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4<head>
5 <title>Writing an LLVM Pass</title>
6 <link rel="stylesheet" href="llvm.css" type="text/css">
7</head>
8<body>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]9
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]10<div class="doc_title">
11 Writing an LLVM Pass
12</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]13
14<ol>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]15 <li><a href="#introduction">Introduction - What is a pass?</a></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]16 <li><a href="#quickstart">Quick Start - Writing hello world</a>
17 <ul>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]18 <li><a href="#makefile">Setting up the build environment</a></li>
19 <li><a href="#basiccode">Basic code required</a></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]20 <li><a href="#running">Running a pass with <tt>opt</tt>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]21 or <tt>analyze</tt></a></li>
22 </ul></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]23 <li><a href="#passtype">Pass classes and requirements</a>
24 <ul>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]25 <li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]26 <li><a href="#Pass">The <tt>Pass</tt> class</a>
27 <ul>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]28 <li><a href="#run">The <tt>run</tt> method</a></li>
29 </ul></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]30 <li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a>
31 <ul>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]32 <li><a href="#doInitialization_mod">The <tt>doInitialization(Module
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]33 &amp;)</tt> method</a></li>
34 <li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]35 <li><a href="#doFinalization_mod">The <tt>doFinalization(Module
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]36 &amp;)</tt> method</a></li>
37 </ul></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]38 <li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
39 <ul>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]40 <li><a href="#doInitialization_fn">The <tt>doInitialization(Function
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]41 &amp;)</tt> method</a></li>
42 <li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt>
43 method</a></li>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]44 <li><a href="#doFinalization_fn">The <tt>doFinalization(Function
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]45 &amp;)</tt> method</a></li>
46 </ul></li>
Brian Gaekecab8b6f2003-07-17 18:53:20 +0000[diff] [blame]47 <li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt>
48 class</a>
Brian Gaeke6a33f362003-07-22 20:53:20 +0000[diff] [blame]49 <ul>
50 <li><a href="#runOnMachineFunction">The
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]51 <tt>runOnMachineFunction(MachineFunction &amp;)</tt> method</a></li>
52 </ul></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]53 </ul>
54 <li><a href="#registration">Pass Registration</a>
55 <ul>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]56 <li><a href="#print">The <tt>print</tt> method</a></li>
57 </ul></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]58 <li><a href="#interaction">Specifying interactions between passes</a>
59 <ul>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]60 <li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt>
61 method</a></li>
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]62 <li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired&lt;&gt;</tt> and <tt>AnalysisUsage::addRequiredTransitive&lt;&gt;</tt> methods</a></li>
63 <li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved&lt;&gt;</tt> method</a></li>
64 <li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li>
65 <li><a href="#getAnalysis">The <tt>getAnalysis&lt;&gt;</tt> and <tt>getAnalysisToUpdate&lt;&gt;</tt> methods</a></li>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]66 </ul></li>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]67 <li><a href="#analysisgroup">Implementing Analysis Groups</a>
68 <ul>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]69 <li><a href="#agconcepts">Analysis Group Concepts</a></li>
70 <li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li>
71 </ul></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]72 <li><a href="#passmanager">What PassManager does</a>
73 <ul>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]74 <li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li>
75 </ul></li>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]76 <li><a href="#debughints">Using GDB with dynamically loaded passes</a>
77 <ul>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]78 <li><a href="#breakpoint">Setting a breakpoint in your pass</a></li>
79 <li><a href="#debugmisc">Miscellaneous Problems</a></li>
80 </ul></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]81 <li><a href="#future">Future extensions planned</a>
82 <ul>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]83 <li><a href="#SMP">Multithreaded LLVM</a></li>
84 <li><a href="#ModuleSource">A new <tt>ModuleSource</tt> interface</a></li>
85 <li><a href="#PassFunctionPass"><tt>Pass</tt>es requiring
86 <tt>FunctionPass</tt>es</a></li>
87 </ul></li>
88</ol>
Chris Lattner38c633d2002-08-08 20:23:41 +0000[diff] [blame]89
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]90<div class="doc_author">
91 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]92</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]93
94<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]95<div class="doc_section">
96 <a name="introduction">Introduction - What is a pass?</a>
97</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]98<!-- *********************************************************************** -->
99
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]100<div class="doc_text">
101
102<p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]103passes are where the interesting parts of the compiler exist. Passes perform
104the transformations and optimizations that make up the compiler, they build
105the analysis results that are used by these transformations, and they are, above
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]106all, a structuring technique for compiler code.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]107
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]108<p>All LLVM passes are subclasses of the <tt><a
109href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Pass.html">Pass</a></tt>
110class, which implement functionality by overriding virtual methods inherited
111from <tt>Pass</tt>. Depending on how your pass works, you may be able to
112inherit from the <tt><a
113href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1FunctionPass.html">FunctionPass</a></tt>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]114or <tt><a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]115href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1BasicBlockPass.html">BasicBlockPass</a></tt>,
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]116which gives the system more information about what your pass does, and how it
117can be combined with other passes. One of the main features of the LLVM Pass
118Framework is that it schedules passes to run in an efficient way based on the
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]119constraints that your pass has.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]120
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]121<p>We start by showing you how to construct a pass, everything from setting up
122the code, to compiling, loading, and executing it. After the basics are down,
123more advanced features are discussed.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]124
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]125</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]126
127<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]128<div class="doc_section">
129 <a name="quickstart">Quick Start - Writing hello world</a>
130</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]131<!-- *********************************************************************** -->
132
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]133<div class="doc_text">
134
135<p>Here we describe how to write the "hello world" of passes. The "Hello" pass
136is designed to simply print out the name of non-external functions that exist in
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]137the program being compiled. It does not modify the program at all, just
138inspects it. The source code and files for this pass are available in the LLVM
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]139source tree in the <tt>lib/Transforms/Hello</tt> directory.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]140
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]141</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]142
143<!-- ======================================================================= -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]144<div class="doc_subsection">
145 <a name="makefile">Setting up the build environment</a>
146</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]147
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]148<div class="doc_text">
149
150<p>First thing you need to do is create a new directory somewhere in the LLVM
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]151source base. For this example, we'll assume that you made
152"<tt>lib/Transforms/Hello</tt>". The first thing you must do is set up a build
153script (Makefile) that will compile the source code for the new pass. To do
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]154this, copy this into "<tt>Makefile</tt>":</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]155
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]156<hr>
157
158<pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]159# Makefile for hello pass
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]160
Chris Lattner17a4c3e2002-08-14 20:06:13 +0000[diff] [blame]161# Path to top level of LLVM heirarchy
Chris Lattner7ce83e52002-08-14 20:07:01 +0000[diff] [blame]162LEVEL = ../../..
