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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;
				268	Hello() : FunctionPass((intptr_t)&ID) {}
				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;
				322	Hello() : FunctionPass((intptr_t)&ID) {}
				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>
				494	subclasses, no optimization can be done for their execution. A module pass
				495	can use function level passes (e.g. dominators) using getAnalysis interface
Devang Patel	7a9a48f	2008-11-06 19:47:49 +0000	[diff] [blame]	496	<tt> getAnalysis<DominatorTree>(Function)</tt>, if the function pass
				497	does not require any module passes. </p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	498
				499	<p>To write a correct <tt>ModulePass</tt> subclass, derive from
				500	<tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the
				501	following signature:</p>
				502
				503	</div>
				504
				505	<!-- _______________________________________________________________________ -->
				506	<div class="doc_subsubsection">
				507	<a name="runOnModule">The <tt>runOnModule</tt> method</a>
				508	</div>
				509
				510	<div class="doc_text">
				511
				512	<div class="doc_code"><pre>
				513	<b>virtual bool</b> runOnModule(Module &M) = 0;
				514	</pre></div>
				515
				516	<p>The <tt>runOnModule</tt> method performs the interesting work of the pass.
				517	It should return true if the module was modified by the transformation and
				518	false otherwise.</p>
				519
				520	</div>
				521
				522	<!-- ======================================================================= -->
				523	<div class="doc_subsection">
				524	<a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
				525	</div>
				526
				527	<div class="doc_text">
				528
				529	<p>The "<tt><a
				530	href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>"
				531	is used by passes that need to traverse the program bottom-up on the call graph
				532	(callees before callers). Deriving from CallGraphSCCPass provides some
				533	mechanics for building and traversing the CallGraph, but also allows the system
				534	to optimize execution of CallGraphSCCPass's. If your pass meets the
				535	requirements outlined below, and doesn't meet the requirements of a <tt><a
				536	href="#FunctionPass">FunctionPass</a></tt> or <tt><a
				537	href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from
				538	<tt>CallGraphSCCPass</tt>.</p>
				539
				540	<p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p>
				541
				542	<p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p>
				543
				544	<ol>
				545
				546	<li>... <em>not allowed</em> to modify any <tt>Function</tt>s that are not in
				547	the current SCC.</li>
				548
				549	<li>... <em>not allowed</em> to inspect any Function's other than those in the
				550	current SCC and the direct callees of the SCC.</li>
				551
				552	<li>... <em>required</em> to preserve the current CallGraph object, updating it
				553	to reflect any changes made to the program.</li>
				554
				555	<li>... <em>not allowed</em> to add or remove SCC's from the current Module,
				556	though they may change the contents of an SCC.</li>
				557
				558	<li>... <em>allowed</em> to add or remove global variables from the current
				559	Module.</li>
				560
				561	<li>... <em>allowed</em> to maintain state across invocations of
				562	<a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li>
				563	</ol>
				564
				565	<p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases
				566	because it has to handle SCCs with more than one node in it. All of the virtual
				567	methods described below should return true if they modified the program, or
				568	false if they didn't.</p>
				569
				570	</div>
				571
				572	<!-- _______________________________________________________________________ -->
				573	<div class="doc_subsubsection">
				574	<a name="doInitialization_scc">The <tt>doInitialization(CallGraph &)</tt>
				575	method</a>
				576	</div>
				577
				578	<div class="doc_text">
				579
				580	<div class="doc_code"><pre>
				581	<b>virtual bool</b> doInitialization(CallGraph &CG);
				582	</pre></div>
				583
				584	<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
				585	<tt>CallGraphSCCPass</tt>'s are not allowed to do. They can add and remove
				586	functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
				587	is designed to do simple initialization type of stuff that does not depend on
				588	the SCCs being processed. The <tt>doInitialization</tt> method call is not
				589	scheduled to overlap with any other pass executions (thus it should be very
				590	fast).</p>
				591
				592	</div>
				593
				594	<!-- _______________________________________________________________________ -->
				595	<div class="doc_subsubsection">
				596	<a name="runOnSCC">The <tt>runOnSCC</tt> method</a>
				597	</div>
				598
				599	<div class="doc_text">
				600
				601	<div class="doc_code"><pre>
				602	<b>virtual bool</b> runOnSCC(const std::vector<CallGraphNode *> &SCCM) = 0;
				603	</pre></div>
				604
				605	<p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and
				606	should return true if the module was modified by the transformation, false
				607	otherwise.</p>
				608
				609	</div>
				610
				611	<!-- _______________________________________________________________________ -->
				612	<div class="doc_subsubsection">
				613	<a name="doFinalization_scc">The <tt>doFinalization(CallGraph
				614	&)</tt> method</a>
				615	</div>
				616
				617	<div class="doc_text">
				618
				619	<div class="doc_code"><pre>
				620	<b>virtual bool</b> doFinalization(CallGraph &CG);
				621	</pre></div>
				622
				623	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				624	called when the pass framework has finished calling <a
				625	href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
				626	program being compiled.</p>
				627
				628	</div>
				629
				630	<!-- ======================================================================= -->
				631	<div class="doc_subsection">
				632	<a name="FunctionPass">The <tt>FunctionPass</tt> class</a>
				633	</div>
				634
				635	<div class="doc_text">
				636
				637	<p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a
				638	href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
				639	subclasses do have a predictable, local behavior that can be expected by the
				640	system. All <tt>FunctionPass</tt> execute on each function in the program
				641	independent of all of the other functions in the program.
