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				2	<html><head><title>Writing an LLVM Pass</title></head>
				3
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	4	<body bgcolor=white>
				5
				6	<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				7	<tr><td>  <font size=+3 color="#EEEEFF" face="Georgia,Palatino,Times,Roman"><b>Writing an LLVM Pass</b></font></td>
				8	</tr></table>
				9
				10
				11	<ol>
				12	<li><a href="#introduction">Introduction - What is a pass?</a>
				13	<li><a href="#quickstart">Quick Start - Writing hello world</a>
				14	<ul>
				15	<li><a href="#makefile">Setting up the build environment</a>
				16	<li><a href="#basiccode">Basic code required</a>
				17	<li><a href="#running">Running a pass with <tt>opt</tt>
				18	or <tt>analyze</tt></a>
				19	</ul>
				20	<li><a href="#passtype">Pass classes and requirements</a>
				21	<ul>
Chris Lattner	f004f9a	2002-09-25 22:31:38 +0000	[diff] [blame]	22	<li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	23	<li><a href="#Pass">The <tt>Pass</tt> class</a>
				24	<ul>
				25	<li><a href="#run">The <tt>run</tt> method</a>
				26	</ul>
				27	<li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a>
				28	<ul>
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	29	<li><a href="#doInitialization_mod">The <tt>doInitialization(Module
				30	&)</tt> method</a>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	31	<li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a>
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	32	<li><a href="#doFinalization_mod">The <tt>doFinalization(Module
				33	&)</tt> method</a>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	34	</ul>
				35	<li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
				36	<ul>
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	37	<li><a href="#doInitialization_fn">The <tt>doInitialization(Function
				38	&)</tt> method</a>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	39	<li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	40	<li><a href="#doFinalization_fn">The <tt>doFinalization(Function
				41	&)</tt> method</a>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	42	</ul>
Brian Gaeke	cab8b6f	2003-07-17 18:53:20 +0000	[diff] [blame^]	43	<li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt>
				44	class</a>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	45	</ul>
				46	<li><a href="#registration">Pass Registration</a>
				47	<ul>
				48	<li><a href="#print">The <tt>print</tt> method</a>
				49	</ul>
				50	<li><a href="#interaction">Specifying interactions between passes</a>
				51	<ul>
				52	<li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
				53	<li><a href="#getAnalysis">The <tt>getAnalysis</tt> method</a>
				54	</ul>
Chris Lattner	7991070	2002-08-22 19:21:04 +0000	[diff] [blame]	55	<li><a href="#analysisgroup">Implementing Analysis Groups</a>
				56	<ul>
				57	<li><a href="#agconcepts">Analysis Group Concepts</a>
				58	<li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a>
				59	</ul>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	60	<li><a href="#passmanager">What PassManager does</a>
				61	<ul>
				62	<li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a>
				63	</ul>
Chris Lattner	480e2ef	2002-09-06 02:02:58 +0000	[diff] [blame]	64	<li><a href="#debughints">Using GDB with dynamically loaded passes</a>
				65	<ul>
				66	<li><a href="#breakpoint">Setting a breakpoint in your pass
				67	<li><a href="#debugmisc">Miscellaneous Problems
				68	</ul>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	69	<li><a href="#future">Future extensions planned</a>
				70	<ul>
				71	<li><a href="#SMP">Multithreaded LLVM</a>
				72	<li><a href="#ModuleSource">A new <tt>ModuleSource</tt> interface</a>
				73	<li><a href="#PassFunctionPass"><tt>Pass</tt>'s requiring <tt>FunctionPass</tt>'s</a>
				74	</ul>
Chris Lattner	38c633d	2002-08-08 20:23:41 +0000	[diff] [blame]	75
				76	<p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></b><p>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	77	</ol><p>
				78
				79
				80
				81	<!-- *********************************************************************** -->
				82	<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				83	<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
				84	<a name="introduction">Introduction - What is a pass?
				85	</b></font></td></tr></table><ul>
				86	<!-- *********************************************************************** -->
				87
				88	The LLVM Pass Framework is an important part of the LLVM system, because LLVM
				89	passes are where the interesting parts of the compiler exist. Passes perform
				90	the transformations and optimizations that make up the compiler, they build
				91	the analysis results that are used by these transformations, and they are, above
				92	all, a structuring technique for compiler code.<p>
				93
				94	All LLVM passes are subclasses of the <tt><a
				95	href="http://llvm.cs.uiuc.edu/doxygen/classPass.html">Pass</a></tt> class, which
				96	implement functionality by overriding virtual methods inherited from
				97	<tt>Pass</tt>. Depending on how your pass works, you may be able to inherit
				98	from the <tt><a
				99	href="http://llvm.cs.uiuc.edu/doxygen/structFunctionPass.html">FunctionPass</a></tt>
				100	or <tt><a
				101	href="http://llvm.cs.uiuc.edu/doxygen/structBasicBlockPass.html">BasicBlockPass</a></tt>,
				102	which gives the system more information about what your pass does, and how it
				103	can be combined with other passes. One of the main features of the LLVM Pass
				104	Framework is that it schedules passes to run in an efficient way based on the
				105	constraints that your pass has.<p>
				106
				107	We start by showing you how to construct a pass, everything from setting up the
				108	code, to compiling, loading, and executing it. After the basics are down, more
				109	advanced features are discussed.<p>
				110
				111
				112	<!-- *********************************************************************** -->
				113	</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				114	<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
				115	<a name="quickstart">Quick Start - Writing hello world
				116	</b></font></td></tr></table><ul>
				117	<!-- *********************************************************************** -->
				118
				119	Here we describe how to write the "hello world" of passes. The "Hello" pass is
				120	designed to simply print out the name of non-external functions that exist in
				121	the program being compiled. It does not modify the program at all, just
				122	inspects it. The source code and files for this pass are available in the LLVM
				123	source tree in the <tt>lib/Transforms/Hello</tt> directory.<p>
				124
				125
				126	<!-- ======================================================================= -->
				127	</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
				128	<tr><td> </td><td width="100%">
				129	<font color="#EEEEFF" face="Georgia,Palatino"><b>
				130	<a name="makefile">Setting up the build environment
				131	</b></font></td></tr></table><ul>
				132
				133	First thing you need to do is create a new directory somewhere in the LLVM
				134	source base. For this example, we'll assume that you made
				135	"<tt>lib/Transforms/Hello</tt>". The first thing you must do is set up a build
				136	script (Makefile) that will compile the source code for the new pass. To do
Chris Lattner	fd21476	2002-09-16 22:37:56 +0000	[diff] [blame]	137	this, copy this into "<tt>Makefile</tt>":<p>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	138
				139	</ul><hr><ul><pre>
				140	# Makefile for hello pass
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	141
Chris Lattner	17a4c3e	2002-08-14 20:06:13 +0000	[diff] [blame]	142	# Path to top level of LLVM heirarchy
Chris Lattner	7ce83e5	2002-08-14 20:07:01 +0000	[diff] [blame]	143	LEVEL = ../../..
