariable"><tt>GlobalVariable</tt></a>s or <a
 href="#Function"><tt>Function</tt></a>s) are the only LLVM values that are
 visible in the bodies of all <a href="#Function"><tt>Function</tt></a>s.
 Because they are visible at global scope, they are also subject to linking with
 other globals defined in different translation units.  To control the linking
 process, <tt>GlobalValue</tt>s know their linkage rules. Specifically,
 <tt>GlobalValue</tt>s know whether they have internal or external linkage, as
 defined by the <tt>LinkageTypes</tt> enumeration.</p>

 <p>If a <tt>GlobalValue</tt> has internal linkage (equivalent to being
 <tt>static</tt> in C), it is not visible to code outside the current translation
 unit, and does not participate in linking.  If it has external linkage, it is
 visible to external code, and does participate in linking.  In addition to
 linkage information, <tt>GlobalValue</tt>s keep track of which <a
 href="#Module"><tt>Module</tt></a> they are currently part of.</p>

 <p>Because <tt>GlobalValue</tt>s are memory objects, they are always referred to
 by their <b>address</b>. As such, the <a href="#Type"><tt>Type</tt></a> of a
 global is always a pointer to its contents. It is important to remember this
 when using the <tt>GetElementPtrInst</tt> instruction because this pointer must
 be dereferenced first. For example, if you have a <tt>GlobalVariable</tt> (a
 subclass of <tt>GlobalValue)</tt> that is an array of 24 ints, type <tt>[24 x
 i32]</tt>, then the <tt>GlobalVariable</tt> is a pointer to that array. Although
 the address of the first element of this array and the value of the
 <tt>GlobalVariable</tt> are the same, they have different types. The
 <tt>GlobalVariable</tt>'s type is <tt>[24 x i32]</tt>. The first element's type
 is <tt>i32.</tt> Because of this, accessing a global value requires you to
 dereference the pointer with <tt>GetElementPtrInst</tt> first, then its elements
 can be accessed. This is explained in the <a href="LangRef.html#globalvars">LLVM
 Language Reference Manual</a>.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_GlobalValue">Important Public Members of the <tt>GlobalValue</tt>
   class</a>
 </div>

 <div class="doc_text">

 <ul>
   <li><tt>bool hasInternalLinkage() const</tt><br>
     <tt>bool hasExternalLinkage() const</tt><br>
     <tt>void setInternalLinkage(bool HasInternalLinkage)</tt>
     <p> These methods manipulate the linkage characteristics of the <tt>GlobalValue</tt>.</p>
     <p> </p>
   </li>
   <li><tt><a href="#Module">Module</a> *getParent()</tt>
     <p> This returns the <a href="#Module"><tt>Module</tt></a> that the
 GlobalValue is currently embedded into.</p></li>
 </ul>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="Function">The <tt>Function</tt> class</a>
 </div>

 <div class="doc_text">

 <p><tt>#include "<a
 href="/doxygen/Function_8h-source.html">llvm/Function.h</a>"</tt><br> doxygen
 info: <a href="/doxygen/classllvm_1_1Function.html">Function Class</a><br>
 Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
 <a href="#Constant"><tt>Constant</tt></a>,
 <a href="#User"><tt>User</tt></a>,
 <a href="#Value"><tt>Value</tt></a></p>

 <p>The <tt>Function</tt> class represents a single procedure in LLVM.  It is
 actually one of the more complex classes in the LLVM heirarchy because it must
 keep track of a large amount of data.  The <tt>Function</tt> class keeps track
 of a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, a list of formal
 <a href="#Argument"><tt>Argument</tt></a>s, and a
 <a href="#SymbolTable"><tt>SymbolTable</tt></a>.</p>

 <p>The list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s is the most
 commonly used part of <tt>Function</tt> objects.  The list imposes an implicit
 ordering of the blocks in the function, which indicate how the code will be
 layed out by the backend.  Additionally, the first <a
 href="#BasicBlock"><tt>BasicBlock</tt></a> is the implicit entry node for the
 <tt>Function</tt>.  It is not legal in LLVM to explicitly branch to this initial
 block.  There are no implicit exit nodes, and in fact there may be multiple exit
 nodes from a single <tt>Function</tt>.  If the <a
 href="#BasicBlock"><tt>BasicBlock</tt></a> list is empty, this indicates that
 the <tt>Function</tt> is actually a function declaration: the actual body of the
 function hasn't been linked in yet.</p>

