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				6	<title>Writing an LLVM Pass</title>
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				10
				11	<div class="doc_title">
				12	Writing an LLVM Pass
				13	</div>
				14
				15	<ol>
				16	<li><a href="#introduction">Introduction - What is a pass?</a></li>
				17	<li><a href="#quickstart">Quick Start - Writing hello world</a>
				18	<ul>
				19	<li><a href="#makefile">Setting up the build environment</a></li>
				20	<li><a href="#basiccode">Basic code required</a></li>
				21	<li><a href="#running">Running a pass with <tt>opt</tt></a></li>
				22	</ul></li>
				23	<li><a href="#passtype">Pass classes and requirements</a>
				24	<ul>
				25	<li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li>
				26	<li><a href="#ModulePass">The <tt>ModulePass</tt> class</a>
				27	<ul>
				28	<li><a href="#runOnModule">The <tt>runOnModule</tt> method</a></li>
				29	</ul></li>
				30	<li><a href="#CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
				31	<ul>
				32	<li><a href="#doInitialization_scc">The <tt>doInitialization(CallGraph
				33	&)</tt> method</a></li>
				34	<li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li>
				35	<li><a href="#doFinalization_scc">The <tt>doFinalization(CallGraph
				36	&)</tt> method</a></li>
				37	</ul></li>
				38	<li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a>
				39	<ul>
				40	<li><a href="#doInitialization_mod">The <tt>doInitialization(Module
				41	&)</tt> method</a></li>
				42	<li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li>
				43	<li><a href="#doFinalization_mod">The <tt>doFinalization(Module
				44	&)</tt> method</a></li>
				45	</ul></li>
				46	<li><a href="#LoopPass">The <tt>LoopPass</tt> class</a>
				47	<ul>
				48	<li><a href="#doInitialization_loop">The <tt>doInitialization(Loop *,
				49	LPPassManager &)</tt> method</a></li>
				50	<li><a href="#runOnLoop">The <tt>runOnLoop</tt> method</a></li>
				51	<li><a href="#doFinalization_loop">The <tt>doFinalization()
				52	</tt> method</a></li>
				53	</ul></li>
				54	<li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
				55	<ul>
				56	<li><a href="#doInitialization_fn">The <tt>doInitialization(Function
				57	&)</tt> method</a></li>
				58	<li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt>
				59	method</a></li>
				60	<li><a href="#doFinalization_fn">The <tt>doFinalization(Function
				61	&)</tt> method</a></li>
				62	</ul></li>
				63	<li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt>
				64	class</a>
				65	<ul>
				66	<li><a href="#runOnMachineFunction">The
				67	<tt>runOnMachineFunction(MachineFunction &)</tt> method</a></li>
				68	</ul></li>
				69	</ul>
				70	<li><a href="#registration">Pass Registration</a>
				71	<ul>
				72	<li><a href="#print">The <tt>print</tt> method</a></li>
				73	</ul></li>
				74	<li><a href="#interaction">Specifying interactions between passes</a>
				75	<ul>
				76	<li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt>
				77	method</a></li>
				78	<li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a></li>
				79	<li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a></li>
				80	<li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li>
Duncan Sands	4e0d6a7	2009-01-28 13:14:17 +0000	[diff] [blame]	81	<li><a href="#getAnalysis">The <tt>getAnalysis<></tt> and
				82	<tt>getAnalysisIfAvailable<></tt> methods</a></li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	83	</ul></li>
				84	<li><a href="#analysisgroup">Implementing Analysis Groups</a>
				85	<ul>
				86	<li><a href="#agconcepts">Analysis Group Concepts</a></li>
				87	<li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li>
				88	</ul></li>
				89	<li><a href="#passStatistics">Pass Statistics</a>
				90	<li><a href="#passmanager">What PassManager does</a>
				91	<ul>
				92	<li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li>
				93	</ul></li>
				94	<li><a href="#registering">Registering dynamically loaded passes</a>
				95	<ul>
				96	<li><a href="#registering_existing">Using existing registries</a></li>
				97	<li><a href="#registering_new">Creating new registries</a></li>
				98	</ul></li>
				99	<li><a href="#debughints">Using GDB with dynamically loaded passes</a>
				100	<ul>
				101	<li><a href="#breakpoint">Setting a breakpoint in your pass</a></li>
				102	<li><a href="#debugmisc">Miscellaneous Problems</a></li>
				103	</ul></li>
				104	<li><a href="#future">Future extensions planned</a>
				105	<ul>
				106	<li><a href="#SMP">Multithreaded LLVM</a></li>
				107	</ul></li>
				108	</ol>
				109
				110	<div class="doc_author">
				111	<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
				112	<a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
				113	</div>
				114
				115	<!-- *********************************************************************** -->
				116	<div class="doc_section">
				117	<a name="introduction">Introduction - What is a pass?</a>
				118	</div>
				119	<!-- *********************************************************************** -->
				120
				121	<div class="doc_text">
				122
				123	<p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM
				124	passes are where most of the interesting parts of the compiler exist. Passes
				125	perform the transformations and optimizations that make up the compiler, they
				126	build the analysis results that are used by these transformations, and they are,
				127	above all, a structuring technique for compiler code.</p>
				128
				129	<p>All LLVM passes are subclasses of the <tt><a
				130	href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>
				131	class, which implement functionality by overriding virtual methods inherited
				132	from <tt>Pass</tt>. Depending on how your pass works, you should inherit from
				133	the <tt><a href="#ModulePass">ModulePass</a></tt>, <tt><a
				134	href="#CallGraphSCCPass">CallGraphSCCPass</a></tt>, <tt><a
				135	href="#FunctionPass">FunctionPass</a></tt>, or <tt><a
				136	href="#LoopPass">LoopPass</a></tt>, or <tt><a
				137	href="#BasicBlockPass">BasicBlockPass</a></tt> classes, which gives the system
				138	more information about what your pass does, and how it can be combined with
				139	other passes. One of the main features of the LLVM Pass Framework is that it
				140	schedules passes to run in an efficient way based on the constraints that your
				141	pass meets (which are indicated by which class they derive from).</p>
				142
				143	<p>We start by showing you how to construct a pass, everything from setting up
				144	the code, to compiling, loading, and executing it. After the basics are down,
				145	more advanced features are discussed.</p>
				146
				147	</div>
				148
				149	<!-- *********************************************************************** -->
				150	<div class="doc_section">
				151	<a name="quickstart">Quick Start - Writing hello world</a>
				152	</div>
				153	<!-- *********************************************************************** -->
				154
				155	<div class="doc_text">
				156
				157	<p>Here we describe how to write the "hello world" of passes. The "Hello" pass
				158	is designed to simply print out the name of non-external functions that exist in
				159	the program being compiled. It does not modify the program at all, it just
				160	inspects it. The source code and files for this pass are available in the LLVM
				161	source tree in the <tt>lib/Transforms/Hello</tt> directory.</p>
				162
				163	</div>
				164
				165	<!-- ======================================================================= -->
				166	<div class="doc_subsection">
				167	<a name="makefile">Setting up the build environment</a>
				168	</div>
				169
				170	<div class="doc_text">
				171
				172	<p>First, you need to create a new directory somewhere in the LLVM source
				173	base. For this example, we'll assume that you made
				174	<tt>lib/Transforms/Hello</tt>. Next, you must set up a build script
				175	(Makefile) that will compile the source code for the new pass. To do this,
				176	copy the following into <tt>Makefile</tt>:</p>
				177	<hr/>
				178
				179	<div class="doc_code"><pre>
				180	# Makefile for hello pass
				181
				182	# Path to top level of LLVM heirarchy
				183	LEVEL = ../../..
				184
				185	# Name of the library to build
				186	LIBRARYNAME = Hello
				187
				188	# Make the shared library become a loadable module so the tools can
				189	# dlopen/dlsym on the resulting library.
				190	LOADABLE_MODULE = 1
				191
				192	# Tell the build system which LLVM libraries your pass needs. You'll probably
				193	# need at least LLVMSystem.a, LLVMSupport.a, LLVMCore.a but possibly several
				194	# others too.
				195	LLVMLIBS = LLVMCore.a LLVMSupport.a LLVMSystem.a
				196
				197	# Include the makefile implementation stuff
				198	include $(LEVEL)/Makefile.common
				199	</pre></div>
				200
				201	<p>This makefile specifies that all of the <tt>.cpp</tt> files in the current
				202	directory are to be compiled and linked together into a
				203	<tt>Debug/lib/Hello.so</tt> shared object that can be dynamically loaded by
				204	the <tt>opt</tt> or <tt>bugpoint</tt> tools via their <tt>-load</tt> options.
				205	If your operating system uses a suffix other than .so (such as windows or
				206	Mac OS/X), the appropriate extension will be used.</p>
				207
				208	<p>Now that we have the build scripts set up, we just need to write the code for
				209	the pass itself.</p>
				210
				211	</div>
				212
				213	<!-- ======================================================================= -->
				214	<div class="doc_subsection">
				215	<a name="basiccode">Basic code required</a>
				216	</div>
				217
				218	<div class="doc_text">
				219
				220	<p>Now that we have a way to compile our new pass, we just have to write it.