Chris Lattner	17a4c3e	2002-08-14 20:06:13 +0000	[diff] [blame]	144
				145	# Name of the library to build
Chris Lattner	7ce83e5	2002-08-14 20:07:01 +0000	[diff] [blame]	146	LIBRARYNAME = hello
Chris Lattner	17a4c3e	2002-08-14 20:06:13 +0000	[diff] [blame]	147
				148	# Build a dynamically loadable shared object
				149	SHARED_LIBRARY = 1
				150
				151	# Include the makefile implementation stuff
				152	include $(LEVEL)/Makefile.common
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	153	</pre></ul><hr><ul><p>
				154
				155	This makefile specifies that all of the <tt>.cpp</tt> files in the current
				156	directory are to be compiled and linked together into a
				157	<tt>lib/Debug/libhello.so</tt> shared object that can be dynamically loaded by
				158	the <tt>opt</tt> or <tt>analyze</tt> tools.<p>
				159
				160	Now that we have the build scripts set up, we just need to write the code for
				161	the pass itself.<p>
				162
				163
				164	<!-- ======================================================================= -->
				165	</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
				166	<tr><td> </td><td width="100%">
				167	<font color="#EEEEFF" face="Georgia,Palatino"><b>
				168	<a name="basiccode">Basic code required
				169	</b></font></td></tr></table><ul>
				170
				171	Now that we have a way to compile our new pass, we just have to write it. Start
				172	out with:<p>
				173
				174	<pre>
				175	<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
				176	<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Function_8h-source.html">llvm/Function.h</a>"
				177	</pre>
				178
				179	Which are needed because we are writing a <tt><a
				180	href="http://llvm.cs.uiuc.edu/doxygen/classPass.html">Pass</a></tt>, and we are
				181	operating on <tt><a
				182	href="http://llvm.cs.uiuc.edu/doxygen/classFunction.html">Function</a></tt>'s.<p>
				183
				184	Next we have:<p>
				185
				186	<pre>
				187	<b>namespace</b> {
				188	</pre><p>
				189
				190	... which starts out an anonymous namespace. Anonymous namespaces are to C++
				191	what the "<tt>static</tt>" keyword is to C (at global scope). It makes the
				192	things declared inside of the anonymous namespace only visible to the current
				193	file. If you're not familiar with them, consult a decent C++ book for more
				194	information.<p>
				195
				196	Next, we declare our pass itself:<p>
				197
				198	<pre>
				199	<b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
				200	</pre><p>
				201
				202	This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
				203	href="http://llvm.cs.uiuc.edu/doxygen/structFunctionPass.html">FunctionPass</a></tt>.
Chris Lattner	d6ea926	2002-09-09 03:48:46 +0000	[diff] [blame]	204	The different builtin pass subclasses are described in detail <a
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	205	href="#passtype">later</a>, but for now, know that <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s
				206	operate a function at a time.<p>
				207
				208	<pre>
				209	<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
				210	std::cerr << "<i>Hello: </i>" << F.getName() << "\n";
				211	<b>return false</b>;
				212	}
				213	}; <i>// end of struct Hello</i>
				214	</pre>
				215
				216	We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method, which
				217	overloads an abstract virtual method inherited from <a
				218	href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed
				219	to do our thing, so we just print out our message with the name of each
				220	function.<p>
				221
				222	<pre>
				223	RegisterOpt<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
				224	} <i>// end of anonymous namespace</i>
				225	</pre><p>
				226
				227	Lastly, we register our class <tt>Hello</tt>, giving it a command line argument
				228	"<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>". There are several
				229	different ways of <a href="#registration">registering your pass</a>, depending
				230	on what it is to be used for. For "optimizations" we use the
				231	<tt>RegisterOpt</tt> template.<p>
				232
				233	As a whole, the <tt>.cpp</tt> file looks like:<p>
				234
				235	<pre>
				236	<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
				237	<b>#include</b> "<a href="http://llvm.cs.uiuc.edu/doxygen/Function_8h-source.html">llvm/Function.h</a>"
				238
				239	<b>namespace</b> {
				240	<b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
				241	<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
				242	std::cerr << "<i>Hello: </i>" << F.getName() << "\n";
				243	<b>return false</b>;
				244	}
				245	};
				246
				247	RegisterOpt<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
				248	}
				249	</pre><p>
				250
				251	Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
				252	command in the local directory and you should get a new
				253	"<tt>lib/Debug/libhello.so</tt> file. Note that everything in this file is
				254	contained in an anonymous namespace: this reflects the fact that passes are self
				255	contained units that do not need external interfaces (although they can have
				256	them) to be useful.<p>
				257
				258
				259	<!-- ======================================================================= -->
				260	</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
				261	<tr><td> </td><td width="100%">
				262	<font color="#EEEEFF" face="Georgia,Palatino"><b>
				263	<a name="running">Running a pass with <tt>opt</tt> or <tt>analyze</tt>
				264	</b></font></td></tr></table><ul>
				265
				266	Now that you have a brand new shiny <tt>.so</tt> file, we can use the
				267	<tt>opt</tt> command to run an LLVM program through your pass. Because you
				268	registered your pass with the <tt>RegisterOpt</tt> template, you will be able to
				269	use the <tt>opt</tt> tool to access it, once loaded.<p>
				270
				271	To test it, follow the example at the end of the <a
				272	href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
				273	LLVM. We can now run the bytecode file (<tt>hello.bc</tt>) for the program
				274	through our transformation like this (or course, any bytecode file will
				275	work):<p>
				276
				277	<pre>
				278	$ opt -load ../../../lib/Debug/libhello.so -hello < hello.bc > /dev/null
				279	Hello: __main
				280	Hello: puts
				281	Hello: main
				282	</pre><p>
				283
				284	The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your pass
				285	as a shared object, which makes '<tt>-hello</tt>' a valid command line argument
				286	(which is one reason you need to <a href="#registration">register your
				287	pass</a>). Because the hello pass does not modify the program in any
				288	interesting way, we just throw away the result of <tt>opt</tt> (sending it to
				289	<tt>/dev/null</tt>).<p>
				290
				291	To see what happened to the other string you registered, try running
				292	<tt>opt</tt> with the <tt>--help</tt> option:<p>
				293
				294	<pre>
				295	$ opt -load ../../../lib/Debug/libhello.so --help
				296	OVERVIEW: llvm .bc -> .bc modular optimizer
				297
				298	USAGE: opt [options] <input bytecode>
				299
				300	OPTIONS:
				301	Optimizations available:
				302	...
				303	-funcresolve - Resolve Functions
				304	-gcse - Global Common Subexpression Elimination
				305	-globaldce - Dead Global Elimination
				306	<b>-hello - Hello World Pass</b>
				307	-indvars - Cannonicalize Induction Variables
				308	-inline - Function Integration/Inlining
				309	-instcombine - Combine redundant instructions
				310	...
				311	</pre><p>
				312
				313	The pass name get added as the information string for your pass, giving some
				314	documentation to users of <tt>opt</tt>. Now that you have a working pass, you
				315	would go ahead and make it do the cool transformations you want. Once you get
				316	it all working and tested, it may become useful to find out how fast your pass
				317	is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command
				318	line option (<tt>--time-passes</tt>) that allows you to get information about
				319	the execution time of your pass along with the other passes you queue up. For
				320	example:<p>
				321
				322	<pre>
				323	$ opt -load ../../../lib/Debug/libhello.so -hello -time-passes < hello.bc > /dev/null
				324	Hello: __main
				325	Hello: puts
				326	Hello: main
				327	===============================================================================
				328	... Pass execution timing report ...
