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 <div class="doc_title">
   Writing an LLVM Pass
 </div>

 <ol>
   <li><a href="#introduction">Introduction - What is a pass?</a></li>
   <li><a href="#quickstart">Quick Start - Writing hello world</a>
     <ul>
     <li><a href="#makefile">Setting up the build environment</a></li>
     <li><a href="#basiccode">Basic code required</a></li>
     <li><a href="#running">Running a pass with <tt>opt</tt></a></li>
     </ul></li>
   <li><a href="#passtype">Pass classes and requirements</a>
      <ul>
      <li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li>
      <li><a href="#ModulePass">The <tt>ModulePass</tt> class</a>
         <ul>
         <li><a href="#runOnModule">The <tt>runOnModule</tt> method</a></li>
         </ul></li>
      <li><a href="#CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
         <ul>
         <li><a href="#doInitialization_scc">The <tt>doInitialization(CallGraph
                                            &amp;)</tt> method</a></li>
         <li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li>
         <li><a href="#doFinalization_scc">The <tt>doFinalization(CallGraph
                                            &amp;)</tt> method</a></li>
         </ul></li>
      <li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a>
         <ul>
         <li><a href="#doInitialization_mod">The <tt>doInitialization(Module
                                             &amp;)</tt> method</a></li>
         <li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li>
         <li><a href="#doFinalization_mod">The <tt>doFinalization(Module
                                             &amp;)</tt> method</a></li>
         </ul></li>
      <li><a href="#LoopPass">The <tt>LoopPass</tt> class</a>
         <ul>
         <li><a href="#doInitialization_loop">The <tt>doInitialization(Loop *,
                                             LPPassManager &amp;)</tt> method</a></li>
         <li><a href="#runOnLoop">The <tt>runOnLoop</tt> method</a></li>
         <li><a href="#doFinalization_loop">The <tt>doFinalization()
                                             </tt> method</a></li>
         </ul></li>
      <li><a href="#RegionPass">The <tt>RegionPass</tt> class</a>
         <ul>
         <li><a href="#doInitialization_region">The <tt>doInitialization(Region *,
                                             RGPassManager &amp;)</tt> method</a></li>
         <li><a href="#runOnRegion">The <tt>runOnRegion</tt> method</a></li>
         <li><a href="#doFinalization_region">The <tt>doFinalization()
                                             </tt> method</a></li>
         </ul></li>
      <li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
         <ul>
         <li><a href="#doInitialization_fn">The <tt>doInitialization(Function
                                              &amp;)</tt> method</a></li>
         <li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt>
                                        method</a></li>
         <li><a href="#doFinalization_fn">The <tt>doFinalization(Function
                                          &amp;)</tt> method</a></li>
         </ul></li>
      <li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt>
                                         class</a>
         <ul>
         <li><a href="#runOnMachineFunction">The
             <tt>runOnMachineFunction(MachineFunction &amp;)</tt> method</a></li>
         </ul></li>
      </ul>
   <li><a href="#registration">Pass Registration</a>
      <ul>
      <li><a href="#print">The <tt>print</tt> method</a></li>
      </ul></li>
   <li><a href="#interaction">Specifying interactions between passes</a>
      <ul>
      <li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt>
                                      method</a></li>
      <li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired&lt;&gt;</tt> and <tt>AnalysisUsage::addRequiredTransitive&lt;&gt;</tt> methods</a></li>
      <li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved&lt;&gt;</tt> method</a></li>
      <li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li>
      <li><a href="#getAnalysis">The <tt>getAnalysis&lt;&gt;</tt> and
 <tt>getAnalysisIfAvailable&lt;&gt;</tt> methods</a></li>
      </ul></li>
   <li><a href="#analysisgroup">Implementing Analysis Groups</a>
      <ul>
      <li><a href="#agconcepts">Analysis Group Concepts</a></li>
      <li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li>
      </ul></li>
   <li><a href="#passStatistics">Pass Statistics</a>
   <li><a href="#passmanager">What PassManager does</a>
     <ul>
     <li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li>
     </ul></li>
   <li><a href="#registering">Registering dynamically loaded passes</a>
     <ul>
       <li><a href="#registering_existing">Using existing registries</a></li>
       <li><a href="#registering_new">Creating new registries</a></li>
     </ul></li>
   <li><a href="#debughints">Using GDB with dynamically loaded passes</a>
     <ul>
     <li><a href="#breakpoint">Setting a breakpoint in your pass</a></li>
     <li><a href="#debugmisc">Miscellaneous Problems</a></li>
     </ul></li>
   <li><a href="#future">Future extensions planned</a>
     <ul>
     <li><a href="#SMP">Multithreaded LLVM</a></li>
     </ul></li>
 </ol>

 <div class="doc_author">
   <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
   <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="introduction">Introduction - What is a pass?</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM
 passes are where most of the interesting parts of the compiler exist.  Passes
 perform the transformations and optimizations that make up the compiler, they
 build the analysis results that are used by these transformations, and they are,
 above all, a structuring technique for compiler code.</p>

 <p>All LLVM passes are subclasses of the <tt><a
 href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>
 class, which implement functionality by overriding virtual methods inherited
 from <tt>Pass</tt>.  Depending on how your pass works, you should inherit from
 the <tt><a href="#ModulePass">ModulePass</a></tt>, <tt><a
 href="#CallGraphSCCPass">CallGraphSCCPass</a></tt>, <tt><a
 href="#FunctionPass">FunctionPass</a></tt>, or <tt><a
 href="#LoopPass">LoopPass</a></tt>, or <tt><a
 href="#RegionPass">RegionPass</a></tt>, or <tt><a
 href="#BasicBlockPass">BasicBlockPass</a></tt> classes, which gives the system
 more information about what your pass does, and how it can be combined with
 other passes.  One of the main features of the LLVM Pass Framework is that it
 schedules passes to run in an efficient way based on the constraints that your
 pass meets (which are indicated by which class they derive from).</p>

 <p>We start by showing you how to construct a pass, everything from setting up
 the code, to compiling, loading, and executing it.  After the basics are down,
 more advanced features are discussed.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="quickstart">Quick Start - Writing hello world</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>Here we describe how to write the "hello world" of passes.  The "Hello" pass
 is designed to simply print out the name of non-external functions that exist in
 the program being compiled.  It does not modify the program at all, it just
 inspects it.  The source code and files for this pass are available in the LLVM
 source tree in the <tt>lib/Transforms/Hello</tt> directory.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="makefile">Setting up the build environment</a>
 </div>

 <div class="doc_text">

   <p>First, you need to create a new directory somewhere in the LLVM source
   base.  For this example, we'll assume that you made
   <tt>lib/Transforms/Hello</tt>.  Next, you must set up a build script
   (Makefile) that will compile the source code for the new pass.  To do this,
   copy the following into <tt>Makefile</tt>:</p>
   <hr/>

 <div class="doc_code"><pre>
 # Makefile for hello pass

 # Path to top level of LLVM hierarchy
 LEVEL = ../../..

 # Name of the library to build
 LIBRARYNAME = Hello

 # Make the shared library become a loadable module so the tools can
 # dlopen/dlsym on the resulting library.
 LOADABLE_MODULE = 1