				221	Start out with:</p>
				222
				223	<div class="doc_code"><pre>
				224	<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
				225	<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
				226	</pre></div>
				227
				228	<p>Which are needed because we are writing a <tt><a
				229	href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>, and
				230	we are operating on <tt><a
				231	href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s.</p>
				232
				233	<p>Next we have:</p>
				234	<div class="doc_code"><pre>
				235	<b>using namespace llvm;</b>
				236	</pre></div>
				237	<p>... which is required because the functions from the include files
				238	live in the llvm namespace.
				239	</p>
				240
				241	<p>Next we have:</p>
				242
				243	<div class="doc_code"><pre>
				244	<b>namespace</b> {
				245	</pre></div>
				246
				247	<p>... which starts out an anonymous namespace. Anonymous namespaces are to C++
				248	what the "<tt>static</tt>" keyword is to C (at global scope). It makes the
				249	things declared inside of the anonymous namespace only visible to the current
				250	file. If you're not familiar with them, consult a decent C++ book for more
				251	information.</p>
				252
				253	<p>Next, we declare our pass itself:</p>
				254
				255	<div class="doc_code"><pre>
				256	<b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
				257	</pre></div><p>
				258
				259	<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
				260	href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>.
				261	The different builtin pass subclasses are described in detail <a
				262	href="#passtype">later</a>, but for now, know that <a
				263	href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate a function at a
				264	time.</p>
				265
				266	<div class="doc_code"><pre>
				267	static char ID;
Dan Gohman	c74a197	2009-02-18 05:09:16 +0000	[diff] [blame]	268	Hello() : FunctionPass(&ID) {}
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	269	</pre></div><p>
				270
				271	<p> This declares pass identifier used by LLVM to identify pass. This allows LLVM to
				272	avoid using expensive C++ runtime information.</p>
				273
				274	<div class="doc_code"><pre>
				275	<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
				276	llvm::cerr << "<i>Hello: </i>" << F.getName() << "\n";
				277	<b>return false</b>;
				278	}
				279	}; <i>// end of struct Hello</i>
				280	</pre></div>
				281
				282	<p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method,
				283	which overloads an abstract virtual method inherited from <a
				284	href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed
				285	to do our thing, so we just print out our message with the name of each
				286	function.</p>
				287
				288	<div class="doc_code"><pre>
				289	char Hello::ID = 0;
				290	</pre></div>
				291
				292	<p> We initialize pass ID here. LLVM uses ID's address to identify pass so
				293	initialization value is not important.</p>
				294
				295	<div class="doc_code"><pre>
Devang Patel	3aab76e	2008-03-19 21:56:59 +0000	[diff] [blame]	296	RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>",
				297	false /* Only looks at CFG */,
				298	false /* Analysis Pass */);
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	299	} <i>// end of anonymous namespace</i>
				300	</pre></div>
				301
				302	<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>,
				303	giving it a command line
Devang Patel	3aab76e	2008-03-19 21:56:59 +0000	[diff] [blame]	304	argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>".
				305	Last two RegisterPass arguments are optional. Their default value is false.
				306	If a pass walks CFG without modifying it then third argument is set to true.
				307	If a pass is an analysis pass, for example dominator tree pass, then true
				308	is supplied as fourth argument. </p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	309
				310	<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
				311
				312	<div class="doc_code"><pre>
				313	<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
				314	<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
				315
				316	<b>using namespace llvm;</b>
				317
				318	<b>namespace</b> {
				319	<b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
				320
				321	static char ID;
Dan Gohman	c74a197	2009-02-18 05:09:16 +0000	[diff] [blame]	322	Hello() : FunctionPass(&ID) {}
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	323
				324	<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
				325	llvm::cerr << "<i>Hello: </i>" << F.getName() << "\n";
				326	<b>return false</b>;
				327	}
				328	};
				329
Devang Patel	8e46f05	2007-07-25 21:05:39 +0000	[diff] [blame]	330	char Hello::ID = 0;
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	331	RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
				332	}
				333	</pre></div>
				334
				335	<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
				336	command in the local directory and you should get a new
				337	"<tt>Debug/lib/Hello.so</tt> file. Note that everything in this file is
				338	contained in an anonymous namespace: this reflects the fact that passes are self
				339	contained units that do not need external interfaces (although they can have
				340	them) to be useful.</p>
				341
				342	</div>
				343
				344	<!-- ======================================================================= -->
				345	<div class="doc_subsection">
				346	<a name="running">Running a pass with <tt>opt</tt></a>
				347	</div>
				348
				349	<div class="doc_text">
				350
				351	<p>Now that you have a brand new shiny shared object file, we can use the
				352	<tt>opt</tt> command to run an LLVM program through your pass. Because you
				353	registered your pass with the <tt>RegisterPass</tt> template, you will be able to
				354	use the <tt>opt</tt> tool to access it, once loaded.</p>
				355
				356	<p>To test it, follow the example at the end of the <a
				357	href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
				358	LLVM. We can now run the bitcode file (<tt>hello.bc</tt>) for the program
				359	through our transformation like this (or course, any bitcode file will
				360	work):</p>
				361
				362	<div class="doc_code"><pre>
				363	$ opt -load ../../../Debug/lib/Hello.so -hello < hello.bc > /dev/null
				364	Hello: __main
				365	Hello: puts
				366	Hello: main
				367	</pre></div>
				368
				369	<p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your
				370	pass as a shared object, which makes '<tt>-hello</tt>' a valid command line
				371	argument (which is one reason you need to <a href="#registration">register your
				372	pass</a>). Because the hello pass does not modify the program in any
				373	interesting way, we just throw away the result of <tt>opt</tt> (sending it to
				374	<tt>/dev/null</tt>).</p>
				375
				376	<p>To see what happened to the other string you registered, try running
				377	<tt>opt</tt> with the <tt>--help</tt> option:</p>
				378
				379	<div class="doc_code"><pre>
				380	$ opt -load ../../../Debug/lib/Hello.so --help
				381	OVERVIEW: llvm .bc -> .bc modular optimizer
				382
				383	USAGE: opt [options] <input bitcode>
				384
				385	OPTIONS:
				386	Optimizations available:
				387	...
				388	-funcresolve - Resolve Functions
				389	-gcse - Global Common Subexpression Elimination
				390	-globaldce - Dead Global Elimination
				391	<b>-hello - Hello World Pass</b>
				392	-indvars - Canonicalize Induction Variables
				393	-inline - Function Integration/Inlining
				394	-instcombine - Combine redundant instructions
				395	...
				396	</pre></div>
				397
				398	<p>The pass name get added as the information string for your pass, giving some
				399	documentation to users of <tt>opt</tt>. Now that you have a working pass, you
				400	would go ahead and make it do the cool transformations you want. Once you get
				401	it all working and tested, it may become useful to find out how fast your pass
				402	is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command
				403	line option (<tt>--time-passes</tt>) that allows you to get information about
				404	the execution time of your pass along with the other passes you queue up. For
				405	example:</p>
				406
				407	<div class="doc_code"><pre>
				408	$ opt -load ../../../Debug/lib/Hello.so -hello -time-passes < hello.bc > /dev/null
				409	Hello: __main
				410	Hello: puts
				411	Hello: main
				412	===============================================================================
				413	... Pass execution timing report ...