				329	===============================================================================
				330	Total Execution Time: 0.02 seconds (0.0479059 wall clock)
				331
				332	---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name ---
				333	0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bytecode Writer
				334	0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction
				335	0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier
				336	<b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b>
				337	0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL
				338	</pre><p>
				339
				340	As you can see, our implementation above is pretty fast :). The additional
				341	passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
				342	that the LLVM emitted by your pass is still valid and well formed LLVM, which
				343	hasn't been broken somehow.
				344
				345	Now that you have seen the basics of the mechanics behind passes, we can talk
				346	about some more details of how they work and how to use them.<p>
				347
				348
				349
				350	<!-- *********************************************************************** -->
				351	</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				352	<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
				353	<a name="passtype">Pass classes and requirements
				354	</b></font></td></tr></table><ul>
				355	<!-- *********************************************************************** -->
				356
				357	One of the first things that you should do when designing a new pass is to
				358	decide what class you should subclass for your pass. The <a
				359	href="#basiccode">Hello World</a> example uses the <tt><a
				360	href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we
				361	did not discuss why or when this should occur. Here we talk about the classes
				362	available, from the most general to the most specific.<p>
				363
Chris Lattner	7991070	2002-08-22 19:21:04 +0000	[diff] [blame]	364	When choosing a superclass for your Pass, you should choose the <b>most
				365	specific</b> class possible, while still being able to meet the requirements
				366	listed. This gives the LLVM Pass Infrastructure information neccesary to
				367	optimize how passes are run, so that the resultant compiler isn't unneccesarily
				368	slow.<p>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	369
				370
Chris Lattner	f004f9a	2002-09-25 22:31:38 +0000	[diff] [blame]	371	<!-- ======================================================================= -->
				372	</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
				373	<tr><td> </td><td width="100%">
				374	<font color="#EEEEFF" face="Georgia,Palatino"><b>
				375	<a name="ImmutablePass">The <tt>ImmutablePass</tt> class
				376	</b></font></td></tr></table><ul>
				377
				378	The most plain and boring type of pass is the "<tt><a
				379	href="http://llvm.cs.uiuc.edu/doxygen/structImmutablePass.html">ImmutablePass</a></tt>"
				380	class. This pass type is used for passes that do not have to be run, do not
				381	change state, and never need to be updated. This is not a normal type of
				382	transformation or analysis, but can provide information about the current
				383	compiler configuration.<p>
				384
				385	Although this pass class is very infrequently used, it is important for
				386	providing information about the current target machine being compiled for, and
				387	other static information that can affect the various transformations.<p>
				388
				389	<tt>ImmutablePass</tt>'s never invalidate other transformations, are never
				390	invalidated, and are never "run".<p>
				391
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	392
				393	<!-- ======================================================================= -->
				394	</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
				395	<tr><td> </td><td width="100%">
				396	<font color="#EEEEFF" face="Georgia,Palatino"><b>
				397	<a name="Pass">The <tt>Pass</tt> class
				398	</b></font></td></tr></table><ul>
				399
				400	The "<tt><a href="http://llvm.cs.uiuc.edu/doxygen/classPass.html">Pass</a></tt>"
				401	class is the most general of all superclasses that you can use. Deriving from
				402	<tt>Pass</tt> indicates that your pass uses the entire program as a unit,
				403	refering to function bodies in no predictable order, or adding and removing
				404	functions. Because nothing is known about the behavior of direct <tt>Pass</tt>
				405	subclasses, no optimization can be done for their execution.<p>
				406
				407	To write a correct <tt>Pass</tt> subclass, derive from <tt>Pass</tt> and
				408	overload the <tt>run</tt> method with the following signature:<p>
				409
				410	<!-- _______________________________________________________________________ -->
				411	</ul><h4><a name="run"><hr size=0>The <tt>run</tt> method</h4><ul>
				412
				413
				414	<pre>
				415	<b>virtual bool</b> run(Module &M) = 0;
				416	</pre><p>
				417
				418	The <tt>run</tt> method performs the interesting work of the pass, and should
				419	return true if the module was modified by the transformation, false
				420	otherwise.<p>
				421
				422
				423
				424	<!-- ======================================================================= -->
				425	</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
				426	<tr><td> </td><td width="100%">
				427	<font color="#EEEEFF" face="Georgia,Palatino"><b>
				428	<a name="FunctionPass">The <tt>FunctionPass</tt> class
				429	</b></font></td></tr></table><ul>
				430
				431	In contrast to direct <tt>Pass</tt> subclasses, direct <tt><a
				432	href="http://llvm.cs.uiuc.edu/doxygen/classPass.html">FunctionPass</a></tt>
				433	subclasses do have a predictable, local behavior that can be expected by the
				434	system. All <tt>FunctionPass</tt> execute on each function in the program
				435	independant of all of the other functions in the program.
				436	<tt>FunctionPass</tt>'s do not require that they are executed in a particular
				437	order, and <tt>FunctionPass</tt>'s do not modify external functions.<p>
				438
				439	To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:<p>
				440
				441	<ol>
				442	<li>Modify a Function other than the one currently being processed.
				443	<li>Add or remove Function's from the current Module.
				444	<li>Add or remove global variables from the current Module.
				445	<li>Maintain state across invocations of
				446	<a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)
				447	</ol><p>
				448
				449	Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a
				450	href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s
				451	may overload three virtual methods to do their work. All of these methods
				452	should return true if they modified the program, or false if they didn't.<p>
				453
				454	<!-- _______________________________________________________________________ -->
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	455	</ul><h4><a name="doInitialization_mod"><hr size=0>The
				456	<tt>doInitialization(Module &)</tt> method</h4><ul>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	457
				458	<pre>
				459	<b>virtual bool</b> doInitialization(Module &M);
				460	</pre><p>
				461
				462	The <tt>doIninitialize</tt> method is allowed to do most of the things that
				463	<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	464	functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
				465	is designed to do simple initialization type of stuff that does not depend on
				466	the functions being processed. The <tt>doInitialization</tt> method call is not
				467	scheduled to overlap with any other pass executions (thus it should be very
				468	fast).<p>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	469
				470	A good example of how this method should be used is the <a
				471	href="http://llvm.cs.uiuc.edu/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
				472	pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
				473	platform dependant <tt>malloc()</tt> and <tt>free()</tt> function calls. It
				474	uses the <tt>doInitialization</tt> method to get a reference to the malloc and
				475	free functions that it needs, adding prototypes to the module if neccesary.<p>
				476
				477	<!-- _______________________________________________________________________ -->
				478	</ul><h4><a name="runOnFunction"><hr size=0>The <tt>runOnFunction</tt> method</h4><ul>
				479
				480	<pre>
				481	<b>virtual bool</b> runOnFunction(Function &F) = 0;
				482	</pre><p>
				483
				484	The <tt>runOnFunction</tt> method must be implemented by your subclass to do the
				485	transformation or analysis work of your pass. As usual, a true value should be
				486	returned if the function is modified.<p>
				487
				488	<!-- _______________________________________________________________________ -->
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	489	</ul><h4><a name="doFinalization_mod"><hr size=0>The <tt>doFinalization(Module &)</tt> method</h4><ul>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	490
				491	<pre>
				492	<b>virtual bool</b> doFinalization(Module &M);
Misha Brukman	e3443a6	2003-07-14 17:05:20 +0000	[diff] [blame]	493	</pre></p>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	494
				495	The <tt>doFinalization</tt> method is an infrequently used method that is called
				496	when the pass framework has finished calling <a
				497	href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
				498	program being compiled.<p>
				499
				500
				501
				502	<!-- ======================================================================= -->
				503	</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
				504	<tr><td> </td><td width="100%">
				505	<font color="#EEEEFF" face="Georgia,Palatino"><b>
				506	<a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
				507	</b></font></td></tr></table><ul>
				508
				509	<tt>BasicBlockPass</tt>'s are just like <a
				510	href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
				511	their scope of inspection and modification to a single basic block at a time.