				642	<tt>FunctionPass</tt>'s do not require that they are executed in a particular
				643	order, and <tt>FunctionPass</tt>'s do not modify external functions.</p>
				644
				645	<p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p>
				646
				647	<ol>
				648	<li>Modify a Function other than the one currently being processed.</li>
				649	<li>Add or remove Function's from the current Module.</li>
				650	<li>Add or remove global variables from the current Module.</li>
				651	<li>Maintain state across invocations of
				652	<a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li>
				653	</ol>
				654
				655	<p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a
				656	href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s
				657	may overload three virtual methods to do their work. All of these methods
				658	should return true if they modified the program, or false if they didn't.</p>
				659
				660	</div>
				661
				662	<!-- _______________________________________________________________________ -->
				663	<div class="doc_subsubsection">
				664	<a name="doInitialization_mod">The <tt>doInitialization(Module &)</tt>
				665	method</a>
				666	</div>
				667
				668	<div class="doc_text">
				669
				670	<div class="doc_code"><pre>
				671	<b>virtual bool</b> doInitialization(Module &M);
				672	</pre></div>
				673
				674	<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
				675	<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove
				676	functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
				677	is designed to do simple initialization type of stuff that does not depend on
				678	the functions being processed. The <tt>doInitialization</tt> method call is not
				679	scheduled to overlap with any other pass executions (thus it should be very
				680	fast).</p>
				681
				682	<p>A good example of how this method should be used is the <a
				683	href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
				684	pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
				685	platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It
				686	uses the <tt>doInitialization</tt> method to get a reference to the malloc and
				687	free functions that it needs, adding prototypes to the module if necessary.</p>
				688
				689	</div>
				690
				691	<!-- _______________________________________________________________________ -->
				692	<div class="doc_subsubsection">
				693	<a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
				694	</div>
				695
				696	<div class="doc_text">
				697
				698	<div class="doc_code"><pre>
				699	<b>virtual bool</b> runOnFunction(Function &F) = 0;
				700	</pre></div><p>
				701
				702	<p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do
				703	the transformation or analysis work of your pass. As usual, a true value should
				704	be returned if the function is modified.</p>
				705
				706	</div>
				707
				708	<!-- _______________________________________________________________________ -->
				709	<div class="doc_subsubsection">
				710	<a name="doFinalization_mod">The <tt>doFinalization(Module
				711	&)</tt> method</a>
				712	</div>
				713
				714	<div class="doc_text">
				715
				716	<div class="doc_code"><pre>
				717	<b>virtual bool</b> doFinalization(Module &M);
				718	</pre></div>
				719
				720	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				721	called when the pass framework has finished calling <a
				722	href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
				723	program being compiled.</p>
				724
				725	</div>
				726
				727	<!-- ======================================================================= -->
				728	<div class="doc_subsection">
				729	<a name="LoopPass">The <tt>LoopPass</tt> class </a>
				730	</div>
				731
				732	<div class="doc_text">
				733
				734	<p> All <tt>LoopPass</tt> execute on each loop in the function independent of
				735	all of the other loops in the function. <tt>LoopPass</tt> processes loops in
				736	loop nest order such that outer most loop is processed last. </p>
				737
				738	<p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using
				739	<tt>LPPassManager</tt> interface. Implementing a loop pass is usually
				740	straightforward. <tt>Looppass</tt>'s may overload three virtual methods to
				741	do their work. All these methods should return true if they modified the
				742	program, or false if they didn't. </p>
				743	</div>
				744
				745	<!-- _______________________________________________________________________ -->
				746	<div class="doc_subsubsection">
				747	<a name="doInitialization_loop">The <tt>doInitialization(Loop *,
				748	LPPassManager &)</tt>
				749	method</a>
				750	</div>
				751
				752	<div class="doc_text">
				753
				754	<div class="doc_code"><pre>
				755	<b>virtual bool</b> doInitialization(Loop *, LPPassManager &LPM);
				756	</pre></div>
				757
				758	<p>The <tt>doInitialization</tt> method is designed to do simple initialization
				759	type of stuff that does not depend on the functions being processed. The
				760	<tt>doInitialization</tt> method call is not scheduled to overlap with any
				761	other pass executions (thus it should be very fast). LPPassManager
				762	interface should be used to access Function or Module level analysis
				763	information.</p>
				764
				765	</div>
				766
				767
				768	<!-- _______________________________________________________________________ -->
				769	<div class="doc_subsubsection">
				770	<a name="runOnLoop">The <tt>runOnLoop</tt> method</a>
				771	</div>
				772
				773	<div class="doc_text">
				774
				775	<div class="doc_code"><pre>
				776	<b>virtual bool</b> runOnLoop(Loop *, LPPassManager &LPM) = 0;
				777	</pre></div><p>
				778
				779	<p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do
				780	the transformation or analysis work of your pass. As usual, a true value should
				781	be returned if the function is modified. <tt>LPPassManager</tt> interface
				782	should be used to update loop nest.</p>
				783
				784	</div>
				785
				786	<!-- _______________________________________________________________________ -->
				787	<div class="doc_subsubsection">
				788	<a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a>
				789	</div>
				790
				791	<div class="doc_text">
				792
				793	<div class="doc_code"><pre>
				794	<b>virtual bool</b> doFinalization();
				795	</pre></div>
				796
				797	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				798	called when the pass framework has finished calling <a
				799	href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the
				800	program being compiled. </p>
				801
				802	</div>
				803
				804
				805
				806	<!-- ======================================================================= -->
				807	<div class="doc_subsection">
				808	<a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
				809	</div>
				810
				811	<div class="doc_text">
				812
				813	<p><tt>BasicBlockPass</tt>'s are just like <a
				814	href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
				815	their scope of inspection and modification to a single basic block at a time.