 <p>In addition to a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, the
 <tt>Function</tt> class also keeps track of the list of formal <a
 href="#Argument"><tt>Argument</tt></a>s that the function receives.  This
 container manages the lifetime of the <a href="#Argument"><tt>Argument</tt></a>
 nodes, just like the <a href="#BasicBlock"><tt>BasicBlock</tt></a> list does for
 the <a href="#BasicBlock"><tt>BasicBlock</tt></a>s.</p>

 <p>The <a href="#SymbolTable"><tt>SymbolTable</tt></a> is a very rarely used
 LLVM feature that is only used when you have to look up a value by name.  Aside
 from that, the <a href="#SymbolTable"><tt>SymbolTable</tt></a> is used
 internally to make sure that there are not conflicts between the names of <a
 href="#Instruction"><tt>Instruction</tt></a>s, <a
 href="#BasicBlock"><tt>BasicBlock</tt></a>s, or <a
 href="#Argument"><tt>Argument</tt></a>s in the function body.</p>

 <p>Note that <tt>Function</tt> is a <a href="#GlobalValue">GlobalValue</a>
 and therefore also a <a href="#Constant">Constant</a>. The value of the function
 is its address (after linking) which is guaranteed to be constant.</p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_Function">Important Public Members of the <tt>Function</tt>
   class</a>
 </div>

 <div class="doc_text">

 <ul>
   <li><tt>Function(const </tt><tt><a href="#FunctionType">FunctionType</a>
   *Ty, LinkageTypes Linkage, const std::string &amp;N = "", Module* Parent = 0)</tt>

     <p>Constructor used when you need to create new <tt>Function</tt>s to add
     the the program.  The constructor must specify the type of the function to
     create and what type of linkage the function should have. The <a
     href="#FunctionType"><tt>FunctionType</tt></a> argument
     specifies the formal arguments and return value for the function. The same
     <a href="#FunctionTypel"><tt>FunctionType</tt></a> value can be used to
     create multiple functions. The <tt>Parent</tt> argument specifies the Module
     in which the function is defined. If this argument is provided, the function
     will automatically be inserted into that module's list of
     functions.</p></li>

   <li><tt>bool isExternal()</tt>

     <p>Return whether or not the <tt>Function</tt> has a body defined.  If the
     function is "external", it does not have a body, and thus must be resolved
     by linking with a function defined in a different translation unit.</p></li>

   <li><tt>Function::iterator</tt> - Typedef for basic block list iterator<br>
     <tt>Function::const_iterator</tt> - Typedef for const_iterator.<br>

     <tt>begin()</tt>, <tt>end()</tt>
     <tt>size()</tt>, <tt>empty()</tt>

     <p>These are forwarding methods that make it easy to access the contents of
     a <tt>Function</tt> object's <a href="#BasicBlock"><tt>BasicBlock</tt></a>
     list.</p></li>

   <li><tt>Function::BasicBlockListType &amp;getBasicBlockList()</tt>

     <p>Returns the list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s.  This
     is necessary to use when you need to update the list or perform a complex
     action that doesn't have a forwarding method.</p></li>

   <li><tt>Function::arg_iterator</tt> - Typedef for the argument list
 iterator<br>
     <tt>Function::const_arg_iterator</tt> - Typedef for const_iterator.<br>

     <tt>arg_begin()</tt>, <tt>arg_end()</tt>
     <tt>arg_size()</tt>, <tt>arg_empty()</tt>

     <p>These are forwarding methods that make it easy to access the contents of
     a <tt>Function</tt> object's <a href="#Argument"><tt>Argument</tt></a>
     list.</p></li>

   <li><tt>Function::ArgumentListType &amp;getArgumentList()</tt>

     <p>Returns the list of <a href="#Argument"><tt>Argument</tt></a>s.  This is
     necessary to use when you need to update the list or perform a complex
     action that doesn't have a forwarding method.</p></li>