				414	===============================================================================
				415	Total Execution Time: 0.02 seconds (0.0479059 wall clock)
				416
				417	---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name ---
				418	0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer
				419	0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction
				420	0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier
				421	<b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b>
				422	0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL
				423	</pre></div>
				424
				425	<p>As you can see, our implementation above is pretty fast :). The additional
				426	passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
				427	that the LLVM emitted by your pass is still valid and well formed LLVM, which
				428	hasn't been broken somehow.</p>
				429
				430	<p>Now that you have seen the basics of the mechanics behind passes, we can talk
				431	about some more details of how they work and how to use them.</p>
				432
				433	</div>
				434
				435	<!-- *********************************************************************** -->
				436	<div class="doc_section">
				437	<a name="passtype">Pass classes and requirements</a>
				438	</div>
				439	<!-- *********************************************************************** -->
				440
				441	<div class="doc_text">
				442
				443	<p>One of the first things that you should do when designing a new pass is to
				444	decide what class you should subclass for your pass. The <a
				445	href="#basiccode">Hello World</a> example uses the <tt><a
				446	href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we
				447	did not discuss why or when this should occur. Here we talk about the classes
				448	available, from the most general to the most specific.</p>
				449
				450	<p>When choosing a superclass for your Pass, you should choose the <b>most
				451	specific</b> class possible, while still being able to meet the requirements
				452	listed. This gives the LLVM Pass Infrastructure information necessary to
				453	optimize how passes are run, so that the resultant compiler isn't unneccesarily
				454	slow.</p>
				455
				456	</div>
				457
				458	<!-- ======================================================================= -->
				459	<div class="doc_subsection">
				460	<a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a>
				461	</div>
				462
				463	<div class="doc_text">
				464
				465	<p>The most plain and boring type of pass is the "<tt><a
				466	href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>"
				467	class. This pass type is used for passes that do not have to be run, do not
				468	change state, and never need to be updated. This is not a normal type of
				469	transformation or analysis, but can provide information about the current
				470	compiler configuration.</p>
				471
				472	<p>Although this pass class is very infrequently used, it is important for
				473	providing information about the current target machine being compiled for, and
				474	other static information that can affect the various transformations.</p>
				475
				476	<p><tt>ImmutablePass</tt>es never invalidate other transformations, are never
				477	invalidated, and are never "run".</p>
				478
				479	</div>
				480
				481	<!-- ======================================================================= -->
				482	<div class="doc_subsection">
				483	<a name="ModulePass">The <tt>ModulePass</tt> class</a>
				484	</div>
				485
				486	<div class="doc_text">
				487
				488	<p>The "<tt><a
				489	href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>"
				490	class is the most general of all superclasses that you can use. Deriving from
				491	<tt>ModulePass</tt> indicates that your pass uses the entire program as a unit,
				492	refering to function bodies in no predictable order, or adding and removing
				493	functions. Because nothing is known about the behavior of <tt>ModulePass</tt>
Daniel Dunbar	b7750c5	2009-07-01 23:38:44 +0000	[diff] [blame]	494	subclasses, no optimization can be done for their execution.</p>
				495
				496	<p>A module pass can use function level passes (e.g. dominators) using
				497	the getAnalysis interface
				498	<tt>getAnalysis<DominatorTree>(llvm::Function *)</tt> to provide the
				499	function to retrieve analysis result for, if the function pass does not require
				500	any module passes. Note that this can only be done for functions for which the
				501	analysis ran, e.g. in the case of dominators you should only ask for the
				502	DominatorTree for function definitions, not declarations.</p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	503
				504	<p>To write a correct <tt>ModulePass</tt> subclass, derive from
				505	<tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the
				506	following signature:</p>
				507
				508	</div>
				509
				510	<!-- _______________________________________________________________________ -->
				511	<div class="doc_subsubsection">
				512	<a name="runOnModule">The <tt>runOnModule</tt> method</a>
				513	</div>
				514
				515	<div class="doc_text">
				516
				517	<div class="doc_code"><pre>
				518	<b>virtual bool</b> runOnModule(Module &M) = 0;
				519	</pre></div>
				520
				521	<p>The <tt>runOnModule</tt> method performs the interesting work of the pass.
				522	It should return true if the module was modified by the transformation and
				523	false otherwise.</p>
				524
				525	</div>
				526
				527	<!-- ======================================================================= -->
				528	<div class="doc_subsection">
				529	<a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
				530	</div>
				531
				532	<div class="doc_text">
				533
				534	<p>The "<tt><a
				535	href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>"
				536	is used by passes that need to traverse the program bottom-up on the call graph
				537	(callees before callers). Deriving from CallGraphSCCPass provides some
				538	mechanics for building and traversing the CallGraph, but also allows the system
				539	to optimize execution of CallGraphSCCPass's. If your pass meets the
				540	requirements outlined below, and doesn't meet the requirements of a <tt><a
				541	href="#FunctionPass">FunctionPass</a></tt> or <tt><a
				542	href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from
				543	<tt>CallGraphSCCPass</tt>.</p>
				544
				545	<p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p>
				546
				547	<p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p>
				548
				549	<ol>
				550
				551	<li>... <em>not allowed</em> to modify any <tt>Function</tt>s that are not in
				552	the current SCC.</li>
				553
				554	<li>... <em>not allowed</em> to inspect any Function's other than those in the
				555	current SCC and the direct callees of the SCC.</li>
				556
				557	<li>... <em>required</em> to preserve the current CallGraph object, updating it
				558	to reflect any changes made to the program.</li>
				559
				560	<li>... <em>not allowed</em> to add or remove SCC's from the current Module,
				561	though they may change the contents of an SCC.</li>
				562
				563	<li>... <em>allowed</em> to add or remove global variables from the current
				564	Module.</li>
				565
				566	<li>... <em>allowed</em> to maintain state across invocations of
				567	<a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li>
				568	</ol>
				569
				570	<p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases
				571	because it has to handle SCCs with more than one node in it. All of the virtual
				572	methods described below should return true if they modified the program, or
				573	false if they didn't.</p>
				574
				575	</div>
				576
				577	<!-- _______________________________________________________________________ -->
				578	<div class="doc_subsubsection">
				579	<a name="doInitialization_scc">The <tt>doInitialization(CallGraph &)</tt>
				580	method</a>
				581	</div>
				582
				583	<div class="doc_text">
				584
				585	<div class="doc_code"><pre>
				586	<b>virtual bool</b> doInitialization(CallGraph &CG);
				587	</pre></div>
				588
				589	<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
				590	<tt>CallGraphSCCPass</tt>'s are not allowed to do. They can add and remove
				591	functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
				592	is designed to do simple initialization type of stuff that does not depend on
				593	the SCCs being processed. The <tt>doInitialization</tt> method call is not
				594	scheduled to overlap with any other pass executions (thus it should be very
				595	fast).</p>
				596
				597	</div>
				598
				599	<!-- _______________________________________________________________________ -->
				600	<div class="doc_subsubsection">
				601	<a name="runOnSCC">The <tt>runOnSCC</tt> method</a>
				602	</div>
				603
				604	<div class="doc_text">
				605
				606	<div class="doc_code"><pre>
				607	<b>virtual bool</b> runOnSCC(const std::vector<CallGraphNode *> &SCCM) = 0;
				608	</pre></div>
				609
				610	<p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and
				611	should return true if the module was modified by the transformation, false
				612	otherwise.</p>
				613
				614	</div>
				615
				616	<!-- _______________________________________________________________________ -->
				617	<div class="doc_subsubsection">
				618	<a name="doFinalization_scc">The <tt>doFinalization(CallGraph
				619	&)</tt> method</a>
				620	</div>
				621
				622	<div class="doc_text">
				623
				624	<div class="doc_code"><pre>
				625	<b>virtual bool</b> doFinalization(CallGraph &CG);
				626	</pre></div>
				627
				628	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				629	called when the pass framework has finished calling <a
				630	href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
				631	program being compiled.</p>
				632
				633	</div>
				634
				635	<!-- ======================================================================= -->
				636	<div class="doc_subsection">
				637	<a name="FunctionPass">The <tt>FunctionPass</tt> class</a>
				638	</div>
				639
				640	<div class="doc_text">
				641
				642	<p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a
				643	href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
				644	subclasses do have a predictable, local behavior that can be expected by the
				645	system. All <tt>FunctionPass</tt> execute on each function in the program
				646	independent of all of the other functions in the program.