				512	As such, they are <b>not</b> allowed to do any of the following:<p>
				513
				514	<ol>
				515	<li>Modify or inspect any basic blocks outside of the current one
				516	<li>Maintain state across invocations of
				517	<a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a>
				518	<li>Modify the constrol flow graph (by altering terminator instructions)
				519	<li>Any of the things verboten for
				520	<a href="#FunctionPass"><tt>FunctionPass</tt></a>'s.
				521	</ol><p>
				522
				523	<tt>BasicBlockPass</tt>'s are useful for traditional local and "peephole"
				524	optimizations. They may override the same <a
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	525	href="#doInitialization_mod"><tt>doInitialization(Module &)</tt></a> and <a
				526	href="#doFinalization_mod"><tt>doFinalization(Module &)</tt></a> methods that <a
				527	href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:<p>
				528
				529	<!-- _______________________________________________________________________ -->
				530	</ul><h4><a name="doInitialization_fn"><hr size=0>The
				531	<tt>doInitialization(Function &)</tt> method</h4><ul>
				532
				533	<pre>
				534	<b>virtual bool</b> doInitialization(Function &F);
				535	</pre><p>
				536
				537	The <tt>doIninitialize</tt> method is allowed to do most of the things that
				538	<tt>BasicBlockPass</tt>'s are not allowed to do, but that
				539	<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed
				540	to do simple initialization type of stuff that does not depend on the
				541	BasicBlocks being processed. The <tt>doInitialization</tt> method call is not
				542	scheduled to overlap with any other pass executions (thus it should be very
				543	fast).<p>
				544
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	545
				546	<!-- _______________________________________________________________________ -->
				547	</ul><h4><a name="runOnBasicBlock"><hr size=0>The <tt>runOnBasicBlock</tt> method</h4><ul>
				548
				549	<pre>
				550	<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0;
				551	</pre><p>
				552
				553	Override this function to do the work of the <tt>BasicBlockPass</tt>. This
				554	function is not allowed to inspect or modify basic blocks other than the
				555	parameter, and are not allowed to modify the CFG. A true value must be returned
				556	if the basic block is modified.<p>
				557
				558
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	559	<!-- _______________________________________________________________________ -->
				560	</ul><h4><a name="doFinalization_fn"><hr size=0>The <tt>doFinalization(Function
				561	&)</tt> method</h4><ul>
				562
				563	<pre>
				564	<b>virtual bool</b> doFinalization(Function &F);
Misha Brukman	e3443a6	2003-07-14 17:05:20 +0000	[diff] [blame]	565	</pre></p>
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	566
				567	The <tt>doFinalization</tt> method is an infrequently used method that is called
				568	when the pass framework has finished calling <a
				569	href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the
				570	program being compiled. This can be used to perform per-function
				571	finalization.<p>
				572
				573
Brian Gaeke	cab8b6f	2003-07-17 18:53:20 +0000	[diff] [blame^]	574	<!-- ======================================================================= -->
				575	</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
				576	<tr><td> </td><td width="100%">
				577	<font color="#EEEEFF" face="Georgia,Palatino"><b>
				578	<a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class
				579	</b></font></td></tr></table><ul>
				580
				581	A <tt>MachineFunctionPass</tt> executes on the machine-dependent
				582	representation of each LLVM function in the program,
				583	independent of all of the other functions in the program.
				584	A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
				585	the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
				586	<tt>MachineFunctionPass</tt>es also have additional restrictions. In
				587	particular, <tt>MachineFunctionPass</tt>es are not allowed to do any of
				588	the following:
				589
				590	<ol>
				591	<li>Modify any LLVM Instructions, BasicBlocks or Functions.
				592	<li>Modify a MachineFunction other than the one currently being processed.
				593	<li>Add or remove MachineFunctions from the current Module.
				594	<li>Add or remove global variables from the current Module.
				595	<li>Maintain state across invocations of
				596	<a href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global data)
				597	</ol><p>
				598
Chris Lattner	d0713f9	2002-09-12 17:06:43 +0000	[diff] [blame]	599
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	600	<!-- *********************************************************************** -->
				601	</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				602	<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
				603	<a name="registration">Pass registration
				604	</b></font></td></tr></table><ul>
				605	<!-- *********************************************************************** -->
				606
				607	In the <a href="#basiccode">Hello World</a> example pass we illustrated how pass
				608	registration works, and discussed some of the reasons that it is used and what
				609	it does. Here we discuss how and why passes are registered.<p>
				610
				611	Passes can be registered in several different ways. Depending on the general
				612	classification of the pass, you should use one of the following templates to
				613	register the pass:<p>
				614
				615	<ul>
				616	<li><b><tt>RegisterOpt</tt></b> - This template should be used when you are
				617	registering a pass that logically should be available for use in the
				618	'<tt>opt</tt>' utility.<p>
				619
				620	<li><b><tt>RegisterAnalysis</tt></b> - This template should be used when you are
				621	registering a pass that logically should be available for use in the
				622	'<tt>analysis</tt>' utility.<p>
				623
				624	<li><b><tt>RegisterLLC</tt></b> - This template should be used when you are
				625	registering a pass that logically should be available for use in the
				626	'<tt>llc</tt>' utility.<p>
				627
				628	<li><b><tt>RegisterPass</tt></b> - This is the generic form of the
				629	<tt>Register*</tt> templates that should be used if you want your pass listed by
				630	multiple or no utilities. This template takes an extra third argument that
				631	specifies which tools it should be listed in. See the <a
				632	href="http://llvm.cs.uiuc.edu/doxygen/PassSupport_8h-source.html">PassSupport.h</a>
				633	file for more information.<p>
				634	</ul><p>
				635
				636	Regardless of how you register your pass, you must specify at least two
				637	parameters. The first parameter is the name of the pass that is to be used on
				638	the command line to specify that the pass should be added to a program (for
				639	example <tt>opt</tt> or <tt>analyze</tt>). The second argument is the name of
				640	the pass, which is to be used for the <tt>--help</tt> output of programs, as
				641	well as for debug output generated by the <tt>--debug-pass</tt> option.<p>
				642
				643	If you pass is constructed by its default constructor, you only ever have to
				644	pass these two arguments. If, on the other hand, you require other information
				645	(like target specific information), you must pass an additional argument. This
				646	argument is a pointer to a function used to create the pass. For an example of
				647	how this works, look at the <a
				648	href="http://llvm.cs.uiuc.edu/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations.cpp</a>
				649	file.<p>
				650
				651	If a pass is registered to be used by the <tt>analyze</tt> utility, you should
				652	implement the virtual <tt>print</tt> method:<p>
				653
				654	<!-- _______________________________________________________________________ -->
				655	</ul><h4><a name="print"><hr size=0>The <tt>print</tt> method</h4><ul>
				656
				657	<pre>
				658	<b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>;
				659	</pre><p>
				660
				661	The <tt>print</tt> method must be implemented by "analyses" in order to print a
				662	human readable version of the analysis results. This is useful for debugging an
				663	analysis itself, as well as for other people to figure out how an analysis
				664	works. The <tt>analyze</tt> tool uses this method to generate its output.<p>
				665