   <li><tt><a href="#BasicBlock">BasicBlock</a> &amp;getEntryBlock()</tt>

     <p>Returns the entry <a href="#BasicBlock"><tt>BasicBlock</tt></a> for the
     function.  Because the entry block for the function is always the first
     block, this returns the first block of the <tt>Function</tt>.</p></li>

   <li><tt><a href="#Type">Type</a> *getReturnType()</tt><br>
     <tt><a href="#FunctionType">FunctionType</a> *getFunctionType()</tt>

     <p>This traverses the <a href="#Type"><tt>Type</tt></a> of the
     <tt>Function</tt> and returns the return type of the function, or the <a
     href="#FunctionType"><tt>FunctionType</tt></a> of the actual
     function.</p></li>

   <li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>

     <p> Return a pointer to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
     for this <tt>Function</tt>.</p></li>
 </ul>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="GlobalVariable">The <tt>GlobalVariable</tt> class</a>
 </div>

 <div class="doc_text">

 <p><tt>#include "<a
 href="/doxygen/GlobalVariable_8h-source.html">llvm/GlobalVariable.h</a>"</tt>
 <br>
 doxygen info: <a href="/doxygen/classllvm_1_1GlobalVariable.html">GlobalVariable
  Class</a><br>
 Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
 <a href="#Constant"><tt>Constant</tt></a>,
 <a href="#User"><tt>User</tt></a>,
 <a href="#Value"><tt>Value</tt></a></p>

 <p>Global variables are represented with the (suprise suprise)
 <tt>GlobalVariable</tt> class. Like functions, <tt>GlobalVariable</tt>s are also
 subclasses of <a href="#GlobalValue"><tt>GlobalValue</tt></a>, and as such are
 always referenced by their address (global values must live in memory, so their
 "name" refers to their constant address). See
 <a href="#GlobalValue"><tt>GlobalValue</tt></a> for more on this.  Global
 variables may have an initial value (which must be a
 <a href="#Constant"><tt>Constant</tt></a>), and if they have an initializer,
 they may be marked as "constant" themselves (indicating that their contents
 never change at runtime).</p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_GlobalVariable">Important Public Members of the
   <tt>GlobalVariable</tt> class</a>
 </div>

 <div class="doc_text">

 <ul>
   <li><tt>GlobalVariable(const </tt><tt><a href="#Type">Type</a> *Ty, bool
   isConstant, LinkageTypes&amp; Linkage, <a href="#Constant">Constant</a>
   *Initializer = 0, const std::string &amp;Name = "", Module* Parent = 0)</tt>

     <p>Create a new global variable of the specified type. If
     <tt>isConstant</tt> is true then the global variable will be marked as
     unchanging for the program. The Linkage parameter specifies the type of
     linkage (internal, external, weak, linkonce, appending) for the variable. If
     the linkage is InternalLinkage, WeakLinkage, or LinkOnceLinkage,&nbsp; then
     the resultant global variable will have internal linkage.  AppendingLinkage
     concatenates together all instances (in different translation units) of the
     variable into a single variable but is only applicable to arrays.  &nbsp;See
     the <a href="LangRef.html#modulestructure">LLVM Language Reference</a> for
     further details on linkage types. Optionally an initializer, a name, and the
     module to put the variable into may be specified for the global variable as
     well.</p></li>

   <li><tt>bool isConstant() const</tt>

     <p>Returns true if this is a global variable that is known not to
     be modified at runtime.</p></li>

   <li><tt>bool hasInitializer()</tt>

     <p>Returns true if this <tt>GlobalVariable</tt> has an intializer.</p></li>

   <li><tt><a href="#Constant">Constant</a> *getInitializer()</tt>

     <p>Returns the intial value for a <tt>GlobalVariable</tt>.  It is not legal
     to call this method if there is no initializer.</p></li>
 </ul>

 </div>


 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="BasicBlock">The <tt>BasicBlock</tt> class</a>
 </div>