Chris Lattner17a4c3e2002-08-14 20:06:13 +0000[diff] [blame]163
164# Name of the library to build
Chris Lattner7ce83e52002-08-14 20:07:01 +0000[diff] [blame]165LIBRARYNAME = hello
Chris Lattner17a4c3e2002-08-14 20:06:13 +0000[diff] [blame]166
167# Build a dynamically loadable shared object
168SHARED_LIBRARY = 1
169
170# Include the makefile implementation stuff
171include $(LEVEL)/Makefile.common
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]172</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]173
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]174<p>This makefile specifies that all of the <tt>.cpp</tt> files in the current
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]175directory are to be compiled and linked together into a
176<tt>lib/Debug/libhello.so</tt> shared object that can be dynamically loaded by
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]177the <tt>opt</tt> or <tt>analyze</tt> tools. If your operating system uses a
178suffix other than .so (such as windows of Mac OS/X), the appropriate extension
179will be used.</p>
John Criswellf9c78652004-01-26 21:26:54 +0000[diff] [blame]180
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]181<p>Now that we have the build scripts set up, we just need to write the code for
182the pass itself.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]183
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]184</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]185
186<!-- ======================================================================= -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]187<div class="doc_subsection">
188 <a name="basiccode">Basic code required</a>
189</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]190
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]191<div class="doc_text">
192
193<p>Now that we have a way to compile our new pass, we just have to write it.
194Start out with:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]195
196<pre>
197<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
198<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Function_8h-source.html">llvm/Function.h</a>"
199</pre>
200
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]201<p>Which are needed because we are writing a <tt><a
202href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Pass.html">Pass</a></tt>, and
203we are operating on <tt><a
204href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Function.html">Function</a></tt>'s.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]205
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]206<p>Next we have:</p>
Jonathan Manton65acb302004-06-30 18:10:30 +0000[diff] [blame]207<pre>
208<b>using namespace llvm;</b>
209</pre>
210<p>... which is required because the functions from the include files
211live in the llvm namespace.
212</p>
213
214<p>Next we have:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]215
216<pre>
217<b>namespace</b> {
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]218</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]219
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]220<p>... which starts out an anonymous namespace. Anonymous namespaces are to C++
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]221what the "<tt>static</tt>" keyword is to C (at global scope). It makes the
222things declared inside of the anonymous namespace only visible to the current
223file. If you're not familiar with them, consult a decent C++ book for more
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]224information.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]225
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]226<p>Next, we declare our pass itself:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]227
228<pre>
229 <b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
230</pre><p>
231
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]232<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
233href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1FunctionPass.html">FunctionPass</a></tt>.
Chris Lattnerd6ea9262002-09-09 03:48:46 +0000[diff] [blame]234The different builtin pass subclasses are described in detail <a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]235href="#passtype">later</a>, but for now, know that <a
236href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate a function at a
237time.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]238
239<pre>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]240 <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &amp;F) {
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]241 std::cerr &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
242 <b>return false</b>;
243 }
244 }; <i>// end of struct Hello</i>
245</pre>
246
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]247<p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method,
248which overloads an abstract virtual method inherited from <a
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]249href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed
250to do our thing, so we just print out our message with the name of each
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]251function.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]252
253<pre>
254 RegisterOpt&lt;Hello&gt; X("<i>hello</i>", "<i>Hello World Pass</i>");
255} <i>// end of anonymous namespace</i>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]256</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]257
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]258<p>Lastly, we register our class <tt>Hello</tt>, giving it a command line
259argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>". There are
260several different ways of <a href="#registration">registering your pass</a>,
261depending on what it is to be used for. For "optimizations" we use the
262<tt>RegisterOpt</tt> template.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]263
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]264<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]265
266<pre>
267<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
268<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Function_8h-source.html">llvm/Function.h</a>"
269
Jonathan Manton65acb302004-06-30 18:10:30 +0000[diff] [blame]270<b>using namespace llvm;</b>
271
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]272<b>namespace</b> {
273 <b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]274 <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &amp;F) {
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]275 std::cerr &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
276 <b>return false</b>;
277 }
278 };
279
280 RegisterOpt&lt;Hello&gt; X("<i>hello</i>", "<i>Hello World Pass</i>");
281}
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]282</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]283
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]284<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]285command in the local directory and you should get a new
286"<tt>lib/Debug/libhello.so</tt> file. Note that everything in this file is
287contained in an anonymous namespace: this reflects the fact that passes are self
288contained units that do not need external interfaces (although they can have
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]289them) to be useful.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]290
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]291</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]292
293<!-- ======================================================================= -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]294<div class="doc_subsection">
295 <a name="running">Running a pass with <tt>opt</tt> or <tt>analyze</tt></a>
296</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]297
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]298<div class="doc_text">
299
John Criswellf9c78652004-01-26 21:26:54 +0000[diff] [blame]300<p>Now that you have a brand new shiny shared object file, we can use the
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]301<tt>opt</tt> command to run an LLVM program through your pass. Because you
302registered your pass with the <tt>RegisterOpt</tt> template, you will be able to
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]303use the <tt>opt</tt> tool to access it, once loaded.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]304
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]305<p>To test it, follow the example at the end of the <a
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]306href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
307LLVM. We can now run the bytecode file (<tt>hello.bc</tt>) for the program
308through our transformation like this (or course, any bytecode file will
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]309work):</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]310
311<pre>
312$ opt -load ../../../lib/Debug/libhello.so -hello &lt; hello.bc &gt; /dev/null
313Hello: __main
314Hello: puts
315Hello: main
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]316</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]317
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]318<p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your
319pass as a shared object, which makes '<tt>-hello</tt>' a valid command line
320argument (which is one reason you need to <a href="#registration">register your
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]321pass</a>). Because the hello pass does not modify the program in any
322interesting way, we just throw away the result of <tt>opt</tt> (sending it to
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]323<tt>/dev/null</tt>).</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]324
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]325<p>To see what happened to the other string you registered, try running
326<tt>opt</tt> with the <tt>--help</tt> option:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]327
328<pre>
329$ opt -load ../../../lib/Debug/libhello.so --help
330OVERVIEW: llvm .bc -&gt; .bc modular optimizer
331
332USAGE: opt [options] &lt;input bytecode&gt;
333
334OPTIONS:
335 Optimizations available:
336...
337 -funcresolve - Resolve Functions
338 -gcse - Global Common Subexpression Elimination
339 -globaldce - Dead Global Elimination
340 <b>-hello - Hello World Pass</b>
Chris Lattner065a6162003-09-10 05:29:43 +0000[diff] [blame]341 -indvars - Canonicalize Induction Variables
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]342 -inline - Function Integration/Inlining
343 -instcombine - Combine redundant instructions
344...
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]345</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]346
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]347<p>The pass name get added as the information string for your pass, giving some
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]348documentation to users of <tt>opt</tt>. Now that you have a working pass, you
349would go ahead and make it do the cool transformations you want. Once you get
350it all working and tested, it may become useful to find out how fast your pass
351is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command
352line option (<tt>--time-passes</tt>) that allows you to get information about
353the execution time of your pass along with the other passes you queue up. For
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]354example:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]355
356<pre>
357$ opt -load ../../../lib/Debug/libhello.so -hello -time-passes &lt; hello.bc &gt; /dev/null
358Hello: __main
359Hello: puts
360Hello: main
361===============================================================================
362 ... Pass execution timing report ...