				816	As such, they are <b>not</b> allowed to do any of the following:</p>
				817
				818	<ol>
				819	<li>Modify or inspect any basic blocks outside of the current one</li>
				820	<li>Maintain state across invocations of
				821	<a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li>
				822	<li>Modify the control flow graph (by altering terminator instructions)</li>
				823	<li>Any of the things forbidden for
				824	<a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li>
				825	</ol>
				826
				827	<p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole"
				828	optimizations. They may override the same <a
				829	href="#doInitialization_mod"><tt>doInitialization(Module &)</tt></a> and <a
				830	href="#doFinalization_mod"><tt>doFinalization(Module &)</tt></a> methods that <a
				831	href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p>
				832
				833	</div>
				834
				835	<!-- _______________________________________________________________________ -->
				836	<div class="doc_subsubsection">
				837	<a name="doInitialization_fn">The <tt>doInitialization(Function
				838	&)</tt> method</a>
				839	</div>
				840
				841	<div class="doc_text">
				842
				843	<div class="doc_code"><pre>
				844	<b>virtual bool</b> doInitialization(Function &F);
				845	</pre></div>
				846
				847	<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
				848	<tt>BasicBlockPass</tt>'s are not allowed to do, but that
				849	<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed
				850	to do simple initialization that does not depend on the
				851	BasicBlocks being processed. The <tt>doInitialization</tt> method call is not
				852	scheduled to overlap with any other pass executions (thus it should be very
				853	fast).</p>
				854
				855	</div>
				856
				857	<!-- _______________________________________________________________________ -->
				858	<div class="doc_subsubsection">
				859	<a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
				860	</div>
				861
				862	<div class="doc_text">
				863
				864	<div class="doc_code"><pre>
				865	<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0;
				866	</pre></div>
				867
				868	<p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This
				869	function is not allowed to inspect or modify basic blocks other than the
				870	parameter, and are not allowed to modify the CFG. A true value must be returned
				871	if the basic block is modified.</p>
				872
				873	</div>
				874
				875	<!-- _______________________________________________________________________ -->
				876	<div class="doc_subsubsection">
				877	<a name="doFinalization_fn">The <tt>doFinalization(Function &)</tt>
				878	method</a>
				879	</div>
				880
				881	<div class="doc_text">
				882
				883	<div class="doc_code"><pre>
				884	<b>virtual bool</b> doFinalization(Function &F);
				885	</pre></div>
				886
				887	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				888	called when the pass framework has finished calling <a
				889	href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the
				890	program being compiled. This can be used to perform per-function
				891	finalization.</p>
				892
				893	</div>
				894
				895	<!-- ======================================================================= -->
				896	<div class="doc_subsection">
				897	<a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a>
				898	</div>
				899
				900	<div class="doc_text">
				901
				902	<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that
				903	executes on the machine-dependent representation of each LLVM function in the
				904	program. A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
				905	the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
				906	<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular,
				907	<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p>
				908
				909	<ol>
				910	<li>Modify any LLVM Instructions, BasicBlocks or Functions.</li>
				911	<li>Modify a MachineFunction other than the one currently being processed.</li>
				912	<li>Add or remove MachineFunctions from the current Module.</li>
				913	<li>Add or remove global variables from the current Module.</li>
				914	<li>Maintain state across invocations of <a
				915	href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global
				916	data)</li>
				917	</ol>
				918
				919	</div>
				920
				921	<!-- _______________________________________________________________________ -->
				922	<div class="doc_subsubsection">
				923	<a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
				924	&MF)</tt> method</a>
				925	</div>
				926
				927	<div class="doc_text">
				928
				929	<div class="doc_code"><pre>
				930	<b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0;
				931	</pre></div>
				932
				933	<p><tt>runOnMachineFunction</tt> can be considered the main entry point of a
				934	<tt>MachineFunctionPass</tt>; that is, you should override this method to do the
				935	work of your <tt>MachineFunctionPass</tt>.</p>
				936
				937	<p>The <tt>runOnMachineFunction</tt> method is called on every
				938	<tt>MachineFunction</tt> in a <tt>Module</tt>, so that the
				939	<tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent
				940	representation of the function. If you want to get at the LLVM <tt>Function</tt>
				941	for the <tt>MachineFunction</tt> you're working on, use
				942	<tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but
				943	remember, you may not modify the LLVM <tt>Function</tt> or its contents from a
				944	<tt>MachineFunctionPass</tt>.</p>
				945
				946	</div>
				947
				948	<!-- *********************************************************************** -->
				949	<div class="doc_section">
				950	<a name="registration">Pass registration</a>
				951	</div>
				952	<!-- *********************************************************************** -->
				953
				954	<div class="doc_text">
				955
				956	<p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how
				957	pass registration works, and discussed some of the reasons that it is used and
				958	what it does. Here we discuss how and why passes are registered.</p>
				959
				960	<p>As we saw above, passes are registered with the <b><tt>RegisterPass</tt></b>
				961	template, which requires you to pass at least two
				962	parameters. The first parameter is the name of the pass that is to be used on
				963	the command line to specify that the pass should be added to a program (for
				964	example, with <tt>opt</tt> or <tt>bugpoint</tt>). The second argument is the
				965	name of the pass, which is to be used for the <tt>--help</tt> output of
				966	programs, as
				967	well as for debug output generated by the <tt>--debug-pass</tt> option.</p>
				968
				969	<p>If you want your pass to be easily dumpable, you should
				970	implement the virtual <tt>print</tt> method:</p>
				971
				972	</div>
				973
				974	<!-- _______________________________________________________________________ -->
				975	<div class="doc_subsubsection">
				976	<a name="print">The <tt>print</tt> method</a>
				977	</div>
				978
				979	<div class="doc_text">
				980
				981	<div class="doc_code"><pre>
Edwin Török	72a8fd2	2008-10-28 17:29:23 +0000	[diff] [blame]	982	<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]	983	</pre></div>
				984
				985	<p>The <tt>print</tt> method must be implemented by "analyses" in order to print
				986	a human readable version of the analysis results. This is useful for debugging
				987	an analysis itself, as well as for other people to figure out how an analysis
				988	works. Use the <tt>opt -analyze</tt> argument to invoke this method.</p>
				989
				990	<p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on,
				991	and the <tt>Module</tt> parameter gives a pointer to the top level module of the
				992	program that has been analyzed. Note however that this pointer may be null in
				993	certain circumstances (such as calling the <tt>Pass::dump()</tt> from a
				994	debugger), so it should only be used to enhance debug output, it should not be
				995	depended on.</p>
				996
				997	</div>
				998
				999	<!-- *********************************************************************** -->
				1000	<div class="doc_section">
				1001	<a name="interaction">Specifying interactions between passes</a>
				1002	</div>
				1003	<!-- *********************************************************************** -->
				1004
				1005	<div class="doc_text">
				1006
John Criswell	a99e43f	2007-12-03 19:34:25 +0000	[diff] [blame]	1007	<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]	1008	that passes interact with each other correctly. Because <tt>PassManager</tt>
				1009	tries to <a href="#passmanager">optimize the execution of passes</a> it must
				1010	know how the passes interact with each other and what dependencies exist between
				1011	the various passes. To track this, each pass can declare the set of passes that
				1012	are required to be executed before the current pass, and the passes which are
				1013	invalidated by the current pass.</p>
				1014
				1015	<p>Typically this functionality is used to require that analysis results are
				1016	computed before your pass is run. Running arbitrary transformation passes can
				1017	invalidate the computed analysis results, which is what the invalidation set
				1018	specifies. If a pass does not implement the <tt><a
				1019	href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not
				1020	having any prerequisite passes, and invalidating <b>all</b> other passes.</p>
				1021
				1022	</div>
				1023
				1024	<!-- _______________________________________________________________________ -->
				1025	<div class="doc_subsubsection">
				1026	<a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
				1027	</div>
				1028
				1029	<div class="doc_text">
				1030
				1031	<div class="doc_code"><pre>
				1032	<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>;
				1033	</pre></div>
				1034
				1035	<p>By implementing the <tt>getAnalysisUsage</tt> method, the required and
				1036	invalidated sets may be specified for your transformation. The implementation
				1037	should fill in the <tt><a
				1038	href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
				1039	object with information about which passes are required and not invalidated. To
				1040	do this, a pass may call any of the following methods on the AnalysisUsage
				1041	object:</p>
				1042	</div>
				1043
				1044	<!-- _______________________________________________________________________ -->
				1045	<div class="doc_subsubsection">
				1046	<a name="AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a>
				1047	</div>
				1048
				1049	<div class="doc_text">
				1050	<p>
				1051	If your pass requires a previous pass to be executed (an analysis for example),
				1052	it can use one of these methods to arrange for it to be run before your pass.