				666	The <tt>ostream</tt> parameter specifies the stream to write the results on, and
				667	the <tt>Module</tt> parameter gives a pointer to the top level module of the
				668	program that has been analyzed. Note however that this pointer may be null in
				669	certain circumstances (such as calling the <tt>Pass::dump()</tt> from a
				670	debugger), so it should only be used to enhance debug output, it should not be
				671	depended on.<p>
				672
				673
				674	<!-- *********************************************************************** -->
				675	</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				676	<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
				677	<a name="interaction">Specifying interactions between passes
				678	</b></font></td></tr></table><ul>
				679	<!-- *********************************************************************** -->
				680
				681	One of the main responsibilities of the <tt>PassManager</tt> is the make sure
				682	that passes interact with each other correctly. Because <tt>PassManager</tt>
				683	tries to <a href="#passmanager">optimize the execution of passes</a> it must
				684	know how the passes interact with each other and what dependencies exist between
				685	the various passes. To track this, each pass can declare the set of passes that
				686	are required to be executed before the current pass, and the passes which are
				687	invalidated by the current pass.<p>
				688
				689	Typically this functionality is used to require that analysis results are
				690	computed before your pass is run. Running arbitrary transformation passes can
				691	invalidate the computed analysis results, which is what the invalidation set
				692	specifies. If a pass does not implement the <tt><a
				693	href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not
				694	having any prerequisite passes, and invalidating <b>all</b> other passes.<p>
				695
				696
				697	<!-- _______________________________________________________________________ -->
				698	</ul><h4><a name="getAnalysisUsage"><hr size=0>The <tt>getAnalysisUsage</tt> method</h4><ul>
				699
				700	<pre>
				701	<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>;
				702	</pre><p>
				703
				704	By implementing the <tt>getAnalysisUsage</tt> method, the required and
				705	invalidated sets may be specified for your transformation. The implementation
				706	should fill in the <tt><a
				707	href="http://llvm.cs.uiuc.edu/doxygen/classAnalysisUsage.html">AnalysisUsage</a></tt>
				708	object with information about which passes are required and not invalidated. To do this, the following set methods are provided by the <tt><a
				709	href="http://llvm.cs.uiuc.edu/doxygen/classAnalysisUsage.html">AnalysisUsage</a></tt> class:<p>
				710
				711	<pre>
				712	<i>// addRequires - Add the specified pass to the required set for your pass.</i>
				713	<b>template</b><<b>class</b> PassClass>
				714	AnalysisUsage &AnalysisUsage::addRequired();
				715
				716	<i>// addPreserved - Add the specified pass to the set of analyses preserved by
				717	// this pass</i>
				718	<b>template</b><<b>class</b> PassClass>
				719	AnalysisUsage &AnalysisUsage::addPreserved();
				720
				721	<i>// setPreservesAll - Call this if the pass does not modify its input at all</i>
				722	<b>void</b> AnalysisUsage::setPreservesAll();
				723
Chris Lattner	8291e04	2002-10-21 19:57:59 +0000	[diff] [blame]	724	<i>// setPreservesCFG - This function should be called by the pass, iff they do not:
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	725	//
				726	// 1. Add or remove basic blocks from the function
				727	// 2. Modify terminator instructions in any way.
				728	//
				729	// This is automatically implied for <a href="#BasicBlockPass">BasicBlockPass</a>'s
				730	//</i>
Chris Lattner	8291e04	2002-10-21 19:57:59 +0000	[diff] [blame]	731	<b>void</b> AnalysisUsage::setPreservesCFG();
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	732	</pre><p>
				733
				734	Some examples of how to use these methods are:<p>
				735
				736	<pre>
				737	<i>// This is an example implementation from an analysis, which does not modify
				738	// the program at all, yet has a prerequisite.</i>
				739	<b>void</b> <a href="http://llvm.cs.uiuc.edu/doxygen/structPostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				740	AU.setPreservesAll();
				741	AU.addRequired<<a href="http://llvm.cs.uiuc.edu/doxygen/structPostDominatorTree.html">PostDominatorTree</a>>();
				742	}
				743	</pre><p>
				744
				745	and:<p>
				746
				747	<pre>
				748	<i>// This example modifies the program, but does not modify the CFG</i>
				749	<b>void</b> <a href="http://llvm.cs.uiuc.edu/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
Chris Lattner	8291e04	2002-10-21 19:57:59 +0000	[diff] [blame]	750	AU.setPreservesCFG();
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	751	AU.addRequired<<a href="http://llvm.cs.uiuc.edu/doxygen/classLoopInfo.html">LoopInfo</a>>();
				752	}
				753	</pre><p>
				754
				755	<!-- _______________________________________________________________________ -->
				756	</ul><h4><a name="getAnalysis"><hr size=0>The <tt>getAnalysis<></tt> method</h4><ul>
				757
				758	The <tt>Pass::getAnalysis<></tt> method is inherited by your class,
				759	providing you with access to the passes that you declared that you required with
				760	the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method. It takes
				761	a single template argument that specifies which pass class you want, and returns
				762	a reference to that pass.<p>
				763
				764	<pre>
				765	<b>template</b><<b>typename</b> PassClass>
				766	AnalysisType &getAnalysis();
				767	</pre><p>
				768
				769	This method call returns a reference to the pass desired. You may get a runtime
				770	assertion failure if you attempt to get an analysis that you did not declare as
				771	required in your <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
				772	implementation. This method can be called by your <tt>run*</tt> method
				773	implementation, or by any other local method invoked by your <tt>run*</tt>
				774	method.<p>
				775
Chris Lattner	7991070	2002-08-22 19:21:04 +0000	[diff] [blame]	776	<!-- *********************************************************************** -->
				777	</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				778	<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
				779	<a name="analysisgroup">Implementing Analysis Groups
				780	</b></font></td></tr></table><ul>
				781	<!-- *********************************************************************** -->
				782
				783	Now that we understand the basics of how passes are defined, how the are used,
				784	and how they are required from other passes, it's time to get a little bit
				785	fancier. All of the pass relationships that we have seen so far are very
				786	simple: one pass depends on one other specific pass to be run before it can run.
				787	For many applications, this is great, for others, more flexibility is
				788	required.<p>
				789
				790	In particular, some analyses are defined such that there is a single simple
				791	interface to the analysis results, but multiple ways of calculating them.
				792	Consider alias analysis for example. The most trivial alias analysis returns
				793	"may alias" for any alias query. The most sophisticated analysis a
				794	flow-sensitive, context-sensitive interprocedural analysis that can take a
				795	significant amount of time to execute (and obviously, there is a lot of room
				796	between these two extremes for other implementations). To cleanly support
				797	situations like this, the LLVM Pass Infrastructure supports the notion of
				798	Analysis Groups.<p>
				799
				800	<!-- _______________________________________________________________________ -->
				801	</ul><h4><a name="agconcepts"><hr size=0>Analysis Group Concepts</h4><ul>
				802
				803	An Analysis Group is a single simple interface that may be implemented by
				804	multiple different passes. Analysis Groups can be given human readable names
				805	just like passes, but unlike passes, they need not derive from the <tt>Pass</tt>
				806	class. An analysis group may have one or more implementations, one of which is
				807	the "default" implementation.<p>
				808
				809	Analysis groups are used by client passes just like other passes are: the
				810	<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods.