363===============================================================================
364 Total Execution Time: 0.02 seconds (0.0479059 wall clock)
365
366 ---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name ---
367 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bytecode Writer
368 0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction
369 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier
370 <b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b>
371 0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]372</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]373
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]374<p>As you can see, our implementation above is pretty fast :). The additional
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]375passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
376that the LLVM emitted by your pass is still valid and well formed LLVM, which
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]377hasn't been broken somehow.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]378
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]379<p>Now that you have seen the basics of the mechanics behind passes, we can talk
380about some more details of how they work and how to use them.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]381
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]382</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]383
384<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]385<div class="doc_section">
386 <a name="passtype">Pass classes and requirements</a>
387</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]388<!-- *********************************************************************** -->
389
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]390<div class="doc_text">
391
392<p>One of the first things that you should do when designing a new pass is to
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]393decide what class you should subclass for your pass. The <a
394href="#basiccode">Hello World</a> example uses the <tt><a
395href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we
396did not discuss why or when this should occur. Here we talk about the classes
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]397available, from the most general to the most specific.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]398
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]399<p>When choosing a superclass for your Pass, you should choose the <b>most
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]400specific</b> class possible, while still being able to meet the requirements
Misha Brukman5560c9d2003-08-18 14:43:39 +0000[diff] [blame]401listed. This gives the LLVM Pass Infrastructure information necessary to
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]402optimize how passes are run, so that the resultant compiler isn't unneccesarily
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]403slow.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]404
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]405</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]406
Chris Lattnerf004f9a2002-09-25 22:31:38 +0000[diff] [blame]407<!-- ======================================================================= -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]408<div class="doc_subsection">
409 <a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a>
410</div>
Chris Lattnerf004f9a2002-09-25 22:31:38 +0000[diff] [blame]411
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]412<div class="doc_text">
413
414<p>The most plain and boring type of pass is the "<tt><a
415href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>"
Chris Lattnerf004f9a2002-09-25 22:31:38 +0000[diff] [blame]416class. This pass type is used for passes that do not have to be run, do not
417change state, and never need to be updated. This is not a normal type of
418transformation or analysis, but can provide information about the current
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]419compiler configuration.</p>
Chris Lattnerf004f9a2002-09-25 22:31:38 +0000[diff] [blame]420
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]421<p>Although this pass class is very infrequently used, it is important for
Chris Lattnerf004f9a2002-09-25 22:31:38 +0000[diff] [blame]422providing information about the current target machine being compiled for, and
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]423other static information that can affect the various transformations.</p>
Chris Lattnerf004f9a2002-09-25 22:31:38 +0000[diff] [blame]424
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]425<p><tt>ImmutablePass</tt>es never invalidate other transformations, are never
426invalidated, and are never "run".</p>
Chris Lattnerf004f9a2002-09-25 22:31:38 +0000[diff] [blame]427
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]428</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]429
430<!-- ======================================================================= -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]431<div class="doc_subsection">
432 <a name="Pass">The <tt>Pass</tt> class</a>
433</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]434
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]435<div class="doc_text">
436
437<p>The "<tt><a
438href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Pass.html">Pass</a></tt>"
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]439class is the most general of all superclasses that you can use. Deriving from
440<tt>Pass</tt> indicates that your pass uses the entire program as a unit,
441refering to function bodies in no predictable order, or adding and removing
442functions. Because nothing is known about the behavior of direct <tt>Pass</tt>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]443subclasses, no optimization can be done for their execution.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]444
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]445<p>To write a correct <tt>Pass</tt> subclass, derive from <tt>Pass</tt> and
446overload the <tt>run</tt> method with the following signature:</p>
447
448</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]449
450<!-- _______________________________________________________________________ -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]451<div class="doc_subsubsection">
452 <a name="run">The <tt>run</tt> method</a>
453</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]454
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]455<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]456
457<pre>
458 <b>virtual bool</b> run(Module &amp;M) = 0;
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]459</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]460
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]461<p>The <tt>run</tt> method performs the interesting work of the pass, and should
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]462return true if the module was modified by the transformation, false
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]463otherwise.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]464
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]465</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]466
467<!-- ======================================================================= -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]468<div class="doc_subsection">
469 <a name="FunctionPass">The <tt>FunctionPass</tt> class</a>
470</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]471
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]472<div class="doc_text">
473
474<p>In contrast to direct <tt>Pass</tt> subclasses, direct <tt><a
475href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]476subclasses do have a predictable, local behavior that can be expected by the
477system. All <tt>FunctionPass</tt> execute on each function in the program
Misha Brukmanef6a6a62003-08-21 22:14:26 +0000[diff] [blame]478independent of all of the other functions in the program.
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]479<tt>FunctionPass</tt>'s do not require that they are executed in a particular
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]480order, and <tt>FunctionPass</tt>'s do not modify external functions.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]481
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]482<p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]483
484<ol>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]485<li>Modify a Function other than the one currently being processed.</li>
486<li>Add or remove Function's from the current Module.</li>
487<li>Add or remove global variables from the current Module.</li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]488<li>Maintain state across invocations of
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]489 <a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li>
490</ol>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]491
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]492<p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]493href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s
494may overload three virtual methods to do their work. All of these methods
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]495should return true if they modified the program, or false if they didn't.</p>
496
497</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]498
499<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]500<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]501 <a name="doInitialization_mod">The <tt>doInitialization(Module &amp;)</tt>
502 method</a>
503</div>
504
505<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]506
507<pre>
508 <b>virtual bool</b> doInitialization(Module &amp;M);
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]509</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]510
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]511<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]512<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]513functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
514is designed to do simple initialization type of stuff that does not depend on
515the functions being processed. The <tt>doInitialization</tt> method call is not
516scheduled to overlap with any other pass executions (thus it should be very
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]517fast).</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]518
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]519<p>A good example of how this method should be used is the <a
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]520href="http://llvm.cs.uiuc.edu/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
521pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
Misha Brukmanef6a6a62003-08-21 22:14:26 +0000[diff] [blame]522platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]523uses the <tt>doInitialization</tt> method to get a reference to the malloc and
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]524free functions that it needs, adding prototypes to the module if necessary.</p>
525
526</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]527
528<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]529<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]530 <a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
531</div>
532
533<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]534
535<pre>
536 <b>virtual bool</b> runOnFunction(Function &amp;F) = 0;
537</pre><p>
538
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]539<p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do
540the transformation or analysis work of your pass. As usual, a true value should
541be returned if the function is modified.</p>
542
543</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]544
545<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]546<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]547 <a name="doFinalization_mod">The <tt>doFinalization(Module
548 &amp;)</tt> method</a>
549</div>
550
551<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]552
553<pre>
554 <b>virtual bool</b> doFinalization(Module &amp;M);
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]555</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]556
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]557<p>The <tt>doFinalization</tt> method is an infrequently used method that is
558called when the pass framework has finished calling <a
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]559href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]560program being compiled.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]561
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]562</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]563
564<!-- ======================================================================= -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]565<div class="doc_subsection">
566 <a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
567</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]568
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]569<div class="doc_text">
570
571<p><tt>BasicBlockPass</tt>'s are just like <a
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]572href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
573their scope of inspection and modification to a single basic block at a time.