				1053	LLVM has many different types of analyses and passes that can be required,
				1054	spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>.
				1055	Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will
				1056	be no critical edges in the CFG when your pass has been run.
				1057	</p>
				1058
				1059	<p>
				1060	Some analyses chain to other analyses to do their job. For example, an <a
				1061	href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a
				1062	href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In
				1063	cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be
				1064	used instead of the <tt>addRequired</tt> method. This informs the PassManager
				1065	that the transitively required pass should be alive as long as the requiring
				1066	pass is.
				1067	</p>
				1068	</div>
				1069
				1070	<!-- _______________________________________________________________________ -->
				1071	<div class="doc_subsubsection">
				1072	<a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a>
				1073	</div>
				1074
				1075	<div class="doc_text">
				1076	<p>
				1077	One of the jobs of the PassManager is to optimize how and when analyses are run.
				1078	In particular, it attempts to avoid recomputing data unless it needs to. For
				1079	this reason, passes are allowed to declare that they preserve (i.e., they don't
				1080	invalidate) an existing analysis if it's available. For example, a simple
				1081	constant folding pass would not modify the CFG, so it can't possibly affect the
				1082	results of dominator analysis. By default, all passes are assumed to invalidate
				1083	all others.
				1084	</p>
				1085
				1086	<p>
				1087	The <tt>AnalysisUsage</tt> class provides several methods which are useful in
				1088	certain circumstances that are related to <tt>addPreserved</tt>. In particular,
				1089	the <tt>setPreservesAll</tt> method can be called to indicate that the pass does
				1090	not modify the LLVM program at all (which is true for analyses), and the
				1091	<tt>setPreservesCFG</tt> method can be used by transformations that change
				1092	instructions in the program but do not modify the CFG or terminator instructions
				1093	(note that this property is implicitly set for <a
				1094	href="#BasicBlockPass">BasicBlockPass</a>'s).
				1095	</p>
				1096
				1097	<p>
				1098	<tt>addPreserved</tt> is particularly useful for transformations like
				1099	<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop
				1100	and dominator related analyses if they exist, so it can preserve them, despite
				1101	the fact that it hacks on the CFG.
				1102	</p>
				1103	</div>
				1104
				1105	<!-- _______________________________________________________________________ -->
				1106	<div class="doc_subsubsection">
				1107	<a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a>
				1108	</div>
				1109
				1110	<div class="doc_text">
				1111
				1112	<div class="doc_code"><pre>
				1113	<i>// This is an example implementation from an analysis, which does not modify
				1114	// the program at all, yet has a prerequisite.</i>
				1115	<b>void</b> <a href="http://llvm.org/doxygen/classllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1116	AU.setPreservesAll();
				1117	AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1PostDominatorTree.html">PostDominatorTree</a>>();
				1118	}
				1119	</pre></div>
				1120
				1121	<p>and:</p>
				1122
				1123	<div class="doc_code"><pre>
				1124	<i>// This example modifies the program, but does not modify the CFG</i>
				1125	<b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1126	AU.setPreservesCFG();
				1127	AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>();
				1128	}
				1129	</pre></div>
				1130
				1131	</div>
				1132
				1133	<!-- _______________________________________________________________________ -->
				1134	<div class="doc_subsubsection">
Duncan Sands	4e0d6a7	2009-01-28 13:14:17 +0000	[diff] [blame^]	1135	<a name="getAnalysis">The <tt>getAnalysis<></tt> and
				1136	<tt>getAnalysisIfAvailable<></tt> methods</a>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1137	</div>
				1138
				1139	<div class="doc_text">
				1140
				1141	<p>The <tt>Pass::getAnalysis<></tt> method is automatically inherited by
				1142	your class, providing you with access to the passes that you declared that you
				1143	required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
				1144	method. It takes a single template argument that specifies which pass class you
				1145	want, and returns a reference to that pass. For example:</p>
				1146
				1147	<div class="doc_code"><pre>
				1148	bool LICM::runOnFunction(Function &F) {
				1149	LoopInfo &LI = getAnalysis<LoopInfo>();
				1150	...
				1151	}
				1152	</pre></div>
				1153
				1154	<p>This method call returns a reference to the pass desired. You may get a
				1155	runtime assertion failure if you attempt to get an analysis that you did not
				1156	declare as required in your <a
				1157	href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This
				1158	method can be called by your <tt>run*</tt> method implementation, or by any
				1159	other local method invoked by your <tt>run*</tt> method.
				1160
				1161	A module level pass can use function level analysis info using this interface.
				1162	For example:</p>
				1163
				1164	<div class="doc_code"><pre>
				1165	bool ModuleLevelPass::runOnModule(Module &M) {
				1166	...
				1167	DominatorTree &DT = getAnalysis<DominatorTree>(Func);
				1168	...
				1169	}
				1170	</pre></div>
				1171
				1172	<p>In above example, runOnFunction for DominatorTree is called by pass manager
				1173	before returning a reference to the desired pass.</p>
				1174
				1175	<p>
				1176	If your pass is capable of updating analyses if they exist (e.g.,
				1177	<tt>BreakCriticalEdges</tt>, as described above), you can use the
Duncan Sands	4e0d6a7	2009-01-28 13:14:17 +0000	[diff] [blame^]	1178	<tt>getAnalysisIfAvailable</tt> method, which returns a pointer to the analysis
				1179	if it is active. For example:</p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1180
				1181	<div class="doc_code"><pre>
				1182	...
Duncan Sands	4e0d6a7	2009-01-28 13:14:17 +0000	[diff] [blame^]	1183	if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) {
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1184	<i>// A DominatorSet is active. This code will update it.</i>
				1185	}
				1186	...
				1187	</pre></div>
				1188
				1189	</div>
				1190
				1191	<!-- *********************************************************************** -->
				1192	<div class="doc_section">
				1193	<a name="analysisgroup">Implementing Analysis Groups</a>
				1194	</div>
				1195	<!-- *********************************************************************** -->
				1196
				1197	<div class="doc_text">
				1198
Chris Lattner	942c395	2007-11-16 05:32:05 +0000	[diff] [blame]	1199	<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]	1200	used, and how they are required from other passes, it's time to get a little bit
				1201	fancier. All of the pass relationships that we have seen so far are very
				1202	simple: one pass depends on one other specific pass to be run before it can run.