 # Include the makefile implementation stuff
 include $(LEVEL)/Makefile.common
 </pre></div>

 <p>This makefile specifies that all of the <tt>.cpp</tt> files in the current
 directory are to be compiled and linked together into a
 <tt>Debug+Asserts/lib/Hello.so</tt> shared object that can be dynamically loaded by
 the <tt>opt</tt> or <tt>bugpoint</tt> tools via their <tt>-load</tt> options.
 If your operating system uses a suffix other than .so (such as windows or
 Mac OS/X), the appropriate extension will be used.</p>

 <p>Now that we have the build scripts set up, we just need to write the code for
 the pass itself.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="basiccode">Basic code required</a>
 </div>

 <div class="doc_text">

 <p>Now that we have a way to compile our new pass, we just have to write it.
 Start out with:</p>

 <div class="doc_code"><pre>
 <b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
 <b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
 <b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>"
 </pre></div>

 <p>Which are needed because we are writing a <tt><a
 href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>,
 we are operating on <tt><a
 href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s,
 and we will be doing some printing.</p>

 <p>Next we have:</p>
 <div class="doc_code"><pre>
 <b>using namespace llvm;</b>
 </pre></div>
 <p>... which is required because the functions from the include files
 live in the llvm namespace.
 </p>

 <p>Next we have:</p>

 <div class="doc_code"><pre>
 <b>namespace</b> {
 </pre></div>

 <p>... which starts out an anonymous namespace.  Anonymous namespaces are to C++
 what the "<tt>static</tt>" keyword is to C (at global scope).  It makes the
 things declared inside of the anonymous namespace only visible to the current
 file.  If you're not familiar with them, consult a decent C++ book for more
 information.</p>

 <p>Next, we declare our pass itself:</p>

 <div class="doc_code"><pre>
   <b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
 </pre></div><p>

 <p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
 href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>.
 The different builtin pass subclasses are described in detail <a
 href="#passtype">later</a>, but for now, know that <a
 href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate a function at a
 time.</p>

 <div class="doc_code"><pre>
      static char ID;
      Hello() : FunctionPass(&amp;ID) {}
 </pre></div><p>

 <p> This declares pass identifier used by LLVM to identify pass. This allows LLVM to
 avoid using expensive C++ runtime information.</p>

 <div class="doc_code"><pre>
     <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &amp;F) {
       errs() &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
       <b>return false</b>;
     }
   };  <i>// end of struct Hello</i>
 </pre></div>

 <p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method,
 which overloads an abstract virtual method inherited from <a
 href="#FunctionPass"><tt>FunctionPass</tt></a>.  This is where we are supposed
 to do our thing, so we just print out our message with the name of each
 function.</p>

 <div class="doc_code"><pre>
   char Hello::ID = 0;
 </pre></div>

 <p> We initialize pass ID here. LLVM uses ID's address to identify pass so
 initialization value is not important.</p>

 <div class="doc_code"><pre>
   INITIALIZE_PASS(Hello, "<i>hello</i>", "<i>Hello World Pass</i>",
                         false /* Only looks at CFG */,
                         false /* Analysis Pass */);
 }  <i>// end of anonymous namespace</i>
 </pre></div>

 <p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>,
 giving it a command line
 argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>".
 Last two arguments describe its behavior.
 If a pass walks CFG without modifying it then third argument is set to true.
 If  a pass is an analysis pass, for example dominator tree pass, then true
 is supplied as fourth argument. </p>

 <p>As a whole, the <tt>.cpp</tt> file looks like:</p>

 <div class="doc_code"><pre>
 <b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
 <b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
 <b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>"

 <b>using namespace llvm;</b>

 <b>namespace</b> {
   <b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {

     static char ID;
     Hello() : FunctionPass(&amp;ID) {}

     <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &amp;F) {
       errs() &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
       <b>return false</b>;
     }
   };

   char Hello::ID = 0;
   INITIALIZE_PASS(Hello, "<i>Hello</i>", "<i>Hello World Pass</i>", false, false);
 }

 </pre></div>

 <p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
 command in the local directory and you should get a new
 "<tt>Debug+Asserts/lib/Hello.so</tt> file.  Note that everything in this file is
 contained in an anonymous namespace: this reflects the fact that passes are self
 contained units that do not need external interfaces (although they can have
 them) to be useful.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="running">Running a pass with <tt>opt</tt></a>
 </div>

 <div class="doc_text">

 <p>Now that you have a brand new shiny shared object file, we can use the
 <tt>opt</tt> command to run an LLVM program through your pass.  Because you
 registered your pass with the <tt>INITIALIZE_PASS</tt> macro, you will be able to
 use the <tt>opt</tt> tool to access it, once loaded.</p>

 <p>To test it, follow the example at the end of the <a
 href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
 LLVM.  We can now run the bitcode file (<tt>hello.bc</tt>) for the program
 through our transformation like this (or course, any bitcode file will
 work):</p>

 <div class="doc_code"><pre>
 $ opt -load ../../../Debug+Asserts/lib/Hello.so -hello &lt; hello.bc &gt; /dev/null
 Hello: __main
 Hello: puts
 Hello: main
 </pre></div>

 <p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your
 pass as a shared object, which makes '<tt>-hello</tt>' a valid command line
 argument (which is one reason you need to <a href="#registration">register your
 pass</a>).  Because the hello pass does not modify the program in any
 interesting way, we just throw away the result of <tt>opt</tt> (sending it to
 <tt>/dev/null</tt>).</p>

 <p>To see what happened to the other string you registered, try running
 <tt>opt</tt> with the <tt>-help</tt> option:</p>

 <div class="doc_code"><pre>
 $ opt -load ../../../Debug+Asserts/lib/Hello.so -help
 OVERVIEW: llvm .bc -&gt; .bc modular optimizer

 USAGE: opt [options] &lt;input bitcode&gt;

 OPTIONS:
   Optimizations available:
 ...
     -funcresolve    - Resolve Functions
     -gcse           - Global Common Subexpression Elimination
     -globaldce      - Dead Global Elimination
     <b>-hello          - Hello World Pass</b>
     -indvars        - Canonicalize Induction Variables
     -inline         - Function Integration/Inlining
     -instcombine    - Combine redundant instructions
 ...
 </pre></div>

 <p>The pass name get added as the information string for your pass, giving some
 documentation to users of <tt>opt</tt>.  Now that you have a working pass, you
 would go ahead and make it do the cool transformations you want.  Once you get
 it all working and tested, it may become useful to find out how fast your pass
 is.  The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command
 line option (<tt>--time-passes</tt>) that allows you to get information about
 the execution time of your pass along with the other passes you queue up.  For
 example:</p>

 <div class="doc_code"><pre>
 $ opt -load ../../../Debug+Asserts/lib/Hello.so -hello -time-passes &lt; hello.bc &gt; /dev/null
 Hello: __main
 Hello: puts
 Hello: main
 ===============================================================================
                       ... Pass execution timing report ...
 ===============================================================================
   Total Execution Time: 0.02 seconds (0.0479059 wall clock)

    ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Pass Name ---
    0.0100 (100.0%)   0.0000 (  0.0%)   0.0100 ( 50.0%)   0.0402 ( 84.0%)  Bitcode Writer
    0.0000 (  0.0%)   0.0100 (100.0%)   0.0100 ( 50.0%)   0.0031 (  6.4%)  Dominator Set Construction
    0.0000 (  0.0%)   0.0000 (  0.0%)   0.0000 (  0.0%)   0.0013 (  2.7%)  Module Verifier
  <b>  0.0000 (  0.0%)   0.0000 (  0.0%)   0.0000 (  0.0%)   0.0033 (  6.9%)  Hello World Pass</b>
    0.0100 (100.0%)   0.0100 (100.0%)   0.0200 (100.0%)   0.0479 (100.0%)  TOTAL
 </pre></div>

 <p>As you can see, our implementation above is pretty fast :).  The additional
 passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
 that the LLVM emitted by your pass is still valid and well formed LLVM, which
 hasn't been broken somehow.</p>

 <p>Now that you have seen the basics of the mechanics behind passes, we can talk
 about some more details of how they work and how to use them.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="passtype">Pass classes and requirements</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>One of the first things that you should do when designing a new pass is to
 decide what class you should subclass for your pass.  The <a
 href="#basiccode">Hello World</a> example uses the <tt><a
 href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we
 did not discuss why or when this should occur.  Here we talk about the classes
 available, from the most general to the most specific.</p>

 <p>When choosing a superclass for your Pass, you should choose the <b>most
 specific</b> class possible, while still being able to meet the requirements
 listed.  This gives the LLVM Pass Infrastructure information necessary to
 optimize how passes are run, so that the resultant compiler isn't unnecessarily
 slow.</p>

 </div>

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

 <div class="doc_text">

 <p>The most plain and boring type of pass is the "<tt><a
 href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>"
 class.  This pass type is used for passes that do not have to be run, do not
 change state, and never need to be updated.  This is not a normal type of
 transformation or analysis, but can provide information about the current
 compiler configuration.</p>

 <p>Although this pass class is very infrequently used, it is important for
 providing information about the current target machine being compiled for, and
 other static information that can affect the various transformations.</p>

 <p><tt>ImmutablePass</tt>es never invalidate other transformations, are never
 invalidated, and are never "run".</p>

 </div>

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

 <div class="doc_text">

 <p>The "<tt><a
 href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>"
 class is the most general of all superclasses that you can use.  Deriving from
 <tt>ModulePass</tt> indicates that your pass uses the entire program as a unit,
 referring to function bodies in no predictable order, or adding and removing
 functions.  Because nothing is known about the behavior of <tt>ModulePass</tt>
 subclasses, no optimization can be done for their execution.</p>

 <p>A module pass can use function level passes (e.g. dominators) using
 the getAnalysis interface
 <tt>getAnalysis&lt;DominatorTree&gt;(llvm::Function *)</tt> to provide the
 function to retrieve analysis result for, if the function pass does not require
 any module or immutable passes. Note that this can only be done for functions for which the
 analysis ran, e.g. in the case of dominators you should only ask for the
 DominatorTree for function definitions, not declarations.</p>

 <p>To write a correct <tt>ModulePass</tt> subclass, derive from
 <tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the
 following signature:</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="runOnModule">The <tt>runOnModule</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> runOnModule(Module &amp;M) = 0;
 </pre></div>

 <p>The <tt>runOnModule</tt> method performs the interesting work of the pass.
 It should return true if the module was modified by the transformation and
 false otherwise.</p>

 </div>

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

 <div class="doc_text">

 <p>The "<tt><a
 href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>"
 is used by passes that need to traverse the program bottom-up on the call graph
 (callees before callers).  Deriving from CallGraphSCCPass provides some
 mechanics for building and traversing the CallGraph, but also allows the system
 to optimize execution of CallGraphSCCPass's.  If your pass meets the
 requirements outlined below, and doesn't meet the requirements of a <tt><a
 href="#FunctionPass">FunctionPass</a></tt> or <tt><a
 href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from
 <tt>CallGraphSCCPass</tt>.</p>

 <p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p>

 <p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p>

 <ol>

 <li>... <em>not allowed</em> to modify any <tt>Function</tt>s that are not in
 the current SCC.</li>

 <li>... <em>not allowed</em> to inspect any Function's other than those in the
 current SCC and the direct callees of the SCC.</li>

 <li>... <em>required</em> to preserve the current CallGraph object, updating it
 to reflect any changes made to the program.</li>

 <li>... <em>not allowed</em> to add or remove SCC's from the current Module,
 though they may change the contents of an SCC.</li>

 <li>... <em>allowed</em> to add or remove global variables from the current
 Module.</li>

 <li>... <em>allowed</em> to maintain state across invocations of
     <a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li>
 </ol>

 <p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases
 because it has to handle SCCs with more than one node in it.  All of the virtual
 methods described below should return true if they modified the program, or
 false if they didn't.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doInitialization_scc">The <tt>doInitialization(CallGraph &amp;)</tt>
   method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doInitialization(CallGraph &amp;CG);
 </pre></div>

 <p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
 <tt>CallGraphSCCPass</tt>'s are not allowed to do.  They can add and remove
 functions, get pointers to functions, etc.  The <tt>doInitialization</tt> method
 is designed to do simple initialization type of stuff that does not depend on
 the SCCs being processed.  The <tt>doInitialization</tt> method call is not
 scheduled to overlap with any other pass executions (thus it should be very
 fast).</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="runOnSCC">The <tt>runOnSCC</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> runOnSCC(CallGraphSCC &amp;SCC) = 0;
 </pre></div>

 <p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and
 should return true if the module was modified by the transformation, false
 otherwise.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doFinalization_scc">The <tt>doFinalization(CallGraph
    &amp;)</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doFinalization(CallGraph &amp;CG);
 </pre></div>

 <p>The <tt>doFinalization</tt> method is an infrequently used method that is
 called when the pass framework has finished calling <a
 href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
 program being compiled.</p>

 </div>

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

 <div class="doc_text">

 <p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a
 href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
 subclasses do have a predictable, local behavior that can be expected by the
 system.  All <tt>FunctionPass</tt> execute on each function in the program
 independent of all of the other functions in the program.
 <tt>FunctionPass</tt>'s do not require that they are executed in a particular
 order, and <tt>FunctionPass</tt>'s do not modify external functions.</p>

 <p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p>

 <ol>
 <li>Modify a Function other than the one currently being processed.</li>
 <li>Add or remove Function's from the current Module.</li>
 <li>Add or remove global variables from the current Module.</li>
 <li>Maintain state across invocations of
     <a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li>
 </ol>

 <p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a
 href="#basiccode">Hello World</a> pass for example).  <tt>FunctionPass</tt>'s
 may overload three virtual methods to do their work.  All of these methods
 should return true if they modified the program, or false if they didn't.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doInitialization_mod">The <tt>doInitialization(Module &amp;)</tt>
   method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doInitialization(Module &amp;M);
 </pre></div>