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]574As such, they are <b>not</b> allowed to do any of the following:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]575
576<ol>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]577<li>Modify or inspect any basic blocks outside of the current one</li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]578<li>Maintain state across invocations of
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]579 <a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li>
580<li>Modify the constrol flow graph (by altering terminator instructions)</li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]581<li>Any of the things verboten for
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]582 <a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li>
583</ol>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]584
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]585<p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole"
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]586optimizations. They may override the same <a
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]587href="#doInitialization_mod"><tt>doInitialization(Module &amp;)</tt></a> and <a
588href="#doFinalization_mod"><tt>doFinalization(Module &amp;)</tt></a> methods that <a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]589href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p>
590
591</div>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]592
593<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]594<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]595 <a name="doInitialization_fn">The <tt>doInitialization(Function
596 &amp;)</tt> method</a>
597</div>
598
599<div class="doc_text">
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]600
601<pre>
602 <b>virtual bool</b> doInitialization(Function &amp;F);
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]603</pre>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]604
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]605<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]606<tt>BasicBlockPass</tt>'s are not allowed to do, but that
607<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed
608to do simple initialization type of stuff that does not depend on the
609BasicBlocks being processed. The <tt>doInitialization</tt> method call is not
610scheduled to overlap with any other pass executions (thus it should be very
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]611fast).</p>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]612
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]613</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]614
615<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]616<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]617 <a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
618</div>
619
620<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]621
622<pre>
623 <b>virtual bool</b> runOnBasicBlock(BasicBlock &amp;BB) = 0;
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]624</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]625
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]626<p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]627function is not allowed to inspect or modify basic blocks other than the
628parameter, and are not allowed to modify the CFG. A true value must be returned
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]629if the basic block is modified.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]630
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]631</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]632
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]633<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]634<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]635 <a name="doFinalization_fn">The <tt>doFinalization(Function &amp;)</tt>
636 method</a>
637</div>
638
639<div class="doc_text">
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]640
641<pre>
642 <b>virtual bool</b> doFinalization(Function &amp;F);
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]643</pre>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]644
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]645<p>The <tt>doFinalization</tt> method is an infrequently used method that is
646called when the pass framework has finished calling <a
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]647href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the
648program being compiled. This can be used to perform per-function
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]649finalization.</p>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]650
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]651</div>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]652
Brian Gaekecab8b6f2003-07-17 18:53:20 +0000[diff] [blame]653<!-- ======================================================================= -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]654<div class="doc_subsection">
655 <a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a>
656</div>
Brian Gaekecab8b6f2003-07-17 18:53:20 +0000[diff] [blame]657
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]658<div class="doc_text">
659
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]660<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that
661executes on the machine-dependent representation of each LLVM function in the
662program. A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
Brian Gaekecab8b6f2003-07-17 18:53:20 +0000[diff] [blame]663the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]664<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular,
665<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p>
Brian Gaekecab8b6f2003-07-17 18:53:20 +0000[diff] [blame]666
667<ol>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]668<li>Modify any LLVM Instructions, BasicBlocks or Functions.</li>
669<li>Modify a MachineFunction other than the one currently being processed.</li>
670<li>Add or remove MachineFunctions from the current Module.</li>
671<li>Add or remove global variables from the current Module.</li>
672<li>Maintain state across invocations of <a
673href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global
674data)</li>
675</ol>
Brian Gaekecab8b6f2003-07-17 18:53:20 +0000[diff] [blame]676
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]677</div>
Chris Lattnerd0713f92002-09-12 17:06:43 +0000[diff] [blame]678
Brian Gaeke6a33f362003-07-22 20:53:20 +0000[diff] [blame]679<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]680<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]681 <a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
682 &amp;MF)</tt> method</a>
683</div>
684
685<div class="doc_text">
Brian Gaeke6a33f362003-07-22 20:53:20 +0000[diff] [blame]686
687<pre>
688 <b>virtual bool</b> runOnMachineFunction(MachineFunction &amp;MF) = 0;
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]689</pre>
Brian Gaeke6a33f362003-07-22 20:53:20 +0000[diff] [blame]690
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]691<p><tt>runOnMachineFunction</tt> can be considered the main entry point of a
692<tt>MachineFunctionPass</tt>; that is, you should override this method to do the
693work of your <tt>MachineFunctionPass</tt>.</p>
Brian Gaeke6a33f362003-07-22 20:53:20 +0000[diff] [blame]694
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]695<p>The <tt>runOnMachineFunction</tt> method is called on every
Brian Gaeke6a33f362003-07-22 20:53:20 +0000[diff] [blame]696<tt>MachineFunction</tt> in a <tt>Module</tt>, so that the
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]697<tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent
698representation of the function. If you want to get at the LLVM <tt>Function</tt>
699for the <tt>MachineFunction</tt> you're working on, use
700<tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but
701remember, you may not modify the LLVM <tt>Function</tt> or its contents from a
702<tt>MachineFunctionPass</tt>.</p>
703
704</div>
Brian Gaeke6a33f362003-07-22 20:53:20 +0000[diff] [blame]705
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]706<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]707<div class="doc_section">
708 <a name="registration">Pass registration</a>
709</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]710<!-- *********************************************************************** -->
711
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]712<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]713
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]714<p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how
715pass registration works, and discussed some of the reasons that it is used and
716what it does. Here we discuss how and why passes are registered.</p>
717
718<p>Passes can be registered in several different ways. Depending on the general
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]719classification of the pass, you should use one of the following templates to
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]720register the pass:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]721
722<ul>
723<li><b><tt>RegisterOpt</tt></b> - This template should be used when you are
724registering a pass that logically should be available for use in the
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]725'<tt>opt</tt>' utility.</li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]726
727<li><b><tt>RegisterAnalysis</tt></b> - This template should be used when you are
728registering a pass that logically should be available for use in the
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]729'<tt>analyze</tt>' utility.</li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]730
731<li><b><tt>RegisterPass</tt></b> - This is the generic form of the
732<tt>Register*</tt> templates that should be used if you want your pass listed by
733multiple or no utilities. This template takes an extra third argument that
734specifies which tools it should be listed in. See the <a
735href="http://llvm.cs.uiuc.edu/doxygen/PassSupport_8h-source.html">PassSupport.h</a>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]736file for more information.</li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]737
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]738</ul>
739
740<p>Regardless of how you register your pass, you must specify at least two
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]741parameters. The first parameter is the name of the pass that is to be used on
742the command line to specify that the pass should be added to a program (for
743example <tt>opt</tt> or <tt>analyze</tt>). The second argument is the name of
744the pass, which is to be used for the <tt>--help</tt> output of programs, as
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]745well as for debug output generated by the <tt>--debug-pass</tt> option.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]746
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]747<p>If a pass is registered to be used by the <tt>analyze</tt> utility, you
748should implement the virtual <tt>print</tt> method:</p>
749
750</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]751
752<!-- _______________________________________________________________________ -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]753<div class="doc_subsubsection">
754 <a name="print">The <tt>print</tt> method</a>
755</div>
756
757<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]758
759<pre>