				647	<tt>FunctionPass</tt>'s do not require that they are executed in a particular
				648	order, and <tt>FunctionPass</tt>'s do not modify external functions.</p>
				649
				650	<p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p>
				651
				652	<ol>
				653	<li>Modify a Function other than the one currently being processed.</li>
				654	<li>Add or remove Function's from the current Module.</li>
				655	<li>Add or remove global variables from the current Module.</li>
				656	<li>Maintain state across invocations of
				657	<a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li>
				658	</ol>
				659
				660	<p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a
				661	href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s
				662	may overload three virtual methods to do their work. All of these methods
				663	should return true if they modified the program, or false if they didn't.</p>
				664
				665	</div>
				666
				667	<!-- _______________________________________________________________________ -->
				668	<div class="doc_subsubsection">
				669	<a name="doInitialization_mod">The <tt>doInitialization(Module &)</tt>
				670	method</a>
				671	</div>
				672
				673	<div class="doc_text">
				674
				675	<div class="doc_code"><pre>
				676	<b>virtual bool</b> doInitialization(Module &M);
				677	</pre></div>
				678
				679	<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
				680	<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove
				681	functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
				682	is designed to do simple initialization type of stuff that does not depend on
				683	the functions being processed. The <tt>doInitialization</tt> method call is not
				684	scheduled to overlap with any other pass executions (thus it should be very
				685	fast).</p>
				686
				687	<p>A good example of how this method should be used is the <a
				688	href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
				689	pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
				690	platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It
				691	uses the <tt>doInitialization</tt> method to get a reference to the malloc and
				692	free functions that it needs, adding prototypes to the module if necessary.</p>
				693
				694	</div>
				695
				696	<!-- _______________________________________________________________________ -->
				697	<div class="doc_subsubsection">
				698	<a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
				699	</div>
				700
				701	<div class="doc_text">
				702
				703	<div class="doc_code"><pre>
				704	<b>virtual bool</b> runOnFunction(Function &F) = 0;
				705	</pre></div><p>
				706
				707	<p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do
				708	the transformation or analysis work of your pass. As usual, a true value should
				709	be returned if the function is modified.</p>
				710
				711	</div>
				712
				713	<!-- _______________________________________________________________________ -->
				714	<div class="doc_subsubsection">
				715	<a name="doFinalization_mod">The <tt>doFinalization(Module
				716	&)</tt> method</a>
				717	</div>
				718
				719	<div class="doc_text">
				720
				721	<div class="doc_code"><pre>
				722	<b>virtual bool</b> doFinalization(Module &M);
				723	</pre></div>
				724
				725	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				726	called when the pass framework has finished calling <a
				727	href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
				728	program being compiled.</p>
				729
				730	</div>
				731
				732	<!-- ======================================================================= -->
				733	<div class="doc_subsection">
				734	<a name="LoopPass">The <tt>LoopPass</tt> class </a>
				735	</div>
				736
				737	<div class="doc_text">
				738
				739	<p> All <tt>LoopPass</tt> execute on each loop in the function independent of
				740	all of the other loops in the function. <tt>LoopPass</tt> processes loops in
				741	loop nest order such that outer most loop is processed last. </p>
				742
				743	<p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using
				744	<tt>LPPassManager</tt> interface. Implementing a loop pass is usually
				745	straightforward. <tt>Looppass</tt>'s may overload three virtual methods to
				746	do their work. All these methods should return true if they modified the
				747	program, or false if they didn't. </p>
				748	</div>
				749
				750	<!-- _______________________________________________________________________ -->
				751	<div class="doc_subsubsection">
				752	<a name="doInitialization_loop">The <tt>doInitialization(Loop *,
				753	LPPassManager &)</tt>
				754	method</a>
				755	</div>
				756
				757	<div class="doc_text">
				758
				759	<div class="doc_code"><pre>
				760	<b>virtual bool</b> doInitialization(Loop *, LPPassManager &LPM);
				761	</pre></div>
				762
				763	<p>The <tt>doInitialization</tt> method is designed to do simple initialization
				764	type of stuff that does not depend on the functions being processed. The
				765	<tt>doInitialization</tt> method call is not scheduled to overlap with any
				766	other pass executions (thus it should be very fast). LPPassManager
				767	interface should be used to access Function or Module level analysis
				768	information.</p>
				769
				770	</div>
				771
				772
				773	<!-- _______________________________________________________________________ -->
				774	<div class="doc_subsubsection">
				775	<a name="runOnLoop">The <tt>runOnLoop</tt> method</a>
				776	</div>
				777
				778	<div class="doc_text">
				779
				780	<div class="doc_code"><pre>
				781	<b>virtual bool</b> runOnLoop(Loop *, LPPassManager &LPM) = 0;
				782	</pre></div><p>
				783
				784	<p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do
				785	the transformation or analysis work of your pass. As usual, a true value should
				786	be returned if the function is modified. <tt>LPPassManager</tt> interface
				787	should be used to update loop nest.</p>
				788
				789	</div>
				790
				791	<!-- _______________________________________________________________________ -->
				792	<div class="doc_subsubsection">
				793	<a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a>
				794	</div>
				795
				796	<div class="doc_text">
				797
				798	<div class="doc_code"><pre>
				799	<b>virtual bool</b> doFinalization();
				800	</pre></div>
				801
				802	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				803	called when the pass framework has finished calling <a
				804	href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the
				805	program being compiled. </p>
				806
				807	</div>
				808
				809
				810
				811	<!-- ======================================================================= -->
				812	<div class="doc_subsection">
				813	<a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
				814	</div>
				815
				816	<div class="doc_text">
				817
				818	<p><tt>BasicBlockPass</tt>'s are just like <a
				819	href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
				820	their scope of inspection and modification to a single basic block at a time.
				821	As such, they are <b>not</b> allowed to do any of the following:</p>
				822
				823	<ol>
				824	<li>Modify or inspect any basic blocks outside of the current one</li>
				825	<li>Maintain state across invocations of
				826	<a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li>
				827	<li>Modify the control flow graph (by altering terminator instructions)</li>
				828	<li>Any of the things forbidden for
				829	<a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li>
				830	</ol>
				831
				832	<p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole"
				833	optimizations. They may override the same <a
				834	href="#doInitialization_mod"><tt>doInitialization(Module &)</tt></a> and <a
				835	href="#doFinalization_mod"><tt>doFinalization(Module &)</tt></a> methods that <a
				836	href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p>
				837
				838	</div>
				839
				840	<!-- _______________________________________________________________________ -->
				841	<div class="doc_subsubsection">
				842	<a name="doInitialization_fn">The <tt>doInitialization(Function
				843	&)</tt> method</a>
				844	</div>
				845
				846	<div class="doc_text">
				847
				848	<div class="doc_code"><pre>
				849	<b>virtual bool</b> doInitialization(Function &F);
				850	</pre></div>
				851
				852	<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
				853	<tt>BasicBlockPass</tt>'s are not allowed to do, but that
				854	<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed
				855	to do simple initialization that does not depend on the
				856	BasicBlocks being processed. The <tt>doInitialization</tt> method call is not
				857	scheduled to overlap with any other pass executions (thus it should be very
				858	fast).</p>
				859
				860	</div>
				861
				862	<!-- _______________________________________________________________________ -->
				863	<div class="doc_subsubsection">
				864	<a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
				865	</div>
				866
				867	<div class="doc_text">
				868
				869	<div class="doc_code"><pre>
				870	<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0;
				871	</pre></div>
				872
				873	<p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This
				874	function is not allowed to inspect or modify basic blocks other than the
				875	parameter, and are not allowed to modify the CFG. A true value must be returned
				876	if the basic block is modified.</p>
				877
				878	</div>
				879
				880	<!-- _______________________________________________________________________ -->
				881	<div class="doc_subsubsection">
				882	<a name="doFinalization_fn">The <tt>doFinalization(Function &)</tt>
				883	method</a>
				884	</div>
				885
				886	<div class="doc_text">
				887
				888	<div class="doc_code"><pre>
				889	<b>virtual bool</b> doFinalization(Function &F);
				890	</pre></div>
				891
				892	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				893	called when the pass framework has finished calling <a
				894	href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the
				895	program being compiled. This can be used to perform per-function
				896	finalization.</p>
				897
				898	</div>
				899
				900	<!-- ======================================================================= -->
				901	<div class="doc_subsection">
				902	<a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a>
				903	</div>
				904
				905	<div class="doc_text">
				906
				907	<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that
				908	executes on the machine-dependent representation of each LLVM function in the
				909	program. A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
				910	the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
				911	<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular,
				912	<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p>
				913
				914	<ol>
				915	<li>Modify any LLVM Instructions, BasicBlocks or Functions.</li>
				916	<li>Modify a MachineFunction other than the one currently being processed.</li>
				917	<li>Add or remove MachineFunctions from the current Module.</li>
				918	<li>Add or remove global variables from the current Module.</li>
				919	<li>Maintain state across invocations of <a
				920	href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global
				921	data)</li>
				922	</ol>
				923
				924	</div>
				925
				926	<!-- _______________________________________________________________________ -->
				927	<div class="doc_subsubsection">
				928	<a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
				929	&MF)</tt> method</a>
				930	</div>
				931
				932	<div class="doc_text">
				933
				934	<div class="doc_code"><pre>
				935	<b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0;
				936	</pre></div>
				937
				938	<p><tt>runOnMachineFunction</tt> can be considered the main entry point of a
				939	<tt>MachineFunctionPass</tt>; that is, you should override this method to do the
				940	work of your <tt>MachineFunctionPass</tt>.</p>
				941