 <p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
 <tt>FunctionPass</tt>'s are not allowed to do.  They can add and remove
 functions, get pointers to functions, etc.  The <tt>doInitialization</tt> method
 is designed to do simple initialization type of stuff that does not depend on
 the functions being processed.  The <tt>doInitialization</tt> method call is not
 scheduled to overlap with any other pass executions (thus it should be very
 fast).</p>

 <p>A good example of how this method should be used is the <a
 href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
 pass.  This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
 platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls.  It
 uses the <tt>doInitialization</tt> method to get a reference to the malloc and
 free functions that it needs, adding prototypes to the module if necessary.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> runOnFunction(Function &amp;F) = 0;
 </pre></div><p>

 <p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do
 the transformation or analysis work of your pass.  As usual, a true value should
 be returned if the function is modified.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doFinalization_mod">The <tt>doFinalization(Module
   &amp;)</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doFinalization(Module &amp;M);
 </pre></div>

 <p>The <tt>doFinalization</tt> method is an infrequently used method that is
 called when the pass framework has finished calling <a
 href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
 program being compiled.</p>

 </div>

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

 <div class="doc_text">

 <p> All <tt>LoopPass</tt> execute on each loop in the function independent of
 all of the other loops in the function. <tt>LoopPass</tt> processes loops in
 loop nest order such that outer most loop is processed last. </p>

 <p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using
 <tt>LPPassManager</tt> interface. Implementing a loop pass is usually
 straightforward. <tt>Looppass</tt>'s may overload three virtual methods to
 do their work. All these methods should return true if they modified the
 program, or false if they didn't. </p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doInitialization_loop">The <tt>doInitialization(Loop *,
                                                  LPPassManager &amp;)</tt>
   method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doInitialization(Loop *, LPPassManager &amp;LPM);
 </pre></div>

 <p>The <tt>doInitialization</tt> method is designed to do simple initialization
 type of stuff that does not depend on the functions being processed.  The
 <tt>doInitialization</tt> method call is not scheduled to overlap with any
 other pass executions (thus it should be very fast). LPPassManager
 interface should be used to access Function or Module level analysis
 information.</p>

 </div>


 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="runOnLoop">The <tt>runOnLoop</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> runOnLoop(Loop *, LPPassManager &amp;LPM) = 0;
 </pre></div><p>

 <p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do
 the transformation or analysis work of your pass.  As usual, a true value should
 be returned if the function is modified. <tt>LPPassManager</tt> interface
 should be used to update loop nest.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doFinalization();
 </pre></div>

 <p>The <tt>doFinalization</tt> method is an infrequently used method that is
 called when the pass framework has finished calling <a
 href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the
 program being compiled. </p>

 </div>

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

 <div class="doc_text">

 <p> <tt>RegionPass</tt> is similar to <a href="#LoopPass"><tt>LoopPass</tt></a>,
 but executes on each single entry single exit region in the function.
 <tt>RegionPass</tt> processes regions in nested order such that the outer most
 region is processed last.  </p>

 <p> <tt>RegionPass</tt> subclasses are allowed to update the region tree by using
 the <tt>RGPassManager</tt> interface. You may overload three virtual methods of
 <tt>RegionPass</tt> to implementing your own region pass is usually. All these
 methods should return true if they modified the program, or false if they didn not.
 </p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doInitialization_region">The <tt>doInitialization(Region *,
                                                  RGPassManager &amp;)</tt>
   method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doInitialization(Region *, RGPassManager &amp;RGM);
 </pre></div>

 <p>The <tt>doInitialization</tt> method is designed to do simple initialization
 type of stuff that does not depend on the functions being processed.  The
 <tt>doInitialization</tt> method call is not scheduled to overlap with any
 other pass executions (thus it should be very fast). RPPassManager
 interface should be used to access Function or Module level analysis
 information.</p>

 </div>


 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="runOnRegion">The <tt>runOnRegion</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> runOnRegion(Region *, RGPassManager &amp;RGM) = 0;
 </pre></div><p>

 <p>The <tt>runOnRegion</tt> method must be implemented by your subclass to do
 the transformation or analysis work of your pass.  As usual, a true value should
 be returned if the region is modified. <tt>RGPassManager</tt> interface
 should be used to update region tree.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doFinalization_region">The <tt>doFinalization()</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doFinalization();
 </pre></div>

 <p>The <tt>doFinalization</tt> method is an infrequently used method that is
 called when the pass framework has finished calling <a
 href="#runOnRegion"><tt>runOnRegion</tt></a> for every region in the
 program being compiled. </p>

 </div>


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

 <div class="doc_text">

 <p><tt>BasicBlockPass</tt>'s are just like <a
 href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
 their scope of inspection and modification to a single basic block at a time.
 As such, they are <b>not</b> allowed to do any of the following:</p>

 <ol>
 <li>Modify or inspect any basic blocks outside of the current one</li>
 <li>Maintain state across invocations of
     <a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li>
 <li>Modify the control flow graph (by altering terminator instructions)</li>
 <li>Any of the things forbidden for
     <a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li>
 </ol>

 <p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole"
 optimizations.  They may override the same <a
 href="#doInitialization_mod"><tt>doInitialization(Module &amp;)</tt></a> and <a
 href="#doFinalization_mod"><tt>doFinalization(Module &amp;)</tt></a> methods that <a
 href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doInitialization_fn">The <tt>doInitialization(Function
   &amp;)</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doInitialization(Function &amp;F);
 </pre></div>

 <p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
 <tt>BasicBlockPass</tt>'s are not allowed to do, but that
 <tt>FunctionPass</tt>'s can.  The <tt>doInitialization</tt> method is designed
 to do simple initialization that does not depend on the
 BasicBlocks being processed.  The <tt>doInitialization</tt> method call is not
 scheduled to overlap with any other pass executions (thus it should be very
 fast).</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> runOnBasicBlock(BasicBlock &amp;BB) = 0;
 </pre></div>

 <p>Override this function to do the work of the <tt>BasicBlockPass</tt>.  This
 function is not allowed to inspect or modify basic blocks other than the
 parameter, and are not allowed to modify the CFG.  A true value must be returned
 if the basic block is modified.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="doFinalization_fn">The <tt>doFinalization(Function &amp;)</tt>
   method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> doFinalization(Function &amp;F);
 </pre></div>

 <p>The <tt>doFinalization</tt> method is an infrequently used method that is
 called when the pass framework has finished calling <a
 href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the
 program being compiled.  This can be used to perform per-function
 finalization.</p>

 </div>

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

 <div class="doc_text">

 <p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that
 executes on the machine-dependent representation of each LLVM function in the
 program.</p>

 <p>Code generator passes are registered and initialized specially by
 <tt>TargetMachine::addPassesToEmitFile</tt> and similar routines, so they
 cannot generally be run from the <tt>opt</tt> or <tt>bugpoint</tt>
 commands.</p>

 <p>A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
 the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
 <tt>MachineFunctionPass</tt>es also have additional restrictions. In particular,
 <tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p>