760 <b>virtual void</b> print(std::ostream &amp;O, <b>const</b> Module *M) <b>const</b>;
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]761</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]762
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]763<p>The <tt>print</tt> method must be implemented by "analyses" in order to print
764a human readable version of the analysis results. This is useful for debugging
765an analysis itself, as well as for other people to figure out how an analysis
766works. The <tt>analyze</tt> tool uses this method to generate its output.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]767
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]768<p>The <tt>ostream</tt> parameter specifies the stream to write the results on,
769and the <tt>Module</tt> parameter gives a pointer to the top level module of the
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]770program that has been analyzed. Note however that this pointer may be null in
771certain circumstances (such as calling the <tt>Pass::dump()</tt> from a
772debugger), so it should only be used to enhance debug output, it should not be
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]773depended on.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]774
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]775</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]776
777<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]778<div class="doc_section">
779 <a name="interaction">Specifying interactions between passes</a>
780</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]781<!-- *********************************************************************** -->
782
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]783<div class="doc_text">
784
785<p>One of the main responsibilities of the <tt>PassManager</tt> is the make sure
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]786that passes interact with each other correctly. Because <tt>PassManager</tt>
787tries to <a href="#passmanager">optimize the execution of passes</a> it must
788know how the passes interact with each other and what dependencies exist between
789the various passes. To track this, each pass can declare the set of passes that
790are required to be executed before the current pass, and the passes which are
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]791invalidated by the current pass.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]792
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]793<p>Typically this functionality is used to require that analysis results are
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]794computed before your pass is run. Running arbitrary transformation passes can
795invalidate the computed analysis results, which is what the invalidation set
796specifies. If a pass does not implement the <tt><a
797href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]798having any prerequisite passes, and invalidating <b>all</b> other passes.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]799
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]800</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]801
802<!-- _______________________________________________________________________ -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]803<div class="doc_subsubsection">
804 <a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
805</div>
806
807<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]808
809<pre>
810 <b>virtual void</b> getAnalysisUsage(AnalysisUsage &amp;Info) <b>const</b>;
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]811</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]812
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]813<p>By implementing the <tt>getAnalysisUsage</tt> method, the required and
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]814invalidated sets may be specified for your transformation. The implementation
815should fill in the <tt><a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]816href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
817object with information about which passes are required and not invalidated. To
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]818do this, a pass may call any of the following methods on the AnalysisUsage
819object:</p>
820</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]821
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]822<!-- _______________________________________________________________________ -->
823<div class="doc_subsubsection">
824 <a name="AU::addRequired">The <tt>AnalysisUsage::addRequired&lt;&gt;</tt> and <tt>AnalysisUsage::addRequiredTransitive&lt;&gt;</tt> methods</a>
825</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]826
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]827<div class="doc_text">
828<p>
829If you pass requires a previous pass to be executed (an analysis for example),
830it can use one of these methods to arrange for it to be run before your pass.
831LLVM has many different types of analyses and passes that can be required,
832spaning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>.
833requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will
834be no critical edges in the CFG when your pass has been run.
835</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]836
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]837<p>
838Some analyses chain to other analyses to do their job. For example, an <a
839href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a
840href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In
841cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be
842used instead of the <tt>addRequired</tt> method. This informs the PassManager
843that the transitively required pass should be alive as long as the requiring
844pass is.
845</p>
846</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]847
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]848<!-- _______________________________________________________________________ -->
849<div class="doc_subsubsection">
850 <a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved&lt;&gt;</tt> method</a>
851</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]852
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]853<div class="doc_text">
854<p>
855One of the jobs of the PassManager is to optimize how and when analyses are run.
856In particular, it attempts to avoid recomputing data unless it needs to. For
857this reason, passes are allowed to declare that they preserve (i.e., they don't
858invalidate) an existing analysis if it's available. For example, a simple
859constant folding pass would not modify the CFG, so it can't possible effect the
860results of dominator analysis. By default, all passes are assumed to invalidate
861all others.
862</p>
863
864<p>
865The <tt>AnalysisUsage</tt> class provides several methods which are useful in
866certain circumstances that are related to <tt>addPreserved</tt>. In particular,
867the <tt>setPreservesAll</tt> method can be called to indicate that the pass does
868not modify the LLVM program at all (which is true for analyses), and the
869<tt>setPreservesCFG</tt> method can be used by transformations that change
870instructions in the program but do not modify the CFG or terminator instructions
871(note that this property is implicitly set for <a
872href="#BasicBlockPass">BasicBlockPass</a>'s).
873</p>
874
875<p>
876<tt>addPreserved</tt> is particularly useful for transformations like
877<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop
878and dominator related analyses if they exist, so it can preserve them, despite
879the fact that it hacks on the CFG.
880</p>
881</div>
882
883<!-- _______________________________________________________________________ -->
884<div class="doc_subsubsection">
885 <a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a>
886</div>
887
888<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]889
890<pre>
891 <i>// This is an example implementation from an analysis, which does not modify
892 // the program at all, yet has a prerequisite.</i>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]893 <b>void</b> <a href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]894 AU.setPreservesAll();
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]895 AU.addRequired&lt;<a href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1PostDominatorTree.html">PostDominatorTree</a>&gt;();
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]896 }
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]897</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]898
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]899<p>and:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]900
901<pre>
902 <i>// This example modifies the program, but does not modify the CFG</i>
903 <b>void</b> <a href="http://llvm.cs.uiuc.edu/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
Chris Lattner8291e042002-10-21 19:57:59 +0000[diff] [blame]904 AU.setPreservesCFG();
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]905 AU.addRequired&lt;<a href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>&gt;();
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]906 }
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]907</pre>
908
909</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]910
911<!-- _______________________________________________________________________ -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]912<div class="doc_subsubsection">
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]913 <a name="getAnalysis">The <tt>getAnalysis&lt;&gt;</tt> and <tt>getAnalysisToUpdate&lt;&gt;</tt> methods</a>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]914</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]915
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]916<div class="doc_text">
917
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]918<p>The <tt>Pass::getAnalysis&lt;&gt;</tt> method is automatically inherited by
919your class, providing you with access to the passes that you declared that you
920required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
921method. It takes a single template argument that specifies which pass class you
922want, and returns a reference to that pass. For example:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]923
924<pre>
Chris Lattnera5776302004-03-17 21:33:32 +0000[diff] [blame]925 bool LICM::runOnFunction(Function &amp;F) {
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]926 LoopInfo &amp;LI = getAnalysis&lt;LoopInfo&gt;();
927 ...
928 }
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]929</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]930
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]931<p>This method call returns a reference to the pass desired. You may get a
932runtime assertion failure if you attempt to get an analysis that you did not
933declare as required in your <a
934href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This
935method can be called by your <tt>run*</tt> method implementation, or by any
936other local method invoked by your <tt>run*</tt> method.</p>
937
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]938<p>
939If your pass is capable of updating analyses if they exist (e.g.,
940<tt>BreakCriticalEdges</tt>, as described above), you can use the
941<tt>getAnalysisToUpdate</tt> method, which returns a pointer to the analysis if
942it is active. For example:</p>
943
944<pre>
945 ...
946 if (DominatorSet *DS = getAnalysisToUpdate&lt;DominatorSet&gt;()) {
947 <i>// A DominatorSet is active. This code will update it.</i>
948 }
949 ...