				1203	For many applications, this is great, for others, more flexibility is
				1204	required.</p>
				1205
				1206	<p>In particular, some analyses are defined such that there is a single simple
				1207	interface to the analysis results, but multiple ways of calculating them.
				1208	Consider alias analysis for example. The most trivial alias analysis returns
				1209	"may alias" for any alias query. The most sophisticated analysis a
				1210	flow-sensitive, context-sensitive interprocedural analysis that can take a
				1211	significant amount of time to execute (and obviously, there is a lot of room
				1212	between these two extremes for other implementations). To cleanly support
				1213	situations like this, the LLVM Pass Infrastructure supports the notion of
				1214	Analysis Groups.</p>
				1215
				1216	</div>
				1217
				1218	<!-- _______________________________________________________________________ -->
				1219	<div class="doc_subsubsection">
				1220	<a name="agconcepts">Analysis Group Concepts</a>
				1221	</div>
				1222
				1223	<div class="doc_text">
				1224
				1225	<p>An Analysis Group is a single simple interface that may be implemented by
				1226	multiple different passes. Analysis Groups can be given human readable names
				1227	just like passes, but unlike passes, they need not derive from the <tt>Pass</tt>
				1228	class. An analysis group may have one or more implementations, one of which is
				1229	the "default" implementation.</p>
				1230
				1231	<p>Analysis groups are used by client passes just like other passes are: the
				1232	<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods.
				1233	In order to resolve this requirement, the <a href="#passmanager">PassManager</a>
				1234	scans the available passes to see if any implementations of the analysis group
				1235	are available. If none is available, the default implementation is created for
				1236	the pass to use. All standard rules for <A href="#interaction">interaction
				1237	between passes</a> still apply.</p>
				1238
				1239	<p>Although <a href="#registration">Pass Registration</a> is optional for normal
				1240	passes, all analysis group implementations must be registered, and must use the
				1241	<A href="#registerag"><tt>RegisterAnalysisGroup</tt></a> template to join the
				1242	implementation pool. Also, a default implementation of the interface
				1243	<b>must</b> be registered with <A
				1244	href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p>
				1245
				1246	<p>As a concrete example of an Analysis Group in action, consider the <a
				1247	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
				1248	analysis group. The default implementation of the alias analysis interface (the
				1249	<tt><a
				1250	href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
				1251	pass) just does a few simple checks that don't require significant analysis to
				1252	compute (such as: two different globals can never alias each other, etc).
				1253	Passes that use the <tt><a
				1254	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
				1255	interface (for example the <tt><a
				1256	href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do
				1257	not care which implementation of alias analysis is actually provided, they just
				1258	use the designated interface.</p>
				1259
				1260	<p>From the user's perspective, commands work just like normal. Issuing the
				1261	command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be
				1262	instantiated and added to the pass sequence. Issuing the command '<tt>opt
				1263	-somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the
				1264	<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a
				1265	hypothetical example) instead.</p>
				1266
				1267	</div>
				1268
				1269	<!-- _______________________________________________________________________ -->
				1270	<div class="doc_subsubsection">
				1271	<a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
				1272	</div>
				1273
				1274	<div class="doc_text">
				1275
				1276	<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
				1277	group itself as well as add pass implementations to the analysis group. First,
				1278	an analysis should be registered, with a human readable name provided for it.
				1279	Unlike registration of passes, there is no command line argument to be specified
				1280	for the Analysis Group Interface itself, because it is "abstract":</p>
				1281
				1282	<div class="doc_code"><pre>
				1283	<b>static</b> RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>");
				1284	</pre></div>
				1285
				1286	<p>Once the analysis is registered, passes can declare that they are valid
				1287	implementations of the interface by using the following code:</p>
				1288
				1289	<div class="doc_code"><pre>
				1290	<b>namespace</b> {
				1291	//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
				1292	RegisterPass<FancyAA>
				1293	B("<i>somefancyaa</i>", "<i>A more complex alias analysis implementation</i>");
				1294
				1295	//<i> Declare that we implement the AliasAnalysis interface</i>
				1296	RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> C(B);
				1297	}
				1298	</pre></div>
				1299
				1300	<p>This just shows a class <tt>FancyAA</tt> that is registered normally, then
				1301	uses the <tt>RegisterAnalysisGroup</tt> template to "join" the <tt><a
				1302	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
				1303	analysis group. Every implementation of an analysis group should join using
				1304	this template. A single pass may join multiple different analysis groups with
				1305	no problem.</p>
				1306
				1307	<div class="doc_code"><pre>
				1308	<b>namespace</b> {
				1309	//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
				1310	RegisterPass<<a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>>
				1311	D("<i>basicaa</i>", "<i>Basic Alias Analysis (default AA impl)</i>");
				1312
				1313	//<i> Declare that we implement the AliasAnalysis interface</i>
				1314	RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, <b>true</b>> E(D);
				1315	}
				1316	</pre></div>
				1317
				1318	<p>Here we show how the default implementation is specified (using the extra
				1319	argument to the <tt>RegisterAnalysisGroup</tt> template). There must be exactly
				1320	one default implementation available at all times for an Analysis Group to be
				1321	used. Only default implementation can derive from <tt>ImmutablePass</tt>.
				1322	Here we declare that the
				1323	<tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
				1324	pass is the default implementation for the interface.</p>
				1325
				1326	</div>
				1327
				1328	<!-- *********************************************************************** -->
				1329	<div class="doc_section">
				1330	<a name="passStatistics">Pass Statistics</a>
				1331	</div>
				1332	<!-- *********************************************************************** -->
				1333
				1334	<div class="doc_text">
				1335	<p>The <a
				1336	href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a>
				1337	class is designed to be an easy way to expose various success
				1338	metrics from passes. These statistics are printed at the end of a
				1339	run, when the -stats command line option is enabled on the command
				1340	line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details.