 <ol>
 <li>Modify or create any LLVM IR Instructions, BasicBlocks, Arguments,
     Functions, GlobalVariables, GlobalAliases, or Modules.</li>
 <li>Modify a MachineFunction other than the one currently being processed.</li>
 <li>Maintain state across invocations of <a
 href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global
 data)</li>
 </ol>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
   &amp;MF)</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual bool</b> runOnMachineFunction(MachineFunction &amp;MF) = 0;
 </pre></div>

 <p><tt>runOnMachineFunction</tt> can be considered the main entry point of a
 <tt>MachineFunctionPass</tt>; that is, you should override this method to do the
 work of your <tt>MachineFunctionPass</tt>.</p>

 <p>The <tt>runOnMachineFunction</tt> method is called on every
 <tt>MachineFunction</tt> in a <tt>Module</tt>, so that the
 <tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent
 representation of the function. If you want to get at the LLVM <tt>Function</tt>
 for the <tt>MachineFunction</tt> you're working on, use
 <tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but
 remember, you may not modify the LLVM <tt>Function</tt> or its contents from a
 <tt>MachineFunctionPass</tt>.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="registration">Pass registration</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how
 pass registration works, and discussed some of the reasons that it is used and
 what it does.  Here we discuss how and why passes are registered.</p>

 <p>As we saw above, passes are registered with the <b><tt>INITIALIZE_PASS</tt></b>
 macro.  The first parameter is the name of the pass that is to be used on
 the command line to specify that the pass should be added to a program (for
 example, with <tt>opt</tt> or <tt>bugpoint</tt>).  The second argument is the
 name of the pass, which is to be used for the <tt>-help</tt> output of
 programs, as
 well as for debug output generated by the <tt>--debug-pass</tt> option.</p>

 <p>If you want your pass to be easily dumpable, you should
 implement the virtual <tt>print</tt> method:</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="print">The <tt>print</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual void</b> print(std::ostream &amp;O, <b>const</b> Module *M) <b>const</b>;
 </pre></div>

 <p>The <tt>print</tt> method must be implemented by "analyses" in order to print
 a human readable version of the analysis results.  This is useful for debugging
 an analysis itself, as well as for other people to figure out how an analysis
 works.  Use the <tt>opt -analyze</tt> argument to invoke this method.</p>

 <p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on,
 and the <tt>Module</tt> parameter gives a pointer to the top level module of the
 program that has been analyzed.  Note however that this pointer may be null in
 certain circumstances (such as calling the <tt>Pass::dump()</tt> from a
 debugger), so it should only be used to enhance debug output, it should not be
 depended on.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="interaction">Specifying interactions between passes</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>One of the main responsibilities of the <tt>PassManager</tt> is to make sure
 that passes interact with each other correctly.  Because <tt>PassManager</tt>
 tries to <a href="#passmanager">optimize the execution of passes</a> it must
 know how the passes interact with each other and what dependencies exist between
 the various passes.  To track this, each pass can declare the set of passes that
 are required to be executed before the current pass, and the passes which are
 invalidated by the current pass.</p>

 <p>Typically this functionality is used to require that analysis results are
 computed before your pass is run.  Running arbitrary transformation passes can
 invalidate the computed analysis results, which is what the invalidation set
 specifies.  If a pass does not implement the <tt><a
 href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not
 having any prerequisite passes, and invalidating <b>all</b> other passes.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual void</b> getAnalysisUsage(AnalysisUsage &amp;Info) <b>const</b>;
 </pre></div>

 <p>By implementing the <tt>getAnalysisUsage</tt> method, the required and
 invalidated sets may be specified for your transformation.  The implementation
 should fill in the <tt><a
 href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
 object with information about which passes are required and not invalidated.  To
 do this, a pass may call any of the following methods on the AnalysisUsage
 object:</p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="AU::addRequired">The <tt>AnalysisUsage::addRequired&lt;&gt;</tt> and <tt>AnalysisUsage::addRequiredTransitive&lt;&gt;</tt> methods</a>
 </div>

 <div class="doc_text">
 <p>
 If your pass requires a previous pass to be executed (an analysis for example),
 it can use one of these methods to arrange for it to be run before your pass.
 LLVM has many different types of analyses and passes that can be required,
 spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>.
 Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will
 be no critical edges in the CFG when your pass has been run.
 </p>

 <p>
 Some analyses chain to other analyses to do their job.  For example, an <a
 href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a
 href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes.  In
 cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be
 used instead of the <tt>addRequired</tt> method.  This informs the PassManager
 that the transitively required pass should be alive as long as the requiring
 pass is.
 </p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved&lt;&gt;</tt> method</a>
 </div>

 <div class="doc_text">
 <p>
 One of the jobs of the PassManager is to optimize how and when analyses are run.
 In particular, it attempts to avoid recomputing data unless it needs to.  For
 this reason, passes are allowed to declare that they preserve (i.e., they don't
 invalidate) an existing analysis if it's available.  For example, a simple
 constant folding pass would not modify the CFG, so it can't possibly affect the
 results of dominator analysis.  By default, all passes are assumed to invalidate
 all others.
 </p>

 <p>
 The <tt>AnalysisUsage</tt> class provides several methods which are useful in
 certain circumstances that are related to <tt>addPreserved</tt>.  In particular,
 the <tt>setPreservesAll</tt> method can be called to indicate that the pass does
 not modify the LLVM program at all (which is true for analyses), and the
 <tt>setPreservesCFG</tt> method can be used by transformations that change
 instructions in the program but do not modify the CFG or terminator instructions
 (note that this property is implicitly set for <a
 href="#BasicBlockPass">BasicBlockPass</a>'s).
 </p>

 <p>
 <tt>addPreserved</tt> is particularly useful for transformations like
 <tt>BreakCriticalEdges</tt>.  This pass knows how to update a small set of loop
 and dominator related analyses if they exist, so it can preserve them, despite
 the fact that it hacks on the CFG.
 </p>
 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <i>// This is an example implementation from an analysis, which does not modify
   // the program at all, yet has a prerequisite.</i>
   <b>void</b> <a href="http://llvm.org/doxygen/classllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
     AU.setPreservesAll();
     AU.addRequired&lt;<a href="http://llvm.org/doxygen/classllvm_1_1PostDominatorTree.html">PostDominatorTree</a>&gt;();
   }
 </pre></div>

 <p>and:</p>

 <div class="doc_code"><pre>
   <i>// This example modifies the program, but does not modify the CFG</i>
   <b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
     AU.setPreservesCFG();
     AU.addRequired&lt;<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>&gt;();
   }
 </pre></div>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="getAnalysis">The <tt>getAnalysis&lt;&gt;</tt> and
 <tt>getAnalysisIfAvailable&lt;&gt;</tt> methods</a>
 </div>

 <div class="doc_text">

 <p>The <tt>Pass::getAnalysis&lt;&gt;</tt> method is automatically inherited by
 your class, providing you with access to the passes that you declared that you
 required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
 method.  It takes a single template argument that specifies which pass class you
 want, and returns a reference to that pass.  For example:</p>

 <div class="doc_code"><pre>
    bool LICM::runOnFunction(Function &amp;F) {
      LoopInfo &amp;LI = getAnalysis&lt;LoopInfo&gt;();
      ...
    }
 </pre></div>

 <p>This method call returns a reference to the pass desired.  You may get a
 runtime assertion failure if you attempt to get an analysis that you did not
 declare as required in your <a
 href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation.  This
 method can be called by your <tt>run*</tt> method implementation, or by any
 other local method invoked by your <tt>run*</tt> method.