950</pre>
951
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]952</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]953
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]954<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]955<div class="doc_section">
956 <a name="analysisgroup">Implementing Analysis Groups</a>
957</div>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]958<!-- *********************************************************************** -->
959
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]960<div class="doc_text">
961
962<p>Now that we understand the basics of how passes are defined, how the are
963used, and how they are required from other passes, it's time to get a little bit
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]964fancier. All of the pass relationships that we have seen so far are very
965simple: one pass depends on one other specific pass to be run before it can run.
966For many applications, this is great, for others, more flexibility is
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]967required.</p>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]968
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]969<p>In particular, some analyses are defined such that there is a single simple
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]970interface to the analysis results, but multiple ways of calculating them.
971Consider alias analysis for example. The most trivial alias analysis returns
972"may alias" for any alias query. The most sophisticated analysis a
973flow-sensitive, context-sensitive interprocedural analysis that can take a
974significant amount of time to execute (and obviously, there is a lot of room
975between these two extremes for other implementations). To cleanly support
976situations like this, the LLVM Pass Infrastructure supports the notion of
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]977Analysis Groups.</p>
978
979</div>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]980
981<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]982<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]983 <a name="agconcepts">Analysis Group Concepts</a>
984</div>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]985
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]986<div class="doc_text">
987
988<p>An Analysis Group is a single simple interface that may be implemented by
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]989multiple different passes. Analysis Groups can be given human readable names
990just like passes, but unlike passes, they need not derive from the <tt>Pass</tt>
991class. An analysis group may have one or more implementations, one of which is
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]992the "default" implementation.</p>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]993
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]994<p>Analysis groups are used by client passes just like other passes are: the
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]995<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods.
996In order to resolve this requirement, the <a href="#passmanager">PassManager</a>
997scans the available passes to see if any implementations of the analysis group
998are available. If none is available, the default implementation is created for
999the pass to use. All standard rules for <A href="#interaction">interaction
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1000between passes</a> still apply.</p>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1001
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1002<p>Although <a href="#registration">Pass Registration</a> is optional for normal
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1003passes, all analysis group implementations must be registered, and must use the
1004<A href="#registerag"><tt>RegisterAnalysisGroup</tt></a> template to join the
1005implementation pool. Also, a default implementation of the interface
1006<b>must</b> be registered with <A
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1007href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1008
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1009<p>As a concrete example of an Analysis Group in action, consider the <a
1010href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1011analysis group. The default implementation of the alias analysis interface (the
1012<tt><a
1013href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
1014pass) just does a few simple checks that don't require significant analysis to
1015compute (such as: two different globals can never alias each other, etc).
1016Passes that use the <tt><a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1017href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1018interface (for example the <tt><a
Misha Brukmand15516e2004-06-03 23:39:36 +0000[diff] [blame]1019href="http://llvm.cs.uiuc.edu/doxygen/structGCSE.html">gcse</a></tt> pass), do
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1020not care which implementation of alias analysis is actually provided, they just
1021use the designated interface.</p>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1022
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1023<p>From the user's perspective, commands work just like normal. Issuing the
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1024command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be
1025instantiated and added to the pass sequence. Issuing the command '<tt>opt
1026-somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the
1027<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1028hypothetical example) instead.</p>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1029
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1030</div>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1031
1032<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]1033<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1034 <a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
1035</div>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1036
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1037<div class="doc_text">
1038
1039<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1040group itself as well as add pass implementations to the analysis group. First,
1041an analysis should be registered, with a human readable name provided for it.
1042Unlike registration of passes, there is no command line argument to be specified
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1043for the Analysis Group Interface itself, because it is "abstract":</p>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1044
1045<pre>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1046 <b>static</b> RegisterAnalysisGroup&lt;<a href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>&gt; A("<i>Alias Analysis</i>");
1047</pre>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1048
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1049<p>Once the analysis is registered, passes can declare that they are valid
1050implementations of the interface by using the following code:</p>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1051
1052<pre>
1053<b>namespace</b> {
1054 //<i> Analysis Group implementations <b>must</b> be registered normally...</i>
1055 RegisterOpt&lt;FancyAA&gt;
1056 B("<i>somefancyaa</i>", "<i>A more complex alias analysis implementation</i>");
1057
1058 //<i> Declare that we implement the AliasAnalysis interface</i>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1059 RegisterAnalysisGroup&lt;<a href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, FancyAA&gt; C;
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1060}
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1061</pre>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1062
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1063<p>This just shows a class <tt>FancyAA</tt> that is registered normally, then
1064uses the <tt>RegisterAnalysisGroup</tt> template to "join" the <tt><a
1065href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1066analysis group. Every implementation of an analysis group should join using
1067this template. A single pass may join multiple different analysis groups with
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1068no problem.</p>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1069
1070<pre>
1071<b>namespace</b> {
1072 //<i> Analysis Group implementations <b>must</b> be registered normally...</i>
1073 RegisterOpt&lt;<a href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>&gt;
1074 D("<i>basicaa</i>", "<i>Basic Alias Analysis (default AA impl)</i>");
1075
1076 //<i> Declare that we implement the AliasAnalysis interface</i>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1077 RegisterAnalysisGroup&lt;<a href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, <a href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>, <b>true</b>&gt; E;
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1078}
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1079</pre>
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1080
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1081<p>Here we show how the default implementation is specified (using the extra
Chris Lattner79910702002-08-22 19:21:04 +0000[diff] [blame]1082argument to the <tt>RegisterAnalysisGroup</tt> template). There must be exactly
1083one default implementation available at all times for an Analysis Group to be
1084used. Here we declare that the <tt><a
1085href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1086pass is the default implementation for the interface.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1087
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1088</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1089
1090<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1091<div class="doc_section">
1092 <a name="passmanager">What PassManager does</a>
1093</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1094<!-- *********************************************************************** -->
1095
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1096<div class="doc_text">
1097
1098<p>The <a
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1099href="http://llvm.cs.uiuc.edu/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1100<a
1101href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1PassManager.html">class</a>
1102takes a list of passes, ensures their <a href="#interaction">prerequisites</a>
1103are set up correctly, and then schedules passes to run efficiently. All of the
1104LLVM tools that run passes use the <tt>PassManager</tt> for execution of these
1105passes.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1106
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1107<p>The <tt>PassManager</tt> does two main things to try to reduce the execution
1108time of a series of passes:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1109
1110<ol>
1111<li><b>Share analysis results</b> - The PassManager attempts to avoid
1112recomputing analysis results as much as possible. This means keeping track of
1113which analyses are available already, which analyses get invalidated, and which
1114analyses are needed to be run for a pass. An important part of work is that the
1115<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing
1116it to <a href="#releaseMemory">free memory</a> allocated to holding analysis
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1117results as soon as they are no longer needed.</li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1118
1119<li><b>Pipeline the execution of passes on the program</b> - The
1120<tt>PassManager</tt> attempts to get better cache and memory usage behavior out
1121of a series of passes by pipelining the passes together. This means that, given
1122a series of consequtive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
1123will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on
1124the first function, then all of the <a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1125href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function,
1126etc... until the entire program has been run through the passes.