				811	In order to resolve this requirement, the <a href="#passmanager">PassManager</a>
				812	scans the available passes to see if any implementations of the analysis group
				813	are available. If none is available, the default implementation is created for
				814	the pass to use. All standard rules for <A href="#interaction">interaction
				815	between passes</a> still apply.<p>
				816
				817	Although <a href="#registration">Pass Registration</a> is optional for normal
				818	passes, all analysis group implementations must be registered, and must use the
				819	<A href="#registerag"><tt>RegisterAnalysisGroup</tt></a> template to join the
				820	implementation pool. Also, a default implementation of the interface
				821	<b>must</b> be registered with <A
				822	href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.<p>
				823
				824	As a concrete example of an Analysis Group in action, consider the <a
				825	href="http://llvm.cs.uiuc.edu/doxygen/structAliasAnalysis.html">AliasAnalysis</a>
				826	analysis group. The default implementation of the alias analysis interface (the
				827	<tt><a
				828	href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
				829	pass) just does a few simple checks that don't require significant analysis to
				830	compute (such as: two different globals can never alias each other, etc).
				831	Passes that use the <tt><a
				832	href="http://llvm.cs.uiuc.edu/doxygen/structAliasAnalysis.html">AliasAnalysis</a></tt>
				833	interface (for example the <tt><a
				834	href="http://llvm.cs.uiuc.edu/doxygen/classGCSE.html">gcse</a></tt> pass), do not care which implementation
				835	of alias analysis is actually provided, they just use the designated
				836	interface.<p>
				837
				838	From the user's perspective, commands work just like normal. Issuing the
				839	command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be
				840	instantiated and added to the pass sequence. Issuing the command '<tt>opt
				841	-somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the
				842	<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a
				843	hypothetical example) instead.<p>
				844
				845
				846	<!-- _______________________________________________________________________ -->
				847	</ul><h4><a name="registerag"><hr size=0>Using <tt>RegisterAnalysisGroup</tt></h4><ul>
				848
				849	The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
				850	group itself as well as add pass implementations to the analysis group. First,
				851	an analysis should be registered, with a human readable name provided for it.
				852	Unlike registration of passes, there is no command line argument to be specified
				853	for the Analysis Group Interface itself, because it is "abstract":<p>
				854
				855	<pre>
				856	<b>static</b> RegisterAnalysisGroup<<a href="http://llvm.cs.uiuc.edu/doxygen/structAliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>");
				857	</pre><p>
				858
				859	Once the analysis is registered, passes can declare that they are valid
				860	implementations of the interface by using the following code:<p>
				861
				862	<pre>
				863	<b>namespace</b> {
				864	//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
				865	RegisterOpt<FancyAA>
				866	B("<i>somefancyaa</i>", "<i>A more complex alias analysis implementation</i>");
				867
				868	//<i> Declare that we implement the AliasAnalysis interface</i>
				869	RegisterAnalysisGroup<<a href="http://llvm.cs.uiuc.edu/doxygen/structAliasAnalysis.html">AliasAnalysis</a>, FancyAA> C;
				870	}
				871	</pre><p>
				872
				873	This just shows a class <tt>FancyAA</tt> that is registered normally, then uses
				874	the <tt>RegisterAnalysisGroup</tt> template to "join" the <tt><a
				875	href="http://llvm.cs.uiuc.edu/doxygen/structAliasAnalysis.html">AliasAnalysis</a></tt>
				876	analysis group. Every implementation of an analysis group should join using
				877	this template. A single pass may join multiple different analysis groups with
				878	no problem.<p>
				879
				880	<pre>
				881	<b>namespace</b> {
				882	//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
				883	RegisterOpt<<a href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>>
				884	D("<i>basicaa</i>", "<i>Basic Alias Analysis (default AA impl)</i>");
				885
				886	//<i> Declare that we implement the AliasAnalysis interface</i>
				887	RegisterAnalysisGroup<<a href="http://llvm.cs.uiuc.edu/doxygen/structAliasAnalysis.html">AliasAnalysis</a>, <a href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>, <b>true</b>> E;
				888	}
				889	</pre><p>
				890
				891	Here we show how the default implementation is specified (using the extra
				892	argument to the <tt>RegisterAnalysisGroup</tt> template). There must be exactly
				893	one default implementation available at all times for an Analysis Group to be
				894	used. Here we declare that the <tt><a
				895	href="http://llvm.cs.uiuc.edu/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
				896	pass is the default implementation for the interface.<p>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	897
				898
				899	<!-- *********************************************************************** -->
				900	</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				901	<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
				902	<a name="passmanager">What PassManager does
				903	</b></font></td></tr></table><ul>
				904	<!-- *********************************************************************** -->
				905
				906	The <a
				907	href="http://llvm.cs.uiuc.edu/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
				908	<a href="http://llvm.cs.uiuc.edu/doxygen/classPassManager.html">class</a> takes
				909	a list of passes, ensures their <a href="#interaction">prerequisites</a> are set
				910	up correctly, and then schedules passes to run efficiently. All of the LLVM
				911	tools that run passes use the <tt>PassManager</tt> for execution of these
				912	passes.<p>
				913
				914	The <tt>PassManager</tt> does two main things to try to reduce the execution
				915	time of a series of passes:<p>
				916
				917	<ol>
				918	<li><b>Share analysis results</b> - The PassManager attempts to avoid
				919	recomputing analysis results as much as possible. This means keeping track of
				920	which analyses are available already, which analyses get invalidated, and which
				921	analyses are needed to be run for a pass. An important part of work is that the
				922	<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing
				923	it to <a href="#releaseMemory">free memory</a> allocated to holding analysis
				924	results as soon as they are no longer needed.<p>
				925
				926	<li><b>Pipeline the execution of passes on the program</b> - The
				927	<tt>PassManager</tt> attempts to get better cache and memory usage behavior out
				928	of a series of passes by pipelining the passes together. This means that, given
				929	a series of consequtive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
				930	will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on
				931	the first function, then all of the <a
				932	href="#FunctionPass"><tt>FunctionPass</tt></a>'s on the second function,
				933	etc... until the entire program has been run through the passes.<p>
				934
				935	This improves the cache behavior of the compiler, because it is only touching
				936	the LLVM program representation for a single function at a time, instead of
				937	traversing the entire program. It reduces the memory consumption of compiler,
				938	because, for example, only one <a
				939	href="http://llvm.cs.uiuc.edu/doxygen/structDominatorSet.html"><tt>DominatorSet</tt></a>
				940	needs to be calculated at a time. This also makes it possible some <a
				941	href="#SMP">interesting enhancements</a> in the future.<p>
				942
				943	</ol><p>
				944
				945	The effectiveness of the <tt>PassManager</tt> is influenced directly by how much
				946	information it has about the behaviors of the passes it is scheduling. For
				947	example, the "preserved" set is intentionally conservative in the face of an
				948	unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method.