				1341
				1342	</div>
				1343
				1344
				1345	<!-- *********************************************************************** -->
				1346	<div class="doc_section">
				1347	<a name="passmanager">What PassManager does</a>
				1348	</div>
				1349	<!-- *********************************************************************** -->
				1350
				1351	<div class="doc_text">
				1352
				1353	<p>The <a
				1354	href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
				1355	<a
				1356	href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a>
				1357	takes a list of passes, ensures their <a href="#interaction">prerequisites</a>
				1358	are set up correctly, and then schedules passes to run efficiently. All of the
				1359	LLVM tools that run passes use the <tt>PassManager</tt> for execution of these
				1360	passes.</p>
				1361
				1362	<p>The <tt>PassManager</tt> does two main things to try to reduce the execution
				1363	time of a series of passes:</p>
				1364
				1365	<ol>
				1366	<li><b>Share analysis results</b> - The PassManager attempts to avoid
				1367	recomputing analysis results as much as possible. This means keeping track of
				1368	which analyses are available already, which analyses get invalidated, and which
				1369	analyses are needed to be run for a pass. An important part of work is that the
				1370	<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing
				1371	it to <a href="#releaseMemory">free memory</a> allocated to holding analysis
				1372	results as soon as they are no longer needed.</li>
				1373
				1374	<li><b>Pipeline the execution of passes on the program</b> - The
				1375	<tt>PassManager</tt> attempts to get better cache and memory usage behavior out
				1376	of a series of passes by pipelining the passes together. This means that, given
				1377	a series of consequtive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
				1378	will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on
				1379	the first function, then all of the <a
				1380	href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function,
				1381	etc... until the entire program has been run through the passes.
				1382
				1383	<p>This improves the cache behavior of the compiler, because it is only touching
				1384	the LLVM program representation for a single function at a time, instead of
				1385	traversing the entire program. It reduces the memory consumption of compiler,
				1386	because, for example, only one <a
				1387	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]	1388	needs to be calculated at a time. This also makes it possible to implement
				1389	some <a
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1390	href="#SMP">interesting enhancements</a> in the future.</p></li>
				1391
				1392	</ol>
				1393
				1394	<p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how
				1395	much information it has about the behaviors of the passes it is scheduling. For
				1396	example, the "preserved" set is intentionally conservative in the face of an
				1397	unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method.
				1398	Not implementing when it should be implemented will have the effect of not
				1399	allowing any analysis results to live across the execution of your pass.</p>
				1400
				1401	<p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line
				1402	options that is useful for debugging pass execution, seeing how things work, and
				1403	diagnosing when you should be preserving more analyses than you currently are
				1404	(To get information about all of the variants of the <tt>--debug-pass</tt>
				1405	option, just type '<tt>opt --help-hidden</tt>').</p>
				1406
				1407	<p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see
				1408	how our <a href="#basiccode">Hello World</a> pass interacts with other passes.
				1409	Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p>
				1410
				1411	<div class="doc_code"><pre>
				1412	$ opt -load ../../../Debug/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null
				1413	Module Pass Manager
				1414	Function Pass Manager
				1415	Dominator Set Construction
				1416	Immediate Dominators Construction
				1417	Global Common Subexpression Elimination
				1418	-- Immediate Dominators Construction
				1419	-- Global Common Subexpression Elimination
				1420	Natural Loop Construction
				1421	Loop Invariant Code Motion
				1422	-- Natural Loop Construction
				1423	-- Loop Invariant Code Motion
				1424	Module Verifier
				1425	-- Dominator Set Construction
				1426	-- Module Verifier
				1427	Bitcode Writer
				1428	--Bitcode Writer
				1429	</pre></div>
				1430
				1431	<p>This output shows us when passes are constructed and when the analysis
				1432	results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that
				1433	GCSE uses dominator and immediate dominator information to do its job. The LICM
				1434	pass uses natural loop information, which uses dominator sets, but not immediate
				1435	dominators. Because immediate dominators are no longer useful after the GCSE
				1436	pass, it is immediately destroyed. The dominator sets are then reused to
				1437	compute natural loop information, which is then used by the LICM pass.</p>
				1438
				1439	<p>After the LICM pass, the module verifier runs (which is automatically added
				1440	by the '<tt>opt</tt>' tool), which uses the dominator set to check that the
				1441	resultant LLVM code is well formed. After it finishes, the dominator set
				1442	information is destroyed, after being computed once, and shared by three
				1443	passes.</p>
				1444
				1445	<p>Lets see how this changes when we run the <a href="#basiccode">Hello
				1446	World</a> pass in between the two passes:</p>
				1447
				1448	<div class="doc_code"><pre>
				1449	$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
				1450	Module Pass Manager
				1451	Function Pass Manager
				1452	Dominator Set Construction
				1453	Immediate Dominators Construction
				1454	Global Common Subexpression Elimination
				1455	<b>-- Dominator Set Construction</b>
				1456	-- Immediate Dominators Construction
				1457	-- Global Common Subexpression Elimination
				1458	<b> Hello World Pass
				1459	-- Hello World Pass
				1460	Dominator Set Construction</b>
				1461	Natural Loop Construction
				1462	Loop Invariant Code Motion
				1463	-- Natural Loop Construction
				1464	-- Loop Invariant Code Motion
				1465	Module Verifier
				1466	-- Dominator Set Construction
				1467	-- Module Verifier
				1468	Bitcode Writer
				1469	--Bitcode Writer
				1470	Hello: __main
				1471	Hello: puts
				1472	Hello: main
				1473	</pre></div>
				1474
				1475	<p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
				1476	Dominator Set pass, even though it doesn't modify the code at all! To fix this,
				1477	we need to add the following <a
				1478	href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p>
				1479
				1480	<div class="doc_code"><pre>
				1481	<i>// We don't modify the program, so we preserve all analyses</i>
				1482	<b>virtual void</b> getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1483	AU.setPreservesAll();
				1484	}
				1485	</pre></div>
				1486
				1487	<p>Now when we run our pass, we get this output:</p>
				1488
				1489	<div class="doc_code"><pre>
				1490	$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
				1491	Pass Arguments: -gcse -hello -licm
				1492	Module Pass Manager
				1493	Function Pass Manager
				1494	Dominator Set Construction
				1495	Immediate Dominators Construction
				1496	Global Common Subexpression Elimination
				1497	-- Immediate Dominators Construction
				1498	-- Global Common Subexpression Elimination
				1499	Hello World Pass
				1500	-- Hello World Pass
				1501	Natural Loop Construction
				1502	Loop Invariant Code Motion
				1503	-- Loop Invariant Code Motion
				1504	-- Natural Loop Construction
				1505	Module Verifier
				1506	-- Dominator Set Construction
				1507	-- Module Verifier
				1508	Bitcode Writer
				1509	--Bitcode Writer
				1510	Hello: __main
				1511	Hello: puts
				1512	Hello: main
				1513	</pre></div>
				1514
				1515	<p>Which shows that we don't accidentally invalidate dominator information
				1516	anymore, and therefore do not have to compute it twice.</p>
				1517
				1518	</div>
				1519
				1520	<!-- _______________________________________________________________________ -->