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1127
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1128<p>This improves the cache behavior of the compiler, because it is only touching
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1129the LLVM program representation for a single function at a time, instead of
1130traversing the entire program. It reduces the memory consumption of compiler,
1131because, for example, only one <a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1132href="http://llvm.cs.uiuc.edu/doxygen/structllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1133needs to be calculated at a time. This also makes it possible some <a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1134href="#SMP">interesting enhancements</a> in the future.</p></li>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1135
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1136</ol>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1137
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1138<p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how
1139much information it has about the behaviors of the passes it is scheduling. For
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1140example, the "preserved" set is intentionally conservative in the face of an
1141unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method.
1142Not implementing when it should be implemented will have the effect of not
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1143allowing any analysis results to live across the execution of your pass.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1144
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1145<p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1146options that is useful for debugging pass execution, seeing how things work, and
1147diagnosing when you should be preserving more analyses than you currently are
1148(To get information about all of the variants of the <tt>--debug-pass</tt>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1149option, just type '<tt>opt --help-hidden</tt>').</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1150
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1151<p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see
1152how our <a href="#basiccode">Hello World</a> pass interacts with other passes.
1153Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1154
1155<pre>
1156$ opt -load ../../../lib/Debug/libhello.so -gcse -licm --debug-pass=Structure &lt; hello.bc &gt; /dev/null
1157Module Pass Manager
1158 Function Pass Manager
1159 Dominator Set Construction
1160 Immediate Dominators Construction
1161 Global Common Subexpression Elimination
1162-- Immediate Dominators Construction
1163-- Global Common Subexpression Elimination
1164 Natural Loop Construction
1165 Loop Invariant Code Motion
1166-- Natural Loop Construction
1167-- Loop Invariant Code Motion
1168 Module Verifier
1169-- Dominator Set Construction
1170-- Module Verifier
1171 Bytecode Writer
1172--Bytecode Writer
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1173</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1174
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1175<p>This output shows us when passes are constructed and when the analysis
1176results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that
1177GCSE uses dominator and immediate dominator information to do its job. The LICM
1178pass uses natural loop information, which uses dominator sets, but not immediate
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1179dominators. Because immediate dominators are no longer useful after the GCSE
1180pass, it is immediately destroyed. The dominator sets are then reused to
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1181compute natural loop information, which is then used by the LICM pass.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1182
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1183<p>After the LICM pass, the module verifier runs (which is automatically added
1184by the '<tt>opt</tt>' tool), which uses the dominator set to check that the
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1185resultant LLVM code is well formed. After it finishes, the dominator set
1186information is destroyed, after being computed once, and shared by three
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1187passes.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1188
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1189<p>Lets see how this changes when we run the <a href="#basiccode">Hello
1190World</a> pass in between the two passes:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1191
1192<pre>
1193$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure &lt; hello.bc &gt; /dev/null
1194Module Pass Manager
1195 Function Pass Manager
1196 Dominator Set Construction
1197 Immediate Dominators Construction
1198 Global Common Subexpression Elimination
1199<b>-- Dominator Set Construction</b>
1200-- Immediate Dominators Construction
1201-- Global Common Subexpression Elimination
1202<b> Hello World Pass
1203-- Hello World Pass
1204 Dominator Set Construction</b>
1205 Natural Loop Construction
1206 Loop Invariant Code Motion
1207-- Natural Loop Construction
1208-- Loop Invariant Code Motion
1209 Module Verifier
1210-- Dominator Set Construction
1211-- Module Verifier
1212 Bytecode Writer
1213--Bytecode Writer
1214Hello: __main
1215Hello: puts
1216Hello: main
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1217</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1218
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1219<p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1220Dominator Set pass, even though it doesn't modify the code at all! To fix this,
1221we need to add the following <a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1222href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1223
1224<pre>
1225 <i>// We don't modify the program, so we preserve all analyses</i>
1226 <b>virtual void</b> getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
1227 AU.setPreservesAll();
1228 }
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1229</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1230
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1231<p>Now when we run our pass, we get this output:</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1232
1233<pre>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1234$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure &lt; hello.bc &gt; /dev/null
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1235Pass Arguments: -gcse -hello -licm
1236Module Pass Manager
1237 Function Pass Manager
1238 Dominator Set Construction
1239 Immediate Dominators Construction
1240 Global Common Subexpression Elimination
1241-- Immediate Dominators Construction
1242-- Global Common Subexpression Elimination
1243 Hello World Pass
1244-- Hello World Pass
1245 Natural Loop Construction
1246 Loop Invariant Code Motion
1247-- Loop Invariant Code Motion
1248-- Natural Loop Construction
1249 Module Verifier
1250-- Dominator Set Construction
1251-- Module Verifier
1252 Bytecode Writer
1253--Bytecode Writer
1254Hello: __main
1255Hello: puts
1256Hello: main
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1257</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1258
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1259<p>Which shows that we don't accidentally invalidate dominator information
1260anymore, and therefore do not have to compute it twice.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1261
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1262</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1263
1264<!-- _______________________________________________________________________ -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1265<div class="doc_subsubsection">
1266 <a name="releaseMemory">The <tt>releaseMemory</tt> method</a>
1267</div>
1268
1269<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1270
1271<pre>
1272 <b>virtual void</b> releaseMemory();
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1273</pre>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1274
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1275<p>The <tt>PassManager</tt> automatically determines when to compute analysis
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1276results, and how long to keep them around for. Because the lifetime of the pass
1277object itself is effectively the entire duration of the compilation process, we
1278need some way to free analysis results when they are no longer useful. The
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1279<tt>releaseMemory</tt> virtual method is the way to do this.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1280
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1281<p>If you are writing an analysis or any other pass that retains a significant
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1282amount of state (for use by another pass which "requires" your pass and uses the
1283<a href="#getAnalysis">getAnalysis</a> method) you should implement
1284<tt>releaseMEmory</tt> to, well, release the memory allocated to maintain this
1285internal state. This method is called after the <tt>run*</tt> method for the
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1286class, before the next call of <tt>run*</tt> in your pass.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1287
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1288</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1289
1290<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1291<div class="doc_section">
1292 <a name="debughints">Using GDB with dynamically loaded passes</a>
1293</div>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1294<!-- *********************************************************************** -->
1295
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1296<div class="doc_text">
1297
1298<p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1299should be. First of all, you can't set a breakpoint in a shared object that has
1300not been loaded yet, and second of all there are problems with inlined functions
1301in shared objects. Here are some suggestions to debugging your pass with
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1302GDB.</p>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1303
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1304<p>For sake of discussion, I'm going to assume that you are debugging a
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1305transformation invoked by <tt>opt</tt>, although nothing described here depends
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1306on that.</p>
1307
1308</div>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1309
1310<!-- _______________________________________________________________________ -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1311<div class="doc_subsubsection">
1312 <a name="breakpoint">Setting a breakpoint in your pass</a>
1313</div>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1314
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1315<div class="doc_text">
1316
1317<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1318
1319<pre>
1320$ <b>gdb opt</b>
1321GNU gdb 5.0
1322Copyright 2000 Free Software Foundation, Inc.
1323GDB is free software, covered by the GNU General Public License, and you are
1324welcome to change it and/or distribute copies of it under certain conditions.
1325Type "show copying" to see the conditions.