 A module level pass can use function level analysis info using this interface.
 For example:</p>

 <div class="doc_code"><pre>
    bool ModuleLevelPass::runOnModule(Module &amp;M) {
      ...
      DominatorTree &amp;DT = getAnalysis&lt;DominatorTree&gt;(Func);
      ...
    }
 </pre></div>

 <p>In above example, runOnFunction for DominatorTree is called by pass manager
 before returning a reference to the desired pass.</p>

 <p>
 If your pass is capable of updating analyses if they exist (e.g.,
 <tt>BreakCriticalEdges</tt>, as described above), you can use the
 <tt>getAnalysisIfAvailable</tt> method, which returns a pointer to the analysis
 if it is active.  For example:</p>

 <div class="doc_code"><pre>
   ...
   if (DominatorSet *DS = getAnalysisIfAvailable&lt;DominatorSet&gt;()) {
     <i>// A DominatorSet is active.  This code will update it.</i>
   }
   ...
 </pre></div>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="analysisgroup">Implementing Analysis Groups</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>Now that we understand the basics of how passes are defined, how they are
 used, and how they are required from other passes, it's time to get a little bit
 fancier.  All of the pass relationships that we have seen so far are very
 simple: one pass depends on one other specific pass to be run before it can run.
 For many applications, this is great, for others, more flexibility is
 required.</p>

 <p>In particular, some analyses are defined such that there is a single simple
 interface to the analysis results, but multiple ways of calculating them.
 Consider alias analysis for example.  The most trivial alias analysis returns
 "may alias" for any alias query.  The most sophisticated analysis a
 flow-sensitive, context-sensitive interprocedural analysis that can take a
 significant amount of time to execute (and obviously, there is a lot of room
 between these two extremes for other implementations).  To cleanly support
 situations like this, the LLVM Pass Infrastructure supports the notion of
 Analysis Groups.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="agconcepts">Analysis Group Concepts</a>
 </div>

 <div class="doc_text">

 <p>An Analysis Group is a single simple interface that may be implemented by
 multiple different passes.  Analysis Groups can be given human readable names
 just like passes, but unlike passes, they need not derive from the <tt>Pass</tt>
 class.  An analysis group may have one or more implementations, one of which is
 the "default" implementation.</p>

 <p>Analysis groups are used by client passes just like other passes are: the
 <tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods.
 In order to resolve this requirement, the <a href="#passmanager">PassManager</a>
 scans the available passes to see if any implementations of the analysis group
 are available.  If none is available, the default implementation is created for
 the pass to use.  All standard rules for <A href="#interaction">interaction
 between passes</a> still apply.</p>

 <p>Although <a href="#registration">Pass Registration</a> is optional for normal
 passes, all analysis group implementations must be registered, and must use the
 <A href="#registerag"><tt>INITIALIZE_AG_PASS</tt></a> template to join the
 implementation pool.  Also, a default implementation of the interface
 <b>must</b> be registered with <A
 href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p>

 <p>As a concrete example of an Analysis Group in action, consider the <a
 href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
 analysis group.  The default implementation of the alias analysis interface (the
 <tt><a
 href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
 pass) just does a few simple checks that don't require significant analysis to
 compute (such as: two different globals can never alias each other, etc).
 Passes that use the <tt><a
 href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
 interface (for example the <tt><a
 href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do
 not care which implementation of alias analysis is actually provided, they just
 use the designated interface.</p>

 <p>From the user's perspective, commands work just like normal.  Issuing the
 command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be
 instantiated and added to the pass sequence.  Issuing the command '<tt>opt
 -somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the
 <tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a
 hypothetical example) instead.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
 </div>

 <div class="doc_text">

 <p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
 group itself, while the <tt>INITIALIZE_AG_PASS</tt> is used to add pass
 implementations to the analysis group.  First,
 an analysis group should be registered, with a human readable name
 provided for it.
 Unlike registration of passes, there is no command line argument to be specified
 for the Analysis Group Interface itself, because it is "abstract":</p>

 <div class="doc_code"><pre>
   <b>static</b> RegisterAnalysisGroup&lt;<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>&gt; A("<i>Alias Analysis</i>");
 </pre></div>

 <p>Once the analysis is registered, passes can declare that they are valid
 implementations of the interface by using the following code:</p>

 <div class="doc_code"><pre>
 <b>namespace</b> {
   //<i> Declare that we implement the AliasAnalysis interface</i>
   INITIALIZE_AG_PASS(FancyAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>somefancyaa</i>",
                      "<i>A more complex alias analysis implementation</i>",
                      false, // <i>Is CFG Only?</i>
                      true,  // <i>Is Analysis?</i>
                      false, // <i>Is default Analysis Group implementation?</i>
                     );
 }
 </pre></div>

 <p>This just shows a class <tt>FancyAA</tt> that
 uses the <tt>INITIALIZE_AG_PASS</tt> macro both to register and
 to "join" the <tt><a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
 analysis group.  Every implementation of an analysis group should join using
 this macro.</p>

 <div class="doc_code"><pre>
 <b>namespace</b> {
   //<i> Declare that we implement the AliasAnalysis interface</i>
   INITIALIZE_AG_PASS(BasicAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>basicaa</i>",
                      "<i>Basic Alias Analysis (default AA impl)</i>",
                      false, // <i>Is CFG Only?</i>
                      true,  // <i>Is Analysis?</i>
                      true, // <i>Is default Analysis Group implementation?</i>
                     );
 }
 </pre></div>

 <p>Here we show how the default implementation is specified (using the final
 argument to the <tt>INITIALIZE_AG_PASS</tt> template).  There must be exactly
 one default implementation available at all times for an Analysis Group to be
 used.  Only default implementation can derive from <tt>ImmutablePass</tt>.
 Here we declare that the
  <tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
 pass is the default implementation for the interface.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="passStatistics">Pass Statistics</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">
 <p>The <a
 href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a>
 class is designed to be an easy way to expose various success
 metrics from passes.  These statistics are printed at the end of a
 run, when the -stats command line option is enabled on the command
 line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details.

 </div>


 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="passmanager">What PassManager does</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>The <a
 href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
 <a
 href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a>
 takes a list of passes, ensures their <a href="#interaction">prerequisites</a>
 are set up correctly, and then schedules passes to run efficiently.  All of the
 LLVM tools that run passes use the <tt>PassManager</tt> for execution of these
 passes.</p>

 <p>The <tt>PassManager</tt> does two main things to try to reduce the execution
 time of a series of passes:</p>

 <ol>
 <li><b>Share analysis results</b> - The PassManager attempts to avoid
 recomputing analysis results as much as possible.  This means keeping track of
 which analyses are available already, which analyses get invalidated, and which
 analyses are needed to be run for a pass.  An important part of work is that the
 <tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing
 it to <a href="#releaseMemory">free memory</a> allocated to holding analysis
 results as soon as they are no longer needed.</li>

 <li><b>Pipeline the execution of passes on the program</b> - The
 <tt>PassManager</tt> attempts to get better cache and memory usage behavior out
 of a series of passes by pipelining the passes together.  This means that, given
 a series of consequtive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
 will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on
 the first function, then all of the <a
 href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function,
 etc... until the entire program has been run through the passes.