				1521	<div class="doc_subsubsection">
				1522	<a name="releaseMemory">The <tt>releaseMemory</tt> method</a>
				1523	</div>
				1524
				1525	<div class="doc_text">
				1526
				1527	<div class="doc_code"><pre>
				1528	<b>virtual void</b> releaseMemory();
				1529	</pre></div>
				1530
				1531	<p>The <tt>PassManager</tt> automatically determines when to compute analysis
				1532	results, and how long to keep them around for. Because the lifetime of the pass
				1533	object itself is effectively the entire duration of the compilation process, we
				1534	need some way to free analysis results when they are no longer useful. The
				1535	<tt>releaseMemory</tt> virtual method is the way to do this.</p>
				1536
				1537	<p>If you are writing an analysis or any other pass that retains a significant
				1538	amount of state (for use by another pass which "requires" your pass and uses the
				1539	<a href="#getAnalysis">getAnalysis</a> method) you should implement
				1540	<tt>releaseMEmory</tt> to, well, release the memory allocated to maintain this
				1541	internal state. This method is called after the <tt>run*</tt> method for the
				1542	class, before the next call of <tt>run*</tt> in your pass.</p>
				1543
				1544	</div>
				1545
				1546	<!-- *********************************************************************** -->
				1547	<div class="doc_section">
				1548	<a name="registering">Registering dynamically loaded passes</a>
				1549	</div>
				1550	<!-- *********************************************************************** -->
				1551
				1552	<div class="doc_text">
				1553
				1554	<p><i>Size matters</i> when constructing production quality tools using llvm,
				1555	both for the purposes of distribution, and for regulating the resident code size
				1556	when running on the target system. Therefore, it becomes desirable to
				1557	selectively use some passes, while omitting others and maintain the flexibility
				1558	to change configurations later on. You want to be able to do all this, and,
				1559	provide feedback to the user. This is where pass registration comes into
				1560	play.</p>
				1561
				1562	<p>The fundamental mechanisms for pass registration are the
				1563	<tt>MachinePassRegistry</tt> class and subclasses of
				1564	<tt>MachinePassRegistryNode</tt>.</p>
				1565
				1566	<p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of
				1567	<tt>MachinePassRegistryNode</tt> objects. This instance maintains the list and
				1568	communicates additions and deletions to the command line interface.</p>
				1569
				1570	<p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain
				1571	information provided about a particular pass. This information includes the
				1572	command line name, the command help string and the address of the function used
				1573	to create an instance of the pass. A global static constructor of one of these
				1574	instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>,
				1575	the static destructor <i>unregisters</i>. Thus a pass that is statically linked
				1576	in the tool will be registered at start up. A dynamically loaded pass will
				1577	register on load and unregister at unload.</p>
				1578
				1579	</div>
				1580
				1581	<!-- _______________________________________________________________________ -->
				1582	<div class="doc_subsection">
				1583	<a name="registering_existing">Using existing registries</a>
				1584	</div>
				1585
				1586	<div class="doc_text">
				1587
				1588	<p>There are predefined registries to track instruction scheduling
				1589	(<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>)
				1590	machine passes. Here we will describe how to <i>register</i> a register
				1591	allocator machine pass.</p>
				1592
				1593	<p>Implement your register allocator machine pass. In your register allocator
				1594	.cpp file add the following include;</p>
				1595
				1596	<div class="doc_code"><pre>
				1597	#include "llvm/CodeGen/RegAllocRegistry.h"
				1598	</pre></div>
				1599
				1600	<p>Also in your register allocator .cpp file, define a creator function in the
				1601	form; </p>
				1602
				1603	<div class="doc_code"><pre>
				1604	FunctionPass *createMyRegisterAllocator() {
				1605	return new MyRegisterAllocator();
				1606	}
				1607	</pre></div>
				1608
				1609	<p>Note that the signature of this function should match the type of
				1610	<tt>RegisterRegAlloc::FunctionPassCtor</tt>. In the same file add the
				1611	"installing" declaration, in the form;</p>
				1612
				1613	<div class="doc_code"><pre>
				1614	static RegisterRegAlloc myRegAlloc("myregalloc",
				1615	" my register allocator help string",
				1616	createMyRegisterAllocator);
				1617	</pre></div>
				1618
				1619	<p>Note the two spaces prior to the help string produces a tidy result on the
				1620	--help query.</p>
				1621
				1622	<div class="doc_code"><pre>
				1623	$ llc --help
				1624	...
				1625	-regalloc - Register allocator to use: (default = linearscan)
				1626	=linearscan - linear scan register allocator
				1627	=local - local register allocator
				1628	=simple - simple register allocator
				1629	=myregalloc - my register allocator help string
				1630	...
				1631	</pre></div>
				1632
				1633	<p>And that's it. The user is now free to use <tt>-regalloc=myregalloc</tt> as
				1634	an option. Registering instruction schedulers is similar except use the
				1635	<tt>RegisterScheduler</tt> class. Note that the
				1636	<tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from
				1637	<tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p>
				1638
				1639	<p>To force the load/linking of your register allocator into the llc/lli tools,
				1640	add your creator function's global declaration to "Passes.h" and add a "pseudo"
				1641	call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p>
				1642
				1643	</div>
				1644
				1645
				1646	<!-- _______________________________________________________________________ -->
				1647	<div class="doc_subsection">
				1648	<a name="registering_new">Creating new registries</a>
				1649	</div>
				1650
				1651	<div class="doc_text">
				1652
				1653	<p>The easiest way to get started is to clone one of the existing registries; we
				1654	recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>. The key things to modify
				1655	are the class name and the <tt>FunctionPassCtor</tt> type.</p>
				1656
				1657	<p>Then you need to declare the registry. Example: if your pass registry is
				1658	<tt>RegisterMyPasses</tt> then define;</p>
				1659
				1660	<div class="doc_code"><pre>
				1661	MachinePassRegistry RegisterMyPasses::Registry;
				1662	</pre></div>
				1663
				1664	<p>And finally, declare the command line option for your passes. Example:</p>
				1665
				1666	<div class="doc_code"><pre>
				1667	cl::opt<RegisterMyPasses::FunctionPassCtor, false,
Dan Gohman	8e58bc5	2008-10-14 17:00:38 +0000	[diff] [blame]	1668	RegisterPassParser<RegisterMyPasses> >
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1669	MyPassOpt("mypass",
				1670	cl::init(&createDefaultMyPass),
				1671	cl::desc("my pass option help"));
				1672	</pre></div>
				1673
				1674	<p>Here the command option is "mypass", with createDefaultMyPass as the default
				1675	creator.</p>
				1676
				1677	</div>
				1678
				1679	<!-- *********************************************************************** -->
				1680	<div class="doc_section">
				1681	<a name="debughints">Using GDB with dynamically loaded passes</a>
				1682	</div>
				1683	<!-- *********************************************************************** -->
				1684
				1685	<div class="doc_text">
				1686
				1687	<p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it
				1688	should be. First of all, you can't set a breakpoint in a shared object that has
				1689	not been loaded yet, and second of all there are problems with inlined functions
				1690	in shared objects. Here are some suggestions to debugging your pass with
				1691	GDB.</p>
				1692
				1693	<p>For sake of discussion, I'm going to assume that you are debugging a
				1694	transformation invoked by <tt>opt</tt>, although nothing described here depends
				1695	on that.</p>
				1696
				1697	</div>
				1698
				1699	<!-- _______________________________________________________________________ -->