				942	<p>The <tt>runOnMachineFunction</tt> method is called on every
				943	<tt>MachineFunction</tt> in a <tt>Module</tt>, so that the
				944	<tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent
				945	representation of the function. If you want to get at the LLVM <tt>Function</tt>
				946	for the <tt>MachineFunction</tt> you're working on, use
				947	<tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but
				948	remember, you may not modify the LLVM <tt>Function</tt> or its contents from a
				949	<tt>MachineFunctionPass</tt>.</p>
				950
				951	</div>
				952
				953	<!-- *********************************************************************** -->
				954	<div class="doc_section">
				955	<a name="registration">Pass registration</a>
				956	</div>
				957	<!-- *********************************************************************** -->
				958
				959	<div class="doc_text">
				960
				961	<p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how
				962	pass registration works, and discussed some of the reasons that it is used and
				963	what it does. Here we discuss how and why passes are registered.</p>
				964
				965	<p>As we saw above, passes are registered with the <b><tt>RegisterPass</tt></b>
				966	template, which requires you to pass at least two
				967	parameters. The first parameter is the name of the pass that is to be used on
				968	the command line to specify that the pass should be added to a program (for
				969	example, with <tt>opt</tt> or <tt>bugpoint</tt>). The second argument is the
				970	name of the pass, which is to be used for the <tt>--help</tt> output of
				971	programs, as
				972	well as for debug output generated by the <tt>--debug-pass</tt> option.</p>
				973
				974	<p>If you want your pass to be easily dumpable, you should
				975	implement the virtual <tt>print</tt> method:</p>
				976
				977	</div>
				978
				979	<!-- _______________________________________________________________________ -->
				980	<div class="doc_subsubsection">
				981	<a name="print">The <tt>print</tt> method</a>
				982	</div>
				983
				984	<div class="doc_text">
				985
				986	<div class="doc_code"><pre>
Edwin Török	72a8fd2	2008-10-28 17:29:23 +0000	[diff] [blame]	987	<b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>;
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	988	</pre></div>
				989
				990	<p>The <tt>print</tt> method must be implemented by "analyses" in order to print
				991	a human readable version of the analysis results. This is useful for debugging
				992	an analysis itself, as well as for other people to figure out how an analysis
				993	works. Use the <tt>opt -analyze</tt> argument to invoke this method.</p>
				994
				995	<p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on,
				996	and the <tt>Module</tt> parameter gives a pointer to the top level module of the
				997	program that has been analyzed. Note however that this pointer may be null in
				998	certain circumstances (such as calling the <tt>Pass::dump()</tt> from a
				999	debugger), so it should only be used to enhance debug output, it should not be
				1000	depended on.</p>
				1001
				1002	</div>
				1003
				1004	<!-- *********************************************************************** -->
				1005	<div class="doc_section">
				1006	<a name="interaction">Specifying interactions between passes</a>
				1007	</div>
				1008	<!-- *********************************************************************** -->
				1009
				1010	<div class="doc_text">
				1011
John Criswell	a99e43f	2007-12-03 19:34:25 +0000	[diff] [blame]	1012	<p>One of the main responsibilities of the <tt>PassManager</tt> is to make sure
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1013	that passes interact with each other correctly. Because <tt>PassManager</tt>
				1014	tries to <a href="#passmanager">optimize the execution of passes</a> it must
				1015	know how the passes interact with each other and what dependencies exist between
				1016	the various passes. To track this, each pass can declare the set of passes that
				1017	are required to be executed before the current pass, and the passes which are
				1018	invalidated by the current pass.</p>
				1019
				1020	<p>Typically this functionality is used to require that analysis results are
				1021	computed before your pass is run. Running arbitrary transformation passes can
				1022	invalidate the computed analysis results, which is what the invalidation set
				1023	specifies. If a pass does not implement the <tt><a
				1024	href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not
				1025	having any prerequisite passes, and invalidating <b>all</b> other passes.</p>
				1026
				1027	</div>
				1028
				1029	<!-- _______________________________________________________________________ -->
				1030	<div class="doc_subsubsection">
				1031	<a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
				1032	</div>
				1033
				1034	<div class="doc_text">
				1035
				1036	<div class="doc_code"><pre>
				1037	<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>;
				1038	</pre></div>
				1039
				1040	<p>By implementing the <tt>getAnalysisUsage</tt> method, the required and
				1041	invalidated sets may be specified for your transformation. The implementation
				1042	should fill in the <tt><a
				1043	href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
				1044	object with information about which passes are required and not invalidated. To
				1045	do this, a pass may call any of the following methods on the AnalysisUsage
				1046	object:</p>
				1047	</div>
				1048
				1049	<!-- _______________________________________________________________________ -->
				1050	<div class="doc_subsubsection">
				1051	<a name="AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a>
				1052	</div>
				1053
				1054	<div class="doc_text">
				1055	<p>
				1056	If your pass requires a previous pass to be executed (an analysis for example),
				1057	it can use one of these methods to arrange for it to be run before your pass.
				1058	LLVM has many different types of analyses and passes that can be required,
				1059	spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>.
				1060	Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will
				1061	be no critical edges in the CFG when your pass has been run.
				1062	</p>
				1063
				1064	<p>
				1065	Some analyses chain to other analyses to do their job. For example, an <a
				1066	href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a
				1067	href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In
				1068	cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be
				1069	used instead of the <tt>addRequired</tt> method. This informs the PassManager
				1070	that the transitively required pass should be alive as long as the requiring
				1071	pass is.
				1072	</p>
				1073	</div>
				1074
				1075	<!-- _______________________________________________________________________ -->
				1076	<div class="doc_subsubsection">
				1077	<a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a>
				1078	</div>
				1079
				1080	<div class="doc_text">
				1081	<p>
				1082	One of the jobs of the PassManager is to optimize how and when analyses are run.
				1083	In particular, it attempts to avoid recomputing data unless it needs to. For
				1084	this reason, passes are allowed to declare that they preserve (i.e., they don't
				1085	invalidate) an existing analysis if it's available. For example, a simple
				1086	constant folding pass would not modify the CFG, so it can't possibly affect the
				1087	results of dominator analysis. By default, all passes are assumed to invalidate
				1088	all others.
				1089	</p>
				1090
				1091	<p>
				1092	The <tt>AnalysisUsage</tt> class provides several methods which are useful in
				1093	certain circumstances that are related to <tt>addPreserved</tt>. In particular,
				1094	the <tt>setPreservesAll</tt> method can be called to indicate that the pass does
				1095	not modify the LLVM program at all (which is true for analyses), and the
				1096	<tt>setPreservesCFG</tt> method can be used by transformations that change
				1097	instructions in the program but do not modify the CFG or terminator instructions
				1098	(note that this property is implicitly set for <a
				1099	href="#BasicBlockPass">BasicBlockPass</a>'s).
				1100	</p>
				1101
				1102	<p>
				1103	<tt>addPreserved</tt> is particularly useful for transformations like
				1104	<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop
				1105	and dominator related analyses if they exist, so it can preserve them, despite
				1106	the fact that it hacks on the CFG.
				1107	</p>
				1108	</div>
				1109
				1110	<!-- _______________________________________________________________________ -->
				1111	<div class="doc_subsubsection">
				1112	<a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a>
				1113	</div>
				1114
				1115	<div class="doc_text">
				1116
				1117	<div class="doc_code"><pre>
				1118	<i>// This is an example implementation from an analysis, which does not modify
				1119	// the program at all, yet has a prerequisite.</i>
				1120	<b>void</b> <a href="http://llvm.org/doxygen/classllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1121	AU.setPreservesAll();
				1122	AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1PostDominatorTree.html">PostDominatorTree</a>>();
				1123	}
				1124	</pre></div>
				1125
				1126	<p>and:</p>
				1127
				1128	<div class="doc_code"><pre>
				1129	<i>// This example modifies the program, but does not modify the CFG</i>
				1130	<b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1131	AU.setPreservesCFG();
				1132	AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>();
				1133	}
				1134	</pre></div>
				1135
				1136	</div>
				1137
				1138	<!-- _______________________________________________________________________ -->
				1139	<div class="doc_subsubsection">
Duncan Sands	4e0d6a7	2009-01-28 13:14:17 +0000	[diff] [blame]	1140	<a name="getAnalysis">The <tt>getAnalysis<></tt> and
				1141	<tt>getAnalysisIfAvailable<></tt> methods</a>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1142	</div>
				1143
				1144	<div class="doc_text">
				1145
				1146	<p>The <tt>Pass::getAnalysis<></tt> method is automatically inherited by
				1147	your class, providing you with access to the passes that you declared that you
				1148	required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
				1149	method. It takes a single template argument that specifies which pass class you
				1150	want, and returns a reference to that pass. For example:</p>
				1151
				1152	<div class="doc_code"><pre>
				1153	bool LICM::runOnFunction(Function &F) {
				1154	LoopInfo &LI = getAnalysis<LoopInfo>();
				1155	...
				1156	}
				1157	</pre></div>
				1158
				1159	<p>This method call returns a reference to the pass desired. You may get a
				1160	runtime assertion failure if you attempt to get an analysis that you did not
				1161	declare as required in your <a
				1162	href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This
				1163	method can be called by your <tt>run*</tt> method implementation, or by any
				1164	other local method invoked by your <tt>run*</tt> method.
				1165
				1166	A module level pass can use function level analysis info using this interface.
				1167	For example:</p>
				1168
				1169	<div class="doc_code"><pre>
				1170	bool ModuleLevelPass::runOnModule(Module &M) {
				1171	...
				1172	DominatorTree &DT = getAnalysis<DominatorTree>(Func);
				1173	...
				1174	}
				1175	</pre></div>
				1176
				1177	<p>In above example, runOnFunction for DominatorTree is called by pass manager
				1178	before returning a reference to the desired pass.</p>
				1179
				1180	<p>
				1181	If your pass is capable of updating analyses if they exist (e.g.,
				1182	<tt>BreakCriticalEdges</tt>, as described above), you can use the
Duncan Sands	4e0d6a7	2009-01-28 13:14:17 +0000	[diff] [blame]	1183	<tt>getAnalysisIfAvailable</tt> method, which returns a pointer to the analysis
				1184	if it is active. For example:</p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1185
				1186	<div class="doc_code"><pre>
				1187	...