 <p>This improves the cache behavior of the compiler, because it is only touching
 the LLVM program representation for a single function at a time, instead of
 traversing the entire program.  It reduces the memory consumption of compiler,
 because, for example, only one <a
 href="http://llvm.org/doxygen/classllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a>
 needs to be calculated at a time.  This also makes it possible to implement
 some <a
 href="#SMP">interesting enhancements</a> in the future.</p></li>

 </ol>

 <p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how
 much information it has about the behaviors of the passes it is scheduling.  For
 example, the "preserved" set is intentionally conservative in the face of an
 unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method.
 Not implementing when it should be implemented will have the effect of not
 allowing any analysis results to live across the execution of your pass.</p>

 <p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line
 options that is useful for debugging pass execution, seeing how things work, and
 diagnosing when you should be preserving more analyses than you currently are
 (To get information about all of the variants of the <tt>--debug-pass</tt>
 option, just type '<tt>opt -help-hidden</tt>').</p>

 <p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see
 how our <a href="#basiccode">Hello World</a> pass interacts with other passes.
 Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p>

 <div class="doc_code"><pre>
 $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -licm --debug-pass=Structure &lt; hello.bc &gt; /dev/null
 Module Pass Manager
   Function Pass Manager
     Dominator Set Construction
     Immediate Dominators Construction
     Global Common Subexpression Elimination
 --  Immediate Dominators Construction
 --  Global Common Subexpression Elimination
     Natural Loop Construction
     Loop Invariant Code Motion
 --  Natural Loop Construction
 --  Loop Invariant Code Motion
     Module Verifier
 --  Dominator Set Construction
 --  Module Verifier
   Bitcode Writer
 --Bitcode Writer
 </pre></div>

 <p>This output shows us when passes are constructed and when the analysis
 results are known to be dead (prefixed with '<tt>--</tt>').  Here we see that
 GCSE uses dominator and immediate dominator information to do its job.  The LICM
 pass uses natural loop information, which uses dominator sets, but not immediate
 dominators.  Because immediate dominators are no longer useful after the GCSE
 pass, it is immediately destroyed.  The dominator sets are then reused to
 compute natural loop information, which is then used by the LICM pass.</p>

 <p>After the LICM pass, the module verifier runs (which is automatically added
 by the '<tt>opt</tt>' tool), which uses the dominator set to check that the
 resultant LLVM code is well formed.  After it finishes, the dominator set
 information is destroyed, after being computed once, and shared by three
 passes.</p>

 <p>Lets see how this changes when we run the <a href="#basiccode">Hello
 World</a> pass in between the two passes:</p>

 <div class="doc_code"><pre>
 $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure &lt; hello.bc &gt; /dev/null
 Module Pass Manager
   Function Pass Manager
     Dominator Set Construction
     Immediate Dominators Construction
     Global Common Subexpression Elimination
 <b>--  Dominator Set Construction</b>
 --  Immediate Dominators Construction
 --  Global Common Subexpression Elimination
 <b>    Hello World Pass
 --  Hello World Pass
     Dominator Set Construction</b>
     Natural Loop Construction
     Loop Invariant Code Motion
 --  Natural Loop Construction
 --  Loop Invariant Code Motion
     Module Verifier
 --  Dominator Set Construction
 --  Module Verifier
   Bitcode Writer
 --Bitcode Writer
 Hello: __main
 Hello: puts
 Hello: main
 </pre></div>

 <p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
 Dominator Set pass, even though it doesn't modify the code at all!  To fix this,
 we need to add the following <a
 href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p>

 <div class="doc_code"><pre>
     <i>// We don't modify the program, so we preserve all analyses</i>
     <b>virtual void</b> getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
       AU.setPreservesAll();
     }
 </pre></div>

 <p>Now when we run our pass, we get this output:</p>

 <div class="doc_code"><pre>
 $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure &lt; hello.bc &gt; /dev/null
 Pass Arguments:  -gcse -hello -licm
 Module Pass Manager
   Function Pass Manager
     Dominator Set Construction
     Immediate Dominators Construction
     Global Common Subexpression Elimination
 --  Immediate Dominators Construction
 --  Global Common Subexpression Elimination
     Hello World Pass
 --  Hello World Pass
     Natural Loop Construction
     Loop Invariant Code Motion
 --  Loop Invariant Code Motion
 --  Natural Loop Construction
     Module Verifier
 --  Dominator Set Construction
 --  Module Verifier
   Bitcode Writer
 --Bitcode Writer
 Hello: __main
 Hello: puts
 Hello: main
 </pre></div>

 <p>Which shows that we don't accidentally invalidate dominator information
 anymore, and therefore do not have to compute it twice.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="releaseMemory">The <tt>releaseMemory</tt> method</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><pre>
   <b>virtual void</b> releaseMemory();
 </pre></div>

 <p>The <tt>PassManager</tt> automatically determines when to compute analysis
 results, and how long to keep them around for.  Because the lifetime of the pass
 object itself is effectively the entire duration of the compilation process, we
 need some way to free analysis results when they are no longer useful.  The
 <tt>releaseMemory</tt> virtual method is the way to do this.</p>

 <p>If you are writing an analysis or any other pass that retains a significant
 amount of state (for use by another pass which "requires" your pass and uses the
 <a href="#getAnalysis">getAnalysis</a> method) you should implement
 <tt>releaseMemory</tt> to, well, release the memory allocated to maintain this
 internal state.  This method is called after the <tt>run*</tt> method for the
 class, before the next call of <tt>run*</tt> in your pass.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="registering">Registering dynamically loaded passes</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p><i>Size matters</i> when constructing production quality tools using llvm,
 both for the purposes of distribution, and for regulating the resident code size
 when running on the target system. Therefore, it becomes desirable to
 selectively use some passes, while omitting others and maintain the flexibility
 to change configurations later on. You want to be able to do all this, and,
 provide feedback to the user. This is where pass registration comes into
 play.</p>

 <p>The fundamental mechanisms for pass registration are the
 <tt>MachinePassRegistry</tt> class and subclasses of
 <tt>MachinePassRegistryNode</tt>.</p>

 <p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of
 <tt>MachinePassRegistryNode</tt> objects.  This instance maintains the list and
 communicates additions and deletions to the command line interface.</p>

 <p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain
 information provided about a particular pass.  This information includes the
 command line name, the command help string and the address of the function used
 to create an instance of the pass.  A global static constructor of one of these
 instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>,
 the static destructor <i>unregisters</i>. Thus a pass that is statically linked
 in the tool will be registered at start up. A dynamically loaded pass will
 register on load and unregister at unload.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsection">
   <a name="registering_existing">Using existing registries</a>
 </div>