1326There is absolutely no warranty for GDB. Type "show warranty" for details.
1327This GDB was configured as "sparc-sun-solaris2.6"...
1328(gdb)
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1329</pre>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1330
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1331<p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1332time to load. Be patient. Since we cannot set a breakpoint in our pass yet
1333(the shared object isn't loaded until runtime), we must execute the process, and
1334have it stop before it invokes our pass, but after it has loaded the shared
1335object. The most foolproof way of doing this is to set a breakpoint in
1336<tt>PassManager::run</tt> and then run the process with the arguments you
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1337want:</p>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1338
1339<pre>
1340(gdb) <b>break PassManager::run</b>
1341Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
Chris Lattnerfafe6302003-10-23 19:48:11 +0000[diff] [blame]1342(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/lib/Debug/[libname].so -[passoption]</b>
1343Starting program: opt test.bc -load $(LLVMTOP)/llvm/lib/Debug/[libname].so -[passoption]
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1344Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
134570 bool PassManager::run(Module &amp;M) { return PM-&gt;run(M); }
1346(gdb)
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1347</pre>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1348
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1349<p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are
1350now free to set breakpoints in your pass so that you can trace through execution
1351or do other standard debugging stuff.</p>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1352
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1353</div>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1354
1355<!-- _______________________________________________________________________ -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1356<div class="doc_subsubsection">
1357 <a name="debugmisc">Miscellaneous Problems</a>
1358</div>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1359
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1360<div class="doc_text">
1361
1362<p>Once you have the basics down, there are a couple of problems that GDB has,
1363some with solutions, some without.</p>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1364
1365<ul>
1366<li>Inline functions have bogus stack information. In general, GDB does a
1367pretty good job getting stack traces and stepping through inline functions.
1368When a pass is dynamically loaded however, it somehow completely loses this
1369capability. The only solution I know of is to de-inline a function (move it
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1370from the body of a class to a .cpp file).</li>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1371
1372<li>Restarting the program breaks breakpoints. After following the information
1373above, you have succeeded in getting some breakpoints planted in your pass. Nex
1374thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
1375and you start getting errors about breakpoints being unsettable. The only way I
1376have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
1377already set in your pass, run the program, and re-set the breakpoints once
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1378execution stops in <tt>PassManager::run</tt>.</li>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1379
1380</ul>
1381
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1382<p>Hopefully these tips will help with common case debugging situations. If
1383you'd like to contribute some tips of your own, just contact <a
1384href="mailto:sabre@nondot.org">Chris</a>.</p>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1385
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1386</div>
Chris Lattner480e2ef2002-09-06 02:02:58 +0000[diff] [blame]1387
1388<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1389<div class="doc_section">
1390 <a name="future">Future extensions planned</a>
1391</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1392<!-- *********************************************************************** -->
1393
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1394<div class="doc_text">
1395
1396<p>Although the LLVM Pass Infrastructure is very capable as it stands, and does
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1397some nifty stuff, there are things we'd like to add in the future. Here is
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1398where we are going:</p>
1399
1400</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1401
1402<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]1403<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1404 <a name="SMP">Multithreaded LLVM</a>
1405</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1406
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1407<div class="doc_text">
1408
1409<p>Multiple CPU machines are becoming more common and compilation can never be
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1410fast enough: obviously we should allow for a multithreaded compiler. Because of
1411the semantics defined for passes above (specifically they cannot maintain state
1412across invocations of their <tt>run*</tt> methods), a nice clean way to
1413implement a multithreaded compiler would be for the <tt>PassManager</tt> class
Misha Brukmanbc0e9982003-07-14 17:20:40 +0000[diff] [blame]1414to create multiple instances of each pass object, and allow the separate
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1415instances to be hacking on different parts of the program at the same time.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1416
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1417<p>This implementation would prevent each of the passes from having to implement
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1418multithreaded constructs, requiring only the LLVM core to have locking in a few
1419places (for global resources). Although this is a simple extension, we simply
1420haven't had time (or multiprocessor machines, thus a reason) to implement this.
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1421Despite that, we have kept the LLVM passes SMP ready, and you should too.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1422
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1423</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1424
1425<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]1426<div class="doc_subsubsection">
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1427 <a name="ModuleSource">A new <tt>ModuleSource</tt> interface</a>
1428</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1429
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1430<div class="doc_text">
1431
1432<p>Currently, the <tt>PassManager</tt>'s <tt>run</tt> method takes a <tt><a
Tanya Lattnera3da7772004-06-22 08:02:25 +0000[diff] [blame]1433href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1Module.html">Module</a></tt>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1434as input, and runs all of the passes on this module. The problem with this
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1435approach is that none of the <tt>PassManager</tt> features can be used for
1436timing and debugging the actual <b>loading</b> of the module from disk or
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1437standard input.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1438
Misha Brukmanefc182e2004-03-08 23:06:46 +0000[diff] [blame]1439<p>To solve this problem, eventually the <tt>PassManager</tt> class will accept
1440a <tt>ModuleSource</tt> object instead of a Module itself. When complete, this
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1441will also allow for streaming of functions out of the bytecode representation,
1442allowing us to avoid holding the entire program in memory at once if we only are
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1443dealing with <a href="#FunctionPass">FunctionPass</a>es.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1444
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1445<p>As part of a different issue, eventually the bytecode loader will be extended
1446to allow on-demand loading of functions from the bytecode representation, in
1447order to better support the runtime reoptimizer. The bytecode format is already
1448capable of this, the loader just needs to be reworked a bit.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1449
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1450</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1451
1452<!-- _______________________________________________________________________ -->
Chris Lattner89256272004-03-17 21:09:55 +0000[diff] [blame]1453<div class="doc_subsubsection">
Misha Brukmanefc182e2004-03-08 23:06:46 +0000[diff] [blame]1454<a name="PassFunctionPass"><tt>Pass</tt>es requiring <tt>FunctionPass</tt>es</a>
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1455</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1456
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1457<div class="doc_text">
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1458
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1459<p>Currently it is illegal for a <a href="#Pass"><tt>Pass</tt></a> to require a
1460<a href="#FunctionPass"><tt>FunctionPass</tt></a>. This is because there is
1461only one instance of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>
1462object ever created, thus nowhere to store information for all of the functions
1463in the program at the same time. Although this has come up a couple of times
1464before, this has always been worked around by factoring one big complicated pass
1465into a global and an interprocedural part, both of which are distinct. In the
1466future, it would be nice to have this though.</p>
1467
1468<p>Note that it is no problem for a <a
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1469href="#FunctionPass"><tt>FunctionPass</tt></a> to require the results of a <a
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1470href="#Pass"><tt>Pass</tt></a>, only the other way around.</p>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1471
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1472</div>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1473
1474<!-- *********************************************************************** -->
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1475<hr>
1476<address>
1477 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
1478 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
1479 <a href="http://validator.w3.org/check/referer"><img
1480 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!" /></a>
Chris Lattnerc6bb8242002-08-08 20:11:18 +0000[diff] [blame]1481
Misha Brukmanc5402402004-01-15 18:34:11 +0000[diff] [blame]1482 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
1483 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
1484 Last modified: $Date$
1485</address>
1486
1487</body>
1488</html>