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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Writing an LLVM Pass</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<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>
or <tt>analyze</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(Module
&)</tt> method</a></li>
<li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li>
<li><a href="#doFinalization_scc">The <tt>doFinalization(Module
&)</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
&)</tt> method</a></li>
<li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li>
<li><a href="#doFinalization_mod">The <tt>doFinalization(Module
&)</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
&)</tt> method</a></li>
<li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt>
method</a></li>
<li><a href="#doFinalization_fn">The <tt>doFinalization(Function
&)</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 &)</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<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a></li>
<li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a></li>
<li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li>
<li><a href="#getAnalysis">The <tt>getAnalysis<></tt> and <tt>getAnalysisToUpdate<></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="#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>
<li><a href="#PassFunctionPass"><tt>ModulePass</tt>es requiring
<tt>FunctionPass</tt>es</a></li>
</ul></li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:[email protected]">Chris Lattner</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="#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/>
<pre>
# Makefile for hello pass
# Path to top level of LLVM heirarchy
LEVEL = ../../..
# Name of the library to build
LIBRARYNAME = Hello
# Build a dynamically linkable shared object
SHARED_LIBRARY = 1
# 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>
<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/lib/Hello.so</tt> shared object that can be dynamically loaded by
the <tt>opt</tt> or <tt>analyze</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>
<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>"
</pre>
<p>Which are needed because we are writing a <tt><a
href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>, and
we are operating on <tt><a
href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s.</p>
<p>Next we have:</p>
<pre>
<b>using namespace llvm;</b>
</pre>
<p>... which is required because the functions from the include files
live in the llvm namespace.
</p>
<p>Next we have:</p>
<pre>
<b>namespace</b> {
</pre>
<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>
<pre>
<b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
</pre><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>
<pre>
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
std::cerr << "<i>Hello: </i>" << F.getName() << "\n";
<b>return false</b>;
}
}; <i>// end of struct Hello</i>
</pre>
<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>
<pre>
RegisterOpt<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
} <i>// end of anonymous namespace</i>
</pre>
<p>Lastly, we register our class <tt>Hello</tt>, giving it a command line
argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>". There are
several different ways of <a href="#registration">registering your pass</a>,
depending on what it is to be used for. For "optimizations" we use the
<tt>RegisterOpt</tt> template.</p>
<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
<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>using namespace llvm;</b>
<b>namespace</b> {
<b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
std::cerr << "<i>Hello: </i>" << F.getName() << "\n";
<b>return false</b>;
}
};
RegisterOpt<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
}
</pre>
<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/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> or <tt>analyze</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>RegisterOpt</tt> template, 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 bytecode file (<tt>hello.bc</tt>) for the program
through our transformation like this (or course, any bytecode file will
work):</p>
<pre>
$ opt -load ../../../Debug/lib/Hello.so -hello < hello.bc > /dev/null
Hello: __main
Hello: puts
Hello: main
</pre>
<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>
<pre>
$ opt -load ../../../Debug/lib/Hello.so --help
OVERVIEW: llvm .bc -> .bc modular optimizer
USAGE: opt [options] <input bytecode>
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>
<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>
<pre>
$ opt -load ../../../Debug/lib/Hello.so -hello -time-passes < hello.bc > /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%) Bytecode 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>
<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 unneccesarily
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,
refering 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>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">
<pre>
<b>virtual bool</b> runOnModule(Module &M) = 0;
</pre>
<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>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(Module &)</tt>
method</a>
</div>
<div class="doc_text">
<pre>
<b>virtual bool</b> doInitialization(Module &M);
</pre>
<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">
<pre>
<b>virtual bool</b> runOnSCC(const std::vector<CallGraphNode *> &SCCM) = 0;
</pre>
<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(Module
&)</tt> method</a>
</div>
<div class="doc_text">
<pre>
<b>virtual bool</b> doFinalization(Module &M);
</pre>
<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 &)</tt>
method</a>
</div>
<div class="doc_text">
<pre>
<b>virtual bool</b> doInitialization(Module &M);
</pre>
<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">
<pre>
<b>virtual bool</b> runOnFunction(Function &F) = 0;
</pre><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
&)</tt> method</a>
</div>
<div class="doc_text">
<pre>
<b>virtual bool</b> doFinalization(Module &M);
</pre>
<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="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 &)</tt></a> and <a
href="#doFinalization_mod"><tt>doFinalization(Module &)</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
&)</tt> method</a>
</div>
<div class="doc_text">
<pre>
<b>virtual bool</b> doInitialization(Function &F);
</pre>
<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">
<pre>
<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0;
</pre>
<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 &)</tt>
method</a>
</div>
<div class="doc_text">
<pre>
<b>virtual bool</b> doFinalization(Function &F);
</pre>
<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. 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 any LLVM Instructions, BasicBlocks or Functions.</li>
<li>Modify a MachineFunction other than the one currently being processed.</li>
<li>Add or remove MachineFunctions from the current Module.</li>
<li>Add or remove global variables from the current Module.</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
&MF)</tt> method</a>
</div>
<div class="doc_text">
<pre>
<b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0;
</pre>
<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>Passes can be registered in several different ways. Depending on the general
classification of the pass, you should use one of the following templates to
register the pass:</p>
<ul>
<li><b><tt>RegisterOpt</tt></b> - This template should be used when you are
registering a pass that logically should be available for use in the
'<tt>opt</tt>' utility.</li>
<li><b><tt>RegisterAnalysis</tt></b> - This template should be used when you are
registering a pass that logically should be available for use in the
'<tt>analyze</tt>' utility.</li>
<li><b><tt>RegisterPass</tt></b> - This is the generic form of the
<tt>Register*</tt> templates that should be used if you want your pass listed by
multiple or no utilities. This template takes an extra third argument that
specifies which tools it should be listed in. See the <a
href="http://llvm.org/doxygen/PassSupport_8h-source.html">PassSupport.h</a>
file for more information.</li>
</ul>
<p>Regardless of how you register your pass, you must specify at least two
parameters. 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 <tt>opt</tt> or <tt>analyze</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 a pass is registered to be used by the <tt>analyze</tt> utility, 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">
<pre>
<b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>;
</pre>
<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. The <tt>analyze</tt> tool uses this method to generate its output.</p>
<p>The <tt>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 the 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">
<pre>
<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>;
</pre>
<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<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></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<></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.