 <div class="doc_text">

 <p>There are predefined registries to track instruction scheduling
 (<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>)
 machine passes.  Here we will describe how to <i>register</i> a register
 allocator machine pass.</p>

 <p>Implement your register allocator machine pass.  In your register allocator
 .cpp file add the following include;</p>

 <div class="doc_code"><pre>
   #include "llvm/CodeGen/RegAllocRegistry.h"
 </pre></div>

 <p>Also in your register allocator .cpp file, define a creator function in the
 form; </p>

 <div class="doc_code"><pre>
   FunctionPass *createMyRegisterAllocator() {
     return new MyRegisterAllocator();
   }
 </pre></div>

 <p>Note that the signature of this function should match the type of
 <tt>RegisterRegAlloc::FunctionPassCtor</tt>.  In the same file add the
 "installing" declaration, in the form;</p>

 <div class="doc_code"><pre>
   static RegisterRegAlloc myRegAlloc("myregalloc",
     "  my register allocator help string",
     createMyRegisterAllocator);
 </pre></div>

 <p>Note the two spaces prior to the help string produces a tidy result on the
 -help query.</p>

 <div class="doc_code"><pre>
 $ llc -help
   ...
   -regalloc                    - Register allocator to use (default=linearscan)
     =linearscan                -   linear scan register allocator
     =local                     -   local register allocator
     =simple                    -   simple register allocator
     =myregalloc                -   my register allocator help string
   ...
 </pre></div>

 <p>And that's it.  The user is now free to use <tt>-regalloc=myregalloc</tt> as
 an option.  Registering instruction schedulers is similar except use the
 <tt>RegisterScheduler</tt> class.  Note that the
 <tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from
 <tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p>

 <p>To force the load/linking of your register allocator into the llc/lli tools,
 add your creator function's global declaration to "Passes.h" and add a "pseudo"
 call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p>

 </div>


 <!-- _______________________________________________________________________ -->
 <div class="doc_subsection">
   <a name="registering_new">Creating new registries</a>
 </div>

 <div class="doc_text">

 <p>The easiest way to get started is to clone one of the existing registries; we
 recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>.  The key things to modify
 are the class name and the <tt>FunctionPassCtor</tt> type.</p>

 <p>Then you need to declare the registry.  Example: if your pass registry is
 <tt>RegisterMyPasses</tt> then define;</p>

 <div class="doc_code"><pre>
 MachinePassRegistry RegisterMyPasses::Registry;
 </pre></div>

 <p>And finally, declare the command line option for your passes.  Example:</p>

 <div class="doc_code"><pre>
   cl::opt&lt;RegisterMyPasses::FunctionPassCtor, false,
           RegisterPassParser&lt;RegisterMyPasses&gt; &gt;
   MyPassOpt("mypass",
             cl::init(&amp;createDefaultMyPass),
             cl::desc("my pass option help"));
 </pre></div>

 <p>Here the command option is "mypass", with createDefaultMyPass as the default
 creator.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="debughints">Using GDB with dynamically loaded passes</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it
 should be.  First of all, you can't set a breakpoint in a shared object that has
 not been loaded yet, and second of all there are problems with inlined functions
 in shared objects.  Here are some suggestions to debugging your pass with
 GDB.</p>

 <p>For sake of discussion, I'm going to assume that you are debugging a
 transformation invoked by <tt>opt</tt>, although nothing described here depends
 on that.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="breakpoint">Setting a breakpoint in your pass</a>
 </div>

 <div class="doc_text">

 <p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>

 <div class="doc_code"><pre>
 $ <b>gdb opt</b>
 GNU gdb 5.0
 Copyright 2000 Free Software Foundation, Inc.
 GDB is free software, covered by the GNU General Public License, and you are
 welcome to change it and/or distribute copies of it under certain conditions.
 Type "show copying" to see the conditions.
 There is absolutely no warranty for GDB.  Type "show warranty" for details.
 This GDB was configured as "sparc-sun-solaris2.6"...
 (gdb)
 </pre></div>

 <p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
 time to load.  Be patient.  Since we cannot set a breakpoint in our pass yet
 (the shared object isn't loaded until runtime), we must execute the process, and
 have it stop before it invokes our pass, but after it has loaded the shared
 object.  The most foolproof way of doing this is to set a breakpoint in
 <tt>PassManager::run</tt> and then run the process with the arguments you
 want:</p>

 <div class="doc_code"><pre>
 (gdb) <b>break llvm::PassManager::run</b>
 Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
 (gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]</b>
 Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]
 Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
 70      bool PassManager::run(Module &amp;M) { return PM-&gt;run(M); }
 (gdb)
 </pre></div>

 <p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are
 now free to set breakpoints in your pass so that you can trace through execution
 or do other standard debugging stuff.</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="debugmisc">Miscellaneous Problems</a>
 </div>

 <div class="doc_text">

 <p>Once you have the basics down, there are a couple of problems that GDB has,
 some with solutions, some without.</p>

 <ul>
 <li>Inline functions have bogus stack information.  In general, GDB does a
 pretty good job getting stack traces and stepping through inline functions.
 When a pass is dynamically loaded however, it somehow completely loses this
 capability.  The only solution I know of is to de-inline a function (move it
 from the body of a class to a .cpp file).</li>

 <li>Restarting the program breaks breakpoints.  After following the information
 above, you have succeeded in getting some breakpoints planted in your pass.  Nex
 thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
 and you start getting errors about breakpoints being unsettable.  The only way I
 have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
 already set in your pass, run the program, and re-set the breakpoints once
 execution stops in <tt>PassManager::run</tt>.</li>

 </ul>

 <p>Hopefully these tips will help with common case debugging situations.  If
 you'd like to contribute some tips of your own, just contact <a
 href="mailto:sabre@nondot.org">Chris</a>.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="future">Future extensions planned</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>Although the LLVM Pass Infrastructure is very capable as it stands, and does
 some nifty stuff, there are things we'd like to add in the future.  Here is
 where we are going:</p>

 </div>

 <!-- _______________________________________________________________________ -->
 <div class="doc_subsubsection">
   <a name="SMP">Multithreaded LLVM</a>
 </div>

 <div class="doc_text">

 <p>Multiple CPU machines are becoming more common and compilation can never be
 fast enough: obviously we should allow for a multithreaded compiler.  Because of
 the semantics defined for passes above (specifically they cannot maintain state
 across invocations of their <tt>run*</tt> methods), a nice clean way to
 implement a multithreaded compiler would be for the <tt>PassManager</tt> class
 to create multiple instances of each pass object, and allow the separate
 instances to be hacking on different parts of the program at the same time.</p>

 <p>This implementation would prevent each of the passes from having to implement
 multithreaded constructs, requiring only the LLVM core to have locking in a few
 places (for global resources).  Although this is a simple extension, we simply
 haven't had time (or multiprocessor machines, thus a reason) to implement this.
 Despite that, we have kept the LLVM passes SMP ready, and you should too.</p>

 </div>

 <!-- *********************************************************************** -->
 <hr>
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