Duncan Sands	4e0d6a7	2009-01-28 13:14:17 +0000	[diff] [blame]	1188	if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) {
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1189	<i>// A DominatorSet is active. This code will update it.</i>
				1190	}
				1191	...
				1192	</pre></div>
				1193
				1194	</div>
				1195
				1196	<!-- *********************************************************************** -->
				1197	<div class="doc_section">
				1198	<a name="analysisgroup">Implementing Analysis Groups</a>
				1199	</div>
				1200	<!-- *********************************************************************** -->
				1201
				1202	<div class="doc_text">
				1203
Chris Lattner	942c395	2007-11-16 05:32:05 +0000	[diff] [blame]	1204	<p>Now that we understand the basics of how passes are defined, how they are
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1205	used, and how they are required from other passes, it's time to get a little bit
				1206	fancier. All of the pass relationships that we have seen so far are very
				1207	simple: one pass depends on one other specific pass to be run before it can run.
				1208	For many applications, this is great, for others, more flexibility is
				1209	required.</p>
				1210
				1211	<p>In particular, some analyses are defined such that there is a single simple
				1212	interface to the analysis results, but multiple ways of calculating them.
				1213	Consider alias analysis for example. The most trivial alias analysis returns
				1214	"may alias" for any alias query. The most sophisticated analysis a
				1215	flow-sensitive, context-sensitive interprocedural analysis that can take a
				1216	significant amount of time to execute (and obviously, there is a lot of room
				1217	between these two extremes for other implementations). To cleanly support
				1218	situations like this, the LLVM Pass Infrastructure supports the notion of
				1219	Analysis Groups.</p>
				1220
				1221	</div>
				1222
				1223	<!-- _______________________________________________________________________ -->
				1224	<div class="doc_subsubsection">
				1225	<a name="agconcepts">Analysis Group Concepts</a>
				1226	</div>
				1227
				1228	<div class="doc_text">
				1229
				1230	<p>An Analysis Group is a single simple interface that may be implemented by
				1231	multiple different passes. Analysis Groups can be given human readable names
				1232	just like passes, but unlike passes, they need not derive from the <tt>Pass</tt>
				1233	class. An analysis group may have one or more implementations, one of which is
				1234	the "default" implementation.</p>
				1235
				1236	<p>Analysis groups are used by client passes just like other passes are: the
				1237	<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods.
				1238	In order to resolve this requirement, the <a href="#passmanager">PassManager</a>
				1239	scans the available passes to see if any implementations of the analysis group
				1240	are available. If none is available, the default implementation is created for
				1241	the pass to use. All standard rules for <A href="#interaction">interaction
				1242	between passes</a> still apply.</p>
				1243
				1244	<p>Although <a href="#registration">Pass Registration</a> is optional for normal
				1245	passes, all analysis group implementations must be registered, and must use the
				1246	<A href="#registerag"><tt>RegisterAnalysisGroup</tt></a> template to join the
				1247	implementation pool. Also, a default implementation of the interface
				1248	<b>must</b> be registered with <A
				1249	href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p>
				1250
				1251	<p>As a concrete example of an Analysis Group in action, consider the <a
				1252	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
				1253	analysis group. The default implementation of the alias analysis interface (the
				1254	<tt><a
				1255	href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
				1256	pass) just does a few simple checks that don't require significant analysis to
				1257	compute (such as: two different globals can never alias each other, etc).
				1258	Passes that use the <tt><a
				1259	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
				1260	interface (for example the <tt><a
				1261	href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do
				1262	not care which implementation of alias analysis is actually provided, they just
				1263	use the designated interface.</p>
				1264
				1265	<p>From the user's perspective, commands work just like normal. Issuing the
				1266	command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be
				1267	instantiated and added to the pass sequence. Issuing the command '<tt>opt
				1268	-somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the
				1269	<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a
				1270	hypothetical example) instead.</p>
				1271
				1272	</div>
				1273
				1274	<!-- _______________________________________________________________________ -->
				1275	<div class="doc_subsubsection">
				1276	<a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
				1277	</div>
				1278
				1279	<div class="doc_text">
				1280
				1281	<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
				1282	group itself as well as add pass implementations to the analysis group. First,
				1283	an analysis should be registered, with a human readable name provided for it.
				1284	Unlike registration of passes, there is no command line argument to be specified
				1285	for the Analysis Group Interface itself, because it is "abstract":</p>
				1286
				1287	<div class="doc_code"><pre>
				1288	<b>static</b> RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>");
				1289	</pre></div>
				1290
				1291	<p>Once the analysis is registered, passes can declare that they are valid
				1292	implementations of the interface by using the following code:</p>
				1293
				1294	<div class="doc_code"><pre>
				1295	<b>namespace</b> {
				1296	//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
				1297	RegisterPass<FancyAA>
				1298	B("<i>somefancyaa</i>", "<i>A more complex alias analysis implementation</i>");
				1299
				1300	//<i> Declare that we implement the AliasAnalysis interface</i>
				1301	RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> C(B);
				1302	}
				1303	</pre></div>
				1304
				1305	<p>This just shows a class <tt>FancyAA</tt> that is registered normally, then
				1306	uses the <tt>RegisterAnalysisGroup</tt> template to "join" the <tt><a
				1307	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
				1308	analysis group. Every implementation of an analysis group should join using
				1309	this template. A single pass may join multiple different analysis groups with
				1310	no problem.</p>
				1311
				1312	<div class="doc_code"><pre>
				1313	<b>namespace</b> {
				1314	//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
				1315	RegisterPass<<a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>>
				1316	D("<i>basicaa</i>", "<i>Basic Alias Analysis (default AA impl)</i>");
				1317
				1318	//<i> Declare that we implement the AliasAnalysis interface</i>
				1319	RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, <b>true</b>> E(D);
				1320	}
				1321	</pre></div>
				1322
				1323	<p>Here we show how the default implementation is specified (using the extra
				1324	argument to the <tt>RegisterAnalysisGroup</tt> template). There must be exactly
				1325	one default implementation available at all times for an Analysis Group to be
				1326	used. Only default implementation can derive from <tt>ImmutablePass</tt>.
				1327	Here we declare that the
				1328	<tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
				1329	pass is the default implementation for the interface.</p>
				1330
				1331	</div>
				1332
				1333	<!-- *********************************************************************** -->
				1334	<div class="doc_section">
				1335	<a name="passStatistics">Pass Statistics</a>
				1336	</div>
				1337	<!-- *********************************************************************** -->
				1338
				1339	<div class="doc_text">
				1340	<p>The <a
				1341	href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a>
				1342	class is designed to be an easy way to expose various success
				1343	metrics from passes. These statistics are printed at the end of a
				1344	run, when the -stats command line option is enabled on the command
				1345	line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details.
				1346
				1347	</div>
				1348
				1349
				1350	<!-- *********************************************************************** -->
				1351	<div class="doc_section">
				1352	<a name="passmanager">What PassManager does</a>
				1353	</div>
				1354	<!-- *********************************************************************** -->
				1355
				1356	<div class="doc_text">
				1357
				1358	<p>The <a
				1359	href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
				1360	<a
				1361	href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a>
				1362	takes a list of passes, ensures their <a href="#interaction">prerequisites</a>
				1363	are set up correctly, and then schedules passes to run efficiently. All of the
				1364	LLVM tools that run passes use the <tt>PassManager</tt> for execution of these
				1365	passes.</p>
				1366
				1367	<p>The <tt>PassManager</tt> does two main things to try to reduce the execution
				1368	time of a series of passes:</p>
				1369
				1370	<ol>
				1371	<li><b>Share analysis results</b> - The PassManager attempts to avoid
				1372	recomputing analysis results as much as possible. This means keeping track of
				1373	which analyses are available already, which analyses get invalidated, and which
				1374	analyses are needed to be run for a pass. An important part of work is that the
				1375	<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing
				1376	it to <a href="#releaseMemory">free memory</a> allocated to holding analysis
				1377	results as soon as they are no longer needed.</li>
				1378
				1379	<li><b>Pipeline the execution of passes on the program</b> - The
				1380	<tt>PassManager</tt> attempts to get better cache and memory usage behavior out
				1381	of a series of passes by pipelining the passes together. This means that, given
				1382	a series of consequtive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
				1383	will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on
				1384	the first function, then all of the <a
				1385	href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function,
				1386	etc... until the entire program has been run through the passes.
				1387
				1388	<p>This improves the cache behavior of the compiler, because it is only touching
				1389	the LLVM program representation for a single function at a time, instead of
				1390	traversing the entire program. It reduces the memory consumption of compiler,
				1391	because, for example, only one <a
				1392	href="http://llvm.org/doxygen/classllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a>
John Criswell	8a72615	2007-12-10 20:26:29 +0000	[diff] [blame]	1393	needs to be calculated at a time. This also makes it possible to implement
				1394	some <a
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1395	href="#SMP">interesting enhancements</a> in the future.</p></li>
				1396
				1397	</ol>
				1398
				1399	<p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how
				1400	much information it has about the behaviors of the passes it is scheduling. For
				1401	example, the "preserved" set is intentionally conservative in the face of an
				1402	unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method.
