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