				949	Not implementing when it should be implemented will have the effect of not
				950	allowing any analysis results to live across the execution of your pass.<p>
				951
				952	The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line
				953	options that is useful for debugging pass execution, seeing how things work, and
				954	diagnosing when you should be preserving more analyses than you currently are
				955	(To get information about all of the variants of the <tt>--debug-pass</tt>
				956	option, just type '<tt>opt --help-hidden</tt>').<p>
				957
				958	By using the <tt>--debug-pass=Structure</tt> option, for example, we can see how
				959	our <a href="#basiccode">Hello World</a> pass interacts with other passes. Lets
				960	try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:<p>
				961
				962	<pre>
				963	$ opt -load ../../../lib/Debug/libhello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null
				964	Module Pass Manager
				965	Function Pass Manager
				966	Dominator Set Construction
				967	Immediate Dominators Construction
				968	Global Common Subexpression Elimination
				969	-- Immediate Dominators Construction
				970	-- Global Common Subexpression Elimination
				971	Natural Loop Construction
				972	Loop Invariant Code Motion
				973	-- Natural Loop Construction
				974	-- Loop Invariant Code Motion
				975	Module Verifier
				976	-- Dominator Set Construction
				977	-- Module Verifier
				978	Bytecode Writer
				979	--Bytecode Writer
				980	</pre><p>
				981
				982	This output shows us when passes are constructed and when the analysis results
				983	are known to be dead (prefixed with '<tt>--</tt>'). Here we see that GCSE uses
				984	dominator and immediate dominator information to do its job. The LICM pass uses
				985	natural loop information, which uses dominator sets, but not immediate
				986	dominators. Because immediate dominators are no longer useful after the GCSE
				987	pass, it is immediately destroyed. The dominator sets are then reused to
				988	compute natural loop information, which is then used by the LICM pass.<p>
				989
				990	After the LICM pass, the module verifier runs (which is automatically added by
				991	the '<tt>opt</tt>' tool), which uses the dominator set to check that the
				992	resultant LLVM code is well formed. After it finishes, the dominator set
				993	information is destroyed, after being computed once, and shared by three
				994	passes.<p>
				995
				996	Lets see how this changes when we run the <a href="#basiccode">Hello World</a>
				997	pass in between the two passes:<p>
				998
				999	<pre>
				1000	$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
				1001	Module Pass Manager
				1002	Function Pass Manager
				1003	Dominator Set Construction
				1004	Immediate Dominators Construction
				1005	Global Common Subexpression Elimination
				1006	<b>-- Dominator Set Construction</b>
				1007	-- Immediate Dominators Construction
				1008	-- Global Common Subexpression Elimination
				1009	<b> Hello World Pass
				1010	-- Hello World Pass
				1011	Dominator Set Construction</b>
				1012	Natural Loop Construction
				1013	Loop Invariant Code Motion
				1014	-- Natural Loop Construction
				1015	-- Loop Invariant Code Motion
				1016	Module Verifier
				1017	-- Dominator Set Construction
				1018	-- Module Verifier
				1019	Bytecode Writer
				1020	--Bytecode Writer
				1021	Hello: __main
				1022	Hello: puts
				1023	Hello: main
				1024	</pre><p>
				1025
				1026	Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
				1027	Dominator Set pass, even though it doesn't modify the code at all! To fix this,
				1028	we need to add the following <a
				1029	href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:<p>
				1030
				1031	<pre>
				1032	<i>// We don't modify the program, so we preserve all analyses</i>
				1033	<b>virtual void</b> getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1034	AU.setPreservesAll();
				1035	}
				1036	</pre><p>
				1037
				1038	Now when we run our pass, we get this output:<p>
				1039
				1040	<pre>
				1041	$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
				1042	Pass Arguments: -gcse -hello -licm
				1043	Module Pass Manager
				1044	Function Pass Manager
				1045	Dominator Set Construction
				1046	Immediate Dominators Construction
				1047	Global Common Subexpression Elimination
				1048	-- Immediate Dominators Construction
				1049	-- Global Common Subexpression Elimination
				1050	Hello World Pass
				1051	-- Hello World Pass
				1052	Natural Loop Construction
				1053	Loop Invariant Code Motion
				1054	-- Loop Invariant Code Motion
				1055	-- Natural Loop Construction
				1056	Module Verifier
				1057	-- Dominator Set Construction
				1058	-- Module Verifier
				1059	Bytecode Writer
				1060	--Bytecode Writer
				1061	Hello: __main
				1062	Hello: puts
				1063	Hello: main
				1064	</pre><p>
				1065
				1066	Which shows that we don't accidentally invalidate dominator information
				1067	anymore, and therefore do not have to compute it twice.<p>
				1068
				1069
				1070	<!-- _______________________________________________________________________ -->
				1071	</ul><h4><a name="releaseMemory"><hr size=0>The <tt>releaseMemory</tt> method</h4><ul>
				1072
				1073	<pre>
				1074	<b>virtual void</b> releaseMemory();
				1075	</pre><p>
				1076
				1077	The <tt>PassManager</tt> automatically determines when to compute analysis
				1078	results, and how long to keep them around for. Because the lifetime of the pass
				1079	object itself is effectively the entire duration of the compilation process, we
				1080	need some way to free analysis results when they are no longer useful. The
				1081	<tt>releaseMemory</tt> virtual method is the way to do this.<p>
				1082
				1083	If you are writing an analysis or any other pass that retains a significant
				1084	amount of state (for use by another pass which "requires" your pass and uses the
				1085	<a href="#getAnalysis">getAnalysis</a> method) you should implement
				1086	<tt>releaseMEmory</tt> to, well, release the memory allocated to maintain this
				1087	internal state. This method is called after the <tt>run*</tt> method for the
				1088	class, before the next call of <tt>run*</tt> in your pass.<p>
				1089
				1090
				1091	<!-- *********************************************************************** -->
				1092	</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				1093	<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
Chris Lattner	480e2ef	2002-09-06 02:02:58 +0000	[diff] [blame]	1094	<a name="debughints">Using GDB with dynamically loaded passes
				1095	</b></font></td></tr></table><ul>
				1096	<!-- *********************************************************************** -->
				1097
				1098	Unfortunately, using GDB with dynamically loaded passes is not as easy as it
				1099	should be. First of all, you can't set a breakpoint in a shared object that has
				1100	not been loaded yet, and second of all there are problems with inlined functions
				1101	in shared objects. Here are some suggestions to debugging your pass with
				1102	GDB.<p>
				1103
				1104	For sake of discussion, I'm going to assume that you are debugging a
				1105	transformation invoked by <tt>opt</tt>, although nothing described here depends
				1106	on that.<p>
				1107
				1108	<!-- _______________________________________________________________________ -->
				1109	</ul><h4><a name="breakpoint"><hr size=0>Setting a breakpoint in your pass</h4><ul>
				1110
				1111	First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:<p>
				1112
				1113	<pre>
				1114	$ <b>gdb opt</b>
				1115	GNU gdb 5.0
				1116	Copyright 2000 Free Software Foundation, Inc.
				1117	GDB is free software, covered by the GNU General Public License, and you are
				1118	welcome to change it and/or distribute copies of it under certain conditions.