				1403	Not implementing when it should be implemented will have the effect of not
				1404	allowing any analysis results to live across the execution of your pass.</p>
				1405
				1406	<p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line
				1407	options that is useful for debugging pass execution, seeing how things work, and
				1408	diagnosing when you should be preserving more analyses than you currently are
				1409	(To get information about all of the variants of the <tt>--debug-pass</tt>
				1410	option, just type '<tt>opt --help-hidden</tt>').</p>
				1411
				1412	<p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see
				1413	how our <a href="#basiccode">Hello World</a> pass interacts with other passes.
				1414	Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p>
				1415
				1416	<div class="doc_code"><pre>
				1417	$ opt -load ../../../Debug/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null
				1418	Module Pass Manager
				1419	Function Pass Manager
				1420	Dominator Set Construction
				1421	Immediate Dominators Construction
				1422	Global Common Subexpression Elimination
				1423	-- Immediate Dominators Construction
				1424	-- Global Common Subexpression Elimination
				1425	Natural Loop Construction
				1426	Loop Invariant Code Motion
				1427	-- Natural Loop Construction
				1428	-- Loop Invariant Code Motion
				1429	Module Verifier
				1430	-- Dominator Set Construction
				1431	-- Module Verifier
				1432	Bitcode Writer
				1433	--Bitcode Writer
				1434	</pre></div>
				1435
				1436	<p>This output shows us when passes are constructed and when the analysis
				1437	results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that
				1438	GCSE uses dominator and immediate dominator information to do its job. The LICM
				1439	pass uses natural loop information, which uses dominator sets, but not immediate
				1440	dominators. Because immediate dominators are no longer useful after the GCSE
				1441	pass, it is immediately destroyed. The dominator sets are then reused to
				1442	compute natural loop information, which is then used by the LICM pass.</p>
				1443
				1444	<p>After the LICM pass, the module verifier runs (which is automatically added
				1445	by the '<tt>opt</tt>' tool), which uses the dominator set to check that the
				1446	resultant LLVM code is well formed. After it finishes, the dominator set
				1447	information is destroyed, after being computed once, and shared by three
				1448	passes.</p>
				1449
				1450	<p>Lets see how this changes when we run the <a href="#basiccode">Hello
				1451	World</a> pass in between the two passes:</p>
				1452
				1453	<div class="doc_code"><pre>
				1454	$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
				1455	Module Pass Manager
				1456	Function Pass Manager
				1457	Dominator Set Construction
				1458	Immediate Dominators Construction
				1459	Global Common Subexpression Elimination
				1460	<b>-- Dominator Set Construction</b>
				1461	-- Immediate Dominators Construction
				1462	-- Global Common Subexpression Elimination
				1463	<b> Hello World Pass
				1464	-- Hello World Pass
				1465	Dominator Set Construction</b>
				1466	Natural Loop Construction
				1467	Loop Invariant Code Motion
				1468	-- Natural Loop Construction
				1469	-- Loop Invariant Code Motion
				1470	Module Verifier
				1471	-- Dominator Set Construction
				1472	-- Module Verifier
				1473	Bitcode Writer
				1474	--Bitcode Writer
				1475	Hello: __main
				1476	Hello: puts
				1477	Hello: main
				1478	</pre></div>
				1479
				1480	<p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
				1481	Dominator Set pass, even though it doesn't modify the code at all! To fix this,
				1482	we need to add the following <a
				1483	href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p>
				1484
				1485	<div class="doc_code"><pre>
				1486	<i>// We don't modify the program, so we preserve all analyses</i>
				1487	<b>virtual void</b> getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1488	AU.setPreservesAll();
				1489	}
				1490	</pre></div>
				1491
				1492	<p>Now when we run our pass, we get this output:</p>
				1493
				1494	<div class="doc_code"><pre>
				1495	$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
				1496	Pass Arguments: -gcse -hello -licm
				1497	Module Pass Manager
				1498	Function Pass Manager
				1499	Dominator Set Construction
				1500	Immediate Dominators Construction
				1501	Global Common Subexpression Elimination
				1502	-- Immediate Dominators Construction
				1503	-- Global Common Subexpression Elimination
				1504	Hello World Pass
				1505	-- Hello World Pass
				1506	Natural Loop Construction
				1507	Loop Invariant Code Motion
				1508	-- Loop Invariant Code Motion
				1509	-- Natural Loop Construction
				1510	Module Verifier
				1511	-- Dominator Set Construction
				1512	-- Module Verifier
				1513	Bitcode Writer
				1514	--Bitcode Writer
				1515	Hello: __main
				1516	Hello: puts
				1517	Hello: main
				1518	</pre></div>
				1519
				1520	<p>Which shows that we don't accidentally invalidate dominator information
				1521	anymore, and therefore do not have to compute it twice.</p>
				1522
				1523	</div>
				1524
				1525	<!-- _______________________________________________________________________ -->
				1526	<div class="doc_subsubsection">
				1527	<a name="releaseMemory">The <tt>releaseMemory</tt> method</a>
				1528	</div>
				1529
				1530	<div class="doc_text">
				1531
				1532	<div class="doc_code"><pre>
				1533	<b>virtual void</b> releaseMemory();
				1534	</pre></div>
				1535
				1536	<p>The <tt>PassManager</tt> automatically determines when to compute analysis
				1537	results, and how long to keep them around for. Because the lifetime of the pass
				1538	object itself is effectively the entire duration of the compilation process, we
				1539	need some way to free analysis results when they are no longer useful. The
				1540	<tt>releaseMemory</tt> virtual method is the way to do this.</p>
				1541
				1542	<p>If you are writing an analysis or any other pass that retains a significant
				1543	amount of state (for use by another pass which "requires" your pass and uses the
				1544	<a href="#getAnalysis">getAnalysis</a> method) you should implement
Dan Gohman	d72d8ea	2009-06-15 18:22:49 +0000	[diff] [blame]	1545	<tt>releaseMemory</tt> to, well, release the memory allocated to maintain this
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1546	internal state. This method is called after the <tt>run*</tt> method for the
				1547	class, before the next call of <tt>run*</tt> in your pass.</p>
				1548
				1549	</div>
				1550
				1551	<!-- *********************************************************************** -->
				1552	<div class="doc_section">
				1553	<a name="registering">Registering dynamically loaded passes</a>
				1554	</div>
				1555	<!-- *********************************************************************** -->
				1556
				1557	<div class="doc_text">
				1558
				1559	<p><i>Size matters</i> when constructing production quality tools using llvm,
				1560	both for the purposes of distribution, and for regulating the resident code size
				1561	when running on the target system. Therefore, it becomes desirable to
				1562	selectively use some passes, while omitting others and maintain the flexibility
				1563	to change configurations later on. You want to be able to do all this, and,
				1564	provide feedback to the user. This is where pass registration comes into
				1565	play.</p>
				1566
				1567	<p>The fundamental mechanisms for pass registration are the
				1568	<tt>MachinePassRegistry</tt> class and subclasses of
				1569	<tt>MachinePassRegistryNode</tt>.</p>
				1570
				1571	<p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of
				1572	<tt>MachinePassRegistryNode</tt> objects. This instance maintains the list and
				1573	communicates additions and deletions to the command line interface.</p>
				1574
				1575	<p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain
				1576	information provided about a particular pass. This information includes the
				1577	command line name, the command help string and the address of the function used
				1578	to create an instance of the pass. A global static constructor of one of these
				1579	instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>,
				1580	the static destructor <i>unregisters</i>. Thus a pass that is statically linked
				1581	in the tool will be registered at start up. A dynamically loaded pass will
				1582	register on load and unregister at unload.</p>
				1583
				1584	</div>
				1585
				1586	<!-- _______________________________________________________________________ -->
				1587	<div class="doc_subsection">
				1588	<a name="registering_existing">Using existing registries</a>
				1589	</div>
				1590
				1591	<div class="doc_text">
				1592
				1593	<p>There are predefined registries to track instruction scheduling
				1594	(<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>)
				1595	machine passes. Here we will describe how to <i>register</i> a register
				1596	allocator machine pass.</p>
				1597
				1598	<p>Implement your register allocator machine pass. In your register allocator
				1599	.cpp file add the following include;</p>
				1600
				1601	<div class="doc_code"><pre>
				1602	#include "llvm/CodeGen/RegAllocRegistry.h"
				1603	</pre></div>
				1604
				1605	<p>Also in your register allocator .cpp file, define a creator function in the
				1606	form; </p>
				1607
				1608	<div class="doc_code"><pre>
				1609	FunctionPass *createMyRegisterAllocator() {
				1610	return new MyRegisterAllocator();
				1611	}
				1612	</pre></div>
				1613
				1614	<p>Note that the signature of this function should match the type of
				1615	<tt>RegisterRegAlloc::FunctionPassCtor</tt>. In the same file add the
				1616	"installing" declaration, in the form;</p>
				1617
				1618	<div class="doc_code"><pre>
				1619	static RegisterRegAlloc myRegAlloc("myregalloc",
				1620	" my register allocator help string",
				1621	createMyRegisterAllocator);
				1622	</pre></div>
				1623
				1624	<p>Note the two spaces prior to the help string produces a tidy result on the
				1625	--help query.</p>
				1626
				1627	<div class="doc_code"><pre>
				1628	$ llc --help
				1629	...