 <div class="doc_text">

 <p><tt>#include "<a
 href="/doxygen/BasicBlock_8h-source.html">llvm/BasicBlock.h</a>"</tt><br>
 doxygen info: <a href="/doxygen/structllvm_1_1BasicBlock.html">BasicBlock
 Class</a><br>
 Superclass: <a href="#Value"><tt>Value</tt></a></p>

 <p>This class represents a single entry multiple exit section of the code,
 commonly known as a basic block by the compiler community.  The
 <tt>BasicBlock</tt> class maintains a list of <a
 href="#Instruction"><tt>Instruction</tt></a>s, which form the body of the block.
 Matching the language definition, the last element of this list of instructions
 is always a terminator instruction (a subclass of the <a
 href="#TerminatorInst"><tt>TerminatorInst</tt></a> class).</p>

 <p>In addition to tracking the list of instructions that make up the block, the
 <tt>BasicBlock</tt> class also keeps track of the <a
 href="#Function"><tt>Function</tt></a> that it is embedded into.</p>

 <p>Note that <tt>BasicBlock</tt>s themselves are <a
 href="#Value"><tt>Value</tt></a>s, because they are referenced by instructions
 like branches and can go in the switch tables. <tt>BasicBlock</tt>s have type
 <tt>label</tt>.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="m_BasicBlock">Important Public Members of the <tt>BasicBlock</tt>
   class</a>
 </div>

 <div class="doc_text">
 <ul>

 <li><tt>BasicBlock(const std::string &amp;Name = "", </tt><tt><a
  href="#Function">Function</a> *Parent = 0)</tt>

 <p>The <tt>BasicBlock</tt> constructor is used to create new basic blocks for
 insertion into a function.  The constructor optionally takes a name for the new
 block, and a <a href="#Function"><tt>Function</tt></a> to insert it into.  If
 the <tt>Parent</tt> parameter is specified, the new <tt>BasicBlock</tt> is
 automatically inserted at the end of the specified <a
 href="#Function"><tt>Function</tt></a>, if not specified, the BasicBlock must be
 manually inserted into the <a href="#Function"><tt>Function</tt></a>.</p></li>

 <li><tt>BasicBlock::iterator</tt> - Typedef for instruction list iterator<br>
 <tt>BasicBlock::const_iterator</tt> - Typedef for const_iterator.<br>
 <tt>begin()</tt>, <tt>end()</tt>, <tt>front()</tt>, <tt>back()</tt>,
 <tt>size()</tt>, <tt>empty()</tt>
 STL-style functions for accessing the instruction list.

 <p>These methods and typedefs are forwarding functions that have the same
 semantics as the standard library methods of the same names.  These methods
 expose the underlying instruction list of a basic block in a way that is easy to
 manipulate.  To get the full complement of container operations (including
 operations to update the list), you must use the <tt>getInstList()</tt>
 method.</p></li>

 <li><tt>BasicBlock::InstListType &amp;getInstList()</tt>

 <p>This method is used to get access to the underlying container that actually
 holds the Instructions.  This method must be used when there isn't a forwarding
 function in the <tt>BasicBlock</tt> class for the operation that you would like
 to perform.  Because there are no forwarding functions for "updating"
 operations, you need to use this if you want to update the contents of a
 <tt>BasicBlock</tt>.</p></li>

 <li><tt><a href="#Function">Function</a> *getParent()</tt>

 <p> Returns a pointer to <a href="#Function"><tt>Function</tt></a> the block is
 embedded into, or a null pointer if it is homeless.</p></li>

 <li><tt><a href="#TerminatorInst">TerminatorInst</a> *getTerminator()</tt>

 <p> Returns a pointer to the terminator instruction that appears at the end of
 the <tt>BasicBlock</tt>.  If there is no terminator instruction, or if the last
 instruction in the block is not a terminator, then a null pointer is
 returned.</p></li>

 </ul>

 </div>


 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="Argument">The <tt>Argument</tt> class</a>
 </div>

 <div class="doc_text">

 <p>This subclass of Value defines the interface for incoming formal
 arguments to a function. A Function maintains a list of its formal
 arguments. An argument has a pointer to the parent Function.</p>

 </div>

 <!-- *********************************************************************** -->
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   <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
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