				1700	<div class="doc_subsubsection">
				1701	<a name="breakpoint">Setting a breakpoint in your pass</a>
				1702	</div>
				1703
				1704	<div class="doc_text">
				1705
				1706	<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>
				1707
				1708	<div class="doc_code"><pre>
				1709	$ <b>gdb opt</b>
				1710	GNU gdb 5.0
				1711	Copyright 2000 Free Software Foundation, Inc.
				1712	GDB is free software, covered by the GNU General Public License, and you are
				1713	welcome to change it and/or distribute copies of it under certain conditions.
				1714	Type "show copying" to see the conditions.
				1715	There is absolutely no warranty for GDB. Type "show warranty" for details.
				1716	This GDB was configured as "sparc-sun-solaris2.6"...
				1717	(gdb)
				1718	</pre></div>
				1719
				1720	<p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
				1721	time to load. Be patient. Since we cannot set a breakpoint in our pass yet
				1722	(the shared object isn't loaded until runtime), we must execute the process, and
				1723	have it stop before it invokes our pass, but after it has loaded the shared
				1724	object. The most foolproof way of doing this is to set a breakpoint in
				1725	<tt>PassManager::run</tt> and then run the process with the arguments you
				1726	want:</p>
				1727
				1728	<div class="doc_code"><pre>
				1729	(gdb) <b>break llvm::PassManager::run</b>
				1730	Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
				1731	(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]</b>
				1732	Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]
				1733	Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
				1734	70 bool PassManager::run(Module &M) { return PM->run(M); }
				1735	(gdb)
				1736	</pre></div>
				1737
				1738	<p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are
				1739	now free to set breakpoints in your pass so that you can trace through execution
				1740	or do other standard debugging stuff.</p>
				1741
				1742	</div>
				1743
				1744	<!-- _______________________________________________________________________ -->
				1745	<div class="doc_subsubsection">
				1746	<a name="debugmisc">Miscellaneous Problems</a>
				1747	</div>
				1748
				1749	<div class="doc_text">
				1750
				1751	<p>Once you have the basics down, there are a couple of problems that GDB has,
				1752	some with solutions, some without.</p>
				1753
				1754	<ul>
				1755	<li>Inline functions have bogus stack information. In general, GDB does a
				1756	pretty good job getting stack traces and stepping through inline functions.
				1757	When a pass is dynamically loaded however, it somehow completely loses this
				1758	capability. The only solution I know of is to de-inline a function (move it
				1759	from the body of a class to a .cpp file).</li>
				1760
				1761	<li>Restarting the program breaks breakpoints. After following the information
				1762	above, you have succeeded in getting some breakpoints planted in your pass. Nex
				1763	thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
				1764	and you start getting errors about breakpoints being unsettable. The only way I
				1765	have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
				1766	already set in your pass, run the program, and re-set the breakpoints once
				1767	execution stops in <tt>PassManager::run</tt>.</li>
				1768
				1769	</ul>
				1770
				1771	<p>Hopefully these tips will help with common case debugging situations. If
				1772	you'd like to contribute some tips of your own, just contact <a
				1773	href="mailto:sabre@nondot.org">Chris</a>.</p>
				1774
				1775	</div>
				1776
				1777	<!-- *********************************************************************** -->
				1778	<div class="doc_section">
				1779	<a name="future">Future extensions planned</a>
				1780	</div>
				1781	<!-- *********************************************************************** -->
				1782
				1783	<div class="doc_text">
				1784
				1785	<p>Although the LLVM Pass Infrastructure is very capable as it stands, and does
				1786	some nifty stuff, there are things we'd like to add in the future. Here is
				1787	where we are going:</p>
				1788
				1789	</div>
				1790
				1791	<!-- _______________________________________________________________________ -->
				1792	<div class="doc_subsubsection">
				1793	<a name="SMP">Multithreaded LLVM</a>
				1794	</div>
				1795
				1796	<div class="doc_text">
				1797
				1798	<p>Multiple CPU machines are becoming more common and compilation can never be
				1799	fast enough: obviously we should allow for a multithreaded compiler. Because of
				1800	the semantics defined for passes above (specifically they cannot maintain state
				1801	across invocations of their <tt>run*</tt> methods), a nice clean way to
				1802	implement a multithreaded compiler would be for the <tt>PassManager</tt> class
				1803	to create multiple instances of each pass object, and allow the separate
				1804	instances to be hacking on different parts of the program at the same time.</p>
				1805
				1806	<p>This implementation would prevent each of the passes from having to implement
				1807	multithreaded constructs, requiring only the LLVM core to have locking in a few
				1808	places (for global resources). Although this is a simple extension, we simply
				1809	haven't had time (or multiprocessor machines, thus a reason) to implement this.
				1810	Despite that, we have kept the LLVM passes SMP ready, and you should too.</p>
				1811
				1812	</div>
				1813
				1814	<!-- *********************************************************************** -->
				1815	<hr>
				1816	<address>
				1817	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman	947321d	2008-12-11 17:34:48 +0000	[diff] [blame]	1818	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]	1819	<a href="http://validator.w3.org/check/referer"><img
Misha Brukman	5c1cc64	2008-12-11 18:23:24 +0000	[diff] [blame]	1820	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]	1821
				1822	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
				1823	<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
				1824	Last modified: $Date$
				1825	</address>
				1826
				1827	</body>
				1828	</html>