				1630	-regalloc - Register allocator to use: (default = linearscan)
				1631	=linearscan - linear scan register allocator
				1632	=local - local register allocator
				1633	=simple - simple register allocator
				1634	=myregalloc - my register allocator help string
				1635	...
				1636	</pre></div>
				1637
				1638	<p>And that's it. The user is now free to use <tt>-regalloc=myregalloc</tt> as
				1639	an option. Registering instruction schedulers is similar except use the
				1640	<tt>RegisterScheduler</tt> class. Note that the
				1641	<tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from
				1642	<tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p>
				1643
				1644	<p>To force the load/linking of your register allocator into the llc/lli tools,
				1645	add your creator function's global declaration to "Passes.h" and add a "pseudo"
				1646	call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p>
				1647
				1648	</div>
				1649
				1650
				1651	<!-- _______________________________________________________________________ -->
				1652	<div class="doc_subsection">
				1653	<a name="registering_new">Creating new registries</a>
				1654	</div>
				1655
				1656	<div class="doc_text">
				1657
				1658	<p>The easiest way to get started is to clone one of the existing registries; we
				1659	recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>. The key things to modify
				1660	are the class name and the <tt>FunctionPassCtor</tt> type.</p>
				1661
				1662	<p>Then you need to declare the registry. Example: if your pass registry is
				1663	<tt>RegisterMyPasses</tt> then define;</p>
				1664
				1665	<div class="doc_code"><pre>
				1666	MachinePassRegistry RegisterMyPasses::Registry;
				1667	</pre></div>
				1668
				1669	<p>And finally, declare the command line option for your passes. Example:</p>
				1670
				1671	<div class="doc_code"><pre>
				1672	cl::opt<RegisterMyPasses::FunctionPassCtor, false,
Dan Gohman	8e58bc5	2008-10-14 17:00:38 +0000	[diff] [blame]	1673	RegisterPassParser<RegisterMyPasses> >
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1674	MyPassOpt("mypass",
				1675	cl::init(&createDefaultMyPass),
				1676	cl::desc("my pass option help"));
				1677	</pre></div>
				1678
				1679	<p>Here the command option is "mypass", with createDefaultMyPass as the default
				1680	creator.</p>
				1681
				1682	</div>
				1683
				1684	<!-- *********************************************************************** -->
				1685	<div class="doc_section">
				1686	<a name="debughints">Using GDB with dynamically loaded passes</a>
				1687	</div>
				1688	<!-- *********************************************************************** -->
				1689
				1690	<div class="doc_text">
				1691
				1692	<p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it
				1693	should be. First of all, you can't set a breakpoint in a shared object that has
				1694	not been loaded yet, and second of all there are problems with inlined functions
				1695	in shared objects. Here are some suggestions to debugging your pass with
				1696	GDB.</p>
				1697
				1698	<p>For sake of discussion, I'm going to assume that you are debugging a
				1699	transformation invoked by <tt>opt</tt>, although nothing described here depends
				1700	on that.</p>
				1701
				1702	</div>
				1703
				1704	<!-- _______________________________________________________________________ -->
				1705	<div class="doc_subsubsection">
				1706	<a name="breakpoint">Setting a breakpoint in your pass</a>
				1707	</div>
				1708
				1709	<div class="doc_text">
				1710
				1711	<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>
				1712
				1713	<div class="doc_code"><pre>
				1714	$ <b>gdb opt</b>
				1715	GNU gdb 5.0
				1716	Copyright 2000 Free Software Foundation, Inc.
				1717	GDB is free software, covered by the GNU General Public License, and you are
				1718	welcome to change it and/or distribute copies of it under certain conditions.
				1719	Type "show copying" to see the conditions.
				1720	There is absolutely no warranty for GDB. Type "show warranty" for details.
				1721	This GDB was configured as "sparc-sun-solaris2.6"...
				1722	(gdb)
				1723	</pre></div>
				1724
				1725	<p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
				1726	time to load. Be patient. Since we cannot set a breakpoint in our pass yet
				1727	(the shared object isn't loaded until runtime), we must execute the process, and
				1728	have it stop before it invokes our pass, but after it has loaded the shared
				1729	object. The most foolproof way of doing this is to set a breakpoint in
				1730	<tt>PassManager::run</tt> and then run the process with the arguments you
				1731	want:</p>
				1732
				1733	<div class="doc_code"><pre>
				1734	(gdb) <b>break llvm::PassManager::run</b>
				1735	Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
				1736	(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]</b>
				1737	Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]
				1738	Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
				1739	70 bool PassManager::run(Module &M) { return PM->run(M); }
				1740	(gdb)
				1741	</pre></div>
				1742
				1743	<p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are
				1744	now free to set breakpoints in your pass so that you can trace through execution
				1745	or do other standard debugging stuff.</p>
				1746
				1747	</div>
				1748
				1749	<!-- _______________________________________________________________________ -->
				1750	<div class="doc_subsubsection">
				1751	<a name="debugmisc">Miscellaneous Problems</a>
				1752	</div>
				1753
				1754	<div class="doc_text">
				1755
				1756	<p>Once you have the basics down, there are a couple of problems that GDB has,
				1757	some with solutions, some without.</p>
				1758
				1759	<ul>
				1760	<li>Inline functions have bogus stack information. In general, GDB does a
				1761	pretty good job getting stack traces and stepping through inline functions.
				1762	When a pass is dynamically loaded however, it somehow completely loses this
				1763	capability. The only solution I know of is to de-inline a function (move it
				1764	from the body of a class to a .cpp file).</li>
				1765
				1766	<li>Restarting the program breaks breakpoints. After following the information
				1767	above, you have succeeded in getting some breakpoints planted in your pass. Nex
				1768	thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
				1769	and you start getting errors about breakpoints being unsettable. The only way I
				1770	have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
				1771	already set in your pass, run the program, and re-set the breakpoints once
				1772	execution stops in <tt>PassManager::run</tt>.</li>
				1773
				1774	</ul>
				1775
				1776	<p>Hopefully these tips will help with common case debugging situations. If
				1777	you'd like to contribute some tips of your own, just contact <a
				1778	href="mailto:sabre@nondot.org">Chris</a>.</p>
				1779
				1780	</div>
				1781
				1782	<!-- *********************************************************************** -->
				1783	<div class="doc_section">
				1784	<a name="future">Future extensions planned</a>
				1785	</div>
				1786	<!-- *********************************************************************** -->
				1787
				1788	<div class="doc_text">
				1789
				1790	<p>Although the LLVM Pass Infrastructure is very capable as it stands, and does
				1791	some nifty stuff, there are things we'd like to add in the future. Here is
				1792	where we are going:</p>
				1793
				1794	</div>
				1795
				1796	<!-- _______________________________________________________________________ -->
				1797	<div class="doc_subsubsection">
				1798	<a name="SMP">Multithreaded LLVM</a>
				1799	</div>
				1800
				1801	<div class="doc_text">
				1802
				1803	<p>Multiple CPU machines are becoming more common and compilation can never be
				1804	fast enough: obviously we should allow for a multithreaded compiler. Because of
				1805	the semantics defined for passes above (specifically they cannot maintain state
				1806	across invocations of their <tt>run*</tt> methods), a nice clean way to
				1807	implement a multithreaded compiler would be for the <tt>PassManager</tt> class
				1808	to create multiple instances of each pass object, and allow the separate
				1809	instances to be hacking on different parts of the program at the same time.</p>
				1810
				1811	<p>This implementation would prevent each of the passes from having to implement
				1812	multithreaded constructs, requiring only the LLVM core to have locking in a few
				1813	places (for global resources). Although this is a simple extension, we simply
				1814	haven't had time (or multiprocessor machines, thus a reason) to implement this.
				1815	Despite that, we have kept the LLVM passes SMP ready, and you should too.</p>
				1816
				1817	</div>
				1818
				1819	<!-- *********************************************************************** -->
				1820	<hr>
				1821	<address>
				1822	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman	947321d	2008-12-11 17:34:48 +0000	[diff] [blame]	1823	src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1824	<a href="http://validator.w3.org/check/referer"><img
Misha Brukman	5c1cc64	2008-12-11 18:23:24 +0000	[diff] [blame]	1825	src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1826
				1827	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
				1828	<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
				1829	Last modified: $Date$
				1830	</address>
				1831
				1832	</body>
				1833	</html>