				1119	Type "show copying" to see the conditions.
				1120	There is absolutely no warranty for GDB. Type "show warranty" for details.
				1121	This GDB was configured as "sparc-sun-solaris2.6"...
				1122	(gdb)
				1123	</pre><p>
				1124
				1125	Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
				1126	time to load. Be patient. Since we cannot set a breakpoint in our pass yet
				1127	(the shared object isn't loaded until runtime), we must execute the process, and
				1128	have it stop before it invokes our pass, but after it has loaded the shared
				1129	object. The most foolproof way of doing this is to set a breakpoint in
				1130	<tt>PassManager::run</tt> and then run the process with the arguments you
				1131	want:<p>
				1132
				1133	<pre>
				1134	(gdb) <b>break PassManager::run</b>
				1135	Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
				1136	(gdb) <b>run test.bc -load /shared/lattner/cvs/llvm/lib/Debug/[libname].so -[passoption]</b>
				1137	Starting program: /shared/lattner/cvs/llvm/tools/Debug/opt test.bc
				1138	-load /shared/lattner/cvs/llvm/lib/Debug/[libname].so -[passoption]
				1139	Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
				1140	70 bool PassManager::run(Module &M) { return PM->run(M); }
				1141	(gdb)
				1142	</pre></p>
				1143
				1144	Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are now
				1145	free to set breakpoints in your pass so that you can trace through execution or
				1146	do other standard debugging stuff.<p>
				1147
				1148
				1149	<!-- _______________________________________________________________________ -->
				1150	</ul><h4><a name="debugmisc"><hr size=0>Miscellaneous Problems</h4><ul>
				1151
				1152	Once you have the basics down, there are a couple of problems that GDB has, some
				1153	with solutions, some without.<p>
				1154
				1155	<ul>
				1156	<li>Inline functions have bogus stack information. In general, GDB does a
				1157	pretty good job getting stack traces and stepping through inline functions.
				1158	When a pass is dynamically loaded however, it somehow completely loses this
				1159	capability. The only solution I know of is to de-inline a function (move it
				1160	from the body of a class to a .cpp file).<p>
				1161
				1162	<li>Restarting the program breaks breakpoints. After following the information
				1163	above, you have succeeded in getting some breakpoints planted in your pass. Nex
				1164	thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
				1165	and you start getting errors about breakpoints being unsettable. The only way I
				1166	have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
				1167	already set in your pass, run the program, and re-set the breakpoints once
				1168	execution stops in <tt>PassManager::run</tt>.<p>
				1169
				1170	</ul>
				1171
				1172	Hopefully these tips will help with common case debugging situations. If you'd
				1173	like to contribute some tips of your own, just contact <a
				1174	href="mailto:sabre@nondot.org">Chris</a>.<p>
				1175
				1176
				1177	<!-- *********************************************************************** -->
				1178	</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
				1179	<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	1180	<a name="future">Future extensions planned
				1181	</b></font></td></tr></table><ul>
				1182	<!-- *********************************************************************** -->
				1183
				1184	Although the LLVM Pass Infrastructure is very capable as it stands, and does
				1185	some nifty stuff, there are things we'd like to add in the future. Here is
				1186	where we are going:<p>
				1187
				1188	<!-- _______________________________________________________________________ -->
				1189	</ul><h4><a name="SMP"><hr size=0>Multithreaded LLVM</h4><ul>
				1190
Chris Lattner	ed2e7a9	2002-09-17 16:47:06 +0000	[diff] [blame]	1191	Multiple CPU machines are becoming more common and compilation can never be
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	1192	fast enough: obviously we should allow for a multithreaded compiler. Because of
				1193	the semantics defined for passes above (specifically they cannot maintain state
				1194	across invocations of their <tt>run*</tt> methods), a nice clean way to
				1195	implement a multithreaded compiler would be for the <tt>PassManager</tt> class
Misha Brukman	bc0e998	2003-07-14 17:20:40 +0000	[diff] [blame]	1196	to create multiple instances of each pass object, and allow the separate
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	1197	instances to be hacking on different parts of the program at the same time.<p>
				1198
				1199	This implementation would prevent each of the passes from having to implement
				1200	multithreaded constructs, requiring only the LLVM core to have locking in a few
				1201	places (for global resources). Although this is a simple extension, we simply
				1202	haven't had time (or multiprocessor machines, thus a reason) to implement this.
				1203	Despite that, we have kept the LLVM passes SMP ready, and you should too.<p>
				1204
				1205
				1206	<!-- _______________________________________________________________________ -->
				1207	</ul><h4><a name="ModuleSource"><hr size=0>A new <tt>ModuleSource</tt> interface</h4><ul>
				1208
				1209	Currently, the <tt>PassManager</tt>'s <tt>run</tt> method takes a <tt><a
				1210	href="http://llvm.cs.uiuc.edu/doxygen/classModule.html">Module</a></tt> as
				1211	input, and runs all of the passes on this module. The problem with this
				1212	approach is that none of the <tt>PassManager</tt> features can be used for
				1213	timing and debugging the actual <b>loading</b> of the module from disk or
				1214	standard input.<p>
				1215
				1216	To solve this problem, eventually the <tt>PassManger</tt> class will accept a
				1217	<tt>ModuleSource</tt> object instead of a Module itself. When complete, this
				1218	will also allow for streaming of functions out of the bytecode representation,
				1219	allowing us to avoid holding the entire program in memory at once if we only are
				1220	dealing with <a href="#FunctionPass">FunctionPass</a>'s.<p>
				1221
				1222	As part of a different issue, eventually the bytecode loader will be extended to
				1223	allow on-demand loading of functions from the bytecode representation, in order
				1224	to better support the runtime reoptimizer. The bytecode format is already
				1225	capable of this, the loader just needs to be reworked a bit.<p>
				1226
				1227
				1228	<!-- _______________________________________________________________________ -->
				1229	</ul><h4><a name="PassFunctionPass"><hr size=0><tt>Pass</tt>'s requiring <tt>FunctionPass</tt>'s</h4><ul>
				1230
				1231	Currently it is illegal for a <a href="#Pass"><tt>Pass</tt></a> to require a <a
				1232	href="#FunctionPass"><tt>FunctionPass</tt></a>. This is because there is only
				1233	one instance of the <a href="#FunctionPass"><tt>FunctionPass</tt></a> object
				1234	ever created, thus nowhere to store information for all of the functions in the
				1235	program at the same time. Although this has come up a couple of times before,
				1236	this has always been worked around by factoring one big complicated pass into a
				1237	global and an interprocedural part, both of which are distinct. In the future,
				1238	it would be nice to have this though.<p>
				1239
				1240	Note that it is no problem for a <a
				1241	href="#FunctionPass"><tt>FunctionPass</tt></a> to require the results of a <a
				1242	href="#Pass"><tt>Pass</tt></a>, only the other way around.<p>
				1243
				1244
				1245	<!-- *********************************************************************** -->
				1246	</ul>
				1247	<!-- *********************************************************************** -->
				1248
				1249	<hr><font size-1>
Chris Lattner	480e2ef	2002-09-06 02:02:58 +0000	[diff] [blame]	1250	<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	1251	<!-- Created: Tue Aug 6 15:00:33 CDT 2002 -->
				1252	<!-- hhmts start -->
Misha Brukman	bc0e998	2003-07-14 17:20:40 +0000	[diff] [blame]	1253	Last modified: Mon Jul 14 12:12:53 CDT 2003
Chris Lattner	c6bb824	2002-08-08 20:11:18 +0000	[diff] [blame]	1254	<!-- hhmts end -->
				1255	</font></body></html>